Analysis of Phenolic Content in Grape Seeds and Skins by Means of a Bio-Electronic Tongue.

A bio-electronic tongue has been developed to evaluate the phenolic content of grape residues (seeds and skins) in a fast and easy way with industrial use in mind. A voltammetric electronic tongue has been designed based on carbon resin electrodes modified with tyrosinase combined with electron mediators. The presence of the phenoloxydase promotes the selectivity and specificity towards phenols. The results of multivariate analysis allowed discriminating seeds and skins according to their polyphenolic content. Partial least squares (PLS) has been used to establish regression models with parameters related to phenolic content measured by spectroscopic methods i.e., total poliphenol content (TPC) and Folin-Ciocalteu (FC) indexes. It has been shown that electronic tongue can be successfully used to predict parameters of interest with high correlation coefficients (higher than 0.99 in both calibration and prediction) and low residual errors. These values can even be improved using genetic algorithms for multivalent analysis. In this way, a fast and simple tool is available for the evaluation of these values. This advantage may be due to the fact that the electrochemical signals are directly related to the phenolic content.

Biosensors Platform Based on Chitosan/AuNPs/Phthalocyanine Composite Films for the Electrochemical Detection of Catechol. The Role of the Surface Structure.

Biosensor platforms consisting of layer by layer films combining materials with different functionalities have been developed and used to obtain improved catechol biosensors. Tyrosinase (Tyr) or laccase (Lac) were deposited onto LbL films formed by layers of a cationic linker (chitosan, CHI) alternating with layers of anionic electrocatalytic materials (sulfonated copper phthalocyanine, CuPcS or gold nanoparticles, AuNP). Films with different layer structures were successfully formed. Characterization of surface roughness and porosity was carried out using AFM. Electrochemical responses towards catechol showed that the LbL composites efficiently improved the electron transfer path between Tyr or Lac and the electrode surface, producing an increase in the intensity over the response in the absence of the LbL platform. LbL structures with higher roughness and pore size facilitated the diffusion of catechol, resulting in lower LODs. The [(CHI)-(AuNP)-(CHI)-(CuPcS)]2-Tyr showed an LOD of 8.55∙10-4 µM, which was one order of magnitude lower than the 9.55·10-3 µM obtained with [(CHI)-(CuPcS)-(CHI)-(AuNP)]2-Tyr, and two orders of magnitude lower than the obtained with other nanostructured platforms. It can be concluded that the combination of adequate materials with complementary activity and the control of the structure of the platform is an excellent strategy to obtain biosensors with improved performances.

Exposure to ultrafine particles in children until 18 years of age: A systematic review.

Airborne ultrafine particles (UFP) have been related to adverse health effects, but exposure in vulnerable population groups such as children is still not well understood. We aim to review the scientific literature regarding personal exposure to UFP in different microenvironments in populations until 18 years of age. The bibliographical search was carried out in July 2019 using the online database PubMed and was completed with references in articles found in the search. We selected the studies that used continuous counters and measured UFP levels in both specific microenvironment (houses, schools, transport, etc) and personal exposure. Finally, 32 studies fulfilled the criteria: of these, 10 analyzed personal exposure and 22 examined UFP levels in the microenvironment (especially in schools or nurseries (18/22)) and five in various microenvironments (including dwellings and means of transport, where exposure levels were higher). The characteristics of the microenvironments with the greatest levels of UFP were being close to heavy traffic or near cooking and cleaning activities. This review revealed the wide differences in exposure assessment methodologies that could lead to a lack of uniform and comparable information about the real UFP exposure in children.

Electrochemical Sensors Modified with Combinations of Sulfur Containing Phthalocyanines and Capped Gold Nanoparticles: A Study of the Influence of the Nature of the Interaction between Sensing Materials.

