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Cationic ionic liquids organic ligands based metal-organic frameworks for fabrication of core-shell microspheres for hydrophilic interaction liquid chromatography Qian Dai, Junqian Ma, Siqi Ma, Shengyu Wang, Lijun Li, Xianghui Zhu, and Xiaoqiang Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04756 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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ACS Applied Materials & Interfaces

Cationic Ionic Liquids Organic Ligands Based Metal-Organic Frameworks for Fabrication of Core-Shell Microspheres for Hydrophilic Interaction Liquid Chromatography

Qian Dai,† Junqian Ma,† Siqi Ma,† Shengyu Wang,† Lijun Li,‡ Xianghui Zhu,† Xiaoqiang Qiao,*,†

†

College of Pharmacy, Key Laboratory of Medicinal Chemistry & Molecular Diagnosis,

Ministry of Education, Hebei University, Baoding 071002, China ‡

College of Chemistry and Environmental Science, Hebei University, Baoding 071002,

China

Corresponding Author: *Associate Professor Dr. Xiaoqiang Qiao Tel.: +86-312-5971107 Fax: +86-312-5971107 E-mail: [email protected]; [email protected]

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ABSTRACT In this study, new metal-organic frameworks (MOFs) nanocrystals modified SiO2 core-shell microspheres

were

designed

with

cationic

ionic

liquids

(ILs)

1,3-bis(4-carboxybutyl)imidazolium bromide (ILI) as organic ligands. By further adjustment the growth cycles, the new ILI-01@SiO2 core-shell stationary phase was facilely fabricated. The developed stationary phase was respectively characterized via element analysis, thermogravimetric analysis, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry. Because the introduction of cationic imidazolium-based ILs ILI for fabrication of the MOFs nanocrystals shell, the new stationary phase exhibits the retention mechanism of hydrophilic interaction liquid chromatography (HILIC). Many polar samples, such as amides, vitamins, nucleic acid bases and nucleosides, were utilized to investigate the performance of the prepared ILI-01@SiO2 column. Compared to the conventional aminosilica column, the new ILI-01@SiO2 column displays high separation selectivity in a shorter separation time. Furthermore, the new ILI-01@SiO2 column was also used for detection of illegal melamine addition in the baby formula. All the above results demonstrate the new ILI-01@SiO2 core-shell stationary phase is of good potentials for high-selectivity separation the polar samples.

KEYWORDS: HILIC, MOFs nanocrystals, Ionic liquids, Core-shell stationary phase, High-efficiency separation

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1. INTRODUCTION As a novel type of hybrid inorganic-organic porous crystal materials, metal-organic frameworks (MOFs) are prepared by self-assembly by metal clusters/ions bridging organic ligands via coordination bonds.1-3 Since first proposed by Yaghi in 1995, they have been receiving increasing attention because of its unique characteristics, such as the high thermal stability, the larger surface area, various structures and pore topologies as well as the tunable surface properties.4-7 All these characteristics render them attractive materials for gas storage,8 adsorption,9-11 separation,12-16 catalysis,17 magnetism,18,19 and drug delivery.20 Furthermore, the unique properties of MOFs also render them attractive candidates for chromatographic stationary phases. In the early research, MOFs as the gas chromatography (GC) stationary phases were largely reported.21-25 However, the applications of MOFs for the high performance liquid chromatography (HPLC) separation are largely lagging behind. Up till now, high stable MOFs in various of solvent system have been exploited as the HPLC stationary phases, such as MOF-5,26 MIL-53,27,28 MIL-101,29,30 MIL-47,31 MIL-100,32 HKUST-1,33 ZIF-8,34,35 and UiO-66.36,37 In the early research, these MOFs particles are often directly packed into the HPLC column. However, the irregular morphology and wide particle size distribution render the packed column with low column efficiency and asymmetric peak shapes,38,39 which largely restrict the applications of the MOFs-based stationary phases for analysis of complex samples. The fabrication of MOFs-based composites with uniform spherical structure could overcome the shortcomings of the directly packed MOFs-based stationary phases. Recently, MOFs modified silica-based stationary phases which could combine both the merits of MOFs and the excellent packed characteristics of spherical silica (SiO2) were reported.33,34,37,40 For example, Yan et al.34 reported to prepare core-shell ZIF-8 modified SiO2 (ZIF-8@SiO2) materials as the HPLC stationary phase by continuous growing of ZIF-8 shell structure on the

