DOI: 10.1002/elan.201400494

Solid-Contact Potentiometric Sensor Based on Polyanilineand Unsubstituted Pillar[5]AreneEkaterina E. Stoikova,[a] Michail I. Sorvin,[a] Dmitry N. Shurpik,[b] Herman C. Budnikov,[a] Ivan I. Stoikov,[b]

and Gennady A. Evtugyn*[a]

1 Introduction

The development of potentiometric sensors with the solidcontact eliminating internal filling solution has found in-creasing attention in the past decade due to simpledesign, durability, miniaturization prospects and low-costmanufacture [1]. In conventional ion-selective electrodesthe membrane containing ionophore contacts with theinner reference electrode via solution with a constantconcentration of a primary ion. Leaching of the internalsolution and low mechanical robustness of the plasticmembrane are considered as a weak point of such an as-sembly. Direct contact of the membrane with the electriccontact requires implementation of the materials withmixed electronic and ionic conductivity which serve asion-to-electron transducers. Besides oxides and complexesof transient metals, conducting polymers are intensivelyapplied in solid-contact potentiometric devices. PANI,polypyrrole and polythiophene derivatives are describedin the assembly of the solid-contact sensors for the detec-tion of alkali and alkali-earth metals [2–5], transientmetals [6–9], inorganic anions [10,11] and charged organ-ic species [12].

The electropolymerized layers in solid-contact poten-tiometric sensors are often covered with conventionalPVC membranes containing plasticizer, lipophilic salt andionophore. Meanwhile the mass and ratio of componentsof such membranes can differ from those applied in thesensors with internal filling. In some cases, lipophilic saltscan be excluded because of the ion exchange propertiesof conductive polymer [9]. For PANI based sensors,chemically synthesized polymer is also used in addition toelectropolymerization product. Although the electropoly-merized PANI forms thinner regular films, chemicallysynthesized material has higher purity and extendeddoping abilities. This eliminates drawbacks of PANI ap-plication, e.g., pH sensitivity of the potential and swellingwith pH shift [9, 10]. On the other hand, conductive poly-

mers do not show well defined redox potential and insome cases demonstrate slow conformational changescaused by redox transformations. This can result in hyste-resis phenomena observed for current�voltage functionsand relaxation properties [13]. The morphology of thefilms and their penetrability for ions depend on the fabri-cation protocol, nature of a counter ion and storage con-ditions [14,15]. This complicates the fabrication of solid-contact sensors because of insufficient reproducibility andlong-term drift of their potential. The behavior of poten-tiometric sensors based on electropolymerized materialsexerting mixed potential with protonation and complexa-tion stages was considered in [16, 17] and the conditionsfor extracting ion-exchange contribution established.

Pillar[5]arene (Scheme 1) is a representative of a newclass of macrocyclic molecules that is made up of hydro-

[a] E. E. Stoikova, M. I. Sorvin, H. C. Budnikov, G. A. EvtugynAnalytical Chemistry Department of Kazan FederalUniversity18 Kremlevskaya Street, Kazan, 420008, Russian Federation*e-mail: Gennady.Evtugyn@kpfu.ru

[b] D. N. Shurpik, I. I. StoikovOrganic Chemistry Department of Kazan Federal University18 Kremlevskaya Street, Kazan, 420008, Russian Federation

Abstract : A novel potentiometric sensor based on screen-printed carbon electrode covered with electropolymerizedpolyaniline (PANI) and unsubstituted pillar[5]arene asionophore has been developed and tested in potentiomet-ric measurements of pH and metal ions. The introductionof pillar[5]arene improved the reversibility of the pH re-

sponse in the range from 2.0 to 9.0 with the slope of45 mV/pH. Among metal cations, the response to Fe3 +

and Ag+ ions was referred to PANI redox conversionwhereas the signal toward Cu2+ in the range from 1.0 �10�6 to 1.0 �10�2 M (limit of detection (LOD) 3.0 �10�7 M) to specific interaction with the macrocycle.

Keywords: Potentiometric sensor · Solid-contact sensor · Pillar[5]arene · Polyaniline · Electropolymerization

Scheme 1. Structure of Pillar[5]arene.

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quinone units linked with methylene bridges at para posi-tion [18]. Pillararenes show complexation propertiestoward various cationic species (paraquat [19], Na+ andK+ ions [20] which can be modified by introduction ofvarious substituents at hydroxyl groups of the macrocycle.Pillar[5]arene can be reversibly oxidized by PANI similar-ly to hydroquinone [21] and hence it can participate inthe PANI electron exchange reactions followed by poten-tial shift.

However, no information on the implementation of pil-lar[5]arene in the assembly of solid-contact sensors andtheir compatibility with PANI is available. The redox ac-tivity of pillar[5]arene and its ability to supramolecularbinding can improve the electrochemical characteristicsof appropriate sensors and extend the number of poten-tial analytes determined.

