Observation of Redox-State-Dependent Reversible Local Structural

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Observation of Redox-State-Dependent Reversible Local Structural Change of Ferrocenyl-Terminated Molecular Island by Electrochemical Frequency Modulation AFM Ken-ichi Umeda† and Ken-ichi Fukui*,‡,§ †

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan, ‡Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan, and §PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan Received December 20, 2009. Revised Manuscript Received January 23, 2010 Electroactive ferrocenylundecanethiol (FcC11H22SH) islands embedded in an n-decanethiol (C10H21SH) selfassembled monolayer (SAM) matrix on Au(111) were studied under potential control in 0.1 M HClO4 aqueous solution using newly developed electrochemical frequency-modulation atomic force microscopy (EC-FM-AFM). The apparent height of the Fc islands from the surface of the matrix SAM increased by about 0.44 nm accompanied by the oxidation of the terminal Fc groups. This potential-dependent reversible change can be explained by formation of an ionic double layer where ClO4- ions are strongly bound on the Fcþ groups. Simultaneous measurements of energy dissipation, which corresponds to the energy to keep the cantilever’s vibrational amplitude constant, revealed distinct change in the magnitude by the oxidation state of the Fc groups. These results indicate that localized charge at an electrode/electrolyte solution interface can be identified, and microscopic information on the electric double layer at the interface is available by using EC-FM-AFM.

1. Introduction Intermolecular electron transfer in solution and resultant redox reactions are quite significant processes in biology as well as in chemistry. Many important biological functions such as cellular respiration (energy gain as ATP) and photosynthesis (solar energy conversion) comprise complex multiple steps involving the transfer of electrons and protons. In a more specific process, an oxidoreductase, which is an enzyme that catalyzes the transfer of electrons from one molecule to another, is working for the conversion of molecules.1,2 In the 1950s, Marcus established a theoretical formula for the intermolecular electron transfer reaction rate. His theory indicates that changes take place in the structure by electron transfer, both in the reacting molecules and in those of the surrounding medium. Because of all these changes, the energy of the molecular system rises temporarily and enables the electron to jump between the molecules. It corresponds to the energy barrier for the reaction. The Marcus theory has been successfully applied to wide range of phenomena including intermolecular electron transfer in solution.3 Nevertheless, microscopic information for the important process available by experiments was quite limited in particular for the relation between the local structure of mesoscopic scale molecules and distribution of reactants, ions, and solvent molecules during the reaction. Electrochemistry has been used in wide range of science and technology because of easy control of electrochemical potentials of interfaces and adsorbed molecules, which initiate electron transfer and redox reactions. It can provide us a way to regulate the redox process for studying the basic phenomena how the local *Corresponding author: e-mail [email protected]; Fax þ816-6850-6235. (1) Campbell, N. A.; Reece, J. B. Biology, 7th ed.; Benjamin Cummings: San Francisco, CA , 2004. (2) Bioelectronics; Willner, I., Katz, E., Eds.; Wiley-VCH: Weinheim, 2005. (3) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.

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environment responds to the electron transfer process. Ferrocenyl-terminated self-assembled monolayers (Fc-terminated SAMs) have been extensively studied as a typical redox-active system tethered on the flat electrode surface.4-14 It is because the Fc group is robust against single-electron oxidation-reduction cycles (Fc0 a Fcþ) in the potential window of aqueous solution. In general, kinds of electrolyte in solution are an important factor for the redox reaction rate because it alters the stabilization energy of the reactants and products; i.e., it changes the reaction Gibbs free energy, ΔrG, of the system. Actually, the redox peak potential of an Fc-terminated SAM was greatly shifted depending on the kinds of counterions that can stabilize oxidized Fcþ moieties.4 As is evident from the Marcus theory, different kinds of electrolytes also affect the redox reaction rate because the arrangement, concentration, etc., of the electrolyte at the moment of electron transfer are included in the parameter of reorganization energy.15 Thus, the system is regarded as a prototype of complex redox systems. (4) Valincius, G.; Niaura, G.; Kazakeviciene, B.; Talaikyte, Z.; Kazemekaite, M.; Butkus, E.; Razumas, V. Langmuir 2004, 20, 6631. (5) Guo, Y.; Zhao, J. W.; Zhu, J. J. Thin Solid Films 2008, 516, 3051. (6) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438. (7) Ye, S.; Haba, T.; Sato, Y.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653. (8) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (9) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (10) Ohtsuka, T.; Sato, Y.; Uosaki, K. Langmuir 1994, 10, 3658. (11) Uematsu, T.; Kuwabata, S. Anal. Sci. 2008, 24, 307. (12) Yao, X.; Wang, J. X.; Zhou, F. M.; Wang, J.; Tao, N. J. J. Phys. Chem. B 2004, 108, 7206. (13) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 372, 117. (14) Nishiyama, K.; Ueda, A.; Tanoue, S.; Koga, T.; Taniguchi, I. Chem. Lett. 2000, 930. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001.

