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Extracting And Analysing Silver Nanoparticles From Natural Environments

Congress: 2015
Author(s): Azibar Rodriguez, Margaret Graham, Geert Cornelis, Helfrid Schulte-Herbruggen
Department of Analytical Chemistry, University of the Basque Country, Basque Country, Spain1, School of GeoSciences, The University of Edinburgh, UK2, Department of Chemistry and Molecular Biology, Gothenburgh University, Sweden3, School of Engineering, The University of Edinburgh, UK4

Keyword(s): Sub-theme 13: Non-conventional sources of water,
AbstractIntroduction

Nanoparticles are defined as having at least one dimension within 1-100 nm. These materials have unique properties due to their high reactive surface area (compared to bulk material) but difference in behaviour and potentially toxicity compared to dissolved ions of the same material. Due to their antimicrobial properties, silver nanoparticles (AgNP) are increasingly being used in a variety of personal care products (sun screens, medical creams), textiles (especially sports wear) as well as for water and waste water treatment. However, little is known about AgNP fate in aquatic environments, let alone their toxic effects, which makes it extremely difficult for regulation of such materials (Aschberger et al. 2011).

Some of the challenges in nanoparticle research include i) the low concentrations at which toxic effects may occur which require sensitive analysis techniques that measure sizes of particles, as well as concentrations ii) nanoparticles in natural water environments are likely to end up in the sediments, however methods to extract the nanoparticles without dissolving or transforming the particles, do not currently exist, making both qualitative and quantitative analysis of nanoparticles difficult in sediments.

To address these challenges, laboratory experiments have been carried out to 1) synthesise silver nanoparticles 2) test the stability of the nanoparticles subjected to different potential extraction solutions.

The aim was to identify a suitable extraction method of AgNPs from sediments which then allow analysis of the AgNPs using "single-particle ICP-MS", an analytical tool with the ability to detect the very low concentrations of AgNPs at relevant environmental conditions and at the same time measure size and distinguish dissolved versus nanoparticulate Ag.

Materials and methods

Four synthesis methods of AgNPs were tested, all using silver nitrate (0.001 M) as a pre-cursor. The overview of different reducing agents and stabilising agents used are given in Table 1, and the methods are described below (1-4).

Table 1. Chemicals used for AgNP synthesis in four methods tested.

1) Sodium borohydride was cooled in an ice bath under continuous stirring, until the solution reached 0°C. 10 ml of silver nitrate was added drop-wise, under continuous stirring.
2) Sodium borohydride and SDS (SDS:silver nitrate at a weight ratio = 10) were mixed and stirred for 30 minutes. 10 ml of silver nitrate was added drop-wise while stirring continuously. The solution was stirred for a further hour.
3) As in 2), but 50 ml silver nitrate added.
4) 50 ml silver nitrate was heated to boiling, while continuously stirred. At boil trisodium citrate was added drop-wise to the solution. As the solution turned yellow, the solution was removed and cooled to room temperature.

The stability of the synthesised nanoparticles were tested using UV-vis at 391 nm (Varian Cary 100 UV-Visible spectrophotometer) and a particles size analyser (90 Plus/BI-MAS Multi Angle Particle Sizing) for 15 days. It was concluded that the most stable particles were synthesised using method 2. These particles were selected for further testing in order to identify a suitable extraction method of AgNP from sediments.

The extraction solutions tested were: 1) de-ionised water, 2) dilute nitric acid (pH 2), dilute hydrochloric acid (pH 2), dilute sodium hydroxide (pH 12), sodium chloride solution (3.5% and 1.75% by weight), magnesium chloride (1M) and acetic acid (0.11M), again analysing the AgNPs using UV-vis and the particle size analyser over time. Results showed that the AgNPs showed greatest stability in sodium hydroxide, de-ionised water and acetic acid.

Conclusions

The results from the experiments suggested that de-ionised water and acetic acid are the most suitable extraction methods. These methods will be further tested on natural sediment samples, while methods using single-particle ICP-MS will be developed to analyse nanoparticles in concentrations likely to be present in the natural environment.

The research addresses a crucial gap in analysis of nanoparticles in water and sediments, where fundamental methods and data for the understanding of the consequences of nanoparticle pollution are urgently needed in order to understand the transport, fate and toxicology of nanoparticles in the natural water environment. This information is needed to inform policy makers for setting of guidelines and regulations . Aschberger, K., Micheletti, C., Sokull-Klüttgen, B., Christensen F.M. Analysis of currently available data for characterising the risk of engineered nanomaterials to the environment and human health – Lessons learned from four case studies. Environment International 37 (2011) 1143-1156.

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