Distance-Independent Charge-Transfer Resistance at Gold Electrodes

J. Phys. Chem. C 2008, 112, 10153–10160

10153

Distance-Independent Charge-Transfer Resistance at Gold Electrodes Modified by Thiol Monolayers and Metal Nanoparticles Christopher R. Bradbury, Jianjun Zhao, and David J. Fermı´n* Departement fu¨r Chemie and Biochemie, UniVersita¨t Bern, Freiestrasse 3, CH-3012 Berne, Switzerland ReceiVed: October 19, 2007; ReVised Manuscript ReceiVed: March 10, 2008

The electrochemical properties of Au electrodes sequentially modified by self-assembled monolayers (SAM) of carboxyl-terminated alkane thiols, ultrathin poly-L-lysine (PLL) film, and diluted monolayers of Au nanoparticles are investigated by electrochemical impedance spectroscopy (EIS). The phenomenological chargetransfer resistance (Rct) for the hexacyanoferrate redox couple at the equilibrium potential exhibited an exponential increase with increasing methylene units (x) in the SAM. The increase of Rct between x ) 1 and 10 was described by a well-defined decay parameter β ) 1.16 ( 0.04 per methylene unit. This behavior suggests that the kinetics of electron transfer is controlled by coherent electron tunneling across the carboxylterminated SAM. Adsorption of the PLL brings about an average 2.5 times decrease in Rct independent of x. The ultrathin PLL film (thickness less than 1 nm) induces an increase of the surface concentration of the redox couple without affecting the β value observed for the SAM-terminated electrodes. Diluted monolayers of Au nanoparticles with an average 19.2 ( 2.1 nm diameter generate significant changes in the dynamics of electron transfer. In contrast to the behavior in the absence of nanoparticles, a distance-independent Rct was observed for x > 5. Detailed analysis of the electrochemical responses as a function of the particle number density revealed that the rate-determining step is the charging of the nanoparticles by the redox species. It is concluded that the electronic communication between the nanoparticles and the electrode surface over distances as large as 13 Å originates from electron transport through the trapped redox probe. The several orders of magnitude changes of the apparent Rct upon nanoparticle adsorption further suggest that electron transport through the film does not occur via a classical hopping mechanism. A mechanism based on nonthermalized electron transport via the density of the redox probe at the Fermi energy (hot electron transport) is proposed to account for the experimental observations. 1. Introduction Generation of electroactive hybrid devices composed of polymeric materials, nanoscopic objects, and supramolecular components requires fundamental knowledge of the dynamics of charge transport and injection to electrical contacts. A particular issue extensively studied in this field is the distance dependence of electron transfer between redox-active components and electrode surfaces.1 The dynamics of charge transfer are affected not only by the nature of the “redox-active” component but also by the electronic properties of the bridging element.2 A typical example is given by thiol-based monolayers featuring a redox-active terminal group self-assembled at Au surfaces.3–10 In these systems, the exponential decrease of the electron-tunneling probability with the length of the spacer is characterized by the decay constant β. By varying parameters such as degree of saturation and electronic delocalization in the bridge, β values ranging from 1 to 0.1 Å-1 have been reported.1,2 The behavior observed at modified extended surfaces has also been observed at the nanoscale11,12 and even at the singlemolecular levels.13–18 The tunneling decay constant between nanostructures and metal electrodes has been investigated by electrochemical measurements and scanning probe microscopy. Chen and Pei estimated a β ) 0.8-0.9 per methylene unit for charge injection * To whom correspondence should be addressed. Current address: School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. Phone: +44 1179288981. Fax: +44 1179250612. E-mail: David.Fermin@ bristol.ac.uk.

into thiol-alkane-protected nanoparticles.19 Vanmaekelbergh and co-workers estimated a value of 0.5 Å-1 in Q-CdSe assembled at Au surfaces via rigid dithiol spacer featuring cyclohexyl units linked by double bonds.20,21 Frisbie and co-workers estimated β ≈ 1.1 per methylene unit for alkanethiols and alkanedithiols adsorbed on Au electrodes using conducting atomic force microscopy (c-AFM).12,22 These authors concluded that neither the work function of the c-AFM tip nor the nature of the molecular contact (physisorbed or chemisorbed) has a significant effect on the observed β. There have been numerous reports on enhancement of the electron-transfer rate constant to redox species mediated by selfassembled metal nanoparticles.23–28 Recently, Jensen et al. have shown that Au nanoparticles can act as an efficient redox relay for cytochrome c assembled at Au electrodes.29 Despite the experimental evidence gathered so far, it is not entirely clear what the role of the nanoparticles in the enhancement of the interfacial electron transfer is. Significant advances can be gained in this area if the information obtained from electrochemical measurements can be compared to those extracted from local conductivity studies employing scanning probe methods. These comparisons can be approached by quantification of the apparent charge-transfer resistance (Rct) at the electrode surface as a function of the number density of adsorbed nanoparticles. In the present paper, the phenomenological Rct between Au nanoparticles and Au electrodes separated by a self-assembled monolayer (SAM) of carboxyl-terminated alkane thiol is investigated as a function of the molecular chain length.

