Scalable enhancement of graphene oxide properties by thermally driven phase transformation

Bioinspired graphene-based metal oxide nanocomposites for photocatalytic and electrochemical performances: an updated review

Considering the rapidly increasing population, the development of new resources, skills, and devices that can provide safe potable water and clean energy remains one of the vital research topics for the scientific community. Owing to this, scientific community discovered such material for tackle this issue of environment benign, the new materials with graphene functionalized derivatives show significant advantages for application in multifunctional catalysis and energy storage systems. Herein, we highlight the recent methods reported for the preparation of graphene-based materials by focusing on the following aspects: (i) transformation of graphite/graphite oxide into graphene/graphene oxide via exfoliation and reduction; (ii) bioinspired fabrication or modification of graphene with various metal oxides and its applications in photocatalysis and storage systems. The kinetics of photocatalysis and the effects of different parameters (such as photocatalyst dose and charge-carrier scavengers) for the optimization of the degradation efficiency of organic dyes, phenol compounds, antibiotics, and pharmaceutical drugs are discussed. Further, we present a brief introduction on different graphene-based metal oxides and a systematic survey of the recently published research literature on electrode materials for lithium-ion batteries (LIBs), supercapacitors, and fuel cells. Subsequently, the power density, stability, pseudocapacitance charge/discharge process, capacity and electrochemical reaction mechanisms of intercalation, and conversion- and alloying-type anode materials are summarized in detail. Furthermore, we thoroughly distinguish the intrinsic differences among underpotential deposition, intercalation, and conventional pseudocapacitance of electrode materials. This review offers a meaningful reference for the construction and fabrication of graphene-based metal oxides as effective photocatalysts for photodegradation study and high-performance optimization of anode materials for LIBs, supercapacitors, and fuel cells.

Confined and spontaneously transformed oxidation structures due to the intrinsic heterogeneous surface morphology of C3N monolayer

Identifying the oxidation structure of two-dimensional interfaces is crucial to improve surface chemistry and electronic properties. Beyond graphene with only phenyl rings, a novel carbon-nitrogen material, C3N, presents an intrinsic heterogeneous surface morphology where each phenyl ring is encircled by six nitrogen atoms, yet its atomistic oxidation structure remains unclear. Here, combining a series of density functional theory calculations and ab initio molecular dynamics simulations, we demonstrate that thermodynamically favorable oxidation loci are confined to the phenyl ring, and kinetic transformations of oxidation structures are feasible along the phenyl ring, whereas those toward nitrogen atoms are proven to be extremely difficult. These results are attributed to the lower barrier of oxygen atom migration along the phenyl ring, while the significantly high barriers toward nitrogen atoms are due to the heterogeneous potential energy surface for oxygen-C3N interaction. This work highlights the significance of surface morphology on the characteristics of oxidation structure, offering insights into tunable electronic properties via confined interfacial oxidation.

Highly emissive blue graphene quantum dots with excitation-independent emission via ultrafast liquid-phase photoreduction

Graphene oxide quantum dots (GOQDs) are promising candidates for biomedical applications since they have lower toxicity and higher biocompatibility than traditional semiconductor quantum dots. However, oxygen functional groups such as epoxy and hydroxyl groups usually induce nonradiative relaxation, which leads to GOQDs exhibiting nonemissive properties. For the enhancement of the emission efficiency of GOQDs, the number of nonradiative relaxation sites should be reduced. This paper reports the synthesis of highly luminescent reduced GOQDs prepared by liquid-phase photoreduction (LPP-rGOQDs). First, GOQDs was fabricated from single-walled carbon nanotubes through chlorate-based oxidation and separation after acoustic cavitation. Subsequently, LPP-rGOQDs were obtained by liquid-phase photoreduction of the GOQD suspension under intense pulsed light irradiation. Liquid-phase photoreduction selectively reduced epoxy groups present on the basal plane of GOQDs, and hydrogenated the basal plane without removal of carbonyl and carboxyl groups at the edges of the GOQDs. Such selective removal of oxidative functional groups was used to control the reduction degree of GOQDs, closely related to their optical properties. The optimized LPP-rGOQDs were bright blue in color and showed quantum yields up to about 19.7%, which was 10 times the quantum yield of GOQDs. Furthermore, the LPP-rGOQDs were utilized to image a human embryonic kidney (HEK293A), and a low cytotoxicity level and satisfactory cell imaging performance were observed.

Enhancing the seawater desalination performance of multilayer reduced graphene oxide membranes by introducing in-plane nanopores: a molecular dynamics simulation study

In this study, using MD simulation, the effect of creating in-plane nanopores in a reduced graphene oxide (rGO) membrane and the formation of a reduced nanoporous graphene oxide (rNPGO) membrane is proposed to increase salt rejection and water flux. To this end, the desalination performance of r1NPGO, r2NPGO and r4NPGO membranes, which have 1, 2 and 4 pore(s), respectively, with a diameter of 0.9 nm and the r1NPGO-3 nm membrane, which has 1 pore with an approximate diameter of 3.0 nm, was investigated and compared from a molecular point of view. The simulation results show that in the rNPGO membranes, by increasing the number of pores from 1 to 4, water flux increases by ∼6 times compared to the rGO membrane. Meanwhile, upon increasing the pore size from 0.9 to 3.0 nm, water flux is enhanced by ∼16 times compared to the rGO membrane. The simulation results also demonstrate that the rGO membrane has two paths for water penetration, which are called the interlayer pathway and in-slit pathway. Moreover, pores in the rNPGO membranes provide another additional path for water transfer by shortening the lateral size of the membranes. This path is referred to as the in-pore pathway. By increasing the size of the pore in the r1NPGO-3 nm membrane, the contribution of the in-pore pathway increases and plays an important role. Furthermore, the simulation results show that in all rGO and rNPGO membranes, the interlayer space acts as a barrier for ions. Therefore, complete salt rejection is observed. Interestingly, by increasing the pore size in the r1NPGO-3 nm membrane, this membrane still maintains complete salt rejection. The observed phenomenon can be a result of very high water flux in this membrane. By increasing water flux, the presence of water molecules around Na+ and Cl- ions decreases. As a result, the formation of Na+Cl- ionic clusters is strengthened in such a way that these clusters do not have the ability to pass through large pores of 3.0 nm.

