Growth and Characterization of Anodic Nanoporous and Nanotubular Structures of Zinc Oxide for Chemical Sensing
Since the time of ancient Greek physicians, it has been known that the odors of body fluids carry information about human health. Advanced studies conducted in recent decades have revealed the identities of hundreds of volatile organic compounds (VOCs) present in exhaled breath. Detecting and monitoring them through breath analysis facilitate non-invasive disease diagnosis. The technology currently uses analytical instruments that are expensive and inappropriate for common point of care services. Chemical sensors are a low-cost alternative for breath gas/VOC detection. The objective of this dissertation work was to develop unique nanostructures of metal oxides and study their characteristics for their potential use as chemiresistive sensors for medical diagnosis. Zinc oxide (ZnO) is one of the most promising materials for chemical sensing. It changes the electrical resistance in relation with concentrations of appropriate chemical species in the environment. Low sensitivity to analyte concentrations in low parts per million level or below and insufficient selectivity are common problems found with this material. Use of low dimensional architectures with unique properties is an effective strategy to address this problem. In this dissertation work, efforts were focused on developing novel ordered nanotube array structures of ZnO, tailoring the properties and applying them for the detection of VOCs and gases that are biomarkers of diseases. Nanostructures of ZnO were synthesized on Zn substrates through a controlled electrochemical anodization process. The anodization conditions were discovered for developing novel nanotubular architectures of ZnO. The influence of process parameters, including voltage, time, temperature, and electrolyte composition, were investigated. Electrical, structural, thermal and surface characterizations were performed for obtaining a scientific understanding of the growth process in different electrolytes. In order to carry out the sensing studies, a new test chamber was fabricated. A setup and a protocol were established for studying multiple sensors at a time. The study conducted at various temperatures and environments revealed remarkable sensitivity of the material to hydrogen and specific VOCs that are biomarkers of diseases, particularly breast cancer. The material exhibited fast response and recovery. The experiments conducted on animals showed promise toward utilizing the material for early-stage breast cancer detection.