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Comparative Study of the Electrodeposition of Poly(3-octylthiophene) Films on Gold Electrodes: Bare and Modified with Dodecanethiol Monomolecular Layer Maciej Mazur and Paweł Krysin´ski* Laboratory of Electrochemistry, Department of Chemistry, University of Warsaw, 02-093 Warsaw, Pasteura 1, Poland Received March 8, 2000. In Final Form: June 19, 2000 Poly(3-octylthiophene) (p3OT) was deposited potentiostatically on bare and dodecanethiol-coated polycrystalline gold electrodes. The obtained polymer layers were investigated by cyclic voltammetry with use of hydrophobic (ferrocene) and hydrophilic (Ru(NH3)63+) redox probes. The study of the morphology of obtained layers was also carried out by scanning tunneling microscopy in air. It was found that while p3OT deposited on bare gold contains a number of defects (pores, holes), the polymer deposited on thiolcoated electrode is characterized by more compact properties. It was proposed that in this case a complex system is formed where we can differentiate between a “mixed” inner layer consisting of polymer and thiol molecules and an outer layer of “pure” poly(3-octylthiophene). By “mixed” we understand the possibility of both filling the existing defects in the monolayer by the polymer and, eventually, replacing, with immediate entrapment into the matrix, some thiol molecules.
Introduction The organization of monomolecular assemblies at solid surfaces allows for the fabrication of interfaces with welldefined composition and thickness. In particular, the spontaneously adsorbed monolayer films formed from alkanethiols and their functionalized analogues at gold surface have been extensively examined. They were used as model molecular systems for elucidating the structurereactivity relationships for a variety of interfacial chemical processes (e.g., wetting, adhesion, molecular recognition, catalysis).1 In an ordered monolayer, the surface properties are determined by the chemical nature of the terminal group of the thiol at the monolayer/ambient interface, so the hydrophobicity, acid-base properties, and the reactivity of the modified electrode can be adjusted by the choice of the end group.2,3The organic monolayers can be used as platforms to carry out chemical and electrochemical reactions which lead to novel two- and three-dimensional molecular architectures of enhanced performance. For example, the physical characteristics of electropolymerized materials can be altered by preadsorbing an organic monolayer at working electrode. Deposition of conducting polymers4 on thiol-coated electrodes has been investigated recently in a number of publications. Patterned thiol monolayers have been used as molecular resists at the surface for selective deposition of conjugated polymers.6-12 Monomers covalently or ioni(1) Ulman, A. An Introduction to Ultrathin Organic Films: Academic Press: New York, 1991. (2) Finklea, H. O. Electroanalytical Chemistry; Bard, A. J., Rubinstein, J., Eds.: Marcel Dekker: New York, 1996; Vol. 19, p 109. (3) Aisenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495. (4) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, Basel, Hong Kong, 1998. (5) Rozsnay, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993. (6) Nishizawa, M.; Miwa, Y.; Matsue, T.; Uchida, I. J. Electrochem. Soc. 1993, 140, 1650. (7) Rozsnay, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. (8) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526. (9) Rozsnay, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309.
