Dear Eduardo,

Generally, milling, homogenization or sonication are the methods used to reduce a material particles size from micro to nano. The approach you have used (sonication) is a good one, however, the efficiency of the process and the quality of the solid particles are largely dependent on the the number of sonication's cycles, sonication time and temperature of the medium placed in the sonicator.

Attached is a file containing a study in which sonication was used to reduce particles size of a drug from micro to nano.

Hoping this will be helpful,

Rafik

Dear Rafik,

Thanks a lot for your answer and for the attached document. I will read it and proceed with the most suitable experiment!!.

By the way, this is the way it looks the "colloidal" suspension...which is more likely a precipitate. Still wondering why after the reaction I got this type of material

It seems that a coagulation took place during the process. The triethoxysilyl groups may form extensive interparticular networks.

If that is the case, try to use dimethylethoxysilane groups, if possible, instead of triethoxysilane groups to avoid this problem.

 dear Atef

thanks a lot for your response. What is surprising is that I am repeating almost the same procedure described in my group. However, in my case I purified the crude of the reaction which is leading to the triethoxysilane graftable moiety. So basically I guess that my material is much more pure than the reported one (in my group), therefore  the concentration needed is much less... could it be the reason?

If you want smaller particles at the end, why not just start with smaller particles and coat those using the same procedure? Getting below 100 nm for existing particles can be tricky. Sometimes the energy put in for grinding or milling will cause the particle size to revert back into the micron region.

Dear Christopher

Thank you for your advice, however I have doubts about decreasing the size of my starting nanoparticles. I reached between 80-100 nm size Silicon particles...sonicating for almost 24 hours...I have a problem of overheating during the sonication process, so I have to change the water every 3 hours. In order to avoid oxidation of the particles...

I meant purchase commercial silica nanoparticles that are much less than 100 nm diameter.

For example:

http://www.admatechs.co.jp/en/product-admanano.html