Journal of Chromatography A, 1339 (2014) 185–191

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Evaluation of ionic liquids-coated carbon nanotubes modified chiral separation system with chondroitin sulfate E as chiral selector in capillary electrophoresis Qi Zhang a , Yingxiang Du a,b,c,∗ , Shuaijing Du d a

Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, PR China Key Laboratory of Drug Quality Control and Pharmacovigilance (Ministry of Education), China Pharmaceutical University, Nanjing 210009, PR China c State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing 210009, PR China d College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China b

a r t i c l e

i n f o

Article history: Received 16 August 2013 Received in revised form 1 February 2014 Accepted 3 February 2014 Available online 8 February 2014 Keywords: Capillary electrophoresis Enantioseparation Carbon nanotubes Ionic liquids Ionic liquids-coated carbon nanotubes Chondroitin sulfate E

a b s t r a c t Nanoparticles (NPs) and ionic liquids (ILs) have been extensively studied and have aroused considerable interest in separation science; however, the employment of ILs-dispersed NPs as buffer modifiers for CE chiral separation has not been previously studied. In this work, we describe a new CE method using ILs dispersed multi-walled carbon nanotubes (ILs-MWNTs) as a modifier for enantioseparation with a polysaccharide, chondroitin sulfate E (CSE), as the chiral selector. As observed, significantly improved separations, including better enantioselectivity and improved peak shapes, were obtained in the ILs-MWNTs modified separation system for all drug enantiomers compared to the single CSE system. Several parameters affecting the enantioseparation, such as the choice of ILs and carbon nanoparticles, ILs-MWNTs concentration, chiral selector concentration, buffer pH and applied voltage, were systematically investigated. Satisfactory separations were achieved when 2.4 ␮g/mL ILs-MWNTs were introduced into the 20 mM Tris/H3 PO4 buffer solution containing 2.5% CSE at pH 2.8–3.4 with a 15 kV applied voltage. A brief mechanism of the enhanced enantioseparation capability of the ILs-MWNTs modified chiral separation system was also discussed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Enantiomeric analysis is a topic of interest in pharmaceutical, toxicological and clinical analysis. This interest stems from the fact that enantiomers of a racemic drug often differ in their pharmacological activity and pharmacokinetic profile [1,2]. Various analytical techniques have been developed over the last few decades for enantioseparation. Among them, Capillary electrophoresis (CE) has emerged as an efficient and versatile technique due to its several advantages such as high separation efficiency, high resolving power, rapidity, as well as low consumption of sample, solvent and chiral selector [3–7]. In CE, chiral separation is mainly achieved by the direct method in which chiral selector is simply added to the background electrolyte (BGE). In some cases, however, satisfactory separation could not be obtained in these conventional separation

∗ Corresponding author at: Department of Analytical Chemistry, China Pharmaceutical University, No. 24 Tongjiaxiang, Nanjing, Jiangsu 210009, PR China. Tel.: +86 25 83221790; fax: +86 25 83221790. E-mail address: du [email protected] (Y. Du). http://dx.doi.org/10.1016/j.chroma.2014.02.003 0021-9673/© 2014 Elsevier B.V. All rights reserved.

systems without any modification. Consequently, researchers have been carrying out experiments to enhance CE enantioselectivity by introducing different types of additives into the BGE. Applications of nanoparticles (NPs) are of rising interest in the past few years. The small size of nanomaterial is responsible for their peculiar advantages, such as favorable surface-to-volume ratio, good chemical stability, significant mechanical strength, ease of modification as well as their applicability in miniaturization [8–10]. Wallingford and Ewing proposed the first use of nanoparticles as pseudostationary phase (PSP) in CE [11]. Since then, a variety of nanoparticle types including carbon nanotubes (CNTs), silica, gold, polymers and molecularly imprinted NPs have to date been employed in CE [10,12–14]. However, only a few papers have reported the application of NPs in CE for chiral separation [13–17]. Among several types of nanomaterials, research on CNTs has rapidly grown over the past few years for their peculiar chemical, mechanical, thermal and electrical characteristics [18]. It can be described as graphite sheets (sp2 carbon) rolled up into tiny tubes up to a few centimeters long and a few to tens of a nanometer in diameter, which are usually caped by a fullerene-like structure [19]. There are two main types of CNTs, namely multi-walled

