If mechanoluminescent materials are subjected to mechanical stress from outside, they emit visible or invisible light. Such excitation can occur, for example, through buckling or gentle pressure, but also completely contact-free via 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 closely monitor the temperatures that develop in the process. Materials scientists at Friedrich Schiller University in Jena have now developed a mechanoluminescent material that they can use to not only generate local heat input using ultrasound, but that also provides feedback on the local temperature at the same time. They report on their research results in the renowned scientific journal ,,Advanced Science".
Semiconductors and rare earths
During their work, the scientists in Jena often deal with the mechanical properties of inorganic materials, in particular with how it is possible to observe mechanical processes optically.Mechanically induced light emission can provide us with many details about the response of a material to mechanical stress," explains Lothar Wondraczek from the University of Jena.But in order to expand the field of applications, it is sometimes necessary to obtain additional information about the temperature prevailing locally during the load - especially when the excitation is performed by ultrasound. Here, we were initially quite simply interested in sensor materials in the form of ultra-fine particles, which - introduced into an environment to be investigated - can both act on their environment through external ultrasound excitation and also feed back information about this action."
For this purpose, the Jena scientists have combined an oxysulfide semiconductor with the rare earth erbium oxide. The semiconducting structure then absorbs the mechanical excitation by ultrasound - the erbium oxide provides the light emission. The temperature can then be read from the spectrum of the emitted light using so-called optical thermometry.This means we always have full control over the temperature development in the material, which can also be influenced by the ultrasound," explains Wondraczek. "We can excite a temperature increase from the outside, measure it by the light emission and thus set up a complete control circuit."
Use in photodynamic therapy
The remote-controlled emission of light combined with temperature control opens 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 help the organism heal," says the Jena materials scientist. 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. Since biological tissue is transparent to the emitted infrared light, it is possible to set and control a desired temperature during auen treatment.However, such ideas are still very much in their infancy.Very extensive research and study work is still needed to put them into practice."
Other applications are closer at hand, in which light and heat are to be brought to dark places in a targeted manner. For example, photosynthesis or other light-driven reactions could be specifically triggered, observed and controlled. Likewise - back to the beginning - the material can be used as a sensor for generating or observing material changes or even as an invisible, coded marking on material surfaces.