Environmental Science: Nano Blog

As nanoparticles find their way into more products, consumers and scientists alike are concerned about the impact their spread may have on our health. When answering this question, it is important to consider not just our direct interaction with nanoparticles through consumer products that incorporate them, but also the ways they might indirectly make their way into our environment. For instance, nanoparticles in the soil could be taken up by plants that we might later eat.

As a global food staple, maize is an ideal candidate for a comprehensive investigation of this topic. In a recent study published in Environmental Science: Nano, a team of researchers investigated the extent to which maize plants take up zinc oxide (ZnO) nanoparticles—one of the most widely used nanomaterials—and the pathways by which they do so. Their results suggest that ZnO nanoparticles dissolve into Zn2+ ions to make their way into the epidermis and roots of the plants, but rarely translocate to the shoots.

The researchers grew maize hydroponically, adding different concentrations of ZnO nanoparticles or Zn2+ ions to the water. Unsurprisingly, higher concentrations of zinc in the growth medium correlated with higher concentrations of zinc in the plants. The zinc content in the maize plants was virtually identical whether the plants were grown in ZnO solution or Zn2+ solution, suggesting that most ZnO nanoparticles make their way into maize plants by first dissolving into Zn2+, instead of being taken up whole. Zinc taken up by this pathway tended to form phosphate complexes inside the plants, largely preventing it from moving upwards into the shoots.

However, TEM imaging of plants treated with fluorescently labeled ZnO nanoparticles showed that some intact nanoparticles did find their way into the maize plants. These nanoparticles accumulated mostly in the root cortex, occasionally making their way into the vascular tissue. As with the dissolved zinc, though, the zinc oxide nanoparticles were often biotransformed to zinc phosphate and prevented from moving into the shoots.

It seems that in the case of maize, zinc oxide nanoparticles do not directly impact the parts of the plant that we would eat, but excessive accumulation of zinc compounds could potentially affect the plant’s overall health. It is unclear from this study whether the findings can be generalized to interactions between other crops and other types of nanoparticles, or even whether the pathway holds for soil-grown (as opposed to hydroponic) maize plants. Nevertheless, it provides a first step towards a comprehensive understanding of plants’ responses to and defenses against nanoparticles.

To access the full article, download a copy for free* by clicking the link below:

Liked this blog post? Read Laurel’s previous entry on how rare earth elements trace nanoparticles through the environment.

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Ceria nanoparticles, also known as nanoceria (cerium IV oxide, CeO₂) are quickly climbing the nanotechnology popularity ladder.  Ten years ago, there were hardly any academic publications using the term “nanoceria” and now there are dozens of publications per year on the subject. However, the consumer market still only has a couple of products that advertise to contain this nanomaterial.

So, could it be that we are finally getting ahead of the curve in attempting to understand environmental impacts of this nanomaterial before it becomes widely popular?

ES Nano recently published a special themed collection on this interesting nanomaterial, whose main property as a catalyst brings promise for a variety of applications. Two other Environmental Science: Nano blog posts have focused on the health effects of nanoceria and its biodistribution in rats.

Nanoceria is a powerful catalyst because its chemical structure shows an oxygen vacancy, so oxygen atoms can move around it while oxidizing and reducing molecules in its vicinity.

It absorbs reactive oxygen species (ROS), also known as free radicals, which brings a potential cosmetic and medical application. This material also absorbs UV radiation, so it might be used to replace titanium dioxide and zinc oxide in sunscreens in the future.

Since nanoceria has the potential to be a widely used in medical and cosmetic applications, it is pivotal to understand its behavior in biologically relevant environments. A recent paper by Sudipta Seal and colleagues discusses the environmental factors that can alter the properties of nanoceria and thus dictate its behavior in biological systems.

According to the authors, properties such as size, surface chemistry, surface stabilizers of nanoceria may affect its behavior in biological systems, but important issues remain to be addressed: Do slight variations in size and physico-chemical properties dictate fundamentally different behaviors? Are observed variations due to fundamentally different nanoparticles or did those particles undergo transformations? How should particles be appropriately prepared for relevant environmental and toxicology studies?

Although these general questions can be asked about a number of other nanomaterials, they are particularly relevant to nanoceria, since so little is known about this trending and promising material so far.

To access the full article, download a copy for free* by clicking the link below:
Behavior of nanoceria in biologically-relevant environments
Amit Kumar, Soumen Das, Prabhakaran Munusamy, William Self, Donald R. Baer, Dean C. Sayle and Sudipta Seal
Environ. Sci.: Nano, 2014, 1, 516-532
DOI: 10.1039/C4EN00052H

Did you like this article?
Find out more about Marina in her first Environmental Science: Nano blog article on carbon nanotubes.

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You and I are not friends, for you are a fish and I am a clam. We are uneasy neighbours. The river water is your playground, the sediment – my sanctuary. If we were living in the Elk River in West Virginia, both of us would have been worried on January 9, 2014. Something changed in the water that day, as crude 4-methylcyclohexanemethanol (MCHM) poured into the river from a leaky storage tank. Would the MCHM have dissolved in the water or bound to sediment? Which of us would have been in more danger – the resident of the water or the denizen of the sediment?

But you are no fish and I am no clam. So, we could try to answer these questions by doing some batch experiments: Take some sediment sample in a bottle, add some water and MCHM, close the bottle tight and shake it for some time. Then measure the concentrations of MCHM and to figure out whether it loves the water or prefers the sediment. We could also run column experiments: Mix some MCHM in water, flow through a packed column and measure the concentrations at the column inlet and outlet to see how much of the MCHM attaches to the column and how much is still left in water at the outlet.

The information gleaned from these experiments regarding the relative concentrations in the sediment, the water or the packed column, are called fate descriptors. Researchers have calculated fate descriptors for thousands of chemicals to figure out whether they partition into the water or the sediment, or volatilize into the air. Fate descriptors help us predict who faces the greatest danger – the clam, the fish or the dragonfly hovering over the water.

Can we run these batch and column experiments to reliably predict the behaviour of nanoparticles in the environment? The concept of fate descriptors calculated from batch and column experiments was established for molecular chemicals.

But nanoparticles have distinctly different properties and do not behave as single molecules. The use of some previously established fate descriptors to predict the environmental fate and transport of nanoparticles has come under criticism.

In a recent perspective article, Dr. Geert Cornelis has discussed the challenges in developing environmentally relevant fate descriptors for nanoparticles. In the same vein, at the 3^rd Sustainable Nanotechnology Organization Conference, Dr. Mark Wiesner and Dr. Greg Lowry stressed on the need to develop functional assays that provide nano-specific fate descriptors.

Like the batch and column experiments, these functional assays need to be operationally simple and of relatively short duration. Indeed, as mentioned in in the perspective article, “[t]he most appropriate method is most likely a compromise between technical accuracy and operational simplicity…”.

You can read more about these challenges and potential solutions in the full article for free*:
“Fate descriptors for engineered nanoparticles: the good, the bad, and the ugly”
Geert Cornelis
Environ. Sci.: Nano, 2015, Advance Article
DOI: 10.1039/C4EN00122B

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About the webwriter

Paramjeet Pati is a PhD Candidate at the Virginia Tech Center for Sustainable Nanotechnology (@VTSuN).
You can find more articles by him in the VTSuN blog, where he writes using the name coffeemug.

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Multi-walled carbon nanotubes (MWCNT) are the preferred choice of nanotubes for many applications as they has a lower cost than the single walled carbon nanotubes (SWCNT). Therefore the chances of them getting into air, natural water systems and soil is extremely probable. Although many beneficial effects are postulated for MWCNTs direct applications such as incorporation into fertilizer to enhance water uptake, seed germination and cell growth can increase their levels in the environment, especially in soil. Since there is evidence of some negative affects on soil microbial communities as well as plants it is always better to have means of monitoring and controlling their levels in the environment. However, methods to detect and quantify MWCNTs in soil and sediments are still not well established. Therefore, Alexander Gogos and co-workers from the Agroscope, Institute for Sustainability Sciences in Switzerland have developed and evaluated a novel approach using asymmetric field flow fractionation (A4F) coupled with multi-angle light scattering (MALS) to differentiate MWCNTs in soil.

Here the high aspect ratios of MWCNT’s have been exploited to differentiate between MWCNTs, soot and native soil particles. The shape factors (ρ) for these materials were calculated by taking the ratio between the radius of gyration (rg) and the hydrodynamic radius (rh). Simply, the rg corresponds to the weighted average of all possible radii of a particle from its center of mass and rh is approximated for non-spherical particles as the radius of a sphere with same diffusion behavior. Elaborately, presence of MWCNTs in the mixtures resulted in increased ρ-values. The fractions of MWCNTs in the mixtures were calculated using the ρ-values obtained from A4F-MALS. They were cross-validated by comparing with the results obtained from automated electron microscopy analysis and were found to be in reasonable agreement. Since natural soils exhibited lower ρ-values consistently this method can be used in specific identification of MWCNTs as well as other high aspect ratio nanomaterials in soil.

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Capabilities of asymmetric flow field-flow fractionation coupled to multi-angle light scattering to detect carbon nanotubes in soot and soil.
Alexander Gogos, Ralf Kaegi, Renato Zenobi, Thomas D. Bucheli
DOI: 10.1039/C4EN00070F

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