Pyrroloquinoline quinone exhibiting peroxidase-mimicking property for colorimetric assays

Small molecule mimics offer the advantages of easy preparation, good thermodynamic stability, and reproducible catalytic activity. However, most of the reported organic artificial mimics face challenges including low catalytic activity, oxidative self-destruction, and auto-aggregation into inactive dimers. Therefore, novel organic mimics with high catalytic activity as well as good thermal and environmental stability are highly desirable.

We found that pyrroloquinoline quinone (PQQ), a nutritionally important growth factor, displayed significant peroxidase-like activity to catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB) by H₂O₂. Its catalytic activity surpassed that of other organic small molecules reported previously, including fluorescein derivatives and adenosine/guanosine triphosphate. Based on the reduction of oxTMB by the enzymatically generated ascorbic acid (AA), the PQQ/TMB-based system was used to monitor alkaline phosphatase (ALP) activity with ascorbic acid 2-phosphate (AAP) as the substrate. This sensing system allowed for the determination of ALP with a detection limit of 1 mU/mL. Furthermore, we discovered that PQQ can coordinate with Cu²⁺ to form metal-organic nanoparticles. The building blocks of two active cofactors endowed the nanoparticles with impressive functionalities for biocatalytic applications. With Cu-PQQ nanoparticles as signal labels, colorimetric immunoassays of prostate specific antigen (PSA) were achieved with a lowest detectable concentration down to 0.1 pg/mL.

This work is valuable for the design of novel biosensors by utilizing PQQ as an artificial enzyme because of its high catalytic activity, good stability, ease of storage, and simple chemical structure.