Dissolution of silver nanowires and nanospheres dictates their toxicity to Escherichia coli

Particle-Driven Effects at the Bacteria Interface: A Nanosilver Investigation of Particle Shape and Dose Metric

Design criteria for controlling engineered nanomaterial (ENM) antimicrobial performance will enable advances in medical, food production, processing and preservation, and water treatment applications. In pursuit of this goal, better resolution of how specific ENM properties, such as nanoparticle shape, influence antimicrobial activity is needed. This study probes the antimicrobial activity toward a model Gram-negative bacterium, Escherichia coli (E. coli), that results from interfacial interactions with differently shaped silver nanoparticles (AgNPs): cube-, disc-, and pseudospherical-AgNPs. The EC50 value (i.e., the concentration of AgNPs that inactivates 50% of the microbial population) for each shape is identified and presented as a function of mass, surface area, and particle number. Further, shifts in relative potency are identified from the associated dose-response curves (e.g., shifts left, to lower concentrations, indicate greater potency). When using a mass-based dose metric, the disc-AgNPs present the highest antimicrobial activity of the three shapes (EC50: 2.39 ± 0.26 μg/mL for discs, 2.99 ± 0.96 μg/mL for cubes, 116.33 ± 6.43 μg/mL for pseudospheres). When surface area and particle number are used as dose metrics, the cube-AgNPs possess the highest antimicrobial activity (EC50-surface area: 4.70 × 10-5 ± 1.51 × 10-5 m2/mL, EC50-particle: 5.97 × 109 ± 1.92 × 109 particles/mL), such that the relative trend in potency becomes cubes > discs > pseudospheres and cubes ≫ discs ⩾ pseudospheres, respectively. The results reveal that the antimicrobial potency of disc-AgNPs is sensitive to the dose metric, significantly decreasing in potency (∼5-30×) upon conversion from a mass-based concentration to surface area and particle number and influencing the conclusions drawn. The shift in relative particle potency highlights the importance of investigating various dose metrics within the experimental design and signals different particle parameters influencing shape-based antimicrobial activity. To probe shape-dependent behavior, we use a unique empirical approach where the physical and chemical properties (ligand chemistry, surface charge) of the AgNP shapes are carefully controlled, and total available surface area is equivalent across shapes as made through modifications to particle size and concentration. The results herein suggest that surface area alone does not drive antimicrobial activity as the different AgNP shapes at equivalent particle surface area yield significantly different magnitudes of antimicrobial activity (i.e., 100% inactivation for cube-AgNPs, <25% inactivation for disc- and pseudospherical-AgNPs). Further, the particle shapes studied possess different crystal facets, illuminating their potential influence on differentiating interactions between the particle surface and the microbe. Whereas surface area may partly contribute to antimicrobial activity in certain ENM shapes (i.e., disc-AgNPs in relation to the pseudospherical-AgNPs), the different magnitudes of antimicrobial activity across shape provide insight into the likely role of other particle-specific factors, such as crystal facets, driving the antimicrobial activity of other shapes (i.e., cube-AgNPs).

What function of nanoparticles is the primary factor for their hyper-toxicity?

Nanomaterials have applications in environmental protection, hygiene, medicine, agriculture, and the food industry due to their enhanced bio-efficacy/toxicity as science and technology have progressed, notably nanotechnology. The extension in the use of nanoparticles in day-to-day products and their excellent efficacy raises worries about safety concerns associated with their use. Therefore, to understand their safety concerns and find the remedy, it is imperative to understand the rationales for their enhanced toxicity at low concentrations to minimize their potential side effects. The worldwide literature quotes different nanoparticle functions responsible for their enhanced bio-efficacy/ toxicity. Since the literature on the comparative toxicity study of nanoparticles of different shapes and sizes having different other physic-chemical properties like surface areas, surface charge, solubility, etc., evident that the nanoparticle's toxicity is not followed the fashion according to their shape, size, surface area, surface charge, solubility, and other Physico-chemical properties. It raises the question then what function of nanoparticle is the primary factor for their hyper toxicity. Why do non-spherical and large-sized nanoparticles show the same or higher toxicity to the same or different cell line or test organism instead of having lower surface area, surface charge, larger size, etc., than their corresponding spherical and smaller-sized nanoparticles? Are these factors a secondary, not primary, factor for nanoparticles hyper-toxicity? If so, what function of nanoparticles is the primary function for their hyper-toxicity? Therefore, in this article, literature related to the comparative toxicity of nanoparticles was thoroughly studied, and a hypothesis is put forth to address the aforesaid question, that the number of atoms/ions/ molecules per nanoparticles is the primary function of nanoparticles toxicity.

Motion Detecting, Temperature Alarming, and Wireless Wearable Bioelectronics Based on Intrinsically Antibacterial Conductive Hydrogels

Hydrogels have attracted considerable interest in developing flexible bioelectronics such as wearable devices, brain-machine interface products, and health-monitoring sensors. However, these bioelectronics are always challenged by microbial contamination, which frequently reduces their service life and durability due to a lack of antibacterial property. Herein, we report a class of inherently antibacterial conductive hydrogels (ACGs) as bioelectronics for motion and temperature detection. The ACGs were composed of poly(N-isopropylacrylamide) (pNIPAM) and silver nanowires (AgNWs) via a two-step polymerization strategy, which increased the crosslink density for enhanced mechanical properties. The introduction of AgNWs improved the conductivity of ACGs and endowed them with excellent antibacterial activity against both Gram-positive and -negative bacteria. Meanwhile, pNIPAM existed in ACGs and exhibited a thermal responsive behavior, thereby inducing sharp changes in their conductivity around body temperature, which was successfully employed to assemble a temperature alarm. Moreover, ACG-based sensors exhibited excellent sensitivity (within a small strain of 5%) and the capability of capturing various motion signals (finger bending, elbow bending, and even throat vibrating). Benefiting from the superiority of ACG-based sensors, we further demonstrated a wearable wireless system for the remote control of a vehicle, which is expected to help disabled people in the future.

Uptake, intracellular dissolution, and cytotoxicity of silver nanowires in cell models

The uptake, intracellular dissolution, and cytotoxicity of silver nanowires (AgNWs) in two cell models (human keratinocytes - HaCaT cells and murine macrophages) were systemically investigated for the first time. Cellular uptake of AgNWs occurred mainly via pathways of clathrin-dependent endocytosis, caveolae-dependent endocytosis, and phagocytosis. AgNWs could be internalized by two types of cells with numerous lysosomal vesicles detected in close vicinity to AgNWs. Meanwhile, AgNWs exposure caused lysosomal permeabilization and release of cathepsisn B into cytoplasm. Furthermore, for the first time, this study found that AgNWs exposure inhibited the transmembrane ATP binding cassette (ABC) efflux transporter activity, which could make AgNWs as chemosensitizers to increase the toxicity of other xenobiotic pollutants. Toxicity assays evaluating reactive oxygen species production and mitochondrial activity indicated that cytotoxicity differed for different cell types and particles. The intracellular presence of AgNWs with different diameters induced similar toxic events but to different extents. AgNWs were absorbed by macrophages more efficiently than HaCaT cells, while AgNWs exhibited only marginal cytotoxicity towards macrophages compared to HaCaT cells. Using an Ag⁺ fluorescence probe, it was found that a fraction of AgNWs was dissolved inside the lysosomes. A higher amount of released Ag⁺ was detected in HaCaT cells than in macrophages, which might partially contribute to their higher cytotoxicity in HaCaT cells. The toxicity of AgNWs in HaCaT cells and macrophages is due to the high-aspect nature of the nanowires rather than the extracellular release of Ag⁺. This study may be useful for risk assessments of AgNWs in their practical applications in the biomedical field.

