Enzymatically Cross-linked Graphene Oxide/Natural Polymer Nanocomposite Hydrogels And Its Applications

Graphene-based nanocomposite hydrogel is defined by a 3D hybrid crosslinked network prepared by incorporating graphene and its derivative nanomaterials into a hydrogel.The nanocomposite hydrogel exhibited extraordinary properties in comparsion with the conventional hydrogel,such as high mechanical strength,good thermal performance and near infrared light（NIR）thermal response performance and so on.It has shown broad application prospects in drug release,biosensors,water purification treatment,microfluidic switches,supercapacitors,tissue engineering,biomimetic materials and catalyst carrier.Graphene-based nanocomposite hydrogels are usually prepared by self-assembly,solution or in-situ polymerization.However,the agglomeration between graphene molecμLes in solution due to the strong van der Waals force and hydrophobic interaction resμLted in hydrogel an inhomogeneous internal structure of the prepared nanocomposite.More importantly,the residual toxic monomers,solvents,crosslinkers and other components during the preparation process gave rise to the poor biocompatibility of hydrogel.Therefore,the graphene-based nanocomposite hydrogels prepared by these conversional methods exhibited inherent disadvantage for the application as drug release carriers or water purification treatment materials.Recently,enzymatically catalyzed hydrogelation has attracted much attention owing to the mild reaction conditions,fast reaction rate,high chemical selectivity,regioselectivity and stereoselectivity,ease of handling and processability.Horseradish peroxidase（HRP）,an enzyme produced from the horseradish,has been used widely to prepare hydrogels with tunable gelation rate and crosslinking density by the HRP-catalyzed crosslinking reaction of polymer-phenol conjugates.The photothermal responsive nanocomposite hydrogels prepared by combining temperature-sensitive hydrogels with photothermally responsive graphene-based material are ideal carriers for controlling drug release.The release profile of loaded drug from the photothermal responsive nanocomposite hydrogels can be tunable using near-infrared light（NIR）as an intelligent control switch.In the first part,photothermally responsive nanocomposite hydrogels（HA-Plu-Tyr-GO）were synthesized by using pluronic F127 functionalized graphene oxide（Plu-GO）as a photothermal conversion element.The Plu-GO was dispersed in a solution of tyramine modified hyaluronic acid（HA-Tyr）and the enzymatically nanocomposite hydrogels were formed by HRP and H₂O₂.The fast gelation within 100s was obtained under physiological conditions.The gelation time and structure of hydrogel can be controlled by controlling the content of HRP,Plu-Tyr,H₂O₂ and GO.The nanocomposite hydrogels exhibited high water absorption and the highest swelling ratio was almost 25at room temperature.Rheological analysis showed that the storage modμLus（G?）of HA-Plu-Tyr-GO hydrogel increased from 1000Pa to 2350 Pa with increasing the GO content from 0.05%to 0.1%.The cumμLative drug release from HA-Plu-Tyr-GO hydrogel using topotecan hydrochloride（TPT）as a model drug was more than 95%under different pH conditions.The HA-Plu-Tyr-GO hydrogels exhibited excellent photothermal responsiveness,and the temperature of hydrogels raised from room temperature to 55.1℃in 50 min under NIR irradiation.The TPT release rate can be significantly increased from 20%to more than 80%within 4 hours under NIR irradiation.The HA-Plu-Tyr-GO nanocomposite hydrogel can be used as NIR-triggered drug delivery carriers.The treatment of dye wastewater is an important environmental problem.Bio-enzyme catalysis can catalyze the decolorization and degradation of dyes rapidly under mild conditions.However,the use of free enzymes in the treatment of dye wastewater has been limited due to the considerably unstable nature and poor repeatability of the free enzyme.Immobilized enzymes exhibited greater stability and catalytic activity,reusability,and easy to separate from wastewater.The polymer hydrogel with good hydrophilicity,enzyme compatibility and network structure tunability provides a hydrophilic and non-toxic microenvironment for the enzyme,and is an ideal immobilized enzyme carrier.In the second part,the double-cross-linked sodium alginate/graphene oxide nanocomposite microgels（SA-Tyr-GO）with in-situ embedded enzymes（HRP）were prepared by the combinationwith Ca-ion cross-linking and HRP/H₂O₂ enzaymatically catalyzed method.The influence of temperature,pH,initial dye and H₂O₂ concentration on the activity of immobilized enzyme was studied using azo dye（Congo red）as the model.The the thermal stability and acid-base adaptability of immobilized HRP were significantly improved.The more than 40%of catalytic activity immobilized HRP was still maintained under the extreme conditions（at pH 2 or 70℃）,while the free HRP were almost completely inactivated at the same conditions.The adsorption kinetics,adsorption isotherm and degradation kinetics of Congo red treated with immobilized enzyme system under different conditions were investigated.The immobilized enzyme microgels can be applied to the efficient decolorization and degradation of dye wastewater.The dye decolorization rate was more than 82%within 150 min.The immobilized enzyme microgels exhibited excellent reusability,and the decolorization rate decreased less than 17%after 12 use cycles.The ion and enzyme-catalyzed cross-linked SA-Tyr-GO hybrid hydrogel by combining the efficiently adsorption of enzyme on GO and the in-siut embedding of enzyme can be used to adsorption with high capacity and efficiently degradation of dyes.