Silver nanoparticle impact on bacterial growth: effect of pH, concentration, and organic matter

Silver nanoparticles as antimicrobial therapeutics: current perspectives and future challenges

Utility of silver metal in antimicrobial therapy is an accepted practice since ages that faded with time because of the identification of a few silver resistant strains in the contemporary era. A successive development of antibiotics soon followed. However, due to an indiscriminate and unregulated use coupled with poor legal control measures and a dearth of expertise in handling the critical episodes, the antibiotics era has already seen a steep decline in the past decades due to the evolution of multi-drug resistant 'superbugs' which pose a sizeable challenge to manage with. Due to limited options in the pipeline and no clear strategy in the forefront, the aspirations for novel, MDR focused drug discovery to target the 'superbugs' arose which once again led to the rise of AgNPs in antimicrobial research. In this review, we have focused on the green routes for the synthesis of AgNPs, the mode of microbial inhibition by AgNPs, synergistic effect of AgNPs with antibiotics and future challenges for the development of nano-silver-based therapeutics.

Impact of chlorination on silver elution from ceramic water filters

Applying silver nanoparticles (nAg) or silver nitrate (AgNO₃) to ceramic water filters improves microbiological efficacy, reduces biofilm formation, and protects stored water from recontamination. A challenge in ceramic filter production is adding sufficient silver to achieve these goals without exceeding the maximum recommended silver concentration in drinking water. Silver release is affected by silver type, application method, and influent water chemistry. Despite a lack of data, there is an assumption that chlorinated water should not be used as influent water because it may increase silver elution. Thus, the objective of this work was to systematically evaluate the impact of chlorinated water (0-4 mg/L free chlorine residual, FCR) on silver release from ceramic filter disks painted with casein-coated nAg, painted with AgNO₃, or containing fired-in nAg over a range of ionic strength (IS = 0-10 mM as NaNO₃) in the presence or absence of natural organic matter (NOM). Influent deionized water containing chlorine increased silver release 2-5-fold compared to controls. However, this effect of chlorine was mitigated at higher IS (≥1 mM) or in the presence of NOM (3 mg C/L). For filter disks painted with nAg or AgNO₃, silver release increased with increasing IS (with or without chlorine), and effluent concentrations remained above the World Health Organization (WHO) guideline of 0.1 mg/L even after 30 h (80 pore volumes, PVs) of flow with a background solution of 10 mM NaNO₃. Silver speciation (nAg vs. Ag⁺) was monitored in effluent samples from painted or fired-in nAg filter disks. Results indicated that in general, greater than 90% of the eluted silver was due to Ag⁺ dissolution rather than nAg release. Additionally, a filter disk prepared with fired-in nAg exhibited a lower % released in the nanoparticle form (nAg = 5% of total Ag in effluent) compared to painted on nAg (nAg = 14% of total Ag in effluent). The findings of this study suggest that chlorinated influent water has minimal impact on silver elution from ceramic filters under simulated natural water conditions, and thus, the recommendation to avoid the use of chlorinated water with ceramic filters is not necessary under most conditions.

Influence of soil porewater properties on the fate and toxicity of silver nanoparticles to Caenorhabditis elegans

Engineered nanoparticles (NPs) entering the environment are subject to various transformations that in turn influence how particles are presented to, and taken up by, organisms. To understand the effect of soil properties on the toxicity of nanosilver to Caenorhabditis elegans, toxicity assays were performed in porewater extracts from natural soils with varying organic matter content and pH using 3-8 nm unfunctionalized silver (Ag 3-8Unf), 52-nm polyvinylpyrrolidone (PVP)-coated Ag NPs (Ag 52PVP), and AgNO₃ as ionic Ag. Effects on NP agglomeration and stability were investigated using ultraviolet-visible (UV-vis) spectroscopy and asymmetric flow field-flow fractionation (AF4); Ag⁺ showed greater overall toxicity than nanosilver, with little difference between the NP types. Increasing soil organic matter content significantly decreased the toxicity of Ag 3-8Unf, whereas it increased that of AgNO₃ . The toxicity of all Ag treatments significantly decreased with increasing porewater pH. Dissolution of both NPs in the porewater extracts was too low to have contributed to their observed toxic effects. The UV-vis spectroscopy revealed low levels of agglomeration/aggregation independent of soil properties for Ag 3-8Unf, whereas higher organic matter as well as low pH appeared to stabilize Ag 52PVP. Overall, both soil organic matter content and pH affected NP fate as well as toxicity to C. elegans; however, there appears to be no clear connection between the measured particle characteristics and their effect. Environ Toxicol Chem 2018;37:2609-2618. © 2018 SETAC.

Antibacterial and biodegradable tissue nano-adhesives for rapid wound closure

BACKGROUND

Although various organic tissue adhesives designed to facilitate would healing are gaining popularity in diverse clinical applications, they present significant inherent limitations, such as rejection, infections, toxicity and/or excessive swelling. It is highly desirable to develop efficient, biocompatible and anti-bacterial tissue adhesives for skin wound healing.

PURPOSE

Inspired by the fact that inorganic nanoparticles can directly glue tissues through the "nanobridging effect", herein disulfide bond-bridged nanosilver-decorated mesoporous silica nanoparticles (Ag-MSNs) was constructed as an effective and safe tissue adhesive with antibacterial and degradable properties for wound closure and healing.

MATERIALS AND METHODS

Ag-MSNs was fabricated by controlled reduce of ultrasmall nanosilvers onto the both surface and large pore of biodegradable MSNs. The obtained MSNs were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and measurement of size distribution, zeta potential, and mesopore properties. Furthermore, adhesion strength test, anti-bacterial assessment, mouse skin wound model, and MTT assays were used to investigate the tissue adhesive property, antibacterial effect, biodegradability and biocompatibility of the Ag-MSNs.

RESULTS

Ag-MSNs exhibited not only strong adhesive properties but also excellent antibacterial activities than that of MSNs. Importantly, this antibacterial nano-adhesive achieved rapid and efficient closure and healing of wounds in comparison to sutures or MSNs in a mouse skin wound model. Furthermore, Ag-MSNs with fast degradable behavior caused little cellular toxicity and even less systemic toxicity during wound healing.

CONCLUSION

Our findings suggest that biodegradable Ag-MSNs can be employed as the next generation of nano-adhesives for rapid wound closure and aesthetic wound healing.

