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Electrochemical Properties of Electrode Modified withLangmuir-Blodgett Film of p-tert-Butylcalix[4]arene Derivativesand Its Application in Determining of SilverLu Wang,a Bang-Tun Zhao,b Bao-Xian Yea*a Department of Chemistry, Zhengzhou University, Zhengzhou 450052, P. R. Chinab Department of Chemistry, Luoyang Normal University, Luoyang 471022, P. R. China*e-mail: yebx@zzu.edu.cn

Received: November 7, 2006Accepted: December 27, 2006

AbstractA new type of voltammetric sensor, Langmuir-Blodgett (LB) film of 5,11,17,23-tetra-tert-butyl-25,27-di(3-thiadiazole-propanoxy)-26,28-dihydroxycalix[4]arene modified glassy carbon electrode (LBTZCA –GCE), was prepared. Theelectrochemical properties of LBTZCA –GCE were researched in detail and its recognizing mechanism for silver ion inaqueous solution was discussed. Using this voltammetric sensor, a new stripping voltammetric method for determiningof Agþ was erected with good sensitivity, selectivity, reproducibility and recovery. The detection limit was 8� 10�9 Mat accumulation time of 180 s. By this method, real samples (lake water, tap water and synthesis sample) wereanalyzed and the results obtained were well satisfactory.

Keywords: Langmuir-Blodgett film, Calix[4]arene derivatives, Thiadiazole, Silver, Stripping voltammetry

DOI: 10.1002/elan.200603770

1. Introduction

Calixarenes are cyclic oligomers synthesized by condensa-tion of a p-alkylated phenol and formaldehyde and recog-nized to be important materials in the supramolecularchemistry field. Calixarenes and their derivatives haveattracted much attention over the past decade as the basisfor molecular and ionic recognition because of theirconformational and structural flexibility [1 – 2]. They offerdifferent sizes of platforms for a wide choice of chemicalmodifications at the upper rim or at the lower rim. As aresult, they have been used widely in analytical chemistry,such as ion selective electrode [3 – 4], capillary electro-phoresis [5], chromatographic stationary phase [6], phasetransfer catalyst [7], Langmuir-Blodgett( LB) membranes[8] and so on. Many researchers reported the application ofcalixarenes and their derivatives in electrochemistry[9 – 11].Electrochemical recognition of metal ions by calixareneshas some reported too. Sun Kil Kang et al. studied theelectrochemical recognition of Ca2þ ion in basic aqueousmedia using quinone-derivatized calix[4]arene [12]. Dong-PingZhan et al. reported the electrochemical recognition ofalkali metal ions at the micro-water 1,2-dichloroethaneinterface using 5,11,17,23-butyl-25,26,27,28-tetra-(ethanoxy-carbonyl)-methoxy-calix[4]arene [13].Chemically modified electrodes (CME) have received an

increasing attention in recent years, especially in the fields ofelectroanalysis due to well recognized advantages in com-parison with the conventional electrodes [14]. Calixarenes

are reported as chemically modified reagents on electrodestoo. Stuart D. Collyer and his co-workers had studied theelectrochemistry of the ferri/ferrocyanide redox couple atgold electrodes modified with calix[4]resorcinarenetetra-thiol [15]. Stripping voltammetry with electrodes modifiedwith calixarenes derivatives were used to determinatecadmium [16], mercury [17], and lead [18]. These reportedmodifying method was by hand-spread, which were asym-metrical distributing of calixarenes on electrode surface.Recently, we have proposed a new idea of using calixarenesLB film to modify electrode as voltammetric sensors andsucceed in recognizing metal ions. By this idea, we havereported of determining trace of Cu2þ [19], Pb2þ and Cd2þ

[20] with p-tert-butylthiacalix[4]arene LB film modifyingGCelectrodes and trace ofHg2þ [21] , Cd2þ andTlþ [22] withp-allylcalix[4]arene LB film modifying GC electrodesrespectively.In this paper, we wish to report our new research of

recognizing and determining trace Agþ based on a novelglassy carbon electrode modified by LB film of a p-tert-butylcalix[4]arene derivative, that is, 5,11,17,23-tetra-tert-butyl-25,27-di(3-thiadiazole-propanoxy)-26,28-dihydroxy-calix[4]arene (TZCA) [23], which provides extra-cavitydimensions and coordination sites of N, S atoms ofthiadiazole compared with parent calixarenes. In addition,it can be prepared as the Z-type LB film ascribing topresence of its hydrophile andhydrophobe groups.Basedonthe factors above, our work acquired good sensitivity,selectivity, reproducibility and recovery.