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Fig. 1. Unit structure of chondroitin sulfate E.

carbon nanotubes (MWNTs), and single-walled carbon nanotubes (SWNTs). Unique properties of nanostructured materials made them extremely attractive for numerous potential applications. In many cases (especially in analytical chemistry), the application of CNTs requires them to be homogeneously dispersed in organic or aqueous solvents. However, it is difficult to disperse CNTs stably in aqueous solution because the wettability of water and CNTs is very poor, and they tend to aggregate into packed ropes or entangled networks. To solve this problem, different approaches have been developed to enhance their dispersability in solvent, including sidewall functionalization [20,21], polymer wrapping [22], modification through – stacking with aromatic molecules [23], and addition of surfactants [24,25]. However, the dispersion capability of these methods is still limited and they always need some cumbersome physical and chemical reaction. Ionic liquids (ILs) are a group of organic salts with melting points (mp) below 100 ◦ C or more often even close to room temperature, and they have received increasing attention in separation science. ILs present unique physical and chemical properties, including lack of negligible vapor pressure, high thermal stability, relatively high ionic conductivity and, especially, strong solubility power [26–30]. Several papers have reported the application of ILs in CE for enantioseparation [31–36], and in these chiral separation systems, the attractive feature of ILs makes them possible to be performed in various modes, e.g. the modification of conventional chiral separation system with achiral ILs, to produce synergistic effect with conventional chiral selectors, or acting as the sole chiral selectors for direct enantioseparation, etc. ILs are good solvents for a lot of inorganic and organic materials as well as polymers. Recently, it is found that CNTs can be easily dispersed in some of the imidazolium-based ILs [37]. In the dispersion process, ILs adsorb tightly on the surface of CNTs. The adsorption would result in polar and charged surface of CNTs, which led to their dispersion in water. The formed ILs-coated CNTs (ILs-CNTs) have attracted considerable attention because it facilitates the application of CNTs in analytical chemistry. Cao et al. [38] studied the use of 1-dodecyl-3-methylimidazolium chloride ([C12 mim]Cl) coated CNTs as a novel pseudostationary phase (PSP) in electrokinetic chromatography for the separation of some flavonoids, phenolic acids and saponins. As reported, improved separation of target compounds can be obtained with the addition of [C12 mim]Cl-CNTs in borate running buffer. As far as we know, the use of NPs modified conventional chiral separation system with polysaccharides as chiral selector in CE was reported in only one paper [17]. Chondroitin sulfates are negatively charged polysaccharides composed of the N-acetylgalactosamine and glucuronic acid residues. Several chondroitin sulfates and related derivatives have already been utilized as one of the most successful types of chiral selectors in CE [39–42]. In our recent work, we first reported the use of Chondroitin Sulfate E (CSE, its structure is shown in Fig. 1) as a novel polysaccharides selector for enantioseparation of some chiral compounds [43]. The evaluation of the enantiorecognition capability of CSE was performed in an uncoated fused-silica capillary by capillary zone electrophoresis (CZE) mode without any modification. As reported, it exhibited a high enantiorecognition capability and a variety of drug enantiomers were resolved in Tris/H3 PO4 buffer with 5.0% (w/v) CSE

concentration. However, all of the analyte peaks suffered from broadening and tailing in the previous work. In this paper, we describe a new CE method using ILs-MWNTs as a modifier for enantioseparation of some representative drug enantiomers with CSE as the chiral selector. As expected, significantly improved separations, including better enantioselectivity and improved peak shapes, were obtained in the ILs-MWNTs modified separation system for all drug enantiomers compared to the single CSE system. To the best of our knowledge, the use of ILs coated NPs for CE application has been reported in only one paper [38], and the employment of ILs-coated NPs as buffer modifiers for CE chiral separation has not been previously reported.

