Organic polymer monoliths as stationary phases for capillary HPLC

Modern rigid porous polymer monoliths were conceived as a new class of stationary phases in classical columns in the early 1990s and later extended to the capillary format. These monolithic materials are typically prepared using a simple molding process carried out within the confines of the capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials with large through-pores that enable applications in a rapid flow-through mode. Since all the mobile phase must flow through the monolith, convection considerably accelerates mass transport within the monolithic separation medium and improves the separations. As a result, monolithic columns perform well even at very high flow rates. Various mechanisms including thermally and UV initiated free radical polymerization as well as ring opening metathesis copolymerizations were demonstrated for the preparation of monolithic capillary columns. The versatility of these preparation techniques was demonstrated by their use with hydrophobic (styrene, divinylbenzene, butyl methacrylate, ethylene dimethacrylate), hydrophilic (2-hydroxyethyl methacrylate, methacrylamide, methylenebisacrylamide), ionizable (vinylsulfonic acid, 2-acrylamido-2-methyl-propanesulfonic acid), and tailor-made (norborn-2-ene, 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene) monomers. Variation of polymerization conditions enables control of the porous properties of the monolith over a broad range and mediates the hydrodynamic properties of the monolithic columns. The applications of polymer-based monolithic capillary columns are demonstrated for numerous separations in the microHPLC mode.     

Less common applications of monoliths: I. Microscale protein mapping with proteolytic enzymes immobilized on monolithic supports

This review summarizes the recent contributions to the rapidly growing area of immobilized enzymes employing both silica and synthetic polymer-based monoliths as supports. Focus is mainly on immobilized proteolytic enzyme reactors designed for studies in proteomics. Porous monoliths emerged first as a new class of stationary phases for HPLC in the early 1990s. Soon thereafter, they were also used as supports for immobilization of proteins and preparation of both stationary phases for bioaffinity chromatography and enzymatic reactors. Organic polymer-based monoliths are typically prepared using a simple molding process carried out within the confines of a "mold" such as chromatographic column or capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials. In contrast, silica-based monoliths are first formed as a rigid rod from tetraalkoxysilane in the presence of PEG and subsequently encased with a plastic tube. Both types of monolith feature large through-pores that enable a rapid flow-through. Since all the solutions must flow through the monolith, the convection considerably accelerates mass transfer within the monolith. As a result, reactors including enzyme immobilized on monolithic support exhibit much higher activity compared to the reactions in solution.     

Preparation and HPLC applications of rigid macroporous organic polymer monoliths

Rigid porous polymer monoliths are a new class of materials that emerged in the early 1990s. These monolithic materials are typically prepared using a simple molding process carried out within the confines of a closed mold. For example, polymerization of a mixture comprising monomers, free-radical initiator, and porogenic solvent affords macroporous materials with large through-pores that enable applications in a rapid flow-through mode. The versatility of the preparation technique is demonstrated by its use with hydrophobic, hydrophilic, ionizable, and zwitterionic monomers. Several system variables can be used to control the porous properties of the monolith over a broad range and to mediate the hydrodynamic properties of the monolithic devices. A variety of methods such as direct copolymerization of functional monomers, chemical modification of reactive groups, and grafting of pore surface with selected polymer chains is available for the control of surface chemistry. Since all the mobile phase must flow through the monolith, the convection considerably accelerates mass transport within the molded material, and the monolithic devices perform well, even at very high flow rates. The applications of polymeric monolithic materials are demonstrated mostly on the separations in the HPLC mode, although CEC, gas chromatography, enzyme immobilization, molecular recognition, advanced detection systems, and microfluidic devices are also mentioned.     

Synthesis of zirconia monoliths for chromatographic separations

The aim of this work is to join the advantages of two different kinds of stationary phases: monolithic columns and zirconia-based supports. On the one hand, silica monolithic columns allow a higher efficiency with a lower back-pressure than traditional packed columns. On the other hand, chromatographic stationary phases based on zirconia have a higher thermal and chemical stability and specific surface properties. Combining these advantages, a zirconia monolith with a macroporous framework could be a real improvement in separation sciences. Two main strategies can be used in order to obtain a zirconia surface on a monolithic skeleton: coating or direct synthesis. The coverage by a zirconia layer of the surface of a silica-based monolith can be performed using the chemical properties of the silanol surface groups. We realized this coverage using zirconium alkoxide and we further grafted n-dodecyl groups using phosphate derivatives. Any loss of efficiency was observed and fast separations have been achieved. The main advance reported in this paper is related to the preparation of zirconia monoliths by a sol-gel process starting from zirconium alkoxide. The synthesis parameters (hydrolysis ratio, porogen type, precursor concentration, drying step, etc.) were defined in order to produce a macroporous zirconia monoliths usable in separation techniques. We produced various homogeneous structures: zirconia rod 2 cm long with a diameter of 2.3 mm, and zirconia monolith inside fused silica capillaries with a 75 microm I.D. These monoliths have a skeleton size of 2 microm and have an average through pore size of 6 microm. Several separations have been reported.     

