Response of a thickness-shear-mode acoustic wave sensor to the adsorption of lipoprotein particles
by Snellings, S. L.; Fuller, J.; Paul, D. W.
The properties of three different lipoprotein films were investigated using a thickness-shear-mode acoustic wave sensor in conjunction with an oscillator circuit that provides an automatic gain control (AGC) output. The oscillator records a frequency shift that is dependent upon both the mass and dissipative properties of the film and associated overlayers, but the shift in AGC voltage (Delta AGC) depends only upon the dissipative properties of the film. The total recorded frequency shift (DeltaF(T)) is treated as a sum of two terms: DeltaF(M), the mass contribution of the frequency shift, and DeltaF(D), the frequency shift attributed to energy dissipation. An empirical relationship between DeltaF(D) and AGC is found by measuring both parameters in purely dissipative fluids, which were mixtures of glycerol and water in the experiments presented here, and establishing a calibration curve from the corresponding data, DeltaF(T) and AGC changes were measured during the formation of lipoprotein films on the thickness shear mode (TSM) acoustic wave sensor. The AGC change measured for a lipoprotein film is converted to DeltaF(D). From this, the DeltaF(M) for the film can then be determined by finding the difference between the total measured frequency shift for the film and DeltaF(D); the mass of the lipoprotein films is estimated using the Sauerbrey equation. The information provided by the AGC feature of the oscillator circuit reveals that films formed from the low-density lipoprotein fraction are rigid and dense as compared to films formed from the high and very-low-density lipoprotein fractions.