Synthesis of Nd³⁺/Yb³⁺ sensitized upconversion core-shell nanocrystals with optimized hosts and doping concentrations

Development of upconversion nanocrystals (UCNs) under 808 nm excitation rather than 980 nm is very important to biological applications for avoiding tissue over-heating. Nd³⁺ and Yb³⁺ dual-sensitized UCNs are proven to be promising candidates but how to select host materials to construct core-shell UCNs with strong UC emissions still remains unexplored. Herein, we prepare a series of homogeneous and heterogeneous core-shell UCNs using NaYF₄ and NaGdF₄ as the core-host and/or as the shell-host, respectively, through the seed-mediated synthetic approach. Our results show that selecting the core-host in the core-shell UCNs plays the key role in determining their final UC intensities. Furthermore, homogeneous core-shell UCNs can give stronger UC fluorescence than the heterogeneous ones due to the low crystal defects at the core-shell interface. Moreover, the concentration effect of activator ions and sensitizer ions in these core-shell UCNs on their UC emissions is analyzed, and optimal doping under different NIR excitation (808 nm, 980 nm or 808 nm/980 nm) is achieved. © The Royal Society of Chemistry 2015.