Smaller than ever nanoparticles can be studied using new microscopy methods.

The smallest single metal nanoparticle that has been detected is 1.4 nanometres in diameter.  Michel Orrit at the University of Leiden, the Netherlands, and colleagues at the University of Mainz, Germany, and University of Bordeaux, France, say this limit is about to be pushed to even smaller sizes.

The researchers say advances in the detection of single metal nanoparticles have come with new techniques based on optical microscopy.

One very sensitive method detects nanoparticles through changes in the particles' immediate surroundings. Excited metal nanoparticles absorb light very efficiently. This causes the temperature of the particle's surroundings to increase resulting in a detectable change in the refractive index.

'This method is very promising for bioscience applications such the development of new, non-fluorescent labels for single biomolecules such as proteins,' said Orrit. 

According to Orrit, scientists will be able to study the more unusual physical properties of single nanoparticles, which 'often differ spectacularly from those of the same materials at larger scales.' Studying isolated particles 'eliminates possible interaction effects between particles and prevents a problem always present in ensemble measurements; averaging over different sizes, shapes and structural defects,' he said.

Gregory Hartland, professor of chemistry at the University of Notre Dame, Indiana, US, agreed. 'Single molecule spectroscopy is one of the few research areas where new science is being discovered,' said Hartland. 'These are very exciting developments, and much more will surely be done in the coming years by many groups around the world.'

Susan Batten