Dimitrios Gournis

Decorating carbon nanotubes with metal or semiconductor nanoparticles

Due to their large chemically active surface and stability at high temperatures carbon nanotubes (CNTs) have been used as a support material for the dispersion and stabilization of metal and semiconductor nanoparticles (NPs). These hybrid materials have found several applications in catalysis, nanoelectronics, optics, nanobiotechnology, etc. Several ways have been described in the literature to immobilize NPs on CNTs and they can be divided into two main pathways: (a) the formation (and stabilization) of metal NPs directly on the carbon nanotube surface, and (b) the connection of chemically modified NPs to carbon nanotubes or to modified CNTs. A plethora of methods for the synthesis of different NPs have been very recently developed. This know-how is now available for the generation of a large variety of new hybrid products in combination with CNTs. A selection of representative examples of the synthesis, properties and applications of NP–CNTs is here reported and discussed.

Multipurpose Organically Modified Carbon Nanotubes: From Functionalization to Nanotube Composites

We show that covalent functionalization of carbon nanotubes (CNTs) via 1,3-dipolar cycloaddition is a powerful method for enhancing the ability to process CNTs and facilitating the preparation of hybrid composites, which is achieved solely by mixing. CNTs were functionalized with phenol groups, providing stable dispersions in a range of polar solvents, including water. Additionally, the functionalized CNTs could easily be combined with polymers and layered aluminosilicate clay minerals to give homogeneous, coherent, transparent CNT thin films and gels.

Catalytic synthesis of carbon nanotubes on clay minerals

Novel clay–carbon tube composites were synthesized by catalytic decomposition of acetylene over iron-catalyst centers supported on montmorillonite surfaces by ion-exchange. TEM and SEM micrographs show the growth of carbon tubes rooted to the clay surfaces, while the iron-nanoparticles (which catalyze the formation of carbon-nanotubes) are detected and characterized by Mossbauer spectroscopy, mainly as ferromagnetic cementite (Fe3C). In the hybrid materials the clay retains its exchange properties making possible the preparation of clay–carbon nanotube derivatives that are valuable for various technological applications.

Development of effective nanobiocatalytic systems through the immobilization of hydrolases on functionalized carbon-based nanomaterials

In this study we report the use of functionalized carbon-based nanomaterials, such as amine-functionalized graphene oxide (GO) and multi-walled carbon nanotubes (CNTs), as effective immobilization supports for various lipases and esterases of industrial interest. Structural and biochemical characterization have revealed that the curvature of the nanomaterial affect the immobilization yield, the catalytic behavior and the secondary structure of enzymes. Infrared spectroscopy study indicates that the catalytic behavior of the immobilized enzymes is correlated with their α-helical content. Hydrolases exhibit higher esterification activity (up to 20-fold) when immobilized on CNTs compared to GO. The covalently immobilized enzymes exhibited comparable or even higher activity compared to the physically adsorbed ones, while they presented higher operational stability. The enhanced catalytic behavior observed for most of the hydrolases covalently immobilized on amine-functionalized CNTs indicate that these functionalized nanomaterials are suitable for the development of efficient nanobiocatalytic systems.

Gd(III)-doped carbon dots as a dual fluorescent-MRI probe

We describe the synthesis of Gd(III)-doped carbon dots as dual fluorescence-MRI probes for biomedical applications. The derived Gd(III)-doped carbon dots show uniform particle size (3–4 nm) and gadolinium distribution and form stable dispersions in water. More importantly, they exhibit bright fluorescence, strong T1-weighted MRI contrast and low cytotoxicity.

Graphene-based nanobiocatalytic systems: recent advances and future prospects.

Graphene-based nanomaterials are particularly useful nanostructured materials that show great promise in biotechnology and biomedicine. Owing to their unique structural features, exceptional chemical, electrical, and mechanical properties, and their ability to affect the microenvironment of biomolecules, graphene-based nanomaterials are suitable for use in various applications, such as immobilization of enzymes. We present the current advances in research on graphene-based nanomaterials used as novel scaffolds to build robust nanobiocatalytic systems. Their catalytic behavior is affected by the nature of enzyme-nanomaterial interactions and, thus, the availability of methods to couple enzymes with nanomaterials is an important issue. We discuss the implications of such interactions along with future prospects and possible challenges in this rapidly developing area.

Lipase immobilization on smectite nanoclays: Characterization and application to the epoxidation of α-pinene

The immobilization of lipase B from Candida antarctica on smectite group nanoclays (Laponite, SWy-2 and Kunipia), as well as on their organically modified derivatives, was investigated. A combination of techniques, namely X-ray diffraction, thermal analysis, X-ray photoelectron and FT-IR spectroscopy, was used for characterization of the novel immobilized biocatalyst. Structural and biochemical characterization have revealed that the hydrophobic microenvironment created by the organo-modified clays induces minor changes on the secondary structure of the enzyme, resulting in enhanced catalytic behaviour in hydrophobic media. The immobilized lipase on such modified nanoclays can be effectively applied for the indirect epoxidation of alpha-pinene using hydrogen peroxide as substrate. The amount of alpha-pinene epoxide produced in a single-step biocatalytic process is up to 3-fold higher than that of free enzyme or enzyme immobilized in non-modified clays. Moreover, lipase immobilized in modified clays retains up to 90% of its initial activity, even after 48h of incubation in the presence of oxidant, and up to 60% after four reaction cycles, while other forms of the enzyme retain less than 10%.

Graphene-Based Nafion Nanocomposite Membranes: Enhanced Proton Transport and Water Retention by Novel Organo-functionalized Graphene Oxide Nanosheets

Novel nanostructured organo-modified layered materials based on graphene oxide carrying various hydrophilic functional groups (-NH(2), -OH, -SO(3)H) are prepared and tested as nanofillers for the creation of innovative graphene-based Nafion nanocomposites. The hybrid membranes are characterized by a combination of analytical techniques, which show that highly homogeneous exfoliated nanocomposites are created. The pulsed field gradient NMR technique is used to measure the water self-diffusion coefficients. Remarkable behavior at temperatures up to 140 °C is observed for some composite membranes, thereby verifying the exceptional water retention property of these materials. Dynamic mechanical analysis shows that hybrid membranes are much stiffer and can withstand higher temperatures than pure Nafion.

Revealing the ultrafast process behind the photoreduction of graphene oxide

Effective techniques to reduce graphene oxide are in demand owing to the multitude of potential applications of this two-dimensional material. A very promising green method to do so is by exposure to ultraviolet irradiation. Unfortunately, the dynamics behind this reduction remain unclear. Here we perform a series of transient absorption experiments in an effort to develop and understand this process on a fundamental level. An ultrafast photoinduced chain reaction is observed to be responsible for the graphene oxide reduction. The reaction is initiated using a femtosecond ultraviolet pulse that photoionizes the solvent, liberating solvated electrons, which trigger the reduction. The present study reaches the fundamental time scale of the ultraviolet photoreduction in solution, which is revealed to be in the picosecond regime.

Large‐Yield Preparation of High‐Electronic‐Quality Graphene by a Langmuir–Schaefer Approach

Graphene was discovered less than five years ago and proved the existence of pure two-dimensional systems, thought physically impossible in the past. It appeared very quickly that this exceptionalmaterial showedmany outstanding properties. Since electrons and holes in graphene have potential for high carrier mobilities, this novel material has become an exciting new playground for physicists; properties such as the halfinteger quantum Hall effect at room temperature, spin transport, high elasticity, electromechanicalmodulation, and ferromagnetism all contribute to the fame of graphene. Since the first experiments conducted five years ago on micromechanically cleaved graphite (the renowned but lowyield adhesive tape method), the growing appeal of graphene’s properties has focused much of the research attention towards the conception of a reliable method for large-scale production. Recent advances using chemical vapor deposition and successful transfer of the prepared films to arbitrary substrates brought impressive results in terms of crystalline quality of the layers and consequent electrical and mechanical properties. Notwithstanding these results, truly controllable singleor multilayer large-scale deposition is still a pressing issue and a method for depositing high-quality graphene at variable coverage on an arbitrary surface is not yet available. Moreover, for practical application or simply for fundamental research purposes, good adhesion of graphene to the substrate is of great importance.