Bifunctional antibody and copper-based metal-organic framework nanocomposites for colorimetric α-fetoprotein sensing

Cu²⁺ are found to greatly reduce the photoinduced oxidase activity of fluorescein and then inhibit the chromogenic reaction catalyzed by fluorescein. A simple colorimetric assay for Cu²⁺ is established. Based on this, bifunctional nanocomposites of α-fetoprotein (AFP) antibody (Ab) and copper-based metal-organic framework (Ab2@Cu-MOF) are synthesized by the simple self-assembly of AFP Ab2, Cu²⁺, and 4,4'-dipyridyl: the binding site of AFP Ab2 exposed on the surface of the nanocomposites can specifically recognize AFP antigen; Cu²⁺ in nanocomposites can inhibit the visible light-induced activity of fluorescein. The structure of Ab2@Cu-MOF disintegrate and Cu²⁺ is released in an acetate buffer solution. The higher the amount of AFP antigens, the more significant the inhibitory effect. Thus, the Ab2@Cu-MOF immunoassay for AFP determination is established using 3,3',5,5'-tetramethylbenzidine as chromogenic substrate with a detection limit of 35 pg.mL^-1. This simple, cheap, and sensitive method sheds substantial light on practical clinical diagnosis. Meanwhile, the mechanism of inhibition is revealed to facilitate the targeted selection of enzyme regulators. Graphical abstract Diagrammatic illustration of Cu²⁺ detection (part a) and Ab2@Cu-MOF immunoassay for sensing α-fetoprotein based on the synthesized Ab2@Cu-MOF nanocomposites (parts a and b).