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2. Experimental

2.1. Apparatus

All the voltammetric measurements were carried out on theCHI 650 Electrochemical analyzer (CH Instruments Com-pany, USA). LB films were formed and deposited on GCEby using JML-04 LB trough (Shanghai Zhongchen Com-pany, China). A disk-type glassy carbon electrode (3 mmdiameter) coated LB film of 5,11,17,23-tetra-tert-butyl-25,27-di(3-thiadiazole-propanoxy)-26,28-dihydroxycalix-[4]arene (TZCA) served as theworking electrode(LBTZCA –GCE), with a saturated calomel electrode (SCE) andplatinum wire acting as reference and counter electrodes,respectively. All potentials were reported versus SCE. Allmeasurements were conducted in solutions deaerated bybubbling N2 for 10 min and performed under room temper-ature.

2.2. Reagents and Solutions

All reagents were of analytical grade and were used withoutfurther purification. The aqueous solutions were preparedusing redistilled water. TZCA shown in Figure 1 was self-synthesized [23].

2.3. Preparation of LB Film

Before coating, pretreatment of GC electrode was carriedout according to normal method, and then electrochemi-cally pretreated by cycling the electrode in 0.5 M H2SO4

until a stable CV curve was obtained.Measurement of p –A isotherm and preparation of LB

monolayer were performed with a JML-04 LB trough. Thespreading solutions were prepared by dissolving about1.4 mg of TZCA in 10 mL of dichloromethane. All spread-ing solutions were kept under 5 8C and renewed every 2weeks. The solution was spread onto the surface of redis-tilled water with a syringe. Thirty minutes were given forevaporation of the spreading solvent. Then aZ-typeLB filmwas gained by a horizontal deposition method.

3. Results and Discussion

3.1. p –A Isotherms

The p –A isotherms of TZCA are presented in Figure 2. Itexhibits a typical characteristic of a condensed monolayerwhere only a vertical condensed phase is observed. Thestable monolayer reveals a collapse pressure of 31 mN ·m�1

and an extrapolatedmolecular area of 1.0 nm2 permolecule.

3.2. Voltammetric Behavior of LBTZCA-GCE

3.2.1. Voltammetric Behavior and Effects by SupportingElectrolyte and pH

In aqueous solution electrolyte, LBTZCA-GCE representedelectro-action only in the solution pH range of 2 – 9, andelectrode reaction was not observed while pH> 9 and pH< 2. A pair of redox peaks was appeared by cyclic scan theLBTZCA-GCE in potential windowof�0.3 V to 0.4 V in nearneutral solution. For getting the maximum voltammetricresponse, various supporting electrolytes, such as Britton-Robinson buffer solution, NH4Cl-HCl, NaH2PO4-Na2HPO4, NH4OH-NH4Cl and HAc-NaAc were tested.The results show that NH4Cl-HCl was the best choice, andthe largest stripping peak current, the lowest backgroundcurrent and the best peak shapewere obtained in 0.1 MNH4

Cl-HCl (pH 4.5) buffer solution, which was chosen to beemployed in following experiments.

3.2.2. Cyclic Voltammogram of LBTZCA-GCE

In 0.1 M NH4Cl-HCl (pH 4.5) buffer solution, a pair ofredox peaks appeared with the peak potentials of Epa¼0.124 V and Epc¼�0.021 V at scan rate of 100 mV/s. Forincreasing of the scan rate, the difference of peak potentialsDEp¼Epa –Epc was increscent and the peak currents waslinear augmentation with equation of: ipa(mA) ¼ 0.0182v(V/s)þ0.9409 (g¼ 0.997). This means that the LBTZCA-GCE

Fig. 1. The structure of TZCA.

Fig. 2. Pressure-area isotherms of TZCA.

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reaction process is a quasi-reversible redox reaction drivenby adsorption.

3.2.3. Effects of Layer Number of LB Film.

The peak currents (ipa and ipc) of two layer films aresignificantly higher than that of others. This can attribute tothe defects during film building up process. There may besomedefects exist in the one layerLB film, and these defectsmay be remedied by the second layer. The impedance mayplay an important action and block the electron transferbetween GCE and TZCA when LB films are three layersand more. So, the peak currents reach the maximum valuesat a two layer LB films, and the peak shape are the best.Therefore, a double layer film was chosen in all followingexperiments.

