Metal–organic framework-derived nitrogen-doped carbon nanotube cages as efficient adsorbents for solid-phase microextraction of polychlorinated biphenyls

Journal Pre-proof Metal–organic framework-derived nitrogen-doped carbon nanotube cages as efficient adsorbents for solid-phase microextraction of polychlorinated biphenyls Yuheng Guo, Xue He, Chuanhui Huang, Hui Chen, Qiaomei Lu, Lan Zhang PII:

S0003-2670(19)31233-4

DOI:

https://doi.org/10.1016/j.aca.2019.10.023

Reference:

ACA 237157

To appear in:

Analytica Chimica Acta

Received Date: 21 July 2019 Revised Date:

12 October 2019

Accepted Date: 14 October 2019

Please cite this article as: Y. Guo, X. He, C. Huang, H. Chen, Q. Lu, L. Zhang, Metal–organic framework-derived nitrogen-doped carbon nanotube cages as efficient adsorbents for solid-phase microextraction of polychlorinated biphenyls, Analytica Chimica Acta, https://doi.org/10.1016/ j.aca.2019.10.023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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Metal–organic framework-derived nitrogen-doped carbon nanotube cages as

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efficient adsorbents for solid-phase microextraction of polychlorinated biphenyls

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Yuheng Guo, Xue He, Chuanhui Huang, Hui Chen, Qiaomei Lu, Lan Zhang*

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Key Laboratory for Analytical Science of Food Safety and Biology (Ministry of

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Education & Fujian Province), College of Chemistry, Fuzhou University, Fuzhou,

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Fujian, 350116, China

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Corresponding author: Lan Zhang

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Postal address: College of Chemistry, Fuzhou University,

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Fuzhou, Fujian, 350116, China

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Tel: 86-591-22866135

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Fax: 86-591-22866135

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E-mail: [email protected] (L. Zhang)

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ABSTRACT: An efficient and stable adsorbent is of critical importance for

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solid-phase microextraction (SPME). In this study, we prepared metal–organic

24

framework-derived nitrogen (N)-doped carbon (C) nanotube cages (N-CNTCs) with

25

unique N-doped active sites and C-rich nanotubes to coat SPME adsorbents. This new

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material was obtained via a simple thermal treatment with ZIF-67, and exhibited high

27

porosity and excellent chemical and thermal stability. Compared with commercial

28

fibers and traditional C nanotube-coated fiber (15 nm), N-CNTC-coated fiber

29

exhibited better extraction properties, mainly due to its π–π interactions, abundant

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active sites, and hollow cage structure, which is composed of interconnected

31

crystalline N-doped C nanotubes. N-CNTC-coated fiber exhibited better extraction

32

performance and shorter extraction equilibrium time than the solid N-doped C-coated

33

fiber due to its hollow cage structure. The N-CNTC-coated fiber was then used to

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identify polychlorinated biphenyls (PCBs) with wide linear range (0.3–1000.0 ng L–1),

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low limits of detection (0.10–0.22 ng L–1), good repeatability (intra-day, 2.6–3.8%;

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inter-day, 3.3–4.8%), and good reproducibility (< 8.6%). We then successfully applied

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the N-CNTC-coated fiber to detect PCBs in river water samples from six cities in

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Fujian Province and obtained satisfactory recovery levels. Thus, the novel N-CNTCs

39

coating proposed in this study is a promising candidate for SPME coating.

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Keyword: Nitrogen-doped carbon nanotubes cages, Solid-phase microextraction, Gas

41

chromatography-mass spectrometry, Polychlorinated biphenyls

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2

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1. Introduction

45

Due to the complexity and diversity of sample matrices, the development of an

46

efficient and stable method for direct detection of target analytes in complex matrices

47

is of great importance. Solid-phase microextraction (SPME) is a rapid and effective

48

sample pretreatment method that is easy to operate, requires almost no organic

49

solvents during analysis, and is easily coupled to gas chromatography (GC) and

50

high-performance liquid chromatography (HPLC) systems. Therefore, SPME

51

technology has been widely applied in environmental [1], biological [2],

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pharmaceutical [3] and food analyses [4]. The general principle of SPME is based on

53

equilibrium of target analyte distribution between the fiber coating and sample matrix.

54

Therefore, fiber coating is a key factor that directly affects the selectivity, sensitivity,

55

and application of SPME. However, commercial SPME fibers have some limitations,

56

such as poor thermal stability and solvent resistance, low adsorption capacity, limited

57

coating options and time-consuming extraction process, which limit their practical

58

application [5, 6]. Thus, the development of high-sensitivity and high-stability coating

59

materials is in urgent demand.

60

To date, diverse advanced materials have been used as SPME fiber coatings.

61

Among these materials, hollow nanostructure materials are valued because they

62

improve adsorption equilibrium and adsorption capacity [7, 8]. Carbon (C)-based

63

nanomaterials exhibit excellent extraction performance for non-polar or weakly polar

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targets and have been used in SPME for their intrinsic hydrophobicity [9, 10].

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Nitrogen (N)-doped nanomaterials have attracted much research attention due to high

3

66

porosity and abundant active sites [11, 12]. The incorporation of N atoms into the C

67

nanostructure could significantly enhance the adsorption, energy storage, and

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mechanical properties of nanomaterials [13-15]. However, combining the desirable

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physical and chemical properties of these materials to achieve rapid equilibration and

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high extraction capacity in SPME remains challenging. Metal–organic frameworks

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(MOFs), which exhibit permanent nanoscale porosities, tunable composition, and

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diverse structures represent a new platform for easily synthesizing functionalized C

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nanomaterials. The microstructure and functionalization of MOF-derived C materials

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can be adjusted by altering the composition and morphology of MOF precursors, and

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by controlling the annealing temperature/time, gas atmosphere, and heating rate

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during pyrolysis process. Previous studies of MOF-derived nanoporous C have

77

successfully performed magnetic solid-phase extraction of some neonicotinoid

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insecticides [16], organochlorine pesticides [17], phenylurea herbicides [18], and

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flunitrazepam [19]. However, few studies have explored MOF-derived N-doped C

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cage materials as SPME adsorbents for enrichment pollutant enrichment, despite their

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great promise for practical applications in analytical chemistry.

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In this study, we synthesized the N-doped C nanotube cages (N-CNTCs) by

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thermal treatment of zeolitic imidazole framework-67 (ZIF-67) particles. ZIF-67

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derived N-CNTCs were applied as a coating on stainless steel wire as SPME fibers

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using a gluing method. N-CNTC-coated fiber was then used to identify seven

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polychlorinated biphenyls (PCBs). We compared the performance of N-CNTCs with

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that of commercial fibers, C nanotube (CNT)-coated fibers (15 nm), and solid

4

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N-doped C (SNC)-coated fiber, and explored the application of N-CNTCs as a

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possible extraction mechanism of the N-CNTCs for PCBs. Finally, we coupled the

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novel N-CNTC-coated fibers with SPME/gas chromatography–mass spectrometry

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(GC–MS) to monitor PCBs from environmental waters from various cities in Fujian

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Province.

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2. Experimental

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2.1. Reagents and materials

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Standard solution (10 mg·L–1) of PCBs (congener numbers: 28, 52, 101 153, 138, 180,

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194), PCB(28), and PCB(101) were purchased from Aladdin Reagent Co. Ltd

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(Shanghai, China), the structure was shown in Table 1. Cobalt nitrate hexahydrate

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(Co(NO3)2·6H2O), 2-methylimidazole (mIM, 99%), dichloromethane (CH2Cl2) and

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hydrofluoric acid (HF) were purchased from China Pharmaceutical Reagent Co., Ltd

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(Shanghai, China). Acetone (CH3COCH3), sodium chloride (NaCl), methanol (MeOH)

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and ethanol (EtOH) were purchased from Fuchen Chemical Reagent Factory (Tianjin,

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China). Sulfuric acid (H2SO4) was purchased from Zhejiang Sanying Chemical

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Reagent Co., Ltd. (Zhejiang, China). Ultrapure water (resistivity, 18.2 MΩ·cm) was

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obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA).

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All of the reagents were at least of analytical grade.

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2.2. Instruments

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An HP 6890N gas chromatograph (Agilent, Palo Alto, USA) equipped with a 5973

108

MS detector (Agilent, Palo Alto, USA) was used for all experiments. A DB-5 MS GC

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capillary column (30 m × 0.25 mm × 0.25 µm) (Agilent, Palo Alto, USA ) was used

5

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for GC separation. The carrier gas was high purity helium (purity > 99.999%) and the

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flow rate was 1.0 mL min–1; All the measurements were carried out at an injector

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temperature of 270 °C under the splitless mode. The oven temperature program, initial

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at 130 °C for 3 min, then increased to 280°C at a speed of 8°C min–1 and held for 6

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min. The total analysis time was 27.75 min. The MS conditions were performed as

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follows: electron impact mode with 70 eV energy; the interface, ion source and

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quadrupole temperature were set at 300 °C, 230 °C and 150 °C, respectively; the data

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of extraction optimized partial was acquired based on full scan mode (Scan) with a

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solvent delay of 11 min; The mass scan was studied in the range of 50-550 amu (m/z).

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Data for the quantitation section was acquired using the selective ion detection mode

120

which was shown in Table 1.

121

Scanning electron microscopy (SEM) images were recorded using a JSM-6300F

122

SEM instrument (JEOL, Tokyo, Japan); Transmission electron microscopy (TEM)

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images

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X-ray powder diffraction (XRD) data were measured by X'Pert Pro MPD

125

Diffractometer (Philips, Netherlands); Micromeritics ASAP 2020 nitrogen adsorption

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apparatus (Micromeritics, Norcross, Georgia, USA) was employed for the

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N2 adsorption and desorption isotherm and Brunauer–Emmett–Teller (BET) surface

128

area of N-CNTCs; ESCALAB 250X-ray Photoelectron spectrometer (XPS) was used

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to evaluate the structure and properties of N-CNTCs; Fourier transform infrared

130

(FT-IR) spectroscopy was carried out using a 360 Fourier infrared spectrometer (IR)

131

(Nicololi, USA); An IKA RET magnetic stirrer (IKA, Guangdong, China) was used

were

obtained

on

Tecnai

G2

6

F20

S-Twin

(FEI,

200

kV);

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for extraction conditions optimization.

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2.3. Synthesis of N-CNTCs

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N-CNTCs were synthesized via a simple thermal treatment with ZIF-67 particles

135

following the method of the literature [20]. Briefly, 1.97 g mIM and 1.746 g

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Co(NO3)2 ·6H2O were dissolved in a 40 mL of solution methanol and ethanol (VMeOH :

137

VEtOH = 1 : 1). The mixture solution was stirred continuously in a beaker for 10 s, and

138

then stored at room temperature for 20 h. ZIF-67 particles were collected by

139

centrifugation, washed three times with ethanol and dried at 80°C under a vacuum.

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ZIF-67 particles were then calcined at 350°C for 1.5 h in a tube furnace. The

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furnace temperature was increased at a rate of 1°C min-1 to 700°C, and maintained for

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4 h, and then gradually decreased to room temperature. During pyrolysis, the furnace

143

atmosphere was Ar/H2 (95%/5% , v/v). Finally, the resulting material was treated with

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0.5 M H2SO4 for 6 h to remove accessible cobalt (Co) nanoparticles. The material was

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washed several times with ultrapure water, collected by centrifugation, and dried at

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80°C under a vacuum to obtain the N-CNTCs. For comparison, we prepared solid

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N-doped C (SNC) by the same method, but in an N2 atmosphere.

148

2.4. Fabrication of the SPME fiber

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To prepare the N-CNTC-coated SPME fiber, stainless steel wire (length, 2.0 cm) was

150

roughened by immersion in a 30% hydrofluoric acid solution at 70°C for 10 min. The

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corroded surface was cleaned with ultrapure water and ethanol, and air-dried. The

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treated stainless steel wire was dipped into epoxy resin for 5 s and then rotated on a

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clean weighing paper to remove superfluous epoxy resin and ensure a sufficiently

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small thickness. The stainless steel wire was then rotated in the prepared N-CNTCs

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powder and cured at 100°C for 30 min. This procedure was performed twice. The

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resulting fiber was conditioned at 250°C for 2 h in the GC inlet under an N2

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atmosphere until a clean blank was obtained. To obtain a uniform coating with a

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thickness of approximately 30 µm. This result was confirmed through SEM.

