Graphene-based Surfaces Research Articles

Hydrogen adsorption and dissociation, together with a multifractal transition into graphene-based surfaces

Ab-initio molecular dynamics simulations, together with multifractal detrended fluctuation analysis (MF-DFA) were used to study hydrogen adsorption into a five-vacancy graphene surface. This surface was chosen because it presents active bonds, due to a lack of carbon ions and may thus be appropriate for molecular hydrogen adsorption and storage. Hydrogen dissociation and chemical hydrogen adsorption, together with a multifractal transition were observed at the beginning of molecular dynamics simulations. Multifractal phase transition is a consequence of hydrogen dissociation and adsorption and was observed in the multifractal spectrum, as a fishtail-like behavior. Once the molecular hydrogen was adsorbed and dissociated, a weak multifractal spectrum was observed in time-series. Weak multifractal behavior was adduced by comparing to fractional Brownian motion (FBM). Multifractal characterization indicates irregular signals and anti-persistence, caused by temperature. A fractional Brownian motion region for forecasting and benchmarking, based on statistical indicators and multifractal characterization of the test region is proposed. The results obtained in this investigation may be significant in the search for surfaces and strategies for hydrogen storage.

In-silico mechanistic analysis of adsorption of Iodinated Contrast Media agents on graphene surface

The study aims at assessing the potential of graphene-based adsorbents to reduce environmental impacts of Iodinated Contrast Media Agents (ICMs). We analyze an extensive collection of ICMs. A modeling approach resting on molecular docking and Density Functional Theory simulations is employed to examine the adsorption process at the molecular level. The study also relies on a Quantitative Structure-Activity Relationship (QSAR) modeling framework to correlate molecular properties with the adsorption energy (Ead) of ICMs, thus enabling identification of the key mechanisms underpinning adsorption and of the key factors contributing to it. A collection of distinct QSAR-based models is developed upon relying on Multiple Linear Regression and a standard genetic algorithm method. Having at our disposal multiple models enables us to take into account the uncertainty associated with model formulation. Maximum Likelihood and formal model identification/discrimination criteria (such as Bayesian and/or information theoretic criteria) are then employed to complement the traditional QSAR modeling phase. This has the advantage of (a) providing a rigorous ranking of the alternative models included in the selected set and (b) quantifying the relative degree of likelihood of each of these models through a weight or posterior probability. The resulting workflow of analysis enables one to seamlessly embed DFT and QSAR studies within a theoretical framework of analysis that explicitly takes into account model and parameter uncertainty. Our results suggest that graphene-based surfaces constitute a promising adsorbent for ICMs removal, π-π stacking being the primary mechanism behind ICM adsorption. Furthermore, our findings offer valuable insights into the potential of graphene-based adsorbent materials for effectively removing ICMs from water systems. They contribute to ascertain the significance of various factors (such as, e.g., the distribution of atomic van der Waals volumes, overall molecular complexity, the presence and arrangement of Iodine atoms, and the presence of polar functional groups) on the adsorption process.

Modulation of Electrokinetic Potentials Using Graphene-Based Surfaces and Variable Substrate Charge Density.

Enhanced electrokinetic phenomena, manifested through the observation of a large streaming potential (Vs), were obtained in microchannels with single-layer graphene (SLG)-coated and few-layer graphene (FLG)-coated surfaces. In comparison to silicon microchannels, the Vs obtained for a given pressure difference along the channel (ΔP) was higher by 75% for the graphene-based channels, with larger values in the SLG case. Computational modeling was used to correlate the surface charge density, tuned through plasma processing, and related zeta potential to measured Vs. The implications related to deploying lower dimensional material surfaces for modulating electrokinetic flows were investigated.

An hydrogen adsorption study on graphene-based surfaces with core–shell type catalysts

An hydrogen adsorption study on graphene-based surfaces with core–shell type catalysts

Different temperature- and pressure-effects on the water-mediated interactions between hydrophobic, hydrophilic, and hydrophobic-hydrophilic nanoscale surfaces.

