Quantification of Quercetin Using Glassy Carbon Electrodes Modified with Multiwalled Carbon Nanotubes Dispersed in Polyethylenimine and Polyacrylic Acid

Quantification of Quercetin Using Glassy Carbon ElectrodesModified with Multiwalled Carbon Nanotubes Dispersed inPolyethylenimine and Polyacrylic Acid

Fabiana Guti�rrez,a Gabriela Ortega,b Jos� Luis Cabrera,b Mar�a D. Rubianes,*a Gustavo A. Rivas*a

a INFIQC, Departamento de F�sico Qu�mica, Facultad de Ciencias Qu�micas, Universidad Nacional de C�rdoba, CiudadUniversitaria, 5000 C�rdoba, Argentinaphone: +54-351-4334169/80; fax: +54-351-4334188

b IMBIV, Farmacognosia – Departamento de Farmacia, Facultad de Ciencias Qu�micas, Universidad Nacional de C�rdoba. CiudadUniversitaria, 5000 C�rdoba, Argentina

*e-mail: [email protected]

Received: May 6, 2010;&Accepted: June 22, 2010

AbstractThis work reports the highly sensitive detection of quercetin using glassy carbon electrodes modified with multiwallcarbon nanotubes (MWCNT) dispersed in polyethylenimine and poly(acrylic acid). The adsorptive stripping withsquare wave voltammetric transduction allowed the detection of 7.5 nM quercetin after 1.0 min accumulation atopen circuit potential. The amperometric detection at 0.150 V is reported as an easier and simpler alternative to de-termine quercetin, with detection limits of 0.2 mM. The careful selection of the working conditions also made possi-ble the detection of submicromolar levels of quercetin in the presence of excess of rutin. The proposed methodolo-gy was successfully used to quantify quercetin in onion samples.

Keywords: Carbon nanotubes, Glassy carbon, Electrochemical sensor, Quercetin, Flavonoids, Rutin, Poly(acrylicacid), Polyethylenimine

DOI: 10.1002/elan.201000291

1. Introduction

Carbon nanotubes (CNT), one of the allotropic forms ofcarbon, consist of cylindrical graphene sheets rolled updelimiting a hollow tube with nanometer diameter anddifferent conductivity depending on the structure [1–3].The unique electronic properties of CNT have made pos-sible their successful application as electrode material,providing a new and interesting alternative to design elec-trochemical sensors. [4].

Since CNTs form bundles due to their important Vander Waals interactions they are non soluble in usual sol-vents. Different strategies have been proposed to disperseCNTs in order to make possible their application for thedevelopment of electrochemical sensors [5–9] and electro-chemical detectors in CE [10–12]. Dispersion in surfac-tants [13,14], solvents [15] polymers [16–23], mineral oil[24–27], and ionic liquids [28] have been successfully per-formed. Polymers wrap around the CNTs, decreasing theintrinsic Van der Waals interactions of CNTs, and makingpossible the dispersion. The noncovalent functionalizationrepresents an excellent alternative because it does notproduce changes in the unique electronic properties ofCNTs. Our group reported for the first time the advan-tages of polyethylenimine (PEI) to disperse multi-walled

carbon nanotubes (MWCNT) [21]. IR spectra demon-strated that the interaction PEI-MWCNT is non covalent[29]. SEM pictures showed that the dispersion fullycovers the glassy carbon surface, although some areaswith higher amounts of CNTs are also evident [21,29].Zhou et al. [30–32] reported the highly successful disper-sion of single-walled carbon nanotubes (SWCNT) withpoly(acrylic acid) (PAA). SEM images show a completecoverage of the electrode surface and homogeneous dis-tribution. Spectroscopic techniques demonstrate that theinteraction of PAA with SWCNT is non covalent.

In this work we report the electrochemical behavior ofquercetin at glassy carbon electrodes modified with multi-walled carbon nanotubes (MWCNT) dispersed in differ-ent polyelectrolytes and the analytical application ofthese electrodes for the quantification of quercetin. Quer-cetin is a flavonol that belongs to the family of flavonoidsand contains several hydroxyl groups located in the differ-ent rings [33] (Scheme 1). Quercetin is highly distributedin fruits and vegetables, and has received great attentiondue to its anticancer [34–36], anti-inflammatory [37], andanti-allergic activity [38], mainly connected with their an-tioxidant properties [39].

