Size-Based Analysis of Au NPs by Online Monolithic Capillary

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Size-based Analysis of Au NPs by Online Monolithic Capillary Microextraction-ICPMS Xiaolan Liu, Bei-Bei Chen, Yabing Cai, Man He, and Bin Hu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03532 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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Analytical Chemistry

Size-based Analysis of Au NPs by Online Monolithic Capillary Microextraction-ICPMS Xiaolan Liu, Beibei Chen, Yabing Cai, Man He, Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, P R China

*

Corresponding author: Fax: +86-27-68754067. Tel: +86-27-68752162. E-mail: [email protected] 1

ACS Paragon Plus Environment


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Abstract An

online

hydrophilic

polymer

monolithic

capillary

microextraction

(CME)-inductively coupled plasma mass spectrometry (ICPMS) analytical system was developed for the size-based analysis of Au NPs in the range of 3-40 nm. Au NPs with different particle sizes retained on the poly(acrylamide-vinylpyridine-methylene bis-(acrylamide)) (poly(AA-VP-Bis)) monolithic capillary showed different elution behavior when cysteamine was used as the eluent, and the napierian logarithm of the critical eluent concentration (Ln C (cysteamine)) was linearly dependent on the particle size of Au NPs. This means that the unknown particle size of Au NPs can be deduced based on their critical eluent concentrations. The developed method was successfully used for the baseline separation of 3 nm and 20 nm Au NPs, as well as 10 nm and 30 nm Au NPs. Compared with chromatographic based methods for the size analysis of Au NPs, the developed online poly(AA-VP-Bis) monolithic CME-ICPMS method is featured with simultaneous separation and enrichment of Au NPs, simple operation, low cost, high analytical speed and high adsorption capacity.

Keywords: Au NPs; size based separation; monolithic capillary microextraction; inductively coupled plasma mass spectrometry

2


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Introduction Nanoparticles (NPs), especially metal NPs have been widely used in different areas including industry, medicine and scientific research because of their physical and chemical characteristics.1,2 However, the increasing use of NPs inevitably results in their accumulation in the environment which may lead potential hazard to human and other organisms. As a result, in addition to pay attention to the use value of NPs, concerns about their adverse effects have also been raised. Several studies have demonstrated the adverse impact of Au NPs3-5 and Ag NPs6-8 to a variety of organisms. Although there is no exact elucidation for the toxic properties of metal NPs, several studies have demonstrated that besides the concentration, the particle size of metal NPs also impacts their toxicity.9,10 Hence, it is of great importance to develop new methods for the analysis of trace metal NPs (especially the size-based analysis of metal NPs) in environmental samples. Among various elemental specific detection techniques,11-13 inductively coupled plasma mass spectrometry (ICPMS) is the most powerful tool due to its extremely low limits of detection (LODs), low mass interference, wide linear range and so on. In 2006, Degueldre et al.14 found the feasibility of single particle (SP)-ICPMS for the analysis of NPs. Recently, some studies demonstrated the suitability of this methodology but the precision of the method needs to be improved.15 When using the common model ICPMS, the direct determination of metal NPs in real-world samples is difficult, because complex sample matrix would affect the accuracy, the concentration of metal NPs is very low, and metal NPs with different size distributions and their corresponding ions cannot be distinguished. Therefore, an appropriate separation/ preconcentration technique is necessary prior to ICPMS determination to concentrate target metal NPs, separate metal NPs from their corresponding ions, and eliminate or decrease the matrix interference in real-world sample analysis. The size fractionation of metal NPs were often provided by chromatographic or electrophoretic based techniques such as field-flow fractionation (FFF),16-20 capillary 3