Voltametric sensors formed by the combination of a sulfur-substituted zinc phthalocyanine (ZnPcRS) and gold nanoparticles capped with tetraoctylammonium bromide (AuNPtOcBr) have been developed. The influence of the nature of the interaction between both components in the response towards catechol has been evaluated. Electrodes modified with a mixture of nanoparticles and phthalocyanine (AuNPtOcBr/ZnPcRS) show an increase in the intensity of the peak associated with the reduction of catechol. Electrodes modified with a covalent adduct-both component are linked through a thioether bond-(AuNPtOcBr-S-ZnPcR), show an increase in the intensity of the oxidation peak. Voltammograms registered at increasing scan rates show that charge transfer coefficients are different in both types of electrodes confirming that the kinetics of the electrochemical reaction is influenced by the nature of the interaction between both electrocatalytic materials. The limits of detection attained are 0.9 × 10-6 mol∙L-1 for the electrode modified with the mixture AuNPtOcBr/ZnPcRS and 1.3 × 10-7 mol∙L-1 for the electrode modified with the covalent adduct AuNPtOcBr-S-ZnPcR. These results indicate that the establishment of covalent bonds between nanoparticles and phthalocyanines can be a good strategy to obtain sensors with enhanced performance, improving the charge transfer rate and the detection limits of voltammetric sensors.

In situ synthesis, stabilization and activity of protein-modified gold nanoparticles for biological applications.

Herein, we demonstrate the use of lysozyme (Lys) as a model to fabricate a protein carrier system based on gold nanoparticles (AuNPs) via the Layer-by-Layer (LbL) technology. Poly(ethyleneimine) (PEI) and poly(sodium 4-styrenesulfonate) (PSS) were used as cationic and anionic polymers respectively to grow oppositely charged layers. Mild aqueous conditions were utilized to avoid protein denaturation and activity instead of organic solvents that have been used in other encapsulation systems. Two different strategies were used: (A) lysozyme acting as a reducing and stabilizing agent in the formation of AuNPs at a temperature of 45 ± 2 °C followed by only two subsequent polymeric layers deposited by LbL, and (B) citrate acting as a reducing agent prior to stabilization of the AuNPs by mercaptoundecanoic acid. Dynamic light scattering, UV-vis spectroscopy, IR spectroscopy and transmission electron microscopy were used to characterize the nanoconjugates. Furthermore, the enzymatic activity of the resulting protein/nanoparticle conjugates was evaluated using the bacteria Micrococcus lysodeikticus as a substrate.

Discrimination of Milks with a Multisensor System Based on Layer-by-Layer Films.

A nanostructured electrochemical bi-sensor system for the analysis of milks has been developed using the layer-by-layer technique. The non-enzymatic sensor [CHI+IL/CuPcS]2, is a layered material containing a negative film of the anionic sulfonated copper phthalocyanine (CuPcS) acting as electrocatalytic material, and a cationic layer containing a mixture of an ionic liquid (IL) (1-butyl-3-methylimidazolium tetrafluoroborate) that enhances the conductivity, and chitosan (CHI), that facilitates the enzyme immobilization. The biosensor ([CHI+IL/CuPcS]2-GAO) results from the immobilization of galactose oxidase on the top of the LbL layers. FTIR, UV⁻vis, and AFM have confirmed the proposed structure and cyclic voltammetry has demonstrated the amplification caused by the combination of materials in the film. Sensors have been combined to form an electronic tongue for milk analysis. Principal component analysis has revealed the ability of the sensor system to discriminate between milk samples with different lactose content. Using a PLS-1 calibration models, correlations have been found between the voltammetric signals and chemical parameters measured by classical methods. PLS-1 models provide excellent correlations with lactose content. Additional information about other components, such as fats, proteins, and acidity, can also be obtained. The method developed is simple, and the short response time permits its use in assaying milk samples online.

Monitoring the Phenolic Ripening of Red Grapes Using a Multisensor System Based on Metal-Oxide Nanoparticles.

The maturity of grapes is usually monitored by means of the sugar concentration. However, the assessment of other parameters such as the phenolic content is also important because the phenolic maturity has an important impact on the organoleptic characteristics of wines. In this work, voltammetric sensors able to detect phenols in red grapes have been developed. They are based on metal oxide nanoparticles (CeO2, NiO, and TiO2,) whose excellent electrocatalytic properties toward phenols allows obtaining sensors with detection limits in the range of 10-8 M and coefficients of variation lower than 7%. An electronic tongue constructed using a combination of the nanoparticle-based sensors is capable to monitor the phenolic maturity of red grapes from véraison to maturity. Principal Component Analysis (PCA) can be successfully used to discriminate samples according to the ripeness. Regression models performed using Partial Least Squares (PLS-1) have established good correlations between voltammetric data obtained with the electrochemical sensors and the Total Polyphenolic Index, the Brix degree and the Total Acidity, with correlation coefficients close to 1 and low number of latent variables. An advantage of this system is that the electronic tongue can be used for the simultaneous assessment of these three parameters which are the main factors used to monitor the maturity of grapes. Thus the electronic tongue based on metal oxide nanoparticles can be a valuable tool to monitor ripeness. These results demonstrate the exciting possible applications of metal oxide nanoparticles in the field of electronic tongues.

