Intracellular pH levels can now be measured with a protein that glows red.

The red fluorescent protein has been dubbed pHRed by its designers, Gary Yellen and his colleagues at Harvard Medical School in Boston, Massachusetts. Light-detecting molecules of pHRed are preferentially excited by light at a wavelength of either 440 nanometres in basic conditions or 585 nanometres in acidic conditions, and the excitation ratio of the two gives a read-out of pH.

pHRed's fluorescence lifetime — the time between excitation and emission — also responds to pH. The authors used this feature in two-photon microscopy to track second-by-second pH changes in living cells.

A red pH sensor leaves the often-used green fluorescence wavelength available for simultaneous study of other cellular properties, such as energy metabolism, during multicolour imaging.

J. Am. Chem. Soc. doi:10.1021/ja202902d (2011)