Development of Second Near-Infrared Photoacoustic Imaging Agents

Photoacoustic imaging (PAI) has attracted increasing attention both in basic research and clinical applications. PAI in the second near-infrared (NIR-II) window shows deeper penetration depth, better spatial resolution, higher maximum permissible exposure, and lower background noise than PAI in the first near-infrared (NIR-I) window. By introducing contrast agents, NIR-II PAI enables visualization of biological processes occurring at depths of several centimeters. Herein, we review a variety of contrast agents ranging from inorganic materials to organic materials and their further development as activatable NIR-II PAI probes to change their signals in response to a biomarker of interest. The physical and biochemical characteristics of the existing photoacoustic contrast agents are discussed, with their potential applications highlighted. Finally, potential challenges and prospects in the investigation of NIR-II PAI contrast agents in biomedical research are discussed.