**Table 3.** *Cont.*

Compared to selenite and selenate, SeNPs are more biocompatible and less toxic to animal organisms [14]. Nevertheless, high doses or long-term supplementation of SeNPs may lead to adverse effects in animal organisms and can be toxic. Several in vivo studies were conducted to measure NPs toxicity. Urbankova et al. reported that SeNPs supplementation had fewer negative effects in rats compared to the standard form. In contrast, supranutritional doses of SeNP administration (0.2, 0.4, and 0.8 mg/kg of body weight) showed a positive effect on reproductive functions and immune and antioxidant capacity. Other experiments on mice and rats supplemented with SeNPs demonstrated the hepatotoxic effect of SeNPs, which were also confirmed by further histological examination [47,62,137,138,142]. Damage to the liver parenchyma and intestinal epithelium in rats was reported after 0.5, 1.5, 3, and 5 mg/kg DM of SeNP supplementation [136]. The authors sugges<sup>t</sup> that short-term SeNP supplementation can be safer and more beneficial in specific treatments. This unfavorable effect could be related to the tested animals' metabolisms, biological characteristics, and the correlation between animal weight and the dosage of NPs administrated. The toxicity of NPs can largely vary among different species [153]. SeNP hyperaccumulation in *Pangasius hypophthalmus* liver, brain, and muscles was observed after SeNP supplementation (2.5–4 mg/L), which caused oxidative stress and toxicity in fish [154]. In another study, SeNP (100 μg Se/L) supplementation in *Oryzias latipes* enhanced oxidative stress caused by the hyperaccumulation of Se in the liver [155].

Based on the studies mentioned above, SeNP supplementation can have many health benefits (e.g., improved production performance, growth, feed efficiency, antioxidant status, and immune status) when present in animal diets compared to inorganic Se sources. Nevertheless, high doses of SeNPs can cause the hyperaccumulation of Se in tissues and oxidative stress or toxicity. Therefore, SeNPs should be included in animal diets in optimum doses to formulate nutritionally balanced feeds. The mechanism of nano-sized Se conversion remains unclear, and the gu<sup>t</sup> microbiota is thought to play a key role in this process. The application of SeNPs showed promising results in improving the oxidative status of the cell induced by a reduction in glutathione (GSH), superoxide dismutase (SOD) levels [156], and GPx activities [157]. Whereas the grea<sup>t</sup> advantage of SeNP application compared to sodium selenite can be increased availability of the element [135], on the other hand, this advantage could be turned into a disadvantage through uncontrolled SeNP penetration across cellular membranes, which might be harmful to animal health. According to Surai et al. [35], the metabolism and assimilation of nano-sized Se could be disadvantageous in the animal diet when Se's main mechanism of biological activity is mediated *via* selenoprotein synthesis. Moreover, the effect of dietary SeNPs on gu<sup>t</sup> health and the formation of the accumulated nano-sized Se in animal tissues after supplementation is still unknown and needs further investigation. Furthermore, the topic of whether SeNPs supplementation may increase Se stores in the body remains unanswered.

#### *4.1. Green Synthesis of SeNPs*

Over the past decade, the biological method of producing NPs has become an emerging trend in nanotechnology and was developed as a sustainable way to overcome the disadvantages of chemical-based NP synthesis (e.g., high cost and toxic chemicals usage) [104]. Green synthesis provides a new possibility to synthesize NPs via an eco-friendly approach using simple unicellular or multicellular biological entities (e.g., bacteria, fungi, yeast, algae, and plant tissues) as natural reducing and stabilizing agents. The biologically synthesized nanostructures offer substantially different properties such as good adhesion, tribologically good properties, optical and electrical properties, and many promising applications. In NP synthesis, reducing and capping agents play an important role in impacting useful NP properties such as size, morphology, stabilizing, and protecting the NPs' surface, preventing aggregation and uncontrolled growth [158]. Chemical components (i.e., polyethylene glycol, formaldehyde, polyethyleneimine, and polyacrylic acid) used as capping, reducing, stabilizing agents, or solvents in the procedures of chemical NP synthesis are hazardous and extremely toxic [159,160]. To be easily utilized in the living

systems and not cause cellular toxicity, capping agents should be nontoxic, biodegradable, biocompatible, biosoluble, and well dispersed [161]. Green capping agents (e.g., amino acids and polysaccharides) are environmentally friendly. They may lead to designing NPs with unique morphologies and sizes, which can improve, for example, drug delivery via NPs, thereby enhancing NPs' antifungal, antiviral, and antibacterial activity. Nutrients in the form of nanoparticles can be encapsulated in nanocapsules and carried via GIT into the bloodstream and then into body organs, where they enhance the bioavailability of delivered nutrients [162]. Biological synthesis was successfully used to produce different metal NPs, such as AuNPs, FeNPs, and AgNPs [160,163–166]. Green synthesis was also employed to produce SeNPs [167–170], and their antimicrobial, antifungal properties, and cytotoxicity were tested in various in vitro studies.

#### *4.2. Antimicrobial Potential of SeNp Produced via Green Synthesis*

Antimicrobial resistance (AMR) represents a major global problem, which significantly affects human and animal health. The wide use of antibiotics as growth promoters in animal farming has caused the development of increased antibiotic resistance in numerous bacterial strains. As a consequence, in 2003, the use of antibiotics in livestock diets was banned in the EU [171]. AMR adversely affects animal health, leading to the poor quality of products of animal origin and economic losses. Therefore, finding a new solution to overcome antibiotic usage is strongly needed. In recent years, nanotechnology enabled the manufacture of effective antimicrobial agents from nano-scaled materials, particularly metals. Many studies confirmed the antioxidant, antibacterial, anticancer, and antifungal activities of metallic NPs [86,172–174]. Whereas selenium nanoparticles have attracted scientific interest primarily as a result of research into their anticancer properties; this nanomaterial's antibacterial potential has recently been identified. NPs have a large surface area, which increases the area of interaction with pathogenic microorganisms. Furthermore, due to the nano size, they are more likely to enter bacterial surfaces.

Although most of the studies on the antimicrobial potential of biogenic SeNPs were conducted in vitro (Table 4), the results of these studies showed noticeable antibacterial, antifungal, and anticancer SeNPs activities against many important humans and animal pathogens and their potential for future applications in nanomedicine and veterinary. Furthermore, SeNPs produced through the green way show lower cytotoxicity, greater bioavailability, and reactivity than inorganic and organic Se, which makes them an attractive candidate for future therapeutic applications. The therapeutic effect of biogenic SeNPs (2.5, 5.0, and 10.0 mg) was also confirmed in an in vitro experiment in a mouse model infected with *Toxoplasma gondii* with no cytotoxicity observed [107]. Based on in vitro studies (Table 4), SeNPs represent a viable approach to inhibit bacterial growth without using antibiotics and to overcome the drawbacks of synthetic methods that employ toxic chemicals. Interesting results were obtained by Cremonini et al. [175] who demonstrated the significantly better antibacterial activity of biogenic SeNPs in comparison with chemically produced NPs.


**Table 4.** Effects of biogenic SeNPs tested in vitro with potential use in nutrition.

