Phase For Open-tubular Capillary Electrochromatography Research Articles

Layer-by-layer self-assembly of polydopamine/gold nanoparticle/thiol coating as the stationary phase for open tubular capillary electrochromatography.

Much attention has been paid to utilizing polydopamine (PDA) as the stationary phase in open-tubular capillary electrochromatography (OT-CEC) owing to its diverse properties, such as strong adhesion to various surfaces, latent reactivity toward amine and thiol groups and metal ion chelating/redox activities. In this study, a novel open-tubular capillary column coated with polydopamine/gold nanoparticles/thiols (PDA/Au NPs/thiols) has been fabricated based on the multiple properties of PDA for the first time. The capillary inner surface was firstly functionalized with a layer of PDA/Au NPs using the strong adhesive and metal ion redox properties of PDA. Thiols were then introduced and covalently reacted with the hybrid coating based on the Michael addition reaction of PDA and thiols and also Au-S bonds. Moreover, benefitting from the porosity of PDA, layer-by-layer (LBL) self-assembly was further applied to increase the amounts of stationary phase (Au NPs and thiols), which can significantly enhance the separation effectiveness and stability of the coated column. The formation of the PDA/Au NP/thiol coating in the capillary was confirmed and characterized by scanning electron microscopy (SEM), Energy Dispersive Spectrometry (EDS) and AFM (Atomic Force Microscopy). Then the separation effectiveness of the PDA/Au NP/thiol@capillary was verified by the separation of alkylbenzenes, which can achieve baseline separation easily with high column efficiency. In addition, the column showed long lifetime and good stability. The relative standard deviations (RSDs) for intra-day and inter-day repeatability of the PDA/Au NP/thiol@capillary were lower than 5%. Therefore, the layer-by-layer self-assembly of PDA/Au NPs/thiols on the capillary inner-surface could be an effective capillary modification strategy.

Room temperature fabrication of post-modified zeolitic imidazolate framework-90 as stationary phase for open-tubular capillary electrochromatography

Metal–organic frameworks (MOFs) are attractive as porous stationary phase for open-tubular capillary electrochromatography (OT-CEC) due to their fascinating structures and unusual properties. Here we report a directly covalent bonding approach to prepare uniform and dense MOF film on the inner wall of fused silica capillary at room temperature for OT-CEC. Zeolitic imidazolate framework-90 (ZIF-90) as a model MOF because it not only possesses large surface area and high stability but also provides the free aldehyde groups to bond to the inner surface of capillary via covalent bond. X-ray diffraction, scan electron microscopy, and UV–vis spectrophotometry were used to confirm the bonding of the ZIF-90 to the inner wall of the silica capillary. The ZIF-90 coating not only increased the phase ratio of open-tubular column, but also improved the interactions of tested analytes and the coating. Owing to the porous structure of ZIF-90 and hydrophobic interactions between the analytes and the organic ligands of ZIF-90, three groups of isomers, neutral and basic compounds and nonsteroidal anti-inflammatory drugs were well separated on the ZIF-90 bonded column. The precisions (relative standard deviation, RSD) of retention time, half peak width and peak area for three consecutive runs were 0.3–1.2%, 1.3–6.0% and 1.5–5.2%, respectively. The run-to-run, day-to-day, and column-to-column precisions (RSDs) for the electroosmotic flow of the ZIF-90 bonded column were 0.2%, 0.4%, and 1.9%, respectively. Moreover, the ZIF-90 bonded column could stand more than 230 runs without observable change in the separation efficiency.

Application of graphene as the stationary phase for open-tubular capillary electrochromatography

We reported the application of graphene as a novel stationary phase in open-tubular capillary electrochromatography (OTCEC). The OT column was easily fabricated by electrostatic assembly of poly(diallydimethylammonium chloride) followed by self-adsorption of negatively charged graphene. The chromatographic properties of graphene-coated OT column were investigated by separation of nitroaniline isomers through variation of the buffer pH, sodium dodecyl sulfate concentration and separation voltage. The results demonstrate the application of graphene in OTCEC greatly improves the separation by largely enhancing the surface area thus increasing stationary/mobile phase interaction. The proposed OT column exhibits good repeatability with run-to-run RSD of retention time as low as 1.5% (n=5), and excellent stability for at least two-week usage with total ∼200runs. The method was successfully applied for analysis of nitroaniline isomers in hair dye samples.

