Bar-codes. Normally the most important and interesting thing they can do is tell apart a bottle of bleach from a bag of crisps (which, to be fair, can be pretty important).

But using tags such as bar-codes means that all sorts of things, not just groceries, can be tracked as they are transported all over the world. However, counterfeiting bar-codes is relatively easy, so invisible tags such as SmartWaterTM have been developed to keep track of more valuable or more sensitive items.



In a new study, scientists in the US have demonstrated a new tagging system that they hope can be used to track and identify explosives. They use tiny nanoparticles, of cleverly-designed metal alloys in order to create unique ‘bar-codes’ of melting temperatures that can be easily identified.

So, what’s the point?

While using a super-hi-tech invisible tag might not make economic sense for labelling bacon and toilet duck in supermarkets, the researchers believe it can be used to track high-value ‘objects of interest’ such as explosives, drugs and the chemical precursors for making them.

In this study, the scientists actually mixed capsules full of their nanoparticles into an explosive material, the idea being that the tag becomes part of the material itself and so difficult to change or remove, whereas a barcode on the packaging can be covered up or peeled off by opportunistic shoplifters or international terrorists alike.

What did they do?

Each set of nanoparticles is designed to melt at a very specific temperature. When analysed by DSC (differential scanning calorimetry) they can be told apart from one another by their melting temperatures. Different combinations of nanoparticles will produce different ‘fingerprints’ of melting points – and so many different tags can be made.

However, while pure metals will generally show a ‘sharp’ melting peak, alloys (solid mixtures of metals) may not necessarily do the same. If the melting peaks are too broad, they can overlap each other and be hard to tell apart.

The problem is that if you use only pure metals, you only have a small ‘library’ of tags, whereas by mixing them together, you produce far more possible combinations, so making alloys with sharp melting points is important for this technology to work.

The reason why many alloys have a broad melting point is because its constituent metals each have different melting points. This means that part of the alloy will start to melt before the rest of it and so the transition from full solid to full liquid happens over a range of temperatures rather than at one specific temperature (see diagram).

Fortunately, however, there is a particular mixing ratio for an alloy known as a eutectic composition. Eutectic (which is from the Greek meaning ‘easy melting’) alloys are special because the whole solid melts at the same time and so the transition from full solid to full liquid occurs over a narrow range of temperatures, producing a sharp melting point.

The researchers made nanoparticles out of four different metals (bismuth, indium, tin and lead), encapsulated them inside tiny silica shells and mixed them into DNT (dinitrotoluene, an explosive similar to TNT). They used a computer program to help them to design alloys of these four metals to make sure they were eutectic.

They then simulated identifying the DNT by analysing the nanoparticles that were mixed into it.

To do this, they had to separate the tags from the explosive before finding the melting points (heating the explosive would not be the best idea in the world!), which they did by dissolving a small sample of the explosive in ethanol. The explosive dissolves, but the silica capsules do not, so they can then be filtered out and tested.

Another method they used to separate out the silica capsules was actually to set off the explosive! They did this in a small container and then collected and analysed the debris.

Did they prove anything?

From their four metals, the scientists made 12 different types of nanoparticles. Each of these had a sharp melting point – the eutectic alloys had worked – meaning they could all be told apart from one-another.

They also found that not only were they able to tell apart specific nanoparticles when they were filtered out from the explosive, but also when collected from the explosion debris.

So, what does it mean?

Their results show that their idea seems to have worked – certainly they showed that their eutectic alloys had sharp, distinct melting points and so could be told apart. Separating out the nanoparticles by dissolving the explosive in ethanol also seemed a pretty straightforward solution to removing them for analysis.

Also, because the nanoparticles could be used to identify the DNT after it was detonated, the researchers suggest that their system could be used forensically to trace the origin of an explosive.

However, their explosion was carried out inside a vessel under controlled conditions – in a real explosion, collecting the nanoparticles may not be so straightforward.

This study is interesting because the alloys have been cleverly designed to have unique and easily identifiable (i.e. sharp) melting points. Exploiting the fact that these eutectic alloys have different melting points to create a unique ‘bar-code’ is also a clever move.

Whether or not it catches on remains to be seen, but in a few years time, you may well see something like this on CSI. Who knows, it might even be used to catch real criminals.

Original article in Scientific Reports Jun 2014

All images are open-source/Creative Commons licence.

Credit: B Duong et al. (First and Fifth); L Haywood (Second); Developed from Duong et al. (Third); Developed from Bantman (Fourth)

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