Transient chemical and structural changes in graphene oxide during ripening

Graphene oxide (GO)-the oxidized form of graphene-is actively studied in various fields, such as energy, electronic devices, separation of water, materials engineering, and medical technologies, owing to its fascinating physicochemical properties. One major drawback of GO is its instability, which leads to the difficulties in product management. A physicochemical understanding of the ever-changing nature of GO can remove the barrier for its growing applications. Here, we evidencde the presence of intrinsic, metastable and transient GO states upon ripening. The three GO states are identified using a [Formula: see text] transition peak of ultraviolet-visible absorption spectra and exhibit inherent magnetic and electrical properties. The presence of three states of GO is supported by the compositional changes of oxygen functional groups detected via X-ray photoelectron spectroscopy and structural information from X-ray diffraction analysis and transmission electron microscopy. Although intrinsic GO having a [Formula: see text] transition at 230.5 ± 0.5 nm is stable only for 5 days at 298 K, the intrinsic state can be stabilized by either storing GO dispersions below 255 K or by adding ammonium peroxydisulfate.

Nanowire-assisted electrochemical perforation of graphene oxide nanosheets for molecular separation

Two-dimensional nanosheets, e.g., graphene oxide (GO), have been widely used to fabricate efficient membranes for molecular separation. However, because of poor transport across nanosheets and high width-to-thickness ratio, the permeation pathway length and tortuosity of these membranes are extremely large, which limit their separation performance. Here we report a facile, scalable, and controllable nanowire electrochemical concept for perforating and modifying nanosheets to shorten permeation pathway and adjust transport property. It is found that confinement effects with locally enhanced charge density, electric field, and hydroxyl radical generation over nanowire tips on anode can be executed under low voltage, thereby inducing confined direct electron loss and indirect oxidation to reform configuration and composition of GO nanosheets. We demonstrate that the porous GO nanosheets with a lot of holes are suitable for assembling separation membranes with tuned accessibility, tortuosity, interlayer space, electronegativity, and hydrophilicity. For molecular separation, the prepared membranes exhibit quadruple water permeance and higher rejections for salts (>91%) and small molecules (>96%) as/than original ones. This nanowire electrochemical perforation concept offers a feasible strategy to reconstruct two-dimensional materials and tune their transport property for separation.

Fluorescent dual-mode sensor for the determination of graphene oxide and catechin in environmental or food field

The novel fluorescent sensor is proposed in this work based on the competitive interactions between the known bioactive compounds, riboflavin and catechin, which act as guests, and graphene oxide (GO) material that acts as host. Specifically, this proposal is based on an indicator displacement assay which allows the detection of GO and catechin (fluorescence quenching of riboflavin signal by GO and increase in fluorescence by catechin on the GO-riboflavin system). Three GO structures with different lateral dimensions and thickness were synthesized and tested, being able to be the three differentiated depending on the attenuation capacity of the fluorescent signal that each one possesses. The environmental analytical control of GO is more and more important, being this method sensitive and selective in the presence of other potential interferent substances. On the other hand, the other sensing capacity of the sensor also allows the determination of catechin in food samples based on the formation of riboflavin-GO complex. It is a rapid, simple and non-expensive procedure. Thus, the same 2D nanomaterial (GO) is seen to play a double role in this sensing strategy (analyte and analytical tool for the determination of another compound).

Selective Photonic Gasification of Strained Oxygen Clusters on Graphene for Tuning Pore Size in the Å Regime

Controlling the size of single-digit pores, such as those in graphene, with an Å resolution has been challenging due to the limited understanding of pore evolution at the atomic scale. The controlled oxidation of graphene has led to Å-scale pores; however, obtaining a fine control over pore evolution from the pore precursor (i.e., the oxygen cluster) is very attractive. Herein, we introduce a novel "control knob" for gasifying clusters to form pores. We show that the cluster evolves into a core/shell structure composed of an epoxy group surrounding an ether core in a bid to reduce the lattice strain at the cluster core. We then selectively gasified the strained core by exposing it to 3.2 eV of light at room temperature. This allowed for pore formation with improved control compared to thermal gasification. This is because, for the latter, cluster-cluster coalescence via thermally promoted epoxy diffusion cannot be ruled out. Using the oxidation temperature as a control knob, we were able to systematically increase the pore density while maintaining a narrow size distribution. This allowed us to increase H2 permeance as well as H2 selectivity. We further show that these pores could differentiate CH4 from N2, which is considered to be a challenging separation. Dedicated molecular dynamics simulations and potential of mean force calculations revealed that the free energy barrier for CH4 translocation through the pores was lower than that for N2. Overall, this study will inspire research on the controlled manipulation of clusters for improved precision in incorporating Å-scale pores in graphene.

Unraveling the Oxidation of a Graphitic Lattice: Structure Determination of Oxygen Clusters

Unraveling the oxidation of graphitic lattice is of great interest for atomic-scale lattice manipulation. Herein, we build epoxy cluster, atom by atom, using Van der Waals' density-functional theory aided by Clar's aromatic π-sextet rule. We predict the formation of cyclic epoxy trimers and its linear chains propagating along the armchair direction of the lattice to minimize the system's energy. Using low-temperature scanning tunneling microscopy on oxidized graphitic lattice, we identify linear chains as bright features that have a threefold symmetry, and which exclusively run along the armchair direction of the lattice confirming the theoretical predictions.

Assessing the evolution of oxygenated functional groups on the graphene oxide surface upon mild thermal annealing in water

Graphene oxide (GO) is known to be a 2D metastable nanomaterial that can be reconstructed under thermal annealing into distinct oxidized and graphitic phases. Up to now, such phase transformation, mainly related to epoxide and hydroxyl functional groups, has been usually achieved by thermally treating layers of GO in the solid state. Here, we present the mild annealing of GO dispersed in an aqueous medium, performed at two temperatures, 50 °C and 80 °C, for different intervals of time. We show experimental evidences of the epoxide instability in the presence of water by means of XPS, cyclic voltammetry and Raman spectroscopy, demonstrating the reorganization of epoxide and hydroxyl moieties initiated by water molecules. In fact, at 50 °C an increase in oxygen content is detected in all annealed samples compared to untreated GO, with a transformation of epoxide groups into vicinal diols. On the other hand, at 80 °C the oxygen content decreases towards the initial value since the vicinal diols, previously formed, transform into single hydroxyls and C[double bond, length as m-dash]C bonds. Moreover, the higher temperature annealing likely favours oxygenated functional groups rearrangements and clustering, in accordance with the literature, leading to a higher electron affinity and conductivity of the graphenic network.

