Chiral discrimination between alanine enantiomers by field effect transistor with a homocysteine monolayer-modified gate

The application of field effect transistor (FET) to chiral discrimination was investigated. An Au film vapor-deposited on a self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane, which was formed on the SiO₂ gate of FET as an adhesive and insulating layer, stabilizes the drain current-gate voltage (I_d-V_g) property of FET. The modification by homocysteine (Hcy) SAM on the surface of Au-coated gate makes it possible for the FET to distinguish between the enantiomers of alanine (Ala). Namely, after the sequential addition of Ala and Cu(II) to a K₂SO₄ solution in this system, it was confirmed that the lateral shift of I_d-V_g curves for the FET corresponded to the chirality of Ala. With the l-Hcy SAM-modified gate, a notable negative shift was observed for l-Ala, whereas the shift observed with d-Ala was much smaller. In contrast, opposite results were obtained with d-Hcy SAM. Results of quartz crystal microbalance measurement suggested that such an FET response was originated from the enantioselective formation of diastereomeric Cu complexes with Ala molecules on the Hcy SAM. This system was demonstrated to respond quantitatively to one enantiomeric form of Ala in mixed solutions of two enantiomers as well as in pure enantiomeric solutions.