Abstract
Freely-dissolved silicon quantum dots were prepared by thermal hydrosilation of 1-undecene at high-porosity porous silicon under reflux in toluene. This reaction produces a suspension of alkyl-capped silicon quantum dots (alkyl SiQDs) with bright orange luminescence, a core Si nanocrystal diameter of about 2.5 nm and a total particle diameter of about 5 nm. Previous work has shown that these particles are rapidly endocytosed by malignant cell lines and have little or no acute toxicity as judged by the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for viability and the Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay for apoptosis. We have extended this work to the CACO-2 cell line, an established model for the human small intestinal mucosa, and demonstrate that neither acute nor chronic (14 days) toxicity is observed as judged by cell morphology, viability, ATP production, ROS production and DNA damage (single cell gel electrophoresis) at doses of 50–200 g mL. Quantitative assessment of the extent of uptake of alkyl SiQDs by CACO-2, HeLa, HepG2, and Huh7 cell lines by flow cytometry showed a wide variation. The liver cell lines (HepG2 and Huh7) were the most active and HeLa and CACO-2 showed comparable activity. Previous work has reported a cholesterol-sensitivity of the endocytosis (HeLa), which suggests a caveolin-mediated pathway. However, gene expression analysis by quantitative real–time polymerase chain reaction (RT-PCR) indicates very low levels of caveolins 1 and 2 in HepG2 and much higher levels in HeLa. The data suggest that the mechanism of endocytosis of the alkyl SiQDs is cell-line dependent.
Highlights
Quantum dots are as well-established as luminescent labels in cell biology [1,2]
The sol may be evaporated to dryness under vacuum and the alkyl Silicon quantum dots (SiQDs) redisperse in nonpolar solvents
Alkyl-capped SiQDs were internalized by CACO-2 without observable toxic effects for the longest durations of exposure studied (14 days)
Summary
Quantum dots are as well-established as luminescent labels in cell biology [1,2] Their advantages over molecular dyes include: resistance to photofading, broad absorption spectra and narrow, bright photoluminescence spectra [3]. 1-undecene at porous silicon layers prepared from low-doped Si, etched at high current density near the threshold for electropolishing [14,15] These quantum dots are stable to aqueous environments for periods of several days [16]. Macrophage cells (RAW 264.7) were treated with two types of silicon particles with diameter 3 nm (nanodots/SiQDs) and 100–300 nm (microdots) and some effects on the cellular inflammatory responses were found. This paper investigates the toxicity and the mechanism of internalization of silicon quantum dots in human cell lines.
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