Controllable Transformation of Aligned ZnO Nanorods to ZIF-8 as

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Controllable transformation of aligned ZnO nanorods to ZIF-8 as solid-phase microextraction coatings with tunable porosity, polarity and conductivity Jing-Bin Zeng, Yulong Li, Xiaofu Zheng, Zizhou Li, Teng Zeng, Wei Duan, Qing Li, Xiao Shang, and Bin Dong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05419 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Analytical Chemistry

Controllable transformation of aligned ZnO nanorods to ZIF-8 as solid-phase microextraction coatings with tunable porosity, polarity and conductivity Jingbin Zeng, *,† Yulong Li,† Xiaofu Zheng,† Zizhou Li,† Teng Zeng,‡ Wei Duan, † Qing Li,† Xiao Shang,† Bin Dong† † College

of Science, China University of Petroleum (East China), Qingdao 266580, China of Civil and Environmental Engineering, Syracuse University, 151 Link Hall, Syracuse, NY 13244, United States Corresponding author, Tel: 86-532-86984530; Email: [email protected] (J. Zeng) ‡ Department

ABSTRACT: While a growing number of solid-phase microextraction (SPME) coatings have been developed, a generalized protocol is still needed to tailor-make SPME coatings with desirable properties for efficient extraction of diverse analytes from sample matrices. In this work, we developed a versatile approach to prepare SPME coatings with tunable properties by controllable in-situ transformation of well-aligned ZNRs into zeolitic imidazolate frameworks-8 (ZIF-8) via reaction with 2-methylimidazole (2-MI). During this process, ZNRs supplied Zn2+ and served as a “hard template” for the in-situ growth of well-aligned ZIF-8 with enhanced surface area for adsorption. Since ZNRs and ZIF-8 exhibit markedly different properties, we obtained a series of ZNRs/ZIF-8 hybrid composites, whose morphology, porosity, polarity and charge transfer resistance can be fine-tuned by simply controlling the concentration of 2-MI. Preparing ZNRs/ZIF-8 SPME coatings with desired properties enabled effective extraction of a wide range of polar and non-polar compounds including aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, alcohols, phenols, anilines and ionic drugs. KEYWORDS: Solid-phase microextraction, ZnO nanorods/zeolitic imidazolate frameworks-8 composite, coating

INTRODUCTION Among all sample preparation techniques, solid-phase microextraction (SPME) has attracted considerable attention due to its compatibility with analytical instruments, simple operation, limited consumption of organic solvents and high enrichment capability.1 This nonexhaustive technique has been broadly applied to the extraction of analytes in biological, environmental, food and pharmaceutical samples.2-7 It has long been recognized that coating materials determine the sensitivity and selectivity of SPME methods. Several types of SPME coatings such as polydimethylsiloxane (PDMS), polyacrylate and Carboxen/PDMS have been commercialized by Supelco®, which has greatly promoted the wider implementation of SPME techniques. Nevertheless, the application of these commercial coatings is somewhat limited due to their chemical instability and the lack of extraction sensitivity and selectivity. To address these limitations, various materials such as carbon nanomaterials,8-10 metal nanoparticles,11-13 aerogels,14, 15 molecularly imprinted polymers,16 biosorbents,17, 18 ionic liquids,19 organic polymers20-23 and metal organic frameworks24-27 have been exploited as alternative SPME coatings. These coatings are usually prepared with enhanced chemical stability, sensitivity and/or selectivity, but the synthesis protocols are oftentimes tailored to target one specific class of analytes.

