Environmental Science: Nano Blog

Silver nanoparticles (AgNPs) are becoming increasingly popular due to their antimicrobial properties. In fact, silver compounds have been used to treat or prevent infections since before penicillin was ever discovered. The ability of AgNPs to kill microorganisms brings great potential for treating drinking water in situations when traditional water treatment is not possible.

Roughly 10% of the world’s population does not have access to safe, clean drinking water because basic sanitation is lacking. In some cases, a point-of-use (aka in-your-home) water treatment method that is easy and affordable might make a great difference in preventing or reducing the incidence of diseases caused by water-borne microbes, such as cholera and poliomyelitis. A water filter that is embedded with a small amount of AgNPs is a great example of a point-of-use treatment method that can be easily distributed to homes in developing countries and used with minimal training.

In this paper, T. Dankovich, from the University of Virginia, presents us an elegant method for creating paper filters that are embedded with silver nanoparticles. Her method can be considered more environmentally friendly than usual silver nanoparticle synthesis techniques because the reducing agent is glucose (that’s right, sugar!) and the heating technique involved nothing more than a domestic microwave oven (that’s right, a kitchen microwave!). In this technique, silver nanoparticles were synthesized directly on the paper filters, as opposed to being synthesized in a liquid suspension and then applied onto the filter. This method avoids the potential pitfall of nanoparticle aggregation during application onto the filter.

The paper filters were successful treating water containing two different types of bacteria (E. coli, and E. faecalis). Moving forward, it will be interesting to know the efficiency of this type of filter in treating real surface water samples and the filtering capacity, or how many liters of water each filter is capable of treating before it needs to be replaced.

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Microwave-assisted incorporation of silver nanoparticles in paper for point-of-use water purification
DOI: 10.1039/c4en00067f
Theresa A. Dankovich

Liked this blog? Find out more about Marina in her first Environmental Science Nano blog on carbon nanotubes.

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Nanocellulose (NC) provides a readily available and biodegradable substrate that can act as a novel template or carrier for a range of different nanomaterials (NMs) including carbonaceous, mineral and metal particles. NC is an ideal platform for inorganic NMs due to its high specific surface area, highly porous structure and high mechanical strength. The resulting NC combines the characteristics of both constituents therefore displaying synergistic properties useful for a variety of uses.

Peter Vikesland and colleagues from Virginia Tech and Duke University, USA provide a tutorial review discussing recent advances in the preparation of NC-based nanocomposites and their potential uses in addressing current and future environmental challenges. This is the first review of its kind to discuss the use of these novel nanocomposites in the context of environmental sciences and engineering applications.

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The authors describe:

• different forms in which NC is produced,
• how the NC can be derived both from plant materials and bacterial processes,
• how NC can be modified into a number of useful forms.

Furthermore, in-depth discussion of the different methods used in the preparation of NC and different types of composites is provided. This includes a discussion of how the guest NM can be incorporated in/on to the NC structure, guidance regarding the challenges faced in these processes and how researchers have address these problems, and instructions on the best available methods currently known for these procedures.

The review also details the uses of NC-based nanocomposities in key environmental science/engineering applications and summarises the practical considerations and advantages these provide over more conventional NMs. This focuses on four principal areas:

1) The better incorporation of antimicrobial materials such as Ag NMs into NC-based filters for air and drinking water purification.

2) The use of NC as a support for photocatalysts and metal catalysts used in the degradation of organic pollutants in water remediation.

3) The use of Au NP/NC biosensors for monitoring of water-borne pathogens and organic contaminants.

4) The use of NC-based nanocomposites in the design of superior energy conversion devices such as fuel cells, solar cells and Li-ion battery manufacturing.

Finally, potential directions of further research in the field of NC nanocomposites are highlighted. Specifically, researching methods to better control the size and distribution of NMs on or within the NC substrate, investigating how the loading of NMs influence the potential applications, and ways to prolong the lifetime and/or regenerate NMs to ensure their sustainability.

