Heterogeneous Consecutive Electron Transfer at Graphite Electrodes

Anal. Chem. 2010, 82, 8598–8603

Heterogeneous Consecutive Electron Transfer at Graphite Electrodes under Steady State Xiaoquan Lu,* Ping Sun, Dongna Yao, Bowan Wu, Zhonghua Xue, Xibing Zhou, Ruiping Sun, Li Li, and Xiuhui Liu Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China In this report, the theory based on thin-layer cyclic voltammetry (TLCV) for consecutive electron transfer (ET) across the interface between two immiscible electrolyte solutions (ITIES) is well developed and experimentally verified. The voltammetric responses to multistep electron transfer at the ITIES are predicted by numerical simulations. Moreover, the impact of empirical parameters on the shape of the multistep current-voltage curve has been examined. The results obtained not only give information regarding the effect of the concentration ratio of the reactants in two phases and the thin-layer thickness on multistep electron transfer, but also prove the excellent agreement between simulations and experiments. The model system of two-step electron transfer of ZnTPP/ [Fe(CN)6]4- was studied, indicating that the BulterVolmer (B-V) theory is suitable for the consecutive electron transfer. Thus, TLCV is demonstrated to be a useful means for investigating the kinetics of heterogeneous consecutive ET. The electron transfer (ET) across the interface between two immiscible electrolyte solutions (ITIES) is one of the fundamental physicochemical processes known to occur in nature.1,2 The emerging interest in studies of ET processes at the ITIES during the last several years3-8 is not only due to its fundamental importance, but also a number of novel applications from artificial photosynthesis9 to liquid redox extraction.10 Electrochemistry at an ITIES has been proved to be an efficient tool to monitor the interfacial electron transfer. Consequently, * To whom correspondence should be addressed. Phone: +86-931-7971276. Fax: +86-931-7971323. E-mail: [email protected]. (1) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265–322. (b) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148–13168. (2) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984, 106, 3047–3049. (3) (a) Girault, H. H.; Schiffrin, D. J. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 1. (b) Girault, H. H. Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., White, R. E., Eds.; Plenum Press: New York, 1993; Vol. 25, p 1. (4) Liquid/Liquid Interfaces; Volkov, A. G., Deamer, D. W., Eds.; CRC Press: Boca Raton, Fl, 1996. (5) Shi, C.; Anson, F. C. Anal. Chem. 1998, 70, 3114–3118. (6) Shi, C.; Anson, F. C. Anal. Chem. 2001, 73, 337–342. (7) Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1996, 100, 17881–17888. (8) Liu, B.; Mirkin, M. V. J. Am. Chem. Soc. 1999, 121, 8352–8355. (9) Sun, K.; Mauzerall, D. J. Phys. Chem. B. 1998, 102, 6440–6447. (10) Clark, J. F.; Clark, D. L.; Strauss, S. H. Environ. Sci. Technol. 1996, 30, 3124–3127.

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the development of electrochemical technique will play a key role in studying ET. In 1977, Samec et al.11 introduced the concept of a four-electrode potentiostat, which essentially led the way to the modern studies of both interfacial structures and charge transfer reactions across the ITIES. Based on conventional electrochemical methods by Schiffrin and Girault,12 the understanding of electron transfer kinetics at the liquid/liquid (L/L) interfaces has been developed. In the 1990s, scanning electrochemical microscopy (SECM) was adopted to study the processes of charge transfer by Bard and others.7,8,13,17 Meanwhile, some other electrochemical techniques, including ac impedance,18 thin-layer cyclic voltammetry (TLCV),5,19-24 microelectrochemical measurements at expanding droplets (MEMED),25 spectroscopic and photoelectrochemical tech(11) Samec, Z.; Marecˇek, V.; Koryta, J.; Khalil, M. W. J. Electroanal. Chem. 1977, 83, 393–397. (12) (a) Girault, H. H. J.; Schiffrin, D. J. J. Electroanal. Chem. 1984, 161, 415– 417. (b) Girault, H. H. J.; Schiffrin, D. J. J. Electroanal. Chem. 1988, 244, 15–26. (c) Geblewicz, G.; Schiffrin, D. J. J. Electroanal. Chem. 1988, 244, 27–37. (d) Cheng, Y.; Schiffrin, D. J. J. Electroanal. Chem. 1991, 314, 153– 163. (e) Cheng, Y.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2517–2523. (13) (a) Wei, C.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1995, 99, 16033– 16042. (b) Solomon, T.; Bard, A. J. J. Phys. Chem. 1995, 99, 17487–17489. (c) Ding, Z. F.; Quinn, B. M.; Bard, A. J. J. Phys. Chem. B 2001, 105, 6367– 6374. (d) Shao, Y.; Mirkin, M. V.; Rusling, J. F. J. Phys.Chem. B 1997, 101, 3202–3208. (e) Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2341–2347. (f) Zhang, J.; Barker, A. L.; Unwin, P. R. J. Electroanal. Chem. 2000, 483, 95–107. (14) Kim, J.; Xiong, H.; Hofmann, M.; Kong, J.; Amemiya, S. Anal. Chem. 2010, 82, 1605–1607. (15) Wang, L. Q.; Kranz, C.; Mizaikoff, B. Anal. Chem. 2010, 82, 3132–3138. (16) Lu, X. Q.; Li, M. R.; Yang, C. H.; Zhang, L. M.; Li, Y. F.; Jiang, L.; Li, H. X.; Liu, C. M.; Hu, W. P. Langmuir 2006, 22, 3035–3039. (17) Lu, X. Q.; Zhang, H. R.; Hu, L. N.; Zhao, C. Y.; Zhang, L. M. Electrochem. Commun. 2006, 8, 1027–1034. (18) Cheng, Y.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1993, 89, 199– 205. (19) (a) Shi, C.; Anson, F. C. J. Phys. Chem. B 1998, 102, 9850–9854. (b) Barker, A. L.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2330–2340. (c) Shi, C.; Anson, F. C. J. Phys. Chem. B 2001, 105, 8963–8969. (20) Lu, X. Q.; Hu, L. N.; Wang, X. Q. Electroanalysis 2005, 17, 953–958. (21) Liu, X. H.; Hu, L. N.; Zhang, L. M.; Liu, H. D.; Lu, X. Q. Electrochim. Acta 2005, 51, 467–473. (22) Lu, X. Q.; Nan, M. N.; Zhang, H. R.; Liu, X. H.; Yuan, H. Q.; Yang, J. J. Phys. Chem. C 2007, 111, 14998–15002. (23) Liu, X. H.; Dong, C. W.; Zhang, K.; Zhi, F. P.; Ding, Z.; Lu, X. Q. Chin. Chem. Lett. 2009, 20, 1115–1118. (24) Liu, X. H.; Zhang, L. M.; Hu, L. N.; Lu, X. Q. J. Chin. Anal. Chem. 2006, 1, 135–139. (25) Liu, X. H.; Yang, J.; Zuo, G. F.; Zhang, K.; Dong, C. W.; Lu, X. Q. J. Phys. Chem. C 2008, 112, 148–152. 10.1021/ac1016997  2010 American Chemical Society Published on Web 09/20/2010

niques,26 also have been employed to study ET processes. Among these teachniques, SECM, one of the newest and the most powerful experimental methods, resolves many of the difficulties associated with traditional electrochemical methods including the IR drop, charging current, and distinction between electron transfer and ion transfer processes.13a Numerous applications of this method have been published.13-16 However, this technique also has its own limitations, due mainly to the construction of the apparatus and the necessity to simulate the kinetic parameters by comparison of experimental and theoretical data. Furthermore, a possible drawback for SECM is that it is essentially indistinguishable from consecutive ET when single-step ET is being analyzed by feedback mode. Thus, this has limited the theoretical treatment of SECM to heterogeneous multistep ET. In contrast to SECM, TLCV reported by Anson and Shi5 is a less complex approach. The remarkable features of this method, especially its simplicity and requirement of a limited amount of reactant, make it suitable for the study of the electrochemical properties of a highly hydrophobic compound. Also, no ion transport across the interface accompanies the cross-phase electron transfer when steady-state currents are measured.19a In general, the interfacial ET is classified into the one-step ET and the multistep ET. Compared with the single-step ET, the multistep ET is more complex and more important. Furthermore, in nature, almost every life cycle involves the process of multistep ET, for example, photosynthesis, the respiratory chain, free-radical scavenging antioxidants,27 and so forth. Although there is a few report28-30 concerning multistep electron transfer, no theoretical treatment has been reported until now. With this motivation, a new theory based on TLCV for the evaluation and identification of cross-phase multistep ET is proposed in the present study. The use of the method merits consideration for several reasons. First, the kinetics analysis could be achieved at low solute concentrations. Second, the method is readily adaptable to existing instrumentation capable of cyclic voltammetry. Third, the data display format is familiar to electrochemists who currently make extensive use of cyclic voltammetry for compound characterization. At the same time, we provide the theoretical basis of TLCV to determine the multistep interfacial electron transfer. The effect of empirical parameters on the shape of the current-voltage curve will be discussed. Moreover, the applicability of TLCV to obtain ET rate constants will be illustrated for the two-step ET of the Fe(CN)64-/ZnTPP. This system provides an interesting example of how the relatively new theory of TLCV is used to probe more complex biological redox chemistry. EXPERIMENTAL SECTION Chemicals and Apparatus. NaCl, NaClO4, K4Fe(CN)6 (AR, Beijing Chemical Reagent Co. Ltd.), and nitrobenzene (NB) (26) (a) Ding, Z. F.; Fermıń, D. J.; Brevet, P. F.; Girault, H. H. J. Electroanal. Chem. 1998, 458, 139–148. (b) Eugster, N.; Fermıń, D. J.; Girault, H. H. J. Phys. Chem. B 2002, 106, 3428–3433. (27) Osakai, T.; Jensen, H.; Nagatani, H.; Fermıń, D. J.; Girault, H. H. J. Electroanal. Chem. 2001, 510, 43–49. (28) Xu, J.; Frcic, A.; Clyburne, J. A. C.; Gossage, R. A.; Yu, H. Z. J. Phys. Chem. B 2004, 108, 5742–5746. (29) Wang, R.; Okajima, T.; Kitamura, F.; Matsumoto, N.; Thiemann, T.; Mataka, S.; Ohsaka, T. J. Phys. Chem. B 2003, 107, 9452–9458. (30) Helfrick, J. C., Jr.; Bottomley, L. A. Anal. Chem. 2009, 81, 9041–9047.

Scheme 1. Simplified Two-Step Electron Transfer Process of Respiratory Chain in the Mitochondria and the Corresponding Reaction at the Electrodea

a

enzyme;

The coenzyme; the reductive enzyme; the oxiditive the semiquinone; the dihydroubiquinone.

(Shanghai Chemical Reagent Co. Ltd.) were of the highest purity and were used as received. Tetrabutylammonium perchlorate (TBAClO4) was synthesized according to ref 7, and 5, 10, 15, 20-tetraphenylporphyrin zinc (ZnTPP) was synthesized in our lab.31 All the electrochemical experiments were carried out in CHI-832 working station (CHI instrument Co. Ltd., Austin, TX). A three-electrode configuration (EPG, auxiliary and reference) was used in all experiments. The pyrolytic graphite electrode (EPG) with 0.30 cm2 of the edges of the graphite planes was prepared. Ag/AgCl/KCl reference electrode and Pt counter electrode was used. Experiments were conducted at room temperature (22 ± 2 °C). Procedures. The EPG was pretreated as described.5 The resulting surface was moderately hydrophobic as indicated by the failure of a drop of water to spread across its surface. When thin layers of NB solutions containing interest reactants (ZnTPP) and supporting electrolytes (TBAClO4) were applied to the graphite electrode held in an upside-down position by transferring 1.5 µL of the solutions to the electrode surface with a microsyringe, the organic liquid spread spontaneously across the surface of the graphite electrode. The electrode was then turned over and immediately immersed in the aqueous solution. Method. Numerical simulations were achieved using the explicit finite-difference method to calculate the current-potential behavior and the concentration profiles of the species in the two phases.32,33 The program for the simulation was written in MATLAB 7.0 by ourselves. Theoretical Analysis for Consecutive Interfacial Electron Transfer. Here, we employed the two-step redox reaction occurring in the living organism as an example. Scheme 1A shows the simplified ET process of a respiration chain in the mitochondria. At first, coenzyme Q was reduced to the semiquinone by reductive enzyme. Then, some of the semiquinone was reduced to dihy(31) Lu, X. Q.; Geng, Z. X.; Wang, Y. S.; Lv, B. Q.; Kang, J. W. Synth. React. Inorg. Met.-Org. Chem. 2002, 32, 843–851. (32) Barker, A. L.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2330–2340. (33) Britz, D. Digital Simulation in Electrochemistry, 2nd, revised, and extended ed.; Springer-Verlag: Berlin, Germany, 1988.


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droubiquinone, while the other semiquinone was resumed to coenzyme Q. Similarly, the dihydroubiquinone was oxidized to the semiquinone. The intricate reactions of Scheme 1A could be also simulated by the corresponding reaction at the electrode as shown in Scheme 1B. One redox-active species (A) contained in the organic thin layer is initially present on the electrode. Then, the A at the electrode is oxidized to B. At the same time, an aqueous containing the redox species (O) is insoluble in the organic thin-layer. When the first step ET process between the oxidized form (B) and reduced form (O) was finished, the first feedback cycle is established in which A is regenerated in the organic phase, at the interface between two immiscible electrolyte solutions, and can diffuse back to the electrode, enhancing the current signal. Subsequently, the C oxidized from B continued to be reduced by O, and the second step ET process was performed. The reactions were involved in the system implemented as eqs 1, 2, and 3: -e

-e

AfBfC (EPG)

(1)

B + O f A + O- (ITIES)

(2)

C + O f B + O- (ITIES)

(3)

The quantitative analysis theory of the TLCV is shown as the following equations (see the Supporting Information). 1 1 1 ) + iobs iD iET

(4)

* DNB1 /d iD ) nFAcNB

(5)

* * cH iET ) nFAketcNB 2O

(6)

* * * -1 DNB) + (nFAcNB )-1(cH )-1ket (iobs)-1 ) d/(nFAcNB 2O

(7) A plot of (iobs)-1 versus (cH2O)-1 should be linear with a slope (nFAc*NBket-1) that can be used to calculate ket and an intercept equal to (iD)-1, where C *NB is the initial bulk concentration of reactant contained in organic phase, C*H2O is the initial bulk concentration of reactant dissolved in the aqueous phase, ket is the bimolecular rate constant for the redox reaction at the interface between two immiscible electrolyte solutions, d is the thickness of the thin layer, n is the number of electrons transfer, F is Faraday’s constant, A is the electrode area, DNB is the diffusion coefficient of reactant in the organic, iD is a steady-state limit-diffusion current, iET is a characteristic current corresponding to the electron-transfer process across the phases, and iobs is the observed plateau current. However, it is noteworthy that the concentration of reactant contained in organic phase for the subsequent step ET process was no longer the initial bulk concentration. In other words, with the achievement of each step ET, the concentration of reactant contained in organic phase was gradually decreased. Consequently, for the multistep cross-phase ET, there is a great need 8600


Figure 1. Voltammetric observation of electron transfer between ZnTPP and K4Fe(CN)6. (A) Cyclic voltammogram for 10 mM K4Fe(CN)6 at an uncoated EPG electrode: supporting electrolyte, 0.1 M NaClO4 + 0.1 M NaCl. (B) Repeat of part A after the electrode surface was covered with 1.5 µL of NB. (C) Cyclic voltammogram with the electrode covered with 1.5 µL of NB containing 1 mM ZnTPP and supporting electrolyte (0.01 M TBAClO4). The aqueous solution contained only supporting electrolyte (0.1 M NaClO4 + 0.1 M NaCl). (D) Repeat of part C with 10 mM K4Fe(CN)6 present in the aqueous phase; 1.5 µL of NB containing 1 mM ZnTPP and supporting electrolyte (0.01 M TBAClO4). Scan rate, 5 mV s-1. The inset shows the cyclic voltammogram obtained at the EPG electrode with the solution of NB cotaining 1 mM ZnTPP and 0.01 M TBAClO4.

to measure the concentration of reactant in the organic phase for the subsequent step ET. It is clear that the calculation of concentrations of the organic species for the subsequent step is dependent on initial bulk concentration of reactant dissolved in the organic phase. This was calculated via eq 5, by determining the ratio of the diffusioncontrolled current (iD) of the voltammetric response under steady-state conditions. Provided that the concentration of c*NB1 for the first step can be assumed to be constant and equal to its initial bulk value, and that iD1 and iDn are calculated from the intercept of the plot of (iobs)-1 versus (cH2O)-1, the concentration of c*NBn for the subsequent step is obtainable as follows: iD1 iDn

)

* cNB n

* cNB 1

* cNB n iDn * ) × cNB (n > 1) 1 iD1

(8)

Therefore, when the concentration of the organic species for the subsequent step ET is be obtained by eq 8, the rate constant will be calculated by eq 7. RESULTS AND DISCUSSION Shown in Figure 1A is a cyclic voltammogram recorded at a bare edge plane EPG electrode in a 10 mM aqueous solution of Fe(CN)64- with 0.1 M NaClO4 + 0.1 M NaCl as the supporting electrolyte. When a thin layer of NB was placed on the electrode surface, the voltammetric response from the Fe(CN)63-/4- couple disappeared because the Fe(CN)64- anions could no longer reach the electrode surface (Figure 1B). Figure 1C shows the response obtained when ZnTPP was dissolved in the NB before the thin layer was formed on the electrode, while the aqueous solution contained only supporting electrolyte. The

Figure 2. (A) Simulated voltammograms for d ) 50 µm, DZnTPP ) DZnTPP+ ) DZnTPP2+ ) 8.5 × 10-6 cm2 s-1, DK4Fe(CN)6 ) 5.5 × 10-6 cm2 s-1, c*ZnTPP ) 0.55 mM, A ) 0.30 cm,2 υs ) 5 mV s-1, k1 ) 0.12 cm M-1 s-1, k2 ) 0.15 cm s-1 M-1, and Kr takes the values (a) 40, (b) 30, (c) 20, (d) 10. (B) Voltammetric observation of ET from 1 mM/L ZnTPP (supporting electrolyte, 0.01 M TBAClO4) in a thin layer of NB to Fe(CN)64- in an aqueous phase (supporting electrolyte, 0.1 M/L NaClO4 + 0.1 M/L NaCl), υs ) 5 mV s-1. c*Ox2 is (a) 40, (b) 30, (c) 20, (d) 10.

two reversible couples near 0.75 and 1.05 V correspond to the two-step electron oxidations of ZnTPP:34,35 ZnTPP f ZnTPP+ f ZnTPP2+. This response was quite stable: repetitive scanning of the potential between +1.1 and -0.1 V produced no significant decrease in the peak currents. When Fe(CN)64- was added to the aqueous phase, the voltammetric response was changed, as shown in Figure 1D. The two-well enhanced anodic currents at the potential where the ZnTPP+, ZnTPP2+ are reduced respectively to ZnTPP and ZnTPP+, which indicates that ZnTPP engages in cross-phase electron transfer to Fe(CN)64- anions in the aqueous phase. The inset is the voltammogram of NB solution containing 1 mM ZnTPP and 10 mM TBAClO4. The apparent formal potentials of the two pairs redox, as calculated from E0′NB ) (1/2)(Epa + Epc), are 695 mV (ZnTPP/ZnTPP+) and 1053 mV (ZnTPP+/ZnTPP2+), respectively, with respect to a Ag/AgCl reference electrode in the aqueous phase. The difference between the two (∆E ) 358 mV) is close to the solution electrochemistry of ZnTPP (∆E ) 880 - 528.5 ) 351.5 mV vs Ag/AgCl reference electrode). This is consistent with the result obtained by scanning electrochemical microscopy (SECM).7,36 Such a large formal potential difference is of great (34) Osakai, T.; Ichikawa, S.; Hotta, H.; Nagatani, H. Anal. Sci. 2004, 20, 1567– 1573. (35) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982–2991. (36) Sugihara, T.; Kinoshita, T.; Aoyagi, S.; Tsujino, Y.; Osakai, T. J. Electro. Chem. 2008, 612, 241–246.

interest for the study of ET kinetics across an ITIES, as it provides a unique opportunity for easy modulation of the overall driving force for interfacial electron transfer reactions. The appearance of anodic plateau instead of a current peak indicates that a steady-state concentration profile is generated within the NB layer. With a fixed concentration of ZnTPP in the NB phase, the concentrations of Fe(CN)64- in the aqueous phase were increased from 10 to 40 mM (Figure 2B). Values of iobs measured at several concentrations of Fe(CN)64- are plotted in Figure 3A. The fitted curve showed that iobs would become independent of the sufficiently high concentration of Fe(CN)64in the aqueous phase. That is, with the sufficiently high concentration of Fe(CN)64-, the concentration of ZnTPP and ZnTPP+ in the NB phase becomes negligibly small, and the corresponding anodic plateau current, iD, is limited by the diffusion of ZnTPP, ZnTPP+, and ZnTPP2+ across the NB layer. By using eq 7, a plot of iobs-1 versus CK4Fe(CN)6-1 is presented in Figure 3B. The plots are reasonably linear, and the slopes of the lines correspond to values of ket that are in reasonable agreement, 0.12 and 0.15 cm s-1 M-1, respectively. The intercept of the lines in Figure 3 corresponds to a value of 10.91 and 9.61 µA for iD, which is in agreement with the limiting value of the current at the highest concentration of Fe(CN)64- in Figure 3A. The overall driving force for the electron-transfer is composed of the difference in the formal potentials of the two redox couples in their respective phases and ∆wo φ, the Galvani potential difference present at the liquid/liquid interface.3a,19c The value of ∆wo φ was varied by changing the concentration of CClO4- (a potential-determining ion that moves across the liquid/liquid interface) in the aqueous phase while maintaining its concentration fixed in the nonaqueous phase. Under such conditions, the formal potential is expected to negative change by about W 4- 37 , which is shown 59 mV with each decade increase in CClO in Figure 4. The resulting rate constants for different concentrations of supporting electrolyte, given in Table 1, show that ket depends on ∆wo φ in the more dilute supporting electrolytes (0.01 M NaClO4) but is only less dependent on the changes in ∆wo φ as the supporting electrolyte concentration increases (0.1-1 M NaClO4). The analogical direction was observed in the process of one-step electron transfer between DMFc in the organic and Fe(CN)63- in the aqueous by Shi and Anson.19c Moreover, the overall driving force in Table 1 varies by about 400 mV, while the rate constants obtained for the various reactions differ by less than an order of magnitude. This apparent insensitivity might be rationalized by appealing to the likely diffuse nature of the region separating the pure (or mutually saturated) phases, as proposed by Schmickler.38,39 The formation of precursor complexes within the interfacial region40 could be a rate-determining step that would be influenced very little by the driving force for the subsequent, faster, electron-transfer step. The increase of the thermodynamic driving force leads to a faster electron transfer at the interface, which is consistent with the results of previous (37) Zhang, J.; Barker, A. L.; Unwin, P. R. J. Electroanal. Chem. 2000, 483, 95–107. (38) Schmickler, W. Interfacial Electrochemistry; Oxford University Press: New York, 1996; Chapter 12. (39) Markin, V. S.; Volkov, A. G. J. Colloid Interface Sci. 1989, 131, 382–392. (40) Girault, H. H. J.; Schiffrin, D. J. J. Electroanal. Chem. 1988, 244, 15–26.


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Figure 3. (A) (a) Variation of the steady-state currents (iobs) with the concentration of K4Fe(CN)6 in the aqueous phase for the first step ET; (b) variation of the steady-state currents (iobs) with the concentration of K4Fe(CN)6 in the aqueous phase for the second step ET. (B) (a) Reciprocal observed currents vs [K4Fe(CN)6]-1 for the first step ET; (b) reciprocal observed currents vs [K4Fe(CN)6]-1 for the second step ET. Table 1. Bimolecular Rate Constants for Two-Step Cross-Phase Electron Transfer between ZnTPP and Fe(CN)64- with Different Concentrations of Supporting Electrolyte supporting electrolyte in H2O I II I II I

Figure 4. Potential dependence of the various concentrations of common ion (ClO4-) in water. CwClO4- ) 1000, 100, 10 mM (from right to left).

investigations.13c,19c,30 For example, Shi and Anson observed a obvious dependence of the bimolecular electron transfer rate constants on the overall driving force when using either [DMFc]+ or [ZnTPP]+ (zinc tetraphenylporphyrin) in the organic phase and a different reducing agent in the aqueous phase.19c A comprehensive set of data using the different redox couples in the aqueous phase and ZnPor in the organic phase is obtained by Ding et al.,13c which indicates the ET rate constant increases upon increased overall driving force. The interrelation between ket and the overall driving force in this system is qualitatively in accord with the conventional Butler-Volmer treatment. 8602


II

1 M NaClO4 + 0.1 M NaCl 1 M NaClO4 + 0.1 M NaCl 0.1 M NaClO4 + 0.1 M NaCl 0.1 M NaClO4 + 0.1 M NaCl 0.01 M NaClO4 + 0.1 M NaCl 0.01 M NaClO4 + 0.1 M NaCl

EfH2Oa (mV)

EfNBb (mV)

overall driving forcec (mV)

ket (cm s-1 M-1)

276

634

358

0.20

276

1030

754

0.27

238

695

457

0.12

238

1053

815

0.15

239

763

524

0.13

239

1113

874

0.34

a Formal potential of the reactant redox couple in the H2O phase vs a saturated Ag/AgCl electrode as measured by cyclic voltammetry at the bare EPG electrode in the H2O phase. b Apparent formal potential of the reactant redox couple in the NB phase vs a saturated Ag/AgCl electrode in the H2O phase containing the indicated supporting electrolyte as measured by cyclic voltammetry. c The overall driving force of the reaction: (EfH2O - EfNB).

The effect of varying the thin-layer thickness on the voltammetric response was also investigated. Shown in Figure 5B, the appearance of the peak current instead of the plateau current gradually decreases with the volumes of the organic phase increased from 1 to 4 µL, which is different from one-step electron transfer.21 The smaller of the thin-layer thickness is the larger of the concentration gradient between the EPG electrode and ITIES,

reactants in two phases (c*w/c*o). The thin layer of NB adhered to the EPG contained 1 mM ZnTPP. The value of ket obtained above experiments was employed. The effect on the voltammetric response of varying the Kr was shown in Figure 2. A key feature of the simulated voltammograms in Figure 2A is that the deviation of the voltammetric response from that predicted by the constant-composition model increases as Kr is decreased, indicating that diffusional limitations of Fe(CN)64- in aqueous phase become more important for multistep ET. Shown in Figure 2B are cyclic voltammograms recorded at an EPG with the same conditions as simulation. It can be seen that the result from experiments is consistent with simulations. Furthermore, it proves that our theory is reasonable. Figure 5 showed the effect of varying the thin-layer thickness (d) on the multistep ET. For a given value of concentration of the aqueous species, with the increase of d, the current peak instead of the anodic plateau was established gradually, which shows that the slower mass transfer across thicker film results in a lower rate of regeneration of redox species at the L/L interface via the ET process and an increased depletion of the redox species in the aqueous phase at the interface. The total agreement between simulation and the experiment implies that the reliable rate constants can be obtained by selecting appropriate the thickness of the thin layers. As a result, the TLCV theory should be suitable for the consecutive electron transfer. Figure 5. (A) Simulated voltammograms for Kr ) 30. In each case, DZnTPP ) DZnTPP+ ) DZnTPP2+ ) 8.5 × 10-6 cm2 s-1, DK4Fe(CN)6 ) 5.5 × 10-6 cm2 s-1, c*ZnTPP ) 1 mM, A ) 0.30 cm2, υs ) 5 mV s-1, k1 ) 0.12 cm M-1 s-1, k2 ) 0.15 cm s-1 M-1, and d takes the values (a) 10, (b) 32, (c) 50, (d) 100 µm. (B) Voltammetric observation of electron-transfer from 1 mM ZnTPP (supporting electrolyte, 10 mM TBAClO4) in a thin layer of NB to 10 mM Fe(CN)64- in water (supporting electrolyte, 0.1 M NaClO4 + 0.1 M NaCl); scan rate, 5 mV s-1 throughout. The thin-layer volume is (a) 1, (b) 2, (c) 3, (d) 4 µL (from top to bottom).

the faster of the diffusion rate of the species in the thin layer, and the larger of the observed current. As the thin layer becomes thicker, the rapider consumption of Fe(CN)64- close to the ITIES and the slower diffusion of the organic redox species decrease the rate of regeneration ZnTPP and ZnTPP+ at the ITIES via the ET process, indicating the steady-state concentration profile in the thin layer is not achieved. As a result, the emergence of the peak current is mainly controlled by the rate of reaction between ZnTPP, ZnTPP+, and Fe(CN)64- across the NB/H2O interface, as well as the diffusional limitation of reactants in aqueous. In a word, the experimental results reveal the strong effect aroused by thin-layer thickness exists in the multistep electron transfer compared to one-step electron transfer. Evaluation of TLCV for Measuring the Rates of Multistep Electron Transfer across L/L Interfaces. The aim of this section is to assess the applicability of the method in analyzing the experimental data. In this paper, the electrode had an area of 0.30 cm2, the diffusion coefficients of coreactions were measured by cyclic voltammetry to be DZnTPP ) DZnTPP+ ) DZnTPP2+ ) 8.5 × 10-6 cm2 s-1, and DFe(CN)64- ) 5.5 × 10-6 cm2 s-1, respectively. t was estimated as the time elapsed between electrode potentials of 0.6 and 1.15 V, which amounted to 103 s at a scan rate of 5 mV s-1. Kr is the concentration ratio of the

CONCLUSIONS In the present study, the theory for the consecutive ET is proposed. According to the theory, which a complex multistep ET processes can be simplified as several one-step electron transfer process, the rate constants for each step will be obtained. The effects of varying the Kr and the thin-layer thickness on the multistep electron transfer were investigated by simulations and experiments. The reasonable agreement between simulations and experiments showed that the rates of multistep ET at L/L interfaces can be measured expediently using the TLCV. By the method, the bimolecular rate constants for the twostep one-electron transfer process of ZnTPP/[Fe(CN)6]4- were obtained. The larger overall driving force lead to a faster electron transfer, indicating that conventional Bulter-Volmer theory is also suitable for the consecutive ET. Additionally, extensive studies for the consecutive electron transfer across the ITIES are currently underway in our laboratory. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (nos. 20775060, 20875077, and 20927004), the Natural Science Foundation of Gansu (no. 0701RJZA109), and Key Laboratory of Polymer Science of Gansu Province, China. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review June 28, 2010. Accepted August 28, 2010. AC1016997


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