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Graphite oxide

Graphite oxide (GO), also known as graphitic oxide or graphite oxyhydroxide, is a non-stoichiometric, layered material obtained by the chemical oxidation of graphite, featuring carbon sheets with a disrupted hexagonal lattice due to the incorporation of oxygen-containing functional groups such as hydroxyl (-OH), epoxy (-O-), carbonyl (=O), and carboxyl (-COOH) moieties attached to both basal planes and edges.^[1] These groups introduce a mixture of sp² (aromatic) and sp³ (aliphatic) hybridized carbon atoms, transforming the hydrophobic, conductive graphite into a hydrophilic, electrically insulating substance with an interlayer spacing of approximately 0.7–1.0 nm and a typical carbon-to-oxygen ratio of 2.1–2.9.^[2] First prepared in the mid-19th century through oxidation with strong acids and oxidants, graphite oxide exhibits variable composition depending on synthesis conditions, enabling its exfoliation into single- or few-layer graphene oxide sheets that are pivotal in nanotechnology, composites, and energy storage applications.^[3]

History and Synthesis

Discovery and early methods

The discovery of graphite oxide is attributed to British chemist Benjamin Collins Brodie, who in 1859 first isolated the material while investigating the atomic weight of graphite. Brodie achieved this by oxidizing flake graphite through the addition of potassium chlorate (KClO₃) to a slurry in fuming nitric acid (HNO₃), resulting in a product he described as forming paper-like foils approximately 0.05 mm thick. This method, now known as the Brodie process, marked the initial chemical route to graphite oxide but suffered from inconsistent results due to the vigorous reaction conditions.^[4] In 1898, Ludwig Staudenmaier refined Brodie's approach to improve oxidation efficiency and yield. Staudenmaier's method involved mixing graphite with a combination of concentrated sulfuric acid (H₂SO₄) and fuming nitric acid, followed by the gradual addition of potassium chlorate to mitigate explosion risks associated with rapid mixing. This modification enhanced the degree of oxidation, producing graphite oxide with a higher oxygen content compared to Brodie's original product, though it retained the use of hazardous fuming acids.^[5] Ulrich Hofmann contributed another early variant in 1934, employing fuming nitric acid as the primary oxidant in conjunction with potassium chlorate, but without the sulfuric acid component central to Staudenmaier's procedure.^[6] This Hofmann method aimed to simplify the synthesis but was limited by poor scalability, as the reliance on fuming nitric acid led to difficulties in handling larger quantities and inconsistent product purity.^[7] A significant advancement came in 1958 with the Hummers-Offeman method, developed by William S. Hummers Jr. and Richard E. Offeman, which prioritized safety and speed over previous techniques. The process oxidizes graphite using a mixture of concentrated sulfuric acid, sodium nitrate (NaNO₃), and potassium permanganate (KMnO₄) at controlled temperatures, followed by treatment with hydrogen peroxide (H₂O₂) to quench residual permanganate and yield a yellow-brown graphite oxide with an empirical formula approximating C_{2.1-2.9}O.^[8] This approach avoided fuming nitric acid, reducing toxicity risks, and achieved higher yields, making it the standard for decades.^[9] Early synthesis efforts, including those by Brodie, Staudenmaier, and Hofmann, faced persistent challenges such as low yields—often below 50% due to incomplete oxidation—and the use of hazardous reagents like fuming acids and chlorates, which posed explosion hazards and generated toxic nitrogen oxide gases.^[10] These limitations spurred later innovations toward safer, more efficient methods.

Modern preparation techniques

Modern preparation techniques for graphite oxide emphasize enhanced safety, reduced environmental impact, and scalability compared to earlier chemical oxidation approaches. These methods build on foundational oxidation principles but incorporate milder reagents, alternative energy inputs, and process controls to minimize toxic byproducts like permanganate residues and explosive gases. Key innovations since the late 20th century include modified wet chemistry, electrochemical processes, and green synthesis routes, enabling higher yields and tunable material quality. The Tour method, introduced in 2010, represents a significant improvement over traditional Hummers' procedures by replacing sodium nitrate with phosphoric acid to eliminate permanganate-related toxic waste and enhance oxidation efficiency. In this approach, graphite flakes are pretreated with a 9:1 mixture of sulfuric and phosphoric acids, followed by gradual addition of potassium permanganate at controlled temperatures below 50°C to prevent overheating; the reaction is quenched with hydrogen peroxide, yielding a highly dispersible graphite oxide with a C/O ratio of approximately 2.2. This method produces up to 100% more hydrophilic material than conventional Hummers' variants while maintaining interlayer spacing around 0.8-1.2 nm, making it suitable for large-scale applications.^[11] Electrochemical exfoliation, developed and refined throughout the 2010s, offers a controlled, oxidant-free route via anodic oxidation of graphite electrodes in aqueous or organic electrolytes. The process involves applying a voltage (typically 10 V) in solutions like sulfuric acid or sulfate salts, where anions intercalate between graphite layers, generating oxygen species that oxidize and exfoliate the material into single- or few-layer graphite oxide sheets; a simplified representation is the transformation of the graphite electrode to GO under electrolytic conditions. Optimized parameters, such as electrolyte composition and current density, achieve yields up to 80% with C/O ratios of 2.1-2.9, providing precise control over oxidation degree and reducing chemical waste compared to batch chemical methods.^[12] Green synthesis routes prioritize sustainability by using biomass-derived precursors and mild oxidants. The Tang-Lau method, reported in 2012, employs a bottom-up self-assembly process where glucose serves as the carbon source and ferric chloride as the oxidant in an aqueous medium at 80°C, forming large-scale graphite oxide nanosheets through hydrothermal condensation without strong acids or permanganates. This approach yields multilayer sheets with tunable thickness (1-1500 nm) and C/O ratios around 2.5, offering an environmentally friendly alternative that avoids hazardous reagents while achieving high purity.^[13] Recent advances from 2023-2025 have integrated energy-efficient enhancements for faster and scalable production. Ultrasound-enhanced methods further improve exfoliation by generating cavitation bubbles that disrupt interlayer bonds during oxidation, enhancing yield and sheet dispersion in hybrid chemical-electrochemical setups. Additionally, 2024 studies on continuous flow reactors enable kg-scale production by processing graphite rolls through electrolytic or oxidative streams, achieving steady-state outputs of up to 1 kg/hour with minimal batch variability and C/O ratios maintained at 2.2-2.9. Across these techniques, oxidation degrees (C/O 2.1-2.9) directly influence yield and dispersibility, with electrochemical variants often outperforming others in efficiency (up to 80% yield) due to tunable voltage control.^[14]^[15]^[16]

