A dual-mode fluorescent and colorimetric immunoassay based on in situ ascorbic acid-induced signal generation from metal-organic frameworks

A novel dual-mode enzyme-linked immunosorbent assay (ELISA) has been developed based on Fe(III)-containing metal–organic frameworks (Fe-MOFs), which possesses unique properties of an ascorbic acid (AA)-responsive fluorescence emission and an oxidase-like activity inhibition. In this design, bimodal signals, a remarkable emission enhancement and a noticeable color change from the oxidation inhibition of a chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB), are generated from the Fe-MOFs probe with the increase of AA concentration. The change in the fluorescent and colorimetric signal is easily regulated by controlling AA level, which in turn is catalytically adjusted by the alkaline phosphatase (ALP) used in ELISA. Under an optimal condition, a limit of detection (LOD) at 180 pg mL−1 is achieved with the proposed dual-mode ELISA using human prostate specific antigen (PSA) as a model analyte. Furthermore, the mechanism of AA-induced emission enhancement of Fe-MOFs is also investigated systematically. The results show that the enhancement is attributed the AA-induced inhibition of ligand-metal charge transfer (LMCT) from the fluorescent ligand 2-amino-1, 4-benzenedicarboxylic acid (BDC-NH2) to Fe(III). When Fe(III) in Fe-MOF is reduced into Fe(II) by AA, the LMCT is partially inhibited and the fluorescence of BDC-NH2 is consequently recovered, thus resulting in a fluorescence enhancement of the Fe-MOFs.