Highly sensitive flexible SERS substrates with a sandwich structure for rapid detection of trace pesticide residues

Compared to rigid surface-enhanced Raman scatting (SERS) substrates, flexible SERS substrates offer significant advantages for the detection of pesticide residues on vegetable and fruit surfaces, primarily due to their convenient sampling process and shorter detection times. However, for paper-based SERS substrates, gold nanoparticles (AuNPs) are easy to form ineffective dispersion in the three-dimensional network structure of paper, resulting in unsatisfactory SERS enhancement. To address this challenge and achieve high-performance flexible SERS substrates, a novel sandwich structure is proposed. Specifically, AuNPs are aggregated on expanded graphite (EG) through a chemical enhancement process facilitated by LiCl. EG is found to improve the adsorption capacity of targeted molecules while preventing signal attenuation caused by filter papers. By employing an aggregating agent at specific concentrations, it is possible to significantly increase the amounts of hot spots to provide a more pronounced enhancement effect. Furthermore, the developed flexible SERS substrates are applied to qualitatively and quantitatively detect pesticide residues, including Thiram, Nitrofural and Dinotefuran. The flexible SERS substrates demonstrate high accuracy and robustness, making them easily storable and suitable for practical applications.