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Ligand-less surfactant assisted emulsification microextraction and total reflection X-ray fluorescence analysis for ionic gold traces quantification in aqueous samples and extracts containing gold nanoparticles Zekeriyya Bahadir, Murat Yazar, and Eva Margui Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04717 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018
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Analytical Chemistry
Ligand-less surfactant assisted emulsification microextraction and total reflection X-ray fluorescence analysis for ionic gold traces quantification in aqueous samples and extracts containing gold nanoparticles Zekeriyya Bahadir†, Murat Yazar‡, Eva Marguíǁ* † Department of Chemistry, Faculty of Arts and Sciences, Giresun University, 28100, Giresun, Turkey ‡ Department of Chemistry, Faculty of Sciences, Karadeniz Technical University, 61080, Trabzon, Turkey ǁǁ Department of Chemistry, University of Girona, C/ Maria Aurèlia Capmany 69, 17003, Girona, Spain. ABSTRACT: Due to the current use of gold nanoparticles (AuNPs) in many fields and their potential dissolution/transformation into ionic gold (Au+3), there is an increasing interest in methods enabling the discrimination of Au+3 from AuNPs in environmental samples. In this contribution, the combination of a novel ligand-less surfactant assisted emulsification microextraction procedure (LL-SAEME) with total reflection X-ray fluorescence spectrometry (TXRF) is proposed for the isolation and preconcentration of Au+3 in aqueous extracts containing AuNPs. The method is fast, simple and involves low operating costs and low consumption of reagents in comparison with other spectroscopic methods. It is based on the formation of a gold hydrophobic compound with the cationic surfactant cetyltrimethylammonium bromide (CTAB) which is extracted in few microliters of 1,2-dichloroethane. After shaking the solution by hand for 5 seconds, the mixture is centrifuged for 3 minutes at 2000 rpm and 5 µL of the organic phase containing the gold ions are deposited on a quartz reflector to carry out the TXRF analysis. Using this approach, the limit of detection for gold was 0.05 µg/L and a good linearity (R2>0.99) was assessed in the range of 1-500 µg/L. Moreover, no matrix effects were observed when ionic gold was extracted from different types of water such as river, mineral and tap waters as well as in synthetic aqueous solutions containing other ions, AuNPs and dissolved organic matter. As study case, the developed LLSAEME-TXRF method was applied to monitor AuNPs stability in soils in laboratory controlled experiments by means of Au+3 monitoring overtime.
INTRODUCTION. Because of its chemical and physical features, gold is widely used in most electronic devices, catalytic converters, electricity, metallurgy, health, medicine and many more applications1-3. Moreover, with the development of nanomedicine in the past decade, nano-scaled materials including gold nanoparticles (AuNPs) have widely applied in biomedical imaging, cancer therapy and diagnostics, and biological and chemical sensing4-5. Food supplements and cosmetics labelled to cotain nanogold are also commercially available6. Therefore, as a result of the rise up of AuNPs usage, a new concern has emerged about their potential occurrence in the environment7. Although the toxicity mechanism of metal nanoparticles has not been clearly elucidated, recent studies pointed that their potential dissolution/transformation into the corresponding metal ions can have an important role6. For this reason there is an increasing interest in analytical methods enabling the quantification of trace amounts of ionic gold (Au+3) as well as its discrimination from AuNPs in aqueous solutions8-9. Commonly used techniques for Au determination include inductively coupled plasma-mass spectrometry (ICP-MS)10 and graphite furnace atomic absorption spectrometry (ETAAS)11. Although these techniques present suitable sensitivity for Au determination at trace levels, the discrimination between Au+3 and AuNPs species is not possible. In the case of ICP-MS, selective quantification of
AuNPs can be only performed if working under single-particle ICP-MS conditions (SP-ICPMS)12. Another approach to isolate gold ions in aqueous solutions is the application of an extraction procedure before the spectroscopic analysis. Several methods have been developed for such purpose including solid phase extraction as well as different types of liquid-phase microextraction procedures13-15. However, most of these extraction methods are used to isolate ionic gold from the aqueous matrix and to improve limits of detection when using less sensitive techniques such as flame atomic absorption spectrometry (FAAS) but none of them is focus on the possibility to determine trace amounts of ionic gold in presence of AuNPs16-18. In the present study, a novel sensitive and selective ligandless surfactant assisted emulsification microextraction (LLSAEME) method for the isolation and preconcentration of Au+3 in water samples and aqueous extracts has been developed. For the first time, the cationic surfactant cetyltrimethylammonium bromide (CTAB) is used in a microextraction procedure for the selective separation of trace amount of gold ions in the presence of AuNPs. After the extraction procedure, since a small volume of an organic solvent is got, a microanalytical technique is required. In this sense, total reflection X-ray fluorescence spectrometry (TXRF) has been selected as analytical technique. TXRF is a commonly used technique for multielement analysis of liquid