大孔型疏水整体柱的制备及其表征

利用液体致孔剂正庚烷,将双甲基丙烯酸乙二醇酯(EDMA)和甲基丙烯酸缩水甘油酯(GMA)聚合成含有环氧基团的大孔型整体柱,用正丁胺修饰制备成疏水性整体柱。压汞法分析表明,疏水整体柱的孔隙率为60.2%,500nm以上的大孔占疏水整体柱孔隙率的65.7%。在2890cm/h的高流速下,疏水整体柱的背压只有10.9MPa。以牛血清白蛋白(BSA)为研究对象,动态吸附容量为14.4mg/g介质,在一定范围内,分离效果不受操作流速的影响。此外,在1445cm/h的流速下,3min内即可对细胞色素C、RNase A、溶菌酶和鸡卵清蛋白进行基线分离。结果表明,大孔型疏水整体柱可用于蛋白质的快速分离和分析。     

Preparation and characterization of imprinted monolith with metal ion as pivot

This report provided the first example of using pivot concept to prepare monolithic molecularly imprinted polymers (MIPs) with ketoprofen (KET) imprints, in which metal ions were employed as mediator between the functional monomer and the template to achieve higher fidelity of imprint. To solve metal ions in pre-polymerization system, a new ternary porogen of dimethyl sulfoxide-toluene-isooctane was developed for preparation of MIP monoliths with high porosity and good permeability. The effect of polymerization parameters such as the nature of metal ions, the ratio of template to metal ion and the degree of crosslinking, on the permeability, morphology and affinity of the metal ion mediated MIP monolith were studied. The experiments demonstrated that Ni(2+), Co(2+) and Zn(2+) can be applied as pivot to prepare KET-imprinted monolith. Relative to monolithic MIP without metal ions, all the ion-mediated macropore MIP monoliths showed enhanced permeability, capacity factor and selectivity factor. High permeability (1.06×10(-7)mm(2)) was obtained on the Co(2+)-mediated MIP monolith and great selectivity factor (3.84) was achieved on the Ni(2+)-mediated one. The stoichiometric displacement model was constructed to investigate the recognition mechanism of metal-ion mediated MIP. The results indicate that metal ion as pivot not only improves the affinity but also allows the fine-tuning on the macroporous structure of MIP monolith.     

Preparation and applications of hybrid organic-inorganic monoliths: a review

This review presents an overview of the properties of hybrid organic-inorganic monolithic materials and summarizes the recent developments in the preparation and applications of these hybrid monolithic materials. Hybrid monolithic materials with porosities, surface functionalities, and fast dynamic transport have developed rapidly, and have been used in a wide range of applications owing to the low cost, good stability, and excellent performance. Basically, these materials can be divided into two major types according to the chemical composition: hybrid silica-based monolith (HSM) and hybrid polymer-based monolith (HPM). Compared to the HPM, HSM monolith has been attracting most wide attentions, and it is commonly synthesized by the sol-gel process. The conventional preparation procedures of two type's hybrid organic-inorganic monoliths are addressed. Applications of hybrid organic-inorganic monoliths in optical devices, capillary microextraction (CME), capillary electrochromatography (CEC), high performance liquid chromatography (HPLC), and chiral separation are also reviewed.     

Silver‐coated monolithic columns for separation in radiopharmaceutical applications

In this study, we demonstrate the preparation of a macroporous monolithic column containing anchored silver nanoparticles and its use for the elimination of excess radioiodine from the radiolabeled pharmaceutical. The poly(glycidyl methacrylate‐co‐ethylene dimethacrylate) monolith was first functionalized with cystamine and the free thiol groups liberated by reaction with borohydride. In‐house‐prepared silver nanoparticles were then attached by interaction with the surface thiols. The deiodization process was demonstrated with the commonly used radiopharmaceutical m‐iodobenzylguanidine labeled with radionuclide iodine‐125.     

