[jeffmoonchop]

Has anyone had any experience in extracting particles from a solid colloidal suspension? I have produced the particles of different organic acids such as citric acid, adipid acid, oxalic acid etc which are suspended in 50:50 water:ethanol. The particle size varies between 300-700nm. I have tried to use filtration using regular filter paper by vacuum and gravity but I get nothing. Ive also tried centrifuge but the particles just redissolve. The life of the particles is not very long due to them redissolving due to the solvent system I have to use. Which means when they are first produced I need to quickly extract them for analysis.

any help would be appreciated. Cheers!

[P]

I have never isolated nano particles so I am guessing...  but these are my first thoughts:-

Can the solvents be rotavaped off? 
Will reduced temperature stop the re-dissolution? (use an ice bath or dry ice in IPA). 
Can they be precitpitated into a lesser solvent without them coalescing onto larger particles? (very small drop wise addition into a constantly stirred liquid that will crash it out of solution).

Hope this helps.. probably wont, but you never know. :-) 

[jeffmoonchop]

thanks for the suggestions. I never thought of rotavaporation however the increased temperature required for this would most likely dissolve the particles before the solvent has chance to decrease in volume (and water takes ages to evap). Once the particles are gone I have to use microfluidics to synthesise them again.

Decreasing the temperature would help, however this would be temporary as id need to remove it from a bath to then try isolate them. But definitely would help in delaying dissolution.

The particles are themselves already a precipitant and remain a stable size for a couple of hours with minimal coalescing. Not sure what you mean about crashing them out because that would involve the particle size changing I would ideally like to keep them monodisperse. Maybe you mean adding more antisolvent? this would increase the supersaturation of the remaining solution and so increase the particle size.

Really im just after a fine dry powder. Are there any special filter papers I could use for trapping small particles?

Grateful for the suggestions would love to hear more.

[P]

The rotavap uses reduced pressure so it should strip the solvent/water off pretty quickly.

You can try doubling or trebling up on the filter paper, but if they are of nano scale dimentions they still might go through. Get the finest grade. :-)

[Arkcon]

If you want nano particles, then you can't use standard filters.  Standard filter membranes for removing bacteria from media or fine particles from HPLC eluents come in 0.45 or 0.2 micron disposable nylon filters.  HPLC solvents are usually filtered through 0.1 micron filters, but such filters are made from precision prepared steel, so they withstand the pressure needed to overcome surface tension.  But those are 100 x the size of nanoparticles, by definition. 

This topic has come up before, and I hadn't heard before or since, of the existence of sub nano filters for nanoparticle filtration.  What would they be made of?  How much pressure would you need to drive the liquid through?  The technology to make micron holes in membranes uses controlled gamma irradiation and chemical etching, or solvent extraction of a specifically prepared co-polymer.  Or for the 0.1 micron steel filters, controlled laser ablation.  I don't know how sub nano holes will be made reliably.

[jeffmoonchop]

Thanks again. I was pretty sure filter papers would be inadequate so thanks for clarifying.

[P]

So - I guess that brings us back to finding a way to drive off the solvent using reduced pressure and heat then?

[mjc123]

Yes, but 300nm is 0.3µm, so the finest filters ought to stop them. I remember having to filter suspensions of ca. 500nm particles using ordinary filter paper (we wanted the particles to get through, but take out larger impurities). The particles clogged the holes and a lot of them were left in the filter paper (and the filtration took ages!). That was with a couple of litres of quite concentrated suspension. It might be worth trying with a very fine filter, if you have enough material to do some experiments with, and are prepared to accept <100% recovery.

[jeffmoonchop]

I've just had a bit of a look online and ultrafiltration looks like it might be quite nice.

https://www.sartorius.co.uk/en/product-family/product-family-detail/m-vivacell-70/VS6011/51268/?no_cache=1&cHash=d3768a0342fb678bd8e6312c80cbb9a6

Adding a filter to centrifugation seems like a great idea.

[Arkcon]

In your case, mjc123:, what you may have wanted was diafiltration, a combination of dialysis and filtration.  For example, its known that large biological macro-molecules will form a gel layer on a filter that prevents further filtration, but my maintaining a laminar flow and repeatedly re-filtering, we can remove ions and concentrate macro-molecules.

Now that I think about, this may be what the O.P. wants: to partition the particles into greater than nano-sized, nano-sized and soluble molecules and ions, so they can reject the first and third fractions and concentrate the middle fraction.  Maybe diafiltration, or simple dialysis is what's needed.

[jeffmoonchop]

thanks for the suggestions I've got a few things I can look into. Cheers

[MOTOBALL]

Freeze-drying, aka Lyophilization

Regards,

Motoball

Edit: sorry, that will not separate the nanoparticles.

[flick-a-bic]

Take you NP solution and 'freeze' it in a centrifuge tube by holding it in liquid nitrogen for an extended period of time (your judgement) then centrifuge as fast as your machine can go. Centrifuge a short period of time. Then get the bottom/side of the tube where the precipitate is present into the LN2 as quickly as you can and actually freeze it.

The cold will help them crash out, then keeping just the precipitate cold will allow you to pour of the remaining solvent.