Immobilization of chitosan in sol–gel phases for chiral open-tubular capillary electrochromatography

Three different approaches for immobilizing cross-linked chitosan molecules (CS-s) in sol–gel phases to form chiral OT-CEC capillaries were comparatively investigated in this study. To synthesize column I, a bare capillary was first silanized with triethoxysilane (TEOS) and then reacted with the reaction product of 3-glycidyloxypropyltrimethoxysilane (GTS) and CS-s. Column II was prepared by the silanization of a bare capillary with a mixture of TEOS and GTS silanes followed by reaction with CS-s. To obtain column III, all the reagents, including TEOS, GTS, and CS-s were reacted together in a bare capillary. The SEM images showed that the column I phase consisted of two distinct layers, GTS and TEOS sol–gel films, while column II and III phases were homogeneous phases. By elemental analysis, the chitosan contents of the columns were found to decrease in the order column I > II > III, which corresponded to the order of the electroosmotic mobility values obtained from the measurements of the electroosmotic flow in the columns. The retention factor and the selectivity for the chiral separation of phenylglycine enantiomers in the optimized Tris running buffer (100 mM, pH 7.5) also followed this decreasing order. Besides the strength of the interaction with the immobilized functional chitosan, the hydrophobicity of the column affected the resolution of enantiomeric samples. The hydrophilic alanine sample could only be resolved by column III, but the hydrophobic tryptophan and catechin enantiomers were better separated by columns I and II. A reverse-phase mechanism has been found in the separations. Furthermore, the resolution and analysis time of column I and II phases were superior to the phase simply bonded with molecular chitosan.     

Sulfated and sulfonated polysaccharide as chiral stationary phases for capillary electrochromatography and capillary electrochromatography–mass spectrometry

The applications of polysaccharide phenyl carbamate derivatives as chiral stationary phases (CSPs) for capillary electrochromatography (CEC) are often hindered by longer retention times, especially using a normal-phase (NP) eluent due to very low electroosmotic flow (EOF). Therefore, in this study, we propose an approach for the aforementioned problems by introducing two new types of negatively charged sulfate and sulfonated groups for polysaccharide CSPs. These CSPs were utilized to pack CEC columns for enantioseparation with a NP eluent. Compared to conventional cellulose tris(3,5-dimethylphenyl carbamate) or CDMPC CSPs, the sulfated CDMPC CSP (sulfur content 4.25%, w/w) shortened the analysis time up to 50% but with a significant loss of enantiomeric resolution (∼60%). On the other hand, the sulfonated CDMPC CSP (sulfur content 1.76%, w/w) not only provided fast throughput but also maintained excellent resolving power. In addition, its synthesis is much more straightforward than the sulfated one. Furthermore, we studied several stationary phase parameters (CSP loading and silica gel pore size) and mobile phase parameters (including type of mobile phase and its composition) to evaluate the throughput and enantioselectivity. Using the optimized conditions, a chiral pool containing 66 analytes was screened to evaluate the enantioselectivity under three different mobile phase modes (i.e., NP, polar organic phase (POP) and reversed-phase (RP) eluents). Among these mobile phase modes, the RP mode showed the highest success rate, whereas some degree of complementary enantioselectivity was observed with NP and POP. Finally, the feasibility of applying this CSP for CEC–MS enantioseparation using internal tapered column was evaluated with NP, POP and RP eluents. In particular, the NP-CEC–MS provided significantly enhanced sensitivity when methanol was replaced with isopropanol in the sheath liquid. Using aminoglutethimide as model chiral analyte, all three modes of CEC–MS demonstrated excellent durability as well as excellent reproducibility of retention time and enantioselectivity.     

