The reduction of graphene oxide

Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

This Review focuses on noncovalent functionalization of graphene and graphene oxide with various species involving biomolecules, polymers, drugs, metals and metal oxide-based nanoparticles, quantum dots, magnetic nanostructures, other carbon allotropes (fullerenes, nanodiamonds, and carbon nanotubes), and graphene analogues (MoS2, WS2). A brief description of π–π interactions, van der Waals forces, ionic interactions, and hydrogen bonding allowing noncovalent modification of graphene and graphene oxide is first given. The main part of this Review is devoted to tailored functionalization for applications in drug delivery, energy materials, solar cells, water splitting, biosensing, bioimaging, environmental, catalytic, photocatalytic, and biomedical technologies. A significant part of this Review explores the possibilities of graphene/graphene oxide-based 3D superstructures and their use in lithium-ion batteries. This Review ends with a look at challenges and future prospects of noncovalently modified graph...

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Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications

The emerging graphene quantum dots (GQDs) and carbon dots (C-dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications.

Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications

Graphene quantum dots (GQDs) are great promising in various applications owing to the quantum confinement and edge effects in addition to their intrinsic properties of graphene, but the preparation of the GQDs in bulk scale is challenging. We demonstrated in this work that the micrometer sized graphene oxide (GO) sheets could react with Fenton reagent (Fe2+/Fe3+/H2O2) efficiently under an UV irradiation, and, as a result, the GQDs with periphery carboxylic groups could be generated with mass scale production. Through a variety of techniques including atomic force microscopy, X-ray photoelectron spectroscopy, gas chromatography, ultraperformance liquid chromatography–mass spectrometry, and total organic carbon measurement, the mechanism of the photo-Fenton reaction of GO was elucidated. The photo-Fenton reaction of GO was initiated at the carbon atoms connected with the oxygen containing groups, and C–C bonds were broken subsequently, therefore, the reaction rate depends strongly on the oxidization extent ...

Photo-Fenton Reaction of Graphene Oxide: A New Strategy to Prepare Graphene Quantum Dots for DNA Cleavage

Bulk-scale production of individual graphene sheets is still challenging although several methodologies have been developed. We report here a rapid and cost-effective approach to reduction of graphene oxide (GO) using hydroxylamine as a reductant. We demonstrated that the reduction of GO with hydroxylamine could take place quickly under a mild condition, and the as-produced graphene sheet showed high electrical conductivity, fair crystalline state, and admirable aqueous dispersibility without using any stabilizing reagents. A mechanism for removal of epoxide and hydroxyl groups from GO by hydroxylamine has been proposed. Comparing with other reported methods, the reduction of GO with hydroxylamine should be a preferable route to bulk-scale production of the graphene because it is simple, efficient, and cost-effective.

Reducing Graphene Oxide via Hydroxylamine: A Simple and Efficient Route to Graphene

Biochemical and biomedical applications of graphene oxide (GO) critically rely on the interaction of biomolecules with it. It has been previously reported that the biological activity of the GO-enzyme conjugate decreases due to electrostatic interaction between the enzymes and GO. Herein, the immobilization of horseradish peroxidase (HRP) and oxalate oxidase (OxOx) on chemically reduced graphene oxide (CRGO) are reported. The enzymes can be adsorbed onto CRGO directly with a tenfold higher enzyme loading than that on GO, and maximum enzyme loadings reach 1.3 and 12 mg mg(-1) for HRP and OxOx, respectively. Significantly, the more CRGO is reduced, the higher the enzyme loading. The CRGO-HRP conjugates also exhibit higher enzyme activity and stability than GO-HRP. Excellent properties of the CRGO-enzyme conjugates are attributed to hydrophobic interaction between the enzymes and the CRGO. The hydrophobic interaction mode of the CRGO-enzyme conjugates can be applied to other hydrophobic proteins, and thus could dramatically improve the performance of immobilized proteins. The results indicate that CRGO is a potential substrate for efficient enzyme immobilization, and is an ideal candidate as a macromolecule carrier and biosensor.

Assembly of graphene oxide-enzyme conjugates through hydrophobic interaction.

Due to the high peroxidase-like activity and small lateral size of graphene quantum dots (GQDs), the covalently assembled GQDs/Au electrode exhibits great performance and stability in H2O2 detection. It is better or comparable to some enzyme-immobilized electrodes, and thus could be useful in sensing H2O2 changes in biological systems.

Graphene quantum dots/gold electrode and its application in living cell H2O2 detection

Graphene quantum dots (GQDs) are graphene sheets with lateral sizes less than 100 nm, and have a higher electron conjugate state and a better dispersion ability in aqueous solution compared to micrometer-sized graphene oxide (GO) sheets. Therefore they can overcome the drawbacks of GO and are an ideal candidate for nano-composites. In this work, composites of GQDs and Fe3O4 nanoparticles (NPs) (GQDs/Fe3O4) were prepared via a one-step co-precipitation approach. The as-prepared GQDs/Fe3O4 composites showed superb peroxidase-like activities, which were much higher than composites of micrometer sized GO and Fe3O4 NPs (GO/Fe3O4), individual GQDs, and individual Fe3O4 NPs. The excellent peroxidase activities of the GQDs/Fe3O4 composites can be attributed to the unique properties of GQDs and the synergistic interactions between the GQDs and Fe3O4 NPs. The GQDs/Fe3O4 composites also exhibited a higher stability and reusability than natural peroxidases. The application of a GQDs/Fe3O4 composite as a catalyst for the removal of phenolic compounds from aqueous solutions was explored with nine phenolic compounds, and showed better or comparable removal efficiencies for some phenolic compounds compared to native horseradish peroxidase (HRP) under the same conditions. The extraordinary catalytic performance and physical properties of the as-prepared GQDs/Fe3O4 composite render it practically useful for industrial wastewater treatment.

Haixia Wu

Papers

Photo-Fenton Reaction of Graphene Oxide: A New Strategy to Prepare Graphene Quantum Dots for DNA Cleavage

Reducing Graphene Oxide via Hydroxylamine: A Simple and Efficient Route to Graphene

Assembly of graphene oxide-enzyme conjugates through hydrophobic interaction.

Graphene quantum dots/gold electrode and its application in living cell H2O2 detection

Composite of graphene quantum dots and Fe3O4 nanoparticles: peroxidase activity and application in phenolic compound removal