Rare-Earth-Based Nanoparticles: From Microwave-Assisted Synthesis to Potential Applications
Creators: name: Liu, Nan
Publisher: name: Université d'Ottawa | University of Ottawa
Year: 2024
Subjects: Lanthanide-based nanoparticles
Microwave-assisted synthesis
bioimaging probes

description: Lanthanide (Ln3+)-based nanoparticles (NPs) are promising candidates for various applications, e.g., bioimaging, therapy, catalysis, and energy conversion, due to their unique optical and magnetic properties. Ln3+-based NPs are capable of emitting UV and visible light (upconversion, UC) as well as near-infrared (NIR) light (downshifting) when excited with NIR light (e.g., 980 nm). Microwave (MW)-assisted synthesis of sodium rare-earth (RE = Y + Gd) tetrafluoride, NaREF4, NPs has attracted increasing attention due to its significantly shorter reaction time and high reproducibility compared to the conventional thermal decomposition or solvothermal methods. NaREF4 can crystallize in two crystalline phases, namely the cubic (α) and the hexagonal (β) crystal structure. The high-symmetry α-phase is more efficient for the magnetic properties, while the lower-symmetry β-phase is more suitable for the optical properties. The present thesis aims to (i) explore the MW-assisted approach towards the synthesis of the whole family of NaREF4 NPs, and evaluate their crystalline phase accessibility (Chapter 4), to (ii) control the architecture of core/multi-shell NaREF4 NPs seeking multimodal functionalities (Chapter 5), and subsequently to (iii) expand the synthesis approach to other fluoride materials beyond NaREF4 NPs, i.e., NaMnF3 (Chapter 6). More precisely, a series of sub-10 nm NaREF4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) were prepared using a MW-assisted approach. The accessibility of these NaREF4 NPs in α- and β-phase as a function of the RE ion choice was investigated. The results showed that using the MW-assisted approach, α-NaREF4 NPs could be obtained for the whole RE family, while the β-NaREF4 NPs were only accessible for the lighter RE ions (Pr to Dy). An adapted ligand removal procedure using a HCl solution of pH 1.5 was developed to render these NPs water dispersible, and the chemical stability of the obtained NPs under the acidic conditions was investigated. Interestingly, upon acidic treatment, some of the α/β-NaREF4 NPs underwent a phase transformation from NaREF4 to REF3, pointing towards challenges with respect to chemical stability. Indeed, a correlation between the thermodynamic stability of the α/β-NaREF4 and the hexagonal/orthorhombic REF3 phases – as a function of RE ion choice – and the chemical stability of the NPs was found (Chapter 4). The design of core/shell/shell (CSS) architectures allows engineering different Ln3+ ions in separate layers in one NP, optimizing their optical properties and generating new functionalities. Having gained new knowledge from the synthesis of NaREF4 core NPs, further optimization of the MW-assisted synthesis approach to control the architecture of CSS NPs was conducted. For example, opto-magnetic CSS NPs were explored as potential multimodal imaging probes. Therefore, β-NaGdF4:Yb3+,Er3+/NaGdF4/NaDyF4 CSS NPs were synthesized (Chapter 5). Careful tuning of the MW reaction conditions allowed for tuning of the inner shell thickness from ca. 3 to 6 nm. Such control is crucial to physically separate the luminescent Er3+/Yb3+ ion pair in the core from the magnetic Dy3+ ions (emission quencher) in the outer shell, ultimately preventing UC loss. The mechanism of UC loss was investigated in detail by evaluating the Er3+ UC intensities and the excited state lifetimes of Yb3+ and Er3+ as a function of the inner shell thickness. The results demonstrated that a 4 nm thick NaGdF4 inner shell did not only restore but enhanced the UC emission of Er3+. In addition, the effect of the outer NaDyF4 shell thickness on the particles’ magnetic and X-ray computed tomography (CT) performance was investigated. Careful shell thickness tuning unveiled that the CSS NPs with the thickest outer shell thickness (4 nm) possessed superior magnetic resonance imaging (MRI) T2 and CT contrast effects than that of other MRI and CT contrast agents (Chapter 5). Inspired by the promising CSS architecture, β-NaGdF4:Yb3+,Er3+/NaGdF4:Tm3+/NaGdF4 CSS NPs were synthesized to investigate their thermometric properties (Chapter 7). This is the latest work in progress, exploring future research directions. The versatility of the developed MW-assisted approach was further explored to synthesize NaMnF3 particles. NaMnF3 particles have been reported as alternative candidates for MRI T1 contrast agents. A stringent adjustment of the reaction conditions was conducted, including the Na+-to-Mn2+ metal ion ratio, nucleation and growth temperatures, as well as reaction time. Interestingly, this MW-assisted method enabled the production of NaMnF3 particles with specific morphologies (nanorods and ribbons) beyond well-established plates by varying the Na+-to-Mn2+ metal ion ratio. Moreover, the obtained NaMnF3 particles exhibited promising contrast effect as MRI T1 contrast agents (Chapter 6). Overall, the MW-assisted approach led to promising achievements for the synthesis of NaREF4 NPs and could be extended to other types of fluoride materials, i.e., NaMnF3. The concept of fabricating multilayer NPs by use of MW-assisted routes that offered control over shell thicknesses provided insights for the future architecture design of Ln3+-based materials for their e.g., biomedical applications and beyond. Further extension of these studies is expected to provide knowledge valuable for the future design of fluoride-related materials for their diverse applications.