Electrochemical processes of bio-analytical importance at micro- and nano-liquid | liquid interfaces

The work presented herein describes the voltammetric characterisation of arrays of miniaturised (either to the micro- or nano-scale) interfaces between two immiscible electrolyte solutions (ITIES), by ion transfer (IT) electrochemistry of the model tetraethylammonium cation (TEA⁺) via cyclic voltammetry, and the use of macro- and micro-scale ITIES as a novel approach to study the behaviour of, and improve the analytical sensitivity to, analytes of biological importance.

The organic phase was gellified with poly (vinyl chloride) to increase the mechanical stability of the microITIES arrays (confined within 100 micrometer deep silicon pores). IT of TEA⁺ revealed the liquid-organogel interface as essentially inlaid on the aqueous side of the micropore channel, the TEA⁺ diffusion coefficient reduced 9-fold in the organogel, and only microITIES array designs with sufficiently large pore-to-pore separations achieved steady-state behaviour. The microITIES arrays were characterised with TEA⁺ under hydrodynamic conditions and hydrodynamic voltammograms created for a mixture of oligopeptides. Using an organogel allowed implementation of voltammetric stripping techniques at the microITIES arrays greatly improving the detection limits and sensitivities for a selection of oligopeptides compared with those attained at the macroITIES.

The first reported use of geometrically regular solid-state nanopore arrays, fabricated in 100 nm thick Si₃N₄ membranes, to support nanoscale ITIES is presented. IT of TEA⁺ for a variety of nanoITIES array designs (in terms of pore radii, pore-to-pore separation, and array pore numbers) is outlined. The results suggest that nanoITIES arrays exhibiting overlapping diffusion zones behave as one mITIES.

The electrochemical response of hen-egg-white lysozyme at the macroITIES was dominated by the charge of lysozyme in the aqueous phase and the nature of the organic phase anion. It is proposed that lysozyme does not transfer across the ITIES itself but absorbs on the aqueous side of the ITIES facilitating the transfer of the organic anion.