Ionic Liquids: Potential Electrolytes for Electrochemical Applications · 2019. 8. 7. · ionic liquids in electrochemical applications. The “Ionic Liquids” themed issue contains

International Journal of Electrochemistry

Guest Editors: S. Zein El Abedin, K. S. Ryder, O. Höfft, and H. K. Farag

Ionic Liquids: Potential Electrolytes for Electrochemical Applications

Ionic Liquids: Potential Electrolytes forElectrochemical Applications

International Journal of Electrochemistry


Guest Editors: S. Zein El Abedin, K. S. Ryder, O. Hofft,and H. K. Farag

Copyright © 2012 Hindawi Publishing Corporation. All rights reserved.

This is a special issue published in “International Journal of Electrochemistry.” All articles are open access articles distributed under theCreative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.

Editorial Board

Maria Carmen Arevalo, SpainShen-Ming Chen, TaiwanAbel Cesar Chialvo, ArgentinaJean-Paul Chopart, FranceA. Rodrigues de Andrade, BrazilSergio Ferro, ItalyGerd-Uwe Flechsig, GermanyRubin Gulaboski, GermanyShengshui Hu, ChinaMehran Javanbakht, IranJiye Jin, Japan

Emilia Kirowa-Eisner, IsraelBoniface Kokoh, FranceEmmanuel Maisonhaute, FranceGrzegorz Milczarek, PolandValentin Mirceski, MacedoniaMohamed Mohamedi, CanadaAngela Molina, SpainDavood Nematollahi, IranK. I. Ozoemena, South AfricaMarıa Isabel Pividori, SpainMiloslav Pravda, Ireland

Manfred Rudolph, GermanyBenjamın R. Scharifker, VenezuelaAuro Atsushi Tanaka, BrazilGermano Tremiliosi-Filho, BrazilHamilton Varela, BrazilJay D. Wadhawan, UKJose H. Zagal, ChileSheng S. Zhang, USAJiujun Zhang, CanadaXueji Zhang, USA

Contents

Ionic Liquids: Potential Electrolytes for Electrochemical Applications, S. Zein El Abedin, K. S. Ryder,O. Hofft, and H. K. FaragVolume 2012, Article ID 978060, 2 pages

Correlation between Quantumchemically Calculated LUMO Energies and the Electrochemical Windowof Ionic Liquids with Reduction-Resistant Anions, Wim Buijs, Geert-Jan Witkamp, and Maaike C. KroonVolume 2012, Article ID 589050, 6 pages

Reversible Deposition and Dissolution of Magnesium from Imidazolium-Based Ionic Liquids,QingSong Zhao, Yanna NuLi, Tuerxun Nasiman, Jun Yang, and JiuLin WangVolume 2012, Article ID 701741, 8 pages

Modification of Nafion Membranes by IL-Cation Exchange: Chemical Surface, Electrical and InterfacialStudy, V. Romero, M. V. Martınez de Yuso, A. Arango, E. Rodrıguez-Castellon, and J. BenaventeVolume 2012, Article ID 349435, 9 pages

Applications of Ionic Liquids in Electrochemical Sensors and Biosensors, Virendra V. Singh,Anil K. Nigam, Anirudh Batra, Mannan Boopathi, Beer Singh, and Rajagopalan VijayaraghavanVolume 2012, Article ID 165683, 19 pages

Electrochemical Behaviour of Actinides and Fission Products in Room-Temperature Ionic Liquids,K. A. Venkatesan, Ch. Jagadeeswara Rao, K. Nagarajan, and P. R. Vasudeva RaoVolume 2012, Article ID 841456, 12 pages

Potentiometric Multisensory Systems with Novel Ion-Exchange Polymer-Based Sensors for Analysis ofDrugs, Olga V. Bobreshova, Anna V. Parshina, and Ksenia A. PolumestnayaVolume 2012, Article ID 392735, 10 pages

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2012, Article ID 978060, 2 pagesdoi:10.1155/2012/978060

Editorial


S. Zein El Abedin,1, 2 K. S. Ryder,3 O. Hofft,2 and H. K. Farag4

1 Electrochemistry and Corrosion Laboratory, National Research Centre, Dokki, Cairo 12622, Egypt2 Institute of Particle Technology, Clausthal University of Technology, Arnold-Sommerfeld-Street 6,38678 Clausthal-Zellerfeld, Germany

3 Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK4 Inorganic Chemistry Department, National Research Centre, Dokki, Cairo 12622, Egypt

Correspondence should be addressed to S. Zein El Abedin, [email protected]

Received 12 August 2012; Accepted 12 August 2012

Copyright © 2012 S. Zein El Abedin et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

In recent years, ionic liquids (ILs) have received great interestsince they have unconventional physical properties superior,in many respects, to those of common molecular liquids.They are usually nonvolatile, nonflammable, less toxic, andare good solvents for both organics and inorganics. Theyhave extremely large electrochemical windows, up to sevenvolts and high thermal stability, up to 300–400◦C. Most ofthe liquid known hitherto have very low vapor pressure (e.g.,at or near room temperature around 10−11–10−10 mbar)which makes new experiments under vacuum conditionspossible. Low vapor pressure also reduces solvent losses inbulk processing and so ILs are the subject of much interestand speculation in respect of green chemistry. Moreover, theionic liquids are “designable” as it is possible to design ILswith the necessary properties for specific applications by aproper choice of cations and anions. In addition to theirextraordinary features, ionic liquids also typically havereasonable conductivities and viscosities contributing totheir potential as neoteric solvents for a wide variety of appli-cations including electrodeposition, electrocatalysis, elec-tropolymerization, electrochemical capacitors, sensors, sepa-rations, and organic synthesis. They are also being developedfor energy devices such as batteries, fuel cells, and solarphotoelectrochemical cells. Furthermore, they are beingexplored for possible applications in the nuclear fuel cycle.The present themed issue demonstrates the potentialities ofionic liquids in electrochemical applications.

The “Ionic Liquids” themed issue contains some inter-esting papers exploring the high potential of ionic liquids aselectrochemical solvents. As the electrochemical stability ofionic liquids is an important characteristic for their appli-cations as electrolytes, quantum chemical calculations areemployed to predict the electrochemical stability of ionicliquids. It is shown in this issue that the quantum chemicalcalculation is an excellent approach to predict the electro-chemical window of ionic liquids. A comprehensive overviewon the electrochemical applications of ionic liquids in spentfuel reprocessing and nuclear waste management is pre-sented. The current status of the research on the electro-chemical recovery of actinides and fission products usingionic liquids is systematically described.

The importance of ionic liquids for the synthesis ofconducting polymers and nanoparticles is highlighted inanother review paper with the focus on the electrochemicalsensors and biosensors based on ionicliquid-/composite-modified electrodes. The modification of a typical cation-exchange polymeric membrane by inclusion of the IL cationinto the matrix structure and its effect on chemical, thermal,and charge transport is also presented in this issue. Theionic-liquid-modified membranes are interesting for a widevariety of applications, particularly for electrochemicaldevices. The electrodeposition of magnesium from differentimidazolium-based ionic liquids is also described. Theelectrodeposition of Mg is a good example for the prospects

2 International Journal of Electrochemistry

of ionic liquids in the electrodeposition of reactive metalswhich in the past were only accessible using high temperaturemolten salts or, in part, organic solvents.

S. Zein El AbedinK. S. Ryder

O. HofftH. K. Farag


Research Article

Correlation between Quantumchemically Calculated LUMOEnergies and the Electrochemical Window of Ionic Liquids withReduction-Resistant Anions

Wim Buijs,1 Geert-Jan Witkamp,1 and Maaike C. Kroon2

1 Laboratory for Process Equipment, Department of Process & Energy, Faculty of Mechanical, Maritime and Materials Engineering,Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

2 Separation Technology Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology,Den Dolech 2, 5612 AZ, P.O. Box 513, STO 1.22, 5600 MB Eindhoven, The Netherlands

Correspondence should be addressed to Wim Buijs, [email protected]

Received 29 January 2012; Accepted 7 March 2012

Academic Editor: Sherif Zein El Abedin

Copyright © 2012 Wim Buijs et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Quantum chemical calculations showed to be an excellent method to predict the electrochemical window of ionic liquidswith reduction-resistant anions. A good correlation between the LUMO energy and the electrochemical window is observed.Surprisingly simple but very fast semiempirical calculations are in full record with density functional theory calculations and are avery attractive tool in the design and optimization of ionic liquids for specific purposes.

1. Introduction

In the last two decades ionic liquids [1–3] have receivedmuch interest for use as water-free electrolytes. They mightcombine the advantages of the conventional high-tempera-ture molten salt electrolytes and aqueous electrolytes. Ionicliquids have wide electrochemical [4–6] and temperature[1, 2] windows, high ionic conductivities [6, 7], can dissolvemost metal salts [1–3], and allow several metals conven-tionally obtained from high-temperature molten salts to bedeposited at room temperature without corrosion problems[8–11]. Moreover, they might posses lower toxicity, flam-mability, and volatility compared to conventional electrolytesystems [12]. Applications include the use of ionic liquidsas electrolytes in battery systems [13], solar cells [14], andelectrochemical capacitors [15–17]. In principle, it is possi-ble to tune the properties of ionic liquids [2]. However, task-specific design of ionic liquids is not straightforward. Re-asons for that are that the synthesis of a large variety of ionicliquids is still cumbersome, and experimental measurementson the properties of ionic liquids are relatively scarce. Mole-cular modeling can be a useful tool to establish both qual-itative and quantitative relations between the properties ofionic liquids and their structure [18].

In this work quantum chemical calculations are usedto predict the electrochemical stability of ionic liquids withreduction-resistant anions. The electrochemical window [1]of such ionic liquids depends primarily on the resistance ofthe cation against reduction and the resistance of the anionagainst oxidation [12]. Previously, Koch et al. have correlatedthe electrochemical oxidation potentials of several anionswith their respective highest occupied molecular orbital(HOMO) energies [19], and an excellent fit was obtained.In this study the electrochemical stability of a series of ionicliquids is correlated with the energy levels of the lowest un-occupied molecular orbital (LUMO) of the cations.

2. Experimental

All calculations were carried out using the Spartan’10 mole-cular modeling suite of programs [20]. Of all ionic liquidstructures built, the best conformer was selected using mole-cular mechanics. These conformers were fully geometry opti-mized at the B3LYP level using the corresponding PM3 struc-tures as input.

The underlying assumptions are that major properties ofan ionic liquid can be obtained from just a single cation/anion pair or in several cases even from the single cation


Table 1: ELUMO of several ionic liquids and ELUMO of the cations only (calculated at PM3 level).

Ionic liquid ELUMO (ionic liquid) [eV] ELUMO (cation) [eV]

1-butylpyridinium tetrafluoroborate −2.31 −5.78

1-butyl-3-methylimidazolium tetrafluoroborate −1.39 −4.95

1-butyl-2,3-dimethylimidazolium tetrafluoroborate −1.46 −4.84

1,1-butylmethylpyrrolidinium tetrafluoroborate −0.63 −4.21

Figure 1: Structure of [bmim+][BF4−] showing the electrostatic

potential projected on the van der Waals surface (surface = electrondensity of 0.002 e/au3, property = electrostatic potential). The elec-tron density is high under the red positions and low under the bluepositions, indicative for negative and positive parts of the molecularensemble.

or anion. Furthermore the application of HOMO/LUMOtheory for electrochemical oxidation/reduction reactionsassumes that there is no specific anodic/cathodic interactionwith the anion/cation of the ionic liquid or internal reactionbetween the cation and the anion and that the electrochem-ical reactions can be described as outer sphere electron trans-fer processes, obeying the Franck-Condon principle [21].

3. Results and Discussion

The structure of the ionic liquid 1-butyl-3-methylimidazoli-um tetrafluoroborate ([bmim+][BF4

−]) was calculated atB3LYP level. It shows nonbonded interactions (C—H· · ·Fhydrogen bonds) between the BF4

− and (i) the hydrogen atring position C2 and (ii) a methyl group hydrogen as can beseen in Figure 1.

The electrostatic potential map indicates that the neg-ative charge (red colored) is located on the formal anion(BF4

−) and the positive charge (blue colored) on the formalcation (bmim+). The structure is nearly identical to previouscalculations [22, 23]. Thereafter, the position of the LUMOon [bmim+][BF4

−] was determined. This position wascompared to the position of the LUMO on the cation onlyas shown in Figure 2. From Figure 2 it can be concluded thatthe LUMO of this ionic liquid is totally located on the cation.

Because the LUMO is located completely on the cation,the resistance of the cation against reduction determinesthe stability with respect to reduction of the ionic liquidpredominantly. However, because of obvious interactions

Table 2: Calculated LUMO energies (B3LYP) for several cations.

Cation ELUMO

(B3LYP) [eV]

1-butylpyridinium [bpyrid+] −6.46

1-butyl-2-methylpyrazolium [bmpyraz+] −5.28

1-ethyl-3-methylimidazolium [emim+] −4.92

1-butyl-3-methylimidazolium [bmim+] −4.82

1-octyl-3-methylimidazolium [omim+] −4.80

1-ethyl-2,3-dimethylimidazolium [edmim+] −4.65

1-butyl-2,3-dimethylimidazolium [bdmim+] −4.54

1-octyl-2,3-dimethylimidazolium [odmim+] −4.51

trimethylpropylammonium [N(1113)+] −3.22

trimethylhexylammonium [N(1116)+] −3.13

trimethylpropylphosphonium [P(1113)+] −2.95

trimethylhexylphosphonium [P(1116)+] −2.85

1,1-ethylmethylpyrrolidinium [empyrrol+] −2.80

1,1-butylmethylpyrrolidinium [bmpyrrol+] −2.82

1,1-ethylmethylpiperidinium [empip+] −2.75

1,1-butylmethylpiperidinium [bmpip+] −2.64

between the cation and anion, the absolute quantitative ener-gy level of the LUMO of the total ionic liquid will be differ-ent from the energy level of the LUMO of the cation solely,but these energy levels should be correlated. To test theexistence of this correlation, the LUMO energy level ofseveral tetrafluoroborate ionic liquids and that of their ca-tions only was calculated. Results are shown in Table 1 andFigure 3, which clearly indicate that both LUMO energy lev-els are correlated. The correlation coefficient is 0.99. There-fore, the assumption that the LUMO energy level of thecation solely determines the resistance of these ionic liquidsagainst reduction seems to be appropriate.

The energy level of the LUMO of several cations wascalculated on the B3LYP level, and results are shown inTable 2. Higher LUMO energies of the cations lead to morestable ionic liquids. Therefore, from Table 2 it can be con-cluded that the order in resistance against reduction is piperi-dinium > pyrrolidinium > quaternary phosphonium > qua-ternary ammonium > imidazolium > pyrazolium > pyri-dinium. This order correlates to experimental data [1, 6, 13].

The experimentally determined electrochemical windowof several ionic liquids with tetrafluoroborate and trifluoromethylsulfonate anions was thereafter correlated to theLUMO energy level of the cation. When the anion type isfixed, the width of the electrochemical window will only

International Journal of Electrochemistry 3

Figure 2: Structures of [bmim+][BF4−] and [bmim+] cation, showing the electrostatic potential projected on the van der Waals surface

(surface = electron density of 0.002 e/au3, property = electrostatic potential) and the position of the LUMO on [bmim+][BF4−] and on the

cation [bmim+] only. Red and blue in the orbitals refer to the phase only, and not to charge.

Table 3: Calculated LUMO energy (B3LYP) of the cation of several ionic liquids with the same anion and their experimentally determinedelectrochemical windows.

Cation ELUMO (B3LYP) [eV]Electrochemical window [V]

ReferencesAnion = [BF4

−] Anion = [(CF3SO2)2N−]

[bpyrid+] −6.46 3.4 — [1]

[bmpyraz+] −5.28 4.1 — [1]

[emim+] −4.92 4.15 4.3 [7, 13, 17, 25]

[bmim+] −4.82 4.25 4.3 [4, 7, 26]

[edmim+] −4.65 — 4.4 [7]

[N(1113)+] −3.22 — 5.2 [13]

[bmpyrrol+] −2.82 — 5.5 [27]

−4

−4.5

−5

−5.5

−6−2.5 −2 −1.5 −1 −0.5

[bmim+]

[bdmim+]

[bmpyrrol+]

ELU

MO

of c

atio

n (

eV)

ELUMO of ionic liquid (eV)

[bpyrid+]

Figure 3: ELUMO of ionic liquid (anion = [BF4−]) versus ELUMO of

cation (calculated at PM3 level). The correlation coefficient is 0.99.

be determined by the stability of the cation with respect toreduction.

In Table 3 the calculated LUMO energy level (on B3LYPlevel) of the cation of several ionic liquids with the sameanion and their experimentally determined electrochemicalwindows are given.

From Figure 4 it can be seen that the correlation is excel-lent with correlation coefficients of 0.98 and 0.99 and thatthe influence of these type of anions on the electrochemical

Ele

ctro

chem

ical

win

dow

(V

)

6

5.5

5

4.5

4

3.5

3−7 −6 −5 −4 −3 −2

ELUMO (B3LYP) (eV)

Figure 4: Plot of calculated ELUMO (B3LYP method) of the cationversus electrochemical window of the ionic liquid: blue: anion BF4

−;pink: anion (CF3SO2)2N−. The correlation coefficients are 0.98(anion = BF4

−) and 0.99 (anion = ((CF3SO2)2N−).

windows is rather small. From Tables 2 and 3 some remarksshould be made. Ionic liquids consisting of cations withlonger alkyl side chains are more stable with respect toreduction. This is consistent with experimental observations[4, 5, 24] but the extension of alkyl side chains also leads to alower conductivity [24]. Therefore it does not seem practical


Table 4: Comparison of calculated ELUMO of several cations by B3LYP and PM3 calculations.

Cation ELUMO [eV] (DFT/B3LYP) ELUMO [eV] (semiempirical/PM3)

1-butylpyridinium −6.46 −5.78

1-butyl-2-methylpyrazolium −5.28 −5.41

1-butyl-2,3-dimethylimidazolium −4.54 −4.84

trimethylpropylphosphonium −2.95 −4.51

trimethylpropylammonium −3.22 −4.45

1,1-butylmethylpyrrolidinium −2.82 −4.21

1,1-butylmethylpiperidinium −2.64 −4.16

Table 5: Calculated PM3 LUMO energy of the cation of several ionic liquids with the same anion and their experimentally determinedelectrochemical windows.

Cation ELUMO (PM3) [eV]Electrochemical window [V]

ReferencesAnion = [BF4

−] Anion = [(CF3SO2)2N−]

[bpyrid+] −5.78 3.4 — [1]

[bmpyraz+] −5.41 4.1 — [1]

[emim+] −4.98 4.15 4.3 [7, 13, 17, 25]

[bmim+] −4.95 4.25 4.3 [4, 7, 26]

[edmim+] −4.88 — 4.4 [7]

[N(1113)+] −4.45 — 5.2 [13]

[bmpyrrol+] −4.21 — 5.5 [27]

to increase the length of the alkyl chain. Furthermore, theelectrochemical stability towards reduction of the nonaro-matic N-containing cations (pyrrolidinium, piperidinium,and tetraalkylammonium) is higher than that of the aromaticcations, which is also consistent with literature data [5, 6,13, 24–27]. Alkylation of the aromatic ring improves theelectrochemical stability [24]. However, nonaromatic cationshave high melting points compared to aromatic cation-basedionic liquids [6]. They need a long alkyl side chain and/orcombination with the bis(trifluoromethylsulfonyl) imideanion to have a melting point lower than room tempera-ture [6]. Finally, it can be noticed that an increasing Lewisacidity of the ionic liquid shifts the reduction potentialpositively (less stable with respect to reduction), which wasalso shown in previous experiments [7, 15, 19]. For ex-ample, the more negative reduction potential of 2-methyl-imidazolium cations compared to 1,3-dialkylimidazoliumcations is linked to their weaker Lewis acidity [7].

So far, the LUMO energy levels of the cations are allcalculated at the B3LYP level. Semiempirical calculations onthe other hand are still much simpler and faster, but definitelyunreliable for energetic comparisons. Because the PM3structures were already available, it was tried to correlatethe PM3 and B3LYP LUMO energies. Table 4 shows that thesemiempirical results are in full record with the B3LYP re-sults. The correlation coefficient between the semiempiricalresults and the B3LYP results is 0.96 (Figure 5).

Because the PM3 calculations show the same trend inLUMO energies of the cation as the B3LYP calculations, itwas also tried to correlate the electrochemical window ofionic liquids with the LUMO energy at PM3 level. The resultsare shown in Table 5 and Figure 6.

−2

−3

−4

−5

−6

−7−7 −6 −5 −4 −3 −2

ELUMO (B3LYP) (eV)

ELU

MO

(PM

3) (

eV)

Figure 5: Comparison of calculated ELUMO of several cations byB3LYP and PM3 calculations. The correlation coefficient is 0.97.

From Figure 6 it can be seen that the correlation is goodwith correlation coefficients of 0.91 and 0.99. Thus PM3calculations have similar predictive capabilities for estimat-ing the electrochemical window of ionic liquids as B3LYPcalculations. Therefore, semiempirical calculations can verywell be used to predict the electrochemical window of ionicliquids with reduction-resistant anions.


−6 −5.5 −4.5 −4

6

5

4

3

2−5

ELUMO (PM3) (eV)

Ele

ctro

chem

ical

win

dow

(V

)

Figure 6: Plot of calculated ELUMO (PM3) of the cation versuselectrochemical window of the ionic liquid: blue: anion BF4

−; pink:anion (CF3SO2)2N−. The correlation coefficients are 0.91 (anion =BF4

−) and 0.99 (anion = (CF3SO2)2N−).

4. Conclusions

The LUMO of ionic liquids with reduction-resistant anions istotally located on the cation. The energy level of the LUMOof the cation is a good predictor for the width of the elec-trochemical window. Semiempirical PM3 calculations are infull record with B3LYP calculations; however they requirevery little computational resources, and thus should be pre-ferred to predict the electrochemical window of ionic liquids.The results obtained here are another demonstration of theversatile power of the old HOMO-LUMO concept to makein principal very complicated issues understandable in rathereasy way.

Acknowledgments

The authors would like to acknowledge J. van Spronsen, R.A. Penners, and M. van den Brink for their help.

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[26] U. Schroder, J. D. Wadhawan, R. G. Compton et al., “Water-induced accelerated ion diffusion: voltammetric studies in 1-methyl-3-[2,6-(S)-dimethylocten-2-yl]imidazolium tetraflu-oroborate, 1-butyl-3-methylimidazolium tetrafluoroborateand hexafluorophosphate ionic liquids,” The New Journal ofChemistry, vol. 24, no. 12, pp. 1009–1015, 2000.

[27] D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, and M. Forsyth,“Pyrrolidinium imides: a new family of molten salts andconductive plastic crystal phases,” Journal of Physical Chem-istry B, vol. 103, no. 20, pp. 4164–4170, 1999.


Research Article

Reversible Deposition and Dissolution of Magnesium fromImidazolium-Based Ionic Liquids

QingSong Zhao, Yanna NuLi, Tuerxun Nasiman, Jun Yang, and JiuLin Wang

Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Correspondence should be addressed to Yanna NuLi, [email protected]

Received 19 December 2011; Revised 10 February 2012; Accepted 23 February 2012


Copyright © 2012 QingSong Zhao et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The electrochemical performance of six imidazolium cation-based ionic liquids (ILs) containing 0.3 mol L−1 Mg(CF3SO3)2 as theelectrolytes for magnesium deposition-dissolution was examined by cyclic voltammogramms and constant current discharge-charge techniques. Scanning electron microscopy and energy dispersive X-ray spectroscopy measurements were conductedto characterize the morphologies and components of the deposits. The cathodic satiability of imidazolium cations can beimproved by increasing the length of alkyls at the 1-position and introducing methyl group at the 2-position of the imidazoliumcations. A reversible magnesium deposition-dissolution can be achieved at room temperature. After adding appreciate amountof tetrahydrofuran (THF) organic solvent, the conductivity and the peak currents for Mg deposition and dissolution can besignificantly improved. The potential polarization of deposition-dissolution process is decreased using Mg powder electrode.

1. Introduction

Increasing depletion of fossil resources, serious industrialpollution, and ecological destruction have cried for low-costand high energy density rechargeable batteries for electricvehicles, load leveling, and storage of energy from renewablesources [1]. Due to higher theoretical capacity (2205 mAh/g),higher negative potential (about−2 V versus standard hydro-gen electrode in aprotic solutions), low cost, safe to handleand environmentally friendly nature, metallic magnesiumis an attractive candidate for the active material of highenergy density batteries [2–4]. But in many nonaqueoussolutions, a reversible process of electrochemical depositionand dissolution of magnesium is hard to achieve becauseof the formation of compact passive film [5]. It is knownthat electrochemical Mg deposition is impossible fromsolutions containing simple ionic Mg salts (such as MgCl2,Mg(ClO4)2, etc.) in commonly used aprotic solvents (suchas alkyl carbonates, esters, and acetonitrile) [6, 7]. However,magnesium can be reversibly deposited electrochemically inthe systems without the passivating phenomena, such asethereal solutions of Grignard reagents (RMgX, R = alkyl,aryl groups; X = halide: Cl, Br) [7–10], amidomagnesium

halides [11, 12], Mg(BR2R′2)2 (R = alkyl and R′ = aryl group)[2, 11], Mg(AX4−n Rn′R′n′′)2 (A = Al, B; X = Cl, Br; R, R′ =alkyl or aryl groups, and n′ + n′′ = n) [13, 14], andPhMgCl-AlCl3 [15]. However, those electrolyte systems stillsuffer from the problems of safety and reliability due to theflammability and high vapor pressure of the ethereal solvents.

