Gold Nanoparticles-based Colorimetric Sensor for Cysteine Detection

A simple, sensitive and selective colorimetric method for the detection of cysteine was demonstrated with unmodified gold nanoparticles (AuNPs) as probes. In this approach, the synthesized AuNPs solution was stabilized by the citrate anions as their repulsion protected the AuNPs from aggregation. Cysteine was added to AuNPs solution and was incubated to react for 3 min. The resulting mixture color changes dramatically from red-purple-blue because cysteine induced the nanoparticle aggregation. These processes were studied and characterized by UV–vis spectroscopy, zeta potential and dynamic light scattering. Several parameters including AuNPs size, reaction time and media pH that governed the analytical performance of the method have been studied in detail and optimized. Under the optimized experimental conditions, cysteine could be selectively detected in a concentration range from 0.1 to 0.6 ppm with a limit of detection as 0.01 ppm at a signal-to-noise ratio of 3. The sensitivity was calculated as 1.474 Abs/ppm. Some common interferents such as Na+, Cu2+, Cl− and urea showed no interference in the determination of cysteine by using AuNPs.