Tunable optical and semiconducting properties of eco-friendly-prepared reduced graphene oxide

Wide bandgap oxidized graphenes have garnered particular interest among the materials explored for these applications because of their exceptional semiconducting and optical properties. This study aims to investigate the tunability of the related properties in reduced graphene oxide (rGO) for potential use in energy conversion, storage, and optoelectronic devices. To accomplish this, we scrutinized crucial parameters of the synthesis process such as reduction time and temperature. Our findings demonstrate that controlling these parameters makes it possible to customize the optical bandgap of reduced graphene oxide within a range of roughly 2.2 eV-1.6 eV. Additionally, we observed that reduced graphene oxide has strong and superior absorption in the visible region, which is attributable to the existence of OFGs and defects. Notably, our results indicate that the absorption coefficients of reduced graphene oxide are up to almost three times higher (7426 ml mg-1 m-1) than those observed in dispersions of exfoliated graphene and graphene oxide (GO). To complement our findings, we employed several spectroscopic and morphological characterizations, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and electrical measurements. The implications of our results are significant for the development and design of future semiconductors for energy conversion and optoelectronic applications.

Temperature-Dependent Optical Properties of Oxidized Graphenes

In this study, we investigate how changing important synthesis-related parameters can affect and control the optical characteristics of graphene oxide (GO) and reduced graphene oxide (rGO). These parameters include drying time and reduction time at two different temperatures. We obtain an understanding of their impact on optical transitions, optical bandgap, absorption coefficient, and absorbance spectrum width by analyzing these factors. Accordingly, GO has an optical bandgap of about 4 eV, which is decreased by the reduction process to 1.9 eV. Both GO and rGO display greater absorption in the visible spectrum, which improves photon capture and boosts efficiency in energy conversion applications. Additionally, our results show that GO and rGO have higher absorption coefficients than those previously reported for dispersions of exfoliated graphene. Defects in GO and rGO, as well as the presence of functional oxygen groups, are the main contributors to this increased absorption. Several measurements are carried out, including spectroscopic and morphological studies, to further support our findings.

Optical properties of graphene oxide

The study of the optical properties of graphene oxide (GO) is crucial in designing functionalized GO materials with specific optical properties for various applications such as (bio) sensors, optoelectronics, and energy storage. The present work aims to investigate the electronic transitions, optical bandgap, and absorption coefficient of GO under different conditions. Specifically, the study examines the effects of drying times ranging from 0 to 120 h while maintaining a fixed temperature of 80°C and low temperatures ranging from 40℃ to 100℃ , with a constant drying time of 24 h. Our findings indicate that exposing the GO sample to a drying time of up to 120 h at 80°C can lead to a reduction in the optical bandgap, decreasing it from 4.09 to 2.76 eV. The π - π * transition was found to be the most affected, shifting from approximately 230 nm at 0 h to 244 nm after 120 h of drying time. Absorption coefficients of 3140-5507 ml mg-1 m-1 were measured, which are similar to those reported for exfoliated graphene dispersions but up to two times higher, confirming the improved optical properties of GO. Our findings can provide insights into determining the optimal temperature and duration required for transforming GO into its reduced form for a specific application through extrapolation. The study is complemented by analyzing the elemental composition, surface morphology change, and electrical properties.

Unravelling the pseudocapacitive origin of graphene oxide-based aerogels by comparative insights

We reveal the intrinsic pseudocapacitive center of graphene-oxide-based aerogels by investigating different modified graphene skeletons from various approaches. A high proportion of carbonyl groups in carbon networks is shown to optimize the construction of rational pseudocapacitive sites by triggering reversible proton-induced surface reactions, leading to satisfactory electrochemical performance.

Enhancing Cementitious Composites with Functionalized Graphene Oxide-Based Materials: Surface Chemistry and Mechanisms

Graphene oxide-based materials (GOBMs) have been widely explored as nano-reinforcements in cementitious composites due to their unique properties. Oxygen-containing functional groups in GOBMs are crucial for enhancing the microstructure of cementitious composites. A better comprehension of their surface chemistry and mechanisms is required to advance the potential applications in cementitious composites of functionalized GOBMs. However, the mechanism by which the oxygen-containing functional groups enhance the response of cementitious composites is still unclear, and controlling the surface chemistry of GOBMs is currently constrained. This review aims to investigate the reactions and mechanisms for functionalized GOBMs as additives incorporated in cement composites. A variety of GOBMs, including graphene oxide (GO), hydroxylated graphene (HO-G), edge-carboxylated graphene (ECG), edge-oxidized graphene oxide (EOGO), reduced graphene oxide (rGO), and GO/silane composite, are discussed with regard to their oxygen functional groups and interactions with the cement microstructure. This review provides insight into the potential benefits of using GOBMs as nano-reinforcements in cementitious composites. A better understanding of the surface chemistry and mechanisms of GOBMs will enable the development of more effective functionalization strategies and open up new possibilities for the design of high-performance cementitious composites.

