Synthesis of Silica Shell/Gold Core Nanoparticles and Plasmonic Characterization of the Aggregates

Abstract

<p>Differences in the wavelengths of the surface plasmon band of gold nanoparticles (AuNP) – before and after particle aggregation – are widely used in bioanalytical assays. However, the gold surfaces in such bioassays can suffer from exchange and desorption of non-covalently bound ligands and from non-specific adsorption of bio-molecules. Silica shells on the surfaces of the gold can extend the available surface chemistries for bioconjugation and potentially avoid these issues. Therefore, silica was grown on gold surfaces primed with polyvinylpyrrolidone (PVP) using either hydrolysis/condensation of tetraethyl orthosilicate under basic conditions or diglycerylsilane at neutral pH. The former precursor permitted slow, controlled growth of shells from about 1.7 to 4.3 nm thickness. By contrast, silica shells formed within an hour using diglyceroxysilane; the thickness was insensitive to changes in silane concentration and incubation time and could be tuned using different molecular weight PVP to prime the particles. The control over shell thickness is discussed with respect to the PVP interface, the electrical double layer, and interpenetrating organic-inorganic hybrid structures. Within the range of shell thicknesses synthesized, the presence of a silica shell on the gold nanoparticles did not significantly affect the absorbance maximum (~ 5 nm) of unaggregated particles. However, the change in absorbance wavelength upon aggregation of the particles was highly dependent on the thickness of the shell. With silica shells coating the AuNP, there was a significant decrease in the absorbance maximum of the aggregated particles, from ~578 to ~536 nm, as the shell thicknesses increased from ~1.7 to ~4.3 nm, due to increased distance between adjacent gold cores. These studies provide guidance for the iv development of colorimetric assays using silica coated AuNP. Such particles also show potential for application in 2D and 3D nanostructured assemblies.</p>