The fact that the suspension was milky (and faintly yellow) - scattering light in all directions - tells you that the particles in it were not < 100 nm but likely in the 0.3 - 5 um (yes, microns) or so range which you filtered out completely with your 0.22 um filter  A colloidal suspension of 5 - 13 nm particles would be clear, transparent and indistinguishable from water.  Your powder is a collection of fused sub-micron and post-micron particles. There are no free, unattached, independent particles of 5 - 13 nm in a powder.  See for example:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507

http://tinyurl.com/hpywsge

ad the earlier webinar references given on slide 3 of the above. Also research a paper of mine called 'Micron sized nano-materials' Powder Technology 174(1):6 - 9 · April 2007  DOI: 10.1016/j.powtec.2006.10.012 

Even dispersing for 1 hour at 600 W won't produce nanosized material.   In order to produce material in the < 50 nm range, you need to start with a bottom up process and keep the material in colloidal suspension - never dry it...

#Adam - the clusters are fused aggregates not agglomerates (the latter can be dispersed by sonication but the former cannot) - see the quote by Rucker on slide 59...

Here is what I think may have happened. You measured the particle size by dynamic light scattering using a highly diluted sample of the suspension, out of necessity. In that dilute solution, the particles were all individual. In the concentrated stock solution, however, the particles self-associated into larger agglomerates, resulting in a turbid solution. When you passed this stock solution through a 0.22 µm filter, the agglomerates were filtered out. The filtrate may contain a much lower concentration of particles, now dilute enough to remain separate, or none at all. This can easily be tested by dynamic light scattering.

Dear Adam,

I just sonicated the particle at 50watt strength for 1hour then after filtering they became clear solution is it still because of the agglomeration?

The fact that the suspension was milky (and faintly yellow) - scattering light in all directions - tells you that the particles in it were not < 100 nm but likely in the 0.3 - 5 um (yes, microns) or so range which you filtered out completely with your 0.22 um filter  A colloidal suspension of 5 - 13 nm particles would be clear, transparent and indistinguishable from water.  Your powder is a collection of fused sub-micron and post-micron particles. There are no free, unattached, independent particles of 5 - 13 nm in a powder.  See for example:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507

http://tinyurl.com/hpywsge

ad the earlier webinar references given on slide 3 of the above. Also research a paper of mine called 'Micron sized nano-materials' Powder Technology 174(1):6 - 9 · April 2007  DOI: 10.1016/j.powtec.2006.10.012 

Even dispersing for 1 hour at 600 W won't produce nanosized material.   In order to produce material in the < 50 nm range, you need to start with a bottom up process and keep the material in colloidal suspension - never dry it...

#Adam - the clusters are fused aggregates not agglomerates (the latter can be dispersed by sonication but the former cannot) - see the quote by Rucker on slide 59...

It seems that filter sterilization is not going to be an option with these particles. Would it be possible for you to prepare them under sterile conditions?

what was tha PDI value and Zetapotential of particle? What was the z average size before filtration by .22? May be me give u solution!

Of course best to check particle size distribution before and after filtration. (result of measurement before filtration may lead you to not use 0.22 nm filter).

Besides the obvious things Alan mentioned, part of the story is also diameter of your tubes leading to less turbid appearance and that by filtration you will always also remove particles smaller the nominal cut value. Further, there might be also particle adsorption to the filter.

Dear All,

Thank you very much for your valuable inputs and I found out that these particles are not individual they form a network so they cannot be filtered so while preparing them I must use the sterile water. 

You must use Di-water or mili q water for preparation of your nanoformulations for  stay away all the ions and salts, because ions and salts enhance the aggregation and agglomeration  of our materials. Other thing, try to find out the PDI value by DLS if the value is less than 0.5/0.4 then the particles are in mono disperse (means most of the particles are in same size. Try to use all the vessels very clean, wash them properly with Di-water. Also find out the zeta-potential, if zeta-potential has high value above 20-25mV then your particles remains stable. What I feel, 0.22 micron filtration not allow the bigger particles to pass and your filtrate become transparent. It means your formulation is not mono disperse, it is poly-disperse. In other words they may have high PDI value.

@ Vinod I have to disagree with your statement.  Generally speaking DI water is a very poor dispersant liquid to stabilize nanomaterials. It washes out surfactants and stabilizers that are used to enhance the particle-particle repulsions - for example, phosphates for inorganic oxide stabilization.  However, for any ionic additive there is an optimum amount above which salting out will occur as the electrical double layer is destroyed.  This optimum concentration can be found empirically or scientifically (e.g. zeta potential - additive titration).  That des not mean, of course, that river or tap water can be used to prepare such materials - yes, all solutions need to be clean and DI water is the best starting medium - but with controlled additives/stabilizers and at a known pH that optimizes the zeta potential for stability.

Particles tend to mondispersity ('mondisperse' is an oxymoron)  with a PDI < 0.05.  PDI's of 0.5 and above are pretty broad distributions and probably settling - laser diffraction rather than DLS may be a better option in these cases.   A standard latex material (prepared by emulsion polymerization) may have a PDI of 0.02 or better.

20 - 20 mV is almost certainly not stable for nanomaterials (i.e. 100% < 100 nm).  A zeta potential of 30 mV or so does not confer stability on 300 nm TiO2 particles - it needs to be below - 60 to 65 mV or so.  For materials in the 10 nm region one would be aiming for an order in the region of100 mV or better.  Obviously one route to modify zeta potential is with pH change.  I believe Preethi has realized that his material is a collection of sub and post-micron fused aggregates which are completely filtered out by the 0.22 um filter. You'll find such pictures and comments in the webinar (plus the references within) I highlighted above.

Dear Vinoth and Alan,

It is being prepared in milli q. I have no idea about the zeta potential and stuffs as I am not from that background and the nanoparticle is provided by our collaborative lab may be they can answer your questions as they know physics and material science. I have shared the link with them let us wait for their response. Thank You

thanks Alan for some novel informations.hope Preethi may get solution for next time

to remind, zeta potential may or may not play a role depending on whether stabilization is due electrostatics or not.

Yes, Titus is correct.  Steric stabilization is another mechanism to be considered.

I agree Alan With your lines.Its new information for me.I must try in my lab with out Di water for nano formulation.Lets see some interesting things!

fused bigger aggregates may have filtered out and solvent in the falcon tube may contain very smaller size particles in a very low concentration.