Previously we have shown that PANI based potentio-metric sensors with tetrasubstituted thiacalix[4]arenes al-lowed potentiometric detection of Fe3+, Ag+ and Hg2+

ions [14]. The selectivity of the response was achieved bymodification of the substituents at the lower rim with het-erocyclic groups binding appropriate cations. The use ofthiacalix[4]arene ionophores also influenced the pH sensi-tivity of the PANI potential by their own buffering prop-erties and localization of internal transfer of hydrogenions participated in the redox transformation of PANIlayer.

In this work, for the first time the solid-contact poten-tiometric sensor based on combination of electropolymer-ized PANI and unsubstituted pillar[5]arene has been de-veloped and investigated to characterize the factors af-fecting potential and to specify analytes determined. Fordemonstration, the determination of Cu2+ ions in variousconditions has been considered.

2 Experimental

2.1 Reagents

Unsubstituted pillar[5]arene was synthesized from com-mercially available 1,4-dimethoxybenzene and removal ofprotective methoxyl group as described elsewhere[22,23]. For further purification pillar[5]arene was repeat-edly washed with chloroform and water. The structureand purity of pillar[5]arene were confirmed by NMR 1Hand MALDI-TOF mass spectroscopy.

Pillar[5]arene. Product yield: 91%. The decompositionwas observed at 230 8C without melting. 1H NMR(CD3COCD3) d, ppm: 3.66 (10H, s, �CH2�), 6.64 (10H, s,ArH), 7.99 (10H, s, �OH). MALDI-TOF MS C35H30O10:calc. [M+] m/z=610.2, found [M+Na]+ m/z=633.1,[M+K]+ m/z=649.2.

To avoid oxidation with atmospheric oxygen, the pil-lar[5]arene preparations were stored under Ar at �20 8C.The pillar[5]arene solutions were prepared with de-aerat-ed solvents and used immediately after preparation. Priorto electropolymerization, aniline (Sigma) was distilledunder vacuum and stored under Ar at 4oC. All other

chemicals were of analytical-reagent grade. Metal nitratesand sulfates used for the selectivity investigation were dis-solved in Millipore water. The elimination of interferingeffect of Fe3+ and Ag+ ions was performed by NaF solu-tion (Sigma). In some experiments, the electrodes weretreated with EDTA (Sigma) between the measurements.All the materials for electrode screen-printing were pur-chased from Gwent Electronic Materials Ltd. (UK) andDelta Paste NPP (Russia).

2.2 Apparatus

The voltammetric experiments including aniline polymer-ization were performed with AUTOLAB PGSTAT 302Npotentiostat and EIS measurements with FRA module(Metrohm Autolab b.v., the Netherlands). Randles equiv-alent circuit (Scheme 2) was used for curve fitting andcalculation of the charge transfer resistance Ret.

Here RS is electrolyte resistance, ZW is the Warburg im-pedance and C is the capacitance of the electrode sur-face/solution interface. The dimensionless “roughnessindex” n was higher than 0.88 in all the experiments, sothat pure capacitance C could be used instead of the con-stant phase element expressing non-ideal behavior of thecharge transfer on the interface.

Galvanostatic coulometric measurements were per-formed with coulometric analyzer “Expert-006” (Econix-Expert, Moscow, Russia). Potentiometric measurementswere conducted with digital four-channel ionometer Ecot-est-001 (Econix-Expert, Moscow, Russia).

Screen-printed carbon electrodes were manufacturedusing screen-printer DEK-248 (DEK International, Eng-land) on polycarbonate support. The electrodes consistedof silver tracks covered with curable graphite paste hard-ened at 90 8C. Each electrode set contained a disk work-ing electrode 3 mm in diameter and axial semicircularauxiliary electrode. Double-junction Ag/AgCl referenceelectrode (AUTOLAB cat. No 6.0726.100) was used as anexternal reference electrode.

The UV spectra were recorded on the UV-3600 UV-spectrometer (Shimadzu) in the mixture of 30.0 mM pil-lar[5]arene and 0.1�0.001 M CuSO4.

2.3 Preparation of Potentiometric Sensor

Screen-printed carbon electrode was first cleaned electro-chemically by potentiostatic polarization in 0.2 M H2SO4

Scheme 2. Randles equivalent circuit.

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at 0.8 V during 10 min. The electropolymerization of ani-line was performed in the mixture of 0.07 M aniline and0.2 M H2SO4 by multiple cycling of the potential between�0.2 and 1.1 V at 50 mV/s. Twenty cycles were performedto obtain dense PANI film with reproducible potential.After that, 10 mL of 1�10 mg mL�1 pillar[5]arene solutionin acetone were distributed among the working electrodeand dried at ambient temperature. The nominal surfaceconcentration of pillar[5]arene was varied from 0.14 to14.0 mg cm�2 (10–1000 ng per electrode). The potentiomet-ric sensors prepared were stored in dry conditions at 4 8C.Prior to use, they were conditioned in 10.0 mM HNO3 for60 min.

