Correlation of the Structural Decomposition and Performance of

silver and copper substrates by surface-enhanced Raman spectroscopy. P Corio , G.F.S Andrade , I.C.N Diógenes , I.S Moreira , F.C Nart , M.L.A Te...
2 downloads 0 Views 308KB Size
Download PDF
736

Langmuir 1997, 13, 736-741

Correlation of the Structural Decomposition and Performance of Pyridinethiolate Surface Modifiers at Gold Electrodes for the Facilitation of Cytochrome c Heterogeneous Electron-Transfer Reactions Brian D. Lamp,†,‡ Daisuke Hobara,§ Marc D. Porter,*,† Katsumi Niki,§,| and Therese M. Cotton† Ames Laboratory-USDOE, Department of Chemistry, and Microanalytical Instrumentation Center, Iowa State University, Ames, Iowa, 50011, and Department of Physical Chemistry, Yokohama National University, Yokohama, Japan Received June 26, 1996. In Final Form: December 2, 1996X This paper describes the results of an electrochemical and spectroscopic (infrared reflection and X-ray photoelectron spectroscopies) investigation of the modified gold electrode surfaces prepared from dilute ethanolic solutions of 4-mercaptopyridine (PySH) and 4,4′-dipyridyl disulfide (PySS). Both precursors have been used extensively as facilitators for the electron transfer of redox proteins like cytochrome c (cyt c). During the course of an investigation of the interfacial architectures formed from the two different precursors, a previously unreported structural instability in the adlayers was discovered. This instability manifests itself as a decrease in the ability of the modified surfaces to facilitate the electron transfer of cyt c that correlates with an increase of the immersion time in the precursor solutions. Results are presented that delineate the decrease in facilitator performance and probe the structural changes resulting in the decrease in performance. Together, the electrochemical and surface spectroscopic findings reveal that the modified surfaces spontaneously decompose to yield an adlayer composed largely of adsorbed atomic and oligomeric sulfur, an adlayer that we found to be ineffective in the facilitation of the electron transfer reaction of cyt c. The implications of these findings on the use of this type of modifier to studies of electron transfer reactions of redox proteins and to issues of the general structural stability of organosulfur-based monolayers are briefly discussed.

Introduction Unraveling the mechanistic pathways of the heterogeneous electron transfer reactions of redox proteins has been an intensely active research area for many years.1 Most redox proteins, however, unfold as a consequence of their strong adsorption at bare electrode surfaces and lose their inherent electron transfer characteristics as a result. Thus, the interactions between proteins and electrode surfaces can have a strong impact on assessments of the rates of this important class of electron transfer reactions. Recent reports have indicated that some types of surface modifications counteract the electron transfer complications often caused by the adsorption of redox proteins at bare electrodes.2 These modifications are viewed as providing an interface that reduces an adsorption-induced unfolding of the protein, improving assessments of the inherent rates of the electron transfer reactions of such * To whom correspondence should be addressed. † Ames Laboratory-USDOE and Iowa State University. ‡ Present address: Department of Chemistry, University of South Dakota, Vermillion, SD 57069. § Yokohama National University. | Present address: Department of Chemistry, Iowa State University, Ames, IA 50011. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) (a) Taniguchi, I.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1982, 140, 187-193. (b) Armstrong, F. A. Struct. Bonding (Berlin) 1990, 72, 137-221. (c) Zhang, D.; Wilson, G. S.; Niki, K. Anal. Chem. 1994, 66, 3873-3881. (d)Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 1847-1849. (e) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. Phys. Chem. 1993, 97, 6564-6572. (f) Cullison, J. K.; Hawkridge, F. M.; Nakashima, N.; Yoshikawa, S. Langmuir 1994, 10, 877-882. (2) (a) Hinnen, C.; Parsons, R.; Niki, K. J. Electroanal. Chem. 1983, 147, 329-337. (b) Hobara, D.; Niki, K.; Chumanov, G.; Cotton, T. M. Colloids Surf. 1994, 93, 241-250. (c) Kuznetsov, B. A.; Mestechkina, N. M.; Shumakovich, G. P. Bioelectrochem. Bioenerg. 1977, 4, 1-17. (d) Kuznetsov, B. A.; Shumakovich, G. P.; Mestechkina, N. M. Bioelectrochem. Bioenerg. 1977, 4, 512-521.

