Si QDots: Where Does Photoluminescence Come From?

Abstract

Silicon nanoparticles (Si NPs) with sizes in the order of bulk exciton Bohr radius [1, 2] present interesting optical properties for fl uorescent labeling in biological imaging applications with their potential nontoxicity [3 – 6] . However, the origin of their photoluminescence has been subjected to intense debate for almost two decades. This debate has been focused on whether quantum confi nement effects or atomic scale defects at the surface of the nanocrystals are responsible for the light emission [7] . There have been reports of Si NPs produced by electrochemical etching of Si wafer, laser pyrolysis, plasma deposition and solution phase synthesis [8] . The last approach seems to be the most promising method because the size of nanoparticles can be controlled by the use of inverse micelles as nanoreactors in solution phase [3, 4] . Moreover, these synthesis methods allow chemical functionalization of Si NPs surfaces with relative ease, to offer numerous stabilization, solubility and bioconjugation options [4, 8 – 13] . In recent years, Wilcoxon ’ s and Tilley ’ s groups have synthesized water dispersible Si NPs by using LiAlH 4 reduction of silicon halide [3] , and then modifi ed the nanoparticles ’ surface with allylamine by using a platinum catalyst [4] . However, we have found that reproducibility and scalability of these reported methods of synthesis have not been completely controlled yet due to many factors, such as kinetics, oxidation, contamination, etc. For example, since LiAlH 4 is a very strong reducing agent, it is diffi cult to control the speed of the reducing reaction required for good reproducibility. It has also been previously reported that H 2 PtCl 6 can be reduced to platinum nanoparticles by methanol or other reducing agent [14] . The presence of other types of nanoparticles as impurities could introduce uncertainties in the origin of the fl uorescence. A possible solution for this problem could be the use of UV irradiation instead of H 2 PtCl 6 to functionalize the nanoparticle surface. Since all these methods involve the evaporation of the initial nonpolar solvent before redispersing in water, the possibility of contamination due to noneliminated toluene must be taken into account, as the optical properties of this solvent have been previously mistaken in the literature with those of Si NPs [15] .