dipyridine SAM on a Au(111) - American Chemical Society

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Structural Changes of 4,4′-(Dithiodibutylene)dipyridine SAM on a Au(111) Electrode with Applied Potential and Solution pH and Influence of Alkyl Chain Length of Pyridine-Terminated Thiolate SAMs on Cytochrome c Electrochemistry Soichiro Yoshimoto,†,‡ Yuta Ono,§ Yutaka Kuwahara,‡,§ Katsuhiko Nishiyama,‡,§ and Isao Taniguchi*,‡,# †

Priority Organization for Innovation and Excellence, Kumamoto University, 2−39−1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡ Kumamoto Institute for Photo-Electro Organics (Phoenics), 3−11−38 Higashi-machi, Higashi-ku, Kumamoto 862-0901, Japan § Graduate School of Science and Technology, Kumamoto University, 2−39−1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ABSTRACT: From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship between the electron transfer reaction and alkyl chain length of pyridine-terminated self-assembled monolayers (SAMs) at the molecular level. In the present study, SAMs of 4,4′(dithiodibutylene)dipyridine ((PyC 4 S) 2 ) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry (CV), surface-enhanced infrared absorption spectroscopy (SEIRAS), and in situ scanning tunneling microscopy (STM). Structural changes in the PyC4S-SAM were driven by changes in the applied electrode potential or pH of the electrolyte solution. In 0.05 M HClO4, a structural change in PyC4S-SAM was drastically induced by the potential manipulation from the open circuit potential to 0.10 V versus a reversible hydrogen electrode (RHE). A high-resolution STM image showed that the PyC4S-SAMs formed a p(12 × √3 R−30°) adlattice at ∼0.15 V versus RHE. A semisquarely arranged adlattice was newly formed at 0 V versus RHE. The reductive peaks that appeared in the CV profile were thus attributed to be a phase transition in the PyC4S-SAM. By changing the electrolyte in the solution from HClO4 to KClO4, PyC4S-SAM having a p(12 × √3 R−30°) lattice was changed to a p(8 × √3 R−30°) adlattice in 0.05 M KClO4 at pH 5.5, indicating that rearrangement of PyC4S-SAM took place. This difference in the adlayer structure between acidic and neutral solutions resulted from the protonation and deprotonation of the pyridine moieties in PyC4S-SAMs. Electrochemical investigations on the electron transfer reaction of cytochrome c revealed that the alkyl chain length in a Py-terminated thiolate SAM plays an important role in the heterogeneous electron transfer rate constant, ko′, but not in the diffusion coefficient, D, of cytochrome c. These results show that the structure of PyC4S-SAM can be controlled by changing the applied electrode potential and pH of the electrolyte solution. In addition, the effect of alkyl chain length in the Py-terminated SAM on cytochrome c electrochemistry is clearly demonstrated. Long-range electron transfer kinetics is discussed on the basis of the Marcus theory.

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INTRODUCTION Molecular design and control of functionalized surfaces play an important role in the formation of highly ordered and stable monomolecular adlayers, and the connection of various organic and/or organometallic components in nanotechnology applications.1−3 In particular, self-assembled monolayers (SAMs) of thiols and their disulfides on metal substrates have been recognized as a simple approach for preparing monolayers on the surfaces.4−11 SAMs have been extensively studied in electrochemistry in order to develop the functional electrodes modified with thiols.12−25 Currently, SAMs are widely used as a platform to create controlled nanospaces, such as surfacesupported metal−organic frameworks (SurfMOFs)26,27 and redox-active molecular wires via layer-by-layer coordination on functionalized surfaces.28,29 © XXXX American Chemical Society

In the research on SAMs, scanning tunneling microscopy (STM) has been accepted as a powerful tool not only for structural investigation at the molecular level30−40 but also for electronic conductivity measurements of single molecules between the substrate and STM tip.41−44 In fact, there are many STM studies on SAMs covering the formation process to highly ordered adlayers such as (√3 × √3)R30° and the related structure, the c(4 × 2) superlattice.31−37 The formation of gold adatoms sandwiched between two sulfur atoms was Special Issue: Kohei Uosaki Festschrift Received: November 29, 2015 Revised: January 18, 2016

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DOI: 10.1021/acs.jpcc.5b11666 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

information, we have been investigating the influence of the alkyl chain length of the adlayer in Py-terminated SAMs on cytochrome c electrochemistry.76 In the present study, the structural change of 4,4′(dithiodibutylene)dipyridine ((PyC4S)2; see Chart 1) SAM

