Lanthanide-based upconversion core-shell NaGdF4 nanocrystals with strong upconversion luminescence and biocompatibility were synthesized by the solvothermal method.The multicolor upconversion emission of these NaGdF4 nanoparticles could be easily obtained by controlling the core-shell compositions.These multicolor core-shell NaGdF4 upconversion nanocrystals could be employed as fluorescent probes for imaging the mouse hair,by which the porous and scalelike structure of the mouse hair were presented clearly.Meanwhile,it was directly shown by fluorescent signals that the mouse hair could resist the corrosion of the strong acid even when the concentration of hydrochloric acid was increased to 36.5%,but could not avoid the carbonization at high temperature of 400 oC.This procedure based on upconversion fluorescent nanoprobes opens a novel route for investigating the basic physical structure and chemical properties of biological tissue and organism.
Lanthanide doped bifunctional materials are potentially important for developing multifunctional devices. Here, NaLuF4:Yb3+/Tm3+/Gd3+/Sm3+ optical-magnetic bifunctional microcrystals were successfully synthesized by hydrothermal method, which could emit ~480 nm blue light from the1G4→3H6 electronic transition and ~800 nm infrared light from the3H4→3H6electronic transition of Tm3+ ion, under the excitation of 980 nm infrared light. By doping Sm3+ ion into NaLuF4:Yb3+/Tm3+/Gd3+, the infrared emission peak centered at 800 nm would shift obviously to longer wavelength. This indicated that Sm3+ ion could efficiently tune the energy level gaps of Tm3+ ions in NaLuF4 host which was demonstrated based on the crystal field theory. In addition, these NaLuF4:Yb3+/Tm3+/Gd3+/Sm3+ microcrystals presented unique ferromagnetic property instead of usually reported paramagnetic prop-erty. Importantly, the ferromagnetic property decreased with increasing the concentration of Gd3+ ion. This was in good agreement with Swift’s theoretical investigation that the coexistence of light rare earth (Gd3+) and heavy rare earth (Yb3+/Tm3+) would lead to the anti-ferromagnetic coupling in the sub-lattices.
Upconversion NaLuF4 nanoparticles were synthesized by the solvothermal method which could emit multicolor visible light under the excitation of 980 nm near-infrared (NIR) photons. These upconversion nanoparticles (UCNPs) with an acidic ligand could rapidly capture the basic rhodamine-B (RB) in plant cells to generate a close UCNPs@RB system. RB could efficiently absorb the green fluorescence from NaLuF4:18 mol.%Yb3+,2 mol.%Er3+ UCNPs and then emitted red light in the UCNPs@RB system by a robust luminescence resonance energy transfer (LRET) from UCNPs to RB. The detection limit of RB with these upconversion fluo-rescent nanoprobes could reach 0.25μg/cm3 in plant cell even under an ultra low excitation power source of 0.2 W/mm2. This LRET phenomenon was also extended to NaLuF4:18 mol.%Yb3+,0.5 mol.%Tm3+@Sodium fluorescein (SF) system. In addition, the differ-ential imaging could be achieved by successively incubating plant cells with fluorescent dyes and UCNPs. The fluorescent dyes ag-gregated in cell wall while UCNPs with surface modification distributed both in cell wall and cytoplasm, so that UCNPs@Dyes formed in cell walls which could emit multicolor light by LRET which was different from the emission in cytoplasm with only UCNPs.
YbF（2.357, YbF3, Ba2 YbF7, and Ba 2 upconversion nanocrystals doped with emitter Er^3＋ ion were synthesized in the same solvent system just with changing the molar ratio of Ba^2＋ to Yb^3＋ in the precursor, which corresponed to the crystal phases of rhombohedral, orthorhombic, tetragonal, and cubic, respectively. All the samples emitted both 660 nm red light and 543/523 nm green light which originated from Er^3＋-4f^n electronic transitions ~4F（9/2-~4I（15/2 and ~4S（3/2/~2H（11/2-~4I（15/2, respectively. It was worth mentioning that YbF 3：Er^3＋, Ba2 YbF7：Er^3＋, and BaF2：Er^3＋ could emit dazzlingly bright light even under the excitation of a 980 nm CW laser with output power of 0.1 W. Upconversion emission mechanism analysis indicated that the intensity ratio of red to green light highly depended on the synergistic effect of crystal structure, concentration quenching, and particle size, but were not sensitive to crystallinity as previously reported for NaL nF4（Ln=lanthanide.
Lanthanide doped NaYF4 microcrystals were synthesized via a facile hydrothermal method. Multicolor upconversion luminescence was observed in NaYF4 microcrystals doped with yb^3＋/Er^3＋, yb^3＋/Tm^3＋, and yb^3＋/Er^3＋/Zm^3＋ under the excitation of 980 nm infrared light. Importantly, the excitation power density dependence of upconversion emission intensity indicated clearly the energy transfer from Tm^3＋ to Er^3＋ ions under the excitation of low power density （5× 10^2× 10^2 W/cm^2）. Meanwhile, the inverse energy transfer from Er^3＋ to Tm^3＋ ions under the excitation of relatively higher power density （4.1 × 10^4.9× 10^4 W/cm^2） was also revealed. This was a direct evidence for reversible energy transfer between Er^3＋ and Tm^3＋ ions. Under the excitation of high power density （4.1 ×10^4-4.9× 10^4 W/cm^2）, dark sensitizers were also motivated so that the bottleneck effect of high concentration Yb^3＋ ion doping was broken. This was the main reason for realizing high upconversion efficiency of the samples with heavy doping of Yb^3＋ ion.
