Discovered in mid-1817 by Jöns Jacob Berzelius, selenium, belonging to Group 16 of the periodic table is an essential trace element for human and animal health, due to its biocompatibility and bioavailability. Additionally, it is known for having different oxidation states, which allows it to interact with distinct chemical elements to form various compounds. Selenium exhibits two forms, organic and inorganic; the latter is known for its genotoxicity. Selenium nanoparticles have been investigated as an alternative to mitigate the toxicity of this element. With antidiabetic, antiviral, chemopreventive, and antimicrobial properties, SeNPs possess significant biomedical potential and can be synthesized using chemical, physical, or green methods, offering new solutions for combating microbial resistance and other diseases. This review discusses the historical discovery of selenium, preparation methods, the versatility of combinations for synthesis, morphological characteristics, and sizes, as well as the impact of SeNP applications obtained through different approaches against medically relevant microorganisms, particularly those exhibiting resistance to conventional antimicrobials.

Background/Objectives: Optimizing synthesis parameters is essential to ensure the quality and stability of nanostructures. This study aimed to optimize the synthesis of selenium nanoparticles (SeNPs) by chemical reduction, using sodium selenite (Na₂SeO₃), ascorbic acid (AA), and polyvinyl alcohol (PVA) at different concentrations, volumes, and molar ratios. The effects of reduction time, purification steps, and variations in the concentration of the precursor and reducing agent, as well as in the volume of the stabilizer, on the characteristics of SeNPs were investigated to ensure their long-term stability, maintenance of their properties, and biological applicability. Methods: The SeNPs were analyzed by UV/Vis absorption spectroscopy, Dynamic Light Scattering (DLS), and Transmission Electron Microscopy (TEM), and were also evaluated for antifungal activity against the SC5314 strain of Candida albicans. Results/Conclusions: Monodisperse SeNPs were obtained under high concentrations of Na₂SeO₃ and AA, short reduction time, higher volumes of PVA (2–4 mL), and purification at 24.300× g, presenting a spherical morphology, hydrodynamic diameter of 137.0–171.7 nm, dry diameter of 20–120 nm, polydispersity index of 0.049–0.306, Zeta potential of −7.79 to −19.6 mV, and stability for up to 180 days. In the absence or presence of 1 mL of PVA, the SeNPs were predominantly amorphous. Regarding biological activity, the SeNPs did not exhibit antifungal activity under the experimental conditions in the tested strain. Together, this study provides a comprehensive update on the synthesis of SeNPs under different conditions and their stability over time, contributing to the consolidation of knowledge in the field.

Background/Objectives: The biogenic synthesis of silver nanoparticles (AgNPs) is a promising alternative method, driven by the presence of metabolites in plant matrices capable of acting as reducing and stabilizing agents. Seasonality is a key factor that influences the phytochemical composition of plants and can directly impact the yield, physicochemical characteristics, stability, and bioactivities of the obtained AgNPs. This study aimed to synthesize AgNPs using aqueous extracts from Paullinia cupana leaves collected during dry and rainy seasons, prepared by two different methods (agitation or infusion), to evaluate the impact of these variables on the biosynthesis and properties of the nanostructures. Methods: The extracts were characterized by UHPLC-HRMS/MS, and their total phenolic compound (TPC) content and antioxidant potential against DPPH and ABTS radicals were determined. The AgNPs were characterized by UV/Vis spectrophotometry, dynamic light scattering (DLS), zeta potential (ZP), nano-particle tracking analysis (NTA), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). Results: The metabolic profile results showed a predominance of alkaloids and flavonoids in all extracts, with greater phytochemical diversity in samples prepared by infusion. TPC indicated superior phenolic extraction in extracts prepared by infusion during the rainy season, correlating with greater antioxidant potential via the elimination of free radicals. The evolution of AgNP synthesis was accompanied by a gradual change in the color of the suspensions and the formation of plasmon bands between 410 and 430 nm, characteristic of spherical AgNPs. The nanostructures presented hydrodynamic diameters between 37.49 and 145.5 nm, PdI between 0.222 and 0.755, and Zeta potential between −11.3 and −39.9 mV, suggesting satisfactory colloidal stability. Morphological analyses revealed predominantly spherical particles with average diameters ranging from 33.61 to 48.86 nm and uniform distribution, while EDX spectra confirmed the presence of silver. Conclusions: Thus, our results demonstrate that both seasonality and the method of extract preparation influence the phytochemical composition and, consequently, the morphology, stability, and optical properties of AgNPs, with subtle emphasis on collections made during the rainy season and extracts prepared by infusion. Such knowledge contributes to the advancement of more reproducible and purpose-oriented syntheses in the field of green nanotechnology, enabling applications in various sectors.

The objective of this study was to reuse agroindustrial waste from orange peels to obtain an orange peel aqueous extract (OPEa), used in the biosynthesis and characterization of selenium nanoparticles (SeNPs). The OPEa acted as a reducing and stabilizing agent in the synthesis of SeNPs, due to its polyphenolic compounds, such as flavonoids, alkaloids, terpenoids, and coumarins, identified by UHPLC-HRMS/MS. The synthesis of SeNPs was conducted in a microwave reactor, at 120 °C and 150 °C, for 30 min, using different volumetric proportions/concentrations between OPEa and sodium selenite (Na₂SeO₃) (5 and 10 mM), in ratios of 1:4, 2:3, 1:1, 3:2, and 4:1 (OPEa _(mL) :Na ₂ SeO _3(mL) ). The SeNPs were characterized by visual inspection, UV/Vis, DLS, pH, TEM, SEM, and EDX. The results showed that SeNPs were successfully biosynthesized under all conditions, exhibiting distinct characteristics depending on the synthesis conditions. The formation of SeNPs was confirmed by the chromatic transition from greenish tones to light yellow to intense red and by UV/Vis spectrometry, with absorption peaks in the range of 250–340 nm. The SeNPs exhibited a hydrodynamic diameter (HD) of 213.6–1296 nm, with mono- and polydispersity characteristics (PdI: 0.033–0.885) and Zeta potential (ZP) between − 24.2 to -54.9 mV. Among the conditions tested, SeNPs with the best characteristics presented more homogeneous UV/Vis bands, lower HD (273.2–282.8 nm), and PdI (0.089–0.231), high ZP (-34.4 to -36.5 mV), stable pH (6), clear, translucent, and pellet-free colloidal solution, spherical morphology, and selenium accounted for 20.17% of its elemental composition.