reported in a recent study on single crystal X-ray structures of thiolate-protected Au clusters45 and on SAMs on a flat Au surface.46−49 Electrochemical STM investigations on electrochemical reductive desorption of SAMs from a Au surface are an attractive target for electrochemists.50−52 In contrast to alkanethiolate SAMs, STM investigations on aromatic thiolate SAMs have also been reported. 4-Mercaptopyridine (PySH) is of particular interest,53−59 because PySH and its corresponding disulfide, 4,4′-dipyridyldisulfide (PySSPy), a popular modifier molecule, were shown in the early 1980s to be an electron transfer promoter for cytochrome c.12,60−63 From the perspective of surface coordination chemistry, PyS-SAM also plays an important role as a linker platform for the coordination of cyano complexes such as [Ru(CN)5(PyS)]4−,64,65 metallophthalosyanine,66 and the electrodeposition of novel metal complexes.67−70 Thus, aromatic thiolate SAMs are attractive for controlling charge transfer and tailoring electronic functionality,27 because cytochrome c electrochemistry is well established. However, the preparation conditions for PyS-SAM are complicated. For example, the purity of PySH is important for its attachment to the Au(111) electrode surface. We found that a small amount of sulfide strongly affects the formation of PyS-SAM, i.e., the replacement of PySH with sulfide easily takes place on Au(111) during modification.71,72 The problem with small impurities of sulfide can be overcome if other crystal planes such as Au(100) and Au(110) are used. The dependence of the stability of the PyS-SAM on Au(111), Au(100), and Au(110) was found using the electron transfer reaction of cytochrome c as a monitor reaction.71 On Au(111), we proposed a simple method for preparation of well-defined PyS-modified surfaces by controlling the applied potential and pH of the solution during modification.72 Salvarezza’s group has also reported on the stability of PyS-SAMs on Au(111).73 They found that C−S bond cleavage between the pyridine C atom at the 4-position and the thiolate S atom occurred while the sample was stored in pure ethanol.73 Thus, a careful examination of the modification conditions is needed when PySH is used as a surface modifier. To overcome such modification difficulties, more stable pyridine-terminated SAMs have been explored. Recently, several interesting Py-terminated thiols were designed and their SAMs investigated by STM. For example, a structural investigation of 3-(4-pyridine-4-ylphenyl)-propane-1-thiolate SAMs on Au(111) was reported by Silien et al., where a highly ordered structure with (2√3 × √3) unit cells was formed for SAMs prepared in an ethanolKOH mixed solution at 345 K.74 Wöll’s group investigated a series of SAMs consisting of Py-terminated phenyl unit-linker thiols on Au(111) under ultrahigh vacuum (UHV).75 The adlayer structure of these SAMs depended on the number of methylene spacers between the SH group and the aromatic moiety, i.e., the (3 × 5√3)rect structure, which has a rather large unit-cell, was found for an even number of methylene units, whereas a smaller unit cell, (2√3 × √3)R30° structure, is assigned for an odd number of methylene units.27,75 It is important to anneal the substrate itself at a high temperature for the formation of highly ordered SAMs. From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship on the molecular level between the electron transfer reaction and the alkyl chain length of the Py-terminated SAMs. Understanding the protonation and deprotonation reaction of the Py moiety is still under discussion. On the basis of this background

Chart 1. Chemical Structure of (PyC4S)2

on the Au(111) electrode surface induced by the applied potential and pH of the solution, and the electrochemical phase transition in PyC4S-SAMs was successfully observed at the molecular level by EC-STM investigation. Also, the electrochemical responses of cytochrome c on PyCnS-modified Au(111) electrodes (n = 0, 1, 2, and 4) were examined and are discussed.

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EXPERIMENTAL SECTION Synthesis of 4-(1-Chlorobuthyl)-pyridine. To a solution of 4-methylpyridine (16.8 g, 0.225 mol) in dry THF (100 mL) N-butyllitium was added (16.0 g, 0.25 mol) slowly at −60 °C. The mixture was stirred at room temperature for 2 h, and then dry THF (100 mL) was added. Then, 1-bromo-3-chloropropane (23.4 mL, 0.19 mol) was added to this solution and stirred for 2 h at −60 °C, and then at 0 °C for 2 h. The reaction mixture was quenched with ice, extracted with ethyl acetate, and dried over MgSO4. The products obtained were purified by column chromatography using silica gel (200 mesh) and a hexane/ethyl acetate mixture. 1H NMR data (CDCl3, 400 MHz, TMS) follow: 1.58−1.84 (m, 4H, aliphatic), 3.35−3.44 (t, 2H), 3.62−3.68 (t, 2H, CH2Cl), 7.36−7.42 (d, 2H, aromatic), 8.56−8.63 (d, 2H, aromatic) ppm; Anal. Calcd for C9H12NCl: C, 63.72; H, 7.13; N, 8.26. Found: C, 63.85; H, 7.12; N, 8.20. Synthesis of 4,4′-(Dithiodibutylene)dipyridine. An aqueous solution of 4-(1-chlorobuthyl)-pyridine (2.5 g 0.015 mol) and sodium thiosulfate (3.1 g, 0.02 mol) was heated under reflux for 2 h. After 2 mL of concentrated HCl solution was added, the mixture was refluxed for 30 min. The solution was cooled to room temperature, and NaOH solution (2.2 g of NaOH in 3 mL of H2O) was added and stirred for 2 h. The solution was extracted with ethyl acetate and dried over MgSO4. The solvent was removed using an evaporator. The crude product was purified by column chromatography using silica gel (200 mesh) and a hexane/ethyl acetate mixture. 1H NMR data (CDCl3, 400 MHz, TMS) follow: 1.56−1.82 (m, 8H, aliphatic), 2.50−2.62 (t, 4H, CH2S), 3.33−3.45 (t, 4H), 7.36−7.42 (d, 4H, aromatic), 8.56−8.63 (d, 4H, aromatic) ppm; Anal. Calcd for C9H12NS2: C, 65.01; H, 7.28; N, 8.42. Found: C, 65.55; H, 7.32; N, 8.30. Materials. HClO4 and KOH (Cica-Merck, ultrapure grade) were used as received. A 0.05 M KClO4 solution (approximately pH 5.5) was prepared by mixing 0.1 M HClO4 and 0.1 M KOH. NaClO4 was used as received form Wako Pure Chemicals. Butanethiol and 4-pyridineethanethiol hydrochloride were purchased from TCI and used as received. All solutions were prepared using ultrapure water purified by a B


