Response of a thickness-shear-mode acoustic wave sensor to the adsorption of lipoprotein particles

The properties of three different lipoprotein films were investigated using a thickness-shear-mode acoustic wave sensor in conjunction with an oscilla tor circuit that provides an automatic gain control (AGC) output. The oscil lator records a frequency shift that is dependent upon both the mass and di ssipative properties of the film and associated overlayers, but the shift i n AGC voltage (Delta AGC) depends only upon the dissipative properties of t he 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 empiri cal relationship between DeltaF(D) and AGC is found by measuring both param eters in purely dissipative fluids, which were mixtures of glycerol and wat er 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) acous tic wave sensor. The AGC change measured for a lipoprotein film is converte d to DeltaF(D). From this, the DeltaF(M) for the film can then be determine d by finding the difference between the total measured frequency shift for the film and DeltaF(D); the mass of the lipoprotein films is estimated usin g the Sauerbrey equation. The information provided by the AGC feature of th e oscillator circuit reveals that films formed from the low-density lipopro tein fraction are rigid and dense as compared to films formed from the high and very-low-density lipoprotein fractions.