Polymer microgels: reactors for semiconductor, metal, and magnetic nanoparticles

We report a strategy for the production of materials with structural hierarchy. The approach employs polymer microgels as templates for the synthesis of semiconductor, metal, or magnetic nanoparticles (NPs). We show that NPs with predetermined dimensions and size-dependent properties can be synthesized by using a very delicate balance between the reaction conditions, the composition and the structure of microgel templates, and the concentration of NPs in the microgel. Postheat treatment of microgels doped with semiconductor nanoparticles reduces NP polydispersity and allows control of their photoluminescence. Microgel templates are particularly beneficial in the synthesis of polymer microspheres heavily loaded with monodisperse superparamagnetic Fe(3)O(4) NPs. Hybrid submicrometer-size microgels have promising potential applications in photonics, catalysis, and separation technologies.     

Phototactic Poly(N-isopropyl acrylamide) Microgels with Photoresponsive Property

Smart functional microgels hold great potential in a variety of applications, especially in drug transportation. However, current drug carriers based on physiological internal stimuli cannot efficiently orientate to designated locations. Therefore, it is necessary to introduce the self-propelled particles to the drug release of the microgels. In order to study self-propulsion of microgels induced by light, it is also a challenge to prepare micron-sized microgels so that they can be observed directly under optical microscopes. In this work, phototactic microgels with photoresponsive properties are prepared. The microgel particles can be observed by confocal laser scanning microscopy. The photoresponsive properties of microgels are fully investigated by various instruments. Light can also regulate the state of the microgel solution, making it switch between turbidity and clarity. The phototaxis of particles irradiated by UV light was studied, which may be used for microgels enrichment and drug transportation and release.     

Aqueous microgels for the growth of hydroxyapatite nanocrystals

In present article, we demonstrate that aqueous microgels can be used as containers for the in-situ synthesis of hydroxyapatite. The hydroxyapatite nanocrystals (HAp NCs) become integrated into microgels forming hybrid colloids. The HAp NCs loaded in the microgel can be varied over a broad range. The HAp NCs are localized within the microgel corona. The deposition of the inorganic nanocrystals decreases the colloidal stability of the microgels and leads to particle aggregation at high HAp NCs loading. Because of the strong interactions between HAp NCs and polymer chains, the swelling degree of microgels decreases, and temperature-sensitive properties disappear at high loading of the inorganic component. We demonstrate that hybrid colloids can be used as building blocks for the preparation of nanostructured films on solid substrates.     

Application of microgels as polymer supports for organic synthesis: preparation of a small phthalide library,a scavenger,and a borohydride reagent

Microgel polymers containing a series of functional groups have been prepared. These microgels were composed of cross-linked poly(styrene) and were prepared by radical polymerization in solution. The microgel polymers exhibit good solubility in an array of different organic solvents, and in addition, they can be efficiently precipitated by the addition of methanol and isolated by filtration. A nine-member phthalide library was synthesized using an aminomethyl-functionalized microgel 5. To further demonstrate the versatility of these microgel polymers, tris(2-aminoethyl)amino microgel 11 was examined as a scavenger reagent to remove unreacted isocyanate after a urea synthesis. Finally, a microgel-supported ammonium borohydride reagent 14 was successfully prepared and used as a reducing agent. Notable features of these microgels are that in all applications the progress of the reaction could be monitored by standard NMR techniques and their preparation is performed using common glassware and techniques found in all organic laboratories.     

POLY（N-ISOPROPYL ACRYLAMIDE） MICROGEL DOPED WITH LUMINESCENT PBS QUANTUM DOTS

Thiol-stabilized PbS quantum dots （QDs） with dimensions 3-5 nm capped with a mixture of 1-thioglycerol/dithioglycerol （TGL/DTG） were coUoidally prepared at room temperature. Room temperature photoluminescence quantum efficiency of freshly prepared PbS QDs （7%-11%） remained higher than 5% upon aging for three weeks when the nanocrystals （NCs） were stored in an ice-bath in the dark, and higher than 5%for at least five weeks when extra DTG ligands were introduced into the nanocrystal solution followed by stirring every two weeks. Poly（N-isopropyl acrylamide） （PNIPAM） microgels were produced via precipitation polymerization with dimensions of ca. 230 nm and polydispersity of 3-5%. Incorporation of PbS QDs into PNIPAM microgels indicated that PbS can be incorporated into the interior of microgel particles and not at the microgel interface. The combination of reasonable room temperature quantum efficiency and strong, efficient luminescence covering the 1.3-1.55 μm telecommunication window makes these nanoparticles promising materials in optical devices and telecommunications.     

