New Insights into the Stability of Silver Sulfide Nanoparticles in Surface Water: Dissolution through Hypochlorite Oxidation

Use of Metallic Nanoparticles Against Eimeria-the Coccidiosis-Causing Agents: A Comprehensive Review

Coccidiosis is a protozoan disease caused by Eimeria species and is a major threat to the poultry industry. Different anti-coccidial drugs (diclazuril, amprolium, halofuginone, ionophores, sulphaquinoxaline, clopidol, and ethopabate) and vaccines have been used for their control. Still, due to the development of resistance, their efficacy has been limited. It is continuously damaging the economy of the poultry industry because under its control, almost $14 billion is spent, globally. Recent research has been introducing better and more effective control of coccidiosis by using metallic and metallic oxide nanoparticles. Zinc, zinc oxide, copper, copper oxide, silver, iron, and iron oxide are commonly used because of their drug delivery mechanism. These nanoparticles combined with other drugs enhance the effect of these drugs and give their better results. Moreover, by using nanotechnology, the resistance issue is also solved because by using several mechanisms at a time, protozoa cannot evolve and thus resistance cannot develop. Green nanotechnology has been giving better results due to its less toxic effects. Utilization of metallic and metallic oxide nanoparticles may present a new, profitable, and economical method of controlling chicken coccidiosis, thus by changing established treatment approaches and improving the health and production of chickens. Thus, the objective of this review is to discuss about economic burden of avian coccidiosis, zinc, zinc oxide, iron, iron oxide, copper, copper oxide, silver nanoparticles use in the treatment of coccidiosis, their benefits, and toxicity.

A smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor for visual detection of mercury ions and l-penicillamine

Establishment of a rapid, sensitive, visual, accurate and low-cost fluorescence detection system to detect multiple targets was of great significance in food safety evaluation, ecological environment monitoring and human health monitoring. In this work, a smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor was proposed based on metal-organic framework (NH₂-MIL-101(Fe)) and CdTe quantum dots (CdTe QDs) for visual detection of mercury ions (Hg²⁺) and L-penicillamine (L-PA), in which NH₂-MIL-101(Fe) was used as the reference signal and CdTe QDs was used as the response signal. The down-conversion fluorescence system at excitation wavelength of 300 nm (ex: 330 nm) was used to detect Hg²⁺ and L-PA, in which the detection limit of Hg²⁺ was 0.053 nM with the fluorescence color changed from green to blue, and the detection limit of L-PA was 1.10 nM with the fluorescence color changed from blue to green. Meanwhile, the up-conversion fluorescence system at excitation wavelength of 700 nm (ex: 700 nm) was used to detect Hg²⁺ and L-PA. The detection limits of Hg²⁺ and L-PA were 0.11 nM and 2.93 nM, respectively. The detection of Hg²⁺ and L-PA were also carried out based on the color extraction RGB values identified by the smartphone with a detection limit of 0.091 nM for Hg²⁺ and 8.97 nM for L-PA. In addition, the concentrations of Hg²⁺ and L-PA were evaluated by three-dimensional dynamic analysis in complex environments. The smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor system provides a new strategy for detection Hg²⁺ and L-PA in food safety evaluation, environmental monitoring and human health monitoring.

Protein corona-induced extraction coupled to Fenton oxidation for selective and non-destructive preconcentration of Ag2S nanoparticles from waters

Sulfidation of silver nanoparticles (AgNPs) to generate silver sulfide nanoparticles (Ag₂S-NPs) significantly influences their fate and toxicity in natural environments. However, the correlational research in this field was limited by the lack of methods for speciation analysis of Ag₂S-NPs. To address this challenge, a novel protocol for the selective Ag₂S-NP extraction from real waters was developed using protein corona-induced extraction coupled to Fenton oxidation of AgNPs with Fe³⁺/H₂O₂ reagents. The ability of various concentrations of Fe³⁺/H₂O₂ to selectively dissociate AgNPs into ions was first evaluated. Then, selective separation and preconcentration of the remaining Ag₂S-NPs was established by optimizing the parameters that may affect the protein corona-induced extraction efficiency, followed by quantification with inductively coupled plasma mass spectrometry (ICP-MS), enabling an ultrahigh enrichment factor of 10,000 and extremely low detection limit (LOD) of 1.8 ng/L. The presence of humic acid (HA), inorganic salts and particles at the environmentally relevant levels had limited effects on Ag₂S-NP extraction. As demonstrated by transmission electron microscopy (TEM) analysis and single particle ICP-MS (spICP-MS), the sizes, shapes, and compositions of Ag₂S-NPs extracted with the proposed method remain in intact. Good recoveries of 83.7-105% were achieved for the Ag₂S-NPs spiked in four natural waters at the level of 97.8 ng/L. Due to the high yields and applicability to Ag₂S-NPs at environmentally relevant concentrations, this proposed method is particularly suitable to track the generation and transformation of Ag₂S-NPs in various scenarios.

Insights into the lower trophic transfer of silver ions than silver containing nanoparticles along an aquatic food chain

Silver nanoparticles (AgNPs) released into the environment are subject to environmental transformation processes before accumulating in aquatic organisms and transferring along the food chain. Lack of understanding on how environmental transformation affects trophic transfer of AgNPs hinders accurate prediction of the environmental risks of these widely present nanomaterials. Here we discover that pristine AgNPs as well as their sulfidation products (Ag₂S-NPs) and dissolution products (Ag⁺) tend to be accumulated in Daphnia magna and subsequently transferred to zebrafish. In D. magna, Ag⁺ exhibits the highest bioaccumulation potential whereas Ag₂S-NPs show the lowest bioaccumulation. Surprisingly, the biomagnification factor of Ag⁺ along the D. magna-zebrafish food chain appears to be significantly lower relative to AgNPs and Ag₂S-NPs, likely due to the limited release of Ag from D. magna to zebrafish during digestion. Moreover, AgNPs and their transformation products mainly accumulate in the internal organs, particularly intestine, of zebrafish. Adsorption of AgNPs on the surface of the intestinal cell membrane mitigates depuration of AgNPs and, at least in part, leads to the larger biomagnification factor of AgNPs, relative to their transformation products. This research highlights the necessity of considering environmental transformation processes of nanomaterials in assessing their bioavailability and risk.

Evaluation of antibacterial activities of silver nanoparticles on culturability and cell viability of Escherichia coli

Nanoparticles released into the environment are attracting increasing concern because of their potential toxic effects. Conventional methods for assessing the toxicity of nanoparticles are usually confined to cultivable cells, but not applicable to viable but non-culturable (VBNC) cells. However, it remains unknown whether silver nanoparticles (AgNPs), a typical antimicrobial agent, could induce bacteria into a VBNC state in natural environments. In this work, the viability of E. coli, an indicator bacterium widely used for assessing the antibacterial activity of AgNPs, was examined through coupling plate counting, fluorescence staining and adenosine triphosphate (ATP) production. AgNPs were found to have a considerable antibacterial ability, which resulted in less than 0.0004% of culturable cells on plates. However, more than 80% of the cells still maintained their cell membrane integrity under the stress of 80 mg/L AgNPs. Meanwhile, the residue of ATP production (0.6%) was 1500 times higher than that of the culturable cells (< 0.0004%). These results clearly demonstrate that when exposed to AgNPs, most of cells fell into a VBNC state, instead of dying. Environmental factors, e.g., Cl^- and illumination, which could change the dissolution, hydrophilicity and zeta potential of AgNPs, eventually influenced the culturability of E. coli. Inhibition of dissolved Ag⁺ and reactive oxygen species was found to facilitate the mitigation of the strain into a VBNC state. Our findings suggest the necessity of re-evaluating the environmental effects and antibacterial activities of AgNPs.

Effects of extracellular polymeric substances on silver nanoparticle bioaccumulation and toxicity to Triticum aestivum L

The potential effects of extracellular polymeric substances (EPS) on the behavior and toxicity of silver nanoparticle (Ag-NPs) and silver sulfide nanoparticle (Ag₂S-NPs) remains ambiguous. The interaction of EPS from Bacillus subtilis with Ag₂S-NPs, metallic Ag-NPs, or ionic Ag, and the associated plant safety had been examined in this study. The biological impacts of Ag-NPs and Ag₂S-NPs were Ag form-dependent and highly influenced by microbial EPS. Compared with metallic Ag-NPs, Ag₂S-NPs exert inert biological impacts, as revealed by 3.44 times lower Ag bioaccumulation in wheat (Triticum aestivum L.) seedlings and nearly reduce plant biomass when wheat was subjected to 1.0 mg-Ag L^-1 of Ag-NPs and Ag₂S-NPs with the transfer factors of 151.56-930.87 vs. 12.52-131.81, respectively. These observations were coincident with the low dissolved Ag ([Ag]_diss) in the Ag₂S-NPs treatment than the Ag-NPs treatment (114.0 vs. 0.0791, μg L^-1). Compared with the enhanced toxicity of Ag₂S-NPs to wheat, Bacillus subtilis EPS significantly alleviate the phytotoxicity of Ag-NPs, as revealed by the relative root elongation (7.15-45.40% decrease vs. 2.39-11.75% increase), and malondialdehyde (1.47-83.22% increase vs. 8.57-25.25% decrease) and H₂O₂ (11.27-71.78% increase vs. 5.16-36.67% decrease) contents. These constrasting plant responses of B. subtilis EPS are mainly caused by their complexation property with toxic Ag⁺ and nutrient elements for wheat stressed by Ag-NPs and Ag₂S-NPs, respectively. Our findings highlight the importance of rhizospheric EPS in affecting the biogeochemistry and ecotoxicity of metal nanoparticles including Ag-NPs and Ag₂S-NPs in agricultural systems.

Oxidation process of lead sulfide nanoparticle in the atmosphere or natural water and influence on toxicity toward Chlorella vulgaris

Lead sulfide nanoparticle (nano-PbS) released into environment can cause hazards to human or ecosystem. Nano-PbS potentially undergoes oxidation in the environment, but oxidation mechanism is not understood yet. Herein, oxidation kinetics and products of nano-PbS by ozone (O₃), hydrogen peroxide (H₂O₂) and hydroxyl radical (HO·) in the atmosphere or natural water were investigated. Results show that oxidation process of nano-PbS can be divided into three stages, producing sulfate, ions and oxides of lead in sequence. O₃ or HO·leads to faster release of ionic lead from nano-PbS in the initial stage than H₂O₂, but causes significant decrease of ionic lead by transforming divalent lead to tetravalent lead oxides in the second or third stage. Toxicity determined taking Chlorella Vulgaris as an example follows an order of PbO₂ < Pb₃O₄ < nano-PbS < PbO < PbSO₄. Toxicity of lead particles is mainly determined by sizes influencing cellular uptake and solubility product constant (K_sp) related with dissolution of lead in cells. The results indicate that the toxicity of nano-PbS increases in an initial oxidation stage and decreases in further oxidation stages. This study provides new insights into environmental behavior of nano-PbS and mechanism understandings for assessing ecological risks of nano-PbS.

Impacts of Ag and Ag2S nanoparticles on the nitrogen removal within vertical flow constructed wetlands treating secondary effluent

In this study, the effects of silver (Ag NPs) and sliver sulfide nanoparticles (Ag₂S NPs) on nitrogen removal and nitrogen functional microbes in constructed wetlands were investigated. The obtained results demonstrated that inhibition extent on nitrogen removal relied on NPs types and high concentrations NPs showed higher negative effects. 0.5 mg/L Ag NPs had no influence on NH₄⁺-N removal, amoA and nxrA gene copies, whereas Ag₂S NPs and Ag⁺ decreased NH₄⁺-N removal by reducing abundances of nitrifying genes. The concentrations of NO₃^--N and TN in all 0.5 mg/L obviously increased compared with control, resulting from decreasing functional genes and denitrifying bacteria. And 0.5 mg/L Ag NPs exhibited largest inhibitory effects, with the highest NO₃^--N effluent concentrations. 2 mg/L Ag NPs decreased NH₄⁺-N removal, but adverse effects gradually vanished with extension of time, whereas both Ag₂S NPs and Ag⁺ at 2 mg/L influenced NH₄⁺-N transformation and decreased the abundance of amoA and nxrA genes and the AOB Nitrosomonas in CWs. Moreover, 2 mg/L of Ag NPs reduced NO₃^--N removal by decreasing abundance of nirS and key denitrifying bacteria. To sum up, the inhibition mechanisms concluded from current results were possibly in that Ag NPs exhibited nanotoxicity rather than ionic toxicity.

The role of natural organic matter in the silver release from sludge generated from coagulation of wastewater spiked with silver nanoparticles

Sludge is an integral part in the migration pathway of silver nanoparticles (AgNPs) from manufacture to the terrestrial environment. However, the detailed information on the role of natural organic matters (NOMs) remains limited. In this study, the sludge generated from coagulation of wastewater spiked with AgNPs (denoted as sludge_C-AgNPs) was taken as the model. Effects of humic acid (HA), alginate (AA) and bovine serum albumin (BSA) on the release amount, dynamics and speciation of silver from the sludge_C-AgNPs were investigated by a series of leaching experiments. The results showed that HA, AA and BSA in the leaching solution could enhance the silver release from the sludge_C-AgNPs. The concentrations of the dissolved and colloidal silver in the BSA solution were the highest at the initial stage of dynamic leaching. The controlling step of the silver release was internal diffusion in the HA and AA solution, while the release of dissolved silver was controlled by both chemical reaction and internal diffusion in the BSA solution. In addition, the released colloidal silver fractions in the BSA solution contained more particles with size >50 nm compared with the HA and AA solutions. The results suggested that the properties of NOMs may be the key factor affecting the transfer of AgNPs from the sludge to the terrestrial environment.

Combined impact of silver nanoparticles and chlorine on the cell integrity and toxin release of Microcystis aeruginosa

Silver nanoparticles (AgNPs) have shown to be toxic to freshwater cyanobacterial species, and sodium hypochlorite (NaOCl) is a common oxidant for the treatment of cyanobacterial cells. AgNPs have a high possibility of co-existing with the cyanobacterial cells in the aqueous environments leading to its exposure to NaOCl during water treatment; however, their combined effects on the cyanobacterial cells are largely undocumented. This work compares the individual and combined effect of AgNP and NaOCl on the integrity and toxin (microcystins) release of Microcystis aeruginosa at varying levels. The results show that the AgNP (0.2-0.6 mg/L) alone has negligible effects on the cell lysis, while NaOCl alone shows concentration-dependent (0.2 < 0.4 < 0.6 mg/L) rupturing of cells. In contrast, the AgNP + NaOCl (0.2-0.6 mg/L) samples show increasing loss in cell integrity at higher AgNP (0.4 and 0.6 mg/L) levels than the NaOCl only samples. NaOCl exposure results in increasing dissolution of AgNPs with time, releasing silver ions (Ag⁺), affecting its size and morphology. The cell-associated total Ag declines over time with an increase in NaOCl levels, maybe due to increasing cell-lysis or NaOCl induced oxidative dissolution of AgNPs. The cell-associated total Ag and released Ag⁺ possibly weaken the cellular membrane, thus assisting NaOCl in faster cell-lysis. The combined exposure of AgNP and NaOCl also results in a higher release of toxin from the cells. This work collectively reveals that the AgNPs combined with NaOCl can enhance the cell lysis and release of toxins.

Silver Ion Release Accelerated in the Gastrovascular Cavity of Hydra vulgaris Increases the Toxicity of Silver Sulfide Nanoparticles

Silver nanoparticles (Ag-NPs) streamed into aquatic environments are chemically transformed into various forms, and one of the predominant forms is silver sulfide NPs (Ag₂ S-NPs). Because of the lower dissolution rate of silver ions (Ag⁺ ), the toxicity of Ag₂ S-NPs could be lower than that of Ag-NPs. However, the toxicity of Ag₂ S-NPs has been observed to be restored under oxidative or acidic conditions. In the present study, 4 aquatic organisms, Pseudokirchneriella subcapitata (algae), Daphnia magna (crustacean), Danio rerio (fish), and Hydra vulgaris (cnidarian), were exposed to Ag₂ S-NPs transformed from Ag-NPs using Na₂ S under anoxic conditions; and acute toxicity was evaluated. The acute toxicity of Ag₂ S-NPs was rarely observed in algae, crustaceans, and fish, whereas it was significantly restored in cnidarians. Although the dissolution rate was low in the medium exposed to Ag₂ S-NPs, high Ag⁺ was detected in H. vulgaris. To understand the mechanisms of Ag₂ S-NP toxicity in cnidarians, transcriptional profiles of H. vulgaris exposed to Ag-NPs, Ag₂ S-NPs, and AgNO₃ were analyzed. As a result, most of the genes that were significantly changed in the Ag₂ S-NPs group were also found to be significantly changed in the AgNO₃ group, indicating that the toxicity of Ag₂ S-NPs was caused by Ag⁺ dissolved by the acidic condition in the gastrovascular cavity of H. vulgaris. This finding is the first in an aquatic organism and suggests the need to reconsider the stability and safety of Ag₂ S-NPs in the aquatic environment. Environ Toxicol Chem 2021;40:1662-1672. © 2021 SETAC.

Reduction of Ionic Silver by Sulfur Dioxide as a Source of Silver Nanoparticles in the Environment

The natural formation of silver nanoparticles (AgNPs) via biotic and abiotic pathways in water and soil media contributes to the biogeochemical cycle of silver metal in the environment. However, the formation of AgNPs in the atmosphere has not been reported. Here, we describe a previously unreported source of AgNPs via the reduction of Ag(I) by SO₂ in the atmosphere, especially in moist environments, using multipronged advanced analytical and surface techniques. The rapid reduction of Ag(I) in the atmospheric aqueous phase was mainly caused by the sulfite ions formed from the dissolution of SO₂ in water, which contributed to the formation of AgNPs and was consistent with the Finke-Watzky model with a major contribution of the reduction-nucleation process. Sunlight irradiation excited SO₂ to form triplet SO₂, which reacted with water to form H₂SO₃ and greatly enhanced Ag(I) reduction and AgNP formation. Different pH values affected the speciation of Ag(I) and S(IV), which were jointly involved in the reduction of Ag(I). The formation of AgNPs was also observed in the atmospheric gas phase via direct reduction of Ag(I) by SO₂(gas), which occurred even in 50 ppbv SO₂(gas). The natural occurrence of AgNPs in the atmosphere may also be involved in silver corrosion, AgNP transformation and regeneration, detoxification of gaseous pollutants, and the sulfur cycle in the environment.

