Piezoelectric quartz crystal monitoring of surface interactions.
Quartz crystal microbalances, (QCM), are high frequency oscillators, capable
of nanogram mass resolution in both air and liquid environments. ~ work has
produced data showing the feasibility of using the QCM for monitoring interactions in
liquids for several types of systems and has allowed comparison with surface plasmon
resonance (SPR) where appropriate.
Bulk phase changes in viscosity and density have been used in the development
of a QCM agglutination assay for the Staphylococcus epidermidis infection which has
reduced diagnosis periods by a factor of twelve.
Direct interactions at the crystal electrode have been employed when studying
bacterial adhesion to protein treated gold surfaces. It was shown that suspensions
containing as little as I x 10-2 cellsml-1 could be recognised using the QCM system.
A novel boronic acid - vicinal diol interaction mechanism has been employed as
a model for receptor-ligand binding. New boronic acid disulphide and short chain thiol
derivatives have been synthesised and the formation of self assembled monolayers of
the~e compounds monitored, both on the gold QCM electrodes and on the gold films
of SPR slides, the assembly mechanism being shown to fit a two stage model shown by
other workers for straight chain thiols. Monolayer orientation was confirmed using
SPR, by the binding of a range of saccharides and the diol containing enzyme cofactor
nicotinamide adenine dinucleotide.
The enzymes lactate dehydrogenase and glucose-6-phosphate dehydrogenase
have been shown to interact with bound NAD using both a novel flow injection system
with QCM detection and SPR. The low molecular weight saccharide, glucose, was
shown to bind reversibly to GDH on the surface of a QCM and the potential for kinetic
stu4ies recognised. This was taken one step further with a preliminary investigation
into sensing within real fluids, using diluted and spiked human seruin samples.