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Chiral metal–organic framework used as stationary phases for capillary electrochromatography
Accepted Manuscript Title: Chiral metal–organic framework used as stationary phases for capillary electrochromatography Author: Zhi-Xin Fei Mei Zhang Jun-Hui Zhang Li-Ming Yuan PII: DOI: Reference:
S0003-2670(14)00508-X http://dx.doi.org/doi:10.1016/j.aca.2014.04.054 ACA 233233
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
11-12-2013 17-4-2014 28-4-2014
Please cite this article as: Zhi-Xin Fei, Mei Zhang, Jun-Hui Zhang, Li-Ming Yuan, Chiral metalndashorganic framework used as stationary phases for capillary electrochromatography, Analytica Chimica Acta http://dx.doi.org/10.1016/j.aca.2014.04.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chiral metal–organic framework used as stationary phases for capillary electrochromatography
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Zhi-Xin Fei a, Mei Zhang b, Jun-Hui Zhang b, and Li-Ming Yuana,b,*
Department of Chemistry, Yunnan Normal University, Kunming 650500, P.R. China
b
Department of Chemistry, East China Normal University, Shanghai 200241, P.R. China
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a
Corresponding author: Li-Ming Yuan: E-mail: [email protected]; Tel: 86-871-65941088;
an
Fax: 86-871-65941088
Highlights
This is a report, for the first time, that the enantioseparations are carried out
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by MOFs in CEC.
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M
Graphical abstract
A novel metal organic framework coated chiral OT capillary column was developed.
Excellent enantioseparation for flavanone and praziquantel were achieved.
The isomers of nitrophenols and ionones were also separated.
Abstract
Metal–organic frameworks (MOFs) have received great attention as novel media in separation sciences because of their fascinating structures and unusual properties. 1
Page 1 of 30
However, to the best of our knowledge, there has been no attempt to utilize chiral MOFs as stationary phases in capillary electrochromatography (CEC). In this study, a homochiral helical MOF [Zn2(D-Cam)2(4,4′-bpy)]n (D-Cam = D-(+)-Camphoric Acid,
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4,4'-bpy = 4,4'-bipyridine) was explored as the chiral stationary phase in open tubular capillary electrochromatography (OT-CEC) for separation of chiral compounds and
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isomers. The MOFs coated column has been developed using a simple procedure via
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MOFs post-coated on the sodium silicate layer. The baseline separations of flavanone and praziquantel were achieved on the MOFs coated column with high resolution of
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more than 2.10. The influences of pH, organic modifier content and buffer
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concentration on separation were investigated. Besides, the separations of isomers (nitrophenols and ionones) were evaluated. The relative standard deviations (RSDs)
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for the retention time of run-to-run, day-to-day and column-to-column were 1.04%,
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2.16% and 3.07%, respectively. The results demonstrated that chiral MOFs are promising for enantioseparation in CEC. Key words: Metal-organic framework, Stationary phase, Open tubular capillary electrochromatography, Chiral separation
1. Introduction Metal–organic frameworks (MOFs) are a class of novel crystalline materials consisting of clusters or chains of metal ions and organic linkers [1–3], which have been shown great promise for applications in gas storage [4], separation [5], catalysis 2
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[6], luminescent materials [7], drug delivery [8] as well as chromatography [9] due to their unusual properties, such as high surface area, good thermal stability, tunable pore sizes. In particular, MOFs as novel stationary phases for chromatographic
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separation have attracted much attention [10]. Recently, many MOFs have been explored as stationary phases for high performance liquid chromatography (HPLC)
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[11–15] and gas chromatography (GC) [16–21]. Besides, there are four reports about
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MOFs used in CEC for isomers or several groups of aromatic analytes mixture separation [22-25]. With the increasing demand for enantioselective separation and
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chiral MOF crystal materials, a large number of chiral MOFs have been synthesized
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[26-28]. In chiral separation field, the applications of chiral MOFs have been focused on the HPLC [29-32] and GC [33-35]. To the best of our knowledge, however, no
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has been reported so far.
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work on the utilization of chiral MOFs as stationary phases for CEC enantioseparation
CEC is a rapidly emerging analytical technique, which combines the high
separation efficiency of capillary electrophoresis (CE) with the selectivity offered by LC. Three types of columns including open-tubular (OT) capillary column, conventional particulate packed capillary column and monolithic capillary column have been employed in CEC. Among the various formats used in CEC, OT-CEC columns are promising for the enantioseparation owing to simple column preparation, stable EOF application, and no bubble formation, etc. Therefore, with the OT columns, some chiral compounds have been successfully separated [36-40]. However, the OT-CEC also suffers from some problems of low phase ratio (the ratio of stationary 3
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phase to mobile phase) and low sample capacity. To overcome the above problems, a series of strategies, such as sol–gel coating, inner-wall etching and depositing nanoparticle phases are applied in the process of preparing OT columns to increase
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the surface area available for CEC separations [41-43]. Therefore, chiral MOFs with versatility and high pore volume can be used in CEC to solve the above problems, and
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obtained enantioseparation at the same time.
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A homochiral helical MOF [Zn2(D-Cam)2(4,4′-bpy)]n consisting of homochiral layers pillared by bipyridine ligands to form two types of unusual three-dimensional
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six-connected self-penetrating architectures was reported [44]. Its unique structures,
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high surface area, good solvent stability, make [Zn2(D-Cam)2(4,4′-bpy)]n an attractive candidate for the enantioseparation of chiral compounds in OT-CEC.
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Here, we first report the utilization of [Zn2(D-Cam)2(4,4′-bpy)]n as the chiral
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stationary phase (CSP) for OT-CEC separation of chiral drugs and positional isomers. The MOF was coated to a capillary which had been pretreated by sodium silicate solution to granulate the inner wall and obtain a stable stationary phase particles layer. The baseline separation of two racemates on the [Zn2 (D-Cam)2(4,4′-bpy)]n-coated open capillary column with high resolution and good column efficiency were achieved. The isomers of nitrophenols and ionones were also separated on the MOF coated capillary column. The results indicate that the enantioseparation on the chiral MOF-coated column is practical in CEC.
2. Experimental
4
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2.1 Chemicals and materials All chemicals and reagents used were at least of analytical grade without further treatment. Fused-silica capillaries (375 μm o.d. × 50 μm i.d.) were from Yongnian
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Optic Fiber Plant (Handan, China). Water was deionized. 4,4'-bipyridine(≥98 %) and D-(+)-camphoric acid (≥99 %) (Adamas-beta, Shanghai, China) were used to prepare
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[Zn2 (D-Cam)2(4,4′-bpy)]n. Flavanone (Acros Organics, Geel, Belgium), praziquantel
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(Sigma-Aldrich, St. Louis, MO, USA), m-nitrophenol, p-nitrophenol, o-nitrophenol and α-ionone, β-ionone (Aladdin, Shanghai, China) were used as the analytes. Sodium
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carbonate(Na2CO3), thiourea, sodium dihydrogen phosphate (NaH2PO4), disodium
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hydrogen phosphate (Na2HPO4), acetonitrile (ACN) and sodium silicate (Na2SiO3)
(Tianjin, China).
