Grigoris Mountrichas

Properties, applications and functionalisation of carbon nanohorns

Functionalisation of carbon nanohorns (CNHs), by covalent bonding or non-covalent supramolecular interacting organic moieties, overcomes insolubility problems and enhances manipulation and processibility. The present review paper summarises our recent accomplishments in the field of chemical modification of CNHs together with various critical and important aspects related to their synthesis, morphology, properties and applications. Moreover, the design and preparation of some novel hybrid materials consisting of CNHs and electron donors will be presented.

Polyplexes based on cationic polymers with strong nucleic acid binding properties

Cationic polymers have been studied for nucleic acid delivery both in vitro and in vivo. However, many polymer-based formulations suffer from lack of stability in biologic fluids due to interactions with anionic biomacromolecules such as proteins and polysaccharides. Likely, the stronger the electrostatic interactions between a cationic polymer and nucleic acids, the higher the stability of the polyplexes in biologic fluids will be. To get evidence for this hypothesis, quaternized poly[3,5-bis(dimethylaminomethylene)-p-hydroxyl styrene] (QNPHOS) with two permanently charged cationic sites per monomer unit as well as its block copolymer with PEG were synthesized and compared with the standard transfectant pDMAEMA, in terms of nucleic acid binding strength, gene silencing and transfection activities of the complexes which these polymers form with siRNA and plasmid DNA, respectively. It was shown that siRNA complexes based on QNPHOS and QNPHOS-PEG dissociate in the presence of a fourfold higher heparin concentration than necessary to destabilize pDMAEMA complexes. Under the same conditions, complexes of DNA and QNPHOS or QNPHOS-PEG did not show any dissociation, in contrast to pDMAEMA polyplexes. The DNA polyplexes based on QNPHOS or QNPHOS-PEG did not show transfection activity, which might be ascribed to their high physicochemical stability. On the other hand, siRNA complexes based on QNPHOS and QNPHOS-PEG showed a low cytotoxicity and an improved siRNA delivery and high gene silencing activity, even higher than those based on pDMAEMA. This might be due to the excellent binding characteristics of QNPHOS and QNPHOS-PEG to siRNA which in turn is ascribed to the presence of two permanently charged cationic groups per monomer unit. Based on the results of this study, it is concluded that formation of strong siRNA complexes with polymers containing double charges per monomer is advantageous.

Photoinduced electron transfer in aqueous carbon nanotube/block copolymer/CdS hybrids: application in the construction of photoelectrochemical cells

Pristine and shorted multi-walled carbon nanotubes (pMWCNT and sMWCNT, respectively) non-covalently modified with a block copolymer (poly[sodium (2-sulfamate-3-carboxylate) isoprene-b-styrene]—abbreviated as CSI) are used for the formation of CdS semiconductor nanohybrids. The CdS nanoparticles are preferentially localized on the surface of the nanotubes due to specific interactions with the polymer chains. In these nanohybrid materials photoinduced electron transfer phenomena are found to occur from the photoexcited CdS nanoparticles to the nanotubes as evidenced by the efficient fluorescence emission quenching of CdS nanoparticles. Nanosecond transient absorption spectroscopy sheds light on the transient species formed by charge separation, namely (pMWCNT)˙−/CSI/(CdS)˙+ and (sMWCNT)˙−/CSI/(CdS)˙+. The MWCNT/CSI/CdS nanohybrid materials are deposited onto ITO electrodes by the drop-casting method. The ITO/sMWCNT/CSI/CdS electrode exhibit an incident photon to photocurrent efficiency (IPCE) of 7% at an applied bias of +0.2 V vs.SCE in a standard three-compartment electrochemical cell. Direct electron injection from the reduced nanotubes to ITO electrode after the photoinduced charge separation is responsible for the photocurrent generation.

Polyelectrolyte-surfactant complexes formed by poly[3,5-bis(trimethylammoniummethyl)4-hydroxystyrene iodide]-block-poly(ethylene oxide) and sodium dodecyl sulfate in aqueous solutions.

Formation of polyelectrolyte-surfactant (PE-S) complexes of poly[3,5-bis(trimethylammoniummethyl)-4-hydroxystyrene iodide]-block-poly(ethylene oxide) (QNPHOS-PEO) and sodium dodecyl sulfate (SDS) in aqueous solution was studied by dynamic and electrophoretic light scattering, small-angle X-ray scattering (SAXS), atomic force microscopy, and fluorometry, using pyrene as a fluorescent probe. SAXS data from the QNPHOS-PEO/SDS solutions were fitted assuming contributions from free copolymer, PE-S aggregates described by a mass fractal model, and densely packed surfactant micelles inside the aggregates. It was found that, unlike other systems of a double hydrophilic block polyelectrolyte and an oppositely charged surfactant, PE-S aggregates of the QNPHOS-PEO/SDS system do not form core-shell particles and the PE-S complex precipitates before reaching the charge equivalence between dodecyl sulfate anions and QNPHOS polycationic blocks, most likely because of conformational rigidity of the QNPHOS blocks, which prevents the system from the corresponding rearrangement.

Complexes between High Charge Density Cationic Polyelectrolytes and Anionic Single- and Double-Tail Surfactants

Polyelectrolyte/surfactant complexes formed between well-defined linear flexible polyelectrolytes, namely, quaternized poly[3,5-bis(dimethylaminomethylene)hydroxystyrene] (Q-N-PHOS), bearing two cationic sites on each repeating unit, and two different anionic surfactants, namely, sodium dodecyl sulfate (SDS) with one hydrocarbon tail and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) with two hydrocarbon chains, are studied by means of fluorescence spectroscopy, electrophoretic, dynamic and static light scattering, and atomic force microscopy. Depending on the surfactant state in initial solutions (i.e., below or above nominal critical micelle concentration, cmc) and final (-/+) charge ratio, self-assembly in nanoparticles of variable size, stability, and effective charge is possible. Spherical, rather polydispserse complexes are formed in all cases. Critical aggregation concentrations (cac) depend on the surfactant type, while hydrophobicity of the main polyelectrolyte chain plays a role in colloidal stability of the complex nanoparticles.

pH-dependent self-assembly of polystyrene-block-poly((sulfamate-carboxylate)isoprene) copolymer in aqueous media.

The amphiphilic polystyrene- block-poly((sulfamate-carboxylate)isoprene) (PS-PISC) diblock copolymer was synthesized from the precursor diblock copolymer polystyrene- block-isoprene by reaction with chlorosulfonyl isocyanate. The structure and behavior of self-assembled PS-PISC nanoparticles was studied in alkaline and acidic aqueous solutions by a combination of static and dynamic light scattering, analytical ultracentrifugation, atomic force and cryogenic transmission electron microscopies, NMR spectroscopy, potentiometric titration, and fluorometry using pyrene as a polarity-sensitive fluorescent probe. It was found that PS-PISC exists in aqueous solutions in the form of micellar aggregates. The aggregation tendency increases with decreasing effective charge density in the shell, that is, with decreasing pH of the solution, and aggregates found in alkaline aqueous media have much smaller molar masses than those formed in acidic media. The latter are dense, collapsed structures with immobile PISC domains in which most of the COOH and NH 2 (+)SO 3 (-) groups are buried inside of the nanoparticles. The swelling of PISC domains and disentanglement of PISC chains after addition of a base are slow processes occurring on the time scale of days.

Stable responsive diblock copolymer micelles for rheology control

Stable block copolymer micelles with crosslinked rubbery cores serve as model soft colloids with tunable rheology. We demonstrate this by studying systematically two such micelles: one with a small polyisoprene core and long polystyrene hairs that dominate the response of the system, and another with a larger polybutadiene core and shorter polystyrene hairs, exhibiting colloid-like behavior. In addition to mass concentration, we use temperature as a means to vary the volume fraction and show that achieving the same volume fraction by these two different routes yields different macroscopic properties. The number density of the particles and osmotic pressure effects are the origin of the difference. The response of these micelles to varying solvent quality conditions has unexpected rheological consequences: remarkably, for concentrated colloid-like micelles, the viscosity exhibits a non-monotonic temperature dependence. This is attributed to particle swelling and changing particle interactions with temperature. It is thus evident that such systems provide ways for tailoring the flow of soft colloids.

Association of Poly(4-hydroxystyrene)-block-Poly(Ethylene oxide) in Aqueous Solutions: Block Copolymer Nanoparticles with Intermixed Blocks

Association behavior of diblock copolymer poly(4-hydroxystyrene)-block-poly(ethylene oxide) (PHOS-PEO) in aqueous solutions and solutions in water/tetrahydrofuran mixtures was studied by static, dynamic, and electrophoretic light scattering, (1)H NMR spectroscopy, transmission electron microscopy, and cryogenic field-emission scanning electron microscopy. It was found that, in alkaline aqueous solutions, PHOS-PEO can form compact spherical nanoparticles whose size depends on the preparation protocol. Instead of a core/shell structure with segregated blocks, the PHOS-PEO nanoparticles have intermixed PHOS and PEO blocks due to hydrogen bond interaction between -OH groups of PHOS and oxygen atoms of PEO and are stabilized electrostatically by a fraction of ionized PHOS units on the surface.

Solution Behavior of Poly(sodium(sulfamate-carboxylate)isoprene), a pH Sensitive and Intrinsically Hydrophobic Polyelectrolyte

The solution properties of a novel pH sensitive and intrinsically hydrophobic polyelectrolyte poly(sodium(sulfamate-carboxylate)isoprene) (SCPI) were investigated by means of dynamic, static, and electrophoretic light-scattering and fluorescence spectroscopy techniques. Because of the pH dependent charge density of the polyelectrolyte chain, the dynamics and structure of the solution were studied at both pH 7 (high charge density) and pH 3 (low charge density) and at low ionic strength conditions for two samples of different molecular weights. In all cases, a fast and a slow diffusive mode were observed. The fast mode is attributed to the diffusion of isolated polyelectrolyte chains, while the slow mode denotes the presence of multichain domains in solution, formed by electrostatic and/or hydrophobic interactions. The size and density of the multichain domains were reduced with decreasing charge density and molecular weight. Effective charge of the particles does not depend appreciably on solution pH, while fluorescence spectroscopy revealed the presence of hydrophobic domains in the studied systems. It was found that increasing temperature resulted in the compaction of both isolated chains and multichain domains due to the hydrophobic effect. Furthermore the increase of the ionic strength of the solution led to a partial dissociation of the multichain domains at short times and at pH 7, while it led to increased aggregation and precipitation at pH 3 and long times. The performed experiments allowed for a separation of the electrostatic and hydrophobic contributions to the self-assembly of the particular polyelectrolyte systems.

Polymer Covalent Functionalization of Carbon Nanohorns Using Bulk Free Radical Polymerization

Herein, we report on a facile approach for the covalent functionalization of carbon nanohorns, using in situ bulk free radical polymerization of methacrylic acid. The obtained material is soluble in aqueous media, facilitating its processability and has been fully characterized by means of complementary spectroscopic techniques, electron microscopy, thermogravimetric analysis, and light scattering. Simultaneously, the material has been used as a template for the synthesis of gold nanoparticles on the surface of the polymer-decorated carbon nanohorns.