含铅聚合物微凝胶与硫化氢反应前后在良溶剂中的分子扩散行为

用溶液聚合法制备出轻度交联的含铅微凝胶,用光子相关光谱技术测定其在良溶剂中与H2S反应前、后的扩散行为,由外推法得到在浓度无限稀时的分子扩散系数,给出微凝胶的流体力学半径。结果表明:相同量的含铅微凝胶在不同的初始浓度下与H2S反应,生成含PbS纳米微粒的凝胶；但其体积增大不同,这说明H2S与含铅微凝胶的反应既可以在分子内也可以在分子间进行,分子间的反应使含硫化铅微凝胶扩散系数随浓度的变化曲线的线性范围变小。     

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.     

UCST-like hybrid PAAm-AA/Fe3O4 microgels. Effect of Fe3O4 nanoparticles on morphology, thermosensitivity and elasticity

The incorporation of metal oxide nanoparticles into microgels forming hybrid systems gives additional functionalities to the system and widens the field of potential application in biomedicine, biotechnology, and other fields. In particular, there have been very few investigations regarding UCST-like hybrid microgels. In connection with this, we report the preparation of UCST-like hybrid microgels of magnetite nanoparticles (Fe(3)O(4)) encapsulated in poly(acrylamide-acrylic acid) microgel matrix via an inverse emulsion polymerization method. The key factor in the preparation of hybrid microgels is the need to divide in two the aqueous phase of the emulsion and feed them separately in order to avoid the aggregation of magnetic nanoparticles prior to polymerization reaction. The morphology, size, and spherical shape of hybrid microgels are determined by scanning electron microscopy. The encapsulation of magnetite nanoparticles within the polymer matrix is confirmed by transmission electron microscopy. Dynamic light scattering is employed to study both the swelling UCST-like behavior and the surface charge of the hybrid microgels. Swelling measurements confirm that the incorporation of magnetite does not affect the thermosensitivity of the system. In order to highlight the rheological behavior that can affect the final potential applications of these hybrid systems, a deep study of the viscoelastic properties is carried out by means of an oscillatory rheometer. The dependence of G' and G' of the microgel dispersions with the frequency suggests a gel-like behavior and hence the occurrence of structural organization. In order to understand this structure formation and the influence of the magnetite in the interaction between hybrid microgels, scaling theory was applied. In terms of rheology, the addition of magnetite leads to a change in the interaction between hybrid microgels giving rise to an increase in the elasticity of the system.     

含铅聚合物微凝胶与硫化氢反应前后在良溶剂中的分子扩散行为

用溶液聚合法制备出轻度交联的含铅微凝胶,用光子相关光谱技术测定其在良溶剂中与H₂S反应前、后的扩散行为,由外推法得到在浓度无限稀时的分子扩散系数,给出微凝胶的流体力学半径。结果表明:相同量的含铅微凝胶在不同的初始浓度下与H₂S反应,生成含PbS纳米微粒的凝胶;但其体积增大不同,这说明H₂S与含铅微凝胶的反应既可以在分子内也可以在分子间进行,分子间的反应使含硫化铅微凝胶扩散系数随浓度的变化曲线的线性范围变小。     

Distribution of CdSe quantum dots within swollen polystyrene microgel particles using confocal microscopy

CdSe quantum dots (QDs) are semiconducting nanoparticles that fluoresce when stimulated by visible light. This property has been exploited in their use as tracer particles in biomedical applications. In this study, confocal microscopy has been used to determine the distribution of QDs within polystyrene microgel particles, dispersed in an organic solvent. It was found that the extent of microgel swelling affected the penetration of the QDs into the particles. Only when the microgel particles were swollen to their maximum extent were the QDs able to penetrate into the central core region of the particles.     

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.     

Effect of initial substrate concentration of the Belousov-Zhabotinsky reaction on self-oscillation for microgel system

Self-oscillation for the microgel particles ( approximately 200 nm in diameter) was studied by changing initial substrate concentrations (i.e., malonic acid, sodium bromate, and nitric acid) of the Belousov-Zhabotinsky (BZ) reaction that is used for chemical energy for the self-oscillation. The cross-linked microgels are composed of N-isopropylacrylamide and ruthenium tris(2,2'-bipyridine), Ru(bpy) 3, which is a catalyst for the BZ reaction. Comparing with the homogeneous, stirred solution of the bulk solution for the BZ reaction, swelling/deswelling oscillation of the microgels showed longer induction period, different dependence of initial substrate concentrations on oscillation period, and different oscillation rhythm. The change in oscillation for the microgels can be understood by considering the microgel network effect.     

