Graphene quantum dot-based hydrogels for photocatalytic degradation of organic dyes

Zero-dimensional graphene nanoparticles have been extensively researched in the past few years. In particular, graphene quantum dots (GQDs), characterized by high biocompatibility, ease of synthesis and tunable photoluminescence, have been identified as potential candidates for numerous applications, including bioimaging and catalysis. Research shows that GQDs can be potentially used as an alternative artificial enzyme to natural enzymes, thus increasing research interests in this area due to their lower cost and higher stability. Recently, carbon nanomaterials have been used as catalysts in various enzymatic reactions, as for example the degradation of toxic compounds. Specifically, GQD can replace horseradish peroxidase catalyst widely used in the oxidation of organic compounds by oxygen peroxide.

Good progress has been made in understanding the underlying mechanism of GQD in peroxide reactions. Most research on peroxidase activity of GQD has been conducted in solution, thus providing a comprehensive understanding of the relationship between the reaction activity and the GQD composition. However, the obtained results are still insufficient as there are very few studies reporting the immobilization of GQD on protecting and supporting scaffolds.

Hydrogels have emerged as promising candidates for use as immobilization support for enzymes. They do not dissolve in water due to their cross-linked polymer chains and their hydrophilic nature allows water to diffuse freely. Even though different GQD-based composite hydrogels have been developed for various applications, no GQD composite hydrogel with enzymatic peroxidase activity has been reported so far. On this account, researchers at CIDETEC, member of the Basque Research and Technology Alliance (BRTA): Antton Ibarbia (Ph.D. student), Dr. Laura Sánchez-Abella, Dr. Hans J. Grande, Dr Virginia Ruiz, together with Professor Luis Lezama from the University of the Basque Country developed graphene quantum dot-based hydrogels and investigated their application in photocatalytic degradation of organic dyes. The work is published in the journal Applied Surface Science.

In their approach, graphene oxide quantum dots (GOQD) functionalized with cross-linkable 3-(triethoxysilyl)propyl methacrylate (MPS) units were covalently immobilized in poly[2-(Methacryloyloxy)ethyl]trimethylammonium]-co-(3-Sulfopropylmethacrylate) 50:50 (A50coS50) hydrogels fabricated through free radical polymerization. A50coS50 hydrogels are known to have antiadhesive properties derived from their pseudo-zwitterionic nature. The properties of the resulting composite hydrogels, especially their peroxidase activity was investigated. The composite hydrogel was utilized to photodegrade organic rhodamine B (RhB) dye to validate its feasibility as an artificial peroxidase enzyme.

The authors showed that the GOQD-A50coS50 composite hydrogels showed peroxidase mimetic activity in the presence of hydrogen peroxide. This allowed efficient catalytic oxidation of chromogenic peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB). Similarly, they showed remarkably high catalytic activity for RhB dye degradation, an activity that was also improved by visible light irradiation. The degradation reaction was highly dependent on the OH radicals, the active species generated from the decomposition of water and hydrogen peroxide. Besides, the yield of the RhB degradation due to the catalytic reaction varied with the hydrogel volume and GOQD loading. It was worth noting that the composite hydrogels retained their high photocatalytic activity after several months of drying and rehydration and months of wet storage.

In summary, the study reported a new approach for the production of GOQD-based hydrogels with remarkably high and stable photocatalytic activity for organic dye degradation. A significant enhancement in the peroxidase-like enzymatic activity of the GOQD-A50coS50 composite hydrogel, attributed to functionalization with cross-linkable MPS, was reported. Moreover, the peroxidase mimetic activity improvement indicated that the A50coS50 hydrogel could be potentially used as enzyme immobilizing support. In a statement to Advance in Engineering, the authors explained their newly developed composite hydrogels could be further explored for potential application in organic pollutant degradation, water treatment, biosensors and other biomedical applications. CIDETEC is currently targeting their use as antimicrobial coatings.