When you dissolve them they them become solution and not MONP's.... You also have to specify what metal oxide(s).

As Alan mentioned, one must distinguish between the solvent and the solution. Also, do you want really to prepare a solution or a dispersion?

Please refer to:

https://en.m.wikipedia.org/wiki/Solvent

https://en.m.wikipedia.org/wiki/Solution

https://en.m.wikipedia.org/wiki/Dispersion_(chemistry)

It definitely depends on the type of you nanoparticles. I assume they are iron oxide because they are the most common nanoparticles in bio and other applications. Check this:

https://www.springer.com/us/book/9781848826700

Here, you can find a thread for it:

https://www.researchgate.net/post/Can_any_one_suggest_any_proper_solvent_for_dissolving_iron_oxide_nanoparticles

Thank you so much for valuable answer.

First try to understand the preparative methods, conditions, and about the nature of the surface properties of MONPs. Concentrate on the stabilization conditions of the MONPs. If the MONPs surface contains huge hydrophilic groups, they must be easily dispersible in polar solvents. Whereas the surface of the MONPs is strongly passivated with highly hydrophobic matters, they would be dispersible in hydrophobic solvents. Best of luck.