Our group has identified and improved by rational mutagenesis the first chloride-sensitive red fluorescent protein (RFP).
This protein presents a cis–trans isomerization of the chromophore that produces a dual emission band (green and red) with different chloride sensitivities, which allowed us to develop a ratiometric methodology to measure intracellular chloride concentrations. You can have a look here, DOI: 10.1021/acssensors.1c00094
Check out the manuscript:
New Red-Emitting Chloride-Sensitive Fluorescent Protein with Biological Uses . Salto et al. ACS Sensors 2021 6 (7), 2563-2573. DOI: 10.1021/acssensors.1c00094.