Environ. Sci. Technol. 2010, 44, 2169–2175
Ion Release Kinetics and Particle Persistence in Aqueous Nano-Silver Colloids J I N G Y U L I U † A N D R O B E R T H . H U R T * ,‡,§ 1 Department of Chemistry, Division of Engineering, Institute for Molecular and Nanoscale Innovation, Brown University, Providence, Rhode Island
Received November 23, 2009. Revised manuscript received January 30, 2010. Accepted February 8, 2010.
Many important aspects of nanosilver behavior are influenced by the ionic activity associated with the particle suspension, including antibacterial potency, eukaryotic toxicity, environmental release, and particle persistence. The present study synthesizes pure, ion-free, citrate-stabilized nanosilver (nAg) colloids as model systems, and measures their time-dependent release of dissolved silver using centrifugal ultrafiltration and atomic absorption spectroscopy. Ion release is shown to be a cooperative oxidation process requiring both dissolved dioxygen and protons. It produces peroxide intermediates, and proceeds to complete reactive dissolution under some conditions. Ion release rates increase with temperature in the range 0-37 °C, and decrease with increasing pH or addition of humic or fulvic acids. Sea salts have only a minor effect on dissolved silver release. Silver nanoparticle surfaces can adsorb Ag+, so even simple colloids contain three forms of silver: Ag0 solids, free Ag+ or its complexes, and surface-adsorbed Ag+. Both thermodynamic analysis and kinetic measurements indicate that Ag0 nanoparticles will not be persistent in realistic environmental compartments containing dissolved oxygen. An empirical kinetic law is proposed that reproduces the observed effects of dissolution time, pH, humic/fulvic acid content, and temperature observed here in the low range of nanosilver concentration most relevant for the environment.
Introduction Of the 1015 nanotechnology-based consumer products or product lines available on the market in August 2009, products containing nanosilver (nAg) are the largest (25%) and fastest growing category (1). The proliferation of nAg is due to its extraordinary usefulness as a broad-spectrum antimicrobial agent (2, 3). Silver is generally believed to be of low toxicity to humans, though several cases have been reported of argyria (irreversible pigmentation of the skin) and/or argyrosis (irreversible pigmentation of eyes) after chronic ingestion of colloidal silver (4), and Mirsattari et al. (5) have reported a clinical fatality likely involving neurotoxicity following chronic ingestion of colloidal silver. A key area of concern for high-volume nAg production and use is the natural environment (6). When nAg-containing products are washed, abraded, or discarded, silver can enter * Corresponding author phone: 401-863-2685; fax: 863-9120; e-mail:
[email protected]. † Department of Chemistry. ‡ Division of Engineering. § Institute for Molecular and Nanoscale Innovation. 10.1021/es9035557
2010 American Chemical Society
Published on Web 02/22/2010
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the environment either as nanoparticles, nanoparticle aggregates, or soluble ions (7). Silver is reported to be highly toxic to some aquatic organisms (8) and as an antibiotic may also damage or alter beneficial microbial communities in the environment (9, 10). Silver is also known to bioaccumulate in phytoplankton and some marine invertebrates (11, 12), and has been shown to be toxic to zebrafish embryos at concentrations as low as 10 ng/L (13). Much of the data on silver ecotoxicity predates the nanomanufacturing era (11, 14, 15), and thus relates to Ag+ and its complexes rather than nAg directly. The understanding of nAg environmental effects is complicated by the coexistence of the particle and ionic forms (10, 16), which will likely exhibit different fate and transport characteristics, and may have independent or synergistic toxicity pathways. Dissolution may thus be a critical process that determines nAg affects in the environment and within organisms (17). Several recent studies have recognized the importance of particle/ion partitioning and have measured Ag+ concentrations associated with nAg colloids under specific experimental conditions (7, 8, 10, 18-20). There is need for a systematic study of the origin of Ag+ in nAg suspensions and of the kinetics and mechanisms of Ag+ release from nAg particle surfaces. A central question for regulation is whether nAg remains a particle in the environment, where it may pose unique new risks associated with its nanoscale dimension, or whether it dissolves to produce Ag+ and soluble complexes, whose environmental risks can be more reliably estimated from existing data and field experience on conventional silver forms. In general, it will be difficult to interpret, predict, or control the biological and environmental behavior of nAg without fundamental knowledge of particle/ion partitioning. Here we present quantitative thermodynamic and kinetic data on dissolved silver release from a citrate-stabilized nAg particle surfaces in well-defined aqueous media. The effects of dissolved oxygen, pH, temperature, ocean salts, and natural organic matter are investigated, and the data used to discuss release kinetics, mechanisms, and the persistence of the nanoparticle form.
Materials and Methods Preparation of nAg Stock Suspensions. Citrate stabilized nAg was synthesized by the method of Jana et al. (21) with minor modification. A 59.2 mL solution containing 0.6 mM trisodium citrate (Fisher) and 2 mM NaBH4 (Alfa Aesar) was prepared in deionized (DI) water (Millipore, 18.3 MΩ · cm) and stirred vigorously in an ice bath. As 0.8 mL of 15 mM AgClO4 (Alfa Aesar) was added into the mixture, the solution turned to yellow, indicating formation of nAg. Following 3 h of additional stirring at room temperature, soluble byproducts were removed by centrifugal ultrafiltration (Amicon Ultra15 3K, Millipore, MA) and DI water addition in two cycles, after which the nAg stock suspensions at 40 mg/L were stored at 4 °C for later use. Characterization of nAg. The morphology and size of nAg were determined by transmission electron microscopy (TEM) on a Philips EM420 at 120 kV. TEM samples were prepared by placing a drop of fresh nAg water suspension on copper grids with a continuous carbon film coating, followed by solvent evaporation at room temperature overnight. The surface charge and size distribution of nAg were evaluated by ζ-potential and dynamic light scattering (DLS), respectively, using a Zetasizer Nano ZS system (Malvern Instruments). The UV-vis spectra from 300-700 nm were obtained using a Shimadzu UV-1700 PharmaSpec, VOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Morphology, size distribution and stability of nAg samples used in this study. (A) TEM image after drying, (B) in situ size distribution by dynamic light scattering, (C) UV-vis absorption spectra of nAg suspended in DI water: (a) freshly synthesized; (b) stored at 4 °C for 5 months. The peak absorbance at 390 nm remains unchanged indicating size and shape stability. and the total silver concentration was quantified by graphite furnace atomic absorption (AA) spectrometry (Perkin Elmer 4100ZL GFAAs) after HNO3 digestion. Ion Release Rates. Ionic release was quantified by diluting nAg stock suspension with DI water to a desired starting concentration, and then tracking the appearance of dissolved silver (Agdis) by graphite-furnace AA analysis. Prior to AA analysis, nAg particles were removed using Amicon centrifugal ultrafilter devices containing porous cellulose membranes with a nominal particle size limit of 1-2 nm. Suspensions were centrifuged for 30 min at 3500 rpm (Allegra X-15R, Beckman Coulter Inc.). The clear filtrates showed no detectable plasmon resonance optical absorption peak, and at early times (before ion release) had very low concentrations of total silver (
