Small‐Molecule Targeted and Activatable Photoacoustic Probes for In Vivo Imaging: Design Principles and Recent Advances

Photoacoustic (PA) imaging has emerged as a powerful modality for in vivo molecular imaging, synergistically combining high optical contrast with deep-tissue penetration of ultrasound imaging. Exogenous PA contrast agents have further expanded this capability by enabling visualization of biological events with improved imaging depth and resolution. However, the background signals from endogenous chromophores often limit the imaging contrast. To address this challenge, diverse strategies have been developed, including tissue- or disease-targeted probes and activatable probe designs that respond to biological environments or disease-relevant biomarkers. In this review, we highlight PA probes that integrate targeted and activatable designs to enhance in vivo PA imaging. We discuss how the disease- or tissue-associated receptors, transporters, and enzymes can be employed to promote selective accumulation, as well as analyte- or enzyme-responsive activation mechanisms of PA probes, highlighting recent advances with demonstrated in vivo imaging performance. Finally, we summarize current achievements and remaining challenges, then provide an outlook for future directions. Collectively, these advances underscore the potential of targeted and activatable PA probes as versatile tools for high-contrast in vivo molecular imaging.