Graphene oxide: a nonspecific enhancer of cellular growth

Safety Assessment of Graphene-Based Materials: Focus on Human Health and the Environment

Graphene and its derivatives are heralded as "miracle" materials with manifold applications in different sectors of society from electronics to energy storage to medicine. The increasing exploitation of graphene-based materials (GBMs) necessitates a comprehensive evaluation of the potential impact of these materials on human health and the environment. Here, we discuss synthesis and characterization of GBMs as well as human and environmental hazard assessment of GBMs using in vitro and in vivo model systems with the aim to understand the properties that underlie the biological effects of these materials; not all GBMs are alike, and it is essential that we disentangle the structure-activity relationships for this class of materials.

Bacterial response to graphene oxide and reduced graphene oxide integrated in agar plates

There are contradictory reports in the literature regarding the anti-bacterial activity of graphene, graphene oxide (GO) and reduced graphene oxide (rGO). This controversy is mostly due to variations in key parameters of the reported experiments, like: type of substrate, form of graphene, number of layers, type of solvent and most importantly, type of bacteria. Here, we present experimental data related to bacterial response to GO and rGO integrated in solid agar-based nutrient plates-a standard set-up for bacterial growth that is widely used by microbiologists. Bacillus subtilis and Pseudomonas aeruginosa strains were used for testing bacterial growth. We observed that plate-integrated rGO showed strong anti-bacterial activity against both bacterial species. By contrast, plate-integrated GO was harmless to both bacteria. These results reinforce the notion that the response of bacteria depends critically on the type of graphene material used and can vary dramatically from one bacterial strain to another, depending on bacterial physiology.

Interaction of graphene oxide nano-sheets and landfill leachate bacterial culture

The interaction of graphene oxide (GO) and municipal landfill leachate bacterial culture was investigated. In aerobic conditions, stepwise addition of GO concentration to 60 mg/L increased the oxygen uptake rate coefficient. However, increasing the GO concentration to 100 mg/L slightly decreased the oxygen uptake rate coefficient. When anoxic conditions developed, GO suspensions decreased the total bacterial activity and the substrate consumption. Raman spectroscopy confirmed the chemical reduction of GO during exposure to bacteria; nonetheless, the extent of reduction in anoxic conditions was more than that in aerobic conditions. The chemical composition of landfill leachate showed to be able to also slightly reduce the GO after 10 days exposure, but the role of bacterial activity in the reduction process seemed to be dominant. As far as it could be seen under a light microscope, while the gram-positive bacteria were more vulnerable in anoxic assays, GO suspension did not affect the morphology and size of active cultures in the aerobic atmosphere.

Protein Nanofibril Assemblies Templated by Graphene Oxide Nanosheets Accelerate Early Cell Adhesion and Induce Osteogenic Differentiation of Human Mesenchymal Stem Cells

Bombyx mori silk fibroin (SF) is a promising natural biocompatible protein. However, its interaction with graphene oxide (GO) has never been studied and the resultant SF/GO matrix has not been used to direct stem cell fate. Herein, we found out that mixing SF molecules and GO nanosheets in an aqueous solution can trigger the assembly of SF nanoparticles into oriented nanofibrils due to the guidance of GO nanosheets, forming SF/GO films with unique nanotopographies and improved modulus upon the removal of the solvent. When GO mass percentage in the SF/GO films is 2 and 10%, the SF assemblies are necklace-like nanofibrils (assembled from loosely linked SF nanoparticles) and solid nanofibrils (assembled from densely linked SF nanoparticles) in the resultant films, termed SG2 and SG10, respectively. GO nanosheets guided the SF assembly into nanofibrils by triggering the structural change of SF molecules from random coils to β-sheets, as confirmed by Fourier transform infrared spectroscopy and circular dichroism measurements. Furthermore, oxidative groups in the GO nanosheets were reduced by the reducing groups in SF during the nanofibril formation according to X-ray photoelectron spectroscopy and Raman spectroscopy. The reduction of the oxidative groups in GO by SF was further verified by the good cell viability on the SF/GO films. The unique nanotopographies of the SF/GO films were found to accelerate the early cell adhesion and induce the osteogenic differentiation of human mesenchymal stem cells (MSCs) even in the absence of additional inducers in the medium. More importantly, SG10 presents a stronger capability in promoting early MSC adhesion by promoting F-actin assembly, increasing cell spreading area, and inducing the osteogenic differentiation of the MSCs by the unique SF/GO nanofibrous matrix. To the best of our knowledge, it is the first report that the SF/GO substrates can induce the osteogenic differentiation of MSCs in the absence of osteogenic differentiation medium. Therefore, SF/GO composite materials would have a potential application in the field of bone tissue engineering.

Nanoarchitectonic-Based Material Platforms for Environmental and Bioprocessing Applications

The challenges of pollution, environmental science, and energy consumption have become global issues of broad societal importance. In order to address these challenges, novel functional systems and advanced materials are needed to achieve high efficiency, low emission, and environmentally friendly performance. A promising approach involves nanostructure-level controls of functional material design through a novel concept, nanoarchitectonics. In this account article, we summarize nanoarchitectonic approaches to create nanoscale platform structures that are potentially useful for environmentally green and bioprocessing applications. The introduced platforms are roughly classified into (i) membrane platforms and (ii) nanostructured platforms. The examples are discussed together with the relevant chemical processes, environmental sensing, bio-related interaction analyses, materials for environmental remediation, non-precious metal catalysts, and facile separation for biomedical uses.

