Tag Archives: Hong Kong University of Science and Technology (HKUST)

City University of Hong Kong (CityU) and its anti-bacterial graphene face masks

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This looks like interesting work and I think the integration of visual images and embedded video in the news release (on the university website) is particularly well done. I won’t be including all the graphical information here as my focus is the text.

A Sept. 10, 2020 City University of Hong Kong (CityU) press release (also on EurekAlert) announces a greener, more effective face mask,

Face masks have become an important tool in fighting against the COVID-19 pandemic. However, improper use or disposal of masks may lead to “secondary transmission”. A research team from City University of Hong Kong (CityU) has successfully produced graphene masks with an anti-bacterial efficiency of 80%, which can be enhanced to almost 100% with exposure to sunlight for around 10 minutes. Initial tests also showed very promising results in the deactivation of two species of coronaviruses. The graphene masks are easily produced at low cost, and can help to resolve the problems of sourcing raw materials and disposing of non-biodegradable masks.

The research is conducted by Dr Ye Ruquan, Assistant Professor from CityU’s Department of Chemistry, in collaboration with other researchers. The findings were published in the scientific journal ACS Nano, titled “Self-Reporting and Photothermally Enhanced Rapid Bacterial Killing on a Laser-Induced Graphene Mask“.

Commonly used surgical masks are not anti-bacterial. This may lead to the risk of secondary transmission of bacterial infection when people touch the contaminated surfaces of the used masks or discard them improperly. Moreover, the melt-blown fabrics used as a bacterial filter poses an impact on the environment as they are difficult to decompose. Therefore, scientists have been looking for alternative materials to make masks.

Converting other materials into graphene by laser

Dr Ye has been studying the use of laser-induced graphene [emphasis mine] in developing sustainable energy. When he was studying PhD degree at Rice University several years ago, the research team he participated in and led by his supervisor discovered an easy way to produce graphene. They found that direct writing on carbon-containing polyimide films (a polymeric plastic material with high thermal stability) using a commercial CO2 infrared laser system can generate 3D porous graphene. The laser changes the structure of the raw material and hence generates graphene. That’s why it is named laser-induced graphene.

Graphene is known for its anti-bacterial properties, so as early as last September, before the outbreak of COVID-19, producing outperforming masks with laser-induced graphene already came across Dr Ye’s mind. He then kick-started the study in collaboration with researchers from the Hong Kong University of Science and Technology (HKUST), Nankai University, and other organisations.

Excellent anti-bacterial efficiency

The research team tested their laser-induced graphene with E. coli, and it achieved high anti-bacterial efficiency of about 82%. In comparison, the anti-bacterial efficiency of activated carbon fibre and melt-blown fabrics, both commonly-used materials in masks, were only 2% and 9% respectively. Experiment results also showed that over 90% of the E. coli deposited on them remained alive even after 8 hours, while most of the E. coli deposited on the graphene surface were dead after 8 hours. Moreover, the laser-induced graphene showed a superior anti-bacterial capacity for aerosolised bacteria.

Dr Ye said that more research on the exact mechanism of graphene’s bacteria-killing property is needed. But he believed it might be related to the damage of bacterial cell membranes by graphene’s sharp edge. And the bacteria may be killed by dehydration induced by the hydrophobic (water-repelling) property of graphene.

Previous studies suggested that COVID-19 would lose its infectivity at high temperatures. So the team carried out experiments to test if the graphene’s photothermal effect (producing heat after absorbing light) can enhance the anti-bacterial effect. The results showed that the anti-bacterial efficiency of the graphene material could be improved to 99.998% within 10 minutes under sunlight, while activated carbon fibre and melt-blown fabrics only showed an efficiency of 67% and 85% respectively.

The team is currently working with laboratories in mainland China to test the graphene material with two species of human coronaviruses. Initial tests showed that it inactivated over 90% of the virus in five minutes and almost 100% in 10 minutes under sunlight. The team plans to conduct testings with the COVID-19 virus later.

Their next step is to further enhance the anti-virus efficiency and develop a reusable strategy for the mask. They hope to release it to the market shortly after designing an optimal structure for the mask and obtaining the certifications.

Dr Ye described the production of laser-induced graphene as a “green technique”. All carbon-containing materials, such as cellulose or paper, can be converted into graphene using this technique. And the conversion can be carried out under ambient conditions without using chemicals other than the raw materials, nor causing pollution. And the energy consumption is low.

“Laser-induced graphene masks are reusable. If biomaterials are used for producing graphene, it can help to resolve the problem of sourcing raw material for masks. And it can lessen the environmental impact caused by the non-biodegradable disposable masks,” he added.

Dr Ye pointed out that producing laser-induced graphene is easy. Within just one and a half minutes, an area of 100 cm² can be converted into graphene as the outer or inner layer of the mask. Depending on the raw materials for producing the graphene, the price of the laser-induced graphene mask is expected to be between that of surgical mask and N95 mask. He added that by adjusting laser power, the size of the pores of the graphene material can be modified so that the breathability would be similar to surgical masks.

A new way to check the condition of the mask

To facilitate users to check whether graphene masks are still in good condition after being used for a period of time, the team fabricated a hygroelectric generator. It is powered by electricity generated from the moisture in human breath. By measuring the change in the moisture-induced voltage when the user breathes through a graphene mask, it provides an indicator of the condition of the mask. Experiment results showed that the more the bacteria and atmospheric particles accumulated on the surface of the mask, the lower the voltage resulted. “The standard of how frequently a mask should be changed is better to be decided by the professionals. Yet, this method we used may serve as a reference,” suggested Dr Ye.

Laser-induced graphene (LIG), Rice University, and Dr. Ye were mentioned here in a May 9, 2018 titled: Do you want that coffee with some graphene on toast?

Back to the latest research, read the caption carefully,

Research shows that over 90% of the E. coli deposited on activated carbon fibre (fig c and d) and melt-blown fabrics (fig e and f) remained alive even after 8 hours. In contrast, most of the E. coli deposited on the graphene surface (fig a and b) were dead. (Photo source: DOI number: 10.1021/acsnano.0c05330)

Here’s a link to and a citation for the paper,

Self-Reporting and Photothermally Enhanced Rapid Bacterial Killing on a Laser-Induced Graphene Mask by Libei Huang, Siyu Xu, Zhaoyu Wang, Ke Xue, Jianjun Su, Yun Song, Sijie Chen, Chunlei Zhu, Ben Zhong Tang, and Ruquan Ye. ACS Nano 2020, 14, 9, 12045–12053 DOI: https://doi.org/10.1021/acsnano.0c05330 Publication Date:August 11, 2020 Copyright © 2020 American Chemical Society

This paper is behind a paywall.

A method for producing two-dimensional quasicrystals from metal organic networks

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A July 13, 2016 news item on ScienceDaily highlights an advance where quasicrystals are concerned,

Unlike classical crystals, quasicrystals do not comprise periodic units, even though they do have a superordinate structure. The formation of the fascinating mosaics that they produce is barely understood. In the context of an international collaborative effort, researchers at the Technical University of Munich (TUM) have now presented a methodology that allows the production of two-dimensional quasicrystals from metal-organic networks, opening the door to the development of promising new materials.

A July 13, 2016 TUM press release (also on EurekAlert), which originated the news item, explains further,

Physicist Daniel Shechtman [emphasis mine] merely put down three question marks in his laboratory journal, when he saw the results of his latest experiment one day in 1982. He was looking at a crystalline pattern that was considered impossible at the time. According to the canonical tenet of the day, crystals always had so-called translational symmetry. They comprise a single basic unit, the so-called elemental cell, that is repeated in the exact same form in all spatial directions.

Although Shechtman’s pattern did contain global symmetry, the individual building blocks could not be mapped onto each other merely by translation. The first quasicrystal had been discovered. In spite of partially stark criticism by reputable colleagues, Shechtman stood fast by his new concept and thus revolutionized the scientific understanding of crystals and solid bodies. In 2011 he ultimately received the Nobel Prize in Chemistry. To this day, both the basic conditions and mechanisms by which these fascinating structures are formed remain largely shrouded in mystery.

