Single-step synthesis of Er3+ and Yb3+ ions doped molybdate/Gd2O3 core–shell nanoparticles for biomedical imaging

Nanostructures as color-tunable luminescent markers have become major, promising tools for bioimaging and biosensing. In this paper separated molybdate/Gd2O3 doped rare earth ions (erbium, Er3+ and ytterbium, Yb3+) core–shell nanoparticles (NPs), were fabricated by a one-step homogeneous precipitation process. Emission properties were studied by cathodo- and photoluminescence. Scanning electron and transmission electron microscopes were used to visualize and determine the size and shape of the NPs. Spherical NPs were obtained. Their core–shell structures were confirmed by x-ray diffraction and energy-dispersive x-ray spectroscopy measurements. We postulated that the molybdate rich core is formed due to high segregation coefficient of the Mo ion during the precipitation. The calcination process resulted in crystallization of δ/ξ (core/shell) NP doped Er and Yb ions, where δ—gadolinium molybdates and ξ—molybdates or gadolinium oxide. We confirmed two different upconversion mechanisms. In the presence of molybdenum ions, in the core of the NPs, Yb3+–${{{\rm{MoO}}}_{4}}^{2-}$ (mid2F7/2, 3T2〉) dimers were formed. As a result of a two 980 nm photon absorption by the dimer, we observed enhanced green luminescence in the upconversion process. However, for the shell formed by the Gd2O3:Er, Yb NPs (without the Mo ions), the typical energy transfer upconversion takes place, which results in red luminescence. We demonstrated that the NPs were transported into cytosol of the HeLa and astrocytes cells by endocytosis. The core–shell NPs are sensitive sensors for the environment prevailing inside (shorter luminescence decay) and outside (longer luminescence decay) of the tested cells. The toxicity of the NPs was examined using MTT assay.The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08) and was partially supported by a grant from the Polish National Science Center 2013/11/B/N21/00089 and partially supported by the grant DEC-2012/07/B/ST5/02080 of the National Science Center of Poland and Center of Excellence. This work has been done in the NanoFun laboratories co-financed by the European Regional Development Fund within the Innovation Economy Operational Program, the Project no. POIG.02.02.00-00-025/09/. This research has been co-finananced with the European Union funds by the European Social Fund and was partially supported by the cluster of Biomedical Engineering Center co-financed by European Union funds under the Operational Programme Innovative Economy (project number UDA-POIG.05.01.00-00). The research was partially financed by the project Sonata from the National Science Centre, UMO-2014/15/D/ST5/02604. The research was partially supported by the Foundation for Polish Science through the International Research Agenda Programme co-financed by the European Union within Smart Growth Operational Program. The research was supported by the National Science Centre (Poland) through Grants No. DEC-2014/14/M/ST3/00484.Peer reviewe