Comment on "Water-Soluble Fluorescent Probe with Dual Mitochondria/Lysosome Targetability for Selective Superoxide...

Recently, a new water-soluble, fluorescein-based probe for the detection of superoxide radical anion in aqueous media was developed by Lu et al. (ACS Sens. 2018, 3, 59-64). The probe was proven to be selective for superoxide and was used successfully also in cells and zebrafish embryos. To characterize the response of the probe to superoxide, Lu et al. used KO₂ dissolved in deionized water as a surrogate. In testing this probe in different applications, we repeated some of these experiments and came to realize that the fluorescence signal observed by the Authors in their experiments with KO₂ was incorrectly attributed to the reaction of the probe with superoxide and is due instead to its reactions with HO^- and HO₂^-. We show that indeed under the conditions used in these assays KO₂ undergoes very fast reaction with water to form HO^- and HO₂^-. On the other hand, by using a proper surrogate, namely, KO₂ dissolved in DMSO, and spin trapping experiments, we confirmed the ability of the probe to detect superoxide.