Precious Gultiano The usual reason for using a UV-VIS spectrometer with nano particles is to estimate a mean size. Thus, the system should never have been dried and the colloidal sol measured in its prepared state. With a powder, you’ll have a collection of fused sub- and post-micron aggregates and agglomerates. There will be no free, independent, discrete particles < 100 nm in such a system. You’ll find it virtually impossible to disperse the system back to its original nano (< 100 nm) state.

For further information, please view these webinars (free registration required):

Dispersion and nanotechnology

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

Silver colloids and invisible ink

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150902SilverColloidInvisibleInk.html

Note that the surface of your 'silver' nanoparticles is fully oxidized - silver is in the +1 oxidation state and not present as the metal.

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.

Precious Gultiano

Look at the preparation of samples in the articles

Article Investigation of interaction between silver ions and rutin i...

Article Preparation and bactericidal properties of silver nanopartic...