Alkylthiol gold nanoparticles in sol-gel-based open tubular capillary electrochromatography

A novel open-tubular capillary electrochromatography (OTCEC) column was prepared by immobilizing dodecanethiol gold nanoparticles on prederivatised fused-silica capillary columns with sol-gel technology. 3-Mercaptopropyl-trimethoxysilane (MPTMS) was selected as sol-gel precursor to develop a sol-gel layer on the inner wall of the capillary, prior to assembly of dodecanethiol gold nanoparticles onto the generated sol-gel layer through specific interaction between the gold nanoparticles and surface terminating thiol groups. The electrochromatographic behaviour of the sol-gel gold nanoparticle capillary was compared with a gold nanoparticle capillary prepared via MPTMS surface functionalisation, through variation of the percentage of the organic modifier, pH, and separation voltage. Efficient separation for a "reversed-phase" test mixture of thiourea, naphthalene, and biphenyl and for selected polycyclic aromatic hydrocarbons (PAHs) was obtained on the sol-gel based gold nanoparticle capillaries. OTCEC separations of three selected drug substances (propiophenone, benzoin, and warfarin) were also demonstrated. Scanning electron microscopy was used for the characterization of the sol-gel gold nanoparticle capillary surface. The results confirm that dodecanethiol gold nanoparticles, bound on the sol-gel-based inner layer of fused-silica capillary, can provide sufficient solute-bonded phase interactions for OTCEC with reproducible retention as well as characteristic reversed-phase behaviour.     

Enantioselective open-tubular capillary electrochromatography using cyclodextrin-modified gold nanoparticles as stationary phase

Enantioselective open-tubular CEC (OTCEC) with thiolated β-CD modified gold nanoparticles (CD-GNPs) as stationary phase was developed. The enantioselective OT capillary column was fabricated by electrostatic assembly of poly(diallydimethylammonium chloride) (PDDA) followed by self-adsorption of negatively charged CD-GNPs. The enantioselective capillary column has a steady EOF mobility over a wide pH range of 3.0 to 9.2 (RSD 4.8%), and is quite stable over 240 min with very good column to column reproducibility. Efficient enantioseparation of the presented method was demonstrated by analyzing three drug enantiomers. Our results show that the column exhibits good run-to-run repeatability for enantioseparations and can maintain the enantioselectivity for more than 1 month if the column was stored in CD-GNPs solution at 4 °C.     

Analytical separations in open-tubular capillary electrochromatography

This review represents a summary of recent progress in open-tubular capillary electrochromatography (OT-CEC) for chiral and achiral separations. The OT-CEC approach is an alternative to packed-CEC that could eliminate the problems associated with retaining frits and silica particles. In OT-CEC, the stationary phase is immobilized on the inner walls of the capillary. Preparation of the stationary phase is critical for OT-CEC. The preparation methods for capillary columns include (i) adsorption, (ii) covalent bonding and/or cross-linking, (iii) porous layers, (iv) chemical bonding after etching, (v) sol-gel, and (vi) molecular imprinting. Major developments, potential applications, technical difficulties and advantages associated with these wall coatings in OT-CEC are presented. In addition, the coupling of OT-CEC with mass spectrometry (MS) is briefly reviewed. Several applications of this hyphenated technique for analytical separations are also summarized.     

Recent trends in open-tubular capillary electrochromatography

Capillary electrochromatography (CEC), which combines the advantages of the high efficiency of capillary electrophoresis (CE) and the high selectivity of liquid chromatography (LC), has recently received considerable attention. Most CEC experiments have been performed with capillary columns packed with small LC packing materials (1.5–5 μm particle diameter). However, problems such as difficulties in packing the small LC packing materials and fabricating the frits still exist in preparing the CEC column. The use of open-tubular columns in CEC is therefore an alternative approach that can eliminate the problems encountered in packed-column CEC. So far, several types of open-tubular columns have been proposed for CEC separations and in this article recent progress in this area is reviewed.     

