High Nd(III)-Sensitizer Concentrations for 800 nm Wavelength Excitation Using Isotropic Core–Shell Upconversion Nanoparticles

Upconverting nanoparticles (UCNPs) are potentially useful for biological applications, if they are capable of high-intensity emission. This requires the highest absorption efficiencies of wavelengths not absorbed or scattered by tissues. 800 nm is considered to be a "biobenign" wavelength because it effectively minimizes signal attenuation and reduces detrimental overheating, while maintaining deep tissue penetration. Neodymium (Nd3+) substitution for ytterbium (Yb3+) in lanthanide-based UCNPs successfully shifts absorption from 980 nm to 800 nm, where water does not show absorption. High Nd3+ concentrations are desired because the more the sensitizer ions, the higher the absorption and thus the upconversion (UC) emission. However, high Nd3+-sensitized UCNPs, above 30 mol % Nd3+, have been limited because of lattice distortions observed in heavily doped core–shell nanoparticles (CS NPs). Here, we overcome this hurdle by introducing a tensile-strained NaLuF4 shell while still ensuring a complete and thicker isotropic shell. We report 50 mol % Nd3+-sensitized CS NPs that effectively release lattice strain between the core and shell. The doping concentration of 50 mol % Nd3+ provided 13-fold UC enhancement compared to CS NPs without Nd3+ in the shell, independent of the activators examined in this study. This exceptional enhancement in UC emission is due to the maintenance of structural uniformity. We demonstrate cell tolerability by PEGylating CS NPs and incubating the NPs with several cell types to show the potential for biological applications.