CrystEngComm Blog

Nano-sized crystals (or nanocrystals) have better solubilities and dissolution characteristics than larger crystals do.   Preparation of nanocrystals of drug molecules is therefore of interest to the pharmaceutical industry, particularly in cases where poor solubility is an issue.  However, preparation of crystals of the desired size and crucially, the correct polymorph, is not straightforward.  Problems include the long times required and the unwanted formation of amorphous material. One promising method of nanocrystal preparation  is to grow crystals in pores where the size of the pores limits the size of the crystals formed.

A recent paper in CrystEngComm by Myerson and co-workers reports the growth of three nanocrystals of pharmaceutical ingredients (APIs) – ibuprofen, fenofibrate and griseofulvin – using silica, where the pores of the silica structure provide so-called rigid confinement. The formation process is simple, involving loading of the API into the silica, washing to remove any API adhering to the surface (rather than inside the pores, see diagram below), crystallisation and drying. These steps can be varied to optimise the outcomes.

The nanocrystals exhibit enhanced solubilities and improved stabilities, due to the protection offered from e.g. moisture by the pores. The pores also limit possible reorganisations which would result in undesired crystal forms. The authors also highlight that the samples can be directly formulated into capsules without any additional steps, decreasing the formulation time significantly.

Read the full paper for more information:

Formation of organic molecular nanocrystals under rigid confinement with analysis by solid state NMR
X. Yang, T. C. Ong, V. K. Michaelis, S. Heng, J. Huang, R. G. Griffin and A. S. Myerson
CrystEngComm, 2014, DOI: 10.1039/C4CE01087F, Paper

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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. She has recently published a book on chemicals from plants.

A new paper by Behrens and co-workers in CrystEngComm details the modulated syntheses of a Zinc-fumarate MOF in both water and DMF. The authors performed kinetic studies for each synthesis, showing that, contrary to what was expected, the modulator increased the rate of formation of the MOF in DMF.

Modulating agents (usually monocarboxylic acids) are added to a MOF reaction mixture to increase the reproducibility and crystallinity of the final product. In coordination modulation, the modulating agent competes with the organic linkers in binding to the metal centre, preventing the formation of impurities but slowing down the reaction. In this current work, the authors chose formic acid as their modulator and performed in situ energy dispersive x-ray diffraction, which allowed for quantitative kinetic data to be produced.

When the authors carried out the synthesis in water, the modulating agent behaved as expected, decreasing the nucleation and growth rates as the formic acid concentration increased. However, when formic acid was added to the DMF-containing reaction mixture, the rate of growth increased. The authors theorise this occurs due to trace water in their commercial formic acid which they investigated by keeping the formic acid concentration constant but increasing water content. This showed remarkable results, increasing the rate constant by 2 orders of magnitude.

By observing that both the presence of a modulator and the water concentration have a large effect on the crystal formation, the authors added to the body of evidence that successful MOF syntheses are highly dependent on subtle changes in reagents and conditions.

Read the full article to find out more

Insight into the mechanism of modulated syntheses: in situ synchrotron diffraction studies on the formation of Zr-fumarate MOF
Gesa Zahn, Philip Zerner, Jann Lippke, Fabian L. Kempf, Sebastian Lilienthal, Christian A. Schröder, Andreas M. Schneidera and Peter Behrens
CrystEngComm, 2014, 16, 9198-9207

Josh Campbell is a PhD student, currently at the University of Southampton, UK studying crystal structure prediction of organic semiconductors. He received his BSc from the University of Bradford.

The quality of protein crystals can sometimes make study of their structure by X-ray crystallography challenging.   Producing higher quality crystals could be facilitated by a better understanding of the crystal growth process.

One method of achieving this  involves inserting a small fluorescent dye into a protein (to form an F-protein) and adding this labelled protein to the crystallising protein.  The distribution, orientation and incorporation efficiency of the F-protein during the growth of the crystals can be studied optically using techniques including polarisation microscopy (see diagram below).

A new paper describes the use of three different F-proteins, each incorporating the dye DY-632-01 NHS ester, to study the crystallisation of the three unlabelled proteins.  This enabled visualisation of the crystal growth habits, symmetry and history as well as the distribution of the F-proteins.

In some cases, the F-proteins are preferentially incorporated into the growing crystal and can act as tracers.  Alternatively, they may not be incorporated at all and provide information about the biophysical factors which affect crystal growth (such as charge distribution and hydrophobicity).

As different F-proteins behave differently during the crystallisation of a given unlabelled protein, repeating the crystallisation experiment with different F-proteins can provide complementary information.

The distribution of F-proteins is not uniform throughout the crystal and authors conclude that that the diffraction quality of the crystal is position dependent. The incorporation of F-proteins may allow areas of higher diffraction quality to be identified.

