Covalent organic frameworks modified with silver nanoparticles as substrates for label-free SERS detection of fungicides.

Despite prohibition in many countries, malachite green (MG) and crystal violet (CV) are illegally used in aquaculture, endangering both human health and the environment. Therefore, it is crucial to develop a method to accurately detect MG and CV. Surface-enhanced Raman spectroscopy (SERS) technology based on noble metal materials has attracted considerable attention for detecting food contaminations. However, noble metal materials are of poor stability and have a very low affinity for organic molecules. Herein, in this study, covalent organic frameworks (COFs) modified with silver nanoparticles (Ag NPs) were used as SERS substrates for the adsorption and detection of fungicides. COFs prepared by the Schiff base reaction of 1,3,5-tris (4-aminophenyl) benzene (TAPB) and 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMTA) had a huge specific surface area and could provide sufficient adsorption sites and Ag NPs grown on the surface of COFs not only generated dense SERS "hot spots" but also improved their stability. The results showed that COF@Ag NPs exhibited a high enhancement factor (1.4 × 10⁶), high reproducibility (RSD = 6.18%), and good stability within 50 days (RSD = 8.88%). Regression analyses in the ranges of 0.0001–10.0 mg/L and 0.0002–10.0 mg/L showed that the limits of detection (LOD) for the standard solutions of MG and CV were 3.8 × 10⁻⁵ mg/L and 1.8 × 10⁻⁵ mg/L, respectively. Furthermore, for MG and CV in fish samples, significant recoveries of 90.83–107.91% and 98.60–116.69% were achieved. Hence, it is hoped that this work could expand the application of COF materials for trace detection in SERS analyses. [ABSTRACT FROM AUTHOR]