Potentiometric Response of Calix[4]pyrrole Liquid MembraneElectrode Towards Neutral Nitrophenols

Tomasz Piotrowski,+ Hanna Radecka,+ Jerzy Radecki,*+ Stefaan Depraetere,++ and Wim Dehaen++

+ Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, Division of Food Sciences, Tuwima 10, PL-10-747 Olsztyn,

Poland; e-mail: radecki@food.irzbz.pan.olsztyn.pl++ University of Leuven, Department of Chemistry, Celestsijnenlaan 200F, B-3001 Heverlee (Leuven), Belgium

Received: May 15, 2000

Final version: July 31, 2000

Abstract

Calix[4]pyrroles were applied as a new class of ligands of potentiometric sensors for neutral nitrophenol isomers. Calix[4]pyrrole-con-taining liquid membranes exhibit a very high af®nity to proton uptake. These membranes, in protonated form, showed very high selectivitytowards para-nitrophenol in the presence of other nitrophenols and dihydroxybenzene isomers. The probable mechanism of the poten-tiometric signal generation of the membrane investigated upon stimulation by nitrophenol isomers existing in neutral form is discussed.

Keywords: Potentiometric sensor, Calix[4]pyrroles, Neutral nitrophenols, Proton af®nity

1. Introduction

A great effort has been devoted to the development ofpotentiometric sensors for inorganic and organic anions [1±4],with the selectivity pattern that deviates from the Hofmeisterseries which is based on the order of lipophilicity [5]. To ful®lthis goal, a speci®c interaction must occur between the guest ionand the ionophore at the interface [6±9].

In comparison to the state of the art of potentiometric sensorsselective toward ionic species, the study on potentiometricsensing of neutral molecules is still an open challenge foranalytical chemists.

The ®rst potentiometric sensor based on macrocyclic andaliphatic amines for discrimination of neutral phenol derivativeshas been developed by Umezawa and co-workers [10,11]. Theyconcluded that the mechanism of signal generation consists onthe interaction between protonated host molecules and neutralphenolic guests, and subsequent proton ejection from themembrane phase to the aqueous phase, which leads to a decreaseof membrane potential (anionic response). The in¯uence of hostlipophilicity, host±guest structure complementarity, and electrondensity on the nitrogen atoms of the macrocyclic host on theabove sensing mechanism were investigated with ISEs based onlipophilic ditopic macrocyclic hexaamines [12].

The calix[4]pyrroles, discovered in 1886 by Baeyer [13], haverecently attracted a lot of attention, because of their ability tobinding anionic and neutral substrate [14±16]. Sessler andco-workers [17] used calix[4]pyrrole, for construction of ISEssensitive toward inorganic ions. To our knowledge, this is the ®rstexample of an application of lipophilic calix[4]pyrroles as asensory element of a potentiometric sensor sensitive to neutralnitrophenols. The selectivity pattern and a hypothesis on the signalgeneration mechanism of this unique sensor will be discussed.

2. Experimental

2.1. Reagents

Calix[4]pyrroles used in this study (Figure 1) were synthe-sized by us as reported [18]. Bis(2-ethylhexyl) phthalate [dioctyl

phthalate (DOP)] used as a membrane solvent and poly(vinylchloride) (PVC; nav� 1100) used as a polymer matrix werepurchased from Wako Pure Chemical, Japan. Ortho-, meta- andpara-isomers of dihydroxybenzene, ortho-, meta-, para-isomersof nitrophenol, para-aminophenol, citric acid, lithium acetate,hydrochloric acid, sodium chloride, (N-[2-hydroxyethyl]piperazine-N0-[2-ethanesulfonic acid]) (HEPES buffer) werepurchased from Sigma-Aldrich Co., PoznanÂ, Poland. Chloroform(for chromatography, stabilized with 2-methyl-2-butene),purchased from Merck, Germany, was puri®ed just before usedby passing through column with alumina±WB-2-basic type(Sigma).

