**Exploring NIR Aza-BODIPY-Based Polarity Sensitive Probes with ON-and-OFF Fluorescence Switching in Pluronic Nanoparticles**

**Bahar Saremi 1,2, Venugopal Bandi 3,**†**, Shahrzad Kazemi 3, Yi Hong 2,4, Francis D'Souza 3,\* and Baohong Yuan 1,2,\***


Received: 18 December 2019; Accepted: 20 February 2020; Published: 2 March 2020

**Abstract:** Because of their deep penetration capability in tissue, red or near infrared (NIR) fluorophores attract much attention in bio-optical imaging. Among these fluorophores, the ones that respond to the immediate microenvironment (i.e., temperature, polarity, pH, viscosity, hypoxia, etc.) are highly desirable. We studied the response of six NIR aza-BODIPY-based and structurally similar fluorophores to polarity and viscosity for incorporation inside Pluronic nanoparticles as switchable fluorescent probes (SFPs). Based on our results, all of these fluorophores were moderately to strongly sensitive to the polarity of the microenvironment. We concluded that attaching amine groups to the fluorophore is not necessary for having strong polarity sensitive probes. We further studied the response of the fluorophores when embedded inside Pluronic nanoparticles and found that four of them qualified as SFPs. We also found that the switching ratio of the fluorophore-encapsulated Pluronic nanoparticles (ION-to-IOFF) is related to the length of the hydrophobic chain of the Pluronic tri-block copolymers. As such, the highest switching ratio pertained to F-68 with the lowest hydrophobic block poly (propylene oxide) (PPO chain of only 30 units).

**Keywords:** aza-BODIPY; environment-sensitive; polarity-sensitive; near-infrared fluorescence imaging; temperature-sensitive; ultrasound switchable fluorescence probe; Pluronic; F-127; F-96; F-68; thermosensitive
