I use to coat the nanoparticles with organic tethers (citrate) or surfactants (CTAB) during the synthesis of the nanoparticles. You are basically making the surface of nanoparticle more polar (disperse better in polar solvent, water).

I use to coat the nanoparticles with organic tethers (citrate) or surfactants (CTAB) during the synthesis of the nanoparticles. You are basically making the surface of nanoparticle more polar (disperse better in polar solvent, water).

I have tried the method that Saravanan mentioned above (making the surface of nanoparticles more polar), and my particles become more dispersible in DI water, comparing to in other non-polar or weak-polar solvents like acetone.

When mentioning coating, is it by dipping or self-assembly method? If not, whatelse methods.

Self assembly. Also it depends on what nanostructure and what coating molecule you use. I used to mix the gold nanoparticles and the desired organic compound (that has polar groups like ether chain, and a functional group that can bond with gold nanoparticle surface) overnight in ethanol water solution.

You can also try coating the nanostructue with SiO2, which will be a polar shell to your nanostructure core. You can also control the thickness of the SiO2 shell.

As Ahmed has written, the classic methods involve steric protection wilth coatings like PEG and/or electrostatic stabilization with charged species such as citrate or other water-soluble charged species. You can either do this directly or in some type of exchange process.