In Situ Hydrothermally Grown TiO2@C CoreâShell Nanowire

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In situ hydrothermal grown of TiO@C core-shell nanowire coating for high sensitive solid phase microextraction of polycyclic aromatic hydrocarbons Fuxin Wang, Juan Zheng, Junlang Qiu, Shuqin Liu, Guosheng Chen, Yexiang Tong, Fang Zhu, and Gangfeng Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14748 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

In situ hydrothermal grown of TiO2@C core-shell nanowire coating for high sensitive solid phase microextraction of polycyclic aromatic hydrocarbons

Fuxin Wang, Juan Zheng, Junlang Qiu, Shuqin Liu, Guosheng Chen, Yexiang Tong, Fang Zhu* and Gangfeng Ouyang*

MOE Key Laboratory of Aquatic Product Safety/KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China.

*

Corresponding author. Tel. & Fax: +86-20-84110845

E-mail: [email protected] (F. Zhu); [email protected] (G. Ouyang).

ACS Paragon Plus Environment


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ABSTRACT Nanostructured materials have great potential for solid phase microextraction (SPME) on account of their tiny size, distinct architectures and superior physical and chemical properties. Herein, a core-shell TiO2@C fiber for SPME was successfully fabricated by the simple hydrothermal reaction of a titanium wire and subsequent amorphous carbon coated. The readily hydrothermal procedure afforded in-situ synthesis of TiO2 nanowires on a Titanium wire and provided a desirable substrate for further coating of amorphous carbon. Benefiting from the much larger surface area of subsequent TiO2 and the good adsorption property of amorphous carbon coating, the core-shell TiO2@C fiber was utilized for the SPME device for the first time and proved to have better performance in extraction of polycyclic aromatic hydrocarbons. In comparison to the PDMS and PDMS/DVB fiber for commercial use, the TiO2@C fiber obtained GC responses 3-8 times as high as those obtained by the commercial 100 µm PDMS and 1-9 times higher than 65 µm PDMS/DVB fiber did. Under the optimized extraction conditions, the low detection limits were obtained in the range of 0.4-7.1 ng L-1 with wider linearity in the range of 10-2000 ng L-1. Moreover, the fiber was successfully used for the determination of polycyclic aromatic hydrocarbons in the Pearl River water, which demonstrated the applicability of the core-shell TiO2@C fiber. KEYWORDS: gas chromatography-mass spectrometry; solid phase microextraction; core-shell; high sensitive; polycyclic aromatic hydrocarbons


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INTRODUCTION As a promising sample pretreatment technique, Solid phase microextraction (SPME) has been rapidly developed since it was introduced in the 1990s 1-2 and widely applied to different fields, such as environmental,3-5 biological,6 forensic,7 crude oil8 and food analysis.9 SPME has many advantages over the traditional technique for instance liquid-liquid extraction and solid-phase extraction by integrating sample extraction, enrichment and injection (commonly to gas chromatography or high performance liquid chromatography) into one step with less solvent consumption.10 Generally, fiber coating is deemed to be the core part of the SPME technique, which determines the selectivity, sensitivity and reproducibility of the technique.11 Currently, most of the commercial SPME fibers coating materials are organic polymers, such as polydimethylsiloxane, polyacrylate, divinylbenzene, carboxen, carbowax and their copolymers.12-13 Nevertheless, most of the commercially SPME fibers are made of fused silica and polymer, which are not only costly, but also easy to bend and swell in organic solvents.14-15 So it is urgent to find a kind of fibers with thermal, chemical stability and excellent selectivity, sensitivity to overcome these problems. In the past two decades, many studies have focused on high-strength metal substrates such as aluminum wire,16 silver wire,17 zinc wire,18 platinum wire,19 titanium wire,20 copper wire

21

and stainless steel wire,22-23 which exhibit good bending property and great

convenience. At the same time, new coating materials such as polyaniline, carboxylated

solid

carbon

spheres,

TiO2/CNT,

Yb–MOF,

[Cu3(µ3-O)(µ-OH)(triazolate)2]+ and graphene were further investigated, which are



