I have gone through several literature where all of them prepare powder from nano particle solution after preparation of nanoparticles solution using plant extract. If we think about the application of nanoparticles, Why we create powder rather using the liquid solution (Aqueous solution can be considered) ?
Md. Shoebul Islam While powders are convenient for transport and storage there are no discrete, independent, separate particles < 100 nm in a powder. This is due to solid-solid diffusion at the nano scale. To retain the nano nature (100% < 100 nm) of the particles then they must be preserved in colloidal dispersion. Try measuring the specific surface area of the powder by BET. It should be greater than 60 m2/cm3 for the particles to be considered less than 100 nm. For further information view this webinar (free registration required):
Dispersion and nanotechnology
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat
Also see plenty of discussions on Research Gate on this topic.
2 quotes from those much greater than I:
'I think dry nanotechnology is probably a dead-end' Rudy Rucker Transhumanity Magazine (August 2002)
‘If the particles are agglomerated and sub-micron it may be impossible to adequately disperse the particle… ‘The energy barrier to redispersion is greater if the particles have been dried. Therefore, the primary particles must remain dispersed in water...’ J H Adair, E. Suvaci, J Sindel, “Surface and Colloid Chemistry” Encyclopedia of materials: Science and Technology pp 8996 - 9006 Elsevier Science Ltd. 2001 ISBN 0-08-0431526
See also a recent question:
https://www.researchgate.net/post/Which_solvents_could_contribute_to_a_realistic_measurement_of_the_hydrodynamic_diameter_via_dynamic_light_scattering_DLS
As it was mentioned by Mr. Rawle, storage and logistics with powder more convinient and cheaper. Also, some nanoparticles should be functionalized, or take part in functionalization. Water media for some of these processes is not compatible. Moreover, not every organic media would be fine solution.
Has mentioned by @Alan F Rawle, powder form may increase the storage efficiency, we need to also consider the fact that keeping the nanoparticles in the aqueous form for a long time may also alter the size of the particles especially in the presence of the reductant.
Thank you all for your nice explanation. Is there any reference or have you all any idea about the shelf life of nanoparticles in aqueous solution?
Alan F Rawle Saheed Ibraheem Egor Ryabhenko
Md. Shoebul Islam In colloidal suspension then nanoparticles can be virtually stable indefinitely. A good example is the gold colloids made by Michael Faraday in the 1850’s which were perfectly stable until World War 2 when the building they were housed in was destroyed by German bombs.
Alan F Rawle Exactly you are. But do you think if a nanoparticle is prepared using plant extract and the solvent is water (and prepare silver nanoparticle using silver nitrate); Then how many days it will remain as colloid or in a nano nature.
Md. Shoebul Islam There is no reason why correctly stabilized Ag nanoparticles can be indefinitely stored in aqueous environment. One important thing to note is that the oxidation state of such particles on the surface is +1. It's the Ag+ ion that's the bactericide, not Ag metal. I assume you have access to a surface sensitive technique such as ESCA/XPS or Auger. See the following webinar (free registration required):
Silver colloids and invisible ink
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150902SilverColloidInvisibleInk.html
One way to sterically stabilize such systems is to absorb AgNO3 (usually in the ammoniacal form) onto an 'inactive' substrate such as silica, alumina etc and then carry out the reduction. In that way stable 'islands' of Ag can be produced in a form suitable for a number of applications such as catalysis.
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