Preparation of porous hydrophilic monoliths: Effect of the polymerization conditions on the porous properties of poly (acrylamide-co-N,N′-methylenebisacrylamide) monolithic rods

Molded macroporous monoliths with pores sizes up to 1000 nm have been prepared by copolymerization of the hydrophilic monomers, acrylamide, and N,N′-methylenebisacrylamide, in the presence of a porogenic diluent. A combination of dimethylsulfoxide and 2-heptanol was selected from a broad spectrum of solvents and water soluble polymers to achieve the optimum composition of the porogenic mixture. In addition to the composition of the porogen the porous properties of the monolithic rods can also be optimized through changes in the percentage of both N,N′-methylene-bisacrylamide (crosslinking monomer) and azobisisobutyronitrile (free radical initiator) used for the polymerization. The hydrophilic monoliths may be used in the separation of biological polymers, solid-phase extraction, or for immobilization of proteins. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 1013–1021, 1997     

Sponge-nested polymer monoliths: Versatile materials for the solid-phase extraction of bisphenols

Polymer monoliths are promising materials for sample preparation due to their high porosity, pH stability, and simple preparation. The use of melamine formaldehyde foams has been reported as an effective support to prepare highly robust silica and polymer monoliths. Herein, divinylbenzene monoliths based on a 50:50 (%, w/w) crosslinker/porogen ratio have been nested within a melamine-formaldehyde sponge, resulting in monoliths with a surface area higher than 400 m²/g. The extraction performance of these monoliths was evaluated for the extraction of endocrine-disrupting bisphenols from aqueous solutions. We evaluated for the first time the versatility of sponge-nested polymer monoliths by comparing three different extraction modes (vortex mixing, magnetic stirring, and orbital shaking). Vortex mixing showed a comparable recovery of bisphenols (39%–81%) in a shorter extraction time (30 min, instead of 2 h). In addition, the robustness of the sponge-nested polymer monoliths was demonstrated for the first time by reshaping a larger monolithic cube (0.125 cm³) into four smaller pieces (4 × 0.03125 cm³) leading to a 16%–21% increase in extraction efficiency. This effect was attributed to an increase in the effective contact area with the sample, obtaining a higher analyte extraction capacity.     

多孔整体材料在固相微萃取中的应用研究进展

作为新型的样品前处理技术,固相微萃取由于具有操作简便、使用灵活、样品用量少、环境友好以及便于与分析仪器联用等优点而受到人们的广泛青睐。多孔整体材料具有通透性好、传质速度快、制备简单和易于改性等优点,目前被广泛用于包括样品前处理在内的诸多领域。文章结合作者的研究工作,对近几年整体材料在固相微萃取中的应用研究进行综述,并对其发展方向进行了展望。     

Temperature effects on retention and efficiency of butyl and lauryl acrylate porous polymer monoliths in capillary electrochromatography

The development of organic porous polymer monoliths represents an alternative approach to stationary phase design. The use of these materials has helped to rekindle interest in capillary electrochromatography. Although a large number of investigations have explored different monolith recipes, polymerization conditions, and application challenges, few investigations have addressed the fundamentals of this separation mode with this type of material. This study addresses the thermodynamics of the reversed phase retention mechanism on 100% butyl acrylate and 1:3 butyl:lauryl acrylate (volume/volume ratio) porous polymer monoliths used in a capillary electrochromatography mode. Linear van't Hoff plots yield enthalpies of retention of ?3.9 to ?14.3 kJ/mol on two different, but related columns for five selected hydrophobic analytes across a thirty degree temperature range. Minimum plate heights were only moderately impacted over this temperature range.     