Recombinant Escherichia coli BL21-pET28a-egfp Cultivated with Nanomaterials in a Modified Microchannel for Biofilm Formation

The application of whole cells as catalytic biofilms in microchannels has attracted increasing scientific interest. However, the excessive biomass formation and structure of biofilms in a reactor limits their use. A microchannel reactor with surface modification was used to colonize recombinant Escherichia coil BL21-pET28a-egfp rapidly and accelerated growth of biofilms in the microchannel. The segmented flow system of 'air/culture medium containing nanomaterials' was firstly used to modulate the biofilms formation of recombinant E. coil; the inhibitory effects of nanomaterials on biofilm formation were investigated. The results indicated that the segmental flow mode has a significant impact on the structure and development of biofilms. Using the channels modified by silane reagent, the culture time of biofilms (30 h) was reduced by 6 h compared to unmodified channels. With the addition of graphene sheets (10 mg/L) in Luria-Bertani (LB) medium, the graphene sheets possessed a minimum inhibition rate of 3.23% against recombinant E. coil. The biofilms cultivated by the LB medium with added graphene sheets were stably formed in 20 h; the formation time was 33.33% shorter than that by LB medium without graphene. The developed method provides an efficient and simple approach for rapid preparation of catalytic biofilms in microchannel reactors.

Humic acid attenuation of silver nanoparticle toxicity by ion complexation and the formation of a Ag3+ coating

The use of silver nanoparticles (AgNPs) result in an inevitable contact with aquatic environments. Here we study the behavior of AgNPs and the developmental toxicity in zebrafish embryos exposed to these nanoparticles (0-10 mg/L) with and without the presence of HA (20 mg/L), using zebrafish facility water (ZFW) and zebrafish growing media (ZGM). The presence of cations and HA gave rise to a decrease in Ag ion release and ζ-potential, an increase in the hydrodynamic diameter and oxidation of the AgNP surface. The results show that the presence of HA and cations in the media, as well as the silver speciation, i.e., the unusual presence of Ag³⁺, decreases the toxicity of AgNPs (LC50_AgNPs: 1.19 mg/L; LC50_{AgNPs + HA}: 3.56 mg/L), as well as silver bioavailability and toxicity in zebrafish embryos. Developmental alterations and the LC50 (1.19 mg/L) of AgNPs in ZFW were more relevant (p ≤ 0.05) than for AgNPs in ZGM (LC50 ˃ 10 mg/L). It was demonstrated that the bioaccumulation and toxicity of AgNPs depends on several factors including AgNPs concentration, nanoparticle aggregation, dissolved silver ions, speciation of silver ions, the amount of salt in the environment, the presence of humic substances and others, and different combinations of all of these factors.

Toxicity of silver nanoparticles to Lumbriculus variegatus is a function of dissolved silver and promoted by low sediment pH

Toxicity of silver nanoparticles (AgNPs) to benthic organisms is a major concern. The use of AgNPs in industry and consumer products leads to increasing release of AgNPs into the aquatic environment-sediments being the major sink. Effects of sediment pH on the toxicity of AgNPs to benthic oligochaeta Lumbriculus variegatus were studied in a 23-d toxicity test. Artificially prepared sediments (pH 5 and 7) were spiked with varying concentrations of uncoated AgNP, polyvinylpyrrolidone (PVP)-coated AgNP, and silver nitrate (AgNO₃ ) as dissolved Ag reference. Number of individuals and biomass change were used as endpoints for the toxicity. The toxic effects were related to the bioaccessible concentration of dissolved Ag in the sediments, assessed with a 2-step extraction procedure. The toxicity of 2 AgNPs was similar and greatly enhanced in the acidic sediment. Because the toxic effects were well related to the bioaccessible concentration of dissolved Ag in the sediments, the toxicity of sediment-associated AgNPs to L. variegatus is suggested to be a function of dissolved Ag rather than a result from NP-specific modes of toxicity. Environ Toxicol Chem 2018;37:1889-1897. © 2018 SETAC.

Differential behaviors of silver nanoparticles and silver ions towards cysteine: Bioremediation and toxicity to Phanerochaete chrysosporium

Potential transformations of silver nanoparticles (AgNPs) upon interaction with naturally ubiquitous organic ligands in aquatic environments influence their transport, persistence, bioavailability, and subsequent toxicity to organisms. In this study, differential behaviors of AgNPs and silver ions (Ag⁺) towards cysteine (Cys), an amino acid representative of thiol ligands that easily coordinate to Ag⁺ and graft to nanoparticle surfaces, were investigated in the aspects of bioremediation and their toxicity to Phanerochaete chrysosporium. Total Ag removal, 2,4-dichlorophenol (2,4-DCP) degradation, extracellular protein secretion, and cellular viability were enhanced to some extent after supplement of various concentrations of cysteine under stress of AgNPs and Ag⁺. However, an obvious decrease in total Ag uptake was observed after 5-50 μM cysteine addition in the groups treated with 10 μM AgNPs and 1 μM Ag⁺, especially at a Cys:Ag molar ratio of 5. More stabilization in uptake pattern at this ratio was detected under Ag⁺ exposure than that under AgNP exposure. Furthermore, in the absence of cysteine, all Ag⁺ treatments stimulated the generation of reactive oxygen species (ROS) more significantly than high-dose AgNPs did. However, cysteine supply under AgNP/Ag⁺ stress aggravated ROS levels, albeit alleviated at 100 μM Ag⁺, indicating that the toxicity profiles of AgNPs and Ag⁺ to P. chrysosporium could be exacerbated or marginally mitigated by cysteine. The results obtained were possibly associated with the lability and bioavailability of AgNP/Ag⁺-cysteine complexes.

Partitioning of Ag and CeO2 nanoparticles versus Ag and Ce ions in soil suspensions and effect of natural organic matter on CeO2 nanoparticles stability

This study examined the solid-liquid distribution of 14.8-nm Ag and 6.2-nm CeO₂ nanoparticles in soil suspensions and compared it to that of Ag⁺ and Ce³⁺ ions, to better understand their environmental behaviour and fate. After 24 h incubation, more than 51% or 29% of the spiked amounts of Ag or CeO₂ nanoparticles, respectively, can be retrieved in the liquid phase of (re)suspended soils. The Ag or Ce concentration remaining in solution depends on the incubation time and was influenced by soil properties. Significant correlations are obtained between, on the one hand, the relative amounts of Ag or CeO₂ nanoparticles in suspension and the soil-pH, CEC, texture, suspended matter, nitrogen, phosphorus, TOC and main and trace elements content on the other hand. The presence of dissolved natural organic matter stabilizes CeO₂ nanoparticles in the aqueous phase. In soil suspensions, Ag⁺ and Ce³⁺ ions seemingly interact more strongly with soil constituents compared to their nanoparticle counterparts, rendering the Ag and CeO₂ nanoparticles to be more stable and potentially bioavailable.

