This review highlights cutting-edge strategies for probing ion dynamics within organelles using fluorescent indicators. It discusses key advances in calibration techniques for absolute ion quantification and their impact on understanding organellar physiology, and concludes with future directions and applications in decoding ion-regulated processes in health and disease.

Ion dynamics within cellular organelles are fundamental to numerous biochemical processes, maintaining homeostasis and enabling critical cellular functions. Despite continuous ion movement across organelle membranes, stable ionic gradients are preserved, creating optimal microenvironments for organelle-specific activities such as ATP production in mitochondria, lysosomal degradation, Golgi-mediated protein modifications, and DNA processing in the nucleus. These gradients are regulated by specialized membrane proteins, including ion channels and transporters, which facilitate selective and controlled ion flux. Dysfunction in these regulatory proteins is linked to various diseases, including neurodegenerative disorders, cardiovascular conditions, immune dysfunctions, and cancers. Understanding ion regulation mechanisms at the molecular level is not only essential for basic cell biology but also crucial for revealing pathological pathways and identifying therapeutic targets. Recent technological advances—such as fluorescent probes based on green fluorescent protein, small molecules, and DNA nanodevices—have significantly enhanced our ability to study ion dynamics with high spatial and temporal resolution. These tools enable both qualitative and quantitative analyses, offering insights into ion transport mechanisms and their physiological relevance. A comprehensive overview of the principles underlying functional imaging of ion dynamics is provided, current challenges in quantitative assessment are highlighted, and future directions in organelle-specific ion regulation are discussed.