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 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 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.