Functional Interface of Ferric Ion Immobilized on Phosphonic Acid

Feb 5, 2009 - José Luis Olloqui-Sariego , Galina S. Zakharova , Andrey A. Poloznikov , Juan José Calvente , Dmitry M. Hushpulian , Lo Gorton , and R...
0 downloads 0 Views 376KB Size
Download PDF
3746

J. Phys. Chem. C 2009, 113, 3746–3750

Functional Interface of Ferric Ion Immobilized on Phosphonic Acid Terminated Self-Assembled Monolayers on a Au Electrode for Detection of Hydrogen Peroxide Yu Chen,† Feng-Bin Wang,‡ Li-Rong Guo,† Li-Min Zheng,*,† and Xing-Hua Xia*,‡ State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, and Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: NoVember 13, 2008; ReVised Manuscript ReceiVed: December 25, 2008

It is reported that inorganic ferric ion immobilized onto a phosphonic acid terminated functional interface can communicate an electron with the electrode and its electrochemistry is very similar to that of the enzymes (proteins) containing heme groups (such as microperoxidase, horseradish peroxidase, hemoglobin, and myoglobin). For example, its formal potential (E0′) is very close to that of the above-mentioned enzymes (proteins) in neutral solution; analogous to the direct electrochemistry of enzymes (proteins), the formal potential and electron-transfer rate of the immobilized inorganic ferric ion show a strong dependence on solution pH in cyclic voltammetry measurements. In comparison to the traditional enzyme (proteins)-based electrochemical biosensors, the ferric ion modified electrode shows more prominent electrocatalytic activity toward the reduction of hydrogen peroxide due to its high loading, fast electron-transfer rate, and excellent selection toward the reduction of hydrogen peroxide. Importantly, the iron ion, as an inorganic material, is very stable in high temperature and economical to produce. Therefore, the kind of iron ion modified electrode can be used to construct a new generation of biosensors with high performance, and it is very hopeful to substitute for enzyme-based biosensors for detection of hydrogen peroxide. 1. Introduction In recent years, the direct electrochemistry of redox proteins has received considerable attention due to fundamental importance in life sciences and potential application in the fabrication of biosensors, bioelectronics, biomedical devices, and biofuel cells. Many redox enzymes (proteins) containing iron element in their reaction centers (such as microperoxidase,1 horseradish peroxidase,2 hemoglobin,3 and myoglobin4) have been widely used to fabricate electrochemical biosensors for detection of H2O2 due to their remarkable advantages such as high substrate specificities and high efficiency. However, these enzymes (proteins) are very expensive and often subjected to denature in some extreme condition (such as high temperature, high/or low pH). Although numerous iron oxide nanocrystallines,5 metal porphyrins,6 metallophthalocyanine,7 and other metalloorganic complexes8 have been used extensively in the biomimetic studies, up to now, there are only a few studies dedicated to exploring the direct electrochemistry of inorganic iron ion.9 In the case of iron oxide nanoparticles, the transformation of the FeIII/FeII redox couple is hard to confirm by spectroscopic characterizations because only a small amount of iron oxide surface species at the surface of iron oxide nanoparticles gives electrochemical activity. In this paper, we discover that simple iron(III) ion immobilized onto a phosphonic acid terminated functional interface can communicate an electron with the electrode very quickly and its electrochemistry is very similar to that of the abovementioned enzymes (proteins). Meanwhile, X-ray photoelectron * To whom correspondence should be addressed. E-mail: lmzheng@ nju.edu.cn (L.-M.Z.); [email protected] (X.-H.X.). Fax: +86-25-83314502. † State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute. ‡ Key Laboratory of Analytical Chemistry for Life Science.

