hi,

to the best of my knowledge,if MNPs are prepared by copercipitation method you should filter or centrifuge them bare mnps are  so unstable than the coated ones for having the efficient dispersal mnps try PEG or PVA as coated materials. good luck

Thanks a lot Dr. Kamyar for your help.

I tried to coat it with chitosan but still magnetic nanoparticles settle down and not well dispersed

You should try another method for preparation e.g: hydrothermal method or CVD. but if you insist to use co-precipitation method first sonicate your solution then filter supernatant to gain stable particle. 

I actually synthesize my particles using hydrothermal synthesis method.

There are two possible reasons for what is generally called instability or insufficient dispersal: flocculation or sedimentation. Sedimentation may be followed by flocculation but usually when shaken the suspension becomes temporarily stable.

It seems you are having problems with sedimentation. In that case your particles are too heavy: buoyancy is insufficient. It requires more details on your particles to come up with a further advice: what is the mass of the particles and what polymers can you use. Also, size polydispersity can play a role: more bigger particles.

Perhaps attached papers may help you. You may slightly modified the method as per your requirement but essentially it will be similar.

Another paper for the stable dispersion.

Thanks a lot Dr Rasbindu I will definitely read them.