Development of a disposable and highly sensitive paper-based immunosensor for early diagnosis of Asian soybean rust.

Soybean is one of the most important crops and plays a key role in the whole food chain production. Soybean crops are very susceptible to the fungus Phakopsora Pachyrhizi, the agent responsible by the Asian soybean rust. The spore of the fungus is easily disseminated by wind with adequate environment, leaf wetness, high humidity and temperatures, the crop can be totally lost within few days. A high sensitive, specific and easy test is the key for early diagnosing the soybean rust and therefore save the crop. Here we present a paper-based immunosensor for early stage diagnosis of soybean rust that can be performed by unskilled operators on-site. Nitrocellulose membrane was chosen as the substrate to stick the antigen due to its high binding properties. Polyclonal antibodies labeled with fluorescent nanoparticles were employed as the recognizers. An analytical curve with spiked samples shows a linear response range from 0.0032 to 3.2 µg/mL. This immunosensor presents a very low detection limit of 2.2 ng/mL, which corresponds approximately to 8-12 spores/mL. The paper-based sensor reachs the detection range of ELISA and PCR based test systems, and outranges the available commercial test kits by two order of magnitude. We believe this immunosensor has a great potential as a point-of-care device for the early diagnosis of Asian soybean rust.

Electrostatic contributions in the increased compatibility of polymer blends.

Successful blending of different polymers to make a structural or functional material requires overcoming limitations due to immiscibility and/or incompatibility that arise from large polymer-polymer interfacial tensions. In the case of latex blends, the combination of capillary adhesion during the blended dispersion drying stage with electrostatic adhesion in the final product is an effective strategy to avoid these limitations, which has been extended to a number of polymer blends and composites. This work shows that adhesion of polymer domains in blends made with natural rubber and synthetic latexes is enhanced by electrostatic adhesion that is in turn enhanced by ion migration, according to the results from scanning electric potential microscopy. The additional attractive force between domains improves blend stability and mechanical properties, broadening the possibilities and scope of latex blends, in consonance with the "green chemistry" paradigm. This novel approach based on electrostatic adhesion can be easily extended to multicomponent systems, including nonpolymers.

Clay platelet partition within polymer blend nanocomposite films by EFTEM.

Transmission electron microscopy (TEM) is the main technique used to investigate the spatial distribution of clay platelets in polymer nanocomposites, but it has not often been successfully used in polymer blend nanocomposites because the high contrast between polymer phases impairs the observation of clay platelets. This work shows that electron spectral imaging in energy-filtered TEM (EFTEM) in the low-energy-loss spectral crossover region allows the observation of platelets on a clear background. Separate polymer domains are discerned by imaging at different energy losses, above and below the crossover energy, revealing the material morphology. Three blends (natural rubber [NR]/poly(styrene-butyl acrylate) [P(S-BA)], P(S-BA)/poly(vinyl chloride) [PVC], and NR/starch) were studied in this work, showing low contrast between the polymer phases in the 40-60 eV range. In the NR/P(S-BA) and P(S-BA)/PVC blend nanocomposites, the clay platelets accumulate in the P(S-BA) phase, while in the P(S-BA)/PVC nanocomposites, clay is also found at the interfaces. In the NR/starch blend, clay concentrates at the interface, but it also penetrates the two polymer phases. These observations reveal that nanostructured soft materials can display complex morphochemical patterns that are discerned thanks to the ability of EFTEM to produce many contrast patterns for the same sample.

Molecular mapping by low-energy-loss energy-filtered transmission electron microscopy imaging.

Structure-function relationships in supramolecular systems depend on the spatial distribution of molecules, ions, and particles within complex arrays. Imaging the spatial distribution of molecular components within nanostructured solids is the objective of many recent techniques, and a powerful tool is electron spectroscopy imaging in the transmission electron microscope (ESI-TEM) in the low-energy-loss range, 0-80 eV. This technique was applied to particulate and thin film samples of dielectric polymers and inorganic compounds, providing excellent distinction between areas occupied by various macromolecules and particles. Domains differentiated by small changes in molecular composition and minor differences in elemental contents are clearly shown. Slight changes in the molecules produce intensity variations in molecular spectra that are in turn expressed in sets of low-energy-loss images, using the standard energy-filtered transmission electron microscopy (EFTEM) procedures. The molecular map resolution is in the nanometer range and very close to the bright-field resolution achieved for the same sample, in the same instrument. Moreover, contrast is excellent, even though sample exposure to the electron beam is minimal.