A fluorescent, pH-independent and ratiometric Cl– sensor, allowing sensing of chloride ions. Clensor can precisely measure the activity and location of subcellular chloride channels. It can target the endosome, lysosome, and trans-golgi network in-vitro or in-cellulo and is relevant to diseases such as Niemann-Pick’s. In this figure (Saha et al., Nat. Nanotechnol., 2015), Clensor highlighted endosomal changes based on different chloride concentrations.