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2.2 Apparatus
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were obtained from Tianjin Fengchuan Chemical Reagent Technology Company
All the experiments were performed on a HPCE system (CL1020, Beijing Cailu Instrumental Co., Beijing, China) equipped with a UV detector (190-700 nm). Data acquisition and processing were controlled by HW-2000 chromatography workstation (Qianpu Software, Shanghai, China). The scanning electron microscopy (SEM) images were performed on S-3000N (Hitachi Science Systems, Japan). The X-ray diffraction (XRD) patterns were obtained from DX-2700 (Dandong Fangyuan Instrument, China). The thermogravimetric analysis (TGA) experiment was performed on a ZRY-1P Simultaneous Thermal Analyzer (Shanghai, China) from room temperature to 700 ºC at a ramp rate of 10 ºC min-1. 5
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2.3 Synthesis of [Zn2(D-Cam)2(4,4′-bpy)]n Crystals were synthesized according to a procedure by Jian Zhang et al [44], Typically, Zn(NO3)2·6H2O, Na2CO3, 4,4'-bipyridine, and D-(+)-camphoric acid in a
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molar ratio of 1.5:1:1:1 were mixed with ultrapure water in a Teflon-lined bomb. The Teflon-lined bomb was then sealed and placed in an oven at 120 ºC for 2 days. After
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being cooled to room temperature, the regular transparent colorless crystals were
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washed thoroughly with ethanol, and collected by centrifugation at 6000 rpm for 10 min. The procedure was repeated three times. Samples for XRD and TGA analysis
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were dried at 120 ºC in air.
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The synthesized [Zn2(D-Cam)2(4,4′-bpy)]n was dispersed in phosphate buffer solution (35 mM, pH 6.5) at room temperature for 24 h, then the suspensions were
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centrifuged at 6000 rpm for 10 min, and the solid was collected and washed with
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ultrapure water for three times and dried at 120 ºC for XRD characterization. 2.4 Preparation of OT-CEC capillary column Firstly, a 60 cm untreated fused-silica capillary was sequentially rinsed with 1M NaOH for 3 h, ultrapure water for 1 h, 0.1 M HCl for 1 h, ultrapure water for 1 h, and then purged with nitrogen for 4 h at 120 ºC. Secondly, the saturated sodium silicate solution was passed through the capillary for 5 min and the capillary was allowed to stand for another 60 min. Then, a thin layer of the sodium silicate solution was completed after rinsed with 0.01M HCl to remove the excess sodium silicate solution. Thirdly, the coating of MOF was referred to the dynamic coating method in the literature [16]. Namely, 2 mL ethanol suspension of the [Zn2 (D-Cam)2(4,4′-bpy)]n (2 6
Page 6 of 30
mg mL-1) was introduced into the capillary under gas pressure, and then pushed through the column at a rate of 50 cm min-1 to leave a MOF coating layer on the sodium silicate layer. Finally, the coated capillary column was flushed for 20 min
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with nitrogen and then conditioned from 30 to 280 ºC at a rate of 1 ºC min-1, and finally kept at 280 ºC for 8 h.
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2.5. CEC procedures
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The mobile phases were prepared by mixing phosphate buffer with acetonitrile (ACN) in different volume ratios, and the phosphate buffer being adjusted to the
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desired pH with disodium hydrogen phosphate. All of the solutions were passed
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through a 0.45 μm membrane filter and subsequently degassed in an ultrasonic bath for 10 min prior to their use. Thiourea was used as a marker for electroosmotic flow
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(EOF). The obtained MOF coated capillary column was conditioned with the mobile
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phases until a stable current was achieved prior to the CEC separation processes. All open-tubular capillary columns were rinsed with ultrapure water for 2 min and mobile phases for 2 min between consecutive runs. Detection was carried out on-column at 254 nm (ionone at 230 nm).
The number of theoretical plates (N/m) and the resolution (RS) were calculated by
the following equation:
5.54 t R N m L W1/2
RS 1.18
2
(1)
t2 t1 W1/2(1) W1/2(2)
(2)
where tR is the retention time and W1/2 is the peak width at half height，L is the 7
Page 7 of 30
total length of column in meters, t1 and t2 are the retention times and W1/2(1) and W1/2(2) are the half height peak widths of the first and second eluted enantiomers, respectively.
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3.1 Characterization of Synthesized MOF [Zn2(D-Cam)2(4,4′-bpy)]n
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3. Results and discussion
The XRD patterns of the synthesized [Zn2(D-Cam)2(4,4′-bpy)]n was in good
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agreement with the corresponding simulated one, indicating the successful preparation
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of [Zn2(D-Cam)2(4,4′-bpy)]n (Fig. 1A-b,c). [Zn2(D-Cam)2(4,4′-bpy)]n displayed good stability in the examined phosphate buffer solution as all the XRD peaks belonging to
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simulated one after treatment (Fig 1A-a,c). Thermal gravimetric analysis (TGA) was
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also employed to prove the stability of [Zn2 (D-Cam)2(4,4′-bpy)]n (Fig. 1B ).
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Fig. 1 in here
3.2 Characterization of MOF-coated OT capillary and measurements of the EOF In this work, the synthesized chiral MOF was introduced into fused silica capillary which have been coated a sodium silicate solution layer. In order to form a stable MOF particles layer, the MOF-coated capillary was heated at 280 ºC to dehydrate between silanol groups of capillary and the sodium silicate. Fig. 2 shows the SEM photographs of bare capillary column (A, B); sodium silicate coated capillary column(C, D) and MOF coated capillary column (E, F). Clearly, the inner wall of the bare capillary column was very smooth as in Figure 2A and 2B. As shown in Fig. 2C and 2D, there is no significant change observed on the surface after the coating of sodium silicate capillary column. Whereas, the MOF coated capillary column 8
Page 8 of 30
displayed rough inner surfaces with ca. 1μm thick layer of MOF particles (Fig. 2E and 2F). Fig. 2 in here
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In CEC, EOF acts as the driving force for the separation. The effective measurement of the EOF in CEC plays a major role in the separation of enantiomers.
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In the work reported here, thiourea was used as an EOF marker to investigate the
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effect of the pH on the EOF of different capillary columns (bare capillary column, sodium silicate-coated capillary column and MOF-coated capillary column) in the
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range of 3.5 to 7.5 (Fig. 3). It is obvious that the EOF of all the three studied columns
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were dependent on the pH of the buffer and the coated capillaries showed different EOF behavior compared to bare capillary. These results showed that the silanol
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groups which produce EOF of coated column were different from the uncoated
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column. For the bare fused-silica capillary, the EOF values increased significantly when the pH values increased, which because of the inner walls of the capillaries were negatively charged and numerous silanol groups on the surface of the capillary would dissociate in this pH range. For the coated capillary, the maximum EOF values was obtained by capillary of coated with sodium silicate, while the coating of the capillary with MOF on the sodium silicate layer was found to have been reduced dramatically, but the tendency of change was basically consistent with the former, demonstrating the successful coating of MOF on the inner surface of capillary. As the OT capillary column prepared in this work was first immobilized, a layer of sodium silicate that likely possesses highly abundant silanol groups on the surface, and then 9
Page 9 of 30
the neutral MOF was post-coated on the sodium silicate layer to form the CSP which led to a reduction in the overall negative charge and a reduction in the EOF. These results definitely indicated that the major source to generate EOF from the sodium
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silicate layer, and the MOF provided negatively charged groups rarely on the prepared chiral OT capillary column.
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Fig. 3 in here
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The dependence of the applied voltage and the produced current were also investigated on MOF modified OT capillary column in Fig. 4. The produced current
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linearly increased from 27 to 75 μA with a correlation coefficient (r) of 0.9993 by
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increasing the applied voltage from 8 to 18 kV. This confirmed that the Joule heating associated problems seemed not to be a cause for concern.
