Journal of Chromatography A, 1343 (2014) 188–194
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Room temperature fabrication of post-modified zeolitic imidazolate framework-90 as stationary phase for open-tubular capillary electrochromatography夽 Li-Qing Yu 1 , Cheng-Xiong Yang, Xiu-Ping Yan ∗ State Key Laboratory of Medicinal Chemical Biology (Nankai University), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), and Research Center for Analytical Sciences, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China
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Article history: Received 24 January 2014 Received in revised form 13 March 2014 Accepted 1 April 2014 Available online 12 April 2014 Keywords: Stationary phase Metal–organic frameworks Open tubular capillary electrochromatography Zeolitic imidazolate framework-90
a b s t r a c t Metal–organic frameworks (MOFs) are attractive as porous stationary phase for open-tubular capillary electrochromatography (OT-CEC) due to their fascinating structures and unusual properties. Here we report a directly covalent bonding approach to prepare uniform and dense MOF film on the inner wall of fused silica capillary at room temperature for OT-CEC. Zeolitic imidazolate framework-90 (ZIF-90) as a model MOF because it not only possesses large surface area and high stability but also provides the free aldehyde groups to bond to the inner surface of capillary via covalent bond. X-ray diffraction, scan electron microscopy, and UV–vis spectrophotometry were used to confirm the bonding of the ZIF-90 to the inner wall of the silica capillary. The ZIF-90 coating not only increased the phase ratio of open-tubular column, but also improved the interactions of tested analytes and the coating. Owing to the porous structure of ZIF-90 and hydrophobic interactions between the analytes and the organic ligands of ZIF-90, three groups of isomers, neutral and basic compounds and nonsteroidal anti-inflammatory drugs were well separated on the ZIF-90 bonded column. The precisions (relative standard deviation, RSD) of retention time, half peak width and peak area for three consecutive runs were 0.3–1.2%, 1.3–6.0% and 1.5–5.2%, respectively. The run-to-run, day-to-day, and column-to-column precisions (RSDs) for the electroosmotic flow of the ZIF-90 bonded column were 0.2%, 0.4%, and 1.9%, respectively. Moreover, the ZIF-90 bonded column could stand more than 230 runs without observable change in the separation efficiency. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) are novel microporous crystalline materials consisting of clusters or chains of metal ions connected by organic ligands [1–3]. Owing to their diverse structures and accessible tunnels and cages, they are increasingly in demand for application in gas storage [4], catalysis [5], separation [6], and drug delivery [7]. The unusual properties such as high surface areas, good thermal and solvent stability, adsorption affinities and the availability of in-pore functionality and outer-surface
夽 Invited paper for the Honor Issue of Professor Peichang Lu’s 90th birthday. ∗ Corresponding author. Tel.: +86 22 23506075; fax: +86 22 23506075. E-mail addresses: [email protected] (L.-Q. Yu), [email protected], [email protected] (X.-P. Yan). 1 On leave from College of Chemical and Environmental Sciences, Hebei University, Baoding 071002, China. http://dx.doi.org/10.1016/j.chroma.2014.04.003 0021-9673/© 2014 Elsevier B.V. All rights reserved.
modification make MOFs attractive as separation media in analytical chemistry [8–10]. One of the most promising applications of MOFs seems to be as novel stationary phases for chromatography [11]. Various MOFs have been explored as the stationary phase in liquid chromatography (LC) or gas chromatography, the pseudostationary phase in electrokinetic chromatography [12], and the MOF-organic polymer hybrid monolithic stationary phase for microbore LC and capillary electrochromatography (CEC) [13]. Open tubular CEC (OT-CEC) has attracted increasing attention due to its advantages of ease of column preparation and no need for end frits and particles packing [14]. Although open tubular columns have been widely applied in chromatographic separations, it is still challenging to achieve high phase ratio, high sample capacity, and good stability due to the lack of fabrication techniques [15]. To date, several approaches including sol–gel-derived phases [16], etched capillary [17,18], porous silica layers [19,20], and nanoparticle phases [21–27] have been developed to address this issue. Since MOFs possess numerous structures, high surface areas, and
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adsorption affinities, the bonding of MOF to the capillary inner wall not only increases the phase ratio of open tubular column, but also improves the interactions of solute molecules and stationary phase. The combination of the unique properties of MOFs and the excellent features of open tubular column has been received attention in OT-CEC. Recently, an in situ, layer-by-layer self-assembly approach was employed to fabricate MIL-100(Fe) coated capillary column [28] and a dynamic coating method was used to prepare CAU-1-polymethyl methacrylate composite coated capillary column [29] for OT-CEC. However, previous approaches for the preparation of MOF-coated open tubular column either were off-line uncontrollable procedure or a thin MOF layer was obtained through many cycles [28,29]. Generally, direct bonding of MOF film to the inner wall of capillary should increase the phase ratio and stability due to the existence of covalent bond between the coating and the surface of capillary. Here, we report a directly covalent bonding approach to prepare firm MOF film on the inner wall of capillary at room temperature for OT-CEC. Zeolitic imidazolate framework-90 (ZIF-90) was employed as a