Recent Publications Feed (ADMIN)

The fabrication and use of a multifunctional electrochemical probe incorporating two independent carbon working electrodes and two electrolyte-filled barrels, equipped with quasi-reference counter electrodes (QRCEs), in the end of a tapered micron-scale pipet is described. This ‘quad-probe’ (4-channel probe) was fabricated by depositing carbon pyrolytically into two diagonally opposite barrels of a laser-pulled quartz quadruple-barrelled pipet. After filling the open channels with electrolyte solution, a meniscus forms at the end of the probe and covers the two working electrodes. The two carbon electrodes can be used to drive local electrochemical reactions within the meniscus while a bias between the QRCEs in the electrolyte channels provides an ion conductance signal that is used to control and position the meniscus on a surface of interest. When brought into contact with a surface, localized high resolution amperometric imaging can be achieved with the two carbon working electrodes with a spatial resolution defined by the meniscus contact area. The substrate can be an insulating material or (semi)conductor, but herein we focus mainly on conducting substrates that can be connected as a third working electrode. Studies using both aqueous and ionic liquid electrolytes in the probe, together with gold and individual single walled carbon nanotube samples, demonstrate the utility of the technique. Substrate generation-dual tip collection measurements are shown to be characterized by high collection efficiencies (approaching 100%). This hybrid configuration of scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) should be powerful for future applications in electrode mapping, as well as in studies of insulating materials as demonstrated by transient spot redox-titration measurements at an electrostatically charged Teflon surface and at a pristine calcite surface, where a functionalized probe is used to follow the immediate pH change due to dissolution.

Nanopipets are versatile tools for nanoscience, particularly when used in scanning ion conductance microscopy (SICM) to determine, in a noncontact manner, the topography of a sample. We present a new method, applying an oscillating bias between a quasi-reference counter electrode (QRCE) in the SICM nanopipet probe and a second QRCE in the bulk solution, to generate a feedback signal to control the distance between the end of a nanopipet and a surface. Both the amplitude and phase of the oscillating ion current, induced by the oscillating bias and extracted using a phase-sensitive detector, are shown to be sensitive to the probe–surface distance and are used to provide stable feedback signals. The phase signal is particularly sensitive at high frequencies of the oscillating bias (up to 30 kHz herein). This development eliminates the need to physically oscillate the probe to generate an oscillating ion current feedback signal, as needed for conventional SICM modes. Moreover, bias modulation allows a feedback signal to be generated without any net ion current flow, ensuring that any polarization of the quasi reference counter electrodes, electro-osmotic effects, and perturbations of the supporting electrolyte composition are minimized. Both feedback signals, magnitude and phase, are analyzed through approach curve measurements to different surfaces at a range of distinct frequencies and via impedance measurements at different distances from a surface. The bias modulated response is readily understood via a simple equivalent circuit model. Bias modulated (BM)-SICM is compared to conventional SICM imaging through measurements of substrates with distinct topographical features and yields equivalent results. Finally, BM-SICM with both amplitude and phase feedback is used for topographical imaging of subtle etch features in a calcite crystal surface. The 2 modes yield similar results, but phase-detection opens up the prospect of faster imaging.

The effect of systematic anodic pre-treatments of pristine single walled carbon nanotube (SWNT) forests on the electrochemical response towards a variety of redox processes is investigated. An experimental arrangement is adopted whereby a microcapillary containing the solution of interest and a quasi reference-counter electrode is brought into contact with a small portion of the forest to enable measurements on the surface before and after controlled anodic polarisation (AP). AP of the surface is found to both improve the voltammetric response (faster apparent heterogeneous electron transfer kinetics) of surface sensitive redox processes, such as Fe^2+/3+, and enhance the electrocatalytic response of the SWNTs towards oxygen reduction; the extent of which can be carefully controlled via the applied anodic potential. AP is expected to remove any trace organic (atmospheric) contaminants that may accumulate on the forest over extended periods as well as allowing the controlled introduction of defects, as confirmed by micro-Raman spectroscopy.