10.1021/jp710165d CCC: $40.75  2008 American Chemical Society Published on Web 06/17/2008

10154 J. Phys. Chem. C, Vol. 112, No. 27, 2008 Following our previous studies, surface modification involves the sequential adsorption of the carboxyl-terminated SAM, an ultrathin layer of poly-L-lysine (PLL), and the citrate-stabilized Au nanoparticles. This approach has allowed us to quantify the apparent Rct as a function of the particle number density. The results described herein clearly demonstrate that Rct is independent of the thiol length even for layer thicknesses larger than 13 Å. This remarkably fast electronic communication between nanoparticles and the electrode surface is rationalized in terms of a resonant process through “localized levels” at the Fermi energy. The levels involved in the electronic wiring of the nanoparticle to the electrode are associated with redox probes trapped in the polyelectrolyte film. 2. Experimental Section 2.1. Chemicals. Mercaptoacetic acid 99% (MAA), 3-mercaptopropionic acid 99% (MPA), 6-mercaptohexanoic acid 90% (MHA), 8-mercaptooctanoic acid 96% (MOA), 11-mercaptoundecanoic acid 95% (MUA), hydrogen tetrachloroaurate(III) hydrate (HAuCl4 · 3H2O, 99.999%), sodium citrate dihydrate (C6H5Na3O7 · 2H2O, 99%), and poly-L-lysine hydrobromide (Mw, 30 000-70 000) were purchased from Sigma-Aldrich. Anhydrous GR-grade potassium ferrocyanide (K4Fe(CN)6 · 3H2O), potassium ferricyanide (K3Fe(CN)6), and sodium sulfate (Na2SO4) were obtained from Merck. All chemicals were used as received. Millipore filtered ultrapure water (MilliQ water, resistivity > 18 MΩ cm-1) was used to prepare all aqueous solutions and for rinsing. 2.2. Synthesis of Au Nanoparticles. The gold nanoparticles were obtained by reduction of HAuCl4 in the presence of citrate.28,30 The one-step synthesis involves injection of 10 cm3 of 1% (w/w) trisodium citrate into 190 cm3 of 2.5 × 10-4 mol dm-3 HAuCl4 · 3H2O under reflux and strong stirring. The solution was refluxed for 1 h, yielding Au nanoparticles with an average diameter of 19.2 ( 2.1 nm as estimated from acoustic AFM and TEM. 2.3. Modification of the Au Electrodes. Evaporated polycrystalline gold films with a nominal thickness of 200 nm were prepared on glass slides with a thin Cr adhesion layer following a procedure previously described.28 These electrodes are immersed into an ethanol solution of the thiols with a concentration of 1 × 10-3 mol dm-3 for 16 h. The excess of thiol was removed by copiously rinsing with absolute ethanol followed by Milli-Q water. This simple preparation generates well-ordered monolayers of MPA and MUA on Au (111) surfaces as probed by infrared31 and in situ STM measurements.32 The electrodes were dried under high-purity nitrogen (N2) flow and subsequently dipped into 1 mg cm-3 of PLL (pH ≈ 6.0) in order to generate an ultrathin film of the polycation. The electrode was maintained in the PLL solution for 20 min, followed by rinsing with copious amounts of Milli-Q water and drying under N2 flow. Adsorption of the Au particles was accomplished by dipping the PLLterminated surface into the colloid solution for 20 min, followed by the same rinsing and drying procedure. Important parameters in these studies are the adsorption time and nanoparticle concentration in the colloidal solution. In the present experimental conditions, the 20 min adsorption time not only allows easy quantification of the number of particles by AFM but also prevents formation of aggregates at the surface and confines the electrochemical kinetics to an accessible time scale. Our previous work demonstrated that Rct is strongly affected by the particle number density (Γ), which is determined by the adsorption time of the colloid.28 AFM analysis was performed with a Molecular Imaging Pico LE in acoustic mode. Nanosen-

Bradbury et al.

Figure 1. Cyclic voltammograms of carboxyl-terminated alkane thiolmodified electrodes in the presence of 1 × 10-3 mol dm-3 Fe(CN)63-/ Fe(CN)64- and 0.1 mol dm-3 Na2SO4. The scan rate was 50 mV s-1. The voltammogram of the clean Au electrode (black dotted line) is contrasted to the responses obtained for thiols with different numbers of methylene units (x).

