Shining example

It might look fine on the surface, but hidden inside a product's nanoscale nooks and crannies could be detrimental contaminants and aggregates.

Several methods have been used over the years to visualise and study product nanoparticles before they leave the manufacturing line but many of these techniques give only an average of particle size distributions in product samples — meaning some contaminants may get through undetected.

A UK company may have solved the problem with the development of an instrument for individually measuring and visualising all nanoparticles within a sample in suspension in real-time. The technology, from

, has already been used by a major international defence contractor for virus detection, and the Wiltshire company is in the process of expanding its services to a broad range of manufacturing industries.

The device can be used close to the point of manufacture, without the need for lengthy sample preparation procedures. A sample can be viewed and analysed from the production line by suspending and diluting its particles in water, for instance, and loading 0.5ml via syringe into the device.

The technology comprises a metallised optical plate illuminated by an 80 micrometre-diameter laser beam. Nanoscale particles in suspension can be directly visualised, sized and counted in real-time on the plate surface using a conventional optical microscope fitted with a low-cost camera and an analytical software package.

The system can be used to study the contents of a wide range of products such as ink and paint pigments, metal oxides in magnetic storage media, cosmetics, polymers, emulsions and colloids.

Nanosight's testing method is similar to Photon Correlation Spectroscopy (PCS), which has been the industry standard for particle testing for almost 30 years. In this method, a cloud of particles is illuminated in a liquid, and a PCS system detector collects light from the cloud of particles and analyses the way the intensity fluctuates. As the particles move, the light intensity flickers, and the detector analyses the speed at which the flicker occurs. From this, the system is able to calculate the path and size of all the particles, giving an average size of the population.

Where Nanosight's system differs, however, is that it does away with a detector looking at light scattered from a dark cloud. Instead, it looks into a beam of light with a camera, and videos all the particles (magnified by 1,000) in the path of the beam, tracking each individual particle simultaneously. From this, the system's software can calculate the measurements of each individual particle in a sample.

Bob Carr, Nanosight's chief technical officer, said individual profiles give much more useful information than average profiles because average figures are often weighted toward the biggest and brightest particles in a sample.

He gave a hypothetical scenario in which a PCS instrument operator is looking for a 70nm particle in a sample that is determined to have an average particle size of 500nm. The danger is 'it might just be bits of 500nm fluff or rubbish floating around, and they never get to see the 70nm particle they were looking for because the 500nm particle blinds the detector to everything but that,' he said.

Yet, Carr admitted there are some limitations to Nanosight's technology. 'We can't see particles as small as PCS can,' he said. 'Our lower size limit is about 10-15nm, while PCS can go down to about five.

'But there aren't that many people that work that low. Most, like those in the paints and pigments industries, work in a 30nm-plus range.'

Nanosight's total nano-particle analysis range is 10-600nm.

The system's technology is also limited in that particles are measured only 2D, while the particles move in 3D within the sample. However, said Carr, this is irrelevant. 'We still get the right information. We just modified our software to accommodate this.'

With cost being one of the most important considerations for firms deciding on testing equipment, Carr said his company's relatively simple equipment means the system cost significantly less than PCS, which uses large, expensive correlators.

'We're about £13,000,' he said, 'whereas an entry-level PCS will be at least twice that.'