An Off-Null Ellipsometer with Lateral Scanning Capability for Kinetic Studies at Liquid-Solid Interfaces

Abstract

A null ellipsometer was equipped with an automatic sample scanning device and used for the measurement of adsorption and desorption of proteins at the liquid/solid interface on surface wettability gradients on silicon wafers. To follow the processes along the wettability gradient, "off-null" ellipsometry was used. The polarizer and analyzer were set at extinction and the light intensity was recorded. The square root of the light intensity from the off-null ellipsometry measurements was found to be proportional to the surface concentration as determined by null ellipsometry measurements. The kinetics of adsorption and nonionic surfactant induced desorption varied considerably between fibrinogen and IgG, and along the wettability gradient. The possibility of following the kinetics of the desorption of protein along the wettability gradient in the presence of a nonionic surfactant gave new information. The nonionic surfactant induced desorption was rapid at the hydrophobic part (advancing contact angle against water, θ ⩾ 90°). In the intermediate region the desorption was slower, but still too rapid to be detectable with ordinary null ellipsometry. In the hydrophilic region very little protein desorption was seen when using the nonionic surfactant. At present, one off-null scan takes 19 s, the limitation being the paper speed of the recorder. The data recording is now improved to allow scans every 1.5 s. For measurements at one point only a time resolution of 1/1000 s will be possible.