Investigations of Sensors Based on New Molecular Architectonics: Synthesis, Fabrication and Application

Abstract

Rapid industrialization and expansion of urban landscapes in large cities generate maximum polluting ingredients to the surrounding environment. While organic contaminants are naturally occurring, the increase in human and geogenic activities has contributed to an upsurge in inorganic pollutants in groundwater resources. These harmful by-products should be detected and eliminated at the source of contamination before they reach the environment. Monitoring these inorganic pollutants is critical to avoiding their negative effects on the environment and human health. So far, many detection techniques have been documented; however, their broad deployment is hampered by limited selectivity, specificity, and an uncompetitive lower limit of detection. Furthermore, the highly technological and advanced methods available for heavy-metal contamination detection demand the intervention of skilled professionals for reliable assessment. Given that traditional colorimetric and/or fluorimetric approaches to detecting these analytes are widely used, there is a significant limitation in detection limit and simplicity of use. Solid-state sensors are a preferable option in this context considering they operate on less electrical power and have a higher detection limit and reliability. To that end, this dissertation addresses the development and evaluation of solid-state sensors for the detection of analytes such as nitrates and heavy metals such as hexavalent chromium and trivalent iron. Metal-oxide-based solid-state sensors have frequently demonstrated poor selectivity and specificity. As a result, a large portion of this thesis is dedicated to a low-cost alternative, bandgap-engineered organic conjugated molecular sensors. A thiourea-based carbon nanocomposite has been designed to selectively detect nitrate ions in water with a detection limit of 10 ppm. The architecture of the device is that of a chemiresistor. A major bottleneck in the detection of nitrate ions is the competitive interference from the fluoride ion. In view of this, the molecule is judiciously designed to minimize interference from such anions. The data acquired from the sensor performance evaluation is further subjected to a statistical dimensional-reduction technique known as “Principal Component Analysis” (PCA) to accomplish pattern identification while minimizing data loss. Furthermore, fluorimetric and solid-state sensing approaches to detecting mutagenic hexavalent chromium have been proposed. By employing a carbon black-based nanocomposite and a highly functionalized variant of the guanidine molecule with an active sensing core, we have engineered a solid-state prototype sensor to detect hexavalent chromium. To attain the highest optimized response from these devices, in addition to excellent reliability and repeatability, two distinct strategies for device fabrication have been explored and compared: drop-casting and doctor-blading methods. The chemiresistive sensor based on carbon nanocomposite exhibited a detection limit of 1 ppm. To further enhance the detection limit, the organic molecule has been investigated for its potential as a fluorimetric sensor for hexavalent chromium. The fluorimetric titration approach demonstrated a detection limit of ~3 ppb, which is lower than the permissible limits set by most water management agencies. Following that, we have developed a lesser-explored phenolphthalein-structured Schiff-base molecular library for the objective study of colorimetric and fluorimetric detection of iron and hexavalent chromium, respectively. Theoretical models utilizing density functional theory provided substantiation for the molecular design method and sensing mechanisms. In addition, the ground state geometrical optimizations and bandgap values have been computed by implementing density functional theory. Electrostatic potential maps have been examined effectively to suggest possible sensing mechanisms for the optical sensors demonstrated throughout this thesis.