Photoswitching enzymatic activity of horseradish peroxidase by graphene oxide for colorimetric immunoassay.

In contrast to the conventional means that the activity of horseradish peroxidase (HRP) is initiated and terminated by the additives of peroxides and strongly acidic stop solutions, this study demonstrates that the enzymatic activity of HRP is switched through the visible light irradiated graphene oxide (GO). And this visible light driven activity of HRP can realize time-precise control without the aids of peroxides (typically H2O2) and acidic stop solutions. The superoxide anions (O2•-) and photogenerated holes (h+) produced by the photo irradiated GO are responsible for activating HRP and the subsequent oxidation of the typical substrates, i.e., 3, 3', 5, 5'-tetramethylbenzidine (TMB) and 2, 2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). It is also validated that the photoswitchable HRP-GO mixture can act as an efficient signal reporter of bioassays by taking the sandwich immunoassay of alpha-fetoprotein (AFP) as an example. The AFP can be detected sensitively and selectively in the linear range from 0.2 fg/mL to 1.0 ng/mL, with a very low detection limit of 0.1 fg/mL. Advantages of the photoswitchable HRP-GO mixture include high catalytic ability, precise time control, and free of additionally harmful reagents.