New optical method for the determination of β-galactosidase and α-fetoprotein based on oxidase-like activity of fluorescein.

Fluorescein has been found as an efficient visible-light-induced oxidase mimic and its catalytic performance is group-dependent. Herein, a facile colorimetric strategy for β-galactosidase (β-gal) was developed using fluorescein di β-D-galactopyranoside (FDG) as a probe based on the analyte induced change in oxidase mimicking activity of fluorescein derivatives. FDG doesn't possess any visible-light-induced oxidase activity and can generate fluorescein and fluorescein mono β-D-galactopyranoside (FMG) in the presence of β-gal. The in situ generated fluorescein and FMG possess high oxidase-like activities under visible-light illumination and could catalyze the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) upon short irradiation by light-emitting diode (LED) lamp. Thus, the β-gal activity can be selectively detected in linear range from 0.10 to 12.9 μg mL-1 with a limit of detection (LOD) of 0.04 μg mL-1. We further integrated with the visual detection of α-fetoprotein antigen (AFP) based on the corresponding colorimetric signal induced by β-gal-linked colorimetric immunoassay, a LOD of 0.08 ng mL-1 could be achieved. Significantly, our proposed assay provides a facile sensing platform based on the change in enzyme mimicking activity induced by analytes. In addition, this optical method works without complex synthesis procedure and efficiently avoids participation of unstable H2O2 as an oxidant. Therefore, the present work not only shows the excellent assay performance in β-gal and tumor biomarker detection, but also opens up a new avenue for its application in practical optical sensing.