Characterisation of fatty acid multilayers using a TSM biosensor

Abstract

Thickness shear mode (TSM) biosensors have many potential applications within the pharmaceutical sciences as a means of measuring mass changes in the nanogram range, film thickness, viscosity and shear moduli. This study addresses the possible use of the TSM sensor as a biosensor for measuring drug partition coefficients. In order to realise this potential, some fundamental understanding is required of the behaviour of lipid films on the sensor. The present study characterises the behaviour of fatty acid multilayers as a suitable model chemical system. Frequency shifts and impedance spectra are presented for multilayers of three fatty acid films coated on to the sensor using a Langmuir-Blodgett trough. The results indicate that the frequency shift is non-linear at lower numbers of fatty acid layers but the response is Sauerbrey-like at higher numbers of layers. Also at high numbers of layers, changes in the impedance spectra indicate viscoelastic behaviour in thicker membranes. An inverse relationship is observed between chain length and frequency shift, which is attributed to variations in the topography of the sensor surface. This work demonstrates the importance of fully characterising the physical behaviour of the lipid multilayers prior to using these systems for the measurement of drug partition coefficients.