A far-red emitting probe for unambiguous detection of mobile zinc in acidic vesicles and deep tissue† †Electronic supplementary information (ESI) available: General methods, synthetic procedures, NMR spectra, kinetic data, pH profile, metal selectivity graph, cell culture and staining protocols, wide-field microscopy experiments, tissue specimen preparation, and two-photon imaging techniques. See DOI: 10.1039/c4sc03388d Click here for additional data file.

Imaging mobile zinc in acidic environments remains challenging because most small-molecule optical probes display pH-dependent fluorescence. Here we report a reaction-based sensor that detects mobile zinc unambiguously at low pH. The sensor responds reversibly and with a large dynamic range to exogenously applied Zn2+ in lysosomes of HeLa cells, endogenous Zn2+ in insulin granules of MIN6 cells, and zinc-rich mossy fiber boutons in hippocampal tissue from mice. This long-wavelength probe is compatible with the green-fluorescent protein, enabling multicolor imaging, and facilitates visualization of mossy fiber boutons at depths of >100 µm, as demonstrated by studies in live tissue employing two-photon microscopy.