Abstract The immobilization of the enzyme tyrosinase (Tyr) in lipid matrices can be explored to produce biosensors for detecting polyphenols, which is relevant for the food industry. Herein, we shall demonstrate the importance of the lipid composition to immobilize the enzyme tyrosinase in Langmuir-Blodgett (LB) films. Tyr could be incorporated into Langmuir monolayers of arachidic acid (AA), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1’-rac-glycerol) (sodium salt) (DPPG), having as the main effect an expansion in the monolayers. Results from polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) pointed to electrostatic interactions between the charged residues of Try and the lipid headgroups, in addition to changes in the order of lipid chains. The interaction between Tyr and DPPC in Langmuir monolayers can be correlated with the superior performance of DPPC/Tyr LB films used as biosensors to detect catechol by cyclic voltammetry. The molecular-level interactions assessed via PM-IRRAS are therefore believed to drive an immobilization process for Tyr in the lipid LB matrix and may serve as a general criterion to identify matrices that preserve enzyme activity.

Interface effects between metal electrodes and organic films are crucial for the overall performance of organic electronics devices. We investigate effects from gold electrodes deposited on spin-coated films of poly(p-phenylene vinylene) (PPV). While a thin Au layer (16 nm) did not affect the absorption and emission spectra of PPV, a 64 nm thick Au layer induced blue shifts in both spectra owing to the reduction in the effective conjugation degree of PPV segments. Upon combining photoluminescence and Raman scattering spectroscopies, we noted that the Au clusters interact preferentially with the phenyl rings of the polymer chain, leading to shifts in the vibrational modes at 1100 and 555 cm-1 and a significant change in the electron-phonon coupling inferred from the Huang-Rhys parameters. These results are consistent with theoretical predictions in ab initiocalculations, which imply that the final properties of polymeric devices may be tuned with adequate conditions for electrode deposition.

In this work we take advantage of the polyelectrolyte character of some Brazilian native gums to fabricate electrically conductive, nanostructured films. The gums Sterculia urens, (caraia), Sterculia striata (chicha) or Anadenanthera macrocarpa Benth were assembled in conjunction with poly(o-methoxyaniline) (POMA) in the form of layered nanostructured films using the layer-by-layer (LbL) technique. All the LbL films displayed a well-defined electroactivity, as confirmed via cyclic voltammetry. In comparison to POMA LbL films fabricated with conventional polyelectrolytes (viz. poly(vinyl sulfonic acid)-PVS), the presence of the gums in the LbL films increased remarkably the electrochemical stability of the films.

O complexo [{RuCl3(dppb)}2(µ-4,4'-bipy)] (dppb = PPh2(CH2)4PPh2; 4,4'-bipy= 4,4'-bipiridil) foi obtido por meio da reação entre 4,4'-bipiridina e mer-[RuCl3(dppb)].H2O, em bom rendimento. As características do complexo binuclear obtido, estudadas por medidas de FT-IR, UV-Vis, EPR e análise elementar foram similares às do complexo [RuCl3(dppb)L] (L= N-heterociclo). [{RuCl3(dppb)}2(µ-4,4'-bipy)] é solúvel em solventes fracamente polares, facilitando a sua utilização para produzir filmes de Langmuir e Langmuir-Blodgett (LB). Enquanto que filmes cast e o complexo em solução são vermelhos, filmes LB são azul-esverdeados, provavelmente devido à oxidação da bisfosfina. A natureza nanoestruturada dos filmes LB, se manifestou nas medidas eletroquímicas, já que estes filmes exibem maior corrente de pico do que os filmes cast e dip-coated. As respostas eletroquímicas destes filmes são caracterizadas pelo processo RuIII RuIII/RuII RuII, o qual difere do complexo em solução. Neste último caso, um processo irreversível é observado devido à formação de novas espécies. A eletroatividade dos filmes LB/[{RuCl3(dppb)}2(µ-4,4' -bipy)] indica a possibilidade de utilização em dispositivos eletroquímicos.

[{RuCl3(dppb)}2(µ-4,4'-bipy)] was prepared from the reaction of the 4,4'-bipyridine and mer-[RuCl3(dppb)].H2O, in good yield. The characteristics of the binuclear complex studied with FT-IR, UV-Vis, EPR and elemental analysis were similar to those of the [RuCl3(dppb)L] (L= N-heterocycle). [{RuCl3(dppb)}2(µ-4,4'-bipy)] is soluble in weakly polar organic solvents, facilitating its use in the production of Langmuir and Langmuir-Blodgett (LB) films production. While the cast films and the complex in solution were red, the LB films were green-bluish, probably due to oxidation of the bisphosphine. The nanostructured nature of the LB films was manifested in electrochemical measurements, as these films exhibited higher peak currents than cast or dip-coated films. For all films, the electrochemical response was dominated by the RuIII RuIII/RuII RuII process, which differed from that of the binuclear compound. For the latter, an irreversible process appeared due to formation of new species. The electroactivity of LB films from [{RuCl3(dppb)}2(µ-4,4'-bipy)] also pointed to their possible use in electrochemical devices.

Adsorption processes in layer-by-layer films are discussed using poly(o-methoxyaniline) (POMA) as a case study and also comparing with results for other polymers from the literature. This follows a brief overview of the materials and characterization techniques employed for self-assembled films, including their possible applications. The original paradigm of the self-assembly method is associated with spontaneous adsorption of oppositely charged polymer layers. While this rationale has been successful in explaining adsorption mechanisms for some polyelectrolytes, for polyanilines other interactions must be included. For POMA, in particular, at least three types of interactions are identified, namely van der Waals forces, ionic interactions and H-bonding. Furthermore, H-bonding is responsible for a number of effects even for charged POMA where electrostatic attraction was expected to predominate. Such effects include POMA dedoping upon contact with a glass substrate at early stages of adsorption, and the non-linear increase in the adsorbed amount with the number of POMA/poly(ethenesulfonic acid) (PVS) bilayers deposited in a multilayer structure. Adsorption of a POMA layer on a glass substrate or on an already formed POMA/PVS film occurs in two steps: a fast, first-order kinetics process with a characteristic time of a few seconds and a slower process represented by a Johnson-Mehl-Avrami function with a characteristic time of hundreds of seconds. These correspond basically to nucleation and growth mechanisms which is corroborated by atomic force microscopy measurements. The amount of material adsorbed in any given layer depends on experimental parameters, especially polymer concentration and pH, owing to the different extents of H-bonding that may allow POMA to adsorb on itself or on PVS molecules.