Selenium and Nano-Selenium Biofortification for Human Health: Opportunities and Challenges

Selenium is an essential micronutrient required for the health of humans and lower plants, but its importance for higher plants is still being investigated. The biological functions of Se related to human health revolve around its presence in 25 known selenoproteins (e.g., selenocysteine or the 21st amino acid). Humans may receive their required Se through plant uptake of soil Se, foods enriched in Se, or Se dietary supplements. Selenium nanoparticles (Se-NPs) have been applied to biofortified foods and feeds. Due to low toxicity and high efficiency, Se-NPs are used in applications such as cancer therapy and nano-medicines. Selenium and nano-selenium may be able to support and enhance the productivity of cultivated plants and animals under stressful conditions because they are antimicrobial and anti-carcinogenic agents, with antioxidant capacity and immune-modulatory efficacy. Thus, nano-selenium could be inserted in the feeds of fish and livestock to improvise stress resilience and productivity. This review offers new insights in Se and Se-NPs biofortification for edible plants and farm animals under stressful environments. Further, extensive research on Se-NPs is required to identify possible adverse effects on humans and their cytotoxicity.


Introduction
The discovery of selenium in 1817 triggered a huge amount of innovative scientific inquiry into human health. Selenium is an essential micronutrient for humans and animals as well as some lower plants, but it still needs more investigation to establish whether or not it is essential for higher plants [1,2]. Selenium (Se) plays a vital role in the metabolism of humans, animals, and many prokaryotes as well as some algae [1]. This micronutrient is the only metalloid that is incorporated into specific proteins, called "selenoproteins", and genetically encoded as well as forming a constitutive part of selenocysteine (SeCys), "the 21st amino acid" [3]. In total, 25 selenoproteins have been identified in the human proteome and are often oxido-reductases, including SeCys as a catalytic residue [4,5]. These selenoproteins mainly have wide redox functions that are vital for regulating human immunity [6], mediating thyroid disorders [7], and for the health of the reproductive system [1,3,8,9]. The essential role of selenium in human health has been confirmed by several researchers [10][11][12][13][14][15][16]. A major distinguishing feature of Se is the narrow margin between Se-deficiency (<40 µg day −1 ) and toxicity (>400 µg day −1 ) [17]. The recommended daily dose for adults is 55 µg day −1 in the USA and 55 to 70 µg day −1 in Europe [18]. Selenium is called the "the essential poison" and characterized as "the double-edged sword" due to its biological effects under deficiency and toxicity [13].
Selenium and nano-selenium (nano-Se) or (Se-NPs) have been used in the maintenance of human health [19]. They can be applied in biomedical and drug delivery [20] dietary supplements, therapeutic agents [18], and nano-medicine applications [21]. The antimicrobial and anticancer properties of both Se and Se-NPs have been confirmed [19,22]. The biofortification of edible foods [23,24] and feeds [25,26] with Se and Se-NPs is an important approach to support human and livestock health.
The primary natural source of Se in foods is crop uptake from soil [13,27]. There are wide geographic variations in the Se content of soils, meaning some regions face Se deficiencies while others have Se toxicity issues based on the Se content in their crops, with both situations having negative impacts on human health [28]. Crops grown in Se deficient soils can be biofortified, including the use of both soil-based and foliar fertilizers to correct the deficiency [27]. Food crops that are commonly biofortified include cereals [29], leafy vegetables like spinach [30] and lettuce [31], and fruits like strawberry [24,32,33] and pomegranate [23]. Due to their lower toxicity, strong capacity to scavenge free radicals, higher bioavailability, and stimulation effect, Se-NPs have been recently used in the production of plants [23,24,[34][35][36], fish [37][38][39][40], livestock [41], and poultry [42][43][44][45][46][47][48].
Therefore, this review explored available information on the use of Se and Se-nanoparticles in biofortification. The use of selenium and nano-selenium to promote human health is discussed, including the biofortification of crops through soil and other amendments. We also investigated the role of Se and nano-Se to support crops under stress.

