I did DLS size measurement with 10 acquisition got average size is 120 nm and STD is 10 while SEM gave me 100 nm and STD is 35 with exactly 1000 NPs. I am sure about the size of DLS should be smaller beacuse its in solution while SEM its in solid but I think STD should be similar to each other. Any expert opinion would be appreciated. Thank you.
> the size of DLS should be smaller
Is it a typo? You've meant DLS size is larger due to it's hydrodynamic nature, right?
Back to the main topic. What quantity do you calculate as standard deviation (SD) for DLS measurements? Is it the SD for 10 consecutive measurements of mean size of some kind (Z-average or Peak size)? In this case SD has a meaning of repeatability for the mean size.
On the other hand, with SEM you've measured each particle, then calculated the mean and SD. In this case, SD is no longer the measure of repeatability, but instead characterizes the polydispersity of your particles, in other words, it is related to how broad particle size distribution is.
Two totally different techniques. One is inherently number-weighted, the other is intensity-weighted. One is single particle based, the other is ensemble based.
With DLS, standard deviation is estimated from the cumulants fit (basically the deviation of the correlation function from an exponential curve). SEM will estimate the diameter of individual particles and calculate a standard deviation. Also, SEM is notoriously bad for sizing - use TEM.
This is due to the fundamental differences in measurement principles and interpretation between the two techniques. The observed differences in STD could be explained by several reasons such as the measurement basis (i.e., DLS measures hydrodynamic diameter while SEM directly measures the physical diameter of the particle), sampling volume, and even the different sensitivity to aggregation of the two techniques is also suggested.
The basic difference in standard deviations between the two techniques could be due to differences in sample preparation (During Gold coating in SEM), sensitivity, nature of the sample.
Mohammad Forhad Khan
Dilip K. Patel
Benyl John Altiche Arevalo
John Francis Miller
Evgeniy Evtushenko
I would like to draw the scientists' attention to the fact that such discussions are meaningless for surfactant micelles, liposomes, proteins... They are formed by hydrophobic interactions mediated by water. The change in the state of water is responsible for the aggregation of molecules. Therefore, neither DLS nor SEM are the methods that describe the true picture of the aggregation of molecules. For this, it is better to use small-angle X-ray scattering or cryo-ТEM.
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