Fluorescence‐based BiosensorsUpdate based on original article by John D. Brennan, Encyclopedia of Analytical Chemistry, ©2000, John Wiley & Sons Ltd.
Authors: Schäferling Michael 
Published: December 2011

Biosensors, as defined by Pure and Applied Chemistry, are ‘chemical sensors in which the recognition system utilizes a biochemical mechanism. The biological recognition system translates information from the biochemical domain, usually an analyte concentration, into a chemical or physical output signal with a defined sensitivity’.(1)It is also appointed that chemical or biological sensors contain two basic components connected in series: a chemical or biomolecular recognition system (receptor) and a physicochemical transducer. According to this prerequisite, this overlook is confined to sensor devices that combine a biomolecular recognition element with an optical signal transducer. Homogeneous or intracellular assays using fluorescent molecular probes or nanoparticles are not considered, although they are frequently termed asmolecular sensorsornanosensorsin the literature. Fluorescence‐based biosensors are generalized as those devices that derive an analytical signal from a photoluminescent (either fluorescence or phosphorescence) emission process. Chemi‐ or bioluminescent detection systems are only briefly discussed in this review.

Biosensors are used for a wide variety of tasks, including detection of compounds of biomedical.(2)environmenta.(3)or defense interes.(4); on‐line monitoring for process contro.(5); quality control of foodstuff.(6); selective detection of compounds undergoing a chemical separatio.(7); and screening of drug compounds.(8)Advantages of such devices include high selectivity, rapid response times, reusability, amenability to remote analysis, and immunity to electrical interferences.(9)The selective nature of complexation between biomolecule and analyte and the small size of sensor devices can be combined with advanced detection techiques such as total internal reflection (TIR) spectroscopy. This results in an ability to measure analytes in complex matrices with unsurpassed sensitivity. Such samples may include highly scattering components such as milk or whole blood.(11)or relatively inaccessible locations such as groundwater wells, or even intracellular environments.(12)The key limitation of such devices mainly centers on the poor stability of biological compounds, which can lead to a substantial drift in instrumental response over time. The so‐called Cambridge definition appoints another characteristic property of sensors.(13)Therein, they are defined as ‘miniaturized devices which can deliver real‐time and on‐line information on the presence of specific compounds or ions in even complex samples’. Accordingly, a sensor is expected to respond reversibly and continuously. With the exception of some enzymatic sensors, these conditions are not fulfilled in case of most biosensors. Particularly, in devices where immunological reagents or DNA are used as recognition elements, they show a lack of reversibility and operate only as a ‘one‐shot’ screen, without the potential for continuous, quantitative analysis. Nevertheless, the designationsimmunosensorsorDNA sensorsbecame accepted for such analytical or diagnostic tools.