sor tips (PPP-NCHR) with a resonance frequency of 300 kHz were used for these measurements. 2.4. Electrochemical Measurements. Cyclic voltammetry and electrochemical impedance spectroscopy were performed in a glass single-compartment cell at room temperature. The area of the working electrode was controlled using a chemically inert adhesive Teflon tape (3M) exposing an area of 0.071 cm2. A graphite rod and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All potentials in this paper are referred to the SCE. The measurements were carried out in 0.1 mol dm-3 Na2SO4 solution containing 1 × 10-3 mol dm-3 K4Fe(CN)6 and 1 × 10-3 mol dm-3 K3Fe(CN)6. The solution was purged with argon for 10 min before each measurement. The electrochemical experiments were performed with an Autolab PGSTAT30 featuring a Frequency Response Analyzer module (Eco Chemie B.V.). The impedance spectra were recorded at the equilibrium potential, Eeq) 0.19 ( 0.01 V, with an amplitude of 10 mV rms and in the frequency range between 10 mHz and 10 kHz. Electrochemical measurements for each modification step were performed on at least four freshly prepared electrodes. 3. Results 3.1. Electrochemical Responses as a Function of Thiol Length. Cyclic voltammograms measured at 50 mV s-1 for the various thiol-modified electrodes in the presence of 1 × 10-3 mol dm-3 Fe(CN)63-/4- are shown in Figure 1. The thiol length is indicated in terms of the number of methylene groups in the alkyl chain (x), e.g., MUA corresponds to x ) 10. It is observed that the peak current decreases for x ) 1 and 2, and no welldefined peaks are observed at this scan rate for x g 5. The behavior observed in Figure 1 indicates that thiols with a chain as short as x ) 1 do affect the electrochemical responses in the presence of the hexacyanoferrate probe. This effect can be rationalized in terms of an increase in the average tunneling distance with increasing thiol length or a decrease in the effective area of the electrode surfaces. The effect of the thiol length on the electrochemical impedance at the equilibrium potential is displayed in Figure 2. The so-called Bode plots allow visualizing the changes observed in the impedance which expands over 4 orders of magnitude. The results show that for frequencies higher than 10 Hz, the impedance responses describe a single RC time constant associated with Rct and the interfacial capacitance (Cint) at the equilibrium potential. As observed in Figure 2A, the logarithm of the amplitude (Z) is characterized by an inflection point as the frequency of modulation decreases. The magnitude Z at the

Charge-Transfer Resistance at Modified Electrodes

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10155

Figure 2. Impedance amplitude (A) and phase shift (B) as a function of the frequency of potential modulation for the carboxyl-terminated SAM on Au electrodes. The composition of the electrolyte solution is as indicated in Figure 1.

inflection point increases exponentially with increasing values of x. In addition, the maximum value of the phase (φ) shifts toward lower frequencies as x increases. For shorter thiols (x < 5), an increase of Z and φ with decreasing frequencies is observed in the range below 10 Hz. These responses are associated with the semi-infinite linear diffusional impedance. The impedance responses over a wide range of frequencies can be accurately fitted using the classical Randles circuit displayed in Scheme 1A. The parameters Ru and W correspond to the uncompensated resistance and Warburg impedance for semi-infinite linear diffusion. The former parameter was initially estimated from the high frequency limit, while the latter is calculated from the concentration of the redox species, the diffusion coefficient, and the geometrical area of the electrode.33 The phenomenological parameters Rct and Cint were the main adjustable parameters in all fits. In order to differentiate the values obtained for the modified electrodes terminated in SAM, PLL, and nanoparticles, the subscripts “sam”, “film”, and “array” will be adopted, respectively. The continuous lines in Figure 2 were obtained from the fitting of this equivalent circuit over the whole frequency range. Self-consistent results were obtained for all spectra, although small deviations from the typical Warburg impedance are observed in the phase responses at frequencies below 1 Hz. These deviations could be associated with nonlinear diffusion phenomena originating from localized charge transfer at the modified electrode.34 However, the key discussions in this paper will be concentrated on the behavior of Rct and Cint, whose estimation is unaffected by the deviations from the Warburg impedance at low frequencies. Consequently, we shall not further discuss theses deviations of our approach to the experimental data. The x dependence of the phenomenological charge-transfer resistance (Rsam) and interfacial capacitance (Csam) of the SAMterminated electrodes is illustrated in Figure 3. The increase of Rsam with increasing values of x can be described by a singleexponential function over the whole range of x values (Figure 3A). This behavior strongly suggests that the process is controlled by the coherent electron tunneling across the SAM layer, which can be phenomenologically expressed in terms of the tunneling decay constant β,2 ° Rsam ) Rsam

exp(βx) ° Rsam

(1)

where the pre-exponential factor is affected by several parameters associated with the redox probe and the effect of the thiol bonding on the electronic properties of the Au surface. The slope obtained from the plot in Figure 3A yields a decay constant β ) 1.16 ( 0.04 per methylene unit. This value is entirely consistent with previous studies on alkane thiol-modified electrodes, not only in the presence of free redox species in

solution35 but also for surface-confined redox species.8 Considering the relationship between the tunneling distance and the number of methylene units, the observed β corresponds to a decay constant of close to 0.8 Å-1.1 A particularly remarkable result is that the decay constant accurately fit the data even for monolayers comprising 1 methylene unit. Previous studies have shown that a defect in the molecular packing of alkane thiols increases for x < 5.36 Other groups have reported changes in the properties of the modified electrodes with the degree of ionization of the carboxyl termination.37–39 We conducted experiments in the present of phosphate buffer solutions with pH values between 5 and 9 (experiments not shown). Although changes in the values of Rsam were observed in these experiments, the trends shown in Figure 3A remained unaffected. We conclude from these results that the electrochemical kinetics involving the hexacyanoferrate redox couple is rather insensitive to structural defects in carboxyl-terminated SAM. 3.2. Effect of the PLL Layer on the Thiol-Modified Electrode. Adsorption of the PLL layer on the SAM generates small changes in the electrochemical responses. The impedance spectra for x ) 7 in the presence and absence of the PLL film is given in Figure 4. The Nyquist plot shows a decrease in the apparent Rct upon adsorption of the PLL. However, the main features of the impedance responses remain qualitatively similar to those depicted in Figure 2. This behavior arises from the fact that the hexacyanoferrate redox couple can penetrate this ultrathin polyelectrolyte film and the electrochemical responses are controlled by the electron tunneling across the underlying thiol monolayer. The strong affinity of this redox probe to the polyelectrolyte film was also clearly demonstrated in our recent studies on PLL/poly-L-glutamic acid multilayers grown on MUA-modified Au electrodes.40 Accurate fits of the impedance spectra were also obtained with the equivalent circuit displayed in Scheme 1A (Randles circuit). The observed charge-transfer resistance (Rfilm) and interfacial capacitance (Cfilm) for PLL-terminated electrodes are displayed in Figure 3. The dependence of the phenomenological Rfilm on the number of methylene groups is identical to that observed for the SAM-terminated electrode (Figure 3A). The fact that the same decay constant β was obtained in the presence of the PLL film confirms that the Rct is essentially controlled by the underlying SAM monolayer. It is also observed that Rfilm is smaller than Rsam, further indicating that the PLL layer does not increase the average tunneling distance at the modified electrode. The difference between Rfilm and Rsam can be rationalized in terms of the effective concentration of the redox probe at the surface. The relationship between the phenomenological Rct and the surface concentration of the oxidized ([ferri]s) and reduced ([ferro]s) is established via the exchange current (io)

