CMOS open-gate ion sensitive field effect transistors for femto-molar dopamine detection

Open-gate ion-sensitive field-effect transistors (ISFET) are presented in this work to provide real-time ultrasensitive dopamine (DA) detection in the femto-molar (fM) range. The polysilicon gates of p-type FETs fabricated in a 0.35-μm CMOS (complementary metal-oxide-semiconductor) process were removed by a convenient post-CMOS process to expose the gate oxide for surface functionalization and biomolecule immobilization. Measured current value increased due to the produced negative charges from binding of the 4-carboxyphenylboronic acid (CPBA) and dopamine molecules. The thin gate oxide as the sensing interface significantly enhances the detection limit, which is comparable to or better than most nanowire-based ISFETs. A self oscillating readout circuit was used to convert the ISFET current to a digital output for measurement of multiple sensors, showing the strength of the CMOS-based approach for sensor integration.