Proton nuclear magnetic resonance (NMR) was used to measure the rate constant and activation energy barrier for electron self-exchanges of the phenylethanethiolate-protected nanoparticle couple [Au25(SC2Ph)18]0/1−. The thiolate ligand α-methylene proton resonances of electrolytically prepared CD2Cl2 solutions of the oxidized (Au250) and reduced (Au251−) nanoparticles exhibit characteristic chemical shifts and line-shapes. That for the α-CH2 protons in Au250 is shifted ∼2 ppm downfield from Au251− and has an increased line-width reflecting the odd electron count of the nanoparticle core. Solution mixtures of Au250 and Au251− exhibit further peak broadening and intermediate values of α-CH2 proton chemical shifts, effects quantitatively consistent with an electron self-exchange process in the fast-exchange regime. Analysis of changes in peak broadening at varied total nanoparticle concentration and at varied temperatures produces a rate constant for [Au25(SC2Ph)18]0/1− self-exchange of 3.0(±0.1) × 107 M−1s−1 at 22 °C and an activation barrier energy EA = 25.0 (±1.5) kJ/mol. This barrier energy is much larger than the calculated estimate of outer-sphere reorganization energy, implying the presence of a significant inner-sphere reorganization energy. The latter is confirmed by a detected difference in the Raman Au−S bond stretch energies of Au250 and Au251− nanoparticles.

We describe the formation of stable, adherent, mesoporous films of 2 nm diameter IrIVOx nanoparticles on glassy carbon electrodes, by a previously unreported method of controlled potential electro-flocculation from pH 13 nanoparticle solutions. These films initiate O2 evolution from water oxidation and then achieve 100% current efficiency, at overpotentials only ∼0.15 and ∼0.25 V higher, respectively, than the reversible H2O/O2 potential. The overpotentials, measured at ∼0.5 mA/cm2, are independent of pH and are the smallest yet reported for electrochemical water oxidation, a property important in possible uses in electrochemical solar cells. The films appear to be mesoporous and microscopically accessible, since O2 evolution currents increase proportionately to multilayer nanoparticle film coverage but without a concurrent increase in overpotential.

Preparation of Ru(II) polypyridyl–iridium oxide nanoparticle (IrOX NP) chromophore–catalyst assemblies on an FTO|nanoITO|TiO2 core/shell by a layer-by-layer procedure is described for application in dye-sensitized photoelectrosynthesis cells (DSPEC). Significantly enhanced, bias-dependent photocurrents with Lumencor 455 nm 14.5 mW/cm2 irradiation are observed for core/shell structures compared to TiO2 after derivatization with [Ru(4,4′-PO3H2bpy)2(bpy)]2+ (RuP2) and uncapped IrOX NPs at pH 5.8 in NaSiF6 buffer with a Pt cathode. Photocurrents arising from photolysis of the resulting photoanodes, FTO|nanoITO|TiO2|−RuP2,IrO2, are dependent on TiO2 shell thickness and applied bias, reaching 0.2 mA/cm2 at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long-term photolysis in the NaSiF6 buffer results in a marked decrease in photocurrent over time due to surface hydrolysis and loss of the chromophore from the surface. Long-term stability, with sustained photocurrents, has been obtained by atomic layer deposition (ALD) of overlayers of TiO2 to stabilize surface binding of −RuP2 prior to the addition of the IrOX NPs.

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.

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 EPEAK at ca. +0.52 V and an irreversible reduction with EPEAK 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.

The electronic conductivity of films of iridium oxide (IrOx) composed of ca. 2 nm nanoparticles (NPs) is strongly dependent on the film oxidation state. The IrIVOx NPs can be electrochemically converted to several oxidation states, ranging from IrIII to IrV 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 IrIII state, the apparent conductivity falls to 10–6 S cm–1.

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)180/1–, Au25(SR)181+/0, and Au144(SR)601+/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 × 106 M–1 s–1 for Au25(SR)180/1–, ∼3 × 105 M–1 s–1 for Au25(SR)181+/0, and ∼1 × 108 M–1 s–1 for Au144(SR)601+/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 > Au250/1– > Au251+/0.

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.

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 cm2/s) and effective ferrocene concentration (C = 0.60 mM) in acetonitrile solutions were determined using ratios of DC and D1/2C data measured by microdisk voltammetry and chronoamperometry. The D result compares favorably to an Einstein–Stokes estimate (2.1 × 10–6 cm2/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.

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, Au25L18. 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 Au25L18 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. Au25L18 is a success story in this regard; however, researchers initially mislabeled this NP as Au28L16 and as Au38L24 before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theoretical investigations. In addition, Au25L18’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 Au25L18. Researchers have determined the single crystal structure of both its “native” as-prepared form, a [N((CH2)7CH3)41+][Au25(SCH2CH2Ph)181−] salt, and of the neutral, oxidized form Au25(SCH2CH2Ph)180. A density functional theory (DFT) analysis correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au13 core stabilized by six Au2(SR)3 semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au25 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.

The highly cationic nanoparticle [Au225(TEA-thiolate+)22(SC6Fc)9] adsorbs so strongly on Pt electrodes from CH3CN/Bu4NClO4 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(CH2)11N(CH2CH3)3+; SC6Fc = S(CH2)6-ferrocene; Fc = ferrocene). The Fc+/0 redox couple’s voltammetry is used to detect the adsorption. The apparent formal potential (E°′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°′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”.

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.

We report reactivity of the gold nanoparticle [TOA+] [Au25(SC2Ph)18]1− (TOA+ = tetraoctylammonium; SC2Ph = phenylethanethiolate = L; [Au25(SC2Ph)18]1− = Au25L181−) with Ag+, Cu2+, and Pb2+ ions. Titration of solutions of Au25L181− in CH2Cl2 with one and two equivalents of Ag+ produces changes in absorbance spectra with isosbestic points, and a titration curve break at 1:1 mol ratio, indicating a stoichiometric interaction. Similar effects are seen with Cu2+ and Pb2+ additions, but the break occurs at 0.5:1 mol ratio metal/nanoparticle. Changes in Au25L181− absorbance and fluorescence spectra are qualitatively similar to those accompanying oxidation of the Au25L181− nanoparticle anion, but the spectra of the stoichiometric products differ slightly according to the metal ion. Addition of higher excess of Ag+ or Cu2+ causes loss of characteristic [Au25(SC2Ph)18]1− UV−vis spectral fine structure and apparent irreversible refining into larger nanoparticles. Voltammetric currents for nanoparticle 0/-1 and +1/0 redox waves are depressed by Ag+ addition. Electrospray ionization mass spectra of products of addition of up to two equivalents Ag+ show prominent peaks for [Au25(SC2Ph)18]1− but also peaks corresponding to bimetal nanoparticles [Au24Ag(SC2Ph)18]2+, [Au23Ag2(SC2Ph)18]2+, and [Au22Ag3(SC2Ph)18]2+. We propose a redox model of reaction of the Au25L181− nanoparticle with metal ions, in which the Au25L181− nanoparticle acts as a reductant toward the metal ion, forming Au25M(SC2Ph)18 adducts that become oxidatively dissociated in the mass spectral cationization environment to yield the bimetals observed.

A single phase (THF) synthesis of monodisperse [Oct4N+][Au25(SR)18−] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct4N+Br− salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au25(SR)18−, as opposed to a partially oxidized state, Au25(SR)180). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au25 formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au25(SR)18; while bromide does not seem to affect Au25 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 Au25− moiety as an end point and possibly involves degradation of larger nanoparticles by hydroperoxides formed from THF and oxygen.

The progress of ligand exchange reactions between the ligands of Au25(SR)18− nanoparticles (SR = S(CH2)2Ph) and thiols with electron-withdrawing substituents (HSPh-p-X; X = Br, NO2) was monitored using 1H nuclear magnetic resonance. As the reactions proceed, the introduction of the electron withdrawing −SPhX ligands into the nanoparticle ligand shell causes a shift of the nanoparticle redox waves (Au251+/0 and Au250/1−) to more positive potentials. Combining the NMR and electrochemical results reveals a nearly linear shift of the redox formal potentials as a function of the average number of exchanged ligands: ∼42 and 25 mV/ligand for X = −NO2 and −Br, respectively. Using a simple model electron-withdrawing ligand (−SCH2Cl), density functional theory (DFT) was used to study in detail the effects on the nanoparticle electronic structure caused by exchange of this ligand for −SCH3. The calculations show how the electronegative −X group changes the polarization of the nanoparticle and the charge distribution among the ligands, the protecting (−SR−Au−SR−Au−SR−) semirings, and the Au13 core. The HOMO−LUMO gap is unchanged by the ligand exchanges; both states are equally stabilized by the presence of each incoming ligand, by ∼60 mV/ligand. Charge analysis suggests no significant changes in the Au13 core, even after complete exchange. Rather, the charge is transferred inside the ligands, mostly from nearest-neighbor atoms of the semirings.

We describe the first example of redox catalysis using a dissolved electroactive nanoparticle, based on the oxidation of water by electrogenerated IrOx nanoparticles containing IrVI states, in pH 13 solutions of 1.6 ± 0.6 nm (dia.) IrIVOx nanoparticles capped solely by hydroxide. At potentials (ca. +0.45 V) higher than the mass transport-controlled plateau of the nanoparticle IrV/IV wave, rising large redox catalytic currents reflect electrochemical generation of IrVI 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 O2. O2 production at +0.55 V corresponds to an overpotential η of only 0.29 V, relative to thermodynamic expectations of the four electron H2O→O2 reaction. The Ir site turnover frequency (TO, mol O2/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 IrOx nanoparticles, providing the first comparison of electrocatalysis by nanoparticle films with redox catalysis by dissolved, diffusing nanoparticles.

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 Au144L60 and Au146L59, present in the sample as a mixture. The improved resolution is based on lowering m/z by exchanging multiple [−SC11H22N(CH2CH3)3+] ligands into the original [−S(CH2)5CH3] 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.

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 NaxAu25(SC2H4Ph)18−y(S(C2H4O)5CH3)y centrally involve the semi-ring Au2L3 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 [Na2AuL2]1+, one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na2Au2L3]1+, represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu3L3]1+ and [NaAu4L4]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 Au4L4 in ESI-TOF of a much larger thiolate-protected Au144 nanoparticle under non-CID conditions.

The robust, irreversible adsorption of ω-ferrocene hexanethiolate-protected gold nanoparticles (composition ca. {Au225(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 ω-sulfonate, carboxylate, and methyl termini, and in different Bu4N+X− electrolyte (X− = C7H7SO3−, ClO4−, CF3SO3−, PF6−, NO3−) solutions in CH2Cl2. The nanoparticle surface coverage (ΓNP) 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.

We describe the electrochemistry of 15 nm diameter silica nanoparticles densely functionalized with ferrocene (FcSiO2) through siloxane couplings. Each nanoparticle bears ∼600 Fc sites, as measured by potentiometric titration (590 Fc) and diffusion-controlled voltammetry (585 Fc) and estimated by XPS (630 Fc). The nanoparticle ferrocene coverage amounts to ca. a complete monolayer of ferrocene sites, which react electrochemically without mutual interactions and which are apparently fully accessible for diffusion-controlled electrode reactions. Diffusion-controlled voltammetry of the FcSiO2 nanoparticles was observed in dilute methanol dispersions and in more concentrated slurry phases formed in methanol/acetonitrile mixtures. Electrochemical studies reveal interesting behavior in the dilute and more concentrated solutions. Because of the large nanoparticle surface area/volume ratio, the ferrocene-coated silica nanoparticles are capable of storing up to 5 × 107 C/m3 of redox charge as dry phases and 6 × 105 C/m3 in the concentrated slurries.

The molecular ion of the nanoparticle Au25(SCH2CH2Ph)18 (A25(SR)18) is observed at 7394 Da in fast atom bombardment (FAB, Xe atoms) ionization mass spectrometry using a 3-nitrobenzyl alcohol matrix. A distinctive pattern of positive fragment ions is evident in the mass interval 5225−7394 Da, where peaks are seen for successive mass losses equivalent to R2S entities. Because the Au25(SCH2CH2Ph)18 nanoparticle structure is crystallographically known to consist of a centered Au13 icosahedral core surrounded by six Au2(SR)3 semirings, the R2S loses are proposed to represent serial rearrangements and decompositions of the semiring structures. Mass losses equivalent to R2S2 and R2 entities also appear at the lower end of this mass interval. The most intense spectral peak, at m/z = 5246 Da, is assigned to the fragment Au25S10, from which all of the CH2CH2Ph organic units have been cleaved but from which no gold atoms have been lost. A different pattern of fragmentation is observed at lower masses, producing ions corresponding to serial losses of one gold atom and varied numbers of sulfur atoms, which continues down to a Au9S2 fragment. FAB mass spectra of the Au nanoparticle are much easier to interpret than laser desorption/ionization spectra, but they show more extensive fragmentation than do electrospray and low laser pulse intensity MALDI spectra. The loss of R2S fragmentation in FAB is distinctive and unlike that seen in the other ionization modes. The FAB spectrum for the nanoparticle Au25(S(CH2)9CH3)18 is also reported; its fragmentation parallels that for Au25(SCH2CH2Ph)18, implying that this nanoparticle has the same surprising stellated (staples) structure.

Ligand exchange reactions of Au25(SCH2CH2Ph)18 with hexanethiol (HSC6) and thiophenol (HSPh) as incoming ligands, and Brust reaction nanoparticle syntheses using mixtures of thiols (HSCH2CH2Ph and HSC6), produce nanoparticles having different, ideally statistically determined, relative populations of the two thiolate ligands (X and Y), i.e., Au25(X)m(Y)m′, where m and m′ vary but always sum to 18. By choice of reactant concentrations, the exchange reaction can reach an equilibrium state or a near-complete exchange of ligands or be at a kinetically determined (nonequilibrium) mixed population, at the time of reaction quenching and subsequent matrix-assisted laser desorption ionization−time-of-flight (MALDI-TOF) mass spectrometric examination. With the assumption that the reactivities of the 18 ligand sites are identical and independent, the equilibrium distributions of ligand populations of the mixed monolayer exchange products should adhere to a binominal distribution. A simulated kinetic model for ligand exchange shows that mixed ligand distributions in nanoparticles not at exchange equilibrium also conform to the binominal distribution. The theory successfully describes MALDI mass spectrometrically determined experimental ligand populations produced in the ligand exchange reaction between Au25(SCH2CH2Ph)18 and hexanethiol, while that between Au25(SCH2CH2Ph)18 and thiophenol yields a more narrow distribution than predicted by random exchanges and no interactions between ligands. Previous nanoparticle ligand analyses by methods such as nuclear magnetic resonance, gas−liquid chromatography, and infrared spectroscopies yield average ligand populations in mixed monolayers and are incapable of detecting such nonrandom ligand distributions.

The electron transport properties of 12 imidazolium ionic liquids (ILs) in which ferrocene groups have been attached to the imidazolium center by different linkers have been investigated. Cobalticenium is alternatively attached in another ionic liquid. The electron transport is measured in the neat (undiluted) ILs, which are very concentrated and viscous, so that electron transport (“electron diffusion”, DE) occurs by electron self-exchange (hopping) reactions of the Fc+/0 couple. The ionic conductivities are also measured and converted into counterion diffusion coefficients DION assuming that the small counterions (PF6− or BF4−) carry most of the ionic current. Converting the electron diffusion rate into an electron exchange rate constant, kEX gives only an apparent rate constant value, and electron transport is instead demonstrably controlled by the values of DION, as expected from a previously introduced transport model called ion atmosphere relaxation. The fluctuational rate of counterion motions around a donor−acceptor pair governs formation of a precursor complex within which electron transfer takes place. In keeping with the model, values of kEX and DION vary in a parallel fashion, and indeed DE and DION are nearly numerically equal. Activation barriers for ionic conductivity and electron transport are also equal. Log−log plots of kEX vs DION are linear with slopes of one, and all of the imidazolium transport data fall onto a common correlation line. The generality of the ion atmosphere relaxation explanation for electron transport control is shown by a log−log coplot of the imidazolium IL data with previous data for PEG-based ILs, where the results also all fall onto a common correlation line.