Metal-to-ligand charge-transfer: Applications to visual detection of β-galactosidase activity and sandwich immunoassay

In this work, we report a novel use of the distinctive metal-to-ligand charge-transfer (MLCT) absorption properties of the chromogenic Fe(BPDS)(3)(4-) (BPDS=bathophenanthroline disulfonic acid) reporter for the visual detection of beta-galactosidase (beta-Gal) activity and sandwich immunoassay. The enzymatic hydrolysis of the substrate p-aminophenyl-beta-D-galactopyranoside can switch on the reduction of Fe3+ to Fe2+ and the subsequent complexation of Fe2+ with the BPDS ligand to generate the Fe(BPDS)(3)(4-) reporter, leading to the appearance of the intense MLCT absorption band at 535 nm and the colorless-to-red color change of the solution. Simply through a single step, the activity of beta-Gal can be sensitively and selectively detected within the dynamic range of 0-220 mU mL(-1) with a limit of detection (LOD) of 1.69 mU mL(-1). This approach is applicable for the visual detection of beta-Gal activities in the presence of complex human serum samples. Besides, when integrated with the sandwich immunoassay of carcinoembryonic antigen, a LOD of 1.16 ng mL(-1) can be achieved. In light of its prominent simplicity and practicality, our MLCT-based approach holds great potential in diagnostic and analytical applications.