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characterized by high extraction efficiency, good stability as well as excellent selectivity and sensitivity.24-29 By contrast, owning to the advantages of well-defined structure, large surface area and unique properties, inorganic nanostructures are promising candidates for SPME fibers. With regard to inorganic materials, TiO2 is considered to be a promising substrate of fiber on account of its high chemical, thermal stability,30 biocompatibility31 and nontoxicity,32 which has been widely applied in photocatalysis,33 solar cells34 and sensing devices35. Recently, nanostructured TiO2-based SPME fibers have been fabricated through in situ oxidation of Ti wires with hydrogen peroxide or with ethylene glycol and ammonium fluoride.36 Due to their attractive structure, these substrates provide much larger active sites. These novel TiO2-based SPME fibers provide excellent substrates for further fabrication of high efficiency fibers. Carbon-based materials as a kind of good adsorbent, have been widely applied to trapping and separating organic compounds. A part of carbon materials such as graphene oxide (GO), polycrystalline graphite, carbon nanotubes (CNTs) and their functionalized forms, have been successfully used as fiber for SPME device due to their good chemical stability and excellent adsorption property.37-39 However, the usage of amorphous carbon as the SPME coating is less reported. And to our knowledge, amorphous carbon also has good chemical stability and excellent adsorption property. Hence, a good extraction performance is expected through the use of the advantage of the TiO2 substrate and the amorphous carbon coating. In this work, we developed a readily and quick method for the in situ synthesis of a


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fiber made from Ti wire coated with TiO2@C coating for the first time. The as-prepared TiO2@C SPME fiber as thick as approximately 3.5 µm possessed better extraction ability toward polycyclic aromatic hydrocarbons (PAHs), obtaining GC responses 3-8 times as high as those obtained by the commercial 100 µm PDMS and 1-9 times higher than 65 µm PDMS/DVB fiber did. Under the optimized extraction conditions, the core shell TiO2@C fiber managed to obtain the lower detection limits (0.4-7.1 ng L-1) and wider linearity (10-2000 ng L-1). Finally the fiber was successfully applied to the real sample analysis.

EXPERIMENTAL SECTION Reagents and materials. Ti wire (purity 99.9%, φ 0.127 mm) was purchased from Alfa Aesar. Glucose, Acetone, Ethanol, Potassium hydroxide (KOH, A.R.), hydrochloric acid (HCl, A.R.), and sodium chloride (NaCl, A.R.) were purchased from Guangzhou Chemical Reagent Factory. Six polycyclic aromatic hydrocarbons standards,

which

contained

naphthalene

(99.5%),

acenaphthylene

(99.5%),

acenaphthene (99.5%), fluorine (98.5%), anthracene (98.5%) and phenanthrene (99%), were purchased from Dr. Ehrensorfor GmbH (Germany). The commercial PDMS (100 µm), PA (85µm) and PDMS/DVB (65 µm) fiber were purchased from Supelco (Bellefonte, PA, USA). Instruments. The analysis of the analytes was conducted with an Agilent Technologies 6980 GC with 5975 MS equipped with a HP-5 MS capillary column (30 m × 0.32 mm i.d. × 0.25 µm). Ultrapure helium was used as the carrier gas at a constant flow rate of 1.2 mL /min. For the chromatographic separation, the GC-MS



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column temperature was programed as follows: the initial oven temperature was set at 50 °C and held for 0.5 min. Next, the oven was programmed at a rate of 20 °C/min to 150 °C and then at a rate of 15 °C / min to 250 °C, held for 6 min. Finally, the oven was programmed to 270 °C at 30 °C / min, held for 8 min. The run time of the whole program was 35.8 min. Preparation of the novel core-shell TiO2@C fiber. The novel core-shell TiO2@C fiber was fabricated by a two-step hydrothermal method. Before use, the Ti wire was cleaned by acetone, ethyl alcohol, and water in turn with ultrasound. Then, 20 ml of KOH solution was placed in a 25 ml Teflon-lined stainless steel autoclave followed by a 3 cm cleaned Ti wire. The autoclave was placed in an oven and heated at 180 °C for 16 h. After completion of the reaction, the autoclave was opened at the room temperature and the TiO2 nanowire (TiO2 NWs) was washed with DI water. Next, the TiO2 NWs were annealed in air at 550 °C for 1h. Then 20 ml of 0.1 M glucose aqueous solution and TiO2 NWs were placed in a Teflon-lined stainless autoclave (25 ml volume) at 180 °C for 4h.40-41 Then, the fibers were annealed in N2 atmosphere at 800 °C for 1h. Finally, the novel core-shell TiO2@C fiber was obtained. According to the experimental results, the concentration of glucose could control the thickness of shell carbon. It is found out that TiO2 nanowires could not be fully coated with amorphous carbon at low concentration. SPME procedure. Before use, all the fibers was aged in the GC injection at 250 °C for 30 min to avoid any contamination. All the experiments were performed in headspace solid phase microextraction (HP-SPME) with 10 mL deionized water and