Disinfection of waterborne viruses using silver nanoparticle-decorated silica hybrid composites in water environments

Silver nanoparticles (AgNPs) have been reported as an effective alternative for controlling a broad-spectrum of pathogenic viruses. We developed a micrometer-sized silica hybrid composite decorated with AgNPs (AgNP-SiO₂) to prevent the inherent aggregation of AgNPs, and facilitated their recovery from environmental media after use. The production process had a high-yield, and fabrication was cost-effective. We evaluated the antiviral capabilities of Ag30-SiO₂ particles against two model viruses, bacteriophage MS2 and murine norovirus (MNV), in four different types of water (deionized, tap, surface, and ground). MNV was more susceptible to Ag30-SiO₂ particles in all four types of water compared to MS2. Furthermore, several water-related factors, including temperature and organic matter content, were shown to affect the antimicrobial capabilities of Ag30-SiO₂ particles. The modified Hom model was the best-fit disinfection model for MNV disinfection in the different types of water. Additionally, this study demonstrated that the effects of a certain level of physical obstacles in water were negligible in regards to the use of Ag30-SiO₂ particles. Thus, effective use of AgNPs in water disinfection processes can be achieved using our novel hybrid composites to inactivate various waterborne viruses.

Comparison of Iris pseudacorus wetland systems with unplanted systems on pollutant removal and microbial community under nanosilver exposure

Rapidly developing industry raises concerns about the environmental risks of silver nanoparticles (AgNPs), but the effects of AgNPs on the performance and microbial community in the constructed wetlands remain unclear. In this study, long-term exposure of AgNPs in two VFCWs was conducted to determine the effects of AgNPs on the pollutant removal and microbial community structure. Before exposing AgNPs, the water quality of effluent was better in planted wetland (CW2), compared with unplanted wetland (CW1). After continuous exposure of 100μg/L AgNPs, the COD (chemical oxygen demand) removal of two CWs had no difference. However, addition of AgNPs reduced the nitrogen and phosphorus removal in two CWs, with decreasing average removal efficiencies of ammonia nitrogen from 46.31% to 32.09% and 59.66% to 51.06%, total nitrogen from 57.76% to 43.78% and 67.35 to 60.58%, total phosphorus from 71.29% to 59.31% and 67.35% to 60.58%, respectively. The vegetable wetlands showed higher resistances to AgNPs loading than unplanted wetlands. In addition, AgNPs accumulated in the wetland substrate, especially in the soil layer with the silver concentration of approximately 4.32μg/g. The small portion of silver was found in plant tissues, and plants played a minor role to remove the AgNPs from wastewater. Moreover, the constructed wetlands could effectively remove the AgNPs from the synthetic wastewater. The illumine high-throughput sequencing results demonstrated the variations of the bacterial community structure at the exposure of AgNPs. The results showed that the dominant phyla were Proteobacteria, Acidobacteria and Bacteroidetes. Compared with unplanted wetlands, the contents of several nitrifying bacteria such as Candidatus Nitrososphaera (AOA) and Nitrospira (NOB) at genus level increased, leading to the higher nitrogen removal in the planted wetlands.

Toxicity of lanthanum oxide nanoparticles to the fungus Moniliella wahieum Y12T isolated from biodiesel

Moniliella wahieum Y12^T, isolated from biodiesel was used as a model organism to assess the use of lanthanum oxide (La₂O₃) (60-80 nm) and silver oxide (AgO) (10-40 nm) nanoparticles as potential fungal inhibitors. This is the first study to investigate the use of nanoscale La₂O₃ as a eukaryotic bio-inhibitor. The AgO nanoparticles were relatively effective at inhibiting the growth of M. wahieum Y12^T. The half maximal effective concentration (EC₅₀) for AgO was 0.012 mg/mL as compared with 4.63 mg/mL of La₂O₃. Fluorescein diacetate analysis showed that AgO nanoparticles significantly reduced metabolic activity in M. wahieum Y12^T. The results of this study indicated that AgO nanoparticles can be a nonspecific inhibitor for the treatment of M. wahieum Y12^T, a eukaryotic biodiesel contaminant.

Effects of Chloride Ions on Dissolution, ROS Generation, and Toxicity of Silver Nanoparticles under UV Irradiation

This work investigates the effect of chloride ion (Cl^-) on dissolution, reactive oxygen species (ROS) generation, and toxicity of citrate-coated silver nanoparticles (AgNPs) under UV irradiation. The dissolution rate was decreased by 0.01 M Cl^- due to AgCl passivation on the AgNP surface. By contrast, high concentrations of Cl^- (0.1 or 0.5 M) promoted dissolution due to the formation of soluble Ag-Cl complexes (AgCl _x^{1- x}). The generation of O₂^•- in the AgNPs/Cl^-/UV system was promoted by 0.01 M Cl^-, whereas it was retarded by 0.1 or 0.5 M Cl^-, which was probably because the aggregation of AgNPs at high ionic strength reduced the nanoparticles' surface areas for radical formation. Additionally, Cl^- contributed to •OH generation in the AgNPs/Cl^-/UV system, in which the produced •OH concentrations increased with increasing Cl^- concentrations. The reduction reaction between silver ions and O₂^•- resulted in lower dissolution rates of AgNPs/Cl^- mixtures under UV irradiation than those in the dark. The phototoxicity of AgNPs toward E. coli with different concentrations of Cl^- followed the order of 0.5 M > 0 M > 0.1 M > 0.01 M. Both ROS and dissolved Ag played significant role in the phototoxicity of AgNPs. This work demonstrates the potential importance of anions in the fate and biological impact of AgNPs.

Comparative Persistence of Engineered Nanoparticles in a Complex Aquatic Ecosystem

During nanoparticle environmental exposure, presence in the water column is expected to dominate long distance transport as well as initial aquatic organism exposure. Much work has been done to understand potential ecological and toxicological effects of these particles. However, little has been done to date to understand the comparative persistence of engineered particles in realistic environmental systems. Presented here is a study of the water column lifetimes of 3 different classes of nanoparticles prepared with a combination of surface chemistries in wetland mesocosms. We find that, when introduced as a single pulse, all tested nanoparticles persist in the water column for periods ranging from 36 h to 10 days. Specifically, we found a range of nanoparticle residence times in the order Ag > TiO₂ > SWCNT > CeO₂. We further explored the hypothesis that heteroaggregation was the primary driving factor for nanoparticle removal from the water column in all but one case, and that values of surface affinity (α) measured in the laboratory appear to predict relative removal rates when heteroaggregation dominates. Though persistence in the water column was relatively short in all cases, differences in persistence may play a role in determining nanoparticle fate and impacts and were poorly predicted by currently prevailing benchmarks such as particle surface preparation.

Threshold Concentrations of Silver Ions Exist for the Sunlight-Induced Formation of Silver Nanoparticles in the Presence of Natural Organic Matter

Sunlight-induced photoformation of silver nanoparticles (nAg), mediated by natural organic matter (NOM), is significantly affected by the concentration of Ag(I) and chloride. The initial photoformation rates of nAg in Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM) solutions were examined under simulated sunlight irradiation. A critical induction concentration (CIC) of Ag(I) (10 mg/L for SRHA and 5 mg/L for SRNOM, respectively) was observed, below which the nAg formation was minimal. The threshold is attributed to the interplay of reduction and oxidation reactions mediated by NOM, reflecting the need to achieve sufficiently fast growth of silver clusters to outcompete oxidative dissolution. The CIC can be reduced by scavenging oxidative radicals or be increased by promoting singlet oxygen and hydrogen peroxide generation. The presence of chloride effectively reduced the CIC by forming AgCl, which facilitates reduction reactions and provides deposition surfaces. SRNOM is more efficient in mediating photoformation of nAg than SRHA, owing to their differed phototransient generation. These results highlight prerequisites for the photoformation of nAg mediated by NOM, in which the photochemistry and solution chemistry are both important.

