We report the emission and upconversion spectra and the lifetimes of Er³⁺ doped Y₂O₃ nanocrystalline and bulk samples. We found that when the nanocrystal and bulk samples of Y_1.80Er_0.20O₃ were excited at 815 nm, the overall emission intensity was stronger for the bulk sample. However, the relative intensity of the (²H_11/2, ⁴S_3/2)→⁴I_15/2/⁴F_9/2→⁴I_15/2 transitions is 2:3 and 3:2 for the nanocrystal and bulk sample, respectively. The decay times obtained for the nanocrystalline samples are in general significantly faster than those observed for the bulk sample. We attribute this to the adsorption of CO₂ on the surface of the nanocrystalline samples. The effect of Er³⁺ concentration on the decay time of the nanocrystalline samples is also discussed.

We report on the efficient incorporation of non-functionalized NaYF₄ : Er³⁺, Yb³⁺ nanoparticles inside HeLa live cancer cells by direct endocytosis. The efficient two-photon excited near-infrared-to-visible upconversion fluorescence of these nanoparticles is then used to obtain high-contrast intracellular fluorescence images of single cells. These images reveal a redistribution of the nanoparticles inside the cell as the incubation time increases. Thus, non-functionalized NaYF₄ : Er³⁺, Yb³⁺ nanoparticles emerge as very promising fluorescence probes for real-time imaging of cellular dynamics.

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%/KGdF₄ core–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.