Tetrahydrofuran was obtained from POCH-Gliwice, Polandand was distilled from solid NaOH just before use. All thesamples and buffer solutions were prepared with deionized waterwith a resistance of 18.2 MO cm and bubbled with nitrogen for30 min just before the potentiometric measurements in order toprotect phenolic analytes from oxidation.

2.2. Electrode Preparation and Potential Measurements

The composition of the PVC matrix liquid membranes based oncalix[4]pyrroles were as follows: 1 wt. % host, 66 % DOP, 33 %PVC. The membranes were prepared according to the procedure ofAmemiya et al. [19]. All components were dissolved in ca. 2 mLfreshly distilled THF. The resulting solution was placed into a glassring of 30 mm diameter and left standing for 24 h to allow evapo-ration of the solvent. The resulting membrane was cut and mountedon a Tacussel Ag=AgCl electrode body. A membrane free of ligandwas prepared in a similar way. The newelectrodes were conditionedovernight, and during ca. 2 h just before each set of experiments, in1.061072 M buffer solution (at the appropriate pH ).

The membrane potential was measured at room temperature(ca. 20 �C) with a pH-meter Hanna Instruments HI 9321 or a pH-meter Tacussel LPH 33OT.

The electrode cell for the potential measurements was asfollows:

Ag/AgCl j 10ÿ1 M KCl jmembrane j sample solution j1 M CH3 COOLi j 3 M KCl jAg=AgCl

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The effect of the pH on the membrane potential was measured byaddition of NaOH solution, containing 1.061072 M Na2SO4 , toa solution of 1.061072 M H2SO4 and by addition of H2SO4

aqueous solution containing 1.061072 M NaOH, to a solutionof 1.061072 M NaOH. The selectivity coef®cients were deter-mined by the matched potential method [20±22].

2.3. Extraction Experiment

1072 M NaCl was vigorously shaken with CHCl3. The pH ofthe aqueous phase was adjusted to 4.52 with hydrochloric acid(1 M). These two phases, organic and aqueous, saturated witheach other, were used for further experiments. Calix[4]pyrrole 1

(4.13 mg) was dissolved in 0.9 mL of organic phase (to get7.861073 M concentration). The calix[4]pyrrole containingorganic phase was shaken with 1 mL of aqueous phase (pH 4.52).The pH of the aqueous phase was measured again and was 5.85,because of proton uptake by the calix[4]pyrrole. Keeping thesetwo phases in contact, the pH of the aqueous one was adjusted to4.15 by adding 20mL of a 1073 M HCl solution prepared on thebase of aqueous phase (1072 NaCl aq. saturated with CHCl3) inorder to obtain saturation of calix[4]pyrrole with protons. Next,100mL of a 1072 M solution of para-nitrophenol, prepared on thebase of the aqueous phase, was added to obtain a 0.961073 Mconcentration. Two phases were vigorously shaken during 1 min.The pH of aqueous solution was measured again. The back-ground experiment was carried out by using the same solution,but without calix[4]pyrrole in the organic phase.

3. Results and Discussion

3.1. Potential Response of a Calix[4]pyrrole Membrane

Electrode Towards Protons

Figure 2 shows the membrane potential of an electrodecontaining ligand 1 (curve A and B) in relation to the pH ofaqueous solution. With decreasing pH the membrane potentialincreased with the slope ca. 33 mV per pH unit (curve A). Almostthe same curve was obtained for an electrode with ligand 2(results are not shown). The changes of potential value in pHrange 9.0±3.5 were 180 mV and 190 mV, for membrane 1 andmembrane 2, respectively. The observed process was reversible.The potential of the same membrane decreases with increasingpH value (Figure 2, curve B). For so-called `̀ solvent'' membrane,without calix[4]pyrrole, the potential change upon pH wasnegligible (Figure 2, curve C).