2.4 Potentiometric Measurements

All the measurements were performed at ambient tem-perature (23�2 8C). The cell set up was as follows:screen-printed electrode jPANI – pillar[5]arene j test solu-tion j salt bridge (0.1 M NaNO3) j j3 M KCl jAgCl, Ag.The potentiometric selectivity coefficients log KCu/j

pot

were determined by the separate solution method (SSM)with 0.01 M solutions of Cu2+ ion and an interfering ionin non-buffered aqueous solutions and 0.5 mM nitric acid[24]. The potential was measured after injection of theprimary or interfering ion solution after reaching of thedrift rate of 1.0 mV per minute. The activities of ionswere calculated from the modified form of the Debye�H�ckel equation. All the measurements were performedin six replications.

3 Results and Discussion

3.1 Electrochemical Oxidation of Pillar[5]arene inSolution and in PANI – Pillar[5]arene Layer

The pillar[5]arene molecule consists of five hydroquinonerings connected via methylene bridge. From the formalpoint of view, the redox conversion of each hydroquinoneunit to benzoquinone should be independent from eachother. Indeed, the coulometric measurements with elec-trogenerated bromine in 50% aqueous dimethylsulfoxideshowed the transfer of 9.5–10 electrons per each pillar[5]-arene molecule. Similar results obtained with electrogen-erated [Fe(CN)6]

3� ions excluded alternative brominationof aromatic rings. The blank experiment with the solventwas performed to quantify interfering influence of thesolvent side reactions with an oxidant. When stored inthe presence of an oxygen, the pillar[5]arene is partiallyoxidized and the number of electrons transferred in cou-lometric experiment tends to decrease to final 5–6 elec-trons per molecule. In similar experiment performed withstoichiometric amount of bromine dissolved in chloro-form, after reaching 40 % conversion the reaction isstopped by spontaneous precipitation of insoluble productof partial oxidation.

The electrochemical behavior of pillar[5]arene wasstudied with cyclic voltammetry by comparison of voltam-

mograms of pillar[5]arene and hydroquinone dissolved in50% aqueous acetone containing Na2SO4 (Figure 1). Theredox activity of hydroquinone and pillar[5]arene differsin potential from that common for aqueous buffer solu-tions (about 0.4 V). The oxidation peak of pillar[5]areneis slightly shifted against that of hydroquinone to a loweranodic potential. The modest peak potential difference(DEp =104 mV) and similar height of anodic and cathodicpeaks (Ia/Ic =0.82) confirm quasi-reversible electron ex-change. Regarding hydroquinone, a broadened smallcathodic peak appeared for high concentration of hydro-quinone. Its shape as well as the peak resolution (DEp =38 mV) suppose a surface limited electrode reaction ofhydroquinone oxidation. This might be either due to slowH+ exchange or the influence of acetone added to dis-solve the pillar[5]arene. The irreversible cathodic peak at�600 mV is related to the reduction of dissolved oxygenand significantly decreased after additional Ar purging.

The comparison of the peak currents recorded at differ-ent concentrations of pillar[5]arene and hydroquinoneconfirmed the preliminary conclusion on about 40% con-version of the macrocycle in electrochemical and chemi-cal oxidation. This stoichiometry of oxidation correspondsto formation of alternating oxidized (quinone) and re-duced (hydroquinone) units which are stabilized by hy-drogen bonds (Scheme 3). Although the geometry ofsemi-oxidation product differs from that of quinhydrone,the spatial distribution of intramolecular hydrogen bondspromotes the configuration favorable for such interac-tions.

Similar interactions are referred for the stabilization ofoligomeric regular structures self-assembled from pil-lar[5]arene dissolved in organic solvents [25]. The solubil-ity of quinhydrone is much lower than that of reduced pil-lar[5]arene and this explains the precipitation of insolubleproduct of chemical oxidation of pillar[5]arene from reac-tion media. The consistent stabilization of oxidized andreduced units can also take place by interaction of neigh-boring pillar[5]arene molecules aggregated on the elec-

Fig. 1. Cyclic voltammograms recorded on screen-printed elec-trode in 50% acetone containing 0.1 M Na2SO4, in the presenceof pillar[5]arene or hydroquinone, scan rate 50 mV/s.

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trode surface as reported elsewhere [21,26]. Analogousredox reaction with chemically polymerized PANI wasobserved in organic solvent between the emeraldine saltand pillar[5]arene by optical spectroscopy and cyclic vol-tammetry [21,26].

The redox conversion of pillar[5]arene contacted withemeraldine form of PANI was confirmed by EIS. The Ny-quist diagrams recorded on glassy carbon electrode con-secutively covered with the PANI and pillar[5]arene withferricyanide ions [Fe(CN)6]

3�/4� as redox probe are shownin Figure 2.

As could be seen, the deposition of pillar[5]arene onthe PANI layer progressively increased the charge trans-fer resistance. This coincides with previously describedredox transformation that decreased the PANI conductiv-ity. At low macrocycle concentration, the redox character-istics of the surface layer remain about constant due toincomplete coverage of the polymer layer with pillar[5]ar-ene aggregates.