Polymer-brush stationary phases for open-tubular capillary electrochromatography

Synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from the inside of silica capillaries by surface-initiated atom transfer radical polymerization (ATRP) yields unique stationary phases for open-tubular capillary electrochromatography (OT-CEC). Although PHEMA brushes have only a small effect on the separation of a set of phenols and anilines, derivatization of PHEMA with ethylenediamine (en) allows baseline resolution of several anilines that co-elute from bare silica capillaries. Derivatization of PHEMA with octanoyl chloride (C8-PHEMA films) affords even better resolution in the separation of a series of phenols and anilines. Increasing the thickness of C8-PHEMA coatings by a factor of 2 enhances resolution for several solute pairs, presumably because of an increase in the effective stationary phase to mobile phase volume ratio. Thus, this work demonstrates that thick polymer brushes provide a tunable stationary phase with a much larger phase ratio than is available from monolayer wall coatings. Through appropriate choice of derivatizing reagents, these polymer brushes should allow separation of a wide range of neutral molecules as well as compounds with similar electrophoretic mobilities.     

Metallomesogenic stationary phase for open-tubular capillary electrochromatography

A synthetic coppermesogenic polymer is prepared and then covalently bonded to the siloxane-based deactivated column as the stationary phases of open-tubular CEC with essentially high phase ratio. The EOF generated from the modified phase is surveyed through conventional aqueous buffers and hydroorganic mobile phases. Zeta potentials, which are computed from the EOF data and the ratio of dielectric constant to viscosity, are plotted as a function of pH, ionic molarity, and compositional range. These plots responsible for the electroosmotic characteristic of the bonded phases are found to be like those of bare fused-silica or deactivated columns through decreasing or increasing the ACN content in the mobile phase, respectively. This two-phase characteristic is basically derived from the polymeric configuration with carboxylato ligands attached onto the polysiloxane backbone. Phthalates and amino acids are suitable probes to examine the two phenomena, more-polar and less-polar mediums, respectively, and to judge whether the chromatographic retention is the major source of separation mechanism. With the mixing modes of Lewis acid-base interaction, dispersive force, and shape discrimination, the chromatographic partition adequately accomplishes the uneasily resolved separations by only CZE mode, although the electrophoretic migration is truly somewhat involved.     

Permanent gold nanoparticle coatings on polyelectrolyte multilayer modified capillaries for open-tubular capillary electrochromatography

This paper reports on a new strategy to coat fused silica capillaries through ionic adsorption of gold nanoparticles (AuNPs) on a polyelectrolyte multilayer (PEM) modified capillary wall. The coating was constructed in situ by alternating rinses with positively charged poly(diallydimethylammonium chloride), negatively charged poly(sodium-4-styrenesulfonate), and positively charged AuNPs. After self-assembly of n-octadecanethiol onto the surface of AuNPs, the modified capillary was investigated as a new medium for the separation of neutral analytes and proteins in open-tubular capillary electrochromatography (OT-CEC). The surface coverage of the capillary wall was increased using the high density of AuNPs which were dynamically capped with 4-dimethylaminopyridine (DMAP). The chromatographic performance of the column coated with positively charged AuNPs was remarkably improved compared with a column modified with negatively charged AuNPs. The coating was robust over more than 810 runs in this study and also showed high stability against 0.01 M NaOH, 0.01 M HCl, and electrolyte concentrations up to 70 mM. The run-to-run, day-to-day, and capillary-to-capillary reproducibilities of electroosmotic flow were satisfying with relative standard deviation values of less than 1% in all cases. The AuNP-coated PEM modified capillary column not only showed good performance for neutral analytes but also was suitable for the analysis of both basic and acidic proteins.     

Comparison of separation behavior of benzodiazepines in packed capillary electrochromatography and open-tubular capillary electrochromatography

Packed column capillary electrochromatography (CEC), open-tubular CEC and microcolum liquid chromatography (LC) using a cholesteryl silica bonded phase have been studied to compare the retention behavior for benzodiazepines. It has been found that packed column CEC gives better resolution, faster analysis time than microcolumn LC for benzodiazepines maintaining similar selectivity except for some solutes which are charged species under the separation conditions. However, open-tubular CEC gave different selectivities to a larger extent for charged benzodiazepines from that which should be produced by the chromatographic properties of the cholesteryl silica phase. Charged species migration times are mainly influenced by electrophoretic mobility rather than the chromatographic interactions.     

Open-tubular capillary electrochromatography using a capillary coated with octadecylamine-capped gold nanoparticles

Octadecylamine-capped gold nanoparticles (ODA-Au-NPs) were prepared and characterized by using UV-Vis adsorption spectrum, transmission electron chromatography (TEM), SEM, and FT-IR. A simple but robust hydrophobic coating was easily developed by flushing a capillary with a solution of ODA-Au-NPs, because the positive charges were carried by the nanoparticles which strongly adsorb to the negatively charged inner surface of a fused-silica capillary via electrostatic and hydrophobic interactions. The chromatographic characteristics of the coated capillary was investigated by varying the experimental parameters such as buffer pH, buffer concentration, and percentage of organic modifier in the mobile phase. The results show that (i) resolution between thiourea and naphthalene is almost the same when comparing the electrochromatograms obtained using pH 7 buffer as mobile phase after and before the capillary column was operated using pH 11 and 3 mobile phase; (ii) no significant changes in retention time and deterioration in peak efficiency were found after 60 runs of test aromatic mixtures; and (iii) column efficiency up to 189 000 theoretical plates/meter for testosterone was obtained. All of the results indicated that the coating could act as a stable stationary phase for open tubular CEC as well as for bioanalysis.     

A new type of capillary column for open-tubular electrochromatography

A new type of open-tubular C(18) ester-bonded electrochromatographic column was prepared with sol-gel technology, followed by an on-column octadecyl silylation reaction. Glycidoxypropyltrimethoxysilane, a widely used and important silane agent, was used as the sol-gel precursor to form a thin coating layer on the wall of the fused-silica capillary. The C(18) groups were introduced into the coating layer by on-column esterification reaction with stearic acid. The electrochromatography behavior of the column was evaluated in terms of the separation of peptides. A high efficiency of 4.8x10(5) plates/m was achieved for a basic pentapeptide using the C(18 )ester-bonded column. In comparison with bare capillaries and glycidoxypropyltrimethoxysilane sol-gel-coated capillaries, the C(18) ester-bonded column showed the best separation of a mixture of seven pentapeptides under identical conditions of buffer, pH, and applied voltage.     

Recent highlights in stationary phase design for open-tubular capillary electrochromatography

This review examines the most recent innovations made to achieve high performance in open-tubular capillary electrochromatography (OT-CEC) separations, focusing on the ingenious chemical and physical solutions made to increase the surface area and equip the stationary phase with exploitable selectivity. Among the approaches taken are chemically bonded ligands, etching with chemical bonding, sol-gels, molecularly imprinted polymers, porous layers, physically attached or adsorbed phases, and nanoparticle coatings. Particularly noteworthy are modern developments with macrocyclic receptor ligands, nanoparticles and open channel electrochromatography on-chip.     

Quantitation and on-line concentration of enantiomers in open-tubular capillary electrochromatography.

The feasibility of open-tubular capillary electrochromatography (OTCEC) with UV detector for quantitation of enantiomers is explored, and a simple on-line sample concentration method to improve detection sensitivity of negatively charged enantiomers more than 1000-fold is described. With a capillary of 25 microm ID, the limits of detection (LODs) for absolute concentration and for enantiomeric ratio are 10(-6) M and 0.6-0.8% (signal-to-noise ratio S/N = 10). Good linearity and reproducibility are observed. The detection sensitivity is enhanced by combination with field-enhanced sample injection (FESI). A water plug is introduced hydrodynamically into the capillary inlet end and then the sample solution prepared with water is introduced with electrokinetic injection. With this concentration technique, the LOD for absolute concentration is reduced to a 10(-9) M level. On the other hand, due to the peak-sharpening effect of FESI, the LOD for enantiomeric ratio for the first-eluted enantiomer is significantly improved, being 0.3%. Effects of the injection conditions, such as length of water plug, buffer concentration, injection voltage, and injection time on the enrichment efficiency are investigated. Online concentration of a racemic compound with two chiral centers is demonstrated.     

开管毛细管电色谱分析3种解热镇痛药物

针对生物体液样品开展药物的绿色高效毛细管电泳分离分析具有重要的研究意义。该研究以3种解热镇痛药(4-氨基安替比林、氨基比林及非那西汀)为研究对象,以嵌段聚合物为涂层,建立了药物的开管毛细管电色谱(OT-CEC)分析新策略。首先,采用活性/可控自由基可逆加成-断裂链转移聚合方法,合成制备得到了两亲性嵌段聚合物-聚(苯乙烯-甲基丙烯酸缩水甘油酯)(P(St-GMA)),并将其涂覆到毛细管内壁;其次,通过考察影响OT-CEC分离效率的关键因素,包括嵌段聚合物的聚合时间、涂覆毛细管嵌段聚合物的浓度、电泳运行缓冲液的种类和pH值、有机溶剂添加剂等,优化了3种解热镇痛药物的OT-CEC分离条件;最终发现,不需添加任何有机溶剂及表面活性剂,仅采用50.0 mmol/L乙酸钠-乙酸(pH 5.7)作为OT-CEC的缓冲溶液,就能实现3种解热镇痛药物的基线分离。在8.0～2.5×10³μmol/L范围内,分析物峰面积与其对应的浓度呈现良好的线性关系,相关系数(R²)均大于0.995,检出限为1.0～2.5μmol/L。结果表明:P(St-GMA)在溶液中自组装...     

Multi-walled carbon nanotube composites with polyacrylate prepared for open-tubular capillary electrochromatography

A new phase containing immobilized carbon nanotubes (CNTs) was synthesized by in situ polymerization of acid-treated multi-walled CNTs using butylmethacrylate (BMA) as the monomer and ethylene dimethacrylate as the crosslinker on a silanized capillary, forming a porous-layered open-tubular column for CEC. Incorporation of CNT nanomaterials into a polymer matrix could increase the phase ratio and take advantage of the easy preparation of an OT-CEC column. The completed BMA-CNT column was characterized by SEM, ATR-IR, and EOF measurements, varying the pH and the added volume organic modifier. In the multi-walled CNTs structure, carboxylate groups were the major ionizable ligands on the phase surface exerting the EOF having electroosmotic mobility, 4.0 × 10(4) cm2 V(-1)1 S(-1)1, in the phosphate buffer at pH 2.8 and RSD values (n=5), 3.2, 4.1, and 4.3%, for three replicate capillaries at pH 7.6. Application of the BMA-CNT column in CEC separations of various samples, including nucleobases, nucleosides, flavonoids, and phenolic acids, proved satisfactory upon optimization of the running buffers. Their optima were found in the borate buffers at pH 9.0/50 mM, pH 9.5/10 mM/50% v/v ACN, and pH 9.5/30 mM/10% v/v methanol, respectively. The separations could also be used to assess the relative contributions of electrophoresis and chromatography to the CEC mechanism by calculating the corresponding velocity and retention factors. Discussions about interactions between the probe solutes and the bonded phase included the π-π interactions, electrostatic repulsion, and hydrogen bonding. Furthermore, a reversed-phase mode was discovered to be involved in the chromatographic retention.     