Cellulose nanofibrils and silver nanowires active coatings for the development of antibacterial packaging surfaces

An active ink composed of cellulose nanofibrils and silver nanowires was deposited on flexible and transparent polymer films using the bar coating process, achieving controlled thicknesses ranging from 200 nm up to 2 μm. For 350 nm thick coating on polyethylene terephthalate films, high transparency (75.6% transmittance) and strong reduction of bacterial growth equal to 89.3% and 100% was noted respectively against Gram-negative Escherichia Coli and Gram-positive Staphylococcus Aureus bacteria using AATCC contact active standard test. Retained antibacterial activity was found with films produced by reverse gravure roll-to-roll process, showing the promising capability of this antibacterial solution to be deployed industrially. Finally, the same ink was also deposited on polylactic acid substrate to investigate barrier properties: for 350 nm thick coating, a reduction of 49% of oxygen transmission rate (dry conditions) and 47% reduction of water vapor transmission rate was noted, proving the enhanced barrier properties of the coatings.

Enhancement of Antibacterial Performance of Silver Nanowire Transparent Film by Post-Heat Treatment

Silver nanomaterials (AgNMs) have been applied as antibacterial agents to combat bacterial infections that can cause disease and death. The antibacterial activity of AgNMs can be improved by increasing the specific surface area, so significant efforts have been devoted to developing various bottom-up synthesis methods to control the size and shape of the particles. Herein, we report on a facile heat-treatment method that can improve the antibacterial activity of transparent silver nanowire (AgNW) films in a size-controllable, top-down manner. AgNW films were fabricated via spin-coating and were then heated at different temperatures (230 and 280 °C) for 30 min. The morphology and the degree of oxidation of the as-fabricated AgNW film were remarkably sensitive to the heat-treatment temperature, while the transparency was insensitive. As the heat-treatment temperature increased, the AgNWs spontaneously broke into more discrete wires and droplets, and oxidation proceeded faster. The increase in the heat-treatment temperature further increased the antibacterial activity of the AgNW film, and the heat treatment at 280 °C improved the antibacterial activity from 31.7% to 94.7% for Staphylococcus aureus, and from 57.0% to 98.7% for Escherichia coli. Following commonly accepted antibacterial mechanisms of AgNMs, we present a correlation between the antibacterial activity and surface observations of the AgNW film.

Evaluation of Antibacterial and Cytotoxicity Properties of Silver Nanowires and Their Composites with Carbon Nanotubes for Biomedical Applications

In this work, we prepared silver nanowires (AgNWs) via the polyol method in the presence or absence of single wall carbon nanotubes (CNTs) and tested their physicochemical, antibacterial and cytotoxic properties. Results showed that the introduction of CNTs lead to the formation of AgNWs at lower temperature, but the final product characteristics of AgNWs and AgNWs-CNT were not significantly different. AgNWs exhibited antibacterial properties against all the studied bacterial species via the formation of oxygen reactive species (ROS) and membrane damage. Furthermore, AgNWs exhibited a dose-dependent and time-dependent toxicity at concentrations ≥ 10 µg/mL. Fibroblasts appeared to be more resistant than human colorectal adenocarcinoma (Caco-2) and osteoblasts to the toxicity of AgNWs. The cytotoxicity of AgNWs was found to be related to the formation of ROS, but not to membrane damage. Overall, these results suggest that AgNWs are potential antibacterial agents against E. coli, S. aureus, MRSA and S. saprophyticus, but their dosage needs to be adjusted according to the route of administration.

Dynamic nano-Ag colloids cytotoxicity to and accumulation by Escherichia coli: Effects of Fe3+, ionic strength and humic acid

Released Ag ions or/and Ag particles are believed to contribute to the cytotoxicity of Ag nanomaterials, and thus, the cytotoxicity and mechanism of Ag nanomaterials should be dynamic in water due to unfixed Ag particle:Ag⁺ ratios. Our recent research found that the cytotoxicity of PVP-Ag nanoparticles is attributable to Ag particles alone in 3 hr bioassays, and shifts to both Ag particles and released Ag⁺ in 48 hr bioassays. Herein, as a continued study, the cytotoxicity and accumulation of 50 and 100 nm Ag colloids in Escherichia coli were determined dynamically. The cytotoxicity and mechanisms of nano-Ag colloids are dynamic throughout exposure and are derived from both Ag ions and particles. Ag accumulation by E. coli is derived mainly from extracellular Ag particles during the initial 12 hr of exposure, and thereafter mainly from intracellular Ag ions. Fe³⁺ accelerates the oxidative dissolution of nano-Ag colloids, which results in decreasing amounts of Ag particles and particle-related toxicity. Na⁺ stabilizes nano-Ag colloids, thereby decreasing the bioavailability of Ag particles and particle-related toxicity. Humic acid (HA) binds Ag⁺ to form Ag⁺-HA, decreasing ion-related toxicity and binding to the E. coli surface, decreasing particle-related toxicity. HA in complex conditions showed a stronger relative contribution to toxicity and accumulation than Na⁺ or Fe³⁺. The results highlighted the cytotoxicity and mechanism of nano-Ag colloids are dynamic and affected by environmental factors, and therefore exposure duration and water chemistry should be seriously considered in environmental and health risk assessments.

