MetalâOrganic Framework-Derived Hollow Carbon Nanocubes for

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Functional Nanostructured Materials (including low-D carbon)

MOF-derived hollow carbon nanocubes for fast solidphase microextraction of polycyclic aromatic hydrocarbons Xingru Hu, Chaohai Wang, Jiansheng Li, Rui Luo, Chao Liu, Xiuyun Sun, Jinyou Shen, Weiqing Han, and Lianjun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02281 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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MOF-derived hollow carbon nanocubes for fast solid-phase microextraction of polycyclic aromatic hydrocarbons Xingru Hu, Chaohai Wang, Jiansheng Li,* Rui Luo, Chao Liu, Xiuyun Sun, Jinyou Shen, Weiqing Han and Lianjun Wang* Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environment and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Tel: +(86) 025-84315351 E-mail: [email protected]; [email protected]

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ABSTRACT: Developing novel coating materials for fast and sensitive solid-phase microextraction (SPME) is highly desired but few achieved. In this work, a new material of metal-organic framework-derived hollow carbon nanocubes (HCNCs) was prepared as a fiber coating material for SPME. The HCNCs coated fiber (denoted as HCNCs-F) exhibited a better enrichment performance than solid carbon nanocubes (SCNCs) coated fiber (denoted as SCNCs-F) and commercial fibers based on the abundant active sites of the hollow structure, hydrophobic interactions and π-π interactions. Moreover, due to the reduced mass-transport lengths of the hollow mesoporous structure, the HCNCs-F demonstrated a faster mass transfer compared with the SCNCs-F. The HCNCs-F was used to determine the six hydrophobic PAHs with wide linear ranges (10-2000 ng L-1 for Nap and 5-2000 ng L-1 for the other five analytes), good reproducibility (relative standard deviation < 8.8%) and low detection limits (0.03-0.70 ng L-1). Finally, the HCNCs-F was successfully applied for the determination of PAHs from the real water samples. It can be concluded from the results that MOF-derived hollow carbon materials are promising candidates for the fast SPME and can be used for practical application in analytical chemistry. KEYWORDS: solid-phase microextraction, gas chromatography-mass spectrometry, hollow carbon nanocubes, fast mass transfer, polycyclic aromatic hydrocarbons

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INTRODUCTION Currently, the direct detection of target analytes from complex matrices is still a great challenge considering the restriction of selectivity and sensitivity. Solid-phase microextraction (SPME), as a green analytical technique, has aroused tremendous interest in the field of isolation and concentration of analytes from environmental,1 agricultural,2 and biological samples.3 The most commonly used format among the different geometries of SPME is the fiber. The small volume of extraction phase coated on the fiber requires high adsorption capacity of adsorbent. Another principle of SPME is the mass transfer process that leads to the partitioning equilibria of extracted analyte between sample matrix and the adsorbate. The two factors show crucial significance to the sensitivity and applicability, which is dependent on the properties and structure of coating materials.4,5 However, the limited commercially available materials along with time-consuming extraction process is a main technical obstacle, which dramatically hinders the practical application of SPME technology.

6-9

In order to

address these challenges, different types of carbon-based materials have been developed as adsorbents for the extraction of organic compounds via non-covalent forces.10 Their pore structure could be designed easily to satisfy the needs of the SPME coating. Therefore, exploring novel carbon-based coating materials with short equilibration time and high enrichment factors is highly desired.11 Metal-organic frameworks (MOFs), a new class of crystalline materials, have been regarded as ideal templates and precursors for synthesizing nanoporous carbons.12-14 Due to their large specific surface area, easy functionalization and high porosity, several kinds of MOFs-derived carbons have been explored as adsorbents for the extraction of organic pollutants.15-19 Although the extraction capacities of MOF-derived carbon materials have been increased, the mass transport was significantly limited because of the solid and monotonously microporous structure, leading to relatively long equilibration time and low enrichment factors. On the other hand, hollow nanostructure, with large surface area and low apparent density is promising and attractive material in the areas of catalysis,20,21 energy storage,22 environmental remediation,23 and biomedicine.24 For example, Guo et al. initially used hollow mesoporous carbon spheres (HMCSs) as adsorbent for bilirubin. The HMCSs exhibited a 3


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remarkably higher adsorption rate and capacity as compared to the commercial activated carbon (AC).25 Zhang et al. reported that mesoporous carbon capsules had more effective removal capacity and higher adsorbing rate for organic pollutants than the commercial AC.23 These results demonstrated that the hollow structure with hierarchical pores played an important role in improving the adsorption equilibration and adsorption capacity.26 However, to the best of our knowledge, utilization of MOF-derived hollow carbon materials as coating materials for SPME with fast equilibration and high enrichment factors is not achieved. Herein, MOF-derived hollow carbon nanocubes were prepared and further coated on the surface of stainless steel wires by physical adhesion as SPME fibers for the determination of polycyclic aromatic hydrocarbons (PAHs). The extraction efficiency of hollow carbon nanocubes (HCNCs) coated fiber (denoted as HCNCs-F) was found superior as compared to solid carbon nanocubes (SCNCs) coated fiber (denoted as SCNCs-F) and commercial fibers. It is due to their unique hollow structure, hydrophobic and π-π interactions. The adsorption kinetic result shows that HCNCs possess faster kinetic rate than the SCNCs. The equilibration time of HCNCs-F was shorter than the SCNCs-F. These results demonstrate the significance of the unique hollow structure and richer mesoporous of HCNCs. Finally, the HCNCs-F has also been applied for the extraction of PAHs from the real water samples successfully. EXPERIMENTAL SECTION Chemicals and Reagents 2-methylimidazole, tannic acid, hexadecyl trimethyl ammonium bromide (CTAB), and the analytes: naphthalene (Nap), acenaphthene (Ace), fluorene (Flu), phenanthrene (Phe), fluoranthene (FlA), pyrene (Pyr), dimethylphthalate (DMP), diethyl phthalate (DEP) and o-xylene (o-X) were all obtained from Aladdin (Nanjing, China). Zn(NO3)2·6H2O, hydrofluoric acid (HF) (40%) and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Corporation, Nanjing, China. Neutral silicone glue was taken from Dow Corning (USA). The 5 µL-microliter syringe was supplied by Gaoge Industrial and Trade, Shanghai, China. The stainless steel wires (SSW, φ 0.30 mm) were bought from RongHua Scientific Equipment Corporation (Nanjing, China). 4



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Instrumentation The prepared solid powder (1 mg) was dissolved in 5 mL absolute ethanol under sonication, after 30 min, a drop of the dispersion was dropped on the copper wire mesh for the analysis of TEM (TECNAI G2 20 LaB6 electron microscope operated at 200 kV). The FEI 250 system was used to obtain the scanning electron microscopy (SEM) analysis of samples at 20 kV. The nitrogen adsorption-desorption isotherms were performed on a physisorption apparatus (ASAP-2020, Micromeritics, America). The X-ray diffraction (XRD, Bruker AXS D 8 advance powder diffraction system) and X-ray photoelectron spectroscopy (XPS, PHI Quantera II ESCA System) were used to analyze the synthesized samples. Gas chromatography (GC) analysis was performed on 7980 GC equipped with 5975 mass spectrometry (MS), Agilent. Helium (99.999%) was employed as carrier gas (flow rate: 1.0 ml min-1). The HP-5 MS capillary column (30 m × 0.25 mm i.d. × 0.25 µm) was used for separation of the analytes. The GC oven temperature program was set as follows: 50 °C for 1 min; 10 °C min-1 to 200 °C for 2 min; 20 oC min-1 to 300 oC. Synthesis of HCNCs The HCNCs were prepared by a controlled etching approach modified from the reported study.27 Firstly, Zn(NO3)2·6H2O (700 mg) was dissolved in 100 mL deionized water (denoted as A), so was the 2-methylimidazole (10.8 g) (denoted as B). After that 4.4 mL CTAB (0.01M) was added in solution B for 10 min under continuous stirring. Afterwards, the mixture B was transferred into the solution A rapidly, followed by a vigorous agitation for 5min. After putting this mixture for 3 hours at room temperature, the white precipitate (ZIF-8 nanocrystals) was obtained by centrifugation, washed with methanol and dried overnight at 100 oC. Secondly, the white powder and 10 g L-1 tannic acid solution (w/v = 2) were mixed and aged for 5 min. The reaction was stopped and the product was cleaned with water and methanol to achieve the hollow ZIF-8 nanocubes. Lastly, the ZIF-8 nanocrystals and the hollow ZIF-8 nanocubes were carbonized under N2 flow at 900 °C for 3 h in a tube furnace to get ZIF-8 SCNCs and ZIF-8 HCNCs, respectively. Fabrication of the HCNCs-F 5


