Amino-functionalized Graphene Quantum Dots Research Articles

Retinal Degeneration Response to Graphene Quantum Dots: Disruption of the Blood-Retina Barrier Modulated by Surface Modification-Dependent DNA Methylation.

Graphene quantum dots (GQDs) are used in diverse fields from chemistry-related materials to biomedicines, thus causing their substantial release into the environment. Appropriate visual function is crucial for facilitating the decision-making process within the nervous system. Given the direct interaction of eyes with the environment and even nanoparticles, herein, GQDs, sulfonic acid-doped GQDs (S-GQDs), and amino-functionalized GQDs (A-GQDs) were employed to understand the potential optic neurotoxicity disruption mechanism by GQDs. The negatively charged GQDs and S-GQDs disturbed the response to light stimulation and impaired the structure of the retinal nuclear layer of zebrafish larvae, causing vision disorder and retinal degeneration. Albeit with sublethal concentrations, a considerably reduced expression of the retinal vascular sprouting factor sirt1 through increased DNA methylation damaged the blood-retina barrier. Importantly, the regulatory effect on vision function was influenced by negatively charged GQDs and S-GQDs but not positively charged A-GQDs. Moreover, cluster analysis and computational simulation studies indicated that binding affinities between GQDs and the DNMT1-ligand binding might be the dominant determinant of the vision function response. The previously unknown pathway of blood-retinal barrier interference offers opportunities to investigate the biological consequences of GQD-based nanomaterials, guiding innovation in the industry toward environmental sustainability.

Amino-functionalized graphene quantum dots as a photoluminescent probe for in vitro cellular imaging of human lung cancer cell lines

Graphene-based fluorescent materials, particularly graphene quantum dots (GQDs), have emerged as a new class of biomedical agents. In the present study, functionalised GQDs have a better chance of being employed in a broader range of bioapplications because of their proven low toxicity, outstanding biocompatibility, and enhanced fluorescence properties. For this purpose, amino-functionalised GQDs (AF-GQDs) were synthesised via hydrothermal treatment by treating graphene oxide in ammonia with heat and water. The as-prepared AF-GQDs samples were characterised using a variety of techniques, including x-ray diffraction analysis (XRD), high-resolution transmission electron microscopy (HR-TEM), and photoluminescence (PL) spectroscopy. The results show that AF-GQDs are merely hexagonal, with an average size of about 8 nm. Also, AF-GQDs are highly soluble in water and display excellent luminescence behaviour. After 48 h of incubation, the MTT results showed that more than 63% of the cells were still alive, even at high concentrations (500 g ml−1) of AF-GQDs. In addition, the AO/EB staining results also showed that the AF-GQDs had the most robust green fluorescence (viable cells). This makes them a promising agent for biomedical imaging because they have good optical properties, are readily soluble in water, are biocompatible, and are not toxic.

Antibacterial property of dual-network hydrogel filled with amino functionalized graphene quantum dots

Bacterial infections seriously impair wound healing, so the development of wound dressings with high antimicrobial activity is of critical importance. In this study, amino functionalized graphene quantum dots (His-GQDs) were prepared via thermal pyrolysis of citric acid and histidine, and doped into a dual-network gelatin/sodium alginate (His-GQDs/G/SA) hydrogel to improve the antibacterial activity. The His-GQDs/G/SA hydrogel exhibited excellent antibacterial activity after 6 h of contact with bacteria. Simultaneously, the composite hydrogel could effectively alleviate the cytotoxicity of His-GQDs. The results of antimicrobial and biocompatibility tests demonstrate that the His-GQDs/G/SA dual-network hydrogel is a promising wound dressing to promote healing of infected wounds.

Controllable functionalization of amino-functionalized graphene quantum dots as fluorescent probe for detection of Cu(Ⅱ) ions detection

Herein, the amino-functionalized graphene quantum dots (GQDs) can be applied for selective detection of Cu(Ⅱ) ions in solutions via photoluminescence (PL) measurement, the PL emission intensities of the GQDs gradually decreased with the increase of Cu2+ concentration (0–100 μm), the PL intensity (quenching effect) decreases almost linearly with increasing Cu2+ concentration. Its correlation coefficient R2 is 0.9529. Quenching of the PL emission by the presence of copper(Ⅱ) ions is attributed to the surface functional groups attached to the GQDs, which can interact with metal ions so that the electrons in the excited state of the GQDs are transferred into the unoccupied orbits of Cu(Ⅱ) ions. Corresponding physical mechanism is proposed to gain an understanding about how the presence of the Copper(Ⅱ) ions can affect the PL emission from GQDs.

