Langmuir 1996, 12, 4249-4252
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Hexacyanoferrate Modification of Gold Electrode through Monolayer Approach D. N. Upadhyay, V. Yegnaraman, and G. Prabhakara Rao* Central Electrochemical Research Institute, Karaikudi-630 006, India Received May 10, 1995. In Final Form: May 6, 1996X Hexacyanoferrate modification of gold electrode has been achieved by immobilizing the species in a self-assembled monolayer of 3,3′-thiodipropionic acid anchored to the gold surface. The surface coverage of hexacyanoferrate is estimated from the voltammetric data point to be about one monolayer, and the modified electrode exhibits excellent surface redox behavior. Further, it shows good electrocatalytic influence on the oxidation of hydrazine and ascorbic acid.
Introduction Chemical modification of electrodes to incorporate desired surface functionality/activity has been the fascinating theme for many researchers in electrochemistry.1 Of late, molecular level modification of electrodes using Langmuir-Blodgett films or self-assembly techniques is gaining importance, since it enables fine-tuning of interfacial structures which will ultimately find applications in diverse fields such as molecular electronics,2-5 molecular recognition,6-10 and sensors.11 Tethering of redox centers on electrode surfaces using the monolayer approach has been widely reported for investigations of facile electron transfer kinetics.12-16 Mono/submonolayer level modification of electrodes with Prussian Blue (PB)12 and its nickel analogue10 using self-assembled monolayers (SAMs) of sulfur compounds as templates has been reported. The former finds application in electrocatalysis while the latter exhibits molecular recognition characteristics. During the course of our investigations to obtain molybdenum hexacyanoferrate (on which information is scarce), using the monolayer approach, we came across an interesting observation that involves immobilization of hexacyanoferrate (HCF) as a monolayer in the presence of the molybdenum ions on the electrode surface. The HCF monolayer was found to exhibit very good surface redox characteristics and was further characterized through its catalytic properties. These observations are considered X
Abstract published in Advance ACS Abstracts, July 1, 1996.
(1) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: NewYork, 1984; Vol. 13, Chapter 3. (2) Carter, F. L., Ed. Molecular Electronic Devices; Dekker: New York, 1982, 1987; Vols. I & II. (3) Hong, F. T., Ed. Biosensors and Biocomputers; Plenum Press: New York, 1992; Chapter I. (4) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (5) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir, 1993, 9, 1614. (6) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (7) Cheng, Q.; Toth, A. B. Anal. Chem. 1992, 64, 1998. (8) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37. (9) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, I. J. Am. Chem. Soc. 1991, 113, 5176. (10) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir, 1995, 11, 666. (11) Sawaguchi, T.; Matsue, T.; Uchida, I. Electrochemical Society Spring Meeting, Washington, DC, 1991; Extended Abstr. Vol. 91-1, p 923. (12) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (13) Miller, C.; Cuendet, P.; Cratzel, M. J. Phys. Chem. 1991, 95, 877. (14) Li,T. T. T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (15) Finklea, H. D.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (16) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, L. C. E. D. J. Am. Chem. Soc. 1987, 109,3559.
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important in light of the fact that the earlier attempts to incorporate HCF onto the electrode surface, met with only limited success.17-20 Moreover, the reported modified electrodes showing poor reversibility leave much scope for improvement. Results of our investigations on monolayer modifications of Au electrode with HCF that exhibit excellent reversible surface redox properties are presented in this article. Experimental Section 3,3′-Thiodipropionic acid (TPA) (Merck), MoO3 (May and Baker), K3Fe(CN)6 (Merck), and other chemicals (KCl, HCl, HNO3, ascorbic acid, and hydrazine) were of Suprapure grade and used without further purification. For electrochemical investigations a Potentioscan (Wenking Model POS 73) coupled to an X-Y/t recorder (Rikadenki Model 201 T) was employed. A conventional three electrode assembly consisting of an Au disk working electrode (supplied by BioAnalytical Systems,Inc.; area 0.036 cm2), a Pt counter electrode, and a normal calomel electrode (NCE) was used. All the potentials are referred to the NCE.