For more information, read the full paper:

Illuminating protein crystal growth using fluorophore-labelled proteins
Alaa Adawy, Willem J. P. van Enckevort, Elisabeth S. Pierson, Willem J. de Grip and Elias Vlieg
CrystEngComm, 2014, DOI: 10.1039/C4CE01281J

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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors

The use of fossil fuel-powered vapour compressors for the allocation of hot and cold air makes a significant contribution to global warming. A greener alternative involves reversible adsorption and desorption of a working fluid (often water) in adsorption heat pumps (AHPs) or adsorption chillers (ADCs), concepts originally devised by Michael Faraday in 1848.

The limiting factor when using MOF-based AHPs and ADCs is the rate of heat transfer. In this light, Al-based MOFs provide an attractive target as Al can not only provide a heat-conducting surface, but is also naturally abundant and of low toxicity.

MOFs for heat transfer

In their recent paper in CrystEngComm, de Lange, Gascon and co-workers evaluate a series Al-based MOFs for use in AHPs and ADCs. Of all the materials they tested, the most favourable characteristics were shown by the compound designated CAU-10-H, a material comprised of [Al–OH]²⁺ chains linked together by isophthalic acid, (CAU is Christian-Albrechts-Universität, where the compounds were first developed). 

In the presence of hydrochloric acid, CAU-10-H can be grown directly on to γ-alumina beads or metallic aluminium. These systems show good water adsorption and stability.  Up to 38kJ of heat can be withdrawn in the evaporator of an AHP/ADC per square metre of Al-coated surface, suggesting further study and development of Al-MOFs is worthwhile.

For more details, read the full paper:

Crystals for sustainability – structuring Al-based MOFs for the allocation of heat and cold
M. F. de Lange, C. P. Ottevanger, M. Wiegman, T. J. H. Vlugt, J. Gascon and F. Kapteijn
CrystEngComm, 2014, DOI: 10.1039/C4CE01073F

Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors

Zinc oxide (ZnO) is an important semiconductor material which can be used for gas sensing.  The sensing property relies on an oxidation-reduction reaction on the ZnO surface between the detected gas and surface oxygen molecules which causes the resistance of the sensor to change.  The nature of exposed facets on the sensor is crucial to its performance however, control of the growth, number, and morphology of these features has so far proved difficult.

In their recent paper in CrystEngComm, Xiang, Xu and co-workers report a simple synthesis of ZnO which allows for exposed facet control.  They discovered a two-step hydrothermal synthesis does not require use of any templates or surfactants but achieves structure control simply by adjusting pH.  In this way, hexagonal-pyramids, -prisms, -prismoids and -disks could be formed (see below)/

The authors tested the materials for gaseous ethanol sensing and the sensitivity was found to vary in the order disks > prismoids > prisms > pyramids.  They showed that the sensitivity of the sensors increased with exposure of (0001) crystal planes as these polar facets can provide more active sites for oxygen absorption than other facets, increasing the gas sensor response.   Their new findings are significant for the future development of high performance gas sensors.

For more information, read the full paper:

Evolution of ZnO microstructures from hexagonal disk to prismoid, prism and pyramid and their crystal facet-dependent gas sensing properties
Nan Qin, Qun Xiang, Hongbin Zhao, Jincang Zhang and Jiaqiang Xu
CrystEngComm, 2014, DOI: 10.1039/C4CE00637B

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Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors. She has recently published a book on chemicals from plants.

Cheap and effective storage of renewable energy is a key challenge for consumers in the future. Lithium ion batteries (LIBs) are one class of materials that meet the important requirements of high energy density, low cost, good power capacity and efficient cycling.

Recent research on LIB materials has focused on maximising their favourable properties to bring them closer to commerical use.

A new paper by Jun Liu and co-workers (Central South University, Changsha, China) describes the preparation and testing of a new anode material based on MoO₃ and graphene oxide (GO). The former material is naturally abundant, has good chemical stability and a high storage capability but also exhibits poor conductivity and lithium ion diffusion. GO has good conductivity, a large surface area and is highly stable, making it an attractive material for composite material formation.  

The authors prepared the new material by first synthesising GO and α-MoO₃ nanoribbons before modifying the surface of the latter to produce a positive charge. This allowed the MoO₃ material to assemble onto the GO.  They then applied heat to form the product, α-MoO₃@GNS (GNS refers to graphene nanosheet), and fabricated the material to form an anode.

These robust nanocomposites exhibit greatly enhanced Li transport efficiency compared to other MoO₃-based materials, as well as high electrical conductivity and good cycling efficiency.

The authors concluded that the two components work synergistically to produce the observed properties and suggested the composite as a potential anode material for high performance LIBs.

 To find out more, read the full article:

Graphene nanosheets encapsulated α-MoO₃ nanoribbons with ultrahigh lithium ion storage properties
Pei-Jie Lu, Ming Lei and Jun Liu
CrystEngComm, 2014, DOI: 10.1039/C4CE00252K

Gwenda Kyd has a PhD in metallocarborane chemistry from the University of Edinburgh. Other research work includes the spectroscopic study of the structure of glasses and organometallic electron-transfer reactions and the preparation of new inorganic phosphors.  She has recently published a book on chemicals from plants.