Published on Web 02/10/2010

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Electrochemical scanning tunneling microscopy (EC-STM) is an established method to study the electrode surface at atomicscale resolution and single molecule electron transfer in electrochemical environments.16,17 The electrochemical potentials of the sample and the tip are controlled independently against the reference electrode by a potentiostat. The substrate potential determines the redox state of molecules tethered on the substrate electrode, and the difference between the tip potential and the substrate potential sets the bias voltage for electron tunneling. The advantage to obtain the local density of states (LDOS) by electron tunneling, however, sometimes restricts the information on local structure, which is important for the electron-transfer processes as noted above. As an example, we have studied on origin of current enhancement through Fc-terminated molecular islands using EC-STM and concluded that the conduction path opened at a determined potential range is the predominant factor for the enhancement.18 In another viewpoint, we could not find any differences that can be attributed to the different redox states of the terminal Fc groups. Electrochemical atomic force microscopy (EC-AFM) is another candidate to study the changes in the local structure of the sample surface depending on the potential.19 The static contact mode or the dynamic ac mode (tapping mode or intermittent contact mode) has been applied for EC-AFM measurements in the literature. Usually, the potential of the tip (and cantilever) has not been controlled for these measurements. However, there is no doubt that the electric double layer surrounding the tip should affect the measurements. For example, improvement of image resolution by applying bias voltage between the conductive tip and the sample was reported.20 The control of the tip potential becomes more important when smaller interaction is necessary for imaging to study on the electrolyte-dependent stabilizations and the reorganization energy of the molecules, etc. Recently, frequency modulation AFM (FM-AFM), which has mainly been used for high-resolution imaging of the solid surfaces in ultrahigh vacuum,21 has been successfully adapted in a liquid environment for imaging soft materials at lower loading forces.22,23 We have developed an electrochemical FM-AFM (EC-FM-AFM) that can be applicable to analyses of electrochemical systems by independent potential control of the sample and the tip with an advantage in high spatial resolution at low loading forces. In this study, we have examined the redox-state-dependent local structural change of the typical electroactive component tethered on the electrode surface in solution by EC-FM-AFM. The redox state of molecular islands of 11-ferrocenyl-1-undecanethiol (FcC11H22SH) embedded in an n-decanethiol (C10H21SH) SAM matrix on Au(111) in 0.1 M HClO4 aqueous solution was successfully recognized in topography and mapping of energy dissipation signals.

2. Experimental Section All reagents were purchased from commercial suppliers and used without further purification. An evaporated gold film on (16) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (17) Zhang, J. D.; Kuznetsov, A. M.; Medvedev, I. G.; Chi, Q. J.; Albrecht, T.; Jensen, P. S.; Ulstrup, J. Chem. Rev. 2008, 108, 2737. (18) Yokota, Y.; Fukui, K.; Enoki, T.; Hara, M. J. Phys. Chem. C 2007, 111, 7561. (19) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (20) Frederix, P. L. T. M.; Gullo, M. R.; Akiyama, T.; Tonin, A.; de Rooij, N. F.; Staufer, U.; Engel, A. Nanotechnology 2005, 16, 997. (21) Noncontact Atomic Force Microscopy; Morita, S., Wiesendanger, R., Meyer, E., Eds.; Springer-Verlag: Berlin, 2002. (22) Fukuma, T.; Kimura, M.; Kobayashi, K.; Matsushige, K.; Yamada, H. Rev. Sci. Instrum. 2005, 76, 053704. (23) Fukuma, T.; Higgins, M. J.; Jarvis, S. P. Phys. Rev. Lett. 2007, 98, 106101.

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Figure 1. Schematic of electrochemical frequency modulation AFM (EC-FM-AFM). Electrochemical potentials of the sample and the tip on the cantilever are controlled by a potentiostat against the potential of the reference electrode. The cantilever is vibrated by a piezo actuator always at its resonant frequency (f 0 ) at constant amplitude (A) by a digital PLL controller. Topography (z piezo voltage), frequency shift (Δf ), energy dissipation (voltage amplitude for cantilever excitation), and optionally cantilever amplitude are recorded as image signals. mica with a (111) oriented surface was used as the substrate. The Au(111) substrates were annealed in a butane flame and immersed in 1 mM C10H21SH (Tokyo Chemical Industry) ethanol solution for at least 24 h to prepare saturation coverage C10H21SH SAMs. The C10H21SH SAMs, which were rinsed with pure ethanol and dried with N2 gas, were immersed into 0.1 mM solution of FcC11H22SH (Dojindo Molecular Technologies) in acetone for 10-40 min to fabricate the Fc-embbeded islands structure, then rinsed with pure acetone, and dried with N2 gas.18 Electrolyte solution of 0.1 M HClO4 used in the whole experiments was prepared using ultrapure grade HClO4 (Cica-Merck) and Milli-Q water (Nihon Millipore). Electrochemical FM-AFM measurements were performed by an original instrument developed based on an electrochemical tapping-mode AFM (PicoScan 2500, Agilent Technology). A schematic of the EC-FM-AFM system is shown in Figure 1. To oscillate the cantilever always at its resonant frequency while keeping a constant amplitude and to accurately detect the frequency shift due to interactions, a digital oscillation controller using a phase locked loop (PLL) (OC4, Nanonis) was used (Digital PLL controller in Figure 1). All the FM-AFM images presented in this paper are topographies obtained at a constant frequency shift and at constant vibrational amplitude. Pt-coated cantilevers (ElectriTap300, BudgetSensors) with a force constant of around 40 N m-1 and a tip radius less than 25 nm were used as the force sensor, except for the image in Figure 2B. A conductive Si cantilever (PPPNCHAuD, Nonosensors) with Au coating at the backside of the cantilever for higher reflectance, a force constant of around 40 N m-1, and a tip radius less than 10 nm was used for the image in Figure 2B. The resonant frequency of the cantilevers was 130-160 kHz in aqueous solution, which was approximately half of the value in air. A Pt wire and an AuOx wire were used as the counter electrode and the reference electrode, respectively. All electrochemical potentials are presented with respect to the Au/AuOx reference electrode, which has a potential of 1.15 V vs reversible hydrogen electrode (RHE) in our measurements by comparing the formal potential of single component redox-active SAMs determined by cyclic voltammetry DOI: 10.1021/la904797h

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Figure 2. In situ FM-AFM images of FcC11H22SH islands embedded in C10H21SH SAMs at different coverage of FcC11H22SH at substrate potential (Es) = tip potential (Et) = -0.8 V vs Au/ AuOx. (A) At lower coverage (197  197 nm2, Δf = þ530 Hz, Ap-p = 0.68 nm). The FcC11H22SH islands were observed as vague protrusions. Etch pits, which are depressions of single Au layer height accompanied by relaxation of stressed Au surface atoms by chemisorptions of C10H21SH, were clearly observed. Inset shows a magnified image (34  34 nm2) of the etch pits. (B) At higher coverage (381  381 nm2, Δf = þ88 Hz, Ap-p = 0.76 nm). Randomly distributed FcC11H22SH islands were observed as bright protrusions at higher number density. (C) Schematic of the FcC11H22SH islands embedded in C10H21SH SAM. (CV) in this system with those in the literature. The value was close to the reported one of 1.20 V vs RHE.24 The potential of the cantilever and the Au substrate can be set independently by using a potentiostat (Agilent Technology), but a common potential was adopted for both electrodes in the present study.

3. Results The major difficulty in high-resolution imaging in liquid is a diminished quality factor (Q factor) of the cantilever resonance vibration. To achieve the high resolution, reduction of the noise from the cantilever deflection sensor is essentially important.22,25 By following the procedures in literature to reduce the deflection noise density of the optical beam deflection method,22,25 we have succeeded in clearly observing a peak of cantilever thermal Brownian motion in water. The thermal Brownian motion peak of the cantilever we used in the present study appeared at about 150 kHz with a Q factor around 10 in water. The ratio of the peak value:the noise floor was 6:1 at the optimized condition, which was good enough by comparison with the values in the literature where atomic resolution FM-AFM images of a cleaved mica surface in water were obtained.22,25 The modifications include exchange of the laser to a more intense one (45 mW at its maximum output) with modulation of the laser power at ca. 360 MHz to reduce the mode hopping noise, improvement of laser focus at the cantilever with a collimation lens and an aperture, increase of the bandwidth of the amplifier of the photodiodes to 1.6 MHz, and so on. (24) Hiesgen, R.; Eberhardt, D.; Meissner, D. Surf. Sci. 2005, 597, 80. (25) Fukuma, T.; Jarvis, S. P. Rev. Sci. Instrum. 2006, 77, 043701.

