Ferrofluids are magnetic nanoparticles, usually iron oxide, with small molecules called surfactants stuck to their surface. These stop the nanoparticles aggregating. Eventually the surfactants themselves will come off, causing aggregation. Lumps of iron oxide precipitate out of the liquid and sink to the bottom. No doubt better surfactants will be developed that take a very long time to degrade.
Could you confirm or deny that you are using the word "nanoparticles" carefully?
I think that the ferrous particles in ferrofluids are mere particles. They'd have to be very, very small to be classified as nano, and we've seen before that making particles of an ordinary material that small can change its properties, ala "transparent titanium dioxide".
"Ferrofluids are composed of nanoscale particles (diameter usually 10 nanometers or less) of magnetite, hematite or some other compound containing iron." - Wikipedia
