Ytterbium-Based Energy Transport of Multicolor Control and Applications in Rare Earth Doped Nanoparticles

Abstract

Lanthanide-doped up-conversion nanoparticles can convert near-infrared light into ultraviolet and visible light emission, showing advantages of large anti-Stokes shift, sharp emission peak, low toxicity, and high chemical stability, which provide a solid foundation for their biological and photonic applications, such as bioimaging, anti-counterfeiting safety, super-resolution imaging, laser and photodynamic therapy. In order to meet the different needs in practical applications, it is necessary to control optical properties of up-conversion materials.

The physical mechanism of lanthanide ion doped upconversion on the nanoscale was deeply studied. Despite the progress, there are many unresolved fundamental problems which hinder the in-depth understanding of rare earth upconversion kinetics, such as the small extinction coefficient of rare earth ions, the weak excitation light absorption of up-conversion nanoparticles, and concentration quenching at high doping concentrations. In addition, the energy mechanisms of rare earth ions for some processes are still unclear, the various crystal structures and morphologies are lacking. Recent research suggested that Yb³⁺ has new functions instead of being a sensitizer in the up-conversion system, such as the tuning of the emission colors and lifetimes. So, in this thesis, we focus on the in-depth physical mechanism of Yb³⁺ energy transfer for the fine spectral control and luminescence dynamics of upconversion systems.

The thesis begins with a review of recent progress in the development of synthesizing high-quality core–shell lanthanide-doped nanoparticles along with the mechanistic analysis of the preparation of special shaped nanoparticles. Moreover, further introduction on tuning luminescence through nanostructure engineering opened up exciting opportunities for various technological frontier applications such as anti-counterfeiting, bioimaging, optical thermometers and so on, which are also summarized in this chapter.

In Chapter 4, we investigate the role of Yb as sensitizer in optical properties of the Nd/Yb coupled up-conversion system under 808 nm excitation and have an in-depth understanding of the role of Yb³⁺ in this coupled system. By adjusting the concentration of Yb³⁺ in the middle layer of NaYF₄:Yb/Er@NaYF₄:Yb@NaYF₄:Nd/Yb nanoparticles, the energy transfer effect on the Yb³⁺ sublattice has been studied to promote the transmission process of sensitizer to activator, and improve up-conversion luminous intensity. At the same time, the physical mechanism of Yb³⁺ energy transfer on the Nd³⁺ sub-lattice is analyzed. According to the energy transfer properties of Yb-A(A = Er, Tm, Ho), the interface energy transfer process of Yb³⁺ at the interface is further studied. By designing NaYF₄:Er@NaYF₄:Yb nanoparticles, it is explored that the interface energy transfer enhances the upconversion emission process of Er. By using infrared dye ICG molecules to further sensitize the nanoparticles, the upconversion luminescence intensity is significantly enhanced. These research contents provide in-depth understanding of the mechanism of energy transfer interaction in lanthanides.

In Chapter 5, the Yb³⁺ ion is neither a sensitizer nor an activator but acts as an energy trapper, and an in-depth analysis of the Yb³⁺-mediated excitation energy cycling transfer is performed to optimize the self-sensitized up-conversion luminescence phenomenon of highly doped Nd³⁺. We designed a NaGdF₄@NaGdF₄:Yb/Nd@NaGdF₄ nanostructure, sandwiching the active light-emitting layer between inert layers to reduce the loss of energy transfer in space and the effect of surface quenching. It is found that highly doped Yb³⁺ can well mediate and utilize the excitation energy in the energy cycle of Nd³⁺→Yb³⁺→Nd³⁺, resulting in a significant increase in the content of Nd³⁺, thereby effectively avoiding concentration quenching. Nd³⁺ 586 nm upconversion emission at ²G_7/2 energy level can be enhanced by two orders of magnitude. By analyzing the results of downshift spectra under 808 nm excitation, it was determined that the effective energy transfer of Nd→Yb is the key to the effective visible upconversion emission of Nd³⁺. The phonon assisted energy transfer of Yb→Nd under the excitation of 980 nm is studied, and the process of phonon assisted energy transfer is sensitive to temperature, so we carried out temperature detection and analysis on the sample and found that the sample showed a trend of thermal enhancement with increasing temperature. This system has a good application prospect in the field of optical sensing, and the temperature probe within 303-393 K has been successfully prepared, and the maximum absolute sensitivity at 303 K is 3.6 K^-1.

In Chapter 6, the influence of the Yb³⁺ migration layer on the upconversion emission of the highly doped Er³⁺ system was systematically studied. The thickness of the Yb³⁺ migration layer in NaErF₄:Yb/Tm@NaYF₄:Yb nanoparticles was adjusted, and it was found that the thickness of the migration layer can change the emission of Er³⁺ from red to green emission under 980 nm excitation. By designing appropriate reaction conditions, adjusting the reaction temperature, the amount of rare earth ions, and the amount of shell precursors, we have developed a general method for synthesizing uniform morphology and hexagonal "flower-like" nanoparticles. Based on the incomplete coating of flower-like nanoparticles, and by further fine-tuning and controlling the energy transfer path between the sensitizer and activator ions, we synthesized NaErF₄:Yb/Tm@NaYF₄:Yb@NaGdF₄:Yb/Nd flower-like nanoparticles, the sample will adjust the different energy levels of the same activator ion along a specific path at a specific excitation wavelength, with different emission wavelengths under excitation at 808 nm, 980 nm, and 1530 nm, and very sensitive to 980 nm excitation power, just change the power range from 0.078 to 2.643 W, the color of the up-conversion emission can change from red to yellow, and then to green. At the same time, the flower-like nanoparticles NaGdF₄:Ce/Tb@NaGdF₄:Tb exhibited obvious fluorescence quenching in the presence of dopamine. The detection range of dopamine was 0.05-250 μM, and the detection limit is design at 6.8 nM. It provides new ideas and references for the development of a new generation of high-sensitivity molecular detection probes.

Date of Award	8 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Bo Zhou (External Supervisor) & Feng WANG (Supervisor)