A toolbox for quasicrystals

Now a group of scientists led by Wilhelm Auwärter and Johannes Barth, both professors in the Department of Surface Physics at TU Munich, in collaboration with Hong Kong University of Science and Technology (HKUST, Prof. Nian Lin, et al) and the Spanish research institute IMDEA Nanoscience (Dr. David Écija), have developed a new basis for producing two-dimensional quasicrystals, which might bring them a good deal closer to understanding these peculiar patterns.

The TUM doctoral candidate José Ignacio Urgel made the pioneering measurements in the course of a research fellowship at HKUST. “We now have a new set of building blocks that we can use to assemble many different new quasicrystalline structures. This diversity allows us to investigate on how quasicrystals are formed,” explain the TUM physicists.

The researchers were successful in linking europium – a metal atom in the lanthanide series – with organic compounds, thereby constructing a two-dimensional quasicrystal that even has the potential to be extended into a three-dimensional quasicrystal. To date, scientists have managed to produce many periodic and in part highly complex structures from metal-organic networks, but never a quasicrystal.

The researchers were also able to thoroughly elucidate the new network geometry in unparalleled resolution using a scanning tunnelling microscope. They found a mosaic of four different basic elements comprising triangles and rectangles distributed irregularly on a substrate. Some of these basic elements assembled themselves to regular dodecagons that, however, cannot be mapped onto each other through parallel translation. The result is a complex pattern, a small work of art at the atomic level with dodecagonal symmetry.

Interesting optical and magnetic properties

In their future work, the researchers are planning to vary the interactions between the metal centers and the attached compounds using computer simulation and experiments in order to understand the conditions under which two-dimensional quasicrystals form. This insight could facilitate the future development of new tailored quasicrystalline layers.

These kinds of materials hold great promise. After all, the new metal-organic quasicrystalline networks may have properties that make them interesting in a wide variety of application. “We have discovered a new playing field on which we can not only investigate quasicrystallinity, but also create new functionalities, especially in the fields of optics and magnetism,” says Dr. David Écija of IMDEA Nanoscience.

For one, scientists could one day use the new methodology to create quasicrystalline coatings that influence photons in such a manner that they are transmitted better or that only certain wavelengths can pass through the material.

In addition, the interactions of the lanthanide building blocks in the new quasicrystals could facilitate the development of magnetic systems with very special properties, so-called “frustrated systems”. Here, the individual atoms in a crystalline grid interfere with each other in a manner that prevents grid points from achieving a minimal energy state. The result: exotic magnetic ground states that can be investigated as information stores for future quantum computers.

The researchers have made an image available,

The quasicrystalline network built up with europium atoms linked with para-quaterphenyl–dicarbonitrile on a gold surface (yellow) - Image: Carlos A. Palma / TUM

The quasicrystalline network built up with europium atoms linked with para-quaterphenyl–dicarbonitrile on a gold surface (yellow) – Image: Carlos A. Palma / TUM

Here’s a link to and a citation for the paper,

Quasicrystallinity expressed in two-dimensional coordination networks by José I. Urgel, David Écija, Guoqing Lyu, Ran Zhang, Carlos-Andres Palma, Willi Auwärter, Nian Lin, & Johannes V. Barth. Nature Chemistry 8, 657–662 (2016) doi:10.1038/nchem.2507 Published online 16 May 2016

This paper is behind a paywall.

For anyone interested in more about the Daniel Schechter story and how he was reviled for his discovery of quasicrystals, there’s more in my Dec. 24, 2013 posting (scroll down about 60% of the way).

