Chiral Separation of Binaphthol Enantiomers on Molecularly Imprinted Polymer Monolith by Capillary Electrochromatography

A Selective Joint Determination of Salicylic Acid in Actinidia chinensis Combining a Molecularly Imprinted Monolithic Column and a Graphene Oxide Modified Electrode

A new combination between selective polymer monolith microextraction (PMME) and sensitive differential pulse voltammetry (DPV) was developed for the determination of the phytohormone salicylic acid (SA) in Actinidia chinensis. A molecularly imprinted monolithic column (MIMC) thermally in-situ polymerized in a micropipette tip by using SA as a template, 4-vinyl pyridine (4-VP) as a functional monomer and ethylene glycol dimethacrylate (EGDMA) as a cross-linker in the mixed porogen of toluene and dodecanol, was employed for the microextraction of SA. The prepared MIMC was characterized by a Fourier transform infrared spectrometer (FI-TR), scanning electron microscope (SEM) and thermo gravimetric analysis (TGA). The results confirmed the binary continuous structure of the porous network. The extracted SA was determined by DPV on a graphene oxide (GO) modified electrode. The joint conditions between MIMC and DPV were investigated practically. Under the optimum conditions, SA could be determined selectively and sensitively in a linear range from 0.1 to 60.0 μg g^-1. The limit of detection was 0.03 μg g^-1 and the recoveries were between 86.2 and 105.2%. The proposed joint method was successfully used to determine SA in Actinidia chinensis.

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

Aptamers and molecularly imprinted polymers as artificial biomimetic receptors in affinity capillary electrophoresis and electrochromatography

Artificial biomimetic receptors, such as aptamers and molecular-imprinted polymers, show antibody-like properties which are due to molecular recognition phenomena characterized by high affinity and selectivity. These binding features have made them suitable in all those application fields in which selective recognition is required. Thus, it is not surprising that they are finding applications in affinity CE as well. Recently, a variety of ACE methods have shown themselves to be suitable tools to provide a detailed quantitative characterization of the thermodynamic and kinetic aspects of binding. At the same time, affinity CE can exploit the peculiarities of these binding interactions to set up CE-based analytical tools for the separation and the determination of specific target molecules in microscale formats. This review will provide a detailed description of affinity CE methods recently reported in the literature and related to these topics.

Monolithic enantiomer-selective stationary phases for capillary electrochromatography

Monolithic materials have become a well-established format for stationary phases in the field of capillary electrochromatography. Four types of monoliths, namely particle-fixed, silica-based, polymer-based, and molecularly imprinted monoliths, have been utilized as enantiomer-selective stationary phases in CEC. This review summarizes recent developments in the area of monolithic enantiomer-selective stationary phases for CEC. The preparative procedure and the characterization of these columns are highlighted. In addition, the disadvantages and limitations of different monolithic enantiomer-selective stationary phases in CEC are briefly discussed.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Advances in chiral separation using capillary electromigration techniques

This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.

Molecularly imprinted polymers as a tool for separation in CEC

Molecularly imprinted polymers (MIPs) are synthesized in the presence of a template which results in the formation of specific recognition cavities complementary to the template in shape and chemical functionality. One of the most successful application areas of MIPs is chromatographic sorbents, which are tailor-made synthetic polymers for a given analyte. However, low efficiency of MIP columns is often observed because of slow kinetics of the template. CEC-based MIPs are thought to improve efficiency of MIP-based separation due to the enhanced flow dynamics of CEC. Another attractive feature is the miniaturized format of CEC, so that fewer templates or monomers for the molecular imprinting are consumed, a characteristic desired for 'green chemistry'. The small dimensions of a capillary demand the development of novel polymer formats that can be applied to a miniaturized system. This review discusses the various formats, i.e., the micro- or nanoparticle, the coating and the monolith, for application in CEC as well as the use in MIP syntheses and characteristics.

Ice Chromatography. Characterization of Water−Ice as a Chromatographic Stationary Phase

Water-ice has been characterized as a stationary phase for liquid chromatography. Solutes having two or more polar groups are retained on this stationary phase with THF/hexane as the mobile phase, suggesting that multipoint interactions are required for measurable solute retention. Chromatographic separation of phenols or crown ethers on water-ice is possible. The ice surface is expected to provide two different adsorption sites coming from the OH and O dangling bonds. Although the solute partition into the quasiliquid layer is also considered, the dependence of the retention times on the THF concentration implies that the interaction of solutes with the water-ice surface rather than the partition into the quasiliquid layer is responsible for solute retention. A retention model suggests that the number of adsorption sites for a crown ether depends on its ring size, whereas two sites are involved for the retention of phenols having two hydroxyl groups. Although hydroxyl groups can act as both a hydrogen bond donor and an acceptor, the interaction with the ice OH sites, which are exposed to the surroundings in comparison with the ice O sites, is more important. However, when an acyclic polyether is added to the mobile phase, its adsorption onto the water ice surface allows the creation of the O sites that phenols can approach without steric hindrance. In the presence of the polyethers adsorbed on the ice surface, the retention of phenols is enhanced, whereas crown ethers become less retained due to the competitive adsorption of the polyethers.

Chiral separation of 1,1′-bi-2-naphthol and its analogue on molecular imprinting monolithic columns by HPLC

Two molecular imprinting polymer (MIP) monolithic columns with (S)-(-)-1,1'-bi-2-naphthol and (R)-(+)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol as the templating molecules, respectively, have been prepared by in situ polymerization using 4-vinylpyridine and ethylene dimethacrylate as functional monomer and cross-linker, respectively. The columns with good flow-through properties were obtained by changing the molar ratio of the functional monomer and the template molecule. The effects of mobile-phase composition on separation of enantiomers were systematically investigated. The results indicate that hydrophobic interaction in aqueous solution and hydrogen-bonding interaction in ACN between the enantiomers and polymers could play important roles in the retention and resolution. The effects of chromatographic conditions, such as flow rate, column temperature, sample loading, on the enantioseparation were also studied. Further, these two MIP columns show a cross-reactivity.

Mechanism of molecular recognition on molecular imprinted monolith by capillary electrochromatography

The recognition mechanism of molecularly imprinted polymer (MIP) in capillary electrochromatography (CEC) is complicated since it possesses a hybrid process, which comprises the features of chromatographic retention, electrophoretic migration and molecular imprinting. For an understanding of the molecular recognition of MIP in CEC, a monolithic MIP in a capillary with 1,1'-binaphthyl-2,2'-diamine (BNA) imprinting was prepared by in situ copolymerization of imprinted molecule, methacrylic acid and ethylene glycol dimethacrylate in porogenic solvent, a mixture of toluene-isooctane. Strong recognition ability and high column performance (theory plates was 43,000 plates/m) of BNA were achieved on this monolithic MIP in CEC mode. In addition, BNA and its structural analogue, 1,1'-bi-2, 2'-naphthol, differing in functional groups, were used as model compounds to study imprinting effect on the resultant BNA-imprinted monolithic column, a reference column without imprinting of BNA and a open capillary. The effects of organic modifier concentration, pH value of buffer, salt concentration of buffer and column temperature on the retention and recognition of two compounds were investigated. The results showed that the molecular recognition on MIP monolith in CEC mode mainly derived from imprinting cavities on BNA-imprinted polymer other than chromatographic retention and electrophoretic migration.

Recent developments in the field of monolithic stationary phases for capillary electrochromatography

This review summarizes the contributions to the rapidly growing area of monolithic columns based on both silica and synthetic polymers for capillary electrochromatography and chip electrochromatography, with a focus on those published during the year 2004. A wide variety of both modified approaches to the "old" monoliths and new monoliths have been reported despite the very short period of time covered. This demonstrates that monolithic stationary phases have become a well-established format in the field of electrochromatography. The simplicity of their preparation as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials.

Molecular imprinting technology in capillary electrochromatography

Molecularly imprinted polymers (MIPs) are tailor-made synthetic polymers with a predetermined selectivity for a given analyte, or group of structurally related compounds, that make them ideal materials for use as stationary phases in affinity chromatography. However, extensive peak broadening and tailing, especially of the more retained compound (normally the template) are often observed. Thus, huge efforts have been made during recent years to use MIPs in capillary electrochromatography, which is inherently a more efficient chromatographic technique than conventional HPLC. Accordingly, this paper gives an overview of the attempts carried out in the recent past to improve the chromatographic performance of MIPs in capillary electrochromatography as well as more recent applications. It is concluded that MIPs are very promising materials for use as selective stationary phases in CEC.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Separation of biomolecules using electrophoresis and nanostructures

A large number of nanostructures have the potential to be used together with electrophoresis as separation media or separation additive in capillary electrophoresis, micellar electrokinetic chromatography, capillary electrochromatography, and other analytical techniques. Among those structures are nanotubes, nanocavities, nanowires, nanoposts, nanocones, nanospheres, molecular imprints, nanoparachutes (conical monodendrons), and general nanoparticles with random structures. This review is focused only on publications describing experimental works using molecular imprints, nanoposts, and nanospheres that are fabricated and applied for the purpose of separation media in electrophoresis-driven separations. The review follows an approximate chronological order in each section. As shown, the most popular are those resulting from molecular imprinting technologies. These biomimetic receptors are used in a great variety of fields, which includes electrophoresis, micellar electrokinetic chromatography, capillary electrochromatography, and other fields not reviewed in this work. A few examples of these other fields are, e.g., liquid chromatography, membranes, extractor or preconcentration techniques, immunosorbent assays, and sensing devices. The second topic scanned in the present work is the nanostructures that are used as obstacles to replace gels or polymers solutions in electrophoresis. Finally, the nascent field of nanospheres of gold and other materials as separation media is also reviewed.