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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 Pb 2+ 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 (Ca 2+, Cu 2+, Cd 2+, Hg 2+, Mg 2+, Ni 2+, and Zn 2+ ) 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 Pb 2+ ions. One of the significant features of the present system is its ability to detect micromolar quantities (ppm level) of Pb 2+ ions in the presence of other metal cations in water. Further, we have theoretically modeled the interaction between the newly synthesized nanoparticles and the Pb 2+ 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.
Yoosaf et al. (2007) studied this question.