TY - JOUR
AU - Xu,, Hong
AB - Abstract Au nanoparticles (AuNPs) (10−15 nm in size) were prepared and deposited on the surfaces of silica particles functionalized using 3-aminopropyltriethoxysilane as the seeds under mild conditions. Then, Au seeds grew further and formed nanosheets by the method of gold chloride hydrate reduction. 3, 5-dimethylphenyl isocyanate derivative of cellulose as chiral selector was coated on the surfaces of SiO2/Au. The obtained spheres possessed a sandwich structure in which silica bead, the packed Au NPs monolayer and cellulose derivative were the core, the interlayer and the shell, respectively. The resultant packing material was evaluated by high-performance liquid chromatography (HPLC) as chiral stationary phase (CSP). The separations of nine pairs of enantiomers were achieved in the normal-phase liquid chromatography mode. The results showed that the new CSP has sufficient interaction with the analytes due to the existence of AuNPs on silica surfaces compared with coated cellulose-silica column. Introduction Chirality is fundamental phenomenon in nature and chemical systems. Due to their different biological, pharmacological and toxicological effects, there are great demands and special concerns for the separation of enantiomeric compounds in analytical chemistry. A well-established view is that one of the enantiomers often induces undesirable pharmacological and biological side effects, sometimes even fatal. For example, S-configuration is the effective ingredient in ibuprofen and is widely used as the antipyretic analgesics and anti-inflammatory drugs, while the R-configuration has no other activity but increasing the burden of metabolism in vivo. The fundamental basis for distinction of enantiomers, be it in a biological or chromatographic system, is transformation of enantiomers to diastereomers or creation of a diastereomeric relationship between ligated enantiomers (selectand, SA) and a receptor (chiral selector, CS) (1, 2). High-performance liquid chromatography (HPLC) methods have been established as important and highly efficient technique in chiral analysis (3–6). Direct liquid chromatographic enantiomer separation with chiral stationary phase (CSP) is the popular and highly selective method. Nowadays, polysaccharide-based CSPs meet the above mentioned requirements causing the fact that these materials are currently applied for more than 80% of all reported analytes and above 90% of all chromatography-based preparative and product-scale separation of enantiomers (7, 8). For polysaccharide-based CSPs, besides the CS providing enantioselectivity, carriers are also very critical factors affecting the dynamics and kinetics of the separation process, as well as the permeability of the phases and, as a final result, resolution and speed of separation (7). Porous spherical silica for polysaccharide-based columns is the most widely applied chromatographic carrier. Although other carriers have also been reported including zirconia (9), organic materials (10), ordered mesoporous silica (11), zirconized and titanized silica (12), and so on, these carriers did not show any obvious advantages over silica. Nanoparticles (NPs) paid more and more attention for their potential application in various fields such as chemistry, physics, material science, pharmaceutical industries and so on, due to their unique physicochemical properties, high stability and large surface-to-volume ratio (13–16). Nowadays, they have also been used in separation science. It has been found that NPs can improve separation selectivity and efficiency, and increase column capacity for chromatographic enantiomer separation (17–19). Nucleotide-capped Ag NPs were used as an ultrahigh efficiency enantioseparation and detection platform for D- and L-cysteine by Ye et al. (20). The results showed that the aggregation of Ag NPs was selectively induced by an enantiomer of cysteine, thus resulting in enantioseparation. Li et al. (21) prepared thiolated β-cyclodextrin modified Au NPs for enantioseparation, based on pseudostationary phase-capillary electrochromatography. The results showed that NPs could have sufficient interaction with analytes, resulting in significant improvement of enantioseparation. Bonding C60 on silica and using it as stationary phase for HPLC, the retention behavior demonstrated that C60-bonded phases possessed unique molecular recognition capabilities and shape selectivities for PAHs, PCBs and quinines (22). It has been proved that using NPs as stationary phases have many advantages. Undoubtedly, further research is greatly needed in the field. In this paper, a polysaccharide-based CSP, based on gold (Au)-coated silica gels carrier, was prepared, characterized and evaluated by HPLC, due to their ease of preparation, controllable particle size, narrow size distribution, good solubility in buffer and convenient modification via the Au-S bond. The goal of this study was to investigate the effects of Au NPs on enantioseparation and the feasibility of preparing polysaccharide-based CSP by coating core–shell silica and to perform initial comparative studies on this CSP and a CSP prepared by coating totally porous silica of same particle size. Experimental Reagents and materials Spherical silica (5 μm particle size; 10 nm pore size; 320 m2 g−1 surface area) was purchased from Fuji Silysia Chemical (Aichi, Japan). 3-Aminopropyltrimethoxysilane (APS) was obtained from Alfa Aesar (Karlsruhe, Germany). Trisodium citrate, flavanone, 6-hydroxyflavanone and gold(III) chloride hydrate (HAuCl4.4H2O, ≥99%) were purchased from CIVI-CHEM Technology Co., Ltd. (Shanghai, China). Hydroxylamine hydrochloride, potassium carbonate and sodium borohydride were purchased from Kermiou Chemical Reagent Co., Ltd. (Tianjin, China). Fenthion, cypermethrin, olaquindox, malathion, diazinon, chlorpyrifos were gifts from Zhenjiang Entry-exit Inspection and Quarantine Bureau (Zhenjiang, China). Ibuprofen, anhydrous pyridine, 3,5-dimethylphenyl isocyanate, tetrahydrofuran and microcrystalline cellulose were purchased from Aladdin reagent database Inc. (Shanghai, China). Isopropanol and n-hexane of HPLC grade were from Dima Technology (Richmond Hill, ONT, Canada). All other reagents were of analytical-reagent grade (Tianjin Chemicals, China). Deionized water (>18 MΩcm−1) was used throughout the experiments. Instrumentation and chromatographic conditions Transmission electron microscopy (TEM) images were obtained on a Hitachi H-7500 TEM spectrometer (Hitachi, Japan). UV-visible spectra were acquired with a Shimadzu UV-2400 spectrophotometer. X-ray diffraction (XRD) pattern was recorded on a D8 Advance X-ray diffractometer that uses graphite-monochromated Cu K radiation. IR spectra were obtained on a Nicolet 20 NEXUS 670 FT-IR (Madison, USA) using KBr pellets. Elemental analysis was measured on a Vario EL elemental analysis system (Elementar, Germany). Field emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S-4800 (Hitachi, Japan). The new separation material was slurry-packed into a 150 mm × 4.6 mm I.D. stainless steel column and a hexane and isopropanol (90/10,V/V) mixture was used as the packing solvent at 60 MPa pressure. SiO2-cellulose derivative CSP column (150 mm × 4.6 mm, 5 μm particle diameter, 10 nm pore size) was prepared by our laboratory. The chromatographic evaluations of the SiO2/Au/cellulose derivative CSP column were carried out using FL2200 HPLC system (Wenling, China). All the test probes with a concentration of 200 μg/mL were prepared in hexane/isopropanol (90/10,v/v). The injected volume was 5 μL. Elution was performed in isocratic mode at a mobile-phase flow rate of 1.0 mL/min. If not otherwise stated, column temperature was 25 °C. Immobilization of APS on silica surfaces The silica (2.0 g) was suspended in 30 mL anhydrous toluene, and APS (1.5 mL) was added with a stirring. The reaction mixture was heated under reflux with