Time resolved emission spectroscopy of poly(2,5-dicyano-p-phenylene-vinylene) films

Films of poly (2,5-dicyano-p-phenylene vinylene), DCNPPV, were obtained by electrochemical synthesis over gold thin layer (20 nm) transparent electrode deposited on a glass plate. The DCNPPV films of 4 µm thickness were produced by electropolymerization process of α,α,α',α'-tetrabromo-2-5-dicyano-p-xilene at different applied potentials (-0.15, -0.25, -0.40, -0.60, -0.80, and -1.0 V) using 0.1 mol L-1 of tetraethylammonium bromide in acetonitrile as the supporting electrolyte. The emission decays have three exponential components: a fast component in the picosecond range (200-400 ps), and two other of about one and five nanoseconds at 293 K. The fluorescence quenching process seems to occur by exciton trapping in a low-energy site and quenching by residual bromine monomer attached at the end of the polymer chain. However, the electrochemical synthesis generates entrapped bromide or ion pairs during the growth step of the film which also contributes to the deactivation. The change of the electrolyte from bromide to perchlorate reduces significantly this additional quenching effect by allowing ion exchange of formed bromide with the nonquenching perchloride anion.Filmes finos de poli(2,5-diciano-p-fenileno vinileno), DCNPPV, foram produzidos por síntese eletroquímica com variação do potencial aplicado de-0,15 até-1,0 V, e depositados sobre camada fina de ouro sobre vidro. A cinética de estado excitado destes materiais foi investigada por medidas de decaimentos de fluorescência. Os filmes apresentam decaimentos com três componentes, uma rápida da ordem de 200-400 picossegundos, e outra duas componentes de aproximadamente um e cinco nanossegundos, na temperatura de 293 K. O decaimento de fluorescência ocorre pela desativação em sítios de baixa energia na cadeia polimérica conjugada e por supressão do estado excitado por monômeros bromados terminais da cadeia e íons brometo aprisionados durante o crescimento eletroquímico do filme. A mudança do ânion do eletrólito suporte de brometo para perclorato reduziu de modo significativo essa contribuição de supressão do estado excitado como resultado da troca iônica por uma espécie não supressora.FAPESPCNP

Electroactive Nanostructured Membranes (ENM): Synthesis and Electrochemical Properties of Redox Mediator-Modified Gold Nanoparticles Using a Dendrimer Layer-by-Layer Approach

The layer-by-layer (LbL) self-assembly process has become an important tool in the fabrication of nanostructured devices for electrochemical applications, especially in those cases where control at the molecular level is required. In this paper we present a system based on electroactive nanostructured membranes (ENM) with ITO-PVS/PAMAM-Au LbL electrodes, in which a redox mediator (Me) is electrodeposited around the Au nanoparticles to form an ITO-PVS/PAMAM-Au@Me system. The redox mediators used were Co, Fe, Ni and Cu hexacyanoferrates. The 3-bilayer ITO-PVS/PAMAM-Au@Me system was characterized electrochemically by cyclic voltammetry and electrochemical impedance spectroscopy. All hexacyanoferrate modified electrodes showed electrocatalytic activity towards hydrogen peroxide, thus demonstrating that this new approach can be used in biosensors and nanodevices, where a redox mediator is required

Discutindo a educação ambiental no cotidiano escolar: desenvolvimento de projetos na escola formação inicial e continuada de professores

A presente pesquisa buscou discutir como a Educação Ambiental (EA) vem sendo trabalhada, no Ensino Fundamental e como os docentes desta escola compreendem e vem inserindo a EA no cotidiano escolar., em uma escola estadual do município de Tangará da Serra/MT, Brasil. Para tanto, realizou-se entrevistas com os professores que fazem parte de um projeto interdisciplinar de EA na escola pesquisada. Verificou-se que o projeto da escola não vem conseguindo alcançar os objetivos propostos por: desconhecimento do mesmo, pelos professores; formação deficiente dos professores, não entendimento da EA como processo de ensino-aprendizagem, falta de recursos didáticos, planejamento inadequado das atividades. A partir dessa constatação, procurou-se debater a impossibilidade de tratar do tema fora do trabalho interdisciplinar, bem como, e principalmente, a importância de um estudo mais aprofundado de EA, vinculando teoria e prática, tanto na formação docente, como em projetos escolares, a fim de fugir do tradicional vínculo “EA e ecologia, lixo e horta”.Facultad de Humanidades y Ciencias de la Educació

In Situ Scanning Tunneling Microscopy Study of 5,6-Dimethyl Uracil on Au (111)

The hydrogen evolution mediated 5,6-dimethyluracil (5,6-DMU) film disordering and subsequent reordering was studied in situ in sulfuric acid solution on Au (111) using STM and cyclic voltammetry. A stable, condensed physisorbed film was formed between the limits of the hydrogen evolution (at -0.3 V/SCE) and the lifting of the surface reconstruction (at 0.4 V/SCE) in the bulk solution concentration used in this study. The onset of hydrogen evolution led to the formation of "bubble" like structures which disrupted the stability of the film causing a disordering process which takes place within the whole scanned area. Stepping the potential back to the region of stability allowed the real time observation of the film reorganization. An anisotropic evolution of the film was found. The surface step edges were not found to be preferential nucleation sites. The molecular packing structure was inferred from high-resolution STM images and the existence of two distinct adsorbate states was established

On the adsorption of sulfate on low Miller index Rh(hkl) electrodes: an in situ vibrational analysis

The adsorption of sulfate ion on Rh(111), Rh(100) and Rh(110) electrodes was studied using in situ FTIR reflection spectroscopy. For wavenumbers above 1200 cm-1, only one strong vibrational band is observed for adsorbed sulfate on the three low index rhodium surfaces. A weak band at 960 cm-1 is also observed for Rh(111). The strong band above 1200 cm-1 is potential dependent with a tuning rate of 50 cm-1 V-1. No differences were observed upon changing the solution acidity in order to provide sulfate or bisulfate in solution. This result is interpreted in terms of complete dissociation of the ions, giving only adsorbed SO4(2-). The surface orientation affects only the amount of the adsorbed sulfate species. It seems that the same adsorbed sulfate is present on Rh(111), Rh(110) and Rh(100)

Enhanced Charge Transport and Incorporation of Redox Mediators in Layer-by-Layer Films Containing PAMAM-Encapsulated Gold Nanoparticles

In this work, we exploit the molecular engineering capability of the layer-by-layer (LbL) method to immobilize layers of gold nanoparticles on indium tin oxide (ITO) substrates, which exhibit enhanced charge transfer and may incorporate mediating redox substances. Polyamidoamine (PAMAM generation 4) dendrimers were used as template/stabilizers for Au nanoparticle growth, with PAMAM-Au nanoparticles serving as cationic polyelectrolytes to produce LbL films with poly(vinylsulfonic acid) (PVS). The cyclic voltammetry (CV) of ITO-PVS/PAMAM-Au electrodes in sulfuric acid presented a redox pair attributed to Au surface oxide formation. The maximum kinetics adsorption is first-order, 95% of the current being achieved after only 5 min of adsorption. Electron hopping can be considered as the charge transport mechanism between the PVS/PAMAM-Au layers within the LbL films. This charge transport was faster than that for nonmodified electrodes, shown by employing hexacyanoferrate(III) as the surface reaction marker. Because the enhanced charge transport may be exploited in biosensors requiring redox mediators, we demonstrate the formation of Prussian blue (PB) around the Au nanoparticles as a proof of principle. PAMAM-Au@PB could be easily prepared by electrodeposition, following the ITO-PVS/ PAMAM-Au LbL film preparation procedure. Furthermore, the coverage of Au nanoparticles by PB may be controlled by monitoring the oxidation current

On-line mass spectrometry of the electro-oxidation of methanol in acidic media on tungsten carbide

The electro-oxidation of methanol at supported tungsten carbide (WC) nanoparticles in sulfuric acid solution was studied using cyclic voltammetry, potentiostatic measurements, and differential electrochemical mass spectroscopy (DEMS). The catalyst was prepared by a sonochemical method and characterized by X-ray diffraction. Over the WC catalyst, the oxidation of methanol (1 M in a sulfuric acid electrolyte) begins at a potential below 0.5 V/RHE during the anodic sweep. During potentiostatic measurements, a maximum current of 0.8 mA mg(-1) was obtained at 0.4 V. Measurements of DEMS showed that the methanol oxidation reaction over tungsten carbide produces CO2 (m/z=44); no methylformate (m/z=60) was detected. These results are discussed in the context of the continued search for alternative materials for the anode catalyst of direct methanol fuel cells.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[04/03051-6]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Cientfico e Tecnologico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq