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.
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