a N2 atmosphere at 95–100 °C for 24 h. The obtained APS-bonded silica (APS-Sil) was filtered and washed with toluene, acetone and ethanol, respectively, and then dried under vacuum at 60 °C for 12 h. Preparation of SiO2/Au particles Synthesis of AuNPs Au nanoparticles (AuNPs) were synthesized by the following method: 3 mL of HAuCl4.4H2O (0.024 M) and 500 mL of trisodium citrate (2.5 × 10−4 M) were added to a flask with stirring, and then sodium borohydride solution (15 mL, 0.1 M) prepared freshly was added quickly to the mixture (23). The solution changed from colorless to a red color. The solution was centrifuged with deionized water, and the precipitate was redispersed in the deionized water. Preparation of SiO2/Au particles In order to prepare speckled SiO2/Au particles, a three-step procedure described below was used (24, 25). Firstly, 100 mL (0.15 M) of AuNPs solution and 10 mL (5 mg/mL) of APS-bonded silica in water were mixed and stirred at room temperature for 10 h. Au seeds were formed on the surfaces of the APS-bonded silica. The solution was centrifuged, washed with purified water to remove unreacted materials, and redispersed in water. Secondly, 153 mg of K2CO3, 500 mL of purified water and 3 mL (0.15 mM) of HAuCl4 solution were poured in a 1000-mL beaker, stirred, and aged in the dark for 24 h. Finally, SiO2–Au seeds solution in step 1 and gold hydroxide solution in step 2 were mixed and stirred with hydroxylamine hydrochloride (0.2 M). This produced the desired speckled SiO2/Au particles. The precipitate dispersed in 10 mL purified water was typically used in further experiments. Preparation of SiO2/Au/cellulose derivative Synthesis of 3,5-dimethylphenyl isocyanate derivative of cellulose Cellulose (0.72 g) in a 50 mL three-necked round-bottomed flask was dried at 80 °C for 4 h under vacuum condition, then added anhydrous pyridine (15 mL) and an excess of 3,5-dimethylphenyl isocyanate. The mixture was refluxed with constant stirring for 24 h. The solids were filtered, washed and dried. White 3,5-dimethylphenyl isocyanate derivative of cellulose was obtained. Coating of 3,5-dimethylphenyl isocyanate derivative of cellulose on the surfaces of SiO2/Au 3,5-Dimethylphenyl isocyanate derivative of cellulose (0.72 g) was dissolved in tetrahydrofuran and standed overnight. Then, SiO2/Au material (3.0 g) was added and obtained SiO2/Au/cellulose CSP by rotary evaporation. The routes for synthesis of the new separation material are shown in Figure 1. Figure 1 Open in new tabDownload slide The routes for synthesis of the new separation material. Figure 1 Open in new tabDownload slide The routes for synthesis of the new separation material. Figure 2 Open in new tabDownload slide (a) TEM image of Au NPs (b) SEM image of Au NPs on the surface of silica gel. Figure 2 Open in new tabDownload slide (a) TEM image of Au NPs (b) SEM image of Au NPs on the surface of silica gel. Preparation of SiO2- cellulose derivative CSP 3,5-Dimethylphenyl isocyanate derivative of cellulose (0.72 g) was dissolved in tetrahydrofuran and standed overnight. Then, SiO2 (3.0 g) was added and obtained SiO2- cellulose derivative CSP by rotary evaporation. Results Firstly, APS-Sil was synthesized. The results of elemental analysis of APS-Sil (C 7.04%，N 2.14%，H 2.01%) clearly indicated that APS-Sil had been prepared successfully. Then, AuNPs were synthesized. The formation of AuNPs was confirmed by TEM measurement. Figure 2a shows a typical TEM image of the overall morphology of the AuNPs, which were mostly of spherical shape and have a size distribution from 10 to 15 nm. After immobilization and growth of AuNPs on silica gel surfaces, the corresponding SEM image is presented in Figure 2b. Compared with silica gel, Figure 2b reveals a distinctly different surface with many NPs and shows a coating layer due to the presence of the AuNPs on the surfaces of the silica gel. Further evidence for the presence of the AuNPs on silica gel surfaces came