physically synthesized SeNPs, but also from NPs produced by other types of microorganisms (Oremland et al. 2004). In most cases, the described biogenic NPs are nanospheres; however, nanowires of the elemental Se also are found (Jain et al. 2014). The size of the formed NPs depends on the type of microorganism, the duration of the synthesis, the amount of biomaterial, and the cultivation conditions (temperature, pH, concentration of Se‐containing anions and oxygen) (Jain et al. 2014; Tugarova and Kamnev 2017).
The advantages of the biological method of NPs’ synthesis include low toxicity, higher biocompatibility and biodegradability of the resulting products compared to NPs synthesized by chemical and physical methods. Besides, the synthesis process is simple, of relatively low cost, does not require extreme conditions and can be carried out at ambient temperature and pressure, does not require specialized equipment and expensive chemicals, and is not a source of toxic waste. At the same time, the disadvantages of this method include the polydispersity of the obtained NPs, the rather large sizes of SeNPs, which limits their use, the need for additional purification of NPs in the case of intracellular synthesis, and the duration of the process up to several days (Oremland et al. 2004; Jain et al. 2014; Tugarova and Kamnev 2017).
Thus, the variety of modern methods of physical, chemical, and biological synthesis allows the researcher to choose a sufficiently effective and affordable method that will allow the synthesis of SeNPs with the necessary characteristics.
3.3 Toxicity of Selenium Nanoparticles
The majority of studies indicate that elemental nanoparticle Se is less toxic to mammals than inorganic or organic Se (Sun et al. 2019). For example, from experiments on Kunming mice, data showed that chemically synthesized SeNPs were 7 times less toxic than selenite (LD50 was 113 and 15 mg Se per kg body weight, respectively), and 3.5 times less toxic than selenomethionine (LD50 was 92.1 and 25.6 mg Se per kg, respectively) (Zhang et al. 2001, 2005; Wang et al. 2007). It is known that at low concentrations, SeNPs do not have a toxic effect on the animal organism (the values of the controlled parameters do not significantly differ from the control group). High concentrations above 3 mg per kg of animal weight cause changes similar to the effects of high doses of inorganic Se, although less pronounced (Jia et al. 2005; Zhang et al. 2005; Shakibaie et al. 2013; Kuršvietienė et al. 2020).
A study of the acute and subchronic toxicity of SeNPs coated with a polysaccharide–protein complex revealed low acute oral toxicity in ICR mice and Sprague–Dawley rats. Assessment of subchronic toxicity showed that the nanocomposite in the oral administration does not have a visible toxic effect up to a concentration of 200 μg Se per kg of body weight per day, which is approximately 30 times higher than the permissible upper level of human consumption of Se. With the per os exposition of this nanocomposite, there were no signs of damage to major organs, including the liver, spleen, heart, kidneys, and lungs (Zhang et al. 2019).
A classic symptom of the toxic effects of Se on the body is the inhibition of growth, which is observed when exposed to high concentrations of SeNPs. For example, animal weight loss was demonstrated in an experiment with Sprague–Dawley rats when administered orally for two weeks at concentrations exceeding 2 mg Se per kg body weight of SeNPs about 80 nm in size, obtained by reducing sodium selenite with ascorbic acid in the presence of chitosan. At the same time, the administration of these NPs in concentrations of 0.2 and 0.4 mg Se per kg of body weight stimulated the growth of animals, and a dose of 8.0 mg Se per kg of body weight inhibited the growth of animals after the first week of administration (He et al. 2014). SeNPs (size 20–60 nm) obtained by chemical reduction of selenite by glutathione also inhibited the growth of Kunming mice at a concentration of 4 and 6 mg Se per kg body weight, although less effective than selenite. In this case, the administration of SeNPs at a concentration of 6 mg Se per kg of weight completely suppressed growth during the first three days, after which the growth rate was restored (Zhang et al. 2005).
The administration SeNPs (size 20–60 nm) at a concentration of 4 and 5 ppm Se to Sprague–Dawley rats for 13 weeks led to a decrease in body weight in males from the eighth week of the experiment, and in females from the sixth and fifth weeks, respectively, although the effect was less pronounced than when exposed to selenite or a Se‐enriched protein (Jia et al. 2005). However, the study indicates that during the preliminary experiment, when rats were injected with SeNPs at a concentration of 6 ppm for 13 weeks, the death of animals was observed, while there were no significant differences in mortality between the administration of SeNPs, selenite or selenium‐enriched protein (Jia et al. 2005).
Other researchers found no difference between the toxic effects of SeNPs or sodium selenite. Female rats were orally administered for 28 days with either 0.05, 0.5, or 4 mg Se/kg body weight/day as SeNPs, 20 nm in size, or 0.05 or 0.5 mg Se/kg body weight/day in the form of sodium selenite. Male rats were administered 4 mg Se/kg body weight per day as SeNPs. Clear toxicity was observed at high doses of SeNPs. At all doses of SeNPs and a dose of sodium selenite 0.5 mg Se/kg body weight per day, a decrease in body weight was observed compared to the control. Relative liver mass was increased with Se at a dose of 0.5 mg Se/kg body weight per day, both as SeNPs and as sodium selenite. At the same time, no effect on brain neurotransmitters or hematological parameters was found. From the data obtained, the authors conclude that SeNPs and ionic Se have similar toxicity (Hadrup et al. 2019).
SeNPs of 80–220 nm in size were synthesized biologically using Bacillus sp. MSh‐1. When these SeNPs at a concentration of 20 mg Se per kg body weight were orally administered for 14 days to Swiss mice, the animals had a weight loss and a mortality rate of 20%. The administration of lower concentrations did not cause growth retardation and did not lead to death. In this experiment, biogenic SeNPs were 26 times less toxic than Se dioxide (LD50 was 198.1 and 7.35 mg Se per kg body weight, respectively) (Shakibaie et al. 2013). Other researchers noted that even high doses of SeNPs do not inhibit growth and do not cause damage to the liver and kidneys, as well as changes in the hematological parameters of healthy animals (Sun et al. 2019).
Hepatotoxicity is another widely discussed toxic effect on exposure to high Se concentrations. Oral administration of SeNPs for two weeks at a concentration of 8.0 mg Se per kg of body weight in rats leads to an increase in the liver, also histopathological changes such as focal necrosis and hepatocyte degeneration. In addition, the activity of liver enzymes increased in animal blood: alanine transaminase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALP), which also indicated liver damage (He et al. 2014). Similarly, an increase in the relative mass of the liver and an increase in ALT activity in the blood of males and females rats was demonstrated with long‐term use of SeNPs at a concentration of 5 ppm. Exposure to concentrations of 4 and 5 ppm also showed pathological changes: mottled liver surface and vacuolar degeneration of hepatocytes (Jia et al. 2005). According to other data, no histological changes in the liver of animals were observed upon administration of 0.05, 0.5, or 4 mg Se/kg body weight/day in the form of 20 nm SeNPs to female rats orally for 28 days (Hadrup et al. 2019). In addition to liver pathology in a 14 days study with Sprague–Dawley rats, changes were found in other organs, indicating a different degree of increase in kidneys, lungs, and spleen caused by exposition of SeNPs at a concentration exceeding 2 mg Se/kg body weight body per day. At the same time, morphological changes in the heart, testicles, and thymus indicated atrophy of these organs in animals treated with SeNPs at a daily concentration of 8 mg Se/kg body weight. Histological sections of the kidneys showed signs of glomerulonephritis and necrobiosis of individual renal tubule cells. Hemorrhages were observed on lung sections with filling of the intra‐alveolar and bronchial spaces with red blood cells, as well as with thickening of the epithelial septa. The thymus cortex zone in rats administered with high SeNPs concentrations was thinner than in the control samples, and the border between the cortical and medullary zones was unclear. Testicular microscopy revealed atrophy of the seminiferous tubules, impaired spermatogenesis, as well as an increase in the number of damaged seminiferous tubules. In addition, detecting DNA fragmentation by labeling the 3′‐hydroxyl termini in the double‐strand DNA breaks showed that the number of apoptotic cells was higher in rats administered with SeNPs at concentrations of 4 and 8 mg of Se per kg of body weight (He et al. 2014). An increase in the mass of the spleen, brain, and heart was also observed in male rats and the spleen, and heart in female rats that administered