In situ synthesis of metal nanoparticles and selective naked-eye detection of lead ions from aqueous media

Abstract

A novel one step synthesis of water soluble Au and Ag nanoparticles has been reported at room temperature using a naturally occurring bifunctional molecule, namely, gallic acid. The mechanistic details of nanoparticle formation were elucidated by carrying out control experiments using a variety of model compounds. The newly synthesized nanoparticles are extremely stable in the pH range of 4.5-5.0, due to (i) the strong electrostatic interaction of the carboxylate anion of the capping agent with the surface of the nanoparticle and (ii) a very high ζ potential (-45 mV). Under these pH conditions, it is difficult to bring nanoparticles in proximity due to strong interparticle electrostatic repulsion. However, the unique coordination behavior of Pb²⁺ ions (coordination number up to 12, flexible bond length and geometry) allows the formation of a stable supramolecular complex resulting in plasmon coupling and a visual color change. Because of the rigid coordination geometry, other metal cations (Ca²⁺, Cu²⁺, Cd²⁺, Hg²⁺, Mg²⁺, Ni²⁺, and Zn²⁺) interact only with lesser numbers of ligands, leaving the nanoparticles isolated; hence, no spectral change was observed under the experimental conditions. The ratiometric plots of the aggregated to the isolated forms indicate a high sensitivity as well as selectivity of Au and Ag nanoparticles toward Pb²⁺ ions. One of the significant features of the present system is its ability to detect micromolar quantities (ppm level) of Pb²⁺ ions in the presence of other metal cations in water. Further, we have theoretically modeled the interaction between the newly synthesized nanoparticles and the Pb²⁺ ion, and various optimized geometries are evaluated. On the basis of the experimental and theoretical studies, a tentative structure of the supramolecular complex leading to a strong interparticle interaction is provided.