Development of a source–drain electrode coated with an insulation layer for detecting concentration changes in a nitrate ion solution

A chip-mounted source–drain electrode coated with an insulator layer was investigated with respect to its ability for monitoring in a concentrated nitrate ion solution. Exposing the electrode surface to highly concentrated nitrate ion solutions resulted in an increase in the detection voltage, while weakly concentrated solutions caused the detection voltage to decrease. Semi-circular shaped microelectrodes consisted of an Au/Cr film of 1/0.1 μm thickness on a glass chip, constructed using photolithography. A novolac resin or fluoro-olefin vinyl ether copolymer was used as the precursor of an insulator layer on the source–drain electrode. NaNO3 solutions of 1.0 × 10−6 to 1.0 mol l−1 (1 μl) were applied to the insulated electrode, and the chemical sensitivity was evaluated by measuring the detection voltage. The effects of thickness, insulator type, solution, and sensor material, on the chemical sensitivity were investigated. For a novolac resin insulator layer of thickness <2.5 μm, the sensor voltage was noticeably unstable, whereas a thicker insulator layer (>5 μm) resulted in a sensor voltage that showed a linear response depending on the NaNO3 concentration in the range of 1.0 × 10−6 to 1.0 × 10−3 mol l−1. The responsiveness of the sensor was improved with increasing insulator layer thickness. Optimization of the insulator thickness is therefore necessary if an effective sensor is to be realized. Sensor detection was also effected by the kind of solution and electrode material. The placing of a liquid droplet on the insulator surface resulted in the formation of an electric double layer at the boundary surface. Here, the molecules within the insulator become polarized, and a charge is formed at the boundary surface between the liquid and solid, and on the reverse side of the insulator. In this case, the relationship between the source–drain electrode current and the sensor voltage follows Ohm's law. Based on this principle, a source–drain electrode produces a signal in response to changes in current originating from polarization of the insulator layer.