Fabrication of Hybrid Materials from Titanium Dioxide and Natural Phenols for Efficient Radical Scavenging against Oxidative Stress

Oxidative stress caused by free radicals is one of the great threats to inflict intracellular damage. Here, we report a convenient approach to the synthesis, characterization, and evaluation of the radical activity of titanium-based composites. We have investigated the potential of natural antioxidants (curcumin, quercetin, catechin, and vitamin E) as radical scavengers and stabilizers. The titanium oxide composites were prepared via three steps including sol-gel synthesis, carboxylation, and esterification. The characterization of the titanium-phenol composites was carried out by FTIR, PXRD, UV-vis and SEM methods. The radical scavenging ability of the novel materials was evaluated using DPPH and an in vitro LPO assay using isolated rat liver mitochondria. The novel materials exhibit both a higher stability and an antioxidant activity in comparison to bare TiO₂. It was found that curcumin and quercetin based composites show the highest antioxidant efficiency among the composites under study followed by catechin and vitamin E based materials. The results from an MTT assay carried out on the Caco-2 cell line indicate that the composites do not contribute to the cytotoxicity in vitro. This study demonstrates that a combination of powerful antioxidants with titanium dioxide can change its functional properties and provide a convenient strategy against oxidative stress.

A photochemical approach for a fast and self-limited covalent modification of surface supported graphene with photoactive dyes

Herein, we report a simple method for a covalent modification of surface supported graphene with photoactive dyes. Graphene was fabricated on cubic-SiC/Si(001) wafers due to their low cost and suitability for mass-production of continuous graphene fit for electronic applications on millimetre scale. Functionalisation of the graphene surface was carried out in solution via white light induced photochemical generation of phenazine radicals from phenazine diazonium salt. The resulting covalently bonded phenazine-graphene hybrid structure was characterised by scanning tunnelling microscopy (STM) and spectroscopy (STS), Raman spectroscopy and density functional theory (DFT) calculations. It was found that phenazine molecules form an overlayer, which exhibit a short range order with a rectangular unit cell on the graphene surface. DFT calculations based on STM results reveal that molecules are standing up in the overlayer with the maximum coverage of 0.25 molecules per graphene unit cell. Raman spectroscopy and STM results show that the growth is limited to one monolayer of standing molecules. STS reveals that the phenazine-graphene hybrid structure has a band gap of 0.8 eV.

Nanoclusters and nanolines: the effect of molybdenum oxide substrate stoichiometry on iron self-assembly

The growth of Fe nanostructures on the stoichiometric MoO₂/Mo(110) and oxygen-rich MoO_2+x /Mo(110) surfaces has been studied using low-temperature scanning tunnelling microscopy (STM) and density functional theory calculations. STM results indicate that at low coverage Fe nucleates on the MoO₂/Mo(110) surface, forming small, well-ordered nanoclusters of uniform size, each consisting of five Fe atoms. These five-atom clusters can agglomerate into larger nanostructures reflecting the substrate geometry, but they retain their individual character within the structure. Linear Fe nanocluster arrays are formed on the MoO₂/Mo(110) surface at room temperature when the surface coverage is greater than 0.6 monolayers. These nanocluster arrays follow the direction of the oxide rows of the strained MoO₂/Mo(110) surface. Slightly altering the preparation procedure of MoO₂/Mo(110) leads to the presence of oxygen adatoms on this surface. Fe deposition onto the oxygen-rich MoO_2+x /Mo(110) surface results in elongated nanostructures that reach up to 24 nm in length. These nanolines have a zigzag shape and are likely composed of partially oxidised Fe formed upon reaction with the oxygen-rich surface.

White light induced covalent modification of graphene using a phenazine dye

A photochemical method to covalently modify graphene with a dye was developed. The hybrid material has a band-gap of 1.95 eV and emits light at 591 nm.

Transport Gap Opening and High On-Off Current Ratio in Trilayer Graphene with Self-Aligned Nanodomain Boundaries

Trilayer graphene exhibits exceptional electronic properties that are of interest both for fundamental science and for technological applications. The ability to achieve a high on-off current ratio is the central question in this field. Here, we propose a simple method to achieve a current on-off ratio of 10(4) by opening a transport gap in Bernal-stacked trilayer graphene. We synthesized Bernal-stacked trilayer graphene with self-aligned periodic nanodomain boundaries (NBs) on the technologically relevant vicinal cubic-SiC(001) substrate and performed electrical measurements. Our low-temperature transport measurements clearly demonstrate that the self-aligned periodic NBs can induce a charge transport gap greater than 1.3 eV. More remarkably, the transport gap of ∼0.4 eV persists even at 100 K. Our results show the feasibility of creating new electronic nanostructures with high on-off current ratios using graphene on cubic-SiC.

Homolytic cleavage of molecular oxygen by manganese porphyrins supported on Ag(111)

Oxygen binding and cleavage are important for both molecular recognition and catalysis. Mn-based porphyrins in particular are used as catalysts for the epoxidation of alkenes, and in this study the homolytic cleavage of O2 by a surface-supported monolayer of Mn porphyrins on Ag(111) is demonstrated by scanning tunneling microscopy, X-ray absorption, and X-ray photoemission. As deposited, {5,10,15,20-tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) adopts a saddle conformation with the average plane of its macrocycle parallel to the substrate and the axial Cl ligand pointing upward, away from the substrate. The adsorption of MnClTPP on Ag(111) is accompanied by a reduction of the Mn oxidation state from Mn(III) to Mn(II) due to charge transfer between the substrate and the molecule. Annealing the Mn(II)ClTPP monolayer up to 510 K causes the chlorine ligands to desorb from the porphyrins while leaving the monolayer intact. The Mn(II)TPP is stabilized by the surface acting as an axial ligand for the metal center. Exposure of the Mn(II)TPP/Ag(111) system to molecular oxygen results in the dissociation of O2 and forms pairs of Mn(III)OTPP molecules on the surface. Annealing at 445 K reduces the Mn(III)OTPP complex back to Mn(II)TPP/Ag(111). The activation energies for Cl and O removal were found to be 0.35 ± 0.02 eV and 0.26 ± 0.03 eV, respectively.

Rotated domain network in graphene on cubic-SiC(001)

The atomic structure of the cubic-SiC(001) surface during ultra-high vacuum graphene synthesis has been studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. Atomically resolved STM studies prove the synthesis of a uniform, millimeter-scale graphene overlayer consisting of nanodomains rotated by ±13.5° relative to the left angle bracket 110 right angle bracket-directed boundaries. The preferential directions of the domain boundaries coincide with the directions of carbon atomic chains on the SiC(001)-c(2 × 2) reconstruction, fabricated prior to graphene synthesis. The presented data show the correlation between the atomic structures of the SiC(001)-c(2 × 2) surface and the graphene/SiC(001) rotated domain network and pave the way for optimizing large-area graphene synthesis on low-cost cubic-SiC(001)/Si(001) wafers.

Atomically resolved STM imaging with a diamond tip: simulation and experiment

The spatial resolution of a scanning tunneling microscope (STM) can be enhanced using light element-terminated probes with spatially localized electron orbitals at the apex atom. Conductive diamond probes can provide carbon atomic orbitals suitable for STM imaging with sub-Ångström lateral resolution and high apex stability crucial for the small tunneling gaps necessary for high-resolution experiments. Here we demonstrate that high spatial resolution can be achieved in STM experiments with single-crystal diamond tips, which are generally only considered for use as probes for atomic force microscopy. The results of STM experiments with a heavily boron-doped, diamond probe on a graphite surface; density functional theory calculations of the tip and surface electronic structure; and first-principles tunneling current calculations demonstrate that the highest spatial resolution can be achieved with diamond tips at tip-sample distances of 3-5 Å when frontier p-orbitals of the tip provide their maximum contribution to the tunneling current. At the same time, atomic resolution is feasible even at extremely small gaps with very high noise in the tunneling current.

Homologous size-extension of hybrid vanadate capsules - solid state structures, solution stability and surface deposition

The dimensions and cavity sizes of the molecular capsules with the general formula [V10O18L4](10-) can be controlled modularly through the nature of the bifunctional, rigid organophosphonate ligands L(1) and L(2) (L(1) = bis(4-phosphonatophenyl)ethyne and L(2) = bis(4-phosphonatophenyl)butadiyne); the solution stability of the molecular entities as demonstrated by ESI-MS studies permits their assembly on the Au(111) surface on a sub-monolayer scale giving rise to a 2D supramolecular structure that is comparable to the packing arrangements of the capsules in the crystal structures.

Correlation between charge-transfer and rotation of C60 on WO2/W(110)

Understanding molecular switching between different charge states is crucial to further progress in molecule-based nano-electronic devices. Herein we have employed scanning tunnelling microscopy to visualize different charge states of a single C60 molecule within a molecular layer grown on the WO2/W(110) surface. The results obtained demonstrate that individual C60 molecules within the layer switch between neutral and negatively charged states in the temperature range of 220-260 K over the time scale of the experiment. The charging of the C60 causes changes in the local density of electron states and consequently a variation in tunnelling current. Using density functional theory calculations, it was found that the charged state corresponds to the negatively charged C60(-), which has accepted an electron. The switching of the molecule into the charged state is triggered continuously by tunnelling electrons when the STM tip is static above an individual C60 molecule with a bias applied. Molecular movement accompanies the molecule's switching between these states.

Control of the axial coordination of a surface-confined manganese (III) porphyrin complex

The organization and thermal lability of chloro(5,10,15,20-tetraphenyl porphyrinato)manganese(III) (Cl-MnTPP) molecules on the Ag(111) surface have been investigated under ultra-high vacuum conditions, using scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy. The findings reveal the epitaxial nature of the molecule-substrate interface, and moreover, offer a valuable insight into the latent coordination properties of surface-confined metalloporphyrins. The Cl-MnTPP molecules are found to self-assemble on the Ag(111) surface at room temperature, forming an ordered molecular overlayer described by a square unit cell. In accordance with the threefold symmetry of the Ag(111) surface, three rotationally equivalent domains of the molecular overlayer are observed. The primitive lattice vectors of the Cl-MnTPP overlayer show an azimuthal rotation of ±15° relative to those of the Ag(111) surface, while the principal molecular axes of the individual molecules are found to be aligned with the substrate (0(-)11) and ((-)211) crystallographic directions. The axial chloride (Cl) ligand is found to be orientated away from the Ag(111) surface, whereby the average plane of the porphyrin macrocycle lies parallel to that of the substrate. When adsorbed on the Ag(111) surface, the Cl-MnTPP molecules display a latent thermal lability resulting in the dissociation of the axial Cl ligand at ~423 K. The thermally induced dissociation of the Cl ligand leaves the porphyrin complex otherwise intact, giving rise to the coordinatively unsaturated Mn(III) derivative. Consistent with the surface conformation of the Cl-MnTPP precursor, the resulting (5,10,15,20-tetraphenyl porphyrinato)manganese(III) (MnTPP) molecules display the same lattice structure and registry with the Ag(111) surface.

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule-substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.

Self-limited growth of triangular PtO2 nanoclusters on the Pt(111) surface

The high temperature oxidation of the Pt(111) surface with molecular oxygen has been studied using scanning tunnelling microscopy and low energy electron diffraction (LEED). Results indicate a self-limited growth of well-ordered PtO2 nanoclusters which have an O-Pt-O trilayer structure. Each nanocluster has a triangular shape and nucleates at the Pt(111) surface step edge due to the mobility of Pt atoms. The triangular PtO2 nanoislands on the upper and lower Pt(111) terraces represent two mirror domains with the mirror plane perpendicular to the substrate and aligned along the [Formula: see text] direction of the latter. LEED data obtained from the nanostructured PtO2/Pt(111) surface show a characteristic (2 x 2) pattern. Different oxidation conditions lead to the formation of chemisorbed oxygen on the Pt(111) surface alongside PtO2 nanoclusters. Oxygen adsorbs on the surface forming a variety of structures which result in (3 x 3), (5 x 5) and (7 x 7) LEED patterns.

Ni(II) porphine nanolines grown on a Ag(111) surface at room temperature

The room temperature growth and ordering of (porphyrinato)nickel (II) (or nickel (II) porphine, NiP) molecules on a Ag(111) surface have been investigated using scanning tunnelling microscopy and low-energy electron diffraction (LEED). At a coverage of one monolayer, NiP molecules form a well-ordered molecular layer, having a hexagonal structure, on the Ag(111) surface. Porphyrin molecules have a flat orientation in this overlayer with the molecular plane lying parallel to the substrate. LEED data obtained from one monolayer of the NiP on the Ag(111) surface show the formation of two mirror domains each rotated either clockwise or anticlockwise by 6 degrees with respect to the substrate. NiP molecules forming a second layer self-assemble into well-ordered and uniformly separated nanolines at room temperature. These nanolines consist of hexagonally ordered NiP molecules and are found to be 1-4 molecules wide, depending on the molecular coverage. The completed second monolayer preserves the same planarity and hexagonal ordering as the first molecular layer but with a 4% lateral relaxation which produces a periodic modulation of approximately 5 nm.

An x-ray absorption and photoemission study of the electronic structure of Ni porphyrins and Ni N-confused porphyrin

Investigations of chemical bonding and electronic structure features for polycrystalline (porphyrinato)nickel (II) (NiP, the simplest Ni porphyrin), (5,10,15,20-tetraphenylporphyrinato)nickel (II) (NiTPP) and (2-aza-21-carba-5,10,15,20-tetraphenylporphyrinato)nickel (II) (N-confused NiTPP, NiNCTPP) have been performed by means of high-resolution soft x-ray absorption and x-ray photoemission spectroscopy. The Ni 2p(3/2) x-ray absorption spectra show strong π-back-bonding in these compounds leading to a high-energy shift (1.2 eV for the NiP and NiTPP) of the entire absorption structure compared to Ni metal. It has been found that the main absorption line of the Ni 2p(3/2) spectrum of the NiNCTPP is shifted by an additional 0.5 eV to higher energies in comparison with those for other nickel porphyrins. This shift is evidence of stronger back-donation (metal-to-ligand charge transfer) and a smaller effective number of 3d electrons on the central Ni atom in the NiNCTPP as compared to other Ni porphyrins. The confused N atom in the NiNCTPP is of pyrrolic type (protonated nitrogen), which was confirmed by the N 1s absorption and core-level photoemission spectra.