The in vivo effects of silver nanoparticles on terrestrial isopods, Porcellio scaber, depend on a dynamic interplay between shape, size and nanoparticle dissolution properties

The present work aims to study the effects that acute exposure to low concentrations of silver nanoparticles (AgNPs) cause in digestive glands of terrestrial isopods (Porcellio scaber). The experiments were designed to integrate different analytical techniques, such as transmission electron microscopy, atomic absorption spectroscopy, proton induced X-ray emission, and Fourier transform IR imaging (FTIRI), in order to gain a comprehensive insight into the process from the AgNPs' synthesis to their interaction with biological tissues in vivo. To this aim, terrestrial isopods were fed with AgNPs having different shapes, sizes, and concentrations. For all the tested conditions, no toxicity at the whole organism level was observed after 14 days of exposure. However, FTIRI showed that AgNPs caused detectable local changes in proteins, lipids, nucleic acids and carbohydrates at the tissue level, to an extent dependent on the interplay of the AgNPs' properties: shape, size, concentration and dissolution of ions from them.

Size- and shape-dependent toxicity of silver nanomaterials in green alga Chlorococcum infusionum

Silver nanomaterials (AgNMs) of different shapes and sizes are potentially toxic to aquatic organisms. However, studies on the toxicity of AgNMs and on their shape-dependent effects on algae are scarce. The present study evaluated the effects of three AgNMs (silver nanospheres, AgNPs; silver nanowires, AgNWs; silver nanoplates, AgPLs) with different shapes coated with polyvinylpyrrolidone on the growth and photosynthetic performance of an alga, Chlorococcum infusionum. We used growth measurements and determined the photosynthetic parameters based on chlorophyll fluorescence transients in the algal cells exposed to different concentrations of the three AgNMs. The effective concentrations at 50% (EC₅₀) of AgNPs, AgNWs, and AgPLs were calculated to be 0.1, 0.045, and 0.021 mg/L, respectively. The results showed that the toxicity of AgNMs in C. infusionum was in the order, AgPLs (40 nm diameter) > AgNWs (21,000 nm length × 42 nm diameter) > AgNPs (57 nm diameter), based on the decrease in growth and three photosynthetic activities. We propose that the toxic potential of AgNMs is primarily dependent on their diameter and secondarily on their shape. Overall, this study provides, for the first time, a comparison of the growth and photosynthetic activities of C. infusionum exposed to AgNMs of three different shapes.

AgNPs Change Microbial Community Structures of Wastewater

Due to their strong antimicrobial activity, silver nanoparticles (AgNPs) are massively produced, applied, consumed and, as a negative consequence, released into wastewater treatment plants. Most AgNPs are assumed to be bound by sludge, and thus bear potential risk for microbial performance and stability. In this lab-scale study, flow cytometry as a high-throughput method and 16S rRNA gene amplicon Illumina MiSeq sequencing were used to track microbial community structure changes when being exposed to AgNPs. Both methods allowed deeper investigation of the toxic impact of chemicals on microbial communities than classical EC₅₀ determination. In addition, ecological metrics were used to quantify microbial community variations depending on AgNP types (10 and 30 nm) and concentrations. Only low changes in α- and intra-community β-diversity values were found both in successive negative and positive control batches and batches that were run with AgNPs below the EC₅₀ value. Instead, AgNPs at EC₅₀ concentrations caused upcoming of certain and disappearance of formerly dominant subcommunities. Flavobacteriia were among those that almost disappeared, while phylotypes affiliated with Gammaproteobacteria (3.6-fold) and Bacilli (8.4-fold) increased in cell abundance in comparison to the negative control. Thus, silver amounts at the EC₅₀ value affected community structure suggesting a potential negative impact on functions in wastewater treatment systems.