S0743-7463(96)00637-3 CCC: $14.00

systems.3 One type of adsorbate-substrate combination that has been used effectively to this end involves the spontaneous adsorption of monolayer films from 4-mercaptopyridine (PySH) and 4,4′-dipyridyl disulfide (PySS) at gold electrodes.1a,2b,4 Studies have found that cytochrome c (cyt c) exhibits rapid electron transfer at the PySH- or PySS-derived electrodes and that the native conformation of the protein is effectively maintained upon its adsorption at these modified surfaces.2b During the course of a study aimed at extending the structural descriptions of this type of modified surface, we discovered a previously unreported instability in the layers formed from both precursors, a situation not found at modified surface prepared using other types of precursors (e.g., carboxylic acid-terminated alkanethiols)1d,2a This paper describes the results of our electrochemical and spectroscopic characterizations (i.e., infrared and X-ray photoelectron spectroscopies) of these systems that led to the discovery of the noted instability. We also propose possible adsorption products of the instability and examine the impact of the instability on the utility of these adsorbates for facilitating the electron transfer processes of redox proteins. Experimental Section A. Materials. 4-Mercaptopyridine (90%), 2-mercaptopyridine (99%), 4,4′-dipyridyl disulfide (98%), 2-mercaptopyrimidine (99%), and potassium hydroxide (99.99%) were purchased from Aldrich. Prior to use, cyt c (horse heart, Sigma type IV) was purified on (3) Sagara, T.; Murakami, H.; Igrashi, I.; Sato, H.; Niki K. Langmuir 1991, 7, 3190-3196. (4) (a) Allen, P. M.; Hill, H. A. O.; Walton, N. J. J. Electroanal. Chem. 1984, 178, 69-86. (b) Niwa, K.; Furukawa, M.; Niki, K. J. Electroanal. Chem. 1988, 245, 275-285. (c) Hinnen, C.; Niki, K. J. Electroanal. Chem. 1989, 264, 157-165. (d) Sagara, T.; Niwa, K.; Sone, A.; Hinnen, C.; Niki, K. Langmuir 1990, 6, 254-262. (e) Bond, A. M.; Hill, H. A. O.; Komorsky-Lovric, S.; Lovric, M.; McCarthy, M. E.; Psalti, I. S. M.; Walton, N. J. J. Phys. Chem. 1992, 96, 8100-8105.

© 1997 American Chemical Society

Modified Gold Electrode Surfaces

Langmuir, Vol. 13, No. 4, 1997 737

Figure 1. Cyclic voltammetry of cytochrome c at gold electrodes immersed for various times in 1.0 mM ethanolic PySH: electrolysis conditions, 100 µM cyt c in 30 mM pH 7 phosphate buffer; sweep rates, 50, 100, and 150 mV/s; immersion times, 1 min (a), 10 min (b), 20 min (c), 30 min (d), 40 min (e), 50 min (f), 60 min (g). a cation exchange column. All other chemicals were used as received. Water was deionized using a Millipore purification system. B. Substrate Preparation. Electrodes were prepared by the resistive evaporation of 300 nm of gold (0.4 nm/s deposition rate) onto either freshly cleaved mica (The Mica Company) or glass microscope slides (Fisher Scientific). All preparations were carried out in a cryogenically pumped Edwards Model E306A coating system at a base pressure of 10-6 Torr. The mica substrates were loaded into the evaporator immediately after cleaving. Upon removal from the evaporation system, the micasupported gold samples were annealed for 4 h at 300 °C in a muffle furnace at ambient pressure. The glass substrates, after cleaning with water, acetone, and hexane, were primed with 15 nm of chromium to improve the adherence of the gold films and were used directly after removal from the coating system. Characterization data related to the roughness and crystallinity of both types of surfaces have appeared elsewhere.5 C. Electrochemical Measurements. Electrochemical measurements utilized a conventional three-electrode electrochemical cell with the geometric area of the working electrode defined by an inert elastomer O-ring (0.62 cm2 area). An Ag/AgCl(saturated KCl) reference electrode was used; all applied potentials are reported with respect to this electrode. Experiments were conducted using a BAS CV-27 potentiostat (Bioanalytical Systems) and either an X-Y recorder or a computer-controlled data acquisition system (Labtech Notebook, Laboratory Technologies). D. Infrared Spectroscopy. Infrared spectra were acquired using a Nicolet Instruments Model 750 Fourier transform (5) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114.