model MOF because it possesses not only large surface area and high stability [30], but also ease of preparation at room temperature [31]. ZIF-90 shows a new sodalite topology with permanent porosity containing a narrow size of the six-membered ring ˚ and especially possesses the free aldehyde group in the pores (3.5 A) framework that allows the covalent functionalization with amine groups through an imine condensation reaction [32,33]. In this work, ZIF-90 was prepared at room temperature [31], and modified with 3-aminopropyltriethoxysilane (APTES). The APTES modified ZIF-90 was then directly immobilized onto the capillary wall via covalently bonding at room temperature. The performance of the ZIF-90 bonded capillary column was evaluated as a new stationary phase for OT-CEC separation of nonsteroidal anti-inflammatory drugs, anilines, and the isomers of xylene, chlorotoluene, and dichlorobenzene. 2. Experimental 2.1. Chemicals and reagents All chemicals and reagents used were at least of analytical grade unless otherwise stated. Ultrapure water was purchased from Tianjin Wahaha Foods Co. Ltd. Sodium formate (99%, NaCO2 H) and zinc nitrate hexahydrate (99%, Zn(NO3 )2 ·6H2 O) were purchased from Tianjin Standard Science and Technology Co. (Tianjin, China). Imidazolate-2-carboxyaldehyde (99%), methanol (MeOH), 3-aminopropyltriethoxysilane (APTES) (98%), and isomers (xylene, dichlorobenzene, chlorotoluene) were purchased from Aladdin Reagent Co. (Shanghai, China). Acetanilide, aniline, 2-nitroaniline, and 1-naphthylamine were purchased from Tianjin No. 1 Chemical Reagent Plant (Tianjin, China), Tianjin Huayue Chemical Reagent Plant (Tianjin, China), Shanghai No. 3 Chemical Reagent Plant (Shanghai, China), and Beijing Chemical Plant (Beijing, China), respectively. Thiourea, naphthalene, anthracene, sodium dihydrogen phosphate, and disodium hydrogen phosphate were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Ibuprofen, naproxen and ketoprofen were purchased from Zhejiang Xianju Pharmaceutical Corporation (Xianju, China). Fused silica capillary (375 m o.d. × 75 m i.d.) was obtained from Yongnian Optic Fiber Plant (Handan, China). 2.2. Synthesis and APTES-modification of ZIF-90 ZIF-90 was synthesized as reported previously [31]. Typically, a solution of 20 mmol of NaCO2 H and 20 mmol of imidazolate-2carboxyaldehyde in 50 mL of MeOH was heated to 50 ◦ C until it
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became clear, then cooled to room temperature. Another solution of 5 mmol of Zn (NO3 )2 ·6H2 O and 50 mL of ultrapure water was poured into the above solution and allowed to stir at room temperature for 1 h. The resulting milky mixture was centrifuged at 11,200 × g for 5 min, and the precipitate was washed with 45 mL of MeOH for three times. The ZIF-90 crystals were dried in an oven at 85 ◦ C. For the APTES-modification, the as-prepared ZIF-90 crystals were immersed in the solution of MeOH and APTES, and refluxed at 110 ◦ C for 30 min. The prepared APTES modified ZIF-90 crystals were washed with MeOH several times and then dried in air at room temperature over night. 2.3. Fabrication of ZIF-90 bonded capillary columns The as-bought capillary was washed with 1 M NaOH for 2 h, ultrapure water for 1 h and 0.1 M HCl for 2 h, then washed with ultrapure water again until the outflow reached pH 7.0. The capillary was dried with nitrogen purging at 150 ◦ C in a gas chromatographic oven overnight. To produce a thin layer of ZIF-90, the APTES modified ZIF-90 was dispersed in MeOH under ultrasonication to give a homogeneous suspension. The suspension was then introduced into the preconditioned fused silica capillary via syringe injection, after which the capillary was sealed at both ends for 12 h at room temperature. 2.4. Characterization The X-ray diffraction (XRD) experiments were performed on a D/max-2500 diffractometer (Rigaku, Japan) using CuK␣ radiation ˚ Scan electron microscopy (SEM) images of the bare (=1.5418 A). capillary and the ZIF-90 bonded capillary were recorded on a SS-550 scanning electron microscope at 15.0 kV (Shimadzu, Japan). Fourier transform infrared (FT-IR) spectra (4000–400 cm−1 ) in KBr plate were obtained on Magna-560 spectrometer (Nicolet, Madison, WI, USA). Solid UV–vis absorption spectra (200–800 nm) were collected on a V-550 spectrometer (JASCO, Japan). 2.5. CEC separation CEC experiments were carried out on a P/ACE MDQ capillary electrophoresis system (Beckman, Fullerton, CA, USA) equipped with a diode array detector (DAD) at 25 ◦ C. Data acquisition and processing was controlled by the Beckman ChemStation software. The mobile phase was obtained by mixing the phosphate buffer solution (PBS) with the appropriate amount of water and MeOH. Prior to separation, all solutions were filtered through a 0.45 m filter and degassed under ultrasonication and injected electrokinetically at 3.45 kPa for 5 s. The ZIF-90 bonded capillary column (total length, 31.2 cm; effective length, 20.0 cm) was rinsed with MeOH (15 min), ultrapure water (4 min), and PBS (10 mM, pH 7.4) (2 min) before the first use, and with running buffer containing 10 mM PBS (2 min) between consecutive runs. The column was then installed in the CEC instrument and equilibrated at 15 kV until a stable current and baseline was achieved. 3. Results and discussion 3.1. Fabrication and characterization of the ZIF-90 bonded capillary Fig. 1 shows the schematic illustration for the fabrication of the ZIF-90 bonded capillary. ZIF-90 crystals were prepared from imidazolate-2-carboxyaldehyde and Zn(NO3 )2 ·6H2 O at room temperature according to Thompson et al. [31]. The good agreement between the experimental and the simulated XRD patterns shows
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Fig. 1. Schematic illustration for the fabrication of the ZIF-90 covalently bond capillary column. RT = room temperature.