Chiral metal–organic framework used as stationary phases for capillary electrochromatography

Metal–organic frameworks (MOFs) have received great attention as novel media in separation sciences because of their fascinating structures and unusual properties. However, to the best of our knowledge, there has been no attempt to utilize chiral MOFs as stationary phases in capillary electrochromatography (CEC). In this study, a homochiral helical MOF [Zn₂(D-Cam)₂(4,4′-bpy)]_n (D-Cam = D-(+)-camphoric acid, 4,4′-bpy = 4,4′-bipyridine) was explored as the chiral stationary phase in open tubular capillary electrochromatography (OT-CEC) for separation of chiral compounds and isomers. The MOFs coated column has been developed using a simple procedure via MOFs post-coated on the sodium silicate layer. The baseline separations of flavanone and praziquantel were achieved on the MOFs coated column with high resolution of more than 2.10. The influences of pH, organic modifier content and buffer concentration on separation were investigated. Besides, the separations of isomers (nitrophenols and ionones) were evaluated. The relative standard deviations (RSDs) for the retention time of run-to-run, day-to-day and column-to-column were 1.04%, 2.16% and 3.07%, respectively. The results demonstrated that chiral MOFs are promising for enantioseparation in CEC.     

Preparation and evaluation of tandem stationary phases for μ-HPLC and capillary electrochromatography

Tandem separation beddings were prepared within the capillary tube with a photopolymerized monolith initially formed by sol-gel technology in combination with microspheric octadecylsilane material by slurry packing. The chromatographic performance of the tandem stationary phases was evaluated in detail by varying the flow rate and the composition of the mobile phase using a self-installed capillary HPLC system. For the tandem stationary phases with a forepart monolithic length of 11 cm and 1 cm the lowest theoretical plate height for the retained component was 26 µm and 34 µm, respectively. After evaluation by capillary electrochromatography, enhanced chromatographic performance was obtained using a column with 1 cm monolithic inlet frit with a theoretical plate height up to 7.20 µm. A scanning electron micrograph with different cross-sections of the column showed that a porous network formed in the center of the capillary and a homogenous slurry packing of C₁₈ was obtained at the back part.     

Metal organic framework–organic polymer monolith stationary phases for capillary electrochromatography and nano-liquid chromatography

In this study, metal organic framework (MOF)–organic polymer monoliths prepared via a 5-min microwave-assisted polymerization of ethylene dimethacrylate (EDMA), butyl methacrylate (BMA), and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) with the addition of various weight percentages (30–60%) of porous MOF (MIL-101(Cr)) were developed as stationary phases for capillary electrochromatography (CEC) and nano-liquid chromatography (nano-LC). Powder X-ray diffraction (PXRD) patterns and nitrogen adsorption/desorption isotherms of these MOF–organic polymer monoliths showed the presence of the inherent characteristic peaks and the nano-sized pores of MIL-101(Cr), which confirmed an unaltered crystalline MIL-101(Cr) skeleton after synthesis; while energy dispersive spectrometer (EDS) and micro-FT-IR spectra suggested homogenous distribution of MIL-101(Cr) in the MIL-101(Cr)–poly(BMA–EDMA) monoliths. This hybrid MOF–polymer column demonstrated high permeability, with almost 800-fold increase compared to MOF packed column, and efficient separation of various analytes (xylene, chlorotoluene, cymene, aromatic acids, polycyclic aromatic hydrocarbons and trypsin digested BSA peptides) either in CEC or nano-LC. This work demonstrated high potentials for MOF–organic polymer monolith as stationary phase in miniaturized chromatography for the first time.     

