We have previously talked about new software features of the Zetasizer Advance range. What about new hardware? Well the Zetasizer Pro and Ultra instruments give access to an optical feature wheel within the instrument. This gives access to more versatile and complex dynamic light scattering (DLS) measurements.

Fluorescence, we got this…

We can use this wheel to easily set up a particle size measurement for fluorescent samples. Any light emitted by the sample other than laser scattering will decrease our signal to noise. The fluorescence filter on the optical wheel can block that for us. The wheel will move the filter out of the optical path when not needed so we maintain full system sensitivity for other sample types that do need the filter.

Particle size measured for florescent quantum dots. Without the filter, the signal to noise is so poor that a reliable particle size can not be measured.

Polarization- what is it and why does it matter?

A wave such as light will may oscillate in a specific plane or be random. This property is polarization. For a quality laser beam this will be in a defined plane. When interaction of light with other materials is dependent on it’s polarization.

The Zetasizer uses a laser with vertical polarization. A sample scatters the light but what happens to the polarization? Most scattered light will also have vertical polarization, but some maybe depolarized. Any depolarized light detected will increase the noise in our measurement. This is why high quality quartz or glass cuvettes are preferable for some measurements, as plastic cuvettes may depolarize the light.

Adding a polarizing filter can clean up the optical signal and remove any depolarized light. For our vertically polarized laser, the polarizer should also be vertical.

The vertical polariszer can therefore improve the signal to noise in measurements using a plastic cell.

Repeat measurements of Z Average size recorded using different cuvettes, with and without polarizers.

The polarizers can also help clean up other sources of noise during a measurement. A recent paper in Scientific Reports shows the use of polarizers in the Zetasizer to remove noise caused by multiple scattering.

Doesn’t the Zetasizer have two polarizers?

Yes. As well as a vertical polarizer, the Zetasizer Pro and Ultra have a second polarizer which is aligned horizontally. With this filter in place, the only light reaching the detector will be depolarized.

You may be wondering why we would want to measure this depolarized signal?

One of the mechanisms that can depolarize the scattered light is rotation of particles we are measuring. In a traditional dynamic light scattering measurement we calculate a diffusion coefficient and convert this to particle size. This is a translational diffusion coefficient and describes how the particles move around.

Particles which are not spherical, or have surface differences, also have a rotational diffusion coefficient which tells us how the particles is tumbling around. By detecting a depolarized DLS signal we can therefore detect rotational diffusion.

What does rotational diffusion tell us?

Qualititively, we may be able to assess some differences in particle shape using this depolarized DLS, or DDLS method.

In the example below, we can see two apparent particle sizes although the sample was expected to be monomodal. Switching to a measurement using the horizontal polarizer shows that the intensity of the smaller component signal is much larger. From this we can infer that this represents rotational diffusion from a non-spherical particle. TEM imaging of these particles did indeed show that some of these particles were rod like.

Measured particle size distribution for a sample of gold nano particles. Without a polarizer we see an extra size population at around 20 nm. The intensity of this signal is much stronger when measured using a horizontal polarizer showing that this is caused by rotational diffusion. TEM images show some rod like particles in this sample.

Can DLS measure particle shape then?

Converting rotational diffusion to length and width is not straight forward. Calculating even an aspect ratio needs some additional information and assumptions which do not apply to all samples. This means that the Zetasizer Advance software doesn’t include any tools to calculate particle shape. If you would like to give these calculations a go, the the software will give you diffusion coefficients you need once you have measured your sample with the two different polarizers.

Additional resources