Cellulose is an abundant, cheap and renewable resource. Nanocellulose is shown to form useful nanocomposites with inorganic nanomaterials, which display valuable optical, catalytic, electrical properties. This review provides guidance to researchers in the field of environmental sciences and engineering on the production and uses for this type of nanocomposites to address current and emerging environmental challenges.

Environmental science and engineering applications of nanocellulose-based nanocomposites
Haoran Wei, Katia Rodriguez, Scott Renneckar and Peter J. Vikesland
Environ. Sci.: Nano, 2014,1, 302
DOI: 10.1039/c4en00059e

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Carbon nanotubes (CNTs) are promising nanomaterials because of their many interesting properties: CNTs are flexible yet super strong and practically impossible to break. Depending on their atomic arrangement, CNTs can be semiconductors or very strong conductors of electricity. When it comes to transporting heat, this amazing material can move it very quickly along its length, while behaving as an insulator from side to side. CNTs can also be doped or decorated with different chemical elements, compounds, or nanoparticles and serve as a transport carrier for those materials. For these reasons, along with many others, CNTs are being developed for a myriad of applications, ranging from electronics to drug delivery. Because on this increasing interest in applications of CNTs, we can expect their global production to grow over time, which may lead to an increase in the potential for CNTs to be released into the environment or to be put into contact with people.

Jie Li and colleagues explored the possibility that CNTs may serve as a “Trojan Horse” by carrying heavy metal ions that were incorporated onto their surface and then releasing those ions when they reach the environment. Imagine the giant Greek wooden horse, arriving the gates of the city of Troy, full of soldiers in its belly. Now, scratch that and imagine a carbon nanotube arriving in your local river, its back teeming with heavy metals. Will these metals dismount the CNTs and impact the wildlife of your local river? Will they go through the local treatment plant and arrive in your home, via your tap water? Or will these metals stay forever stuck onto the CNTs and not cause any harm?

To answer this question, researchers from China studied different types of CNTs with a variety of metal ions: copper, hexavalent chromium (a known carcinogen), and arsenic (a known toxic chemical).

They found that the absorption and desorption of heavy metals from CNTs do not occur at the same pace. In practical terms, the metal ions come off the CNTs slower than the rate at which they are put onto the CNTs. When comparing different types of CNTs, they found that multi-walled CNTs and double-walled CNTs have a higher capacity to carry arsenic and chromium cations than single walled CNTs or oxidized CNTs. For copper (an anion), they observed exactly the opposite.

The sorption of heavy metal ions onto the surface of CNTs is a reversible process.Therefore it is possible that once CNTs enter the environment, they release the metals which they were carrying. The higher the concentration of metals, the more reversible this process might be. The release of metals from CNTs occurs differently for negatively versus positively charged ionic metals and varies greatly among different types of CNTs. This means that we cannot predict what will happen to CNTs (and the metals they carry) in the environment without understanding their structure and knowing exactly what types of chemicals are sorbed onto their surface.

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About the webwriter

Marina Vance is a research scientist at Virginia Tech and associate director of the Virginia Tech Center for Sustainable Nanotechnology Her work focuses on people’s exposure to nanomaterials.

Follow her on twitter: @marinavance

Environmental transformation of engineered nanomaterials makes it extremely difficult for us to track them under natural conditions because their identity and the related properties are changing constantly. Therefore, much research is being conducted, simulating natural environmental conditions, to understand these nanoparticle transformations and the subsequent behavior. One such parameter which is constantly under investigation is the effect of sun light or more specifically the visible (400-700 nm) – lower energy UVA light (300-400 nm). Since higher energy UV-light (UVB and UVC) is efficiently absorbed by the ozone layer and the atmosphere, not much attention has been given to the transformations caused by them. However, UVC radiation is used heavily in municipal drinking and wastewater treatment plants for water disinfection from microorganisms. Given the increasing usage of carbon nanotubes (CNTs) and their potential release into water systems, Julie L. Bitter and co-workers of John Hopkins University, Baltimore in USA have conducted a thorough investigation on the transformations occurring on oxidized multiwalled CNTs (O-MWCNTs) upon the exposure to UVC (254 nm) irradiation.

