yes it is important. Its value will suggest you that whether your nanoparticles is stable in that system or not. It means whether it is going to agglomerate or not we can know by this. we can have idea that whether the obtained result is effect of separate separate one one particle or of agglomerated one. Both nicely dispersed and agglomerated will give different effect in plant. I will suggest you to do DLS too with zeta potential so that you can know your nanoparticles hydrodynamic size in that medium or liquid.

yes it is important. Its value will suggest you that whether your nanoparticles is stable in that system or not. It means whether it is going to agglomerate or not we can know by this. we can have idea that whether the obtained result is effect of separate separate one one particle or of agglomerated one. Both nicely dispersed and agglomerated will give different effect in plant. I will suggest you to do DLS too with zeta potential so that you can know your nanoparticles hydrodynamic size in that medium or liquid.

You can, but you do not need to do zeta potential measurement (maybe misleading regarding colloidal stability, because electrostatic stabilization is one option only). What is mandatory is to check the dispersed state directly, e.g. by determination of particle size distribution under the conditions of your experiment. What you should measure is metal ion concentration in solution along with a blank measurement with metal ions present.

Dear Ms Narges Ghasemi Siani, The charge of the particles will surely also affect their bioactivity, since biomembranes have negative charge. One example is gentherapy, where the negative charges of the DNA-plasmids have to be neutralized, or better being net positively charged in complexes with cationic polymers och lipids, to obtain high levels of cellular transfection.

Yes, Mahd Qasim has an important point. Nanoparticles are prone to aggregation, due to their large specific surface area. Therefor, doing experiments with these small particles require determination of their charges and size distributions, to detect possible formation of aggregates. Myself, I was working on a suspension for parenteral administration and everything looked fine, until I added NaCl.  Actually, it was my young daughter, Sofia, that told med to check for possible effects of electrolytes. Her suggestion was good, since, when using Dynamic Light Scattering, DLS, we found that aggregates bigger than microns formed, as NaCl was added. So, it's not enough, just to beleive that you've got nanoparticles. You must check it, before you can claim they are. /  Johan Carlfors  at :  www.ontargetchemistry.com  ,  Uppsala,  Sweden.

As mentionned above Electrostatic interaction is not necessarilly the only reason of interaction between particles and substrate.

Key point when studying Zeta potential is to understand that it does not characterize the particle itself but the particle in the bulk. 

Dispersion stability  will be affected by surrounding medium. (pH,ionic strength ...) . This can be studied by Zeta potential.  The stabilty will affect particle size distribution.

Phytotoxicity will certainly be affected by both parameters size/zeta potential.

Whether there is a direct link or not will depend what is tested as phytotoxicity - response of algae, seed germination or plant growth. I would assume in almost all cases effect of nanoparticle will be due to dissolution of Zn-ions.