有机-硅胶杂化整体柱的制备及应用研究进展

有机-硅胶杂化整体柱结合了有机聚合物整体柱和硅胶整体柱的优势, 具有制备简单、机械强度高和通透性好等优点, 近年来备受关注. 有机-硅胶杂化整体柱的制备方法主要有常规溶胶-凝胶法、“一锅法”和其它聚合方法. 目前, 杂化整体柱已被广泛应用于微纳尺度分离分析、样品预处理和固定化酶反应器基质中. 本文综述了有机-硅胶杂化整体柱的制备方法及应用研究进展, 并展望了其今后的发展前景.     

Porous Monoliths: Stationary Phases of Choice for High Performance Liquid Chromatography in Various Formats

 Modern porous monoliths have been conceived as a new class of stationary phases for high performance liquid chromatography (HPLC) in classical columns in the early 1990s and later extended to the capillary format. These monolithic materials are prepared using simple processes carried out in an external mold (inorganic monoliths) or within the confines of the column (organic monoliths and all capillary columns). These methods afford macroporous materials with large through-pores that enable applications in a rapid flow-through mode. Since all the mobile phase must flow through the monolith, the convection considerably accelerates mass transport within the monolithic separation medium and improves the separations. As a result, the monolithic columns perform well even at very high flow rates. The applications of monolithic capillary columns are demonstrated on numerous separations in the HPLC mode.     

One‐pot synthesis of a new high vinyl content hybrid silica monolith dedicated to nanoliquid chromatography

A new vinyltrimethoxysilane‐based hybrid silica monolith was developed and used as a reversed‐phase capillary column. The synthesis of this rich vinyl hybrid macroporous monolith, by cocondensation of vinyltrimethoxysilane with tetramethoxysilane, was investigated using an unconventional (formamide, nitric acid) porogen/catalyst system. A macroporous hybrid silica monolith with 80% in mass of vinyltrimethoxysilane in the feeding silane solution was obtained and compared to a more conventional low vinyl content hybrid monolith with only of 20% vinyltrimethoxysilane. Monoliths were characterized by scanning electron microscopy, ²⁹Si nuclear magnetic resonance spectroscopy and N₂ adsorption–desorption. About 80% of the vinyl precursor was incorporated in the final materials, leading to 15.9 and 61.5% of Si atoms bonded to vinyl groups for 20% vinyltrimethoxysilane and 80% vinyltrimethoxysilane, respectively. The 80% vinyltrimethoxysilane monolith presents a lower surface area than 20% vinyltrimethoxysilane (159 versus 551 m²/g), which is nevertheless compensated by a higher vinyl surface density. Chromatographic properties were evaluated in reversed‐phase mode. Plots of ln(k) versus percentage of organic modifier were used to assess the reversed‐phase mechanism. Its high content of organic groups leads to high retention properties. Column efficiencies of 170 000 plates/m were measured for this 80% vinyltrimethoxysilane hybrid silica monolith. Long capillary monolithic columns (90 cm) were successfully synthesized (N = 120 000).     

银纳米颗粒负载阶层多孔二氧化硅块体的制备及表征

以溶胶-凝胶伴随相分离法制备的阶层多孔二氧化硅作为载体,3-氨丙基三乙氧基硅烷(APTES)为改性剂,乙醇为还原剂,在阶层多孔二氧化硅固体骨架上进行银纳米颗粒均匀负载.利用扫描电镜(SEM)、透射电镜(TEM)、X射线衍射(XRD)、汞压、N2吸附/脱附、X射线光电子能谱(XPS)等测试技术对银纳米颗粒负载阶层多孔二氧化硅进行了表征,探讨了APTES表面改性、乙醇还原机理以及银纳米颗粒负载块体的孔结构特征变化规律.结果表明:APTES表面改性将氨基接枝于阶层骨架上,氨基与银离子形成银氨离子,银氨离子经乙醇还原后将平均粒径约16 nm的银纳米颗粒成功负载于二氧化硅的大孔及介孔内部;负载后的阶层多孔块体的大孔骨架未受到破坏,但其比表面积由418 m2·g-1下降到254m2·g-1,两次还原负载能提高银纳米颗粒的负载量.     

