PhD Oral Exam - Zubi Sadiq, Chemical and Materials Engineering

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

Point-of-use sensors are essential for environmental monitoring, food quality assessment, and disease diagnostics, particularly in remote and in-field settings. However, sensitivity, user-friendliness, and portability challenges limit their commercial availability, highlighting the need for improved sensing platforms. Gold nanoparticles (AuNPs) are promising for assay development due to their rapid synthesis, functionalization, and naked-eye colorimetric response, making them ideal for point-of-use applications.

This thesis introduces a novel, low-cost, portable, and user-friendly detection platform using dextran-gold nanoparticles (dAuNPs), selected for their enhanced stability and simple synthesis. Both plasmonic and nanozyme properties of dAuNPs contribute towards a highly sensitive detection system. The dAuNPs-based tablet platform was developed using a pipette-out technique, ensuring solid, stable tablet formation (dAuNPs-Tablet). The pH-responsive behaviour of the tablet was assessed in various ionic solutions; the concentration of dextran was optimized; and different solid supports (e.g., composite, powder) were tested for optimal performance. The tablet-based approach offers superior portability, storage, and transportation compared to solution-phase dAuNPs (dAuNPs-Solution), quick production, easy handling, long-term stability (over four years), and low-cost (~1.22 CAD/100 tablets). Spectroscopic and microscopic analyses confirmed that dAuNPs retain their morphology during tablet formation, making dAuNPs-Tablet a robust and user-friendly alternative. A plasmonic tablet sensor was developed using dAuNPs-Solution, prepared under reflux conditions with post-synthetic dextran addition to facilitate tablet formation. This sensor detected reactive small species like hypochlorite and hydrogen peroxide. Hypochlorite was tested in swimming pool water, while hydrogen peroxide as an oxidative stress biomarker, was analyzed in urine sample using Fenton chemistry. Additionally, the antioxidant effect of green tea in reducing oxidative stress was explored.

Next, the tablet platform was advanced by developing a dual-functional system. We directly transformed dAuNPs-Solution into a tablet form, eliminating reflux conditions and post-synthetic dextran addition, resulting in the formation of a "direct tablet". This dual-functional tablet detected uric acid via its plasmonic response and glucose through its nanozyme activity, producing colorimetric signals within biologically relevant range, making it highly suitable for point-of-care applications. Thus, this thesis presents a transformative tablet-based detection platform with multiplexed capabilities, advancing point-of-use assays. The biocompatibility of dextran in dAuNPs-Tablets further highlights their potential for biomedical applications.