Long-Range Electronic Communication between Metal Nanoparticles

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J. Phys. Chem. C 2008, 112, 6832-6841

Long-Range Electronic Communication between Metal Nanoparticles and Electrode Surfaces Separated by Polyelectrolyte Multilayer Films Jianjun Zhao, Christopher R. Bradbury, and David J. Fermıń*,† Departement fu¨r Chemie und Biochemie, UniVersita¨t Bern, Freiestrasse 3, CH-3012 Berne, Switzerland ReceiVed: October 19, 2007; In Final Form: December 22, 2007

The dynamics of electron transfer across Au electrodes modified by ultrathin polyelectrolyte multilayers (PEM) and a diluted monolayer of Au nanoparticle was investigated as a function of the film thickness. Au electrodes were sequentially modified by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), followed by alternate adsorption of poly-L-lysine (PLL) and poly-L-glutamic acid (PGA) layers. Submonolayer coverage of citrate stabilized 19.2 ( 2.1 nm Au nanoparticles was achieved by electrostatic adsorption on PLL terminated surfaces. In the absence of nanoparticles, cyclic voltammetry and electrochemical impedance spectroscopy of the hexacyanoferrate redox probe showed that the charge-transfer resistance is independent of the number of adsorbed polyelectrolyte layers. These results revealed that the redox species can penetrate the PEM film and the electrochemical responses are controlled by the electron tunneling across the initial monolayer of MUA. The phenomenological charge-transfer resistance decreased by more than 2 orders of magnitude upon adsorption of the Au nanoparticles. Normalization of the electrochemical responses with the number density of particles revealed that the PEM thickness introduces insignificant effects on the charge-transfer resistance. The effective distance independent electron-transfer kinetic was observed for film thickness up to 6.5 nm. Furthermore, in situ atomic force microscopy studies show that the Au nanoparticles do not introduce measurable local deformation (compression) of the PEM films. The unique long-range electronic communication in this system is interpreted in terms of a resonant transport process involving the density of states of trapped redox species at the redox Fermi energy.

1. Introduction Control over the dynamics of charge transport across hybrid materials featuring functional polymers and nanostructures is a key challenge toward the generation of ultrathin functional devices. The assembly of nanoparticles on electrode surfaces can be achieved by typical bifunctional linkers such as di-thiols and alkyl-diisocyano monolayers.1-11 These linkers rely on the formation of covalent bonding with the metal surface and the nanoparticles. On the other hand, electrostatic adsorption employing charged colloidal particles and polyelectrolyte modified electrodes is by far the simplest, most versatile, and controllable way to prepared submonolayers.12-18 Our previous works show that key properties of electrodes such as electrochemical reactivity and work function can be controlled by electrostatic adsorption of various metallic nanostructures.19-21 The electrochemical responses of metal electrodes modified by PEM have been investigated by several groups.22-29 The kinetics of electrochemical responses appears connected to the transport properties of the redox probe across the PEM. In the presence of ionic redox species, the permeation rate is strongly determined by the charge of the outermost polyelectrolyte layer in the film.30 Silva and co-workers have examined the ion transport properties of PEM films by electrochemical impedance spectroscopy (EIS).27,28 Their analysis implicitly considers preferential transport through “defects” in the film, although no microscopic features in the structure of the film related to defects were identified. Alternative models consider sequential barriers associated with each of the layers in the films.25 * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +44 117 9288981. Fax: +44 117 9250612. † Current affiliation: School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.

However, experiments have shown that the properties of a buried polyelectrolyte layer can be strongly influenced by the outer layers.31 Consequently, the collective behavior of the films cannot be generally expressed as a linear combination of the number of layers. Further complications are introduced by the dependence of the thickness and film structure on parameters such as temperature, ionic strength, pH, pKa of the polyelectrolytes, surface charge density of the substrate, and nature of the counter-ions.31-34 Adsorption of metal nanostructures at the modified electrode surface commonly leads to an increase in the electrochemical reactivity of the assembly.3,17,35-38 The observed enhancement of the electron-transfer kinetics brings about fundamental questions on electronic communication between nanostructures and electrode surfaces. Hicks et al. estimated heterogeneous electron-transfer rate constants of the order of 100 s-1 between Au electrodes and Au nanoparticles immobilized by carboxylateion bridges.3 The work by Chen and Pei has dealt with the dynamics of charging small metal clusters protected by alkane thiolate ligands confined to electrode surfaces.39 These authors reported that the dynamics of charge injection is dependent on the thiol length with a tunneling decay constant (β) between 0.8 and 0.9 per methylene unit. Vanmaekelbergh and co-workers found β values of 0.5 Å-1 for CdSe quantum dots linked to Au surfaces by non-saturated dithiols.10,40 More recently, Wang et al. reported β as low as 0.4 Å-1 from conductivity measurements in arrays of 2 nm Au nanoparticles featuring saturated alkyl spacers.41 Our group have reported the first quantitative study on the apparent charge-transfer resistance as a function of the number density of metal nanoparticles at modified electrodes.20 In this work, electron transfer involving the hexacyanoferrate redox probe was effectively blocked by a self-assembled monolayer

10.1021/jp710167y CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008

Long-Range Electron Transfer across PEM SCHEME 1: Graphical Illustration of the PEM Film Terminated with a Nanoparticle Monolayera

a The Au electrode is initially modified with a self-assembled monolayer of mercaptoundecanoic acid (MUA). The PEM film is grown by sequential electrostatic adsorption of poly-L-lysine (PLL) and polyL-glutamic acid (PGA). The layer-by-layer growth is always terminated by a PLL layer. The structure of the film is denoted with the number of PLL-PGA bilayers grown (n), e.g., n ) 2 in this schematic illustration. The electrochemical responses of the hexacyanoferrate probe are examined as a function of n in the presence and absence of the nanoparticle assembly. The diagram is not drawn to scale.

