Reagentless fluorescent biosensors based on proteins for continuous monitoring systems

There is a lack of commercially available efficient and autonomous systems capable of continuous monitoring of (bio)chemical data for clinical, environmental, food, or industrial samples. The weakest link in the design of these systems is the (bio)chemical receptor (bCR). The bCR should have transducer ability, the recognition event should be a single reaction, and the bCR should be easily regenerated. Transport proteins and enzymes are well placed as bCR for optical continuous monitoring systems (OCMS). In this paper we review quantitative aspects and the main transducer strategies which have been developed for transport proteins, using periplasmic binding proteins (linking an environmentally sensitive fluorophore or FRET between two fluorophores) and concanavalin A (competitive reversible assays) as representative examples. Efficient immobilization systems and implementation in OCMS are also reviewed. Some kinds of enzymes can fulfil the necessary requirements to be appropriate bCR. Strategies using flavoenzymes chemically modified with fluorophores can be successfully implemented in OCMS and they are, in our opinion, the most appropriate option.