Characterization of ion-sensitive extended-gate field effect transistor coated with functional self-assembled monolayer

Abstract

Extended-gate field effect transistors (FETs) were characterized by modifying self-assembled monolayers (SAMs) of alkanethiols with functional groups (–CH3, –COOH, –NH2, and –OH) on the gate surface. The SAMs with a polar group such as –COOH or –NH2 on the Au gate surface of the FET showed potential responses in the pH range of 5 to 10. In particular, the carboxylated SAM-coated gate FET showed higher sensitivities of 42 to 56 mV/pH, which are close to those of a Nernstian response. Moreover, the pH dependence of the carboxylated SAM-coated gate FET was maintained even in a solution with a higher salt concentration of 500 mM, which was used as the measurement solution. The effect of the ion strength in the solution on the pH response using the SAM-coated gate FET should be considered for the change in the concentration of hydrogen ions within the Debye length, which can be explained using the site-binding model for the functional groups at the terminus of the alkyl chains. In this case, the Debye length is assumed to be the distance from the packed alkyl chain layer of SAMs, which are hydrophobic. From these results, the construction of the SAMs with various functional groups on extended-gate FETs enables the FETs to be applied in ion-sensitive FET biosensors depending on the chemical characteristics of biological targets.