An acidic pH-triggered polymeric micelle for dual-modality MR and optical imaging

A dual-modality imaging probe was developed for magnetic resonance imaging (MRI) and optical imaging, based on a Fe₃O₄-encapsulated pH-responsive block copolymer, which was conjugated with a red fluorescent dye Sulforhodamine 101 (SR101). The block copolymer, containing methoxy poly(ethylene glycol) (PEG) as a hydrophilic segment and poly(β-amino ester) (PAE) with ionizable tertiary amine groups on its backbone as a pH-responsive segment, can be dissolved in response to an acidic pH environment, or form self-assembled micelles in aqueous media at the physiological pH (∼pH 7.4) due to the hydrophobic deionized PAE segment acting as a micellar core. The polymeric micelle conjugated with SR101 was found to have a red-fluorescent emission at 612 nm, a sharp pH transition of pH 6.8-7.0, indicating that it can be an ideal pH-triggered carrier in response to the acidic biological environment. Subsequently, the Fe₃O ₄-encapsulated micelle was evaluated by dynamic light scattering (DLS), cryogenic transmission electron microscope (cryo-TEM), and pH-dependent Fe₃O₄ release. The confocal laser scanning microscopy (CLSM) observation demonstrated the cellular uptake of SR101-labeled polymeric micelles by breast cancer cells.