infrared spectrometer equipped with a liquid-nitrogen-cooled narrow band MCT detector. Transmission spectra were obtained using KBr dispersions. Reflection spectra were collected using p-polarized light incident on the sample at 80° with respect to the surface normal. All reflection spectra are presented as -log(R/R0), where R is the reflectance of the PySH- or PySSderived samples and R0 is the reflectance of a perdeuterooctadecanethiolate monolayer adsorbed at gold. E. X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were acquired using a Physical Electronics Model 5500 multitechnique surface analysis system equipped with a hemispherical analyzer, torroidal monochromator, and a multichannel detector. Excitation used monochromatic Al KR radiation at 300 W, with detection at 45°.

Results and Discussion A. Voltammetry of Cytochrome c at a Gold Electrode Modified by Immersion into Dilute PySH Solutions. We began our investigation by exploring the effects of immersion time on the ability of the adlayers formed at gold by PySH and PySS to influence the efficiency of the electron transfer reaction of cyt c. The results of the study for the PySH-derived surface modifiers are presented by the series of cyclic voltammetric currentpotential curves in Figure 1 (scan rates of 150, 100, and 50 mV/s), with the cathodic (Ep,c) and anodic (Ep,a) peakcurrent potentials and the separations between the peakcurrent potentials for the scans at 100 mV/s summarized in Table 1. These curves were obtained using electrodes prepared by altering the immersion time of the gold

738

Langmuir, Vol. 13, No. 4, 1997

Lamp et al.

Table 1. Cathodic and Anodic Peak Potentials (Ep,c and Ep,a) and Separations between Peak-Current Potentials (|∆Ep,|) for the Electron Transfer Reaction of Cytochrome c as a Function of Immersion Time in a 1 mM Ethanolic Solution of PySH (scan rate 100 mV/s) immersion time (min)

Ep,c (V)

Ep,a (V)

|∆Ep| (V)

