Designing and characterizing rare-earth upconverting nanoparticles as nanoNewton mechanical sensors.

Non-invasive nanoscale mechanical force sensors will enable the monitoring of nanoNewton-scale forces in biological and condensed matter systems. Due to their near-infrared excitation with visible emission and robust host lattices, rare-earth upconverting nanoparticles (UCNPs) are a promising technology for mechanical force sensing. When engineered correctly, UCNPs exhibit a ratiometric color response to applied pressure but maintaining brightness high enough to detect this ratiometric color change at the single-particle level remains an outstanding challenge in the field. To create bright and force responsive particles, we explore alkaline earth host lattices (CaLuF, SrLuF, BaLuF). All particles studied are sub-15nm in diameter, core-shell, and are doped with 30% Yb³⁺ and 2.9% Er³⁺. Ensemble measurements with a diamond anvil cell yield a red-to-green intensity ratio (I_R/I_G) pressure responses of 20 ± 1.2, 16.2 ± 0.5, 12.7 ± 0.8, and 40 ± 4 % (I_R/I_G)/GPa for CaLuF, SrLuF, BaLuF, and NaYF₄, the control sample, respectively. We find that SrLuF-based nanoparticles respond to mechanical pressures changes of 37 MPa or 27 nN of force. Using a custom confocal-AFM microscope, we explore the brightness of single UCNPs and are developing techniques to explore their force sensitivity.