Compared with the photoacoustic imaging (PAI) in the first near-infrared window (NIR-I, 700-1000 nm), the PAI in the second near-infrared window (NIR-II, 1000-1700 nm) has the advantages of less background noise, deeper tissue penetration, and larger maximum permissible exposure, which has been become a research hotspot. In this review, the recent advances in nanomaterials for NIR-II PAI are summarized, including the advantages of NIR-II PAI and the applications of nanomaterials-based NIR-II PAI in biomedical field. The nanomaterials-based PA contrast agents are classified into three categories, including organic (such as assembled small molecules, conjugated polymers, semiconducting polymers, and phthalocyanines), inorganic (such as metal sulfides, noble metals, carbon nanomaterials, quantum dots, 2D transition metal carbides and nitrides [MXenes], and semimetals), or organic/inorganic hybrid nanomaterials, which are mainly used for imaging and diagnosis of tumor tissues, brain vasculature, inflammation, and other tissues, as well as the monitoring of catalytic reaction processes in vivo. Moreover, the opportunities and challenges of NIR-II PAI are also expounded.
Keywords: biomedical applications; nanomaterials; photoacoustic contrast agents; photoacoustic imaging; second near-infrared windows.
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