Morphology-dependent magneto-plasmonic coupling in Au-Fe₃O₄ nanostructures for magnetic-enriched SERS detection

Magneto-plasmonic nanostructures combining magnetic and plasmonic functionalities provide promising platforms for sensitive bioanalytical detection. Here, we report a morphology-controlled strategy for synthesizing Au-Fe₃O₄ hybrid nanostructures through seed-mediated growth. Controlled Au nucleation and deposition enabled the evolution from Au-Fe₃O₄ heterodimers to Fe₃O₄@Au core-shell structures and faceted nanocuboids. TEM, XRD, and UV-Vis analyses confirmed the structural evolution, coexistence of crystalline Fe₃O₄ and Au phases, and development of plasmonic activity. The hybrid nanostructures retained superparamagnetic behavior despite a decrease in saturation magnetization with increasing Au coverage. SERS measurements for a novel aCD19 biomolecule revealed a morphology-dependent response, with Fe₃O₄@Au nanocuboids exhibiting the strongest enhancement due to enhanced electromagnetic-field localization. Quantitative analysis yielded a maximum enhancement factor of approximately 6.1 × 10⁵, detectable aCD19 signals down to 10^-10 M, a concentration-dependent response with R² = 0.96, and an RSD of 9.85% across 10 measurement positions. These results demonstrate the potential of Fe₃O₄@Au nanocuboids as sensitive, reproducible, and magnetically responsive SERS substrates for bioanalytical detection.