Sensitive detection of choline in infant formulas by SERS marker transformation occurring on a filter-based flexible substrate

In this study, we developed a simple and sensitive surface-enhanced Raman scattering (SERS) technique for the detection of choline in infant formulas based on a filter-based flexible substrate. The flexible substrate with excellent SERS activity and reproducibility was fabricated using a syringe filter and Ag triangular nanoplates. The Raman marker, 3-mercaptophenylboronic acid, was oxidised to 3-hydroxythiophenol in the presence of H₂O₂, which caused a decrease in the SERS peak intensity ratio (I_{1021 cm}⁻¹: I_{996 cm}⁻¹). Utilising the enhanced flexible substrate, a quantitative detection method for H₂O₂ was developed. Additionally, choline could be hydrolysed by choline oxidase to generate betaine and H₂O₂. Thus, the SERS method was successfully applied to detect choline in the concentration range of 10^-3–10^-7 M with a beneficial limit of detection as low as 8.36 nM. Importantly, the recovery of choline from infant formula milk powder was approximately 99 %. The excellent performance of the flexible substrate-based SERS method demonstrated its potential for application in H₂O₂-generated oxidase-based biosensing.