spectroscopy (XPS) measurements confirm the transformation of the FeIII/FeII redox couple. In addition, the iron(III) ion modified electrode shows a prominent electrocatalytic activity toward the reduction of hydrogen peroxide. The present functional interface possessing high sensitivity, high selectivity, and high catalytic activity can be used to construct a new generation of sensors with high performance. 2. Experimental Section 2.1. Materials. All the chemicals were of analytical grade (AR) and used without further purification. The 5 mM phosphate buffer solution (PBS, pH 7.0) containing 250 mM KCl was prepared by using K2HPO4, KH2PO4, and KCl. 3-Mercaptopropylphosphonic acid [HS-(CH2)3-PO3H2, MPPA] was prepared according to the modified procedure by using 3-bromopropylene as the starting material.10 2.2. Preparation of the MPPA/Au Electrode. After a Au electrode (CHI, 2 mm in diameter) was pretreated by the oxidation/reduction cycling in the potential region of hydrogen and oxygen evolution in a 0.5 M H2SO4 solution, it was immersed in a 4 mM MPPA solution in the dark for 24 h at room temperature (20 ( 2 °C), followed by thoroughly rinsing with Millipore water, 0.1 M KOH, 0.1 M HClO4, and Millipore water, consecutively. The process ensures the formation of a monolayer of MPPA on the Au electrode. 2.3. Preparation of the FeIII/MPPA/Au Electrode. Immobilization of iron(III) ion on the MPPA/Au electrode was accomplished by immersing the MPPA/Au electrode in a FeCl3 solution (50 mM, containing 100 mM HCl, pH 1.03) for 20 min at room temperature (20 ( 2 °C). After the FeIII/MPPA/ Au electrodes were rinsed with 100 mM HCl and Millipore water, consecutively, electrochemical measurements were carried out.

10.1021/jp809998h CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

Ferric Ion Modified MPPA/Au Electrode

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3747

Figure 1. (A) Schematic representation of the fabrication procedure of a FeIII/MPPA/Au electrode. XPS spectra of the (a) MPPA/Au and (b) FeIII/MPPA/Au electrodes in the (B) Fe2p and (C) P2p region.

2.4. Apparatus. The cyclic voltammetry (CV) measurements were performed by using a CHI 660 C electrochemical analyzer. All potentials refer to the saturated calomel reference electrode (SCE). The electrolyte was purged with high-purity nitrogen for at least 10 min prior to measurements for removing the dissolved oxygen. All electrochemical experiments were performed at room temperature (20 ( 2 °C). The high-resolution XPS measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer. The binding energy was calibrated by the Au 4f7/2 peak energy of 84.0 eV. 3. Results and Discussion 3.1. Characterization of the FeIII/MPPA/Au Electrode. The MPPA-modified Au electrode (MPPA/Au) and iron(III) ion modified MPPA/Au electrode (FeIII/MPPA/Au) were obtained by the self-assembly method (Figure 1A). Phosphonic acid (-PO3H2) groups are good complex-forming reagents.11 They can interact strongly with metal ion mostly through the formation of stable M-O-P bonds to produce metal phosphonate with low solubility;12 therefore, successful immobilization of iron(III) on the -PO3H2 group terminated functional interface is obtained. As revealed by the XPS measurements, the appearance of the FeIII2p1/2 peak (711.6 eV) and FeIII2p3/2 peak (725.1 eV) confirms the successful immobilization of inorganic iron(III) ion on the surface of the MPPA/Au electrode (Figure 1B). The phosphonic acid groups of the MPPA/Au electrode are completely protonated in the FeCl3 solution containing 100 mM HCl (pH 1.03). Therefore, the electrostatic attractions between -PO3H2 groups and iron(III) ion is ruled out. As shown in Figure 1C, the P2p peak of the FeIII/MPPA/Au electrode was negatively shifted with comparison to the MPPA/Au electrode, further indicating that the interaction between -PO3H2 groups and inorganic iron(III) is coordination rather than electrostatic attraction. In the absence of an MPPA monolayer, the iron(III) ion cannot adsorb onto a bare Au electrode, which has been verified by electrochemical measurement (data not shown). 3.2. Direct Electron Transfer of the FeIII/MPPA/Au Electrode. The cyclic voltammograms (CVs) of the FeIII/MPPA/ Au electrode show a pair of well-defined and nearly reversible redox peaks (E0′ ) -365 ( 7 mV, ∆Ep ) 105 ( 10 mV, ipa ≈

