1e:T534,Ag₂S nanoparticles are increasingly important in biomedicine, such as in cancer imaging. However, there has been only limited success in the exploration of theranostic Ag₂S nanoparticles for photoinduced cancer imaging and simultaneous therapy. Here we report size-dependent Ag₂S nanodots (NDs) with well-defined nanostructure as a theranostic agent for multimodal imaging and simultaneous photothermal therapy. The NDs are precisely synthesized through carefully controlled growth of Ag₂S in hollow human serum albumin nanocages. These NDs produce effective fluorescence in second near-infrared (NIR-II) region, distinct photoacoustic intensity, and good photothermal conversion in a size-dependent manner under light irradiation, thereby generating sufficient in vivo fluorescence and photoacoustic signals as well as potent hyperthermia at tumors. Moreover, Ag₂S NDs possess ideal resistance to photobleaching, effective cellular uptake, preferable tumor accumulation, and in vivo elimination, thus facilitating NIR-II fluorescence/photoacoustics imaging with both ultrasensitivity and microscopic spatial resolution and simultaneous photothermal tumor ablation. These findings provide insight into the clinical potential of Ag₂S nanodots for cancer theranostics.1f:T531,Photoacoustic (PA) imaging holds great promise for preclinical research and clinical practice. However, most studies rely on the laser wavelength in the first near-infrared (NIR) window (NIR-I, 650-950 nm), while few studies have been exploited in the second NIR window (NIR-II, 1000-1700 nm), mainly due to the lack of NIR-II absorbing contrast agents. We herein report the synthesis of a broadband absorbing PA contrast agent based on semiconducting polymer nanoparticles (SPN-II) and apply it for PA imaging in NIR-II window. SPN-II can absorb in both NIR-I and NIR-II regions, providing the feasibility to directly compare PA imaging at 750 nm with that at 1064 nm. Because of the weaker background PA signals from biological tissues in NIR-II window, the signal-to-noise ratio (SNR) of SPN-II resulted PA images at 1064 nm can be 1.4-times higher than that at 750 nm when comparing at the imaging depth of 3 cm. The proof-of-concept application of NIR-II PA imaging is demonstrated in in vivo imaging of brain vasculature in living rats, which showed 1.5-times higher SNR as compared with NIR-I PA imaging. Our study not only introduces the first broadband absorbing organic contrast agent that is applicable for PA imaging in both NIR-I and NIR-II windows but also reveals the advantages of NIR-II over NIR-I in PA imaging.20:T5da,

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