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.