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An acoustic technique was used to monitor the interaction of the pore-forming cytolytic toxin CytB with a positively charged supported lipid bilayer. The acoustic device, which is based on a waveguide geometry, is sensitive to changes in the mass of the supported bilayer. The specificity of the interaction, rate and extent of the association, reversibility and effect of previous depositions of toxin were investigated. The CytB was found to bind irreversibly to the lipids at all fractional coverages even when the protein-to-lipid ratio was high enough to imply that the protein was associating with the external surface of the bilayer. The CytB formed stable structures with the bilayer at high protein surface concentrations and did not appear to disrupt the bilayer in the manner of a detergent. The rate of association with the bilayer was found to be directly proportional to the solution concentration of CytB at higher concentrations but appeared to be low at a CytB solution concentration of 5 μg mL^-1, leading to relatively low amounts of CytB being associated with the bilayer.

The aim of this work is to quantify the viscoelastic properties and calculate the density of a protein layer deposited on a solid/liquid interface. Devices, which use shear-horizontal surface acoustic waves propagating in a waveguide configuration at 108 and 155 MHz, were employed in the detection of specific binding and adsorption of IgG antibodies to protein A modified surfaces and gold, respectively. To obtain both viscoelastic and mass information about the biological layers, each device was first calibrated to viscosity changes by introducing a series of glycerol−water solutions to the sensing surface. Via this calibration, an equivalent viscosity of 1.00−1.30 cP for IgG layers in solution was obtained. By modeling the proteins as solid noninteracting particles, the quantitative viscoelastic information was then used to calculate protein surface coverages of 10−300 ng/cm², which was in good agreement with surface plasmon resonance measured surface coverages. Modeling results yield information about the mechanical and chemical interactions between proteins in surface layers. Furthermore, for the first time, simultaneous quantitative, real-time mass and viscoelastic information of protein layers at a solid/ liquid interface was achieved by this technique.

An acoustic wave device was used to study the deposition of lipid vesicles on solid surfaces of different hydrophilicity. The device operated at 103 MHz and consisted of a quartz substrate with a polymer overlayer acting as an acoustic waveguide and a thin layer of gold. The hydrophilicity of the device surface was controlled by modifying the gold surface with hexadecanethiol (HDT) for 16 h and mercaptoundecanol (MUO) for 10 min and 16 h in order to obtain a hydrophobic, relatively hydrophilic, and hydrophilic surface, respectively. The interaction of a vesicle suspension of 2-oleoylpalmitoyl-sn-glycero-3-phosphocholine (POPC) in PBS with each surface was monitored by recording the phase and amplitude of the acoustic wave in real time. The acoustic signal detected on the hydrophobic (HDT) and hydrophilic (16 h MUO) surfaces indicated the formation of a supported monolayer and bilayer, respectively, while a vesicle layer was detected on the less hydrophilic (10 min MUO) surface. The above findings were confirmed by using ¹⁴C-labeled lipids and by monitoring the nonspecific binding of BSA on each surface. These experiments clearly showed that the hydrophilic properties of the solid surface are very important for designing a two dimensionsional (bilayer) or three-dimensional (vesicle) membrane-type interface layer. Furthermore, the simultaneous monitoring of the phase and amplitude of the acoustic wave was shown to provide complementary information related to mass and viscoelastic interfacial changes.

In this work we present a novel pulse mode Love wave biosensor that monitors both changes in amplitude and phase. A series of concentrations of 3350 molecular weight poly(ethylene glycol) (PEG) solutions are used as a calibration sequence for the pulse mode system using a network analyzer and high frequency oscilloscope. The operation of the pulse mode system is then compared to the continuous wave network analyzer by showing a sequence of deposition and removal of a model mass layer of palmitoyl-oleoyl-sn-glycerophosphocholine (POPC) vesicles. This experimental apparatus has the potential for making many hundreds of measurements a minute and so allowing the dynamics of fast interactions to be observed.

Simultaneous operation of a Love wave biosensor at the fundamental frequency and third harmonic, including the optimisation of IDT metallisation thickness, has been investigated. Data is presented showing a sequence of deposition and removal of a model mass layer of palmitoyl-oleoyl-sn-glycerophosphocholine (POPC) vesicles while frequency hopping between 110 and 330 MHz.