of 11-mercatoundecanoic acid (MUA) and poly-L-lysine (PLL) on Au electrodes. Citrate stabilized Au nanoparticles electrostatically adsorbed on the PLL film exhibited a behavior analogous to a two-dimensional (2D) array of nanoelectrodes. The estimated interfacial first-order electron-transfer rate constant for individual nanoparticles was found to be identical to the one observed for macroscopic clean gold surfaces. Recently, we have also observed that the fast electronic communication between particles and the electrode surface appears independent of the length of the carboxyl terminated alkane thiol assembled at the Au surface.42 These observations are strikingly evident in the presence of thiols with more than 5 methylene groups, allowing us to conclude that the charge transport occurs via an unconventional mechanism involving the confined redox probes. The present paper explores the dynamics of nanoparticle mediated electron transfer across ultrathin PEM as a function of the film thickness. As illustrated in Scheme 1, the electrochemical properties of ultrathin films formed by sequential adsorption of PLL and poly-L-glutamic acid (PGA) on MUA modified Au electrodes are investigated in the presence of 19.2 ( 2.1 nm Au particles assembled at the outer layer. In situ atomic force microscopy (AFM) was employed to investigate the film thickness, particle number density and local changes in the film structure upon particle adsorption. The results consistently show that the electron can be effectively channeled through the metal nanoparticles across films with thickness as large as 6.5 nm. Furthermore, the apparent kinetic of electron transfer is controlled by the electron exchange between the particles and the redox species rather than the charge transfer across the film. 2. Experimental Section 2.1. Chemicals. 11-Mercaptoundecanoic acid 95% (MUA), poly-L-lysine hydrobromide Mw ) 30 000-70 000 (PLL), polyL-glutamic acid Mw ) 50 000-100 000 (PGA), poly(ethylene-

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6833 imine) Mw ) 25 000 (PEI), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O), and sodium citrate dihydrate (99%) were purchased from Sigma-Aldrich. Potassium ferrocyanide (K4Fe(CN)6‚3H2O), potassium ferricyanide (K3Fe(CN)6), and sodium sulfate (Na2SO4) anhydrous GR grade were purchased from Merck. All chemicals were used as received. Millipore ultrapure water (Milli-Q) was used to prepare all aqueous solutions and for rinsing. 2.2. Synthesis of Au Nanoparticles. The gold nanoparticles were synthesized in aqueous solution by the reduction of 2.5 × 10-4 mol dm-3 of HAuCl4‚3H2O (190 cm3) in the presence of 10 cm3 of 1% (w/w) trisodium citrate for 1 h under reflux and strong stirring.20,43 The nanoparticles size obtained from this method is 19.2 ( 2.1 nm as estimated from acoustic AFM images. 2.3. Assembly of Polyelectrolyte Multilayer Film and Au Nanoparticles. The ultrathin PEM films were generated by the electrostatic layer-by-layer method,44 employing aqueous solutions of the polyelectrolyte with no added electrolytes.34,45,46 Polycrystalline evaporated gold electrodes were immersed into 1 × 10-3 mol dm-3 MUA solution in ethanol for over 16 h. The excess of MUA was removed by rinsing with a copious amount of absolute ethanol and Milli-Q water. The electrodes were dried under a stream of high purity N2. Subsequently, the gold electrodes were alternately dipped into 1 mg cm-3 of PLL (pH ≈ 6.0) and 1 mg cm-3 PGA aqueous solution (pH ≈ 7.0) for 20 min, respectively.45 Between each deposition step, the modified electrodes were rinsed with copious amounts of Milli-Q water and dried under N2 flow. The deposition of Au particles was accomplished by dipping PLL terminated PEM films into the colloid solution for a fixed period of 20 min, followed by the same rinsing and drying procedure. For the sake of clarity, the composition of the PEM is associated with the number of PLL-PGA bilayers (n) as illustrated in Scheme 1. For instance, a PEM film labeled as n ) 0 and 5 consists of MUA-PLL and MUA-(PLL-PGA)5-PLL, respectively. 2.4. Electrochemical Measurements. Electrochemical measurements were performed in a single compartment cell. The area of the working electrode, 0.071 cm2, was controlled using chemically inert adhesive Teflon tape (3M). A platinum foil of 0.34 cm2 and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All potentials in this paper are referred to the SCE. Measurements were carried out in solutions containing 0.1 mol dm-3 Na2SO4 solution, 1 × 10-3 mol dm-3 K4Fe(CN)6, and 1 × 10-3 mol dm-3 K3Fe(CN)6. The solution was purged with N2 for 20 min before each measurement. Cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were measured 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, between the frequency range of 30 kHz to 130 mHz and 10 mV rms amplitude. The electrochemical parameters for each electrode modification were averaged from at least 3 different freshly prepared samples. 2.5. AFM Analysis. The AFM measurements were conducted using a Molecular Imaging Pico LE in contact and acoustic mode. Ex situ acoustic mode measurements were executed with silicon tips (Nanosensor PPP-NCHR) featuring a resonance frequency of 300 kHz. In situ contact experiments were conducted with silicon nitride tips from Budget Sensor SiNi, while silicon nitride tips from Veeco DNP-S20 were used for the in situ acoustic mode experiments. Optical examination of the samples was performed with a CCD camera focused onto

6834 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Figure 1. Cyclic voltammograms of the MUA and PEM modified electrode in the presence of 1 × 10-3 mol dm-3 Fe(CN)64-/Fe(CN)63at 50 mV s-1. The self-assembled MUA monolayer strongly blocked the electron-transfer responses at the Au electrode. The adsorption of the PEM induces an increase of the faradaic current. Very weak dependence of the current on the number of polyelectrolyte layer (n ) 2 and 10) is observed.