A robust approach to customize different SPME coatings involves the use of polymers, sol-gels or organic adhesives as binders to immobilize previously explored SPE adsorbents onto solid substrate.28-30 This approach allows analysts to choose suitable materials from the rich libraries of SPE sorbents based on the analyte properties. Nevertheless, it is inconvenient to prepare different coating materials using different chemical reagents and reaction conditions. It is therefore desirable to develop a generalized protocol, in which the reagents and reaction conditions used are similar for synthesis of SPME coatings with desired properties. One-dimensional ZnO nanorods (ZNRs) and nanowires have found broad applicability in the fields of solar cells, piezoelectronic nanogenerators and nanosensors, owing to its wide bandgap, special electrical property, and excellent mechanical stability.31 ZNRs have been prepared as an SPME coating which shows excellent affinity towards polar compounds, such as aldehydes and pheonls due to its hydrophilic nature.32-36 ZNRs can also be used for extraction of less polar compounds when loaded with hydrophobic ligands or polymers.37-39 Zeolitic imidazolate framework-8 (ZIF-8) is a type of metal organic framework synthesized by the reaction between Zn2+ and 2methylimidazole (2-MI) under mild conditions. ZIF-8 is hydrophobic and microporous with large surface area, and has been shown to have special affinity towards

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nonpolar compounds like benzenes, n-alkanes and PAHs.40, 41 Taken together, ZNRs are hydrophilic and conductive with minimal porosity, while ZIF-8 is hydrophobic and non-conductive with abundant porosity. Considering that the ZNRs possesses Zn2+ on their surface, it is viable to in-situ grow ZIF-8 by reacting the ZNRs with 2-MI. Hence, by regulating the degree of transformation from ZNRs into ZIF-8, we anticipate to obtain a series of fiber coatings with different polarity, porosity, specific surface area and conductivity. To achieve this goal, aligned ZnO nanorods (ZNRs) were grown from porous polyaniline (PANI) network, and then in-situ transformed to ZIF-8 by reacting with 2-MI, as shown in Fig. 1. PANI is a porous polymer that can be easily coated onto metal wires using electrochemical approaches, so it is a suitable substrate for the subsequent seeded growth of well-aligned ZNRs. Different amounts of 2-MI were used to react with ZNRs to in-situ produce ZNRs/ZIF-8 composites with different weight ratios. To investigate the fundamental structure-property relationships of these coatings, we tested their extraction performance using various model compounds spanning across a wide range of polarity, e.g. n-alkanes, polycyclic aromatic hydrocarbons (PAHs), alcohols, phenols, aromatic amines and illicit basic drugs. Our results indicated that the proposed coating preparation method is capable of producing a series of fiber coatings to meet the analytical demands for analytes with different properties.

Figure 1. Schematic illustration of PANI/ZNRs/ZIF-8 coating preparation

EXPERIMENTAL SECTION Chemicals and Instrumentation. information)

(see supporting

Preparation of PANI/ZNRs coated fibers. Stainless steel wires (SSWs) (17 cm of length and 0.15 mm of diameter) were used as the substrate of SPME coatings. A layer of PANI was first deposited onto SSWs by a cyclic voltammetry method. Briefly, a three-electrode system consisting of an SSW as the working electrode (WE), a saturated calomel electrode (SCE) as the reference electrode (RE) and a platinum wire as the counter electrode (CE) was placed into a beaker containing H2SO4 (0.8 mol L-1) and aniline (0.5 mol L-1). The three-electrode system was connected to an electrochemical analyzer, which applied voltages ranging from 0 to 0.8 V at a scanning rate of 100 mV s-1 for 15 cycles. In this process, a

