Fluorescence-Based Optical Biosensors

Biosensors integrate biological molecules with a signal transduction device to produce a signal when a molecular recognition event occurs. This tutorial will focus entirely on biosensors that employ optical devices and incorporate fluorescent mechanisms for signal transduction. Compared with the electrochemical glucose sensors found in pharmacies world wide, optical biosensors are often more complex and more costly. However, optical biosensors are better suited for repetitive analysis or continuous monitoring, for interrogation of complex fluids, and for measuring binding events in real time. Optical imaging methods have also been widely adapted to measuring microarrays of recognition events; this experience provides a base for the development of highly multiplexed optical biosensors.

Optical biosensors not included in the following discussion are, nonetheless, worth mentioning. This group of biosensors is primarily directed at detection of a target without the requirement for a label. Thus performing the assay is simplified by requiring only the exposure of the sample (containing target) to the biological recognition molecule. Noteworthy among these methods are interferometry, surface plasmon resonance, resonant and antiresonance reflectometry, and cantilever-based systems [1, 2]. All of these sensors measure a change in optical properties, usually refractive index, at the sensing surface where the recognition molecules are immobilized. Convenient to use, they are excellent tools for measuring reactions in well-defined fluids. They tend to be less sensitive to very small targets or to very large targets that have most of their mass outside the sensing region; improvements in waveguide technology are minimizing these problems. However, nonspecific adsorption of components from complex samples can reduce the sensitivity by creating a significant background signal that must be accurately subtracted.