Improving surface plasmon resonance sensor and nanoaperture optical tweezers for biomolecule analysis
Date
2023-08-30
Authors
Babaei, Elham
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Abstract
This thesis explores innovative approaches to improve sensitivity of surface plasmon resonance (SPR) sensors and nanoaperture optical tweezers (NOT) technique for biomolecule analysis.
A significant enhancement in the sensitivity of surface plasmon sensors by 3.3 times and a quadrupling of resolution are presented compared to conventional SPR sensors. The optimal design parameters for generating short-range modes on a gold film (period: 250 nm, gap size: 40 nm, thickness: 10 nm) are identified using rigorous coupled wave analysis (RCWA) to achieve minima for incident angle and wavelength, following the same configuration as conventional SPR sensors which employ a standard 50 nm thick gold film and Kretschmann-Raether coupling with a light wavelength of 760 nm. Finite difference time domain simulations confirm the correspondence of short-range surface plasmon modes to localized surface plasmons (LSP). By using the field confinement capability of short-range surface plasmon (SRSP) modes, higher sensitivity in SPR is achieved, facilitating the characterization of biomolecule interactions.
The second study introduces novel approaches to enhance the colloidal lithography technique commonly used. The simplification of nanoaperture detection and characterization is achieved by charge coupled device (CCD) images and polarization-dependent transmission, eliminating the need for a scanning electron microscope (SEM), which otherwise makes the process time-consuming and costly. By employing polarization analysis, configurations of the holes, including single, trimers, and other clusters, along with their orientations, can be identified. Furthermore, changing the substrate of the sample from glass to polyvinyl chloride (PVC) results in a seven-fold decrease in the minimum required power for trapping 20 nm polystyrene beads, attributed to reduced surface repulsion. Lastly, the utilization of tape exfoliation instead of sonication in ethanol is presented, which preserves the uniform apertures on the gold surface.
In the third study, an innovative method is presented for rapidly trapping single, unlabeled proteins in a NOT system. By integrating the principles of dielectrophoresis and NOT, a significant 10-fold reduction in trapping time is achieved, along with the successful trapping of Neuropeptide Y, which has a molecular weight of only 4kDa. This improvement is obtained by placing the counter electrode on a glass substrate outside the solution, thereby creating a fringe field that enhances the trapping performance. Placing the counter electrode directly in the solution does not generally lead to faster protein trapping. However, it is observed that electrophoresis can expedite the trapping of polystyrene spheres, especially with increasing applied voltage.
This effect is attributed to changes in the repulsive surface potential. This voltage-dependent trapping enhancement is only observed with positive applied voltages. Furthermore, in other projects, extraordinary acoustic Raman (EAR) spectroscopy was used to trap a 20 nm polystyrene sphere. An optimal power level is observed for exciting the vibrational modes in the nanoparticle and simultaneously obtaining sharp peaks in the normalized standard deviation (NSTD) of the noise versus beat frequency spectrum. Additionally, PR65, which is a subunit of the protein phosphatase 2A (PP2A), is trapped, and its acoustic vibration modes are excited using the EAR technique. The experimental results demonstrate that the vibrational modes occur at frequencies of 9.29 GHz, 19.28 GHz, 30.23 GHz, and 41.18 GHz, which align with the results obtained through normal mode analysis (NMA). It is noteworthy that this technique offers the advantage of being single-molecule and label-free compared to conventional methods used for biomolecule characterization. The characterization of PR65 serves as a valuable model for understanding the structural regulation of various repeat proteins.
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Keywords
Optical tweezers, Dielectrophoresis, Electrophoresis, Short range surface plasmon biosensor