ipc) at a scan rate of 0.2 V s-1 (Figure 2A). The formal potential (E0′) is very close to that of the above-mentioned enzymes (proteins) in similar experiment condition.1-4 In comparison with the result of the MPPA/Au electrode, it is safe to state that the redox peaks of the FeIII/MPPA/Au electrode originate from the characteristics of the FeIII/FeII redox couple. In addition, after the FeIII/MPPA/Au electrode was electrolyzed at -0.45 V, appearance of the FeII2p1/2 peak (711.6 eV) and FeII2p3/2 peak (725.1 eV) confirms the successful transformation between iron(III) ion and iron(II) ion (Figure 2B). With the increase of scan rate, the E0′ of the FeIII/MPPA/Au electrode is almost unchanged, while the anodic and cathodic peak currents increase linearly (inset in Figure 2A), showing a surface-controlled electrode reaction. In this process, the surface concentration (Γ) of the iron(III) ion is estimated to be 1.8 × 10-10 mol cm-2 by integrating the CV oxidation peak and applying the formula of Q ) nFAΓ, where Q is the integrated charge of the oxidation peak, n is the number of electrons (assuming n ) 1), A is the effective area of the bare Au electrode (0.0732 cm2), and F is the Faraday constant. When taking into account the hydrated diameters of Fe(H2O)63+ ion (d ) 0.90 nm), the theoretical monolayer coverage (Γ*) for close-packed iron(III) ion is 1.87 × 10-10 mol cm-2, which is close to the present coverage of iron(III) ion on the MPPA/Au electrode (i.e., 1.80 × 10-10 mol cm-2). In comparison to the above-mentioned enzymes (proteins), it is worth noting that this value is almost 10 times higher than the theoretical monolayer coverage of these enzymes (proteins) due to the smaller diameter of inorganic iron(III) ion (e.g., the theoretically compacted monolayer coverage for hemoglobin is 1.89 × 10-11 mol cm-2 3e,13). Thus, the high loading of ferric ion electroactive species can significantly improve the sensitivity of electrochemical signal and enhance electrocatalytic capacity. The charge-transfer rate constant (ks) of this redox couple is estimated from the difference of peak potential (∆Ep) based on Laviron’s approach for a diffusionless thin-layer voltammetry,14 giving an average ks value of 68.9 ( 6.0 s-1 at a scan rate of 10 V s-1. This value is much larger than those obtained for enzymes (proteins) immobilized on various electrode materials,1-4 which is likely attributed to the shorter electron-transfer path of iron(III) to electrode as

3748 J. Phys. Chem. C, Vol. 113, No. 9, 2009

Chen et al.

Figure 2. (A) CVs of the FeIII/MPPA/Au electrode in 5 mM pH 7.0 phosphate buffer containing 250 mM KCl at 20 °C. The scan rates are 1.0, 0.8, 0.6, 0.4, and 0.2 V s-1 from outer to inner. The innermost line is the voltammogram of an MPPA/Au electrode at a scan rate of 0.2 V s-1. Inset: plots of the corresponding cathodic and anodic peak currents vs scan rate. (B) XPS spectra of the FeIII/MPPA/Au electrode (a) without the pretreatment and (b) with the pretreatment of potentiostatic polarization at -0.45 V in 5 mM PBS (pH 7.0) for 10 s in the Fe2p region. (C) Anodic peak current (ipa) of the FeIII/MPPA/Au electrode as a function of immersion time of the MPPA/Au electrode in FeIII ion solution. (D) Dependence of the ∆Ep of the FeIII/MPPA/Au electrode on the ionic strength of the buffer solution (the ionic strength was adjusted with suitable amounts of 1 M KCl).

compared to that of the heme center of enzymes (proteins) to the electrode because their heme centers are usually buried deeply in the large three-dimensional structure of enzymes (proteins). Figure 2C shows the effect of FeIII ion immersion time on oxidation peak current for the FeIII ion immobilized on the MPPA/Au electrode. A rapid increase in peak current is observed when the immersion time increases from 0.13 to 20 min and then reaches a near plateau from 20 to 120 min. Therefore, 20 min was chosen as the immersion time for FeIII ion adsorption in this work. Figure 2D shows the influence of ionic strength of solution on the ∆Ep values of the FeIII/MPPA/ Au electrode. It is clear that increase of ionic strength results in decrease of the ∆Ep values, indicating electron-transfer rate of the immobilized FeIII ion increases with the increase of the solution’s ionic strength since smaller ∆Ep corresponds to larger electron-transfer rate. The present results demonstrate that the change of activity coefficients of the immobilized FeIII ion plays an important role in determining the redox parameters of the immobilized FeIII ion. Analogous to the direct electrochemistry of those enzymes (proteins),1-4 CVs of the FeIII/MPPA/Au electrode also show a strong dependence on solution pH (Figure 3A). The formal potential E0′ of the FeIII/MPPA/Au electrode shifts linearly with a slope of -61 mV pH-1 in the pH range of 5.0-10.7 (Figure 3B), suggesting that one proton transfer is coupled to each electron transfer in the electrochemical reaction. Moreover, change of the peak potential with pH is reversible. The continuous CVs of the FeIII/MPPA/Au electrode show relative stability, indicating the fast breakdown of the Fe-O-P bond

