Sample cells were preassembled prior to use. Two thin strip spacers (thickness $200 mm) were positioned $5 mm apart on the center of a pre-cleaned microscope slide, and epoxy applied at the outer edges of the spacers. A streptavidin functionalized 24 Â 40 mm, no. 1.5 coverglass (Xenopore) was then put on the top of the slide. Before introducing the bead and RNA mixture, the sample cell was surface-coated with acetylated BSA by incubation with binding buffer for 30 min at room temperature and washed with 1 ml of the binding buffer, to prevent any sticking of beads to the surface. The bead and RNA mixture was then introduced into the sample cell, and incubated for 30 min at room temperature, and finally washed with 1 ml of binding buffer to remove any unbound beads and RNA.