A pH-independent, DNA-based ratiometric fluorescent reporter of pH and HOCl, that simultaneously detects pH and HOCl. It can target the phagosome, endosome and lysosome in-vitro in macrophages, neutrophils and monocytes. It is relevant to several diseases such as multiple sclerosis, acute coronary syndrome. This figure (Thekkan et al., Nat. Chem. Biol., 2018) demonstrates representative images of HOCl sensor phagocytosed by human neutrophils, monocytes and macrophages.