“…To broaden the applications of PAI modality and to improve the sensitivity, specificity, and contrast of PAI further, several exogenous contrast agents with strong NIR absorption and high photothermal conversion efficiency were developed. We discussed the recently reported PA exogenous contrast agents, such as metallic nanomaterials (Au/Ag (T. Kim et al, ), AuPB (J. Zhou, Jiang, et al, )), QDs (MoO 3â x ; Ding et al, ), small molecule ICG dye (P. K. Upputuri & Pramanik, ), IRâ1061 dyeâloaded lipid NPs (Q. Chen, Chen, et al, ), biodegradable Biliverdin NPs (Fathi et al, ), organic dyed droplets (P. Zhang et al, ), semiconducting polymer nanoparticles (B. Guo et al, ; Jiang, Upputuri, et al, ; Miao & Pu, ; P. K. Upputuri et al, ; L. Zhou, Zhou, & Wu, ). We classified these PA contrast agents into inorganic and organic materials based on their chemical structure, and summarized their advantages and disadvantages in Table .…”