A quantitative fluorescent probe that responds to changes in temperature is highly desirable for studies of biological environments, particularly in cellulo. Here, we report new cellpermeable fluorescence probes based on the BODIPY moiety that respond to environmental temperature. The new probes were developed on the basis of a well-established BODIPY-based viscosity probe by functionalization with cyclopropyl substituents at α and β positions of the BODIPY core. In contrast to the parent BODIPY fluorophore, α-cyclopropyl-substituted fluorophore displays temperature-dependent time-resolved fluorescence decays showing greatly diminished viscosity dependence, making it an attractive sensor to be used with fluorescence lifetime imaging microscopy (FLIM). We performed theoretical calculations that help rationalize the effect of the cyclopropyl substituents on the photophysical behavior of the new BODIPYs. In summary, we designed an attractive new quantitative FLIM-based temperature probe that can be used for temperature sensing in live cells.

Understanding viscosity in complex environments remains a largely unanswered question despite its importance in determining reaction rates in vivo. Here, time‐resolved fluorescence anisotropy imaging (TR‐FAIM) is combined with fluorescent molecular rotors (FMRs) to simultaneously determine two non‐equivalent viscosity‐related parameters in complex heterogeneous environments. The parameters, FMR rotational correlation time and lifetime, are extracted from fluorescence anisotropy decays, which in heterogeneous environments show dip‐and‐rise behavior due to multiple dye populations. Decays of this kind are found both in artificially constructed adiposomes and in live cell lipid droplet organelles. Molecular dynamics simulations are used to assign each population to nano‐environments within the lipid systems. The less viscous population corresponds to the state showing an average 25° tilt to the lipid membrane normal, and the more viscous population to the state showing an average 55° tilt. This combined experimental and simulation approach enables a comprehensive description of the FMR probe behavior within viscous nano‐environments in complex, biological systems.

No abstract