As we know, ionic liquids (ILs) have been widely re-searched and applied in organic synthesis, catalysis, sepa-rations, and electrochemistry owing to the superior perfor-mance such as nonflammability, wide liquid-phase range,low vapour pressure, lack of volatility, wide electrochemicalwindow, and great thermal and electrochemical stability[16–20]. ILs were used as electrolytes for electrochemicallyreversible deposition and dissolution of lithium [21, 22]. Inparticular, ILs have been regarded as attractive candidatesfor lithium-battery electrolyte [23–25]. We first reported theelectrodeposition of magnesium in imidazolium-based andpiperidine-based ionic liquids [26–29]. Cheek et al. studiedthe electrodeposition of magnesium in imidazolium-basedILs and pyrrole-based ILs containing a Grignard reagentor several inorganic magnesium salts [30]. Recently, Moritaet al. [31, 32] reported Mg electrodeposition from the elec-trolyte consisted of quaternary ammonium-based ILs with


N N

TFSI−

EMImBF4 BMImBF4 BMImPF6 BMMImBF4 HMMImBF4 BMMImTFSI

+ + + + +

BF4− BF4

− PF6−

N N N N N N N N N N+

BF4− BF4

−

Figure 1: Chemical structures of the imidazolium-based ILs used in this work.

Grignard reagents or simple Mg salt, and they also synthe-sized a series of imidazolium-based ILs and examined theelectrodeposition of magnesium from the solution mixed theILs with a Grignard reagent [33].

In this paper, we systematically studied the relationbetween the chemical structure of imidazolium-based ILscontaining Mg(CF3SO3)2 and the ionic conductivity, elec-trochemical window, and Mg deposition and dissolution.Furthermore, tetrahydrofuran (THF) as an additive wasadded in the electrolytes to improve the electrochemicalperformance of the electrolytes.

2. Experimental

2.1. Preparation of Electrolytes. 1-Ethyl-3-methylimidazoli-um tetrafluoroborate (EMImBF4, >99.0%), 1-butyl-3-meth-ylimidazolium tetrafluoroborate (BMImBF4, >99.0%),1-butyl-3-methylimidazolium hexafluorophosphate(BMImPF6, >99.0%), 1-butyl-2,3-dimethylimidazolium tet-rafluoroborate (BMMImBF4, >99.0%), 1-hexyl-2,3-dimeth-ylimidazolium tetrafluoroborate (HMMImBF4,>99.0%), 1-butyl-2,3-dimethylimidazolium bis((trifluoromethyl)sulfonyl)imide (BMMImTFSI, >99.0%) were purchased from Centerfor Green Chemistry and Catalysis (LICP, CAS, China) andused as received. Figure 1 shows the chemical structure ofthese imidazolium-based ILs. Mg(CF3SO3)2 (Alfa Aesar) wasstored under an argon atmosphere and used without furthertreatment. Tetrahydrofuran (Aladdin, 99.9%) was redistilledto receive lower water content. 0.3 mol L−1 solutions wereprepared by adding appropriate amount of Mg(CF3SO3)2

to imidazolium-based ILs in an airtight flask inside anargon-filled glove box (Mbraun, Unilab, Germany) andstirring the mixtures inside the box at room temperature forabout 12 h. Then, appropriate amount of THF with differentvolume ratio was added and stirred about several hours.

2.2. Measurement Procedures and Apparatus. The specificconductivity of the solutions was measured using an FE30conductivity meter and the InLab 710 conductivity mea-suring cell (Mettler Toledo, Switzerland). FTIR spectroscopyof the solutions was conducted with Paragon 1000 (PerkinElmer, Inc., USA) at 4000–450 cm−1, resolution 0.1 cm−1.The samples were prepared by evenly spreading solution onKBr disc.

Cyclic voltammograms (CVs) of three-electrode cellswere conducted in the argon-filled glove box at room tem-perature using an electrochemical instrument of CHI604AElectrochemical Workstation (Shanghai, China). The work-ing electrode was Pt disk (geometric area = 3.14× 10−2 cm2),and magnesium ribbon (1 mm diameter) (Aldrich) served as

Table 1: Conductivity and electrochemical window of theimidazolium-based ILs used in this work.

Ionic liquidsConductivity/mS

cm−1 (25◦C)

Electrochemical window ofionic liquids on Pt

electrode/V versus Mg

EMImBF4 14.46 0.3–3.0

BMImBF4 3.36 −0.4–3.0

BMImPF6 1.33 −0.6–3.5

HMMImBF4 0.43 −1.2–3.2

BMMImTFSI 1.94 −1.0–3.2

BMMImBF4 2.05 (40◦C) −0.8–3.2

counter and reference electrodes. All of the electrodes werepolished before use.

Electrochemical magnesium deposition-dissolutioncycles were examined with CR2025 coin-type cells. Copperfoil (Φ12 mm) was used as a working electrode (substrate)for the deposition and dissolution of magnesium. Magne-sium strip or a mixture of 90 wt.% Mg powder (99%, Sin-opharm Chemical Reagent Co., Ltd.) ball-milled 350 rmpfor 10 h and sieved (300 mesh), 3 wt.% carbon black, and7 wt.% polytetrafluoroethylene (Aldrich) binder pressed onCu foil as a counter electrode. Glass fiber membrane wasused as a separator. The cells were assembled in the glovebox. Magnesium was deposited onto the substrate for fixedperiods of 30 min followed by stripping to a fixed potentiallimit of 0.8 V versus Mg at a constant current density of0.1 mA cm−2. There was a 30 s rest between deposition anddissolution. The magnesium deposition and dissolutionon the substrate was referred as the discharge and chargeprocess, respectively. The time of charge divided by thetime of discharge was defined as the deposition-dissolutionefficiency.

The surface morphology and element analysis of theelectrodeposits were examined on a JEOL JSM-6460 scan-ning electron microscope (SEM) equipped with an energydispersive X-ray spectroscopy (EDS). The samples weredeposited for 10 h on copper substrate at 0.05 mA cm−2 andwashed carefully with drying THF solvent to remove solubleresidue in the glove box. Then, the samples were transferredout of the box and kept carefully without exposure to theatmosphere.


The ionic conductivity and the electrochemical window ofseveral imidazolium-based ILs are summarized in Table 1.The conductivity tends to decrease with increasing the lengthof alkyls at the 1-position and introducing methyl group


Table 2: Conductivity of the solutions dissolving 0.3 mol L−1 Mg(CF3SO3)2 in ILs without or with THF by different volume ratios for ILsand THF.

Solutions without THF BMImBF4 BMImPF6 HMMImBF4 BMMImTFSI BMMImBF4

Conductivity/mS cm−1 (25◦C) 3.08 1.31 0.32 1.79 1.18 (40◦C)

Solutions with THFBMImBF4

(6 : 1)BMImBF4

(3 : 1)BMImBF4

(1 : 1)BMMImBF4

(6 : 1)BMMImBF4

(3 : 1)BMMImBF4

(1 : 1)

Conductivity/mS cm−1 (25◦C) 5.72 8.03 9.33 1.97 3.65 5.56

at the 2-position of the imidazolium ring. The cathodicand anodic limits of the electrochemical window depend onthe anions and cations of the ILs, respectively. The cationstability is as follows: EMIm+ < BMIm+ < BMMIm+ <HMMIm+. Increasing the length of alkyls at the 1-positionof the imidazolium ring seems to lead to a lower potentialfor the cathodic limit. Furthermore, the cathodic stabilityis improved by introducing methyl group at the 2-positionbecause the proton at 2-position has stronger reactivity. Theresult suggests that the reduction potential of EMImBF4 isabove 0 V versus Mg, and EMIm-based ILs cannot be usedfor magnesium deposition electrolyte. On the other hand,the stability of different anions is as follows: BF4

− < TFSI− <PF6

−. The anodic stability tends to improve using TFSI− andPF6

− anions. However, the reversible deposition-dissolutionof magnesium cannot be obtained in the solutions ofBMImPF6 or BMMImTFSI containing Mg(CF3SO3)2. Itprobably relates with the strong electronegativity of TFSI−

and PF6− anions, which may form a passive film with

magnesium ions.The ionic conductivity of 0.3 mol L−1 Mg(CF3SO3)2 di-

ssolving in the ILs with or without THF is summarized inTable 2. The dissolution of Mg(CF3SO3)2 in ILs gives slightlylower conductivity than ILs. Furthermore, the conductivityof the solutions significantly increases after adding THFbecause the small molecule organic ether solvent reduces theviscosity of the ILs. High volatility of THF solvent can alsobe partly suppressed by mixing the nonvolatile ILs due toso-called “dilution effect.” However, excess THF cannot beadded considering the solubility of Mg(CF3SO3)2 in THF.

Figure 2 shows FTIR spectra (transmittance mode) ofBMImBF4, BMImBF4 + THF (with 3 : 1 volume ratio),0.3 mol L−1 Mg(CF3SO3)2/BMImBF4, and 0.3 mol L−1

Mg(CF3SO3)2/BMImBF4 + THF (3 : 1). The peaks in therange of 3600–3300 cm−1 can be attributed to νO−H vibrationfrom trace H2O. 2840–3000 cm−1 and 1365–1470 cm−1

belong to νC−H and δC−H vibration modes of imidazoliumcation, respectively. Those around 3100 cm−1 and 1630 cm−1

are related to νC−H and δC=C vibration mode of imidazoliumring, respectively. 1574 cm−1 can be attributed to thevibration of imidazolium ring, and the peaks between1250 cm−1 and 1360 cm−1 belong to νC−N vibration. Thepeak of 750 cm−1 is related to the C–H plane swing vibrationand 1170 cm−1 to νC−C framework vibration. 840 cm−1

belongs to H–C=C–H nonplanar angle vibration and 500–650 cm−1 to far-infrared absorption frequency. The peaksof 521 cm−1 and 1059 cm−1 are the characteristic frequencyof BF4

−. Little difference appears between the spectra ofBMImBF4 and BMImBF4 + THF, Mg(CF3SO3)2/BMImBF4,

4000 3500 3000 2500 2000 1500 1000 500

(a)

(b)

(c)

(d)

Tran

smit

tan

ce (

%)

Wavenumber (cm−1)

Figure 2: FTIR spectra of (a) BMImBF4, (b) BMImBF4 +THF (3 : 1), (c) 0.3 mol L−1 BMImBF4/Mg(CF3SO3)2, and (d)0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1).

4000 3500 3000 2500 2000 1500 1000 500

(a)

(b)

(c)

(d)Tran

smit

tan

ce (

%)

Wavenumber (cm−1)

Figure 3: FTIR spectra of (a) BMImPF6, (b) 0.3 mol L−1

Mg(CF3SO3)2/BMImPF6, (c) BMMImTFSI, and (d) 0.3 mol L−1

Mg(CF3SO3)2/BMMImTFSI.

and Mg(CF3SO3)2/BMImBF4 + THF. It is suggested thatthe addition of a certain amount of THF has little impacton chemical constitutions of the solutions. After adding ofa certain amount of Mg(CF3SO3)2, no characteristic peaksof CF3SO3

− appear, and the characteristic peak of BF4− at

4 International Journal of ElectrochemistryC

urr

ent

den

sity

(m

A c

m−2

)

4

2

0

−2

−4

−6−0.5 0 0.5 1 1.5 2

4

2

0

−2

−4

−60 1 2 3C

urr

ent

den

sity

(m

A c

m−2

)

Potential (V) versus Mg RE


(a)

Cu

rren

t de

nsi

ty (

mA

cm−2

)

4

2

0

−2

−4

−6 −0.5 0 0.5 1 1.5 2


(b)

Cu

rren

t de

nsi

ty (

mA

cm−2

)

4

2

0

−2

−4

−6−0.5 0 0.5 1 1.5 2


(c)

Cu

rren

t de

nsi

ty (

mA

cm−2

)

4

2

0

−2

−4

−6−0.5 0 0.5 1 1.5 2


(d)

Figure 4: Typical cyclic voltammograms of Mg deposition-dissolution on Pt disk electrode from (a) 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4,inset is the CV of BMImBF4, (b) 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (6 : 1), (c) 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1),and (d) 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (1 : 1). The scanning rate was 50 mV s−1.

1059 cm−1 and the peak of νC−N vibration at 1280 cm−1

become double peaks, and a new peak at 1226 cm−1 isfound. It is suggested that some complex reaction takes placebetween Mg(CF3SO3)2 and BMImBF4, which may restrainthe formation of passive film, which is favorable for the re-versible deposition-dissolution of magnesium.

Figure 3 shows FTIR spectra of BMImPF6, 0.3 mol L−1

Mg(CF3SO3)2/BMImPF6, BMMImTFSI, and 0.3 mol L−1

Mg(CF3SO3)2/BMMImTFSI. The peak of 830 cm−1 belongsto the characteristic frequency of PF6

− shown in Figures 3(a)and 3(b), respectively. In Figures 3(c) and 3(d), 1350 cm−1

and 1192 cm−1 can be attributed to the characteristic peaksof –CF3 and –SO3, respectively. The peaks of 3496 cm−1,1669 cm−1, and 1253 cm−1 are related to CF3SO3

−. Afteradding Mg(CF3SO3)2, the characteristic peaks of PF6

− andCF3SO3

− remain (shown in Figures 3(a) and 3(b)). It issuggested that Mg2+ is not involved in the complex reaction

because the strong electronegativity of PF6− may destroy

the reaction. Similar phenomenons appear in ILs containingTFSI− anions, as shown in Figures 3(c) and 3(d). It can besupposed that the passive film may take place in both ofsystems and no reversible magnesium deposition-dissolutionappears.

We examined the reversible deposition-dissolution behavior of magnesium in Mg(CF3SO3)2/BMImBF4 solutionand the impact of different THF volume ratio on theelectrochemical performance. Figure 4 compares the cyclicvoltammograms (CVs) on platinum disk electrodefrom BMImBF4, 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4,0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (6 : 1),0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1),and 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (1 : 1)at 50 mV s−1. Compared to pure BMImBF4 (inset ofFigure 4(a)), a redox wave at −0.2 V and 0.6 ∼ 1.0 V

International Journal of Electrochemistry 5i p

a(m

A c

m−2

)

0.5

0.4

0.3

0.2

0.1

00 0.05 0.1 0.15 0.2

v1/2 (Vs−1)1/2

Figure 5: A plot of the anodic peak currents against square rootof the scan rates for the striping of Mg on platinum disk electrodefrom 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (6 : 1).

0 1 2 3

0

1

2

3

−1

−1

Cu

rren

t de

nsi

ty (

mA

cm−2

)

(a)(b)


Figure 6: Typical cyclic voltammograms of Mg deposition-dissolution on Pt disk electrode from (a) BMMImBF4 + THF(3 : 1), (b) 0.3 mol L−1 Mg(CF3SO3)2/BMMImBF4 + THF (3 : 1).The scanning rate was 50 mV s−1.

versus Mg appears in 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4

(Figure 4(a)), which may correspond to the couple ofcathodic deposition and anodic dissolution of Mg. Thepeak current density is low due to a low conductivity ofthe solution. Adding THF can improve the conductivity,and the peak current density increases with the increaseof THF amount (Figures 4(b), 4(c), and 4(d)). However,the overpotential between reduction and oxidation peaksbecomes larger when the ratio of BMImBF4 and THFreaches to 1 : 1 (Figure 4(d)). Because of the lower solubilityof Mg(CF3SO3)2 in THF, excess THF may affect thesolubility of Mg(CF3SO3)2 in the ILs and the electrochemicalperformance of the solution. The plot of the anodic peakcurrents versus square root of the scan rates on platinum

SEI 20 µm

(a)

325

260

195

130

65

00.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Energy (keV)

Cu

Mg

(b)

Figure 7: SEM image of magnesium deposit and correspondingEDS result on copper substrate from 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1) solution. The charge amount is 1.8 C cm−2.

disk electrode from 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4

+ THF (6 : 1) is shown in Figure 5. The linear plot provesthat the mass-transport process is most likely diffusion ofelectroactive species from the electrolyte to the electrodeinterface.

As shown in Table 1, the stability of imidazolium cationof BMImBF4 can be improved when active H in thecation is substituted by methyl. Figure 6 shows typical cyclicvoltammograms on Pt disk electrode from BMMImBF4 +THF (3 : 1) and 0.3 mol L−1 Mg(CF3SO3)2/BMMImBF4 +THF (3 : 1) at 50 mV s−1. The reduction potential of blanksolution of BMMImBF4 + THF arrives to −0.8 V versus Mgand oxidation potential to 2.5 V versus Mg. The additionof Mg(CF3SO3)2 causes the appearance of a reduction


95 100 105 110 115

0

1

2

3

−1

4

5

Time (h)

1

0.5

0

−0.5

−1110.5 111 111.5

Time (h)

Vol

tage

(V

) ve

rsu

s M

g

Vol

tage

(V

) ve

rsu

s M

g

(a)

0 20 40 60 800

20

40

60

80

100

Cycle number

Cyc

ling

effici

ency

(b)

Figure 8: Typical galvanostatic cycling curves (a) Inset shows one of the cycle curves, and cycling efficiencies (b) for Mg deposition-dissolution on Cu substrate in 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1). The counter electrode is Mg sheet.

0

1

2

3

4

Time (h)

72 76 80 84

0.8

0.6

0.4

0.2

0

−0.2

71 71.2 71.4 71.6 71.8 72Time (h)

Vol

tage

(V

) ve

rsu

s M

g

Vol

tage

(V

) ve

rsu

s M

g

Figure 9: Typical galvanostatic cycling curves for Mg deposition-dissolution on Cu substrate in 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4

+ THF (3 : 1) inset shows one of the cycle curves. The counterelectrode is magnesium powder electrode.

process at −0.11 V and a oxidation process at 0.91 V,with current significantly higher than the background cur-rent in this potential range. Though the reversibility ofMg2+ in BMMImBF4 is improved compared to BMImBF4,the peak current density is smaller due to the lowerconductivity.

The surface morphology and composition of the elec-trodeposited layer was investigated by means of scanningelectron microscope equipped with an energy dispersive X-ray spectroscopy. Figure 7 shows the SEM image and corre-sponding EDS result of the deposit on copper substrate from0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1) solution,the deposited charge amount is 1.8 C cm−2. Although thedeposit is sparse and uneven, micrometric size crystals canbe observed. EDS analysis shows that the deposit containsmagnesium, in addition to copper that resulted from the

substrate. In order to know whether the deposit containscrystalline magnesium or not, XRD analysis was performed.However, the layer is too thin to produce good XRDpattern.

Figure 8 presents typical magnesium deposition-disso-lution cycles on Cu substrate in 0.3 mol L−1 Mg(CF3SO3)2/BMImBF4 + THF (3 : 1) and the cycling efficiencies (cal-culated according to the ratio of the charge amount ofmagnesium dissolution to that of magnesium deposition)during initial 85 cycles. The counter electrode is Mg sheet.As shown in the Figure 8(a), the deposition potential ofMg on Cu substrate is about −0.5 V and the correspondingdissolution potential is 0.5 V. The cycling efficiencies duringinitial 20 cycles are low because of the complex adsorptionand diffusion of electrolyte. After 20 cycles, the cyclingefficiencies can reach about 96%. When Mg sheet electrodeis replaced with Mg powder electrode, the potentials ofdeposition and dissolution are significantly reduced (shownin Figure 9). It is suggested that the increase of the electrodesurface area has a marked effect on decreasing the interfacialresistance and suppressing the potential polarization. Thisis mostly attributed to the porous characters and stable andhomogeneous interface of the powder electrode [34, 35].

4. Conclusions

Several imidazolium-based ILs containing 0.3 mol L−1

Mg(CF3SO3)2 for electrochemical magnesium depositionand dissolution were systematically studied. The chemicalstructure of imidazolium cations has a remarkable impacton the ionic conductivity and the electrochemical windowof the solutions. After adding of a certain amount of THF,the conductivity of the solution, the performance for Mgdeposition-dissolution can be improved. Replacing Mgsheet electrode with Mg powder electrode, the potentialpolarization of deposition-dissolution process is decreased.


Acknowledgment

This work is supported by the National Nature ScienceFoundation of China (Project no. 20603022, 20973112).

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Research Article

Modification of Nafion Membranes by IL-Cation Exchange:Chemical Surface, Electrical and Interfacial Study

V. Romero,1 M. V. Martınez de Yuso,2 A. Arango,2 E. Rodrıguez-Castellon,2 and J. Benavente1

1 Departamento de Fısica Aplicada I, Facultad de Ciencias, Universidad de Malaga, 29071 Malaga, Spain2 Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad de Malaga, 29071 Malaga, Spain

Correspondence should be addressed to J. Benavente, j [email protected]

Received 4 September 2011; Revised 19 December 2011; Accepted 2 January 2012

Academic Editor: Oliver Hofft

Copyright © 2012 V. Romero et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Bulk and surface changes in two proton-exchange membranes (Nafion-112 and Nafion-117) as a result of the incorporation of theIL-cation n-dodecyltriethylammonium (or DTA+) by a proton/cation exchange mechanism after immersion in a DTA+ aqueoussolution were analysed by impedance spectroscopy (IS), differential scanning calorimetry (DSC), X-ray photoelectron spectro-scopy (XPS), and contact angle measurements performed with dry samples of the original Nafion and Nafion-DTA+-modifiedmembranes. Only slight differences were obtained in the incorporation degree and surface chemical nature depending on the mem-brane thickness, and DTA+ incorporation modified both the hydrophobic character of the original Nafion membranes and theirthermal stability. Electrical characterization of the dry Nafion-112 membrane was performed by impedance spectroscopy whiledifferent HCl solutions were used for membrane potential measurements. A study of time evolution of the impedance curvesmeasured in the system “IL aqueous solution/Nafion-112 membrane/IL aqueous solution” was also performed. This study allowsus monitoring the electrical changes associated to the IL-cation incorporation in both the membrane and the membrane/IL solu-tion interface, and it provides supplementary information on the characteristic of the Nafion/DTA+ hybrid material. Moreover,the results also show the significant effect of water on the electrical resistance of the Nafion-112/IL-cation-modified membrane.

1. Introduction

As it is well known, ionic liquids are room temperaturemolten salts (RTILs) with very low vapour pressure and com-posed only of ions. These salts are characterized by weak in-teractions owing to the combination of a large cation and acharge-delocalized anion. Ionic liquids exhibit very interest-ing properties such as the solubilization of a large range oforganic molecules and transition metal complexes or thepossibility of their repeated use due to reduced environmen-tal loss, while their good thermal stability tolerates reactiveprocesses at high temperatures favouring a faster kinetic [1–4]. Various kinds of salts can be used to design an ionicliquid with tailoring properties for a given application, buttypical IL-cations are imidazolium and quaternary ammo-nium salts, while hexafluorophosphate is a common anion.Due to their particular characteristics, the use of ILs inmany areas of technology and science has increased lately

and they can be used as solvents for organic reactions andcatalysis, CO2 capture devices, selective separation of chem-ical species, dissolution of natural biomaterials, energy con-version, bioscience, and so forth [5–7]. Miscibility of ionicliquids with water is also a point of interest and it seems tobe mostly determined by the associated anion. RTILs withanions such as halides, BF4

−, nitrate, and perchlorate, arewater miscible, while those with PF6

− AsF6−, and so forth

are immiscible. The activity of water seems to be higher inthe hydrophobic ionic liquid than in the hydrophilic one andthus water will partition to the gas-liquid interface of the im-miscible ionic liquid more than in the miscible one [8].

The conductivity of ionic liquids is also being studied fortheir application in batteries and fuel cells. As it is well knowna fuel cell basically consists of two electrodes sandwichinga proton-conducting membrane (or MEA). Nafion is thepolymeric material widely used as a reference in proton fuelcell (PEMFCs) due to its high ionic conductivity as well as its


mechanical and chemical stability at room temperature, andit consists of a backbone of polytetrafluoroethylene (PTFE)with sidechains of perfluorovinylether (PFAE) with a sul-fonic group end. Nafion combines the high hydrophobicityof the PTFE with the great hydrophilicity of the sulfonicgroups at intervals along the chain, which facilitate the trans-port of ions and give it the character of a polymer electrolytestructure [9, 10]. The mechanism of transport across theNafion membranes seems to be associated to both the pres-ence of water and its state within the membrane structure[11, 12], which is a point of significant importance for itsapplication in PEMFCs due to the conductivity decreasefound in Nafion membranes at temperature above 80◦C,which is associated to the Nafion membrane water loss. Forthis reason some authors have proposed the modification ofNafion membranes by incorporation of ILs as a way to reducethe Nafion limitation at temperature higher than 120◦C asa result of water loss and/or to minimize other kinds ofelectrode/membrane surface interactions due to electrode orcatalyst changes [13–16].

In this context, it should be pointed out the significantnumber of papers published in recent years dealing with ori-ginal and modified proton-exchange membranes conductiv-ity and permeability, which are considered key parametersfor their applications in proton and methanol (DMFCs) fuelcells, respectively [16–22]; particularly, a reduction of aroundtwo orders of magnitude in the methanol permeability acrossNafion/IL-modified membranes attributed to the decrease ofthe methanol crossover associated to the presence of the IL-cation has already been reported [23]. However, a more re-duced number of papers on surface study of both originaland modified membranes are published, although mem-brane surface represents the interface between two differenttransport media, which can be solid/solid in the case offuel cells or solid/liquid when supported liquid membranes(SLMs) or other electrochemical devices proposed for selec-tive solute separation are considered [15, 24, 25].

This work studies both surface and bulk phase changescaused in two polytetrafluoroethylene sulfonate Nafionmembranes (both in protonated form but different thick-ness) as a result of the incorporation of a IL-cation (n-dode-cyltriethylammonium or DTA+) by cation-exchange mech-anism after their immersion in a water DTA+ (Nafion/DTA+ samples). Surface changes associated to IL-cation in-corporation in the Nafion membranes were studied by X-rayphotoelectron spectroscopy (XPS) analysis and contact anglemeasurements, while its effect on the thermal stability of theNafion membranes was determined by analyzing the DSCcurves. Electrical modifications in dry Nafion/IL-cation sam-ples were determined by impedance spectroscopy (IS) mea-surements, but proton transport numbers across the mem-brane were obtained from membrane potential measure-ments carried out with the membranes in contact with HClaqueous solutions. Moreover, IS technique was also usedfor the electrical characterization of the IL solution/Nafion112 membrane/IL solution system, which was performed atdifferent times (0 ≤ t(h) ≤ 40) as a way of monitoring kineticchanges in the membrane and the membrane/IL-solutioninterface and to estimate of interfacial effects.

2. Experimental

2.1. Materials. A protonated Nafion membrane fromDupont (USA), Nafion-112, was modified by incorpora-tion of the IL-cation n-dodecyltrimethylammonium (DTA+)through H+/cation exchange mechanism [15]. The mem-brane has the following characteristics given by the manufac-turer: nominal thickness of 51 μm, density 2000 kg/m3, ionicconductivity 8.3 S/m, acid capacity 0.89 meq/g, and initialwater content 5 wt%. To see the effect of membrane thicknesson the IL-cation incorporation degree and membrane sur-face characteristics another similar membrane with a thick-ness of 180 μm (Nafion-117 sample) was also studied. TheDTA+ (C12H25N(CH3)3

+) IL-cation characteristics are mole-cular weight of 228 g/mol and molar volume (estimated bySchroeder’ method [26]) of 350 cm3/mol and it was suppliedby TCI Instrument (England). Although DTA+Cl− is not anionic liquid but a quaternary amine, the DTA+ cation is typi-cal of common ILs.

Samples of Nafion-112 and Nafion-117 membraneswithout any previous pretreatment were immersed in a 60%DTA+Cl− aqueous solution and variations in the liquid ofpH and conductivity were measured using a pH conductivitymeter (model 720 A, Orion) and a conductivity meter (mod-el 960, Schott Instruments), respectively. Both the pH andthe conductivity were measured before and after DTA+ in-corporation to determine the concentration of ions insidethe membranes. The incorporation degree, ID (mol/m3), wasdetermined by the following expression: ID = (Kt−K0)/λH

+,whereK0 andKt represent the conductivity of the IL-aqueoussolution at times 0 and t, respectively, while λH

+ is the pro-ton conductivity; taking into account the solution volumeand the ion-exchange capacity of a given membrane the per-centage of cation incorporation (mol/membrane mass) wasdetermined. All these measurements were performed at thelaboratory of Professor J. Crespo, REQUIMTE/CQFB, FCT,Universidade Nova de Lisboa (Portugal).

2.2. XPS Measurements. The chemical characterisation ofthe surface of the studied modified membranes and theNafion-112 was performed by XPS. A Physical electronicsspectrometer (PHI 5700) was used, with X-ray Mg Kαradiation (300 W, 15 kV, 1253.6 eV) as the excitation source.High-resolution spectra were recorded at a given take-offangle of 45◦ by a concentric hemispherical analyser operatingin the constant pass energy mode at 29.35 eV, using a 720 μmdiameter analysis area. Under these conditions, the Au 4 f7/2line was recorded with 1.16 eV FWHM at a binding energy of84.0 eV. The spectrometer energy scale was calibrated usingCu 2p3/2, Ag 3d5/2, and Au 4 f7/2 photoelectron lines at 932.7,368.3, and 84.0 eV, respectively. Charge referencing was doneagainst –CF2-carbon of Nafion (C 1s, 292.0 eV).

Membranes were mounted on a sample holder withoutadhesive tape and kept overnight at high vacuum in thepreparation chamber before being transferred to the analysischamber of the spectrometer for testing. Each spectral regionwas scanned for several sweeps until a good signal-to-noiseratio was observed. The pressure in the analysis chamberwas maintained lower than 5 × 10−6 Pa. PHI ACCESS


ESCA-V6.0 F software package was used for acquisition anddata analysis. A Shirley-type background was subtractedfrom the signals. Recorded spectra were always fitted usingGauss-Lorentz curves and following the methodology descri-bed in detail elsewhere [24], in order to determinate moreaccurately the binding energy (BE) of the different elementcore levels. Atomic concentration percentages of the charact-eristic elements of the surfaces were determined taking intoaccount the corresponding area sensitivity factor [25] forthe different measured spectral regions. Membrane sampleswere irradiated for a maximum time of 20 min to minimizepossible X-ray damage.

2.3. Contact Angle Measurements. The hydrophobic/hydro-philic character of the membrane surfaces was determinedfrom contact angles measurements, and they were performedwith a Teclis T2010 instrument equipped with a video sys-tem. Membrane samples were mounted on glass slides toprovide a flat surface for analysis. The drop method was usedto measure the contact angle of deionized water on the sur-face of the membranes at ambient temperature. Dry sam-ples of Nafion-112, Nafion-117, Naf-112/DTA+, and Naf-117/DTA+ membranes with any pretreatment were analyzed.

2.4. Thermal Analysis. TG/DSC analysis was performed witha thermobalance TGA/DSC1 of Mettler. A working air flowof 50 cm3/min was used for measurements carried out undermost similar atmospheric conditions. The TG profile wascollected in the 30 < T (◦C) < 750 temperature range witha heating rate of 10◦C/min, using an open platinum pan of70 μL. The thermal analysis was performed with dry samplesof all the membranes with any pretreatment.

2.5. Impedance Spectroscopy and Membrane Potential Mea-surements. Impedance spectroscopy (IS) measurements withdry Naf-112/DTA+ and Naf-117/DTA+ samples at equilib-rium IL-cation incorporation were performed in a cell con-sisting of a Teflon support on which two Pt electrodes wereplaced and screwed down [27]. However, an electrochemicalcell similar to that described in [28], which consists in twoglass half cells with the membranes placed in the middle andseparating the IL-solutions was used for IS measurementswith both the IL-solution and the IL-solution/membrane/IL-solution. Time evolution of the impedance curves measuredin this latter system allows the monitoring of dynamicalchanges in both the membrane bulk phase and the mem-brane/IL-solution interface.

With both cell systems, IS measurements were performedusing a response frequency analyzer (Solartron 1260, Eng-land) controlled by a computer. 100 different frequencies inthe range 1 Hz–107 Hz at a maximum voltage of 0.01 V weremeasured, and the experimental data were corrected by soft-ware, the influence of connecting cables, and other parasitecapacitances.

The electromotive force, ΔE, between two HCl solutionsof concentrations c1 and c2 at both sides of the membranescaused by the concentration gradient was measured bytwo reversible Ag/AgCl electrodes connected to a digital

0 10 20 30 40 500

20

40

60

80

Naf-117/DTA+

Naf-112/DTA+

Inco

rpor

atio

nde

gree

(%)

Time (h)

Figure 1: Time dependence of DTA+ incorporation into the struc-ture of (�) Nafion-112 membrane and (�) Nafion-117 membrane.

voltmeter (Yokohama 7552, 1 GΩ input resistance). Mea-surements were carried out by keeping the concentration ofthe solution at one membrane side constant (c1 = 0.001 M)and gradually changing the concentration of the solution atthe other side, c2, from 2× 10−4 M to 7.5× 10−3 M [27].


3.1. Incorporation of DTA+ Cation in the Nafion Membranes.The dynamic of IL-cation inclusion into Nafion-112 andNafion-117 membranes as a function of the contact timeis shown in Figure 1, where the incorporation degree is ex-pressed as a % of the cationic exchange capacity of Nafionsamples. Differences in the kinetic of DTA+ incorporationdepending on the thickness of the Nafion membrane thick-ness were obtained, which seems to be related to the time ne-cessary to reach an equilibrium state (around 5 h for Nafion-112 and 30 h for Nafion-117), while the equilibrium valuefor DTA+ incorporation hardly depends on the membrane(59.6% for Naf-112/DTA+ and 61.4% for Naf-117/DTA+),which allows the estimation of an average value of (60.5 ±0.5)% for Nafion-DTA+ aqueous solution system.

3.2. Membrane Surface Characterization. Surface chemicalchanges associated to the inclusion of the IL-cation DTA+

into the structure of the Nafion membranes was studiedby XPS analysis, which allows the determination of theatomic concentration percentage (A.C. %) of the elementspresent on the surfaces of both Nafion and Nafion/IL-modified membranes, which were obtained from highresolution spectra of the main photoelectron peaks (C1s, O 1s, F 1s, S 2p, and N 1s). Figure 2 shows thechemical structure of Nafion, while Table 2 presents theatomic concentration percentages (A.C.) of the characteristicelements found on the surface of the Nafion-112 andNafion-117 membranes and the Nafion/IL-cation modifiedsamples at equilibrium state, that is, after more than40 h in contact with the IL-cation aqueous solution; small


CF2 CF2

CF2 CF2CF2

CF2 CF

CFO O

SO3CF3

M

nx

m

Figure 2: Chemical structure of Nafion.

Table 1: Atomic concentration percentages of the characteristicelements found on the membranes surfaces.

Sample C% O% F% S% N%

Nafion (theoretical) 30.8 7.7 60.0 1.5 —

Naf-112 39.0 6.6 53.4 1.0 0.2

Naf-117 41.4 6.4 50.5 0.9 0.4

Naf-112/DTA+ 54.5 11.5 27.2 1.1 2.8

Naf-117/DTA+ 49.5 9.8 36.2 1.1 1.5

Table 2: Average values of the contact angles for original and IL-modified membranes.

Membrane Average contact angle (◦)

Nafion-112 91± 2

Nafion-117 91 ± 3

Naf-112/DTA+ 74 ± 6

Naf-117/DTA+ 85 ± 5

percentages <0.5% of other elements such as chlorine andsilicon were also found and they are associated to sur-face contamination. For comparison, the theoretical valuesfor the Nafion membrane calculated by its chemical for-mulae are also indicated in Table 2, and only slight dif-ferences between theoretical and experimental values forboth Nafion membranes were obtained, which could be dueto membranes surfaces contamination, particularly to thepresence of adventitious carbon [29].

As can be observed in Table 1, the concentration offluorine (Nafion characteristic element) in both modifiedmembranes is significantly lower than in the original samplesdue to the membrane surface coverage by the IL; however,high experimental carbon percentage was obtained for bothmodified samples, which could be associated to DTA+ com-ponent, but the result of surface contamination indicatedabove might affect the A.C. of this element.

This point can be discriminated by considering the XPSspectra, which give information on the chemical interactionsamong the different elements on the solid surface. Figure 3shows the C 1s core level spectrum for Nafion-112 and bothNafion/IL-cation modified membranes (Naf-112/DTA+ andNaf-117/DTA+) and certain differences can be observed. TheNafion-112 sample shows three different photoemissions atbinding energies (BE) of 291.8–292.0 eV (associated to CF2

and CF–O) and at 284.8–285.0 eV (C–H bond), plus a smallshoulder at 294.0 eV (CF3). The C 1s spectra for both Nafion/DTA+-modified membranes also present those peaks andshoulder, but another shoulder at 286.4 eV associated to C–Ncan also be observed [29, 30].

295 290 2850

0.2

0.4

0.6

0.8

1 C–H

C–N

CF3

CF2, CF–O

Naf-112/DTA+

Naf-117/DTA+

Naf-112

I(a

.u.)

Binding energy (eV)

Figure 3: Comparison of the C 1s core level spectra obtained forNafion-112, Naf-112/DTA+, and Naf-117/DTA+, membranes (bothmodified membranes after 48 h in contact with the IL-solution).

Surface modification of the Nafion membranes associ-ated to the incorporation of the IL-cation might also affectits hydrophobic character, which can be determined by con-tact angle measurements. Figure 4 shows differences in theform of the water drops on the surface of the Nafion-112membrane and the Naf-112/DTA+ sample after 40 h of im-mersion in the solution, which is a clear indication ofthe superficial changes due to DTA+ incorporation. Table 2shows the average contact angle values obtained for thestudied membranes; as can be observed practically the samevalues were obtained for original Nafion-112 and Nafion-117 membranes, but small differences were found betweenboth IL-cation-modified samples presenting membrane Naf-112/DTA+ a slightly more hydrophilic character, which alsoagrees with XPS results.

Since the results presented in this section only showslight chemical surface differences for both Nafion-IL-modi-fied membranes related to the sample thickness, in the fol-lowing section dedicated to the electrical effect of IL-cationmodification only the Naf-112/DTA+ membrane will beconsidered.

3.3. Thermal Characterization of Original and IL-Cation-Modified Membranes. The thermogravimetric profiles of theNafion-112 and Nafion-117 pristine membranes show aweight loss of 79.76% between 320 and 470◦C (for Nafion-117) and 72.8% between 310 and 480◦C (for Nafion-112).The profile obtained for the Nafion-117 sample is similar tothat observed by Di Noto et al. [31], although measurementswere performed under different conditions (N2 atmospherein [31] and in air in this paper). Upon incorporation ofDTA+, the weight losses occur at higher temperatures: 85.8%between 340 and 520◦C for Nafion-117/DTA+ and 77.8%between 350 and 550◦C for Nafion-112/DTA+, which alsoincludes the degradation of the DTA+. According to DSCresults the incorporation of DTA+ seems to communicatehigher thermal stability to the studied membranes, which isreflected by the higher finishing decomposition temperatures


(a)

(b)

Figure 4: Differences in the hydrophobic character of (a) Nafion-112 and (b) Naf-112/DTA+ membranes by the comparison of water-membrane surfaces contact angles.

0 100 200 300 400 500 600 700

0

−0.003

−0.002

−0.001

0.001

Temperature (◦C)

DTG curve

dW/dt

(mg◦

C−1

)

Figure 5: DTG profiles for Nafion-117 (solid line) and Nafion-117/DTA+ (dashed line) membranes.

for membranes containing DTA. The DTG profiles also con-firm this point as can be observed in Figure 5, where a com-parison between Nafion-117 and Nafion-117/DTA+ mem-branes is presented. The minima of the DTG curve forNafion-117/DTA+ membrane are shifted to higher temper-atures in comparison to those observed in the DTG curve forthe pristine Nafion-117.

3.4. Electrical Characterization of the Naf-112/DTA+ Mem-brane. The evaluation of the electric modification of Nafion-112 membrane by IL-cation incorporation was performed by

0

1

1.5

2

0 1 1.5 2

×106

×106

0.5

0.5

Equivalent circuit: (RmQm)

Zreal (Ω)

−Zim

g(Ω

)

Figure 6: Nyquist plot (−Z img versus Zreal) and equivalent circuitfor Naf-112/DTA+ dry sample (�) and Naf-112/DTA+(w) hydratedsample (�), that means, after 24 h submerged in distilled water.

impedance spectroscopy and membrane potential measure-ments. Since these latter measurements are carried with HCl-aqueous solutions, the effect of water on membrane electricalparameters is also considered by comparing impedancecurves for Naf-112/DTA+ and after 24 h submerged indistilled water (Naf-112/DTA+(w)) samples. Figure 6 showsa comparison of the Nyquist (−Zimag versus Zreal) plot ob-tained for both samples, where significant differences canbe observed, although in both cases the equivalent circuitcorresponds to a parallel association of a resistance (Rm) anda constant phase element (Qm) or nonideal capacitor [32,33]. The fitting of the impedance date by a nonlinear pro-gram allows the estimation of Rm [34] and the followingvalues were obtained: Rm(Naf-112/DTA+) = 2 × 106 ohmand Rm(Naf-112/DTA+(w)) = 2.8× 105 ohm, this representspractically one order of magnitude lower for the hydrat-ed membrane. However, no indication of membrane con-ductivity (σ) can be determined from these results sincethe well-known and commonly used relationship betweenelectrical resistance and conductivity, R = L/σ · S, is onlyvalid for homogeneous conductors, and it is not applic-able to the modified membranes since no evidence of homo-geneous distribution of the IL-cation throughout the Nafionstructure exists. In any case, the obtained values show the sig-nificant increase of electrical resistance caused in the Nafionmembrane by the inclusion of the IL-cation but mem-brane rehydration clearly reduce this effect.

This point can be more clearly observed in Figures 7(a)and 7(b) where a comparison of the Bode diagrams (Zreal

versus frequency and −Z imag versus frequency, resp.) forNafion-112, Naf-112/DTA+, and Naf-112/DTA+(w) samplesis presented. As can be observed, significantly lower valuesfor the real part of the impedance (directly related to elec-trical resistance) were obtained for the original Nafion-112 membrane in comparison with Naf-112/DTA+(w) and


102 104

102

01 3

01 4

01 5

01 6

01 6100

f (Hz)

101

Zre

al(Ω

)

(a)

Interface

Membrane

102

104

106

102 104 106100

100

Zimg (Ω)

Zreal (Ω)

Zreal (Ω)

−Zim

g(Ω

)

(b)

Figure 7: Bode plots, (a) Zreal versus frequency and (b) Zimg versus frequency, for Nafion-112 (♦), Naf-112/DTA+ dry (�) and Naf-112/DTA+(w) (�) membranes.

−1.5 0 1.5 3−40

−20

0

20

40

ΔΦ

mem

br(m

V)

ln (cv/c f )

Figure 8: Membrane potentials versus solution concentrationsratio for Nafion-112 (�) and Naf-112/DTA+ (�) in contact withHCl solutions.

Naf-112/DTA+ samples (3 or 4 orders of magnitude, resp.),while the shift to lower values of the maximum relaxationfrequency obtained for the IL-modified samples might berelated to a more compact charge packing.

Membrane potential, ΔΦmemb, is the electrical potentialdifference between both sides of a membrane separating twosolutions of the same electrolyte but different concentrations(c1 and c2) [35]. Figure 8 shows the variation of membranepotentials with the logarithm of the solution concentrationsratio, where a practically linear relationship can be observed;

for comparison, membrane potential for an ideal cation-exchange membrane is also shown in Figure 7 (dotted line)and these values hardly differ from those corresponding toboth Naf-112/DTA+ and Nafion-112 membranes in agree-ment with the well-known electronegative character of thissample.

Ion transport number (ti) is the parameter commonlyused to characterize the effect of membrane charge on thetransport of ions since it represents the fraction of the elec-tric current transported by an ion i (ti = Ii/IT, then

∑i ti = 1;

t+ + t− = 1 for single salts). For an ideal cation-exchang-er membrane t+ = 1, and the ΔE for the concentrationcell reaches the maximum value [35]: ΔEmax = −(2RT/F) ln(c2/c1); then, the value of the cation transport numberfor two given concentrations (c1 and c2) can be obtained as:t+ = ΔE/ΔEmax. Cation transport numbers across Nafion-112 and Naf-112/DTA+ membranes were calculated and thefollowing average values were obtained: 〈t+ (Nafion-112)〉 =(0.946± 0.006) and 〈t+(Naf-112/DTA+)〉 = (0.973± 0.012),which hardly differentiate one from each other although theyseem to indicate a slightly higher proton transport in the IL-modified membrane. Cation transport number values ob-tained in this paper hardly differ from those determined in[18] for a Nafion membrane and different electrolyte solu-tions.

3.5. Monitoring IL-Cation Inclusion Electrical Effects. As wasalready established in the previous sections, the H+/DTA+

exchange mechanism taking place when the Nafion-112membrane was in contact with the IL solution causes changesin the membrane and the membrane solution interface. Timeevolution of the impedance spectroscopy curves can giveinformation on such changes. Figure 9 shows the impedancediagrams (Bode plots) obtained for the system electrode//IL-solution/Nafion-112/IL-solution//electrode at different time


102 104 106

103

104

105

100

f (Hz)

Zre

al(Ω

)

(a)

102 104 106100

10

100

1000

Interface

Membrane

IL

f (Hz)

1000022 h

40 h

1 h

6 h

0 h

(x)DTA+-Aqueous solution

−Zim

g(Ω

)

(b)

Figure 9: (a) Variation of the (a) real part of the impedance versus frequency and (b) imaginary part of the impedance versus frequency as afunction of the Nafion-112 membrane contact time with the IL-solution. (o) t = 0, (�) t = 1 h, (�) t = 6 h, (♦) t = 22 h, and (∇) t = 40 h.Impedance results for the IL-solution alone (×), this means without any membrane in the measuring cell are also drawn.

instances. For comparison, the impedance values measuredwith the IL solution (without any membrane placed in thecell) are also indicated in Figure 9. These results show theseparate effect of DTA+ inclusion (and H+ lost) on both thereal part of the impedance (related to the charge movementthrough the electrical resistance) and the imaginary partof the impedance (associated to charge storage by thecapacitance). Figure 9 (a) shows the increase of Zreal valuesfor the membrane/IL-solution system with time, where thereare higher values for 22 h than for 40 h, but these results alsoinclude interfacial effects, which seem to be developed after10 h of membrane/IL-aqueous solution contact. This point ismore clearly observed in Figure 9(b), where different relaxa-tions depending on the system and/or time period con-sidered can be observed. As expected, the IL-solution onlypresents a relaxation at high frequencies ( fmax ≈ 3 ×106 Hz) since only free or “solution” charges are involved;however, the membrane/IL-solution system exhibits differentbehaviours depending on the time period which is associ-ated to the exchange kinetic: (a) at the beginning of theexchange process the main effect observed corresponds tothe increase in of membrane characteristic parameters (elec-trical resistance and capacitance) due to the replace of thesmall and high conductive proton (H+) by the wider andheavy DTA+-cation and two relaxation processes, one asso-ciated to the membrane ( fmax ≈ 104 Hz) and another tothe electrolyte solution placed between the electrodes andthe membrane surfaces ( fmax ≈ 2-3 × 106 Hz) can be ob-served; (b) after some hours of membrane immersion in theIL-solution (around 10 h) a structured profile seems to bedeveloped at the membrane interface and then three differ-ent relaxation processes with maximum frequencies around200 Hz (interface), 104 Hz (membrane), and 2-3 × 106 Hz

(electrolyte solution) are obtained. These latter curves aresimilar to those obtained with composite membranes (asthose used for reverse osmosis or nanofiltration processes) incontact with common electrolytes solutions (NaCl or KCl)and are associated to the typical double layer (dense-activelayer/porous-support) structure of that kind of membranes[36, 37].

The fitting of the impedance data obtained for each timeallows the estimation of the electrical resistance and capa-citance associated to both bulk membrane and interface andits dependence with membrane immersion time in the IL-solution is shown in Figure 10. A significant increase in themembrane electrical resistance associated to the IL-cationincorporation (similar to that shown in Figure 1) can beobserved in Figure 10, but an interfacial process seems to bedeveloped after 6–8 h, which practically corresponds to thetime at which the proton/cation exchange mechanism forIL-cation incorporation reaches the plateau; this interfacialprocess is time decreasing probably due to counter-diffusioneffect.

These results show the complexity of IL-cation incorpo-ration into Nafion membranes associated to the kinetic ofproton/IL-cation exchange as well as the possibility of usingimpedance spectroscopy technique to discriminate the effectsassociated to both the own membrane and the interface.

4. Conclusions

The modification of a typical cation-exchange polymericmembrane by inclusion of the IL-cation into the matrixstructure by a proton/cation exchange mechanism and itseffect on chemical, thermal, and charge transport was pre-sented. Membrane thickness seems to affect equilibrium time


0 20 40 600

1

2

3×105

Ri

(Ω)

t (h)

Figure 10: Time dependence of the electrical resistance of (�) bulkmembrane, (�) interface.

but hardly modifies the incorporation degree at equilibrium,which presents an average value of (60.5 ± 0.5)% for theNafion-DTA+ aqueous solution studied.

Electrical, thermal and surface characterizations of thestudied systems were basically carried out by impedancespectroscopy, DSC, XPS, and contact angle measurements.DTA+ incorporation seems to reduce the hydrophobic char-acter of the original Nafion membranes and to increase theirthermal stability. The effect of water in the electrical resis-tance of the modified samples was also shown, which is sig-nificant point for many electrochemical applications. More-over, this work also presents the direct correlation betweencation incorporation in the membrane and modifications inits electrical resistance but also associated interfacial effects.

The diversity of available ILs made it possible to choosethat the use of IL-modified membranes’ is more adequate fora specific application opening for a wide variety of applica-tions, particularly those related to electrochemical systems.

Acknowledgments

V. Romero acknowledges the financial support to MICINN,Spain, through the FPU scholarship. The authors thank toProf. J. Crespo, Prof. M. I. Coelhoso and Dr. A. Neves, Dpt.Chemical Engineering at the Universidade Nova de Lisboafor their supervision and help during cation-incorporationin membranes measurements, and to CICYT (ProjectCTQ2011-27770, FEDER funds) for financial support.

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Review Article

Applications of Ionic Liquids in ElectrochemicalSensors and Biosensors

Virendra V. Singh,1 Anil K. Nigam,1 Anirudh Batra,2 Mannan Boopathi,1

Beer Singh,1 and Rajagopalan Vijayaraghavan1

1 Defence Research and Development Establishment, DRDO, Gwalior 474 002, India2 Amity University, Noida 201303, India

Correspondence should be addressed to Mannan Boopathi, [email protected]

Received 10 August 2011; Accepted 14 November 2011


Copyright © 2012 Virendra V. Singh et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Ionic liquids (ILs) are salt that exist in the liquid phase at and around 298 K and are comprised of a bulky, asymmetric organiccation and the anion usually inorganic ion but some ILs also with organic anion. ILs have attracted much attention as a replacementfor traditional organic solvents as they possess many attractive properties. Among these properties, intrinsic ion conductivity, lowvolatility, high chemical and thermal stability, low combustibility, and wide electrochemical windows are few. Due to negligible ornonzero volatility of these solvents, they are considered “greener” for the environment as they do not evaporate like volatile organiccompounds (VOCs). ILs have been widely used in electrodeposition, electrosynthesis, electrocatalysis, electrochemical capacitor,lubricants, plasticizers, solvent, lithium batteries, solvents to manufacture nanomaterials, extraction, gas absorption agents, andso forth. Besides a brief discussion of the introduction, history, and properties of ILs the major purpose of this review paper isto provide an overview on the advantages of ILs for the synthesis of conducting polymer and nanoparticle when compared toconventional media and also to focus on the electrochemical sensors and biosensors based on IL/composite modified macrodiskelectrodes. Subsequently, recent developments and major strategies for enhancing sensing performance are discussed.

1. Introduction

ILs are a class of materials which have attracted manyscientists as holding a great promise for green chemistryapplications [1–3]. Paul Walden gave a definition to ILsthat is still acknowledged today [4]. They are “materialscomposed of cations and anions, those melt around 100◦Cor below as an arbitrary temperature limit.” This definitionidentifies the difference from molten salts that have also beenknown for a long time and are inorganic salts with highmelting temperatures [5]. A typical IL has a bulky organiccation (e.g., N-alkylpyridinium, N-N′-dialkylimidazolium)that is weakly coordinated to an organic or inorganic anion,such as BF4

−, Cl−, I−, CF3SO3−, and AlCl4

−. The bigdifference in the size of a bulky cation and a small aniondoes not allow packing of lattice, which happens in manyinorganic salts; instead, the ions are disorganized. This resultsin that some of these salts remain liquid at the room

temperature [2]. ILs are liquid electrolytes composed entirelyof ions. Due to the high ionic conductivity, nonvolatility,low vapor pressure, thermal stability, hydrophobicity, andwide electrochemical window that ionic liquids possess,these compounds have become a novel solution to problemsencountered with organic solvents and these molecules area prospective solution to the limitations encountered inelectrochemical systems [6, 7]. This new chemical groupcan reduce the use of hazardous and polluting organicsolvents due to their unique characteristics as well astaking part in various new syntheses. Due to these uniqueproperties, ILs have been widely used in different field ofapplications (Figure 1). These physical properties can bevaried by selecting different combinations of ions [8, 9].Since the electrochemical window of the pure ILs dependson the electrochemical stability of the cation and/or anion,understanding the ion behavior at the electrode surface leadsto improvement and implementation of the IL to the desired


Chemistry

Coatings

Energy

Lonic liquids

Chemical engineering

Electro- chemistry

Biotechnology

Figure 1: Applications of ILs.

system [10]. The presence of an abundance of charge carriersmeans that when ILs are used as solvents, no supportingelectrolyte is required for electrochemical experiments andthis minimizes waste towards greener site [9].

2. History of ILs

The field of ILs has been reviewed by several authors,including Welton [8], Holbrey and Seddon [9], and Seddon[2]. Moreover, ILs have been known for a long time, buttheir extensive use as solvents in chemical processes forsynthesis and catalysis has recently become significant. Thediscovery date of the first IL is disputed, along with theidentity of its discoverer. Ethanolammonium nitrate (m.p.52–55◦C) was reported in 1888 by Gabriel and Weiner [11].The first RTIL [EtNH3][NO3] (melting point 12◦C) wasdiscovered in 1914 [12]. This material is probably the firstdescribed in the literature that fulfills the definition of ILsused today. In this context it should be noted that at thattime Walden had of course no idea of this definition or thewhole concept of ILs. Consequently, it is not surprising thatat time no attention was paid to the potential of this classof materials. Moreover, Walden had reported the physicalproperties of ethylammonium nitrate, [C2H5NH3] NO3,which has a melting point of 12◦C, formed by the reaction ofethylamine with concentrated nitric acid, but interest did notdevelop until the discovery of binary ILs made from mixturesof aluminum(III) chloride and N-alkylpyridinium [13] or1,3-dialkylimidazolium chloride [14].

In 1970s and 1980s, Osteryoung et al. [13, 15] and Husseyet al. [14, 16, 17] carried out extensive research on organic

chloride-aluminium chloride ambient temperature ILs andthe first major review of ILs was written by Hussey [18]. TheILs based on AlCl3 can be regarded as the first generation ofILs.

In the late 1990s, ILs became one of the most promisingchemicals as solvents. An important property of theimidazolium halogenoaluminate salts is that their physicalproperties such as viscosity, melting point, and aciditycould be adjusted by changing the alkyl substituents andthe imidazolium/pyridinium and halide/halogenoaluminateratios [19]. Two major drawbacks for some applications weremoisture sensitivity and acidity/basicity. In 1992, Wilkes andZaworotko obtained second generation of ILs with “neutral”weakly coordinating anions such as hexafluorophosphate(PF6

−) and tetrafluoroborate (BF4−), allowing a much wider

range of applications [20]. After the first reports on thesynthesis and applications of air stable ILs such as 1-n-butyl-3-methlyimidazolium tetrafluoroborate ([BMIm][BF4])and 1-butyl-3-methlyimidazolium hexafluorophosphate([BMIm][PF6]), the number of air and water stable ILs hasstarted to increase rapidly [21]. Unlike the chloroaluminateILs, these ILs could be prepared and safely stored outsideof an inert atmosphere. Generally, these ILs are waterinsensitive; however, the exposure to moisture for a longtime can cause some changes in their physical and chemicalproperties. Therefore, ILs based on more hydrophobicanions such as tri-fluoromethanesulfonate (CF3SO3

−),bis(trifluoromethanesulfonyl)imide [(CF3SO2)2N−], andtris(trifluoromethanesulfonyl)methide [(CF3SO2)3C−] havebeen developed [22–24]. These ILs have received extensiveattention not only because of their low reactivity with waterbut also because of their large electrochemical windows.Usually these ILs can be well dried to the water contents ofbelow as 1 ppm under vacuum at temperatures between 100and 150◦C.

3. Electrochemical Properties of ILs

ILs are made of positively and negatively charged ions,whereas water and organic solvents, such as toluene anddichloromethane, are made of molecules. The structure ofILs is similar to the table salt such as sodium chloride whichcontains crystals made of positive sodium ions and negativechlorine ions, not molecules. While salts do not melt below800◦C, most of ILs remains liquid at room temperature.Since these conventional molten salts exhibit high meltingpoints, their use as solvents in applications is severely limited.However, ILs are liquid generally up to 200◦C. ILs have a wideliquid ranges.

Researchers explained that ILs remain liquid at roomtemperature due to the reason that their ions do not packwell [25]. Combination of bulky and asymmetrical cationsand evenly shaped anions forms a regular structure, namely,a liquid phase. The physical and chemical properties of ILscan be varied over a wide range by the selection of suitablecations and anions. Some of the properties that dependon the cation and anion selection include melting point,viscosity, and solubility characteristics [26]. Most widely


Table 1: List of some abbreviations.

Abbreviation Name

[BEIm] 1-Butyl-3-ethylimidazolium

[BMIm]BF4 1-Butyl-3-methylimidazolium tetrafluoroborate

[BMIm]PF6 1-Butyl-3-methylimidazolium hexafluorophosphate

[BMIm]Tf2N/NTf2 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[BMP]Tf2N/NTf2 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

[BMMIm]Tf2N/NTf2 1-Butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

[BMIm]TfO 1-Butyl-3-methylimidazolium trifluoromethanesulfonate

[EMIm]BF4 1-Ethyl-3-methylimidazoliumtetrafluoroborate

[EMIm]Cl 1-Ethyl-3-methylimidazolium chloride

[EMIm]PF6 1-Ethyl-3-methylimidazolium hexafluorophosphate

[OMIm]Tf2N/NTf2 1-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[PMP] 1-Propyl-1-methylpyrrolidinium

TFA Trifluoroacetate

Tf2N/NTf2 Bis(trifluoromethylsulfonyl)imide

TfO Trifluoromethanesulfonate

BF4 Tetrafluoroborate

PF6 Hexafluorophosphate

used ILs and their structures are given in Table 1. As solvents,ILs posses several advantages over conventional organicsolvents, which make them environmentally compatible [27–34].

(i) A diverse range of organic, inorganic, and organo-metallic compounds are soluble in ILs. The solubilityof gases such as O2, benzene, nitrous oxide, ethylene,ethane, and carbon monooxide is also good, whichmakes them attractive solvent system for catalytichydrogenations, carbonylations, hydroformylation,and aerobic oxidations.

(ii) Due to the large electrochemical window, they areespecially suitable as reaction media for electrochem-ical (and also chemically induced) polymerizationprocesses leading to conducting polymers.

(iii) ILs are highly polar.

(iv) ILs consist of loosely coordinating bulky ions.

(v) Most of ILs have a liquid window of up to 200◦Cwhich enables wide kinetic control.

(vi) ILs have high thermal conductivity.

(vii) ILs are immiscible with many organic solvents.

(viii) ILs are nonaqueous polar alternatives for phasetransfer processes.

(ix) The solvent properties of ILs can be tuned fora specific application by varying the anion cationcombinations.

(x) ILs tend to have good thermal stability and can beliquid over a range of 300◦C. This wide liquid rangeis distinct advantages over traditional solvent systemthat have a much narrower liquid range; for example,water has a liquid range of 100◦C or toluene 206◦C.

(xi) The majority of ILs have low volatility. This propertymakes them easy to contain, use, and transfer andin addition they can be used under high vacuumconditions. This is an important feature that reduceschronic exposure to solvent vapors.

(xii) ILs can be recycled. Recovery and recycling of thecatalyst are also possible with the ILs, thus keepingproduction of waste and loss of valuable catalysts to aminimum.

Properties such as nonflammability, high ionic conduc-tivity, and electrochemical and thermal stability of ILs makethem ideal electrolytes in electrochemical devices like inbatteries [35–38], capacitors [39–41], fuel cells [42], photo-voltaics [43–48], actuators [49], and electrochemical sensors.Recently, ILs have captured the attention of the analyststo use them in different analytical applications as well. ILscan improve separation of complex mixtures of both polarand nonpolar compounds when used either as stationaryphase or as additives in gas-liquid chromatography [50–53],liquid chromatography [52], and capillary electrophoresis[54]. They are also used in optical sensors [55, 56] and alsoto enhance the analytical performance of the matrix-assistedlaser desorption ionization mass spectrometry (MALDI-MS)[57]. The use of ILs in different applications is determined bytheir intrinsic properties.

3.1. Physical Properties of ILs

3.1.1. Conductivity. In electrochemical experiments, theconductivity of a solvent is of vital importance. ILs havereasonably good ionic conductivities compared with thoseof organic solvents/electrolyte systems (up to 10 mS cm−1)[58]. At elevated temperatures of, for example, 200◦C aconductivity of 0.1Ω−1 cm−1 can be achieved for somesystems. ILs have advantages over traditional organic solvents


as their conductivity is intrinsic and they do not requirethe addition of supporting electrolyte. However, at roomtemperature their conductivities are usually lower than thoseof concentrated aqueous electrolytes. Based on the fact thatILs are composed solely of ions, it would be expected thatILs have high conductivities. This is not the case since theconductivity of any solution not only depends on the numberof charge carriers but also on their mobility. The largeconstituent ions of ILs reduce the ion mobility which, inturn, leads to lower conductivities. Furthermore, ion pairformation and/or ion aggregation lead to reduced conduc-tivity. In fact, the change in conductivity with compositionin ILs can be attributed almost directly to changes in theviscosity [59]. Hence, ILs of higher viscosity exhibit lowerconductivity. Increasing the temperature results in increasein conductivity and decrease in viscosity.

3.1.2. Viscosity. The viscosity of a solvent is an importantelectrochemical property since high viscosities limit someapplications and slow down the rate of diffusion-controlledchemical reactions [60].

As a result of the strong electrostatic and other interac-tion forces the viscosity of ILs is typically 10 to 100 timeshigher than that of water or organic solvents [61–64]. Theviscosity of IL is affected by the nature of both the cationsand anions. Alkyl chain lengthening in the cation leads toan increase in viscosity. This is due to stronger van derwaals forces between cations leading to increase in the energyrequired for molecular motion. The nature of the anion alsoaffects the viscosity of the IL, particularly through relativebasicity and the ability to participate in hydrogen bonding.It is found that PF6

− and BF4− anions form much more

viscous IL due to strong H. . ..F interactions than those ILformed with the weakly basic [NTf2]− anion, in which thenegative charge is quite delocalized over the two sulfoxidegroups [65]. Differences in viscosities may also be causedby the size, shape, and molar mass of anion with smaller,lighter, and more symmetric anions leading to more viscousILs.

3.1.3. Density. ILs are composed only of ions; almost all ILsare denser than water, from 1.0 to 1.6 g cm−3 depending ontheir ion structure. The density has been found to decreasewith increasing alkyl chain length on the imidazolium cation[66, 67]. Similarly, in the ammonium and sulfonium salts,the density decreases with increasing alkyl chain length.This clearly shows that the charged ion unit is heavierthan the hydrocarbon chain. Accordingly, the density ofILs is tunable to some extent. The density of aromaticonium salts is higher than that of aliphatic ammoniumsalts. Generally, density decreases in the order of pyridiniumsalts > imidazolium salts > aliphatic ammonium salts andpiperidinium salts. The densities of ILs are also affected bythe anion species. Similarly to the trends for cations, thedensity of ILs decreases with increasing alkyl chain length ofthe anion. The density of ILs is increased on the introductionof a heavy chain such as fluoroalkyl chains. For example,1-ethyl-3-methylimidazolium (EMIm) salts became heavier

with the following anion species: CH3SO3− < BF4

− andCF3COO− < CF3SO3

− < (CF3SO2)2N− < (C2F5SO2)2N−. Itis easy to understand this order as an effect of formula weightof the ions. However, these tendencies are still empirical anda perfect correlation between ion structure and density is notavailable.

3.1.4. Thermal Stability and Decomposition Temperature.ILs can be thermally stable up to temperatures of 450◦Cwith decomposition temperatures around 300–500◦C. Thethermal stability of ILs is limited by the strength of theirheteroatom-carbon and their heteroatom-hydrogen bonds,respectively [68]. The thermal decomposition temperaturedecreases as the anion hydrophobicity increases. Halideanions reduce the thermal stability of ILs, with decom-position occurring at least 373 K below corresponding ILswith nonhalide anions. Relative anion stabilities have beensuggested by Huddleston et al. [69] as PF6

− [BMIm]PF6 > NTf2

− ≈ BF4− > halides. The ILs [BMIm][PF6],

[BMIm][NTf2], and [BMIm][BF4] have decomposition tem-perature of 373 K higher than the corresponding halide IL[BMIm][I]. The trend of thermal stability with respect tocation species appears to go as follows: phosphonium >imidazolium > tetraalkyl ammonium pyrrolidinium [70].

3.1.5. Electrochemical Potential Window. The electrochemi-cal potential window (potential region without significantbackground current) is a key criterion to be consideredwhen any medium is used in electrochemical measurements.A general common feature of ILs is their inherent redox-robustness, because of the robustness of cations and anionsemployed for their preparation. ILs generally exhibit a widepotential window, which is highly desirable property forapplying the ILs as electrochemical solvents. Typical windowsof 4.5–5 V have been reported for the ILs [71, 72] andeven an enlarged electrochemical window up to 7 V wasfound for some IL such as 1-butyl-3-methyl-imidazoliumtetrafluoroborate [73]. On the whole, this potential windowrange is equal to slightly wider than that observed inconventional organic electrolytes but largely exceeds thataccessible in aqueous electrolytes.

4. Applications of ILs in Conducting Polymerand Nanoparticle Syntheses

The utilization of ILs for the synthesis and use of conductingpolymers (CPs) brings together two of the most excitingand promising areas of research from recent years. CPsare organic materials that can display electronic, magnetic,and optical properties similar to metals, but that also havethe mechanical properties and low density of a polymer.They have the potential to allow the design and fabricationof a vast number of electrochemical devices includingphotovoltaics, batteries, chemical sensors, supercapacitors,conducting textiles, electrochromics, and electromechanicalactuators [74–77].

Research into CPs has been increasingly intense for thelast 3 decades, since MacDiarmid, Heeger, and Shirakawa


published their seminal work on polyacetylene, whichdemonstrated that the conductivity of these materials canbe increased by several orders of magnitude by doping withanions [78, 79]. The importance of these materials andthe progress made in this field is reflected in the award ofthe Nobel Prize for chemistry in 2000 to these foundingresearchers in this area.

ILs are new solvents for polymerization reactions.The potential benefits of using ILs as electrolytes in CPsdevices have been investigated by a number of authorsfor applications such as actuators [80–88], supercapacitors[89–91], electrochromic devices [83, 92], and solar cells[93] with significant improvements in lifetimes and deviceperformance reported.

The potential of the ILs as media for electrosynthesis ofCPs was first demonstrated in the late 1980s for polyfluorene[94, 95], polythiophene [96], or polyphenylene [97] inchloroaluminate ILs. But the high sensitivity of these ILstowards water produced HCl that led to rapid decompositionof the polymer. One should remember that electrooxidativepolymerization involves the coupling between two radicalcations of the monomer or of the produced oligomers [98].It is likely that stabilizing interactions should play a favorablerole in the coupling between two charged species, explainingthe good results noticed in ILs.

Electrosynthesis of polypyrrole (pPy) in [BMIm][PF6]on iron and Pt [99, 100] and in [EMIm][OTf],[EMIm][BF4], and [EMIm][PF6] on Pt [101], poly(3,4-eth-ylenedioxythiophene) (PEDOT) in [EMIm][NTf2], [BMIm][PF6], and [BMIm][BF4] on Pt [102–104], or in[EMIm][BF4] on “K-glass” of poly(3-(4-fluorophenyl)thiophene) in [EMMIm][NTf2] and [EEMIm][NTf2] on Pt[105], poly(paraphenylene) (PPP) in 1-hexyl-3-methyl-imidazolium tris(pentafluoroethyl) trifluorophosphate onPt [106], poly(3-chlorothiophene) in [BMIm][PF6] on Pt[107], and of polythiophene in [EMIm][NTf2] on Pt [108]has been studied.

Lu et al. [109] reported significant improvements indevice performance when the ILs 1-ethyl-3-methylimida-zolium hexafluorophosphate, [C2mim][PF6], and 1-ethyl-3-methylimidazolium tetrafluoroborate, [C2mim][BF4], wereused as supporting electrolytes for pPy and poly(aniline)actuators and for PEDOT in electrochromic devices, respec-tively. For the PEDOT study, the IL was also used as thegrowth medium for the electropolymerization.

Easier or more efficient preparation of CPs films in neatILs as compared with conventional media was highlighted insome works [102, 107, 110, 111]. The electropolymerizationof benzene could be achieved with milder conditions in ILsthan in the usually employed concentrated sulfuric acid orliquid SO2 [110]. As mentioned by the authors, the use ofILs would enable further studies on the nanoscale with insitu scanning tunneling microscopy (STM) that would betotally precluded in aggressive media like 18 M sulfuric acidor liquid SO2 [110]. Comparison of electropolymerizationof pyrrole in neat [EMIm][OTf], as well as in [EMIm][OTf]diluted in CH3CN or H2O (0.1 mol L−1), demonstrated thatthe electropolymerization is more efficient in the neat IL[110]. This result strengthens the idea that ILs are powerful

media for the electrochemical generation of CPs films in agreener way.

The influence of the nature of the ILs towards thepreparation, the morphology, and the electrochemical activ-ity of polymers has been investigated [101, 102, 108].The electropolymerization of pyrrole was found to bemore efficient in [EMIm][OTf] than in [EMIm][BF4] and[EMIm][PF6], with a formation of smoother and morehighly doped polymer films than those from the latterILs. This result indicates a significant influence of theanion [103]. Thiophene, bithiophene, and terthiophenehave been polymerized in imidazolium- and pyrrolidinium-based [NTf2] ILs. Whatever the monomer, use of thepyrrolidinium salts led to polythiophene films that weresmoother and denser and had a lower electroactive surfacethan those from the imidazolium salts [108]. Very strikingdifferences between films grown in conventional solventsand those grown in ILs have been observed using scanningelectron microscopy (SEM) [102, 112–116]. Generally, thefilms grown from ILs appear to be considerably smoother,which may also result in improved conductivities. SEManalysis of poly(thiophene) grown from [C2mim][NTf2] and[C4mpyr][NTf2] reveals a slightly smoother morphologyfor the poly(thiophene) films from the pyrrolidinium ionicliquid [105].

Dong et al. [117] have investigated the electro-chemical polymerization of 1,2-methylenedioxybenzene(MDOB) in an IL 1-butyl-3-methylimidazolium hex-afluorophosphate [BMIm][PF6]. This polymer poly(1,2-methylenedioxybenzene) (PMDOB) showed good redoxactivity and stability even in concentrated sulfuric acid.In contrast to acetonitrile containing 0.1 mol/L Bu4NBF4,BMImPF6 serves as both the growth medium and anelectrolyte. Hence, enhanced electrochemical stability ofPMDOB can be easily obtained on repetitive redox cycling;as formed PMDOB represented good electrochromic prop-erties from green grass to opalescent between doped anddedoped states. SEM results demonstrated that smooth andcompact PMDOB films composed of ordered nanostructureswere obtained, implying their possible utilizations in ion-sieving films, ion-selective, and matrices for catalyst particles.

Sekiguchi et al. [118] reported the electropolymerisationof pyrrole in 1-ethyl-3-methylimidazolium triflate, bothneat and as a 0.1 M solution in either acetonitrile, orwater and observed an improvement in the morphol-ogy and electrochemical capacity of the films. Moreover,Pringle et al. [119] reported the electrochemical synthe-sis of inherently CPs such as polypyrrole in a molecu-lar solvent/electrolyte system such as acetonitrile/lithiumperchlorate and IL as well. We use ILs 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methy-limidazolium bis(trifluoromethanesulfonyl)amide, and N,N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)amide, both as the growth medium and as an electrolyte forthe electrochemical cycling of polypyrrole films. Use of theionic liquid as the growth medium results in significantlyaltered film morphologies and improved electrochemicalactivities over traditional organic solvents.


Pyrrole was electropolymerized in ILs using a platinumwire that was vertically placed into the cell, completelyimmersed in the ILs and touching the bottom of the cell.It was observed that if the polymerization started as aninitial layer on a section of the working electrode, prolongedelectrosynthesis led to a growth of the polymer film along thesurface of the ionic liquid. This solution-surface electropoly-merization was found to occur with a large range of ILs [120].Fibrils of polypyrrole, polythiophene, and PEDOT wereobtained from closely related interfacial polymerization.Using this time a chemical polymerization route, a biphasicionic liquid/water system with the oxidant dissolved in theaqueous phase and the monomers dissolved in the ILs, allowsthe polymerization to occur at this RTIL/water interface. Theresulting polymers consisted of fibrils about 50 nm wide andseveral hundreds of nanometers long [121].

The introduction of the metal nanoparticles into thesensing interface to facilitate the electron transfer can signif-icantly improve the sensitivity [122]. Nanoparticles of noblemetals have attracted special interest because they differfrom their bulk metal counterparts. Their size controllability,chemical stability, large surface area, thermal stability, goodbiocompatibility, high catalysis activity, surface tenability,and suitability for many surface immobilization mechanismsmake them very advantageous for application in sensors[121, 123]. For example, CaCO3 nanoparticles can offer alarge surface area for adsorption of substances of interest dueto their porous structure.

ILs possess pre-organized structures, mainly throughhydrogen bonds [124–126] which induce 3D structuredimensionality in these systems. Conversely, aggregates ofclassical salts display charge-ordered structures. Since ILscan form extended hydrogen bond networks at the liquidstate, therefore they demonstrate this very unique propertyof high self-organization on the nanomolecular scale andcan be classified as supramolecular fluids. This nanoscalestructural organization of ILs can be used to drive thespontaneous extended ordering of nanomaterials [127, 128].Some of the classical examples that ILs as designer solventsfor the synthesis of metal nanoparticles have illustratedthis concept include IL-mediated synthesis of orderedmesoporous materials and microporous aluminophosphates,wherein ILs served as both the solvent and structure-directing agents [129, 130]. One another example in thiscategory includes synthesis of protein and silica nanocapsulesin [BMIm][BF4] IL via self-organization process, as wasdemonstrated recently by Suarez et al. [131]. It has also beenestablished that the properties of imidazolium-based ILs aredependent on their organized nanoaggregates, rather thanmerely on their isolated cations and anions [132, 133]. The3D arrangement of the imidazolium-based ILs is generallyformed through chains of the cationic imidazolium rings,which generates supramolecular channels in which anionsare typically accommodated as chains [126]. The formationof this 3-D ionic network entails high directional polar-izability, which provides an opportunity to adopt a rangeof external species in either hydrophilic or hydrophobicregions of ILs [134]. Therefore, size and shape of the metalnanoparticles synthesized in ILs are typically modulated by

the volume of these 3-dimensionally arranged regions withinIL environment. It would probably not be an overstatementto classify ILs as nanostructured solvents, with a potential todirect the tailored synthesis of nanoscale materials.

A series of studies in the literature suggests that ILsinteract relatively strongly with the surface of metal nanopar-ticles, which have been summarized in a recent report [135].In fact, transition metal nanoclusters and nanoparticlesstabilization by imidazolium-based ILs are now considered asa classical stabilization method [135]. Therefore, in accountof the previous points, unambiguously ILs are an appealingmedium for the formation and stabilization of catalyticallyactive transition metal nanoparticles.

5. Applications of IL in Gas Sensor

Extensive efforts have been made to develop new materialsand transducers for gas sensing both at room and athigh temperatures with particular emphasis on optimizinginterface properties among the gas phase, the sensitivematerials, and the transducer. IL thin films perform wellas sensor interfaces and provide additional control overselectivity and sensitivity when interacting with analytes inthe gas phase.

Due to the entire ionic composition, which eliminates theneed to add supporting electrolyte, the intrinsic conductivityand negligible vapor pressure ILs are unique compoundsto be used in the development of stable electrochemicalsensors for gaseous analytes such as O2, CO2, and NH3

[136–142]. The superoxide radical (O2•−) which is generated

in situ by the reduction of O2 was found to be stable inILs at glassy carbon, gold (Au), or platinum (Pt) electrodes[136–138]. This makes the amperometric detection of O2

possible and the reported solid-state O2 gas sensor basedon porous polyethylene supported [EMIm][BF4] membranehas a wide detection range, high sensitivity, and excellentreproducibility [136]. With increasing levels of CO2 in thesample, cyclic voltammetry shows an increased cathodicpeak current from the production of O2

•− radicals togetherwith the decreased peak current from the reverse scan ofoxidation. This indicates that the generated O2

•− radicalreacts irreversibly with CO2 to form peroxydicarbonate ion,C2O6

2− [138].

Cai et al. [143] have developed an SO2 gas sensorresembling the Clark model that employs an IL as theelectrolyte, while Wang and coworkers [136, 144] havereported a supported IL membrane-coated oxygen sensor,incorporating 1-ethyl-3-methylimidazolium tetrafluorobo-rate ([C2MIm][BF4]) into a polyethylene membrane.

