In living organisms, ClO− is generated from hydrogen peroxide and chloride ion in activated neutrophils, catalyzed by myeloperoxidase (MPO).1,5,6 ClO− is commonly used in disinfectants and bleaches.7,8 However, excessive formation of hypochlorite can also cause tissue damage and a series of human diseases, such as atherosclerosis, arthritis and even cancers, etc.9,10 Therefore, the rapid and sensitive detection of ClO− is important in biological samples. Here, MPO is linked to arthritic joint disease.