Production of reduced graphene oxide via hydrothermal reduction in an aqueous sulphuric acid suspension and its electrochemical behaviour

Reduced graphene oxide: Biofabrication and environmental applications

Green synthesis of high-quality reduced graphene oxide (rGO) from agro-industrial waste resources remains attractive owing to its outstanding environmental benefits. The remarkable properties of rGO include excellent morphology, uniform particle size, good optical properties, high conductivity, nontoxicity, and extraordinary chemical stability. Traditional methods for the synthesis of rGO nanomaterials involve several chemical reactions including oxidation, carbonization, toxic solvent, and pyrolysis which produce harmful byproducts. Green preparation of rGO is an emerging area of research in graphene technology which is cost-effective and sustainable in the procedure. Owing to the uniform particle rGO particle size, these smart nanomaterials have wide applicability, including in metal ions and pollutant sensing and adsorption, photocatalysis, optoelectrical devices, medical diagnosis, and drug delivery. Here we review the physicochemical properties of rGO, the biowaste sources and green methods of rGO synthesis, and the diverse applications of rGO, including in water purification and the biomedical fields. With this review, covering more than 200 research articles published on rGO in the last eight years ending in 2022, we aim to provide a quick guide for researchers seeking up-to-date information on the properties, production, and applicability of rGO, with special attention to rGO applications in water purification and the biomedical fields.

Facile electrooxidation of ethanol on reduced graphene oxide supported Pt-Pd bimetallic nanocomposite surfaces in acidic media

Development of electrocatalysts with extended homogeneity and improved metal-support interactions is of urgent scientific need in the context of electrochemical energy applications. Herein, bimetallic Pt-Pd nanoparticles with good homogeneity are fabricated using a convenient solution phase chemical reduction method onto a reduced graphene oxide (rGO) support. X-ray diffraction studies revealed that Pt-Pd/rGO possesses the crystallite size of 3.1 nm. The efficacies of Pt-Pd/rGO catalyst (20 wt% Pt + 10 wt% Pd on rGO support, Pt:Pd atomic ratio = 1:1) towards ethanol electrooxidation reaction (EOR) are evaluated in acidic conditions by cyclic voltammetry using catalyst-coated glassy carbon electrode as a working electrode. With the better dispersion on rGO support the Pt-Pd/rGO nancomposite catalyst exhibit highest mass specific activity (0.358 mA/µg-Pt) which is observed to be 1.9 times of similarly synthesized 20 wt% Pt/rGO (0.189 mA/µg-Pt) and 2.5 times of commercial 20 wt% Pt/C (0.142 mA/µg-Pt), respectively. Apart from the observed improved EOR activity, the Pt-Pd/rGO catalyst exhibited better stability than Pt/rGO and Pt/C catalysts. Strong synergy offered by Pt, Pd and rGO support could contribute to the observed higher EOR activity of Pt-Pd/rGO.

Bio-carbon-layered CuO-catalyzed decarboxylative alkenylation of cyclic ethers

An efficient methodology for the direct decarboxylative functionalization of cinnamic acid derivatives with cyclic ethers has been developed by using biogenic CuO/C nanoparticles. This protocol is compatible with broad range of substrates.

Application of some graphene derivatives to increase the efficiency of stem cell therapy

Graphene and its derivatives have application potential in many areas such as environmental technology, catalysis, biomedicine, and in particular, stem cell-based differentiation and regenerative therapies. Mesenchymal stem cell transplantation has emerged as a potential therapy for some diseases, such as acute kidney damage, liver failure and myocardial infarction. However, the poor survival of transplanted stem cells in such applications has significantly limited their therapeutic effectiveness. Graphene-based materials can improve the therapeutic efficacy of stem cells as they prevent the death of implanted cells by attaching them prior to implantation and increasing their paracrine secretion. In this review, we will highlight a number of recent studies that have investigated the potential use of graphene or its derivatives in stem cell applications and the prevention of transplanted stem cells from cell death, thereby improving their therapeutic efficacy.

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoO_x nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoO_x electrodes, in the presence or not of Fe²⁺ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoO_x electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm² direct current for 40 min using the N-doped-CNF-Co/CoO_x electrodes loaded with 25 wt% of Co/CoO_x NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoO_x electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

Au101-rGO nanocomposite: immobilization of phosphine-protected gold nanoclusters on reduced graphene oxide without aggregation

