Ultra-thin CoAg-MOFNs for SERS for the rapid analysis of imidacloprid pesticide residues in citrus

In this study, ultra-thin metal–organic framework nanosheets (MOFNs) were used for preparing an enhanced substrate to rapidly detect imidacloprid residues in citrus based on surface-enhanced Raman scattering (SERS) technology. The ultrasonic stripping method was used to prepare Co-MOFNs. Subsequently, a CoAg-MOFN-enhanced substrate was prepared by loading AgNPs onto the Co-MOFNs to enhance the SERS signal. The effects of the concentration of AgNPs, the ratio of Co-MOFNs to AgNPs, and the mixing time between the as-prepared enhanced base and imidacloprid standard solution on the SERS peak intensity were investigated, which showed that when the concentrated volume of AgNPs was 20 times and the mixed volume ratio of Co-MOFNs and AgNPs was 1 : 1, the prepared CoAg-MOFNs exhibited a good SERS enhanced effect. Notably, SERS response was the best when the CoAg-MOFN nanocomposite was mixed with an imidacloprid pesticide for 1 min. Utilizing density functional theory (DFT), four Raman characteristic peaks, specifically at 764, 784, 942 and 1035 cm⁻¹, were identified for the qualitative and quantitative assessment of imidacloprid pesticide residues in citrus fruits. A quantitative analysis curve representing imidacloprid residues was developed, correlating the intensity at 1035 cm⁻¹ over a range of 0.5–50 mg kg⁻¹ (y = 2.7964x + 26.211, R² = 0.9923), which demonstrated a strong linear relationship. The limit of detection (LOD) was 0.5 mg kg⁻¹ and the limit of quantitation (LOQ) was 0.5 mg kg⁻¹, which could meet the maximum residue limit (MRL) of 1 mg kg⁻¹ stipulated by the Codex Alimentarius Commission (CAC) and European Union (EU). The recovery rates were 93.02–101.07%, and the RSD was 2.57–7.71%, indicating that the prepared CoAg-MOFN nanocomposite combined with the SERS method is suitable for the analysis of imidacloprid pesticide residues in citrus.