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Category: Nanotechnology

A Graphene Superconductor That Plays More Than One Tune

Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that’s thinner than a human hair but has a depth of special traits. It easily switches from a superconducting material that conducts electricity without losing any energy, to an insulator that resists the flow of electric current, and back again to a superconductor — all with a simple flip of a switch.

“Usually, when someone wants to study how electrons interact with each other in a superconducting quantum phase versus an insulating phase, they would need to look at different materials. With our system, you can study both the superconductivity phase and the insulating phase in one place,”

said Guorui Chen, the study’s lead author and a postdoctoral researcher in the lab of Feng Wang, who led the study. Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division, is also a UC Berkeley physics professor.

Trilayer Graphene Superlattice

The graphene device is composed of three atomically thin (2-D) layers of graphene. When sandwiched between 2-D layers of boron nitride, it forms a repeating pattern called a moiré superlattice.

The material could help other scientists understand the complicated mechanics behind a phenomenon known as high-temperature superconductivity, where a material can conduct electricity without resistance at temperatures higher than expected, though still hundreds of degrees below freezing.

In a previous study, the researchers reported observing the properties of a Mott insulator in a device made of trilayer graphene. A Mott insulator is a class of material that somehow stops conducting electricity at hundreds of degrees below freezing despite classical theory predicting electrical conductivity.

But it has long been believed that a Mott insulator can become superconductive by adding more electrons or positive charges to make it superconductive, Chen explained.

Superconductivity

For the past 10 years, researchers have been studying ways to combine different 2-D materials, often starting with graphene — a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

“So for this study we asked ourselves, ‘If our trilayer graphene system is a Mott insulator, could it also be a superconductor?'”

said Chen.

Working with David Goldhaber-Gordon of Stanford University and the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory, and Yuanbo Zhang of Fudan University, the researchers used a dilution refrigerator, which can reach intensely cold temperatures of 40 millikelvins — or nearly minus 460 degrees Fahrenheit — to cool the graphene/boron nitride device down to a temperature at which the researchers expected superconductivity to appear near the Mott insulator phase, said Chen.

Mott Insulator

Once the device reached a temperature of 4 kelvins (minus 452 degrees Fahrenheit), the researchers applied a range of electrical voltages to the tiny top and bottom gates of the device.

As they expected, when they applied a high vertical electrical field to both the top and bottom gates, an electron filled each cell of the graphene/boron nitride device. This caused the electrons to stabilize and stay in place, and this “localization” of electrons turned the device into a Mott insulator.

The graphene/boron nitride moiré superlattice
The graphene/boron nitride moiré superlattice material is composed of three atomically thin (2D) layers of graphene (gray) sandwiched between 2D layers of boron nitride (red and blue) to form a repeating pattern called a moiré superlattice. Superconductivity is indicated by the light-green circles.
Credit: Guorui Chen et al./Berkeley Lab

Then, they applied an even higher electrical voltage to the gates. To their delight, a second reading indicated that the electrons were no longer stable. Instead, they were shuttling about, moving from cell to cell, and conducting electricity without loss or resistance.

In other words, the device had switched from the Mott insulator phase to the superconductor phase.

Chen explained that the boron nitride moiré superlattice somehow increases the electron-electron interactions that take place when an electrical voltage is applied to the device, an effect that switches on its superconducting phase. It’s also reversible — when a lower electrical voltage is applied to the gates, the device switches back to an insulating state.

A Tiny Playground

The multitasking device offers scientists a tiny, versatile playground for studying the exquisite interplay between atoms and electrons in exotic new superconducting materials with potential use in quantum computers — computers that store and manipulate information in qubits, which are typically subatomic particles such as electrons or photons — as well as new Mott insulator materials that could one day make tiny 2-D Mott transistors for microelectronics a reality.

“This result was very exciting for us. We never imagined that the graphene/boron nitride device would do so well,” Chen said. “You can study almost everything with it, from single particles to superconductivity. It’s the best system I know of for studying new kinds of physics,”

Chen said.

This study was supported by the Center for Novel Pathways to Quantum Coherence in Materials (NPQC), an Energy Frontier Research Center led by Berkeley Lab and funded by the DOE Office of Science. NPQC brings together researchers at Berkeley Lab, Argonne National Laboratory, Columbia University, and UC Santa Barbara to study how quantum coherence underlies unexpected phenomena in new materials such as trilayer graphene, with an eye toward future uses in quantum information science and technology.

Original Study: Signatures of tunable superconductivity in a trilayer graphene moiré superlattice

Top Image: Guorui Chen/Berkeley Lab

Programmed Nanorobots Seek And Destroy Cancer Tumors

Nanorobots have been successfully programmed to shrink tumors by cutting off their blood supply. This major advancement came from Arizona State University (ASU) scientists, in collaboration with researchers from the National Center for Nanoscience and Technology (NCNST), of the Chinese Academy of Sciences.

