Recent Publications Feed (ADMIN)

Double potential step chronoamperometry (DPSC) is demonstrated as a technique for investigating partitioning between a solute in aqueous solution and non-polar oil droplet(s) immobilised at an electrode. Here a species in aqueous solution which does not partition into the oil phase is converted at the electrode surface into another species which either does not or does partition into the oil drop. The first case is investigated experimentally by considering generation of the ionic redox species, FcTMA²⁺ from FcTMA⁺, while the second case is exemplified by studies of Br₂ generation from Br⁻. The case of molecular partitioning at the three phase interface has received little attention hitherto. To maintain oil droplet stability a boron-doped diamond electrode is employed functionalised with Pt nanoparticles to impart electrocatalytic activity on the electrode towards Br₂ production. An arrangement is utilised where the droplet(s) sit(s) on (but does not cover) the electrode surface. We show both experimentally and through finite element simulation how the charge–time profile for the generation and collection of electroactive species can be used to obtain information on the extent of partitioning and how this is affected by factors such as the number and size of droplets. Finally, we highlight the suitability of this approach for investigating reactions which take place within the droplet.

The development of a novel analytical technique, electrochemical X-Ray fluorescence (EC-XRF) is described and applied to the quantitative detection of heavy metals in solution, achieving sub-ppb limits of detection (LOD). In EC-XRF electrochemical preconcentration of a species of interest onto the target electrode is achieved by cathodic electrodeposition. Unambiguous elemental identification and quantification of metal concentration is then made using XRF. This simple electrochemical preconcentration step improves the LOD of energy dispersive XRF by over four orders of magnitude (for similar sample preparation timescales). Large area free-standing boron doped diamond grown using microwave plasma chemical vapor deposition techniques is found to be ideal as the electrode material for both electrodeposition and XRF due to its wide solvent window, transparency to the XRF beam and ability to be produced in mechanically robust thin film form. During electrodeposition it is possible to vary both the deposition potential (Edep) and deposition time (tdep). For the metals Cu2+ and Pb2+ the highest detection sensitivities were found for Edep = -1.75 V and tdep = 4000 s with LODs of 0.05 ppb and 0.04 ppb achieved, respectively. In mixed Cu2+/Pb2+ solutions EC-XRF shows that Cu2+ deposition is unimpeded by Pb2+, across a broad concentration range, but this is only true for Pb2+ when both metals are present at low concentrations (10 nM); boding well for trace level measurements. In a dual mixed metal solution EC-XRF can also be employed to either selectively deposit the metal which has the most positive formal reduction potential or exhaustively deplete it from solution, enabling uninhibited detection of the second metal.

The requirement to separate topographical effects from surface electrochemistry information is a major limitation of scanning electrochemical microscopy (SECM). With many applications of SECM involving the study of (semi)conducting electrode surfaces, the hybridisation of SECM with scanning tunnelling microscopy (STM) or a surface conductance probe would provide the ultimate topographical imaging capability to SECM, but previous attempts are limited. Here, the conversion of a general scanning electrochemical probe microscopy (SEPM) platform to facilitate contact electrical conductance (C)‐ and electron tunnelling (T)‐SECM measurements is considered. Measurements in air under ambient conditions with a Pt/Ir wire tip are used to assess the performance of the piezoelectric positioning system. A hopping‐mode imaging protocol is implemented, whereby the tip approaches the surface at each pixel until a desired current magnitude is exceeded, and the corresponding z position (surface height) is recorded at a set of predefined xy coordinates in the plane of the surface. At slow tip approach rates, the current shows an exponential dependence on tip‐substrate distance, as expected for electron tunnelling. For measurements in electrochemical environments, in order to overcome well‐known problems with leakage currents at coated‐wire tips used for electrochemical STM, Pt‐sensitised carbon nanoelectrodes are used as tips. The hydrogen evolution reaction on 2D Au nanocrystals serves as an exemplar system for the successful simultaneous mapping of topography and electrochemical activity.