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PAHs standard sample in a 20 mL vial. Then, the prepared vial was placed in the magnetic stirrers, which could control the extraction temperature and agitation speed. Next, the GERSTEL MPS auto-sampler inserted the fiber into the vial and fixed the fiber coating above the water sample. After a certain period time, the fiber was removed from the vial and immediately inserted into the GC inlet for thermal desorption and analysis. Characterization. The morphologies of fibers were investigated by field-emission scanning electron microscope (SEM. FEI, Quanta 400F). The microstructure was further investigated by the Transmission electron microscopy (TEM, TecnaiTM G2 F30, 300 kV). X-Ray diffractometer (XRD), Raman spectroscopy and X-ray Photoelectron Spectroscopy were used to identify the crystal structure and the phase purity of the samples. Specific surface area (SBET) of amorphous carbon shell was calculated by BET theory with the nitrogen adsorption and desorption measurement isotherms by using a Micrometrics ASAP 2020 analyzer at 77 K. RESULTS AND DISCUSSION The fabrication process of the novel core-shell TiO2@C fiber is schematically illustrated in Figure 1a. Firstly, the aligned TiO2 nanowires were directly in situ grown on Ti wire (Figure 1b and e) via a mild hydrothermal method. As shown in Figure 1c and f, it is clearly shown that the entire surface of Ti wire is fully and uniformly covered with TiO2 nanowires with a diameter of about 85 nm and a thickness of about 3.5 µm (Figure S1). Subsequently, the surface of the aligned TiO2 nanowires was covered with a thin carbon shell by a glucose-assisted hydrothermal method. SEM



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images of TiO2 nanowire arrays before and after coating a thin film of carbon shell is shown in Figure 1c, f and d, g, respectively. This indicates that the nanowire arrays were uniformly grown on the surface of titanium wire and the integration of carbon shell into the array does not deteriorate the ordered structure but increases the diameter of the nanowires and the roughness of the wire surface. In order to confirm X-ray diffraction patterns of the samples, XRD experiment was conducted. The result is displayed in Figure 2a, which shows that the TiO2 nanowires are rutile phase (JCPDF # 65-0192) and the carbon shell is amorphous. In order to obtain the internal structure composition and phase of the sample, transmission electron microscopy (TEM) measurement was conducted. A typical TEM analysis of the nanowires is displayed in Figure 2b, which obviously demonstrates the core shell structure of the TiO2@C sample. The high resolution TEM (HRTEM) image in Figure 2c clearly shows that there is no visible lattices in carbon shell where the well-resolver lattice spacing around 0.32 nm. The lattice spacing of TiO2 is corresponding to (110) planes of the rutile TiO2 (JCPDF # 65-0192) and the carbon shell is amorphous. Additionally, as shown Figure 2c, the TiO2 nanowires are obviously coated with the amorphous carbon shell, which is about 7.5 nm in thickness. The corresponding selected area electron diffraction (SAED) pattern in the Figure 2d shows the bright and sharp diffraction spots , which are corresponding to (001), (111) and (110) planes of the rutile TiO2 (JCPDF # 65-0192). No diffraction spots is corresponding to carbon shell, which was again confirm that the TiO2 nanowires are well crystalline and the carbon shell is amorphous. The electron energy loss spectroscopy (EELS) mapping reveals


