Biosensors for Environmental Monitoring and Water Control: Review - State of the Art
Context This paper reviews the use of biosensors for environmental biomonitoring and especially for the detection of water pollutants. These systems are developed in view of on-line applications, continuous and real time analysis. The principle and the design of the different systems proposed for this purpose are described with their performances deduced from pilot or in situ studies carried out up to now. Automation and autonomy, sensitivity and specificity are critical points that will determine the success of their applications in biomonitoring and the kind of application that can be envisaged. It is necessary they require minimal human intervention for maintenance and working . The more sensitive systems can be used for the monitoring of drinking and ground waters, the less sensitive ones for the monitoring of complex effluents, more heavily contaminated. Biosensors can be distinguished on the basis of the type of biocatalyst associated with thetransducer: the biological signal delivered by the biocatalyst is transmitted to a detector, also called transducer. The transducer, which may be an optical, electrochemical or piezoelectrical detector, transforms the biological response into an electric signal. This signal can be easily amplified and interpreted in terms of the toxicity and level of pollution of the analyzed sample. Three categories of biosensors can be defined: - biosensors using aquatic vertebrates and invertebrates: fish, microcrustacea, bivalves. Their behavior in the tested medium is studied as the criterion for toxicity; - cellular sensors, measuring physiological and biochemical functions such as respiration, bioluminescence, and photosynthesis, in microorganisms immobilized on the transducer (bacteria, yeast, microalgae,..) or suspended in the tested medium (activated sludge); - biosensors measuring an "affinity" response and a specific binding between enzyme/substrate or antibody/antigen. These systems use enzymes or antibodies immobilized in close contact with the transducer; they may detect the (analogs of) enzymatic substrates and inhibitors, or the (analogs of) antigenic substances binding to the antibody. These systems appear promising on the basis of their sensitivity. At present they can be applied for the detection of triazines and phenols. Such systems need to be developed and extended to other pollutants in order to cover the wide range of aquatic contaminants. User-friendliness, attendance and maintenance requirements, and service life are other critical aspects affecting the performances of a biosensor. These qualities need to be evaluated during the validation step of the equipment. In situ validation is essential for evaluating the relevance of the system in environmental biomonitoring and its applications. It is probable that among the numerous systems proposed as biosensors, only a few will be considered as suitable tools for on-line monitoring of waters.