sure it is sensitive, but you need a very good modern SEM (I have recently used an SEM which was capable of 250k magnification, and we saw "everything" I wanted to know about my nanoparticles).

spin coat the sample on a substrate and dry them then take SEM analysis

If you don't have a spin coater, you could just place a droplet on a sample holder stub, then dry the solvent away from the suspension in a drybox. Depending on the concentration of the colloids in the suspension, you would want to repeat this process multiple times in order to ensure a decent coverage of the particles on the sample stub. If the particles are insulating, remember to coat the samples with C or Au. Hope that helps.

directly put one drop on the stub and dry the solvent

I can use at first ultrasound to prevent agglomeration of nanoparticles. Use holey films or other carbon film for electron microscopy , mount it on a stub using carbon tape, put one drop on the holey film and dry the solvent.

you could sonicate the emulsion to prevent agglomeration and directly put the emulsion droplet on the SEM sample holder. Air dry the solvent, sputter it and subject it to SEM analysis.

I would freeze-dry the colloidal nanoparticle dispersion (lyophilisation), you will find some surprise!

Why you are looking for nanoparticles with SEM? I think this technique is not so sensitive for NPs detection..

sure it is sensitive, but you need a very good modern SEM (I have recently used an SEM which was capable of 250k magnification, and we saw "everything" I wanted to know about my nanoparticles).

I would drop cast or spin coat on a very flat substrate. Doing so directly on an SEM stub probably has too many crevices and is not flat on the nm scale. So......... Take a small piece of a Si wafer, plasma clean, spin coat or drop-cast the NP suspension, evap solvent, sputter coat, SEM. Another substrate that may be used in place of the Si wafer fragment is a Cu grid used for TEM analysis.

 @Mr. Bernhard

I did your method, I left my samples in the freezer even for many days but nothing happens it remains liquid.

@Dr. R. K. Singh, I did your method too. I am finding a lot of crap along with my flakes unfortunately it doesn't work at all.

@Mr. Bhaskaran.. I did your method but all the flakes get aggregated around the periphery of silicon substrate.

So, please let me know if any one have  modified or some new approach.

By the way I have chemically exfoliated Graphene and Phosphorene having a size not less then 1micron.

Thanks.

@Mohammad Wasil Malik - sorry, I was laughing when I read your answer, I am very sorry :-) But I suggested to "freeze-dry", not to put it in refrigerator!

You need to search for some laboratory which has a freeze-drying apparatus, freeze-drying is done at low temperature under vacuum.

Ohh really...??

I understand just put into freezer..hahah

you should laugh..

but I think i will not get the rid of crap? or I should get?

first, send your sample to the freeze-drying lab, then send us the SEMs which you get, then we discuss; not now with all the concerns and no facts

It is not important to collect the particle on the side of the wafer, in high magnification it is not evident which part of the wafer is seen.

I also used a lot of sem by wafer- based method, it is working

Usually when you prepare the samples, it is usual to collect on the sides.