Researchers are continuously on the lookout for materials which are more environmentally friendly and sustainable in order to improve emerging technologies. Nanoparticles (NPs) are the basis for a variety of emerging technologies used for industrial, biomedical and environmental applications, but their release into the environment is still a cause for concern. What if a nanomaterial, that has minimal negative environmental impact, was available? Jared Bozich from the University of Wisconsin and colleagues have published a research paper demonstrating that surface chemistry has the potential to increase or decrease negative biological impacts of NPs.

The surfaces of NPs are typically modified with surface functional groups that control properties such as stability. In this research, the acute and chronic toxicity of well characterized gold nanoparticles (AuNPs), functionalized with ligands of differing charges were investigated in Daphnia magna. D. magna (more commonly called water fleas) are widely accepted as a model organism for assessing the toxicity of environmental contaminates and experience reduced reproduction, growth and increased mortality with exposure to toxic substances. D.magna were exposed to concentrations of four types of functionalized AuNps.

The results showed that initial particle charge significantly impacted overall toxicity, with positively charged particles being more toxic than their negatively-charged counterparts. This could be explained by the increased cellular uptake of positively charged particles due to their high levels of attraction with cellular membrane. This creates a hole in the membrane due to the densely populated charge on the NP surface – allowing the particle to enter the intracellular matrix and continue to cause damage. Another interesting result of this study was that the smaller the particle, the higher the toxicity. This can be explained by a similar theory; after cellular uptake the smaller particles are able to cross the gut lumen of the daphnis, potentially further interacting with D. magna cells and causing damage.

The results of this study identify mechanisms for AuNP toxicity by examining NP toxicity with different charges, using an environmental relevant organism. NPs have the potential to be highly beneficial to society, but in order to minimize the environmental implications the mechanisms that govern the toxicity of NPs need to be betters elucidated. This study demonstrates how the charge and identity of a ligand can influence AuNp toxicity.

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Surface chemistry, charge and ligand type impact the toxicity of gold nanoparticles to Daphnia magna

Jared Bozich, Samuel E Lohse, Marco D Torelli, Catherine Murphy, Robert Hamers and Rebecca Klaper

DOI: 10.1039/C4EN00006D

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With the level of engineered nanomaterials (EMNs) in the environment continuously increasing, there are rising concerns with regards to their potential environmental impact. In recent years a more accurate understanding of particle behaviour in complex systems has been gained.  Numerous studies have investigated the environmental hazards of ENMs, but the link between these two aspects is less developed. There are still considerable knowledge gaps with respect to ENMs bioavailability in the environment.

A review by Nadia Von Moos, from the University of Geneva, and colleagues provides an overview of what is currently known about environmental transformations of nanomaterials as well as their interactions with, and their toxicity towards bacteria and microalgae.

This diagram shows the processes at the medium – bio-interface underlying the bioavailability of ENMs to aquatic microorganisms (AMO). The bioavailability is dependent on many processes, such as chemical and physical transformations, adsorption and desorption, internalization, intracellular fate, agglomeration, dissolution and surface transformations. The importance of ENMs’ material characteristics has been reviewed before, but this review emphasises the environmental factors affecting the above processes. A quantitative understanding of ENM bioavailability requires insights into their behaviour during transport from the ambient medium to the AMO interface and of the processes underlying adsorption, internalization as well as intracellular fate – all of which are discussed in this review.

This review clearly concludes that there are still considerable knowledge gaps with respect to the effects of agglomeration on bioavailability, exact uptake routes, intracellular compartmentalization as well as dissolved organic matter-protein competition on the surface of internalized engineered nanoparticles. This review can be used to guide future research efforts in nanomaterial hazard and risk assessment. To read more, download your free* copy by clicking the link below:

Bioavailability of inorganic nanoparticles to planktonic bacteria and aquatic microalgae in freshwater

Nadia Von Moos, Paul Bowen and Vera I Slaveykova

DOI: 10.1039/C3EN00054K

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Who doesn’t like power? We all do. That’s why we love modifying nanomaterial surfaces. It gives us the power to control their properties. In the case of Fe nanoparticles (Fe NP), which are heavily used in environmental remediation, partial deposition of secondary metals such as Pd, Ni, Cu or Pt results in bimetallic clusters with enhanced reaction efficiencies towards treating contaminated groundwater and soils. Nevertheless one has to be cautious about how these modifications affect the nanoparticle toxicity. In fact, Fe NPs are known to possess antibacterial and antifungal properties; potential mechanisms of toxicity are membrane disruption and oxidative damage from the reactive oxygen species (ROS).

