Synthesis, Characterization, and Tunable Optical Properties of Hollow Gold Nanospheres

Abstract

Nearly monodisperse hollow gold nanospheres (HGNs) with tunable interior and exterior diameters have been synthesized by sacrificial galvanic replacement of cobalt nanoparticles. It is possible to tune the peak of the surface plasmon band absorption between 550 and 820 nm by carefully controlling particle size and wall thickness. Cobalt particle size is tunable by simultaneously changing the concentration of sodium borohydride and sodium citrate, the reducing and capping agent, respectively. The thickness of the gold shell can be varied by carefully controlling the addition of gold salt. With successful demonstration of ensemble as well as single HGN surface-enhanced Raman scattering, these HGNs have shown great potential for chemical and biological sensing applications, especially those requiring nanostructures with near-IR absorption.

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…These structures, which are typically made of spherical silica cores surrounded by a layer of noble metal aggregates, have variation in size, shape and surface morphology due to the random nature of aggregation [64,65]. The polydispersity found within a given ensemble of shells will lead to a broadening of the extinction line width [66]. This results in reduced overlap with incident light and lower efficiency for processes such as photothermal conversion.…”

“…HGNs are comprised of a hollow, solvent filled dielectric core and a polycrystalline gold shell [66,[75][76][77]. Both the core width and shell thickness can be tuned through synthesis to produce a range of overall diameters (20-125 nm) and aspect ratios.…”

“…These particles also possess large surface to volume (S/V) ratios, tunable plasmon resonance, pinholes that act as hot spots to further intensify EM surface energy and, as hollow structures, are known to be more sensitive to the refractive index of their surroundings than their solid counterparts [15,66,[81][82][83][84]. Additionally, their scattering and absorption cross-sections can be adjusted through synthesis to maximize efficiency for specific applications, and since the first hyperpolarizabilities (ˇ) of hollow nanoparticles are much larger than those of solid NPs having the same size, HGNs are excellent candidates for any application involving non-linear optics [85,86].…”

“…Expanding on the work of Liang et al, Schwartzberg et al used the same redox pair [cobalt and gold] to tune the LSPR of HGNs across the entire visible spectrum and out to the NIR [66]. It was recognized that control of the cobalt sacrificial template diameter was the key to producing larger HGNs with red shifted absorptions.…”

“…Since the concentration of capping agent not only stabilizes the cobalt nanoparticle seed-mediated growth but also affects the number of nucleation sites generated post-reduction, lower concentrations of capping agent lead to a smaller number of larger seed nuclei being formed [112]. This resulted in cobalt nanoparticles, and therefore HGNs, with larger diameters [66].…”