I want to measure A647 fluorescence in sera, but I won't have the time to do it right away. Is that possible to di it later after freezing the samples ?
You have not explained the details, but it seems that you use labeling with 647 nm fluorophore like AlexaFluor 647. Such conjugates are quite stable. So after freezing you can keep your samples, perhaps for weeks, and measure the fluorescence later (I don’t say about biological activity). In principle, the chromophores of most common fluorescent dyes will be sufficiently stable in frozen aqueous medium in the dark.
So basically, I am doing gavage of mice with A647 coupled molecules and trying to assay which ones are reaching the blood circulation, by measuring A647 in sera.