Stress Sensing Based on Upconversion Piezo-Luminescence Effect

Upconversion nanoparticles (UCNPs) offer unique advantages in stress sensing, such as noncontact operation and immunity to electromagnetic interference. However, their stress response mechanism remains unclear, limiting practical applications. In this study, core-shell structured NaGdF₄/Er@NaYbF₄@NaYF₄ UCNPs were uniformly dispersed in a PDMS elastomer to fabricate a flexible piezo-luminescent composite sensing material. Experiments demonstrate that under uniaxial compressive stress, PDMS deformation induces local concentration changes of UCNPs, thereby modulating the intensity ratio of the violet peak (408 nm) to the green peak (540 nm) (I₄₀₈/I₅₄₀) in the upconversion luminescence spectrum. This ratio depends linearly on applied stress (0-5 kPa) in different stages, yielding sensitivities from 0.261 kPa^-1 to 0.032 kPa^-1, enabling quantitative and continuous stress detection. Concentration-gradient experiments, power-dependent tests, and theoretical analysis confirm that stress alters the interparticle distance, affecting the Yb³⁺ → Er³⁺energy transfer efficiency. The violet-emitting level (²H_9/2) exhibits higher sensitivity to distance variations, leading to selective changes in the violet-green ratio. This work provides new insights into the application of UCNPs in flexible stress sensing and clarifies the underlying piezo-luminescent mechanism.