The oxidations of halides, Cl− and Br−, display some dif-ferent behaviors in ILs than those observed in organic media.Oxidation of Cl− in [BMIm][PF6], with large concentrationsof [BMIm][Cl], displays an irreversible process on bothplatinum and graphite electrode. The main feature was thatthis oxidation does not lead to chlorine gas evolution but tothe formation of oxidation products that stay in the ILs in theform of complexes between Cl2 molecule and chloride ionwhere the Cl3

− is the major product [145–147]. Moreover,


the electrochemical oxidation of the nitrite ion (NO2−)

and nitrogen dioxide gas (NO2) in [EMIm][Tf2N] has beenstudied by cyclic voltammetry on Pt electrodes of varioussizes [148]. From chronoamperometric measurements, thefollowing solubility values were calculated: 7.5 mM for NO2

−

and 51 mM for NO2, indicating that this IL is a potentialmedia for sensing NO2 gas.

Determination of ammonia based on the electrooxida-tion of hydroquinone in dimethylformamide (DMF) and[EMIm][Tf2N] has also been reported [141]. Ammonia canremove protons from the hydroquinone molecules reversiblyand thus facilitate the oxidation process giving a new waveat less positive potentials in the cyclic voltammogram (CV).Similar responses were found in both dimethylformamideand [EMIm][Tf2N]. The detection limit of ammonia basedon this method is 4.2 ppm in DMF. Solubility and thermo-dynamic properties of different gases such as CO2, H2, andO2 in [BMIm][PF6] have been thoroughly studied [149]. ILsare also utilized as sensing materials for detection of organicvapors by using the quartz crystal microbalance (QCM)technique [150–152].

Yu et al. [153] have developed an integrated sensor thatcombines electrochemical and piezoelectric transductionmechanisms into a single miniaturized platform for explo-sives. The IL [BMIm][BF4] was used as both the electrolyteand the sorption solvent for the two-dimensional electro-chemical and piezoelectric gas sensors. Jin and coworkersdeveloped a high-temperature sensor array using a QCM-based sensor [151]. Thin films of seven ILs were employedto provide sensitivity to concentrations of various flammableorganic vapors (i.e., ethanol, dichloromethane, benzene,heptane).

Schafer and coworkers [154] developed a QCM-ILsensor for use as an artificial nose using the ubiquitous[C4C1Im][PF6]. The IL was spin coated onto the surfaceof a 10 MHz AT-cut quartz crystal with gold electrode. Thework specifically studied the response of the sensor to ethylacetate. The deposition of the IL on the surface of theelectrode decreased the resonance frequency of the QCM by2017 Hz, demonstrating the popularity of QCMs for sensorapplications. Goubaidoulline and coworkers opted to entrapIL within nanoporous alumina deposited on the surface ofthe QCM to eliminate dewetting of the QCM and solving theproblem of the soft IL surface [152]. The detection limits forthe organic vapors ranged from 321 to 7634 mg/m3. Sensorresponse times were on the order of minutes.

6. Application of IL in Electrochemical Sensors

By far the most highly investigated aspect of IL-basedelectrochemical sensors is in the realm of electrochemistry,with most of them being based on IL modified electrodes[155–158] wherein the IL typically serves as both binderand conductor. Common attributes observed when incor-porating ILs into electrodes include higher conductivity,good catalytic ability, long-term stability (including stabilityat elevated temperature), superior sensitivity, improvedlinearity, and better selectivity. Different electrochemical

sensors have been developed by using ILs. Several groupshad used different kinds of ILs in electrode modifica-tion for the design of new electrochemical sensors or asnovel electrocatalytic materials [159–164]. Lu et al. [165]used a novel chitosan/1-butyl-3-methylimidazolium hex-afluorophosphate (BMIMPF6) composite material as a newimmobilization matrix to entrap the proteins and studied theelectrochemistry behaviors of hemoglobin (Hb) on the glassycarbon electrode. Maleki et al. developed an IL-modifiedcarbon paste electrode (CPE) with 1-octylpyrridinium hex-afluorophosphate (OPFP) as binder and further used it forthe detection of some electroactive molecules [166–168]. Sunet al. also applied the IL-modified CPE for the investigationof the direct electrochemistry of hemoglobin [169–171].

Sun et al. [172] have used hydrophilic IL 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIm][BF4] as amodifier in the CPE to make a new kind of IL-modifiedcarbon paste electrode (IL-CPE). The fabricated IL-CPEshowed good electrocatalytic behaviors towards the oxida-tion of metol with the enhancement of the redox peakcurrent and the decrease of the peak-to-peak separation witha detection limit of 2.0 × 10−6 mol L−1 (3σ). The establishedmethod was successfully applied to the synthetic samples andphotographic solutions detection with good recovery.

Xiao et al. [173] described the carbon nanotube(CNT)/Ruthenium (Ru), Pd, and Au/IL composite electrodefor the fabrication of a nonenzymatic glucose sensor.Electrochemical experiments show that the PtRu- (1 : 1,i.e., ratio of (H2PtCl6)/(RuCl3)) multiwalled carbonnanotube- (MWNT-) IL nanocomposite-modified glassycarbon electrode (PtRu(1 : 1)-MWNT-IL/GCE) has smallerelectron transfer resistance and larger active surfacearea than PtRu(1 : 1)/GCE, PtRu(1 : 1)-MWNT/GCE,PtPd(1 : 1)-MWNT-IL/GCE, and PtAu(1 : 1)-MWNT-IL/GCE. The PtRu(1 : 1)-MWNT-IL/GCE also presentsstronger electrocatalytic activity towards the glucoseoxidation than other electrodes. At −0.1 V, the electroderesponds linearly to glucose up to 15 mM in neutralmedia, with a detection limit of 0.05 mM (S/N = 3) anddetection sensitivity of 10.7 μAcm−2 mM−1. Meanwhile, theinterference of ascorbic acid, uric acid, acetamidophenol,and fructose is effectively avoided. The as-made sensorwas applied to the determination of glucose in serum andurine samples. The results agreed closely with the resultsobtained by a hospital. This novel nonenzyme sensor thushas potential application in glucose detection.

Jabbar et al. [174] have found the excellentelectrocatalytic reductive dechlorination of 1,1 bis(p-chlorophenyl) 2,2,2-trichloroethane (DDT) in the IL 1-butyl-3-ethylimidazolium tetrafluoroborate ([BMIm][BF4])in the presence of a cobalamin derivative afforded 1,19-(ethylidene) bis(4-chlorobenzene) (DDO) and 1,19-(ethenylidene) bis(4-chlorobenzene) (DDNU) with 1,19-(2-chloroethylidene) bis(4-chlorobenzene) (DDMS); theenhanced reactivity, as well as the recyclability of thecobalamin derivative catalyst in IL, makes the present systemmore efficient for the development of green technologies.

Safavi et al. [168] have constructed a a nonenzymaticcomposite electrode by mixing the nanoscale Ni(OH)2 with


graphite powder and [Opyr][PF6], which showed excellentelectocatalytic activity towards oxidation of glucose in analkaline solution.

The mechanism of this catalytic detection is as follows:

Ni(OH)2 + OH− −→ NiO(OH) + H2O + e− (1)

NiO(OH) + glucose −→ Ni(OH)2 + glucono-δ-lactone (2)

Ding et al. [175] have used IL, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF4]), asthe supporting electrolyte for direct electrochemical responseof Myoglobin (Myb) at the basal plane graphite (BPG)electrode. Both anodic and cathodic peak currents increasedlinearly with the potential scan rate. Compared with thesupporting electrolyte of phosphate buffer, [HEMIm][BF4]played an obvious promotion for the direct electron transferbetween Myb and the BPG electrode. Further investigationsuggested that Myb was adsorbed tightly on the surfaceof the BPG electrode in the presence of [HEMIm][BF4]to form a stable, approximate monolayer Myb film. Mybadsorbed on the BPG electrode surface showed a remarkableelectrocatalytic activity for the reduction of oxygen in a[HEMIm][BF4] aqueous solution. Based on these, a third-generation sensor could be constructed to directly detect theconcentration of oxygen in aqueous solution with a limit ofdetection of 2.3× 10−8 M.

Tu and coworkers [176] developed a composite mate-rial based on single-walled carbon nanotube (SWCNT),a water-insoluble porphyrin (hydroxyferriprotoporphyrin,hematin), and [BMIm][PF6]. This composite materialwas used to modify a GC electrode in order to studythe direct electrochemistry and electrochemical propertyof porphyrin. The porphyrin dissolved in [BMIm][PF6]can be self-assembled on SWCNT by π-π noncovalentinteraction, which leads to good dispersion of the SWC-NTs in the [BMIm][PF6] and a direct electrochemi-cal response corresponding to the Fe3+/2+ redox cou-ple. The presence of SWCNT and [BMIm][PF6] pro-duces a synergic effect that accelerates the electron-transferbetween porphyrin and the electrode. As a consequence,the porphyrin/SWCNT/[BMIm][PF6]-modified GC showedexcellent electrocatalytic activity towards the reduction oftrichloroacetic acid (TCA). This study provides a facile wayfor preparing biofunctional materials, accelerating electron-transfer, and extending the application of porphyrins/IL-based composite materials in sensor applications.

Singh et al. [177] developed a greener electrochemicalmethod for a chemical warfare agent (CWA) using triethylsulfonium bis(trifluoromethylsulfonyl)imide (TSBTSI).Addition of CWA nitrogen mustard-2 (NM-2) to TSBTSIshowed a new peak in addition to an enhancementin peak current than observed for blank TSBTSI. Thecalculated diffusion coefficient for NM-2 in TSBTSI andAcetonitrile-TBAP was found to be 2.14 × 10−7 cm2 s−1 and1.57 × 10−4 cm2 s−1, respectively. The linear response wasobtained in the range of 2.94 × 10−5 to 1.17 × 10−3 M withthe correlation coefficient 0.9977 and the detection limit of1.47× 10−5 M (s/n = 3) with chronoamperometric method.Electrochemical impedance spectroscopy results revealed an

enhancement of solution conductivity after the addition ofNM-2 to TSBTSI. The large number of available IL basedon the combination of anion and cation and their uniqueproperties allows scope for the possible electrochemicaldetection of toxic CWAs in a greener way.

7. Application of IL inElectrochemical Biosensors

Biosensors are small devices employing biochemical molecu-lar recognition properties as the basis for a selective analysis.The major processes involved in any biosensor systemare (i) analyte recognition, (ii) signal transduction, and(iii) readout. In an electrochemical biosensor, a molecularsensing device couples a biological recognition elementto an electrode transducer, which converts the biologicalrecognition event into an electrical signal.

ILs have shown good compatibility with biomoleculesand enzymes and even whole cells are active in variousILs. [BMIm][Cl] was found miscible with silk, which is anattractive biomaterial with excellent mechanical propertiesand biocompatibility. The patterned films cast from thissilk-IL solution supported normal cell proliferation anddifferentiation [178]. Recently, some authors have reportedincreased stability of enzymes in ILs compared with stabilityin some organic solvents [179–181]. ILs were also foundto act as agents to stabilize proteins effectively at elevatedtemperatures [182]. Laszlo and Compton have also reportedthe catalysis of hemin activated by an electron acceptor inIL solutions and it was found that the activity of heminincreased with the enhanced amount of IL in the methanol-IL system [179].

Dramatically enhanced activity and thermal stabilityof horseradish peroxide (HRP) were obtained when itwas immobilized in the [BMIm][BF4]-based sol-gel matrix[183]. The IL was used as a template solvent for thesilica gel matrix via a simple sol-gel method via hydrolysisof tetraethyl orthosilicate in [BMIm][BF4]. This particularHRP-immobilized sol-gel matrix was further used in amper-ometric biosensors [184]. The novel amperometric hydrogenperoxide biosensor based on this ionogel exhibited excellentstability and sensitivity. The detection limit for hydrogenperoxide is reported to be 1.1 μM.

Direct electrochemical reduction of hemin has beenstudied by cyclic voltammetry and chronocoulometryin the ILs, [BMIm][PF6] and [OMIm][PF6] [185]. N-Methylimidazole- (NMI-) ligated hemin had a lower E1/2

than pyridine-ligated hemin in both ILs, which is consistentwith the stronger electron donor characteristic of NMI. Itwas further discovered that while hemin is electrochemicallyactive in IL, its behavior is modified by the ligand fieldstrength and surface adsorption phenomena at the workingelectrode. Electrochemistry and electrocatalysis of a numberof heme proteins entrapped in agarose hydrogel films in[BMIm][PF6] have also been investigated [186]. UV-Visand FTIR spectroscopy show that the heme proteins retaintheir native structure in agarose film. CV shows that thedirect electron transfer between the heme proteins and glassy


carbon electrode (GCE) is quasireversible in [BMIm][PF6].The redox potentials for hemoglobin (Hb), myoglobin, HRP,cytochrome c, and catalase were found to be lower than thosein aqueous solution, indicating the catalytic effect in theIL matrix. The heme proteins can catalyze electroreductionof trichloroacetic acid and tert-butyl hydroperoxide in[BMIm][PF6] [186]. Direct electrochemical response of HRP[187], myoglobin [188], and Hb [169] has been observed onIL-modified electrodes.

Zhao et al. [155] mixed MWCNTs with the 1-octyl 3-methylimidazolium hexafluoro phosphate, by grinding themtogether in a mortar to create a gel-like paste which was thenapplied to the surface of a cleaned GC electrode. Using aplatinum wire and a saturated calomel electrode as auxiliaryand reference electrodes, respectively, CVs were measured fordopamine in phosphate buffer (PB) for both the MWCNT-IL-modified GC electrode and a bare GC electrode. In bothcases, two pairs of redox peaks characteristic to dopaminewere observed. An immediate advantage of the MWCNT-IL-modified electrode (ME) was a larger peak current withsmaller peak separations, an indication of faster electrontransport to the electrode surface. Similar measurementsfor ascorbic acid and uric acid revealed that the anodicpeak potentials were, respectively, shifted more negative (by∼0.31 V) and more positive (∼0.02 V), when employingthe MWCNT-IL-modified electrode compared with GC.This feature helps to eliminate overlap between dopamine’sanodic peak and the anodic peaks of ascorbic acid and uricacid as occured at a GC electrode. Further resolution wasachieved by using differential pulse voltammetry. At pH 7.08,the ascorbic acid and uric acid peaks are separated fromdopamine by 0.20 and 0.15 V, respectively. Hence, dopaminecould be determined in the presence of uric acid and ascorbicacid in 100-fold excess. The detection limit of dopamine wasdetermined to be 1.0 × 10−7 M with a linear dynamic rangeup to 1.0 × 10−4 M.

In fabricating electrochemical biosensing layers, theCNT/IL-based composite materials are advantageous dueto the following reasons: (i) retain inherent mechanical,electrical, and thermal properties of CNT (e.g., large surfacearea, good electroconductive nanowires, etc.), (ii) bettersolvent and conductivity property of ILs (e.g., large potentialwindow, ion conductor, etc.), and (iii) proper interac-tion between CNT and ILs. Direct physical adsorption isone of the most widely used approaches to immobilizebiomolecules on CNT/IL-modified sensing layers. However,the preparation of CNT/IL pastes and composites by directmixing of the CNT with a suitable IL has been limitedby the very high background currents which would limitmonitoring the faradaic current, and hence, limit the useof such a combination. This increased capacitive chargingcurrent can be reduced using steady-state linear sweepvoltammetry at rotating disk electrodes, as suggested byCompton group [189].

Most of carbon paste electrodes (CPEs) reported forelectroanalysis are based on incorporation of a sensingmaterial into the carbon paste. The carbon paste usuallyconsists of graphite powder dispersed in a nonconductivemineral oil such as paraffin. Incorporation of mineral oil

gives some disadvantages. Mineral oil is not componentfixed since it is derived from refining of petroleum andprocessing of crude oil. As a result, contaminants or matrixcomponents may unpredictably effect on detection andanalysis. In addition, the mechanical stability of CPEs restssomewhere between that of liquid membrane electrodesand solid state electrodes. CPEs have attracted attentionas ion selective electrodes mainly due to their improvedrenewability, stable response, and low ohmic resistance whencompared to membrane electrodes [190–193].

Musameh and Wang [194] have prepared IL-carboncomposite glucose biosensor with the help of n-octyl-pyridinium hexafluorophosphate (nOPPF6) and graphitepowder and they found that the electrocatalytic propertiesof the ILs are not impaired by their association withthe graphite powder. The marked electrocatalytic activitytowards hydrogen peroxide permits effective amperometricbiosensing of glucose in connection with the incorporationof glucose oxidase within the three-dimensional IL/graphitematrix. The prepared composite film shows acceleratedelectron transfer with low background current and improvedlinearity. A comparison was made between IL-based bio-composite devices and conventional mineral oil/graphitebiocomposite and it is found that IL-based biocompositedevices performed well with good linearity. In this work theinfluence of the IL and glucose oxidase (GOx) loading wasalso studied towards the amperometric and voltammetricresponses.

Kachoosangi et al. [195] developed a new compositematerial based on MWCNT and [Opyr][PF6], which showedan extremely low capacitive and background current com-pared to graphite and mineral oil-based CPEs. The newcomposite material combines the unique and attractiveelectrocatalytic behaviour of CNTs and [Opyr][PF6], withvery low background current and mechanically robuststructure compared to many other forms of compositeand paste electrodes made from CNTs and other ILs.Wang et al. [196] prepared a composite material based on[BNIm][BF4], MWCNT, and chitosan successfully used forthe electrochemical oxidation of NADH.

CNT and carbon microbeads were incorporated into thewater-immiscible [BMIm][PF6] [197]. In particular, CNThas been demonstrated to be physically cross-linked in theviscous ILs, forming gels [198]. These carbon-compositematerials mixed with HRP, Hb, or glucose oxidase weredropped onto gold or GCEs. The direct electrochemistryof heme proteins or glucose oxidase immobilized at thesecarbon materials has been investigated and this promisingbiointerface promoted direct electron transfer and biocat-alytic performance [198–200].

CNT-modified GCEs showed enhanced sensitivity andstability in the oxidation of phenolic compounds [199]. TheCNT-modified electrodes were also found to show goodelectrocatalytic ability to biomolecules such as dopamine(DA), ascorbic acid (AA), and dihydronicotinamide ade-nine dinucleotide (NADH) [199–203]. CNTs usually existas tangled bundles that are difficult to dissolve in bothaqueous and organic media. However, they can be wellindividualized in ILs due to “cation-π” interactions and a


CNTs-IL gel can easily be prepared using IL as the binder[204], where the CNTs greatly enhance the conductivity ofthe system. Electrochemical functionalization of single wallcarbon nanotubes in [BMIm][PF6] was used to make aglucose sensor by covalently binding glucose oxidase (GOD)to the modified nanotube [205].

Zhang et al. [206] have prepared a novel polyaniline-IL-carbon nanofiber (PANI-IL-CNF) composite by in situone-step electropolymerization of aniline in the presenceof IL and CNF for fabrication of amperometric biosensorsfor phenol. This biosensor exhibited a wide linear responseto catechol ranging from 4.0 × 10−10 to 2.1 × 10−6 Mwith a high sensitivity of 296 ± 4 AM−1 cm−2, a limit ofdetection down to 0.1 nM at the signal-to-noise ratio of 3,and applied potential of−0.05 V. According to the Arrheniusequation, the activation energy for enzymatic reaction wascalculated to be 38.8 kJ mol−1 using catechol as the substrate.The apparent Michaelis-Menten constants of the enzymeelectrode were estimated to be 1.44, 1.33, 1.16, and 0.65 μMfor catechol, p-cresol, phenol, and m-cresol, respectively. Thefunctionalization of CNF with PANI in IL provided goodbiocompatible platform for biosensing and biocatalysis.

Tao et al. [156] compared the interaction of[BMIm][BF4] with three different types of carbonaceousmaterial for use in MEs: acid-treated MWCNTs(AMWCNTs), pristine MWCNTs (PMWCNTs), and pyro-lytic graphite powder (PGP). To prepare the MEs, eachcarbon material was ground with the water-miscible IL[BMIm][BF4], resulting in AMWCNT-[BMIm][BF4],PMWCNT-[BMIm][BF4], and PGP-[BMIm][BF4] compos-ites which were then applied on to a polished GC electrodesurface. The composite films were presumably attachedthrough electrostatic adsorption between the negativelycharged surface and the positive component of the IL. Theconductivities and optical properties of each composite werecompared using ac impedance and Raman spectroscopy.[BMIm][BF4] formed a gel when ground with MWCNTs,whereas a viscous liquid resulted from the admixture of[BMIm][BF4] with PGP. Raman spectroscopy demonstratedthat both AMWCNs and PMWCNs electrostaticallyinteract with [BMIm][BF4] while no such interactionoccurs between PGP and [BMIm][BF4], accounting forthe fact that PGP-[BMIm][BF4] blend fails to form gel.Data from ac impedance and CV measurements (usingpotassium ferricyanide, K3[Fe(CN)6]) demonstrated thatthe ME conductivity increases in the following order: PGP-[BMIm][BF4] < PMWCNT-[BMIm][BF4] < AMWCNT-[BMIm][BF4]. Thus, the AMWCNT-[BMIm][BF4] MEsystem was selected for additional bioelectrochemicalstudies. For these experiments, the redox biocatalyst (i.e.,hemin, hemoglobin (Hb), or HRP) was initially mixed withthe IL prior to grinding with AMWCNTs and the ensuinggel fixed to the GC electrode by rubbing. CV measurementsshowed a pair of reversible peaks attributed to the hemeFe3+/Fe2+ redox couple for all three MEs. The hemin, Hb,and HRP-AMWCNT-[BMIm][BF4] MEs all demonstratedelectrocatalytic behavior towards H2O2, with the fastestelectron transfer rate observed for the hemin-based ME. TheMEs were reproducibly fabricated and reportedly exhibited

good stability, retaining ≥60% of the peak current after 20cycles.

A novel amperometric biosensor was fabricated basedon the immobilization of cholesterol oxidase (ChOx) intoa cross-linked matrix of chitosan- (Chi-) IL (1-butyl-3-methylimidazolium tetrafluoroborate) by Gopalan et al.[207]. In this study, the surface of bare electrode (indium tinoxide-coated glass) was modified with the electrodepositionof Au particles onto thiol- (SH-) functionalized MWNTs.The presence of Au particles in the matrix of CNTs providesan environment for the enhanced electrocatalytic activities.The MWNT(SH)-Au/Chi-IL/ChOx biosensor exhibited alinear response to cholesterol in the concentration rangeof 0.5–5 mM with a correlation coefficient of 0.998, goodsensitivity (200 μAM−1), a low response time (approximately7s), repeatability (R.S.D value of 1.9%), and long-termstability (20 days with a decrease of 5% response). Thesynergistic influence of MWNT(SH), Au particles, Chi, andIL contributes to the excellent performance for the biosensor.

Kachoosangi et al. [195] prepared a new compositeelectrode using MWCNT and the IL n-octylpyridinumhexafluorophosphate (OPFP). One major advantage of thiselectrode compared to other electrodes using carbon nan-otubes and other ILs is its extremely low capacitance andbackground currents. A 10% (w/w) loading of MWCNT wasselected as the optimal composition based on voltammetricresults, as well as the stability of the background responsein solution. The new composite electrode showed goodactivity towards hydrogen peroxide and NADH, with thepossibility of fabricating a sensitive biosensor for glucose andalcohol using glucose oxidase and alcohol dehydrogenase,respectively, by simply incorporating the specific enzymewithin the composite matrix. The marked electrode stabilityand antifouling features towards NADH oxidation was muchhigher for this composite compared to a bare glassy carbonelectrode. While a loading of 2% MWCNT showed very poorelectrochemical behavior, a large enhancement was observedupon gentle heating to 70◦C, which gave a response similar tothe optimum composition of 10%. The ease of preparation,low background current, high sensitivity, stability, and smallloading of nanotubes using this composite can create newnovel avenues and applications for fabricating robust sensorsand biosensors for many important species.

The composite film based on Nafion and hydropho-bic IL 1-butyl-3-methyl-imidazolium hexafluorophosphate[BMIm][PF6] was explored by Chen et al. [208]. In thiswork, nafion was used as a binder to form nafion-ILscomposite film and help [BMIm][PF6] effectively adheredon GCE. X-ray photoelectron spectroscopy (XPS), CV, andelectrochemical impedance spectroscopy (EIS) were used tocharacterize this composite film, showing that the compositefilm can effectively adhere on the GC electrode surfacethrough Nafion interacting with [BMIm][PF6] and GCelectrode. Meanwhile, doping [BMIm][PF6] in Nafion canalso effectively reduce the electron transfer resistance ofnafion. The composite film can be readily used as an immo-bilization matrix to entrap horseradish peroxidase (HRP).A pair of well-defined redox peaks of HRP was obtainedat the HRP/Nafion-[BMIm][PF6] composite film-modified


GC electrode through direct electron transfer between theprotein and the underlying electrode. HRP can still retainits biological activity and enhance electrochemical reductiontowards O2 and H2O2. It is expected that this compositefilm may find more potential applications in biosensors andbiocatalysis.

An organophosphorus hydrolase (OPH) immobilizedon MWCNT/IL where IL = [BMIm][BF4], [BMIm][PF6], or[BMIm][NTf2]-modified composite electrodes showedhigher sensitivity and better stability in detectingorganophosphate [209]. The modification of MWCNTswith these different ILs allowed high dispersion of CNTbundles and the formation of a 3D-network structure, goodcompatibility with OPH, and accelerated electron-transferreaction at the interface. The electron-transfer rate of thethree modified electrodes increased after the incorporationof OPH in the following order: OPH/[BMIm][PF6] <OPH/[BMIm][BF4] < OPH/[BMIm][NTf2]. Sun et al.[210] developed two methods, namely, casting and rubbingmethods to prepare the Fc-filled SWCNT-modified GCelectrode which showed excellent mediation of H2O2 basedon the Fc/Fc+ couple used as electron-transfer mediator foroxidation of H2O2 to O2 and reduction of H2O2 to H2O.