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carboxyl-silica microspheres. The new core-shell MOFs-based materials were utilized to separate pesticides, endocrine-disruption chemicals and the column efficiency for separation of bisphenol A reached 23,000 plates/m. Ding and coauthors37 further utilized UiO-66 to grow MOFs shell on the surface of aminosilica so as to fabricate UiO-66@SiO2 core-shell stationary phase. Since the new materials possess both molecular sieving effect and reverse shape selectivity, it is suitable for separation of structural isomers. Ethybenzene and xylene were separated with high efficiency and low backpressure with the prepared core-shell stationary phase. Furthermore, by further in-depth investigation of the retention mechanisms of the previously reported MOFs-based stationary phases, MOFs for reversed-phase liquid chromatography (RPLC) or normal-phase liquid chromatography (NPLC) separation were often reported and many weakly polar compounds as well as isomers were efficiently separated via shape selectivity, hydrophobicity, π-π, size-exclusion and/or hydrogen bonding interactions.37,41 However, the existing MOFs-based materials which can be used to separate polar compounds are little reported. Ionic liquids (ILs) are the salts in the liquid form. The outstanding characteristics of ILs, such as the good solvating properties, high charge nature and excellent stability, render they are usable in a variety of fields.42-44 Moreover, the cationic structure of the ILs renders them excellent organically functionalized reagents for fabrication of stationary phases with the retention mechanism of hydrophilic interaction liquid chromatography (HILIC) which can be used to separate water-soluble or polar analytes.45 However, to the best of our knowledge, cationic ILs as organic ligands for fabrication of MOFs-based stationary phases have not been

reported

to

date.

Herein,

for

the

first

time,

with

cationic

ILs

1,3-bis(4-carboxybutyl)imidazolium bromide (ILI) as organic ligands and zinc nitrate hexahydrate as metal ions, ILI-01 based MOFs nanocrystals were facilely prepared and

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further exploited to grow on the surface of carboxyl-silica. By adjusting the numbers of growth cycles, ILI-01@SiO2 core-shell microspheres were facilely fabricated. The new stationary phase exhibits HILIC interaction mechanism. Many hydrophilic samples, for example amides, vitamins, nucleic acid bases and nucleosides, were separated with high efficiency and good peak shapes. 2. EXPERIMENTAL SECTION 2.1. Materials 1-(Trimethylsilyl)imidazole,

zinc

nitrate

hexahydrate

(Zn(NO3)2·6H2O),

methyl

4-bromobutyrate, and glutaric anhydride were supplied via Adamas (Shanghai, China). Bare silica and aminosilica microspheres (5 µm) were supplied via Bonna-Agela (Tianjin, China). N,N-Dimethylformamide,

thioacetamide,

iodoacetamide,

N,N-dimethylacetamide,

N,N´-methylenebisacrylamide, urea and melamine were supplied via Alfa Aesar (Shanghai, China). Nicotinamide, pyridoxine hydrochloride, cyanocobalamin and harpagide were from Unimicro Technologies (Shanghai, China). Uracil and cytosine were provided by J&K Scientific (Beijing, China). Adenosine and cytidine were acquired from Tokyo Chemical Industry (Shanghai, China). Acetonitrile (ACN, HPLC-grade) was provided by Thermo Fisher Scientific (New York, USA). 1-Hydroxy-2-naphthoic acid (with the pKa of 2.7) and benzoic acid (with the pKa of 4.2) were all purchased from Chengjie Chemical (Shanghai, China). Baby formula was gifted by the Huiyou Supermarket (Baoding, China). 2.2. Synthesis of ILI The synthetic procedure for ILI is as follows. In brief, 1-(trimethylsilyl)imidazole (4.4 g, 31.3 mmol) and methyl 4-bromobutyrate (11.3 g, 62.6 mmol) were firstly mixed in a flask (volume: 100 mL). After stirring at 60 °C for 24 h via N2 protection, intermediate product 1,3-bis(4-methyl butyrate)imidazolium bromide was obtained. The product was thoroughly washed by ether in order to discard the excess reactants. The obtained 1,3-bis(4-methyl

