What are practical bulk filtration methods for suspended nanoparticles under 100nm, preferably under 20 nm? By bulk, i mean filtering more than 1 ml at a time. Glass filters I have seen are mainly applicable in the micron range. Centrifugal separation and extraction of the top surface for further refining is also something I saw, but I'm not sure how to further filter that. There were also some experimental methods involving electroosmotic flow.The nanoparticles in question are Silica synthesized by variations of the Stober method in ethanol. Thanks for the help!
Syringe filters are available in 100nm and 200nm pore sizes from a range of manufacturers.
For thorough filtering of solvents and very small nanoparticles, 20nm filters are available, and Whatman Anotop filters are commonly used.
http://www.sigmaaldrich.com/catalog/product/aldrich/z747637?lang=en®ion=GB
http://www.sigmaaldrich.com/catalog/product/aldrich/z747777?lang=en®ion=GB&cm_sp=Insite-_-prodRecCold_xorders-_-prodRecCold2-1
These come in two sizes, 10mm and 25 mm diameter, which can filter up to 10 ml or 100 ml respectively.
When using syringe filters, it is important to remember to discard the first few drops of solvent that passes through the liquid as this initial volume will was away material left from the manufacturing process of the filter.
Besides the already mentioned syringe filters, ultrafiltration (http://www.merckmillipore.com/DE/en/product/MF-Millipore%E2%84%A2-Membrane-Filters,MM_NF-C152#specifications) or dialysis are established methodes for purification of nanoparticles. Both are rather expensive, especially the initial costs for the ultrafiltration setup are quiet high.
Centrifugation with repeated exchange of the supernatant is a very quick and reliable way, the only problem might be aggregation of the particles.
Including above, dialysis membranes with various pore sizes are also available with molecular weight cutoff , in order to filter nanostructures.
instead of filtering you may try size selective precipitation
https://www.researchgate.net/publication/272436214_Classification_of_Zinc_Sulfide_Quantum_Dots_by_Size_Insights_to_the_Particle_Surface-Solvent_Interaction_of_Colloids
Article Classification of Zinc Sulfide Quantum Dots by Size: Insight...
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