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SERS of Langmuir-Blodgett Monolayers: Coverage Dependence R. Aroca* and D. Battisti Department of Chemistry & Biochemistry, University of Windsor, Windsor, Ontario, Canada, N9B 3P4 Received December 19,1988.I n Final Form: July 24, 1989 The coverage dependence of surface-enhanced Raman scattering (SERS) and surface-enhanced resonant Raman scattering (SERRS) was studied by using mixed Langmuir-Blodgett (LB) monolayers of tetra-tert-butylvanadylphthalocyanine((t-Bu),VOPc) and arachidic acid. Ag-coated Sn spheres (which provide a broad range of electromagnetic resonances in the visible), Ag, and Au island films were used to enhance the inelastic scattering. LB layers of neat (t-Bu),VOPc and mixed monolayers with 20 mol % of arachidic acid and with various other molar ratios of arachidic acid:dye as high as 1OO:l were prepared to obtain submonolayer dye coverage of the metal surface and to determine the coverage effect of the adsorbed dye on the Raman intensities.
Introduction One of the key parameters in the understanding of the SERS phenomena is the surface coverage dependence of the SERS intensity and the enhancement factors (EF).' Experimental measurements of SERS intensity as a function of surface coverage have been explained, for instance, in terms of active sites that were thought to be responsible for the enhancement on a metal surface. Therefore, after the saturation of these sites by adsorption of molecular species, the SERS signal should reach a plateau. The latter argument was used to explain experimental results of pyridine adsorbed onto rough Ag surfaces within the "adatom" hypothesis,2 whereby SERS was due t o molecules adsorbed at special atomic scale roughness sites in the evaporated Ag films. The electromagnetic model of SERS intensitF4made possible a quantitative approach for the interpretation of the surface coverage effect on SERS. For example, Murray and Bodoffd proposed a depolarization model that accounted for their observations of SERS coverage dependence from cyanide on Ag island films. Recently, Zeman e t a1.6 carried out experimental and computational studies of coverage dependence of SERS for cobalt phthalocyanine (CoPc) on CaF,-roughened Ag films. They found that the EF decreased with increasing CoPc thickness and that the maximum intensity was obtained a t submonolayer coverage. It was suggested that the negative dependence of the surface-enhanced resonant Raman scattering (SERRS) with surface coverage was a consequence of damping of the plasmon resonance by the adsorbed layer. In the present report, the variation of the SERS (excitation outside the electronic adsorption of the dye) intensities with the number of mixed LB layers coating a Sn/Ag substrate was measured. Similarly, the variation of SERRS intensities from evaporated films of (tBu),VOPc on Ag-coated Sn spheres and from LB layers
* Author to whom correspondence should be directed.
(1) Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982. (2) Pockrand, I.; Otto, A. Solid State Commun. 1980,35,861. (3) Moskovits, M. Reu. Mod. Phys. 1985,57,783.McCall, S.;Platzman, P. M.; Wolff, P. A. Phys. Lett. 1980,A77, 381. Gersten, J. I., Nitzan, A. J . Chem. Phys. 1980,73,3023. (4) Aroca, R.;Kovacs, G. J. In Vibrational Spectra and Structure; Durig, J., Ed.; Elsevier: Amsterdam, Vol. 18, 1989. (5) Murray, C. A.; Bodoff, S. Phys. Reu. 1985,832,671. (6) Zeman, E.J.; Carron, K. T.; Schatz, G. C.; Van Duyne, R. P. J . Chem. Phys. 1987,87,4189.
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on Sn/Ag and Au island films is reported. Multilayer coats of mixed LB layers with low dye concentration are shown to be very efficient for the achievement of maximum surface electromagnetic enhancement.