from XRD analysis given in Figure 3. A weak diffraction peak can be indexed to SiO2 in addition to the peaks from AuNPs. It is evident that AuNPs had been immobilized onto the surfaces of the silica gel by Au-N bonds. Figure 3 Open in new tabDownload slide XRD pattern of SiO2/Au particles. Figure 3 Open in new tabDownload slide XRD pattern of SiO2/Au particles. Before and after AuNPs was immobilized on the surfaces of the silica gel, the UV adsorption peak of AuNPs has a shift from 510 to 522 nm with an increase of the AuNPs size. When the SiO2-cores were covered with AuNPs, the core-shell SiO2/Au system was formed. The localized surface plasmon resonances of the core-shell system can be explained by plasmon hybridization theory (26, 27). This theory has been used to explain the resonant modes of a core–shell particle or core like SiO2 and Au shell in which bonding and antibonding plasmon modes result. It is demonstrated that the bonding mode which is observable in the visible range is red shifted, our study is very well consistent with the prediction of this theory qualitatively (25). Figure 4 Open in new tabDownload slide Infrared spectra of (1) 3,5-dimethylphenyl isocyanate derivative of cellulose， (2) cellulose. Figure 4 Open in new tabDownload slide Infrared spectra of (1) 3,5-dimethylphenyl isocyanate derivative of cellulose， (2) cellulose. Figure 5 Open in new tabDownload slide Molecular structures of the test enantiomers. Figure 5 Open in new tabDownload slide Molecular structures of the test enantiomers. After cellulose was modified by 3,5-dimethylphenyl isocyanate, its FT-IR spectrum was shown in Figure 4. In FT-IR spectrum of cellulose, strong absorption band at 3,380 cm−1 was attributed to O-H stretching. After being derivatized, the stretching vibration of O-H groups at 3,380 cm−1 was greatly weakened and the absorption peak almost disappeared. The absorption peaks of C=O groups at 1,750 cm−1 and aromatic rings at 1,600–1,700 cm−1 appeared. The results indicated that O-H groups of cellulose had been esterified. Discussion Evaluation of chiral separation on CSP The retention of optically active substances on chiral adsorbents is determined by a combination of enantioselective and nonselective interactions. It is impossible to separate these contributions using linear chromatography methods (28), but certain conclusions can be drawn by comparing the chromatographic characteristics in a series of related compounds. The CSP’s enantioseparation toward chiral compounds was evaluated using six pesticides including malathion, cypermethrin, olaquindox, chlorpyrifos, fenthion and diazinon and three other enantiomers. Chiral pesticide analysis has attracted broad interest, due to their different toxicity and toxicology characteristics. Their enantioseparation is very important for the synthesis of chiral pesticide, monitoring the quality of products during production, pharmacological studies and detecting chiral pesticide in food and environment fields. Other tested chiral compounds included flavanone, 6-hydroxyflavanone and ibuprofen. The molecular structures of these enantiomers are presented in Figure 5. The chromatographic parameters including retention factor (k’), selectivity (α) and resolution (Rs) were evaluated with n-hexane/isopropanol as the mobile phase. The results were listed in Table I. Most of the racemates were separable into enantiomers under normal phase liquid chromatography (NPLC) mode and the values of α and Rs do also follow the same trend as k’. Values of k’ of other compounds on SiO2/Au/cellulose CSP were smaller than those on SiO2-cellulose derivative CSP under the same conditions except chlorpyrifos and fenthion. Moreover, the retention time for ibuprofen, diazinon, flavanone and 6-hydroxyflavanone was too long to find any peak in limited time on SiO2-cellulose derivative CSP. The possible reason was that the surface properties of the new CSP underwent changes and it had a weaker