Methylene Blue: An FDA-Approved NIR-II Fluorogenic Probe with Extremely Low pH Responsibility for Hyperchlorhydria Imaging

Methylene blue (MB) is an FDA (Food and Drug Administration)-approved contrast agent with donor–acceptor (D–A) structure integrated with carbonyl-containing nitrogen-heterocycles. MB can be converted into MBH (protonated MB) by protonation, which not only induces the fluorescence emission red-shifted from the first near-infrared window (NIR-I, 650–950 nm) to the second near-infrared window (NIR-II, 1000–1700 nm) but also achieves ACQ-to-AIE conversion. MB has been successfully demonstrated in hyperacidemia imaging with an extremely low pH value (<1).