Monitoring the pH in tumor lesions provides abundant physiological information. However, currently developed pH sensors only achieve sensitive detection in the settled response region around the pH transition point (pH_t). To realize tumor pH monitoring with high sensitivity within a wider response region, reported here are serial pH_t adjustable sensors (pTAS) that simply regulate the component ratio of second near‐infrared (NIR‐II) emission aza‐BODIPY (NAB) donor and pH sensitive rhodamine‐based pre‐acceptor (NRh) in Förster resonance energy transfer system. Combining the pH response regions of pTAS, a twofold widened pH detection range (6.11–7.22) is obtained compared to the pH_t settled sensor (6.38–6.94). With an adjustable pH_t, in vivo tumor pH increase and decrease processes could be dynamically visualized through dual‐channel ratiometric bioimaging within the NIR‐II window, with a coefficient of variation under 1 % compared to the standard pH meter.

Fig 1. (a) Chemical Structure of NIR-II FRET donor and acceptor. (b)  pH response illustration of pTAS. Channel 1–2 represent for the signal collection regions during ratiometric fluorescence imaging. c) Illustration of of pH_t regulation in pTAS. d) Illustration of the in vivo dynamic pH monitor at tumor by ratiometric fluorescence imaging.

https://onlinelibrary.wiley.com/doi/10.1002/anie.202012021