Optical probes and transducers

Biosensors are by definition a combination of a biological receptor compound and a physical or physicochemical transducer. Therefore, the transducing structure is a critical part of every biosensor. In the development of new and improved biosensing layers the importance of the transducing structure is not restricted to the substrate to which biological structures have to be coupled. A field of even greater importance is the use of transducers as probes providing information on the structure and function of biosensing layers, and their relation to a transducer surface. The aim of this paper is to give an overview on optical transducer principles and optical (surface) analytical techniques relevant as part of biosensing structures as well as probes in the development and optimisation of biosensing layers. Categories discussed are basic optical effects, materials involved, surface chemistry, the principal and technological limits of spatial resolution, and sensitivity. The intimate relation between the spatial resolution of a probe, the resulting size of interaction areas, and the feasibility of array structures is pointed out. Two interferometric methods are presented in principle, and their application to biosensing and some results are discussed in detail. The necessity to characterise receptor layers to get detailed information about the interaction process is pointed out. The close relationship between optimal characterisation of layers by selection of adequate probe technologies and improvement of probe performance, and the development of new biosensing layers is discussed. Finally, an outlook is given for future aspects of improved spatial resolution and multianalyte detection.