Tag Archives: Feng Wang

A graphene ‘camera’ and your beating heart: say cheese

Comparing it to a ‘camera’, even with the quotes, is a bit of a stretch for my taste but I can’t come up with a better comparison. Here’s a video so you can judge for yourself,

Caption: This video repeats three times the graphene camera images of a single beat of an embryonic chicken heart. The images, separated by 5 milliseconds, were measured by a laser bouncing off a graphene sheet lying beneath the heart. The images are about 2 millimeters on a side. Credit: UC Berkeley images by Halleh Balch, Alister McGuire and Jason Horng

A June 16, 2021 news item on ScienceDaily announces the research,

Bay Area [San Francisco, California] scientists have captured the real-time electrical activity of a beating heart, using a sheet of graphene to record an optical image — almost like a video camera — of the faint electric fields generated by the rhythmic firing of the heart’s muscle cells.

A University of California at Berkeley (UC Berkeley) June 16, 2021 news release (also on EurekAlert) by Robert Sanders, which originated the news item, provides more detail,

The graphene camera represents a new type of sensor useful for studying cells and tissues that generate electrical voltages, including groups of neurons or cardiac muscle cells. To date, electrodes or chemical dyes have been used to measure electrical firing in these cells. But electrodes and dyes measure the voltage at one point only; a graphene sheet measures the voltage continuously over all the tissue it touches.

The development, published online last week in the journal Nano Letters, comes from a collaboration between two teams of quantum physicists at the University of California, Berkeley, and physical chemists at Stanford University.

“Because we are imaging all cells simultaneously onto a camera, we don’t have to scan, and we don’t have just a point measurement. We can image the entire network of cells at the same time,” said Halleh Balch, one of three first authors of the paper and a recent Ph.D. recipient in UC Berkeley’s Department of Physics.

While the graphene sensor works without having to label cells with dyes or tracers, it can easily be combined with standard microscopy to image fluorescently labeled nerve or muscle tissue while simultaneously recording the electrical signals the cells use to communicate.

“The ease with which you can image an entire region of a sample could be especially useful in the study of neural networks that have all sorts of cell types involved,” said another first author of the study, Allister McGuire, who recently received a Ph.D. from Stanford and. “If you have a fluorescently labeled cell system, you might only be targeting a certain type of neuron. Our system would allow you to capture electrical activity in all neurons and their support cells with very high integrity, which could really impact the way that people do these network level studies.”

Graphene is a one-atom thick sheet of carbon atoms arranged in a two-dimensional hexagonal pattern reminiscent of honeycomb. The 2D structure has captured the interest of physicists for several decades because of its unique electrical properties and robustness and its interesting optical and optoelectronic properties.

“This is maybe the first example where you can use an optical readout of 2D materials to measure biological electrical fields,” said senior author Feng Wang, UC Berkeley professor of physics. “People have used 2D materials to do some sensing with pure electrical readout before, but this is unique in that it works with microscopy so that you can do parallel detection.”

The team calls the tool a critically coupled waveguide-amplified graphene electric field sensor, or CAGE sensor.

“This study is just a preliminary one; we want to showcase to biologists that there is such a tool you can use, and you can do great imaging. It has fast time resolution and great electric field sensitivity,” said the third first author, Jason Horng, a UC Berkeley Ph.D. recipient who is now a postdoctoral fellow at the National Institute of Standards and Technology. “Right now, it is just a prototype, but in the future, I think we can improve the device.”

Graphene is sensitive to electric fields

Ten years ago, Wang discovered that an electric field affects how graphene reflects or absorbs light. Balch and Horng exploited this discovery in designing the graphene camera. They obtained a sheet of graphene about 1 centimeter on a side produced by chemical vapor deposition in the lab of UC Berkeley physics professor Michael Crommie and placed on it a live heart from a chicken embryo, freshly extracted from a fertilized egg. These experiments were performed in the Stanford lab of Bianxiao Cui, who develops nanoscale tools to study electrical signaling in neurons and cardiac cells.

The team showed that when the graphene was tuned properly, the electrical signals that flowed along the surface of the heart during a beat were sufficient to change the reflectance of the graphene sheet.

“When cells contract, they fire action potentials that generate a small electric field outside of the cell,” Balch said. “The absorption of graphene right under that cell is modified, so we will see a change in the amount of light that comes back from that position on the large area of graphene.”

In initial studies, however, Horng found that the change in reflectance was too small to detect easily. An electric field reduces the reflectance of graphene by at most 2%; the effect was much less from changes in the electric field when the heart muscle cells fired an action potential.

Together, Balch, Horng and Wang found a way to amplify this signal by adding a thin waveguide below graphene, forcing the reflected laser light to bounce internally about 100 times before escaping. This made the change in reflectance detectable by a normal optical video camera.