Highly sensitive fluorescent probe for selective detection of hypochlorite ions using nitrogen-fluorine co-doped carbon nanodots

Hypochlorite ions (ClO^-) are widely used in bleaching agents and disinfectants. However, high concentrations of chloride species are harmful to human health. Therefore, effective methods for the detection of ClO^- ions are required. In this study, using 4-fluorophthalic acid and glycine, nitrogen-fluorine co-doped carbon nanodots (N,F-CDs) were synthesized by one-pot hydrothermal synthesis for use as a fluorescent probe for the fluorometric detection of ClO^- in aqueous media, based on the inhibition of n → π* transitions. The excitation and emission peak centers of the N,F-CDs are at 387 and 545 nm, respectively. The N,F-CDs show a fast quenching response (<1 min) for ClO^- and can be used in a wide pH range (pH 4-13). Under optimal conditions, the fluorescence intensity decreased with increase in the ClO^- concentration from 0 to 35 μM, and a low limit of detection (9.6 nM) was achieved. This probe possesses excellent selectivity and high sensitivity and was used to analyze standardized samples of piped water, achieving a satisfactory recovery. Thus, this nitrogen-fluorine co-doped nanodot probe is promising for the detection of pollutants.

Cellular, Molecular and Biochemical Impacts of Silver Nanoparticles on Rat Cerebellar Cortex

BACKGROUND

The excessive exposure to silver nanoparticles (Ag-NPs) has raised concerns about their possible risks to the human health. The brain is a highly vulnerable organ to nano-silver harmfulness. The aim of this work was to evaluate the impacts of Ag-NPs exposure on the cerebellar cortex of rats.

METHODS

Rats were assigned to: Control, vehicle control and Ag-NP-exposed groups (at doses of 10 mg and 30 mg/kg/day). Samples were processed for light and electron microscopy examinations. Immunohistochemical localization of c-Jun N-terminal kinase (JNK), nuclear factor kappa beta (NF-κB) and calbindin D28k (CB) proteins was performed. Analyses of expression of DNA damage inducible transcript 4 (Ddit4), flavin containing monooxygenase 2 (FMO2) and thioredoxin-interacting protein (Txnip) genes were done. Serum levels of inflammatory cytokines were also measured.

RESULTS

Ag-NPs enhanced apoptosis as evident by upregulation of Ddit4 gene expressions and JNK protein immune expressions. Alterations of redox homeostasis were verified by enhancement of Txnip and FMO2 gene expressions, favoring the activation of inflammatory responses by increasing NF-κB protein immune expressions and serum inflammatory mediator levels. Another cytotoxic effect was the reduction of immune expressions of the calcium regulator CB.

CONCLUSION

Ag-NPs exposure provoked biochemical, cellular and molecular changes of rat cerebellar cortex in a dose-dependent manner.

Insights into Mechanism of Hypochlorite-Induced Functionalization of Polymers toward Separating BFR-Containing Components from Microplastics

Surface functionalization of polymers is significant for an emerging flotation technique for separation of microplastics toward the recycling of plastic wastes. In this study, the hypochlorite-induced functionalization of polymers, including ABS, PMMA, PS, and PVC polymers, was intensively investigated. Afterward, its emerging application in flotation separation of microplastic mixtures was assessed based on a Box-Behnken design of the response surface methodology. The functionalization favorably induced decreases in the contact angle and zeta potential of polymers, suggesting hydrophilic and negatively charged surfaces. Particularly, the functionalization of ABS polymers was the most effective, leading to the obviously decreased contact angle (from 92.5° to 67.8°) and zeta potential (from -26.4 mV to -41.7 mV) at neutral condition. The major mechanism for these variations was the oxidation of the sp³-C and butenyl group by hydroxyl radical and the hydrolysis of cyano group, which introduced the hydroxyl, carboxyl, and amide groups and rough topographies on the surface of ABS polymers. Oxygen functionalities introduced on the surfaces of other polymers were far less than those of ABS polymers. This selectivity inspired us to apply the functionalization in flotation separation of ABS microplastics from microplastic mixtures. After functionalization, ABS microplastics showed a significantly decreased floatability in flotation tests since the hydrophilic surface was repulsive to the adhesion of air bubbles. An empirical model was built to optimize the separation efficiency using the overall desirability function. Under optimum conditions, ABS microplastics were efficiently separated, and their removal rate, recovery, and purity were 99.8%, 99.8%, and >99.9%, respectively. These findings provide significant insights into the mechanism of the functionalization of polymers and show a promising prospect for pollution control of plastic wastes.

Coexposed nanoparticulate Ag alleviates the acute toxicity induced by ionic Ag+in vivo

Health concerns of silver nanoparticles (AgNPs) emerged with the increase of their industrial and biomedical application and thus human exposure. The highly dynamic properties of AgNPs lead to coexposure to nanoparticulate and ionic silver, and the combined effects of different Ag species might alter their individual toxicity. Herein, the toxicity of AgNPs combined with ionic Ag⁺ toward the rat was investigated after intravenous (i.v.) exposure to either AgNPs (5 mg/kg), Ag⁺ (5 mg/kg), or a mixture of Ag⁺ and AgNPs (5 mg/kg for both). Comparable results by histopathological and biochemical studies revealed that the exposure to individual AgNPs causes no apparent toxicity in rats, while Ag⁺ ions at the same dose induced marked acute toxicity. More importantly, while there was a negligible combined effect on the Ag accumulation, the less toxic AgNPs ameliorated Ag⁺ induced toxicity to rat organs after coexposure to the mixture of Ag⁺ and AgNPs, which might result from the complexation of Ag⁺ with the thiols like metallothioneins. Therefore, the combined toxicity of particulate and ionic Ag was complicated by their individual toxicities and also their interaction with intracellular detoxification biomolecules, regardless of differences in Ag accumulation. Although further investigations are still needed for the potential toxic mechanisms of the coexposed AgNPs and Ag⁺, considerations of the combined toxicity of different Ag species will reflect more accurate assessments of their health impacts.

