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

Abstract

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 NaHoF 4 :Yb 3+ core-only and NaHoF 4 @NaYbF 4 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 NaHoF 4 @NaYbF 4 UCNPs. Meanwhile, high-quality red-upconversion luminescence with low G/R (green/red) ratio and high intensity from Ho 3+ ions were obtained. An innovative theory of F c and F q , 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 NaHoF 4 material from MR imaging to photo-induced therapy.

Insight into the Luminescence Alternation of Sub‐30 nm Upconversion Nanoparticles with a Small NaHoF4 Core and Multi‐Gd3+/Yb3+ Coexisting Shells

“…). [68] It could negatively connect activators to surface quenchers (SQs) outside nanoparticle surface with the help of EBT processes (here EBT-EM-SQ processes for short), thus drastically decreasing the UCL intensity. With these energy processes, the Ho 3+ ions near the coreshell interface are inclined to emit green light but also vulnerable due to tremendous EBT processes that ruthlessly cut down the 5 F 4 / 5 S 2 excited states.…”

“…One is the EBT process between Ho 3+ and Yb 3+ , that is, 5 F 4 / 5 S 2 (Ho 3+ ) + 2 F 7/2 (Yb 3+ ) → 5 I 6 (Ho 3+ ) + 2 F 5/2 (Yb 3+ ) or 5 F 5 (Ho 3+ ) + 2 F 7/2 (Yb 3+ ) → 5 I 7 (Ho 3+ ) + 2 F 5/2 (Yb 3+ ); the other one is the energy migration (EM) process which could engage numerous Yb 3+ in to form energy flow chains: 2 F 5/2 (Yb 1 3+ ) + 2 F 7/2 (Yb 2 3+ ) → 2 F 7/2 (Yb 1 3+ ) + 2 F 5/2 (Yb 2 3+ ). [ 68 ] It could negatively connect activators to surface quenchers (SQs) outside nanoparticle surface with the help of EBT processes (here EBT‐EM‐SQ processes for short), thus drastically decreasing the UCL intensity. With these energy processes, the Ho 3+ ions near the core–shell interface are inclined to emit green light but also vulnerable due to tremendous EBT processes that ruthlessly cut down the 5 F 4 / 5 S 2 excited states.…”

“…Detailed steps could be found in the previous work. [68] Synthesis of NaHoF 4 UCNPs: 1 mmol of HoCl 3 , 6 mL of OA, and 15 mL of ODE were weighed and put in a 100 mL three-necked flask. The air was removed and the temperature was gradually increased to 90 °C with reflux and magnetic stirring.…”

“…Still and all, we managed to achieve dorminant emission shift through alteration of NaHoF 4 core size and NaYbF 4 shell thickness for NaHoF 4 @NaYbF 4 (Ho@Yb) featured UCNPs in our former work. [ 68 ] The NaHoF 4 @NaYbF 4 @NaGdF 4 sample possessed even better UCL intensity than that of traditional recipe. Some basic characterizations relevant to biouse were proceeded as well, hinting at a further fulfillment of versatility.…”

A novel photostable near‐infrared‐to‐near‐infrared fluorescent nanoparticle for in vivo imaging

“…Fluorescence imaging has been acting as a mainstream observation technique in the clinic diagnosis since it is minimally invasive and has the ability to image targeted structures by using fluorescence labeling strategy (Bruchez Jr & Moronne, 1998; Chan & Nie, 1998; Gai, Yang, Yang, et al, 2018; Gulzar, Xu, Wang, et al, 2019; Kuang, Xu, Wang, et al, 2019; Larson, Zipfel, & Williams, 2003; Li, Zhao, & Qin, 2016; Vinegoni, Swisher, & Feruglio, 2016). However, the fluorescence imaging approaches suffer from the scattering and absorption of the light by tissues.…”