2. Experimental 2.1. Chemicals and reagents Chondroitin sulfate E (90%) was purchased from Wuhan Yuancheng Gong-chuang Technology Co., Ltd (Wuhan, China). SWNT (od.1–2 nm, length 5–20 ␮m) and MWNT (od.10–20 nm, length 5–30 ␮m) were purchased from Alpha Nano Technology (Chengdu, China). A variety of ILs, including 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4 ), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6 ), 1-dodecyl-3-methylimidazolium chloride ([C12 mim]Cl) and 1aminoethyl-3-methylimidazolium bromide ([C2 NH2 mim]Br) were purchased from Shanghai Cheng Jie Chemical Co., Ltd (Shanghai, China). Tris (hydroxymethyl) aminomethane (Tris) was purchased from Shanghai Huixing Biochemistry Reagent (Shanghai, China). Nylon filters (0.45 ␮m), methanol and ethanol, all of HPLC grade, were purchased from Jiangsu Hanbon Sci. & Tech. Co., Ltd. (Nanjing, China). Phosphoric acid, sodium hydrogen and 2-butanol were of analytical reagent from Nanjing Chemical Reagent (Nanjing, China). Double distilled water was used throughout all the experiments. Laudanosine (LAU) and propranolol hydrochloride (PRO) were purchased from Sigma (St. Louis, MO, USA). Amlodipine besylate (AML), Citalopram hydrobromide (CIT) and Nefopam hydrochloride (NEF) were supplied by Jiangsu Institute for Food and Drug Control (Nanjing, China). All these drug samples were racemic mixtures. Their structures are shown in Fig. 2.

2.2. Apparatus Electrophoretic experiments were performed with an Agilent 3D CE system (Agilent Technologies, Waldbronn, Germany), which consisted of a sampling device, a power supply, a photodiode array UV detector (wavelength range from 190 to 600 nm) and a data processor. The whole system was driven by Agilent ChemStation software (Revision B.02.01) for system control, data collection and analysis. It was equipped with a 50 cm (41.5 cm effective length) × 50 ␮m id uncoated fused-silica capillary (Hebei Yongnian County Reafine Chromatography Ltd., Hebei, China). Sample injections were performed by pressure (50 mbar, 4 s). All separations were carried out at 15 ◦ C using a voltage in a range of 10–20 kV. The wavelength for detection was 237 nm (AML and CIT), 230 nm (LAU), 215 nm (NEF), or 225 nm (PRO). The CE system was operated in the conventional mode with the anode at the injector end of the capillary. A new capillary was first rinsed with 1.0 M NaOH for 20 min, followed by the 0.1 M NaOH for 20 min, water for 10 min, and BGE for 10 min. At the beginning of each day, the capillary was flushed with 0.1 M NaOH (10 min) followed by water (10 min). Between consecutive injections, the capillary was rinsed with 0.1 M NaOH, water for 3 min and running buffer for 5 min.

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Fig. 2. Structures of racemic drugs studied in this work.

