Visualizing redox-active metal ions, such as Fe ²⁺ and Fe ³⁺ ions, are essential for understanding their roles in biological processes and human diseases. Despite the development of imaging probes and techniques, imaging both Fe ²⁺ and Fe ³⁺ simultaneously in living cells with high selectivity and sensitivity has not been reported. Here, we selected and developed DNAzyme-based fluorescent turn-on sensors that are selective for either Fe ²⁺ or Fe ³⁺ , revealing a decreased Fe ³⁺ /Fe ²⁺ ratio during ferroptosis and an increased Fe ³⁺ /Fe ²⁺ ratio in Alzheimer’s disease mouse brain. The elevated Fe ³⁺ /Fe ²⁺ ratio was mainly observed in amyloid plaque regions, suggesting a correlation between amyloid plaques and the accumulation of Fe ³⁺ and/or conversion of Fe ²⁺ to Fe ³⁺ . Our sensors can provide deep insights into the biological roles of labile iron redox cycling.