Morphology manipulation of NaYbF4:Er3+ nano/microstructures by hydrothermal synthesis and enhanced upconversion red emission for bioimaging

The synthesis of NaYbF4 crystals was successfully accomplished by a facile hydrothermal method. The phase and the morphology of the products were governed by reaction times, tuning the PH values and double solvents co-doping. The products with diverse morphologies, including quasi-spherical nanoparticles, hollow microtubes, microrods, nanorods, flower-like microprisms, and branch-like irregular shapes, were prepared under different external parameters. The experimental results demonstrate that the reaction time, PH value and organic additives play vital roles in the phase and morphology of the products. Photoluminescence measurements demonstrate that the relative upconversion (UC) photoluminescence intensity of as-prepared products strongly depends on the crystal phase, morphology, size, crystallinity, surface organic group, as well as surface defects. Impressively, the energy transfer mechanisms are systematically clarified, and the enhancement of red emission is ascribed to the high Yb3+ concentration. The emission colors might be finely tuned from green to orange-red. Furthermore, the luminescence decay lifetime is related to morphologies, sizes, defects, emission intensity, and Red/Green (R/G) ratio. Particularly, through simple cytotoxicity test and in vivo bioimaging, the synthesized UC red emission nanospheres have broad application prospects in the field of biochemistry. •The α-NaYbF4 and β-NaYbF4 can be precisely tuned by reaction time and PH value of the initial solution.•The β-NaYbF4 with outstanding luminescence property can be prepared for PEG co-doped with CA.•The energy-transfer mechanisms are systematically clarified and the enhancement of red emission intensity is ascribed to the high Yb3+ concentration.•The luminescence decay lifetime is positively correlated with luminescence intensity and R/G ratio.•The synthesized UC red emission nanospheres have been proved to have broad application prospects in the field of biochemistry.