Cytotoxicity mitigated silica entrapped copper doped Zinc Sulfide Quantum dots as luminescent nanoprobes for biolabeling

Abstract Background
Quantum dots (QDs) are luminescent semiconductor nanoparticles with unique optical properties that facilitate their use in sensing, biological labelling, optical imaging, and diagnostics. Wider band gap materials, such as Zinc Sulfide, are extensively employed as quantum dot nanoprobes since they offer higher photostability, higher quantum yield, larger molar extinction coefficients, and longer fluorescence lifetimes than conventional organic fluorescent dyes used in bioassays. Tunable multiphoton emission in quantum dots is accomplished by doping with transition metals, of which copper is the most beneficial owing to its comparable ionic radius, intense emission, and composition-variable spectral broadening. However, an overdose of Cu is toxic to the cells, leading to apoptosis. This cytotoxicity impedes the utilization of Cu-doped ZnS quantum dots for biolabeling.
Methods
The present work deals with the diminution of copper cytotoxicity in Cu-doped ZnS Q-dots by means of silica entrapment, equipping them for in vitro and in vivo bioassays in the future. Copper-doped ZnS Q-dots were synthesized by chemical precipitation method and overlaid with silica by sol-gel method. Cytotoxicity investigation was performed on L929 Mouse fibroblast cells.
Results
X-ray diffraction studies confirmed that the prepared Q-dots were approximately 2 nm in size and were in the cubic phase. High Resolution Transmission Electron Microscopy revealed the spherical morphology of Q-dots. Micro-Raman Analysis was used to determine the Raman modes of the samples. Band gap energy was computed using UV-Visible Spectroscopy. Photoluminescence Spectroscopy demonstrated two emission peaks around 418 nm and 455 nm due to sulphur vacancy and copper trap levels, respectively, for Cu:ZnS Q-dots with hiked PL intensity on silica coating. In vitro cell toxicity studies performed on the as-prepared Q-dots by microscopic observation of treated cells, as well as by MTT colorimetric assay, manifested the attenuation of cytotoxicity in silica overspread copper-doped Q-dots.
Conclusions
Silica entrapment subsided the copper-induced cytotoxicity by minimizing the photochemical oxidation of the Q-dots surface together with making them hydrophilic. Furthermore, silica coating boosted the photoluminescence intensity of the Q-dots. Such Q-dots could be a potent alternative to fluorescent organic pigments for biolabelling.