Engineering Bright and Mechanosensitive Alkaline-Earth Rare-Earth Upconverting Nanoparticles.

Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb and Er, which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/√Hz) of these particles. The SrYbErF@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 ± 0.04 GPa/√Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.