Cefalexin fingerprinting in water using graphene oxide/gold nanoparticle-modified silica photonic microsphere SERS substrates.

Cefalexin residues in aquatic environments require detection methods that are fast, label-free, and spectroscopically reliable. Surface-enhanced Raman spectroscopy (SERS) can provide molecular fingerprints, but its analytical performance depends on analyte adsorption and the spatial consistency of the enhanced response. Here, a three-dimensional silica photonic microsphere (SPM) substrate was modified with graphene oxide (GO) and gold nanoparticles (AuNPs) to form an SPM@GO@AuNPs substrate for cefalexin fingerprinting in water. GO served as an AuNP-anchoring layer and as an adsorption interface that enriched cefalexin near SERS-active AuNP junctions, whereas the monodisperse SPM provided a curved, readily handled three-dimensional support. Cefalexin bands at 768, 833, 996, 1022, and 1189 cm-1 were resolved, with the 1189 cm-1 band used for quantification. Under optimized conditions, the method exhibited a linear range of 0.01-10 μg/mL, a limit of detection of 0.41 μg/mL, and a 10 min binding step. Cefalexin was distinguished from four cephalosporin analogues using the full spectral fingerprint rather than a single band. Spiked tap-water analysis gave mean recoveries of 86.99-96.83%, supporting the feasibility of direct aqueous screening. The method is best suited to rapid fingerprint-based screening of cefalexin in relatively simple water matrices.