Holographic pattern photobleaching apparatus for the measurement of lateral transport processes at interfaces—Design and performance

Abstract

A new instrument was developed which allows the detection and characterization of lateral transport processes at the fluid–solid interface. It is based on holographic photobleaching combined with a laterally resolved fluorescence detection by a photon-counting camera. The geometry of the holographic pattern was chosen such that by ‘‘on-chip’’ integration of photoelectrons in the liquid-nitrogen-cooled camera the images are integrated along the symmetry axis of the pattern and subsequently stored on the chip. Time-dependent changes in the pattern caused by directed motion or by diffusion are then qualified by Fourier analysis. Lateral diffusion coefficients between 10−12 and 10−6 cm2/s and velocities between 10−10 and 10−4 m/s are measurable. In order to discriminate processes at the surface and in the volume, evanescent field illumination can be chosen. The penetration depth of the evanescent field can be varied from 96 to 800 nm. The performance of the instrument is demonstrated on selected examples.