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The fabrication of HCNCs-F by physical adhesion involved the following processes. (a) The SSW was sonicated to clean in acetone for 30 min. After drying, the anterior segment of the SSW was immerged in 40% HF solution for 30 min, washed with water and dried in a vacuum oven for further use. (b) 0.5 g neutral silicone glue and 1 mL toluene were added in a 2 mL centrifugal tube and mixed entirely by ultrasound. The cleaned SSW was dipped into the mixed solution for 5 seconds then taken out and wiped by a piece of filter paper. Subsequently, the wire was rotated into the powder of ZIF-8 HCNCs. (c) The coated fiber was placed in an oven at 100 oC for 20 min to obtain the HCNCs-F. After that, the fiber was fixed on a microsyringe (5-µL) and then aged in the GC injector for 60 min at 280 °C. Enrichment procedure and sample preparation The stock solution of PAHs (200 mg L-1) was prepared in methanol and stored in a refrigerator. The PAH working solutions at the concentrations of 5, 10, 50, 100, 500, 1000, 2000 ng L-1 were prepared by diluting the stock solution with ultrapure water step by step. The aqueous sample of different concentrations (10 mL) and 1.0 g NaCl (10% w/v) were placed into a 25 ml glass bottle, under stirring at the rate of 800 rpm. A thermostatic water bath was used to control the extraction temperature at 50 oC. The HCNCs-F was immersed into the sample solution directly to perform the extraction. After the extraction time of 30 min, it was removed from the vial and inserted into the GC inlet immediately for 3 min at 280 oC for desorption and analysis. The real water samples were collected freshly from three sampling sites, which are illustrated in Figure S1. The resulted samples were filtered through a 0.22 µm filter membrane. The above mentioned SPME procedure was employed to extract the PAHs from the treated water samples. Finally, a spiked level of 100 ng L-1 for real samples were used for the analytical performance assessment. RESULTS AND DISCUSSION SEM and TEM images were employed to investigate the structure of the as-prepared materials. As shown in Figure 1, both SCNCs and HCNCs possess uniform and cubic morphology, which is similar to their parent nanocubes (Figure S2a,b), indicating 6



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carbonization has negligible effect on the final morphology. The hollow structure can be clearly observed from Figure 1d and Figure 1e. The formation of hollow structure is due to the etching treatment of tannic acid. The protons released from the tannic acid can break the coordination bonds of ZIF-8 frameworks and reach the inside of the frameworks. Meanwhile, the tannic acid can act as a protective agent on the surface of ZIF-8 to prevent proton further etching.

27,28

The size of SCNCs and HCNCs is about 150 nm and 120nm, respectively.

