Article pubs.acs.org/ac
Chemometric Analytical Approach for the Cloud Point Extraction and Inductively Coupled Plasma Mass Spectrometric Determination of Zinc Oxide Nanoparticles in Water Samples Seyed Mohammad Majedi,† Hian Kee Lee,*,†,‡ and Barry C. Kelly§ †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore National University of Singapore Environmental Research Institute, T-Lab Building #02-01, 5A Engineering Drive 1, Singapore 117411, Singapore § Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore ‡
S Supporting Information *
ABSTRACT: Cloud point extraction (CPE) with inductively coupled plasma mass spectrometry (ICPMS) was applied to the analysis of zinc oxide nanoparticles (ZnO NPs, mean diameter ∼40 nm) in water and wastewater samples. Five CPE factors, surfactant (Triton X-114 (TX-114)) concentration, pH, ionic strength, incubation temperature, and incubation time, were investigated and optimized by orthogonal array design (OAD). A three-level OAD, OA27 (313) matrix was employed in which the effects of the factors and their contributions to the extraction efficiency were quantitatively assessed by the analysis of variance (ANOVA). Based on the analysis, the best extraction efficiency (87.3%) was obtained at 0.25% (w/v) of TX-114, pH = 10, salt content of 15 mM NaCl, incubation temperature of 45 °C, and incubation time of 30 min. The results showed that surfactant concentration, pH, incubation time, and ionic strength exert significant effects on the extraction efficiency. Preconcentration factors of 62 and 220 were obtained with 0.25 and 0.05% (w/v) TX-114, respectively. The relative recoveries of ZnO NPs from different environmental waters were in the range 64−123% at 0.5−100 μg/L spiked levels. The ZnO NPs extracted into the TX-114-rich phase were characterized by transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDS) and UV−visible spectrometry. Based on the results, no significant changes in size and shape of NPs were observed compared to those in the water before extraction. The extracted ZnO NPs were determined after microwave digestion by ICPMS. A detection limit of 0.05 μg/L was achieved for ZnO NPs. The optimized conditions were successfully applied to the analysis of ZnO NPs in water samples. 0.058 μg/L and 0.74 μg/L, respectively. On the basis of ecotoxicological studies, the risk quotient (RQ) (which is defined as the ratio of PEC to PNEC (predicted no effect concentration)) were 0.32 and 7.7, respectively. The PEC value for STP sludge was much higher and reached 64.7 mg/kg. The behavior of ENMs is significantly different from their bulk forms as a result of their particular physical and chemical properties.10 Several studies of the toxicity of ZnO NPs and the corresponding released zinc ion to various living organisms, such as zebrafish,11 algae,12 invertebrate,13 phytoplankton,14 and bacteria,15 have been reported, and ZnO NPs have been described as some of the most toxic nanomaterials.11,15 The half maximal effective concentration (EC50) of ZnO NPs was
N
anosized materials have attracted attention considerably in the past few years. Their increasing applicability in different areas due to their high surface area and reactivity has given rise to the vast production of engineered nanomaterials (ENMs), which may reach 58 000 tonnes in the next 10 years.1 Among ENMs, zinc oxide nanoparticles (ZnO NPs) have found applications in pigments, sensors, cosmetics, sunscreens, optoelectronic devices, and as coatings and catalysts.2−5 They have also been used in paints, textiles, and plastics on account of their antibacterial and antifungal properties.6−8 In view of their inclusion in a variety of industrial and consumer products, the occurrence of ZnO NPs in the environment is worthy of attention. Their predicted environmental concentrations (PECs) have been modeled in different regions of the world and reported in various environmental compartments.9 The maximum PEC values for surface water and sewage treatment plant (STP) effluent were found to be © 2012 American Chemical Society
Received: March 27, 2012 Accepted: June 27, 2012 Published: June 27, 2012 6546
dx.doi.org/10.1021/ac300833t | Anal. Chem. 2012, 84, 6546−6552
Analytical Chemistry
Article
