Poly(N,N-dimethylaminoethyl methacrylate)/graphene oxide hybrid hydrogels: pH and temperature sensitivities and Cr(VI) adsorption

Different amounts of graphene oxide (GO) were incorporated to N,N-dimethylaminoethyl methacrylate (DMAEMA), fabricating a series of pH and temperature dual sensitive PDMAEMA/GO hybrid hydrogels by in situ polymerization. Their microscopic network structures as well as swelling properties and Cr(VI) adsorption were characterized. The equilibrium swelling ratios (ESR) of hydrogels increased significantly with 0.5 wt% GO feeding of DMAEMA amount, and then decreased with further GO loading increasing. All hydrogels showed obvious deswelling when pH value of swelling mediums increased from 5 to 10 gradually. At pH 7, hydrogels revealed slight ESR increment with temperature up to 50 °C, above which obvious deswelling occurred. In pH 8 buffer, 0.5 wt% of GO loading triggered lower critical solution temperature (LCST) to decrease by 3 °C, and 2–7 °C increment was observed when 1–6 wt% of GO was loaded, as compared with that of GO-free PDMAEMA hydrogel. Cr(VI) adsorption of hydrogels was also improved by the introduction of GO to some extent, and the maximum Cr(VI) adsorption of 180 mg/g was realized, indicating that the obtained PDMAEMA/GO hybrid hydrogels possess excellent adsorption performance.     

Efficient immobilization of lipase from Candida rugosa by entrapment into poly(N-isopropylacrylamide-co-itaconic acid) hydrogels under mild conditions

Temperature and pH-sensitive hydrogels, based on N-isopropylacrylamide and itaconic acid, with varying comonomer ratios and crosslinking agent content, were prepared by free radical crosslinking copolymerization. The immobilization of lipase from Candida rugosa was carried out by post-loading entrapment method at different temperatures until equilibrium swelling was achieved. The effects of the hydrogel composition and the immobilization temperature on the hydrogel-binding capacity, immobilized lipase specific activity, as well as the enzyme leakage were studied. It was found that the NiPAAm/IA ratio, crosslinking agent concentration and the temperature at which the entrapment was performed significantly affected the hydrogel-binding capacity. The biocatalysts obtained by entrapment into hydrogel with the highest itaconic acid content at 5?°C exhibited both highest binding capacity and the highest specific activity, but appeared to be less suitable for repeated uses than those obtained at 25?°C.     

Immobilization of Candida rugosa lipase on modified natural wool fibers

A method has been developed to immobilize lipase from Candida rugosa on modified natural wool fibers by means of graft copolymerization of poly ethylacrylate in presence of potassium persulphate and Mohr’s salt redox initiator. The activities of free and immobilized lipase have been studied. FTIR spectroscopy, scanning electron microscopy, and the Bradford method were used to characterize lipase immobilization. The efficiency of the immobilization was evaluated by examining the relative enzymatic activity of free enzyme before and after the immobilization of lipase. The results showed that the optimum temperature of immobilized lipase was 40 °C, which was identical to that of the free enzyme, and the immobilized lipase exhibited a higher relative activity than that of free lipase over 40 °C. The optimal pH for immobilized lipase was 8.0, which was higher than that of the free lipase (pH 7.5), and the immobilization resulted in stabilization of enzyme over a broader pH range. The kinetic constant value (km) of immobilized lipase was higher than that of the free lipase. However, the thermal and operational stabilities of immobilized lipase have been improved greatly.     

