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.