Application of pH-responsive poly(2-dimethyl-aminoethylmethacrylate)-block-poly(acrylic acid) coatings for the open-tubular capillary electrochromatographic analysis of acidic and basic compounds

Chiral ligand-exchange capillary electrochromatography with thermo-responsive poly(N,N-dimethylacrylamide) as coating for efficient separation of D,L-amino acids

Herein, for addressing the issue in enantioseparation and being inspired by regulating hydrophobic/hydrophilic interactions in smart polymer through varying external environmental conditions, we demonstrated a chiral ligand exchange-capillary electrochromatography-UV (CLE-CEC-UV) system by introducing a thermo-responsive poly(styrene-maleic anhydride-N,N-dimethylacrylamide) (PSMDMA) to the coating for enantioseparation of dansylated D,L-amino acids (Dns-D,L-AAs). As metal ions-chiral ligands complexes in running buffer played the role of chiral selectors, the proposed CLE-CEC-UV system exhibited significantly enhanced enantioseparation efficiency for the first time, especially, 15-pairs were baseline separated at 40 °C, compared to 4-pairs baseline separated at 20 °C among 16-pairs of Dns-D,L-AAs. It was attributed to the high hydrophobic interactions between Dns-D,L-AAs and PSMDMA coating at high temperature. Additionally, the formation of strong coordinated complexes between metal ions, Dns-D,L-AAs and chiral ligands was also a key to achieve high CLE-CEC-UV performance. The proposed method showed an excellent linear dependence relation between concentration of L-alanine and UV absorbance intensity, ranging from 0.1 to 3.0 mmol L-1, with limit of detection (LOD) of 10.0 μmol L-1. Further evaluation of alanine aminotransferase activity in various mice organs with L-alanine as the substrate confirmed the potential application of the CLE-CEC-UV technique. Moreover, the coated capillary exhibited good repeatability with relative standard deviations less than 1.86 % for migration time and less than 3.94 % for resolution. This work highlights and integrates the advantages of smart polymer coatings and metal ion-chiral ligand complexes, encouraging development of more unique CLE-CEC systems for efficient enantioseparation and practical application in living bio-systems.

In situ growth of dual-responsive polymer as coating for open tubular capillary electrochromatographic separation of epimedins

Recently, open tubular capillary electrochromatography (OT-CEC) has captured considerable interest; its efficient separation capability hinges on the interactions between analytes and polymer coatings. However, in situ growth of stimuli-responsive polymers as coatings has been rarely studied and is crucial for expanding the OT-CEC technique and its application. Herein, following poly(styrene-maleicanhydride) (PSM) chemically bonded onto the inner surface of the capillary, a dual pH/temperature stimuli-responsive block copolymer, P(SMN-COOH), was prepared by in situ polymerizing poly(N-isopropylacrylamide) carboxylic acid terminated [P(N-COOH)] in PSM. An OT-CEC protocol was first explored using the coated capillary for epimedins separation. As a proof of concept, the developed OT-CEC system facilitated hydrogen bonding and tuning the hydrophilic/hydrophobic interactions between the test analytes and the P(SMN-COOH) coating by varying buffer pH and environmental temperature. Four epimedins with similar chemical structures were baseline separated under 40 °C at pH 10.0, exhibiting dramatical improvement in separation efficiency in comparison to its performance under 25 °C at pH 4.0. In addition, the coated capillary showed good repeatability and reusability with relative standard deviations for migration time and peak area between 0.7 and 1.7% and between 2.9 and 4.6%, respectively, and no significant changes after six runs. This work introduces a paradigm for efficient OT-CEC separation of herbal medicines through adjusting the interactions between analytes and smart polymer coatings, addressing polymer coating design and OT-CEC challenges.

Open tubular capillary electrochromatography with dual-responsive polymer as coating for separation of chromones

Fabricating polymeric coatings that are responsive to multiple/dual stimuli is crucial and remains a major challenge in the development of highly efficient open tubular capillary electrochromatography (OT-CEC). In this study, a pH and temperature-responsive block copolymer, poly(styrene-maleic anhydride 2-dimethylamino ethyl methacrylate), P(St-MAn-DMAEMA), was designed and synthesized. Using P(St-MAn-DMAEMA) as the coating, an OT-CEC protocol was constructed for the analysis of chromones. The morphology and hydrophobicity-hydrophilicity of the polymeric coating could change via varying the environmental conditions, affecting the separation efficiency of OT-CEC. Interestingly, the best performance of OT-CEC was achieved at pH 9.7 and 45 °C via tuning the interactions between the coating and the analytes. Additionally, the proposed OT-CEC method exhibited a good linear range for the detection of the three test chromones from 10.0 to 100.0 μM, with all correlation coefficients (R2) >0.997. The coatings also had good stability and reusability. This work provides an approach for the preparation of new multiple-stimuli-responsive polymeric coatings for the establishment of OT-CEC systems.

On-line preconcentration and determination of sulfadiazine in food samples using surface molecularly imprinted polymer coating by capillary electrophoresis

In this study, on-line preconcentration and selective determination of the trace sulfadiazine (SDZ) existing in milk and hen egg white samples were realized by the capillary electrophoresis using molecularly imprinted polymer (MIP) coated capillary. The capillary coated with MIP was firstly prepared through the surface imprinted techniques, using SDZ as template molecule and dopamine as function monomer and crosslinker, and then amine-terminated poly(2-methyl-2-oxazoline) (PMOXA-NH₂) was introduced onto polydopamine layer to reduce the non-specific adsorption. Successful preparation of SDZ-MIP-PMOXA coating was verified by zeta potential, as well as water contact angle. The SDZ-MIP-PMOXA coated capillary performed well on-line preconcentration of SDZ and the obtained peak area of SDZ was 46 times higher than that one obtained in bare capillary using the same procedure. Then the proposed on-line preconcentration method was fully validated and displayed good linear behavior in the concentration from 5.0 to 100.0 ng/mL, with the limit of detection was low to 1.5 ng/mL; and this method presented excellent accuracy and robustness. The prepared SDZ-MIP-PMOXA coated capillary also showed high selectivity with the imprinting factor of 5.85 and good repeatability during five consecutive runs with the relative standard deviation value of peak area was 1.6%. At last, the application of the prepared SDZ-MIP-PMOXA coated capillary in the detection of SDZ in spiked food samples was investigated, and good recoveries of 98.7-109.3% were obtained.

Preparation of pH-responsive block copolymers for separation of cephalosporin antibiotics by open-tubular capillary electrochromatography

Stimuli-responsive block copolymers have exhibited their feasibility for drug delivery and analysis of biomolecules. However, study of the electrophoretic behavior of antibiotics by open tubular capillary electrochromatography (OT-CEC) based on smart block copolymers coatings is still a substantial challenge. Herein, we reported an OT-CEC protocol for analysis of cephalosporin antibiotics with pH-responsive block copolymers as coatings. By using the reversible addition-fragmentation chain-transfers radical polymerisation technique, the smart poly(styrene-maleic anhydride-acrylic acid) (P(St-MAn-AA)) was synthesized and subsequently chemical bonded onto the inner walls of amino-grafted capillaries. The pH induced changes in the stretch/curl states of P(St-MAn-AA) chains were used to generate an adjustable hydrophobic/hydrophilic interaction and hydrogen bonds between the polymer coatings and the analytes. The OT-CEC performance was evaluated by varying the monomer ratios, polymer coating amounts and layers, buffer concentrations and pH values. Baseline separation of the three-test antibiotics was achieved at pH 8.0. The proposed OT-CEC technique was further applied to the determination of rat serum antibiotics in the metabolic processes. The present work demonstrates an enhancement in antibiotics separation efficiency, and shows a great potential for the preparation of stimuli-responsive block copolymers coatings and in OT-CEC analysis of real samples in living bio-systems.

Determination of pepsin by capillary electrophoresis using mixed polymer coated capillary with switchable properties towards protein adsorption/desorption

In this work, a simple on-line preconcentration method for quantitative detection of pepsin was realized by using the binary mixed polymer brushes coated capillary with switchable properties towards protein adsorption. Firstly, the binary mixed polymer brushes were prepared by grafting poly(2-methyl-2-oxazoline) and poly(4-vinylpyridine) onto the inner wall of the capillary through polydopamine anchor. Then the coatings were characterized by X-ray photoelectron spectrometer and electroosmotic flow measurement. The results indicated that the composition of coating could be controlled by varying the feed ratio of poly(2-methyl-2-oxazoline) to poly(4-vinylpyridine) and the inner surface charge could be tuned toward the change of pH and ionic strength. The results showed when poly(2-methyl-2-oxazoline)/poly(4-vinylpyridine) mass ratio was 80/20, the highest on-line preconcentration effect was obtained and the sensitivity enhancement factor was 6.3. Moreover, satisfactory sensitivity (limit of detection: 7.5 ng/mL) and good repeatability were obtained with on-line preconcentration method. The polymer coated capillary was still stable for on-line preconcentration and detection of pepsin after 50 consecutive runs. Lastly, the proposed method was used successfully to on-line preconcentrate pepsin in saliva matrix. This article is protected by copyright. All rights reserved.

Formulation and Characterization of Stimuli-Responsive Lecithin-Based Liposome Complexes with Poly(acrylic acid)/Poly(N,N-dimethylaminoethyl methacrylate) and Pluronic® Copolymers for Controlled Drug Delivery

Polymer–liposome complexes (PLCs) can be efficiently applied for the treatment and/or diagnosis of several types of diseases, such as cancerous, dermatological, neurological, ophthalmic and orthopedic. In this work, temperature-/pH-sensitive PLC-based systems for controlled release were developed and characterized. The selected hydrophilic polymeric setup consists of copolymers of Pluronic^®-poly(acrylic acid) (PLU-PAA) and Pluronic^®-poly(N,N-dimethylaminoethyl methacrylate) (PLU-PD) synthesized by atom transfer radical polymerization (ATRP). The copolymers were incorporated into liposomes formulated from soybean lecithin, with different copolymer/phospholipid ratios (2.5, 5 and 10%). PLCs were characterized by evaluating their particle size, polydispersity, surface charge, capacity of release and encapsulation efficiency. Their cytotoxic potential was assessed by determining the viability of human epithelial cells exposed to them. The results showed that the incorporation of the synthesized copolymers positively contributed to the stabilization of the liposomes. The main accomplishments of this work were the innovative synthesis of PLU-PD and PLU-PAA by ATRP, and the liposome stabilization by their incorporation. The formulated PLCs exhibited relevant characteristics, notably stimuli-responsive attributes upon slight changes in pH and/or temperature, with proven absence of cellular toxicity, which could be of interest for the treatment or diagnosis of all diseases that cause some particular pH/temperature change in the target area.

Recent advancements in open-tubular liquid chromatography and capillary electrochromatography during 2014-2018

This review critically discusses the developments on open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) during 2014-2018. An appropriate Scopus search revealed 5 reviews, 4 theoretical papers on open-tubular format chromatography, 29 OT-LC articles, 68 OT-CEC articles and 4 OT-LC/OT-CEC articles, indicating a sustained interest in these areas. The open-tubular format typically uses a capillary column with inner walls that are coated with an ample layer or coating of solid stationary phase material. The ratio between the capillary internal diameter and coating thickness (CID/CT) is ideally ≤ 100 for appropriate chromatographic retention. We, therefore, approximated the CID/CT ratios and found that 22 OT-LC papers have CID/CT ratios ≤100. The other 7 OT-LC papers have CID/CT ratio >100 but have clearly demonstrated chromatographic retention. These 29 papers utilised reversed phase or ion exchange mechanisms using known or innovative solid stationary phase materials (e.g. metal organic frameworks), stationary pseudophases from ionic surfactants or porous supports. On the other hand, we found that 68 OT-CEC papers, 7 OT-LC papers and 4 OT-LC & OT-CEC papers have CID/CT ratios >100. Notably, 44 papers (42 OT-CEC and 2 OT-LC & OT-CEC) did not report the retention factor and/or effective electrophoretic mobility of analytes. Considering all covered papers, the most popular activity was on the development of new chromatographic materials as coatings. However, we encourage OT-CEC researchers to not only characterise changes in the electroosmotic flow but also verify the interaction of the analytes with the coating. In addition, the articles reported were largely driven by stationary phase or support development and not by practical applications.

Advances in capillary electro-chromatography

Capillary electrochromatography (CEC) is a micro-scale separation technique which is a hybrid between capillary electrophoresis (CE) and liquid chromatography (LC). CEC can be performed in packed, monolithic and open-tubular columns. In recent three years (from 2016 to 2018), enormous attention for CEC has been the development of novel stationary phases. This review mainly covers the development of novel stationary phases for open-tubular and monolithic columns. In particular, some biomaterials attracted increasing interest. There are no significant breakthroughs in technology and principles in CEC. The typical CEC applications, especially chiral separations are described.

Amplification of lysozyme signal detected in capillary electrophoresis using mixed polymer brushes coating with switchable properties

In this work, a mixed polymer brushes based on poly(2-methyl-2-oxazoline) (PMOXA) and poly(acrylic acid) (PAA) coated capillary with switchable protein adsorption/desorption properties was developed and applied for on-line extraction and preconcentration of lysozyme. The study of electroosmotic flow (EOF) and fluorescence microscope showed that the inner surface charge of PMOXA/PAA mixed brush coated capillary displayed the switchable behavior toward the change of pH value and ionic strength (I), and PMOXA/PAA mixed brushes coated capillary could adsorb high amounts of lysozyme at pH 7 (I = 10^-5 M), and the most of adsorbed lysozyme could then be desorbed at pH 3 (I = 10^-1 M). Subsequently, this coated capillary with switchable lysozyme adsorption/desorption ability was applied for on-line extraction and preconcentration of lysozyme during capillary electrophoresis (CE) performance. Under the process of on-line preconcentration, the detection signal (peak area) of lysozyme obtained in PMOXA/PAA coated capillary was 26 times that obtained in bare capillary under normal CE while the contour chain length of PAA was 1.56 times that of PMOXA. Moreover, the value of low detection limit (LOD) of lysozyme using above coated capillary under on-line preconcentration method reached to 4.5 × 10^-9 mg/mL, and 1 × 10⁵-fold sensitivity enhancement was realized for lysozyme as compared with the bare capillary under normal CE.

Recent developments in open tubular capillary electrochromatography from 2016 to 2017

Interest in open-tubular capillary electrochromatography (OT-CEC) continues to thrive because of the inherent advantage of OT-CEC combining the high efficiency of capillary electrophoresis and the high selectivity of high performance liquid chromatography. For the period 2016 to 2017, novel materials have been developed as first-time stationary phases for OT-CEC and are grouped in this review as polymer-based materials, frameworks, nanoparticles, graphene-based materials, and biomaterials. Coating and fabrication methods mostly rely on covalent coating strategies while non-covalent immobilisation strategies like electrostatic assembly are notably still being employed. The concern of overcoming phase ratio challenges in OT-CEC coatings have also generated adoption of combined coating strategies including multi-layering, layer-by-layer self-assembly and methods adapted from nanofilm fabrications like epitaxial growth, liquid phase deposition, or nucleation of crystal growth. The emergence of non-conventional coating characterisation methods such as transmission electron microscopy, X-ray diffraction or X-ray photoelectron spectroscopy is also discussed.

Stability effect of cholesterol-poly(acrylic acid) in a stimuli-responsive polymer-liposome complex obtained from soybean lecithin for controlled drug delivery

The development of polymer-liposome complexes (PLCs), in particular for biomedical applications, has grown significantly in the last decades. The importance of these studies comes from the emerging need in finding intelligent controlled release systems, more predictable, effective and selective, for applications in several areas, such as treatment and/or diagnosis of cancer, neurological, dermatological, ophthalmic and orthopedic diseases, gene therapy, cosmetic treatments, and food engineering. This work reports the development and characterization of a pH sensitive system for controlled release based on PLCs. The selected hydrophilic polymer was poly(acrylic acid) (PAA) synthesized by atom transfer radical polymerization (ATRP) with a cholesterol (CHO) end-group to improve the anchoring of the polymer into the lipid bilayer. The polymer was incorporated into liposomes formulated from soybean lecithin and stearylamine, with different stearylamine/phospholipid and polymer/phospholipid ratios (5, 10 and 20%). The developed PLCs were characterized in terms of particle size, polydispersity, zeta potential, release profiles, and encapsulation efficiency. Cell viability studies were performed to assess the cytotoxic potential of PLCs. The results showed that the liposomal formulation with 5% of stearylamine and 10% of polymer positively contribute to the stabilization of the complexes. Afterwards, the carboxylic acid groups of the polymer present at the surface of the liposomes were crosslinked and the same parameters analyzed. The crosslinked complexes showed to be more stable at physiologic conditions. In addition, the release profiles at different pHs (2-12) revealed that the obtained complexes released all their content at acidic conditions. In summary, the main accomplishments of this work are: (i) innovative synthesis of cholesterol-poly(acrylic acid) (CHO-PAA) by ATRP; (ii) stabilization of the liposomal formulation by incorporation of stearylamine and CHO-PAA; (iii) new approach for CHO-PAA crosslinking, resulting in more stable PLCs at physiological conditions; (iv) destabilization of PLCs upon slight changes of pH, showing their pH sensitivity; and (v) the PLCs do not exhibit cellular toxicity.