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 K_sp-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 NIR_in−NIR_out luminescent profile deserves an expectable vision for the subsequent clinical applications of UCL imaging for deep biological tissues.