Electrochemical Properties of Th(IV)-Hexacyanoferrate Sol-Gel Carbon Composite Electrode: Electrocatalytic Oxidation of Dopamine and Ascorbic Acid

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1 1034 Journal of the Chinese Chemical Society, 2008, 55, Electrochemical Properties of Th(IV)-Hexacyanoferrate Sol-Gel Carbon Composite Electrode: Electrocatalytic Oxidation of Dopamine and Ascorbic Acid Khalil Farhadi, a * Farshad Kheiri a and Mir Maqsood Golzan b a Department of Chemistry, Faculty of Science, Urmia University, Urmia, Iran b Department of Physics, Faculty of Science, Urmia University, Urmia, Iran A new sol-gel carbon composite electrode using hexacyanoferrate (HCF)-Th(IV) ion pair as a suitable modifier is fabricated in the present study. The Th(IV)-HCF-sol-gel carbon composite electrode (THCF- CCE) has been prepared by mixing methyl trimethoxysilan (MTMOS) sol-gel precursor and carbon powder with ion pair and then to fix in a plastic tube. Cyclic voltammetry and chronoamperometry were employed to study the electrochemical and electrocatalytic properties of proposed electrode. The apparent charge transfer rate constant, k s, and transfer coefficient,, for electron transfer between ion-pair and sol-gel CPE were calculated as s -1 and 0.52, respectively. The THCF-CCE showed a significant electrocatalytic activity towards oxidation of ascorbic acid (AA) and dopamine (DA) in 0.l M acidic phosphate buffer solutions (ph 3) containing KCl as a supporting electrolyte. The mean value of the diffusion coefficients for ascorbic acid and dopamine were found and (cm 2 s -1 ), respectively. High stability, good reproducibility, rapid response, easy surface regeneration and fabrication are the important characteristics of the proposed sensor. The resulting peaks from the electrocatalytic oxidation of AA and DA were well resolved with good sensitivity. A linear response was observed for AA and DA in the concentration range of to Mand to M, respectively. Keywords: Sol-Gel; MTMOS; (HCF)-Th(IV) ion pair; Ascorbic acid; Dopamine. INTRODUCTION Dopamine (DA) is one of the most important neurotransmitters and it plays a significant role in the functioning of the central nervous, renal, and hormonal systems. 1 The development of voltammetric sensors for detection of neurotransmitters in the intracellular fluids of the central nervous system has received much interest in the past few decades. Several methods of electrode pretreatment and electrode modification have been reported to increase selectivity for DA oxidation. 2,3 Similar to DA, ascorbic acid (AA) is also a biologically important compound. Recent clinical studies have demonstrated that the content of AA in biological fluids can be used to assess the amount of oxidation stress in human metabolism, 4 and excessive oxidative stress has been linked to cancer, diabetes mellitus and hepatic disease. Hence, a considerable effort has been devoted to the development of voltammetric methods for the determination of AA. However, it is almost too difficult to determine this component electrochemically by direct oxidation on a conventional electrode because of surface fouling by oxidation products and sluggish electron transfer. 5,6 One of the major problems in the detection of DA (typical concentration is M) is the presence of ascorbic acid (AA), which is oxidized at almost the same potential as DA on standard electrodes. For solving this problem, different modified electrodes such as self-assembled monolayer modified electrode, 7 self-assembled bilayer membrane modified glassy carbon electrode, 8 sol-gel composite electrode, 9 lipoic acid-coated carbon fiber microelectrodes, 10 cationic and anionic surfactant modified electrode, 11 carbon paste electrode modified with iron(ii) phthalocyanine complexes 12 and tetrabromo-p-benzoquinone modified carbon past electrode 13 were fabricated. Among the carbon electrodes, the sol-gel modified electrodes are of the particular importance. The ease and speed of the preparation and of obtaining a new reproducible surface, the low residual current, porous surface and low-cost * Corresponding author. Fax: ; khalil.farhadi@yahoo.com; kh.farhadi@mail.urmia.ac.ir (Khalil Farhadi)

