University of Bielefeld, Germany

A chemist finds beauty in molecules that resemble an early model of the Solar System.

Since Plato's time, people have been fascinated by the beauty of highly symmetrical objects. The symmetry of the C60 buckyball surely contributed to scientists' tremendous interest in this spherical molecule. Indeed, I was convinced that the discovery of C60 would induce a rush among chemists to search for other symmetrical structures.

That rush may not have happened, but scientists have still turned up some surprising highly symmetrical structures. A recent report from researchers at Xiamen University in China (X.-J. Kong et al. J. Am. Chem. Soc. 129, 7016–7017; 2007) describes a cluster in which beauty cages beauty; it consists of an icosidodecahedron of nickel ions, having 20 triangular faces and 12 pentagonal faces, inside of which sits a dodecahedron of lanthanum ions.

The team describes the magnificent structure as 'Keplerate', a term that I and my colleagues first used around ten years ago to describe structures that contain Platonic and Archimedean solids (regular polyhedra, and polyhedra with two types of face, respectively) one inside another, like Russian dolls. It honours Johannes Kepler, who in the sixteenth century developed a model of the cosmos in which "the radii of the successive planetary orbits are proportional to the radii of spheres that are successively circumscribed around and inscribed within the five Platonic solids".

Another recent report found these same shapes — the icosidodecahedron and dodecahedron — in Keplerate-type arrangements in quasicrystals (H. Takakura et al. Nature Mater. 6, 58–63; 2007). Such crystals are still poorly understood. I hope that future work will correlate these materials' properties with their beauty.

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University of Hyderabad, India

A chemist applauds an algorithm able to predict crystal structures from chemical composition alone.

I work in crystal engineering, a field that involves designing and constructing crystals with desired physical, chemical or pharmaceutical properties from small organic molecules. It is an experimental science based on pattern recognition and retrosynthetic strategies, in which the structure is considered as the sum of smaller, simpler parts.

Improvements to computational crystal-structure prediction could make design protocols more reliable. But this is such a difficult problem that only a handful of groups in the field work on it. In this context, I found a recent paper presenting a seemingly reliable method to be thought-provoking (A. R. Oganov and C. W. Glass J. Chem. Phys. 124, 244704; 2006).

Typically, crystal-structure prediction involves computer generation of putative crystal structures using a force field, which represents the interactions between atoms in neighbouring molecules. The correct structure is presumed to be that which minimizes the crystal's energy.

The procedure is problematic because the force fields may not be well tailored to the molecules being studied, and because the experimental structure may not be the lowest-energy arrangement. It is also impossible to explore all conceivable structures, which are mind-boggling in number.

Oganov and Glass use an evolutionary algorithm to localize the search to the most promising structures. Their approach is attractive in that it requires no system-specific knowledge — the input is just the molecule's chemical composition, not even its structure — and their ability to predict the unusual tetragonal structure of urea is impressive.

Is this the long-awaited breakthrough in crystal engineering? Perhaps not, but surely it's an important step forward.

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Nagoya University, Japan

A theoretical chemist compares love to hydrogen bonds.

Water molecules assemble into ice "palm to palm", like Romeo and Juliet on their first encounter. Each molecule reaches out to four neighbours, forming hydrogen bonds that lock the molecules into a tetrahedral network. And like the love of Shakespeare's pair, water's hydrogen bonds are resilient. Ice contrives to keep its network, even in the tightest of spaces.

Researchers recently predicted that ice constrained by a carbon nanotube's wall will form either tubular structures or intricate arrangements of double- and quadruple-stranded helices, depending on temperature, pressure and nanotube diameter (J. Bai et al. Proc. Natl Acad. Sci. USA 103, 19664–19667; 2006).

I have spent many years studying the structure and dynamics of water, but am still amazed by these luxuriant ice structures. Had computer simulations not shown how strenuously ice's network can adapt for its molecules to keep their four hands touching, we could hardly have imagined such structures would be possible.

Simulations have also predicted that confined ice can have two symmetrically different phases, which become deformed and indistinguishable when put under pressure (K. Koga et al. Nature 412, 802–805; 2001). So we expect that one type of ice will easily transform into the other through collective motion of its hydrogen bonds.

My prediction is that confined liquid water, which has a disordered network of hydrogen bonds, will undergo similar structural rearrangements. Molecular mechanisms may cause large changes to the network structure of water trapped in proteins or at membrane surfaces, for example. These studies could therefore help us begin to understand another intimate relationship — the relationship between water and life.