Surface modification of macroporous poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) resins for improved Candida antarctica lipase B immobilization

Crosslinked macroporous hydrophilic poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) [abbreviated poly(GMA-co-EGDMA)] with identical chemical structure (60% of glycidyl methacrylate) but with varied average pore sizes (from 30 to 560 nm), specific surface areas (from 13.2 to 106.0 m²/g), specific volumes (from 0.755 to 1.191 cm³/g) and particle sizes (<100 μm–630 μm) were synthesized via suspension polymerization. Modifications of poly(GMA-co-EGDMA) with various diamines (1,2-diaminoethane, 1,4-diaminobutane, 1,6-diaminohexane and 1,8-diaminooctane), 2-fluoroethylamine, glutaraldehyde and cyanuric chloride were carried out. The influence of the interaction between Candida antarctica lipase B (Cal-B) and various carriers during immobilization on the loading and hydrolytic activity (hydrolysis of para-nitrophenyl acetate) of the immobilized Cal-B were studied. Immobilization of Cal-B was performed at different temperatures and pH values. Cal-B immobilized at 30 °C and pH 6.8 was leading to increased activities. Purely physical adsorption between enzyme and copolymer was observed on carriers in which amine or fluorine groups were introduced into the carrier structure by modification with various diamines or 2-fluoroethylamine. As a consequence enzyme loading and activity decreases. In contrary, modification of the poly(GMA-co-EGDMA) with glutaraldehyde and cyanuric chloride results in a covalent connection between enzyme and carrier. The obtained results show a significant increase in Cal-B activity. The influence of the amount of glutaraldehyde and cyanuric chloride used for modification was screened. Increasing the amount of glutaraldehyde or cyanuric chloride used for modification resulted in an increase of the enzyme loading. Consequently, higher amount of glutaraldehyde used led to a higher fraction of the enzyme molecules that are covalently connected on to the carrier. As the amount of glutaraldehyde or cyanuric chloride used for modifications increases, activity of immobilized C. antarctica lipase B primarily increases, showing the highest value for 0.66% and 0.050% w/w, respectively, and subsequently decreases. We could show that Cal-B immobilized on epoxy-containing copolymer modified with glutaraldehyde and cyanuric chloride performs higher activity than free enzyme powder.     

Lipase Catalyzed Synthesis of Medium-chain Biodiesel from Cinnamonum camphora Seed Oil

The non-edible camphor tree seed oil was extracted and catalyzed by immobilized lipase for biodiesel production. The oil yield from camphor tree seeds reached 35.2% of seed weight by twice microwave-assisted extractions. Gas chromatography showed that free fatty acid content in camphor tree seed oil was 1.88%, and the main fatty acids were capric acid (53.4%) and lauric acid (38.7%). With immobilized lipase Candida sp. 99–125 as catalyst, several important factors for reaction conditions were examined through orthogonal experiments. The optimum conditions were obtained: water content and enzyme loading were both 15% with a molar ratio of 1:3.5 (oil/ethanol), and the process of alcoholysis was in nine steps at 40 °C for 24 h, with agitation at 170 r·min^− 1. As a result, the medium-chain biodiesel yield was 93.5%. The immobilized lipase was stable when it was used repeatedly for 210 h.     

Freezing as a path to build macroporous structures: Superfast responsive polyacrylamide hydrogels

Macroporous polyacrylamide (PAAm) hydrogels were prepared from acrylamide monomer and N,N′-methylene(bis)acrylamide (BAAm) crosslinker in frozen aqueous solutions. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at a gel preparation temperature of −6 °C. The hydrogels prepared below −6 °C exhibit a heterogeneous morphology consisting of pores of sizes 10–70 μm, while those formed at higher temperatures have a non-porous structure. PAAm networks with largest pores were obtained at −18 °C. The pore size of the networks increased while the thickness of the pore walls decreased by decreasing the monomer concentration. The hydrogels formed below −6 °C exhibit superfast swelling and deswelling properties as well as reversible swelling–deswelling cycles in water and in acetone, respectively.     

Immobilization of β-glucosidase and its aroma-increasing effect on tea beverage

β-Glucosidase was effectively immobilized on alginate by the method of crosslinking–entrapment–crosslinking. After optimization of the immobilized conditions, the activity recovery of immobilized β-glucosidase achieved to 46.0%. The properties of immobilized β-glucosidase were investigated. Its optimum temperature was determined to be 45 °C, decreasing 10 °C compared with that of free enzyme, whereas the optimum pH did not change. The thermal and pH stabilities of immobilized β-glucosidase increased to some degree. The K_m value for immobilized β-glucosidase was estimated to be 1.97 × 10^?3 mol/L. The immobilized β-glucosidase was also applied to treat the tea beverage to investigate its aroma-increasing effect. The results showed that after treated with immobilized β-glucosidase, the total amount of essential oil in green tea, oolong tea and black tea increased by 20.69%, 10.30% and 6.79%, respectively. The storage stability and reusability of the immobilized β-glucosidase were improved significantly, with 73.3% activity retention after stored for 42 days and 93.6% residual activity after repeatedly used for 50 times.     

