A highly sensitive and selective sensor for the quantitative assay of inorganic pyrophosphatase (PPase) activity was developed based on a fluorescence “turn–off” strategy. Carbon quantum dots@Cu(II)-based metal–organic framework nanotubes (CQDs@Cu-MOF) with length less than 300 nm and width less than 20 nm were synthesized. CQDs in the nanotubes exhibited weak fluorescence owing to static quenching. The coordination reaction between pyrophosphate ion (PPi) and Cu(II) decomposed CQDs@Cu-MOF and led to the release of CQDs, of which the fluorescence recovered. In the presence of PPase, the hydrolysis of PPi generated phosphate ion (Pi). CQDs@Cu-MOF remained their structural stability and the fluorescence turned off. The fluorescence intensity difference of the mixture of CQDs@Cu-MOF and PPi in the absence and presence of PPase (-ΔF) was proportional to the PPase concentration from 0.1 to 5 mU mL⁻¹ and that from 5 to 50 mU mL⁻¹, and a limit of detection at 0.03 mU mL⁻¹ was obtained. PPase activity in human serum was analyzed using the proposed fluorescence sensor and the recovery values were found to vary from 95.0% to 104 %.