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3.3 Separation of isomers
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Fig. 4 in here
To evaluate the performance of the MOF-coated capillary column, the separation
of nitrophenols were firstly tested. In the presence of chial MOF, nitrophenols were baseline separated in 15 min (Fig. 5A-b), especially for o-nitrophenol and p-nitrophenol (RS=7.63). Peak broadening was inconspicuous with the column efficiencies were above 184000 plates m-1. To show whether the MOF played an important role in the separation, we tested bare capillary column under the same conditions (Fig. 5A-a). However, the peaks for the separation of o-nitrophenol and p-nitrophenol showed a significant overlap on bare column. The low resolution can be explained by the loss of interaction between the MOF stationary and the phenolic 10
Page 10 of 30
hydroxyl groups the solute [24]. The migration time for all of the analytes was increased with MOF-coated OT capillary columns. It was ascribed to the reduction of the EOF in MOF-coated OT capillary columns. What’s more, the strong interaction
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between the analytes and the inner surface of the MOF-coated OT capillary columns was another important factor. To gain further insight into the separation of positional
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isomers on chiral MOF-coated OT capillary column, ionones were used as test
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analytes. As can be seen from Fig. 5B, partly separation of α, β-ionone was achieved on MOF-coated capillary column with the resolution of 0.69. The above results
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demonstrated that the MOF exhibited good selectivity for isomers.
3.4 Separation of chiral compounds
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Fig. 5 in here
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The most important advantage of this stationary phase in CEC is its enantioselective and resolving abilities. The performance of the chiral MOF-coated OT capillary column was demonstrated for the enantioseparation of two pair racemates flavanone and praziquantel. As shown in Table 1, the MOF-coated OT capillary column with 35 mM phosphate buffer / ACN (90:10) as the mobile phase and applied voltages 12 kV gave high-resolution separation of flavanone and praziquantel only within 16 min at room temperature. Typically, the high resolution (RS=3.99) and the column efficiency of 58000 plates m-1 for flavanone were obtained. The performance of praziquantel resolution (RS=2.10) higher than the stationary phases of Chiralcel OD, OJ, and Chiralpak AD, AS in CEC [45, 46]. However, usually separation of these chiral compounds in CEC used silica stationary phase 11
Page 11 of 30
modified by chemically bonding or coated cyclodextrin and other chiral selector was needed to achieve good separation [47-49]. Table 1 in here
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It is known that the concentration of organic modifier has a great influence on the resolution. In CEC, ACN is usually used to adjust the polarity of the mobile phase and
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to tune the interaction between the mobile phase and the analyte. In this work, ACN
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were selected as the organic modifiers to study the influence on separation for two pair racemates. It is worth noting that the analytes were not eluted with pure
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phosphate buffer as the mobile phase even in 50 min (Fig. 6) which indicated the
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presence of strong interaction between the analytes and the open metal sites of MOF. As the content of ACN in the mobile phase increased, all analytes were eluted within
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16 min and baseline separation of the two racemates was achieved on the
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MOF-coated capillary column with 10% (v/v) ACN in the mobile phase. When the content of ACN in the mobile phase increased, accompany with remarkable reduction in the resolution of the two enantiomers were observed (Fig. 6, 10%-30% ACN). The above mobile phase influence of enantioseparation most probably as a consequence of a distinct competition from solvent adsorption [32]. As the added of the content of ACN in the mobile phase of phosphate buffer / ACN enhanced the competition of ACN for the adsorption on the open metal sites, and thus weakened the interaction between the open metal sites and the analytes. Therefore, a suitable amount of ACN in the mobile phase could obtain better separation. So, the optimum concentration of ACN in the mobile phase for chiral separations was determined to be 10% (v/v). 12
Page 12 of 30
Fig. 6 in here The concentration of the buffer is another key factor which can also significantly affect the enantioseparation of analytes. Hence, the influence of buffer concentration
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on enantioseparations of selected analytes was studied by gradually increasing phosphate buffer from 25 mM to 40 mM. As can be seen from Fig. 7, the migration
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times of the two enantiomers gradually increases with augmenting buffer
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concentration, probably due to decrease of the EOF with increasing ionic strength of the buffer. Moreover, the resolution of the two enantiomers initially increased as the
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phosphate buffer concentration increased up to 35 mM. However, further increased in
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the phosphate buffer concentration did not lead to increase in the separation efficiency for both racemates. As a consequence, the optimum concentration of phosphate buffer
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concentration in the mobile phase for chiral separation was found to 35 mM.
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Fig. 7 in here
The effect of change pH on the separation was displayed in Fig. 8. As expected,
the migration times of the two enantiomers displayed an increasing tendency with the increment of pH value from 4.5 to 7.5. Moreover, the greatest of resolution were achieved by increasing the pH to 6.5. The pH value of phosphate buffer had effects not only on migration time but also on separation. Thus, pH=6.5 was selected for all of the subsequent chiral separation experiments. Fig. 8 in here As the influence of the chiral microenvironment on the chiral properties of chromatographic systems is complicated [33], it is difficult for the chiral recognition 13
Page 13 of 30
mechanism of enantioseparation on MOF-coated OT columns to be understood completely. MOF-coated column has excellent chiral recognition ability toward racemates due to the influence of the chiral microenviroment of [Zn2
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(D-Cam)2(4,4′-bpy)]n. The crystal structure of MOF [Zn2 (D-Cam)2(4,4′-bpy)]n was provided in Fig. 9. The left- and right-handed helices formed by alternating D-Cam
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ligands and metal sites are joined together through the common dinuclear metal
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clusters exist in the homochiral layer (the interlayer distance is 7.756 Å). Although the ratio between the left- and right-handed helices is 1:1, the resulting 3-D framework is
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homochiral because of the presence of the enantiopure building block [44]. Therefore,
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chiral recognition mostly depends on the interaction of the analytes with the 3-D framework enantiopure building block and the helical homochiral layer surface of
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[Zn2 (D-Cam)2(4,4′-bpy)]n in which the best steric fit is the main interactive force [50].
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Besides, a weak hydrogen bonding between the organic ligands of [Zn2 (D-Cam)2(4,4′-bpy)]n and the racemates also likely enhances the interaction between the chiral stationary and the analytes. The π–π stacking interaction between the pyridine rings on the outer crystal surface of [Zn2 (D-Cam)2(4,4′-bpy)]n and the benzene
ring
of
the
analytes
should
also
contribute
a
lot
to
the
hydrophobic–hydrophobic interaction between [Zn2 (D-Cam)2(4,4′-bpy)]n and the analytes. Fig. 9 in here 3.5 Repeatability and stability of MOF-coated OT capillary column The repeatability and stability properties of the MOF-coated capillary column 14
Page 14 of 30
were evaluated using flavanone with 35 mM phosphate buffer with 10% (v/v) acetonitrile as mobile phase (pH=6.5). The endurance of the coating was found to be more than 10 days and the relative standard deviations (RSDs) for the retention time
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of analyte were less than 2.16% (interday, n=5), which indicated the good stability of the OT column. The run-to-run RSD (n=5) and the column-to-column RSD (n=5)
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were 1.04% and 3.07%, respectively, which confirmed the good reproducibility of the
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column preparation.
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4. Conclusion
In summary, we have reported the first example of using chiral MOFs as the
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stationary phase fabricated chiral MOF-coated OT capillary column for CEC
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separation of chiral compounds and positional isomers. The MOFs coated capillary
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column gave baseline separation of flavanone and praziquantel with high column efficiency, various chromatographic parameters such as pH, organic modifier composition and buffer concentration were carefully optimized to get the best separation. In addition, the isomers of nitrophenols and ionones were also separated on the MOF coated capillary column. The results show that chiral MOFs were used in OT-CEC is very attractive for chiral compounds and positional isomers. We believe that these findings will open a new avenue for broad applications of MOFs in chiral chromatography separations.