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Fibers based on ZIF-67, SNC and CNT-15 nm were prepared using the same

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methods for comparison.

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2.5. SPME procedure

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Stock solutions (1.0 mg mL–1) of PCBs were prepared in acetone. We prepared

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working standard solutions of PCBs by diluting the stock solution stepwise with

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ultrapure water. All extraction experiments were performed in 25-mL glass vials; 20

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mL working standard solution or actual sample solution was added into the vial. The

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N-CNTC-coated fiber was inserted into the standard or sample solution for extraction,

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and the vial was simultaneously immersed in a water bath with a magnetic stirrer (Fig.

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S1) to maintain the optimum temperature of 50°C and agitation speed of 750 rpm for

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30 min. Following extraction, the fiber was removed from the vial and immediately

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inserted into the GC inlet for thermal desorption at 270°C for 4 min and subsequent

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MS analysis. The fiber was cleaned everyday by leaving it in the injection port for 30

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min at 300°C to eliminate any carry-over of analytes from the previous extraction.

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Before starting a set of experiments, a blank analysis was performed to confirm the

174

absence of contaminants were desorbed from the fiber.

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2.6. Sample pretreatment

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River water samples were collected from six cities (Fujian, China) for analysis. Water

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samples were filtered through a 0.45-µm filter membrane (Tianjin Jinteng Experiment

178

Equipment Co. Ltd., Tianjin, China), and stored in brown glass bottles at 4°C for

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subsequent experiments

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3. Results and Discussion

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3.1. Synthesis and characterization of N-CNTCs coating

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The process of synthesizing N-CNTCs and fabricating N-CNTC-coated fiber is

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presented in Figure 1a. As described in Section 2.3, ZIF-67 dodecahedron crystals

184

were synthesized by mixing mIM and Co(NO3)2·6H2O at room temperature.

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Following the Co metal autocatalysis pyrolysis process under Ar/H2 (95%/5%, v/v),

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ZIF-67 was converted to morphology-preserved thin N-doped C nanotube

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(N-CNT)-assembled structures with uniform hollow cages. N-CNTCs were then

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directly coated onto stainless steel wire as SPME fibers using a gluing method. The

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N-CNTC-coated fiber exhibited enhanced extraction properties in practical

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application.

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Surface morphology of the prepared materials was characterized by TEM and

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SEM. The ZIF-67 nanoparticles are dodecahedron crystals with uniform micrometer

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size (Fig. 1b). The N-CNTCs have a cage-like structure that faces inward to form an

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opening with an average size of 1 µm (Fig. 1c and 1d). The outer diameter of the thin

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multi-walled N-CNTs ranged from 10 to 20 nm, with an inner diameter of ~5 nm (Fig.

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1c, inset). These small hollow cages with uniform particle size attached

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homogeneously to the surface of the fibers. SEM images confirm that

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N-CNTC-coated fibers were successfully synthesized and evenly distributed on the

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fiber surface (Fig. 1d and 1e). Elemental mapping of N-CNTCs revealed the uniform

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distribution of C, N, oxygen (O), and Co throughout the particles (Fig. 1f).

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The porous nature of the coating material had a significant effect on extraction

202

and adsorption. The specific surface area and porosity of N-CNTCs crystals were

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measured by N adsorption–desorption at 77 K. The N2 adsorption–desorption

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isotherms of N-CNTCs formed a Type IV adsorption isothermal curve with a

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pronounced hysteresis loop (Fig. 2a), indicating that the N-CNTCs consisted of

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micropores and mesopores. Our experimental results showed that the BET specific

207

surface area of the N-CNTCs was 399 m2 g–1. This high surface area could facilitate

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the improvement of extraction performance, making the material suitable as a coating

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material for SPME.

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To investigate the form and mode of action of each element in the N-CNTCs

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coating, we examined the N-CNTCs chemical composition by XPS and observed

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characteristic peaks for C, N, O, and Co (Fig. 2b). The high-resolution C 1s spectrum

213

was divided into three peaks, which correspond to C-C at 283.8 eV, C=N at 285 eV,

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and C=O at 288.2 eV (Fig. S2b). N content reached 13.86%; the high-resolution N 1s

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XPS spectrum can be attributed to two types of N species: pyridine N at 397.7 eV and

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pyrrole N at 399.4 eV (Fig. S2a). Doped N could enhance intermolecular forces such

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as hydrogen bonding and stacking. In particular, Dai [21] demonstrated that C

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nanotubes contribute to a single-electron p system whereas N exists as a pyridinic site

219

and a 2-p electron system in a pyrolytic site; therefore, strongly promoting π–π

10

220

conjugation for analytes. The O element was derived from oxidation of the C surface

221

by residual O in the original MOF [22].

222

Then the crystalline structure of N-CNTCs was investigated by XRD patterns

223

(Fig. 2c). The characteristic C peak of the graphite phase was at 26.6°, corresponding

224

to the (002) crystal plane of graphite-phase C. Our comparison of these peaks between

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graphite-phase C and metal cobalt demonstrated successful synthesis of the

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N-CNTCs.

227

Raman spectroscopy showed that C in the N-CNTCs was in an ordered graphite

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phase (Fig. 2d), with two peaks having C characteristics, the G (~1600 cm–1) and D

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(~1350 cm–1) bands, which are related to graphite sp2 hybrid C and disordered or

230

defective C, respectively. These diagrams show that the N-CNTCs IG/ID ratio was

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much greater than 1, indicating good graphite sp2 hybrids and an ordered graphite

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phase. Thus, the N-CNTCs coating offers a strong π–π stacking interaction with

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analytes.

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3.2. Optimization of PCBs extraction conditions

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To achieve ideal PCBs extraction efficiency by the N-CNTCs coating in SPME, the

236

effects of several parameters were studied and optimized, including extraction

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temperature, extraction time, desorption temperature, desorption time, agitation speed,

238

salt concentration and PH (Fig. 3). The optimization experiments were performed

239

using 1.0 ng mL–1 PCBs solution with a total volume of 20 mL.

