Characterization of Ferrocene-Modified Electrode Using

May 24, 2014 - procedure.28 The gold was vapor-deposited (VPC-410A, Ulvac) on a freshly cleaved mica template, which was glued on the disk with the go...
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Characterization of Ferrocene-Modified Electrode Using Electrochemical Surface Forces Apparatus Motohiro Kasuya† and Kazue Kurihara*,†,‡ †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan WPI-AIMR, Tohoku University, Sendai 980-8577, Japan

‡

S Supporting Information *

ABSTRACT: A electrochemical surface forces apparatus (ECSFA) was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) and oxidized ferrocene (ferrocenium cation, Fc+-SAM) in a 1 mM aqueous electrolyte. The double-layer repulsion in both cases of the Fc-SAM and Fc+-SAM electrodes was observed. The surface charge density (σ) evaluated from the double-layer repulsions between the Fc+-SAM electrodes in 1 mM aqueous KClO4 was 0.0027 C/m2, which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m2. The σ values of the Fc+-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m2 for NO3−, SO42−, and CF3SO3−, respectively. The degrees of dissociation (αd) between the ferrocenium cation and these counteranions were obtained from σ and the density of the ferrocenium on the electrode. The αd value of CF3SO3−, 4.1%, was the highest, followed in the order, SO42− > NO3− > ClO4−, indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.

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INTRODUCTION Chemically modified electrodes have been extensively studied because of the broad interests in their applications such as photovoltaics,1 artificial photoelectrochemical cells,2 and molecular sensing.3 The surface potential and surface charge density of the electrodes are important properties in electrochemistry because they determine the performance of electrodes, and sensitively influence the interactions between the electrode surface and ions in the electrolyte solutions. Surface forces measurements have been regarded as a promising tool for evaluating the surface potential and the surface charge density based on the electric double-layer forces.4−6 Naturally, the development of an electrochemical surface forces apparatus (EC-SFA) has been reported.7−10 However, the previous studies used mica as one surface and mercury7 or gold electrodes8,9 as another because FECO (fringes of equal chromatic order)4 used for the distance determination in these studies required at least one surface to be transparent. These researchers reported the evaluation of the surface potential under various applied electrochemical potentials by fitting the force curves with the DLVO (Derjaguin−Landau−Verwey−Overbeek) theory. However, quantitative analysis of the interaction is more difficult compared to the measurement for two identical surfaces. The report by Fan and Bard used two platinum electrode surfaces in an SFA chamber; however, only the electrochemical currents were measured at various separations and no force profile was determined.10 Electrochemical atomic forces microscopy (EC© 2014 American Chemical Society

AFM) was also employed; however, it was also applied to measure forces between the dissimilar surfaces.11,12 Recently, we developed a new surface forces apparatus, called the twin-path SFA, using two-beam (twin-path) interferometry for measuring the interactions between nontransparent substrates.13 On the basis of this apparatus, we have developed a new EC-SFA, which enabled us to perform the forces measurements on identical gold electrode surfaces as a function of the surface separation under the applied potential.14 Ion adsorption on electrodes is one of the major research targets to understand electrochemical processes on electrodes. Particularly, actively studied is the ion pairing of the redox species on the electrode with counterions in the electrolyte solution because it reduces the effective charge density, thus the effective potential of the electrode.15−23 Therefore, various surface-sensitive analytical methods,17−23 and a molecular dynamic simulation,24 were applied for obtaining microscopic views of this phenomenon. Ellipsometry,17,18 surface plasmon resonance (SPR),19 infrared reflection−absorption spectroscopy (IR-RAS), 20 surface-enhanced Raman spectroscopy (SERS), 21 and electrochemical quartz microbalance (EQCM)22 were also applied. Nevertheless, a few studies using quantitative analytical tools, such as EQCM22 and SERS,21 were reported for obtaining the binding constant of Received: March 11, 2014 Revised: May 19, 2014 Published: May 24, 2014 7093

