Colorimetric detection of mercury, lead and copper ions simultaneously using protein-functionalized gold nanoparticles

A simple, cost-effective and rapid colorimetric method for any or all of Hg2+, Pb2+ and Cu2+ detection using papain-functionalized gold nanoparticles (P-AuNPs) has been developed. Papain is a protein with seven cystein residues, which can selectively bind with Hg2+, Pb2+ and Cu2+. We functionalized gold nanoparticles with papain. The P-AuNPs could be used to simultaneously detect Hg2+, Pb2+ and Cu2+, and showed different responses to the three ions in an aqueous solution based on the aggregation-induced color change of gold nanoparticles. The P-AuNPs displayed the most obvious response to mercury ions in water in contrast to lead and copper ions, and the real water sample analysis verified the conclusion. The sensitivity of the detection system was influenced by the pH of the P-AuNPs solution, the concentration of P-AuNPs and the size of gold nanoparticles, and we found that larger gold nanoparticles contributed to more sensitive results. The detection system can detect as low as 200 nM Hg2+, Pb2+ or Cu2+ using 42 nm gold nanoparticles. We expect our approach to have wide-ranging applications in the developing region for monitoring water quality in some areas.