PhD Thesis Colloquium: Shweta Shekar (22/05/23)

Thesis title:

Investigations of sensors based on new molecular architectonics: synthesis, fabrication, and application

Faculty advisor(s):

Prof. Praveen C Ramamurthy


22nd May, 2023 (Monday), 4:00 PM (India Standard Time)


KPA Auditorium, Department of Materials Engineering


Rapid industrialization and expansion of urban landscapes in large cities generate maximum pollutants to the surrounding environment. While organic pollutants occur naturally, the rise in anthropogenic and geogenic activities has contributed to an increase in inorganic pollutants in groundwater resources. These toxic by-products should be detected and remedied from the start of the polluting source itself before it reaches out to the ecosystem. Monitoring these inorganic pollutants is imperative to avoid their adverse impacts on ecological and human health. Thus far, variegated detection systems have been reported; however, restricted selectivity, specificity and uncompetitive lower limit of detection impede their extensive deployment. Additionally, sophisticated, and advanced instruments available for the identification of heavy-metal toxins require skilled technicians for appropriate interpretation. Although the traditional colorimetric and/or fluorimetric methods of detecting these analytes are common, there is an underlying hindrance in the detection limit, and ease of usage.

Contextually, solid-state sensors are a better alternative as they operate on low power, provide a better detection limit and have higher reliability. To this extent, my thesis focuses on the design and testing of solid-state sensors to detect analytes such as nitrates, fluorides, and the mutagenic carcinogenic heavy metal hexavalent chromium. Metal-oxide-based solid-state sensors have often exhibited a lack of selectivity and specificity. Hence, the work presented here explores a low-cost alternative viz., bandgap engineered organic conjugated molecular sensors. Biomimetic-based molecular architecturing of a thiourea-based carbon nanocomposite has been developed to selectively detect nitrate ions in water with a detection limit of 10 ppm. The device architecture is that of a chemiresistor. The notorious inorganic contaminant, nitrate, has a high interference from fluoride ion in the detection process. Consequently, it is imperative to judiciously design a selective receptor, which shows a limited response to nitrate ions. A statistical dimensional-reduction method called ‘Principal Component Analysis’ (PCA) has been employed to achieve pattern recognition by minimizing the loss of data for chemiresistive.

Furthermore, solid-state and fluorimetric sensing methods have been established to detect mutagenic hexavalent chromium ions. A carbon-black nanocomposite of metformin-Schiff-base exhibited selective detection of hexavalent chromium, with a limit of detection of 1 ppm. The fluorimetric sensor, however, exhibited a detection limit of 2.9 ppb of Cr(VI). We have then developed a lesser-explored phenolphthalein-structured Schiff base molecular library for the objective study of colorimetric and fluorimetric detection of trivalent iron and hexavalent chromium, respectively. Molecular simulations have been carried out for the ground state geometrical optimizations; bandgap is determined through TDDFT calculations using Gaussian software. 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 implementing density functional theory. Electrostatic potential maps have been examined effectively to suggest possible sensing mechanisms for the optical sensors demonstrated in this thesis.