240

Extraction time was an important factor that affected the development of the

241

SPME method. As extraction time increases, the amount of analyte extracted

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increases until adsorption equilibrium is reached [23]. In this study, extraction time

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was investigated from 10 to 50 min. As extraction time increased, the extraction

244

efficiency of all PCBs increased from 10 to 30 min, followed by a slight decline in

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PCB(180), whereas the other PCBs remained almost unchanged from 30 to 50 min

246

(Fig. 3a). To increase extraction efficiency, we selected 30 min as the optimal

247

extraction time for further optimization.

248

Extraction temperature has a dual effect on the extraction process. High

249

temperature can accelerate the rate of diffusion; however, increasing the temperature

250

will reduce the extraction rate because the extraction process is exothermic [24]. The

251

effect of extraction temperature was tested from 30 to 70°C; the highest extraction

252

efficiency was achieved at 50°C for all analytes (Fig. 3b). For most PCBs, extraction

253

efficiency decreases above the optimal temperature, because the partial coefficients of

254

the analytes decrease at high temperatures. Thus, we selected an extraction

255

temperature of 50°C for further experiments.

256

Proper desorption conditions can improve the sensitivity of the SPME method

257

and prolong the lifespan of the fiber [25]. We observed variation in desorption time

258

(1–6 min) and temperature (250–290°C) (Fig. 3c and 3d). These results indicate

259

optimum desorption conditions of 4 min and 270°C.

260

Agitation enhances the diffusion of analytes between the fiber coating and the

261

sample solution [26]. We examined the effects of various agitation speeds from 0 to

262

1000 rpm. For all analytes, the peak areas reached a maximum at 750 rpm (Fig. 3e).

263

Therefore, we selected an agitation speed of 750 rpm for our experiments.

12

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In this study, the influence of salt addition on SPME was investigated by

265

changing the NaCl concentration from 0 to 0.2 g mL–1. The peak areas of three target

266

PCBs increased as NaCl concentration increased from 0 to 0.05 g mL–1, and then

267

decreased with further addition of NaCl (Fig. 3f). The peak areas of three other PCBs

268

decreased continually as NaCl concentration increased.This phenomenon can be

269

attributed to salting out and precompetitive effects [27]. On the one hand, salts can

270

affect the solubility of analytes and increase the distribution coefficient in aqueous

271

solutions. On the other hand, salts can affect the surface properties of the adsorbent

272

and high salt concentrations can lead to competitive adsorption with the target analyte,

273

reducing adsorption efficiency. Therefore, we selected an NaCl concentration of

274

0.05 g mL–1 in the sample solution for this study.

275

An appropriate pH can improve SPME extraction performance. In this study, the

276

effect of sample pH on PCBs extraction efficiency was investigated by adjusting the

277

pH from 2.0 to 12.0. The peak areas of all seven target PCBs increased significantly

278

as pH increased from 2.0 to 4.0 (Fig. 3g). Subsequently, the peak areas of PCBs (28)

279

and (52) remained nearly constant as the pH increased from 4.0 to 12.0. The peak area

280

of PCB(101) did not change significantly within a pH range of 2.0–10.0, but

281

decreased rapidly when pH exceeded 10.0. By contrast, the peak areas of PCBs (138),

282

(154), (180), and (194) had maximum values at pH 4–8, and then decreased rapidly as

283

the pH increased from 8 to 12. The peak area of all analytes did not change

284

significantly within a pH range of 4.0-8.0. This is due to the excellent chemical

285

stability of N-CNTCs. Considering that the typical pH of lakes in the study areas

13

286

ranges from 5.0 to 7.0, we did not adjust the sample pH.

287

From the experimental results, the optimum extraction conditions of PCBs were

288

obtained as follows: extraction temperature: 50°C; extraction time: 30 min; desorption

289

temperature: 270°C; desorption time: 4 min; agitation speed: 750 rpm; salt

290

concentration (CNaCl ): 0.05 g mL–1.

291

3.3. Extraction performance and stability of N-CNTC-coated fiber

292

Under optimal extraction conditions, we compared the PCBs extraction performance

293

of the N-CNTC-coated fiber with that of two commercial SPME fibers (65 µm

294

PDMS/DVB and 75 µm CAR/PDMS) and traditional C nanotube (CNT-15 nm)

295

SPME fiber (30 µm thickness). For all analytes, the N-CNTC-coated fiber exhibited

296

higher extraction efficiency than the traditional C nanotube fiber with the same

297

thickness (Fig. 4a). However, although the thickness of the N-CNTCs coating was

298

30 µm, it showed much better extraction performance than the commercial fibers

299

among all analytes. Furthermore, we prepared ZIF-67-derived solid N-doped C (SNC)

300

following the same method used to prepare N-CNTCs, except in an N2 atmosphere.

301

SEM and TEM revealed that the SNC materials were solid, with a dodecahedron

302

structure (Fig. S3). The ZIF-67-, SNC-, and N-CNTC-coated fibers were then used to

303

extract analytes. The N-CNTCs exhibited more sensitive PCBs extraction

304

performance than SNC or ZIF-67 (Fig. 4b).

305

To further confirm the structure advantages of N-CNTCs for PCBs extraction,

306

we conducted an extraction experiment using PCBs (28) and (101) as the target

307

molecules using N-CNTC- and SNC-coated fibers. As shown in Fig. S4, the

14

308

equilibrium times for N-CNTC-coated fiber extraction of PCBs (28) and (101) were

309

30 and 45 min, respectively. However, those for extraction by SNC-coated fiber were

310

50 and 55 min, respectively. These results indicate that the hollow cage structure

311

composed of N-doped C nanotubes significantly improved the rate of extraction of the

312

target analytes, because mass transfer is faster for this structure than for solid

313

nanoparticles.

314

Under optimal conditions, the stability of the N-CNTCs coating was tested with

315

different organic solvents (acetone, dichloromethane, or methanol) and different pH

316

levels (pH 6 or 8). The N-CNTC-coated fiber can be applied in both polar and

317

non-polar solvent (Fig. 4c). Its excellent stability indicates that N-CNTC-coated fiber

318

is suitable for measurements in experimental samples.