the successful preparation of ZIF-90 crystals (Fig. 2A). APTES functionalization of ZIF-90 crystals is crucial for the successful fabrication of ZIF-90 layer on the inner capillary wall to provide abundance ethyoxyl groups for subsequent covalent bonding to the inner capillary wall. ZIF-90 was post-functionalized via the imine condensation reaction between the free aldehyde groups of the ZIF-90 and the amino group of APTES. FT-IR spectra further confirm the post-functionalization of ZIF-90 with APTES (Fig. 2B) as the C O band of the aldehyde at 1675 cm−1 was replaced by the C N bond of the imine at 1637 cm−1 . The presence of the Si O band at 1080 cm−1 also reveals that ZIF-90 was successfully modified with APTES [34,35]. After post-functionalization, all XRD peaks of the ZIF-90 remain (Fig. 2A). In addition, no obvious change in the XRD pattern of ZIF-90 was observed after the pretreatment with pure water and PBS at pH 3 and 9 for 48 h (Fig. 2C), showing the good stability of ZIF-90 in water and PBS. The APTES modified ZIF-90 was covalently bond to the inner capillary wall via the reaction between the silanol on the surface of capillary and the ethyoxyl in APTES modified ZIF-90 framework.
Fig. 2. (A) XRD patterns of the simulated ZIF-90, the synthesized ZIF-90, the APTES modified ZIF-90 and the capillary coated with ZIF-90. (B) FT-IR spectra of the asprepared ZIF-90, and the APTES modified ZIF-90. (C) XRD patterns of the simulated ZIF-90 and ZIF-90 treated with aqueous PBS (10 mM) and pure water for 48 h. The simulated XRD of ZIF-90 was generated from the crystallographic data reported in ref. 30 using the Mercury 3.1 software.
Fig. 3. SEM images: (A) the cross section of the bare capillary pretreated with 1 M NaOH for 2 h; (B) the cross section of the capillary coated with ZIF-90 thin layer.
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Fig. 4. UV–vis spectra: (A) bare quartz; (B) APTES; (C) ZIF-90; (D) the quartz coated with the APTES-modified-ZIF-90.
SEM images show a ∼0.4 m thick ZIF-90 layer coated on the inner capillary wall (Fig. 3B cf. A). The assembly of ZIF-90 coating onto the capillary wall was further monitored by solid UV–vis absorption spectroscopy. Since it is not convenient to obtain the UV–vis spectra of ZIF-90 on the inner capillary wall, fused silica capillary was replaced with quartz slide whose nature is the same as the fused silica capillary. The absorption spectra of the quartz, APTES modified quartz, ZIF-90, and the APTES modified ZIF-90 coated quartz are shown in Fig. 4. The ZIF-90 coated quartz exhibited a characteristic strong absorption of the imidazolium rings in the ZIF-90 framework with a maximum wavelength at 280 nm (curves C and D in Fig. 4). The existence of the characteristic peaks of ZIF-90 in the XRD pattern of the ZIF-90 coated capillary also shows the successful fabrication of ZIF-90 layer on the inner capillary wall (Fig. 2A).
Fig. 5. (A) Effect of MeOH content in PBS (10 mM, pH 7.4) on the EOF for bare and ZIF-90 columns. (B) Effect of pH* (apparent pH) on the EOF in MeOH-PBS (10 mM) (50:50, v/v) for bare and ZIF-90 columns. Separation conditions: separation voltage, 20 kV; detection wavelength, 254 nm.
3.2. Effect of organic modifier and pH on the electroosmotic flow (EOF) 3.3. Effect of organic modifier on separation OT-CEC has received great attention because of its advantages of high selectivity and efficiency, short analysis time, low sample and mobile phase consumption. The stationary phase in CEC has a double role: providing efficient chromatographic separation and generating reliable EOF. To evaluate the effect of organic modifier and pH on the EOF, thiourea was used as the EOF marker and MeOH was selected as the organic modifier. The effect of the MeOH content in running electrolyte solution (10 mM PBS, pH 7.4) on the EOF for the bare capillary and ZIF-90 bond capillary columns is shown in Fig. 5A. The EOF decreased as the MeOH content increased for both bare capillary and ZIF-90 capillary, but ZIF-90 bond capillary column always gave lower EOF than the bare capillary column due to the reduction of the negative charge density from the bond ZIF90 layer. The effect of pH on the EOF for the bare and ZIF-90 bond capillary columns was investigated in MeOH-PBS (10 mM) (50:50, v/v) (Fig. 5B). Here, the pH measured in MeOH-PBS (10 mM) (50:50, v/v) is actually an apparent pH (pH*) due to the organic solvent effect [36]. Both of the bare capillary and the ZIF-90 bond capillary columns display a similar EOF-pH* profile. The EOF increased in the pH* range from 3.0 to 8.0 partially due to the increased ionization of the silanols of the columns. In addition, change in pH* from 3.0 to 8.0 also led to variation in the ionic strength of the running electrolyte solution, which should also contribute to the increase of EOF. Again, the ZIF-90 on the capillary column gave lower EOF than the bare capillary column due to the coverage of the ZIF-90 (Fig. 5B).
