Optical microscope images of U937 cells. (a) Untreated U937 cells. The cells tend to be asymmetrical with a dark center. (b) U937 cells treated with FT after 72 hours. Notice the pronounced swelling and appearance of vacuoles (white dots in the middle). (c) U937 cells treated with 200 nm diameter SM-PNs after 72 hours of incubation. Nanowires are visible in the background with the presence of cellular debris. (d) U937 cells treated with 35 nm diameter SM-PNs after 72 hours of incubation. The nanowires are too small to be viewed at this magnification. Cellular debris from burst cells appear as specs in the background. Scale bar = 10 m.