Structure and Composition

Atomic and molecular structure

Graphite consists of stacked layers of sp²-hybridized carbon atoms arranged in a hexagonal lattice, where each layer is held together by strong σ-bonds and adjacent layers interact via weak van der Waals forces, resulting in an interlayer distance of 0.335 nm.^[17] Oxidation of graphite transforms this ordered structure into graphite oxide, causing a significant expansion of the interlayer spacing to approximately 0.7–1.2 nm due to the intercalation of oxygen species between the layers and the introduction of disrupted sp³-hybridized carbon regions that pucker the originally planar sheets.^[18] This expansion arises from the chemical disruption of the π-conjugated system, leading to a more irregular and less crystalline arrangement compared to pristine graphite.^[19] Several theoretical models describe the atomic and molecular structure of graphite oxide. The early Hofmann model proposed a relatively flat layered structure with epoxy bridges connecting adjacent carbon atoms across the sheets.^[4] In contrast, the influential Lerf-Klinowski model depicts graphite oxide as a mosaic of unoxidized aromatic sp² domains interspersed with oxidized sp³ regions, where the base carbon lattice is modified by attachments that induce local distortions.^[19] The stacking order in graphite oxide shows considerable variability, characterized by turbostratic disorder—in which layers exhibit random rotations relative to one another—and defects that generate amorphous-like regions within the otherwise layered architecture.^[20] Density functional theory (DFT) simulations corroborate these features, optimizing structures for different oxidation levels and yielding bond lengths such as C-O ≈ 1.4 Å in oxidized regions and C=C ≈ 1.42 Å in preserved sp² domains, highlighting the hybrid sp²/sp³ nature of the material.^[21]^[22]

Functional groups and variability

Graphite oxide features a variety of oxygen-containing functional groups that disrupt the pristine graphite lattice, primarily including epoxy (C-O-C) bridges, hydroxyl (-OH) groups, carbonyl (C=O) functionalities, and carboxyl (-COOH) groups. These groups are predominantly distributed on the basal planes and edges of the carbon sheets, with epoxy and hydroxyl groups accounting for approximately 50% of the oxygenation on the basal planes, while carboxyl groups constitute 10-20% and are mainly located at the sheet edges.^[23]^[24] The empirical formula of graphite oxide is approximately C₂HO (with hydrogen content varying from 0.8 to 1.0), reflecting a C/O atomic ratio that typically ranges from 2.1 to 2.9, depending on the extent of oxidation.^[25] This variability in composition arises from differences in synthesis conditions, such as the choice of oxidizing agents and reaction parameters. For instance, the Hummers method, which employs potassium permanganate in sulfuric acid, promotes higher oxidation levels that increase the proportion of epoxy groups relative to other functionalities.^[26] The reduction potential of these groups can be assessed through X-ray photoelectron spectroscopy (XPS), where the C 1s binding energy for C-O bonds associated with epoxy and hydroxyl groups appears at approximately 286.5 eV.^[27] These functional groups significantly influence the reactivity of graphite oxide, enabling covalent modifications such as amidation reactions with the carboxyl groups to form amide bonds for further derivatization.^[24] Recent structural models from 2023 to 2025, informed by nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics simulations, have refined assignments of these groups and revealed dynamic rearrangements, such as epoxy-to-hydroxyl conversions, particularly in aqueous solutions where hydration drives group mobility and interconversion.^[28]^[29]

Characterization Methods

Spectroscopic techniques

Fourier-transform infrared (FTIR) spectroscopy is widely employed to identify and quantify the oxygen-containing functional groups in graphite oxide, providing vibrational fingerprints of molecular bonds. Characteristic absorption peaks include a broad band at approximately 3400 cm⁻¹ attributed to O-H stretching from hydroxyl groups, a sharp peak at around 1720 cm⁻¹ corresponding to C=O stretching in carbonyl and carboxyl functionalities, and a band near 1050 cm⁻¹ indicative of C-O stretching in epoxy groups.^[30] These peaks allow for the assessment of oxidation extent and functional group distribution, with variations in intensity reflecting differences in synthesis methods or sample purity.^[31] Raman spectroscopy offers insights into the structural disorder and defect density in graphite oxide through the analysis of key vibrational modes of carbon atoms. The D band at about 1350 cm⁻¹ arises from breathing modes of sp³-hybridized carbon atoms at defects or edges, while the G band at approximately 1580 cm⁻¹ originates from in-plane stretching of sp²-hybridized carbon atoms. The intensity ratio I_D/I_G, typically ranging from 0.9 to 1.2, serves as a quantitative measure of defect density, with higher values indicating greater disruption of the graphitic lattice due to oxidation.^[31]^[30] X-ray photoelectron spectroscopy (XPS) provides elemental composition and chemical state information at the surface of graphite oxide by analyzing core-level binding energies. In the C 1s spectrum, the sp² carbon peak appears at 284.5 eV, while oxygenated carbons are deconvoluted into components at 286–288 eV for C-O (hydroxyl/epoxy) and C=O (carbonyl/carboxyl) bonds, revealing the degree of functionalization. The O 1s spectrum features a peak at around 532 eV, primarily from C-O and C=O oxygen atoms.^[31]^[30] These spectra enable calculation of the C/O atomic ratio, often around 2–3, highlighting the high oxygen content.^[4] Nuclear magnetic resonance (NMR) spectroscopy, particularly solid-state ¹³C NMR, elucidates the carbon environments in insoluble graphite oxide samples, supporting models like the Lerf-Klinowski structure with oxidized and aromatic domains. Key signals include resonances at 60–70 ppm for C-O carbons in epoxy and hydroxyl groups, and at 130–140 ppm for aromatic sp² carbons, with additional peaks around 170 ppm for carboxyl carbons in some variants.^[30] This technique distinguishes between functional group types and assesses the proportion of oxidized versus pristine graphitic regions without requiring dissolution.^[32] Ultraviolet-visible (UV-Vis) spectroscopy probes the electronic structure and conjugation extent in graphite oxide dispersions, revealing transitions associated with π-conjugated systems. A strong absorption peak at 230 nm corresponds to the π-π* transition of C=C bonds in aromatic regions, while a shoulder at about 300 nm arises from the n-π* transition involving carbonyl oxygen lone pairs.^[30] The position and intensity of these bands indicate the restoration of conjugation upon reduction, with graphite oxide showing restored visible color due to extended π-systems.^[33] These spectroscopic methods complement imaging techniques to provide a comprehensive understanding of graphite oxide's chemical bonding and structure.

