Schematic representation of single-molecule flow-stretching bead assay. (A) A setup based on a conventional optical microscope. A buffer solution including target proteins flows through the flow chamber with constant rate by a syringe pump. (B) ~300 beads (2.8 μm in diameter) attached to tethered-DNA molecules on the surface under 10× magnification objective. (C) Bead-DNA is immobilized on the streptavidin-coated surface that is passivated with biotin-PEG/PEG (1:100). A laminar flow stretched bead-DNA. (D) Intensity profile of a bead image shows a Gaussian distribution.