Potential Regulation of the Spectroelectrochemical Response of

Potential Regulation of the Spectroelectrochemical Response of Monolayer-Protected Gold ... electrode-supported MPC films in aqueous media in the pres...
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J. Phys. Chem. C 2007, 111, 4277-4284

4277

Potential Regulation of the Spectroelectrochemical Response of Monolayer-Protected Gold Cluster Films by Electrolyte Composition Virginia Ruiz,* Alvaro Colina, Maria Ara´ nzazu Heras, and Jesu´ s Lo´ pez-Palacios Department of Chemistry, UniVersity of Burgos, Pza. Misael Ban˜ uelos s/n, E-09001 Burgos, Spain ReceiVed: September 26, 2006; In Final Form: December 15, 2006

The spectroelectrochemistry of multilayer films of hexanethiolate-protected Au147 clusters (C6-Au147 MPCs) supported on gold electrodes has been investigated by UV-vis reflectance spectroelectrochemical quartz crystal microbalance in aqueous solutions containing electrolyte anions with different degrees of hydrophobicity (PF6-, ClO4-, BF4-, and NO3-). The multiresponse technique has enabled us to measure simultaneously potential-dependent reflectance and gravimetric changes in the film during the stepwise charging of the C6Au147 MPCs in different electrolytic media. Changes in the film reflectance, due to quantized charging of the C6-Au147 MPCs, proved to be very sensitive to both the hydrophobicity and concentration of the electrolyte anion used. Thus, the onset potential for the charging current and consequently, the reflectance attenuation, shifted anodically with decreasing concentration/hydrophobicity of the electrolyte anion. Moreover, the magnitude of the film reflectance changes resulting from each single-electron-transfer process varied between electrolytes. Electrochemical quartz crystal microbalance (EQCM) measurements enabled us to rationalize the differences noted in the spectroelectrochemical responses in terms of dissimilar ion-binding strengths of the electrolyte anions with the hydrophobic MPCs. Hence, the results presented here demonstrate the feasibility of regulating the electro-optical performance of C6-Au147 MPCs by simple chemical manipulation of the sequential electron transfer by solution ions.

Introduction Clusters of few tens of Au atoms coated with a monolayer of thiolate ligands, so-called monolayer protected clusters (MPCs),1 have been the focus of extensive research over the past decade.1-33 Their unique chemical, electrical, and optical properties make them potential candidates as building blocks in nanodevices for different electronic, optical, and sensor applications.2-7 In particular, enormous attention has been devoted to the remarkable electrochemical responses exhibited by these near-molecular entities,8-23 which possess molecular capacitances in the sub-aF range due to their small size. As a result of this small capacitance, it is possible to detect the successive one-electron double-layer charging of the MPC cores as a series of peaks in cyclic voltammograms, a phenomenon known as quantized double layer (QDL) charging.8 MPC electrochemistry and electron-transfer dynamics have been extensively investigated, both dissolved in electrolyte solutions8-14 or attached to electrode surfaces as monolayers15-19 or multilayers.20,21,24 Among these studies, the phenomenon of ioninduced rectification of the MPC-quantized capacitance charging is of particular interest. This rectification has been reported from electrode-supported MPC films in aqueous media in the presence of hydrophobic electrolyte ions.17 Moreover, it has been demonstrated that these rectification effects, both in monolayers17,18 and multilayers,21 are directly related to the electrolyte ion hydrophobicity and hence ion-binding strength, manifested by the cathodic (anodic) shift of the onset potential with anions (cations) of increasing hydrophobicity. Thus, there have been numerous reports by Chen’s group aimed at controlling the ionbinding chemistry and hence the rectified MPC charging with * To whom correspondence should be addressed. E-mail: vrfernandez@ ubu.es. Phone: +34 947 258817. Fax: +34 947 258831.

the aim of exploring their possible application in the development of single-electron molecular diodes.25 Motivated by this interesting electrochemical phenomenon, we have recently investigated the spectroelectrochemical changes arising from the rectified quantized charging of MPC assemblies in aqueous solutions.26-28 Compared to the vast bibliography devoted to the electrochemistry of Au MPCs, there are relatively fewreportsconcerningtheiroptical29-30 andspectroelectrochemical31-33 properties. While the existing reports examine the spectroelectrochemistry of Au MPCs dissolved in organic solvents, we were recently able to resolve electronic transitions promoted by discrete anodic charging of electrode-supported C6-Au147 MPC films in aqueous solutions26 that were comparable to those observed by Murray’s group for MPC solutions.33 Moreover, the potential-induced spectral changes encountered in C6-Au147 MPC films supported on optically transparent electrodes (ITO) were linearly dependent on the MPC core charge state up to MPC+5 in perchlorate solutions and, hence, could be reversibly switched upon MPC discharging.26 Lately, it has been possible to resolve a staircase-shaped increment of the film absorbance with potential during MPC quantized charging, which was remarkably similiar to the voltammetric profile, providing clear proof that the spectral changes were induced by the discrete charging of the MPCs.27 With the aim of shedding more light on this intriguing phenomenon, we recently employed so-called multiresponse techniques capable of providing information of a different nature on electrochemical processes simultaneously. In particular, UV-vis reflectance spectroelectrochemical quartz crystal microbalance (SEQCM, a combination of UV-vis near-normal incidence reflectance spectroelectrochemistry34 and EQCM) was used to investigate the charge dependent spectral features

10.1021/jp066304a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007

4278 J. Phys. Chem. C, Vol. 111, No. 11, 2007 exhibited by ensembles of C6-Au147 MPCs.28 The versatility and powerfulness of the in situ combination of gravimetric (EQCM) and spectroelectrochemical techniques to examine electrochemical systems undergoing changes in color and mass simultaneously is well-illustrated by studies of diverse interfacial phenomena, such as adsorption,35 electropolymerization processes,36,37 doping-dedoping of conducting polymer films,38,39 electrodeposition and stripping processes,40 electrochemically induced conformational changes,41,42 and desorption of monolayers from electrodes,35 etc. In the case of C6-Au147 MPC films supported on a gold-plated quartz piezoelectric crystal, the joint technique (SEQCM) enabled us to detect changes in the film reflectance that were dependent on the MPC core charge in NH4ClO4 aqueous solutions, in agreement with those measured on ITO-supported MPC films by absorption spectroelectrochemistry. Simultaneously, analysis of the complementary gravimetric signal made it possible to assess the amount of electrolyte ions involved in compensating MPC charge by ionpair formation. Here, the spectroelectrochemical behavior of Au-supported multilayer films of C6-Au147 MPCs investigated by SEQCM in the presence of electrolyte solutions containing anions with different degrees of hydrophobicity is reported. Since the film spectral response is intimately linked to the rectified MPC charging features and these, in turn, are affected by the hydrophobicity (and hence ion-binding strength) of the electrolyte anions, we can in principle expect to be able to tune the spectroelectrochemistry of these films through choice of the surrounding electrolyte media. Thus, cyclic voltammetry and potential step charging experiments of C6-Au147 MPC films were performed in different electrolytic media and followed by SEQCM. The analysis of three complementary responses (electrochemical, gravimetric, and optical) supplied simultaneously by SEQCM will provide deeper insight on the MPCion binding chemistry and its inter-relationship with the induced reflectance changes in electrolyte solutions with anions of varied hydrophobicity. Experimental Section Chemicals. Ammonium hexafluorophosphate, NH4PF6 (Fluka, >98%), ammonium perchlorate, NH4ClO4 (Panreac, 99%), ammonium tetrafluoroborate, NH4BF4 (Fluka, >98%), ammonium nitrate, NH4NO3 (Merck, >99%), and sodium acetate, NaAc (Merck, 99.5%) were all used as received. Other chemicals were reagent grade and used as received. Aqueous solutions were prepared using high-quality water (MilliQ gradient A10 system, Millipore, Bedford, MA). Preparation of C6-Au147 MPC Films. C6-Au147 MPCs were synthesized according to the two-phase Brust method43 using the modifications proposed by Murray and co-workers to obtain MPCs of small core radius (r ) 0.81 nm, Au147) and better monodispersity.44 MPC core size (1.6 nm diameter) was determined from the peak spacing (245 ( 3 mV) in the differential pulse voltammetric (DPV) response of C6-Au147 MPCs in 0.01 M bis(triphenylphosphoranylidene) ammonium tetrakis(pentafluoro phenyl)borate/0.1 M 1,2-dichloroethane (DCE) solutions. C6-Au147 MPCs films were formed by casting a few drops of concentrated chloroform solutions of C6-Au147 MPCs (2 mg mL-1 of Au MPCs) onto the gold-plated quartz crystal electrode followed by drying under a nitrogen stream. The number of MPC layers was estimated from voltammetric experiments. Typical drop-cast films investigated here varied from few tens to around one hundred MPC layers.

Ruiz et al. Instrumentation and Procedures. The instrumental setup for performing reflectance SEQCM measurements has been depicted and described in detail elsewhere.28 Briefly, a conventional three-electrode cell was used in all experiments, consisting of a homemade Ag/AgCl/KCl (3 M) microreference electrode, a Pt foil counter electrode, and a commercial gold film (A ) 1.3 cm2) vapor-deposited on a 5 MHz AT-cut quartz crystal (12.8 mm diameter) as working electrode (Maxtek, Inc., USA). Highly reflective gold electrodes were used (polished to a mirrorlike finish by the manufacturer) in order to maximize the optical sensitivity. The reference and counter electrodes were placed inside holes drilled in a Teflon screw cap fitted to the crystal holder, thereby enabling proximity to the working electrode. The potential was controlled by an Autolab PGSTAT 20 potentiostat (Eco Chemie B.V., The Netherlands), and the crystal resonance frequency was measured with a plating monitor (PM-700, Maxtek, Inc., USA) with a sensitivity of 0.5 Hz at 5 MHz (or 0.01 µg cm-2). The EQCM was calibrated using the Ag deposition method45 to obtain the correspondence between changes in the quartz crystal resonance frequency and mass uptake/loss, and the obtained calibration constant was 0.036 Hz ng-1. The light beam, supplied by a deuterium-halogen light source (Avalight-DH-S, Avantes, The Netherlands) was both conducted to and collected from the spectroelectrochemical cell by a reflection probe (FCR-7UV200, Avantes, The Netherlands). The reflection probe, a bifurcated bundle consisting of six 200-µm illumination fibers around one central read fiber, was placed in a hole on the Teflon cap, facing the surface of the gold-plated quartz crystal at a distance of approximately 2 mm. In this way, the beam is incident perpendicularly to the electrode surface and the reflected light (which samples a spot of approximately 1 mm2) is collected by the central read fiber of the reflection probe and conducted to an S2000 fiber optic spectrometer (Ocean Optics, USA), made up of a 2048-element diode array. Electrochemical/gravimetric and spectroscopic responses were synchronized by means of an external trigger and recorded by two PCs separately. Changes in the film reflectance will be given with respect to the pristine film prior to applying a potential bias and calculated as ∆R/R ) (R - R0)/R, where R0 is the initial intensity of the reflected beam and R the intensity at different applied potentials. Results and Discussion The electrochemical, gravimetric, and spectroelectrochemical responses obtained for a representative C6-Au147 MPC multilayer film (3.6 × 10-9 mol cm-2) supported on a Au-plated quartz crystal working electrode during cyclic voltammetric experiments will be investigated first in solutions of varied PF6concentration and, second, in the presence of electrolyte anions of different hydrophobicity: PF6-, ClO4-, BF4-, and NO3-. Finally, the effect of the anion hydrophobicity on the three responses supplied by SEQCM during potential step charge/ discharge experiments will also be analyzed. Cyclic Voltammetric Experiments: Influence of the Electrolyte Concentration. Typical cyclic voltammograms for a C6-Au147 MPC film in electrolyte solutions with different NH4PF6 concentrations (containing also varied concentrations of NaAc to keep the overall solution ionic strength constant at 0.1 M) are shown in Figure 1a. A series of four well-defined evenly spaced quantized charging peaks can be observed in the voltammograms recorded at the different NH4PF6 concentrations, and these rectified charging features are clearly sensitive

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Figure 1. (a) Cyclic voltammograms recorded at a C6-Au147 MPC film/Au electrode in electrolyte solutions of different compositions: x M NH4PF6 and (0.1 - x) M NaAc. From left to right, x ) 0.1, 0.05, 0.025, 0.01, and 0.005 M. Scan rate: 0.025 V s-1. The inset shows the variation of formal potentials for the four charging steps with the natural logarithm of PF6- concentration. Symbols indicate experimental values and lines are the corresponding linear regressions. (b) Electrical charge, (c) mass changes (divided by the molecular weight of the electrolyte anion, PF6-), and (d) variation of the film reflectance at 700 nm recorded simultaneously during the anodic scan of the CV experiments shown in panel (a).

to electrolyte composition. As expected, the MPC voltammetric profiles shift negatively with increasing PF6- concentration21,25

Ef ) E0′ +

( )()

( )

K2 RT RT ln - (pz - pz-1) ln[anion] naF K1 na F

(1)

where Ef and E0′ are the formal potentials in the presence and absence of ion binding, respectively, na is the effective number of electron transfer, K1 and pz-1 (K2 and pz) are the equilibrium constant and the number of anions bound to the reduced (oxidized) forms of the MPC molecules, respectively, and other parameters have their usual significance. Despite the peak

potential shift, the potential spacing between consecutive peaks (∆V) remains practically constant in the concentration range investigated, with an average value of 0.138 ( 0.002 V, which corresponds to a MPC capacitance (CMPC) of 1.16 aF. However, at sufficiently low NH4PF6 concentrations ( 0.99 in all cases) for all the charging steps. Furthermore, the slopes ∆m-Q for the four charging steps were practically identical in each solution and only very small differences were observed between concentrations. Hence, a plot of ∆m divided by the molecular weight of PF6- (that is, the number of moles of PF6-, NPF6-) vs Q divided by the Faraday constant (which gives z×NMPC, NMPC being the number of moles of MPCs) will enable a direct calculation of the number

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Figure 2. (a) Cyclic voltammograms recorded at a C6-Au147 MPC film/Au electrode in 0.1 M solutions of the supporting electrolytes indicated in the figure legend. Scan rate: 0.05 V s-1. (b) Electrical charge, (c) mass changes (divided by the molecular weight of each electrolyte anion), and (d) variation of the film reflectance at 700 nm recorded simultaneously during the CV experiments shown in panel (a). Anodic and cathodic scans depicted by solid and dashed lines, respectively.

of MPC-bound PF6- ions (pz ) NPF6-/NMPC) for each MPC core charge (z) in the different solutions. From the slope of the linear regressions, pz:z ratios were extracted. These ratios ranged between 1.25:1 and 1.50:1 (for all z values up to MPC+4) with increasing PF6- concentration and could be explained by uptake of one PF6- molecule solvated with 2-4 water molecules, respectively. Hence, both voltammetric and gravimetric data seem to point to the 1:1 ratio for the four charging steps in PF6- solutions previously reported.21 Reflectance Measurements. Figure 3a shows difference reflectance spectra for the C6-Au147 MPC film (with respect to the spectrum of the unbiased film) observed during the voltammetric scan in 0.1 M NH4PF6 at anodic potentials where MPC core charge state varied from 0 (top spectrum) to +4 (bottom spectrum). As was the case in NH4ClO4 solutions,28 the film reflectance decreases in the whole visible range as the MPC core charge is made increasingly anodic and the magnitude of the reflectance drop is proportional to the MPC redox state.

Ruiz et al. The reflectance attenuation is more pronounced at long wavelengths, with a broad band stretching between 700 and 850 nm. Moreover, the reflectance changes in the presence of this electrolyte anion were also found to be reversible upon neutralization of the Au-supported MPC film during the cathodic scan. It is worth noting that when the bare Au surface (in absence of MPC film) was scanned through the same potential region in 0.1 M NH4PF6, reflectance changes were less than 5% of those observed with MPC films (the CV and the potential dependence of the reflectance at 700 nm of the bare Au substrate are displayed in the Supporting Information). Analogous spectra were recorded in solutions with different NH4PF6 concentrations, the only difference being the formal potentials for each MPC charging event. The effect of PF6concentration on the film spectroelectrochemical response can be better portrayed by plotting the evolution of the reflectance changes at one wavelength (for instance, 700 nm) vs the applied potential for the different solutions (Figure 1d). As was the case in the EQCM response, monotonic changes of ∆R/R with potential were observed at the scan rates used here (the feasibility of resolving a staircase shape in the spectroelectrochemical response by scanning the potential at very slow rates has been demonstrated previously27). Interestingly, the voltammetric and spectroelectrochemical features are equally affected by the concentration of PF6-, as manifested by the comparable shifts in onset potentials for both responses. The complete agreement between the three signals is evident in parts b-d of Figure 1, highlighting the remarkable similarity between the cathodic shifts in charging current onset, mass rise, and reflectance drop with increasing PF6- concentration. Indeed, the formal potential values for each charging step determined from the reflectance changes (following the method described in ref 28) exhibited discrepancies of less than 3 mV with respect to those extracted from the voltammetric response. However, despite the negative shift in formal potential with increasing PF6- concentration, analogous reflectance drops were measured at equal MPC charge states in media of differing PF6concentration, that is, ∆R/R is directly related to z. As the potential at which the electron transfer, and consequently the spectral change occurs shifts cathodically with increasing PF6- concentration, the magnitude of ∆R/R attained at a given applied potential increases with PF6- concentration (Figure 1d). These results prove that differing optical responses for a C6-Au147 MPC film can be induced at a given bias potential simply by varying the concentration of the hydrophobic electrolyte anion. Cyclic Voltammetric Experiments: Influence of the Electrolyte Anion Hydrophobicity. After having demonstrated the possibility of manipulating the spectroelectrochemical response of the C6-Au147 MPC films by the electrolyte concentration, the effect of supporting electrolytes with varied degrees of hydrophobicity on the spectroelectrochemical features was investigated. Since it is known that the rectification of MPC quantized charging is more sensitive to electrolyte anions than to cations,18,25 in the present study supporting electrolytes with the same cation (NH4+ ) and different anions (PF6-, ClO4-, BF4-, and NO3-) were used to examine the influence of the anion hydrophobicity. Cyclic voltammograms recorded in 0.1 M solutions of each electrolyte are shown in Figure 2a. Four well-defined quantized charging peaks are apparent in the voltammetric profiles for the most hydrophobic anions (PF6-, ClO4-, and BF4-) and only two in the presence of the least hydrophobic one (NO3-) at this scan rate, 0.05 V s-1 (three peaks are clearly resolved at lower scan rates). The onset

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Figure 3. Difference reflectance spectra (with respect to the uncharged film) for a C6-Au147 MPC film/Au electrode recorded during voltammetric scans at anodic potentials where the MPC cores are in the charge states indicated in the figure. The composition of the electrolyte solutions was: (a) 0.1 M NH4PF6, (b) 0.1 M NH4ClO4, (c) 0.1 M NH4BF4, and (d) 0.1 M NH4NO3. Scan rate: 0.05 V s-1.

potential for the rectified charging currents showed the expected18,21 anodic shift with decreasing hydrophobicity of the anions (PF6- > ClO4- > BF4- > NO3-), the formal potentials for the first charging peak thus increasing as follows: +0.280 V (PF6-), +0.391 V (ClO4-), +0.477 V (BF4-), and +0.612 V (NO3-). Hence, this observed anodic shift of the MPC-rectified charging features with decreasing anion hydrophobicity seems to be associated with the weaker interactions occurring between the anions and the hydrophobic MPCs as the ion hydration increases. These weaker interactions of the least hydrophobic anion (NO3-) with the MPCs lead to a decrease in the MPC molecular capacitance, as evidenced from the slightly larger potential spacing between consecutive charging peaks encountered in this electrolyte: 0.136 V (PF6-), 0.135 V (ClO4-), 0.137 V (BF4-), and 0.143 V (NO3-). These ∆V correspond to CMPC of 1.17 aF (PF6-), 1.18 aF (ClO4-), 1.16 aF (BF4-), and 1.12 aF (NO3-). The surface coverage of the C6-Au147 MPC film, whose cyclic voltammograms are shown in Figure 2a, was estimated from the slope of the linear dependence of the first anodic peak current on the scan rate in solutions of the different electrolytes. Thus, comparable surface coverage values were obtained in the four media: 3.6 × 10-9 mol cm-2 (PF6-), 3.7 × 10-9 mol cm-2 (ClO4-), 3.8 × 10-9 mol cm-2 (BF4-), and 3.8 × 10-9 mol cm-2 (NO3-), which corresponds to about 110-115 layers of MPCs. As in the case of PF6- (previous section), the formal potentials for all charging steps were found to vary linearly with the natural logarithm of the anion concentration in the four electrolytic media. The ratios of the number of MPC-bound anions to the MPC charge state (pz:z) were determined from the slope of the linear regressions obtained for each electrolyte solution. Thus, two different types of ion-binding interactions were observed. For the most hydrophobic anions (ClO4-, BF4-), the slopes for all charging steps (-26.3 mV (0/+1), -27.6 mV (+1/ +2), -26.7 mV (+2/+3), and -28.2 mV (+3/+4) for ClO4- and -25.4 mV (0/+1), -25.1 mV (+1/+2), -26.5 mV (+2/+3),

and -30.2 mV (+3/+4) for BF4-) were very close to the -25 mV expected for a 1:1 ratio of pz:z, as was also the case in PF6- solutions. In contrast, the slopes obtained in NO3- solutions (-30.0 mV (0/+1), -43.6 mV (+1/+2)) indicate that the number of NO3- ions bound to MPCs (pz) is only 1 at z ) +1 but increases to almost 3 at z ) +2. These two types of ion-binding interactions for anions of different hydrophobicity have been previously reported for MPC multilayer films and ascribed to the above-mentioned differences in the effective interactions with the MPCs as a consequence of dissimilar degrees of ion hydration.21 That is, since NO3- is the most solvated anion, interactions with the MPCs are less effective and more ions are needed to compensate the MPC anodic charge. These results were corroborated by the mass variations measured by EQCM in each electrolyte media shown in Figure 2c (normalized by the molecular weight of each anion for ready comparison). Simultaneously monitoring the gravimetric response for the four electrolytes, together with the Q-E plot (Figure 2b), provided evidence of different ion-pairing trends. While the electrical charge in the anodic scan increases with potential with nearly identical slopes in the four electrolytes (although at different onset potentials), a corresponding agreement in the ∆m vs E responses for the four charging processes is only observed in the presence of the most hydrophobic anions (PF6-, ClO4-, and BF4-). In contrast, in NO3- solutions, the slope of the ∆m vs E plot is similar to that obtained for the other electrolytes only during the first charging step (MPC+1, +0.55 < E < +0.70 V) but increases significantly during the second charging process (+0.70 < E < +0.85 V). Thus, plots of ∆m divided by the molecular weight of each anion (MA-) vs Q/F data (that is, NA- vs z×NMPC) measured during the anodic scans in the four electrolytes exhibited excellent linearity (R2 > 0.99) for all the charging steps. However, the slopes for the linear regressions at each charging step, which represent the increment for the number of MPC-bound anions ac-

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Ruiz et al.

TABLE 1: Reflectance Changes at 700 nm of a C6-Au147 MPC Film at Different Charge States/Different Applied Potentials in Electrolyte Solutions of Varied Compositions MPC charge state (z) NH4PF6 0.1 M NH4ClO4 0.1 M NH4BF4 0.1 M NH4NO3 0.1 M

E (V)

+1

+2

+3

+4

+0.4 V

+0.6 V

+0.8 V

-0.014 -0.020 -0.015 -0.017

-0.029 -0.035 -0.030 -0.037

-0.045 -0.059 -0.047

-0.066 -0.081 -0.062

-0.019 -0.015 -0.004 -0.001

-0.045 -0.037 -0.022 -0.007

-0.067 -0.074 -0.047 -0.038

companying each oxidation process (pz-pz-1), differ between anions. Thus, while pz:z ratios of roughly 1:1 were obtained for all z for the most hydrophobic anions (PF6-, ClO4-, and BF4-), in NO3- solutions the ratio was only 1:1 for z ) +1 and increased to roughly 3:2 for z ) +2, as indicated by the higher slope (1.8) of the linear regression for the +1/+2 charging step. Hence, the ion-binding interactions directly quantified from EQCM measurements were consistent with those calculated from voltammetry data. The C6-Au147 MPC film difference reflectance spectra monitored during the voltammetric scan at anodic potentials corresponding to MPC charge states ranging from 0 to +4 (+2) in PF6-, ClO4-, and BF4- (NO3-) solutions, respectively, are shown in Figure 3. The shape of the reflectance spectra recorded in the different electrolyte solutions did not differ significantly nor did the dependence of the reflectance changes with the MPC redox state. However, the variation of reflectance (at a given wavelength, 700 nm) with the applied potential in each electrolyte is clearly influenced by the hydrophobicity of the anions (and hence ion-binding strength), as Figure 2d shows. The positive onset of the quantized electron-transfer reactions with decreasing hydrophobicity gives rise to a comparable shift of the spectroelectrochemical responses. Moreover, the magnitude of the reflectance changes with each electron-transfer process now varies with the electrolyte (Table 1), in contrast with the case studied above (in solutions of varied PF6concentrations), where each charging step resulted in equal reflectance drops. From the table, it can be seen that the magnitude of ∆R/R at this wavelength increases with z for the four anions with two different slopes: NH4PF6 ≈ NH4BF4 < NH4ClO4 ≈ NH4NO3. This seems to indicate that the intensity of reflectance changes measured at the film/solution interface might not only be dictated by the hydrophobicity (and hence amount of anions compensating charge) but also by the chemical nature of the anions (F- vs O-containing ions). In summary, the effect of the anion hydrophobicity on the film spectroelectrochemical response is twofold: first, a given MPC charge state is attained at a different potential in each electrolyte; second, the intensity of the reflectance drop for a certain MPC charge state also varies with the electrolyte anion. The combined effect makes it possible to induce, at a given applied potential, more pronounced changes of the optical response of a C6-Au147 MPC film by controlling not only the concentration but also the hydrophobicity of the electrolyte anion (Table 1). Potential Step Experiments: Influence of the Electrolyte Anion Hydrophobicity. Finally, the effect of the hydrophobicity of the anion on both the gravimetric and spectroelectrochemical response of the C6-Au147 MPC film was also investigated during double potential step chronocoulometric experiments, where the film was successively charged/discharged to increasing oxidation states. Given the positive onset of the peak potentials in the voltammetric profiles in the order PF6- < ClO4- < BF4- < NO3-, the electrode potential was stepped from -0.1 V to a series of different anodic values in each media in order to achieve the following MPC charge states: +1 e z e +4

Figure 4. Electrical charge (solid line) and changes in mass (dotted line) and film reflectance at 700 nm (dashed line) recorded simultaneously during successive double potential steps for charge-discharge of a C6-Au147 MPC film to increasingly anodic MPC charge states (in 0.1 M NH4PF6 aqueous solution). Applied oxidation potentials: +0.34 V (MPC+1), +0.48 V (MPC+2), +0.61 V (MPC+3), +0.74 V (MPC+4). Neutralization potential: -0.1 V. Duration of each potential step: 30 s. Inset: the mass increment during the first 4 s of each oxidation step (divided by the molecular weight of the electrolyte anion, PF6-) is plotted vs the corresponding electrical charge (divided by the Faraday constant).

(in 0.1 M solutions of PF6-, ClO4-, and BF4-) and +1 e z e +3 (in 0.1 M NO3-). The duration of each oxidation step was 30 s, after which the potential was reversed to -0.1 V for 30 s in order to discharge the film prior to the next oxidation step to a higher anodic potential. A representative plot of the behavior observed in the four electrolytes is given in Figure 4 showing the time evolution of the three responses (Q, ∆m, and ∆R/R at 700 nm) measured simultaneously during the four charge/discharge steps in 0.1 M NH4PF6. The reflectance spectra recorded in each electrolyte with the MPC film at differring oxidation states are comparable to those displayed in Figure 3. Hence the wavelength used in the voltammetric measurements was chosen to illustrate the optical behavior of the film. Overall, the shape of the three responses in NH4PF6 (and also in NH4BF4 and NH4 NO3) is akin to that observed in NH4ClO4 solutions previously.28 The main differences between electrolytes are highlighted in parts a and b of Figure 5 and will be discussed below. Film reflectance drops were observed during the anodic steps in the presence of all electrolytes and increased in magnitude with the MPC oxidation state (Figure 5a). As in the voltammetric experiments, the magnitude of the reflectance attenuations noted for each charging process varied between electrolytes. For instance, at 700 nm, two different attenuation rates with the MPC oxidation state were encountered: a shallower one in NH4PF6 and NH4BF4 and a steeper one in NH4ClO4 and NH4NO3. EQCM measurements (vide infra) will indicate that the different spectroelectrochemical responses cannot be solely accounted for

Tuning Spectroelectrochemistry of Au MPC Films

Figure 5. (a) Reflectance changes at 700 nm and (b) variation of mass (divided by the molecular weight of the electrolyte anion) during the double potential step charge/discharge of the C6-Au147 MPC film in 0.1 M solutions of the different supporting electrolytes (indicated in the figure legend).

by differences in the anion hydrophibicity. However, the maximum reflectance attenuation was reached within the first 3-4 s of each anodic pulse in all the electrolytes. The reflectance value remained constant during the rest of the oxidation steps and returns immediately to the initial value in the cathodic pulses. Thus, the spectroelectrochemical response, which is intimately linked to the MPC quantized electron-transfer processes, does not reveal significant differences in the kinetics of charge and discharge of the multilayer film in electrolyte anions of varied hydrophobicity (Figure 5a). This is further corroborated if we estimate the amount of MPCs in the film from the chronocoulometric data in the four electrolyte solutions as NMPC ) Q/zF, using Q values measured at the time when the maximum ∆R/R is reached in each oxidation step (between 3 and 4 s in all cases). The average NMPC values from the four (three in NO3-) charging steps were (5.0 ( 0.5) × 10-9 mol (in PF6-), (4.8 ( 0.9) × 10-9 mol (in ClO4-) (4.9 ( 0.3) × 10-9 mol (in BF4-), and (4.7 ( 0.5) × 10-9 mol (in NO3-). The agreement of these values with those estimated from the voltammetric experiments at varied scan rates (NMPC )AΓ) suggests that full film oxidation is achieved within that short time scale. Likewise, mass uptake only occurred concurrently with the reflectance drop in each anodic step, with a remarkable similarity to the chronocoulometric signal in the initial seconds (Figure 4). Both optical and gravimetric responses seem to indicate that QDL charging of the MPC film is the dominating contribution in this region of the chronocoulogram. Hence, we have taken only ∆m and ∆Q values corresponding to this time scale in order to investigate the ion-pairing interactions for each electrolyte anion. The inset of Figure 4 shows the excellent linearity of the plots of ∆m/MA- vs ∆Q/F (that is, z×NMPC) for all the charging steps recorded in the presence of PF6- (for 0 e t e 4 s). Similar linear plots were obtained for the other

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4283 electrolytes but with different slopes, as can be inferred from Figure 5b. The slopes of the linear regressions yielded pz:z ratios for each anion comparable to those found in the voltammetric experiments. Thus, a roughly 1:1 ratio (+1 e z e +4) was encountered in the most hydrophobic media (PF6-, ClO4-, and BF4-), whereas the pz:z ratios increased with z in NO3- solutions from 1:1 to 3:2 and 6:3, as given by the increasing slope values: 1.2 (z ) +1), 1.6 (z ) +2), and 2.2 (z ) +3). These values, determined from direct EQCM measurements, further confirm that more NO3- anions are needed to compensate the MPC charge due to the less effective interaction of this more solvated anion with the MPCs. The differences in anion hydrophobicity also give rise to different trends in the gravimetric responses during the rest of the anodic steps (once the spectral signal reaches a stable value) as seen in Figure 5b. While in the presence of hydrophobic anions (PF6-, ClO4-, and BF4-), a small mass loss was detected (that is more pronounced at z ) +3 and +4), in solutions of the most hydrated anion (NO3-), the mass value attained in the first seconds remained roughly constant during the whole anodic step. The mass loss could be explained by water repellence from the film caused by ion-pairing of the MPCs with the most hydrophobic anions. In view of the gravimetric signal, the different spectroelectrochemical responses cannot be solely interpreted on the basis of the amount of adsorbed anions and water at the film/solution interface. Rather, the dependence of the resulting ∆R/R on the electrolyte anion is more complex and seems to be also determined by the chemical nature of the anions. More studies are in progress to investigate this further. Here we have primarily demonstrated the possibility of tuning the electro-optical response of C6-Au147 MPC films by varying the electrolyte composition, with evident implications in the development of sensor nanodevices. Conclusions The results presented in this paper further illustrate the versatility of SEQCM as a powerful probe to investigate the spectroelectrochemical behavior of C6-Au147 MPC films in electrolyte solutions with anions of varied hydrophobicity (PF6-, ClO4-, BF4-, and NO3-). The analysis of the three simultaneous responses (electrochemical, gravimetric, and optical) supplied by SEQCM during cyclic voltammetry and potential step charging experiments has provided a deeper insight into the inter-relationship of the potential-induced reflectance changes and the MPC-ion binding chemistry in different electrolyte solutions. We have demonstrated that it is possible to control changes in the film reflectance by chemical manipulation (with the solution ions) of the discrete electron transfers to C6-Au147 MPC cores, i.e., the potential regulation of MPC rectified quantized charging by anions of different binding strength manifested in the spectroelectrochemical response of the MPC film. Furthermore, while changes of the electrolyte anion concentration only shifted the onset potential for the film reflectance attenuation, varying the hydrophobicity of the electrolyte anion affected both the onset potential and the magnitude of the spectroelectrochemical changes. Hence, this double effect makes it possible to induce, at a given applied potential, different optical responses for the MPC film by simply varying the nature of the supporting electrolyte anion. Moreover, the potential-induced reflectance changes are fast and reversible in the different electrolytic media, as shown by the doublepotential step charge/discharge experiments. Hence, we believe that the remarkable anion-sensitive electro-optical performance of these films offers great promise for sensor applications.

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