A) Enzyme-substrate-buffer (E+S+B) enters into the left inlet, and enzyme-buffer (E+B) is injected to the right inlet. In this way, the enzyme concentration is uniform all over the entire chip, while the substrate concentration is higher in the left side (blue) gradually decreasing towards the right (white). The dashes rectangle indicates the area where the enzyme movement is studied in great detail using STED-FCS microscopy technology. B) The concentration of the enzyme urease (black) and the concentration of urea (its substrate, gray) are plotted against the position within the FCS zone, between 0 and 2.5 millimeters from the laser beam. The scale at each point is an error bar, showing the standard deviation over five repeated measurements. C) The diffusion coefficient (Da), which indicates how fast the enzyme is moving, does not change in the absence of substrate, but it increases when the substrate is present, meaning that the enzyme is moving faster to the right.