N-vinylpyrrolidone/acrylic acid/2-acrylamido-2-methylpropane sulfonic acid based hydrogels: Synthesis,characterization and their application in the removal of heavy metals

In this work, the synthesis of N-vinylpyrrolidone/acrylic acid/2-acrylamido-2-methylpropane sulfonic acid (NVP/AAc/AMPS) based hydrogels by UV-curing technique was studied and their swelling behavior, heavy metal ion recovery capabilities were investigated. The structures of hydrogels were characterized by FT-IR analysis and the results were consistent with the expected structures. Thermal gravimetric analysis of hydrogels showed that the thermal stability of hydrogel decreases slightly with incorporation of AMPS units into the structure. In addition, the morphology of the dry hydrogel sample was examined by SEM. According to swelling experiments, hydrogels with higher AMPS content gave relatively higher swelling ratio compared to neat hydrogel. These hydrogels were used for the separation of Cd(II), Cu(II) and Fe(III) ions from their aqueous solutions. The influence of the uptake conditions such as pH, time and initial feed concentration on the metal ion binding capacity of hydrogel was also tested. The selectivity of the hydrogel towards the different metal ions tested was Cd(II) > Cu(II) > Fe(III). It was observed that the specific interaction between metal ions and ionic co-monomers in the hydrogel affected the metal binding capacity of the hydrogel. The recovery of metal ions was also investigated in acid media.     

Synthesis and properties of biodegradable hydrogels of κ-carrageenan grafted acrylic acid-co-2-acrylamido-2-methylpropanesulfonic acid as candidates for drug delivery systems

Novel types of highly swelling hydrogels were prepared by grafting crosslinked polyacrylic acid-co-poly-2-acrylamido-2-methylpropanesulfonic acid (PAA-co-PAMPS) chains onto κ-carrageenan through a free radical polymerization method. Here, we propose a mechanism for κ-carrageenan-g-PAA-co-PAMPS formation and confirm the hydrogel structure using FTIR spectroscopy. The effect of grafting variables (i.e. concentration of methylenebisacrylamide (MBA), acrylic acid/-2-acrylamido-2-methylpropanesulfonic acid (AA/AMPS) weight ratio, ammonium persulfate (APS), κ-carrageenan, neutralization percent and reaction temperature) were systematically optimized to achieve a hydrogel with a maximum swelling capacity. The maximum water absorbency of the optimized final product was 1238 g/g, while poly-2-acrylamido-2-methylpropanesulfonic acid-g-κ-carrageenan and polyacrylic acid-g-κ-carrageenan hydrogels swelled to a range of 135–800 g/g. The swelling of superabsorbent hydrogels was measured in various solutions with pH values ranging from 1 to 13. In addition, the pH reversibility, on–off switching behavior and swelling kinetics in distilled water were preliminarily investigated.     

Lipase Immobilization onto the Surface of PGMA-b-PDMAEMA-grafted Magnetic Nanoparticles Prepared via Atom Transfer Radical Polymerization

A block copolymer of 2-dimethylaminoethyl methacrylate (DMAEMA) and glycidyl methacrylate (GMA) was grafted onto the surface of magnetic nanoparticles (Fe₃O₄) via atom transfer radical polymerization. The resultant PGMA-b-PDMAEMA-grafted-Fe₃O₄ magnetic nanoparticles with amino and epoxy groups were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, thermo-gravimetric analysis, and scanning electron microscopy. Lipase from Burkholderia cepacia was successfully immobilized onto the magnetic nanoparticles by physical adsorption and covalent bonding. The immobilization capacity of the magnetic particles is 0.5 mg lipase per mg support, with an activity recovery of up to 43.1% under the optimum immobilization condition. Biochemical characterization shows that the immobilized lipase exhibits improved thermal stability, good tolerance to organic solvents with high lg P, and higher pH stability than the free lipase at pH 9.0. After six consecutive cycles, the residual activity of the immobilized lipase is still over 55% of its initial activity.     

Stimuli responsive carbon nanocomposite hydrogels with efficient conducting properties as a precursor to bioelectronics

Stimuli responsive conducting carbon nanocomposite hydrogels were synthesized from glycerol methacrylate, ethylene glycol methacrylate and diethylene glycol methacrylate by thermal polymerization techniques. Carbon nanoparticles of size <50 nm were incorporated into the polymer at a concentration of 0.005% (w/w) during synthesis. The hydrogels were characterized by Fourier Transform-Infrared Spectroscopy, thermogravimetric analysis, Scanning Electron Microscopy techniques and X-ray diffraction study. The hydrogels have excellent absorption properties in aqueous solvents which is sensitive to pH, temperature, ionic strength, etc. For instance, the pH sensitive swelling behavior of Polyglycerylmethacrylate-carbon nanocomposite increases from acidic medium (SR∼24, pH = 3) to basic medium (SR∼130, pH = 12). The hydrogels also showed significant swelling behavior in presence of different biological samples such as folic acid, uric acid, DNA and RNA. The electrical impedance value decreases to a large extent after the addition of carbon nanoparticles into the gels. It was observed that the conductivity rises to a maximum of about 1000 folds in the nanocomposite hydrogels. The increase in electrical conductivity is also verified by current–voltage measurements.     

Synthesis and swelling properties of poly[acrylamide-co-(crotonic acid)] superabsorbents

Superabsorbent polymers of acrylamide (AAm)/crotonic acid (CA) were synthesized by foamed polymerization in an aqueous solution of AAm with CA as a comonomer, initiated by an initiator couple of ammonium persulfate and N,N,N′N′-tetramethylethylenediamine. A crosslinking agent N,N′-methylenebisacrylamide, a foaming agent sodium bicarbonate, and a foam stabilizer, a triblock copolymer of polyoxyethylene/polyoxypropylene/polyoxyethylene, were used in the polymerization. The influences of the relative contents of CA, crosslinking agent, and initiator, on the swelling properties of the superabsorbent polymer systems were examined. The superabsorbent polymer synthesized with an AAm/CA ratio of 98:2 by mole, 0.5 wt.% of N,N′-methylenebisacrylamide and 1 wt.% of ammonium persulfate at 250 rpm and 50 °C for 30 min of polymerization time produced the highest water absorption of 211 ± 9 times its dried weight and could absorb water up to 162 ± 4 g g⁻¹ of the dry copolymer within 10 min. The electrochemical reaction for acrylamide–crotonic acid polymerization was investigated by cyclic voltammetry. The anodic current indicated that acrylamide acting as an electron donor whereas crotonic acid performed as an electron receiver, then providing the cathodic current. The diffusion of water into the superabsorbent polymer was non-Fickian (case II and anomalous). Acrylamide–crotonic acid superabsorbents containing various crosslinker concentrations had a water swelling in the range of 79–289 g g⁻¹. The diffusion coefficients varied between 6.9 × 10⁻⁹ and 5.1 × 10⁻⁸ cm² s⁻¹. Adsorption of the basic dye by the superabsorbent was a monolayer evaluated by the Langmuir isotherm. The superabsorbents can thus be used to adsorb cationic dyes in textile industry.     

Molecularly imprinted hydrogels for fibrinogen recognition

Molecularly imprinted hydrogels (MIHs) composed of N-isopropylacrylamide (NIPA) and maleic acid (MA) monomers and fibrinogen (Fbg, MW: 340 kDa, pI: 5.8) imprinted molecule were fabricated in pH buffer solutions (pH_prep: 4.0, 5.8 and 8.0). The non-imprinted hydrogels were also prepared in the same conditions without Fbg imprinted molecule. The adsorption equilibrium constant and the maximum adsorption capacity of the MIH prepared in pH 4.0 were evaluated to 9.61 mL mg^?1 and 38.99 mg protein g^?1 dry gel in pH 4.0 buffer solution, respectively. The selectivity tests were also performed by using two reference molecules as bovine serum albumin (BSA, MW: 67 kDa, pI: 4.78) and urease (MW: 480 kDa, pI: 5.99). The MIHs have 2.37–4.23 times higher selectivity for the Fbg imprinted molecule than the non-imprinted hydrogels prepared in the same conditions. Easy preparation of the MIHs, their high stability and ability to recognize small and large proteins, as well as to discriminate molecules with small variations in charge, make this approach attractive and broadly applicable in biotechnology, assays and sensors.     

Effect of reaction parameters on conversion of krill (Euphausia superba) oil by immobilized lipase ethanolysis

Monoglyceride and diglyceride were produced by performing ethanolysis of krill oil with immobilized lipase and the influence of various parameters on the enzymatic ethanolysis was assessed. As an immobilized lipase, lipozyme TL-IM (thermonuces lanuginose) was used. Ethanolysis was done in non-pressurized and pressurized system to compare the reaction rate and yield. The optimal condition was found at 2.0 of ethanol mole ratio, temperature of 60 °C, lipases amount of 5 wt% in non-pressurized system. At pressurized system the optimal temperature and pressure was found at 50 °C and 10 MPa. However, at 50 °C monoglyceride was higher in pressurized system than in non-pressurized system.     

Preparation and application of epoxy–chitosan/alginate support in the immobilization of microbial lipases by covalent attachment

The aim of this work was the preparation and application of highly hydrophobic epoxy–chitosan/alginate as a support to immobilize microbial lipases from Thermomyces lanuginosus commercially available as Lipolase® (TLL1) and Lipex® 100L (TLL2) and Pseudomonas fluorescens (PFL). The catalytic properties of the biocatalysts were assayed in olive oil hydrolysis and butyl butyrate synthesis. The results indicated that 12 h was enough for TLL1 to be immobilized on the support. Covalent attachment of TLL1 turned biocatalysts highly active and around 6-fold more stable than free lipase. Based on the results, a time of incubation of 24 h was selected for further studies about the maximum immobilized protein amount and butyl butyrate synthesis. Maximum protein loading immobilized was found to be 25.4 mg g^?1 support for TLL1, followed by TLL2 (20.5 mg g^?1) and PFL (15.5 mg g^?1) offering 80 mg protein g^?1 support. The immobilization of TLL1 and TLL2 resulted in highly active biocatalysts (around 1300 IU g^?1 gel), almost fivefold higher than PFL (272.4 IU g^?1 gel). In butyl butyrate synthesis, PFL showed similar activity to TLL1 and TLL2 derivatives, up to 60 mmol L^?1. The biocatalysts displayed high activity after five successive cycles, retaining around 95% of the initial activity.     

Carbonaceous–siliceous composite materials as immobilization support for lipase from Alcaligenes sp.: Application to the synthesis of antioxidants

Two carbonaceous–siliceous composite materials, produced by hydrothermal and carbonization processes, were evaluated as immobilization support for lipase from Alcaligenes sp. These materials exhibited similar chemical characteristics but their carbon content and porous characteristics were different, which explain the catalytic behavior and stability of the biocatalysts immobilized on them. Higher activity and immobilization selectivity was achieved with the microporous material that had higher carbon content. The lipase immobilized on the mesoporous material had a higher thermal stability at 55 °C, pH 7.0 or at 40 °C in tert-butanol, simulating the reaction conditions required for organic synthesis. Both biocatalysts were tested in the synthesis of palmitoyl ascorbate and they were compared with the commercial biocatalyst QLC. The synthesis conversions with the lipase immobilized in mesoporous materials and with the biocatalyst QLC were similar (50%), but only the former could be reused. These are promising biocatalysts for industrial applications.     