(i.e., FeIII-O-P + H2O ) FeIIIOH + H+ + -O-P) is impossible. Therefore, the formation of hydroxo complexes (i.e., ionization of the water molecule coordinated to the central iron) may exclusively explain the pH-dependent CVs.

FeIII(O - P)x(H2O)y(OH)z ) FeIII(O P)x(H2O)y-1(OH)z+1 + H+

(chemical reaction)

(1)

FeIII(O - P)x(H2O)y-1(OH)z+1 + e- ) FeII(O P)x(H2O)y-1(OH)z+1

(electrochemical reaction)

(2)

where (O-P), (H2O), (OH) stand for ligands. In general, the iron(III) ion easily forms a six-coordination complex; therefore, x + y + z ) 6. It is well-known that the iron atom of the porphyrin center in heme proteins (such as microperoxidase, horseradish peroxidase, hemoglobin, and myoglobin) possesses a water molecule ligand, which is similar to that of iron(III) ion immobilized on MPPA. From the electrochemical results of the FeIII/MPPA/Au electrode, it is reasonably expected that ionization of the water molecule coordinated to the iron center of proteins rather than protonation/deprotonation of the propionate groups attached to the porphyrin center is mainly responsible for the pH dependency of the E0′ of heme proteins. In addition, ∆Ep of the FeIII/MPPA/Au electrode shows a strong dependence on solution pH. As shown in Figure 3C, the dependence of ∆Ep on solution pH shows a bell-shaped form. It reaches a maximum near pH 8.0 to ∼9.4. With increasing pH value, the ionization degree of the water molecule coordi-

Ferric Ion Modified MPPA/Au Electrode

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3749

Figure 3. (A) CVs of the FeIII/MPPA/Au electrode in 5 mM pH 7.0 phosphate buffer containing 250 mM KCl at different pH values of 6.05, 8.53, and 10.7 (from right to left) at a scan rate of 0.2 V s-1. (B) Effect of pH on the E0′. (C) Effect of pH on the ∆Ep.

Figure 4. (A) CVs of MPPA/Au (a and b) and the FeIII/MPPA/Au electrode (c and d) in the absence (a and c) and presence (b and d) of 0.3 mM H2O2 in 5 mM pH 7.0 phosphate buffer containing 250 mM KCl at a scan rate of 0.2 V s-1. (B) Amperometric responses of the MPPA/Au and FeIII/MPPA/Au electrodes to H2O2. Conditions: the electrode potential was controlled at -0.3 V; successive addition of 15.5 mM H2O2 to 10 mL of 5 mM pH 7.0 phosphate buffer containing 250 mM KCl under continuous stirring condition. Inset: plots of catalytic current at the FeIII/MPPA/ Au electrode vs H2O2 concentration. (C) Amperometric response of the Fe FeIII/MPPA/Au electrode on injection of 0.1 mM H2O2, 0.5 mM uric acid (UA), and 0.1 mM ascorbic acid (AA) to 10 mL of 5 mM pH 7.0 phosphate buffer containing 250 mM KCl at the applied potential of -0.3 V under continuous stirring condition.

nated to the central iron should increase because the iron(III) is easily hydrolyzed, resulting in a change in the metal ligand from water to hydroxy. This result could reflect a fact that the change

of the iron ligand makes a significant effect on the electrontransfer rate of the FeIII/MPPA/Au electrode, since the smaller ∆Ep value corresponds to a faster charge-transfer rate.

