Raising the bar
(appeared on 16th April 2014)

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To see a world in a grain of sand may be realized with nano-particles, , says S.Ananthanarayanan.

Barcoding has brought about revolution in material handling. Just attach a tag printed with a series of dark or light lines to a product and the product can be identified, out of thousands or others, by a hand-held sensor, and a computer can use the information to create an invoice or bill, a list of items scanned, alert some process, create entries in a data base, like the books taken out from a library, and so on. But there are places where the numbers run into millions, or more, or the conditions are physically difficult, and there is need for a more compact, reliable and hard-wearing coding medium.

Patrick S Doyle and his team at MIT report in the journal, Nature Materials, a technique of coding synthetic micro-particles with a pattern of different crystals that glow in different colours in infra red light. The technique would allow creating barcodes that could encode billions of objects, or pieces of information, with economy and efficiency, and also with great endurance in harsh conditions.

Barcoding

The simple black and white strip coding that we find on grocery or library books relies on a sensor reading the strips in the barcode, as representing the numbers, ‘0’, for a dark strip, or ‘1’ for a light strip. Thus, each strip codes a choice out of two possibilities and two strips would code for one of 2x2 = 4 possibilities. A code with ten strips would then code for a choice out of 2x2x2x2x2x2x2x2x2x2 = 1024 possibilities. Twenty strips would code for a choice out of a million possibilities, and so on.

Once the codes for different objects are decided, it is routine to print out the barcode and attach it to an object, along with additional information, like, in the case of a package in the post, the place of origin, destination, weight, category, quality of service paid for, etc. And at every stage of passage, the object can be spotted and a log kept of its progress.

In many industrial applications, however, the numbers are very large and there is need for the label to be compact, apart from being easy to apply and fast to read, with accuracy, and also to survive corrosive environments or high temperatures. While simple barcoding cannot satisfy the requirements, a variation uses a two dimensional array of dark and light dots, but the capacity is limited to a few thousands. Using a series of colours for coding increases the range, but presents problems in correct colour reproduction and sensing. Combining colour and graphics, using printing with magnetic inks, has also failed for similar reasons. In this context, the use of micro-particles, which contain markers to serve as the carrier of information, is attractive, so long as the need for ease of creation, decoding and durability can be met.

The authors of the paper in Nature Materials describe their innovation, of constructing micro-particles using chains of molecules, with control of the rate of formation of the chains. The method, called Stop Flow Lithography, amounts to punctuating the growth of the micro-particle using exposure to ultra violet light shone through a stencil that controls where the light falls. The method combines the high resolution, or detail, that is possible with optical methods with the fast production of micro-particles when the material of the particles is allowed to form long chains of molecules while in rapid flow.

Upconversion

The markers that were incorporated into the micro-particles were crystals of material that emit visible light when excited by infrared light. This is called upconversion and is the opposite of the more familiar phenomenon of fluorescence, as seen in the domestic tube light. In fluorescence, higher energy photons of ultra violet light excite atoms of the material, which then decay in two steps, emitting lower energy photons of visible light in the process.

In upconversion, it is the excitation of the atoms that takes place in two steps, by the low energy photons of infrared light and the atoms are excited to a doubly high energy state. The atoms then de-excite in a single step, emitting higher energy photons in the visible region.

The elements whose atoms allow two step excitation, are the so-called rare earths (they are quite abundant, in fact) whose atoms have an electron configuration that allows transitions of electrons in shells other than the outermost shell

The MIT group prepared micro-particles by creating polymers, or molecule chains, with periodic insertion of crystals of materials that support upconversion of infrared photons. The method used, of flow lithography, enables speedy creation of the particles, and the specific colours of emission enable decoding using simple hand-held devices. The variety of coding crystals that is possible, and the spacing, allows a label of very small dimensions to code for over a million distinct combinations and the error rate is found to be less than one in a billion.

Accuracy and capacity

The authors note that faithful colour reproduction is the key requirement of the coding to be efficient and reliable. It was found that the specific colours emitted were quite independent of differences in the process of formation of the particles. This ensures that coding carried out in different facilities would still be read in the same way by different decoding devices.

To demonstrate the high capacity created, the authors describe a method with almost limitless coding ability, and with great robustness. A number of uniquely encoded micro-particles were laminated into the surface of objects, using ultra violet light for hardening the surface, a common process in industrial packaging, with no injury to the micro-particles. While each micro-particle could encode some hundreds of thousands of unique objects, the capacity of a series of such particles, each one unique, would run into many trillions. As the particles are tiny, large numbers can be embedded in an object in a highly covert manner. “Randomly embedding 10 particles from a set of just 1,000 unique asymmetric particles yields an encoding capacity of _(1,000)10, or 1030, enough to uniquely barcode every manufactured product on Earth,” say the authors in the paper. This kind of capacity could be used to mark pharmaceutical products, for instance, to differentiate the genuine from counterfeits.

“The mere ability to tune particle material properties without impacting encoding performance unlocks a vast potential for immediate in-line integration of encoded particles into complex manufacturing processes or even consumer products. With modest expansion of the available colour palette or number of stripes per particle, for which no foreseeable impediment exists, single-particle encoding capacities will increase very rapidly,” the authors say.

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