2 Electrochemistry of Th(IV)-Hexacyanoferrate Sol-Gel J. Chin. Chem. Soc., Vol. 55, No. 5, of carbon paste sol gel modified electrode are some advantages of over all other electrodes. Therefore, the sol-gel modified electrodes provide a suitable electrode substrate for the preparation of the modified electrodes. Application of redox species as the electron mediator to modify electrodes is an interesting field in analytical chemistry. One of the most important effects of mediator is lowering the overpotential required for electrochemical reaction and enhancement of sensitivity and selectivity of the method. 14 Sol-gel technology has aroused great interest in the design and application of electrochemical sensors due to its simplicity, physical rigidity, transparency, porosity and permeability, versatility and flexibility in the fabrication procedure. 15,16 Silver and copper dispersed composite electrodes prepared by sol-gel derived ceramic graphite containing 3-mercaptopropyl trimethoxysilane have been used for simultaneous determination of ascorbic acid and dopamine. 17,18 A fluorescent sol-gel biosensor has been also used for ascorbic acid determination. 19 The modified electrodes prepared with sol-gel technique have some disadvantageous such as low detection sensitivity and consuming specific and expensive reagents and materials for their preparation. Therefore, it is necessary to develop an electrochemical sensor based on sol-gel technique, which is free from the above problems. To the best of our knowledge, the modified electrodes prepared by sol-gel technique have not shown better performance in peak separation and simultaneous determination of AA, and neurotransmitters. Among the various mediators used for electrode modification, solid metal hexacyanoferrates (MHCFs) were used as suitable modifier due to excellent electron transfer properties. Therefore, various transition metal cations have been used with hexacyanoferrate to fabricate MHCF modified electrodes, where M represents iron(iii), 20,21 cadmium, 22 chromium, 23 cobalt, 24 copper, gallium, 28 indium, 29,30 lanthanum, 31 manganese, 32 molybdenium, 33 nickel, palladium, 37 platinum, 38 samarium(iii), 39 silver, 40 titanium dioxide, 41 vanadium, 42 zinc, 43,44 zirconium, 45 ruthenium oxide, 46 terbium, 47 cerium, 48 neodymium, 49 gadolinium 50 or yttrium. 51 Most of these modified electrodes have been used to reduce the overvoltage, overcome the slow kinetics of many electrode processes and determination of one species as an analytical application. To the best of our knowledge, Th(IV)-hexacyanoferrate ion association (THCF) has not been used as a mediator in construction of modified electrodes. In this work, the electrochemical behavior of a THCF sol-gel carbon composite electrode (CCE) prepared by mixing methyl trimethoxysilan (MTMOS) sol-gel precursor, carbon powder and Th(IV)-hexacyanoferrate ion pair was studied and its applicability in the simultaneous determination of ascorbic acid and dopamine from acidic medium was evaluated. EXPERIMENTAL Chemicals Th(NO 3 ) 4.5H 2 O, K 4 Fe(CN) 6, graphite fine powder, methyl trimethoxysilan (MTMOS) were obtained from Merck and ascorbic acid (AA), dopamine (DA), were obtained from Fluka and used as received. All other chemicals used in this investigation were of analytical grade. All solutions were prepared with twice distilled water. The buffer solution (0.1 M) was made up from H 3 PO 4 +KH 2 PO 4, and the ph was adjusted with 0.1 M H 3 PO 4 and 2.0 M KOH. AA and DA solutions were prepared just prior to use and all experiments were carried out at the ambient temperature of the laboratory (25 C). All solutions tested were deaerated by passing highly pure nitrogen (99.999%) before the electrochemical experiments and a continuous flow of nitrogen was maintained over the sample solution during the experiments. Apparatus The CV measurements were performed by using Autolab PGSTAT30 Potentiostat/Galvanostat equipped with a frequency response analyzer (FRA4.9) and controlled by general purpose electrochemical system (GPES4.9) software (Eco Chemie, Utrecht, The Netherlands). The cell used was equipped with a modified carbon paste disk as the working electrode, a graphite electrode as an auxiliary electrode and with a saturated calomel electrode (SCE) as a reference electrode. All potentials in the text are quoted versus this reference electrode. A personal computer was used for data storage and processing. The ph was measured with a Zag Chemie model PTR79 ph/mv meter. The morphology study was performed by LEO 440i (30 kv). Preparation of sol-gel carbon composite electrode To a 20 ml acidic aliquot of a 0.1 M aqueous thorium nitrate solution was slowly added a 20 ml potassium hexacyanoferrate solution (0.1 M) with a continuous stirring until the precipitation was completed. The resulting ion-pair precipitate was isolated by filtration, washed thoroughly with deionized water and dried at 40 C. The

