Do refractive index and absorption matter for nanoparticles?

Abstract: Refractive Index of Gold, Silver, Titanium and Other NanoParticles.

FAQ: How important are refractive index and absorption for nanoparticles?

A Zetasizer scientist is looking at a sample of nanoparticles to determine if the refractive index might be necessary. It can be measured, but often is not required when the size is below 100 nanometers.In general, the optical properties of the scattering material have a tremendous influence on the observed scattering behavior. Mie theory can fully describe these phenomena. Therefore Mie theory is the best choice. For instance, the refractive index n and absorption k of the scattering material affect the scattering intensity. As a result, for many researchers interested in the nanomaterial size range, the optical properties of the material itself are unknown. What can we do in such a case?

Things to consider for optical properties of nanoparticles (Zetasizer)

  • Firstly, the amount of scattering is directly related to the properties of the material. However, for dynamic light scattering (DLS), the material properties – although often requested in the setup of an experiment – may be irrelevant. But if only an average size by intensity and an average polydispersity (PDI) by intensity are required, then it does not matter which material produced the intensity. The material properties do come into play when the intensity size distribution is transformed into a volume or number distribution. In that case we have to know exactly how much light is scattered by each nanoparticle. And in order to predict that, Mie theory requires the refractive index and absorption of that particle.
  • For small nanoparticles of less than 100nm the material properties will not even matter. As a result, the volume distribution obtained via DLS will not change significantly in that case.

Try out a value and see the effect!

In addition to the above two points, you can model refractive index values and see what happens:

  • For example, you can convince yourself of the effect of the material properties. To do this, first edit an existing data record (high-light the record, right-click, edit record). After that give it a new sample name [for example “Tungsten sample with n=1.6 and abs=0.01”] and now edit the Material properties [click on the … box next to it, Add, enter a material name and its associated refractive index and absorption] and then OK OK. As a result, a copy of your original record with the new analysis parameters will appear in your file. By highlighting both records [press the Ctrl-key and highlight both] the results with the two different methods can now be overlayed and subsequently compared.
    You can then confirm that there is no difference in the intensity distribution result (and the z-average and polydispersity for that matter).
    Therefore, you can directly observe the influence that a change in the material properties of the scattering material may have on the volume distribution.
  • Some optical properties for nanomaterials can be found via google. For example, here is a shortlist of refractive index and absorption values for common nanomaterials below. To clarify, this is for a Helium-Neon laser with λ=632nm (the wavelength in the Zetasizer).

Select Refractive index properties of nanoparticles

In the table below we list a few common materials. Some – however not all – are part of the standard software parameter list.

Table of select material properties

  Sample material   refractive index   absorption
Liposomes #
Phospholipids n=1.45 k=0.001
Exosomes n=1.37 – 1.39* k=0.01
Microvesicles (> .2µm) n=1.40* k=0.01
Nanoparticles and Colloids
Gold [Au] n=0.20 k=3.32
Silver [Ag] n=0.135 k=3.99
Platinum [Pt] n=2.32 k=4.16
Palladium [Pd] n=1.77 k=4.29
TiO2 n=2.41 k=0.001
SiO2 n=1.54 k=0.00
PFOB emulsions n=1.305 k=0.10
Nanodiamonds n=2.42 k=0.00
Macromolecules
Proteins n=1.45 k=0.001
Polystyrene n=1.59 k=0.01

In summary, the short answer is: even without the parameters, we can obtain useful information from nanoparticles with DLS.

#Optical characterization of liposomes by right-angle light scattering and turbidity” Biochimica et Biophysica Acta (BBA) – Biomembranes 1467, 1,  219-226 (2000)
* “Measurement of refractive index by nanoparticle tracking analysis reveals heterogeneity in extracellular vesicles” Journal of Extracellular Vesicles 2014, 3:25361 DOI: 10.3402/jev.v3.25361 (2014)

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If you have any questions, please email me at ulf.nobbmann@malvern.com. Thanks!