This paper reports the development of a simple and new photoelectrochemical (PEC) aptamer-based sensor for ultrasensitive determination of the concentration of diazinon (DZN) using the surface plasmon resonance effect (SPR) of gold nanoparticles (AuNPs) deposited on a titanium dioxide ( TiO2) film. A thin layer of flourin tin-oxide (FTO) was covered on the surface of glass plates during the spray pyrolysis process, and the resulting FTO plates were modified layer by layer with TiO2 film and AuNPs as the photoactive nanomaterials. The AuNPs were utilized to increase the absorption rate of visible light through the formation of hot electrons. The prepared AuNPs/TiO2 nanocomposite revealed a higher photoelectro catalytic activity compared to the pure TiO2. In order to improve the selectivity of the proposed PEC sensor, the thiolated aptamer was conjugated to the AuNPs/TiO2 nanocomposite through S–Au bonds. Upon exposition of the fabricated PEC apatasensor to DZN molecules, the formation of the aptamer-DZN complex restricted the electron transfer at the surface of the PEC sensor; therefore, the photocurrent signal decreased. The simultaneous usage of the PEC technique and aptamer led to the improvement of the analytical performance of the proposed sensor in terms of sensitivity, selectivity, reproducibility, and stability for the quantitative determination of diazinon with a wide linear range of 0.2 to 1000 nM and a low detection limit of 0.04 nM. In addition, the prepared PEC aptasensor was used for the determination of diazinon in water and biological samples, and satisfactory results were obtained which confirms the practical application of the proposed PEC aptasensor.

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