Recent strategies to enhance the performance of polymer monoliths for analytical separations

This review summarizes recent developments made in the incorporation of functional materials into organic polymer monoliths, together with new monolithic forms and formats, which enhance their application as supports and stationary phase materials for sample preparation and chromatographic separations. While polymer monoliths are well‐known supports for the separation of large molecules, recent developments have been made to improve their features for the separation of small molecules. The selectivity and performance of organic polymer monoliths has been improved by the incorporation of different materials, such as metal‐organic frameworks, covalent organic frameworks, or other types of nanostructured materials (carbon nanohorns, nanodiamonds, polyoxometalates, layered double hydroxides, or attapulgite). The surface area of polymer monoliths has been significantly increased by polymer hypercrosslinking, resulting in increased efficiency when applied to the separation of small molecules. In addition, recent exploration of less conventional supports for casting polymer monoliths, including photonic fibres and 3D printed materials, has opened new avenues for the applications of polymer monoliths in the field of separation science. Recent developments made in these topics are covered, focusing on the strategies followed by the authors to prepare the polymer monoliths and the effect of these modifications on the developed analytical applications.     

Incorporation of ionic liquid into porous polymer monoliths to enhance the separation of small molecules in reversed‐phase high‐performance liquid chromatography

An ionic liquid was incorporated into the porous polymer monoliths to afford stationary phases with enhanced chromatographic performance for small molecules in reversed‐phase high‐performance liquid chromatography. The effect of the ionic liquid in the polymerization mixture on the performance of the monoliths was studied in detail. While monoliths without ionic liquid exhibited poor resolution and low efficiency, the addition of ionic liquid to the polymerization mixture provides highly increased resolution and high efficiency. The chromatographic performances of the monoliths were demonstrated by the separations of various small molecules including aromatic hydrocarbons, isomers, and homologues using a binary polar mobile phase. The present column efficiency reached 27 000 plates/m, which showed that the ionic liquid monoliths are alternative stationary phases in the separation of small molecules by high‐performance liquid chromatography     

Preparation of new dioxygen-active triphenylantimony(V) catecholate-containing porous polymer

Novel porous polymers with 4-ethoxy-3,6-di-tert-butyl-o-benzoquinone grafted to the porous surface have been prepared via the secondary functionalization of porous polymer monoliths obtained by the photopolymerization of oligo(carbonate dimethacrylate) and hydroxyethyl methacrylate in methanol solution. These quinone-functionalized polymers have been applied for the synthesis of triphenylantimony(V) catecholate-containing polymers by the oxidative addition reaction of quinone moieties with SbPh₃. The obtained antimony-containing porous polymeric material is able to reversibly bind molecular oxygen in nearly quantitative yield. The rate of molecular oxygen sorption is 900 times higher for the porous material than that for the same film material and comparable with the process rate in solution. Copyright © 2016 John Wiley & Sons, Ltd.     

High temperature liquid chromatography of intact proteins using organic polymer monoliths and alternative solvent systems

Alternative approaches to conventional acetonitrile gradient methods for reversed-phase liquid chromatographic analysis of intact proteins have been investigated using commercial poly(styrene-co-divinylbenzene) monolithic columns (Dionex ProSwift™ RP-2H and RP-4H). Alternative solvents to acetonitrile (2-propanol and methanol) coupled with elevated temperatures demonstrated complementary approaches to adjusting separation selectivity and reducing organic solvent consumption. Measurements of peak area at increasing isothermal temperature intervals indicated that only minor (<5%) decreases in detectable protein recovery occurred between 40 and 100 °C on the timescale of separation (2–5 min). The reduced viscosity of a 2-propanol/water eluent at elevated temperatures permitted coupling of three columns to increase peak production (peaks/min) by 16.5%. Finally, narrow-bore (1 mm i.d.) columns were found to provide a more suitable avenue to fast, high temperature (up to 140 °C) separations.     