reported to be in the range from tens of μg/L to several mg/ L.16,17 The colloidal stability and solubility of ENMs contribute significantly in their ecotoxicity and environmental fate and transport. Natural organic matters (NOMs), pH, type of capping agent and surfactant, size and shape of nanoparticle, ionic strength, and type of electrolyte all influence the stability and dissolution of ZnO NPs in water.18−21 Several microscopic, spectroscopic, and separation techniques have been employed for the detection of ENMs such as gold, silver, zinc oxide, and quantum dot nanoparticles.22−27 However, they are insufficient to monitor actual environmental concentrations of ENMs.28 There have been few quantitative methods introduced for the determination of ENMs.29−33 While several published papers have focused on the toxicity and bioavailability (in particular, solubility) assessment of ZnO NPs, data on the determination of ZnO NPs are scarce.23 Techniques for the separation of ZnO NPs, such as solvent extraction34,35 and dialysis,36 are focused on the purification of synthetic ZnO NPs and the elimination of impurities. Thus, these approaches are not directly involved with the quantitative analysis of these materials. Since ZnO NPs exhibit high environmental occurrence and impact, an efficient analytical approach for the speciation, characterization, and determination of ZnO NPs is desirable. Cloud point extraction (CPE) was earlier described by Watanabe et al.37 and further developed by Hinze et al.38 CPE using Triton X-114 (TX-114) as surfactant for analysis of trace nanoparticles was first introduced by Liu et al. for the thermo-reversible separation of various nanoparticles and, in particular, gold nanoparticles in the aqueous phase.39 It was developed for the preconcentration and speciation of silver nanoparticles in environmental waters40 and antibacterial products,41 and removal of copper oxide nanoparticles from wastewater.42 In these studies, the TX-114-rich phase was separated and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), and UV− visible spectroscopy, and determined by inductively coupled plasma mass spectrometry (ICPMS) after microwave digestion. High extraction efficiency, low detection limit, good reproducibility, and enrichment factor were reported for the determination of silver nanoparticles. In this work, we report the applicability of CPE for the separation and speciation analysis of ZnO NPs in environmental waters. Orthogonal array design (OAD) as an efficient statistical method is used to optimize pertinent CPE conditions and two-variable interaction parameters and to evaluate their simultaneous influence on the extraction efficiency. OAD permits fewer numbers of experiments to be conducted compared to univariate approaches. A three-level OA27 (313) OAD matrix is employed to optimize the CPE conditions, and contributions of CPE factors and two-variable interactions are estimated by the analysis of variance (ANOVA). With the aid of this chemometric approach, the effects of experimental conditions on the incorporation of ZnO NPs with surfactant micelles are described.
um salt), and humic acid (sodium salt, dissolved organic carbon (DOC) ∼39% (w/w)) were obtained from Sigma-Aldrich (St. Louis, MO, U.S.A.). Nitric acid (65% w/v) and zinc standard solution (1000 mg/L) were bought from Merck (Darmstadt, Germany). Hydrogen peroxide (30−35% w/v) was obtained from Kanto (Tokyo, Japan). Ultrapure water (18.2 MΩ) was produced with a PURELAB Option-Q water purification system (ELGA LabWater, Marlow, U.K.). Stock suspensions of ZnO NPs (100 mg/L) were prepared by dispersion of commercial ZnO NPs in ultrapure water. The stock suspension was stabilized by 10 mg/L humic acid (DOC ∼3.9 mg C/L) at pH = 7.0, and sonicated for 15 min at 0.27 kW of power in an ultrasonic bath (50 kHz, SoniClean, Thebarton, SA, Australia) to avoid aggregation of ZnO NPs in the water.20 The exact ZnO NP concentration in the stock suspension was measured as follows: the suspension was centrifuged at 14000 rpm for 30 min,20,41 and the sedimented phase was then subjected to microwave digestion and analyzed by ICPMS. The supernatant was imaged using TEM to confirm the absence of nanoparticles. It was analyzed directly by ICPMS to measure the dissolved portion.43 Regarding the average size of the commercial ZnO NPs (∼40 nm), the total released zinc ion did not exceed 2% of the initial concentration upon dispersion of ZnO NPs in water. Thus, the stock suspension was prepared freshly. The working suspensions of ZnO NPs were prepared by diluting the stock suspension with ultrapure water and determined by ICPMS routinely. It is worth mentioning that since ZnO NPs may be adsorbed on the vessels made of materials such as Teflon and plastic,27 borosilicate glass containers were used for all experiments. The adsorption of ZnO NPs on the vessels was negligible (
