Luminescent markers to fight against counterfeiting and better recycle materials

The impact of counterfeiting on the economy, ecology and consumer safety is an increasingly significant phenomenon. For example, the WHO considers that one medicine in ten is counterfeit in the world, and even one in four in developing countries. One of the ways to combat this scourge consists in marking the products by incorporating a luminescent marker, that is to say a marker which emits visible light in response to certain stimuli, for example irradiation with ultraviolet rays.
In addition to its effectiveness in the field of the fight against counterfeiting, the use of luminescent markers is also of great interest in the field of recycling materials. Indeed, to be perfectly effective, recycling must be carried out on materials that are as pure as possible. For example, mixing several types of plastic leads to a recycled product with degraded properties, unusable in many applications. Marking each type of plastic with a luminescent marker of a different color would allow ultra-fast and reliable identification and sorting of waste. This optimized recycling of materials has an obvious ecological interest, by making it possible to drastically reduce the mass of waste that is ultimately incinerated and the need for raw materials.
“Rare-earth-based coordination polymers” are compounds capable of serving as a luminescent marker in these two fields: that of optimizing the recycling of materials and that of the fight against counterfeiting. It is even conceivable that the same marking could meet both concerns, by ensuring the traceability of the material from its production to its marketing and then by allowing its recycling at the end of its life.

What is a "rare earth"?

Rare earths are the chemical elements between lanthanum (La) and lutetium (Lu) in the periodic table (lanthanide series), to which yttrium (Y) is added. They are the longest series of elements with similar chemical properties in the periodic table (15 elements).
Periodic Table of Chemical Elements. Rare earths are circled in black.
ExplorersInternational/Pixabay, modified by Carole Daiguebonne, Olivier Guillou, Provided by the author
Although chemically similar, these metals have different physical properties, particularly optical and magnetic. Due to their unique physical properties, rare earths are found in many applications such as high performance magnets, lighting or displays, for example.
Reputed to be non-toxic, rare earths are widespread in the earth's crust. However, at present, their extraction is overwhelmingly carried out in China.

What is a luminescent “coordination polymer”?

Coordination polymers are one-, two- or three-dimensional structures in which metals (rare earths, for example) are bound together by organic molecules called "ligands".
Schematic representation of coordination polymers. The rods symbolize the ligands and the spheres the metals.
Carole Daiguebonne, Olivier Guillou, Provided by the author
These ligands play an important role in the luminescence of coordination polymers. They are the ones that absorb the excitation ultraviolet radiation, transmit this energy to the metal which, in turn, de-excites by emitting light in the visible or infrared. This excitation/de-excitation mechanism is called the “antenna effect”. And it is particularly interesting in the field of labeling, because it makes it possible to excite (and therefore visualize) all the rare earths with a single and same excitation signal.

Schematic representation of the antenna effect.

Carole Daiguebonne, Olivier Guillou, Provided by the author
Interest of coordination polymers based on rare earths in the marking of materials
It is possible to make several metals of different natures cohabit within the same coordination polymer. When these metals are rare earths, they are distributed randomly (because of their similar chemical properties) forming real "molecular alloys". It is therefore possible to form extremely numerous families of coordination polymers having the same physico-chemical properties (thermal and chemical stability, particle size, etc.), but different physical properties, in particular optical ones.
Emission colors of compounds based on terbium and europium belonging to the same family.
Carole Daiguebonne, Olivier Guillou, Provided by the author
For example, if we consider a series of coordination polymers based on europium (which emits in the red) and terbium (which emits in the green), they will emit in all the colors between red and green, in function of the relative contents of europium and terbium.
One of these families has been shown to consist of 4.1015 different coordination polymers! It is therefore possible to change the marker as often as necessary without modifying the method of manufacturing the material which it is desired to mark. This is particularly interesting, as it prevents the marker itself from being copied and allows the vintage imaging of materials (a marker composition corresponding to a batch number, for example).

“Heart-shell” formatting

Of course, to be technologically acceptable, these markers must exhibit as intense a luminescence as possible, so that they can be used at levels as low as possible. Indeed, a low content of marker makes it possible to limit the cost of the marking and to make its identification difficult.
Compounds of this type currently marketed in the form of “molecular alloys” are used at contents of the order of a few ppm (1 ppm=1 g of marker per ton of materials). Unfortunately, certain compositions of the alloy lead to energy transfers between the different rare earths which cause a decrease in luminescence intensity.
Schematic representation of a core-shell type coordination polymer.

Carole Daiguebonne, Olivier Guillou, Author provided

To avoid this, it has recently been shown that it is possible to manufacture particles of coordination polymers based on rare earths of the "core-shell" type, that is to say particles whose internal and external parts do not have the same composition. In these particles, the energy transfers between the metals of the core and those of the shell are drastically reduced and the overall luminescence intensity is greatly increased.
This new generation of luminescent markers based on rare earths opens up new perspectives in the fields of the fight against counterfeiting and the recyclability of materials.

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