Upconversion NaYF4 nanoparticles for size dependent cell imaging and concentration dependent detection of rhodamine B

Abstract

Upconversion nanoparticles (UCNPs) based on NaYF4 nanocrystals with strong upconversion luminescence are synthesized by the solvothermal method. The emission color of these NaYF4 upconversion nanoparticles can be easily modulated by the doping. These NaYF4 upconversion nanocrystals can be employed as fluorescence donors to pump fluorescent organic molecules. For example, the efficient luminescence resonant energy transfer (LRET) can be achieved by controlling the distance between NaYF4:Yb3+/Er3+ UCNPs and Rhodamine B (RB). NaYF4:Yb3+/Er3+ UCNPs can emit green light at the wavelength of 540 nm while RB can efficiently absorb the green light of 540 nm to emit red light of 610 nm. The LRET efficiency is highly dependent on the concentration of NaYF4 upconversion fluorescent donors. For the fixed concentration of 3.2 μg/mL RB, the optimal concentration of NaYF4:Yb3+/Er3+ UCNPs is equal to 4 mg/mL which generates the highest LRET signal ratio. In addition, it is addressed that the upconversion nanoparticles with diameter of 200 nm are suitable for imaging the cells larger than 10 μm with clear differentiation between cell walls and cytoplasm.