2.3. Procedures Aqueous dispersions of [C12 mim]Cl-CNTs were prepared by adding 25 mL of 0.5% (w/v) [C12 mim]Cl solution containing 10% (v/v) of 2-butanol to 1 mg of pristine MWNTs or SWNTs placed into a round-bottomed flask. The dispersion was then sonicated with an ultrasonic bath during 1 h for the CNTs suspension [38]. [C2 NH2 mim]Br-CNTs were prepared following the literature procedures [44]: 10 mg of pristine CNTs and 1.0 g of [C2 NH2 mim]Br were placed into a 25 mL flask, and the mixture was stirred for 1 h at room temperature. This was then dissolved in 40 mL of anhydrous ethanol and treated by sonication for 10 min. The solid was separated from the solution by vacuum filtration through a 0.2 mm Millipore membrane and the same washing procedure was then repeated. The final [C2 NH2 mim]Br-CNTs composite was obtained after drying at 60 ◦ C for 3 h in a blast oven. Aqueous dispersions of [C2 NH2 mim]Br-CNTs were prepared by dissolving desired amount of [C2 NH2 mim]Br-CNTs composite in distilled water and sonicated with an ultrasonic bath during 10 min. The BGE consisted of 20 mM Tris solution, adjusted to specified pH value with a small volume of H3 PO4 (10% v/v). The running buffer solutions were freshly prepared by adding appropriate amounts of CSE and ILs-dispersed CNTs in the BGE solution, and then adjusting pH exactly to a desired value by adding a small volume of H3 PO4 (10% v/v) using a microsyringe. The racemic sample solutions (1.0 mg/mL) were dissolved in methanol (LAU) or distilled water (others). Running buffers and sample solutions were filtered with a 0.45 ␮m pore membrane filter and degassed by sonication prior to use.

2.4. Calculations The resolution (Rs ) and selectivity factor (˛) of the enantiomers were calculated from Rs = 1.18(t2 − t1 )/(w1 + w2 ) and ˛ = t2 /t1 , where t1 and t2 are the migration times of the two enantiomers, and w1 and w2 are the peak widths at half-height of the first and second eluting enantiomer. The electroosmotic flow (EOF) and apparent mobilities (app ) were expressed by the equation, eof = (L × l)/(V × t0 ), and app = (L × l)/(V × tm ), where L, l, V, t0 and tm are total capillary length, effective capillary length, applied voltage, migration time of acetone (a neutral marker) and migration

time of the first migrating enantiomer in the presence of chiral additives, respectively. Effective mobilities (eff ) were calculated from eff = app − eof .

3. Results and discussion 3.1. The choice of ILs and carbon nanoparticles It is reported that CNTs can be easily coated with some of the short aliphatic chain ILs ([bmim]BF4 or [bmim]PF6 ) by mechanical milling, forming a thermally stable gel (bucky gel) [37,45]. We reproduced the experiment and obtained the bucky gel. However, the use of this composite as buffer additives was unsuitable for their extremely high viscosity, which would lead to the obstruction of capillary. Instead, a long aliphatic chain IL ([C12 mim]Cl) was investigated for its water-miscible property and amphiphilic molecular structure. It could be observed that pristine CNTs were readily dispersed in water with the aid of very small amounts of [C12 mim]Cl ILs. However, deformed peaks and unstable baseline were observed in this study when [C12 mim]Cl-coated CNTs was added into the buffer solution containing CSE for enantioseparation. Based on these results, we selected an amino IL, namely [C2 NH2 mim]Br, to continue our work. As observed, the pristine CNTs were well dispersed in water with the coating of [C2 NH2 mim]Br IL. It has been established theoretically and experimentally that both the amino group and the imidazolium ions of [C2 NH2 mim]Br have a significant affinity for physisorption along CNT walls [46,47]. The adsorption of the ILs resulted in polar and ionic surface of the CNTs, and thus led to the dispersion of CNTs in water [44]. We carried out further experiments to investigate the performance of [C2 NH2 mim]Br-CNTs modified CSE chiral separation system. As expected, favorable enhancement of enantioseparation was observed when [C2 NH2 mim]Br-CNTs was employed as buffer modifier. Thus, [C2 NH2 mim]Br was selected for subsequent work. For the choice of carbon nanoparticles, two main types of CNTs, namely MWNTs and SWNTs, were investigated simultaneously in our experiment. We found that the suspension of [C2 NH2 mim]Br-MWNTs remained stable after several day’s deposition. In contrast, some precipitation was observed in the suspension of [C2 NH2 mim]Br-SWNTs. Based on these considerations,

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Fig. 3. Electropherograms illustrating the influence of ILs-MWNTs on the enantioseparation of five drug enantiomers: (a) without ILs-MWNTs in the running buffer; (b) containing 2.4 ␮g/mL [C2 NH2 mim]Br-MWNTs in the running buffer. Conditions: 20 mM Tris/H3 PO4 buffer solution containing 2.5% CSE; pH, 2.8 (3.4 for NEF); applied voltage, 15 kV; other conditions as in Section 2.

[C2 NH2 mim]Br coated MWNTs was selected as buffer modifier in this work for enantioseparation.

the adsorption of ILs-MWNTs onto the capillary wall [17]. This is critical to improve the peak shape (peak tailing) that might arise from the interaction of the analytes with the capillary inner wall. The decreased effective mobilities of selected enantiomers likely

3.2. Enantioseparation performance of the ILs-MWNTs modified system Several racemic drugs (AML, LAU, NEF, CIT and PRO) were selected as the model analytes. A comparative study between the performance of single CSE system and ILs-MWNTs modified separation system was conducted to evaluate the effect of ILs-MWNTs on enantioseparation. Fig. 3 shows the comparative electropherograms of the chiral separations of five studied drugs obtained without (a) and with (b) the [C2 NH2 mim]Br-MWNTs modification. As can be seen, significantly improved separations, including increased Rs and ˛, as well as improved peak shapes, were obtained in the ILs-MWNTs modified separation system for all drug enantiomers compared to the single CSE system. Table 1 shows the effect of ILs-MWNTs on EOF, as well as the electrophoresis behavior of four representative analytes, AML, LAU, CIT and PRO. We found that the presence of ILs-MWNTs efficiently reduced the electroosmotic flow (EOF), mainly owing to

Table 1 Effects of ILs-MWNTs on EOF, and the electrophoresis behavior of selected enantiomers. Drugs

Buffer without ILs-MWNTsa

Buffer with ILs-MWNTsb

eof = 1.07

eof = 0.81

a

AML LAU CIT PRO

app b

eff c

app

eff

1.97 2.02 1.77 1.64

0.90 0.95 0.70 0.57

1.58 1.49 1.48 1.24

0.77 0.68 0.67 0.43

Conditions: 20 mM Tris/H3 PO4 buffer solution containing 2.5% CSE; pH 2.8; applied voltage, 15 kV; (A), without ILs-MWNTs in the running buffer; (B), containing 2.4 ␮g/mL ILs-MWNTs in the running buffer; other conditions as in Section 2. a Electroosmotic flow, cm2 /s/V × 10−4 . b Apparent mobilities, cm2 /s/V × 10−4 . c Effective mobilities, cm2 /s/V × 10−4 .

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Fig. 4. Effects of ILs-MWNTs concentration on the resolution (A) and enantioselectivity (B) of drug enantiomers. Conditions: 20 mM Tris/H3 PO4 buffer solution containing 2.5% CSE; pH, 2.8 (3.4 for NEF); applied voltage, 15 kV; other conditions as in Section 2.

resulted mainly from the enhanced extent of reaction between analytes and selectors. Previous studies have demonstrated that chondroitin sulfates and some other polysaccharides have favorable interactions with nanotubes, leading to a sufficient adsorption of polysaccharides molecule on the surface of CNTs [48,49]. Thus, we speculate that the adsorption of chiral selector to ILs-MWNTs would provide a better contact, and thus allowing sufficient complexation between CSE and analytes [14]. 3.3. Effect of ILs-MWNTs concentration It is reported that a low concentration of nanoparticles is sufficient to provide adequate adsorption sites [14]. Thus, in this work the effect of ILs-MWNTs concentration on the separation system was investigated ranging from 0 to 3.2 ␮g/mL. As seen in Fig. 4,

the Rs and ˛ value of drug enantiomers first improved gradually with the ILs-MWNTs concentration increasing in steps. This may be explained by the fact that the NPs would provide a large surface for CSE adsorption, thus allowing sufficient complexation between chiral selectors and the analytes. The best separation in terms of Rs was obtained at the ILs-MWNTs concentration of 2.4 ␮g/mL. At higher concentrations, the Rs and ˛ value generally tended to decrease, or barely increased. We speculate that when the localization sites are saturated for both enantiomers, any further increase of the ILsMWNTs concentration would not improve separation. On the other hand, high concentrations of ILs-MWNTs would give rise to large ropes, bundles and agglomerates in the running buffer due to strong van der Waals interactions and the hydrophobic nature, which would result in unstable buffer solutions and increased ultraviolet spectrum background absorbance. Accordingly, an ILs-MWNTs

Fig. 5. Effect of CSE concentration on the resolution (A) and enantioselectivity (B) of drug enantiomers. Conditions: 20 mM Tris/H3 PO4 buffer solution containing 2.4 ␮g/mL [C2 NH2 mim]Br-MWNTs; buffer pH, 2.8 (3.4 for NEF); applied voltage, 15 kV; other conditions as in Section 2.

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Table 2 Effects of running buffer pH on enantioseparation in the presence of ILs-MWNTs. Drugs

Buffer pH 3.8

AML LAU NEF CIT PRO

3.4

3.1

2.8

2.5

t2 /t1 (min)

Rs /˛

t2 /t1 (min)

Rs /˛

t2 /t1 (min)

Rs /˛

t2 /t1 (min)

Rs /˛

t2 /t1 (min)

Rs /˛

9.294/9.148 9.556/9.409 9.050/8.878 – 12.893

1.11/1.016 1.37/1.016 1.94/1.019 – NS

11.602/11.386 10.182/10.014 11.937/11.669 12.288/12.219 13.270/13.152

1.66/1.019 1.54/1.017 2.00/1.023 0.40/1.006 0.51/1.009

14.394/13.896 12.277/12.025 12.920/12.634 14.210/14.082 17.827/17.632

2.40/1.036 1.85/1.021 1.82/1.023 0.82/1.009 0.63/1.011

15.186/14.564 15.859/15.486 13.622/13.339 15.745/15.593 18.829/18.602

3.03/1.043 2.01/1.024 1.70/1.021 0.95/1.010 0.98/1.012

17.084/16.591 16.552/16.165 15.822/15.536 16.728/16.552 20.715/20.468

2.58/1.030 1.78/1.024 1.34/1.018 0.77/1.011 0.85/1.012

Conditions: 20 mM Tris/H3 PO4 buffer solution containing 2.4 ␮g/mL [C2 NH2 mim]Br-MWNTs and 2.5% CSE; applied voltage, 15 kV; “–”: deformed peak; NS: no separation; other conditions as in Section 2.

concentration of 2.4 ␮g/mL was considered to be optimum for the separation system.

3.4. Effect of CSE concentration The concentration of chiral selector is another essential factor influencing the enantioseparation. In order to acquire the optimum CSE concentration for the ILs-MWNTs modified separation system, the effect of CSE concentrations (1–3%, w/v) on Rs and ˛ of five analytes were investigated with 20 mM Tris/H3 PO4 buffer solution containing 2.4 ␮g/mL [C2 NH2 mim]Br-MWNTs. We observed that the migration times for all the solutes tended to increase as the concentration of CSE increased, naturally owing to the intensive chiral selector–analyte interaction as well as the increased viscosity of the running buffer. As shown in Fig. 5, the Rs and ˛ of drug enantiomers increased as the CSE concentration rose from 1.0% to 2.5%. However, the Rs and ˛ seemed to gradually reach a limiting value or even slightly decreased for some analytes (PRO and CIT) when the CSE concentrations were above 2.5%, presumably due to the saturated complexation between analytes and the chiral selector. The further increase in CSE concentration was eventually limited by increased current and viscosity, which would lead to an unstable baseline and obstruction of the capillary. Therefore, taking satisfactory resolution, small consumption of chiral selector and short analysis times for consideration, a concentration of 2.5% was determined to be appropriate for the ILs-MWNTs modified CSE separation system.

