Dear Zohreh,

I confess to no working experience with MCM-41 mesoporous silica.  

That said, if you need it dispersed into water, would you be willing to first contact the powder with some isopropanol and then add water to that wet paste?  This might help wet out some pores that currently have air in them.  The resultant isopropanol level could be kept quite low to your water level and might not impede future process steps you plan to take with this material.

Otherwise you could pre-contact with a small amount of surfactant or low molecular weight polycarboxylate polymer at high pH, if your targeted application can tolerate their presence.

You could attempt ultrasonication for a few moments after either of the above conditions unless you fear the material is quite friable.

Best regards,

Mike Bender

yes, The poor solubility of the particles may be due to hydrophobic surface.

I agree with Dr. Mike blender, you add some surfcant with MCM-41, Then go fo vigorous magnetic stirring and ultasonic cavitation for 40-45 minute for better dispersion in water.

All the best

Did you dried the nanoparticles during the synthesis process? If so, you may have formed aggregates and you will not be able to redisperse them in water. I suggest you to keep them all the time in EtOH and transfer them into water once you need them in this medium. Also, silica nps (I don't know for those with gold core) are not very stable over time in water and will aggregate too.

For silica surface, water solubility is determined mainly by  surface charge arising from ionised silanol groups on the surface. Owing to the relatively heavy gold core inside the silica shell, you need more surface charge to disperse them in water compared to there mesoporous counterparts. Here come the important note, how did you remove the CTAB template? If you used EtOH extraction, it would give the decent amount of silanol groups and must be dispersible in water with good ultrasonication, however, if you used thermal treatments you remove CTAB, if would cost you the surface silanol, thus render poor solubility. In that case, you need to passivate the surface with a mild base or with water-soluble functional groups such as propylamine groups etc.