layer of PANI was coated on the surface of SSW. Then, a seed-mediated method was used to grow aligned ZNRs onto the PANI coating to prepare a PANI/ZNRs coated fiber. In detail, the PANI coating was vertically dipped into an aqueous mixture containing zinc nitrate (0.5 mol L-1) and methenamine (0.5 mol L-1) for 30 s and placed in an oven for conditioning at 180 °C for 2 min. The abovementioned step was repeated for 5 times to ensure sufficient deposition of ZnO seeds into the PANI network. After that, the coating was dipped into the mixture of zinc nitrate (0.05 mol L-1) and methenamine (0.05 mol L-1) and heated at 95 °C for 2 h in an oven to obtain a PANI/ZNRs coated fiber. Preparation of PANI/ZNRs/ZIF-8 coated fibers. A hydrothermal method was used to in-situ grow ZIF-8 onto PANI/ZNRs coated fibers for the preparation of PANI/ZNRs/ZIF-8 coated fibers. Specifically, a PANI/ZNRs coated fiber was vertically immersed into a 2MI solution (DMF/H2O =3:1) and heated at 90 °C for 8 h in an oven. During this period, ZNRs were etched by 2-MI leading to the in-situ formation of ZIF-8. To control the extent of transformation from ZNRs to ZIF-8, the concentration of 2-MI was varied at 0.0125, 0.125 and 1.25 mol L-1 to obtain PANI/ZNRs/ZIF-8 (0), PANI/ZNRs/ZIF-8 (1) and PANI/ZNRs/ZIF-8 (2) coated fibers, respectively. Electro-enhanced (EE)-SPME experiments. A 15-mL standard solution containing a mixture of aromatic amines at 1 μg mL-1 was loaded into a sample vial sealed with a rubber pad. The sample solution was adjusted to pH 3 using HCl (1 mol L-1). A three-electrode system including an as-prepared SPME fiber (WE), a saturated calomel electrode (RE) and a platinum wire (CE) was placed in the sample vial and connected to an electrochemical analyzer. A constant voltage of -2.0 V was applied to the three-electrode system. The extraction was performed at room temperature (25 ◦ C) with a constant stirring rate of 750 rpm for 20 min. Following the extraction, the fiber was introduced into the GC injector and desorbed at 260 ◦C for 4 min. SPME experiments were conducted under the same conditions as described for EESPME without applying a voltage. For comparison, PANI/ZNRs, PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF8(2) coated fiber were used for both SPME and EE-SPME experiments. For EE-SPME and SPME of ionic drugs, the same procedures were followed using a standard solution mixture of basic drugs (ephedrine, MDA, caffeine, atropine, methadone, cocaine, codeine, acetylcodeine, heroine and papaverine) at 10 μg mL-1 without pH adjustment. Real sample analysis. Sewage water samples were collected from an oil refinery, filtered and diluted twice before use. A PANI/ZNRs/ZIF-8(2) coated fiber was used to extract samples (15 mL) under stirring for 40 min, followed by GC-FID analysis. Recovery tests were carried out by spiking a mixed standard with 1methylnaphthalene and 2-methylnaphthalene at 10 ng mL-1 and the other PAHs at 1 ng mL-1.

RESULTS AND DISCUSSION


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Analytical Chemistry is advantageous since ZNRs can serve as a direct source of Zn2+ for the in-situ growth of ZIF-8.

Figure 2. SEM images of (a, b) PANI (c, d) PANI/ZNRs (eh) PANI/ZNRs/ZIF-8(1) and (i-l) PANI/ZNRs/ZIF-8(2) coated SPME fibers. Characterization of SPME coatings. As shown in Fig. 2a-b, a porous layer of PANI was coated onto the SSW. Then, a seed-mediated growth method was used to grow the aligned ZNRs from the porous network. Fig. 2c-d show that almost vertically aligned ZNRs were grown from the porous network. To in-situ convert ZNRs into ZIF-8, 2-MI was added to react with Zn2+ of ZNRs. The concentration of 2-MI was adjusted between 0.0125 and 1.25 M to convert ZNRs into ZIF-8 with different extent. As shown in Fig. S1, the morphology of ZNRs barely changed when the concentration of 2-MI was fixed at 0.0125 M. In addition, no characteristic diffraction peaks for ZIF-8 can be found from the XRD patterns (Fig. S2). These results indicate that the reaction between Zn2+ and 2-MI was barely initiated when the concentration of 2-MI was low. As the concentration of 2-MI was increased to 0.125 M, it can be observed from Fig. 2e-h that ZNRs were partially etched from inside, resulting in the formation of a nanotube structure composed by a hybrid of ZNRs and some emerging crystals, which were confirmed to be ZIF8 based on the XRD results (Fig. 3b). As the concentration of 2-MI was further increased to 1.25 M, the structure of ZNRs collapsed with the formation of additional ZIF-8 crystals (Fig. 2i-l), indicating a higher degree of transformation from ZNRs to ZIF-8. Such conclusion is further supported by the XRD patterns (Fig. 3b), in which the diffraction peaks for ZNRs decreased while those for ZIF-8 gradually increased from PANI/ZNRs/ZIF-8(1) to PANI/ZNRs/ZIF-8(2). Moreover, SEM-EDX results (Fig. 3a) show the ratio of C and N elements increased and the ratio of Zn and O decreased, respectively, demonstrating the dissolution of ZNRs by 2-MI and the generation of ZIF-8. Unlike traditional MOF-based SPME coatings, the arrangement of the as-prepared ZIF-8 was more organized because it was in-situ grown from the aligned ZNRs. Such alignment reduces the extent of overlap among MOFs, thereby increasing the available surface area of the coating. Furthermore, our fabrication protocol

Figure 3. (a) SEM-EDX analysis results based on weight and atom percentage obtained from PANI/ZNRs/ZIF-8(1) (upper row) and PANI/ZNRs/ZIF-8(2) (lower row) (b) XRD patterns, (c) N2 adsorption-desorption isotherms and (d) pore size distribution profiles of PANI/ZNRs, PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF-8(2). To investigate the porosity change during the transformation of ZNRs into ZIF-8, N2 adsorptiondesorption measurements were performed. As shown in Fig. 3c, the N2 adsorption-desorption curve for ZNRs belongs to type III isotherm, suggesting the nonporous or macroporous structural features of this material. Taking into account its low specific surface area (64 m² g-1), we inferred that ZNRs are nonporous. The isotherm curve of PANI/ZNRs/ZIF-8(0) is similar to that of PANI/ZNRs (Fig. S3). With regard to PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF-8(2), both curves follow typical type II adsorption, in which a sharp increase of adsorption was observed at the initial stage, suggesting the predominant presence of micropores in these two materials. Based on the pore size distribution curves (Fig. 3d), the pore sizes of PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF-8(2) were both centered at 0.79 nm. The BET specific surface areas of PANI/ZNRs, PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF8(2) were 64, 256 and 436 m²/g, respectively, further verifying the progressive transformation from ZNRs to ZIF-8. To examine changes in the wettability from ZNRs to ZIF-8, contact angle measurements were conducted by depositing droplets of water on the surface of ZNRs, ZNRs/ZIF-8(1) and ZNRs/ZIF-8(2). For ZNRs, the contact angle decreased from 55.4 to 30.7˚ in 60 s (Fig. 4a and d); while for ZNRs/ZIF-8(1) and ZNRs/ZIF-8(2), the contact angle were both greater than 90˚ and barely decreased in 60 s (Fig. 4b, c, e and f). Specifically, the contact angles for ZNRs/ZIF-8(1) and ZNRs/ZIF-8(2) are 104.5 and 134.6 ˚,



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respectively. These results indicate that the hydrophilicity of these three materials follows the order of ZNRs> Table 1. The structures, n-octanol/water partition constant (log Kow) and EFs of the analytes. EFsa Analytes

Structure

cyclopentane 2-methylpentane CH3

2-methylnaphthalene

LogKow

PANI/ZNRs

PANI/ZNRs

PANI/ZNRs

/ZIF8(1)

/ZIF8(2)