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The Journal of Physical Chemistry C Millipore Milli-Q (Advantage) system. Horse heart cytochrome c was purchased from Sigma and purified by using column chromatography before use in electrochemical measurements.77 SAM Preparation. Au(111) single crystal electrodes were prepared using Clavilier’s method from a high-purity Au wire 0.8 mm in diameter (99.999%: Tanaka Kikinzoku Kogyo K.K.).78 Prior to each experiment, the Au(111) electrode was annealed for 30 s in a hydrogen flame and quenched in ultrapure water saturated with H2 gas. The PyC4S-SAM was prepared by immersing a freshly annealed Au(111) electrode into ∼10 μM (PyC4S)2 aqueous solution for 20 min. Note that the pH of a (PyC4S)2 aqueous solution is considered slightly acidic (pH 4−6), although we did not check its pH. If the pKa value of (PyC4S)2 in water is ∼5, the ratio of the protonated and unprotonated forms is ∼1:1. After modification, the Au(111) electrode surface was thoroughly washed with ultrapure water and further ultrasonicated in ultrapure water for 15 s. The modified Au(111) electrode was then transferred into an electrochemical cell or electrochemical STM cell filled with electrolyte solution. Characterization. Electrochemical reductive desorption of the modifiers on the Au(111) electrodes was carried out in 0.1 M KOH at a scan rate of 50 mV s−1 at 20 °C using a Toho Giken PS-06 potentiostat with a function generator. A Pt wire, used as the counter electrode, and either an Ag/AgCl (sat. KCl) or reversible hydrogen electrode (RHE) reference electrodes were used for cyclic voltammetry. Heterogeneous electron transfer rate constants were obtained from a simulated CV by DigiSim.77 Electrochemical STM measurements were performed in either 0.05 M HClO4 or 0.05 M KClO4 using a Nanoscope E system (Digital Instruments, Santa Barbara) with a tungsten tip (0.25 mm diameter) etched in 1 M KOH. Tips were coated with a transparent nail polish to minimize the faradaic current.63,79 STM images were taken using the constant-current mode with a high-resolution scanner (Vecco, HD-0.5I). All potentials were referred to the RHE for in situ STM. A Bio-Rad FTS-6000 FT-IR spectrometer equipped with an MCT detector and a single-reflection accessory was used to record spectra. The spectrometer was operated in the rapidscanning mode with a resolution of 4 cm−1. Thin-film electrodes (Au) for surface-enhanced infrared spectroscopy (SEIRAS) were prepared according to the procedures reported by Osawa et al.80,81

Figure 1. (a) Cyclic voltammograms of bare (dotted line) and PyC4Smodified (solid line) Au(111) electrodes in 0.05 M HClO4, and (b) cyclic voltammograms of butanethiol (C4S)-modified (dotted line) and PyC4S-modified (solid line) Au(111) electrodes in 0.1 M KOH for reductive desorption recorded at a scan rate of 20 mV s−1 for (a) and 50 mV s−1 for (b). The C4S-SAM was prepared by immersion in a 1 mM butanethiol/ethanol solution for 10 h.

PyC4 S − Au + e− → PyC4S− + Au

(1)

The electronic charge consumed by the reductive desorption of PyC4S in 0.1 M KOH was calculated to be 49.6−58.7 μC cm−2, leading to a surface excess of (5.6 ± 0.5) × 10−10 mol cm−2. In addition, as we reported previously, the surface excess for PyS- and PyC2S-modified Au(111) was calculated to be (5.0 ± 0.5) × 10−1071,72 and (5.9 ± 0.5) × 10−10 mol cm−2,76 respectively (Table 1).20 However, as pointed out by the Lipkowski group, the influence of the electronic double-layer charging contribution during reductive desorption of a SAM should be considered. They proposed a modified reaction equation, taking the substitution reaction between water and thiolate into consideration, instead of the simpler eq 1. According to their report, the electronic charge due to the charging current constitutes 15−20% of the total electronic charge consumed in the reductive desorption of the SAM.83 Although the surface excess was roughly estimated using these values in the present study, a difference due to the alkyl chain length was found in the CV profile. Compared to these values, the surface excess of PyC4S is slightly higher than that of PyS and PyC2S, suggesting the formation of a higher densely packed adlayer for PyC4S-SAM. The reductive peak potential of PyC4S was more negative than that of PyS and PyC2S by 0.36 and 0.04 V, respectively (see Table 1). In addition, the peak potential for the reductive desorption of butanethiolate (C4S)-SAM, as shown by a dashed line in Figure 1b, is −0.85 V. The surface excess, calculated from the electronic charge consumed by the reductive desorption, is (8.5 ± 0.5) × 10−10 mol cm−2, which is consistent with the formation of the (√3 × √3)R30° adlattice.30,31,33,84,85 The negative shift of the reductive peak potential of PyC4S-SAM relative to that of the C4S-SAM on the

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RESULTS AND DISCUSSION Figure 1a shows a typical cyclic voltammogram of a PyC4Smodified Au(111) electrode in 0.05 M HClO4. Potential scanning was started at or near the open circuit potential (OCP) (which is equal to 0.75−0.80 V versus RHE). In the potential region between 0.80 and 0.20 V, a featureless CV profile is observed, whereas two peaks are observed during the cathodic scan at 0.01 and −0.13 V. Taking the electronic charges consumed by the reductive peak area into consideration, these reductive peaks are associated with phase transitions in PyC4S-SAM rather than successive desorption of PyC4S molecules from the Au(111) surface. In contrast to 0.05 M HClO4, a reductive peak potential was observed at −0.91 V vs Ag/AgCl (sat. KCl) in 0.1 M KOH for the PyC4Smodified Au(111) electrode, as shown in Figure 1b. PyC4SSAM can be electrochemically desorbed from the Au surface in an alkaline solution, as described by eq 1.82 C


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Table 1. Peak Potential of Reductive Desorption, Surface Excess Evaluated from CV, and Adlattice Constants via HighResolution STM Images adlattice Ep, red (V vs Ag/AgCl) PyS PyC1S PyC2S PyC4S

−0.53 −0.85 −0.86 −0.91

2

surface excess (Γ) (mol/cm ) (5.0 (5.1 (5.9 (5.6

± ± ± ±

0.2) 0.5) 0.5) 0.5)

× × × ×

10−10 10−10 10−10 10−10

in acidic solution

in neutral solution

reference

5 × √3 5 × √3 10 × √3 12 × √3

5 × √3 n/d 5 × √3 8 × √3

53, 55, 58, 71, 72 unpublished data 76 present work

atom. As shown in Figure 2a,b, bands attributed to the pyridine ring are not present. The relatively broad band at ∼1650 cm−1 corresponds to the bending mode of the H2O molecule. The formation of PyC4S-SAM was confirmed by the results of STM and cyclic voltammetry. The reason no bands were observed for the pyridine ring in PyC4S-SAM could be the long distance between the pyridine moiety and the electrode surface. Although an accurate angle for the PyC4 unit relative to the surface has not been obtained, the PyC4 units could orient by a tilt angle of ∼30° with respect to the surface normal in the SAM, as previously reported for butanethiol and related SAMs studies.8,34,48,73 The distance between the S atom and the pyridine N atom is estimated to be ∼0.91 nm based on the molecular structure calculated by Chem3D. Therefore, the enhanced factor becomes small, and no bands attributed to the pyridine ring in PyC4S-SAM were observed by SEIRAS. On the basis of the CV profile shown in Figure 1a, a potential-dependent STM observation was carried out in 0.05 M HClO4. Figure 3a shows a typical STM image of as-prepared PyC4S-SAM on a Au(111) surface at 0.60 V versus RHE. Etching pits formed during the modification were observed on the atomically flat Au(111) terrace, indicating that a closely packed SAM is perfectly formed through the chemical bond between the thiolate S and Au. However, the characteristic

Au(111) electrode indicates that PyC4S-SAM is more stable than the C4S-SAM on the Au(111) surface. This difference is due to the hydrophobic interaction between butyl moieties and the π−π interaction between nearest-neighbor pyridine rings in PyC4S-SAM. Figure 2a,b show typical SEIRA spectra for the PyC4S-SAM on a Au/Si substrate in 0.1 M HClO4 and 0.1 M NaClO4 at 0 V

Figure 2. Surface-enhanced IR spectra of PyC4S-modified Au evaporated on a Au/Si electrode in (a) 0.1 M HClO4 and (b) 0.1 M NaClO4 (pH 6). (c) bulk spectrum of (PyC4S)2 in a KBr pellet.

versus Ag/AgCl, respectively. Figure 2c shows the FT-IR spectrum of (PyC4S)2 dispersed in KBr. In Figure 2c, several characteristic peaks are observed, all of which correspond to the vibration modes of the pyridine ring.86 In a previous paper, we reported that the peaks at 1604 and 1435 cm−1 in the SEIRA spectrum for PyC2S-SAM in 0.1 M NaClO4 correspond to the vibration modes of the pyridine ring.76 There are many factors that determine the absorbance for SEIRAS. The distance between the electrode surface and the moiety of the molecule of interest (here, pyridine ring) is one of the most important factors.81 The impact of this factor decreases rapidly as the distance increases. The absorbance corresponding to the pyridine ring was smaller than that of 4-pyridinethiol SAM (PyS-SAM) because the pyridine moiety is separated from the surface by an ethylene chain. Here, we used (PyC4S)2, which has a butylene chain between the pyridine moiety and the sulfur

Figure 3. Time-dependent STM images (40 × 40 nm2) of PyC4SSAM on a Au(111) surface obtained at (a) 0.60 V and (b-d) 0.13 V versus RHE in 0.05 M HClO4 after manipulating the potential from 0.40 V: (b) 1 min, (c) 2 min, and (d) 3 min after (a), respectively. The tip potential and tunneling current were 0.22 V and 0.70 nA. D


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The Journal of Physical Chemistry C ordered structure of the PyC4S-SAM was not observed in the potential region between 0.80 and 0.35 V versus RHE, as have been observed for SAMs of thiol having aromatic units modified at room temperature. Temperature control during SAM formation containing aromatic units in solution is important to obtain a highly ordered structure.74,87 On the other hand, pyridineethanethiolate (PyC2S)-SAM formed highly ordered arrays even at or near 0.80 V versus RHE.76 The reason for the difference in the STM image at OCP between PyC2S- and PyC4S-SAMs is as follows: For PyC2SSAM, the π−π interaction between the nearest-neighbor pyridine rings is stronger than the hydrophobic interaction between nearest-neighbor ethyl chains because the ethyl chains are short. The hydrophobic interaction between the butyl groups in PyC4S-SAM is stronger than the π−π interaction between pyridine rings. In general, it is known that an nalkanethiolate SAM forms the (√3 × √3)R30° structure and its related lattice, the c(4 × 2) superlattice.31−37 For PyC4SSAM, it is difficult to form the (√3 × √3)R30° lattice because the pyridine moiety as a terminated group is bulky. Indeed, the surface excess was estimated to be 5.6 × 10−10 mol cm−2 via electrochemical reductive desorption. This value is lower than the (√3 × √3)R30° structure for which the surface excess is 7.6 × 10−10 mol cm−2. When the potential was manipulated in the negative direction, a drastic change in PyC4S-SAM was observed at or near 0.10 V versus RHE. As shown in Figure 3b−d, the time-dependent STM images show that well-ordered domains were gradually formed on the terrace. Ordered domains gradually grew on the terrace after small patches of an ordered portion appeared, as shown in Figure 3b. With continuous scanning for 5−10 min at 0.13 V versus RHE, the terraces were completely covered with highly ordered domains, as shown in Figure 4a. Stripes consisting of alternately arranged bright and dark rows are clearly seen in the domains. As shown in the close-up view (Figure 4b), molecular rows with dark gaps are clearly observed. The direction of each molecular row is parallel to the [112̅], the so-called √3 direction. The stripes in the STM image of PyC4S-SAM are similar to that obtained for the PyC2S-SAM on Au(111).76 However, a careful inspection revealed a difference in the gap width between PyC2S- and PyC4S-SAMs. Based on the cross-sectional profile for the bright and dark site (not shown), the difference in the height was found to be 0.03 nm. The intermolecular spacing between PyC4S molecules aligned in the [11̅0] and √3 directions were found to be 3.45 ± 0.10 nm and 0.50 ± 0.03 nm, respectively, in 0.05 M HClO4. These distances correspond to 12 and √3 times the Au lattice constant. Therefore, the adlattice of PyC4SSAM on Au(111) acquired at 0.13 V in 0.05 M HClO4 can be assigned as a p(12 × √3 R−30°) structure. The gap length of PyC4S-SAM (3.45 nm) is longer than that obtained for PyC2SSAM (2.88 nm).76 If the p(12 × √3R−30°) unit cell includes four PyC4S molecules, the surface excess is calculated to be 3.8 × 10−10 mol cm−2. This value is not consistent with that calculated (5.6 × 10−10 mol cm−2) from the CV profile for the reductive desorption of PyC4S-SAM. This discrepancy can be explained by considering an overestimation from the contribution of the electronic double-layer charging current. The surface pKa of PyS-SAM on Au for the protonation of Py moieties is reported to change as a function of the electrode potentials, even in acidic solutions. 88,89 The potential manipulation in the negative direction will accelerate formation of protonated Py moieties in PyC4S-SAM, resulting in the phase transition of PyC4S molecules from the Au surface. A

Figure 4. (a) Large-scale (75 × 75 nm2) and (b) high-resolution (25 × 25 nm2) STM images of PyC4S-SAM on Au(111) acquired at 0.13 V versus RHE in 0.05 M HClO4. The tip potential was 0.22 V and the tunneling current was 0.65 nA for (a) and 0.80 nA for (b). The set of three arrows indicates Au lattice directions. (c) The proposed model for the PyC4S-SAM adlattice on a Au(111) surface with a p(12 × √3 R−30°) unit cell.

