Kinetics of pH-Induced formation and dissociation of polymeric vesicles assembled from a water-soluble zwitterionic diblock copolymer

The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompatible zwitterionic diblock copolymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC- b-PDPA), was investigated in detail via a combination of stopped-flow light scattering and laser light scattering (LLS). Upon jumping from pH 2 to 10, stopped-flow light scattering reveals three distinct relaxation processes for the early stages of vesicle self-assembly (0-40 s). Kinetic sequences associated with the obtained three characteristic relaxation times have been tentatively proposed. Moreover, the kinetics of vesicle formation in the later stage (from 3 min onward) was investigated by dynamic LLS. It was found that both the intensity-averaged hydrodynamic radius, R h, and the polydispersity, mu2/Gamma (2), decrease exponentially, yielding a characteristic relaxation time of approximately 350 s. To our knowledge, this is the first report on the kinetics of the unimer-to-vesicle transition of a stimulus-responsive diblock copolymer. The kinetics of vesicle dissociation for a pH jump from 12 to 2 was also investigated. The breakdown of polymeric vesicles is extremely fast and is independent of polymer concentration; it is complete within approximately 5 ms and is in marked contrast to the much slower rate of vesicle formation.     

pH-responsive vesicles based on a hydrolytically self-cross-linkable copolymer

A new type of shape-persistent, pH-responsive vesicle was prepared by the self-assembly of a novel poly(ethylene oxide)-block-poly[2-(diethylamino)ethyl methacrylate-stat-3-(trimethoxysilyl)propyl methacrylate], PEO-b-P(DEA-stat-TMSPMA), copolymer. Vesicles were formed spontaneously in aqueous THF solution, with the hydrophilic PEO chains forming the corona and the pH-sensitive P(DEA-stat-TMSPMA) blocks being located in the membrane walls. Hydrolytic cross-linking within the hydrophobic membrane walls fixed the vesicle morphology. The resulting colloidally stable vesicles were characterized by 1H NMR, transmission electron microscopy (TEM), dynamic laser light scattering (DLS), and stopped-flow fluorescence experiments. The latter technique indicated that the permeability of the vesicle walls was sensitive to the pH of the aqueous solution, as expected. Gold-decorated vesicles were obtained by in situ reduction of AuCl4- anions to produce gold nanoparticles within the vesicle walls. (Yellow, hydrophilic PEO; green, pH-responsive DEA residues; blue, hydrolytically self-cross-linkable TMSPMA residues.)     

Preparation of primary amine-based block copolymer vesicles by direct dissolution in water and subsequent stabilization by sol-gel chemistry

A new amphiphilic biocompatible diblock copolymer, poly(epsilon-caprolactone)-block-poly(2-aminoethyl methacrylate), PCL-b-PAMA, was synthesized in three steps by (i) ring-opening polymerization of epsilon-caprolactone, (ii) end-group modification by esterification, and (iii) atom transfer radical polymerization (ATRP) of 2-aminoethyl methacrylate hydrochloride (AMA) in its hydrochloride salt form. This copolymer forms block copolymer vesicles with the hydrophobic PCL block forming the vesicle membrane. Unusually, these vesicles are easily prepared by direct dissolution in water without using organic co-solvents, pH adjustment, or even stirring. These vesicles can be stabilized by aqueous sol-gel chemistry using tetramethyl orthosilicate (TMOS) as the silica precursor. It is well-known that cationic polymers can catalyze silica formation, but in this particular case, it seems that the TMOS precursor is solubilized within the hydrophobic PCL membrane. Thus, the neutral membrane actually directs silica formation, rather than the cationic PAMA chains. The final vesicle morphology and the silica content depend on the silicification conditions. Provided that the TMOS/AMA molar ratio does not exceed 10:1, silicification is solely confined within the PCL membrane. At higher ratios, silica nanoparticles (5-12 nm) are also observed on the outer surface of the silicified vesicles. However, these nanoparticles appear to be only weakly adsorbed, since they can be easily removed by dialysis. The mean hydrodynamic diameter of the silicified vesicles varies from 175 to 205 nm with solution pH due to (de)protonation of the externally expressed PAMA chains. Calcination of the silicified vesicles at 800 degrees C leads to the formation of hollow silica particles. 1H NMR, transmission electron microscopy (TEM), dynamic light scattering (DLS), aqueous electrophoresis, and thermogravimetric analysis (TGA) were employed to characterize the vesicles, both before and after silicification.     

Solution pH-regulated interfacial adsorption of diblock phosphorylcholine copolymers

Spectroscopic ellipsometry has been used to examine the pH-responsive interfacial adsorption of a series of biocompatible diblock copolymers incorporating 2-methacryloyloxyethyl phosphorylcholine-based (MPC) residues and 2-(dialkylamino)ethyl methacrylate residues, with a specific focus on 2-(diethylamino)ethyl groups (referred to as MPCm-DEAn, where m and n refer to the mean degrees of polymerization of each block) at the hydrophilic silicon oxide/water interface. For all the copolymers studied the surface excess shows only weak concentration dependence. Increasing the length of the DEA block has little effect on the dynamic or equilibrated adsorption at pH 7, indicating that the DEA block adopts a flat conformation on the silicon oxide surface at this pH. With increasing pH, however, the surface excess shows a dramatic increase, followed by a subsequent decline. The observed maximum in surface excess represents a balance between charge over-compensation of the copolymer with the oppositely charged surface and the subsequently reduced charge density of the copolymer. Variations in the observed maxima for various MPCm-DEAn diblock copolymers indicate different surface conformations at high pH. Salt addition does not affect copolymer adsorption. This behavior is attractive for biomedical applications in which the ionic strength is variable. It was also found that the preadsorbed diblock copolymers immobilized DNA from solution to an extent that is proportional to the relative charge ratio between the anionic DNA and the cationic DEA block of the copolymer.     

Facile synthesis of highly biocompatible poly(2-(methacryloyloxy)ethyl phosphorylcholine)-coated gold nanoparticles in aqueous solution

Diblock copolymers comprising a highly biocompatible poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) block and a poly(2-(dimethylamino)ethyl methacrylate) (PDMA) block were evaluated for the synthesis of sterically stabilized gold nanoparticles in aqueous solution. The PDMA block becomes partially protonated on addition of HAuCl4, and the remaining nonprotonated tertiary amine groups reduce the AuCl4- counterion to zerovalent gold in situ. This approach results in the adsorption of the PDMA block onto the gold nanoparticle surface while the PMPC chains serve as a stabilizing block, producing highly biocompatible gold sols in aqueous solution at ambient temperature without any external reducing agent. The size and shape of gold nanoparticles could be readily controlled by tuning synthesis parameters such as the block composition and the relative and absolute concentrations of the PMPC-PDMA diblock copolymer and HAuCl4. These highly biocompatible gold sols have potential biomedical applications.     

Effects of copolymer concentration and chain length on the pH-responsive behavior of diblock copolymer micellar films

The pH-responsive behavior of adsorbed diblock copolymer films of PDMA-PDEA (poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate)) on silica has been characterized using a quartz crystal microbalance with dissipation monitoring (QCM-D), an optical reflectometer (OR) and an atomic force microscope (AFM). The copolymer was adsorbed at pH 9 from various copolymer concentrations; QCM-D measurements indicate that the level of desorption when rinsed at pH 9 depends on the initial copolymer concentration. The adsorbed films produced at pH 9 generally have low charge densities; adjusting the solution pH to 4 results in a significant protonation of the constituent copolymers and a related interfacial structural change for the copolymer film. OR studies show no significant change during pH cycling, while QCM-D measurements indicate that the adsorbed mass and dissipation alter dramatically in response to the solution pH. The difference between the QCM-D adsorbed masses and dissipation values at pH 4 and 9 were found to be dependent on the initial copolymer concentration. This is due to differences in the initial conformations within the adsorbed copolymer layers at pH 9. The effect of the PDMA chain length on the pH-responsive behavior has also been studied; both the QCM-D adsorbed mass and dissipation of PDMA54-PDEA24 (shorter PDMA block) at pH 4 and 9 were observed to be greater than those of PDMA9X-PDEA2Y (longer PDMA block). This suggests that the normal extension of the adsorbed PDMA54-PDEA24 copolymer films is more significant than that of the PDMA9X-PDEA2Y films on silica.     

