SypHer3s: a genetically encoded fluorescent ratiometric probe with enhanced brightness and an improved dynamic range.

We designed a genetically encoded ratiometric fluorescent probe, SypHer3s, with enhanced brightness and optimized pKa, which responds to pH changes in different cellular compartments. SypHer3s was successfully utilized for imaging the pH dynamics in mitochondria of living neurons and in quantitative pH measurement in zebrafish embryos.