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butyrate)imidazolium bromide was further dissolved in 7 mL of HCl (37%), followed by further refluxing at 100 °C for 2 h to hydrolyze the diester groups. The product was dried. After it was sequentially washed by ether/acetone for three times, respectively, the final product ILI was obtained. The synthesized ILI was characterized by Bruker apex ultra 7.0 T FT-MS and AVANCE III 600-MHz NMR spectrometer (Bremen, Germany), respectively. 1H NMR (600 MHz, CDCl3) δ=2.29-2.26 (m, 4H), 2.50 (t, 4H, J=8.1 Hz), 4.36 (t, 4H, J=7.0 Hz), 6.61 (s, 2H),6.64 (s, 1H). HRMS, m/z: 241.1181. 2.3. Preparation of ILI-01 MOFs Nanocrystals For preparation of ILI-01 nanocrystals, Zn(NO3)2·6H2O (8.1 g, 27.2 mmol), ILI (10.1 g, 31.3 mmol) and methanol (10 mL) were placed in a flask. After stirring for 24 h at 70 °C via N2 protection, Zn(NO3)2·6H2O (8.1 g, 27.2 mmol) and ILI (10.1 g, 31.3 mmol) were further added to the above suspension and refluxed for 24 h at 70 °C via N2 protection. After being cooled to 25 °C, the obtained ILI-01 nanocrystals were thoroughly washed with methanol and then dried at 60 °C to obtain the ILI-01 nanocrystals, for further analysis. Element analysis for C, H and N was determined by the EAI CE-440 elemental analyzer (Chicago, USA) while for Zn was determined via the method of EDTA titration. Element analysis: C, 12.57%; H, 2.16%; N, 0.43%; Zn, 34.29%. 2.4. Preparation of ILI-01@SiO2 Core-Shell Microspheres The ILI-01@SiO2 core-shell microspheres were prepared with two steps. Firstly, carboxylate-terminated silica microspheres were prepared based on the previous report,34 with some modifications. In brief, aminosilica (3.8 g) was suspended in 6 mL of N,N-dimethylformamide. Then, glutaric anhydride (8.0 g, 70.1 mmol) was placed in the above suspension solution. After stirring for 24 h under 30 °C via N2 protection, the resulting product was centrifuged, thoroughly washed by ethanol and then dried at 60 °C, to obtain the carboxyl-silica microspheres.

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Secondly, ILI-01@SiO2 core-shell microspheres were fabricated. Typically, carboxyl-silica microsphere (3.8 g) was placed in a flask (volume: 100 mL). After they were suspended in 10 mL of methanol, Zn(NO3)2·6H2O (8.1 g, 27.2 mmol) was placed in the above suspension solution. After stirring for 2 h at 70 °C, ILI (10.1 g, 31.3 mmol) was further introduced and reacted via stir for 24 h at 70 °C to obtain the one layer-coated core-shell microspheres. After the microspheres were thoroughly washed with ethanol, totally six cycles of alternative reaction with Zn(NO3)2·6H2O and ILI were performed to grow the ILI-01 shell on the surface of silica to obtain the new stationary phases. Element analysis: C, 6.56%; H, 1.19%; N, 0.79%; Zn, 7.05%. 2.5. Characterization Bruker Vertex 70 spectrometer (Ettlingen, Germany) was applied to record the Fourier transform infrared (FT-IR) spectra in the region of 400-4000 cm-1 in the transmission mode. Bruker D8 X-ray diffractometer (Karlsruhe, Germany) was utilized to obtain the X-ray diffraction (XRD) signals via Ni-filtered CuKa irradiation (λ=1.5406 Å) and the data were recorded in the 2θ range of 10-80o. PerkinElmer TGA/SDTA851E equipment (Boston, USA) was used for thermogravimetric analysis (TGA) of the materials from 30 to 800 °C with the heating-rate at 10 °C/min under dry air. JSM-7500 field-emission scanning electron microscope apparatus (Japan Electron Optics Laboratory, Tokyo, Japan) was utilized to acquire the scanning electron micrograph (SEM) data of the materials. Nitrogen adsorption-desorption isotherms were recorded by the Micromeritics tristar II 3020 porosimeter (Norcross, USA). HPLC columns were packed via a GLK1000 packed column system (Galak Chromatography Technology, Wuxi, China). 2.6. Column Packing For the column packing, ILI-01@SiO2 core-shell microspheres, aminosilica microspheres or bare silica microspheres (2.1 g) were firstly dispersed in chloroform/cyclohexanol (2:3, v/v)

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to make slurry by ultrasonication for about 1 min. After that, the slurry was introduced into the HPLC column jackets (150 mm × 4.6 mm i.d.) at the pressure of 35 MPa for 20 min with isopropanol/methanol (1:1, v/v) as the propellent solution. Lastly, the prepared columns were rinsed for 2 h with ACN as the eluent (1 mL/min), before use. 2.7. HPLC Analysis The Elite P230II HPLC system (Dalian, China), containing a UV detector, two P230II pumps, and an Rheodyne injection valve (Cotati, USA), was used to evaluate the new ILI-01@SiO2 column. All the samples were separated by the fabricated ILI-01@SiO2 column and the aminosilica column via isocratic elution, respectively. Moreover, the test results for nucleic acid bases and nucleosides via the fabricated ILI-01@SiO2 column were also compared with the bare silica column. ACN/H2O was utilized as the mobile phase system and the separation was performed at the ambient temperature. The flow-rate was set at 1 mL/min for the separation of melamine, baby formula sample, vitamins, nucleic acid bases and nucleosides while for amides is 0.8 mL/min. The detection wavelengths are 214 nm (for amides), 240 nm (for melamine and baby formula), 254 nm (for nucleic acid bases and nucleosides), and 272 nm (for vitamins). In the present work, benzene was used to determine the dead time of the fabricated ILI-01@SiO2 column with ACN as the eluent. The resolution of the two adjacent chromatographic peaks was calculated based on the formular R= 2

t 2 − t1 w2 + w1

In the formular, W1 and W2 are the width of peak 1 and peak 2 while t1 and t2 are the retention time of the peak 1 and peak 2. It can be facilely calculated via the workstation equipped within the HPLC system.

2.8. Preparation of the Baby Formula Sample The procedures for pretreating the baby formula sample are as follows.46 Firstly, 1% formic acid water solution (5 mL ) was used to dilute the baby formula sample (0.4 g). Subsequently, 8


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the sample was vortexed at room temperature for 1 min, followed by adding of 10 mL of ACN. The suspension was filtered through 0.22 µm film and then centrifuged for about 10 min at 15,000 r/min. After that, the supernatant was collected and repeatedly filtered and centrifuged for 3 times, ready for subsequent usage.

3. RESULTS AND DISCUSSION 3.1. Design the ILI-01@SiO2 Core-Shell Stationary Phase MOFs-based materials are a novel type of promising HPLC stationary phases because of its fascinating structures and unique characteristics, such as the large surface area, ordered structured cavities, excellent thermostability, and permanent nanoscale porosity.47 However, due to the restriction of the existing MOFs materials, the development of MOFs-based stationary phases suitable for separation of polar compounds have been rarely reported. In the previous report, Dyson et al.48 firstly reported an imidazolium-based ILs bearing two carboxylic groups. Herein, a similar imidazolium-based ILs ILI was designed and synthesized with two steps, as shown in Fig. 1a. The aromatic imidazolium group and dicarboxylate structures of ILI render it an excellent organic ligand to fabricate MOFs nanocrystals. Furthermore, the cationic structure of ILI render it with high hydrophility which benefit for the preparation of stationary phase with HILIC retention mechanism. Therefore, cationic ILI organic ligand based MOFs nanocrystals were designed, prepared and further used to grow on the surface of silica so as to fabricate a new ILI-01@SiO2 core-shell stationary phase. The new stationary phase was prepared with two steps, as shown in Fig. 1b. By alternative reaction of carboxyl-silica microspheres with Zn(NO3)2·6H2O and ILI, the novel core-shell stationary phase could be facilely prepared.

