I observed frequently that hydrodynamic size taken by DLS is subsequently greater than 100 nm, then why do we call these are nano-particles,even i think these are not nano-particles according to the definition i.e. (nano-particles < 100 nm). can we use the word nano-particle for this arbitrarily? justify!
If you are referring to non-spherical nanoparticles, they could have feature sizes less than 100 nm even when DLS gives the value of hydrodynamic radius (quasi) as over 100 nm. I use quasi because DLS would not give you the correct values for Hydrodynamic sizes. It needs to be corrected for the effects of rotational diffusion in the case of non-spherical particles.
The hydrodynamic size refers to the total size of the (nano)particle system that undergoes Brownian motion. In other words, the core particle, together with the surface molecules/ions and the solvation layer. If the stabilizer is a polymer (especially one that swells in the solvent), then the swollen polymer and the solvation layer really dominates the hydrodynamic size. The difference between hydrodynamic size (wet) and size by electron microscopy (dry) can vary substantially, depending on the specific system. Your concern (> 100 nm) is valid if it is the size determined by electron microscopy (or crystallite size by XRD). It is common to get >100 nm hydrodynamic size for colloidal nanoparticles that are smaller than 30 - 40 nm (as seen in a TEM for e.g.).
in few words... Hydrodynamic size is not the true size of the true nanoparticle...
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