3.2.4. Electrode Process Dynamics Parameters

For the adsorptive-driven redox peaks, based on theLavironKs [24] theory, there is the following relationequation between peak current and scan rate:

ip¼ n2F2vAG*/4RT¼ nFQv/4RT

Where, Q¼ nFAG* represents the peak area of CV. This ismeans that the electron transfer number n can be calculatedas long as theCVpeak areaQ is obtained under certain scanrate. From this, scan rates 20, 40, 60, 80, 100, 110, 140, 180,and 200 mV/s, respectively were preformed and n¼ 2 wereobtained under all scan rates.The overlay amountG* of TZCAon electrode surface can

be calculated from Q¼ nFAG*. Under all scan ratesmentioned above,G*¼ 6.86� 10�8 mol cm�2 were obtained.The peak potentials (Epa and Epc) were negative shift by

increasing pH of solution. From theNernstian equation, theproton number taken part in reaction could be calculated:

Ep ¼ E0 þ RTnF

lnO½ � Hþ½ �@

R½ �

¼ E0 þ RTnF

lnO½ �R½ � þ @

RTnF

ln Hþ½ �

From this, we calculated out that there were two protonsparticipate in such electrode reaction process. Hence, weknow that the ratio value of proton and electron taken partin electrode reaction is 1.For a quasi-reversible interfacial reaction, the charge

transfer coefficient a can be obtained from followingequation [25]:

Ep ¼ kþ RTanF

ln v

From the slope of Ep – lnn straight curve, a¼ 0.91 wasobtained.

Based on the LavironKs theory [24], the apparent rateconstant ks of electrode reaction can be calculated byfollowing equation:

lg ks¼a lg (1�a)þ (1�a) lg aþ lg (RT/nFv)�a (1�a)nFDEp/2.3RT

The average apparent rate constant ks obtained for thisreaction was 1.10� 10�2 s�1.

3.3. LBTZCA-GCE Recognizing Agþ and AnalyticalApplication

In 0.1 M HNO3 solution, LBTZCA-GCE represented noelectro-action in potential range of 0.4 V– to 0.3 V (Fig. 3,curve a). If the LBTZCA-GCE was immerged in 1� 10�6 MAgþ for a moment and then transferred it in 0.1 M HNO3

solution for cyclic scan, a pair of redox peaks wererepresented (Fig. 3, curve b). This means that the LBTZCA-

Fig. 3. Voltammograms of LBTZCA-GCE (a) and LBTZCA-GCEimmerged in 1� 10�6 M Agþ(b). Buffer Solution: 0.1 M HNO3,scan rate: 100 mV/s.

Fig. 4. Differential pulse stripping voltammogram of LBTZCA-GCE with Agþ concentration of 1� 10�6 M. Voltammetric con-dition: Buffer Solution of 0.1 M HNO3, deposition time of 180 s at�0.2 V (vs. SCE), pulse width of 50 ms, and amplitude of 50 mV.

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GCE possesses recognizing function for Agþ ion. As can beseen from Figure 3b, the oxidation peak current is moresensitive to reduction peak current. Based on this, a differ-ential pulse stripping voltammetry was established fordetermining of traceAgþ using the oxidation process (Fig. 4).

3.3.1. Selection of the Optimal Operation Conditions

Because the oxidation peak current is more sensitive toreduction peak current (Fig. 3, curve b), a negative potentialis used in accumulation step to reduce Agþ to Ag0, and thenaccumulatedAg0 be oxidized by a positive differential pulsescan potential in the stripping step. The detecting mecha-nism can be expressed as:

AgþþCalfilm!Agþ�Calfilm (accumulation)

Agþ�Calfilmþ e!Ag0filmþCalfilm (accumulation)

Ag0film!Agþsolutionþ e (stripping)

