LAUSR

We have demonstrated an efficient synthetic route with crystal structures for the construction of acidic pH‐triggered visible‐to‐NIR interchangeable ratiometric fluorescent pH sensors. This bioresponsive probe exhibits pH‐sensitive reversible absorption/emission features, low cytotoxicity, a huge 322 nm bathochromic spectral shift with augmented quantum yield from neutral to acidic pH, high sensitivity and selective targeting ability of live‐cell lysosomes with ideal pKa, off‐to‐on narrow NIR absorption/fluorescence signals with high molar absorption coefficient at acidic lysosomal lumen, and in‐situ live‐cell pH‐activated ratiometric imaging of lysosomal pH. Selective staining and ratiometric pH imaging in human carcinoma live‐cell lysosomes were monitored by dual‐channel confocal laser scanning microscope using a pH‐activatable organic fluorescent dye comprising a morpholine moiety for lysosome targeting and an acidic pH openable oxazolidine ring. Moreover, real‐time tracking of lysosomes, 3D, and multicolor live‐cell imaging have been achieved using the synthesized pH‐activatable probe.