Bioelectronics with nanocarbons

Characterizing the electrical activity of cardiomyocytes and neurons is crucial in understanding the complex processes in the heart and brain tissues, both in healthy and diseased states. Micro- and nanotechnologies have significantly improved the electrophysiological investigation of cellular networks. Carbon-based nanomaterials or nanocarbons, such as carbon nanotubes (CNTs), nanodiamonds (NDs) and graphene are promising building blocks for bioelectronics platforms owing to their outstanding chemical and physical properties. In this review, we discuss the various bioelectronics applications of nanocarbons and their derivatives. Furthermore, we touch upon the challenges that remain in the field and describe the emergence of carbon-based hybrid-nanomaterials that will potentially address those limitations, thus improving the capabilities to investigate the electrophysiology of excitable cells, both as a network and at the single cell level.

Effect of graphene oxide on the bioactivities of nitrifying and denitrifying bacteria in aerobic granular sludge

With the widespread application of graphene oxide (GO), it would be inevitably released into wastewater treatment plants (WWTPs) and get involved in the biochemical process. So far, there are controversies on the effects of low GO concentration (0.05-0.1 g/L) on the nitrogen removal process. Therefore, this study essentially investigates any potential effects of GO on wastewater microbial communities functions. In present study, the nitrifying and denitrifying batch tests were introduced to investigate the influence of 0.06 g/L of GO on bacteria. The results showed that GO could be easily combined with the aerobic granular sludge (AGS), and NH₄⁺-N was sharply absorbed, which directly promoted the bioactivities of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) and extracellular polymeric substances (EPS) production. The influence of GO on the denitrifying bacteria was negligible, which resulted in the stable EPS production. Furthermore, as inferred from the near maximum chemical reaction rates, there were no obvious changes on the microbial community functions during nitrogen removal process.

Graphene Materials in Antimicrobial Nanomedicine: Current Status and Future Perspectives

Graphene materials (GMs), such as graphene, graphene oxide (GO), reduced GO (rGO), and graphene quantum dots (GQDs), are rapidly emerging as a new class of broad-spectrum antimicrobial agents. This report describes their state-of-the-art and potential future covering both fundamental aspects and biomedical applications. First, the current understanding of the antimicrobial mechanisms of GMs is illustrated, and the complex picture of underlying structure-property-activity relationships is sketched. Next, the different modes of utilization of antimicrobial GMs are explained, which include their use as colloidal dispersions, surface coatings, and photothermal/photodynamic therapy agents. Due to their practical relevance, the examples where GMs function as synergistic agents or release platforms for metal ions and/or antibiotic drugs are also discussed. Later, the applicability of GMs in the design of wound dressings, infection-protective coatings, and antibiotic-like formulations ("nanoantibiotics") is assessed. Notably, to support our assessments, the existing clinical applications of conventional carbon materials are also evaluated. Finally, the key hurdles of the field are highlighted, and several possible directions for future investigations are proposed. We hope that the roadmap provided here will encourage researchers to tackle remaining challenges toward clinical translation of promising research findings and help realize the potential of GMs in antimicrobial nanomedicine.

Antimicrobial and Antibiofilm Efficacy of Graphene Oxide against Chronic Wound Microorganisms

Chronic wounds represent an increasing problem worldwide. Graphene oxide (GO) has been reported to exhibit strong antibacterial activity toward both Gram-positive and Gram-negative bacteria. The aim of this work was to investigate the in vitro antimicrobial and antibiofilm efficacy of GO against wound pathogens. Staphylococcus aureus PECHA 10, Pseudomonas aeruginosa PECHA 4, and Candida albicans X3 clinical isolates were incubated with 50 mg/liter of GO for 2 and 24 h to evaluate the antimicrobial effect. Optical and atomic force microscopy images were performed to visualize the effect of GO on microbial cells. Moreover, the antibiofilm effect of GO was tested on biofilms, both in formation and mature. Compared to the respective time controls, GO significantly reduced the S. aureus growth both at 2 and 24 h in a time-dependent way, and it displayed a bacteriostatic effect in respect to the GO t = 0; an immediate (after 2 h) slowdown of bacterial growth was detected for P. aeruginosa, whereas a tardive effect (after 24 h) was recorded for C. albicans Atomic force microscopy images showed the complete wrapping of S. aureus and C. albicans with GO sheets, which explains its antimicrobial activity. Moreover, significant inhibition of biofilm formation and a reduction of mature biofilm were recorded for each detected microorganism. The antibacterial and antibiofilm properties of GO against chronic wound microorganisms make it an interesting candidate to incorporate into wound bandages to treat and/or prevent microbial infections.

