Energy transfer between rare earth-doped ceramic nanoparticles for gauging strain and temperature in elastic polymers

The field of self-reporting materials is gaining increased attention recently. These materials can convey the forces applied to them without the need to integrate external devices. The ability to report strain exerted onto polymers is especially important, as it might give indication of high levels of force that may damage the material. These materials could also be applied to in-dwelling medical devices such as artificial tendons and heart valves. In this work, we designed a self-reporting system based on energy transfer (ET) between rare earth-doped ceramic nanoparticles (RED-CNPs). When external strain and heat are applied, the efficiency of the ET changes, resulting in changes in the emission spectra. We chose particles that are excited by 808 nm radiation and emit in the over-thousand-nanometer near infrared, since this range offers high penetration to biological tissues. Nanoparticles (NPs) were synthesized using the thermal decomposition method and then extensively characterized. X-ray diffraction analysis revealed that the doped NaYF₄ crystals are in the β-phase, while inductively coupled plasma emission spectrometry demonstrated the existence of two types of NPs: NaYF₄: Nd³⁺, Yb³⁺ and NaYF₄: Er³⁺. The absorption and emission of the particles showed no ET between the two kinds of NPs when simply mixed together. In order to reduce the interparticle distance and allow ET, RED-CNPs were coated with polydopamine (PDA) and then conjugated with a polymeric linker. Scanning electron microscopy, dynamic light scattering and absorption analysis showed the successful coating with PDA and the creation of interconnected NP networks. Emission from the conjugated system showed evidence of ET, while changes in the emission lifetime of Yb as measured by time-gated imaging further suggested ET between Yb and Er. Finally, the conjugated system was integrated into poly(dimethylsiloxane) and it was shown that as strain or heat are exerted onto it, the emission spectra under 808 nm excitation is varied.