Assembled growth of 3D Fe₃O₄@Au nanoparticles for efficient photothermal ablation and SERS detection of microorganisms

The development of a single agent with multifunctionality for the rapid detection and inhibition of the spread of pathogenic microorganisms is of great importance for environmental hygiene as well as water and food safety. For this purpose, we integrate plasmonic nano-Au, near-infrared (NIR)-activated Fe₃O₄ nanoclusters, and NIR-absorbing polydopamine to form a leukocyte-like Fe₃O₄@Au nanostructure to treat microorganisms. Through high-temperature reduction of HAuCl₄ with l-dopamine, Au atoms are spontaneously generated along with Fe₃O₄ nanoclusters via a site-selected atom deposition process between the Au(111) and Fe₃O₄(222) lattice planes. Combining the magnetic properties of Fe₃O₄ and the optical functionality of gold nanoparticles, the Fe₃O₄@Au nanohybrid exhibits effective photothermal conversion and magnetism-guided aggregation to improve the molecular surface-enhanced Raman scattering (SERS) signal, achieving a limit of detection on the micromolar to nanomolar level for methylene blue (MB) and 4-aminothiophenol (4-ATP). After magnetism-assisted adsorption, we adopt Escherichia coli (E. coli) as a model analyte and demonstrate label-free SERS sensing of bacterial cell molecular structures based on optical fingerprints and recyclable photothermal ablation of bacterial pathogens (Gram-positive, Gram-negative, and anaerobic bacteria).