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Solutions of monodisperse monolayer-protected clusters (MPCs) of gold can beused 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)[SUB6][SUP3-/4-], Ru(NH[SUB3])[SUB6][SUP3+], and Ru(CN)[SUB6][SUP4-]) 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. [ABSTRACT FROM AUTHOR]

Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (∼2 to 4 nanomaters 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 radox-active macromoleculer species with the combined optical and charging properties of semiconductor quantum dots. [ABSTRACT FROM AUTHOR]

A novel method to readily determine the lipophilicity of electrogenerated charged species is reported. This is achieved by local electrolysis at a Pt coated micropipette and, subsequently, driving the electrogenerated species to transfer across the liquid–liquid interface supported at the tip of the micropipette under potential control. The formal potential of ion transfer can then be used to give a measure of its relative lipophilicity. The method proposed is facile and enables the study of potentially unstable charged products of electron transfer reactions. [Copyright &y& Elsevier]

The article investigates electron transfer kinetics between a solution redox mediator and the nanocrystal core using a monolayer of alkanethiolate protected gold nanocrystals and reports the first quantitative estimate for the rate of electron transfer at the nanocrystal/solution interface. The rate of electron transfer was found to be depended on the redox couple used, which shows the importance of molecule specific properties such as the effect of their relative hydrophilicities.

The article focuses on the electrodeposition noble metals on single-walled carbon nanotubes (SWNTs). The SWNT serves a dual function: initially as the electrodeposition template and subsequently as a wire to electrically connect the deposited Au, Pt and Pd nanoparticles. SWNT electrodes suitable for electrochemical measurements were produced via patterned SWNT growth following standard microfabrication techniques. Metal electrodeposition was performed potentiostatically in a two-electrode arrangement where the deposition potential, duration of the pulse and the concentration of the metal salt (HAuCl4, K,PtCl4, (NH4)2PdCl4) were varied.

The article throws light on the time-resolved electrochemical detection of discrete adsorption events. It reports the amperometric detection of single carboxylated micro and nanospheres adsorbing on electrodes as well-resolved, discrete steps in the current-time response. Detection at the single-event level is a necessary requirement for a range of single-molecule experiments and can greatly enhance existing sensor technologies. Electrochemical detection of micron-sized latex beads at arrays of microelectrodes has also been proposed as a sensing technique for use in sandwich-type immunoassays, and it was deduced that single-bead detection could be realized.

Thiol-protected gold nanoparticles, so-called monolayer protected clusters (MPCs), are unique in that they can be repeatedly isolated and dispersed without irreversible aggregation. This extraordinary stability is due to the strength of the gold-thiol bond. From extensive research on 2D self-assembled thiol monolayers on Au (SAMs), the gold-sulfur bond is broken only under extreme oxidative conditions, at high temperatures or by electrochemical reduction. MPCs are essentially 3D SAMs and have been termed freely diffusing nanoelectrodes.

Describes the preparation of hexanethiol-capped Au (C6S-Au) particles by the Brust method using the modification proposed by Murray and colleagues to obtain particles of small core radius and improved monodispersity. Methods used to investigate the monolayer protected clusters (MPC) redox properties; Current response; Differential pulse voltammetry response; Oxidation states for monolayer protected gold nanoparticles.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Examines the potential of ring-disk ultramicroelectrodes for scanning electrochemical microscopy. Investigation on the partitioning of iodine across liquid-liquid interface; Problems associated with double layer charging; Collection of electrode-generated species.

The article differentiates ion solvation in the film and electrolyte phase in controlling redox properties of model electroactive thin films. Ion transfer across the film/solution interface has received little attention to date, and here we will demonstrate how it can effectively shut off the redox response of nanoparticle films.