Hard Template-Directed Synthesis

Abstract

Silica is presently widely used as a template for the development of various nanostructures due to its characteristic high surface area, thermal and chemical resistance, the presence of reactive silanol groups (Si–OH), and nanopores. Nontraditional nanostructures consisting of noble metals are often fabricated by depositing thin layers of metals (or their precursors) on some silica beads existing templates. A straightforward sol-gel method can be utilized for the preparation of noble metal nanoparticles and their composites via in situ-doped silica aerogels. The metal dispersion and size distribution obtained by this approach depends on the kind of metal, the reaction conditions, and the metal loading. Another approach consisted of making use of the pore channels of hexagonal mesoporous silica, as matrixes for controlling the nanoparticles size. It was demonstrated that noble metal nanoparticles could be coated with a silica shell via the reaction of citrate-stabilized gold nanoparticles and alkylaminotrimethoxysilane, followed by polymerization after the addition of a sodium silicate solution. The spherical silica nanoparticles were used as the templates of interior cores of metal nanoshells and as the templates for the deposition of the external noble metal shell layer by layer. A controlled growth of silver nanoparticles on TiO2 templates is expected in tuning the optical response of the silver–TiO2 hybrids. The growth model can explain the phenomenon of silver nanoplates lying on the small silver nanoparticles. A simple and reproducible method for the activation of monodispersed silver nanoparticles was insertion of the concentrated NaCl, NaBr, and NaI solution into the silver nanoparticle dispersion. It was also demonstrated the entrapment of thiol-coated gold particles, and believe that the technique is straightforwardly applicable also for other hydrophobically coated nanoparticles. The presence of a substrate influences the resultant frequency and bandwidth of the surface plasmon resonance and more generally its multipole distribution. By combining two different materials or different particle shapes of the same material within the same single nanostructure, new properties of the coupled system can be obtained. An interesting approach to introduce theranostic functionalities into a nanosystem is to covalently attach a metal-porphyrin chelate to mesoporous silica nanoparticles (MSNs) which are readily taken up by cells. Bioconjugates based on MSNs are used in a wide array of applications, including chemical catalysis, drug delivery, controlled release of therapeutics, and cell labeling and killing. UV-photoexcited TiO2 nanoparticles and their conjugates in aqueous solution form various reactive oxygen species, mainly highly reactive hydroxyl (OH), peroxy (HO2) radicals, and singlet oxygen, highly reactive hydroxyl radicals, electrons and superoxide ions able to deactivate of bacteria, algae, viruses and kill cancer cells.