Green synthesis and antimicrobial mechanism of nanoparticles: applications in agricultural and agrifood safety.

The growing demand for food and its safety are a challenge for agriculture and agrifood. This has led to the incorporation of alternatives such as organic agriculture, the use of biocontrollers, the development of transgenic plants resistant to pathogens and the incorporation of nanotechnology. In this sense, agrochemicals based on nanoparticles (NPs) have been developed. Recently, the green synthesis of NPs has grown rapidly and, for this reason, molecules, microorganisms, fungi and plants are used. Synthesis from plant extracts offers a broad spectrum and, despite the fact that NPs are usually dispersed in size and shape, extensive antimicrobial effectiveness has been demonstrated at nanomolar concentrations. It has been shown that the mechanism of action can be through the dissipation of the driving force of the protons, the alteration of cellular permeability, the formation of bonds with the thiol group of the proteins, the generation of reactive species of oxygen, and the hyperoxidation of DNA, RNA and even the cell membrane. To improve the efficiency of NPs, modifications have been made such as coating with other metals, the addition of antibiotics, detergents and surfactants, as well as the acidification of the solution. Consequently, NPs are considered as a promising method for achieving safety in the agricultural and agrifood area. However, it is necessary to investigate the side effects of NPs, when applied in agroecological systems, on the textural, nutriment and sensory properties of food, as well as the impact on human health. © 2022 Society of Chemical Industry.

Millimeter-long carbon nanotubes: outstanding electron-emitting sources.

We are reporting the fabrication of a very efficient electron source using millimeter-long and highly crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.17 V/µm and reach threshold emission at 0.24 V/µm. In addition, these electron sources are very stable and can achieve a peak current density of 750 mA cm(-2) at only 0.45 V/µm. In order to demonstrate intense electron beam generation, these devices were used to produce visible light by cathodoluminescence. Finally, density functional theory calculations were used to rationalize the measured electronic field emission properties in open carbon nanotubes of different lengths. The modeling establishes a clear correlation between length and field enhancement factor.

Efficient vapor sensors using foils of dispersed nitrogen-doped and pure carbon multiwalled nanotubes.

We fabricated vapor sensors using nitrogen-doped (CNx) and pure multi-walled carbon nanotubes (MWNTs), and compared their performance. The sensors were constructed by dispersing the nanotube materials in methanol so as to form millimeter-long foils (nanotube paper), consisting of compact arrays of crisscrossing nanotubes. The devices were characterized by electrical resistance measurements and SEM studies. For CNx-based sensors, we observed that low concentrations of vapors such an acetone, ethanol, and chloroform were efficiently detected within 0.1-0.3 seconds via a physisorption mechanism. This physisorption is explained in terms of a weak interaction of the vapor molecules with the pyridinic sites (N bonded to two carbon atoms) present in the doped tubes. We believe that the methanol used for preparing the foils has a strong effect in saturating substitutional N atoms (N atoms bonded to three carbon atoms) that are also located in the CNx tubes. However, when pure carbon MWNTs were tested as sensors, we witnessed chemisorption of these vapors. First-principles density functional calculations confirmed that the gaseous molecules are able to interact with N-doped carbon nanotubes, via a physisorption mechanism, in which pyridine sites play a crucial role.