Surface Functionalization of Silicon Nanoparticles

If silicon nanoparticles are to be used in a biological environment, then, at minimum, their surface must be modified to make them dispersible in aqueous solutions and to protect them from quenching by compounds present in biological fluids. Surface chemistry is an area where silicon nanoparticles potentially have significant advantages over compound semiconductors. Because silicon forms strong covalent bonds with carbon and oxygen, one can covalently link organic entities to a silicon surface. This contrasts with compound semiconductors where surface functionalization relies on adsorption of bifunctional linker molecules or hydrophobic interactions between surfactants on the nanocrystal surface and a polymer or biomolecule. Covalent attachment not only provides stronger and more robust linkages between the nanoparticle and the molecules attached to it, but also reduces the size of the overall organically capped particle by allowing for shorter linkages between the nanocrystal and the functional groups that make it water dispersible and allow biological functionalization of it.

There are two primary means of attaching organic molecules to a silicon surface: silicon-carbon bonds formed by hydrosilylation reactions, or silicon-oxygen-silicon-carbon linkages formed by sila-nization reactions. These will be considered separately in Section 4.4.1 and Section 4.4.2. Other reactions may also be effective, and will be considered briefly at the end of this section. The surface chemistry of organic molecules on silicon and germanium surfaces has recently been reviewed by both Buriak [84] and Bent [85]. Wayner and Wolkow [86] presented a review that considered only hydrogen-terminated silicon surfaces. Although surface reactions on single-crystalline silicon wafers are often carried out under vacuum, using vapor-phase reagents, this is generally not possible for silicon nanocrystals. Thus, here we consider only solution-phase chemistry. On silicon nanoparticles, it is also not generally possible to carry out reactions that are particular to a certain crystal surface, such as the cycloaddition of al-kenes or dienes with silicon dimers on the Si(100) surface. Thus, such reactions are not considered here.

An important fact to keep in mind when considering the attachment of molecules onto freestanding nanocrystals is the effect on the overall size of the structure. Particularly for silicon nanocrystals, which are typically in the 1 to 5 nm diameter range, the volume of the organic molecules attached to the surface can be comparable to, or even significantly greater than, the volume of the crystalline core. Figure 4.3 shows a model representation of a silicon nanocrystal, roughly 2 nm in diameter (containing 323 silicon atoms) that has been covered only on one hemisphere with C10H20 alkyl chains to illustrate the difference in diameter between the coated and the uncoated nanocrystal. The majority of the overall volume of a fully coated nanocrystal like this is made up of the alkyl chains.

FIGURE 4.3 Model of a ~2 nm silicon nanocrystal with one hemisphere covered with alkyl chains to illustrate the effect of surface functionalization on overall nanoparticle size.
0 0

Post a comment