It is required to reduce the excitation volume of fluorescence with enhanced field intensity to apply fluorescence correlation spectroscopy (FCS) technique to the investigation of biological molecules. Field localization induced by plasmonic nanostructures enables measurements of molecular dynamics under high concentration exhibiting high signal-to-noise (SNR) ratio. To achieve this goal, we have investigated the feasibility of plasmonic monomer and dimer nanostructures for FCS techniques. We have studied field enhancement and localization induced by different gold monomer arrays whose shapes were circle, rhombus and triangle and also gold dimer arrays which had a gap of 10 nm. These plasmnoic nanostructures were considered to be on the gold film and glass substrate with chrome adhesion layer. We could shift the peak wavelength of field enhancement by changing the dimensions of nanostructures to spectrally overlap the field enhancement to excitation and emission spectrum of fluorophores. In the case of dimer configuration having a 90-nm diameter and a 20-nm height, we have induced the near-field localization with a light source at 671 nm whose dimension was 18×6×6 nm^3 with an enhanced field intensity by 500 times in comparison with a incident light. The field distribution was analyzed numerically and experimentally using finite-difference-time-domain method and near-field scanning optical microscope. We could measure the diffusion coefficients of 50-nm fluorescent beads with improved SNR which was found to be 44.6×10-14 m^2/s. Results would include the diffusion mapping of fluorescence molecules using imaging FCS technique to show the plasmnoic nanostructures is applicable to nanoscale FCS for study of molecular biology.
Kiheung Kim, Hongki Lee, and Donghyun Kim, "Nanoscale light confinement and fluorescence excitation using plasmonic metal nanostructures (Conference Presentation)," Proc. SPIE 10506, Nanoscale Imaging, Sensing, and Actuation for Biomedical Applications XV, 105060O (Presented at SPIE BiOS: January 31, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2291420.5752194731001.
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