Storage Wars: how citrate-capped silver nanoparticle suspensions are affected by not-so-trivial decisions

A critical but often overlooked component of silver nanoparticle (AgNP) suspensions involves their behavior following short- and long-term storage. The current study investigates the integrity of citrate-capped AgNP suspensions, nominally 20 nm in average diameter, in a series of distinct storage conditions, based on possible combinations of reasonable decisions researchers make, both nanoparticle-based (AgNP and relative citrate concentration) and environmental-based (solution oxygenation and ambient light or dark). AgNP integrity was determined by monitoring single particle stability, aggregation/agglomeration, and oxidation for 104 days. We demonstrate that AgNP suspensions lose their physical and chemical integrity by two distinct processes: (1) oxidation only (light-independent) and (2) oxidation followed by photo-reduction (light-dependent), following initial dilution from a concentrated (and newly synthesized) AgNP stock solution. Optical spectroscopy indicates that the effects of oxidation are readily observed while the effects of photo-reduction are less obvious, leading to a greater increase in average particle diameter, the formation of new, metallic nanoparticles, and the oxidation of the parent citrate capping agent. In general, the overall integrity of citrate-capped AgNP suspensions are best maintained when these solutions are purged with nitrogen gas and stored in the dark at the highest AgNP and citrate concentrations. This study outlines a strategy for both assessing and monitoring the integrity of AgNP suspensions in an effort to harmonize long-term experiments and promote inter-laboratory consistency.