What is the importance and relationship of the hydrodynamic size of nanoparticles measured with DLS to the size measured with TEM?
Good question. I’ll try and answer your question part by part from what I understand. Firstly, as we already know that by DLS we get the hydrodynamic radius of the particle whereas by TEM we get an estimation of the projected area diameter. So as far as DLS is concerned, the theory states that when a dispersed particle moves through a liquid medium, a thin electric dipole layer of the solvent adheres to its surface. This layer influences the movement of the particle in the medium. Thus the hydrodynamic diameter gives us information of the inorganic core along with any coating material and the solvent layer attached to the particle as it moves under the influence of brownian motion. While estimating size by TEM, this hydration layer is not present hence, we get information only about the inorganic core. The projected area diameter estimated by TEM is theoretically defined as the area of a sphere having the same area as the projected area of the particle resting in a stable positon. Sometimes due to poor contrast in TEM the size measurement of the coating layer if present could be underestimated or missed. Hence, the hydrodynamic diameter is always greater than the size estimated by TEM.
Talking about the importance of the hydrodynamic diameter, many studies suggest that it is an important paramater for understanding and optimizing the nanoparticles’ performance in biological assays as well as understanding the in vitro migration of the particles.
Well, in my opinion, both are “true” size. They are just two different things, with TEM you measure the core size and with DLS you have into account the surfactant and the interaction with solvent molecules. Of course TEM is important but for me, working on nanoparticles for bioimaging, is much more important the hydrodynamic size. A nanoparticle with 5 nm size and homogeneous in TEM that once in water is 1000 nm is worthless.