Water-mediated interactions (WMIs) are responsible for diverse processes in aqueous solutions, including protein folding and nanoparticle aggregation. WMI may be affected by changes in temperature and pressure, and hence, they can alter chemical/physical processes that occur in aqueous environments. Traditionally, attention has been focused on hydrophobic interactions while, in comparison, the role of hydrophilic and hybrid (hydrophobic-hydrophilic) interactions have been mostly overlooked. Here, we study the role of T and P on the WMI between nanoscale (i) hydrophobic-hydrophobic, (ii) hydrophilic-hydrophilic, and (iii) hydrophilic-hydrophobic pairs of (hydroxylated/non-hydroxylated) graphene-based surfaces. We find that hydrophobic, hydrophilic, and hybrid interactions are all sensitive to P. However, while hydrophobic interactions [case (i)] are considerably sensitive to T-variations, hydrophilic [case (ii)] and hybrid interactions [case (iii)] are practically T-independent. An analysis of the entropic and enthalpic contributions to the potential of mean force for cases (i)-(iii) is also presented. Our results are important in understanding T- and P-induced protein denaturation and the interactions of biomolecules in solution, including protein aggregation and phase separation processes. From the computational point of view, the results presented here are relevant in the design of implicit water models for the study of molecular and colloidal/nanoparticle systems at different thermodynamic conditions.

Dimensionally controlled graphene-based surfaces for photothermal membrane crystallization

Membrane-based photothermal crystallization - a pioneering technology for mining valuable minerals from seawater and brines - exploits self-heating nanostructured interfaces to boost water evaporation, so achieving a controlled supersaturation environment that promotes the nucleation and growth of salts.This work explores, for the first time, the use of two-dimensional graphene thin films (2D-G) and three dimensional vertically orientated graphene sheet arrays (3D-G) as potential photothermal membranes applied to the dehydration of sodium chloride, potassium chloride and magnesium sulfate hypersaline solutions, followed by salt crystallization. A systematic study sheds light on the role of vertical alignment of graphene sheets on the interfacial, light absorption and photothermal characteristics of the membrane, impacting on the water evaporation rate and on the crystal size distribution of the investigated salts.Overall, 3D-G facilitates the crystallization of the salts because of superior light-to-heat conversion leading to a 3-fold improvement of the evaporation rate with respect to 2D-G. The exploitation of sunlight graphene-based interfaces is demonstrated as a potential sustainable solution to aqueous wastes valorization via recovery in solid phase of dissolved salts using renewable solar energy.

Magnetic transitions in CO adsorption on graphene-based surfaces

Density functional theory was used to study carbon monoxide adsorption on several graphene-based surfaces. The surfaces were based on oxygen ions adsorbed (and without oxygen adsorption) on double- and triple-vacancy graphene and on a particular N-doped graphene surface. Magnetic transition, the largest adsorption energy (-6.5 eV) and CO reduction, occur in a proposed and more sensitive stationary point of the triple-vacancy graphene surface after CO adsorption. Oxygen adsorption on the former triple-vacancy graphene surface produces magnetic transition, chemisorption (-2.4 eV) forming an ester and CO oxidizes all after CO adsorption. The magnetic transition observed in both systems is correlated with charge transfer. Following the results of molecular dynamics the magnetic order prevails even at room temperature (T=300 K) in the former two systems. The results presented in this work could be useful for the development of magnetic CO sensors.

Graphene-based materials enhance cardiomyogenic and angiogenic differentiation capacity of human mesenchymal stem cells in vitro – Focus on cardiac tissue regeneration

Cell-based therapies have recently emerged as promising strategies for the treatment of cardiovascular disease. Mesenchymal stem cells (MSCs) are a promising cell type that represent a class of adult stem cells characterized by multipotency, high proliferative capacity, paracrine activity, and low immunogenicity. To improve the functional and therapeutic efficacy of MSCs, novel biomaterials are considered as scaffolds/surfaces that promote MSCs growth and differentiation. One of them are graphene-based materials, including graphene oxide (GO) and reduced graphene oxide (rGO). Due to the unique physical, chemical, and biological properties of graphene, scaffolds comprising GO/rGO have been examined as novel platforms to improve the differentiation potential of human MSCs in vitro. We verified different i) size of GO flakes, ii) reduction level, and iii) layer thickness to select the most suitable artificial niche for MSCs culture. The results revealed that graphene-based substrates constitute non-toxic substrates for MSCs. Surfaces with large flakes of GO as well as low reduced rGO are the most biocompatible for MSCs propagation and do not affect their proliferation and survival. Interestingly, small GO flakes and highly reduced rGO decreased MSCs proliferation and induced their apoptosis. We also found that GO and rGO substrates did not alter the MSCs phenotype, cell cycle progression and might modulate the adhesive capabilities of these cells. Importantly, we demonstrated that both materials promoted the cardiomyogenic and angiogenic differentiation capacity of MSCs in vitro. Thus, our data indicates that graphene-based surfaces represent promising materials that may influence the therapeutic application of MSCs via supporting their pro-regenerative potential.

Theoretical analysis of doped graphene as cathode catalyst in Li-O2 and Na-O2 batteries – the impact of the computational scheme

Understanding the reactions in M-O2 cells (M = Li or Na) is of great importance for further advancement of this promising technology. Computational modelling can be helpful along this way, but an adequate approach is needed to model such complex systems. We propose a new scheme for modelling processes in M-O2 cells, where reference energies are obtained from high-level theory, CCSD(T), while the interactions of reaction intermediates with catalyst surfaces are extracted from computationally less expensive DFT. The approach is demonstrated for the case of graphene-based surfaces as model catalysts in Li-O2 and Na-O2 cells using the minimum viable mechanism. B-doped graphene was identified as the best catalyst amongst considered surfaces, while pristine graphene performs poorly. Moreover, we show that the inclusion of dispersion corrections for DFT has a significant impact on calculated discharge and charge potentials and suggests that long-range dispersion interactions should always be considered when graphene-based materials are modelled as electrocatalysts. Finally, we offer general guidelines for designing new ORR catalysts for M-O2 cells in terms of the optimization of the interactions of catalyst surface with reaction intermediates.

Controllable dewetting transition on graphene-based nanotextured surfaces

Controllable wetting state on nanotextured surfaces is of fundamental importance in diverse applications, including self-cleaning, droplet manipulation, and lubrication. The Cassie–Baxter (CB) state, which has attracted broad interests due to the dramatic reduction of solid/liquid contact, can easily break down and the inverse transition (dewetting) is highly challenging. Herein, via molecular dynamics simulation, we show that the regulation of geometrical parameters characterizing the graphene-based surfaces enables one to control the wetting states. Our results demonstrate that decoration of carbon nanopillars on graphene could favor dewetting transition as solid surface fraction increases to a critical value. Furthermore, the ability repelling water for solid surface is highly sensitive to the local density distribution of water droplet along the direction perpendicular to nanopillared graphene. Importantly, the mechanistic pathway of self-recovery of CB state is illustrated in details. Dewetting initiates with the vapor cavity nucleation at the basal surface associated with the bottom water-graphene adhesion force, then the ascend of droplet exhibits strong dependence on the subsequent spontaneous expand of vapor cavity which is necessary to achieve the dewetting transition. Our results provide a rational way to achieve dewetting process, which could guide further design of engineering devices with controllable wetting-related applications.

Altering the reactivity of pristine, N- and P-doped graphene by strain engineering: A DFT view on energy related aspects

For carbon-based materials, in contrast to metal surfaces, a general relationship between strain and reactivity is not yet established, even though there are literature reports on strained graphene. Knowledge of such relationships would be extremely beneficial for understanding the reactivity of graphene-based surfaces and finding optimisation strategies which would make these materials more suitable for targeted applications. Here we investigate the effects of compressive and tensile strain (up to ±5%) on the structure, electronic properties and reactivity of pure, N-doped and P-doped graphene, using DFT calculations. We demonstrate the possibility of tuning the topology of the graphene surface by strain, as well as by the choice of the dopant atom. The reactivity of (doped) strained graphene is probed using H and Na as simple adsorbates of great practical importance. Strain can both enhance and weaken H and Na adsorption on (doped) graphene. In case of Na adsorption, a linear relationship is observed between the Na adsorption energy on P-doped graphene and the phosphorus charge. A linear relationship between the Na adsorption energy on flat graphene surfaces and strain is found. Based on the adsorption energies and electrical conductivity, potentially good candidates for hydrogen storage and sodium-ion battery electrodes are discussed.

Influence of surface hydrophilicity and hydration on the rotational relaxation of supercooled water on graphene oxide surfaces.

Hydration or interfacial water present in biomolecules and inorganic solids has been shown to exhibit a dynamical transition upon supercooling. However, understanding the extent of the underlying surface hydrophilicity as well as the local distribution of hydrophilic/hydrophobic patches on the dynamical transition is unexplored. Here, we use molecular dynamics simulations with a TIP4P/2005 water model to study the translational and rotational relaxation dynamics of interfacial water on graphene surfaces. The purpose of this study is to investigate the influence of both the surface chemistry and the extent of hydration on the rotational transitions of interfacial water on graphene oxide (GO) surfaces in the deeply supercooled region. We have considered three graphene-based surfaces: a GO surface with equal proportions of oxidized and pristine graphene regions in a striped topology, a fully oxidized surface and a pristine graphene surface. The dipole relaxation time of interfacial water (high hydration) shows a strong-to-strong transition, strong nature, and strong-to-strong transition on these surfaces, respectively, in the temperature range of 210-298 K. In contrast, bulk water shows a fragile-to-strong rotational transition upon supercooling. In all these cases at high hydration, interfacial water co-exists with a thick water film with bulk-like properties. To investigate the influence of bulk water on dynamical transitions, we simulated a low hydration regime where only bound water (surface water) is present on the GO surfaces and found that the rotational relaxation of surface water on both the GO and fully oxidized surfaces shows a single Arrhenius temperature dependence. Bulk water is found to have a greater influence on the rotational relaxation in the presence of a hydrophobic surface and the dipole angular distributions show distinct differences on the surfaces upon supercooling. Our results indicate that not only does the local extent of surface hydrophilicity play a role in determining the energy landscape explored by the water molecules upon supercooling, but the presence of bulk water also modulates the dynamic transition.

Antimicrobial mechanism of reduced graphene oxide-copper oxide (rGO-CuO) nanocomposite films: The case of Pseudomonas aeruginosa PAO1.

The antimicrobial properties of two-dimensional materials such as graphene-based surfaces are vital for environmental and biomedical applications. Here, the improvement of the antibacterial property of reduced graphene oxide by the preparation of rGO-CuO nanocomposite films was reported. The rGO-CuO nanocomposites were synthesized via a simple hydrothermal method, and the nanocomposite films were fabricated by filtering through a polytetrafluoroethylene (PTFE) filter with the assistance of a vacuum filtration unit. After characterization of the nanocomposite films, the antibacterial properties were tested against Pseudomonas aeruginosa PAO1. The fabricated rGO-CuO nanocomposite films exhibited excellent antibacterial activity, leading to complete bacterial inactivation upon contact. The antibacterial properties were closely linked to the reactive oxygen species (ROS) independent pathway rather than the ROS-dependent pathway. This work provides an insight into the antibacterial mechanisms of reduced GO and copper oxide composite film for water treatment systems and the potential application of these nanocomposites in biomedicine.

Impact of Graphene-Based Surfaces on the Basic Biological Properties of Human Umbilical Cord Mesenchymal Stem Cells: Implications for Ex Vivo Cell Expansion Aimed at Tissue Repair.

The potential therapeutic applications of mesenchymal stem/stromal cells (MSCs) and biomaterials have attracted a great amount of interest in the field of biomedical engineering. MSCs are multipotent adult stem cells characterized as cells with specific features, e.g., high differentiation potential, low immunogenicity, immunomodulatory properties, and efficient in vitro expansion ability. Human umbilical cord Wharton’s jelly-derived MSCs (hUC-MSCs) are a new, important cell type that may be used for therapeutic purposes, i.e., for autologous and allogeneic transplantations. To improve the therapeutic efficiency of hUC-MSCs, novel biomaterials have been considered for use as scaffolds dedicated to the propagation and differentiation of these cells. Nowadays, some of the most promising materials for tissue engineering include graphene and its derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO). Due to their physicochemical properties, they can be easily modified with biomolecules, which enable their interaction with different types of cells, including MSCs. In this study, we demonstrate the impact of graphene-based substrates (GO, rGO) on the biological properties of hUC-MSCs. The size of the GO flakes and the reduction level of GO have been considered as important factors determining the most favorable surface for hUC-MSCs growth. The obtained results revealed that GO and rGO are suitable scaffolds for hUC-MSCs. hUC-MSCs cultured on: (i) a thin layer of GO and (ii) an rGO surface with a low reduction level demonstrated a viability and proliferation rate comparable to those estimated under standard culture conditions. Interestingly, cell culture on a highly reduced GO substrate resulted in a decreased hUC-MSCs proliferation rate and induced cell apoptosis. Moreover, our analysis demonstrated that hUC-MSCs cultured on all the tested GO and rGO scaffolds showed no alterations of their typical mesenchymal phenotype, regardless of the reduction level and size of the GO flakes. Thus, GO scaffolds and rGO scaffolds with a low reduction level exhibit potential applicability as novel, safe, and biocompatible materials for utilization in regenerative medicine.

Implications of Chemical Reduction Using Hydriodic Acid on the Antimicrobial Properties of Graphene Oxide and Reduced Graphene Oxide Membranes.

The antimicrobial properties of graphene-based membranes such as single-layer graphene oxide (GO) and modified graphene oxide (rGO) on top of cellulose ester membrane are reported in this study. rGO membranes are made from GO by hydriodic acid (HI) vapor treatment. The antibacterial properties are tested after 3 h contact time with selected model bacteria. Complete bacterial cell inactivation is found only after contact with rGO membranes, while no significant bacterial inactivation is found for the control i) GO membrane, ii) the mixed cellulose ester support, and the iii) rGO membrane after additional washing that removes the remaining HI. This indicates that the antimicrobial effect is neither caused by the graphene nor the membrane support. The antimicrobial effect is found to be conclusively linked to the HI eliminating microbial growth, at concentrations from 0.005%. These findings emphasize the importance of caution in the reporting of antimicrobial properties of graphene-based surfaces.

Surface morphology-dependent spontaneous bacterial behaviors on graphene oxide membranes

The need for biofouling control in membrane-based water treatment has prompted the use of novel materials for membrane fabrication. Recently, engineered graphene oxide (GO) membranes produced by layer-by-layer stacking of GO nanosheets have been considered to be an economical and easy fabrication method with potential for biofouling control. However, contradictory evidence exists on the interaction between graphene-based surfaces and microorganisms in the literature. To clarify to some extent these discrepancies, the role of GO layer morphology in microbial behaviors on membrane surfaces was investigated by using Pseudomonas aeruginosa PAO1 as a model bacterium. As a result, we found a strong adhesion of P. aeruginosa PAO1 on GO membranes at an elevated GO thickness (>1.71429 mg/cm2). This behavior is attributed to changes in the surface morphology, which helps harbor and trap bacterial cells. Our results reveal the significance of GO layer morphology in bacterial behaviors on membrane surfaces. Those findings suggests that GO layers with considerable roughness and curvature may serve as an absorbent for bacterial cells and potentially reduce biofouling in membrane-based water and wastewater treatment systems when used in the pre-treatment unit.

(Invited) On the Active Sites in the Graphene-Catalyzed Oxygen Reduction Reaction

A critical issue in the impressive worldwide fuel cell research efforts is the possibility of using graphene-based materials in the oxygen reduction reaction (ORR). In the overwhelming literature that has accumulated, much controversy (and even more confusion) exists regarding some of the essential aspects of the reaction mechanism. A consensus can emerge only if interpretations of the abundant experimental evidence are consistent with what is known about the behavior of graphene-based materials, both the novel and the traditional ones, as catalysts in the absence of an electric field, as well as with the well understood gas-phase behavior of sp2-hybridized carbon materials in the presence of dioxygen. From this standpoint, the key issue is whether a reasonable and instructive analogy exists between the mechanism of electron-transfer catalysis on the carbon surface, e- + C(O2) = C(O2 -) C(O2 -) + H2O(aq) = OH-(aq) + C(OOH) C(OOH) = C=O + OH, and the proposed mechanism of electrocatalysis: Ar-C: + O2 → Ar-C(O2) Ar-C(O2) + H+(aq) → Ar-C(OOH) Ar-C(OOH) → Ar-C + HO2 - A clarification can be sought by analyzing the results of the search for ever more effective (electro)catalysts. In this context, structural modifications of the graphene surface are particularly enlightening. Thus, for example, the superior electrocatalytic activity of nitrogen-doped graphene is attributed to the presence of either quaternary or pyridinic nitrogen. Boron is of interest as well, if for nothing else then for its contribution of one electron less than the carbon atoms to graphene’s pi-system. And the presence, or formation, of an embedded MN4 moiety – where M is most often a transition metal such as cobalt or iron – is also of great interest because of the obvious analogy with porphyrin or phthalocyanine structures. Using a heretofore unexplored approach to examine these widely varying graphene-based surfaces, and unifying the evidence based on density functional theory, a compelling case is presented here for the experimentally elusive identification of the active sites (AS) that are responsible for the technologically and economically crucial electron transfer at the fuel cell cathode: AS + O2 + 4e-(aq) + 4H+(aq) = AS + 2H2O. It is also argued, not unexpectedly, that these are essentially the same sites at which analogous oxygen transfer processes - carbon oxidation/combustion, graphene functionalization and oxygen spillover - also occur. Their thermodynamic driving force is shown to be crucially dependent on the details of the electron density distribution and spin pairing processes at the active sites and in their vicinity.

Microarrays with a pillared patterned double-layer graphene substrate: A molecular dynamics simulation approach

Microarray technology plays an important role in detecting the sequence of the genome. In the production of microarrays, the formation of a homogeneous and uniform spot due to biodroplet pining is of great importance. In this work, using the molecular dynamics simulation method, it is possible to create homogeneous spots on the surface of graphene-based surfaces. Hence, the behavior of five different types of DNA including homogeneous sequences of adenine, cytosine, guanine, and thymine as well as heterogeneous sequence of bases in the water droplets, was subjected to a double-layer graphene substrate with a pillar structure. The results showed that double layer graphene with pillar pattern are successful in pinning of biodroplet and this substrate leads to the differnet placement of Oligonucleotide probes. Also, the results of wettability demonstrated that the formation of hydrogen bond between a single strand and water molecules reduces the spreading and increases the contact angle and height of biodroplet containing oligonucleotides with respect to the water droplet without oligonucleotides on the surface of the double layer graphene in pillar pattern. The results of the simulation of single-stranded DNA after equilibration showed that single strand of adenine and heterogeneous single strands had the least folding compared to other single strands, and the single strand of adenine produced the most stable structure. Hence, single-stranded adenine is stretched more compared to other single strands, which is due to the lack of tendency of this single strand for internal hydrogen bonds and regular spiral structure resulting from a strong internal π-π stacking interaction. As a result, single strand of adenine is more accessible to detect target single strands for use in microarrays.

A Facile Approach to Achieve Tunable Wettablility of Graphene-based Films from Superhydrophilicty to Superhydrophobicity

A facile, safe and low cost thermal approach was developed to regulate wettability of graphene-based surfaces from superhydrophilicity to superhydrophobicity, which promises mass production on surface technology in various industrial applications. The long-lasting stable superhydrophobicity was attained by hydrogenating graphene with chem-absorbed functional groups. Meanwhile, the approach paves the way for producing fully hydrogenated graphene, which is an entirely novel material called graphene.

Interfacial stability of graphene-based surfaces in water and organic solvents

The mass production of graphene and graphene oxide (GO) is essential for its use in commercial products. To improve its processing in the solution, dispersion behavior of graphene-based materials and their colloidal stability must be further understood. This study used all-atom molecular dynamics simulations to understand how electrostatics, van der Waals interactions, and hydrogen bonding affect the exfoliation and stability of three-layered graphene as a function of oxidation and solvent. Water, methanol, and ethanol were chosen as solvents due to their various dispersion behaviors. Our study indicated that (1) both surface oxidation level and solvent type can heavily influence the stability and (2) a decrease in interlayer vdW interactions, an increase in GO–solvent electrostatic interactions, and an increase in GO–solvent hydrogen bonding are important factors that can facilitate the dissolution of GO.