Grid pattern of nanothick microgel network

A novel grid pattern of two kinds of nanothick microgels was developed by alternate patterning using photolithography. At first, 100-microm-wide nanothick PAAm microgel stripes were grafted on a polystyrene surface by UV irradiation of the photoreactive azidobenzoyl-derivatized polyallylamine-coated surface through a photomask with 100-microm-wide stripes. Then, a second set of 100-microm-wide nanothick PAAc microgel stripes were grafted across the PAAm-grated polystyrene surface by UV irradiation of the photoreactive azidophenyl-derivatized poly(acrylic acid)-coated surface through a photomask placed perpendicularly to the first set of PAAm microgel stripes. The PAAc microgel stripe pattern was formed over the PAAm microgel stripe pattern. The cross angle of the two microgel stripes could be controlled by adjusting the position of the photomask when the second microgel pattern was prepared. Swelling and shrinking of the microgels were investigated by scanning probe microscopy (SPM) in an aqueous solution. SPM observation indicated that the thickness of the gel network was 100 to 500 nm. The regions containing PAAm, PAAc, and the PAAc-PAAm overlapping microgels showed different swelling and shrinking properties when the pH was changed. The PAAm microgel swelled at low pH and shrank at high pH whereas the PAAc microgel swelled at high pH and shrank at low pH. However, the PAAc-PAAm overlapping microgel did not change as significantly as did the two microgels, indicating that the swelling and shrinking of the two gels was partially offset. The pH-induced structural change was repeatedly reversible. The novel grid pattern of nanothick microgels will find applications in various fields such as smart actuators, artificial muscles, sensors, and drug delivery systems as well as in tissue engineering and so forth.     

Poly(N-vinylcaprolactam-co-glycidyl methacrylate) aqueous microgels labeled with fluorescent LaF3:Eu nanoparticles

We describe the synthesis and properties of functional microgel particles based on poly(N-vinylcaprolactam-co-glycidyl methacrylate) (PVCL/PGMA) copolymer. A series of colloidally stable microgel particles with a range of glycidyl methacrylate content were prepared by surfactant-free heterophase polymerization in water. The microgel particles obtained had hydrodynamic radii between 250 and 350 nm and were fairly monodisperse in size; however, a broadening of the particle size distribution was observed for samples with a low GMA content. The PVCL/PGMA microgel particles exhibit thermally responsive reversible changes in diameter in water, and the swelling degree increased with the PVCL fraction in the copolymer structure. These microgels were then modified with photoluminescent europium-doped lanthanum fluoride nanoparticles (LaF3:Eu-AEP) through reaction of the 2-aminoethyl phosphate surface ligands with epoxy groups present in the microgel. These hybrid microgels were colloidally stable and thermally responsive in aqueous solution.     

Hollow,pH-Sensitive Microgels as Nanocontainers for the Encapsulation of Proteins

Depending on their architectural and chemical design, microgels can selectively take up and release small molecules by changing the environmental properties, or capture and protect their cargo from the surrounding conditions. These outstanding properties make them promising candidates for use in biomedical applications as delivery or carrier systems. In this study, hollow anionic p(N-isopropylacrylamid-e-co-itaconic acid) microgels are synthesized and analyzed regarding their size, charge, and charge distribution. Furthermore, interactions between these microgels and the model protein cytochrome c are investigated as a function of pH. In this system, pH serves as a switch for the electrostatic interactions to alternate between no interaction, attraction, and repulsion. UV–vis spectroscopy is used to quantitatively study the encapsulation of cytochrome c and possible leakage. Additionally, fluorescence-lifetime images unravel the spatial distribution of the protein within the hollow microgels as a function of pH. These analyses show that cytochrome c mainly remains entrapped in the microgel, with pH controlling the localization of the protein – either in the microgel's cavity or in its network. This significantly differentiates these hollow microgels from microgels with similar chemical composition but without a solvent filled cavity.     

Temperature-Responsive Poly(N-Isopropylacrylamide-Acrylamide-Phenylboronic Acid) Microgels for Stabilization of Silver Nanoparticles

Poly(N-isopropylacrylamide-acrylamide-phenylboronic acid) [P(NIPAM-AAm-PBA)] microgels of uniform size were prepared by the chemical reaction of 3-aminophenylboronic acid with poly(N-isopropylacrylamide-acrylamide-acrylic acid) [P(NIPAM-AAm-AA)] microgels in aqueous medium in the presence of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride catalyst via carbodiimide coupling. Silver (Ag) nanoparticles were prepared using seed-mediated growth method and stabilized in P(NIPAM-AAm-PBA)] microgels. Ag nanoparticles and hybrid microgels were characterized by transmission electron microscopy, UV–visible, and dynamic light scattering techniques. The temperature-responsive behavior of hybrid microgels was found to be similar to that of the pure microgels. The value of volume transition temperature of hybrid microgels was found to be slightly higher than that of pure microgels due to shielding effect of Ag nanoparticles present on the surface of microgel particle. The decrease in the size of hybrid microgels as compared to that of pure microgels in swollen state is due to physical cross-linking by Ag nanoparticles inside the network of microgels. The stable hybrid polymer microgel system has a potential to be used for different applications.     