Therefore, E-J. Kim and colleagues from the Pohang University of Science and Technology have investigated the mechanism of toxicity for 4 different bimetallic Fe NPs: Fe/Cu, Fe/Ni, Fe/Pd and Fe/Pt, toward Escherichia coli along with bare Fe NPs. Synthesis of NPs was completed in-house and characterization data showed 50-70 nm primary particles with homogenous secondary metal coatings and zerovalent Fe cores.

The initial experiments consisted of testing cell viability upon exposure to NPs using CFU assay, ROS production using DCF-DA fluorescence dye (3hr post exposure), peroxidase activity using antioxidant enzyme glutathione peroxidase (GPx) and NP dissolution to determine the role of oxidative stress on the cell death. The results obtained are summarized below.

Therefore, the team concluded an alternative mechanism of toxicity via membrane disruption using a spectroscopic approach with FTIR spectroscopy, TEM imaging and anion release profiles. The TEM images showed higher levels of uptake for Fe/Cu in contrast to Fe/Pd. FTIR spectra of bacterial cells showed peaks corresponding to the C-O/C-O-C, PO₂– and CH₃– stretches that disappear upon exposure to NPs. Finally, the anion release profiles were found to be the most consistent with the cell viability indicating that NP-mediated membrane permeability and/or membrane damage is the major mechanism of toxicity. Higher amounts of PO₃^2- and Cl^– were observed for cell cultures exposed to Fe/Cu while they were lowest for Fe/Pd and Fe/Pt. Interestingly Fe/Ni caused the highest release of SO₄^2- indicating the disruption of sulfate transport system.

Overall, Fe/Pd was concluded to have the least toxicity and superior performance in terms of environmental decontamination while Fe/Cu NPs remained at the other end of the spectrum. The authors suggest this work may provide a platform for environmental engineers to design treatment strategies for environmental remediation with less harmful side effects over unmodified Fe NPs.

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Comparative toxicity of bimetallic Fe nanoparticles toward Escherichia coli: mechanism and environmental implications

Eun-Ju Kim, Thao Le Thanh and   Yoon-Seok Chang

DOI: 10.1039/C3EN00057E

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Iron-containing nanoparticles are beneficial in a wide range of applications, but little is known about the effects they have on bacterial pathogens.  Jennifer Borcherding and colleagues from the University of Iowa, including Vicki Grassian, the Environmental Science: Nano Editorial Board Chair, have completed research to show that iron oxide nanoparticles can induce bacterial growth and biofilm formation as well as inhibit antimicrobial peptide (AMP) function.

Iron-containing nanoparticles are generally thought to be non-toxic; however these studies suggest that the effect of co-exposures of nanoparticles with known pathogens and their impact on host innate immunity should be taken into consideration when screening nanoparticle toxicity. The use of iron-containing nanoparticles in industry is increasing, therefore the potential for iron-containing particle exposure, as particulate matter in air, is also increasing. This is a concern to human health, as particulate matter has been associated with increased respiratory exacerbations, pneumococcal infections, otitis media and eye infections.

Three main experiments were carried out to investigate the effect of iron-containing nanoparticles on bacterial pathogens.

1. Pseudomonas aeruginosa (PA01), a known pathogen to humans, animals and plants, was exposed to iron oxide nanoparticles of different size ranges. These results showed that the smallest particles induced the greatest amount of growth.
2. Biofilms were grown in the presence of iron oxide particles and aluminium oxide particles of similar sizes. These results showed that biofilm formation was increased more in the presence of iron oxide particles than in the presence of aluminium oxide particles.
3. The effects of iron-containing particles on AMP activity was determined by incubating physiologically relevant concentration of AMPs and iron oxide particles, of different sizes, and testing the effects. It was shown that the smaller iron-containing particles provided the greatest amount of bioavailable iron and inhibited AMP activity the most.

This study showed that the smaller the particle, the larger the effect for bacterial growth, biofilm formation and AMP function impairment. This is because the smaller particles have large surface areas and increased dissolution. The research has demonstrated that iron oxide nanoparticles provide a source of bioavailable iron and play an important role in bacterial growth.

