The potential of traditionally used medicinal plants for the synthesis of selenium nanoparticles

Abstract Selenium nanoparticles (SeNPs) have the potential to be used in many applications. In recent years green synthesis using plant extracts has gained popularity, due to the use of non-toxic solvents. In this work, the application of plantain (Plantago lanceolata L.), yarrow (Achilea millefolium L.) and nettle (Urtica dioica L.) in the synthesis of SeNPs is presented. The obtained nanoparticles were characterized in terms of size and antioxidant activity. A strong correlation between the dimensions of synthesized nanoparticles and their ability to scavenge hydroxyl radicals was established. Graphical Abstract


Introduction
In recent years, nanoparticles of elemental selenium (SeNPs) have attracted great attention. SeNPs differ from the properties of its corresponding bulk materials, similar to other nanoparticles, and their toxicity is significantly lower in comparison to inorganic and organic forms of selenium (Bhattacharjee et al. 2019). SeNPs found the wide range of applications due to their unique properties and potential bioactivities (Bisht et al. 2022). For this reason various methods of their synthesis have been described in the literature (Pyrzy nska and Sentkowska 2021; Bisht et al. 2022). The commonly applied method of preparing nanoselenium is chemical reduction of its salts such as sodium selenite, sodium selenosulfate or selenious acid in the presence of stabilizing agent. The chemical methodologies are criticized due to the use of toxic chemicals in the synthesis protocol. That was the main reason for involving non-toxic reagents like ascorbic acid or sugars (Viera et al. 2017;Bartosiak et al. 2019). However, chemically synthesized SeNPs are often unstable, aggregating, and precipitating in aqueous solutions, that lowering their bioactivity (Bisht et al. 2022). The addition of a stabilizer entails the use of another, not necessarily non-toxic, substance.
Green chemistry methods employing either microogranism or plant extracts have emerged as viable substitutes to chemical synthesis methods . The advantages of green synthesis over the classical one are among others slower kinetics, environmental compatibility, control over crystal growth and simplicity do scale up. Many successful green syntheses of metallic nanoparticles have been described, including synthesis of silver nanoparticles, magnetite nanoparticles or titanium dioxide nanoparticles (Heydari and Rashidipour 2022;Bahmani et al. 2019;Heydari et al. 2019). Recent studies show the promising potential of Sargassum wighitii extracts and seaweed synthesized ZnO NP as an eco-friendly tool against pests of agricultural and medicinal importance (Murugan et al. 2018). Also, seaweed-synthesized AgNP were highly effective against the malaria vector Anopheles stephensi and the crop pest Plutella xylostella even tested at low doses (Roni et al. 2015). A good alternative seems to be the use of plant extracts for the synthesis of SeNPs (Viera et al. 2017). The extract naturally contains compounds that can act as both selenium reducers and stabilizers, thus, this approach fits perfectly into the "green synthesis" trend. Medicinal plants has long been used in the food, cosmetics and pharmaceutical industries due to its nutritional and health potential (Finimundy et al. 2020). This study uses three herbal infusions commonly used for medicinal purposes: plantain (Plantago lanceolata L.), yarrow (Achilea millefolium L.), and nettle (Urtica dioica L.). Their polyphenolic profile and antioxidant activity were measured. This is a new approach because most of the work published so far uses locally grown plants, for example, avaram (Cassia auriculata) or arauna (Terminalia arjuna), both growing in India and Sri Lanka (Prasad and Selvaraj 2014;Anu et al. 2020). This definitely makes it difficult to repeat such a synthesis of SeNPs due to the limited availability of plant material. The plants used in the described research are common, which makes the synthesis with their use more universal. The obtained SeNPs were studied in terms of their size, shape and antioxidant activity. The correlation between the mean nanoparticles size value and the ability to scavenge hydroxyl radicals was also established.

Results and discussion
All plants used in the study are widely used in folk medicine, mainly due to the high content of polyphenols. This was also confirmed by Hydrophilic Interaction Liquid Chromatography coupled to mass spectrometry detection (HILIC-MS) analysis and the results are presented in Table S1. It turned out that all of the extracts are rich in polyphenolic compounds, mainly polyphenolic acids. This is an additional argument for their use in the synthesis of nanoparticles. Synthetic gallic acid was described for the preparation of iron oxide NPs, but it was still a chemical approach (Shah et al. 2017). In the extracts of the studied plants, gallic acid was detected in rather small concentrations in comparison to other polyphenolic acids. In yarrow extract it was below the limit of detection (below 0.01 mg/L), however this extract is a great source of p-hydroxybenzoic acid (pHBA) as well as chlorogenic acid (123.4 ± 0.85 and 65.8 ± 0.43 mg/L, respectively). Also nettle extract was rich in pHBA and quite high concentration of protocatechuic and ferulic acid. pHBA was also the main polyphenolic acid in plantain extract. Among polyphenols, epicatechin was the dominant in all of studied extract in concentration range of 2.5-3.7 mg/L. Other detected compounds were present in the extract in the concentrations below 1 mg/L. It should be highlighted that ascorbic acid, which is commonly used in the chemical synthesis of SeNPs was also detected in all used herbal extracts. The higher concentration was found in yarrow (7.08 ± 0.11).
The studied herbal extracts also differed in their ability to scavenge hydroxyl radicals, which are the reactive oxygen species, commonly formed in vivo and can cause serious damage to biomolecules. The smallest value was established for plantainonly 25%. For yarrow and nettle, these values were much higher equal 78% and 86% respectively. Such characterized extracts were involved into SeNPs synthesis.
As a result of each of the syntheses, nanoparticles with average dimensions from 140.5 ± 42.4 for nettle to 177.3 þ 41.2 nm for plantain were obtained ( Figure S1). The SeNPs obtained from synthesis with the use of yarrow extract was in size of 156.4 ± 52.4 nm. This research has confirmed that the synthesized SeNPs using plant materials are a very good antioxidant. The antioxidant activity of synthesized SeNPs were significantly higher than those obtained for the appropriate extract. In the case of synthesis with the use of plantain, an almost four-fold increase in the ability to scavenge hydroxyl radicals was observed. The obtained value was equal to 84%. For yarrow and nettle extracts these values were comparable and equal to 91% and 92%, respectively. A strong correlation was observed between the mean size of the nanoparticles and their ability to scavenge hydroxyl radicals. The smaller is the dimensions of SeNPs, the greater is their antioxidant capacity. The correlation coefficient for this relation is À0.948. It should be highlighted that Se (IV) used for the synthesis did not show the ability to scavenge hydroxyl radicals. The measurement was made for the same concentration as used in the synthesis, so it can be concluded that the antioxidant capacity of selenium strictly depends on the chemical form in which it occurs.

Conclusion
Plant extracts have been successfully used in the synthesis of selenium nanoparticles. The obtained SeNPs showed a higher ability to scavenge hydroxyl radicals than the extracts themselves, which confirms the validity of calling SeNPs as a nano-antioxidant. The existence of a large correlation between the antioxidant capacity and the size of nanoparticles has been shown. The smaller the mean value of the nanoparticle size is, the greater the antioxidant capacity they exhibit. The use of plant extracts in the synthesis of nanoparticles is a promising alternative to the classic, chemical method of their synthesis. SeNPs obtained in this manner exhibit great potential in biomedical applications such as cancer therapy, targeted chemotherapy or drug delivery. They can also found their applications in food and pharmaceutical industry. Due to the less toxicity of green synthesized SeNPs, it is expected that the drugs based on them may be commercially available.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The author(s) reported there is no funding associated with the work featured in this article.