1 10 20 30 40 50 60

+0.06 +0.06 +0.06 +0.05 +0.05 +0.05 +0.04

+0.12 +0.13 +0.14 +0.15 +0.15 +0.16 +0.17

0.06 0.07 0.08 0.10 0.10 0.11 0.13

electrodes in a 1.0 mM ethanolic solution of PySH from 1 to 60 min. The electrolytic solution was 100 µM in cyt c and 30 mM in phosphate buffer (pH 7), and the scans were initiated at +0.30 V and reversed at -0.10 V. At short immersion times (e.g., 1 min), the curves for cyt c are characterized by a well-defined wave in both the cathodic and anodic sweeps. These waves correspond to the redox transformation of adsorbed and of solution forms of cyt c.1,4 The formal potential for this system, estimated as the average values of Ep,c and Ep,a, is +0.09 V. Furthermore, the nearly ideal values of the peak separations (e.g., ∼60 mV at the sample with the 1-min immersion time) are indicative of facile electron transfer kinetics at this voltammetric time-scale.6 The shapes of the voltammetric curves for cyt c obtained at bare gold electrodes have much larger peak separations (e.g., ∼500 mV) and almost undetectable reoxidative faradaic currents;4d,e this type of response is diagnostic of a decrease in the rate of electron transfer. These results demonstrate the improvements in electron transfer rates realized by the presence of the PySH-derived monolayer at gold electrode surfaces and are consistent with literature precedents.1a,4 An interesting and previously undetected trend develops if the ability of the modified electrode to facilitate the electron transfer reaction of cyt c is monitored as a function of immersion time in the modifier solutions. As described above, the electron transfer reaction for cyt c is comparatively facile at gold electrodes that are modified at short immersion times using PySH. With increases in immersion time (e.g., 30 and 60 min), however, the voltammetric curves deteriorate dramatically. These changes are evident by the increase in the current flow superimposed on the redox process for cyt c in Figure 1 as well as by the increase in the difference between the values of Ep,c and Ep,a in Table 1. These changes indicate a diminution in the ability of the modified electrode to facilitate the heterogeneous electron transfer reaction of cyt c. Further increases in immersion time result in a continued loss in the performance of the modified surface toward the electrolysis of cyt c. For example, the waves for the electrolysis of cyt c for samples immersed in the PySH solution for ∼24 h (not shown in Figure 1) were only barely distinguishable from the background currents. We have found a similar trend for the performance degradation of electrodes prepared in dilute (1.0 mM) ethanolic solutions of PySS. The combined weight of the results in Figure 1 therefore reveals a notable decrease in the performance of the two types of modified surfaces as facilitators for the heterogeneous electron transfer reaction of cyt c. It follows, in view of the large body of evidence in the literature for related sulfur-based monolayers at gold,7,8 that the change (6) Nicholson, R. S. Anal. Chem. 1965, 37, 1351. (7) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assmbly; Academic: Boston, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463.

Figure 2. Infrared transmission spectrum of PySH dispersed in KBr (a); S ) 0.5 AU. Infrared reflection spectra for the adlayer formed by immersion in 1.0 mM ethanolic solution of PySH for 1 min (b), 15 min (c), 1 h (d), 5 h (e), 24 h (f); S ) 0.0005 AU.

in performance is possibly related to a change in the composition and/or structure of the PySH- and PySSderived surfaces as a function of immersion time. We further speculate that the changes in the two systems have similar origins since both precursors are expected to form the same type of chemisorbed adlayer, i.e., a goldbound thiolate.1,9 As such, the results of an investigation of the structural evolution of these modifier systems comprise the balance of this paper, focusing on findings from characterizations of the PySH-derived system. We note that similar results were obtained for the PySSderived system but are omitted for brevity. B. Infrared Characterization. As a starting point for probing for possible immersion time dependencies on the composition and/or structure of the two types of modified surfaces, characterizations using infrared reflection spectroscopy were conducted. Figure 2 presents the results for the PySH-derived adlayer between 2000 and 700 cm-1 and includes a spectrum of PySH dispersed in KBr for comparative purposes. The KBr spectrum is dominated by several strong bands characteristic of a pyridyl moiety. For example, the modes between 1600 and 1400 cm-1 correspond to the coupled in-plane CdC and CdN stretching modes (ν(CdC + CdN)) of the pyridyl ring. Modes attributable to the C-H groups are also evident, with an in-plane C-H bend (δ(CH)ip) at 1229 (8) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (b) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (9) (a) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi J. Electroanal. Chem. 1984, 175, 341-348. (b) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi J. Electroanal. Chem. 1985, 186, 299-307. (c) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (d) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-588. (e) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (f) Garrell, R. L.; Szafranski, C.; Tanner, W. SPIE Raman an Luminescence Spectroscopies in Technology II 1990, 1336, 264-271. (g) Szafranski, C. A.; Tanner, W.; Laibinis, P.; Garrell, R. L. Submitted for publication in Langmuir.

