Short-Wave\nInfrared Upconverting Nanoparticles

Optical\ntechnologies enable real-time, noninvasive analysis of\ncomplex systems but are limited to discrete regions of the optical\nspectrum. While wavelengths in the short-wave infrared (SWIR) window\n(typically, 1700–3000 nm) should enable deep subsurface penetration\nand reduced photodamage, there are few luminescent probes that can\nbe excited in this region. Here, we report the discovery of lanthanide-based\nupconverting nanoparticles (UCNPs) that efficiently convert 1740 \nor 1950 nm excitation to wavelengths compatible with conventional\nsilicon detectors. Screening of Ln<sup>3+</sup> ion combinations by\ndifferential rate equation modeling identifies Ho<sup>3+</sup>/Tm<sup>3+</sup> or Tm<sup>3+</sup> dopants with strong visible or NIR-I\nemission following SWIR excitation. Experimental upconverted photoluminescence\nexcitation (U-PLE) spectra find that 10% Tm<sup>3+</sup>-doped NaYF<sub>4</sub> core/shell UCNPs have the strongest 800 nm emission from\nSWIR wavelengths, while UCNPs with an added 2% or 10% Ho<sup>3+</sup> show the strongest red emission when excited at 1740 or 1950 nm.\nMechanistic modeling shows that addition of a low percentage of Ho<sup>3+</sup> to Tm<sup>3+</sup>-doped UCNPs shifts their emission from\n800 to 652 nm by acting as a hub of efficient SWIR energy acceptance\nand redistribution up to visible emission manifolds. Parallel experimental\nand computational analysis shows rate equation models are able to\npredict compositions for specific wavelengths of both excitation and\nemission. These SWIR-responsive probes open a new IR bioimaging window,\nand are responsive at wavelengths important for vision technologies.