Biotransducer

A biotransducer is the recognition-transduction component of a biosensor system. It consists of two intimately coupled parts; a bio-recognition layer and a physicochemical transducer, which acting together converts a biochemical signal to an electronic or optical signal. The bio-recognition layer typically contains an enzyme or another binding protein such as antibody. However, oligonucleotide sequences, sub-cellular fragments such as organelles (e.g. ) and receptor carrying fragments (e.g. cell wall), single whole cells, small numbers of cells on synthetic scaffolds, or thin slices of animal or plant tissues, may also comprise the bio-recognition layer. It gives the biosensor selectivity and specificity. The physicochemical transducer is typically in intimate and controlled contact with the recognition layer. As a result of the presence and biochemical action of the analyte (target of interest), a physico-chemical change is produced within the biorecognition layer that is measured by the physicochemical transducer producing a signal that is proportionate to the concentration of the analyte. The physicochemical transducer may be electrochemical, optical, electronic, gravimetric, pyroelectric or piezoelectric. Based on the type of biotransducer, biosensors can be classified as shown to the right.

Electrochemical biosensors contain a biorecognition element that selectively reacts with the target analyte and produces an electrical signal that is proportional to the analyte concentration. In general, there are several approaches that can be used to detect electrochemical changes during a biorecognition event and these can be classified as follows: amperometric, potentiometric, impedance, and conductometric.

Amperometric transducers detect change in current as a result of electrochemical oxidation or reduction. Typically, the bioreceptor molecule is immobilized on the working electrode (commonly gold, carbon, or platinum). The potential between the working electrode and the reference electrode (usually Ag/AgCl) is fixed at a value and then current is measured with respect to time. The applied potential is the driving force for the electron transfer reaction. The current produced is a direct measure of the rate of electron transfer. The current reflects the reaction occurring between the bioreceptor molecule and analyte and is limited by the mass transport rate of the analyte to the electrode.

Potentiometric sensors measure a potential or charge accumulation of an electrochemical cell. The transducer typically comprises an ion selective electrode (ISE) and a reference electrode. The ISE features a membrane that selectively interacts with the charged ion of interest, causing the accumulation of a charge potential compared to the reference electrode. The reference electrode provides a constant half-cell potential that is unaffected by analyte concentration. A high impedance voltmeter is used to measure the electromotive force or potential between the two electrodes when zero or no significant current flows between them. The potentiometric response is governed by the Nernst equation in that the potential is proportional to the logarithm of the concentration of the analyte.

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