Synthesis, Electrochemistry, and Excited-State Properties of Three Ru(II) Quaterpyridine Complexes.

The complexes [Ru(qpy)LL'](2+) (qpy = 2,2':6',2″:6″,2‴-quaterpyridine), with 1: L = acetonitrile, L'= chloride; 2: L = L'= acetonitrile; and 3: L = L'= vinylpyridine, have been prepared from [Ru(qpy) (Cl)2]. Their absorption spectra in CH3CN exhibit broad metal-to-ligand charge transfer (MLCT) absorptions arising from overlapping (1)A1 → (1)MLCT transitions. Photoluminescence is not observed at room temperature, but all three are weakly emissive in 4:1 ethanol/methanol glasses at 77 K with broad, featureless emissions observed between 600 and 1000 nm consistent with MLCT phosphorescence. Cyclic voltammograms in CH3CN reveal the expected Ru(III/II) redox couples. In 0.1 M trifluoroacetic acid (TFA), 1 and 2 undergo aquation to give [Ru(II)(qpy)(OH2)2](2+), as evidenced by the appearance of waves for the couples [Ru(III)(qpy)(OH2)2](3+)/[Ru(II)(qpy)(OH2)2](2+), [Ru(IV)(qpy)(O)(OH2)](2+)/[Ru(III)(qpy)(OH2)2](3+), and [Ru(VI)(qpy)(O)2](2+)/[Ru(IV)(qpy)(O)(OH2)](2+) in cyclic voltammograms.

Electronic conductivity of films of electroflocculated 2 nm iridium oxide nanoparticles.

The electronic conductivity of films of iridium oxide (IrO(x)) composed of ca. 2 nm nanoparticles (NPs) is strongly dependent on the film oxidation state. The Ir(IV)O(x) NPs can be electrochemically converted to several oxidation states, ranging from Ir(III) to Ir(V) oxides. The NP films exhibit a very high apparent conductivity, e.g., 10(-2) S cm(-1), when the NPs are in the oxidized +4/+5 state. When the film is fully reduced to its Ir(III) state, the apparent conductivity falls to 10(-6) S cm(-1).

Temperature dependence of solid-state electron exchanges of mixed-valent ferrocenated Au monolayer-protected clusters.

Electron transfers (ETs) in mixed-valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, the presence of ~1:1 mixed-valent ferrocenated thiolates in the organothiolate ligand shells of <2 nm diameter Au225, Au144, and Au25 monolayer-protected clusters (MPCs) exerts a retarding effect on ET between them at and below room temperature. Near room temperature, in dry samples, bimolecular rate constants for ET between organothiolate-ligated MPCs are diminished by the addition of ferrocenated ligands to their ligand shells. At lower temperatures (down to ~77 K), the thermally activated (Arrhenius) ET process dissipates, and the ET rates become temperature-independent. Among the Au225, Au144, and Au25 MPCs, the temperature-independent ET rates fall in the same order as at ambient temperatures: Au225 > Au144 > Au25. The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands and are primarily determined by the Au nanoparticle core size.

Solution voltammetry of 4 nm magnetite iron oxide nanoparticles.

The voltammetry of solution-dispersed magnetite iron oxide Fe3O4 nanoparticles is described. Their currents are controlled by nanoparticle transport rates, as shown with potential step chronoamperometry and rotated disk voltammetry. In pH 2 citrate buffer with added NaClO4 electrolyte, solution cyclic voltammetry of these nanoparticles (average diameter 4.4 ± 0.9 nm, each containing ca. 30 Fe sites) displays an electrochemically irreversible oxidation with E(PEAK) at ca. +0.52 V and an irreversible reduction with E(PEAK) at ca. +0.2 V vs Ag/AgCl reference electrode. These processes are presumed to correspond to the formal potentials for one-electron oxidation of Fe(II) and reduction of Fe(III) at their different sites in the magnetite nanoparticle structure. The heterogeneous electrode reaction rates of the nanoparticles are very slow, in the 10(-5) cm/s range. The nanoparticles are additionally characterized by a variety of tools, e.g., TEM, UV/vis, and XPS spectroscopies.

Kinetics and low temperature studies of electron transfers in films of small (<2 nm) Au monolayer protected clusters.

This work examines the temperature dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)18(0/1-), Au25(SR)18(1+/0), and Au144(SR)60(1+/0), where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temperature and for ca. 1:1 mol:mol mixed valencies, the bimolecular ET rate constants (assuming a cubic lattice model) are ~2 × 10(6) M(-1) s(-1) for Au25(SR)18(0/1-), ~3 × 10(5) M(-1) s(-1) for Au25(SR)18(1+/0), and ~1 × 10(8) M(-1) s(-1) for Au144(SR)60(1+/0). Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, respectively. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temperature but, among the different MPCs, fall in the same order of ET rate: Au144(+1/0) > Au25(0/1-) > Au25(1+/0).

Formation of iridium(IV) oxide (IrOX) films by electroflocculation.

Films of iridium(IV) oxide nanoparticles (IrOX NPs) become deposited on electrodes from nanoparticle solutions when potentials sufficient to initiate water oxidation are applied. Evidence is given that the film-forming mechanism is nanoparticle precipitation. Following an induction period during which a significant amount of charge is passed, the NPs begin to deposit as islands. It appears that the proton release that accompanies nanoparticle oxidation triggers the nanoparticle electroflocculation and subsequent precipitation. Flocculation from nanoparticle solutions can also be induced by the addition of a chemical oxidant (Ce(IV)). The film formation is followed by cyclic voltammetry (CV), rotated ring disk voltammetry (RRDE), and electrochemical quartz crystal microbalance (eQCM) measurements, supplemented with AFM and SEM microscopies.

Synthesis and electrochemistry of 6 nm ferrocenated indium-tin oxide nanoparticles.

Indium-tin oxide (ITO) nanoparticles, 6.1 ± 0.8 nm in diameter, were synthesized using a hot injection method. After reaction with 3-aminopropyldimethylethoxysilane to replace the initial oleylamine and oleic acid capping ligands, the aminated nanoparticles were rendered electroactive by functionalization with ferrocenoyl chloride. The nanoparticle color changed from blue-green to light brown, and the nanoparticles became more soluble in polar solvents, notably acetonitrile. The nanoparticle diffusion coefficient (D = 1.0 × 10(-6) cm(2)/s) and effective ferrocene concentration (C = 0.60 mM) in acetonitrile solutions were determined using ratios of DC and D(1/2)C data measured by microdisk voltammetry and chronoamperometry. The D result compares favorably to an Einstein-Stokes estimate (2.1 × 10(-6) cm(2)/s), assuming an 8 nm hydrodynamic diameter in acetonitrile (6 nm for the ITO core plus 2 nm for the ligand shell). The ferrocene concentration result is lower than anticipated (ca. 1.60 mM) based on a potentiometric titration of the ferrocene sites with Cu(II) in acetonitrile. Cyclic voltammetric data indicate tendency of the ferrocenated nanoparticles to adsorb on the Pt working electrode.

Electron transfer dynamics of iridium oxide nanoparticles attached to electrodes by self-assembled monolayers.

Self-assembled monolayers (SAMs) of carboxylated alkanethiolates (-S(CH(2))(n-1)CO(2)(-)) on flat gold electrode surfaces are used to tether small (ca. 2 nm d.) iridium(IV) oxide nanoparticles (Ir(IV)O(X) NPs) to the electrode. Peak potential separations in cyclic voltammetry (CV) of the nanoparticle Ir(IV/III) wave, in pH 13 aqueous base, increase with n, showing that the Ir(IV/III) apparent electron transfer kinetics of metal oxide sites in the nanoparticles respond to the imposed SAM electron transfer tunneling barrier. Estimated apparent electron transfer rate constants (k(app)(0)) for n = 12 and 16 are 9.8 and 0.12 s(-1). Owing to uncompensated solution resistance, k(app)(0) for n = 8 was too large to measure in the potential sweep experiment. For the cathodic scans, coulometric charges under the Ir(IV/III) voltammetric waves were independent of potential scan rate, suggesting participation of all of the iridium oxide redox sites (ca. 130 per NP) in the NPs. These experiments show that it is possible to control and study electron transfer dynamics of electroactive nanoparticles including, as shown by preliminary experiments, that of the electrocatalysis of water oxidation by iridium oxide nanoparticles.

The story of a monodisperse gold nanoparticle: Au25L18.

Au nanoparticles (NPs) with protecting organothiolate ligands and core diameters smaller than 2 nm are interesting materials because their size-dependent properties range from metal-like to molecule-like. This Account focuses on the most thoroughly investigated of these NPs, Au(25)L(18). Future advances in nanocluster catalysis and electronic miniaturization and biological applications such as drug delivery will depend on a thorough understanding of nanoscale materials in which molecule-like characteristics appear. This Account tells the story of Au(25)L(18) and its associated synthetic, structural, mass spectrometric, electron transfer, optical spectroscopy, and magnetic resonance results. We also reference other Au NP studies to introduce helpful synthetic and measurement tools. Historically, nanoparticle sizes have been described by their diameters. Recently, researchers have reported actual molecular formulas for very small NPs, which is chemically preferable to solely reporting their size. Au(25)L(18) is a success story in this regard; however, researchers initially mislabeled this NP as Au(28)L(16) and as Au(38)L(24) before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theoretical investigations. In addition, Au(25)L(18)'s accessibility in pure form and molecule-like properties make it an attractive research target. The properties of this NP include a large energy gap readily seen in cyclic voltammetry (related to its HOMO-LUMO gap), a UV-vis absorbance spectrum with step-like fine structure, and NIR fluorescence emission. A single crystal structure and theoretical analysis have served as important steps in understanding the chemistry of Au(25)L(18). Researchers have determined the single crystal structure of both its "native" as-prepared form, a [N((CH(2))(7)CH(3))(4)(1+)][Au(25)(SCH(2)CH(2)Ph)(18)(1-)] salt, and of the neutral, oxidized form Au(25)(SCH(2)CH(2)Ph)(18)(0). A density functional theory (DFT) analysis correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au(13) core stabilized by six Au(2)(SR)(3) semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au(25) NP result from these semiring structures. This overview of the identification, structure determination, and analytical properties of perhaps the best understood Au nanoparticle provides results that should be useful for further analyses and applications. We also hope that the story of this nanoparticle will be useful to those who teach about nanoparticle science.

Interfacial ion transfers between a monolayer phase of cationic Au nanoparticles and contacting organic solvent.

The highly cationic nanoparticle [Au(225)(TEA-thiolate(+))(22)(SC6Fc)(9)] adsorbs so strongly on Pt electrodes from CH(3)CN/Bu(4)NClO(4) electrolyte solutions that films comprised of 1-2 monolayers of nanoparticles can be transferred to nanoparticle-free electrolyte solutions without desorption and ferrocene voltammetry stably observed. (TEA-thiolate(+) = -S(CH(2))(11)N(CH(2)CH(3))(3)(+); SC6Fc = S(CH(2))(6)-ferrocene; Fc = ferrocene). The Fc(+/0) redox couple's voltammetry is used to detect the adsorption. The apparent formal potential (E(o)'(APP)) of the Fc(+/0) couple depends on the electrolyte--its anion, cation, and concentration--in the contacting nanoparticle-free solution. A 10-fold change in electrolyte concentration shifts the Fc(+/0) E(o)'(APP) by 48-67 mV, depending on the electrolyte. The dependency is interpreted to reflect the energetics of transfer of charge-compensating anions from the electrolyte solution to the monolayer nanoparticle "phase", promoted by the formation of Fc(+) sites in the nanoparticle film. This interpretation is supported by electrochemical quartz crystal microbalance results. Some further aspects of the results suggest adsorption of electrolyte cations at the nanoparticle film/electrolyte solution interface. The interface mimics a liquid/liquid interface between immiscible electrolyte solutions, in which the ion transfer approaches permselective behavior. The experimental results show that even 1-2 monolayers of highly ionic nanoparticles can behave as a polyelectrolyte "phase".

Mass spectrometry of ligand exchange chelation of the nanoparticle [Au25(SCH2CH2C6H5)18]1- by CH3C6H3(SH)2.

Mass spectrally detected products of ligand exchange reactions of the nanoparticle [Au25(SC2H4C6H5)18](1-), (abbrev. Au25(SC2Ph)18), where the dithiol is toluene-3,4-dithiol, CH3C6H3(SH)2, include nanoparticles containing both doubly (bidentate, or chelating) and singly bonded dithiol. The bidentate binding displaces two of the original -SC2Ph ligands, and singly bonded dithiol displaces one -SC2Ph ligand, while maintaining, for mass spectrally detected species, occupancy of 18 ligation sites. Extended exchange reaction times result in an apparent maximum of six chelated dithiolates. In the Au25(SC2Ph)18 nanoparticle, six semi-rings of -S(R)-Au-S(R)-Au-S(R)- act as the protecting ligand shell surrounding a Au13 core; the chelation is suggested to involve binding of dithiolates to adjacent semi-rings, rather than to a single semi-ring. Both high resolution ESI and lower resolution MALDI spectra support the product assignments. A minor extent of bidentate ligand incorporation is sufficient to severely compromise the well-known Au25(SC2Ph)18 UV-vis fine structure and to alter its voltammetric pattern, reflecting either associated semi-ring distortion and/or decay of the exchange product.

Synthesis of monodisperse [Oct4N(+)][Au25(SR)18(-)] nanoparticles, with some mechanistic observations.

A single phase (THF) synthesis of monodisperse [Oct(4)N(+)][Au(25)(SR)(18)(-)] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct(4)N(+)Br(-) salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au(25)(SR)(18)(-), as opposed to a partially oxidized state, Au(25)(SR)(18)(0)). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au(25) formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au(25)(SR)(18); while bromide does not seem to affect Au(25) formation, but it may play a role in maintaining the -1 oxidation state. Conducting the nanoparticle synthesis and "aging" period in the absence of dioxygen (under Ar) does not produce small nanoparticles, providing insights into the pathway of reaction product "aging" in the synthesis solvent, THF. The "aging" process favors the Au(25)(-) moiety as an end point and possibly involves degradation of larger nanoparticles by hydroperoxides formed from THF and oxygen.

Electrogenerated IrO(x) nanoparticles as dissolved redox catalysts for water oxidation.

We describe the first example of redox catalysis using a dissolved electroactive nanoparticle, based on the oxidation of water by electrogenerated IrO(x) nanoparticles containing Ir(VI) states, in pH 13 solutions of 1.6 +/- 0.6 nm (dia.) Ir(IV)O(x) nanoparticles capped solely by hydroxide. At potentials (ca. +0.45 V) higher than the mass transport-controlled plateau of the nanoparticle Ir(V/IV) wave, rising large redox catalytic currents reflect electrochemical generation of Ir(VI) states, which by +0.55 V and onward to +1.0 V are shown by rotated ring disk electrode experiments to lead with 100% current efficiency to the oxidation of water to O(2). O(2) production at +0.55 V corresponds to an overpotential eta of only 0.29 V, relative to thermodynamic expectations of the four electron H(2)O-->O(2) reaction. The Ir site turnover frequency (TO, mol O(2)/Ir sites/s) is 8-11 s(-1). Controlled potential coulometry shows that all Ir sites in these nanoparticles (average 66 Ir each) are electroactive, meaning that the nanoparticles are small enough to allow the required electron and proton transport throughout. Both the overpotential and TO values are nearly the same as those observed previously for films electroflocculated from similar IrO(x) nanoparticles, providing the first comparison of electrocatalysis by nanoparticle films with redox catalysis by dissolved, diffusing nanoparticles.

Redox site-mediated charge transport in a Hg-SAM//Ru(NH(3))(6)(3+/2+)//SAM-Hg junction with a dynamic interelectrode separation: compatibility with redox cycling and electron hopping mechanisms.

This paper describes the formation and electrical properties of a new Hg-based metal-molecules-metal junction that incorporates charged redox sites into the space between the electrodes. The junction is formed by bringing into contact two mercury-drop electrodes whose surfaces are covered by COO(-)-terminated self-assembled monolayers (SAMs) and immersed in a basic aqueous solution of Ru(NH(3))(6)Cl(3). The electrical behavior of the junction, which is contacted at its edges by aqueous electrolyte solution, has been characterized electrochemically. This characterization shows that current flowing through the junction on the initial potential cycles is dominated by a redox-cycling mechanism and that the rates of electron transport can be controlled by controlling the potentials of the mercury electrodes with respect to the redox potential of the Ru(NH(3))(6)(3+/2+) couple. On repeated cycling of the potential across the junction, the current across it increases by as much as a factor of 40, and this increase is accompanied by a large (>300 mV) negative shift in the formal potential for the reduction of Ru(NH(3))(6)(3+). The most plausible rationalization of this behavior postulates a decrease in the size of the gap between the electrodes with cycling and a mechanism of conduction dominated by physical diffusion of Ru(NH(3))(6)(3+/2+) ions (at larger interelectrode spacing), with a possible contribution of electron hopping to charge transport (at smaller interelectrode spacing). In this rationalization, the negative shift in the formal potential plausibly reflects extrusion of the solution of electrolyte from the junction and an increase in the effective concentration of negatively charged species (surface-immobilized COO(-) groups) in the volume bounded by the electrodes. This junction has the characteristics required for use in screening and in exploratory work, involving nanogap electrochemical systems, and in mechanistic studies involving these systems. It does not have the stability needed for long-term technological applications.

Electrospray ionization mass spectrometry of intrinsically cationized nanoparticles, [Au(144/146)(SC(11)H(22)N(CH(2)CH(3))(3)(+))(x)(S(CH(2))(5)CH(3))(y)](x+).

Electrospray ionization triple-quadrupole mass spectrometry of ca. 1.6 nm diameter thiolate-protected gold nanoparticles has been achieved at higher resolution than in previous reports. The results reveal the presence of nanoparticles with formulas Au(144)L(60) and Au(146)L(59), present in the sample as a mixture. The improved resolution is based on lowering m/z by exchanging multiple [-SC(11)H(22)N(CH(2)CH(3))(3)(+)] ligands into the original [-S(CH(2))(5)CH(3)] ligand shell. The nanoparticles are thus intrinsically cationized and appear as a series of 10+ to 15+ mass spectral peaks. The assigned state of charge was confirmed by a collision-induced dissociation measurement.

Tandem mass spectrometry of thiolate-protected Au nanoparticles Na(x)Au25(SC2H4Ph)(18-y)(S(C2H4O)5CH3)(y).

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticle's protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.

Ferrocenated au nanoparticle monolayer adsorption on self-assembled monolayer-coated electrodes.

The robust, irreversible adsorption of omega-ferrocene hexanethiolate-protected gold nanoparticles (composition ca. {Au(225)(SC6Fc)(43)}) on electrodes provides an opportunity to investigate their submonolayer and monolayer films in nanoparticle-free solutions. Observations of nanoparticle adsorption on unmodified electrodes are extended here to Au electrodes having more explicitly controlled surfaces, namely self-assembled monolayers (SAMs) of alkanethiolates with omega-sulfonate, carboxylate, and methyl termini, and in different Bu(4)N(+)X(-) electrolyte (X(-) = C(7)H(7)SO(3)(-), ClO(4)(-), CF(3)SO(3)(-), PF(6)(-), NO(3)(-)) solutions in CH(2)Cl(2). The nanoparticle surface coverage (Gamma(NP)) and the stability of the adsorbed nanoparticle film to repeated ferrocene/ferrocenium redox cycling decrease in the order of sulfonate > carboxylate > methyl terminated SAM, with increasing hydrophobicity of X(-) and with increasing alkyl chain length. The results are consistent with the proposal that the strong surface adsorption is jointly associated with the polyfunctional character of the nanoparticles, analogous to entropically driven adsorptions of polymeric ions on charged surfaces, and with lateral, ion-bridged nanoparticle-nanoparticle interactions.

Gold nanoparticles: past, present, and future.

This perspective reviews recent developments in the synthesis, electrochemistry, and optical properties of gold nanoparticles, with emphasis on papers initiating the developments and with an eye to their consequences. Key aspects of Au nanoparticle synthesis have included the two-phase synthesis of thiolated nanoparticles, the sequestration and reduction of Au salts within dendrimers, the controlled growth of larger particles of well-defined shapes via the seeded approach, and the assembling of a variety of nanoparticle networks and nanostructures. The electrochemistry of thiolated Au nanoparticles is systemized as regions of bulk-continuum voltammetry, voltammetry reflective of quantized double-layer charging, and molecule-like voltammetry reflective of molecular energy gaps. These features are principally determined by the nanoparticle core. Interesting multielectron Au nanoparticle voltammetry is observed when the thiolate ligand shell has been decorated with redox groupings. Another development is that Au nanoparticles were discovered to exhibit unanticipated properties as heterogeneous catalysts, starting with the low-temperature oxidation of CO. Substantial progress has also been made in understanding the surface plasmon spectroscopy of Au nanoparticles and nanorods. The need to investigate the optical properties of metal particles of a single, well-defined shape and size has motivated the development of a number of new techniques, leading to the study of electron transfer and redox catalysis on single nanoparticles.

Anion-induced adsorption of ferrocenated nanoparticles.

Au nanoparticles fully coated with omega-ferrocenyl hexanethiolate ligands, with average composition Au225(omega-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing submonolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and there exhibit stable ferrocene voltammetry over long periods of time. The kinetics of forming the robustly adsorbed layer are slow; monolayer and submonolayer deposition can be described by a rate law that is first order in nanoparticle concentration and in available electrode surface. The adsorption mechanism is proposed to involve entropically enhanced (multiple) ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by scanning to positive potentials (through the ferrocene wave) and by high concentrations of Bu4N+ X- electrolyte (X- = ClO4(-), PF6(-)) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is not appreciably diminished by the adsorbed ferrocenated nanoparticles, which are gradually desorbed by scanning to potentials more negative than the electrode's potential of zero charge. At very slow scan rates, voltammetric current peaks are symmetrical and nearly reversible, but exhibit E(fwhm) considerably narrower (typically 35 mV) than ideally expected (90.6 mV, at 298 K) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is qualitatively explicable by a surface-activity effect invoking large, attractive lateral interactions between nanoparticles and, or alternatively, by a model in which ferrocene sites react serially at formal potentials that become successively altered as ion-pair bridges are formed. At faster scan rates, both deltaE(peak) and E(fwhm) increase in a manner consistent with a combination of uncompensated ohmic resistance of the electrolyte solution and of the adsorbed film, as distinct from behavior produced by slow electron transfer.