Multivalent Interactions between 2D Nanomaterials and Biointerfaces

2D nanomaterials, particularly graphene, offer many fascinating physicochemical properties that have generated exciting visions of future biological applications. In order to capitalize on the potential of 2D nanomaterials in this field, a full understanding of their interactions with biointerfaces is crucial. The uptake pathways, toxicity, long-term fate of 2D nanomaterials in biological systems, and their interactions with the living systems are fundamental questions that must be understood. Here, the latest progress is summarized, with a focus on pathogen, mammalian cell, and tissue interactions. The cellular uptake pathways of graphene derivatives will be discussed, along with health risks, and interactions with membranes-including bacteria and viruses-and the role of chemical structure and modifications. Other novel 2D nanomaterials with potential biomedical applications, such as transition-metal dichalcogenides, transition-metal oxide, and black phosphorus will be discussed at the end of this review.

Mechano-bactericidal mechanism of graphene nanomaterials

Growing interest in the bactericidal effect of graphene and graphene-derived nanomaterials has led to the investigation and effective publication of the bactericidal effects of the substratum, many of which present highly conflicting material. The nature of bacterial cell death on graphene bio-interfaces, therefore, remains poorly understood. Here, we review recent findings on the bactericidal effect of graphene and graphene-derived nanomaterials, and proposed mechanisms of cell inactivation, due to mechanical contact with graphene materials, including lipid extraction, physical damage to membranes and pore formation.

Graphene and its derivatives as biomedical materials: future prospects and challenges

Graphene and its derivatives possess some intriguing properties, which generates tremendous interests in various fields, including biomedicine. The biomedical applications of graphene-based nanomaterials have attracted great interests over the last decade, and several groups have started working on this field around the globe. Because of the excellent biocompatibility, solubility and selectivity, graphene and its derivatives have shown great potential as biosensing and bio-imaging materials. Also, due to some unique physico-chemical properties of graphene and its derivatives, such as large surface area, high purity, good bio-functionalizability, easy solubility, high drug loading capacity, capability of easy cell membrane penetration, etc., graphene-based nanomaterials become promising candidates for bio-delivery carriers. Besides, graphene and its derivatives have also shown interesting applications in the fields of cell-culture, cell-growth and tissue engineering. In this article, a comprehensive review on the applications of graphene and its derivatives as biomedical materials has been presented. The unique properties of graphene and its derivatives (such as graphene oxide, reduced graphene oxide, graphane, graphone, graphyne, graphdiyne, fluorographene and their doped versions) have been discussed, followed by discussions on the recent efforts on the applications of graphene and its derivatives in biosensing, bio-imaging, drug delivery and therapy, cell culture, tissue engineering and cell growth. Also, the challenges involved in the use of graphene and its derivatives as biomedical materials are discussed briefly, followed by the future perspectives of the use of graphene-based nanomaterials in bio-applications. The review will provide an outlook to the applications of graphene and its derivatives, and may open up new horizons to inspire broader interests across various disciplines.

Highly Stable Graphene-Based Nanocomposite (GO-PEI-Ag) with Broad-Spectrum, Long-Term Antimicrobial Activity and Antibiofilm Effects

Various silver nanoparticle (AgNP)-decorated graphene oxide (GO) nanocomposites (GO-Ag) have received increasing attention owing to their antimicrobial activity and biocompatibility; however, their aggregation in physiological solutions and the generally complex synthesis methods warrant improvement. This study aimed to synthesize a polyethyleneimine (PEI)-modified and AgNP-decorated GO nanocomposite (GO-PEI-Ag) through a facile approach through microwave irradiation without any extra reductants and surfactants; its antimicrobial activity was investigated on Gram-negative/-positive bacteria (including drug-resistant bacteria) and fungi. Compared with GO-Ag, GO-PEI-Ag acquired excellent stability in physiological solutions and electropositivity, showing substantially higher antimicrobial efficacy. Moreover, GO-PEI-Ag exhibited particularly excellent long-term effects, presenting no obvious decline in antimicrobial activity after 1 week storage in physiological saline and repeated use for three times and the lasting inhibition of bacterial growth in nutrient-rich culture medium. In contrast, GO-Ag exhibited a >60% decline in antimicrobial activity after storage. Importantly, GO-PEI-Ag effectively eliminated adhered bacteria, thereby preventing biofilm formation. The primary antimicrobial mechanisms of GO-PEI-Ag were evidenced as physical damage to the pathogen structure, causing cytoplasmic leakage. Hence, stable GO-PEI-Ag with robust, long-term antimicrobial activity holds promise in combating public-health threats posed by drug-resistant bacteria and biofilms.

Functionalized 2D nanomaterials with switchable binding to investigate graphene-bacteria interactions

Graphene and its derivatives have recently attracted much attention for sensing and deactivating pathogens. However, the mechanism of multivalent interactions at the graphene-pathogen interface is not fully understood. Since different physicochemical parameters of graphene play a role at this interface, control over graphene's structure is necessary to study the mechanism of these interactions. In this work, different graphene derivatives and also zwitterionic graphene nanomaterials (ZGNMs) were synthesized with defined exposure, in terms of polymer coverage and functionality, and isoelectric points. Then, the switchable interactions of these nanomaterials with E. coli and Bacillus cereus were investigated to study the validity of the generally proposed "trapping" and "nano-knives" mechanisms for inactivating bacteria by graphene derivatives. It was found that the antibacterial activity of graphene derivatives strongly depends on the accessible area, i.e. edges and basal plane of sheets and tightness of their agglomerations. Our data clearly confirm the authenticity of "trapping" and "nano-knives" mechanisms for the antibacterial activity of graphene sheets.

