What is the problem with the SEM image? Few years ago I made Ag NPs in the dendrimer polyacrylamide matrices and the SEM was perfect because of very different transparency of polymer and metal nanoparticles....

I agree with what Mykola said. The polymer should be mostly transparent in the SEM, based on my work with polymer coated Au NPs.

What size are your NPs? Can you image them without a problem if they are not coated with PEI?

If you want to remove the PEI, you can dry the NPs on a substrate and then use an oxygen plasma to selectively remove the PEI. The oxygen plasma will etch away the organic material and will leave the metal unaffected.

I hope that helps!

There are several method to prove the encapsulation of your nanoparticles. You can use TEM instead of SEM. Also you can use FTIR analyses to see the dendrimer bond on the surface of you methal nanoparticles. Another methods can be TGA and XPS analyses by which you even prove the presence and the W/W amount of dendrimer and metal nanoparticles. The size and surface charge changes (DLS and Zeta potential) also can be other methods to prove your encapsulation. For the separation of dendrimer encapsulated NPs you may use centrifugation or filteration depending on your core metal nanoparticles and dendrimer type.

Hope that this will be helpful

I never found any difficulty in SEM analysis with PEI coated Au nanoparticles. Calcination is  one way for the removal of PEI and it is already known factor that these particles are stable at this higher temperature