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The van der Waals attraction has great contribution for FMAFM imaging in vacuum, but this is quite attenuated in liquid because polarizable solvent shields the electrostatic forces. Therefore, the attractive region does not necessarily appear close to the surface in liquid. Repulsive interaction (positive frequency shift) was used to obtain FM-AFM images for the present study. Origins of the repulsion other than the steric repulsion at contact region will be discussed later. By using the EC-FM-AFM, redoxacitive FcC11H22SH islands embedded in C10H21SH SAMs were analyzed at the potentials where they are reduced or oxidized. The FcC11H22SH islands were prepared by a method first demonstrated by Weiss’s group.26,27 By exposing the preformed SAM to a solution of guest molecules at low concentration for a short time, the guest molecules insert into the SAM at its local defect sites. The FM-AFM images in Figure 2 show bright protrusions or spots close to the dark depressions. It is a characteristic commonly observed in previous STM measurements on mixed SAMs prepared by this method.27 The dark depressions are well-known to be formed with single Au layer depth sometimes called etch pits as a result of relaxation of the stressed surface Au atoms of Au(111) by chemisorptions of alkanethiols. So the etch pit is surrounded by a step edge, where insertion or exchange of the guest molecules is easier. Thus, the bright protrusions or spots in Figure 2 probably correspond to inserted FcC11H22SH molecules. Therefore, our EC-FM-AFM can clearly image the single atomic layer pit and discriminate molecules with different length shown in Figure 2c. When the surface coverage of FcC11H22SH was higher, bright protrusions corresponding to FcC11H22SH islands became more evident as shown in Figure 2B. As was pointed in literature, the coverage was not necessarily proportional to the time of immersion to FcC11H22SH solution.27 It may rather depend on the local density of the preformed C10H21SH SAM. Following measurements and analyses will be performed on the surface with rather high coverage because of convenience for statistical treatments to draw tendencies from the complex system. Cyclic voltammograms (CV) of the Fc-terminated alkanethiols embedded in shorter alkanethiol SAMs have been reported by several groups. As were summarized in recent literature,5,6 the embedded Fc-terminated alkanethiols gave more negative apparent formal potential voltages than those of fully covered ones. For example, a FcC12H24SH embedded in C10H21SH SAM on Au(111) at a mole fraction of 0.1 gave a peak at 0.19 V vs Ag/AgCl (0.39 V vs RHE); however, the single component fully covered FcC12H24SH SAM gave a major peak at 0.36 V vs Ag/AgCl (0.56 V vs RHE).6 The former peak was assigned to the oxidation of the Fc moiety in rather an isolated state, and the latter peak was attributed to a clustered state where the interaction from the neighboring Fcþ ions inhibited the oxidation.5,6 We obtained similar CV curves, and a peak at -0.7 to -0.74 V vs Au/AuOx was obtained for the FcC11H22SH embedded C10H21SH SAMs in 0.1 M HClO4 (Figure S1 in the Supporting Information). Thus, in our measurements the Fc moieties were fully reduced to Fc0 or fully oxidized to Fcþ at -0.8 V vs Au/AuOx (0.35 V vs RHE) and at -0.4 V vs Au/AuOx (0.75 V vs RHE), respectively. Figure 3 shows series of in situ FM-AFM images of the FcC11H22SH embedded C10H21SH SAM at different electrochemical potentials. They were obtained at the same region (a circle in each image shows the same protrusion). At -0.8 V vs Au/AuOx in Figure 3A, reduced Fc0 islands were observed as bright protrusions (26) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (27) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1.

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Figure 3. Series of in situ FM-AFM images (Δf = þ528 Hz, Ap-p = 1.00 nm) of FcC11H22SH islands embedded in C10H21SH SAM (176  159 nm2) in 0.1 M HClO4 at different electrochemical potentials: (A) substrate potential (Es) = tip potential (Et) = -0.8 V, (B) Es = Et = -0.4 V, and (A0 ) Es = Et = -0.8 V, vs Au/AuOx, respectively. Image (A0 ) was recorded after acquisition of (B), showing the reproducibility of the image contrast at the FcC11H22SH islands. (D) A comparison of line profiles at a selected FcC11H22SH island indicated by a circle and a dotted line on each image.

Figure 4. A histogram of the height of FcC11H22SH islands in the reduced state (-0.8 V) and in the oxidized state (-0.4 V). To compare the height in different redox states at a similar tip condition, four FM-AFM images at the same area (Figure 3A and another one for the reduced state and Figure 3B and another one for the oxidized state) were used for the analysis. Twenty-seven islands were labeled as shown in Figure S2 in the Supporting Information, and the height of each island from the surrounding C10H21SH layer was measured from the corresponding line profile. Calculated Gaussian parameters and fitted curves for both states are indicated in the figure.

