A colourful way to size up nanoparticles

A property of nanoparticles commonly exploited in a wide range of  applications can be turned on the nanoparticles themselves to give scientists an estimate of the particles' size. Researchers in China have now developed a straightforward light-scattering technique to estimate the size of gold nanoparticles in the 35 to 110nm range.

Nanoparticles composed of noble metals are at the forefront of advances, both experimental and practical, optics, sensors and biomedical research. In nanotechnology, size really does matter and techniques for measuring nanoparticles are essential to progress.

Yi-Tao Long, of East China University of Science and Technology in Shanghai, and colleagues have exploited the colour of light scattered from the surface of gold nanoparticles due to plasmon resonance. This chrominance can be viewed using dark-field microscopy. The peak levels of red, green and blue light scattered by the nanoparticles correlate directly with their diameters. A Matlab program developed by the team does the number crunching to convert wavelengths into particle size.

Nanoparticles can be sized thanks to the localised surface plasmon resonance (LSPR) effect observed in gold, silver and copper particles. This phenomenon arises as incident light interacts with surface electrons and is scattered. It forms the basis of many of the applications of nanoparticles in photonics, for instance.

The team explains that the approach is more convenient and less time-consuming than other techniques. They add that it is possible to use the technique in real-world environments rather than requiring pristine laboratory conditions and a scanning electron microscope. And they have now validated their size estimates against precise measurements and claim that an error margin of less than 5 nm is possible.

'I do believe this work represents a very elegant new method for estimating gold nanoparticle size according to their light scattering optical properties,' Qun Huo of the University of Central Florida tells Chemistry World. Huo specialises in the synthesis and properties of nanoparticles. 'The principle behind the method has been known within the community for a while,' she adds. 'However, the authors of this paper have managed to introduce mathematical software to analyse the optical properties and so correlate the information with nanoparticle size. This new tool could be very valuable in further advancing gold nanoparticle-enabled bioimaging and chemical sensing applications.'

Long says: 'In order to extend the application, we will improve the theoretical calculations for other metallic nanoparticles, including gold, silver, copper nanoparticles of different size and shape.' The method will be limited by the scattering intensity of light, which is directly proportional to the sixth power of the nanoparticle's diameter, Long explains. 'For smaller nanoparticles, the scattering light is very insignificant,' he says. 'We could observe smaller nanoparticles by improving the detector sensitivity and the light source power.'

David Bradley