A "turn-on" polymer nanothermometer based on aggregation induced emission for intracellular temperature sensing.

Temperature measurements at the nanoscale facilitate the understanding of physiological processes related to heat in cells. Herein, we prepare a tetraphenylethylene-functionalized fluorophore (TPPEBr) with dual characteristics of twisted intramolecular charge transfer (TICT) and aggregation induced emission (AIE). It is polymerized with a thermo-responsive unit NIPAM to construct a fluorescent polymer nanothermometer (PNIPAM-TPPEBr). The phase transition behavior of PNIPAM from dispersed chains to dense spheres in aqueous media promotes the aggregation of TPPEBr fluorophores, which makes the fluorescence of PNIPAM-TPPEBr enhance with increasing temperature. Furthermore, the phase transition of PNIPAM is accompanied by a significant decrease in the polarity of the microenvironment, resulting in a blue shift in the emission wavelength of TPPEBr. Varying the ratio of NIPAM and TPPEBr can regulate the thermo-responsiveness of PNIPAM-TPPEBr in the physiological temperature range (31-38 °C), and the maximum relative thermal sensitivity reaches 13.2 % °C. The thermo-responsive performance of this nanothermometer is independent of the intracellular microenvironment, and it is successfully applied in the temperature imaging of A549 cells. Under the stimulation of ionomycin and oxidative phosphorylation inhibitor, the cell temperature increased by ca. 1.5 °C and ca. 1.0 °C, respectively.