pH-dependent response of citrate capped Au nanoparticle to Pb2+ ion

Au nanoparticle prepared by reduction of HAuCl4 with citrate was found to show pH-dependent response to Pb2+ ion. At pH value of 8.2, the citrate capped Au nanoparticles show response to a variety of heavy metal ions which have strong affinity to the carboxyl groups of citrate ligand, such as Pb2+, Cu2+, Fe3+ etc. At pH value of 11.2, other heavy metal ions with strong affinity to the carboxyl groups become less effective to induce the color change of the Au nanoparticles due to their suppressed coordination with the ligand on the particle surface upon increased concentration of hydroxyl group. However, the Au nanoparticles are still sensitive to Pb2+ ion under the pH of 11.2 due to its amphoteric character. Such pH-dependent response character of the Au nanoparticles allows the selective detection of Pb2+ ion at the pH of 11.2, for example, the citrate capped Au nanoparticles are qualified for the quantitative detection of Pb2+ ions in the range of concentrations from 0.2 μM to 15 μM in the presence of 0.04 M KCl.