Hydrophilic Polymer Monolithic Capillary Microextraction Online

9 Jan 2015 - ... in the next 10 years.4,5 Research shows that many properties including shape, ... show that polymer monolithic CME is also useful for...
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Hydrophilic Polymer Monolithic Capillary Microextraction Online Coupled to ICPMS for the Determination of Carboxyl GroupContaining Gold Nanoparticles in Environmental Waters Lin Zhang, Beibei Chen, Man He, Xiaolan Liu, and Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan, Hubei 430072, P. R. China S Supporting Information *

ABSTRACT: In this study, the hydrophilic polymer monolithic capillary (poly(acrylamide-vinylpyridine-methylene bis(acrylamide)), poly(AA-VP-Bis)) was prepared for the separation and enrichment of carboxyl group-containing gold nanoparticles (Au NPs) from environmental waters followed by online ICPMS determination. The extraction mechanism of the prepared poly(AA-VP-Bis) monolithic capillary for Au NPs is based on the static electrical and hydrogen bond interactions between the carboxyl group on the surface of Au NPs and pyridine/amide groups on the surface of the monolith. Under the optimal conditions, a detection limit of 24.2 fmol L−1 and a sample throughput of 6 h−1 were achieved for 3 nm citrate stabilized Au NPs, and the original morphology of the Au NPs could be maintained during the extraction process. The developed method was successfully applied for the analysis of carboxyl groupcontaining Au NPs in environmental water samples, such as tap water, the Yangtze River water, and the East Lake water, with recoveries in the range of 77−103%. Compared with the reported approaches for analysis of Au NPs, this method is an online strategy for carboxyl group-containing Au NPs determination and has the merits of low detection limit, small sample consumption, fast extraction/desorption kinetics, wide linear range, high selectivity, and high throughput.

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anoparticles (NPs), due to their special properties,1 have been widely used in the fields of industry, medicine, and materials. Gold nanoparticles (Au NPs), in particular, have been extensively used in light catalysis, optical microscope probes, biosensors, targeted drug delivery, and so on due to the virtues of higher electron density, dielectric properties, and the catalytic effect as well as good biocompatibility.2,3 However, the extensive use of NPs resulting in their increasing release into the environment will cause harmful risks to the environment and organisms. It is predicted that the concentration of Au NPs and silver nanoparticles (Ag NPs) in environmental water could be as high as 140 and 80 ng L−1, respectively, in the next 10 years.4,5 Research shows that many properties including shape, size, and different coatings of the NPs would have an impact on the adverse effects and even toxicity of NPs in the organisms.6−8 Therefore, the detection of trace Au NPs in environmental water samples is significant for environmental pollution monitoring and biological safety assessment. At present, the detection methods for NPs include atomic optical/mass spectrometry, electrochemical methods, UV spectroscopy, and Raman spectroscopy. 9 Among them, elemental specific inductively coupled plasma mass spectrometry (ICPMS) has the advantages of low detection limit and wide linear range and not being dependent on the different species. ICPMS with the Babington nebulizer is able to directly analyze particles with a diameter less than 2 μm.10 For Au NPs © 2015 American Chemical Society

with a size less than 20 nm, there was no significant difference between the results obtained by direct suspension sampling and that obtained by solution sampling after digestion.11 ICPMS with direct suspension sampling mode for NPs determination eliminates possible sample loss and pollution which would occur during the digestion process. However, we need to keep in mind that the prerequisite for this ICPMS direct suspension sampling is that NPs should be well dispersed in the sample solution without any aggregation. Additionally, the inherent properties of NPs, their low concentration levels, and the very complex sample matrix make it a challenging task to determine NPs in real-world samples without sample pretreatment. Traditional sample pretreatment methods such as centrifugation, filtration, and dialysis have the drawbacks of being time-consuming, having a troublesome operation, and easily causing aggregation of NPs. To solve these problems, some new methods such as chemical derivatization,12 microliquid liquid extraction (MLLE),13 cloud point extraction (CPE),14−18 solid phase extraction (SPE),19,20 and magnetic solid phase extraction (MSPE)21 have been proposed in recent years. By means of oxidation of Ag NPs into Ag+, an indirect determination of Ag NPs can be Received: October 2, 2014 Accepted: January 9, 2015 Published: January 9, 2015 1789