similar to Figure 2B. When the potential was increased to -0.4 V vs Au/AuOx (Figure 3B), oxidized Fcþ islands were observed brighter (higher in topography at the constant frequency shift). This change in contrast was in principle reversible against the potential as is shown in Figure 3A0 . A small difference in image resolution between Figures 3A and 3A0 was probably due to a slight change in the state of the tip. As is clearly shown in the line profiles of a Fc island marked in each image, the apparent height of the Fc island from the surrounding C10H21SH SAM became larger by oxidation to Fcþ and was recovered to the original level by reduction to Fc0 (Figure 3C). By measuring the apparent height of each island from the surrounding SAM, the height distributions for both redox states are summarized as a histogram in Figure 4. The increase in island height by oxidation to Fcþ can be confirmed from the histogram. Although the height distribution was wider for the oxidized state, the average height increased by 0.44 nm. Experimental evidence that indicated the increase in the thickness of Fc-terminated SAMs by electrochemical oxidation of the Fc moieties was reported by using various techniques.7-13 The origin of the height change of the Fc-islands will be discussed later. Lateral sizes of the Fc islands also seem to be increased by oxidation. We should be careful enough, however, in analyses of the apparent lateral size measured by SPM as has repeatedly been discussed in the literature.28 The lateral size of an protruding object measured by a finite size tip is always larger than the real size by a geometrical reason. When a sample with a disk of the thickness d on a flat substrate is measured by a tip of radius R and it is assumed that the minimum distance between the tip surface and the sample surface is constant, the edge of the disk becomes blurred and the apparent lateral size increases by 2(2Rd - d2)1/2.28 If R = 20 nm (close to the data sheet value of the tip used in our experiments) is assumed for measurements of protruding islands of 0.55 and 1.3 nm high, the lateral size may be measured larger by

9.3 and 14.2 nm, respectively. The lateral size increase in accordance to the height increase of the Fc island in the line profiles in Figure 3C is close to the value of this simple estimation. Therefore, we do not deal with the change in lateral size in the following analyses and discussion. Interesting and informative results were obtained by measurements on magnitude of energy dissipation simultaneously with the topography. During imaging the surface by keeping the frequency shift constant, the amplitude of the vibrating cantilever was also kept constant. When a dissipative interaction works for the vibration, more energy is necessary to keep the constant amplitude. Thus, the value includes the information what kinds of interaction are working during the measurements.29 Generally, adhesive hystereses, fluctuation of the system by the Brownian motion, vibration in viscous fluid, etc., cause the energy dissipation. Possible reasons for the energy dissipation at the electrode interface will be discussed later. Figures 5A and 5B show energy dissipation images of the FcC11H22SH embedded in C10H21SH SAM simultaneously recorded with the topography images in Figures 3A and 3B, respectively. The voltage applied to the piezo actuator for vibrating the cantilever at a constant amplitude (Ap-p = 1.00 nm) was recorded as the energy dissipation signal. Higher brightness (larger voltage) in the image indicates larger energy dissipation. These images and corresponding topography images apparently indicate two things. One is that the energy dissipation is smaller at the Fc islands than at the surrounding C10H21SH SAM. The islands were observed as depressions in Figure 5A. The other is that the energy dissipation is smaller at the oxidized Fcþ islands than at the neutral Fc0 islands. The Fcþ islands were imaged as deeper depressions in Figure 5B. It is also confirmed by the comparison of line profiles of an Fc island in the neutral state and in the oxidized state shown in Figure 5C.

(28) Fukui, K.; Iwasawa, Y. Surf. Sci. 1999, 441, 529.

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(29) Gauthier, M.; Kantorovich, L.; Tsukada, M. In Noncontact Atomic Force Microscopy; Morita, S., Wiesendanger, R., Meyer, E., Eds.; Springer-Verlag: Berlin/ Heidelberg, 2002; p 371.

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Figure 6. Histogram of the energy dissipation difference (depression) at the FcC11H22SH islands in the reduced state (-0.8 V) and in the oxidized state (-0.4 V). The depth of each FcC11H22SH island from the surrounding C10H21SH layer in an energy dissipation image was measured from the corresponding line profile. Twenty-seven islands labeled in Figure S2 were analyzed for four energy dissipation images at the same area (Figure 5A and another one for the reduced state and Figure 5B and another one for the oxidized state) which correspond to the topographic images used for the analysis in Figure 4. Calculated Gaussian parameters and fitted curves for both states are indicated in the figure.

Figure 5. Energy dissipation images of FcC11H22SH islands embedded in C10H21SH SAM (176  159 nm2) (A) at the reduced state (-0.8 V) and (B) at the oxidized state (-0.4 V), which were recorded simultaneously with topography images in Figures 3A and 3B, respectively. The voltage applied to the piezo actuator for vibrating the cantilever at the constant amplitude (Ap-p = 1.00 nm) was imaged as contrast. Thus, energy dissipation is larger for brighter region. (C) A comparison of energy dissipation line profiles at the FcC11H22SH island (indicated by a circle a dotted line in each image), whose topographic line profiles are shown in Figure 3C.

The magnitude of depression at each Fc island (energy dissipation difference from surrounding C10H21SH SAM) was measured from the corresponding line profile, and their distributions for both redox states are summarized as a histogram in Figure 6. Several factors seem to be included as the origins of the contrast change, which are important to understand the local environment of the redox-active species tethered at the electrode surface under electrochemical control.

