Some of the major methods for structural and physicochemical characterization of iron oxide nanoparticles : x-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), nuclear magnetic resonance spectroscopy (NMR), and atomic absorption spectroscopy (AAS) are discussed in this paper : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3097763/
I suppose your iron oxide nanoparticles are suspended in a liquid host, and you are correctly trying to get info with the easiest technique, that is UV-Vis spectroscopy.
II am sure there is a vast literature available on the net, but you can get a first idea from the following link: