Can Carbon Nanotubes Deliver on their Promise in Biology? Harnessing Unique Properties for Unparalleled Applications

Carbon nanotubes (CNTs) are cylindrical sheets of hexagonally ordered carbon atoms, giving tubes with diameters on the order of a few nanometers and lengths typically in the micrometer range. They may be single- or multiwalled (SWCNTs and MWCNTs respectively). Since the seminal report of their synthesis in 1991, CNTs have fascinated scientists of all stripes. Physicists have been intrigued by their electrical, thermal, and vibrational potential. Materials scientists have worked on integrating them into ultrastrong composites and electronic devices, while chemists have been fascinated by the effects of curvature on reactivity and have developed new synthesis and purification techniques. However, to date no large-scale, real-life biotechnological CNT breakthrough has been industrially adopted and it is proving difficult to justify taking these materials forward into the clinic. We believe that these challenges are not the end of the story, but that a viable carbon nanotube biotechnology is one in which the unique properties of nanotubes bring about an effect that would be otherwise impossible. In this Outlook, we therefore seek to reframe the field by highlighting those biological applications in which the singular properties of CNTs provide some entirely new activity or biological effect as a pointer to "what could be"

Three-dimensional reconstruction of individual helical nano-filament structures from atomic force microscopy topographs

Atomic force microscopy, AFM, is a powerful tool that can produce detailed topographical images of individual nano-structures with a high signal-to-noise ratio without the need for ensemble averaging. However, the application of AFM in structural biology has been hampered by the tip-sample convolution effect, which distorts images of nano-structures, particularly those that are of similar dimensions to the cantilever probe tips used in AFM. Here we show that the tip-sample convolution results in a feature-dependent and non-uniform distribution of image resolution on AFM topographs. We show how this effect can be utilised in structural studies of nano-sized upward convex objects such as spherical or filamentous molecular assemblies deposited on a flat surface, because it causes ‘magnification’ of such objects in AFM topographs. Subsequently, this enhancement effect is harnessed through contact-point based deconvolution of AFM topographs. Here, the application of this approach is demonstrated through the 3D reconstruction of the surface envelope of individual helical amyloid filaments without the need of cross-particle averaging using the contact- deconvoluted AFM topographs. Resolving the structural variations of individual macromolecular assemblies within inherently heterogeneous populations is paramount for mechanistic understanding of many biological phenomena such as amyloid toxicity and prion strains. The approach presented here will also facilitate the use of AFM for high-resolution structural studies and integrative structural biology analysis of single molecular assemblies

Supramolecular Behaviour and Fluorescence of Rhodamine-Functionalised ROMP Polymers

Inherently fluorescent polymers are of interest in materials and medicine. We report a ring-opening metathesis polymerisation (ROMP) platform for creation of amphiphilic block copolymers in which one block is formed from rhodamine B-containing monomers. The polymers self-assemble into well-defined micelles which are able to sequester molecular dyes and further interact with them by energy transfer. Despite incorporating a cationic dye known to bind DNA, the polymer micelles do not interact with DNA, indicating that they are potentially safe for use in bioanalytical applications

Intermolecular interactions in bromo-, methyl-, and cyanoimidazole derivatives

Materials containing bistable N-H�N hydrogen bonds, such as imidazole crystals, show promise for applications in electronics. Herein, we examine the effect of imidazole functionalization upon structural parameters relating to proton transfer, molecular rotation, and order-disorder transitions. Three different substituents are studied: methyl-, bromo-, and cyano-, resulting in steric, electronic, and supramolecular modification of the imidazole core. © 2013 American Chemical Society

A meta-xylenediamide macrocycle containing rotaxane anion host system constructed by a new synthetic clipping methodology

A novel rotaxane containing a meta-xylenediamide macrocycle is prepared by a new clipping methodology. Upon anion metathesis to the non-coordinating hexafluorophosphate salt, the rotaxane host system was shown to bind chloride and bromide anions more strongly than the basic oxoanions dihydrogen phosphate and acetate in the competitive solvent system 1:1 CDCl 3:CD 3OD. © 2011 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique

Extending the family of heteroditopic calix[4]diquinone receptors for cooperative and ion-pair recognition

New heteroditopic calix[4]diquinone receptors with different anion binding units and calix[4]diquinone scaffolds have been synthesised. Ion-pair binding studies using UV/visible and 1H NMR spectroscopies reveal that receptors 1, 2 and 4 are cooperative AND ion-pair receptors, which display little affinity for 'free' ions but enhanced binding of the ion-pairs NaCl, NH 4Cl and KCl. The more preorganised receptor 2 binds the ion-pairs more weakly than receptor 1. Varying the nature of the calix[4]diquinone scaffold appears to have little effect on ion-pair binding, although the calix[4]diquinone framework is more conformationally flexible than the tert-butylcalix[4]diquinone one and can be synthesised using the milder oxidant chlorine dioxide. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique

Chloride anion triggered motion in a bis-imidazolium rotaxane

We report the first bis-imidazolium-containing rotaxane, synthesised via anion templated self-assembly. Its co-conformation is controlled by a chloride anion recognition mechanism, thus demonstrating the viability of this protocol as a stimulus for shuttling molecular motion. © 2011 The Royal Society of Chemistry

Cation-induced molecular motion of spring-like [2]catenanes

The syntheses and cation recognition studies of two novel heteroditopic [2]catenanes that are capable of reversible rotary motion are described. Prepared by chloride anion templation, both catenanes possess a calixdiquinone unit able to bind Na+, K+, NH4 + and Ba2+ cations. Following characterization of thesalts of both catenanes by NMR, mass spectrometry and crystal structure determination, hexafluorophosphate salts were investigated for their behaviour upon addition of cations. Ba2+ complexation caused a partial intra-ring rotation of the two macrocycles in both species, which was reversed upon addition of the sequestering SO4 2- anion. © The Royal Society of Chemistry 2011

Iodo-imidazolium salts: Halogen bonding in crystals and anion-templated pseudorotaxanes

The 2-iodoimidazolium group is exploited in the anion-templated assembly of pseudorotaxanes with isophthalamide containing macrocycles. Crystallographic and solution-phase studies illustrate that the iodo-imidazolium motif is a potent halogen bond donor, forming the most stable interpenetrated assemblies in solution with the chloride anion template. © 2013 The Royal Society of Chemistry

A new synthetic route to chloride selective [2]catenanes

A novel anion templation route has been developed to synthesise two new catenanes, which are observed to selectively complex chloride in protic solvent media. © 2011 The Royal Society of Chemistry