Browsing by Subject "Upconversion"

The ability to interrogate structure-function photophysical properties on lanthanide-doped nanoscale materials will define their utility in next-generation applications and devices that capitalize on their size, light-conversion efficiencies, emissive wavelengths, syntheses, and environmental stabilities. The two main topics of this dissertation are (i) the interrogation of laser power-dependent quantum yield and total radiant flux metrics for a homogeneous, solution phase upconversion nanocrystal composition under both continuous wave and femtosecond-pulsed excitation utilizing a custom engineered absolute measurement system, and (ii) the synthesis, characterization, and power-dependent x-ray excited scintillation properties of [Y₂O₃; Eu] nanocrystals, and their integration into a fiber-optic radiation sensing device capable of in vivo dosimetry.

Presented herein is the laser power-dependent total radiant flux and absolute quantum yield measurements of homogeneous, solution-phase [NaYF₄; Yb (15%), Er (2%)] upconversion nanocrystals, and further compares the quantitative total radiant flux and absolute quantum yield measurements under both 970 nm continuous-wave and 976 nm pulsed Ti-Sapphire laser excitation (140 fs pulse-width, 80 MHz). This study demonstrates that at comparable excitation densities under continuous-wave and fs-pulsed excitation from 42 - 284 W/cm2, the absolute quantum yield, and the total radiant flux per unit volume, are within a factor of two when spectra are integrated over the 500 - 700 nm wavelength regime. This study further establishes the radiant flux as the true unit of merit for quantifying emissive output intensity of upconverting nanocrystals for application purposes, especially given the high uncertainty in solution phase upconversion nanocrystal quantum yield measurements due to their low absorption cross-section. Additionally, a commercially available bulk [NaYF₄; Yb (20%), Er (3%)] upconversion sample was measured in the solid-state to provide a total radiant flux and absolute quantum yield standard. The measurements were accomplished utilizing a custom-engineered, multi-detector integrating sphere measurement system that can measure spectral sample emission in Watts on a flux-calibrated (W/nm) CCD-spectrometer, enabling the direct measurement of the total radiant flux without need for an absorbance or quantum yield value.

Also presented is the development and characterization of a scintillating nanocrystalline composition, [Y_2-xO₃; Eu_x, Li_y], in which Eu and Li dopant ion concentrations were systematically varied in order to define the most emissive compositions under specific x-ray excitation conditions. It is shown that these optimized [Y_2-xO₃; Eu_x, Li_y] compositions display scintillation responses that: (i) correlate linearly with incident radiation exposure at x-ray energies spanning from 40 - 220 kVp, and (ii) manifest no evidence of scintillation intensity saturation at the highest evaluated radiation exposures [up to 4 Roentgen per second]. X-ray excitation energies of 40, 120, and 220 kVp were chosen to probe the dependence of the integrated emission intensity upon x-ray exposure-rate in energy regimes where either the photoelectric or the Compton effect governs the scintillation mechanism on the most emissive [Y_2-xO₃; Eu_x, Li_y] composition, [Y_1.9O₃; Eu_0.1, Li_0.16]. These experiments demonstrate for nanoscale [Y_2-xO₃; Eu_x], that for comparable radiation exposures, when scintillation is governed by the photoelectric effect (120 kVp excitation), greater integrated emission intensities are recorded relative to excitation energies where the Compton effect regulates scintillation (220 kVp excitation).

The nanoscale [Y_1.9O₃; Eu_0.1, Li_0.16] was further exploited as a detector material in a prototype fiber-optic radiation sensor. The scintillation intensity from a [Y_1.9O₃; Eu_0.1, Li_0.16]-modified optical fiber tip, recorded using a CCD-photodetector or a Si-photodiode, was correlated with radiation exposure using a Precision XRAD 225Cx small-animal image guided radiation therapy (IGRT) system, an orthovoltage cabinet-irradiator, and a clinical X-ray Computed Tomography (CT) machine. For all x-ray energies tested from 80 - 225 kVp, this near-radiotransparent device recorded scintillation intensities that tracked linearly with total radiation exposure, highlighting its capability to provide alternately accurate dosimetry measurements for both diagnostic imaging and radiation therapy treatment. Because Si-based CCD and photodiode detectors manifest maximal sensitivities over the emission range of nanoscale [Y_1.9O₃; Eu_0.1, Li_0.16], the timing speeds, sizes, and low power-consumption of these devices, coupled with the detection element's linear dependence of scintillation intensity with radiation dose, demonstrates the opportunity for next-generation radiation exposure measuring devices for in/ex vivo applications that are ultra-small, inexpensive, and accurate.

Lanthanide-doped materials have been studied for decades for their unique photophysical properties derived from their f-block orbitals. More recently, the prominence of techniques allowing for the study and characterization of nanoscale materials has led to a renewed interest in these materials on the nanoscale. This work has led to a wealth of studies on the synthesis and characterization of a number of lanthanide doped nanomaterials as well as numerous proposals and preliminary demonstrations of their applications. This dissertation focuses on three key aspects of this field: (1) a synthetic means to controlling the relative intensities of the emissive bands in upconversion nanoparticles (UCNPs) (2) the development of a 3D imaging modalities that leverage the unique photophysical properties of upconverting nanomaterials and (3) the application of scintillating nanoscale radiation detection materials as an alternative to the single-crystal bulk lanthanide doped materials and plastic scintillators currently employed.

In many proposed applications of upconverting nanomaterials, the relative intensities of emission bands are a key component. For instance, security inks rely on measuring the ratio of the green and red emission peaks to distinguish between authentic and counterfeit materials. While multiple upconversion compositions have been identified with varying relative emission intensities, there has been little study into effective means of synthetically manipulating the intensities of these emission bands within a single composition. Research presented here focuses on understanding the fundamental influences on the coprecipitation reaction method and ultimately manipulating the synthetic techniques to achieve meaningful changes in the emission properties without changing the overall composition of the nanoparticle. An injection reaction method is developed that allows for manipulating the interionic distance between sensitizer and activator pairs resulting in a significant decrease in the red emission. It is further shown that this change in interionic distance is a likely overlooked contributing factor in the emission properties of other compositions that have been studied. This work represents the first steps towards designing application specific emission properties via synthetic control rather than developing applications specific nanomaterial compositions.

Owing to their NIR excitation, lack of tissue autofluorescence, and photostability UCNPs are a promising target for bio-imaging applications. Past studies have demonstrated the potential for UCNPs for cellular, tissue and whole body imaging. However, an imaging system that fully incorporates these materials and leverages their properties to improve upon existing imaging capabilities has yet to be shown. An optical emission computed tomography imaging (OECT) system is modified to enable NIR excitation and imaging of the upconversion signal. Using this system, the first 3D modeled upconversion imaging is demonstrated. By coupling the transmission and upconversion signals in this imaging modality, precise structural imaging is possible and background free images of both the bronchial pathways in the lungs as well as white pulp structures in the spleen are shown using upconversion enhanced OECT.

In addition to upconversion processes, the f-block orbitals can also be utilized in scintillation processed to convert ionizing radiation into visible light. Typically, lanthanide based single crystals are used in the bulk for developing large radiation detectors. However, recent advances in the Therien lab have seen the development of nanoscale [Y2O3] nanomaterials and their incorporation into a fiber optic detector (nanoFOD). This detector is shown here to function as a quality assurance tool in monitoring brachytherapy radiation treatments. Further, using the 3D printing method of fused deposition modeling, this scintillating nanomaterial is incorporated into a first of its kind low-cost radiation-imaging screen. In both applications, the nanomaterial is demonstrated to be highly effective at detecting and monitoring radiation. The low cost, and ease of manufacture for these devices as well as their instantaneous detection of radiation dose rates are a significant improvement over the current detector technologies employed.