Nanoparticles are a hot area of chemical research, and for good reason. Spheres have high surface area. Materials with a high surface area are more reactive in chemical reactions than materials with a low surface area; all the action takes place on the surface, and so the more room that is available, the more reactions that take place. If a sphere is shrunk down to microscopic levels – a nanoparticle – it becomes the most active, highest surface-area material available per unit weight / unit volume. Chemists are therefore very excited about transforming their tried and true precious metal catalyst such as gold into nanoparticles, particularly if those nanoparticles can be incorporated into plastics and polymers. This would make the catalyst much easier to recover after a chemical reaction was complete, so that the precious metal wouldn’t be thrown away but rather could be recycled.
However, gold nanoparticles are spheres, and so trying to give the polymer / plastic a chemical handhold to latch onto has been difficult. Polymer production requires a well-defined, well-known number of attachment points in order to get the best quality polymer. Nanoparticles were thought to be too random and too accessible to allow this type of exact measurement. However, a recent article published in the top journal Science has outlined a method of accomplishing this task. The breakthrough relies on an old mathematical principle, which can be understood in terms of thinking about fur. If you were to coat a soccer ball in fur and begin combing the hairs so they laid down flat, you wouldn’t be able to coax 100% of the fur to lie down smooth. There would be two regions, opposite from each other, where turbulence would require tiny swirls of the hair to stick out straight.
Carrying this idea onto the nanoparticles, chemists coated the gold nanoparticles with long chains of molecules called thiols. They have a sulfur atom in their structure and the sulfurs form weak bonds at the surface of the gold nanoparticle, with their long “tails” floating out behind them – just like hairs / hair follicles behave. Similar to the soccer ball example, at the north and south poles of the nanoparticles existed two single molecules of thiols, sticking straight out – the hair that refused to lie flat, no matter how much you combed it. These two particular thiol molecules were less stable than the other thiols coating the nanoparticle, as they are not stabilized by any favorable intermolecular interactions (dispersion forces) between the thiol chains. They can therefore be plucked out and replaced by thiols that contain a carboxylic acid at the end of their chain.
On the molecular level, then, you would see a “fuzzy” gold nanoparticle whose surface was covered by thiols, and two carboxylic acid groups sticking out of this thiol “beard” at opposite ends of the molecule. Carboxylic acids are easy to polymerize (nylons are prepared in this fashion) and so after polymerization, the result is a gold nanoparticle which has been incorporated into a polymer chain using covalent bonds. Unlike the heavily crosslinked, inexact structure that you might expect, the compound is very well behaved and is the same as any other difunctional monomer.
Nanoparticle research continues to evolve. Fundamental to this field of study is the idea that the shape of the particle and the environment in which it is in play key roles in the reactivity of the nanoparticle. It is safe to assume that placing these nanoparticles into polymers will lead to materials with new and exciting physical and chemical properties, and it will be interesting to see how other researchers pick up on this idea. In a sense, nanoparticles are now no different from other polymer starting materials, but they possess an additional level of sophistication and reactivity not previously seen in the plastics industry.
The source of this article can be found at:
“Divalent Metal Nanoparticles”.
Gretchen A. DeVries, Markus Brunnbauer, Ying Hu, Alicia M. Jackson, Brenda Long, Brian T. Neltner,Oktay Uzun, Benjamin H. Wunsch, Francesco Stellacci.
Science, a publication of the American Association for the Advancement of Science.