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  • Open access
  • 46 Reads
Colloidal Characteristics of Molybdenum Blue Nanoparticles Dispersion for Catalytic Applications

Molybdenum blue are oxygen-containing compounds of molybdenum of variable composition in which molybdenum is in oxidation states + 5 and + 6. Molybdenum-oxygen clusters represent a large class of polyoxometalates. Under certain conditions, it is possible to synthesize clusters of a certain shape and size. The nanosized of the clusters and their monodispersity make it possible to consider molybdenum blue as promising precursors for the synthesis of various catalysts, especially for preparation of refractory compounds - molybdenum carbides or nitrides.

For the synthesis of supported catalysts using dispersions of nanoparticles (sols), it is necessary to know their main colloidal-chemical properties, the most important of which are electro-surface characteristics, rheological properties and the conditions for maintaining aggregative stability.

The paper presents the results of a study of the colloidal-chemical properties of molybdenum blue, the dispersed phase of which is represented by toroidal particles of the class Mo154-x. It was found that aggregate stable dispersions exist in the pH range from 0.8 to 3.0 pH units. In the investigated pH range, molybdenum blue particles are negatively charged, and the electrokinetic potential does not exceed 30 mV. Molybdenum blues have high aggregate stability and can be concentrated to a high concentration of the dispersed phase (20-30 wt%); at higher concentrations, a transition of the sol into a gel is observed. In a wide range of concentrations molybdenum blues are Newtonian liquids, the viscosity of which mainly depends on the concentration of the dispersed phase. It was found that dispersions are characterized by the presence of electro-viscous effects. The nature of the aggregate stability of the dispersions of molybdenum blue is discussed.

  • Open access
  • 48 Reads
Molybdenum-tungsten blue nanoparticles as a precursor for ultrafine binary carbides

Binary or mixed molybdenum-tungsten carbides with high surface area and small particle size are of great interest for catalytic applications, for example dry reforming of methane (DRM) or hydrogen evolution reduction (HER). The promising method for synthesis of ultafine carbide particles is sol-gel method using dispersions of molybdenum-tungsten nanoparticles. For further use it is necessary to know main properties of molybdenum-blue nanoparticles, including size, structure and stability under different conditions.

The synthesis of dispersions of molybdenum-tungsten blue was carried out as a result of the reduction of molybdate and tungstate ions in an acidic medium. Ascorbic acid was used as a reducing agent and acted as a carbon source.Dispersions and nanoparticles were investigated by UV/vis, infrared (FTIR) and X-ray photoelectron (XPS ) spectroscopy, transmission electronic microscopy (TEM) and dynamic light scattering (DLS).

It was shown, that molybdenum-tungsten blues are highly dispersed systems based on polyoxometalate complexes of molybdenum and tungsten. A unique property of POM is monodispersity and small particle size about 3-5 nm. The effect of molar ratio reducing agent/ metal (molybdenum and tungsten) and molar ratio molybdenum/tungsten on the properties of dispersions was investigated. It was shown, that stable nanoparticles were formed in the range of molar ratio reducing agent/metal from 0.8 to 1.0 and at the molar ratio molybdenum/tungsten [Mo]:[W]= 95:5; 90:10; 80:20; 50:50.

  • Open access
  • 102 Reads
Self-assembly of nanoclusters in molybdenum blue dispersions in the presence of organic reducing agent

Molybdenum blue nanoparticles belong to the polyoxometalates (POM) containing molybdenum in oxidation states + 5 and + 6. Polyoxometalates exhibit remarkable physicochemical properties, structural flexibility and highly reactivity. These systems are considered as the most promising clusters for production of hybrid materials, drug delivery systems, nanoreactors and catalytic materials. The synthesis of nanoclusters of polyoxomolybdates is self-assembly from various initial building blocks and identification of its structure.

A study of the processes leading to the formation of particles in stable colloidal systems is necessary for more complete understanding of the synthesis of molybdenum blue as colloidal systems. The aim of this paper was to synthesize stable dispersions of molybdenum blue using organic reducing agents and investigate the self-assembly process.

Molybdenum blue dispersions were synthesized by reducing an acidic molybdate solution with glucose, hydroquinone and ascorbic acid. The influence of the H/Mo molar ratio on the rate of formation of molybdenum particles was established. For each reducing agent, the conditions for the formation of aggregative stable dispersion of nanoclusters with the maximum concentration of particles are determined. The dispersed phase is presented by toroidal molybdenum oxide nanoclusters of the class Mo154-x, which was confirmed by the results of UV-Vis, FTIR and XPS spectroscopy, DLS and TEM.

