Nanoplates get in shape
A quick, easy way to make silver nanoplates just a few atoms thick has been devised by chemists in China.
Visible light can interact with free electrons in silver nanoplates - making them promising candidates for applications such as optical probes or contrast agents in biomedical imaging. But controlling their shape and size is critical if they are to be put to use.
Triangular (left) and hexagonal (right) silver nanoplates
Previously, creating uniform silver nanoplates has required costly or dangerous reagents and long reaction times. Now, a team led by Jiannian Yao at the Beijing National Laboratory for Molecular Sciences, China, has developed a reaction that occurs at room temperature and is complete in less than 15 minutes.
Yao's process reduces silver oxide with hydrazine in the presence of trisodium citrate (TSC) and EDTA, a multi-dentate ligand. The EDTA forms complexes with the Ag+ ions in solution, slowing down the reduction reaction and allowing nanoplates of silver metal to form. Meanwhile, the TSC molecules prevent the tiny plates, which are 60-200nm wide and only around 11nm thick, from sticking together into bigger clumps.
By varying the amount of EDTA used in the reaction, the team reports that the size and shape of nanoplates can be fine-tuned. This is important because differently shaped nanoplates interact with different wavelengths of light - giving them different properties and colours.
'This is a nice addition to the existing pool of synthesis techniques,' says Luis Liz-Marzán, an expert on silver nanoparticles at the University of Vigo, Galecia, Spain. But more work must be done to ensure that triangular or hexagonal plates can be made very selectively, he adds.
Aibing Yu, who works on nanoplate synthesis at the University of New South Wales in Sydney, Australia, agrees. 'This work is exciting as it highlights the link between particle morphology and functional properties,' he says. 'The fundamentals underlying the growth mechanism and shape control should be further explored.'
Z Cao et al, J. Mater. Chem., 2008, DOI: 10.1039/b800691a