Fig. 4.

Simultaneous filtration of two low molecular mass fluorophores with positive and negative charges through ITE SiN membranes. The membranes were obtained by etching SiN films irradiated Bi ions for 40 min. (A) Fluorescent image of passage of the Alexa dye through a negatively charged SiN membrane. (Left) Initial frame taken immediately after introduction of the feeding solution on top of the membrane; (Right) image was acquired 4 min later. See animations in Movies S1–S5. It is clear that the initial and the final frames are almost identical: the membrane rejects the negative dye very efficiently. (B) Passage of rhodamine through a negatively charged SiN membrane. Since this dye is positive, it diffuses freely through the membrane thus forming a bright final frame. (C) Kinetics of the passage of the Alexa (red ♦) and rhodamine (■) dyes. The fluorescence intensity was measured from the level at the position 50 μm away from the membrane edge (see dashed line in D), and normalized to the intensity in the first frame measured in the middle of the membrane. (D) Cross-section of the final frames taken after 4 min shown on A and B; note that the fluorescence intensity of the Alexa dye (red line) is very low compared with the rhodamine trace (black line).