Investigations on the development of a novel hybrid sensor for environmental monitoring applications.

Bhavsar, Kaushalkumar

Abstract

Heavy metal toxicity is a major environmental problem world-wide. Increased spreading and high concentration levels of the toxic heavy metals in water environments have posed a severe threat to human health and the ecosystem. Over the years, to improve the drinking water quality standards, safe threshold concentrations of these highly toxic pollutants are constantly being lowered by the governmental and environmental bodies. Current instrumental techniques used to detect these low levels of heavy metal ions are laboratory based, use sophisticated instruments, expensive, time consuming and require trained personnel. There is a constant need for the development of in-situ, rapid, highly sensitive and selective sensors to monitor the very low concentration levels. Various approaches for improving sensitivity and selectivity have been investigated over the years involving multiple detection techniques. In general, optical approaches provide higher sensitivity along with simplicity while electrochemical sensors provide better selectivity. In the last decade, nanomaterials have emerged as a key element in their sensitivity improvement. Combining all these advantages, a novel hybrid sensor has been envisaged integrating optical and electrical fields in addition to nanomaterials. This thesis reports investigations on enhancing the sensitivity/selectivity through optical, nanomaterials and electrochemical routes, and then integrating these to realise a hybrid sensor. A novel optical sensor has been developed using the phenomena of evanescent waves in optical fibre with dithizone to detect heavy metal ions. A U-bent sensor geometry has been investigated to enhance the optical sensitivity of the sensor through higher evanescent field near the surface. Further, optical field confinement to the surface has been investigated through thin film coating to improve the sensitivity. A new inverted trench design based sensor has been developed, and sensitivity enhancement has been achieved through this novel design and confirmed using modelling work accompanied by experimental results. Large surface to volume ratio of nanomaterials, such as ZnO nanowires, on the sensor surface can provide enhanced surface interactions leading to higher sensitivity. But, surfaces modified with ZnO nanostructures tend to be hydrophobic in nature. A new remote and non-contact method to tune the wettability of the ZnO nanostructures using LEDs has been developed. Higher sensitivity has been achieved by tuning the wettability of ZnO nanowires using the developed method. An electrochemical sensor has been developed in order to understand the potential effects of the electric field on the near surface molecular dynamics and thereby, effects on the optical detection. Effects of parameters such as deposition time, scan frequency, concentration, electrode materials and their surface area have been investigated to improve the sensitivity and selectivity. Multi-ions selectivity has been achieved by simultaneous detection of copper, mercury and lead ions in buffer solution. Higher sensitivity has been obtained by modifying the gold electrode using graphene flakes. Further, to integrate the optical field with this sensor to realize the hybrid sensor, thickness of the gold electrode has been optimised to allow the penetration of evanescent field onto the electrode surface. Under optimised conditions evanescent field resonantly couples to the surface plasmons of the gold electrode. Computational investigations have been carried out to study the effect of number of graphene layers on the sensitivity of the surface plasmon resonance (SPR) based optical sensor integrated with the electrochemical sensor. Preliminary investigations on the developed hybrid sensor show that the electric field complements the optical field. Investigations have shown that application of electric field enhances the sensitivity for optical detection by attracting more ions on the electrode and also, provides the multi-ion selectivity. These investigations have opened up new possibilities for the real-time monitoring of highly sensitive and selective molecular interactions, showing strong potential in a range of applications areas such as environmental sensing, biosensing and agricultural sensing.

Citation

BHAVSAR, K. 2016. Investigations on the development of a novel hybrid sensor for environmental monitoring applications. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-1677500

• Thesis Type: Thesis
• Deposit Date: May 31, 2022
• Publicly Available Date: May 31, 2022
• DOI: https://doi.org/10.48526/rgu-wt-1677500
• Keywords: Heavy metal ions; Electrochemical sensors; Fibre optic sensors; Surface plasmon resonance
• Public URL: https://rgu-repository.worktribe.com/output/1677500
• Award Date: Jun 30, 2016