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electrophoresis (CE),21-24 gel electrophoresis (GE),25 size-exclusion chromatography (SEC),26,27 hydrodynamic chromatography (HDC),28 high performance liquid chromatography (HPLC),29,30 and thin layer chromatography (TLC).31 FFF possesses several different separation models including electrical, magnetic, thermal and flow FFF to provide size-based separation of NPs in a wide size distribution. Engelhard et al.23 used micellar electrokinetic chromatography (MEKC) coupled to ICPMS for the separation of Au NPs in the size range of 5-50 nm. CE has high separation efficiency, but it suffers from low anti-interference ability and complex interface with following ICPMS detection. LC based techniques (such as SEC, HDC, HPLC and TLC) are also reported in the separation of NPs. Among them, SEC is commonly used to get the size-based fractionation information of NPs. Although chromatographic or electrophoretic based techniques are capable for size-based separation of NPs, they suffer from the matrix interference, and no preconcentration of NPs involved also limited their applications in real-world sample analysis. Therefore, it is essential to develop novel pretreatment methods to realize the size-based separation, the preconcentration of target metal NPs and the matrix removal as well. Recently,

different

kinds

of

sample

pretreatment

methods

including

ligand-assisted extraction,32 cloud point extraction (CPE),33-39 solid phase extraction (SPE),40 magnetic solid phase extraction (MSPE)41 and monolithic capillary microextraction (CME)42 have been used for the preconcentration of NPs. Liu et al.38 developed a Triton X-114 based CPE as an efficient separation approach for the speciation analysis of Ag NPs and Ag+. The method was then extended to the analysis of Au NPs,37 CuO NPs33 and ZnO NPs.39 Li et al.40 employed an ionic exchange resin as adsorbents for the effective and selective extraction of noble metal NPs from real environmental

water.

Su

et

al.41

prepared

Al3+

immobilized

Fe3O4@SiO2@iminodiacetic acid NPs as MSPE sorbents for the analysis of Au NPs and Au ions in environmental waters with high enrichment factors. Zhang et al.42 prepared

a

hydrophilic

polymer

monolithic

capillary

(poly(acrylamide-vinylpyridine-methylene bis(acrylamide)) (poly(AA-VP-Bis)) for the preconcentration of carboxyl group-containing Au NPs from environmental waters 4


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with high adsorption capacity and high selectivity. In these extraction based methods, the metal NPs could be well preconcentrated and separated from their corresponding ions and other sample matrix. However, to the best of our knowledge, no extraction based method has been reported for size-based separation of metal NPs till now. In our previous study,42,43 we found an interesting phenomenon that Au NPs with different particle size showed quite different desorption behavior on poly(AA-VP-Bis) monolithic capillary when 4% cysteamine was employed as the eluent. It gives us a hint that poly(AA-VP-Bis) monolithic CME has the potential to fractionate Au NPs based on their particle sizes. Therefore, in this work, the elution behavior of Au NPs with different particle sizes from poly(AA-VP-Bis) monolithic capillary was studied very carefully to find the relationship between the elution condition and the particle size of Au NPs, and a new strategy for the size-based analysis of Au NPs by online monolithic CME-ICPMS detection was proposed.

Experimental Section Instrumentation and Reagents The experiment was performed by a quadrupole (Q) ICPMS (Model Agilent 7500a, Hewlett-Packard, Yokogawa Analytical Systems, Tokyo, Japan) with a Babington nebulizer, and the optimum operation conditions are summarized in Table S1. Fused silica capillary (530 µm i.d. × 680 µm o.d.) was obtained from Yongnian Optical Fiber Factory (Hebei, China). HAuCl4·4H2O (Sigma-Aldrich, MO, USA), 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. The 40 nm Au NPs was purchased from Nanjing Ji Cang Nano Science and Technology Ltd. Au NPs Certified Reference Material of 10 nm (RM 8011) were purchased from National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). The information on other instruments and reagents is detailed in the Supporting Information 5



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Preparation of Poly(AA-VP-Bis) Monolithic Capillary The procedure for the preparation of poly(AA-VP-Bis) monolithic capillary was according to the Ref.44 with minor modifications. Briefly, 165 mg acrylamide (AA), 185 mg methylene bis(acrylamide) (Bis), 110 µL 4-vinylpyridine (4-VP), 900 µL dodecanol and 4.5 mg 2,2-azobisisobutyronitrile (AIBN) were dissolved in 1125 µL dimethyl sulfoxide (DMSO). And the mixture was homogenized by vortex and sonication for 15 min. The prepolymerization solution was then filled in the above activated capillary (length of 12 cm) by a syringe. The capillary was sealed at both ends with rubber septum and immersed in a water bath at 60 °C for 12 h. Then the monolithic capillary was washed with ethanol to remove porogens and unreacted components, and cut into 3 cm for further use.

