Discriminating the states of matter in metallic nanoparticle transformations: what are we missing?

A limiting factor in assessing the risk of current and emerging nanomaterials in biological and environmental systems is the ability to accurately detect and characterize their size, shape, and composition in broad product distributions and complex media. Asymmetric flow field-flow fractionation (A4F) is capable of separation without stationary phase interactions or large applied forces. Here, we demonstrate unprecedented A4F fractionation of metallic nanoclusters with core diameters near 1 nm and with high resolution. The isolated nanocluster populations were characterized online with UV-vis absorption and inductively coupled plasma mass spectrometry (ICP-MS). We apply our methodology to a model system, poly(N-vinyl-2-pyrrolidone)-protected silver nanoparticles with an excess of tripeptide-glutathione (GSH). The temporal evolution of the initial silver nanoparticle distribution in the presence of excess GSH results in the appearance and persistence of a continuum of matter states (e.g., Ag(+) nanoclusters and nanoparticles) that could be fractionated with A4F, characterized by their optical signatures and diffusion coefficients, and quantified with ICP-MS. The results suggest that our methodology is generally applicable to metallic systems when appropriate online detection is coupled to the A4F. Because we extend the capability of the coupled A4F system to reliably detect, characterize, and quantify metallic populations in the sub-5 nm regime, the opportunity exists to survey the formation and transformation products of nanomaterials in more relevant biological and environmental systems. Thus, individually assessing the risks associated with specific ion, nanocluster, and nanoparticle populations is achievable, where such populations may have previously been misrepresented.