Cascade selective recognition of H2O2 and ascorbic acid in living cells using carbon-based nanozymes with peroxidase-like activity

The design of optical nanosensors based on carbon nanozymes has gained considerable interest in analyte detection. In this study, a simple and rapid fluorescence/colorimetry probe, NH2GQDs/o-phenylenediamine (OPD), was constructed by combining amino-functionalized graphene quantum dots (NH2GQDs) and o-phenylenediamine. In the presence of NH2GQDs/H2O2, OPD was oxidized to 2,3-diaminophenazine (DAP), giving fluorescent and colorimetric signals, which can be used to quantitatively measure H2O2. Furthermore, NH2GQDs/OPD/ H2O2 exhibited high selectivity and sensitivity for detecting ascorbic acid (AA) with a detection limit of 0.062 μM using the fluorescent method. Its selectivity for AA was ascribed to the formation of the condensation compound of DAP with dehydroascorbic acid rather than the quinoxaline path for dopamine and glucose. The colorimetric sensing system NH2GQDs/OPD displayed yellow in the presence of H2O2, while it changed to green upon adding AA. This sensing system was successfully applied for imaging H2O2 and AA in human cervical cancer cells and onion tissues. Additionally, the flexible sensing system can be used to fabricate the logic gate “AND.” The results provide a valuable method for its potential application in the biosensing, clinical medicine, and food industries.

Cellulose circular economy: Amino-functionalized graphene quantum dots as highly sensitive vaccine indicators

Cellulose is a highly content natural polymer that can be extracted from plants. Rice straw is a valuable resource because it is rich in cellulose. This study aims to develop a technology to extract and utilize cellulose from rice straw, which would otherwise be discarded. Cellulose derived from rice straw is used to synthesize low-biotoxicity, high-performance graphene quantum dots. Cellulose is hydrolyzed into glucose and processed into graphene quantum dots using the hydrothermal method, which is a green synthetic route. The 3-(4,5-dimethylthiazol-2-yl)− 2,5-diphenyltetrazolium bromide assay shows that the natural biomass (cellulose)-derived graphene quantum dots have low biological toxicity and cell viability that exceeds 100% (48 h, 400 micrograms per milliliter). Their luminescence properties, chemical stability, and biological toxicity are significantly improved by subsequent reduction and amination. Subsequently, amino-functionalized graphene quantum dots are used as a display agent for vaccine detection, which quantified secondary antibody concentration with a detection limit of 2 pg/mL, excellent linearity (r2 = 0.9982), and a low coefficient of variation of < 0.04%, outperforming the existing 3,3′5,5′-tetramethylbenzidine. This study demonstrates that graphene quantum dots synthesized from natural polymers are suitable for low-toxicity applications, such as vaccine indicators.

Single-sized N-functionality graphene quantum dot in tunable dual-modality near infrared–I/II illumination detection and photodynamic therapy under multiphoton nonlinear excitation

Doping sorted graphene quantum dots (GQDs) with heteroatoms and functionalizing them with amino acid could improve their radiative recombination and two-photon properties—including their excitation-wavelength-independent photoluminescence from the ultraviolet to the near-infrared-I (NIR-I) region, absorption, quantum yield, absolute cross section, lifetime, and radiative-to-nonradiative decay ratio—under two-photon excitation (TPE) at a low excitation energy and short photoexcitation duration, as determined using a self-made optical microscopy system with a femtosecond Ti–sapphire laser. Four types of sorted GQDs were investigated: undoped GQDs, nitrogen-doped GQDs (N-GQDs), amino-functionalized GQDs (amino-GQDs), and N-doped and amino-functionalized GQDs (amino-N-GQDs). Among them, the sorted amino-N-GQDs are effective as a two-photon photosensitizer and generate the highest quantity of reactive oxygen species for the elimination of multidrug-resistant cancer cells through two-photon photodynamic therapy (PDT). Larger amino-N-GQDs result in a greater number of C–N and N–functionalities, leading to a superior photochemical effect and more favorable intrinsic luminescence properties, making the dots effective contrast agents for tracking and localizing cancer cells during in-depth bioimaging in a three-dimensional biological environment under TPE in the NIR-II region. Overall, this study highlights the potential of large amino-N-GQDs as a material for future application to dual-modality two-photon PDT and biomedical imaging.

