Quantised charging of monolayer-protected nanoparticles.

Metal nanoparticles coated with an organic monolayer, so-called monolayer protected clusters (MPCs), can show quantised charging at room temperature due to their sub-attofarad capacitance arising from the core size and the nature of the protecting monolayer. In this tutorial review, we examine the factors affecting the energetics of MPC charging. In the first section, the underlying physics of quantised charging is outlined and we give an overview of the various methods that can be used to measure single electron transfer to nanoparticles. In the subsequent sections, we discuss how electrochemical measurements can be used to give information on the quantised charging of freely diffusing and films of immobilised MPCs. The predictions of models used to determine MPC capacitance are compared with experimental data from the literature.

Synthesis and stability of monolayer-protected Au38 clusters.

A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust-Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = -1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au-S complexes.

Ion permeability of SAMs on nanoparticle surfaces.

This article reports on the ion permeability of self-assembled monolayers (SAMs) formed on the surface of charged alkanethiol-protected gold nanoparticles, so-called monolayer-protected clusters (MPCs). The capacitance and thus the charging energy required to add/remove an electron from the metal core are extremely sensitive to ions entering the monolayer, and the extent of ion penetration can be tuned by the charge and size of the ions and the permittivity of the solvent. Experimentally, this effect is comparable to ion association with conventional redox molecules, indicating that MPCs despite their large size and the fundamentally differing nature of the electron transfer process can be treated analogously to redox molecules.

Experimental observation of nonlinear ionic transport at the nanometer scale.

Nanometer-sized electrodes are used to probe the transport of ions in liquid by monitoring heterogeneous electrochemical reactions. We observe pronounced nonlinearities of ion flux versus concentration when transport is localized within a region smaller than 10 nm. We show that these observations cannot be explained using conventional continuum, mean-field descriptions of ionic transport. The data indicate that these deviations are caused by the high flux of charged species that is achieved at nanometer-sized electrodes.

Resolving electron transfer kinetics at the nanocrystal/solution interface.

The kinetics of electron transfer between individual gold nanocrystals and a solution redox species is quantified. The observed rate is dependent on the extent of electronic coupling between nanocrystals in the monolayer indicating the effect of Coulomb blockade on electrochemical kinetics.

Electrodeposition of noble metal nanoparticles on carbon nanotubes.

Noble metal nanoparticles can be electrodeposited on carbon nanotubes under potential control. The nanotube sidewalls serve both as the electrodeposition template and as the wire electrically connecting the deposited nanoparticles.

Time-resolved electrochemical detection of discrete adsorption events.

Individual binding events are observed using amperometric detection. Discrete steps in the microelectrode amperometric response correspond to the adsorption of single microspheres on the electrode surface.

Reductive desorption of thiolate from monolayer protected gold clusters.

The "electrochemical potential window" of monolayer-protected gold cluster (MPC) nanoelectrodes is probed where the electrified liquid-liquid interface is used as the detector. The first observation of the reductive desorption of thiolate at negative MPC core charge is reported.

Electron transport in two-dimensional arrays of gold nanocrystals investigated by scanning electrochemical microscopy.

This article reports the use of the scanning electrochemical microscope (SECM) to investigate the electronic properties of Langmuir monolayers of alkane thiol protected gold nanocrystals (NCs). A substantial increase in monolayer conductivity upon mechanical compression of the Au NC monolayer is reported for the first time. This may be the room temperature signature of the insulator to metal transition previously reported for comparable silver NC monolayers. Factors influencing the conductivity of the monolayer NC array are discussed.

Electrochemical resolution of 15 oxidation states for monolayer protected gold nanoparticles.

The first observation of 15 voltammetric quantized charging peaks for a solution of hexanethiol-capped gold nanoparticles (so-called monolayer protected clusters MPCs) at room temperature is reported where the variation in peak spacing with increasing charge stored in the metal core is discussed in terms of MPC capacitance.

Interfacial reactivity of monolayer-protected clusters studied by scanning electrochemical microscopy.

Solutions of monodisperse monolayer-protected clusters (MPCs) of gold can be used as multivalent redox mediators in electrochemical experiments due to their quantized double-layer charging properties. We demonstrate their use in scanning electrochemical microscopy (SECM) experiments wherein the species of interest (up to 2-electron reduction or 4-electron oxidation from the native charge-state of the MPCs) is generated at the tip electrode, providing a simple means to adjust the driving force of the electron transfer (ET). Approach curves to perfectly insulating (Teflon) and conducting (Pt) substrates are obtained. Subsequently, heterogeneous ET between MPCs in 1,2-dichloroethane and an aqueous redox couple (Ce(IV), Fe(CN)63-/4-, Ru(NH3)63+, and Ru(CN)64-) is probed with both feedback and potentiometric mode of SECM operation. Depending on the charge-state of the MPCs, they can accept/donate charge heterogeneously at the liquid-liquid interface. However, this reaction is very slow in contrast to ET involving MPCs at the metal-electrolyte interface.

Disk-generation/ring-collection scanning electrochemical microscopy: theory and application.

The potential of ring-disk ultramicroelectrodes (RD UMEs) as probes for scanning electrochemical microscopy (SECM) was investigated both theoretically and experimentally. In particular, the disk-generation/ring-collection (DG/RC) mode of operation was considered. In this case, the interaction of two species with the substrate under investigation can be followed simultaneously from single tip current-distance measurement (approach curve) to the substrate. Theoretical approach curves for DG/RC were calculated by numerical methods. Such approach curves to both insulating and conducting substrates indicate a strong tip response dependence on the ring radius while the response was relatively insensitive to ring thickness and overall tip radius. The RD tip was characterized by fitting experimental approach curves recorded at insulating and conducting substrates to simulated curves for a given tip geometry. DG/RC SECM was then applied to investigate the partitioning of iodine across a liquid-liquid interface.

Electrochemistry and electrogenerated chemiluminescence from silicon nanocrystal quantum dots.

Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (approximately 2 to 4 nanometers in diameter) was observed by differential pulse voltammetry (DPV) in N,N'-dimethylformamide and acetonitrile. The electrochemical gap between the onset of electron injection and hole injection-related to the highest occupied and lowest unoccupied molecular orbitals-grew with decreasing nanocrystal size, and the DPV peak potentials above the onset for electron injection roughly correspond to expected Coulomb blockade or quantized double-layer charging energies. Electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redox-active coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak maximum at 640 nanometers, a significant red shift from the photoluminescence maximum (420 nanometers) of the same silicon NC solution. These results demonstrate that the chemical stability of silicon NCs could enable their use as redox-active macromolecular species with the combined optical and charging properties of semiconductor quantum dots.