319

3.4. Possible extraction mechanism of the N-CNTC-coated fiber

320

Adsorption kinetics is a important character that provides insights into possible

321

adsorption mechanisms. We conducted an adsorption experiment of 0.5 mg N-CNTCs

322

or SNC were used as the SPME coating to extract PCBs (28) and (101) (2 ng mL–1) in

323

20 mL working standard solution, respectively. The results were shown in Fig. S5.

324

Compared with SNC, the adsorption equilibrium of analytes on N-CNTCs was

325

achieved in shorter contact time. This indicates that N-CNTCs have a faster

326

adsorption rate to analytes than SNC. Two frequently used kinetic models,

327

pseudo-first-order kinetic model (1) and pseudo-second-order kinetic model (2), were

328

applied to describe the adsorption kinetic of onto N-CNTCs and SNC.

329

ln(Qe - Qt) = lnQe - k1t

(1)

15

330

t/Qt = 1/k2Qe 2 + t/Qe

331

where Qe (mg g–1) and Qt (mg g–1) is equilibrium adsorption capacity and adsorption

332

capacity at time t (min), respectively. k1 (min–1) is the pseudo-first-order rate constant.

333

k2 (g mg–1 min–1) is the pseudo-second-order rate constant.

(2)

334

The correlation coefficients and relevant parameters of the two kinetic models

335

are shown in Table S1. The pseudo-second-order model showed a higher correlation

336

coefficient R than pseudo-first-order, suggesting that pseudo-second-order kinetic

337

model can well describes the adsorption of PCBs on N-CNTCs and SNC. The result

338

also indicates the presence of chemisorption during the adsorption process.

339

Furthermore, the pseudo-second-order rate constant (k2) reveals that adsorption

340

kinetics of PCBs on N-CNTCs was faster than that of SNC. These results are due to

341

the fact that the hollow cage structure of N-CNTCs facilitates the entry of target

342

molecules into the center to accelerate mass transfer, and the abundant active sites

343

contribute to adsorption.

344

Adsorption equilibrium isotherm is critically important to understand elucidate

345

the interaction the interaction between adsorbate molecules and the adsorbent surface.

346

0.5 mg N-CNTCs were used as the SPME coating to extract different concentrations

347

of PCBs in 20 mL working standard solution. To well-understand the adsorption

348

process, two typical adsorption models, Langmuir (3) and Freundlich isotherm models

349

(4), are used to fit the adsorption isotherms data of PCBs by using N-CNTCs.

350

Ce/Qe = 1/KLQm + Ce/Qm

351

lnQe = lnKF + lnCe/n

(3) (4)

16

352

where Qe (mg g–1) is equilibrium adsorption capacity. Qm is the maximum adsorbed

353

amount. Ce (mg L–1) is the equilibrium concentration of the adsorbate. KL (L mg–1)is

354

the Langmuir constant. KF ((mg g–1) (L mg–1)1/n) and n are Freundlich constants.

355

Figure S6 and Table S2 shown adsorption isotherms and fitting curves regression

356

parameters, respectively. The extraction amount of the N-CNTCs leveled off when the

357

concentration reached 30 ng mL–1. The equilibrium extraction amount for PCBs (28)

358

and (101) were 0.70 and 0.64 mg g–1 on N-CNTCs, respectively. The adsorption datas

359

onto N-CNTCs are in good agreement with Freundlich model, indicating that the

360

adsorption

361

heterogeneous surface. The Freundlich Parameter 1/n less than 1 indicates than

362

adsorption is a favorable process.

of

PCBs

on

N-CNTCs

was

a

multilayer adsorption

in

a

363

The remarkable extraction properties of the N-CNTCs could be attributed to π–π

364

interactions, abundant active sites, and the hollow cage structure, which is composed

365

of interconnected crystalline N-doped C nanotubes. Abundant graphitic C in the

366

N-CNTCs results in strong π–π stacking interactions between the N-CNTCs and PCB

367

aromatic ring [13]. Additionally, after N doping, numerous N atoms act as adsorption

368

sites on N-CNTCs, enhancing the capacity of the fiber coating to adsorb PCBs, as

369

previously reported [21, 28]. Finally, the large specific surface area, large mesopores,

370

and the hollow cage structure of N-CNTCs promote the accessibility of target

371

molecules, accelerate diffusion, and enable high-exposure active sites to promote

372

adsorption performance, thereby greatly decreasing extraction time and increasing

373

extraction efficiency.

17

374

3.5. Analytical performance and real sample detection

375

3.5.1 Analytical performance

376

The performance of the developed SPME method in water sample PCBs

377

determination was evaluated under the optimal conditions. We plotted the working

378

curve of the spiked water sample and determined the linear range, linear correlation

379

coefficient (R), and limits of detection (LODs) and limits of quantitation (LOQs) of

380

the standard curves of the seven PCBs extracted from ultrapure water (Table 2). This

381

method showed broad linear ranges (0.3–1000.0 ng L–1) with good correlation

382

coefficients (R > 0.9977) for PCBs. The LODs ranged from 0.10 to 0.22 ng L–1 based

383

on a signal to noise ratio (S/N) of 3, and the LOQs ranged from 0.33 to 0.72 ng L–1

384

based on an S/N of 10. These results indicate that the new method successfully

385

detected PCBs. We then added a 750.0 ng L–1 solution to ultrapure water as a standard

386

solution for intra- and inter-day precision experiments. Relative standard deviations

387

(RSDs) among and between fibers were calculated three times by parallel extraction

388

and desorption. RSDs of three replicate experiments ranged from 2.6% to 4.8%. The

389

fiber-to-fiber reproducibility of three fibers ranged from 5.6% to 8.6%. Thus, the

390

analytical method had sufficient precision for the detection of trace amounts of PCBs.

391

We also compared the extraction performance of the prepared N-CNTC-coated fiber

392

after multiple cycles of extraction/desorption (Fig. S7). The fiber extraction

393

performance for PCBs remained unchanged after 200 cycles of extraction/desorption.

394

Thus, the prepared N-CNTC-coated fiber was stable and the preparation method was

395

feasible.

18

396

The method proposed in this study was then compared with other methods

397

described in previous studies; the results are shown in Table 3. The proposed method

398

achieved a much wider linear range and much lower LODs than other PCBs

399

extraction methods, and produced efficient extraction and simultaneous analysis of

400

multiple residues.

401

3.5.2 Real samples detection

402

The N-CNTC-coated fibers were then used for the detection and analysis of PCBs in

403

six river water samples via the SPME process. None of the PCBs were found in the

404

Fuzhou or Nanping river water samples (Table 4). However, we detected 4.6 ng L–1

405

PCB(28), 1.9 ng L–1 PCB(101), 12.6 ng L–1 PCB(153), and 7.2 ng L–1 PCB(138) in

406

Xiamen. In the Longyan river water samples, we detected 6.3 ng L–1 PCB(52),

407

18.4 ng L–1 PCB(101), and 26.2 ng L–1 PCB(153); in Putian river water samples, we

408

detected 9.3 ng L–1 PCB(28), 3.1 ng L–1 PCB(101), and 10.2 ng L–1 PCB(153); and in

409

Zhangzhou river water samples, we detected 7.5 ng L–1 PCB(101) and 13.4 ng L–1

410

PCB(153). The coating showed good recovery of PCBs (80.3–112.6%) (Table 4). To

411

better reflect the reproducibility of the experimental analysis method, we calculated

412

the recovery rate of the method by adding different concentrations of PCBs (50.0,

413

100.0, and 200.0 ng L–1) to river water samples from the six cities in Fujian Province.

414

A typical chromatogram of the river water samples is shown in Figure 5. At low

415

sample concentration (50.0 ng L–1), the sample peak showed a very high response; the

416

coating was therefore little affected by matrix effects of the river water samples. Thus,

417

N-CNTCs exhibited a strong extraction effect and good recovery and reproducibility

19

418

using SPME with GC–MS.

419

4. Conclusion

420

In this study, a simple and easily operated pyrolysis method was introduced to prepare

421

N-CNTCs for use as SPME fiber coating. The resulting uniform N-CNTCs displayed

422

a hollow structure with high specific surface area, high porosity, and good chemical

423

stability. Abundant active sites, π–π interactions, and a hollow cage structure

424

composed of interconnected crystalline N-doped C nanotubes are the main attributes

425

contributing to the excellent extraction performance of N-CNTCs. Based on the fiber

426

properties of N-CNTCs, their application in SPME showed satisfactory extraction of

427

PCBs for determination by GC–MS. This method was successfully used to extract and

428

detect trace PCBs from river samples collected in six cities in Fujian Province. The

429

results of this study provide an effective method for PCBs detection in water samples,

430

and are expected to shed light on potential further applications of MOF-derived

431

N-doped C cage material in analytical chemistry.

432 433 434

Notes The authors declare no competing financial interest.

435 436

Acknowledgements

437

The authors are grateful for the National Nature Sciences Foundation of China

438

(21575028, 21705026), the Nature Sciences Funding of Fujian Province

439

(2016J01051), the Program for Changjiang Scholars and Innovative Research Team

20

440

in University (No. IRT15R11), China, Collaborative Innovation Center of Chinese

441

Oolong Tea Industry-Collaborative Innovation Center (2011) of Fujian Province. Xue

442

He and Yuheng Guo contributed equally to this work.

443 444

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phenylurea herbicides, Anal. Chim. Acta 870 (2015) 67-74.

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Carbon as an Efficient Adsorbent for Preconcentration of Flunitrazepam from

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Beverage Samples, Food Anal. Methods 10 (2017) 2772-2780.

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framework-derived bifunctional oxygen electrocatalyst, Nat. Energy 1 (2016) 15006.

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framework-derived carbons: Preparation from ZIF-8 and application in the adsorptive

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removal of sulfamethoxazole from water, Catal. Today 301 (2018) 90-97.

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Metal-organic framework-coated stainless steel fiber for solid-phase microextraction

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of polychlorinated biphenyls, J. Chromatogr. A 1570 (2018) 10-18.

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polymer nanoparticles as the solid-phase microextraction fiber coating for the

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extraction of organochlorines, J. Chromatogr. A 1556 (2018) 47-54.

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[25] G.F. Ouyang, D. Vuckovic, J. Pawliszyn, Nondestructive sampling of living

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systems using in vivo solid-phase microextraction, Chem. Rev. 111 (2011) 2784-2814.

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[26] Q.Q. Wang, H.H. Wu, F.Y. Lv, Y.T. Cao, Y. Zhou, N. Gan, A headspace sorptive

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extraction method with magnetic mesoporous titanium dioxide@covalent organic

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frameworks composite coating for selective determination of trace polychlorinated

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biphenyls in soils, J. Chromatogr. A 1572 (2018) 1-8.

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core-conjugated microporous polymer coating for highly sensitive solid-phase

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microextraction of polar phenol compounds in water samples, Anal. Chim. Acta 1015

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M.Oshima, Y. Harada, Lewis Basicity of Nitrogen-Doped Graphite Observed by CO2

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Chemisorption, Nanoscale Res. Lett. 11 (2016) 127.

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[29] G.H. Wang, Y.Q. Lei, H.C. Song, Exploration of metal-organic framework

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MOF-177 coated fibers for headspace solid-phase microextraction of polychlorinated

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biphenyls and polycyclic aromatic hydrocarbons, Talanta 144 (2015) 369-374.

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[30] Y.Y. Wu, C.X. Yang, X.P. Yan, Fabrication of metal-organic framework MIL-88B

537

films on stainless steel fibers for solid-phase microextraction of polychlorinated

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biphenyls, J. Chromatogr. A 1334 (2014) 1-8.

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540

nanotubes: An adsorbent for magnetic solid-phase extraction of polychlorinated

541

biphenyls from environmental and biological samples, J. Chromatogr. A 1449 (2016)

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[32] F.Y. Lv, N. Gan, Y.T. Cao, Y. Zhou, R.J. Zuo, Y.R. Dong, A molybdenum

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disulfide/reduced

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chromatography-mass spectrometry for the saponification-headspace solid-phase

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microextraction of polychlorinated biphenyls in food, J. Chromatogr. A 1525 (2017)

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[33] C. Hu, M. He, B.B. Chen, B. Hu, Simultaneous determination of polar and apolar

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compounds in environmental samples by a polyaniline/hydroxyl multi-walled carbon

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nanotubes composite-coated stir bar sorptive extraction coupled with high

551

performance liquid chromatography, J. Chromatogr. A 1394 (2015) 36-45.

