What is the best method to distinguish and identify metal nanoparticles dispersed in water, given their very small size ( 3-5 nanometer ), while ensuring that the method used does not alter or affect their size?
The best method is transmission electron microscope. You can use a scanning electron microscope, small-angle X-ray diffractometry, dynamic light scattering. The latter method reports the hydrodynamic diameter of the nanoparticle.