Nanomaterials for photoacoustic imaging in the second near-infrared window

Photoacoustic imaging (PAI) is a rapidly developing imaging technique for both fundamental research and clinical applications. Recent studies revealed that PAI in the second near-infrared (NIR-II) region exhibits enhanced deep-tissue imaging capability, which benefits from reduced photon scattering, minimized background noise and increased applicable power in comparison to PAI in the first near-infrared (NIR-I) region. This review focuses on the latest achievements on PAI in the NIR-II region. The advantages of shifting PAI from NIR-I to NIR-II is first compared, followed by discussions on nanomaterials as contrast agents for NIR-II PAI. In the end, the challenges and perspectives of PAI in the NIR-II region are also elaborated.