Gold Nanoparticle-Catalyzed Clock Reaction of Methylene Blue and Hydrazine for Visual Chronometric Detection of Glutathione and Cysteine

A visual chronometric assay for glutathione (GSH) and cysteine (Cy) is reported based on a gold nanoparticle (AuNP)-catalyzed clock reaction of methylene blue and hydrazine. The blue MB is reduced to colorless leucomethylene blue (LMB) by hydrazine in the presence of AuNPs as the nanocatalysts, and it is again oxidized back to MB by oxygen under shaking. Therefore, a periodic oscillation is observed between blue MB and colorless LMB. However, when GSH or Cy is added to the clock reaction system, the catalytic activity of AuNPs decreases because of the inhibition of the formation of gold-hydride species, leading to a long reaction time from MB to LMB. The chronometric assay displays a good linear range of 10 to 330 μM for GSH and 20 to 400 μM for Cy, respectively. The limit of detection is estimated to be 8 and 10 μM at a 3σ, respectively. Different concentrations of GSH/Cy can be visually detected by observing the height of the blue solution under different reaction time points. In addition, the present assay shows good selectivity, and it is successfully applied for detecting GSH/Cy in fetal bovine serum.