Silvio D. Rodríguez

A model for the dependence of the electrical conductance with the applied stress in insulating-conducting composites

A model for the dependence of the electrical conductance, G, with the strain induced by external mechanical stress in conducting particles-polymer composites is presented. The model assumes that the percolation probability between neighboring particles must depart from a scale-invariant behavior but saturate at moderated-high strains, reaching percolation path’s saturation, with sigmoid dependence. This dependence is obtained by proposing a dynamic picture where contacts or bonds between neighboring particles are created but also destructed when a stress is applied and relatively moderated or high strains, e, are produced in the composite. The electrical conductance of prepared graphite-polydimethylsiloxane composites were measured as function of the applied pressure and fitted by the presented model. The elastic response to the uniaxial compression was studied using a texture analyzer. The possibility of nonuniversal effects in the conduction critical exponent, t, was taken into account. It is concluded ...

Violet ZnSe/ZnS as an alternative to green CdSe/ZnS in nanocrystal-fluorescent protein FRET systems.

Fluorescent proteins from the green fluorescent protein family strongly interact with CdSe/ZnS and ZnSe/ZnS nanocrystals at neutral pH. Green emitting CdSe/ZnS nanocrystals and red emitting fluorescent protein dTomato constitute a 72% efficiency FRET system with the largest alteration of the overall photoluminescence profile, following complex formation, observed so far. The substitution of ZnSe/ZnS for CdSe/ZnS nanocrystals as energy donors enabled the use of a green fluorescent protein, GFP5, as energy acceptor. Violet emitting ZnSe/ZnS nanocrystals and green GFP5 constitute a system with 43% FRET efficiency and an unusually strong sensitized emission. ZnSe/ZnS-GFP5 provides a cadmium-free, high-contrast FRET system that covers only the high-energy part of the visible spectrum, leaving room for simultaneous use of the yellow and red color channels. Anisotropic fluorescence measurements confirmed the depolarization of GFP5 sensitized emission.

Fast and Efficient Food Quality Control Using Electronic Noses: Adulteration Detection Achieved by Unfolded Cluster Analysis Coupled with Time-Window Selection

The objective of this work is to report the improvements obtained in the discrimination of complex aroma samples with subtle differences in odor pattern, by the use of a fast procedure suitable for the cases of measurements in the field demanding decision-making in real time using a portable electronic nose. This device consists of a sensor array which records changes in conductivity as a function of time when aroma molecules reach the sensors. The core of the method consists of applying unfolded cluster analysis to selected time windows (UCATW) within the temporal evolution of the aroma profile recorded by the gas sensors, yielding an efficient, fast, and reliable data analysis tool that is easy to perform for electronic nose users. The performance of this data handling was tested in two case studies of food adulteration. The results demonstrated that this methodology enables to discriminate highly similar samples, herewith reducing the probability of achieving a wrong grouping due to the use of flawed data. The automation of this type of analysis is simple and improves the efficiency of the device significantly, herewith reducing the time of sensor’s signal recording that is necessary for a reliable assessment of the studied system. The results were validated by clustering the sample component scores that are obtained by applying parallel factor analysis (PARAFAC) to the original three-dimensional data array. An additional validation was obtained by means of a leave-one-out resampling procedure.

Host-Guest Molecular Interactions in Vanillin/Amylose Inclusion Complexes

The interaction of 4-hydroxy-3-methoxybenzaldehyde (vanillin) and Hylon VII due to the formation of an inclusion complex is studied using Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and circular dichroism (CD). The results confirm the close interaction among the different functional groups of vanillin and its host. In addition, a second case study was carried out with an amylose from a different source (100% amylose [APT III]). As a result, remarkable differences were found in the vanillin complexation capability of this amylose, which is only shown in solution by circular dichroism spectroscopy studies through a clear Cotton effect. This finding confirms the value of using CD studies, which shows that, depending on the amylose source, inclusion complexes can be found in solution, or both in solution and the coexisting precipitates, as shown using other techniques, such as X-ray diffraction (XRD) or differential scanning calorimetry (DSC). Moreover, solubility assays and complexation of both starches with iodine and subsequent absorption spectroscopy studies gives more information regarding the possible source of the starch encapsulation capability. Thus, Hylon VII shows higher capacity as vanillin encapsulant than APT III, showing the formation of inclusion complexes both in solution and solid phase, whereas APT III complexes are only perceivable in solution.

Detection of quinoa flour adulteration by means of FT-MIR spectroscopy combined with chemometric methods

Quinoa flour has been receiving an increasing attention as a substitute for wheat flour in bread formulations due to immuno-nutritional features. This growing interest in quinoa has increased the demand and consequently the prices, being a target for possible adulterations with cheaper cereals. Fourier transform Mid-infrared spectroscopy (FT-MIR) was used in the present work as a fingerprinting technique to detect the presence of three adulterants (soybean, maize and wheat flours). Partial least squares discriminant analysis (PLS-DA) and soft independent modelling of class analogy (SIMCA) models were used to classify pure from adulterated samples. 414 samples were measured, including pure quinoa flour, pure adulterant flours and adulterated quinoa flours using three different proportions (10, 5 and 1% w/w). PLS-DA showed better classification results than SIMCA, with error rates from 2 to 8% for the three strategies used to detect the presence of adulterants.

Assessing Changes in Enriched Maize Flour Formulations After Extrusion by Means of FTIR, XRD, and Chemometric Analysis

Spectral analysis employing multivariate techniques was employed to differentiate plain maize flours from formulations containing maize with added milled chia or quinoa seeds for producing cereal breakfast extrudates. The physicochemical changes of the enriched formulations due to processing stages and formulation were evaluated by using FTIR and chemometric analysis, which allowed a rapid and non-destructive discrimination between sample processing and compositional aspects. Specific IR frequencies were selected which provided highest sample discrimination. Selected IR absorbance relationships at those specific wavenumbers were useful to track changes promoted by extrusion for carbohydrates, proteins, and lipids. The complexes between amylose and lipids, that takes place during extrusion, underwent distinctive changes as confirmed by XRD. The crystallinity loss, after extrusion (with an average value of 50%), shows evidence of amylose-lipid complexes formation of type Eh and Vh. Correlations between the textural behavior, composition, and selected FTIR indices were obtained.