Modified Gold Electrode Surfaces

cm-1 and out-of-plane C-H bends (δ(CH)op) at 813 and 800 cm-1.10 In contrast to that for PySH in KBr, a smaller number of bands are detected in the infrared reflection spectrum for the electrode immersed in the PySH solution for 1.0 min. Importantly, the 1-min spectrum contains several of the bands observed in the KBr spectrum, including the ν(CdC + CdN) bands at 1613 and 1473 cm-1 and the δ(CH)ip band at 1210 cm-1. The presence of these bands qualitatively confirms the formation of an adlayer of the expected composition. There are, however, some notable differences between the KBr and 1-min immersion spectrum. Several of the strong bands present in the KBr spectrum (e.g., 1111 cm-1) are comparatively weaker or absent in the adlayer spectrum. Furthermore, a band at 1564 cm-1 is present in the adlayer spectrum but not in the KBr spectrum. We believe that this band is a ν(CdC + CdN) but are as yet uncertain as to how chemisorption at gold results in its appearance or how this feature is related to the expected structure of the adlayer. Because of the differences between the KBr and 1-min spectra, coupled with the low signal-to-noise ratio in the adlayer spectrum, an orientation analysis using the infrared surface selection rule11 was not undertaken.12 In addition to the above, the spectra in Figure 2 reveal that increases in immersion time have a profound effect upon the structure of the adlayer. The spectra show a strong temporal decay in the absorption strengths of the bands diagnostic of a pyridyl functionality. Subtle changes are evident in comparing the spectrum for samples at 15-min and 1-min immersion times. Even more dramatic differences are found after 5-h and 24-h immersion times, with the pyridyl bands only slightly above background levels in the 24-h spectrum. We believe that these changes reflect a degradation in the composition of the adlayer. The basis for this assertion rests with our earlier findings concerning the instability of the adlayers formed from the chemisorption of mercaptoethanol13 and other short chain length alkanethiols14 at gold electrodes. For example, both electrochemical and XPS characterizations indicate that the adlayer formed from mercaptoethanol at gold rapidly converts to adsorbed atomic and oligomeric sulfur species. The evolution of the spectral data in Figure 2 therefore suggests that a similar decomposition may be operative. It follows that this decomposition may be linked to the decrease in the ability of the PySH- and PySS-derived modifiers to facilitate the electron transfer reaction of cyt c. The electrochemical and XPS characterizations in the next two sections provide strong support for this assertion. C. Electrochemical Characterization. We have previously described a method by which a range of thiolate monolayers can be desorbed electrochemically from gold electrodes by the reaction in eq 1.15 For adsorbates similar to those expected to form by the chemisorption of PySH (10) (a) Katritzky, A. R.; Gardner, J. N. J. Chem. Soc. 1958, 21982204. (b) Katritzky, A. R. Q. Rev. 1959, 13, 353-373. (c) Kline, C. H., Jr.; Turkevich, J. J. Chem. Phys. 1944, 12, 300-309. (d) Christensen, P. A.; Hamnett, A.; Blackham, I. J. Electroanal. Chem. 1991, 318, 407410. (11) (a) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315. (b) Pearce, H. A.; Sheppard, N. Surf. Sci., 1976, 59, 205-217. (12) We note that recent studies on this (refs 10g and 10d. Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753-756.) and related systems (Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979-9984. Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981.) have concluded that the plane of the pyridyl ring is oriented toward the surface normal, with the C2v symmetry axis canted to a small extent with respect to the surface normal. (13) Weisshaar, D. E.; Walczak, M. M., Porter, M. D. Langmuir 1993, 9, 323-329. (14) Zhong, C. J., Porter, M. D. Unpublished results. (15) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.

Langmuir, Vol. 13, No. 4, 1997 739

RSAu + e- f RS- + Au

(1)

and PySS at annealed (i.e. Au(111)) gold electrodes,16 the application of a linear voltage sweep in basic solution generally produces a single cathodic wave for the reductive desorption of the thiolate adlayer. We have also found that the potential for the reductive desorption process, Erd, is dependent on both the strength of the gold-sulfur interaction and the extent of the intermolecular interactions between adjacent adsorbates.15,17-19 Additionally, the desorption process can be used to determine the surface concentration of the adlayer, Γ, through eq 2, where Q is

Q ) nFAΓ

(2)

the charge consumed in the reductive desorption process,15 n is the number of electrons involved in the electrode reaction in eq 1, F is Faraday’s constant, and A is the electrode surface area. The typical value of Q for the desorption of alkanethiolates is ∼75 µC cm-2 20 and corresponds closely to that expected for the x3×x3 adlayer formed by this class of adsorbates at a Au(111) surface.7 Figure 3 presents the results from a linear sweep voltammetric characterization of the adlayer formed at Au(111) electrodes as a function of the immersion time in a 1.0 mM ethanolic solution of PySH. The scans were initiated at -0.20 V with a sweep rate of 100 mV/s and terminated at -1.2 V; 0.5 M KOH(aq) was used as the supporting electrolyte. For the 1-min immersion, the voltammetric curve is dominated by a cathodic wave with an Erd of -0.55 V. The charge consumed by this process (∼50 µC cm-2) is much less than that observed for the desorption of monolayers formed from n-alkanethiols at Au(111). We attribute the lower charge primarily to the packing limitation imposed by the larger size of a pyridyl group in comparison to a fully extend alkyl chain. Recent orientational analyses on related systems,21,22 as well as on thiophenol-derived adlayers,23 support this conclusion. In contrast to the data for the 1-min immersion time, the voltammetric curves for samples prepared for more extended immersion times undergo a marked evolution. Figure 3 shows that as immersion time increases, a wave at more negative values of applied voltage (-0.90 V) appears and that this wave grows at the expense of the wave observed for the 1-min immersion time. Within 5 h of immersion, only the wave at -0.90 V is detected. The magnitude of the charge consumed by the more negative wave (180 µC cm-2) is also much greater than that consumed by the wave at -0.55 V for the 1-min immersion as well as for those found for n-alkanethiol-derived monolayers.24,25 These findings, coupled with the spectroscopic data in Figure 3, clearly indicate that the (16) Zhong, C. J.; Porter, M. D. Submitted to J. Am. Chem. Soc. (17) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (18) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (19) Zhong, C. J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 1161611617. (20) After correction for surface roughness factor of 1.1 (see ref 5). (21) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955-963. (22) Gui, J. Y.; Stern, D. A.; Lin, C.-H.; Gao, P.; Hubbard, A. T. Langmuir 1991, 7, 3183-3189. (23) Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981. (24) Voltammograms at intermediate times are less reproducible than those at each time extreme, most likely a result of variable solution concentration and electrode activity. (25) To confirm the potential for this process, experiments were condicted with 1 mM PySH in the electrolyte solution. A single redox couple is evident in these voltammograms, characteristic of the reversible deposition/desorption of the thiolate at the electrode surface.18 The couple present in these data is centered at -0.53 V and has a charge for both the anodic deposition and cathodic desorption of 50 µC/cm2.

740

Langmuir, Vol. 13, No. 4, 1997

Figure 3. Voltammetric desorption curves as a function of immersion time in a 1 mM ethanolic PySH solution: immersion times, (a) 1 min, (b) 30 min, (c) 60 min, (d) 300 min, (e) 1440 min; electrolyte, 0.5 M KOH(aq); scan rate, 100 mV/s.

structure of the adlayer is strongly affected by the extent of immersion in the PySH solution. The transformation of the voltammetric data in Figure 3 is reminiscent of those found earlier for the adlayers formed at Au(111) from dilute solutions of mercaptoethanol and mercaptoacetic acid.13 In these latter systems, we attributed the transformation to an adsorption-induced activation of the oxidative cleavage of the C-S bond and the resulting formation of an adlayer composed largely of atomic and oligomeric forms of sulfur. We believe that a similar process is operative with the PySH- and PySSderived adlayers. The next section presents the results of an XPS characterization aimed at furthering our insights into the structural evolution of these adlayers, after which the findings from electrochemical and XPS analyses of the modified surface formed by the immersion of Au(111) in a dilute solution of Na2S are described. D. X-ray Photoelectron Spectroscopy. Insights into the compositional details of the PySH- and PySSderived adlayers were also sought using the diagnostic capabilities of XPS. The results of these characterizations are presented for the S(2p) and N(1s) regions in Figure 4 for samples prepared at 1-min (Figure 4a) and 24-h (Figure 4b) immersion times in the PySH solution. For the 1-min immersion, the features evident in both regions are in agreement with the expected composition of the adlayer. That is, the N(1s) band at 399.0 eV is consistent with position for nitrogen incorporated within a pyridine ring,26 and the S(2p1/2) and S(2p3/2) couplet at 161.2 and 162.4 eV, respectively, with the positions of sulfur as a gold-bound thiolate.9c,19,27-29 We also note that the atomic sulfur-to-nitrogen ratio in the adlayer, after accounting for the differences in the cross sections for the two species,30 is close to the 1:1 ratio expected from the PySH precursor. (26) Nordberg, R.; Albridge, R. G.; Bergmark, T.; Ericson, U.; Hedman, J.; Nordling, C.; Siegbahn, K.; Lindberg, B. J. Ark. Kemi 1967, 28, 257278. (27) Lamp, B. D.; Alves, C. A.; Franek, J. E.; Porter, M. D. Manuscript in preparation. (28) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (29) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (30) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mailenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin Elmer: Eden Prairie, MN, 1978.

