José A. Martín-Gago

Fullerenes from aromatic precursors by surface-catalysed cyclodehydrogenation

Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C60 from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C60 and the triazafullerene C57N3 from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.

Label-free detection of DNA hybridization based on hydration-induced tension in nucleic acid films

The properties of water at the nanoscale are crucial in many areas of biology, but the confinement of water molecules in sub-nanometre channels in biological systems has received relatively little attention. Advances in nanotechnology make it possible to explore the role played by water molecules in living systems, potentially leading to the development of ultrasensitive biosensors. Here we show that the adsorption of water by a self-assembled monolayer of single-stranded DNA on a silicon microcantilever can be detected by measuring how the tension in the monolayer changes as a result of hydration. Our approach relies on the microcantilever bending by an amount that depends on the tension in the monolayer. In particular, we find that the tension changes dramatically when the monolayer interacts with either complementary or single mismatched single-stranded DNA targets. Our results suggest that the tension is mainly governed by hydration forces in the channels between the DNA molecules and could lead to the development of a label-free DNA biosensor that can detect single mutations. The technique provides sensitivity in the femtomolar range that is at least two orders of magnitude better than that obtained previously with label-free nanomechanical biosensors and with label-dependent microarrays.

The interaction of Pt with TiO2(110) surfaces: a comparative XPS, UPS, ISS, and ESD study

The interaction of platinum with (110) single crystal surfaces of titanium dioxide (TiO2) has been studied by means of X-ray and ultraviolet photoemission spectroscopies (XPS and UPS), ion backscattering spectroscopy (ISS), Auger electron spectroscopy (AES), and electron-stimulated desorption (ESD) for Pt coverages in the sub- and monolayer range. The latter have been evaporated thermally under ultra-high vacuum conditions. We found that Pt overlayers are thermodynamically stable at room temperature with respect to the formation of Ti suboxides. At submonolayer coverages, Pt atoms adsorb preferentially on top of five-fold coordinated Ti surface atoms which act as nucleation sites for the subsequent three-dimensional growth of Pt clusters. Metallic Pt clusters are formed at the surface at elevated temperatures. UPS difference spectra show additional Pt-derived electronic states below the Fermi energy. Chemical bonds between Pt and TiO2 surface atoms are not formed under these conditions. On pre-reduced TiO2(110) surfaces, in contrast, a localized electronic charge transfer is found between Ti3+ states and Pt surface atoms.

Synthesis of cobalt ferrite core/metallic shell nanoparticles for the development of a specific PNA/DNA biosensor

Controlled synthesis of cobalt ferrite superparamagnetic nanoparticles covered with a gold shell has been achieved by an affinity and trap strategy. Magnetic nanoparticles are functionalized with a mixture of amino and thiol groups that facilitate the electrostatic attraction and further chemisorption of gold nanoparticles, respectively. Using these nanoparticles as seeds, a complete coating shell is achieved by gold salt-iterative reduction leading to monodisperse water-soluble gold-covered magnetic nanoparticles, with an average diameter ranging from 21 to 29 nm. These constitute a versatile platform for immobilization of biomolecules via thiol chemistry, which is exemplified by the immobilization of peptide nucleic acid (PNA) oligomers that specifically hybridize with complementary DNA molecules in solution. Hybridation with DNA probes has been measured using Rhodamine 6G fluorescence marker and the detection of a single nucleotide mutation has been achieved. These results suggest the PNA-nanoparticles application as a biosensor for DNA genotyping avoiding commonly time-consuming procedures employed.

Strain-driven Moire superstructures of epitaxial graphene on transition metal surfaces.

STM images of multidomain epitaxial graphene on Pt(111) have been combined with a geometrical model to investigate the origin of the coincidence Moiré superstructures. We show that there is a relation between the appearance of a particular Moiré periodicity and the minimization of the absolute value of the strain between the graphene and the substrate for the different orientations between both atomic lattices. This model predicts all the stable epitaxial graphene structures that can be grown on transition metal surfaces, and we have made use of it for reproducing previously published data from different authors. Its validity suggests that minimization of the strain within the coincident graphene unit-cell due to a strong local interaction is the driving force in the formation of Moiré superstructures.

Electrochemical growth of Acidithiobacillus ferrooxidans on a graphite electrode for obtaining a biocathode for direct electrocatalytic reduction of oxygen

An aspect in microbial fuel cell research that is currently of great interest is the development of bacterial cathodes. Bacterial cathodes that catalyze oxygen reduction to water at low pH have the advantage of overcoming the kinetic limitations due to the requirement of 4 protons per molecule reduced. In this work we have studied the performance of a biocathode using as electrocatalyst an acidophile microorganism: Acidithiobacillus ferrooxidans. Growth of the microorganism directly on the electrode took place using an applied voltage of 0 V vs. SCE as the only energy source and without adding redox mediators to the solution. Current densities of up to 5 A m(-2) were measured for O2 reduction in the At. ferrooxidans cathode at pH 2.0 and the electrocatalytic wave was shifted 300 mV to higher potential compared to the control graphite electrodes without the bacterium.

Graphene functionalisation with a conjugated poly(fluorene) by click coupling: striking electronic properties in solution.

Graphene flakes covalently modified with a conjugated polymer, poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis-(6-azidohexyl)fluorene)] (PFA), were efficiently synthesised by a Cu-catalysed Huisgen 1,3-dipolar cycloaddition between alkyne-modified graphene and an azide-functionalised polymer. Two approaches for the modification of graphene with alkyne groups were investigated (coupling with a diazonium salt generated in situ or an amidation reaction) and the optimum conditions determined. The success of the click-coupling approach was confirmed by FTIR, (1)H NMR, Raman, and X-ray photoelectron spectroscopy (XPS). The absorption and emission spectra of the click product show a strong solvent dependency.

Tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo)dehydrogenation of heteroaromatics.

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest.

Deposition of PVD solar absorber coatings for high-efficiency thermal collectors

Abstract In this contribution, we present results of a research project for the development of new layered and graded black solar absorbers by magnetron sputtering techniques to be included as selective surfaces in very high-efficiency thermal solar collectors. Solar absorber based in layered and graded cermet coating of Mo–Al2O3 was deposited by DC reactive magnetron sputtering. The ceramic and metallic components of the cermets were deposited in a sub-layer system consisting of alternating metallic and oxide sub-layers. In a way to change the metal volume fraction, the thickness of the sub-layers is changed. The effect of metallic fraction and the number of sub-layers in optical properties of the films were studied and good results were obtained. The optimum film selectivity achieved was a solar absorptance of 94% and a thermal emmissivity of 5%. The magnetron sputtering technique has demonstrated that it is a potential method to produce high reproducible spectrally selective coatings with graded structure since it is possible to control in an atomic level the addition of metal phases to the dielectric matrix. These composite coatings are attractive candidates for use in photothermal solar energy conversion.

Weakly interacting molecular layer of spinning C60 molecules on TiO2 (110) surfaces.

The adsorption of C(60), a typical acceptor organic molecule, on a TiO(2) (110) surface has been investigated by a multitechnique combination, including van der Waals density functional calculations. It is shown that the adsorbed molecules form a weakly interacting molecular layer, which sits on the fivefold-coordinated Ti that is confined between the prominent bridging oxygen rows (see figure).