“One way of thinking about it is that the more times that light bounces off of graphene as it propagates through this little cavity, the more effects that light feels from graphene’s response, and that allows us to obtain very, very high sensitivity to electric fields and voltages down to microvolts,” Balch said.

The increased amplification necessarily lowers the resolution of the image, but at 10 microns, it is more than enough to study cardiac cells that are several tens of microns across, she said.

Another application, McGuire said, is to test the effect of drug candidates on heart muscle before these drugs go into clinical trials to see whether, for example, they induce an unwanted arrhythmia. To demonstrate this, he and his colleagues observed the beating chicken heart with CAGE and an optical microscope while infusing it with a drug, blebbistatin, that inhibits the muscle protein myosin. They observed the heart stop beating, but CAGE showed that the electrical signals were unaffected.

Because graphene sheets are mechanically tough, they could also be placed directly on the surface of the brain to get a continuous measure of electrical activity — for example, to monitor neuron firing in the brains of those with epilepsy or to study fundamental brain activity. Today’s electrode arrays measure activity at a few hundred points, not continuously over the brain surface.

“One of the things that is amazing to me about this project is that electric fields mediate chemical interactions, mediate biophysical interactions — they mediate all sorts of processes in the natural world — but we never measure them. We measure current, and we measure voltage,” Balch said. “The ability to actually image electric fields gives you a look at a modality that you previously had little insight into.”

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

Graphene Electric Field Sensor Enables Single Shot Label-Free Imaging of Bioelectric Potentials by Halleh B. Balch, Allister F. McGuire, Jason Horng, Hsin-Zon Tsai, Kevin K. Qi, Yi-Shiou Duh, Patrick R. Forrester, Michael F. Crommie, Bianxiao Cui, and Feng Wang. Nano Lett. 2021, XXXX, XXX, XXX-XXX OI: https://doi.org/10.1021/acs.nanolett.1c00543 Publication Date: June 8, 2021 © 2021 American Chemical Society

This paper is behind a paywall.

Better and greener oil recovery

A June 27, 2016 news item on phys.org describes research on achieving better oil recovery,

As oil producers struggle to adapt to lower prices, getting as much oil as possible out of every well has become even more important, despite concerns from nearby residents that some chemicals used to boost production may pollute underground water resources.

Researchers from the University of Houston have reported the discovery of a nanotechnology-based solution that could address both issues – achieving 15 percent tertiary oil recovery at low cost, without the large volume of chemicals used in most commercial fluids.

A June 27, 2016 University of Houston news release (also on EurekAlert) by Jeannie Kever, which originated the news item, provides more detail,

The solution – graphene-based Janus amphiphilic nanosheets – is effective at a concentration of just 0.01 percent, meeting or exceeding the performance of both conventional and other nanotechnology-based fluids, said Zhifeng Ren, MD Anderson Chair professor of physics. Janus nanoparticles have at least two physical properties, allowing different chemical reactions on the same particle.

The low concentration and the high efficiency in boosting tertiary oil recovery make the nanofluid both more environmentally friendly and less expensive than options now on the market, said Ren, who also is a principal investigator at the Texas Center for Superconductivity at UH. He is lead author on a paper describing the work, published June 27 [2016] in the Proceedings of the National Academy of Sciences.

“Our results provide a novel nanofluid flooding method for tertiary oil recovery that is comparable to the sophisticated chemical methods,” they wrote. “We anticipate that this work will bring simple nanofluid flooding at low concentration to the stage of oilfield practice, which could result in oil being recovered in a more environmentally friendly and cost-effective manner.”

In addition to Ren, researchers involved with the project include Ching-Wu “Paul” Chu, chief scientist at the Texas Center for Superconductivity at UH; graduate students Dan Luo and Yuan Liu; researchers Feng Wang and Feng Cao; Richard C. Willson, professor of chemical and biomolecular engineering; and Jingyi Zhu, Xiaogang Li and Zhaozhong Yang, all of Southwest Petroleum University in Chengdu, China.

The U.S. Department of Energy estimates as much as 75 percent of recoverable reserves may be left after producers capture hydrocarbons that naturally rise to the surface or are pumped out mechanically, followed by a secondary recovery process using water or gas injection.

Traditional “tertiary” recovery involves injecting a chemical mix into the well and can recover between 10 percent and 20 percent, according to the authors.

But the large volume of chemicals used in tertiary oil recovery has raised concerns about potential environmental damage.

“Obviously simple nanofluid flooding (containing only nanoparticles) at low concentration (0.01 wt% or less) shows the greatest potential from the environmental and economic perspective,” the researchers wrote.

Previously developed simple nanofluids recover less than 5 percent of the oil when used at a 0.01 percent concentration, they reported. That forces oil producers to choose between a higher nanoparticle concentration – adding to the cost – or mixing with polymers or surfactants.

