Georgina Fabregat

Nanostructured conducting polymer for dopamine detection

In this work, we demonstrate the ability of poly(N-methylpyrrole) to form nanostructures and to detect very low concentrations of dopamine, an important neurotransmitter. Poly(N-methylpyrrole) hollow particles of controlled thickness have been prepared using the layer-by-layer assembly technique and polystyrene core-shell particles as templates, which are subsequently eliminated to yield free-standing hollow microspheres with a layer thickness of 30 nm. The morphology and composition of these structures have been evaluated by scanning electron microscopy, transmission electron microscopy, FTIR, Raman and X-ray photoelectron spectroscopies. Results demonstrate that intact hollow spheres can be obtained controlling the number of polymer deposition cycles. Furthermore, two kind of sensors were constructed by immobilizing poly(N-methylpyrrole)/Au nanocomposites and poly(N-methylpyrrole) nanomembranes on the surface of a glassy carbon electrode. Electrochemical techniques were employed to evaluate the ability of poly(N-methylpyrrole) to absorb/immobilize dopamine molecules. It was found that systems based on this conducting polymer are highly sensitive to the neurotransmitter concentration, presenting a very fast response even when the concentration of the dopamine is very low.

Selective detection of dopamine combining multilayers of conducting polymers with gold nanoparticles

Electrodes based on the combination of three-layered films formed by two different conducting polymers and gold nanoparticles have been developed for the selective voltammetric determination of dopamine in mixtures with ascorbic acid and uric acid and human urine samples with real interferents. Voltammetric studies of solution mixtures indicate that electrodes formed by alternated layers of poly(3,4-ethylenedioxithiophene) (internal and external layer) and poly(N-methylpyrrole) (intermediate layer) show the best performance in terms of sensitivity and resolution. Furthermore, the sensitivity of such three-layered electrodes increases only slightly after coating its surface with gold nanoparticles (AuNPs), indicating that the catalytic effect typically played by AuNPs in the oxidation of dopamine is less effective in this case. Electrochemical pretreatments based on the application of consecutive oxidation-reduction cycles to electrodes before the detection process have been found to improve the selectivity without altering the sensitivity. On the other hand, the flux of dopamine to the three-layered surface increases linearly with the scan rate. The detection limit for these electrodes is around 10 μM DA in mixtures with uric acid, ascorbic acid, and cetaminophen, decreasing to 2-3 μM in the absence of such interferents. The utility of three-layered electrodes as sensors has also been demonstrated by determining DA in human samples with real interferents.

An electroactive and biologically responsive hybrid conjugate based on chemical similarity

Synthetic amino acids have become very important tools for the design of new materials. In this work, an electroactive polymer–amino acid hybrid material has been synthesized by conjugating poly(3,4-ethylenedioxythiophene) (PEDOT), a well known conducting polymer, with a synthetic amino acid bearing 3,4-ethylenedioxythiophene, which has been explicitly designed and prepared for such a purpose. Nanometric films have been electrochemically generated using a two-step procedure to evaluate the properties and potential applications of the resulting hybrid material. The successful incorporation of the amino acid as end-capping of the PEDOT chains has been proved by FTIR, energy dispersive X-ray and X-ray photoelectron spectroscopies. The fabrication of the hybrid material using an engineered tissue has allowed us to preserve not only morphological and structural characteristics of the conducting polymer but also, and most importantly, to preserve the electrical conductivity, electroactivity, electrochemical stability and specific capacitance. Finally, the behavior of the hybrid material as a cellular matrix has been compared with that of PEDOT using cellular adhesion and proliferation assays. Results obtained in this work represent the success of a new strategy for the preparation of peptide-conducting polymer hybrid materials, which is currently being improved upon by transforming the 3,4-ethylenedioxythiophene-containing amino acid into a cell adhesive peptide.

Hybrid materials consisting of an all-conjugated polythiophene backbone and grafted hydrophilic poly(ethylene glycol) chains

Organic hybrid materials consisting of an all-conjugated polythiophene backbone and well-defined poly(ethylene glycol) (PEG) grafted chains have been prepared by anodic polymerization of chemically synthesized macromonomers. The latter consist of a pentathiophene sequence in which the central ring bears a PEG chain with Mw = 1000 or 2000 at the 3-position. The influence of the polymerization potential, the length of the PEG branches and the dopant agent on the structure and properties of the graft copolymers has been examined. The chemical structure of the grafted materials has been corroborated by FTIR and X-ray photoelectron spectroscopies. Scanning electron microscopy and atomic force microscopy studies reveal that the morphology and topography of these materials are influenced by the above mentioned factors, even though homogeneous films showing a compact distribution of nanoaggregates, very flat surfaces (i.e. roughness < 15 A) and nanometric thickness (i.e. 100–500 nm) were obtained in all cases. Cyclic voltammetry assays have been used to determine the presence of charged species, the electroactivity, the electrostability and the formation of cross-links. The electrochemical stability of the copolymer with grafted PEG chains of Mw = 1000 has been found to increase with the number of consecutive oxidation–reduction cycles (self-electrostabilizing behavior). Finally, a preliminary investigation into the applicability of these hybrid materials as active surfaces for the selective adsorption of proteins is presented.

Detection of Dopamine Using Chemically Synthesized Multilayered Hollow Microspheres

Microspheres made of alternating layers of two different conducting polymers, poly(3,4-ethylenedioxythiophene) and poly(N-methylpyrrole), have been found to be sensitive to dopamine (DA) oxidation, presenting a very well-defined and linear response in the range of DA concentrations from 0.5 to 2 mM. The novelty of the present study is the use of doped multilayered hollow microspheres, which are prepared by successive oxidative chemical polymerizations in FeCl3 aqueous solution. The multilayered microspheres were characterized by FTIR and UV-visible spectroscopies, scanning and transmission electron microscopies, and atomic force microscopy. The UV-visible bands confirm that the multilayered system is not well doped with FeCl4(-) counterions. Therefore, the doping level was increased by further oxidation with LiClO4 before DA electrochemical detection. Despite that the range of concentration detection was limited from 0.5 mM to 2 mM, doped hollow multilayered microspheres show a very good anodic peak current response compared to single-layer films fabricated with an individual conducting polymer and activated by gold nanoparticles.

Polythiophene-g-poly(ethylene glycol) graft copolymers for electroactive scaffolds

The properties, microscopic organization and behavior as the cellular matrix of an all-conjugated polythiophene backbone (PTh) and well-defined poly(ethylene glycol) (PEG) grafted chains have been investigated using different experimental techniques and molecular dynamic simulations. UV-vis spectroscopy has been used to determine the optical band gap, which has been found to vary between 2.25 and 2.9 eV depending on the length of the PEG chains and the chemical nature of the dopant anion, and to detect polaron → bipolaron transitions between band gap states. The two graft copolymers have been found to be excellent cellular matrices, their behavior being remarkably better than that found for other biocompatible polythiophene derivatives [e.g. poly(3,4-ethylenedioxythiophene)]. This is fully consistent with the hydrophilicity of the copolymers, which increases with the molecular weight of the PEG chains, and the molecular organization predicted by atomistic molecular dynamics simulations. Graft copolymers tethered to the surface tend to form biphasic structures in solvated environments (i.e. extended PTh and PEG fragments are perpendicular and parallel to the surface, respectively) while they collapse onto the surface in desolvated environments. Furthermore, the electrochemical activity and the maximum of current density are remarkably higher for samples coated with cells than for uncoated samples, suggesting multiple biotechnological applications in which the transmission with cells is carried out at the electrochemical level.

Incorporation of a Clot-Binding Peptide into Polythiophene: Properties of Composites for Biomedical Applications

Biocomposites formed by a pentapeptide (CREKA), which recognizes clotted plasma proteins, entrapped into the poly(3,4-ethylenedioxythiophene) (PEDOT) matrix have been prepared using three very different procedures. X-ray photoelectron spectroscopy analyses indicate that PEDOT-CREKA films, prepared by chronoamperometry in basic aqueous solution (pH = 10.3) and deposited onto a PEDOT internal layer, present the higher concentration of peptide: one CREKA molecule per six polymer repeat units. The surface of this bilayered system shows numerous folds homogeneously distributed, which have been exhaustively characterized by scanning electron microscopy and atomic force microscopy. Indeed, the morphology and topography of such bilayered films is completely different from those of biocomposite-prepared acid aqueous and organic solutions as polymerization media. The impact of the entrapped peptide molecules in the electrochemical properties of the conducting polymer has been found to be practically negligible. In contrast, biocompatibility assays with two different cellular lines indicate that PEDOT-CREKA favors cellular proliferation, which has been attributed to the binding of the peptide to the fibrin molecules from the serum used as a supplement in the culture medium. The latter assumption has been corroborated examining the ability of PEDOT-CREKA to bind fibrin. The latter ability has been also used to explore an alternative strategy based on the treatment of PEDOT-CREKA with fibrin to promote cell attachment and growth. Overall, the results suggest that PEDOT-CREKA is appropriated for multiple biomedical applications combining the electrochemical properties of conducting polymer and the ability of the peptide to recognize and bind proteins.

Water absorbed by polyaniline emeraldine tends to organize, forming nanodrops

Interactions, in terms of both binding energies and microscopic organization, of water molecules absorbed by hydrophilic polyaniline emeraldine base have been investigated using quantum mechanical calculations, molecular dynamics simulation, FTIR spectroscopy, and (1)H NMR. From an enthalpic point of view, water molecules interact more favorably with imine nitrogen atoms than with amine ones, even though the latter are entropically favored with respect to the former because of their two binding sites. Quantum mechanical results show that interaction energies of water molecules reversibly absorbed but organized individually around a binding site range from 3.0 to 6.3 kcal/mol, which is in good agreement with activation energies of 3-5 kcal/mol previously determined by thermodynamic measurements. The irreversible absorption of water to produce C-OH groups in rings of diimine units has been examined considering a three steps process in which water molecules act as both acidic and nucleophilic reagent. Although calculations predict that the whole process is disfavored by 5-8 kcal/mol only, FTIR and (1)H NMR detected the existence of reversibly absorbed water but not of C-OH groups. Both the binding energies and the structural information provided by molecular dynamics simulations have been used to interpret the existence of two types of physisorbed water molecules: (i) those that interact individually with polymer chains and (ii) those immersed in nanodrops that are contained within the polymeric matrix. The binding energies calculated for these two types of water molecules are fully consistent with the thermodynamic activation energies previously reported.

Polyaniline Emeraldine Salt in the Amorphous Solid State: Polaron versus Bipolaron

The polaronic and bipolaronic forms of polyaniline emeraldine salt (PAni-ES) in the amorphous solid state have been simulated using classical molecular dynamics (MD) and hybrid quantum mechanical/molecular mechanical-molecular dynamics (QM/MM-MD) approaches. It should be remarked that the electronic state of PAni-ES has been theoretically investigated in the gas phase, solution phase, and crystalline state, but this is the first study in the amorphous solid state, which is the most typical for this conducting polymer. MD simulations were carried out using force-field parametrizations explicitly developed for polaronic and bipolaronic models. QM/MM-MD calculations were performed using a quantum mechanical zone defined by four repeat units. In addition of the structural and electronic characteristics of the two forms of PAni-ES, MD and QM/MM-MD simulations indicate that the bipolaronic is the most stable state of amorphous PAni-ES. Complementary studies have been carried out using different experimental techniques. Although the morphology and topography of doped and undoped PAni are very similar, comparison of their UV-vis spectra supports the preference toward the bipolaronic form of PAni-ES.

Controlling the Morphology of Poly(N-cyanoethylpyrrole)

The morphology of poly(N-cyanoethylpyrrole) has been controlled through the polymerization process. This polymer has been prepared by anodic polymerization, chemical oxidative polymerization in emulsion medium, and layer-by-layer templating polymerization. Anodic polymerization using LiClO4 as supporting electrolyte provides compact films, in which the oxidation degree is controlled through the thickness, useful for the microdetection of dopamine. Chemical polymerization using FeCl3 as oxidant agent results in very well-defined microspheres with porous internal structure, which may be useful in molecular loading and transport processes. Finally, the layer-by-layer templating technique produces core-shell particles of controlled size and thickness. Moreover, these core-shell particles can be easily converted in hollow microspheres by removing the template.