Synthesis of transition metal doped zinc selenide nanoparticles for bioimaging

Due to tunable luminescence and absorption spectra, high quantum yield, compatibility with various technologies and materials colloidal quantum dots (QDs) are attracting much interest in various fields of science and engineering, such as light emitting devices, photovoltaics and catalysis. Because of superior photostability compared to organic fluorophores and wide possibilities for chemical modification and bioconjugation colloidal nanocrystals are well suited for bioimaging. In this respect another advantageous feature of QDs is the extensive absorption spectrum that allows performing multicolor detection using one excitation source. The most common material for preparation of visible light emitting QDs is CdSe [1]. Being an important issue for safety of QDs application (especially for biomedical studies) cytotoxicity of cadmium leads to development of colloidal QDs emitting in visible range, which do not have highly toxic elements in their composition. One of such material groups is zinc chalcogenides (ZnS, ZnSe) doped with 3d transition metal atoms. Doping shifts UV-blue photoluminescence maximum of the host material to longer wavelengths leading to green emission in the case of copper dopant and yellow to orange luminescence for manganese-doped nanocrystals [2]. Despite certain difficulties with emission band tunability and larger full width at half maximum parameter such quantum dots retain many advantageous features of cadmium chalcogenide QDs such as high quantum yield and small hydrodynamic size. Other characteristic features are: a large Stokes shift and a higher thermal stability of photoluminescence [2]. Colloidal QDs of ZnSe doped with Mn²⁺ were synthesized in aqueous solution employing co-nucleation doping strategy. Nanocrystals exhibit strong photoluminescence under UV excitation with maximum near wavelength of 600 nm which is ascribed to radiative relaxation via Mn²⁺ d-orbital states inside host bandgap. Samples were characterized by means of X-ray diffraction analysis, photoluminescence and absorption spectroscopy (Fig. 1). In order to secure dopant ions inside the nanocrystal and increase quantum yield ZnS shell was deposited using thiourea as a source of sulfur species.