3750 J. Phys. Chem. C, Vol. 113, No. 9, 2009 3.3. Electrocatalytic Activity of the FeIII/MPPA/Au Electrode toward the Reduction of H2O2. Interestingly, the FeIII/ MPPA/Au functional interface shows electrocatalytic activity toward the reduction of hydrogen peroxide as in the case of heme proteins. As shown in Figure 4A, curve d, addition of H2O2 results in a significant increase of the cathodic peak current, while the anodic peak current decreases. We should bear in mind that the FeII/FeIII couple in solution (Fenton’s reagent) catalyzes the breakdown of H2O2 chemically.15 Although both iron(III) and iron(II) can catalyze the decomposition of H2O2, it is generally considered that the reaction rate between H2O2 and iron(II) is much higher than that of H2O2 and iron(III). Thus, though the exact electrocatalysis mechanism is not yet clear by far, we reasonably presume the electrochemically generated iron(II) at low potentials is responsible for the obvious reduction current of H2O2. Likely, the two-electron-two-proton reduction of H2O2 to give H2O on electrochemically generated iron(II) centers, which are being transformed into Fe(III). The Fe(III) is then electrochemically rereduced back into Fe(II), and the corresponding current is monitored.5c In addition, the amperometric response of the FeIII/MPPA/ Au electrode is significant and very quick upon addition of H2O2 (Figure 4B). The modified electrode achieves 95% of its maximum steady-state current in less than 0.7 s. Such a quick response originates from the rapid mass transport of analytes because the diffusion of reactants to the electrode surface almost is not blocked in the SAMs system. The corresponding calibration plot shows the current response and the concentration of H2O2 have a linear relationship in the concentration range from 1.5 × 10-6 to 2.7 × 10-4 M with a correlation coefficient of 0.9989 (inset in Figure 4B) and a detection limit of 4.0 × 10-7 M at a signal-to-noise ratio of 3. As comparison, performance of the present FeIII/MPPA/Au sensor for hydrogen peroxide detection is much better than that of hemoglobin immobilized on MPPA/Au biosensors under the same experimental conditions.3e On the other hand, the response of H2O2 on the MPPA/Au electrode is negligible (Figure 4B), indicating the observed amperometric responses on the FeIII/MPPA/Au electrode are due to the immobilized iron(III) ion. It is also found that other electroactive species of ascorbic acid (AA) and uric acid (UA) do not interfere the detection of hydrogen peroxide (Figure 4C), indicating the FeIII/MPPA/Au electrode possesses excellent selection toward the reduction of hydrogen peroxide. In addition, the FeIII/MPPA/Au interface shows highly thermal stability, e.g., after it was incubated at 50 °C for 1 h, the electrocatalytic activity of the electrode for H2O2 remained. 4. Conclusion In summary, it is reported that the immobilized inorganic ferric ion possesses intrinsic enzyme mimetic activity. The corresponding FeIII/MPPA/Au electrode shows prominent electrocatalytic activity toward the reduction of hydrogen peroxide due to its high loading, fast electron-transfer rate, and excellent selectivity toward the reduction of hydrogen peroxide. Importantly, the iron ion, as an inorganic material, is more stable than proteins in high temperature and easier or more economical to produce. This study shows that the ferric ion based sensor is

