Multifunctional CoFe2O4/APTES/Ag Hybrids Integrating Magnetic Separation, Molecular Trapping, and Charge Transfer for Point-of-Care Surface-Enhanced Raman Scattering Detection of Anthrax Biomarkers

This work addresses the challenge of achieving point-of-care (POC) surface-enhanced Raman scattering (SERS) detection of small-molecule biomarkers in real samples without pretreatment. We synthesized multifunctional CoFe2O4/APTES/Ag hybrids through a simple chemical approach for label-free SERS detection of 2,6-pyridinedicarboxylic acid (DPA), a key biomarker of Bacillus anthracis in food samples. The hard-magnetic CoFe2O4 component enabled rapid magnetic separation, reaching enrichment equilibrium in just 5 min while reducing matrix interference. Simultaneously, the semiconducting properties of CoFe2O4 (narrow 1.35 eV bandgap) synergized with plasmonic Ag nanoparticles to enhance SERS activity via an improved charge transfer effect. Further functionalization with APTES promoted selective DPA trapping through hydrogen-bonding interactions. The resulting hybrids achieved high sensitivity (10 nM detection limit with first-order derivative processing), excellent reproducibility, and rapid detection in untreated complex matrices. Quantitative SERS analysis of DPA in milk and water samples yielded recoveries of 98.89–108.3%. Even at 0.1 ppm concentration, reliable in situ detection was possible using a portable Raman spectrometer after a brief magnetic enrichment. This integrated approach demonstrates significant potential for developing multifunctional SERS substrates capable of POC detection of biomarkers in practical applications all-in-one.