2-D melting and surfacing premelting of a single particle

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Scientists at the Hong Kong University of Science and Technology (HKUST) and the University of Amsterdam (in the Netherlands) have measured surface premelting with single particle resolution. From a March 15, 2016 HKUST news release on EurekAlert,

The surface of a solid often melts into a thin layer of liquid even below its melting point. Such surface premelting is prevalent in all classes of solids; for instance, two pieces of ice can fuse below 0°C because the premelted surface water becomes embedded inside the bulk at the contact point and thus freeze. Premelting facilitates crystal growth and is critical in metallurgy, geology, and meteorology such as glacier movement, frost heave, snowflake growth and skating. However, the causative factors of various premelting scenarios, and the effect of dimensionality on premelting are poorly understood due to the lack of microscopic measurements.

To this end, researchers from the Hong Kong University of Science and Technology (HKUST) and University of Amsterdam conducted a research where they were able to measure surface premelting with single-particle resolution for the first time by using novel colloidal crystals. They found that dimensionality is crucial to bulk melting and bulk solid-solid transitions, which strongly affect surface melting behaviors. To the surprise of the researchers, they found that a crystal with free surfaces (solid-vapor interface) melted homogenously from both surfaces and within the bulk, in contrast to the commonly assumed heterogeneous melting from surfaces. These observations would provide new challenges on premelting and melting theories.

The research team was led by associate professor of physics Yilong Han and graduate student Bo Li from HKUST. HKUST graduate students Feng Wang, Di Zhou, Yi Peng, and postdoctoral researcher Ran Ni from University of Amsterdam in Netherlands also participated in the research.

…

Micrometer sized colloidal spheres in liquid suspensions have been used as powerful model systems for the studies of phase transitions because the thermal-motion trajectories of these “big atoms” can be directly visualized under an optical microscope. “Previous studies mainly used repulsive colloids, which cannot form stable solid-vapor interfaces,” said Han. “Here, we made a novel type colloid with temperature-sensitive attractions which can better mimic atoms, since all atoms have attractions, or otherwise they cannot condense into stable solid in air. We assembled these attractive spheres into large well-tunable two-dimensional colloidal crystals with free surfaces for the first time.

“This paves the way to study surface physics using colloidal model systems. Our first project along this direction is about surface premelting, which was poorly understood before. Surprisingly, we found that it is also related to bulk melting and solid-solid transitions,” Han added.

The team found that two-dimensional (2D) monolayer crystals premelted into a thin layer of liquid with a constant thickness, an exotic phenomenon known as incomplete blocked premelting. By contrast, the surface-liquid thickness of the two- or three-layer thin-film crystal increased to infinity as it approaches its melting point, i.e. a conventional complete premelting. Such blocked surface premelting has been occasionally observed, e.g. in ice and germanium, but lacks theoretical explanations.

“Here, we found that the premelting of the 2D crystal was triggered by an abrupt lattice dilation because the crystal can no longer provide enough attractions to surface particles after a drop in density.” Li said. “Before the surface liquid grew thick, the bulk crystal collapsed and melted due to mechanical instability. This provides a new simple mechanism for blocked premelting. The two-layer crystals are mechanically stable because particles have more neighbors. Thus they exhibit a conventional surface melting.”

As an abrupt dilation does not change the lattice symmetry, this is an isostructural solid-solid transition, which usually occurs in metallic and multiferroic materials. The colloidal system provides the first experimental observation of isostructural solid-solid transition at the single-particle level.

The mechanical instability induced a homogenous melting from within the crystal rather than heterogeneous melting from the surface. “We observed that the 2D melting is a first-order transition with a homogeneous proliferation of grain boundaries, which confirmed the grain-boundary-mediated 2D melting theory.” said Han. “First-order 2D melting has been observed in some molecular monolayers, but the theoretically predicted grain-boundary formation has not been observed before.”

Here’s a link to and a citation for the paper,

Imaging the Homogeneous Nucleation During the Melting of Superheated Colloidal Crystals by Ziren Wang, Feng Wang, Yi Peng, Zhongyu Zheng, Yilong Han. Science  05 Oct 2012:
Vol. 338, Issue 6103, pp. 87-90 DOI: 10.1126/science.1224763

This paper is behind a paywall.