The effect of the pore structure and zeta potential of porous polymer monoliths on separation performance in ion-exchange mode

Most often, in bioseparations involving charged macromolecules, the chromatographic systems have low Reynolds and high Peclet numbers. For such systems, an expression is developed and presented in this work for evaluating the throughput in polymeric monoliths where ion-exchange adsorption occurs, as a function of (i) the pressure drop along the length of the monolith, (ii) the functional form and width of the throughpore-size distribution of the monolith, and (iii) the magnitude of the zeta potential on the surface of the throughpores of the monolith. Gaussian and log-normal throughpore-size distributions whose mean throughpore-size and standard deviation values are based on experimentally measured throughpore-size distribution data by mercury porosimetry employed on polymeric monoliths are used in this work, and their effect on the throughput relative to that obtained from a polymeric monolith having a uniform throughpore-size distribution is studied for different values of the ratio of the standard deviation to the mean throughpore-size. The results indicate that relatively modest increases in the throughput, when compared with the throughput that could be achieved in a polymeric monolith having a uniform throughpore-size distribution, could be obtained in polymeric monoliths having disperse throughpore-size distributions, and the magnitude of the increase becomes larger when the disperse distribution is skewed to larger throughpore sizes. Furthermore, the results of this work indicate that, under certain conditions, relatively modest increases in the throughput of a charged analyte could also be achieved by altering the value of the zeta potential on the surface of the throughpores of the monolith. Due to the difficulties inherent in controlling the functional form and width of the throughpore-size distribution during the synthesis of polymeric monoliths, it would appear to be more practical to increase the value of the throughput of a charged analyte by altering the value of the zeta potential through prudent selection of the ion-exchange surface functional groups and fine-tuned with the pH of the mobile phase. Thus, for ion-exchange chromatography systems, the zeta potential could be considered an important parameter for column designers and operators to manipulate, since its alteration could increase the through-put of a charged analyte in polymeric monoliths or in columns packed with charged particles.     

Advances in the preparation of porous polymer monoliths in capillaries and microfluidic chips with focus on morphological aspects

Porous polymer monoliths have emerged as unique materials for many applications, including liquid-chromatographic analyses at an unrivaled speed, solid-phase extraction, and enzyme immobilization in capillary and microfluidic chip format. This article reviews the state of the art in the preparation of monoliths in narrow-bore capillaries and microfluidic chips and their miniaturization under conditions of spatial confinement. New developments in their preparation mainly using free radical polymerization techniques with a focus on morphological aspects in view of homogeneous porous materials are described. The suitability of monoliths for analysis of both large and small molecules is also discussed.     

Preparation and characterization of macroporous monoliths imprinted with erythromycin

The synthesis of macroporous molecularly imprinted monoliths was performed using the monomers system 2‐hydroxyethyl methacrylate‐ethylene glycol dimethacrylate and erythromycin as a template. The copolymerization was carried out in situ inside 50 mm × 4.6 mm i.d. stainless‐steel tubing. The morphology of the monoliths was examined with scanning electron microscopy. The porous characteristics were determined both from the data of hydrodynamic permeability of monoliths and by means of mercury intrusion porosimetry. The retention parameters of target substance (erythromycin), values of calculated imprinting factors and apparent dynamic dissociation constants were obtained for monoliths prepared with the application of different amount of template (4, 8 and 12 mol%). The separations of the mixtures azithromycin/erythromycin and ciprofloxacin/erythromycin were demonstrated. Additionally, the possibility of erythromycin quantification in human blood plasma was shown.     

Optimization of binary porogen solvent composition for preparation of butyl methacrylate monoliths in capillary liquid chromatography

Butyl methacrylate monolithic columns in 320 microm i.d. fused silica capillaries for reversed-phase capillary liquid chromatography were prepared by radical polymerization initiated thermally with azobisisobutyronitrile (AIBN). Polymerization mixture contained butyl methacrylate (BMA) as the function monomer and ethylene dimethacrylate (EDMA) as the crosslinking agent with 1,4-butanediol and 1-propanol as a binary porogen solvent. Ratio of 1,4-butanediol to 1-propanol in the porogen solvent was optimized regarding the monolithic column efficiency and performance. Total porosity, column permeability, separation impedance, Walters hydrophobicity index, retention factors, peak asymmetry factors, height equivalents to a theoretical plate and peak resolutions were used for characterization of the prepared monolithic columns. The polymerization mixture consisting of 17.8% of BMA, 21.8% of EDMA, 18.0% of 1,4-butanediol, 42.0% of 1-propanol and 0.4% AIBN generated monolithic columns of the best performance having a sufficient permeability and the lowest separation impedance. It was also demonstrated that monolithic columns of this composition exhibited good preparation reproducibility and an excellent pressure resistance when applied in capillary liquid chromatography.     

Fine control of the porous structure and chromatographic properties of monodisperse macroporous poly(styrene-co-divinylbenzene) beads prepared using polymer porogens

The porous structure of monodisperse macroporous beads can be controlled by using soluble polymers with well-defined structural characteristics as part of the porogenic mixture. In general, the use of linear polystyrene as a porogen in the preparation of poly (styrene-co-divinylbenzene) beads shifts the pore size distribution towards larger pores. While a direct correlation between pore size and molecular weight of the porogen has been established, the chemical composition of the polymer porogen has no effect on the porous and chromatographic properties of the beads. These findings suggest that the average molar volume of the porogenic system is important while the miscibility of the polymer porogen with the crosslinked polymer that is formed is of little relevance. © 1994 John Wiley & Sons, Inc.     

Reversed-phase electrochromatography of amino acids and peptides using porous polymer monoliths

Efficient harvest and recovery of high-purity monoclonal antibodies was achieved using hydrophobic charge induction chromatography (HCIC). Both simple and complex feedstocks were studied, including protein-free cell culture supernatant and the clarified/concentrated milk of transgenic goats. Viral clearance studies demonstrated a 4-log reduction of MVM virus (minute virus of mice), along with substantial reduction of DNA content. Sorbent characterization studies confirmed that HCIC is based on the pH-dependent behavior of a dual-mode, ionizable ligand. Binding, based on hydrophobic interaction, was achieved under near-physiological conditions, and in the absence of lyotropic salt. Desorption was accomplished under mild conditions--pH 4.0. At this pH, both ligand and antibody carry a net positive charge, and desorption occurs on the basis of electrostatic charge repulsion. pH-based control of chromatographic function was demonstrated. Chromatography on this antibody-selective HCIC sorbent was evaluated as a cost-effective, process-compatible alternative to affinity chromatography protein A sorbents.     

Enzymatic activity of oxalate oxidase and kinetic measurements by optical methods in transparent sol-gel monoliths

Novel synthetic techniques are used for the encapsulation of the enzymes oxalate oxidase and peroxidase in stable, optically transparent porous silica glass matrices. The large enzymes are fully immobilized in the porous glass but small molecules such as oxalate ions pass readily through the pores in the glass. The enzymes catalyze the reactions leading to the formation of a colored dye product. Upon exposure of the doped glass to oxalate solutions, a colored glass is formed. The absorption spectrum of the colored product and the changes of absorbance with time are measured within the glass matrix. The sensitivity and the time-dependence of the response are discussed.     

Development of an online solid‐phase extraction with liquid chromatography method based on polymer monoliths for the determination of dopamine

Porous polymer monoliths have been used to develop an online solid‐phase extraction with liquid chromatography method for determination of dopamine in urine as well as for a continuous monitoring of dopamine in flowing system. A polymerization mixture containing 4‐vinylphenylboronic acid monomer has been used to prepare a trapping column based on specific ring formation reaction with dopamine cis‐diol functionality. Additionally, a monolithic stationary phase with zwitterion functionality has been used to prepare capillary column for the separation of dopamine. Experimental conditions including molarity, pH, and flow rate of the loading buffer together with a valve switching time have been optimized to provide the highest recovery for dopamine. Experimental setup has been used to determine dopamine in a urine. By using both calibration curve and standard addition method, the dopamine level was determined to be 1.19 and 1.28 mg/L, respectively. Further, we have used experimental design to optimize coupling of two extraction monolithic loops to separation capillary column with monolithic phase for a comprehensive monitoring of dopamine. After multivariate analysis, sample loading flow‐rate and a flow‐rate of flushing buffer were selected as the most significant variables. Optimized experimental setup was applied to continuously monitor dopamine degradation.     