4. Discussion Structural change of Fc-terminated SAMs by electrochemical oxidation has been studied extensively. Ye et al. measured in situ infrared reflection absorption spectroscopy (IRRAS) of FcC11H22SH SAMs on Au(111) and found the increase of the ν(C-H) intensity of the cyclopentadienyl (Cp) ring in ferrocenyl group and decrease of the ν(C-H) intensity of methylene in alkyl chains upon oxidation to ferrocenium cation (Fcþ).7,8 They considered these changes can be explained by decrease in the tilt angle of the alkyl chains from the surface normal and selective detection of the vibrational dipoles along the surface normal by the IRRAS. Electrostatic repulsion between the Fcþ cations and the positively charged Au electrode was proposed as a reason for 9108 DOI: 10.1021/la904797h

such a reorientation. Another factor being considered was the ion pair formation between the Fcþ cation and a perchlorate anion (ClO4-). Viana et al. also studied Fc-terminated alkyl thiols with different lengths by infrared spectroscopy and concluded that the flipping of the ferrocenyl group while fixing the tilt angle of the alkyl chain was the major structural change upon oxidation.9 A little contradiction remains for the tilt angle of the alkyl chains, but the reorientation of the Fc group was commonly observed. From surface plasmon resonance (SPR) measurements, Yao et al. estimated that the thickness change of FcC11H22SH SAMs in 0.1 M HClO4 was 0.09 nm.12 It must be noted, however, that the thickness estimation by SPR or ellipsometry is not straightforward particularly for the system that may bind other species to the layer. In the case of FcC11H22SH SAMs, binding of counteranions was suggested when Fc ligand was oxidized to Fcþ. As a summary, the estimated increase of the thickness in accordance with the oxidation in HClO4 solution has variation from 0.09 to 0.3 nm.10-12 In contrast, no noticeable change in thickness was observed for alkanethiol SAMs without redox-active ligands, when the potential was swept in the same range.11,12 The concentration of ions changes at the alkanethiol SAM/solution interface when the potential is swept; however, such non-sitespecific adsorption at low concentration does not affect the thickness measurements. The effect of the change of the ion species at the interface will be discussed later. Therefore, the increase in height of the FcC11H22SH islands embedded in the C10H21SH SAM is expected to be in the range from 0.09 to 0.3 nm, if we postulate the embedded Fc islands show the same response against the potential change as the FcC11H22SH SAM. Our FMAFM results summarized in Figure 4 and Figure S3 indicated that the height increase was ∼0.44 nm on average. Ion pairing of the perchloric anion (ClO4-) to the oxidized Fcþ cation was suggested by the electrochemical quartz crystal microbalance (EQCM) measurements.7,13 In accordance with the oxidation current by a potential sweep in HClO4 aqueous solution, the mass of the Au electrode modified with FcC11H22SH Langmuir 2010, 26(11), 9104–9110

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Figure 7. Schematic representation of the local structure change of the FcC11H22SH island embedded in C10H21SH SAM by oxidation. As oxidation of Fc group to ferrocenium cation (Fcþ) proceeds, hydrophobic ClO4- anions coordinates to the Fcþ. The Fcþ islands result in a more rigid and stable structure.

SAMs increased. From the frequency shift value of the EQCM and the corresponding CV data, they concluded that estimated increase of mass roughly corresponded to one ClO4- anion against one Fcþ cation. More direct evidence of ion-pairing was provided by surface-enhanced Raman spectroscopy (SERS) measurements.4,14 A peak at 933 cm-1, which can be assigned to the Cl-O stretching band of ClO4- bound to the Fc-terminated SAM, was observed when the sample potential was set positive enough to oxidize the Fc groups. An important point from the observations was that the ion-pairing depended on the kind of anions. SERS signals for ion pairs were observed when hydrophobic anions like PF6-, ClO4-, etc., were used but not observed when hydrophilic anions like SO42- and NH2SO3- were used.4 This was qualitatively explained by the different adsorption energies of anions. The ClO4- was proposed to be bound between Fcþ cations to reduce Coulomb repulsion forming a 1:1 in-plane network structure.4 Recently redox-state-dependent actuation of the FcC11H22SH-SAM-covered cantilever caused by lateral expansion of the oxidized SAM by formation of the 1:1 network structure in HClO4 solution was reported.30 However, these experimental results do not necessarily restrict the adsorption site of ClO4- to the in-plane position. Figure 7 shows a slightly modified model of the 1:1 network structure, where ClO4- ions are bound on flipped Fcþ groups. This model includes the flipping of the Fcþ group, strong binding of ClO4-, and thickness increase indicated by macroscopic spectroscopies. Lateral expansion can occur depending on how deep the ClO4- ions are buried in the Fcþ layer. Recent FM-AFM measurements at the water/lipid bilayer interface in buffer solution showed hydration layers can be imaged by choosing an appropriate range of the frequency shift.23,31 As indicated in frequency shift vs distance curves measured during approach of a tip to the fully covered single-component FcC11H22SH SAM at different electrochemical potential (Figure S4 in the Supporting Information), the force acting between the tip and the sample to image Figure 3 was in the range of a few hundred piconewtons. It is too large to image rather weakly bound hydration layers. If the standard bond lengths and angles, a 30° tilt of (30) Norman, L. L.; Badia, A. J. Am. Chem. Soc. 2009, 131, 2328. (31) Fukuma, T.; Higgins, M. J.; Jarvis, S. P. Biophys. J. 2007, 92, 3603. (32) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.

