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Particle/Surface Interactions Investigated with TIRM G. Brügger, L. Schaffner, J. Ríčka, and M.Frenz Dept. of Biomedical Photonics, University of Bern; [email protected]; [email protected] Evanescent Wave Illumination/TIRM • total internal reflection (TIRM) when angle of incidence qi higher than critical angle qc → “evanescent wave” is generated • Intensity of evanescent wave gets exponentially weaker with increasing distance to surface and particles scatter light proportional to this intensity → evanescent wave illuminates only particles very close to the surface → possible to determine particle/surface separation by particle brightness Optical Tweezers • particles with refractive index higher than surrounding medium can be trapped with a gaussian laser beam • depending on beam shape, particles will be trapped around focus or pushed away Possible Measurements • observation of ad- and desorption of single particles to surface → deposition statistics, particle counting [1] • total fluorescence signal → determination of surface mass density of fluorescence marked molecules [2] • interaction between single particle (trapped by optical tweezers) and surface, intensity of scattered light contains information about weak repulsive and attractive forces → interaction potential can be restored [3] Selected Measurements • particle adsorption on different surfaces • glass Supported Phospholipid Bilayer (SPB) encourages particle adsorption • protein adsorption onto glass • adsorbed amount can be determined absolutely from subsequent flushing at the end of the experiment • single particle pushed by optical trap against glass • fluctuations due to Brownian motion allow determination of interaction potential References 1. L. Schaffner et al., “Surfactant mediated adsorption of negatively charged latex particles to a cellulose surface”, Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006), 39–50 2. V. Hlady, D. R. Reinecke, J. D. Andrade, “Fluorescence of Adsorbed Protein Layers”, Journal of Colloid and Interface Science 111 (1986), 555-569 3. Dennis C. Prieve, “Measurement of colloidal forces with TIRM”, Advances in Colloid and Interface Science 82 (1999), 93-125