2, Institut für Physikalische Chemie und Elektrochemie, Leibniz University, Hannover, , Germany
The American Cancer Society lists cadmium and its compounds as group 1 carcinogens in humans as exposure to these species through food, water, and the air is known to increase the lifetime risk of cancer. In this work, we report a field-effect transistor (FET) based electrolyte
insulator semiconductor(EIS) sensor with a CdSe doped silica gel used as the sensing membrane. The EIS structure has a metallic layer as the back contact and the sensing membrane as the top layer with a pseudo-reference platinum electrode as top contact. The electrolyte consists of spiked cadmium acetate solutions and 0.1M KNO3. The back contact (150 nm Al) was first deposited, with a Cr/Au(10nm/50nm) capping layer using thermal evaporation at 4.2 x 10-6 Torr, followed by a rapid thermal annealing step at 450°C. Silica gel synthesis was carried out using hydrolysis and condensation of tetraethylorthosilicate in ethanol, distilled water, and HCl. Thin film X-ray diffraction (XRD) measurements on spin-coated silica gel revealed amorphous silica sol-gel formation. CdSe nanoparticles (~3.4nm) with photoluminescence (PL) and absorbance peaks at 570nm and 562nm, respectively, were mixed with the silica sol to form a stable ink. This ink was spin-coated over a silicon wafer with 100 nm of thermal oxide. Capacitance-voltage (CV) measurements were carried out using an AC voltage of 50mV superposed on a DC voltage ramp in the range -1.5V to +1.5V at the 100 kHz. The CV curve was detected in current mode with the Stanford Research SR830 lock-in amplifier (gain = 1M Ohm). Threshold voltage shifts (△VT) were extracted from the CV curve was monitored as a function of varying Cd concentration (1M to 10-7M) in solution. The relationship between the shift and the concentration is linear down to the detection limit of 1 ppm in this initial study, which establishes this baseline FET-based platform for real-time detection of Cd ions in the environment in a low-cost and sustainable manner.