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Nanoscale Building Method Could Power Anti-Counterfeiting Tools

Scientists have developed a new nanoscale building technique that could help create exclusive anti-counterfeiting and chemical sensing tools, which can be used with the human eyes.

Heyou Zhang and collaborator Calum Kinnear working in the NanoScience Laboratory at The University of Melbourne’s School of Chemistry. Image Credit: Gavan Mitchell and Michelle Gough (University of Melbourne/Exciton Science).

For the first time, researchers from the ARC Centre of Excellence in Exciton Science have successfully arranged tiny gold rods in accurate patterns, and in numbers largely sufficient for practical use. The study findings have been reported in the Advanced Functional Materials journal.

It is possible to arrange these gold rods to produce a range of colors, which change based on how they are observed. This renders them a great anti-counterfeiting feature. For instance, if used on a passport or banknote, they could be useful for customs agents or cashiers.

Moreover, they can be altered to turn into various colors when chemicals are present, serving as a warning for hazardous levels of carbon monoxide and other gases. These effects have been noted earlier, but it was not feasible to make them at a size that can be observed by the naked eye. A new chemical assembly technique was required.

It is easy to get the bricks in a house to match. On a smaller scale, children can do the same with Lego. But how can things be built precisely at the nanoscale?

A nanometre is roughly one-billionth the size of 1 m. From a different point of view, the thickness of a sheet of paper is 100,000 nm, and the fingernails of a person grow nearly 1 nm every second. Therefore, one must shrink to the subatomic level to achieve this difficult task.

However, Heyou Zhang, the lead author of the study and a PhD candidate at The University of Melbourne, has employed a method termed electrophoretic deposition (EPD).

The whole idea of my PhD is to be able to better control single nanoparticles. Builders construct houses, brick by brick, and they can put each brick where they want. I want to use nanoparticles in a similar way. But at the nanometre scale, you can’t move nanoparticles yourself. They are invisible. You need to use a method to drive or push the particle into a certain position.

Heyou Zhang, PhD Candidate, The University of Melbourne

In the EPD technique, an electric field of specific strength is applied to the materials, and the separation of positive and negative charges is used to push the rods into place.

You have a positive potential and, if the particle is negative, they attract each other. If I have the positive potential on the side of a wall and I have some holes on the wall, the particle can only be attracted to those holes.

Heyou Zhang, PhD Candidate, The University of Melbourne

Using the technique, Heyou and his team could build collections of more than one million nanorods per square millimeter, in patterns of their choice.

Apart from anticounterfeiting and chemical sensing, the assembly method could find applications in efficient lighting, smartphones, renewable energy, and laptops.

Journal Reference:

Zhang, H., et al. (2020) Direct Assembly of Vertically Oriented, Gold Nanorod Arrays. Advanced Functional Materials.

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