Aptamer-Functionalized Gold Nanoparticles and Sulfur-Doped Graphene Quantum Dots for Ultrasensitive Dual-Mode Surface-Enhanced Raman Spectroscopy/Fluorescence Detection of Omethoate

The development of selective, ultrasensitive, and reliable sensing strategies for trace-level detection of hazardous pesticides is crucial for ensuring food safety and effective environmental monitoring. Herein, we report a dual-mode fluorescence/surface-enhanced Raman spectroscopy (SERS) aptasensing platform for omethoate (OM) detection based on thiol-functionalized aptamer-modified gold nanoparticles (Au-Apt) and sulfur-doped graphene quantum dots (S-GQDs). The sensing system is constructed through strong Au–S affinity interactions, resulting in the efficient fluorescence quenching of S-GQDs in the assembled state. Upon OM-specific aptamer recognition, structural switching of the aptamer induces the formation of a Au-Apt/OM complex and the simultaneous release of S-GQDs, leading to fluorescence recovery and the generation of a strong SERS signal from OM localized within plasmonic hotspots. The fluorescence mode exhibits a linear response over the OM concentration range of 1–20 ppb with a limit of detection (LOD) of 1.7 ppb, while the SERS mode achieves an ultralow LOD of 0.05 ppb, representing a significant improvement over most previously reported fluorescence- or SERS-based OM sensors. The dual-mode platform exhibits excellent signal reproducibility, effective probe recoverability, and reliable analytical performance in complex matrices, as demonstrated through real-sample analysis of water and fruit extracts. By integration of complementary fluorescence and SERS readouts with distinct linear response ranges, this work provides a robust and cross-validated sensing strategy that minimizes false-negative results and enhances analytical confidence. The proposed dual-mode aptasensor offers a promising approach for practical pesticide residue monitoring and can be readily extended to other targets by rational aptamer selection.