Introducing bifunctional metal-organic frameworks to the construction of a novel ratiometric fluorescence sensor for screening acid phosphatase activity

Restricted to single sensing mechanisms, most luminescent metal-organic framework (LMOF)-based sensors were constructed for detection of limited targets. Here, a new biosensor is described for screening acid phosphatase (ACP) activity via bifunctional NH2-MIL-101 MOFs acting as both fluorescent indicator and biomimetic catalyst. NH2-MIL-101 possesses an inherent fluorescence emission at 456 nm (F456). As a peroxidase-like nanozyme, it catalyzes oxidation of o-phenylenediamine (OPD) by H2O2 to generate fluorescent 2,3-diaminophenazine with the maximum emission at 556 nm (F556). Upon introducing NH2-MIL-101 into a mixture of OPD and H2O2, F456 is quenched, while F556 increases. The ACP sensing is based on pyrophosphate ion (PPi) mediated fluorescence tuning of the NH2-MIL-101/OPD/H2O2 system. PPi inhibits the NH2-MIL-101 catalytic ability by specific binding to its Fe center, while ACP addition recovers the activity by hydrolyzing PPi. Upon addition of PPi and ACP into the NH2-MIL-101/OPD/H2O2 system, a ratiometric luminescence signal (F556/F456) is obtained, and a ratiometric fluorescent sensor can be developed for the sensitive detection of PPi and for screening ACP activity. Plots of F556/F456 vs. ACP concentration were linear over 0.01-30 U/L, with a detection limit of 0.005 U/L. The proposed sensor was successfully used for ACP detection in serum samples. This ratiometric fluorescence assay will open new applications for LMOF-based biosensors.