The effects of silver nanomaterial shape and size on toxicity to Caenorhabditis elegans in soil media

Silver nanomaterials (AgNMs) of various shapes and sizes are manufactured for different purposes and used as ingredients in a wide variety of products and applications. Recently, the toxicity of AgNMs has attracted significant attention. However, the effect of the shape of AgNMs (particles, wires, plates) on their toxicity in soil ecosystems remains poorly understood. In this study, we added AgNMs of different shapes and sizes (silver nanoparticles, AgNPs; 10 μm silver nanowires, 10-AgNWs; 20 μm silver nanowires, 20-AgNWs; silver nanoplates, AgPLs) to natural soil and determined their effect on the growth and reproduction of the free-living nematode, Caenorhabditis elegans. AgNPs and AgPLs were found to inhibit the growth and reproduction of C. elegans, whereas AgNWs had a negligible effect. Among these AgNMs, the results of this study suggest AgNPs are the most toxic. This confirms that the shape of AgNPs plays a significant role in their toxicity level. To the best of our knowledge, this is the first comparative analysis of the shape-dependent toxicity of AgNMs in the soil using nematode C. elegans. This study provides a scientific reference for assessing shape-dependent soil nanotoxicity.

Rapid in situ assessment of Cu-ion mediated effects and antibacterial efficacy of copper surfaces

Release of metal ions from metal-based surfaces has been considered one of the main drivers of their antimicrobial activity. Here we describe a method that enables parallel assessment of metal ion release from solid metallic surfaces and antimicrobial efficacy of these surfaces in a short time period. The protocol involves placement of a small volume of bioluminescent bacteria onto the tested surface and direct measurement of bioluminescence at various time points. In this study, two recombinant Escherichia coli strains, one expressing bioluminescence constitutively and applicable for general antimicrobial testing, and the other induced by Cu ions, were selected. Decrease in bioluminescence of constitutive E. coli on the surfaces showed a good correlation with the decrease in bacterial viability. Response of Cu-inducible E. coli showed a correlation with Cu content in the tested surfaces but not with Cu dissolution suggesting the role of direct bacteria-surface contact in Cu ion-driven antibacterial effects. In summary, the presented protocol enables the analysis of microbial toxicity and bioavailability of surface-released metal ions directly on solid surfaces within 30-60 min. Although optimized for copper and copper alloy surfaces and E. coli, the method can be extended to other types of metallic surfaces and bacterial strains.

Current Knowledge on the Use of Computational Toxicology in Hazard Assessment of Metallic Engineered Nanomaterials

As listed by the European Chemicals Agency, the three elements in evaluating the hazards of engineered nanomaterials (ENMs) include the integration and evaluation of toxicity data, categorization and labeling of ENMs, and derivation of hazard threshold levels for human health and the environment. Assessing the hazards of ENMs solely based on laboratory tests is time-consuming, resource intensive, and constrained by ethical considerations. The adoption of computational toxicology into this task has recently become a priority. Alternative approaches such as (quantitative) structure-activity relationships ((Q)SAR) and read-across are of significant help in predicting nanotoxicity and filling data gaps, and in classifying the hazards of ENMs to individual species. Thereupon, the species sensitivity distribution (SSD) approach is able to serve the establishment of ENM hazard thresholds sufficiently protecting the ecosystem. This article critically reviews the current knowledge on the development of in silico models in predicting and classifying the hazard of metallic ENMs, and the development of SSDs for metallic ENMs. Further discussion includes the significance of well-curated experimental datasets and the interpretation of toxicity mechanisms of metallic ENMs based on reported models. An outlook is also given on future directions of research in this frontier.

