Chemical Science Blog

Metal-organic framework (MOF) nanoparticles combine with carbon microfibres to make large-scale materials with many possible applications

Assembling very small-scale nanoparticles into larger structures, commonly known as macroarchitectures, offers opportunities to exploit the nanoparticles’ unique chemical and physical properties while they are embodied within much larger components. Researchers in China, Australia and Japan developed a method that readily combines nanoparticles called metal-organic frameworks (MOFs) and micron-sized carbon fibres into versatile macroarchitecture materials.

The team, at Nanjing University of Science and Technology, China, the University of Queensland, Australia and the JST-ERATO Yamauchi Materials Space Tectonics Project, Japan, report their innovation in an article in the open access journal Chemical Science.

“While retaining the characteristics of the nanomaterial they are built from, our macroarchitectures also add in many new kinds of features such as high surface areas, high mechanical strength and low density,” says Professor Yusuke Yamauchi of the University of Queensland group.

From nano to macro in a simpler process

The manufacturing procedure begins with the MOF nanoparticles, which consist of metal ions or metallic clusters connected by organic (carbon-based) linker groups. Varying the metallic and organic components can generate a wide variety of MOFs with different chemical and physical properties.

The MOFs are then combined with hollow carbon-based fibres to form much larger centimetre-scale aerogels, which are highly porous and have extremely low densities. These unique materials can be generated in a variety of desired shapes, and possess great elasticity and compressibility, combined with chemical stability and physical strength.

Existing methods for making similar materials are problematic as their assembly usually yields products with relatively poor mechanical properties, and requires the use of adhesives or templates which have to be removed in additional steps. In contrast, the new method causes ‘zeolitic imidazolate framework (ZIF-8)-polyacrylonitrile nanofibers’ to directly assemble into centimetre-sized aerogels with controllable shapes and tuneable properties.

“The materials integrate the properties of one-dimensional nanofibers and three-dimensional carbon aerogels,” says Yamauchi.

Many possible uses

The macroarchitectures composed of three-dimensional porous interconnected networks could have commercial applications in many fields. The initial key to unlocking a wide range of practical uses is to design MOF structures that will achieve specific functions in each resulting aerogel. These could involve adsorbing specific chemicals into the pores of the final structure, catalysing chemical processes, or converting and storing energy, including electrical energy within capacitors.

Laboratory-scale trials have already demonstrated that some of the porous structures – which the researchers describe as “somewhat resembling that of a loofah sponge” – have impressive oil-retaining properties when exposed to mixtures of oil and water. This effect could be exploited to clean oil from polluted water. One version of the materials also has catalytic properties that could be useful for chemically degrading a variety of other pollutants.

The aerogels also have an impressive ability to absorb light and convert it into heat at high efficiency, which could be used to prepare drinkable water by desalinating seawater. “We believe that in the future our materials could be used for several large-scale and cost-effective water purification applications,” says Yamauchi.

The researchers now aim to develop the potential for moving from laboratory scale proof-of-concept demonstrations to commercially useful applications.

Chemical Science is open and free for both readers and authors.

Article details:

Zhang, Z. et al: “Modular assembly of MOF-derived carbon nanofibers into macroarchitectures for water treatment.” Chem. Sci., 2022, 13, 9159-9164

A single molecule magnet could help us understand the biochemistry of health and disease

A single molecule that behaves like a powerful magnet could help chemists determine the structure of many other molecules. Researchers based in Italy and Brazil describe the development and potential of their unusually powerful Nuclear Magnetic Resonance (NMR) shift agent in the open access journal Chemical Science.

NMR uses a strong magnetic field to split the spin levels of the nuclei at the centre of some atoms. Monitoring the splitting can reveal the chemical environment surrounding individual atoms, allowing the structure of entire molecules, including large biological macromolecules to be determined.

NMR is based on the same physical principles as medical MRI imaging, but instead of generating images of bodies it creates graphical read-outs of atomic interactions that can be readily deciphered by experts. It has been a fundamental tool of chemistry research since long before the widespread application of MRI imaging.

One problem, however, is that the signals from atoms in large molecules can overlap and interfere in ways that blur the data. This can be resolved by introducing a tiny magnetic tag into a region of interest within a large molecule. The magnetism of the tag shifts the signals from nearby atoms in a predictable way, separating them out from signals from other regions that are not of immediate interest.

The tags are called shift agents and many are available, but researchers are seeking more powerful and effective shift agents to make NMR signals ever clearer and allow them to reveal new details of molecular structure within larger molecules.

The researchers in Italy and Brazil found inspiration for their new shift agent in an unusual place – chemicals used in research towards quantum technologies.

“By searching molecular materials designed for the miniaturisation of information storage and quantum technologies, we have identified and re-designed a molecule that shifts the NMR signals of the neighbouring atoms twice as much as the currently used molecules,” says researcher Roberta Sessoli at the University of Florence. Sessoli and her colleagues in Italy collaborated with researchers at the Federal University of Parana, Brazil.

The molecule they devised has a cage-like arrangement of organic (carbon-based) chemical groups holding an atom of the rare-earth element dysprosium at its centre. It was produced by a relatively simple chemical modification that hugely increased the desirable magnetic properties of the molecule the team began with. Experiments and computational modelling showed that this design modification ensures the new shift agent has a very high and directional magnetic field while being sufficiently stable to be used in solution at room temperature.

The researchers hope that their shift agent can contribute to the worldwide effort to understand the structure of the very complex biomolecules that control the chemistry of life.

“The more we can learn about the structure and functions of proteins, for example, the better and faster we will be able to design new therapies for old and new diseases,” Sessoli says.

Chemical Science is open and free for both readers and authors.

Article details:

Santana, F. S. “A dysprosium single molecule magnet outperforming current pseudocontact shift agents.” Chemical Science, 2022, 13, 5680-5871

Thin films of novel semiconductors could open a new window for optoelectronics

An international research team describe how to make thin films of semiconductor materials composed of calcium copper and phosphorus (CaCuP) in the open access journal Chemical Science.

“We have synthesised CaCuP thin films for the first time, and found them to be semiconductors with very high p-type electrical conductivity,” says Andrea Crovetto at the Technical University of Denmark. He worked on the research with colleagues in the UK, USA and Germany.

Crovetto explains that p-type conductivity (p for positive) is a form of semi-conduction in which electric current is carried by the movement of positively charged “holes” rather than by the mobile electrons of n-type (n for negative) conductivity. “The holes can be thought of as bubbles of missing electrons moving around a sea of inactive electrons,” he says.

High performance and transparent p-type conducting materials are keenly sought by researchers as they are expected to offer improved efficiency and new design opportunities for optoelectronic devices that work by inter-converting light and electrical energy in either direction. Applications could include solar cells generating electrical power, sensors driven by or responding to light, and also transparent electronics.

The challenge to find efficient p-type conducting films inspired Crovetto to write a research proposal and to visit the group led by Andriy Zakutayev at the National Renewable Energy Laboratory in the USA. He knew that Zakutayev’s team had a unique growth chamber that could create a wide range of phosphide films. He also sought guidance and assistance from David Scanlon at University College, London, UK, who was an expert on the theoretical aspects CaCuP phosphide films. His is the type of work that had predicted such films could have novel and very useful characteristics.

The researchers and other colleagues then collaborated to predict in more detail the properties of specific CaCuP films and then eventually to make and test them. PhD student Joe Willis from Scanlon’s group led this new round of theoretical predictions.

“CaCuP had never been made before as a thin film and I was afraid it might not be stable in air, so it was exciting to finally see it synthesised and find that it was not degrading when taking it out of the growth chamber,” says Crovetto.

The electrical properties proved to be close to what was expected. One challenge for the future, however, is to make the films more transparent than was initially achieved. Crovetto says he was disappointed to see that the first films made were not as transparent as the team had hoped. They believe that this might be resolved by future developments in the chemistry of growing the films, which will be a priority task for their ongoing investigations.

Crovetto says that combining transparency with good p-type electrical conduction is not easily achieved using conventional materials like oxides or binary semiconductors. Successful incorporation of good transparency into the new CaCuP could therefore be a very significant step forwards. “Our work could open up a new field of phosphide materials discovery,” he concludes.

Chemical Science is open and free for both readers and authors.

Article details:

Willis, J. et al:  Prediction and realisation of high mobility and degenerate p-type conductivity in CaCuP thin films Chemical Science (2022).

Block copolymers (BCPs) consist of amphiphilic molecules that can self-assemble in selective solvents and generate various types of nano- and micro-dimensional structures. The unique self-assembly process is low-cost and relatively straight forward in solution phase.  The final structures have morphological diversity and complexity.  These self-assembled structures have been widely used in various applications such as drug delivery, catalysis, and water purification. The self-assembly process follows a heating step for dissolution of all the components and a subsequent cooling step. Both the steps and the parameters are vital for final structural characteristics of the assembled structures. A group of scientists from two esteemed universities in Canada recently studied the effects of rate of cooling in the self-assembly process.

Schematic representation of how cooling rate can change the morphology of formed micellar structures using PFS BCP and homopolymers

The authors used a systematic approach to explain the influence of cooling rate on micelle morphologies for a series of PFS based BCPs. The cooling rate greatly influences the size and the shape of colloidal structures. Rapid cooling increases branching and opens a new avenue to manipulate micelle morphologies. The study finds that rapid cooling reduces crystallinity, as polymer chains do not have enough time to pack in ordered structures.

The authors standardized sample preparation protocol and then varied the cooling times, with quick cooling of 2.5 min producing flower like structures and median cooling time of 50 mins leading to the same structural features with larger size. Co-self-assembly of homopolymer BCP mixtures with variable cooling rate also shows that quick cooling generates uniform sized branched micellar structures with elongated central platelets whereas slow cooling led to a long single fiber with a dark circle platelet in the centre.

With several examples and optimization conditions, the effect of cooling in the formation of self-assembled micellar structures has been evaluated. The main outcome of this study is that the cooling rate is another parameter to manipulate crystallization-driven self-assembly and to control micelle morphologies. There exists a lot of possibilities to use the findings and apply them to generate BCPs with a crystallizable block with important optical or electronic properties.

For details, please visit the entire article at https://doi.org/10.1039/D1SC05937H

About the author:

Dr Damayanti Bagchi is a postdoctoral researcher in Irene Chen’s lab at University of California, Los Angeles, United States. She obtained her PhD in Physical Chemistry from Satyendra Nath Bose National Centre for Basic Sciences, India. Her research is focused on spectroscopic studies of nano-biomaterials. She is interested in exploring light enabled therapeutics. She enjoys food and experimenting with various cuisines, which she found resembles products/ side products of chemical reactions!