Digestive Ripening-Mediated Growth of NaYbF4:Tm@NaYF4 Core-Shell Nanoparticles for Bioimaging

Size and size distribution control along with the excitation/emission wavelength manipulation are the most indispensable for both fundamental research and applications of upconverting nanophosphors. In contradiction to the usual Ostwald ripening process, digestive ripening-mediated growth of core-shell nanostructures is observed in a lanthanide-doped ternary fluoride system. Time-dependent size evolution of NaYbF4:Tm@NaYF4 nanoparticles is systematically investigated, and a Ksp-involved growth mechanism is proposed to better understand this unusual phenomenon. The upconversion luminescence (UCL) intensity and branch ratio of near-infrared to visible emission (NIR/VIS) dramatically increase due to the NaYF4 coating but subsequently undergo a decline when an additional SiO2 layer grows outside. As a comparison, an alternant type of core-shell-shell nanostructure, NaYbF4:Tm@SiO2@NaYF4, is further constructed, in which both the UCL intensity and NIR/VIS ratio exhibit an inverse trend. The comparative analysis on the basis of steady and transient spectroscopic data from NaYbF4:Tm, and alternatively coated by silica and NaYF4 nanoshells, respectively, demonstrates that the UCL properties can be regulated through core-shell engineering once the optical behavior of the surface states can be manipulated. The results provide a promising alternative strategy to fabricate a uniform core-shell nanostructure with multicompositions, and this NIRin-NIRout luminescent profile deserves an expectable vision for the subsequent clinical applications of UCL imaging for deep biological tissues.