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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 SeNPs at a concentration of 13 ppm for 13 weeks (Jia et al. 2005).

In addition to changes in the liver enzymes activity, high concentrations of SeNPs can cause changes in other blood biochemical parameters. For example, there is an increase in the activity of creatine phosphokinase (Shakibaie et al. 2013); an increase (Shakibaie et al. 2013) or decrease (Jia et al. 2005) in the number of red blood cells and hemoglobin; and an increase in the total bilirubin, creatinine, urea, triglycerides (Shakibaie et al. 2013), and cholesterol (Shakibaie et al. 2013; He et al. 2014). Also in animals, there was an increase in the number of leukocytes, platelets, and hematocrit (Shakibaie et al. 2013). After the administration of SeNPs, many indicators of the oxidative stress and the activity of the tissue detoxification enzymes may change. There is a decrease of malondialdehyde and reduced glutathione, twofold increase of the glutathione S‐transferase activity compared to the control (Zhang et al. 2005), a decrease in the oxidation of lipids and proteins, an increase of glutathione peroxidase, superoxide dismutase, and catalase (Bai et al. 2020b). It indicates systemic changes caused by exposure to SeNPs and affecting all body systems. After local injection of the Se‐arabinogalactan nanocomposite into the tibia fracture zone in rabbits at 9–21 days after the operation, a significant increase in skin temperature in the area of the surgical wound was noted compared with the control values. These changes may be a manifestation of the metabolic processes activation in the area of bone repair due to the presence of an additional source of Se (Rodionova et al. 2015b, 2016). At the same time, in the area of traumatic bone damage, a low intensity of mineralization of the forming bone callus is observed (Rodionova et al. 2014, 2015a; Shurygina et al. 2015). Although Se in the nanoparticle form at high concentrations causes changes in almost all organs, its effect on the mammalian body is less toxic than when exposed in the form of inorganic or organic compounds.

Fish, in contrast, have a different toxic reaction to Se compounds. SeNPs’ toxicity is higher than selenite toxicity for fish Pangasius hypophthalmus and Oryzias latipes. The NPs synthesized by Labeo rohita most often caused the death of pangasius hypophthalmos. After 96 hours of exposure, the LC₅₀ for SeNPs was 3.97 mg/l and for inorganic Se 5.82 mg/l (Kumar et al. 2018). A study of the SeNPs (size 20–60 nm) synthesized by Latipes Oryzias showed that SeNPs caused 100% mortality at a concentration of 3.2 mg Se per liter, while selenite at a concentration of 2 mg/l caused only to 10%, and 8 mg/l to 80% fish mortality. The calculated LC₅₀ after 48 hours for SeNPs was 1.0 mg Se/l, and for selenite 4.7 mg Se/l (Li et al. 2008). Assessment of the effect of chemically synthesized SeNPs in different sizes (25–90 nm and 50–250 nm) obtained as a result of the recovery of selenite by glutathione and biologically synthesized SeNPs (100–350 nm) on fish embryos of the Danio rerio species showed a higher toxicity of chemically synthesized SeNPs. The study demonstrated that in 72 hours after fertilization, the highest mortality was observed during incubation with chemically synthesized SeNPs. They caused 56% mortality in fry even at a Se concentration of 1 mg/l, while incubation with selenite or biologically synthesized SeNPs did not result in death during this incubation period at the same concentration (Mal et al. 2017). The toxicity of chemically and biologically synthesized NPs for Danio rerio embryos was also evaluated by the decrease in the yield of embryos from eggs. In this test, chemically synthesized SeNPs with smaller particles showed the highest toxicity. The output rate slowed down even at low (0.2 mg/l) concentration of chemically synthesized SeNPs and in 72 hours after fertilization (the period with 100% yield of embryos in the control), only 75% of the embryos were released. The toxic effect of SeNPs with larger particles was observed only at concentrations above 2 mg/l Se. With an increase in the concentration of chemically synthesized SeNPs, the yield of embryos decreased to a greater extent. Only 25% of the embryos left the eggs after 72 hours at a concentration of 5 mg Se/l. The authors associated a decrease in the yield with a possible inhibition of the incubation enzyme chorionase by NPs (Mal et al. 2017).

In addition to directly affecting the viability of fish, SeNPs affect the functioning of their organs and systems. So, Oryziaslatipes after 10 days of incubation with SeNPs at a concentration of 100 μg Se/l had hyperaccumulation of Se in the liver. It was six times higher than during incubation with selenite in the equivalent concentration. SeNPs also caused more efficient accumulation of Se in the gills and muscles compared to selenite, with differences ranging from two to four times. When studying the clearance of Se from the fish body (after 10 days of exposure to selenite or SeNPs, the fish were placed in Se‐free water for 7 days), its concentration in the muscles decreased (42.5% for selenite and 36.5% for SeNPs), whereas in the liver and gills did not change significantly (Mal et al. 2017). In elimination experiments, the rate of Se clearance for seven days was 2.34 μg Se/(kg·h) for selenite and 8.7 μg Se/(kg·h) for SeNPs. The authors attribute this to a lower incorporation of Se from NPs into selenoproteins. However, despite a higher level of clearance, SeNPs provided an active accumulation of Se in the liver and gills of fish, and therefore, the concentration of Se in these organs after seven days of elimination remained still high (Mal et al. 2017).

Hyperaccumulation of Se also occurred in the liver, gills, and brain tissue in an experiment with fish of the species Pangasius hypophthalmus when exposed to SeNPs in concentrations from 2.5–4.0 mg Se/l. A similar effect was demonstrated during incubation with inorganic Se, however, only at higher concentrations from 4.5 to 6.0 mg/l. At the same time, in muscles, Se from NPs accumulated less than inorganic Se and was actively excreted from the body. After exposure to SeNPs or inorganic Se, the liver of Pangasius hypophthalmus had several morphological changes, such as cloudy swelling, focal necrosis, interstitial edema, hemorrhage, hepatocyte hypertrophy, the presence of pyknotic nucleus and large vacuoles. Changes were also observed in the gills, such as thickening of primary lamellae epithelium, curling of secondary lamellae, epithelial hyperplasia, and fusion of secondary lamellae (Kumar et al. 2018).

For the effects of Se in the form of NPs on the fish organism of Pangasius hypophthalmus (Kumar et al. 2018), it was demonstrated that SeNPs affect many enzyme systems. The effect on the antioxidant system was manifested in the form of an increase in the activity of catalase by 244–514%, and inhibition of the activity of superoxide dismutase in the liver, gills, and brain tissue. The effect on the detoxification system was demonstrated by an increase in the activity of glutathione S‐transferase by 153–374% in the liver, gills, and brain tissue. An increase in enzyme activity associated with tissue damage was also detected (AST and ALT activity increased in muscles, liver, gills, and brain tissue by 181–689% and by 94–315%, respectively). The change in enzyme activity occurred upon exposure to both low and high concentrations of SeNPs. A similar inhibition of superoxide dismutase activity was also observed in the liver of Oryzias latipes, with SeNPs showing more potent inhibitory properties than selenite. In relation to glutathione S‐transferase, the effect of SeNPs caused an increase of the enzyme activity in the liver, but in contrast, selenite had no registered effect on it (Mal et al. 2017). However, SeNPs showed a full improvement of hematological and biochemical parameters more than that of sodium selenite in the elimination of Ag NPs’ toxicity (Ibrahim 2020).

From the published data, we can conclude that elemental Se in the form of NPs can affect immunoregulation, reproduction, kidney function, and, to a greater extent, the liver; modulate the activity of the antioxidant and detoxification systems; as well as in high concentrations (above 2 mg Se per kg of animal weight) cause the development of Se‐induced toxicity in both mammals and fish. Moreover, the toxicity of SeNPs is higher than inorganic Se and causes a more acute reaction to exposure even at low concentrations. It possibly associates with hyperaccumulation of Se in tissues, which again reminds us of the need to take into account the problems of ecotoxicity for Se nanocomposites (Shurygina et al. 2018). The toxic effects of SeNPs are as shown in Figure 3.2.