The optical setup of a dark-field optical section microscope. The light source was a 60 W metal halide light. It illuminated the samples through a hemisphere glass lens. The illumination light had a large incident angle in the medium, only the scattering light was collected by the objective lens and imaged by a color CCD. We recorded the scattering images at different focal positions by using a quickly linear scan of the objective lens along the depth direction. The left image is a dark-field CCD image for a HeLa cell without any AuNPs.