Optical transducers
In this approach, both catalytic and affinity reactions can be measured. The reaction can be measured. The reaction causes a change in fluorescence or absorbance resulting due to change in the refractive index of the surface between two media which differ in density. For instance, if antibodies bind on a metal layer, the refractive index of the medium in contact with this layer will change. Similarly, when antigen binds to antibodies already bound on this layer, the refractive index will change further. These changes in refractive index due to reactions are measured by fluorescence/absorbance. A popular example is a biosensor, which makes use of luciferase enzyme for detection of bacteria in food. The bacteria are lysed to release ATP, which is used by luciferase in the presence of O₂, to produce light that is measured by the biosensors. Pharmacia Biosensors AB (Sweden) marketed a device called BIAcore^TM, which measures antibody-antigen interactions for the study of drug-receptor interactions.