You will need to put your sample on a conducting substrate (e.g. carbon tape, directly on the aluminum stub, silicon wafer, etc) otherwise it will charge up under the SEM beam. If you put it onto a glass slide, this will form a barrier to conduction, unless you coat the sample. If there is not too much organic coating, you probably won't need to sputter coat the samples to avoid charging if you use a conductive substrate. You should probably try to clean them too, maybe using rinsing and centrifugation before depositing them onto a substrate for imaging, or the excess reactants may still be present. Your silver particles may be too small to see well in SEM, so you may want to consider TEM instead. TEM could also be used to produce electron diffraction data, that may be helpful in characterising the crystallinity of your Ag nanoparticles.

I would deposit onto either a Si wafer for FEG-SEM or onto a carbon film TEM grid for STEM or TEM. I would recommend cleaning with UV or ozone. This should strip away the organic residue. Do not coat with gold, the grain size is too large and will obscure your sample. If you are using Si wafer then use a very low accelerating voltage, if you have it, use deceleration or gentle beam, or whatever your SEM manufacturer calls it. 420nm is quite big for a FEG-SEM, so you should have no problem seeing these. As for keeping it overnight, this should be ok if stored under vacuum, then ozone clean before observation. You will get higher resolution with STEM or TEM but again if you are at 420nm it should not be an issue.

If you can wash out your organic components (as suggested by Johanna) and place your particles on conductive substrate, then coating is not needed. Your particles are rather big, so coating will not significantly alter your images anyway. I see no problems for SEM observation.