Functionalization of channels with PLL and PEM. Microfluidic channels were fabricated using a two-step process described above in fabrication section. An oxygen plasma treatment was used to ac�vate the surfaces of the microfluidic devices, a�er which we rinsed the devices (10x its internal volume) with a solu�on of PLL in water (0.5 g L -1 ). A�er equilibra�on (5-10 min), excess solu�on in the channels was thoroughly washed with water (50x its internal volume). Further func�onaliza�on were performed using layer-by-layer technique, for which we used CETONI Elements so�ware to automate the above men�oned procedure. PLL and PSS solu�ons were prepared (0.5 g L -1 in water) and transferred into glass syringes and mounted on low-pressure pumps. For details, see Supplementary Informa�on, S5.