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Two-dimensional transition metal dichalcogenides (TMDCs), such as MoS2 and WS2, have recently emerged as nanomaterials with potential use in biomedicine. An attractive means to favor their interaction with biological media is the use of proper biomolecules as exfoliating/dispersing agents. Here, MoS2 flakes were stabilized with different small functional biomolecules such as adenosine monophosphate (AMP), guanosine monophosphate (GMP) and flavin mononucleotide (FMN) through the strong nucleotide−MoS2 interaction of Lewis acid-base type, rather than just on the weak dispersive and hydrophobic forces commonly associated with the use of many surfactants. The impact of the nucleotide-stabilized MoS2 flakes on the viability and cell proliferation, on the production of intracellular reactive oxygen species (ROS), and on the preosteoblast differentiation process (early stage) has been also evaluated, as well as the incorporation and intracellular localization of the nanomaterials by MC3T3-E1 and Saos-2 cells. The nucleotide-stabilized MoS2 flakes were found to exhibit excellent biocompatibility. Furthermore, their incorporation did not affect the integrity of the cell plasma membrane, which makes them ideal candidates for delivering drug/gene directly into cells. The in vitro cell response of tumor cells to these nanomaterials differs from that of undifferentiated cells, which provides the basis for their potential use in cancer therapy.

Due to its large surface area, high electrical conductivity as well as mechanical and thermal stability, pristine graphene has the potential to be an excellent support for metal nanoparticles (NPs), but the scarce amount of intrinsic chemical groups/defects in its structure that could act as anchoring sites for the NPs hinders this type of use. Here, a simple strategy based on the stabilization of pristine graphene in aqueous dispersion with the assistance of a low amount of flavin mononucleotide (FMN) is shown to yield a material that combines high electrical conductivity and abundance of extrinsic anchoring sites, so that pristine graphene–metal (Pd and Pt) NP hybrids with good dispersion and metal loading can be obtained from FMN–stabilized graphene. The activity of these hybrids towards the methanol oxidation reaction (MOR) both in acidic and alkaline media is studied by cyclic voltammetry (CV) and their stability investigated by chronoamperometry. The pristine graphene–Pt NP hybrid prepared by this simple, eco–friendly protocol is demonstrated to outperform most previously reported pristine graphene– and reduced graphene oxide–metal NP hybrids as electrocatalyst for the MOR, both in terms of catalytic activity and stability, avoiding at the same time the use of harsh chemicals or complex synthetic routes.

Electrolytic – usually referred to as electrochemical – exfoliation of graphite in water under anodic potential holds enormous promise as a simple, green and high-yield method for the mass production of graphene, but currently suffers from several drawbacks that hinder its widespread adoption, one of the most critical being the oxidation and subsequent structural degradation of the carbon lattice that is usually associated with such a production process. To overcome this and other limitations, we introduce and implement the concept of multifunctional electrolytes. The latter are amphiphilic anions (mostly polyaromatic hydrocarbons appended with sulfonate groups) that play different relevant roles as (1) an intercalating electrolyte to trigger exfoliation of graphite into graphene flakes, (2) a dispersant to afford stable aqueous colloidal suspensions of the flakes suitable for further use, (3) a sacrificial agent to prevent graphene oxidation during exfoliation and (4) a linker to promote nanoparticle anchoring on the graphene flakes, yielding functional hybrids. The implementation of this strategy with some selected amphiphiles even furnishes anodically exfoliated graphenes of a quality similar to that of flakes produced by direct, ultrasound- or shear-induced exfoliation of graphite in the liquid phase (i.e., almost oxide- and defect-free). These high quality materials were used for the preparation of catalytically efficient graphene–Pt nanoparticle hybrids, as demonstrated by model reactions (reduction of nitroarenes). The multifunctional performance of these electrolytes is also discussed and rationalized, and a mechanistic picture of their oxidation-preventing ability is proposed. Overall, the present results open the prospect of anodic exfoliation as a competitive method for the production of very high quality graphene flakes.

Due to its large surface area, high electrical conductivity as well as mechanical and thermal stability, pristine graphene has the potential to be an excellent support for metal nanoparticles (NPs), but the scarce amount of intrinsic chemical groups/defects in its structure that could act as anchoring sites for the NPs hinders this type of use. Here, a simple strategy based on the stabilization of pristine graphene in aqueous dispersion with the assistance of a low amount of flavin mononucleotide (FMN) is shown to yield a material that combines high electrical conductivity and abundance of extrinsic anchoring sites, so that pristine graphene–metal (Pd and Pt) NP hybrids with good dispersion and metal loading can be obtained from FMN–stabilized graphene. The activity of these hybrids towards the methanol oxidation reaction (MOR) both in acidic and alkaline media is studied by cyclic voltammetry (CV) and their stability investigated by chronoamperometry. The pristine graphene–Pt NP hybrid prepared by this simple, eco–friendly protocol is demonstrated to outperform most previously reported pristine graphene– and reduced graphene oxide–metal NP hybrids as electrocatalyst for the MOR, both in terms of catalytic activity and stability, avoiding at the same time the use of harsh chemicals or complex synthetic routes.