Lamp et al.

Figure 4. Dependence of XPS spectra in the nitrogen 1s and sulfur 2p regions on immersion time in a 1 mM ethanolic PySH solution: immersion time, (a) 1 min, (b) 24 h.

Longer immersion times give rise to a much different set of XPS signatures. Increases in immersion time result in a gradual broadening of the features in both the N(1s) and S(2p) regions. The evolution of these features is exemplified by the spectra in Figure 4b. In both regions, the features are clearly broader than those for the sample from the 1-min immersion time. These changes are also accompanied by an increase (a factor of ∼2) in the strength of the bands in the S(2p) region as well as by a loss of signal in the N(1s) region. We note that the sample-tosample variability was greater in the N(1s) region than in the S(2p) region, with the atomic sulfur-to-nitrogen ratio ranging between 2.5:1 and 3.0:1. More importantly, the decrease in signal strength in the N(1s) region is in general agreement with the loss of infrared spectral features diagnostic of a pyridyl group in Figure 2, providing further evidence for the decomposition of the adlayer with increasing immersion time. The next section, which discusses the results from analyses of samples modified in a Na2S solution, also examines the structural implications of the broadening the XPS data. E. Characterizations of Au(111) Electrodes Modified by Immersion into a Dilute Solution of Na2S. The experiments described in this section were designed to test whether the evolution of the structure of the adlayer formed at gold electrodes from PySH could be ascribed to the growth of an atomic and/or oligomeric sulfur adlayer. To this end, samples were prepared by the immersion of Au(111) electrodes in a 1.0 mM Na2S (unbuffered) solution for 30 min. The results of the voltammetric and XPS characterizations are shown in parts a and b of Figure 5, respectively. The voltammetric curve is dominated by a large cathodic wave at -0.90 V and has an integrated charge of ∼150 µC cm-2. This curve, which is similar to that reported in a earlier study of Na2S at gold,31 has a strong correspondence to those found for the samples in Figure 3 with the 5- and 24-h immersion times. (31) (a) Wierse, D. G.; Lohrengel, M. M.; Schultze, J. W. J. Electroanal. Chem. 1978, 92, 121-131. (b) Baltruschat, H.; Staud, N.; Heitbaum, J. J. Electroanal. Chem. 1988, 239, 361-374. (c) Buckley, A. N.; Hamilton, I. C.; Woods, R. J. Electroanal. Chem. 1987, 216, 213-227. (d) Lezna, R. O.; de Tacconi, N. R.; Arvia, A. J. J. Electroanal. Chem. 1990, 283, 319-336.

Modified Gold Electrode Surfaces

Figure 5. Characterization of electrodes immersed for 30 min in 1 mM Na2S(aq): (a) voltammetric desorption curve (conditions are as in Figure 3); (b) XPS spectrum in the sulfur 2p region (conditions are as in Figure 4).

