Bright, Mechanosensitive Upconversion with Cubic-Phase\nHeteroepitaxial Core–Shell Nanoparticles

Abstract

Lanthanide-doped\nnanoparticles are an emerging class of optical\nsensors, exhibiting sharp emission peaks, high signal-to-noise ratio,\nphotostability, and a ratiometric color response to stress. The same\ncentrosymmetric crystal field environment that allows for high mechanosensitivity\nin the cubic-phase (α), however, contributes to low upconversion\nquantum yield (UCQY). In this work, we engineer brighter mechanosensitive\nupconverters using a core–shell geometry. Sub-25 nm α-NaYF₄:Yb,Er cores are shelled with an optically inert surface passivation\nlayer of ∼4.5 nm thickness. Using different shell materials,\nincluding NaGdF₄, NaYF₄, and NaLuF₄, we study how compressive to tensile strain influences the nanoparticles’\nimaging and sensing properties. All core–shell nanoparticles\nexhibit enhanced UCQY, up to 0.14% at 150 W/cm², which\nrivals the efficiency of unshelled hexagonal-phase (β) nanoparticles.\nAdditionally, strain at the core–shell interface can tune mechanosensitivity.\nIn particular, the compressive Gd shell results in the largest color\nresponse from yellow-green to orange or, quantitatively, a change\nin the red to green ratio of 12.2 ± 1.2% per GPa. For all samples,\nthe ratiometric readouts are consistent over three pressure cycles\nfrom ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly\nimproves signal brightness without compromising the core’s\nmechano-sensing capabilities and further, promotes core–shell\ncubic-phase nanoparticles as upcoming in vivo and in situ optical\nsensors.