Quantum dots are among the most promising tools in the nanomedicine area. The potential uses of these colloidal semiconductor nanocrystals include bioengineering, bioanalytical detection, bioimaging, as well as targeted drug delivery, in vivo monitoring, and photodynamic therapy. The possibility of such wide applications is attributed to their unique size-dependent photophysical properties, like narrow and symmetrical emission spectra, broad absorption spectra, high quantum yield, and simultaneous excitation by a single light source. However, toxicity of the QDs (especially those based on Cd) is a serious limitation in biomedical applications. Synthesizing biocompatible nanocrystals, composed of weakly toxic ZnO became a leading route in quantum dots nanofabrication.

However, uncapped ZnO QDs tend to aggregate and are not stable due to high surface reactivity. In this work, we present various approaches which can be useful to eliminate these drawbacks. Introducing of inorganic shell made of higher band-gap semiconductor as well as stabilizing with oleic acid were the routes to obtain better stability and protection of the ZnO core. Oleate-capped ZnO QDs as well as ZnO/MgO and ZnO/ZrO₂ core-shell QDs synthesized in a sol-gel process were characterized by UV-vis and fluorescence spectroscopies, transmission electron microscopy (TEM), and powder X-ray diffraction (XRD), showing good stability and preserved luminescent properties.

As water-stabilityis the fundamental criterion to apply these nanoparticles in biological applications, several strategies were evaluated to disperse primarily hydrophobic ZnO QDs in aqueous media. One of the methods used by us was silanization with 3-aminopropyltrimethoxysilane and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane, which resulted in exchange of hydrophobic ligands into hydrophilic ones. In terms of bioimaging such modified QDs were further conjugated with small selected biomolecules (folic acid, L-glutathione or gluconic acid) used as targeting ligands. The effects of structural properties of biomolecules on the formation, stabilization, photoluminescence of ZnO-core QDs have been evaluated. The obtained results demonstrated that aminosiloxane-capped ZnO QDs coupled either with folic acid, glutathione or gluconic acid show good optical properties.

Obtained results encourage our team to work on cytotoxicity of modified-ZnO QDs. First tests have demonstrated that synthesized materials are very promising and should be more developed to be successfully used in medical applications, such as e.g. cancer therapy.  

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