Thin film nanocomposite polyamide membrane doped with amino-functionalized graphene quantum dots for organic solvent nanofiltration

Thin film nanocomposite (TFN) organic solvent nanofiltration (OSN) membrane with improved separation performance was fabricated via interfacial polymerization reaction between amine-functionalized graphene quantum dots (af-GQDs) doped aqueous monomer solution of m-phenylenediamine (MPD) and n-hexane solution containing trimesoyl chloride (TMC) monomer. We emphasize the employment of extra-low content for both af-GQDs and MPD, so that we could maximize the function of these nanomaterials and eliminate their aggregation at the same time. The af-GQDs nanomaterials could regulate the interfacial polymerization process and could be strongly anchored in the polyamide film via covalent bonding, hence could prevent their loss while providing more channels for faster solvent penetration. As a result, the optimized OSN membrane, TFN-50, shows a super-smooth surface, with a surface roughness as low as 4.3 ± 0.2 nm, and excellent pure solvent permeances which reach 135.8, 112.7, and 69.8 L m−2 h−1 MPa−1 for methanol, DMF and ethanol, respectively, as well as a very high Rhodamine B (RDB, 479 Da) rejection of up to 99.2%, and an ethanol permeance of 61.4 L m−2 h−1 MPa−1. Furthermore, the TFN-50 OSN membrane still shows excellent filtration performance after being submerged in DMF for 336 h or continuous crossflow filtration with Rose bengal (RB, 1017 Da)/DMF solution for 136 h, demonstrating its excellent solvent resistance.

Antifouling and Chemical-Resistant Nanofiltration Membrane Modulated by a Functionalized MWCNT Interlayer for Efficient Dye/Salt Separation at High Salinity.

Dye/salt separation in textile wastewater is of great importance. Membrane filtration technology is an environmentally friendly and effective approach to solve this issue. In this study, a thin-film composite membrane with a tannic acid (TA)-modified carboxylic multiwalled carbon nanotube (MWCNT) interlayer (M-TA) was prepared by interfacial polymerization with amino-functionalized graphene quantum dots (NGQDs) acting as aqueous monomers. The addition of the M-TA interlayer favored the formation of a thinner, more hydrophilic, and smoother selective skin layer for the composite membrane. The pure water permeability of the M-TA-NGQDs membrane was ∼9.32 L m-2 h-1 bar-1, which was higher than that of the NGQDs membrane without the interlayer. Meanwhile, the M-TA-NGQDs membrane presented better methyl orange (MO) rejection (97.79%) than the NGQDs membrane (87.51%). The optimal M-TA-NGQDs membrane exhibited excellent dye rejection (Congo red (CR): 99.61%; brilliant green (BG): 96.04%) and low salt rejection (NaCl < 15%). Noticeably, the M-TA-NGQDs membrane displayed effective selective separation performance (CR and BG > 99%) for dye/NaCl mixed solutions even at a high NaCl concentration of 50,000 mg/L. Furthermore, the M-TA-NGQDs membrane presented high water permeability recovery ratio values (91.02-98.20%). Importantly, the M-TA-NGQDs membrane showed excellent chemical stability (acid/alkali resistance). Generally, the fabricated M-TA-NGQDs membrane exhibited a great prospect for applications in dye wastewater treatment and water recycling, especially for the effective selective separation of dye/salt mixtures for high-salinity textile dyeing wastewater.

