Functionalization of graphenic surfaces by oxygen plasma toward enhanced wettability and cell adhesion: experiments corroborated by molecular modelling

Graphenic materials attract huge attention because of their outstanding properties, and have a wide range of applications as, i.e., components of biomaterials. Due to their hydrophobic nature, however, the surfaces need to be functionalized to improve wettability and biocompatibility. In this study, we investigate the functionalization of graphenic surfaces by oxygen plasma treatment, introducing surface functional groups in a controlled way. The AFM images and LDI-MS results clearly show that the graphenic surface exposed to plasma is decorated with -OH groups, whereas the surface topography remains intact. The measured water contact angle decreases significantly after oxygen plasma treatment from 99° to ca. 5°, making the surface hydrophilic. It is also reflected in the surface free energy values which increase from 48.18 mJ m^-2 to 74.53 mJ m^-2 when the number of surface oxygen groups reaches 4 -OH/84 Ų. The DFT (VASP) molecular models of unmodified and oxygen-functionalized graphenic surfaces were constructed and used for molecular interpretation of water-graphenic surface interactions. The computational models were validated by comparison of the theoretically determined water contact angle (based on the Young-Dupré equation) to the experimentally determined values. Additionally, the VASPsol (implicit water environment) results were calibrated against the explicit water models that can be used in further research. Finally, the biological role of functional groups on the graphenic surface was examined in terms of cell adhesion with the use of mouse fibroblast cell line (NIH/3T3). The obtained results illustrate the correlation between surface oxygen groups, wettability, and biocompatibility providing the guidelines for the molecular level-driven design of carbon materials for various applications.

From crystal phase mixture to pure metal-organic frameworks - Tuning pore and structure properties

In this study, a sonochemical route for the preparation of a new Hf-MIL-140A metal-organic framework from a mixture of UiO-66/MIL-140A is presented. The sonochemical synthesis route not only allows the phase-pure MIL-140A structure to be obtained but also induces structural defects in the MIL-140A structure. The synergic effect between the sonochemical irradiation and the presence of a highly acidic environment results in the generation of slit-like defects in the crystal structure, which increases specific surface area and pore volume. The BET-specific surface area in the case of sonochemically derived Zr-MIL-140A reaches 653.3 m2/g, which is 1.5 times higher than that obtained during conventional synthesis. The developed Hf-MIL-140A structure is isostructural to Zr-MIL-140A, which was confirmed by synchrotron X-ray powder diffraction (SR-XRD) and by continuous rotation electron diffraction (cRED) analysis. The obtained MOF materials have high thermal and chemical stability, which makes them promising candidates for applications such as gas adsorption, radioactive waste removal, catalysis, and drug delivery.

Zirconium-Based Metal-Organic Frameworks as Acriflavine Cargos in the Battle against Coronaviruses─A Theoretical and Experimental Approach

In this study, we present a complementary approach for obtaining an effective drug, based on acriflavine (ACF) and zirconium-based metal-organic frameworks (MOFs), against SARS-CoV-2. The experimental results showed that acriflavine inhibits the interaction between viral receptor-binding domain (RBD) of spike protein and angiotensin converting enzyme-2 (ACE2) host receptor driving viral cell entry. The prepared ACF@MOF composites exhibited low (MOF-808 and UiO-66) and high (UiO-67 and NU-1000) ACF loadings. The drug release profiles from prepared composites showed different release kinetics depending on the local pore environment. The long-term ACF release with the effective antiviral ACF concentration was observed for all studied ACF@MOF composites. The density functional theory (DFT) calculations allowed us to determine that π-π stacking together with electrostatic interaction plays an important role in acriflavine adsorption and release from ACF@MOF composites. The molecular docking results have shown that acriflavine interacts with several possible binding sites within the RBD and binding site at the RBD/ACE2 interface. The cytotoxicity and ecotoxicity results have confirmed that the prepared ACF@MOF composites may be considered potentially safe for living organisms. The complementary experimental and theoretical results presented in this study have confirmed that the ACF@MOF composites may be considered a potential candidate for the COVID-19 treatment, which makes them good candidates for clinical trials.

Covalently bonded surface functional groups on carbon nanotubes: from molecular modeling to practical applications

The aim of this work was to investigate how chemical functionalization affects the electronic properties of multi-walled carbon nanotubes, altering the electrophoretic deposition process: a method of choice for the fabrication of high quality, all-carbon nanotube (CNT) layers. Wet chemistry methods were applied to modify the surfaces of CNTs by insertion of various oxygen- and nitrogen-containing groups. Transmission electron microscopy revealed no significant changes in the material morphology, while X-ray photoelectron spectroscopy and Raman spectroscopy showed that changes in the chemical composition did not translate to the changes in the structure. Molecularly modelled optimized surface functional group geometries and electron density distributions allowed the calculation of the dipole moments (-COOH = 0.77; -OH = 1.65; -CON(CH₃CH₂)₂ = 3.33; -CONH₂ = 2.00; -NH₂ = 0.78). Due to their polarity, the introduction of surface functional groups resulted in significant modifications of the electronic properties of CNTs, as elucidated by work function measurements via the Kelvin method and ultraviolet photoelectron spectroscopy. The work function changed from 4.6 eV (raw CNTs) to 4.94 eV for the -OH functionalized CNTs and 4.3 eV for the CNTs functionalized with -CON(CH₃CH₂), and was inversely proportional to the dipole moment values. Finally, using CNT dispersions, electrophoretic deposition was conducted, allowing the correlation of the work function of CNTs and the measured electrophoretic current with the impact on the deposits' qualities. Thus, a rational background for the development of carbon-based biomaterials was provided.

Structural Studies of Aluminated form of Zeolites-EXAFS and XRD Experiment, STEM Micrography, and DFT Modelling

In this article, the results of computational structural studies on Al-containing zeolites, via periodic DFT + D modelling and FDM (Finite Difference Method) to solve the Schrödinger equation (FDMNES) for XAS simulations, corroborated by EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy and PXRD (powder X-ray diffractometry), are presented. The applicability of Radial Distribution Function (RDF) to screen out the postulated zeolite structure is also discussed. The structural conclusions are further verified by HR-TEM imaging.

Multiscale exploration of hydrocarbon adsorption and hopping through ZSM-5 channels - from Monte Carlo modelling to experiment

In this article the results of statistical MC modelling corroborated by the FT-IR spectroscopy and gravimetric adsorption studies of low aliphatic hydrocarbons in ZSM-5 (Si/Al = 28 or silicalite) are presented. The extension of the existing Dubbeldam's forcefield towards inclusion of the finite aluminium-containing zeolites is proposed and its applicability is verified experimentally for the sorption of linear and branched hydrocarbons. The FT-IR spectroscopy applicability to follow the kinetics of small hydrocarbon adsorption has been successfully verified by the application of the Crank solution for diffusion to spectroscopy derived results.

Towards the understanding of the gold interaction with AIII-BV semiconductors at the atomic level

AIII-BV semiconductors have been considered to be a promising material for decades in overcoming the limitations of silicon semiconductor devices. One of the important aspects within the AIII-BV semiconductor technology is gold-semiconductor interactions on the nanoscale. We report on the investigations into the basic chemical interactions of Au atoms with AIII-BV semiconductor crystals by the investigation of the nanostructure formation in the process of thermally-induced Au self-assembly on various AIII-BV surfaces by means of atomically resolved High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) measurements. We have found that the formation of nanostructures is a consequence of the surface diffusion and nucleation of adatoms produced by Au induced chemical reactions on AIII-BV semiconductor surfaces. Only for InSb crystals we have found that there is efficient diffusion of Au atoms into the bulk, which we experimentally studied by Machine Learning HAADF STEM image quantification and theoretically by Density Functional Theory (DFT) calculations with the inclusion of finite temperature effects. Furthermore, the effective number of Au atoms needed to release one AIII metal atom has been estimated. The experimental finding reveals a difference in the Au interactions with the In- and Ga-based groups of AIII-BV semiconductors. Our comprehensive and systematic studies uncover the details of the Au interactions with the AIII-BV surface at the atomic level with chemical sensitivity and shed new light on the fundamental Au/AIII-BV interactions at the atomic scale.

Attaching titania clusters of various size to reduced graphene oxide and its impact on the conceivable photocatalytic behavior of the junctions-a DFT/D + U and TD DFTB modeling

DFT/D  +  U and density functional based tight binding (DFTB) molecular modeling was used to investigate the role of the structural, electronic and optical properties of reduced graphene oxide surface (r-GO), hybridized with hydrated TiO₂ moieties of various size, ranging from small molecular Ti₂O₄ clusters into extended Ti₄₃O₈₆ rutile type nanocrystals of ~5 nm diameter. The calculated adhesion energies, varying from  -5.048 eV (r-GO|Ti₂O₄), -12.159 eV (r-GO|Ti₅O₁₀), -18.499 eV (r-GO|Ti₁₅O₃₀) to  -42.484 eV (r-GO|Ti₄₃O₈₆), indicate high stability of these composites. It was shown that electronic interactions at the r-GO|(1 1 0)TiO₂ interface give rise to net charge flow from the r-GO substrate towards the TiO₂ moieties, analyzed in terms of the partial charge density 3D plots and an interfacial dipole moment formation. The DOS structure of the composites was calculated by means of the time dependent DFTB approach, and the position and composition of the VB and CB edges, along with the presence of weak mid-gap 2p  C states originating from the intact graphene-like patches in the r-GO substrate were discussed in detail in the context of conceivable photocatalytic activity of the composites. The constructed band alignment diagram implies formation of the staggered type II scheme, with the electric field offset that is sensitive to the titania cluster size. In the case of the nano-reticular TiO₂, where only a fraction of the Ti atoms is engaged in the Ti-O-C linkers formation, recombination of the photogenerated charges is inhibited owing to favorable spatial separation effect. For small molecular TiO₂ clusters with all Ti cations anchored to the r-GO layer fast cross-relaxation quenches the beneficial interfacial charge separation effect, since the strong hybridization of the oxygen and carbon states provides a convenient pathway for the efficient electronic coupling between the CB edge states of r-GO and the VB edge states of the TiO₂ moieties. A phenomenological model of the molecular r-GO|Ti₂O₄ and the reticular r-GO|Ti₄₃O₈₆ composites was constructed in account for different photocatalytic behavior of both junctions.

Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H₂ gas, as well as the formation of carbidic and graphitic surface carbon.

A comparative computational study on hydrogen adsorption on the Ag(+), Cu(+), Mg(2+), Cd(2+), and Zn(2+) cationic sites in zeolites

In this article the interaction between H2 and Ag(+), Cu(+), Mg(2+), Cd(2+), and Zn(2+) cations in cluster models of several sizes has been studied computationally. Depending on the changes imposed by the adsorption process on the H2 molecule the activation can vary in a wide range - from only slight weakening of the H-H bond to complete dissociation of the H2 molecule. The NOCV (Natural Orbitals for Chemical Valence) analysis allowed for decomposition of the electron density distortion into contributions easier for interpretation. Three essential factors have been identified (i-iii). In the case of bare cations the main contribution is a donation from σH2 to the cation (i). When a zeolite framework surrounding the cation is introduced, it hinders σ-donation and enhances π-backdonation from the cation to the antibonding orbital of the molecule (ii). For Cu(i) and Ag(i) sites π-backdonation becomes dominant, while for Mg(ii), Cd(ii), and Zn(ii) cations, the σ-donation, albeit diminished, still remains a dominant contribution. Calculations showed that the localization and coordination of Zn(ii) have crucial influence on its interaction with H2. We identified a Zn(2+) position at which the H2 molecule dissociates - here the interaction between H2 and oxygen framework (iii) plays a crucial role. Based on the calculations the mechanism of H2 transformation has been proposed. Upon heterolytic dissociation of H2 the Zn(0) moiety and two OH groups can be formed. Eventually, in two elementary steps, the H2 molecule can be restored. In this case, the ability of the site to activate/dissociate hydrogen is caused by the low coordination number of the zinc cation and the geometry of the site which allows positively charged H2 to interact with framework oxygen what enhances the formation of OH and Z-O-(ZnH)(+) groups.

Spin-resolved NOCV analysis of the zeolite framework influence on the interaction of NO with Cu(i/ii) sites in zeolites

In the present work the function of a zeolite framework in modifying the properties of copper sites interacting with NO has been studied.

On-surface polymerization on a semiconducting oxide: aryl halide coupling controlled by surface hydroxyl groups on rutile TiO2(011)

The covalent coupling of aryl halide monomers on the rutile TiO₂(011)-(2 × 1) surface requires the presence of surface hydroxyl groups and is controlled by their density.

Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.