The decay of silver nanoparticles in preoxidation process

To investigate the fate of metal-based nanoparticles in water oxidation treatment processes, the decay of Ag-NPs in the presence of three kinds of water treatment preoxidants, sodium hypochlorite (NaClO), hydrogen peroxide (H₂O₂) and potassium permanganate (KMnO₄), was investigated in this work. Dissolution of Ag-NPs into silver ions (Ag⁺) was found to occur under exposure to NaClO, H₂O₂ and KMnO₄. The morphology of Ag-NPs changed after reacting with NaClO, H₂O₂ and KMnO₄. Factors affecting the decay of Ag-NPs, i.e., the dosage of oxidants, pH, the presence of humic acid, typical ions in water, and the size of the nanoparticles, were investigated. A higher dosage of oxidants, the presence of calcium ions, and lower size of Ag-NPs promoted the decay of Ag-NPs. The presence of humic acid and sulfide ions inhibited the decay of Ag-NPs. The decay of Ag-NPs under exposure to oxidants was significantly affected by the pH. The mechanism of the Ag-NPs in the presence of oxidants under different environmental conditions is also discussed.

High conversion synthesis of <10 nm starch-stabilized silver nanoparticles using microwave technology

A microwave reaction to convert 99 ± 1% of Ag+ to silver nanoparticles (AgNPs) of size <10 nm within 4.5 min with a specific production rate and energy input of 5.75 mg AgNP L-1 min-1 and 5.45 W mL-1 reaction volume was developed. The glucose reduced and food grade starch stabilized particles remained colloidally stable with less than a 4% change in the surface plasmon resonance band at 425-430 nm at t > 300 days. TEM determined the size of AgNPs, while TEM-EDS and XRD verified elemental composition. The conversion was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and thermal gravimetric analysis (TGA). Additionally, the required silver to starch input mass ratio, 1.0:1.3, to produce colloidally stabilized AgNPs is significantly reduced compared to previous studies. The antibacterial activity of freshly prepared AgNPs and AgNPs aged >300 days was demonstrated against E. coli as determined by agar diffusion assays. This result, corroborated by spectrophotometric and TEM measurements, indicates long-term colloidal stability of the product. Thus, this study sustainably produced antibacterial AgNPs from minimal inputs. In the broader context, the current work has quantified a sustainable platform technology to produce sphere-like inorganic nanoparticles with antimicrobial properties.

Distinct toxicity of silver nanoparticles and silver nitrate to Daphnia magna in M4 medium and surface water

Toxicity of silver nanoparticles (AgNPs) has been studied in various culture media. However, these media notably differ from the natural aquatic system, thus the conclusions may be inapplicable for real environment condition. The toxicity and its underlying mechanism of AgNPs in surface waters warrant more investigations. This study investigated the acute toxicity, chronic toxicity, bioaccumulation, and alga-daphnia food chain transfer of citrate-coated AgNPs (C-AgNPs) and Ag⁺ (from AgNO₃) to D. magna in a culture medium (M4) and a surface water sample. Results show that the acute toxicity in the surface water was significantly lower than that in the M4 medium and the toxicity of Ag⁺ was greatly higher than that of C-AgNPs. The 48h median effect concentration (EC₅₀) of C-AgNPs to D. magna in the M4 medium and the surface water was 110±9.3μg/L and 270±26μg/L, respectively, while that of Ag⁺ was 1.8±0.7μg/L and 8.0±0.6μg/L, respectively. The released Ag⁺ contributed to but not dominated the acute toxicity of C-AgNPs. At the EC₅₀ of C-AgNPs, the contribution of released Ag⁺ was 35.7% and 28.0% to the apparent nanotoxicity in the M4 medium and the surface water sample, respectively. The chronic toxicity of C-AgNPs and Ag⁺ was also lower in the surface water sample than in the M4 medium as indicated by the significantly higher survival of daphnia in the surface water during the 21d exposure. The daphnia took up less but depurated more Ag in the surface water than in the M4 medium, which could account for the lower toxicity in the surface water. Biological magnification of Ag through the alga-daphnia food chain was not observed. These findings will be helpful for assessing the environmental risk of AgNPs and understanding the mechanism of nanotoxcity.

A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver Nanoparticles on Hydrated Mineral Surfaces

Although minerals are known to affect the environmental fate and transformation of heavy-metal ions, little is known about their interaction with the heavily exploited silver nanoparticles (AgNPs). Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric methods for imaging the distribution of AgNPs on various mineral surfaces and their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was tested on two macro- and microsized mineral models, muscovite [KAl₂(AlSi₃O₁₀)(OH)₂] and corundum (α-Al₂O₃), under key environmental conditions (ionic strength and pH). Both AgNPs^- and AgNPs⁺ were found to covalently attach to corundum (pH_pzc = 9.1) through the formation of Ag-O-Al- bonds and thereby to potentially experience reduced environmental mobility. Because label-free Raman imaging showed no molecular interactions between AgNPs^- and muscovite (pH_pzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce spatial distribution of AgNPs^- on such mineral surfaces. Raman maps comprising of n = 625-961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source software, and the results were confirmed via ICP-OES, AFM, and SEM-EDX. The proposed Raman-based imaging requires minimum to no sample preparation; is sensitive, noninvasive, cost-effective; and might be extended to other environmentally relevant systems.

Organic matter modifies biochemical but not most behavioral responses of the clam Ruditapes philippinarum to nanosilver exposure

Adsorption of dissolved organic matter (DOM) can alter the environmental fate, bioavailability and toxicity of silver nanoparticles (Ag NPs). However, a number of questions remain about DOM's ability to modify nanotoxicity. Here, we examine the impact of humic acid (HA, as a model DOM) on the toxicity of Ag NPs (10 μg L^-1) in the marine clam Ruditapes philippinarum. Results showed that DOM additions to Ag NP treatments reduce clam silver tissue burdens and the oxidative stress response. However, HA does not significantly affect the impact of Ag NPs on clam acetylcholinesterase activity and feeding behavior (measured as filtration rate). Overall, the integrated biological response index supports the conclusion that humic acid reduces the toxicity of Ag NPs, clearly indicating the importance of considering environmental factors when assessing potential risks posed by nanomaterials in natural settings.

Contradictory effects of silver nanoparticles on activated sludge wastewater treatment

Increased amount of nano-silver will be released into domestic and industrial waste streams due to its extensive application. However, great controversy still exists on the effects of silver nanoparticle (Ag-NP) on biological wastewater treatment processes and a toxicology model has not been built yet. Four sequencing batch reactors with activated sludge has been run for over three months with different silver species at a concentration of 1mg Ag/L in influent. Both freshly prepared Ag-NPs and aged Ag-NPs were tested with released silver ion as control. Results in this study showed that Ag-NPs, especially freshly prepared Ag-NPs, can help to maintain or even increase the diversity of microbial community in activated sludge and the biomass concentration even under long-term treatment. It indicates that the hormesis model need to be considered for the toxicology of Ag-NPs.

Impact of water chemistry on the particle-specific toxicity of copper nanoparticles to Daphnia magna

Toxicity of metallic nanoparticle suspensions (NP_(total)) is generally assumed to result from the combined effect of the particles present in suspensions (NP_(particle)) and their released ions (NP_(ion)). Evaluation and consideration of how water chemistry affects the particle-specific toxicity of NP_(total) are critical for environmental risk assessment of nanoparticles. In this study, it was found that the toxicity of Cu NP_(particle) to Daphnia magna, in line with the trends in toxicity for Cu NP_(ion), decreased with increasing pH and with increasing concentrations of divalent cations and dissolved organic carbon (DOC). Without the addition of DOC, the toxicity of Cu NP_(total) to D. magna at the LC50 was driven mainly by Cu NP_(ion) (accounting for ≥53% of the observed toxicity). However, toxicity of Cu NP_(total) in the presence of DOC at a concentration ranging from 5 to 50mg C/L largely resulted from the NP_(particle) (57%-85%), which could be attributable to the large reduction of the concentration of Cu NP_(ion) and the enhancement of the stability of Cu NP_(particle) when DOC was added. Our results indicate that water chemistry needs to be explicitly taken into consideration when evaluating the role of NP_(particle) and NP_(ion) in the observed toxicity of NP_(total).

Chemical transformation of silver nanoparticles in aquatic environments: Mechanism, morphology and toxicity

Silver nanoparticles (Ag NPs) have been inevitably introduced into ecological environment during their extensive applications in daily human life. Thermodynamically, Ag NPs are unstable and transform into other species under various aqueous conditions. Ag NPs and their transformation products pose potential threats to environment and humans. However, the complex environmental conditions and transformations of Ag NPs complicate their human health and environmental risk assessment. To bridge the knowledge gap, four essential environmental transformations, oxidative dissolution, sulfidation, chlorination and photoreduction, of Ag NPs are reviewed herein. The mechanism, morphology and size change, as well as the toxicity of Ag NPs during these transformations under certain aqueous conditions are detailed. In particular, these environmental transformations have shown strong correlations that are discussed. The transformation, fate, bioavailability, morphology and toxicity of Ag NPs are critical factors and should be considered in a complete human health and environmental risk assessment of Ag NPs. The fluctuation of these factors in the realistic environment is also vital and should be considered.

Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects

Dissolved organic matter (DOM) is ubiquitous in the environment and has high reactivity. Once engineered nanoparticles (ENPs) are released into natural systems, interactions of DOM with ENPs may significantly affect the fate and transport of ENPs, as well as the bioavailability and toxicity of ENPs to organisms. However, because of the complexity of DOM and the shortage of useful characterization methods, large knowledge gaps exist in our understanding of the interactions between DOM and ENPs. In this article, we systematically reviewed the interactions between DOM and ENPs, discussed the effects of DOM on the environmental behavior of ENPs, and described the changes in bioavailability and toxicity of ENPs caused by DOM. Critical evaluations of published references suggest further need for assessing and predicting the influences of DOM on the transport, transformation, bioavailability, and toxicity of ENPs in the environment.

Acute ecotoxicity of coated colloidal goethite nanoparticles on Daphnia magna: Evaluating the influence of exposure approaches

Synthesized iron oxide nanoparticles have been proposed as an alternative to non-dispersed iron oxides for in situ environmental remediation. Their colloidal properties enable their injection into porous media, i.e. soils and aquifers, and offer a higher efficiency in removing contaminants. However, this dispersed state is also the cause of concerns over their environmental fate and toxicity, e.g., by increasing the exposure time to aquatic organisms in groundwater remediation activities. Therefore, the objective of in situ groundwater remediation is to establish local reactive barriers in the subsurface by injection by means of reactive colloids with a controllable mobility under in situ conditions and present as colloids as shortly as possible. In this work, we examined the toxicity of humic acid-coated colloidal goethite nanoparticles in Daphnia magna. The adaptation of the ecotoxicological standard tests for nanomaterials is intensely discussed to increase comparability and reliability of results. In the present study, the effect of different exposure conditions on goethite nanoparticles colloidal behaviour and acute Daphnia immobilization effects was investigated. For this purpose, iron concentration in the water column, aggregation state and acute effects were studied in: i) a standard test, ii) test design with exposure dispersions incubated for a week and iii) water accommodated fraction. Despite the different aggregation and settling of the particles found between the approaches tested, no differences in toxicity were observed. Coated nanoparticles were found clogging up the filtering apparatus, and/or adhered to the exoskeleton, hindering the swimming and molting, and causing the immobilization and death of the organisms at doses of ≥943mg/L (EC50). The data suggests that the toxic potential of these nanoparticles is mainly related to the physical interaction with the daphnids.

Influence of Interaction Between α-Fe2O3 Nanoparticles and Dissolved Fulvic Acid on the Physiological Responses in Synechococcus sp. PCC7942

The ecotoxicity of α-Fe₂O₃ nanoparticles (NPs) and its interaction with a typical natural organic matter (NOM), fulvic acid (FA) on the physiological responses of Synechococcus sp. PCC7942 was studied. α-Fe₂O₃ NPs inhibited the algae growth at concentration higher than 10 mg L^-1 and induced oxidative stress, indicated by enhanced antioxidant enzymes activities, elevated protein and sugar content. FA could efficiently recover cell growth and reduce antioxidant enzyme activities which induced by α-Fe₂O₃ NPs, indicating the toxicity of NPs was alleviated in the presence of FA. α-Fe₂O₃ NPs could form large aggregates coating on cell surface and inhibit cell growth. FTIR spectra verified FA interacted with α-Fe₂O₃ NPs through carboxyl groups, partly replaced the binding sites of α-Fe₂O₃ NPs on algal cell walls, thus reduced NPs aggregates coating on cell surface. This favors reducing the oxidative stress caused by direct contact and increasing light availability, thus mitigate NPs toxicity.