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the molecular axis from the surface normal, and attachment of ball-like Fc of 0.66 nm on its end32 are assumed for FcC11H22SH, the estimated height difference between the surrounding C10H21SH SAM and the embedded FcC11H22SH island is about 0.55 nm. Thus, the freedom for the Fc group in the C10H21SH matrix is rather small. The height of Fc islands was measured to be 0.51 ( 0.17 nm (Figure 4), which is consistent with the expected value. The height increase by oxidation of Fc groups estimated in Figure 4 was ca. 0.44 ( 0.20 nm, which was slightly smaller than a van der Waals diameter of the ClO4- anion, which is about 0.53 nm. Embedment of ClO4- between the Fcþ cations is supposed to be restricted by surrounding the C10H21SH SAM matrix. Thus, strongly bound ClO4- anion layer on the Fcþ cations is a reasonable explanation for the increase in the apparent height of the Fc islands. When we consider the force acting between two working electrodes of the sample and the tip, we have to know the apparent charge on each electrode. Potential of zero charge (pzc) for an electrode is defined as the voltage where no free excess electronic charge is present on the electrode surface. If the electrode voltage is more positive (negative) than the pzc, positive (negative) charge is induced at the electrode surface. Stern layer, which is formed by specific adsorption of counterions at close proximity of the charged electrode surface, and diffuse electric double layer, which has higher density of counterions than the neutral bulk solution extending in typical decay length (Debye length) of about 1 nm for 0.1 M HClO4, compensate for the charge on each electrode. The pzc of C10H21SH SAMs on Au(111) in 0.1 M HClO4 was reported to be -0.53 V vs Ag/AgCl (-0.33 V vs RHE).33 A stepped Pt surface is reasonable as a model of the Pt-coated tip used in our measurements. The pzc of a Pt(111) surface was estimated to be ca. 0.2 V vs RHE in 0.1 M HClO4,34 and this value shifted to negative direction when step density increased.35 The potential range of our measurements (from -0.8 to -0.4 V vs Au/AuOx (from 0.35 to 0.75 V vs RHE)) was more positive than the pzc values of C10H21SH SAMs on Au(111) and the Pt-coated cantilever tip; thus, both electrode surfaces were always positively charged. Therefore, repulsive interaction between the tip and the sample in our measurements was partially due to the double-layer repulsion. As far as we know, there is no report on the pzc of FcC11H22SH SAMs on Au(111). The work function of a FcC11H22SH SAM on Au(111) measured in UHV was þ36 mV against a bare Au(111) surface,36 whose pzc value in 0.1 M HClO4 was reported to be 0.33 V vs Ag/AgCl (0.53 V vs RHE).33 Thus, from a correlation between the pzc value and the work function, it may be supposed that the pzc of the FcC11H22SH SAM on Au(111) is close to 0.6 V vs RHE, which is much more positive than that of C10H21SH SAMs on Au(111) (-0.33 V vs RHE). When the Fc islands are neutral in charge at 0.75 V vs RHE, positive charge induced at the electrode surface at the Fc islands should be much less than that at the surrounding C10H21SH SAM. In Figure 5, the contrast of energy dissipation on the Fc islands showed remarkable difference depending on the charged state of Fc. Several origins of energy dissipation in FM-AFM have been pointed out in the literature.29,37 One of them is the interaction (33) Ramirez, P.; Andreu, R.; Cuesta, A.; Calzado, C. J.; Calvente, J. J. Anal. Chem. 2007, 79, 6473. (34) Weaver, M. J. Langmuir 1998, 14, 3932. (35) Gomez, R.; Climent, V.; Feliu, J. M.; Weaver, M. J. J. Phys. Chem. B 2000, 104, 597. (36) Watcharinyanon, S.; Moons, E.; Johansson, L. S. O. J. Phys. Chem. C 2009, 113, 1972. (37) Gauthier, M.; Perez, R.; Arai, T.; Tomitori, M.; Tsukada, M. Phys. Rev. Lett. 2002, 89, 146104.

DOI: 10.1021/la904797h

9109

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showing hysteresis; i.e., the potential felt by the tip from the sample is different during approach and retraction within a vibration of the cantilever. To keep the amplitude constant against the damping, larger energy is necessary. Another example is Joule heating due to displacement current by changing the capacitance between the tip and the sample in each vibration. In liquid environment, viscous dissipation by the surrounding fluid is significant. Finite element analyses revealed that damping arises due to combination of squeeze film effects and viscous shear near the edges of the cantilever during approaching closer to the sample surface.38 A recent report using amplitude-modulated AFM (AM-AFM) for the solid/liquid interfaces in KNO3 aqueous solution showed that effective damping began to increase at some distance from the solid surfaces and the onset distance decreased by increasing the electrolyte concentration.39 The authors concluded it was due to the thickness decrease of diffuse electric double layer (reduction of Debye length). They also measured the dependence of the damping magnitude on the potential of the gold electrode surface (although without potential control of the tip). The damping magnitude in the double layer at the potential close to the pzc of a gold surface was smaller than those at more positive or more negative potentials.39 From these results, the authors deduced that the damping originated from the displacement of the counterions by the tip. The counterions have higher concentration in diffuse electric double layer than the neutral bulk solution as the electrode potential is separated from the pzc. Here we would like to consider if displacement of counteranions ClO4- in diffuse electric double layer can explain the energy dissipation contrast observed in the present study. The excess amount of counteranions ClO4- in the diffuse double layer is supposed to be smaller on the Fc0 islands than on the surrounding C10H21SH SAM because less positive charge should be induced for the former region as discussed above. When the Fc moieties are oxidized, the ionic layer formed by binding of ClO4- to Fcþ gives downward electric dipoles whose magnitude depends how deep the ClO4- ions are buried in the Fc layer. These electric dipoles contribute to compensation of the potential difference between the positively charged electrode and the bulk solution, thus further reducing the ClO4concentration in the diffuse electric double layer on the Fc islands. Thus, the concentration of ClO4- in the diffuse electric double layer in each region is expected to be in the order of C10H21SH SAM > Fc0 islands . Fcþ islands, which coincides (38) Basak, S.; Raman, A.; Garimella, S. V. J. Appl. Phys. 2006, 99, 114906. (39) Wu, Y.; Gupta, C.; Shannon, M. A. Langmuir 2008, 24, 10817.

9110 DOI: 10.1021/la904797h

Umeda and Fukui

with the order of energy dissipation magnitude in Figures 5 and 6. As a whole, the ion-pairing network structure in Figure 7 is a reasonable model to explain our EC-FM-AFM results which showed increase in apparent height as well as decrease in energy dissipation at Fc islands by oxidation of the Fc groups.

5. Conclusions We demonstrated that FM-AFM measurements under electrochemical potential control were efficiently applicable to analyses of the redox-state-dependent local structural change at the solid/liquid interfaces. Electroactive FcC11H22SH islands embedded in the C10H21SH SAM matrix showed reversible change in local structure by EC-FM-AFM measurements. Line-profile analyses of each Fc island revealed that the apparent height increased by about 0.44 nm accompanied by the oxidation of the terminal Fc group to Fcþ. A structural model, where ClO4- ions are strongly bound on the flipped Fcþ groups forming an ionic double layer, was proposed as a modification of a previous model. The major factor for the increase in the apparent height can be attributed to the formation of the double layer in the oxidized state. Energy necessary to keep the cantilever’s vibrational amplitude constant (i.e., energy dissipation) was reduced on the Fc0 islands and further reduced on the oxidized Fcþ islands. The concentration of counterions in the diffuse electric double layer was deduced as a possible factor for the difference. The most significant contribution of the present work is that we have shown that the change in localized charge emerged at the solid/liquid interface dependent on an external parameter can be detected through the change in local structure (and energy dissipation) by using potential regulated FM-AFM. It may open a new stage for characterization of electron transfer reactions in nanometer scale spatial resolution. Acknowledgment. This work was financially supported by the Asahi Glass Foundation and the Kurata Memorial Hitachi Science and Technology Foundation. K.U. thanks Prof. H. Yamada of Kyoto University and Prof. T. Fukuma in Kanazawa University for kind suggestions to reduce the possible noises of the FM-AFM. Supporting Information Available: A typical CV of FcC11H22SH islands embedded in the C10H21SH SAM on Au(111), labels of twenty-seven Fc islands in Figure 3, frequency shift vs distance curves during tip approached to the fully covered single component FcC11H22SH SAM on Au(111) at different electrochemical potentials. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(11), 9104–9110