Graphene-Assisted Synthesis of 2D Polyglycerols as Innovative Platforms for Multivalent Virus Interactions

2D nanomaterials have garnered widespread attention in biomedicine and bioengineering due to their unique physicochemical properties. However, poor functionality, low solubility, intrinsic toxicity, and nonspecific interactions at biointerfaces have hampered their application in vivo. Here, biocompatible polyglycerol units are crosslinked in two dimensions using a graphene-assisted strategy leading to highly functional and water-soluble polyglycerols nanosheets with 263 ± 53 nm and 2.7 ± 0.2 nm average lateral size and thickness, respectively. A single-layer hyperbranched polyglycerol containing azide functional groups is covalently conjugated to the surface of a functional graphene template through pH-sensitive linkers. Then, lateral crosslinking of polyglycerol units is carried out by loading tripropargylamine on the surface of graphene followed by lifting off this reagent for an on-face click reaction. Subsequently, the polyglycerol nanosheets are detached from the surface of graphene by slight acidification and centrifugation and is sulfated to mimic heparin sulfate proteoglycans. To highlight the impact of the two-dimensionality of the synthesized polyglycerol sulfate nanosheets at nanobiointerfaces, their efficiency with respect to herpes simplex virus type 1 and severe acute respiratory syndrome corona virus 2 inhibition is compared to their 3D nanogel analogs. Four times stronger in virus inhibition suggests that 2D polyglycerols are superior to their current 3D counterparts.     

Chitosan‐Based Heparan Sulfate Mimetics Promote Epidermal Formation in a Human Organotypic Skin Model

Biomimetic materials that replicate biological functions have great promise for use as therapeutics in regenerative medicine applications. Heparan sulfate (HS) is the natural binding partner for growth factors enabling longer half‐lives and potentiation of their signaling. In this study a water soluble chitosan‐arginine is modified with sulfate moieties in order to mimic the structure of HS. Sulfated chitosan‐arginine with a degree of sulfation of 58% binds fibroblast growth factor 2 with higher affinity than HS. The sulfated chitosan‐arginine also promotes epithelial cell migration and supports the formation of an expanded epidermis in an organotypic skin model. Furthermore, sulfated chitosan‐arginine promotes the expression of the HS proteoglycan, perlecan, by both epithelial and fibroblast cells. Perlecan itself modulates the activity of mitogens and is essential for the formation of the epidermis. The synthesized sulfated Ch‐Arg derivatives mimic HS, support formation of the epidermis, and thus have the potential to assist in wound healing.     

Regulating Cellular Behavior on Few‐Layer Reduced Graphene Oxide Films with Well‐Controlled Reduction States

Understanding the effect of graphene on cellular behavior is important for enabling a range of new biological and biomedical applications. However, due to the complexity of cell responses and graphene surface states, regulating cellular behaviors on graphene or its derivatives is still a great challenge. To address this challenge we have developed a novel, facile route to regulate the cellular behaviors on few‐layer reduced graphene oxide (FRGO) films by controlling the reduction states of graphene oxide. Our results indicate that the surface oxygen content of FRGO has a strong influence on cellular behavior, with the best performance for cell attachment, proliferation and phenotype being obtained in moderately reduced FRGO. Cell performance decreased significantly as the FRGO was highly reduced. Moderate performance was found in non‐reduced pure graphene oxide and control glass slides. Our results highlight the important role of surface physicochemical characteristics of graphene and its derivatives in their interactions with biocomponents, and may have great potential in enabling the utility of graphene based materials in various biomedical and bioelectronic applications.     

