Various Au Nanoparticle Organizations Fabricated through SiO2 Monomer Induced Self-Assembly

A novel method has been developed to fabricate the assembly of Au colloidal nanoparticles (NPs) using SiO2 monomers. The key strategy was the use of a controlled sol−gel procedure including hydrolysis, deposition, and condensation of tetraethyl orthosilicate (TEOS). Namely, the assembly of Au NPs was created by the anisotropic deposition of SiO2 monomers and subsequent permanent fixing by the growth of a SiO2 shell. Various assemblies of Au NPs such as dimer, trimer, and pearl-chain morphology were fabricated by systematically changing the concentration and injection speed of TEOS. A longitudinal plasmon resonance band was observed as a result of the assembly of Au NPs and can be tuned from visible to near-infrared by altering the length of pearl-chain morphology. In addition, single Au NP was homogeneously coated with a SiO2 shell by means of controlling the deposition rate of SiO2 monomers during a Stöber synthesis without the use of a silane coupling agent or bulk polymer as the surface primer to render the Au surface vitreophilic. The Au NPs (mean size 11.4 nm in diameter) were thus encapsulated into SiO2 beads with a wide range of sizes (from 20 to 50 nm in diameter). These pure SiO2-coated Au beads with tunable shell thickness should be crucial for biosensors, particularly as Raman-tag particles.