Experimental Section Langmuir-Blodgett (LB) monolayers of (t-Bu),VOPc and mixed monolayers containing 20 mol % of arachidic acid and with other molar ratios of arachidic acid to Pc up to 1001 were prepared at room temperature and transferred to Corning 7059 glass slides or slides coated with a metal island film. A Lauda trough, equipped with an electronically controlled film deposition device, was used for film compression and transfer. The compression speed was 0.1 A2/moleculeper s, and the film transfer was carried out at 4.8 mm/min at a constant pressure of 10 dyn/cm. Experiments with mixed monolayers were reproduced by using constant pressures for film transfer of 10 and 15 dyn/cm. Monolayers were formed by substrate withdrawal (Z-deposition). During the transfer of the monolayer onto the metal island film, the surface area measured at the trough was larger than the corresponding area of the glass slide, in agreement with the fact that a greater amount of material would be needed to cover a rough metal island surface. For spreading onto the water surface, the solutes were dissolved in toluene. The film pressure for neat (t-Bu),VOPc was measurable at about 100 A2/molecule, while the area per molecule derived from the isotherm was about 60 A2/molecule. The area per (tBu),VOPc molecule derived from the isotherm of the mixed monolayer (1O:l molar ratio), assuming 20 A2/arachidic acid molecule, was 70 A' for a surface pressure between 10 and 40 dyn/cm. The isotherm for a neat (t-Bu),VOPc monolayer and the corresponding isotherm for a mixed monolayer (101ratio) are given in Figure 1. The absorption spectra of the LB monolayers showed two well-defined bands at 348 and 706 nm. Experiments carried out with the 647.1-nm Kr+ laser line, in resonance with the red absorption band, resulted in the excitation of RRS and SERRS. The 514.5-nm laser was conveniently outside the red absorption for RS and SERS excitation. Ag-coated Sn spheres' were formed by evaporating 100-nm mass thickness of Sn at a rate of 0.5 nm/s onto glass substrate heated at 120 "C;100 nm of Ag was then overlaid with the substrate held at room temperature. A particle distribution study of the Sn/Ag substrate was carried out, and the results are shown in Figure 2, where the width of each bar is 130 nm. The starting point for the first bar was 380 nm. The average value was 530 nm. The inset in Figure 2 shows the scanning electron micrograph of the film. The Ag-coated Sn spheres,with the particle distribution shown (7) Aroca, R.;Kovacs, G. J. J. Mol. Struct. 1988,174,53.
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Figure 2. Particle distribution for the Ag-coated Sn spherea film. The bar width is 130 nm, and the first interval starts at 380 nm. in Figure 2, can support surface plasmons in a broad range of the visible spectrum, which allowed the observation of strong SERS signals with the 514.5 and 647.1-nm laser lines. Ag and Au island films were formed by the evaporation of 10 nm of Ag and 4 nm of Au at a rate of 0.1 nm/s onto glass substrate kept at 200 O C . The absorption spectrum of the Ag films showed a maximum at 490 nm, and the Au films had a broad absorption centered at 650 nm. In a separate evaporator used for preparation of organic films, overlayers of 1-10-nm mass thickness of (t-Bu),VOPc were evaporated onto the SERS-active surfaces. Four polarized spectra were routinely measured: SS,SP, PP, and PS. Raman shifts were measured with a Spex-1403 and a Ramanor-U-1000 double monochromator
Results and Discussion The vibrational characterization of (t-Bu),VOPc as thin films and as LmgmuirBlodgett monolayers has been given elsewhere." SERS and SERRS spectra of one mixed monolayer on Sn/Ag spheres and on a Au island film are given in Figures 3 and 4. For comparison, in Figure 5 the SERRS of three mixed monolayers (molar ratio 101) on Sn/Ag, detached from the surface by one spacer layer of arachidic acid, and the RRS spectrum of three LB layers of the neat (t-Bu),VOPc on glass are given. The following observations were made from the surface-enhanced spectra of neat and mixed monolayers of (t-Bu),VOPc: (i) Fundamental vibrational frequencies measured in the SERS and SERRS spectra of neat and mixed monolayers were observed unshifted from their values in the LB assemblies (three or more layers) or bulk spectra, an indication of physisorption (as opposed to chemisorption) of (8) A m . R.;Battisti. D. In Recent Developments in Molecular Spectmscopy; Jordanov. B.,K i m , N.. Simova. P.. Eds.; World %entine: Singapore, 1989; p 213.
Figure 4. SERRS of one mixed (20 mol k)LB monolayer of (f-Bu),VOPc on Sn/Ag spheres (upper trace) and of one mixed LB layer ( 1 0 1 mole ratio) on 4-nm Au island film (lower trace). Both obtained with 50 mW of the 647.1-nm laser line. Intensity in arbitrary units.