3 1036 J. Chin. Chem. Soc., Vol. 55, No. 5, 2008 Farhadietal. composition of the formed ion pair was confirmed as Th[Fe(CN) 6 ] by the chemical analysis. A portion about 150 mg of prepared ion-pair was added to 3.5 ml MTMOS solution (10% V-V in methanol) containing 45 LHCl11M and the gel mixture was stirred for 2 min. Two milliliters of this homogenous gel was mixed well with graphite powder (380 mg) for more than 10 min. The resulting ion-pair solgel composite was packed tightly into the cavity (3.0 mm inner diameter) of a Teflon tube with a copper wire base for contact and dried under ambient conditions (25 C) for 30 h. The surface of the modified electrode was polished with a smooth weighing paper and rinsed well in ultra-pure water. The ceramic composite did not shrink on drying and exhibited good adhesion to the Teflon capillary wall and to the copper wire base. An unmodified composite was prepared in a similar fashion without the addition of ion-pair. After polishing, the surface of the Th(IV)-HCF carbon composite electrode (THCF-CCE) and the unmodified CCE were smooth and shiny. RESULTS AND DISCUSSION Electrochemical Behavior of THCF-CCE According to the previous studies, there is no report on the study of the electrochemical properties and specially the electrocatalytic activity of Th(IV)-HCF modified solgel carbon-ceramic electrode in aqueous media. Therefore, we were interested in studying the electrochemical behavior of THCF-CCE in buffer aqueous solutions. Cyclic voltammograms of THCF-CCE in 0.l M phosphate buffer solution (ph 3.0) at different low scan rates are shown in Fig. 1A. As can be seen, the cyclic voltammograms exhibit a pair of well-defined anodic and cathodic peaks correspond to Fe(CN) 6 3- /Fe(CN) 6 4- redox system (with E pa =0.40V,E pc = V, E 1/2 = V vs. Ag/AgCl, KCl sat and Ep = 150 mv, potential scan rate = 20 mv s -1 ). The cyclic voltammogram of unmodified CPE in pure supporting electrolyte shows no anodic or cathodic peaks. The plots of the anodic and cathodic peak currents were linearly dependent on Fig. 1. (A) Cyclic voltammograms of THCF-CCE in 0.1 M phosphate buffer (ph 3) at various scan rates: curves 1-5 correspond to 10, 20, 30, 40 and 50 mv s -1 scan rates, respectively. (B) Variations of I p versus scan rates. (C) Variation of E p versus the logarithm of the high scan rates.

4 Electrochemistry of Th(IV)-Hexacyanoferrate Sol-Gel J. Chin. Chem. Soc., Vol. 55, No. 5, square root of the sweep rate ( 1/2 ) with a correlation coefficient of at all scan rates ( mv s -1 ) (Fig. 1B). This behavior indicates that the nature of redox process is diffusion controlled. A potential scan rates higher than 40 mv s -1, peak separation increases, indicating the limitation arising from charge transfer kinetics (Fig. 1C). The effect of ph on the electrochemical behavior of the THCF-CCE was investigated by cyclic voltammetry in 0.1 M phosphate buffer solution of various ph values (2-10) containing 0.1 M KCl as supporting electrolyte. The obtained results showed a slightly decrease of the current response with increasing ph value due to a lower stability of the hexacyanoferrate-th(iv) association. The mediator coating is stable at ph lower than 6.0. The stability of THCF-CCE as the rate of loss of electrochemical activity was examined by repetitive scans in a 0.l M phosphate buffer solution (ph 3.0). This rate was evaluated by noting any decrease in anodic or cathodic charge in consecutive potential scan cycles. The obtained results showed that the anodic and cathodic peak currents of THCF-CCE decrease 5% after 100 cycles at potential scan rates 20 mv s -1 without any considerable in potential peak separation. Furthermore, no change in activity of THCF-CCE was observed after storing in air for two months. An approximate estimate of the surface coverage of the electrode was made by adopting the method used by Laviron. 52 According to this method, total amount of charge (Q) is related to the surface concentration of electroactive species, *, by the following equation: * Q (1) nfa where n represents the number of the electrons involved in reaction, A is the surface area of the electrode, * (mol cm -2 ) is the surface coverage. Q is a total amount of the charge which can be obtained from the integration of cyclic voltammetry peak, and F represents Faraday constant. The average amount of electrode surface coverage for anodic peaks recorded at 40 mv s -1 (as a low scan rate) with surface area cm 2 was (mol cm -2 ) for n = 1. At higher potential scan rates, the peak separation increases and the apparent coverage is slightly diminished probably due to drop in charge transfer through a modified layer, which becomes rate-limiting at higher scan rates. The charge transfer coefficient,, and the heterogeneous charge transfer rate constant, k s, of a surface-confined redox couple can be evaluated from cyclic voltammetric experiments and using the variation of anodic and cathodic peak potentials with scan rate, according to the procedure of Laviron. The obtained results showed