Weak but necessary
Supramolecular assemblies formed by reversible non-covalent bonds have a range of unique optical, chemical and rheological properties that are of interest in fields such as biomedicine and pharmacology. Achieving long-range order over multiple length scales is essential if the properties of these assemblies are to be harnessed on the macroscopic level. But such ideal order has been difficult to achieve because the self-organization processes responsible are not yet fully understood. Laurent Simon and colleagues (Physical Review Letters 94, 066103; 2005) have now followed the individual steps of self-organization of a supramolecular assembly by scanning tunnelling microscopy (STM). The researchers find that long-range ordering in films of bis-urea-substituted toluene on a Au(111) surface is reliant on weak lateral coupling between the supramolecular polymers that result from hydrogen bonding between the molecules. At the onset of the ordering process, the pairs of polymers are randomly oriented with respect to the substrate, indicating that the coupling to the substrate is fairly weak. Cooperative behaviour of large supramolecular assemblies of such pairs of polymers then leads to a 'locking in' with the substrate lattice. This occurs without perturbing either the pairing of the polymers or the underlying reconstruction of the gold surface. Intriguingly, the large-scale ordered assembly is transparent to electron tunnelling under specific voltage and current conditions, revealing both the molecular assembly and the underlying substrate in STM images.
Collagen in a bind
There is increasing interest in the use of synthetic tissues and organ-replacement therapies, which often necessitates the use of collagen. Allen Wang and colleagues (Journal of the American Chemical Society; http://dx.doi.org/10.1021/ja0431915) have developed a 'physical' technique to functionalize collagen, which is based on its ability to form a unique right-handed triple-helical architecture. Functionalization is usually achieved by chemically coupling desired molecules to amino acid side chains, but this proves difficult to control, and not suitable for collagen scaffolds that contain live cells or tissues. The method by Wang et al. uses collagen mimetic peptides (CMPs) that are composed of multimers of peptides known to form triple helices. The CMPs were found to bind to natural or denatured collagen by a strand-exchange reaction and triple-helix association. Because CMPs also bind to other bioactive components, the CMP-collagen link provides a convenient way to add further functionality to collagen, and therefore a new method for engineering tissues.
Painless tooth restoration
Bid farewell to painful dentist sessions and unsightly tooth fillings. If dentists take up the new hydroxyapatite paste developed by Yamagishi and colleagues in Japan (Nature. 433, 819; 2005), caries could be stopped at a very early stage with a quick treatment that simply entails masterly application of the paste. This paste is easily prepared by mixing a fluoride-enriched apatite powder with a phosphoric acid solution containing hydrogen peroxide. The authors watched the progressive growth of new hydroxyapatite crystals on tooth enamel with an atomic force microscope, and saw the surface completely covered with new crystals after just three minutes. After 15 minutes the paste forms a 20-μm-thick layer that is hard and resistant to acid attack. Given that acidity is at the origin of caries, this treatment is not only a soft painless way to catch and halt the early stages of tooth decay — when the drilling and removal procedure is not ideal — but may also prevent their reoccurrence.
The 299,792,458 metres per second at which light travels through free space makes it ideal for transmitting large amounts of information over long distances and at high speed. But when it comes to processing this information at either end of an optical transmission line, it is often desirable to be able to slow it down to a more leisurely pace — such as to avoid problems caused by heavy traffic on an optical network. Writing in Physical Review Letters (94, 073903; 2005). Henkjan Gersen and colleagues not only manage to slow light down by a thousand times by passing it through a photonic-crystal waveguide, but actually record its progress through this material by a series of high-speed images taken with a near-field scanning optical microscope. As well as providing an elegant and unequivocal demonstration of the slowing of light in a photonic crystal, these images provide important insight into the interaction of light and matter in this and similar structures. For instance, for more than 3.6 ps after the passage of a light pulse through their waveguide, the authors observe a persistent localized optical mode, which could represent a new means to trap and manipulate light.
The controlled deflection of cantilevers by external stimuli is currently being investigated for the development of miniaturized devices such as robots, machines, valves or sensors. In a biological context, the coupling of an amplification path to recognition events is particularly attractive as a general method to enhance the sensitivity of biosensors. Itamar Willner and colleagues now report in Nano Letters (http://dx.doi.org/10.1021/nl050204j) on an amplified magnetomechanical methodology used for the highly sensitive detection of specific activities of endonucleases. Nucleic acids are immobilized on gold-coated cantilevers and hybridized with complementary nucleic acids associated with magnetic particles. The cantilevers positioned in a flow cell are subjected to an external magnetic field, leading to the deflection of the cantilevers. Addition of endeonucleases result in the sequence-specific scission of the respective endeonuclease DNA, and to the release of the magnetic particles, a process that leads to the retraction of the cantilever to its original state.
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From painless teeth to binding collagen. Nature Mater 4, 263 (2005). https://doi.org/10.1038/nmat1370