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Reinforcement of silica aerogels, remarkable lightweight mesoporous materials with outstanding insulation performance, is still a challenging research topic. Among the strategies used to overcome their brittleness, one of the most effective is the manufacturing of aerogel composites with embedded fibres. In this work, the incorporation of nanofibres together with microfibres in a tetraethoxysilane-vinyltrimethoxysilane matrix is investigated for the first time for the development of novel aerogel nanocomposites. The nanofibres, synthesized from different aramid fibres, including Kevlar® pulp, Technora®, Teijinconex® and Twaron® fibres, were used in different combinations with microaramids and the resulting nanocomposites were thoroughly investigated for their physicochemical and thermomechanical features. The properties depended on the type and amount of the nano/microfibre used. While the microfibres exhibited low interaction with the silica matrix, the higher surface of the nanofibres ensured increased contact with the gel matrix. A low bulk density of 161 kg m-3 and thermal conductivity of 38.3 mW m-1 K-1 (Hot Disk®) was achieved when combining the nanofibres obtained from Kevlar® pulp with the Technora® or Teijinconex® long fibres. The nanofibres showed higher dispersion and random orientation and in combination with microfibres led to the improvement by a factor of three regarding the mechanical properties of the aerogel nanocomposites reinforced only with microfibres. The scale-up process of the samples and simulated tests of thermal cycling and vacuum outgassing successfully conducted indicate good compliance with space applications.

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A novel amperometric sensor for the direct determination of toxic mercury ions, Hg2+, based on the organic chelator ionophore N, N di (2-hydroxy-5-[(4-nitrophenyl)diazenyl]benzaldehyde) benzene-1,2-diamine (NDBD), and multiwalled carbon nanotubes (MWCNT) immobilized on a glassy carbon electrode surface was developed. The parameters influencing sensor performance including the ionophore concentration, the applied potential, and electrolyte pH were optimized. The sensor response to Hg2+ was linear between 1-25 µM with a limit of detection of 60 nM. Interferences from other heavy metal ions were evaluated and the sensor showed excellent selectivity towards Hg2+. The method was successfully applied to the determination of mercury ions in milk and water samples.

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A highly sensitive glucose oxidase (GOx) electrochemical biosensor for the determination of the biotoxic trace metal ions Hg2+, Cd2+, Pb2+ and CrVI by enzyme inhibition has been developed. GOx was immobilized on a novel sensing platform consisting of poly(brilliant green) films formed by potential cycling electropolymerization in sulfuric acid doped ethaline deep eutectic solvent on multiwalled carbon nanotube modified glassy carbon electrodes. Polymer films produced in this medium presented more uniform morphology and better electrochemical sensing properties than those prepared in aqueous solution. The inhibitor concentration necessary to give 50% inhibition, I50, was used for the determination of the type of reversible inhibition, and the relationship between I50 and the inhibition constant Ki is discussed. The new biosensor was successfully used for the determination of biotoxic trace metal ions with a nanomolar limit of detection, lower than in the literature, very good repeatability, stability and selectivity, and was applied successfully to detection of the toxic trace metal species in milk samples.

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The aim of the present work was to develop an amperometric biosensor for tyramine (Tyr) measurement in food and beverages. The biosensor architecture is based on tyrosinase (Tyrase) immobilization on glassy carbon electrode modified by a nanocomposite consisting of gold nanoparticles (AuNP) synthesized by a green method and poly(8-anilino-1-naphthalene sulphonic acid) modified glassy carbon electrode. Under optimized experimental conditions for fixed potential amperometric detection, the biosensor exhibited a linear response to tyramine in the range 10-120 µM and the limit of detection was estimated to be 0.71 µM. The novel platform showed good selectivity, long-term stability, and reproducibility. The strong interaction between tyrosinase and the nanocomposite was revealed by the high value of the Michaelis-Menten constant (79.3 µM). The fabricated biosensor was successfully applied to the determination of Tyr in dairy products and fermented drinks with good recoveries, which makes it a promising biosensor for quantification of tyramine.

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A novel impedimetric sensor for the determination of tyramine (Tyr), a biogenic amine, on the surface of gold nanoparticle-poly-(8-anilino-1-napthalene sulphonic acid), AuNP-PANSA modified gold electrode (AuE) is presented for the first time. The AuNP were successfully synthesized by a green synthesis method. Their characterization and optimization were conducted using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, electrochemical impedance spectroscopy and cyclic voltammetry. Under optimal conditions, the impedimetric sensor revealed a relatively broad linear range from 0.8 to 80 µM similar to more complex architectures found in the literature and the limit of detection of 0.04 µM was the lowest achieved until now. In order to test the reliability of the proposed method, real sample application studies were conducted using dairy products and fermented drinks. It was found that the sensor presented a good selectivity and recovery. Furthermore, the impedimetric sensor shows good reproducibility, stability, selectivity and very small interferences which augur well for its application in food safety control processes.

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A novel and simple biosensor for the determination of bisphenol A (BPA) based on xanthine oxidase (XOD) enzymatic inhibition has been developed. The biosensor was prepared from xanthine oxidase immobilised by crosslinking with glutaraldehyde, with hypoxanthine as enzyme substrate, and was successfully applied to the determination of BPA using fixed potential amperometry. Biosensor performance was optimised with respect to the applied potential, influence of pH of the electrolyte solution, XOD loading and the substrate concentration. The enzyme inhibition mechanism was evaluated from Cornish-Bowden plus Dixon plots and was found to be reversible and competitive with an apparent inhibition constant of 8.15 nM. Under optimised conditions, the determination of BPA can be achieved in the linear range up to 41 nM with a detection limit of 1.0 nM, which is equal to the lowest reported in the literature, with very good repeatability and reproducibility. The selectivity of the biosensor was evaluated by performing an interference study and found to be excellent; and stability was investigated. It was successfully applied to the detection of BPA in mineral water and in river water.

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A new amperometric hydrogen peroxide enzyme inhibition biosensor for the indirect determination of toxic mercury ions, Hg2+, based on catalase immobilized on a glassy carbon electrode surface by cross-linking with glutaraldehyde and bovine serum albumin, is reported. The parameters influencing biosensor performance were optimized, including enzyme loading, the amount of hydrogen peroxide, the applied potential and electrolyte pH. It was shown that the inhibition of catalase by Hg2+ species is irreversible, with a linear inhibition response between 5×10-11 and 5×10-10M. The limit of detection calculated as 10% inhibition was 1.8×10-11M and is the lowest reported until now. Electrochemical impedance spectroscopy was successfully used as a diagnostic of inhibition. Interferences from other heavy metal ions and organic pesticides were evaluated and the inhibition showed very good selectivity towards Hg2+. The method was successfully applied to the determination of mercury ions in different types of water sample.

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A novel amperometric biosensor for choline determination has been developed, exploiting the electrocatalytic properties of multiwalled carbon nanotubes (MWCNT) and gold nanoparticles (GNP). Chitosan (Chit), a natural biocompatible polymer, was used to disperse CNT, then Chit-MWCNT was dropped on the surface of a glassy carbon electrode (GCE), followed by GNP; finally, choline oxidase (ChOx) was immobilized by glutaraldehyde crosslinking. The ChOx/(GNP)4/MWCNT/GCE exhibited linear response to choline from 3 to 120µM, the sensitivity was 204µAcm-2mM-1 and the detection limit was 0.6µM. The biosensor exhibited good intra and inter-electrode precision, and excellent selectivity and stability. Electrochemical impedance spectroscopy (EIS) was also used to measure choline at 0.0V and this is the first report on choline determination by EIS. Successful measurement in milk samples was performed.

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Phenolic compounds detection in fruit extracts from: açai, bacuri, buriti, blackberry, black mulberry, blueberry, juçara, physalis, raspberry, and tamarillo, have been performed by reverse-phase high performance liquid chromatography with electrochemical detection (RP-HPLC-EC), using two detectors in series: a wall-jet detector flow cell with a glassy carbon electrode, and a thin-layer flow cell detector with a boron doped diamond electrode. This methodology, in gradient elution mode, was successfully used to detect seventeen phenolic compounds in the fruit extracts. The total antioxidant capacity of the fruit extracts by the electrochemical quantitative index (EI) and the method of capture of diphenilpicrilhydrazil (DPPH(●)) free radical "efficient concentration" (EC50), was evaluated. A very good correlation between EI and EC50 assays has been obtained, the fruit with the highest total antioxidant capacity being blackberry, while physalis exhibited the lowest antioxidant power.

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A rapid electrochemical procedure for the determination of dipyrone was successfully developed at a carbon nanotube modified graphite-epoxy resin composite (GrEC) electrode. The composite electrode was used as support on which multi-walled carbon nanotubes (MWCNT) were immobilised by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS) in a chitosan (Chit) matrix. The electrochemical behaviour of dipyrone at this electrode in different buffer electrolytes with pH values between 5.0 and 8.0 was explored using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy, and comparison with a conventional glassy carbon electrode was made. Dipyrone was best determined by differential pulse voltammetry with a low limit of detection of 1.4 microM. Application to commercial samples was demonstrated.

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The effectiveness of immobilization of functionalized carbon nanotubes into chitosan using different cross-linking agents has been evaluated. The cross-linkers used were glyoxal (GO), glutaraldehyde (GA), epichlorohydrin (ECH), and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS), and the nanotubes were retained on graphite epoxy resin composite electrodes. The nanotube modified electrodes have been characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Using CV and EIS in the presence of potassium hexacyanoferrate(III), the electroactive area of all types of electrodes was determined and the redox process analyzed, leading to the conclusion that ECH and EDC-NHS are better for immobilization of functionalized carbon nanotubes inside the chitosan matrix. The modified electrodes were successfully applied to the determination of hydrogen peroxide by fixed potential amperometry at -0.1 V vs SCE, the highest response being exhibited when using ECH.

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Improved biosensors for acetaldehyde determination have been developed using a bienzymatic strategy, based on a mediator-modified carbon film electrode and co-immobilisation of NADH oxidase and aldehyde dehydrogenase. Modification of the carbon film electrode with poly(neutral red) mediator resulted in a sensitive, low-cost and reliable NADH detector. Immobilisation of the enzymes was performed using encapsulation in a sol-gel matrix or cross-linking with glutaraldehyde. The bienzymatic biosensors were characterized by studying the influence of pH, applied potential and co-factors. The sol-gel and glutaraldehyde biosensors showed a linear response up to 60 microM and 100 microM, respectively, with detection limits of 2.6 microM and 3.3 microM and sensitivities were 1.7 microA mM(-1) and 5.6 microA mM(-1). The optimised biosensors showed good stability and good selectivity and have been tested for application for the determination of acetaldehyde in natural samples such as wine.

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A new, improved sensor for the electrocatalytic determination of ascorbate has been developed that has both a low applied operating potential and a low detection limit. The sensor was constructed by depositing copper hexacyanoferrate film either electrochemically or chemically onto carbon film electrode, and it was then characterised by cyclic voltammetry and electrochemical impedance spectroscopy. Chemically deposited films were shown to be the best for ascorbate determination and were used as an amperometric sensor at +0.05 V versus SCE to determine ascorbate in wines and juice. The linear range extended to 5 mM with a limit of detection of 2.1 microM, the sensor was stable for more than four months, and it could be used continuously for at least 20 days.