Persistent Multilayer Electrode Adsorption of Polycationic Au

Oct 8, 2010 - Tessa Carducci , Royce Murray ... Laura Cabo-Fernández , Dan F. Bradley , Simon Romani , Simon J. Higgins ... Sardar, Beasley and Murra...
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J. Phys. Chem. C 2010, 114, 18384–18389

Persistent Multilayer Electrode Adsorption of Polycationic Au Nanoparticles Christopher A. Beasley, Rajesh Sardar, Neil M. Barnes, and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina, 27599 ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: September 17, 2010

Multilayers of mixed monolayer thiolate-protected Au nanoparticles with polycationic ligand shells are strongly and persistently adsorbed on Pt electrodes from dilute CH3CN/electrolyte solutions of the nanoparticles. The adsorption is so robust that the electrodes can be transferred, with rinsing, to nanoparticle-free CH3CN/ electrolyte (and other organic solvents) and their voltammetry observed without significant desorption. The nanoparticles have the average composition [Au225(TEA-thiolate+)X(SC6Fc)Y][ClO4-lX, where X ) 18, 22, or 27. The cation sites are a quaternary ammonium-terminated ligand (TEA-thiolate+ ) -S(CH2)11N(CH2CH3)3+). The ferrocene content of the ligand shell (Y) allows voltammetric detection of the adsorption, and also affects it. This work describes how the extent of nanoparticle adsorption (surface coverage, ΓNP mol/cm2) depends on the manner of exposure of the electrode (with potential scanning, at a fixed potential, or at open circuit) to the nanoparticle solution, and on the nanoparticle concentration, the electrolyte and its concentration, and on the mixed monolayer composition. Increase of the cationic TEA-thiolate+ component of the mixed monolayer from X ) 18 to 27 increases, for a given exposure mode, the attained amount of adsorption from < one monolayer to > two monolayers, and broadens the voltammetric wave. The adsorption phenomenon is interpreted as induced by multiple ion-pair bridges between cation sites and electrolyte anions, and as such represents an entropic consequence of multiple interactions between nanoparticles and between nanoparticles and the electrode. 1. Introduction The electrochemistry of thiolate-protected Au nanoparticles (NPs) and the chemistry of their interactions with electrode surfaces and with other nanoparticles is an actively expanding topic. The Au NPs have an important impact in molecular1,2 and electronic sensor applications3,4 and in device fabrication.5,6 Electrochemistry intrinsic to the Au nanoparticle itself has been delineated7 into bulk-continuum, quantized double layer charging, and molecule-like forms of voltammetry. Using the thiolate ligation chemistry, the Au NPs can be functionalized with redox moieties to produce interesting multielectron reactivities.7-10 Films on planar substrates based on metal nanoparticles as building blocks have attracted substantial interest for their optical, electrical11-13 and stability14 properties. Applications of Au NP films in chemiresistors and in biological sensing have been reported by various groups.7,15-19 Many of the envisioned applications of NPs (Au and others) involve attaching them to electrode surfaces, either as submonolayer or thicker films, and so film-making chemistry has seen particular attention. The attachment chemistries have exploited both covalent and ionic interactions.15,20-24 Ionic interactions are used in “layer-by-layer” (LBL) depositions13,21,25-29 wherein electrostatic ionic interactions between functionalized nanoparticles and polymer electrolytes, in serial exposures to solutions of each, promote the formation of thicker NP films. Multilayer Au NP films can also be prepared based on ligand/ metal ion/ligand cross-linking.20,22,29 We have recently contributed to Au NP multielectron and film-forming chemistry using solely electrostatic interactions, in reports of the irreversible adsorption of fully ferrocenated and mixed monolayer Au NPs on bare electrodes and on * To whom correspondence should be addressed. E-mail: rwm@ email.unc.edu.

