SPQ is a fluorescent chloride indicator. Its fluorescence is rapidly quenched by chloride in vitro. It has been used to assess voltage-sensitive chloride transport in placental microvillus vesicles. It has also been used to continuously quantify intracellular calcium efflux induced by parathyroid hormone, forskolin , and phorbol 12-myristate 13-acetate (PMA; ) in isolated human erythrocytes. In addition, SPQ has been used to determine the type of ion conductance induced by the cystic fibrosis transmembrane conductance regulator (CFTR) in HEK293 cells expressing pTrial10-CRTR2. SPQ displays excitation maxima of 320 and 350 nm and an emission maximum of 445 nm.