In the accumulation step, the silver ions were complexationwith TZCA at the LB film electrode surface and reducedsimultaneously to Ag0 by adding a sufficiently negativepotential. In the stripping step, the resulting oxidation peakconstituted the analytical signal.Accumulation potential is an important parameter for

stripping techniques and has non-negligible influence on theefficiency of preconcentration and the sensitivity of deter-mination. The effect of accumulation potential on thestripping peak current of Agþ was examined over thepotential range of 0.1 V to �0.4 V (Fig. 5). The strippingpeak currents increased gradually when the accumulationpotential shifted from 0.1 to �0.2 V, and a well-definedstripping peak corresponding to Agþ was obtained. Furthernegative shift of the accumulation potential caused peakcurrents changed very slightly. However, the backgroundcurrent increased simultaneously. Thus the potential of�0.2 V was chosen as the accumulation potential infollowing studies.The accumulation time is another important parameter in

stripping analysis. General, the longer accumulation time,the higher sensitivity is gotten in stripping voltammetry, butnarrow detect range is emerged. For a detecting solutioncontaining 1.0� 10�6 M Agþ, accumulating saturation wasreached by preconcentration of 180 s (Fig. 6). In this study,180 s was chosen as the accumulation time in further studies.The peak currents were linearly relationship with Agþ

concentrations in about two quantitative orders under180 s preconcentration time.

3.3.2. Linear Range, Detection Limit, and Reproducibility

Series concentrations of standard solutions of Agþ weredetected under the optimized working conditions describedabove. The peak currents responded were linearly relation-ship with Agþ concentrations in the range of 2� 10�8 – 1�10�6 M (Fig. 7). The linear equation was:

ip(mA)¼�0.316þ 5.784� 107 C (g¼ 0.998).

The detection limit was 8� 10�9 M at accumulation time of180 s. The measurements of 1� 10�6 M Agþ were parallelcarried out for 10 times and the relative deviations was3.4%. The results indicated that the LBTZCA modified GCEelectrode had excellent reproducibility.

3.3.3. Interference

In order to evaluate the selectivity of the proposed method,possible interference by coexisting metal ions was inves-tigated by the addition of the interfering ion to a solutioncontaining 1� 10�6 M of Ag þ. The results showed that 103-fold of alkalinemetal ions, earthmetal ions, 200-fold of Fe3þ,Fe2þ, Mn2þ, Co2þ, Ni2þ, Zn2þ, Cr3þ, Cd2þ did not significantlyinfluence the height of the peak currents. However, 60-foldPb2þ and 40-fold Cu2þ interferes the determination bydecreasing the Agþ signal reduced about 5%. For reasonsascribed as they also form complexes with TZCA andcompete with the complex formation of Agþ at GCEsurface. On the other hand, 10-fold Hg2þ was found to

Fig. 5. The effect of accumulation potential.

Fig. 6. The effect of accumulation time.

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increase the Agþ response current about 5%. This may beexplained as mercury ions were reduced simultaneously toform a thin mercury film, which increased the accumulationefficiency of Agþ. Adding the shelter reagent can eliminatethe interference of Pb2þ, Cu2þ and Hg2þ.

3.3.4. Analysis of Real Samples

Unfortunately, suitable sullage sample was not found in ourarea. To investigate the applicability of the proposeddetermination method described above, tap water, lakewater and synthesized water samples were employed todetermining silver by standard addition technique (Table 1).The standard silver solutions were added to the samplesolution after preparation of the sample and before analysis.The analytical results and recovery are listed in Table 1.

4. Conclusions

In this work, we prepared a new chemically modifiedelectrode based on a calixarene derivative LB film. Theelectrochemical properties of LBTZCA –GCE and recogni-tion mechanism of this electrode for silver ion in aqueoussolutionwas discussed, at the same time, some experimentalparameters were optimized. A good sensitivity and low

detection limits for detecting silver ion were obtained. Thefabricated electrode of LB film exhibited excellent sensi-tivity, selectivity and reproducibility. The use of thisvoltammetric sensor for other metal species and organicmolecule is currently in progress.

5. Acknowledgement

The financial support provided byNational Natural ScienceFoundation of China (20475050, 20575060) is greatlyappreciated.

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Fig. 7. The relationship between peak currents and concentra-tion.

Table 1. Recovery for silver obtained on lake water, tape waterand synthesized water samples. ND: not detected

Samples Originalconcentration(10�8 M)

Added(10�8 M)

Found(10�8 M)

Recovery(%)

Tap water ND 5 4.85 97Lake water ND 5 5.15 103Syn solution 4 5 8.64 96

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Electroanalysis 19, 2007, No. 9, 923 – 927 www.electroanalysis.wiley-vch.de C 2007 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim