Grafeno y otros materiales bidimensionales en dispersión coloidal: preparación, caracterización y formación de híbridos

Tesis doctoral presentada en el Departamento de Ciencia y Tecnología de Materiales de la Universidad de Oviedo. Octubre de 2016.[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 towards methanol oxidation and oxygen reduction. Concerning MoS2, 2D nanosheets of this material were prepared in aqueous medium based on two different methods: exfoliation by means of lithium intercalation and direct ultrasound-assisted exfoliation using DNA and RNA nucleotides as dispersants. In the former case, the obtained single-layer nanosheets were demonstrated to possess a high catalytic activity in reduction reactions (nitroarenes, dyes), which was ascribed to the presence of 1T metallic domains generated during intercalation. In the latter case, the DNA/RNA nucleotides were shown to be unusually efficient dispersants for MoS2, which was attributed to the presence of relatively strong, specific nucleotide-MoS2 interactions of Lewis acid-base type that are not present in other 2D materials such as graphene. The nucleotide-stabilized MoS2 nanosheets exhibited good catalytic activity in the reduction of nitroarenes, despite the absence of 1T domains, electrocatalytic activity towards the hydrogen evolution reaction, as well as a good biocompatibility.[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 de grafeno. Dichos híbridos presentaron una buena actividad catalítica en la reducción de nitroarenos y electrocatalítica en la reacción de oxidación de metanol y reducción de oxígeno. Respecto al MoS2, se han preparado láminas 2D de este material en medio acuoso mediante dos métodos diferentes: exfoliación por intercalación de litio y exfoliación directa por ultrasonidos usando nucleótidos del ADN y ARN como dispersantes. En el primer caso, se demostró que las láminas monocapa obtenidas poseían una alta actividad catalítica en reacciones de reducción (nitroarenos, colorantes), atribuida a la presencia de dominios de fase metálica 1T generados durante el proceso de intercalación. En el segundo caso, se demostró que los nucleótidos de ADN/ARN son dispersantes inusualmente eficientes del MoS2, lo cual se pudo atribuir a la presencia de interacciones nucléotido-MoS2 específicas y relativamente fuertes de tipo ácido-base Lewis, que no están presentes en otros materiales 2D como el grafeno. Las láminas de MoS2 estabilizadas por nucléotidos presentaron buena actividad catalítica en la reducción de nitroarenos, a pesar de la ausencia de dominios 1T, electrocatalítica para la reacción de evolución de hidrógeno, así como una buena biocompatibilidad.Peer reviewe

Grafeno y otros materiales bidimensionales en dispersión coloidal: preparación, caracterización y formación de híbridos

Tesis doctoral presentada en el Departamento de Ciencia y Tecnología de Materiales de la Universidad de Oviedo. Octubre de 2016.[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 towards methanol oxidation and oxygen reduction. Concerning MoS2, 2D nanosheets of this material were prepared in aqueous medium based on two different methods: exfoliation by means of lithium intercalation and direct ultrasound-assisted exfoliation using DNA and RNA nucleotides as dispersants. In the former case, the obtained single-layer nanosheets were demonstrated to possess a high catalytic activity in reduction reactions (nitroarenes, dyes), which was ascribed to the presence of 1T metallic domains generated during intercalation. In the latter case, the DNA/RNA nucleotides were shown to be unusually efficient dispersants for MoS2, which was attributed to the presence of relatively strong, specific nucleotide-MoS2 interactions of Lewis acid-base type that are not present in other 2D materials such as graphene. The nucleotide-stabilized MoS2 nanosheets exhibited good catalytic activity in the reduction of nitroarenes, despite the absence of 1T domains, electrocatalytic activity towards the hydrogen evolution reaction, as well as a good biocompatibility.[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 de grafeno. Dichos híbridos presentaron una buena actividad catalítica en la reducción de nitroarenos y electrocatalítica en la reacción de oxidación de metanol y reducción de oxígeno. Respecto al MoS2, se han preparado láminas 2D de este material en medio acuoso mediante dos métodos diferentes: exfoliación por intercalación de litio y exfoliación directa por ultrasonidos usando nucleótidos del ADN y ARN como dispersantes. En el primer caso, se demostró que las láminas monocapa obtenidas poseían una alta actividad catalítica en reacciones de reducción (nitroarenos, colorantes), atribuida a la presencia de dominios de fase metálica 1T generados durante el proceso de intercalación. En el segundo caso, se demostró que los nucleótidos de ADN/ARN son dispersantes inusualmente eficientes del MoS2, lo cual se pudo atribuir a la presencia de interacciones nucléotido-MoS2 específicas y relativamente fuertes de tipo ácido-base Lewis, que no están presentes en otros materiales 2D como el grafeno. Las láminas de MoS2 estabilizadas por nucléotidos presentaron buena actividad catalítica en la reducción de nitroarenos, a pesar de la ausencia de dominios 1T, electrocatalítica para la reacción de evolución de hidrógeno, así como una buena biocompatibilidad.Peer reviewe

Self-paced cycling improves cognition on institutionalized older adults without known cognitive impairment: A 15-month randomized controlled trial

This study aimed at identifying the effects of self-paced cycling on the cognitive and functional status and fall risk on institutionalized older adults without cognitive impairment. A total of 39 individuals were randomly assigned to an exercise group or to a control group. The exercise group participants cycled at their self-selected intensity at least for 15 min daily during 15 months. The control group participants performed recreational activities. The Mini-Mental State Examination, Fuld object memory evaluation, and symbol digit modality test were used for cognitive assessments. The Katz index, the timed "Up & Go" test, and the World Health Organization questionnaire were used to assess functional independence, mobility, and fall risk. Significant improvements were observed in the exercise group for global cognition and attention, visual scanning, and processing speed. Long-term self-paced cycling training seems to have a protective effect on cognitive status and attention, visual scanning, and processing speed in older institutionalized individuals

The effect of Brain Gym on cognitive function in older people: A systematic review and meta-analysis

This review aimed to systematically evaluate and meta-analyze the available data on the effects of Brain Gym (BG) on cognitive function in older people. Six electronic databases were searched systematically using: “Brain Gym” AND “elderly, “Brain Gym” AND “older people”. The PEDro and MINORS scales were used to evaluate methodological quality. For the meta-analysis, inverse variance or generic inverse variance was used and heterogeneity was assessed with the Chi2 test and I2 test. Ten research studies with a high to low quality. Significant changes intra- and inter-group were observed for neurocognitive outcomes in the BG groups. Findings from the metaanalysis indicated changes in the BG groups, on cognitive function by means of the Mini–Mental State Examination, were not greater than those reported in the control/comparison groups. BG will not lead to improvements in cognitive function in people with and without cognitive impairment, supported by low to high evidence

MoS2 flakes stabilized with DNA/RNA nucleotides: in vitro cell response

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.publishe

Impact of Covalent Functionalization on the Aqueous Processability, Catalytic Activity, and Biocompatibility of Chemically Exfoliated MoS2 Nanosheets

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.Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (FEDER) through project MAT2015-69844-R is gratefully acknowledged, as well as partial support of the ERDF/FEDER. Operative Program of the Región de Murcia 2007–2013. We also acknowledge partial funding by Plan de Ciencia, Tecnología e Innovación 2013-2017 del Principado de Asturias and FEDER through grant GRUPIN14-056.Peer reviewe

Excess weight in relation to lifestyle habits in spanish first-year university students: differences between pre- and post-COVID-19—a serial cross-sectional study based on uniHcos project

The objective of this research was to study the relationship between the body weight and diet, physical activity, and other habits among freshmen students by sex, and to determine whether these habits have changed during the post-era of the COVID-19 pandemic. A serial cross-sectional study with data from 11 Spanish universities was carried out. In total, 10,096 first-year university students (73.2% female, mean age = 19.0 ± 1.5 years) completed an online self-administered questionnaire between 2012 and 2022. For some analyses, questionnaires were categorized by the year in which the survey was filled out as Before COVID-19, Lockdown, and New Normal. In total, 72.9% of participants were within the normal weight range, and 17.7% of men and 11.8% of women were overweight (p < 0.001). The students who did not meet the WHO criteria of physical activity, spent more than 7 h per day sitting, and skipped breakfast had a higher prevalence of obesity (p < 0.05). According to the period of study, the prevalence of overweight/obesity Before COVID-19 was 16.1% (95% CI: 15.4–16.9%), while in Lockdown the prevalence was significantly higher (20.2, 95% CI: 17.1–23.8) and in New Normal it was 18.9% (CI: 15.7–22.5). Moreover, the study suggests that during the Lockdown period, there was a reduction in the practice of physical activity and an increase in the prevalence of a healthy diet. For all these, it is necessary to propose public health interventions that improve the lifestyles of university students.Ministerio de Sanidad, Servicios Sociales e Igualdad | Ref. 2010|145Ministerio de Sanidad, Servicios Sociales e Igualdad | Ref. 2013|034Ministerio de Sanidad, Servicios Sociales e Igualdad | Ref. 2020|030Ministerio de Economía y Competitividad | Ref. PI16/0194

Efficient Pt electrocatalysts supported onto flavin mononucleotide–exfoliated pristine graphene for the methanol oxidation reaction

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.Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (FEDER) through projects MAT2015-69844-R and MAT2016-76595-R is gratefully acknowledged. We also acknowledge partial funding by Plan de Ciencia, Tecnología e Innovación 2013-2017 del Principado de Asturias and FEDER through grant GRUPIN14-056. M.A-V. is thankful to MINECO and for his pre-doctoral contract. R.R.R acknowledges financial support from MINECO through “Juan de la Cierva” program (JCI-2012-12664)

Impact of the COVID-19 Pandemic on the Evolution of Prevalence and Patterns of Cannabis Use among First-Year University Students in Spain—UniHcos Project

Among university students there has been evidence that the COVID-19 pandemic increased their psychological distress, exacerbated by social restrictions. The main objective of this study was to explore the impact of the COVID-19 pandemic on the prevalence and patterns of cannabis use among university students, in contrast to previous trends since 2012. Data from 10,522 first-year university students (73.3% female, Mage 19 (SD = 1.6)) from eleven Spanish universities collected between 2012 and May 2022 was analysed. Prevalences of cannabis use and their differences by sex were studied, as well as changes in patterns of use and its use for coping during the pandemic. It was found that during lockdown, all prevalence rates of cannabis use decreased in both sexes, showing no statistically significant differences and increasing again in the new normal period in both. Among regular cannabis users, 79.7% reported maintaining or increasing their cannabis use during the pandemic, and of these, half reported using cannabis to cope. Moreover, cannabis use in the usual household increased during the lockdown. These results show that although the overall prevalence of cannabis use was reduced during the lockdown, regular users tended to maintain or increase cannabis use. This could imply two different patterns of use among students, one social and occasional versus the other regular, providing new lines of research for prevention and the implementation of social policies.This research was funded by the National Drug Plan from the Ministry of Health, Social Services and Equality of Spain (Grant numbers: 2010|145, 2013|034 and 2020|030) and Instituto de Salud Carlos III (Grant number: PI16/01947)