Thickness and viscosity of organic thin films probed by combined surface acoustic Love wave and surface plasmon resonance

Direct detection biosensors aim at detecting molecu- lar (antibody-antigen, DNA hybridation, cell attachment) binding events by means of electrical, mechanical or optical effects. A quantitative analysis of the amount of material bound to the surface requires the knowledge of the physical properties of the layer, namely optical index/dielectric constant, density, thickness, and a proper model of the interaction of this layer with the probing field (acoustic or optical). We here focus on the in- situ identification of the physical properties of thin organic (polymer and protein) layers bound to a substrate supporting the propagation of surface acoustic waves. In order to resolve some uncertainty on the resulting acoustic parameters, we propose the simultaneous probing of the same bound layer by optical methods (surface plasmon resonance) in a combined instrument as a means to uniquely identify the physical properties of the layer, namely the density, optical index, viscosity and thickness of the layer. We illustrate this technique for protein layers of collagen and fibrinogen. We then propose two models - transmission line and harmonic admittance computation - for analyzing these data and extract a quantitative viscosity information.