Considering the importance of this flavonoid in differ-ent areas, the development of analytical methodologies

2650 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 22, 2650 – 2657

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that make possible a highly sensitive detection is highlyrequired. In this sense, electrochemical methodologiesdemonstrated to be very successful. Oliveira-Brett andGhica [40] demonstrated that the electrooxidation ofquercetin is complex and pH dependent, and that thefinal product passivates the electrode surface. Zeng et al.[41] reported the voltammetric quantification of quercetinat nM levels by adsorption at carbon nanotubes pasteelectrode (CNTPE) prepared with paraffin oil as aggluti-nant. Kim and Xu [42] informed about the stable, repro-ducible and sensitive quantification of quercetin at pH 3.8using a glassy carbon electrode (GCE) modified withCNTs dispersed in Nafion. Lin et al. [43] proposed thedetection of quercetin at submicromolar levels from theoxidation current at CNTPE prepared with nujol as dis-persant agent. A highly sensitive adsorptive voltammetricstripping determination of quercetin with a detectionlimit of 4.8 nM was proposed by Jin et al. [44]. Jung et al.[45] reported the simultaneous square wave voltammetry(SWV) detection of rutin and quercetin by using a GCEmodified with MWCNTs and succinyl-B-cyclodextrin.

In the following sections we report the electrochemicalbehavior of quercetin at GCE modified with CNTs dis-persed in PAA and PEI as well as the optimization oftwo strategies for the highly sensitive quercetin quantifi-cation, by amperometry and adsorptive stripping withmedium exchange and SWV detection.

2. Experimental

2.1. Reagents

Polyethylenimine (PEI, Average MW 750000, Catalognumber P-3143) was purchased from Sigma. Poly(acrylicacid) (PAA, Average MW 450000, Catalog number18,128-5) was received from Aldrich. Multiwalled carbonnanotubes powder (MWCNT (30�15) nm diameter, (1–5) m length) were obtained from NanoLab, Inc. (MA,USA). Other chemicals were reagent grade and usedwithout further purification. Quercetin (C15H10O7) wasobtained by acidic hydrolysis of quercetin-3,7,3’,4’-tetra-sulfate, a flavonoid isolated from leaves of F. bidentis[46]. Rutin (C27H30O16) was obtained from leaves of Pro-sopis alpataco and purified and identified according to[47]. Quercetin and rutin were identified by 1H NMR(Br�ker Advance II 400) and UV-vis spectroscopy (Shi-madzu 160A). The stock solutions of quercetin and rutin(1.0 mM) were prepared in pure ethanol. They remainedstable for one month at 4 8C. The working solutions wereobtained by dilution of the stock solution with 0.10 Mphosphate buffer solution pH 7.40. Ultrapure water (1=18 MW cm) from a Millipore-MilliQ system was used forpreparing all the solutions. A 0.10 M phosphate buffer so-lution pH 7.40 was employed as supporting electrolyte.

2.2. Apparatus

The measurements were performed with a TEQ 02 andEpsilon (BAS) potentiostats. The electrodes were insertedinto the cell (BAS, Model MF-1084) through holes in itsTeflon cover. A platinum wire and Ag/AgCl, 3 M NaCl(BAS, Model RE-5B) were used as counter and referenceelectrodes, respectively. All potentials are referred to thelatter. A magnetic stirrer provided the convective trans-port during the amperometric measurements.

2.3. Preparation of Modified GCE

2.3.1. Preparation of Dispersions

CNT-PEI: the dispersion was obtained by mixing 1.0 mgof MWCNTs with 1.0 mL of 1.0 mg/mL PEI solution(prepared in 50 : 50 v/v ethanol/water) followed by sonica-tion for 15 min (Testlab 160 W).

CNT-PAA: the dispersion was obtained by mixing1.0 mg of MWCNTs with 1.0 mL of 1.0 mg/mL PAA solu-tion (prepared in 50 :50 v/v ethanol/water) followed bysonication for 40 min. Once prepared, the dispersion wascentrifuged twice at 9000 rpm for 15 min and finally fil-tered through a Munktell paper (pore size 8–12 mm).

CNT in 50 :50 v/v ethanol/water: the dispersion was ob-tained by mixing 1.0 mg of MWCNTs with 1.0 mL of50 :50 v/v ethanol/water followed by sonication for15 min.