In this work, O-MWCNTs suspended in ultrapure water was subjected to UVC irradiation under two configurations;

(1) large batch volumes,

(2) small batch volumes.

Samples treated under large batch volumes were used in characterization studies and mass loss measurements while the latter was used in particle sizing and concentration measurements. Furthermore, the effect of water quality parameters was investigated by varying the solution pH and the ionic strength.

The results indicated that O –MWCNTs surface undergo decarboxylation inducing aggregation causing the particles to settle out of the solution. This process was found to be dominated by one photon, direct excitation mechanism instead of the mechanism mediated by reactive oxygen species. Surface characterization with XPS analysis as well as chemical derivatization showed a significant reduction in the distribution of oxygen-containing functional groups upon irradiation, supporting the above observation. Furthermore, it was interesting to note that aggregation was resisted until a sufficient number of carboxylic acid groups were removed where the electrostatic repulsions between the O-MWCNTs were no longer strong enough to prevent aggregation. This UVC induced aggregation was observed at all the light intensities and under both oxic and anoxic conditions. The resistance towards photo-induced aggregation however, was enhanced under high pH and low ionic strength conditions.

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Transformations of oxidized multiwalled carbon nanotubes exposed to UVC (254 nm) irradiation

Julie L. Bitter,   Jin Yang,   Somayeh Beigzadeh Milani,  Chad T. Jafvert and   D. Howard Fairbrother

DOI: 10.1039/C4EN00073K

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Plants: basic components of the ecosystem and vulnerable to nanoparticle exposure. It is important to understand the interactions between nanoparticles and plants, especially when herbivorous consumers introduce these plants into our food chain.

As CeO₂ NPs are widely used in many applications, their interactions with the ecosystem are inevitable. It has previously been shown that CeO₂ NPs can inhibit root elongation of plants in aqueous suspensions. Dr Zhiyong Zhang et al investigated the toxicity of CeO₂ NPs on asparagus lettuce in a plant agar medium, a semisolid, soil-like medium which provides a more realistic environment for plant growth.

A variety of parameters were investigated to understand the plant’s defence and response to abiotic stress caused by CeO₂ NPs. Although the agar medium limited the bioavailability of CeO₂ NPs, they were still more toxic to asparagus lettuce in the agar medium than in aqueous solution. This could be caused by the production of excess reactive oxygen species causing oxidative stress to the plants.

The increased phytotoxicity of CeO₂ NPs in a soil like medium can also be explained by the biotransformation of CeO₂ NPs. It has previously been demonstrated that Ce³⁺ released from CeO₂ NPs can cause species-specific toxicity. This study showed that in an agar medium more than 20% of the Ce in the roots was transformed to Ce³⁺, whereas in aqueous solution only 6% of CeO₂ was reduced to Ce³⁺. It is therefore reasonable to postulate that the phytotoxicity of CeO₂ NPs is also attributed to the release of Ce³⁺.

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Effect of Cerium Oxide Nanoparticles on Asparagus Lettuce Cultured in an Agar Medium
Di Cui,   Peng Zhang,   yuhui Ma,   Xiao He,   Yuanyuan Li,  Jing Zhang,   Yuechun Zhao and   Zhiyong Zhang
DOI: 10.1039/C4EN00025K

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Nanoparticles – they are really small; we can’t see them, hear them or feel them and therefore they are pretty hard to detect! Manuel David Montaño, from Colorado School of Mines, and colleagues have made improvements in the detection and characterization of engineered nanoparticles, simply by reducing the time of detection for each nanoparticle – the dwell time.

