Silica Coating of Metal-Related Nanoparticles and Their Properties

Abstract

This chapter describes our studies on the silica coating of metal-related nanoparticles, or the fabrication of core-shell particles composed of metal-related nanoparticle cores and silica shells, along with their various properties. Various metal-related nanoparticles such as metallic nanoparticles, metallic alloy nanoparticles, and nanoparticles containing metallic nanoparticles were successfully coated with silica by a sol-gel process. Nanoparticles of metallic cobalt (Co) and metallic copper (Cu) are easily oxidized in air and in aqueous solutions exposed to air. Nevertheless, the crystallization of metallic Co nanoparticles with no oxidation of the metallic Co was boosted by annealing even in the atmosphere, and the crystallized nanoparticles exhibited magnetic properties. For metallic Cu, an absorption peak derived from the surface plasmon resonance (SPR) of metallic Cu nanoparticles was caught for a colloidal solution of silica-coated metallic Cu nanoparticles. Accordingly, silica coating was confirmed to chemically stabilize the core nanoparticles via the physical barrier of the silica shells. For metallic gold (Au) and metallic silver (Ag), the optical spectra of silica-coated metallic nanoparticle colloidal solutions coincided with the forecast of Mie theory, which confirmed that most metallic nanoparticles were coated with silica shells with no aggregation. The silica-coated metallic Au nanoparticle colloidal solution exhibited strong X-ray absorption, which is applicable to X-ray imaging. In addition to the silica coating of nanoparticles with a single metal component, silica coating was also successfully performed on nanoparticles of an alloy of metallic platinum (Pt) and metallic ruthenium, nanoparticles of an alloy of metallic Co and metallic Pt, and multilayered nanoparticles composed of metallic Au, silica, and gadolinium compound, which exhibited catalytic activity, magnetic properties, and magnetic resonance imaging ability, respectively. The abovementioned core nanoparticles were spherical. In contrast, the silica coating of metallic Au nanorods (AuNRs), which are not spherical, was also performed successfully. The silica-coated AuNR colloidal solution had an SPR peak in the near-infrared region, so that cancer cells in culture medium died from a temperature rise given by photothermal conversion via near-infrared laser irradiation.