Detection of L-cysteine in urine samples based on CdS/TiO₂-modified extended-gate field-effect transistor...

A novel extended-gate field-effect transistor (FET) photoelectrochemical (EGFET PEC) sensor was designed for highly sensitive detection of L-cysteine (L-Cys). TiO₂ was initially modified on the ITO electrode by the sol-gel dip-coating method and calcined to produce TiO₂/ITO. Then, CdS was synthesized on the TiO₂ surface by hydrothermal method to obtain the CdS-TiO₂ heterojunction material. CdS/TiO₂/ITO was connected to the gate of the FET to obtain an EGFET PEC sensor. Under the irradiation of a xenon lamp simulating visible light, the CdS/TiO₂ heterojunction composite absorbs light energy to produce photogenerated electron-hole pairs, which have strong photocatalytic oxidation activity and oxidize L-Cys covalently identified by Cd(II) through CdS covalent. These pairs generate a photovoltage that controls the current between the source and the drain to detect L-Cys. Under the optimized experimental conditions, the optical drain current (I_D) of the sensor exhibited a good linear relationship with the logarithm of L-Cys in the range of 5.0 × 10^-9-1.0 × 10^-6 mol/L, and the detection limit was 1.3 × 10^-9 mol/L (S/N = 3), which is lower than the values reported by other detection methods. Results showed that the CdS/TiO₂/ITO EGFET PEC sensor revealed high sensitivity and good selectivity. The sensor has been used to determine L-Cys in urine samples.