A novel sol-gel calix[4]arene-modified capillary column for open-tubular capillary electrochromatography

A novel open-tubular CEC (OT-CEC) column was prepared by immobilized 5,11,17,23-tetra-tert-butyl-25,27-diethoxy-26,28-dihydroxy-calix[4]arene (Calix[4]) on fused-silica capillary column with sol-gel technology. Calix[4] was initially reacted with gamma-glycidoxypropyltrimethoxysilane (KH-560) to form a new sol-gel precursor (calix[4]-KH-560), and then mixed with another precursor, namely tetraethoxysilane (TEOS). After hydrolysis and condensation, a sol-gel matrix was obtained, in which calix[4] was incorporated. Successful calix[4]-modified sol-gel coating was suggested by infrared (IR) spectra and greatly decreased EOF. In comparison with OT columns prepared by the sol-gel method with TEOS and KH-560 only, the calix[4]-modified sol-gel column showed greatly improved separation of isomeric toluidines, nitrophenols, picolines and neurotransmitters with structural similarity. Migration time and theoretical plate number reproducibility were satisfactory with RSDs less than 1 and 6% each for within column runs and not more than 3 and 7% each for column-to-column measurements, using toluidine and nitrophenol isomers as test solutes on this column. High separation efficiencies (96,000-300,000 plates/m) for basic toluidine isomers were obtained. This column was also successfully employed to combine extraction procedure for the determination of dopamine (DA) and norepinephrine (NE) in Portulaca oleracea L. The recoveries of DA and NE were 93.3 and 94.2%, respectively.     

Advances in sol-gel based columns for capillary electrochromatography: sol-gel open-tubular columns

The development of sol-gel open-tubular column technology in capillary electrochromatography (CEC) is reviewed. Sol-gel column technology offers a versatile means of creating organic-inorganic hybrid stationary phases. Sol-gel column technology provides a general approach to column fabrication for microseparation techniques including CEC, and is amenable to both open-tubular and monolithic columns. Direct chemical bonding of the stationary phase to the capillary inner walls provides enhanced thermal and solvent stability to sol-gel columns. Sol-gel stationary phases inherently possess higher surface area, and thus provide an effective one-step alternative to conventional open-tubular column technology. Sol-gel column technology is applicable to both silica-based and transition metal oxide-based hybrid stationary phases, and thus, provides a great opportunity to utilize advanced material properties of a wide range of nontraditional stationary phases to achieve enhanced selectivity in analytical microseparations. A wide variety of stationary phase ligands can be chemically immobilized on the capillary inner surface using a single-step sol-gel procedure. Sol-gel chemistry can be applied to design stationary phases with desired chromatographic characteristics, including the possibility of creating columns with either a positive or a negative charge on the stationary phase surface. This provides a new tool to control electroosmotic flow (EOF) in the column. Column efficiencies on the order of half a million theoretical plates per meter have been reported for sol-gel open-tubular CEC columns. The selectivity of sol-gel stationary phases can be easily fine-tuned by adjusting the composition of the coating sol solution. Open-tubular columns have significant advantages over their packed counterparts because of the simplicity in column making and hassle-free fritless operation. Open-tubular CEC columns possess low sample capacity and low detection sensitivity. Full utilization of the analytical potential of sol-gel open-tubular columns will require a concomitant development in the area of high-sensitivity detection technology.     

Multi-wall carbon nanotubes bonding on silica-hydride surfaces for open-tubular capillary electrochromatography

Prepared multi-wall carbon nanotube (MWNT) materials, including untreated MWNT, HNO₃-treated MWNT and HNO₃-HCl-treated MWNT were covalently attached onto a silica-hydride-modified capillary by hydrosilation, using the abundant double bonds between the pentagon carbons in the MWNT structure. These MWNT-incorporated capillaries were characterized by SEM, ATR-IR and electroosmotic flow (EOF) measurements in phosphate buffers with a pH range of 3.7–9.3 and in the mixtures of acetonitrile modifier. The untreated capillary was assumed to carry some carboxylate groups formed on the non-acid-treated MWNTs, as it had higher EOF values than the hydride capillary. As the MWNTs were treated with HNO₃ and HCl solutions, the capillaries had increasingly higher EOF values. To examine the existence of an electrochromatography mechanism in the modified capillaries, a mixture of nucleosides and thymine was probed to check the velocity factor and retention factor. In addition to the π–π interaction between the probe solutes and the MWNT immobilized stationary phases; a reversed-phase mechanism could contribute to the chromatographic retention. For acidic tetracyclines, increasing the loadability of MWNTs resulted in a high retention factor and improved the separation resolution.     