  • Open access
  • 44 Reads
Spontaneous emission spectrum of a WS2 monolayer under strong coupling conditions

Two-dimensional materials allow for extreme light confinement, thus becoming important candidates for all optical application platforms. Monolayers of transition-metal dichalcogenides are direct band gap semiconducting 2D materials featuring bandgaps in the visible and near-IR range, strong excitonic resonances, and high oscillator strengths, among other properties, as well as, supporting exciton polaritons. The optical properties of quantum emitters, such as molecules or quantum dots, near single or multilayer transition-metal dichalcogenides have been investigated, where the relaxation rate of the quantum emitter increases or decreases. The studies on the coupled quantum emitter - transition-metal dichalcogenides remain so far in the weak light-matter coupling regime. In this work, we study the spontaneous emission spectrum of a two-level quantum emitter near a WS2 layer, in which case the Purcell factor of the QE can take values up to 104. We further study the Rabi splitting in the spontaneous emission spectrum at room temperature for a quantum emitter with free-space decay times in the 10 ps to 500 ps range. We observe that at close distance of the quantum emitter to the WS2 layer, combined with short decay times, the spectrum can feature several peaks. In such cases, the Rabi splitting lies between 0.25 eV and 0.05 eV, for increasing free-space decay times, indicating strong coupling conditions for the light-matter interaction between the quantum emitter and the WS2 layer. Moreover, no simple relation between the inverse free-space decay time and the corresponding Rabi splitting value has been found. As the distance between the quantum emitter and the layer increases farther, the light-matter interaction coupling enters the weak coupling regime, which leads to vanishing Rabi splitting in the spontaneous emission spectrum for free-space decay times larger than a few tenths of ps.

  • Open access
  • 204 Reads
'Self-Delivery' Using Anti-Inflammatory Hydrogels: Biological Evaluation of NSAID-Dehydrodipeptide Conjugates

Supramolecular hydrogels where the hydrogelator molecule is endowed with intrinsic pharmacological properties can potentially fulfil a dual function in drug delivery systems as nanocariers for incorporated drugs and as active drugs themselves. In this present study, we investigated the pharmacological activities of a panel of naproxen-dehydrodipeptide conjugates, previously studied for their hydrogelation ability and as nanocarriers for drug-delivery applications. A library of dehydrodipeptides, containing N-terminal canonical amino acids (Phe, Tyr, Trp, Ala, Asp, Lys, Met) N-capped with naproxen and linked to a C-terminal dehydroaminoacid (ΔPhe, ΔAbu), were evaluated for their anti-inflammatory and anti-cancer activities, as well as for their cytotoxicity to non-cancer cells, using a variety of enzymatic and cellular assays. All compounds except one were able to significantly inhibit lipoxygenase (LOX) enzyme at a similar level to naproxen. One of the compounds was able to inhibit the cyclooxygenase-2 (COX-2) to a greater extent than naproxen, without inhibiting cyclooxygenase-1 (COX-1), and therefore is a potential lead in the search for selective COX-2 inhibitors. This hydrogelator is a potential candidate for dual COX/LOX inhibition as an optimised strategy for treating inflammatory conditions.

  • Open access
  • 63 Reads
Multilayer graphene supported flexible optoelectronic devices

Here, we demonstrate the synthesis process of large area graphene, grown by Atmospheric Pressure Chemical Vapour Deposition technique and transferred on polyethylene terephthalate PET substrates. Quality assessments were confirmed by Scanning Electron Microscopy (SEM), Raman analysis and optical spectroscopy. Stability of the sheet resistance after 1500 bending tests of graphene /PET is demonstrated. Next, graphene was implemented as transparent conductive electrode in flexible devices: several graphene-based Polymer Dispersed Liquid Crystal (PDLC) devices have been fabricated and their electro-optical properties, such as voltage-dependent transmittance, response time and flexibility were measured and presented by video demonstrations. The obtained results open a great potential of graphene integration into the next generation ITO-free flexible and stretchable optoelectronics.

  • Open access
  • 104 Reads
Chemical Vapor Transport Growth and Characterization of WTe2 Crystals

Single crystals of Tungsten Ditelluride WTe2 having a 2D layered structure are grown by chemical vapor transport method using bromine as the transporting agent. The structure and properties of the WTe2 single crystals as well as exfoliated thin film samples were characterized using XRD single crystal and powder X-ray diffraction, EDX, Raman spectroscopy and spectroscopic ellipsometry. The quality of the samples relation to growth conditions is further accessed and possible applications discussed.

  • Open access
  • 198 Reads
Optimization of the crosslinking process with glutaraldehyde vapor in PVA based electrospun membranes to wound dressings applications

Chronic wounds (CW) are characterized by delay or non-healing after 4 weeks of treatment. CW have become more prevalent, lead to a huge burden on healthcare and social systems, requiring specialized care. Wound dressings with specific properties capable to promote regeneration and avoid infections are highly needed. However, their efficacies are still limited due to the complexity of wound healing process and inadequate functions of wound dressings. To the effect, mats prepared at 10 w/v% concentration in glacial acetic acid and water in a 75/25 v/v% ratio based at different ratios of PVA/CA (100/0, 90/10 and 80/20 v/v%) were produced via electrospinning technique. The electrospinning setup consisted of a syringe and metal needle, an aluminum collecting plate, and a high voltage power supply. Conditions were optimized to obtain uniform, bead free mats, with a flexible structure and mechanical resilience. To overcome the instant solubilization of the PVA portion of the mat in aqueous media, a crosslinking process was employed. Crosslinking implies the formation of chemical bonds between different molecular chains to generate strong, stable and water insoluble 3D networks. For PVA, the crosslinking process can be accomplished via chemical or physical reactions, being the dialdehydes, diisocyanates, dicarboxylic, tricarboxylic and boric acids the most frequent chemical agents applied. Even though there are various options, glutaraldehyde (GA) is by far the most common crosslinker used in PVA processing due to its efficiency, ease of access and processing, and low cost. Further, in comparison with other crosslinking agents, GA in its vapor form has demonstrate reduced or no cytotoxic effect. Therefore, GA vapor was the crosslinker chosen and various parameters of process as the amount of GA, time, temperature and drying methods to its elimination were tested. GA vapor at 25% in water for 7 h at 60ºC, using 6 mL of solution per 130×120 mm2 of mat proved to be the most efficient option. To eliminate GA from mats and stored the crosslinked mats, the mats were stored in in a controllable environment of 41% RH and 19ºC until usage.

  • Open access
  • 71 Reads
Green reduction of silver ions to silver nanoparticles using aqueous plant extracts

Silver nanoparticles are used in numerous scientific fields due to their versatile properties and because their surface can be functionalized with different biological molecules. Their synthesis follows both conventional and unconventional routes and, in recent years, the methods that start from plants are constantly detaching from the hazardous, time consuming chemical methods. This paper presents the green synthesis of silver nanoparticles from St. Benedict's herb (Geum urbanum) and its corresponding aqueous extracts via two different temperature conditions: room temperature, for 24 hours with no additional stirring and 500C, for 30 minutes under a cosntant stirring of 600 rpm. Silver nanoparticles were then characterized using UV-Vis spectroscopy at well - established time intervals in the range of 250 -650 nm and FTIR spectra were recorded to show the presence of different functional groups. DLS technique was used to investigate the particle size and zeta potential was also measured to analyze the stability of the green synthesized silver nanoparticles. TEM miscroscopy revealed a spherical - shape profile of the green synthesized silver nanoparticles and optical microscopy images were also recorded. Also, the antioxidant activity was determined using the DPPH method and compared to that of the crude aqueous extracts.

  • Open access
  • 54 Reads
Graphene quantum dots: physico-chemical characterization and in vitro biological effects

Graphene quantum dots (GQDs) represents nanoscale structures with strong quantum property and exceptional photoluminescence properties. These particles have promising applications in nanomedicine, specifically for diagnostics, cargo delivery, photothermal therapy and bioimaging. In this context, we aimed to characterize GQDs available on the market for a further utilization for in vivo purposes. Transmission and scanning electron microscopy (TEM and SEM), and energy dispersive X-ray spectroscopy (EDX), were used to characterize the morphology and elemental composition of GQDs. In addition, the hydrodynamic size and the zeta potential were measured for these nanoparticles. Their biocompatibility was investigated on human fibroblast lung cells (MRC-5 cell line) after 24 and 72 hours of incubation with concentrations up to 200 μg/mL of GQDs. TEM images showed graphene sheets with few wrinkle structures, the dots having uniform diameter in the range between 1.0 and 5.0 nm. SEM examination revealed the three-dimensional structure with a sponge-like aspect and pores of various sizes. Their tendency to aggregate provided the formation of aggregates with sizes of hundreds of nanometers, as it was revealed by the hydrodynamic diameter of about 270 nm. A negative zeta potential of -16 mV confirmed the anionic character of GQDs. Concentrations up to 50 μg/mL exhibited a low toxicity in lung cells as revealed by MTT assay and fluorescent microscopy of actin cytoskeleton after both time intervals, confirming a potential further testing on animals for clinical purposes. However, the high doses of GQDs induced cell death and must be avoided in future. Given the new experimental evidences obtained on GQDs, more knowledge has been achieved, which is very useful for prospective research to revolutionize the future of nanomedicine and biotechnology.

Acknowledgments M.S. Stan acknowledges the support of the Operational Programme Human Capital of the Ministry of European Funds through the Financial Agreement 51668/09.07.2019, SMIS code 124705.