Non-covalent reconfigurable microgel colloidosomes with a well-defined bilayer shell

Microgels are extremely interfacially active and are widely used to stabilize emulsions. However, they are commonly used to stabilize oil-in-water emulsions due to their intrinsic hydrophilicity and initially dispersed in water. In addition, there have been no attempts to control microgel structural layers that are formed at the interface and as a result it limits applications of microgel in advanced materials. Here, we show that by introducing octanol into poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels, octanol-swollen microgels can rapidly diffuse from the initially dispersed oil phase onto the water droplet surface. This facilitates the formation of microgel-laden interfacial layers with strong elastic responses and also generates stable inverse water-in-oil Pickering emulsions. These emulsions can be used as templates to produce microgel colloidosomes, herein termed ‘microgelsomes’, with shells that can be fine-tuned from a particle monolayer to a well-defined bilayer. The microgelsomes can then be used to encapsulate and/or anchor nanoparticles, proteins, vitamin C, bio-based nanocrystals or enzymes. Moreover, the programmed release of these substances can be achieved by using ethanol as a trigger to mediate shell permeability. Thus, these reconfigurable microgelsomes with a microgel-bilayer shell can respond to external stimuli and demonstrate tailored properties, which offers novel insights into microgels and promise wider application of Pickering emulsions stabilized by soft colloids.

Inverse W/O Pickering emulsions and reconfigurable microgelsomes with a well-defined bilayer structure are prepared from octanol-swollen PNIPAM-co-MAA microgels and the combination of binary microgels, which promise wider application of soft colloids.     

Recyclability of poly (N-isopropylacrylamide) microgel-based assemblies for organic dye removal from water

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels and their aggregates have been shown to effectively remove organic dye molecules from aqueous solutions. Here, we investigate the reusability of these microgel-based systems by exposing them to the organic azo dye molecule, 4-(2-hydroxy-1-naphthylazo) benzenesulfonic acid sodium salt (Orange II). Following exposure, the microgels are isolated, and added to methanol to extract the trapped Orange II from the microgels, followed by a subsequent isolation. The isolated microgels were then exposed to Orange II once again, and the uptake efficiency of the recycled microgels determined. We found that the microgels and their aggregates could be reused to remove the organic dye with little loss in extraction efficiency with the number of recycling cycles.     

Photo‐triggered microgel aggregation using o‐nitrobenzaldehyde as aggregating power source

In this work, cationic and anionic microgels which are mainly formed from thermal responsive polymer, poly(N‐isopropylacrylamide), are prepared and mixed in water. These microgels interact with each other due to the electrostatic interaction, and aggregate voluntarily. By applying the microgel aggregating system, photo‐responsive aggregating system is constructed by using o‐nitrobenzaldehyde (NBA), which reacts and releases hydrogen triggered by photo stimuli. The microgel aggregates in an aqueous solution of NBA re‐disperse depending on the irradiation time of UV light. In addition, by masking the UV irradiated area, the resultant shapes of microgel aggregates are controlled. The aggregated microgel shows rapid and drastic volume changes in response to heat. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1317‐1322     

Synthesis,characterization, and influence of synthesis parameters on particle sizes of a new microgel family

Poly(p‐nitrophenylacrylate‐co‐methacrylamide) and poly(p‐Nitrophenylacrylate‐co‐N,N′‐isopropylacrylamide) reactive microgels were synthesized by precipitation polymerization. The process was followed qualitatively by infrared spectroscopy (ATR‐FTIR) and microgels composition was determined by nuclear magnetic resonance (¹H NMR). Scanning electron microscopy of obtained colloidal particles showed strictly spherical morphologies with a moderate polydispersity. The average hydrodynamic particle diameter and particle size distributions were measured by quasi‐elastic light scattering and the particle size distributions obtained ranged from 100 to 600 nm. Several synthetic parameters affect the particle size of these materials and thus, indirectly, their properties and future applications. In this article, we report the influence of different polymerization reaction conditions in the final microgel dimensions. For example, we observed that the different solvent‐comonomer affinity induced a significant change in swollen particle size of the copolymeric microgels. On the other hand, the crosslinking density limited the particle sizes, but an excess of crosslinker content in the reaction mixture resulted in the opposite effect. Finally, we also studied the influence of initiator content in the mean particle size. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3833–3842, 2007     

Selenium‐Modified Microgels as Bio‐Inspired Oxidation Catalysts

Active colloidal catalysts inspired by glutathione peroxidase (GPx) were synthesized by integration of catalytically active selenium (Se) moieties into aqueous microgels. A diselenide crosslinker (Se X‐linker) was successfully synthesized and incorporated into microgels through precipitation polymerization, along with the conventional crosslinker N,N′‐methylenebis(acrylamide) (BIS). Diselenide bonds within the microgels were cleaved through oxidation by H₂O₂ and converted to seleninic acid whilst maintaining the intact microgel microstructure. Through this approach catalytically active microgels with variable amounts of seleninic acid were synthesized. Remarkably, the microgels exhibited higher catalytic activity and selectivity at low reaction temperatures than the molecular Se catalyst in a model oxidation reaction of acrolein to acrylic acid and methyl acrylate.     

