Chemistry
The power of attraction
Hybrid organic-inorganic materials can self-assemble into tiny doughnut-like structures.


Coordination-driven self-assembly leads to the formation of micrometer-scale toroids.
© 2016 KAUST Xavier Pita
Engineered nanometer- and micrometer-scale structures have a vast array of uses in electronics, sensors and biomedical applications. Because these are difficult to fabricate, KAUST researchers are trying a bottom-up philosophy, which harnesses the natural forces between atoms and molecules such that microstructures form themselves.
This approach, a departure from the usual top-down approach, involves the etching away of material to leave the desired sculpted structure behind; however, because this approach can be tricky, expensive and time consuming, KAUST researchers became motivated to find a new approach.
Associate Professor of Chemical Science Niveen Khashab and her team and colleagues from the Imaging and Characterization Core Lab and the Max-Planck-Institute of Colloids and Interfaces in Germany demonstrated this bottom-up approach in the selfassembly of microscale toroids (doughnut-shaped forms), made of both inorganic and organic materials1.
A number of forces can bring atoms and molecules together. These include surface tension, electrostatic attraction and repulsion, and a weak fundamental force known as van der Waals interactions. The toroids created by Khashab’s team were formed via metal coordination. A metallic sodium chloride atom, an amphiphilic (both hydrophilic and lipophilic) molecule called saponin and a polymer known as chitosan were combined and formed weak chemical bonds.
This is a result of what is known as coordination-driven selfassembly,” explained Khashab. “The metal ions interact with different chemical motifs, leading to the formation of novel frameworks and morphologies.”
Within just a few minutes, coordination bonding between the iron atoms and the oxygen and the hydrogen in the molecules initially drives the selfassembly of star-like nanostructures. Repulsive electrostatic and hydrophobic interactions then lead to the formation of toroids.
The toroids were approximately 3.9 to 4.8 micrometers in diameter and held their shape even a month after fabrication. Disassembly of the microstructures required five hours of mechanically stirring the solution.
There are numerous naturally occurring biological structures that take a toroid shape; for example, proteins and DNA of some types of viruses and bacteria self-assemble in this way. Many of these are known to play an important role in the formation of pores in biomembranes.
This research could help to build a better understanding of how these complex biostructures are created and provide a way of mimicking them at the molecular level.
“Next, we hope to prepare a new generation of these hybrid structures with a temperature-responsive gap size,” said Khashab. “These toroid structures could be used as pockets for active catalysis and separation.”
References
- Al-Rehili, S., Fhayli, K., Hammami, M. A. … Khashab, N. M. Anisotropic self-assembly of organic-inorganic hybrid microtoroids. Journal of the American Chemical Society advance online publication, 24 October 2016.| article
You might also like

Chemistry
Saving desalination membranes from minerals and microbes

Chemistry
Sensing water for smarter agriculture

Chemistry
Tapping into seawater's energetic potential

Chemistry
Taking salt out of the water equation

Chemistry
Getting more out of light

Chemistry
Molybdenum caught holding the hydrogen

Chemistry
Shaping the future of purification

Chemistry