Multifunctional amino functionalized graphene quantum dots wrapped upconversion nanoparticles for photodynamic therapy and X-ray CT imaging

Multifunctional amino functionalized graphene quantum dots wrapped upconversion nanoparticles for photodynamic therapy and X-ray CT imaging

The synergistic antioxidant effect of polydopamine coating with amino-functionalized graphene quantum dots on osteoblast protection against oxidative stress

Oxidative stress is a major pathological feature in patients with metabolic bone diseases and isn’t conducive to postoperative osseointegration of implants. Although the free-radical scavenging activities of polydopamine (PDA) have been reported, its antioxidant-active catechol structure is susceptible to oxidation. Here, amino-functionalized graphene quantum dots (GQDs) were introduced into PDA to protect catechol structure from oxidation and expand its free-radical scavenging capacity. Compared to the clinically used hydroxyapatite (HAp) coating and the PDA-HAp coating, the GQD/PDA-HAp coating exhibited higher capacity to scavenge ABTS·+, DPPH and O2·-. The enhanced antioxidant effect of the GQD/PDA-HAp coating was ascribed to the combined effects of the sp2-hybridized carbon in GQDs and more catechol groups in PDA, indicating the synergistic antioxidant relationship between GQDs and PDA. In addition, the amino-functionalized GQDs increased the catechol content and the reducibility of the GQD/PDA coating as confirmed by Folin-Ciocalteu colorimetric assay and mediated electrochemical probing. In vitro studies showed that the GQD/PDA-HAp coating minimized intracellular reactive oxygen species production and protected osteoblast viability against H2O2-induced oxidative stress. Taken together, introducing amino groups in PDA is an efficient approach to preserve its structural related antioxidant properties which may broaden its application in treating oxidative stress-related diseases.

YAP/TFRC/ALOXE3 signaling is involved in ferroptosis and neuroinflammation triggered by amino-functionalized graphene quantum dots

Recently, graphene quantum dots (GQDs) have been attracting extensive interests in a widespread application, but the little information on the risk assessments of GQDs could impose a restriction on their contributions in the neuroscience. In this study, the intranasal administration of amino group functionalized GQDs (A-GQDs) was observed to enhance the number of neuronal cell death, trigger ferroptosis activities, i.e. ferrous iron overload and lipid peroxidation as well as cause inflammatory reactions in hippocampus. Meanwhile, these adverse effects were found in BV2 microglial cells treated with A-GQDs, which were all attenuated by a ferroptosis-specific inhibitor ferrostain-1 (Fer-1). The inflammatory responses to A-GQDs in BV2 cells were evidenced as the increased secretion of pro-inflammatory cytokine TNF-α that was attributed to the high mobility group box 1 (HMGB1) released from ferroptosis-occurred cells. The nuclear transference of yes-associated protein (YAP), an effector in Hippo signaling pathway, regulated by the phosphorylation of p38 mitogen-activated protein kinase (MAPK) was then found to play a critical role in A-GQDs triggering ferroptosis through transcriptionally mediating transferrin receptor protein (TFRC) and arachidonate lipoxygenase 3 (ALOXE3) to increase levels of ferrous iron and lipid peroxides, respectively. The findings not only highlight the importance of risk assessments on QDs containing low toxic component, but also provide toxic biomarkers for the interventions of GQDs in the brain.

Penetration of submicron amino-functionalized graphene quantum dots in plant stomata, implication for the depollution of atmospheric soot particles

Penetration of submicron amino-functionalized graphene quantum dots in plant stomata, implication for the depollution of atmospheric soot particles

Blue luminescent amino-functionalized graphene quantum dots as a responsive material for potential detection of metal ions and malathion

Large amounts of hazardous and toxic substances in the environment require non-toxic, cheap, easy, rapid, and sensitive methods for their detection. Blue luminescent graphene quantum dots (GQDs) were produced by electrochemical cleavage of graphite electrodes followed by gamma irradiation in the presence of ethylenediamine (EDA). Modified dots were able to detect metal ions (Co2+, Pd2+, Fe3+) due to photoluminescence quenching. The highest sensitivity was detected for the sample irradiated at a dose of 25 kGy. The limits of detection (LODs) were 1.79, 2.55, and 0.66 μmol L−1 for Co2+, Fe3+, and Pd2+, respectively. It was observed that GQDs irradiated at 200 kGy act as an ultra-sensitive turn-on probe for Malathion detection with LOD of 94 nmol L−1. Atomic force microscopy images proved the aggregation of GQDs in the presence of the investigated metal ions. Results obtained by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and LIVE/DEAD cytotoxicity test indicated that GQDs irradiated with EDA are not toxic towards MRC-5 cells, which makes them a promising, eco-friendly and safe material for sensing application.

