Date of Award
Upconversion, Passivation Layers, Nanoparticles, and Lanthanides
Chemistry | Inorganic Chemistry | Materials Chemistry | Physical Chemistry
Upconversion (UC), is a phenomenon that occurs when low-energy excitation (usually near-infrared (NIR)) results in higher-energy emission. Upconversion nanoparticles (UCNPs) are particles less than 100 nm in size that are synthesized using rare earth metals such as Yb, Er, Tm, and Y. UCNPs have important applications in a variety of fields, including bio-imaging, security printing, and latent fingerprint development. Traditional methods for latent fingerprint development such as fluorescent powder dusting have several drawbacks including low contrast, high background interference, and autofluorescence. NIR-to-NIR UCNPs are composed of NaYF4: Yb, Tm and emit 800 nm light under 980 nm excitation due to the absorbance of Yb3+ ions. NIR excitation produces no background emission from substrates and ambient lighting does not hinder the collection of the 800 nm emission from the nanoparticles. Exciting using NIR light reduces fluorescence from the substrate and improves the contrast of luminescent images created using the particles. These features make NIR-to-NIR UCNPs attractive for latent fingerprint development. Here, NIR-to-NIR UCNPs of various Yb doping with passive shell layers of NaYF4 were synthesized. Passive shells are those that do not participate in upconversion but assist in covering surface defects that may have occurred during synthesis and decreases the effect of surface quenching. The goal of this research was to optimize and increase the brightness of the NIR-to-NIR UCNPs by adjusting the doping concentrations of Yb, while also observing the effect of surface quenching before and after the addition of the inert NaYF4 shells to the UCNPs. The brightness of the nanoparticles was analyzed using internal quantum efficiency measurements at varying excitation power densities. In addition to the NIR-to-NIR UCNPs, NIR-to-Green UCNPs with both an active shell of 10% Yb and 10% Nd and passive shell of NaYF4 were synthesized. NIR-to-Green UCNPs are composed of NaYF4: Yb, Er, and convert NIR excitation to shorter-wavelength green emission. The 980 nm excitation wavelength required by traditional Yb, Er UCNPs overlaps a weak absorbance band of water, interfering with excitation and increasing the chance of overheating tissues while imaging. However, addition of the active shell allows for the NIR-to-Green UCNPs to be excited by either 800 nm or 980 nm light while also covering surface defects and decreasing surface quenching. Exciting the particles with 800 nm, which is in a transparency window for biological samples, greatly reduces the danger of overheating tissue while maintaining the high penetration depth for bio-imaging. For security printing, these particles can also be used to print two different but overlapping images which can be viewed individually using different excitation sources.
Chov, Ashleigh M., "Enhancing Upconversion nanoparticle performance of NaYF4: Yb,Tm with doping optimization and shell addition and of NaYF4: Yb,Er with Yb,Nd shell addition" (2020). Honors Thesis. 113.