Gels of acid treated multiwall CNTs with water-miscibleIL [BMIm][BF4] were made by grinding them together[156]. GCE modified by this gel was used in direct electro-chemical study of heme proteins (Hb and HRP). The hemeproteins entrapped in CNTs-[BMIm][BF4] gel exhibit goodbiocatalytic activity towards H2O2 due to the biocompatibil-ity of [BMIm][BF4]. This composite material has shown tokeep the bioactivity and to facilitate direct electron transferof heme proteins. Carbon fiber microelectrode has also beenmodified by CNTs-IL gel and used in bioelectrochemicalstudies [158]. The carbon fiber microelectrode modified byCNTs-IL gel promotes greatly the direct electron transferof glucose oxidase and exhibits effective catalytic activityto biomolecules, such as DA, AA, and NADH. IL hasalso been used as binder to modify electrode surface ofmultiwall carbon nanotubes. This construction has betterelectrochemical properties than by chitosan or Nafion-modified electrodes. The GOD adsorbed at this modifiedelectrode shows good stability and electrocatalytic activity toglucose with a broad linear concentration range up to 20 mM[155].

Rahimi et al. [211] developed a nanocomposite materialconsisting of amine functionalized multiwalled carbonnan-otubes and an IL, 1-butyl-3-methylimidazolium tetrafluo-roborate and further it was used in construction of a novelcatalase-based biosensor for the measurement of hydrogenperoxide. The modified electrode exhibited a quasireversibleCV corresponding to the Fe2+/Fe3+ redox couple in theheme prosthetic group of catalase with a formal potentialof −460 mV in 0.1 M phosphate buffer solution at pH =7.0. The nanocomposite film showed an obvious promotionof the direct electron transfer between catalase and theunderlying electrode. The apparent charge transfer rate con-stant and transfer coefficient for electron transfer betweenthe electrode surface and enzyme were reported as 2.23 s−1

and 0.45, respectively. The immobilized catalase exhibited

a relatively high sensitivity (4.9 nA/nM) towards hydrogenperoxide.

Graphene [212], known as single layer graphite, hasattracted intensive scientific interest in recent years due toits two-dimensional and unique physical properties, suchas high intrinsic carrier mobility at room temperature(≈105 cm2/V·s), excellent mechanical strength, and electricaland thermal conductivity comparable to the in-plane valueof graphite [213]. Owing to the unique properties [214, 215]graphene/IL-based composite materials have been widelyused in fabricating electrochemical biosensors [216].

Shan et al. [217] reported the first graphene-basedglucose biosensor with graphene/polyethylenimine-functionalized IL nanocomposites-modified electrodewhich exhibits wide linear glucose response (2 to 14 mM,R = 0.994), good reproducibility (relative standard deviationof the current response to 6 mM glucose at −0.5 V was 3.2%for 10 successive measurements), and high stability (responsecurrent +4.9% after 1 week). In this work, direct electron-transfer of GOx and electrocatalytic activity towards thereduction of O2 and H2O2 at the polyvinylpyrrolidone-protected graphene/polyethyleniminefunctionalizedIL/GOx-modified electrode has been demonstrated.

IL-graphene-based biosensors have been reported fordirect electron transfer and detection of different targetsalso such as NADH and guanine [218, 219]. These resultssuggest that IL-graphene hybrid nanosheets due to good dis-persibility and long-term stability are expected to be usefulmaterials for enhancing the electrochemical performance forthe detection of different target molecules in electroanalyticalapplications.

8. Conclusions

ILs are solvents that can be designed for special applicationseither by synthesizing new ILs or by paring different cationsand anions to fine-tune the properties of a particular IL.In this review article we have tried to give an overviewon the importance of ionic liquids for the synthesis ofconducting polymers and nanoparticle. Moreover, electro-chemical methods such as potentiometry, voltammetry, andQCM have been used in ILs to develop new generation ofion selective sensors, voltammetric devices, gas sensors, andelectrochemical biosensors.

The good catalytic ability of IL-based composite mate-rials together with the simple preparation procedure greatlypromotes the development of highly sensitive, selective, andreproducible microelectrode to be used in electrochemicalbiosensors and other bioelectrochemical devices. Takingadvantage of the fact that ILs could be used as bothimmobilizing matrices to entrap heme proteins and enzymesas the electron-transfer promoters to achieve direct electron-transfer between the redox proteins and underlying elec-trodes hence provided a good electrocatalytic sensing plat-form for various substrates. The fundamental understandingof the underlying biochemistry, surface chemistry, electro-chemistry, material chemistry, and technological advancesis needed in order to enhance the capabilities and improve


the reliability, portability, and functionality of IL-basedbiosensors to bring the biosensors in real applications. Thestill rising interest in ILs in various fields of chemistry willsurely lead to a rising output of products papers, stimulatingfurther studies. It can be expected that ILs in electrochemicalsensing are a promising and exciting area of research anddefinitely need further exploration. Combination of ILs withelectrochemical sensors has the potential to broaden or evenrevolutionize the range of analytical methods.

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Review Article

Electrochemical Behaviour of Actinides and Fission Products inRoom-Temperature Ionic Liquids

K. A. Venkatesan, Ch. Jagadeeswara Rao, K. Nagarajan, and P. R. Vasudeva Rao

Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

Correspondence should be addressed to K. A. Venkatesan, [email protected]

Received 20 April 2011; Accepted 6 September 2011

Academic Editor: Oliver Hofft

Copyright © 2012 K. A. Venkatesan et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

In the recent past, room-temperature ionic liquids (RTILs) are being explored for possible applications in nuclear fuel cycle. RTILsare being studied as an alternative to the diluent, n-dodecane (n-DD), in aqueous reprocessing and as possible substitute to high-temperature molten salts in nonaqueous reprocessing applications. This paper deals with the current status of the electrochemicalresearch aimed at the recovery of actinides and fission products using room-temperature ionic liquid as medium. The dissolutionof actinide and lanthanide oxides in ionic liquid media and the electrochemical behavior of the resultant solutions are discussedin this paper.

1. Introduction

Nuclear reactors employ natural or enriched uranium as thefuel [1]. The spent nuclear fuel discharged from the nuclearreactor is composed of fissile elements such as plutoniumand depleted uranium and several other elements formedby fission reaction, known as fission products. The spentnuclear fuel is, therefore, reprocessed to recover these fissileelements for the fabrication of nuclear fuel for future reac-tors. There are two different technologically viable methodsavailable for reprocessing of spent nuclear fuel. The first oneis the aqueous-based, industrially well-established, PUREX(plutonium uranium recovery by extraction) process [2]and the other is nonaqueous-based pyrochemical process[3]. The PUREX process involves the dissolution of spentnuclear fuel in nitric acid medium followed by the selectiveextraction of uranium and plutonium in a solution of tri-n-butylphosphate (TBP) in n-dodecane (DD). However,the undesirable limitation of the PUREX process is thechemical and radiolytic degradation of the solvent systemand the generation of large volumes of secondary wastes.The flammability of hydrocarbon employed in the PUREXprocess is also another disadvantage.

In contrast to this method, the nonaqueous pyrochemicalmethod for reprocessing of spent nuclear fuel has several

advantages, such as minimum waste generation, low critical-ity concern and feasibility to reprocess the high burn-up, andshort cooled fuels. This method exploits the differences inthe thermodynamic stabilities of various actinides and fissionproducts for the dissolution of spent nuclear fuel in theinorganic molten salt media followed by the electrochemicalrecovery of actinides [4]. Usually, the inorganic molten salt iscomposed of an eutectic of alkali or alkaline earth chloride.Therefore, the processing temperature in this method isinvariably above 800 K depending upon the composition ofchosen eutectic. In view of this, the pyrochemical methodalso offers a few challenges with respect to the plantdesign, requirement of inert atmosphere, high-temperatureoperation and corrosion problems, and so forth.

2. Room-Temperature Ionic Liquids

Substitution of unsymmetrical organic cation in place ofalkali metal cation of the inorganic chloride salt dramat-ically reduces the melting point of the resultant organicchloride to near ambient temperatures. For instance, themelting temperature of NaCl is 1074 K and that of 1-butyl-3-methylimidazolium chloride (bmimCl) is 343 K. Suchorganic salts are known as room-temperature ionic liquids(RTILs). According to the most accepted definition, RTILs


Table 1: Some of the cations and anions used to form room-temperature ionic liquids.

CATIONS

N+

N

CH3

R

N+

CH3

H3C

H3C

H3C

N+

CH3

H3C

N+

CH3

H3C

HO

OCH3

N+

CH3

CH3

N+ R

R

R

R

P+ R

R

R

R

C2H5

C5H11

C2H5

H5C2 P+

C2H5

C2H5

H5C2 CH2OCH3P+

ANIONS

O

S

F

N−O

O

O

S

F

F

FF

F

CN

N−

CN

P−F

FF

F

F

F

O

S

F

O−

OF

FF

B− F

F

F

Cl−, Br−,

are composed fully of dissociated ions and melt at temper-ature lower than 373 K [5–7]. Some examples of the cation-anion combinations for making RTILs are shown in Table 1.They have several attractive properties suitable for industrialexploitation such as insignificant vapour pressure, largeliquidus range, good electrical conductivity and thermalstability, wide electrochemical window, and amazing abilityto dissolve organic and inorganic compounds In view of this,RTILs are being explored for various applications [7–13].

3. Electrochemical Stability of RTILs

In the recent past, room-temperature ionic liquids arereceiving increased attention for possible electrochemicalapplications in the area of nuclear fuel reprocessing andwaste management. For such applications, it is desirable touse an ionic liquid that has good solubility of actinides,excellent thermal and radiation stability, and wide elec-trochemical window with extended cathodic stability. The

knowledge of electrochemical stability can be derived fromthe measurement of the electrochemical window of RTIL.The electrochemical window [7, 14, 15] is defined as thepotential range in which the ionic liquid does not undergoany oxidation and reduction reactions. Table 2 shows theelectrochemical windows of some ionic liquids offeringelectrochemical window more than 2 V. The abbreviationsof the ionic liquids reported are tabulated in Table 3. Itis observed that many RTILs offer cathodic stability up to−3.3 V and electrochemical window >4 V. Some RTILs offerwindow more than 6 V, which is indeed desirable for theelectrodeposition of metals such as lithium. Simka et al. [16]reviewed the electrodeposition of highly electropositive met-als in ionic liquid media and reported the feasibility of usingionic liquids for such applications. Table 4 summarises thelist of metals for which electrodeposition has been reportedfrom RTIL medium [16, 17]. It is observed that several metalsranging from S- block to P- block through the transitionmetals have been studied. However, the studies related to


Table 2: The electrochemical windows of some ionic liquids GC-glassy carbon, VC- vitreous carbon, RDE-rotating disc electrode, andFc-ferrocene.

Name of the ionicliquid

Cathodiclimit, V

Anodiclimit, V

Electrochemical window, V(reference electrode)

Workingelectrode

References

bmimCl −1.38 0.74 2.12 (Pd) GC [18]

bmimBF4

−2.2 2.2 4.4 (Ag) GC [19]

−2.2 2.4 4.6 (Ag) Au [19]

−2.4 2.5 4.9 (Ag) Pt [19]

bmimPF6

−2.1 +2.5 4.6 (SCE) — [8]

−2.50 3.85 6.35 (Pt) VC RDE [20]

−2.1 >5.0 >7.1 (Pt) W RDE [20]

bmimNTf2 ∼ −1.9 ∼2.3 4.2 (Ag) Pt [21]

bmimCF3CO2 ∼ −2.0 ∼0.7 2.7 (SCE) — [8]

hmimCl −0.88 0.75 1.63 (Pd) GC [22]

bdmimBF4

−1.8 2.4 4.2 (Ag) GC [19]

−2.3 2.3 4.6 (Ag) Au [19]

−2.0 2.3 4.3 (Ag) Pt [19]

−2.4 3.0 5.4 (Ag) Ta [19]

HbetNTf2 −1.16 1.2 2.4 (Fc/Fc+) GC [23]

BMPyNTf2

−2.91 2.65 5.56 (Fc/Fc+) GC [22, 24]

∼ −3.1 ∼3.2 6.3 (Pt) GC [25]

−3.0 2.5 5.5 (Ag/Ag+) GC [26]

MPPiNTf2−2.71 2.26 4.97 (Fc/Fc+) GC [22]

−3.3 2.3 5.6 (Fc/Fc+) GC [27]

P2225DCA ∼ −3.2 ∼2.6 5.8 (Ag/Ag+) Pt [28]

P222(101)DCA ∼ −2.5 ∼3.0 5.5 (Ag/Ag+) Pt [28]

N222(101)DCA ∼ −2.6 ∼2.4 5.0 (Ag/Ag+) Pt [28]

P222(201)DCA ∼ −2.5 ∼2.6 5.1 (Ag/Ag+) Pt [28]

P2225NTf2 ∼ −3.2 ∼3.0 6.2 (Fc/Fc+) Pt [29]

P222(101)NTf2 ∼ −3.0 ∼2.7 5.7 (Fc/Fc+) Pt [29]

P2228NTf2 ∼ −3.2 ∼3.0 6.2 (Fc/Fc+) Pt [29]

P222(201)NTf2 ∼ −3.1 ∼2.2 5.3 (Fc/Fc+) Pt [29]

bmim(CF3)2PO2 ∼ −2.3 ∼2.2 4.5 (Ag) Pt [21]

bmimOTf ∼ −2.2 ∼2.8 5.0 (Pt) GC [25]

emimBF4∼ −2.1 ∼1.9 4.0 (Pt) GC [25]

∼0.9 ∼5.0 4.1 (Li/Li+) GC [30]

mpimBF4 ∼1.0 ∼5.1 4.1 (Li/Li+) GC [30]

Bu4NNO3 ∼ −3.0 ∼1.5 4.5 (SCE) — [8]

Bu4PNO3 ∼ −2.0 ∼1.5 3.5 (SCE) — [8]

lanthanides and actinides are less. In addition, the metallicelectrodeposit obtained for lanthanides are not usually stableand undergoes oxidation to their respective oxides duringelectrolysis.

4. Electrochemical Behavior ofLanthanides and Actinides in Ionic Liquids

The electrochemistry of actinides was indeed reported afew decades ago using chloroaluminate-based RTIL as themedium. However, these studies were not directed towardsthe electrodeposition of actinides, but involved only the

basic understanding of coordination chemistry, solutionchemistry, and electrochemical behaviour of metals. In fact,this was due to the inadequacy in the electrochemicalwindow and poor cathodic stability of chloroaluminates.The earliest work on the electrochemistry of uranium inacidic and basic chloroaluminate melt was reported byD’Olieslanger and coworkers [31, 32]. In acidic 2 : 1 melts(AlCl3:N-(n-butyl) pyridinium chloride), the reduction ofU(IV) to U(III) at glassy carbon electrodes was reportedto be irreversible. The formal standard potential of theU(IV)/U(III) redox couple as a function of the melt aciditywas determined. U(III) in the melt was found to exist as


Table 3: The abbreviations of the ionic liquids.

Name of the ionic liquid Abbreviation

1-butyl-3-methylimidazolium chloride bmimCl

1-butyl-3-methylimidazolium tetrafluoroborate bmimBF4

1-butyl-3-methylimidazolium hexafluorophosphate bmimPF6

1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide bmimNTf2

1-butyl-3-methylimidazolium trifluoromethanecarbonate bmimCF3CO2

1-butyl-3-methylimidazolium dicyanamide bmimDCA

1-hexyl-3-methylimidazolium chloride hmimCl

1-butyl-2,3-dimethylimidazolium chloride bdmimBF4

Betaine bis(trifluoromethanesulfonyl)imide HbetNTf2

N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide BMPyNTf2

N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide MPPiNTf2

Triethylpentylphosphonium dicyanamide P2225DCA

Triethyldimethyletherphosphonium dicyanamide P222(101)DCA

Triethyldimethyletherammonium dicyanamide N222(101)DCA

Triethylethylmethyletherphosphonium dicyanamide P222(201)DCA

Triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide P2225NTf2

Triethyldimethyletherphosphonium bis(trifluoromethanesulfonyl)imide P222(101)NTf2

Triethyloctylphosphonium bis(trifluoromethanesulfonyl)imide P2228NTf2

Triethylethylmethyletherphosphonium bis(trifluoromethanesulfonyl)imide P222(201)NTf2

1-butyl-3-methylimidazolium bis(trifluoromethane)phosphinate bmim(CF3)2PO2

1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonate bmimOTf

1-ethyl-3-methylimidazolium chloride emimCl

1-ethyl-3-methylimidazolium tetrafluoroborate emimBF4

1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide emimNTf2

1-methyl-3-propylimidazolium tetrafluoroborate mpimBF4

Tetrabutylammonium nitrate Bu4NNO3

Tetrabutylphosphonium nitrate Bu4PNO3

Tri-n-butylmethyl ammonium bis(trifluoromethanesulfonyl)imide Bu3mebu3NNTf2

Tri-n-methylbutyl ammonium bis(trifluoromethanesulfonyl)imide Me3NBuNTf2

1-octyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide OMPyNTf2

N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide demmaNTf2

Tricaprylmethylammonium nitrate TOMAN

1-butylpyridinium chloride BuPyCl

1-butylpyridinium tetrafluoroborate BuPyBF4

Tricaprylmethylammonium chloride Aliquat 336

1-butyl-3-methylimidazolium nonafluorobutanesulfonate BmimNfO

1-[2-(2-methoxyethoxy)ethyl]-3-methylimidazolium chloride C5O2ImCl

1-[2-(2-methoxyethoxy)ethyl]-3-methylimidazolium hexafluorophosphate C5O2ImPF6

Table 4: List of some metals studied for the electrodeposition from room-temperature ionic liquid medium [16, 17].

S. No. Group Metals deposited

1 S-block elements Li, Mg, Cs, and Sr

2 P-block elements Sn, Al, Al-In, Al-Sb, Al-In-Sb, Al-Si, Ge, Ga, Te, Tl, Pb, Bi, and Si

3 Transition elementsTi, Ta, Mn, Zn, Zn-Mn alloy, Ru, Rh, Pd, Ag, Au, Co, Co-Zn

alloy, Pd-Ag alloy, Pt-Zn alloy, Pt, Pd-In alloy, Ni-Zn alloy,

Ni, Zn-Sn alloy, Cd-Te, Cu, Fe, Hg, Cr, and Nb

4 Lanthanides La, Eu, Nd, and Sm

5 Actinides U and Th


free U3+ ion, and U(IV) as chlorocomplexes, UClx(4−x)+ with

3 ≥ x ≥ 1. The oxidation of U(IV) to U(V) at glassy carbonelectrodes was irreversible and dependent on the melt acidity.Further oxidation of U(V) to U(VI) was not feasible dueto the anodic instability of the melt. In contrast to acidicmelt, U(VI) in basic melt [33] (AlCl3 + BuPyCl), existed aschloro complexes UO2Cl4+x

(2+x)−, which underwent a single-step irreversible two-electron transfer to soluble UCl6

2− asshown in (1). The diffusion coefficient was determined tobe 2.65 × 10−7 cm2/s in the basic melt (mole ratio ofAlCl3 : BuPyCl= 0.82 : 1)

[UO2Cl(4+x)

](2+x)− + 2 AlCl4− + 2e−

−→ UCl62− + 2 AlOCl2

− + (2 + x)Cl−.(1)

In addition, U(IV) could not be reduced to U(III) inbasic melt due to the cathodic instability of BuPy+ cation,which undergoes reduction in preference to U(IV). However,U(VI) could be reduced to U(III) in basic AlCl3-emimCl [34,35], as the 1-ethyl-3-methylimidazolium cation (emim+)is more difficult to reduce than the BuPy+ cation. Theuranyl ion (UO2

2+) underwent a two electron transfer inbasic AlCl3-emimCl melt followed by the transfer of oxygento chloroaluminate complex of ionic liquid leading to theformation of [UCl6]2− as shown in (2) and (3). [UCl6]2− wasthen reduced to [UCl6]3− in the melt

[UO2Cl4]2− + 2 AlCl4− + 2e−

−→ 2{AlOCl2}− + [UCl6]2− + 2Cl−,(2)

[UCl6]2− + e− −→ [UCl6]3−. (3)

Electrochemical behavior of U(VI) in bmimCl was re-ported by Giridhar et al. [18]. U(VI) in bmimCl underwenta single-step two-electron transfer reduction to uraniumoxide (UO2) deposit at glassy carbon working electrode.They reported that the electrochemical reduction was notonly governed by the diffusion of U(VI), but also by chargetransfer kinetics at the working electrode. Thermal analysisof the uranium oxide deposit obtained by the electrolysis ofU(VI) from bmimCl revealed the entrapment of nearly 5%bmimCl during electrodeposition.

Ikeda and coworkers [36] developed a new approachfor reprocessing spent nuclear fuel using room-temperatureionic liquid as medium. The method involved oxidativedissolution of the spent nuclear fuel by using Cl2, followedby the electrochemical reduction of U(VI) to UO2. By thismethod, the electrochemical behavior of UO2

2+ in variousionic liquids such as bmimCl, bmimBF4, and bmimNfOwas studied and the two-step reduction of UO2

2+ toU(IV) through U(V) was reported. The electrochemicalreduction of UO2

2+ in bmimNfO at−1.0 V (versus Ag/AgCl)produced uranium oxide deposit at carbon electrode. Theelectrochemical behaviour of [UO2Cl4]2− in bmimCl ionicliquid was also reported by Ikeda et al. [37]. The uraniumcomplex, [UO2Cl4]2−, was dissolved in bmimCl by usingCs2UO2Cl4 or UO2Cl2·nH2O compounds and reported

the quasireversible reduction of [UO2Cl4]2− to [UO2Cl4]3−.Similarly, Ikeda and coauthors [38] studied the dissolutionof uranium fluoride (UF4) in bmimCl by oxidation of U(IV)to U(VI) and the UO2

2+ present in bmimCl was reduced toUO2 by a two-step one electron transfer.

The electrochemical behaviour of neptunium in acidicand basic AlCl3–1-n-butylpyridinium chloride melt at 313 Kwas studied by Schoebrechts and Gilbert [39]. In acidicand basic melts, the reduction of Np(IV) to Np(III) atglassy carbon electrode was quasireversible. In basic melt,Np(III) and Np(IV) exist as NpCl6

3− and NpCl62−, whereas

in acidic melt it exist in the form of solvated Np3+ andNpClx

(4−x)−(3 ≥ x ≥ 1). The study showed that the acidicmelt acted as a poor solvating medium for neptunium.

Choppin and coworkers [40] have investigated the elec-trochemistry and spectroscopy of UO2

2+ in acidic chloroa-luminate medium. UO2

2+ in acidic AlCl3-emimCl melt wasgradually reduced to U(V) and it was facilitated only byAlCl4

− and Al2Cl7− species by the sequence of reactions

shown in the following:

UO22+ + Al2Cl7

− −→ UO4+ + AlCl4− + AlOCl3

2−, (4)

UO4+ + Al2Cl72− −→ U6+ + AlCl4

− + AlOCl32−, (5)

U6+ + AlCl4− −→ U5+ + AlCl3 + 1/2Cl2. (6)

Deetlefs et al. studied [41] the electrochemistry of variousuranium halide complexes in basic and acidic bromoalumi-nate (III) imidazolium ionic liquid. U(IV) in the basic ionicliquid was reduced to U(III) and U(VI) to U(IV). Lin andHussey [42] studied the electrochemical and spectroscopicproperties of Ce(III) in basic AlCl3-emimCl and reporteda quasireversible reduction of Ce(IV) to Ce(III) at glassycarbon, Pt and W electrodes.

The redox behavior of actinides in haloaluminate ionicliquids (1st generation) demonstrate that the electrochemicalwindow of haloaluminates permits the redox conversion ofAn(III) to An(VI) and vice versa, irrespective of the compo-sition of RTIL. Reduction of An(III) to An(0) was not feasibledue to inadequate cathodic stability of haloaluminates. Asimilar behavior could be expected for lanthanides also, asthe characteristic trivalents are hard to reduce to metallicstate than actinides. The studies reported by Schoebrechtset al. [43, 44] also confirmed that some trivalent lanthanidesions such as Sm(III), Eu(III), Tm(III), and Yb(III) dissolvedin acidic AlCl3-BuPyCl ionic liquid were reduced only to thecorresponding divalent ions. Due to these limitations, RTILsdid not gain popularity in nonaqueous reprocessing earlier.

Moisture stable ionic liquids were discovered in late1990s. They are known as 2nd generation ionic liquids.Several RTILs having electrochemical window as large as∼6 V and extended cathodic stability [45–47] were iden-tified. This factor gave a renaissance to the use of ionicliquids in nonaqueous processing applications. Nikitenkoand coworkers [48–50] investigated the spectroscopic andelectrochemical aspects of U(IV)-hexachloro complexes inbmimNTf2, Bu3MeNNTf2 hydrophobic ionic liquids. Due to


large electrochemical window of RTIL, it was possible tostudy the oxidation and reduction behaviour, up to themetallic form. The uranium redox values were reporteddepend strongly on the type of RTIL cation. [UCl4]2−

in Bu3MeNNTf2 can be converted to metallic uranium at−3.12 V (versus Fc/Fc+) at 333 K. Nikitenko and Moisy [50]further investigated the coordination behaviour of Np(IV)and Pu(IV) in bmimNTf2. The [NpCl6]2− and [PuCl6]2−

complexes are electrochemically inert in bmimNTf2 atglassy carbon electrode. However, the addition of bmimClfacilitated the quasireversible electrochemical reduction ofNp(IV)/Np(III) and Pu(IV)/Pu(III) and oxidation of Np(IV)and Pu(IV). These redox reactions were reported only whenCl−/An(IV) ratio exceeded six in ionic liquid medium.

