How are you deciding that you have nanoparticles or not?  They will need preparing and stabilizing under appropriate conditions of pH and ionic additives in order to prevent aggregation.  See for example:

Nov 11th, 2008. Dispersion and nanotechnology  http://tinyurl.com/hpywsge

Because the plant extract has many biomolecules that cover the nanoparticle surface and that causes that the reactivity changes.

I repeat my question.  How do you know you have iron particles < 100 nm in the system?

Thanks Dario and Alan,

Is it just a matter of the surface of the nanoparticle being covered in bio-molecules from the extract or is it due to the specific species that is reducing the iron salts in the first place? I am fairly sure the iron nanoparticles generated from plant extract mediated synthesis results in iron oxide nanoparticles whereas the sodium borohydride reduction of iron salts generates Fe0 (zero valent iron) nanoparticles in anaerobic conditions.

Furthermore, plant extract generated nanoparticles i.e. green tea extract will generate non-magnetic nanoparticles however the sodium borohydride reduction of iron salts generates magnetic nanoparticles. 

A few studies have shown zero valent iron generated from plant extracts via XPS however there is no confirmation of this via XRD. 

Hi Alan,

I characterize my particles via TEM, SEM+EDS, XRD, zeta potential and FTIR.

Thanks,

Adam

Hi Adam OK, so you're sure you have primary particles of < 100 nm.

You then have a stabilization issue - perhaps pH or if you've formed Fe3O4 the long range magnetic forces always overcome the short range van der Waals (only steric stabilization would be possible here). Take a look here for a general overview especially slides 45 onwards:

Nov 11th, 2008. Dispersion and nanotechnology  http://tinyurl.com/hpywsge

iron salt solution and NaBH4 will produce fresh nanoscale zero-valent iron. I am not sure what plant extract reduction of Fe salts will make. But several things will affect NP's reactivity, including surface areas, particle size, and iron oxide layer thickness. 

Thanks Alan,

I am actually immobilizing them into solid support matricies and they have good eveness if size and have a good surface area. 

Thanks Yanlai,

What you say is true however I believe that reduction is a broad term and the reduction mechanisms of NaBH4 are different to plant extract mediated reduction leading to a different species of iron. This will ultimately show different intrinsic reactivity between particles.