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EETE APR 2014

Sensor designs All biosensors usually involve minimal sample preparation as the biological sensing component is highly selective for the analyte concerned. The signal is produced by electrochemical and physical changes in the conducting polymer layer due to changes occurring at the surface of the sensor. Field effect transistors (FETs), in which the gate region has been modified with an enzyme or antibody, can also detect very low concentrations of various analytes as the binding of the analyte to the gate region of the FET causes a change in the drain-source current. Many recent advances in biosensors have come through the use of graphene, which offers unique physiochemical properties, and superior mechanical, thermal, and electrical properties. Graphene-based biosensors offer the potential for high sensitivity because graphene is a two-dimensional single atomic layer of graphite that can maximize the interaction between the surface dopants and adsorbates. Graphene has much lower Johnson noise (the noise in a resistive material caused by thermal motion of charge carriers) than carbon nanotubes that are functionalized for bio-detection applications. Therefore, a very small variation in the carrier concentration in a graphene biosensor can cause a notable variation in electrical conductivity that can be measured. Depending on the analyte and bioreceptor, the transducer portion of a biosensor could employ any of these mechanisms: Amperometric: Amperometric devices detect changes in current. They measure currents generated when electrons are exchanged between a biological system and an electrode. Potentiometric: Some reactions cause a change in voltage (potential at constant current) between electrodes that can be detected or measured. Conductive: Conductimetric devices detect changes in conductivity between two electrodes. Resistive: Resistivity is the inverse of conductivity, and can be measured with similar methods. Capacitive: When the biorecognition reaction causes a change in the dielectric constant of the medium near the bioreceptor, capacitance measurement method can be used as a transducer. Piezoelectric: In a piezoelectric material, there is a coupling between its mechanical and electrical properties. Fig. 1: A generic biosensor structure. Fig. 2: An SMU instrument configured as a constant current source and a voltmeter. It can be used to create an electrical oscillator whose frequency can be varied and measured by varying a mass applied to its surface. In the case of a biosensor, that mass can change due to the reaction taking place on the surface. Thermal: These devices measure changes in temperature. Optical: Optical biosensors correlate changes in concentration, mass, or number of molecules to direct changes www.electronics-eetimes.com Electronic Engineering Times Europe April 2014 31


EETE APR 2014
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