We present both experimental and theoretical studies for investigating DNA molecules attached on metallic nanospheres. We have developed an efficient and accurate numerical method to investigate light scattering from plasmonic nanospheres on a substrate covered by a shell, based on the Green’s function approach with suitable spherical harmonic basis. Next, we use this method to study optical scattering from DNA molecules attached to metallic nanoparticles placed on a substrate and compare with experimental results. We obtain fairly good agreement between theoretical predictions and the measured ellipsometric spectra. The metallic nanoparticles were used to detect the binding with DNA molecules in a microfluidic setup via spectroscopic ellipsometry (SE), and a detectable change in ellipsometric spectra was found when DNA molecules are captured on Au nanoparticles. Our theoretical simulation indicates that the coverage of Au nanosphere by a submonolayer of DNA molecules, which is modeled by a thin layer of dielectric material (which may absorb light), can lead to a small but detectable spectroscopic shift in both the Ψ and Δ spectra with more significant change in Δ spectra in agreement with experimental results. Our studies demonstrated the ultrasensitive capability of SE for sensing submonolayer coverage of DNA molecules on Au nanospheres. Hence the spectroscopic ellipsometric measurements coupled with theoretical analysis via an efficient computation method can be an effective tool for detecting DNA molecules attached on Au nanoparticles, thus achieving label-free, non-destructive, and high-sensitivity biosensing with nanoscale resolution.
We performed experimental measurements and theoretical simulation based on an efficient half-space Green’s function
method to investigate the diffraction patterns of light scattering from silicon and ZnO microspheres on a substrate. The
microscopic ellipsometry image for s- and p-polarized reflectance and their phase difference (Rs, Rp, and Δ) was taken
by a modified Optrel MULTISKOP system with rotating compensator configuration for various angles of incidence and
wavelengths ranging from 450nm to 750nm. An 80X objective was used and the pixel size for our image is around
200nm. The images obtained display clear diffraction patterns, which is analyzed by an efficient full-wave simulation
based on half-space Green’s function method. The near-field distributions obtained theoretically are then converted to
far-field images by filtering out the evanescent waves and propagating waves which cannot reach the objective. The
experimental results are then compared with simulated images to gain better understanding of the image patterns. Some
prominent peaks are observed and attributed to resonances related to whispering gallery modes.
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