Dissertations, Theses, and Capstone Projects

Date of Degree

2-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Chemistry

Advisor

Alexander Greer

Committee Members

Orrette Wauchope

Ryan Murelli

Lesley Davenport

Subject Categories

Chemistry

Keywords

Au thiolate glutathione nanoclusters, photodynamic, hydrophobic and hydrophillic silica nanoparticles, near infrared (NIR) phosphorescence, slope inflection angle, particle as a proxy for quantification of singlet oxygen in cells, shades of phototoxicity in in fluorescent imaging agents

Abstract

This thesis comprises five chapters as enumerated below.

Chapter 1 describes our findings on the reactions of Au glutathione nanocluster (Au20SG16) with singlet oxygen (1O2) in D2O. We noticed it underwent a self-photooxidation reaction. Leading us to mechanistic studies using photophysical, photochemical, theoretical, and indirect trapping methods. We found rapid total quenching of singlet oxygen (1O2) by ground-state Au20(SG)16, with evidence for dioxygen insertion into the nanocluster. Supported by analyses with IR, ESI-MS, and density functional theory, we propose the formation of Au–O–O–SG bonds in the Au nanocluster. The expansion of the staple motif from dioxygen insertion is attributed to heightened lability and blebbing by the O–O group. We then demonstrated that the self-photooxidized Au20(SG)16 undergoes oxygen-atom transfer to a phosphine trap in the dark.

Chapter 2 discusses the air-borne singlet oxygen photooxidation of prenyl phenol coated silica nanoparticles. We uncovered significant selectivity for dihydrofuran formation over allylic hydroperoxide formation. The hydrophobic particle causes prenyl phenol to produce an iso-hydroperoxide intermediate with an internally protonated oxygen atom, which leads to dihydrofuran formation as well as O-atom transfer. In contrast, hydrophilic particles cause prenyl phenol to produce allylic hydroperoxide, due to phenol OH hydrogen bonding with SiOH surface groups. Mechanistic insight is provided by air/nanoparticle interface coated with the prenyl phenol, in which product yield were 6-fold greater on the hydrophobic nanoparticles compared to the hydrophilic nanoparticles and total rate constants (ASI-kT) of 1O2 were 13-fold greater on the hydrophobic vs hydrophilic nanoparticles. A slope intersection method (SIM) method was also developed that uses the airborne 1O2 lifetime (τairborne) and surface-associated 1O2 lifetime (τsurf) to quantitate 1O2 transitioning from volatile to non-volatile and surface boundary (surface···1O2). Further mechanistic insight on the selectivity of the reaction of prenyl phenol with 1O2 was provided by DFT calculations.

Chapter 3 describes our study of the mechanism of association of airborne 1O2 at the air/surface interface by using 1O2’s near infrared (NIR) phosphorescence and geometric analysis based on slope inflection angle (θ) of air-to-particle transfer. This offered insight to 1O2-surface binding as opposed to conventional kinetic analysis. Two 9,10-disubstituted anthracene quenchers were adsorbed to the particle surface, producing θ ranging from ~91° (greater quenching) to ~99° (less quenching) due to the reduction of airborne 1O2 lifetime (τairborne) by 43% to 95%. A more efficient (lower θ) 1O2 quenching is observed in the order dimethylanthracene-coated particle > anthracene dianion-coated particle > native silica. The anthracene dianion charges and surface silanols did not enhance the 1O2 surface quenching. Indeed, the quenching of airborne 1O2 by native silica was minimal, in which a slight reduction in its surface lifetime (τsurf) was observed (0-5%). This θ approach opens up opportunities in fields such as surface oxidation processes in nanoplastics that is an emerging concern.

Chapter 4 describes a strategy that enables the numerical quantification of the 1O2 molecules that are required to oxidize a particle as a proxy for the number of 1O2 molecules that are lethal to a cell. Few studies have focused on how to estimate the number of 1O2 molecules required to kill a cell. This estimate is needed since 1O2 is thought to account for 75% of the photodynamic effect instead of oxygen radicals and ions. The particle system discussed in this chapter is ~9-fold larger than the diameter of the radiation-induced fibrosis (RIF) tumor cell, but the number of 1O2 reactive sites are nearly identical, each ~1.4 × 1011 per particle and per cell. Interestingly, the combined 1O2 reactive amino acids, unsaturated lipid, and guanine sites is 50- to 180-fold less than the 1.2 × 109 1O2 molecules needed to kill each cell in vivo, where [1O2]ss » 1 mM. Here, we advance an understanding of mechanistic details of 1O2 reactions at both solid particle and cell surfaces. We find that comparison is better suited on a per particle and per cell basis because the particle volume is 670-fold greater than the cell volume and particles are packed ~105 per cm3, whereas cells are 108 per cm3.

Chapter 5 highlights a paper by Huang et al. in an issue of Photochemistry and Photobiology in 2024. It describes shades of phototoxicity in fluorescent imaging agents that are not intended to be phototoxic. Phototoxicity was assessed using a modified neutral red uptake (NRU) in vitro assay with mean photo-effects (MPE) for the fluorescent agents IRdye800, indocyanine green (ICG), proflavine, and methylene blue (MB), with comparisons to known phototoxic agents benzoporphyrin derivative (BPD) and rose bengal (RB). The experimental conditions were aimed to mimic clinical settings, using not only visible light, but also near infrared light for insight to photosafety and deep tissue damage. Molecular mechanisms underlying the phototoxicities were not sought, but IRdye800 and ICG were mainly deemed to be safe, whereas proflavine and MB would require precautions since phototoxicity can overshadow their utility as fluorescent imaging agents.

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