Figure 5. Resonant Raman spectrum of three LB layers of neat (I-Bu),VOPc on glass (lower trace). SERRS of three LB mixed layers (molar ratio 101) detached from the Ag-coated Sn spheres by one LB spacer layer of arachidic acid. the Pc molecules to the metal surface. (ii) For this Pc molecule, as for many other polyatomic molecules with similar geometries in the ground and excited states, the RRS is mainly due to Henberg-Teller contributions, and no overtone progressions of vibrational fundamentals were observed. (iii) The relative intensities in the SERRS spectra (see figures a t different dilutions in the fatty acid matrix) showed no deviation from relative intensity pattern observed in the RRS spectra. S E R R S Coverage Dependence of Evaporated Films. With a vacuum evaporator, mass thicknesses of 1 nm and higher of (t-Bu),VOPc were deposited onto the Ag-coated Sn spheres substrate at room temperature. A simple calculation shows that the 1-nm deposition would be below a monolayer coverage by approximately the roughness factor of the Ag/Sn substrate. The results of these experiments are presented in Figure 6.
Aroca and Battisti
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The relative intensity of the 685-cm-l band was compared in all spectra, and the lowest number of counts was taken as unity. Thicker films were grown by evaporation onto the same substrate, in an effort to keep constant the enhancing properties of the Ag/Sn surface. Despite the experimental uncertainties involved when trying to measure intensities under identical conditions, the trend shown in Figure 6 is genuine and reproducible. Furthermore, results with the same negative dependence have also been obtained in our laboratory for the SERRS of other phthalocyanine derivatives. Qualitatively, the conclusion was that the SERRS enhancement factor is maximized when the amount of substance deposited on the surface is minimized (“the analytical chemist’s dream”). It is known that a continuous alteration of the local electromagnetic environment of Ag island films can be made by coating the film with an increasing thickness of a strongly absorbing dye molecule.4’11 The dye coating will change the behavior of the plasma resonances and therefore alter the SERS intensity. In particular, a dye absorbing at the frequency of excitation (RRS) could strongly damp the plasma resonances and diminish the enhanced radiation from the radiative plasmons.6 SERS and SERRS Coverage Dependence of LB Layers. The resonant Raman spectrum (647.1-nm excitation) for three LB monolayers of neat (t-Bu),VOPc on glass can be seen in Figure 5 (lower trace). It should be pointed out that the intensity of the RRS signal increases with the number of LB layers, and there was little evidence for strong self-absorption. Inelastic scattering produced by excitation (at 514.5 nm) outside the visible absorption band was below the detection level. Here, the coverage dependence of the surface-enhanced Raman signal measured for a Sn/Ag surface coated with mixed L B containing (t-Bu),VOPc and 20 mol 70 of arachidic acid is discussed. In these experiments, different sections of the glass slide with the SERS-active substrate were covered with one, two, and three monolayers of the arachidic-(t-Bu),VOPc mixture. The Raman spectrum for each of the sections was measured with the 514.5and 647.1-nm laser lines, respectively. All instrumental parameters were kept constant during measurements with each laser line. SERS coverage dependence is shown in the inset of Figure 6, where the minimum intensity has been given the value 2. Included in the same figure, the intensity of the 685-cm-‘ band obtained when a 5-nm film was evaporated onto a Sn/Ag surface is shown. This point is not strictly comparable with those of the LB sam(9) Eagen, C. F. A p p l . Opt. 1981,20, 3035.
(IO) Weitz, D. A,; Garoff, S.; Gramila, T. J. Appl. Opt. 1982, 7, 168. (11) Craighead, H. G., Glass, A. M. Opt. Lett. 1981, 6, 248.
