Mechanically strong, functional hybrid hydrogel networks with upper critical solution temperature (UCST)-type thermosensitivity for volume change and temperature-controlled fluorescence are presented. Thermosensitive poly(N-acryloyl glycinamide)/poly(glycidyl methacrylate) (PNAGA/PGMA) interpenetrating network (IPN) functional hydrogels were prepared sequentially by photoinitiated radical polymerization. Amine moieties were introduced by a ring-opening reaction of GMA, followed by subsequent coupling of fluorescein isocyanate (FITC). Fourier transform infrared and differential scanning calorimetry were deployed to prove the successful incorporation of the PGMA network and the introduction of amino groups. Equilibrium swelling at different temperatures in pure water and phosphate-buffered saline (PBS) was conducted to study the thermosensitive properties of the IPN hydrogels. The IPN hydrogels retained their thermophilic swelling properties after the introduction of PGMA and showed increased mechanical strength. The fluorescence properties of the IPN hydrogels were studied by fluorescence spectroscopy at temperatures from 10 to 40 °C in water and PBS and under acidic and alkaline conditions. Fluorescence activity of FITC-coupled IPN hydrogels with a weight ratio (PGMA/PNAGA) of around 0.5 showed an increased temperature dependence in aqueous medium compared to dissolved FITC. Thermosensitive fluorescent PNAGA IPN hydrogels may function as thermometers or thermosensitive devices.
Functional thermoresponsive copolymers poly(N-acryloyl glycinamide-coglycidyl methacrylate) (PNAGA-co-GMA) and poly(N-acryloyl glycinamide-co-N-(methacrylate)succinimide) (PNAGA-co-MNHS) were prepared by free-radical polymerization. With increasing functional comonomer content, the phase transition temperature increased. The functional copolymers reacted with the enzyme α-amylase by W/O emulsion to form water-soluble biohybrid nanogels. The hydrodynamic radius of biohybrid nanogels increased with temperature due to thermophilic swelling. Biohybrid nanogels of PNAGA-co-GMA and PNAGA-co-MNHS revealed a 1.2-and 1.5-fold increase of enzyme activity at 40 and 15 °C, respectively. Free α-amylase showed a 1.1-fold increase in comparison. Therefore, nanogels of these reactive thermoresponsive copolymers could be used to modulate the enzyme activity of various enzymes. As reactive polymers, they could be used as an initiation site for grafting polymerization to introduce functionality to PNAGA copolymers in further work.
Dual-responsive nano-structured poly(N-acryloyl glycinamide) (NSG PNAGA) hydrogels were prepared in a cross-linking polymerization reaction of activated poly(N-isopropylacrylamide) (PNIPAM) nanogels with N-acryloyl glycinamide (NAGA). Reactive double bonds on the nanogel were accessed by prematurely stopping the precipitation polymerization of PNIPAM nanogels. The nano-structured hydrogels retained a high mechanical strength (storage modulus G′ ≥ 10,000 Pa and elasticity modulus E mod ∼ 100 kPa), elasticity (L ≥ 600%), and lower (LCST) and upper critical solution temperature (UCST)-type swelling properties. Turbidity measurements showed LCST-type behavior from 0 to 34 °C and UCST-type behavior from 34 to 50 °C. The rheological behavior of NSG PNAGA hydrogels follows a dual-responsive UCST- and LCST-type behavior. At the LCST, a leap of storage modulus G′ of up to 3700 Pa was observed. Scanning electron microscopy showed a distinct morphology compared to the neat PNAGA hydrogel due to the incorporation of PNIPAM into the network. The self-healing properties of NSG PNAGA hydrogels were successfully demonstrated. The nano-structured PNAGA hydrogels could be used as temperature sensors or biological scaffolds in future applications.
Smart hydrogels hold much potential for biocatalysis, not only for the immobilization of enzymes, but also for the control of enzyme activity. We investigated upper critical solution temperature‐type poly N‐acryloyl glycinamide (pNAGA) hydrogels as a smart matrix for the amine transaminase from Bacillus megaterium (BmTA). Physical entrapment of BmTA in pNAGA hydrogels results in high immobilization efficiency (>89 %) and high activity (97 %). The temperature‐sensitiveness of pNAGA is preserved upon immobilization of BmTA and shows a gradual deswelling upon temperature reduction. While enzyme activity is mainly controlled by temperature, deactivation tended to be higher for immobilized BmTA (≈62–68 %) than for free BmTA (≈44 %), suggesting a deactivating effect due to deswelling of the pNAGA gel. Although the deactivation in response to hydrogel deswelling is not yet suitable for controlling enzyme activity sufficiently, it is nevertheless a good starting point for further optimization.
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