Imaging and structural analysis

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are widely used to visualize the nanoscale morphology of graphite oxide, revealing characteristic wrinkled sheet-like structures with lateral dimensions typically ranging from 1 to 10 μm and thicknesses of 1-2 nm for individual layers.^[34]^[35] These techniques often capture the crumpled and folded appearance of the sheets, attributed to the oxidative functionalization and exfoliation process, with SEM particularly effective for observing larger-scale aggregation and surface topography.^[36] Edge-on TEM views provide insights into the interlayer spacing, showing expanded distances compared to pristine graphite due to oxygen incorporation, typically around 0.7-1.0 nm between adjacent layers.^[37] Atomic force microscopy (AFM), especially in tapping mode, complements TEM/SEM by offering height profiles that confirm the distinction between monolayer graphite oxide sheets (~1 nm thick) and multilayer stacks (several nm).^[37] This mode minimizes tip-sample interactions, enabling measurement of surface roughness values of approximately 0.5-1 nm, which highlights the irregular, functionalized topography without significant deformation.^[38] X-ray diffraction (XRD) analysis of graphite oxide exhibits a broad peak corresponding to the (002) plane at a d-spacing of 0.8-1.2 nm, significantly larger than the 0.34 nm in graphite, indicating disrupted stacking and high exfoliation potential.^[39] The broadening of this peak allows estimation of lateral crystallite size using the Scherrer equation:

D = \frac{K \lambda}{\beta \cos \theta}

where D is the crystallite size, K = 0.9 is the shape factor, \lambda is the X-ray wavelength, \beta is the full width at half maximum, and \theta is the Bragg angle; this yields sizes on the order of a few nanometers, reflecting the nanoscale domains in graphite oxide.^[40] Recent advances, including cryogenic electron microscopy (cryo-EM) applications, have enabled visualization of hydrated graphite oxide structures, revealing dynamic swelling behaviors where interlayer distances expand to approximately 1.1–1.3 nm in aqueous environments due to water intercalation.^[41] These techniques preserve the native wet-state morphology, contrasting with dehydrated samples. A key limitation in imaging graphite oxide arises from sample preparation artifacts, particularly differences between dry and wet states; drying can induce collapse of swollen layers and artificial wrinkling, while wet preparation risks aggregation or incomplete exfoliation during transfer to substrates.^[42] Such variations necessitate in situ or environmental imaging methods to mitigate discrepancies. Spectroscopic techniques can briefly confirm the chemical features observed in these images.

Properties

Surface and colloidal properties

Graphite oxide, often denoted as graphene oxide (GO) in contemporary literature, possesses an amphiphilic nature stemming from its chemically heterogeneous structure. The basal planes feature hydrophilic oxygen-containing groups, primarily hydroxyl and epoxy functionalities, which render these regions water-attracting, while the unoxidized aromatic sp²-hybridized carbon domains exhibit hydrophobic characteristics. This asymmetry enables GO sheets to self-assemble at oil-water interfaces, functioning as a two-dimensional surfactant to stabilize emulsions by reducing interfacial tension and preventing droplet coalescence.^[43]^[44] The surface charge of GO plays a pivotal role in its colloidal behavior, with zeta potential values typically ranging from -20 to -40 mV in aqueous dispersions at pH greater than 4. This negative charge originates from the deprotonation of carboxyl groups predominantly located at the sheet edges, generating repulsive electrostatic forces that hinder aggregation. At lower pH values, partial protonation reduces the magnitude of the zeta potential, potentially leading to diminished stability.^[45]^[46] GO dispersions demonstrate robust stability in water, achieving concentrations up to 10 mg/mL through a combination of electrostatic repulsion and steric hindrance provided by the solvated functional groups. However, exposure to high ionic strength environments, such as those containing elevated salt concentrations, screens the surface charges, promoting flocculation and sedimentation of the nanosheets. This sensitivity underscores the importance of controlling ionic conditions for maintaining colloidal integrity.^[47]^[48] Wettability assessments of GO-modified surfaces reveal water contact angles between 30° and 60°, signifying moderate hydrophilicity attributable to the oxygenated moieties. This property can be precisely modulated via post-synthesis functionalization, such as esterification or amidation of carboxyl groups, to tailor surface energy for specific interfacial applications. Recent investigations from 2023 to 2025 have highlighted GO's efficacy in forming Pickering emulsions that serve as robust supports for heterogeneous catalysis, where the sheets anchor catalysts at emulsion interfaces to enhance selectivity and recyclability.^[49]^[50]^[51]