ple. However, it was included to show that the evaporated samples and the LB samples produced results of similar intensity. The dependence of the SERRS intensity with the surface coverage is shown as an inset in Figure 1. Here, the relative intensity of the 1523-cm-’ band was used for plotting, and the minimum number of counts per second was assumed as unity. First, the advantages of the LB experiment for the present study should be pointed out. For example, it was accurately known that the Sn/Ag surface was covered with one or more LB monolayers containing a mixture of arachidic acid-(t-Bu),VOPc. Secondly, the stacking of two and three monolayers on the SERS-active surface allowed one to take advantage of the long-range effect of the electromagnetic enhancement,12 which would increase the SERS signal due to the presence of a second layer not directly physisorbed onto the Sn/Ag surface. This effect is clearly illustrated in Figures 1 and 6, where an increase in the SERS and the SERRS signal is evident upon addition of a second LB monolayer. The addition of a third monolayer had the net effect of reducing the overall intensity. It is assumed that the total measured intensity is the result of a number of competitive processes, and with the addition of the third LB layer there was a larger negative contribution. The RRS of monolayer assemblies shown above indicated that self-absorption (that may become an important contribution) could not be used to explain the reduction observed in the intensity pattern. A calculation using the particle plasmon model13 for a silver prolate (and oblate) spheroid in a host medium of dielectric constant greater than 1 (air) produces a considerable red shift of the plasmon absorption, which would also change the enhancement factor. Calculations within this simple model also showed a splitting of the peak, for the enhancement factor as a function of the incident wavelength, in a medium with dielectric constant greater than l.ll It should be pointed out that in SERRS experiments for mixed LB layers on Ag the P-polarized light was, in all cases, more effective for spectral excitation than the S-polarized light. This contrasted with what was observed in the RRS experiments, where I , was greater than I,. This final observation supports the concept that P-polarized light was more efficient for surface plasmon excitation. However, in the intensity measurements on Au island films, I , > I , was found (similar to RRS), which could be rationalized by assuming predominant oblate spheroidal shapes in the particle distribution of the Au film. SERRS at Submonolayer Coverage. Two metal substrates were used to study the inelastic scattering intensities of mixed arachidic acid-(t-Bu),VOPc at submonolayer coverage: Ag-coated Sn spheres and Au island films. The (t-Bu),VOPc, as most of the Pc molecules, is a very stable molecule. However, at low surface coverage the photostability of the Pc molecule seems to be a factor. The signals of the samples on Sn/Ag showed a significant time dependence, which made the direct scanning measurements unsuitable. To avoid this problem, Raman measurements at submonolayer coverage were carried out with a sample rotator,’, and all of the slides were rotated with the same frequency. The mixed arachidic acid:(& Bu),VOPc = 1O:l mole ratio was used as an internal reference, and one section of each film was always coated with one LB monolayer of the internal reference. The SERRS intensity of the 685-cm-l band was measured in (12) Kovacs, G. J.; Loutfy, R. 0.;Vincett, P. S.; Jennings, C.; Aroca, R. Langmuir 1986, 2,689. (13) Aroca, R., Martin, F. J . Raman Spectrosc. 1985, 16, 156. (14) Kiefer, W.; Bernstein, H. J. A p p l . Spectrosc. 1971, 25, 500.
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SERS of Langmuir-Blodgett Monolayers 33
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Figure 7. Submonolayer coverage dependence of SERRS intensities on S n / A and Au substrates. Solid line data (counts/s) of t h e 685-cm- band normalized by the same band in the 1O:l mixed layer. T h e dashed line corresponds to the same data
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mixed LB layers with a mole ratio from 1:l to 1OO:l. The data presented in Figure 7 (solid line) have been normalized with respect to the internal reference (1O:l mole ratio LB layer). The intensity of each sample was further multiplied by the corresponding dilution factor, giving the second set of points in the figures (dashed lines). The results obtained on Au were reproducible, and the bars in Figure 7 indicate the region in which the experimental values of different measurements were found. The results on Sn/Ag spheres were more scattered; nevertheless, the trend shown in Figure 7 seems to be valid. Minimal changes in the absorption spectra of the Au films were observed after coating them with LB layers in the 1:l to 1O:l mole ratio. An important feature in the present results was the fact that the relative intensity of the vibrational fundamentals in the SERRS spectra was the same for all dilutions, as can be seen in Figures 4 and 5. Zeman et a1.,6 however, have found that their theoretical models predicted a different coverage dependence for the 683and the 1544-cm-' vibrations. Since this particular behavior was not observed for (t-Bu),VOPc, further computational and experimental work in this area is presently in progress. It was evident that SERRS obtained in the submonolayer coverage experiments was mainly due to an electromagnetic enhancement. This conclusion was supported by the fact that the vibrational spectra of arachidic acid-(t-Bu),VOPc monolayers at all dilutions indicated a very weak interaction with both Ag and Au surfaces, and no frequency shifts were observed (a clear case of physisorption). The results presented in Figure 7 indicate that the variation of the SERRS intensities at submonolayer coverage of a Ag surface with ( t Bu),VOPc is different from the variation observed on a Au surface. It is possible that the function representing the change of the SERRS intensity with the surface coverage would be characteristic of the metal-dye pair. Experiments with other dye molecules would help one understand the specificity or generality of the submonolayer coverage dependence function. The general idea of plasmon damping6 by the adsorbates should be considered as a factor in the explanation of surface coverage variations of SERRS intensities.
SERRS of Mixed Multilayers. A SERS-active surface coated with a mixed monomolecular layer with an arachidic acid:@-Bu),VOPc molar ratio of 1 0 1 would have a maximum coverage of 0.1 monolayer of (t-Bu),VOPc. Three different sets of experiments were carried out with the 1O:l mixed LB layers. In the first set of experiments, different sections of a glass slide with a 10-nmthick Ag island film were coated with one through four mixed LB monolayers. Plasmon frequencies of Ag films coated with the neat (t-Bu),VOPc monolayer were strongly affected. However, coating with the mixed 1O:l monolayers did not change the absorption spectrum of the 10nm Ag film. SERRS intensities measured for the 685and the 1523-cm-' vibrational frequencies increased steadily with the number of LB layers. Due to the longrange resonant surface plasmon enhancement, large increases in intensity were observed for the film section containing two and three layers, and a smaller positive increase was seen for the fourth layer. The second set of experiments consisted in the preparation of Ag-coated Sn film coated with one LB layer. Sections of the same film were then coated with two through four LB layers. SERRS measured with the 647.1nm line on each of these sections showed the same intensity pattern observed for the 10-nm Ag film. The SERRS intensity increased steadily with the number of layers, with an indication of reaching a plateau after the fourth layer. For instance, the intensity of the 1523-cm-l band in the three layer section was 2 times that of the twolayer section. However, the intensity of the same band in the section with four layers was only 1.2 times that of the three-layer section. The last experiment was designed to check the long-range SERRS enhancement in the absence of dye directly attached to the Ag surface. In this case, a Ag-coated Sn film was first coated with one monolayer of arachidic acid as a spacer layer. Again, different sections of the slide were then coated with one through four mixed LB layers. The spectrum of this sample in the section with three mixed LB layers and one spacer layer is shown in Figure 5 (upper trace). Although the intensities were much lower than in the previous two samples due to the absence of the first mixed LB layer (major contributor to the total intensity), the pattern of increase in intensity with the number of layers was maintained, and the intensity reached a plateau at the fourth layer. Similar results were obtained when the 1O:l molar ratio sample (without spacer) was excited with the 514.5nm laser line (SERS). Since the excitation occurred outside the molecular electronic absorption, the Raman scattering for the slide section with one LB layer was detectable only with a long accumulation time. However, the intensity of the signal measured for the macrocycle vibration at 685 cm-' increased steadily with the number of mixed LB layers covering the Ag substrate. Conclusion Qualitatively, the coverage dependence of the SERRS signal obtained from evaporated (t-Bu),VOPc onto a Agcoated Sn sphere surface showed a maximum at submonolayer coverage. For a quantitative determination of the coverage dependence of SERRS intensities, monolayer and multilayer LB coats on Ag and Au surfaces were used. The LB technique provided a measure of the horizontal surface coverage by using mixed layers with different dye concentrations and allowed the exact determination of the number of monomolecular layers that covered the SERS-active substrate. Studies a t submonolayer coverage of Sn/Ag and Au surfaces, carried out by using Langmuir-Blodgett monolayers of mixed compo-
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Langmuir 1990, 6, 254-262
sition arachidic acid-(t-Bu),VOPc, showed a pattern in the change of the SERRS intensities which was characteristic for each metal. LB experiments with two consecutive LB layers (containing 20% arachidic acid) showed an increase in the SERS and SERRS signals. The contribution of the second layer is clearly due to the longrange nature of the electromagnetic enhancement. However, further LB coverage decreased the SERS and SERRS intensities. Since all SERS and SERRS signals observed were mainly due to electromagnetic enhancement, variation in intensity may be attributed to the fact that the dye layer strongly damps the resonance of the metal particle. Experiments with multilayers of arachidic acid:dye = 101molar ratio showed a continuous increase in SERRS intensity with the number of mixed layers coating the Ag surface. The effect tapered off after the fourth layer,
in agreement with a long-range enhancement due to surface plasmons. The mix* LB layer technique emerged as a very powerful analytical tool for the study of strongly absorbing dyes using SERRS. For practical applications, the most efficient SERRS signal may be obtained by working with two or three mixed LB layers of dye (in the 1O:l to 5:l region of mole ratio) on a SERS-active surface. Acknowledgment. We thank Dr. R. 0. Loutfy from Xerox Research Centre of Canada for providing a purified sample of (t-Bu),VOPc. Financial assistance from NSERC of Canada is gratefully acknowledged. Registry No. (t-Bu),VOPc, 95865-59-1; Ag, 7440-22-4; Au, 7440-57-5; Sn, 7440-31-5; arachidic acid, 506-30-9.