Selenium and Nano Selenium: General Information
Although Selenium and its nanoparticles share some common and general properties, they important differences based on their unique chemical, physical, and biological properties (Table 1). For example, bulk elemental Se is not water soluble, but Se-NPs are partially water soluble ( Figure 1). The behavior and biological features of Se and Se-NPs in the nutrition of higher plants and humans may differ. The role of Se in human nutrition has been confirmed, whereas the biological effects, recommended daily intake and toxicity/deficiency levels of Se-NPs still need more investigations [3,8,10,11,19,49]. Indications of the general role of Se-NPs on human nutrition have been distinguished through studies on fertilization of crops [50][51][52][53], poultry [42][43][44][46][47][48]54], and human health [18,19,21,[55][56][57]. There are many studies on Se and Se-NPs concerning their potential impact on human health, but the situation is different for higher plants, where much effort is still needed. Uptake from the soil and translocation as well as transformation of Se-NPs in higher plants needs more research. Will these nanoparticles be transformed into toxic forms? What will happen if Se nanoparticles are added or co-applied with another nanoparticle? What are the conditions that control Se-NPs transformations in the rhizosphere? What are the expected ecotoxicological effects of applied Se-NPs in the rhizosphere? At present, there are limited studies of the role of Se-NPs in plant nutrition [23,24,[58][59][60], but the biogeochemistry of Se and Se-NPs in agroecosystems and their speciation in cultivated plants are important issues in terms of biofortification of crops for human health [13,61].
Major questions exist regarding Se and Se-NPs biofortification. Do biological Se-NPs have the ability to replace mineral Se-fertilizer in the framework of sustainable agriculture [62,63]? Will it be possible to find standard levels for deficiency and toxicity of Se-NPs as has been done for Se for humans, animals and higher plants? It is important to understand the different forms of Se, including inorganic (i.e., selenate, selenite, selenide, and elemental nano-Se) and organic (i.e., selenomethionine and selenocysteine), as these are important for Se behavior, especially in soil environments ( Figure  1). These forms might control Se availability for plant uptake with contributions to the biofortification process [64][65][66]. There are many studies on Se and Se-NPs concerning their potential impact on human health, but the situation is different for higher plants, where much effort is still needed. Uptake from the soil and translocation as well as transformation of Se-NPs in higher plants needs more research. Will these nanoparticles be transformed into toxic forms? What will happen if Se nanoparticles are added or co-applied with another nanoparticle? What are the conditions that control Se-NPs transformations in the rhizosphere? What are the expected ecotoxicological effects of applied Se-NPs in the rhizosphere? At present, there are limited studies of the role of Se-NPs in plant nutrition [23,24,[58][59][60], but the biogeochemistry of Se and Se-NPs in agroecosystems and their speciation in cultivated plants are important issues in terms of biofortification of crops for human health [13,61].
Major questions exist regarding Se and Se-NPs biofortification. Do biological Se-NPs have the ability to replace mineral Se-fertilizer in the framework of sustainable agriculture [62,63]? Will it be possible to find standard levels for deficiency and toxicity of Se-NPs as has been done for Se for humans, animals and higher plants? It is important to understand the different forms of Se, including inorganic (i.e., selenate, selenite, selenide, and elemental nano-Se) and organic (i.e., selenomethionine and selenocysteine), as these are important for Se behavior, especially in soil environments ( Figure 1). These forms might control Se availability for plant uptake with contributions to the biofortification process [64][65][66]. Table 1. The biological features of selenium and nano-selenium and the possible roles in plant and human nutrition.

