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Polydispersity from a Gold Standard

31 May 2018 No Comment

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How good can a gold standard be?

 

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, under ideal circumstances, when all other interferences are minimized. This is especially relevant at the smaller nanoscale where fewer standards are available, and to those 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 National Institute of Standards and Technology, the Reference Material RM8011 with nominal 10nm diameter, citrate-stabilized Au nanoparticles in an aqueous buffer. This is their smallest nanomaterial reference. It is delivered in a 5mL ampoule, there are mostly monomers in the preparation according to the NIST RM8011 Report of Investigation.

With a Zetasizer Nano ZSP, looking at a 20nm filtered sample, in a glass cuvette, the “best” PDI obtained was 0.05 ± 0.01 (standard deviation from 20 repeat measurements in automatic mode). For different aliquots and cuvettes, the results are essentially the same, albeit with a slightly larger standard deviation 0.05 ± 0.03.

While ISO22412 advises considering only z-average and PDI we also investigated the peak parameters as reported from a standard general purpose NNLS size distribution analysis due to consistent repeatable data quality of the correlation functions.

The findings are summarized in the table below.

 

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 with 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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If you have any questions, please email me at ulf.nobbmann@malvernpanalytical.com. Thanks! While opinions expressed are generally those of the author, some parts may have been modified by our editorial team.