Since the report on Ca2+ indicators by Tsien and co-workers,[1] ratiometric fluorescent probes have come to be recognized as a promising tool for biosensing and bioimaging studies.[2] Significant efforts have been devoted to develop this class of probes for a wide range of analytes including metal ions,[3] inorganic and organic anions,[4] enzyme activities,[5] and nucleic acids.[6–8] A typical way to produce the ratiometric signal is by ...