Hydration behavior and solubility

Graphite oxide (GO) displays pronounced hydrophilicity owing to its abundance of oxygen-containing functional groups, such as hydroxyl and epoxide moieties, which enable water molecules to intercalate into the interlayer spaces primarily through hydrogen bonding. This intercalation leads to significant swelling of the layered structure, with the basal d-spacing expanding from approximately 0.6 nm in the dry state to around 1.2 nm in the fully hydrated state at 100% relative humidity.^[52] The exfoliation of GO in aqueous environments proceeds via osmotic pressure and solvation effects, where water molecules and ions penetrate the interlayer galleries, weakening van der Waals attractions and ultimately yielding single-layer GO sheets upon sufficient dispersion. This process is modeled by the swelling increment \Delta d = n_w \times 0.25 \, \mathrm{nm}, where n_w represents the number of intercalated water layers, typically 2–3 under ambient conditions, resulting in an overall expansion of 0.5–0.75 nm.^[41] The solubility of GO in water is highly pH-dependent, with stable dispersions achieved across a broad range of pH 3–10 due to the deprotonation of carboxylic acid groups at the sheet edges, which imparts negative surface charge and electrostatic repulsion. In pure water (pH ≈ 7) without sonication, GO tends to aggregate into multilayer stacks, though mild agitation can promote redispersion.^[53]^[46] GO membranes exhibit exceptional selective transport of water vapor, attributed to the hydrophilic nanochannels formed by intercalated water layers, enabling high flux rates on the order of $10^3–$10^4 , \mathrm{g/m^2 \cdot day}$ under ambient conditions while restricting larger molecules.^[54] Recent investigations into GO thin films have revealed humidity-induced structural transitions, where increasing relative humidity from 10% to 98% triggers a shift from disordered pore structures to more ordered, slit-like configurations, accompanied by interlayer spacing increases up to 1.3 nm and enhanced swelling driven by surface charge modulation.^[55] The surface charge of GO sheets contributes to colloidal stability during these hydration processes by promoting repulsion in aqueous media.^[53]

Optical, electrical, and thermal properties

Graphite oxide, often referred to as graphene oxide (GO) in dispersed forms, exhibits distinctive optical properties arising from its oxidized structure, which introduces sp³-hybridized carbon atoms and disrupts the extended π-conjugation of pristine graphite. Thin films of GO demonstrate high transmittance in the visible range, typically around 80-95%, making them suitable for transparent applications.^[56] This transparency stems from the material's low absorption in the 400-700 nm region, though prolonged drying can increase absorbance and reduce transmittance slightly.^[57] The optical bandgap of GO is estimated to be in the range of 2.5-4 eV, significantly wider than that of graphene (near zero), due to the oxidation-induced localization of π-electrons.^[57] This bandgap can be quantified using the Tauc plot method for direct transitions, where the relation is given by:

(\alpha h \nu)^2 = A (h \nu - E_g)

Here, \alpha is the absorption coefficient, h \nu is the photon energy, A is a constant, and E_g is the bandgap energy; extrapolation of the linear portion to the energy axis yields E_g.^[58] GO displays photoluminescence peaks in the 400-600 nm range, attributed to radiative recombination involving oxygen functional groups and defect states. Additionally, GO displays third-order optical nonlinearity with a susceptibility \chi^{(3)} \approx 10^{-13} esu, enabling potential uses in laser modulation.^[59] Electrically, GO behaves as an insulator with resistivity ranging from $10^2 to $10^6 \Omega \cdot \mathrm{cm}, in stark contrast to the high conductivity of graphene (~10^6 S/m), owing to the breaking of sp² networks by oxygen-containing groups.^[60] This insulating nature arises from structural defects that trap charge carriers, though partial reduction can tune the conductivity to 10-100 S/m by restoring some π-conjugation.^[61] Thermally, GO maintains stability up to approximately 200°C, beyond which it decomposes, releasing CO and CO₂ from the elimination of oxygen functional groups.^[62] Its thermal conductivity is low, typically 0.1-5 W/m·K, much reduced compared to graphite's in-plane value of ~2000 W/m·K, due to phonon scattering at oxidized sites and defects.^[63] In thin films, thermal transport exhibits anisotropy, with higher in-plane conductivity (up to several W/m·K) than out-of-plane (sub-1 W/m·K), reflecting the layered structure.^[64] These properties are influenced by the degree of oxidation and hydration, though intrinsic bulk behaviors dominate.^[65]

Applications

Production of graphene and derivatives

Graphite oxide serves as a key precursor for producing graphene and its derivatives through an exfoliation-reduction process. The material is first dispersed in water or organic solvents and exfoliated into individual graphene oxide (GO) sheets using ultrasonication or high-speed centrifugation, which overcomes the interlayer van der Waals forces weakened by oxidation-induced spacing. This yields stable colloidal suspensions of single- to few-layer GO sheets with lateral dimensions typically ranging from hundreds of nanometers to micrometers. Subsequent reduction removes oxygen-containing functional groups, partially restoring the sp² carbon network of pristine graphene. Common reduction methods include chemical treatment with hydrazine hydrate at 100°C, following the reaction GO + N₂H₄ → rGO + N₂, which effectively deoxygenates the sheets while minimizing structural damage; thermal annealing at temperatures between 200°C and 1000°C, where rapid heating causes simultaneous deoxygenation and exfoliation via gas evolution; and photochemical reduction using UV or visible light in the presence of sensitizers, which offers milder conditions for preserving sheet integrity.^[66]^[37]^[67] The resulting reduced graphene oxide (rGO) exhibits restored graphitic character, with approximately 70-90% of carbon atoms reverting to sp² hybridization and a C/O atomic ratio exceeding 10, as determined by X-ray photoelectron spectroscopy. However, residual defects, such as vacancies and persistent oxygen groups, lead to an I_D/I_G Raman intensity ratio of around 1.0, indicating moderate disorder compared to pristine graphene. These properties make rGO a versatile, albeit imperfect, graphene analog suitable for composite materials and films.^[68] Variants of this process enable the synthesis of specialized graphene derivatives. For instance, spin-coating GO suspensions onto substrates followed by thermal or chemical reduction produces uniform thin films or microlens arrays, leveraging the optical properties of GO for structured devices. Additionally, GO intermediates can be fluorinated using fluorine precursors like HF or SF₆ during exfoliation, yielding fluorographene with a wide bandgap and high chemical stability.^[69]^[70] The solution-processable nature of GO provides scalability advantages over vapor-phase methods like chemical vapor deposition, enabling large-area production without high-vacuum equipment. In 2024, electrochemical reduction techniques have advanced mass production, facilitating scalable production with yields up to 100 g/day of high-quality rGO through controlled cathodic processes in aqueous electrolytes.^[15]^[71]