Rct )

RT RT ) nFio n2F2k A[ferri]R[ferro]1-R et s s

(2)

where ket is the heterogeneous electron-transfer rate constant, A is the surface area of the electrode, and R is the transfer coefficient. It follows that at the formal transfer potential, [ferri]s ) [ferro]s ) [redox]s, and eq 2 is simplified to

Rct )

RT n F ketA[redox]s 2 2

(3)

Assuming that ket is determined by the tunneling probability across the underlying SAM, the ratio of the intercepts obtained

10156 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Bradbury et al.

SCHEME 1: Randles (A) and Dual-Charge-Transfer Pathway (B) Equivalent Circuits for Rationalizing the Impedance Responses of Modified Electrodesa

a The phenomenological parameters in the Randles circuit correspond to the uncompensated resistance (Ru), the interfacial capacitance (Cint), the charge-transfer resistance (Rct), and the Warburg impedance (W). Independently of electrode modification, the impedance data were accurately described by the Randles circuit. The phenomenological parameters obtained from fitting circuit A to the SAM-, PLL-, and nanoparticle-terminated electrodes are identified by the subscripts sam, film, and array, respectively. Circuit B represents charge transfer via the nanoparticle as a double barrier junction, where Rnano, Cnano, and Rfilm′Cfilm′ are the time constants for charge injection from the redox species to the nanoparticles and from the nanoparticles to the electrode, respectively.

observed that the average capacitance of the PLL-terminated surface is lower than for the SAM-modified electrode, particularly for shorter thiols length. This behavior can be rationalized by defining Cfilm in terms of two parallel plate capacitors connected in series. These capacitors are associated with the electrode|SAM boundary (CAu|sam) and the SAM|PLL boundary (Cpll). It follows

1 1 1 ) + Cfilm CAu|sam Cpll Figure 3. Phenomenological charge-transfer resistance (A) and interfacial capacitance (B) as a function of the number of methylene units in the SAM. The values were obtained from fitting the Randles equivalent circuit (Scheme 1A) to the SAM- (0) and PLL- (red circle) terminated electrode surfaces. The composition of the electrolyte solution is as indicated in Figure 1. The exponential increase of Rsam and Rfilm with x was interpreted in terms of a decrease in the coherent tunneling probability with a decay parameter β ) 1.16 ( 0.04 per methylene unit. The decrease of the interfacial capacitance is related to the increase of the thickness of a dielectric layer associated with SAM.

Figure 4. Nyquist representation of the impedance spectra of the x ) 7 SAM-modified electrode in the presence (red circle) and absence (0) of the PLL film. Experimental conditions are identical to those indicated in Figures 1 and 2. Continuous lines are associated with the fits of the Randles equivalent circuit (Scheme 1A). These results show that the PLL film does not increase the average tunneling distance between the redox species and the electrode surface. The lower Rct obtained for the PLL-terminated electrode suggests that the interfacial concentration of the redox probe is higher with respect to the SAM-terminated surface.

in Figure 3A establishes that [redox]s is on average 2.5 times larger in the presence of PLL. As observed in the case of Csam, the value of Cfilm decreases with increasing number of methylene groups (Figure 3B). It is

(4)

Considering that CAu|sam is mainly determined by the permittivity and thickness of the compact alkyl monolayer, it can be obtained from eq 4 that Cpll ) (4.9 ( 2.9) × 10-5 F cm-2. This capacitance is somewhat larger than the characteristic Helmholtz capacitance at the metal|electrolyte interface (CH ≈ 1 × 10-5 F cm-2). This could be taken as an indication of the high density of charges accumulated in the ultrathin dimensions of the films, which is estimated to be less than 1 nm thick.40,41 However, interpretation of this figure should be treated rather cautiously due to the large oversimplification involved in the potential distribution across these complex interfaces. The behavior of Rct and Cint upon sequential modification of the surface by the SAM and PLL reveals that the electrochemical kinetics is mainly controlled by the coherent electron-tunneling probability across the SAM. The PLL film does not provide an additional barrier for electron transfer but generates an increase in the surface concentration of the redox probe. As discussed in the next section, the redox species “trapped” inside the PLL film appears to play a fundamental role in the electronic coupling between electrostatically adsorbed metal nanoparticles and the electrode surface. 3.3. Nanoparticle-Modified Electrodes. Diluted monolayers of citrate-stabilized Au nanoparticles (19.2 ( 2.1 nm) were adsorbed on PLL-modified electrode via electrostatic adsorption from the crude colloidal solution. The adsorption time was fixed at 20 min, generating randomly distributed nanoparticles as exemplified in the AFM image displayed in Figure 5. The average nanoparticle number density obtained over a large ensemble of samples was Γ ) (7.0 ( 3.0) × 109 cm-2. This value is consistent with our other reports under similar conditions.27,28 We also observed that the dependence of Γ on the adsorption time on PLL-modified surfaces is little affected by the substrate employed and the composition of the film underneath the PLL layer.42

Charge-Transfer Resistance at Modified Electrodes

Figure 5. Typical 1 µm × 1 µm acoustic AFM image of Au nanoparticle-terminated electrode surface. The condition for the electrostatic adsorption (see text) consistently generates a diluted monoloyer of Au nanoparticles with an average Γ ) (7.0 ( 3.0) × 109 cm-2. In this case, the Au surface was modified by a self-assembled monolayer of x ) 10.

Figure 6. Cyclic voltammograms of the nanoparticle-terminated electrodes as a function of the thiol length. The experimental conditions are as described in Figure 1. Unlike the behavior observed for the SAMmodified electrodes, the faradaic current is effectively independent of the thiol length for x g 5. For shorter thiols, the voltammetric features resemble the features observed for clean Au electrodes.

The cyclic voltammograms at 50 mV s-1 obtained for different thiol lengths and constant Γ are displayed in Figure 6. These results show that the reactivity of the electrode increases as compared to the voltammograms shown in Figure 1. As the x value increases, the enhancement of the faradaic current upon adsorption of the nanoparticles becomes more significant. For the thiols featuring x < 5, the voltammogram in the presence of the Au nanoparticles exhibits the characteristic features of a quasi-reversible electron transfer with a peak-to-peak potential difference ∆Epeak) 90 mV and the same current density obtained for the clean Au electrode under identical conditions. Remarkably, the faradaic current becomes independent of the thiol length for x g 5. This behavior is significantly different than the one observed in the absence of the nanoparticles, where strong suppression of the current is observed for the longer thiols (see Figure 1). These experimental results provide unique evidence of a change in mechanism for the charge-transfer reaction induced by Au nanoparticles. In other words, the independence of the faradaic current on x demonstrates that the rate-determining step of the process is no longer the coherent electron tunneling across the SAM. The effect of nanoparticle adsorption on Rct is clearly observed in the family of Bode plots in Figure 7. Very few differences are observed in the impedance responses for x ) 1 and 2, which are comparable to the results obtained without nanoparticles (Figure 2). As anticipated from the voltammetric

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10157

Figure 7. Impedance amplitude (A) and phase shift (B) as a function of the frequency of potential modulation for the nanoparticle-terminated Au electrodes. As illustrated by the continuous lines, the frequency dependence of both parameters can be described in terms of the Randles equivalent circuit (Scheme 1A). These results clearly show that electrochemical kinetics becomes independent of the number of methylene units at the SAM for x g 5.

responses in Figure 6, the impedance responses are effectively independent of the thiol length for x g 5. This behavior sharply contrasts with the results obtained in the absence of the nanoparticles, in which an exponential increase of Rsam and Rfilm was observed with increasing x (Figure 3A). On the other hand, our previous studies have shown that the apparent chargetransfer resistance in the presence of the Au nanoparticles (Rarray) is strongly dependent on the particle number density.28 These results provide further qualitative evidence that the electrontransfer process is essentially “channeled” through the nanoparticles for the electrodes modified with long thiol chains. In the case of x ) 1 and 2, direct transfer through the film appears to dominate the electrochemical responses. An interesting observation from the impedance data in Figure 7 is the presence of a single time constant in the frequency range above 10 Hz. At lower frequencies, a characteristic Warburg signal is observed independently of the thiol length. For thiols featuring x g 5, the impedance responses are orders of magnitude lower in the presence of the Au nanoparticles. The presence of a single time constant indicates that the charge transfer “channeled through” the nanoparticles is controlled by either (i) the transfer kinetics between the nanoparticles and the electrode surface or (ii) the charging of the nanoparticles by the redox species. The fact that the impedance responses are effectively independent of the thiol length for x between 5 and 10, while exhibiting Warburg behavior at low frequencies, strongly suggests that the rate-determining step is the charge injection from the redox species to the Au nanoparticles. Fits of the Randles equivalent circuit (Scheme 1A) to the data in Figure 7 allowed extracting the values of Rarray. The continuous lines in Figure 7 confirm that the frequencydependent impedance can be accurately described by this simple circuit. The fitting iterations converged to a consistent set of values for the adjusting parameters. The Warburg component exhibited the same value obtained for the short length thiols and the bare Au electrode. However, the previous discussion concluded that charge transfer occurs via two parallel pathways, long-range tunneling through the thiol barrier or via the metal nanoparticles. Consequently, the phenomenological Rarray and Carray are functions of the various elementary processes involved in the electrochemical responses. Including the transfer kinetics between the nanoparticles and the electrode surface, the main processes contributing to the impedance responses are represented by the equivalent circuit in Scheme 1B. The faradaic impedance associated with charge transfer via the nanoparticles is described by the two RC

10158 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Bradbury et al.

Figure 8. Distance dependence of the phenomenological chargetransfer resistance for nanoparticle- (∆), PLL- (red line), and SAM(black line) terminated electrodes. For x e 5, Rarray shows comparable values to Rsam. As the length of the thiols increases, Rarray reaches a constant value which is orders of magnitude smaller than in the absence of the nanoparticle. The behavior observed for longer thiols indicates that the rate-determining step of the process is no longer direct coherent tunneling across the thiol-modified electrode. As indicated in the text, the rate-determining step becomes charging of the nanoparticle by the redox species.

components in series which resembles the classical doubletunnel barrier junction.43,44 The element Rnano accounts for transfer from the redox species to the particles, while Rfilm′ is related to transfer from the particles to the electrode. The capacitance Cnano is determined by the fraction of the surface area covered by the metal nanoparticles (θ). Similarly, Cfilm′ is related to the boundary region between the metal nanoparticles and the electrode surface. In this model, the diffusion impedance is coupled via a single Warburg element in series to the transfer via the film (Rfilm) and the nanoparticles. This is a simplification based on the fact that diffusion to an array of 20 nm nanoparticles with a number density of approximately 1010 cm-2 can be effectively described in terms of semi-infinite linear diffusion for f < 50 kHz.27 In view of the distance independence of Rarray for electrodes modified with long thiols, the contribution of the Rfilm′Cfilm′ time constant can be neglected. Consequently, in the frequency range where the Warburg component is negligible (f > 10 Hz), the phenomenological Rarray and Carray are given by

Rarray )

RnanoRfilm Rnano + Rfilm

Carray ) θCAu + (1 - θ)Cfilm

(5) (6)

where CAu is the Helmholtz capacitance per unit of area of bulk Au (approximately 10-5 F cm-2). Equation 5 indicates that the smallest charge-transfer resistance will provide the most significant contribution to Rarray. On the other hand, the contribution of the nanoparticles to Carray is expected to be rather small due to the low coverage of the nanoparticles. Taking the average Γ ) (7.0 ( 3.0) × 109 cm-2, it can be estimated that the value of θ is below 10%. Therefore, the expected difference between Carray and Cfilm is smaller than the dispersion of the experimental data. Figure 8 contrasts the thiol length dependence of Rarray with the trends observed for Rsam and Rfilm. As already mentioned, the presence of Au nanoparticles has little effect on the phenomenological Rct for thiols with less than five methylene units. In this case, electrode modification generates a relatively weak barrier for the charge transfer and the kinetics is dominated by the electron tunneling across the SAM. A rather unexpected result was obtained upon fitting Rarray as a function of x based on eq 5. It emerged that Rarray approaches values of Rsam rather

than Rfilm as the thiol length decreases. It could be argued that adsorption of the citrate-stabilized Au nanoparticles generates a local concentration polarization of the redox species at the interface, leading to an increase of Rct. Our previous Kelvin probe studies have shown that the changes in the outer potential (ψfilm - ψnano) for a similar particle number density is approximately -0.1 V.27,45 At this stage it is rather difficult to establish whether this change in outer potential can generate a significant effect on the surface concentration of the probe in such a heterogeneous system. The most significant result arising from the analysis in Figure 8 is that the behavior of Rarray can be described by a constant value Rnano ) (1.4 ( 0.1) × 104 Ω. Invoking the classical expressions for partially blocked electrodes, Rnano can be rationalized in terms of θ as:34,46,47

Rnano )

RT RT ) (7) n2F2θketo A[redox]s n2F2Γπr2nanoketo A[redox]s

Evaluating eq 7 with the Γ values measured from AFM, it follows that k°et ) (1.4(0.1)×10-2 cm-1. The estimated k°et is consistent with previously reported values for the hexacyanoferrate redox couple on macroscopic metal electrodes.48–50 3.4. Resonant Charge Transport through Adsorbed Redox Probes. Analysis of Rnano confirms that the kinetics of the electrochemical reaction is controlled by the charge transfer from the redox species to the adsorbed nanoparticle. The distance independence of Rnano strongly suggests that charge transfer from the particle to the electrode is not governed by a direct coherent tunneling mechanism. The tunneling probability sharply decreases as the length of the thiol increases over 1 nm. This is the reason behind the exponential increment of Rsam and Rfilm with increasing number of methylene units in the thiol molecules. Our recent studies on polyelectrolyte multilayers have shown that Rnano remains constant for distances as large as 6.5 nm.40 Furthermore, we reported identical enhancement of the electron-transfer kinetics in assemblies where the Au nanoparticles are separated not only by MUA and a polyelectrolyte film but also by a redox permeable layer of SiO2 nanoparticles.51 These two set of data exclude the possibility that the decrease of Rct arises from direct physical contact between the metal nanoparticles and the electrode surface. A conventional diffusion (hopping) mechanism involving the redox species is unlikely to be the main transport mechanism between the nanoparticles and the electrode. This mechanism implies that the charges reside in the redox species confined to the ultrathin PLL film before the transfer step to the electrode surface. As Rfilm is several orders of magnitude larger than Rnano for x > 5, the effective activation energy for electron transfer across the SAM is expected to be significantly smaller in the presence of the particles. Consequently, the transport process across the SAM cannot involve relaxation of the nuclear coordinates (outer reorganization energy) of the redox probe to their solvated equilibrium states inside the PLL. In other words, the electrochemical responses in the presence of the Au nanoparticles can be a manifestation of a hot electron-transport process. Our interpretation of the long-range transport in this system is based on a resonant electron-transfer step at the Fermi energy from the particle to the electrode surface.52 This mechanism can be regarded similarly to resonant tunneling transport from a STM tip to a surface modified by redox-active species in the low-bias regime. The density of states at the bridge is provided by the hexacyanoferrate species confined to the PLL film. At this stage, we do not have quantitative information on the

Charge-Transfer Resistance at Modified Electrodes concentration of the redox species trapped in the PLL layer. However, this could reach values significantly larger than in the bulk of the electrolyte solution considering the ultrathin nature of the film. From this point of view, the role of the nanoparticles is to increase the density of states at the redox Fermi energy, while the trapped redox probe creates a narrow energetic path for charge transfer across the SAM. It should be stressed that we are not claiming distanceindependent charge-transfer kinetics between the nanoparticle and the electrode surface. We cannot comment on the rate of this step as the electrochemical measurements are only sensitive to the rate-determining process, i.e., charging of the nanoparticle by the redox species. However, these results clearly show that charge transport across the modified electrode is at least 2 orders of magnitude faster upon adsorption of the metal nanoparticles. Further investigations are currently underway in our laboratory to unravel more details of the unique transport properties in this system. 5. Conclusions The dynamics of charge transfer across carboxyl-terminated thiols and ultrathin PLL films have been investigated in the presence and absence of Au nanoparticles. An exponential increase of the Rct for the hexacyanoferrate redox couple was observed for thiols between 1 and 10 methylene units. This behavior was described by a single tunneling decay constant β ) 1.16 ( 0.04 per methylene unit over the entire range of spacers. These results strongly suggest that the electron-transfer kinetics is determined by the coherent tunneling probability across the SAM. In addition, electrostatic repulsion between the carboxyl termination of the SAM and the redox species renders this system less sensitive to charge transfer via defects in the monolayer. The electrostatic adsorption of an ultrathin layer of PLL (less than 1 nm thick) on the SAM generates an average 2.5 times decrease of Rct independently of the length of the thiol. Consequently, the PLL layer does not affect the phenomenological value of β obtained for the SAM-modified electrode. This result clearly suggests that the PLL does not alter the effective tunneling distance between the redox species and the electrode surface. The decrease of Rct upon adsorption of the PLL was rationalized in terms of an increase in the interfacial concentration of the redox probe. Diluted monolayers of citrate-stabilized Au nanoparticles, Γ ) (7.0 ( 3.0) × 109 cm-2, were electrostatically adsorbed on the PLL-terminated surface. For assemblies featuring thiols shorter than five methylene units, the adsorbed nanoparticles had little effect on the dynamic electrochemical measurements. For longer thiols, the apparent Rct becomes independent of the number of methylene units. This rather unique observation clearly indicates that the electron-transfer process is no longer determined by coherent tunneling from the redox species to the electrode. Analysis of the impedance data demonstrate that the electron-transfer process is determined by the charging of the metal nanoparticles via the redox species. The constant value of Rct up to distances as large as 13 Å cannot be explained in terms of a direct tunneling phenomena between the nanoparticles and the electrode surface. The key aspect in this efficient electronic communication is linked to redox species “trapped” in the ultrathin PLL film. However, the role of the trapped hexacyanoferrate does not correspond to the conventional redox mediators typically used in enzymemodified electrodes.53 The orders of magnitude changes in Rct suggest that the activation energy for electron transfer across

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10159 the blocking layer is significantly lower in the presence of the nanoparticles. We rationalize this behavior in terms of isoenergetic electron transport at the Fermi redox energy mediated by the density of states of the trapped redox species. This phenomenon resembles a resonant transport rather than a hopping-type mechanism. Acknowledgment. We are grateful to Prof. Thomas Feurer and Mr. Beat Locher from the Department of Physics (Universita¨t Bern) for the assistance on the preparation of the gold electrodes, as well as to Prof. Laurence Peter (University of Bath) for helpful discussion. We gratefully acknowledge the financial support by the Swiss National Science Foundation (projects: PP002-68708, PP002-116898/1 and 200021-105238), the Portland-Zementstiftung and the Stiftung zur Fo¨rderung der Wissenschaftlichen Forschung an der Universita¨t Bern. References and Notes (1) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668. (2) Newton, M. D.; Smalley, J. F. Phys. Chem. Chem. Phys. 2007, 9, 555. (3) Chidsey, C. E. D. Science 1991, 251, 919. (4) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (5) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (6) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y. P. J. Phys. Chem. 1995, 99, 13141. (7) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563. (8) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (9) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J. Y.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059. (10) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (11) McCreery, R. L.; Wu, J.; Kalakodimi, R. P. Phys. Chem. Chem. Phys. 2006, 8, 2572. (12) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 2970. (13) Xu, B. Q.; Tao, N. J. J. Science 2003, 301, 1221. (14) Xu, B. Q.; Xiao, X. Y.; Tao, N. J. J. Am. Chem. Soc. 2003, 125, 16164. (15) Haiss, W.; Nichols, R. J.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2004, 6, 4330. (16) Li, X. L.; He, J.; Hihath, J.; Xu, B. Q.; Lindsay, S. M.; Tao, N. J. J. Am. Chem. Soc. 2006, 128, 2135. (17) He, J.; Sankey, O.; Lee, M.; Tao, N. J.; Li, X. L.; Lindsay, S. Faraday Trans. 2006, 131, 145. (18) Chen, F.; Hihath, J.; Huang, Z. F.; Li, X. L.; Tao, N. J. Annu. ReV. Phys. Chem. 2007, 58, 535. (19) Chen, S. W.; Pei, R. J. J. Am. Chem. Soc. 2001, 123, 10607. (20) Bakkers, E.; Marsman, A. W.; Jenneskens, L. W.; Vanmaekelbergh, D. Angew. Chem., Int. Ed. 2000, 39, 2297. (21) Bakkers, E.; Roest, A. L.; Marsman, A. W.; Jenneskens, L. W.; de Jong-van Steensel, L. I.; Kelly, J. J.; Vanmaekelbergh, D. J. Phys. Chem. B 2000, 104, 7266. (22) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D. J. Am. Chem. Soc. 2004, 126, 14287. (23) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (24) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2003, 107, 4844. (25) Chirea, M.; Garcı´a-Morales, V.; Manzanares, J. A.; Pereira, C.; Gulaboski, R.; Silva, F. J. Phys. Chem. B 2005, 109. (26) Chirea, M.; Pereira, C. M.; Silva, F. J. Phys. Chem. C 2007, 111, 9255. (27) Kakkassery, J. J.; Abid, J.-P.; Carrara, M.; Fermı´n, D. J. Faraday Trans. 2004, 125, 157. (28) Zhao, J.; Bradbury, C. R.; Huclova, S.; Potapova, I.; Carrara, M.; Fermı´n, D. J. J. Phys. Chem. B 2005, 109, 22985. (29) Jensen, P. S.; Chi, Q.; Grumsen, F. B.; Abad, J. M.; Horsewell, A.; Schiffrin, D. J.; Ulstrup, J. J. Phys. Chem. C 2007, 111, 6124.

10160 J. Phys. Chem. C, Vol. 112, No. 27, 2008 (30) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (31) Nuzzo, R. G.; Dubois, L.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (32) Azzaroni, O.; Vela, M. E.; Martin, H.; Creus, A. H.; Andreasen, G.; Salvarezza, R. C. Langmuir 2001, 17, 6647. (33) Bard, A. J.; Faulkner, C. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons Inc.: New York, 2000. (34) Tokuda, K.; Gueshi, T.; Matsuda, H. J. Electroanal. Chem. 1979, 102, 41. (35) Miller, C.; Cuendet, P.; Graetzel, M. J. Phys. Chem. 1991, 95, 877. (36) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (37) Dai, Z.; Ju, H. Phys. Chem. Chem. Phys. 2001, 3, 3769. (38) Kakiuchi, T.; Iida, M.; Imabayashi, S.-i.; Niki, K. Langmuir 2000, 16, 5397. (39) Schweiss, R.; Werner, C.; Knoll, W. J. Electroanal. Chem. 2003, 540, 145. (40) Zhao, J.; Bradbury, C. R.; Fermı´n, D. J. J. Phys. Chem. C 2008, 112, 6832. (41) Cheng, Y.; Corn, R. M. J. Phys. Chem. B 1999, 103, 8726. (42) Bradbury, C. R. Ph.D. Thesis, University of Berne, 2007.

Bradbury et al. (43) Reincke, F.; Hickey, S. G.; Kelly, J. J.; Braam, T. W.; Jenneskens, L. W.; Vanmaekelbergh, D. J. Electroanal. Chem. 2002, 522, 2. (44) Shekhah, O.; Busse, C.; Bashir, A.; Turcu, F.; Yin, X.; Cyganik, P.; Birkner, A.; Schuhmann, W.; Wo¨ll, C. Phys. Chem. Chem. Phys. 2006, 8, 33751. (45) Carrara, M.; Kakkassery, J. J.; Abid, J.-P.; Fermı´n, D. J. ChemPhysChem 2004, 5, 571. (46) Davies, T. J.; Moore, R. R.; Banks, C. E.; Compton, R. G. J. Electroanal. Chem. 2004, 574, 123. (47) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (48) Krysinski, P.; Brzostowska-Smolska, M. J. Electroanal. Chem. 1997, 424, 61. (49) Janek, R. P.; Fawcett, W. R.; Ulman, A. Langmuir 1998, 14, 3011. (50) Peter, L. M.; Duerr, W.; Bindra, P.; Gerischer, H. J. Electroanal. Chem. 1976, 71, 31. (51) Zhao, J.; Wassem, M.; Bradbury, C. R.; Fermı´n, D. J. J. Phys. Chem. C 2008, 112, 7284. (52) Schmickler, W. Chem. Phys. 2003, 289, 349. (53) Bard, A. J. Integrated Chemical Systems: A Chemical Approach to Nanotechnology; John Wiley & Sons Inc.: New York, 1994.

JP710165D