The demonstration of the technology, the first-of-its-kind study in mammals, used breast cancer, melanoma, ovarian and lung cancer mouse models.

“We have developed the first fully autonomous, DNA robotic system for a very precise drug design and targeted cancer therapy. Moreover, this technology is a strategy that can be used for many types of cancer, since all solid tumor-feeding blood vessels are essentially the same,”

said Hao Yan, director of the ASU Biodesign Institute’s Center for Molecular Design and Biomimetics and the Milton Glick Professor in the School of Molecular Sciences.

DNA Origami

Yan is an expert in the field of DNA origami, which in the past two decades, has developed atomic-scale manufacturing to build more and more complex structures.

The bricks to build their structures come from DNA, which can self-fold into all sorts of shapes and sizes  – all at a scale one thousand times smaller than the width of a human hair – in the hopes of one day revolutionizing computing, electronics and medicine.

That one day may be coming a bit faster than anticipated.

Nanomedicine is a new branch of medicine that seeks to combine the promise of nanotechnology to open up entirely new avenues for treatments, such as making minuscule, molecule-sized nanoparticles to diagnose and treat difficult diseases, especially cancer.

Nanoscale robots
Image courtesy of ASU Biodesign Institute

Until now, the challenge to advancing nanomedicine has been difficult because scientists wanted to design, build and carefully control nanorobots to actively seek and destroy cancerous tumors, while not harming any healthy cells.

The international team of researchers overcame this problem by using a seemingly simple strategy to very selectively seek and starve out a tumor.

Drug Nanocarriers

The work was initiated about 5 years ago. The NCNST researchers first wanted to specifically cut-off of tumor blood supply by inducing blood coagulation with high therapeutic efficacy and safety profiles in multiple solid tumors using DNA-based nanocarriers.

Prof. Hao Yan’s expertise has upgraded the nanomedicine design to be a fully programmable robotic system, able to perform its mission entirely on its own.

“These nanorobots can be programmed to transport molecular payloads and cause on-site tumor blood supply blockages, which can lead to tissue death and shrink the tumor,”

said Baoquan Ding, a professor at the NCNST, located in Beijing.

To perform their study, the scientists took advantage of a well-known mouse tumor model, where human cancer cells are injected into a mouse to induce aggressive tumor growth.

Once the tumor was growing, the nanorobots were deployed to come to the rescue.

Each nanorobot is made from a flat, rectangular DNA origami sheet, 90 nanometers by 60 nanometers in size. A key blood-clotting enzyme, called thrombin, is attached to the surface.

Thrombin

Thrombin can block tumor blood flow by clotting the blood within the vessels that feed tumor growth, causing a sort of tumor mini-heart attack, and leading to tumor tissue death.

First, an average of four thrombin molecules was attached to a flat DNA scaffold. Next, the flat sheet was folded in on itself like a sheet of paper into a circle to make a hollow tube.

They were injected with an IV into a mouse, then traveled throughout the bloodstream, homing in on the tumors.

The key to programming a nanorobot that only attacks a cancer cell was to include a special payload on its surface, called a DNA aptamer. The DNA aptamer could specifically target a protein, called nucleolin, that is made in high amounts only on the surface of tumor endothelial cells, and not found on the surface of healthy cells.

Once bound to the tumor blood vessel surface, the nanorobot was programmed, like the notorious Trojan horse, to deliver its unsuspecting drug cargo in the very heart of the tumor, exposing an enzyme called thrombin that is key to blood clotting.

The nanorobots worked fast, congregating in large numbers to quickly surround the tumor just hours after injection.

Safe and Effective

First and foremost, the team showed that the nanorobots were safe and effective in shrinking tumors.

“The nanorobot proved to be safe and immunologically inert for use in normal mice and, also in Bama miniature pigs, showing no detectable changes in normal blood coagulation or cell morphology,”

said Yuliang Zhao, also a professor at NCNST and lead scientist of the international collaborative team.

Most importantly, there was no evidence of the nanorobots spreading into the brain where it could cause unwanted side effects, such as a stroke.

“The nanorobots are decidedly safe in the normal tissues of mice and large animals,”

said Guangjun Nie, another professor at the NCNST and a key member of the collaborative team.

The treatment blocked tumor blood supply and generated tumor tissue damage within 24 hours while having no effect on healthy tissues. After attacking tumors, most of the nanorobots were cleared and degraded from the body after 24 hours.

by two days, there was evidence of advanced thrombosis, and 3 days, thrombi in all tumor vessels were observed.

thrombin molecule
Image courtesy of ASU Biodesign Institute

The key is to trigger thrombin only when it is inside tumor blood vessels. Also, in the melanoma mouse model, 3 out of 8 mice receiving the nanorobot therapy showed complete regression of the tumors. The median survival time more than doubled, extending from 20.5 to 45 days.

They also tried their system in a test of a primary mouse lung cancer model, which mimics the human clinical course of lung cancer patients. They showed shrinkage of tumor tissues after a 2-week treatment.

Important Milestone

For Yan, the important study milestone represents the end of the beginning for nanomedicine.

“The thrombin delivery DNA nanorobot constitutes a major advance in the application of DNA nanotechnology for cancer therapy,” said Yan. “In a melanoma mouse model, the nanorobot not only affected the primary tumor but also prevented the formation of metastasis, showing promising therapeutic potential.”

Yan and his collaborators are now actively pursuing clinical partners to further develop this technology.

“I think we are much closer to real, practical medical applications of the technology,” said Yan. “Combinations of different rationally designed nanorobots carrying various agents may help to accomplish the ultimate goal of cancer research: the eradication of solid tumors and vascularized metastases. Furthermore, the current strategy may be developed as a drug delivery platform for the treatment of other diseases by modification of the geometry of the nanostructures, the targeting groups and the loaded cargoes.”

The work was supported by grants from National Basic Research Plan of China, the National Natural Science Foundation of China, the National Distinguished Young Scientists program, Innovation Research Group of National Natural Science Foundation, Beijing Municipal Science & Technology Commission, CAS Interdisciplinary Innovation Team, Key Research Program of Frontier Sciences, CAS, and US National Institute of Health Director’s Transformative Research Award.

Original Study: A DNA nanorobot functions as a cancer therapeutic in response to a molecular trigger in vivo

Top Image: Jason Drees, Arizona State University

DNA Nanorobots Pick Up And Sort Molecules

An autonomous molecular machine, made of a single strand of DNA, that can autonomously “walk” around a surface, pick up certain molecules and drop them off in designated locations has been developed by scientists at California Institute of Technology.

Lulu Qian, assistant professor of bioengineering, explains:

“Just like electromechanical robots are sent off to faraway places, like Mars, we would like to send molecular robots to minuscule places where humans can’t go, such as the bloodstream. Our goal was to design and build a molecular robot that could perform a sophisticated nanomechanical task: cargo sorting.”

The researchers constructed three basic building blocks that could be used to assemble a DNA robot: a “leg” with two “feet” for walking, an “arm” and “hand” for picking up cargo, and a segment that can recognize a specific drop-off point and signal to the hand to release its cargo. Each of these components is made of just a few nucleotides within a single strand of DNA.

Sorting Scattered Molecules

In principle, these modular building blocks could be assembled in many different ways to complete different tasks. A DNA robot with several hands and arms, for example, could be used to carry multiple molecules simultaneously.

The Qian group built a robot that could explore a molecular surface, pick up two different molecules — a fluorescent yellow dye and a fluorescent pink dye — and then distribute them to two distinct regions on the surface. Using fluorescent molecules enabled the researchers to see if the molecules ended up in their intended locations.

The robot successfully sorted six scattered molecules, three pink and three yellow, into their correct places in 24 hours. Adding more robots to the surface shortened the time it took to complete the task.

“Though we demonstrated a robot for this specific task, the same system design can be generalized to work with dozens of types of cargos at any arbitrary initial location on the surface. One could also have multiple robots performing diverse sorting tasks in parallel,”

said former graduate student Anupama Thubagere, who led the work.

Designing DNA Machines

The key to designing DNA machines is the fact that DNA has unique chemical and physical properties that are known and programmable. A single strand of DNA is made up of four different molecules called nucleotides — abbreviated A, G, C, and T — and arranged in a string called a sequence.

These nucleotides bond in specific pairs: A with T, and G with C. When a single strand encounters a so-called reverse complementary strand—for example, CGATT and AATCG—the two strands zip together in the classic double helix shape.

nanorobot dna concept
Credit: Ella Maru Studio, scientific-illustrations.com

A single strand containing the right nucleotides can force two partially zipped strands to unzip from each other. How quickly each zipping and unzipping event happens and how much energy it consumes can be estimated for any given DNA sequence, allowing researchers to control how fast the robot moves and how much energy it uses to perform a task.

Additionally, the length of a single strand or two zipped strands can be calculated. Thus, the leg and foot of a DNA robot can be designed for a desired step size—in this case, 6 nanometers, which is about a hundred millionth of a human’s step size.

Using these chemical and physical principles, researchers can design not only robots but also “playgrounds,” such as molecular pegboards, to test them on.