3.5. Effect of other separating parameters It is well known that buffer pH is always a critical and sensitive parameter for CE chiral separation optimization as it governs the charge of chiral selector (ionizable compounds, e.g. anionic polysaccharides) and analytes, which in turn influences the interactions between the selector and analytes (e.g. electrostatic interaction and hydrogen bonding). In addition, the pH of buffer solution has influence on the dispersion state of ILs-MWNTs because the surface modification of nanotube is very sensitive to the variation of surrounding environment [38]. In this study, low pH conditions are desirable for basic analytes because protonation of the analyte occurs. The negatively charged polysaccharides often show the best resolving power toward analytes with the opposite charge due to strong electrostatic interactions between the analytes and selector. Accordingly, we investigated the influence of buffer pH on the enantiomeric separations of five basic compounds in the pH range of 2.5–3.8 using 20 mM Tris/H3 PO4 buffer solution containing 2.5% CSE and 2.4 ␮g/mL [C2 NH2 mim]Br-MWNTs. As illustrated in Table 2, the optimum separations in terms of Rs were obtained at pH 2.8 for AML, LAU, CIT, PRO, and pH 3.4 for NEF. As the pH dropped from 3.8 to 2.5, the Rs of enantiomers first increased due to the enhancement of complexation between analytes and chiral selector, and

subsequently tended to decrease due to peak broadening or the weaker complexation at lower pH. The influence of the applied voltage on enantiomeric separation was also studied over the range of 10–20 kV. It was observed that an increase of applied voltage brought a general decrease of the migration times of all the enantiomers, naturally because of the high migration velocity of both analytes and EOF. As applied voltage increased from 10 to 15 kV, the Rs of drug enantiomers tended to increase owing to the rise of capillary column efficiency; however, the continued increase in applied voltage brought a decrease in Rs for all the tested enantiomers because of the shorter reaction time, as well as the decreased separation efficiency caused by joule heating. Taking account of the good resolution and short migration time simultaneously, an applied voltage of 15 kV was therefore selected as the optimum for the chiral separation. 4. Concluding remarks In the present paper, the potential of ionic liquid-coated MWNTs modified chiral separation systems for CE enantioseparation with CSE as the chiral selector has been demonstrated. Compared to the single CSE system, significantly improved separations, including better enantioselectivity and improved peak shapes, were obtained in the ILs-MWNTs modified separation system for all drug enantiomers. We speculate that the presence of ILs-MWNTs in the running buffer dramatically suppressed the adsorption of analytes onto the capillary wall and, thus, improved the peak shapes of enantiomers. Also, the dispersed NPs would form large surface area platforms for CSE adsorption so as to provide sufficient contact and interaction between CSE and the analytes. The choice of ILs and carbon nanoparticles was detailedly discussed. Some primary parameters, including ILs-MWNTs concentration, CSE concentration, buffer pH and applied voltage, were systematically investigated and optimized. The best separation results were obtained at buffer pH 2.8–3.4 using 20 mM Tris/H3 PO4 containing 2.5% CSE and 2.4 ␮g/mL ILs-MWNTs with a 15 kV applied voltage. It is the first time that ILs-CNTs modified chiral separation systems are evaluated for their enantioseparation capability. The presented NP-modified system still has some aspects to be improved, such as the relatively high costs compared to the single chiral selector CZE system. Nevertheless, the development of novel NPs modified chiral separation system will facilitate the continued growth of the application of ILs and NPs in CE chiral separation. Conflict of interest statement The authors have declared no conflict of interest. Acknowledgments This work was supported by the Project of National Natural Science Foundation of China (Nos. 81373378 and 81072610).

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Evaluation of ionic liquids-coated carbon nanotubes modified chiral separation system with chondroitin sulfate E as chiral selector in capillary electrophoresis.

Nanoparticles (NPs) and ionic liquids (ILs) have been extensively studied and have aroused considerable interest in separation science; however, the e...
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