Energy storage and conversion

Graphite oxide (GO) and its reduced form (rGO) have been integrated into flexible electrodes for lithium-ion batteries, particularly through composites with manganese dioxide (MnO₂). These GO/rGO-MnO₂ hybrids leverage the high electrical conductivity and mechanical flexibility of rGO to buffer volume expansion during lithium insertion/extraction, enhancing cycle stability and rate performance. For instance, MnO₂ nanorods anchored on rGO deliver a reversible specific capacity of 600 mAh g⁻¹ at 0.5 A g⁻¹ after over 650 cycles, retaining 168 mAh g⁻¹ even at high rates of 5 A g⁻¹.^[72] In supercapacitors, GO and rGO contribute pseudocapacitance via Faradaic redox reactions at oxygen-containing functional groups, such as hydroxyl, epoxy, and carbonyl moieties, which enable reversible charge transfer in addition to electric double-layer capacitance. These reactions occur at the electrode-electrolyte interface, boosting overall energy storage. Recent rGO-metal oxide composites, like rGO with cobalt phosphide, exhibit specific capacitances around 350 F g⁻¹ at low scan rates, with excellent cycling retention due to the synergistic conductivity and pseudocapacitive effects. The basic capacitance mechanism is described by the equation:

C = \frac{Q}{\Delta V}

where C is capacitance, Q is charge, and \Delta V is potential difference. GO-based scaffolds also facilitate hydrogen storage through physisorption on oxygen functional sites and defects, achieving uptake capacities of 1-2 wt% at 77 K and moderate pressures (up to 50 bar). Thermal reduction of GO creates strained structures that enhance adsorption without chemical bonding, making it suitable for lightweight storage systems.^[73] For energy conversion, GO and rGO serve as electron transport layers in dye-sensitized solar cells (DSSCs), reducing charge recombination and improving electron mobility when composited with TiO₂. Devices incorporating 8 wt% rGO in TiO₂ nanowire photoanodes achieve power conversion efficiencies of approximately 5%, a 46% improvement over pure TiO₂ counterparts. Specific efficiencies remain in the 5-10% range.^[74] The advantages of GO in these applications stem from its flexibility, enabling bendable devices, and its theoretical surface area of about 2400 m² g⁻¹, which maximizes active sites for ion adsorption and charge transfer—far exceeding practical values in bulk materials but establishing its potential scale. Graphene derivatives from GO reduction briefly enhance these components by restoring sp² networks for better conductivity.

Environmental remediation and purification

Graphite oxide (GO) has emerged as a promising material for water purification, particularly in desalination processes where GO-based membranes achieve high NaCl rejection rates exceeding 95% through precise size sieving enabled by interlayer spacing of approximately 0.7 nm (7 Å).^[75] These membranes leverage the nanoscale spacing between GO nanosheets to selectively permit water molecules while blocking hydrated salt ions, offering enhanced permeability compared to traditional reverse osmosis systems.^[76] Additionally, GO excels in heavy metal adsorption from aqueous solutions, with capacities for Pb²⁺ reaching up to 500 mg/g, primarily through chelation with carboxyl functional groups on the GO surface.^[77] In pollutant removal, recent 2025 studies highlight GO's efficacy against emerging contaminants such as microplastics, where GO composites demonstrate superior adsorption due to their high surface area and tunable surface chemistry.^[78] For organic dyes, GO adsorbents exhibit capacities exceeding 200 mg/g for methylene blue, facilitating efficient wastewater treatment in textile industries.^[79] Biochar-GO hybrids further extend these applications to soil remediation, combining GO's adsorption prowess with biochar's stability to immobilize heavy metals and organic pollutants in contaminated soils, thereby preventing leaching into groundwater.^[80] GO also finds utility in agricultural fertilizer applications, where thin GO films enable sustained release of trace elements like Zn²⁺ over periods up to 30 days, reducing nutrient loss by approximately 50% compared to conventional fertilizers through controlled diffusion mechanisms.^[81] This approach minimizes environmental runoff and enhances crop uptake efficiency, promoting sustainable farming practices.^[82] The adsorption mechanisms of GO involve electrostatic attraction between its oxygen-containing groups and charged pollutants, complemented by π-π stacking interactions with aromatic compounds, which enhance selectivity and capacity.^[83] These processes are often modeled using the Langmuir isotherm, which describes monolayer adsorption on a homogeneous surface:

q_e = \frac{q_m K_L C_e}{1 + K_L C_e}

where q_e is the equilibrium adsorption capacity (mg/g), q_m is the maximum adsorption capacity (mg/g), K_L is the Langmuir constant (L/mg), and C_e is the equilibrium concentration (mg/L).^[84] GO's colloidal stability further aids its dispersion in aqueous solutions, ensuring uniform application in remediation setups.^[85] For sustainability, magnetic GO composites enable facile recycling through external magnetic fields, allowing repeated use in adsorption cycles with minimal loss of efficiency, thus reducing waste and operational costs in large-scale environmental cleanup.^[86]

