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 β-galactosidase (β-Gal) activity and sandwich immunoassay . The enzymatic hydrolysis of the substrate p -aminophenyl-β-D-galactopyranoside can switch on the reduction of Fe 3+ to Fe 2+ and the subsequent complexation of Fe 2+ 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 β-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 β-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.