Plant Nutrition
Essentiality NOT yet confirmed, but it is a beneficial nutrient at low concentrations [1] NOT yet confirmed, but may have a positive impact on levels of bio-compounds beneficial for human health in treated plants [60] Main uptake form Selenate (SeO 4 2− ) through sulfate transporter (e.g., SULTR1;1 and SULTR1;2), selenite (SeO 3 2− ) via phosphate transport (like OsPT2), and silicon transporter (OsNIP2;1) in roots [67] Unclear (may be through a passive diffusion process), Se-NPs are soluble, highly stable, have low toxicity, and high bioavailability [23] Converted form after uptake Uptake is only by roots, both selenate and selenite will be converted into organic forms like SeCys, SeMet, and MeSeCys [68]. SeMet and MeSeCys are the most dominant species in Se-enriched plants There is bioavailability of Se-NPs in plants, Se-NPs uptake could occur by roots, then transform into organic Se compounds like SeCys, SeMet, and MeSeCys, with dominance of SeMet [68] Translocation from roots to shoots Chemical Se-NPs and selenite have similar translocation of Se from roots to shoots during the longer exposure period (72 h), whereas biological Se-NPs rarely translocate to shoots [68] A few Se-NPs may transport from roots to shoots due to their rapid assimilation into selenite and organic forms [68] Main functions in plant Selenium may increase plant growth and biomass; protect plants from abiotic/biotic stresses; deter herbivores via volatile Se (dimethyl selenide) [69] Se-NPs (especially 5-200 nm), increase activities of some enzymes like GSH-Px, TrxR, and GST could scavenge free radicals, have excellent bio-availability, low toxicity, and high biological activity in plants [23] Toxicity level For agricultural crops < 50 mg Se kg −1 [23], for most angiosperm species > 10-100 mg Se kg -1 DM [70] About 100 mg kg −1 is not toxic for most cultivated crops [71], 275 mg L −1 is the toxic level for sorghum [34] Deficiency level Se content (µg kg −1 ) < 20 for severely deficient areas and 30-50 for deficient areas [72] NOT yet reported Selenium as a contaminant for plants At concentrations > 10 mg kg −1 soil, Se may cause oxidative stress for plants [73] Few publications addressed Se-NPs as a contaminant [74]. SeNPs can remove Hg in soil [69] Human Nutrition Essentiality and absorption Confirmed forms of soluble selenium are mainly absorbed in lower part of the small intestine [18] May be essential. Se-NPs may absorb and be metabolized in the gastrointestinal tract [18] Main dietary sources Cereals or grains, poultry, breads, fish, eggs, meat, nuts, and broccoli [10,75] SeNPs could be used as dietary supplementation due to their therapeutic properties, such as being an anti-carcinogen [18] Main applications or uses Biomedical and drug delivery [20], biofortification of edible crops, and animals for human health [76] Therapeutic or nanomedicine applications [21,56] Main Se-forms for human intake Se-methionine, Se-cysteine, and Se-methyl-selenocysteine [77] Se-NPs in biological or chemical form could be used in nutritional supplements [18] or to combat cancer [57] Main components in humans Selenoproteins or the 21st protein-ogenic amino acid selenocysteine, e.g., glutathione peroxidases [16] NOT yet established Main functions in humans Regulates the immune system, mediates thyroid disorders and the health of the human reproductive system [1,9] Se-NPs may have higher antioxidative capacity compared to other Se-forms (inorganic or organic) and be a more effective therapeutic agent against MeHg neurotoxicity than other forms of Se [78] Toxicity level The upper intake level may be more than 400 µg day −1 [53], mortality results from 1 to 100 mg Se kg −1 body weight [79] NOT yet established The biofortification of cultivated crops using nano-Se may be an important strategy [30] that could be adapted to minimize environmental problems, in particular problems that resulted from the over-use of mineral fertilizers. This is particularly true because Se is rare in the Earth's crust. Nano-Se and Se biofortified edible crops still need more research from different points of view, such as environmental, economic, human health, and animal health perspectives [81][82][83]. Nano-Se has the potential to protect animals against oxidative stress [84], ameliorate heavy metal stress [85], or function as an effective cancer therapy [86]. The use of nano-Se or Se to support cultivated plants under different stresses is an important strategy due to the ameliorative effects of Se and nano-Se in enhancing the productivity of cultivated crops under harsh conditions such as heat stress [34], nitrate stress [87], pathogen (like Alternaria solani) stress [59], NaCl stress [60], and soil salinity stress [24].

Biofortification of Selenium and Nano-Selenium for Human Health
Realization of the relationship between Se as a nutrient and human health started with the discovery of the essentiality of this micronutrient in 1817. Many studies have confirmed that Se is essential for human health due to its role in preventing many chronic diseases such as cancer, neurodegenerative diseases, and cardiovascular disease as an essential component of more than 25 enzymes in humans [53]. The level of Se or nano-Se can be increased in foods through the biofortification approach [29,36,52,[88][89][90][91][92]. Products from farm animals can also be enriched with Se [41,45,93]. The biofortification of cereal crops including wheat, rice and maize as well as some main vegetable crops including tomato, potato and lettuce will be reviewed in this section.
Malnutrition and micronutrient deficiencies have become a global issue and improving the nutrition of millions of people around the world may be achieved using staple crops and appropriate agronomic practices [105,131]. In the past biofortification mainly involved the main cereal crops (e.g., rice, maize, and wheat) and then moved to include pulse crops as well as some animal-based foods such as milk and cheese [132], meat [133], and eggs [134]. The Se-biofortification of cereal crops depends on Se forms, method of application, the efficacy of Se-fertilizers [118], the time of application, and plant growth stage [83,135]. It also depends on soil properties, in particular soil pH, salinity content, redox potential, organic matter content, and the soil microbial community [13,69,116,[136][137][138][139]. The Se-biofortification of cereal crops including wheat, rice, and maize could be evaluated under different applied Se-forms and different growth conditions (Table 2). A review of the literature led to the following conclusions: 1.

2.
The recommended Se-dose for biofortification of cereal crops mainly depends on the plant species and its variety or cultivar, the application method (seed coating and priming, foliar, or soil application), the growth media (e.g., soil, hydroponics, artificial growth media), the growth conditions (open field, controlled greenhouse, or in vitro experiment), the Se-form (inorganic, organic, or nano form), nano-Se characterization (the method of preparing, the size and color of nanoparticles), the background Se content in the soil, and the agricultural management practices [69,85].

3.
For wheat crops, the recommended Se-dose under field experiments was 21 g Se ha −1 as a foliar application [89], while an applied dose of up 120 g Se ha −1 did not cause any visible phytotoxicity symptoms [140]. Under pot experiments, an applied Se-dose of 2.5 mg Se kg −1 soil was a suitable dose for Se-fortification of grain wheat [136].

4.
For rice crops, Se-foliar fertilization up to 100 g Se ha −1 as sodium selenite produces safe and high converting levels of Se into general rice proteins under field experiments when there was an initial low total soil Se content up to 0.1 mg Se kg −1 soil [141]. The best method to fortify the rice plants was to use 6 mg Se L −1 under NaCl stress as a combination of foliar spraying and seed priming [73]. The recommended applied Se-dose for the growth of rice clearly depends on the growth stage (the seedling, tillering, booting, full heading, and mature stage). Foliar application of sodium selenite (10 mg L −1 ) at the booting and full heading stages enhanced the accumulation of SeMet, confirming that the previous Se rate is the ideal level for Se-biofortification of rice [115].

5.
For maize crops, biofortification with Se could be achieved under field conditions through a fertigation system at an application rate of 100-200 g of Se ha −1 as sodium selenite. The applied Se might enhance the nutraceutical value and antioxidant content of maize grains without any leaching of Se into groundwater [142,143]. Ngigi et al. [125] reported that the Se biofortification level (0.3 mg kg −1 ) could be achieved in three field locations in Kenya using a foliar Se-dose of 20 g ha −1 as sodium selenate, whereas Wang et al. [64] indicated that the Se-level may be up to 30 g Se ha −1 in China.  Vegetables are a major source of nutrients and phytochemicals that support human health and nutritional sustainability [148]. Vegetables routinely grown for human consumption include allium, cruciferous, legumes, yellow-orange-red, and leafy green vegetables [149]. These vegetables are important in biofortification programs due to their importance for human health and short growth period (Table 3). Several vegetable crops have already been used in biofortification programs, including vegetables enriched in Se such as tomato [150,151], potato [85,94,120], lettuce [91,126,152], onion [153], garlic [154,155], cabbage [139], carrot, broccoli [156,157], asparagus [158], radish [66,159,160], and spinach [30,158]. Seedlings fortified with Na 2 SeO 3 at 0.5 mg Se L −1 , with and without 5 mg I L −1 In greenhouse, seeds sown into mineral wool plugs, seedlings placed into the NFT hydroponic method, salicylic acid improved bio-fortification efficiency using Se and I Seeds of lettuce (Rijk Zwaan, the Netherlands, [165]) Applied Se at 0.1, 0.5, 5, 10, and 50 µmol L -1 ) as Na 2 SeO 3 for 30 days Hydroponic system, the 0.5 µmol L -1 Se may be used to reduce nitrate content in leaves with increasing lettuce yield, inducing assimilation of NO 3 For tomato crops, the Se-biofortification program may differ depending on the growth media. A Se concentration of 0.05 mg L −1 with selenate and selenite as dual Se sources may be optimum for tomato fruits grown under the hydroponic technique [69], but under drip irrigation this dose may be up 1.5 mg Se L −1 [151]. For potato crops, foliar applied Se at 100 g ha −1 at the tuber bulking stage led to the highest tuber Se-content [85], whereas a low Se dose (0.75 mg kg −1 as selenate) for pots filled with tropical soil (pH 4.8; clay 71%; total Se content 0.065 mg kg −1 ) was the most efficient source for potato biofortification under tropical conditions [94]. The hydroponic system is a common technique for producing lettuce under greenhouse conditions and the Se-bioavailability in a hydroponic system was higher for selenite (40 µmol L −1 Se) compared with selenate due to its fast bio-transformation into organic forms in plant cells [126]. Under field conditions, a foliar of rate of 100 mg kg −1 may be the proper Se dose for lettuce biofortification in salt-affected soils (pH: 8.65; EC: 4.49 dS m −1 ; [152]).

Interaction of Selenium and Nano-Selenium with Environmental Conditions
Plant growth and development are mainly controlled by environmental conditions (e.g., water, nutrients, air, light, etc.). These conditions include both normal and stressful environments (i.e., biotic and abiotic stresses). Plants have the ability to alleviate these stresses using exogenous and endogenous anti-stressors or tools (through their defense system) such as plant growth-promoting rhizobacteria [167], silicon [168][169][170], nanoparticles of selenium, or silicon [171] and selenium [2]. Several studies have confirmed the identified role of Se in promoting cultivated plant growth under a variety of stresses [24,83,86,172,173]. There is a growing body of literature that shows the potential of Se-nanoparticles to promote plant growth under different conditions (Table 4), but this still needs more investigation, particularly under stressful conditions. Comparing the potential of Se-NPs as revealed in human and animal studies with plant investigations, much more progress has been achieved in the human and animal fields.
It is well documented that biological nano-Se has desirable characteristics such as high biosafety and bioactivity properties, low toxicity, high solubility, and high mobility due to their large surface/volume ratio compared with soluble inorganic Se salts, mainly selenate and selenite [24,34]. The uptake of Se-NPs by plants primarily depends on the synthesis method (bio-synthesis or chemo-synthesis) and the size of the nanoparticles [141].

Plant Cultivar (Country, Reference) Nano-Se and Added Rate Experimental Conditions and Se-Biofortified Dose
Strawberry (Fragaria ananassa Duch.), cv. Kurdistan (Iran, [24]) Foliar applied Se-NPs (10- Seedlings were cultivated in greenhouse in plastic containers for 6 weeks, treatments were exposed to Se for 72 h, Se-NPs could be applied as a new fertilizer to produce Se-biofortified plants with best root uptake for 40 nm Se-NPs Spinach: Spinacia oleracea L., Stoik variety (Russia, [30]) Se was foliar applied in three forms (220 g Se ha −1 ) including Se-NPs (<10 nm), selenite and selenate, harvested after 60 days In field experiment in heavy loamy soil, pH 6.8, Se-NPs can be used in the biofortification of spinach with Se due to higher growth-promotion compared to other Se forms

Selenium and Nano-Selenium Reduce the Toxicity of As, Cd, and Hg
Selenium has long been of great interest in a wide range of fields including the medical, pharmaceutical, agricultural, and industrial sectors. Recently, a considerable literature has developed around the potential of Se and Se-NPs to help humans and animals deal with environmental stresses, but much less research has been done regarding Se, Se-NPs, and environmental stress in plants [175]. Plant related soil and environmental stresses include salinity, drought, waterlogging, heat, and heavy metal stress, which represent serious constraints for global crop productivity [109]. Heavy metals and potential toxic trace metals that may exist in soil include arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), and selenium, which can be introduced into the soil through human activities such as mining [176], industrialization [177], urbanization [33], and agricultural practices [178,179]. These metals can also naturally occur in soils at levels that may cause problems [28,105].
Several investigations suggest that Se can play an important role for plants growing under stressful conditions, although further work on Se is required to confirm its essentiality for higher plants. Many studies conducted on Se and its potential in higher plants have investigated its uptake, translocation, metabolism, and toxicity [17,160,180,181]. Biofortification studies with Se and Se-NPs are considered promising because Se is essential for human and animal health [53]. Selenium biofortification and phytoremediation are at the same level of importance from the environmental protection point of view [81]. Based on the work done to date, some general comments can be made concerning the role of Se and nano-Se in higher plants under different stresses: The ameliorative role of Se when plants are stressed by soil heavy metal content has been investigated in several studies, including how Se protects plants against heavy metal stress, whereas there is little work investigating the use of Se-NPs in this context [34,78].

2.
Selenium has already been investigated by many researchers as a way to ameliorate As stress on rice plants [2,92,[182][183][184][185][186][187][188][189][190][191]. These studies primed rice seed with Se during germination under As-stress. The ameliorative role of Se under As-oxidative stress occurred through the modulation of thiol (R-SH) and antioxidant enzymes in rice or Se-modulating the level of phenolics and nutrients alleviating the toxicity of arsenic in the rice plants. 3.
The most important studies investigating Se and its role under Cd stress on rice plants include the application of Se to mitigate Cd accumulation in high-Cd-contaminated soils [192], the behavior of Se at different planting times [115], the effects of Se-forms and application methods on modulation of rice growth [83], how Se reduces the uptake and translocation of Cd from contaminated soils [193], and reducing oxidative stress induced by Cd [33,[194][195][196][197][198]. 4.
Selenium has been shown to moderate the impacts of mercury (Hg) stress on cultivated rice [78,138,[199][200][201]. Chapman et al. [202] studied how native plants in a mined field were able to grow under Hg and As soil pollution as well as how selenium promoted the growth of the plant seedlings by decreasing Hg and As bioaccumulation in these plants. Selenium also has the ability to decrease rice plant uptake of methylmercury in Hg-contaminated soils while increasing the uptake of other nutrient elements [78].
Selenium nanoparticles are also thought to behave like Se in protecting cultivated plants under heavy metals stress but only a few studies have been published. Investigations that have been conducted regarding Se-NPs and stressful environments include the role of Se-NPs in enhancing the growth of some cultivated plants under stress such as sorghum under high temperature stress [34], strawberry under salinity stress [24] and rice under Cd and Pb toxicity [171].

Are Selenium and Nano-Selenium Emerging Pollutants?
With widespread interest in Se biofortification, it is possible that the environment (i.e., soil, water, air, and plants) might become contaminated with Se due to extensive Se applications, which in turn could create a risk for human health [28,210] (Figure 2). The main anthropogenic sources of Se-pollution include agricultural and industrial activities [211][212][213]. The most serious problem resulting from Se-pollution is contamination of drinking water sources; hence, remediation requires effective techniques to remove Se from natural waters [214,215]. Environmental Se-contamination may be bio-remediated using proper microbial adaptations like alkylation [216] or Se-transformation, bioavailability, mobility and volatilization into the atmosphere [15].
In addition to anthropogenic Se contamination problems, soils that are naturally high in Se content (seleniferous soils) are found in several places worldwide such as Punjab, India [217][218][219], Pine Ridge, Fort Collins, Colorado, USA [220], Western Colorado, USA [221], Enshi in China [222,223], Saskatchewan in Canada, Queensland in Australia, Irapuato in Mexico, the State of Boyaca in Colombia, and the Deog-Pyoung area in Korea [224]. Therefore, to address both anthropogenic and natural Se problems in soil, there is an urgent need to study Se-biofortification and phytoremediation through hyper-accumulating plants, which have the ability to uptake huge amounts of Se from soils and transfer it into the atmosphere by volatilization [67]. The agronomic Se biofortification of edible plants, pasture and forage crops should be improved through judicious biofortification programs without excessive applications or applications of Se in places that do not experience Se deficiency in the soil. Over-biofortification of products with selenium should be avoided, even in areas with Se-deficiencies, to prevent the build-up of Se to toxic levels in food and related products.
Further investigations are needed to monitor the behavior of Se-NPs in different environments including agricultural soils, waters, and farm animals. Se-NPs have already been used in the remediation of soils and waters that were contaminated with heavy metals like mercury [225][226][227]. This work confirmed that soils and groundwater contaminated with elemental mercury could be remediated via biogenic Se-NPs based on mechanisms like the immobilization of elemental mercury [226,227], applied hetero-aggregation of soil particulate organic matter [225], applied biofilm-coated quartz sand [228], and applied goethite colloids [138] in the presence of Se-NPs [64]. Further investigations are needed to monitor the behavior of Se-NPs in different environments including agricultural soils, waters, and farm animals. Se-NPs have already been used in the remediation of soils and waters that were contaminated with heavy metals like mercury [225][226][227]. This work confirmed that soils and groundwater contaminated with elemental mercury could be remediated via biogenic Se-NPs based on mechanisms like the immobilization of elemental mercury [226,227], applied hetero-aggregation of soil particulate organic matter [225], applied biofilm-coated quartz sand [228], and applied goethite colloids [138] in the presence of Se-NPs [64].

Conclusions
Selenium is an essential micronutrient for the nutrition of humans, animals, and lower plants, but whether or not it is essential for higher plants has yet to be confirmed. Nano-selenium is one of the most potentially useful Se-forms and has fascinating properties that lead to its use in nanomedicine applications, drug delivery, therapeutic applications, biomedicalue applications, and cancer prevention. Several places worldwide have a Se-deficiency problem due to low Se soils and should implement programs that guarantee safe and proper Se-supplementation levels for human health. The biofortification approach has shown particular promise for dietary supplementation of Se so that humans and/or animals have Se present in the right form, place, dose, and time. Due to the changing world and environmental stressors, there is an urgent need to document how and when Se helps plants and animals overcome stresses, including the identification of suitable Se and Se-NP fertilizers, application timing, and application rates. Several studies have demonstrated the ameliorative effects of Se and nano-Se on plants and farm animals under stress, but the use of Se-NPs in biofortification programs still needs more research. Despite all the documented benefits of Se, the difference between Se deficiency and Se toxicity is very small. We also need to understand the possible adverse effects and cytotoxicity of these Se-NPs on humans and effects on crops. It is also Figure 2. Possible fates of selenium in the environment. Selenium could be released into the aquatic environment from industrial and agricultural activities. The excessive application of Se as fertilizer may lead to environmental hazards, which may be controlled by immobilization and mobilization reactions in soil, driven by soil clay and organic matter content as well as microbial activities. These Se-pathways may threaten human and animal health.

Conclusions
Selenium is an essential micronutrient for the nutrition of humans, animals, and lower plants, but whether or not it is essential for higher plants has yet to be confirmed. Nano-selenium is one of the most potentially useful Se-forms and has fascinating properties that lead to its use in nanomedicine applications, drug delivery, therapeutic applications, biomedicalue applications, and cancer prevention. Several places worldwide have a Se-deficiency problem due to low Se soils and should implement programs that guarantee safe and proper Se-supplementation levels for human health. The biofortification approach has shown particular promise for dietary supplementation of Se so that humans and/or animals have Se present in the right form, place, dose, and time. Due to the changing world and environmental stressors, there is an urgent need to document how and when Se helps plants and animals overcome stresses, including the identification of suitable Se and Se-NP fertilizers, application timing, and application rates. Several studies have demonstrated the ameliorative effects of Se and nano-Se on plants and farm animals under stress, but the use of Se-NPs in biofortification programs still needs more research. Despite all the documented benefits of Se, the difference between Se deficiency and Se toxicity is very small. We also need to understand the possible adverse effects and cytotoxicity of these Se-NPs on humans and effects on crops. It is also important to understand Se and Se-NP accumulation, transformation, and transport through soil. Due to the intensive use of Se and its forms in many fields, Se and nano-Se have become potential emerging pollutants in agroecosystems. This issue has increasingly been recognized as a potentially serious, worldwide public health concern.
Author Contributions: This project was conceived and led by H.E.R. All authors contributed to writing the paper, interpreting information presented, and have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.