Asymmetric cyanine dyes of Formula I bind nucleic acid but not red blood cell membrane, and function as photosensitizers when rigidly bound but not when free in solution. Unbound dye thus causes minimal oxidative damage. The dyes do not substantially accumulate in red blood cells, thereby minimizing hemolysis due to oxidative damage. Biological fluids can be disinfected by mixing the fluid with these asymmetric cyanine dye that binds to nucleic acid, irradiating the mixture, and recovering clinically significant components from the biological fluid and/or assaying the fluid for pathogens.