How do we measure the CV Values and reveal the real size distributions?

Through the years of working with nano-/microspheres, our researchers and technicians fully understand the importance of the size distributions from batch to batch. In the field of size characterizations, there are mainly two methods, scanning/transmission/scanning transmission electron micrographs (SEM/TEM/STEM) and dynamic light scattering (i.e. DLS). Unfortunately, we found DLS are unsuitable for accurately determine the size distributions of nano-/microspheres when compared to SEM/TEM/STEM techniques. 
 

• DLS results are closely related to colloidal stability, and as an inevitable result, the size distributions will not be accurate for large-sized nanoparticles. For nanoparticles below 100 nm, the hydrodynamic sizes become relevant. However, for nanoparticles around or above a few hundred nanometers, colloidal stability is greatly affected by the role of gravity. In comparison, the SEM/TEM/STEM images are based on dry samples only where higher reproducibility and reliability have been seen through numerous studies in our case and in the literature.

• DLS cannot distinguish between nanoparticles themselves and the aggregates of surfactants. As most of our colloidal nano-/microspheres have a trace of surfactants purposely added in the system for longer lifetime, DLS becomes unsuitable for direct size measurements unless the nano-/microspheres are washed at least three times with the Milli-Q water or de-ionized water to fully remove any trace of surfactants. Similarly, the nano-/microspheres due to inefficient re-suspension also distort the DLS results; for instance, a couple of aggregated 100 nm nanoparticles might be seen as 200 nm or 300 nm nanoparticles under DLS measurements.

• DLS cannot give any insight into the shape or the spherical degree of nano-/microspheres. Irregularly-shaped particles and perfectly spherical particles can be interpreted the same under DLS measurements.

As a result, we use SEM/TEM/STEM techniques instead of DLS measurements as our routine quality control method. We routinely take at least 10 electron micrographs and process them with ImageJ for size distributions as well as the CV value. This is also the reason that we typically include a couple of representative SEM/TEM/STEM images with our products when delivered to our clients. We wish to reveal the real size distributions and let the clients visualize the images of nano-/microspheres directly instead of viewing the fitted curves indirectly.