Due to using probe sonicator for re-dispersing silica nanoparticles, I am getting black depositions at the bottom of the falcon tube. Can anyone suggest me any method to re-disperse silica nanoparticles?
It depends on solvent and on temperature.
If at a given solvent (e.g. DMF, NMP) the storage temperature drops below 15C or higher 45C - the agglomeration of silica nanoparticles becomes irreversible - from MSDS for silicasoles.
Regards, Leonid
In my note (see above) I referred to Materials Data Safety Sheet (MSDS) where very thoroughly is described the process of how to recover/revive the silica colloid.
As mentioned above - depending on temperature of storage - there are situations when the silicasol can NOT be revived. In other cases - to reach DE-agglomeration MSDS - recommends mixing by very gentle rocking of the vessel/container. And, yes !
the latter does work.
That is why (again referring to MSDS) - an intensive sonication - may bring negative results.
Leonid
You can try to use vortex. With silica you should get not get any black deposits. This black ppt must be something else. Try ethanol and gentle sonication or vortex.
Good luck
@ Ahmed A. Farghaly- Sir, I already tried to re-disperse silica nanoparticles using vortex but vortexing could not re-disperse silica. I tried with ethanol also.
Making disperse system of colloidal silica is related to its stability and in case of hydrophilic-type sol it depends on water film on silica particle mainly, so that if you use ethanol you'll remove this protective layer and silica tend to form gel-state. So that may you use simply water and ultrasonics?
If silica sols are dried or gelled, they become irreversibly aggregated. Adding strong alkali could dissolve the silica into waterglass but will not give you the colloidal silica product back in its original state. Advice; go for a new sample.
Along with the valuable advice in the previous answers I would also suggest resuspension in water at slightly higher pH 8-9. The would add additional stability to your silica colloid if it gets redispersed. Ultrasonication should resuspend the particles if the aggregation is reversible.
An alternative to using a probe sonicator could be to use an ultrasonic bath. This means you can avoid 'contaminating' your NPs with the probe.
However, from my experience, the use of an ultrasonic bath for silica NP redispersion can be user dependent. I have found that I have to move my tube around in the water until I find a 'hot spot' where the NPs become vigorously redispersed. The tube can even begin to vibrate in my hand if the 'hot spot' is strong. If the tube is not located in the 'hot spot' then redispersion is poor. I have notice that people often leave a tube of silica NPs in the an ultrasonic bath for 10 minutes and walk away assuming redispersion will be fine, but when they remeasure the size of their NPs they remain poorly redispersed.
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