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conductive epoxy (CW2400, Chemtronics), and then the connected area was covered with epoxy resin. This electrode was used as the working electrode (WE) using a potentiostat (ALS/CH Instruments electrochemical analyzer-model 700C, BAS) for controlling the WE potential. The counter electrode (CE) was a Pt wire, and the reference electrode (RE) was a Ag/ AgCl (saturated KCl) electrode (BAS). A salt bridge made of agar gel was used for connecting the twin-path SFA chamber and the RE. To avoid the contamination by Cl− ions from the salt bridge, we used salts containing ions, same as in electrolyte solutions for the salt bridges. This arrangement allowed us to run electrochemical measurements in the three-electrodes cell arrangement inside the chamber for the twin-path SFA. The SFA chamber was filled with the 1 mM aqueous electrolyte solutions. Before all the measurements, argon (99.9999%) was bubbled through the solutions for more than 30 min for deaeration, and the experiments were done under an argon atmosphere. The interaction force (F) between the gold electrode surfaces was measured as a function of the surface separation (D) in an aqueous solution following a previously reported procedure.13 The spring constant of the cantilever was in the range of 180− 240 N/m. We continuously changed the surface separation between the gold surfaces at a constant approaching rate (20 nm/s) when we obtained the force curves. We calculated the contribution of the hydrodynamic forces based on the work by Horn and Chan.29 The value was ca. 0.08 mN/m at D = 10 nm in the case for a KClO4 solution. Therefore, we may ignore the hydrodynamic forces because it is significantly smaller than the observed electric double-layer repulsion (ca. 0.4 mN/m at D = 10 nm). This was supported by the experimental decay length of the observed repulsion similar to the theoretical Debye length as described in Table 1. The obtained force was

the ion pairing, which is necessary for designing wellperforming modified electrodes. These techniques could not provide an accurate binding constant. EQCM evaluated the mass changes by the ion pairing; however, the dependencies of the pairing on anion species, possibly because of the influences of the hydration of ions, were not studied.22 SERS could provide only limited quantitative analyses because enhancement factors for SERS varied with the distances between the ions and electrode surfaces.25 Surface forces measurement enable us to evaluate the surface charge densities on the electrodes. It is quantitatively possible to evaluate the degrees of the ion pairing using EC-SFA for the redox modified electrodes. In this study, we performed the forces measurements on the interactions between ferrocene terminated self-assembled monolayers (Fc-SAM) on gold electrodes employing EC-SFA based on the twin-path SFA. The Fc-SAM on a gold surface is one of the most studied chemically modified electrodes.15−17,19−22,24,26,27 Quantitative evaluation of the ion pairing between Fc+ and various counteranions in electrolyte solutions (ClO4−, NO3−, SO42−, and CF3SO3−) revealed the surface charge densities (σ) and the degrees of dissociation (αd). We choose the counterions expected to exhibit different affinities to the Fc-SAM by following the previous studies.15−17,19,21,27 The obtained αd values indicated that most of the positive charges of Fc+-SAM were compensated by the formation of the ion pair with counteranions in the cases of all four anions.

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EXPERIMENTAL SECTION Materials. 6-(Ferrocenyl)hexanethiol (FcC6SH, Aldrich), KClO4 (99.99%, Aldrich), KNO3 (99.995%, Merck), K2SO4 (99.99%, Aldrich), and KCF3SO3 (98%, TCI) were used as received. Cyclohexane (99.5%, Nacalai Tesque) was distilled prior to use. Water was pure water (NANOpureII, Barnstead, 18 MΩ/cm resistance). Gold (99.99%) and Pt wire (99.9999, ϕ = 0.2 mm) were from Tanaka Kikinzoku. Surface Forces Measurement. A schematic illustration of the measurement system is shown in Figure 1. The gold

Table 1. Analyzed Data from Surface Forces Shown in Figure 3

counteranion ClO4− NO3− SO42− CF3SO3−

applied potential (V)

decay length, 1/κ (nm)

Debye length (nm)

surface charge density, σ (C/m2)

0.8 0 0.8 0 0.8 0 0.8 0

10.3 9.1 9.5 8.7 5.5 5.0 9.3 9.6

9.6 9.6 9.6 9.6 5.6 5.6 9.6 9.6

0.0027 0.0011 0.0048 0.0011 0.0040 0.0014 0.0104 0.0010

degree of dissociation, αd (%) 0.5 1.1 2.0 4.1

normalized by the radius R of the surface curvature using the Derjaguin approximation,4 F/R = 2πGf, where Gf is the interaction free energy per unit area between two flat surfaces. We used 20 ± 2 mm for R which is a typical value when we measured R of surfaces using the FECO SFA in our laboratory. We performed the surface forces measurement on at least three different samples at each condition, more than three times for each sample (See Supporting Information). The averaged value and error were obtained from these measurements.

