PHOTO-INDUCED, PHYSICO-CHEMICAL TRANSFORMATIONS OF SILVER NANOPARTICLE SUSPENSIONS
Justin M. Gorham, Robert I. MacCuspie, D. Howard Fairbrother, and R. David Holbrook
The amount of consumer products containing nanomaterials has been rapidly increasing over the past 2 decades. The most prevalent nanomaterial used today is the silver nanoparticle (AgNP) due to its antimicrobial properties. The eventual release of AgNPs into the ecosystem from normal use and disposal has raised numerous environmental, health and safety (EHS) questions, such as the toxicological impact. Additionally, AgNPs released into the natural environment have the potential to be transformed through exposure to a variety of conditions (i.e. UV radiation, pH, NOM). These transformations are relatively unexplored and have the potential to greatly impact EHS studies. To address this question, the focus of the present study is to investigate the UV-induced, physico-chemical transformations of differently sized, citrate-capped AgNP suspensions. Samples were exposed to 300 nm light, representative of the high energy portion of the solar spectrum. AgNP suspensions ((20, 40, 60 and 80) nm) were observed to visibly transform from yellow to colorless after prolonged irradiation. Using UV-vis spectroscopy to monitor the loss of the surface plasmon resonance (SPR) absorbance, plots were obtained for each AgNP suspension that fit well to an exponential decay, thereby indicating pseudo-first order kinetics. Additionally, the rates of AgNP photolysis exhibited a linear dependence with the specific surface area (cm2 x g-1) and a non-linear dependence on irradiance. Atomic force microscopy and dynamic light scattering measurements revealed the presence of a size decrease in exposed AgNPs, while addition of NaBH4 recovered the characteristic SPR absorbance in irradiated AgNP suspensions. These studies suggest that the UV-induced transformations of AgNP suspensions progress via a surface dominated, thermal process resulting in fragmentation and oxidation.