interaction with the analytes due to the existence of AuNPs on the new CSP’ surfaces, leading to a decrease in the retention. Therefore, the new CSP has potential application in fast separation for chiral compounds. The retention for ibuprofen including a carboxyl group was the strongest, which may be due to the stronger electrostatic attraction and hydrogen bond interaction ascribed to the -OH groups from the cellulose. Rs of fenthion, chlorpyrifos and olaquindox on the new CSP were higher than on SiO2-cellulose derivative CSP. The reason is that they have special groups interacting with Au NPs to increase chiral separation, especially chlorpyrifos and fenthion with thiophosphate group in addition to common aromatic ring. According to the literature (29), either D- or L-cysteine-modified AuNPs was employed for enantioselective adsorption of propylene oxide. The enantioselective adsorption process was monitored by optical rotation measurements, which were based on the rotation of polarized light by (R)- and (S)-propylene oxide being enhanced through interaction with AuNPs. The results demonstrated that L-cysteine (D-cysteine)-modified AuNPs could selectively adsorb the (R)-propylene oxide ((S)-propylene oxide). When exposed to racemic propylene oxide, the chiral AuNPs selectively adsorbed one enantiomer and gave an enantiomeric excess in the solution phase, thereby inducing enantioselective separation. Additionally, AuNPs can provide sufficient interaction with the analytes through the increased surface area. Therefore, Au NPs and molecular structures play important roles in the retention and enantioseparation of the racemates on SiO2/Au/cellulose derivative CSP. Representative chromatograms are shown in Figure 6. Table I Separation Results of the Test Enantiomers on the CSPs Racemates . Mobile phase (V/V) n-hexane/isopropanol . SiO2/Au/cellulose derivative CSP . SiO2- cellulose derivative CSP . . k1′ . α . Rs . k1′ . α . Rs . Flavanone 85/15 5.90 1.30 1.06 — — — Cypermethrin 85/15 0.78 1.29 0.58 0.86 2.38 1.05 6-Hydroxyflavanone 95/5 2.61 1.27 0.87 — — — Olaquindox 98/2 2.70 1.50 2.30 3.67 1.31 1.00 Malathion 95/5 1.72 1.63 1.06 8.54 1.11 1.16 Diazinon 90/10 1.80 1.30 1.50 — — — Chlorpyrifos 90/10 1.34 1.33 1.26 0.10 4.38 0.71 Fenthion 55/45 0.24 2.58 1.24 0.12 4.93 0.68 Ibuprofen 70/30 22.06 1.97 2.20 — — — Racemates . Mobile phase (V/V) n-hexane/isopropanol . SiO2/Au/cellulose derivative CSP . SiO2- cellulose derivative CSP . . k1′ . α . Rs . k1′ . α . Rs . Flavanone 85/15 5.90 1.30 1.06 — — — Cypermethrin 85/15 0.78 1.29 0.58 0.86 2.38 1.05 6-Hydroxyflavanone 95/5 2.61 1.27 0.87 — — — Olaquindox 98/2 2.70 1.50 2.30 3.67 1.31 1.00 Malathion 95/5 1.72 1.63 1.06 8.54 1.11 1.16 Diazinon 90/10 1.80 1.30 1.50 — — — Chlorpyrifos 90/10 1.34 1.33 1.26 0.10 4.38 0.71 Fenthion 55/45 0.24 2.58 1.24 0.12 4.93 0.68 Ibuprofen 70/30 22.06 1.97 2.20 — — — Open in new tab Table I Separation Results of the Test Enantiomers on the CSPs Racemates . Mobile phase (V/V) n-hexane/isopropanol . SiO2/Au/cellulose derivative CSP . SiO2- cellulose derivative CSP . . k1′ . α . Rs . k1′ . α . Rs . Flavanone 85/15 5.90 1.30 1.06 — — — Cypermethrin 85/15 0.78 1.29 0.58 0.86 2.38 1.05 6-Hydroxyflavanone 95/5 2.61 1.27 0.87 — — — Olaquindox 98/2 2.70 1.50 2.30 3.67 1.31 1.00 Malathion 95/5 1.72 1.63 1.06 8.54 1.11 1.16 Diazinon 90/10 1.80 1.30 1.50 — — — Chlorpyrifos 90/10 1.34 1.33 1.26 0.10 4.38 0.71 Fenthion 55/45 0.24 2.58 1.24 0.12 4.93 0.68 Ibuprofen 70/30 22.06 1.97 2.20 — — — Racemates . Mobile phase (V/V) n-hexane/isopropanol . SiO2/Au/cellulose derivative CSP . SiO2- cellulose derivative CSP . . k1′ . α . Rs . k1′ . α . Rs . Flavanone 85/15 5.90 1.30 1.06 — — — Cypermethrin 85/15 0.78 1.29 0.58 0.86 2.38 1.05 6-Hydroxyflavanone 95/5 2.61 1.27 0.87 — — — Olaquindox 98/2 2.70 1.50 2.30 3.67 1.31 1.00 Malathion 95/5 1.72 1.63 1.06 8.54 1.11 1.16 Diazinon 90/10 1.80 1.30 1.50 — — — Chlorpyrifos 90/10 1.34 1.33 1.26 0.10 4.38 0.71 Fenthion 