Particle size distributions of silver nanoparticles at environmentally relevant conditions

Abstract

Silver nanoparticles (Ag NPs) are becoming increasingly popular as antimicrobial agents in consumer goods with consequent risk to environmental health from discharges. Environmentally relevant fate and transport investigations are limited but essential to gain understanding towards bioavailability and toxicology. In this study, monodisperse 15 nm citrate-stabilised Ag NPs were synthesised, characterised and then fractionated by flow field-flow fractionation (FlFFF) at environmentally relevant conditions (pH 5 or 8, presence of natural organic macromolecules (NOM) and presence of sodium or calcium). At low ionic strength, Ag NPs particle size increased as pH increased from 5 to 8. However, changing the ionic strength from 10⁻³ to 10⁻² M Na increased instability of the Ag NPs, and loss of peak at pH 5 but in the presence of humic substance (HS), a reduction in NP size was seen, most likely due to a reduction in the diffuse layer. The presence of Ca²⁺ ions, at the higher ionic strengths caused complete loss of the solution Ag NPs with or without HS, most likely due to aggregation. At the lower Ca²⁺ ionic strength the Ag NPs were still unstable, but again, in the presence of HS the NPs were largely dispersed. The presence of HS improved stability of Ag NPs under these conditions by forming a surface coating resulting in both steric and charge stabilisation. This work implies that Ag NPs could have long residence times in aquatic systems in the presence of HS potentially resulting in increased bioavailability.

Introduction

Nanoparticles (NPs) are generally defined as being between 1 and 100 nm and are currently of great interest, commercially and in industrial processes. Silver NPs (Ag NPs) are the main NP type in use currently [1] and are used in increasing quantities in industry as they have powerful anti-microbial properties [2] and show great potential in medicine and health-related areas. Silver toxicity to many organisms has long been known in the dissolved or ionic form [3], with concerns previously centering on discharges from photographic industry and mining [4]. More recently, significant concerns have been expressed about the potential risk of Ag NPs, due to the current and projected high exposure [1], [5] and their likely high hazard and toxicity in the environment [6]. Indeed, a number ecotoxicology studies of silver nanoparticles in algae bacteria, invertebrates, fish and humans in both in vivo and in vitro studies [7], [8], [9], [10], [11], [12], [13] have been conducted, with mechanisms of action including oxidative stress, binding to thiol groups on proteins, cell wall pitting, changes in membrane permeability and effects on the proton motive force and ATP generation [14], [15]. In addition, lowest observed effect concentrations (LOECs) range from a few ng L⁻¹ to tens of mg L⁻¹, with the differences due to a number of factors such as species differences and solution conditions. However, one of the major uncertainties is in accurate dose measurement, appropriate characterisation and the use of commercially available nanoparticles which are polydisperse, heterogeneous and not easily dispersible. In addition, most studies use high concentrations over short exposure periods which are not environmentally realistic.

This lack of data at environmentally realistic conditions, including relevant pH, ionic strength and accounting for the influence of natural organic macromolecules (NOM) such as humic substances (HS) is one of the major limitations on current NP ecotoxicological and environmental research. Research by the authors [16], [17] and others [18], [19], has shown that HS and solution conditions such as pH, affect surface properties and aggregation of fullerene, carbon nanotubes, gold and iron oxide NPs. Short term bacterial toxicity of fullerene NPs is reduced [20] and this coincides with our work on Ag NPs [21].

The objective of the work reported here is thus to produce well defined Ag NPs, to characterise them appropriately and to examine the change of size in relation to solution conditions and washing procedures. The detailed analysis performed here is a fundamental requirement for further toxicology and transport studies.