Synthesis, Functionalization, and Characterization of Dominant UV Emitting Upconverting Nanocrystals and Absolute Quantum Yield and Power Dependence Metrics for the Elucidation of Upconversion Mechanisms
The discovery, formulation, and characterization of novel compositions of matter for aid in the diagnosis and treatment of disease has ever been a compelling force behind nanomaterials development. In instances of disease originating from oncogenic mutation, proliferation, and metathesis; cancer has long been a most difficult dysfunction to diagnosis and treat in virtue of its innate alteration and disregulation of otherwise well-managed and healthful cellular processes. To date, cancer therapies have relied largely on highly toxic chemotherapy or radiation treatments, addressing the overarching problem of individual cellular mutations in a global sense, often deleterious to the overall health of the patient. Ever-progressing work on nanomaterial-based applications to either promote cancer diagnosis or implement novel therapeutic means of drug delivery, activation, or the precisely-targeted destruction of cancer cell lines has been afforded much attention in the integrated biological and materials science fields. Recent developments in nanosized laser materials incorporating lanthanide-doped sensitizer and activator pairs and the development of numerous crystallographic, co-dopant, morphological, and/or surface-appended optimizations to these materials have given rise to a novel class of nanomaterials, with unique photophysical properties that have direct import into light-based activation of chemical processes, triggered non-invasively through biological tissues, and merging intra-cellularly targetable nanocrystalline compositions and ex vivo light activation. Upconverting nanocrystals (UCNCs) are one such class of nanomaterial wherein near-infrared (NIR) light, at the nadir of tissue absorption, can serve to sequentially or cooperatively excite long-lived lanthanide (Ln3+) 4f excited states and, through various energy transfer processes coupled between both the UCNC material composition and its integral Ln3+ dopants, are capable of building an excited state population capable of emitting in higher frequencies than its incident NIR excitation.
In the study of these UCNCs, the prospect of activating intra-cellular photodynamic processes or drugs of low cellular toxicity, until light activated in a precisely localized regime (e.g. the nucleus of a cell), has motivated extensive research into the generation of novel UCNC materials, in multiple compositions and on multiple size scales to direct the mechanisms of upconversion (UC) to produce high fluence ultraviolet (UV) photons upon NIR (972 nm) excitation. Continuing optimizations have yielded a high ytterbium (Yb) sensitizer, cubic α-NaYbF4 UCNC composition, codoped with a thulium activator, to generate excited state saturated UV transitions, 1I6 → 3F4 (349 nm) and 1D2 → 3H6 (362 nm), and their refinement to afford dominant UV emissive spectral signatures at low NIR laser excitation. Their photophysical dynamics are sparsely described in the literature, breaking from both fields of laser photonics and conventional inorganic nanoscience, and require renewed emphasis to be afforded in exacting crystallographic, photophysical, and size dependent effect characterization, heavily directing the structure-function relationships of luminescent Ln3+ dopants and their host crystal matrices. Requisite in this study is a call for the optimization of uniform, monodisperse, and reproducible preparations of unique UCNCs and precise characterization of the properties they display and the origins thereof.
Offered herein are the enveloping efforts to more fully understand the mechanistic processes of UC of both poorly characterized, literature standard materials, novel UCNCs tuned for enhancement of UC emission in the UV, and the adaptations to each that ultimately affect their photophysical dynamics. A tandem course of this research follows from inorganic shelling, passivation methodologies to ameliorate crystallographic surface defects and UC luminescence quenching sites to overall enhance the dominant UV emissivity of novel co-doped UCNC. These state-of-the-art UC materials are: 1) α-NaYbF4: Tm3+, interlaced with gallium, chromium, yttrium, and other trivalent metal ions, serving to finely modulate UC mechanistic processes and enhance luminescent properties and 2) sodium co-doped LaF3 and BaLaF4 (0.5%Tm, 20%Yb), displaying 3 and 2 orders of magnitude enhancement of UV emissions due to controlled perturbation of the local crystal field environment. The Core @ Shell architectural derivatives of these materials exhibit an eminent departure from classical luminescent fluorophores, phosphors, or quantum confined luminescent nanomaterials, in both degree of luminescent flux generation and the complicated mechanistic processes they are derived from.
To a great extent, this work attempts to establish testable grounds for comparison of UCNCs; extending from interrogation of photophysical lifetime measurements, excitation versus emissive flux power dependence studies, high resolution X-ray photoelectron spectroscopy (HR-XPS) and power diffraction (HR-XRD) assessments of crystallographic defects and perturbations on the atomic scale, and the establishment of new metrics of radiant flux versus absolute quantum yield for use in comparison of UCNCs towards their applicability in areas of variable or limited excitation flux and the ultimate utility of discerning hit-to-lead UCNC materials for medical nanodevice compositions. A salient component affecting these metrics is the direct surface interactions with respect to solvents, coordinating ligands, and appended functional moieties for enhancement of UCNCs towards specific applications; largely directed towards cancer biology and medical study. In a confluence of inquisition of UCNCs and their high energy, UV luminescent properties, interfacing with the surface presenting effects of solublization and bio-targeting molecular functionalization; literature standard, β-NaYF4 (2%Er, 20%Yb) UCNCs have been generated in highly uniform compositions to assess the size-dependent effects with respect to luminescent quenching surrounding a UCNC surface and functionalization methodologies have been offered as a proof of concept towards the construction of an optimized biomolecular targeting nanodevice, with known limits and predictable interactions, both to NIR excitation light and potential intra-cellular biological environments.
The ultimate goal of these explorations is the innovative fusion of the above concepts into a nanotherapeutic device involving: 1) the generation of a well-studied and predictable NIR-absorbing and dominant UV-emissive UCNC, with defined co-dopant optimizations and employing an optimal Core @ Shell architecture, 2) the requisite surface functionalization needed to afford aqueous solubility and a means of covalently conjugating targeting molecules of interest, and 3) the ultimate and equal assessment of such a composite system with respect to possible alternate materials in the literature and novel UCNCs currently under development. To date, no such convergent study has been conducted to any degree of reproducibility or certainty of desired and defined functionality. In this work is described in detail each optimized component for such a device or potentially one marked by differing, but assessable conditions for alternate applications. The optimization of a sub-10 nm, dominant UV-emissive UCNC, the crystallographic and photophysical origins of its UC mechanism under varied conditions, and the optimal means of their employment (both in terms of establishing equivalent metrics and utility in cancer nanotherapeutics), assessment, and readdressing of, as yet undiscovered limits to these materials are presented.
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