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that C is uniformly distributed in the whole nanowires (Figure 2e-h) and the corresponding EDS line scan of the TiO2@C sample also reveals the core-shell structure of the sample (Figure S2 and S3). Since the amorphous carbon is too thin to provide enough materials for the N2 adsorption analysis, we prepared a sample in which only glucose was included in the hydrothermal reaction. The BET surface area (SBET) was measured to be as high as 515.4 m2 g-1 (Figure S4). To investigate the chemical composition of the synthesized TiO2@C nanowires, Raman spectra and X-ray photoelectron spectroscopy were further characterized. Figure 3a presents the typical Raman spectra of the TiO2 and TiO2@C samples. Both of them show the characteristic peaks of rutile TiO2 at around 145, 236, 446 and 609 cm-1.42-43 However, the TiO2@C sample exhibited two new sharp peaks at around 1348 and 1538 cm-1, which were well assigned to the D and G bands of carbon and suggested that the carbon shell was coated on the surface of the TiO2 nanowires as expected. Next, the XPS survey spectra (Figure 3b) was used to characterize the detailed chemical composition of the TiO2@C nanowires, which confirmed the existence of Ti, O, and C elements. The atomic percent of C, Ti and O is respectively 73.64 %, 22.37 % and 4.09 %. The core-level Ti 2p spectrum (Figure 3c) exhibits two intense peaks at about 459.5 eV for the Ti 2p3/2 and 465.2 eV for Ti 2p1/2.44 Figure S5 shows the O 1s XPS spectrum of the TiO2@C sample. The C 1s XPS spectrum of the TiO2@C sample shown in Figure 3d can be fitted into three components. These three main peaks at 284.8 eV, 285.9 eV, 287.3 eV can be corresponding to the carbon bond of C-C, C-OH and C=O bond. Therefore, the amorphous C is successfully coated on



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the surface of the single crystalline TiO2 nanowire arrays, which is in line with the results of XRD and Raman. In order to investigate the extraction ability of the TiO2@C nanowire as fiber, analytical measurements were conducted in a GC-MS equipped with a GERSTEL MPS auto-sampler. As shown in Figure S6, the TiO2 made by this paper has no adsorption to the PAHs, which only acts as a stable substrate. To achieve the optimal extraction efficiencies of the TiO2@C fiber for PAHs, the effect factors of extraction temperature, deposition temperature, extraction time, and salt concentration were studied and the optimization results were shown in Figure S7. Under the optimized extraction conditions, the core shell TiO2@C fiber and two commercial fibers (100 µm PDMS and 65 µm PDMS/DVB fibers) were conducted to extract the 2 µg L-1 PAHs solution. As shown in Figure 4, the GC responses of the ultra-thin TiO2@C fiber were respectively 3-8 times and 1-9 times higher than those of the commercial 30 µm PDMS and 65 µm PDMS/DVB fibers did. It is clearly shown that the as-prepared TiO2@C fiber has excellent extraction ability for PAHs as compared to the commercial fibers. As a whole, the as-prepared TiO2@C fiber has a great potential for being applied to the extraction of PAHs, due to its unique ordered structure and excellent adsorption performance. Then, the linearity, correlation coefficient, enrichment factors (EFs), relative standard deviations (RSD), and limits of detection (LOD) of the proposed method were studied under the optimized extraction conditions. As shown in Table 1, good linearity for all the six analytes was obtained in the range of 10-2000 ng L-1, with the


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corresponding R2 ranging from 0.9931 to 0.9991. The EFs of the six PAHs were ranged from 1059 to 4830. Under the optimized conditions, the accuracy of the TiO2@C fiber was investigated by five repeated extractions and the RSDs (n=5) ranged from 3.2% to 12.8%. Moreover, the reproducibility of fiber-to-fiber was studied and the RSDs ranged from 3.8% to 11.6%, which indicated the good repeatability of the fiber. The limits of detection (LODs) were between 0.41 ng L-1 and 7.07 ng L-1 for PAHs. The LODs of this work were compared with other works and the results are shown in Table S1. Although the as-prepared TiO2@C fiber was very thin (only 3.5 µm), it strongly indicated that the as-prepared TiO2@C fiber could be used for the detection of PAHs for SPME. The low LODs indicated the high sensitivity of the proposed method and its great potential for being efficiently applied in SPME analysis PAHs in real samples. To evaluate the practicability of the proposed method in real water samples, water from the Pearl River water (Guangzhou, China) was analyzed with the as-prepared TiO2@C fiber and the proposed method in this work. As shown in Table 2, anthracene (45.8 ng L-1) and phenathrene (30.9 ng L-1) were detected in the water samples. Good recoveries, ranging from 86.3 % to 129.8 %, were performed by spiking 50 ng L-1 standards into the real water sample, which proved the as-established method is practical and effective. CONCLUSIONS In this study, the novel TiO2@C fiber was well-content synthesized by the mild hydrothermal method and utilized as SPME fiber. The as-prepared ultrathin TiO2@C



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fiber (3.5 µm) proved to have good thermal, chemical stability and excellent extraction performance for the six PAHs as compared to the commercial 100 µm PDMS and 65 µm PDMS/DVB fiber. Coupled to GC-MS analysis, the novel core shell TiO2@C fiber and the proposed method can obtain low detection limits, low RSDs, wider linearity and good recoveries of six PAHs. With these advantages, the novel core-shell TiO2@C fiber can be efficiently applied in SPME analysis and desirably replace commercial fibers for the detection of polycyclic aromatic hydrocarbons in water samples. Moreover, the as-prepared TiO2@C fiber was successfully used for the determination of the PAHs in the Pearl River water. The design concept reported in this work may hopefully open up a new insight into the design of TiO2-based fiber for the application in SPME.


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ASSOCIATED CONTENT Supporting Information available: Figure S1: Low-magnification SEM image of the section of the TiO2 fiber; Figure S2: EDS line scan of the TiO2@C sample; Figure S3: EDS spectra of the TiO2@C sample; Figure S4: N2 adsorption-deposition isotherm of the amorphous carbon; Figure S5: The O 1s core level XPS spectrum of TiO2@C sample; Figure S6: The chromatogram of TiO2 and TiO2@C fiber at the same condition. Figure S7: Effects of SPME conditions on the extraction efficiencies of TiO2@C fiber. Table S1. The comparison of LODs between the novel core-shell TiO2@C fiber and the other works. AUTHOR INFORMATION Corresponding Author. Email: [email protected] (F. Zhu); [email protected] (G. Ouyang). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by projects of National Natural Science Foundation of China (21377172, 21225731, 21477166, 21527813), and the NSF of Guangdong Province (S2013030013474).

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Figure Captions Figure. 1. (a) Schematic illustration of the fabrication process of the novel core shell TiO2@C fiber. (b), (c) and (d) Low-magnification SEM images of the Ti wire, TiO2,TiO2@C fiber. (e), (f) and (g) are the corresponding magnified SEM images.

Figure. 2. (a) XRD Spectra of the Ti wire, TiO2, TiO2@C fiber. (b) TEM and (c) HRTEM images of the TiO2@C core-shell nanowires. (d) and (e-h) The corresponding selected area electron diffraction (SADE) pattern and EELS mapping image.

Figure. 3. (a) Raman spectra of the pristine TiO2 and TiO2@C sample. (b) XPS survey. (c) Ti 2p core level XPS spectra of as-prepared TiO2@C nanowires. (d) C 1s core level XPS spectrum of TiO2@C sample.

Figure. 4. Comparison of extraction efficiencies using TiO2@C fiber, two commercial fibers (100 µm PDMS and 65 µm PDMS/DVB fiber). Sample volume, 10 mL; concentration, 2 µg L-1.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Table. 1. Analytical performance of the TiO2@C fiber. Ananlytes

Line range (ng L-1)

R2

EF

LODa (ng L-1)

LOQ (ng L-1)

RSD (%)

Single fiber, n=5

Fiber-to-fiber, n=3

Naphthalene

10-2000

0.9966

1059

7.07

23.6

3.2

3.8

Acenaphthylene

10-2000

0.9931

2192

1.02

3.4

3.9

11.6

Acenaphthene

10-2000

0.9965

2471

0.48

1.6

4.8

7.6

Fluorene

10-2000

0.9991

4830

0.41

1.37

6.8

4.6

Anthracene

10-5000

0.9976

4054

4.0

13.3

8.1

9.7

Phenathrene

10-5000

0.9991

3850

3.24

10.8

12.8

11.0

a

Sample volume, 10ml; extraction time, 60 min; extraction temperature, 45 °C； salt concentration, 24% (w/v); deposition temperature

250 °C, deposition time 300 s.


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Table. 2. The results for the determination of the PAHs in river using the proposed SPME-GC-MS method Substance

Naphthalene

Concentration of river

RSD

Spiled concentration (ng

Detected

(ng L-1)

n=3)

(%,

L-1)

L-1)

nd

-

50

48.8

concentration

(ng

Relative recovery (%)

97.6

Acenaphthylene

nq

-

50

64.9

129.8

Acenaphthene

nq

-

50

52.3

104.6

Fluorene

nq

-

50

51.2

102.4

Anthracene

45.8

9.2

50

85.8

89.6

Phenathrene

30.9

10.3

50

69

86.3

.



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For TOC only


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Graphical abstract We developed a readily and qui 223x86mm (300 x 300 DPI)


In Situ Hydrothermally Grown TiO2@C CoreâShell Nanowire

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In Situ Hydrothermally Grown TiO2@C CoreâShell Nanowire