Stability and toxicity of differently coated selenium nanoparticles under model environmental exposure settings

Aggregation and stability of selenium nanoparticles: Complex roles of surface coating, electrolytes and natural organic matter

The application of selenium nanoparticles (SeNPs) as nanofertilizers may lead to the release of SeNPs into aquatic systems. However, the environmental behavior of SeNPs is rarely studied. In this study, using alginate-coated SeNPs (Alg-SeNPs) and polyvinyl alcohol-coated SeNPs (PVA-SeNPs) as models, we systematically investigated the aggregation and stability of SeNPs under various water conditions. PVA-SeNPs were highly stable in mono- and polyvalent electrolytes, probably due to the strong steric hindrance of the capping agent. Alg-SeNPs only suffered from a limited increase in size, even at 2500 mmol/L NaCl and 200 mmol/L MgCl₂, while they underwent apparent aggregation in CaCl₂ and LaCl₃ solutions. The binding of Ca²⁺ and La³⁺ with the guluronic acid part in alginate induced the formation of cross-linking aggregates. Natural organic matter enhanced the stability of Alg-SeNPs in monovalent electrolytes, while accelerated the attachment of Alg-SeNPs in polyvalent electrolytes, due to the cation bridge effects. The long-term stability of SeNPs in natural water showed that the aggregation sizes of Alg-SeNPs and PVA-SeNPs increased to several hundreds of nanometers or above 10 µm after 30 days, implying that SeNPs may be suspended in the water column or further settle down, depending on the surrounding water chemistry. The study may contribute to the deep insight into the fate and mobility of SeNPs in the aquatic environment. The varying fate of SeNPs in different natural waters also suggests that the risks of SeNPs to organisms living in diverse depths in the aquatic compartment should be concerned.

Influence of Garlic (Allium sativum) Clove-Based Selenium Nanoparticles on Status of Nutritional, Biochemical, Enzymological, and Gene Expressions in the Freshwater Prawn Macrobrachium rosenbergii (De Man, 1879)

Selenium (Se) is one of the essential micronutrients for performing vital body functions. This study aims at examining the influence of dietary supplementation of garlic clove-based green-synthesized selenium nanoparticles (GBGS-SeNPs, 48-87 nm) on carcass minerals and trace elements, and growth, biochemical, enzymological, and gene expression analyses in the freshwater prawn, Macrobrachium rosenbergii post larvae (PL). The 96 h LC₅₀ of this GBGS-SeNPs to M. rosenbergii PL was 52.23 mg L^-1. Five different artificial diets without supplementation of GBGS-SeNPs (control, 0.0 mg kg^-1) and with supplementations of GBGS-SeNPs starting from 100 times lower than the LC₅₀ value (0.5, 1.0, 1.5, and 2.0 mg kg^-1) were prepared and fed to M. rosenbergii PL for 90 days. A dose-dependent accumulation of Se was observed in the carcass of experimental prawns. GBGS-SeNPs, up to 1.5 mg kg^-1 significantly influenced the absorption of other trace elements (Ca, Cu, and Fe) and mineral salts (K, Mg, Na, and Zn). GBGS-SeNPs-supplemented diets showed efficient food conversion ratio (FCR) of 1.32 g against 2.71 g, and therefore enhanced the survival rate (85.6% against 78.8% in control) and weight gain (WG) of 1.41 g against 0.46 g of control prawn. GBGS-SeNPs significantly elevated the activities of protease, amylase, and lipase, and the contents of total protein, essential amino acids (EAA), total carbohydrate, total lipid, monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), and ash. These indicate the growth promoting potential of GBGS-SeNPs in prawn. The insignificantly altered activities of glutamic oxaloacetate transaminase (GOT), glutamic pyruvate transaminase (GPT), superoxide dismutase (SOD), and catalase, and the content of malondialdehyde (MDA) up to 1.5 mg kg^-1 suggest its acceptability in prawn. Moreover, a respective down- and upregulated myostatin (MSTN) and crustacean hyperglycemic hormone (CHH) genes confirmed the influence of GBGS-SeNPs on the growth of prawn. In contrast, 2.0 mg kg^-1 GBGS-SeNPs supplementation starts to produce negative effects on prawn (FCR, 1.76 g; survival rate, 82.2%; WG, 0.84 g against respective values of 1.32 g, 85.6%; and 1.41 g observed in 1.5 mg kg^-1 of GBGS-SeNPs-supplemented diet fed prawn). This study recommends a maximum of 1.5 mg kg^-1 GBGS-SeNPs as dietary supplement to attain sustainable growth of M. rosenbergii. This was confirmed through polynomial and linear regression analyses.

A green and fast microwave-assisted synthesis of selenium nanoparticles and their characterization under gastrointestinal conditions using mass spectrometry

We present a novel microwave-assisted green synthesis of selenium nanoparticles (SeNPs) using yeast extract as source of a non-toxic reducing and capping agents. Effects of synthesis and gastrointestinal digestion conditions on the biogenic Se particle size distribution and number concentration using SP ICP MS were evaluated. The median equivalent diameter of SeNPs varied depending on the synthesis conditions. Upon incubation in simulated gastric juice, the increase of SeNPs size was observed, whereas after simulated intestinal juice addition, their size came back close to the initial value. The biomolecules contained in yeast extract, which play predominant role in the synthesis of SeNPs, were identified by non-targeted qualitative analysis using LC Orbitrap ESI MS. The use of the state-of-the-art MS techniques allowed both the comprehensive assessment of the processes leading to the SeNPs formation and the evaluation of their behavior under gastrointestinal conditions which is of utmost importance for their use as a novel selenium source.

Using Machine Learning to make nanomaterials sustainable

Sustainable development is a key challenge for contemporary human societies; failure to achieve sustainability could threaten human survival. In this review article, we illustrate how Machine Learning (ML) could support more sustainable development, covering the basics of data gathering through each step of the Environmental Risk Assessment (ERA). The literature provides several examples showing how ML can be employed in most steps of a typical ERA.A key observation is that there are currently no clear guidance for using such autonomous technologies in ERAs or which standards/checks are required. Steering thus seems to be the most important task for supporting the use of ML in the ERA of nano- and smart-materials. Resources should be devoted to developing a strategy for implementing ML in ERA with a strong emphasis on data foundations, methodologies, and the related sensitivities/uncertainties. We should recognise historical errors and biases (e.g., in data) to avoid embedding them during ML programming.

Amelioration Strategies for Silver Diamine Fluoride: Moving from Black to White

Topical cariostatic agents have become a reasonable alternative for managing dental caries in young children. Silver diamine fluoride (SDF) is a practical topical approach to arrest caries and avoid extensive and risky dental treatment. However, the literature demonstrates a parental hesitation towards accepting SDF because of black unaesthetic tooth discolouration following application. The rapid oxidation of ionic silver darkens demineralised tooth structure permanently. In this regard, nano-metallic antimicrobials could augment or substitute for silver, and thereby enhance SDF aesthetic performance. Recently, biomedical research has drawn attention to selenium nanoparticles (SeNPs) due to their antimicrobial, antioxidant, and antiviral potencies. Various in vitro studies have examined the effect of SeNPs on the virulence of bacteria. This narrative review explores practical issues when using SDF and suggests future directions to develop it, focusing on antimicrobial metals. Several methods are described that could be followed to reduce the discolouration concern, including the use of nanoparticles of silver, of silver fluoride, or of selenium or other metals with antimicrobial actions. There could also be value in using remineralising agents other than fluoride, such as NPs of hydroxyapatite. There could be variations made to formulations in order to lower the levels of silver and fluoride in the SDF or even to replace one or both of the silver and fluoride components completely. Moreover, since oxidation processes appear central to the chemistry of the staining, adding SeNPs which have antioxidant actions could have an anti-staining benefit; SeNPs could be used for their antimicrobial actions as well. Future research should address the topic of selenium chemistry to optimise how SeNPs would be used with or in place of ionic silver. Incorporating other antimicrobial metals as nanoparticles should also be explored, taking into account the optimal physicochemical parameters for each of these.

Strategic nanoparticle-mediated plant disease resistance

Nanotechnology is a promising means for development of sustainable agriculture while the study of nanoparticle-mediated plant disease resistance is still in its primary stage. Nanotechnology has shown great promise in regulating: the content of secondary metabolites, inducing disease resistance genes, delivering hormones, delivering biomolecules (such as: nucleotides, proteins, and activators), and obtaining transgenic plants to resist plant diseases. In this review, we conclude its versatility and applicability in disease management strategies and diagnostics and as molecular tools. With the advent of new biotechnologies (e.g. de novo regeneration, CRISPR/Cas9, and GRF4-GIF1 fusion protein), we discuss the potential of nanoparticles as an optimal platform to deliver biomolecules to plants for genetic engineering. In order to ensure the safe use and social acceptance of plant nanoparticle technology, its adverse effects are discussed, including the risk of transferring nanoparticles through the food chain.

Embryonic exposure to selenium nanoparticles delays growth and hatching in the freshwater snail Lymnaea stagnalis

Selenium nanoparticles (SeNPs) have been applied in the biomedical and biocidal domain which may have potential environmental risks for aquatic systems. However, the knowledge of its toxicity and the role of functionalization on aquatic invertebrates are scarce. Thus, the present study aimed to analyze the embryotoxicity of two types of SeNPs coated with Sodium carboxymethyl cellulose (CMC-SeNPs) and Chitosan (CS-SeNPs) to the freshwater snail Lymnaea stagnalis in lake water, focusing on embryonic development. The influence of surface coatings and ions release, on the embryonic development of SeNPs to freshwater snail L. stagnalis was investigated. For this end, the snails were exposed to different concentrations of SeNPs and Se ions (0.05-1 mg L^-1) during 7 days and multiple endpoints were analyzed, including developmental stage frequency, morphological alterations, embryos mortality and hatching success. The results showed that both Se forms promoted the developmental delay, mortality, morphological changes, and hatching inhibition in snail embryos in a concentration-dependent manner. CMC-SeNPs are 2.6 times more embryotoxic compared to CS-SeNPs indicating the importance of surface coating on the embryotoxicity. Moreover, the results revealed that although both forms of Se inhibited the embryo development and reduced the hatching of L. stagnalis, the mode of action on the embryogenesis was different. SeNPs had a higher toxicity to snails' embryos compared to their dissolved counterparts. Despite significant dissolution, by comparing the SeNPs with their dissolved fraction, the results suggest SeNPs inhibition effect on the snail development could be caused by both SeNPs and Se⁴⁺, and SeNPs might be the major development retardation driver rather than Se ions. The present study evidenced by the first time the toxicity effects of SeNPs on the snail embryogenesis, and highlighted how SeNPs intrinsic properties influence their transformation and toxicity in environmental relevant scenarios.

Biogenic synthesis of selenium and tellurium nanoparticles by marine bacteria and their biological activity

Selenium (SeNPs) and tellurium nanoparticles (TeNPs) were synthesized by green technology using the three new bacterial marine isolates (strains PL 2476, AF 2469 and G 2451). Isolates were classified as Pseudoalteromonas shioyasakiensis according to 16S rRNA sequence analysis, morphological characteristics, and biochemical reactions. The bioreduction processes of isolates were studied in comparison with the previously described Alteromonas macleodii (strain 2328). All strains exhibited significant tolerance to selenite and tellurite up to 1000 µg/mL. A comparative analysis of the bioreduction processes of the isolates demonstrated that the strains have a high rate of reduction processes. Characterization of biogenic red SeNPs and black TeNPs using scanning electron microscopy (SEM), EDX analysis, Dynamic Light Scattering, and micro-Raman Spectroscopy revealed that all the isolates form stable spherical selenium and tellurium nanoparticles whose size as well as elemental composition depend on the producer strain. Nanoparticles of the smallest size (up to 100 nm) were observed only for strain PL 2476. Biogenic SeNPs and TeNPs were also characterized and tested for their antimicrobial, antifouling and cytotoxic activities. Significant antimicrobial activity was shown for nanoparticles at relatively high concentrations (500 and 1000 µg/mL), with the antimicrobial activity of TeNPs being more significant than SeNPs. In contrast, against cell cultures (breast cancer cells (SkBr3) and human dermal fibroblasts (HDF) SeNPs showed greater toxicity than tellurium nanoparticles. Studies have demonstrated the high antifouling effectiveness of selenium and tellurium nanoparticles when introduced into self-polishing coatings. According to the results obtained, the use of SeNPs and TeNPs as antifouling additives can reduce the concentration of leachable biocides used in coatings, reducing the pressure on the environment.