Interlayered thin-film nanocomposite membrane with synergetic effect of COFs interlayer and GQDs incorporation for organic solvent nanofiltration

The separation performance of current organic solvent nanofiltration (OSN) membranes are still not satisfied. In this work, an ultra-thin TpHz covalent organic frameworks (COFs) interlayer was in situ constructed in advance on polyimide substrate, then an ultra-thin polyamide separation layer incorporated with amino-functionalized graphene quantum dots (af-GQDs) was constructed via interfacial polymerization (IP). A high performance interlayered thin-film nanocomposite (iTFN) OSN membrane with integrally covalent bonding structure, ultra-thin and ultra-smooth surface was obtained after post crosslinking and activation. The effect of ultra-low concentrations for the polyamide reaction monomers, the COFs monomer Tp and the doped af-GQDs nanoparticle reveals that they could help to form a very thin (7–8 nm in thickness) and defect-free skin layer with very smooth surface (about 2.7 nm in roughness). The iTFN OSN membrane shows excellent solvent permeance and solute selectivity, with an ethanol permeance reaching 97.6 L m−2 h−1 MPa−1 and an RDB rejection higher than 99%. After a 180 h long-term filtration with 100 mg L−1 RB/DMF solution, it remained an RB rejection of higher than 99%, demonstrating its superior resistance to strong polar solvent. Our work paves a unique way for the fabrication of novel OSN membrane with both in situ constructed nanomaterials interlayer and nanomaterials incorporated in skin layer.

Efficient multi-component PL emission tunning in single emissive layer (SEL) by amino-functionalized graphene quantum dots in type-II ZnSe/af-GQDs soft core/shell interface as an efficient phosphor

This research demonstrates first report on successful formation of ZnSe/amino functionalized graphene quantum dots (af-GQDs), type-II core-shell structures. The structures having potential as a single emissive layer (SEL), were synthesized via a facile and eco-friendly hot injection method. We achieved a highly efficient tuning of PL emission spectral components in visible region ranging from (∼458 to 750nm). The functional groups attached with af-GQDs rendered soft shell feature to the structures and the soft core-shell interface significantly passivated the strain-induced defect states and reduced Auger recombination (AR). The PL emission properties of ZnSe/af-GQDs were studied through steady state and temperature dependent photoluminescence (TDPL) spectroscopy within the temperature range (80-300k), using 457nm excitation laser. Three samples including bare ZnSe QDs, bare af-GQDs and ZnSe/af-GQDs core/shell structures were prepared. The steady state PL emission spectra revealed a weak blue and pronounced green, yellow, orange and red emission, whereas the full width half maximum (FWHM) of PL spectra distinctly decreased in ZnSe/af-GQDs. The carrier activation and escape energies of radiative and non-radiative recombination respectively were estimated through Arhenious and Brethelot fitting functions for TDPL measurements. The calculated values clearly indicated a decrease in activation energies and an increase in escape energies in ZnSe/af-GQD X-ray photon spectroscopy (XPS) measurements revealed the presence of Zn 2p and Se (3d) O1s and NIs orbitals of constituent elements including Zn, Se, C, O and N. Tight binding model was applied to estimate the valance and conduction band energies that suggested formation of type-II core-shell band alignment in the structures. The relative PLQY was estimated using comparative method and Rhodamine-B was used as reference and calculated PLQY are 58%, 61% and 83% for ZnSe, af-GQDs and ZnSe/af-GQDs. A mechanism of formation of a soft core-shell interface for ZnSe/af-GQDs is clarified. We suggest that ZnSe-/af-GQDs structures synthesized in this report have a great potential for LEDs and display devices in which multi-color components are of paramount importance, such as white light emission. This report may open a gate way for more and vivid possibilities to apply af-GQDs for optoelectronic devices.

Fabrication of polyarylate thin-film nanocomposite membrane based on graphene quantum dots interlayer for enhanced gas separation performance

Interlayered-thin film nanocomposite (i-TFN) membranes with nanomaterials as the interlayer has attracted more and more attention of researchers in recent years due to the effective regulation of the separation layer structure in the membrane separation field. In this work, the amino-functionalized graphene quantum dots (N-GQDs) were synthesized and applied as an interlayer to modify the polyethersulfone (PES) ultrafiltration substrate. Then interfacial polymerization (IP) between trimesoyl chloride (TMC) and β-cyclodextrin (β-CD) was performed to prepare novel i-TFN polyarylate membranes for CO2/N2 separation. The effect of the N-GQDs deposition amount on the structure and gas separation performance of the composite membrane was studied thoroughly. The results indicated that the i-TFN polyarylate membrane presented a particular sandwich structure and provided an effective transport channel for the diffusion of CO2. Moreover, the affinity of the N-containing groups (e. g. amino and amide groups) in the i-TFN membrane and the CO2 molecules helped to increase the CO2/N2 separation selectivity. At the optimal preparation condition, the obtained i-TFN membrane showed CO2/N2 selectivity of 23.3 with CO2 permeance of 174.5 GPU. This work proposes a novel nanomaterial interlayer to effectively improve the gas permselectivity by regulating the structure of i-TFN membranes, which has significant instruction for the preparation of high-performance i-TFN by exploring the interlayer of other nanomaterials.

The Synergistic Antioxidant Effect of Polydopamine Coating with Amino-Functionalized Graphene Quantum Dots on Osteoblast Protection Against Oxidative Stress

The Synergistic Antioxidant Effect of Polydopamine Coating with Amino-Functionalized Graphene Quantum Dots on Osteoblast Protection Against Oxidative Stress

One-pot alkali cutting-assisted synthesis of fluorescence tunable amino-functionalized graphene quantum dots as a multifunctional nanosensor for sensing of pH and tannic acid

Herein, a one-pot alkali cutting-assisted synthesis approach has been developed to gain fluorescence (FL) tunable amino functionalized GQDs (NH2-GQDs), which exhibit concentration- and excitation-dependent FL behaviors, due to the self-assembled J-type aggregation effect and different electronic transitions governed by graphene basal plane and functional groups. While NH2-GQDs possess brighter FL emission than pristine GQDs, owning to the functionalization of amino groups with strong electron withdrawing ability. Particularly, the pH-dependent FL behavior of NH2-GQDs further reflects the FL emission mechanism originated from the intrinsic zigzag sites and introduced amino and carboxylic groups, which is available for pH sensing. Moreover, the NH2-GQDs also show a FL quenching upon reaction with tannic acid (TA), resulting in the construction of a FL turn-off TA sensing platform. A good linear relationship is obtained between logarithm of FL intensity (log F) and TA concentration in a linear dynamic range of 1–40 μM and a limit of detection of 43.3 nM (3σ/s, n = 9) is achieved, with a precision of 0.08% RSD at a concentration level of 5 μM (n = 9). This work features a simple and direct approach to acquire multifunctional nanosensor, providing great potential for further applications in chem/biosensing.

Identification of potential circRNA-miRNA-mRNA regulatory networks in response to graphene quantum dots in microglia by microarray analysis

Along with the increasing application of graphene quantum dots (GQDs) in the fields of biomedicine and neuroscience, it is important to assess the probably adverse effects of GQDs in the central nervous system (CNS) but their underlying toxic mechanisms is still unclear. In this study, we evaluate the molecular mechanisms associated with circular RNAs (circRNAs) of nitrogen-doped GQDs (N-GQDs) and amino-functionalized GQDs (A-GQDs) damaging the cell viability and cellular structure in microglia by an integrative analysis of RNA microarray. The differentially expressed circRNA (DEcircRNAs)-miRNA- differentially expressed mRNA (DEmRNAs) regulatory networks were conducted in BV2 microglial cells treated with 25 µg/mL N-GQDs, 100 µg/mL N-GQDs and 100 µg/mL A-GQDs. Based on that, the protein-coding genes in each ceRNA network were collected to do bio-functional analysis to evaluate signaling pathways that were indirectly mediated by circRNAs. Some pathways that could play indispensable roles in the neurotoxicity of N-GQDs or both two kinds of GQDs were found. Low-dosed N-GQDs exposure mainly induced inflammatory action in microglia, while high-dosed N-GQDs and A-GQDs exposure both affect olfactory transduction and GABAergic synapse. Meanwhile, several classical signaling pathways, including mTOR, ErbB and MAPK, could make diverse contributions to the neurotoxicity of both two kinds of GQDs. These circRNAs could be toxic biomarkers or protective targets in neurotoxicity of GQDs. More importantly, they emphasized the necessity of comprehensive analysis of latent molecular mechanisms through epigenetics approaches in biosafety assessment of graphene-based nanomaterials.