A DNA-based pH sensor triggered by protons based on fluorescence resonance energy transfer (FRET) inside living cells. It can work in-vitro or target the endo-lysosomal pathway, and trans-golgi network in cultured cells or inside a living organism. Change in pH is relevant to diseases such as Niemann-Pick’s and Alzheimer’s. In this figure (Modi et al., Nat. Nanotechnol., 2009), a pH sensor was able to map endosomal spatiotemporal changes.