A voltammetric investigation of the sulfidation of silver nanoparticles by zinc sulfide

Silver nanoparticles (Ag NPs) are among the most common forms of nanoparticles in consumer products, yet the environmental implications of their widespread use remain unclear due to uncertainties about their fate. Because sulfidation of Ag NPs results in the formation of a stable silver sulfide (Ag₂S) product, it is likely an important removal mechanism of bioavailable silver in natural waters. In addition to sulfide, the complete conversion of Ag NPs to Ag₂S will require dissolved oxygen or some other oxidant so dispersed metal sulfides may be an important pool of reactive sulfide for such reactions in oxygenated systems. The reaction of Ag NPs with zinc sulfide (ZnS) was investigated using a voltammetric method, anodic stripping voltammetry (ASV). ASV provided sensitive, in situ measurements of the release of zinc (Zn²⁺) cations resulting from the cation exchange reaction between Ag NPs and ZnS. The effects of Ag NP size and surface coatings on the initial rates of sulfidation by ZnS were examined. Sulfidation of smaller Ag NPs generally occurred faster and to a greater extent due to their larger relative surface areas. Sulfidation of Ag NPs capped by citrate and lipoic acid occurred more rapidly relative to polyvinylpyrrolidone (PVP) and branched polyethylene (BPEI). This study demonstrates the utility of voltammetry for such investigations and provides insights into important factors controlling Ag NP sulfidation such as availability of dissolved oxygen, Ag NP size and Ag NP surface coating. Furthermore, this work demonstrates the importance of cation exchange reactions between silver and metal sulfides, and how the environmental release of Ag NPs could alter the speciation of other metals of environmental significance.

Copper(I) Promotes Silver Sulfide Dissolution and Increases Silver Phytoavailability

Metal sulfides, including acanthite (Ag₂S), are persistent in the environment. In colloidal form, however, they can serve as a "Trojan horse", facilitating the mobility of trace metal contaminants. The natural processes that lead to the in situ dissolution of colloidal metal sulfides in soil are largely unknown. In this study, the dissolution of colloidal Ag₂S in topsoil and Ag phytoavailability to wheat were examined in Ag₂S-Cu(II)-thiosulfate systems. Cu(II) and thiosulfate strongly increased silver release (up to 83% of total Ag) from Ag₂S in the dark. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Cu K-edge X-ray absorption spectroscopy identified Cu(I) as the driving force of Ag₂S dissolution. Density functional theory calculations further demonstrated the ability of Cu(I) to substitute for surface Ag on Ag₂S in an energetically favorable manner. However, excess Cu(II) could enhance the formation of precipitates containing Cu(I), Ag, and S. Our results indicate that at ambient temperature and in the dark, Cu(I) can promote the dissolution of Ag₂S and act as a precipitating agent. These findings reveal previously unrecognized biogeochemical processes of colloidal Ag₂S and their importance in determining the fate of metal sulfides in the environment and probably also in vivo.

Highly efficient removal of thallium(I) from wastewater via hypochlorite catalytic oxidation coupled with adsorption by hydrochar coated nickel ferrite composite

In this study, tannery wastewater was used as carbon source to hydrothermally synthesize magnetic carbon-coated nickel ferrite composite (NiFe₂O₄@C), which was employed as a catalyst for thallium (Tl) oxidation by hypochlorite and simultaneously as an adsorbent for Tl removal from wastewater. Compared with NiFe₂O₄@C adsorption or hypochlorite oxidation alone, the combination of NiFe₂O₄@C and hypochlorite substantially enhanced the rate and efficiency of Tl(I) removal. In addition, this process was highly effective for Tl(I) removal over a wide pH range (6-12). The maximum Tl(I) removal capacity was 1699 mg/g at pH 10, which is the highest one reported so far. Electron spin resonance spectra suggested the formation of hypochlorite-based free radicals induced by the NiFe₂O₄@C composite, which enhanced the Tl(I) oxidation and removal. Oxidation-induced surface precipitation and surface complexation were found to be the main Tl(I) removal mechanisms. Consecutive cyclic regeneration tests implied robust regeneration and reuse performance of the composite. Moreover, it was effective for Tl(I) removal from real industrial wastewater. Therefore, the hypochlorite catalytic oxidation coupled with adsorption by the magnetic NiFe₂O₄@C composite is a promising technique for Tl(I) removal from wastewater. This hybrid process also has great potential for the removal of other pollutants.

A field study of the fate of biosolid-borne silver in the soil-crop system

Land application of biosolids is a major route for the introduction of silver (Ag) into the terrestrial environment. Previous studies have focused on the risks from Ag to the human food chain but there is still a lack of quantitative information on the flow of biosolid-borne Ag in the soil-crop system. Two long-term field experiments were selected to provide contrasting soil properties and tillage crops to investigate the fate of Ag from sequentially applied biosolids. Biosolid-borne Ag accumulated in the soil and < 1‰ of applied Ag was taken up by the crops. The biosolid-borne Ag also migrated down and accumulated significantly (p < 0.05) in the soil profile to a depth of 60-80 cm at an application rate of 72 t biosolids ha^-1. Soil texture significantly affected the downward transport of biosolid-borne Ag and the migration of Ag appeared to be more pronounced in a soil profile with a low clay content. Moreover, loss of Ag by leaching may not be related to the biosolid application rate. Leaching losses of Ag may have continued for some time after biosolid amendment was suspended. The results indicate that soil texture may be a key factor affecting the distribution of biosolid-borne Ag in the soil-crop system.

