Can anyone please explain if the size determination using DLS is exact size of nanoparticle or it is bigger than actual size of nanoparticle.
(I am giving an example of nickel metal nanoparticle which have been synthesized in colloidal form using binary protective agents PVP and one more and solvent is ethylene glycol.)
Dear Manoj Yadav :
DLS measures the hydrodynamic radius of the sphere. The radius of the nanoparticles with adsorbed ions and solvent molecules. He is a little larger than the actual radius. See attached file.
Best regards
Dear Manoj Yadav :
DLS measures the hydrodynamic radius of the sphere. The radius of the nanoparticles with adsorbed ions and solvent molecules. He is a little larger than the actual radius. See attached file.
Best regards
Hi Manoj,
As Yuri has explained, DLS measures the hydrodynamic size of the particle. Another way to explain this, is to realise that DLS measures diffusion of the nanoparticles, and then from this, a size is generated by effectively quantifying what size of hard sphere would diffuse in the same manner.
This is not the same as the physical size that might be recorded by electron microscopy due to charge interactions. Also, SEM is typically done with dry samples and some colloids will swell suspended in a solvent.
Caveats to be aware of in interpreting DLS data. There are several methods to analyse the correlation function measured in DLS. Cumulants analysis generates the Z Average size, which represents the average size of your sample, and is therefore sensitive to the presence of large transient particles such as dust or aggregates and may therefore give a larger answer.
A variety of distribution based techniques exist, but these results are usually intensity weighted, and given that scattering intensity is proportional to the 6th power of the particle radius, size results may be skewed towards larger sizes.
One additional point.
DLS measurements lead to diffusion coefficient (diffusion coefficient distributions).
Stokes Einstein relation is used to calculate the radius (radii distributions).
The viscosity of ethylene glycol is a function of the water content (reduce with increasing content). Both the diffusion coefficient and the viscosity are within the denominator of the SE relation.
Hello Manoj,
the DSL technique is based on the measurements of time dependent light intensity fluctuations caused by the Brown movement of particles in the dispersions and Michael made a good point here. We were measuring particle size of ZnS stabilized by CTAB by different techniques. Size of ZnS nanoparticles was about 10 nm, but by measuring DLS we observed that the particles with the hydrodynamic diameters were ranging from 23 nm to 130 nm. The DLS method provided the size of ZnS nanoparticles including the complex structures of adsorbed CTA+. This could be your case as well with PVP and other stuff around your NPs. So yes, your particles should be much smaller than you observed by DLS measurement.
Yes Laislav !! thank you...but how to get actual size of nanoparticles..or how can we predict actual size of nanoparticles?
If you want to know real sizes, you should get good TEM images of your NPs and manually subtract values.
DLS-based particle size analyzers provide a distribution of hydrodynamic diameter of particles in liquid. It is a statistical parameter for large sample (a lot of particles).
Microscopy provide size and morphology (it is important!) for a moderate amount of particles. Obviously, it is technically impossible to calculate a size distribution for millions particles. In addition, there are some artifacts of drying - particles can aggregate and the liquid can produce a thin film on theirs surface after the drying.
A good way is use of both methods and results comparison.
Good luck!
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