Near infrared (NIR) fluorescent dyes that are widely used for cancer imaging usually suffer from their hydrophobicity. To overcome this problem, a water-suspendable and biodegradable NIR-light-activating aza-BODIPY (AZB-NO₂) encapsulated in polymeric nanoparticles was prepared as a new class of deep-tissue imaging agent. AZB-NO₂ possesses an intense, broad NIR absorption band (600-800 nm) with a remarkably high fluorescent quantum yield. After being encapsulated with a biodegradable polycaprolactone (PCL) and a Kolliphor P188 surfactant by emulsification-solvent evaporation method, the AZB-NO₂ formed a spherical shape as observed in scanning electron micrographs (SEM) with a hydrodynamic average size of 201 nm (average PDI = 0.185). The results from transmission electron micrographs (TEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping indicated that the AZB-NO₂ homogeneously distributed in the polymeric shell. UV-visible-NIR and fluorescence spectra of the obtained nanoparticles, AZB-NO₂@PCL, revealed that the nanoparticles prepared by using 0.8 mg dye loading exhibited the highest fluorescence quantum yield. These nanoparticles were then applied for fluorescence imaging in human glioblastoma cell line (U-251). After the cells were exposed to AZB-NO₂@PCL, the materials appeared to be localized inside U-251 cells within 3 h and the fluorescence signal enhanced along with the increased incubation times. Moreover, 3D cell culture was used in this study to mimic in vivo tumor environments. The AZB-NO₂@PCL exhibited bright fluorescence from U-251 cells inside 3D Ca-alginate scaffolds after 24 h incubation. Our study successfully demonstrated that the encapsulation of hydrophobic aza-BODIPY dye could enhance the water-suspendability of the dye yielding biocompatible nanoparticles efficiently used in cancer cell imaging applications.

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