Self-Assembly Silver Nanoparticles Decorated on Gold Nanoislands for Label-Free Localized Surface Plasmon Resonance Biosensing

A nanostructure of self-assembly silver nanoparticles decorated on gold nanoislands (SAM Ag@AuNIs) is proposed in this article as a sensitive sensing medium for localized surface plasmon resonance (LSPR) biosensing applications. A wide tunable plasmonic sensing range from 472 to 552 nm is found by adjusting the Au–Ag atomic ratio. Biotin is employed as an indicator to investigate the potential biosensing applicability and site-specific bioactivity of the SAM Ag@AuNIs. In virtue of a white light common-path sensing system, direct adsorption of biotin on the SAM Ag@AuNIs is measured in label-free manner with detection limit (LOD) of 2.9 pg mL⁻¹ (11.87 × 10⁻¹² m). It is 10⁵-fold better than that of TiN film, which is previously reported to provide excellent LSPR LOD after functionalization with biotin. The experimental and computational results indicate that the biotin molecule can selectively immobilize onto the Ag nanoparticles (AgNPs) surface through S–Ag bonding, resulting in site-specific functionalization. Meanwhile, the geometry of AgNPs/AuNIs provided a coupling effect that helps to improve the sensitivity. Here, AgNPs are, for the first time, reported to be directly functionalized with biotin. The SAM Ag@AuNIs can serve as a desirable plasmonic medium in the biosensing community based on the exceptionally good LOD of biotin.