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Supporting Information Table S1. An FIA-3110 flow injection system (Jitian Instruments Co. Ltd., Beijing, China) composed of two peristaltic pumps and an eight-channel multifunctional valve was used for online preconcentration of Au NPs. Characterization techniques employed in this work include an X-650 scanning electron microscope (SEM, Hitachi, Tokyo, Japan), JEM-2010 transmission electron microscope (TEM, Tokyo, Japan), UV−vis spectroscopy (Shimadzu, Japan), and energy dispersive X-ray analyzer (EDX, Hitachi, Tokyo, Japan). A glass capillary (1 mm i.d.) was purchased from Instrument Plant of West China Medical University (Chengdu, China). A Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, China) was used for the adjustment of the pH values. Standard Solutions and Reagents. HAuCl4·4H2O (Sigma-Aldrich, MO, USA) was used for the preparation of Au NPs and a free Au ions standard solution. Sodium citrate (Sigma-Aldrich, MO, USA) and NaBH4 (Sinopharm, Shanghai, China) were used to prepare Au NPs. The synthesis of Au NPs with different particle sizes is detailed in the Supporting Information. Cysteamine hydrochloride was purchased from Sigma (MO, USA). Mercaptosuccinic acid (MSA) and 11-mercaptoundecanoic acid (MUA) were from Aladdin Company (Shanghai, China), and humic acid was from Shanghai Reagent Company (Shanghai, China). The standard working solution and the mixed solutions of metal ions were obtained by stepwise diluting the stock solution. Methacryloxypropyl trimethoxysilane (WD-70) was purchased from Chemical Plant of Wuhan University (Wuhan, China). Acrylamide (AA, Amresco, NJ, USA), vinylpyridine (VP, Aladdin, Shanghai, China), and methylene bis(acrylamide) (Bis, Sigma-Aldrich, MO, USA) were chosen as monomers and cross-linker, respectively. Dimethyl sulfoxide (DMSO, Sinopharm, Shanghai, China) and dodecanol (Aladdin, Shanghai, China) were used as porogens. All other reagents had an analytical pure level or better. Ultra pure water (Milli-Q Element, 18.2 MΩ·cm, Molsheim, France) was used for all experiments. Preparation of Poly(AA-VP-Bis) Monolithic Capillary. Poly(AA-VP-Bis) was prepared by in situ polymerization methods.26 AA (110 mg), Bis (120 mg), VP (0.08 mL), dodecanol (0.6 mL), and azobis(isobutyronitrile) (AIBN, 1% of the total mass of monomers and cross-linking agent) were dissolved in DMSO (0.75 mL), and the resulting solution was sonicated for 10 min to obtain a homogeneous solution. The obtained prepolymerization solution was filled in the activated capillary, and the polymerization was allowed to proceed at 60 °C for 16 h with both ends of the capillary being sealed. After that, the capillary was washed with methanol. Finally, the monolithic capillary was cut into 2.5 cm long segments and used for the subsequent experiments. Experimental Methods. The experimental setup of CMEICPMS is illustrated in Figure 1, and the specific experimental process is shown in Table 1. Briefly, 1 mL of Au NPs solution was passed through the poly(AA-VP-Bis) monolithic capillary with the flow injection system (Figure 1A). Afterward, the Au NPs retained on the monolithic column were online eluted with 0.1 mL of 4% cysteamine for further ICPMS detection (Figure 1B). After extraction, the poly(AA-VP-Bis) monolithic capillary was sequentially washed with eluent (for avoidance of any memory effect) and water (for re-equilibrium) for the next run. A typical online CME-ICPMS spectrum is shown in Figure

