1.6. Transducers Used
Conventional Transducers. Majority of biosensors existing today use three types of transducers for converting the action of the bioreceptor molecule into measurable signal. These are: (1) amperometry based on H2O2 or O2 measurement; (2) potentiometry based on pH or pIon measurement; (3) photometry utilizing optical fibers (see Fig. 11).
Biorecognition reactions often generates chemical species that can be measured by electrochemical methods. In amperometry (Fig. 1.11a), typically the reaction product is H2O2 (or the reactant is O2) which can be measured by a pair of electrodes (Fig. 1.11a). When a suitable voltage is impressed on one of the electrode against a reference electrode (typically Ag/AgCl or Calomel), the target species (H2O2 or O2) is reduced at the electrode and this generates electrical current (hence the name 'amperometry'). In potentiometry, a glass membrane or other membrane electrode is used for measuring the membrane potential (hence the name potentiometry) resulting from the difference in the concentrations of H+ or other positive ions across the membrane. In photometry (Fig. 11c), the light from an indicator molecule is the measured signal. For this method to work, one of the reactants or products of the biorecognition reaction has to be linked to colorimetric, fluorescent or luminescent indicator molecules. Usually, an optical fiber is
Fig. 10. Possible bioreceptor molecules and molecular assemblies for biosensor applications; their requirements for structural integrity and signals generated.
Fig. 1.11. Three conventional transducers used for biosensor development: (a) photometric; (b) potentiometric (based on pH sensor); (c) amperometric (based on Clark oxygen sensor).
Table 1.5. Other transducers used in biosensors.
|Piezoelectric||change in mass|
|Capacitive||dielectric constant||antibody sensors|
used for guiding the light signals from the source to the detector. Adaptation and exploitation of these three routes (photometric, potentiometric and amperometric) where user acceptability is already established, has been an obvious approach to the development of reagentless biosensor devices with a high specificity and selectivity.
Piezoelectric Transducers. The transducer of a biosensor is not restricted to the three described above. In principle, any variable which is affected by the biorecognition reaction can be used to generate the transduced signal.The piezoelectric and surface acoustic wave devices utilize a surface which is sensitive to changes in mass. These transducers have been used where the biorecognition reaction causes a change in mass.
Conductimetry. Monitoring solution conductance was originally applied as a method of determining reaction rates. The technique involves the measurement of changes in conductance due to the migration of ions. Many enzyme-linked reactions result in a change in total ion concentration and this would imply that they are suitable for conductimetric biosensors.
Capacitance Measurement. When the biorecognition reaction causes a change in the dielectric constant of the medium in the vicinity of the bioreceptor, capacitance measurement method can be used as a transducer. Antigen-antibody reaction is a good example. Suppose antibody molecules are immobilized between two metal electrodes of known area. When antigen is added and binds with the antibody, the dielectric constant of the medium between the two electrodes is expected to change significantly. This change translates into a change in capacitance.
Thermometry All chemical reactions are accompanied by the absorption (endothermic) or evolution (exothermic) of heat. Measurements of H, the enthalpy of reaction at different temperatures allows one to calculate S (entropy) and G (Gibbs free energy) for a reaction and therefore collect basic thermodynamic data. The hydrolysis of ATP for example is exothermic:
ATP4- + H2O~ADP3- + HPO4- + H+; DH298 = - 22.2 kJ (pH 7)
or the immunoreaction between anti-HSA and its antigen HSA yields -30.5 kJ/mol. For this latter reaction, the total increase in temperature for 1 mol of antibody is of the order of 10-5 K, but many enzyme-catalysed reactions have greater H, and produce more easily measurable changes in temperature.
Enzyme Thermistor. For a biosensor device, the biorecognition compound must be immobilized on a temperature-sensing element capable of detecting very small temperature changes. The major initiative in this area has come from the Mosbach group at the University of Lund. Initially, they immobilized glucose oxidase or penicillinase in a small column, so that temperature changes in the column effluent were monitored by thermistors to give an enzyme thermistor sensitive to glucose and penicillin, respectively. They have also applied the technique to other substrates and to immunoassay using an enzyme-labeled antigen.
FET as a Transducer. As advances are made in biosensors, there was a need for miniaturization and mass production. Field effect transistors (FET) used extensively in semiconductor industry in memory chips and logic chips respond to change in electric field (in front of the 'gate' of an FET). An FET is thus capable of detecting changes in ion concentration when the gate is expose to a solution that contains ions. Therefore, pH and ion concentration can be measured with an FET. The advantage of this transducer is that it can be incorporated directly to the electronic signal processing circuitry. In fact, pen-size FET based pH sensor is being marketed commercially.