Introduction
Accurate data on engineered nanoparticle (ENP) environmental behavior and the interplay between ENP size, dissolution rate, agglomeration, and interaction with the sample matrix is critical to appropriately characterize the risks these novel materials may pose to environmental health. The advancement of the single particle ICP-MS (SP-ICP-MS) technique is a great benefit for the study of ENPs in natural systems at environmentally relevant (ng/L) concentrations. Previous studies may have obscured environmentally-relevant transformations because of artificially high ENP concentrations used in the experiments1. Therefore, the SP-ICP-MS method is at the forefront to garner the type of information most relevant for environmental risk assessments, namely the precise tracking of changes in ENP size, associated dissolved metal concentration, and determining polydispersity of an ENP sample, all at dilute concentrations in complex solutions. Because dissolution rate is surface-area controlled, the time to complete dissolution is highly dependent on the initial and (potentially stable) intermediate particle sizes. By measuring the change in particle size, as well as the evolution of Ag+(aq) in solution, using SP-ICP-MS, potential pitfalls related to loss of Ag+ to experimental materials and to other environmental surfaces, such as suspended sediments or biota in the case of complex matrices, may be avoided.
SP-ICP-MS Technique
The theoretical basis of detecting and measuring single particles by SP-ICP-MS has been well-studied in recent years1-8. This basis relies on the assumption that at sufficiently short dwell times and low particle number concentrations, detected pulses represent individual particle events. As a result, analysis in single particle mode uses thousands of fast, individual readings with the goal of capturing one (or a slice of one) ENP event. The particle mass can then be determined by the intensity of the ICP-MS response. If the ENP’s element is also present as a dissolved species (i.e. dissolved silver vs. a silver nanoparticle), an increase of the baseline is observed in single particle mode. This increase is directly proportional to the instrument’s calibration curve of the dissolved species. In this study, we used the Syngistix™ Nano Application Module for particle measurement/detection and automated data treatment. Determination of the transport efficiency (i.e. the percentage of particles in solution that are detected) is critical to determining the ENP size when using calibrations based on dissolved standards. To avoid coincidence (i.e. two particles being detected in the same pulse), particle concentrations were adjusted so that no more than 1500 particles were detected in 60 s acquisition time2,6.
Experimental
Materials Ag ENPs (100 nm diameter, NanoXact, NanoComposix, USA) with polyvinylpyrrolidone (PVP) as a capping agent were examined. ENP suspensions were made by diluting stock solutions (20 mg/L Ag ENPs) with water to yield a final concentration of 50 ng/L Ag ENPs. To match the peak intensities observed by SP-ICP-MS, dissolved Ag standards (High-Purity Standards; QC-7-M) were used for calibration and diluted in 2% HNO3 (Optima grade) for final concentrations ranging from 0.1-1 μg/L. For determination of nebulization efficiency, 100 nm Au NPs were obtained from BBI™ Solutions (Cardiff, UK) and prepared daily as a 100 ng/L ENP solution in distilled (DI) water. Water samples analyzed included deionized water (DI, 18.3 M-ohm cm Nanopure), tap water (Colorado School of Mines campus, Golden CO) and surface water. The surface water sample, collected in June 2012 from Clear Creek in Golden, CO, was taken just beneath the water surface, approximately 1 m from the creek bank, and passed through a 0.45-micron filter. The sample was stored in a polyethylene bottle at 20°C prior to use. The tap water contained approximately 1 mg/L free chlorine, as tested by the Golden, Colorado water treatment facility.
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