The content of organic modifier in the mobile phase not only influences the EOF but also the chromatographic resolution. Generally, the separation of charged analytes depends on their electrophoretic and electroosmotic mobilities and the partition interaction between the mobile phase and the stationary phase. However, the separation for neutral analytes only depends on the partition interaction between the analytes and the ZIF-90 coating on the inner walls of the capillary column. Naphthalene and anthracene were used as the tested analytes and MeOH was used as the organic modifier to investigate the effect of the organic modifier content on the separation on ZIF-90 bond capillary column. The resolution and retention of the analytes decreased with the increasing of the MeOH content (Fig. S1 in Supplementary data), which may be attributed to the increased solubility of the analytes in reduced polarity of mobile phase, the decreased viscosity, and the increased interaction between the analytes and the mobile phase. The elution of naphthalene and anthracene followed an increasing the number of the aromatic rings, indicating that the retention mechanism is consistent with the increasing of hydrophobic interaction between the analyte and the imidazolium ring in the ZIF-90 framework. 3.4. Loading capacity and reproducibility of ZIF-90 bond column The loading capacity of ZIF-90 bond column is defined as the amount of sample injected which makes the corresponding peak
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Fig. 6. Loading capacity test on ZIF-90 bond capillary column using naphthalene as the analyte and MeOH-PBS (10 mM, pH 7.4) (40:60, v/v) as the mobile phase. Separation conditions: separation voltage, 25 kV; detection wavelength, 214 nm.
width at half-height (W1/2 ) increase to 10% over the W1/2 at low sample amounts [25]. The loading capacity was studied by injecting naphthalene in the range from 0.05 to 0.8 g L−1 while the injected sample volume was fixed to about 15 nL. As shown in Fig. 6, the W1/2 for naphthalene at 0.7 g L−1 increased by 10% over the peak width at 0.1 g L−1 . Thus, the loading capacity of ZIF-90 bond column for naphthalene was 0.7 g L−1 . The higher loading capacity of ZIF-90 bond capillary column can be ascribed to the high surface area of ZIF-90 and compact ZIF-90 layer on the capillary wall. The electrochromatograms for the three replicate separations of aniline, acetanilide, o-nitroaniline and 1-naphthylamine and nonsteroidal anti-inflammatory drugs on the ZIF-90 bond column are shown in Fig. 7. The precisions (relative standard deviation, RSD) of retention time, W1/2 , and peak area for the analytes were 0.3–1.2%, 1.3–6.0% and 1.5–5.2%, respectively. Besides, the run-to-run, dayto-day, and column-to-column reproducibility (RSD) for the EOF was 0.2% (n = 5), 0.4% (n = 5), and 1.9% (n = 3), respectively. The recoveries of three nonsteroidal anti-inflammatory drugs spiked at 3 mg L−1 level in a local lake water sample ranged from 91% to 109% (Fig. S3 and Table S2 in Supplementary data). Moreover, the ZIF-90 bond column can be used for 230 runs without obvious change in separation efficiency. The long lifetime can be attributed to the existence of chemical bond between ZIF-90 and the inner capillary wall. 3.5. OT-CEC separation on ZIF-90 column The separation for xylene isomers is challenging due to their coherent boiling points and dimensions [9,37,38]. We first tested the separation of xylene isomers to evaluate the performance of ZIF-90 bond column. All three isomers in the mixture of xylene isomers were separated on the ZIF-90 bond column (Fig. 8A). However, no separation of the xylene isomers was achieved on the bare capillary column (Fig. 8B). The improvement of the separation efficiency was attributed to the porous structure of ZIF-90 and hydrophobic interactions between the analytes and the imidazolium rings in ZIF-90 framework. We further demonstrated the applicability of ZIF-90 bond capillary column for the separation of dichlorobenzene and chlorotoluene isomers. The ZIF-90 capillary column also gave excellent separation of dichlorobenzene and chlorotoluene isomers (Fig. 8A) but the bare capillary column gave no separation of these iosomers (Fig. 8B). Furthermore, the ZIF-90 bond column gave preferable retention for the ortho-isomers, allowing selective separation
Fig. 7. Electrochromatograms for three replicate separations of the tested compounds on ZIF-90 bond column: (A) nonsteroidal anti-inflammatory drugs in MeOH-PBS (10 mM, pH 4.5) (40:60, v/v); (B) aniline, acetanilide, o-nitroaniline and 1-naphthylamine in MeOH-PBS (10 mM, pH 7.8) (55:45, v/v). Separation condition: applied voltage, 15 kV; detection wavelength, 214 nm for (A) and 235 nm for (B).
of ortho-isomers from the other substituted position isomers. Besides, the ZIF-90 bond columns offered higher column efficiency and comparable precisions for the separation of the tested isomers compared with other methods with different columns [13,39,40] (Table 1 and Table S1 in Supplementary data). The effect of the MeOH content in the mobile phase on the retention factor (k) decreased as the MeOH content increased (Fig. S2 in Supplementary data), showing a reversed phase retention mechanism.
Table 1 Precision for five replicate separations and column efficiency (N) for the three groups of isomers on ZIF-90 bond column. Analyte
p-Xylene m-Xylene o-Xylene m-Dichlorobenzene p-Dichlorobenzene o-Dichlorobenzene m-Chlorotoluene p-Chlorotoluene o-Chlorotoluene
RSD (%) (n = 5)
N (plates/m)
Retention time
Peak area
0.3 0.3 0.3 0.2 0.3 0.2 0.5 0.6 0.5
0.6 0.5 1.2 0.5 0.6 0.7 1.4 1.6 1.6
65,456 39,845 21,006 49,251 39,841 19,687 25,639 20,065 15,649
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Fig. 8. Electrochromatograms for the separation of the tested isomers: (A) ZIF-90 bond capillary column; (B) bare capillary column. Separation conditions for both capillary columns: mobile phase, MeOH-PBS (10 mM, pH 7.4) (40:60, v/v); separation voltage, 15 kV; detection wavelength, 254 nm.
