Collot, M. et al. eLife 4, e05808 (2015).

Despite the advent of genetically encoded calcium indicators, calcium dyes remain indispensable for a variety of neuroscience imaging applications, especially in the red spectrum. Collot et al. developed a red calcium indicator called CaRuby-Nano, which is part of a larger family of dyes based on X-Rhodamine and the calcium-binding moiety BAPTA. CaRuby-Nano's sensitivity is in the nanomolar range and therefore similar to that of established green calcium indicators. This high affinity makes the dye most suitable for the detection of small calcium changes or single action potentials. CaRuby-Nano can be used for functional imaging in vivo or in vitro: for example, using two-photon imaging. Furthermore, the dye can be combined with genetically encoded reporters that emit in the green spectrum, which Collot et al. demonstrate in the mouse olfactory bulb.