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Preferential Interaction of Na+ over K+ to Carboxylate-functionalized Silver Nanoparticles
Citation:
Pokhrel, L., C. Andersen, P. Rygiewicz, AND M. Johnson. Preferential Interaction of Na+ over K+ to Carboxylate-functionalized Silver Nanoparticles. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 490:11-18, (2014).
Impact/Purpose:
Engineered nanoparticles (ENPs) have been recognized as valuable components of novel technologies and are currently being used in a variety of consumer products due to their unique physical, chemical, and electrical properties. The properties that make these particles functionally unique also influence their behavior and interactions with biotic and abiotic components of the environment. One of the challenges scientists face when examining the potential environmental health and safety aspects of ENPs is to understand the underlying chemistry of these interactions in order to better predict and model potential environmental effects. WED scientists are examining the toxicity of several ENPs and how their physico-chemical properties may influence toxicity to terrestrial organisms and ecosystems. Because of their small size and high reactivity, ENPs can readily bind with cations and anions in the environment, influencing their behavior and toxicity. Classical chemistry allows the prediction of binding preference based on the Hofmeister series, which is a measure of salts’ (cation and anion) ability to precipitate egg white proteins; for many ENPs, it is unknown whether binding to monovalent cations (Na+, K+) follows Hofmeister principle, or whether they behave differently due to their unique properties. WED scientists examined carboxylate-functionalized silver nanoparticle (AgNP) in order to determine if its’ binding behavior conformed to the Hofmeister series assessing six different physico-chemical characteristics: surface Plasmon resonance/optical absorbance, conductivity, pH, hydrodynamic diameter, electrophoretic mobility, and surface charge. Results showed that, despite both being monovalent, Na+ interacted differently than K+ with AgNPs, indicating local Na+ pairing with carboxylate on AgNP surfaces is quick and remarkably favored over K+. This study suggests that AgNPs may transform into micron-size aggregates upon release into the natural environment, where Na+ and K+ are prevalent, potentially altering their toxicity to biological organisms. In addition to providing better understanding of the underlying chemistry of AgNPs as they interact with naturally ubiquitous cations (Na+/K+), the results may provide insight into how to engineer or alter ENPs’ surfaces to lower their potential toxicity to biological organisms.
Description:
Elucidating mechanistic interactions between specific ions (Na+/ K+) and nanoparticle surfaces to alter particle stability in polar media has received little attention. We investigated relative preferential binding of Na+ and K+ to carboxylate-functionalized silver nanoparticles (carboxylate–AgNPs) to determine if binding preference followed the Hofmeister series. We hypothesized that Na+ will show greater affinity than K+ to pair with carboxylates on AgNP surfaces, thereby destabilizing the colloidal system. Destabilization of carboxylate–AgNPs by Na+ or K+ binding is assessed by measuring six different physico-chemical characteristics: surface Plasmon resonance/optical absorbance, conductivity, pH, hydrodynamic diameter, electrophoretic mobility, and surface charge. We show, for the first time, that the cations behave differently, indicating local Na+ pairing with carboxylate on AgNP surfaces is quick and remarkably favored over K+. Our results suggest that AgNPs may transform into micron-size aggregates upon release into aqueous environments and that the fate of such aggregates may need consideration when assessing environmental risk.