A fullerene-rhodamine B photosensitizer with pH-activated visible-light absorbance/fluorescence/photodynamic therapy

The development of smart photosensitizers with tumor microenvironment-activable fluorescence turn-on and singlet oxygen generation plays an important role in tumor bioimaging and photodynamic therapy. Herein, a pH-activable heavy-atom-free photosensitizer (C₆₀-RB) has been successfully synthesized through introducing a fullerene unit onto rhodamine B hydrazide. Under acidic conditions, C₆₀-RB, having a spirolactam structure, can be activated to its ring-opened structure C₆₀-RB-H and thus, visible-light absorbance enhancement, fluorescence turn-on and triplet excited state generation can be accomplished. In the presence of hydrion, the fullerene unit in C₆₀-RB C₆₀-RB-H acting as an intramolecular spin converter can cause good intersystem crossing, and the energy gap between S₁ and T₁ (ΔE_ST) is lowered to 0.017 eV. Through encapsulation with amphiphilic DSPE-mPEG₂₀₀₀, water-soluble nanoparticles (NPs) of C₆₀-RB are obtained. In vitro experiments indicate that C₆₀-RB NPs are capable of universal cellular uptake and lysosomal activation (pH 4.5-5.0), and they also exhibit excellent photodynamic therapeutic effect. The fluorescence turn-on and efficient singlet oxygen generation enabled by pH-activated C₆₀-RB NPs testify their great potential applications for cancer diagnosis and treatment.