A zwitterionic red-emitting water-soluble conjugated polymer with high resistance to nonspecific binding for two-photon cell imaging and good singlet oxygen production capability

Abstract

Water-soluble conjugated polymers (WSCPs) with the dual capability of two-photon imaging and singlet oxygen (¹O₂) production have become one of the most promising agents for noninvasive imaging guided photodynamic therapy (PDT). To obtain good imaging and PDT effects, two-photon excited (TPE) red-emitting WSCPs with good biocompatibility and ¹O₂ production capability are highly needed. Herein, a novel zwitterionic red-emitting WSCP P1′′ and its cationic analogue P1′ were synthesized and studied by comparison. The backbones of both polymers consisted of electron-rich poly[9,9′-bis(2-(2-(2-bromoethoxy)ethoxy)ethyl)fluorene-2,7-ylene vinylene] doped with a small amount of electron-deficient red-emitting unit, 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT). Both P1′ and P1′′ achieved TPE red-emission through fluorescence resonance energy transfer (FRET). However, when compared with P1′, P1′′ exhibited higher water-solubility and resistance to nonspecific binding in the physiological environment, which could lead to a 5-fold increase in cellular uptake. Moreover, two-photon cell imaging experiments indicated that more of P1′′ entered into HeLa cells than P1′ and located in the cytoplasm instead of adhering near the cell membrane. Furthermore, P1′′ displayed a ¹O₂ quantum yield of 61%, which was much higher than that of P1′ (39%). These results could be mainly attributed to the anionic sulfonate groups within the zwitterionic side chains, which probably not only further enhanced the water solubility, but also promoted the generation of ¹O₂ by facilitating the transfer of the singlet-to-triplet excited state. Hence, zwitterionic P1′′ was demonstrated to show potential as an excellent TPE red-emitting photosensitizer for two-photon imaging-guided PDT.