Novel Membrane Adsorbers with Grafted Zwitterionic Polymers Synthesized by Surface-Initiated ATRP and Their Salt-Modulated Permeability and Protein Binding Properties

A novel zwitterionic polymer functionalized porous membrane adsorber was obtained by grafting poly­(N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine) (polySPE) to poly­(ethylene terephthalate) (PET) track-etched membrane surface via surface-initiated atom transfer radical polymerization (SI-ATRP). The ATRP conditions were optimized, the thus established grafting was well-controlled, and the degree of grafting could be adjusted. Functionalized membranes with a degree of grafting of about 3.5 μg/cm² relative to the specific surface area showed almost zero values of zeta potential estimated from the trans-membrane streaming potential measurements. Typical “anti-polyelectrolyte” effect was observed for the polySPE grafted membranes. Flux through the membrane was reduced by adding chaotropic chloride and perchlorate salts to the solution which extended the polySPE chains grafted on the membrane pore wall. Perchlorate salt exhibited much stronger effect on polySPE chain conformation than chloride salt and for a membrane with a degree of grafting of 2.7 μg/cm², even 2 mM KClO₄ could extend the thickness of the polymer layer to more than two times (∼43 nm) of that in pure water (∼20 nm). On the contrary, small amounts of kosmotropic ions (10 mM SO₄^2‑) further “salted out” the polySPE chains and led to a slightly increased flux. PolySPE grafted PET membranes with different degree of grafting were then used as membrane adsorber for protein binding. Human IgG was used as model protein and the binding capacity was evaluated under both static (no convective flow through the membrane) and dynamic conditions (flow-through conditions). Static adsorption experiments showed that IgG could be loaded to the membrane at medium salt concentration and 85–95% of bound protein could be eluted at either low (zero) or very high salt concentrations. Dynamic flow-through experiments then revealed the influences of salt concentration and salt type on IgG binding. Effects of two chaotropic salts, NaCl and NaClO₄, were evaluated. Slight but not negligible binding of IgG from pure water was suppressed by adding NaCl. IgG binding was then increased in the NaCl concentration range of 100–500 mM and reached a maximum binding capacity value at about 500 mM. Further increase of NaCl concentration led to a decreased binding again. KClO₄ showed similar effects onto IgG binding, but this salt functions in a much lower and much narrower concentration range. All results with respect to grafted layer swelling and protein binding followed the empirical Hofmeister series

Dispersions of Various Titania Nanoparticles in Two Different Ionic Liquids

The dispersibility of different lab-made and commercial TiO₂ nanoparticles prepared by gas-phase processes in room temperature ionic liquids was for the first time studied by dynamic light scattering and advanced rheology. The characterization of the nanopowders has been done with transmission electron microscopy, X-ray diffraction analysis, nitrogen adsorption, and Brunauer–Emmett–Teller (BET) analysis and FT-IR spectroscopy. The colloidal stabilities of the resulting dispersions were strongly influenced by particle characteristics such as aggregation level, mean particle size, and surface functionality. The period of the ultrasound treatment, the powder concentration in the dispersion, and the hydrophilicity of the ionic liquid were also important influences. It was found that most types of powders disperse better in the hydrophilic ionic liquid because of the hydroxyl groups and adsorbed water present on the powders' surfaces. The best dispersions over a broader concentration range were obtained for a lab-made powder produced by chemical vapor synthesis (aerosol method) which had the smallest nonaggregated particles

Ionic Liquid-Based Route for the Preparation of Catalytically Active Cellulose–TiO₂ Porous Films and Spheres

The present work evaluates the possibilities of processing cellulose with ionic liquids and functional nanoparticles like TiO₂ toward a new generation of porous nanocomposites, shaped as films or spheres, which may find direct application in water purification, catalysis, and self-cleaning materials. The focus was set on the factors controlling the formation of the porous film structure during the nonsolvent induced phase separation process from polymer solutions in ionic liquids via immersion in water and during the porous film drying step. Temperature and cosolvent addition facilitate cellulose solubilization and help control the phase separation by improving the mass transfer. The complex relation between the catalytic activity of the porous TiO₂–cellulose nanocomposite materials obtained under different processing conditions and their structure has been studied during the photodegradation of model organic dyes like rhodamine B and methylene blue. After drying, the catalytic activity of the nanocomposites decreases as a consequence of the reformation of the intra- and intermolecular hydrogen bonds in cellulose which diminish the flexibility and the mobility of the fine cellulose fibrils network

