38 Basic principles of a biosensor The development of biosensors is a very multidisciplinary research field. It requires the integration of disciplines like biochemistry, microelectronics, biology, surface chemistry and physics at the micro- and nanoscale. In general, a sensor can be defined as a system that generates a specific electronic signal as a result of an external stimulus, allowing the quantification of certain physical (temperature, pressure, mass,…) or chemical (pH, O2,…) properties. Sensors where biological components such as proteins, oligonucleotides, cells and tissue are included in the system and used for the generation of a specific signal towards the target component, are denoted as biosensors. A schematic representation of the working principle of a biosensor is depicted in Figure 1. A specific biosensor is characterized by three main aspects: (i) biological recognition element, (ii) transducer system, and (iii) integration of this recognition element with the transducer system. CHEMICAL COMPONENT BIOLOGICAL RECOGNITION ELEMENT INTEGRATION TRANSDUCER SYSTEM BIOSENSOR SIGNAL GENERATION Figure 1. Schematic representation of the working principle of a biosensor. A (bio)chemical component (DNA/RNA strands, proteins, low molecular weight compounds,…) reacts with a biosensor, composed of three essential elements: (i) a biological recognition molecule, specific for the target molecule, (ii) a transducer system, and (iii) the integration of the biorecognition molecule with the transducer system. The accuracy of a biosensor depends on the specificity and the selectivity of the biorecognition molecule in relation to the target. The biosensor is mostly restricted to the quantification of one specific (or one specific class of) target(s).
There is a broad spectrum of biological recognition molecules available to act as capturing agent for the target molecule. Enzymatic biosensors generate a signal by means of an enzymatic conversion of the target molecule into an optically or electrochemically detectable component. Examples include the detection of pesticides and herbicides in fruits and vegetables and drinking water (Mello et al., 2002). Antibodies are commonly used for the detection of antigens in the socalled immunoassays. These assays are reported highly performant for the detection of toxins and pathogens in food. DNA and RNA bioreceptors are used for the quantification of complementary DNA or RNA strands (e.g. pathogen detection in food). In the last few years, the possibility to use cell structures and tissue as specific recognition molecules, has been investigated for the detection of toxic compounds in food samples (Mello et al., 2002). Aptamers are a recently new and promising class of biorecognition molecules. Aptamers are oligonucleotides which can be designed with a receptor function for a multitude of target molecules. Selection of these molecules happens through an iterative in vitro selection procedure. Aptamers are considered mainly as an interesting alternative to antibodies in immunoassays. Compared to antibodies, aptamers are very stable and can be produced in large quantities with very reproducible characteristics. In addition, they can easily be chemically modified to improve the bioreceptor performance in food samples. A crucial point in the design of a biosensor is the integration of the bioreceptor with the transduction system. This happens through surface chemistry. Nanotechnology has contributed substantially in creating and characterising bioreceptor layers. The choice of the immobilisation strategy is crucial to create a stable and sensitive biosensor surface that avoids aspecific binding of unwanted food components. The latter results in false positive results. Different immobilisation strategies and surface characterisation methods have been described in the literature ranging from covalent linkage, cross-linking, adsorption, adsorption-cross-linking or encapsulation. Depending on the requested performance of the biosensor one of these methods is selected. As mentioned above, a transducer translates the interaction between a bioreceptor and its target into an electric signal. A multitude of bioreceptors has been described in the literature. In general, differentiation is made between three different groups of transducer systems: electrochemical, piezo-electrical and optical systems. The electrochemical transducer systems, based on 39