Cellulose Nanocrystal Particle Size measured by AFM was appeared different, what was found from DLS measurement. Average DLS values for dispersed particles are higher than the values measured by AFM. Bimodal distributions were easily identified with AFM, but DLS results were skewed toward larger particles. How to correlate those results?
you read this paper
https://www.andreas-brinkmann.net/PDF/Brinkmann-2016-Langmuir-32-6105.pdf
Hello M. Nuruzzaman Khan
As TEM is similar to AFM in the detection of the morphology of NPs, this picture describes in detail the difference between TEM & DLS.
Also, this is a nice paper about different techniques for NPs characterizations
https://pubs.rsc.org/en/content/articlelanding/2018/nr/c8nr02278j#!divAbstract
Size via AFM is (I assume) based on a length measurement (diameter) whereas DLS is based on the 4th power of the diameter. Thus, unless you have a perfectly monodisperse material, DLS will always give a larger answer.
Also, AFM is a single particle based technique whereas as DLS is an ensemble technique. Because of this and the diameter weighting, it will pick up larger particles that AFM misses. It may only require one large particle in many thousands of small ones to affect DLS and chances are you won't see it via AFM.
In my experience, AFM is more reliable at measuring particle size distributions than DLS. Typically, standard DLS equipment does not allow multi-angle measurements, and the calculations use the Mie theory, which is designed for the spherical shape of particles, which can differ significantly from real particles. This leads to large measurement errors in contrast to AFM.
Precision refers to how closely grouped a set of consecutive measurements are. Accuracy refers to how close to an accepted or certified value the measurement is. Both will depend on the material as well as the measurement technique and protocol.
John Francis Miller
Scattering intensity is proportional to the 6th power of the radius in the Rayleigh region. You're thinking of the inverse 4th power for the wavelength.
Apples are being compared to pears. AFM is measuring a number-based distribution (for which 10000 particles will be needed to obtain the mean to 1% standard error) and DLS gives an intensity-based distribution, measuring many millions or billions of particles simultaneously (at relatively low resolution).
Alexander Loskutov I would ask you what unambiguous dimension is measured in AFM for irregular particles? You need more than one number to define an irregular particle if you don't use some form of equivalent (usually spherical or circular). If you have a cylinder is the diameter measured or the length (occasionally)? Interpretation is still an issue here and material is rarely sold and specified on the number of particles. In the Rayleigh region (~ < 100 nm) the scattering is isotropic with no angular dependence, In the region > 100 nm, then laser diffraction works well (and this is where Mie theory may be employed). DLS does not measure size directly but a diffusion coefficient. If you don't want to make any assumptions, then don't convert the diffusion coefficient to hydrodynamic diameter based on the Stokes-Einstein equation. Finally, both techniques have international standards associated with them so the comment 'This leads to large measurement errors in contrast to AFM' is nonsense. Both techniques measure appropriate international certified reference materials correctly.
If one has 1 million 1 nm particles then they make the same mass as a single 100 nm particle. Imagine they're particles of gold. Would you ignore the single large particle (as 'not typical')? It makes up 50% of the value of the system...
I would also add the following.
How many times are AFM results repeated on the same sample to get a handle on the repeatability and reproducibility? Polydispersity is the bane of all particle size measurement techniques.
We should see the techniques as complementary, not competitive...
Alan F Rawle very wisely said "We should see the techniques as complementary, not competitive..."
Absolutely. One of the golden rules of particle sizing. The more complementary techniques you can use, the better the understanding of your sample.
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