N-methyl Mesoporphyrin IX is a widely used G-quadruplex DNA-specific fluorescent binder that effectively monitors Aβ fibrillation. N-methyl Mesoporphyrin IX is sensitive to G-quadruplex DNA and unreactive to duplex, triplex, and single-stranded forms of DNA. N-methyl Mesoporphyrin IX needs to stack with Aβ assemblies to emit strong fluorescence. Meanwhile N-methyl Mesoporphyrin IX is an in situ inhibitor and ex situ monitor of Aβ amyloid production in vitro and intracellularly.