A highly selective “off-on” fluorescent sensor for subcellular visualization of labile iron(III) in living cells

Abstract

A highly selective fluorescent sensor “RhHPA” derived from a condensation reaction between rhodamine hydrizine and 6-hydroxymetyl-pyridine-2-aldehyde has been developed for selective Fe3+-imaging in living cells. The sensor exhibits marked selectivity and sensitivity towards Fe3+, with a chelation-induced “off-on” response. The sensor binds to Fe3+ at 1:1 stoichiometry with high affinity (apparent binding constant 1.27 × 107 M−1) and displays a distinct change in colour and fluorescence upon the alteration of free Fe3+ levels with a reversible response and little interference with other biological relevant metal ions. The sensor works at pH 5–9 that covers most of the physiological pH range. Cell imaging studies demonstrated that the sensor is capable of detecting basal level Fe3+ as well as the dynamic changes in Fe3+ levels in live cells at subcellular resolution, with two labile Fe3+ pools identified in mitochondria and endosomes/lysosomes in bovine aortic endothelial cells (BAEC) for the first time.