Dry nanoparticle samples can be stored for as long as u want at room temperature. Dark storage conditions are preferable. Prepare aqueous solution when doing any assay or characterization.

The best way to keep the green synthesized nanoparticles solution at 4 C

Dried nanoparticles are no longer nanoparticles. Anyway keep the suspension always cold. As for how long that is something you need to determine experimentally. If they absorb light you can use that to see changes in the suspension.

Dear Sir Omar,

How are dried nanoparticles "no longer" nanoparticles? Would you like to elaborate a little?

Once you remove water from the system, van der Waals forces will kick in between the nanoparticles. Good luck trying to redisperse them.

Yes, no doubt redispersion is an issue, but my point was, they still remain nanoparticles even after drying. Most of the technicians operating XRD, SEM, TEM and FT-IR in our region runs dry samples only, be it the nanoparticles or any other substance for which nanoparticle drying is imperative.