Biomedical and sensing applications

Graphite oxide (GO), also known as graphene oxide, has emerged as a versatile nanomaterial in biomedical applications due to its high surface area, functional groups for conjugation, and biocompatibility when properly modified. In drug delivery, GO serves as an efficient nanocarrier for chemotherapeutic agents, leveraging non-covalent interactions such as π-π stacking to achieve high loading capacities. For instance, doxorubicin (DOX) can be loaded onto GO at ratios exceeding 100% w/w, enabling substantial drug payloads relative to the carrier mass.^[87] This loading mechanism exploits the aromatic structure of DOX aligning with the sp² domains of GO, resulting in stable complexes suitable for targeted delivery.^[88] A key advantage in cancer therapy is the pH-responsive release profile of GO-based systems, which exploits the acidic microenvironment of tumors (pH ≈ 5.0–6.5) compared to physiological conditions (pH ≈ 7.4). Protonation of GO's oxygen-containing groups at low pH weakens π-π interactions and enhances electrostatic repulsion, facilitating controlled DOX release at tumor sites while minimizing premature leakage in healthy tissues.^[89] Studies have demonstrated up to 80% DOX release within 24 hours at pH 5.0, versus less than 20% at pH 7.4, improving therapeutic efficacy and reducing systemic toxicity.^[90] In precision medicine, functionalized GO enables targeted therapies by conjugating ligands that bind specific receptors overexpressed on cancer cells. Folate-conjugated GO, for example, targets folate receptors abundant on various tumor cells, enhancing cellular uptake through receptor-mediated endocytosis. This approach has shown selective delivery of DOX to folate receptor-positive cancer cells, with uptake efficiencies increasing by over 5-fold compared to non-targeted GO.^[91] Such modifications support personalized treatment strategies, including combined chemo-photothermal therapy where GO's photothermal properties amplify drug effects under near-infrared irradiation.^[92] GO's role extends to electrochemical sensing for biomarkers, exemplified by 2024 developments in glucose monitoring. A GO-modified electrode integrated with nickel-cobalt catalysts enables non-enzymatic detection in physiological ranges (0.1–25 mM) with a low detection limit of 0.02 mM.^[93] This configuration benefits from GO's conductivity and large surface area, which facilitate electron transfer and enzyme-free operation, crucial for implantable or wearable devices in diabetes management. For biosensing, GO-based field-effect transistor (FET) sensors provide ultrasensitive detection of biomolecules like DNA and proteins. These devices exploit changes in GO's electrical properties upon biomolecule binding, achieving detection limits as low as 1 nM for DNA hybridization via peptide nucleic acid probes.^[94] Protein detection, such as streptavidin, similarly reaches 1 nM sensitivity through specific antibody immobilization on GO channels, enabling label-free, real-time monitoring.^[95] Recent 2025 advances highlight GO in wearable sensors for non-invasive biomarker detection, integrating reduced GO with flexible substrates for continuous health monitoring. These sensors detect sweat-based biomarkers like cortisol or lactate with limits of detection in the nM range, offering stretchability up to 100% strain and stability over 10,000 cycles.^[96] Such innovations support point-of-care diagnostics, with GO's tunable conductivity enhancing signal transduction in epidermal electronics.^[97] Biocompatibility tuning is essential for in vivo applications, where unmodified GO may aggregate due to protein corona formation. PEGylation, involving polyethylene glycol grafting, sterically stabilizes GO nanosheets, reducing aggregation and prolonging circulation times in blood. PEGylated GO exhibits over 90% cell viability in vivo at doses up to 20 mg/kg, compared to 50% for pristine GO, by minimizing immune recognition and opsonization.^[98] This modification has been pivotal in preclinical trials for extended tumor retention.^[99] Underlying many sensing mechanisms is GO's exceptional fluorescence quenching ability, which enables Förster resonance energy transfer (FRET)-based detection. GO quenches fluorophores attached to probes (e.g., dyes on DNA aptamers) through π-π stacking or energy transfer, with quenching efficiencies exceeding 95%. Upon target binding, spatial separation restores fluorescence, allowing sensitive detection of analytes like microRNAs at femtomolar levels.^[100] This platform has been adapted for multiplexed biosensing in clinical samples.^[101]

Materials and coatings

Graphite oxide, commonly referred to as graphene oxide (GO), serves as a versatile component in advanced materials and coatings, leveraging its layered structure and functional groups for enhanced performance. In anticorrosion applications, GO-based coatings on metals, such as steel and magnesium alloys, achieve inhibition efficiencies exceeding 90% by forming a physical barrier that restricts the diffusion of corrosive species like chloride ions and oxygen.^[102] This barrier effect is particularly effective in epoxy-GO hybrid systems, where the incorporation of GO nanosheets improves adhesion and impermeability, leading to long-term protection in harsh environments.^[103] Furthermore, GO-polymer hybrids, such as those combined with polyurethane or acrylic resins, exhibit superior scratch resistance due to the uniform dispersion of GO sheets, which distribute mechanical stress and prevent surface abrasion.^[104] In composite materials, GO acts as a nanofiller to reinforce polymer matrices, notably in epoxies, where low loadings (0.5-2 wt%) can increase tensile strength by 20-50% through strong interfacial interactions and load transfer mechanisms.^[105] This enhancement stems from GO's high aspect ratio and covalent bonding with the epoxy matrix, resulting in improved modulus and fracture toughness without compromising flexibility.^[106] Recent advancements as of 2025 have expanded GO's role in catalysis within composites, where functionalized GO (e.g., with sulfonic acid or metal nanoparticles) facilitates organic transformations such as Suzuki-Miyaura couplings and oxidation reactions, achieving yields over 90% under mild conditions due to the synergistic active sites on GO edges.^[107] These catalytic composites enable in-situ reactions during material processing, promoting sustainable manufacturing. For optical devices, GO enables the fabrication of ultrathin flat lenses capable of subwavelength focusing across visible to near-infrared wavelengths, with numerical apertures around 0.3 that support applications in compact imaging systems.^[108] The lens performance arises from GO's tunable refractive index and low dispersion, allowing precise phase control via thickness gradients in films as thin as 200 nm.^[109] Additionally, GO's strong third-order optical nonlinearity, on the order of 0.45 cm²/GW at telecommunication wavelengths, facilitates all-optical switching devices with ultrafast response times under picosecond pulses, enabling signal modulation without electronic components.^[110] A key processing method for these applications is layer-by-layer (LbL) assembly, which deposits alternating GO layers with oppositely charged polymers to form uniform thin films with controllable thicknesses of 10-100 nm, depending on the number of bilayers.^[111] This technique ensures defect-free stacking and scalability for large-area coatings. GO-incorporated materials also offer inherent advantages, including robust UV protection by absorbing harmful radiation through π-π* transitions, thereby preventing photodegradation in polymers.^[112] In terms of flame retardancy, GO acts as a char former, increasing the limiting oxygen index (LOI) above 30% in coated substrates by promoting intumescent barriers that isolate heat and oxygen during combustion.^[113] The electrical properties of GO further enhance conductivity in these composites, bridging insulating matrices for antistatic applications.^[114]

Toxicity and Safety

Biological and cellular effects

Graphene oxide (GO), the exfoliated form of graphite oxide, demonstrates significant cytotoxicity in vitro primarily through the generation of reactive oxygen species (ROS) and physical disruption of cell membranes. Toxicity studies primarily address GO, which exhibits higher reactivity due to its nanoscale dimensions compared to bulk graphite oxide. In human cervical cancer (HeLa) cells, GO exposure induces ROS production, leading to oxidative stress markers such as increased malondialdehyde (MDA) levels and decreased superoxide dismutase (SOD) activity, with cell viability decreasing notably at concentrations exceeding 20 μg/mL after 24 hours.^[115] Cytotoxicity assays, including MTT and LDH release, reveal IC50 values for GO in HeLa and similar epithelial cells typically ranging from 50 to 100 μg/mL, depending on exposure duration and GO sheet size.^[115]^[116] The sharp edges of GO nanosheets contribute to membrane damage by direct mechanical piercing and wrapping around cells, causing cytoskeletal destabilization and loss of integrity.^[117] In vivo studies highlight GO's potential to induce inflammatory responses and systemic distribution in animal models. Intratracheal or intravenous administration of GO to mice at doses above 40 mg/kg provokes acute lung inflammation, characterized by pulmonary edema, granuloma formation, and recruitment of inflammatory cells. Following intravenous injection, GO accumulates primarily in the liver and spleen, with biodistribution influenced by particle size—smaller sheets (less than 100 nm) showing higher uptake in these organs compared to larger aggregates that lodge in the lungs.^[118] The toxicological mechanisms of GO are largely size- and dose-dependent, with smaller nanosheets (under 1 μm) exhibiting greater cellular uptake and toxicity due to enhanced penetration across biological barriers. Recent investigations confirm genotoxicity through oxidative stress-mediated DNA damage, as evidenced by increased comet tail moments and oxidized DNA bases in human bronchial epithelial cells at concentrations of 10–40 μg/mL.^[119] Functionalization plays a key role in modulating effects: reduced GO (rGO) is generally less toxic than pristine GO owing to diminished oxygen content and ROS production, while carboxyl-rich GO variants promote greater cellular adhesion but may reduce overall cytotoxicity in some contexts. Due to its persistence in biological tissues—with retention observed in mouse lungs for over three months post-exposure—and associated health risks, GO has not received FDA approval for human therapeutic use as of November 2025, limiting its application to preclinical research.^[115]

Environmental impact and sustainable practices

The production of graphite oxide, particularly via the traditional Hummers' method, generates hazardous waste acid effluent containing Mn²⁺ ions, which poses significant environmental pollution risks if not properly managed.^[120] This manganese contamination arises from the use of potassium permanganate as an oxidant, leading to effluent that requires extraction and recycling strategies, such as oxidation precipitation into Mn₃O₄ nanoparticles, to mitigate ecological harm.^[120] Additionally, the release of graphite oxide nanomaterials into aquatic environments during production or use can induce toxicity in algae, with a 96-hour EC₅₀ value of approximately 52 mg/L for species like Microcystis aeruginosa, primarily through adsorption effects that disrupt organic matter dynamics.^[121] Sustainability challenges in graphite oxide processing include the high energy demands of thermal reduction methods to produce reduced forms, where electricity consumption dominates environmental impacts and contributes substantially to the overall carbon footprint.^[122] Life cycle assessments indicate that conventional graphite oxide production can result in a global warming potential of around 8-46 kg CO₂ equivalent per kg, depending on the synthesis route, highlighting gaps in energy efficiency and emissions control as noted in recent reviews.^[123]^[124] To address these issues, green synthesis approaches have emerged, such as flow-chemistry methods using periodate as an oxidant, which enable rapid production in minutes while reducing pollution and safety hazards compared to permanganate-based processes.^[125] Recyclable techniques, including the reuse of sulfuric acid in Couette–Taylor flow reactors, further enhance sustainability by cutting water usage for washing by up to 75% per gram of product, alongside high acid recovery rates exceeding 97%.^[126] Regulatory frameworks play a key role in managing risks, with the European Union's REACH regulation (No. 1907/2006) requiring registration of graphite oxide as a nanomaterial, including detailed physical-chemical data, while the CLP regulation (No. 1272/2008) mandates hazard classification and safety data sheets.^[127] Emerging standards for nanomaterial waste emphasize safe disposal and recycling to prevent environmental release, though specific protocols remain under development globally.^[128] Looking ahead, circular economy strategies offer promise, such as upcycling graphite waste from lithium-ion battery recycling into graphite/reduced graphite oxide composites for electrocatalysts, which not only diverts waste but also yields high-performance materials with power densities up to 100 mW/cm² in applications like zinc-air batteries.^[129]