The XPS spectrum in Figure 5b, which shows the S(2p) region, also corresponds strongly to that for the sample from the 24-h immersion in the PySH solution in Figure 4b. The feature in Figure 5b covers the same energy range (i.e., centroid at ∼162 eV) and has a similar shape and strength as the feature in Figure 4b. We therefore attribute the broadening of the S(2p) feature in Figure 4b primarily to the overlap of the S(2p) couplets from a distribution of adsorbed atomic and oligomeric forms of sulfur species at the gold surface, with the broadening a reflection of a combination of sulfur-sulfur and goldsulfur interactions.5 Conversion of the modifier surfaces to a more sulfurlike species is also consistent with the facilitator performance of gold electrodes coated in the Na2S solution. Voltammetric scans for cyt c using such a coating are similar to those observed for long term immersions (e.g., 24 h) in the PySH solution. On the basis of the combined weight of the above results, we conclude that the gradual decrease in the ability of the PySH- and PySS-derived electrodes to facilitate the electron transfer reaction of cyt c is a consequence of the evolution of the adlayer from pyridinethiolate to a structure composed largely of various froms of adsorbed atomic and oligomeric sulfur. Conclusions The above results have revealed a previously undetected structural conversion in the monolayers formed from PySH and PySS precursors immobilized at gold electrodes. This conversion manifests itself as a structural evolution of an adsorbed monolayer of pyridinethiolate to an adlayer composed extensively of various forms of atomic and oligomeric sulfur, the presence of which increases with an increase in the immersion time in dilute solutions of the precursors. As a consequence, the performance of the modified electrodes to facilitate the efficiency of the redox transformation of cyt c markedly diminishes.

Langmuir, Vol. 13, No. 4, 1997 741

Questions therefore emerge concerning the mechanism of the conversion process, the factors of importance to the conversion, and the types of adsorbates susceptible to the conversion. While experiments to develop insights into these issues are underway, we have conducted a few preliminary studies that provide a framework for the factors probably of importance. In one such study, we have tested the effects of oxygen in the precursor solution. Results from voltammetric characterizations indicate that dissolved oxygen does not play a detectable role. This finding reflects experiments where samples prepared in sealed vessels of argon-purged precursor solutions transformed to a sulfided surface at roughly the same rate as those prepared in unpurged solutions. In another study, samples formed at short immersion times were exposed to the laboratory ambient for extended periods. A voltammetric analysis of these samples revealed that such exposures also led to a sulfurization of the gold surface; however, the rate of the conversion was much slower, with an air-exposed sample requiring ∼24 h to reach the same extent of conversion as observed for a sample under 1-h of continuous immersion. We also examined the dependence of the instability the adlayer on the structure of the precursor by examining the voltammetry for samples prepared from 1 mM solutions of 2-mercaptopyridine (2-PySH), 2,2′-mercaptopyrimidine (2-PyR), and thiophenol (TP). In the case of 2-PySH, the immersion dependence proceeded at a similar rate as found with the PySH-derived samples. Samples immersed in 2-PyR, however, do not exhibit voltammetric features indicative of surface sulfurization even after an immersion time of 4 days. There was also little change in the voltammetric responses for immersion dependence experiments using TP. Our preliminary studies suggest that there are clear structural factors of importance to the stability of this type of organosulfur-derived monolayer. Whether the stability can be correlated to differences in the donoracceptor strength of the chemical groups in close proximity of the sulfur-carbon bond and a subsequent destabilization of this bond upon binding to gold is one the several questions that we are presently pursuing. We are also seeking to determine whether this instability has relevance to the suggestion that layers formed at gold from long chain alkanethiols exist as a surface-bond disulfide (as opposed to a thiolate), since a surface composed in part of oligomeric sulfur would give rise to the observed S-S separation distances found in grazing incidence X-ray diffraction results.32 In summary, our results point to an important issue that needs to be addressed in the design and application of these types of facilitator surfaces. Acknowledgment. The expert assistance of Jim Anderegg in the collection and analysis of the XPS data is gratefully acknowledged. K.N. and D.H. express their appreciation for the support of the Japan Society for Promotion of Science through the US-Japan Cooperative Science Program. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. w-7405-eng-82. This work was supported by the Office of Basic Energy Research, Chemical Sciences Division of the U.S. Department of Energy. LA960637P (32) (a) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (b) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536.