Increasing depth penetration in biological tissue imaging using 808-nm excited Nd³⁺/Yb³⁺/Er³⁺-doped upconverting nanoparticles

Ytterbium (Yb³⁺)-sensitized upconverting nanoparticles (UCNPs) are excited at 975 nm causing relatively high absorption in tissue. A new type of UCNPs with neodymium (Nd³⁺) and Yb³⁺ codoping is excitable at a 808-nm wavelength. At this wavelength, the tissue absorption is lower. Here we quantify, both experimentally and theoretically, to what extent Nd³⁺-doped UCNPs will provide an increased signal at larger depths in tissue compared to conventional 975-nm excited UCNPs.