Ultrasensitive colorimetric detection of Cu2+ ion based on catalytic oxidation of l-cysteine

As an essential element, copper ion (Cu2+) plays important roles in human beings for its participation in diverse metabolic processes as a cofactor and/or a structural component of enzymes. However, excessive uptake of Cu2+ ion gives rise to the risk of certain diseases. So, it is important to develop simple ways to monitor and detect Cu2+ ion. In this study, a simple, facile colorimetric sensor for the ultrasensitive determination of Cu2+ ion was developed based on the following principle: l-cysteine and 1-chloro-2,4-dinitrobenzene (CDNB) could be conjugated to form the yellow product 2,4-dinitrophenylcysteine (DNPC), which was measurable at 355 nm; however, upon addition of Cu2+ ion, the absorbance of DNPC would be decreased owing to the Cu2+ ion catalytic oxidation of l-cysteine to l-cystine in the presence of O2. Thus, the colorimetric detection of Cu2+ ion could be achieved. The optimal pH, buffer, temperature and incubation time for the colorimetric sensor were obtained of pH 6.8 in 0.1 M HEPES solution, 90 °C and 50 min, respectively. A good linearity within the range of 0.8-10 nM (r=0.996) was attained, with a high detectability up to 0.5 nM. Analyses of Cu2+ ion in drinking water, lake water, seawater and biological samples were carried out and the method performances were found to agree well with that obtained by ICP-MS. The developed simple colorimetric sensor proved applicable for Cu2+ ion determination in real samples with high sensitivity and selectivity.