Demonstration of the enhancement of gate bias and ionic strength in electric-double-layer field-effect-transistor biosensors

In this study, we investigate the mechanism of enhanced electric-double-layer (EnEDL) field-effect-transistor (FET)-based biosensors, which can successfully detect proteins in high ionic strength solution, such as in 1XPBS. This EnEDL FET biosensor performs good sensitivity in high ionic strength solution by creating enhanced capacitance in the EDL with higher gate voltage or higher salt concentration, and operating in a large transconductance of the FET to amplify the signal. AC impedance measurement is also introduced and identifies that the imaginary part dominates the sensitivity of C-reactive protein (CRP) detection, indicating that the EnEDL capacitance plays the key role in the sensing mechanism. Extended gate connecting to the FET and a separated gate electrode is fabricated and biased with the gate voltage for the EnEDL FET biosensors. The test solution is placed between the two electrodes, and the voltage drop across the solution caused by CRP concentration was analyzed for different gate voltages and salt concentrations for the EnEDL FET biosensors. The results show that as the EDL is enhanced by higher gate bias or higher salt concentration, the sensitivity is increased, either in AC impedance analysis or voltage drop in the test solution. To utilize the enhanced EDL, the maximum transconductance of the FET should be located in a larger gate voltage. This technology can allow direct protein detection in 5 min. without complex dilution of clinical samples, which is promising for portable devices for personal healthcare.