If mechano­luminescent materials are subjected to external mechanical stress, they emit visible or invisible light. Such excitation can occur due to bending or gentle pressure, for example, but also completely contact-free through ultra­sound. In this way, the effect can be triggered remotely and light can be brought to places that normally tend to be in the dark, for example in the human body. If the ultrasound treatment is to be used at the same time to generate local heat, it is important in such a sensitive environment to observe closely the temperatures that occur. Material scientists at Friedrich Schiller University Jena, Germany have now developed a mechano­luminescent material that can not only be used to generate a local heat input by means of ultrasound, but also provides feedback on the local temperature at the same time.

In their work, the Jena scientists often deal with the mechanical properties of inorganic materials, in particular with how one can observe mechanical processes optically. “Mechani­cally induced light emission can provide us with many details about a material’s response to mechanical stress,” explains Lothar Wondraczek of the University of Jena. “But in order to expand the field of applications, it is sometimes also necessary to obtain addi­tional information about the local temperature – especially when the excitation is carried out by means of ultrasound. Here, we were initially interested in sensor materials in the form of ultra-fine particles, which – intro­duced into the environment to be studied – can provide feedback infor­mation about how ultra­sound interacts with this environment.” 

For this purpose, the Jena researchers have combined an oxysulphide semi­conductor with the rare earth erbium oxide. The semi­conducting structure absorbs mechanical energy provided by ultrasound excitation, with the erbium oxide providing the light emission. The temperature can then be read from the spectrum of the emitted light by means of optical thermo­metry. “This means that we can stimulate a temperature increase from the outside, measure it from the charac­teristics of light emission, and thus establish a complete control circuit,” explains Wondraczek.

The remote-controlled light emission, combined with temperature control, could open up completely new areas of appli­cation for such mechano­luminescent materials, for example in medicine. “One possible field of application could be photodynamic therapy, in which light is used to control photo­physical processes that can support the organism in healing,” says materials scientist Wondraczek.

With multi-responsive mechano­luminescent materials in the form of very fine particles, not only could light and heat be generated at a desired location, but they could also be controlled in a targeted manner. As biological tissue is transparent to the infrared light emitted, it is possible to set and control a desired tempera­ture from the outside during treatment. “However, such ideas are still very much in their infancy. Very extensive research and study are still needed in order to put them into practice.”

More accessible are other applications in which light and heat need to be brought to dark places in a targeted fashion. For example, photo­synthesis or other light-driven reactions could be specifically triggered, observed and controlled. Likewise, going back to the beginning, the material can be used as a sensor for generating or obser­ving material changes, or also as an invisible, coded marking on material surfaces. (Source: FSU Jena)

Reference: Y. Ding et al.: Ultrasound-Induced Mechanoluminescence and Optical Thermometry Toward Stimulus-Responsive Materials with Simultaneous Trigger Response and Read-Out Functions, Adv. Sci., online 16 June 2022; DOI: 10.1002/advs.202201631

Link: Laboratory of Glass Science, Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Jena, Germany