Fluorescence complexation could hold the key to more detailed monitoring of airborne nanomaterials. Researchers at Texas A&M University, USA describe the successful development of a new method of simultaneous online quantification and characterization.

The increased manufacture and use of nanomaterials has led to increased concerns about their associated health risks, particularly in the occupational setting. One of the main barriers to addressing uncertainties in this field is a poor understanding of personal exposure levels. There is currently a lack of sufficient dynamic data regarding the main exposure routes for airborne nanomaterials so appropriate exposure guidelines, profiles and models cannot be established.

It is important, therefore to establish effective means of measuring the levels and chemical/physical properties of nanomaterials in real-time. Preferred traditional means of nanomaterial analysis e.g. mass spectrometry (MS) and electron microscopy (EM) cannot be easily adapted to use in an online capacity. This means exposure analysis is generally slow and can only be carried out for relatively short, unrepresentative time-frames.

In this study Fanxu Meng and co-workers introduce and demonstrate an integrated methodology that allows continuous online monitoring of the levels and characterization of airborne nanomaterials. This method combines ultra high flow sampling with a sensitive fluorescence-based detection system.

The sampling system comprised a modified wetted wall cyclone (WWC) collector which has an ultra high flow rate (>1000 L min^-1) combined with a continuous flow microfluidic network allowing online detection capability. The detection system utilized florescence generated by the combination of a suspension of collected nanoparticles and a tracer dye solution. The resulting dye-nanoparticle complex produces an intense and easily detectible fluorescent signature, dependent on the quantity and the physical/chemical properties of the collected nanomaterials.

The integrated system was tested using prepared suspensions of Al₂O₃. A scanning mobility particle sizer (SMPS) was used to quantify the concentration and size distribution of nanoparticles inside the test chamber. It was shown that florescence displays a characteristic pattern, dependent on the concentration but also the size and particle surface area of the nanomaterials. A linear correlation between florescence intensity and airborne concentration of Al₂O₃ at concentrations up to 1.0-1.2 wt% at flow rates of 0.2 and 0.02 mL min^-1 was observed.

This work provides a platform for continuous measurement of airborne nanomaterials. Simultaneous sampling and compound characterization can enable better time-resolved assessment of the transport and fate of released nanomaterials and identification of hazardous releases. The method described is capable of sampling a broad range of air volumes (representative of the work place environment) and allows online detection and analysis. The authors highlight potential further innovations for this work, possibly leading to more detailed exposure profiles for nanomaterials; establishing fluorescence “fingerprints” for a range of different nanoparticles; and analysis of biological material.

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Localized Fluorescent Complexation Enables Rapid Monitoring of Airborne Nanoparticles
Fanxu Meng, Maria D. King, Yassin A. Hassan, and Victor M. Ugaz
DOI: 10.1039/C4EN00017J

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A study by Gregory Lowry and colleagues from Carnegie Mellon University suggests that Copper Oxide (CuO) nanoparticles (NPs) are sulfidized in the environment, affecting their resulting properties. Some of the affected properties, such as solubility are relevant to the toxicity of these nanoparticles in the environment.

Many nanoparticles are transformed in the environment; it is often the transformed materials which cause concern with regards to nanoparticle toxicity. There have been several papers highlighting how important it is to research the properties of nanoparticle transformations. A recent review by Nasia Von Moos provides an overview of what is currently known about environmental transformations of nanomaterials in freshwater systems, a recent paper by Julián A. Gallego-Urrea discusses the transformations which TiO₂ nanoparticles undergo once they reach the aquatic environment and this research paper reports that sulfidation is an important transformation for some metal oxide nanoparticles, such as CuO NPs.

Why is sulfidation in the environment important?

Copper-based NPs are being used in semiconductors, heat transfer fluids, catalysts, batteries and many more products and technologies. Their wide spread uses will likely lead to subsequent release into the environment, raising concerns about their potential toxicity. It has already been demonstrated that CuO NPs are toxic to many organisms including crustaceans, algae and fish; although Cu²⁺ is more toxic to most of these organisms. It is therefore essential to determine what the products of sulfidized CuO NPs are and if they are more or less toxic to the environment when compared with pristine CuO.

Cuo NPs were characterized and sulfidized in water by inorganic sulfide. Characterization of the resulting products showed that CuO is sulfidized to several copper sulphide species including crystalline CuS (covellite), amorphous (Cu_xS_y) species and copper sulphate hydroxide species. In previous studies it has been demonstrated shown that sulfidation decreases the solubility and metal availability of Ag and ZnO NPs. This study however shows that the sulfidation of CuO NPs breaks the trend. Sulfidation actually increased the dissolved fraction of copper compared to pristine CuO NPs. This increased release of Cu²⁺ and CuS nanoclusters from sulfidized NPs compared to CuO suggests that toxicity studies with pristine CuO may be misleading in environments where sulfidation is likely to occur, demonstrating that it is prudent to use environmentally transformed nanoparticles in fate, transport and toxicitiy studies rather than focusing soley on the prisitne materials. Access the full article for free* by clicking the link below.

Sulfidation of copper oxide nanoparticles and properties of resulting copper sulfide
Rui Ma, John Stegemeier, Clement Levard, James Dale, Clinton W Noack, Tittany Yang, Gordon Brown and Gregory Lowry
DOI: 10.1039/C4EN00018H

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The author recommends further studies which are still needed to:

1. Identify the nature of the Cu_xS_y nanoclusters.
2. Assess the toxicity of sulfidized CuO NPs and Cu_xS_y nanoclusters.
3. Assess the stability of very small metal sulphide clusters (Ag, Zn and Cu) against oxidation under environmental and biological conditions.
4. Assess how sulfidation of CuO NPs occurs in situ at relevant CuO/S concentration ratios and how this affects their bioavailability under realistic exposure scenarios.

Sean Lehman and Sarah Larsen from the University of Iowa review zeolite and mesoporous silica nanomaterials with emphasis on connections to the environment.

In recent years, there has been a great deal of interest in zeolites and mesoporous silica nanomaterials (MSNs). Zeolites are widely used in industry for applications such as catalysis, separations and gas adsorption, however the authors believe that these porous nanomaterials have a largely unrealized commercial potential for environmental applications.

This review article covers three major areas:

1. Greener synthesis of zeolite and MSNs
2. Potential of zeolite and MSNs for environmental applications
3. The biological toxicity of zeolite and MSNs

Due to cost and reduced thermal stability MSNs are not as extensively applied as zeolite; however they are currently being investigated for potential environmental and biomedical applications. Their varied physiochemical properties open up a wide range of potential applications. The more applications that these porous nanomaterials have in industry, the greater the interested in developing greener synthesis for them and reducing their toxicity. With two measurements which are on the nanoscale, pore size as well as particle size, zeolites and MSN make very interesting nanomaterials.

This review describes both the environmental applications, including environmental catalysis and adsorption of environmental contaminants, and implications of zeolite and MSNs. Due to concerns that increased use of these materials translates to increased exposures, toxicity studies of both nanomaterials are also reviewed.

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Zeolite and Mesoporous Silica Nanomaterials: Greener Syntheses, Environmental Applications and Biological Toxicity

Sean E Lehman and Sarah C Larsen
DOI: 10.1039/C4EN00031E, Critical Review

Zeolites and MSNs are silicate or aluminosilicate nanomaterials with well-defined pore networks; there are however some differences between the two porous nanomaterials.

Properties of zeolites:

• Crystalline aluminosilicates (or silicates)
• Regular arrangements of micropores
• High surface areas
• Exchangeable cations

Properties of MSN:

• Amorphous silica materials
• Regualr arrangement of mesopores
• Very high surface area

The first area discussed in this review is the synthesis of zeolite and MSNs using green synthetic routes. The green strategies can be organized into three main categories: solvent, template and heating. The diagram below demonstrates the strategies for the greener synthesis of zeolites and mesoporous silica.

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