Enzyme-cofactor-assisted photochemical synthesis of Ag nanostructures and shape-dependent optical sensing of Hg(II) ions

Abstract

We describe a rapid green photochemical route for the synthesis of polyhedral Ag nanostructures, using the enzyme cofactor reduced nicotinamide adenine dinucleotide (NADH) and their analytical application in the optical sensing of environmentally hazardous Hg(II) ions at parts per billion (ppb) levels. Our synthetic methodology involves the reduction of Ag(I) ions by triplet-state NADH in aqueous solution. Quasi-spherical nanoparticles with an average size of 25 nm were obtained at the initial stage of the reaction. Light induces the transformation of quasi-spherical nanoparticles into a polyhedral nanostructure, with sizes ranging from 30 nm to 40 nm. The polyhedral nanostructures are highly stable for months in darkness and display two distinct surface plasmon bands, at 400 and 630 nm, corresponding to the quadrupole and dipole in-plane plasmon resonance, respectively. The polyhedral nanostructures are mainly composed of a (111) lattice plane with twinned boundaries. The thermodynamically favorable redox reaction of colloidal polyhedral Ag nanostructures with Hg(II) ions has been exploited for the optical sensing of Hg(II) in aqueous solution. The oxidative etching of polyhedral Ag nanostructures by Hg(II) ions strongly influences their optical characteristics: the deep green color of the colloidal Ag nanostructures changes to bright yellow. The surface plasmon band at the longer-wavelength side disappears in the presence of Hg(II) ions. The polyhedral Ag nanostructures transform to spherical nanoparticles because of oxidative etching. Sensing of Hg(II) ions at the ppb level has been achieved without any additional reagents. Our sensing method is very simple and highly selective; other common metal cations do not interfere with the measurement of Hg(II). The optical sensing capability of the Ag nanostructure is strongly dependent on their shape. The polyhedral Ag nanostructures have higher activity than the conventional spherical nanoparticles. Electrochemical studies supports the remarkable reactivity of polyhedral nanostructures toward Hg(II) ions. The polyhedral nanostructures are more susceptible to the oxidative etching reaction. The shape of the nanoparticle controls the oxidation potential of Ag nanostructures. The high reactivity of the polyhedral nanostructures is ascribed to the existence of defect sites.