Wet Spinning of Graphene Oxide Fibers with Different MnO2 Additives

We present the fabrication of graphene oxide (GO) and manganese dioxide (MnO₂) composite fibers via wet spinning processes, which entails the effects of MnO₂ micromorphology and mass loading on the extrudability of GO/MnO₂ spinning dope and on the properties of resulted composite fibers. Various sizes of rod and sea-urchin shaped MnO₂ microparticles have been synthesized via hydrothermal reactions with different oxidants and hydrothermal conditions. Both the microparticle morphology and mass loading significantly affect the extrudability of the GO/MnO₂ mixture. In addition, the orientation of MnO₂ microparticles within the fibers is largely affected by their microscopic surface areas. The composite fibers have been made electrically conductive via chemical or thermal treatments and then applied as fiber cathodes in Zn-ion battery prototypes. Thermal annealing under an argon atmosphere turns out to be an appropriate method to avoid MnO₂ dissolution and leaching, which have been observed in the chemical treatments. These rGO/MnO₂ fiber cathodes have been assembled into prototype Zn-ion batteries with Zn wire as the anode and xanthan-gum gel containing ZnSO₄ and MnSO₄ salts as the electrolyte. The resulted electrochemical output depends on the annealing temperature and MnO₂ distribution within the fiber cathodes, while the best performer shows stable cycling stability at a maximum capacity of ca. 80 mA h/g.

Synthesis of Graphene Oxide from Sugarcane Dry Leaves by Two-Stage Pyrolysis

Natural or synthetic graphite as precursors for the preparation of graphene oxide (GO) have constraints due to their limited availability, high reaction temperature for processing of synthetic graphite and higher generation cost. The use of oxidants, long reaction duration, the generation of toxic gases and residues of inorganic salts, the degree of hazard and low yield are some of the disadvantages of the oxidative-exfoliation methods. Under these circumstances, biomass waste usage as a precursor is a viable alternative. The conversion of biomass into GO by the pyrolysis method is ecofriendly with diverse applications, which partially overcomes the waste disposal problem encountered by the existing methods. In this study, graphene oxide (GO) is prepared from dry leaves of sugarcane plant through a two-step pyrolysis method using ferric (III) citrate as a catalyst, followed by treatment with conc. H₂SO₄. The synthesized GO is analyzed by UV-Vis., FTIR, XRD, SEM, TEM, EDS and Raman spectroscopy. The synthesized GO has many oxygen-containing functional groups (-OH, C-OH, COOH, C-O). It shows a sheet-like structure with a crystalline size of 10.08 nm. The GO has a graphitic structure due to the Raman shift of G (1339 cm^-1) and D (1591 cm^-1) bands. The prepared GO has multilayers due to the ratio of 0.92 between I_D and I_G. The weight ratios between carbon and oxygen are examined by SEM-EDS and TEM-EDS and found to be 3.35 and 38.11. This study reveals that the conversion of sugarcane dry leaves into the high-value-added material GO becomes realistic and feasible and thus reduces the production cost of GO.

Green and Sustainable Ultrasound-Assisted Anodic Electrochemical Preparation of Graphene Oxide Dispersions and Their Antioxidant Properties

A fast method for preparing aqueous graphene oxide (GO) dispersions by electrochemical oxidation of a graphite anode without preliminary intercalation with oxidizing agents is proposed. Ultrasonic probing was used in the modulation mode of ultrasonic waves (work/rest) for more efficient graphite oxidation-exfoliation. It is shown that the 4/2 s mode of ultrasonic modulation is the most effective due to the probe material's low corrosion while maintaining the optimum synthesis temperature not exceeding 30-35 °C and achieving the best characteristics of the resulting product. Three cases of anodic oxidation of graphite to obtain graphene oxide were considered: (1) a combined cathode-anode compartment, (2) a split cathode-anode salt-bridged compartment, and (3) separated anode compartment with a 3.5 kDa dialysis membrane. It was determined that the approach to synthesis with a divided cathode-anode compartment makes it possible to obtain GO sheets with fewer defects compared to chemical methods or methods with a combined cathode-anode compartment and makes it possible to control the oxidation degree of the material (C:O ratio) by varying the current density. The prepared samples showed good stability for more than six months. The spectral and morphological characteristics were studied. Using chemiluminometry in the luminol/Co(II)/H₂O₂ system, the antioxidant properties concerning three key reactive oxygen species (H₂O₂, superoxide anion radical, and hydroxyl radical) were demonstrated. It was also shown that the prepared GO dispersions do not induce lipid and phospholipid peroxidation.

Tuning the electrical properties of graphene oxide through low-temperature thermal annealing

During the last fifteen years, the reduction of electrically insulating graphene oxide (GO) through the elimination of oxygen containing functional groups and the restoration of sp² conjugation yielding its conducting form, known as reduced graphene oxide (rGO), has been widely investigated as a scalable and low-cost method to produce materials featuring graphene-like characteristics. Among various protocols, thermal annealing represents an attractive green approach compatible with industrial processes. However, the high temperatures typically required to accomplish this process are energetically demanding and are incompatible with the use of plastic substrates often desired for flexible electronics applications. Here, we report a systematic study on the low-temperature annealing of GO by optimizing different annealing conditions, i.e., temperature, time, and reduction atmosphere. We show that the reduction is accompanied by structural changes of GO, which affect its electrochemical performance when used as an electrode material in supercapacitors. We demonstrate that thermally-reduced GO (TrGO) obtained under air or inert atmosphere at relatively low temperatures (<300 °C) exhibits low film resistivities (10^-2-10^-4 Ω m) combined with unaltered resistance after 2000 bending cycles when supported on plastic substrates. Moreover, it exhibits enhanced electrochemical characteristics with a specific capacitance of 208 F g^-1 and a capacitance retention of >99% after 2000 cycles. The reported strategy is an important step forward toward the development of environmentally friendly TrGO for future electrical or electrochemical applications.

ZnO/Graphene Composite from Solvent-Exfoliated Few-Layer Graphene Nanosheets for Photocatalytic Dye Degradation under Sunlight Irradiation

ZnO/graphene nanocomposites were prepared using a facile approach. Graphene nanosheets were prepared by ultrasonication-based liquid phase exfoliation of graphite powder in a low boiling point organic solvent, 1,2-Dichloroethane, for the preparation of ZnO/graphene nanocomposites. Structural properties of the synthesized ZnO/graphene nanocomposites were studied through powder XRD and micro-Raman analysis. The characteristic Raman active modes of ZnO and graphene present in the micro-Raman spectra ensured the formation of ZnO/graphene nanocomposite and it is inferred that the graphene sheets in the composites were few layers in nature. Increasing the concentration of graphene influenced the surface morphology of the ZnO nanoparticles and a flower shape ZnO was formed on the graphene nanosheets of the composite with high graphene concentration. The efficiencies of the samples for the photocatalytic degradation of Methylene Blue dye under sunlight irradiation were investigated and 97% degradation efficiency was observed. The stability of the nanocomposites was evaluated by performing five cycles, and 92% degradation efficiency was maintained. The observed results were compared with that of ZnO/graphene composite derived from other methods.

