Techniques to evaluate the mass sensitivity of Love mode surface acoustic wave biosensors

We investigated the mass sensitivity of Love mode SAW biosensors by experimental and theoretical techniques. A first experimental approach is based on the analysis of the dispersion curve of the sensor, which helps to determine by derivation a value of the mass sensitivity. In order to obtain the dispersion curve, we have performed a chemical wet etching procedure, which enables the continuous monitoring of the transfer function during the etching of the entire guiding layer. A second experimental approach is based on the addition and removal of layers in known quantities. The mass sensitivity is estimated in different cases: etching of a thin gold layer, copper electrodeposition, and surface adsorption of an ionic surfactant. The results obtained by these techniques are compared to a theoretical model. In the theoretical model, the layered structure of the acoustic sensor is described in terms of mechanical transmission lines and the model is used to determine the sensitivity of the acoustic device to density and viscosity variations of thin films and bulk materials. The attenuation and phase shifts are quantified and analyzed to identify the type of interaction between the adsorbed biomolecules layer and the sensor. We conclude that the Love mode SAW biosensor must be considered and optimized as a viscosity sensor and an independent layer thickness measurement is needed to apply adequately the theoretical model.