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My Research


My research focuses on the development of new electrochemical methods for the visualization of ion fluxes at various interfaces. These techniques could allow spatially resolved visualization and quantification of ion fluxes associated with various physiochemical and biological processes aiding to understand the mechanism and kinetics of such processes along with mapping their heterogeneity.

Quad-Barrel Multifunctional Electrochemical and Ion Conductance Probe for Voltammetric Analysis and Imaging

Quad-barrel


This work describes 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 for localized high resolution amperometric and potentiometric measurements at conducting and insulating surfaces. 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. 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.

B. P. Nadappuram, K. McKelvey, J. C. Byers, A. G. Güell, A. W. Colburn, R. A. Lazenby, P. R. Unwin, Anal. Chem., 2015, 87, 3566-3573.


Fabrication and Characterization of Dual Function Nanoscale pH-Scanning Ion Conductance Microscopy (SICM) Probes for High Resolution pH Mapping


pH-SICM


The measurement of local pH is hugely valuable in explaining complex interfacial reactions, which produce or consume protons and alter the pH near an interface. Additionally, many biological processes result in either intracellular or extracellular pH changes, and the quantitative measurement of these pH changes with high spatial resolution would aid in understanding the mechanisms involved. This work reports an easy fabrication and use of nanoscale dual function pH-scanning ion conductance microscopy (SICM) probes. These probes incorporate an iridium oxide coated carbon electrode for pH measurement and an SICM barrel for distance control, enabling simultaneous pH and topography mapping. The pH-SICM probes were employed in both a hopping mode and a constant separation mode for simultaneous mapping of pH and topography of a calcite microcrystal during dissolution to demonstrate their suitability for quantitative high spatial resolution measurements at surfaces and interfaces.

B. P. Nadappuram, K. McKelvey, R. Al-Botros, A. W. Colburn, and P. R. Unwin, Anal. Chem., 2013, 85, 8070–8074.



Fabrication, Characterization, and Functionalization of Dual Carbon Electrodes as Probes for Scanning Electrochemical Microscopy (SECM)

dual carbon

This work describes the fabrication and use of a dual carbon electrode for scanning electrochemical microscopy. Dual carbon electrodes (DCEs) are quickly, easily, and cheaply fabricated by depositing pyrolytic carbon into a quartz theta nanopipet. The size of DCEs can be controlled by adjusting the pulling parameters used to make the nanopipet. When operated in generation/collection (G/C) mode, the small separation between the electrodes leads to reasonable collection efficiencies of ca. 30%. A three-dimensional finite element method (FEM) simulation is developed to predict the current response of these electrodes as a means of estimating the probe geometry. Voltammetric measurements at individual electrodes combined with generation/collection measurements provide a reasonable guide to the electrode size. DCEs are employed in a scanning electrochemical microscopy (SECM) configuration, and their use for both approach curves and imaging is considered. G/C approach curve measurements are shown to be particularly sensitive to the nature of the substrate, with insulating surfaces leading to enhanced collection efficiencies, whereas conducting surfaces lead to a decrease of collection efficiency. As a proof-of-concept, DCEs are further used to locally generate an artificial electron acceptor and to follow the flux of this species and its reduced form during photosynthesis at isolated thylakoid membranes. In addition, 2-dimensional images of a single thylakoid membrane

are reported and analyzed to demonstrate the high sensitivity of G/C measurements to localized surface processes. It is finally shown that individual nanometer-size electrodes can be functionalized through the selective deposition of platinum on one of the two electrodes in a DCE while leaving the other one unmodified. This provides an indication of the future versatility of this type of probe for nanoscale measurements and imaging.

K. McKelvey, B. P. Nadappuram, P. Actis, Y. Takahashi, Y. E. Korchev, T. Matsue, C. Robinson, and P. R. Unwin, Anal. Chem., 2013, 85, 7519–7526

A new Approach for the Fabrication of Microscale Lipid Bilayers at Glass Pipets: Application to Quantitative Passive Permeation Visualization

A new method of planar bilayer lipid membrane (BLM) formation is described in this work, that allows stable, solvent-free lipid bilayers exhibiting high seal resistances to be formed rapidly, easily and reproducibly. Using these bilayers the passive permeation of a series of carboxylic acids is investigated, to determine quantitatively the trend in permeability with lipophilicity of the acid. BLMs are formed at the tip openings of pulled theta pipets, and the rate of permeation of each carboxylic acid across the bilayer, from within the pipet into the bulk solution is determined. This is achieved through spatially-resolved measurements of the pH change that occurs upon the permeation of the weak acid, visualized using a pH-sensitive fluorophore with a confocal laser scanning microscope. For bilayers formed in this way, the weak acids show increasing permeability with lipophilicity. Furthermore, the arrangement allows the effect of a trans-membrane electric field on permeation to be explored. For both propanoic and hexanoic acid it is found that an applied electric field enhances molecular transport, which is attributed to the formation of pores within the membrane.

K. E. Meadows, B. P. Nadappuram, P.R. Unwin, Soft Matter. 2014,10, 8433–8441