Polarity-Independent Temperature-Induced Reversible Fluorescence Switching of Organic Nanoparticles: Application to Intracellular Temperature Imaging

Carbazole-based amphiphilic molecules have been synthesized that can form fluorescent organic nanoparticles in the aqueous medium and generate distinct optical responses via restricted intermolecular rotation (RIR). The nanoparticles showed polarity-independent, reversible, fluorescence switching between cyan and blue on temperature change. Unlike other fluorescent systems, here we observe ratiometric, reversible response with temperature. Such kind of fluorescence intensity ratio measurement instead of analysis at a single wavelength is always beneficial as it can nullify (or minimize) the effects of concentration and inner-filter effects or other optical artifacts. The temperature-sensitive fluorescence response was observed only in the case of nanoparticles, not with molecularly dissolved probe molecules. The mechanistic investigation indicates a transition of the physical state from aggregates to monomers with increasing temperature. Also, the compound showed a very similar fluorescence response upon the temperature change of a solid surface. Since the compound was found to be biocompatible and fairly stable against digestive enzymes, we employ the system for monitoring intracellular temperature change.