Designing Er<sup>3+</sup>/Ho<sup>3+</sup>-Doped Near-Infrared (NIR-II) Fluorescent Ceramic Particles for Avoiding Optical Absorption by Water

Abstract

Rare-earth-doped ceramics, which are near-infrared (NIR) fluorescent materials, have applications as photonic materials in various fields, including medical biology. The NIR wavelength range in which these rare-earth-based materials function is highly transparent in biological tissues and is suitable for deep-tissue imaging. However, the most commonly used 980-nm excitation light is partially absorbed by the water involved in the observation system. In the present study, we focused on the wavelength bands of 800-nm excitation and 1150-nm emission, where no water absorption exists, and investigated the rare-earth doping composition to obtain a ceramic material, NaYF4, with the above excitation and emission wavelengths. NaYF4 particles co-doped with rare-earth ions Er3+ (sensitizer to 800 nm) and Ho3+ (emitter at 1150 nm) were designed for over-1000 nm NIR fluorescent probes. The fluorescence intensity at 1150 nm was maximum at a rare-earth composition of Y3+: Er3+: Ho3+ = 82:14:4 (mol%). The obtained fluorescent Er3+/Ho3+-doped NaYF4 particles exhibited a long fluorescence lifetime (3300 ± 120 μs at 25℃), which is characteristic of rare-earth elements. Er3+/Ho3+-doped NaYF4 is expected to have applications as a long-lifetime NIR-II fluorescent material with the highest transparency in biological tissues.