Conjugation of the photoluminescent silicon nanoparticles to streptavidin

We have covalently attached multiple photoluminescent silicon nanoparticles (SNs) to streptavidin molecules. Conjugation of SNs to a target protein is achieved using the multistage photoassisted procedure. In a first step, the terminal hydrogen in the freshly prepared SNs is substituted with an alkane monolayer that serves as a platform for chemical linkage to a heterobifunctional cross-linker: 4-azido-2,3,5,6-tetrafluorobenzoic acid, succinimidyl ester. A resulting surface coating stabilizes nanoparticles against oxidation and aggregation. Next, an open end of bifunctional cross-linker-diazirine succinimidyl ester is reacted with carboxyl moieties of streptavidin and forms an amide bond. Gel and capillary electrophoresis of the SN-streptavidin complex demonstrated separate elution of the conjugation product and unreacted protein. Then, the number of SNs per protein molecule was determined by measuring complex charge variation by capillary electrophoresis. Conjugate functionality was tested by allowing it to interact with biotinylated polystyrene microbeads. Intense photoluminescence at carefully washed microbeads demonstrated selective binding of silicon nanoparticle bearing streptavidin to biotinylated microbeads. The high quantum yield of streptavidin-SN conjugate in combination with the small size and biocompatibility of silicon nanoparticles presents an attractive platform for the fluorescence labeling in diverse bioassays.