Small-molecule photoacoustic probes for in vivo imaging

Photoacoustic (PA) imaging, a non-invasive and non-radioactive biological modality, has emerged as a powerful tool for in vivo bioimaging, offering high-spatial-resolution capabilities in deep tissues. Recent advancements in small molecule-based PA probes have significantly expanded the utility of PA imaging, enabling high-spatiotemporal-resolution visualization of diverse biological processes and thereby advancing the field from anatomical to molecular applications. This comprehensive review provides an overview of small molecule-based PA probes tailored for in vivo imaging. We delve into their fundamental design strategies, photophysical properties, and intricate response mechanisms, aiming to facilitate future innovations in biological sensing and imaging. Probes are systematically categorized based on their utility in detecting and imaging critical bioactive species, including metal ions, reactive oxygen species, reactive nitrogen species, hypoxia, biothiols, enzymes, pH, and polarity. Finally, we discuss current limitations and outline future prospects to inspire continued research across the interdisciplinary domains of chemistry, chemical biology, and biomedicine. This review serves as a valuable resource and guide for the development of next-generation small molecule-based PA probes for practical molecular imaging applications.