Claudia R. Rivarola

Functionalised conjugated materials as building blocks of electronic nanostructures

Two different approaches towards conjugated material (carbon nanotubes, conjugated polymers) functionalisation are presented: covalent bonding of functional groups and covalent interaction with soluble polymers. Covalent functionalisation of carbon nanotubes is made by reaction of the aromatic ring with aryl radicals, produced by reduction of diazonium ions. In the case of conducting polymers, covalent functionalisation is brought about by reaction of polyanilines with diazotized aromatic amines (including amino terminated azo dyes). The non covalent functionalisation of carbon nanotubes is made by wrapping the nanotubes with soluble conducting polyanilines. The functionalised materials are characterised by FTIR spectroscopy, X-ray diffraction, dynamic light scattering, ultraviolet-visible absorption and emission spectroscopy, transmission electron microscopy, cyclic voltammetry, differential electrochemical mass spectroscopy and conductivity measurements. The materials are to build ionic self assembled multilayers using a layer-by-layer deposition process. The charge transport and electrocatalytic behaviour of the assemblies, relevant to the application of the assemblies in nanostructured electrochemical biosensors, are evaluated using different redox molecules and/or its intrinsic electroactivity as probes.

Smart surfaces: reversible switching of a polymeric hydrogel topography

Patterns imprinted on smart surfaces are fabricated by direct laser interference patterning (DLIP) of thick films based on poly(N-isopropylacrylamide) (PNIPAM) doped with suitable dyes. Optical and atomic force (AFM) microscopy images reveal that the pattern imprinted on the dry hydrogel film (line-arrays) becomes flat due to swelling of the hydrogel upon immersion in water. The pattern re-emerges after drying the hydrogel. Heating the hydrogel above the phase transition temperature of PNIPAM (ca. 32 °C) also restores the pattern by hydrogel volume collapse. This behaviour suggests that this patterning technique would allow us to produce surfaces useful for technological application.

Nanocomposite synthesis by absorption of nanoparticles into macroporous hydrogels. Building a chemomechanical actuator driven by electromagnetic radiation

Macroporous hydrogels irreversibly absorb solid nanoparticles from aqueous dispersions. A nanocomposite is made using a macroporous thermosensitive hydrogel (poly(N-isopropylacrylamide-co-(2-acrylamido-2-methyl propane sulfonic acid)) (poly(NIPAm-co-AMPS)) and conductive polymer (polyaniline, PANI) nanoparticles (PANI NPs). Macroporous gels of poly(NIPAm-co-AMPS) were made by a cryogelation technique. NPs of PANI were produced by precipitation polymerization. It is found that PANI NPs are easily absorbed into the macroporous hydrogels while conventional non-porous hydrogels do not incorporate NPs. It is shown that PANI NPs, dispersed in water, absorb NIR laser light or microwave radiation, increasing their temperature. Upon irradiation of the nanocomposite with microwaves or NIR laser light, the PANI NPs heat up and induce the phase transition of the thermosensitive hydrogel matrix and the internal solution is released. Other nano-objects, such as gold nanorods and PANI nanofibers, are also easily incorporated into the macroporous gel. The resulting nanocomposites also suffer a phase transition upon irradiation with electromagnetic waves. The results suggest that, using a thermosensitive matrix and conducting nanoparticles, mechanical/chemical actuators driven at a distance by electromagnetic radiation can be built. The sensitivity of the nanocomposite to electromagnetic radiation can be modulated by the pH, depending on the nature of the incorporated nanoparticles. Additionally, it is possible to make systems which absorb either NIR or microwaves or both.

Organic Chemistry of Polyanilines: Tailoring Properties to Technological Applications~!2008-08-19~!2008-09-11~!2008-11-28~!

The use of organic chemistry reactions to introduce additional functional groups on polyanilines is described. Among the reactions discussed are: electrophilic aromatic substitution, nucleophilic addition to the aromatic rings, nu- cleophilic substitution on the amine groups and reactions on pendant groups. The use of combinatorial chemistry tech- niques, by coupling of combinatorially synthesised diazonium salts with polyaniline, to produce a functionalized polyani- lines library is also reviewed. The modification of polyaniline introduces or alters different properties of the materials: solubility, self-doping and redox coupled ion exchange. The tailoring of those properties to technical applications is there- fore examined.

Evidence of Hydrophobic Interactions Controlling Mobile Ions Release from Smart Hydrogels

A water soluble cationic metal complex (tris(2,2′-bipyridine)ruthenium(II)) can be loaded into hydrogels containing acrylamide and/or acrylic acid units. The cationic complex is retained in polymer containing acrylic acid units at high pH and released at low pH. This is likely due to electrostatic interactions of the cation with the carboxylate anions, present at high pH, which are converted into neutral carboxylic acid at low pH, releasing the metal complex. Since the gel contract at low pH, the water soluble cation is also released with the water expelled from the gel. However, a strong retention of the cation inside the gels is observed when acrylamide units are present. A possible explanation is a hydrophobic interaction of the large metal complex with the polyacrylamide network. Using the counteracting electrostatic and hydrophobic interactions of probe molecules with smart hydrogel matrixes it is possible to tune the pH of maximum release away from the pKa of the ionizable group.

Physicochemical properties of ionic and non-ionic biocompatible hydrogels in water and cell culture conditions: Relation with type of morphologies of bovine fetal fibroblasts in contact with the surfaces

Cationic, anionic and non-ionic hydrogels having acrylamide polymer backbones were synthesized via free radical polymerization with N,N-methylenebisacrylamide (BIS) as crosslinker. The chemical structures of the hydrogels were characterized by Fourier Transform Infrared Spectroscopy (FTIR). Physicochemical properties such as swelling kinetic, maximum swelling capacity, volume phase transition temperature (VPTT) and wettability (static water contact angle) of hydrogels swollen in aqueous and cell culture medium, at room and cell culture temperatures were studied. In order to correlate the surface properties of the hydrogels and cellular adhesivity of bovine fetal fibroblasts (BFFs), cellular behaviour was analyzed by inverted fluorescence optical microscopy and atomic force microscopy (AFM). MTT assay demonstrated that the number of viable cells in contact with hydrogels does not significantly change in comparison to a control surface. Flattened and spindle-shaped cells and cell spheroids were the adopted morphologies during first days of culture on different hydrogels. Cell spheroids were easily obtained during the first 5days of culture in contact with PNIPAM-co-20%HMA (poly (N-isopropylacrylamide-co-20%N-acryloyl-tris-(hydroxymethyl)aminomethane)) hydrogel surface. After 15days of culture all hydrogels showed high adhesion and visual proliferation. According to obtained results, non-ionic and hydrophilic surfaces with moderated wettability induce the formation of BFFs cell spheroids. These hydrogel surfaces could be used in clinical and biochemical treatments at laboratory level to cell growth and will allow generating the base for future biotechnologic platform.

Synthesis, Properties and Applications of Conducting Polymer Nano-Objects

Two different approaches are used to produce conducting polymer nano-objects. One is a “top-down” approach which involves laser ablation of conducting polymer films using laser light interference patterns (direct laser interference patterning, DLIP) to produce various surface shapes, including nanowires and nanodots. Polyaniline(PANI) and polypyrrole (PPy) nanostructures could be easily produced by ablation of films, previously formed by in-situ polymerization of the aromatic monomers. The other is a “bottom-up” approach involving the controlled nucleation and growth during monomer polymerization. This is achieved by performing the polymerization at the interface of two immiscible solvents. Both kinds of nanomaterials are characterized using dynamic light scattering, TEM, EDAX, FTIR, UV-vis and fluorescence spectroscopy. The structures are studied by SEM-FIB, optical and fluorescence microscopy along with water contact angle. It is shown that nanometric sized structures can be made by both methods. The chemical structures are quite similar or identical to that of the bulk polymer. While PANI nanofibers are dispersed in acid media, due to the surface charge related with chain protonation, they agglomerate in neutral media. In the interest of biological applications, different soluble polymers are used to help disperse the nanofibers at neutral pH. Both the dispersing agent and PANI nanofibers have to be innocuous to biological cells and higher organisms, like frog larvae. The successful intake of PANI nanofibers into cancer line cells and frog larvae prompts its application as NIR radiation absorbers in photothermal or photoacoustic tumor therapy and/or tomography.

Improving the retention and reusability of Alpha-amylase by immobilization in nanoporous polyacrylamide-graphene oxide nanocomposites

Alpha-amylase was immobilized inside three different polymeric matrices: polyacrylamide hydrogel (PAAm), polyacrylamide-graphene oxide nanocomposite (PAAm-GO) and alginate in order to study and compare the effect of the matrix on the catalytic performance. The morphology, swelling, mechanical properties, retention efficiency, and the catalytic behavior of these newly supported biocatalysts were studied. Nanocomposite made of PAAm-GO matrix incorporated 98% of the enzyme, likely through a cooperative effect, while alginate gels incorporated only 30%. Moreover, the enzyme retention using PAAm-GO reached a value of 97.5%. Starch hydrolysis catalyzed by the immobilized enzyme in PAAm-GO matrix showed similar kinetics profiles up to 5 cycles suggesting that the enzymatic activity is retained. These results compare very favorably with conventional immobilization in alginate where almost no activity was observed after 3 cycles. All results suggest that the PAAm matrices protect the biocatalyst allowing its reusability. Moreover, the improvements in enzyme catalytic properties via immobilization made this system as an excellent candidate in bio-industrial applications such as bioethanol production. Furthermore, the synthesized catalyst could produce a high yield of bioethanol by using enzymes and yeast immobilized in the same PAAm matrix. In this way, it is possible to produce sequential or simultaneous saccharification and fermentation.