Nanostruktury krzemionkowo-metaliczne. II. Otrzymywanie, właściwości i zastosowanie

In the past decade the silica-metal nanostructures consisting of siliceous cores and metallic nanoshells have been intensively studied. The second part of this review article presents recent advances in the synthesis, characterization and application of silica-gold core-shell nanostructures. A special emphasis is given to the nanostructures composed of the silica core and gold or silver nanoshell. Nowadays gold is often used metal for the formation of nanoshells. The reason for this interest is a great application potential of SiO2-Au nanostructures in catalysis, chemical and biological detection of various substances, optoelectronics, photonic crystals, plasmonics, and in analytical techniques utilizing surface enhanced Raman spectroscopy. Silver nanoshells formed on siliceous cores show similar properties as those of gold nanoshells. Silica-gold nanostructures can be prepared using various methods, for instance, by growing up gold nanoshells on the siliceous cores with deposited gold nanoparticles, by precipitating gold nanoparticles with their simultaneous deposition on modified silica cores, by reducing gold ions on Sn-modified silica particles, or by forming gold nanoshells under ultrasonic conditions. This article presents several methods for the formation of silica-metal nanostructures. A special emphasis is given to the method of growing up gold nanoshell on the modified silica core with deposited gold nanoparticles. This method assures a good control of the nanoshell thickness as well as its surface properties. In this method the organically modified silica particles are initially covered with gold nanoparticles, which facilitate a further growth of gold nanoshell by reduction of tetrachloroauric acid with agents such as formaldehyde. In the case of aminopropyl-modified silica particles, about 25–30% coverage of the silica surface with gold nanoparticles can be achieved. The effect of other than aminopropyl organic groups on the coverage of the silica surface with gold nanoparticles was studied showing that amino and mercaptopropyl groups assure about 30% surface coverage with gold, while this coverage is very small when methyl and diphenylphospine groups are on the silica surface. The aforementioned reduction of gold ions and growth of gold nanoparticles depend on the uniformity of the initial coverage of the silica surface with gold nanoparticles, the concentration of reduced gold ions as well as the nature and the concentration of reducing agent. The most often used reducing agents are formaldehyde and sodium borohydride, although hydroxylamine hydrochloride, carbon monoxide, hydrogen peroxide and trisodium citrate are also used. Silver is the next popular metal after gold used for the formation of nanoshells. The other metal used are platinum, palladium, nickel and copper. The final sections of this review are devoted to a brief presentation of various techniques used for characterization of core-shell nanostructures as well as to their applications. The most often used methods include scanning (SEM) and transmission (TEM) electron microscopy, wavelength (WDS) and energy (EDS, EDX) dispersion spectroscopy, photoacoustic spectoscopy (PAS), dynamic light scattering (DLS), surface plasmon resonance (SPR), powder X-ray diffraction (XRD), IR and UV-Vis spectroscopy, Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimery (TG) and adsorption (ADS). As regards applications of silica-metal core-shell nanostructures, the prospects are enormous ranging from catalysis, optoelectronics, to drug delivery, and other medical applications.