Sensitivity enhancement in the colorimetric detection of lead(II) ion using gallic acid–capped gold nanoparticles: Improving size distribution and minimizing interparticle repulsion

We have developed a colorimetric assay for the highly sensitive and selective detection of Pb2+ by narrowing the size distribution of gallic acid–capped gold nanoparticles (GA–AuNPs) and minimizing electrostatic repulsion between each GA–AuNP. We unveil that the particle size and size distribution of GA–AuNPs could be controlled by varying the pH of HAuCl4 with fixed concentrations of HAuCl4 and GA. When the pH of the precursor solution (i.e., HAuCl4) was adjusted from 2.2 to 11.1, the average diameter of GA–AuNPs was decreased from 75.1 nm to 9.3 nm and their size distribution was reduced from 56.6−93.6 nm to 9.0−9.6 nm. The colorimetric sensitivity of the Pb2+-induced aggregation of GA–AuNPs could be improved using narrow size distribution of GA–AuNPs. Moreover, further enhancement of the colorimetric sensitivity of GA–AuNPs toward Pb2+ could be achieved by adding NaClO4 to minimize electrostatic repulsion between GA–AuNPs, which provide a small energy barrier for Pb2+ to overcome. Under the optimum conditions (1.0 mM NaClO4 and 20 mM formic acid at pH 4.5), the selectivity of 9.3 nm GA–AuNPs for Pb2+ over other metal ions in aqueous solutions is remarkably high, and its minimum detectable concentration for Pb2+ is 10 nM. We demonstrate the practicality of 9.3 nm GA–AuNPs for the determination of Pb2+ in drinking water. This approach offers several advantages, including simplicity (without temperature control), low cost (no enzyme or DNA), high sensitivity, high selectivity, and a large linear range (10.0–1000.0 nM).