Zou, P. et al. Nat. Commun. 5, 4625 (2014).

Expanding the color range of genetically encoded sensors for neuronal activity enables combinatorial expression of different sensors and thus more flexibility in the types of experiments that can be performed. Zou et al. developed a series of voltage sensors whose emissions cover the visible spectrum. These voltage sensors are based on electrochromic fluorescence resonance energy transfer, meaning that the brightness of a fluorescent protein is influenced by the fluorescent voltage sensor to which it is fused. The most sensitive versions of this type of sensor are fusions between QuasAr2 and Citrine, mOrange2 or mRuby2. They allow the detection of action potentials in cultured hippocampal neurons and show fluorescence changes of 8–13% per 100 millivolts. It will be interesting to see their performance in vivo and in combination with other optogenetic tools.