Peroxidase enzymes are of critical importance within the scientific community for their applications in biosensing assays. In a living system, natural peroxidases function as catalysts in the oxidation of peroxide (e.g., H2O2) - a harmful byproduct of aerobic processes and convert them into harmless compounds. Such an ability allows peroxidases to serve as labels in biosensing assays, where they are conjugated to antibodies and accurately produce a detection signal by catalyzing substrates. However, due to intrinsic limitations, namely instability, Peroxidase made of proteins substantially inhibit broader applications. Alternatively, nanoparticles produced from noble metals have been found to exhibit peroxidase-like abilities and, therefore, can be used as synthetic enzymes with the potential to replace their natural counterparts. Given that the stability of most peroxidase mimics is already much better than their natural counterparts, in this field, the principal challenge has been creating substantial improvements to the catalytic efficiency of the mimics. This study sought to create a cage-like nanostructure ( denoted as nanocages) consisting of two platinum group metals. This experiment uses Galvanic replacement as a mechanism to hollow all Nanocages formed. Galvanic replacement has been primarily demonstrated using coinage metals ( Ex. Ag and Au). This experiment seeks to show that this process is viable for other Nobel metals, as well. In particular, palladium cubes were used as scaffolds or sacrificed templates to induce the reaction with a precursor containing a secondary Nobel metal (Platinum, Rhodium, or Ruthenium). Once viable samples where produced and checked via TEM ( Transmission Electron microscope), the peroxidase-like activity was compared to the activity of a non-hollowed nanostructure of the same material composition using TMB colorimetric assay.

Platinum-Group Metal (PGM) nanostructures as advantageous alternatives to natural peroxidases have drawn great attention because of their superior catalytic activities, which can effectively enhance performance of enzyme-based in vitro diagnostics. The catalytic activity of metal nanoparticle peroxidase mimics can depend on their size, shape, elemental composition, and surface ligand of PGM nanostructures. Therefore, to develop optimal peroxidase mimics for a few bioanalytical and diagnostic applications, such as enzyme-linked immunosorbent assay (ELISA), it is important to investigate how structural aspects of PGM nanoparticles correlate with the ability of the nanoparticles to serve as functional mimics of protein peroxidase enzymes. In summary, this dissertation has studied: 1) iridium (Ir), platinum (Pt) and Ir/Pt bimetallic nanowire structures as peroxidase mimics, and the effect of different wires' length on their peroxidase-like activities and certain application of sandwich ELISA for the detection of carcinoembryonic antigen (CEA, a cancer biomarker); 2) ultra-small Ir nanoparticles, with an average size of 1.1 nm, supported by WO2.72 nanowire with high catalytic activity. Those Ir nanoparticles were applied to sandwich ELISA and competitive ELISA for sensitive detection of CEA and aflatoxin B1 (AFB1, a carcinogenic toxin), respectively; 3) the size effect of peroxidase mimics on their catalytic activities and performance in biosensing application, where Pd-Ir core-shell nanoparticles were used as a type of model peroxidase mimics. These studies may significantly stimulate further investigations of PGM nanostructures as peroxidase mimics and other potential applications in in vitro diagnostics.

Reactive oxygen species (ROS) such as superoxide radical anion (O2•¯), hydroxylradical (OH•), hydrogen peroxide (H2O2) and peroxynitrite (ONOO-) that are produced during the metabolism of oxygen under oxidative stress in aerobic organisms destroy several key biomolecules and lead to a number of disease states. Mammalian systems possess several effective defense mechanisms including antioxidant enzymes to detoxify these ROS. The selenocysteine-containing Glutathione peroxidase (GPx) is particularly an efficient enzyme in the detoxification of H2O2 and other hydroperoxides by using glutathione (GSH) as cofactor. The chemistry at the active siteof GPx has been extensively investigated with the help of synthetic selenium compounds. Although the anti-inflammatory compound ebselen(2-phenyl-1,2-benzoisoselenazol-3(2H)-one) is undergoing phase III clinical trial as antioxidant, the chemistry of ebselen is still not understood. The present study on a number of ebselen derivatives with various N-substitutions reveals that the substitution at the N atom is important for the antioxidant activity. This study also suggests that the nature for thiol cofactor has a dramatic effect on the GPx activity of ebselen derivatives. It has been shown that ebselen exhibits very poor catalytic activity in the presence of aromatic thiols mainly due to strong Se….O nonbonded interactions that lead to extensive thiol exchange reactions in the selenenyl sulfide intermediate. To prevent the se….O interactions, a series of tertiary amide-based diselenides have been synthesized along with their secondary amide counterparts. Detailed structure-activity correlation studies reveal that the GPx-like activity of the sec-amide-based compounds can be significantly enhanced by the substitution at the free-NH group of sec-amide functionality. The N,N-dialkylbenzylamine-based diselenides exhibit their catalytic activities via the generation of selenols which was confirmed by the reaction with anti-arthritic gold(I) compounds. Interestingly, the replacement of the hydrogen atom at the 6th position of the benzene ring of N,N-dialkylbenzylamine-based diselenides by a methoxy group prevents the thiol exchange reactions mainly be weakening the Se…N interactions and thus enhances the GPx activity. On the other hand, the catalytic activity of the tert-amine-based diselenides can also be increased by replacing the tert-amino groups with the corresponding sec-amine moieties. It has been observed that the basic amino group in the amine-based diselenides deprotonates the selenol and also the thiol cofactor, which is crucial for the higher catalytic activities of the amine-based compounds. Peroxynitrite (PN, ONOO), a strong nitrating agent, is known to inactivate a number of proteins, enzymes and other biomolecules by nitration of tyrosine residues. In this study, we have shown that the commonly used antithyroid drugs and their analogues inhibit protein tyrosine nitration. This study reveals that antithyroid agents having PN scavenging activity may be beneficial of hyperthyroidism as these compounds may protect the thyroid gland from nitrative or nitrosative stress.