Abstract
Gold-gold sulfide nanoparticles are of interest for drug delivery, biomedical imaging, and photothermal therapy applications due to a facile synthesis method resulting in small particles with high near-infrared (NIR) absorption efficiency. Previous studies suggest that the NIR sensitivity of these nanoparticles was due to hexagonally shaped metal-coated dielectric nanoparticles that consist of a gold sulfide core and gold shell. Here, we illustrate that the conventional synthesis procedure results in the formation of polydisperse samples of icosahedral gold particles, gold nanoplates, and small gold spheres. Importantly, through compositional analysis, via UV/vis absorption spectrophotometry, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS), we show that all of the nanoparticles exhibit identical face center cubic (FCC) gold crystalline structures, thus suggesting that sulfide is not present in the final fabricated nanoparticles. We show that icosahedrally shaped nanoparticles result in a blue-shifted absorbance, with a peak in the visible range. Alternatively, the nanoplate nanoparticles result in the characteristic NIR absorbance peak. Thus, we report that the NIR-contributing species in conventional gold-gold sulfide formulations are nanoplates that are comprised entirely of gold. Furthermore, polydisperse gold nanoparticle samples produced by the traditional one-step reduction of HAuCl4 by sodium thiosulfate show increased in vitro toxicity, compared to isolated and more homogeneous constituent samples. This result exemplifies the importance of developing monodisperse nanoparticle formulations that are well characterized in order to expedite the development of clinically beneficial nanomaterials.
Highlights
Gold-based nanoparticles have received immense research attention due to their unique optical properties, biocompatibility, and ease of surface modification [1]
Based on the original published synthesis procedures, GGS gold-gold sulfide nanoparticle (NP) can be characterized as having two surface plasmon resonance (SPR) bands: a 520 nm peak attributed to small spherical gold nanoparticles and an 800 nm peak contributed by a heterogeneous combination of predominately icosahedra, nanoplates, and irregularly shaped asymmetric nanoparticles [6]
Formation, and Elemental Composition of Icosahedra gold-gold sulfide nanoparticles (GGS NP) Original models based upon the Mie theory have suggested that the NIR SPR associated with GGS NP are due to a hexagonal core-shell structure [10]
Summary
Gold-based nanoparticles have received immense research attention due to their unique optical properties, biocompatibility, and ease of surface modification [1]. Additional efforts have focused on eliminating small spherical gold impurities (with an SPR absorbance at ~520 nm) using physical methods such as filtration and dialysis [21, 22] While these distinct fabrication procedures are in various stages of research and development with their own advantages, we proceed with a detailed analysis of conventional GGS NP fabrication to clarify unknown issues for potential future nanomedicine applications, namely, a thorough characterization and elucidation of the structure of these nanoparticles and their elemental composition, confirmation of the nanoparticulate species that predominately contributes to the NIR peak, and a closer examination of sample polydispersity and cellular toxicity. This will lead to future studies aimed at creating optimal monodisperse solutions in order to minimize undesirable toxicity [17, 23]
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