Development of ICP-MS based nanometrology techniques for characterization of silver nanoparticles in environmental systems

Abstract

The ubiquitous use of goods containing nanoparticles (NPs) will lead inevitably to environmental release and interaction with biota. Methods to detect, quantify, and characterize NPs in environmental matrices are highlighted as one of the areas of highest priority research in understanding potential environmental and health risks. Specifically, techniques are needed to determine the size and concentration of NPs in complex matrices. Particular analytical challenges include distinguishing NPs from other constituents of the matrix (i.e. natural particles, humic substances, and debris), method detection limits are often higher than exposure concentrations, and differentiating dissolved metal and NPs. This work focuses on the development and optimization of two methods that address a number of challenges for nanometrology: single particle (sp)ICP-MS and asymmetrical flow field flow fractionation (AF4)-ICP-MS. Advancements in the spICP-MS method included systematic studies on distinction between ionic and NP fractions, resolution of polydisperse NP samples, and defining the techniques' dynamic range (in terms of both particle size and concentration). Upon application of the technique, silver (Ag) NPs were discovered in raw wastewater treatment plant influent and effluent. Furthermore, methodical Ag NP stability studies determined the influence of particle capping agents and water chemistry parameters in a variety of synthetic, natural and processed waters. Method development for AF4-ICP-MS revolved around optimizing run conditions (i.e. operational flows, carrier fluid, membrane choice) to study detection limits, sample recovery, and resolution of polydisperse samples. Practical studies included sizing Ag NP in a sediment-dwelling, freshwater oligochaete (Lumbriculus variegatus) and the kinetics of accumulation of protein bound Ag+. In direct comparison, spICP-MS was found to be more versatile with less sample preparation and lower total analyte detection limit (ng/L vs. [micron]g/L), though AF4-ICP-MS could detect smaller particle sizes (2 nm vs. 25 nm) and elucidate NP/matrix interactions for changes in particle hydrodynamic diameter. Additionally, spICP-MS afforded us the opportunity to determine the kinetic rate of Ag NP dissolution rate kinetics at environmentally relevent concentrations, the first study of its kind. We found significantly variable dissolution rates for differently capped NPs in addition to water chemistries. Tannic acid capping agent was least resistant to dissolution compared to citrate and PVP, while high concentrations of natural organic matter seemed to stabilize the particles over time in comparison to DI water. The residual chlorine in tap water increased the dissolution rates of all particles dramatically, which we hypothesize to be due to residual chlorine. Herein is described method development protocol and results of aforementioned studies comparing sp and AF4 ICP-MS and supporting their use as choice nanometrology techniques for quantitative environmental and toxicological studies.