Wide Visible-Range Activatable Fluorescence ZnSe:Eu3+/Mn2+@ZnS Quantum Dots: Local Atomic Structure Order and Application as a Nanoprobe for Bioimaging

The development of QDs based fluorescence bionanoprobe for cellular imaging fundamentally relies upon the precise knowledge of particle-cell interaction, optical properties of QDs inside and outside of the cell, movement of a particle in and out of the cell and the fate of particle. We reported engineering and physicochemical characterization of water-dispersible Eu3+/Mn2+ co-doped ZnSe@ZnS core/shell QDs and studied their potentials as bionanoprobes for biomedical applications in RAW 264.7 macrophages by evaluating their biocompatibility, fluorescence imaging capability, time-dependent uptake, endocytosis and exocytosis. The oxidation state and local atomic structure of the Eu dopant studied by X-ray absorption fine structure (XAFS) analysis manifested that the Eu3+ ions occupied sites in both ZnSe and ZnS lattices for the core/shell QDs. A novel approach was developed to relieve the excitation constraint of wide bandgap ZnSe by co-incorporation of Eu3+/Mn2+ dopants, enabling the QDs to be excited at a wide UV-visible range. The QDs displayed tunable emission colors by a gradual increase in Eu3+ concentration at a fixed amount of Mn2+, systematically enhancing the Mn2+ emission intensity via energy transfer from the Eu3+ to Mn2+ ion. The QDs presented high cell viability above 85%, inducing no cell activation. The detailed analyses of QDs treated cells by dual mode fluorescence CytoViva microscopy confirmed the systematic color-tunable fluorescence and its intensity enhances as a function of incubation time. The cells internalized the QDs predominantly via micropinocytosis and other lipid raft-mediated endocytic pathways, retaining an efficient amount for 24 h. The unique color tunability and consistent high intensity emission of QDs would be useful for developing multiplex fluorescent bionanoprobe in the wide-visible region