Bhatt et al. [51–53] studied the electrochemistry of somelanthanide ions, La(III), Sm(III), and Eu(III) in R4XNTf2,where X = N, P, and As. The RTIL, Me4XNTf2, exhibiteda large electrochemical window (∼6 V). They also studied[52] the reduction of La(III), Sm(III), and Eu(III) to metallicstate in the RTIL, Me3BuNNTf2, and reported similar resultsas in the previous case. However, it was reported that thedeposits were not stable and underwent very fast oxidationto their respective oxides during the course of washing treat-ment. Moreover, the standard reduction potentials of theselanthanides are nearer or negative to the reduction potentialof the ionic liquids. Therefore, the lanthanides electrodepositobtained using these ionic liquids were not very useful. Thevoltammetric behavior of [Th(NTf2)4(HNTf2)]·2H2O wasalso studied [53] in Me3BuNNTf2. Th(IV) in this ionic liquidwas reduced to Th(0) by a single-step reduction. The E0 valuefor the reduction of Th(IV) to Th(0) was determined to be−2.20 V (versus (Fc+/Fc), −1.80 V versus SHE). Due to thepresence of moisture in ionic liquid, the reduced productTh(0) was converted to ThO2.

Legeai et al. [54] reported the electrodeposition oflanthanum in OMPyNTf2 ionic liquid. The authors reportedthat the wide electrochemical window (4.8 V) and low hygro-scopic character of OMPyNTf2 allowed the electrodepositionof lanthanum without the need of an inert atmosphere.About 350 nm thick lanthanum film on Pt electrode wasdeposited. Nagaishi et al. [55] studied the physicochem-ical behaviour, spectroscopic and electrochemical behav-ior of Eu(III) as a function of water in demmaNTf2

and bmimNTf2ionic liquids. The variations in the diffu-sion coefficient of Eu(III) in dehydrated and water satu-rated conditions were reported by the authors. Yamagataet al. [56] studied the electrochemical behaviour of samar-ium(III), europium(III) and ytterbium(III) in BMPyNTf2,and emimNTf2 ionic liquids. The quasireversible reductionof Sm(III), Eu(III) and Yb(III) to divalent state was reported.The electrochemical behaviour of Eu(III), Sm(III), andCe(III) in bmimCl ionic liquid at Pt electrode at 373 Kwas also studied by Jagadeeswara Rao et al. [57]. Again,the reduction of Eu(III), Sm(III), and Ce(III) to the cor-responding divalent ions was quasireversible. The apparentstandard potentials and the thermodynamic parameters forthe reduction reaction were determined and reported in thispaper. Matsumiya et al. [58] reported the electrochemical

behavior of Eu(III) and Sm(III) in ionic liquids, P2225NTf2

and N2225NTf2. From the redox potentials, the authorsreported that the donor property of phosphonium-basedionic liquids was slightly larger than that of nitrogen-basedionic liquids.

The electrochemical behavior of Eu(III) [24] and U(IV)[59] in BMPyNTf2 and MPPiNTf2, respectively, was studiedby Jagadeeswara Rao et al. The europium oxide (Eu2O3)and uranium oxide (UO2) were dissolved in RTIL mediumand the electrochemical behavior of Eu(III) in BMPyNTf2

and U(IV) in MPPiNTf2 was studied by various transientelectrochemical techniques. The metallic nature of europiumand uranium deposit obtained in the study was confirmedby XRD and EDXRF techniques. The study established thefeasibility of dissolving the lanthanide and actinide oxidesin ionic liquid medium and recovery in metallic form byelectrodeposition. Joseph et al. [60] studied the lithium-assisted electrochemical reduction of U3O8 in MPPiNTf2 toexplore the feasibility of using RTILs for direct electrochemi-cal reduction of uranium oxide at near ambient temperature.The electrochemical behavior of Li+ in MPPiNTf2 at stainlesssteel electrode was investigated by cyclic voltammetry andchronoamperometry. Electrodeposition of metallic lithiumon U3O8 particles contained in a stainless steel basket wascarried out to examine the feasibility of reducing U3O8

to metallic form. The results indicated the feasibility ofreducing bulk of U3O8 to UO2 at near ambient temperature.However, reduction of UO2 to metallic form was notobserved under the present conditions.

5. Electrochemical Behavior ofFission Products in Ionic Liquids

5.1. Cesium and Strontium. Cesium-137 and Strontium-90are the major radiotoxic fission product isotopes presentin high-level liquid waste. Chen and Hussey [61] studiedthe electrochemistry of the electropositive fission product,Cs, at mercury electrode using Bu3MeNNTf2 by variousvoltammetric techniques. The reduction of Cs+ at mercuryelectrode was quasireversible and a diffusion coefficient of∼10−8 cm2/s at 303 K was reported. Deposition/strippingwas conducted at mercury film electrode with recovery of97% of loaded cesium.

Chen and Hussey [62] also reported the selective extrac-tion of Cs+ and Sr2+ from aqueous solutions by usingthe ionophores calix[4]arene-bis(tert-octylbenzo-crown-6)(BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6),respectively, present in the hydrophobic, ionic liquid,Bu3MeNNTf2. The electrochemistry of Cs+ coordinatedby BOBCalixC6 and Sr2+ coordinated by DCH18C6 wasexamined at mercury film electrode (MFE) by usingcyclic staircase voltammetry, sampled current voltam-metry at a rotating electrode, and chronoamperometry.Both BOBCalixC6·2Cs+ and DCH18C6·Sr2+ exhibited well-defined reduction waves at approximately −2.4 and −2.9 Vversus the ferrocene/ferrocenium (Fc/Fc+) couple, respec-tively. The coordinated metals were reduced to their respec-tive amalgams, permitting the recycling of the ionophores.


Chen [63] reported the extraction of Sr2+ and Cs+

from aqueous solutions by using the ionophores dicyclo-hexano-18-crown-6 (DCH18C6) and calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6), respectively, in the hy-drophobic, ionic liquid, Bu3MeNNTf2. The possibility ofusing the electrodeposition technique to recycle the ionicliquid and the ionophores employed for extraction of Sr2+

and Cs+ ions was explored.

5.2. Platinum Group Metals. Spent nuclear fuel is composedof strategic metals such as uranium, and plutonium andpotentially useful fission byproducts such as palladium,ruthenium, and rhodium. Significant quantities of platinumgroups metals (PGMs) are produced as fission products[64]. Most of the fission PGM isotopes in the spent nuclearfuel are nonradioactive or very weakly radioactive. Thesefission PGM’s are routed to high-level liquid waste (HLLW)during reprocessing of the spent nuclear fuel. Therefore, theHLLW as well as the spent nuclear fuel inself are the valubleresources of man-made noble metals.

Recovery of valuable PGMs from nuclear HLLW wasextensively studied [65–70] in the last two decades withparticular interest in the separation of palladium. Severalauthors have reported the electrochemical recovery of PGMsfrom nitric acid medium [71–75]. Kirshin and Pokhitonov[73] studied the electrolytic recovery of palladium fromnitric acid solutions and reported the efficiency of theprocess in the presence of HNO3, NaNO3, uranium, andother admixtures. Varentsov and Varentsova [74] reportedthe electrodeposition of rhodium on carbon fiber electrodesfrom nitric acid and confirmed that speciation of rhodiumstrongly affects the deposition behavior. Koizumi and Kawata[75] reported the electrolytic extraction of fission platinoidsfrom nitric acid medium. Recoveries of 90%, 23%, and 10%were reported, respectively, for the deposition of Pd, Rh, andRu, and the deposition rates were reported to decrease withincrease in nitric acid concentration.

The electrochemical behavior of PGMs, namely, ruthe-nium (III), rhodium (III), and palladium (II) in HLLW aswell as ionic liquid medium was studied by Jayakumar etal. [76, 77]. Palladium (II) present in nitric acid mediumunderwent an irreversible single-step two-electron transferto metallic palladium at stainless steel electrode, and it wasquantitatively recovered by electrolysis at –0.5 V (versus Pd).However, several complications aggravated in the presenceof interfering metal ions such as silver, nitrate, and iron thatare likely to present in HLLW during electrolysis. As a result,the recovery and Faradaic efficiency was dropped below 30%when electrolysis was carried out with simulated wastes.

Later a novel approach, extraction-electrodeposition(EX-EL) method, was developed by the same group for thequantitative recovery of palladium from simulated HLLWusing room-temperature ionic liquid as medium [78]. In-itially, Giridhar et al. [79] reported the extraction-elec-trodeposition (EX-EL) for the separation and recovery ofpalladium from nitric acid medium using TOMAN ionicliquid. Later, Jayakumar et al. [78] studied the EX-EL processfor the recovery of palladium from high-level liquid waste in

detail. The process exploited a few remarkable properties ofroom-temperature ionic liquid, namely, liquid ion exchangebehavior and wide electrochemical window to develop asimplified procedure for quantitative recovery of palladium.More than 60% of palladium was extracted using 0.5 MTOMAN/CHCl3 in a single contact and complete extractionwas achieved in five contacts. The extracted palladium wasquantitatively recovered by electrodeposition at stainless steelelectrode.

The electrochemical behaviour of Pd(II), Ru(III) andRh(III) in ionic liquid is required to understand the fea-sibility of recovering these noble metals directly from thespent nuclear fuel by nonaqueous processing routes. Inthis context, the electrochemical behavior of palladium(II)in the basic aluminium chloride + emimCl ionic liquidat glassy carbon, tungsten, and platinum electrodes wasreported by Sun and Hussey [80]. A single-step two-electronreduction of Pd(II) to Pd(0) was reported. In addition, anucleation loop was also observed. Hussey et al. [81] re-ported an electrochemical study of the ruthenium(III) and(IV) hexachlorometallates in a basic room-temperature chlo-roaluminate molten salt. Ruthenium(IV) showed a coupleof reduction waves in the cyclic voltammogram. The firstwave corresponds to the reduction of Ru(IV) to Ru(III) andthe second reduction was attributed to the multielectronreduction of Ru(III) to other unknown ruthenium com-plexes. De long et al. [82, 83] studied the electrodeposition ofpalladium from AlCl3-emimCl ionic liquid and adsorptionof palladium chloride on solid electrodes. Electrodepositionof palladium in the melt was dependent on the mole fractionof AlCl3 and the reduction potential was shifted to ∼+2.0 V,when the melt was changed from basic to acidic.

Crisp et al. [84] reported the reduction of dioxotetra-chlororuthenate(VI) to hexachlororuthenate(IV) in a basic1-butylpyridinium chloride-aluminum(III) chloride ionicliquid. The authors have indicated that the redox system wasthe first irreversible transfer of an oxide ion from an tetra-chloroaluminate in an ambient temperature ionic liquid.Electrodeposition of palladium-silver [85] and palladium-indium [86] from a Lewis basic emimCl/emimBF4 ionicliquid was reported by Sun and coworkers. Since the reduc-tion potentials of Ag(I) and Pd(II) were very close to eachother and alloy formation was observed [87]. In the case ofpalladium-indium [86], over potential deposition (OPD) ofpalladium was reported in the presence of indium; however,underpotential deposition was observed for indium in thepresence of palladium.

Bando et al. [88] investigated the electrodeposition ofpalladium in a hydrophobic BMPyNTf2 room-temperatureionic liquid. The irreversible reduction of PdBr4

2− andPdCl4

2− to Pd(0) was observed in this ionic liquid, andthe authors have reported that the reduction potential ofPdCl4

2− was more negative than that of PdBr42−, reflecting

the difference in the donor property between chloride andbromide. Giridhar et al. studied the feasibility of recov-ering fission palladium from bmimCl medium [85] andextraction-electrodeposition procedure using Aliquat 336ionic liquid [79]. However, the electrochemical behavior ofPd(II), Rh(III), and Ru(III) in bmimCl was studied in detail


by Jayakumar et al. [89–91]. Electrowinning of palladiumwas conveniently carried out at −0.8 V (versus Pd) atstainless steel electrode, whereas the electrodeposition ofrhodium was feasible only at −1.6 V. In contrast to boththese metal ions, ruthenium (III) formed a stable solutionwith bmimCl, and reduction of Ru (III) in to metallicform was not feasible in bmimCl. However, coexistence ofpalladium (II) in bmimCl favored underpotential depositionof ruthenium and rhodium. Initial deposition of palladiumon working electrode seems to shift the deposition potentialsof ruthenium (III) and rhodium (III) and favored underpo-tential deposition by more than 1 V. The study established thefeasibility of using bmimCl ionic liquid as electrolyte for theelectrochemical recovery of fission platinoids at 373 K.

Raz et al. [92] studied the electrodeposition of ruthe-nium on n-type silicon from bmimPF6. The reduction ofruthenium occurred at −2.1 V (versus Pt) and the strippingat 0.2 V (versus Pt). Metallic Ru film of ∼100 nm thick-ness was deposited and characterized by scanning electronmicroscopy (SEM) and X-ray photoelectron spectroscopy(XPS). Mann et al. [93] studied the electrodeposition of ultrathin ruthenium films on Au(111) substrate from bmimDCAionic liquid medium. Matsumiya et al. [58] reported theelectrochemical behavior of [PdCl4]2− and [PdBr4]2− inroom-temperature ionic liquids, P2225NTf2, P222(12)NTf2,and N2225NTf2. They mentioned that the diffusion coeffi-cients of palladium complex in phosphonium-based ionicliquids were slightly larger than those in correspondingammonium counterparts and close to those in BMPyNTf2.

5.3. Thermal and Radiation Stability. Ionic liquids with goodradiation and thermal stability are indeed necessary fornuclear fuel cycle applications. Moura Ramos et al. [94]investigated the glass transition of imidazolium-based RTILssuch as C5O2ImCl and C5O2ImPF6

− and reported theheat capacity jump during glass transition. Zhang et al.[95, 96] determined the molar heat capacities of BuPyBF4

and bmimBF4 using adiabatic calorimeter and reported theenthalpy and entropy change during glass transition of RTIL.Reddy and coworkers [97] reported some thermodynamicproperties of bmimCl determined by using techniques suchas TG, DTA, and DSC. Jagadeeswara Rao et al. [98] reportedthe thermal stabilities and heat capacities of various ionicliquids such as MPPiNTf2, BMPyNTf2, and HbetNTf2. Theionic liquids, MPPiNTf2, and BMPyNTf2, were reported tobe thermally stable up to 650 K, whereas HbetNTf2 was stableonly up to 560 K.

Allen et al. [99] investigated the radiation stability of1, 3-dialkylimidazolium nitrate/chloride ionic liquids andreported that their stabilities are comparable with that ofbenzene and more stable than the mixtures of tributylphos-phate and odourless kerosene under similar irradiationconditions. These ionic liquids were reported as radiationresistant up to 400 kGy, and it was attributed to the com-bination of properties of a salt and aromaticity. Berthon etal. [100] studied the gamma radiolysis of hydrophobic ionicliquids bmimPF6 and bmimNTf2. It was reported that theproperties such as density, surface tension, and refractive

index were unchanged upon irradiation; however, significantincrease in viscosities and lowering of conductivity wasobserved in case of irradiated RTILs. Moreover, the overallconcentrations of radiolysis products did not exceed 1 mol%when irradiated up to 1200 kGy. Jagadeeswara Rao et al. [22]also reported a similar observation for the ionic liquids suchas, bmimCl, hmimCl, HbetNTf2, BMPyNTf2, MPPiNTf2,and aliquat 336 ionic liquids upon gamma irradiation.

Lall-Ramnarine et al. [101] investigated pulse radiolysisof bis(oxalato)borate anion-based ionic liquids. Efficientscavenging of radiolytically generated electrons was observedin these ionic liquids. It was suggested that these boratedionic liquids may find useful applications during handlingof concentrated fissile elements. Tarabek et al. [102] studiedthe electron beam radiolysis of several NTf2

−-based ionicliquids. The study revealed that the formation of hydrogengas due to radiolysis of ionic liquids was relatively lowcompared to aliphatic and aromatic organic compounds.Ionic liquids with imidazolium and pyridinium cations werereported to give smallest hydrogen yields. Yuan et al. [103]studied the gamma radiolysis of bmimNTf2. It was reportedthat the absorbance of ionic liquids at 290 nm increasedwith increase of absorbed dose, and it was attributed tothe radiolysis of bmim+ cation. Qi et al. [104] studied thegamma radiation effect on bmimBF4. They reported thatthe irradiation-induced darkness and increased the UVabsorption, but it did not affect the glass transition pointof bmimBF4. They concluded that the radiation stability ofbmimBF4 was higher than bmimPF6.

6. Conclusions

RTILs initially introduced as a substitute to the moleculardiluent, n-dodecane, in aqueous reprocessing applications,are now receiving an upsurge in the studies related tononaqueous reprocessing. The discovery of the moisturestable ionic liquids in late 1990s having wide electrochemicalwindow (∼6 V) and excellent cathodic stability indeedboosted the ionic liquid research for such applications. Theelectrochemical behavior of actinides and fission products inmoisture stable ionic liquids reported so far are encouraging,and, going by the research work published recently, it appearsthat ionic liquids have good potential for nonaqueousprocessing applications. While these new results and noveltechnologies exhibit a great potential, the physical andchemical properties of the ionic liquid needs to be tuned tosuit the ionic liquid for robust applications. It is, therefore,recommended to study the fundamental properties of ionicliquids that govern electrochemical stability, solubility ofactinides and fission products, viscosity, and so forth todesign RTILs having all the required properties for thedesirable applications.

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Research Article

Potentiometric Multisensory Systems with Novel Ion-ExchangePolymer-Based Sensors for Analysis of Drugs

Olga V. Bobreshova, Anna V. Parshina, and Ksenia A. Polumestnaya

Department of Analytical Chemistry, The Voronezh State University, Universitetskaya pl. 1, Voronezh 394006, Russia

Correspondence should be addressed to Olga V. Bobreshova, [email protected]

Received 20 June 2011; Accepted 7 September 2011

Academic Editor: Karl Scott Ryder

Copyright © 2012 Olga V. Bobreshova et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

This paper examines potentiometric multisensory systems that consist of novel cross-sensitive PD-sensors (Potential Donnan-sensors). The analytical signal of PD-sensors is the Donnan potential at the ion-exchange polymer/electrolyte test solutioninterface. The use of novel sensors for the quantitative analysis of multicomponent aqueous solutions of amino acids, vitaminsand medical substances is based on protolytic and ion-exchange reactions at the interfaces of ion-exchangers and test solutions.The potentiometric sensor arrays consist of PD-sensors and ion-selective electrodes. Such systems were developed for themulticomponent quantitative analysis of lysine monohydrochloride, thiamine chloride and novocaine hydrochloride solutions thatcontained salts of alkaline and alkaline-earth metals, as well as for mixed solutions of nicotinic acid and pyridoxine hydrochloride.Multivariate methods of analysis were used for sensor calibration and the analysis of the total response of sensor arrays. The errorsof measurement of the electrolytes in aqueous solutions did not exceed 10%. The developed multisensory systems were used todetermine the composition of a therapeutic “Mineral salt with low content of sodium chloride” and to determine concentrationsof novocaine in sewage samples from a dental clinic.

1. Introduction

UV spectrophotometry [1], spectrofluorimetry [2], andHPLC [3] are commonly used methods for the quantitativeanalysis of amino acids, vitamins, and medical substances inaqueous solutions.

The advantages of potentiometric methods include thepossibility of rapid, in situ analysis, the automation and sep-aration of measurements, the simplicity of the technique,and the absence of any probe preparation [4–6]. The directpotentiometric determination of ions in aqueous solutionsis based on either the measurement of membrane potentialor oxidation-reduction potential of ion-selective electrodes(ISEs). Classical representations of ISEs are based on the the-ory of a glass electrode [7–10]. The Nicolsky-Eisenman equa-tion can be applied to the description of a sensor response inmulticomponent solutions. The influences of interfering ionsare considered by means of selectivity constants, which arestrictly reasonable only for binary systems. The use of ISEs in

real systems is currently limited by their low selectivity andaccuracy in multicomponent systems.

Modern investigations in the field of potentiometric sen-sors have taken the following approaches: the search fornovel materials useful for the construction of ISEs [11–13],the miniaturisation of sensors [14, 15], the elimination of atransmembranous stream of defined ions from an ISEinternal reference solution in test solutions [16, 17], thedevelopment of a theory describing ISEs without restrictionsconcerning equilibrium or steady state [6, 18] and researchand development of multisensitive systems for the analysis ofmulticomponent liquid environments [19–25].

A multisensitive system includes an array of cross-sensi-tive sensors (i.e., sensors that are sensitive to several compo-nents in a given solution) and algorithms for processing themultidimensional data from a sensor array [19–26]. Ref-erences [21–25] offer criteria for an estimation of cross-sensitivity, including the average inclination (i.e., sensitivityfactor), the stability factor, and the nonselectivity factor.


The sensitivity factor S = (1/n)∑Si is the average value of

lean angles from calibrations Si of all sensors in an array inindividual solutions of defined components. The stabilityfactor K = (1/n)

∑(Si/Di) is the average value of a lean angle

Si relative to the distribution of lean angles Di from cal-ibrations of all sensors in individual solutions of definedcomponents. The nonselectivity factor F = S/D is the averageinclination S relative to a distribution D of the average in-clinations. The response of an array of cross-sensitive sensorsis difficult because it contains information from variouscomponents that are present in the test solution, in additionto their interactions. Multivariate calibrations reduce the un-certainty of the analysis and reveal internal hidden interac-tions between variables as a result of increasing concen-trations [27]. Established potentiometric multisensitive sys-tems, such as the “electronic tongue,” only permit qualitativeand semiquantitative analyses of foodstuffs and pharmaceu-tical products [19–26].

Recently, we described the development of a novel poten-tiometric sensor (PD sensor), which measures the Donnanpotential at an ion-exchange polymer (IEP)/electrolyte testsolution interface [28–33]. The Donnan potential is theGalvani potential between two points outside the externalinterfaces of double electrical layers (DELs) at the IEP/testsolution interface [34–37]. Consequently, it is impossible todirectly measure the Donnan potential; however, it is possibleto estimate its value. An attempt to define the Donnanpotential is known. For this purpose, the electromotive force(EMF) of the electrochemical circuit is measured, whichincludes two reference cells with reference electrodes, tworeference solutions, and two salt bridges. One salt bridge isin contact with a test solution of the inorganic salt, and thesecond is in contact with a membrane surface. Additionally,the membrane is also in contact with a test solution [38]. Onedisadvantage of this method includes the short time ofpotential stability (1.5–2.0 min) due to transmembranoustransport. Similarly, we determine the Donnan potential bymeasuring the EMF of the electrochemical circuit, but it ismeasured from the potential jump at the individual ion-exchanger/test solution interface [28–31]. The use of themembrane potential equilibrium constant as an analyticalsignal, which is the Donnan potential at the IEP/test solutioninterface, allows us to eliminate issues related to migrationand diffusion in ionophore-based potentiometric sensors[16, 17]. This process ultimately increases the accuracy, sta-bility and sensitivity of organic and inorganic ion measure-ments.

We have previously described the application of the PDsensor for the selective determination of lysine in the pres-ence of neutral amino acids, ammonium ions [28, 32], andPD sensors, which are cross-sensitive in multicomponentsolutions of some amino acids, vitamins, medicinal substan-ces, and inorganic salts [29, 30, 33].

The aim of this paper was to develop potentiometric mul-tisensory systems with novel polymer-based ion-exchangePD sensors for the determination of inorganic ions and var-ious ionic forms of organic electrolytes in multicomponentaqueous solutions. Specifically, the analytes of interest wereamino acids, vitamins, and medical substances.

2. Experimental

2.1. Reagents. All chemicals were of analytical reagent grade.All solutions were prepared using distilled water with a resist-ance of 0.35 MΩ·cm. The following analytes of interest weredissolved in aqueous solutions: lysine monohydrochloride(LysHCl), thiamine chloride (ThiaminCl), pyridoxine hy-drochloride (PyridoxinHCl), nicotinic acid (Niacin), novo-caine chloride (NovHCl), and inorganic electrolytes (NaCl,KCl, CaCl2, and MgSO4). Concentrations of the varioussolution components ranged from 1.0 × 10−4 to 1.0 M. ThepH values of LysHCl + KCl + NaCl + MgSO4, ThiaminCl +KCl + NaCl, NovHCl + KCl + NaCl, and PyridoxinHCl +Niacin solutions were 5.27 ± 0.05, (3.46–4.65) ± 0.04, 4.5 ±0.4, and (3.12–4.40) ± 0.05, respectively.

Systems containing sulphocation-exchange polymerswith different structures (i.e., homogeneous perfluorinatedsulphocation-exchange MF-4SK membranes and tubes,which are Russian analogues of Nafionc, and heterogeneoushydrocarbonic MC-40 membranes) and individual solutionsof inorganic electrolytes (i.e., HCl, NaCl, and KCl) were pre-viously researched for the selection of ion-exchange materi-als.

The structure of perfluorinated sulphocation-exchangepolymers (PSPs) is formed from a system of nanopipes (10–17/5–10/3–5 nm) and pores (0.75–1.25 nm) with hydropho-bic walls and hydrophilic sulphonate ionic groups withinthe channel volumes. The structural units of the hydrocar-bonic polymers are represented by macroclusters of micro-pores with radii of 2-3 nm. The structural units of thehydrocarbonic polymers include ion-exchange groups andhydrophilic regions in a matrix that is divided by meso- andmacropores with 5 ÷ 500 nm radii. The meso- and macrop-ores are filled with test solution and include polymeric chainsand an inert material [39].

PSPs are characterised by optimal selective properties asa result of fewer numbers of mesopores and the completeabsence of macropores. Thus, the presence of hydrophobic(i.e., polytetrafluoroethylene chains) and hydrophilic (i.e.,sulphonate ionic groups) regions in such polymers providesthe matrix with labile structural components, which allowsfor the electrochemical properties of PSPs to be controlledby changing the ionic form of the polymers. Therefore, thehydrophobicity of the matrix from PSP and the absenceof macropores together define a greater interface transitionactivation energy of hydrated ions compared to hetero-geneous hydrocarbonic polymers. Hence, the use of PSPsin PD sensors provides increased signal, sensitivity, andaccuracy in comparison with hydrocarbonic polymers. Thecomparison of the sensitivity of PSP-based PD sensors andMK-40 membranes for a number of inorganic ions is shownin Figure 1.