Preparation of cellulose-based conductive hydrogels with ionic liquid

Conductive hydrogel composed of microcrystalline cellulose (MCC) and polypyrrole (PPy) was prepared in ionic liquid; and the resulting hydrogel was characterized with FT-IR, SEM, XRD and TGA. By doping with TsONa, the MCC/PPy composite hydrogels showed relatively high electrical conductivity, up to 7.83 × 10⁻³ S/cm, measured using a four-probe method. The swelling kinetics of the composite hydrogels indicated that the swelling process was mainly influenced by the cellulose content; and the equilibrium swelling ratio decreased as the increasing of MCC content in the hydrogels. In addition, the MCC/PPy composite hydrogels exhibited significantly enhanced mechanical property in contrast to MCC hydrogel.     

Comparison between cobalt and cobalt–platinum supported on zeolite NaY: Cobalt reducibility and their catalytic performance for butane hydrogenolysis

This work investigated reducibility of cobalt species in monometallic Co/NaY and bimetallic CoPt/NaY catalysts with various Co loading (1, 6 and 10 wt.%) and fixed Pt loading (1 wt.%). The form and environment of Co species after reduction was determined by X-ray absorption spectroscopy including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The cobalt species in the mono- and bimetallic catalyst with Co loading of 1 wt.% was not reducible whereas those with Co loading of 6 and 10 wt.% were partially reduced. The extent of reduction increased with Co loading and enhanced by the presence of Pt. Catalytic performance for n-butane hydrogenolysis mono- and bimetallic catalysts were compared. The higher extent of Co reduction in 6CoPt/NaY and 10CoPt/NaY resulted in higher conversions than the monometallic counterpart. Sequential hydrogenolysis was favored on the monometallic catalysts because methane was the only product. The presence of Pt suppressed such reaction resulting in ethane and propane. The effect of Pt on such effect was most prominent in 6CoPt/NaY.     

Improved mechanical and swelling behavior of the composite hydrogels prepared by ionic monomer and acid-activated Laponite

A composite hydrogel (CH) with much improved mechanical and swelling properties was prepared using an ionic monomer and acid-activated Laponite XLS which was used as a cross-linking agent. Addition of acid-activated clay solved the gelation problem when ionic monomers were added to clay mineral dispersions. Reaction of Laponite XLS with sulfuric acid yielded amorphous silica. A dispersion of the acid-activated Laponite and the monomers was used to synthesize composite hydrogels by in-situ polymerization. The FT-IR spectra and rheological results of the composite hydrogels demonstrated the formation of a network. The equilibrium swelling ratios of composite hydrogels (> 6000 g/g) were more than 18 times larger than traditional organic cross-linked hydrogels. The moduli G′ and G″ in the observed frequency range were about 4 and 10 times larger than those of organic cross-linked hydrogel (OR gel). The improvement in both the equilibrium swelling ratio and mechanical strength was attributed to the homogeneous cross-linked network structure.     

High tensibility and pH-responsive swelling of nanocomposite hydrogels containing the positively chargeable 2-(dimethylamino)ethyl methacrylate monomer

Positively chargeable nanocomposite hydrogels (NC gels) were synthesized by in situ copolymerization of acrylamide (AM) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) in an aqueous suspension of hectorite clay Laponite XLS. The stability of the Laponite suspension containing DMAEMA was monitored by its transmittance, viscosity and zeta potential. The polymerization was initiated either by redox or UV radiation to fabricate the NC gels. Elongation at break and tensile strength decreased with increasing DMAEMA content in monomers up to 17 mol%. As expected from the protonation of DMAEMA, the NC gels containing more than 5 mol% DMAEMA showed pH-responsive swelling; the gels were swollen at pH < 4 and shrunken at pH > 4. In comparison with the redox-initiated NC gels, the UV-initiated NC gels possessed a more homogeneous structure with higher transmittance, better mechanical properties, and a larger equilibrium swelling ratio at pH < 4.