CF® Dye Maleimides are thiol-reactive fluorescent dyes. Maleimides are commonly used to label proteins on free thiol groups. CF® dyes are Biotium's line of next-generation fluorescent dyes with advantages in brightness, photostability, and conjugate specificity compared to other fluorescent dyes. UV-excitable blue fluorescent CF®350 dye has excitation/emission at 347/448 nm.