Chain-like 2-amino-4-thiazoleacetic acid tethered AuNPs as colorimetric and spectrophotometric probe for organophosphate pesticide in water and fruit samples

This investigation describes the detection of quinalphos (QP), one of the organophosphate pesticides using 2-amino-4-thiazoleacetic acid anchored AuNPs (ATA-AuNPs) via naked eye and spectrophotometry. The ATA-AuNPs were synthesized by a bottom-up approach and confirmed by UV–vis, XRD, zeta potential and HR-TEM analyses. The ATA-AuNPs exhibited absorption maximum at 521 nm and the HR-TEM images revealed the formation of chain-like morphology of ATA-AuNPs with an average size of 6.4 nm. The zeta potential value of −38.5 mV suggested that the –NH₂ groups of ATA adsorbed on AuNPs surface and the acetic acid group was free from binding, which prevented the aggregation of AuNPs by electrostatic repulsion. The ATA-AuNPs solution color was changed to violet from wine-red upon the addition of 3.5 µM QP whereas the absorption at 521 nm was diminished with the appearance of a new peak around 650 nm. The ATA-AuNPs started to aggregate after the addition of 7.5 µM of QP and the size of ATA-AuNPs was increased to 10.7 nm due to aggregation. Since the free acid group of ATA was involved in hydrogen bonding with the sulphur atom of QP the ATA-AuNPs were aggregated. It is obvious from the solution color change that naked eye detection of 2.5 µM QP can be possible. The ATA-AuNPs showed excellent anti-interference ability towards QP even with 2850-fold higher concentration of major interferents. A good linearity was observed in the detection of 0.5 to 3.5 µM QP (R² = 0.9971) with the lowest LOD of 14.37 µg L⁻¹ (S/N = 3). The practicability of the developed colorimetric and spectrophotometric methods was examined by determining the QP levels in QP treated field water samples and fruit samples.