ABSTRACT
In the development of biosensors, optical transduction methods have, from the beginning, attracted a great deal of interest and research effort. The reasons for this become clear when one considers the constraints imposed by the biosensor format: biosensors use receptor molecules (often antibodies or DNA strands) attached to a solid transducer surface which is in contact with the sample to be analysed. The presence of analyte in the sample is detected by measuring the binding of analyte molecules to the surface-bound receptors. The number of receptor molecules that are bound to the transducer surface is normally very low (for a medium-sized protein such as streptavidin, 1012 molecules per square centimetre represents a good surface coverage), which implies that the method used to detect the binding of the analyte to the surface must be very sensitive. In addition, the detection technique should be surface specific – sensitive only to molecules at or very close to the surface – so that interference from the bulk solution is eliminated. This is particularly important if washing steps are to be reduced or eliminated from the assay. Finally, the choice of detection method is further limited by the requirement that measurements be performed in aqueous solution, and that the method be non-destructive so that real-time measurements giving kinetic information can be performed. Optical methods fulfil all these requirements and have the additional advantage of a long history of use in biochemical and chemical analysis. Examples such as UV-vis and infrared absorbance spectroscopy, fluorescence spectroscopy and Raman spectroscopy demonstrate the extent to which optical analysis methods are established and the wide range of information about molecular structure, conformation and environment that they can give.