Metal nanocrystals incorporated within pH-responsive microgel particles

Cross-linked sterically stabilized latexes of approximately 250 nm diameter were synthesized by emulsion polymerization of 2-(diethylamino)ethyl methacrylate using a bifunctional oligo(propylene oxide)-based diacrylate cross-linker and a poly(ethylene oxide)-based macromonomer as the stabilizer at pH 9. These particles exhibit reversible swelling properties in water by adjusting the solution pH. At low pH, they exist as swollen microgels as a result of protonation of the tertiary amine units. Deswelling occurs above pH 7 [the effective pK(a) of poly(2-(diethylamino)ethyl methacrylate)], leading to the formation of the original compact latex particles. The swollen microgels can be used as nanoreactors: efficient impregnation with Pt nanoparticles can be achieved by incorporating precursor platinum compounds, followed by metal reduction. Dynamic light scattering was used to compare two methods of Pt nanoparticle impregnation with respect to the size and stability of the final Pt-loaded microgel particles. In the first method, the H2PtCl6 precursor was added to hydrophobic latex particles at high pH, followed by metal reduction. In the second method, H2PtCl6 was added to hydrophilic swollen microgel particles at low pH, and then this metal salt was reduced in situ using NaBH4 and the pH was raised by the addition of base. Both the Pt salt-loaded (metalated) microgels and the final Pt nanoparticle-loaded microgels had well-defined structures that were independent of the synthesis route. Polymer-metal interactions were investigated by UV-visible absorption spectroscopy, which confirmed that the Pt salt was completely reduced to zero-valent Pt. Transmission electron microscopy and X-ray diffraction studies verified the formation of nanometer-sized Pt nanoparticles within these microgels, which can be used as recoverable colloidal catalyst supports for various organic reactions.     

Hybrid microgels with reversibly changeable multiple brilliant color

We report reversibly color changeable hybrid microgels that tune multiple brilliant colors due to interparticle interactions of SPR using several structured nanoparticles. The interparticle interactions were brought out using the thermosensitive swelling/deswelling property of microgel. We employ N-isopropylacrylamide (NIPAM) and glycidyl methacrylate (GMA) copolymerized microgels (NG microgels) as templates for in situ synthesis of Au nanoparticles. The seed Au nanoparticles could be stably grown by successive reduction of Au and Ag in the microgels. Interestingly, the hybrid microgels were able to exhibit multiple brilliant colors by attaching Au/Ag multiple core/shell bimetallic nanoparticles in the microgels, and the color change reversibility of each hybrid microgel was accomplished by adjusting the nanoparticles' sizes. Obtained microgels shown in this study will find important applications such as in biomedical and electronic devices.     

Flow of microgel capsules through topographically patterned microchannels

We investigated the flow dynamics of microgel capsules in topographically patterned microfluidic devices. For microgels flowing through channel constrictions, or orifices, we observed three phenomena: (i) the effect of confinement, (ii) the role of interactions between the microgels and the channel surface, and (iii) the effect of the velocities of microgels prior to their passage through an orifice. We studied negatively charged alginate microgels and positively charged alginate microgels coated with N-(2-hydroxy)propyl-3-trimethylammonium chitosan chloride (HTCC). Aqueous dispersions of microgels were driven through poly(dimethyl siloxane) microchannels carrying a weak negative surface charge. The velocity of the continuous phase, and hence, the velocity of the microgels increased as they passed through topographically patterned orifices. Alginate microgels were observed to have a larger increase in velocity relative to HTCC-coated alginate microgels. This effect, which was attributed to electrostatic attraction or repulsion, was found to be strongest for orifices with dimensions close to the microgel diameter. For example, when 75 microm-diameter microgels flowed through a 76 microm orifice, alginate gels (negatively charged) experienced a 2x greater increase in velocity than HTCC-coated (positively charged) microgels. This effect was exaggerated at lower initial flow rates. For example, when 75 microm-diameter microgels flowed through an 80 microm orifice, a two-fold difference in the velocity changes of the two microgel types was observed when the initial flow rate was 275 microm s(-1), while a three-fold difference in velocity changes was observed when the initial flow rate was 130 microm s(-1). We speculate that these studies will be useful for modeling the flow of suspensions of cells or other biologically relevant particles for a wide range of applications.