Modification of the Electrode Gold electrode was pretreated by cycling between 0.1 and 1.2 V at 100 mV‚s-1 in 0.5 M H2SO4 for about 15 min, rinsed thoroughly with water, and then dipped in aqueous 0.1 M TPA solution for 3 h. Afterwards, the electrode was washed well with water, allowed to dry and then immersed in molybdenum chloride solution [prepared by dissolving 1.66 g of MoO3 in the minimum amount of HNO3-HCl mixture (1:3)] for about 1 h. Subsequently, the electrode was thoroughly washed and cycled between -0.2 and 0.6 V in 0.5 M KCl (pH 4.0) containing 10 mM K3Fe(CN)6 at 100 mV‚s-1 for 15 min. The electrode was then rinsed and used for voltammetric studies. Results and Discussion Attempts to modify the electrode in the absence of TPA were unsuccessful. Similarly, HCF modification obtained after treatment of the TPA monolayer with molybdenum chloride solution yielded a coverage (cf. Figure 1b) that was marginally lower than that resulting in the absence of molybdenum chloride (cf. Figure 1a). Moreover, the redox peaks were broader and the formal potential (estimated as the average of the anodic and cathodic peak potentials21) of the surface redox wave was about 50 mV (17) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 247. (18) Kuo, K. N.; Murray, R. W. J. Electroanal. Chem. 1982, 131, 37. (19) Oyama, N.; Shimomura, T.; Shigehara, K.; Anson, F. C. J. Electroanal. Chem. 1980, 112, 271. (20) Geno, P. W.; Ravichandran, K. and Baldwin, R. P. J. Electroanal. Chem. 1985, 183, 155.
© 1996 American Chemical Society
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Figure 2. Plot of anodic peak current (ip,a) against potential sweep rate; based on data from cyclic voltammetric measurements on the HCF-modified Au electrode (modified as per conditions described in Figure 1b) in 0.5 M KCl (pH ) 4) at different sweep rates.
Figure 1. Voltammetric response of the HCF-modified electrode showing the formation of sharp redox peaks due to molybdenum chloride treatment of the TPA monolayer on Au. Cyclic voltammogram of the modified electrode in 0.5 M KCl (pH ) 4) at 100 mV‚s-1; (a) without the molybdenum chloride treatment of the monolayer (dashed line) and (b) with treatment (solid line).
more anodic than that resulting in the presence of molybdenum. The voltammetric studies, described subsequently in this communication, relate to the Au electrode subjected to HCF modification after successive treatment in solutions of TPA and molybdenum chloride. The electrode thus modified is referred to as the Au/HCF electrode in the following discussions. The Au/HCF electrode was cycled between 0.6 and -0.2 V in 0.5 M KCl buffered to pH 4.0, and a typical CV recorded at 100 mV‚s-1 is depicted in Figure 1. Successful incorporation of the redox groups in the modified electrode is evidenced by the presence of a pair of well-defined anodic (0.1 V) and cathodic (0.07 V) peaks. Moreover, the CV behavior studied as a function of scan rate in the range 2-200 mV‚s-1 exhibited, at all sweep rates, sharp redox peaks analogous to those in Figure 1. The plot of anodic peak current (ip,a) vs sweep rate yielded a straight line (cf. Figure 2). This, along with the following observations, (i) absence of a diffusion tail on the descending region of both the anodic and cathodic peaks and (ii) the surface coverage estimated by integrating the charge under the voltammetric peak (corresponding to 250 ( 20 µC‚cm-2) being practically the same for both the anodic and cathodic peaks, confirms that the voltammetric behavior noticed in Figure 1 is due to a surface redox reaction. Further, continuous cycling of the modified electrode in a KCl background between +0.6 and -0.2 V for about 90 min results in about 20% reduction only in the peak currents. This suggests that the modification is fairly stable. The values of the formal potential were estimated from the CV data as the average of the anodic and cathodic peak potentials.21 The formal potential of the surface redox (21) Reference 1 above, p 206.
