Why does the AgNP size measurement appear smaller in DLS (13nm) compared to TEM (43nm)? Attached is a TEM image displaying the nanoparticles with an approximate diameter of 43nm, while DLS indicates a diameter around 13nm. Could someone provide an explanation for this discrepancy? I would greatly appreciate any insights or suggestions.
Yiqin Li This is unusual - normally the intensity-based ensemble DLS method gives larger results than the particle-by-particle number-based statistics of TEM.
I'd look at representative sampling. How did you prepare your sample for TEM? The DLS technique has looked at billions of particles simultaneously. You'd need to examine 10000 particles under the TEM to get a standard error of 1%. How many did you examine? If, for example, you took the TEM sample from the top of a container, the larger particles could have settled or be settling and you'd extract a sample with smaller particles. In the whole history of TEM not more than a few g of total sample has been measured. If your TEM sample has been produced by microtoming then you'd have a set of discs, the maximum of which would be the actual diameter of your particle (consider slicing a monodisperse distribution of spheres - what would you get?).
Agree with Alan F Rawle; DLS measures hydrodynamic size of metal nanoparticles usually consisting of both metal cores and ligand shells while TEM measures sizes of metal cores, leading to relatively larger sizes in TEM than DLS method.
One possibility is that metal nanoparticles aggregated during the drying process on TEM grid (also from this TEM image, these AgNPs are likely to aggregate).
Hello Yiqin Li ,
colloidal gold can occasionally show an additional contribution from rotational diffusion. See for example Fig 5 in https://www.malvernpanalytical.com/en/learn/knowledge-center/application-notes/an180516goldnanoparticleszetasizerultra
In general here are some potential additional causes for incorrect size by DLS https://www.malvernpanalytical.com/en/learn/knowledge-center/insights/why-does-dls-say-that-my-particles-are-too-small/
Yiqin Li Your TEM picture is very crowded. And how do you get '43 nm' from this picture? If you carried out the DLS experiment at a similar concentration then multiple scattering would cause an apparently decreased size, but I'm not sure to the large extent you display. That's why the international standards (ISO and ASTM) recommend a concentration/dilution ladder to look at such effects. Have you carried out such an experiment?
Yiqin Li, check the solvent viscosity value. If the value is higher than it should be, the Rh value will be lower.
However, most probably, you are facing with interparticle interactions, the influence of second virial coefficient, A2. Try diluting solution 10x, or making a series of concentrations, followed by extrapolation to infinite dilution, c=0.
It should work.
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