Bulgarian Academy of Sciences

Multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) were embedded into poly(vinylidene fluoride) (PVDF) electrospun fibers with varying mass fractions (0–4 wt%). In addition, the hybrid combinations at 1.5 wt.% GNP/MWCNT/PVDF were fabricated to evaluate potential synergistic effects. The electrospun fiber mats were thermally post-treated at 145°C for 3 and 6 h to assess the influence of annealing on the microstructure, crystalline phase composition, and functional performance. The morphological, structural, mechanical, and piezoelectric properties were examined in relation to both the distinct geometries of the fillers and the thermal annealing. The combination of annealing and nanofiller-induced nucleation significantly improved the crystallinity of the fibers. The tensile strength increased up to 11.1 MPa for 0.5 wt.% MWCNT/PVDF and 15.7 MPa for 1.5 wt.% GNP/PVDF after 6 h of annealing. Structural analysis revealed a pronounced α→β phase transition in the 1.5 wt. % hybrid compositions, reaching a β/α ratio up to 7.8 substantially greater than the transitions observed for mono-filler systems: 4.7 for 1.5 wt.% GNP/PVDF and 3.0 for 1.5 wt.% MWCNT/PVDF. The effect of the high β-phase content was further confirmed at the nanoscale by piezoresponse force microscopy (PFM), which showed a consistent piezoelectric response through the fibers: a coercive voltage of approximately ± 40 V, which decreased to ± 20 V, ascribed to the dispersed nanofillers within the fiber. These results demonstrate that combining hybrid carbon fillers with controlled annealing enables tunable crystalline structures and enhanced electromechanical performance in electrospun PVDF nanofibers. • hybrid GNP/MWCNT fillers drive synergetically β-phase nucleation mechanisms. • The interfacial interaction between PVDF and carbon fillers was quantified using Piezoresponse Force Microscopy (PFM) measurements. • PFM confirmed a consistent piezoelectric response with coercive voltage decreasing from ± 40 V to ± 20 V due to dispersed nanofillers • Hybrid fillers and annealing tailor PVDF crystallinity and functional performance.

Galanthamine, an AChE inhibitor marketed for symptomatic treatment of mild to moderate Alzheimer's disease, is produced by both chemical synthesis and extraction from plant of the Amaryllidoideae subfamily. There are a few validated GC-MS and HPLC-MS methods for its quantification in plant material. All of these methods apply multi-step sample preparation procedures including toxic solvents. The assessment of plant material and development of extraction methodologies from new plant sources (e.g. Hippeastrum papilio) requires validation of quantitative analytical methods considering the species alkaloid pattern. The extraction of galanthamine from H. papilio raw material was optimized selecting 0.072% of HCl water solution as the most effective extractant. An aliquot (1 µL) of the total extract (1 mL) was directly injected into the GC-MS system avoiding any purification steps with toxic solvents. Selectivity, linearity, sensitivity, precision, accuracy, stability and robustness were determined. The LLOQ and LLOD in SIM mode were found at a concentration of 10 ng/mL and 2 ng/mL, respectively. The method was used for assessment of plant raw material, alkaloid fractions and extraction effectiveness. The method describes the fastest and simplest extraction procedure and direct aqueous injection into the GC-MS for quantification of galanthamine in plant material. As compared to the LC-MS, the use of EIMS detector provides important information and advantage in the assessment of alkaloid fractions from new plant sources of galanthamine.

Low-cost air quality sensors enable dense urban monitoring networks but require calibration against reference-grade instruments. While machine learning calibration is well-established for co-located sensor pairs, applying these calibrations to sensors deployed far from any reference station—the operational reality for most network sensors—lacks systematic methodology. We address this gap using 24 months of hourly data (August 2023–July 2025) from Sofia, Bulgaria, where five official reference stations (Executive Environmental Agency) operate alongside 22 AirThings low-cost sensors, four of which are co-located. Random Forest models achieved R2∈(0.53,0.75) across PM2.5, PM10, NO2, and O3, representing from 40% (for O3) to 408% (for PM2.5) improvement over Multiple Linear Regression baselines. Using leave-one-station-out spatial cross-validation, we derived pollutant-specific uncertainty growth rates (α) from 3.84% to 5.62% per km, characterizing how calibration uncertainty increases with distance from reference stations (statistically significant for PM10 and O3, p<0.05). Applied to 18 non-co-located sensors, the framework generated 1.2 million calibrated hourly measurements with 95% prediction intervals over the study period. Co-location sites spaced 6 km apart achieve a less than 30% uncertainty increase at network midpoints, within EU Air Quality Directive thresholds for indicative monitoring. These empirically derived α parameters enable network planners to predict measurement reliability at arbitrary sensor locations without ground-truth validation, providing evidence-based guidance for cost-effective hybrid monitoring network design.

The recent endeavor to establish a sustainable society, with respect to environmental protection and occupational health prevention, imposes the need for the development of environmentally friendly anodization electrolytes. In addition, these electrolytes should be composed of biocompatible organic acids derived from renewable sources. In response to these challenges, a need arises to seek environmentally conscious alternatives to the widely used sulfuric acid anodization electrolytes. Accordingly, a comparative study was performed on the anodic polarization of AA2024-T3 aircraft alloy samples for 30 min at 0 or 20 °C. The respective electrolytes were composed of 0.5 M solutions of oxalic, citric, tartaric acids, or glycine. The comparative analysis included optical metallographic microscopy (OMM), scanning electron microscopy (SEM), determination of color characteristics and wettability, chemical composition analysis by X-ray photoelectron spectroscopy (XPS), and assessment of the corrosion protective properties of the obtained layers. The latter were defined by the application of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scanning (PDS) after 24 h of exposure to a 0.5% NaCl solution. Among the most important conclusions is that the barrier properties of the layers obtained in citric and tartaric acid electrolytes remarkably exceed those of the film obtained in oxalic acid. The use of glycine does not result in film formation at all. The process temperature did not have as strong an effect as the electrolyte composition.

Two arithmetic operations are introduced and some of their properties are studied. It is proved that they can be operations of semi-groups, but not of monoids. It is shown that their reverse operations are not one-valued. Some connections between the new operations and the well-known arithmetic functions φ, Ψ, σ are shown.

The four-color theorem in standard mathematics and the four-letter theorem in Hilbert mathematics are juxtaposed. The former is a topological theorem thus not allowing for its elementary metric proof as the latter. This is interpreted as an example of “Modernity’s bottle” forcing mathematics not to use any tools entering reality for complying with the taboo for the gap between mathematical models and reality. The same relation in Hilbert arithmetic is interpreted as passing through the “threshold” between two and three dimensions after the Gleason and Kochen - Specker theorems. The representation of mathematical duality and physical complementarity is discussed. The philosophical concept of “the totality” translated in ontomathematics implies for mathematical models to be “concave” in order to obey the absoluteness of the totality. A principle of the equivalence of “concave” (philosophical or “metaphysical”) and “convex” (proper mathematical) explanations being inherent from the viewpoint of ontomathematics is investigated by the example of the correspondence of the Bohmian (non-local and metaphysic: thus “concave”) versus “Copenhagen” (local and agnostic: thus, “convex”) interpretations of quantum mechanics. The eventual representation of quantum contextuality/ holism as fractal structures/ functional (Hausdorff) dimensionality is conjectured. The third dimension of reality (in the sense of the cited theorems) is researched in its two versions: “concave” (metaphysic, in particular, Bohmian) and “convex” (traditional and mathematical, particularly, “Copenhagen”). An illustration by the “four letters” of DNA (RNA) is considered. An interpretation of Peano arithmetic as the “half” of Boolean algebra unifying propositional logic and set theory is linked.

In this study, sol–gel-synthesized nanoparticles were characterized by various physicochemical techniques, including scanning electron microscopy (SEM), X-ray powder diffraction (XRD), UV-Vis spectrophotometry, and thermogravimetric analysis (DTA/TG). The as-obtained powders were tested for their antimicrobial activity against the Gram-positive bacteria Staphylococcus aureus and Enterococcus faecalis, as well as the fungal strains Candida albicans and Saccharomyces cerevisiae. Additionally, the photocatalytic performance of the samples was evaluated under simulated solar light. The results are promising for possible environmental applications. The antimicrobial assessment also revealed notable effects, with varying degrees of growth inhibition observed across the tested microorganisms. The main approach in this study consists of the combination of physicochemical characterization with antibacterial and photocatalytic evaluations, resulting in promising multifunctional materials.