Effects of silver nanowire length and exposure route on cytotoxicity to earthworms

To accurately evaluate the environmental toxicity of silver nanowires (AgNWs), it is necessary to characterize how the cytotoxicity of these nanomaterials is affected by the route of exposure. However, few studies have addressed the exposure route or mechanism of toxicity of nanomaterials, particularly of nanowires, in living organisms. In this study, we therefore analyzed the main exposure route of AgNWs in vitro, using earthworms (Eisenia andrei) as a model system, via flow cytometry. We subsequently examined the in vivo toxicity of AgNWs to earthworms in soil. These tests revealed that intracellular esterase activity was correlated with adsorption of the nanowires to the surfaces of coelomocytes in vitro, and that in vivo cytotoxicity resulted mainly from oral, rather than dermal, exposure to the nanowires. Overall, shorter AgNWs (10 μm) were more toxic than longer AgNWs (20 μm). To our knowledge, this study is the first report regarding the ecotoxicity of nanowires to earthworms in soil. Our findings provide important information to help assess the risk of toxic AgNW contamination of soil ecosystems.

Silver Nanoparticle-Directed Mast Cell Degranulation Is Mediated through Calcium and PI3K Signaling Independent of the High Affinity IgE Receptor

Engineered nanomaterial (ENM)-mediated toxicity often involves triggering immune responses. Mast cells can regulate both innate and adaptive immune responses and are key effectors in allergic diseases and inflammation. Silver nanoparticles (AgNPs) are one of the most prevalent nanomaterials used in consumer products due to their antimicrobial properties. We have previously shown that AgNPs induce mast cell degranulation that was dependent on nanoparticle physicochemical properties. Furthermore, we identified a role for scavenger receptor B1 (SR-B1) in AgNP-mediated mast cell degranulation. However, it is completely unknown how SR-B1 mediates mast cell degranulation and the intracellular signaling pathways involved. In the current study, we hypothesized that SR-B1 interaction with AgNPs directs mast cell degranulation through activation of signal transduction pathways that culminate in an increase in intracellular calcium signal leading to mast cell degranulation. For these studies, we utilized bone marrow-derived mast cells (BMMC) isolated from C57Bl/6 mice and RBL-2H3 cells (rat basophilic leukemia cell line). Our data support our hypothesis and show that AgNP-directed mast cell degranulation involves activation of PI3K, PLCγ and an increase in intracellular calcium levels. Moreover, we found that influx of extracellular calcium is required for the cells to degranulate in response to AgNP exposure and is mediated at least partially via the CRAC channels. Taken together, our results provide new insights into AgNP-induced mast cell activation that are key for designing novel ENMs that are devoid of immune system activation.

Effect of Direct Contact on the Phytotoxicity of Silver Nanomaterials

The increasing use of silver nanomaterials (AgNMs) in consumer products will result in an increased amount entering the environment, where AgNMs were recently found to cause phytotoxicity in the model plant Lolium multiflorum. To better understand the causes of the phytotoxicity, we have designed a new set of experiments to study the effect of AgNM dissolution. Dissolution of AgNMs was measured over a 1-month period to determine if dissolution alone caused phytotoxicity. Very little dissolution was observed over the testing period, suggesting a different mechanism caused the majority of the toxicity. To further confirm this hypothesis, AgNMs were physically separated from the seeds and plants by a dialysis membrane. Toxicity was ameliorated in AgNM-exposed plants, showing that direct contact between AgNMs and plant seeds/roots is a required condition for the observed phytotoxicity in plant models. Probing further, a surface reactivity assay showed increased surface reactivity of silver nanoparticles (AgNPs) and silver nanocubes (AgNCs) corresponded to increased toxicity compared to silver nanowires (AgNWs). The work here can help build the knowledge base regarding shape control of nanomaterials and reducing unintended side effects.

Developments in Antibacterial Disinfection Techniques

One of the most daunting challenges facing nations today is controlling the spread of increasingly lethal bacteria. Today, a handful of bacteria can no longer be treated with traditional antibiotics and show antibacterial resistance. In this regard, nanotechnology possesses tremendous potential for the development of novel tools which help prevent and combat the spread of unwanted microorganisms. These tools can provide unique solutions for the challenges of the traditional disinfection methods, such as increased antibacterial activity, cost reduction, biocompatibility and personalized treatment. Despite its great potential, nanotechnology remains in its infancy and continued research efforts are required to achieve its full potential. In this chapter, traditional methods and their associated limitations are reviewed for their efficacy against microbial spread, and potential solutions in nanotechnology are described. A review of the state of the art disinfection techniques using nanotechnology is presented, and promising new areas in the field are discussed.

Electrochemical monitoring of colloidal silver nanowires in aqueous samples

Silver nanowires (NWs) are increasingly utilized in technological materials and consumer products, but an effective analytical technique is not yet available to measure their concentration in the environment. Here, we present an electrochemical method to quantify Ag NWs suspended in aqueous solution. Using linear sweep voltammetry, the Ag NWs are identified by the peak potential while their concentration is revealed by the intensity of the peak current. The peak current varies linearly with the Ag NW concentration with a low detection limit of 3.50 ng mL(-1). This method is also successfully applied to quantify Ag NWs in mixtures with nanoparticles, through their specific oxidation behavior, and in wastewater obtained after the Ag NW film preparation process.

Reducing Environmental Toxicity of Silver Nanoparticles through Shape Control

The use of antibacterial silver nanomaterials in consumer products ranging from textiles to toys has given rise to concerns over their environmental toxicity. These materials, primarily nanoparticles, have been shown to be toxic to a wide range of organisms; thus methods and materials that reduce their environmental toxicity while retaining their useful antibacterial properties can potentially solve this problem. Here we demonstrate that silver nanocubes display a lower toxicity toward the model plant species Lolium multiflorum while showing similar toxicity toward other environmentally relevant and model organisms (Danio rerio and Caenorhabditis elegans) and bacterial species (Esherichia coli, Bacillus cereus, and Pseudomonas aeruginosa) compared to quasi-spherical silver nanoparticles and silver nanowires. More specifically, in the L. multiflorum experiments, the roots of silver nanocube treated plants were 5.3% shorter than the control, while silver nanoparticle treated plant roots were 39.6% shorter than the control. The findings here could assist in the future development of new antibacterial products that cause less environmental toxicity after their intended use.

Aquatic Toxicity Comparison of Silver Nanoparticles and Silver Nanowires

To better understand the potential ecotoxicological impact of silver nanoparticles (AgNPs) and silver nanowires (AgNWs) released into freshwater environments, the toxicities of these nanomaterials were assessed and compared using Organization for Economic Cooperation and Development (OECD) test guidelines, including a "Daphnia sp., acute immobilization test," "Fish, acute toxicity test," and "freshwater alga and cyanobacteria, growth inhibition test." Based on the estimated median lethal/effective concentrations of AgNPs and AgNWs, the susceptibility to the nanomaterials was different among test organisms (daphnia > algae > fish), suggesting that the AgNPs are classified as "category acute 1" for Daphnia magna, "category acute 2" for Oryzias latipes, and "category acute 1" for Raphidocelis subcapitata, while the AgNWs are classified as "category acute 1" for Daphnia magna, "category acute 2" for Oryzias latipes, and "category acute 2" for Raphidocelis subcapitata, according to the GHS (Globally Harmonized System of Classification and Labelling of Chemicals). In conclusion, the present results suggest that more attention should be paid to prevent the accidental or intentional release of silver nanomaterials into freshwater aquatic environments.

Nanomaterials and neurodegeneration

The increasing application of nanotechnology in various industrial, environmental, and human settings raises questions surrounding the potential adverse effects induced by nanosized materials to human health, including the possible neurotoxic and neuroinflammatory properties of those substances and their capability to induce neurodegeneration. In this review, a panel of metal oxide nanoparticles (NPs), namely titanium dioxide, silicon dioxide, zinc oxide, copper oxide, iron NPs, and carbon nanotubes have been focused. An overview has been provided of the in vitro and in vivo evidence of adverse effects to the central nervous system. Research indicated that these nanomaterials (NMs) not only reach the brain, but also can cause a certain degree of brain tissue damage, including cytotoxicity, genotoxicity, induction of oxidative stress, and inflammation, all potentially involved in the onset and progression of neurodegeneration. Surface chemistry of the NMs may play an important role in their localization and subsequent effects on the brain of rodents. In addition, NM shape differences may induce varying degrees of neurotoxicity. However, one of the potential biomedical applications of NMs is nanodevices for early diagnostic and novel therapeutic approaches to counteract age related diseases. In this context, engineered NMs were promising vehicles to carry diagnostic and therapeutic compounds across the blood-brain barrier, thereby representing very timely and attractive theranostic tools in neurodegenerative diseases. Therefore, a careful assessment of the risk-benefit ratio must be taken into consideration in using nanosized materials.

Mutagenic Effects of Nanosilver Consumer Products: a new Approach to Physicochemical Properties

Serious concerns have been expressed about potential health risks of Nano silver containing consumer products (AgNPs) therefore regulatory health risk assessment on such nanoparticles has become mandatory for the safe use of AgNPsinbiomedicalproducts with special concerns to the mutagenic potentials. In this study, we examined the inhibitory and mutagenicity effects of AgNPs in three different sizes of three colloidal AgNPs by Minimal Inhibitory concentration (MIC), Minimal Bactericidal Concentration (MBC) and Bacterial Reverse Mutation Assay (Ames test).All samples were characterized by transmission electron microscopy (TEM), X-Ray Diffraction (XRD) and Dynamic Light Scattering (DLS). DLS analysis showed lack of large agglomeration of the AgNPs and TEM results showed the spherical AgNPswith the average sizes of 15, 19.6, 21.8 nms. Furthermore the XRD analysis showed the crystalline samples with a face centered cubic structure of pure silver.AmestestresultsonColloidal silver nanoparticles showed lack of any mutation in TA100, TA98, YG1029S. typhymuriumstrains.In addition colloidal silver nanoparticles reduced the mutation ratesin all three strains in a concentration dependent manner .This finding creates a new issue in the possible antimutagenic effects of colloidal AgNPsas a new pharmaceutical productwhich should be consideredinfuture studiesby focusing onthephysicochemical properties of AgNPs.

Short versus long silver nanowires: a comparison of in vivo pulmonary effects post instillation

Background

Silver nanowires (Ag NWs) are increasingly being used to produce touchscreens for smart phones and computers. When applied in a thin film over a plastic substrate, Ag NWs create a transparent, highly-conductive network of fibers enabling the touch interface between consumers and their electronics. Large-scale application methods utilize techniques whereby Ag NW suspensions are deposited onto substrates via droplets. Aerosolized droplets increase risk of occupational Ag NW exposure. Currently, there are few published studies on Ag NW exposure-related health effects. Concerns have risen about the potential for greater toxicity from exposure to high-aspect ratio nanomaterials compared to their non-fibrous counterparts. This study examines whether Ag NWs of varying lengths affect biological responses and silver distribution within the lungs at different time-points.

Methods

Two different sizes of Ag NWs (2 μm [S-Ag NWs] and 20 μm [L-Ag NWs]) were tested. Male, Sprague-Dawley rats were intratracheally instilled with Ag NWs (0, 0.1, 0.5, or 1.0 mg/kg). Broncho-alveolar lavage fluid (BALF) and lung tissues were obtained at 1, 7, and 21 days post exposure for analysis of BAL total cells, cell differentials, and total protein as well as tissue pathology and silver distribution.

Results and conclusions

The two highest doses produced significant increases in BAL endpoints. At Day 1, Ag NWs increased total cells, inflammatory polymorphonuclear cells (PMNs), and total protein. PMNs persisted for both Ag NW types at Day 7, though not significantly so, and by Day 21, PMNs appeared in line with sham control values. Striking histopathological features associated with Ag NWs included 1) a strong influx of eosinophils at Days 1 and 7; and 2) formation of Langhans and foreign body giant cells at Days 7 and 21. Epithelial sloughing in the terminal bronchioles (TB) and cellular exudate in alveolar regions were also common. By Day 21, Ag NWs were primarily enclosed in granulomas or surrounded by numerous macrophages in the TB-alveolar duct junction. These findings suggest short and long Ag NWs produce pulmonary toxicity; thus, further research into exposure-related health effects and possible exposure scenarios are necessary to ensure human safety as Ag NW demand increases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-014-0052-6) contains supplementary material, which is available to authorized users.