Determination of cysteine via its inhibition of catalytic activity of silver coated ZnO/Fe3O4 composites used for conversion of 4-nitrophenol into 4-aminophenol

Abstract

Cysteine plays vital roles in physiological processes such as scavenging reactive oxygen species to protect for oxidative DNA damages, participating in enzymatic reactions, detoxifying and facilitating the elimination of heavy metals. Therefore, rapid and simple method of detection is highly demanding. In this study, stable silver nanoparticles decorated ZnO/Fe3O4 composites were prepared and used as catalytic signal amplification together with 4-nitrophenol (4-NP) as a colorimetric probe molecule for detection of cysteine. The detection of cysteine is based on tuning the inherent catalytic reaction of 4-NP to 4-aminophenol (4-AP) in the presence of NaBH4. The reduction of 4-NP to 4-AP in the presence of AgNPs/ZnO/Fe3O4 composites is fast. The same reaction mixture in the presence of cysteine, however, suppresses the rate of the reaction and it takes a longer reaction time for a complete reduction of 4-NP to 4-AP. At 1 min reaction time, the absorbance of 4-NP in the presence of a series of cysteine concentration is found to be concentration dependent. This is the bases for the determination of cysteine and the method is able to determine in a wide range of cysteine concentrations with high reproducibility from 0.1 to 150 µM with the detection limit of 90 nM as calculated based on the 3σ rule. The assay shows excellent selectivity against potentially interfering molecules and applied to quantification of cysteine in tap water samples.