A phosphinate-based red fluorescent probe for imaging the superoxide radical anion generated by RAW264.7 macrophages

4',9'-Bis(diphenylphosphinyl)naphthofluorescein (PNF-1) has been designed and synthesized as a highly selective, sensitive, cell-permeable, red fluorescent probe for detecting O-2(center dot-) in biological systems. The design strategy for the probe is based on the nucleophilic mechanism of O-2(center dot-) to mediate deprotection of the probe to naphthofluorescein, the emission spectrum of which is just in the spectral region of low background fluorescence interference in biological systems. Upon treatment with O-2(center dot-), the probe exhibits a strong fluorescence response and high selectivity for O-2(center dot-), rather than other reactive oxygen species or biological compounds. A linear calibration curve for PNF-1 showed a detection limit of 0.1 nm O-2(center dot-). This new type of fluorescent probe allows nanomolar changes in O-2(center dot-) concentrations in living cells to be detected by confocal microscopy.