AgAu bimetallic nanoparticle-decorated ZnO nanoplates for sensitive SERS detection of cypermethrin: plasmonic hotspots and interfacial charge-transfer effects.

A ZnO/AgAu hybrid SERS substrate was developed for the sensitive detection of cypermethrin by integrating the charge-transfer capability of ZnO with the strong plasmonic response of AgAu bimetallic nanoparticles. ZnO nanoplates were synthesized hydrothermally and subsequently decorated with AgAu nanoparticles through the in situ co-reduction of AgNO3 and HAuCl4. Structural, morphological, optical, and surface-chemical analyses confirmed the formation of well-dispersed AgAu bimetallic nanostructures strongly coupled with the ZnO support. By optimizing the Ag/Au molar ratio, the substrate achieved a broad linear response toward cypermethrin over 0.2-10 ppm, a minimum detectable concentration of 0.2 ppm, and an enhancement factor of 8.13 × 107. Density functional theory calculations were employed to optimize the cypermethrin molecular geometry and simulate its Raman spectrum. The calculated vibrational modes agreed well with the experimental Raman bands, enabling reliable peak assignment and supporting the interpretation of the molecular-substrate interaction. The high SERS activity is ascribed to the synergistic combination of localized electromagnetic enhancement generated by AgAu nanoparticles and chemical enhancement arising from interfacial charge transfer across the ZnO/AgAu-cypermethrin system. These results demonstrate that the ZnO/AgAu hybrid provides an effective and robust platform for potential application in cypermethrin sensing.