Thus, deposition of pillar[5]arene on the electropoly-merized PANI results in formation of a hybrid layer withsemi-oxidized form of the macrocycle and reversibleredox properties favorable for potentiometric measure-ments.

3.3 Potentiometric pH Measurements

The PANI based potentiometric sensors have been de-scribed for the measurements of acidity [27,28] and spe-

cies affecting protonation of emeraldine salt (ammoniaand biogenic amines [29,30]). The pH dependence of thePANI potential is related to the involvement of H+ ionsin the reactions reversible transformation of the PANIforms.

Meanwhile the use of screen-printed electrodes astransducers of PANI based pH sensors is often complicat-ed with lower sensitivity of the response and rathernarrow pH range determined in comparison with the pa-rameters of Pt or glassy carbon electrodes covered withelectropolymerized or chemically synthesized PANI. Thiscould cause from less efficiency of electron transductionin such sensors.

The electropolymerization of PANI was performedusing the protocol of multiple cycling previously testedfor the manufacture of solid-contact sensors based onthiacalix[4]arenes [9]. The number of cycles and the con-centration of the monomer were specified to reach fullcoverage of electrode with a thin durable film able to re-versible redox transformation.

The pillar[5]arene deposition on the PANI layer im-proved the reversibility of the pH dependency of elec-trode potential. Figure 3 shows the electrode potentialagainst the pH measured with conventional glass pH elec-trode. The records were started in acidic region and con-ducted continuously by NaOH addition to pH 9.0 andthen back to pH 2.0 by the HCl (the direction of the pHshift is indicated with arrow). The start- and end-points ofthe curve recorded on PANI covered screen-printed elec-

Scheme 3. Formation of alternating oxidized (quinone) and reduced (hydroquinone) units which are stabilized by hydrogen bonds.

Fig. 2. (A) The Nyquist diagram of impedance spectra recorded on screen-printed carbon electrode covered with PANI (1) andPANI – 1.4 mgcm�2 pillar[5]arene (2). Measurements with 0.01 molL�1 K3[Fe(CN)6] and 0.01 molL�1 K4[Fe(CN)6], frequency range0.04 Hz - 100 kHz, amplitude 5 mV. (B) Charge transfer resistance Ret (mean�SD for six electrodes) measured with different quanti-ties of pillar[5]arene in the surface layer.

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trode with no macrocycle differed from each other by90 mV. Contrary to that, no hysteresis of the pH responsewas found with 1.40 mg cm�2 pillar[5]arene coating. Thissurface concentration of the macrocycle coincided to themaximal difference in the charge transfer resistance de-termined prior to and after macrocycle deposition. Theaccuracy of the pH measurement with PANI – pillar[5]ar-ene layer was significantly higher than that of PANIbased sensors (�44.3�1.5 mV/pH vs. �40.9�3.4 mV/pH). The positive effect of pillar[5]arene on the pH re-sponse of appropriate PANI sensor can be due to coordi-nated interaction between hydroquinone/benzoquinoneand emeraldine/leucoemeralidne pairs described earlierin voltammetric investigation of hydroquinone oxidationon the PANI covered electrode [31]. The illustration ofpossible interactions promoting the H+ and electron ex-change within the layer is presented in Scheme 4.

3.4 Potentiometric Testing of Transient Metals

Both PANI and phenolic compounds can bind metal cat-ions in various conditions due to donor-acceptor interac-tions with nitrogen atoms of the PANI chain and pheno-late groups. Similar interactions can cause specific poten-tiometric response of PANI – pillar[5]arene based sensor.All the experiments were performed with 1.40 mg/cm2 pil-

lar[5]arene which showed maximal effect on the PANIcharacteristics. Standard solutions of metal nitrates variedfrom 1.0 � 10�7 to 1.0� 10�1 M were tested. Freshly pre-pared PANI-pillar[5]arene sensors were used to avoidpossible memory effect and accumulation of interferenc-es. The potentiometric sensors were immersed in non-buf-fered solutions of alkali, alkali-earth and transient metalsalts and its potential was recorded to reach stable value.In most cases, changes in the sensor potential were ob-served within 20–30 s incubation.

The characteristics of metal ion detection are summar-ized in Table 1. For comparison, the results obtained withthe PANI based sensors in the absence of pillar[5]areneare also presented. The electrodes covered with PANIonly show low sensitivity toward metal ions with rathernarrow linear range of concentrations within two ordersof their magnitude. The slopes of the curves in Nernstianplots were significantly lower than theoretical value of 59/n mV where n is the charge of the primary ion [32].