Graphene supported transition metal clusters are of great interest for potential applications, such as catalysis, due to their unique properties. In this work, a simple approach to deposit Au₁₀₁(PPh₃)₂₁Cl₅ (Au₁₀₁NC) on reduced graphene oxide (rGO) via an ex situ method is presented. Reduction of graphene oxide at native pH (pH ≈ 2) to rGO was performed under aqueous hydrothermal conditions. Decoration of rGO sheets with controlled content of 5 wt% Au was accomplished using only pre-synthesised Au₁₀₁NC and rGO as precursors and methanol as solvent. High resolution scanning transmission electron microscopy indicated that the cluster size did not change upon deposition with an average diameter of 1.4 ± 0.4 nm. It was determined that the rGO reduction method was crucial to avoid agglomeration, with rGO reduced at pH ≈ 11 resulting in agglomeration. X-ray photoelectron spectroscopy was used to confirm the deposition of Au₁₀₁NCs and show the presence of triphenyl phosphine ligands, which together with attenuated total reflectance Fourier transform infrared spectroscopy, advocates that the deposition of Au₁₀₁NCs onto the surface of rGO was facilitated via non-covalent interactions with the phenyl groups of the ligands. Inductively coupled plasma mass spectrometry and thermogravimetric analysis were used to determine the gold loading and both agree with a gold loading of ca. 4.8-5 wt%. The presented simple and mild strategy demonstrates that good compatibility between size-specific phosphine protected gold clusters and rGO can prevent aggregation of the metal clusters. This work contributes towards producing an agglomeration-free synthesis of size-specific ligated gold clusters on rGO that could have wide range of applications.

Electrochemical sensor based on glassy carbon electrode modified by polymelamine formaldehyde/graphene oxide nanocomposite for ultrasensitive detection of oxycodone

Polymelamine formaldehyde/graphene oxide (PMF/GO) nanocomposite was used, for the first time, to study the ultrasensitive and selective electrochemical detection of oxycodone (OXC). The successful characterization of PMF/GO was verified based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and Raman spectroscopy. The modified GCE (PMF/GO-GCE) proved its electrocatalytic effect on OXC determination according to cyclic, linear sweep, and differential pulse voltammetry (CV, LSV, and DPV) and electrochemical impedance spectroscopy (EIS) studies. The developed sensor under optimal conditions offered a linear relationship in a limited range of  0.01 to 45 μmol L^-1 with the limit of detection (LOD) of 2.0 nmol L^-1. The proposed PMF/GO-GCE sensor was effectively employed for the OXC detection in human urine and serum samples. Graphical abstract.

Recent Advances in Zinc Oxide Nanostructures with Antimicrobial Activities

This article reviews the recent developments in the synthesis, antibacterial activity, and visible-light photocatalytic bacterial inactivation of nano-zinc oxide. Polycrystalline wurtzite ZnO nanostructures with a hexagonal lattice having different shapes can be synthesized by means of vapor-, liquid-, and solid-phase processing techniques. Among these, ZnO hierarchical nanostructures prepared from the liquid phase route are commonly used for antimicrobial activity. In particular, plant extract-mediated biosynthesis is a single step process for preparing nano-ZnO without using surfactants and toxic chemicals. The phytochemical molecules of natural plant extracts are attractive agents for reducing and stabilizing zinc ions of zinc salt precursors to form green ZnO nanostructures. The peel extracts of certain citrus fruits like grapefruits, lemons and oranges, acting as excellent chelating agents for zinc ions. Furthermore, phytochemicals of the plant extracts capped on ZnO nanomaterials are very effective for killing various bacterial strains, leading to low minimum inhibitory concentration (MIC) values. Bioactive phytocompounds from green ZnO also inhibit hemolysis of Staphylococcus aureus infected red blood cells and inflammatory activity of mammalian immune system. In general, three mechanisms have been adopted to explain bactericidal activity of ZnO nanomaterials, including direct contact killing, reactive oxygen species (ROS) production, and released zinc ion inactivation. These toxic effects lead to the destruction of bacterial membrane, denaturation of enzyme, inhibition of cellular respiration and deoxyribonucleic acid replication, causing leakage of the cytoplasmic content and eventual cell death. Meanwhile, antimicrobial activity of doped and modified ZnO nanomaterials under visible light can be attributed to photogeneration of ROS on their surfaces. Thus particular attention is paid to the design and synthesis of visible light-activated ZnO photocatalysts with antibacterial properties.