similar effect on protonation by the potential manipulation was proposed by Kolb’s group for PyS-SAM in 0.1 M H2SO4.58 On the basis of high-resolution STM images, a proposed model is displayed in Figure 4c. It is reasonable that the PyC4S molecules exist in the dark region between the bright molecular rows in Figure 4b, because a careful inspection of the stripes in each domain reveals satellite spots between the bright molecular rows. The difference in the brightness of the molecular rows is caused by the difference in either the adsorption site or molecular conformation. By comparing the results of the pH-driven conformational change in PyC2SSAM,76 it is concluded that the PyC4S molecular rows are formed by the dimer interacting with thiolate S atoms between two PyC4S molecules. The proposed model for S−S dimer formation is also supported by STM studies on the PySSAM.53−56,58,59 In addition, formation of RS−Au−SR species is favored for short and intermediate chain length alkanethiolate SAMs, as reported by Salvarezza’s group.90 Further negative potential manipulations from 0.13 to 0 V made the molecular rows aligned in the √3 direction gradually disappear as shown in Figure 5a. A uniformly arranged different adlayer structure of PyC4S-SAM appeared when the potential was held at 0 V. Figure 5b shows a high-resolution STM image of this new adlayer structure. The adlayer is not commensurate with the Au(111) lattice. The distance between nearestneighbor spots is 0.9−1.0 nm. A possible explanation for the formation of an incommensurate structure at or near the hydrogen evolution potential is the difference in the molecular configuration of the butyl chain in PyC4S. Interestingly, etching pits (vacancy islands) were rarely seen on the terrace. That is, the etching pits were eliminated with the transformation from (12 × √3) to a semisquare structure. This result indicates that E


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stripe (or the spaces between brightest molecular rows) is slightly narrower than that obtained in 0.05 M HClO4 (see Figure 4a). A careful inspection revealed that the number of etching pits decreased versus that obtained in 0.05 M HClO4, and a few defects or disordered regions were observed among the ordered domains. The molecular rows in adjacent domains also cross each other at a rotation angle of 60°. If as-prepared PyC4S-SAM made contact with 0.05 M KClO4, many etching pits would be found on the terrace, as shown in Figure 6b. Although ordered portions partially appeared by the manipulation of potentials more negative than OCP, the surface was not fully covered with ordered domains. This is the case because the electrode was used first in 0.05 M HClO4. The density of PyC4S-SAM decreased before transfer to KClO4 during the negative potential manipulation in HClO4. The protonation of the Py moieties in PyC4−SAM may accelerate a drastic structural change due to the relaxation of the repulsive interaction between protonated PyC4S molecules. At pH 5.5, the condition of observation, the resolution of the STM image often changed during the measurement. The acid−base equilibrium of PyS-SAMs is reported to be a function of the solution pH and the applied potential.89 The surface pKa of PyS-SAMs was estimated to be ∼5.56 The surface pKa value for the related Py-terminated aromatic SAM, 4-(4-pyridyl)phenylmethanethiol SAM, which has a similar length between the Py moiety and thiol S atom as that of PyC4S, was estimated to be 5.2 ± 0.5 from the electrochemical response of [Fe(CN)6]3−/4− in various pH solutions.92 At ∼ pH 5, the ratio of protonated and unprotonated Py moieties is almost 1. A typical high-resolution STM image is shown in Figure 7a, where molecular rows with different directions are clearly observed. This figure shows rotational domains. Each molecular row is running parallel to a √3 direction. The intermolecular spaces between PyC4S molecules aligned in the [110̅ ] and √3 directions are found to be 2.35 ± 0.06 nm and 0.50 ± 0.03 nm, respectively, which correspond to 8 and √3 times the Au lattice constant. Therefore, the p(8 × √3R−30°) structure is assigned to the unit cell for PyC4S-SAMs on Au(111) in 0.05 M KClO4. The p(8 × √3R−30°) lattice including two PyC4S molecules leads to a surface excess of 2.86 × 10−10 mol cm−2. This value is lower than that obtained in 0.05 M HClO4. A careful inspection of this data reveals that the molecular rows shift at domain boundaries, such that each molecular row is composed of a set of two PyC4S molecules, judging from the adlattice constant. Furthermore, it can be seen that the brightest and darker rows are alternately arranged in each molecular row. Therefore, the characteristic molecular rows of PyC4S-SAM on Au(111) obtained in 0.05 M KClO4 might be tentatively assigned to molecular rows consisting of protonated and unprotonated pyridine moieties, as indicated in Figure 7a by red and blue arrows, respectively. Based on the highresolution STM image, the adlayer structure of PyC4S-SAM formed in 0.05 M KClO4 is proposed to be a dimer, as shown in Figure 7b. In the proposed model, protonated +HPy moieties are colored green for N atoms, whereas unprotonated Py moieties are colored with blue for the N atom. If the brighter and darker spots in each molecular row can be assigned as protonated and unprotonated pyridine moieties, respectively, the two satellite spots that appear inside each molecular row are attributed to thiolate S atoms, which are weakly interacted to each other. Indeed, the distance between two spots is ∼0.40 nm. Although we could not find clear evidence for the formation of an S−S dimer in the molecular rows from the

