Ali, Babar (2024) Self-Assembled Nanostructured Substrates for ATR-SEIRA Applications. [Tesi di dottorato]

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Abstract

Infrared (IR) spectroscopy is a versatile tool for point-of-care systems such as biological sensors. Due to small the intrinsic cross-section of a molecular vibration which limits the sensitivity of detection, but it can be addressed in several ways, including improving the IR light source, forming extremely sensitive IR detection devices, by taking advantage of the SEIRA (Surface Enhanced Infrared Absorption) phenomenon. The first two solutions are costlier in comparison to the SERIA phenomenon because it is an inexpensive and efficient way to improve the interaction of light with matter. The key objective of this study was to develop, fabricate, and analyze a low-cost plasmonic substrate for attenuated total reflectance SEIRA (ATR-SEIRA). This is a highly effective method for detecting even the smallest quantities, including conformational flexibility, and improving the vibrational modes of compounds. We have used a FEM (finite element method) based wave optics module in the frequency domain (COMSOL Multiphysics) to numerically analyze plasmonic nanostructure based on a periodic gold Nano disk array to determine its sensitivity and tunability. We developed a numerical model using the Otto configuration to simulate the effect of a SEIRA substrate in ATR-SEIRA instrumentation’s (by including the “prism-effect” in the model). To validate the numerical method, self-assembled Nano-spheres lithography fabrication based SEIRA substrates are characterized using the scanning electron microscope (SEM) and ATR-FTIR (ATR-Fourier Transform IR) spectrometer. Furthermore, we have demonstrated the ATR-SEIRA phenomenon by fictionalizing the SEIRA substrate with a self-assembled bio-absorbed analyte 4-aminothiophenol (4-ATP) as monolayer. The outcomes obtained are in good agreement with the numerical analysis. As a standard benchmark, we have evaluated the performance of our realized SEIRA substrate by calculating the amplification factor, which amounts to 10^5. These results confirm the accuracy of numerical modelling and show that the proposed low-cost ATR-SEIRA substrates are repeatable, homogeneous, and can increase sensitivity. Overall, the proposed SEIRA substrate can be available tool for performing IR absorption spectral analyses, offering better performance compared to standard FTIR approaches.

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