Compare with their parent nanocubes (Figure S2a,c), about 10% shrinkage is observed after carbonization, which is similar to previous reports.29 The XRD patterns before and after the etching process confirm that ZIF-8 nanocubes and hollow nanocubes match well with the simulated spectra indicating their high crystallinity (Figure S3), in accordance with previous report.30 The N2 adsorption-desorption curves of HCNCs and SCNCs are presented in Figure 1c. It can be seen that HCNCs show type IV isotherms, implying the existence of mesopores. However, SCNCs possess typical type I isotherm, indicative of a microporous structure.31 The specific surface area and pore volume of HCNCs and SCNCs are calculated to be 601 m2 g-1 and 0.43 cm3 g-1, and 769 m2 g-1 and 0.13 cm3 g-1, respectively. The pore size distribution curves of SCNCs and HCNCs (Figure 1f and the inset in Figure 1f) shows that the HCNCs has richer mesoporous structure, which is favorable for mass transfer.32,33 To explore the elemental composition of the HCNCs, XPS analysis is performed and the result indicates that the sample HCNCs is composed of carbon, oxygen and nitrogen (Figure S4).

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Figure 1. (a) SEM and (b) TEM of SCNCs; (c) N2 adsorption-desorption isotherms of SCNCs and HCNCs; (d) SEM and (e) TEM of HCNCs; (f) BJH pore size distribution curves of SCNCs and HCNCs and the inset was the magnified BJH pore size distribution curves.

The obtained HCNCs are further coated on the surface of SSW by physical adhesion as SPME fiber (Figure 2a and the inset in Figure 2a ). The coating thickness (t) was calculated to be approximately 15 µm based on the equations t = (D-d)/2, where D and d represent the diameter of the coated and bare SSW. From the adhesion nanoparticles SEM images (Figure 2b), the HCNCs can be observed clearly, which are evenly distributed on the surface of the SSW.34,35 Moreover, the broken particles (white arrow) indicate that the hollow structure is well kept during the coating process.

Figure 2. (a) SEM image of HCNCs-F and the inset was a digital picture of HCNCs-F; (b) high-magnification SEM image of the HCNCs-F.

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PAHs is a serious threat to human health because of their carcinogenic, mutagenic, and teratogenic.36 Herein, six primary hydrophobic PAHs including Nap, Ace, Flu, Phe, FlA and Pyr were selected as standard analytes to evaluate the extraction performance of the self-made HCNCs-F. Before utilization, several factors were needed to be individually optimized. As shown in Figure S5, the extraction time 30 min, extraction temperature 50 oC, desorption time 3 min and salt concentration 10% (w/v) were chosen as the optimal conditions. After optimization, HCNCs-F was applied in the evaluation and detection experiments.

Figure 3. (a) Extraction efficiency comparison for the PAHs of HCNCs-F with SCNCs-F and commercially available SPME fibres, error bar shows the standard deviation for triplicate extractions; (b) Comparison of lowest LOD vs extraction time for HCNCs-F with the other reported materials.

The extraction efficiency of the HCNCs-F was compared with those of SCNCs-F, commercial 100 µm polydimethylsiloxane (PDMS) and 75 µm Carboxen/PDMS fibers. The commercial PDMS coating is homogeneous polymer and the Carboxen/PDMS coating is a type of carbon material. As shown in Figure 3a, the HCNCs-F exhibited higher extraction efficiencies towards PAHs. Typically, the extraction efficiency of HCNCs-F was 2.4 to 6.9 times higher than that of Carboxen/PDMS fiber, 2.0 to 3.6 times for PDMS fiber and 1.1 to 2.2 times for SCNCs-F. Apparently, the HCNCs-F showed superior adsorption abilities towards the selected analytes than the SCNCs-F and commercial fibers. Furthermore, the analytical method incorporating HCNCs-F and GC/MS was established for determination of PAHs. The relevant analytical parameters are listed in Table 1. The linear ranges were observed to be in the wide range of 10-2000 ng L-1 for Nap and 5-2000 ng L-1 for other five analytes, with correlation coefficients (R) higher than 0.9850. The relative standard deviations 9


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(RSDs) (six replicate extractions) were determined to be ranging from 3.7 to 8.2%. Meanwhile, the reproducibility was tested based on three different fibers, and the fiber-to-fiber RSDs were calculated to be 2.6-8.8%. The detection limits (LODs), based on a signal-to-noise ratio of 3, were in the range of 0.03-0.70 ng L-1. From Figure 3b, it was found that the LODs of the established method were lower than most of the other reported works within shorter analysis time. These results revealed that the proposed method could provide reliable and sensitive analysis for PAHs. Table 1 Analytical parameters of the developed SPME-GC-MS method for the determination of the PAHs under the optimized conditions. RSD Correlation LOD Linear range -1 Analyte coefficient (ng L One fiber Fiber-to-fiber (ng L-1 ) S/N= 3) (R) (%, n = 6) (%, n = 3) Nap