pH-responsive nanoaggregation of diblock phosphorylcholine copolymers

We have characterized three diblock copolymers bearing zwitterionic phosphorylcholine and weak tertiary amine groups, namely, poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(dimethylamino)ethyl methacrylate)60] (denoted as MPC30-DMA60, Mn=18,000), poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diethylamino)ethyl methacrylate)60) (denoted as MPC30-DEA60, Mn=20,000), and poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diisopropylamino)ethyl methacrylate)60) (denoted as MPC30-DPA60, Mn=21,000), by studying their surface tension and solution aggregation through a combined approach of surface tension measurement, dynamic light scattering, and small-angle neutron scattering. Our results show that larger tertiary amine substituents lead to an increasing tendency to form micellar aggregates, which is consistent with the increasing copolymer hydrophobicity. Thus, MPC30-DMA60 did not aggregate under the experimental conditions studied. The free chains exist in the form of thin cylinders, whose length decreases with copolymer concentration and solution temperature but increases with solution pH. The diameters of the MPC30-DMA60 cylinders remained almost constant at around 30 A under all the conditions studied. At the lower copolymer concentration of 0.5 wt %, the cylindrical lengths correspond to the persistence length of the copolymer backbone and are close to its full length, indicating a rather high rigidity. Further data analysis showed that, at the two higher concentrations of 2 and 4 wt %, the phosphorylcholine and amine blocks associate, inducing bending of the copolymer backbone. One backbone kink was required to satisfy all the constraints, including the dry volume of the copolymer. MPC30-DEA60 showed a similar trend of pH- and concentration-dependent conformational responses for the free copolymer, but in addition micellar aggregation occurred at pH 9. In contrast, MPC30-DPA60 exhibited significantly reduced solubility associated with strong aggregation, which is consistent with it being the most hydrophobic copolymer in the series.     

pH-responsive behavior of selectively quaternized diblock copolymers adsorbed at the silica/aqueous solution interface

The desorption and subsequent pH-responsive behavior of selectively quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) films at the silica/aqueous solution interface has been characterized. The copolymer films were prepared at pH 9, where micelle-like surface aggregates are spontaneously formed on silica. The subsequent rinse with a copolymer-free electrolyte solution adjusted to pH 9 causes partial desorption of the weakly or non-quaternized copolymers, but negligible desorption for the highly quaternized copolymers. Further rinsing with a pH 4 electrolyte solution results in additional desorption and extension (swelling) of the remaining adsorbed copolymer film normal to the interface. This pH-responsive behavior is reversible for two pH cycles (9-4-9-4) as monitored by both quartz crystal microbalance with dissipation monitoring (QCM-D) and also zeta potential measurements. The magnitude of the pH-responsive behavior depends on the mean degree of quaternization of the PDMA block. Moreover, a combination of contact angle data, zeta potential measurements and in situ atomic force microscopy (AFM) studies indicates that the pH-responsive behavior is influenced not only by the number of cationic binding sites on the adsorbed copolymer chains but also by the adsorbed layer structure.     

Quantitative evaluation of mechanosensing of cells on dynamically tunable hydrogels

Thin hydrogel films based on an ABA triblock copolymer gelator [where A is pH-sensitive poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) and B is biocompatible poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC)] were used as a stimulus-responsive substrate that allows fine adjustment of the mechanical environment experienced by mouse myoblast cells. The hydrogel film elasticity could be reversibly modulated by a factor of 40 via careful pH adjustment without adversely affecting cell viability. Myoblast cells exhibited pronounced stress fiber formation and flattening on increasing the hydrogel elasticity. As a new tool to evaluate the strength of cell adhesion, we combined a picosecond laser with an inverted microscope and utilized the strong shock wave created by the laser pulse to determine the critical pressure required for cell detachment. Furthermore, we demonstrate that an abrupt jump in the hydrogel elasticity can be utilized to monitor how cells adapt their morphology to changes in their mechanical environment.     

新型共聚物涂层毛细管电泳柱及其分离蛋白质的研究

 研究新型共聚物——ＺＢ系列表面键合剂在毛细管电泳中的应用。采用物理吸附的方法制备了ＺＢ－００４,ＺＢ－０１４,ＺＢ－０１６等３种涂层毛细管柱,在ｐＨ３～５范围内,３种涂层均能有效地降低管壁对蛋白质的吸附作用和电渗流,其中亲水性较弱的ＺＢ－００４涂层的分离性能最好。在ｐＨ＜５时,涂层具有较高的稳定性和良好的分析重复性,但在更高的ｐＨ值条件下,仍然存在着峰形畸变和电渗流迅速增加的现象。     

A novel cationic triblock copolymer as noncovalent coating for the separation of proteins by CE