achieved.12,22 This chemical derivatization strategy is simple and has a low cost, but it could not keep the morphology of NPs and had a relatively high limit of detection (LOD, 14 μg L−1 12 and 25 nmol L−1 22). Lopez-Lorente et al.13 developed a method of MLLE with UV/Raman spectra detection for the determination of Au NPs with an LOD of 1.17 fmol L−1 and acceptable precision. Liu’s group14,15 introduced the CPE method into the separation and enrichment of NPs. Ag NPs could be sedimented in the rich-surfactant phase easily and rapidly to achieve their preconcentration. This strategy has the merits of high enrichment factor and strong anti-interference ability; however, acid digestion was used prior to ICPMS determination probably due to the relatively high viscosity of the rich-surfactant phase. Later on, the CPE method was extended to be utilized for the analysis of zinc oxide, Au NPs, and Ag NPs in environmental water.16−18 Recently, Li et al.19 developed a novel method using C-18 silica particles as SPE materials for preconcentration of Au NPs in environmental water. Afterward, they established a method using cation exchange resin as SPE packing material to enrich Au NPs, Ag NPs, and Pd NPs in environmental water.20 These two methods could maintain the morphology of NPs after SPE, but they suffered from long desorption time (more than 3 h). In our recent work, MSPE with self-prepared Al3+ immobilized Fe3O4@SiO2@iminodiacetic acid (IDA) nanoparticles as the adsorbent combined with ICPMS was developed for the determination of Au NPs and Au ions in environmental waters with high enrichment factors and no acid digestion required.21 Although these established methods are all successfully applied to the NPs analysis in real-world samples, they are all off-line methods. Therefore, there is a high demand to develop online and high throughput sample preparation methods for NPs analysis. The polymer monolithic column, as the SPE materials, has the virtues of fast and efficient mass transfer, simple preparation, and good biocompatibility. Accordingly, polymer monolithic capillary microextraction (CME) has the advantages of low sample consumption, short analytical time, and easy online operation and, thus, has been widely applied in the separation and enrichment of inorganic ions, organic small molecules, and biological macromolecules.23 Recent studies show that polymer monolithic CME is also useful for online screening of nanodrugs24 and separation of biological nanoparticles, such as viruses, phages, and cells.25 The above studies give us a hint that polymer monolithic CME has potential application in fast and online preconcentration of NPs. Therefore, the purpose of this work is to establish a new method which integrates polymer monolithic CME and ICPMS for online fast and sensitive analysis of NPs. For this purpose, a hydrophilic methyl acrylamide polymer (poly(acrylamidevinylpyridine-methylene bis(acrylamide)), poly(AA-VP-Bis)) monolithic capillary was prepared. With carboxyl groupcontaining Au NPs as model NPs, the extraction behavior of carboxyl group-containing Au NPs and free Au ions on the prepared poly(AA-VP-Bis) monolith was studied. For validation, the developed method of polymer monolithic CMEICPMS was applied for the analysis of carboxyl groupcontaining Au NPs in environmental water samples.

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EXPERIMENTAL SECTION Instrumentations. Agilent 7500a ICPMS (Agilent, Tokyo, Japan) was used for the determination of Au NPs and free Au ions, and the optimal operation conditions are summarized in 1790

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Factory, Tianjin, China) for removing the particulate matter, adjusted to pH 5.5 with 0.01 mol L−1 HNO3 and 0.01 mol L−1 NH3·H2O, and subjected to poly(AA-VP-Bis) monolithic CME-ICPMS determination. Blank samples were treated by the same procedures with the ultra pure water.

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RESULTS AND DISCUSSION Feasibility of Extraction of Carboxyl Group-Containing Au NPs by Poly(AA-VP-Bis) Monolithic CME. With the use of bare 3 nm Au NPs (stable in citrate aqueous solution for at least 3 weeks, stability study shown in Supporting Information, Figure S1) as the target, the feasibility of poly(AA-VP-Bis) monolithic CME for the extraction of Au NPs was studied. Figure 2A displays the adsorption behavior of

Figure 1. Experimental setup of online CME-ICPMS (A, sample loading; B, sample injection) and a typical online CME-ICPMS spectrum (C) (Au NPs 0.1 μg L−1).

