Single-atom engineering of hemicyanine and its amphiphilic derivative for optimized near infrared phototheranostics

Near-infrared (NIR) dyes are widely used in the field of in vivo phototheranostics. Hemicyanine dyes (HDs) have recently received tremendous attention due to their easy synthesis and excellent NIR features. However, HDs can easily form non-fluorescent aggregates and their potential for phototherapy still needs further exploration due to their poor ability to generate reactive oxygen species (ROS). Herein, a series of hemicyanine dyes with different chalcogen atom (O, S, Se) substitutions were constructed to achieve optimized potential for phototheranostics. By replacing O with the heavy atom Se in the xanthene skeleton, CySe-NEt₂ showed much more favourable features such as extended NIR absorption/emission wavelength, boosted ¹O₂ generation rate and higher photothermal effect. In addition, a poly(ethylene glycol) (PEG) group was introduced into the scaffold and yielded a nanotheranostic agent CySe-mPEG_5K, which easily formed nanoparticles with appealing features such as excellent photostability, effective prevention of unpleasant H-aggregation, fast/selective tumor accumulation and minimum dark toxicity. Solid tumor growth was significantly suppressed through combined photodynamic therapy (PDT) and photothermal therapy (PTT) guided by NIR fluorescence (NIRF) and photoacoustic (PA) imaging. This study not only presents the first example of selenium-substituted hemicyanine dyes, but also offers a reliable design strategy for the development of potent NIR phototheranostic agents with multi-mode imaging-guided combination therapeutic ability.