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Figure 1. Schematic demonstration for synthesis of 1,3-bis(4-carboxybutyl)imidazolium bromide (a) and fabrication of ILI-01@SiO2 core-shell stationary phase (b).

3.2. Characterization of ILI-01@SiO2 Core-Shell Stationary Phase FT-IR, XRD, SEM and TGA were utilized to characterize the prepared ILI-01@SiO2 core-shell stationary phase, respectively. Fig. 2a is the FT-IR spectra of aminosilica, carboxyl-silica, and the ILI-01@SiO2 core-shell stationary phase. Band at about 1100 cm-1 can be simultaneously observed from the spectra of aminosilica, carboxyl-silica, and ILI-01@SiO2 core-shell stationary phase is the stretching vibration of Si-O-Si bond of the SiO2 backbone. After the aminosilica was further reacted with glutaric anhydride, bands 1718 and 1560 cm-1 attributing to =CO (-COOH) and -CO-NH- are present in the spectra of carboxyl-silica demonstrating that the aminosilica was successfully functionalized with 10


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glutaric anhydride. To further verify the ILI-01@SiO2 core-shell stationary phase was successfully fabricated, the FT-IR spectrum of ILI-01 nanocrystals was simultaneously performed. The characteristic bands of 1363, 1318, 740 and 634 cm-1 presenting in ILI-01 nanocrystals could be simultaneously observed from the spectrum of ILI-01@SiO2 core-shell stationary phase, indicating the successful growth of ILI-01 nanocrystals on the surface of silica. The XRD patterns of aminosilica, ILI-01@SiO2 core-shell stationary phase and ILI-01 nanocrystals are shown in Fig. 2b. The presenting of the characteristic signal at 2θ=20-25° of SiO2 and the representing peaks for ILI-01 nanocrystals are well matched with that from the ILI-01@SiO2 core-shell stationary phase. Furthermore, from the SEM image of ILI-01@SiO2 core-shell stationary phase, a uniform coating with some ILI-01 nanocrystals spots could be observed. These results further confirm the successful growing of ILI-01 nanocrystals shell on the surface of silica (Fig. 2c). TGA of the prepared ILI-01@SiO2 core-shell stationary phase was also analyzed (Fig. 2d). The endothermic mass loss of ILI-01@SiO2 core-shell stationary phase was firstly observed from about 100 °C which caused by loss of the H2O molecules adsorbed by the ILI-01 nanocrystals shell. From around 435 °C, the ILI-01@SiO2 core-shell stationary phase begins to rapidly collapse indicating that the new stationary phase is of good stability as high as 400 °C. Furthermore, from the characterization results via nitrogen adsorption-desorption, the average pore size of the prepared ILI-01@SiO2 core-shell stationary phase is about 9.0 nm and the BET surface area is 148.9 m2/g (Fig. S1).

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Figure 2. Characterization of the prepared ILI-01@SiO2 core-shell stationary phase. (a) FT-IR spectra of bare aminosilica, carboxylate-terminated silica, ILI-01@SiO2 core-shell stationary phase, and ILI-01 nanocrystals; (b) XRD patterns of bare aminosilica, ILI-01@SiO2 core-shell stationary phase, and ILI-01 nanocrystals; (c) SEM images of aminosilica and ILI-01@SiO2 core-shell stationary phase; (d) Thermogravimetric curves of aminosilica, carboxylate-terminated silica, and ILI-01@SiO2 core-shell stationary phase.

