Controlled sp(2) Functionalization of Boron Doped Diamond as a Route for the Fabrication of Robust and Nernstian pH Electrodes.

The development of a voltammetric boron doped diamond (BDD) pH sensor is described. To obtain pH sensitivity, laser micromachining (ablation) is utilized to introduce controlled regions of sp(2) carbon into a high quality polycrystalline BDD electrode. The resulting sp(2) carbon is activated to produce electrochemically reducible quinone groups using a high temperature acid treatment, followed by anodic polarization. Once activated, no further treatment is required. The quinone groups show a linear (R(2) = 0.999) and Nernstian (59 mV/(pH unit)) pH-dependent reductive current-voltage response over a large analyzable pH range, from pH 2 to pH 12. Using the laser approach, it is possible to optimize sp(2) coverage on the BDD surface, such that a measurable pH response is recorded, while minimizing background currents arising from oxygen reduction reactions on sp(2) carbon in the potential region of interest. This enables the sensor to be used in aerated solutions, boding well for in situ analysis. The voltammetric response of the electrode is not compromised by the presence of excess metal ions such as Pb(2+), Cd(2+), Cu(2+), and Zn(2+). Furthermore, the pH sensor is stable over a 3 month period (the current time period of testing), can be stored in air between measurements, requires no reactivation of the surface between measurements, and can be reproducibly fabricated using the proposed approach. The efficacy of this pH sensor in a real-world sample is demonstrated with pH measurements in U.K. seawater.

Investigation of molecular partitioning between non polar oil droplets and aqueous solution using double potential step chronoamperometry.

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, FcTMA(2+) 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.