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Polydiacetylenes: Functional supramolecular smart materials

27 Jan 2012

Polydiacetylene polymers (PDAs) are a popular research topic for polymer and materials chemists due to their interesting optical, spectral, electronic and structural properties. Jong-Man Kim and colleagues’ recent Feature Article gives a detailed overview of the diverse range of structural morphologies and the related functional properties featured by PDAs in recent years and highlight their importance in sensor and display technologies.

Interactions between the diacetylene (DA) monomers before polymerisation can directly influence the final polymerised structure. The monomers can be functionalised to contain motifs that encourage non-covalent interactions such as hydrogen bonding, π-stacking, electrostatics and hydrophobic interactions, allowing the DAs to self-assemble into nanostructures. Subsequent polymerisation of the acetylene groups results in cross-linking within the nanostructure, forming new materials with striking properties. One particularly interesting example of this is shown below – the DA monomers are substituted with long hydrophobic chains and polar head groups which assemble in water to form vesicles. Shining UV light on the vesicles causes the diacetylene groups to polymerise, generating PDA vesicles which are blue in colour.

For an in-depth and fascinating overview of the recent conceptual and technological advances in the chemistry of PDAs, download the full Feature Article here.

Posted on behalf of Cally Haynes, web science writer for ChemComm.

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Germanium-bridged polymer for organic solar cells

25 Jan 2012

A team of scientists from the UK and US have made a novel germanium-bridged polymer that shows promise for organic solar cells.

Martin Heeney, at Imperial College London, and colleagues synthesised a heterocyclic monomer bridged with two germanium atoms. They co-polymerised it with an electron-accepting benzothiadiazole to give a polymer that, in contrast to the analogous C-bridged system, is semicrystalline.

Being able to design crystallinity into conjugated polymers in such a way is useful because crystalline polymers are better at transporting charge and hence offer more potential for solar cells. Heeney’s polymer exhibited power conversion efficiencies of over 5 % in bulk heterojunction solar cells. The team are now investigating the use of additives and co-solvents to increase this further.

Find out more – download Heeney’s ChemComm communication.

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Look into my MRI: Non-invasive detection of melanin formation

25 Jan 2012

Magnetic Resonance Imaging (MRI) is used as a medical tool to image soft tissue. It can give insights into the physical state of brain, identify sports injuries and diagnose cancer without ionising radiation. Its main drawback is the low sensitivity of the contrast enhancing probes it uses. These are the paramagnetic metal complexes, often containing gadolinium or manganese, that distinguish between healthy and damaged tissue by altering the relaxation times of the water protons in the body. The challenge is to increase the concentration of the contrast reagents in the tissues.

One solution is to use nanocarriers to deliver a high concentration of the contrast agent to the specific site of interest. Silvio Aime and his team recently used the nanocarrier, apoferritin, to transport solid MnOOH, which was subsequently reduced to paramagnetic Mn²⁺. Their recent ChemComm details how they have now taken this one step further to find a way of generating the Mn²⁺ contrast agent using a naturally occurring reductant. The result: a probe for melanin.

Melanin is produced from the successive oxidation of tyrosine – a process that is up-regulated in malignant melanoma. Aime exploited this oxidation process by introducing MnOOH-loaded apoferritin into melanoma cells – the MnOOH was reduced, generating the contrast agent and enabling the cells to be successfully imaged by MRI.

in vivo MRI images of tumour-bearing mice before and after administration of the contrast agent loaded nanocarrier

The team tested the sensor on melanogenic cells (melanin-forming cells) against non-melanogenic cells. They demonstrated that the signal generated belonged only to the melanin-producing cells which had internalised the Mn(III)OOH–apoferriton payloads. Animal studies revealed enhanced signal intensity in melanogenic tumours.

This interesting research has the potential to provide non-invasive, early diagnoses of skin cancers and evaluate the development of tumours, critical for saving people’s lives. The in vivo sensor may also be used to monitor other processes involving massive oxidative processes. The work of Silvio Aime is one to watch.

To find out more, download the ChemComm article today…

Posted on behalf of Sarah Brown, web science writer for ChemComm.

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Scattering layer for dye-sensitised solar cells

28 Nov 2011

We are all aware how important it is to find new improved ways to generate sustainable energy. One very promising candidate is the dye-sensitised solar cell which is particularly attractive due to its low production cost and mechanical robustness.

