Ternary Chalcogenide Nanosheets with Ultrahigh Photothermal Conversion Efficiency for Photoacoustic Theranostics

2D materials (TDMs) have been explored for photonic theranostics. To achieve deep‐tissue penetration, near‐infrared (NIR) light is essential for photoacoustic (PA) theranostics. However, because the absorption profiles of existing TDMs are generally featureless with no obvious NIR absorption peaks, their PA signals and therapeutic efficacies are limited. This paper herein reports the synthesis and application of ternary chalcogenide nanosheets (Ta 2 NiS 5 ‐P) for PA theranostics. In contrast to the current TDMs for such application, Ta 2 NiS 5 ‐P has a ternary instead of binary composition. This difference brings in the strong and featured NIR for Ta 2 NiS 5 ‐P. To the best of the knowledge, this is the first example using ternary chalcogenide nanosheets for such application; moreover, the photothermal conversion efficiency of Ta 2 NiS 5 ‐P is the highest (35%) among all the reported TDMs based on the same calculation method. These advantages allow Ta 2 NiS 5 ‐P to passively target, effectively delineate, and completely eradicate the tumor of living mice after systemic administration.