Fabrication of porous polymer monoliths covalently attached to the walls of channels in plastic microdevices

UV-initiated grafting of plastic tubes and microfluidic chips with ethylene diacrylate followed by the preparation of porous polymer monoliths has been studied. The first step affords a thin grafted layer of polymer with a multiplicity of pendent double bonds that are then used in the second step for covalent attachment of the monolith to the wall. As clearly seen on scanning electron micrographs, this procedure prevents the formation of voids at the monolith-channel interface a problem that has always plagued approaches involving bulk polymerization in nontreated channels due to the shrinkage of the monolith during the polymerization process and its lack of compatibility with the material of the device. Irradiation with UV light through a photomask allows precise patterning specifying both the area subjected to surface modification and the location of the monolith within specific areas of the device.     

Towards porous polymer monoliths for the efficient, retention-independent performance in the isocratic separation of small molecules by means of nano-liquid chromatography

We have investigated the free-radical copolymerization dynamics of styrene and divinylbenzene in the presence of micro- and macro-porogenic diluents in 100 μm I.D. sized molds under conditions of slow thermal initiation leading to (macro)porous poly(styrene-co-divinylbenzene) monolithic scaffolds. These specifically designed experiments allowed the quantitative determination of monomer specific conversion against polymerization time to derive the porous polymer scaffold composition at each desirable copolymerization stage after phase separation. This was carried out over a time scale of 3h up to 48 h polymerization time, enabling the efficient and repeatable termination of the polymerization reactions. In parallel, the porous and hydrodynamic properties of the derived monolithic columns were thoroughly studied in isocratic nano-LC mode for the reversed-phase separation of a homologous series of small retained molecules. At the optimized initiator concentration, polymerization temperature and time, the macroporous poly(styrene-co-divinylbenzene) monoliths show a permanent mesoporous pore space, which was readily observable by electron microscopy and indicated by nitrogen adsorption experiments. Under these conditions, we consistently find a polymer scaffold composition which suggests a high degree of cross-linking and thus minimum amount of gel porosity. These columns reveal a retention-insensitive plate height in the separation of small retained molecules which only slightly decreases at increased linear mobile phase velocity. As the polymerization progresses, a build-up of less-densely cross-linked material occurs, which is directly reflected in the observed consistent increase in retention and plate heights. This leads to a significant deterioration in overall isocratic separation performance. The decrease in performance is ascribed in particular to the increased mass transfer resistance governing the monoliths' performance over the whole linear chromatographic flow velocity range at polymerization times significantly higher than that of phase separation. The performance of the optimized monoliths only becomes limited by fluid dispersion due to the poorly structured macroporous pore space.     

Characterization of hybrid organo-silica monoliths for possible application in the gradient elution of peptides

We characterized thermally polymerized organo-silica hybrid monolithic capillaries to test their applicability in the gradient elution of peptides. We have used a single-pot approach utilizing 3-(methacryloyloxy)propyltrimethoxysilane (MPTMS), ethylene dimethacrylate (EDMA), and n-octadecyl methacrylate (ODM) as functional monomers. The organo-silica monolith containing MPTMS and EDMA was compared with the stationary phase prepared by adding ODM to the original polymerization mixture. Column prepared using a three-monomer system provided a lower accessible volume of flow-through pores, a higher proportion of mesopores, and higher efficiency. We utilized isocratic and gradient elution data to predict peak widths in gradient elution. Both protocols provided comparable results and can be used for peptide peak width prediction. However, applying gradient elution data for peak width prediction seems simpler. Finally, we tested the effect of gradient time on achievable peak capacity in the gradient elution of peptides with a column prepared with a three-monomer system providing a higher peak capacity. However, the performance of hybrid organo-silica monolithic stationary phases in gradient elution of peptides must be improved compared to other monolithic stationary phases. The limiting factor is column efficiency in highly aqueous mobile phases, which needs to be focused on.