The implementation of pillar[5]arene in the PANI layerextends the linear range of concentrations for most ionstested except alkali metals. However, the slope of calibra-tion curves remains rather low and does not allow analyti-cal application of the potentiometric sensors. The resultsof potentiometric detection could be affected by nonbuf-fered media with low ionic strength especially for lowmetal concentrations. Meanwhile the conclusion aboutrather low sensitivity of the detection of alkali and alkali-earth metal ions with the PANI based sensor was con-firmed by similar experiments previously performed inthe presence of Na2SO4 and nitric acid (pH 1.7) [9,33].

There are three exceptions from the behavior of metalions described above, i.e. Ag+ , Fe3+ and Cu2+ . The Ag+

and Fe3 + ions show super-Nernstian slope of the narrowmiddle part of S-shaped calibration curve. Such a behavioris typical for redox-aided response caused by oxidation ofPANI. Indeed, the addition of some antioxidants, i.e. as-corbic acid, suppressed the signal of Fe3+ ions.

The slope of the calibration curve of copper obtainedin non-buffered media (Table 1) is between those corre-sponded to mono and divalent cations. This might be dueto contribution of partial reduction of Cu2+ ions by themacrocycle. Thus, the UV spectra of pillar[5]arene – Cu2 +

system dissolved in dimethylsulfoxide contained a broadenlow intensive band at 400–600 nm (lmax 460 nm) typicalfor Cu(I) compounds [35]. The products of two-electron

Fig. 3. The pH dependence of the potential of the sensor basedon PANI (black curves) and on PANI – pillar[5]arene(1.40 mgcm�2, gray curves). Arrows show the direction of the pHshift.

Scheme 4. Illustration of possible interactions promoting the H+ and electron exchange within the layer.

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transfer would have adsorption maximum in the area of560 nm. Certainly, such changes were observed in reac-tion conditions different from those of electrochemicalexperiment and for a longer time interval. The reactivityof PANI can also affect the redox conversion of an ana-lyte observed by optical method. Meanwhile the oxida-tion activity of copper ions toward PANI was previouslyfound sufficiently lower than that of Ag+ and Fe3+ ions[9]. Besides, PANI based microsensor was applied for themonitoring of Cu2+ - lactate oscillation system [34]. Thisassumes that altering concentrations of Cu2+/Cu+ speciesdid not interfere with pH dependence of the PANI poten-tial and hence they could not shift the sensitivity of po-tentiometric response against that caused by complexa-tion.

Thus, mixed value of the slope of Cu2+ calibrationcurve is explained by two mechanisms of potential gener-ation, i.e. redox conversion in the pair of Cu2+ - pillar[5]-arene and by complexation of a primary ion with themacrocycle. The product formed in such complexationcan involve coordination of copper ion near the ringformed by hydroxyl and quinone groups coordinated byhydrogen bonds as is outlined in Figure 4.

The increase in the amount of pillar[5]arene loaded onthe PANI layer from 1.40 to 14.0 mg/cm2 as well as higherconcentration of nitric acid able to re-oxidize Cu+ ionssuppress the contribution of redox process and return theslope of calibration curve to the values typical for com-plexation of a divalent ion (about 30 mV). The influenceof the measurement conditions on the copper determina-tion is presented in Table 2.

The use of acetic buffer solution was found inappropri-ate for reliable and sensitive response toward copperions. The slope of calibration curve was below the leveltypical for divalent cations and decreased with increasingbuffer concentration. Probably, acetate ions compete withpillar[5]arene for the Cu2+ ions binding. The characteris-tics of Cu2+ detection were significantly better in nitricacid. This might be due to suppression of partial hydroly-sis of copper ions observed in water and due to pH stabi-lization of the surface layer. In 10.0 mM HNO3 solution,the LOD calculated for S/N=3 ratio decreased down to

0.3 mM. This is lower than that observed in non-bufferedmedia (0.7 mM).

The selectivity of the response toward copper ions isprovided by the ability of Cu2+ ions to form coordinationbonds with vicinal hydroxyl groups known for polyols[36]. The results obtained with electrochemical tech-niques do not allow establishing the stoichiometry of suchcomplexation (1 :1 or 2 : 1, with two copper ions posi-tioned from both sides of a macrocycle). From the gener-al consideration, 1 : 1 complexes seem more probabletaking into account the easiness ability to self-aggregationof the pillar[5]arene molecules.

The dynamic response and reversibility of the Cu2 +

binding for alternating copper concentrations are shownin Figure 5. The response time was estimated as a periodof reaching 95% shift of the potential after the analyteinjection. In 10.0 mM HNO3 it was equal to 15 s andslightly increased with the pH to 25–40 s at pH 5.0 (seealso Figure 5 A for comparison). In the same pH interval,the absolute response toward 10 mM Cu2+ ions decreasedfrom pH 1.7 to pH 5.0 by about 10 %. This is significantlylower than could be expected from the slopes of appro-priate calibration curves (Figure 3 and Table 1). Thus, theuse of nitric acid partially suppressed the pH influence on

Table 1. Characteristics of metal cation detection in non-buffered media.a

Metal PANI based sensor PANI – pillar[5]arene based sensorConcentration range (M) Sensitivity (mV/decade) Concentration range (M) Sensitivity (mV/decade)