Redox Reaction Mechanism of Cytochrome c at Modified Gold Electrodes T. Sagara,t K. Niwa,t A. Sone,t C. Hinnen,$ and K. Niki*pt Department of Physical Chemistry, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan, and Labotatorie d’Electrochimie Interfaciale du C N R S 1, Place Aristide Briand, F-92190 Meudon-Belleveu, France Received October 20, 1988. I n Final Form: July 25, 1989 The roles of surface modifiers (4-pyridyl derivatives) in the electron-transfer reaction of cytochrome c at a gold electrode surface were investigated by using ac impedance and electroreflectance techniques. It was found that there are at least three types of modifiers. Cytochrome c was immobilized on the bis(4-pyridyl) disulfide modified gold electrode, and the electron-transfer reaction between the electrode and cytochrome c in the bulk takes place through these immobilized layers on the electrode. The surface was weak, and the modbinding of cytochrome c to the trans-1,2-bis(4-pyridyl)ethylene-modified ified surface was partially covered by cytochrome c. The electrode reaction takes place through the adsorbed layer of the modifier on the electrode surface. The electrode reaction of cytochrome c at the bare electrode surface also takes place in this case with a formal potential about 440 mV more negative than the potential of native cytochrome c. Cytochrome c was coadsorbed on the gold electrode with 4,4‘-bipyridine. The formal potential of cytochrome c coadsorbed with 4,4‘-bipyridine was about 225 mV more negative than the potential of the native one but more positive than that of cytochrome c adsorbed on the bare electrode surface. The reversible electrode reaction of cytochrome c in the bulk through the adsorbed layer of cytochrome c was very improbable in this case. The electrode reaction of the bulk species may take place through the adsorbed layer of 4,4’-bipyridine,
Introduction Cytochrome c exhibits voltammetric responses ranging from reversible to irreversible at various On the other hand, cytochrome c3 exhibits a reversible voltammetric response at various electrodes without mediators or promoter^.^ Both cytochrome c and cytochrome c3 adsorb strongly on various elecYokohama National University. Laboratorie d’Electrochimie Interfaciale du CNRS. (1) Bowden, E. F.; Hawkridge, F. M.; Blount, H. N. In Comprehensive Treatise of Electrochemistry; Srinivasan, S., Chizmadzhev, Y. A,, Bockris, J. O’M., Conway, E. B., Yeager, E., Eds.; Plenum Press: New York, 1985; p 297. (2) (a) Bowden, E. F.; Hawkridge, F. M.; Blount, H. N. J. Electroanal. Chem. 1984, 161, 355. (b) Reed, D. E., Hawkridge, F. M. Anal. Chem. 1987,59, 2334. t t
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trodes from aqueous solutions, and the voltammetric behavior of these cytochromes in the solution is strongly influenced by the nature of the adsorbed films on the electrodes since the electrode reaction of the bulk species takes place through the adsorbed film^.^.^ It has been shown t h a t t h e formal potential of t h e adsorbed cytochrome c is about 440 mV more negative than the potential of native cytochrome c. On the other hand, (3) (a) Niki, K.; Yagi, T.; Inokuchi, H.; Kimura, K. J.Electrochem.
SOC.1977, 124, 1889. (b) Niki, K.; Yagi, T.; Inokuchi, H.; Kimura, K. J. Am. Chem. SOC.1979,101,3335. (4) (a) Scheller, F. Bioelectrochem. Bioenerg. 1977, 4 , 490. (b) Kuznetsov, B. A.; Schumakovich, G. P.; Mestechkina, N. M. Bioelectrochem. Bioenerg. 1977, 4 , 512. (c) Haladjian, J.; Bianco, P.; Serre, P.-A. Bioelectrochem. Bioenerg. 1979, 6, 555. (5) Hinnen, C.; Parsons, R.; Niki, K. J. Electroanal. Chem. 1983, 147,329.
0 1990 American Chemical Society
