Enzyme monolayer-functionalized field-effect transistors for biosensor applications

A gate surface of an ion-selective field-effect transistor was modified with a monolayer enzyme array that stimulates biocatalytic reactions that control the gate potential. Stepwise assemblage of the biocatalytic layer included primary silanization of the Al2O3-gate with 3-aminopropyltriethoxysilane, subsequent activation of the amino groups with glutaric dialdehyde and the covalent attachment of the enzyme to the functionalized gate surface. Urease, glucose oxidase, acetylcholine esterase and α-chymotrypsin were used to organize the biocatalytic matrices onto the chip gate. The resulting enzyme-based field-effect transistors, ENFETs, demonstrated capability to sense urea, glucose, acetylcholine and N-acetyl-l-tyrosine ethyl ester, respectively. The mechanism of the biosensing involves the alteration of the pH in the sensing layer by the biocatalytic reactions and the detection of the pH change by the ENFET. The major advantage of the enzyme-thin-layered FET devices as biosensors is the fast response-time (several tens of seconds) of these bioelectronic devices. This advantage over traditional thick-polymer-based ENFETs results from the low diffusion barrier for the substrate penetration to the biocatalytic active sites and minute isolation of the pH-sensitive gate surface from the bulk solution.