Scheme 1. Structure of quercetin and rutin.

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Quantification of Quercerin

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2.3.2. Preparation of Glassy Carbon Modified Electrodes

Previous the modification, the GCE was polished withalumina slurries of 1.0, 0.30 and 0.05 mm for 2 min each.

Preparation of GCE modified with CNT dispersed inPEI (GCE/PEI-CNT), PAA (GCE/PAA-CNT) or 50 : 50v/v ethanol/water (GCE/Et-CNT): the electrode was pre-pared by dropping 20 mL of the corresponding dispersionon the top of a polished glassy carbon electrode and fur-ther evaporation of the solvent by exposure to air for90 min.

Preparation of GCE modified with PEI (GCE/PEI)and PAA (GCE/PAA): a volume of 20 mL of 1.0 mg/mLPEI or PAA solutions (prepared in 50 : 50 v/v ethanol/water) was dropped on polished and clean GCE, andthen it was exposed to air until dry.

2.3.3. Preparation of Onion Samples

The onion was obtained from a supermarket. The sampleswere obtained from onion skin (3.6 g) and flesh onion(8.3 g). They were cut into small pieces and dried in a dryoven at 60 8C for 24 h. The dried portions were powderedan extracted with ethanol (300 mL) for 24h at room tem-perature under stirring and dark conditions. The extractswere filtered and 90 % of the solvent was removed byheating at 50 8C under vacuum conditions. The resultingresidues were diluted with ethanol (25 mL) and stored at4 8C under dark conditions until use.

2.4. Procedures

The cyclic voltammetric experiments were performed at0.100 V/s using a 0.100 M phosphate buffer solutionpH 7.40 as supporting electrolyte. All the experimentswere conducted at room temperature.

The adsorptive stripping voltammetric experiments con-sisted of two steps: (I) Quercetin preconcentration (ad-sorption): performed by immersion of the correspondingelectrode (GCE, GCE/PEI-CNT, GCE/PAA-CNT, GCE/Et-CNT, GCE/PEI or GCE/PAA) in a phosphate buffersolution containing quercetin for 1.0 min at open circuitpotential under stirring conditions; (II) Square wave vol-tammetry-stripping : performed in a 0.100 M phosphatebuffer solution pH 7.40 by SWV between �0.300 V and0.300 V with a pulse height of 4 mV, a pulse amplitude of50 mV, a period of 200 ms, frequency 50 Hz, step0.50 mV, without stirring. Between steps I and II the elec-trode was rinsed with the phosphate buffer solution for10 s.

All the experiments were conducted at room tempera-ture.

3. Results and Discussion

3.1. Electrochemical Response of Quercetin

Figure 1 displays cyclic voltammograms for 1.0 �10�4 Mquercetin at bare GCE at 0.100 V/s. When the potential isscanned between �0.300 V and 0.700 V (solid line), thereare two oxidation peaks, one at 0.105 V, due to the oxida-tion of the catechol group of ring B [41], and the other,less defined, at 0.425 V, due to the oxidation of the OHgroup of ring C. In the negative scan there is just onebroad reduction peak at 0.089 V. If the positive scan isstopped at 0.150 V (dashed line), the potential of the re-duction peak slightly shifts to less positive potentialswhile the associated current largely increases, suggestingthat at this potential occurs the reduction of the productgenerated at 0.105 V. In this way, it is possible to obtain aquasireversible couple with peak potential separation of58 mV. This peaks system was selected for further studies.

To evaluate the influence of CNTs on the electrochemi-cal behavior of quercetin, we studied the cyclic voltam-metric response of 1.0 �10�4 M quercetin in a 0.100 Mphosphate buffer solution pH 7.40 at bare GCE, GCE/PEI-CNT and GCE/PAA-CNT. Table 1 summarizes thecorresponding voltammetric parameters. The results indi-cate that, in the presence of CNTs (either at GCE/PEI-CNT or at GCE/PAA-CNT), the oxidation and reductioncurrents increase while the peak potential separations arealmost the same as those obtained at GCE, indicatingthat the enhancement in the currents is mainly due to theincrease of the electroactive area. It is important to men-tion that at GCE/PEI-CNT the capacitive currents aresignificantly higher than at GCE, while in the case ofGCE/PAA-CNT these currents are comparable to thoseat GCE, allowing, in this way, a more clear definition ofthe peaks.