The increase of membrane potential could be explained onthe basis of a successive proton transfer from the aqueous tothe membrane phase by the ligands incorporated there. Thisleads to a charge separation between the electrode surface andthe electrolyte. It is likely that protonation takes place onlyat the surface of modi®ed membranes, so the electroneutralityin the bulk of the membrane still holds. The reversibility ofthe observed process leads to the conclusion that during theprotonation and deprotonation processes, no decomposition orpolymerization of calix[4]pyrroles occurs. Similar phenomenawere observed for liquid membrane electrodes incorporatedwith polyamines [10±12, 23]. In these cases, the chargeseparation was triggered by protonation of the nitrogen atomsfrom host molecule rings. The proton uptake by the nitrogenatoms of the pyrrole derivatives is highly unlikely. The basicityof the a-carbon (pKb�ÿ3.8) and b-carbon (pKb�ÿ5.9) arehigher than the basicity of the pyrrole nitrogen atom(pKb�ÿ10) [24]. Thus, probably, protonation takes place atthe a- or b-carbon atoms of the calix[4]pyrroles, because ofthe higher density of electrons there. A similar way ofprotonation was observed for polypyrroles doped with proticacids [25±27].

Fig. 1. Chemical structure of calix[4]pyrroles.

Fig. 2. Potential vs. pH of electrode incorporated with ligand 1. A) (s)Initial conditions: 1.061072 M H2SO4. pH was adjusted by addingNaOH to aq. solution containing 1.061072 M H2SO4. B) (d) Initialconditions: 1.061072 M NaOH. pH was adjusted by adding of H2SO4

to aq. solution containing 1.061072 M NaOH. C) (m) Potential vs. pHof electrode free of ligand. Measurement conditions as in curve A.

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3.2. Potential Response of a Calix[4]pyrrole Membrane

Electrode Toward Phenol Derivatives

The potentiometric response of calix[4]pyrrole-containingmembranes towards the dihydroxybenzene derivatives was veryweak in all of the circumstances investigated (results not shown).However, the response to nitrophenol isomers was high andselective (Figure 3A, B, C). The EMF measurements were carriedout in the presence of 1072 M citric buffer (pH 4.0 and 6.0) andin the presence of 1072 M HEPES buffer pH 8.0. The EMFresponse of these two buffers were checked in 2 mM NaH2PO4 asa background solution. Both of them have caused no potentialchanged in the concentration range from 1076 M to 1072 M.

At pH 4.0 (1072 M citric buffer), the value of the potentialchange of the membrane modi®ed with ligand 1 was ÿ68.7� 4.2 mV after stimulation by para-nitrophenol and ÿ49.3�1.5 mV after stimulation by meta isomer. The lowest `̀ anionic''signal, ÿ19.0� 6.0 mV, was obtained for ortho isomer (Figure3A). In pH 6.0 (1072 M citric buffer), the respective value were:ÿ71.0� 7.0, ÿ33.3� 3.0, ÿ17.0� 3.5 mV (Figure 3B). Gener-ally, sequence of response value was similar as at pH 4.0, but theselectivity between meta- and para-nitrophenol became moreclear. The membrane with ligand 2 responded to nitrophenols in asimilar manner as the previous one, with ligand 1, but theselectivity was weaker (results not shown). The signal generatedfor the para- and meta-nitrophenols were almost the same in pH4.0 and 6.0. A less signi®cant decrease of membrane potential wasobserved in the presence of the ortho isomer, in both buffers at pH4.0 and pH 6.0. The `̀ solvent'' membrane showed very weakresponse towards nitrophenol guests [11].

The response of both membranes toward para-aminophenolhas also been checked in order to study the in¯uence of acidity ofthe guest on the potential signal generation. This compound withrather low acidity (pKa� 10.46) showed no response in allcircumstances investigated (data not presented).

Generally, at the pH discussed, the sequences of the signalmagnitude generated by the investigated membranes were asfollows:

Membrane with ligand 1: para-nitrophenol > meta-nitro-phenol > ortho-nitrophenol.

