Electrochemical characterisation of microsquare nanoband edge electrode (MNEE) arrays and their use as biosensors
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Date
30/11/2017Author
Piper, Andrew
Metadata
Abstract
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.