A colorimetric/fluorescence dual-mode method using GQDs-regulated Fe3O4@Cu@PLL nanozyme activity for thiamethoxam detection

The newly established lower maximum residue limit (MRL) for thiamethoxam (TMX) necessitates the development of more sensitive detection methods. This study presents a novel colorimetric-fluorescence sensing platform to detect TMX through graphene quantum dot (GQD)-mediated regulation of Fe 3 O 4 @Cu@PLL nanozyme activity. The mechanism investigation revealed that GQDs effectively suppress the peroxidase-like activity of Fe 3 O 4 @Cu@PLL by reducing and scavenging hydroxyl radicals (•OH) generated during catalytic reactions, which constitutes the fundamental principle of the dual-mode detection strategy. The sensing system was constructed through the hybridization of aptamer-functionalized Fe 3 O 4 @Cu@PLL (Fe 3 O 4 @Cu@PLL/Apt) with complementary DNA strand-modified GQDs (CS-GQDs), forming Fe 3 O 4 @Cu@PLL/Apt-CS/GQDs complexes. Following magnetic separation, two distinct detection channels were established: (1) The supernatant showed negligible fluorescence due to CS-GQDs removal, while (2) the precipitates exhibited suppressed peroxidase-like activity owing to GQD-induced inhibition. However, TMX presence triggered the dissociation of CS-GQDs from the complexes through competitive binding with the aptamer. This resulted in two measurable signals: a fluorescence signal recovery at 450 nm in the supernatant containing CS-GQDs, and restored nanozyme activity in the Fe 3 O 4 @Cu@PLL/Apt+TMX precipitates that catalyzed H 2 O 2 -mediated oxidation of TMB to blue-colored oxTMB. The optimized system achieved detection limits of 8 ng/mL (colorimetric) and 6 ng/mL (fluorescence), demonstrating high sensitivity. Validation experiments using three spiked samples (tea, beans, and kumquats) showed satisfactory recoveries ranging from 94.59 % to 108.23 %, confirming the method's reliability for accurate, on-site TMX detection in food matrices.