Table 1. Operating Procedures for Online CME-ICPMS Determination of Au NPs step

pump

flow rate (mL min−1)

1

0.25

1

2

time (min) 4

2

0.05

1 2

0.05 0

2

0.05

2

0.2

2

1 3 2 4

1 2

purpose sample loading washing/ eluent elution waiting washing/ eluent washing/ H2O washing/ H2O

valve position

ICPMS measurement

B

not acquire data

A

acquire data

B

not acquire data

B

not acquire data

Figure 2. Feasibility of poly(AA-VP-Bis) monolithic capillary for the extraction of carboxyl group-containing Au NPs. (A) Effect of pH on the adsorption percentage of Au NPs and free Au ions (concentration of Au NPs and free Au ions: 20 μg L−1 as Au; sample volume: 1 mL). (B) Effect of different eluents on the recovery of Au NPs (1, 2% glycine solution (pH = 7); 2, 2% MSA (pH = 7); 3, 2% cysteine solution (pH = 7); 4, 2% cysteamine (pH = 7); concentration of Au NPs: 20 μg L−1 as Au; sample volume: 1 mL; pH = 5.5; sample flow rate: 0.25 mL min−1).

1C. The signals integrated from 10 to 50 s were used for data processing. The blank sample solution and the series of standard solution were subjected to the same experimental procedure of online CME-ICPMS. All the experimental work was carried out in triplicate, and the average result was presented. Sample Preparation. Tap water, river water, and lake water were collected from the laboratory (Wuhan University, Wuhan, China), the Yangtze River (Wuhan, China), and the East Lake (Wuhan, China), respectively. Immediately after sampling, the water samples were filtered through a 0.45 μm cellulose acetate membrane (Tianjin Jinteng Instrument

Au NPs and free Au ions on the poly(AA-VP-Bis) monolithic capillary when the pH varies from 2 to 8. As can be seen, Au NPs could be quantitatively adsorbed on the monolithic column in the entire studied pH range, while free Au ions were barely adsorbed on the poly(AA-VP-Bis) monolithic column when the pH varied from 2 to 6, and then, when increasing the pH from 6 to 8, its adsorption percentage increased gradually probably due to the hydrolysis of free Au ions in the alkaline 1791

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Figure 3. Characterization of poly(AA-VP-Bis) monolithic capillary. (A) SEM images (the cross section magnified at 80× (i) and 2500× (ii)) and photographs (before (iii) and after (iv) adsorption) of the polymer monolithic capillary. (B) UV−vis spectra of the Au NPs aqueous solution measured before and after the adsorption of NPs onto the monolith. Inset: visual appearance of samples before (i) and after (ii) adsorption. (C) EDX of the polymer monolith with adsorption of Au NPs.

which has a stronger interaction with Au NPs than the carboxyl group is preferred. It is known that the thiol group has a stronger interaction with Au NPs than the carboxyl group, and the amino group has an electrostatic interaction with the carboxyl group. Therefore, different kinds of reagents containing a thiol and/or amino group (MSA, cysteine, glycine, and cysteamine) were investigated as the reagent to elute Au NPs from the poly(AA-VP-Bis) monolithic capillary. The results are shown in Figure 2B. By fixing the eluent pH as 7, the recoveries of Au NPs were less than 40% or 10% when using 2% (m/v) MSA or 2% (m/v) glycine as the eluent, while recoveries of Au NPs were higher than 90% when using 2% cysteine or 2% cysteamine as the eluent. These results reveal that both the thiol and amino group played an important role in the elution of Au NPs from the poly(AA-VP-Bis) monolithic capillary. In neutral and weak alkaline conditions, primary amine groups are deprotonated and exposed to Au NPs, so the elution of Au NPs from the poly(AA-VP-Bis) monolith can be achieved due to the hydrogen bonding and electrostatic interaction between eluent reagent and Au NPs. In addition, the stable Au−S bond formed between the thiol group and Au NPs can also contribute to the fast elution and further stabilize Au NPs in the eluent. The above results demonstrate the feasibility of extraction of the carboxyl group-containing Au NPs by poly(AA-VP-Bis) monolithic CME. Specifically, carboxyl group-containing Au NPs can selectively be adsorbed on the poly(AA-VP-Bis) monolithic capillary in a wide range of pH values and

condition. As for the bare Au NPs, the change of pH value would affect the ionization equilibrium of citrate, causing charge change on the surface of the Au NPs. Since the pKa1 value of citric acid is 2.927 and the zeta potential of Au NPs at different solution pH values indicated that the surface of the Au NPs is negatively charged (−40.3 to −31.8 mV)28 and stable at pH 4− 11,29 Au NPs would be susceptible to agglomeration due to the hydrogen bonds interaction among stabilized citrates when the pH value was below 3. When the pH of Au NPs solution is in the range of pH 3 to pKb of pyridyl (6.0), Au NPs could be adsorbed on the poly(AA-VP-Bis) monolith based on electrostatic interaction. This phenomenon coincided with the adsorption behavior of bare Ag NPs on the pyridine group.27 Besides electrostatic interaction, hydrogen bonds could also contribute to the strong adsorption behavior of Au NPs on the poly(AA-VP-Bis) monolithic capillary. The abundance of carboxyl group on the surface of Au NPs is beneficial for the formation of hydrogen bonds with the acrylamino group from the poly(AA-VP-Bis) monolith. The above experimental results demonstrate that Au NPs can be adsorbed on the monolith and separated from free Au ions in a relatively wide pH range. For separation of Au NPs from free Au ion and preconcentration of Au NPs, pH 5.5 was employed in the subsequent experiments. By fixing sample pH at 5.5, the elution of Au NPs from the poly(AA-VP-Bis) monolithic capillary was studied. As discussed above, the adsorption of Au NPs on the poly(AA-VP-Bis) monolith is based on electrostatic interaction and hydrogen bonds. To elute Au NPs from the monolith, the elution reagent 1792

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polymer monolithic CME process, the original Au NPs solution and the eluent of CME were characterized by TEM. Figure 4A,B shows the TEM images of the Au NPs solution before and

quantitatively be eluted from the column by using cysteine or cysteamine as the eluent. In the following experiments, cysteamine was selected as the eluent because it has a larger solubility than cysteine and the cysteine stabilized Au NPs containing carboxyl group which may result in the aggregation of Au NPs under certain conditions due to the intermolecular hydrogen bonding interaction.30 Optimization of Poly(AA-VP-Bis) Monolithic CME. To obtain the best extraction performance of poly(AA-VP-Bis) monolithic CME for Au NPs, a series of CME conditions affecting the extraction of Au NPs, including sample flow rate and volume and eluent concentration and volume, were studied by using bare Au NPs (stabilized in citrate aqueous solution) as the target unless otherwise specified (details are shown in Supporting Information Figures S2−S4). The sample flow rate was set as 0.25 mL min−1. Quantitative recoveries of Au NPs could be obtained in the entire studied range of sample volume from 1 to 8 mL; we chose a 1 mL sample solution for realworld sample analysis to trade off the enrichment factor and analytical speed. To ensure quantitative elution of Au NPs from the column, 0.1 mL of 4% (m/v) cysteamine was chosen as the eluent and the eluent flow rate was 0.05 mL min−1. Therefore, the theoretical enrichment factor is 10-fold, and the sample throughput was 6 h−1. These features suggest that poly(AA-VPBis) monolithic CME is suitable for online analysis of carboxyl group-containing Au NPs with good enrichment and fast adsorption/desorption kinetics. Evaluation of the Performance of Poly(AA-VP-Bis) Monolithic Capillary. Characterization of Poly(AA-VP-Bis) Monolithic Capillary before and after Adsorption of Au NPs. To address the poly(AA-VP-Bis) monolithic capillary having an excellent extraction ability toward carboxyl group-containing Au NPs, several characterization techniques including SEM, UV−vis spectroscopy, EDX, and TEM were used. The prepared monolithic capillary was characterized by SEM. As shown in Figure 3Ai,ii, the bare poly(AA-VP-Bis) monolithic capillary was uniform and composed of cross-linked micron spheres. Then, the Au NPs solution with a concentration of about 20 mg L−1 was continuously passed through the monolithic bed. After adsorption, the color of the poly(AAVP-Bis) monolithic capillary changed from white into red (see Figure 3Aiii,iv). UV−vis spectroscopy was employed to monitor the UV absorption of the Au NPs solution before and after poly(AAVP-Bis) monolithic CME. For this purpose, an Au NPs solution (about 20 mg L−1) was continuously passed through the polymer monolith and the effluent was collected. Figure 3B is the UV−vis spectra of the original Au NPs solution and the effluent. As can be seen, the original Au NPs solution had a strong absorption peak at about 516 nm while the effluent showed almost no absorption peak, indicating that the Au NPs were completely adsorbed on the poly(AA-VP-Bis) monolithic column. This can be further confirmed by the color of Au NPs solution (red) and the effluent (colorless and transparent) (see the inset of Figure 3B). The red monolithic column (used above) loaded with a large number of Au NPs was characterized by EDX. As shown in Figure 3C, the signal peak of Au could be clearly observed and the mass percentages of Au were as high as 15%, which further demonstrated that the poly(AA-VP-Bis) monolithic column showed high adsorption capacity toward Au NPs. In order to investigate whether the morphology of the Au NPs could be maintained (without agglomeration) during the