Experimental Methods 1 mL aqueous solution of Au NPs (5 µg L-1, calculated as Au) with different size (from 3 to 40 nm) at pH 7 was passed through the prepared monolithic capillary at a flow rate of 0.25 mL min-1. The retained Au NPs was eluted with cysteamine (pH=7) at different concentrations and on-line determined by ICPMS.

Sample Preparation Lake water (collected from the East Lake, Wuhan, China, DOC45 was in the range of 2.98-10.02 mg L-1, pH was 7.98) was filtered through a 0.45 µm cellulose acetate membrane (Tianjin Jinteng Instrument Factory, Tianjin, China) for removing large particulate matter. And then 1 mL of the sample solution was subjected to the on-line CME-ICPMS determination. The information on the spiked experiment was detailed in Supporting Information.

RESULTS AND DISCUSSION Characterization of Au NPs and monolithic column In this work, five groups of Au NPs with different particle sizes (termed as group A, B, C, D and E from smallest to largest) were employed as the representative 6


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analytes. The particle sizes of Au NPs in group A-E were characterized by transmission electron microscope (TEM). From Figure S1(A-E), it can be seen that the particle size of the Au NPs for each group were quite uniform which laid the foundation for further analysis. The statistical results of the average sizes of Au NPs in each group were accomplished by the software of Nano Measurer 1.2 with the particle number of 100, and the sizes of Au NPs were 3.2±0.5 nm, 11.9±1.1 nm, 16.3±1.6 nm, 21.8±3.9 nm and 36.7±3.4 nm for group A, B, C, D and E, respectively. In order to confirm the dispersity of Au NPs with different particle sizes in aqueous solution, dynamic light scatter (DLS) was also employed to characterize the Au NPs. The results in Figure S2 showed a narrow particle size distribution in each group, and the average sizes were 13.0 nm, 17.2 nm, 28.6 nm and 38.1 nm for group B, C, D, and E, respectively, which were in accordance with the results obtained by TEM (the DLS result for group A was not shown because the particle size of Au NPs in group A is close to the size detection limit of the instrument). The monolithic column was characterized by the N2 adsorption and the average skeleton pore size of the monolithic column was 5.7 nm.

Elution Study In our previous work,42,43 it was found that cysteamine (pH=7) showed best elution performance. What’s more, there is an interesting phenomenon that Au NPs with different particle size showed quite different desorption kinetics. As a result, the elution behavior of Au NPs (group A-E) on poly(AA-VP-Bis) monolithic capillary was investigated in detail by using different concentration of cysteamine (pH 7) as eluent in this work. By fixing the flow rate of the eluent at 50 µL min-1, three portions of 100 µL cysteamine with different concentrations were used to elute Au NPs with different particle sizes, and the results were shown in Figure S3(A-E). In group A, the concentration of cysteamine in the range of 0.1-2% (w/v) were investigated, and it was found that 100 µL 0.3% cysteamine was the lowest eluent concentration (termed as critical eluent concentration) for quantitative elution (elution efficiency higher than 85%); In group B and C, 100 µL 1.5% and 2% cysteamine can provide a quantitative 7



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recovery for the Au NPs, respectively; In group D, 100 µL of cysteamine with a much higher concentration of 15% was the critical eluent concentration for Au NPs; For group E, in the studied concentration range (0.3%-18%, higher concentration of cysteamine was not tested because of the incompatibility for ICPMS detection), the Au NPs cannot be quantitatively eluted by the first portions of 100 µL cysteamine, but 200 µL 10% cysteamine was enough for the quantitative elution of Au NPs in group E. As a result, the critical eluent conditions were 100 µL 0.3%, 1.5%, 2% and 15% cysteamine for group A-D, respectively, and 200 µL 10% cysteamine for group E. In order to prove the universal of the developed method, the citrate-Au NPs with different sizes were modified with 11-mercaptoundecanoic acid (MUA) and subjected to the same extraction procedure, the results in Figure S4 in Supporting Information showed that the MUA-Au NPs had a similar elution behavior to the citrate-Au NPs. The trends of the elution of Au NPs in group A-E by 100 µL cysteamine with different concentration were summarized in Figure 1. From Figure 1, it was found that the critical eluent concentration was increased with the particle size of Au NPs sharply. An exponential relationship between the particle sizes and the critical eluent concentration was explored by data processing and fitting. It was found that the napierian logarithm of the critical eluent concentration (Ln C (cysteamine)) was linearly dependent on the particle size of Au NPs which was depicted in the Figure 2, and the correlation coefficient (R2) was 0.9373. This phenomenon could be explained mainly by specific surface area effect and spheres volume effect. When the particle size of Au NPs increased, the interaction surface between the single Au NP and the active site of the monolithic capillary was also increased which means a greater interaction force and results in a high concentration of eluent to break the interaction between the monolith and Au NPs. On the other hand, it may be related to the desorption kinetics of the Au NPs in the porous monolithic capillary. The larger the spheres volume of Au NPs, the slower the desorption kinetics. Therefore, when the elution flow rate and volume was fixed, a high concentration eluent was needed for a larger particle size. What’s more, the fitted linear equation (y=0.1981x-6.5891, R2=0.9373) for the particle size of Au NPs and the Ln C (cysteamine) can be used for 8


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the prediction of the critical eluent concentration for a specific particle size of Au NPs and vice versa.

Separation of Au NPs based on the Size As shown in Figure 1 and S3, the critical elution condition (100 µL of 0.3% cysteamine) for Au NPs in group A can hardly elute Au NPs in group C, D, E, and the critical elution condition (100 µL of 1.5% cysteamine) for Au NPs in group B can hardly elute Au NPs in group D and E. It implies that size-based separation of Au NPs can be achieved by carefully selecting the elution conditions. According to the difference of the elution behavior, the separation of the mixture of Au NPs in group A with C, D or E, as well as the mixture of Au NPs in group B with D or E was conducted by employing the corresponding critical elution conditions, and the results obtained by on-line monolithic CME-ICPMS are shown in Figure 3 (separation of Au NPs in group A with C, as well as B with D), Figure S5 (separation of Au NPs in group A with D or E, as well as B with E) and Table S2 (quantification results). As can be seen, all the studied mixtures of Au NPs could be well separated with good recoveries. Furthermore, to investigate the effect of the concentration ratio of Au NPs from different groups on the separation, different concentration ratios (1:5 and 5:1) were applied. The results shown in Figure S6 and Table S2 demonstrate that the concentration of Au NPs in different groups after separation were in consistent with the original concentration. These results indicated that the proposed online monolithic CME-ICPMS method is capable for the size-based separation of Au NPs with the separation resolution (the minimum size-difference for baseline separation) of ca. 20 nm-difference.

Method Validation The NIST Au NPs Certified Reference Material of 10 nm (CRM 8011) was used for the validation of the method. The results in Figure S7(A) indicate that 100 µL of 1.5% cysteamine was enough for the quantitative elution of the Au NPs. As the particle size of Au NPs in group B is close to the size of the NIST Au NPs (RM 8011), the elution behavior of them were compared and similar elution behavior was 9



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achieved (Figure S7(B)). In addition, the results of NIST Au NPs (RM 8011) were also characterized using the calibration curve (y = 0.1981x-6.5891) mentioned above, and the result shown in Figure S7(C) (red spot) indicates the good accuracy of the proposed method.

Comparison with Other Methods There are different sample pretreatment methods28-38 reported for the analysis of the metal NPs. However, all these reported methods28-38 were only used for determining the concentration of metal NPs, and most of them are offline methods, while the developed monolithic CME-ICPMS is an online analytical method which is capable for size-based separation of Au NPs besides preconcentration. Chromatographic based techniques are the main methods for size-based separation of metal NPs, however, these methods cannot preconcentrate the metal NPs. Table 1 lists different methods including different models of FFF, CE, HDC, TLC and the developed monolithic CME method for the separation of Au NPs in terms of analyzed particle size, approximate size resolution and separation time. Compared with chromatographic based methods, the developed monolithic CME method can not only separate the Au NPs with different size but also preconcentrate the Au NPs although its size resolution is comparable or slightly worse than the reported approaches listed in Table 1. In addition, the developed method, in which a single self-prepared monolithic column with the length of 3 cm was used, is rapid (only 6 min separation time), low cost, simple and straightforward without extra applied voltage.

Sample Analysis The developed method was applied to the analysis of Au NPs with different sizes in East Lake samples. The analytical results along with the recovery for the spiked East Lake samples with the particle sizes of 3 nm and 20 nm, as well as 10 nm and 30 nm are listed in Table S3. It can be seen that the recoveries of Au NPs were between 83% and 118% for the spiked samples, which showed that the developed method can be used in the size-based analysis of Au NPs in environmental water samples.

CONCLUSIONS 10


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A hydrophilic poly(AA-VP-Bis) monolithic capillary microextraction was online coupled with ICPMS for the size-based analysis of Au NPs in the range of 3-40 nm. The method was based on that Au NPs with different particle sizes retained on the poly(AA-VP-Bis) monolithic capillary have different elution behaviors. After careful investigation, an empirical correlation of the critical eluent concentration of cysteamine and the Au NPs particle size was obtained. The developed NPs based separation method has efficiently widened the application areas of monolithic CME. The method used a simple sample pretreatment procedure to achieve the goal of matrix removal, enrichment of Au NPs and their size-based separation simultaneously. It provides a new concept for the size-based separation of NPs.

Supporting Information Additional information on instrumentation and reagents, operating conditions of ICPMS (Table S1), quantification results of Au NPs (Table S2), real sample analysis results (Table S3), synthesis of Au NPs, TEM (Figure S1) and DLS (Figure S2) of Au NPs, effect of eluent concentration (Figure S3(A-E)), recovery of MUA-Au NPs (Figure S4), separation of Au NPs (Figure S5 and S6), information on the recovery of Certified Reference Material (Figure S7) as noted in the text (PDF).

ACKNOWLEDGMENTS Financial support from the National Nature Science Foundation of China (Nos. 21205090, 21175102, 21575107), Science Fund for Creative Research Groups of NSFC (No. 20921062), the Fundamental Research Funds for the Central Universities (No. 2015203020209), 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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Figure captions Figure 1 The total variation trend of eluent concentration on the recovery of Au NPs in group A to E. (the concentration of Au NPs: 5 µg L-1 as Au; sample volume: 1 mL; pH = 7; sample flow rate: 0.25 mL min-1; eluent: 100 µL cysteamine pH=7) Figure 2 The relationship between the sizes and eluent concentrations of Au NPs. Figure 3 Separation of Au NPs of group (1) A and C, as well as (2) B and D with the concentration ratio of 1: 1.

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Figure 1

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Figure 2

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Figure 3

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Table 1. Comparison of Separation Methods Reported in the Literatures for Au NPs with Different Particle Size Method Electrical FFF Asymmetric flow FFF CE CE CE HDC TLC GE Monolithic CME

10, 20, 40

Separation resolution/ nm 10

10, 20, 30, 60

10

45

19

5, 15, 20, 30 5.3, 19 5, 10, 20, 50 5, 10, 20, 50, 100, 250 13, 34, 41, 47, 100 5, 10, 20 3, 10, 20, 30, 40

10 5 20 — 20 — 20

20 7 10 8 25 20 6

21 22 23 28 31 25 This work

Particle size of Au NPs/ nm

—, not entirely separated.

19


Separation time/ min

Ref.

20

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for TOC only

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Size-Based Analysis of Au NPs by Online Monolithic Capillary

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