A novel noncovalent adsorbed coating for CE has been prepared and explored. This coating was based on quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(ethylene oxide)-block-poly(2-(dimethylamino)ethyl methacrylate) (QDED) triblock copolymer which was synthesized by atomic transfer radical polymerization (ATRP) in our laboratory. The polycationic polymer and the negatively charged fused-silica surface attracted each other through electrostatic interactions and hydrogen bonds. It was demonstrated that the coated capillaries provided an electroosmotic flow with reverse direction, and the magnitude of the electroosmotic flow can be modulated by varying the molecular mass of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) block and pH value of the buffer. The effects of the molecular mass of PDMAEMA block in QDED triblock copolymer and pH value of the buffer on the separation of basic proteins were investigated in detail. The triblock copolymer coatings showed higher separation efficiency, better migration time repeatability and would apply to wider range of pH than bare fused-silica capillary when used in separating proteins. Proteins from egg white were also separated through this QDED triblock copolymer-coated capillary. These results demonstrated that the QDED triblock copolymer coatings are suitable for analyzing biosamples.     

CO2-sensitive amphiphilic triblock copolymer self-assembly morphology transition and accelerating drug release from polymeric vesicle

A series of triblock copolymers,containing a CO2-switchable block poly(2-(dimethylamino)ethyl methacrylate) (PDM) block and two symmetrical hydrophilic blocks polyacrylamide (PAM),were synthesized using atom transfer radical polymerization (ATRP) method.The pH and conductivity tests showed that the triblock copolymer exhibited switchable responsiveness to CO2,i.e.a relatively low conductivity of solution could be switched on and off by bubbling and removing of CO2,and the triblock copolymer aqueous solution displayed a CO2-switchable viscosity variation.The changes were all attributed to protonation of tertiary amine groups in PDM blocks and proven by 1H-NMR.Cryogenic transmission electron microscopy and dynamic light scattering characterization demonstrated that the viscosity variation was the result of a unilamellar vesicle-network aggregate structure transition.The release of rhodamine B from the vesicles with and without CO2 stimuli showed the potential application in drug delivery domains;after CO2 bubbling,the drug release rate could be accelerated.Finally,reasonable mechanism of CO2-switchable morphology changes and CO2-induced drug release was proposed.     

Preparation of water dispersible poly(methyl methacrylate)-based vesicles for facile persistent antibacterial applications

We report a facile strategy for incorporating persistent and effective antibacterial property into a widely used polymer, poly(methyl methacrylate)(PMMA), by copolymerizing methyl methacrylate(MMA) with 2-(tert-butylamino)ethyl methacrylate(TA) in one pot via atom transfer radical polymerization(ATRP). The subsequent self-assembly of the resultant poly(methyl methacrylate)-block-poly[(2-tert-butylamino)ethyl methacrylate](PMMA20-b-PTA15) diblock copolymer affords well-defined water-dispersible vesicles, which can be facilely sprayed on the walls in hospitals for effective inhibition and killing of bacteria. 1H-NMR and gel permeation chromatography(GPC) studies confirmed the successful synthesis of welldefined copolymer. Transmission electron microscopy(TEM), atomic force microscopy(AFM) and dynamic light scattering(DLS) studies proved the formation of vesicles with narrow size distribution. DLS studies revealed the excellent stability of vesicles at various temperatures. Antibacterial tests showed effective antibacterial activities of polymer vesicles against both Gram-positive and Gram-negative bacteria. Moreover, this strategy may be extended for preparing a wide range of polymeric materials for facile antibacterial applications in many fields.     

Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.     

Design and proof of reversible micelle‐to‐vesicle multistimuli‐responsive morphological regulations

Multistimuli‐responsive precise morphological control over self‐assembled polymers is of great importance for applications in nanoscience as drug delivery system. A novel pH, photoresponsive, and cyclodextrin‐responsive block copolymer were developed to investigate the reversible morphological transition from micelles to vesicles. The azobenzene‐containing block copolymer poly(ethylene oxide)‐b‐poly(2‐(diethylamino)ethyl methacrylate‐co‐6‐(4‐phenylazo phenoxy)hexyl methacrylate) [PEO‐b‐P(DEAEMA‐co‐PPHMA)] was synthesized by atom transfer radical polymerization. This system can self‐assemble into vesicles in aqueous solution at pH 8. On adjusting the solution pH to 3, there was a transition from vesicles to micelles. The same behavior, that is, transition from vesicles to micelles was also realizable on addition of β‐cyclodextrin (β‐CD) to the PEO‐b‐P(DEAEMA‐co‐PPHMA) solution at pH 8. Furthermore, after β‐CD was added, alternating irradiation of the solution with UV and visible light can also induce the reversible micelle‐to‐vesicle transition because of the photoinduced trans‐to‐cis isomerization of azobenzene units. The multistimuli‐responsive precise morphological changes were studied by laser light scattering, transmission electron microscopy, and UV–vis spectra. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Preparation and characterization of temperature and pH-sensitive chitosan material and its controlled release on coenzyme A