Application of bare gold nanoparticles in open‐tubular CEC separations of polyaromatic hydrocarbons and peptides

In this study, bare gold nanoparticles (GNPs) immobilized in the sol-gel-pretreated fused-silica (FS) capillary as a stationary phase for open-tubular capillary electrochromatography (OT-CEC) are for the first time shown to be able to separate both hydrophobic polyaromatic hydrocarbons (PAHs) as well as hydrophilic cationic antimicrobial peptides. Model mixture of four PAHs, naphthalene, fluorene, phenanthrene, and anthracene, was resolved by OT-CEC in the GNP-modified FS capillaries using the hydro-organic background electrolyte (BGE) composed of 20 mmol/L sodium phosphate buffer, pH 7, modified with ACN at 8:2 v/v ratio. On the other hand, three synthetic analogues of an antimicrobial peptide mastoparan PDD-B, basic tetradecapeptides INWKKLGKKILGAL-NH(2), INSLKLGKKILGAL-NH(2) and NWLRLGRRILGAL-NH(2), were separated in aqueous acidic BGEs, pH 2.1-3.1, composed of weak acids (formic and acetic) or amphoteric amino or imino acids (aspartic or iminodiacetic), utilizing the advantage of a slow reversed (anodic) EOF and slightly positive charge of the GNP-modified FS capillary suppressing the adsorption of cationic peptides on the inner capillary wall and improving their resolution.

Cyclodextrin modified gold nanoparticles-based open-tubular capillary electrochromatographic separations of polyaromatic hydrocarbons

In this study, spherical gold nanoparticles (GNPs) of 14.7 nm diameter, prepared by citrate reduction of a gold(III) salt and characterized by UV–Vis absorption spectrometry and transmission electron microscopy, were modified by a covalent attachment of 6I-O-(3-mercaptopropyl)β-cyclodextrin (β-CD-SH) or per-6-deoxy-per-6-mercapto-β-cyclodextrin (β-CD-SH7). Subsequently, via three alternative approaches, β-CD-modified GNPs were immobilized onto the inner wall of the fused-silica (FS) capillaries and applied as special stationary phases for open-tubular capillary electrochromatography (OT-CEC). The first immobilization procedure was based on pre-derivatization of a FS capillary with (3-mercaptopropyl)trimethoxysilane (MPTMS) followed by subsequent reactions with GNPs and β-CD-SH or β-CD-SH7. The other two preparation protocols took advantage of sol–gel approach gaining a significant increase in the interaction surface for solutes. In both instances, the sol–gel created 3D structure was further covalently modified with GNPs. Serving that purpose, either β-CD-SH7 modified GNPs were used for the immobilization into the sol–gel matrix (“one-step sol–gel technique”) or native GNPs were immobilized first into the sol–gel matrix and subsequently modified with β-CD-SH7 (“two-step sol–gel technique”). The separation performance of CD-GNPs modified FS capillaries was tested by OT-CEC in reversed-phase mode applied to separation of a model mixture of five polyaromatic hydrocarbons. The highest separation efficiencies were obtained with the capillaries prepared by two-step sol–gel technique. However, with respect to the relatively low reproducibility of this method, the first of the above preparation procedures, i.e., a simple pre-derivatization of the FS capillary with MPTMS ensued with β-CD-SH7-GNPs immobilization seems to be more feasible approach providing decent separation efficiency.

Succinyl methacrylate polymerized in porous-layered phases for open-tubular capillary electrochromatography: Comparison with silica hydride monolayered phases

A polymer phase, which was constructed with butyl methacrylate (BMA), an ionizable monomer (mono-(2-(methacryloyloxy)ethyl) succinate (MES)), and a crosslinking agent (ethylene dimethacrylate), was first formed in a porous-layered open-tubular (PLOT) capillary. The PLOT capillary was characterized with SEM and electrophoretic flow as the pH level, ionic strength and addition of organic modifiers in the running buffers changed. In addition, a bare capillary and a silica hydride based capillary (SiH-MES), which bore a monolayered MES phase on it, were used to compared with the BMA-MES capillary. Besides optimizing the capillary electrochromatographic (CEC) conditions for each group of analytes, which were a mixture of nucleosides and thymine, flavonoids, and phenolic acids,comparison of the separation selectivity among analytes between the BMA-MES and SiH-MES capillaries was done according to the velocity and retention factors obtained from the CEC data. Overall, the polymeric phase formed in the PLOT mode was capable of preventing blockage of the columns and was superior to the monolayered phase bonding with the same ionizable ligands for application in CEC as well as to the bare silica phase in CE.

Comparision of succinate- and phthalate-functionalized etched silica hydride phases for open-tubular capillary electrochromatography

Two open-tubular (OT) capillary electrochromatographic (CEC) columns were prepared by chemically bonding ionizable mono-(2-(methacryloyloxy)ethyl) succinate (MES) and phthalate-functionalized (MEP) ligands onto silica hydride-based phases through surface etching, silanization, and hydrosilation reactions, starting with a bare fused-silica tube. An analysis of the effect of performance of electrophoretic flow (EOF) on the changes in pH values, ionic strength, and the amount of acetonitrile modifiers helped to reveal that some silanol groups remained in the surface composite of the modified capillaries and to prove that MEP capillaries actually exerted greater EOF than MES ones. To explore the potential utilization of these two columns in various fields, three categories of samples, which spanned a wide range of polarities, were prepared and analyzed through many systematic trials of optimizing CEC conditions. For the separation of a mixture of nucleosides and thymine, guanine and adenine with purine uncleobases, which exhibit greater aromaticity than pyrimidine nucleobases, performed a higher retention in the MEP capillary through a π–π interaction than in the MES capillary. While four steroids were used as test samples, their migration order revealed that the MES stationary phase is hydrophilic in comparison with the MEP. An addition of methanol modifier (30%, v/v) into 10 mM borate buffer (pH 9.55 for MEP; pH 10.0 for MES) was necessary to accomplish a baseline separation of nine flavonoids in the MEP and MES capillaries. Studies on the elution order of these solutes revealed the presence of chromatographic activity in addition to electrophoretic migration. Especially in the MEP capillary, hydrophobic characteristics and π–π interactions with aromatic solutes were found and further improved to resolve an enantiomeric pair, catechin and epicatechin. Overall, the hydride-based stationary phases with ionizable ligands were successfully applied to the OT-CEC separations, and these results confidently propose an ideal route to the synthesis of a novel OT-CEC column.

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 (C 8-PHEMA films) affords even better resolution in the separation of a series of phenols and anilines. Increasing the thickness of C 8-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.

Alkylthiol gold nanoparticles in open-tubular capillary electrochromatography

Open-tubular columns for capillary electrochromatography (CEC) were formed by immobilising dodecanethiol gold nanoparticles on prederivatised 3-aminopropyl-trimethoxysilane (APTMS) or 3-mercaptopropyl-trimethoxysilane (MPTMS) fused-silica capillaries. The initial stage of this research involved the synthesis and characterisation of dodecanethiol gold nanoparticles, with tunnelling electron microscopy analysis of the dispersed phase of the gold nanoparticles dispersion in CHCl 3 revealing spherical particles. The surface features of an Au-MPTMS coated capillary column were determined using scanning electron microscopy. The electroosmotic flow characteristics of Au-APTMS and Au-MPTMS capillary columns were then determined, by varying the pH and the voltage. The electrochromatographic properties of the gold nanoparticles CEC capillaries were investigated using a “reversed-phase” test mixture of thiourea, benzophenone and biphenyl and selected pyrethroid pesticides. Efficient separations of benzophenone and biphenyl solutes on Au-MPTMS and Au-APTMS capillary columns were obtained, as were linear plots of logarithm capacity factor versus % MeOH. A study of the reproducibility of retention for these solutes on Au-APTMS, Au-MPTMS and on a loosely coated capillary demonstrated the necessity of a coupling agent to prevent the gold nanoparticles from washing-off. These dodecanethiol gold capillary columns are easier to produce and operate than packed capillary columns. The research work confirms the use of gold nanoparticles as a novel phase for open-tubular CEC, demonstrating reproducible retention and characteristic reversed-phase behaviour.

Sol-gel technique for the preparation of beta-cyclodextrin derivative stationary phase in open-tubular capillary electrochromatography.

A novel open-tubular capillary electrochromatography (OT-CEC) column coated with 2,6-dibutyl-beta-cyclodextrin (DB-beta-CD) was prepared using sol-gel technique. In the sol-gel approach, owing to the three-dimensional network of sol-gel and the strong chemical bond between the stationary phase and the surface of capillary columns, good chromatographic characteristics and unique selectivity in separating isomers were shown. We achieved high efficiencies of 5-14 x 10(4) plates/m for the isomeric nitrophenols using the sol-gel-derived DB-beta-CD columns. The migration time reproducibility of the separation of the isomeric nitrophenols was better than 2.2% over five runs and 4.5% from column to column. These sol-gel-coated DB-beta-CD columns have shown improved separations of isomeric aminophenols, isomeric dihydroxybenzenes and isomeric nitrophenols, in comparison with the sol-gel matrix capillary column. The influences of buffer pH and methanol solvent on separation were investigated. The chiral resolution of enantiomers such as ibuprofen and binaphthol was explored primarily.

Preparation of stationary phases for open-tubular capillary electrochromatography using the sol–gel method

Capillary electrochromatography requires the deposition of a stationary phase inside the capillary. In this paper the sol–gel method is proposed for this purpose. The gels were prepared externally and injected into a fused-silica capillary, where anchorage to the capillary wall was possible through condensation reactions between the silanol groups of the capillary wall and the residual silanol groups the gel. Contrary to a commonly used practice, alkaline pretreatment of the inner capillary wall prior to the introduction and anchoring of the gel was found to be only marginally effective in improving the mechanical stability of the column. The influence of various parameters, such as the pH, the water content, the presence of alcohol (ethanol) on the formation of tetraethoxysilane (TEOS)–n-octyltriethoxysilane (C 8-TEOS) hybrid gels of varied composition is discussed. The pH and the amount of water present were found to be the determining factors in the preparation of a stable gel with the desired mechanical and chromatographic properties. By carrying out the gel formation at 80°C, capillary columns could be produced in 2.5 h. While an acidic pH was required during (external) gel formation, subsequent treatment of the gel inside the capillary with an alkaline solution (‘aging’) was found to improve separation and stationary phase capacity significantly. The capillary columns were subsequently used to separate a mixture of polycyclic aromatic hydrocarbons in less than 3 min.