When performing dynamic light scattering (DLS) measurements, we sometimes encounter especially nice data. An interesting issue to consider is the level of data quality one can expect to achieve. What is the best gold standard DLS under ideal circumstances, avoiding interference from sample uncertainty? This is especially relevant at the smaller nanoscale. Here, fewer standards are available. And it is also of interest to researchers who have less practical experience with sizing colloidal nanoparticles by DLS. How monodisperse can a good sample be? What are achievable polydispersity index (PDI) values to aspire to?
What is the lowest PDI and standard deviation in DLS?
In order to find the answer to this question, we bought a sample of colloidal gold from NIST. The National Institute of Standards and Technology Reference Material RM8011 has a nominal 10nm diameter. These gold Au nanoparticles are citrate-stabilized in an aqueous buffer. This standard is their smallest nanomaterial reference. It is delivered in a 5mL ampoule. The vial contains mostly monomers in the preparation according to the NIST RM8011 Report of Investigation.
For this study, we used a Zetasizer Nano ZSP to look at a 20nm filtered sample. With the sample in a glass cuvette, the “best” PDI we obtained was 0.05 ± 0.01 . (The standard deviation is from 20 repeat measurements in automatic mode). In addition, for different aliquots and cuvettes the results are very similar. In plastic, absolute values are essentially the same, albeit with a slightly larger standard deviation 0.05 ± 0.03.
Furthermore, while ISO22412 advises considering only z-average and PDI we also investigated the peak parameters. In order to do this we used a standard general purpose NNLS size distribution analysis on consistently repeatable data quality of the correlation functions.
Summary of “best in DLS” achievements
The table below shows a summary of our findings for the best gold standard DLS. Certainly both z-average and intensity peak were very repeatable (better than +- 1%) in the glass cuvette. On the other hand, in plastic cuvettes results for mean size and PDI are slightly more variable (+- 3%) .
We presented this as a poster recently. The full version of “Investigation of Size and Polydispersity of a Nanoparticle Reference Material by DLS” contains additional details. It includes a comparison to the reported NIST values.
Dynamic Light Scattering can serve as a reliable, rapid size assessment tool for nanotechnology. Typical standard deviations in z-average are ~3% over different cuvettes and can be as low as ~1% in glass. The “best achievable” PDI of this 10nm gold NIST reference is ~0.05.
NB: Lower PDIs are achievable with larger size particles such as latex beads.
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