3.3. Retention Mechanism Uracil,

cytidine,

adenosine,

cytosine,

harpagide,

N,N-dimethylacetamide

and

cyanocobalamin are typically hydrophilic compounds possessing strong polarity. Firstly, the chromatographic retention mechanism of the prepared ILI-01@SiO2 core-shell stationary 12


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phase was investigated with the above-mentioned analytes. As shown in Fig. S2a, it could be seen that, if ACN volume fraction in the eluent system increased from 75% to 95%, the retention time of the above analytes gradually increases. Obviously, the prepared ILI-01@SiO2 core-shell stationary phase exhibits typical HILIC retention mechanism. To further investigate the retention characteristics, the effects of pH value and the content of buffer salt in the mobile phase system on the retention of the prepared ILI-01@SiO2 core-shell stationary phase were also investigated. Fig. S2b is the effect of buffer salt concentration of the mobile phase system on the retention of these compounds. For neutral or basic analytes, such as uracil, cytidine, nicotinamide and melamine, if ammonium formate concentration in the eluent system increased from 10 mM to 30 mM, no obvious effect is observed. However, the retention time of the two acidic compounds, including 1-hydroxy-2-naphthoic acid and benzoic acid, gradually decreases with the increase of the concentration of ammonium formate in the eluent system. In present conditions, the two acids partially ionize. Thus, the attractive electrostatic interaction between the positively-charged imidazolium-based ILI-01@SiO2 core-shell stationary phases and the negatively-charged acids weakens as the increase of the concentration of ammonium formate in the mobile phase system, resulting in decreased retention. Fig. S2c shows the effect of pH value on the retention characteristics of the prepared ILI-01@SiO2 core-shell stationary phase. The influence of the pH value for neutral/basic analytes is slight. While for acidic benzoic acid, the retention sharply decreases if the pH value of the mobile phase system decreased to 3.0. The main reason is that benzoic acid almost exists in the neutral state in pH 3.0. Thus, electrostatic attraction interaction between benzoic acid and the prepared stationary phase sharply weakens, resulting in decreased retention. Furthermore, the charge state of the acidic 1-hydroxy-2-naphthoic acid would not influence by the pH value in the pH range of 3.0-7.0. Therefore, the retention is affected slightly.

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3.4. Chromatographic Performance The chromatographic performance of the prepared ILI-01@SiO2 column was investigated by a variety of hydrophilic samples, including amides, vitamins, nucleic acid bases and nucleosides. For comparison, aminosilica packed column was also used for separation of the above-mentioned samples.

3.4.1. Separation Nucleic Acid Bases & Nucleosides Nucleic acid bases and nucleosides, including cytidine (log P: -2.510), cytosine (log P: -1.730), adenosine (log P: -1.23) and uracil (log P: -0.707), were firstly used to evaluate the developed ILI-01@SiO2 core-shell stationary phase. If 85% ACN/15% H2O was utilized as the eluent, a rapid separation (about 4 min) of the four compounds was achieved (Fig. 3a). Almost all the peaks exhibit with good symmetry and the asymmetry factors are 0.99, 0.90, 0.96 and 0.80 for uracil, adenosine, cytidine and cytosine, respectively. When the packed aminosilica column was used, baseline-separation of the above-mentioned analytes was obtained within 6.5 min with the identical eluent conditions (Fig. 3b). If further increasing the content of H2O in the mobile phase system to 20%, the retention time of peak 3 on the aminosilica column is almost equal with the peak 4 via the developed ILI-01@SiO2 column in Fig. 3a. However, the peaks representing uracil (peak 1) and adenosine (peak 2) overlap together (Fig. 3c). Furthermore, in the aminosilica column, the elution orders are consistent with the hydrophilcity of these compounds. However, in the developed ILI-01@SiO2 column, the elution orders of cytidine and cytosine reverse. The probable reason is that the imidazolium-based organic ligand of ILI-01 nanocrystals could provide stronger hydrophobic and π-π interactions for cytosine, resulting in increased retention via the developed ILI-01@SiO2 column.

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Figure 3. Separation chromatograms of nucleic acid bases and nucleosides on the prepared ILI-01@SiO2 column (a) and aminosilica column (b, c). Conditions: mobile phase, ACN/H2O (85/15, v/v) for (a) and (b), ACN/H2O (80/20, v/v) for (c); flow-rate, 1.0 mL/min; detection wavelength, 254 nm. Peaks: 1, uracil; 2, adenosine; 3, cytidine; 4, cytosine.

