In vivo ion visualization achieved by activatable organic photoacoustic probes

Metal ions participate in many intricate physiological and pathological processes in living systems. Therefore, ion imaging is meaningful for the visualization of biological processes as well as therapeutic intervention in diseases. The emerging noninvasive and nonionized photoacoustic imaging (PAI) technology affords high-resolution spatiotemporal mapping of ions in vivo with excellent biosafety and outstanding tissue penetration depth. This review critically summarizes the state-of-the-art in vivo ion visualization based on activatable organic PA probes. The general principles of designing activatable organic PA probes are pointed out while the ion-recognition mechanisms are highlighted to clarify the structural design strategies for specific ion imaging. The challenges and outlook are also discussed regarding probe properties, quantitative detection, and instrumental limitations.