Dear Scientists,
I would like to consult you about an issue that I have regarding DLS. As we know DLS measures the hydrodynamic diameter of the nanoparticles according to brownian motion. Also, I observed that DLS gives peaks for any liquids even pure water which is so weird! My question is how can we be sure that the DLS peak is real and belongs to the particle size and it is not artifact?
In other words, when we have a solution and we are not sure that contains nanoparticles or not, how can we prove the formation or non-formation of particles using DLS?
Best regards
How do you know your water is free of particles? What kind of filtration do you use to remove any particulates?
It is common to use 0.2 micron filters but those will still permit smaller particles to be present.
Also, water scatters light albeit weakly. With a powerful enough laser and fast enough correlator, you could measure the diffusion. Any thermal gradients in the water will lead to density fluctuations which, in turn, lead to refractive index changes. It is differences in refractive index that leads to scattering. So, the instrument may detect these random fluctuations.
It also depends on the method used to estimate size from the correlation function and how "intelligent" the software developers were at discriminating noise from a real response. You also need to know how the software comes up with a "peak". For example, when using the cumulants method to fit a correlation function, you only obtain an average diameter and a polydispersity index - just two numbers. But some vendors plot of graph of a normal distribution centered on the average with a width derived from the polydispersity - but they don't alert the user to the fact that this is just a visual crutch and not a true distribution.
And then there's the reality of the electronics. Any noise in the electronics that contributes to the processed scattered light intensity signal can interfere and show up as an apparent diffusing particle. Recall, the instrument is designed to analyze random noise.
S.M.Ali Dadfar
In addition to the excellent points of John Francis Miller I would add...
You are measuring noise-on-noise. A computer will usually produce an answer with garbage input. You have to be the 'nut behind the wheel' in understanding what is reasonable or not. This is what science is about. Running standard known materials is one way to demonstrate both instrument performance and getting a feel for good data acquisition. There's a whole bunch of indicators in many software packages that alert you to potential problems such as no sample present (attenuator on 11 is one way)..
Dear John Francis Miller,
Thank you so much for your helpful and fruitful information. It really helped me a lot. Regarding the particles, I am using Milli-Q water which is supposed to be free of particles. What about the attenuator number? what is the optimum number?
Best regards,
Ali
S.M.Ali Dadfar
Reading the manual and Help topics will help. The ideal attenuator position is '7' showing an adequate amount of scattering for measurement. However, measurements can be taken well outside this regime. At high attenuator numbers (10 or 11) then this is weak signal/scattering caused by any one of 3 factors (and you'll need at least one of these to be in your favor in order to be sufficient to get a robust, reproducible result):
- Concentration. Typically double the volume concentration for double the scattering intensity
- Size. In the Rayleigh region there's a (volume)2 or (radius)6 dependence on scattering. Sub-5nm particles scatter very weakly
- Optical contrast (Relative Refractive Index, RRI). A metal colloid will always scatter more photons than a protein, for example
At high concentrations (attenuator 1 or 2) then the possibility of multiple scattering will need investigation. The above factors are why the international standards (ISO22412:2020 & ASTM E2490-09 (reapproved 2021) recommend a concentration ladder to explore these factors.
In terms of filtration, you'll need to filter as good as or better than what you want to measure. For (measurement of) sub-10nm material all continuous phases should be filtered to better than 0.02 microns. The best markers of result quality are the 3R's (Repeatability, Reproducibility, Robustness).
Nothing is ever particle free. As John Francis Miller alludes above, even molecules scatter light. This is the reason for the blue sky overhead and the red sky (longer path length) at dawn and dusk.
Good luck with your research.
Dear Alan F Rawle ,
Thank you so much for your kind reply. Your comment was very helpful.
Best regards,
Ali
S.M.Ali Dadfar As you can see from here:
https://www.emdmillipore.com/US/en/product/Millipak-Filter,MM_NF-C85052?CatalogCategoryID=
Milli-Q systems typically use 0.22um membrane filters.
Hi S.M.Ali Dadfar ,
you may find this blog on "solvent peaks" of interest https://www.materials-talks.com/solvent-peaks-how-to-get-rid-of-the-buffersaltadditive-dls-contribution/
It is also always good to keep relative contributions in terms of derived count rate in mind Especially if you are trying to assess how relevant the signal from the buffer alone might be versus the overall sample. https://www.materials-talks.com/derived-count-rate-what-is-it/
Only way to validate the presence of nanoparticles and their size is to use calibrated solutions of nanoparticles at lower and higher ranges. If your instrument is properly calibrated then check your solution for size of your nanoparticles.
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