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[PDF] Development and validation of a wearable biosensor for continuous ...
The sensor demonstrated a linear response range of 0.1-25 mM with a sensitivity of 22.8 ± 0.9 μA/mM·cm². ... 0-10 mM glucose. Detection Limit. 0.02 mM. S/N ...
[95]
Fabrication of Ultrasensitive Field-Effect Transistor DNA Biosensors ...
... detection limit of 1 nM. Dong ... Ultrasensitive label-free detection of PNA-DNA hybridization by reduced graphene oxide field-effect transistor biosensor.
[96]
Interplay of graphene–DNA interactions: Unveiling sensing potential ...
(c) and (d) Structures of graphene oxide (GO) and reduced graphene oxide (RGO). ... limit of detection (LOD) of 1 nM for the detection of streptavidin.
[97]
Recent Advances of Graphene‐Based Wearable Sensors: Synthesis ...
May 19, 2025 · ... graphene oxide (rGO). Graphene, GO, rGO, and 2D graphene-like ... Biosensors, in particular gas sensors, provide noninvasive biomarker ...
[98]
Enhancement and optimization of a graphene-based biosensing ...
Oct 21, 2025 · By integrating graphene oxide, these sensors achieve higher precision, efficiency, and reliability in detection. Graphene-based technology ...
[99]
Effective PEGylation method to improve biocompatibility of graphene ...
Feb 5, 2020 · Another study has been published Li and coworkers found that PEGylation of graphene reduces retention and promotes the clearance of graphene ...
[100]
Improved In Vitro and In Vivo Biocompatibility of Graphene Oxide ...
Improved In Vitro and In Vivo Biocompatibility of Graphene Oxide through Surface Modification: Poly(Acrylic Acid)-Functionalization is Superior to PEGylation.Missing: aggregation | Show results with:aggregation
[101]
Fluorescence resonance energy transfer between quantum dots and ...
Jul 1, 2010 · Fluorescence resonance energy transfer between quantum dots and graphene oxide for sensing biomolecules. Anal Chem. 2010 Jul 1;82(13):5511-7 ...
[102]
Fluorescence and Sensing Applications of Graphene Oxide and ...
Sep 19, 2018 · Graphene Oxide as a Donor in FRET. In a FRET process, GO can serve as an energy donor where its fluorescence gets quenched by an energy ...
[103]
A review on recent advances in graphene oxide-based composite ...
The first part of the review covers graphene, graphene oxide, and reduced graphene oxide-based coatings for passive and active corrosion protection. The second ...
[104]
Enhanced anti-corrosion characteristics of epoxy-functionalized ...
Functionalized graphene oxide (FGO) was prepared using the procedure reported in previous literature [35,36]. The graphite oxide of 700 mg was suspended in 70 ...
[105]
Tannin-based modified graphene oxide anti-corrosion composite ...
May 1, 2024 · Tannin-based modified graphene oxide anti-corrosion composite coating with favourable corrosion inhibition, self-healing and photothermal ...<|separator|>
[106]
Mechanical properties of graphene oxide (GO)/epoxy composites
In this study, the influence of graphene oxide (GO) on epoxy matrix composite was investigated. Adding 1.5 vol.% GO to the epoxy matrix increased the tensile ...Experimental · Mechanical Testing · Results And Discussion
[107]
Evaluation of physical and mechanical properties of graphene oxide ...
Reinforced epoxy resin composites with modified graphene oxide have a higher tensile strength than epoxy alone. The maximum increase in tensile strength was ...
[108]
Recent Advances in Functionalization of Graphene Oxide and Its ...
Feb 5, 2025 · Recent Advances in Functionalization of Graphene Oxide and Its Role in Catalytic Organic Transformations: A Comprehensive Review (2018–2024).
[109]
Highly efficient and ultra-broadband graphene oxide ultrathin lenses ...
Sep 22, 2015 · We report on a 200-nm-thick graphene oxide (GO) flat lens with 3D subwavelength focusing that is able to tightly focus broadband light from the visible to near ...Results · Broadband Focusing · Femtosecond Laser...
[110]
Ultra-high NA graphene oxide flat lens on a fiber facet with near ...
Jun 27, 2024 · Here we propose an ultrathin (400 nm) graphene oxide (GO) film lens fabricated in situ on a standard single-mode fiber facet using the ...
[111]
Observation of Third-order Nonlinearities in Graphene Oxide Film at ...
Aug 29, 2017 · Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm 2 /GW in graphene oxide thin films at the telecommunication wavelength ...
[112]
Layer-by-Layer Growth of Graphene Oxide-Based Films for ...
The monolayer films consisted of 11-14 Å thick GO sheets, with lateral dimensions between 150 nm and 9 μm. Silicon substrates primed with amine monolayers gave ...
[113]
[PDF] FLAME RETARDANCY AND ULTRAVIOLET RESISTANCE OF SILK ...
Jun 15, 2016 · Silk fabrics were coated by graphene oxide hydrosol in order to improve its flame retardancy and ultraviolet resistance.
[114]
Efficient and Durable Flame-Retardant Coatings on Wood ...
Aug 9, 2022 · Graphene oxide (GO) as a two-dimensional carbon material possesses a unique layered structure and a good specific surface area, making it a heat ...Introduction · Materials and Methods · Results and Discussion · References
[115]
The functional graphene/epoxy resin composites prepared by novel ...
Aug 1, 2024 · This study addresses this issue by preparing graphene oxide via the Hummer method, modifying it with hyperbranched polyamide ester, and reducing it with ...
[116]
https://onlinelibrary.wiley.com/doi/10.1155/2016/5851035
[117]
https://doi.org/10.1073/pnas.1222276110
[118]
https://doi.org/10.1016/j.biomaterials.2013.01.001
[119]
https://www.tandfonline.com/doi/full/10.1080/17435390.2023.2265465
[120]
Efficient Extraction of Mn2+ Ions From the Waste Produced in the ...
Graphene nanosheets are produced in mass by Hummers method, accompanied with the emission of waste acid effluent with Mn2+, which should be reasonably ...
[121]
Impact of Graphene Oxide on Algal Organic Matter of Microcystis ...
Dec 10, 2018 · GO exhibited no significant influence on algal growth and the photosynthetic system. The EC50 of GO to algae was calculated as 52.34 ± 0.05 mg/L ...Introduction · Results and Discussion · Conclusions · Materials and Methods
[122]
Comparison of environmental impacts in the production of graphene ...
Mar 15, 2025 · The results indicate that graphene production from PWW is considerably more environmentally sustainable, reducing the GWP from 4841 kg CO2 ...
[123]
Environmentally Friendly Production of Graphene Oxide via Ball ...
Jul 13, 2025 · A cradle-to-gate life cycle assessment (LCA) was conducted to quantify the environmental footprint associated with each milling duration.Introduction · Experimental Sections · Results and Discussion · Conclusions
[124]
A review on sustainable production of graphene and related life ...
Dec 28, 2021 · This review aims at highlighting the current state of art on sustainable development of graphene-related materials and related environmental impact assessment ...
[125]
Flow-Chemistry Based Green Synthesis of Graphene Oxide at ...
A new oxygen-atom-transfer (OAT) mechanism for periodate oxidizing graphite is revealed, exhibiting controllable reaction activity, strong orbital interaction, ...Missing: bio- | Show results with:bio-
[126]
Toward Green Synthesis of Graphene Oxide Using Recycled ...
Jan 24, 2017 · We propose a green synthesis technique for GO production using recycled sulfuric acid and filter-processed oxidized natural graphite obtained from a Couette– ...Missing: sustainable | Show results with:sustainable
[127]
What is the toxicity of graphene oxide - EcoMundo
Being a nanomaterial, graphene is subject to REACH n°1907/2006 and CLP n°1272/2008 regulations. Any importer or manufacturer of graphene must register its ...
[128]
Safe nanomaterials: from their use, application, and disposal to ...
This review aims to display and context the global applications of nanomaterials, their final disposal, as well as the perspective of the current efforts.
[129]
A Pathway to Circular Economy-Converting Li-Ion Battery Recycling ...
In this study, graphite waste recovered from the recycling of LIBs was successfully upcycled into an active graphite/rGO (reduced graphene oxide) composite ...