Hello, I have some doubts about DLS measuring the size of the nanoparticles.I hope to get answers from you.
I measure TiO2-NPs(60nm，commercialization ) in 0.01M PBS. But Instrument prompt “sample too polydisperse for cumulant analysis” and “sample too polydisperse for distribution analysis” .Correlation Functions show the picture as followed. Why does this happen and how to avoid it ?
All dispersions were filtered with 0.45um filter. Ultrasonic dispersionwas used.
Dear Zhao Shu,
As I see from the distribution curve, the problem seems to be that the DLS detects different particle sizes, from too small to too big. This is a problem for the calculation of the particle size and polydispersity. Often, this ocurrs when your suspension is highly concentrated. You should re-prepare your suspension with lower concentration of the particles and try measure again. Also, you should check the suspension solvent, because you will need to configure the refraction index on the equipment. It is different to measure when the particles are suspended in ethanol, water, or other solvent. Finally, be sure that you disperse really well the particles. This is often done by ultrasonic equipment.
Hope it helps, and good luck!
DLS is based upon measuring the fluctuations of the scattered intensity of a collection of particles. These fluctuations can be turned into a correlation function. Usually, a cumulant fit is performed to yield a diffusion coefficient. The analysis and interpretation of a correlation function becomes inaccurate if the system is very polydisperse. It is anyway risky to derive size distributions from DLS; different distributions may yield a similar correlation function.
Hi Zhao Shu
the correlation function does not look too bad, but there are some larger components in there. Try to centrifuge, let the sample settle for a while, or filter again. Great if dilution solved the problem. You might also like this blog:
https://www.materials-talks.com/why-does-dls-say-that-my-particles-are-too-small/
Another reason could be the ionic strength of your PBS solution which may generate "Debye screening effects". This essentially means that electrostatic stabilization of your particles is decreased, leading to agglomeration and the apparent polydispersity.
See here for more info: https://www.materials-talks.com/debye-screening-how-it-affects-zeta-potential/
"“sample too polydisperse for cumulant analysis” does not make sense. Any correlation function can be analyzed using the cumulants method. It is a deterministic method.
Thanks for everyone! These answers are very helpful for me. And I retried again and then it was solved greatly. The details measures as followed:
1 st, 0.01 mg/ml TiO2-NP was dispersed by sonication.
2 nd, the sample was centrifuged under 2000rpm for 2min.
3 rd, dilluted supernatant and then measured with DLS.
Though it still had some large particles (>1000nm), the results were good and replicated well. Interestly, it showed lots of ultrafine partlces(
one should be careful - it does not make sense to treat (change) the sample until 'it can be measured'. The useful information is what is the sample in original state and after defined steps of treatment.
If you start with a powder it is already strongly aggregated/agglomerated. While agglomeration might be reversed, e.g. by sonication, reversal of aggregation is almost impossible.
Hi! I faced the same issue during my initial studies.
Our samples are nanoemulsions, We altered the dilution for the sample that is used for DLS analysis, and that helped in sorting out the issue!
When we used the nanoemulsion directly and with high concentration, the same issue popped up!
You may use an SBL or NNLS algorithm which would fit for polydisperse samples.
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts