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Anal. Chem. 1903, 65,1910-1915
Investigation of Electrochemical Behavior and Mass-Transfer Process of Ferrocene-Siloxane Polymer Film Using Quartz Crystal Electrode Method Shin Ikeda and Noboru Oyama' Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
The electrochemical behavior of a redox-active ferrocene-modified siloxane polymer (PSF)which is capable of acting as a n electron mediator for flavin enzymes was examined. By cyclic voltammetry, it was revealed that the redox activity of PSF film was strongly affected by anionic species in the soaking solution. While a highly stable current response corresponding to the ferrocene/ ferricinium (Fco/Fc+)redox couple of the film was observed in NaC104 or NaBF4 supporting electrolyte solutions, the current response gradually decreased by cycling of the potential in a n aqueous solution of NaCl, NaN03, sodium ptoluenesulfonate, or phosphate buffer. The result of electrochemical quartz crystal microbalance (EQCM) measurements and dependence of the formal potential of the Fco/Fc+redox couple with activity of the supporting electrolyte anion in NaC104 aqueous solution showed that anion, cation, and solvent move simultaneously across the polymer film/solution interface during the redox reaction. A piezoelectric admittance measurement of the PSF-coated quartz crystal electrode in NaClO4 solution showed that the viscosity of the PSF film is little affected by successive redox cycling and by its redox state; that is, the rigidity of the film is maintained. Whereas, in a solution of NaN03 or phosphate buffer, significant changes in the physical and/or chemical properties of the PSF film, which may be correlated to electrochemical inactivation, were observed.
It is well-known that some ferrocenyl compounds have the capacity to replace oxygen as the electron acceptor for electron-transfer enzymes. Althoughmany studiesconcerning ferrocenyl compounds as electron mediators confined to electrode substrates have been reported, "dissolution" of the mediator is an inherent problem in many cases. Recently, Hale et al.17-19 have reported that a ferrocene-modified siloxane polymer, characterized by a highly flexible structure within a carbon paste matrix, is effective as an electron acceptor for flavin enzyme. They reported that dissolution is absent and excellent sensor characteristics are observed. However, the detailed electrochemistry of this polymer is unknown. More detailed information related to the nature of this polymer film, suchas redoxactivity and charge-transfer process within the film, is needed to realize successful applications as enzyme sensors. In this paper, we examine in detail the electrochemical response of a ferrocene-modifiedsiloxanepolymer (PSF)using various electrochemical methods and the electrochemical quartz crystal microbalance (EQCM) method. The EQCM method is an in situ microgravimetry method based on measurement of the resonant frequency change induced by mass or viscoelasticity changes of a film attached to a quartz crystal substrate and can be used to analyze mass-transfer processes during the redox reaction of thin films.*&z' Furthermore, in this research, we obtain a qualitative measurement of the viscoelasticity changes of the PSF film, (7) Peerce, P. J.; Bard, A. J. J.Electroanal. Chem. 1980, 108, 121. (8) Willman, K. W.; Rocklin, R. D.; Nowak, R. J.; Kuo, K.-N.; Schultz, F. A.; Murray, R. W. J. Am. Chem. SOC.1980,102, 7629. (9) Nowak, R. J.; Schultz, F. A.; Umana, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980,52, 315. (10) Rolison, D. R.; Umana, M.; Burgmayer, P.; Murray, R. W. Inorg. Chem. 1981.20. -2996. --(11) Umana, M.; Denieevich, P.; Rolison,D. R.; Nakahama, S.;Murray, R. W. Anal. Chem. 1981,53, 1170. (12) Pickup, P. G.; Osteryoung, R. A. J.Electrochem. SOC.1983,130, - - r - - ,
INTRODUCTION Polymer-coated electrodes have been increasingly studied in electrochemical research over the last decade since they are widely applicable to electronic/optical devices, electrocatalysis, chemical sensor devices, and energy storage materials. In particular, since redox polymers which contain the ferrocenyl group as an electroactive functionality possess high redox activity and chemical stability, they have attracted the attention of a number of researchers.'-16
* To whom correspondence
should be addressed.
(1) Lenhard, J. R.; Murray, R. W. J.Am. Chem. SOC.1978,100,7870.
(2) Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Haas, 0.;Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J.ElectroanaL Chem. 1978, 87, 429. (3) Mertz, A.; Bard, A. J. J.Am. Chem. SOC.1978, 100, 3222. (4) Oyama, N.; Yap, K. B.; Anson, F. C. J. Electroanal. Chem. 1979, 100, 233. (5) Wrighton, M. S. Acc. Chern. Res. 1979, 12, 303. (6) Bocarsly, A. B.; Walton, E. G.; Wrighton, M. S. J . Am. Chem. SOC. 1980,102, 3390. 0003-2700/93/0365-19 10$04.00/0
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(13) Pickup, P. G.; Murray, R. W. J.Electrochem. SOC.1984,131,833. (14)Carlin, C. M.; Kepley, L. J.; Bard, A. J. J.Electrochem. SOC.1986, 132, 353. (15) Leddy, J.; Bard, A. J. J. Electroanal. Chem. 1985, 189, 203. (16) Hagemeister, M. P.; White, H. S. J. Phys. Chem. 1987,91, 150. (17) Hale, P. D.; Inagaki, T.; Karan, H. 1.; Okamoto, Y.; Skotheim, T. A. J. Am. Chem. SOC.1989,111, 3482. (18) Gorton, L.; Karan, H. I.; Hale, P. D.; Inagaki, T.; Okamoto, Y.; Skotheim, T. A. Anal. Chim. Acta 1990,228,23. (19) Hale, P. D.; Boguslavsky, L. I.; Inagaki, T.; Karan, H. I.; Lee, H. S.; Skotheim, T. A.; Okamoto, Y. Anal. Chem. 1991,63,677. (20) Varineau, P. T.; Buttry, D. A. J. Phys. Chern. 1987,91, 1292. (21) Borjas, R.; Buttry, D. A. J. Electroanul. Chem. 1990,280, 73. (22) Daifuku, H.; Kawagoe, T.; Mateunaga, T.; Yamamoto, N.; Ohsaka, T.; Oyama, N. Synth. Met. 1991,4143, 2897. (23) Hillman, A. R.; Loveday, D. C.; Bruckenstein, S. J.Electroanul. Chem. 1991,300, 67. (24) Mizunuma, M.; Ohsaka, T.; Miyamoto, H.; Oyama, N. Bull. Chem. SOC.Jpn. 1991, 64, 2887. (25) Kelly, A. J.; Oyama, N. J. Phys. Chem. 1991,95,9579. (26) Shimazu, K.; Yagi, I.;Sato, Y.; Uosaki, K. Langmuir 1992,8,1385. (27) Sacks, M. D., Ed. Advanced Composite Materiak; Ceramic Transactions;American Chemical Society: Washington, DC, 1991; Vol. 19, p 389.
0 1993 American Chemical Socbty
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
m:n =1:1 FC: Ferrocene
Fc Flguro 1.
Structure of ferrocenamcdifled siloxane polymer (PSF).
Reference Electrode \
1
Counter Electrode
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with the crystal as the active element of an oscillation circuit driven at 6-V dc. The admittance measurements of the QCE were obtained by a Hewlett-Packard 4192A-LF impedance analyzer at a 0.1-V oscillator level. A polarization unit (Toho Technical Research, PS-06) was used as potentiostat and a function generator (Toho Technical Research, FG-02) was used in the EQCM experiment for potential step measurement. A microcomputer (NEC, PC9801RX) was used to interface the above instruments. The amount of charge passed during the electrochemical measurements was monitored by a coulometer/ ampere hour meter 3320 (Toho Technical Research). Apparent formal potentials for redox reaction of the PSF film were examined using a PSF film-coated basal plane pyrolytic graphite (BPG Union Carbide Co.) electrode (0.2 cm2) as the working electrode. The measurements of UV/light absorbance of the PSF film were carried out by using a UV/visiblespectrophotometer (Ubest55, Jasco). Indium tin oxide (ITO) electrodes coated with PSF film, a spiral Pt wire, and a Ag wire were used as the working, counter, and reference electrodes, respectively.
Au
77-
Quartz Crystal
Screw Cap
-
GlassTube
/
Quartz Crystal / Electrode
Schematicrepresentationof electrochemicalquartz crystal microbalance (EQCM) measurement system.
Flguro 2.
which also influence the frequency change, under several experimental conditions by employing a piezoelectric admittance measurement of the PSF film coated on a quartz crystal electrode.
EXPERIMENTAL SECTION Reagents. Ferrocene-modifiedsiloxanepolymer (PSF Figure l),molecular weight 4000-4500, was synthesized as previously
reported.% The PSF film (thickness ca. 20 nm) was coated onto the electrode substrate (geometric area 0.5 cm2) by droplet evaporation from 5 pL of chloroform stock solution containing 0.1 w t 5% PSF and stored under ambient conditions. All supporting electrolytes used were of guaranteed reagent grade and used as received. Doubly distilled deionized water was used for the aqueous electrolyte solutions. Apparatus. The instrumentation and procedure for electrochemical quartz crystal microbalance (EQCM)measurement were as previously reported.%.” Quartz crystals (5-MHz ATcut; Toyo Kurafuto Ltd.) coated with two metallic layers on both sides, a Cr (ca. 2 nm) as an adhesion layer by vacuum deposition and a Au (ca. 300 nm) layer by sputtering, were used as quartz crystal electrodes (QCE). After the PSF film was coated to one aide, this QCE was attached to a glass tube with a silicone rubber which penetrates the screw cap and used as the working electrode as shown in Figure 2. The screw cap with a QCE was fixed to the bottom of aglass cell. A spiral Pt wire and a sodium chloridesaturated calomel electrode (SSCE) were used as the counter and reference electrodes, respectively. The frequency response was measured on a Hewlett-Packard 5334B universal counter
RESULTS AND DISCUSSION Influence of Supporting Electrolyte on Electrochemical Responses. Figure 3 shows typical cyclic voltammograms obtained for the PSF film-coated BPG electrodes in 0.1 M NaC104,0.1 M phosphate buffer (pH 7.01, and 0.1 M NaCl aqueous solutions. The voltammograms are strongly dependent on the dissolved anionic species. As can be seen in Figure 3a for NaC104, the anodic and cathodic current responses corresponding to the ferrocene/ferricinium (FeO/ Fc+) redox couple gradually increase with the number of potential sweeps. For the first potential sweep, an extremely depressed and broadened anodic response is observed; a steady-state voltammogram is obtained by 10 repetitions of the potential sweep. The first ingress of C104- ion into the film induced by oxidation from the initial state (reduced form) is a very slow process for the presented time scale (50 m V/s). This response is interpreted by considering that a small amount of C104- ion remains in the PSF film after the first potential sweep,changing the hydrophobic nature of the PSF film to a hydrophilic one, and consequently, it facilitates the movements of C104- anion and water through the film for subsequent potential scans. This behavior has been recognized as a “break-in” effect, which was first proposed by Kaufman et al.29 for the redox reaction of functionalized tetrathiafulvalenes (TTF) films. A similar result was also observed in 0.1 M NaBF4 aqueous solution. On the other hand, a different behavior is observedin Figure 3b for NaCl and Figure 3c for phosphate buffer aqueous solutions. As demonstrated, there is a steady decrease in both anodic and cathodic current responses. Similar results were obtained using NaN03 or sodium p-toluenesulfonate as supporting electrolytes. Particularly, in phosphate buffer solution, the film became totally inactive in less than 10 potential sweeps. The “supporting electrolyte dependence” can be qualitatively understood by a difference in degree of hydration of dissolved anionic species. Both Clod- and BF4ions, which lead to high redox activity of the PSF film, possess a regular tetrahedral structure and are well-known as more hydrophobic ionic species than others examined.30 Since the PSF film forms a highly hydrophobic domain on the electrode surface due to the polymeric backbone of the siloxane structure, hydrophobic interaction between the anion inserted during oxidation and the film seems to be an important factor in maintaining stable redox activity. Furthermore, resoaking of the PSF film in NaC104 or NaBF4 aqueous solution once ~
(28) Inagaki, T.; Lee, H.S.; Skotheim, T.A.; Okamoto, Y.J. Chem. SOC.,Chem. Commun. 1989,1181.
(29) Schroeder, A. H.;Kaufman, F. B. J . Electroanal. Chem. 1980, 113, 209. (30) Hindman, J. C. J. Chem. Phys. 1962,36, 1OOO.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
I
,
,
,
,
l ” -
0
“ E v -
P C. Y _ i
o?
--r-
t ‘iVSSSCE E v vs >>,C Typical cyclic voltammograms obtained at PSF film-coated BPG electrodes in 0.1 aqueous solutions. Scan rate, 50 m VIS. Flgure 3.
EI
M
” vs
SSCE
(a) NaCIO,, (b) NaCi, and (c) phosphate buffer
i6C
T
440 I
1
0
420 -
O.ls. That is, it can be imagined that the “nonequilibrium” state of water caused by movement of C104- ion shifts to an “equilibrium” state rapidly, a t the film/solution interface. Hillman et al.23
NO. 14, JULY 15, 1993 1911
have proposed that transport rates with redox of PVF decrease in the order C104- > Na+C104- > HzO from the results of similar EQCM experiments. In the case of PSF, the order of the transport rate of Clod-, Na+, and HzO cannot be clarified. Though the reasons for the difference in the masstransfer process between PVF and PSF films have not been clarified yet, more detailed study is in progress, and will be reported.
ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (04205033 for N.O.) from Ministry of Education, Science and Culture. RECEIVED for review December 9, 1992. Accepted April 9, 1993.
