Small-molecule photoacoustic probes for in vivo imaging

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.