Intrinsic self-healing materials have received significant attention due to the characteristic recovery after damage properties through reversible dynamic covalent and non-covalent interactions. Furthermore, functional recovery with reliable mechanical properties are highly keen as protective coatings, specifically for devices and sensors vulnerable to abrasion in severe environments. Here, we present a functional hierarchical nanostructure capable of multiple micro-sized healings, with enhanced mechanical hardness due to the incorporation of graphene oxide (rGO) nanoplatelets. A self-healing multilayered nanocomposite formed by poly(ethylene imine) (PEI) and poly(acrylic acid) (PAA) was easily assembled by the layer-by-layer (LbL) technique. The addition of the rGO nanoplatelets in the LbL nanostructure resulted in a 13-fold increase in hardness (0.4 ± 0.1 GPa) when compared to the (PEI/PAA) architecture (0.03 ± 0.01 GPa). In addition, the nanocomposite presents an enhanced insulating electrical behavior (∼ 4.10⁻⁸ S/cm) despite the addition of the rGO nanoplatelets. Raman and Zeta Potential analysis indicated a possible wrapping of the rGOs by PEI, justifying the observed insulating electrical characteristics. The nanocomposite presents good hydrophobicity with a water contact angle of 136°, interesting to extend the lifetime and protect underlying layers from humidity, degradation, and encrustation. Therefore, we propose an attractive hydrophobic, electrically insulating, and mechanically resistant multifunctional coating for high-performance electronic interfaces from minor cuts and abrasions, dispensing maintainer intervention.

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Chapter 1 shows a general presentation of catalysts. Chapter 2 presents a cascade system for alcohol oxidase with the enzyme benzaldehyde lyase. The project was successful in producing in one container only a two-step compound; first, alcohol was converted into aldehyde by alcohol oxidase and, sequentially, the aldehyde converted into the compounds of interest. Background crystals are formed in the final product, turning the removal of the material of interest simpler and more effective. In Chapter 3, alcohol oxidase was converted into aldehyde in a flow system, but now using the enzyme aryl alcohol oxidase. As oxidase requires oxygen as an electron acceptor, this is a limiting factor due to the low solubility of O2 in the buffer solution. A flow system becomes more interesting as it can overcome this limitation by improving the oxygen mass transport in buffer solution due to the increased contact area between two fluids. Vigorous agitation is required to achieve similar results in a standard system, which normally compromises the enzymatic structure due to mechanical stress generated by aeration strength. The expected impairment of the structured tertiary enzyme was not observed for the aryl alcohol oxidase enzyme. The findings in Chapter 3 about the mechanical resistance of aryl alcohol oxidase under vigorous motivated us to scale up the system from 50mL to 1L, presented in Chapter 4. It was possible to obtain high catalytic frequency values in this new configuration, but another challenge appeared: the low solubility of both substrate and product in an aqueous medium. This challenge was overcome by the biphasic liquid system composed the organic phase in the upper portion and the buffer in the lower part of the solution. In this system, the aqueous phase was fed with the substrate by the organic phase at all times, and the product removed. The encouraging results showed that aryl alcohol oxidase is a strong candidate for industrial applications. In Chapter 5, we have evaluated carbon compounds deposited on electrodes to produce locally hydrogen peroxide for peroxidase; the idea is getting the halogenation of a model compound by combining catalytic and electrochemical techniques. Here, a fine tune of the voltage or current on a gas diffusion electrode covered with carbon nanotubes enabled the hydrogen peroxide formation. Since the CiVCPO (vanadium chloroperoxidase from Curvularia inaequalis) enzyme used in this project needs hydrogen peroxide, the system is interesting to make the enzymatic reaction always working at its maximum efficiency. Finally, Chapter 6 shows a synthesis of carbon-based materials to support the enzyme immobilization on electrodes, bringing a new possibility of using some of the enzymes studied here in biofuel cells and third-generation biosensors. The main idea is to work with enzymes without mediators using these carbon-based materials to sequester electrons from the enzymes to the electrodes. Higher currents were achieved with the newly synthesized carbon-based materials and glucose oxidase, paving the way to design devices having faster electrochemical responses.@en

Various enzymes utilize hydrogen peroxide as an oxidant. Such “peroxizymes” are potentially very attractive catalysts for a broad range of oxidation reactions. Most peroxizymes, however, are inactivated by an excess of H₂O₂. The electrochemical reduction of oxygen can be used as an in situ generation method for hydrogen peroxide to drive the peroxizymes at high operational stabilities. Using conventional electrode materials, however, also necessitates significant overpotentials, thereby reducing the energy efficiency of these systems. This study concerns a method to coat a gas-diffusion electrode with oxidized carbon nanotubes (oCNTs), thereby greatly reducing the overpotential needed to perform an electroenzymatic halogenation reaction. In comparison to the unmodified electrode, with the oCNTs-modified electrode the overpotential can be reduced by approximately 100 mV at comparable product formation rates.

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The selective oxidation of trans-2-hexen-1-ol to the corresponding aldehyde using a recombinant aryl alcohol oxidase from Pleurotus eryngii (PeAAOx) is reported. Especially using the two liquid phase system to overcome solubility and product inhibition issues enabled to achieve more than 2.200.000 catalytic turnovers for the production enzyme as well as molar product concentrations, pointing towards an economic feasible reaction.

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The biocatalytic preparation of trans-hex-2-enal from trans-hex-2-enol using a novel aryl alcohol oxidase from Pleurotus eryngii (PeAAOx) is reported. As O₂-dependent enzyme PeAAOx-dependent reactions are generally plagued by the poor solubility of O₂ in aqueous media and mass transfer limitations resulting in poor reaction rates. These limitations were efficiently overcome by conducting the reaction in a flow-reactor setup reaching unpreceded catalytic activities for the enzyme in terms of turnover frequency (up to 38 s^-1) and turnover numbers (more than 300000) pointing towards preparative usefulness of the proposed reaction scheme.

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Microfluidic devices based on polydimethylsiloxane shown a plethora of experimental possibilities due to good transparency, flexibility and ability to adhere reversibly and irreversibly to distinct materials. Though PDMS is a milestone in microfluidic developments, its cost and handling directed the field to search for new options. 3D printing technology nowadays starts a revolution offering materials and possibilities that can contribute positively to current methodologies. Here we explored the fused deposition modeling 3D printing technique to obtain integrated, transparent and sealed microchannels made with polylactic acid, a cheap alternative material to set up microfluidic systems. Using a home-made 3D printer, devices could be assembled in a simplified process, enabling the integration of different materials such as paper, glass, wire and polymers within the microchannel. To demonstrate the efficacy of this approach, a 3D-printed electronic tongue sensor was built, enabling the distinction of basic tastes below the human threshold.

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Graphene is a breakthrough 2D material due to its unique mechanical, electrical, and thermal properties, with considerable responsiveness in real applications. However, the coverage of large areas with pristine graphene is a challenge and graphene derivatives have been alternatively exploited to produce hybrid and composite materials that allow for new developments, considering also the handling of large areas using distinct methodologies. For electronic applications there is significant interest in the investigation of the electrical properties of graphene derivatives and related composites to determine whether the characteristic 2D charge transport of pristine graphene is preserved. Here, we report a systematic study of the charge transport mechanisms of reduced graphene oxide chemically functionalized with sodium polystyrene sulfonate (PSS), named as GPSS. GPSS was produced either as quantum dots (QDs) or nanoplatelets (NPLs), being further nanostructured with poly(diallyldimethylammonium chloride) through the layer-by-layer (LbL) assembly to produce graphene nanocomposites with molecular level control. Current-voltage (I-V) measurements indicated a meticulous growth of the LbL nanostructures onto gold interdigitated electrodes (IDEs), with a space-charge-limited current dominated by a Mott-variable range hopping mechanism. A 2D intra-planar conduction within the GPSS nanostructure was observed, which resulted in effective charge carrier mobility (μ) of 4.7 cm² V^-1 s^-1 for the QDs and 34.7 cm² V^-1 s^-1 for the NPLs. The LbL assemblies together with the dimension of the materials (QDs or NPLs) were favorably used for the fine tuning and control of the charge carrier mobility inside the LbL nanostructures. Such 2D charge conduction mechanism and high μ values inside an interlocked multilayered assembly containing graphene-based nanocomposites are of great interest for organic devices and functionalization of interfaces.

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The one-pot multistep enzymatic oxidation of aliphatic and benzylic alcohols to the corresponding aldehydes combined with their subsequent carboligation to chiral α-hydroxy ketones has been exemplarily evaluated in terms of being a “green” biocatalytic approach. Besides the potential to start from bio-derived alcohols, this concept avoids the direct use of the reactive aldehyde intermediates, enables addition of high substrate concentrations in one liquid phase while maintaining enzyme activity and enables a simplified product isolation with diminished waste formation.@en