Nano/macroporous monolithic scaffolds prepared by the sol–gel method

Development of optimal scaffolds for bone tissue engineering and regeneration is still a challenge, since many materials and structures have been proposed but few have reached clinical expectations. This work reports on the preparation and characterization of SiO₂-CaO and SiO₂-CaO-P₂O₅ sol–gel derived monoliths, with potential application as glass scaffolds for bone regeneration, exhibiting a nano/macro trimodal pore size distribution, including pores of ~100’s of micrometers (μm), several microns and just a few nanometers (nm) in size. Interconnected macropores (~20–200 μm) have been obtained in the present work by polymerization-induced spinodal phase separation along with the sol–gel transition, when a water soluble polymer [poly(ethylene oxide)] was added to the sol–gel solution; the several-micron pores are spherical and isolated and might be the result of secondary phase separation by nucleation-growth mechanism; the interconnected nanopore (~5–25 nm) structure of the macroporous gel skeleton, on the other hand, was tailored by solvent exchange procedures. The morphological and textural characterization of these materials was performed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray ultra microscopy (XuM), nitrogen adsorption and mercury intrusion porosimetry. The factors affecting the porosity exhibited by the scaffolds, such as glass composition and solvent exchange conditions, have been assessed.

Ionic liquid-mediated sol–gel coatings for capillary microextraction

Ionic liquid (IL)-mediated sol–gel hybrid organic–inorganic materials present enormous potential for effective use in analytical microextraction. This opportunity, however, has not yet been explored. One obstacle to materializing this prospect arises from high viscosity of ILs significantly slowing down sol–gel reactions. In this work, we developed a method that overcomes this hurdle and provides IL-mediated advanced sol–gel materials for capillary microextraction (CME). We examined two different ILs: (a) a phosphonium-based IL, trihexyltetradecylphosphonium tetrafluoroborate, and (b) a pyridinium-based ionic liquid, N-butyl-4-methylpyridinium tetrafluoroborate. These ILs were evaluated in conjunction with two types of hydroxy-terminated polymers: (a) two Si–OH terminated polymers (PDMS and BMPO), and (b) two C–OH terminated polymers (PEG and polyTHF) that differ in their sol–gel reactivity. Scanning electron microscopy results demonstrate that ILs can serve as porogenic agents in sol–gel reactions. The IL-mediated sol–gel coatings prepared with silanol-terminated polymers provided up to 28 times higher extractions in off-line CME-GC compared to analogous sol–gel coatings prepared without any IL in the sol solution. Contrary to this, the IL-mediated sol–gel coatings prepared with C–OH terminated polymers provided lower extraction efficiencies compared to their IL-free counterparts. These observations were explained by (a) lower sol–gel reactivity of C–OH groups in PEG and polyTHF compared to Si–OH groups in PDMS and in hydrolyzed alkoxysilane precursors and (b) extremely high viscosity of ionic liquids. This study shows that IL-generated porous morphology alone is not enough to provide effective extraction media: careful choice of the organic polymer and the precursor with close sol–gel reactivity must be made to ensure effective chemical bonding of the organic polymer to the created sol–gel material to be able to provide the desired sorbent characteristics. Additionally, IL-mediated sol–gel PDMS coatings provided run-to-run RSD values of 4.2–5.0% and detection limits ranging from 3.2 ng/L to 17.4 ng/L. PDMS sol–gels prepared without ILs provided RSD values of 2.8–14.1%, and detection limits ranging from 4.9 ng/L to 487.0 ng/L.     

Preparation of stationary phases for open tubular capillary electrochromatography on the basis of sulfated β-cyclodextrin intercalated in layered double hydroxides

The capillary column modified by sulfated β-cyclodextrin intercalated in layered double hydroxides was prepared for open tubular capillary electrochromatography and was applied to separate the racemoid of 1-phenyl-1,2-ethanediol under the optimized conditions. The article is published in the original.     

