Thermodynamic analysis of the heterogenous binding sites of molecularly imprinted polymers

The thermodynamic interactions of two polymers, one Fmoc-L-Trp-imprinted (MIP), the other one an unimprinted reference (NIP), with the two Fmoc-tryptophan enantiomers were studied by frontal analysis, which allows accurate measurements of the adsorption isotherms. These isotherms were acquired at temperatures of 40, 50, 60, and 70 degrees C, for sample concentrations ranging between 0.005 and 40 mM. The mobile phase used was acetonitrile with one percent acetic acid as an organic modifier. Within the measured concentration ranges, the tri-Langmuir isotherm model accounts best for the isotherm data of both enantiomers on the MIP, the bi-Langmuir model for the isotherm data of Fmoc-L-Trp on the NIP. These isotherm models were selected using three independent processes: statistical tests on the results from regression of the isotherm data to different isotherm models; calculation of the affinity energy distribution from the raw isotherm data; comparison of the experimental and the calculated band profiles. The isotherm parameters obtained from these best selected isotherm models showed that the enantiomeric selectivity does not change significantly with temperature, while the affinity of the substrates for both the MIP and the NIP decrease considerably with increasing temperatures. These temperature effects on the binding performance of the MIP were clarified by considering the thermodynamic functions (i.e., the standard molar Gibbs free energy, the standard molar entropy of adsorption, and the standard molar enthalpy of adsorption) for each identified type of adsorption sites, derived from the Van't Hoff equation. This showed that the entropy of transfer of Fmoc-L-Trp from the mobile to the MIP stationary phase is the dominant driving force for the selective adsorption of Fmoc-L-Trp onto the enantioselective binding sites. This entropy does not change significantly with increasing temperatures from 40 to 70 degrees C.     

Selective recognition of 4-nitrophenol from aqueous solution by molecularly imprinted polymers with functionalized tetratitanate whisker composites as support

Three kinds of molecularly imprinted polymers (MIPs) were obtained with surface molecular imprinting technique on functionalized potassium tetratitanate whisker (F-PTW). The results of adsorption experiments indicated that MIP prepared using PTW modified with N-(2-aminoethyl)-3-(trimethoxysilyl)propylamine (AAPTS) (F-PTW A) as support [MIP(1)] was superior to the other two polymers, then MIP(1) was selected to analyze the 4-nitrophenol (4-NP) adsorption process from aqueous solution in this study. AAPTS offered hydrophilic exterior that allowed to self-assemble with the template 4-NP through intermolecular interaction rather than based on the interactions between the functional monomers and template. Equilibrium adsorption data were analyzed by the Langmuir and Freundlich isotherm models at various temperatures. Kinetic properties were successfully investigated by pseudo-first-order model, pseudo-second-order model, intraparticle diffusion equation, initial adsorption rate, half-adsorption time. A diffusion-controlled process as the essential adsorption rate-controlling step was also proposed. The performance of such imprinted polymer was further demonstrated by high-performance liquid chromatography, and the results showed that the selectivity of MIP(1) exhibited higher affinity for template 4-NP over competitive phenolic compounds than that of non-imprinted polymer NIP(1). MIP(1) could be reused four times without significant loss in the adsorption capacity.     

Synthesis and Adsorption Study of BSA Surface Imprinted Polymer on CdS Quantum Dots

A new bovine serum albumin (BSA) surface imprinting method was developed by the in-corporation of quantum dots (QDs) into molecularly imprinted polymers (MIP), which can offer shape selectivity. Preparation and adsorption conditions were optimized. Physical ap-pearance of the QDs and QDs-MIP particles was illustrated by scanning electron microscope images. Photoluminescence emission of CdS was quenched when rebinding of the template.The quenching of photoluminescence emissions is presumably due to the fluorescence reso-nance energy transfer between quantum dots and BSA template molecules. The adsorption is compiled with Langmuir isotherm, and chemical adsorption is the rate-controlling step.The maximum adsorption capacity could reach 226.0 mg/g, which is 142.4 mg/g larger than that of undoped BSA MIP. This study demonstrates the validity of QDs coupled with MIP technology for analyzing BSA.     

分子印迹聚合物颗粒在毛细管电色谱中的应用

依据分子印迹技术(MIT)制备的分子印迹聚合物(MIP)颗粒对模板分子及其结构类似物具有特异性识别和选择性吸附作用,同时具有较大的比表面积和快速的传质动力学特性,因而被广泛用作液相色谱固定相和固相萃取材料。将MIP颗粒作为固定相应用于毛细管电色谱(CEC),结合了CEC的快速、高效和MIP的高亲和性、高选择性的特点,成为分析科学领域最具有发展前景的分离技术之一。MIP颗粒在CEC领域有几种不同的应用形式: 作为填充材料填充到毛细管柱中;作为嵌入材料嵌入到毛细管柱内部不同基质的骨架中;作为准固定相添加到CEC运行缓冲溶液中。本文综述了近几年MIP颗粒在CEC领域应用的发展,对该领域今后的发展前景进行了展望。     

Moment analysis of retention equilibrium, mass transfer kinetics, and thermodynamic properties in reversed-phase liquid chromatography using phenyl bonded silica gel

Information about retention equilibrium, mass transfer kinetics, and related thermodynamic properties of chromatography using phenyldimethylsilyl (Ph)-silica gel was derived by moment analysis of pulse response peak profiles. The results for the Ph-silica gel were compared with those for octadecyldimethylsilyl (C18)-silica gel. Some parameters characterizing the chromatographic behavior of the two stationary phases were correlated with the hydrophobic surface area of sample molecules. Surface diffusion had a predominant role for intraparticle diffusion. An enthalpy-entropy compensation was established for both the retention equilibrium and surface diffusion. A linear correlation was observed between the logarithm of surface diffusion coefficient (Ds) and that of retention equilibrium constant (Ka), suggesting the establishment of a linear free energy relationship. The ratio of Ds to molecular diffusivity (Dm) decreased with increasing Ka and was correlated by a single curved line. The value of Ds was of the same order of magnitude with Dm when Ka became negligible. These results suggest the presence of a sort of correlation between surface diffusion and molecular diffusion and the restriction of the molecular mobility by surface diffusion due to the retention strength.     

液晶分子印迹整体柱的制备及其分子识别热力学

液晶分子印迹聚合物(MIPs)因刚性液晶单体的加入而在超低交联度水平下也能印迹和识别模板分子,有效解决了传统MIPs因高交联度造成的位点包埋、结合容量低、传质慢等问题。尽管液晶MIPs具有如此独特的优势,但却面临着由于交联度的大幅度降低而导致印迹效果下降的问题。为了研究液晶MIPs的结合特性,制备具有良好印迹效果的低交联液晶MIPs,该文通过二次接枝聚合,制备了一系列不同交联度的液晶分子印迹整体柱,用高效液相色谱法研究了聚合参数与印迹整体柱亲和性的关系。实验中选用三羟甲基丙烷三甲基丙烯酸酯(TRIM)为交联剂,以甲苯和十二醇为致孔剂合成整体柱骨架,并在此基础上以(S)-萘普生为模板,加入液晶单体4-氰基苯基单环己基乙烯(CPCE)进行二次聚合接枝。实验中系统考察了流动相中乙腈比例及缓冲液pH值对色谱保留的影响,结果发现液晶单体的加入使得MIPs对萘普生保留控制机制由原来的氢键作用变为了疏水作用;通过动态吸附实验得到的突破曲线经前沿分析及对吸附等温线Langmuir、Freundlich和Scatchard分析拟合,发现交联度为15%时液晶MIPs印迹因子最大(3.78)、非均一性最强,且特异性吸附量高于非特异性吸附量。液晶MIPs的计量置换模型(SDM-R)分析表明,液晶印迹整体柱对模板分子的总亲和力(ln A=0.645)明显高于其类似物;而从空间匹配程度看,与液晶印迹整体柱空间匹配程度最高的是酮洛芬而非模板分子,但液晶印迹整体柱对酮洛芬的总亲和力(ln A=0.242)不及模板分子的一半,表明在本低交联液晶印迹系统中,空间效应不是决定印迹系统识别能力的主要因素。进一步的分离热力学研究发现,低交联液晶印迹柱的|ΔΔH|<T|ΔΔS|,而交联度为70%的非液晶MIPs柱的|ΔΔH|>T|ΔΔS|,表明液晶MIPs的分离过程是一个熵控制过程,而常规无液晶MIPs的分离过程是一个焓控制过程。上述结果表明,液晶单体的加入改变了MIPs的识别机制,适当的低交联度可显著提高液晶MIPs的识别性能,因此液晶MIPs这些特质有望使其成为新一代的MIPs。     

Effect of the temperature on the isotherm parameters of phenol in reversed-phase liquid chromatography

Adsorption isotherm data of phenol from an aqueous solution of methanol onto a C18-bonded silica (Symmetry-C18) were acquired by frontal analysis (FA) at six different temperatures, in a wide concentration range. The non-linear fitting of these data provided the bi-Langmuir model as best isotherm model, a conclusion further supported by the results of the calculation of the affinity energy distribution (AED). The isotherm parameters were obtained using several methods, the fitting of FA isotherm data, the calculation of the AED, and the inverse method, that uses overloaded elution band profiles. The different values obtained are in close agreement. They allow a quantitative investigation of the separate properties of the low- and the high-energy sites on the adsorbent surface. Increasing the temperature decreases the saturation capacity of the low-energy adsorption sites and the adsorption constant of the high-energy sites. In contrast, increasing the temperature does not cause any significant changes in either the saturation capacity of the high-energy sites or the adsorption constant of the low-energy sites.     

Retention equilibrium and mass transfer characteristics in reversed-phase liquid chromatography using methanol-water mixtures

Pulse response experiments (i.e., elution chromatography) were made in reversed-phase liquid chromatography (RPLC) using a C18 silica gel column and methanol-water mixtures of different compositions (phi). The moment analysis of the elution peak profiles measured in the RPLC system provided some items of information about four parameters characterizing the retention equilibrium and the mass transfer kinetics in the column, i.e., adsorption equilibrium constant, isosteric heat of adsorption, surface diffusion coefficient and activation energy of surface diffusion. Characteristics of the chromatographic behavior were studied by analyzing the dependence of the four parameters on phi and the correlation between them. It was found that surface diffusion was one of the important processes of molecular migration having a significant contribution to the mass transfer kinetics in the column. Both the adsorption equilibrium constant and the surface diffusion coefficient varied depending on phi. The direction of their changes was approximately opposite, suggesting that the mass transfer in the manner of surface diffusion was restricted owing to the retention of the sample molecules on the stationary phase.     

Adsorption isotherms of the fullerenes C60 and C70 on a tetraphenylporphyrin-bonded silica

The single-component adsorption isotherms of the C60 (from 0 to 15 g/L) and C70 (from 0 to 8 g/L) buckminsterfullerenes on a tetraphenylporphyrin-bonded silica were acquired by frontal analysis, using a solution of toluene-1-methylnaphthalene (40:60, v/v) as the mobile phase. The best isotherm model derived from the fitting of these adsorption data was the bi-Langmuir model, a choice supported by the bimodal affinity energy distribution (AED) obtained for C60. The isotherm parameters derived from the inverse method (IM) of isotherm determination (by fitting calculated profiles to experimental overloaded band profiles of C60 and C70) are in very good agreement with those derived from the FA data. According to the isotherm parameters found by these three methods (FA, AED, IM), the tetraphenylporphyrin-bonded silica can adsorb 54 and 42 mmol/L of C60 and C70 fullerenes, respectively, a result that is consistent with the relative molecular size of these two compounds. The 20% lower surface accessibility for C70 is compensated by a three times higher equilibrium constant on the low-energy sites, giving a selectivity alpha(C70/C60) = 3.6. Large volumes (0.2, 0.8 and 1.7 mL) of mixtures of C60 (3.2 g/L) and C70 (1.3 g/L) were injected and their elution profiles compared to those calculated from the competitive bi-Langmuir model derived from the single-component isotherm data. A good agreement is obtained between calculated and experimental profiles, which supports the two-site adsorption mechanism derived from the single-component adsorption data. The measurements of the influence of the pressure on the retention of C60 and C70 demonstrate that the partial molar volumes of the two buckminsterfullerenes are 12 mL/mol larger in the stationary than in the mobile phase.     

Influence of the particle porosity on chromatographic band profiles

The mass transfer kinetics of butyl benzoate, eluted on a monolithic RPLC column with methanol-water (65:35, v/v) as the mobile phase was investigated, using the perturbation method to acquire isotherm data and the mobile phase velocity dependence of the height equivalent to a theoretical plate of perturbation peaks to acquire kinetics data. The equilibrium isotherm of butyl benzoate is accounted for by the liquid-solid extended multilayer BET isotherm model. The total porosity of the column varies much with the butyl benzoate concentration, influencing strongly the parameters of its mass transfer kinetics and the profiles of the breakthrough curves. Using all these parameters, the general rate model of chromatography predicts band profiles and Van Deemter curves that are in excellent agreement with experimental results provided the influence of concentration on the porosity is properly taken into account. This agreement confirms the validity of the models selected for the isotherm and for the mass transfer kinetics.     

Influence of pressure on the chromatographic behavior of insulin variants under nonlinear conditions

The effect of pressure on the chromatographic behavior of two insulin variants in RPLC was investigated on a YMC-ODS C18 column, under nonlinear conditions. The adsorption isotherm data of porcine insulin and Lispro were measured at average column pressures ranging from 52 to 242 bar. These data fit well to the Toth and the bi-Langmuir isotherm models. The saturation capacity increases rapidly with increasing pressure while the affinity (or equilibrium) constant and the parameter characterizing the surface heterogeneity decrease. It is noteworthy that the distribution coefficient of the insulin variants increases with increasing pressure whereas their equilibrium constant b decreases for porcine insulin and increases for Lispro. The association constant b(ds), which characterizes the adsorption and desorption equilibrium of insulin in the system, increases with increasing pressure. The excellent agreement between the experimental overloaded profiles recorded under different pressures and those calculated using the POR model suggests that the chromatographic behavior of insulin is controlled more by equilibrium thermodynamics than by the mass transfer kinetics. The latter seems to be nearly independent of the average column pressure. Thus, increasing the average column pressure is an efficient, albeit costly, way to increase the loading capacity of the column, hence the production rate in preparative chromatography.     

Study of the mass transfer kinetics in a monolithic column

The purpose of this work is to investigate the mass transfer kinetics of butylbenzoate on a monolithic RPLC column, with methanol-water (65:35, v/v) as the mobile phase. We used the perturbation method, measuring the height equivalent to a theoretical plate (HETP) of the peaks obtained as the response to small pulses of solute injected on a concentration plateau. The equilibrium isotherm of butylbenzoate was previously determined by frontal analysis. It is well accounted for by a liquid-solid extended multilayer BET isotherm model. The equilibrium data derived from the pulse method are in excellent agreement with those of frontal analysis in the accessible concentration range of 0 to 8 g/dm3. Plots of the HETP of small pulses. injected on eight different plateau concentrations, were acquired in a wide range of mobile phase flow velocities. The axial dispersion and the mass transfer kinetic coefficients were derived from these data. The validity of these measurements is discussed. The mass kinetics of butylbenzoate depends strongly on the plateau concentration. Processes involving adsorptive interactions between the solute and the stationary phase, e.g. surface diffusion and adsorption-desorption kinetics, combine in series to the external mass transfer kinetics and to effective pore diffusivity.     

Optimization of polymerization parameters for the sorption of oseltamivir onto molecularly imprinted polymers

Molecularly imprinted polymers (MIPs) are tailor-made polymers with high selectivity for a given analyte, or group of structurally related compounds. The influence of the process parameters (the moles of functional monomer and cross-linker, the selection of functional monomer and solvent) on the preparation of oseltamivir (OS)-imprinted polymers was investigated. A mathematical method for uniform design to optimize these selected parameters and to increase the MIP selectivity for template molecules was applied. The optimal conditions to synthesize MIP were 0.69 mmol 30% acrylamide (AA) + 70% 4-Vinylpyridine (4-VP) and 5.0 mmol ethylene glycol dimethacrylate (EGDMA) copolymerized in 5 ml toluene in the presence of 0.1 mmol OS. MIP showed high affinity and selectivity for separation of the template molecule from other compounds. In the present study, we have established an effective LC-MS/MS method to identify and quantify OS with good sensitivity, accuracy and precision.     

Adsorption Behavior and Dynamic Interactions of Anionic Acid Blue 25 on Agricultural Waste

In this study, adsorption characteristics of a negatively charged dye, Acid Blue 25 (AB25), on pomelo pith (PP) was studied by varying the adsorption parameters, with the aim of evaluating the adsorption mechanism and establishing the role of hydrogen bonding interactions of AB25 on agricultural wastes. The kinetics, intraparticle diffusion, mechanism, and thermodynamics of the AB25 adsorption were systematically evaluated and analyzed by pseudo-first-order and pseudo-second-order kinetic models, the Weber–Morris intraparticle and Boyd mass transfer models, the Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin isotherm models, and the Van’t Hoff equation. It was found that AB25 adsorption followed pseudo-second-order kinetics, governed by a two-step pore-volume intraparticle diffusion of external mass transfer of AB25 onto the PP surface. The adsorption process occurred spontaneously. The adsorption mechanism could be explained by the Langmuir isotherm model, and the maximum adsorption capacity was estimated to be 26.9 mg g⁻¹, which is comparable to many reported adsorbents derived from agricultural wastes. Changes in the vibrational spectra of the adsorbent before and after dye adsorption suggested that AB25 molecules are bound to the PP surface via electrostatic and hydrogen bonding interactions. The results demonstrated that the use of pomelo pith, similar to other agricultural wastes, would provide a basis to design a simple energy-saving, sustainable, and cost-effective approach to remove negatively charged synthetic dyes from wastewater.     

Temperature sensitive dopamine-imprinted (N,N-methylene-bis-acrylamide cross-linked) polymer and its potential application to the selective extraction of adrenergic drugs from urine

A temperature sensitive dopamine-imprinted polymer was prepared in 80% aqueous methanol solution by free-radical cross-linking co-polymerisation of methacrylic acid and acrylamide at 60 degrees C in the presence of N,N-methylene-bis-acrylamide as the cross-linker and dopamine hydrochloride as template molecule. The resulting molecularly imprinted polymer (MIP) formed temperature responsive materials, which could be used for the selective separation of appropriate dopamine and adrenergic compounds from a liquid matrix at ambient temperatures. The thermoresponsive MIP exhibited a swelling-deswelling transition in 80% aqueous methanol solution at about 35 degrees C. The capacity of the thermoresponsive MIP to recognise the template molecule when present in aqueous methanol solution changed with temperature, with the highest selectivity found at 35 degrees C. Additionally, binding parameters obtained from Scatchard analyses indicate that increasing temperature resulted in an increased affinity and binding capacity of specific binding sites, but had less effect on non-selective binding sites. Subsequently, the thermoresponsive MIP was tested for its application as a sorbent material, utilisable in the selective solid-phase extraction (SPE) of dopamine and other adrenergic compounds (epinephrine, isoproterenol, salbutamol and serotonin) from urine samples. It was shown that the compounds that were structurally related to dopamine could be removed by elution, while dopamine and serotonin, the analytes of interest, remained strongly adsorbed to the adsorbent during SPE applications. The thermoresponsive MIP displayed different efficiency in clean-up and enrichments using the SPE protocol at different temperatures.     

Silica particles coated with azobenzene-containing photoresponsive molecule-imprinted skin layer

We developed a novel type of azobenzene-containing photoresponsive molecule-imprinted silica microspheres. Ibuprofen and activated silica particles were used as template molecules and substrates, respectively. Pre-synthesized azobenzene-based monomers were chemically bonded on the surface of silica particles. Template–monomer complexes were formed relying on hydrogen bonding. Then skin layer was formed by graft polymerization of azobenzene-based monomers. After that, ibuprofen molecules were removed from their embedded spaces. The vacant spaces on the surface of particles were easily accessible for the template molecules. Furthermore, photoinduced trans–cis isomerization of azobenzene chromophores within imprinted vacant spaces was able to regulate their substrate affinity. The results demonstrated that the imprinted silica microspheres possessed obvious molecular imprinting effects towards the template ibuprofen, rather fast template rebinding kinetics, and appreciate selectivity over structurally related compounds.     

Kinetic Study of the Mass Transfer of Bovine Serum Albumin on Cibacron Blue Cellulose Membranes by Using the Multi-Plate and Transport Models

The mass transfer kinetics of bovine serum albumin on Cibacron blue F3GA cellulose affinity membranes has been investigated. It was found that the multi-plate (MP) and transport models successfully described experimental breakthrough curves obtained by single-step frontal analysis. The correlation between the two models was used to estimate the rate coefficients of mass transfer from experimental data. The flow rate was found to have little effect on the performance of affinity membrane separations. The improvement of the performance by increasing the thickness of the membranes was limited. The transport model was simplified by approximating the very sharp Langmuir isotherm determined by a rectangular isotherm and an analytical solution obtained although this was found to be unsuited to describe the experimental data.     

Selective Adsorption of Co(Ⅱ)by Mesoporous Silica SBA-15-Supported Surface Ion Imprinted Polymer:Kinetics,Isotherms,and Thermodynamics Studies

A novel surface ion imprinted adsorbent [Co(II)‐IIP] using polyethyleneimine (PEI) as function monomer and ordered mesoporous silica SBA‐15 as support matrix was prepared for Co(II) analysis with high selectivity. The prepared polymer was characterized by Fourier transmission infrared spectrometry, scanning electron microscopy, X‐ray diffraction and nitrogen adsorption‐desorption isotherm. Bath experiments of Co(II) adsorption onto Co(II)‐IIP were performed under the optimum conditions. The experimental data were analyzed by pseudo‐first‐order and pseudo‐second‐order kinetic models. It was found that the pseudo‐second‐order model best correlated the kinetic data. The intraparticle diffusion and liquid film diffusion were applied to discuss the adsorption mechanism. The results showed that Co(II) adsorption onto IIP was controlled by the intraparticle diffusion mechanism, along with a considerable film diffusion contribution. Langmuir, Freundlich and Dubinin‐Radushke‐ vich adsorption models were applied to determine the isotherm parameters. Langmuir model fitted the experiment data well and the maximum calculated capacity of Co(II) reached 39.26 mg/g under room temperature. The thermodynamic data were indicative of the spontaneousness of the endothermic sorption process of Co(II) onto Co(II)‐IIP. Co(II)‐IIP showed high affinity and selectivity for template ion compared with non imprinted polymer (NIP).     

Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk

Batch experiments were carried out for the sorption of methylene blue onto rice husk particles. The operating variables studied were initial solution pH, initial dye concentration, adsorbent concentration, and contact time. Equilibrium data were fitted to the Freundlich and Langmuir isotherm equations and the equilibrium data were found to be well represented by the Langmuir isotherm equation. The monolayer sorption capacity of rice husks for methylene blue sorption was found to be 40.5833 mg/g at room temperature (32 degrees C). The sorption was analyzed using pseudo-first-order and pseudo-second-order kinetic models and the sorption kinetics was found to follow a pseudo-second-order kinetic model. Also the applicability of pseudo second order in modeling the kinetic data was also discussed. The sorption process was found to be controlled by both surface and pore diffusion with surface diffusion at the earlier stages followed by pore diffusion at the later stages. The average external mass transfer coefficient and intraparticle diffusion coefficient was found to be 0.01133 min(-1) and 0.695358 mg/g min0.5. Analysis of sorption data using a Boyd plot confirms that external mass transfer is the rate limiting step in the sorption process. The effective diffusion coefficient, Di was calculated using the Boyd constant and was found to be 5.05 x 10(-04) cm2/s for an initial dye concentration of 50 mg/L. A single-stage batch-adsorber design of the adsorption of methylene blue onto rice husk has been studied based on the Langmuir isotherm equation.