In contrast, they describe recovering 15.2 percent of the oil using their new and simple nanofluid at that concentration – comparable to chemical methods and about three times more efficient than other nanofluids.

Dan Luo, a UH graduate student and first author on the paper, said when the graphene-based fluid meets with the brine/oil mixture in the reservoir, the nanosheets in the fluid spontaneously go to the interface, reducing interfacial tension and helping the oil flow toward the production well.

Ren said the solution works in a completely new way.

“When it is injected, the solution helps detach the oil from the rock surface,” he said. Under certain hydrodynamic conditions, the graphene-based fluid forms a strong elastic and recoverable film at the oil and water interface, instead of forming an emulsion, he said.

Researchers said the difference is due to the asymmetric property of the 2-dimensional material. Nanoparticles are usually either hydrophobic – water-repelling, like oil – or hydrophilic, water-like, said Feng Wang, a post-doctoral researcher who shared first author-duties with Luo.

“Ours is both,” he said. “Ours is Janus and also strictly amphiphilic.”

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

Nanofluid of graphene-based amphiphilic Janus nanosheets for tertiary or enhanced oil recovery: High performance at low concentration by Dan Luo, Feng Wang, Jingyi Zhu, Feng Cao, Yuan Liu, Xiaogang Li, Richard C. Willson, Zhaozhong Yang, Ching-Wu Chu, and Zhifeng Ren. PNAS 2016 doi: 10.1073/pnas.1608135113 published ahead of print June 27, 2016,

This paper is behind a paywall.

2-D melting and surfacing premelting of a single particle

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.

Purple promises and bioimaging from Singapore’s A*STAR

A May 7, 2014 news item on Nanowerk describes a promising new approach to bioimaging,

Labeling biomolecules with light-emitting nanoparticles is a powerful technique for observing cell movement and signaling under realistic, in vivo conditions. The small size of these probes, however, often limits their optical capabilities. In particular, many nanoparticles have trouble producing high-energy light with wavelengths in the violet to ultraviolet range, which can trigger critical biological reactions.

Now, an international team led by Xiaogang Liu from the A*STAR Institute of Materials Research and Engineering and the National University of Singapore has discovered a novel class of rare-earth nanocrystals that preserve excited energy inside their atomic framework, resulting in unusually intense violet emissions …

A May 7, 2014 A*STAR (Agency for Science, Technology and Research) news release (h/t Imagist), which originated the news item, describes the problems with current bioimaging techniques and the new approach in more detail (Note: Links have been removed)

Nanocrystals selectively infused, or ‘doped’, with rare-earth ions have attracted the attention of researchers, because of their low toxicity and ability to convert low-energy laser light into violet-colored luminescence emissions — a process known as photon upconversion. Efforts to improve the intensity of these emissions have focused on ytterbium (Yb) rare-earth dopants, as they are easily excitable with standard lasers. Unfortunately, elevated amounts of Yb dopants can rapidly diminish, or ‘quench’, the generated light.

This quenching probably arises from the long-range migration of laser-excited energy states from Yb and toward defects in the nanocrystal. Most rare-earth nanocrystals have relatively uniform dopant distributions, but Liu and co-workers considered that a different crystal arrangement — clustering dopants into multi-atom arrays separated by large distances — could produce localized excited states that do not undergo migratory quenching.

The team screened numerous nanocrystals with different symmetries before discovering a material that met their criteria: a potassium fluoride crystal doped with Yb and europium rare earths (KYb2F7:Eu). Experiments revealed that the isolated Yb ‘energy clusters’ inside this pill-shaped nanocrystal (see image) enabled substantially higher dopant concentrations than usual — Yb accounted for up to 98 per cent of the crystal’s mass — and helped initiate multiphoton upconversion that yielded violet light with an intensity eight times higher than previously seen.

The researchers then explored the biological applications of their nanocrystals by using them to detect alkaline phosphatases, enzymes that frequently indicate bone and liver diseases. When the team brought the nanocrystals close to an alkaline phosphate-catalyzed reaction, they saw the violet emissions diminish in direct proportion to a chemical indicator produced by the enzyme. This approach enables swift and sensitive detection of this critical biomolecule at microscale concentration levels.

“We believe that the fundamental aspects of these findings — that crystal structures can greatly influence luminescence properties — could allow upconversion nanocrystals to eventually outperform conventional fluorescent biomarkers,” says Liu.

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

Enhancing multiphoton upconversion through energy clustering at sublattice level by Juan Wang, Renren Deng, Mark A. MacDonald, Bolei Chen, Jikang Yuan, Feng Wang, Dongzhi Chi, Tzi Sum Andy Hor, Peng Zhang, Guokui Liu, Yu Han, & Xiaogang Liu. Nature Materials 13, 157–162 (2014) doi:10.1038/nmat3804 Published online 24 November 2013

This paper is behind a paywall but there is a free preview via ReadCube Access.