David J. Schiffrin

Synthesis of thiol-derivatised gold nanoparticles in a two-phase Liquid–Liquid system

Using two-phase (water–toluene) reduction of AuCl4– by sodium borohydride in the presence of an alkanethiol, solutions of 1–3 nm gold particles bearing a surface coating of thiol have been prepared and characterised; this novel material can be handled as a simple chemical compound.

A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups.

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres—chemical species whose oxidation number, and hence electronic structure, can be changed reversibly—support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.

Spontaneous ordering of bimodal ensembles of nanoscopic gold clusters

The controlled fabrication of very small structures at scales beyond the current limits of lithographic techniques is a technological goal of great practical and fundamental interest. Important progress has been made over the past few years in the preparation of ordered ensembles of metal and semiconductor nanocrystals. For example, monodisperse fractions of thiol-stabilized gold nanoparticles have been crystallized into two- and three-dimensional superlattices. Metal particles stabilized by quaternary ammonium salts can also self-assemble into superlattice structures,. Gold particle preparations with quite broad (polydisperse) size distributions also show some tendency to form ordered structures by a process involving spontaneous size segregation,. Here we report that alkanethiol-derivatized gold nanocrystals of different, well defined sizes organize themselves spontaneously into complex, ordered two-dimensional arrays that are structurally related to both colloidal crystals and alloys between metals of different atomic radii. We observe three types of organization: first, different-sized particles intimately mixed, forming an ordered bimodal array (Fig. 1); second, size-segregated regions, each containing hexagonal-close-packed monodisperse particles (Fig. 2); and third, a structure in which particles of several different sizes occupy random positions in a pseudo-hexagonal lattice (Fig. 3).

Surface redox catalysis for O2 reduction on quinone-modified glassy carbon electrodes

Abstract The electrochemical reduction of oxygen on bare glassy carbon (GC) and on electrodes grafted with anthraquinone has been studied using the rotating ring-disk electrode (RRDE) technique. The electrode surface was grafted by the electrochemical reduction of the corresponding diazonium salt. The functionalised electrode showed quasi-reversible redox behaviour in oxygen-free 0.1 M KOH and the cyclic voltammetric response of the surface bound anthraquinone was stable on potential cycling. The covalently attached anthraquinone acts as an electrocatalyst for oxygen reduction and a well defined diffusion limited current plateau was observed in the potential range from −0.75 to −1.25 V (SCE) in 0.1 M KOH. The RRDE results show that oxygen reduction on both electrode surfaces studied stops at the hydrogen peroxide stage. The modified carbon electrodes are potential candidates for the electrochemical production of hydrogen peroxide.

Synthesis and reactions of functionalised gold nanoparticles

Stable functionalised gold nanoparticles are prepared by simultaneous reduction of tetrachloroaurate ions and attachment of bifunctional organic thiol molecules to the growing gold nuclei leading to a material whose chemical behaviour is characterised by the vacant functionality of the bifunctional thiol ligand.

From monolayers to nanostructured materials : an organic chemist's view of self-assembly

Abstract Simple methods are described for the production of gold nanoparticles with narrow size distributions by reduction of tetrachloroaurate solutions in the presence of thiol-containing organic compounds which self-assemble on the gold surface. Stable solutions of somewhat larger particles can be produced if the thiol is absent. The thiol-derivatized materials are stable in air over long periods and can be handled in much the same way as simple organic compounds. Using dithiols as the derivatizing spacer units, ways have been developed for the preparation of materials in three dimensional form and as thin films attached to a solid substrate. Such materials show conductivities that mimic the behaviour of semiconductors and that depend markedly on the structure of the dithiol used to link the gold particles together. The increase in conductivity with increasing temperature probably involves activated electron hopping from particle to particle. Surfaces treated with a coating of the materials show electroreflectance changes with applied potential that also differ according to the structure of the dithiol spacer. Unusual effects have been observed on heterogeneous electron transfer from electrode surfaces treated with layers of the gold nanoparticles and dithiol spacers. Applications for these nanostructured materials can be envisaged, which range from submicroelectronic devices and circuitry to electrical modification of the reflectance of glass. Such applications will require a multidisciplinary approach with a substantial organic chemical research input.

Electrochemical nucleation from molten salts—I. Diffusion controlled electrodeposition of silver from alkali molten nitrates

Abstract The electrodeposition of silver on platinum from solutions in the molten nitrate eutectic NaNO 3 —KNO 3 has been studied using chronopotentiometry, cyclic voltammetry and the potential step technique. In each case, the early stages of metal deposition are shown to be controlled by a nucleation process. Similar results were also obtained from studies of electrodeposition from aqueous solution, ie of silver on platinum and graphite and of mercury on graphite. Unless allowance is made for the effects of nucleation, anomalous diffusion coefficients will be obtained. The potentiostatic growth transients are in accord with a process of instantaneous nucleation and subsequent hemispherical growth, although the observed concentration dependence is at variance with that predicted by existing theories.

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

Electrochemical reduction of oxygen on anthraquinone-modified glassy carbon electrodes in alkaline solution

Abstract The electrochemical reduction of oxygen on glassy carbon (GC) electrodes grafted with anthraquinone (AQ) has been studied using the rotating ring–disk electrode technique. Grafting was achieved by the electrochemical reduction of the corresponding diazonium salt and the effect of AQ surface concentration on the kinetics of oxygen reduction has been investigated. The two-electron reduction of oxygen to hydrogen peroxide was observed for all the quinone-modified electrodes studied and the catalytic activity of the electrodes for O 2 reduction was dependent on the AQ surface concentration. The kinetic parameters of oxygen reduction on GC/AQ electrodes in 0.1 M KOH were determined as a function of AQ surface concentration considering a surface redox catalytic cycle model for quinone-modified electrodes. The rate constant of the chemical reaction between the semiquinone radical anion of AQ and molecular oxygen has been determined.

Integrated optical directional coupler biosensor

We present measurements of biomolecular binding reactions, using a new type of integrated-optical biosensor based on a planar directional coupler structure. The device is fabricated by Ag(+) - Na(+) ion exchange in glass, and definition of the sensing region is achieved by use of transparent fluoropolymer isolation layers formed by thermal evaporation. The suitability of the sensor for application to the detection of environmental pollutants is considered.