Silk fibroin-Yb3+/Er3+:YAG composite films and their thermometric applications based on up-conversion luminescence

Silk Fibroin (SF) is a well-known protein, and has been employed in the development of diverse advanced materials for applications on medical and biological field, drug delivery systems and, also, in the electronic and photonic area. In addition, the up-conversion emission process could be proposed in fibroin-based materials and becomes a challenge for the photonic field based on biocompatible devices obtained from fibroin. To avoid the unwanted upconversion (UC) quenching process, herein, lanthanide ions were used as sensitizer and emitters ions, and them was incorporated into an inorganic particle obtained from spray pyrolysis synthesis. Specifically, we have obtained Yb3+ and Er3+ ions doped YAG (Yttrium Aluminum Garnet, Y3Al5O12) spherical submicrometric particles, without spurious phases. All samples were characterized by X-Ray Diffraction, Scanning Electron Microscopy, and photoluminescence. The spectroscopic studies showed an intense UC emission from Er3+ ions. Furthermore, looking for the development of a new fibroin composite which should be used as sensor or smart ink, we inserted the YAG particles in SF, and a new flexible composite film with controlled amounts of particles and tuned transmittance behavior was obtained. Finally, it was confirmed the characteristic luminescent behavior (as UC emission) of the particles in the composite films. As the emitters levels (2H11/2 and 4S3/2) from Er3+ ions are thermally coupled, all materials (particles and films) were exploited as luminescent thermometer, and data confirmed the thermal sensibility comparable to other systems based on Yb3+/Er3+ ions.