Metal oxide nanoparticles are finding increasing application in various commercial products, leading to concerns for their environmental fate and potential toxicity. It is generally assumed that nanoparticles will persist as small particles in aquatic systems and that their bioavailability could be significantly greater than that of larger particles. The current study using nanoparticulate ZnO (ca. 30 nm) has shown that this is not always so. Particle characterization using transmission electron microscopy and dynamic light scattering techniques showed that particle aggregation is significant in a freshwater system, resulting in flocs ranging from several hundred nanometers to several microns. Chemical investigations using equilibrium dialysis demonstrated rapid dissolution of ZnO nanoparticles in a freshwater medium (pH 7.6), with a saturation solubility in the milligram per liter range, similar to that of bulk ZnO. Toxicity experiments using the freshwater alga Pseudokirchneriella subcapitata revealed comparable toxicity for nanoparticulate ZnO, bulk ZnO, and ZnCl2, with a 72-h IC50 value near 60 microg Zn/ L, attributable solely to dissolved zinc. Care therefore needs to be taken in toxicity testing in ascribing toxicity to nanoparticles per se when the effects may be related, at least in part, to simple solubility.

Currently there are few ideal methods for the characterization of nanoparticles in complex, environmental samples, leading to significant gaps in toxicity and exposure assessments of nanomaterials. Single particle-inductively coupled plasma-mass spectrometry (spICP-MS) is an emerging technique that can both size and count metal-containing nanoparticles. A major benefit of the spICP-MS method is its ability to characterize nanoparticles at concentrations relevant to the environment. This paper presents a practical guide on how to count and size nanoparticles using spICP-MS. Different methods are investigated for measuring transport efficiency (i.e. nebulization efficiency), an important term in the spICP-MS calculations. In addition, an alternative protocol is provided for determining particle size that broadens the applicability of the technique to all types of inorganic nanoparticles. Initial comparison, using well-characterized, monodisperse silver nanoparticles, showed the importance of having an accurate transport efficiency value when determining particle number concentration and, if using the newly presented protocol, particle size. Ultimately, the goal of this paper is to provide improvements to nanometrology by further developing this technique for the characterization of metal-containing nanoparticles.

Sizing engineered nanoparticles in simple, laboratory systems is now a robust field of science; however, application of available techniques to more complex, natural systems is hindered by numerous challenges including low nanoparticle number concentrations, polydispersity from aggregation and/or dissolution, and interference from other incidental particulates. A new emerging technique, single particle inductively coupled plasma-mass spectrometry (spICPMS), has the potential to address many of these analytical challenges when sizing inorganic nanoparticles in environmental matrices. However, to date, there is little beyond the initial feasibility studies that investigates the performance characteristics and validation of spICPMS as a nanoparticle sizing technique. This study compares sizing of four silver nanoparticle dispersions (nominal diameters of 40, 60, 80, and 100 nm) by spICPMS to four established sizing techniques: dynamic light scattering, differential centrifugal sedimentation, nanoparticle tracking analysis, and TEM. Results show that spICPMS is able to size silver nanoparticles, across different sizes and particle number concentrations, with accuracy similar to the other commercially available techniques. Furthermore, a novel approach to evaluating particle coincidence is presented. In addition, spICPMS size measurements were successfully performed on nanoparticles suspended in algal growth media at low concentrations. Overall, while further development of the technique is needed, spICPMS yields important advantages over other techniques when sizing nanoparticles in environmentally relevant media.