The bioelectrochemistry of enzymes and their cofactors at carbon nanotube and nitrogen-doped carbon nanotube electrodes
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This dissertation explores the electrochemical behavior of enzymes and their cofactors at carbon nanotube (CNT) and nitrogen-doped carbon nanotube (N-CNT) electrodes. Two common types of oxidoreductases are considered: flavin adenine dinucleotide (FAD)-dependent oxidases and nicotinamide adenine dinucleotide-dependent (NAD⁺)-dehydrogenases. Chapter 1 presents the oxygen reduction reaction (ORR) at N-CNT electrodes as a way to electrochemically measure enzymatic turnover at the electrode surface. The unique peroxide pathway at N-CNT electrodes, which catalytically disproportionates hydrogen peroxide (H₂O₂) back into oxygen, provides an increased ORR current directly proportional to the rate of enzymatic turnover for H₂O₂ producing enzymes, even in an oxygen saturated solution. Biosensing of L-lactate using the increased ORR current is demonstrated using L-lactate oxidase. Chapter 2 explores the surface bound electrochemical signal of FAD when FAD-dependent enzyme or free FAD is allowed to spontaneously adsorb onto the CNT/N-CNT surface. Specifically, the origin of the enzymatically generated FAD signal and the rate constant of the electron transfer are elucidated. Chapter 3 continues the discussion of the cofactor FAD by demonstrating its use as an informative surface specific redox probe for graphitic carbon surfaces. Primarily, FAD can be used to determine the electroactive surface area and the relative hydrophobicity/hydrophilicity of graphitic surfaces. Chapter 4 changes gears to NAD⁺-dependent dehydrogenases by investigating the electrocatalytic oxidation of NADH at N-CNTs in comparison with conventional carbon electrodes or nondoped CNTs. Biosensing of glucose through the oxidation of NADH is demonstrated using glucose dehydrogenase adsorbed onto the N-CNT surface. Chapter 5 continues the discussion of NAD⁺-dependent dehydrogenases by addressing the reaction kinetics of NADH oxidation at N-CNTs as a tool to measure the enzymatic reduction of NAD⁺.