Nanozyme-Based Colorimetric Sensors for the Detection of Urinary Biomarkers of Early Onset Diabetes and Renal Deterioration

Nanozymes are nanomaterials that possess intrinsic enzyme-mimicking catalytic activity and have the potential to be used in sensing, disease diagnosis, drug delivery, and environmental remediation applications. In sensing applications, nanozymes produce a colorimetric and/or fluorometric output similar to that of enzyme-linked immunosorbent assay (ELISA). This makes them a potential replacement for ELISA in clinical settings, as they can eliminate the use of natural enzymes, which are susceptible to the surrounding molecules and the environment. Nanozymes can potentially provide stability, easy storage, tunability, and catalytic activity across a wide range of conditions. While nanozyme sensors for disease detection use blood as the source of biomarkers, urine, which contains metabolites eliminated by the kidneys, prostate, liver, pancreas, and other organs, is an abundant source of disease biomarkers. Effective monitoring of these urinary biomarkers can potentially identify diseases at an early stage, monitor disease progression, and assess the effectiveness of treatment. This thesis focuses on the development of nanozyme sensors to detect key biomarkers associated with diabetes and diabetes-induced renal deterioration. Diabetes is a serious and chronic condition with no cure. Although early diagnosis can reduce the risks associated with diabetes-related complications, this disease is undiagnosed in over 45% of the global population. Urine contains biomarkers that are indicative of early-onset diabetes. Therefore, this thesis attempts to develop nanozyme sensors to detect acid phosphatase (Chapter 3), glucose (Chapters 4 and 5), and uric acid (Chapter 5) in a physiologically relevant range with minimal or no sample treatment or sample dilution. Chapter 3 shows the potential of Platinum (Pt) nanozyme to detect urinary acid phosphatase (ACP) with high sensitivity and a broad dynamic range. The high oxidase-mimicking activity of Pt nanozymes at pH 5 allows the conventional two-step ACP assay (optimal activity of ACP is also at pH 5) to be performed in a single step, reducing the assay time by half. The ability to detect ACP in the physiologically relevant range in undiluted human urine samples demonstrated the potential of the Pt nanozyme sensor in complex biological fluids. To enhance the commercial potential of nanozyme sensors, Chapters 4 and 5 focus on creating nanozymes on cotton fabric as templates, allowing on-demand tunability of the catalytic reaction. Chapter 4 highlights the importance of creating Ag-based bimetallic nanozymes using a combination of electroless deposition and galvanic replacement reactions. The bimetallic Ag-Pt nanozyme showed peroxidase-mimicking catalytic activity by generating hydroxyl and superoxide radicals, which is ‘atypical’ as peroxidase-mimicking nanozymes are known to only produce hydroxyl radicals. The high catalytic activity of the Ag-Pt nanozyme was used to detect millimolar concentrations of glucose in undiluted human urine samples. Chapter 5 shows the potential of a non-noble metal and amorphous FeOOH nanozyme created on cotton fabrics using electroless plating. As a single metal catalyst, this nanozyme also showed atypical peroxidase-mimicking catalytic activity over a wide pH range (pH 5-7), which enabled the detection of two biomarkers, glucose and uric acid, in undiluted human urine. Overall, this thesis is an attempt to create new nanozyme-based sensors to detect disease-related biomarkers in urine with minimal sample preparation, which makes them viable for the practical deployment of point-of-care diagnostic systems.