2.2. Apparatus. All solutions were analysed at 25 ± 0.05◦Cusing a liquid thermostat TJ-TS-01/12. All potentiometricmeasurements were performed using an Expert-001-3(0.1)fluid analyser. The reported relative error for this devicefor pH and EMF measurements is 2.5% and 1.5%, respec-tively. A potassium-selective electrode (K-SE, ELIS-121 K),


0

10

20

30

40

50

60

MK-40PSP

H+ Na+ K+ Ca2+

H+ > Na+ > K+ > Ca2+

S i(m

V/p

C)

Figure 1: Sensitivity of PD sensors based on PSP and MC-40 incorresponded ionic types in test solutions of HCl, NaCl, KCl, andCaCl2.

calcium-selective electrode (Mg(Ca)-SE, ELIS-121Mg(Ca))with a polyvinylchloride membrane, glass sodium-selectiveelectrode (Na-SE, ELIS-112Na), silver chloride/silver refer-ence electrode (EVS-1M3.1), and glass electrode (ELS-43-07) were all used for the pH control of test solutions. Thesensors were rinsed with distilled water for 30 s betweenmeasurements.

2.3. Organisation Principles of Potentiometric MultisensorySystems with PD Sensors. A scheme of an electrochemical cellfor analysing multicomponent aqueous solutions of organicand inorganic electrolytes [29–31] is presented in Figure 2.

The sensor array included PD sensors (Ai), ISE (Bi), anda silver chloride/silver reference electrode (C). The potentialsof sensors Ai and Bi were measured with reference electrodeC using a high-resistance electronic voltmeter V. Responsesfrom electrode Ai registered after 5–7 min, which was thetime it took to reach a quasiequilibrium state [29–31].

The PD sensor [28] included two plastic encasements,designated 1 and 2, with volumes of 5 and 0.5 cm3, respec-tively. Encasements 1 and 2 were connected to a rubberstopper 3. Encasement 1 was filled with reference solution.Depending on the ionic form of the polymer, 1 mol/l solu-tions of HCl or KCl were used as reference solutions. En-casement 2 prevented the IEP from drying. The internalreference electrode 4, which was a silver wire covered withAgCl, was fixed in encasement 1 and immersed in a 1 M KClsolution. The free end of the 6–8 cm long PSP (both tubeand membrane) 5 was fixed in stoppers 3 and 6 and wasimmersed in the test solution.

The electrochemical circuit (1) for the determination ofthe response of the PD sensor was constructed in the follow-ing configuration [29–31]:

Ag|AgCl, 1 M Cl− |PSP| test solution |sat .KCl, AgCl|Ag,(1)

E = Δϕ0(A/C)Ag/AgCl + Δϕ1MCl−

PSP + Δϕdiff + ΔϕPSPtest-solution

+Δϕtest-solutionsat .KCl − Δϕ0(C)

Ag/AgCl,(2)

V

6

2

3

1

5

4A1 A2. . . B1 B2. . . C

Figure 2: The scheme of the electrochemical cell for the determina-tion of organic electrolytes in multicomponent test solutions: Ai isthe PD sensor; 1, 2 are plastic encasements; 3, 6 are rubber stoppers;4 is internal reference electrode; 5 is PSP in K+-type; 7 is 1 M KCl; 8is the test solution; Bi is ISE; C is silver chloride/silver electrode; Vis high-resistance voltmeter.

where Δϕ0(A/C)Ag/AgCl and Δϕ0(C)

Ag/AgCl are the standard potentials ofan intrinsic reference electrode of the PD sensor (A) andreference electrode (C), respectively; Δϕ1MCl−

PSP is the potentialdifference at the interface between the intrinsic referencesolution of the PD sensor (A) and PSP; Δϕdiff is the diffusionpotential in the PSP phase; ΔϕPSP

test-solution is the Donnan po-tential at the interface PSP/test solution; Δϕtest-solution

sat.KCl is thepotential difference at the test solution/KCl saturated solu-tion interface of the reference electrode (C).

The organisation of the PD sensor is as follows. The sumof all potential jumps in the EMF (1) was negligibly smallcompared to the Donnan potential at the PSP/test solutioninterface. The concentration of the reference solution wascomparable to the concentration of fixed groups in PSP. Theinterfaces of the PSP/test solution and PSP/reference solutionin the PD sensor were separated, which allowed us to neglectthe influence of diffusion and migration processes on theanalytical signal. Thus, the analytical signal of the PD sensorwas the Donnan potential at the PSP/test solution interface.The quantitative consideration of the contributions of poten-tial jumps at all interfaces to the total EMF for determinationof the response of the PD sensor was presented previously[30].

2.4. Multivariate Calibration Methods. The stability, sensi-tivity, and selectivity of sensors were estimated in indi-vidual solutions of analytes. The determination of activitycoefficients in the polyionic systems is a difficult scientificproblem. Therefore, calibration of the sensors was performedin the ΔϕD/pC coordinates. In this case, information aboutthe relationship between activity and ion concentration in


Referencesolution

PSP

Testsolution

K+

K+-type

K+

Xz+

Xz+

Mez+

Mez+

(a)

C5H4NHCOOH+H3O+

H+-type

H2O

X2+Xz+Mez+

X+

H3O+

H3O+

C5H4NHCOO±

H3O+

+

H2O

+

Xz+Mez+

H3O+

H3O+

(b)

Figure 3: The quasiequilibria at the interface of PSP in K-type (a) and H+-type (b) with test and reference solutions in PD sensor: X+ islysine, thiamin, and novocaine cations, Mez+ is cations of alkaline, and alkaline-earth metals.

the phase of a solution and in the phase of the PSP was con-tained in the calibration coefficients.

Multivariate calibration methods were used to deduce thecalibration equations for the calculation of analyte concen-trations in mixed test solutions. The values of factors of con-centrations were changed with a constant step. Initial factorswere encrypted so that the sum of the values of any twofactors from all experiments was equal to zero, which satis-fied the requirements of nondegeneracy and orthogonalityfor the experimental plan. The absence of systematic errorsand insignificant distinction of variances was a necessary re-quirement for the responses of sensors. Statistical modelsthat did not take into account (3) and that did take intoaccount (4), the interference of different components, wereutilised to calculate concentrations of components and wereas follows:

Ek = B0 +3∑

i=1

Bi · pCi, (3)

Ek = B0 +3∑

i=1

Bi · pCi +3∑

i=1

bi j · pCi · pC j , (4)

where Ei is the response of i-sensor, mV; b0, bi, and bi jare coefficients of calibration equations, mV/pC; pCi andpC j are the negative decimal logarithm of concentrations ofinvestigated i, j-components.

The coefficients of multivariate calibration equationswere determined by the method of least squares [27]. Thevalues of the calibration coefficients were compared to thefitting errors to verify their statistical significance. To verifythe adequacy of the calibration equations, the difference be-tween the calculated and experimental response values of thesensors was compared to the distribution results from theduplicated experiments. A spreadsheet was used to calculateregression parameters for the sensor calibration curves.


The PD sensors were organised so that quasi-equilibria,which are formed at PSP/test solution and PSP/reference so-lution interfaces, are stable as a function of time and are

independent from each other. The ionic properties and con-centrations in the solution phase volumes and the ion-exchanger were slightly changed [34], permitting the use ofPSP in the PD sensors, which were not transferred into theelectrolyte test solution. Both inorganic ions of the initialform of an ion-exchanger and various ionic forms of organicelectrolytes take part in the formation of the Donnan poten-tial at the IEP/test multicomponent solution interface.

In systems with inorganic electrolytes, ion-exchange re-actions are potentially defining. In such reactions, a hydratedshell of ions partially breaks up in solution phase and thenreorganises in the PSP phase. Therefore, the sensitivity of thePD sensor to solution phase inorganic ion concentrations in-creases with decreasing charge, crystallographic radius, andincrease in degree of hydration (Figure 1).

Various functional groups (–NH2, –COOH) are presentin the structure of amino acids, vitamins, and medical sub-stances. These functional groups are capable of participatingin ion-exchange and proteolytic reactions in the solutionphase, PSP, and at the interface. The quasi-equilibria at thePSP/multicomponent test solution interface for K+- and H+-type PSPs are shown in Figures 3(a) and 3(b), respectively.

The potential defining reactions of PSP-based K+-typePD sensors are ion-exchange reactions. The PSP-based K+-type PD sensor was characterised by high sensitivity to allorganic (41 ± 2 mV/pLysH, 38 ± 4 mV/pThiaminH, 51 ±4 mV/pNovH, and 36 ± 2 mV/pPyridoxinH) and inorganic(44 ± 2 mV/pNa, 42 ± 2 mV/pK, and 32 ± 2 mV/pMg) ions(Figure 4). Thus, these ion-exchange reactions were the causeof cross-sensitivity of PD sensors in the multicomponent testsolutions, in addition to solutions of ThiaminCl and NovHCl(i.e., the sorption of Thiamin+ and NovH+ ions was difficultbecause of their size). The PSP-based K+-type PD sensor wascharacterised by a higher sensitivity in solutions of the strongelectrolytes LysHCl, ThiaminCl, PyridoxinHCl, NovHCl,KCl, NaCl, and MgSO4 compared to the sensitivity of Niacin(23± 2 mV/pC), which is a weak electrolyte (Figure 4).

In PSP-based H+-type PD sensors, hydronium ionscontributed to the Donnan potential. The hydronium ionscompeted with large organic cations during formation of theDonnan potential at the PSP/test solution interface becauseorganic cations have hydrophilic and hydrophobic groups.


Table 1: The factors for the estimation of the cross-sensitivity of sensors Ak , Bk .

Determiningcomponents

LysHCl, KCl, NaCl, MgSO4 ThiaminCl, KCl, NaCl NovHCl, KCl, NaCl Niacin, PyridoxinHCl

Sensor A1 B1 B2 B3 A1 B1 B2 A1 B1 B2 A1 A2

S = (1/n)∑n

k=1Sk ,mV/pC

40 27 29 13 41 28 44 46 34 42 29 33

K = 1/n∑n

k=1Sk/Dk 9 5 6 3 5 7.5 11 15 4 2 8 16

F = S/D 1.4 0.08 0.05 0.09 2 0.1 0.2 2 0.1 0.2 0.3 17

01020304050

LysH

Cl

Nov

HC

l

Nia

cin

KC

l

NaC

l

MgS

O4

PSP in H+-typePSP in K+-type

S i(m

V/p

C)

Th

iam

inC

l

Pyr

idox

inH

Cl

Figure 4: Sensitivity of PSP-based K-type (a) and H+-type (b)PD sensors in test solutions of LysHCl, ThiaminCl, NovHCl,PyridoxinHCl, Niacin, NaCl, KCl, and MgSO4.

Additionally, the singly charged organic cations LysH+,ThiaminH+, and NovH+ did not participate in ion-exchangereactions. These cations transformed into doubly-chargedions in the polymer phase as a result of a heterogeneous prot-eolytic reaction. Thus, adsorption at the interface of largedoubly charged ions resulted in the decreased contributionof an organic component to the Donnan potential. As aresult, the stability of the analytical signal of the PD sensorin such systems decreased. In some cases, the sensitivity toan organic component also decreased (e.g., by 1.2- and 1.5-times to LysH+ and NovH+, resp.). However, the sensitivityof PSP-based H+-type PD sensors to inorganic ions changedonly slightly in comparison with PSP in salt forms (Figure 4).

The sensitivity of the PSP-based H+-type PD sensors to nic-otinic acid increased by 1.5-times compared to PSP-basedK+-type PD sensors (Figure 4). In the PSP-based H+-type PDsensors, both cations (NiacinH+) and zwitterions (Niacin±)of nicotinic acid contributed to the Donnan potential. Prote-olytic reactions were the cause of Niacin± ion contributions,which resulted in the transfer of cations to the PSP phase.

Thus, the use of PSPs in various ionic forms led todifferent PD sensor sensitivities toward the same organiccomponent. However, the sensitivity of PD sensors towardinorganic ions changed insignificantly. The various influ-ences of ionic forms of PSPs on the sensitivity to vitaminsPyridoxinHCl and Niacin permitted the use of PD sen-sors for their joint determination in mixed aqueous solu-tions.

3.1. Multisensory Systems for Determination of Amino Acids,Vitamins, and Medical Substances. The potentiometric mul-tisensory systems were developed for a multicomponentquantitative analysis of lysine monohydrochloride, thiaminechloride, and novocaine hydrochloride solutions that alsocontained chlorides of potassium and sodium. Additionally,solutions of nicotinic acid and pyridoxine hydrochloridewere also analysed.

The sensor array for the analysis of solutions LysHCl +KCl + NaCl, ThiaminCl + KCl + NaCl, and NovHCl + KCl +NaCl included the PSP-based K+-type PD sensor (A1), K-SE (B1), Na-SE (B2), and silver chloride/silver reference elec-trode (C). The electrochemical circuits for the determinationof the responses of the sensor array are described by

(A1) Ag | AgCl, 1 M KCl | K+-type PSP | test solution | sat. KCl, AgCl | Ag, (C), (5)

(B1) Ag | AgCl, 0.1 M KCl | PVC membrane | test solution | sat. KCl, AgCl | Ag, (C), (6)

(B2) Ag | AgCl, 0.1 M HCl | glass membrane | test solution | sat. KCl, AgCl | Ag, (C). (7)

The sensor array for the analysis of solutions LysHCl +KCl + NaCl + MgSO4 included Mg(Ca)-SE (B3) and sensorsA1, B1, B2, and C. The electrochemical circuits for the

determination of the response of the sensor array are de-scribed by (5)–(8):

(B3) Ag | AgCl, 0.1 M KCl | PVC membrane | test solution | sat. KCl, AgCl | Ag, (C). (8)


According to reference [24], a cross-sensitive sensor mustbe characterised by a sensitivity factor S > 25 mV/pC, astability factor K > 2, and a nonselectivity factor F > 0.5.Shown in Table 1 are the values of the factors S, K , and Fto ions LysH+, K+, Na+, Mg2+; Thiamin+, K+, and Na+, andNovH+, K+, and Na+ for sensors A1 and Bk, respectively. Thevalues of factors S, K , and F were calculated on the basis ofexperimental data (Figure 4) that were obtained by sensorcalibrations in test solutions (Si(mV/pC) = individual anglecoefficient, Di(mV/pC)2 = dispersion of individual anglecoefficient, and D(mV/pC)2 = dispersion of average angle).

Large sensitivity and nonselectivity factors for the PSP-based K+-type PD sensor in the groups of ions (Table 1)predicted commensurable contributions of correspondingions to the PD sensor response in LysHCl + KCl + NaCl,

ThiaminCl + KCl + NaCl and NovHCl + KCl + NaCl solu-tions.

ISEs were not as highly selective in test solutions. Forexample, Mg(Ca)-SE had an average angle lower than25 mV/pC (Table 1). The preferential sensitivity of ISEs tocorresponding inorganic ions caused low nonselectivity fac-tors. However, the main requirement for ISE selection isresponse stability (>2) toward defined components whencross-sensitive PD sensors are present in a sensor array.

The sensor array for the analysis of PyridoxinHCl +Niacin solutions included two PSP-based K+-(A1) and H+-type (A2) PD sensors and a reference electrode (C). The elec-trochemical circuit for the determination of PD sensor re-sponses is described by (5) and (9)

(A2) Ag | AgCl, 1 M HCl | H+-type PSP | test solution | sat. KCl, AgCl | Ag. (9)

Shown in Table 1 are the values of the cross-sensitivitycriteria for PD sensors in individual solutions of Pyridox-inHCl and Niacin. Significant values of factors S, F, andK permitted the use of PSP-based K+- and H+-type PDsensors for the determination of PyridoxinHCl and Niacin intheir mixed aqueous solutions. Thus, the various sensitivitiesof the sensors to defined components (Figure 4) predicteda significant distinction of their contributions to sensorresponses in the mixed solutions.

The mixed test solutions LysHCl + KCl + NaCl, Thi-aminCl + KCl + NaCl, NovHCl + KCl + NaCl, andPyridoxinHCl + Niacin were studied for the multivariatecalibration of the sensor array. All possible combinations offactors pC were examined for each analyte in the range of 2–4 with a constant step of pC = 1. The coefficient estimatesfrom the multivariate calibration equations without takinginto account any interference of components to the responsesof sensors Ak and Bk are presented in Table 2.

The coefficient estimates of the multivariate calibrationequations, taking into account the interference of compo-nents on the responses of sensors Ak and Bk, are presentedin Table 3.

The equations were adequate at a confidence level of0.05. The statistical models that took into account theinteractions of factors reduced the errors of PD sensorsand the errors of Na-SE by 1.3–2.3-times and 1.4–3.2-times,respectively, compared to statistical models that did not takeinto account the interactions of factors. The errors for K-SE and Mg(Ca)-SE did not change. Therefore, to calculateanalyte concentrations, we used (4) for the PD sensor andNa-SE and (3) for the K-SE and Mg(Ca)-SE.

Shown in Tables 4 and 5 are the actual and measuredvalues of the analyte concentrations for some test solutions.

The number of replicate measurements was 6–8. Thestatistical data interpretation was made using a confidencecoefficient of 0.95. The relative error of measurement was 2–10%.

3.2. Determination of Lysine Monohydrochlorides and Novo-caine Hydrochlorides in Therapeutic Products and Sewage froma Dental Clinic. The potentiometric multisensory systemused to analyse LysHCl + KCl + NaCl + MgSO4 solutionswas used for the analysis of therapeutic “mineral salt withlow content of sodium chloride” samples. This productcontained NaCl, KCl, MgSO4, and LysHCl in the followingmass rations (%): 0.35–0.58; 0.31–0.40; 0.05–0.10 and 0.02–0.10 [40]. The responses of sensors A1, B1, B2, and B3

were measured against a reference electrode C in aqueoussolutions of 1 g/l of salt. The defined concentrations of LysH,K+, Na+, and Mg2+ were 1.0 · 10−4 ± 0.4 · 10−4 M (0.054 ±0.004% LysHCl in dry sample), 5.1·10−3±0.3·10−3 M (0.38±0.02% KCl in dry sample), 9.0 · 10−3 ± 0.5 · 10−3 M (0.02 ±0.008% NaCl in dry sample), and 4.5 · 10−4 ± 0.3 · 10−4 M(0.53± 0.04% MgSO4 in dry sample), respectively. Thus, themeasured composition of the therapeutic salt samples was inagreement with the stated product composition.

The potentiometric multisensory system used to analyseNovHCl + KCl + NaCl solutions was used to analyse sewagesamples from a dental clinic. The responses of sensors A1,B1, and B2 were measured against reference electrode C insewage samples. These samples were taken from a sewerknee before and after a patient’s reception. The disparity ofNovH+ concentrations in the dental clinic sewage before andafter the patient’s reception was (0.44 ± 0.01)·10−5 M. Theconcentrations of K+ and Na+ in the sewage were 2.1·10−3 Mand 1.7 · 10−2 M, respectively, that is, in concordance with[41]. Regular drainage of novocaine hydrochloride into thesewage is a serious problem because the medications are toxicsubstances with narcotic activity.

4. Conclusion

Here, the development of potentiometric multisensory sys-tems, in which novel potentiometric PD sensors were cross-sensitive was described. The analytical signal of PD sensors


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Sen

sor

A1

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B2

B3

A1

B1

B2

A1

B1

B2

A1

A2

b 0±Δb 0

31±

635

9±

428±

226

6±

638±

438

1±1

680±

1.6

8±

835

9±

870±

8−4

4±

4−4

0±

4b 1±Δb 1

−6±

1.1

——

−2±

2−1

5±1.

1−3

4±

4−1

2±0.

4−1

4±2

—−1

1±2

−3±

1.1

−8±

1.1

b 2±Δb 2

−11±

1.1

−42±

4−1

3±1

—−1

5±1.

1−1

6±

4−1

5±0.

4−1

3±2

−41±

8−1

2±2

−37±

1.1

−35±

1.1

b 3±Δb 3

−17±

1.1

—−2

7±1

—−1

2±1.

1—

−17±

0.4

−8±

2—

−23±

2—

—b 4±Δb 4

−5±

1.1

——

−20±

2—

——

——

——

—ΔE/E

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0.3

106

710

215

41.

4


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Cl(

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Cl

Nov

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l(pC

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Cl

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cin

(pC

1)

+N

aCl(

pC3)

+M

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4(p

C4)

(pC

2)

+N

aCl(

pC3)

(pC

2)

+N

aCl(

pC3)

Pyr

idox

inH

Cl(

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Sen

sor

A1

B1

B2

B3

A1

B1

B2

A1

B1

B2

A1

A2

b 0±Δb 0

−121±3

035

9±

4−4

8±

426

6±

6−1

23±1

031

9±

35−6

1±

3−1

22±2

035

9±

8−9

0±12

−130±1

0−8

3±1

0b 1±Δb 1

6±

6—

—−2±

224±

5−1

3±10

9±

1.6

14±

8—

15±

625±

57±

5b 2±Δb 2

32±

6−4

2±4

13±

3—

15±

55±

421±

1.6

16±

8−4

1±

814±

4−9±

5−2

0±

5b 3±Δb 3

12±

6—

——

26±

5—

19±

1.6

21±

8—

30±

6—

—b 4±Δb 4

18±

6—

—−2

0±2

——

——

——

——

b 12±Δb 1

2—

——

—−5±

1.7

−7±

6−4±0

.6−6±

4—

—−1

0±

1.7

−5±

1.7

b 13±Δb 1

3−9±

1.1

——

—−4±

1.7

—−4±0

.6−6±

4—

−9±

2—

—b 1

4±Δb 1

4−4±

1.1

——

——

——

——

——

—b 2

3±Δb 2

3−4±

1.1

—−9±1

.5—

−7±

1.7

–−8±0

.6−5±

4—

−9±

2—

—b 2

4±Δb 2

4−4±

1.1

——

——

——

——

——

—b 3

4±Δb 3

42±

1.1

——

——

——

——

——

—ΔE/E

,%2

37

0.2

66

55

24

30.

6


Table 4: The actual and measured values of the analyte concentrations for some test solutions ThiaminCl + KCl + NaCl, NovHCl + KCl +NaCl.

Test solutionAdded, M Found, M

X+ K+ Na+ X+ K+ Na+

Thiamin+,ThiaminH2+(X) K+

Na+Cl−

1.0 · 10−4 1.0 · 10−3 1.0 · 10−2 (1.0 + 0.05) · 10−4 (1.0 + 0.10) · 10−3 (1.0 + 0.12) · 10−2

1.0 · 10−3 1.0 · 10−2 1.0 · 10−3 (1.0± 0.04) · 10−3 (1.0± 0.13) · 10−2 (1.0± 0.04) · 10−3

1.0 · 10−2 1.0 · 10−4 1.0 · 10−3 (1.0± 0.03) · 10−2 (1.0± 0.15) · 10−4 (1.0± 0.04) · 10−3

NovH+(X) K+

Na+Cl−

1.0 · 10−4 1.0 · 10−4 1.0 · 10−4 (1.0± 0.03) · 10−4 (1.0± 0.10) · 10−4 (1.0± 0.20) · 10−4

1.0 · 10−4 1.0 · 10−3 1.0 · 10−2 (1.0± 0.02) · 10−4 (1.0± 0.11) · 10−3 (1.1± 0.10) · 10−2

1.0 · 10−3 1.0 · 10−2 1.0 · 10−3 (1.0± 0.01) · 10−3 (1.1± 0.01) · 10−2 (1.0± 0.02) · 10−3

Table 5: The actual and measured values of the analyte concentrations for some test solutions PyridoxinHCl + Niacin.

Test solutionAdded, M Found, M

PyridoxinH+ Niacin PyridoxinH+ Niacin

PyridoxinH+ 1.0 · 10−4 1.0 · 10−2 (1.0± 0.012) · 10−4 (1.0± 0.015) · 10−2

Niacin+ 1.0 · 10−3 1.0 · 10−3 (1.1± 0.012) · 10−3 (1.0± 0.015) · 10−3

Niacin± Cl− 1.0 · 10−2 1.0 · 10−4 (1.0± 0.012) · 10−2 (1.0± 0.015) · 10−4

is the Donnan potential at the individual PSP/test electrolytesolution interface. The use of the membrane potential equi-librium component as an analytical signal, which is theDonnan potential at the IEP/test solution interface, resultedin the elimination of migration and diffusion problems in-herent in potentiometric sensors. As a result, the accuracyand stability of the analysis subsequently increased. The useof novel sensors for the quantitative analysis of multicom-ponent aqueous solutions of amino acids, vitamins, andmedical substances was based on proteolytic and ion-excha-nge reactions at the PSP/test solution interface. The poten-tiometric sensor arrays were developed for multicomponentquantitative analyses of lysine monohydrochloride, thiaminechloride, and novocaine hydrochloride solutions containingsalts of alkaline and alkaline-earth metals as well as mixedsolutions of nicotinic acid and pyridoxine hydrochloride.The sensor arrays consisted of cross-sensitive PD sensors andISEs. The multivariate methods of the analysis were used forsensor calibrations in addition to the analysis of the totalresponse of the sensor arrays. The relative errors of elec-trolyte measurements in aqueous solutions did not exceed10%.

The developed multisensory systems were then used todetermine the composition of therapeutic “mineral salt withlow content of sodium chloride” and dental clinic sewage.

The PD sensor multisensory systems made it possible toperform quantitative analyses of multicomponent solutionsof different electrolytes, which is in contrast to the majorityof known potentiometric sensors arrays that only permitsemiquantitative analyses.

Acknowledgments

The authors thank cand.ch.sc., the chief of the laboratoryof Membrane Processes OSS “Plastpolymer” (St. Petersburg,Russia) Timofeev Sergey Vasilevich for giving samples of per-fluorinated sulphonic cation-exchange membranes, tubes,

and rods. This study was funded by the Russian Fund ofFundamental Research (09-03-97505 r center a).

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