Fig. 3. DNA hybridization chain reaction (HCR) on the surface of zebrafish embryos. (a) a′, b′, and c′ are regions that are complementary to regions a, b, and c, respectively. Hairpin M1 can be unfolded by hybridization with initiator U4T28, resulting in growing DNA strands. (b) Addition of 6-FAM fluorescently labeled M1 (M1-FAM) and Cy3 fluorescently labeled M2 (M2-Cy3) to the U4T28 pre-treated zebrafish results in DNA HCR and concomitant increase in fluorescence. (c) Fluorescence images of 1 dpf zebrafish embryos after incubation with U4T28 for 1 h and subsequent exposure to 1 μM M1-FAM and M2-Cy3 for 1 h. Green channel: 6-FAM; red channel: Cy3. (d) Normalized fluorescence intensity of attached DNA on the surface of zebrafish embryos. Fluorescence intensities of images (c) and ESI Fig. S3a† were calculated by Image J and plotted as a percentage relative to the fluorescence of M1-FAM or M2-Cy3 of Fig. 3c. The intensities of Fig. 3c were set to 100%.