Scientists from East China Normal University have found a way to increase the efficiency of dye-sensitised solar cells by introducing an additional layer on top of the TiO₂ photoanode. The extra layer, made up of Y₃Al₅O₁₂:Ce down-converting microparticles, causes the conversion efficiency to increase from 6.97% to 7.91%.

SEM image of the Y₃Al₅O₁₂:Ce layer and the J-V curves for the solar cell with and without the additional layer

Likun Pan and his team have attributed this improved performance to the increased light absorbing and scattering properties of the microparticle layer (meaning more suitable photons are available for absorption by the dye) and reduced electron transfer resistance.

Download the ChemComm article to read more about the fabrication of Pan’s solar cell device.

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One nanoparticle, two nanoparticles, three nanoparticles, four!

22 Nov 2011

Nanoparticles might be small but they frequent the pages of many a journal due to the ongoing boom in nanotechnology research. Whilst they are useful in a myriad of fields, it is still difficult to directly characterise these extraordinarily small entities. King among the visualisation techniques is electron microscopy but this often requires the isolation of the sample on a support – hardly sufficient for analysing a dynamic sample in solution! Dynamic light scattering is another potential technique but finds limitations when it comes to much smaller nanoparticle sizes.

Ideally, you want to be able to count and size individual nanoparticles at a rate which produces reliable statistics. To address this challenge, Richard Compton and his team, including Neil Rees and Yi-ge Zhou who conducted the experiments alongside Jeseelan Pillay, Robert Tshikhudo and Sibulelo Vilakazi from Mintek, Randburg, have used anodic particle coulometry (APC) to measure gold nanoparticle collisions with a glassy carbon microelectrode and thus count and size individual nanoparticles.

With the electrode potential set above +1.0 V, they were able to record oxidative Faradaic transients from nanoparticle collisions and calculate an average nanoparticle radius which compared extremely well to the radius obtained from scanning electron microscopy measurements. They were also able to observe nanoparticle aggregation, which holds great promise for monitoring dynamic aggregation reactions.

It shouldn’t be long before this technique is routinely used to gain more information on all sizes of metallic nanoparticles which are currently being used in a variety of applications.

Read the ChemComm article by Compton and team for more.

Posted on behalf of Iain Larmour, web science writer for ChemComm.

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Hybrid biofuel cell

16 Nov 2011

Scientists from Israel have designed a biofuel cell that combines the advantages of both enzymatic and microbial fuel cells.

Biofuel cells use redox enzymes to convert chemical energy into electricity. These cells can be divided into two categories: enzymatic fuel cells which require the enzymes to be purified and microbial fuel cells which make use of an entire microorganism. There are pros and cons to both strategies – enzymatic fuel cells tend to have increased power output whilst microbial fuel cells enable full oxidation of a wider range of fuels.

Yet now, Lital Alfonta and co-workers demonstrate that by designing a hybrid cell, one can have the best of both worlds. The team have modified yeast to display redox enzymes on their surface and then introduced this into both the anode and cathode compartments. This approach removes the need to purify the enzymes and enables regeneration of both fuel compartments.

To find out more about Alfonta’s biofuel cell device, read the ChemComm article today.

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Aromaticity web theme issue – welcoming submissions

02 Nov 2011

We are delighted to announce a high-profile web themed issue on Aromaticity.

Guest editors: Nazario Martín (University Complutense of Madrid), Michael Hayley (University of Oregon) and Rik Tykwinski (University of Erlangen-Nuremberg)

This themed issue will consist of a series of invited Communications and Feature Articles covering work on all aspects of chemistry related with aromaticity – from new fundamental knowledge about aromaticity and theoretically interesting new arene structures to novel applications of aromatics and heteroaromatics which take advantage of their unique optical and electronic attributes.

The level of quality of this issue will be extremely high, and all manuscripts will undergo strict peer review. You are therefore encouraged to report work that you consider to be very important and conceptually significant. Please note that inclusion in the issue is subject to the discretion of the guest editors.

Publication of the peer-reviewed articles will occur without delay to ensure the timely dissemination of the work. The articles will then be assembled on the ChemComm website as a web-based thematic issue.

Submit your work before 31^st May 2012. Please add “aromaticity” in the comments to the editor section.