Membrane with ligand 2: para-nitrophenol � meta-nitro-phenol > ortho-nitrophenol.

So, we can see the ability, specially in the case of membrane withligand 1, to distinguish nitrophenols isomers.

These sequences and the fact that we have obtained a veryweak response for dihydroxybenzene isomers and no responsetoward para-aminophenol, suggest that acidity of the targetmolecules has a strong in¯uence on the process of signalgeneration by membranes modi®ed with calix[4]pyrroles. Theexception is ortho-nitrophenol. This compound showed veryweak potential response, even though its acidity is very similar tothe acidity of para-nitrophenol (pKa: 7.21 and 7.16, respectively).One of the reasons for that might be steric hindrance. On the otherhand, the very close proximity of NO2 and OH groups will inducean intramolecular hydrogen bond. This may be the cause of thelower potentiometric response of this isomer. The selectivitycoef®cients for the membrane electrode with ligand 1, determinedby the matched potential method [20±22] are collected in Table 1together with the acidity constants (pKa) and the partition coef-®cients between octanol and water (log P) [28]. The sensor 1showed extremely high selectivity for para-nitrophenol in thepresence of other nitrophenol and dihydroxybenzenes isomers.

Fig. 3. Potential response of electrode incorporated with ligand1 toward nitrophenol isomers: (�) ortho-nitrophenol; (j) meta-nitrophenol; (m) para-nitrophenol; A) pH 4.0, 1072 M citricbuffer; B) pH 6.0, 1072 M citric buffer; C) pH 8.0, 1072 M HEPESbuffer.

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Electroanalysis 2001, 13, No. 4

The discrimination between para- and meta-nitrophenol wasslightly better in citric buffer pH 6.0 than in pH 4.0. The selec-tivity and sensitivity pattern follows the order of acidity andlipophilicity as well. The best potentiometric signal was observedfor the compound with the highest acidity and lipophilicity.

At pH 8.0 (1072 M HEPES buffer) we can observe dramaticchanges in behavior of the membrane with ligand 1 (Figure 3C).The `̀ anionic'' signal was observed only for the meta isomer(ÿ34.0� 1.7 mV). At pH 8.0 only the meta isomer mainly existsas a neutral molecule (pKa� 8.36), whereas ortho- and para-nitrophenols are almost totally dissociated (pKa� 7.21,pKa� 7.16, respectively). These results suggest, that for thegeneration of the potentiometric signal as we have observed atpH 4.0 and 6.0, the presence of undissociated forms of nitro-phenols investigated are necessary.

Sessler and co-workers [29] reported a 1 : 1 complex formationbetween meso-octamethylcalix[4]pyrrole and 4-nitrophenolateinorganic phase (MeCN or CH2Cl2). Our results showed thatonly nitrophenols in their neutral forms are able to generate thecharge separation which leads to potential change at the interfacebetween the liquid membrane incorporated with calix[4]pyrroleand electrolyte solution.

A characteristic property of the membrane electrodes whichhad calix[4]pyrrole incorporated was their very fast signalgeneration. After changing the concentration of nitrophenolguest, in both buffer solutions, a very rapid (during a fewseconds) a decrease of membrane potential was observed.

3.3 The Attempt of the Evaluation of Signal

Mechanism Generation

In order to evaluate the mechanism of signal generation byother means than potentiometric measurements, an extractionexperiment involving calix[4]pyrrole in the organic phase andpara-nitrophenol in the aqueous one was done. The pH of anaqueous solution (1072 M NaCl) being in equilibrium withCHCl3, after adding to the organic phase calix[4]pyrrole 1 andvigorously shaking, changed from 4.52 (initial value) to5.85� 0.02 (n� 3). This supported the hypothesis concerningthe ability of the calix[4]pyrroles to the proton uptake.