Preparation and characterization of poly (N-isopropylacrylamide)/polyvinylamine core-shell microgels

In this paper, well-defined temperature- and pH-sensitive core-shell microgels were synthesized by graft copolymerization in the absence of surfactant and stabilizer. The microgel particles consisted of poly (N-isopropylacrylamide (NIPAm)) core crosslinked with N, N′-methylene-bisacrylamide (MBA) and polyvinylamine (PVAm) shell. The effect of MBA content and NIPAm/PVAm ratio on microgel size was investigated. SEM showed that the microgels were spherical and had narrow particle-size distribution. TEM images of the microgels clearly displayed well-defined core-shell morphologies. Zeta-potential measurement further elucidated that the microgels possessed positively charged PVAm molecules on the microgel surface. Turbidity measurement and ¹H-nuclear magnetic resonance (NMR) experiments indicated that the VPTT of microgels was the same as the LCST of PNIPAm. ¹H-NMR experiments also inferred that the methyl proton of N-isopropylacrylamide appeared three peaks and responded to hydrogen-bonding interaction including polymer chain with water molecular, intramolecular interaction and intermolecular interaction, respectively.     

Polypyrrole/poly(N‐isopropylacrylamide‐co‐acrylic acid) thermosensitive and electrically conductive composite microgels

The electrically conductive polypyrrole/dodecylbenzene sulfonic acid/poly(N‐isopropylacrylamide‐co‐acrylic acid) (PPy/DBSA/poly(NIPAAm‐co‐AA)) composite microgels were synthesized by a chemical oxidation of pyrrole in the presence of DBSA as the primary dopant, and poly(NIPAAm‐co‐AA) microgels as the polymeric codopant and template, in which APS was used as the oxidant. It was proposed to prepare “intelligent” polymer microgel particles containing both thermosensitive and electrically conducting properties. The polymerization of pyrrole took place directly inside the microgel networks, leading to formation of composite microgels and the morphology was observed by transmission electron microscope. PPy particles interacted strongly with microgels, as the acid groups of microgels acted as the polymeric codopant. The composite microgels thus formed showed electrically conducting behavior dependent on humidity and temperature. At temperatures lower than lower critical solution temperature, the conductivity decreased with increasing the humidity and a small hysteresis phenomenon was observed. The hysteresis became indistinct when temperature was near volume phase transition temperature. However, after the treatment of high temperature and high humidity, the conductivity increased surprisingly due to the structure reorganization inside the composite microgels. The distinctive functionality of the PPy composite microgels was expected to be utilized in many attractive applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1648–1659, 2006     

Effects of Hofmeister anions on the flocculation behavior of temperature-responsive poly(N-isopropylacrylamide) microgels

Effects of some sodium salts (NaCl, NaClO₃, and NaSCN) in the Hofmeister series on deswelling and temperature-induced aggregation behavior of microgels of poly(N-isopropylacrylamide) (PNIPAAM) and PNIPAAM-co-PAA with attached poly(acrylic acid) moieties were investigated with the aid of turbidimetry and dynamic light scattering. Addition of salt in the concentration range 0.1–0.5?M generated aggregation of the PNIPAAM microgel particles at elevated temperatures, but it was no distinct difference between chaotropic and kosmotropic anions. In contrast, the flocculation behavior at high temperatures for PNIPAAM-co-PAA revealed a prominent influence of salinity and type of anion on the formation of aggregates. The aggregation transition was shifted to the highest temperature for the most chaotropic anion (SCN^?), and the aggregation transition at the same salt concentration is consistent with the typical Hofmeister series. The turbidity results from the PNIPAAM-co-PAA microgels disclosed a two-step transition for the considered anions, and both a low and high temperature change in the turbidity data was observed. The high-temperature transition followed the Hofmeister series.     

Fabrication of silver nanoparticles in pH responsive polymer microgel dispersion for catalytic reduction of nitrobenzene in aqueous medium

Copolymer microgels based on N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) have been synthesized by free radical emulsion polymerization using N,N-methylenebisacrylamide (BIS) as a cross-linker. Synthesized microgels were characterized by Fourier transform infrared spectroscopy (FTIR). Then silver nanoparticles were fabricated in the synthesized microgels by in-situ reduction of AgNO₃ with NaBH₄. The formation of silver nanoparticles was confirmed by UV–Vis spectroscopy. The pH sensitivity of the copolymer microgels was investigated using dynamic light scattering technique (DLS). Hydrodynamic radius of P (NIPAM–MAA) microgels increases with increase in pH of the medium at 25°C. Surface plasmon resonance wavelength (λ_SPR) of silver nanoparticles increases with increase in hydrodynamic radius due to change in pH of the medium. The catalytic activity for the reduction of nitrobenzene (NB), an environmental pollutant, into aniline was investigated by UV–Vis spectroscopy in excess of NaBH₄ using hybrid microgels as catalyst. The value of apparent rate constant (k_app) of the reaction was calculated using pseudo first order kinetic model and it was found to be linearly related to the amount of catalyst. The results were compared with literature data. The system was found to be an effective catalyst for conversion of NB into aniline.     

Synthesis and characterization of Poly(N-isopropylacrylamide)/Poly(acrylic acid) semi-IPN nanocomposite microgels

A novel pH- and temperature-sensitive nanocomposite microgel based on linear Poly(acrylic acid) (PAAc) and Poly(N-isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was synthesized by a two-step method. First, PNIPA microgel was prepared via surfactant-free emulsion polymerization by using inorganic clay as a crosslinker, and then AAc monomer was polymerized within the PNIPA microgel. The structure and morphology of the microgel were confirmed by FTIR, WXRD and TEM. The results indicated that the exfoliated clay platelets were dispersed homogeneously in the PNIPA microgels and acted as a multifunctional crosslinker, while the linear PAAc polymer chains incorporated in the PNIPA microgel network to form a semi-interpenetrating polymer network (semi-IPN) structure. The hydrodynamic diameters of the semi-IPN microgels ranged from 360 to 400 nm, which was much smaller than that of the conventional microgel prepared by using N,N′-methylenebis(acrylamide) (MBA) as a chemical crosslinker, the later was about 740 nm. The semi-IPN microgels exhibited good pH- and temperature-sensitivity, which could respond independently to both pH and temperature changes.     

Synthesis and properties of organic-inorganic hybrid P(NIPAM-<Emphasis Type="Italic">co</Emphasis>-AM-<Emphasis Type="Italic">co</Emphasis>-TMSPMA) microgels

Utilizing the hydrolysis and condensation of the methoxysilyl moieties, organic-inorganic hybrid poly(N-isopropylacrylamide-co-acrylamide-co-3-(trimethoxysilyl)propylmethacrylate) P(NIPAM-co-AM-co-TMSPMA) microgels were prepared via two different methods. The first method was that the microgels were post-fabricated from the crosslinkable linear P(NIPAM-co-AM-co-TMSPMA) terpolymer aqueous solutions above the lower critical solution temperature (LCST) of the terpolymer. For the second method, the microgels were directly synthesized by conventional surfactant free emulsion copolymerization of NIPAM, AM, and TMSPMA. The hydrodynamic diameter and stability of the resultant P(NIPAM-co-AM-co-TMSPMA) microgels strongly depend on the pH and temperature of the microgel aqueous solution. The hydrodynamic diameters of the microgels decreased with increasing the measuring temperature. The phase transition temperature of the microgels was found to be around 34°C, which was independent of the initial terpolymer concentration and shifted to lower temperature with increasing the preparation temperature. Increasing the initial amount of AM will enhance the instability of the microgels at high pH values. Moreover, the P(NIPAM-co-AM-co-TMSPMA) microgels obtained from the linear terpolymer had more homogeneous microstructures as compared with the corresponding NIPAM/AM/TMSPMA microgels prepared by one step emulsion copolymerization as revealed by light scattering measurements.     

Glucose sensitive poly (<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microgel based etalons

Thermoresponsive microgels have been shown to be an excellent platform for designing sensor materials. Recently, poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel based etalon materials have been described as direct sensing materials that can be designed to have a single, unique color. These color tunable materials show immense promise for sensing due to their spectral sensitivity and bright visual color. Here, we describe a proof-of-concept for etalon sensing of glucose. We found that aminophenylboronic acid (APBA)-functionalized pNIPAm-co-AAc microgels in an etalon respond to 3 mg/mL glucose concentrations by red shifting their reflectance peaks by 110 nm up to 150 nm. Additionally, APBA-functionalized pNIPAm-co-AAc microgels have a depressed volume phase transition temperature at 18–20 °C, which shifts to 24–26 °C after glucose binding. We also demonstrate that these materials show a marked visual color change, which is a first step towards developing direct read-out sensor devices.     

Solvent exchange kinetics in poly(N-isopropylacrylamide) microgel-based etalons

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based etalons are constructed by depositing thin Au layers (mirrors) on either side of a planar microgel layer. When immersed in water, the microgel layer swells and the etalon exhibits visual color. The thermoresponsivity of the pNIPAm-based microgels allows the Au mirror spacing, and hence the device color, to be dynamically modulated. Necessarily, when the mirror spacing is modulated solvent in the microgel layer must be expelled to the surroundings. Previously, we determined that the etalon deswelling kinetics depended critically on the thickness of the Au layer covering the microgels. Here, we report on solvent exchange kinetics. We found that the time required for solvent entry into the microgel layer is much longer than solvent exit. In addition, the rate was found to again depend critically on the thickness of the Au layer covering the microgel layer; thicker Au layers corresponded to slower solvent exchange kinetics.

Fluorine-containing pH-responsive core/shell microgel particles: preparation,characterization, and their applications in controlled drug release

A kind of novel fluorine-containing pH-responsive core/shell microgels poly(DMAEMA-co-HFMA)-g-PEG were prepared via surfactant-free emulsion polymerization using water as the solvent. The well-defined chemical structure of the copolymers was characterized by FTIR, ¹H-NMR, ¹⁹F-NMR, and elemental analysis. The microgel particles were studied by florescence probe technique, dynamic light scattering, and zeta potential measurement; the microgels displayed a significant pH-responsive behavior. Furthermore, the cytotoxicity assay indicated that the copolymer microgels had low toxicity, and 5-FU-loaded microgels offered a certain killing potency against cancer cells. In addition, the drug loading and in vitro drug release demonstrated that 5-FU was successfully incorporated into polymeric microgels, and the drug-loaded microgels showed a marked pH-dependent drug release behavior. This study suggests that the poly(DMAEMA-co-HFMA)-g-PEG microgels play an important role in the release mechanism stimulated by the change in the pH and have potential applications as a controlled drug release carrier.     

Polyurethane acrylate microgel synthesized by emulsion polymerization using unsaturated self-emulsified polymer

Polyurethane (PU) acrylate microgels were obtained by emulsion polymerization of self-emulsified PU acrylate terminated by 2-hydroxyethyl methacrylate without any extra emulsifier and crosslinker. Moreover, the PU acrylate was also used as stabilizer and crosslinker to synthesize poly(methyl methacrylate) (PMMA)–PU composite microgels via emulsion polymerization, which provided a new method to synthesize PU microgels and their composite microgels. The kinetics of microgel synthesis was studied by gel permeation chromatography. The dynamic rheological behaviors indicated that a crosslinked structure was formed. The frequency dependency of the loss tangent and complex viscosities showed strong relationships with the microgel structure. Those microgels with rigid PMMA core showed higher ability to slide than the soft PU acrylate microgel, which had influence on the changing of loss tangent with frequency. All the microgels swollen in tetrahydrofuran exhibited high viscosities and strong shear-thinning behaviors. As a sort of flexible microgel, the PU microgel was able to form a coherent film at room temperature, which was distinct from hard microgels.     

Spectral time moment analysis of microgel deswelling. Effect of the heating rate

Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through the self-association of amphiphilic HPC molecules and the subsequent cross linking at room temperature. Dynamic Light Scattering was used to study the transport properties of HPC microgels below and above the volume phase transition. Highly nonexponential, multimodal microgel spectra were observed and successfully analyzed by spectral time moment analysis. This article expands earlier results and focuses on the effect of the heating rate on microgel deswelling. During the fast heating two identified microgel modes with apparent hydrodynamic radii (R_H) of 25–30 nm and 400–650 nm collapse into one mode with R_H = 100–150 nm. This indicates the shrinkage of microgel size distribution and an apparent decrease in the radius of larger microgels. During the slow heating, however, both microgel-identified modes remain present above T_c. Although equally represented below the transition, the dominance of larger microgels' mode increases almost two fold with rising temperature above 40°C. Moreover, R_H for this mode increases from 250–300 nm to about 800–850 nm with a multi-step temperature change from 40 to 42.5°C, indicating the growth (and not shrinkage) of microgels. The second mode is represented by the temperature independent R_H, but its contribution goes down from about 50% to less than 10%. © 2008Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2792–2802, 2008     

Impact of microgel morphology on functionalized microgel-drug interactions

The interactions of a range of water-soluble drugs of different charges and hydrophobicities with carboxylic acid-functionalized poly(N-isopropylacrylamide)-based microgels containing different functional group distributions are investigated to determine the impact of drug properties and microgel morphologies on drug uptake and release. The radial distribution of carboxylic acid functional groups in the microgel and the hydrophobicities of the cationic drugs both strongly affect drug partitioning between the solution and microgel phases. Microgels with surface-localized functional group distributions bind less cationic drug than bulk-functionalized microgels, likely due to the formation of a locally collapsed "skin layer" at the acid-base drug binding sites at the microgel surface. In this way, cationic drugs induce a local phase transition that can be used to regulate small molecule diffusion in and out of the gel. As the drug hydrophobicity is increased, the skin layer becomes more condensed and less drug uptake is achieved. In the case of anionic or neutral drugs, high drug uptakes are achieved independent of the functional group distribution within the microgel. High drug uptake is also observed when nonfunctionalized poly(N-isopropylacrylamide) microgels are used as the uptake matrix, suggesting the importance of hydrophobic partitioning in regulating drug-microgel interactions.     

Zwitterionic microgel based anti(-bio)fouling smart membranes for tunable water filtration and molecular separation

Tunable gating polymeric nanostructured membrane with excellent water permeability and precise molecular separation is highly advantageous for smart nanofiltration application. Polymeric nanostructures such as microgels with functionalizable cross-linkable moieties can be an excellent choice to construct membranes with a thin separation layer, functionality, and tunable transport properties. In the present work, we prepared switchable anti(bio)fouling membranes using zwitterionically functionalized antibacterial thermoresponsive aqueous core-shell microgels with a thin separation layer for controlled filtration and separation applications. The microgels were synthesized using a one-step graft copolymerization of poly(N-isopropylacrylamide) and polyethyleneimine (PEI) followed by zwitterionization of free amine groups of PEI chains with 1,3-propane sultone. Microgel synthesis and zwitterionization were confirmed by spectroscopic and elemntal analysis. The obtained microgels were thoroughly characterized to analyze their thermoresponsive behavior, morphology, charge, and antibacterial properties. After that, characterizations were performed to elucidate the surface properties, water permeation, rejection, and flux recovery of the microgel membranes prepared by suction filtration over a track-etched support. It was observed that zwitterionic membrane provides better hydrophilicity, lower bovine serum albumin (BSA) adsorption, and desirable antimicrobial activity. The pure water permeability was directly related to the microgel layer thickness, applied pressure, and temperature of the feed solution. The novel nanostructured membrane leads to an excellent water permeance with a high gating ratio, high flux recovery rate with low irreversible fouling, better rejection for various dyes, and foulant. Most importantly, the long-term performance of the membrane is appreciable as the microgel layer remains intact and provides excellent separation up to a longer period. Owing to easy preparation and well control over thickness, the zwitterionic microgel membranes constitute unique and interactive membranes for various pressure-driven separation and purification applications.     

Preparation of CuS-P(NIPAM-co-MAA)Hybrid Microgels with Controlled Surface Structures

Copper sulfide‐poly(isopropylacrylamide‐co‐methacrylic acid) [CuS‐P(NIPAM‐co‐MAA)] hybrid microgels with patterned surface structures have been synthesized by means of the polymer microgel template technique. The results showed that the surface morphology of the hybrid microgels could be regulated by controlling the decomposition of thioacetamide (TAA) in an acidic medium. The rate of precipitation and the amount of metal sulfide significantly affect the surface structures of the hybrid microgels.     

Controlling biotinylation of microgels and modeling streptavidin uptake

Compared are two approaches for the biotinylation of poly(N-isopropylacrylamide-co-vinylacetic acid) microgels, 300-nm diameter, water swollen particles with a corona of carboxyl groups. The biotinylated microgels are a platform for bioactive water-based ink. Streptavidin binding was measured as a function of biotin density, and the results were interpreted with a new model that predicts the minimum local density of biotins required to capture a streptavidin. An amino-polyethylene glycol derivative of biotin gave higher biotin contents than a biotin hydrazide. However, the streptavidin content versus biotin content results for both biotin derivatives fell on the same master curve with maximum biotin coverage of 0.11?mg of bound streptavidin per milligram of biotinylated microgel. Exclusion experiments showed that streptavidin was too big to penetrate the cross-linked microgel structure; thus, the conjugated streptavidin was restricted to the microgel surface. The colloidal stability of the microgels was preserved, and the biotinylated products showed good hydrolytic stability.