Experimental Verification of an Exact Evanescent Light Scattering Model for TIRM

Abstract

Total internal reflection microscopy (TIRM) is a method for the precise measurement of interaction potentials between a spherical colloidal particle and a wall. The method is based on single-particle evanescent wave light scattering. The well-established model used to interpret TIRM data is based on an exponential relation between scattering intensity and particle wall distance. However, applying this model for a certain range of experimental parameters leads to significant distortions of the measured potentials. Using a TIRM setup based on a two-wavelength illumination technique, we were able to directly measure the intensity distance relation revealing deviations from an exponential decay. The intensity-distance relations could be compared to scattering simulations taking into account exact experimental parameters and multiple reflections between a particle and the wall. Converging simulation results were independently obtained by the T-matrix method and the discrete sources method (DSM) and show excellent agreement with experiments. Using the new scattering model for data evaluation, we could reconstruct the correct potential shape for distorted interaction potentials as we demonstrate. The comparison of simulations to experiment intrinsically yields a new method to determine absolute particle-wall distances, a highly desired quantity in TIRM experiments.

A Colloid Model System for Interfacial Sorption Kinetics

“…40,41 We find that scattering intensity fits an exponential decay fairly well above h > 50 nm with λ = 145 ± 5 nm up to about h ≈ 500 nm, where the signal is too weak to resolve height information.…”

Mobility and height detection of particle labels in an optical evanescent wave biosensor with single-label resolution

“…Intensity ratios that are higher than expected from the maximum bond height displacement, might be explained by a deviation from the idealized exponential decay model. Recently, it has been shown that multiple reflections between a particle and the surface can cause a distortion of the exponential model, especially for large penetration depths (close to half the excitation wavelength) and small particle heights (Helden et al 2006, Hertlein et al 2008. For a small penetration depth of 132 nm, which is on the order of the 95 nm penetration depth in our system, simulations with 1.6 µm diameter polystyrene particles pointed to a reduction in scattered light intensity near the surface of approximately 15%, leading to a reduced measured height change.…”

On the electromagnetic scattering of arbitrary shaped beams by arbitrary shaped particles: A review

“…Also, Borghese et al [49] studied the radiation torque and force on optically linear nanostructures using an angular spectrum representation of a focused laser beam after passing through a high-NA objective lens, with details of the procedure used available from Borghese et al [50] dealing with optical trapping of nonspherical particles in the T-matrix formalism. In 2008, Hertlein et al [51] used a PWS in association with the discrete sources method (DSM) -which may be viewed as a variant of EBCM -to study an evanescent light scattering model for total internal reflection microscopy (TIRM), including an experimental verification. In 2001, a PWS representation consisting of about a hundred components was used by Nieminen et al [52] for the calculation, associated with optical measurements of laser trapping forces on a non-spherical particle.…”