Figure 1. Schematic illustration of electrochemical surface forces apparatus (EC-SFA).

electrode surface was prepared on cylindrical silica disks (curvature radius, R = 20 mm) following a previously reported procedure.28 The gold was vapor-deposited (VPC-410A, Ulvac) on a freshly cleaved mica template, which was glued on the disk with the gold side down using an epoxy resin (Epikote 1004, Shell). Then the mica template was subsequently removed just prior to use. The gold surfaces were dipped in a 0.5 mM cyclohexane solution of 6-(ferrocenyl)hexanethiol for 8 h and washed by cyclohexane and ethanol. A Teflon-coated Cu wire was connected to the Fc-modified gold surface using a

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RESULTS AND DISCUSSION Figure 2a shows a cyclic voltammogram of the Fc-SAM electrodes in 10 mM aqueous KClO4 in conventional electrochemical cells at various scan speeds (v) of the 7094

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Figure 2. (a) Cyclic voltammogram of the ferrocene-modified surfaces in the 10 mM aqueous KClO4 in conventional electrochemical cells. (b) Cyclic voltammogram of the ferrocene-modified surfaces in the 1 mM aqueous KClO4 in the EC-SFA chamber. Scanning rate of the applied potential was 0.1 V/s.

electrochemical potential. A redox wave was observed around 0.4 V (vs Ag/AgCl) at any v’s. The current density (Ip) of this peak was proportional to v. These results were similar to reported ones,15,16,21,26 indicating that this peak was due to the one electron redox reaction of the ferrocene groups immobilized on the electrode. We evaluated the surface concentration of the ferrocene (CFc) by integrating the oxidation peak. The obtained CFc value of the Fc-SAM was 3.1 ± 0.2 molecule/nm2 and similar to the value of the closest packed Fc film (2.6 molecules/nm2) and previously reported values (2.7−3.3 molecules/nm2).15,16,21,26 The former value was calculated from the size of the Fc group (6.6 nm) obtained from the density of the Fc crystal.26 Redox waves similar to Figure 2a were observed in the voltammogram of the Fc-SAM electrodes in the chamber for the twin-path SFA as shown in Figure 2b. The separation of the redox peaks was reported to depend on the electrolyte concentration.16 The larger separation between the redox peaks of Fc-SAM in the SFA chamber (Figure 2b) than that in the conventional system (Figure 2a) should be due to the different concentrations of KClO4, 1 mM for the SFA study and 10 mM for the conventional electrochemical measurement. Figure 3a shows force profiles of interactions between the ferrocene-modified surfaces in 1 mM aqueous KClO4. When the applied potential was 0.8 V vs Ag/AgCl,30 in which the ferrocene molecules were in the cationic state, the observed repulsion followed the exponential function of which the decay length (1/κ) was 10.5 ± 0.5 nm. This decay length was in good

Figure 3. Force profiles between the ferrocene-modified surfaces in 1 mM aqueous electrolyte solution with various applied potential. (a) KClO4, (b) KNO3, (c) K2SO4, and (d) KCF3SO3. The different shapes of symbols in the graphs represent the repeating experiments. Blue symbols represent the data at the applied potential of 0.8 V vs Ag/ AgCl, and green symbols at the applied potential of 0 V vs Ag/AgCl. Solid line denotes the theoretical fits to the Poisson−Boltzmann equation of the electric double-layer force for the constant charge model.

agreement with the theoretical Debye length (9.6 nm) of the double-layer repulsion for the current salt concentration, indicating that the observed repulsion was attributed to the double-layer repulsion. The surface charge density (σ) could be evaluated by fitting to the Poisson−Boltzmann equation of the electric double-layer force for the constant charge model,31,32 which take into account the confinement and double-layer 7095

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respectively. The enhancement factors of SERS depended on the distances between the ions and electrode surfaces.25 Thus, differences between the previous and current results may depend on characteristics of the employed methods and/or difference in the ion adsorption mode such as the location of ions. Future studies are necessary to shed new light on the ion adsorption to electrodes.

