Viscosity Effects on Excited-State Dynamics of Indocyanine Green for Phototheranostic.

The excited-state dynamics of indocyanine green (ICG) fundamentally determine its photophysical properties for phototheranostic. However, its dynamics are predictable to be susceptible toward intracellular viscosity due to its almost freely rotating structure, making the precise phototheranostic very challenging. Therefore, correlating the viscosity with the dynamics of ICG is of great importance and urgency for precise phototheranostic prospects. This study presents systemic investigations on the viscosity-dependent dynamics of ICG for phototheranostic. Femtosecond transient absorption (fs-TA) experiments elucidate a prolonged radiative transition (225 ps vs 152 ps) for ICG in a viscous environment, which benefits fluorescence. High viscosity remarkably extends the triplet excited-state lifetime of ICG but reduces its internal conversion (6.2 ps vs 2.2 ps). The extended triplet lifetime affords sufficient photosensitization time to enhance photodynamic therapy. A moderative internal conversion is unfavorable for heat production, resulting in inferior photothermal therapy. With this clear picture of excitation energy state dissipation in mind, we readily identified the safety laser power density for precise phototheranostic. This work provides an insightful understanding of viscosity-relevant excited-state dynamics toward phototheranostic, which is also beneficial for designing novel ICG derivatives with improved phototheranostic performance.

Thiadiazoloquinoxaline-Based Semiconducting Polymer Nanoparticles for NIR-II Fluorescence Imaging-Guided Photothermal Therapy.

Phototheranostics have gained more and more attention in the field of cancer diagnosis and therapy. Among a variety of fluorophores for phototheranostics, semiconducting polymer nanoparticles (SPNs), which are usually constructed by encapsulating hydrophobic semiconducting polymers (SPs) with amphiphilic copolymers, have shown great promise. As second near-infrared (NIR-II) fluorescence imaging has both higher imaging resolution and deeper tissue penetration compared with first near-infrared (NIR-I) fluorescence imaging, NIR-II fluorescent SPNs have been widely designed and prepared. Among numerous structural units for semiconducting polymers (SPs) synthesis, thiadiazoloquinoxaline (TQ) has been proved as an efficient electron acceptor unit for constructing NIR-II fluorescent SPs by reacting with proper electron donor units. Herein, we summarize recent advances in TQ-based SPNs for NIR-II fluorescence imaging-guided cancer photothermal therapy. The preparation of TQ-based SPNs is first described. NIR-II fluorescence imaging-based and multimodal imaging-based phototheranostics are sequentially discussed. At last, the conclusion and future perspectives of this field are presented.