An ultrasensitive label-free colorimetric assay for glutathione based on Ag regulated autocatalytic oxidation of o-phenylenediamine.

The discriminative determination of glutathione (GSH) over cysteine (Cys) and homocysteine (Hcy) is still challenging in bioassays due to their similar functional groups. Herein, a novel, simple, rapid, sensitive and cost-effective label-free colorimetric method for the selective determination of GSH over Cys and Hcy was proposed. In this assay, firstly, o-phenylenediamine (OPD) could be oxidized by silver ions (Ag) to produce silver nanoparticles (AgNPs) and pale yellow colored 2,3-diaminophenazine (shorted as OPDox) with absorbance peak center at 429 nm. The as-formed AgNPs could further catalyze the redox reaction between OPD and Ag. In the presence of GSH, GSH could chelate with Ag, inhibiting the oxidation ability of Ag, and link with AgNPs, influence the catalytic ability of AgNPs. Furthermore, GSH possesses strong reducibility to reduce OPDox, resulting in color fading of the detection solution and decrease in absorbance intensity. The proposed Ag-OPD based sensing system exhibited a wide linear range from 2 nM to 1 µM for GSH detection, with the limit of detection as low as 1.7 nM. Moreover, this developed method was successfully applied to GSH detection in plasma and urine samples with satisfied results.