Clicking beyond suspensions: understanding thiol-ene chemistry on solid-supported MoS2.

Molecular functionalization of MoS2 has attracted a lot of attention due to its potential to afford fine-tuned hybrid materials that benefit from the power of synthetic chemistry and molecular design. Here, we report on the on-surface reaction of maleimides on bulk and molecular beam epitaxy grown single-layer MoS2, both in ambient conditions as well as ultrahigh vacuum using scanning probe microscopy.

Upscaled Synthesis Protocol for Phase-Pure, Colloidally Stable MXenes with Long Shelf Lives.

MXenes are electrically conductive 2D transition metal carbides/nitrides obtained by the etching of nanolaminated MAX phase compounds, followed by exfoliation to single- or few-layered nanosheets. The mainstream chemical etching processes have evolved from pure hydrofluoric acid (HF) etching into the innovative "minimally intensive layer delamination" (MILD) route. Despite their current popularity and remarkable application potential, the scalability of MILD-produced MXenes remains unproven, excluding MXenes from industrial applications. This work proposes a "next-generation MILD" (NGMILD) synthesis protocol for phase-pure, colloidally stable MXenes that withstand long periods of dry storage. NGMILD incorporates the synergistic effects of a secondary salt, a richer lithium (Li) environment, and iterative alcohol-based washing to achieve high-purity MXenes, while improving etching efficiency, intercalation, and shelf life. Moreover, NGMILD comprises a sulfuric acid (H2 SO4 ) post-treatment for the selective removal of the Li3 AlF6 impurity that commonly persists in MILD-produced MXenes. This work demonstrates the upscaled NGMILD synthesis of (50 g) phase-pure Ti3 C2 Tz MXene clays with high extraction yields (>22%) of supernatant dispersions. Finally, NGMILD-produced MXene clays dry-stored for six months under ambient conditions experience minimal degradation, while retaining excellent redispersibility. Overall, the NGMILD protocol is a leap forward toward the industrial production of MXenes and their subsequent market deployment.

Machine-Learning-Aided NO2 Discrimination with an Array of Graphene Chemiresistors Covalently Functionalized by Diazonium Chemistry.

Boosted by the emerging need for highly integrated gas sensors in the internet of things (IoT) ecosystems, electronic noses (e-noses) are gaining interest for the detection of specific molecules over a background of interfering gases. The sensing of nitrogen dioxide is particularly relevant for applications in environmental monitoring and precision medicine. Here we present an easy and efficient functionalization procedure to covalently modify graphene layers, taking advantage of diazonium chemistry. Separate graphene layers were functionalized with one of three different aryl rings: 4-nitrophenyl, 4-carboxyphenyl and 4-bromophenyl. The distinct modified graphene layers were assembled with a pristine layer into an e-nose for NO2 discrimination. A remarkable sensitivity to NO2 was demonstrated through exposure to gaseous solutions with NO2 concentrations in the 1-10 ppm range at room temperature. Then, the discrimination capability of the sensor array was tested by carrying out exposure to several interfering gases and analyzing the data through multivariate statistical analysis. This analysis showed that the e-nose can discriminate NO2 among all the interfering gases in a two-dimensional principal component analysis space. Finally, the e-nose was trained to accurately recognize NO2 contributions with a linear discriminant analysis approach, thus providing a metric for discrimination assessment with a prediction accuracy above 95 %.

Nanoscale chemical patterning of graphite at different length scales.

Chemical patterning surfaces is relevant in several different domains of science and technology with exciting possibilities in electronics, catalysis, sensing, and photonics. Here, we present a novel strategy for chemical patterning of graphite using a combination of covalent and non-covalent approaches. Building on our previous work, where self-assembled monolayers of linear alkanes were used as sacrificial masks for directing the covalent anchoring of aryl groups to the graphite surface in sub-10 nm arrays, we present a modified design of a template alkane with alkoxy terminal groups which allowed better pattern transfer fidelity in comparison to simple linear alkanes. We also explored the use of chronoamperometry (CA) instead of previously used cyclic voltammetry (CV) for the functionalization process, which enabled patterning of the graphite surface at two-different length scales: few hundred nanometer circular patterns interspersed with sub-10 nm linear arrays. The covalent chemical patterning process has been studied in detail using CV and CA measurements whereas the patterned substrates have been thoroughly characterized using Raman spectroscopy, scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Based on the comparison between the pattern transfer fidelity of previously studied alkanes and newly synthesized alkoxy alkane, we discuss plausible molecular mechanism of pattern transfer.

Impact of the Filling Medium on the Colour and Sensory Characteristics of Canned European Eels (Anguilla anguilla L.).

The different vegetable oils used in canned fish as a filling medium have a preserving effect and contribute to the palatability of the product. In this study, the colour of European eels and the filling medium (sunflower oil, olive oil or spicy olive oil) was measured at different steps of the canning process. The sensorial characteristics of canned eels packed in the different oils were also evaluated. Colour scores (CieLab values) were higher in canned eels packed in sunflower and spicy olive oil than in canned eels packed in olive oil. The changes in colour parameters depended on the type of oil, the stage of the process and the storage time. Colour changes in canned eels packed in olive oil were highest during the sterilization process. Spicy olive oil was the filling medium in which the colour change was greatest, probably due to the migration of some of the spice components into the oil. Organoleptic properties were directly related to the type of oil used as the filling medium. The canned eels packed in sunflower oil were those awarded the highest scores in consumer tests, although the preferences varied depending on the age and gender of the consumers.

A chemisorbed interfacial layer for seeding atomic layer deposition on graphite.

The integration of graphene, and more broadly two-dimensional materials, into devices and hybrid materials often requires the deposition of thin films on their usually inert surface. As a result, strategies for the introduction of surface reactive sites have been developed but currently pose a dilemma between robustness and preservation of the graphene properties. A method is reported here for covalently modifying graphitic surfaces, introducing functional groups that act as reactive sites for the growth of high quality dielectric layers. Aryl diazonium species containing tri-methoxy groups are covalently bonded (grafted) to highly oriented pyrolytic graphite (HOPG) and graphene, acting as seeding species for atomic layer deposition (ALD) of Al2O3, a high-κ dielectric material. A smooth and uniform dielectric film growth is confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurements. Raman spectroscopy showed that the aryl groups gradually detach from the graphitic surface during the Al2O3 ALD process at 150 °C, with the surface reverting back to the original sp2-hybridized state and without damaging the dielectric layer. Thus, the grafted aryl groups can act as a sacrificial seeding layer after healing the defects of the graphitic surface with annealing treatment.

Multicomponent Covalent Chemical Patterning of Graphene.

The chemical patterning of graphene is being pursued tenaciously due to exciting possibilities in electronics, catalysis, sensing, and photonics. Despite the intense efforts, spatially controlled, multifunctional covalent patterning of graphene has not been achieved. The lack of control originates from the inherently poor reactivity of the basal plane of graphene, which necessitates the use of harsh chemistries. Here, we demonstrate spatially resolved multicomponent covalent chemical patterning of single layer graphene using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. This work provides a solid foundation for the fabrication of chemically patterned multifunctional graphene interfaces for device applications.

Covalent functionalization of molybdenum disulfide by chemically activated diazonium salts.

Covalent functionalization is one of the most efficient ways to tune the properties of layered materials in a highly controlled manner. However, molecular chemisorption on semiconducting transition metal dichalcogenides remains a delicate task due to the inertness of their surface. Here we perform covalent modification of bulk and single layer molybdenum disulfide (MoS2) using chemical activation of diazonium salts. A high level of control over the grafting density and yield on MoS2 basal plane can be achieved by this approach. Using scanning probe microscopies and X-ray photoelectron spectroscopy we prove the covalent functionalization of MoS2.

Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modified by Reverse Micelles.

The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efficiency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in conditions of long-term storage but not in the operation of devices with such contacts. In this regard, the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim of the present work is to understand the reactions occurring at the surfactant/metal interface, which may help to improve the fabrication of novel electrodes.

Biochemical Changes during the Manufacture of Galician Chorizo Sausage as Affected by the Addition of Autochthonous Starter Cultures.

In this work, the effect of the use of two autochthonous starter cultures (Lactobacillus sakei LS131 + Staphylococcus equorum SA25 (EQU), or L. sakei LS131 + Staphylococcus saprophyticus SB12 (SAP)) on the physicochemical, microbiological, proteolytic and lipolytic changes taking place during the manufacture of Galician chorizo, a traditional Spanish sausage, was studied. Three different batches (control (CNT), EQU and SAP) were manufactured in triplicate and analysed during the manufacturing process (samples were taken and analysed at 0, 2, 5, 9, 14, 21 and 30 days of ripening) for proximate composition, pH, aw, colour parameters, nitrogen fractions, free amino acids, biogenic amines, fat parameters and free fatty acids. The use of either of these two starter cultures slightly but significantly reduced the pH values during the fermentation and increased the percentage of transformation to nitrosyl-heme pigments as well as the a* and b* values in the final products. The two starters significantly decreased the Enterobacteriaceae counts in the final product, but without this microbial group completely disappearing. Both starter cultures significantly increased the α-amino acidic nitrogen and the total basic volatile nitrogen fractions during manufacturing, also increasing the free amino acid content and reducing the total biogenic amine content by approximately 20%. The SAP starter enhanced the lipolytic processes, increasing the free fatty acid content. Due to their performances, these two starter cultures seem to be suitable for increasing the quality and safety of the Galician chorizo sausage.

Self-limiting covalent modification of carbon surfaces: diazonium chemistry with a twist.

The chemistry of carbon surfaces has regained traction in recent years in view of its applicability towards covalent modification of a variety of (2D) materials. A general requisite is the formation of a dense and well-defined monolayer of aryl groups covalently bound to the surface. Given the use of reactive chemistries however, it is often not easy to achieve precise control over the monolayer growth while maintaining high grafting densities. Here we present a straightforward experimental protocol for the fabrication of well-defined covalent monolayers onto the surface of graphite. Using a combination of surface analytical tools, we demonstrate that the ascorbic acid mediated dediazoniation of aryldiazonium salts leads to self-limiting growth of monolayers with high grafting densities. The aryl radicals preferentially attach to the basal plane of the substrate and once the surface is covered with a covalent monolayer, the surface reaction does not proceed further to an appreciable extent. The layer thickness of the covalent films was measured using atomic force microscopy whereas the grafting efficiencies were assessed using Raman spectroscopy. The chemical composition of the grafted films was studied using X-ray photoelectron spectroscopy whereas scanning tunneling microscopy provided nanometer scale insight into the structure of the covalent films. Mechanistic aspects of the process are also discussed. The self-terminating chemistry described here is a new addition to the synthetic armory for covalent modification of materials and sets a strong foundation for achieving precise nanoscale control over the covalent functionalization process.

Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes.

Lead halide perovskites have attracted tremendous attention in photovoltaics due to their impressive optoelectronic properties. However, the poor stability of perovskite-based devices remains a bottleneck for further commercial development. Two-dimensional perovskites have great potential in optoelectronic devices, as they are much more stable than their three-dimensional counterparts and rapidly catching up in performance. Herein, we demonstrate high-quality two-dimensional novel perovskite thin films with alternating cations in the interlayer space. This innovative perovskite provides highly stable semiconductor thin films for efficient near-infrared light-emitting diodes (LEDs). Highly efficient LEDs with tunable emission wavelengths from 680 to 770 nm along with excellent operational stability are demonstrated by varying the thickness of the interlayer spacer cation. Furthermore, the best-performing device exhibits an external quantum efficiency of 3.4% at a high current density (J) of 249 mA/cm2 and remains above 2.5% for a J up to 720 mA cm-2, leading to a high radiance of 77.5 W/Sr m2 when driven at 6 V. The same device also shows impressive operational stability, retaining almost 80% of its initial performance after operating at 20 mA/cm2 for 350 min. This work provides fundamental evidence that this novel alternating interlayer cation 2D perovskite can be a promising and stable photonic emitter.

Damages at the nanoscale on red blood cells promoted by fire corals.

The hydrocoral Millepora alcicornis, known as fire coral, biosynthesize protein toxins with phospholipase A2 (PLA2) activity as a main defense mechanism; proteins that rapidly catalyse the hydrolysis at the sn-2 position of phosphatidylcholine-type phospholipids of cellular membranes. This hydrolysis mechanism triggers a structural damage in the outer leaflet of the red blood cells (RBC) membrane, by generating pores in the lipid bilayer that leads to a depletion of the cellular content of the damaged cell. A secondary mechanism, tentatively caused by pore-forming proteins toxins (PFTs), has been observed. The use of atomic force microscopy (AFM) has allowed to visualize the evolution of damages produced on the surface of the cells at the nanoscale level along the time.

Highly Stabilized Nanoparticles on Poly-l-Lysine-Coated Oxidized Metals: A Versatile Platform with Enhanced Antimicrobial Activity.

The increasing incidence of infections in implantable devices has encouraged the search for biocompatible antimicrobial surfaces. To inhibit the bacterial adhesion and proliferation on biomaterials, several surface functionalization strategies have been developed. However, most of these strategies lead to bacteriostatic effect and only few of these are able to reach the bactericidal condition. In this work, bactericidal surfaces were designed through the functionalization of titanium surfaces with poly-l-lysine (PLL) as the mediator for the incorporation of antimicrobial silver nanoparticles (AgNPs). This functionalization influences the adsorption of the particles on the substrate impeding the agglomeration observed when bare titanium surfaces are used, leading to a homogeneous distribution of AgNPs on the surfaces. The antimicrobial activity of this surface has been tested against two different strains, namely, Staphylococcus aureus and Pseudomonas aeruginosa. For both strains and different AgNPs sizes, the surface modified with PLL and AgNPs shows a much enhanced antimicrobial activity in comparison with AgNPs deposited on bare titanium. This enhanced antibacterial activity is high enough to reach bactericidal effect, a condition hard to achieve in antimicrobial surfaces. Importantly, the designed surfaces are able to decrease the bacterial viability more than 5 orders with respect to the initial bacterial inoculum. That means that a relative low load of AgNPs on the PLL-modified titanium surfaces reaches 99.999% bacterial death after 24 h. The results of the present study are important to avoid infections in indwelling materials by reinforcing the preventive antibiotic therapy usually dosed throughout the surgical procedure and during the postoperative period.

Activation of the Hog1 MAPK by the Ssk2/Ssk22 MAP3Ks, in the absence of the osmosensors, is not sufficient to trigger osmostress adaptation in Saccharomyces cerevisiae.

Yeast cells respond to hyperosmotic stress by activating the high-osmolarity glycerol (HOG) pathway, which consists of two branches, Hkr1/Msb2-Sho1 and Sln1, which trigger phosphorylation and nuclear internalization of the Hog1 mitogen-activated protein kinase. In the nucleus, Hog1 regulates gene transcription and cell cycle progression, which allows the cell to respond and adapt to hyperosmotic conditions. This study demonstrates that the uncoupling of the known sensors of both branches of the pathway at the level of Ssk1 and Ste11 impairs cell growth in hyperosmotic medium. However, under these conditions, Hog1 was still phosphorylated and internalized into the nucleus, suggesting the existence of an alternative Hog1 activation mechanism. In the ssk1ste11 mutant, phosphorylated Hog1 failed to associate with chromatin and to activate transcription of canonical hyperosmolarity-responsive genes. Accordingly, Hog1 also failed to induce glycerol production at the levels of a wild-type strain. Inactivation of the Ptp2 phosphatase moderately rescued growth impairment of the ssk1ste11 mutant under hyperosmotic conditions, indicating that downregulation of the HOG pathway only partially explains the phenotypes displayed by the ssk1ste11 mutant. Cell cycle defects were also observed in response to stress when Hog1 was phosphorylated in the ssk1ste11 mutant. Taken together, these observations indicate that Hog1 phosphorylation by noncanonical upstream mechanisms is not sufficient to trigger a protective response to hyperosmotic stress.

Role of the Sln1-phosphorelay pathway in the response to hyperosmotic stress in the yeast Kluyveromyces lactis.

The Kluyveromyces lactis SLN1 phosphorelay system includes the osmosensor histidine kinase Sln1, the phosphotransfer protein Ypd1 and the response regulator Ssk1. Here we show that K. lactis has a functional phosphorelay system. In vitro assays, using a heterologous histidine kinase, show that the phosphate group is accepted by KlYpd1 and transferred to KlSsk1. Upon hyperosmotic stress the phosphorelay is inactivated, KlYpd1 is dephosphorylated in a KlSln1 dependent manner, and only the version of KlSsk1 that lacks the phosphate group interacts with the MAPKKK KlSsk2. Interestingly, inactivation of the KlPtp2 phosphatase in a ΔKlsln1 mutant did not lead to KlHog1 constitutive phosphorylation. KlHog1 can replace ScHog1p and activate the hyperosmotic response in Saccharomyces cerevisiae, and when ScSln1 is inactivated, KlHog1 becomes phosphorylated and induces cell lethality. All these observations indicate that the phosphorelay negatively regulates KlHog1. Nevertheless, in the absence of KlSln1 or KlYpd1, no constitutive phosphorylation is detected and cells are viable, suggesting that a strong negative feedback that is independent of KlPtp2 operates in K. lactis. Compared with S. cerevisiae, K. lactis has only a moderate accumulation of glycerol and fails to produce trehalose under hyperosmotic stress, indicating that regulation of osmolyte production is different in K. lactis.

The Role of a Double Molecular Anchor on the Mobility and Self-Assembly of Thiols on Au(111): The Case of Mercaptobenzoic Acid.

The dynamics of the self-assembly process of thiol molecules on Au(111) is affected by the interplay between molecule-substrate and molecule-molecule interactions. Therefore, it is interesting to explore the effect of a second anchor to the gold surface, in addition to the S atom, on both the order and the feasibility of phase transitions in self-assembled monolayers. To assess the role of an additional O anchor, we have compared the adsorption of two mercaptobenzoic acid isomers, 2-mercaptobenzoic acid (2-MBA) and 4-mercaptobenzoic acid (4-MBA), on Au(111). Results from scanning tunneling microscopy, X-ray photoelectron spectroscopy, electrochemical techniques, and density functional theory calculations show that the additional O anchor in 2-MBA hinders surface mobility, reducing domain size and impeding the molecular reorganization involved in phase transition to denser phases on the Au(111) substrates. This knowledge can help to predict the range order and molecular density of the thiol SAM depending on the chemical structure of the adsorbate.

Ineffective Phosphorylation of Mitogen-Activated Protein Kinase Hog1p in Response to High Osmotic Stress in the Yeast Kluyveromyces lactis.

When treated with a hyperosmotic stimulus, Kluyveromyces lactis cells respond by activating the mitogen-activated protein kinase (MAPK) K. lactis Hog1 (KlHog1) protein via two conserved branches, SLN1 and SHO1. Mutants affected in only one branch can cope with external hyperosmolarity by activating KlHog1p by phosphorylation, except for single ΔKlste11 and ΔKlste50 mutants, which showed high sensitivity to osmotic stress, even though the other branch (SLN1) was intact. Inactivation of both branches by deletion of KlSHO1 and KlSSK2 also produced sensitivity to high salt. Interestingly, we have observed that in ΔKlste11 and ΔKlsho1 ΔKlssk2 mutants, which exhibit sensitivity to hyperosmotic stress, and contrary to what would be expected, KlHog1p becomes phosphorylated. Additionally, in mutants lacking both MAPK kinase kinases (MAPKKKs) present in K. lactis (KlSte11p and KlSsk2p), the hyperosmotic stress induced the phosphorylation and nuclear internalization of KlHog1p, but it failed to induce the transcriptional expression of KlSTL1 and the cell was unable to grow in high-osmolarity medium. KlHog1p phosphorylation via the canonical HOG pathway or in mutants where the SHO1 and SLN1 branches have been inactivated requires not only the presence of KlPbs2p but also its kinase activity. This indicates that when the SHO1 and SLN1 branches are inactivated, high-osmotic-stress conditions activate an independent input that yields active KlPbs2p, which, in turn, renders KlHog1p phosphorylation ineffective. Finally, we found that KlSte11p can alleviate the sensitivity to hyperosmotic stress displayed by a ΔKlsho1 ΔKlssk2 mutant when it is anchored to the plasma membrane by adding the KlSho1p transmembrane segments, indicating that this chimeric protein can substitute for KlSho1p and KlSsk2p.

The KlSTE2 and KlSTE3 genes encode MATalpha- and MATa-specific G-protein-coupled receptors, respectively, which are required for mating of Kluyveromyces lactis haploid cells.

Mating in yeast is initiated by binding of pheromone to G-protein-coupled receptors expressed in haploid cells. We analysed the role of KlSte2p and KlSte3p receptors in the Kluyveromyces lactis mating pathway. By sequence analysis, KlSte2p and KlSte3p are the homologues of the Saccharomyces cerevisiae alpha-pheromone and a-pheromone receptors, respectively. However, by expression experiments, we determined that KlSTE2 gene is expressed in the cells typified as MATalpha and therefore is the receptor for the K. lactis a-pheromone and KlSTE3 gene is expressed in the MATa cells and binds the alpha-pheromone. The KlSTE2 gene is silent in MATa cells, while it is highly expressed in MATalpha cells, and conversely the KlSTE3 gene is expressed in MATa cells and repressed in MATalpha cells. Disruption mutants of both genes were found to be sterile, and this defect is reversed by plasmidic copies of each gene. The cytoplasmic C-terminus of KlSte3p interacts strongly with the KlGpa1p (Galpha) subunit, which is involved in the transduction of the pheromone stimulus to induce mating. Remarkably, this same domain does not interact with a constitutive active allele of the Galpha subunit, indicating that the C-terminus is able to discriminate between the active (GTP-bound) and inactive (GDP-bound) forms of the Galpha subunit.

The pheromone response pathway of Kluyveromyces lactis.

The mating pheromone response pathway in Saccharomyces cerevisiae is one of the best understood signalling pathways in eukaryotes. Comparison of this system with pathways in other fungal species has generated surprises and insights. Cloning and targetted disruption of genes encoding components of the pheromone response pathway has allowed the attribution of specific functions to these signal transduction components. In this review we describe current knowledge of the Kluyveromyces lactis mating system, and compare it with the well-understood S. cerevisiae pathway, emphasizing the similarities and differences in the heterotrimeric G protein activity. This mating pathway is controlled positively by both the Galpha and the Gbeta subunits of the heterotrimeric G protein.