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 absorption for nanoparticles?


In general the optical properties of the scattering material have a tremendous influence on the scattering behavior which can be observed. Mie theory can fully describe these phenomena, and is the best choice. For many researchers with interest in the nanomaterial size range, often the optical properties of the material itself may not be known. What can one do in such a case?

  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 for the volume distribution obtained via DLS.
  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).
    You can then 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. For a Helium Neon laser with λ=632nm here is a short list of refractive index and absorption values for commonly encountered nanomaterials.

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, useful information from nanoparticles can be obtained from DLS.

* “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

Previously

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