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[34] Q.N. Jia, G.C. Zhao, Preparation of graphene-based solid phase microextraction

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fiber and its determination of polychlorinated biphenyls. J. Instru. Anal. 5 (2013)

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[35] L. Wang, X. Wang, J.B. Zhou, R.S. Zhao, Carbon nanotube sponges as a

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solid-phase extraction adsorbent for the enrichment and determination of

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polychlorinated biphenyls at trace levels in environmental water samples. Talanta 160

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559 560 561 562 563 564 565 566 567 568 569 570 571

26

572

Figure captions:

573

Fig. 1. (a) The schematic fabrication process of N-CNTCs coated SPME fiber; (b) the

574

TEM image of ZIF-67 dodecahedron crystal; (c) TEM image of hollow dodecahedra

575

N-CNTCs, TEM image of N-CNTs on the surface of N-CNTCs (inset); (d) TEM

576

image of N-CNTCs coated SPME fiber; (e) SEM image of hollow dodecahedra

577

N-CNTCs on the surface of SPME fiber; (f) Elemental mapping images of N-CNTCs,

578

including C, Co, N and O elements. Scale bars represent 500 nm for (b); 200 nm for

579

(c) and 100 nm for insert in (c); 150 µm for (d); 1 µm for (e).

580

Fig. 2. (a) The N2 adsorption–desorption isotherm of the N-CNTCs and the pore size

581

distribution of the N-CNTCs (inset); (b) The XPS pattern of the N-CNTCs; (c) The

582

XRD pattern of the N-CNTCs; (d) The raman spectra of the N-CNTCs.

583

Fig. 3. Influence of the experimental conditions on the extraction efficiency obtained

584

for the N-CNTCs coated SPME fiber including extraction time (Fig. 3a), extraction

585

temperature (Fig. 3b), desorption time (Fig. 3c), desorption temperature (Fig. 3d),

586

agitation speed (Fig. 3e), salt concentration (Fig. 3f) and pH (Fig. 3g). The

587

concentration of the PCBs used in the SPME process was 1.0 ng mL–1. Error bars

588

show the standard deviation of the mean (n=3).

589

Fig. 4. (a) Comparison of the extraction efficiencies for PCBs with 30 µm N-CNTCs,

590

30 µm CNTs-15 nm, 75 µm CAR/PDMS and 65 µm PDMS/DVB fiber. (b)

591

Comparison of the extraction efficiencies for PCBs with 30 µm ZIF-67, NC and

592

N-CNTCs fiber. (c) The stability of the coating tested in organic solvents (acetone,

593

dichloromethane, methanol) and different pH (pH = 6, 8). Conditions: extraction time,

27

594

30 min; extraction temperature, 50°C; desorption time, 4 min; desorption temperature,

595

270 °C; agitation speed, 750 rpm; salt concentration (CNaCl ), 0.05 g mL–1; PCBs

596

concentration, 1.0 ng mL–1. Error bars show the standard deviation of the mean (n=3).

597

Fig. 5. Chromatograms of the PCBs in river water samples (a) Fuzhou city, (b)

598

Xiamen city, (c) Longyan city, (d) Putian city, (e) Nanping city, (f) Zhangzhou city.

599

Spiked with 50.0 ng L–1 PCBs. Peak identity is in order: PCB(28), PCB(52),

600

PCB(101), PCB(153), PCB(138), PCB(180), PCB(194).

601 602 603 604 605 606 607 608 609 610 611 612 613 614 615

28

616

Figures

617

Fig. 1

618 619 620 621 622 623 624 625 626 627 628 629 630 29

631

Fig. 2

632 633 634 635 636 637 638 639 640 641 642 643 30

644

Fig. 3

645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 31

662

Fig. 4

663 664 665 666 667 668 669 670 671 672 673

32

674

Fig. 5

675 676 677 678 679 680 681 682 683 684 685 686 687 688 689

33

690

Table 1. Molecular formula, molecular weight, chemical structures, and ions (m/z) of

691

the analytes. Ions(m/z)

Molecular Analytes

Mol. Wt.

Ret Time

Structure

foamula

C-ion

Q-ion

(min)

256,186,150

256

11.99

292,220,257

292

13.26

326,256,184

326

15.86

360,290,218

360

18.04

360,290,218

360

18.68

394,324,252

394

20.27

430,358,288

430

22.34

Cl

PCB(28)

C12H7Cl3

257.54

Cl

Cl

Cl Cl

PCB(52)

C12H6Cl4

291.99 Cl

Cl

Cl Cl

PCB(101)

C12H5Cl5

326.43

Cl

Cl

Cl

Cl Cl

PCB(153)

C12H4Cl6

360.88

Cl

Cl

Cl

Cl

PCB(138)

C12H4Cl6

360.88

Cl

Cl Cl

Cl

Cl

Cl

Cl

PCB(180)

C12H3Cl7

395.32

Cl

Cl

Cl

Cl

PCB(194)

C12H2Cl8

429.77

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

692 693 694 695 696 697

34

Cl

698

Table 2. Analytical performance used in the SIM mode for SPME-GC/MS

699

determination of the seven PCBs residues compounds. One fiber Linear

Correlation

LOQs LODs

Analytes

range

(ng L-1, S/N =

coefficient -1

(ng L , S/N = 3) (ng L-1)

Fiber to fiber repeatability reproducibility RSD % (n = 3)

(R)

10)

RSD % (n = 3) intra-day

inter-day

PCB(28)

0.5-1000.0

0.9987

0.17

0.57

3.2

4.1

6.3

PCB(52)

0.3-1000.0

0.9986

0.13

0.42

2.7

3.4

7.2

PCB(101)

0.3-1000.0

0.9994

0.14

0.47

3.6

4.6

5.8

PCB(153)

0.3-1000.0

0.9983

0.10

0.33

3.1

4.8

6.7

PCB(138)

0.5-1000.0

0.9997

0.16

0.54

2.6

3.8

8.6

PCB(180)

0.3-1000.0

0.9988

0.14

0.45

3.8

4.5

7.4

PCB(194)

0.5-1000.0

0.9977

0.22

0.72

3.1

3.3

5.6

700 701 702 703 704 705 706 707 708 709 710 711

35

712

Table 3. Comparison the established method based on some other materials and

713

commercially fibers with the reported methods for detection PCBs.