Zero-valent iron-manganese bimetallic nanocomposites catalyze hypochlorite for enhanced thallium(I) oxidation and removal from wastewater: Materials characterization, process optimization and removal mechanisms

Nano zero-valent metals adsorption coupled with advanced oxidation for environmental pollutants removal has been gaining attention recently. In this study, zero-valent iron-manganese (nZVIM) bimetallic nanocomposites were prepared via one-pot borohydride reduction and coupled with hypochlorite (ClO^-) oxidation for enhanced thallium (Tl) removal from wastewater. Amorphous nZVIM nanoparticles were successfully synthesized, with a specific surface area of 106.89 m²/g, and a saturation magnetization of 65.16 emu/g. In comparison with the nZVIM adsorption or ClO^- oxidation alone, the hybrid nZVIM/ClO^- process achieved much faster Tl(I) removal rate over a wide pH range from 6 to 10. Maximum Tl(I) removal capacity was as high as 990.0 mg/g. The oxidation-induced adsorption for Tl(I) removal well followed the pseudo-first kinetic order model. Stable and effective adsorbent regeneration was achieved during the cyclic adsorption-desorption tests. This process also had high resistance to the interference of external cations, can act as an effective pretreatment for Tl(I) removal from the actual saline industrial wastewater. The main mechanisms for Tl(I) removal were found to be oxidation, surface precipitation, pore retention, and surface complexation. The nZVIM coupled with ClO^- approach has great potential for Tl(I) removal from wastewater, and its application in other fields is highly anticipated.

Combination of the BeWo b30 placental transport model and the embryonic stem cell test to assess the potential developmental toxicity of silver nanoparticles

BACKGROUND

Silver nanoparticles (AgNPs) are used extensively in various consumer products because of their antimicrobial potential. This requires insight in their potential hazards and risks including adverse effects during pregnancy on the developing fetus. Using a combination of the BeWo b30 placental transport model and the mouse embryonic stem cell test (EST), we investigated the capability of pristine AgNPs with different surface chemistries and aged AgNPs (silver sulfide (Ag2S) NPs) to cross the placental barrier and induce developmental toxicity. The uptake/association and transport of AgNPs through the BeWo b30 was characterized using ICP-MS and single particle (sp)ICP-MS at different time points. The developmental toxicity of the AgNPs was investigated by characterizing their potential to inhibit the differentiation of mouse embryonic stem cells (mESCs) into beating cardiomyocytes.

RESULTS

The AgNPs are able to cross the BeWo b30 cell layer to a level that was limited and dependent on their surface chemistry. In the EST, no in vitro developmental toxicity was observed as the effects on differentiation of the mESCs were only detected at cytotoxic concentrations. The aged AgNPs were significantly less cytotoxic, less bioavailable and did not induce developmental toxicity.

CONCLUSIONS

Pristine AgNPs are capable to cross the placental barrier to an extent that is influenced by their surface chemistry and that this transport is likely low but not negligible. Next to that, the tested AgNPs have low intrinsic potencies for developmental toxicity. The combination of the BeWo b30 model with the EST is of added value in developmental toxicity screening and prioritization of AgNPs.

Enhanced thallium(I) removal from wastewater using hypochlorite oxidation coupled with magnetite-based biochar adsorption

The development of efficient and regenerable adsorbent coupled with advanced oxidation for enhanced thallium (Tl) removal has been a recent focus on wastewater treatment. In this study, a magnetite-based biochar derived from watermelon rinds was synthesized and used as a sustainable adsorbent and catalyst for hypochlorite oxidation and removal of Tl(I) from wastewater. The addition of hypochlorite substantially enhanced the Tl(I) removal under normal pH range (6-9). Maximum Tl adsorption capacity of 1123 mg/g was achieved, which is 12.3% higher than the highest value previously reported. The magnetic biochar can be regenerated using 0.1 mol/L HNO₃ solution for elution in only 5 min, with a Tl desorption efficiency of 78.9%. The Tl removal efficiency was constantly higher than 98.5% during five consecutive recycle tests, indicating the effective reuse performance of the adsorbent. Oxidation, surface precipitation, pore retention and surface complexation were the main mechanisms for Tl(I) removal. The re-dissolution of Tl compounds and ion exchange of Tl cations with proton were the main mechanisms for adsorbent regeneration. Given the fast oxidation rate, high adsorption capacity, steady reusability and facile separability, this magnetic biochar-hypochlorite technique is a promising means for Tl(I) removal from wastewater. The catalytic hypochlorite oxidation induced by the magnetic biochar has also great potential to the effective removal of other pollutants.

Direct cyanidation of silver sulfide by heterolytic C–CN bond cleavage of acetonitrile

Extraction of silver as silver cyanide from silver sulfide was made possible using acetonitrile as the source of cyanide. The process of cyanidation took place through the oxidation of sulfide to sulfur oxides and cleavage of the C–CN bond of acetonitrile. The reaction was found to be catalyzed by vanadium pentoxide and hydrogen peroxide. The different species involved in the cyanidation process were duly characterized using FTIR, ESI-MS, HRMS, XPS and UV-vis spectroscopic analysis. The mechanism of the cyanidation process was confirmed through in situ FTIR analysis.

Herein, we report the cleavage of the C–CN bond of acetonitrile, catalyzed by vanadium pentoxide, for the direct cyanidation of silver sulfide.

Adverse effects of nanosilver on human health and the environment

Silver and silver nanoparticles (AgNPs) exhibit antimicrobial properties against some bacteria, fungi and viruses, however, the ever-increasing application of nanosilver in consumer products, water disinfection and healthcare settings, have raised concerns over the public health/environmental safety of this nanomaterial. The current ubiquity of nanosilver may result in repeated exposure through various routes (skin, inhalation, or ingestion) which may lead to health complications. While there are a number of review articles and case studies published to date on the subject, an updated coherent review that clearly delineates thresholds and safe doses is lacking. Thus, it is plausible to have an overview of the most recent findings on the threshold limits, safe doses of silver and its related nanoscale forms, and the needed actions to ensure the safety and health of human, terrestrial and aquatic lives. This review provides an account of the effects of nanosilver in our daily lives. STATEMENT OF SIGNIFICANCE: This manuscripts is a review of the toxicity of nanosized silver. With respect to the existing literature, it goes beyond stating that there is a knowledge gap, drawing the attention of a wider readership to the ever-growing evidence of nanosilver toxicity to human and nature, and outlining the dose thresholds based on comprehensive data mining and visualisation. There are nearly 500 consumer products that claim to contain nanosilver. Thus, we trust a review of recent conclusive findings is timely. This manuscript is in line with the scope of the Journal, enabling a better understanding of the biological response to a widely-used bionanomaterial. Moreover, it provides a bigger picture of the link between surface properties and biocompatibility of nanosilver in different forms.

Stability of silver nanoparticle sulfidation products

The adoption of silver nanoparticles in consumer goods has raised concerns about the potential environmental harm of their widespread use. We studied chemical transformations that Ag NPs may undergo as they pass through sulfide-rich conditions common in waste water treatment plants (WWTPs), which may limit the release of Ag⁺ from Ag NPs due to the formation of low-solubility silver sulfide (Ag₂S). However, it is uncertain whether sulfidation is complete and if sulfidized Ag NPs continue to release Ag⁺. To address these uncertainties, we monitored the reaction of Ag NPs with various levels of sulfide with an ion selective electrode and UV/visible spectrophotometry over the course of two months. We characterized the products of the sulfidation reactions with a purge-and-trap acid volatile sulfide (AVS) analysis, which served as a measure of the stability of the sulfidized products because sulfide would be readily lost to oxidation unless it is stabilized as Ag₂S. The Ag NP surface plasmon resonance (SPR) absorbance peak was initially diminished and then returned over the course of several days after reaction with limited amounts of sulfide, suggesting a dynamic system that may retain some characteristics of the pristine Ag NPs. However, ICP-MS analysis of sulfidized Ag NP suspensions over a two-month period demonstrates that sulfidation limits the release of Ag⁺ ions from nanosilver that pass through a WWTP, even when sulfide concentrations are limited relative to silver.

Silver nanomaterials in the natural environment: An overview of their biosynthesis and kinetic behavior

Silver nanomaterials (Ag NMs) are fabricated by many biological components in our environment. Recently, research on their biosynthesis and reactions has become a focus of attention. Due to the complexity of biological systems and samples, specific processes and mechanisms involving Ag NMs are difficult to identify and elucidate on the molecular and chemical-bond level. The microorganisms and composite components of plant extracts are of great interest in many biological syntheses. Although potential biomolecules have been shown to play essential roles in biological systems in Ag NM biosynthesis, the detailed mechanism of the electron transfer process and crucial molecules that control this reaction have only recently come into focus. The reactive behavior of the Ag NMs is of great significance for understanding their overall behavior and toxicity. Additionally, only limited knowledge is available about their kinetics. All reactions involve chemical bond formation, electron transfer, or electrostatic interactions. An overview is presented of the biosynthesis of Ag NMs based on molecular supports including a nitrate reductase/NADH oxidase-involved electron transfer reaction and their mechanisms in Ag⁺ reduction: quinol-mediated mechanism and superoxide-dependent mechanism, and molecular supports in plant extracts, is presented. The environmental reaction kinetics and mechanisms of the interactions of Ag NMs with substances are introduced based on the formation and classification of chemical bonds. The particle-particle reaction kinetics of Ag NMs in the environment are discussed to directly explain their stability and aggregation behavior. The toxicity of Ag NMs is also presented. In addition, future prospects are summarized. This review is the first to provide an insight into the mediating molecules and chemical bonds involved in the biosynthesis, kinetics, and mechanisms of action of Ag NMs.

Re-evaluation of stability and toxicity of silver sulfide nanoparticle in environmental water: Oxidative dissolution by manganese oxide

Stability of silver sulfide nanoparticle (Ag₂S-NP) in the environment has recently drawn considerable attention since it is associated with environmental risk. Although the overestimated stability of Ag₂S-NP in aqueous solution has already been recognized, studies on transformation of Ag₂S-NP in environmental water are still very scarce. Here we reported that Ag₂S-NP could undergo dissolution by manganese(IV) oxide (MnO₂), an important naturally occurring oxidant in the environment, even in environmental water, although the dissolved silver would probably be adsorbed onto the particles (>0.45 μm) in environmental water, mitigating the measurable levels of dissolved silver. The extent and rate of Ag₂S-NP dissolution rose with the increasing concentration of MnO₂. In addition, environmental factors including natural organic matter, inorganic salts and organic acids could accelerate the Ag₂S-NP dissolution by MnO₂, wherein an increase in dissolution extent was also observed. We further documented that Ag₂S-NP dissolution by MnO₂ was highly dependent on O₂ and it was an oxidative dissolution, with the production of SO₄^2-. Finally, dissolution of Ag₂S-NP by MnO₂ affected zebra fish (Danio rerio) embryo viability, showing significant reduction in embryo survival and hatching rates, compared to embryos exposed to Ag₂S-NP, MnO₂ or dissolved manganese alone. These findings would further shed light on the stability of Ag₂S-NP in the natural environment - essential for comprehensive nano risk assessment.

Interpreting the effects of natural organic matter on antimicrobial activity of Ag2S nanoparticles with soft particle theory

Natural organic matter (NOM) ubiquitously exists in natural waters and would adsorb onto the particle surface. Previous studies showed that NOM would alleviate the toxicity of nanomaterials, while the mechanism is seldom quantitatively interpreted. Herein, the effects of humic substances [Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA)] and biomacromolecules [alginate and bovine serum albumin (BSA)] on the aggregation and antimicrobial effects of silver sulfide nanoparticles (Ag₂S-NPs) were investigated. The aggregation kinetics of Ag₂S-NPs in electrolyte solutions were in agreement with the results based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dynamic light scattering (DLS) results showed that the SRFA, SRHA, alginate and BSA molecules coated on the Ag₂S-NPs surfaces. The NOM coating layer prevented salt-induced coagulation of Ag₂S-NPs, and the effects of BSA and SRHA on Ag₂S-NPs stabilizing were more obvious than that of SRFA and alginate. Flow cytometry analysis results suggested that BSA and SRHA were more effective on alleviating the Ag₂S-NPs induced cell (Escherichia coli) membrane damage than SRFA and alginate. After interpreting the electrophoretic mobility (EPM) data of the NOM coated Ag₂S-NPs by Ohshima's soft particle theory, it was found that the thickness of the NOM coating layers followed the orders of BSA > SRHA > alginate > SRFA. The E.coli cell membrane damage level was negatively correlated with the thickness and softness of the coating layer. NOM coating may physically alleviate the contact between NPs and E. coli cells and thus attenuate the extent of cell membrane damage caused by the NP-cell interaction. This work provides a new perspective for quantitatively interpreting the influence of NOM on the environmental behaviors and risks of nanomaterials.

Transformation of AgCl Particles under Conditions Typical of Natural Waters: Implications for Oxidant Generation

The engineered silver nanoparticles (AgNPs) used in consumer products are ultimately released to the environment either as Ag(0), silver sulfide (Ag₂S(s)), silver chloride (AgCl(s)), and/or dissolved Ag(I) complexes. Of these, AgCl(s) and Ag₂S(s) exhibit semiconducting properties and hence may have significant implications to oxidant generation and subsequent redox transformations in natural waters. In this work, we investigate the transformation and photoreactivity of AgCl(s) under simulated natural water conditions with the photoreactivity probed by measuring the oxidation of formate (HCOO^-), a simple compound with a well-defined oxidation pathway. Our results show that AgCl(s) undergoes rapid dissolution in the presence of chloride concentrations representative of seawater (ca. 0.5 M NaCl) forming dissolved Ag(I) complexes but is stable in fresh waters and slightly brackish waters (≤200 mM NaCl). We further show that under these lower salinity conditions in which AgCl(s) is stable, pH has a significant impact on the reactivity of semiconducting AgCl(s). The photoreactivity (measured as initial HCOO^- oxidation rate) of AgCl(s) is relatively constant at pH 4.0 for periods of 24 h or more; however, it decreases rapidly under alkaline conditions. The rapid transformation (or "aging") of AgCl(s) under alkaline conditions suggests that AgCl(s), potentially transported through wastewater effluent to fresh or brackish water environments, may not have a significant impact in such environments. In comparison, in situ formed AgCl(s), potentially formed as a result of the oxidation of high concentrations (≥60 μg Ag·L^-1) of Ag(0) and/or Ag₂S(s), may have significant implications to oxidant generation in natural waters. Our results further show that rapid cycling of Ag between the 0 and +I redox states in sunlit surface waters as a result of the presence of AgNP oxidants (such as H₂O₂ and organic radicals) will further enhance the rate and extent of oxidant generation by AgCl(s).

A Current Overview of the Biological and Cellular Effects of Nanosilver

Nanosilver plays an important role in nanoscience and nanotechnology, and is becoming increasingly used for applications in nanomedicine. Nanosilver ranges from 1 to 100 nanometers in diameter. Smaller particles more readily enter cells and interact with the cellular components. The exposure dose, particle size, coating, and aggregation state of the nanosilver, as well as the cell type or organism on which it is tested, are all large determining factors on the effect and potential toxicity of nanosilver. A high exposure dose to nanosilver alters the cellular stress responses and initiates cascades of signalling that can eventually trigger organelle autophagy and apoptosis. This review summarizes the current knowledge of the effects of nanosilver on cellular metabolic function and response to stress. Both the causative effects of nanosilver on oxidative stress, endoplasmic reticulum stress, and hypoxic stress—as well as the effects of nanosilver on the responses to such stresses—are outlined. The interactions and effects of nanosilver on cellular uptake, oxidative stress (reactive oxygen species), inflammation, hypoxic response, mitochondrial function, endoplasmic reticulum (ER) function and the unfolded protein response, autophagy and apoptosis, angiogenesis, epigenetics, genotoxicity, and cancer development and tumorigenesis—as well as other pathway alterations—are examined in this review.

Stabilization of Ag-Au Bimetallic Nanocrystals in Aquatic Environments Mediated by Dissolved Organic Matter: A Mechanistic Perspective

Gold and silver nanoparticles can be stabilized endogenously within aquatic environments from dissolved ionic species as a result of mineralization induced by dissolved organic matter. However, the ability of fulvic and humic acids to stabilize bimetallic nanoparticles is entirely unexplored. Elucidating the formation of such particles is imperative given their potential ecological toxicity. Herein, we demonstrate the nucleation, growth, and stabilization of bimetallic Ag-Au nanocrystals from the interactions of Ag⁺ and Au³⁺ with Suwannee River fulvic and humic acids. The mechanisms underpinning the stabilization of Ag-Au alloy NPs at different pH (6.0-9.0) values are studied by UV-vis spectrophotometry, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Complexation of free Ag⁺ and Au³⁺ ions with the Lewis basic groups (carbonyls, carboxyls, and thiols) of FA and HA, followed by electron-transfer from redox-active moieties present in dissolved organic matter initiates the nucleation of the NPs. Alloy formation and interdiffusion of Au and Ag atoms are further facilitated by a galvanic replacement reaction between AuCl₄^- and Ag. Charge-transfer from Au to Ag stabilizes the formed bimetallic NPs. A more pronounced agglomeration of the Ag-Au NPs is observed when HA is used compared to FA as the reducing agent. The bimetallic NPs are stable for greater than four months, which suggests the possible persistence and dispersion of these materials in aquatic environments. The mechanistic ideas have broad generalizability to reductive mineralization processes mediated by dissolved organic matter.

Effect of silver sulfide nanoparticles on photochemical degradation of dissolved organic matter in surface water

Silver sulfide nanoparticles (Ag₂SNPs) have shown photocatalytic activity, yet little is known about the effect of Ag₂SNPs on the photochemical degradation of dissolved organic matter (DOM) in surface water, which seriously impairs understanding of Ag₂SNPs' environmental risks. Herein, this study on the basis of electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) showed for the first time that photodegradation of natural organic matter (NOM, 2R101 N) could be accelerated by both bared and polyvinylpyrrolidone (PVP)-coated Ag₂SNPs; the NOM with Ag₂SNPs (e.g., 500 μg/L) exposed to light irradiation for 96 h showed molecular formulas with lower O/C ratios as compared to the NOM alone. Also, added number of points (ranging from 1 to 2 carboxyl groups) having the same Kendrick mass defect (KMD) (COO) values and higher intensity in smaller Kendrick mass (KM) (COO) values were observed in NOM with Ag₂SNPs compared to NOM alone. However, negligible effects of Ag₂SNPs on photodegradation of humic acid (HA, 2S101H) were observed, even when the concentration of Ag₂SNPs was as high as 5 mg/L. Besides molecular characteristics, a great reduction in organic carbon content of NOM within 96 h was only observed in the presence of Ag₂SNPs under light condition. More importantly, the enhanced photodegradation of DOM by Ag₂SNPs even at a concentration of 100 μg/L was also validated in surface water. These findings suggest that Ag₂SNPs have the potential to accelerate the photochemical degradation of DOM in surface water.