Figure 4. TEM images of Au NPs before (A) and after (B) extraction.

after the extraction, respectively. It can be seen that the morphology of the Au NPs could be well maintained during the extraction process. Therefore, the eluent could be directly introduced in ICPMS for subsequent determination without acid digestion. Preparation Reproducibility and Regeneration of Poly(AAVP-Bis) Monolithic Capillary. To evaluate the preparation reproducibility (expressed as relative standard deviations (RSDs)) of the poly(AA-VP-Bis) monolithic capillaries, the extraction efficiencies of citrate stabilized Au NPs on seven segments of monolithic capillary prepared in the same batch and among different batches were investigated with sample solution containing 1 μg L−1 of Au NPs under the optimal CME conditions. It was found that the RSDs (n = 7) were 4.9% in one batch and 5.6% among different batches. The regeneration ability of the self-prepared polymer monolithic capillary was evaluated. It was found that the prepared poly(AA-VP-Bis) monolithic capillary could be easily regenerated by passing through 0.1 mL of 4% (m/v) cysteamine at a flow rate of 0.05 mL min−1. After regeneration, the selfprepared polymer monolithic capillary could be reused 20 times without an obvious decrease of extraction efficiency. Adsorption Capacity of Poly(AA-VP-Bis) Monolithic Capillary. The adsorption capacity of the poly(AA-VP-Bis) monolithic capillary for Au NPs was conducted according to the previous report.31 A 50 mL solution containing 10 mg L−1 citrate stabilized Au NPs was passed through a length of 2.5 cm polymer monolithic capillary, and the effluent was determined by ICPMS. The maximum adsorption capacities, defined as the 10% leakage of analytes, was 480 μg (calculated as Au) for Au NPs. The high adsorption capacity of 2.5 cm monolithic capillary can be attributed to the large specific surface area of 1793

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capillary should be suitable for the extraction of carboxyl stabilized Au NPs. To confirm this hypothesis, Au NPs with different coatings containing carboxyl group were examined including MSA and MUA stabilized Au NPs. As shown in Figure 5, poly(AA-VP-Bis) monolithic capillary provided a

the monolith and abundant functional groups on the surface of the column bed. Interference Study. Anti-interference ability is one of the key factors for a sample pretreatment method being applicable in real-world sample analysis. Therefore, the effect of coexisting ions prevailing in environmental samples, free Au ions, humic acids, and Au NPs with different particle size and coatings on the extraction efficiency of polymer monolithic capillary was studied. Details about the investigation on the effect of coexisting ions, free Au ions, and humic acids were summarized in Supporting Information Figures S5 and S6. In natural waters,32 the average concentrations of common ions are K+, 1.6 mg L−1; Na+, 11 mg L−1; Ca2+, 95 mg L−1; Mg2+, 38 mg L−1; SO42−, 71.5 mg L−1; Cl−, 25 mg L−1; humic acid, 10 mg L−1. The interference study demonstrated that the common coexisting ions, humic acids, and free Au ions in environmental water will not interfere with the analysis of citrate stabilized Au NPs. The particle sizes of NPs naturally have a wide range, so the effect of the particle size of Au NPs on the extraction performance of the prepared poly(AA-VP-Bis) monolithic capillary for Au NPs was investigated to evaluate the applicability of the proposed method. For this purpose, three kinds of citrate stabilized Au NPs with particle sizes of 3, 17, and 40 nm, respectively, were prepared as the targets (TEM images are shown in Supporting Information Figure S7). Table 2 summarizes the adsorption percentage and recovery of these

