Temperature-responsive AuNPs/poly(N-isopropylacrylamide) hydrogel-based sensor for SERS detection of paraquat in fruit juices.

A novel composite SERS substrate was developed based on gold nanoparticles (AuNPs) and poly(N-isopropylacrylamide) (PNIPAM) hydrogel. PNIPAM hydrogel is a temperature-responsive material with a lower critical solution temperature (LCST) of about 32 °C, regulating its swelling-shrinking behavior by changing temperature to evaluate the enrichment efficiency of target molecules. Experimental results demonstrated that the substrate exhibited a swelling ratio of 400-600% at room temperature, facilitating the efficient enrichment of paraquat molecules. The optimal Raman response was observed when the substrate contained a NIPAM concentration of 0.8 mol/L and an AuNPs concentration of 300%. Additionally, the substrate exhibited excellent detection performance, including high sensitivity, good reproducibility, uniformity, and temporal stability. The minimum detectable concentration of paraquat in standard solutions was 0.1 μg/L. In apple juice and orange juice, the detection limit ranged from 0.072 to 0.112 μg/L, with spiked recoveries of 85.55%-110.55% and 88.57%-107.15%, respectively. This study presents a highly selective and reliable SERS sensing strategy for the rapid detection of trace paraquat in juice samples.