The exfoliation and colloidal stabilization of layered transition metal dichalcogenides (TMDs) in an aqueous medium using functional biomolecules as dispersing agents have a number of potential benefits toward the production and practical use of the corresponding two-dimensional materials, but such a strategy has so far remained underexplored. Here, we report that DNA and RNA nucleotides are highly efficient dispersants in the preparation of stable aqueous suspensions of MoS2 and other TMD nanosheets at significant concentrations (up to 5–10 mg mL–1). Unlike the case of common surfactants, for which adsorption on 2D materials is generally based on weak dispersive forces, the exceptional colloidal stability of the TMD flakes was shown to rely on the presence of relatively strong, specific interactions of Lewis acid–base type between the DNA/RNA nucleotide molecules and the flakes. Moreover, the nucleotide-stabilized MoS2 nanosheets were shown to be efficient catalysts in the reduction of nitroarenes (4-nitrophenol and 4-nitroaniline), thus constituting an attractive alternative to the use of expensive heterogeneous catalysts based on noble metals, and exhibited an electrocatalytic activity toward the hydrogen evolution reaction that was not impaired by the possible presence of nucleotide molecules adsorbed on their active sites. The biocompatibility of these materials was also demonstrated on the basis of cell proliferation and viability assays. Overall, the present work opens new vistas on the colloidal stabilization of 2D materials based on specific interactions that could be useful toward different practical applications.

Chemically exfoliated MoS2 (ce-MoS2) has emerged in recent years as an attractive two-dimensional material for use in relevant technological applications, but fully exploiting its potential and versatility will most probably require the deployment of appropriate chemical modification strategies. Here, we demonstrate that extensive covalent functionalization of ce-MoS2 nanosheets with acetic acid groups (∼0.4 groups grafted per MoS2 unit) based on the organoiodide chemistry brings a number of benefits in terms of their processability and functionality. Specifically, the acetic acid-functionalized nanosheets were furnished with long-term (>6 months) colloidal stability in aqueous medium at relatively high concentrations, exhibited a markedly improved temporal retention of catalytic activity toward the reduction of nitroarenes, and could be more effectively coupled with silver nanoparticles to form hybrid nanostructures. Furthermore, in vitro cell proliferation tests carried out with murine fibroblasts suggested that the chemical derivatization had a positive effect on the biocompatibility of ce-MoS2. A hydrothermal annealing procedure was also implemented to promote the structural conversion of the functionalized nanosheets from the 1T phase that was induced during the chemical exfoliation step to the original 2H phase of the starting bulk material, while retaining at the same time the aqueous colloidal stability afforded by the presence of the acetic acid groups. Overall, by highlighting the benefits of this type of chemical derivatization, the present work should contribute to strengthen the position of ce-MoS2 as a two-dimensional material of significant practical utility.

[EN] The main objective of this thesis is the development of methods for the preparation of different two-dimensional (2D) materials, mainly graphene, molybdenum disulfide (MoS2) and graphitic carbon nitride (g-C3N4), based on the liquid-phase exfoliation of the corresponding layered materials, as well as the exploration of their possible applications. The preparation of reduced graphene oxide (RGO) through catalytic reduction of graphene oxide at room temperature assisted by silver nanoparticles (AgNPs) has been studied, the mechanism of which has been probed. This strategy allowed the direct attainment of RGO-AgNPs hybrids. The hybrids exhibited a good catalytic activity in the reduction of 4-nitrophenol and electrocatalytic activity in the reduction of hydrogen peroxide. Likewise, the dispersibility of RGO and g-C3N4 in a broad variety of organic solvents has been examined. The dispersion behavior of these 2D materials was analyzed and rationalized on the basis of the surface tension as well as Hildebrand and Hansen solubility parameters of the solvents, and it was observed that it differed considerably from that of homogeneous 2D materials. It was concluded that it is necessary to take chemical and structural details of both RGO and g-C3N4 into account in order to understand what types of solvents are effective for each material. As an alternative to the graphite oxide route, the preparation of pristine graphene in water using a derivative of vitamin B2 as a stabilizer, namely flavin mononucleotide (FMN), was investigated. Very high dispersed concentrations of high quality graphene could be attained using limited amounts of FMN, which is potentially advantageous with a view to different applications. The presence of FMN adsorbed on the graphene nanosheets also afforded hybrids with metal (Ag, Pt, Pd) NPs well dispersed on the nanosheets. These hybrids displayed a good catalytic activity in the reduction of nitroarenes, as well as electrocatalytic activity toward, [ES] El objetivo central de esta tesis es el desarrollo de métodos de preparación de diferentes materiales bidimensionales (2D), principalmente grafeno, disulfuro de molibdeno (MoS2) y nitruro de carbono grafítico (g-C3N4), basados en la exfoliación en medio líquido de los correspondientes materiales laminares, así como la exploración de sus posibles aplicaciones. Se ha estudiado la preparación de óxido de grafeno reducido (RGO) mediante reducción catalítica de óxido de grafeno a temperatura ambiente asistida por nanopartículas de plata (AgNPs), cuyo mecanismo se ha investigado. Dicha estrategia permitió obtener directamente híbridos RGO-AgNPs. Estos híbridos presentaron buena actividad catalítica en la reducción de 4-nitrofenol y electrocatalítica en la reducción de peróxido de hidrógeno. Asimismo, se ha estudiado la dispersabilidad de RGO y g-C3N4 en una amplia variedad de disolventes orgánicos. El comportamiento en dispersión de estos materiales 2D química y estructuralmente heterogéneos fue analizado y racionalizado en términos de la tensión superficial y los parámetros de solubilidad de Hildebrand y Hansen de los disolventes, observándose que difería notablemente del documentado para materiales 2D homogéneos. Se concluyó que es necesario tener en cuenta detalles químicos y estructurales tanto del RGO como del g-C3N4 para comprender qué tipo de disolventes son efectivos para cada material. Como alternativa a la ruta del óxido de grafito, se investigó la obtención de grafeno prístino en agua usando un derivado de la vitamina B2, concretamente el mononucleótido de flavina (FMN), como agente estabilizante. Fue posible obtener concentraciones muy elevadas de grafeno de alta calidad estructural en dispersión usando cantidades limitadas de FMN, lo cual es potencialmente ventajoso con vistas a diferentes aplicaciones. La presencia de FMN adsorbido en las láminas permitió asimismo la obtención de híbridos con NPs metálicas (Ag, Pd, Pt) bien dispersas sobre las láminas d

Anodic exfoliation of graphite has emerged as an attractive method to access graphene nanosheets in large quantities, but oxidation reactions associated to this process compromise the structural quality of the resulting materials. Here, we demonstrate that the type of starting graphite material impacts the oxygen and defect content of anodically exfoliated graphenes obtained thereof. We investigated highly oriented pyrolytic graphite (HOPG) as well as graphite foil, flakes and powder as electrode in the anodic process. Importantly, materials with low levels of oxidation and disorder (similar to those typically achieved with cathodic exfoliation approaches) could be attained through proper choice of the graphite electrode. Specifically, using graphite foil afforded nanosheets of higher quality than that of HOPG-derived nanosheets. This discrepancy was interpreted to arise from the structural peculiarities of the former, where the presence of folds, voids and wrinkles would make its exfoliation process to be less reliant on oxidation reactions. Furthermore, cell viability tests carried out with murine fibroblasts on thin graphene films suggested that the anodically exfoliated graphenes investigated here (possessing low or high oxidation levels) are highly biocompatible. Overall, control upon the extent of oxidation and disorder should expand the scope of anodically exfoliated graphenes in prospective applications.

The stable dispersion of graphene flakes in an aqueous medium is highly desirable for the development of materials based on this two-dimensional carbon structure, but current production protocols that make use of a number of surfactants typically suffer from limitations regarding graphene concentration or the amount of surfactant required to colloidally stabilize the sheets. Here, we demonstrate that an innocuous and readily available derivative of vitamin B2, namely the sodium salt of flavin mononucleotide (FMNS), is a highly efficient dispersant in the preparation of aqueous dispersions of defect-free, few-layer graphene flakes. Most notably, graphene concentrations in water as high as ∼50 mg mL–1 using low amounts of FMNS (FMNS/graphene mass ratios of about 0.04) could be attained, which facilitated the formation of free-standing graphene films displaying high electrical conductivity (∼52000 S m–1) without the need of carrying out thermal annealing or other types of post-treatment. The excellent performance of FMNS as a graphene dispersant could be attributed to the combined effect of strong adsorption on the sheets through the isoalloxazine moiety of the molecule and efficient colloidal stabilization provided by its negatively charged phosphate group. The FMNS-stabilized graphene sheets could be decorated with nanoparticles of several noble metals (Ag, Pd, and Pt), and the resulting hybrids exhibited a high catalytic activity in the reduction of nitroarenes and electroreduction of oxygen. Overall, the present results should expedite the processing and implementation of graphene in, e.g., conductive inks, composites, and hybrid materials with practical utility in a wide range of applications.

Metal-assisted reduction of graphene oxide (GO) has recently emerged as a fast, efficient and room-temperature method towards the preparation of chemically derived graphene, but according to the mechanisms of reduction that have been proposed, not all relevant metals (e.g., Ag) should be a priori effective for this purpose. Here, we show that aqueous GO dispersions can be very efficiently reduced at room temperature with NaBH4 using Ag nanoparticles (Ag NPs) as catalysts, either generated in situ from appropriate precursors (AgNO3) or added to the dispersions as pre-formed objects. We propose and investigate a reduction mechanism that involves the charging of the Ag NPs with excess electrons obtained from the oxidation of a product of the spontaneous hydrolysis of NaBH4 in the aqueous medium. These excess electrons are then transferred to the GO sheets, triggering their reduction. The catalytic and electrochemical performance of the reduced GO–Ag NP hybrids that result from this process has also been examined. In particular, the hybrids are seen to exhibit very high catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol as a model reaction, and are also effective towards the electrochemical reduction of H2O2.