The formation of silver nanoparticles (AgNPs) via reduction of silver ions (Ag+) in the presence of humic acids (HAs) under various environmentally relevant conditions is described. HAs tested were from Suwannee River (SUW), and samples of three sediment HAs (SHAs), and five soils obtained across the state of Florida. The time required to form AgNPs varied depending upon the type and concentration of HA, as well as temperature. SUW and all three SHAs reduced Ag+ at 22 oC. However, none of the soil HAs formed absorbance-detectable AgNPs at room temperature when allowed to react for a period of 25 days, at which time experiments were halted. The appearance of the characteristic surface plasmon resonance (SPR) of AgNPs was observed by ultraviolet-visible spectroscopy in as few as (2 to 4) days at 22 oC, for SHAs and for SUW. An elevated temperature of 90 oC resulted in the accelerated appearance of the SPR within (45 to 90) mins for SUW and all SHAs. The formation of AgNPs at 90 oC was usually complete within 3 h. Transmission electron microscopy and atomic force microscopy images showed that the AgNPs formed were generally spherical and had a broad size distribution. Dynamic light scattering also revealed polydisperse particle size distributions. HAs appeared to colloidally stabilize AgNPs based on a lack of observable change in the spectral characteristics over a period of two months. The results suggest the potential for direct formation of AgNPs under environmental conditions from Ag+ sources, implying that not all AgNPs observed in natural waters today may be of anthropogenic origins.
Citation: Environmental Science & Technology
Pub Type: Journals
silver nanoparticles, humic acid, environmental nanoparitcles