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Room temperature fabrication of post-modified zeolitic imidazolate framework-90 as stationary phase for open-tubular capillary electrochromatography夽 Li-Qing Yu 1 , Cheng-Xiong Yang, Xiu-Ping Yan ∗ State Key Laboratory of Medicinal Chemical Biology (Nankai University), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), and Research Center for Analytical Sciences, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China
a r t i c l e
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Article history: Received 24 January 2014 Received in revised form 13 March 2014 Accepted 1 April 2014 Available online 12 April 2014 Keywords: Stationary phase Metal–organic frameworks Open tubular capillary electrochromatography Zeolitic imidazolate framework-90
a b s t r a c t Metal–organic frameworks (MOFs) are attractive as porous stationary phase for open-tubular capillary electrochromatography (OT-CEC) due to their fascinating structures and unusual properties. Here we report a directly covalent bonding approach to prepare uniform and dense MOF film on the inner wall of fused silica capillary at room temperature for OT-CEC. Zeolitic imidazolate framework-90 (ZIF-90) as a model MOF because it not only possesses large surface area and high stability but also provides the free aldehyde groups to bond to the inner surface of capillary via covalent bond. X-ray diffraction, scan electron microscopy, and UV–vis spectrophotometry were used to confirm the bonding of the ZIF-90 to the inner wall of the silica capillary. The ZIF-90 coating not only increased the phase ratio of open-tubular column, but also improved the interactions of tested analytes and the coating. Owing to the porous structure of ZIF-90 and hydrophobic interactions between the analytes and the organic ligands of ZIF-90, three groups of isomers, neutral and basic compounds and nonsteroidal anti-inflammatory drugs were well separated on the ZIF-90 bonded column. The precisions (relative standard deviation, RSD) of retention time, half peak width and peak area for three consecutive runs were 0.3–1.2%, 1.3–6.0% and 1.5–5.2%, respectively. The run-to-run, day-to-day, and column-to-column precisions (RSDs) for the electroosmotic flow of the ZIF-90 bonded column were 0.2%, 0.4%, and 1.9%, respectively. Moreover, the ZIF-90 bonded column could stand more than 230 runs without observable change in the separation efficiency. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) are novel microporous crystalline materials consisting of clusters or chains of metal ions connected by organic ligands [1–3]. Owing to their diverse structures and accessible tunnels and cages, they are increasingly in demand for application in gas storage [4], catalysis [5], separation [6], and drug delivery [7]. The unusual properties such as high surface areas, good thermal and solvent stability, adsorption affinities and the availability of in-pore functionality and outer-surface
夽 Invited paper for the Honor Issue of Professor Peichang Lu’s 90th birthday. ∗ Corresponding author. Tel.: +86 22 23506075; fax: +86 22 23506075. E-mail addresses: [email protected] (L.-Q. Yu), [email protected], [email protected] (X.-P. Yan). 1 On leave from College of Chemical and Environmental Sciences, Hebei University, Baoding 071002, China. http://dx.doi.org/10.1016/j.chroma.2014.04.003 0021-9673/© 2014 Elsevier B.V. All rights reserved.
modification make MOFs attractive as separation media in analytical chemistry [8–10]. One of the most promising applications of MOFs seems to be as novel stationary phases for chromatography [11]. Various MOFs have been explored as the stationary phase in liquid chromatography (LC) or gas chromatography, the pseudostationary phase in electrokinetic chromatography [12], and the MOF-organic polymer hybrid monolithic stationary phase for microbore LC and capillary electrochromatography (CEC) [13]. Open tubular CEC (OT-CEC) has attracted increasing attention due to its advantages of ease of column preparation and no need for end frits and particles packing [14]. Although open tubular columns have been widely applied in chromatographic separations, it is still challenging to achieve high phase ratio, high sample capacity, and good stability due to the lack of fabrication techniques [15]. To date, several approaches including sol–gel-derived phases [16], etched capillary [17,18], porous silica layers [19,20], and nanoparticle phases [21–27] have been developed to address this issue. Since MOFs possess numerous structures, high surface areas, and
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adsorption affinities, the bonding of MOF to the capillary inner wall not only increases the phase ratio of open tubular column, but also improves the interactions of solute molecules and stationary phase. The combination of the unique properties of MOFs and the excellent features of open tubular column has been received attention in OT-CEC. Recently, an in situ, layer-by-layer self-assembly approach was employed to fabricate MIL-100(Fe) coated capillary column [28] and a dynamic coating method was used to prepare CAU-1-polymethyl methacrylate composite coated capillary column [29] for OT-CEC. However, previous approaches for the preparation of MOF-coated open tubular column either were off-line uncontrollable procedure or a thin MOF layer was obtained through many cycles [28,29]. Generally, direct bonding of MOF film to the inner wall of capillary should increase the phase ratio and stability due to the existence of covalent bond between the coating and the surface of capillary. Here, we report a directly covalent bonding approach to prepare firm MOF film on the inner wall of capillary at room temperature for OT-CEC. Zeolitic imidazolate framework-90 (ZIF-90) was employed as a model MOF because it