PhD Oral Exam - Mannu Kaur Maken, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Lanthanide upconverting nanoparticles (LnUCNPs), known for their luminescence when irradiated with near-infrared (NIR) light, often face the challenge of limited absorption capabilities, particularly due to the low absorption cross-section of sensitizer ions like Yb3+. Incorporating NIR dyes has shown promise in enhancing the upconversion luminescence of LnUCNPs by efficiently harnessing NIR light and channeling this energy to the sensitizer ions. The higher absorption cross-section of these NIR dyes, compared to Yb3+, amplifies the LnUCNPs' absorption prowess. However, the inherent photoinstability of such organic dyes hinders their practical applications, especially when sustained NIR exposure is essential.

Addressing this challenge, this research work delved into improving the photostability of dyesensitized LnUCNPs. The focus was on IR820 dye, when functionalized with thiophenol groups bearing para substituents of varied electron density, showcased enhanced stability. This enhanced stability was evident when IR820-NO2 sensitized NaGdF4:Er3+, Yb3+/NaGdF4: Yb3+ excited at 808 nm manifested brighter upconversion luminescence due to a better overlap with the Yb3+ ion absorption, and doubling its photostability duration to 90 minutes. In the process, this research also unearthed certain inconsistencies in the prevalent literature pertaining to the 980 nm and 808 nm wavelengths and the energy back-transfer dynamics from Er3+ to the dye.

Progressing further, the IR820-COOH dye was functionalized with APTMS and was incorporated into a silica framework surrounding NaGdF4:Er3+, Yb3+/NaGdF4: Yb3+. An in-depth analysis underscored the superior photostability and upconversion luminescence of this embedded system when pitted against covalent surface-linked and electrostatic variants. This optimized embedded system was also examined vis-à-vis Nd3+ doped nanoparticles, both being excited at 808 nm. Exploiting its enhanced attributes, this system was adapted as an 808 nm-responsive Fenton-type catalyst by coating it with a wide-pored silica shell incorporated with hematite nanoparticles. This culminated in an efficient degradation of the Rhodamine B pollutant, a significant environmental menace introduced by the textile sector.

To encapsulate, this thesis paves the way for bolstering the photostability and versatility of dye-sensitized LnUCNPs, employing intricate dye modifications and embedding strategies, heralding new possibilities for applications.