cally attached to thiol monolayer have been electropolymerized in order to obtain ultrathin, two-dimensional polymer films.13-17 One-component or mixed thiol monolayers were used in conjunction with adsorbate-directed electrochemical deposition to produce arrays of nano- or micrometer-sized features of polymeric material on surfaces.18-21 The effect of different thiol monolayers on the deposition of thicker polymer layers (rate of electropolymerization, adhesion to the substrate, density of the polymer, morphology of the film) has been studied as well.22-28 In this paper we present the comparative study of poly(3-octylthiophene) (p3OT) deposition on bare and dodecanethiol (C12SH)-coated gold electrode. The aim of this study was to provide a very thin, yet compact polymeric film with its electrochemical behavior determined solely (10) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. (11) Sayre, C. N.; Collard, D. M. J. Mater. Chem. 1997, 7, 909. (12) Huang, Z.; Wang, P.-C.; Feng, J.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. M. Synth. Met. 1997, 85, 1375. (13) Willcut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (14) Willcut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (15) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 10733. (16) Michalitsch, R.; Kassmi, A. E.; Yassar, A.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1998, 457, 129. (17) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752. (18) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (19) Hayes, W. A.; Shannon, C. Langmuir 1998, 14, 1099. (20) Mazur, M.; Krysinski, P.; Jackowska, K. Thin Solid Films 1998, 330, 167. (21) Krysinski, P.; Brzostowska-Smolska, M.; Mazur, M. Mater. Sci. Eng. C 1999, 8-9, 551. (22) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (23) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F.; Pireaux, J.-J. Synth. Met. 1998, 96, 165. (24) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F.; Caudano, R. J. Electrochem. Soc. 1999, 146, 2913. (25) Sayre, C. N.; Collard, D. M. Langmuir 1997, 13, 714. (26) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (27) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L.; Vogt, K.; Kohl, P. Synth. Met. 1993, 55, 1171. (28) Rubinstein, I.; Rishpon, J.; Sabatini, E.; Redondo, E.; Gottesfeld, S. J. Am. Chem. Soc. 1987, 112, 6135.
10.1021/la000352j CCC: $19.00 © 2000 American Chemical Society Published on Web 09/15/2000
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by the film properties, without the influence of metal substrate being normally accessible through the pores present in such a thin film deposited without the alkanethiol monolayer. Experimental Section Chemicals. All chemicals were of the highest quality commercially available: 3-octylthiophene (3OT) (Aldrich, 98%), dodecanethiol (Aldrich, 98%), lithium perchlorate (LiClO4) (Aldrich, ACS grade), acetonitrile (Aldrich, anhydrous, 99.8%), ferrocene (Aldrich, 98%), chloroform (Lachema, reagent grade), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3) (Aldrich, 98%), potassium chloride (KCl) (POCh, reagent grade), chromium (Aldrich, 99.99+%), tungsten wire (Aldrich, 99.9+%), potassium hydroxide (KOH) (POCh, reagent grade). Aqueous solutions were prepared from water of high purity (Milli-Q). Instrumentation. Electrochemical measurements were conducted with PC-controlled, custom built potentiostat/galvanostat (KSP Electronics, Poland), using conventional small volume three-electrode cell with Pt wire as counterelectrode. All potentials are quoted versus Ag/AgCl/1 M KCl(aq) reference electrode. Solutions of hexaamineruthenium(III) chloride used for integrity studies of polymer layers were deaerated by bubbling with argon for 10 min before measurements. Commercially available Nanoscope IIIa from Digital Instruments was used to collect all the scanning tunneling microscope (STM) images presented in this paper. The microscope was run in the constant current mode. Tungsten tips were prepared by electrochemical etching of a 0.25-mm diameter wire in 4 M aqueous KOH as described elsewhere.29 Preparation of Electrodes. All polycrystalline gold electrodes (Au, 99.99%) used for electrochemical measurements were cleaned by annealing in the reductive flame of the burner; they were then cyclically polarized in 1 M HClO4 aqueous solution within the -300-1500 mV potential range. Substrates used for STM measurements were microscope glass slides with a evaporated 2-nm layer of chromium and 200 nm of gold (prepared in the Laboratory of Electrochemistry, University of Florence, Italy). Monolayer Formation. Thiol monolayers were prepared by immersing clean gold electrodes into 10 mM dodecanethiol in chloroform solution for about 3 h. Then, the monolayer-modified gold was removed from the solution, rinsed with chloroform, and dried. Poly(3-octylthiophene) Deposition. The electrochemical polymerization of 3-octylthiophene (0.1 M of 3OT in 0.1 M LiClO4 in acetonitrile) was done potentiostatically at 1450 mV for a specified period of time, and then a potential of 0 mV was applied for 40 s in order to transform the polymer into its reduced form.