3.0

105

465

986

3.21

173

844

1756

3.86

160

1273

2316

3.87

196

1245

2178

ZNRs/ZIF-8(2). To explore the electrochemical properties of the three coatings, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis were performed. Fig.4g shows that all three electrodes exhibit symmetric redox peaks, which is indicative of good reaction reversibility. EIS analysis (Fig. 4h) further reveals that the PANI/ZNRs/ZIF(8)-1 coating exhibited the lowest impedance of charge transfer, which can be attributed to the compromise of the conductivity and adsorption ability of the coating. Although the transformation from ZNRs to ZIF-8 inevitably decreases the coating conductivity, improving the adsorption capacity of coatings towards [Fe(CN)6]3- and [Fe(CN)6]4- should alleviate the impedance of charge transfer. Based on the aforementioned results, we can conclude that the conversion from ZNRs into ZIF-8 can be readily controlled by adjusting the concentration of 2-MI to obtain SPME coatings with desired properties such as polarity, porosity, specific surface area and charge adsorption ability. SPME performance. To test the extraction performance of these SPME coatings, a group of chemical compounds spanning across a wide range of polarity including petroleum ethers (containing a large amount of aliphatic hydrocarbons), PAHs, alcohols, phenols, aromatic amines and illicit drugs were selected as model analytes. The PANI/ZNRs/ZIF-8(2) coated fiber showed 5.5-8.2-fold and

1.2-2.8-fold higher extraction efficiencies towards petroleum ethers as compared to PANI/ZNRs and PANI/ZNRs/ZIF-8(1) coated fibers (Fig. 5a).

Figure 4. Images of water droplets spread on (a, d) ZNRs (b, e) PANI/ZNRs/ZIF-8(1) and (c, f) PANI/ZNRs/ZIF-8(2)


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Analytical Chemistry during contact angle measurement; (g) CV and (h) EIS measurements of ZNRs, PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF-8(2) electrodes. This result can be rationalized by the fact that PANI/ZNRs/ZIF-8(2) has a hydrophobic surface, large specific surface area and abundant micropores, making it an excellent candidate for the extraction of less polar compounds. In addition, the molecular sieving effect stemmed from the microporous structures of ZIF-8 also played an important role in the extraction of linear alkanes, which are in good agreement with previous results. 40 Similarly, PANI/ZNRs/ZIF-8(2) also possessed the highest extraction efficiency towards PAHs among all the fibers tested (Fig. 5b), which is mainly attributed to the strong п-п stacking interaction between PAHs and imidazole group of ZIF-8.40, 41 In contrast, for more polar analytes like alcohols and phenols (Fig. 5c), PANI/ZNRs/ZIF-8(1) showed overall the best extraction efficiency probably because it strikes the balance between hydrophilicity and specific surface area. In addition, hydrogen bonding interactions between ZIF-8 and the analytes may also contribute to the enhanced extraction of these analytes.42, 43 For comparison, a commercial PDMS/DVB fiber, which proved to show high affinity towards these analytes, was selected as a benchmark. As indicated in Fig. 5d, the PANI/ZNRs/ZIF-8(1)-coated fiber showed overall comparable extraction efficiency for PAHs to the PDMS/DVB fiber. To gain additional insights into the extraction selectivity of fiber coatings, the structure and Log Kow of typical compounds were correlated with the enrichment factors (EFs) of these analytes using the proposed fiber coatings. As shown in Table 1, the PANI/ZNRs/ZIF-8(2)-coated fiber exhibited the highest EF of 2316 towards 2-methylnaphthalene, which has the lowest polarity based on its LogKow value. The hydrophobic and п-п interaction between the analyte and the coating likely contributed for this high extraction efficiency.41 In contrast, the highest EF for the most hydrophilic analyte (propanol) was obtained by the PANI/ZNRs/ZIF-8(1)-coated fiber, which compromises the hydrophilicity and specific surface area as mentioned above. These results verified our hypothesis that the extraction selectivity of the fiber coatings can be finetuned by controlling the weight ratio between ZNRs and ZIF-8. EE-SPME performance. Previously, we developed an EESPME technique, in which a suitable voltage was applied to the fiber coatings to improve the extraction of ionic species.44, 45 The adsorption capacity and charge transfer resistance of the fiber coating play a key role in this method because the extraction is driven by electrophoresis and electrostatic interaction.

Figure 5. (a) SPME-GC chromatograms of the analysis for petroleum ethers using different SPME fibers (Peak identification: 1. 2,2-dimethybutane, 2. cyclopentane, 3. 2methylpentane, 4. 3-methylpentane, 5. n-hexane, 6. 2,2dimethylpentane, 7. methylcyclopentane, 8. 2,4dimethylpentane, 9. 2,2,3-trimethylbutane, 10. cyclohexane, 11. 2-methylhexane, 12. 2,3-dimethylpentane, 13. 1,1-dimethylcyclopentane, 14. 3-methylhexane,15. cis-1, 3-dimethylcyclopentane, 16. trans-1,3dimethylcyclopentane, 17. trans-1,2-dimethylcyclopentane, 18. n-heptane). Comparison of extraction ability for (b) PAHs (Peak identification: 1. 2-methylnaphthalene, 2. 1methylnaphthalene, 3. pyrene, 4. phenanthrene, 5. chrysene, 6. benzo(a)pyrene) (c) alcohols (Peak identification:1. ethanol, 2. 1-propanol, 3. 1-butanol, 4. glycol) and (d) phenols (Peak identification: 1. phenol, 2. 4-methylphenol, 3. 2,6-dimethylphenol, 4. o-chlorophenol, 5. 2, 4-dichlorophenol) using different SPME fibers. Experimental parameters such as the applied voltage, extraction time, electrolyte concentration and pH, were optimized to maximize the efficiency of EE-SPME (Fig. S4). We then evaluated these three coatings for the extraction of four protonated amines using SPME and EESPME methods and again observed that PANI/ZNR/ZIF8(1) exhibited overall the best EE-SPME efficiency among all these three coatings (Fig. 6a-c). The enhancement factors of EE-SPME over SPME for four anilines using PANI/ZNRs, PANI/ZNRs/ZIF-8(1) and PANI/ZNRs/ZIF8(2) were 1.5-4.5, 5.0-9.7 and 2.4-4.8, respectively (Table S1). These results suggest that PANI/ZNRs/ZIF-8(1)coated fiber shows overall the lowest charge transfer resistance and highest adsorption affinity towards polar amines among the three fiber coatings (Fig. 4h). The PANI/ZNRs/ZIF-8(1)-coated fiber was further used to extract 10 kinds of ionic drugs using SPME and EE-SPME (Fig. 6d). Comparatively, EE-SPME exhibited much better extraction efficiencies towards these drugs over that of SPME, further confirming the suitability of



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Table 2. The analytical data of the developed SPME-GC method for the analysis of PAHs using a PANI/ZNRs/ZIF-8(2) coated fiber Analyte

Linear range (ng

mL-1)

Correlation coefficients

LOD (ng

L-1)

Single fiber reproducibility (n=6, %)

Fiber-to-fiber reproducibilit y (n=5, %)

