A news piece published in the July / August issue of Materials Today (Vol. 16 No. 7 – 8) this month – currently available here: http://digital.materialstoday.com/
(PDF of my article here: JulAug13 – MatTod – 258-259 )
Polymer ‘Stars’ Hold the Key to Monodisperse Colloidal Nanocrystals
A new way to produce high-quality nanocrystals using tiny star-shaped reaction vessels has been developed by researchers at Georgia Tech in the US.
Over the past decade, colloidal nanocrystals have found applications in many fields including drug delivery, electronics and catalysis, and so efforts to produce high-quality crystals have gained momentum. Existing techniques are reliable, but often time-consuming, requiring tightly-defined experimental conditions such as multistep reactions and purifications. And each of the established techniques, such as sol-gel processes, can only produce a single type of nanocrystal, limiting their applicability.
A Nature Nanotechnology paper from a team of materials scientists at Georgia Tech has reported on a new route to producing high-quality nanocrystals using star-shaped block co-polymer structures as reaction vessels. The team have produced nearly monodisperse magnetic, metallic, ferroelectric, semiconductor and luminescent colloidal nanocrystals, all using these tiny star-shaped vessels.
The co-polymer ‘stars’ consist of a central beta-cyclodextrin core to which multiple “arms” – as many as 21 linear chains of poly(acrylic) acid (PAA) – are covalently bonded. Together, they form the aggregate of molecules that serve as a reaction vessel and template for the formation of the nanocrystals.
In this technique, the size of the nanocrystals (between a few nanometers and tens of nanometers) is determined by the length of the hydrophilic PAA chain. More surprisingly was the team’s finding that the volume ratio of the two solvents – dimethlformamide and benzyl alcohol – had a profound influence on the shape uniformity of the nanocrystals. For lead titanate (PbTiO3), a solvent ratio of 9:1 was found to produce a dispersion with a size distribution within 5% of the average size.
Zhiqun Lin and his team further tailored the technique to produce plain, core-shell and hollow nanoparticles that can be made soluble either in organic solvents or water – leading to an interest from biomedical researchers. The colloidal dispersions of these nanocrystals were also found to be stable and homogenous over long durations – as much as two years – without any sign of precipitation
The versatility and robustness of this technique, and the stability of the nanocrystals produced has already won the team wide acclaim, but Ling hopes to extend it further to include more complex structures such as nanorods and crystals with multifunctional shells.
Nature Nanotechnology (2013) doi:10.1038/NNano.2013.85