10-2000

0.9874

0.70

6.1

3.5

Ace

5-2000

0.9944

0.41

7.5

6.3

Flu

5-2000

0.9850

0.29

8.2

2.6

Phe

5-2000

0.9971

0.14

5.4

8.8

FlA

5-2000

0.9935

0.07

3.7

7.0

Pyr

5-2000

0.9883

0.03

6.9

4.3

To study the adsorption mechanism of HCNCs, adsorption experiments of SCNCs and HCNCs for different kinds of analytes under different contact time were conducted. The experimental data for the PAHs (Nap, Ace and Flu), benzenes (o-X) and esters (DMP) were shown in Figure 4a and Figure S6, respectively. The adsorption equilibriums of the analytes on the HCNCs were attained with shorter contact time compared the SCNCs, which indicated that the adsorption rate of HCNCs was obviously faster than SCNCs. Two frequently used kinetic models, including the pseudo-first-order and pseudo-second-order models, were applied to fit the experimental data. The pseudo-second-order model showed higher correlation coefficient R value and the calculated adsorption capacity value was in better agreement with the experimental date (Table S1). This result revealed that the pseudo-second-order kinetic model was more suitable for the adsorption of PAHs on the SCNCs and HCNCs, indicating that chemisorption was involved in the adsorption 10



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process. Moreover, the pseudo-second-order rate constant (k2) of HCNCs was higher than that of SCNCs, demonstrating that HCNCs possessed faster adsorption kinetic for PAHs than SCNCs. As shown in Figure 4b, the target molecules can diffuse into the inner space of HCNCs to accelerate mass transfer compared with SCNCs.

Figure 4. (a) The experimental data of the analytes adsorbed by SCNCs (solid symbol) and HCNCs (hollow symbol); (b) Schematic mechanism of mass transfer in HCNCs and SCNCs.

In order to investigate the effect of the hollow structure on SPME fibres, SCNCs-F and HCNCs-F were employed to analyse target molecules including Nap, Ace and Flu. The extraction experiment was conducted in aqueous samples. As shown in Figure 5, the extraction equilibrium time of Nap, Ace and Flu by using HCNCs-F was determined to be 40, 80 and 140 min, respectively. The corresponding time of Nap, Ace and Flu for SCNCs-F was 70, 100 and 180 min. Furthermore, the extraction experiment was also conducted in non-aqueous samples to avoid the influence of water molecule. The extraction equilibrium time for HCNCs-F was found shorter than SCNCs-F (Figure S7). It can be assessed from the results that hollow mesoporous structure markedly enhanced extraction rate of targeted analytes due to faster mass transfer. Adsorption isotherm modelling can provide a definite insight to understand and elucidate the interaction between the adsorbent and adsorbate. In this sense, two kinds of adsorption isotherm models, including Langmuir and Freundlich models, were used here to fit the equilibrium data of PAHs by using HCNCs. The fitting curves and the corresponding regression parameters are shown in Figure S7 and Table S2, respectively. Freundlich model with higher correlation coefficient was better fitted than 11


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Langmuir model, which shows that the adsorption of PAHs on HCNCs was a heterogeneous and multilayer adsorption process. The Freundlich constants 1/n, smaller than 1, were indicative of a favourable adsorption process. It is worth noting that the KF values increased by increasing the number of benzene rings of the analytes. This revealed that the high adsorption capacity of HCNCs was mainly attributed to the π-π interactions between PAHs and HCNCs.

Figure 5. Extraction time curves of HCNCs-F and SCNCs-F for Nap (a), Ace (b) and Flu (c) in aqueous samples.

