V H Krishna Prasad

There are many answers to this question on RG. A little search would help you enormously. Take a look at:

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

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

Anyway, to keep the particles in the nano form they should never be dried. They will invariably aggregate as sure as night follows day via van der Waals attractive forces and solid-solid diffusion leading to solid bridged particles. A powder with small primary particles is a collection of fused sub- and post-micron aggregates and agglomerates.

According to the international (ASTM & ISO) standards a concentration ladder should be employed to isolate a region of stability (no multiple scattering and adequate data). Zeta potential (ZP) is a holistic property of the system - particles and continuous phase. Thus, a pH-ZP titration is normally employed. It isn't measured directly but inferred from mobility measurements. One needs to avoid hindered mobility (particle-particle interactions) so a true movement is measured.

For more details on attempting to produce a dispersion from a powder (wetting, separation, stabilization), please see the webinar (free registration required):

Dispersion and nanotechnology

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

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

Finally, it is a general rule that if the particle size distribution of a powder is to be measured then it is a laser diffraction experiment, not DLS.