Biosensors are devices that transforms information ranging from the concentration of a particular sample component to entire composition analysis, into an analytical useful signal (Köper, 2014).Chaplin, (2014) defines a biosensor as an analytical device, which can transform a biological response into an electrical signal. The term ‘biosensor’ is commonly used to refer to sensor devices used with the aim of defining the concentration of parameters and other substances of biological interest (Chaplin, 2014).
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There are two differences between an immobilization reaction and bimolecular reaction. One major difference is that in an immobilization reaction only a single reactant is moving while in a biomolecule reaction both reactants are mobile. The other chief difference is that in a bimolecular reaction, the reaction sites are independent whereas in immobilization reaction on surfaces the reaction sites are dependent (Köper, 2014).
There are four main methods for immobilization i.e. adsorption, covalent binding, entrapment and membrane confinement. Adsorption is the simplest method. It entails mixing the enzyme with an appropriate adsorbent, under suitable conditions of ionic strength and pH. After an adequate incubation period, the next step is washing off the loosely bound and unbound enzyme, which produces the immobilized enzyme in a more directly usable form. The dynamic force instigating this binding is frequently due to the amalgamation of hydrophobic effects and the creation of some salt linkages per enzyme molecule. The selection of the adsorbent mainly entails reducing the leakage of the enzyme during use. The physical links between the molecules of the enzymes and the support are strong. However, the reduction may occur due to several divergent factors such as the introduction of the substrate. It is good to be cautious to ensure that there is no weakening of the binding forces during use by unsuitable changes in ionic strength or pH. Suitable adsorbents include the porous carbon, glasses, ion-exchange matrices, clays, polymeric aromatic resins and hydrous metal oxides (Chaplin, 2014). Adsorbed bio-elements are vulnerable to changes in ionic strength, substrate, and pH. The utilization of adsorption methods occurs in cases of short-term investigations e.g. DNA or proteins on mica for Atomic Force Microscopy studies. Nevertheless, it has minimal satisfaction, as there is little control over the attachment site or the orientation (Köper, 2014). The following are the criteria for a good immobilization i.e. retained activity, stable, controlled orientation, accessibility, ‘correct’ amount and close to the surface (Köper, 2014).
The special methods of immobilization include biotin- streptavidin, biotin- linkers and protein A – antibody. Streptavidin, avidin and neutravidin are proteins that have a high affinity for biotin, and these complexes are employed to trap other bio-elements onto the transducer surface (Köper, 2014). The special method that entails utilization of streptavidin and biotin to immobilize proteins is a good example of the cofactor technique. The interface between the streptavidin and biotin is one of the most powerful noncovalent interactions in nature. The formation of streptavidin monolayers involves the immobilization onto a self-assembled biotin monolayer or by forming a Langmuir-Blodgett of lipid-tagged biotin. Since streptavidin comprises four symmetrical biotin-binding sites, after streptavidin immobilization, the two remaining biotin-binding sites can be used to immobilize an additional layer of biotinylated proteins or enzymes. If the attachment of biotin is to the immobilized enzyme at a distinctive site e.g. a surface-exposed cysteine, the orientation of the enzyme is by the biotin-streptavidin interaction. Despite an extremely tight binding interaction, this special immobilization method does not encompass the substrate pocket. Therefore, the enzymes are left free to bind to the substrate and turn over product (Mathur & Singh, 2009).
Examples of biosensors include electrochemical biosensor, electrochemical transducers, field effect transistors (FETs), and Silicon nanowire FETs. Electrochemical biosensors are classified according to the measured electrical parameter into potentiometric (measure potentials at equilibrium), amperometric (measure current), impedimetric (for impedance) and conductometric (measure conductance or resistance). Silicon nanowire FETs currently have gotten incredible attention as a capable tool as biosensors due to their selectivity, sensitivity, real-time detection capabilities.The application FETs that are ion-sensitive occurs in the measurement of concentrations of ions such as H+ in pH measurements. The sample solution is usually in direct contact with the FET gate-electrode material, which determines the gate voltage that in turn controls the source-to-drain current through the FET ( McGrath & Cliodhna Ni Scanaill, 2013). The use of FETs as sensors entails replacement of the gate electrode with e.g. a detection element that can uptake charged species. The aftermath is the change in the electrical field. Change in the electrical diameter of the conduction channel then results in conductance changes between the source and the drain, which is now measured.
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