Synthetic receptor molecules that selectively bind charged guests can store chemical information. The transduction of this information into electronic signals connects the chemical and electronic domains. Field effect transistors (FETs) are attractive transducing elements because these microdevices are able to register and amplify chemical changes at the gate oxide surface of the semiconductor chip.
Integration of molecular receptors and field effect transistors into one chemical system gives a device that can communicate-changes of substrate activities in aqueous solution. Simulations of a system in which the receptor molecules are directly attached to the FET gate oxide indicate serious limitations with respect to sensitivity, dynamic range and extreme requirements for complex stability. Therefore we have concentrated on the integration of covalently attached thin membranes.
The problem of the thermodynamically ill-defined oxidemembrane ipterface has been solved by applying a covalently linked hydrophilic polyhydroxyethylmethacrylate (polyHEMA) gel between the sensing membrane and the silylated gate oxide. A buffered aqueous electrolyte solution in the hydrogel renders the surface potential at the gate oxide constant via the dissociation equilibrium of the residual silanol groups. The subsequent attachment of a polysiloxane membrane that has the required dielectric constant, glass transition temperature Tg, and receptor molecule, provides a stable chemical system that transduces the complexation of cationic species into electronic signals (CHEMFET).
The response to changing K⊕ concentrations in a solution of 0.1 M NaCl is fast (<1 sec) and linear in the concentration range of 10−5–1.0 M (55–58 mV /decade). A reference FET (REFET) based on the same technology is obtained when the intrinsic sensitivity to changes in ion concentration is eliminated by the addition of 2.10−5 mol g−1 of didodecyldimethyl ammonium bromide to the ACE membrane. Differential measurements with a REFET/CHEMFET combination showed excellent linear K⊕ response over long periods of time.
All chemical reactions used are compatible with planar IC technology and allow fabrication on wafer scale.