Figure 5. Effects of the different coatings on the recovery of Au NPs (concentration of Au NPs 20 μg L−1 as Au; sample volume: 1 mL; pH = 5.5; sample flow rate: 0.25 mL min−1; eluent: 0.1 mL of 4% cysteamine).

comparable recovery for MSA or MUA stabilized Au NPs and citrate stabilized Au NPs. Therefore, the developed method could be used for the extraction of carboxyl group-containing Au NPs. Furthermore, the recovery of citrate stabilized Ag NPs on the poly(AA-VP-Bis) monolithic capillary was also examined, and quantitative recovery can be obtained (see in Figure 5). Similar to Au NPs, Ag NPs have a high affinity to the thiol group too and, thus, can be quantitatively eluted by 0.1 mL of 4% cysteamine as well. This result indicates that the proposed method has potential applicability in the analysis of carboxyl group-containing metal NPs that have a high affinity to the thiol group. Analytical Performance. The optimal online CME conditions are as follows. One mL of sample solution (pH adjusted to 5.5) was pumped into the poly(AA-VP-Bis) monolith with the flow rate of 0.25 mL min−1 for adsorption, and 0.1 mL of 4% (m/v) cysteamine was used as the eluent to desorb the retained carboxyl group-containing Au NPs with the flow rate of 0.05 mL min−1. The enrichment factor is 10-fold. According to the IUPAC definition, the LOD (three times of SDs of laboratory procedural blanks by 11 replicates) for carboxyl group-containing Au NPs was 3.97 ng L−1 as Au, which corresponded to 24.2 fmol L−1 as Au NPs with particle size of 3 nm (number of particles was 1.45 × 1010). The online CME-ICPMS spectra of the blank and 3 nm citrate stabilized Au NPs solutions (concentrations 0.02, 0.05, 0.2, and 0.4 μg L−1 corresponding to particle numbers of 7.30 × 1010, 1.83 × 1011, 7.30 × 1011, and 1.46 × 1012, respectively) presented in Figure S8 in the Supporting Information illustrated the high sensitivity of the proposed method. The dependence of 197Au intensities with the Au NPs concentration shown in Figure S9 in the Supporting Information indicated that the good linear relationship with correlation coefficients greater than 0.99 was obtained for carboxyl group-containing Au NPs in the concentration range of 0.02−20 μg L−1. The RSDs for seven replicates online analysis of 0.2 μg L−1 citrate stabilized Au NPs was 5.1% with the sample throughput of 6 h−1.

Table 2. Adsorption and Recovery of Different Particle Size of Au NPs recovery (%) particle size of Au NPs (nm)

adsorption (%)

0.1 mL of 4% cysteamine

0.5 mL of 4% cysteamine

3 17 40

94 ± 4 95 ± 3 92 ± 5

92 ± 4 42 ± 5 15 ± 3

96 ± 3 89 ± 5 58 ± 6

Au NPs on the poly(AA-VP-Bis) monolithic capillary. As can be seen, all three kinds of Au NPs have good adsorption efficiency on the monolith, but the recovery was gradually decreased with the increase of the particle size of Au NPs. Increasing eluent volume from 0.1 to 0.5 mL can increase the recovery of Au NPs. This could be attributed to the stability of Au NPs and transfer resistance in the elution of Au NPs from the monolith. First, the stability of Au NPs decreased with the increase of particle size,33 which might lead to partial aggregation of Au NPs and possible sedimentation in the pores of the monolith during extraction. Second, Au NPs with larger particle size are harder to remove from the pores, resulting in a larger eluent volume required. The results demonstrated that the prepared poly(AA-VP-Bis) monolithic capillary exhibited excellent adsorption performance toward all kinds of the tested Au NPs, while faster desorption kinetics can be established for Au NPs with smaller particle size. It also implied that the prepared poly(AA-VP-Bis) monolithic capillary is a promising material for the removal of Au NPs in real samples and has the potential to fractionate Au NPs based on particle size. The coating of NPs is another important factor affecting their extraction by the poly(AA-VP-Bis) monolithic capillary. On the basis of the extraction principle of electrostatic and hydrogen bond interaction, the prepared poly(AA-VP-Bis) monolithic 1794

