Semiconducting Polymer Nanoparticles for Centimeters‐Deep Photoacoustic Imaging in the Second Near‐Infrared Window

Thienoisoindigo‐based semiconducting polymer with a strong near‐infrared absorbance is synthesized and its water‐dispersed nanoparticles (TSPNs) are investigated as a contrast agent for photoacoustic (PA) imaging in the second near‐infrared (NIR‐II) window (1000–1350 nm). The TSPNs generate a strong PA signal in the NIR‐II optical window, where background signals from endogenous contrast agents, including blood and lipid, are at the local minima. By embedding a TSPN‐containing tube in chicken‐breast tissue, an imaging depth of more than 5 cm at 1064 nm excitation is achieved with a contrast‐agent concentration as low as 40 µg mL −1 . The TSPNs under the skin or in the tumor are clearly visualized at 1100 and 1300 nm, with negligible interference from the tissue background. TSPN as a PA contrast in the NIR‐II window opens new opportunities for biomedical imaging of deep tissues with improved contrast.