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Princeton University, New Jersey, USA

A chemical engineer is struck by the strange properties of 'patchy' colloids.

A recent paper about the behaviour of colloids makes an intriguing prediction — suggesting that they can adopt an 'empty' liquid state.

I study disordered states of matter, such as liquids and glasses. I find colloids interesting because they make phenomena such as crystal nucleation and the glass transition amenable to direct observation. Nanometre- or micrometre-sized particles suspended in liquids are wonderful model atoms. They arrange themselves in the same way that atoms and simple molecules do into solids, liquids or gases.

But controlling the interactions between colloidal particles provides a window into structural and thermodynamic behaviour beyond that found in atomic systems, as this recent theoretical paper shows (E. Bianchi et al. Phys. Rev. Lett. 97, 168301; 2006).

It maps the phase diagrams of 'patchy' colloids. The particles in such colloids are decorated with sticky spots, which tend to bond them together. As the number of bonded neighbours per particle is reduced towards two, the phase diagrams predict liquid states with a vanishing packing fraction. This means the colloidal particles occupy a tiny fraction of the available space — but they still behave as a liquid that is distinct from the gas-like phase of still lower packing fraction.

The low-temperature behaviour of such 'empty' liquids is especially interesting. The calculations suggest that cooling the colloid can freeze in place the empty configuration to give a glassy state of arbitrarily low density.

These predictions have not been tested experimentally. But chemists have already developed techniques for making patchy particles, so the work of Bianchi et al. could guide experimentalists in their exploration of this fascinating form of matter.

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Massachusetts Institute of Technology, Cambridge, USA

A chemist predicts a bright future for sensors based on carbon nanotubes.

I am struck by the parallels between the development of polymer-based chemical sensors and those made from carbon nanotubes.

About ten years ago, I started to develop sensors from conjugated organic polymers, which took advantage of the materials' optical properties, rather than the electrical properties that had been exploited in devices until that time. This work led to fluorescent sensors, which are now being used in Iraq to detect explosives.

As with polymers, early work on nanotube sensors focused on detecting changes in a tube's electrical conduction when it binds to a molecule of interest. But electrical responses are sensitive to stray electric fields, which create interference in the signal.

Now, researchers working with nanotubes are also moving towards optical methods. A demonstration of a biosensor for glucose (P. W. Barone and M. S. Strano Angew. Chem. Int. Edn 45, 8138–8141; 2006) sets the stage.

To make the sensor, the team first attached glucose groups to nanotubes. They then mixed these nanotubes with a large molecule, known as concanavalin A, which can bind to four glucose molecules at once. The glucose-decked nanotubes end up caught in clumps around the concanavalin A, which attenuates their emission. This system is sensitive to glucose because any glucose in solution loosens the nanotube clusters, and so boosts fluorescence.

A significant advantage of nanotubes is that they emit near infrared light, a longer wavelength than that accessible with polymers. And it just happens that human tissue is almost transparent in this spectral region. As a result, sensors based on these materials might be used for in vivo clinical diagnostics.

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Rensselaer Polytechnic Institute, Troy, New York

Childhood memories cause a nanotechnologist to go nuts for plant-derived nanomaterials.

As a child growing up in Kerala, southern India, I marvelled at the unusual cashew fruit, with its kidney-shaped nut dangling from a swollen apple.

Since then, nanotechnology has become my passion. So it was with a curious mix of scientific interest and childhood memories that I read a recent paper describing how nanomaterials could be derived from plant sources such as the cashew nut.

I had never thought of a cashew nut as anything more than a food item. However, a little research reveals that cashew-nut-shell liquid, rich in natural long-chain phenols, already has applications ranging from hydrophobic coatings to anti-ageing creams.

George John and Praveen Kumar Vemula at the City College of New York, in their recent article (G. John & P. K. Vemula Soft Matter 2, 909–914; 2006), show how cashew-nut-shell liquid can also serve as a starting material for a variety of nanostructures.

The oil contains molecules that have phenol groups for heads, and long hydrocarbon tails. These can form structures such as lipid nanotubes and twisted nanofibres.

To make this happen, the molecules' structure is first modified by attaching water-loving sugar groups to the phenols. The cooperative effect of head groups hydrogen bonding and the hydrophobic interactions of the tails leads the molecules to self assemble into bilayers. These then further organize into the fibres and tubes.

Using a similar strategy, it should be possible to develop a wide range of novel soft nanomaterials from other plant resources. The breadth of precursors available in our plants and crops should inspire all nanotechnologists — not just those fond of cashew nuts.

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