Gaining a Critical Mass: A Dose Metric Conversion Case Study Using Silver Nanoparticles

Abstract

Mass concentration is the standard convention to express exposure in ecotoxicology for dissolved substances. However, nanotoxicology has challenged the suitability of the mass concentration dose metric. Alternative metrics often discussed in the literature include particle number, surface area, and ion release (kinetics, equilibrium). It is unlikely that any single metric is universally applicable to all types of nanoparticles. However, determining the optimal metric for a specific type of nanoparticle requires novel studies to generate supportive data and employ methods to compensate for current analytical capability gaps. This investigation generated acute toxicity data for two standard species (Ceriodaphnia dubia, Pimephales promelas) exposed to five sizes (10, 20, 30, 60, 100 nm) of monodispersed citrate- and polyvinylpyrrolidone-coated silver nanoparticles. Particles were sized by various techniques to populate available models for expressing the particle number, surface area, and dissolved fraction. Results indicate that the acute toxicity of the tested silver nanoparticles is best expressed by ion release, and is relatable to total exposed surface area. Particle number was not relatable to the observed acute silver nanoparticle effects.

Assessing nanomaterial exposures in aquatic ecotoxicological testing: Framework and case studies based on dispersion and dissolution

“…Since AgNPs may release toxic ions into test media (Kennedy et al, 2015b), two comparative bioassays were conducted; one that included the settled particles and another that adapted elutriate methods Example durations are based on typical daily water renewals used in bioassay test methods. "Dispersible" is functionally and conservatively defined as !1% of the original concentration.…”

“…ENMs that agglomerate and settle to the bottom of test vessels can potentially entrain test organisms (Arndt et al, 2013;Petersen et al, 2015). ENM settling may also result in a predominance of smaller suspended particles and released ions or other constituent substances in the water column (Hull et al, 2009, Kennedy et al, 2015b, suggesting a need to test three different fractions of the exposure; the entire ENM exposure, the suspended fraction and the settled fraction (Potthoff et al, 2015). The focus of this study was on basic water-column assays, which use test organisms that are highly sensitive to ENMs (Coll et al, 2016;Garner et al, 2015).…”

“…Zeta potential was determined from electrophoretic mobility measurements and PZC determined using a BI-ZTU auto-titrator (90 Plus/BI-MAS, Brookhaven Instruments, Holtsville, NY). Dissolved concentrations were determined by 60 min of ultracentrifugation (Beckman Coulter, Inc., 355603, Brea, CA) at 100,000Â g, as previously described (Poda et al, 2011) and validated (Kennedy et al, 2015b).…”

“…Key testing issues derive from the particulate and fibrous nature of ENMs, and their tendency to agglomerate and settle out of suspension in bioassay media, resulting in significant exposure variation and thus complicated dosimetry. Some ENMs (e.g., AgNPs, CuO, ZnO) dissolve in assay media (Griffitt et al, 2008;Kennedy et al, 2015b) resulting in variable mixtures of particles and dissolved substances. These behaviors present significant challenges for quantifying exposure based on traditional total mass dose metrics, and complicate the use of alternative exposure metrics (e.g.…”

“…These behaviors present significant challenges for quantifying exposure based on traditional total mass dose metrics, and complicate the use of alternative exposure metrics (e.g. surface area, particle or agglomerate number) that may be used to quantify ENM exposure (Hull et al, 2012;Kennedy et al, 2015b). The absence of specific guidance for aquatic ecotoxicity testing of ENMs complicates risk management and assessment for ENM and greatly reduces the value of test results for predicting hazard and risk (Petersen et al, 2015).…”