Near-Infrared Light Absorbing Silver-Coated Hollow Gold Nanostars for Surface-Enhanced Raman Scattering Detection of Bovine Serum Albumin Using Capping Ligand Exchange

Two novel urchinlike plasmonic nanostructures, hollow gold nanostars (HNSs) and silver-coated hollow gold nanostars (AgHNSs), were synthesized using hollow gold nanospheres (HGNs) as templates. The particle morphology was assessed by transmission electron microscopy (TEM), optical properties were characterized by UV–vis spectroscopy, and elemental composition was evaluated by both inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy-dispersive X-ray (EDX) analysis. Both the HNSs and AgHNSs exhibit tunable structural and optical properties. Their unique star-shaped structure provides the desired “hot spots” for enhancing the electromagnetic (EM) field for surface-enhanced Raman scattering (SERS) applications, while their near-infrared (NIR) absorption is ideal for biomedical applications. A comparative analysis of the relative SERS enhancement using rhodamine 6G (R6G) showed that HNSs reported herein have an order of magnitude higher relative enhancement over previously reported HNSs. Moreover, AgHNSs have a 4-fold increase in the SERS signal compared to HNSs. Discrete dipole approximation (DDA) simulations of electric field enhancement were used to corroborate the experimental findings. The comparison between the experimental and theoretical results suggests that the significant increase in SERS enhancement cannot be completely explained by the larger dielectric constant of silver. The branching of individual spikes, or the increased binding efficacy of the analyte to the nanoparticles, likely also plays an important role. To further explore the applicability of these novel structures, successful capping ligand exchange from citrate to pentanethiol was used to switch the surface charge from negative to positive to facilitate the SERS detection of dried bovine serum albumin (BSA). AgHNSs are stable and effective NIR-absorbing substrates for SERS detection of biological samples. Elemental analyses of AgHNSs, in conjunction with DDA simulations, provide insights into their enhancement mechanism.