In some nanotechnology circles, it is almost a mantra that, once released to the environment, nanoparticles will inevitably aggregate or agglomerate into larger masses and thereby lose their nanoscale-related properties and, by implication at least, any associated risks.

But can we count on nanoparticles released to the environment to self-regulate their own risk so conveniently?

For example, the National Nanotechnology Initiative prominently features on its website an article it commissioned, titled "Understanding Risk Assessment of Engineered Nanomaterials:  How can we know what is a risk and what is not?"  In a section designed to lead readers to question published studies that suggest nanomaterials might pose risks, the article says: "In solution or in air, it's quite difficult to keep nanomaterials separate, as they tend to clump in larger aggregates or agglomerates."  This is a point the author of this rather short article felt compelled to repeat twice more.

An FAQ issued by Germany's Federal Institute for Risk Assessment states that "nanoparticles tend to aggregate into larger unions which are generally larger than 100 nm. The toxic effects of nanoparticles linked to their small size and higher reactivity are then no longer relevant."

And a recent post on the blog of the nanotechnology practice group at Porter and Wright asserts that nanoparticles "have been shown to have fewer potential adverse health effects when they occur in cluster form (aggregates and/or agglomerates).  In the 'good news' department, scientists studying aerosol dispersion of nanoparticles have found they tend to cling together when dispersed into the environment." 

I won't even begin to try to lay out here how much more complex and unpredictable than this nanoparticle aggregation and environmental fate and transport are in the real-world.  Instead, let me just cite two excellent papers that do so:  see Maynard and Weisner et al.

But I do want to briefly discuss and cite some recent studies supporting three reasons why we can't count on nanoparticles released to the environment to self-regulate their own risk so conveniently: 

1.  Some nanomaterials can be stabilized as nanoscale particles in solution under environmental conditions.  A number of studies have found that carbon-based nanomaterials – despite their inherently very low water solubility – can be "solubilized," that is, can enter and remain in stable suspensions upon interaction with water or with other common, naturally occurring substances.  The latest study, authored by Salonen et al. and published in the journal Small, finds that C₇₀ fullerenes can form "stable, homogeneous suspensions" in water through interaction with ubiquitous phenolic acids that are present in and released from virtually all plant matter.  It appears that individual C₇₀ fullerenes first become coated with the phenolic acid, and then form small, loose clusters with diameters on the order of a few nanometers.  This study merited a "spotlight" on the Nanowerk website.

Earlier work has found similar behavior:  Fortner et al. identified the formation of stable suspensions of "nanocrystals" of C₆₀ fullerenes in water – nanoscale (25-500 nm diameter) aggregates they call "nano-C₆₀" that have entirely shed the extreme hydrophobicity of the individual fullerenes.  Hyung et al. found that multi-walled carbon nanotubes could be stabilized as individual particles through interaction with natural organic matter found in river waters; the natural material actually worked better than commonly used surfactants selected to serve that same solubilizing function.

2.  For performance reasons, nanoparticles are being actively engineered not to clump.  For most nanomaterial applications, optimal performance depends on minimizing any disordered clumping or even maximizing dispersal, so that the properties of individual nanoparticles or highly ordered nanostructures can fully exert themselves.  For this reason, researchers are working overtime to coat, cap, chemically modify or otherwise force nanoparticles not to aggregate or agglomerate.  See, for example, Yang et al.'s use of special capping agents to prevent aggregation of platinum nanoparticles, and Nadagouda and Varma's similar work with silver and palladium nanoparticles.  Similar efforts have been mounted to chemically modify ceramic nanoparticles to ensure dispersal.

So even to the extent that native or current forms of nanoparticles do readily clump or retain their hydrophobicity, any assumption that engineered nanomaterials entering commerce and the environment will inevitably do so is wholly unwarranted.

3.  Even agglomerated or clumped nanoparticles can be toxic.  The assumption that aggregated nanoparticles lose all of their nanoscale properties or become benign is also unwarranted.  Maynard and Kuempel have amply demonstrated that even large aggregates on individual nanoparticles typically retain many of their nanostructural features and properties.  But what about toxicity?

Fortner et al. found that their fullerene nanocrystals exhibited antimicrobial activity, suppressing bacterial growth and respiration.  In addition to confirming nano-C₆₀'s antibacterial activity, a recent paper by Lyon and Alvarez cited a number of studies demonstrating that the formation of these nanoscale aggregates in water yields a material with high toxicity to aquatic invertebrates, fish and the cells of higher organisms.  The aggregates have also been shown to enter and accumulate in those cells and to adhere to lipids.

Finally, Salonen et al. showed that their phenolic acid-coated C₇₀ clusters could readily translocate across the membranes of human cells in culture and enter the membrane surrounding the cell nucleus.  Moreover, they induced the contraction and ultimate death of those cells – ironically, apparently by aggregating into micro-sized particles through interaction with the cell membranes.

Once again, we find that nanomaterials' actual behavior confounds conventional wisdom and, when approaching their toxicology, forces us to question or abandon our assumptions and biases.

Can nanoparticles get into our drinking water and if so, what's the harm?

Nanoparticles are being used in cosmetics and other personal care products with increasing frequency.  Carbon fullerenes, also known as buckyballs, have recently been touted as imparting age-defying antioxidant benefits when added to skin cream.  And there are some studies that seem to support these claims.  But even if such claimed benefits turn out to be true, this is by no means the end of the story. 

Skin creams are eventually washed off and down the drain.  Once they enter a sewage treatment system, the fate of nanoparticles is largely unknown.  Depending on their physical and chemical properties, engineered nanoparticles may wind up in the sludge – the solids – or they could spill over into the wastewater discharge, and wind up in lakes, rivers and streams.  If the latter occurs, they could end up in our drinking water.

This is not wild speculation.  One of the most hotly debated issues in environmental science is the frequent detection of chemicals from pharmaceuticals and personal care products in our drinking water.

If fullerenes act as antioxidants, then what's the big deal if they get into our drinking water — wouldn't they be beneficial? The very properties that allow antioxidants to scavenge the reactive oxygen species (ROS) that can damage cells also allow these compounds to liberate them, by cycling between uptake and release.  Under certain circumstances, antioxidants are more likely to release ROS than to take them up.  More research is needed to understand under what conditions so-called antioxidants might be likely to contribute to rather than prevent cell damage.

A new study published in Environmental Health Perspectives evaluated the effect of a single oral dose of fullerenes and single-walled carbon nanotubes (SWCNTs) on the extent of oxidative damage to DNA in the liver, colon and lungs of rats.  The authors were duplicating a study that demonstrated oxidative DNA damage in these organs following a single oral dose of the constituents in diesel exhaust.  DNA damage is important, because it is often a precursor to cancer.

In the new study, rats fed either fullerenes or SWCNTs exhibited increased levels of DNA damage in the liver and lungs, sometimes even at quite a low dose.  Neither nanoparticle had a significant effect on the colon at the doses tested, however.

So what does all this mean?  In my view, since we already know that chemicals in personal care products can wind up in drinking water, we should take more of a look-before-we-leap approach to evaluating new chemicals, including engineered nanomaterials, for use in these products.  This need also extends, of course, to the thousands of ingredients already present in personal care products that have not been adequately tested for safety.

Among the questions we need answered up front:  what is the expected fate and transport of such nanoparticles in the environment, who or what might be exposed during this process (including fish, bacteria, and any other critters), and if they wind up in our drinking water, what are the potential health effects?  The study discussed in this post is but one tiny piece of this puzzle.  (Of course, oral exposure to nanoparticles may occur through routes other than drinking water contamination, such as through direct ingestion of nanomaterials in foods or via the mucociliary escalator that removes inhaled particles from the lung to the digestive tract.)

Clearly we need regulators to be asking more of these questions – and requiring companies to provide the answers.  As my colleague Richard Denison pointed out in two of his recent posts, however, this doesn't appear to be happening with new chemical notifications being processed by EPA.  Instead, EPA seems to regard an absence of data as grounds for concluding an absence of risk.  And EPA gives the potential impacts at the downstream end of nanomaterials' lifecycles very short shrift.