Encapsulation-Activated Deep-NIR Triarylmethane Dyes via Steric Hindrance Engineering for Long-Circulating Fluorescence/Photoacoustic Dual-Modal Bioimaging

NIR-II fluorescence/photoacoustic dual-modal imaging offers complementary advantages in sensitivity and penetration depth. However, its efficacy is compromised by aggregation, nonspecific binding, and background interference, which persist even with polymeric encapsulation. To address these limitations, we report herein the rationally designed encapsulation-activated triarylmethane dyes (ED5) functionalized with bulky 1,8-bis(2,4,6-trimethylphenoxy)anthracene (1,8-BMPhA) moieties. The judiciously installed bulky substituents not only impart sufficient steric hindrance to repel protein binding and suppress intermolecular π-π stacking but also favor a locally near-coplanar geometry of the conjugated backbone, thereby preserving a high oscillator strength and a large molar extinction coefficient. Upon DSPE-PEG2000 encapsulation, nanoconfinement restricts intramolecular rotation and suppresses nonradiative decay, triggering a marked turn-on response of both fluorescence and photoacoustic signals. The optimized ED5_3@NPs exhibit a 14-fold fluorescence enhancement, a molar extinction coefficient of 71,000 M-1 cm-1, and a photothermal conversion efficiency of 62%. In vivo, they enable sustained high-quality imaging over a 51-h period, achieve crosstalk-free vascular-lymphatic dual-channel visualization, and afford high-contrast orthotopic liver tumor diagnosis against strong background interference. This encapsulation-activated strategy overcomes the long-standing constraints of NIR-II fluorophores, including low molar extinction coefficient, aggregation propensity, poor water solubility, and low photothermal conversion efficiency. It offers a general and well-defined paradigm for high-fidelity deep-tissue imaging and provides a promising strategy for further development of precision diagnostics.