Trevor J. Davies

Electrochemistry of immobilised redox droplets: Concepts and applications

The use of microdroplet modified electrodes provides a simple methodology with which to study the biphasic electrochemistry of a plethora of species, encouraging the use of such techniques to mimic emulsion media. Furthermore, since the droplets may be miniaturised, this approach may assist in the field of biomimetic electrochemistry. For these reasons, this paper reviews the voltammetry of electrodes modified with electrochemically active droplets. The primary focus of the review is of unsupported droplets, where electron transfer processes occur at the three phase boundary, the base circumference of the individual droplets (of a volume range spanning nine orders of magnitude, going from microlitre to femtolitre volumes). The voltammetry of such systems is categorised via a semi-quantitative appreciation of the voltammetric characteristics. Finally, several topical examples illustrating the potential of application of this technology are described.

Investigation of modified basal plane pyrolytic graphite electrodes: definitive evidence for the electrocatalytic properties of the ends of carbon nanotubes.

The basis of the electrocatalytic nature of multi-wall carbon nanotubes is suggested to reside in electron transfer from the ends of nanotubes, which structurally resemble the behaviour of edge plane (as opposed to basal plane) graphite, and is demonstrated via the comparison of the electrochemical oxidation of epinephrine and the electrochemical reduction of ferricyanide at nanotube-modified electrodes using different types of graphite electrodes and with C(60)-modified electrodes.

Boron-doped diamond microdisc arrays: electrochemical characterisation and their use as a substrate for the production of microelectrode arrays of diverse metals (Ag, Au, Cu)via electrodeposition

A novel boron-doped diamond (BDD) microelectrode array is characterised with electrochemical and atomic force microscopic techniques. The array consists of 40 micron-diameter sized BDD discs which are separated by 250 microns from their nearest neighbour in a hexagonal arrangement. The conducting discs can be electroplated to produce arrays of copper, silver or gold for analytical purposes in addition to operating as an array of BDD-microelectrodes. Proof-of-concept is shown for four separate examples; a gold plated array for arsenic detection, a copper plated array for nitrate analysis, a silver plated array for hydrogen peroxide monitoring and last, cathodic stripping voltammetry for lead at the bare BDD-array.

Surfactant-free emulsion electrosynthesis via power ultrasound: electrocatalytic formation of carbon–carbon bonds

Proof-of-concept of the mediated electrosynthesis of carbon–carbon bonds in totally ‘green’ surfactant-free emulsion media generated by application of power ultrasound to a biphasic water–organic mixture is illustrated by reference to three systems, some requiring further activation by light, and each catalysed by vitamin B12. The voltammetry of aqueous vitamin B12 solutions at an electrode modified with microdroplets of the organic reactant is employed to gain an insight into the electrocatalytic pathway and readily permits the identification of optimum reaction parameters, such as starting material ratios and wavelength of light. The latter are employed in proof-of-concept emulsion electrosynthesis under conditions of triple activation (electron transfer, ultrasound and light).

Photoelectrochemistry of bromonitrobenzenes: mechanism and photoelectrochemically-induced halex reactions

The theory for homogeneous ECrevE, ECrevCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para -bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECrevCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho -bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C � /Br bond occurs reversibly with the competing formation of a C � /Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions. # 2002 Elsevier Science B.V. All rights reserved.

Photoelectrochemical dechlorination of phenols

The photoelectrochemical reductions of 4-chlorophenol and 2,4-dichlorophenol are studied in acetonitrile solution at platinum electrodes. The photoelectrochemical reduction follows a CE-type mechanism with the electrochemical step being the formation of dihydrogen. The photochemistry arises from the excitation of the chlorophenolate anion with subsequent loss of chloride, so suggesting green routes based on photons and electricity only, applicable in both aqueous and non-aqueous solution for the dechlorination of chlorophenols.

Voltammetry of immobilised microdroplets containing p-chloranil on basal plane pyrolytic graphite electrodesElectronic supplementary information (ESI) available: Comparison of the reduction of TCBQ in BN with other organic solvents (DMF, DMSO and PrCN). See http://www.rsc.org/suppdata/cp/b4/b405531d/

The electron transfer properties of p-chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, TCBQ) were investigated in both homogeneous and heterogeneous media. For the homogeneous study, the electrochemical reduction of TCBQ was carried out in different aprotic solvents (namely: benzonitrile (BN), N,N,dimethylformamide (DMF), propylcyanide (PrCN) and dimethylsulfoxide (DMSO)) and revealed two successive one-electron reductions according to a quasi-reversible EE mechanism. For the heterogeneous study, cyclic voltammetry with basal plane pyrolytic graphite electrodes modified with microdroplets of benzonitrile/TCBQ was employed. The droplets were found to be randomly dispersed with a degree of overlapping and average diameters of 5 μm giving the microdroplets individual volumes of ca. 33 fL. The redox processes within the electrically insulating microdroplets were shown to be very sensitive to the nature and concentration of ions in the surrounding aqueous phase as, in order to retain electroneutrality within the unsupported oil phase, electric field-induced migration of ions likely occurs during the Faradaic current flow. Depending on the lipophilicity of the aqueous electrolyte cation uptake into or electrochemical generated anion expulsion from the organic phase containing the electroactive specie TCBQ was induced electrochemically. Alkali metal cation uptake into the microdroplet environment was not observed. However less hydrophilic tetraalkylammonium cations NR4+ (R+ = Bu and Pe) inserted. Proton insertion into the oil phase was also shown to occur as the current|voltage shifted to more positive potentials, making the reductive process more facile, as the pH of the buffer solution was decreased. The higher efficiency of proton insertion as compared with Group I cations insertion was explained in terms of the formation of strong O–H covalent bonds which outweighs the ion phase transfer thermodynamics. Finally, the cross-phase electron transfer across the benzonitrile|water interface was examined when the TCBQ microdroplets were purposely made conductive by addition of a hydrophobic, nonpartitioning electrolyte in the oil phase. Again, the resulted voltammetry was found to change depending on the identity and concentration of the salt dissolved in the surrounding aqueous environment.