Synthesis of graphene oxide-quaternary ammonium nanocomposite with synergistic antibacterial activity to promote infected wound healing

BACKGROUND

Bacterial infection is one of the most common complications in burn, trauma, and chronic refractory wounds and is an impediment to healing. The frequent occurrence of antimicrobial-resistant bacteria due to irrational application of antibiotics increases treatment cost and mortality. Graphene oxide (GO) has been generally reported to possess high antimicrobial activity against a wide range of bacteria in vitro. In this study, a graphene oxide-quaternary ammonium salt (GO-QAS) nanocomposite was synthesized and thoroughly investigated for synergistic antibacterial activity, underlying antibacterial mechanisms and biocompatibility in vitro and in vivo.

METHODS

The GO-QAS nanocomposite was synthesized through amidation reactions of carboxylic group end-capped QAS polymers with primary amine-decorated GO to achieve high QAS loading ratios on nanosheets. Next, we investigated the antibacterial activity and biocompatibility of GO-QAS in vitro and in vivo.

RESULTS

GO-QAS exhibited synergistic antibacterial activity against bacteria through not only mechanical membrane perturbation, including wrapping, bacterial membrane insertion, and bacterial membrane perforation, but also oxidative stress induction. In addition, it was found that GO-QAS could eradicate multidrug-resistant bacteria more effectively than conventional antibiotics. The in vitro and in vivo toxicity tests indicated that GO-QAS did not exhibit obvious toxicity towards mammalian cells or organs at low concentrations. Notably, GO-QAS topically applied on infected wounds maintained highly efficient antibacterial activity and promoted infected wound healing in vivo.

CONCLUSIONS

The GO-QAS nanocomposite exhibits excellent synergistic antibacterial activity and good biocompatibility both in vitro and in vivo. The antibacterial mechanisms involve both mechanical membrane perturbation and oxidative stress induction. In addition, GO-QAS accelerated the healing process of infected wounds by promoting re-epithelialization and granulation tissue formation. Overall, the results indicated that the GO-QAS nanocomposite could be applied as a promising antimicrobial agent for infected wound management and antibacterial wound dressing synthesis.

Graphene-Based Nanomaterials for Tissue Engineering in the Dental Field

The world of dentistry is approaching graphene-based nanomaterials as substitutes for tissue engineering. Apart from its exceptional mechanical strength, electrical conductivity and thermal stability, graphene and its derivatives can be functionalized with several bioactive molecules. They can also be incorporated into different scaffolds used in regenerative dentistry, generating nanocomposites with improved characteristics. This review presents the state of the art of graphene-based nanomaterial applications in the dental field. We first discuss the interactions between cells and graphene, summarizing the available in vitro and in vivo studies concerning graphene biocompatibility and cytotoxicity. We then highlight the role of graphene-based nanomaterials in stem cell control, in terms of adhesion, proliferation and differentiation. Particular attention will be given to stem cells of dental origin, such as those isolated from dental pulp, periodontal ligament or dental follicle. The review then discusses the interactions between graphene-based nanomaterials with cells of the immune system; we also focus on the antibacterial activity of graphene nanomaterials. In the last section, we offer our perspectives on the various opportunities facing the use of graphene and its derivatives in associations with titanium dental implants, membranes for bone regeneration, resins, cements and adhesives as well as for tooth-whitening procedures.

Various Physiological Response to Graphene Oxide and Amine-Functionalized Graphene Oxide in Wheat (Triticum aestivum)

An increasing number of investigations have been performed on the phytotoxicity of carbon-based nanomaterials duo to their extensive use in various fields. In the present study, we investigated the phytotoxicity of unfunctionalized graphene oxide (GO) and amine-functionalized graphene oxide (G-NH₂) on wheat (Triticum aestivum) in the concentration range from 125 to 2000 μg/mL after 9 days of hydroponic culture. Our results found that the incubation with both nanomaterials did not affect the final seed germination rate, despite some influence in the initial stage. Transmission electron microscopy (TEM) observations indicated that exposure to GO at a high concentration (above 1000 μg/mL) resulted in a severe loss of morphology of seedlings, and a decrease in root length, shoot length and relative biomass, along with obvious damage to plant tissue structures (root, stem and leaf) when compared with the control. GO induced increased damage to root cells, which were determined by electrolyte leakage. Conversely, the plant growth was enhanced under G-NH₂ exposure, and the root and stem lengths were increased by 19.27% and 19.61% at 2000 μg/mL, respectively. The plant tissue structures were not affected, and neither GO nor G-NH₂ were observed to accumulate in the wheat plant root cells. The present investigations provide important information for evaluation of the environmental safety of GO and better understanding plant-nanoparticle interactions.

