Bernadette M. Quinn

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.

Counting the atoms in supported, monolayer-protected gold clusters.

Mass spectrometry of supported, monolayer-protected, nominally Au(38) (MP-Au(38)) clusters is performed via quantitative High Angle Annular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) using size-selected Au(N) clusters (N = 25, 38, 55) as mass standards. With the intensity due to the (presumed) 24 hexanethiolate ligands taken into account, the clusters are found to contain 38 +/- 2 Au atoms. The method may also be used to determine the degree of aggregation of deposited nanoparticles.

Micro ring–disk electrode probes for scanning electrochemical microscopy

Abstract The construction and characterisation of ring–disk (RD) microelectrodes suitable for use in scanning electrochemical microscopy (SECM) is reported. Such RD electrodes are proposed as probes for novel generator–collector SECM experiments. In this case, the interaction of both the reactants and products with the substrate under investigation can be followed simultaneously from a single approach curve to the substrate. Examples of such approach curves to conducting and insulating substrates are given to demonstrate the potential of this new mode of SECM operation.

Electron transfer at micro liquid–liquid interfaces

Abstract The novel use of micro ITIES to study electron transfer (ET) has been investigated. Micropipettes and microholes were used to support the interface and the organic solvents considered were 1,2-DCE and o -NPOE. ET involving ferrocene, dimethylferrocene and tetracyanoquinodimethane as the organic redox couples was studied. The ferri/ferrocyanide redox couple was used in all cases as the aqueous redox couple. Micro ITIES were found not to be amenable to kinetic determination. ET is complicated by reactions involving the base electrolytes used, partitioning of redox products, equilibrium reactions and interfacial precipitation, which will need to be considered if accurate kinetic data is to be realised.

Study of ion transfer across phospholipid monolayers adsorbed at micropipette ITIES

Abstract The transfer of tetraethylammonium cations across phospholilpid monolayers deposited at a micropipette ITIES (interface between two immiscible electrolyte solutions) was studied using short potential step techniques. The observed current transients consisted of a capacitive and a faradaic current component, which were separated using a non-linear fit to a model function. Fits were very good. From the capacitive component, the surface coverage of lipids was calculated by applying a simple model for the monolayer configuration. It appeared that the coverage varied between 80 and 99%. The rate of ion transfer was increased slightly after the lipid addition, but no significant variations between different lipids were found. The transfer mechanism is discussed briefly.

Charge injection and lateral conductivity in monolayers of metallic nanoparticles

Scanning electrochemical microscopy has been used to quantify both the charge injection energetics between a solution redox couple and a monolayer of hexanethiol stabilised gold nanoparticles and the subsequent lateral charge transport in the monolayer.

Lipophilicity of ions electrogenerated at a Pt coated micropipette supported liquid-liquid interface

Abstract 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.