The remarkable performance of HCNCs-F may be ascribed to the following reasons: (1) The hydrophobic surface provide “hydrophobic-hydrophobic” interaction between HCNCs and the PAHs. HCNCs-F was used to extract the polar analytes. The extraction efficiency of HCNCs-F towards hydrophobic Nap was 8.7 and 11.4 times higher than that for hydrophilic DEP and DMP (Figure S9), indicating that the hydrophobic interactions play a selective role in SPME. (2) The π-π interaction between the condensed rings of PAHs and π-conjugated groups of the HCNCs coating has an influence on the adsorption affinity. (3) The hollow mesoporous structure was beneficial to mass transport of the analytes. 3.4. Environmental applications Table 2 Analytical results for the determination of the PAHs in environmental water samples. S1 S2 S3 analyte Found

Recovery

-1

RSD

a

Found

(ng L ) (%, n = 3) (%, n = 3)

Recovery

-1

RSD

a

(ng L ) (%, n = 3) (%, n = 3)

Found

Recovery

-1

RSD

a

(ng L ) (%, n = 3) (%, n = 3)

Nap

nd

95.3

7.87

nd

92.4

8.28

nd

116.3

4.13

Ace

2.9

91.2

5.01

nd

102.6

5.26

2.3

107.8

3.34

Flu

nd

94.4

2.36

nd

98.5

4.25

nd

96.0

8.32

Phe

nd

105.0

6.17

42.1

97.3

3.29

24.9

93.8

2.64 12



FlA

11.6

96.4

7.81

37.7

101.7

8.27

12.5

104.2

3.87

Pyr

7.8

97.1

3.05

4.4

92.1

6.26

9.2

95.9

6.36

nd, not detected. a Recovery date for spiked with 100 ng L-1 PAHs solution.

The proposed method using HCNCs-F was further applied for the trace analysis of PAHs from three real water samples. The quantification results are summarized in Table 2. Both FlA and Pyr were detected in the three samples (4.4-37.7 ng L-1). In addition, Nap (2.9 ng L-1) and Phe (42.1 ng L-1) were found in S1 and S2, respectively, which were both detected in S3. Their concentration in S3 was reduced compared with that in S1 and S2. The method validation experiment was conducted by spiking 100 ng L-1 standards into the water sample. The recoveries of S1, S2 and S3 were 91.2-105.0%, 92.1-101.7% and 93.8-116.3%, and the corresponding RSDs were 3.05-7.87%, 3.29-8.28% and 2.64-8.32%, respectively. The typical chromatograms shown in Figure 6 demonstrated that the extraction capacity of HCNCs-F for PAHs was barely influenced by the impurities, which may exist in actual samples. The results showed that the HCNCs-F based SPME-GC/MS method offered effective and practical determination of PAHs from the real water samples. 3 2 3

1

4)

Intensity(x10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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56

4

2

d c

1

b a

0 12

15

18

Time (min)

21

Figure 6. Chromatograms of PAHs from water samples S1 (a), S2 (b), S3 (c) and spiked water sample (d). Peak identity: 1-Nap, 2-Ace, 3-Flu, 4-Phe, 5-FlA, 6-Pyr.

CONCLUSIONS In summary, MOFs based hollow carbon nanocubes were employed as coating materials for SPME. Benefit from the novel hollow structure, HCNCs-F demonstrated a faster mass transfer compared with SCNCs-F, thus leading to shorter analysis time. 13


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The resultant HCNCs-F was explored for the direct immersion SPME of six hydrophobic PAHs coupled with GC/MS. It provided better enrichment ability over commercial fibers due to the hydrophobic interactions, π-π interactions and hollow mesoporous structure. The established method was applied for the determination of PAHs from real water samples with a good recovery. It is expected that HCNCs will be a kind of promising material for analytical chemistry. Supporting Information Investigation of Adsorption Theory; Optimizations for SPME parameters; Abbreviation; Figure S1-S9; Table S1-S2; This information is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51478224), the priority academic program development of Jiangsu higher education institutions.

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