**4. Conclusions**

Six members of the aza-BODIPY-based and structurally related fluorophores with excitation and emission in the red/NIR region were investigated. It was shown that fluorophore 4 was a very strong polarity-sensitive probe, followed by fluorophores 1, 2, 5, 6, and 3. It was also shown that attaching amine groups to the fluorophore is not necessary for having strong polarity-sensitive probes. After encapsulating fluorophores into thermosensitive Pluronic nanoparticles, it was found that fluorophores 4, 2, 3, and 5 can be used as ON-and-OFF fluorescence switches because of the dramatic change in peak emission fluorescence intensity over a narrow range of temperatures (fluorophore 4 had the strongest switching response). It was also shown that the switching ratio of the fluorophore-encapsulated Pluronic nanoparticles (ION-to-IOFF) increased with decreasing the number of hydrophobic chains. As such, Pluronic F-68 with a PPO length of 30 had the highest switching ratio, while the Pluronic F-127 nanoparticles with the PPO length of 65 had the lowest.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/12/3/540/s1, Supplementary Materials S1: Absorbance and Emission Spectra, Supplementary Materials S2: Fluorescence Lifetime Decay Curves, Supplementary Materials S3: Effect of Filtration and Dilution on LCST, Supplementary Materials S4: Compound 2 (Top DMAADPCA) Synthesis Procedure.

**Author Contributions:** Conceptualization, B.Y.; methodology, B.Y., B.S., V.B., S.K.; software, B.S.; validation, B.S.; formal analysis, B.S.; investigation, B.S.; resources, B.Y., F.D., Y.H.; data curation, B.S.; writing—original draft preparation, B.S. and B.Y.; writing—review and editing, B.S., B.Y., V.B., Y.H., F.D.; visualization, B.S., V.B.; supervision, B.Y., Y.H., F.D.; project administration, B.Y.; funding acquisition, B.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported in part by funding from the CPRIT RP170564 (BY) and NSF CBET-1253199 (BY). **Acknowledgments:** We appreciate David Manivanh's help in measuring the viscosity of our samples. Our gratitude goes out to Guido Verbeck and the Laboratory for Imaging Mass Spectrometry at the University of North Texas for MALDI-Orbitrap Mass Spectrometry data.

**Conflicts of Interest:** The authors declare no conflict of interest.
