Date on Master's Thesis/Doctoral Dissertation

12-2020

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Mendes, Sergio B.

Committee Co-Chair (if applicable)

Liu, Jinjun

Committee Member

Liu, Jinjun

Committee Member

Smadici, Serban

Committee Member

Yu, Ming

Author's Keywords

Evanescent; wave; cavity; ring; down spectroscopy

Abstract

This dissertation reports the development of ultra-sensitive platforms based on the laser cavity ring-down spectroscopic (CRDS) technique to enable optical and spectroelectrochemical investigations in the condensed phase of matter at challenging scenarios. Firstly, an electrically-active solid/liquid interface for the evanescent-wave cavity ring-down spectroscopy (EW-CRDS) was developed to specroelectrochemically investigate redox events. By coating the interface of total internal reflection of the EW-CRDS platform with a high quality optically transparent and electrically conductive indium tin oxide thin film (ITO), we demonstrated that sufficiently long ring-down times can be achieved to allow for spectroelectrochemical investigations of redox species at solid/liquid interfaces at low surface coverages. The effects of an applied electric potential on the adsorption behavior of a redox protein onto different interfaces were investigated. For each interface, the adsorption and desorption constants, the surface equilibrium constant, the Gibbs free energy of adsorption, and the surface coverage were optically measured by our electrically-active EW-CRDS tool. Cyclic voltammetry (CV) scans under synchronous optical readout were performed to study the effects of each molecular interface in the redox process of surface-adsorbed protein species. The electro-active EW-CRDS technology is experimentally tested and demonstrated to provide a high-performance platform for studies of electrode-driven redox events of surface-confined molecular species at low submonolayer coverages and at a single diffraction-limited spot. Next, the electrically-active capability of the EW-CRDS device has been extended to develop a bio-sensing strategy based on the combination of the electro-active EW-CRDS platform with a sandwich immunoassay approach for the detection antigens of the influenza A virus (H5N1). Initially, the EW-CRDS was deployed to characterize in-situ and in real-time the formation of the assembly of the immunoassay-based biosensor. Our strategy proceeds in a stepwise manner: in the first step, the surface of the electro-active EW-CRDS device is functionalized with a capture antibody (Ab) aimed at a specific virus antigen. Next, the capture Ab-coated surface is exposed to a target antigen, which after binding to the surface it promotes the immobilization of secondary Ab that has been labeled with a redox-active probe. The redox-active probe methylene blue acts as a transduction element for monitoring molecular binding events and can be electrochemically modulated on the EW-CRDS platform to provide a unique optical interrogation signal. Based on this novel detection strategy, the experimental results have demonstrated an outstanding level of sensitivity in the pico-molar range for the detection of the influenza virus antigen. Finally, we used an electrically modulated optical signal collected with an electro-active EW-CRDS platform under CV potential modulation for fast detection and direct quantification of a target antigen. Such results demonstrate the potential of the electro-active EW-CRDS technology for high sensitive detection of surface-confined biomarkers at a single spot and at very low concentrations, which open the prospect towards an arrayed-detection technology. In addition, the highly sensitive CRDS technology in the liquid phase was applied for trace detection of nitrite ions in aqueous environment. The principle of the analytical method used for the determination of trace nitrite is based on the changing of the chromatic reagent color from purple to yellow due to its reduction reaction of nitrite ions in an acidic medium, which results in a decrease of optical absorbance. The decrease in absorbance is directly proportional to the nitrite concentration and the CRDS technique was used to measure the absorbance decrease at 532 nm. The experimental results show that nitrite ions could be detected accurately within a low detection limit, which can reach as low as 8 ng/ml. With the current CRDS setup, we have achieved an absorption sensitivity of about 1x10-6 cm-1. Finally, we have explored the ultra-sensitivity of the CRDS platform for direct measurements in changes of optical loss in ultra-thin films of a semiconductor material (ITO) during surface treatment with ultra-violet (UV) radiation. The results demonstrated the ability of the CRDS to measure minute absorption changes in the ITO ultra-thin film during UV treatment. Upon inserting a glass slide coated with an ITO film at Brewster’s angle inside the optical cavity, the cavity ring-down time of about 1.6 μs was achieved, which enables measurements of optical absorption loss as low as 3x10-6 cm-1. The ITO ultra-thin film was treated with UV radiation in two ways: after and during the deposition process at different oxygen flow rates. In order to evaluate the effect of surface treatment, the CRDS platform was employed to measure the extinction coefficient for each coated sample with and without UV exposure, while the change in electrical resistivity was monitored simultaneously. Based on the measurements of optical constants and cavity ring-down times, the UV treatment could be tuned to increase the optical efficiency of an ITO ultra-thin film. These applications are demonstrated to provide novel tools to study the interfacial phenomena with high sensitivity and specificity, which are expected to open several opportunities to investigate a wide range of molecular assemblies and sensing applications.

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