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2 2010

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

Jankiewicz B.J., Choma J., Jamioła D., Jaroniec M.

Wiadomości Chemiczne

2010

[Z] 64, 11-12

943-981

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.

Nanostruktury krzemionkowo-metaliczne. I. Otrzymywanie i modyfikacja nanocząstek krzemionkowych

Jankiewicz B.J., Choma J., Jamioła D., Jaroniec M.

Wiadomości Chemiczne

2010

[Z] 64, 11-12

913-942

Silica-metal nanostructures consisting of siliceous cores and metal nanoshells attract recently a lot of attention because of their unique properties, mainly catalytic and spectroscopic. The core of these nanostructures is coated with a thin layer (nanoshell) of another material, often being a noble metal, of a thickness between 1 and 20 nm. The silica-metal nanostructures are highly functional materials of properties different from those of the siliceous core and the metal nanoshell. Already nowadays these nanostructures have found various applications such as for an exaltation of chemical stability of colloids, for an enhancement of luminescence properties of materials, for biosensing, drug delivery and other medical applications. The main goal of this two-part review is the presentation of various methods for the preparation of silica-metal nanostructures, description of the most important physicochemical properties of these materials, and presentation of their potential applications. The first part is focused on the main preparation methods of silica particles being used as cores for the aforementioned core-shell nanostructures and methods for their surface modification. A special emphasis is given on the Stöber method, which is relatively simple, effective and well verified for the synthesis of large silica particles (with diameters from 100 nm to several microns). A typical preparation of silica particles is based on mixing ethanol, ammonia (as a catalyst), and often small amount of water, followed by rapid or gradual addition of tetraethyl orthosilicate (TEOS) under vigorous stirring. This article reviews numerous studies reporting the effects of various factors on the structural properties of silica particles, especially the steps required for controlling their size, assuring narrow particle size distribution and high uniformity of the resulting particles. The surface chemistry of siliceous cores is essential in the process of metal nanoshells formation. Therefore, in this work the main methods for modification of the silica surface are presented in details in order to prepare this surface for the formation of metal nanoshells. This modification is often based on the replacement of silanols with specific organic groups (often aminopropyl and mercaptopropyl groups), which interact strongly with metal nanoparticles. In 1998 Halas and co-workers elaborated a simple method for the preparation of core-shell nanostructures involving attachment of specific organic groups to the silica surface followed by deposition of gold nanoparticles and subsequent reduction of tetrachloroauric acid in order to obtain continuous gold nanoshells. The first modifier of the silica surface was 3-aminopropyltriethoxysilane. Later, several other modifiers have been tested. The aforementioned modification of the silica surface can be singleor two-step process. The single-step process involves hydrolysis and condensation of TEOS in the presence of functional organosilane, which results in silica particles with desired organic groups attached. The two-step process involves the synthesis of silica particles in the first step and their surface modification with organosilanes in the second step. The presented literature survey shows that the proper modification of the surface of silica particles is a necessary condition for the formation of uniform metal nanoshells.