You should strictly disperse them under conditions expected for further use, e.g. it does not make sense to disperse them in a special solvent if its application is in water buffered to a fixed pH value and defined electrolyte content.

I tried distilled water but the issue is that particles don't remain suspended. Particles just settle at the bottom.  How to keep them suspended?

your experiments shows that in water (as one could know from their polarity) interaction between particles is much stronger that towards the water. Therefore for intended use in aqueous media they should have been modified during synthesis or addition of dispersants should be allowed for their application. Potentially you may try ionic or nonionic surfactants or hydrophilic polymers like polyethylene glycol.

Its indeed a useful answer. All I want to know that what can I do at this stage when I have prepared my formulation and its not getting dispersed in water. Suggest me please.

I have read articles in which they disperse ethyl cellulose nanoparticles in water. I am not getting why I am having this issue. 

must be modified particles, addition of dispersants or low degree of substitution. Alternatively you may obtain temporary dispersion (only) in case of high energy input.

Thank you. How to prepare temporary dispersion? Please tell me the name of dispersant which according to you will be suitable for Ethyl cellulose and Hpmc particles. 

Temporary dispersion means that after or even during dispersion, e.g. by sonication particles get reagglomerated.  Time scale also depends on dilution - the higher particle concentration the faster reagglomeration. Principle answer to possible dispersants see my answer of yesterday. To name few examples of popular dispersants - Triton X100, LAS. You may find more effective ones and you will have to tune the optimum concentration.

Thank you. Your answer is very helpful.