Ellipsometric advances for local surface plasmon resonance to determine chitosan adsorption on layer-by-layer gold nanoparticles

The ellipsometric measurement of local surface plasmon resonance (LSPR) caused by the adsorption of chitosan on layer-by-layer gold nanoparticles (Au NPs) was investigated. Six nanometer (6 nm) Au NPs were prepared and layer-by-layer Au NPs were fabricated to shift the LSPR to 520, 540, and 560 nm, respectively, due to the Mie theory. The thicknesses and the fractions of the layer-by-layer Au NPs were measured accurately using a combination of the Fresnel equation and the Maxwell-Garnett equations for ellipsometry. Furthermore, the position of the LSPR was shifted by chitosan. Using trajectory to record the trace of polarized light for ellipsometry resulting from LSPR, it was found that LSPR is predominantly induced when the LSPR position is close to the wavelength of the ellipsometric measurement. The trajectory circle of LSPR is very large for an increase of chitosan adsorption on Au NPs when the LSPR position is close to the detected wavelength. The linear approximation aspect quantifying the trajectory corresponds with the change of LSPR for the adsorption of chitosan, except for cases with low incidence and Brewster angles. The aspects and technologies of ellipsometry will benefit from the findings in this study, with potential applications in the fields of determination of molecular adsorption.