Acknowledgements 15
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The work is supported by the National Natural Science Foundation (No. 21275126, No. 21127012) of China.
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Chem. Soc. 126 (2004) 6106-6114.
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[34] S.M. Xie, X.H. Zhang, Z.J. Zhang, M. Zhang, J. Jia, L.M. Yuan, Anal. Bioanal. Chem. 405 (2013) 3407-3412. [35] S.M. Xie, X.H. Zhang, Z.J. Zhang, L.M. Yuan. Anal. Lett. 46 (2013) 753-763.
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[36] T. Wakita, B. Chankvetadze, C. Yamamoto, Y. Okamoto, J. Sep. Sci. 25 (2002) 167-169. [37] S. Mayer, V. Schurig, J. High. Res. Chromatogr. 15 (1992) 129-131.
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[38] Y.C. Wang, Z.R. Zeng, N. Guan, J.K. Cheng, Electrophoresis 22 (2001) 2167-2172.
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[39] Z. Liu, K. Otsuka, S. Terabe, J. Sep. Sci. 24 (2001) 17-26.
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[40] Y. Yang, J.W. Wu, X.L. Gou, P. Su, Anal. Lett. 5 (2013) 5753-5759. [41] P. Narang, L. A. Colon, J. Chromatogr. A 773 (1997) 65-72.
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[42] M.T. Matyska, J.J. Pesek, A. Katrekar, Anal. Chem. 71(1999) 5508-5514.
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[43] X. Wang, C. Cheng, S. Wang, M. Zhao, P. K. Dasgupta, S. Liu, Anal. Chem. 81 (2009) 7428-7435.
d
[44] J. Zhang, Y.G. Yao, X.H. Bu. Chem. Mater. 19 (2007) 5083-5089.
Ac ce pt e
[45] D. Mangelings, M. Maftouh, D. L. Massart, Y.V. Heyden, Electrophoresis 25 (2004) 2808-2816.
[46] D. Mangelings, J. Discory, M. Maftouh, D. L. Massart, Y.V. Heyden, Electrophoresis 26 (2005) 3930-3941.
[47] S. Fanali, G. D'Orazio, T. Farkas, B. Chankvetadze, J. Chromatogr. A 1269 (2012) 136-142.
[48] S. Fanali, G. D'Orazio, K. Lomsadze, S. Samakashvili, B. Chankvetadze, J. Chromatogr. A 1217 (2010) 1166-1174. [49] X.H. Lai, S.C. Ng, J. Chromatogr. A 1059 (2004) 53-54. [50] T. Ikai, Y. Okamoto, Chem. Rev. 109 (2009) 6077-6101. 19
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Figure captions
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Fig. 1. (A) Comparison of the experimental and simulated XRD patterns of [Zn2(D-Cam)2(4,4′-bpy)]n: (a) treated in phosphate buffer solution for 24 h; (b)
an
curve of the prepared [Zn2(D-Cam)2(4,4′-bpy)]n.
us
simulated from single-crystal X-ray diffraction data, (c) as-synthesized. (B)TGA
Fig. 2. SEM micrographs of open-tubular capillary columns: (A), (B) bare capillary
d
capillary column;
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column; (C), (D) sodium silicate coated capillary column; (E), (F) MOF coated
Ac ce pt e
Fig. 3. Evaluation of pH on the apparent EOF in different capillaries. Separation conditions: mobile phase, 35 mM phosphate buffer of various pH values /ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10cm height for 2 s.
Fig. 4. Plot of current versus applied electric field strength. Conditions: buffer pH 6.5, other conditions are the same as in Fig.3.
Fig. 5. Chromatograms for separation of the isomers: (A) nitrophenol: (a) CZE electropherogram. (b) CEC electropherogram on the MOF coated capillary column. 1=thiourea; 2=m-nitrophenol; 3=o –nitrophenol; 4=p –nitrophenol. (B) ionone. Separation conditions: mobile phase, 35 mM phosphate buffer (pH 6.5)/ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, (A) 13 20
Page 20 of 30
kV, (B) 10 kV; detection wavelength, (A)254 nm, (B)230 nm; injection, 10 cm height for 2 s.
Fig. 6. Effect of the mobile phase composition on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer
ip t
containing different ACN content at pH 6.5. capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10
us
cr
cm height for 8 s.
Fig. 7. Effect of the buffer concentration on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, different concentration of
an
phosphate buffer containing 10% ACN; Other conditions are the same as in Fig.6.
M
Fig. 8. Effect of the buffer pH on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer containing 10% ACN;
Ac ce pt e
d
Other conditions are the same as in Fig.6.
Fig. 9. The schematic of crystal structure of [Zn2 (D-Cam)2(4,4′-bpy)]n. (A) Homochiral 2-D layer; (B) The orderly arrangement of left- and right-handed helices.
Table 1. Separations of Racemates on MOF-coated OT Capillary Column.
Racemates
tR1/min
tR2/min
N1/m
N2/m
RS
flavanone
13.869
15.610
58000
26000
3.99
praziquantel
12.338
13.113
52000
36000
2.10
21
Page 21 of 30
ip t cr us an M
d
Fig. 1. (A) Comparison of the experimental and simulated XRD patterns of
Ac ce pt e
[Zn2(D-Cam)2(4,4′-bpy)]n: (a) treated in phosphate buffer solution for 24 h; (b) simulated from single-crystal X-ray diffraction data, (c) as-synthesized. (B) TGA curve of the prepared [Zn2(D-Cam)2(4,4′-bpy)]n.
22
Page 22 of 30
ip t cr us an M d Ac ce pt e
Fig. 2. SEM micrographs of open-tubular capillary columns: (A), (B) bare capillary column; (C), (D) sodium silicate coated capillary column; (E), (F) MOF coated capillary column;
23
Page 23 of 30
ip t cr us an M d
Fig. 3. Evaluation of pH on the apparent EOF in different capillaries. Separation
Ac ce pt e
conditions: mobile phase, 35 mM phosphate buffer of various pH values /ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10 cm height for 2 s.
24
Page 24 of 30
ip t cr us an M d
Fig. 4. Plot of current versus applied electric field strength. Conditions: buffer pH 6.5,
Ac ce pt e
other conditions are the same as in Fig.3.
25
Page 25 of 30
ip t cr us an
Fig. 5. Chromatograms for separation of the isomers: (A) nitrophenol: (a) CZE
M
electropherogram. (b)CEC electropherogram on the MOF coated capillary column. 1= thiourea; 2=m-nitrophenol; 3=o –nitrophenol; 4=p –nitrophenol. (B) ionone. Separation conditions: mobile phase, 35 mM phosphate buffer (pH 6.5)/ACN = 90/10
d
(v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, (A)13
Ac ce pt e
kV, (B) 10 kV; detection wavelength, (A)254 nm, (B)230 nm; injection, 10 cm height for 2 s.
26
Page 26 of 30
ip t cr us
Fig. 6. Effect of the mobile phase composition on separation of the flavanone (A) and
an
praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer containing different ACN content at pH 6.5. capillary, 50 cm x 50 mm i.d. (effective
Ac ce pt e
d
cm height for 8 s.