Preparation of a zeolite imidazole skeleton–silica hybrid monolithic column for amino acid analysis via capillary electrochromatography

We developed a novel, convenient and low-cost one-pot strategy for preparing a zeolitic imidazolate framework-8 (ZIF-8)–silica hybrid monolithic column by adding ZIF-8 directly to a polymer solution of the silica matrix. The simulated stationary phase and monolithic column prepared under optimal conditions were characterized using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis nitrogen physisorption and zeta potential. The results obtained confirmed the successful introduction of ZIF-8 into the silica monolithic column, and the prepared monolithic column exhibited good permeability and physicochemical stability. A capillary electrochromatography method was developed based on a ZIF-8–silica hybrid monolithic column through which 15 mixed amino acids, 4 neutral compounds, 4 nipagin esters and 2 chlorinated fungicides were separated in 14, 5, 7 and 6 min, respectively, under optimal conditions. The relative standard deviations retention times and column efficiencies in run-to-run, day-to-day and column-to-column varied in the ranges of 1.90%–2.21%, 2.13%–2.51% and 3.08%–6.65%, respectively, which demonstrated that ZIF-8–silica hybrid monolithic column exhibited satisfactory reproducibility and stability. The incorporation of ZIF-8 into a silica monolithic column is a promising method for preparing novel monolithic columns composed of a metal–organic framework.     

Sensing abilities of materials prepared by sol–gel technology

The advantages of the sol–gel technology are undoubtedly simplicity and versatility. It enables to obtain for example oxides in the form of layers, powders, monoliths or fibers. These materials can be successfully applied for sensing purposes due to their properties such as transparency, porosity, and high surface areas. In this article, the basis of operation of mainly optical and semiconductor sensors are presented. A brief overview of various kinds of sensors is submitted. The utility of optical fibers and planar waveguides in these systems is discussed. The paper contains also some results obtained by the authors in the field of thin film-based sensors.     

Mixed ligand monolithic columns for reversed-phase capillary electrochromatography via hydrophobic and π interactions

Novel mixed ligand monoliths (MLM) for capillary electrochromatography (CEC) of a wide range of solutes differing in both polarity and size were introduced. The MLM capillary columns were based on the different compositions of octadecyl acrylate (ODA) and 2-naphthyl methacrylate (NAPM) monomers in the presence of trimethylolpropane trimethacrylate (TRIM) crosslinker and a ternary porogenic solvent made up of cyclohexanol, ethylene glycol, and water. As expected, the magnitude of the electroosmotic flow (EOF) changed with the composition of the MLM. As the percent of the monomer ODA in the polymerization mixture was increased, the EOF increased to a maximum at 50-mol% ODA and then leveled off at 75-mol% and 100-mol% ODA, an indication that the ODA ligand in general exhibited a higher binding for the mobile-phase ions than the NAPM ligand. This is due to the fact that the ODA is an acrylate-based monomer, whereas the NAPM is a methacrylate-based monomer. While ODA provided solely nonpolar interactions, NAPM exhibited both nonpolar and π interactions with certain solutes. It was found out that columns with a given composition of both ligands yielded a unique selectivity for a given set of solutes that was not matched by columns made by either ODA or NAPM alone. Several test mixtures were used in the evaluation of the MLM columns including polycyclic aromatic hydrocarbons, alkyl phenyl ketones, nitroalkanes, alkylbenzenes, toluene derivatives, peptides, and proteins. Peptide mapping of the tryptic digest of the standard lysozyme protein was also studied.     

Evolution of two acid steps sol–gel phases by FTIR

FTIR has been used to follow the evolution of a sol–gel preparation accomplished with two acid/acid steps. The reaction of methyltrimethoxysilane mixed with colloidal silica was taken as an example, aiming at proving the use of IR spectroscopy for the determination of reaction aspects like the kinetic behaviour that is essential for the scale-up of the application. The steps were carried out for 60 min each and samples were taken in each step at variable time intervals. For the spectroscopy analysis the dispersion samples were deposited on a spinning disc of KBr or ZnSe obtaining thin hybrid organic/inorganic films. The signals at 950 cm⁻¹ and 1,000–1,100 cm⁻¹ related to the hydrolysis and condensation reaction, respectively were recorded at variable time and their intensity normalised to the 1,273 cm⁻¹band of Si-CH₃ group not involved in the hydrolysis nor condensation reaction. The effects of pH and temperature have been investigated showing that reliable effective data on the reaction extent of SiO₂-doped sol–gel dispersions can be obtained by the FTIR spectroscopy. The data have been supported and interpreted also by SEM observations. The reactions degree of the (poly)condensation sharply increases after the addition of more alkoxysilane. Short reaction times can be designed that could be compatible with industrial production.     

In-column “click” preparation of hydrophobic organic monolithic stationary phases for protein separation

Two types of macroporous organic polymer monoliths based on glycidyl methacrylate (GMA), 4-vinylbenzyl chloride (VBC) and divinylbenzene (DVB) were prepared inside stainless-steel tubes. Azide functionalities were firstly introduced on the surfaces of poly(GMA-co-DVB) and poly(VBC-co-DVB) monoliths to provide reactive sites for click chemistry. With the application of copper(I)-catalyzed (3 + 2) azide-alkyne cycloaddition, an in-column click-modification approach for covalent attachment of long alkyl chains onto polymer monoliths was developed. The column morphology and surface chemistry of the fabricated monolithic columns were characterized by the scanning electron microscopy, mercury intrusion porosimeter, Fourier transform infrared spectroscopy, and elemental analyses, respectively. The chromatographic performances of the “clicked” stationary phases were demonstrated with the high separation efficiency for a variety of proteins within 4 min.     

Novel β-cyclodextrin derivative functionalized polymethacrylate-based monolithic columns for enantioselective separation of ibuprofen and naproxen enantiomers in capillary electrochromatography

A novel enantioselective polymethacrylate-based monolithic column for capillary electrochromatography was prepared by ring-opening reaction of epoxy groups from poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith with a novel β-cyclodextrin derivative bearing 4-dimethylamino-1,8-naphthalimide functionalities. Conditions for the ring-opening reaction with respect to different reaction parameters were thoroughly optimized to obtain high electroosmotic flow, separation efficiency and enantioselectivity for the analytes. The nonaqueous mobile phase composition regarding acetonitrile–methanol ratio and the concentration of electrolyte were examined to manipulate the hydrophobic inclusion and anion-exchange interaction between the analytes and chiral stationary phase. It was observed that in addition to β-cyclodextrin cavity, the electrostatic interaction exhibited pronounced influence on the enantioseparation of acidic analytes. Acidic enantiomers (ibuprofen and naproxen) could be separated with separation factor (α) values up to 1.08 and a maximum separation efficiency of 86 000 plates/m could be achieved.     

Capillary electrochromatography–mass spectrometry determination of melamine and related triazine by-products using poly(divinyl benzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic stationary phases

In this study, a capillary electrochromatography (CEC) method coupled either with UV or mass spectrometric detection was developed for the detection of trace-amounts of melamine and its related by-products (ammeline, ammelide, and cyanuric acid). A series of poly(divinyl benzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic columns, which were prepared by a simple in situ polymerization with divinyl benzene (DVB), vinylbenzyl trimethylammonium chloride (VBTA) and different types of alkene monomers such as 1-octene, 1-dodecene or 1-octadecene were used as separation columns, with the poly(DVB-1-dodecene-VBTA) monolith as the optimal chromatographic material because it provided a better separation. The detection limits of four melamine derivatives were in the ranged of 0.6–2.18 mg L⁻¹ by the optimal CEC–UV mode, and were reduced from 2.2 to 19.4 μg L⁻¹ by the optimal CEC–MS mode. Finally, the proposed CEC methods successfully determined melamine contaminations (0.1 mg L⁻¹ per analyte) in several dairy products as test samples with analyte recovery range of 69–85% (intra-day) and 68–75% (inter-day), and with peak area reproducibility range of 4.3–8.6% and 8.7–15.6% for intra-day and inter-day, respectively. This is the first report for CEC separation coupled with MS detection applied in trace melamine residue analyses with a faster separation and comparable or even better detection ability than previous GC–MS, CE–MS, as well as LC–MS methods.     

Study of tri-layer antireflection coatings prepared by sol–gel method

Antireflective coatings (ARCs) on tri-layer thin film stacks were studied in this paper. Silica sols have been prepared by acid-catalyzed or base-catalyzed hydrolysis and condensation reactions of tetraethyl orthosilicate. Antireflective nanometric SiO₂/TiO₂ films are formed on both sides of the glass substrates by combining the sol–gel method and the dip-coating technique. Seen from the transmittance spectra of different films, a maximum light transmittance of 99.9% was obtained at the band of 300–800 nm. Scanning electron microscope (SEM) and atomic force microscopy (AFM) confirm the well-covered surface morphology. By the SEM observations we can see that the films are full of coverage on glass surface and containing no voids or cracks. The image root mean square roughness of the two types of ARCs provided by the AFM is 1.21 and 3.04 nm, respectively. Furthermore, a surface profiler was used to determine the thickness of each layer in the obtained multi-layer coating system.     

Evaluation of chiral separation based on bovine serum albumin–conjugated carbon nanotubes as stationary phase in capillary electrochromatography

In this work, we utilized adsorbed BSA and multiwalled carbon nanoparticles (BSA/MWCNTs) as a stationary phase in open tubular (OT) capillary for separation of chiral drugs. (3-Aminopropyl)triethoxysilane was used to assist fabrication of BSA/MWCNTs-coated OT column by covalent bonding. Incorporation of MWCNTs nanomaterials into a polymer matrix could increase the phase ratio and take advantage of the easy preparation of an open tubular CEC column. SEM was carried out to characterize the BSA/MWCNTs OT columns. The electrochromatographic performance of the OT columns was evaluated by separation of ketoprofen, ibuprofen, uniconazole, and hesperidin. The effects of MWCNTs concentration, background solution pH and concentration, and applied voltage on separation were investigated. Chiral separations of ketoprofen, ibuprofen, uniconazole, and hesperidin were achieved using the BSA/MWCNTs-coated OT column with resolutions of 24.20, 12.81, 1.50, and 1.85, respectively. Their optimas were found in the 30 mM phosphate buffers at pH 5.0, 6.5, 7.0, and 6.5, respectively. In addition, the columns demonstrated good repeatability and stability with the run-to-run, day-to-day, and batch-to-batch RSDs of migration times less than 3.5%.     

Status of sol–gel process for nuclear fuels

Sol–gel process provides an alternate route for fabrication of ceramic nuclear fuel. The sol–gel process provides several advantages over the conventional powder pellet fabrication process by eliminating handling of radioactive powders. The sol–gel process uses only fluids or fluid like materials, thus become amenable to remote handling. The sol–gel process has been developed for the production of coated particle fuels for High Temperature Gas Cooled Reactors (HTGRs), as sphere-pac fuel for Fast Breeder Reactors (FBRs) and as SGMP fuel for Thermal Reactors. Internal Gelation Process is one of the most important routes of the sol–gel process and has been accepted as the most promising process route globally. Several countries having plutonium or ²³³U based fuel program have developed sol–gel process for nuclear fuels. In India there is special interest for the development of the sol–gel process for the thorium–uranium fuels keeping in view the large resources of thorium in India. Sol–gel process for fuel fabrication is also very attractive route for closing the nuclear fuel cycle efficiently. Author is BRNS Raja Ramanna Fellow.     

Stellan Hjertén's contribution to the development of monolithic stationary phases

This overview is presented to celebrate the birthday of one of the luminaries of the separation science and my friend - Stellan Hjertén. He made significant contributions to a variety of areas in separation science such as electrophoresis, LC, and CEC to name just a few. Since the scope of his work was enormous, this review will focus only on a single aspect of his scientific activities, the design and applications of monolithic materials. During the years starting from 1989, Stellan Hjertén published many excellent papers concerning the preparation of acrylamide chemistry-based monoliths and their use in both micro-HPLC and CEC. The following text details his works in the field.