Photochromeless photochromic nanomaterials emitting multicolour light for smart displays and inks

by Virginia Greco

A paper in “Materials Horizons” introduces a novel approach, based on solid lipid nanoparticles and nanostructured composites, to produce multicolour-emissive materials responding to temperature changes. Cost-effective, high-tuneable and easily integrable into devices, they can be applied in smart displays and in anti-counterfeiting inks. Dr Claudio Roscini, from the ICN2 Nanostructured Functional Materials Group, is one of the corresponding authors.

Materials that emit light and change colour as an effect of temperature variations find a variety of applications ranging from thermal sensing to smart displays and devices. For the letter, in particular, it is key to achieve multicolour emission, as opposed to cases in which a single colour switch (for example from green to red) is sufficient. Even though thermoresponsive multicolour luminescent systems have already been realised through different methodologies, further developments are still required in order to obtain materials that are cost-effective, scalable and integrable in devices, as well as highly tuneable both in terms of emission colour and temperature range.

In a work recently published in Materials Horizons (and highlighted in the front cover), a team of researchers from the Catalan Institute of Nanoscience and Nanotechnology (ICN2), the Autonomous University of Barcelona (UAB) and the Perugia University (Italy) introduces a novel strategy for the fabrication of multicolour luminescent systems responsive to temperature, starting from commercially available phase change materials (PCMs) and transferring this property to the nanoscale. This study was coordinated by Dr Claudio Roscini, senior postdoctoral researcher in the ICN2 Nanostructured Functional Materials Group, led by Dr Daniel Ruiz-Molina, and Dr Jordi Hernando Campos, from the Chemistry Department of UAB. First author of the paper is Jaume R. Otaegui Rabanal, Doctoral Student at the UAB and the ICN2.

Phase Change Materials (PCMs) exhibit the ability to promote reversible fluorescence changes of dissolved fluorophores –chemical compounds that can re-emit light after excitation— upon solid-liquid transition. Such switch between two colours happens when the melting temperature of the PCM matrix, in which the emitter is dispersed, is reached. In order to obtain a multicolour material, the authors of this study miniaturized the emitter-PCM mixture, melting at different temperatures, into nanometer-sized particles and then combined various types of these nanoparticles in polymeric films or cellulose papers, through printing. This is the first time that such approach is used to this purpose.

Solid lipid nanoparticles (SLPs) loaded with different kinds of emitter-PCM combinations were synthesised and then incorporated into transparent polymer films. The resulting thermally responsive luminescent nanocomposites can be applied for the fabrication of smart displays and sensors. In addition, they can be used to prepare water-based colloidal inks for inkjet printing, which is currently the preferred technique to create luminescent patterns. The latter finds a relevant application, among others, in anti-counterfeiting techniques.

With only one emitter, the researchers were able to provide switching among red, green and intermediate colours (orange, yellow). By adding a second one, having blue luminescence, they could access to other colours. Thus, the combination in different ratios of nanoparticles with either emitter allowed to obtain most of the shades within the standard RGB colour space, including white emission. Finally, they took advantage of the results of a previous work of theirs, in which it was demonstrated that the thermally induced emission of emitter-PCM mixtures combined with plasmonic gold nanoshells can also be triggered using near-infrared (NIR) irradiation. Therefore, they succeeded in producing transparent composites whose fluorescence can be induced or changed through illumination with a collimated NIR laser.

This is extremely interesting, since by introducing plasmonic nanoparticles the authors achieved photochromeless photofluorochromic materials, in other words, materials that change their fluorescence with light without the need for using photochromes –which is the common technique. In addition, they are activated with NIR irradiation, instead of working with UV and visible light.

This important work is part of a series of studies based on the same principles (see references below) developed by the ICN2 Nanostructured Functional Materials Group in collaboration with the UAB and the Perugia University, which also led to the filing of a patent, titled “A Photoinduced Thermochromic or Thermoluminescent Composition”.

Reference articles:

Jaume Ramon Otaegui, Daniel Ruiz-Molina, Loredana Latterini, Jordi Hernando and Claudio Roscini, Thermoresponsive multicolor-emissive materials based on solid lipid nanoparticles. Mater. Horiz., 2021, 8, 3043. DOI: 10.1039/D1MH01050F

Jaume Ramon Otaegui, Pau Rubirola, Daniel Ruiz-Molina, Jordi Hernando, Claudio Roscini, Solid Materials with Near-Infrared-Induced Fluorescence Modulation. Advanced Optical Materials, Vol 8 Issue 21, November 2020, DOI: 10.1002/adom.202001063

Giuseppina Massaro, Jordi Hernando, Daniel Ruiz-Molina, Claudio Roscini and Loredana Latterini Thermally Switchable Molecular Upconversion Emission. Chem. Mater. 2016, 28, 3, 738–745. DOI: 10.1021/acs.chemmater.5b03532