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and solid microsamples 19-21. To work under conditions of total reflection, samples must be prepared as thin films22. For that, 5–50 µL of liquid sample are deposited on a reflective support with a subsequent drying of the resulting drop. The developed LL-SAEME-TXRF method involves low operating costs and low consumption of reagents in comparison with other spectroscopic methods and lies within the green analytical chemistry rules. Factors affecting the extraction system and the subsequent TXRF analysis have been investigated. Analytical figures of merit have been evaluated and the feasibility of the proposed methodology has been assessed by analyzing different Au-spiked water samples. As study case, the developed LL-SAEME-TXRF method has been also applied to monitor AuNPs stability in soils in laboratory controlled experiments by means of Au+3 monitoring overtime. EXPERIMENTAL REAGENTS AND SOLUTIONS. Gold stock solutions of 1000 mg L-1 were prepared by dissolving an appropriate amount of HAuCl4 (Merk, Darmstadt, Germany). Commercial solutions of gold nanoparticles (AuNPs, 70 nm, 50 mg L-1) stabilized with sodium citrate were supplied by NanoComposix (San Diego, USA). Organic solvents such as chloroform (CHCl3), dichloromethane (CH2Cl2), 1,2dichloroethane (C2H4Cl2) and carbon tetrachloride (CCl4) were also purchased from Merck (Darmstadt, Germany). Stock solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB, GmbH, Steinheim, Germany) were prepared freshly by dissolving an appropriate amount of the reagent in pure water. All solutions were prepared using analytical reagent grade chemicals and distilled water, purified through a MilliQ Plussystem (Millipore). WATER SAMPLES. In this study, different types of natural waters as well as two waste water effluents from municipal and industrial treatment plants were sampled and used in Au extraction experiments. (Additional information in Table S-1). All samples were collected in different areas belonging to Girona region (Catalonia, in the northeast of Spain). Water samples were used after filtration using a 0.45 µm pore size cellulose acetate membrane filter. AQUEOUS SOIL EXTRACTS. The introduction of AuNPs in the environment is inevitably due to the increasing use of this type of nanomaterials in many different applications7. The stability, mobility and fate of these nanomaterials can be affected for instance by solid components present in soils23. To expand the knowledge about NPs and soil interactions, lab-controlled experiments are usually carried out24. For that, 0.5 g of soil (additional information about its chemical composition can be found in Table S-2) were putted in contact with 20 mL of an aqueous solution spiked with 500 µg L-1 of AuNPs and mixed at room temperature overnight using a rotary agitator set at 35 rpm. Then, loaded samples were dried into the oven for 24h at 60ºC and were extracted with 10 mL of ultrapure water for 24h (according to the German Standard Method DIN 38414-S4) to evaluate leaching and potential mobility of AuNPs. The leaching process was repeated with other soil aliquots after 3 days, 1 week and 2 weeks. The presence of Au+3 in the extracts was determined in all cases using the developed LL-
SAEME-TXRF method, after a previous filtration of the soil extract using acetate cellulose membrane filters with a pore size of 0.45 µm. LL-SAEME AND TXRF ANALYSIS. As stated, the extracant used in this work is the cationic surface-active agent (CTAB). In solutions of high chloride concentration, Au is present in the form of anionic chlorocomplex (AuCl4-) which can interact with CTAB forming the corresponding ion pair which is extracted in a few microliters of an organic solvent. [AuCl4]-(aq) + CTAB+(org) [AuCl4]- CTAB+(aq) [AuCl4]- CTAB+(aq) + nS(org) [AuCl4]- CTAB+xnS(org) Where aq: aqueous phase, org: organic phase and S: organic solvent The extraction procedure was as follows: 10 mL of aqueous samples were acidified to pH=1.00 (using the suitable amount of hydrochloric acid) and then 100 µL of a 1,2-dichloroethane solution including CTAB cationic agent at a final concentration of 0.005mM were added. After manual shacking of the mixture for 5 s, a cloudy white suspension appeared. Phase separation was performed by centrifugation of the mixture at 2000rpm for 3 min. A small volume (20 µL) of organic phase, containing the AuCl4-CTAB species was placed at the bottom of the tube. Then, 5 µL of the preconcentrated sample were deposited on a quartz reflector and dried using an IR lamp. TXRF analysis were carried out using a commercial benchtop TXRF spectrometer S2 PICOFOX (BrukerNano, Germany) with a low power tungsten X-ray tube (50kV, 1mA) and a Silicon Drift Detector (SSD, Resolution