In order to evaluate the contribution of the polymersused to disperse CNTs on the response of the resulting

Fig. 1. Cyclic voltammograms obtained at GCE for 1.0 � 10�4 Mquercetin in different potential range: up to 0.700 V (solid line)and 0.150 V (dashed line). Scan rate: 0.100 V/s. Supporting elec-trolyte: 0.100 M phosphate buffer solution pH 7.40.

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polymer-CNTs-glassy carbon modified electrodes, westudied the voltammetric behavior of quercetin at GCEmodified with CNT dispersed in a 50/50 v/v ethanol/watersolution (not shown). No peaks were observed at GCE/Et-CNT under our experimental conditions, strongly sug-gesting that the presence of the polymer is necessary toobtain a more efficient dispersion.

A comparison of the cyclic voltammetric behavior ofquercetin at GCE/PEI and GCE/PAA was also per-formed in order to demonstrate, in an indirect way, theadvantages of the presence of CNTs (not shown). In bothcases, the currents were smaller than those obtained atGCE (0.6 and 1.2 mA at GCE/PEI and GCE/PAA, re-spectively), evidencing a passivating effect of the poly-mers. Therefore, the improvement in the response ofquercetin at GCE/polymer-CNT is due to the presence ofCNTs efficiently dispersed in PEI or PAA.

The oxidation peak current for 1.0 �10�4 M quercetinat GCE/PEI-CNT and GCE/PAA-CNT showed a linearrelationship with the scan rate in the range of 0.005 to0.300 V/s, demonstrating an adsorptive control of the re-sponse (i=0.60 mA+98.5�v mA, r=0.9998 for GCE/PEI-CNT, and i=0.54 mA+92.7� v mA, r=0.998 forGCE/PAA-CNT).

Table 2 compares the oxidation currents of quercetinobtained in a 1.0 �10�4 M quercetin solution at differentsurfaces (GCE, GCE-PAA, GCE/PEI, GCE/PEI-CNT,GCE/PAA-CNT, and GCE/Et-CNT) with those obtainedin a 0.10 M phosphate buffer solution pH 7.40 after1.0 min accumulation at open circuit potential from 1.0 �10�4 M quercetin and medium exchange. The effective-ness of the adsorption of quercetin at the electrode surfa-ces containing CNTs is clear. In fact, the adsorption ofquercetin allows an enhancement in the oxidation cur-rents at all the surfaces. The noticeable increase of thesignal at GCE modified either with PEI or CNT-PEIafter the accumulation step demonstrates the fundamen-tal role of the polymer charge in the adsorption event.The adsorption also made possible the definition of theoxidation current peak at GCE/Et-CNT, although the as-sociated currents are considerably smaller than the onesobtained at GCE modified with PEI-CNT or PAA-CNT.The strength of quercetin adsorption demonstrated to behighly dependent on the surface where the accumulationwas performed (Table 2). In fact, the comparison of quer-cetin oxidation currents at the different surfaces beforeand after the medium exchange demonstrated that thestronger adsorption was obtained at GCE/PEI-CNT andGCE/PAA-CNT. Similar experiments performed at GCE/

Et-CNT showed a large decrease in the signal, indicatingthat ethanol/water is not efficient as dispersant agent andthat the polymeric matrix is necessary to ensure an effi-cient adsorption.

3.2. Adsorptive Stripping

Taking into account the strong adsorption of quercetin atGCE modified with PEI-CNTs and PAA-CNTs, we opti-mized an electrochemical methodology for the quantifica-tion of quercetin based on the adsorptive stripping withmedium exchange and square wave voltammetric detec-tion.

Figure 2A displays the effect of the adsorption time atGCE/PEI-CNTs of 1.0� 10�5 M quercetin at open circuitpotential on the oxidation peak current obtained fromSWV performed in a 0.100 M phosphate buffer solutionpH 7.40 after medium exchange. The signal increasesalmost linearly at the beginning and then more slowlydue to a saturation effect, as expected for a process underadsorptive control. A time of 1.0 min was selected for fur-ther studies. Figure 2B shows square wave voltammo-grams obtained at GCE/PEI-CNTs in a 0.100 M phos-phate buffer solution pH 7.40 for increasing concentra-tions of quercetin from 8.0 � 10�7 M to 1.0 �10�5 M after1.0 min accumulation at open circuit potential. The corre-sponding calibration plot is shown as an inset, and the an-alytical parameters are summarized in Table 3. Figure 2Cshows the square wave voltammograms obtained at GCE/PAA-CNTs under conditions identical to those used forFigure 2B for increasing quercetin concentrations from1.0 �10�7 M to 5.0 �10�6 M. The corresponding calibration

Table 1. Voltammetric parameters for the electrooxidation of 1.0 � 10�4 M quercetin at different surfaces. Scan rate: 0.100 V/s. Sup-porting electrolyte: 0.100 M phosphate buffer solution pH 7.40.

Electrode Ipa (mA) Ipc (mA) Epa (V) Epc (V) ~Ep (V)

GCE 2.3 2.1 0.105 0.080 0.058GCE/PEI-CNT 12.5 12.1 0.108 0.040 0.067GCE/PAA-CNT 4.5 5.9 0.126 0.062 0.064

Table 2. Oxidation peak currents for quercetin obtained at dif-ferent surfaces. Experimental conditions: (A) Without accumula-tion; (B) after accumulation for 1.0 min at open circuit potentialand stripping without medium exchange; (C) after accumulationfor 1.0 min at open circuit potential and stripping after mediumexchange in 0.100 M phosphate buffer solution pH 7.40. Querce-tin concentration: 1.0 � 10�4 M.

Electrode Currents (mA)

A B C

GCE 2.3 3.5 2.9GCE/PEI-CNT 12.5 50.9 43.3GCE/PAA-CNT 4.5 7.5 6.5GCE/Et-CNT – 7.6 2.9GCE/PEI 0.6 2.8 2.1GCE/PAA 1.2 2.2 1.7




plot is also included as Inset. In this case, a more clearand better-defined response than at GCE/PEI-CNTs isobtained even for low levels of quercetin. The improve-

ment achieved at GCE/PAA-CNTs is essentially due tothe lower base line currents obtained at this surface. As aconsequence of these advantages, the detection limit is7.5 nM, value comparable to those obtained in previousreports [45] and even better than others [44].

3.3. Amperometric Quantification of Quercetin

We also evaluated the amperometric detection of querce-tin. Figure 3A shows the amperometric recording at0.150 V obtained at GCE/PEI-CNT for successive addi-tions of 2.1� 10�6 M quercetin in a 0.100 M phosphatebuffer solution pH 7.40. The steady-state is rapidlyreached after each addition of quercetin. Figure 3B dis-plays calibration plots obtained from amperometric ex-periments at different platforms: GCE (a), GCE/PAA-CNT (b) and GCE/PEI-CNTs (c). The sensitivity increas-es in the presence of CNTs, although this effect is mostpronounced for GCE/PEI-CNTs, in agreement with thevoltammetric results shown in Table 1. As it is shown inTable 4, the amperometry allows the quantification ofquercetin at submicromolar levels. Even when the re-sponse is not as sensitive as the adsorptive stripping, itrepresents an interesting methodology for further analyti-cal applications.

To evaluate the short time stability of the sensor, wemonitored the current obtained at 0.150 V after one addi-tion of 8.4 �10�6 M quercetin. After 40 min, the quercetinoxidation signal decreased 14% at bare GCE, 7.2 % atGCE/PEI-CNTs, 11 % at GCE/PAA-CNTs, 48% atGCE/PEI, and 20% at GCE/PAA. It is clear that whenCNTs are present, the decrease in the signal is smaller,reinforcing once more the anti-fouling activity of theCNTs present at the electrode surface.

Regarding the reproducibility of the amperometric de-termination of quercetin using the same surfaces, atGCE/PAA-CNTs, the sensitivity remains almost constantup to sixth calibration (the decrease in the sensitivity isjust 10%). At GCE/PEI-CNTs, the sensitivity largely de-creased for successive calibrations (i.e. after 6 calibra-tions, the sensitivity decreased 40% of the original one).