Figure 3. Cyclic voltammogram on a bare Au electrode of 10 mM ferricyanide in 0.5 M KCl (pH ) 4). Potential sweep rate ) 100 mV‚s-1.
behavior is 0.085 V, which is very close to the formal potential (0.06 V) obtained for the same redox system, namely, the ferro/ferricyanide in solution phase (cf. Figure 3, which depicts the CV of 10 mM ferri/ferrocyanide in 0.5 M KCl at 100 mV‚s-1 on the electrode just prior to incorporation of HCF). Thus, the formal potential data obtained above conform to the general expectation that surface formal potential does not vary considerably from the solution formal potential.19,22,23 During the modification treatment, the possibility of the formation of a PB film which may give rise to the observed redox behavior is ruled out on the basis of the following considerations: (i) The cycling of the electrode during modification has been restricted to only between 0.6 and -0.2 V. It is known24 that cycling between 1.5 and -0.4 V is necessary for the formation of a PB film under similar experimental conditions. (ii) The modified electrode when cycled upto 1.0 V in the anodic direction in a KCl background does not show a second pair of redox peaks around 0.8 V which is characteristic of PB. (iii) The midpoint potential of the surface redox behavior on the modified electrode is observed at about 0.085 V, which is more cathodic than 0.2 V, around which the first pair (22) Lenhard, J. R.; Rocklin, R.; Abruna, H.; Willman, K.; Kuo, K. N.; Nowak, R.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 5213. (23) Lennox, J. C.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 3710. (24) Upadhyay, D. N.; Gomathi, H.; Rao, G. P. J. Electroanal. Chem. 1991, 301, 199.
Hexacyanoferrate Modification of Gold Electrode
of redox peaks characteristic of PB normally occurs. Also, the possibility of attributing the redox behavior observed in Figure 1 to the formation of molybdenum hexacyanoferrate is remote, since the formal potential of the most stable redox system of the cyano complexes of molybdenum (involving +4 and +5 oxidation states) happens to be about 0.6 V,25 which is far away from the presently observed midpoint potentials. These considerations, along with the earlier observation20 of 0.12 V as the formal potential of the surface-bound HCF, reveal that the surface redox behavior resulting from the presently described modification should be due to HCF groups immobilized on the electrode surface. Further, detailed studies based on surface sensitive techniques are needed for precise characterization of the surface. The surface coverage estimated by integration of the charge under the voltammetric peaks in Figure 1b is found to be almost the same, as mentioned earlier, for both the anodic and cathodic peaks and corresponds to 250 ( 20 µC‚cm-2. The surface coverage can also be determined independently from the slope of the linear plot of ip vs v (Figure 2), using the following relationship1
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Figure 4. Voltammetric response showing the catalytic effect of the HCF-modified electrode on hydrazine oxidation. Cyclic voltammogram of 5 mM hydrazine in 1 M sulfuric acid on (a) a bare Au electrode and (b) the modified Au electrode (modified as per the conditions described in Figure 1b). Potential sweep rate ) 50 mV‚s-1.
ip ) n2F2AΓv/4RT where v is the scan rate, A is the area of the electrode, Γ is the surface coverage due to redox moieties, and n, F, R, and T have their usual significance. Accordingly, the surface coverage estimated from Figure 2 corresponds to 430 ( 20 µC‚cm-2, which is slightly higher than that obtained from the charge integration of cyclic voltammogram. The agreement between the values arrived at by the two routes is reasonably good considering that the studies involve measurements on solid polycrystalline electrodes. The charge corresponding to the surface-immobilized redox species, as remarked earlier, is estimated from the integration of the voltammetric peak to be 250 ( 20 µC‚cm-2. This works out to a surface coverage of 2.5 × 10-9 mol‚cm-2. The approximate upper limit for the monolayer coverage of a bivalent adsorbate is reported to be on the order of 10-9 mol‚cm-2.26 Further, coverage calculations from underpotential deposition of metals show that a monolayer corresponds to around 2 × 10-9 mol‚cm-2.27 Relatively lower values (on the order of 10-10 mol‚cm-2) for monolayer coverage of surface redox species have been reported;18 however, this pertains to redox species electrostatically trapped inside a polymeric matrix. Thus, the charge estimations in the present studies indicate that the modification yields a coverage which is about one monolayer. The separation between anodic and cathodic peaks (cf. Figure 1) for the surface redox wave (∆Ep) is found to be about 30 mV at 100 mV‚s-1. For an ideal, reversible, surface redox behavior, ∆Ep should be zero and independent of sweep rate.1 However, most of the experimentally reported, nearly reversible surface redox systems are, indeed, characterized by finite values of ∆Ep.22 The studies on ∆Ep variation as a function of scan rate on this electrode indicate that ∆Ep increases to a very little extent (10-35 mV) as v is increased from 5 to 200 mV‚s-1. The very low values of ∆Ep, that also vary insignificantly with change in scan rate show that the modification yields an almost reversible surface-bound redox system. It is pertinent to (25) Sharpe, A. G. The Chemistry of Cyanocomplexes of the Transition Metals; Academic Press: London, 1976; p 57. (26) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1973, 77, 1401. (27) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley & Sons: New York, 1978; Vol. 11, Chapter II.
Figure 5. Voltammetric response showing the catalytic effect of the HCF-modified electrode on ascorbic acid oxidation. Cyclic voltammogram of 5 mM ascorbic acid in 0.5 M KCl on (a) bare Au electrode (dashed line) and (b) the modified Au electrode (solid line; modified as per the conditions described in Figure 1b). Potential sweep rate ) 50 mV‚s-1.
note that earlier attempts to incorporate reversible HCF through electrostatic binding on a PVP-modified carbon paste electrode have yielded only quasireversible modification with ∆Ep values as high as 90 mV20 and 150 mV.28 The half-peak-width (∆E1/2) of the surface redox waves is predicted1 to be 91/n mV under ideal situations where the adsorbates do not interact with each other. The present studies have yielded ∆E1/2 values of 45 and 25 mV for the anodic and cathodic peaks, respectively. Deviations from the ideal values of ∆E1/2 are normally attributed to mutual interactions between the adsorbates resulting in smaller ∆E1/2 and narrower peaks when attractive forces predominate.29 The low ∆E1/2 values noted in the present case are suggestive of attractive interactions in the (28) Bonakdar, M.; Vilchez, J. L.; Mottola, H. A. J. Electroanal. Chem. 1989, 266, 47. (29) (a) Angerstein-Kozlowska, H.; Klinger, J.; Conway, B. E. J. Electroanal. Chem. 1977, 75, 45. (b) Laviron, E. J. Electroanal. Chem. 1974, 52, 395.
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monolayer that need further study. Similarly, the absence of symmetry between the anodic and cathodic peaks in the present case, which is similar to an earlier report30 involving surface-bound polyvinyl ferrocene, demands further understanding of its origin. The foregoing observations can be tentatively understood as follows: TPA molecules by virtue of strong attractive interactions between Au and S atoms can form a SAM on the Au electrode, as evidenced by our earlier studies10 and literature reports.31,32 Because of the short methylene chain-length and polar head groups, namely, carboxylate, the monolayer formed may not be compact and will be permeable to the electrolyte.7,10 The treatment of the TPA monolayer in molybdenum chloride solution, prior to cycling in ferricyanide, possibly allows the incorporation of molybdenum cations into the monolayer through interactions with anionic carboxylate moieties. The presence of cations in the TPA film may facilitate the immobilization of HCF anions through electrostatic interactions and thereby account for the observed surface redox behavior of ferro/ferricyanide. The above electrostatic interactions may also contribute to the stability of the modified film. A screening effect between the adsorbed (30) Merz, A.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 3222. (31) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (32) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426.
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HCF anions by the presence of molybdenum cations in the admonolayer is not ruled out and hence can contribute to attractive interactions, giving rise to the observed narrowing of peaks.29 Further detailed investigations are needed for more comprehensive understanding of the modification. Electrocatalytic Studies with Au/HCF The remarkable electrocatalytic properties exhibited by surface HCF moieties toward oxidation of ascorbic acid and hydrazine are depicted in Figures 4 and 5. It can be seen that, on bare Au, oxidation of hydrazine starts at about 0.8 V, as manifested by an increase in current. However, the rate of the reaction being slow, no clear peak formation could be noticed. On the other hand, on Au/HCF a well-formed peak could be observed at around 0.75 V, with enhanced currents (nearly 50 times). Similarly, it can be seen from Figure 5 in the case of ascorbic acid that the oxidation is catalyzed and the peak (whose value is doubled with sharp features) on Au/HCF is shifted to less anodic potentials by 0.25 V when compared to that on bare gold. Acknowledgment. The authors express their sincere thanks to the Director, Central Electrochemical Research Institute, Karaikudi for his permission to publish this work. LA9503616