银纳米颗粒负载阶层多孔二氧化硅块体的制备及表征

以溶胶-凝胶伴随相分离法制备的阶层多孔二氧化硅作为载体,3-氨丙基三乙氧基硅烷（APTES）为改性剂,乙醇为还原剂,在阶层多孔二氧化硅固体骨架上进行银纳米颗粒均匀负载. 利用扫描电镜（SEM）、透射电镜（TEM）、X射线衍射（XRD）、汞压、N₂吸附/脱附、X射线光电子能谱（XPS）等测试技术对银纳米颗粒负载阶层多孔二氧化硅进行了表征,探讨了APTES表面改性、乙醇还原机理以及银纳米颗粒负载块体的孔结构特征变化规律. 结果表明：APTES表面改性将氨基接枝于阶层骨架上,氨基与银离子形成银氨离子,银氨离子经乙醇还原后将平均粒径约16 nm的银纳米颗粒成功负载于二氧化硅的大孔及介孔内部;负载后的阶层多孔块体的大孔骨架未受到破坏,但其比表面积由418 m²·g^-1下降到254 m²·g^-1,两次还原负载能提高银纳米颗粒的负载量.     

Preparation and evaluation of a macroporous molecularly imprinted hybrid silica monolithic column for recognition of proteins by high performance liquid chromatography

A novel type of macroporous molecularly imprinted hybrid silica monolithic column was first developed for recognition of proteins. The macroporous silica-based monolithic skeleton was synthesized in a 4.6 mm i.d. stainless steel column by a mild sol–gel process with methyltrimethoxysilane (MTMS) as a sole precursor, and then vinyl groups were introduced onto the surface of the silica skeleton by chemical modification of γ-methacryloxypropyltrimethoxysilane (γ-MAPS). Subsequently, the molecularly imprinted polymer (MIP) coating was copolymerized and anchored onto the surface of the silica monolith. Bovine serum albumin (BSA) and lysozyme (Lyz), which differ greatly in molecular size, isoelectric point, and charge, were representatively selected for imprinted templates to evaluate recognition property of the hybrid silica-based MIP monolith. Some important factors, such as template–monomer molar ratio, total monomer concentration and crosslinking density, were systematically investigated. Under the optimum conditions, the obtained hybrid silica-based MIP monolith showed higher binding affinity for template than its corresponding non-imprinted (NIP) monolith. The imprinted factor (IF) for BSA and Lyz reached 9.07 and 6.52, respectively. Moreover, the hybrid silica-based MIP monolith displayed favorable binding characteristics for template over competitive protein. Compared with the imprinted silica beads for stationary phase and in situ organic polymer-based hydrogel MIP monolith, the hybrid silica MIP monolith exhibited higher recognition, stability and lifetime.     

Capillary electrochromatography with monolithic stationary phases. 4. Preparation of neutral stearyl-acrylate monoliths and their evaluation in capillary electrochromatography of neutral and charged small species as well as peptides and proteins

A neutral, nonpolar monolithic capillary column having a relatively strong electroosmotic flow (EOF) yet free of electrostatic interactions with charged solutes was developed for the reversed-phase capillary electrochromatography (RP-CEC) of neutral and charged species including peptides and proteins. The neutral nonpolar monolith is based on the in situ polymerization of pentaerythritol diacrylate monostearate (PEDAS) in a ternary porogenic solvent composed of cyclohexanol, ethylene glycol, and water. PEDAS plays the role of both the cross-linker and the ligand provider, generating a macroporous nonpolar monolith having C17 chains as the chromatographic ligands. Despite the fact that the neutral PEDAS monolith is devoid of fixed charges, the monolithic capillary columns exhibited a relatively strong EOF due to the ability of PEDAS to adsorb sufficient amounts of electrolyte ions from the mobile phase. The adsorbed ions imparted the neutral PEDAS monolith the zeta potential necessary to support the EOF required for mass transport across the monolithic column. The absence of fixed charges on the surface of the neutral PEDAS monolith and in turn the adsorption sites for electrostatic attraction of charged solutes allowed the rapid and efficient separations of proteins and peptides at pH 7.0, with an average plate number of 255,000 and 121,000 plates/m, respectively. To the best of our knowledge, this constitutes the first report on the separation of proteins at neutral pH by RP-CEC using a neutral monolithic column.     

Comparison of different types of outlet frits in slurry‐packed capillary columns

Fused‐silica capillary columns for high‐performance liquid chromatography with 320 and 250 μm inner diameter were prepared by slurry packing with 5 and 3 μm Nucleosil C₁₈ stationary phase. Different types of mechanical and monolithic outlet frits were used and their influence on the resulting column performance was evaluated. Columns with quartz wool exhibited symmetrical peaks and low theoretical plate height, and the preparation time was short. The performance of monolithic frits varied based on type of monolith, length of the frit, and silanization procedure. The best frit performed similarly to the quartz wool ones, but the preparation took several hours. Their main advantage lies in the possibility of on‐column detection, because the detection window can be burnt immediately behind the frit.     

