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Nanomaterial (NM) aggregation is a key process determining their environmental, fate behavior and effects. Nanomaterials are typically engineered to remain kinetically stable; however, in environmental and toxicological media, NMs are prone to aggregation. The aggregation kinetics of NM is typically quantified by measuring their attachment efficiency (α) and critical coagulation concentration (CCC). Several studies measured α and CCC for Ag NMs with a major focus on investigating the effects of ionic strength, ion valency and natural organic matter, with few studies investigating other environmental factors such as light and dissolved oxygen and none investigating the effect of particle size, buffer type and concentration, or surface coverage by capping agent. A survey of recent research articles reporting CCC values for Ag NMs reveals substantial variation in experimental conditions and particle stability with CCC values of monovalent and divalent counterions covering a wide range (ca. 25 to infinity for monovalent counterions and 1.6 to infinity for divalent counterions). Here, we rationalize the differences in the CCC values for Ag NMs based on the variability in the experimental conditions, which includes NM and medium physicochemical properties. Capping agents determines NM stability mechanism with citrate, sodium dodecyl sulfate (SDS), and alginate stabilizing NM by electrostatic mechanism; whereas polyvinylpyrrolidone (PVP), casein, dextrin, tween, branched polyethyleneimine (BPEI), and Gum Arabic stabilizing NMs by steric mechanisms. The CCC values for Ag NMs with different capping agents follow the order citrate∼alginate∼SDS

Environmental context. Studies of manufactured nanoparticles (NPs) in the environment have been performed almost exclusively at high NP concentrations. These data lead to misunderstandings related to NP fate and effects at relevant environmental concentrations, which are expected to be low. A better understanding of the concentration-dependent behaviour of NPs will improve our understanding of their fate and effects under environmentally realistic conditions.Abstract. This rapid communication highlights the importance of nanoparticle concentration in determining their environmental fate and behaviour. Notably, two fate processes have been considered: dissolution and aggregation. The decrease in nanoparticle concentration results in increased dissolution and decreased aggregate sizes, inferring higher potential for environmental transport of nanoparticles. The behaviour (e.g, dissolution, aggregation, disaggregation) and fate (e.g. mobility, fugacity, non-transient (sink) or transient source) of nanoparticles (NPs) in environmental and toxicological media have been investigated for over a decade, typically at high NP concentrations (e.g. milligram per litre range) which are not relevant to the environment, [1] resulting in some potentially misleading assumptions that (i) NP behaviour is dominated by aggregation and thus their fate is dominated by sedimentation and removal from the water column, or, in porous media, deposition and removal from the aqueous phase [2] ; (ii) NP dissolution is limited for many NPs and rarely are all NPs dissolved fully in environmental and biological media over relevant timescales [1] and (iii) many NPs therefore impart little or no toxic risk to pelagic organisms as a result of limited NP dissolution and NP removal by aggregation and sedimentation. [3] Several NP groups (e.g. Ag NPs, Cu NPs, Cd NPs, ZnO) may undergo dissolution and release ions with well known toxic effects. [4] These various issues complicate NP risk characterisation and are exacerbated by the general use of high NP concentrations in NP fate, behaviour and ecotoxicological studies. [2,5] Use of high NP concentrations has been motivated by poor detection limits of available analytical techniques (e.g. dynamic light scattering, laser Doppler electrophoresis, UV-Vis spectroscopy) together with enhanced likelihood of observing more pronounced changes and effects at high NP concentrations. [6] Furthermore, most published nanoecotoxicological data are acute exposure studies, which also drive the high concentration selection bias in order to generate measurable biological responses. Many NPs tested for toxicity to aquatic organisms have been non-toxic on acute time scales until they reach unrealistically high exposure concentrations. Clear predictive linkages between unrealistic high acute exposures and more realistic low chronic exposures have not been established for aquatic systems, and are likely to be further complicated by differing concentration-dependent behaviours of NPs.Despite these concerns, little attention ha...

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Engineered nanoparticle (NP) size and natural organic matter (NOM) composition play important roles in determining NP environmental behaviors. The aim of this work was to investigate how NP size and...

The bioavailability of dissolved Pt­(IV) and polyvinylpyrrolidone-coated platinum nanoparticles (PtNPs) of five different nominal hydrodynamic diameters (20, 30, 50, 75, and 95 nm) was characterized in laboratory experiments using the model freshwater snail Lymnaea stagnalis. Dissolved Pt­(IV) and all nanoparticle sizes were bioavailable to L. stagnalis. Platinum bioavailability, inferred from conditional uptake rate constants, was greater for nanoparticulate than dissolved forms and increased with increasing nanoparticle hydrodynamic diameter. The effect of natural organic matter (NOM) composition on PtNP bioavailability was evaluated using six NOM samples at two nanoparticle sizes (20 and 95 nm). NOM suppressed the bioavailability of 95 nm PtNPs in all cases, and DOM reduced sulfur content exhibited a positive correlation with 95 nm PtNP bioavailability. The bioavailability of 20 nm PtNPs was only suppressed by NOM with a low reduced sulfur content. The physiological elimination of Pt accumulated after dissolved Pt­(IV) exposure was slow and constant. In contrast, the elimination of Pt accumulated after PtNP exposures exhibited a triphasic pattern likely involving in vivo PtNP dissolution. This work highlights the importance of PtNP size and interfacial interactions with NOM on Pt bioavailability and suggests that in vivo PtNP transformations could yield unexpectedly higher adverse effects to organisms than dissolved exposure alone.

Many nanotoxicological studies have assessed the acute toxicity of nanoparticles (NPs) at high exposure concentrations. There is a gap in understanding NP chronic environmental effects at lower exposure concentrations. This study reports life-cycle chronic toxicity of sublethal exposures of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) relative to dissolved silver nitrate (AgNO) for the estuarine meiobenthic copepod, Amphiascus tenuiremis, over a range of environmentally relevant concentrations, i.e., 20, 30, 45, and 75 µg-Ag L. A concentration-dependent increase in mortality of larval nauplii and juvenile copepodites was observed. In both treatment types, significantly higher mortality was observed at 45 and 75 µg-Ag L than in controls. In AgNO exposures, fecundity declined sharply (1.8-7 fold) from 30 to 75 µg Ag L. In contrast, fecundity was not affected by PVP-AgNPs exposures. A Leslie matrix population-growth model predicted sharply 60-86% of decline in overall population sizes and individual life-stage numbers from 30-75 µg-Ag L as dissolved AgNO. In contrast, no population growth suppressions were predicted for any PVP-AgNPs exposures. Slower release of dissolved Ag from PVP-AgNPs and/or reduced Ag uptake in the nanoform may explain these sharp contrasts in copepod response.

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