Soft nanoparticles incorporating a benzo[c]xanthene fluorophore: facile synthesis and ratiometric pH sensing

Polymeric nanoparticles offer considerable potential in resolving key issues related to the delivery of small lipophilic pharmaceutics. However, significant challenges remain with respect to the development and application of multifunctional, easily accessible fluorescent tools for tracing the cellular uptake and trafficking of soft nanoparticles. We have evaluated a platform for this purpose utilizing the pH-responsive molecular sensor seminaphthofluorescein-C (benzo[c]xanthene ‘SNAFL-C’); its chemical conjugation to an asymmetric miktoarm star polymer (AB₂) at its hydrophobic arm terminus; and traceable polymeric nanoparticles through self-assembly of SNAFL-AB₂ or physical encapsulation of the fluorophore. The characteristic high Stokes shift and ratiometric emission behaviour of SNAFL-C were retained in its polymer conjugate as well as in the self-assembled structures. Assemblies of the miktoarm fluorophore conjugate exhibited moderately reduced brightness as a result of aggregation induced quenching, an effect not observed for the physically encapsulated species. Fluorescence quenching experiments probing the partitioning of SNAFL-C in the core shell structures revealed localization of the primary fluorescent species in equilibrium based on their physiochemical properties, providing rationalization of differing aggregation behaviour observed for physically encapsulated and covalently linked formulations. Live cell fluorescence imaging in human glioblastoma cells integrating both SNAFL-C and its nanoformulations demonstrated the utility of the fluorophore in biological imaging applications and highlighted the necessity of detailed and stepwise spectral and photophysical evaluations. Collectively, these systems offer new avenues to explore fluorescence imaging using polymeric nanocarriers, leading to insights of broad importance to drug delivery and theragnostics.