Results and Discussion Comparison of Potentiostatic Deposition of p3OT on Bare and Thiol-Coated Gold. Figure 1 shows the chronoamperometric curves of electrochemical polymerization of 3-octylthiophene (3OT) on bare (a) and dodecanethiol (C12SH)-coated (b) gold electrode. In both cases a very steep decrease of current is observed due to the double layer charging effect. However, the charging currents recorded for the thiol modified gold are significantly smaller, which can be assigned to a smaller capacity of highly hydrophobic thiol monolayer.30 For larger polymerization times both curves pass through a minimum of current, which corresponds with the threshold of monomer oxidation and nucleation of the polymer. The depth of the minimum for thiol-coated gold is larger, because of the blocking properties of the monolayer and more difficult transport of monomers from (29) Heckl, W. M. In Procedures in Scanning Probe Microscopies; Colton, R. J., Engel, A., Frommer, J. E., Gaub, H. E., Gewirth, A. A., Guckenberger, R., Rabe, J., Heckl, W. M., Parkinson, B., Eds.; John Wiley and Sons: Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1998; p 76. (30) Krysinski, P.; Brzostowka-Smolska, M. Bioelectrochem. Bioenerg. 1998, 44, 163.
Figure 1. Chronoamperometry on (a) bare and (b) C12SHcoated gold at 1450 mV in 0.1 M 3-octylthiophene in 0.1 M LiClO4 in acetonitrile.
the solution to the metal surface. However, the blocking properties of the dodecanethiol monolayer in acetonitrile solution, where we performed the experiments, are significantly smaller in comparison with frequently reported extremely high passivating behavior of thiol monolayer in aqueous solutions. This is because the acetonitrile molecules, which are less polar than the water molecules, can penetrate relatively easily the hydrophobic region of alkyl chains of the thiol monolayer. After reaching a minimum on amperometric curves, the current increases to approach a plateau (thiol-coated gold) or a broad maximum (bare gold), after about 10 and 5 s, respectively. Higher polymerization times are characterized by a uniform growth of p3OT (flat part of the curves, 10-25 and 10-40 s for bare and modified gold, respectively) and subsequent further increase of the deposition rate in both cases. Redox Behavior of the Polymer Layers. In Figure 2 the cyclic voltammograms of p3OT electrosynthesized on bare and thiol-modified gold are presented. In both cases a pair of clearly shaped voltammetric peaks around 1000 mV is observed, which can be assigned to the oxidation and reduction of the polymer layers. These cyclic voltammograms are in good agreement with the literature data,31 which means that the polymers of good electronic properties were obtained. Although the voltammetric curves for C12SH/p3OT and p3OT are similar in shape, the absolute values of currents are slightly smaller for C12SH/p3OT layer. We believe that the main reason for this phenomenon is the smaller capacitive currents observed for C12SH/p3OT since this layer is more compact and dense (see discussion in C12SH/p3OT Layer in Aqueous Ru(NH3)6Cl3 Solutions). It is also possible that even though the polymerization charge is the same in the two cases, the steric effects associated with the presence of thiol monolayer could prevent the effective recombination of oxidized monomers and result in somehow smaller amount of deposit. The analogous behavior was reported in the study of galvanostatically deposited poly(3-decylthiophene) on C12SH-coated gold.21 (31) Roncali, J. Chem. Rev. 1992, 92, 711.
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Figure 2. Steady-state cyclic voltammograms for (a) p3OT and (b) C12SH/p3OT modified gold in 0.1 M LiClO4 in acetonitrile (Qpolym ) 10 mC/cm2). Sweep rate ν ) 50 mV/s.
Figure 3. Cyclic voltammograms for (a) p3OT and (b) C12SH/p3OT modified gold in 1 mM ferrocene in 0.1 M LiClO4 in acetonitrile (Qpolym ) 10 mC/cm2). Sweep rate ν ) 50 mV/s.
Compactness of p3OT and C12SH/p3OT Layers. One of the important properties of thin layers covering the electrode surface is their defectness and smoothness. Cyclic voltammetry in the presence of a redox probe in the solution is a convenient technique used to investigate the integrity of blocking layers on the electrode. Through the study of the current-voltage characteristics of filmmodified electrodes, it is possible to derive important information about organization of the layer, e.g., lack or presence of defect sites. The fact that conducting polymers can be switched between the two statessconducting (oxidized) and nonconducting (reduced)sopens up the possibilities for investigation of their integrity properties in the potential range of nonconductance. Thus, the two kinetically fast redox probes (ferrocene in acetonitrile and Ru(NH3)6Cl3 in aqueous solution) were chosen on the basis of their different hydrophilic/hydrophobic properties for such studies of highly hydrophobic p3OT and C12SH/p3OT films on gold. Acetonitrile Solutions of Ferrocene. The integrity studies of p3OT and C12SH/p3OT were carried out in an acetonitrile solution in the presence of a hydrophobic reversible redox probe, ferrocene. The cyclic voltammograms were recorded in the potential region 0-600 mV, where, as one could see from Figure 2, the p3OT was in its reduced, nonconducting form (formal potential of ferrocene is 370 mV, as calculated from CV curve on bare electrode as an average value of anodic and cathodic peak potentials). Under these conditions the polymer should behave rather like a blocking layer on the metal surface, attenuating the oxidation of ferrocene. Figure 3 shows the CV curves of ferrocene redox reaction on p3OT and C12SH/p3OT layers. The voltammograms recorded for p3OT show the oxidation peak at about 500 mV and no reduction peak in the studied potential region. Although the curve is irreversible, it is recognizable that the blocking properties of the polymer layer are not very high (large currents), and ferrocene molecules can be transported relatively easily through the highly hydrophobic layer and then be oxidized. Another explanation of such voltammetric behavior is that even if p3OT is
reduced it can mediate the electron transfer between ferrocene and the bare metal.32 Similar electrochemical behavior is observed for the composite C12SH/p3OT layer, but the increase of ferrocene oxidation current is not associated with the voltammetric peak within this potential range and the currents are smaller. The comparison of the two layers shows that, despite relatively small differences caused by low “sensitivity” of the hydrophobic redox probe, the C12SH/p3OT layer exhibits higher blocking properties. However, it seems that more satisfactory results (more significant differences in the behavior of these two layers) could be obtained with use of hydrophilic redox probe in polar medium. This could allow for more precise detection of eventual defects due to the hindrance of the permeation of such a probe through the polymer or thiol/polymer layers. C12SH/p3OT Layer in Aqueous Ru(NH3)6Cl3 Solutions. Studying the integrity of C12SH/p3OT and p3OT layers with use of the hydrophobic redox probe gave no satisfactory results as was discussed in the paragraph above. This is the reason we decided to choose a hydrophilic, kinetically fast redox probesRu(NH3)6Cl3 in aqueous KCl solution. The hexaamineruthenium(III) redox probe is frequently used in the studies of passivating properties of thiol monolayers.33 We were also using it successfully in our previous studies of self-assembled monolayers modified with conducting polymers.20,21 The formal potential of the ruthenium complex is -160 mV; therefore, in this range of potentials the polymer is in its reduced, nonconducting form. Moreover, poly(3-octythiophene) is practically electrochemically inactive in aqueous solutions due to the high hydrophobicity of the polymer layer.34 This prevents eventual redox reactions between the polymer layer and the probe, which could make it more difficult to elaborate the experimental data. (32) Bobacka, J.; Grzeszczuk, M.; Ivaska, A. J. Electroanal. Chem. 1997, 427, 63. (33) Krysinski, P.; Brzostowska-Smolska, M. J. Electroanal. Chem. 1997, 424, 61. (34) Roncali, J.; Shi, L. H.; Garreau, R.; Garnier, F.; Lemaire, M. Synth. Met. 1990, 36, 267.
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Figure 4. Cyclic voltammograms of C12SH/p3OT-coated gold (for different polymerization charge densities) in 1 mM Ru(NH3)6Cl3 in aqueous 0.5 M KCl. Sweep rate ν ) 50 mV/s.
Thus, Figure 4 shows a set of voltammetric curves of Ru(NH3)6Cl3 reduction recorded on C12SH- and C12SH/ p3OT-modified gold electrodes for different charges passed during the potentiostatic electropolymerization. As it can be seen, all of these curves are characterized by very small currents (in comparison to bare gold, not shown) and by an exponential shape. This extremely irreversible behavior shows that the layers on the electrodes act as barriers for electron transfer between redox species in the solution and metal. This confirms that the layers are essentially free from structural defects (pinholes) and seem to be uniform over the whole surface. The decrease of Ru(NH3)3+ reduction current with the increase of polymerization charge is obviously reasonable: the blocking properties are higher for thicker layers. Figure 5 shows the dependence of the reduction current read from the CV curves at -400 mV as a function of polymerization charge density. Except for the point at Qpolym ) 0 mC/cm2, the other experimental data follow the exponential curve which shows that the current flowing through the blocking layer decreases with the increase of the thickness of the layer. The fact that the current for the unmodified thiol monolayer does not follow the exponential curve derives from the structural changes of the monolayer during the polymerization process. We think that at the initial steps of deposition, the polymer is filling the defects (e.g., collapse sites) of an imperfect thiol monolayer, which results in a more dense and thus more compact structure. On the other hand, the relatively high currents flowing during the electropolymerization process (see Figure 1) can additionally produce a number of new monolayer defects (immediately filled with a polymer), which can be attributed to collapse of the alkyl chains or partial desorption of thiol molecules. On the basis of our experimental data, it is difficult to judge, whether the polymer is incorporated into the monolayer just simply by filling the defects or additionally some thiol molecules are desorbed and entrapped into the forming polymer matrix. Despite the mechanism of the polymer growth at the initial steps, it seems that a thin composite or “mixed” thiol/ polymer layer adjacent to the bare metal is formed, having
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Figure 5. Current density at -400 mV read from cyclic voltammograms of C12SH/p3OT-coated gold in 1 mM Ru(NH3)6Cl3 in aqueous 0.5 M KCl.
Figure 6. Cyclic voltammograms of p3OT-coated gold (for different polymerization charge densities) in 1 mM Ru(NH3)6Cl3 in aqueous 0.5 M KCl. Sweep rate ν ) 50 mV/s.
different blocking properties than the unmodified monolayer. Probably the subsequent growth of the polymer film does not affect this “inner” layer, so even if the morphology of the “outer” polymer layer is the same as of pure p3OT, the blocking properties of the whole structure are not affected. p3OT Layer in an Aqueous Ru(NH3)6Cl3 Solution. Figure 6 shows cyclic voltammograms of Ru(NH3)63+ redox behavior on p3OT-coated gold. As one can see both the magnitude of the current and the shape of the curves change with the thickness (polymerization charge density) of the prepared polymer layers. While the redox signal of Ru(NH3)63+ is only slightly affected by the presence of thin layer of the polymer (Qpolym ) 7.96 mC/cm2), with the
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Figure 7. Dependence of the average distance between defect sites in p3OT versus polymerization charge density.
increase of polymerization charge the observed currents decrease and the shape of the curves becomes sigmoidal (see inset). This behavior is totally different than that of the C12SH/p3OT layers. These results suggest that, at the initial stages of a polymer deposition, the obtained layers are not compact, with large areas of uncovered bare gold. Further growth of the polymer leads to layers with a number of relatively small defects acting as single microelectrodes. The sigmoidal shape of cycling voltammograms for larger polymerization charges confirms the presence of holes or pores in p3OT layers. Additionally the limiting current does not vary with the sweep rate in the range of 5-1000 mV/s (not shown) which strengthens the validity of the derived conclusions. According to the theory of Amatore and co-workers,35 for blocking layers characterized by the sigmoidal voltammograms, the analysis of the magnitude and the position of Ru(NH3)63+ reduction waves with respect to its formal potential on bare electrode, allowed us to obtain an average effective diameter K of the defect sites, and the distance between them, L. Figure 7 presents the dependence of calculated value of L versus the polymerization charge density. The distance between pinholes in the polymer film increases nonlinearly with its thickness and is in the range of tens of micrometers. The effective diameter of defect sites (Figure 8) decreases with polymerization charge, but the changes are rather small; the diameter is about 40 nm. The above results suggest that with the growth of p3OT the amount of defect sites decreases, but the morphology of the film (diameter of pores) does not change significantly. Scanning Tunneling Microscopy of C12SH/p3OTand p3OT-Coated Gold. Figures 9 and 10 present the STM images in air of p3OT deposited on C12SH-coated and bare gold, respectively. Both surfaces look similar and are characterized by rather smooth morphology. The images are analogous to these reported in the literature concerned with thin layers of polythiophenes.36 The (35) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39.
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Figure 8. Dependence of the average defect site diameter in p3OT versus polymerization charge density.
Figure 9. STM image (500 nm × 500 nm, Z scale 30 nm) of C12SH/p3OT-coated gold (Qpolym ) 10 mC/cm2). Imaging conditions: tunneling current, 300 pA; potential bias, 100 mV; scan rate, 1.3 Hz.
surfaces consist of small plumes projecting from the bulk. Plume diameters are relatively uniform, ca. 30 nm. It is not clear, however, whether the observed features of the thin layers (estimated thickness of the layers calculated from the charge consumed for polymerization is ca. 100 nm, assuming density of the film equal to 0.9 g/cm3 37) are immanent characteristics of the polymer or rather reflect the morphology of underlying gold substrate, since one can observe a number of holes and valleys on (36) Heckl, W. M. In Procedures in Scanning Probe Microscopies; Colton, R. J., Engel, A., Frommer, J. E., Gaub, H. E., Gewirth, A. A., Guckenberger, R., Rabe, J., Heckl, W. M., Parkinson, B., Eds.; John Wiley and Sons: Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1998; p 257-260. (37) Bobacka, J.; Lewenstam, A.; Ivaska, A. Talanta 1993, 40, 1437.
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The general conclusion can be drawn, however, that since we do not observe any significant distinctions in morphology on the nanometer scale the reason for different blocking properties of C12SH/p3OT and p3OT layers does not derive from morphological aspects. This rather confirms the hypothesis of formation of a thin, compact inner layer in the case of deposition on a thiol-coated gold.
Figure 10. STM image (500 nm × 500 nm, Z scale 30 nm) of p3OT-coated gold (Qpolym ) 10 mC/cm2). Imaging conditions: tunneling current, 300 pA; potential bias, 100 mV; scan rate, 1.3 Hz.
both C12SH/p3OT and p3OT surfaces. Additionally, in view of discussion conducted above, it is probable that on the heterogeneous, composite surface some features can be simply artifacts deriving from different height of the energy barrier the tunneling electrons have to penetrate in different areas of the electrode surface. This is why we cannot compare the sizes of observed by STM well-like features with pinhole defect diameters calculated on the basis of electrochemical data.
Conclusions We compared the electrochemical behavior of p3OT layers deposited potentiostatically on bare and dodecanethiol-coated polycrystalline gold. We found that, although the morphology and redox behavior of the layers are similar, the significant differences could be observed in their blocking properties against the hydrophilic redox species. The increased blocking properties of C12SH/p3OT can be explained in terms of formation of a composite, “mixed” thiol-polymer inner layer, adjacent to gold, which is more compact than “pure” p3OT. The importance of above systems, especially thin thiol/ polymer layers, derives from the fact that they separate the bare electrode from the bulk of the solution allowing one to carry out the redox reactions exclusively on the polymer and not on metal surface accessible through layer defects. This could find many applications in design of biological component analytical devices and molecular electronics. Acknowledgment. Special thanks are due to Dr. G. Aloisi and Prof. R. Guidelli for the possibility of preparing gold-covered glass slides. The funding for this work was provided by a grant KBN 3 T09A 128 16 from the State Committee for Scientific Research. Funds provided by the Department of Chemistry, University of Warsaw, through the 12-501/68/BW-1453/9/99 grant are also acknowledged. LA000352J
