Dual functional NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, Nd³⁺ core-shell nanoparticles for cell temperature sensing and imaging

Lanthanide-doped up-conversion nanoparticles (UCNPs) provide a remote temperature sensing approach to monitoring biological microenvironments. In this research, the UCNPs of NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, Nd³⁺ with hexagonal (β)-phase were synthesized and applied in cell temperature sensing as well as imaging after surface modification with meso-2, 3-dimercaptosuccinic acid. In the core-shell UCNPs, Yb³⁺ ions were introduced as energy transfer media between sensitizers of Nd³⁺ and activators of Er³⁺ to improve Er³⁺emission and prevent their quenching behavior due to multiple energy levels of Nd³⁺. Under the excitations of 808 nm and 980 nm lasers, the NaYF₄:Yb³⁺, Er³⁺@NaYF₄:Yb³⁺, Nd³⁺ nanoparticles exhibited an efficient green band with two emission peaks at 525 nm and 545 nm, respectively, which originated from the transitions of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 for Er³⁺ ions. We demonstrate that an occurrence of good logarithmic linearity exists between the intensity ratio of these two emission peaks and the reciprocal of the inside or outside temperature of NIH-3T3 cells. A better thermal stability is proved through temperature-dependent spectra with a heating-cooling cycle. The obtained viability of NIH-3T3 cells is greater than 90% after incubations of about 12 and 24 (h), and they possess a lower cytotoxicity of UCNPs. This work provides a method for monitoring the cell temperature and its living state from multiple dimensions including temperature response, cell images and visual up-conversion fluorescent color.