You Can't Coat a Nanosphere Like You Can a Macro One

Imagine you were given the task of coating a spherical object, and you were handed a tennis ball, a can of paint and a paintbrush. After thinking for a minute, you figured you could dispense with the paintbrush and just dip the tennis ball in the paint and  not only make a much quicker job of it but also probably cover the tennis ball in paint more completely.

In that circumstance you would be very clever and be given a pat on your back for your ingenuity. However, if you tried that on the nanoscale you might discover that the coating wasn’t covering your sphere.

Researchers Matthew Lane and Gary Grest at Sandia National Laboratories have determined through the use of computer simulations that when attempting to coat a nanoparticle the coating drops off the sphere, leaving what is described as “louvres” and a not a complete protective coating.

This would appear to be a problem. To prevent nanoparticles from aggregating in an undesired way, the method used has been covering them in a coating so they don’t stick together. It seems this research shows that those coatings may very well have the opposite effect.

In the article cited above, Carlos Gutierrez, manager of Sandia’s Surfaces and Interface Sciences Department, said, “It’s well-known that aggregation of nanoparticles in suspension is presently an obstacle to their commercial and industrial use. The simulations show that even coatings fully and uniformly applied to spherical nanoparticles are significantly distorted at the water-vapor interface.”

The research, which was originally published back in June in the journal Physical Review Letters, demonstrated through simulations that with nanoparticles because the diameter of the sphere is smaller than the thickness of the coating the curvature of the sphere causes the coating to drop off leaving the louvre-like surface.

While this phenomenon may soothe those chemical engineers who had been pulling their hair out trying to get nanoparticles to fully disperse in a solution, they may be worse off than where they started from, now lacking the simple tool of coating the nanoparticles to prevent aggregation.

But the good news is that the characteristics of the patchy coatings are particular to each nanoparticle. As the article describes it, “Though each particle is coated asymmetrically, the asymmetry is consistent for any given set. Said another way, all coated nanoscopic sets are asymmetric in their own way.”

The article goes on to argue that by having predictable variations for each member of a nanoset there is a possibility for new applications.

Unfortunately, what those applications might be is not provided. Instead we get a summation of what this simulation has given us, “What we’ve done here is to put up a large ‘dead end’ sign to prevent researchers from wasting time going down the wrong path,” Lane said. “Increasing surface density of the coating or its molecular chain length isn’t going to improve patchy coatings, as it would for larger particles.” And then a small bit of encouragement, “But there are numerous other possible paths to new outcomes when you can control the shape of the aggregation.”

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY