Smart materials; smart choices?

Why nano?  Why care?  For non-nanotech initiates, an obsession with nanotechnology must sometimes seem a bizarre occupation of the sad and lonely.  And even within the nanotechnology community, who hasn’t had occasional doubts over the legitimacy of singling out “nano” as something special?  Yet occasionally a piece of work comes along that helps put things back into perspective.  For me, a paper just published on-line in the journal Nano Letters did exactly that.

To be quite frank, the paper’s title is not what I would call inspirational.  But dig below the surface, and you unearth an object lesson in what makes nano so intriguing, and why taking a fresh look at possible health and environmental impacts is so important.  First the science though.

The Science

The paper in question is “Controlled Manipulation of Giant Hybrid Inorganic Nanowire Assemblies” by Fung Suong Ou, Manikoth M. Shaijumon, and Pulickel M. Ajayan, published on-line in Nano Letters, May 29 2008.  Unfortunately, a subscription to the journal is needed to view the paper, but the supplemental information is freely available (here), and well worth looking at.  

In brief, the authors used a nanoscale fabrication technique to construct long, straight, carbon nanotubes capped with gold nanowires.  Think “magician’s wand” with the nanotube as the stem and the gold as the white tip, and you will get the idea.  The nano-wands (for want of a better description) were between 100 nm and 150 nm wide, and over 100 mircometres (100,000 nm) long.  Micrographs in the paper show rafts of uniform-length nano-wands stacked side by side, with individual wands fraying off at the edges.

But this is where things get interesting.  These long, straight artificial rods were designed to have one end that was hydrophobic (water-repelling; the carbon end), and one end that was hydrophilic (water-seeking; the gold).  When dispersed in water, these wands formed a uniform suspension.  But when an organic solvent—dichloromethane (DCM)—was added to the mix, the nano-wands assembled into shells around the DCM, with the black carbon nanotubes facing in and the gold tips facing out.  With a bit of shaking and ultrasonic agitation, one large gold-coloured sphere was formed, separating the DCM from the water.  Reversing the process by suspending the nano-wands in DCM and adding water, a large black sphere assembled; separating the water from the organic solvent.  Black, because in this case the carbon nanotube “tails” were pointing outward.

Using the same fabrication technique, the researchers demonstrated a couple of other tricks.  By adding a band of the metal nickel below the gold tip, the nano-wands could be made magnetic—so now the spheres separating the two liquids could be moved around using a magnetic field.  And by adding an ultraviolet light-degradable hydrophobic chemical to the gold end of nano-wands, spheres were constructed that quite literally turned inside-out under UV irradiation.

The Promise

Nanotechnology is all about functionality—making materials and products that behave in new and unusual waysbecause they have been engineered at an incredibly fine scale.  This new and unusual behaviour might in some cases be due to the unusual physics and chemistry of small clusters of atoms (such as the size-related fluorescence of quantum dots).  But it can just as easily arise from engineering a material at such a fine scale that it can be used in new ways (such as making antimicrobial silver particles small enough to be incorporated into a miscellany of products); or constructing materials at the nanoscale with such sophistication that new properties emerge (multi-functional nano-therapeutics for instance).  The nano-wands are most definitely in the latter categories—their functionality arises from their smallness and sophistication.  

The important point here is that, while size matters, performance matters more.  And so while these nano-wands are technically larger than the 100 nm limit usually (and somewhat arbitrarily) imposed on nanotechnology, they nevertheless represent an ability to create a novel functional material through sophisticated engineering at a very fine scale.

And what functionality!  This is a crude material compared to what could be achieved using similar construction techniques, but even so the nano-wands behave in a most unusual way.  Functionally, they are reminiscent of polar molecules, and the spheres they form are analogous to micelles—“capsules” formed by organic molecules with opposing hydrophobic and hydrophilic ends.  But by engineering them at the nanoscale out of inorganic materials, structural and functional possibilities open up that are way beyond the realm of chemistry alone.  

It is easy to imagine how this material could be used to encapsulate and collect chemical spills in the environment.  Or deliver drugs to where they are needed in a very targeted way (only releasing their payload by disassembling when the right signal is received).  Yet the work of Fung Suong Ou and colleagues hints at much greater things.  Using the same basic technology, there is nothing to prevent the construction of multi-component nanomaterials that can assemble and re-assemble in many different ways, depending on their environment and the stimuli they receive.  As the paper’s authors’ conclude, 

“This controlled engineering feat at the nanoscale that allows well-controlled assembly and manipulation could lead to the creation of smart materials that are a cornerstone for the development of nanotechnology-based applications.”

The challenge

But stimulating as the science is, this paper is also an object lesson in why new thinking is needed on possible risks to human health and the environment, if such technologies are to succeed.

First and foremost, the paper comes hot on the heels of Poland et al.’s study linking some forms of multi-walled carbon nanotubes to precursors of mesothelioma—a disease more usually associated with asbestos exposure.  Poland’s research suggests that carbon nanotubes which are thin, longer than 15 – 20 micrometres, straight, and dispersible, could lead to the disease if inhaled.  The nano-wands in the Ou et al. paper are around 150 nm in diameter, something over 100 micrometres long, straight, and apparently dispersible—in other words, exactly the types of fibres which Poland’s work suggests more research is needed on before the possible health implications are understood.

It’s too early to tell whether Ou’s nano-wands will have their own unique risk-profile.  But their inevitable comparison with the nanotubes used in Poland’s study and the possibilities of dispersive use hinted at in the accompanying press release do raise important questions about their safety.  The important point here is not that this particular material might show harmful behaviour, but that there is always the chance that novel behaviour can lead to unanticipated harm—unless the right questions are asked early on.  And this most definitely requires new thinking on what those questions are, and how they might best be answered.

The second object lesson in new challenges concerns regulations.  Unless used as a drug or pesticide, substances are typically regulated according to their chemical makeup.  It’s an approach that was developed at a time when the terms “chemical” and “substance” were interchangeable.  But Ou’s nano-wands challenge this paradigm.  

These nano-wands and other hybrid substances have no unique chemical identity, and so potentially slip through the net of many existing regulations.  Yet they display a functionality that depends on their physical form and complex makeup, which is not predictable from their chemical components.  And regulations are needed that recognize this.  If effective approaches are to be developed to ensure the safe use of this emerging class of material, new thinking is needed on how substances are classified and regulated.

The bottom line

Why nano? As Ou’s work shows, we can potentially do things with nano that are way beyond any other technology at our disposal.  And when nano is combined with other technologies like biotech and information tech, the possibilities become endless.

Why care?  Because nano will change your life, whether you like it or not.  And you might want to make sure that it is a change for the better, not for the worse.

And the nano-wands?  These have tremendous potential as an innovative new material.  Lets hope that their development is matched by equally innovative thinking on using them safely.

Further resources

Paper: Controlled Manipulation of Giant Hybrid Inorganic Nanowire Assemblies

Supplemental Material to the paper

Managing the Effects of Nanotechnology.  J. Clarence Davies

Paper: Carbon nanotubes introduced into the abdominal cavity of mice show asbestos-like pathogenicity in a pilot study

 

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This post first appeared on the SAFENANO blog in May 2008

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