Ultralow Concentration in Parts Per Trillion with Nanomolar Level Detection for Monodispersed Upconversion Nanoparticles of Li-Co-doped NaYF4:Ho3+/Yb3+: Potential Materials in Multimodal Applications

Foreign ion accommodation plays a significant role in altering the behavior of nanomaterials for their crystal structures, morphology, and functionalities. In the present work, the influence of Li⁺ co-doping on crystallinity and the photoluminescence upconversion property of NaYF₄:Ho³⁺/Yb³⁺ has been investigated. The crystallinity gets improved significantly after Li⁺ co-doping into NaYF₄:Ho³⁺/Yb³⁺. An X-ray diffraction study confirmed the formation of β-NaYF₄ nanoparticles. A transmission electron microscopy study confirmed the appearance of monodispersed spherical nanoparticles with an average particle size of 30 nm for the sample without an Li⁺ ion, whereas hexagonal shaped particles with the longest diagonal of 35 nm were observed for the sample with an Li⁺ ion. Under the 980 nm excitation, two prominent upconverting bands at 540 and 650 nm arose due to ⁵S₂/⁵F₄ → ⁵I₈ (green) and ⁵F₅ → ⁵I₈ (red), electronic transitions of Ho³⁺ ions, respectively. Such green and red band emissions can be applied in various applications, such as bioimaging in chicken muscle with 0.5 cm penetration depth and anticounterfeiting. The synthesized upconversion nanoparticles exhibited bright green emission. The detection limit was found in our sample to be 22.6 parts per trillion with molarity (32.1 nM) using integrating sphere technique.