A novel copolymer P(CS–Ma–DMAEMA) was synthesized with chitosan (CS), maleic anhydride (Ma) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) by grafting and copolymerization. The copolymer obtained was analyzed by FT-IR, ¹H NMR and UV, and the molecular weight and polydispersity were determined by gel permeation chromatography (GPC). The average size and distribution of copolymer micelles were determined by dynamic light scattering (DLS). Their aqueous solution properties and controlled coenzyme A delivery were also studied. It was found that the copolymer had temperature sensitivity and pH sensitivity. The factors affecting release behavior, such as concentration, pH and temperature were discussed in this paper. The higher concentration of the copolymer aqueous solution absorbed more coenzyme A than the lower one. The increasing temperature accelerated the drug release from the copolymer. The pH of the copolymer solution had significant impact on the release of coenzyme A. The results suggested that the novel copolymer could be used as drug delivery carrier.     

新型共聚物涂层毛细管电泳柱及其分离蛋白质的研究

研究新型共聚物——ＺＢ系列表面键合剂在毛细管电泳中的应用。采用物理吸附的方法制备了ＺＢ－００４,ＺＢ－０１４,ＺＢ－０１６等３种涂层毛细管柱,在ｐＨ３～５范围内,３种涂层均能有效地降低管壁对蛋白质的吸附作用和电渗流,其中亲水性较弱的ＺＢ－００４涂层的分离性能最好。在ｐＨ＜５时,涂层具有较高的稳定性和良好的分析重复性,但在更高的ｐＨ值条件下,仍然存在着峰形畸变和电渗流迅速增加的现象。     

Development,characterization, swelling,and network parameters of amino acid grafted guar gum based pH responsive polymeric hydrogels

This study includes the synthesis of graft copolymer (GG-g-PAPA) from N-Acryloyl-L-phenylalanine (APA) and guar gum through free radical polymerization. Then, the novel pH sensitive GG-g-PAPA-cl-(PHEA-co-PAMPS) [GGAH] polymeric hydrogels were synthesized by employing free radical cross-linking polymerization using graft copolymer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and 2-hydroxy ethyl acrylate (HEA). The GGAH hydrogels were characterized using FTIR and SEM. Swelling studies of GGAH hydrogels were performed in distilled water, pH 1.2, and pH 7.4 solutions. The network and swelling kinetic parameters of GGAH hydrogels are also calculated.     

Proton diffusion across membranes of vesicles of poly(styrene-b-acrylic acid) diblock copolymers

A study of proton diffusion across membranes of block copolymer vesicles in dilute solution is described. The vesicles were formed by the self-assembly of a diblock copolymer of poly(styrene-b-acrylic acid) (PS(310)-b-PAA(36), where the numbers represent the degree of polymerization for individual blocks). A pH gradient was created across the vesicle membrane with the interior pH (pH(in)) of ca. 2.9 and the exterior pH (pH(out)) of ca. 8.5. The permeability of the polystyrene (PS) membrane was tuned by the addition of different amounts of dioxane (0-40 wt %) to the external aqueous solution. Proton concentrations in the solution outside of the vesicles were followed by monitoring the spectrum of a pH-sensitive fluorescent dye, namely 8-hydroxypyrene-1,3,6-trisulfonate. After the start of the experiment, the proton concentrations increase linearly with the square root of time, while the slopes of the lines increase with dioxane content. To calculate the diffusion coefficients of the protons across the vesicular membrane, the concentration data were fitted using a model, which describes the diffusion of species across the membrane of a reservoir. The apparent diffusion coefficient (D*, which equals the true diffusion coefficient multiplied by the partition coefficient of protons between PS and water) increases from 1.1 x 10(-18) cm(2)/s at 7 wt % dioxane in the external solution to 1.2 x 10(-14) cm(2)/s at 40 wt %. The increase of D* with dioxane content is related to its plasticization of the PS membrane, which can be used as a gating mechanism.