The outbreak of the COVID-19 pandemic has dramatically shaped higher education and seen the distinct rise of e-learning as a compulsory element of the modern educational landscape. Accordingly, this ...study highlights the factors which have influenced how students perceive their academic performance during this emergency changeover to e-learning. The empirical analysis is performed on a sample of 10,092 higher education students from 10 countries across 4 continents during the pandemic's first wave through an online survey. A structural equation model revealed the quality of e-learning was mainly derived from service quality, the teacher's active role in the process of online education, and the overall system quality, while the students' digital competencies and online interactions with their colleagues and teachers were considered to be slightly less important factors. The impact of e-learning quality on the students' performance was strongly mediated by their satisfaction with e-learning. In general, the model gave quite consistent results across countries, gender, study fields, and levels of study. The findings provide a basis for policy recommendations to support decision-makers incorporate e-learning issues in the current and any new similar circumstances.
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/CuPc
₂, is a layered material ...containing a negative film of the anionic sulfonated copper phthalocyanine (CuPc
) 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/CuPc
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.
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•PCL-based sensors were developed by a new environmentally friendly approach.•Proper electrical conductivity was achieved by coating with PEDOT:PSS.•The gas dissolution foaming ...approach proved to improve the sensoring performance.•The foaming process allows an enhanced distribution of PEDOT:PSS within the fibers.•Optimized sensors with a reproducible and stable response were achieved.
A new procedure to enhance the performance of polymer-based electrochemical sensors is proposed in this work. Polycaprolactone (PCL) electrospun fiber mats with tunable fiber morphology are functionalized with a conductive polymer (PEDOT:PSS) by a facile dip-coting process, providing them the necessary electrical conductivity to work as sensors. The modification of the fiber morphology is achieved by an enhanced gas dissolution foaming procedure, an environmentally friendly procedure that employs CO2 as blowing agent and takes advantage of recent advances that allowed extending such procedure to polymeric microfibers. Thus, the enhanced gas dissolution foaming approach was employed both before and after the coating of the fiber mats with PEDOT:PSS, producing in both cases hollow fibers with enhanced surface porosity and area, as well as increased diameter regarding the initial solid PCL fibers. The addition of PEDOT:PSS, both in solid and foamed PCL fibers, allows their use as sensors, as proved by cyclic voltammetry in a KCl solution, as well as calibrated with catechol solutions. Remarkable influence of the foaming procedure on the performance of the sensors have been found, proving by a detailed characterization that the foaming procedure applied after the PEDOT:PSS coating provides an enhanced sensoring response (i.e., increased signal, optimal linearity, decreased LOD) due to their superior surface area and optimal PEDOT:PSS distribution along the fiber mats, not only covering the external surface of the fibers but infusing into the inner regions.
In this work, two sets of voltammetric sensors -prepared using different strategies- have been combined in an electronic tongue to evaluate the complete antioxidant profile of red wines. To this aim, ...wine samples were analyzed with the whole set of sensors. In order to reduce the large dimensionality of the data set while keeping the relevant information provided by the sensors, two different methods of feature selection and data compression were used (the kernels method and Discrete Wavelet Transform feature extraction method). Then, the coefficients obtained were used as the input variables of Principal Component Analysis (to evaluate the capability of discrimination. Partial-least squares regression (PLS) and artificial neural networks (ANNs) were performer to build the quantitative prediction models that allowed the quantification of the antioxidant capacity of the tested wines.
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.
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)
-Tyr showed an LOD of 8.55∙10
μM, which was one order of magnitude lower than the 9.55·10
µM obtained with (CHI)-(CuPcS)-(CHI)-(AuNP)
-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.
The integration of nanomaterials as electron mediators in electrochemical biosensors is taking on an essential role. Due to their high surface-to-volume ratio and high conductivity, metallic ...nanowires are an interesting option. In this paper, silver nanowires (AgNWs) were exploited to design a novel catechol electrochemical biosensor, and the benefits of increasing the aspect ratio of the electron mediator (nanowires vs. nanoparticles) were analyzed. Atomic force microscopy (AFM) studies have shown a homogeneous distribution of the enzyme along the silver nanowires, maximizing the contact surface. The large contact area promotes electron transfer between the enzyme and the electrode surface, resulting in a Limit of Detection (LOD) of 2.7 × 10
M for tyrosinase immobilized onto AgNWs (AgNWs-Tyr), which is one order of magnitude lower than the LOD of 3.2 × 10
M) obtained using tyrosinase immobilized onto silver nanoparticles (AgNPs-Tyr). The calculated K
constant was 122 mM. The simultaneous use of electrochemistry and AFM has demonstrated a limited electrochemical fouling that facilitates stable and reproducible detection. Finally, the biosensor showed excellent anti-interference characteristics toward the main phenols present in wines including vanillin, pyrogallol, quercetin and catechin. The biosensor was able to successfully detect the presence of catechol in real wine samples. These results make AgNWs promising elements in nanowired biosensors for the sensitive, stable and rapid voltammetric detection of phenols in real applications.
This work focuses on studying natural rubber (NR) and epoxidized natural rubber (ENR) foams crosslinked at different absorbed doses and foamed using a chemical blowing agent. The aim is to study if ...the existence of epoxide groups within the natural rubber molecular structure influences the expansion capacity and the cellular structure morphology of these types of foam. This is accomplished by studying the foaming kinetics, that is, by quantifying the evolution of density and cellular structure parameters, such as cell size and cell density, as a function of foaming time. The results show that ENR foams exhibit higher cell nucleation rates and less intense cell degeneration than NR ones, which is because the presence of epoxide groups allows the polymer to reach higher crosslinking levels. Consequently, ENR foams present more homogeneous cellular structures and lower cell sizes.
Cellular materials based on natural rubber and epoxidized natural rubber are studied in this work. The influence of the presence of epoxy groups and the level of absorbed dose are analyzed. The foams produced with epoxidized natural rubber present higher crosslinking density, more homogenous cellular structures, and lower cell sizes than the foams produced with natural rubber.
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