Temperature sensing and bio-imaging applications based on polyethylenimine/CaF2 nanoparticles with upconversion fluorescence

Rare earth ion-doped upconversion nanoparticles (UCNPs) have become a research hotspot for their applications such as nano-thermometry and bio-imaging. Here we report one pot solvothermal synthesis of polyethylenimine (PEI) coated CaF2: 1%Er3+, 2%Yb3+ UCNPs with good water-solubility and biocompatibility. The nanoparticles show a spherical shape with an average size of 40 nm and display intense green light (Yb3+ and Er3+) under the 980 nm irradiation. The maximum sensitivity of temperature sensing in the biological temperature range is obtained to be 0.00215 K-1 using the fluorescence intensity ratio (FIR) technique. Furthermore, after the UCNPs are incubated with Hela cancer cells for 6 h, the obvious green sign from Er3+ can be observed under the excitation at 980 nm. All the experimental results imply that the PEI/CaF2: 1%Er3+, 2% Yb3+ nanoparticles are excellent candidates for nano-thermometry and have good potential for biological applications.