Next, HCl solution was added to the aqueous phase being incontact with the organic one containing calix[4]pyrrole, to reachpH 4.15. To the aqueous phase, prepared in this manner, asolution of para-nitrophenol was added. After vigorous shaking,the pH of the aqueous solution decreased to 3.98� 0.02 (n� 3).This pH decrease indicated on the proton ejection from organic tothe aqueous phase upon calix[4]pyrrole±para-nitrophenol inter-action at the interface. The addition of para-nitrophenol solutionto an identical aqueous solution being in contact with organicphase without calix[4]pyrrole, caused no pH change at all. Thisproves that at these conditions, the dissociation of para-nitro-phenol does not occur.

At the present moment, it is dif®cult to grasp the realmechanism of signal generation of investigated membranes, butthe results obtained could lead to the assumption that the surfacedeprotonation process of the calix[4]pyrrole membrane isinvolved in generation of an `̀ anionic'' potentiometric signal inthe presence of nitrophenol isomers.

A hypothesis on the mechanism of the process investigatedcould consist of the following steps. In the ®rst one, thecalix[4]pyrrole ring is protonated, probably at the a- or b-carbonatoms. The consequence of this step is the charge separationbetween the two phases and the enhancement of hydrogendonation of N±H groups from pyrrole ring. This facilitates thecreation of hydrogen bound between the oxygen atom fromnitrophenol guests and hydrogen atom from N±H group whichtakes place at the border between two phases. This leads to lossof the phenolic hydrogen and the oxygen bound (O±H) in theguest molecule and, ®nally to proton ejection with concomitantcounter anion transport from the interface to the aqueous bulk.These hydrogen ions become solvated in the water phase. Theenergy pro®t from the hydrogen ion solvation process might be adriving force allowing for dissociation of nitrophenols at theinterface, even in acidic media. This event might be responsiblefor generation of observed anionic responses of membranesinvestigated after stimulation by nitrophenols existing in neutralforms. The negligible response, which we observed for dihy-droxybenzenes and para-aminophenol, which are much moreweaker acids than nitrophenol, supports this hypothesis.

4. Conclusions

Calix[4]pyrroles-containing liquid membrane electrodesshowed very high af®nity to uptake of protons and very high sele-ctivity towards para-nitrophenol, occurring in the neutral form, inthe presence of other nitrophenols and dihydroxybenzene isomers.

The hypothesis of the mechanism of the potentiometric signalgeneration of this new sensor, generally, consists on the rever-sible interaction of protonated calix[4]pyrrole membrane surfacewith the neutral nitrophenol guest, and, subsequent protontransfer from the interface to the aqueous solution.

We are aware, that the hypothesis introduced demands moreexperimental evidence. Further research on the above phenomenaare in progress in our laboratories and will be reported in duecourse.

5. Acknowledgements

This work was supported by bilateral Flemish-Polish grantno. BIL 98=33 (17=1999±2001).

Table 1. Potentiometric selectivity coef®cients (log KMPMij ) of membrane

sensor based on calix[4]pyrrole 1 together with acid dissociation con-stants (pKa) and partition coef®cients log Poct for phenol derivativeguests. log KMPM

ij were determined in 1072 M citrate buffer pH 4.0 or pH6.0 by the matched potential method (MPM) [20±22] in mixed solutionwith 1.061073 M para-nitrophenol as the background. Mean valuefrom three repetitions. The concentration of interfering compound in thebackground solution was changed up to 5.061073 M. log Poct: partitioncoef®cients between octanol and water [28]; pKa: acidity constants; *:the cationic potential response was observed.

log KMPMij

Interfering compounds log Poct pKa pH 4.0 pH 6.0

para-Nitrophenol 1.91 7.16 0 0meta-Nitrophenol 2.00 8.36 ÿ0.31� 0.02 ÿ0.69� 0.13ortho-Nitrophenol 1.75 7.21 * ÿ0.86� 0.02ortho-Dihydroxybenzene 0.95 9.36 * *meta-Dihydroxybenzene 0.79 9.44 * *para-Dihydroxybenzene 0.55 9.91 * *

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