A different approach for the analysis of grapes: Using the skin as sensing element.

In this work, an alternative method to monitor the phenolic maturity of grapes was developed. In this approach, the skins of grapes were used to cover the surface of carbon paste electrodes and the voltammetric signals obtained with the skin-modified sensors were used to obtain information about the phenolic content of the skins. These sensors could easily detect differences in the phenolic composition of different Spanish varieties of grapes (Mencía, Prieto Picudo and Juan García). Moreover, sensors were able to monitor changes in the phenolic content throughout the ripening process from véraison until harvest. Using PLS-1 (Partial Least Squares), correlations were established between the voltammetric signals registered with the skin-modified sensors and the phenolic content measured by classical methods (Glories or Total Polyphenol Index). PLS-1 models provided additional information about Brix degree, density or sugar content, which usually used to establish the harvesting date. The quality of the correlations was influenced by the maturation process and the structural and mechanical skin properties. Thus the skin sensors fabricated with Juan García and Prieto Picudo grapes (that showed faster polyphenolic maturation and a higher amount of extractable polyphenols than Mencía), showed good correlations and therefore could be used to monitor the ripening.

Electronic Noses and Tongues in Wine Industry.

The quality of wines is usually evaluated by a sensory panel formed of trained experts or traditional chemical analysis. Over the last few decades, electronic noses (e-noses) and electronic tongues have been developed to determine the quality of foods and beverages. They consist of arrays of sensors with cross-sensitivity, combined with pattern recognition software, which provide a fingerprint of the samples that can be used to discriminate or classify the samples. This holistic approach is inspired by the method used in mammals to recognize food through their senses. They have been widely applied to the analysis of wines, including quality control, aging control, or the detection of fraudulence, among others. In this paper, the current status of research and development in the field of e-noses and tongues applied to the analysis of wines is reviewed. Their potential applications in the wine industry are described. The review ends with a final comment about expected future developments.

Discrimination of Apple Liqueurs (Nalewka) Using a Voltammetric Electronic Tongue, UV-Vis and Raman Spectroscopy.

The capability of a phthalocyanine-based voltammetric electronic tongue to analyze strong alcoholic beverages has been evaluated and compared with the performance of spectroscopic techniques coupled to chemometrics. Nalewka Polish liqueurs prepared from five apple varieties have been used as a model of strong liqueurs. Principal Component Analysis has demonstrated that the best discrimination between liqueurs prepared from different apple varieties is achieved using the e-tongue and UV-Vis spectroscopy. Raman spectra coupled to chemometrics have not been efficient in discriminating liqueurs. The calculated Euclidean distances and the k-Nearest Neighbors algorithm (kNN) confirmed these results. The main advantage of the e-tongue is that, using PLS-1, good correlations have been found simultaneously with the phenolic content measured by the Folin-Ciocalteu method (R² of 0.97 in calibration and R² of 0.93 in validation) and also with the density, a marker of the alcoholic content method (R² of 0.93 in calibration and R² of 0.88 in validation). UV-Vis coupled with chemometrics has shown good correlations only with the phenolic content (R² of 0.99 in calibration and R² of 0.99 in validation) but correlations with the alcoholic content were low. Raman coupled with chemometrics has shown good correlations only with density (R² of 0.96 in calibration and R² of 0.85 in validation). In summary, from the three holistic methods evaluated to analyze strong alcoholic liqueurs, the voltammetric electronic tongue using phthalocyanines as sensing elements is superior to Raman or UV-Vis techniques because it shows an excellent discrimination capability and remarkable correlations with both antioxidant capacity and alcoholic content-the most important parameters to be measured in this type of liqueurs.

Layered composites of PEDOT/PSS/nanoparticles and PEDOT/PSS/phthalocyanines as electron mediators for sensors and biosensors.

The sensing properties of electrodes chemically modified with PEDOT/PSS towards catechol and hydroquinone sensing have been successfully improved by combining layers of PEDOT/PSS with layers of a secondary electrocatalytic material such as gold nanoparticles (PEDOT/PSS/AuNPs), copper phthalocyanine (PEDOT/PSS/CuPc) or lutetium bisphthalocyanine (PEDOT/PSS/LuPc2). Layered composites exhibit synergistic effects that strongly enhance the electrocatalytic activity as indicated by the increase in intensity and the shift of the redox peaks to lower potentials. A remarkable improvement has been achieved using PEDOT/PSS/LuPc2, which exhibits excellent electrocatalytic activity towards the oxidation of catechol. The kinetic studies demonstrated diffusion-controlled processes at the electrode surfaces. The kinetic parameters such as Tafel slopes and charge transfer coefficient (α) confirm the improved electrocatalytic activity of the layered electron mediators. The peak currents increased linearly with concentration of catechol and hydroquinone over the range of 1.5 × 10-4 to 4.0 × 10-6 mol·L-1 with a limit of detection on the scale of µmol·L-1. The layered composite hybrid systems were also found to be excellent electron mediators in biosensors containing tyrosinase and laccase, and they combine the recognition and biocatalytic properties of biomolecules with the unique catalytic features of composite materials. The observed increase in the intensity of the responses allowed detection limits of 1 × 10-7 mol·L-1 to be attained.

Electrochemical behavior of polypyrrol/AuNP composites deposited by different electrochemical methods: sensing properties towards catechol.

Two different methods were used to obtain polypyrrole/AuNP (Ppy/AuNP) composites. One through the electrooxidation of the pyrrole monomer in the presence of colloidal gold nanoparticles, referred to as trapping method (T), and the second one by electrodeposition of both components from one solution containing the monomer and a gold salt, referred to as cogeneration method (C). In both cases, electrodeposition was carried out through galvanostatic and potentiostatic methods and using platinum (Pt) or stainless steel (SS) as substrates. Scanning electron microscopy (SEM) demonstrated that in all cases gold nanoparticles of similar size were uniformly dispersed in the Ppy matrix. The amount of AuNPs incorporated in the Ppy films was higher when electropolymerization was carried out by chronopotentiometry (CP). Besides, cogeneration method allowed for the incorporation of a higher number of AuNPs than trapping. Impedance experiments demonstrated that the insertion of AuNPs increased the conductivity. As an electrochemical sensor, the Ppy/AuNp deposited on platinum exhibited a strong electrocatalytic activity towards the oxidation of catechol. The effect was higher in films obtained by CP than in films obtained by chronoamperometry (CA). The influence of the method used to introduce the AuNPs (trapping or cogeneration) was not so important. The limits of detection (LOD) were in the range from 10(-5) to 10(-6) mol/L. LODs attained using films deposited on platinum were lower due to a synergy between AuNPs and platinum that facilitates the electron transfer, improving the electrocatalytic properties. Such synergistic effects are not so pronounced on stainless steel, but acceptable LOD are attained with lower price sensors.

An Electrochemical Quartz Crystal Microbalance Multisensor System Based on Phthalocyanine Nanostructured Films: Discrimination of Musts.

An array of electrochemical quartz crystal electrodes (EQCM) modified with nanostructured films based on phthalocyanines was developed and used to discriminate musts prepared from different varieties of grapes. Nanostructured films of iron, nickel and copper phthalocyanines were deposited on Pt/quartz crystals through the Layer by Layer technique by alternating layers of the corresponding phthalocyanine and poly-allylamine hydrochloride. Simultaneous electrochemical and mass measurements were used to study the mass changes accompanying the oxidation of electroactive species present in must samples obtained from six Spanish varieties of grapes (Juan García, Prieto Picudo, Mencía Regadío, Cabernet Sauvignon, Garnacha and Tempranillo). The mass and voltammetric outputs were processed using three-way models. Parallel Factor Analysis (PARAFAC) was successfully used to discriminate the must samples according to their variety. Multi-way partial least squares (N-PLS) evidenced the correlations existing between the voltammetric data and the polyphenolic content measured by chemical methods. Similarly, N-PLS showed a correlation between mass outputs and parameters related to the sugar content. These results demonstrated that electronic tongues based on arrays of EQCM sensors can offer advantages over arrays of mass or voltammetric sensors used separately.