Effect of Pretreatment Conditions on the Precise Nanoporosity of Graphene Oxide

Nanoscale pores in graphene oxide (GO) control various important functions. The nanoporosity of GO is sensitive to low-temperature heating. Therefore, it is important to carefully process GO and GO-based materials to achieve superior functions. Optimum pretreatment conditions, such as the pre-evacuation temperature and time, are important during gas adsorption in GO to obtain accurate pore structure information. This study demonstrated that the pre-evacuation temperature and time for gas adsorption in GO must be approximately 333-353 K and 4 h, respectively, to avoid the irreversible alteration of nanoporosity. In situ temperature-dependent Fourier-transform infrared spectra and thermogravimetric analysis-mass spectrometry suggested significant structural changes in GO above the pre-evacuation temperature (353 K) through the desorption of "physically adsorbed water" and decomposition of unstable surface functional groups. The nanoporosity of GO significantly changed above the aforementioned pre-evacuation temperature and time. Thus, standard pretreatment is indispensable for understanding the intrinsic interface properties of GO.

Optical Biosensor Based on Graphene and Its Derivatives for Detecting Biomolecules

Graphene and its derivatives show great potential for biosensing due to their extraordinary optical, electrical and physical properties. In particular, graphene and its derivatives have excellent optical properties such as broadband and tunable absorption, fluorescence bursts, and strong polarization-related effects. Optical biosensors based on graphene and its derivatives make nondestructive detection of biomolecules possible. The focus of this paper is to review the preparation of graphene and its derivatives, as well as recent advances in optical biosensors based on graphene and its derivatives. The working principle of face plasmon resonance (SPR), surface-enhanced Raman spectroscopy (SERS), fluorescence resonance energy transfer (FRET) and colorimetric sensors are summarized, and the advantages and disadvantages of graphene and its derivatives applicable to various types of sensors are analyzed, and the methods of surface functionalization of graphene and its derivatives are introduced; these optical biosensors can be used for the detection of a range of biomolecules such as single cells, cellular secretions, proteins, nucleic acids, and antigen-antibodies; these new high-performance optical sensors are capable of detecting changes in surface structure and biomolecular interactions with the advantages of ultra-fast detection, high sensitivity, label-free, specific recognition, and the ability to respond in real-time. Problems in the current stage of application are discussed, as well as future prospects for graphene and its biosensors. Achieving the applicability, reusability and low cost of novel optical biosensors for a variety of complex environments and achieving scale-up production, which still faces serious challenges.

Amphiphilic Nanointerface: Inducing the Interfacial Activation for Lipase

Graphene-based materials are widely used in the field of immobilized enzymes due to their easily tunable interfacial properties. We designed amphiphilic nanobiological interfaces between graphene oxide (GO) and lipase TL (Thermomyces lanuginosus) with tunable reduction degrees through molecular dynamics simulations and a facile chemical modulation, thus revealing the optimal interface for the interfacial activation of lipase TL and addressing the weakness of lipase TL, which exhibits weak catalytic activity due to an inconspicuous active site lid. It was demonstrated that the reduced graphene oxide (rGO) after 4 h of ascorbic acid reduction could boost the relative enzyme activity of lipase TL to reach 208%, which was 48% higher than the pristine GO and 120% higher than the rGO after 48 h of reduction. Moreover, TL-GO-4 h's tolerance against heat, organic solvent, and long-term storage environment was higher than that of free TL. The drawbacks of strong hydrophobic nanomaterials on lipase production were explored in depth with the help of molecular dynamics simulations, which explained the mechanism of enzyme activity enhancement. We demonstrated that nanomaterials with certain hydrophilicity could facilitate the lipase to undergo interfacial activation and improve its stability and protein loading rate, displaying the potential of the extensive application.

Remarkably enhanced dynamic oxygen migration on graphene oxide supported by copper substrate

The dynamic covalent properties of graphene oxide (GO) are of fundamental interest to a broad range of scientific areas and technological applications. It remains a challenge to access feasible dynamic reactions for reversibly breaking/reforming the covalent bonds of oxygen functional groups on GO, although these reactions can be induced by photonic or mechanical routes, or mediated by adsorbed water. Here, using density functional theory calculations, we demonstrate the remarkably enhanced dynamic oxygen migration along the basal plane of GO supported by copper substrate (GO@copper), with C-O bond breaking reactions and proton transfer between neighboring epoxy and hydroxyl groups. Compared to reactions on GO, the energy barriers of oxygen migrations on GO@copper are sharply reduced to be less than or comparable to thermal fluctuations, and meanwhile the crystallographic match between GO and copper substrate induces new oxygen migration paths on GO@copper. This work sheds light on understanding of the metal substrate-enhanced dynamic properties of GO, and evidences the strategy to tune the activity of two-dimensional-interfacial oxygen groups for various potential applications.

Functionalized graphene oxide nanosheets with folic acid and silk fibroin as a novel nanobiocomposite for biomedical applications

In this paper, a novel graphene oxide-folic acid/silk fibroin (GO-FA/SF) nanobiocomposite scaffold was designed and fabricated using affordable and non-toxic materials. The GO was synthesized using the hummer method, covalently functionalized with FA, and then easily conjugated with extracted SF via the freeze-drying process. For characterization of the scaffold, several techniques were employed: Fourier-transform infrared (FT-IR), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), and thermogravimetric analysis (TGA). The cell viability method, hemolysis, and anti-biofilm assays were performed, exploring the biological capability of the nanobiocomposite. The cell viability percentages were 96.67, 96.35 and 97.23% for 24, 48, and 72 h, respectively, and its hemolytic effect was less than 10%. In addition, it was shown that this nanobiocomposite prevents the formation of Pseudomonas aeruginosa biofilm and has antibacterial activity.