overlap. The weak adhesion (less than 0.6 mN/m) indicated that the van der Waals attraction was quite weakened compared to the gold−gold surface adhesion in water (more than 10 mN/ m). This showed that fitting the observed force by the doublelayer repulsion is more suitable than by the conventional DLVO theory, which provided twice the charge density obtained by the former.32 In the former case, the error caused by neglecting the van der Waals contribution should be less than several percentage points. The evaluated σ for the Fc+SAM in the aqueous KClO4 was 0.0027 ± 0.0002 C/m2. The degree of the dissociation between Fc+ and counteranion (αd) obtained from the ratio of σ/CFc was 0.5 ± 0.1%. This αd value indicated that most of the positive charges of Fc+-SAM were compensated by the formation of the ion pair with ClO4−. This agrees with previous studies by EQCM22 or Raman spectroscopy21 reporting that most of the Fc+ on the film formed ion pairing with the ClO4−. When the applied potential was 0 V vs Ag/AgCl, at which ferrocene molecules on the electrodes were neutral, the observed repulsion also followed the exponential function, as shown in Figure 3a. The decay length (1/κ) of the repulsion was 9.6 ± 0.5 nm, which was in good agreement with the theoretical Debye length (9.6 nm) of the double-layer repulsion, and the amplitude of the repulsion was one-third of the value at 0.8 V. The σ value for the Fc-SAM evaluated by the same method as the Fc+-SAM was 0.0011 ± 0.0001 C/m2, which may be due to ion adsorption on the film. To investigate the sign of the adsorbed ion species on the neutral Fc-SAM, we measured the surface forces between Fc-SAM and negatively charged mica surfaces in 1 mM aqueous KClO4, as shown in Figure S1 in the Supporting Information. Electric double-layer repulsion between the two surfaces was observed, indicating that Fc-SAM was negatively charged with adsorption of the ClO4− anions. Force profiles of the interactions between the Fc+-SAM or Fc-SAM electrodes were repulsive in the 1 mM aqueous KNO3, K2SO4, and KCF3SO3 as shown in parts (b), (c), and (d) of Figure 3. The decay lengths of the repulsions were in good agreement with the theoretical Debye lengths (see Table 1), which indicated that the observed repulsions were the electric double-layer forces. We evaluated the σ of the Fc+-SAM in these solutions using the same method for evaluating those in the case of KClO4. The σ values in the case of KNO3, K2SO4, and KCF3SO3 were 0.0048, 0.0040, and 0.0104 C/m2, respectively. The ratio of the oxidized ferrocene Fc+ to feroccene Fc on the electrodes was reported to depend on the kind of counteranions in electrolyte solutions.21 To estimate realistic dissociation degree, αd = σ/CFc+, the density of Fc+ (CFc+) on the electrode was obtained from the integration of the reduction wave on the cyclic voltammogram: 2.8, 1.3, and 1.6 molecules/nm2 in the case of KNO3, K2SO4, and KCF3SO3. The αd values obtained from the σ and CFc+ were 1.1% ± 0.1%, 2.0% ± 0.2%, and 4.1% ± 0.3% in the case of KNO3, K2SO4, and KCF3SO3, respectively. The order of the degree of dissociation (αd) was CF3SO3− > SO42− > NO3− > ClO4−. These αd values indicated that most of the Fc+ formed the ion pair with all four ions. Our results agreed with the order of the αd values for the various counteranions reported in a previous study using SERS.21 This trend was previously explained by the difference in hydrophilicity of ions.21 The αd values in the present study were much lower than the previously reported values by SERS that the αd values of ClO4−, NO3−, and SO42− were 10%, 72%, and 100%,

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CONCLUSION The interactions between the Fc-SAM electrode surfaces at various applied potentials were directly measured using the electrochemical surface forces apparatus (EC-SFA), which we recently developed. Double-layer forces between the Fc+-SAM electrodes were observed, from which we could evaluate the degrees of dissociation between ferrocenium cation and counteranion (αd). αd in the aqueous KClO4, KNO3, K2SO4, and KCF3SO3 solutions was 0.5%, 1.1%, 2.0%, and 4.1%, respectively. These values indicated that most of the Fc+ formed an ion pair with the counteranions. Applications of Fc-SAM electrodes to biosensors3 and microactuator devices33 have been well-known. Our findings in the ion pairing provide fundamental and quantitative information for these applications of Fc-SAM. Ion adsorption on the electrodes depending on ion species was well-studied also on metal electrodes such as gold and platinum electrodes.34 Our method could provide new knowledge on such ion adsorption, which is under investigation at our laboratory. In this study, we characterized the change in the surface charge of the electrode, which was easy to characterize. A redox reaction such as quinone−hydroquinone does not accompany changes in the surface charge, but the hydrophilicity may be different between quinone and hydroquinone, which could change the adhesion and/or hydration force between electrodes. It should be interesting to study various redox systems in the future using the EC-SFA.

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ASSOCIATED CONTENT

S Supporting Information *

Force profiles between the mica and ferrocene-modified surfaces in 1 mM aqueous KClO4, fitting results by conventional DLVO theory and force profiles between the ferrocenemodified surfaces to show the reproducibility of the measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the CREST program of the Japan Science and Technology Agency (JST). REFERENCES

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