Graphene oxide chemically reduced and functionalized with KOH-PEI for efficient Cr(VI) adsorption and reduction in acidic medium

In this study, graphene oxide (GO), polyethyleneimine (PEI) and potassium hydroxide (KOH) were used to synthesize reduced graphene oxide (rGO/PEI-KOH) nanocomposite. The presence and grafting of PEI molecules on the reduced GO surface were assessed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analyses. The rGO/PEI-KOH nanocomposite was successfully applied for hexavalent chromium, Cr(VI), wastewater elimination. The resulting rGO/PEI-KOH adsorbent was found to be highly effective for Cr(VI) removal at low pH values and achieved a maximum capacity of experimental adsorption of 398.9 mg/g, which is one of the highest sorption capacity of most GO- and PEI-based adsorbents reported in the literature up to date. Studying the adsorption mechanism, the sorption isotherm revealed that the modified-Langmuir model was the best fit and Cr(VI) removal follows a pseudo-second-order kinetics, with the predominance of intraparticle diffusion during the first step of adsorption. XPS analysis indicated the presence of appreciable amount of Cr(III) on the adsorbent surface, which suggests that the adsorbed Cr(VI) ions were effectively reduced to Cr(III) on the rGO/PEI-KOH adsorbent surface (∼70% of the total adsorbed Cr). Cr(VI) adsorption and subsequent reduction to Cr(III) both contributed to the Cr(VI) removal. The results of the present study highlight the benefits of rGO/PEI-KOH like low cost, environmentally friendly, large toxic Cr(VI) ions adsorption capacity and its effective reduction to less-toxic Cr(III).

NH3 Sensor Based on 2D Wormlike Polypyrrole/Graphene Heterostructures for a Self-Powered Integrated System

The rapid development of a NH₃ sensor puts forward a great challenge for active materials and integrated sensing systems. In this work, an ultrasensitive NH₃ sensor based on two-dimensional (2D) wormlike mesoporous polypyrrole/reduced graphene oxide (w-mPPy@rGO) heterostructures, synthesized by a universal soft template method is reported, revealing the structure-property coupling effect of the w-mPPy/rGO heterostructure for sensing performance improvement, and demonstrates great potential in the integration of a self-powered sensor system. Remarkably, the 2D w-mPPy@rGO heterostructrure exhibits preferable response toward NH₃ (ΔR/R₀ = 45% for 10 ppm NH₃ with a detection limit of 41 ppb) than those of the spherical mesoporous hybrid (s-mPPy@rGO) and the nonporous hybrid (n-PPy@rGO) due to its large specific surface area (193 m²/g), which guarantees fast gas diffusion and transport of carriers. Moreover, the w-mPPy@rGO heterostructures display outstanding selectivity to common volatile organic compounds (VOCs), H₂S, and CO, prominent antihumidity inteference superior to most existing chemosensors, superior reversibility and favorable repeatability, providing high potential for practicability. Thus, a self-powered sensor system composed of a nanogenerator, a lithium-ion battery, and a w-mPPy@rGO-based sensor was fabricated to realize wireless, portable, cost-effective, and light-weight NH₃ monitoring. Impressively, our self-powered sensor system exhibits high response toward 5-40 mg NH₄NO₃, which is a common explosive to generate NH₃ via alkaline hydrolysis, rendering it a highly prospective technique in a NH₃-based sensing field.

Capacitance Enhancement of Hydrothermally Reduced Graphene Oxide Nanofibers

Nanocarbon materials present sp2-carbon domains skilled for electrochemical energy conversion or storage applications. In this work, we investigate graphene oxide nanofibers (GONFs) as a recent interesting carbon material class. This material combines the filamentous morphology of the starting carbon nanofibers (CNFs) and the interlayer spacing of graphene oxide, and exhibits a domain arrangement accessible for fast transport of electrons and ions. Reduced GONFs (RGONFs) present the partial removal of basal functional groups, resulting in higher mesoporosity, turbostratic stacking, and surface chemistry less restrictive for transport phenomena. Besides, the filament morphology minimizes the severe layer restacking shown in the reduction of conventional graphene oxide sheets. The influence of the reduction temperature (140-220 °C) on the electrochemical behaviour in aqueous 0.5 M H2SO4 of RGONFs is reported. RGONFs present an improved capacitance up to 16 times higher than GONFs, ascribed to the unique structure of RGONFs containing accessible turbostratic domains and restored electronic conductivity. Hydrothermal reduction at 140 °C results in the highest capacitance as evidenced by cyclic voltammetry and electrochemical impedance spectroscopy measurements (up to 137 F·g-1). Higher temperatures lead to the removal of sulphur groups and slightly thicker graphite domains, and consequently a decrease of the capacitance.

High-Efficiency Omnidirectional Broadband Light-Management Coating Using the Hierarchical Ordered-Disordered Nanostructures with Ultra-Mechanochemical Resistance

High-efficient light-management nanostructures are critical to various optical applications. However, in practical implementation, these structures have been limited by the need to resist mechanical abrasion, erosion, chemical exposure, ultraviolet radiation, and performance deterioration by dust accumulation. To address these critical technological gaps, we herein report a conceptually different approach, employing a hierarchical nanostructure embedded with multilayer LightScribe-etched graphene, capable of omnidirectional broadband light management with both high optical transparency (>90%) and high haze (∼89%), ideal for photovoltaics, which simultaneously demonstrates extraordinary robustness to various environmental challenges ranging from mechanical abrasion, UV exposure, corrosions, outdoor exposures to resistance to dust accumulation. The reported nanostructures can be readily combined to any optoelectrical device's surface, and the practical tests on coated amorphous silicon solar cells show that it outperforms the state-of-the-art commercial coating by maintaining both 10% efficiency improvement along with the prevention of dust accumulation in contrast to 56.2% efficiency degradation with the commercial coating after the 1 month outdoor test.

Nanosized-Zinc-Mediated Self-Gelation of Graphene Oxide under Ambient Conditions

Self-assembly of 3D reduced graphene oxide (rGO) sponges has received increasing attention in recent years. By far, chemical reduction, hydrothermal treatment, template-directed chemical vapor deposition, and electrodeposition are the typical methods. Herein, the utilization of zinc nanoparticles as a reducing agent to fabricate 3D rGO sponges is reported. The relative negative standard electrode potential of zinc to graphene oxide (GO) allowed the spontaneous formation of the rGO-Zn hydrogel. Zinc-free 3D rGO sponges were recovered by acid leaching of the zinc species and freeze-drying. This room-temperature electroless gelation is dependent on pH. The structure and electrochemical performance of the as-synthesized rGO sponges were determined by the mass ratio of Zn to GO. Comprehensive physical and chemical characterizations were utilized to understand the 3D structure evolution of the rGO sponges. The rGO sponge, with an optimized texture, showed high capacitance and good stability in 1 m H₂ SO₄ electrolyte.

Novel sulfonated graphene oxide incorporated polysulfone nanocomposite membranes for enhanced-performance in ultrafiltration process

A novel polysulfone (PSf) nanocomposite ultrafiltration (UF) membrane using sulfonated graphene oxide (SGO) as additives was fabricated and investigated. SGO nanoparticles were chemically synthesized from graphene oxide (GO) by using sulfuric acid (H₂SO₄) and were confirmed by Raman and Fourier transform infrared (FTIR) spectroscopy. The morphology of prepared membranes was characterized by scanning electron microscopy (SEM), energy dispersive x-ray (EDX) and atomic force microscopy (AFM). Results showed that adding small amount (less than 0.3 wt%) of SGO improved wettability, porosity and mean pore size of PSf/SGO membranes compared to the pristine PSf membrane and significantly enhanced the water flux of SGO incorporated PSf membranes. In UF performance, the nanocomposite membrane prepared by adding 1.5 w/w% SGO of PSf (designated as M1.5) showed the highest water flux result, which was 125% higher than the control PSf membrane (no SGO addition). Interestingly, there was no trade-off between water flux and bovine serum albumin (BSA) rejection, i.e more than 98% BSA rejection. The addition of SGO hydrophilic additives also showed better results in long-term BSA separation performance. The enhancement of hybrid membrane's properties was attributed to the hydrophilicity of sulfonic acid group (SO₃H) on the surface of SGO additive. This study suggested that the SGO nanoparticle is a promising candidate to modify the PSf UF membranes.

A process to enhance the specific surface area and capacitance of hydrothermally reduced graphene oxide

The impact of post-synthesis processing in reduced graphene oxide materials for supercapacitor electrodes has been analyzed. A comparative study of vacuum, freeze and critical point drying was carried out for hydrothermally reduced graphene oxide demonstrating that the optimization of the specific surface area and preservation of the porous network are critical to maximize its supercapacitance performance. As described below, using a supercritical fluid as the drying medium, unprecedented values of the specific surface area (364 m² g^-1) and supercapacitance (441 F g^-1) for this class of materials have been achieved.

Graphene-Based Materials for Stem Cell Applications

Although graphene and its derivatives have been proven to be suitable for several biomedical applications such as for cancer therapy and biosensing, the use of graphene for stem cell research is a relatively new area that has only recently started to be investigated. For stem cell applications, graphene has been utilized by itself or in combination with other types of materials such as nanoparticles, nanofibers, and polymer scaffolds to take advantage of the several unique properties of graphene, such as the flexibility in size, shape, hydrophilicity, as well as its excellent biocompatibility. In this review, we will highlight a number of previous studies that have investigated the potential of graphene or its derivatives for stem cell applications, with a particular focus on guiding stem cell differentiation into specific lineages (e.g., osteogenesis, neurogenesis, and oligodendrogenesis), promoting stem cell growth, stem cell delivery/transplantation, and effective monitoring of their differentiation. We hope that this review promotes and accelerates the use of graphene-based materials for regenerative therapies, especially for stem cell-based approaches to cure various incurable diseases/disorders such as neurological diseases (e.g., Alzheimer's disease and Parkinson's disease), stroke, spinal cord injuries, bone/cartilage defects, and cardiovascular diseases.