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Iron Oxide Nanoparticles Induce Pseudomonas Aeruginosa Growth and Inhibit Antimicrobial Peptide Function

Jonas Baltrusaitis, Jennifer Borcherding, Haihan Chen, Larissa Stebounova, Chia-Ming Wu, Gayan Rubasinghege, Imali Mudunkotuwa, Juan Carballo, Joseph Zabner, Vicki Grassian and Alejandro Comellas

DOI: 10.1039/C3EN00029J

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The rapid growth of urbanized and industrialized areas across the globe is an ecological concern. The subject of environmental remediation is an increasingly important focus of research, with much work undertaken to discover ways of effectively treating toxic pollutants in an environmentally sound and sustainable manner. A principal development within this field has been the advent of nanomaterial-based photocatalysts for degrading organic pollutants, which provide a potentially rapid, cheap and relatively green alternative to conventional physical (e.g. adsoption, ultrafiltration) and chemical (UV radiation, H₂O₂ oxidation) methods of treating and removing  pollutants.

BiOX (X= Cl, Br and I) materials are an emerging group of high performance semi-conducting nanomaterials for this purpose thanks to their desirable optical properties. Much research has focussed on the photocatalytic behaviour and performance of these materials under sunlight irradiation. This critical review by Liqun Ye and co-workers at Nanyang Normal University in China provides an overview of the recent developments in the research of heterogeneous chemistry and photochemistry of BiOX, relevant to their use in the photocatalytic removal of contaminants from air and water.

The review provides a concise description of key structural and optical properties that allow BiOX to act as efficient photocatalyts, outlines the different synthetic methods used for the production of these materials and describes how these different synthetic routes influence the morphology and photocatalytic activity of the different BiOX produced.  The synthetic formation mechanisms are also described and depicted.

Additionally, the review describes the photocatalytic mechanisms involved in the treatment of a wide range of pollutants (including VOCs, dyes, alcohols, heavy metals, bacteria NO_x and PVC). This includes discussions of photocatalytic activity, selectivity and stability, as well as providing detailed pollutant degradation pathways and description of intermediate species.

While BiOX displays excellent catalytic behaviour, modifications are still required to optimise their practical application for remediation under natural solar irradiation. These modification methods (e.g. the use of cocatalysts, doping, coupling, dye sensitization, graphene, defects, surface plasmon resonance and solid solutions) are discussed in detail. The review also includes a discussion of different BiOX facet effects and describes methods of facet confirmation within these crystalline structures.

This paper is comprehensive review of the current understanding within an innovative field of research that has the potential to improve environmental safety and protection. Furthermore, the authors provide a guideline for the most efficient production of highly active BiOX photocatalysts and outline key uncertainties and questions remaining in this field, indicating potential directions for future research in this area, in both practical and theoretical perspectives.

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Recent Advances on BiOX (X = Cl, Br and I) Photocatalysts: Synthesis, Modification, Facet Effect and Mechanisms
Liqun Ye, Yurong Su, Xiaoli Jin and Haiquan Xie
DOI: 10.1039/C3EN00098B

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Typically when people drink water they expect it to be clear, however the effluent discharge by many industries contains a considerable amount of dye. A very small amount of dye contaminating water can be highly visible and is undesirable for several reasons:

• Colour influences negative perception of water quality.
• Colour interferes with light penetration, reducing photosynthesis in aquatic plants and therefore destroying aquatic ecosystems.
• Dyes can be toxic, carcinogenic and mutagenic, which is a serious hazard to aquatic organisms as well as human health.
• Dyes are difficult to remove as most dyes are resistant to biological degradation.

Layered double hydroxides (LHZS) have been an attractive candidate for adsorbents to selectively remove pollutants due to their large surface area, ease of preparation, exchangeable interlayer anions, compositional flexibility and low cost. However Shiyao Zhu and colleagues at Jilin University have prepared CuZn hydroxyl double salts (CuZn-HDS) and tested them as an adsorbent for methyl orange (MO) removal.  The adsorption performance of CuZn-HDS was much better than the adsorption performance of LHZS, proving CuZn-HDS to be a promising adsorbent for the removal of dye from wastewater.

Three CuZn-HDS samples were prepared using different water ratios and their adsorption capacities and surface areas were investigated and compared to the adsorption capacities of LHZS.

This picture shows the scanning electron microscope images of LHZS (a) and each CuZn-HDS sample after MO adsorption. CuZn-1 (b) had the highest surface area and the highest adsorption capacity for MO due to its multivalve flower-like structure with stacked nanoplatelets.

Due to the increasing environmental pollution from dye wastewater emissions, an important role for these nanomaterials is as potential adsorbents for the removal of pollutants from wastewater.

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High adsorption capacity for dye removal by CuZn hydroxyl double salts

Shiyao Zhu, Shihui Jiao, Ziwei Liu, Guangsheng Pang and Shouhua Feng

DOI: 10.1039/C3EN00078H

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Titania (TiO₂) nanoparticles are used in a great number of consumer products and therefore the quantity of nanoparticles and their transformation products reaching the environment is increasing. The question is, when they reach the aquatic environment, what happens to their stability?

Nanoparticles in aquatic environments are subject to different types of forces which can lead to aggregation, stabilization or deposition of the particles.

The aggregation of nanoparticles has a strong influence in determining their final fate in the environment. Julián A. Gallego-Urrea and colleagues from the University of Gothenburg have conducted a study to evaluate the effects of natural organic matter (NOM) on the aggregation kinetics of titania nanoparticles.

It is important to be able to predict nanoparticles transformation products in a physically meaningful way for near source emissions, spill situations and ecotoxicity tests, where other particles are less significant for determining ENP fate. When nanoparticles interact with NOM transformation processes can occur. For example, sorption of NOM onto the surface of engineered nanoparticles (ENP) can lead to aggregation via bridging mechanisms or stabilization.

The stability of titania nanoparticles in natural water is influenced not only by their concentration, but also by the physico-chemical characteristics of the receiving water.

In this study the stabilization of synthesized titania nanoparticles obtained at various pH levels, with a variety of electrolytes, was evaluated.

The titania nanoparticles were rapidly mixed with NOM, Sodium Alginate, Fulvic acid or Humic acid and three different electrolytes, NaCl, CaCl₂ and Na₂SO₄

The change in particle size was then monitored with time-resolved dynamic light scattering (TR-DLS) at different concentrations of three different electrolytes and different solutions of pH. By measuring the particle size it could be determined what effects the different systems actually had on the stability of the nanoparticles.

The results showed that:

• The addition of NOM increased the stability of the systems with NaCl and Na₂SO_4.
• NOM had little influence on the CaCl₂ system, suggesting that there is bridging coagulation between the NOM and calcium.

In general, it was demonstrated that a low coverage of NOM on top of bare titania particles can induce aggregation, but further coverage can protect them from aggregation – even at high ionic strengths. The degree of coverage is governed by the concentration ratio between nanoparticles and NOM.

The study showed that the ionic strength, pH, quality and quantity of NOM all have a significant influence on the aggregation behaviour of titania.

To read more about the full experiment, download a copy for free by clicking the link below.

Influence of different types of natural organic matter on titania nanoparticles stability: effects of counter ion concentration and pH

Julian Alberto Gallego-Urrea, Jenny Perez-Holmberg and Martin Hasselloev

DOI: 10.1039/C3EN00106G

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A constant critique of numerous research conducted on the fate and transformation of nanoparticles has been the inability to reach concentrations that are environmentally relevant. Finally, this problem seems to have reached the end of the tunnel. Single particle ICP-MS (spICP-MS) has come to the rescue with the potential of detection and characterization of trace level engineered nanoparticles (ENPs) in complex matrices. Not only that, the analysis is even possible in the presence of known sinks for ENPs such as dissolved organic carbon (DOC), sediments and biota. This is because the measurement is of the primary particle size instead of increasing dissolved ions in the solution.

D.M. Mitrano and the colleagues from Colarado School of Mines, USA explicitly demonstrate this measurement of particle size in their analysis of Ag ENPs dissolution in different matrices using spICP-MS. The study considers two different sized Ag ENPs; 60 and 100 nm with three different polymer coatings: citrate (CA), tannic acid (TA) and polyvinylpyrrolidone (PVP) at predicted environmentally relevant concentrations (ng/L range).  Figure 1, taken from their paper, represents the decrease in the pulse intensity over time as the dissolution of 100nm TA@Ag ENPs at 50ng/L takes place.

Figure 1

This raw intensity data is then used in computing the respective particle sizes based on calibration with dissolved standards.

The effect of water chemistry parameters under these dilute concentrations are hence investigated by quantitative evaluation of the rate and extent of particle dissolution by the using solutions with increasing complexity; deionized water, tap water, surface water and moderately hard reconstituted water to mimic the realistic environmental matrices. Comparisons have also been made between the different polymer coatings.

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Tracking dissolution of silver nanoparticles at environmentally relevant concentrations in laboratory, natural and processed waters using single particle ICP-MS (spICP-MS)
Denise M Mitrano, James Ranville, Anthony Bednar, Karen Kazor, Amanda S Hering and Christopher Higgins
DOI: 10.1039/C3EN00108C

Having recently read several papers regarding the toxicity of nanoparticles to the aquatic environment, I was starting to view nanoparticles as an environmental bad guy. However, this study conducted by Anthony B. Dichiara and colleagues from Rochester Institute of Technology has stopped me in my tracks. It appears that nanoparticles could provide a sustainable solution for future aqueous purification systems – in the form of carbon nanocomposite papers – and we like sustainable solutions!

Activated carbon is currently the most commonly used material for water purification, but carbon nanotubes have higher adsorption capacities. For an adsorption material to be used for large-scale removal of pollutants from water, the regeneration property and reusability must also be taken into consideration.

In this study, the recycling efficiency of Graphene nanoplatelet-single-walled nanontube hybrid papers (graphene cylinders) when saturated with 2,4-dichlorophenoxyacetic acid was investigated.  Figure 1 shows that every time the papers were recycled the uptake of 2,4-dichlorophenoxyacetic acid always exceeded that of the original adsorbents.

Figure 1

Carbon nanocomposite papers are therefore potential adsorbents for future aqueous purification systems due to their larger adsorption capacity and enhanced recovery ability. So perhaps nanoparticles are not too bad after all!

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Enhanced adsorption of carbon nanocomposites exhausted with 2,4-dichlorophenoxyacetic acid after regeneration by thermal oxidation and microwave irradiation, Antony Dichiara, Jordan Benton-Smith and Reginald Rogers

DOI: 10.1039/C3EN00093A

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Cellulose-derived structural supports can improve the performance and the environmental credentials of gold nanoparticle catalysts. This study from the Polymer Research Institute of Sichuan University in China demonstrates the potential of this approach.

Achieving the optimum performance and stability of catalysts is of paramount importance to the development of a wide range of industrial processes and the manufacture of new products and devices. Gold nanoparticles (Au NPs) are widely used as catalysts in many chemical processes. However, these often suffer from instability owing to their large active surface area which can lead to self-aggregation. They therefore require a supporting matrix, commonly using polymeric encapsulation or support on graphene oxide or silica surfaces.

However, greener, more sustainable approaches are now being sought to improve the environmental credentials of these processes. Cellulose is the most common and abundant natural polymer.  Cellulose nanocrystals (CNs) can therefore provide a possible solution as these posses the desired properties (mechanical strength, self-assembly, high specific surface area, nanometirc dispersity and high stability) to allow them to act as an effective catalyst carrier.

However the use of these have previously involved use of toxic reducing agents such as sodium borohydride or hydrazine hydride. A cleaner synthesis is required. In this study Xiaodong Wu and co workers report the first instance of a one-step, environmentally-friendly synthesis of gold NPs (Au NPs) deposited on cellulose nanocrystals and demonstrate the technical and environmental advantages of this approach.

CNs were derived from bulk cellulose using controlled acid hydrolysis of waste cotton fabric. The Au NPs were synthesised and deposited on to CNs by hydrothermal reduction of HAuCl₄. Both Au NP-CN hybrids and unsupported Au NPs were prepared and their performance in the reduction of 4-nitrophenol was compared using UV-Vis spectroscopic analysis. The structure and properties of the resulting particles were characterized using transmission electron microscopy (TEM), X-Ray Diffraction (XRD) and X-Ray photoelectron spectrometric (XPS) techniques.

It was shown that the electron-rich –OH groups, abundant on their surface allow the CNs to provide the dual role of reductant and stabilizer in this process. The larger specific surface area and better dispersity of Au NPs supported on CNs meant these nano-hybrid catalysts displayed enhanced stability and superior catalytic activity than an unsupported Au NP catalyst for the reduction of 4-NP.

This approach offers a number of important advantages over previous traditional techniques. It has a lower overall cost and avoids the use of toxic or dangerous reducing, capping or dispersing agents. Furthermore this process utilises a renewable and biodegradable natural resource since the CNs can be obtained from various plant materials.

This investigation could therefore potentially pave the way to green production of bio-supported organic/inorganic nano-hybrid catalysts with possible further applications in the production of sensors, antibacterial materials and electronic devices.

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Green synthesis and formation mechanism of cellulose nanocrystal-supported gold nanoparticles with enhanced catalytic performance

Xiaodong Wu, Canhui Lu, Zehang Zhou, Guiping Yuan, Rui Xiong and Xinxing Zhang Environ. Sci.: Nano, 2014, 1, 71-79 DOI: 10.1039/C3EN00066D

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