Figure 5. (a) Large-scale (75 × 75 nm2) and (b) high-resolution (25 × 25 nm2) STM images of PyC4S-SAM on Au(111) at 0 V versus RHE in 0.05 M HClO4. The tip potential and tunneling current were 0.39 V and 0.50 nA, respectively. The set of three arrows indicates Au lattice directions.

negative potential manipulation accelerates structural changes of PyC4S-SAM on the Au (111) surface, and the mobility of gold atoms on the Au (111) substrate. A similar phase transition was also observed by Boruget’s group for 4methylbenzenethiolate-SAM on Au(111) in 0.1 M HClO4. They pointed out that such a potential-induced structural change is enhanced by the STM tip because of the weak binding of thiolate to gold at potentials more negative than the potential of zero charge (pzc).91 In contrast to the STM images obtained in 0.05 M HClO4, a clear difference in structure for PyC4S-SAM was found in 0.05 M KClO4 (pH of the solution was adjusted to 5.5−6.0). Figure 6a shows a typical STM image of PyC4S-SAM on Au(111)

Figure 6. Large-scale (50 × 50 nm2) STM images of PyC4S-SAMs on Au(111) obtained at (a) 0.77 V and (b) 0.95 V versus RHE in 0.05 M KClO4 (pH 5.5). The PyC4S-SAM-modified electrodes used were (a) after first used for STM measurements in 0.05 M HClO4 and (b) freshly prepared (as-prepared). The modification of PyC4S was done as described in the experimental section. The tip potential and tunneling current were (a) 0.57 V and 1.25 nA, and (b) 0.56 V and 0.43 nA, respectively.

(which was prepared in 0.05 M HClO4) at 0.77 V versus RHE, which is negative potential versus OCP (where is nearly equal to 1.10 V), in 0.05 M KClO4. Although a featureless surface morphology was obtained at an early stage of observation, ordered portions began to appear by maintaining a slightly negative potential versus OCP. For example, by maintaining 0.77 V versus RHE, the terrace was almost covered with several ordered domains within 30 min, giving an image as shown in Figure 6a. Each molecular row was running parallel to the √3 direction. From the STM image in Figure 6a, the width of the F


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Figure 7. (a) High-resolution (20 × 20 nm2) STM image of PyC4S-SAM on Au(111) at 0.72 V versus RHE in 0.05 M KClO4 (pH 5.5). The tip potential and tunneling current were 0.57 V and 0.65 nA, respectively. (b) The proposed model of a p(8 × √3 R−30°) unit cell. (c) A height-shaded STM image of PyC4S-SAM.

high-resolution STM image shown in Figure 7a, it is likely that the interaction between the S atoms in each molecular row can adopt the proposed model. If the S−S dimer spacing is ∼0.4 nm, the adlayer structure of PyC4S-SAM shown in the STM image can be explained as a “ladder”-like structure. A heightshaded image is shown in Figure 7c, so that the structural assignment can be clearly visualized, where the +HPy, S−S dimer, and Py moieties are assigned with red, green, and blue arrows, respectively. It is noteworthy that the 0.43 nm S−S spacing was proposed in a structural model for PyS-SAM on Au(111) in 0.1 M NaClO4 (pH 5).56 The proposed model shown in Figure 7b is consistent with the STM images shown in Figure 7a,c. The values of the surface excess (Γ) and adlattice constant for Py-terminated SAMs with various alkyl chain lengths are summarized in Table 1. The surface excess increases as the number of the methylene units increases. Although the adlattice constant for every Py-terminated thiolate SAM depends on the electrode potential and pH of the solution, the adlattice constant increases as the number of methylene units increases. Using a PyC4S-SAM-modified Au(111) electrode, the electrochemical response of cytochrome c was examined to understand the effect of the number of alkyl chains between the Py moiety and the S of PyCnS. Figure 8a−c show cyclic voltammograms of 0.10 mM cytochrome c in a 0.10 M phosphate buffer (pH 7) for PyC1S-, PyC2S-, and PyC4Smodified Au(111) electrodes, respectively. Well-defined redox waves of cytochrome c were observed for the corresponding modified Au(111) electrodes in the cyclic voltammograms recorded at scan rates lower than 0.10 V s−1 (not shown). The observed currents are proportional to the square root of the scan rate, indicating that the electrochemical reaction of cytochrome c is diffusion-controlled. The separation of the anodic and cathodic peaks, ΔE, were ∼66, ∼ 67, and ∼70 mV at a scan rate of 50 mV s−1 for PyC1S-, PyC2S-, and PyC4Smodified Au(111) electrodes, respectively, suggesting a quasireversible one-electron transfer reaction. Interestingly, the result showed a clear redox couple for cytochrome c even for the PyC4S-modified Au(111) electrode, despite of the insertion of 4 methylene units between the S atom and the Py moiety. Curve fitting of digitally simulated values for the backgroundsubtracted voltammogram was carried out (Figure 8e,f) to understand better the cytochrome c electrochemistry occurring on the PyCnS-SAM-modified Au(111) electrodes. Using a digital simulation technique, the formal redox potential, E0′, the

Figure 8. Cyclic voltammogram of 0.10 mM cytochrome c in a 0.1 M phosphate buffer solution containing 0.1 M NaClO4 (pH 7) for (a) PyC1S-, (b) PyC2S-, and (c) PyC4S-modified Au(111) electrodes recorded at a scan rate of 50 mV s−1. (d), (e), and (f) correspond to the background subtracted voltammogram with simulated data shown by circles for (a), (b), and (c), respectively.

heterogeneous electron transfer rate constant, ko′, and the diffusion coefficient, D, for cytochrome c were estimated for voltammograms obtained independently on PyCnS-SAMmodified Au(111) electrodes. The estimated values of these parameters for the PyC4S-SAM electrode are summarized in Table 2, together with the data obtained similarly for PyS-, PyC1S-, and PyC2S-modified Au(111) electrodes. The E0′ and D values are almost independent of the modifiers (PyCnS, n = 0, 1, 2, and 4) on the Au(111) electrodes. However, the ko′ value clearly depends on the number of alkyl chain on the modifier. The plot of ln ko′ as a function of the distance G


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√3 R−30°) structure constituted from four molecules and a semisquare array were found at 0.13 and 0 V, respectively, in 0.05 M HClO4. When the electrolyte solution was changed from 0.05 M HClO4 to KClO4 (pH 5.5), the adlayer structure of PyC4S-SAM changed to the p(8 × √3 R−30°) structure constituted from two molecules at potentials more negative than the open circuit potential in 0.05 M KClO4. The difference in the adlayer structure between acidic and neutral solutions resulted from the protonated and unprotonated pyridine moieties in the PyC4S-SAMs. From the electrochemical response of cytochrome c on PyCnS-modified Au(111) electrodes (n = 0, 1, 2, and 4), the heterogeneous electron transfer rate constant, ko′, was found to depend on the number of methylene units in the modifiers. Based on Marcus theory, the β value was determined to be 3.8 nm−1.

Table 2. Electrochemical Parameters of Cytochrome c from CV Simulation SAM

E0′ (mV vs Ag/AgCl)

PyS PyC1S PyC2S PyC4S

65 66 66 65

D (× 10−6 /cm2 s−1)

ko′ (× 10−3 /cm s−1)

± ± ± ±

6.0 5.0 2.7 1.1

1.0 0.9 1.3 0.9

0.3 0.3 0.3 0.3

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. # Professor Emeritus, Kumamoto University

Figure 9. Schematic representations of the PyCnS-SAM-modified Au (111) electrodes and the plot of the heterogeneous electron transfer rate constant, ln ko′, for cytochrome c on PyCnS-SAM-modified Au(111) electrodes, estimated from voltammograms using a digital simulation technique, as a function of the distance between the S atom and the N atom of the pyridine moiety in PyCnS (calculated on the basis of molecular structures using Chem3D) on the electrodes on the basis of Marcus theory.

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency (JST) program, “Special Coordination Funds for Promoting Science and Technology” (to S.Y.) and by a Grant-in-Aid for Scientific Research on Innovative Areas No. 21108005, “Coordination Programming” from MEXT, Japan.

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between the S atom and the N atom in the pyridine moiety of the modifier gives a linear relation with the slope of −3.8 nm−1, as shown in Figure 9. Long-range electron transfer kinetics in the Marcus theory can be given by eq 2: ko′/ko′# = exp( −β(d − d #))

(2)

o #

where k ′ and k ′ are the heterogeneous electron transfer rate constants for the electron transfer path distance of d and d# (on the electrode surface as a standard path), respectively. The β value (3.8 nm−1) obtained from the linear relation suggests that electron transfer via tunneling through the alkyl chain layer is just like that in a vacuum. We examined the effect of alkyl chain length for pyridineterminated thiols on the redox reaction of [Fe(CN)6]4− using Au(111) electrodes modified with PyCnS (n = 0, 1, 2, and 4) to understand better this electron transfer reaction of cytochrome c. The estimated β value (∼3.7 nm−1, not shown) obtained via the CV simulation technique was almost the same as that obtained for cytochrome c. These results suggest that the electron transfer of cytochrome c in the present study on PyCnS-SAMs-modified electrodes (n = 0, 1, 2, and 4) does not take place really through the bonds of alkyl chains of SAMs but through defects (through space tunneling mechanism) in the SAMs. o

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CONCLUSIONS The PyC4S-SAM-modified Au(111) surfaces were investigated in acidic and neutral electrolyte solutions, and the structural changes of PyC4S-SAM were observed by changing the electrode potential and pH of the electrolyte solution. Highresolution STM images of PyC4S-SAM showed that a p(12 × H


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