Mitochondrial-targeted peptide BP19 is a fluorescent probe designed for the selective measurement of labile iron within mitochondria. It demonstrates iron-selective sensing activity, accumulates in mitochondria, and shows reduced fluorescence in iron-loaded cells, with fluorescence recovery upon iron chelation. Mitochondrial-targeted peptide BP19 can be utilized to assess mitochondrial labile iron levels in fibroblasts from Friedreich's ataxia.