An electroactive hybrid biointerface for enhancing neuronal differentiation and axonal outgrowth on bio-subretinal chip

Retinal prostheses offer viable vision restoration therapy for patients with blindness. However, a critical requirement for maintaining the stable performance of electrical stimulation and signal transmission is the biocompatibility of the electrode interface. Here, we demonstrated a functionalized electrode-neuron biointerface composed of an annealed graphene oxide-collagen (aGO-COL) composite and neuronal cells. The aGO-COL exhibited an electroactive 3D crumpled surface structure and enhanced the differentiation efficiency of PC-12 ​cells. It is integrated into a photovoltaic self-powered retinal chip to develop a biohybrid retinal implant that facilitates biocompatibility and tissue regeneration. Moreover, aGO-COL micropatterns fabricated via 3D bioprinting can be used to create neuronal cell microarrays, which supports the possibility of retaining the high spatial resolution achieved through electrical stimulation of the retinal chip. This study paves the way for the next generation of biohybrid retinal implants based on biointerfaces.

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Electroactive aGO-COL composites with 3D crumpled surface structures can effectively improve neuronal cell differentiation.

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The aGO-COL biointerface facilitates the development of a biohybrid retinal implant that integrates neuronal cells.

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The printed aGO-COL micropatterns support the creation of neuronal cell microarrays with specific patterns.

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This achievement reveals an excellent opportunity to bridge the gap between biological and artificial implants.

Enhanced Probe Bonding and Fluorescence Properties through Annealed Graphene Oxide Nanosheets

Graphene oxide (GO) has been widely used in biological sensing studies because of its excellent physical and chemical properties. In particular, the rich functional groups on the surface of GO can effectively enhance the bonding of biomolecules and serve as an efficient sensing substrate. However, when biomolecules are labeled with fluorescence, the GO interface affects the biomolecules by reducing the fluorescence properties and limiting their applications in biosensing. Here, we establish an annealed GO (aGO) substrate through the annealing process, which can effectively increase the bonding amount of a DNA probe because of the accumulation of oxygen atoms on the surface without significantly damaging the nanosheet structure. Furthermore, we prove that the aGO substrate can effectively maintain its fluorescence performance and stability by exposing more graphic domains. Overall, this study successfully verifies that GO's interface annealing modification can be used as an alternative innovative interface application in biosensing.

Super-assembly of freestanding graphene oxide-aramid fiber membrane with T-mode subnanochannels for sensitive ion transport

Biomimetic nacre-like membranes composed of two-dimensional lamellar sheets and one-dimensional nanofibers exhibit high mechanical strength and excellent stability. Thus, they show substantial application in the field of membrane science and water purification. However, the limited techniques for the assembly of two-dimensional lamellar membranes and one-dimensional nanofibers hamper their development and application. Herein, we developed a nacre-like and freestanding graphene oxide/aramid fiber membrane with abundant T-mode subnanochannels by introducing aramid fibers into graphene oxide interlamination via the super-assembly interaction between graphene oxide and aramid fibers. Benefiting from the presence of stable and adjustable sub-nanometer-size ion transport channels, the graphene oxide/aramid fiber composite membrane exhibited excellent mono/divalent ion selectivity of 3.51 (K⁺/Mg²⁺), which is superior to that of the pure graphene oxide membrane. The experimental results suggest that the mono/divalent ion selectivity is ascribed to the subnanochannels in the graphene oxide/aramid fiber composite membrane, electrostatic repulsion interaction and strong interaction between the divalent metal ion and carboxyl groups. Moreover, the composite membrane exhibited remarkable charge selectivity with a K⁺/Cl^- ratio of up to ∼158, indicating that this graphene oxide/aramid fiber composite membrane has great potential for application in energy conversion. This study provides an avenue to prepare freestanding and nacre-like composite membranes with abundant T-mode ion transport channels for ion recognition and energy conversion, which also shows great application prospects in the field of membrane science and water purification.

Demulsification of water-in-crude oil emulsion driven by a carbonaceous demulsifier from natural rice husks

Removing emulsified water from a water-in-crude oil (W/O) emulsion is critically required prior to downstream processing in the petroleum industry. In this work, environmentally friendly and amphipathic rice husk carbon (RHC) demulsifier was prepared by a simple carbonization process in a muffle furnace using rice husks as starting materials. RHC was characterized by field-emission scanning electron microscope, energy dispersive spectrometer, Fourier transform infrared spectrometer, ultraviolet-visible spectrometer, powder X-ray diffraction, zeta potential and synchronal thermal analyzer. The factors such as dosage, temperature, settling time, pH value and salinity were systematically investigated. The results indicated that the dehydration efficiency (DE) reached as high as 96.99% with 600 mg/L of RHC for 80 min at 70 °C. RHC exhibited an optimal DE under neutral condition, but it was also effective under acidic and alkaline conditions. Also, it had an excellent salt tolerance. The possible demulsification mechanism was explored by interfacial properties, different treatment methods for RHC and microexamination. The demulsification of RHC is attributed to its high interfacial activity, oxygen-containing groups and content of silica. It indicates that RHC is an effective demulsifier for the treatment of the W/O emulsion.

Nanomaterials in diagnostics, imaging and delivery: Applications from COVID-19 to cancer

ABSTRACT

In the past two decades, the emergence of nanomaterials for biomedical applications has shown tremendous promise for changing the paradigm of all aspects of disease management. Nanomaterials are particularly attractive for being a modularly tunable system; with the ability to add functionality for early diagnostics, drug delivery, therapy, treatment and monitoring of patient response. In this review, a survey of the landscape of different classes of nanomaterials being developed for applications in diagnostics and imaging, as well as for the delivery of prophylactic vaccines and therapeutics such as small molecules and biologic drugs is undertaken; with a particular focus on COVID-19 diagnostics and vaccination. Work involving bio-templated nanomaterials for high-resolution imaging applications for early cancer detection, as well as for optimal cancer treatment efficacy, is discussed. The main challenges which need to be overcome from the standpoint of effective delivery and mitigating toxicity concerns are investigated. Subsequently, a section is included with resources for researchers and practitioners in nanomedicine, to help tailor their designs and formulations from a clinical perspective. Finally, three key areas for researchers to focus on are highlighted; to accelerate the development and clinical translation of these nanomaterials, thereby unleashing the true potential of nanomedicine in healthcare.

A Blueprint for the Synthesis and Characterization of Thiolated Graphene

Graphene derivatization to either engineer its physical and chemical properties or overcome the problem of the facile synthesis of nanographenes is a subject of significant attention in the nanomaterials research community. In this paper, we propose a facile and scalable method for the synthesis of thiolated graphene via a two-step liquid-phase treatment of graphene oxide (GO). Employing the core-level methods, the introduction of up to 5.1 at.% of thiols is indicated with the simultaneous rise of the C/O ratio to 16.8. The crumpling of the graphene layer upon thiolation without its perforation is pointed out by microscopic and Raman studies. The conductance of thiolated graphene is revealed to be driven by the Mott hopping mechanism with the sheet resistance values of 2.15 kΩ/sq and dependable on the environment. The preliminary results on the chemiresistive effect of these films upon exposure to ethanol vapors in the mix with dry and humid air are shown. Finally, the work function value and valence band structure of thiolated graphene are analyzed. Taken together, the developed method and findings of the morphology and physics of the thiolated graphene guide the further application of this derivative in energy storage, sensing devices, and smart materials.

Sulfur Functionalization via Epoxide Ring Opening on a Reduced Graphene Oxide Surface to Form Metal (II) Organo-bis-[1,2]-oxathiin

The epoxide ring-opening reaction in graphene oxide (GO) by nucleophiles is a very fascinating and advanced methodology to develop novel functional material. Herewith, we report an advanced strategy for opening the epoxide ring on the rGO surface via easily an available nucleophile (Na₂S), which is further functionalized with O atom to obtain four-membered rings (FMRs). The Cd coordination with the S atom puts extra stress on the FMR leading to the C-C bond cleavage of the four-membered heteroatomic rings on the rGO surface. This strategic approach leads to the fabrication of an innovative metal (II) organo-bis-[1,2]-oxathiin (MOBOT) chemical moiety (M = Cd, Zn). The MOBOT compound further shows enhanced H₂ generation activity and hence is promising as a potential photocatalyst for solar hydrogen generation. This compound might also be a potential candidate for optoelectronic applications.

The Combination of 2D Layered Graphene Oxide and 3D Porous Cellulose Heterogeneous Membranes for Nanofluidic Osmotic Power Generation

Salinity gradient energy, as a type of blue energy, is a promising sustainable energy source. Its energy conversion efficiency is significantly determined by the selective membranes. Recently, nanofluidic membrane made by two-dimensional (2D) nanomaterials (e.g., graphene) with densely packed nanochannels has been considered as a high-efficient membrane in the osmotic power generation research field. Herein, the graphene oxide-cellulose acetate (GO-CA) heterogeneous membrane was assembled by combining a porous CA membrane and a layered GO membrane; the combination of 2D nanochannels and 3D porous structures make it show high surface-charge-governed property and excellent ion transport stability, resulting in an efficient osmotic power harvesting. A power density of about 0.13 W/m² is achieved for the sea-river mimicking system and up to 0.55 W/m² at a 500-fold salinity gradient. With different functions, the CA and GO membranes served as ion storage layer and ion selection layer, respectively. The GO-CA heterogeneous membrane open a promising avenue for fabrication of porous and layered platform for wide potential applications, such as sustainable power generation, water purification, and seawater desalination.

Graphene, Carbon Nanotube and Plasmonic Nanosensors for Detection of Viral Pathogens: Opportunities for Rapid Testing in Pandemics like COVID-19

With the emergence of global pandemics such as the Black Death (Plague), 1918 influenza, smallpox, tuberculosis, HIV/AIDS, and currently the COVID-19 outbreak caused by the SARS-CoV-2 virus, there is an urgent, pressing medical need to devise methods of rapid testing and diagnostics to screen a large population of the planet. The important considerations for any such diagnostic test include: 1) high sensitivity (to maximize true positive rate of detection); 2) high specificity (to minimize false positives); 3) low cost of testing (to enable widespread adoption, even in resource-constrained settings); 4) rapid turnaround time from sample collection to test result; and 5) test assay without the need for specialized equipment. While existing testing methods for COVID-19 such as RT-PCR (real-time reverse transcriptase polymerase chain reaction) offer high sensitivity and specificity, they are quite expensive – in terms of the reagents and equipment required, the laboratory expertise needed to run and interpret the test data, and the turnaround time. In this review, we summarize the recent advances made using carbon nanotubes for sensors; as a nanotechnology-based approach for diagnostic testing of viral pathogens; to improve the performance of the detection assays with respect to sensitivity, specificity and cost. Carbon nanomaterials are an attractive platform for designing biosensors due to their scalability, tunable functionality, photostability, and unique opto-electronic properties. Two possible approaches for pathogen detection using carbon nanomaterials are discussed here: 1) optical sensing, and 2) electrochemical sensing. We explore the chemical modifications performed to add functionality to the carbon nanotubes, and the physical, optical and/or electronic considerations used for testing devices or sensors fabricated using these carbon nanomaterials. Given this progress, it is reason to be cautiously optimistic that nanosensors based on carbon nanotubes, graphene technology and plasmonic resonance effects can play an important role towards the development of accurate, cost-effective, widespread testing capacity for the world’s population, to help detect, monitor and mitigate the spread of disease outbreaks.

