Do refractive index and absorption matter for nanoparticles?

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

Crystal-refractive-index-facets-with-rainbowFAQ: How important are refractive index and absorption for nanoparticles?


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, and is the best choice. Here, the refractive index n and absorption k of the scattering material affect scattering intensity. 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?

4 things to consider for optical properties of nano particles (Zetasizer)

  1. First of all, for dynamic light scattering (DLS), the material properties – although often requested in the setup of an experiment – may be irrelevant. Yes, the amount of scattering is directly related to the properties of the material, 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, because now 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.
  2. 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.
  3. The best way to convince yourself of the effect of the choice of material properties is to edit an existing data record (high-light the record, right-click, edit record). Then give it a new sample name [for example “Tungsten sample with n=1.6 and abs=0.01”] and 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. 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 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.
  4. Some optical properties for nano materials can be found via google. As an example, here is a short list of refractive index and absorption values for common nanomaterials. To clarify, this is for a Helium Neon laser with λ=632nm (and the wave length in the Zetasizer).

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)

Resources

Intensity – Volume – Number – Which Size is Correct?

Mie theory: the first 100 years

Dynamic light scattering – common terms defined

Malvern Reference Manual: Sample dispersion and refractive index guide (MAN0396-1-0)

List of dielectric constants of common solvents (from UMass)

Previously

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