Nanoparticles have been subjected to a variety of surface engineering for various applications such as self-assembly of organic components and bioactive species, and dielectric-metal core-shell nanostructures. Core-shell structures can be grown epitaxially and the shell can be considered as an extension of core structure with different chemical compositions.
Nanoshells
The inherent nature of core/shell nanoparticles make them potentially very useful in many areas such as biomedical applications. For example, using a silica core, the plasmon becomes very sensitive to the shell thickness and with silica/gold nanoshells and an 800 nm plasmon, using NIR laser light (808 nm) focused on tumors, nanoshells can be accumulated to localized heat to selectively kill the tumors cells.
AuNi core shell nanocrystals
AuNi core shell nanocrystals can be prepared using a two-step reduction method. First, mixtures of octahedral, triangular and hexagonal plate like, decahedral, and icosahedral Au core seeds can be prepared by reducing HAuCl4•4H2O in ethylene glycol (EG) using microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then, Ni shells are overgrown on Au core seeds by reducing Ni(NO3)2•6H2O in EG with NaOH and PVP using oil bath heating.
Nanoparticle having silica core with a gold shell
The outstanding potential of core/shell nanoparticles is their ability to obtain structures with combinations of properties. Although spherical gold nanoparticles generally have a surface plasmon resonance at a wavelength of about 520 nm, a spherical silica core with a gold shell offers a very highly tunable plasmon wavelength depending on the thickness of the shell and the core diameter.
Conventionally silica core/gold shell nanoparticles can be fabricated by chemical reduction of gold ions. These structures can also be generated by photochemical reduction and by nanosphere lithography. These techniques could provide finer control over the properties of the shell.
Photochemical reduction
The photochemical reduction is a combination of the procedures in which small gold colloid (1-2 nm) can be prepared. Silica nanoparticles 110 nm in diameter can be suspended in ethanol and functionalized with 3-aminopropyltriethoxysilane (APTES). The APTES covered silica is purified and the pH is adjusted. The negatively charged gold seeds are absorbed onto the positively charged amine groups on the silica surface. A solution is prepared of the gold-modified silica and HAuCl4 in ethylene glycol. Irradiation is performed with a mercury xenon lamp and a band filter selecting wavelengths from 230 nm to 400 nm.
This result demonstrates that the photochemical reduction is capable of reducing gold onto the surface of silica nanoparticles. In addition, without the gold seed, no gold is reduced onto the surface of the silica.