Figure 2.

Time for capture of first 10 analyte molecules on a disk-shaped sensor of radius r by 3D diffusion, 3D + 2D diffusion and 3D diffusion followed by directed active transport of analyte. We assume a diffusion constant for 3D-diffusion of 80 μm²/s, 2D-diffusion constant is taken as one-half (∼microtubule diffusion constant on surface) or one-tenth (∼protein diffusion constant in a lipid bilayer) of the 3D-diffusion constant, an analyte concentration C of 1 pM, a compartment radius R of 100 μm, an active transport velocity of 0.5 μm/s, and a capture fraction f of 0.9.