Chemical exfoliation of graphene and its application in organic electronics and energy storage devices

Graphene, the thinnest two-dimensional material possible, is considered as a realistic candidate for the numerous applications in electronic, energy storage and conversion devices due to its unique properties, such as high optical transmittance, high conductivity, excellent chemical and thermal stability. However, the electronic and chemical properties of graphene are highly dependent on their preparation methods. Therefore, the development of novel chemical exfoliation process which aims at high yield synthesis of high quality graphene while maintaining good solution processability is of great concern. This thesis focuses on the solution production of high-quality graphene by wet-chemical exfoliation methods and addresses the applications of the chemically exfoliated graphene in organic electronics and energy storage devices.rnPlatinum is the most commonly used catalysts for fuel cells but they suffered from sluggish electron transfer kinetics. On the other hand, heteroatom doped graphene is known to enhance not only electrical conductivity but also long term operation stability. In this regard, a simple synthetic method is developed for the nitrogen doped graphene (NG) preparation. Moreover, iron (Fe) can be incorporated into the synthetic process. As-prepared NG with and without Fe shows excellent catalytic activity and stability compared to that of Pt based catalysts.rnHigh electrical conductivity is one of the most important requirements for the application of graphene in electronic devices. Therefore, for the fabrication of electrically conductive graphene films, a novel methane plasma assisted reduction of GO is developed. The high electrical conductivity of plasma reduced GO films revealed an excellent electrochemical performance in terms of high power and energy densities when used as an electrode in the micro-supercapacitors.rnAlthough, GO can be prepared in bulk scale, large amount of defect density and low electrical conductivity are major drawbacks. To overcome the intrinsic limitation of poor quality of GO and/or reduced GO, a novel protocol is extablished for mass production of high-quality graphene by means of electrochemical exfoliation of graphite. The prepared graphene shows high electrical conductivity, low defect density and good solution processability. Furthermore, when used as electrodes in organic field-effect transistors and/or in supercapacitors, the electrochemically exfoliated graphene shows excellent device performances. The low cost and environment friendly production of such high-quality graphene is of great importance for future generation electronics and energy storage devices. rn

Preparation and functional characterisation of recombinant silicatein-alpha from the sponge Suberites domuncula

The marine world is an immense source of biodiversity that provides substances with striking potentials in medicinal chemistry and biotechnology. Sponges (Porifera) are marine animals that represent the most impressive example of organisms possessing the ability to metabolise silica through a family of enzymes known as silicateins. Complex skeletal structures (spicules) made of pure biogenic silica (biosilica) are produced under physiological conditions. Biosilica is a natural material comprising inorganic and organic components with unique mechanical, optical, and physico-chemical properties, including promising potential to be used for development of therapeutic agents in regenerative medicine. Unravelling the intimate physiological mechanisms occurring in sponges during the construction of their siliceous spicules is an on-going project, and several questions have been addressed by the studies proposed by our working group. In this doctoral work, the recombinant DNA technology is exploited for functional and structural characterisation of silicatein. Its precursors are produced as fusion proteins with a chaperone tag (named TF-Ps), and a robust method for the overexpression of native soluble proteins in high concentrations has been developed. In addition, it is observed and proven experimentally that the maturation of silicatein is an autocatalytic event that: (i) can be modulated by rational use of protease inhibitors; (ii) is influenced by the temperature of the environment; (iii) only slightly depends on the pH. In the same experimental framework, observations on the dynamics in the maturation of silicateins allow a better understanding of how the axial filaments form during the early stages of spicule construction. In addition, the definition of new distinct properties of silicatein (termed “structure-guiding” and “structure-forming”) is introduced. By homology models and through comparisons with similar proteins (the cathepsins), domains with significant surface hydrophobicity are identified as potential self-assembly mediators. Moreover, a high-throughput screening showed that TF-Ps could generate crystals under certain conditions, becoming promising for further structural studies. With the goal of optimise the properties of the recombinant silicatein, implementation of new production systems are tried for the first time. Success in the expression of silicatein-type proteins in insect and yeast cells, constitute a promising basis for further development, towards the establishment of an efficient method for the production of a high-value pure and soluble protein.

Microwave-assisted wet chemical (MAWC) synthesis of lithium iron phosphate

LiFePO4 is a Co-free battery material. Its advantages of low cost, non-toxic and flat discharge plateau show promising for vehicle propulsion applications. A major problem associated with this material is its low electrical conductivity. Use of nanosized LiFePO4 coated with carbon is considered a solution because the nanosized particles have much shorter path for L+ ions to travel from the LiFePO4 crystal lattice to electrolytes. As other nano material powders, however, nano LiFePO4 could have processing and health issues. In order to achieve high electrical conductivity while maintaining a satisfactory manufacturability, the particles should possess both of the nano- and the microcharacteristics correspondingly. These two contradictory requirements could only be fulfilled if the LiFePO4 powders have a hierarchical structure: micron-sized parent particles assembled by nanosized crystallites with appropriate electrolyte communication channels. This study addressed the issue by study of the formation and development mechanisms of the LiFePO4 crystallites and their microstructures. Microwaveassisted wet chemical (MAWC) synthesis approach was employed in order to facilitate the evolvement of the nanostructures. The results reveal that the LiFePO4 crystallites were directly nucleated from amorphous precursors by competition against other low temperature phases, Li3PO4 and Fe3(PO4)2•8H2O. Growth of the crystalline LiFePO4 particles went through oriented attachment first, followed by revised Ostwald ripening and then recrystallization. While recrystallization played the role in growth of well crystallized particles, oriented attachment and revised Ostwald ripening were responsible for formation of the straight edge and plate-like shaped LiFePO4 particles comprised of nanoscale substructure. Oriented attachment and revised Ostwald ripening seemed to be also responsible for clustering the plate-like LiFePO4 particles into a high-level aggregated structure. The finding from this study indicates a hope for obtaining the hierarchical structure of LiFePO4 particles that could exhibit the both micro- and nano- scale characteristics. Future study is proposed to further advance the understanding of the structural development mechanisms, so that they can be manipulated for new LiFePO4 structures ideal for battery application.

In Situ SHINERS at Electrochemical Single-Crystal Electrode/Electrolyte Interfaces: Tuning Preparation Strategies and Selected Applications

We have studied Au(55 nm)@SiO2 nanoparticles (NPs) on two low-index phases of gold and platinum single crystal electrodes in ClO4– and SO42– ion-containing electrolytes by both electrochemical methods and in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). We showed the blocking of the electrode with surfactants originating from the synthesis of as-prepared SHINERS NPs. We introduce an efficient procedure to overcome this problem, which provides a fundamental platform for the application of SHINERS in surface electrochemistry and beyond. Our method is based on a hydrogen evolution treatment of the SHINERS-NP-modified single-crystal surfaces. The reliability of our preparation strategy is demonstrated in electrochemical SHINERS experiments on the potential-controlled adsorption and phase formation of pyridine on Au(hkl) and Pt(hkl). We obtained high-quality Raman spectra on these well-defined and structurally carefully characterized single-crystal surfaces. The analysis of the characteristic A1 vibrational modes revealed perfect agreement with the interpretation of single-crystal voltammetric and chronoamperometric experiments. Our study demonstrates that the SHINERS protocol developed in this work qualifies this Raman method as a pioneering approach with unique opportunities for in situ structure and reactivity studies at well-defined electrochemical solid/liquid interfaces.

PREPARATION AND CHARACTERIZATION OF COVALENTLY LINKED ENZYME-ANTIBODY CONJUGATES AND THEIR USE IN MEASURING MINUTE QUANTITIES OF PROTEIN ANTIGENS

The discovery and characterization of oncofetal proteins have led to significant advances in early cancer diagnosis and therapeutic monitoring of patients undergoing cancer chemotherapy. These tumor-associated antigens are presently measured by sensitive, specific immunoassay techniques based on the detection of minute amounts of labeled antigen or antibody incorporated into immune complexes, which must be isolated from free antigen and antibody.^ Since there are several disadvantages with using radioisotopes, the most common immunolabel, one major objective was to prepare covalently coupled enzyme-antibody conjugates and evaluate their use as a practical alternative to radiolabeled immune reagents. An improved technique for the production of enzyme-antibody conjugates was developed that involves oxidizing the carbohydrate moieties on a glycoprotein enzyme, then introducing antibody in the presence of polyethylene glycol (PEG). Covalent enzyme-antibody conjugates involving alkaline phosphatase and amyloglucosidase were produced and characterized.^ In order to increase the sensitivity of detecting the amyloglucosidase-antibody conjugate, an enzyme cycling assay was developed that measures glucose, the product of maltose cleavage by amyloglucosidase, in the picomole range. The increased sensitivity obtained by combined usage of the amyloglucosidase-antibody conjugate and enzyme cycling assay was then compared to that of conventional enzyme immunoassay (EIA).^ For immune complex isolation, polystyrene tubes and protein A-bearing Staphylococcus aureus were evaluated as solid phase matrices, upon which antibodies can be immobilized. A sandwich-type EIA, using antibody-coated S. aureus, was developed that measures human albumin (HSA) in the nanogram range. The assay, using an alkaline phosphatase-anti-HSA conjugate, was applied to the determination of HSA in human urine and evaluated extensively for its clinical applicability.^ Finally, in view of the clinical significance of alpha-fetoprotein (AFP) as an oncofetal antigen and the difficulty with its purification for use as an immunogen and assay standard, a chemical purification protocol was developed that resulted in a high yield of immunochemically pure AFP. ^

Dielectrophoretic approaches to sample preparation and analysis

Dielectrophoresis (DEP) has been used to manipulate cells in low-conductivity suspending media using AC electrical fields generated on micro-fabricated electrode arrays. This has created the possibility of performing automatically on a micro-scale more sophisticated cell processing than that currently requiring substantial laboratory equipment, reagent volumes, time, and human intervention. In this research the manipulation of aqueous droplets in an immiscible, low-permittivity suspending medium is described to complement previous work on dielectrophoretic cell manipulation. Such droplets can be used as carriers not only for air- and water-borne samples, contaminants, chemical reagents, viral and gene products, and cells, but also the reagents to process and characterize these samples. A long-term goal of this area of research is to perform chemical and biological assays on automated, micro-scaled devices at or near the point-of-care, which will increase the availability of modern medicine to people who do not have ready access to large medical institutions and decrease the cost and delays associated with that lack of access. In this research I present proofs-of-concept for droplet manipulation and droplet-based biochemical analysis using dielectrophoresis as the motive force. Proofs-of-concept developed for the first time in this research include: (1) showing droplet movement on a two-dimensional array of electrodes, (2) achieving controlled dielectric droplet injection, (3) fusing and reacting droplets, and (4) demonstrating a protein fluorescence assay using micro-droplets. ^

Preparation and characterization of polyester resin-layered silicate containing reactive groups

Polymer nanocomposites, specifically nanoclay-reinforced polymers, have attracted great interest as matrix materials for high temperature composite applications. Nanocomposites require relatively low dispersant loads to achieve significant property enhancements. These enhancements are mainly a consequence of the interfacial effects that result from dispersing the silicate nanolayers in the polymer matrix and the high in-plane strength, stiffness and aspect ratio of the lamellar nanoparticles. The montmorillonite (MMT) clay, modified with organic onium ions with long alkyl chains as Cloisites, has been widely used to obtain nanocomposites. The presence of reactive groups in organic onium ions can form chemical bonds with the polymer matrix which favours a very high exfoliation degree of the clay platelets in the nanocomposite (1,2)

Nerve growth factor acutely reduces chemical transmission by means of postsynaptic TrkA-like receptors in squid giant synapse

Tyrosine phosphorylation has been shown to be an important modulator of synaptic transmission in both vertebrates and invertebrates. Such findings hint toward the existence of extracellular ligands capable of activating this widely represented signaling mechanism at or close to the synapse. Examples of such ligands are the peptide growth factors which, on binding, activate receptor tyrosine kinases. To gain insight into the physiological consequences of receptor tyrosine kinase activation in squid giant synapse, a series of growth factors was tested in this preparation. Electrophysiological, pharmacological, and biochemical analysis demonstrated that nerve growth factor (NGF) triggers an acute and specific reduction of the postsynaptic potential amplitude, without affecting the presynaptic spike generation or presynaptic calcium current. The NGF target is localized at a postsynaptic site and involves a new TrkA-like receptor. The squid receptor crossreacts with antibodies generated against mammalian TrkA, is tyrosine phosphorylated in response to NGF stimulation, and is blocked by specific pharmacological inhibitors. The modulation described emphasizes the important role of growth factors on invertebrate synaptic transmission.

High resolution ultrastructural mapping of total calcium: electron spectroscopic imaging/electron energy loss spectroscopy analysis of a physically/chemically processed nerve-muscle preparation.

We report on a procedure for tissue preparation that combines thoroughly controlled physical and chemical treatments: quick-freezing and freeze-drying followed by fixation with OsO4 vapors and embedding by direct resin infiltration. Specimens of frog cutaneous pectoris muscle thus prepared were analyzed for total calcium using electron spectroscopic imaging/electron energy loss spectroscopy (ESI/EELS) approach. The preservation of the ultrastructure was excellent, with positive K/Na ratios revealed in the fibers by x-ray microanalysis. Clear, high-resolution EELS/ESI calcium signals were recorded from the lumen of terminal cisternae of the sarcoplasmic reticulum but not from longitudinal cisternae, as expected from previous studies carried out with different techniques. In many mitochondria, calcium was below detection whereas in others it was appreciable although at variable level. Within the motor nerve terminals, synaptic vesicles as well as some cisternae of the smooth endoplasmic reticulum yielded positive signals at variance with mitochondria, that were most often below detection. Taken as a whole, the present study reveals the potential of our experimental approach to map with high spatial resolution the total calcium within individual intracellular organelles identified by their established ultrastructure, but only where the element is present at high levels.

Thermogravimetric analysis of the pyrolysis of several materials used in the preparation of commercial cigarettes

Comunicación presentada en forma de póster en el "12th Mediterranean Congress of Chemical Engineering", Barcelona (Spain), November 15-18, 2011

Catalytic effect of MCM-41 in the pyrolysis of tobacco and several substances used as additives in the preparation of commercial cigarettes

Comunicación presentada en forma de póster en el "12th Mediterranean Congress of Chemical Engineering", Barcelona (Spain), November 15-18, 2011.