In order to determine the toxicity, fate and transport of nanoparticles in the environment, we first need to determine the size and quantity of nanoparticles in the environment. In order to determine the size and quantity of nanoparticles in the environment we need to be able to accurately detect these nanoparticles. Single particle ICP-MS (spICP-MS) is already a promising technique to detect and characterize low concentrations of engineered nanoparticles in biological and environmental matrices. Initially developed for aerosol particle analysis, spICP-MS uses time resolved analysis with dwell times of approximately 10miliseconds. So how does it work? A discrete pulse of intensity, origination from nanoparticle vaporization and ionization, can be detected – the signal generated by the ions can then be correlated to nanoparticle mass.

The problem – this method of detection only works on low concentrations of nanoparticles. In high concentrations of nanoparticles, two or more nanoparticles can be detected during the same dwell time giving invalid results. It appears that particles are larger in size and lower in concentration that they really would be in the environment. To overcome this problem, often samples have to be diluted considerably, making the results less environmentally relevant. In this study, instead of diluting the samples, researchers simply reduced the dwell time from milliseconds to microseconds. This improved the resolution and working range of spICP-MS, allowing a greater breadth of environmental samples to be analysed.

You can read the full paper for free* by clicking the link below

Improvements in the detection and characterization of engineered nanoparticles using spICP-MS with microsecond dwell times
M. D. Montaño, H. R. Badiei, S. Bazargan and J. F. Ranville
DOI: 10.1039/C4EN00058G

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Titanium Oxide nanoparticles (TiO₂ NPs) – given their wide applications, exposure scenarios and classification as a class 2B carcinogen (International Agency of Research on Cancer), it is high time for the precise and accurate quantitative analysis of TiO₂ NP contamination in environmental samples. Being one of the metal oxides that is extremely hard to solubilize makes accurate and precise measurement of TiO₂ a challenge. Recent investigations using mixed acid digestion and alkali potassium hydroxide (KOH) fusion has given improved recoveries of TiO₂ when compared to the conventional methods. This is great news for all of us who struggle to quantify TiO₂ nanoparticles in complex environmental matrices! However, as with all nanoscale materials there are complications arising from size and polymorph dependent thermodynamic stabilities as well as chemical reactions between Ti and other sample matrices, especially at elevated temperature and pressure. Thus, R. G. Silva and coworkers of United States Environmental Protection Agency (US-EPA) investigate the digestibility of different polymorphs of TiO₂ NPs; anatase, rutile and brookite. These samples were used for spiking environmental matrices consisting of river sediment and clay minerals (bentonite and kaolinite). Furthermore, a portion of these were subjected to heat (300^OC) and pressure (10.3 bar) treatment to investigate its impact on the Ti recovery from the TiO₂ NPs.

Extensive characterization of all three nanoparticle samples with respect to size, shape, crystallinity and surface area before and after the heat and pressure treatments showed significant changes in the physicochemical properties of anatase and brookite. Rutile on the other hand was resistant to changes. In terms of digestion, acid digestion resulted in relatively lower Ti concentration for the pure TiO₂ NP samples that underwent heat and pressure treatment. In contrast, alkali fusion resulted in increased levels of Ti. Nevertheless, when the TiO₂ NP polymorphs were blended in the environmental matrices, for anatase and brookite the recoveries were similar for both types of digestions. However, for the recovery of rutile the alkali fusion method proved to be superior to that of the mixed acid method. Therefore, this work recommends using the alkali fusion method for the extraction of Ti from TiO₂ NP contaminated unknown environmental samples.

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Polymorph-dependent titanium dioxide nanoparticle dissolution in acidic and alkali digestions

R. G. Silva, M. N. Nadagouda, C. L. Patterson, Srinivas Panguluri, T. P. Luxton, E. Sahle-Demessieb and   C. A. Impellitterib
DOI: 10.1039/C3EN00103B

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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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