Li+ 1 � 10�3–7� 10�2 �15.5 1�10�5–1 �10�2 �15.4Na+ 1 � 10�5–1� 10�1 �3.5 1�10�5–5 �10�2 �12.8K+ 1 � 10�5–5� 10�2 �10.2 1�10�5–5 �10�2 �15.5Ca2+ 1 � 10�4–5� 10�3 �22.2 1�10�5–5 �10�2 �8.2Mg2+ 1 � 10�4–5� 10�3 �19.6 1�10�5–5 �10�2 �9.7Ni2+ 1 � 10�4–5� 10�3 �8.5 1�10�5–5 �10�2 �4.5Cu2+ 1 � 10�4–1� 10�2 �12.8 1�10�6–1 �10�2 �36.8Co2+ 2 � 10�4–2� 10�3 �7.2 2�10�5–5 �10�2 �10.0Pb2+ 1 � 10�5–5� 10�3 �12.2 5�10�5–5 �10�2 �8.7Ag+ 7 � 10�4–1� 10�2 �63.1 7�10�4–1 �10�2 �67.5Fe3+ 1 � 10�4– � 10�3 �231 1�10�4–1 �10�3 �124

Fig. 4. The outline of the possible structure of the complexformed by partially oxidized pillar[5]arene and Cu2+ ions (theparts of the complex are shown not in the same scale).

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the response toward Cu2 + ions. For PANI based potentio-metric sensor in the absence of pillar[5]arene the selectiv-ity of Cu2 + binding was found the same in the pH intervalfrom 2 to 5 [9].

The reversibility of the response toward Cu(II) ionswas examined in the series of measurements in alternat-ing solutions of 1.0 �10�3 and 1.0� 10�5 M CuSO4 per-formed with the same sensor (Figure 5 B). The electrodeswere washed between the measurements with 0.1 mMEDTA. The signal was stabilized after first cycle. Whenwashed with distilled water between measurements, thepotentiometric sensors exert gradual deviation of thesignal by 4–7 mV in each measurement. Probably, the in-troduction of lipophilic salt in the surface layer would im-prove the reversibility but this would complicate andsensor preparation.

The sensitivity of the Cu2+ ions determination corre-sponds to that of other solid-contact sensors reported(Table 3). The coated wire electrodes with PVC mem-brane containing ionophore show similar characteristics,though the deposition of PANI – pillar[5]arene layer pro-posed in this work is much simpler. The only analog ofthe approach proposed involves the polyelectrolyte com-plex of polythiophene derivative with polystyrolsulfonate.It demonstrates rather narrow range of concentrations de-termined and the LOD value which is higher than thatreached in this work. The inclusion of various ionophoresin the carbon paste and its connection with nanomaterialslike metal oxides or multiwalled carbon nanotubes im-proves the performance of potentiometric sensors. Thiscan be related to accumulation of primary ions in theelectrode material. The application of sorptional accumu-

lation leaves space for the further improvement of thesensitivity of the sensor developed but can be complicat-ed with redox conversion of Cu2+ ions and decreased life-time.

No significant influence of counter anion of copper salton sensitivity and selectivity of Cu detection was foundfor chloride, nitrate and sulfate. The influence of acetatewas described above for the experiments performed inthe presence of acetate buffer solution.

3.5 Reproducibility and Selectivity

Each potentiometric sensor makes it possible to performup to 20 measurements with the repeatability of the po-tential of 4.5 % without significant losses of sensitivitytoward the primary ion. Intermediate treatment of poten-tiometric sensor with 1.0 �10�4 M EDTA improves the ac-curacy of the measurement (RSD 3.4 % for 10.0 mMCuSO4). All the calculations were performed for the setof six sensors made with the same reagents. When left indry conditions, the potentiometric sensors retain theirsensitivity for more than three months. Prior to use, thepotentiometric sensor stored in dry conditions weresoaked in 10.0 mM HNO3 for one hour. No incubation inthe solution of primary ion (Cu2+) was required.

For a longer period of time, the sensitivity of the re-sponse decreased by about 15 % of initial value (sixmonths testing). Independent measurements of the resist-ance of screen-printed electrodes showed that at leastpart of the changes can be related to worsen conditionsof the electrode exchange caused by aging of the carbontracks of screen-printed electrodes. Freshly prepared, the

Table 2. Determination of Cu2+ ions with ISE based on screen-printed carbon electrode modified with PANI and pillar[5]arene (sixelectrodes).

