Objective-Type Dark-Field Illumination for Scattering from Microbeads

We introduce a method for detecting and tracking small particles in a solution near a surface. The method is based on blocking the back-reflected illumination beam in an objective-type total internal reflection microscope, leaving unhindered the light scattered by the particles and resulting in dark-field illumination. Using this method, we tracked the motion of 60-nm polystyrene beads with a signal-to-noise ratio of 6 and detected 20-nm gold particles with a signal-to-noise ratio of 5. We illustrate the method's use by following the Brownian motion of small beads attached by short DNA tethers to a substrate.