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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