Rahim Abdur You can't. You should have kept them wet in colloidal suspension without drying. For further information, view this webinar (free registration required):

Dispersion and nanotechnology

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

See also:

https://www.researchgate.net/post/How_to_deagglomerate_the_dry_nano-particle_powder_for_DLS_measurements_How_to_use_a_suitable_surfactant_for_this_purpose

Rahim Abdur

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, you can make dispersion in ethanol, if it not works then add dilute HNO3.

Best wishes

Alan F Rawle Thank you for your valuable suggestion. I also agree with you, but I had to use this process due to some limitations.

You should be able to disperse it in water if the synthesis process was alkaline-precipitation. Try to decrease NP size at least below 20 nm and be sure that you have adequate surface agents such as nitrates and OH groups to promote dispersion. Finally, try to sonicate the dispersion at least for 1 h.