Triphenylamine flanked boron difluoride formazanate for NIR-II fluorescence imaging-guided photothermal therapy

Small molecular theranostic agents play the pivot roles in the biomedical field and hold high clinical translational potential. However, it is still difficult to prepare these theranostic agents through a convenient and straightforward synthetic route. Herein, based on the electron-deficient building block boron difluoride formazanate (BDF), we designed and synthesized a novel NIR-II small molecular fluorescent dye (BDF-Ph) by flanking electron-rich triphenylamine units onto BDF to hamper the intermolecular π-π interaction, thus to achieve high fluorescence quantum yield. Then donor-acceptor-donor (D-A-D) structured BDF-Ph was encapsulated into amphiphilic polystyrene-g-poly (ethylene glycol) (PS-g-PEG) to prepare NIR-II fluorescent BDF-Ph NPs. BDF-Ph NPs possessed an emission peak at 975 nm with a high fluorescence quantum efficiency of 0.32% (IR1061, Φ = 1.7%) and a high photothermal conversion efficiency of 40.1%, making it an excellent NIR-II fluorescent probe for nanotheranostics. In vivo investigation illustrated that BDF-Ph NPs exhibited strong NIR-II fluorescence signals. Meanwhile, upon exposure to 808 nm laser, BDF-Ph NPs could eradicate tumors without apparent adverse effects under the direction of NIR-II fluorescence imaging. These fascinating results confirmed that BDF-Ph NPs as an alternative theranostic nanoplatform could be used for oncotherapy.