User:Krishnasinghaec/sandbox

Electrochemical biotransducers[edit]

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[edit] 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[edit] 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. Impedance[edit] Electrochemical impedance spectroscopy (EIS) involves measuring resistive and capacitive changes caused by a biorecognition event. Typically, a small amplitude sinsusoidal electrical stimulus is applied, causing current to flow through the biosensor. The frequency is varied over a range to obtain the impedance spectrum. The resistive and capacitive components of impedance are determined from in phase and out of phase current responses. Typically, a conventional three-electrode system is made specific to the analyte by immobilizing a biorecognition element to the surface. A voltage is applied and the current is measured. The interfacial impedance between the electrode and solution changes as a result of the analyte binding. An impedance analyzer can be used to control and apply the stimulus as well as measure the impedance changes. Conductometry[edit] Conductometric sensing involves measuring the change in conductive properties of the sample solution or a medium. The reaction between the biomolecule and analyte changes the ionic species concentration, leading to a change in the solution electrical conductivity or current flow. Two metal electrodes are separated at a certain distance and an AC potential is applied across the electrodes, causing a current flow between the electrodes. During a biorecognition event the ionic composition changes, using an ohmmeter the change in conductance can be measured. Optical biotransducers[edit]

Optical biotransducers, used in optical biosensors for signal transduction, use photons in order to collect information about analyte.[2] These are highly sensitive, highly specific, small in size and cost effective. The detection mechanism of optical biotransducer depends upon the enzyme system that converts analyte into products which are either oxidized or reduced at the working electrode.[3] Evanescent field detection principle is most commonly used in an optical biosensor system as the transduction principle. This principle is one of the most sensitive detection methods. It enables the detection of fluorophores exclusively in the close proximity of the optical fiber. [4]