Advances on graphene-based nanomaterials for biomedical applications

Graphene-based nanomaterials, such as graphene oxide and reduced graphene oxide, have been attracting increasing attention in the field of biology and biomedicine over the past few years. Incorporation of these novel materials with drug, gene, photosensitizer and other cargos to construct novel delivery systems has witnessed rapid advance on the basis of their large surface area, distinct surface properties, excellent biocompatibility and pH sensitivity. Moreover, the inherent photothermal effect of these appealing materials enables them with the ability of killing targeting cells via a physical mechanism. Recently, more attentions have been attached to tissue engineering, including bone, neural, cardiac, cartilage, musculoskeletal, and skin/adipose tissue engineering, due to the outstanding mechanical strength, stiffness, electrical conductivity, various two-dimensional (2D) and three-dimensional (3D) morphologies of graphene-based nanomaterials. Herein, emerging applications of these nanomaterials in bio-imaging, drug/gene delivery, phototherapy, multimodality therapy and tissue engineering were comprehensively reviewed. Inevitably, the burgeon of this kind of novel materials leads to the endeavor to consider their safety so that this issue has been deeply discussed and summarized in our review. We hope that this review offers an overall understanding of these nanomaterials for later in-depth investigations.

Graphene oxide-facilitated transport of levofloxacin and ciprofloxacin in saturated and unsaturated porous media

In this work, effects of graphene oxide (GO) on the co-transport of the two typical Fluoroquinolones (FQs) - levofloxacin (LEV) and ciprofloxacin (CIP) in saturated and unsaturated quartz sand media were studied. The adsorption isotherms showed that GO had much larger sorption capacities to LEV and CIP than sand with the largest Langmuir adsorption capacity of 409 mg g^-1 (CIP-GO); while the sorption affinity of the two FQs onto the two adsorbents might follow the order of CIP-sand > LEV-sand > LEV-GO > CIP-GO. GO promoted the mobility of the two FQs in both saturated and unsaturated porous media due to its strong mobility and sorption capacity. The GO-bound LEV/CIP was responsible for the LEV/CIP transport in the porous media, and transport of GO-bound FQs increased with the increasing of initial GO concentration. Under unsaturated conditions, moisture showed little effect on the transport of GO-bound CIP; however, the mobility of GO-bound LEV reduced with the decreasing of moisture content, suggesting the transport of adsorbed LEV from GO to air-water interface. GO sorption reduced the antibacterial ability of the two FQs, but they were still effective in inhibiting E. coli growth.

Synthesis spherical porous hydroxyapatite/graphene oxide composites by ultrasonic-assisted method for biomedical applications

Spherical porous hydroxyapatite (SHA)/graphene oxide (GO) composites with different GO (w/w) content of 16%, 40%, and 71% have been fabricated through a facile and controllable ultrasonic-assisted method at room temperature. The products were characterized by x-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, mechanical testing and biomimetic mineralization. Results showed SHA were covered by GO, and SHA/GO composites had an irregular surface with different degrees of wrinkles. The elastic modulus and hardness of SHA/GO-3 composites were up to 12.45 ± 0.33 GPa and 686.67 ± 26.95 MPa, which indicated that the contents of GO had an effect on SHA/GO composites. And the mechanical properties of SHA/GO-2 composites were similar to SHA particles. The biomimetic mineralization in SBF solution showed the bone-like apatite layer on composites surface, which demonstrated that the SHA/GO materials had osseointegration property. Moreover, in vitro cytocompatibility of SHA/GO-2 composites and pure GO were evaluated by cell adhesion and proliferation tests using MC3T3-E1 cells, which demonstrated that the SHA/GO composites can act as a good template for the cells growth and adhesion. These results suggested that the SHA/GO composites will be a promising material for biomedical application.

The toxicity of graphene and its impacting on bioleaching of metal ions from sewages sludge by Acidithiobacillus sp

The increasing production of graphene raised concerns about their releasing into sewage sludge, however, there is little information about graphene impacting on the growth of bacteria and hence their bioleaching of metal ions from sewages sludge. In this study, we reported that Acidithiobacillus sp., isolated from sewages, were used to bioleach Cu²⁺ and Zn²⁺ from sewages sludge in the presence of graphene. The negative effect on the growth of Acidithiobacillus sp. and dose-dependent were observed in presence of graphene, where the optical density (OD₄₂₀) of the culture decreased from 0.163 to 0.045, while the bioleaching efficiency of Cu²⁺ (70%-16%) and Zn²⁺ (80%-48%) were also reduced when the graphene dose decreased from 50 mg L^-1 to 1 mg L^-1. Furthermore, scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirmed that the direct contacts between graphene and cell at 1 mg L^-1 graphene caused cell membrane disruption, while Acidithiobacillus sp. grew better by forming dense biofilms around the suspended graphene at a 50 mg L^-1. LIVE/DEAD staining further demonstrated that almost no live cells were detected at 1 mg L^-1 graphene. The toxicity of graphene could generally be explained by depending on the concentration of graphene. The new findings provide an insight into dose dependence, which impacted on the growth of Acidithiobacillus sp. and their bioleaching of metal ion from sludge.

Electron Transfer Directed Antibacterial Properties of Graphene Oxide on Metals

Nanomaterials such as silver nanoparticles and graphene-based composites are known to exhibit biocidal activities. However, interactions with surrounding medium or supporting substrates can significantly influence this activity. Here, it is shown that superior antimicrobial properties of natural shellac-derived graphene oxide (GO) coatings is obtained on metallic films, such as Zn, Ni, Sn, and steel. It is also found that such activities are directly correlated to the electrical conductivity of the GO-metal systems; the higher the conductivity the better is the antibacterial activity. GO-metal substrate interactions serve as an efficient electron sink for the bacterial respiratory pathway, where electrons modify oxygen containing functional groups on GO surfaces to generate reactive oxygen species (ROS). A concerted effect of nonoxidative electron transfer mechanism and consequent ROS mediated oxidative stress to the bacteria result in an enhanced antimicrobial action of naturally derived GO-metal films. The lack of germicidal effect in exposed cells for GO supported on electrically nonconductive substrates such as glass corroborates the above hypothesis. The results can lead to new GO coated antibacterial metal surfaces important for environmental and biomedical applications.

Cytokine Profiling of Primary Human Macrophages Exposed to Endotoxin-Free Graphene Oxide: Size-Independent NLRP3 Inflammasome Activation

Graphene-based materials including graphene oxide (GO) are envisioned for a variety of biomedical applications. However, there are conflicting results concerning the biocompatibility of these materials. Here, a question is raised whether GO with small or large lateral dimensions triggers cytotoxicity and/or cytokine responses in primary human monocyte-derived macrophages. GO sheets produced under sterile conditions by a modified Hummers' method are found to be taken up by macrophages without signs of cytotoxicity. Then, multiplex arrays are used for profiling of proinflammatory and anti-inflammatory responses. Notably, GO suppresses the lipopolysaccharide (LPS)-triggered induction of several chemokines and cytokines, including the anti-inflammatory cytokine, interleukin-10 (IL-10). No production of proinflammatory TNF-α is observed. However, GO elicits caspase-dependent IL-1 β expression, a hallmark of inflammasome activation, in LPS-primed macrophages. Furthermore, GO-triggered IL-1 β production requires NADPH oxidase-generated reactive oxygen species and cellular uptake of GO and is accompanied by cathepsin B release and K+ efflux. Using THP-1 knockdown cells, a role for the inflammasome sensor, NLRP3, the adaptor protein, ASC, and caspase-1 for GO-induced IL-1β secretion is demonstrated. Finally, these studies show that inflammasome activation is independent of the lateral dimensions of the GO sheets. These studies provide novel insights regarding the immunomodulatory properties of endotoxin-free GO.

Improving Antibacterial Activity and Biocompatibility of Bioinspired Electrospinning Silk Fibroin Nanofibers Modified by Graphene Oxide

In this article, the silk fibroin (SF)/graphene oxide (GO)-blended nanofibers with one bioinspired nanostructure are fabricated via electrospinning. The morphology, chemical structure, antibacterial activity, and biocompatibility of the blending nanofibers are investigated. The results indicate that GO plays an important role in preparing the distinctive bioinspired structure. The antibacterial activity and in vivo cell culture test demonstrate that blending of GO could improve the antibacterial activity and biocompatibility of SF nanofibers. The blended nanofibers developed in this study may have considerable potential for wound dressing applications.

Graphene oxide conjugated with polymers: a study of culture condition to determine whether a bacterial growth stimulant or an antimicrobial agent?

Background

The results showed that the deciding factor is the culture medium in which the bacteria and the graphene oxide (GO) are incubated at the initial manipulation step. These findings allow better use of GO and GO-based materials more and be able to clearly apply them in the field of biomedical nanotechnology.

Results

To study the use of GO sheets applied in the field of biomedical nanotechnology, this study determines whether GO-based materials [GO, GO-polyoxyalkyleneamine (POAA), and GO-chitosan] stimulate or inhibit bacterial growth in detail. It is found that it depends on whether the bacteria and GO-based materials are incubated with a nutrient at the initial step. This is a critical factor for the fortune of bacteria. GO stimulates bacterial growth and microbial proliferation for Gram-negative and Gram-positive bacteria and might also provide augmented surface attachment for both types of bacteria. When an external barrier that is composed of GO-based materials forms around the surface of the bacteria, it suppresses nutrients that are essential to microbial growth and simultaneously produces oxidative stress, which causes bacteria to die, regardless of whether they have an outer-membrane-Gram-negative-bacteria or lack an outer-membrane-Gram-positive-bacteria, even for high concentrations of biocompatible GO-POAA. The results also show that these GO-based materials are capable of inducing reactive oxygen species (ROS)-dependent oxidative stress on bacteria. Besides, GO-based materials may act as a biofilm, so it is hypothesized that they suppress the toxicity of low-dose chitosan.

Conclusion

Graphene oxide is not an antimicrobial material but it is a general growth enhancer that can act as a biofilm to enhance bacterial attachment and proliferation. However, GO-based materials are capable of inducing ROS-dependent oxidative stress on bacteria. The applications of GO-based materials can clearly be used in antimicrobial surface coatings, surface-attached stem cells for orthopedics, antifouling for biocides and microbial fuel cells and microbial electro-synthesis.

Electronic supplementary material

The online version of this article (10.1186/s12951-017-0328-8) contains supplementary material, which is available to authorized users.

Graphene-Functionalized Biomimetic Scaffolds for Tissue Regeneration

Graphene is a two-dimensional atomic layer of graphite, where carbon atoms are assembled in a honeycombed lattice structure. Recently, graphene family nanomaterials, including pristine graphene, graphene oxide and reduced graphene oxide, have increasingly attracted a great deal of interest from researchers in a variety of science, engineering and industrial fields because of their unique structural and functional features. In particular, extensive studies have been actively conducted in the biomedical and related fields, including multidisciplinary and emerging areas, as their stimulating effects on cell behaviors have been becoming an increasing concern. Herein, we are attempting to summarize some of recent findings in the fields of tissue regeneration concerning the graphene family nanomaterial-functionalized biomimetic scaffolds, and to provide the promising perspectives for the possible applications of graphene family nanomaterial.

Fabrication of the graphene honeycomb structure as a scaffold for the study of cell growth

The good biocompatibility of the graphene honeycomb structure was evaluated by the proliferation of HeLa and MCF-7 cell lines.

PLGA-based nanofibers with a biomimetic polynoradrenaline sheath for rapid in vivo sampling of tetrodotoxin and sulfonamides in pufferfish

PLGA nanofibers with PNA sheath modification achieve enhanced extraction performance and antibiofouling capacity for in vivo sampling in pufferfish.

Boron nitride nanomaterials: biocompatibility and bio-applications

Boron nitride has structural characteristics similar to carbon 2D materials (graphene and its derivatives) and its layered structure has been exploited to form different nanostructures such as nanohorns, nanotubes, nanoparticles and nanosheets.

Systematic toxicity investigation of graphene oxide: evaluation of assay selection, cell type, exposure period and flake size

Understanding the toxicity of nanomaterials is essential for the safe and sustainable development of new applications. This is particularly true for a nanomaterial as widely used as graphene oxide (GO), which is utilized as films for electronics, membranes for filtration, drug carriers and more. Despite this, the current literature presents conflicting results on the overall toxicity of GO. Here, the cytotoxicity of three sizes of commercially available GO was investigated on six cell lines, as values of NOAEL/LOAEL. The effectiveness of four viability assays was also evaluated. The overall toxicity of GO greatly varied between cell lines; the suspension cells showed a greater response to the GO treatment compared to the adherent cell lines. Time dependent cytotoxicity was also cell line dependent, with only one cell line demonstrating obvious dependence. The six cell lines were also tested to evaluate their response to varying GO flake sizes: the suspension/phagocytic cells showed little variation in viability, while a difference was observed for the adherent/non-phagocytic cell lines. By systematically studying the effect of dose, GO size and treatment time for the six cell lines by using commercially available GO samples, we eliminate many of the variables which may result in the conflicting reports on the cytotoxicity of GO in the literature.

Implication of graphene oxide in Cd-contaminated soil: A case study of bacterial communities

The application of graphene oxide (GO) has attracted increasing concerns in the past decade regarding its environmental impacts, except for the impact of GO on a metal-contaminated soil system, due to its special properties. In the present work, the effects of GO on the migration and transformation of heavy metals and soil bacterial communities in Cd-contaminant soil were systematically evaluated. Soil samples were exposed to different doses of GO (0, 1, and 2 g kg^-1) over 60 days. The Community Bureau of Reference (BCR) sequential extraction procedure was used to reflect the interaction between GO and Cd. Several microbial parameters, including enzyme activities and bacterial community structure, were measured to determine the impacts of GO on polluted soil microbial communities. It was shown that Cd was immobilized by GO throughout the entire exposure period. Interestingly, the structure of the bacterial community changed. The relative abundance of the major bacterial phyla (e.g., Acidobacteria and Actinobacteria) increased, which was possibly attributed to the reduced toxicity of Cd in the presence of GO. However, GO exerted an adverse influence on the relative abundance of some phyla (e.g., WD272 and TM6). The diversity of bacterial communities was slightly restricted. The functional bacteria related to carbon and the nitrogen cycling were also affected, which, consequently, may influence the nutrient cycling in soil.

Graphene Quantum Dots for Cell Proliferation, Nucleus Imaging, and Photoluminescent Sensing Applications

We report a simple one-pot microwave assisted "green synthesis" of Graphene Quantum Dots (GQDs) using grape seed extract as a green therapeutic carbon source. These GQDs readily self-assemble, hereafter referred to as "self-assembled" GQDs (sGQDs) in the aqueous medium. The sGQDs enter via caveolae and clathrin-mediated endocytosis and target themselves into cell nucleus within 6-8 h without additional assistance of external capping/targeting agent. The tendency to self-localize themselves into cell nucleus also remains consistent in different cell lines such as L929, HT-1080, MIA PaCa-2, HeLa, and MG-63 cells, thereby serving as a nucleus labelling agent. Furthermore, the sGQDs are highly biocompatible and act as an enhancer in cell proliferation in mouse fibroblasts as confirmed by in vitro wound scratch assay and cell cycle analysis. Also, photoluminescence property of sGQDs (lifetime circa (ca.) 10 ns) was used for optical pH sensing application. The sGQDs show linear, cyclic and reversible trend in its fluorescence intensity between pH 3 and pH 10 (response time: ~1 min, sensitivity -49.96 ± 3.5 mV/pH) thereby serving as a good pH sensing agent. A simple, cost-effective, scalable and green synthetic approach based sGQDs can be used to develop selective organelle labelling, nucleus targeting in theranostics, and optical sensing probes.