Temperature modulates AgNP impacts on microbial decomposer activity

Silver nanoparticles (AgNP)s can have toxic effects on aquatic species and compromise important ecosystem processes. AgNP impacts have been the focus of much research, but their effects under different environmental contexts, such as the increase in global temperature are difficult to predict. The aim of this study was to evaluate the interactive effects of AgNPs and temperature on the activity and diversity of microbial decomposers of plant litter in streams. Litter-associated microbial communities were exposed in microcosms to increased concentrations of AgNPs (50 to 75000μgL^-1) and AgNO₃ (5 to 7500μgL^-1) and kept for 21days at 10°C, 16°C and 23°C. Effects of AgNPs and AgNO₃ were assessed based on leaf mass loss and litter-associated microbial communities by measuring microbial diversity, the activity of fungal extracellular enzymes, and fungal biomass and reproduction. Increase in temperature stimulated leaf mass loss, but not fungal biomass and reproduction. Increased AgNP and AgNO₃ concentrations inhibited fungal reproduction and diversity, particularly at 23°C. Activities of the extracellular enzymes phenol oxidase and β-glucosidase were generally higher at 23°C. Microbial communities were mainly structured by AgNP and AgNO₃ concentrations more than by temperature. The negative effects of nano and ionic Ag on microbial activity were more pronounced at 10 and 23°C. The behavior of AgNPs was more related to water physical and chemical characteristics (pH) than to temperature. Results highlight the importance of considering different environmental scenarios when examining NP toxicity to freshwater biota and ecosystem processes.

Bio-nano interface and environment: A critical review

The bio-nano interface is the boundary where engineered nanomaterials (ENMs) meet the biological system, exerting the biological function for which they have been designed or inducing adverse effects on other cells or organisms when they reach nontarget scenarios (i.e., the natural environment). Research has been performed to determine the fate, transport, and toxic properties of ENMs, but much of it is focused on pristine or so-called as-manufactured ENMs, or else modifications of the materials were not assessed. We review the most recent progress regarding the bio-nano interface and the transformations that ENMs undergo in the environment, paying special attention to the adsorption of environmental biomolecules on the surface of ENMs. Whereas the protein corona has received considerable attention in the fields of biomedics and human toxicology, its environmental analogue (the eco-corona) has been much less studied. A section dedicated to the analytical methods for studying and characterizing the eco-corona is also presented. We conclude by presenting and discussing the key problems and knowledge gaps that need to be resolved in the near future regarding the bio-nano interface and the eco-corona. Environ Toxicol Chem 2017;36:3181-3193. © 2017 SETAC.

Effects of humic acid on the interactions between zinc oxide nanoparticles and bacterial biofilms

The effects of humic acid (HA) on interactions between ZnO nanoparticles (ZnO NPs) and Pseudomonas putida KT2440 biofilms at different maturity stages were investigated. Three stages of biofilm development were identified according to bacterial adenosine triphosphate (ATP) activity associated with biofilm development process. In the initial biofilm stage 1, the ATP content of bacteria was reduced by more than 90% when biofilms were exposed to ZnO NPs. However, in the mature biofilm stages 2 and 3, the ATP content was only slightly decreased. Biofilms at stage 3 exhibited less susceptibility to ZnO NPs than biofilms at stage 2. These results suggest that more mature biofilms have a significantly higher tolerance to ZnO NPs compared to young biofilms. In addition, biofilms with intact extracellular polymeric substances (EPS) showed higher tolerance to ZnO NPs than those without EPS, indicating that EPS play a key role in alleviating the toxic effects of ZnO NPs. In both pure ZnO NPs and ZnO-HA mixtures, dissolved Zn²⁺ originating from the NPs significantly contributed to the overall toxicity. The presence of HA dramatically decreased the toxicity of ZnO NPs due to the binding of Zn²⁺ on HA. The combined results from this work suggest that the biofilm maturity stages and environmental constituents (such as humic acid) are important factors to consider when evaluating potential risks of NPs to ecological systems.

Tracking the Transformation of Nanoparticulate and Ionic Silver at Environmentally Relevant Concentration Levels by Hollow Fiber Flow Field-Flow Fractionation Coupled to ICPMS

It is a great challenge to monitor the physical and chemical transformation of nanoparticles at environmentally relevant concentration levels, mainly because the commonly used techniques like dynamic light scattering and transmission electron microscopy are unable to characterize and quantify trace level nanoparticles in complex matrices. Herein, we demonstrate the on-line coupled system of hollow fiber flow field-flow fractionation (HF5), minicolumn concentration, and inductively coupled plasma mass spectrometry (ICPMS) detection as an efficient approach to study the aggregation and chemical transformation of silver nanoparticles (AgNPs) and ionic Ag species in the aqueous environment at ng/mL levels. Taking advantage of the in-line dialysis of HF5, the selective capture of Ag(I) species by the resin in minicolumn, and the high selectivity and sensitivity of ICPMS detection, we recorded the aggregation of 10 ng/mL AgNPs in complex matrices (e.g., NOM, Na⁺/Ca²⁺), revealing an interesting tiny AgNPs formation process of photoreduction of trace level Ag(I) that is different from larger AgNPs generated at high concentration of Ag(I) by accurate characterization and respectively identifying and quantifying new thiol-complexed Ag(I) and residual Ag(I) in the intertransformation of Ag(I) and AgNPs in domestic wastewater by simultaneously detecting the S and Ag signals via ICPMS.

Stability of single dispersed silver nanoparticles in natural and synthetic freshwaters: Effects of dissolved oxygen

Silver nanoparticles (AgNPs) are increasingly used in various commercial products. This increased use raises ecological concerns because of the large release of AgNPs into the environment. Once released, the local water chemistry has the potential to influence the environmental fates and behaviors of AgNPs. The impacts of dissolved oxygen and natural organic matter (NOM) on the dissolution and stability of AgNPs were investigated in synthetic and natural freshwaters for 7 days. In synthetic freshwater, the aggregation of AgNPs occurred due to the compression of the electric double layer, accompanied by the dissolution of AgNPs. However, once oxygen was removed, the highest dissolved Ag (Ag_dis) concentration decreased from 356.5 μg/L to 272.1 μg/L, the pH of the AgNP suspensions increased from less than 7.6 to more than 8.4, and AgNPs were regenerated by the reduction of released Ag⁺ by citrate. The addition of NOM mitigated aggregation, inhibited oxidative dissolution and induced the transformation of AgNPs into Ag₂S due to the formation of NOM-adsorbed layers, the reduction of Ag⁺ by NOM, and the high affinity of sulfur-enriched species in NOM for Ag. Likewise, in oxygen-depleted natural freshwaters, the inhibition of oxidative dissolution was obtained in comparison with oxygenated freshwaters, showing a decrease in the maximum Ag_dis concentration from 137.6 and 57.0 μg/L to 83.3 and 42.4 μg/L from two natural freshwater sites. Our results suggested that aggregation and dissolution of AgNPs in aquatic environments depend on the chemical composition, where oxygen-depleted freshwaters more significantly increase the colloidal stability. In comparison with oxic conditions, anoxic conditions were more favorable to the regeneration of AgNPs by reducing species (e.g., citrate and NOM) and enhanced the stability of nanoparticles. This indicates that some AgNPs will be more stable for long periods in oxygen-deprived freshwaters, and pose more serious environmental risks than that in oxygenated freshwaters.

Relative importance of humic and fulvic acid on ROS generation, dissolution, and toxicity of sulfide nanoparticles

In this study, the effect of natural organic matter (NOM) composition (humic acid (HA) or fulvic acid (FA)) on dissolution, reactive oxygen species (ROS) generation, and toxicity of sulfide nanoparticles (NPs) was investigated under UV irradiation. NOM acted as a UV filter or antioxidant, decreasing ROS (O₂^-, OH, and ¹O₂) generation by WS₂ and MoS₂ NPs. The higher light-absorbing fractions of HA in NP/HA mixtures and the faster reaction rate of HA with ROS resulted in higher inhibition effect of HA than FA on O₂^- and OH generation by WS₂ and MoS₂ NPs. Both HA and FA completely inhibited ¹O₂ generation by WS₂ and MoS₂ NPs. NOM could transfer electrons to CdS and promote its O₂^- generation. No measurable amount of OH was generated by CdS with or without NOM. FA decreased ¹O₂ generation by CdS more significantly than HA due to the higher reaction rate between FA and ¹O₂. HA showed a higher inhibition effect on the dissolution rate of CdS and WS₂ NPs than FA. Both HA and FA played minor roles in the toxicity of CdS toward Escherichia coli but decreased the toxicity of MoS₂ and WS₂ due to the reduced ROS generation and/or dissolution concentrations.

Response of anaerobic membrane bioreactor to the presence of nano-Bi2WO6: reactor performance, supernatant characteristics, and microbial community

Considering the increasing incorporation of manufactured nano-material into consumer products, there is a concern about its potential impacts in biological wastewater treatment. In this study, the response of anaerobic sludge to the presence of Bi₂WO₆ nano-particles (NPs) was investigated in the anaerobic membrane bioreactor (AnMBR). As the concentration of Bi₂WO₆ in the reactor was controlled around 1 mg/L, there was no significant difference in effluent water quality or bacterial activities before and after NP exposure, partially due to the microbial-induced NP aggregation and stable complex formation. However, with the increasing dosage of Bi₂WO₆ from 5 to 40 mg/L, great influences on the AnMBR performance were observed, including the reduction of COD removal efficiency, inhibition of the mechanization step, increased production of soluble microbial products, and enhanced secretion of extracellular polymer substrates. Additional investigation with high-throughput sequencing was conducted, clearly demonstrating that Bi₂WO₆ NPs induced changes in the bacterial community.

Diospyros assimilis root extract assisted biosynthesised silver nanoparticles and their evaluation of antimicrobial activity

The current research study focuses on biosynthesis of silver nanoparticles (Ag NPs) for the first time from silver acetate employing methanolic root extract of Diospyros assimilis . The UV–Vis absorption spectrum of biologically synthesised nanoparticles displayed a surface plasmon peak at 428 nm indicating the formation of Ag NPs. The influence of metal ion concentration, reaction time and amount of root extract in forming Ag NPs by microscopic and spectral analysis was thoroughly investigated. Structural analysis from transmission electron microscopy confirmed the nature of metallic silver as face‐centered cubic (FCC) crystalline with an average diameter of 17 nm, which correlates with an average crystallite size (19 nm) calculated from X‐ray diffraction analysis. Further, the work was extended for the preliminary examination of antimicrobial activity of biologically synthesised Ag NPs that displayed promising activity against all the tested pathogenic strains.

Effect of Iron Oxide Nanoparticles and Amoxicillin on Bacterial Growth in the Presence of Dissolved Organic Carbon

The impact of emerging contaminants in the presence of active pharmaceutical pollutants plays an important role in the persistence and activity of environmental bacteria. This manuscript focuses on the impact of amoxicillin functionalized iron oxide nanoparticles on bacterial growth, in the presence of dissolved organic carbon (humic acid). The impact of these emerging contaminants individually and collectively on the growth profiles of model gram positive and negative bacteria was tracked for 24 h. Results indicate exposure to subinhibitory concentrations of amoxicillin bound iron oxide nanoparticles, in the presence of humic acid, increase bacterial growth in Pseudomonas aeruginosa and Staphylococcus aureus. Accelerated bacterial growth was associated with an increase in iron ions, which have been shown to influence upregulation of cellular metabolism. Though iron oxide nanoparticles are often regarded as benign, this work demonstrates the distinguishable impact of amoxicillin bound iron oxide nanoparticles in the presence of dissolved organic carbon. The results indicate differential impacts of combined contaminants on bacterial growth, having potential implications for environmental and human health.

Natural marine bacteria as model organisms for the hazard-assessment of consumer products containing silver nanoparticles

Scarce information is available regarding the fate and toxicology of engineered silver nanoparticles (AgNPs) in the marine environment, especially when compared to other environmental compartments. Hence, the antibacterial activity of the NM-300 AgNPs (OECD programme) and a household product containing colloidal AgNPs (Mesosilver) was investigated using marine bacteria, pure cultures and natural mixed populations (microcosm approach). Bacterial susceptibility to AgNPs was species-specific, with Gram negative bacteria being more resistant than the Gram positive species (NM-300 concentration used ranged between 0.062 and 1.5 mg L^-1), and the Mesosilver product was more toxic than the NM-300. Bacterial viability and the physiological status (O₂ uptake measured by respirometry) of the microbial community in the microcosm was negatively affected at an initial concentration of 1 mg L^-1 NM-300. The high chloride concentrations in the media/seawater led to the formation of silver-chloro complexes thus enhancing AgNP toxicity. We recommend the use of natural marine bacteria as models when assessing the environmental relevant antibacterial properties of products containing nanosilver.

Exploring the Behavior and Metabolic Transformations of SeNPs in Exposed Lactic Acid Bacteria. Effect of Nanoparticles Coating Agent

The behavior and transformation of selenium nanoparticles (SeNPs) in living systems such as microorganisms is largely unknown. To address this knowledge gap, we examined the effect of three types of SeNP suspensions toward Lactobacillus delbrueckii subsp. bulgaricus LB-12 using a variety of techniques. SeNPs were synthesized using three types of coating agents (chitosan (CS-SeNPs), hydroxyethyl cellulose (HEC-SeNPs) and a non-ionic surfactant, surfynol (ethoxylated-SeNPs)). Morphologies of SeNPs were all spherical. Transmission electron microscopy (TEM) was used to locate SeNPs in the bacteria. High performance liquid chromatography (HPLC) on line coupled to inductively coupled plasma mass spectrometry (ICP-MS) was applied to evaluate SeNP transformation by bacteria. Finally, flow cytometry employing the live/dead test and optical density measurements at 600 nm (OD600) were used for evaluating the percentages of bacteria viability when supplementing with SeNPs. Negligible damage was detected by flow cytometry when bacteria were exposed to HEC-SeNPs or CS-SeNPs at a level of 10 μg Se mL-1. In contrast, ethoxylated-SeNPs were found to be the most harmful nanoparticles toward bacteria. CS-SeNPs passed through the membrane without causing damage. Once inside, SeNPs were metabolically transformed to organic selenium compounds. Results evidenced the importance of capping agents when establishing the true behavior of NPs.

Silver-Functionalized Bacterial Cellulose as Antibacterial Membrane for Wound-Healing Applications

Bacterial cellulose (BC) functionalized with silver nanoparticles (AgNPs) is evaluated as an antimicrobial membrane for wound-healing treatment. A facile green synthesis of silver nanoparticles inside the porous three-dimensional weblike BC network has been obtained by UV light irradiation. AgNPs were photochemically deposited onto the BC gel network as well as they were chemically bonded to the cellulose fiber surfaces. AgNPs with a narrow size distribution along with some aggregates in the BC network were evidenced from the morphological analyses. A highly crystalline nature of the BC membranes was observed in X-ray diffraction measurements, and the presence of metallic silver confirmed the photochemical reduction of Ag+ → Ag0 in Ag/BC composites. Antibacterial activity of the hybrid composites, such as pellicles, performed against the Gram-negative bacteria (Escherichia coli) by disk diffusion and growth dynamics methods showed high bacteria-killing performance. No significant amount of silver release was observed from the Ag/BC pellicles even after a long soaking time. As composite pellicles are preserved in a moist environment that also favors wound recovery, by combining all of these properties the material could be useful in wound-healing treatments.

Citrate-Coated Silver Nanoparticles Growth-Independently Inhibit Aflatoxin Synthesis in Aspergillus parasiticus

Manufactured silver nanoparticles (Ag NPs) have long been used as antimicrobials. However, little is known about how these NPs affect fungal cell functions. While multiple previous studies reveal that Ag NPs inhibit secondary metabolite syntheses in several mycotoxin producing filamentous fungi, these effects are associated with growth repression and hence need sublethal to lethal NP doses, which besides stopping fungal growth, can potentially accumulate in the environment. Here we demonstrate that citrate-coated Ag NPs of size 20 nm, when applied at a selected nonlethal dose, can result in a >2 fold inhibition of biosynthesis of the carcinogenic mycotoxin and secondary metabolite, aflatoxin B₁ in the filamentous fungus and an important plant pathogen, Aspergillus parasiticus, without inhibiting fungal growth. We also show that the observed inhibition was not due to Ag ions, but was specifically associated with the mycelial uptake of Ag NPs. The NP exposure resulted in a significant decrease in transcript levels of five aflatoxin genes and at least two key global regulators of secondary metabolism, laeA and veA, with a concomitant reduction of total reactive oxygen species (ROS). Finally, the depletion of Ag NPs in the growth medium allowed the fungus to regain completely its ability of aflatoxin biosynthesis. Our results therefore demonstrate the feasibility of Ag NPs to inhibit fungal secondary metabolism at nonlethal concentrations, hence providing a novel starting point for discovery of custom designed engineered nanoparticles that can efficiently prevent mycotoxins with minimal risk to health and environment.

The ability of consortium wastewater protozoan and bacterial species to remove COD in the presence of nanomaterials under varying pH conditions

The aim of this study was to ascertain the survival limit and capability of commonly found wastewater protozoan (Aspidisca sp, Trachelophyllum sp and Peranema sp) and bacterial (Bacillus licheniformis, Brevibacillus laterosporus and Pseudomonas putida) species to remove COD while exposed to commercial nanomaterials under varying pH conditions. The experimental study was carried out in modified mixed liquor media adjusted to various pH levels (pH 2, 7 and 10) and a comparative study was performed to determine the difference between the cytotoxicity effects of commercial zinc oxide (nZnO) and silver (nAg) nanomaterials (NMs) on the target wastewater microbial communities using standard methods. The selected microbial communities were exposed to lethal concentrations ranging from 0.015 g/L to 40 g/L for nZnO and from 0.015 g/L to 2 g/L for nAg for a period of 5 days of incubation at 30°C (100 r/min). Compared with the absence of NMs in wastewater mixed liquor, the relevant environmental concentration ranging between 10 µg/L and 100 µg/L, for both nZnO and nAg caused no adverse effects, but the presence of 20 g of nZnO/L and 0.65 g of nAg/L significantly inhibited microbial growth. Statistical evidence showed that nAg was significantly more toxic compared to nZnO, but there was an insignificant difference in toxicity between microbial communities and pH variations. A significant decrease in the removal of COD by microbial populations was observed in the presence of NMs with a moderate correlation of r = 0.3 to r = 0.7 at all pH levels. It was evident that there was a physical interaction between commercial NMs and target wastewater microbial communities; although not quantitatively assessed, cell morphology and cell death were observed. Such phenomena suggest the high resilience of the microbial community, but it is the accumulation of NMs that will have adverse effects on the performance in terms of COD removal.

Engineered nano particles: Nature, behavior, and effect on the environment

Increased application of engineered nano particles (ENPs) in production of various appliances and consumer items is increasing their presence in the natural environment. Although a wide variety of nano particles (NPs) are ubiquitously dispersed in ecosystems, risk assessment guidelines to describe their ageing, direct exposure, and long-term accumulation characteristics are poorly developed. In this review, we describe what is known about the life cycle of ENPs and their impact on natural systems and examine if there is a cohesive relationship between their transformation processes and bio-accessibility in various food chains. Different environmental stressors influence the fate of these particles in the environment. Composition of solid media, pore size, solution chemistry, mineral composition, presence of natural organic matter, and fluid velocity are some environmental stressors that influence the transformation, transport, and mobility of nano particles. Transformed nano particles can reduce cell viability, growth and morphology, enhance oxidative stress, and damage DNA in living organisms.

Effects of water chemistry and surface contact on the toxicity of silver nanoparticles to Bacillus subtilis

The growing use of silver nanoparticles (AgNPs) has created concerns about its potential impacts on natural microbial communities. In this study, the physicochemical properties of AgNPs and its toxicity on natural bacteria Bacillus subtilis (B. subtilis) were investigated in aqueous conditions. The characterization data showed that AgNPs highly aggregated in aqueous conditions, and the hydrodynamic diameter of AgNPs in aqueous conditions was larger than its primary size. The studied AgNPs was less toxic to B. subtilis in estuarine water as compared to that in Milli-Q water and artificial seawater, which might be due to the observed enhanced aggregation of AgNPs in estuarine water. The toxicity of AgNPs to B. subtilis was greatly reduced when their surface contact was blocked by a dialysis membrane. Scanning electron microscope images showed that exposure contact to AgNPs resulted in damage of the microbial cell wall and enhanced formation of fibrillar structures. These results suggest that particle-cell contact is largely responsible for the observed toxicity of AgNPs in B. subtilis. This study can help to understand the potential impacts of AgNPs to natural microbes, especially in the complex aquatic environments.