3.4. Electrochemical Determination of Quercetin in thePresence of Rutin

Since quercetin can be obtained as a product of rutin hy-drolysis [48], is interesting to evaluate the feasibility todetect quercetin in the presence of rutin (the structure ofrutin is also presented in the Scheme 1). Figure 4 shows

Fig. 2. (A) Oxidation peak currents obtained from square wavevoltammograms performed in a 0.100 M phosphate buffer solu-tion pH 7.40 after different accumulation time in 1.0 �10�4 Mquercetin. Working electrode: GCE/PEI-CNTs. (B) Square wavevoltammograms obtained at GCE/PEI-CNTs in a 0.100 M phos-phate buffer solution pH 7.40 after 1.0 min accumulation in solu-tions of quercetin of different concentration: a) 8.0 �10�7, b)1.0 �10�6, c) 3.0 � 10�6, d) 5.0� 10�6, e) 7.0 �10�6, and f) 1.0 �

10�5 M. (C) Square wave voltammograms obtained at GCE/PAA-CNTs in a 0.100 M phosphate buffer solution pH 7.40 after1.0 min accumulation in solutions of quercetin of different con-centrations: a) 1.0 � 10�7, b) 3.0 � 10�7, c) 5.0 �10�7, d) 8.0 �10�7,e) 1.0� 10�6, f) 2.0 �10�6, and g) 5.0 �10�6 M. The correspondingcalibration plots are also shown as insets in Fig. 2B and 2C. SWVconditions: pulse height: 4 mV, pulse amplitude: 50 mV, period:200 ms, frequency: 50 Hz, step: 0.50 mV.




the voltammetric response of 1.0 �10�4 M rutin at GCEwithout accumulation (solid line). The oxidation occurs at0.231 V while the reduction takes place at 0.177 V, withcurrents of 1.70 and 0.92 mA, respectively, suggesting aquasi-reversible behavior. The accumulation of rutin doesnot produce appreciable enhancement in the oxidationcurrent (not shown). The voltammogram depicted inFigure 4 in dashed line corresponds to the response of themixture of 1.0 � 10�4 M rutin and 1.0 �10�4 M quercetin. Itis evident that, even when the main contributions forrutin and quercetin oxidation are relatively separated andit is possible to have an idea of the presence of a mixtureof quercetin and rutin, the cyclic voltammetry does notrepresent a good alternative to quantify quercetin in thepresence of rutin without an appropriate deconvolutionmethodology.

We evaluate the feasibility to determine quercetin inthe presence of rutin. Figure 5 shows calibration plots ob-tained at GCE/PEI-CNT from amperometric experimentsat 0.100 V for successive additions of quercetin to a0.100 M phosphate buffer solution pH 7.40 containing dif-ferent concentrations of rutin: 0 (a), 5.0 �10�6 (b) and1.0 �10�5 (c) M. The sensitivities are the following: (3.0�0.1) �104; (2.9�0.1) �104 (3.3 % error compared to (a)) ;(2.6�0.1) �104 mA M�1 (13.3 % error compared to (a)),respectively. Thus, it is possible to detect micromolarlevels of quercetin even in the presence of 1.0 �10�5 Mrutin without an appreciable decrease of the signal. If theamperometric determination is performed at 0.150 V,since the oxidation of rutin already starts at this potential,the electrode is passivated by the oxidation products andthe signal for quercetin oxidation decreases. The sensitivi-ty decreases 15.7 % in the presence of 1.0� 10�6 M rutinwhile in the presence of 1.0 �10�5 M rutin it decreases61.8%.

Table 3. Analytical parameters for the quantification of quercetin by adsorptive stripping with SWV detection. Experimental condi-tions: Adsorption step: 1.0 min at open circuit potential, Stripping: after medium exchange in 0.100 M phosphate buffer solutionpH 7.40. SWV conditions: pulse height: 4 mV, pulse amplitude: 50 mV, period: 200 ms, frequency: 50 Hz, step: 0.50 mV.

Electrode Detection limit [a] (M) Linear range Sensitivity (mA M�1)

GCE/PEI-CNT 8.9 � 10�8 8.0� 10�7 M to 7.0� 10�6 M (2.5�0.1) � 106

r=0.993GCE/PAA-CNT 7.5 � 10�9 1.0� 10�7 M to 5.0� 10�6 M (8.7�0.3) � 106

r=0.996

[a] Detection limit according to the 3.3 SDNoise Signal/m criterion, m is the slope of the calibration curve and SD the standard deviation.

Fig. 3. A) Amperometric recordings obtained at 0.150 V atGCE/PEI-CNTs for successive additions of quercetin. (B) Cali-bration plots obtained from amperometric experiments at0.150 V at GCE (a), GCE/PAA-CNTs (b), and GCE/PEI-CNTs(c). Supporting electrolyte: 0.100 M phosphate buffer solutionspH 7.40.

Table 4. Analytical parameters for the amperometric quantification of quercetin. Working potential: 0.100 V.

Electrode Detection limit [a] (M) Linear range Sensitivity (mA M�1)

GCE 1.7 � 10�6 2.0� 10�6 M to 2.3� 10�5 M (1.50�0.03)� 104

r=0.9997GCE/PEI-CNT 1.4 � 10�6 2.0� 10�6 M to 1.5� 10�5 M (7.6�0.3) � 104

r=0.997GCE/PAA-CNT 2.0 � 10�7 2.0� 10�6 M to 1.1� 10�5 M (3.3�0.1) � 104

r=0.9997

[a] Detection limit according to the 3.3 SDNoise Signal/m criterion, m is the slope of the calibration curve and SD the standard deviation.




Since at GCE/PAA-CNT the oxidation of rutin occursat lower potentials, compared to GCE/PEI-CNT, even at0.100 V it was not possible to determine quercetin in thepresence of concentrations as small as 5.0 �10�7 M rutin,since the sensitivity decreased 39.3 %. Lower potentialswere not adequate since the sensitivity for quercetin de-creases. Therefore, GCE/PEI-CNT is the selected elec-trode to quantify quercetin in the presence of rutin.

3.5. Quantification of Quercetin in Onion Samples

The sensor was used to quantify quercetin in onion sam-ples, where most of the flavonoids correspond to querce-tin. The determinations were performed by amperometrywith the external addition of standard. Considering the

results previously obtained for rutin and quercetin mix-tures and those reported by Zielinska et al. [49], and inorder to minimize the interference of glucoside quercetinderivatives, the selected potential was 0.100 V. The con-centration in onion skin was (440�5) mg % (sensitivity=(5.47�0.07) �104 mA M�1, r=0.9991), in agreement withthe results obtained by Biesaga et al. [50]. The quercetinin flesh content was (110�5) mg % (sensitivity= (4.03�0.07) �104 mA M�1, r=0.9989).

4. Conclusions

The advantages of the modification of GCEs with CNTsdispersed in PAA and PEI for the quantification of quer-cetin have been demonstrated. The use of the polymericmatrix to disperse CNTs confers to the resulting electro-chemical sensor, robustness and stability. The presence ofCNTs in the platform is crucial, not only for the signifi-cant enhancement in sensitivity but also for the importantanti-fouling effects that markedly diminishes the passivat-ing effect of quercetin oxidation products.

The stronger adsorption of quercetin at GCE/PEI-CNTs and GCE/PAA-CNTs, has made possible the nano-molar detection of the flavonoid by adsorptive strippingwith medium exchange and SWV transduction.

Amperometry at GCE/PEI-CNTs and GCE/PAA-CNTs represents a more practical strategy for the detec-tion of quercetin. The highly sensitive and selective quan-tification of quercetin in the presence of rutin by amper-ometry was also demonstrated, opening the doors for fur-ther practical applications. The sensor was successfullyused for the quantification of quercetin in onion, openingthe doors to further analytical applications in food prod-ucts.

In summary, the careful selection of the polyelectrolyteused to disperse CNTs is a very important aspect for thedevelopment of highly sensitive and selective sensors forquercetin and metabolically related compounds.

Acknowledgements

The authors thank CONICET, SECyT-UNC, ANPCyT,Ministery of Science and Technology of Cordoba. F. G.acknowledges ANPCyT for the fellowship.

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Fig. 4. Cyclic voltammograms obtained in 1.0 � 10�4 M rutin(solid line) and 1.0 � 10�4 M rutin+1.0 �10�4 M quercetin(dashed line). Supporting electrolyte: 0.100 M phosphate buffersolution pH 7.40.

Fig. 5. Calibration plots obtained from amperometric experi-ments performed at GCE/PEI-CNTs for successive additions of1.0 �10�4 M quercetin to 0.100 M phosphate buffer solutionpH 7.40 containing different concentrations of rutin: (*) 0 M;(&) 5� 10�6 M; (~) 1�10�5 M. Working potential: 0.100 V.




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