Recent Publications Feed (ADMIN)

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 sp² carbon into a high quality polycrystalline BDD electrode. The resulting sp² 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 (R² = 0.999) and Nernstian (59 mV / pH unit) pH dependent reductive current-voltage response over a large analyzable pH range, from pH 2-12. Using the laser approach, it is possible to optimize sp² coverage on the BDD surface, such that a measurable pH response is recorded, whilst minimizing background currents arising from oxygen reduction reactions on sp² 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 Pb²⁺, Cd²⁺, Cu²⁺ and Zn²⁺. Furthermore, the pH sensor is stable over a three month period (the current time period of testing), can be stored in air in between measurements, requires no re-activation 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 UK seawater.

A number of renewable energy systems require an understanding and correlation of material properties and (photo)electrochemical activity on the micro to nanoscale. Among these, conducting polymer electrodes continue to be important materials. In this contribution, an ultrasensitive scanning electrochemical cell microscopy (SECCM) platform is used to electrodeposit microscale thin films of poly(3-hexylthiophene) (P3HT) on an optically transparent gold electrode and to correlate the morphology (film thickness and structural order) with photo-activity. The electrochemical growth of P3HT begins with a thin ordered film up to 10 nm thick, after which a second more disordered film is deposited, as revealed by micro-Raman spectroscopy. A decrease in photo-activity for the thicker films, measured in-situ immediately following film deposition, is attributed to an increase in bulk film disorder that limits charge transport. Higher resolution ex-situ SECCM photo-transient measurements, using a smaller diameter probe, show local variations in photo-activity within a given deposit. Even after aging, thinner, more ordered regions within a deposit exhibit sustained enhanced photocurrent densities compared to areas where the film is thicker and more disordered. The platform opens up new possibilities for high-throughput combinatorial correlation studies, by allowing materials fabrication and high spatial resolution probing of processes in photoelectrochemical materials.

The electrochemical detection of a single nanoparticle (NP) at a support electrode can provide key information on surface chemistry and fundamental electron transfer (ET) properties at the nanoscale. This study employs scanning electrochemical cell microscopy (SECCM) as a fluidic device to both deliver and study the interactions between individual citrate-capped gold nanoparticles (AuNPs) and a range of alkanethiol-modified Au electrodes with different terminal groups, namely, -COOH, -OH and -CH₃. Single NP collisions were detected by the AuNP-mediated ET reaction to Fe(CN)₆^4-/3- in aqueous solution. The collision frequency, residence time and current-time characteristics of AuNPs is greatly affected by the terminal groups of the alkanethiol. Methods to determine these parameters, including the effect of the instrument response function, and derive ET kinetics are outlined. To further understand the interactions of AuNPs with these surfaces, atomic force microscopy (AFM) force measurements were performed using citrate-modified Au-coated AFM tips and the same alkanethiol-modified Au substrates in aqueous solution at the same potential bias as for the AuNP collision experiments. Force curves on OH-terminated surfaces showed no repulsion and negligible adhesion force. In contrast, a clear repulsion (on approach) was seen for COOH- terminated surface and adhesion forces (on retract) were observed for both COOH- and CH₃-terminated surfaces. These interactions help to explain the residence times and collision frequencies in AuNP collisions. More generally, as the interfacial properties probed by AFM appear to be amplified in NP collision experiments, and new features also become evident, it is suggested that such experiments provide a new means of probing surface chemistry at the nanoscale.