possesses not only large surface area and high stability [30], but also ease of preparation at room temperature [31]. ZIF-90 shows a new sodalite topology with permanent porosity containing a narrow size of the six-membered ring ˚ and especially possesses the free aldehyde group in the pores (3.5 A) framework that allows the covalent functionalization with amine groups through an imine condensation reaction [32,33]. In this work, ZIF-90 was prepared at room temperature [31], and modified with 3-aminopropyltriethoxysilane (APTES). The APTES modified ZIF-90 was then directly immobilized onto the capillary wall via covalently bonding at room temperature. The performance of the ZIF-90 bonded capillary column was evaluated as a new stationary phase for OT-CEC separation of nonsteroidal anti-inflammatory drugs, anilines, and the isomers of xylene, chlorotoluene, and dichlorobenzene. 2. Experimental 2.1. Chemicals and reagents All chemicals and reagents used were at least of analytical grade unless otherwise stated. Ultrapure water was purchased from Tianjin Wahaha Foods Co. Ltd. Sodium formate (99%, NaCO2 H) and zinc nitrate hexahydrate (99%, Zn(NO3 )2 ·6H2 O) were purchased from Tianjin Standard Science and Technology Co. (Tianjin, China). Imidazolate-2-carboxyaldehyde (99%), methanol (MeOH), 3-aminopropyltriethoxysilane (APTES) (98%), and isomers (xylene, dichlorobenzene, chlorotoluene) were purchased from Aladdin Reagent Co. (Shanghai, China). Acetanilide, aniline, 2-nitroaniline, and 1-naphthylamine were purchased from Tianjin No. 1 Chemical Reagent Plant (Tianjin, China), Tianjin Huayue Chemical Reagent Plant (Tianjin, China), Shanghai No. 3 Chemical Reagent Plant (Shanghai, China), and Beijing Chemical Plant (Beijing, China), respectively. Thiourea, naphthalene, anthracene, sodium dihydrogen phosphate, and disodium hydrogen phosphate were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Ibuprofen, naproxen and ketoprofen were purchased from Zhejiang Xianju Pharmaceutical Corporation (Xianju, China). Fused silica capillary (375 m o.d. × 75 m i.d.) was obtained from Yongnian Optic Fiber Plant (Handan, China). 2.2. Synthesis and APTES-modification of ZIF-90 ZIF-90 was synthesized as reported previously [31]. Typically, a solution of 20 mmol of NaCO2 H and 20 mmol of imidazolate-2carboxyaldehyde in 50 mL of MeOH was heated to 50 ◦ C until it
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became clear, then cooled to room temperature. Another solution of 5 mmol of Zn (NO3 )2 ·6H2 O and 50 mL of ultrapure water was poured into the above solution and allowed to stir at room temperature for 1 h. The resulting milky mixture was centrifuged at 11,200 × g for 5 min, and the precipitate was washed with 45 mL of MeOH for three times. The ZIF-90 crystals were dried in an oven at 85 ◦ C. For the APTES-modification, the as-prepared ZIF-90 crystals were immersed in the solution of MeOH and APTES, and refluxed at 110 ◦ C for 30 min. The prepared APTES modified ZIF-90 crystals were washed with MeOH several times and then dried in air at room temperature over night. 2.3. Fabrication of ZIF-90 bonded capillary columns The as-bought capillary was washed with 1 M NaOH for 2 h, ultrapure water for 1 h and 0.1 M HCl for 2 h, then washed with ultrapure water again until the outflow reached pH 7.0. The capillary was dried with nitrogen purging at 150 ◦ C in a gas chromatographic oven overnight. To produce a thin layer of ZIF-90, the APTES modified ZIF-90 was dispersed in MeOH under ultrasonication to give a homogeneous suspension. The suspension was then introduced into the preconditioned fused silica capillary via syringe injection, after which the capillary was sealed at both ends for 12 h at room temperature. 2.4. Characterization The X-ray diffraction (XRD) experiments were performed on a D/max-2500 diffractometer (Rigaku, Japan) using CuK␣ radiation ˚ Scan electron microscopy (SEM) images of the bare (=1.5418 A). capillary and the ZIF-90 bonded capillary were recorded on a SS-550 scanning electron microscope at 15.0 kV (Shimadzu, Japan). Fourier transform infrared (FT-IR) spectra (4000–400 cm−1 ) in KBr plate were obtained on Magna-560 spectrometer (Nicolet, Madison, WI, USA). Solid UV–vis absorption spectra (200–800 nm) were collected on a V-550 spectrometer (JASCO, Japan). 2.5. CEC separation CEC experiments were carried out on a P/ACE MDQ capillary electrophoresis system (Beckman, Fullerton, CA, USA) equipped with a diode array detector (DAD) at 25 ◦ C. Data acquisition and processing was controlled by the Beckman ChemStation software. The mobile phase was obtained by mixing the phosphate buffer solution (PBS) with the appropriate amount of water and MeOH. Prior to separation, all solutions were filtered through a 0.45 m filter and degassed under ultrasonication and injected electrokinetically at 3.45 kPa for 5 s. The ZIF-90 bonded capillary column (total length, 31.2 cm; effective length, 20.0 cm) was rinsed with MeOH (15 min), ultrapure water (4 min), and PBS (10 mM, pH 7.4) (2 min) before the first use, and with running buffer containing 10 mM PBS (2 min) between consecutive runs. The column was then installed in the CEC instrument and equilibrated at 15 kV until a stable current and baseline was achieved. 3. Results and discussion 3.1. Fabrication and characterization of the ZIF-90 bonded capillary Fig. 1 shows the schematic illustration for the fabrication of the ZIF-90 bonded capillary. ZIF-90 crystals were prepared from imidazolate-2-carboxyaldehyde and Zn(NO3 )2 ·6H2 O at room temperature according to Thompson et al. [31]. The good agreement between the experimental and the simulated XRD patterns shows
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Fig. 1. Schematic illustration for the fabrication of the ZIF-90 covalently bond capillary column. RT = room temperature.
the successful preparation of ZIF-90 crystals (Fig. 2A). APTES functionalization of ZIF-90 crystals is crucial for the successful fabrication of ZIF-90 layer on the inner capillary wall to provide abundance ethyoxyl groups for subsequent covalent bonding to the inner capillary wall. ZIF-90 was post-functionalized via the imine condensation reaction between the free aldehyde groups of the ZIF-90 and the amino group of APTES. FT-IR spectra further confirm the post-functionalization of ZIF-90 with APTES (Fig. 2B) as the C O band of the aldehyde at 1675 cm−1 was replaced by the C N bond of the imine at 1637 cm−1 . The presence of the Si O band at 1080 cm−1 also reveals that ZIF-90 was successfully modified with APTES [34,35]. After post-functionalization, all XRD peaks of the ZIF-90 remain (Fig. 2A). In addition, no obvious change in the XRD pattern of ZIF-90 was observed after the pretreatment with pure water and PBS at pH 3 and 9 for 48 h (Fig. 2C), showing the good stability of ZIF-90 in water and PBS. The APTES modified ZIF-90 was covalently bond to the inner capillary wall via the reaction between the silanol on the surface of capillary and the ethyoxyl in APTES modified ZIF-90 framework.
Fig. 2. (A) XRD patterns of the simulated ZIF-90, the synthesized ZIF-90, the APTES modified ZIF-90 and the capillary coated with ZIF-90. (B) FT-IR spectra of the asprepared ZIF-90, and the APTES modified ZIF-90. (C) XRD patterns of the simulated ZIF-90 and ZIF-90 treated with aqueous PBS (10 mM) and pure water for 48 h. The simulated XRD of ZIF-90 was generated from the crystallographic data reported in ref. 30 using the Mercury 3.1 software.
Fig. 3. SEM images: (A) the cross section of the bare capillary pretreated with 1 M NaOH for 2 h; (B) the cross section of the capillary coated with ZIF-90 thin layer.
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Fig. 4. UV–vis spectra: (A) bare quartz; (B) APTES; (C) ZIF-90; (D) the quartz coated with the APTES-modified-ZIF-90.
SEM images show a ∼0.4 m thick ZIF-90 layer coated on the inner capillary wall (Fig. 3B cf. A). The assembly of ZIF-90 coating onto the capillary wall was further monitored by solid UV–vis absorption spectroscopy. Since it is not convenient to obtain the UV–vis spectra of ZIF-90 on the inner capillary wall, fused silica capillary was replaced with quartz slide whose nature is the same as the fused silica capillary. The absorption spectra of the quartz, APTES modified quartz, ZIF-90, and the APTES modified ZIF-90 coated quartz are shown in Fig. 4. The ZIF-90 coated quartz exhibited a characteristic strong absorption of the imidazolium rings in the ZIF-90 framework with a maximum wavelength at 280 nm (curves C and D in Fig. 4). The existence of the characteristic peaks of ZIF-90 in the XRD pattern of the ZIF-90 coated capillary also shows the successful fabrication of ZIF-90 layer on the inner capillary wall (Fig. 2A).
Fig. 5. (A) Effect of MeOH content in PBS (10 mM, pH 7.4) on the EOF for bare and ZIF-90 columns. (B) Effect of pH* (apparent pH) on the EOF in MeOH-PBS (10 mM) (50:50, v/v) for bare and ZIF-90 columns. Separation conditions: separation voltage, 20 kV; detection wavelength, 254 nm.
3.2. Effect of organic modifier and pH on the electroosmotic flow (EOF) 3.3. Effect of organic modifier on separation OT-CEC has received great attention because of its advantages of high selectivity and efficiency, short analysis time, low sample and mobile phase consumption. The stationary phase in CEC has a double role: providing efficient chromatographic separation and generating reliable EOF. To evaluate the effect of organic modifier and pH on the EOF, thiourea was used as the EOF marker and MeOH was selected as the organic modifier. The effect of the MeOH content in running electrolyte solution (10 mM PBS, pH 7.4) on the EOF for the bare capillary and ZIF-90 bond capillary columns is shown in Fig. 5A. The EOF decreased as the MeOH content increased for both bare capillary and ZIF-90 capillary, but ZIF-90 bond capillary column always gave lower EOF than the bare capillary column due to the reduction of the negative charge density from the bond ZIF90 layer. The effect of pH on the EOF for the bare and ZIF-90 bond capillary columns was investigated in MeOH-PBS (10 mM) (50:50, v/v) (Fig. 5B). Here, the pH measured in MeOH-PBS (10 mM) (50:50, v/v) is actually an apparent pH (pH*) due to the organic solvent effect [36]. Both of the bare capillary and the ZIF-90 bond capillary columns display a similar EOF-pH* profile. The EOF increased in the pH* range from 3.0 to 8.0 partially due to the increased ionization of the silanols of the columns. In addition, change in pH* from 3.0 to 8.0 also led to variation in the ionic strength of the running electrolyte solution, which should also contribute to the increase of EOF. Again, the ZIF-90 on the capillary column gave lower EOF than the bare capillary column due to the coverage of the ZIF-90 (Fig. 5B).
The content of organic modifier in the mobile phase not only influences the EOF but also the chromatographic resolution. Generally, the separation of charged analytes depends on their electrophoretic and electroosmotic mobilities and the partition interaction between the mobile phase and the stationary phase. However, the separation for neutral analytes only depends on the partition interaction between the analytes and the ZIF-90 coating on the inner walls of the capillary column. Naphthalene and anthracene were used as the tested analytes and MeOH was used as the organic modifier to investigate the effect of the organic modifier content on the separation on ZIF-90 bond capillary column. The resolution and retention of the analytes decreased with the increasing of the MeOH content (Fig. S1 in Supplementary data), which may be attributed to the increased solubility of the analytes in reduced polarity of mobile phase, the decreased viscosity, and the increased interaction between the analytes and the mobile phase. The elution of naphthalene and anthracene followed an increasing the number of the aromatic rings, indicating that the retention mechanism is consistent with the increasing of hydrophobic interaction between the analyte and the imidazolium ring in the ZIF-90 framework. 3.4. Loading capacity and reproducibility of ZIF-90 bond column The loading capacity of ZIF-90 bond column is defined as the amount of sample injected which makes the corresponding peak
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Fig. 6. Loading capacity test on ZIF-90 bond capillary column using naphthalene as the analyte and MeOH-PBS (10 mM, pH 7.4) (40:60, v/v) as the mobile phase. Separation conditions: separation voltage, 25 kV; detection wavelength, 214 nm.