Moreover, the separation performance of the developed ILI-01@SiO2 column was also compared with the bare silica column and the results are shown in Fig. S3. Although baseline-separation the above-mentioned four analytes are also obtained with the identical eluent conditions in Fig. 3a, serious tailing peaks are observed, especially for cytidine and cytosine (Fig. S3b). The asymmetry factors are 3.21 and 3.26, respectively. Even thought the developed ILI-01@SiO2 column is directly compared with a recent reported calixarene ILs modified silica-based stationary phase49, a comparable separation results could also be observed for nucleic acid bases and nucleosides. However, the developed ILI-01@SiO2 15



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column could be used in ion-pair reagents or buffer solution free mobile phase system. From the above-mentioned discussions, the developed ILI-01@SiO2 column can be utilized to separate polar compounds with higher separation selectivity via a shorter separation time. The stability and suitability of the developed ILI-01@SiO2 column was also investigated by checking the retention time of nucleic acid bases and nucleosides. The single column was consecutively injected for 10, 200, 400 and 800 runs and the typical chromatograms are illustrated in Fig. S4. No noticeable change of the chromatographic profiling is observed. Furthermore, the retention time relative standard deviation (RSD) values of the four analytes range from 1.31 to 1.49%. From the above-mentioned discussions, the prepared ILI-01@SiO2 column demonstrate high stability and suitability, benefiting for the future HPLC applications.

3.4.2. Separation Vitamins The vitamin samples, including nicotinamide, pyridoxine hydrochloride and cyanocobalamin, were further used to evaluate the developed ILI-01@SiO2 column and the results were further compared with that separated via the packed aminosilica column. With 80% ACN/20% H2O as the mobile phase, the three compounds were baseline-separated within about 4 min when the developed ILI-01@SiO2 column was used (Fig. 4a). If the packed aminosilica column was utilized, the retention time of cyanocobalamin (peak 3) could reach to about 13 min (Fig. 4b) with the identical eluent conditions in Fig. 4a. If 68% ACN/32% H2O was utilized as the eluent, although the retention time of cyanocobalamin is almost the same as that via the developed ILI-01@SiO2 column, the resolution of pyridoxine hydrochloride and cyanocobalamin is only 1.37 (Fig. 4c). Thus, baseline-separation of pyridoxine hydrochloride and cyanocobalamin could not be achieved.

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Figure 4. Separation chromatograms of vitamins on the prepared ILI-01@SiO2 column (a) and aminosilica column (b, c). Conditions: mobile phase, ACN/H2O (80/20, v/v) for (a) and (b), ACN/H2O (68/32, v/v) for (c); flow-rate, 1.0 mL/min; detection wavelength, 272 nm. Peaks: 1, nicotinamide; 2, pyridoxine hydrochloride; 3, cyanocobalamin.

3.4.3 Separation of Amides The developed ILI-01@SiO2 column has also been proved to be efficient for separation of amides.

A

representative

chromatogram

of

thioacetamide,

iodoacetamide,

N,N´-methylenebisacrylamide, N,N-dimethylacetamide and urea is shown in Fig. 5a. A well separation of these amides was achieved within 8 min when 98% ACN/2% H2O was used as the mobile phase. However, when the packed aminosilica column was utilized to separate the above-mentioned analytes with the identical eluent conditions in Fig. 5a, although the retention time of urea (peak 5) is similar to that via the prepared ILI-01@SiO2 column, the 17



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peaks representing iodoacetamide and N,N´-methylenebisacrylamide overlap together (Fig. 5b). Simultaneously, the elution orders of iodoacetamide/N,N´-methylenebisacrylamide and N,N-dimethylacetamide reverse via the developed ILI-01@SiO2 column.

Figure 5. Separation chromatograms of amides on the prepared ILI-01@SiO2 column (a) and aminosilica column (b). Conditions: mobile phase, ACN/H2O (98/2, v/v); flow-rate, 0.8 mL/min; detection wavelength, 214 nm. Peaks: 1, thioacetamide; 2, iodoacetamide; 3, N,N´-methylenebisacrylamide; 4, N, N-dimethylacetamide; 5, urea.

