Silicon nanocrystals: Biocompatible fluorescent nanolabel

This study discusses a synthesis, characterization, bioconjugation, and biological responses of fluorescent silicon nanocrystals (SiNCs). First, the efficient method of synthesizing fluorescent SiNCs is presented employing an electrochemical reduction of trichloro(octyl)silane. Miligram amount of bright fluorescent SiNCs is obtained with well-defined nanocrystalline structures. Octyl passivation of the surface provides SiNCs a monodispersity, hydrophobicity and physical stability. Second, physical and chemical characteristics of synthesized SiNCs are described based on the nanoparticle size and surface chemistry information measured by different techniques: transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDX), X-ray powder diffraction (XRD), dynamic light scattering (DLS), UV-VIS absorption spectroscopy (Abs), photoluminescence spectroscopy (PL), and Fourier Transform Infrared spectroscopy (FT-IR). Next, potential applications of SiNCs as a fluorescent nanolabel are highlighted by studying the covalent, multiple attachments of fluorescent SiNCs to the target biomolecules. Streptavidin is tagged by several SiNCs through covalent linkage while it retains its characteristic affinity to biotin molecules. SiNCs-Streptavidin-biotin complexes are characterized by FT-IR spectroscopy for each step of conjugation. SiNCs retain their brightness of blue fluorescence after the final conjugation with biotinylated microbeads. Finally, biological responses of SiNCs are assessed by in vitro assay experiments with murine macrophages. Cytotoxicity of SiNCs is not pronounced until their concentration reaches up to 20 µg/ml. Inflammatory responses of SiNCs are not activated because of their small size (< 5nm in diameter) which allows them pass through the macrophage's defense mechanism. SiNCs penetrate the macrophage cells by pinocytosis and are observed by fluorescence microscopy and optical Z-stacks. Presented studies on the synthesis, characterizations, bio-tagging, and biological responses of SiNCs will benefit research in broad areas of nano-bio science and facilitate the development of small, bright, biocompatible, and multifunctional nanolabels for biological applications