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Posted: Feb 19, 2007
Watersoluble, luminescent silicon nanoparticles with high quantum yield
(Nanowerk Spotlight) Current medical and biological fluorescent imaging is limited by the use of dye markers, which are not photostable. The dyes can break down under photoexcitation, room light or higher temperatures. The observation of strong visible emission in porous silicon therefore has triggered substantial interest in exploring the synthesis and characterization of silicon nanoparticles. Due to their biocompatibility, high photoluminescence quantum efficiency and stability against photobleaching, silicon nanoparticles are expected to be an ideal candidate for replacing fluorescent dyes in many biological assays and fluorescence imaging techniques. For instance, they have been proposed as better quantum dots for in vivo applications, potentially replacing quantum dots of highly toxic cadmium. Different synthetic and physical methods have been used to prepare silicon nanoparticles. However, the yields of nanoparticles from these methods are very low and an HF (hydrofluoric acid) etching process is often necessary to obtain photoluminescent, hydrogen-terminated silicon nanoparticles. Now, researchers have developed a new solution route for the production of macroscopic amounts of hydrogen terminated silicon nanoparticles without hazardous material handling. This synthesis route is simple and thus offers great opportunity for scaled-up preparation of semiconductor materials.
For silicon nanoparticles to be used in biomedical applications it is essential that they have high stability, a substantial photoluminescence quantum yield in the visible region, and solubility in aqueous media. While the etching processes do not hinder the applications of these nanoparticles, a direct method for preparing hydrogen terminated silicon nanoparticles with less processing would be an improvement.
"We developed a simple method to synthesize water soluble silicon nanoparticles" Angelique Louie tells Nanowerk. "Our method represents an advance in materials chemistry as it is much simpler than existing technology to prepare silicon nanoparticles, such as laser driven pyrolysis, and results in a water soluble silicon nanoparticle. Most literature reports of silicon nanoparticles are in organic solvent."
Synthetic route for the generation of water-soluble silicon nanoparticles. Synthesized by a "metathesis" reaction of NaSi and NH4Br, the hydride-covered silicon nanoparticles can be modified by a hydrosilylation process to form Si"C bonds on the surface. Finally, an amphiphilic polymer of octylamine-modified poly(acrylic acid) can be coated on the alkyl-terminated silicon nanoparticles to render them water-soluble with a high quantum yield. (Reprinted with permission from IOP Publishing)
The synthesis method of the UC Davis scientists involves reaction of the Zintl salt (NaSi) with ammonium bromide. The formation reaction proceeds via a chemical "metathesis" process, in which hydride-covered elemental silicon is formed with several by-products, e.g. NH3, H2 and NaBr. The yield
is quantitative and results in a hydrogen-terminated silicon nanoparticle.
"The hydride-covered silicon nanoparticles can be modified further by a hydrosilylation process to form chemically robust Si"C bonds on the surface" says Louie. "Finally, a biocompatible and synthetically convenient polymer can be coated on the alkyl-terminated silicon nanoparticles to make the latter water-soluble, while retaining a high quantum yield."
The emission of silicon nanoparticles prepared by this process is in the blue region of the visible spectrum. Blue emission is not ideal for in vivo imaging but is suitable for biological research. Louie and her colleagues are currently working on producing stable green and red emission nanoparticles with high quantum yields with their method.
For future medical applications, the major challenge for the field is to develop either highly stable quantum dots that do not pose any health threat, or to develop quantum dots that are innately nontoxic. Further studies on high-quality silicon nanoparticles, particularly the photophysics of silicon nanoparticles with narrow size distributions, are needed since such silicon nanoparticles with controllable optical properties are of great benefit for further applications such as fluorescent labels.