Biosensor based on surface plasmons has attracted a great deal of attention in recent years due to its simple detection system and high sensitivity. Propagating surface plasmon and localized surface plasmons based biosensors have become a central tool for characterizing and quantifying biomolecular interactions. Biomolecular binding and/or unbinding at the active surface of an SPR biosensor is controlled by various mechanisms, and quantitative analysis of the sensor response to interactions between the studied biomolecule (analyte) and the surface bound receptors requires a state of the art technology with multi-functionality .
Here, we present the development of a label free optical biosensor based on a combination of surface plasmon resonance (SPR) and ellipsometry, called SPR ellipsometry. With high precision measurement of ellipsometry having thickness sensitivity down to ∼ 0.1 A and high sensitivity response of the SPR, this optical tool facilitates to detect changes in the effective thickness of analytes and surface bound receptor at very low concentration. Both prism and grating coupling schemes were implemented in this study. Starting with prism coupling scheme, our proposed optical technique was assembled by using a dove prism integrated with a commercially ellipsometry system. Metal film on glass substrate was fixed on the custom made microfluidic flow cell and finally mounted on the top of the long edge of the dove prism. An index matching liquid was used to suppress the unnecessary prism–slide interference due to the air gap. Single axis alignment of the optical components in our optical setup cannot tune the position of the SPR dip thereby limiting its application. Hence, a tunable SPR biochip in wider spectral range is still desired to extend the applications of our proposed technique. This difficulty can be overcome by utilizing the gold nanoparticle immobilized on the metal surface, which helps to modulate of the surface plasmon dispersion relation. In addition, the gold nanoparticles can lead to strong coupling of incident light to plasmon resonances and enhance the sensitivity of the SPR biosensor. In this prospect, the amplified plasmonic response from various distributions of gold nanoparticles coated on top of gold thin film was studied using our proposed optical tool. Based on this study, we found that the surface plasmon resonance dip can be tuned from the visible to near infrared by simply varying the gold nanoparticle concentration. After these investigations were made, our sensor performance was tested through spectroscopic measurements which further help to choose corresponding wavelength with maximum sensitivity. Dynamic measurements at a fixed wavelength (correspond to maximum sensitivity) allow us to monitor the real-time response to the changes in surface properties on a metallic film. Our study is further extended to study biomolecular interactions. By recording the ellipsometry data in terms of relative changes in the ellipsometric parameters, □ and □ as sensor signals we monitor the biomolecular interactions in dynamic and static modes.
Moving on, we explore the possibilities of using gold nanostructure film (instead of thin film) with ellipsometry to design a localized surface plasmon resonance biosensor so called nanoparticle enhanced ellipsometry. Two different gold nanostructure films, one prepared by immobilizing the colloidal gold nanoparticle on glass substrate and another prepared by thermal annealing of gold film were used in this study. The thermal annealed samples show the formation of the gold nano islands film partially embedded in the glass substrate. Based on the bulk sensitivity test, the thermally annealed film gives one order of magnitude higher in the refractive index (RI) sensitivity as compare to the gold thin film. This gold nanostructure films were used in various kinds of biomolecular interaction studies including DNA hybridization, antigen-antibody interaction, protein-cell interaction etc. By combining spectroscopic ellipsometry measurements with theoretical modeling, our proposed technique allows quantitative analysis of the biomolecules such as effective change in thickness, refractive index, porosity, surface mass density etc. when capture on thin film as well as nanostructure film. Through dynamic measurement, we can understand the adsorption time, interaction time, and the dissociation time about the biomolecules under investigation.
In this thesis, we use a grating coupling scheme to combine the SPR with ellipsometry and used ellipsometry phase information as a sensor signal. A facile way of preparing large area of plasmonic nanostructures through soft nanoimprint lithography is presented. This technique significantly reduces the cost of the fabrication and makes it easy to replicate the samples without using expensive tools such as e-beam lithography, focus ion beam etc. Based on the bulk sensitivity test for imprint plasmonic nanostructures using ellipsometry phase signal, we found that the 1D binary metallic grating gives the best refractive index sensitivity which is comparable to that of the nanoparticle enhance ellipsometry. Finally, this biochip is used in studying protein–protein interaction, as a demonstration we dynamically monitor the interaction between bovine serum albumin (BSA) and anti-BSA protein. Thus, we envision that ellipsometry investigations with these highly sensitive sensing platforms can be used in developing a non-destructive, label free, and highly sensitive biosensor with a sub-nanometer resolution in thickness.

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