Gold nanoparticle-coated capillaries for protein and peptide analysis on open-tubular capillary electrochromatography

We report a new method of immobilization of gold nanoparticles (AuNPs) on a fused-silica capillary through covalent binding. The resulting modified capillary was applied to electrophoretic systems to improve the efficiency of separation and the selectivity of selected solutes. The immobilization of AuNPs on the capillary wall was performed in a very simple and fast way without requiring heating. The surface features of an AuNP-coated capillary column were determined using the scanning electron microscopy. The chromatographic properties of AuNP-coated capillaries were investigated through variation of the buffer pH and separation voltage. Effective separations of synthetic peptides mixture were obtained on the AuNP-coated capillaries. The method shows a remarkable stability since it was reused about 900 times. The capacity factor was duplicated. Therefore, this modification is stable and can be applied to different separation purposes. A complex mixture of tryptic peptide fragments of HSA was analyzed in both the bare- and the AuNP-coated capillaries. Better electrophoretic peptide profile was observed when using the AuNP-coated capillary.     

A novel open-tubular capillary electrochromatography with magnetic nanoparticle coating as stationary phase

A novel open‐tubular capillary electrochromatography (OT‐CEC) with modified core/shell magnetic nanoparticles coating as stationary phase was introduced using external magnetic force to fix magnetic nanoparticles. The magnetic nanoparticles coating inside the capillary columns could be easily regenerated by removing and re‐applying the external magnetic field. Magnetic field intensity, concentration and flow rate of nanoparticles suspension were investigated to achieve simple and stable preparation. Mixture of five organic acids was used as the marker sample to evaluate the OT‐CEC system, and the relative column efficiency of anthranilic acid reaches 220 000 plates/m. The excellent within‐column and between‐column repeatability has been testified with the RSDs of retention time of less than 1.51 and 5.29%, respectively. The aqueous extract of rhizoma gastrodiae was analyzed by the OT‐CEC system, and 23 peaks were eluted in 30 min. Compared with conventional open‐tubular capillary column, this new system shows faster separation speed and higher column efficiency from the larger surface area of nanoparticles. It has great potential in the method development for the analysis of complex samples, since magnetic coating can effectively prolong the column life by expediently replacing stationary phase to eliminate the pollution or irreversible adsorption.     

金纳米粒子修饰的手性毛细管电色谱固定相的制备及性能表征

制备了粒径为15 nm的金纳米粒子(GNPs)并将其修饰到氨基衍生化的硅胶整体柱内,通过化学键合法将牛血清白蛋白(BSA)固载到GNPs的表面作为手性固定相。通过透射电子显微镜、扫描电子显微镜等方法进行表征,结果表明,GNPs分散性良好,并被成功地修饰到毛细管柱内,含量高达17.18%。优化了BSA手性柱的制备条件,最终确定了体积分数为10%的3-氨丙基三乙氧基硅烷(APTES)和15 g/L BSA为最佳反应条件。在毛细管电色谱分离模式下,对缓冲液pH值、电压等分离条件进行了考察,最终选择了10 mmol/L pH 7.4的磷酸缓冲液和15 kV运行电压作为最佳分离条件。手性柱对3种手性化合物(色氨酸、阿替洛尔和麻黄碱)有拆分效果,对色氨酸能实现基线分离。与物理吸附法相比,化学键合法制备的手性柱拆分效果好,分析物无需柱前衍生化,且色谱柱稳定性良好。该文的制备方法也为其他类型手性选择剂的引入提供了良好的思路。