pH-sensitive Microgel Research Articles

Transport of ionic species affected by interactions with a pH-sensitive monolayer of microgel particles attached to electrode surface

In this work, we present the use of pH-sensitive microgel based on acrylic acid crosslinked with N,N′-bis(acryloyl)cystamine in the modification of Au electrode surface through the chemisorption process. The strong interactions between the crosslinker disulfide bridges and the gold surface were employed. The shape and morphology of the microgel particles were determined using scanning electron microscopy (SEM). The gel sensitivity to pH was examined using the dynamic light scattering technique (DLS). The impact of pH on electrode processes at the modified Au surface was studied using cyclic voltammetry (CV). It was found that at appropriate pH the microgel particles formed a loose monolayer on the electrode surface, which, in the swollen state, was able to block the transport of negatively charged redox probe to the electrode surface. On the other hand, when the microgels were in the shrunken state, the electrode surface was partially uncovered, and that allowed the redox probe to reach the electrode surface. The anchoring process of the microgels on the Au electrode surface was monitored with a quartz crystal microbalance with dissipation technique (QCM-D). The electrochemical properties of the modified with the pH-sensitive microgel layer electrodes were examined in the presence of either positively or negatively charged redox probes using CV and electrochemical impedance spectroscopy (EIS). It turned out that the electrochemical signals were strongly affected by both: degree of swelling of the microgel and protonation of the carboxylic groups of acrylic acid in the polymer network. The switching of electrode behavior was fully reversible and relatively fast.

Acid pre-flushing evaluation before pH-sensitive microgel treatment in carbonate reservoirs: Experimental and numerical approach

pH-sensitive microgel flooding is a deep conformance control technique in which the microgel viscosity depends on formation pH. Lowering the formation pH by an acid pre-flushing treatment guarantees easy injection of these microgels. The applications of these microgels were mainly dedicated to sandstones in the literature. However, this study provides an integrated laboratory and numerical modeling approach to find a proper pre-flushing acid in carbonates. The performance of various acids (strong and weak) is evaluated via batch experiments (static tests). The dynamic of acid pre-flushing (dynamic test) and pH-sensitive microgel injection are investigated by flooding experiments. A compositional reactive flow model (advection-diffusion-reaction), developed in Python, is also used to analyze the behavior of the flooding experiments. To evaluate the flooding performance, a 3-D field-scale reservoir simulation model with high-permeability contrast is then implemented. The batch and dynamic tests show that acetic acid can maintain a low-pH environment across the medium, which provides a favorable condition for pH-sensitive microgel transports. The geochemical model can accurately predict the measured equilibrium pH and calcium concentration of effluents at different injection rates and salinities. After the establishment of a near-equilibrium condition, the remaining unionized acids in the flooding experiments result in deep conformance control of pH-sensitive microgels. The pH-sensitive microgel flooding at the field-scale reveals an excellent performance in permeability modification of high-conductive zones. Plugging the first 30% of a high-permeable channel can guarantee the recovery factor increase of this deep conformance control technique.

Highly efficient removal of bisphenols from aqueous solution using environmental-sensitive microgel

A temperature and pH sensitive microgel was synthesized and applied for sorption of five bisphenols (bisphenol A (BPA), bisphenol B (BPB), bisphenol E (BPE), bisphenol F (BPF) and bisphenol S (BPS)) from aqueous solutions. The microgel was synthesized from methacrylate derivatives of poly(ethylene glycol) and acrylic acid using the precipitation polymerization method. Liquid chromatography with UV detection (LC-UV) was used for the quantification of bisphenols. The microgel appeared to be a satisfactory sorbent. The maximum experimental adsorption capacities obtained for all five bisphenols were between two values: 55 mg g−1 and 160 mg g−1. The sequence of bisphenols absorption was the following BPS<BFP<BPE<BPA<BPB. Kinetics of sorption, effects of pH and temperature on sorption capacity were also investigated. The experimental data were well described by the pseudo-second order kinetic model and fitted well the adsorption Langmuir model. Using the analysis of the Langmuir model we reached the maximum sorption capacity of 70 mg g−1 to 210 mg g−1. The microgel can be regenerated effectively using an organic solvent; the adsorption capacity remained constant after multiple times of gel uses. The proposed microgel is a promising sorbent for reasonably fast adsorption of bisphenols (BPs) and might be used for the determination of very low concentrations of BPs present in water.

Investigation of acid pre-flushing and pH-sensitive microgel injection in fractured carbonate rocks for conformance control purposes

Waterflooding in fractured reservoirs is a challenging task due to the presence of high conductive flow pathways such as fractures. Much of the injected water passes through fractures without sweeping the oil in the low permeable area, which results in an early breakthrough. Implementing deep conformance control techniques can be a remedy for this early water breakthrough. pH-sensitive microgel injection is a conformance control method in which the dependency of microgel viscosity to pH guarantees easy injection of these microgels into formations at low pH environments. Because of the geochemical reactions among rock minerals, microgels, and a pre-flushing acid, the microgel pH increases; therefore, these microgels swell and block high conductive fractures. In this study, a designed visual cell containing rock samples is implemented to observe rock–microgel interactions during a pH-sensitive microgel flooding into a fractured carbonate medium. First, the dependency of fracture aperture changes to the acid pre-flush flow rate is examined. Then, we investigate the effect of pH-sensitive microgel concentration on its resistance to block fractures during post-water flooding by studying the gel failure mechanisms (e.g., adhesive separation, cohesive failure). Finally, the effect of an initial aperture of fracture is examined on microgel washout when water injection is resumed. The results showed that both decreasing the acid flow rate and lowering the initial aperture could increase the rate of aperture changes. Moreover, the microgel solution with a concentration of 1 wt% showed the highest resistance (98.2 psi/ft) against post-water injection. Additionally, this microgel concentration had the highest permeability reduction factor. Meanwhile, the smaller initial aperture of fracture contributed to a higher microgel resistance.

Dual Responsive Poly(N-vinylcaprolactam) Based Degradable Microgels for Drug Delivery

Poly(N-vinylcaprolactam)-based biodegradable microgels are prepared for drug delivery application via precipitation polymerization using diacetone acrylamide (DAAM) and dimethyl itaconate (IADME) as comonomers. The microgel particles are subsequently crosslinked by addition of adipic acid dihydrazide, which reacts with the ketone groups of DAAM. Itaconic acid (IA) groups are generated by the hydrolysis of IADME units inside the microgels resulting into both pH and temperature sensitive microgel particles. Volume phase transition temperature of the obtained microgels is influenced by both IA content and pH of the surrounding medium. Due to the incorporation of hydrazone linkages, the microgels show degradation under acidic conditions. These microgels can effectively encapsulate doxorubicin (DOX) as a model drug and show low DOX leakage under physiological conditions while rapid DOX release is observed at low pH. The results of the cytotoxicity assay further display that the DOX-loaded microgels exhibit effective antitumor activity against HeLa cells demonstrating their great potential as drug delivery carriers for cancer therapy.

Spatially dependent diffusion coefficient as a model for pH sensitive microgel particles in microchannels.

Solute transport and intermixing in microfluidic devices is strongly dependent on diffusional processes. Brownian Dynamics simulations of pressure-driven flow of model microgel particles in microchannels have been carried out to explore these processes and the factors that influence them. The effects of a pH-field that induces a spatial dependence of particle size and consequently the self-diffusion coefficient and system thermodynamic state were focused on. Simulations were carried out in 1D to represent some of the cross flow dependencies, and in 2D and 3D to include the effects of flow and particle concentration, with typical stripe-like diffusion coefficient spatial variations. In 1D, the mean square displacement and particle displacement probability distribution function agreed well with an analytically solvable model consisting of infinitely repulsive walls and a discontinuous pH-profile in the middle of the channel. Skew category Brownian motion and non-Gaussian dynamics were observed, which follows from correlations of step lengths in the system, and can be considered to be an example of so-called "diffusing diffusivity." In Poiseuille flow simulations, the particles accumulated in regions of larger diffusivity and the largest particle concentration throughput was found when this region was in the middle of the channel. The trends in the calculated cross-channel diffusional behavior were found to be very similar in 2D and 3D.

Synthesis and characterization of Schiff base contained dextran microgels in water-in-oil inverse microemulsion

Polysaccharide-based microgels with high water content, excellent biocompatibility and controllable particle size have been widely studied as ideal candidates for drug release and delivery. In this study, microgels based on dextran were developed via the Schiff base formation between aldehyded dextran and ethylenediamine in a water-in-oil (W/O) microemulsion. Particle size of the resulted microgel was controllable between 800 and 1100nm by modulating the amount of the employed co-surfactants (Span 80/Tween 80). Furthermore, fluoresceins (e.g., aminofluorescein) and drugs (e.g., doxorubicin) with free amino groups can be conjugated onto the network of the dextran-based microgel via Schiff base linkages. Since the Schiff base linkages are degradable via hydrolysis and their stability decreases with the environmental pH decreases, the resulted Schiff bases contained microgel showed a pH dependent degradation profile. These results indicated that the pH-sensitive microgel based on dextran could be used as promising drug delivery systems for biomedical applications.

Surface activity of amphiphilic cationic pH-responsive poly(4-vinylpyridine) microgel at air/water interface

Abstract In this study, we report the relation between the surface activity parameters of soft, pH sensitive microgel particles based on cationic poly(4-vinyl pyridine), p(4-VP) for the first time and corroborate the variation of their chemical structures and their concentrations adsorbed at the air–water interface. In this respect, p(4-VP) particles were quaternized by modifying with alkyl bromides of different chain length. Additionally, hydroxyl, amino and nitrile functional group-containing alkyl bromides were used to quaternize the crosslinked p(4-VP) to determine the effect of hydrophilic and hydrophobic groups on the surface activities of the prepared microgels. The chemical structures of the prepared microgel were characterized by Fourier transform infrared spectrometry (FT-IR). The solution behavior was investigated by surface tension technique, scanning electron microscopy (SEM), dynamic light scattering (DLS), and zeta potential measurements. The results indicate that the microgels can spontaneously assemble into spherical-shaped micelle aggregates, with different critical aggregation concentrations (cac), and hydrodynamic diameters from about 500–1000 nm at pH 4, 7 and 9, depending on the microgel chemical structure. For the first time, p(4-VP) microgels quaternized with hydroxyethyl bromide showed reduction of surface tension of water using amphiphilic microgels from 72.1 to 31.1 mN/m.

Application of pH-sensitive magnetic nanoparticles microgel as a sorbent for the preconcentration of phenoxy acid herbicides in water samples

Introducing new sorbents is an interesting and debatable issue in the field of sample preparation. In this study, for the first time, a pH-sensitive magnetic nanoparticles microgel, Fe3O4–SiO2–oly(4-vinylpyridine), was introduced as a new sorbent. The operating mechanism of this sorbent is based on changing the pH value of the sample and consequently the structure of this pH-sensitive microgel is changed. So that, at pH 6.0 the microgel was ready to accept and load the analytes (partial swelling), and when the pH was increased to 8.0, the microgel was closed and analytes were trapped inside the sorbent (deswelling). At pH 2.0 the microgel was opened and the analytes were released from the microgel (swelling). As the adsorption and desorption mechanism is based on changing the pH and only aqueous medium is used as the effluent solvent, this method is introduced as a green extraction method. The use of this microgel resulted in excellent figures of merit. The limits of quantitation and detection for herbicides were obtained within the range of 10–30 and 3–10ngmL−1, respectively. Finally, the proposed method was successfully applied to determine the concentration of phenoxy acid herbicides as hazardous materials in water samples.

Dual-stimuli-sensitive microgels as a tool for stimulated spongelike adsorption of biomaterials for biosensor applications.

This work examines the fabrication regime and the properties of microgel and microgel/enzyme thin films adsorbed onto conductive substrates (graphite or gold). The films were formed via two sequential steps: the adsorption of a temperature- and pH-sensitive microgel synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAM) and 3-(N,N-dimethylamino)propylmethacrylamide (DMAPMA) (poly(NIPAM-co-DMAPMA) at the pH-condition corresponding to its noncharged state (first step of adsorption), followed by the enzyme, tyrosinase, adsorption at the pH-condition when the microgel and the enzyme are oppositely charged (second step of adsorption). The stimuli-sensitive properties of poly(NIPAM-co-DMAPMA) microgel were characterized by potentiometric titration and dynamic light scattering (DLS) in solution as well as by atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D) at solid interface. Enhanced deposition of poly(NIPAM-co-DMAPMA) microgel particles was shown at elevated temperatures exceeding the volume phase transition temperature (VPTT). The subsequent electrostatic interaction of the poly(NIPAM-co-DMAPMA) microgel matrix with tyrosinase was examined at different adsorption regimes. A considerable increase in the amount of the adsorbed enzyme was detected when the microgel film is first brought into a collapsed state but then was allowed to interact with the enzyme at T < VPTT. Spongelike approach to enzyme adsorption was applied for modification of screen-printed graphite electrodes by poly(NIPAM-co-DMAPMA)/tyrosinase films and the resultant biosensors for phenol were tested amperometrically. By temperature-induced stimulating both (i) poly(NIPAM-co-DMAPMA) microgel adsorption at T > VPTT and (ii) following spongelike tyrosinase loading at T < VPTT, we can achieve more than 3.5-fold increase in biosensor sensitivity for phenol assay. Thus, a very simple, novel, and fast strategy for physical entrapment of biomolecules by the polymeric matrix was proposed and tested. Being based on this unique stimuli-sensitive behavior of the microgel, this stimulated spongelike adsorption provides polymer films comprising concentrated biomaterial.

A study on dispersion and characterisation of α-mangostin loaded pH sensitive microgel systems

Backgroundα-Mangostin was extracted with methanol from the rind of mangosteen fruit and purified by using silica gel column chromatography technique. The compound is characterised using infrared, 13C and 1H NMR as well as UV–vis spectroscopy. The α-mangostin dispersion in colloidal systems was studied by incorporating it with an ionic microgel, poly (N-Isopropylacrylamide)-co-2VP at different pH.ResultThe DLS result showed the size of microgel-α-mangostin mixture declined from 548 nm to 200 nm upon the increment of the pH. Moreover, it was found the morphology of loaded compound depended largely on the nature of the continuous phase of the microgel system. Interestingly, by manipulating the pH, α-mangostin tends to form crystal at extremely low pH and transforms into spherical shapes at pH 6.ConclusionThis research shows different structures of the α-mangostin particle that are attributed by adjusting the pH using microgel systems as a template.

Characterization of pH-sensitive Polymer Microgel Transport in Porous Media for Improving Oil Recovery

We found that the sizes of poly(acrylic acid) microgels in solution were high enough to be retained by rock matrix. To overcome this problem, we varied the mixing period and mixing rate with and without heat, added NaOH followed by HCl, injected the polymer at the transient state, and added a chemical agent, but without any success. However, the microgel particles were not propagating through the cores and they plugged just the front face of the core. Thus, this kind of polymer can successfully improve oil recovery in fractured systems only, which we proved experimentally to be significant.

Control of number density and swelling/shrinking behavior of P(NIPAM–AAc) particles at solid surfaces

Temperature and pH sensitive microgel particles of poly(N-isopropyl acrylamide) (PNIPAM) and acrylic acid (AAc) comonomers were adsorbed on silicon wafers pre-coated with PEI. The amount of adsorbed microgel particles was controlled by pH and concentration of microgel dispersions and the speed of rotation during the preparation. The swelling/deswelling behaviour of adsorbed P(NIPAM-co-AAc) microgel particles was studied by scanning force microscopy (SFM) and was compared to the temperature-induced volume phase transition in the respective bulk conditions investigated by dynamic light scattering (DLS) and electrophoretic mobility measurements. Also, after the adsorption, the swelling and shrinking is still reversible over several heating and cooling cycles, although the microgel particles are strongly compressed. The particles stay fixed at low ionic strength, up to 10−3M, and intermediate pH. At higher ionic strength or in the acidic or basic regime, they can be moved by the SFM tip or even leave the surface. At low ionic strength the particles swell and shrink in the lateral and vertical directions. At higher ionic strength the particles react more pronouncedly in the vertical direction, and almost not at all in the lateral direction. In contrast to volume measurements, the particles are more strongly swollen at high ionic strength than at low salt concentrations.

Rapid redispersion of pH-sensitive microgel flocculation with poly(lysine)

Rapid redispersion of pH-sensitive microgel flocculation with poly(lysine)

Electrical control of the volume of pH-sensitive micro-gels

The volume of a pH-sensitive polymer gel was electrically controlled in a pH modulator cell composed of a polypyrrole-coated platinum mesh electrode. A pH-sensitive micro-gel of methyl-methacrylate-acrylic acid co-polymer with a diameter of 80 nm in the shrunken state and 140 nm in the swollen state was prepared. The micro-gel volume was reversibly modulated by ca. 30% simultaneously in response to the electrically controlled pH of a solution. The transmittance of the micro-gel suspension was employed to monitor the dynamics of swelling by clarifying the relation of the transmittance to the diameter of the swollen micro-gel by absorbing water.