Direct detection transducers convert a physical phenomenon -- in our case the adsorption of (bio)chemical compounds on a surface -- to a signal representative of the phenomenon under investigation. Such devices have exhibited increasing interest with respect to classical (bio)chemical measurement strategies involving preliminary sample preparation (fluorescent or radioactive labeling) since they allow for continuous monitoring without supervision of the measurement process. Among the most classical techniques, micro-cantilevers and tuning forks are based on a vibration frequency/bending measurement and resonant mirrors on the optical transmission change due to optical index variations.
Our interest will focus on some of the very classical measurement techniques designed for improved signal to noise ratio by using evanescent waves, namely Quartz Crystal Resonators (QCR) and Surface Acoustic Wave (SAW) transducers for acoustic measurement techniques, and Surface Plasmon Resonance (SPR) as an optical technique. Our research activity has focused on the characterization of thin organic films as needed for the quantitative analysis of the recorded signals. Indeed, while a basic conversion from observed signal to adsorbed mass appears valid for small globular proteins, the analysis becomes more complex for longer fibrillar proteins and a detailed analysis is needed to understand discrepancies between optical and acoustic measurement strategies. Our approach in extracting the physical parameters of thin films, beyond the common use of bulk material properties, is based on the combination of multiple measurement techniques to extract individual quantities including optical index, density, viscosity and layer thickness. We shall thus consider atomic force microscopy (AFM) combination with QCR, SAW/SPR, and AFM/SAW.
The latest considerations under investigation involve white light SPR and broadband acoustic transducers (HBAR) allowing for a real time full dispersion relation characterization of the (bio)molecule properties during adsorption on a surface.
Although the research topics involved in the development of a functional biosensor range from biology to engineering through surface chemistry characterization and physics, the presentation will solely focus on the understanding of the author on topics related to the analysis of the recorded data for physical property extraction.
 E. Gizeli and C.R. Lowe, Biomolecular Sensors (2002)  publications available at http://jmfriedt.free.fr/ refs 73, 103, 111, 112 for example