Fluorescent Silicon Nanorods-Based Ratiometric Sensors for Long-Term and Real-Time Measurements of Intracellular pH in Live Cells

Long-term and real-time investigation of the dynamic process of pH_i changes is critically significant for understanding the related pathogenesis of diseases and the design of intracellular drug delivery systems. Herein, we present a one-step synthetic strategy to construct ratiometric pH sensors, which are made of europium (Eu)-doped one-dimensional silicon nanorods (Eu@SiNRs). The as-prepared Eu@SiNRs have distinct emission maxima peaks at 470 and 620 nm under 405 nm excitation. Of particular note, the fluorescence emission intensity at 470 nm decreases along with the increase of pH, while the one at 620 nm is nearly unaffected by pH changes, making Eu@SiNRs a feasible probe for pH sensing ratiometrically. Moreover, Eu@SiNRs are found to be responsive to a broad pH range (ca. 3-9), biocompatible (e.g., ∼100% of cell viability during 24 h treatment) and photostable (e.g., ∼10% loss of intensity after 40 min continuous UV irradiation). Taking advantages of these merits, we employ Eu@SiNRs for the visualization of the cytoplasmic alkalization process mediated by nigericin in living cells, for around 30 min without interruption, revealing important information for understanding the dynamic process of pH_i fluctuations.

Graphene oxide electrodeposited electrode enhances start-up and selective enrichment of exoelectrogens in bioelectrochemical systems

This study seeks to assess the impact that the anodic electrodeposition of graphene oxide (GO) has on the start-up process and on the development of microbial communities on the anode of BESs. The GO electrodeposited electrodes were characterised in abiotic conditions to verify the extent of the modification and were then transferred to a bioelectrochemical reactor. Results showed that the modified electrode allowed for a reduced start-up time compared to the control electrode. After three months, high throughput sequencing was performed, revealing that electrochemically reduced graphene oxide acts as a selective agent toward exoelectrogenic bacteria as Geobacter. Overall, this study shows that GO modified electrodes enhance biofilm build up in BES.

Redox-active nanomaterials for nanomedicine applications

Nanomedicine utilizes the remarkable properties of nanomaterials for the diagnosis, treatment, and prevention of disease. Many of these nanomaterials have been shown to have robust antioxidative properties, potentially functioning as strong scavengers of reactive oxygen species. Conversely, several nanomaterials have also been shown to promote the generation of reactive oxygen species, which may precipitate the onset of oxidative stress, a state that is thought to contribute to the development of a variety of adverse conditions. As such, the impacts of nanomaterials on biological entities are often associated with and influenced by their specific redox properties. In this review, we overview several classes of nanomaterials that have been or projected to be used across a wide range of biomedical applications, with discussion focusing on their unique redox properties. Nanomaterials examined include iron, cerium, and titanium metal oxide nanoparticles, gold, silver, and selenium nanoparticles, and various nanoscale carbon allotropes such as graphene, carbon nanotubes, fullerenes, and their derivatives/variations. Principal topics of discussion include the chemical mechanisms by which the nanomaterials directly interact with biological entities and the biological cascades that are thus indirectly impacted. Selected case studies highlighting the redox properties of nanomaterials and how they affect biological responses are used to exemplify the biologically-relevant redox mechanisms for each of the described nanomaterials.

When stem cells meet graphene: Opportunities and challenges in regenerative medicine

Recent advances in stem cell research and nanotechnology have significantly influenced the landscape of tissue engineering and regenerative medicine. Precise and reproducible control of the fate of stem cells and their lineage specification have, therefore, become more crucial than ever for the success of stem cell-based technologies. Extensive research has been geared towards developing materials that are capable of mimicking the physiological microenvironment of stem cells and at the same time, controlling their eventual fate. An interesting example of these materials is two-dimensional graphene and its related derivatives. A high specific surface area coupled with superior chemical stability, biocompatibility, and flexibility in functionalization render graphene-based nanomaterials one of the most exciting platforms for tissue engineering and regenerative medicine applications, especially for stem cell growth, proliferation, and differentiation. In this review, we discuss the love-hate relationship between stem cells and graphene-based nanomaterials in tissue engineering and regenerative medicine. We first discuss the role and importance of stem cells in tissue engineering and regenerative medicine. We then highlight the use of nanomaterials for stem cell control, the interaction between stem cells and graphene nanomaterials as well as their biocompatibility, biodistribution, and biodegradability considerations. We also offer our perspectives on the various challenges and opportunities facing the use of graphene and its derivatives for stem cell growth and differentiation.