Colorimetric sensing of cysteine using label-free silver nanoparticles

The development of methods for the effective measurement of cysteine in biological fluids is of great physiological and pathological importance. In this study, we present a convenient and economical colorimetric method for the quantitative determination of cysteine using label-free silver nanoparticles (Ag NPs) as color indicators and Ca2+ ions as cross-linking agents. The simultaneous addition of cysteine and Ca2+ to a Ag NPs colloidal solution results in the cross-linking of the Ag NPs, leading to a rapid yellow-to-red color change. In this way, a simple and rapid method for the quantitative assay of cysteine in the linear range of 0.1–1000 μM was obtained through monitoring of absorbance signatures. This convenient technique exhibited high selectivity toward cysteine over a range of other amino acids, as well some thiol-containing compounds. Importantly, our newly developed method was successfully applied to the detection of cysteine in biological fluids, such as serum and artificial cerebrospinal fluid based on changes in the color of these solutions in less than 10 min.