Ti (IV) attached‐phosphonic acid functionalized capillary monolith as a stationary phase for in‐syringe‐type fast and robust enrichment of phosphopeptides

In this study, poly(vinylphosphonic acid‐co‐ethylene dimethacrylate), poly(VPA‐co‐EDMA) capillary monolith was synthesized as a starting material for obtaining a stationary phase for microscale enrichment of phosphopeptides. The chelation of active phosphonate groups with Ti (IV) ions gave a macroporous monolithic column with a mean pore size of 5.4 μm. The phosphopeptides from different sources were enriched on Ti (IV)‐attached poly(VPA‐co‐EDMA) monolith using a syringe‐pump. The monolithic capillary columns exhibited highly sensitive/selective enrichment performance with phosphoprotein concentrations as low as 1.0 fmol/mL. Six different phosphopeptides were detected with high intensity by the treatment of β‐casein digest with the concentration of 1.0 fmol/mL, using Ti (IV)@poly(VPA‐co‐EDMA) monolith. Highly selective enrichment of phosphopeptides was also successfully carried out even at trace amounts, in a complex mixture of digested proteins (molar ratio of β‐casein to bovine serum albumin, 1:1500) and three phosphopeptides were successfully detected. Four highly intense signals of phosphopeptides in human serum were also observed with high signal‐to‐noise ratio and a clear background after enrichment with Ti (IV)@poly(VPA‐co‐EDMA) monolith. It was concluded that the capillary microextraction system enabled fast, efficient and robust enrichment of phosphopeptides from microscale complex samples. The whole enrichment process was completed within 20 min, which was shorter than in the previously reported studies.     

制备方法对CuO-CeO2/Al2O3/FeCrAl整体催化剂结构、粘附稳定性和催化性能的影响

 采用浸渍法、粉末涂覆法、沉积沉淀浸渍法、溶胶高温分解法、原位溶液燃烧法和微乳液法制备了 CuO-CeO2/Al2O3/FeCrAl 整体催化剂, 并运用扫描电镜、X 射线衍射、程序升温还原、超声波振动和热振荡等手段研究了制备方法对活性组分的负载及其分布、催化剂结构、粘附稳定性和催化 CO 优先氧化反应性能的影响. 结果表明, 溶胶高温分解法、原位溶液燃烧法和微乳液法制备的整体催化剂表现出较好的催化 CO 优先氧化反应性能和较高的粘附稳定性.     

Effect of the nature of a porogen on the porous structure of monolithic polydivinylbenzene sorbents

In quartz capillaries, macroporous monolithic sorbents based on divinylbenzene are synthesized and their porous structure is studied via inverse size-exclusion-hydrodynamic chromatography. Either a single-component porogen (a higher alcohol) or a two-component porogen (a mixture of a higher alcohol and mesitylene) is used for the synthesis of monoliths. The removal of a solvent good for a polymer from a porogen results in an increase in the size of flow-through channels and a decrease in the free-space volume inside the monolith; this space is used for the separation of polymer sorbates (the working volume of a column). At the same time, the volume of micro- and mesopores in the monolith structure is practically independent of the content of the good solvent in the porogen. It is inferred that the good porogen plays an active role in formation of the macroporous structure of monoliths. The structure of monoliths obtained on the basis of the two-component porogen with the use of nonanol and mesitylene or toluene is optimum for the molecular-mass analysis of polymers.     

Recent Advances of Polymer Monolithic Columns Functionalized with Micro/Nanomaterials: Synthesis and Application

The field of separation science has recently witnessed an explosion of interest and progress in the design and study of porous polymer monolithic materials. Monolithic columns with their unique structure possess some exceptional characteristics, which make them an excellent tool in the hands of analytical chemists, not only for separation but also for sample pretreatment. As a new member of the polymer monolith family, the micro/nanomaterial-functionalized polymer monolith has attracted considerable attention due to its many distinct characteristics, such as high permeability and selectively tailored surface chemistries. It exhibits great potential in separation science and analytical sample preparation. This review summarizes and highlights recent major advances of the micro/nanomaterial-functionalized polymer monolith, focusing on design considerations and the application of separation and enrichment. A brief overview of the properties of polymer monolithic columns is included, and then specific attention is paid to discuss the methods of fabrication and application of the micro/nanomaterial-functionalized polymer monolith in separation, sample pretreatment and enrichment, and highly sensitive detection. Finally, future possible research directions and challenges in the field are discussed.