Hollow gold-silver double-shell nanospheres: Structure, optical absorption, and surface-enhanced Raman scattering

Hollow gold-silver double-shell nanospheres (AuAgHNSs) have been synthesized and their structural, optical, and single-particle surface-enhanced Raman scattering (SERS) properties characterized. The structure of these AuAgHNSs have been determined by transmission electron microscopy (TEM), revealing a uniform silver outer shell of approximately 3 nm deposited by seed-mediated growth on 28 +/- 6.2 nm hollow gold nanospheres (AuHNSs). Their plasmon resonance combines the energies of both the silver and the gold shells, broadening the absorption profile, while their overall structure maintains a spherical shape. This property is useful for SERS and other applications that require small, spherical nanostructures and broad, tunable absorption. The mechanism of silver growth on AuHNSs and gold NPs is proposed as silver island formation before complete encapsulation. SERS studies of mercaptobenzoic acid (MBA) before and after silver deposition show a 4-5-fold enhancement over AuHNSs with little change in signal consistency, while solid NPs exhibit little or no signal improvement. A change in the relative peak intensity in the MBA Raman spectrum before and after silver coating is attributed to the effect of orientation and binding of thiols to gold and silver surfaces.