electrodes modified with self-assembled monolayers (SAMs).29-33 The strong NP adsorption was attributed to multiple ion-pair formation between electrode-adsorbed anions and ferrocenium sites on the Au NPs. Here, we describe this remarkably strong adsorption in more detail, using Au NPs with mixed monolayers of quaternary ammonium thiolate and ferrocene thiolate ligands (respectively, -S(CH2)11N(CH2CH3)3+ (abbrev. TEA-thiolate+) and -S(CH2)5CH3 (abbrev. SC6Fc). The mixed monolayer Au NPs have cores33 of (avg.) Au225 and are designated [Au225(TEAthiolate+)X(SC6Fc)Y][ClO4-lX where X + Y ) 31 and X ) 18, 22, or 27. The quaternary ammonium sites make the Au NP strongly cationic (equivalent to a NP poly electrolyte), and the ferrocene sites provide a redox label enabling measurement of the extent of NP film formation (as a surface coverage ΓNP). These NPs adsorb on Pt electrodes from dilute CH3CN/ electrolyte solutions spontaneously and so robustly that the electrode plus absorbed NP film can be transferred, with rinsing, to nanoparticle-free CH3CN/electrolyte (and other organic solvents), and examined voltammetrically without significant desorption. A previous report described33 how an approximately monolayer adsorbed film of Au225(TEA-thiolate+)22(SC6Fc)9 can exhibit phase-like behavior, resulting in shifts in the apparent ferrocene+/0 formal potential attributable to electrolyte iontransfer energetics. The present report shows that adsorption of [Au225(TEA-thiolate+)X(SC6Fc)Y][ClO4-lX NPs from CH3CN/ electrolyte solutions occurs spontaneously at any applied (constant) electrode potential, and is lessened (but not eliminated) by negative applied potentials. To explore the adsorption’s dependence on other parameters, a briefer exposure (single cyclical potential scan) was used to show how ΓNP additionally depends on the NP cationic charge, on the chosen electrolyte anion and the electrolyte concentration, and on the use of other

10.1021/jp1065665  2010 American Chemical Society Published on Web 10/08/2010

Adsorption of Polycationic Au Nanoparticles

Figure 1. The cartoon represents adsorption of a film of NPs via ionpair formation between adsorbed anions and quaternary ammonium groups. NP-NP bridging ion-pairs aid in formation of a second adsorbed monolayer. During voltammetric oxidation of ferrocene sites, ion transfer occurs at the film/solution interface.

organic solvents. To assess the dependence of adsorbed ΓNP on the NP concentration in the adsorption solution, adsorption was allowed to occur for a fixed time at electrode “open circuit”. The three above experimental protocolss“constant potential”, “potential scanning”, and “open circuit”sare further detailed in the Experimental Section. The results support and are consistent with the Figure 1 model of an entropically driven NP adsorption based on multiple ion associations of cationic NP sites with electrode-adsorbed electrolyte anions. 2. Experimental Section 2.1. Chemicals. 11-Bromo-1-undecene (>95%), triethylamine (Et3N, >99%), thioacetic acid (>98%), t-butylammonium borohydride (Bu4NBH4, >98%) t-octylammonium bromide (Oct4NBr, >98%), sodium borohydride (NaBH4, >98%), t-butylammonium perchlorate (Bu4NClO4, >99%), lithium perchlorate (LiClO4, >98%), t-butylammonium p-toluenesulfonate (Bu4NC7H7SO3, puress), t-butylammonium hexafluorophosphate (Bu4NPF6, puress) and t-ethylammonium perchlorate (Et4NClO4, >99%) from Aldrich, and toluene (reagent grade), acetonitrile (Optima), methylene chloride (HPLC grade), tetrahydrofuran (HPLC grade), and ethanol (HPLC grade) from Fisher were used as received. HAuCl4 xH2O (from 99.999% pure gold) was synthesized using a literature procedure34 and stored in a freezer at -20 °C. Water was purified using a Barnstead NANOpure system (18 MΩ). 2.2. Synthesis of Ferrocene Hexanethiol. Ferrocene hexanethiol (HSC6Fc) was synthesized as previously described.35 1 H NMR (400 MHz, CD2Cl2) of the thiol gave the appropriate NMR peaks: δ ) 4.0 (Fc, s, 9 H), 2.49 (CH2SH, q, 2H J ) 7.2 Hz), 2.30 (CH2Fc, t, 2H, J ) 7.6 Hz), 1.56 (CH2CH2SH, m, 2 H), 1.46 (CH2CH2Fc, m, 2 H), and 1.32 (m, 4 H) ppm, with no dithiol impurity peaks (t, 4H, 2.66 ppm) present and no significant line-broadening, indicating that nearly all ferrocene groups were in the reduced state. 2.3. Synthesis of N, N,N-Triethyl(11-undecylmercapto)ammonium Chloride (TEA-Thiolate). The thiol was synthesized as previously reported.36,37 2.4. Synthesis of Quaternary Ammonium Thiolate Functionalized Au NPs. The TEA-thiolate+ protected Au NPs were synthesized as previously described.33 The average ligand count per Au225 core is 31, by 1H NMR with an internal calibrant. This is a lower ligand count than seen for fully ferrocenated

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18385 Au225 NP, presumably reflecting a combination of steric and electrostatic effects.30-32 Three different mixed monolayer NPs were prepared by ligand exchange between Au225(TEA-thiolate+)31 and HSC6Fc using 1:0.5, 1:1, or 1:2 mol ratios of HSC6Fc:TEA-thiolate+ ligands, producing respectively, mixed monolayer NPs of average composition Au225(TEA-thiolate+)27Fc4, Au225(TEAthiolate+)22Fc9, and Au225(TEA-thiolate+)18Fc13. The exchange involved stirring a 50-mL ethanol solutions of ∼0.20 g of Au225(TEA-thiolate+)31 and 0.042 g of HSC6Fc for 4 h at room temperature, followed by addition of 0.10 g LiClO4, which precipitated the NPs as a black solid. The solution was stirred for 30 additional min; the solid was then collected on a fine glass frit and washed with ethanol. The extent of ligand exchange was determined by 1H NMR. The isolated NPs are assumed to be ClO4- salts. Most of the experiments described used the [Au225(TEA-thiolate+)22(SC6Fc)9][ClO4-]22 NP salt. 2.5. Electrochemistry. Voltammetry was typically done in 0.05 mM Au225(TEA-thiolate+)22(SC6Fc)9/CH3CN solutions containing various concentrations of electrolyte, using a 1.6 mm diameter diameter Pt disk, Pt wire, and Ag/AgCl/3 M KCl (aq) working, counter, and reference electrodes, respectively. A roughness factor of 2.6 was determined for the 1.6 mm working electrode from the charge under the hydrogen desorption peak in 0.10 M H2SO4.38 Measurements were performed using a CH Instruments (Austin, TX) Model 760C electrochemical analyzer. Adsorption of the NPs was done using three experimental protocols in which the polished, clean electrode was exposed to NP solutions with “potential scanning”, “constant potential”, or at “open circuit”. In all cases, the electrode was repolished between experiments to remove adsorbed NP. “Potential scanning” involved a single cyclical scan at 0.5 V/s from -0.1 to 1.0 V vs Ag/AgCl in unstirred 0.05 mM NP CH3CN solutions containing 1.0, 0.1, or 0.01 M Bu4NClO4, or 1.0 M Bu4N+X(X- ) PF6- or p-toluene sulfonate-). The electrode was then rinsed and transferred to a NP-free solution of the same electrolytesbut sometimes a different solventsat the same concentration for voltammetric study at 0.5 V/s. The quantity of adsorbed NP (surface coverage, ΓNP, mol NP/cm2) was determined from the charge, Q, under the ferrocene oxidation peak using

Q ) nAmFΓNP

(1)

where n is the average number of ferrocenes per NP and Am is the roughness-corrected Pt working electrode area. For comparison, based on an overall NP radius ) ∼3.2 nm and a corresponding 32.2 nm2 geometrical footprint, a model monolayer of Au225(TEA-thiolate+)22(SC6Fc)9 is estimated as ∼5.2 × 10-12 mol NP/cm2. Experimental results to be presented in Figures 2, 4, 5, and 7 were obtained using the potential-scanning protocol. Because the ΓNP measurements do not represent true equilibrium quantities, reproducibility of the data was generally no better than 10-20%, provided experimental conditions were well-replicated. In the “constant potential” adsorption protocol, leading to results of Figure 3, the freshly polished Pt electrode was exposed to 0.05 mM Au225(TEA-thiolate)22(SC6Fc)9 in 1.0 M Bu4NClO4/ CH3CN at a constant applied potential (from -0.1 to -1.0 V vs Ag/AgCl (aq)) for 5 min, rinsed with CH3CN and transferred to NP-free 1.0 M Bu4NClO4/CH3CN. The NP surface coverage was determined by voltammetrically scanning through the ferrocene wave of the adsorbed NP, as above.

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Figure 2. Cyclic voltammetry at V ) 0.5 V/s of adsorbed NPs with varied numbers of SC6Fc ligands (Y) in 1.0 M Bu4NClO4/CH3CN. The adsorbed films were prepared by “potential scanning” protocol (V ) 0.5 V/s) in 0.05 mM Au225(TEA-thiolate+)27(SC6Fc)4 or Au225(TEAthiolate+)22(SC6Fc)9 or Au225(TEA-thiolate+)18(SC6Fc)13 solutions in 1.0 M Bu4NClO4/CH3CN, followed by transfer to NP-free 1.0 M Bu4NClO4/CH3CN electrolyte.

In the open circuit adsorption protocol, the Pt working electrode was exposed to unstirred 0.1 M Bu4NClO4/CH3CN solutions of Au225(TEA-thiolate)22(SC6Fc)9, at NP concentrations from 0.025 to 0.2 mM, for 15 min. The electrode was rinsed with CH3CN, transferred to NP-free 0.1 M Bu4NClO4/CH3CN, and ΓNP determined as above, giving results in Figure 6. While these experiments are conducted using a Pt working electrode, other tests show that adsorption is also induced using Au or carbon electrodes. 3. Results and Discussion The previously presented cartoon (Figure 1) of the Au NP adsorbed film contains several important features. Foremost is the hypothesis that ion pair bridges between electrolyte anions adsorbed to the electrode and the cation-functionalized NP drive the adsorption chemistrysthis idea has been supported by tests in which electrode surface charge is deliberately supplied using thiolate-on-Au self-assembled monolayers (SAMs).32 Second, the ion association linkages in the cartoon need not be individually strong; the effective irreversibility of the adsorption is based on there being multiple linkagessyielding an entropic effect analogous to metal chelation.39 Thus, higher cationic charge on the NP should produce stronger adsorption and putatively, higher surface coverages (ΓNP). These features are further scrutinized in the experiments presented below. The role of electrolyte anion adsorption is examined as a function of the effect of the potential applied during “constant potential” adsorption, and of the specific chosen solvent and electrolyte anion. The role of NP charge is examined by changing the proportion of TEA-thiolate+ ligands on the Au225 NP. We last briefly consider, in turn, effects influencing the voltammetric waveshape, the NP-NP ion pair bridging interactions that lead to multimonolayer surface coverage, and the shifts in apparent E°′ formal potentials of the Fc+/0 couple. We begin with the role of NP and adsorbed anion charges. 3.1. Adsorption of Au225(TEA-Thiolate+)X(SC6Fc)Y with Varied Y, at Different Applied Potentials, With Different Electrolyte Anions, And Stability in Different Solvents. Au NPs were prepared with average mixed monolayer compositions of Au225(TEA-thiolate+)18(SC6Fc)13, Au225(TEA-thiolate+)22(SC6Fc)9, and Au225(TEA-thiolate+)27(SC6Fc)4. Figure 2 shows cyclic voltammograms of these three NPs that have been adsorbed from 1.0 M Bu4NClO4/CH3CN electrolyte onto the

Beasley et al.

Figure 3. Dependence of ΓNP coverage on the potential applied during NP adsorption by “constant potential” adsorption protocol (5 min in unstirred 0.05 mM Au225(TEA-thiolate+)22(SC6Fc)9 in 1.0 M Bu4NClO4/ CH3CN, followed by transfer to NP-free 1.0 M Bu4NClO4/CH3CN electrolyte. ΓNP measured by a -0.1 T 1.0 V cyclical scan at V ) 0.5 V/s.

Pt working electrode in the “potential scanning” protocol (see Experimental Section). The Pt electrode has been transferred to NP-free 1.0 M Bu4NClO4/CH3CN electrolyte solution. The quantity of adsorbed NP, as judged from the charge under the Fc+/0 wave, shows that somewhat under a full monolayer of NP becomes adsorbed when Y ) 13, but ΓNP increases to the equivalent of ∼2 monolayers of NP for Y ) 9 and 4. (Recall that monolayer coverage is estimated as ∼5.2 × 10-12 mol NP/ cm2, that ΓNP is normalized for the number of ferrocene/NP, and that experimental ΓNP is corrected for microscopic electrode area.) It is clear that increased NP ionic charge (X) enhances the extent of adsorption, and drives it above a single monolayer level. In our previous study of Au225 NPs coated solely with ferrocene thiolate ligands on bare electrodes,31 the effective NP charge was less positive because the NPs were adsorbed from a (mostly) neutral ferrocene state, and obtained surface coverages were generally less than a monolayer of NPs. The Au225(TEA-thiolate+)22(SC6Fc)9 NP was chosen for all subsequent adsorption experiments. Enhanced electrolyte anion adsorbability also enhances the extent of adsorption of this NP as shown in Figure 3. Figure 3 presents the quantity of NP adsorption observed when the “constant potential” protocol is used to induce adsorption from a NP solution in 1.0 M Bu4NClO4/CH3CN electrolyte, again transferring the electrode to NP-free 1.0 M Bu4NClO4/CH3CN electrolyte to measure ΓNP using the ferrocene voltammetry. When the adsorption-inducing step is conducted at 0 V vs. Ag/AgCl, ΓNP is equivalent to roughly two monolayers of NPs. As the potential at which adsorption is induced is made more negative, by which the surface population of electrode-adsorbed ClO4- anions should diminish, the resulting ΓNP accordingly also decreases as expected. ΓNP is not depressed to zero, however, implying that the anion adsorption is to a degree nanoparticle-induced and that lateral interactions (Figure 1) are also important. Figure 4 shows that ΓNP is also sensitive to the chosen electrolyte anion, in a “potential scanning” experiment remaining at near two monolayers for PF6- and ClO4- but falling somewhat when using the less polar, better CH3CN-solvated tosylate anion. In contrast, for Au225 NPs coated solely with ferrocene thiolate ligands, ΓNP fell to zero when using tosylate electrolyte.31 It would appear that the larger cationic charge of the Au225(TEAthiolate+)22(SC6Fc)9 NP (via the TEA-thiolate+ ligands) can compensatesin inducing counterion adsorption on the electrodesfor a more weakly adsorbing electrolyte anion (e.g., tosylate).

Adsorption of Polycationic Au Nanoparticles

Figure 4. Cyclic voltammetry of adsorbed Au225(TEA-thiolate+)22(SC6Fc)9 film prepared by potential scanning protocol (0.05 mM Au225(TEA-thiolate+)22(SC6Fc)9 in 1.0 M electrolyte/CH3CN, V ) 0.5 V/s); followed by transfer to NP-free 1.0 M electrolyte/CH3CN, where electrolyte ) Bu4NC7H7SO3, Bu4NClO4, or Bu4NPF6, as indicated.

Figure 5. Cyclic voltammetry of adsorbed Au225(TEA-thiolate+)22(SC6Fc)9 film prepared by potential scanning protocol (0.05 mM Au225(TEA-thiolate+)22(SC6Fc)9 in 0.1 M Bu4NClO4/CH3CN, V ) 0.5 V/s), followed by transfer to NP-free 0.1 M Bu4NClO4/solvent, where the solvent is indicated in the graphic.

The adsorption of Au225(TEA-thiolate+)22(SC6Fc)9 NPs persists in a variety of organic solvents, as shown by the data in Figure 5. These voltammograms of adsorbed NPs were obtained by “potential scanning” adsorption from 0.05 mM Au225(TEAthiolate+)22(SC6Fc)9 in 0.1 M Bu4NClO4/CH3CN, followed by transfer, with rinsing (with CH3CN) to the solvents indicated in the figure. There are obvious differences in uncompensated resistance effects in the different solventss∆EPEAK varies from 28 to 305 mVsbut similar values of ΓNP are observed in all. The voltammograms in the different solvents were stable over multiple cyclical potential scans; ΓNP decreased by only ∼3% after 25 potential cycles as reported in our previous publication.33 However, the NP adsorption was not stable when the electrode was transferred to aqueous electrolyte, dissipating after a few cyclical potential scans. 3.2. Adsorption Isotherm of Au225(TEA-Thiolate+)22(SC6Fc)9. In the experiments above, adsorption was induced with a potential applied to the electrode. In previous work with Au225 NPs coated solely with ferrocene thiolate ligands,31 adsorption occurred when the electrode was simply immersed at open circuit in the NP solution, but ΓNP was appreciably smaller than when adsorption was induced by “potential scanning” through the ferrocene wave. Ferrocenated dendrimers are also known to adsorb at open circuit.40 A crude adsorption isotherm was taken for Au225(TEAthiolate+)22(SC6Fc)9 adsorption on Pt electrode immersed at “open circuit” in unstirred 0.1 M Bu4NClO4/CH3CN solutions of NPs for 15 min. Longer exposure times were not investigated.

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Figure 6. Dependence of ΓNP coverage observed in “open circuit” 15 min. (unstirred) exposure of electrode to 0.025 to 0.20 mM of Au225(TEA-thiolate+)22(SC6Fc)9 in 0.1 M Bu4NClO4/CH3CN, followed by transfer to NP-free 0.1 M Bu4NClO4/CH3CN. ΓNP determined by a -0.1 T 1.0 V cyclical scan at V ) 0.5 V/s.

The amount of adsorption, determined by transfer to fresh NPfree 0.1 M Bu4NClO4/CH3CN, as a function of the NP concentration in the adsorption solution, is shown in Figure 6. The peak shape of the Fc0/+ wave was the same as for films adsorbed using potential scanning experiments. The ΓNP saturated at slightly over 2 equivalent NP monolayers. While the isotherm has a Langmuir-like appearance; the data quality precludes an exacting analysis. Given the multiple parameters inspected above that influence the amount of adsorption, any isotherm parameters would presumably represent only a single specific set of conditions. The isotherm does, however, make clear that the energetics of forming the first NP monolayer, where the anchoring chemistry (Figure 1) is proposed to be ion-pair bridges to electrode-adsorbed electrolyte anions, and of forming a second monolayer, are not sharply different. Otherwise, a double-plateau isotherm might result, as seen in other work.41 The energetic differences between the multiple ion-pair bridges between NP TEA-thiolate+ cation sites and electrode-adsorbed electrolyte anions and NP-NP ion-pair bridges with electrolyte anions must be subtle, not sharply different. Indeed, precipitation from older solutions was occasionally seen in the present work. Further, in unpublished experiments42 with NPs having monolayers terminated with partially neutralized carboxylic acid groups, >400 equivalent monolayers of adsorbed NPs could be seen to accumulate on electrodes upon repeated positive potential excursions, and aggregates of NPs precipitate when the monolayers are further neutralized. Thus, if the factors favoring ionpair bridging association between NPs become sufficiently favorable, then it can be expected43 that extensive associations and ultimately precipitations can occur from the bulk solution phase. Lastly, we remember that the strong and essentially irreversible NP adsorption involves multiple ion association interactions, much like the stabilization factors in polymer electrolyte “layerby-layer” formation of multilayer films on solid supports.43-47 This is understood as basically an entropic effect created by release of multiple solvent and small counterions from the polyelectrolyte structures. 3.3. Ferrocene Waveshapes and Formal Potentials for Adsorbed NPs. Inspection of the voltammetry in the preceding Figures reveals some interesting waveshapes, which are commented upon here. Very narrow voltammetric peaks have been seen31,32 for adsorbed Au225 NPs that bear solely ferrocene thiolate ligandssdown to 35 mV Efwhm. Ideally, for surface confined, noninteracting electroactive sites the theoretical Efwhm for a monolayer is 90.6 mV and is based on a linear relationship

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Beasley et al. explicitly expected from the previous results; that in Figure 2 should ideally from the theory33 be 60 mV per decade of ferrocene site concentration in the adsorbed NP film.50 4. Conclusions

Figure 7. Cyclic voltammetry of adsorbed Au225(TEA-thiolate+)22(SC6Fc)9 film prepared by potential scanning protocol (0.05 mM Au225(TEA-thiolate+)22(SC6Fc)9 in 1.0, 0.1, or 0.01 M Bu4NClO4/ CH3CN, V ) 0.5 V/s); followed by transfer to NP-free Bu4NClO4/ CH3CN at the same electrolyte concentration.

between surface coverage and activity.38 We have discussed30-32 this wave-narrowing effect in terms of various kinds of interactions between the ferrocene ligands on the NPs, both laterally between NPs and intraligand shell, but further work33 has brought out phase-like behavior that may reflect some more coherent film organization. Figure 4 shows that for a given NP, the Efwhm varies with the electrolyte anionsthe PF6- anion promoting narrow peaks and the tosylate anion the opposite. The lower ΓNP for tosylate was discussed above in terms of its expected stronger solvation, relative to the other electrolyte anions; the larger voltammetric Efwhm in tosylate solutions imply correspondingly better solvation (weaker attractive ion-ion interactions31) of an adsorbed NP layer containing this ion as the TEA-thiolate+ counterion. Oppositely, the PF6- electrolyte anion produces waveshapes that are sharper at the peaks than at the base, which could imply47 a variability of ferrocene formal potentials induced by variable anion-induced Fc+-Fc+ associations. Also interesting is the systematic trend of Figure 2, where Efwhm ) 130, 89, and 83 mV for increasing ferrocene content (Y ) 4, 9, and 13, respectively). The voltammetric peaks become more narrow when more ferrocene sites are present, which is expected based on ferrocene-ferrocene interactions observed when the -SC6Fc ligand population is high31,32 and where the terminal groupings were ferrocenes,31,32 a steric circumstance promoting Fc-Fc NP-NP interactions. For the NPs in Figure 2, the short linkers for the SC6Fc ligands, relative to those (C11) of the TEA-thiolate+ ligands, should promote steric screening of NP-NP interactions between ferrocene sites, and the terminal groupings being cationic TEA-thiolate+ sites should promote repulsive NP-NP interactions for increased populations of cationic sites (e.g., decreased X). In the case of fewer ferrocene sites (Y ) 4), the Efwhm ) 130 mV, could represent a repulsive interaction between ferrocenium and cationic TEA-thiolate+ sites. With regards to Fc+/0 formal potentials, systematic shifts in the apparent ferrocene E°′ values can be seen in the voltammetry of Figures 2 and 7. In Figures 2 and 7,31,32 the apparent Fc+/0 E°′ values shift to more positive potentials with, respectively, increasing ferrocene site concentration and with decreasing electrolyte concentration. These shifts are consistent with our previous observations and analysis of ion transfer potentials,33 where the adsorbed NP film acts as a permselective phase, admitting electrolyte anions and associated solvent in concert with ferrocene site oxidation and consequent counterion influx demand for charge neutrality.48,49 The shift in Figure 7 is

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