2-methylnaphthalene

2.5-250

0.997

8.2

6.2

7.3

1-methylnaphthalene

2.5-250

0.988

12.4

7.0

8.0

pyrene

0.25-25

0.994

37.2

9.2

10.0

phenanthrene

0.25-25

0.997

24.6

4.5

4.1

chrysene

0.25-25

0.997

66.5

9.8

9.3

benzo(a)pyrene

2.5-250

0.991

134.4

8.6

10.4

this fiber for EE-SPME application. To sum up, these results support our hypothesis that the extraction performance of PANI/ZNRs/ZIF-8 coated fibers can be fine-tuned by regulating the extent of transformation from ZNRs to ZIF-8 in the coatings. Method validation. To more comprehensively investigate the practical applicability of the fiber coating, a SPME-GC-FID method was developed for the analysis of PAHs using a PANI/ZNRs/ZIF-8(2) coated fiber. SPME and GC parameters including extraction time and temperature, salt concentration, stirring rate, desorption temperature and time were optimized (Fig. S5). Under the optimal conditions, the method was used to analyze a series of standard solutions containing PAHs at concentrations ranging from 0.25 to 250 ng mL-1. As listed in Table 2, the method exhibited good linearity for 1methylnaphthalene, 2-methylnaphthalene and benzo(a)pyrene in the range of 2.5-250 ng mL-1, and for the other PAHs in the range of 0.25-25 ng mL-1. Limits of detection (LODs) were calculated as 3 times of the signalto-noise (S/N). The LODs for the selected PAHs were in the range of 8.2-134.4 ng L-1. The precision of the method was evaluated by analyzing 6 replicates of the PAHs solution, and the relative standard deviations (RSDs) fell between 4.5 and 9.8%. Fiber-to-fiber reproducibility was also studied, and the RSDs ranged from 4.1% to 10.4%. It should be noted that the ZIF-8 in the coating was not very resistant to highly acidic (pH11) solutions, so the fiber coating was not applicable for analyzing samples under extreme pH conditions. A comparison of the linear range, LODs, RSDs, and correlation coefficients of our method to recently published studies is also summarized in Table S2. Real sample analysis. To evaluate its viability for the analysis of real samples, the method was used for the determination of PAHs in sewage water samples from a petroleum refinery, which generally has very complex matrix. As shown in Table S3, all the analytes except benzo(a)pyrene were detected with the concentrations in the range of 0.97-6.5 ng mL-1. The recovery of the six PAHs

spiked in the water samples ranged from 84.5% to 109.8%. A typical chromatogram using the developed SPME-GC method for the analysis of real sample was shown in Fig. S6.

Figure 6. Comparison between SPME-GC and EE-SPMEGC for the analysis of anilines using (a) PANI/ZNRs, (b) PANI/ZNRs/ZIF-8(1) and (c) PANI/ZNRs/ZIF-8(2) coated fibers. (d) Comparison between SPME-GC and EE-SPMEGC for the analysis of basic drugs using a PANI/ZNRs/ZIF-8(1) coated fiber. Black and red lines in ad represent the SPME-GC and EE-SPME-GC analysis, respectively. Peak identification in a-c:1. aniline, 2. Nmethylaniline, 3. N,N-dimethylaniline, 4. N,Ndiethylaniline; Peak identification in d: 1. ephedrine, 2. 3,4-methylenedioxyamphetamine, 3. caffeine, 4. atropine, 5. methadone, 6. cocaine, 7. codeine, 8. acetylcodeine, 9. heroine, 10. papaverine.

CONCLUSIONS In summary, we have developed a protocol to prepare novel SPME coatings consisting of hybrid ZNRs/ZIFs. A


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Analytical Chemistry series of SPME coatings containing different ratios of ZNRs to ZIF-8 were produced by reacting ZNRs with different concentrations of 2-MI. Depending on the properties of target analytes, we can customize SPME coatings with desired polarity, porosity and charge transfer resistance by regulating the ratio of ZNRs to ZIF8 in the SPME coating. This work might open a new avenue for SPME coating preparation since other metal oxide nanomaterials can also serve as “hard templates” to provide metal ions for the in-situ growth of metal oxide/MOFs hybrid nanomaterials, which may introduce a great diversity of adsorbents with variable properties for SPME applications.

ASSOCIATED CONTENT Chemicals and instrumentation; N2 adsorption-desorption isotherms, XRD patterns, and a SEM image of PANI/ZNRs/ZIF-8(0); Enhancement factors between EESPME and SPME; The optimization of EE-SPME for the extraction of anilines; The optimization of SPME for the extraction of PAHs; Method comparison for the analysis of PAHs; Results of real sample analysis. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Jingbin Zeng, e-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21876206) , the Natural Scientific Foundation of Shandong (No. ZR2016BQ23) and the Fundamental Research Funds for the Central Universities (18CX02037A).

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Analytical Chemistry

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Controllable Transformation of Aligned ZnO Nanorods to ZIF-8 as

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