Quantum trapping and rotational self-alignment in triangular Casimir microcavities.

Casimir torque, a rotational motion driven by zero-point energy minimization, is a problem that attracts notable research interest. Recently, it has been realized using liquid crystal phases and natural anisotropic substrates. However, for natural materials, substantial torque occurs only at van der Waals distances of ~10 nm. Here, we use Casimir self-assembly with triangular gold nanostructures for rotational self-alignment at truly Casimir distances (100 to 200 nm separation). The interplay of repulsive electrostatic and attractive Casimir potentials forms a stable quantum trap, giving rise to a tunable Fabry-Pérot microcavity. This cavity self-aligns both laterally and rotationally to maximize area overlap between templated and floating flakes. The rotational self-alignment is sensitive to the equilibrium distance between the two triangles and their area, offering possibilities for active control via electrostatic screening manipulation. Our self-assembled Casimir microcavities present a versatile and tunable platform for nanophotonic, polaritonic, and optomechanical applications.

Eco-Friendly Iron-Humic Nanofertilizers Synthesis for the Prevention of Iron Chlorosis in Soybean (Glycine max) Grown in Calcareous Soil.

Iron deficiency is a frequent problem for many crops, particularly in calcareous soils and iron humates are commonly applied in the Mediterranean basin in spite of their lesser efficiency than iron synthetic chelates. Development and application of new fertilizers using nanotechnology are one of the potentially effective options of enhancing the iron humates, according to the sustainable agriculture. Particle size, pH, and kinetics constrain the iron humate efficiency. Thus, it is relevant to understand the iron humate mechanism in the plant-soil system linking their particle size, characterization and iron distribution in plant and soil using 57Fe as a tracer tool. Three hybrid nanomaterials (F, S, and M) were synthesized as iron-humic nanofertilizers (57Fe-NFs) from leonardite potassium humate and 57Fe used in the form of 57Fe(NO3)3 or 57Fe2(SO4)3. They were characterized using Mössbauer spectroscopy, X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), transmission electron microscopy (TEM) and tested for iron availability in a calcareous soil pot experiment carried out under growth chamber conditions. Three doses (35, 75, and 150 µmol pot-1) of each iron-humic material were applied to soybean iron deficient plants and their iron nutrition contributions were compared to 57FeEDDHA and leonardite potassium humate as control treatments. Ferrihydrite was detected as the main structure of all three 57Fe-NFs and the plants tested with iron-humic compounds exhibited continuous long-term statistically reproducible iron uptake and showed high shoot fresh weight. Moreover, the 57Fe from the humic nanofertilizers remained available in soil and was detected in soybean pods. The Fe-NFs offers a natural, low cost and environmental option to the traditional iron fertilization in calcareous soils.

Gold Decoration and Photoresistive Response to Nitrogen Dioxide of WS2 Nanotubes.

Composites of WS2 nanotubes (NT-WS2 ) and gold nanoparticles (AuNPs) were prepared using aqueous HAuCl4 solutions and subjected to surface analysis. The obtained materials were jointly characterized by X-ray photoelectron (XPS), Raman scattering (RSS), and ultraviolet photoelectron (UPS) spectroscopies. Optical extinction spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscopy regime (STEM-EELS) were also employed to study plasmon features of the nanocomposite. It was found that AuNPs deposition is accompanied by a partial oxidative dissolution of WS2 , whereas Au-S interfacial species could be responsible for the tight contact of metal nanoparticles and the disulfide. A remarkable sensitivity of n-type resistance of NT-WS2 and Au-NT-WS2 to the adsorption of NO2 gas was also demonstrated at room temperature using periodical illumination by a 530 nm light-emitting diode. Au-NT-WS2 nanocomposites are found to possess a higher photoresponse and enhanced sensitivity in the 0.25-2.0 ppm range of NO2 concentration, as compared to the pristine NT-WS2 . This behaviour is discussed within the physisorption-charge transfer model to explore sensing properties of the nanocomposites.

Strong light-matter interaction in tungsten disulfide nanotubes.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

Facile chemical routes to mesoporous silver substrates for SERS analysis.

Mesoporous silver nanoparticles were easily synthesized through the bulk reduction of crystalline silver(I) oxide and used for the preparation of highly porous surface-enhanced Raman scattering (SERS)-active substrates. An analogous procedure was successfully performed for the production of mesoporous silver films by chemical reduction of oxidized silver films. The sponge-like silver blocks with high surface area and the in-situ-prepared mesoporous silver films are efficient as both analyte adsorbents and Raman signal enhancement mediators. The efficiency of silver reduction was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The developed substrates were applied for SERS detection of rhodamine 6G (enhancement factor of about 1-5 × 105) and an anti-ischemic mildronate drug (meldonium; enhancement factor of ≈102) that is known for its ability to increase the endurance performance of athletes.

Silver Eco-Solvent Ink for Reactive Printing of Polychromatic SERS and SPR Substrates.

A new reactive ink based on a silver citrate complex is proposed for a photochemical route to surface-enhanced Raman spectroscopy active substrates with controllable extinction spectra. The drop-cast test of the ink reveals homogeneous nucleation of silver and colloid particle growth originating directly from photochemical in situ reduction in droplets, while the following evaporation of the deposited ink produces small nano- and micron-size particles. The prepared nanostructures and substrates were accurately characterized by electron microscopy methods and optical extinction spectroscopy. Varying the duration of UV irradiation allows tuning the morphology of individual silver nanoparticles forming hierarchical ring structures with numerous "hot spots" for most efficient Raman enhancement. Raman measurements of probe molecules of rhodamine 6G and methylene blue reached the largest signal enhancement of 106 by the resonance effects.

Key Roles of Size and Crystallinity of Nanosized Iron Hydr(oxides) Stabilized by Humic Substances in Iron Bioavailability to Plants.

Availability of Fe in soil to plants is closely related to the presence of humic substances (HS). Still, the systematic data on applicability of iron-based nanomaterials stabilized with HS as a source for plant nutrition are missing. The goal of our study was to establish a connection between properties of iron-based materials stabilized by HS and their bioavailability to plants. We have prepared two samples of leonardite HS-stabilized iron-based materials with substantially different properties using the reported protocols and studied their physical chemical state in relation to iron uptake and other biological effects. We used Mössbauer spectroscopy, XRD, SAXS, and TEM to conclude on iron speciation, size, and crystallinity. One material (Fe-HA) consisted of polynuclear iron(III) (hydr)oxide complexes, so-called ferric polymers, distributed in HS matrix. These complexes are composed of predominantly amorphous small-size components (<5 nm) with inclusions of larger crystalline particles (the mean size of (11 ± 4) nm). The other material was composed of well-crystalline feroxyhyte (δ'-FeOOH) NPs with mean transverse sizes of (35 ± 20) nm stabilized by small amounts of HS. Bioavailability studies were conducted on wheat plants under conditions of iron deficiency. The uptake studies have shown that small and amorphous ferric polymers were readily translocated into the leaves on the level of Fe-EDTA, whereas relatively large and crystalline feroxyhyte NPs were mostly sorbed on the roots. The obtained data are consistent with the size exclusion limits of cell wall pores (5-20 nm). Both samples demonstrated distinct beneficial effects with respect to photosynthetic activity and lipid biosynthesis. The obtained results might be of use for production of iron-based nanomaterials stabilized by HS with the tailored iron availability to plants. They can be applied as the only source for iron nutrition as well as in combination with the other elements, for example, for industrial production of "nanofortified" macrofertilizers (NPK).