width at half-height (W1/2 ) increase to 10% over the W1/2 at low sample amounts [25]. The loading capacity was studied by injecting naphthalene in the range from 0.05 to 0.8 g L−1 while the injected sample volume was fixed to about 15 nL. As shown in Fig. 6, the W1/2 for naphthalene at 0.7 g L−1 increased by 10% over the peak width at 0.1 g L−1 . Thus, the loading capacity of ZIF-90 bond column for naphthalene was 0.7 g L−1 . The higher loading capacity of ZIF-90 bond capillary column can be ascribed to the high surface area of ZIF-90 and compact ZIF-90 layer on the capillary wall. The electrochromatograms for the three replicate separations of aniline, acetanilide, o-nitroaniline and 1-naphthylamine and nonsteroidal anti-inflammatory drugs on the ZIF-90 bond column are shown in Fig. 7. The precisions (relative standard deviation, RSD) of retention time, W1/2 , and peak area for the analytes were 0.3–1.2%, 1.3–6.0% and 1.5–5.2%, respectively. Besides, the run-to-run, dayto-day, and column-to-column reproducibility (RSD) for the EOF was 0.2% (n = 5), 0.4% (n = 5), and 1.9% (n = 3), respectively. The recoveries of three nonsteroidal anti-inflammatory drugs spiked at 3 mg L−1 level in a local lake water sample ranged from 91% to 109% (Fig. S3 and Table S2 in Supplementary data). Moreover, the ZIF-90 bond column can be used for 230 runs without obvious change in separation efficiency. The long lifetime can be attributed to the existence of chemical bond between ZIF-90 and the inner capillary wall. 3.5. OT-CEC separation on ZIF-90 column The separation for xylene isomers is challenging due to their coherent boiling points and dimensions [9,37,38]. We first tested the separation of xylene isomers to evaluate the performance of ZIF-90 bond column. All three isomers in the mixture of xylene isomers were separated on the ZIF-90 bond column (Fig. 8A). However, no separation of the xylene isomers was achieved on the bare capillary column (Fig. 8B). The improvement of the separation efficiency was attributed to the porous structure of ZIF-90 and hydrophobic interactions between the analytes and the imidazolium rings in ZIF-90 framework. We further demonstrated the applicability of ZIF-90 bond capillary column for the separation of dichlorobenzene and chlorotoluene isomers. The ZIF-90 capillary column also gave excellent separation of dichlorobenzene and chlorotoluene isomers (Fig. 8A) but the bare capillary column gave no separation of these iosomers (Fig. 8B). Furthermore, the ZIF-90 bond column gave preferable retention for the ortho-isomers, allowing selective separation
Fig. 7. Electrochromatograms for three replicate separations of the tested compounds on ZIF-90 bond column: (A) nonsteroidal anti-inflammatory drugs in MeOH-PBS (10 mM, pH 4.5) (40:60, v/v); (B) aniline, acetanilide, o-nitroaniline and 1-naphthylamine in MeOH-PBS (10 mM, pH 7.8) (55:45, v/v). Separation condition: applied voltage, 15 kV; detection wavelength, 214 nm for (A) and 235 nm for (B).
of ortho-isomers from the other substituted position isomers. Besides, the ZIF-90 bond columns offered higher column efficiency and comparable precisions for the separation of the tested isomers compared with other methods with different columns [13,39,40] (Table 1 and Table S1 in Supplementary data). The effect of the MeOH content in the mobile phase on the retention factor (k) decreased as the MeOH content increased (Fig. S2 in Supplementary data), showing a reversed phase retention mechanism.
Table 1 Precision for five replicate separations and column efficiency (N) for the three groups of isomers on ZIF-90 bond column. Analyte
p-Xylene m-Xylene o-Xylene m-Dichlorobenzene p-Dichlorobenzene o-Dichlorobenzene m-Chlorotoluene p-Chlorotoluene o-Chlorotoluene
RSD (%) (n = 5)
N (plates/m)
Retention time
Peak area
0.3 0.3 0.3 0.2 0.3 0.2 0.5 0.6 0.5
0.6 0.5 1.2 0.5 0.6 0.7 1.4 1.6 1.6
65,456 39,845 21,006 49,251 39,841 19,687 25,639 20,065 15,649
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Fig. 8. Electrochromatograms for the separation of the tested isomers: (A) ZIF-90 bond capillary column; (B) bare capillary column. Separation conditions for both capillary columns: mobile phase, MeOH-PBS (10 mM, pH 7.4) (40:60, v/v); separation voltage, 15 kV; detection wavelength, 254 nm.
4. Conclusions In summary, we have reported a new method for fabricating ZIF-90 bonded capillary column at room temperature and the application of ZIF-90 as novel stationary phase for OT-CEC. The ZIF-90 bonded column gives excellent performance for the separation of the studied isomers, neutral and basic compounds and nonsteroidal anti-inflammatory drugs, being promising for OT-CEC application. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants 21077057, 20935001) and the Fundamental Research Funds for the Central Universities (Grant 21305071). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.04.003. References [1] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276.
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