Ratiometric Fluorescent Mapping of pH and Glutathione Dictates Intracellular Transport Pathways of Micellar Nanoparticles

Visualization of intracellular transport pathways is crucial to investigate the internalization mechanism and understand the intracellular behavior of nanomaterials. Herein, we rationalized the design of micellar nanopartides (NPs for ratiometric fluorescent mapping of intracellular pH and glutathione (GSH), two essential parameters for maintaining normal cellular functions. Specifically, pH-sensitive naphthalimide-based probe (NPI) and pH-inert rhodamine B (RhB) were covalently labeled to double hydrophilic block copolymers (DHBCs) using the thiolactone chemistry, enabling the covalent attachment of NPI and RhB to one molecule with a redox-responsive disulfide linkage. The dually labeled DHBCs exhibited blue/orange dual emissions in acidic pH, which was further converted into green/orange dual emissions in neutral pH because of the deprotonation of NPI moieties and the sole green emission in the presence of GSH at neutral pH because of the decreased Forster resonance energy transfer efficiency between an NPI donor and an RhB acceptor as a result of GSH-mediated cleavage of disulfide bonds. These remarkable ratiometric fluorescence changes allowed for not only the simultaneous mapping of the intracellular pH and GSH but also the intracellular transport pathways of internalized NPs.