Preety Rohilla

Thank you for your question. A little research on RG will show that it's been asked and answered in different ways many times. Take a look at:

https://www.researchgate.net/post/How_can_I_overcome_dispersion_issues_faced_while_re-suspending_powder_nanoparticles

https://www.researchgate.net/post/How_the_agglomerated_green_nanoparticles_can_restored_into_nano_size_particles

One overall webinar (registration needed) is this one:

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

The other take home is to familiarize yourself with Stokes' Law. Settling will tell you that you have micron-plus sized particles in the system.

'The micron scale is volumetrically 109 times larger than the nanoscale. Confusing microtechnology with molecular technology is like confusing an elephant with a ladybug' K Eric Drexler quoted in Ed Regis Nano: the emerging science of nanotechnology' Little, Brown and Company, 1st Edition pp 207 - 208 (1995)

The short summary is that, in order to preserve the 'nanoness' of the system, one should never, ever dry it. 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 particles……‘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

Dear Dr Preety Rohilla . See the following useful RG link: Article Dispersion of Nanomaterials in Aqueous Media: Towards Protoc...

Kindly see also the following useful link: https://clu-in.org/download/techfocus/nano/Nano-srel15-2-1.pdf

Also check please the following useful RG link: Article Effect of sonication on particle dispersion, administered do...

The following link is also very good: Article A Review of Spectral Methods for Dispersion Characterization...

After preparing gold nanoparticles, it can be confirmed by simple two process, first there will be a color change, and secondly you can use UV-Vis spectrophotometer, it gives abs(max) peak at around 500-550 nm. So if you have to use AuNps in solution form, there is no point of converting them to powder form I guess.