3.5. Analysis Melamine in Baby Formula Sample Melamine is a strongly hydrophilic compound. The nitrogen-containing heterocyclic structure of melamine renders it with high nitrogen content (66%). In the past few years, people have

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illegally added of melamine into the dairy products in China, resulting in high apparent protein content via the method of Kjeldahl. Even though melamine has been proved with low toxicity, it can accumulate within the human body for a long time, leading to kidney failure even death.50 The developed ILI-01@SiO2 column was also utilized to inspect the potential addition of melamine in baby formula. The representative HPLC chromatograms of the standard melamine and baby formula sample via the prepared ILI-01@SiO2 column are shown in Fig 6a and b. With 80% ACN/20% H2O as the mobile phase, melamine standard exhibits strong retention (Fig. 6a). Furthermore, the peak representing melamine is well separated with the matrix of baby formula sample. Obviously, the sample matrix would not affect the detection of melamine via the developed ILI-01@SiO2 column. If the packed aminosilica column was used with the identical eluent conditions in Fig. 6a, the retention time of melamine standard is almost the same as one of the compounds existing in the baby formula sample (Fig. 6c and d). Thus, it is difficult to detect the potential addition of melamine into the sample of baby formula via the aminosilica column. From the above-mentioned discussions, the prepared ILI-01@SiO2 core-shell stationary phase exhibits a good potential for applications in the fields of food inspection.

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Figure 6. Chromatograms of melamine standard and baby formula respectively separated by the prepared ILI-01@SiO2 column (a, b) and aminosilica column (c, d). Conditions: mobile phase, ACN/H2O (80/20, v/v); flow-rate, 1.0 mL/min; detection wavelength, 240 nm.

4. CONCLUSIONS In this work, cationic ILI based MOFs nanocrystals were fabricated and further used for preparing of ILI-01@SiO2 core-shell stationary phase. Since the introduction of cationic imidazolium-based functionalized ILs as the organic ligand of the MOFs shell material, the new stationary phase exhibits HILIC retention mechanism. Polar analytes, including amides, vitamins, nucleic acid bases and nucleosides, were successfully separated with rapid separation time and excellent selectivity. Furthermore, successful detection of illegal addition of the melamine in the baby formula sample could also be achieved. In subsequent research,

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we plan to develop a series of novel HILIC core-shell stationary phases based on MOFs nanocrystals with the introduction of a variety of ILs in order to achieve high-selectivity separation the hydrophilic compounds.

ACKNOWLEDGEMENTS We thank the financial support from Young Talent of Hebei Province, National Natural Science Foundation of China (No. 21205027), Baoding Science and Technology Project (No. 15ZG028), and Hebei University Science Fund for the Distinguished Young Scholar (No. 2015JQ06).

ASSOCIATED CONTENT Supporting Information Available: (1) Information regarding the N2 adsorption-desorption isotherms of the prepared ILI-01@SiO2 stationary phase (Figure S1). (2) Information regarding the effect of ACN, buffer salt concentration, and pH of the mobile phase on the retention of analytes (Figure S2). (3) Information regarding the comparison of the prepared ILI-01@SiO2 column and the bare silica column for separation of nucleic acid bases and nucleosides (Figure S3). (4) Information regarding the characterization of stability and suitability of the prepared ILI-01@SiO2 column via chromatographic separation of nucleic acid bases and nucleosides (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Phone & fax: +86-312-5971107. Author Contributions All authors have approved the final version of the manuscript.

Notes The authors declare no competing financial interest.

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Chromatography. Talanta 2016, 152, 392-400. (50) Hu, K.; Zhang, Y.; Liu, J.; Chen, K.; Zhao, W.; Zhu, W.; Song, Z.; Ye, B.; Zhang, S. Development

and

Application

of

a

New

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(L-phenylalaninemethylester-N-carbonylmethoxy)-26,28-dihydroxy-para-tert-butylcalix[4]are ne Stationary Phase. J. Sep. Sci. 2013, 36, 445-453.

For TOC only

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