Spectroscopic ellipsometry and biology: recent developments and challenges

In surface biology there are numerous studies carried out using single wavelength ellipsometry, especially in the area of macromolecular adsorption on solid surfaces. The results obtained contribute significantly to the understanding of the basic mechanisms of adsorption and surface dynamics of organic molecules, especially of proteins. An example of an area of great importance is biomaterials, where ellipsometry is used as a tool in the process of acquiring knowledge about the biological acceptance of new as well as currently used implant materials. In the area of affinity biosensors, ellipsometry has been suggested as a potential readout principle. Ellipsometry is also a tool in emerging technologies, such as surface molecular engineering with the aim to construct molecular superstructures with predesigned biological functions and to interface biology with electronics. However, in most cases when ellipsometry is applied in biology, it has been used for surface mass determination. The potential in using spectroscopic data for resolving microstructural and dynamic information has not been exploited fully. From the above perspective, this report reviews the use of spectroscopic ellipsometry for studies in surface biology and highlights the advantages it offers. Two main themes are developed. The first is spectroscopy on monolayers of macromolecules with emphasis on determination of their dielectric functions and microstructure. A specific example discussed is ferritin adsorption on gold. The results, including dynamics of both the surface mass and layer microstructure, indicate an adsorption model based on a two-state adsorption mechanism. The second theme is ellipsometrically based biosensor systems. The discussion covers aspects of what imaging ellipsometry can provide in this context and is exemplified by results from affinity biosensor and gas sensor systems.