To further confirm the presence of a rigid shell, a scanning electron microscopy (SEM) was performed. The images obtained (Figure 1) show particles with a diameter of 100–150 nm. Particles were almost perfectly spherical and despite not having being metal-coated, no sign of melting or damage from the electron beam was visible, suggesting a rigid shell surface. The discrepancy between the size results obtained by SEM and DLS could be ascribed to the fact that DLS measures a hydrodynamic diameter, based on the NP diffusion coefficient, and thus it also takes into account the hydration layer surrounding the particles .