Combination of Surface Charge and Size Controls the Cellular Uptake of Functionalized Graphene Sheets

A fundamental issue for biomedical applications of graphene is the correlation between its physicochemical properties and cellular uptake mechanism. However, such studies are challenging due to the intrinsic polydispersity of graphene. In this work, a series of water soluble graphene sheets with the same polymer coverage, density of functional groups, and fluorescence intensity but three different sizes and surface charges are produced. The effect of the latter two factors and their combination on the mechanism of cellular uptake and intracellular pathways of these defined nanosheets is investigated via confocal and Raman microscopies. While positively (? NH₃⁺) and negatively (? OSO₃^?) charged sheets show an energy dependent uptake, their neutral analogs do not show any significant uptake. The cellular uptake efficacy of positively charged graphene sheets is independent of the size and occurs both through phagocytosis and clathrin‐mediated endocytosis pathways. However, cellular uptake efficacy of graphene sheets with negative surface charge strongly depends on the size of the sheets. They cross the membrane mainly through phagocytosis and sulfate‐receptor‐mediated endocytosis. This study demonstrates that the impact of the size of graphene derivatives on their cellular uptake pathways highly depends on their surface charges and vice versa.     

The Exfoliation of Graphene in Liquids by Electrochemical,Chemical, and Sonication‐Assisted Techniques: A Nanoscale Study

The different exfoliation routes of graphite to produce graphene by sonication in solvent, chemical oxidation and electrochemical oxidation are compared. The exfoliation process and roughening of a flat graphite substrate is directly visualized at the nanoscale by scanning probe and electron microscopy. The etching damage in graphite and the properties of the exfoliated sheets are compared by Raman spectroscopy and X‐ray diffraction analysis. The results show the trade‐off between exfoliation speed and preservation of graphene quality. A key step to achieve efficient exfoliation is to couple gas production and mechanical exfoliation on a macroscale with non‐covalent exfoliation and preservation of graphene properties on a molecular scale.     

Hybrid of Iron Nitride and Nitrogen‐Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction

It is extremely desirable but challenging to create highly active, stable, and low‐cost catalysts towards oxygen reduction reaction to replace Pt‐based catalysts in order to perform the commercialization of fuel cells. Here, a novel iron nitride/nitrogen doped‐graphene aerogel hybrid, synthesized by a facile two‐step hydrothermal process, in which iron phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π–π stacking and oxygen‐containing functional groups, is reported. As a result, there exist strong interactions between Fe _x N nanoparticles and graphene substrates, leading to a synergistic effect towards oxygen reduction reaction. It is worth noting that the onset potential and current density of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure nitrogen‐doped graphene aerogel, free Fe _x N and their physical mixtures. The hybrid also exhibits comparable catalytic activity as commercial Pt/C at the same catalyst loading, while its stability and resistance to methanol crossover are superior. Interestingly, it is found that, apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high density of Fe–N–C sties and small size of Fe _x N particles boost charge transfer rate.     

Synergistic CO2‐Sieving from Polymer with Intrinsic Microporosity Masking Nanoporous Single‐Layer Graphene

High‐flux nanoporous single‐layer graphene membranes are highly promising for energy‐efficient gas separation. Herein, in the context of carbon capture, a remarkable enhancement in the CO₂ selectivity is demonstrated by uniquely masking nanoporous single‐layer graphene with polymer with intrinsic microporosity (PIM‐1). In the process, a major bottleneck of the state‐of‐the‐art pore‐incorporation techniques in graphene has been overcome, where in addition to the molecular sieving nanopores, larger nonselective nanopores are also incorporated, which so far, has restricted the realization of CO₂‐sieving from graphene membranes. Overall, much higher CO₂/N₂ selectivity (33) is achieved from the composite film than that from the standalone nanoporous graphene (NG) (10) and the PIM‐1 membranes (15), crossing the selectivity target (20) for postcombustion carbon capture. The selectivity enhancement is explained by an analytical gas transport model for NG, which shows that the transport of the stronger‐adsorbing CO₂ is dominated by the adsorbed phase transport pathway whereas the transport of N₂ benefits significantly from the direct gas‐phase transport pathway. Further, slow positron annihilation Doppler broadening spectroscopy reveals that the interactions with graphene reduce the free volume of interfacial PIM‐1 chains which is expected to contribute to the selectivity. Overall, this approach brings graphene membrane a step closer to industrial deployment.     

A Generic Synthetic Approach to Large‐Scale Pristine‐Graphene/Metal‐Nanoparticles Hybrids

The homogeneous attachment of metal‐nanoparticles (metal‐NPs) on pristine‐graphene surface to construct pristine‐graphene/metal‐NPs hybrids is highly expected for application in many fields such as transparent electrodes and conductive composites. However, it remains a great challenge since the pristine‐graphene is highly hydrophobic. Here, an environmentally friendly generic synthetic approach to large‐scale pristine‐graphene/metal‐NPs hybrids is presented, by a combinatorial process of exfoliating expanded graphite in N‐methyl pyrrolidone via sonication and centrifugation to achieve the pristine‐graphene, and attaching pre‐synthesized metal‐NPs on the pristine‐graphene in ethanol via van der Waals interactions between the metal‐NPs and the pristine‐graphene. Nanoparticles of different metals (such as Ag, Au, and Pd) with various morphologies (such as sphere, cube, plate, multi‐angle, and spherical‐particle assembling) can be homogeneously attached on the defect‐free pristine‐graphene with controlled packing densities. Both the pristine‐graphene and the metal‐NPs preserve their original intrinsic structures. The as‐synthesized pristine‐graphene/Ag‐NPs hybrids show very high surface‐enhanced Raman scattering activity due to the combined effects of large surface area of the pristine‐graphene to adsorb more target molecules and the electromagnetic enhancement of the Ag‐NPs. This large‐scale synthesis of the pristine‐graphene/metal‐NPs hybrids with tunable shape and packing density of metal‐NPs opens up opportunities for fundamental research and potential applications ranging from devices to transparent electrodes and conductive composites.     

Towards van der Waals Epitaxial Growth of GaAs on Si using a Graphene Buffer Layer

Van der Waals growth of GaAs on silicon using a two‐dimensional layered material, graphene, as a lattice mismatch/thermal expansion coefficient mismatch relieving buffer layer is presented. Two‐dimensional growth of GaAs thin films on graphene is a potential route towards heteroepitaxial integration of GaAs on silicon in the developing field of silicon photonics. Hetero‐layered GaAs is deposited by molecular beam epitaxy on graphene/silicon at growth temperatures ranging from 350 °C to 600 °C under a constant arsenic flux. Samples are characterized by plan‐view scanning electron microscopy, atomic force microscopy, Raman microscopy, and X‐ray diffraction. The low energy of the graphene surface and the GaAs/graphene interface is overcome through an optimized growth technique to obtain an atomically smooth low­ temperature GaAs nucleation layer. However, the low adsorption and migration energies of gallium and arsenic atoms on graphene result in cluster‐growth mode during crystallization of GaAs films at an elevated temperature. In this paper, we present the first example of an ultrasmooth morphology for GaAs films with a strong (111) oriented fiber‐texture on graphene/silicon using quasi van der Waals epitaxy, making it a remarkable step towards an eventual demonstration of the epitaxial growth of GaAs by this approach for heterogeneous integration.     

On‐the‐Spot Immobilization of Quantum Dots,Graphene Oxide,and Proteins via Hydrophobins

Class I hydrophobin Vmh2, a peculiar surface active and versatile fungal protein, is known to self‐assemble into chemically stable amphiphilic films, to be able to change wettability of surfaces, and to strongly adsorb other proteins. Herein, a fast, highly homogeneous and efficient glass functionalization by spontaneous self‐assembling of Vmh2 at liquid–solid interfaces is achieved (in 2 min). The Vmh2‐coated glass slides are proven to immobilize not only proteins but also nanomaterials such as graphene oxide (GO) and quantum dots (QDs). As models, bovine serum albumin labeled with Alexa 555 fluorophore, anti‐immunoglobulin G antibodies, and cadmium telluride QDs are patterned in a microarray fashion in order to demonstrate functionality, reproducibility, and versatility of the proposed substrate. Additionally, a GO layer is effectively and homogeneously self‐assembled onto the studied functionalized surface. This approach offers a quick and simple alternative to immobilize nanomaterials and proteins, which is appealing for new bioanalytical and nanobioenabled applications.     

