Piezoelectric enzyme sensors

The piezoelectric sensor is a gravimetric device which measures the mass of a sample deposited on the surface of a piezoelectric crystal by detecting the variation in its characteristic resonance frequency. The selectivity of the sensor is ensured by depositing a substance that has a specific interaction with the analyte on the crystal surface. When a substance is adsorbed or deposited on the surface of the sensor, the resonance frequency of the piezoelectric crystal diminishes [13] according to the expression  

Fipre 4.39 A piezoelectric biosensor uses tiu vibration of a piezoelectric quartz disc and can detect the variation in mass due to coupUng qf the analyte with the protein. 

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Sensitive, selective detection of biochemically active compounds can be achieved by employing antigen-antibody, enzyme-substrate, and other receptor-protein pairs, several of which have been utilized in the development of piezoelectric immunoassay devices. The potential analytical uses of these materials has been reviewed, particularly with respect to the development of biochemical sensors [221-224], The receptor protein (e.g., enzyme, antibody) can be immobilized directly on the sensor surface, or it can be suspended in a suitable film or membrane. An example of the sensitivity and response range that can be... 

To demonstrate a way of the use of aptamers in design of biomimetic sensors, two examples will be cited from the recent literature. The piezoelectric sensor for protein IgE has been developed with the use of commercially available anti-IgE aptamer oligonucleotide.167 The obtained sensor shows specificity and sensitivity equivalent to these of immunosensor, but for aptamer-based sensor a less decrease of sensitivity after consecutive cycles of analyte binding and regeneration, as well as relative heat resistance and stability over several weeks was shown. A more complex mechanism of sensing was employed in adenosine aptamer-based sensor.168 Detection was based on enzymatic activity measurements by fluorescence polarization with the use of aptameric enzyme subunit, which was a DNA aptamer composed of enzyme-inhibiting aptamer and adenosine-binding aptamer. 

Several piezoelectric sensors for the detection of low molecular weight compounds like pesticides (9-11), toxins (12), or drugs (13) have been reported in recent years (Table 1). A piezoelectric biosensor with a reversibly coupled (via His-tag) paraoxon for the binding of tetrameric butyrylcholinesterase to the active site of the enzyme has also been described by us (14). The commercially available devices have recently been reviewed (15, 16). 

In contrast the use of the antibodies for the detection of antigens has not an amplification stage involved and then the affinity reaction should be amplified in order to have a clear transduction. We have two possibilities, one is the use of a bioconjugate involving a bound enzyme, like in the classical ELISA test the second is the inherent amplification given by the mass of the biological element involved, a piezoelectric device (sensitive to mass) can detect minute amount of large proteins (like antibodies) if they are attracted on the surface of the sensor. 

Piezoelectric sensors Quartz crystals may be induced to resonate under electrical control. In this state they are sensitive to the mass of absorbing material. Piezoelectric sensors make use of this effect for the detection of very small amounts of inorganic or biological material, e.g. by use of surface immobilized enzymes or antibodies. 

A piezoelectric biosensor, for the detection of several organophosphorus pesticides was developed (Halamek et al., 2005). The sensor was based on the immobilization of a reversible inhibitor of cholinesterase on the surface of the sensor. The binding of AChE to this inhibitor was monitored with a mass-sensitive piezoelectric quartz crystal. In the presence of an inhibiting substance in the sample, the binding of the enzyme to the immobilized compound was reduced, and the decrease of mass change was proportional to the concentration of the analyte in the sample. This sensor was applied to the determination of pesticides in river water samples. 

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