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The purpose of this study was to develop a sensitive and low-cost optical sensor for screening pymetrozine using unmodified gold nanoparticles (G NPs). The presence of pymetrozine could rapidly induce the aggregation of G NPs, thereby resulting in red to deep blue colour change. The concentration of pymetrozine was determined with a limit of detection lower than 1×10-6 M by the naked eye and was further easily monitored by a UV-vis spectrometer. The highest selectivity was achieved in comparison with other 11 pesticides and the possible underlying mechanism was investigated in detail by the simultaneous addition of glutathione and pymetrozine in G NPs solution.