Synthesis, characterisation and thermo-physical properties of highly stable graphene oxide-based aqueous nanofluids for potential low-temperature direct absorption solar applications

Two types of highly stable 0.1% graphene oxide-based aqueous nanofluids were synthesised and investigated. The first nanofluid (GO) was prepared under the influence of ultrasonic irradiation without surfactant. The second nanofluid was treated with tetra ethyl ammonium hydroxide to reduce the graphene oxide to form reduced graphene oxide (RGO) during ultrasonic irradiation. The GO and RGO powders were characterised by various techniques such as field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman. Also UV–visible absorption spectroscopy was carried out and band gap energies were determined. Optical band gap energies for indirect transitions ranged from 3.4 to 4.4 eV and for direct transitions they ranged between 2.2 and 3.7 eV. Thermal conductivity measurements of the GO-based aqueous nanofluid revealed an enhancement of 9.5% at 40 °C compared to pure water, while the RGO-based aqueous nanofluid at 40 °C had a value 9.23% lower than pure water. Furthermore, the photothermal response of the RGO-based aqueous nanofluid had a temperature increase of 13.5 °C, (enhancement of 60.2%) compared to pure water, the GO-based aqueous nanofluid only displayed a temperature rise of 10.9 °C, (enhancement of 46.6%) after 20 min exposure to a solar irradiance of 1000 W m⁻². Both nanofluid types displayed good long-term stability, with the GO-based aqueous nanofluid having a zeta potential of 30.3 mV and the RGO-based aqueous nanofluid having a value of 47.6 mV after 6 months. The good dispersion stability and photothermal performance makes both nanofluid types very promising working fluids for low-temperature direct absorption solar collectors.

Variable-Angle Spectroscopic Ellipsometry of Graphene-Based Films

A review of the authors’ research works on Variable-Angle Spectroscopy (VASE) of graphene-based films is presented. The interaction of graphene oxide (GO) with magnetron-sputtered metals is a promising research area. VASE optical models of GO thin films deposited on magnetron-sputtered titanium (Ti), silver (Ag) and gold (Au) are discussed. Moreover, the optical properties of graphene nanoplatelet (GNPS) films and reduced graphene oxide (RGO) stabilized with Poly(Sodium 4-Styrenesulfonate) (PSS) films, which are less studied graphene-related materials, are shown. Finally, different optical behaviors of chemical vapor deposition (CVD)-grown monolayer, bilayer, and trilayer graphene films on silicon and polyethylene terephthalate (PET) substrates are recapitulated.

Graphene Oxide Chemistry Management via the Use of KMnO4/K2Cr2O7 Oxidizing Agents

In this paper, we propose a facile approach to the management of graphene oxide (GO) chemistry via its synthesis using KMnO4/K2Cr2O7 oxidizing agents at different ratios. Using Fourier Transformed Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray Absorption Spectroscopy, we show that the number of basal-plane and edge-located oxygenic groups can be controllably tuned by altering the KMnO4/K2Cr2O7 ratio. The linear two-fold reduction in the number of the hydroxyls and epoxides with the simultaneous three-fold rise in the content of carbonyls and carboxyls is indicated upon the transition from KMnO4 to K2Cr2O7 as a predominant oxidizing agent. The effect of the oxidation mixture's composition on the structure of the synthesized GOs is also comprehensively studied by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy, atomic-force microscopy, optical microscopy, and the laser diffraction method. The nanoscale corrugation of the GO platelets with the increase of the K2Cr2O7 content is signified, whereas the 10-100 μm lateral size, lamellar, and defect-free structure is demonstrated for all of the synthesized GOs regardless of the KMnO4/K2Cr2O7 ratio. The proposed method for the synthesis of GO with the desired chemistry opens up new horizons for the development of graphene-based materials with tunable functional properties.

Self-assembled graphene-based microfibers with eclectic optical properties

The construction of graphene-based microfibers with reinforced mechanical and electrical properties has been the subject of numerous researches in recent years. However, the fabrication of graphene-based fibers with remarkable optical features still remains a challenge and has not been addressed so far. This paper aims to report a series of flexible self-assembled fibers, synthesized through a few-minute sonication of thermally oxidized graphene oxide nanosheets, so-called Nanoporous Over-Oxidized Graphene (NOG), in an acidic medium. These free-standing glassy fibers were classified into four distinct morphological structures and displayed a collection of intriguing optical properties comprising high transparency, strong birefringence, fixed body colorations (e.g. colorless, blue, green, and red), tunable interference marginal colorations, UV-visible-near IR fluorescence, and upconversion emissions. Moreover, they exhibited high chemical stability in strongly acidic, basic, and oxidizing media. The foregoing notable attributes introduce the NOG fiber as a promising candidate both for the construction of graphene-based photoluminescent textiles and the development of a wide variety of optical applications.

Graphene-Based Composites as Catalysts for the Degradation of Pharmaceuticals

The incessant release of pharmaceuticals into the aquatic environment continues to be a subject of increasing concern. This is because of the growing demand for potable water sources and the potential health hazards which these pollutants pose to aquatic animals and humans. The inability of conventional water treatment systems to remove these compounds creates the need for new treatment systems in order to deal with these class of compounds. This review focuses on advanced oxidation processes that employ graphene-based composites as catalysts for the degradation of pharmaceuticals. These composites have been identified to possess enhanced catalytic activity due to increased surface area and reduced charge carrier recombination. The techniques employed in synthesizing these composites have been explored and five different advanced oxidation processes-direct degradation process, chemical oxidation process, photocatalysis, electrocatalyis processes and sonocatalytic/sono-photocatalytic processes-have been studied in terms of their enhanced catalytic activity. Finally, a comparative analysis of the processes that employ graphene-based composites was done in terms of process efficiency, reaction rate, mineralization efficiency and time required to achieve 90% degradation.

The critical contribution of oxidation debris on the acidic properties of graphene oxide in an aqueous solution

The contribution of oxidation debris (OD) to the acidity of graphene oxide (GO) was investigated in this study. With Na₂CO₃ as the titrator base, the Boehm titration results showed that the total acidity of GO in an aqueous solution decreased from 9.72 to 2.74 mmol g^-1 after a thorough removal of OD and that the total acidity of OD was 26.45 mmol g^-1. Thermogravimetric analyses showed that the mass ratios of OD and residual graphene sheets (named bwGO) were ∼26 % and ∼73 % of the whole pristine GO, respectively. Based on the quantitative relationships between the mass ratio and acid site density, it was concluded that the total acidity of GO was equal to the sum of the acidity from bwGO and the OD contained in GO. Under alkaline conditions, the splitting and stripping of OD was attributed to the combined effect of the cleavage of H-bonds by nucleophilic attack from OH- and the electrostatic repulsion due to the ionization of carboxylic acids, in which the former became dominant when the pH shifted to neutral and weakly acidic. This study provides an explanation for the origin of GO acidity in aqueous solutions and highlights the role of OD in the chemistry of GO.