Flexible label-free SERS substrate with alginate-chitosan@silver nanocube for in situ nondestructive detection of thiram on apples

The rapid in situ detection of pesticide residues in real samples based on surface-enhanced Raman spectroscopy (SERS) remains a challenge, necessitating an urgent need for a feasible solution that addresses issues such as sample complexity, reproducibility, and SERS substrate stability. This paper proposes a flexible SERS substrate, which consists of a composite gel made of sodium alginate-chitosan loaded with silver nanocubes (SA-CTS@AgNCs). The flexible nature of the SERS substrate enables the analysis of irregular surfaces of apples, dispensing with laborious pretreatment and promoting an effective contact with target molecules. By utilizing the SA-CTS@AgNCs substrate in conjunction with a portable Raman instrument, an exceptional sensitivity was achieved with a detection limit of 0.055 mg/L for thiram in apples. In addition, the stability, homogeneity, and batch-to-batch reproducibility of the substrates were evaluated. The experimental results showed that after 45 days of storage, the substrate still maintained more than 84.40 % SERS activity, demonstrating long-term stability. Within a single substrate, the point-to-point relative standard deviation (RSD) was only 4.2 %, while among different batches of substrates, the RSD was as low as 6.8 %, displaying better homogeneity and reproducibility. Hence, this flexible SERS substrate provides a reliable and convenient platform for rapid detection and on-site monitoring of food safety.