“…Generally, nontargeted PA contrast agents refer to the imaging agents without any targeting unit, which are often used in biomedical research such as in vivo angiography, − biodistribution monitoring, − drug metabolism evaluation, ,, etc. In contrast, targeted PA contrast agents usually consist of a targeting unit such as aptamers, , antibodies, peptides, − and small-molecule ligands, − which connect with the additional PA generation components, enabling the specific readout of the biomarker or biological events in certain locations. Generally, to facilitate the targeted specificity, the biotargets are supposed to be overexpressed in disease sites or show low levels in off-target sites.…”