Measurement conditions Pillar[5]arene E (mV)= (a�Da)�(b�Db)·pC (M) Conc. range (M) LOD (M)(mg/cm2) a�Da b�Db R2

0.01 M acetate buffer solution, pH 5.0 1.4 136.4�2.4 17.4�0.5 0.9923 3.0 � 10�6–5.0� 10�3 1.0� 10�6

0.5 mM nitric acid, pH 3.3 0.14 283.8�2.8 40.4�0.8 0.9954 5.0 � 10�5–1.0� 10�2 5.0� 10�6

0.5 mM nitric acid, pH 3.3 1.4 295.6�5.5 31.0�1.6 0.9789 7.0 � 10�5–1.0� 10�2 3.0� 10�5

0.5 mM nitric acid, pH 3.3 14.0 314.5�3.4 19.5�1.2 0.9574 1.0 � 10�6–1.0� 10�2 3.0� 10�5

10.0 mM nitric acid, pH 1.7 1.4 330.2�2.4 29.5�0.5 0.9974 1.0 � 10�6–1.0� 10�2 3.0� 10�7

Fig. 5. Dynamic response (A) and reversibility (B) of the sensor response toward Cu2+ ions. The final Cu2+ concentration: 1–5.0�10�7, 2–7.0� 10�7, 3–9.0� 10�7, 4–1.1� 10�6, 5–1.5� 10�6, 6–2.0� 10�6, 7–3.0� 10�6, 8–4.0� 10�6, 9–6.0� 10�6 molL�1. Measurements in0.1 mM HNO3.

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electrodes had a resistance of 140–180 W between theelectrical contact point and the working electrode area.After six months storage, the above value increased to320 W.

The selectivity of the response toward Cu2+ ions wasmeasured by SSM with 1.0� 10�2 M solutions of primaryand interfering ions both in non-buffered and acidicmedia. The concentration of nitric acid chosen (5.0 mM)was sufficient for prevention of the hydrolysis of themetal salts. The �log KCu/j

pot values (Table 4) were foundto be higher in the presence of pillar[5]arene and inacidic media against pure PANI and aqueous salt solu-tions, respectively. Minimal pH influence were observedfor alkali and alkali-earth metals but the partial hydroly-sis of Zn2+, Co2+ , Ni2 + and Pb2+ ions decreased the selec-tivity of response. The characteristics of the PANI basedsensor (data in brackets) were shifted in nitric acid to thesame direction as those of the sensors including pillar[5]-arene so that the selectivity of copper determination inwater and acidic media was similar to each other.

The potentiometric sensor involving pillar[5]areneshows remarkable selectivity of Cu2+ detection over allthe metal ions with Nernstian behavior. The increase ofpillar[5]arene loading in the surface layer from 0.14 to1.4 mg/cm2 increased the selectivity coefficients by one-two orders of magnitude. Probably this could be relatedto partial shielding of PANI with the macrocycle mole-cules. This decreased the influence of positively chargedcations on the PANI potential.

Regarding interfering effect of Fe3 + and Ag+ ions ex-erting the PANI oxidation, the addition of sodium fluo-ride to its final concentration of 1.0 mM eliminates theirinfluence on the copper determination.

3.6 Real Sample Analysis

The potentiometric sensor developed was tested in directdetermination of Cu2+ ions in two samples, i.e. polyvita-min drops “Complivit” (“UfaVita”, Russia) and Bor-deaux mixture used as a fungicide in vineyards, fruit-farms and gardens.

The “Complivit” is polyvitamin-mineral complex con-sisting of vitamins A, B, C and E reached with minerals(2.5 mg of FeSO4, 2.9 mg of CuSO4, 217 mg of CaHPO4,0.4 mg of CoSO4, 117 mg of MgHPO4, 11 mg of MnSO4,8.8 mg of ZnSO4 per one pill). After mechanical grinding,the preparation was dissolved in 10.0 mM HNO3, the so-lution was filtered and then adjusted to pH 2.0. However,the solution shifted the sensor potential in the directionopposite to that observed for Cu2+ ions addition probablydue to reduction of PANI with the antioxidants containedin the sample. To confirm this suggestion, the calibrationgraph of ascorbic acid was obtained (Figure 6). The S-shape of the curve corresponds to the redox response re-lated to partial reduction of PANI.

To avoid the influence of antioxidants, the sampletreatment was modified. The pills were first dissolved in50% HNO3 and heated to 50–60 8C for 15 min to oxidizethe interferences. After that, the solution was dissolved

Table 4. Potentiometric selectivity coefficients log KCu/jpot calculated by SSM for 0.01 M solutions of primary and interfering ions

(values in the brackets refer to the screen-printed carbon electrode covered with PANI only).

Interfering ion �log KCu/jpot Interfering ion �log KCu/j

pot

H2O 0.5 mM HNO3 H2O 0.5 mM HNO3

K+ 3.81 (1.32) 3.60 (1.10) Zn2+ 3.20 (1.17) 3.70 (1.70)Na+ 3.75 (0.24) 3.50 (0.30) Co2+ 2.53 (1.43) 3.30 (1.73)Li+ 3.89 (1.32) 3.50 (1.10) Ni2+ 3.26 (1.10) 4.22 (2.16)Ca2+ 3.08 (0.55) 3.52 (0.72) Pb2+ 2.63 (1.14) 4.35 (3.20)Mg2+ 2.89 (0.23) 3.20 (0.55)

Table 3. Comparison of analytical characteristics of various solid-contact potentiometric sensors for Cu(II) determination.

Ionophore/Carrier Conc. range, M LOD (M) Ref.

Bis[(2-(hydroxyethylimino)phenolato] copper(II) in PVC membrane 1.0 �10�6–1.0�10�1 8.3� 10�7 [37]N-hydroxysuccinimide in PVC – o-nitrophenyloctyl ether membrane 1.0 �10�4–1.0�10�2 4.4� 10�6 [38]PVC membrane doped with 1-ethyl-3-methyl imidazolium chloride 1.0 �10�7–1.0�10�1 3.2� 10�8 [39]Poly(3,4-ethylenedioxythiophene) - poly(4-styrenesulfonate) 1.0 �10�4–1.0�10�1 1.0� 10�5 [40]2-((2-(2-(2-(2-Hydroxy-5-methoxybenzylidene amino)phenyl)disufanyl)phenyl-imino)methyl)-4-methoxyphenol Schiff base in PVC membrane

1.2 �10�7–1.0�10�1 9.8� 10�8 [41]

Etioporphyrin I in carbon paste 1.28� 10�6–1.28� 10�2 8.99�10�7 [42]Phenanthroline�tetraphenyl borate in printing ink of screen-printed electrodeor in the carbon paste

1.0 �10�6–1.0�10�2 1.0� 10�6 [43]

Thiosalicylic acid in carbon paste 1.0 �10�6–1.0�10�3 5.0� 10�7 [44]7,16-Diaza-1-thia-4,10,13,19-tetraoxa-6,17-dioxo-2,3;20,21-dinaphtho-cyclounei-cosane in carbon paste

1.0 �10�8–1.0�10�2 7.0� 10�9 [45]

Pillar[5]arene onto PANI layer (measurements in nitric acid) 1.0 �10�6–1.0�10�2 3.0� 10�7 This work

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with water and pH adjusted as described before. The finalconcentration of Cu2+ ions was equal to 1.17� 10�3 M, therecovery calculated for six different sensors was equal to84�14 %.

The 1 % Bordeaux mixture was prepared by dissolutionof copper sulphate in slaked lime followed by dilution ofthe suspension to nominal copper concentration of 1.0�10�4 M and pH correction to pH 2.0. The recovery ofCu2+ determination for six individual sensors was foundto be 75�18%. Higher variation of the results againstthose of Complivit can be related to the heterogeneity ofthe initial sample containing the precipitation of thecopper oxocarbonate. Although the accuracy of thecopper determination is not very high, the potentiometricsensor developed can find application in control of themanufacture of copper containing chemicals, vitaminsand dietary supplements to avoid their overdose and pos-sible toxic effects on a human.

4 Conclusion

The introduction of pillar[5]arene in the PANI layer haschanged the potentiometric response of the sensortoward pH and readily oxidized compounds due to in-volvement of the macrocycle in the electron exchangeand improved reversibility of the above process. In addi-tion to Fe3+ and Ag+ ions which oxidize PANI, specificresponse to Cu2+ ions was found. This was related totheir complexation with pillar[5]arene. The potentiomet-ric detection of Cu2+ was impossible if the potentiometricsensor contained the PANI taken alone. The use of pil-lar[5]arene made it possible to determine from 1.0 �10�6

to 1.0 �10�2 M Cu2+ with the LOD of 3.0 �10�7 M. Theselectivity of the signal toward copper ions is rather highand offers opportunities to detect the primary ions in themixtures with many inorganic salts as was shown bydirect Cu(II) detection in polyvitamin preparation andBordeaux mixture. The selectivity of the signal can be re-ferred to the ability of Cu(II) ions to form complexes via

vicinal hydroxyl groups of the ionophore. The stability ofthe response toward pH shifts is higher in the presence of0.5–10.0 mM HNO3 as supporting electrolyte. Althoughthe manipulations with unsubstituted pillar[5]arene re-quire care handling because of its possible oxidation, theelectrochemical characteristics of the PANI – pillar[5]ar-ene coating are quite stable and allow up to 20 dailymeasurements with at least three month storage period.The redox interactions between PANI and pillar[5]areneimproved the reversibility of the electron exchange andas was shown by cyclic voltammetry and EIS. Being verycheap and simple in manufacture, the potentiometricsensor developed can find application for preliminarycontrol of copper containing products. The possible inter-ference of oxidizable species as well as Fe3 + and Ag+

ions affecting the PANI redox status can be suppressedby treatment with nitric acid and sodium fluoride, respec-tively. The following progress in the sensor assembly canbe expected by implementation of the pillar[5]arene inthe carbon materials used for electrode scree-printing orby introduction of lipophilic salts in the surface film.

Acknowledgement

The financial support by the Russian Science Foundation(Grant 14-13-00058) is gratefully acknowledged.

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Received: September 9, 2014Accepted: October 26, 2014

Published online: December 11, 2014

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