NO.

Coatings

Extraction

Linear range

LODs

time (min)

(ng L-1)

(ng L-1)

Analytical method

RSD(%)

Reference

1

N-CNTCs

SPME-GC-MS

30

0.3-1000.0

0.1-0.22

2.6-8.6

This work

2

MOF-177

SPME-GC-MS

50

1-50

0.69-4.42

1.47-8.67

[29]

3

MIL-88B

SPME-GC-MS

50

5-200

0.45-1.32

4.2-8.7

[30]

MSPE-GC-MS/MS

30

5-1000

0.31-0.49

1.5-8.0

[31]

SPME-GC-MS

40

250-100000

51-93

<4.9%

[32]

SBSE-HPLC-UV

50

100-100000

170-810

6.5-11.6

[33]

SPME-GC-ECD

30

50-3500

4.7-8.8

1.4-8.8

[34]

SPE-GC-MS/MS

--

10-1000

0.72-1.98

2.42-6.60

[35]

4

5

6

7

Fe3O4 @ Co-MONTs

MoS2/RGO

PANi/MWC NTs-OH

Graphene

CNT 8 sponges

714 715 716 717 718 719 720 36

721

Table 4. Analytical results for the determination of the seven PCBs residues

722

compounds in real environmental water samples. NO.

Analytes

PCB(28) Found (ng L-1)

1

a

ND

91.6(3.5)

96.6(2.5)

water

Rb (RSD)

89.7(1.3)

a

85.0(3.9)

86.3(4.5)

91.3(3.9)

94.7(3.9) 109.0(2.6) 103.4(4.9)

102.7(4.5) 99.0(4.2)

96.9(4.6)

93.0(5.3)

94.5(4.3) 104.6(3.4)

93.1(3.0)

4.6

ND

1.9

12.6

7.2

ND

ND

Xiamen river

R (RSD)

89.1(4.3)

90.6(5.3)

90.4(4.1)

88.6(6.2)

93.7(4.4)

84.4(4.1)

103.9(5.1)

water

Rb (RSD)

89.4(3.2)

87.1(6.3)

86.6(5.2)

89.3(2.3)

84.6(4.6)

83.9(5.4)

86.9(4.2)

106.2(4.3) 93.9(3.9) 105.0(4.1) 98.9(1.3)

97.0(5.6)

88.2(4.7)

91.8(4.5)

26.2

ND

ND

ND

84.3(5.1) 103.6(6.7) 89.7(6.3)

86.3(4.8)

96.3(2.3)

89.5(1.5)

86.4(4.2)

84.5(5.3)

96.0(4.2)

99.8(3.1) 108.6(4.0) 105.6(7.3)

R (RSD) -1

Found (ng L ) Longyan river water

Ra (RSD) b

R (RSD) c

R (RSD)

Putian river water

91.4(2.3)

89.7(5.3)

3.1

10.2

ND

ND

ND

Ra (RSD)

87.3(3.7)

99.5(6.9)

92.0(1.9)

90.3(6.4)

89.1(3.6)

89.6(4.5)

91.7(5.9)

83.1(3.1)

85.3(4.9)

89.1(3.4)

90.7(1.3)

89.2(5.1)

96.4(5.8)

90.9(1.6)

102.9(3.2) 90.8(2.9) 103.5(4.2) 87.5(1.9)

86.8(2.7)

99.0(5.9)

95.9(4.9)

b

R (RSD)

Found (ng L )

ND

ND

ND

ND

ND

ND

ND

Ra (RSD)

94.5(5.1)

84.7(1.3)

84.3(2.0)

90.9(5.6)

83.2(2.7)

82.9(6.3)

83.3(4.3)

98.2(3.2)

88.5(3.3)

80.3(5.3)

84.3(5.0)

86.7(4.1)

93.0(2.1)

97.2(5.1)

111.0(5.1) 101.6(2.4) 110.5(4.6) 105.4(3.1) 108.8(5.0) 97.6(2.7)

99.8(4.3)

b

R (RSD) c

R (RSD) Found (ng L-1)

river water

84.0(4.6)

ND

-1

Zhangzhou

18.4

9.3

R (RSD)

river water

6.3

Found (ng L )

c

Nanping

ND

112.6(7.1) 99.6(4.2) 111.3(3.5) 106.0(6.3) 104.4(2.3) 93.5(4.6) -1

6

ND

86.4(4.3)

c

5

ND

93.7(4.2)

Found (ng L )

PCB(194)

ND

82.3(3.9) 103.8(2.3) 84.1(2.3)

-1

4

ND

R (RSD)

R (RSD)

3

ND

Fuzhou river

c

2

ND

PCB(52) PCB(101) PCB(153) PCB(138) PCB(180)

ND

ND

7.5

13.4

ND

ND

ND

a

97.2(2.6)

87.9(4.3)

94.2(2.2)

96.2(3.2)

99.1(4.1)

86.5(7.3)

91.6(4.8)

b

82.9(2.2)

89.3(1.9)

92.7(2.4)

97.2(1.5)

99.0(3.9) 106.1(2.7) 105.4(4.2)

R (RSD) R (RSD) c

R (RSD)

102.5(2.3) 103.7(4.9) 105.1(1.5) 100.8(4.5) 101.4(4.1) 105.7(3.6) 104.4(4.9)

723

Ra = Recovery of this method (spiked with 50.0 ng L-1).

724

Rb = Recovery of this method (spiked with 100.0 ng L-1).

725

Rc = Recovery of this method (spiked with 200.0 ng L-1).

726

ND = Not detected.

37

●Nitrogen-doped carbon nanotubes cages were synthesized via facile pyrolysis method. ●The unique hollow cages structure, π−π interactions and abundant nitrogen-doped active sites were possible mechanism. ●The proposed method based on N-CNTCs coated fiber exhibited low limits of detection. ●The fiber was successfully used for analysis of trace PCBs in real river samples.

Conflict of interest statement The authors declared that they have no conflicts of interest to this manuscript entitled “Metal-organic framework-derived nitrogen-doped carbon nanotubes cages as an efficient adsorbent for solid-phase microextraction of polychlorinated biphenyls”. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the manuscript submitted.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: