Chitosan is on the surface of your nanoparticles. It's basically a broth that bacteria love. Therefore, you need to add an antimicrobial drug to store the broth.
Drying, in general, should not be applied to a nanoparticle suspension. Water or the "solvent" is part of the nanosystem as a suspension. Drying will destroy the nanoparticles as single entities.
I don't think that these particle have any stability issue. However, if there is any, the storage below the -20 degree centigrade after the drying is preferable for long term applications and research usage.