An appropriate animal diet and living environment play a key role in animal health and performance. Thus, optimizing these factors is important for increasing rearing efficiency, which can positively determine the quality of production of animal origin. Over the last decade, nanotechnology has received the attention of many researchers due to its promising agricultural and food applications. Nanotechnology provides new “intelligent” solutions in animal nutrient delivery and health protection, and, indeed, it has the large potential to improve animal production systems [100]. This interest is mainly caused by the unique physicochemical properties of nanoparticles (NPs), which refers to their small size (1–100 nm), high stability, hydrophobicity, and large surface area. NPs’ hydrophobicity is important for good dispersion in water or serum and is also required to enhance their interaction with cell membranes [101]. The NPs’ size affects the cellular intake and allows them to easily pass through the stomach wall and diffuse into body cells quicker than common elements with larger particle sizes. The in vitro absorption of NPs with a diameter of 0.1 µm was found to be higher than 1 and 10 µm NPs [102]. The thickness of gastric mucus layers (total mucus), which continuously cover the gastrointestinal tract’s (GIT) surface, varies from 200 µm in the small intestine to 480–800 µm in the large intestine [14] and could allow the transport of NPs through the layer. According to Corbo et al. [103], NPs, especially nanominerals (e.g., Se and Zn), have a higher surface-area-to-volume ratio, providing more surface area for contact with the mucosal tissues and cells. Better absorption of NPs into the mucosal surface increases the particle residence time in the GIT. When a nanomineral is introduced into a biological medium, such as blood or mucus, proteins adsorb on its surface, giving it a unique “biological identity”, a so-called protein corona, which can have an impact on the NPs’ distribution as well as their potential toxicity [104]. Nanoparticles have been used in animal nutrition for their antibacterial, antifungal, and antioxidant properties as well as probiotics and to maintain general animal performance and health. The antimicrobial activities of metallic NPs (e.g., ZnO, CuO, and AgNPs) and SeNPs have been demonstrated by different researchers [105,106,107,108].

Selenium nanoparticles (SeNPs) are nano-sized (generally <60 nm in diameter) elemental selenium particles with excellent nano-properties [109]. For the NPs’ synthesis, there are two main strategies used: bottom-up (including chemical vapor deposition, hydrothermal and solvothermal methods, chemical reduction, and green synthesis) and top-down (including mechanical milling, laser ablation, etching, sputtering, and electro-explosion). The top-down strategy involves the mechanical breaking down of the bulk material into nanostructured materials. In contrast, the bottom-up method uses chemical reactions to break bulk into several parts to form NPs [110]. Methods of NPs synthesis can also be divided into physical, chemical, and biological (the so-called “green way” or “green synthesis”). The chemical methods of nanoparticle synthesis are the most common approaches commercially employed in various areas of NP applications. Concurrently, plenty of research indicates a potential environmental threat of nanotechnology related to NP toxicity [111,112,113,114,115]. The chemical approach to NP synthesis is related to the use of toxic chemicals, which are hazardous to humans and the environment [116]. Designing NPs via a green route using biological and eco-friendly materials reduces the negative environmental impact [117].

Various studies have investigated the possibilities of using selenium nanoparticles as a new source of selenium. For instance, sodium selenite NPs coated with methacrylate polymers were orally supplemented with ruminants, improving selenium absorption [62]. Shi et al. [118] stated that dietary nano-sized Se improved Se content in the blood and tissues and enhanced ruminal fermentation and feed utilization in sheep, which were fed a basal diet supplemented with 0.3, 3, and 6 g/kg DM of nano-Se. Kojouri et al. [119] reported the positive effect of dietary SeNPs inclusion (0.1 mg/kg DM for 60 days) on the antioxidant activity and weight gain of young lambs. In another experiment, the inclusion of 1 mg/kg DM of nano-sized Se into sheep’s diet exhibited a better antioxidative effect after 20–30 days of supplementation [120]. Xun et al. [121] also reported enhanced rumen fermentation and feed conversion efficiency in sheep supplemented with 4 mg/kg DM of nano-sized Se compared with selenium yeast (SY). In another experiment, supplementation with 0.5 mg/kg DM of nano-sized Se improved hair follicle development and promoted growth in Cashmere goats [122]. Experiments with nano-Se inclusion in broiler chicken diets conducted by Gangadoo et al. [48,123] demonstrated improved gut health and general animal performance; the best results in both experiments were obtained with an SeNP supplementation of 0.9 mg/kg DM with no toxic effect occurring. Previous studies have demonstrated the benefits of using SeNPs in broiler feed, with increased absorption and diffusion of material into organs and tissues, increased antioxidant capacity, and meat quality.

In contrast, Wang et al. [124] did not observe any beneficial effect of SeNP supplementation in terms of enhancing the oxidative status in broilers, but Se improved the survival rates. Gulyas et al. [125] reported changes in the proteome profile in chickens after SeNPs supplementation. These results could be related to the specific patented method of NP preparation used in this study. Several studies reported improvements in the growth performance [126,127,128], intestinal health [129,130], and antioxidant status [131] of aquatic animals supplemented with SeNPs. Se supplementation alleviated the antioxidant balance and enhanced kidneys cells’ resistance to oxidative damage in grass carp [132]. In another study, SeNP supplementation improved intestinal health, feed utilization, and growth performance in Nile tilapia [130]. The enhancement of the growth performance and feed efficiency after SeNP supplementation (0.4–0.8 mg) in Nile tilapia was also observed by Ibrahim et al. [133]. Markedly, the nanoform of Se can enhance growth performance in fish. The recommended dosage of SeNP dietary inclusion ranges from 0.15 to 4 mg/kg depending on the fish species [134].

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 suggest 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 gut 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 great 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 gut 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.