Monodisperse SnO2-Coated Gold Nanoparticles Are Markedly More Stable than Analogous SiO2-Coated Gold Nanoparticles

This manuscript describes the synthesis of uniform monodisperse SnO2-coated gold nanoparticles and examines their colloidal stability as function of pH, with direct comparison to better known and widely used SiO2-coated gold nanoparticles. Aqueous acidic and basic colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions were prepared, and their stability was monitored visually and by UV-vis spectroscopy. Notably, the SnO2-coated Au nanoparticle solutions were stable up to pH 12.5. However, at pH 13 and 14, the SnO2-coated Au nanoparticles underwent aggregation, which could be fully reversed upon neutralization of the solutions. In contrast, the SiO2-coated Au nanoparticle solutions were unstable at pH >10.5, irreversibly producing a precipitate composed of bare Au nanoparticle aggregates having little or no silica coating. Under acidic conditions, sedimentation was observed from both the colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions, but the colloidal solutions could be reconstituted upon neutralization of the acidic solutions. The sedimentation at low pH coincided with the reported isoelectric pH values of SiO2 and SnO2, respectively. From an applications perspective, we are seeking to develop SnO2-coated metal nanoparticles as stable alternatives to the more widely employed SiO2-coated nanoparticles, with a particular emphasis on their use in sensor devices and solar cells.