M.L.A. Gil

New, fast and green procedure for the synthesis of gold nanoparticles based on sonocatalysis

An easy, cheap and green synthetic route, using high-power ultrasounds and sodium citrate dihydrate as non-toxic reducing and stabilizer agent, produces gold nanoparticles in aqueous solution, and at ambient conditions. The time required for the synthesis is 5.5 min. The spherical nanoparticles obtained by this route show a homogeneous size distribution, within the range 5-17 nm, with an average diameter of 10±1 nm. Moreover, 90% of the particles have a diameter ranging from 7 to 13 nm, and their half-life is more than 30 days. The gold nanoparticles synthesized following this route are known as sononanoparticles. Gold sononanoparticles have been characterized by TEM and XRD and their stability has been studied by UV-Vis spectroscopy. Alternative experimental designs are compared to optimize the proposed synthesis procedure.

Sonosynthesis of gold nanoparticles from a geranium leaf extract.

A rapid in situ biosynthesis of gold nanoparticles (AuNPs) is proposed in which a geranium (Pelargonium zonale) leaf extract was used as a non-toxic reducing and stabilizing agent in a sonocatalysis process based on high-power ultrasound. The synthesis process took only 3.5 min in aqueous solution under ambient conditions. The stability of the nanoparticles was studied by UV-Vis absorption spectroscopy with reference to the surface plasmon resonance (SPR) band. AuNPs have an average lifetime of about 8 weeks at 4 °C in the absence of light. The morphology and crystalline phase of the gold nanoparticles were characterized by transmission electron microscopy (TEM). The composition of the nanoparticles was evaluated by electron diffraction and X-ray energy dispersive spectroscopy (EDS). A total of 80% of the gold nanoparticles obtained in this way have a diameter in the range 8-20 nm, with an average size of 12±3 nm. Fourier transform infrared spectroscopy (FTIR) indicated the presence of biomolecules that could be responsible for reducing and capping the biosynthesized gold nanoparticles. A hypothesis concerning the type of organic molecules involved in this process is also given. Experimental design linked to the simplex method was used to optimize the experimental conditions for this green synthesis route. To the best of our knowledge, this is the first time that a high-power ultrasound-based sonocatalytic process and experimental design coupled to a simplex optimization process has been used in the biosynthesis of AuNPs.

Quantitative characterization of desorptive stripping voltammograms complicated by surface dimerization reactions. Application to the reductive desorption of thiols from mercury

Abstract The influence of analyte adsorption and of a follow-up surface dimerization reaction on desorptive stripping voltammograms has been investigated. A modified version of the spline orthogonal collocation technique has been developed to compute the voltammetric response under Langmuirian reversible conditions. Simple analytical expressions derived for surface-confined redox processes are shown to reproduce the stripping voltammograms only in the presence of significant ( θ * R >0.01) analyte adsorption. In the absence of analyte adsorption, asymmetrical waves are obtained as an intrinsic characteristic of the stripping protocol. Empirical equations relating the stripping peak potential and the voltammetric half-height width with scan rate and adsorbed product coverage are presented. Voltammetric scan rate dependence is shown to provide a quantitative estimate of the analyte adsorption extent, whereas its dependence on adsorbed product coverage allows one to identify the presence of surface dimerization processes. Maximum surface excesses and dimerization equilibrium constants can readily be obtained by plotting the reciprocal half-height width versus the surface concentration of stripped species. In the absence of analyte adsorption, a complementary convolutive analysis is also proposed. Application is made to the cathodic stripping of three mercaptocarboxylic acids differing in their hydrocarbon chain-length. The extent of oxidized product dimerization is shown to display a strong dependence on the solution pH, so that higher peak current sensitivities are found in acidic solutions, where oxidized products are present as dimers.

Formation of siliceous sediments in brandy after diatomite filtration.

Brandy is quite a stable spirit but sometimes light sediment appears. This sediment was separated and analysed by IR and SEM-EDX. It was revealed that the sediment is composed mostly of silica and residual organic matter. Silica was present as an amorphous phase and as microparticles. In an attempt to reproduce the formation of the sediment, a diatomite extract was prepared with an ethanol/water mixture (36% vol.) and a calcined diatomite similar to that used in brandy filtration. This extract was added to unfiltered brandy in different amounts. After 1 month, the Si concentration decreased in all samples and sediments with similar compositions and features to those found in the unstable brandy appeared. The amounts of sediment obtained were directly related to the decrease in Si concentration in solution. Consequently, it can be concluded that siliceous sediment in brandy originates from Si released during diatomite filtration.

Diatomite releases silica during spirit filtration

The purpose of this study was to ascertain whether diatomite is an inert filter aid during spirit filtration. Surely, any compound with a negative effect on the spirit composition or the consumers health could be dissolved. In this study different diatomites were treated with 36% vol. ethanol/water mixtures and the amounts and structures of the extracted compounds were determined. Furthermore, Brandy de Jerez was diatomite- and membrane-filtered at different temperatures and the silicon content was analysed. It was found that up to 0.36% by weight of diatomite dissolved in the aqueous ethanol and amorphous silica, in the form of hollow spherical microparticles, was the most abundant component. Silicon concentrations in Brandy de Jerez increased by up to 163.0% after contact with diatomite and these changes were more marked for calcined diatomite. In contrast, reductions of more than 30% in silicon concentrations were achieved after membrane filtration at low temperatures.

Voltammetry of surface redox processes perturbed by a father-son reaction

Abstract The theory for surface redox processes perturbed by a father–son reaction under linear voltammetric conditions, and in the absence of mass transport complications, has been extended to remove the kinetic restrictions of previous treatments. The solution of the initial value problem, when reactants and products remain in the adsorbed state, is shown to be closely related to that corresponding to reactants equilibrated with the bulk solution at any potential. Analytical expressions for the voltammetric waves are derived for the fast reversible and irreversible kinetic limits, and empirical equations are developed to relate the peak coordinates with experimental variables when only numerical solutions are available. Bidirectional kinetics of the father–son reaction leads to two voltammetric waves, even when a single electron is being exchanged in the redox process. A comparison is made with previous results derived for a surface DIM1 electrodimerization mechanism, and diagnostic criteria are outlined in terms of the peak parameter values.

Voltammetry of Surface Redox Processes Perturbed by Dimerization and Adsorption of the Products Application to the Oxidation of 3-Mercaptopropanol on Mercury

Electrochemical evidence shows that the oxidation of thiols on mercury is characterized by an extensive adsorption of the oxidized products on the electrode surface, and by the formation of dimeric R-S-Hg-S-R organometallic complexes as the final oxidation products, The orthogonal collocation technique has been used in this work to solve the simplest voltammetric boundary value problem accounting for these observations. The proposed oxidation mechanism includes a following surface dimerization step and it considers simultaneously the adsorption of the oxidized products. Analytical expressions for the voltammetric wave have been derived when the adsorption strength of the products is either very strong or weak. When the adsorption strength is moderate, numerical results have been used to explore the influence of the dimerization step on the voltammetric shape. Theoretical predictions have been applied to interpret the voltammetric behavior of 3-mercaptopropanol on mercury as a function of the solution pH. Quantitative agreement with the proposed mechanism is only found in the most acid (pH 2) solution, whereas progressive distortion of the voltammetric waves is observed as the solution pH increases, which is tentatively ascribed to the existence of two low- and high-density surface states of the adsorbed products

TiO2-SiO2 Coatings with a Low Content of AuNPs for Producing Self-Cleaning Building Materials

The high pollution levels in our cities are producing a significant increase of dust on buildings. An application of photoactive coatings on building materials can produce buildings with self-cleaning surfaces. In this study, we have developed a simple sol-gel route for producing Au-TiO2/SiO2 photocatalysts with application on buildings. The gold nanoparticles (AuNPs) improved the TiO2 photoactivity under solar radiation because they promoted absorption in the visible range. We varied the content of AuNPs in the sols under study, in order to investigate their effect on self-cleaning properties. The sols obtained were sprayed on a common building stone, producing coatings which adhere firmly to the stone and preserve their aesthetic qualities. We studied the decolourization efficiency of the photocatalysts under study against methylene blue and against soot (a real staining agent for buildings). Finally, we established that the coating with an intermediate Au content presented the best self-cleaning performance, due to the role played by its structure and texture on its photoactivity.