Ultrafast Laser Pulses Enable One‐Step Graphene Patterning on Woods and Leaves for Green Electronics

Fast, simple, cost‐efficient, eco‐friendly, and design‐flexible patterning of high‐quality graphene from abundant natural resources is of immense interest for the mass production of next‐generation graphene‐based green electronics. Most electronic components have been manufactured by repetitive photolithography processes involving a large number of masks, photoresists, and toxic etchants; resulting in slow, complex, expensive, less‐flexible, and often corrosive electronics manufacturing processes to date. Here, a one‐step formation and patterning of highly conductive graphene on natural woods and leaves by programmable irradiation of ultrafast high‐photon‐energy laser pulses in ambient air is presented. Direct photoconversion of woods and leaves into graphene is realized at a low temperature by intense ultrafast light pulses with controlled fluences. Green graphene electronic components of electrical interconnects, flexible temperature sensors, and energy‐storing pseudocapacitors are fabricated from woods and leaves. This direct graphene synthesis is a breakthrough toward biocompatible, biodegradable, and eco‐friendlily manufactured green electronics for the sustainable earth.     

Oxygen‐Free Highly Conductive Graphene Papers

Graphene papers have a potential to overcome the gap from nanoscale graphene to real macroscale applications of graphene. A unique process for preparation of highly conductive graphene thin paper by means of Ar⁺ ion irradiation of graphene oxide (GO) papers, with carbon/oxygen ratio reduced to 100:1, is presented. The composition of graphene paper in terms of carbon/oxygen ratio and in terms of types of individual oxygen‐containing groups is monitored throughout the process. Angle‐resolved high resolution X‐ray photoelectron spectroscopy helps to investigate the depth profile of carbon and oxygen within reduced GO paper. C/O ratios over 100 on the surface and 40 in bulk material are observed. In order to bring insight to the processes of oxygen removal from GO paper by low energy Ar⁺ ion bombardment, the gases released during the irradiation are analyzed by mass spectroscopy. It is proven that Ar⁺ ion beam can be applied as a technique for fabrication of highly reduced graphene papers with high conductivities. Such highly conductive graphene papers have great potential to be used in application for construction of microelectronic and sensor devices.     

Ultrafast Direct Ablative Patterning of HOPG by Single Laser Pulses to Produce Graphene Ribbons

Ultrafast, single step and direct patterning of highly oriented pyrolytic graphite (HOPG) is achieved through pulsed laser interference ablation using a near field transmitting phase mask. Periodic arrays of lines are patterned on the HOPG surface over large areas by spatially modulating the laser intensity through the mask. Thus patterned surface serve as a source for multi and few layer graphene ribbons for transferring onto desired substrates using polydimethylsiloxane as transferring agent. The transferred regions are contained with few layer graphene (5–6 layers) ribbons as well as thick graphitic ribbons (30–40 nm), with widths ～1 μm and lengths of several micrometers. Raman, TEM and electrical measurements have confirmed that the transferred ribbons are highly crystalline in nature. Using combinations of shadow and transmitting phase masks, other patterns such as checker boards and diamond‐shaped pits are produced.     