55/45 0.24 2.58 1.24 0.12 4.93 0.68 Ibuprofen 70/30 22.06 1.97 2.20 — — — Open in new tab Figure 6 Open in new tabDownload slide Chromatograms for some racemic compounds on the new column. (a) Fenthion, conditions: n-hexane/isopropanol (55/45, v/v), UV detection: 252 nm; (b) 6-Hydroxyflavanone, conditions: n-hexane/isopropanol (95/5, v/v), UV detection: 254 nm; (c) Cypermethrin, conditions: n-hexane/isopropanol (85/15, v/v), UV detection: 240 nm; (d) Ibuprofen, conditions: n-hexane/isopropanol (70/30, v/v), UV detection: 250 nm; (e) Olaquindox, conditions: n-hexane/isopropanol (98/2, v/v), UV detection: 372 nm;(f)Flavanone, conditions: n-hexane/isopropanol (85/15, v/v), UV detection: 254 nm; flow rate 1.00 mL/min. Figure 6 Open in new tabDownload slide Chromatograms for some racemic compounds on the new column. (a) Fenthion, conditions: n-hexane/isopropanol (55/45, v/v), UV detection: 252 nm; (b) 6-Hydroxyflavanone, conditions: n-hexane/isopropanol (95/5, v/v), UV detection: 254 nm; (c) Cypermethrin, conditions: n-hexane/isopropanol (85/15, v/v), UV detection: 240 nm; (d) Ibuprofen, conditions: n-hexane/isopropanol (70/30, v/v), UV detection: 250 nm; (e) Olaquindox, conditions: n-hexane/isopropanol (98/2, v/v), UV detection: 372 nm;(f)Flavanone, conditions: n-hexane/isopropanol (85/15, v/v), UV detection: 254 nm; flow rate 1.00 mL/min. The effects of alcohol modifier content The effects of alcohol modifier content on enantiomer separation were studied. Not only the alcohol modifier itself but also the content of a given polar modifier in mobile phases can affect Rs and the elution order of enantiomer (30, 31). Therefore, the influences of mobile phase on the enantioseparation ability of the new CSP were investigated. With decreasing the content of isopropanol, the retention for four organophosphorus insecticides including cypermethrin, olaquindox, chlorpyrifos and diazinon was stronger, meeting NPLC retention mode. However, Rs values did not follow the same trend as k’. It was not surprising that the retention factor and the resolution factor values do not follow the same trend. Indeed, while the former relates to the thermodynamic of retention, the latter to the mass transfer kinetics. Conclusions In this paper, a new CSP, with a sandwich structure in which silica bead, the packed Au NPs monolayer and cellulose derivative are the core, the interlayer and the shell, respectively, was prepared and characterized. Nine racemates were used to evaluate chiral selectivity of the new CSP under NPLC mode. The new CSP demonstrated different enantioseparation capability in comparison to the cellulose derivatives modified silica column, due to the existence of Au NPs. Au NPs and molecular structures play important roles in the retention and enantioseparation of the racemates on SiO2/Au/cellulose derivative CSP. Compliance with ethical standards Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors. Acknowledgements This work was supported by the Research Project for Science and Technology of Higher Education of Ningxia Province of China (NGY 2018014), Natural Science Foundation of Ningxia (2019AAC03023), the East-West Cooperation Project of Ningxia Key R & D Plan (2017BY064 and 2019BFH02014), Zhenjiang Social Development Project of China Fund (SH2019015) and the National First-rate Discipline Construction Project of Ningxia (Chemical Engineering and Technology, NXYLXK2017A04). Conflict of Interest The authors declare no potential conflict of interest. References 1. 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TI - Polysaccharide-Based Chiral Stationary Phases on Gold Nanoparticles Modified Silica Beads for Liquid-Phase Separation of Enantiomers
JF - Journal of Chromatographic Science
DO - 10.1093/chromsci/bmaa042
DA - 2020-08-21
UR - https://www.deepdyve.com/lp/oxford-university-press/polysaccharide-based-chiral-stationary-phases-on-gold-nanoparticles-CmsMvhS8zl
SP - 731
EP - 736
VL - 58
IS - 8
DP - DeepDyve
ER -