Reduction of silver ions to form silver nanoparticles by redox-active organic molecules: coupled impact of the redox state and environmental factors

Abstract

The release of silver ions (Ag⁺) is an important process in determining the environmental fate and toxic effects of silver nanoparticles (AgNPs), but the released Ag⁺ can be reduced to form new AgNPs in environments. Here the reduction of Ag⁺ by redox-active organic molecules to form AgNPs under different lighting and oxygen conditions was investigated using anthraquinonoe-2,6-disulphonate (AQDS) and nicotinamide adenine dinucleotide (NAD⁺) as representatives. In anaerobic environments, oxidized AQDS (AQDS_ox) and NAD⁺ failed to reduce Ag⁺ to form AgNPs, whereas reduced AQDS (AQDS_red) and NAD⁺ (NADH) produced a concentration-dependent AgNP production pattern, underlining a key role of the redox state in Ag⁺ reduction by organics. The presence of O₂ diminished the reducing capacity of NADH and AQDS_red, probably due to their oxidation by O₂. However, under UV light, the AgNP formation rate was dramatically enhanced in AQDS, independent of O₂. Our results revealed that the Ag⁺ photoreduction was mainly attributed to the photoexcited AQDS, not the radical intermediates. For the complex organic humic acid (HA), and consistent with the phenomena in AQDS, the reduced HA (HA_red) reduced Ag⁺ at a much faster rate than the native HA (HA_ox) in anaerobic and dark environments, and the amounts of AgNPs produced by HA_red decreased in oxic environments. UV illumination enabled significant Ag⁺ reduction in both HA_ox and HA_red, and O₂ promoted the AgNP formation. Our study systematically revealed different Ag⁺-reduction pathways to form AgNPs, with an organic's redox state and environmental factors having a combined effect.