M
length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10
27
Page 27 of 30
ip t cr
Fig. 7. Effect of the buffer concentration on separation of the flavanone (A) and
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praziquantel (B). Separation conditions: mobile phase, different concentration of
Ac ce pt e
d
M
an
phosphate buffer containing 10% ACN; Other conditions are the same as in Fig. 6.
28
Page 28 of 30
ip t cr
Fig. 8. Effect of the buffer pH on separation of the flavanone (A) and praziquantel (B).
us
Separation conditions: mobile phase, 35 mM phosphate buffer containing 10% ACN;
Ac ce pt e
d
M
an
Other conditions are the same as in Fig. 6.
29
Page 29 of 30
ip t
Fig. 9. The schematic of crystal structure of [Zn2(D-Cam)2(4,4′-bpy)]n. (A)
Ac ce pt e
d
M
an
us
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Homochiral 2-D layer; (B) The orderly arrangement of left- and right-handed helices.
30
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S0003-2670(14)00508-X http://dx.doi.org/doi:10.1016/j.aca.2014.04.054 ACA 233233
To appear in:
Analytica Chimica Acta
Received date: Revised date: Accepted date:
11-12-2013 17-4-2014 28-4-2014
Please cite this article as: Zhi-Xin Fei, Mei Zhang, Jun-Hui Zhang, Li-Ming Yuan, Chiral metalndashorganic framework used as stationary phases for capillary electrochromatography, Analytica Chimica Acta http://dx.doi.org/10.1016/j.aca.2014.04.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chiral metal–organic framework used as stationary phases for capillary electrochromatography
ip t
Zhi-Xin Fei a, Mei Zhang b, Jun-Hui Zhang b, and Li-Ming Yuana,b,*
Department of Chemistry, Yunnan Normal University, Kunming 650500, P.R. China
b
Department of Chemistry, East China Normal University, Shanghai 200241, P.R. China
us
cr
a
Corresponding author: Li-Ming Yuan: E-mail: [email protected]; Tel: 86-871-65941088;
an
Fax: 86-871-65941088
Highlights
This is a report, for the first time, that the enantioseparations are carried out
Ac ce pt e
by MOFs in CEC.
d
M
Graphical abstract
A novel metal organic framework coated chiral OT capillary column was developed.
Excellent enantioseparation for flavanone and praziquantel were achieved.
The isomers of nitrophenols and ionones were also separated.
Abstract
Metal–organic frameworks (MOFs) have received great attention as novel media in separation sciences because of their fascinating structures and unusual properties. 1
Page 1 of 30
However, to the best of our knowledge, there has been no attempt to utilize chiral MOFs as stationary phases in capillary electrochromatography (CEC). In this study, a homochiral helical MOF [Zn2(D-Cam)2(4,4′-bpy)]n (D-Cam = D-(+)-Camphoric Acid,
ip t
4,4'-bpy = 4,4'-bipyridine) was explored as the chiral stationary phase in open tubular capillary electrochromatography (OT-CEC) for separation of chiral compounds and
cr
isomers. The MOFs coated column has been developed using a simple procedure via
us
MOFs post-coated on the sodium silicate layer. The baseline separations of flavanone and praziquantel were achieved on the MOFs coated column with high resolution of
an
more than 2.10. The influences of pH, organic modifier content and buffer
M
concentration on separation were investigated. Besides, the separations of isomers (nitrophenols and ionones) were evaluated. The relative standard deviations (RSDs)
d
for the retention time of run-to-run, day-to-day and column-to-column were 1.04%,
Ac ce pt e
2.16% and 3.07%, respectively. The results demonstrated that chiral MOFs are promising for enantioseparation in CEC. Key words: Metal-organic framework, Stationary phase, Open tubular capillary electrochromatography, Chiral separation
1. Introduction Metal–organic frameworks (MOFs) are a class of novel crystalline materials consisting of clusters or chains of metal ions and organic linkers [1–3], which have been shown great promise for applications in gas storage [4], separation [5], catalysis 2
Page 2 of 30
[6], luminescent materials [7], drug delivery [8] as well as chromatography [9] due to their unusual properties, such as high surface area, good thermal stability, tunable pore sizes. In particular, MOFs as novel stationary phases for chromatographic
ip t
separation have attracted much attention [10]. Recently, many MOFs have been explored as stationary phases for high performance liquid chromatography (HPLC)
cr
[11–15] and gas chromatography (GC) [16–21]. Besides, there are four reports about
us
MOFs used in CEC for isomers or several groups of aromatic analytes mixture separation [22-25]. With the increasing demand for enantioselective separation and
an
chiral MOF crystal materials, a large number of chiral MOFs have been synthesized
M
[26-28]. In chiral separation field, the applications of chiral MOFs have been focused on the HPLC [29-32] and GC [33-35]. To the best of our knowledge, however, no
Ac ce pt e
has been reported so far.
d
work on the utilization of chiral MOFs as stationary phases for CEC enantioseparation
CEC is a rapidly emerging analytical technique, which combines the high
separation efficiency of capillary electrophoresis (CE) with the selectivity offered by LC. Three types of columns including open-tubular (OT) capillary column, conventional particulate packed capillary column and monolithic capillary column have been employed in CEC. Among the various formats used in CEC, OT-CEC columns are promising for the enantioseparation owing to simple column preparation, stable EOF application, and no bubble formation, etc. Therefore, with the OT columns, some chiral compounds have been successfully separated [36-40]. However, the OT-CEC also suffers from some problems of low phase ratio (the ratio of stationary 3
Page 3 of 30
phase to mobile phase) and low sample capacity. To overcome the above problems, a series of strategies, such as sol–gel coating, inner-wall etching and depositing nanoparticle phases are applied in the process of preparing OT columns to increase
ip t
the surface area available for CEC separations [41-43]. Therefore, chiral MOFs with versatility and high pore volume can be used in CEC to solve the above problems, and
cr
obtained enantioseparation at the same time.
us
A homochiral helical MOF [Zn2(D-Cam)2(4,4′-bpy)]n consisting of homochiral layers pillared by bipyridine ligands to form two types of unusual three-dimensional
an
six-connected self-penetrating architectures was reported [44]. Its unique structures,
M
high surface area, good solvent stability, make [Zn2(D-Cam)2(4,4′-bpy)]n an attractive candidate for the enantioseparation of chiral compounds in OT-CEC.
d
Here, we first report the utilization of [Zn2(D-Cam)2(4,4′-bpy)]n as the chiral
Ac ce pt e
stationary phase (CSP) for OT-CEC separation of chiral drugs and positional isomers. The MOF was coated to a capillary which had been pretreated by sodium silicate solution to granulate the inner wall and obtain a stable stationary phase particles layer. The baseline separation of two racemates on the [Zn2 (D-Cam)2(4,4′-bpy)]n-coated open capillary column with high resolution and good column efficiency were achieved. The isomers of nitrophenols and ionones were also separated on the MOF coated capillary column. The results indicate that the enantioseparation on the chiral MOF-coated column is practical in CEC.
2. Experimental
4
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2.1 Chemicals and materials All chemicals and reagents used were at least of analytical grade without further treatment. Fused-silica capillaries (375 μm o.d. × 50 μm i.d.) were from Yongnian
ip t
Optic Fiber Plant (Handan, China). Water was deionized. 4,4'-bipyridine(≥98 %) and D-(+)-camphoric acid (≥99 %) (Adamas-beta, Shanghai, China) were used to prepare
cr
[Zn2 (D-Cam)2(4,4′-bpy)]n. Flavanone (Acros Organics, Geel, Belgium), praziquantel
us
(Sigma-Aldrich, St. Louis, MO, USA), m-nitrophenol, p-nitrophenol, o-nitrophenol and α-ionone, β-ionone (Aladdin, Shanghai, China) were used as the analytes. Sodium
an
carbonate(Na2CO3), thiourea, sodium dihydrogen phosphate (NaH2PO4), disodium
M
hydrogen phosphate (Na2HPO4), acetonitrile (ACN) and sodium silicate (Na2SiO3)
(Tianjin, China).
Ac ce pt e
2.2 Apparatus
d
were obtained from Tianjin Fengchuan Chemical Reagent Technology Company
All the experiments were performed on a HPCE system (CL1020, Beijing Cailu Instrumental Co., Beijing, China) equipped with a UV detector (190-700 nm). Data acquisition and processing were controlled by HW-2000 chromatography workstation (Qianpu Software, Shanghai, China). The scanning electron microscopy (SEM) images were performed on S-3000N (Hitachi Science Systems, Japan). The X-ray diffraction (XRD) patterns were obtained from DX-2700 (Dandong Fangyuan Instrument, China). The thermogravimetric analysis (TGA) experiment was performed on a ZRY-1P Simultaneous Thermal Analyzer (Shanghai, China) from room temperature to 700 ºC at a ramp rate of 10 ºC min-1. 5
Page 5 of 30
2.3 Synthesis of [Zn2(D-Cam)2(4,4′-bpy)]n Crystals were synthesized according to a procedure by Jian Zhang et al [44], Typically, Zn(NO3)2·6H2O, Na2CO3, 4,4'-bipyridine, and D-(+)-camphoric acid in a
ip t
molar ratio of 1.5:1:1:1 were mixed with ultrapure water in a Teflon-lined bomb. The Teflon-lined bomb was then sealed and placed in an oven at 120 ºC for 2 days. After
cr
being cooled to room temperature, the regular transparent colorless crystals were
us
washed thoroughly with ethanol, and collected by centrifugation at 6000 rpm for 10 min. The procedure was repeated three times. Samples for XRD and TGA analysis
an
were dried at 120 ºC in air.
M
The synthesized [Zn2(D-Cam)2(4,4′-bpy)]n was dispersed in phosphate buffer solution (35 mM, pH 6.5) at room temperature for 24 h, then the suspensions were
d
centrifuged at 6000 rpm for 10 min, and the solid was collected and washed with
Ac ce pt e
ultrapure water for three times and dried at 120 ºC for XRD characterization. 2.4 Preparation of OT-CEC capillary column Firstly, a 60 cm untreated fused-silica capillary was sequentially rinsed with 1M NaOH for 3 h, ultrapure water for 1 h, 0.1 M HCl for 1 h, ultrapure water for 1 h, and then purged with nitrogen for 4 h at 120 ºC. Secondly, the saturated sodium silicate solution was passed through the capillary for 5 min and the capillary was allowed to stand for another 60 min. Then, a thin layer of the sodium silicate solution was completed after rinsed with 0.01M HCl to remove the excess sodium silicate solution. Thirdly, the coating of MOF was referred to the dynamic coating method in the literature [16]. Namely, 2 mL ethanol suspension of the [Zn2 (D-Cam)2(4,4′-bpy)]n (2 6
Page 6 of 30
mg mL-1) was introduced into the capillary under gas pressure, and then pushed through the column at a rate of 50 cm min-1 to leave a MOF coating layer on the sodium silicate layer. Finally, the coated capillary column was flushed for 20 min
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with nitrogen and then conditioned from 30 to 280 ºC at a rate of 1 ºC min-1, and finally kept at 280 ºC for 8 h.
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2.5. CEC procedures
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The mobile phases were prepared by mixing phosphate buffer with acetonitrile (ACN) in different volume ratios, and the phosphate buffer being adjusted to the
an
desired pH with disodium hydrogen phosphate. All of the solutions were passed
M
through a 0.45 μm membrane filter and subsequently degassed in an ultrasonic bath for 10 min prior to their use. Thiourea was used as a marker for electroosmotic flow
d
(EOF). The obtained MOF coated capillary column was conditioned with the mobile
Ac ce pt e
phases until a stable current was achieved prior to the CEC separation processes. All open-tubular capillary columns were rinsed with ultrapure water for 2 min and mobile phases for 2 min between consecutive runs. Detection was carried out on-column at 254 nm (ionone at 230 nm).
The number of theoretical plates (N/m) and the resolution (RS) were calculated by
the following equation:
5.54 t R N m L W1/2
RS 1.18
2
(1)
t2 t1 W1/2(1) W1/2(2)
(2)
where tR is the retention time and W1/2 is the peak width at half height，L is the 7
Page 7 of 30
total length of column in meters, t1 and t2 are the retention times and W1/2(1) and W1/2(2) are the half height peak widths of the first and second eluted enantiomers, respectively.
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3.1 Characterization of Synthesized MOF [Zn2(D-Cam)2(4,4′-bpy)]n
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3. Results and discussion
The XRD patterns of the synthesized [Zn2(D-Cam)2(4,4′-bpy)]n was in good
us
agreement with the corresponding simulated one, indicating the successful preparation
an
of [Zn2(D-Cam)2(4,4′-bpy)]n (Fig. 1A-b,c). [Zn2(D-Cam)2(4,4′-bpy)]n displayed good stability in the examined phosphate buffer solution as all the XRD peaks belonging to
M
simulated one after treatment (Fig 1A-a,c). Thermal gravimetric analysis (TGA) was
d
also employed to prove the stability of [Zn2 (D-Cam)2(4,4′-bpy)]n (Fig. 1B ).
Ac ce pt e
Fig. 1 in here
3.2 Characterization of MOF-coated OT capillary and measurements of the EOF In this work, the synthesized chiral MOF was introduced into fused silica capillary which have been coated a sodium silicate solution layer. In order to form a stable MOF particles layer, the MOF-coated capillary was heated at 280 ºC to dehydrate between silanol groups of capillary and the sodium silicate. Fig. 2 shows the SEM photographs of bare capillary column (A, B); sodium silicate coated capillary column(C, D) and MOF coated capillary column (E, F). Clearly, the inner wall of the bare capillary column was very smooth as in Figure 2A and 2B. As shown in Fig. 2C and 2D, there is no significant change observed on the surface after the coating of sodium silicate capillary column. Whereas, the MOF coated capillary column 8
Page 8 of 30
displayed rough inner surfaces with ca. 1μm thick layer of MOF particles (Fig. 2E and 2F). Fig. 2 in here
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In CEC, EOF acts as the driving force for the separation. The effective measurement of the EOF in CEC plays a major role in the separation of enantiomers.
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In the work reported here, thiourea was used as an EOF marker to investigate the
us
effect of the pH on the EOF of different capillary columns (bare capillary column, sodium silicate-coated capillary column and MOF-coated capillary column) in the
an
range of 3.5 to 7.5 (Fig. 3). It is obvious that the EOF of all the three studied columns
M
were dependent on the pH of the buffer and the coated capillaries showed different EOF behavior compared to bare capillary. These results showed that the silanol
d
groups which produce EOF of coated column were different from the uncoated
Ac ce pt e
column. For the bare fused-silica capillary, the EOF values increased significantly when the pH values increased, which because of the inner walls of the capillaries were negatively charged and numerous silanol groups on the surface of the capillary would dissociate in this pH range. For the coated capillary, the maximum EOF values was obtained by capillary of coated with sodium silicate, while the coating of the capillary with MOF on the sodium silicate layer was found to have been reduced dramatically, but the tendency of change was basically consistent with the former, demonstrating the successful coating of MOF on the inner surface of capillary. As the OT capillary column prepared in this work was first immobilized, a layer of sodium silicate that likely possesses highly abundant silanol groups on the surface, and then 9
Page 9 of 30
the neutral MOF was post-coated on the sodium silicate layer to form the CSP which led to a reduction in the overall negative charge and a reduction in the EOF. These results definitely indicated that the major source to generate EOF from the sodium
ip t
silicate layer, and the MOF provided negatively charged groups rarely on the prepared chiral OT capillary column.
cr
Fig. 3 in here
us
The dependence of the applied voltage and the produced current were also investigated on MOF modified OT capillary column in Fig. 4. The produced current
an
linearly increased from 27 to 75 μA with a correlation coefficient (r) of 0.9993 by
M
increasing the applied voltage from 8 to 18 kV. This confirmed that the Joule heating associated problems seemed not to be a cause for concern.
Ac ce pt e
3.3 Separation of isomers
d
Fig. 4 in here
To evaluate the performance of the MOF-coated capillary column, the separation
of nitrophenols were firstly tested. In the presence of chial MOF, nitrophenols were baseline separated in 15 min (Fig. 5A-b), especially for o-nitrophenol and p-nitrophenol (RS=7.63). Peak broadening was inconspicuous with the column efficiencies were above 184000 plates m-1. To show whether the MOF played an important role in the separation, we tested bare capillary column under the same conditions (Fig. 5A-a). However, the peaks for the separation of o-nitrophenol and p-nitrophenol showed a significant overlap on bare column. The low resolution can be explained by the loss of interaction between the MOF stationary and the phenolic 10
Page 10 of 30
hydroxyl groups the solute [24]. The migration time for all of the analytes was increased with MOF-coated OT capillary columns. It was ascribed to the reduction of the EOF in MOF-coated OT capillary columns. What’s more, the strong interaction
ip t
between the analytes and the inner surface of the MOF-coated OT capillary columns was another important factor. To gain further insight into the separation of positional
cr
isomers on chiral MOF-coated OT capillary column, ionones were used as test
us
analytes. As can be seen from Fig. 5B, partly separation of α, β-ionone was achieved on MOF-coated capillary column with the resolution of 0.69. The above results
an
demonstrated that the MOF exhibited good selectivity for isomers.
3.4 Separation of chiral compounds
M
Fig. 5 in here
Ac ce pt e
d
The most important advantage of this stationary phase in CEC is its enantioselective and resolving abilities. The performance of the chiral MOF-coated OT capillary column was demonstrated for the enantioseparation of two pair racemates flavanone and praziquantel. As shown in Table 1, the MOF-coated OT capillary column with 35 mM phosphate buffer / ACN (90:10) as the mobile phase and applied voltages 12 kV gave high-resolution separation of flavanone and praziquantel only within 16 min at room temperature. Typically, the high resolution (RS=3.99) and the column efficiency of 58000 plates m-1 for flavanone were obtained. The performance of praziquantel resolution (RS=2.10) higher than the stationary phases of Chiralcel OD, OJ, and Chiralpak AD, AS in CEC [45, 46]. However, usually separation of these chiral compounds in CEC used silica stationary phase 11
Page 11 of 30
modified by chemically bonding or coated cyclodextrin and other chiral selector was needed to achieve good separation [47-49]. Table 1 in here
ip t
It is known that the concentration of organic modifier has a great influence on the resolution. In CEC, ACN is usually used to adjust the polarity of the mobile phase and
cr
to tune the interaction between the mobile phase and the analyte. In this work, ACN
us
were selected as the organic modifiers to study the influence on separation for two pair racemates. It is worth noting that the analytes were not eluted with pure
an
phosphate buffer as the mobile phase even in 50 min (Fig. 6) which indicated the
M
presence of strong interaction between the analytes and the open metal sites of MOF. As the content of ACN in the mobile phase increased, all analytes were eluted within
d
16 min and baseline separation of the two racemates was achieved on the
Ac ce pt e
MOF-coated capillary column with 10% (v/v) ACN in the mobile phase. When the content of ACN in the mobile phase increased, accompany with remarkable reduction in the resolution of the two enantiomers were observed (Fig. 6, 10%-30% ACN). The above mobile phase influence of enantioseparation most probably as a consequence of a distinct competition from solvent adsorption [32]. As the added of the content of ACN in the mobile phase of phosphate buffer / ACN enhanced the competition of ACN for the adsorption on the open metal sites, and thus weakened the interaction between the open metal sites and the analytes. Therefore, a suitable amount of ACN in the mobile phase could obtain better separation. So, the optimum concentration of ACN in the mobile phase for chiral separations was determined to be 10% (v/v). 12
Page 12 of 30
Fig. 6 in here The concentration of the buffer is another key factor which can also significantly affect the enantioseparation of analytes. Hence, the influence of buffer concentration
ip t
on enantioseparations of selected analytes was studied by gradually increasing phosphate buffer from 25 mM to 40 mM. As can be seen from Fig. 7, the migration
cr
times of the two enantiomers gradually increases with augmenting buffer
us
concentration, probably due to decrease of the EOF with increasing ionic strength of the buffer. Moreover, the resolution of the two enantiomers initially increased as the
an
phosphate buffer concentration increased up to 35 mM. However, further increased in
M
the phosphate buffer concentration did not lead to increase in the separation efficiency for both racemates. As a consequence, the optimum concentration of phosphate buffer
d
concentration in the mobile phase for chiral separation was found to 35 mM.
Ac ce pt e
Fig. 7 in here
The effect of change pH on the separation was displayed in Fig. 8. As expected,
the migration times of the two enantiomers displayed an increasing tendency with the increment of pH value from 4.5 to 7.5. Moreover, the greatest of resolution were achieved by increasing the pH to 6.5. The pH value of phosphate buffer had effects not only on migration time but also on separation. Thus, pH=6.5 was selected for all of the subsequent chiral separation experiments. Fig. 8 in here As the influence of the chiral microenvironment on the chiral properties of chromatographic systems is complicated [33], it is difficult for the chiral recognition 13
Page 13 of 30
mechanism of enantioseparation on MOF-coated OT columns to be understood completely. MOF-coated column has excellent chiral recognition ability toward racemates due to the influence of the chiral microenviroment of [Zn2
ip t
(D-Cam)2(4,4′-bpy)]n. The crystal structure of MOF [Zn2 (D-Cam)2(4,4′-bpy)]n was provided in Fig. 9. The left- and right-handed helices formed by alternating D-Cam
cr
ligands and metal sites are joined together through the common dinuclear metal
us
clusters exist in the homochiral layer (the interlayer distance is 7.756 Å). Although the ratio between the left- and right-handed helices is 1:1, the resulting 3-D framework is
an
homochiral because of the presence of the enantiopure building block [44]. Therefore,
M
chiral recognition mostly depends on the interaction of the analytes with the 3-D framework enantiopure building block and the helical homochiral layer surface of
d
[Zn2 (D-Cam)2(4,4′-bpy)]n in which the best steric fit is the main interactive force [50].
Ac ce pt e
Besides, a weak hydrogen bonding between the organic ligands of [Zn2 (D-Cam)2(4,4′-bpy)]n and the racemates also likely enhances the interaction between the chiral stationary and the analytes. The π–π stacking interaction between the pyridine rings on the outer crystal surface of [Zn2 (D-Cam)2(4,4′-bpy)]n and the benzene
ring
of
the
analytes
should
also
contribute
a
lot
to
the
hydrophobic–hydrophobic interaction between [Zn2 (D-Cam)2(4,4′-bpy)]n and the analytes. Fig. 9 in here 3.5 Repeatability and stability of MOF-coated OT capillary column The repeatability and stability properties of the MOF-coated capillary column 14
Page 14 of 30
were evaluated using flavanone with 35 mM phosphate buffer with 10% (v/v) acetonitrile as mobile phase (pH=6.5). The endurance of the coating was found to be more than 10 days and the relative standard deviations (RSDs) for the retention time
ip t
of analyte were less than 2.16% (interday, n=5), which indicated the good stability of the OT column. The run-to-run RSD (n=5) and the column-to-column RSD (n=5)
cr
were 1.04% and 3.07%, respectively, which confirmed the good reproducibility of the
us
column preparation.
an
4. Conclusion
In summary, we have reported the first example of using chiral MOFs as the
M
stationary phase fabricated chiral MOF-coated OT capillary column for CEC
d
separation of chiral compounds and positional isomers. The MOFs coated capillary
Ac ce pt e
column gave baseline separation of flavanone and praziquantel with high column efficiency, various chromatographic parameters such as pH, organic modifier composition and buffer concentration were carefully optimized to get the best separation. In addition, the isomers of nitrophenols and ionones were also separated on the MOF coated capillary column. The results show that chiral MOFs were used in OT-CEC is very attractive for chiral compounds and positional isomers. We believe that these findings will open a new avenue for broad applications of MOFs in chiral chromatography separations.
Acknowledgements 15
Page 15 of 30
The work is supported by the National Natural Science Foundation (No. 21275126, No. 21127012) of China.
ip t
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cr
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Page 19 of 30
ip t
Figure captions
cr
Fig. 1. (A) Comparison of the experimental and simulated XRD patterns of [Zn2(D-Cam)2(4,4′-bpy)]n: (a) treated in phosphate buffer solution for 24 h; (b)
an
curve of the prepared [Zn2(D-Cam)2(4,4′-bpy)]n.
us
simulated from single-crystal X-ray diffraction data, (c) as-synthesized. (B)TGA
Fig. 2. SEM micrographs of open-tubular capillary columns: (A), (B) bare capillary
d
capillary column;
M
column; (C), (D) sodium silicate coated capillary column; (E), (F) MOF coated
Ac ce pt e
Fig. 3. Evaluation of pH on the apparent EOF in different capillaries. Separation conditions: mobile phase, 35 mM phosphate buffer of various pH values /ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10cm height for 2 s.
Fig. 4. Plot of current versus applied electric field strength. Conditions: buffer pH 6.5, other conditions are the same as in Fig.3.
Fig. 5. Chromatograms for separation of the isomers: (A) nitrophenol: (a) CZE electropherogram. (b) CEC electropherogram on the MOF coated capillary column. 1=thiourea; 2=m-nitrophenol; 3=o –nitrophenol; 4=p –nitrophenol. (B) ionone. Separation conditions: mobile phase, 35 mM phosphate buffer (pH 6.5)/ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, (A) 13 20
Page 20 of 30
kV, (B) 10 kV; detection wavelength, (A)254 nm, (B)230 nm; injection, 10 cm height for 2 s.
Fig. 6. Effect of the mobile phase composition on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer
ip t
containing different ACN content at pH 6.5. capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10
us
cr
cm height for 8 s.
Fig. 7. Effect of the buffer concentration on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, different concentration of
an
phosphate buffer containing 10% ACN; Other conditions are the same as in Fig.6.
M
Fig. 8. Effect of the buffer pH on separation of the flavanone (A) and praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer containing 10% ACN;
Ac ce pt e
d
Other conditions are the same as in Fig.6.
Fig. 9. The schematic of crystal structure of [Zn2 (D-Cam)2(4,4′-bpy)]n. (A) Homochiral 2-D layer; (B) The orderly arrangement of left- and right-handed helices.
Table 1. Separations of Racemates on MOF-coated OT Capillary Column.
Racemates
tR1/min
tR2/min
N1/m
N2/m
RS
flavanone
13.869
15.610
58000
26000
3.99
praziquantel
12.338
13.113
52000
36000
2.10
21
Page 21 of 30
ip t cr us an M
d
Fig. 1. (A) Comparison of the experimental and simulated XRD patterns of
Ac ce pt e
[Zn2(D-Cam)2(4,4′-bpy)]n: (a) treated in phosphate buffer solution for 24 h; (b) simulated from single-crystal X-ray diffraction data, (c) as-synthesized. (B) TGA curve of the prepared [Zn2(D-Cam)2(4,4′-bpy)]n.
22
Page 22 of 30
ip t cr us an M d Ac ce pt e
Fig. 2. SEM micrographs of open-tubular capillary columns: (A), (B) bare capillary column; (C), (D) sodium silicate coated capillary column; (E), (F) MOF coated capillary column;
23
Page 23 of 30
ip t cr us an M d
Fig. 3. Evaluation of pH on the apparent EOF in different capillaries. Separation
Ac ce pt e
conditions: mobile phase, 35 mM phosphate buffer of various pH values /ACN = 90/10 (v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10 cm height for 2 s.
24
Page 24 of 30
ip t cr us an M d
Fig. 4. Plot of current versus applied electric field strength. Conditions: buffer pH 6.5,
Ac ce pt e
other conditions are the same as in Fig.3.
25
Page 25 of 30
ip t cr us an
Fig. 5. Chromatograms for separation of the isomers: (A) nitrophenol: (a) CZE
M
electropherogram. (b)CEC electropherogram on the MOF coated capillary column. 1= thiourea; 2=m-nitrophenol; 3=o –nitrophenol; 4=p –nitrophenol. (B) ionone. Separation conditions: mobile phase, 35 mM phosphate buffer (pH 6.5)/ACN = 90/10
d
(v/v); capillary, 50 cm x 50 mm i.d. (effective length 41 cm); applied voltage, (A)13
Ac ce pt e
kV, (B) 10 kV; detection wavelength, (A)254 nm, (B)230 nm; injection, 10 cm height for 2 s.
26
Page 26 of 30
ip t cr us
Fig. 6. Effect of the mobile phase composition on separation of the flavanone (A) and
an
praziquantel (B). Separation conditions: mobile phase, 35 mM phosphate buffer containing different ACN content at pH 6.5. capillary, 50 cm x 50 mm i.d. (effective
Ac ce pt e
d
cm height for 8 s.
M
length 41 cm); applied voltage, 12 kV; detection wavelength, 254 nm; injection, 10
27
Page 27 of 30
ip t cr
Fig. 7. Effect of the buffer concentration on separation of the flavanone (A) and
us
praziquantel (B). Separation conditions: mobile phase, different concentration of
Ac ce pt e
d
M
an
phosphate buffer containing 10% ACN; Other conditions are the same as in Fig. 6.
28
Page 28 of 30
ip t cr
Fig. 8. Effect of the buffer pH on separation of the flavanone (A) and praziquantel (B).
us
Separation conditions: mobile phase, 35 mM phosphate buffer containing 10% ACN;
Ac ce pt e
d
M
an
Other conditions are the same as in Fig. 6.
29
Page 29 of 30
ip t
Fig. 9. The schematic of crystal structure of [Zn2(D-Cam)2(4,4′-bpy)]n. (A)
Ac ce pt e
d
M
an
us
cr
Homochiral 2-D layer; (B) The orderly arrangement of left- and right-handed helices.
30
Page 30 of 30