Water dispersible ultra-small multifunctional KGdF₄:Tm³⁺, Yb³⁺nanoparticles with near-infrared to near-infrared upconversion

Ultra-small multifunctional KGdF4:Tm3+,Yb3+nanoparticles with near-infrared to near-infrared upconversion are synthesized. The average sizes of KGdF4:Tm3+2%, Yb3+20% core-only and KGdF4:Tm3+2%, Yb3+20%/KGdF4core-shell nanoparticles are ∼3.7 nm and ∼7.4 nm, respectively, which fall within the reported optimal sizes (<10 nm) for bioimaging probes. The excitation and emission at 980 and 803 nm are favorable to deeper tissue penetration and reduced autofluorescence. The weak upconversion luminescence of the ultra-small core-only nanoparticles is overcome by the use of the core-shell approach. The magnetic mass susceptibility and magnetization of the ultra-small core-only and core-shell nanoparticles were determined, and are close to those of large nanoparticles (26 and 50 nm) used for magnetic resonance imaging and bio-separation. There is no variation in the magnetic properties with the nanoparticles' sizes between the core-only (∼3.7 nm) and core-shell (∼7.4 nm) nanoparticles, which differs from the size-dependent luminescence. The oleate-capped core-shell nanoparticles were further encapsulated with a PEG-phospholipid shell to endow them with dispersibility in water, which is indispensable for future biological applications.