Synthesis and characterization of Au@Ag nanoparticles for multiwavelength SERS biosensing.

Gold-core silver-shell (Au@Ag) nanoparticles are promising substrates for surface-enhanced Raman spectroscopy (SERS) due to their tunable plasmonic properties and enhanced stability. In this study, we synthesized Au@Ag nanoparticles with varying core sizes (13 nm and 23 nm) and silver shell thicknesses, controlled via silver nitrate concentration during a seed-mediated growth process. The nanoparticles were characterized using TEM, UV-vis, DLS, and zeta potential measurements. A significant shift from 528 nm to 405 nm in surface plasmon resonance maxima is observed in forming the Ag shell layer on the Au core. The SERS performance of the nanoparticles was systematically evaluated using 4-mercaptobenzoic acid (4-MBA) across multiple excitation wavelengths (442-830 nm). The results demonstrated that thicker silver shells significantly enhanced SERS signals, achieving an enhancement factor up to 9.4 × 108 at 633 nm excitation. Additionally, biologically relevant ergothioneine was detected in fetal bovine serum with a limit of detection of 0.5 µM, corresponding to physiological concentrations. Spectral shifts observed at varying ergothioneine concentrations suggested adsorption-dependent molecular orientation changes. Stability tests confirmed that thin silver shells provided improved resistance to oxidation and aggregation, while thicker shells offered enhanced SERS activity at the expense of long-term colloidal stability. Overall, these systematically optimized Au@Ag core-shell nanoparticles show substantial potential for sensitive, stable, and versatile SERS-based biosensing and diagnostic applications.