that the E p values are linearly proportional to log, for scan rates higher than 100 mv s -1 ( E p > 200/n mv) (see Fig. 1B). Under these conditions, the following equations could used to determine the electron transfer rate constant between hexacyanoferrate (HCF)-Th(IV) and CCE: Epa Eo Aln[ ( 1 ) ] m Epc Eo Bln[ ] m logk s log( 1 ) ( 1 )log RT nf E p log( ) ( 1 )( ) (4) nfv 23. RT RT RT where: A, B, m RT K s ( )( ) ( 1 ) nf nf F nv According to equations 2 and 3, the plot of E p -E o =f(log ) yields two straight lines with a slope equal to 2.3 RT/ nf for the cathodic peak and 2.3 RT/(1- )nf for anodic peak. Using these slopes and also equation (4), the values of and k s were found to be 0.53 and 3.10 ± 0.1 s -1. Electrocatalytic oxidation of AA at THCF-CCE Figure 2 shows the CVs obtained from the oxidation of AA at unmodified CCE and THCF-CCE. The curves (A) and (B) correspond to 0.1 M buffer phosphate solution (ph 3) in the presence of M AA at unmodified CCE and THCF-CCE, respectively. As can be seen, at the CCE with- Fig. 2. CVs for (A) M AA at CCE without mediator, (B) M AA at THCF-CCE in 0.1 M phosphate buffer ph = 3; scan rate: 20 mv s -1. (2) (3)

5 1038 J. Chin. Chem. Soc., Vol. 55, No. 5, 2008 Farhadietal. out mediator electrode, the CV exhibited a broad peak at potential of 600 mv with poor current response for AA oxidation. In contrast, at the THCF-CCE, the oxidation current increased greatly at a reduced peak potential of 310 mv and the corresponding cathodic peak disappeared on the reverse scan of potential. The anodic peak current increased with the increase in the concentration of AA. The significant negative shift in the peak potential about 300 mv and the enhanced current response observed with the THCF- CCE compared to unmodified CCE indicate a strong electrocatalytic effect of Fe(CN) 6 3- /Fe(CN) 6 4- redox couple in the THCF-CCE towards the oxidation of AA. Fig. 3A shows cyclic voltammograms of THCF-CCE in 0.1 M buffer solution (ph 3.0) in the presence of different concentrations of ascorbic acid. Also, a typical calibration curve with a correlation coefficient near to 1 and a wide linear dynamic rang between and MAAisshowninFig.3B. Electrocatalytic oxidation of DA at THCF-CCE In Fig. 4, curves (A) and (B) correspond to electrooxidation of M DA at CCE without mediator and THCF-CCE, respectively. From these CVs, it can be observed that at the THCF-CCE, the oxidation current increased greatly with a decrease in oxidation potential about 150 mv in comparison to unmodified CCE. These observations showed an efficient electrocatalytic activity for the oxidation of DA using proposed chemically modified carbon composite electrode. A good calibration curve with a wide linear range and high sensitivity ( to M) was obtained from the electrocatalytic of various concentration of DA at THCF-CCE (Figures not shown). Effect of ph on the oxidation of AA and DA The influence of solution ph on the electrochemical response of the THCF-CCE towards the oxidation of AA and DA was studied in ph range from 2 to 9. The obtained results showed that the peak potential for both AA and DA shifted negatively with the increase in ph of the electrolyte. Also, the current responses for AA and DA oxidation were high at acidic ph conditions. The difference between potential shifts for AA and DA oxidation were nearly constant at all tested ph. The obtained current responses were diminished with the increase in ph of the electrolyte. Since the proposed THCF-CCE showed excellent stability as well as reproducible signals in acidic solutions, so, the ph of solutions was fixed at 3.0 for studying the electrocatalytic activity of AA and DA. Chronoamperometric Studies for AA and DA Double potential step chronoamperometry, as well as other electrochemical methods was employed for investigation of electrochemical process at the chemically modified electrodes. Fig. 5 shows the current time curve of THCF-CCE obtained by setting the working electrode potentialat0.60v(atthefirstpotentialstep)andat0.00v(at second potential step) vs. Ag/AgCl for the various concentration of AA in buffered aqueous solutions (ph 3) and also for modified electrode in the absence of AA. Similar figures were obtained from the electrooxidation of DA at THCF-CCE (not shown). For an electroactive material with a diffusion coefficient (D), the current corresponding to the electrochemical reaction (under diffusion controlled) Fig. 3. CVs for (A) different concentration of AA at THCF-CCE (B) calibration curve for ascorbic acidinrange to M respectively in 0.1 M phosphate buffer ph = 3; scan rate: 20 mv s -1. Fig. 4. CVs for (A) M DA at unmodified electrode and (B) M DA at THCF-CCE in 0.1 M phosphate buffer ph = 3; scan rate: 20 mv s -1.

6 Electrochemistry of Th(IV)-Hexacyanoferrate Sol-Gel J. Chin. Chem. Soc., Vol. 55, No. 5, is described by Cottrell equation: 53 I=nFAC 0 D 1/2-1/2 t -1/2 (5) where D is apparent the diffusion coefficient (cm 2 s -1 )and C 0 is apparent the bulk concentration (mol cm -3 ), respectively. Diffusion coefficient is easily calculated from the slop of the plot of I vs. t -1/2. The obtained chronoamperomgrams using modified electrode for solution containing AA and/or DA represented a typical I-t -1/2 curves which indicates that the observed currents have a diffusion control nature. Inset of Fig. 5 illustrate these phenomena. The mean value of the D for ascorbic acid and dopamine were found and (cm 2 s -1 ), respectively. Simultaneous determination of DA and AA Fig. 6 shows the CVs of a mixture of AA and DA at THCF-CCE in 0.1 M phosphate buffer solution (ph 3). As can be seen, two well-defined and separated anodic peaks at the potential of 305 and 400 mv were appeared for electrocatalytic oxidation of mixture MAAand M DA at the proposed THCF-CCE, respectively. The difference between the two peak potentials is about 100 mv and the current response signals increase with the increase in the concentration of AA and/or DA. In fact, AA with a pka of 4.17, can be exist mainly as neutral form in the experimental conditions (ph 3.0) and DA exists in its cationic form resulted from protenation of amino group. Therefore, it may be stated that due to hydrophobic properties of solgel composite, the interaction between neutral AA and mediator is very high in comparison to protonated DA and causes to a negative shift in anodic potential of AA. For the further investigations, the morphology of THCF-CCE was examined with a scanned electron microscope (LEO 440i) operating at 30 kv. The samples were mounted on a metal stub with double adhesive tape and coated under vacuum with gold in an argon atmosphere prior to observation. The response of THCF-CCE is probably related to morphology of the trimethoxysilan (MTMOS) sol-gel and therefore, it is an important factor affecting its performance. Fig. 7 shows the morphologies of MTMOSsol-gel characterized by SEM. The SEM of the trimethoxysilan (MTMOS) sol-gel surface displays a chemically clean three-dimensional uniform porous structure. The aggregates of the sol-gel matrix on the electrode surface produced a very narrow particle size distribution. This uniform open structure probably provides a significant increase of effective electrode surface for electrocatalytic AA in presence of DA. CONCLUSIONS The Th(IV)-HCF film was deposited on the carbon composite ceramic electrode, using a new and simple method compared with those reported previously. A modified car- Fig. 5. Chronoamperograms obtained at THCF-CCE in the absence (1) and presence of 2) 1, 3) 1.5 mm of AA in 0.1 M phosphate buffer solution (ph 3.00). First and second potential steps were 0.50 and 0.00 V versus Ag/AgCl, respectively. InsetA:PlotofIversust -1/2 for ascorbic acid obtained from chronoamperograms 1 and 2. Fig. 6. Oxidation of MAAinpresenceof M DA at THCF-CCE in 0.1 M phosphate buffer (ph 3); scan rate: 20 mv s -1.

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