Electrochemical characterisation of microsquare nanoband edge electrode (MNEE) arrays and their use as biosensors

Nanoelectrodes are defined as electrodes which have a critical dimension on the order of nanometres. Due to their smaller dimensions they have a reduced iR drop and enhanced mass transport, which results in the rapid establishment of an enhanced steady-state diffusion profile and a greater Faradaic current density, along with a smaller relative double layer capacitance, which together give a significantly increased signal to noise ratio compared to macroelectrodes. This potentially makes nanoelectrodes better sensors and analytical tools than macroelectrodes in terms of their having lower limits of detection and faster detection times. However, due to difficulties with fabrication most nanoelectrode designs are highly irreproducible which has inhibited their characterisation and commercial development. The Mount group has previously reported the design, fabrication and characterisation of a novel nanoelectrode design in conjunction with Engineers from the Scottish Microelectronic Centre (SMC). Microsquare Nanoband Edge Electrode arrays (MNEEs) consist of an array of cavities with nanoscale Pt bands (formed by sandwiching the metal between insulating layers) exposed around their perimeter. MNEEs are fabricated using a photolithographic process so can be reproducibly made in large quantities to high fidelity. The purpose of this work is to develop our understanding of the fundamental electrochemical behaviour of MNEEs for biosensing. First, a quantitative analysis of the cyclic voltammograms (CVs) and Electrochemical Impedance Spectroscopy (EIS) of macroelectrodes, microelectrodes and MNEE are compared and discussed. Second, their fundamental response is compared in terms of their biosensing properties by using a pre-established impedimetric biosensing protocol developed on macroelectrodes. This protocol uses a PNA probe to detect the mecA cassette of methicillin resistant staphylococcus aureus (MRSA). The procedure has been optimised and compared for macroelectrodes, microelectrodes and MNEE so as to compare their performances as biosensors. It was observed that MNEE’s: (a) form thiol films faster than electrodes with larger dimensions, determined by kinetic studies of 6-mercaptohexan-1-ol film formation (b) form films with different packing structures dependant on the electrode bulk to edge ratio (c) can detect the same concentration of target in less time than larger electrodes because of their increased sensitivity. The film packing has also been quantitatively investigated using EIS and it can be seen that films formed n MNEE were better able to incorporate target DNA into their more splayed out structure. Unique to this project has been the establishment of a protocol to form heterogeneous carbazole-alanine hydrogel matrices on nanoelectrodes, whose polymerisation is initiated by a pH swing at the electrode surface induced by the oxidation of hydroquinone. The gels growth pattern follows the diffusion field at the electrode and can be monitored using EIS. This also gives a measure of the permeability of the gel by fitting to the correct equivalent circuit. The gel structure has been imaged using light microscopy, confocal microscopy and scanning electrochemical microscopy (SEM). The results give a visual demonstration that MNEE has enhanced diffusion at the corners of the cavities, which is in agreement with previously published simulations, and give evidence as to the onset of hemispherical diffusion and the conditions at which the diffusion field between neighbouring electrodes begin to overlap, a phenomenon which can be observed visually and correlated to changes in the EIS data. Hydrogels have been grown chronopotentiometrically at different currents and the permittivity (through the diffusion coefficients) has been measured of redox couples through gels grown at different speeds. It was found that the hierarchical structure of the hydrogels can be tuned; potentially opening the door to a new breed of tuneable, biocompatible anti-biofouling matrices on bio-functionalised electrodes. The system was characterised using the same MRSA detection protocol as optimised for the MNEE and the target DNA was found to be able to permeate through the hydrogels and bind to the probe, which resulted in a significant change in impedance