Zetasizer Nano and nano particles

The Zetasizer Nano series measures the size of particles in a fluid down to less than a nanometre by observing the thermal movement, or Brownian motion, of the particle. Submicron particle sizes are measured by observing the scattering of laser light from these particles, determining the diffusion speed and deriving the size from this, using the Stokes-Einstein relationship. This method is called Dynamic Light Scattering (DLS).

Measuring the size of nano particles
There are few analytical instruments that can easily measure the size of dispersed particles when the size approaches a nanometre. They are all either imaging techniques, such as electron microscopy, or atomic force microscopes, that produce a scan that looks like an image of the material.

Both of these techniques are expensive in terms of the capital cost of equipment, and in terms of the time and expertise required to produce a useful "image".

They also suffer from the statistical problem that only a few particles are observed, unless very many experiments are done.

One subtlety that is very often missed and causes problems in data interpretation is that if a size is calculated, it is naturally done on the basis of the mean of a number distribution. Calculating the number mean involves adding the number of particles of a particular size to a size ‘bin’, and the average size calculated from this distribution, which gives equal ‘weight’ to each particle.

This means that as there are usually many more small particles than large ones in a real world sample, the result is weighted towards the size of the most numerous particles present, the small ones. This is fine if the size of the small particles is required. However if the exercise requires a mean of the volume of the particles, or is to detect the presence of aggregates, then counting numbers of particles is a poor technique.

Dynamic Light Scattering (DLS)
As Dynamic Light Scattering is sensitive to the intensity of light scattered by particles, and larger particles scatter more light than small particles, then the DLS is very sensitive to the presence of aggregates, and hence this technique is an excellent basis for studying the stability of submicron particle dispersions.

Why do we need a sensitive system?
Sensitivity is important for the measurement of submicron particles, as the amount of light scattered reduces according to the sixth power of the diameter. In other words a 1nm particle scatters one millionth the amount of light than a 10nm particle.

The Zetasizer Nano uses patented NIBS™ technology to increase the detection sensitivity. When combined with the high sensitivity of the Avalanche Photodiode Detector (APD) with a quantum efficiency of greater than 60% (compared to a quantum efficiency of about 4% for traditional photomultiplier detectors), this makes the system the most sensitive available.

Without this level of sensitivity, it would be easy to measure the size of a 20nm `zor 100nm particle, but less than 10nm would be difficult, and 1nm would be impossible.

Even with equal volumes of particles, 1000 time greater sensitivity is required to measure a 1nm particle compared with one of 10nm.

To get the increase in sensitivity provided by the Zetasizer Nano using an increase in laser power, would require the use of a 4 Watt laser! This has serious implications in cost, and for the laboratory as a whole in term of the size of the unit, the power required and the maintenance. It would be far from the ideal of a small low cost bench top unit.

In addition to this, illuminating particles with such high power can affect the particle in a number of ways, one being absorption of power which will affect the diffusion speed and the local viscosity of the medium, both of which will cause errors. So increasing the laser power is not a method of choice.

The sensitivity of the Zetasizer Nano alone is not enough to justify it as a useful technique, it must be easy to use as well. The NIBS technique enables dilute and concentrated systems alike to be measured, with the set-up adjusted entirely automatically.

The software has been designed to provide the features required by a research user, without compromising the usability for everyday size measurements. One of the ways this is achieved is the use of predefined measurement schemes, (SOP’s) which make measurements a one click operation.