Functional Graphenic Materials Via a Johnson−Claisen Rearrangement

Current research in materials has devoted much attention to graphene, with a considerable amount of the chemical manipulation going through the oxidized state of the material, known as graphene oxide (GO). In this report, the hydroxyl functionalities in GO, the vast majority that must be allylic alcohols, are subjected to Johnson?Claisen rearrangement conditions. In these conditions, a [3, 3] sigmatropic rearrangement after reaction with triethyl orthoacetate gives rise to an ester functional group, attached to the graphitic framework via a robust C?C bond. This variation of the Claisen rearrangement offers an unprecedented versatility of further functionalizations, while maintaining the desirable properties of unfunctionalized graphene. The resultant functional groups were found to withstand reductive treatments for the deoxygenation of graphene sheets and a resumption of electronic conductivity is observed. The ester groups are easily saponified to carboxylic acids in situ with basic conditions, to give water‐soluble graphene. The ester functionality can be further reacted as is, or the carboxylic acid can easily be converted to the more reactive acid chloride. Subsequent amide formation yields up to 1 amide in 15 graphene carbons and increases intergallery spacing up to 12.8 Å, suggesting utility of this material in capacitors and in gas storage. Other functionalization schemes, which include the installation of terminal alkynes and dipolar cycloadditions, allow for the synthesis of a highly positively charged, water‐soluble graphene. The highly negatively and positively charged graphenes (zeta potentials of ?75 mV and +56 mV, respectively), are successfully used to build layer‐by‐layer (LBL) constructs.     

A Decaheme Cytochrome as a Molecular Electron Conduit in Dye‐Sensitized Photoanodes

In nature, charge recombination in light‐harvesting reaction centers is minimized by efficient charge separation. Here, it is aimed to mimic this by coupling dye‐sensitized TiO₂ nanocrystals to a decaheme protein, MtrC from Shewanella oneidensis MR‐1, where the 10 hemes of MtrC form a ≈7‐nm‐long molecular wire between the TiO₂ and the underlying electrode. The system is assembled by forming a densely packed MtrC film on an ultra‐flat gold electrode, followed by the adsorption of approximately 7 nm TiO₂ nanocrystals that are modified with a phosphonated bipyridine Ru(II) dye (RuP). The step‐by‐step construction of the MtrC/TiO₂ system is monitored with (photo)electrochemistry, quartz‐crystal microbalance with dissipation (QCM‐D), and atomic force microscopy (AFM). Photocurrents are dependent on the redox state of the MtrC, confirming that electrons are transferred from the TiO₂ nanocrystals to the surface via the MtrC conduit. In other words, in these TiO₂/MtrC hybrid photodiodes, MtrC traps the conduction‐band electrons from TiO₂ before transferring them to the electrode, creating a photobioelectrochemical system in which a redox protein is used to mimic the efficient charge separation found in biological photosystems.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

Large‐Area Synthesis of Continuous and Uniform MoS2 Monolayer Films on Graphene

Heterostructures composed of multiple layers of different atomically thin materials are of interest due to their unique properties and potential for new device functionality. MoS₂‐graphene heterostructures have shown promise as photodetectors and vertical tunnel transistors. However, progress is limited by the typically micrometer‐scale devices and by the multiple alignments required for fabrication when utilizing mechanically exfoliated material. Here, the synthesis of large‐area, continuous, and uniform MoS₂ monolayers directly on graphene by chemical vapor deposition is reported, resulting in heterostructure samples on the centimeter scale with the possibility for even larger lateral dimensions. Atomic force microscopy, photoluminescence, X‐ray photoelectron, and Raman spectroscopies demonstrate uniform single‐layer growth of stoichiometric MoS₂. The ability to reproducibly generate large‐area heterostructures is highly advantageous for both fundamental investigations and technological applications.     

Development and Simulation of Sulfur‐doped Graphene Supported Platinum with Exemplary Stability and Activity Towards Oxygen Reduction

Sulfur‐doped graphene (SG) is prepared by a thermal shock/quench anneal process and investigated as a unique Pt nanoparticle support (Pt/SG) for the oxygen reduction reaction (ORR). Particularly, SG is found to induce highly favorable catalyst‐support interactions, resulting in excellent half‐cell based ORR activity of 139 mA mg_Pt ^?1 at 0.9 V vs RHE, significant improvements over commercial Pt/C (121 mA mg_Pt ^?1) and Pt‐graphene (Pt/G, 101 mA mg_Pt ^?1). Pt/SG also demonstrates unprecedented stability, maintaining 87% of its electrochemically active surface area following accelerated degradation testing. Furthermore, a majority of ORR activity is maintained, providing 108 mA mg_Pt ^?1, a remarkable 171% improvement over Pt/C (39.8 mA mg_Pt ^?1) and an 89% improvement over Pt/G (57.0 mA mg_Pt ^?1). Computational simulations highlight that the interactions between Pt and graphene are enhanced significantly by sulfur doping, leading to a tethering effect that can explain the outstanding electrochemical stability. Furthermore, sulfur dopants result in a downshift of the platinum d‐band center, explaining the excellent ORR activity and rendering SG as a new and highly promising class of catalyst supports for electrochemical energy technologies such as fuel cells.     

Hyperbranched Polyglycerol‐Grafted Superparamagnetic Iron Oxide Nanoparticles: Synthesis,Characterization, Functionalization,Size Separation,Magnetic Properties,and Biological Applications

For biomedical application of nanoparticles, the surface chemical functionality is very important to impart additional functions, such as solubility and stability in a physiological environment, and targeting specificity as an imaging probe and a drug carrier. Although polyethylene glycol (PEG) has been used extensively, here, it is proposed that hyperbranched polyglycerol (PG) is a good or even better alternative to PEG. Superparamagnetic iron oxide nanoparticles (SPIONs) prepared using a polyol method are directly functionalized with PG through ring‐opening polymerization of glycidol. The resulting SPION‐PG is highly soluble in pure water (>40 mg mL^?1) and in a phosphate buffer solution (>25 mg mL^?1). Such high solubility enables separation of SPION‐PG according to size using size exclusion chromatography (SEC). The size‐separated SPION‐PG shows a gradual increase in transverse relaxivity (r₂) with increasing particle size. For biological application, SPION‐PG is functionalized through multistep organic transformations (–OH → –OTs (tosylate) → –N₃ → –RGD) including click chemistry as a key step to impart targeting specificity by immobilization of cyclic RGD peptide (Arg‐Gly‐Asp‐D ‐Tyr‐Lys) on the surface. The targeting effect is demonstrated by the cell experiments; SPION‐PG‐RGD is taken up by the cells overexpressing α_vβ₃‐integrin such as U87MG and A549.     

Strongly‐Coupled Freestanding Hybrid Films of Graphene and Layered Titanate Nanosheets: An Effective Way to Tailor the Physicochemical and Antibacterial Properties of Graphene Film

An effective way to tailor the physicochemical properties of graphene film is developed by combining colloidal suspensions of reduced graphene oxide (rG‐O) nanosheets and exfoliated layered titanate nanosheets for the fabrication of freestanding hybrid films comprised of stacked and overlapped nanosheets. A flow‐directed filtration of such mixed colloidal suspensions yields freestanding hybrid films comprised of strongly‐coupled rG‐O and titanate nanosheets with tunable chemical composition. This is the first example of highly flexible hybrid films composed of graphene and metal oxide nanosheets. The intimate incorporation of layered titanate nanosheets into the graphene film gives rise not only to an increase of mechanical strength but also to increased surface roughness, chemical stability, and hydrophilicity; thus, the physicochemical properties of the graphene film can be tuned by hybridization with inorganic nanosheets. These freestanding hybrid films of rG‐O‐layered titanate show unprecedentedly high antibacterial property, resulting in the complete sterilization of Escherichia coli O157:H7 (≈100%) in the very short time of 15 min. The antibacterial activity of the hybrid film is far superior to that of the pure graphene film, underscoring the beneficial effect of the layered metal oxide nanosheets in improving the functionality of the graphene film.