Utilization of Olive Pomace in Green Synthesis of Selenium Nanoparticles: Physico-Chemical Characterization, Bioaccessibility and Biocompatibility

Olive pomace extract (OPE) was investigated as a potential surface modifier for the development of the green synthesis process of selenium nanoparticles (SeNPs). In order to evaluate them as potential nutraceuticals, the obtained nanosystems were characterized in terms of size distribution, shape, zeta potential, stability in different media, gastrointestinal bioaccessibility and biocompatibility. Systems with a unimodal size distribution of spherical particles were obtained, with average diameters ranging from 53.3 nm to 181.7 nm, depending on the type of coating agent used and the presence of OPE in the reaction mixture. The nanosystems were significantly affected by the gastrointestinal conditions. Bioaccessibility ranged from 33.57% to 56.93% and it was significantly increased by functionalization of with OPE. Biocompatibility was investigated in the HepG2 and Caco2 cell models, proving that they had significantly lower toxicity in comparison to sodium selenite. Significant differences were observed in cellular responses depending on the type of cells used, indicating differences in the mechanisms of toxicity induced by SeNPs. The obtained results provide new insight into the possibilities for the utilization of valuable food-waste extracts in the sustainable development of nanonutraceuticals.

Multi-Biofunctional Properties of Phytofabricated Selenium Nanoparticles From Carica papaya Fruit Extract: Antioxidant, Antimicrobial, Antimycotoxin, Anticancer, and Biocompatibility

The present study focused on phytofabrication of selenium nanoparticles (SeNPs) from Carica papaya extract and exploration of their multi-biofunctional features. Total phenolics and flavonoids of C. papaya fruit extract were determined as 23.30 ± 1.88 mg gallic acid equivalents and 19.21 ± 0.44 mg quercetin equivalents per gram, respectively, which suggested that C. papaya fruit extract could be a competitive reducing and stabilizing agent during phytofabrication of nanoparticles. UV–Vis and FTIR spectroscopy showed the formation of SeNPs from sodium selenite, which could be related to the reducing and stabilizing activities of C. papaya fruit extract. The SeNPs were found to be stable with a Zeta potential of −32 mV. The average hydrodynamic size of SeNPs was found as 159 nm by dynamic light scattering. The SeNPs showed a broader XRD pattern with no sharp Bragg’s peaks and found to be amorphous. SEM showed that SeNPs were spherical in shape and EDX pattern showed that SeNPs were made up of Se (71.81%), C (11.41%), and O (14.88%). The HR-TEM picture showed that SeNPs were spherical in morphology and have a size range of 101–137 nm. The SeNPs exhibited potent antioxidant activity and their EC50 values (effective concentration required to inhibit 50% of radicals) were 45.65 ± 2.01 and 43.06 ± 3.80 μg/ml in DPPH and ABTS assays, respectively. The antimicrobial action of SeNPs was found as a broad spectrum and suppressed microbial pathogens in ascending order: fungi > Gram-positive bacteria > Gram-negative bacteria. The SeNPs have been demonstrated to reduce the growth and ochratoxin A (OTA) of mycotoxigenic Aspergillus ochraceus and Penicillium verrucosum at 40 μg/ml in broth culture, which is noteworthy. The SeNPs reduced cancer cell proliferation (RAW 264.7, Caco-2, MCF-7, and IMR-32) more preferentially than normal cells (Vero), found to be highly biocompatible. Lower doses of SeNPs (up to 50 μg/ml) were shown to be less toxic and did not cause death in Danio rerio (zebrafish) embryos, implying that lower doses of SeNPs could be beneficial for biological purposes. The present study concluded that phytofabricated SeNPs have multiple biofunctional properties, including antioxidant, antimicrobial, antimycotoxin, and anticancer activities, as well as high biocompatibility.

Poly-L-Lysine-Lactobionic Acid-Capped Selenium Nanoparticles for Liver-Targeted Gene Delivery

Liver cancer is currently regarded as the second leading cause of cancer-related mortality globally and is the sixth most diagnosed malignancy. Selenium nanoparticles (SeNPs) have attracted favorable attention as nanocarriers for gene therapy, as they possess beneficial antioxidant and anticancer properties. This study aimed to design, functionalize and characterize SeNPs to efficiently bind, protect and deliver pCMV-Luc DNA to hepatocellular carcinoma (HepG2) cells. The SeNPs were synthesized by ascorbic acid reduction and functionalized with poly-L-lysine (PLL) to stabilize and confer positive charges to the nanoparticles. The SeNPs were further decorated with lactobionic acid (LA) to target the asialoglycoprotein receptors abundantly expressed on the surface of the hepatocytes. All SeNPs were spherical, in the nanoscale range (<130 nm) and were capable of successfully binding, compacting and protecting the pDNA against nuclease degradation. The functionalized SeNP nanocomplexes exhibited minimal cytotoxicity (<30%) with enhanced transfection efficiency in the cell lines tested. Furthermore, the targeted SeNP (LA-PLL-SeNP) nanocomplex showed significant (* p < 0.05, ** p < 0.01, **** p < 0.0001) transgene expression in the HepG2 cells compared to the receptor-negative embryonic kidney (HEK293) cells, confirming receptor-mediated endocytosis. Overall, these functionalized SeNPs exhibit favorable features of suitable gene nanocarriers for the treatment of liver cancer.

Therapeutic Potential and Main Methods of Obtaining Selenium Nanoparticles

This review presents the latest data on the importance of selenium nanoparticles in human health, their use in medicine, and the main known methods of their production by various methods. In recent years, a multifaceted study of nanoscale complexes in medicine, including selenium nanoparticles, has become very important in view of a number of positive features that make it possible to create new drugs based on them or significantly improve the properties of existing drugs. It is known that selenium is an essential trace element that is part of key antioxidant enzymes. In mammals, there are 25 selenoproteins, in which selenium is a key component of the active site. The important role of selenium in human health has been repeatedly proven by several hundred works in the past few decades; in recent years, the study of selenium nanocomplexes has become the focus of researchers. A large amount of accumulated data requires generalization and systematization in order to improve understanding of the key mechanisms and prospects for the use of selenium nanoparticles in medicine, which is the purpose of this review.

Selenium Nanoparticles for Biomedical Applications: From Development and Characterization to Therapeutics

Selenium (Se) is an essential element to human health that can be obtained in nature through several sources. In the human body, it is incorporated into selenocysteine, an amino acid used to synthesize several selenoproteins, which have an active center usually dependent on the presence of Se. Although Se shows several beneficial properties in human health, it has also a narrow therapeutic window, and therefore the excessive intake of inorganic and organic Se-based compounds often leads to toxicity. Nanoparticles based on Se (SeNPs) are less toxic than inorganic and organic Se. They are both biocompatible and capable of effectively delivering combinations of payloads to specific cells following their functionalization with active targeting ligands. Herein, the main origin of Se intake, its role on the human body, and its primary biomedical applications are revised. Particular focus will be given to the main therapeutic targets that are explored for SeNPs in cancer therapies, discussing the different functionalization methodologies used to improve SeNPs stability, while enabling the extensive delivery of drug-loaded SeNP to tumor sites, thus avoiding off-target effects.

Nanoadsorbents in focus for the remediation of environmentally-related contaminants with rising toxicity concerns

Modern lifestyle demands high-end commodities, for instance, cosmetics, detergents, shampoos, household cleaning, sanitary items, medicines, and so forth. In recent years, these products' consumption has increased considerably, being antibiotics and some other pharmaceutical and personal care products (PPCPs). Several antibiotics and PPCPs represent a wide range of emerging contaminants with a straight ingress into aquatic systems, given their high persistence in seawater, effluent treatment plants, and even drinking water. Under these considerations, the necessity of developing new and affordable technologies for the treatment and sustainable mitigation of pollutants is highly requisite for a safer and cleaner environment. One possible mitigation solution is an effective deployment of nanotechnological cues as promising matrices that can contribute by attending issues and improving the current strategies to detect, prevent, and mitigate hazardous pollutants in water. Focused on nanoparticles' distinctive physical and chemical properties, such as high surface area, small size, and shape, metallic nanoparticles (MNPs) have been investigated for water remediation. MNPs gained increasing interest among research groups due to their superior efficiency, stability, and high catalyst activity compared with conventional systems. This review summarizes the occurrence of antibiotics and PPCPs and the application of MNPs as pollutant mitigators in the aquatic environment. The work also focuses on transportation fate, toxicity, and current regulations for environmental safety.

Highly stable selenium nanoparticles: Assembly and stabilization via flagellin FliC and porin OmpF in Rahnella aquatilis HX2

Microorganisms play a critical role in the reduction of the more toxic selenite and selenate to the less toxic elemental selenium. However, the assembly process and stability of selenium nanoparticles (SeNPs) remain understudied. The plant growth-promoting rhizobacterium Rahnella aquatilis HX2 can reduce selenite to biogenic SeNPs (BioSeNPs). Two main proteins, namely flagellin FliC and porin OmpF were identified in the BioSeNPs. The fliC and ompF gene mutation experiments demonstrated that the FliC and OmpF could control the assembly of BioSeNPs in vivo. At the same time, the expressed and purified FliC and OmpF could control the assembly of SeNPs in vitro. BioSeNPs produced by R. aquatilis HX2 exhibited high stability under various ionic strengths, while the chemically synthesized SeNPs (CheSeNPs) showed a high level of aggregation. The in vitro experiments verified that FliC and OmpF could prevent the aggregation of the CheSeNPs under various ionic strengths. This work reports the preparation of highly stable BioSeNPs produced by strain R. aquatilis HX2 and verifies that FliC and OmpF both could control the assembly and stability of BioSeNPs. BioSeNPs with high stability could be suitable as nutritional supplement to remedy selenium deficiency and in nanomedicine applications.

Toxicity and deleterious impacts of selenium nanoparticles at supranutritional and imbalance levels on male goldfish (Carassius auratus) sperm

BACKGROUND

Selenium has a major role in male reproduction and antioxidative mechanisms. Although deficiency of this element can result in damages to the body's organs, this metalloid can induce deleterious effects in organisms by causing oxidative stress. This study assessed the spermatotoxicity of selenium nanoparticles (SeNPs) in goldfish (Carassius auratus) based on genotoxicity, antioxidant status, sperm quality, and histopathology.

METHODS

The fish with an average weight of 70 g (n = 288) were divided into four experimental groups (three replicates) and fed three times a day with SeNPs at different levels of 0, 0.1, 0.5, and 1 mg kg diet for 30 and 60 days.

RESULTS

After 30 and 60 days of feeding trial, compared to the control group, spermatocrit percentage markedly decreased at 1 mg kg SeNPs on day 30 as well as at 0.5 and 1 mg kg on day 60 (p < 0.05). Computer-assisted sperm analysis parameters especially VCL, VSL, and VAP decreased in response to SeNPs (p < 0.05). Percentage of fast speed progressive sperm cells was highest in fish fed with 0.1 mg kg SeNPs following the dietary experiment and significantly reduced in a SeNPs dose-dependent manner (p < 0.05). In addition, the levels of Malondialdehyde and Glutathione peroxidase were significantly elevated in seminal plasma of all SeNPs-treated groups (p < 0.05). On day 60, DNA damage of sperm was greatly increased at 1 mg kg SeNPs (p < 0.05). Moreover, the highest percentage of spermatocyte and spermatid were observed at the highest dose of SeNPs while the highest percentage of spermatozoa was recorded at the lowest and moderate SeNPs doses.

CONCLUSION

These findings suggested that non-optimal doses of SeNPs could reduce sperm quality, induce oxidative stress, and DNA damage in sperm, and disrupt testis development.

Effects of Humic Acids on the Ecotoxicity of Fe3O4 Nanoparticles and Fe-Ions: Impact of Oxidation and Aging

The magnetite nanoparticles (MNPs) are increasingly produced and studied for various environmental applications, yet the information on their ecotoxicity is scarce. We evaluated the ecotoxicity of MNPs (~7 nm) before and after the addition of humic acids (HAs). White mustard Sinapis alba and unicellular ciliates Paramecium caudatum were used as test species. The MNPs were modified by HAs and oxidized/aged under mild and harsh conditions. Bare MNPs proved not toxic to plants (96 h EC₅₀ > 3300 mg/L) but the addition of HAs and mild oxidation increased their inhibitory effect, especially after harsh oxidation (96 h EC₅₀ = 330 mg/L). Nevertheless, all these formulations could be ranked as ‘not harmful’ to S. alba (i.e., 96 h EC₅₀ > 100 mg/L). The same tendency was observed for ciliates, but the respective EC₅₀ values ranged from ‘harmful’ (24 h EC₅₀ = 10–100 mg/L) to ‘very toxic’ (24 h EC₅₀ < 1 mg/L). The ecotoxicity of Fe-ions with and without the addition of HAs was evaluated in parallel: Fe (II) and Fe (III) ions were toxic to S. alba (96 h EC₅₀ = 35 and 60 mg/L, respectively) and even more toxic to ciliates (24 h EC₅₀ = 1 and 3 mg/L, respectively). Addition of the HAs to Fe-ions yielded the respective complexes not harmful to plants (96h EC₅₀ > 100 mg/L) but toxic to ciliates (24 h EC₅₀ = 10–100 mg/L). These findings will be helpful for the understanding of the environmental fate and toxicity of iron-based NPs.