the sample via an optical access in the scanner. The AFM images were processed with the SPIP software (Image Metrology). Measurements of the PEM films thickness were performed in situ on films deposited on poly(ethyleneimine) (PEI) modified mica. Images recorded for PEM depth analysis were only linearly flattened. In order to examine the swelling of the PEM film in the presence of adsorbed nanoparticles, the surface topography was examined in acoustic mode in air as well as in the presence of the electrolyte. Flat Au films evaporated on freshly cleaved mica and annealed at 650 °C for 16 h were employed as substrate. Careful alignment of the sample and scanner, aided with the video microscope, allowed imaging of a specific ensemble of particles after changing the AFM tip. 3. Results 3.1. Cyclic Voltammetry of the Modified Electrodes as a Function of the Number of Adsorbed Polyelectrolyte Layers. Characteristic cyclic voltammograms of the couple Fe(CN)64-/ Fe(CN)63- on Au electrodes modified by PEM films with different number of adsorbed layers are illustrated in Figure 1. As discussed in previous reports,20,37 the self-assembled MUA monolayer introduces a strong barrier to the electron-transfer reaction. The decrease of the electron-transfer rate is determined by (i) decrease in the tunneling probability due to the ordered alkyl chains and (ii) electrostatic forces associated with the ionized carboxyl termination and the anionic redox probe.42 In Figure 1, it is also observed that the PEM film induces a slight increase of the faradaic current. The results suggest that the PEM overcompensate the charge of the carboxyl terminated MUA monolayer, promoting an increase in the surface concentration of the redox probe. The results in Figure 1 also illustrate the weak dependence of the electrochemical responses on the number of adsorbed polyelectrolyte layers (n). Slightly higher currents are observed for n ) 10 with respect to n ) 2 at high overpotentials. It should also be mentioned that these differences are comparable with the dispersion observed for different electrodes prepared under identical conditions. The small dependence of the voltammetric features with the n values indicates that the redox process is effectively controlled by the electron tunneling across the underlying MUA-PLL layer. Under the conditions employed for the layer-by-layer growth of the PEM film, the ionic transport

Zhao et al.

Figure 2. Cyclic voltammograms for the bare Au electrodes and nanoparticle modified PEM films at 50 mV s-1. A substantial increase of the faradaic current for the PEM modified electrode is observed upon adsorption of metal nanoparticles (compare with data in Figure 1). A decrease of the current density and increase of the peak-to-peak potential difference is observed with increasing n values. The voltammogram obtained for a clean Au electrode under identical experimental conditions is also shown for comparison.

of the hexacyanoferrate couple is sufficiently fast that no concentration profiles are generated across the film under the time scale of the experiments. Furthermore, these results confirm that the compact structure of the MUA layer remains unaffected by the PEM adsorption. Our previous STM studies of the MUAPLL ultrathin film suggested that the polycation forms a homogeneous compact layer on top of the carboxyl-terminated thiol.20 More quantitative analysis on the electron-transfer dynamics as function of n will be presented in the next section. The effect of the nanoparticle adsorption on the voltammetric responses at Au electrodes modified with PEM is illustrated in Figure 2. The results clearly show a substantial increase in the electron-transfer rate upon adsorption of the Au nanoparticles (compare to Figure 1). On average, the voltammograms showed an increase of the peak-to-peak difference (∆Epeak) and a decrease of the overall current with increasing values of n. The voltammogram obtained under identical conditions with a clean Au electrode is also illustrated for comparison. For instance, ∆Epeak increases by less than 40 mV upon increasing n from 0 to 10. These small changes are somewhat obscured by the dispersions in the data from sample to sample. The origin of the data dispersion can be connected to the acute dependence of the electrochemical responses on the particle number density (Γ).20 It should also be mentioned that no changes in the formal potential of the redox species was observed, indicating that no significant effects associated with Donnan potentials take place.47 The voltammetric behavior observed in Figure 2 is characteristic of partially blocked electrodes, providing further evidence of a change in mechanism from electron tunneling across the MUA-PLL boundary to a nanoparticle-mediated electron-transfer process. 3.2. Charge-Transfer Resistance for the PEM Modified Electrode as a Function of n. Nyquist representation of the impedance responses for PEM modified electrodes in the absence of Au nanoparticles are illustrated in Figure 3a. At the formal redox potential, the impedance data feature a single RC time constant within the examined frequency range. These responses were successfully analyzed by the conventional Randles circuit displayed in Figure 3b. This circuit is composed of the phenomenological charge-transfer resistance (Rct), the interfacial capacitance (Cint), the uncompensated resistance (Ru), and the Warburg impedance (W). In the case of the MUA and PEM modified electrodes, the Warburg component is excluded

Long-Range Electron Transfer across PEM

Figure 3. Impedance spectra of the MUA and PEM modified electrodes measured at 0.19 V vs SCE (A). The concentration of the redox probe solution was 1 × 10-3 mol dm-3 Fe(CN)64-/Fe(CN)63-. The impedance data is characterized by single RC time constant. The phenomenological charge-transfer resistance (Rct), double layer capacitance (Cint), and the uncompensated resistance (Ru) were estimated by fitting the experimental data to the equivalent circuit shown in (B). A substantial decrease of Rct is observed upon adsorption of the PEM on the MUA monolayer. However, Rfilm exhibited a weak dependence on n. For the sake of consistency in the discussion, the Warburg diffusional impedance (W) is also included the scheme. However, this parameter is not included in the evaluation of the impedance for the MUA and PEM modified electrodes.

from the analysis in the studied frequency range. For the sake of consistency, the sub-indexes “MUA”, “film”, and “array” will be used for differentiating the parameters obtained MUA, PEM, and nanoparticle terminated electrodes. The chargetransfer resistance obtained for the MUA modified electrode (RMUA) is determined by the tunneling probability across the thiol barrier. For the PEM modified electrodes (Rfilm), contributions from the charge transport across the ultrathin film can also be expected. Consequently, evaluating the behavior of Rfilm as a function of n allows the clarification of whether the PEM hinders the transport of the redox probe to the electrode surface. All fittings based on the equivalent circuit in Figure 3b were performed with a nonlinear least-square routine, taking the three elements as adjustable parameters. Under the present conditions, the average value of RMUA is (4.3 ( 2.2) × 106 Ω. The apparent charge-transfer resistance slightly decreases upon adsorption of the ultrathin polyelectrolyte films. For instance, PEM films with n ) 10 showed an average Rfilm of (0.95 ( 0.17) × 106 Ω. A key finding of the present studies is the weak dependence of Rfilm on n. This behavior confirms that the rate of the electrochemical process is controlled, to a large extent, by the electron tunneling across the buried MUA-PLL boundary. Recently, we have shown that Rfilm (n ) 0) exponentially increases with the thickness of the underlying SAM.42 The apparent charge-transfer resistance in

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6835

Figure 4. Impedance spectra of the nanoparticle terminated film for different n values (A). The impedance spectra exhibit a single time constant at high frequencies and a Warburg type response at low frequencies. A generalized equivalent circuit for a two step electrontransfer process is illustrated in (B). The parameters R′film C′film determine the time constant for the electron transfer between the nanoparticles and the Au electrode. The kinetics of electron exchange between the particles and the redox species corresponds to the RnanoCnano time constant. The diffusion impedance of the redox probe is introduced by the Warburg (W) component.

the presence and absence of a single PLL layer exhibited a distance dependence characterized by a decay constant β ) 1.16 ( 0.04 per methylene unit. The value reported for the MUAPLL in our previous work is identical to the average Rfilm obtained for the PEM films.20 3.3. Effect of the Nanoparticle Adsorption on the Electrochemical Impedance Responses. As illustrated in Figure 4a, the adsorption of metal nanoparticles brings about a substantial decrease in the impedance magnitude in comparison to the responses observed for as-grown PEM film (compared to Figure 3). The 2 orders of magnitude decrease in the impedance responses clearly demonstrates a change in the mechanism of electron transfer involving the hexacyanoferrate redox probe. The impedance spectra are characterized by single time constant determined by the phenomenological chargetransfer resistance (Rarray) and the interfacial capacitance (Carray) of the nanostructured electrode. The low-frequency range exhibits a Warburg impedance associated originating from the semi-infinite planar diffusion of the redox probe from the electrolyte solution. A relative weak increase in the faradaic impedance can be observed for high number of multilayers in the assembly. Changes in the measured Rarray with the number of polyelectrolyte layers can be determined by either (i) changes in the nanoparticle number density at the PEM surface or (ii) a decrease in the effective electron-transfer rate constant with increasing n. The fact that the impedance responses in Figure 4 show a single time constant at frequencies above 10 Hz and Warburg type responses at low frequencies allow analyzing the spectra in terms of the Randles circuit (Figure 3b). In this case, the phenomenological Rarray is composed of a series of parameters associated with the charge transfer via the nanoparticles and


Zhao et al.

Rarray )

Figure 5. Phenomenological charge-transfer resistance for the PEM film in the presence (Rarray) and absence (Rfilm) of the adsorbed nanoparticles layer as a function of n. The nanoparticle assembly induces 2 orders of magnitude decrease in the charge-transfer resistance. Both parameters exhibit a very weak dependence on the number of polyelectrolyte layers. These results provide clear evidence of a change in mechanism from tunneling across the MUA-PLL boundary to a nanoparticle-mediated electron transfer.

across the PEM film. The continuous line in Figure 4 demonstrates that this equivalent circuit is a rather good approximation to the experimental results. Consistent fitting were obtained in which the Warburg component approaches the expected value taking into account the concentration of the redox probe and the geometrical area of the electrode. This result confirms that the transition from spherical to planar diffusion geometry for randomly distributed 20 nm particle array with a number density of the order of 1010 cm-2 is outside the frequency range available.20,37 The dependence of the phenomenological Rarray and Rfilm with the number of polyelectrolyte bilayers is displayed in Figure 5. The adsorption of the nanoparticles brings about over a 2 orders of magnitude decrease in the phenomenological charge-transfer resistance even for n values as large as 10. It is also observed that neither Rfilm nor Rarray are affected by the number of polyelectrolyte bilayers in a substantial fashion. In the absence of metal nanoparticles, the self-assembled MUA layer generates a strong blocking effect that controls the dynamics of electron transfer. Consequently, Rfilm for n values between 0 and 10 is effectively the same. The large difference in the magnitude of Rfilm and Rarray has important consequences in the physical interpretation of the latter. As mentioned previously, the adsorption of the metal nanoparticles opens up a new channel for the electron transfer across the PEM modified electrode. The impedance responses associated with the nanostructured electrode can be represented in terms of the equivalent circuit shown in Figure 4b. The charge transfer channeled through the nanoparticles can be represented by a double tunneling barrier junction.48 The time constant for charge injection to the particles by the redox species corresponds to RnanoCnano, while R′film C′film represents the time constant for the charge transfer from the particle to the electrode. The equivalent circuit in Figure 4b generates two semicircles in the complex plane associated with the R′film C′film and RnanoCnano time constants. However, the experimental impedance spectra consistently show a single time constant at high frequencies and a Warburg type response at low frequencies (see Figure 4a). In view of the rather weak dependence of impedance responses on n, the presence of a single time constant strongly suggest that the contribution of R′film C′film is rather insignificant. In the frequency range where the diffusional impedance is negligible, Rarray can be expressed as

RnanoRfilm Rnano + Rfilm

(1)

Taking the average values of Rfilm ) (1.0 ( 0.4) × 106 and Rarray ) (1.0 ( 0.5) × 104 Ω for the different number of PEM bilayers, eq 1 essentially establishes that Rarray ≈ Rnano. These results reveal that the charge transfer from the redox species to the electrode surface is entirely channeled through the metal nanoparticles. Although the PEM offers very little hindrance to the transport of the redox probe, the underneath MUA monolayer provides a strong blocking layer for the direct electron tunneling from the redox species. As discussed further on, the MUA tunneling barrier significantly decreases in the areas of the electrode covered by the nanoparticles. In order to further rationalize the experimental values of Rarray and Carray, the particle number density should be examined as a function of n. Acoustic AFM images of nanoparticles assemblies obtained for 2, 7, and 10 bilayers are illustrated in Figure 6. The height contrast in the image was adjusted in order to emphasize the topography of the metal nanoparticles. The asevaporated gold films were composed of nanoscopic size grains that crowded the untreated topographic images. However, the rather regular size of the grains and the smooth nature of the PEM films allowed a straightforward deconvolution of the topography of the nanoparticles assembly. In addition, particle identification can be enhanced by comparing topographic, amplitude, and phase images. The particle number density (Γ) for various PEM films is displayed in Figure 7. The values of Γ were obtained after examining five different samples in four different areas of each sample. Although fluctuations between 10 and 20% in Γ were observed between the different samples, it can be concluded that the particle number density is independent of the number of adsorbed polyelectrolyte layers. The average value over all the PEM films featuring n > 1 was Γ ) (4.8 ( 1.0)×109 cm-2. This value is smaller than the one obtained for a single PLL film by a factor of 2. The origin of the decrease in Γ for the PEM film with respect to the single PLL layer is unclear. However, the key conclusion of this data is that Γ is effectively determined by the adsorption time for the various PEM films. The interfacial capacitance obtained from the impedance spectra in the presence (Carray) and absence (Cfilm) of the nanoparticles are also displayed in Figure 7. Both parameters are little dependent on n with average values of Cfilm ) (2.12 ( 0.08) × 10-6 F cm-2 and Carray ) (2.25 ( 0.14) × 10-6 F cm-2. As a first approximation, Cfilm can be defined in terms of two capacitors in series associated with the electrode|PEM (CM|film) and the PEM|electrolyte (Cfilm|sol) boundaries

1 1 1 ) + Cfilm CM|film Cfilm|sol

(2)

It is expected that Cfilm|sol exhibits a high capacitance due to the strong interpenetration of the electrolyte into the PEM film. Therefore, the major contribution to Cfilm is given by CM|film which is independent of the film thickness. On the other hand, the contribution of the metal nanoparticles to Carray can be described within the framework of the model for partially blocked electrodes.11 Under this approximation, the capacitance associated with the metal nanoparticles (θCAu) is considered in parallel to the capacitance of the uncovered areas of the electrode ((1 - θ)Cfilm)

Carray ) θCAu + (1 - θ)Cfilm

(3)


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Figure 6. Representative ex situ 2 µm × 2 µm AFM images of nanoparticle assemblies on PEM films featuring 2 (A), 7 (B), and 10 (C) bilayers.

Figure 7. Nanoparticle number density (Γ) as a function of n obtained from AFM measurements. An average value of Γ ) (4.8 ( 1.0)×109 cm-2 was observed for polyelectrolyte films featuring n > 1. The dependence of the interfacial capacitance Carray and Cfilm on n is also illustrated. The small difference between both parameters obtained from the impedance data (Figures 3 and 4) is consistent with the relatively low values of Γ.

where θ is the fraction of the electrode covered by the Au nanoparticles, CAu and Cfilm are the capacitances of the bare Au electrode and MUA-PEM film terminated surface, respectively. Taking the average Γ ) (4.8 ( 1.0) × 109 cm-2, it can be estimated that the value of θ is below 5%. Consequently, in agreement with the experimental data in Figure 7, the low particle number density brings about negligible changes to the double layer capacitance. The fact that Rarray ≈ Rnano, and that the impedance contribution from R′film C′film is negligible, allows analyzing this parameter employing the partially block electrode model. It follows20,49-51

Rnano )

RT RT ) 2 2 ap ap n F θkAuAc n F kAuΓnanoπ(rnano)2Ac 2 2

(4)

where the parameter kap Au is the apparent electron-transfer rate constant for θ )1, A is the geometrical area of the electrode, rnano is the average radius of the nanoparticles and c is the molar concentration of the redox probe. Taking the average value of Rnano ) (1.0 ( 0.5) × 104 Ω, it follows from eq 4 that kap Au ) (0.025 ( 0.012) cm s-1. This value is within the range reported for bulk polycrystalline Au electrodes which is between 0.010 and 0.035 cm s-1.52-55 This result is rather significant as it confirms that the impedance responses can be interpreted without introducing a contribution from R′film C′film. In other words, the metal nanoparticles separated from the electrode surface by a PEM film with as many as 10 bilayers behave as an array of nanoscopic Au electrodes. In order to further analyze

the implications of these unique results, we shall further investigate the properties of the PEM film use as a spacer. 3.4. PEM Film Thickness as a Function of the Number of Adsorption Steps. One of the most striking features of the data illustrated in Figures 5 and 7 is the weak dependence of the electrochemical parameters with the number of adsorbed layers. In order to further rationalize these trends, the thickness of the PEM film (δ) was measured as a function of n employing in situ AFM. These measurements were conducted by removing sections of the PEM film with the AFM tip in contact mode, followed by measuring the depth profile of the generated trough. The experiments were performed on PEI treated mica as the force required to remove the PEM films resulted in indentation into the gold electrodes. Mica has a similar hardness as the AFM tip and the forces required to remove the PEM film did not damage or alter the mica. The PEI layer creates a uniform positive layer on the mica surface on which the PGA-PLL can be adsorbed under the similar conditions used for the PEM growth on the modified Au electrodes. The AFM images were recorded in situ under soft contact mode. The low forces exerted on the sample in this mode do not alter the image of the surface even after repetitive scans. Increasing the tip force by a factor of 10 allows the removal of the PEM film in an area of 2 µm × 2 µm. The use of a relative high force was required due to the strong interaction between the PEM and the mica surface. Interference in the determination of the depth profile by the debris generated from the film scratching was found to be reduced if the scan direction for imaging was rotated by 90° with respect to the scratching direction. Measurements of the depth profile were performed in the presence of the electrolyte solution with different applied forces (i.e., set points). The set point was reduced until just before the tip lost contact with the film. Due to the soft nature of the film, changes in the set point resulted in different estimation of δ. These variations were included in the standard deviations. This method of imaging PEM films has successfully been used by others.33,56 A typical image of a tip-induced trough in a PEM film is illustrated in Figure 8a. Height histograms of selected areas of the trough were generated in order to estimate the average film thickness. The histogram shown in Figure 8b corresponds to the height distribution of the area inside the white rectangle in Figure 8a. The height histograms showed two clear peaks relating to the average height of the mica surface with respect to the average height of the PEM. The difference in these two values was taken as δ.57 Figure 9 displays the average δ for 3, 5, 7, and 9 bilayers obtained from in situ AFM measurements in the presence of 0.1 mol dm-3 Na2SO4, 1 × 10-3 mol dm-3 K4Fe(CN)6, and


Figure 8. 5 µm × 5 µm (full Z range 138 nm) AFM image of a PEM film featuring 9 bilayers grown on PEI treated mica (A). The measurement was performed in the presence of the electrolyte solution in soft-contact mode. A trough of approximately 4 µm2 was generated by applying a high force between the surface and the tip. Height histograms were obtained from the area enclosed by the white box (B). The height difference between the two maxima was taken as the film thickness (δ).

Figure 9. Film thickness as function of the number of polyelectrolyte layers in the presence of the electrolyte solution containing 1 × 10-3 mol dm-3 Fe(CN)64-/ Fe(CN)63- and 0.1 mol dm-3 Na2SO4. The film thickness increases by 40% on average in the presence of the hexacyanoferrate couple.

1 × 10-3 mol dm-3 K3Fe(CN)6. Each point in the graph corresponds to the average of three different samples, in which six troughs were made. The error bars are the standard deviation from the eighteen measurements. The results show a linear dependence of the film thickness with n. Extrapolation of δ to n ) 0 yielded a value of 0.64 ( 0.08 nm. This value corresponds to the thickness of the PEI layer in the presence of the redox electrolyte solution. The thickness of the PEM films in Milli-Q water was difficult to measure as they were significantly thinner and due to interferences arising from the roughness at the

Zhao et al. trough boundaries. On average, it was obtained that the δ values were 40% thinner than compare with the redox electrolyte solution. Film swelling was also observed in 0.1 mol dm-3 Na2SO4, although to a lesser degree than in the presence of the hexacyanoferrate couple. These results suggest that permeation of ions into the film is not only dependent on the ionic strength but also on properties such as the effective ionic charge and polarizability. Although a detailed discussion on ion permeability and swelling of PEM is outside the scope of this work, these results can be taken as a further evidence of the affinity of the hexacyanoferrate couple to the polyelectrolyte film. Previous studies of δ as a function of n for the PLL-PGA system have resulted in a broad range of values. For instance, thickness between 13 and 300 nm have been reported for PEM with n ) 9.33,34,45,58-61 The large dispersion of the literature data indicates the tremendous sensitivity of the film thickness to the composition of the polyelectrolyte solutions and the growing protocol. One of the key parameters is the ionic strength of the polyelectrolyte solution. Schlenoff and Dubas reported δ values in the same range as those in Figure 9 for PEM films of poly(diallyldimethylammonium) (PDADMAC) and polystyrene sulfonate (PSS) in the absence of added electrolytes.62 A quasi-linear dependence of δ on n was also observed under these conditions. These authors observed up to 50 times thicker films upon addition of 1 mol dm-3 NaCl to the polyelectrolyte solutions. It has also been reported that blow drying the films with N2 gas between deposition steps results in thinner films.34 In addition, the pH range in which the PLL and PGA were deposited results in thinner films.60,63 The high uptake of the hexacyanoferrate ion has been reported previously.46,64,65 It has been observed that the affinity of ionic species to the PEM increases with increasing ionic charge.66 The redox species are trapped into the multilayer network, replacing smaller counterions incorporated in the film growth. This effect accounts for the independence of Rfilm on the number of layers (see Figure 5) as well as the decrease in the chargetransfer resistance of the MUA modified electrode upon the PLL adsorption. The ultrathin nature of the film also allows fast diffusion of the redox couple, and therefore the concentration inside the PEM can be considered constant at all potentials. 3.5. Film Swelling in the Presence of Adsorbed Nanoparticles. The trend in Figure 9 clearly shows a linear increase of the film thickness with n. However, a key point relevant to the electrochemical behavior described earlier is whether this thickness represents the distance separating the nanoparticle from the electrode. Our AFM studies of the film thickness were dependent on the set point, suggesting that the film exhibits a soft (compressible) nature. Given the weak dependence of the electrochemical responses on n, it could be argued that the electrostatic adsorption of the nanoparticles generates highly localized forces affecting the film thickness. In order to investigate this issue, the structure of diluted Au nanoparticles monolayers adsorbed on PEM films with n ) 9 were investigated by acoustic AFM in air and in the redox electrolyte solution. In the case that film swelling is significantly hindered by the adsorbed nanoparticles, the average particle height is expected to decrease in the presence of the hexacyanoferrate couple. In these experiments, Au films were evaporated on mica followed by annealing in a muffle furnace at 650 °C for 16 h to generate large (111) terraces. Large atomically flat domains are crucial in these experiments in order to prevent the convolution of the particle height with the surface topography.


J. Phys. Chem. C, Vol. 112, No. 17, 2008 6839 SCHEME 2: Illustration of the PEM Film Swelling Associated with the Intercalation of the Hexacyanoferrate Redox Couplea

a Ex situ and in situ studies of the average nanoparticle height suggest that the swelling is homogeneous even in the areas covered by the nanoparticles. The diagram is not drawn to scale.

Figure 10. 500 nm × 500 nm (Z range 28 nm) ex situ acoustic AFM image of an ensemble of particles adsorbed on a PEM film featuring n ) 9 (A). The same ensemble of particles was imaged in the presence of the redox electrolyte solution. The effective height of the particles measured ex situ and in situ is displayed in (B). The difference in particle heights suggests that the PEM film swells homogeneously even in the areas covered by the nanoparticles.

In addition, the ultrathin nature of the film as well as the nonnegligible size dispersion of the nanoparticles obliged that swelling induced modifications of the films have to be investigated in a specific ensemble of particles. Theses experiments are nontrivial as the properties of the AFM tips required for acoustic mode measurements ex situ and in situ are rather different (see experimental section). Therefore, the selection of the ensemble of particles was performed ex situ, followed by withdrawal and replacement of the tip and relocation of the selected ensemble of particles. This procedure was achieved with the aid of the video microscope, identification of distinctive surface features as well as careful preparation and alignment of the sample holder. Ex situ and in situ imaging was performed with various set points. An ex situ image of a typical ensemble of particles on a PEM film with n ) 9 is illustrated in Figure 10a. The particle number density was kept low to facilitate the determination of the particle height (1 min adsorption time). Each particle was identified with a number and the measured height is displayed in Figure 10b. Measurements performed in the presence of the redox electrolyte solution used in the electrochemical experiments provided a slight increase of the apparent particle height. This apparent increment in the particle height was observed for the whole ensemble of particles as illustrated in Figure 10b. At this stage, it cannot be entirely excluded that this effect is related to tip induced compression of the film in the acoustic mode. However, experiments recorded with different set point consistently show an increase in the particle height. It is unclear the origin of the apparent increase in the particle height in the in situ measurements. However, these results provide strong evidence that the film swelling occurs homogeneously, even in the area covered by the nanoparticles. In the

case that the adsorbed particles prevent the local penetration of the ionic species, the apparent height is expected to decrease by approximately 5 nm for PEM films of n ) 9. Although more accurate experiments would be required to address this issue, e.g., neutron reflectivity, our experiments allowed us to postulate that the nanoparticles are effectively “sitting” on top of the PEM film as illustrated in Scheme 2. If this interpretation is accurate, then the electronic communication between nanoparticles and the electrode surface attached by PEM films is truly remarkable. Our quantitative analysis of the electrochemical responses shows that the effective charge-transfer resistance in this system remains essentially independent of the distance separating the nanoparticles from the electrode surface, even for values as large as 6.5 nm. 4. Discussion The comprehensive studies described in the previous sections concluded that the dynamic electrochemical responses in the presence of the Au nanoparticles are controlled by the electron transfer from the redox species to the metal nanostructure. This condition remains valid even if the particles are separated by distances as larger as 6.5 nm from the electrode surface. In the absence of the nanoparticles, the effective Rct is determined by the electron tunneling across the underneath MUA monolayer and largely independent of the PEM film thickness. Our recent experiments have also shown that carboxyl terminated alkane thiols with chains of less than 5 methylene units generates significant smaller Rct and the enhancement of the electrochemical responses introduced by the metal nanoparticles is less significant.42 Chirea et al. have reported the electrochemical behavior of multilayers containing MUA protected Au nanoparticles electrostatically adsorbed on mercaptosuccinic acid (MSA) modified Au electrodes.38 In comparison to the result shown in Figure 5, the changes of the charge-transfer resistance reported by Chirea et al. are somewhat insignificant. This arises from the fact that the Au particles are protected by strong blocking layer and the overall process is mainly controlled by the permeation of the redox species to the electrode surface. The AFM studies in the previous sections suggested that the nanoparticles do not introduce significant deformation of the

6840 J. Phys. Chem. C, Vol. 112, No. 17, 2008 PEM film. The swelling of the PEM in the presence of the redox probe occurs homogenously across the film even in the presence of Au nanoparticles. We consider these results as further evidence that the charge-transfer mechanism is not connected to diffusion through defects (pin-holes) generated by the electrostatic adsorption of the Au nanoparticles. We have recently shown that the adsorption of a packed monolayer of SiO2 particles of similar size on MUA-(PLL-PGA)3-PLL modified electrode does not introduce any significant changes in Rct.67 Furthermore, the subsequent adsorption of a diluted monolayer of Au nanoparticles on top of the SiO2-PLL layer generates a strong decrease of Rct to the similar level as those shown in Figure 5. In other words, one layer of PGA can be replaced by one layer of SiO2 and the electrochemical signals remain essentially controlled by the number density of Au nanoparticles. These observations entirely exclude the possibilities that the enhancement of the electrochemical kinetics is due to either pin-holes generated by electrostatic adsorption or direct physical contact between the metal nanoparticles and the electrode surface. A key observation in connection to the unusual electron transport properties in this system is the ability of the redox probe to penetrate the electrostatically assembled films. The results in Figure 5 allow to conclude that Rct is largely independent of the PEM thickness even in the absence of the nanoparticles. Consequently, the PEM multilayer offers very little resistance to the transport of the redox electrolyte in comparison to the electron tunneling barrier generated by the underneath MUA monolayer. The average 40% increment in the film thickness in the presence of hexacyanoferrate indicates that there is a large concentration of the redox probe inside the PEM multilayer. Considering that the nanoparticles are physically separated from the electrode surface by distances as large as 6.5 nm, the electronic communication (transport) will involve energy levels associated with “trapped” redox probe. The probability of coherent tunneling between the nanoparticles and the electrode surface are negligible under such large distances. The unusual distance independence of Rct indicates that the charge transport between the particles and the electrode surface cannot be rationalized in terms of a “classical” hopping mechanism via the confined redox probe. This can be demonstrated by normalizing the measured charge-transfer resistance through the film (R*film) and via the nanoparticles (R*nano) per unit of area. Taking the average values from Figure 5, it follows R*film ) (7.1 ( 2.8) × 104 Ω cm2 while R*nano ) 10.7 ( 5.1 Ω cm2. The fact that electron transfer is approximately 10 thousand times faster in the presence of the nanoparticles demonstrates that the charge-transfer barrier associated with electrode modification does not control the electrochemical kinetics. This observation provides evidence that the charge transport through the confined redox species does not involve the reorganization of their solvation shell, i.e., a hot electron transport. The “hot electron-transfer mechanism” can be rationalized in terms of the Gerischer formalism for electron transfer. At equilibrium, the Fermi levels at the electrode and nanoparticles are equalized with the corresponding Fermi energy of the redox probe. In the absence of the metal nanoparticles, the exchange current is determined by the tunneling probability across the modified electrode and the density of states of the oxidized and reduced species at the Fermi energy. The adsorption of the Au nanoparticles brings about a significant increase in the density of states at the Fermi energy. Our results revealed that excess charge in the particle is swiftly transferred iso-energetically to

Zhao et al. the electrode surface via the density of states of the probe at the Fermi energy. In other words, the transported charges do not reside in the film and no reorganization of the dielectric medium occurs around the trapped redox probe. If the latter process does take place, then the electrochemical kinetics in presence and absence of the nanoparticles would be essentially controlled by the tunneling probability across the MUA layer. In summary, we strongly believe that the unique properties observed in this system are directly linked to the blocking nature of the MUA monolayer and the strong affinity of the redox probe to the PEM film. Preliminary experiments have shown that the electrochemical responses are rather sensitive to the properties of the redox probe and the polycation employed. In addition, different behavior could also be expected if the redox species are chemically grafted to the polyelectrolyte film. We are currently directing our efforts to find convenient ways to examine the dynamic electrochemical responses as function of the density of redox species (charge carriers) in the PEM film. 5. Conclusions The electrochemical properties of 2D metal nanoparticle assemblies on PEM films have been investigated as a function of the film thickness. Sequential assembly of PLL and PGA generates ultrathin films which thickness can be controlled by the number of adsorption steps. Au nanoparticle coverage below 5% was obtained by controlling the immersion time of PLL terminated films in the citrate stabilized colloidal solution. Systematic studies of the redox properties of the films in the presence and absence of the nanoparticle submonolayer were performed. The results obtained from dynamic electrochemical measurements and in situ AFM studies open new questions about electron transfer across this class of soft materials. In the absence of Au nanoparticles, the dynamics of electron transfer involving the hexacyanoferrate redox couple was found independent of the PEM thickness grown on MUA modified Au electrodes. We concluded that the charge-transfer resistance is determined by the tunneling probability through the initial MUA monolayer rather than the transport of the redox probe inside the PEM. The facile diffusion of the redox couple into the PEM is also linked to the film swelling as measured from in situ AFM measurements. In the presence of the redox electrolyte, the effective thickness of the PEM layers increases up to 6.5 nm. In spite of the significant changes in the film thickness, the kinetics of electron transfer between adsorbed nanoparticles and the metal electrodes remain effectively independent of n. Quantitative analysis of the charge-transfer resistance clearly indicated that the Au nanoparticles behave as independent Au nanoelectrodes. This conclusion is valid even for particles adsorbed on PEM films containing as much as 10 bilayers. Comparison of the surface topography by ex situ and in situ acoustic AFM measurements suggest that the electrolyte induced PEM swelling occurs homogeneously even in the presence of the metal nanoparticles. The ensemble of the results discussed herein points toward a remarkable long-range electronic communication between metal nanoparticles and PEM modified electrodes. From a phenomenological point of view, this behavior can be associated with an electronic coupling β < 0.1. The origin of the “unhindered” long-range electron transfer is associated with the partial permeation of the redox probe inside the PEM film. The evolution of the dynamic electrochemical signal upon adsorption of the metal nanoparticles demonstrates that the charge transport does not occur by conventional hopping mechanism. It is

Long-Range Electron Transfer across PEM proposed that the electron transport takes place via the density of states of the redox probe at the Fermi energy without relaxation of the nuclear coordinates (outer reorganization energy). The physical aspects in relation to these observations can have an impact beyond current interests in nanoparticle/ polymer heterostructures. Key issues concerning electron transport involving proteins as well as other nanoscopic structures could be manifestation of a similar transport mechanism.68 This work further demonstrates that surface electrochemical reactivity can be effectively modulated by (i) well-defined self-assembled blocking layers, (ii) electrostatic growth of functional hierarchical structures, and (iii) controlled permeation by redox probes. 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. The fruitful discussions with Prof. Laurence M. Peter (University of Bath) are also acknowledged. We are indebted to Ms. Gabriela Kissling and Ms. Christa Bu¨nzli for their assistance in proof-reading and art-work. This work was financially supported by the Swiss National Science Foundation (Projects: PP002-68708 and 200021-105238), the PortlandZementstiftung 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, 6668-6697. (2) Anderson Neil, A.; Lian, T. Ann. ReV. Phys. Chem. 2005, 56, 491. (3) Hicks, J. F.; Zamborini, F. P.; Murray, R. W. J. Phys. Chem. B 2002, 106, 7751. (4) Son, K. A.; Kim, H. I.; Houston, J. E. Phys. ReV. Lett. 2001, 86, 5357. (5) Protsailo, L. V.; Fawcett, W. R. Electrochim. Acta 2000, 45, 3497. (6) Chen, S. J. Phys. Chem. B 2000, 104, 663. (7) Miller, C.; Cuendet, P.; Graetzel, M. J. Phys. Chem. 1991, 95, 877. (8) Nahir, T. M.; Bowden, E. F. Electrochim. Acta 1994, 39, 2347. (9) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (10) 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. (11) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2003, 107, 4844. (12) Liu, Y. J.; Wang, Y. X.; Claus, R. O. Chem. Phys. Lett. 1998, 298, 315. (13) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430. (14) Song, W.; Okamura, M.; Kondo, T.; Uosaki, K. J. Electroanal. Chem. 2003, 554-555, 385. (15) Sagara, T.; Kato, N.; Nakashima, N. J. Phys. Chem. B 2002, 106, 1205. (16) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3, 1203. (17) Chirea, M.; Garcıá-Morales, V.; Manzanares, J. A.; Pereira, C.; Gulaboski, R.; Silva, F. J. Phys. Chem. B 2005, 109. (18) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61. (19) Carrara, M.; Kakkassery, J. J.; Abid, J.-P.; Fermin, D. J. ChemPhysChem 2004, 5, 571. (20) Zhao, J.; Bradbury, C. R.; Huclova, S.; Potapova, I.; Carrara, M.; Fermin, D. J. J. Phys. Chem. B 2005, 109, 22985. (21) Schnippering, M.; Carrara, M.; Foelske, A.; Kotz, R.; Fermin, D. J. Phys. Chem. Chem. Phys. 2007, 9, 725. (22) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (23) Han, S.; Lindholm-Sethson, B. Electrochim. Acta 1999, 45, 845. (24) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (25) Pardo-Yissar, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110.

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Long-Range Electronic Communication between Metal Nanoparticles

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