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Supramolecular assembly of polyoxometalate triangles

31 Oct 2011

Polyoxometalates (POMs) are a diverse class of inorganic materials that are of great interest due to their exciting range of redox, conducting, magnetic and catalytic properties. Recent collaborative work from Professor Garry Hanan in Montreal and Professor Bernold Hasenknopf in Paris reports the inclusion of a Lindqvist-type hexavanadate POM as a component of a self-assembling supramolecular framework.

They designed ligand 1 utilising a triol motif to complex the POM in combination with pyridyl groups to serve as binding sites for a transition metal. The reaction of 1 with a suitable decavanadate yielded complex 2, a hexavanadate POM complex that is itself a structurally rigid and spatially well-defined bi-pyridyl ligand. The 60° angle between the coordination vectors of the pyridyl groups led the authors to predict that the coordination of a trans-PdCl₂ unit by 2 would result in a supramolecular triangle.

Initially the reaction of 2 with [PdCl₂(CH₃CN)₂] in DMAc yielded a complex mixture of products; however, heating to 80 °C for 48 hours led to just a single assembly. This was shown by a number of analytical techniques to be the predicted triangular assembly 3.

This work elegantly uses a classical motif for self-assembly to create a multi-component supramolecular architecture. It is a great step towards the goal of creating functional supramolecular arrays, integrating the desirable properties of POMs into a new framework and bridging the gap between solid state oxides and coordination chemistry.

Researcher’s perspective:

The obtention of discrete coordination-driven POM-based arrays had so far eluded the community, with only a few well characterised coordination polymers. We started this challenging project of a molecular triangle by a simple drawing based on geometrical considerations. A range of conditions were explored, but most lead to complex mixtures except one. This identification of optimal conditions was our first satisfaction. However, we needed solid evidence of the triangular nature of the obtained array to validate our approach as a rational design: a combination of techniques confirmed the successful and selective synthesis of the triangle. As a student, I learnt a lot from the complementary competencies provided by the two groups on a project at the interface of inorganic, organic, supramolecular and coordination chemistry. Dealing with the specific difficulties of each domain has been an exciting challenge that has re-confirmed my choice in academic research. Marie-Pierre Santoni, a student in the research collaboration

To read more about Hanan and Hasenknopf’s work, download their ChemComm article.

Posted on behalf of Cally Haynes, ChemComm web writer.

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Worm-like peptides form gels at physiological pH

18 Oct 2011

Salt can induce gelation of functionalised dipeptides at high pH, say a team of scientists from the UK and France. The finding will significantly expand the utility of hydrogels, they claim.

Hydrogels are formed when molecules assemble into nanofibres, which then entangle and form a gel matrix. They can be used for a variety of applications, including drug delivery and scaffolds for tissue engineering. Peptide-based gelators are of particular interest at present, in part because they are potentially biocompatible.

Gelation is commonly triggered by pH, temperature or solvent composition. Dave Adams, at the University of Liverpool, and colleagues discovered that functionalised dipeptides that adopt a worm-like micelle structure at high pH form gels when salts are added to them. High pH is normally outside the window for gelation so the finding could be very useful for the field. Importantly, now that these gels can be made at physiological pH, they should find increased use in bioapplications.

Graphical abstract: Salt-induced hydrogelation of functionalised-dipeptides at high pH

Read more in Adams’ ChemComm communication, free to access for a limited period.

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Easy xenon capture and release using MOFs

18 Oct 2011

Graphical abstract: Facile xenon capture and release at room temperature using a metal–organic framework: a comparison with activated charcoal Xenon is naturally present in very small amounts in the atmosphere but radioactive forms are released following nuclear detonations, reprocessing and explosions, such as the recent catastrophe at Fukushima Daiichi Nuclear Power Plant in Japan. Xenon is also used in a variety of other applications, from lighting to medical imaging, so capturing and separating it (from its sister noble gas krypton) is important for both commercial uses and atmospheric monitoring.

Praveen Thallapally, at Pacific Northwest National Laboratory, Richland, US, and colleagues made two well-known metal–organic frameworks (MOFs) known as MOF-5 and NiDOBDC and compared their ability to capture and separate xenon with activated carbon. They found that NiDOBDC adsorbs significantly more xenon than MOF-5, and is more selective for xenon over krypton than activated carbon.

To find out more, download Dr Thallapally’s ChemComm communication.

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