Fine-Tuning Ho-Based Red-Upconversion Luminescence by Altering NaHoF₄ Core Size and NaYbF₄ Shell Thickness

Ameliorating upconversion luminescence (UCL) properties of rare-earth (RE)-doped upconvertion nanoparticles (UCNPs), specifically with high doping concentration, is extremely crucial in progressive scientific research. In this work, by high temperature pyrolysis, various NaHoF4:Yb3+ core-only and NaHoF4@NaYbF4 featured core–multishell UCNPs were synthesized using oleate chemicals. X-ray diffraction (XRD), transmission electron microscopy (TEM) scans and mapping, X-ray photoelectron spectroscopy (XPS), UCL spectrum, UCL lifetime, and pump power dependency were used to characterize the samples. CT and MR imaging, DPBF absorbance curves, in vitro cytotoxicity of Ce6 modified UCNPs were examined. By using the method of simple structure alteration of core size and shell thickness, dominant emission shift from green to red was successfully achieved for highly doped NaHoF4@NaYbF4 UCNPs. Meanwhile, high-quality red-upconversion luminescence with low G/R (green/red) ratio and high intensity from Ho3+ ions were obtained. An innovative theory of Fc and Fq, serving as a revised version of traditional concentration quenching and size effect, was introduced and proved that a large core and thick shell would both enhance UCL intensity, but acted conversely in terms of G/R ratio. An extra active or inert shell would effectively increase the UCL intensity under low-power laser excitation and enable UCL color to vary from green to yellow to red. Our study may broaden the application domain for NaHoF4 material from MR imaging to photo-induced therapy.