Magnetoresponsive Poly(ether sulfone)-Based Iron Oxide <i>cum</i> Hydrogel Mixed Matrix Composite Membranes for Switchable Molecular Sieving

Stimuli-responsive membranes that can adjust mass transfer and interfacial properties “on demand” have drawn large interest over the last few decades. Here, we designed and prepared a novel magnetoresponsive separation membrane with remote switchable molecular sieving effect by simple one-step and scalable nonsolvent induced phase separation (NIPS) process. Specifically, poly­(ether sulfone) (PES) as matrix for an anisotropic membrane, prefabricated poly­(<i>N</i>-isopropylacrylamide) (PNIPAAm) nanogel (NG) particles as functional gates, and iron oxide magnetic nanoparticles (MNP) as localized heaters were combined in a synergistic way. Before membrane casting, the properties of the building blocks, including swelling property and size distribution for NG, and magnetic property and heating efficiency for MNP, were investigated. Further, to identify optimal film casting conditions for membrane preparation by NIPS, in-depth rheological study of the effects of composition and temperature on blend dope solutions was performed. At last, a composite membrane with 10% MNP and 10% NG blended in a porous PES matrix was obtained, which showed a large, reversible, and stable magneto-responsivity. It had 9 times higher water permeability at the “on” state of alternating magnetic field (AMF) than at the “off”-state. Moreover, the molecular weight cutoff of such membrane could be reversibly shifted from ∼70 to 1750 kDa by switching off or on the external AMF, as demonstrated in dextran ultrafiltration tests. Overall, it has been proved that the molecular sieving performance of the novel mixed matrix composite membrane can be controlled by the swollen/shrunken state of PNIPAAm NG embedded in the nanoporous barrier layer of a PES-based anisotropic porous matrix, via the heat generation of nearby MNP. And the structure of such membrane can be tailored by the NIPS process conditions. Such membrane has potential as enabling material for remote-controlled drug release systems or devices for tunable fractionations of biomacromolecule/-particle mixtures

How Do Polyethylene Glycol and Poly(sulfobetaine) Hydrogel Layers on Ultrafiltration Membranes Minimize Fouling and Stay Stable in Cleaning Chemicals?

We compare the efficiency of grafting polyethylene glycol (PEG) and poly­(sulfobetaine) hydrogel layer on poly­(ether imide) (PEI) hollow-fiber ultrafiltration membrane surfaces in terms of filtration performance, fouling minimization and stability in cleaning solutions. Two previously established different methods toward the two different chemistries (and both had already proven to be suited to reduce fouling significantly) are applied to the same PEI membranes. The hydrophilicity of PEI membranes is improved by the modification, as indicated by the change of contact angle value from 89° to 68° for both methods, due to the hydration layer formed in the hydrogel layers. Their pure water flux declines because of the additional permeation barrier from the hydrogel layers. However, these barriers increase protein rejection. In the exposure at a static condition, grafting PEG or poly­(sulfobetaine) reduces protein adsorption to 23% or 11%, respectively. In the dynamic filtration, the hydrogel layers minimizes the flux reduction and increases the reversibility of fouling. Compared to the pristine PEI membrane that can recover its flux to 42% after hydraulic cleaning, the PEG and poly­(sulfobetaine) grafted membranes can recover their flux up to 63% and 94%, respectively. Stability tests show that the poly­(sulfobetaine) hydrogel layer is stable in acid, base and chlorine solutions, whereas the PEG hydrogel layer suffers alkaline hydrolysis in base and oxidation in chlorine conditions. With its chemical stability and pronounced capability of minimizing fouling, especially irreversible fouling, protective poly­(sulfobetaine) hydrogel layers have great potential for various membrane-based applications

Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite Films Prepared by a Green Method

The present work expands our previous studies related to cellulose processing with room-temperature ionic liquids and simultaneous integration of functional nanoparticles toward photocatalytically active and easily recyclable nanocomposite porous films based on a renewable matrix material. Porosity can be tuned by the selection of phase separation conditions for the films obtained from the casting solutions of cellulose in ionic liquids or their mixture with an organic co-solvent. TiO₂ nanoparticles confer to the nanocomposite photocatalytic activity, while Fe₃O₄ nanoparticles make it magnetically active. The photocatalytic activity of the cellulose film containing 10 mg of TiO₂ was 1 order of magnitude lower than that of the same amount of pure TiO₂ nanopowder, due to the reduction of the active catalytic surface which can be reached by UV irradiation after embedment in the polymer matrix. However, this fixation in a solid polymer support allows facile recovery of the catalyst after use. The rate constant when using the cellulose nanocomposite doped with TiO₂ and Fe₃O₄ (<i>k</i> ≈ 0.0019 min^–1) is very close to that for the corresponding composite containing only TiO₂ (<i>k</i> ≈ 0.0017 min^–1), suggesting that co-doping with Fe₃O₄ nanoparticles did not diminish the photocatalytic activity of the final composite, which can be easily separated from solution with a magnet. Additionally, by Fe₃O₄ doping, the composite material’s temperature can be homogeneously increased by ∼12 K via exposure to a high-frequency alternating magnetic field (AMF) for 5 min. For an optimal thermal response to AMF, the magnetite nanoparticles have to be homogeneously dispersed within the polymer matrix. The preparation method for the casting solution has been found to play an essential role for the one-step fabrication of multifunctional cellulose-based nanocomposite materials

Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite Films Prepared by a Green Method

The present work expands our previous studies related to cellulose processing with room-temperature ionic liquids and simultaneous integration of functional nanoparticles toward photocatalytically active and easily recyclable nanocomposite porous films based on a renewable matrix material. Porosity can be tuned by the selection of phase separation conditions for the films obtained from the casting solutions of cellulose in ionic liquids or their mixture with an organic co-solvent. TiO₂ nanoparticles confer to the nanocomposite photocatalytic activity, while Fe₃O₄ nanoparticles make it magnetically active. The photocatalytic activity of the cellulose film containing 10 mg of TiO₂ was 1 order of magnitude lower than that of the same amount of pure TiO₂ nanopowder, due to the reduction of the active catalytic surface which can be reached by UV irradiation after embedment in the polymer matrix. However, this fixation in a solid polymer support allows facile recovery of the catalyst after use. The rate constant when using the cellulose nanocomposite doped with TiO₂ and Fe₃O₄ (<i>k</i> ≈ 0.0019 min^–1) is very close to that for the corresponding composite containing only TiO₂ (<i>k</i> ≈ 0.0017 min^–1), suggesting that co-doping with Fe₃O₄ nanoparticles did not diminish the photocatalytic activity of the final composite, which can be easily separated from solution with a magnet. Additionally, by Fe₃O₄ doping, the composite material’s temperature can be homogeneously increased by ∼12 K via exposure to a high-frequency alternating magnetic field (AMF) for 5 min. For an optimal thermal response to AMF, the magnetite nanoparticles have to be homogeneously dispersed within the polymer matrix. The preparation method for the casting solution has been found to play an essential role for the one-step fabrication of multifunctional cellulose-based nanocomposite materials

Systematic Investigation of Dispersions of Unmodified Inorganic Nanoparticles in Organic Solvents with Focus on the Hansen Solubility Parameters

Dispersions of unmodified nanoparticles (titanium dioxide, hydroxyapatite) were prepared by redispersion of nanoparticle powders in organic solvents using an ultrasound treatment. The dispersion quality was judged by dynamic light scattering (DLS) measurements and visual evaluation. Whereas “bad” solvents led to no or unstable dispersions with large particle diameters, dispersions made from the “good” solvents consisted of particles with relatively small diameters and were stable for several days or longer. For titanium dioxide, mixtures from four of the “good” solvents identified after first screening of a large set of solvents were prepared and tested as dispersion agent. Thus obtained dispersions showed superior properties compared to the previous dispersions, with small particles sizes and good long-time stability. Based on a rating of solvent quality and by calculation using the software HSPiP v3, the Hansen solubility parameters of the particles were then determined. Subsequently, entirely new solvent mixtures that could best fit these parameters were selected and found to also exhibit suitable properties as dispersion agent for the nanoparticles. The same iterative and quantitative approach worked also for the preparation of good and stable dispersions of hydroxyapatite. All results show that this is a promising methodology to disperse inorganic nanoparticles into suited organic solvents, for instance for the preparation of new polymeric nanocomposites. Furthermore, the method can be used to indirectly characterize the surface chemistry of nanoparticles

Antifouling and Antibacterial Multifunctional Polyzwitterion/Enzyme Coating on Silicone Catheter Material Prepared by Electrostatic Layer-by-Layer Assembly

The formation of bacterial biofilms on indwelling medical devices generally causes high risks for adverse complications such as catheter-associated urinary tract infections. In this work, a strategy for synthesizing innovative coatings of poly­(di­methyl­siloxane) (PDMS) catheter material, using layer-by-layer assembly with three novel functional polymeric building blocks, is reported, i.e., an antifouling copolymer with zwitterionic and quaternary ammonium side groups, a contact biocidal derivative of that polymer with octyl groups, and the antibacterial hydrogen peroxide (H₂O₂) producing enzyme cellobiose dehydrogenase (CDH). CDH oxidizes oligosaccharides by transferring electrons to oxygen, resulting in the production of H₂O₂. The design and synthesis of random copolymers which combine segments that have antifouling properties by zwitterionic groups and can be used for electrostatically driven layer-by-layer (LbL) assembly at the same time were based on the atom-transfer radical polymerization of di­methyl­amino­ethyl meth­acrylate and subsequent partial sulfo­beta­ini­zation with 1,3-propane sultone followed by quaternization with methyl iodide only or octyl bromide and thereafter methyl iodide. The alternating multilayer systems were formed by consecutive adsorption of the novel polycations with up to 50% zwitterionic groups and of poly­(styrene­sulfonate) as the polyanion. Due to its negative charge, enzyme CDH was also firmly embedded as a polyanionic layer in the multilayer system. This LbL coating procedure was first performed on prefunctionalized silicon wafers and studied in detail with ellipsometry as well as contact angle (CA) and zetapotential (ZP) measurements before it was transferred to prefunctionalized PDMS and analyzed by CA and ZP measurements as well as atomic force microscopy. The coatings comprising six layers were stable and yielded a more neutral and hydrophilic surface than did PDMS, the polycation with 50% zwitterionic groups having the largest effect. Enzyme activity was found to be dependent on the depth of embedment in the multilayer coating. Depending on the used polymeric building block, up to a 60% reduction in the amount of adhering bacteria and clear evidence for killed bacteria due to the antimicrobial functionality of the coating could be confirmed. Overall, this work demonstrates the feasibility of an easy to perform and shape-independent method for preparing an antifouling and antimicrobial coating for the significant reduction of biofilm formation and thus reducing the risk of acquiring infections by using urinary catheters

Design of Thermally Responsive Polymeric Hydrogels for Brackish Water Desalination: Effect of Architecture on Swelling, Deswelling, and Salt Rejection

In this work, we explore the ability of utilizing hydrogels synthesized from a temperature-sensitive polymer and a polyelectrolyte to desalinate salt water by means of reversible thermally induced absorption and desorption. Thus, the influence of the macromolecular architecture on the swelling/deswelling behavior for such hydrogels was investigated by tailor-made network structures. To this end, a series of chemically cross-linked polymeric hydrogels were synthesized via free radical-initiated copolymerization of sodium acrylate (SA) with the thermoresponsive comonomer <i>N</i>-isopropylacrylamide (NIPAAm) by realizing different structural types. In particular, two different polyNIPAAm macromonomers, either with one acrylate function at the chain end or with additional acrylate functions as side groups were synthesized by controlled polymerization and subsequent polymer-analogous reaction and then used as building blocks. The rheological behaviors of hydrogels and their estimated mesh sizes are discussed. The performance of the hydrogels in terms of swelling and deswelling in both deionized water (DI) and brackish water (2 g/L NaCl) was measured as a function of cross-linking degree and particle size. The salt content could be reduced by 23% in one cycle by using the best performing material