Finding a method to assess commercial nanoparticles

Video: Are silica particles toxic for the environment?

 

How are nanoparticles emitted?

Silica, or silicon dioxide (SiO2), is the main constituent of the earth’s crust, making this raw material inexpensive and present practically everywhere. Manufactured silica nanomaterials are used in many applications in society, such as building materials, paints, cosmetics, pharmaceuticals and food, for example.

Synthetic amorphous silica is the form of silicon dioxide that is intentionally manufactured. Roughly 4 million tons per year of amorphous silica materials, including silica gel and colloidal silica, are sold and used globally. Silica nanomaterials are potentially emitted into the environment from the commercial products they are added to via different routes, either from the time of production, through the wear of products as well as in the refuse phase of the product.

To increase our understanding of the potential environmental hazards and risks of manufactured silica nanomaterials, we have evaluated a suite of particles having different particle size and surface chemical properties. We hope to identify critical particle characteristics that are correlated to the environmental fate of the particles, particularly in fresh water systems. One challenge we have is to be able to track nanoparticles in real as well as simulated environments, as silica is so abundant and is found practically everywhere.

In our program, AkzoNobel have supplied the researchers with a wide range of colloidal silica particles of varied sizes and surface modifications, chosen from the existing variety of commercial particles. With the help of a research group at Chalmers (Dr Romain Bordes and Dr Krzysztof Kolman) fluorescent labelled silica particles have been synthesized. With the fluorescent dye built into the silica nanoparticles, they can easily be distinguished from the background of silica in natural waters.

What happens when the nanoparticles are emitted?

Where and how do nanoparticles interact with living organisms? To be able to understand how nanoparticles interact with organic matter, the researchers have been using model molecules for organic material, or NOM (Natural organic matter). The surface of nanoparticles will likely be changed when the particles are released into nature, as biomolecules will bind to them, forming an “eco-corona”.

The eco-corona is dependent on the organisms present, the surrounding conditions (eg pH, temperature), as well as the surface properties of the nanoparticles. Researcher Jörgen Rosenqvist and Assistant Professor Caroline Jonsson at University of Gothenburg wanted to see how both small and large natural organic molecules, naturally found in runoff waters, streams and lakes interact with silica nanoparticles depending on their size and surface charge, as well as the pH and salt concentration of the molecular suspensions.

The researchers found that NOM does not induce silica nanoparticles to aggregate, indicating a lack of binding to the silica nanoparticles. The particles remain stable in water suspensions containing the NOM molecules. The high stability of these particles suggests that they could spread from a point source over quite a large area. At the same time, the particles are also likely to be efficiently diluted by this transport and therefore probably more unlikely to have a large impact on the ecosystem.

Christoph Langhammer and his group at Chalmers University of Technology are developing experimental methods to study how nanomaterials and nanoparticles interact with their surroundings. They use a phenomenon that emerges in metal nanoparticles when they are affected by light.

The light makes electrons in the metal sway, which is called plasmon resonance. The phenomenon is then used to “sense” how the nanoparticles interact with the surroundings. Their first study of palladium nanoparticles clearly shows that nanoparticles are like “individuals” with distinctive character and temperament, even though they were manufactured with the goal to be totally the same. The method has been developed during the recent years, and it is now possible to study individual nanoparticles in an efficient way. In the context of our program, the goal is now to start to understand in what way this difference between nanoparticles affects how they interact with their surroundings, for example proteins and other molecules that would be found in connection to aquatic organisms or living cells, and whether they are harmful or not.

Last year Post doc Rickard Frost, working together with Christoph Langhammar at Chalmers and MEN researcher Tommy Cederwall at Lund University, published in the journal Nanoscale the use of their nanoplasmonic sensor (NPS) to study real-time in situ analysis of biocorona formation occurring on surface-associated nanofabricated metal core– SiO2 shell nanostructures. One benefit of the NPS sensor is that when used under conditions that would normally promote aggregation of SiO2 nanoparticles in suspension, such as the adsorption of certain proteins to the nanoparticles, aggregation of the SiO2 nanostructures at the sensor surface is prohibited. They were therefore able to follow the exchange of proteins at the SiO2 nanoparticle surface, and found that the biocorona formed at curved versus flat (faceted) portions of the nanoparticle surface exhibit different properties.

– It is important to mention that nanomaterials are not only extremely exciting to modern technology, but also pose a risk to humans and the environment. Our participation in the program ensures that we take a holistic view on nanomaterials and not only focus on their potentially groundbreaking properties for new applications, which I find very important, says Christoph Langhammer.

How toxic are the nanoparticles?

 

The same set of silica nanoparticles produced by the program partner AkzoNobel were tested in a variety of ecotoxicological and human toxiocological experimental models in the program. Researchers have tested the hazard of silica particles to bacteria, algae and fish cells.

This setup from Professor Thomas Backhaus’ group at University of Gothenburg is, together with the test models at Lund University, a reflection of the major trophic levels in an aquatic ecosystem (algae as representatives of primary producers, daphnias as primary consumers, fish (cells) as secondary consumers and bacteria as destruents). Substantial effort was invested in order to complement these tests with investigations using fish embryos, an OECD‐standardized assay. Unfortunately, those experiments were unsuccessful so far, due to technical problems.

Frida Book, PhD student at University of Gothenburg, first conducted the tests with the presence of a common biocide, which proved to show a considerable toxicity. The following tests were therefore conducted without the biocide. The tests were made with a concentration-response design. The results of the study show toxic effects only at concentrations that are several orders of magnitude higher then environmentally relevant levels. It can be concluded that manufactured silica nanomaterials currently do not pose any discernable risks to the freshwater aquatic environment.

Post doc Mikael Ekvall, together with Prof Lars-Anders Hanson at Lund University conducted toxicity tests of the same silica nanoparticles on the zooplankton Daphnia magna – which represents another trophic level. The researchers in Lund performed acute tests of 24 hours, and then extended tests of 48 hours. The concentrations of silica were far higher than natural concentrations in the extended test. Despite that, the researchers did not see any toxicity on the zooplankton.

Researchers in the program have also completed tests with silica nanoparticles on human cell culture. They evaluated them for cytotoxicity (how toxic they are to cells) using a human immune cell line as a model.

At Karolinska Institutet (KI) Professor Bengt Fadeel’s group found that some silica particles triggered a dose-dependent toxicity in human cells while others were not toxic. They concluded from these studies that the size of the particles as well as the surface modification played a significant role for the toxicity. In the on-going work, it will be important to understand why some particles have a dose-dependent toxicity while others don’t show this behavior.

Another next step for the researchers at KI is to study how the silica nanoparticles affects human lung cells. This is done to compare the effects on immune cells and lung cells. The results will be published together with results from different model systems, spanning from human and fish cell lines, to zooplankton, bacteria and algae. In this way, the researchers can show the potential impact of nanoparticles across many relevant species.

 

Reproduced with permission from Winkler et al. Toxicology. 2013 Nov 8;313(1):15-23.

 

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