Chen et al. very hopeful to substitute for enzyme-based biosensors for detection of hydrogen peroxide. In addition, it is worth noting that the immobilization strategy of iron ion also shows a potential application in the electro-Fenton system16 due to the reversible electrochemical property of the immobilized iron(III) ion. Acknowledgment. This work is supported by the NSFC (Nos. 20775034, 20535010), the National Science Fund for Creative Research Groups (20521503, 20721002), and the National Basic Research Program of China (2007CB925102, 2007CB936404). References and Notes (1) (a) Xu, Z.; Gao, N.; Chen, H.; Dong, S. Langmuir 2005, 21, 10808. (b) Jiang, L.; Glidle, A.; McNeil, C. J.; Cooper, J. M. Biosens. Bioelectron. 1997, 12, 1143. (c) Behera, S.; Raj, C. R. J. Electroanal. Chem. 2008, 15, 159. (2) (a) Lu, H.; Rusling, J. F.; Hu, N. J. Phys. Chem. B 2007, 111, 14378. (b) Lyon, J. L.; Stevenson, K. J. Anal. Chem. 2006, 78, 8518. (c) Polsky, R.; Harper, J. C.; Dirk, S. M.; Arango, D. C.; Wheeler, D. R.; Brozik, S. M. Langmuir 2007, 23, 364. (d) Liu, S. Q.; Chen, A. C. Langmuir 2005, 21, 8409. (3) (a) Shan, D.; Han, E.; Xue, H.; Cosnier, S. Biomacromolecules 2007, 8, 3041. (b) Lu, Q.; Hu, C.; Cui, R.; Hu, S. J. Phys. Chem. B 2007, 111, 9808. (c) Zhang, Q.; Zhang, L.; Li, J. J. Phys. Chem. C 2007, 111, 8655. (d) Shi, G.; Sun, Z.; Liu, M.; Zhang, L.; Liu, Y.; Qu, Y.; Jin, L. Anal. Chem. 2007, 79, 3581. (e) Chen, Y.; Jin, B.; Guo, L. R.; Yang, X. J.; Zheng, L. M.; Xia, X. H. Chem. Eur. J. 2008, 14, 10727. (f) Paddon, C. A.; Marken, F. Electrochem. Commun. 2004, 6, 1249. (4) (a) Zhang, H.; Hu, N. J. Phys. Chem. B 2007, 111, 10583. (b) Li, N.; Xu, J. Z.; Yao, H.; Zhu, J. J.; Chen, H. Y. J. Phys. Chem. B 2006, 110, 11561. (c) Guto, P. M.; Rusling, J. F. J. Phys. Chem. B 2005, 109, 24457. (5) (a) Gao, L.; Zhuang, J.; Nie, L.; Zhang, J. B.; Zhang, Y.; Gu, N.; Wang, T. H.; Feng, J.; Yang, D. L.; Perrett, S.; Yan, X. Y. Nat. Nanotechnol. 2007, 2, 577. (b) Hrbac, J.; Halouzka, V.; Zboril, R.; Papadopoulos, K.; Triantis, T. Electroanalysis 2007, 19, 1850. (c) Lin, M. S.; Leu, H. J. Electroanalysis 2005, 17, 2068. (d) Kruusma, J.; Mould, N.; Jurkschat, K.; Crossley, A.; Banks, C. E. Electrochem. Commun. 2007, 9, 2330. (e) Zhang, L. H.; Zhai, Y. M.; Gao, N.; Wen, D.; Dong, S. J. Electrochem. Commun. 2008, 10, 1524. (f) Sljukic, B.; Banks, C. E.; Crossley, A.; Comptona, R. G. Electroanalysis 2006, 18, 1757. (6) (a) Shiryaeva, I. M.; Collman, J. P.; Boulatov, R.; Sunderland, C. J. Anal. Chem. 2003, 75, 494. (b) Pilloud, D. L.; Chen, X. X.; Dutton, P. L.; Moser, C. C. J. Phys. Chem. B 2000, 104, 2868. (7) Ozoemena, K. I.; Nyokong, T. Electrochim. Acta 2006, 51, 2669. (8) Gobi, K. V.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 1998, 444, 145. (9) Milsom, E. V.; Dash, H. A.; Jenkins, T. A.; Halliwell, C. M.; Thetford, A.; Bligh, N.; Nogala, W.; Opallo, M.; Marken, F. J. Electroanal. Chem. 2007, 610, 28. (10) (a) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. (b) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (11) (a) Yang, Y. F.; Ma, Y. S.; Guo, L. R.; Zheng, L. M. Cryst. Growth Des. 2008, 8, 1213. (b) Cao, D. K.; Li, Y. Z.; Zheng, L. M. Inorg. Chem. 2007, 46, 7571. (12) (a) Konar, S.; Bhuvanesh, N.; Clearfield, A. J. Am. Chem. Soc. 2006, 128, 9604. (b) Marcinko, S.; Fadeev, A. Y. Langmuir 2004, 20, 2270. (13) (a) Mimica, D.; Zagal, J. H.; Bedioui, F. Electrochem. Commun. 2001, 3, 435. (b) Shen, L.; Hu, N. Biomacromolecules 2005, 6, 1475. (c) Wang, S. F.; Chen, T.; Zhang, Z. L.; Shen, X. C.; Lu, Z. X.; Pang, D. W.; Wong, K. Y. Langmuir 2005, 21, 9260. (14) Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (15) (a) Malik, P. K. J. Phys. Chem. A 2004, 108, 2675. (b) Oliveira, R.; Almeida, M. F.; Santos, L.; Madeira, L. M. Ind. Eng. Chem. Res. 2006, 45, 1266. (16) Zhang, G. Q.; Yang, F. G.; Gao, M. M.; Liu, L. F. J. Phys. Chem. C 2008, 112, 8957.

JP809998H