thanks a lot for your reply. Cy5 and Alexa647 are indeed very prone to be bleached. But I am not sure how fast it is? Ideally, I want something could go away (let's say 90%) in 1 min. Do you think you can bleach them by strong light in 1 min? or is there another way to shut down it, like oxidation? I know there are some photoswitchable dyes, which could be useful for my expt. But they are mostly reversible, which might cause problem later on. Thanks again.