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Table 3. Analytical Results (mean ± s.d., n = 3) for Carboxyl Group-Containing Au NPs in Tap Water, the Yangtze River Water, and the East Lake Water added (3 nm Au NPs) concentration (μg L−1) tap water

Yangtze River water

East Lake water

0 0.06 0.20 2.00 0 0.06 0.20 2.00 0 0.06 0.20 2.00

found concentration (μg L−1)

number of particles 2.19 × 1011 7.30 × 1011 7.30 × 1012 2.19 × 1011 7.30 × 1011 7.30 × 1012 2.19 × 1011 7.30 × 1011 7.30 × 1012

A comparison of the analytical performance of the developed method with that of several other approaches13,18−21 for the determination of Au NPs is shown in Table S2 in the Supporting Information. It can be seen that the LOD of the proposed method was comparable with the reported methods18 and inferior to the method using magnetic SPE,21 but the proposed method has the fastest adsorption/desorption kinetics (analytical time, 10 min) and lowest sample consumption (1 mL) among these methods. Furthermore, the proposed method is an online analytical strategy which exhibits some unique advantages such as high sample throughput, low random error, and no sample digestion required. Sample Analysis. The concentrations of carboxyl groupcontaining Au NPs in tap water, the Yangtze River water, and the East Lake water were determined by the online monolithic CME-ICPMS, and no carboxyl group-containing Au NPs were determined in these natural environmental waters. In order to verify the accuracy of the method, the spiked samples were analyzed and the recoveries for the spiked samples are presented in Table 3. As can be seen, the recovery in the range of 77−103% was obtained for the spiked samples, indicating that the proposed method is capable of analyzing the trace/ultratrace amounts of carboxyl group-containing Au NPs in natural environmental water samples.

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number of particles

recovery, %

2.08 × 1011 6.94 × 1011 6.72 × 1012

95 95 92

1.90 × 1011 6.57 × 1011 6.86 × 1012

87 90 94

1.68 × 1011 6.21 × 1011 7.52 × 1012

77 85 103

N.D. 0.057 ± 0.01 0.19 ± 0.01 1.84 ± 0.08 N.D. 0.052 ± 0.01 0.18 ± 0.01 1.88 ± 0.18 N.D. 0.046 ± 0.01 0.17 ± 0.01 2.06 ± 0.25

ASSOCIATED CONTENT

S Supporting Information *

Additional information on operating conditions of ICPMS (Table S1), synthesis of Au NPs (including Figure S7), stability of Au NPs (including Figure S1), effect of sample flow rate (including Figure S2), sample volume (including Figure S3), eluent concentration and volume (including Figure S4), common coexisting ions, humic acid (including Figure S5), and free Au ions (including Figure S6) on the extraction of carboxyl group-containing Au NPs, analytical performance comparison (Table S2), online CME-ICPMS spectra (Figure S8), and dynamic range for Au NPs (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-27-68752162. Fax: 0086-27-68754067. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Nature Science Foundation of China (Nos. 21375097, 21205090, 21175102), Science Fund for Creative Research Groups of NSFC (No. 20921062), the National Basic Research Program of China (973 Program, 2013CB933900), and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University are gratefully acknowledged.

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CONCLUSIONS In this paper, a hydrophilic polymer monolithic capillary was prepared and its extraction performance for carboxyl groupcontaining Au NPs was studied. The prepared poly(AA-VP-Bis) monolithic capillary has fast adsorption/desorption kinetics and high adsorption capacity toward carboxyl group-containing Au NPs. Furthermore, Au NPs could keep their morphology during the CME process, which is a prerequisite condition for online analysis. On the basis of the above fact, a novel method of online poly(AA-VP-Bis) monolithic CME-ICPMS analysis of trace carboxyl group-containing Au NPs in environmental water samples was developed. The proposed method has the advantages of low LOD, good selectivity, good tolerance to the sample matrix interference, and simple and fast operation and can be extended to the analysis of other carboxyl groupcontaining metal NPs in which the metal has as high affinity to the thiol group as Au due to its main extraction principle of hydrogen bonding and electrostatic interaction.

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