Random Walk

In this study, the DNA robot moves around on a 58-nanometer-by-58-nanometer pegboard on which the pegs are made of single strands of DNA complementary to the robot’s leg and foot. The robot binds to a peg with its leg and one of its feet; the other foot floats freely.

When random molecular fluctuations cause this free foot to encounter a nearby peg, it pulls the robot to the new peg and its other foot is freed. This process continues with the robot moving in a random direction at each step.

It may take a day for a robot to explore the entire board. Along the way, as the robot encounters cargo molecules tethered to pegs, it grabs them with its “hand” components and carries them around until it detects the signal of the drop-off point.

The process is slow, but it allows for a very simple robot design that utilizes very little chemical energy.

“We don’t develop DNA robots for any specific applications. Our lab focuses on discovering the engineering principles that enable the development of general-purpose DNA robots. However, it is my hope that other researchers could use these principles for exciting applications, such as using a DNA robot for synthesizing a therapeutic chemical from its constituent parts in an artificial molecular factory, delivering a drug only when a specific signal is given in bloodstreams or cells, or sorting molecular components in trash for recycling,”

says Qian.

Original Study: A Cargo-sorting Robot

Top Image: Demin Liu

Carbon Dioxide Absorption By Boron Nitride Foam

A light foam created from two-dimensional sheets of hexagonal boron nitride by materials scientists at Rice University absorbs carbon dioxide.

They discovered freeze-drying hexagonal-boron nitride (h-BN) transformed it into a macro-scale foam that disintegrates in liquids. But adding a bit of polyvinyl alcohol (PVA) into the mix turned it into a far more robust and useful material.

Blocks of hexagonal-boron nitride foam
Blocks of hexagonal-boron nitride foam treated with polyvinyl alcohol proved able to adsorb more than three times its weight in carbon dioxide. The reusable material was created at Rice University.
Credit: Ajayan Research Group/Rice University

The foam is highly porous and its properties can be tuned for use in air filters and as gas absorption materials, according to researchers in the Rice lab of materials scientist Pulickel Ajayan. The polyvinyl alcohol serves as a glue. Mixed into a solution with flakes of h-BN, it binds the junctions as the microscopic sheets arrange themselves into a lattice when freeze-dried.

The one-step process is scalable, the researchers said.

Absorbs 340% Its Weight

Co-author and Rice postdoctoral researcher Chandra Sekhar Tiwary said:

“Even a very small amount of PVA works. It helps make the foam stiff by gluing the interconnects between the h-BN sheets – and at the same time, it hardly changes the surface area at all.”

In molecular dynamics simulations, the foam adsorbed 340 percent of its own weight in carbon dioxide. The greenhouse gas can be evaporated out of the material, which can be reused repeatedly, Tiwary said. Compression tests showed the foam got stiffer through 2,000 cycles as well.

And when coated with PDMS, another polymer, the foam becomes an effective shield from lasers that could be used in biomedical, electronics and other applications, he said.

Ultimately, the researchers want to gain control over the size of the material’s pores for specific applications, like separating oil from water.

hexagonal-boron nitride foam
A microscope image shows the high surface area of hexagonal-boron nitride foam glued together with polyvinyl alcohol.
Credit: Ajayan Research Group/Rice University

Simulations carried out by co-author Cristiano Woellner, a joint postdoctoral researcher at Rice and the State University of Campinas, Brazil, could serve as a guide for experimentalists.

“It’s important to join experiments and theoretical calculations to see the mechanical response of this composite,” Woellner said. “This way, experimentalists will see in advance how they can improve the system.”

Original Study: Lightweight Hexagonal Boron Nitride Foam for CO2 Absorption

Top Image: A molecular dynamics simulation shows polyvinyl alcohol molecules of carbon (teal), oxygen (red) and hydrogen (white) binding two-dimensional sheets of hexagonal-boron nitride (blue and yellow). The reusable material created at Rice University can sequester more than three times its weight in carbon dioxide. Credit: Ajayan Research Group/Rice University

Multi-color Electron Microscopy For Biomolecules

An added detector on an electron microscope can aid in determining which molecules are found in which parts of a cell, scientists at the UMCG and Delft University of Technology report. Ben Giepmans, the team leader from Groningen, explains:

“This detector enables us to assign a colour to molecules in a cell. Multicolour electron microscopes are a new addition to medical research, and they could generate interesting results.”

Electron microscopes can zoom in minute detail, making the tiniest structures in a cell visible. They are therefore much more precise than optical microscopes, which have been around for much longer.

“But an electron microscope always shows images in gray scales,” Giepmans says. “We have now demonstrated that you can introduce colour with this detector. You can compare it with Google Earth—satellite images give a good impression of what a small part of the Earth looks like, but if you color the roads and cities, it is much easier to find your bearings. Similarly, if you colour molecules, you make it easier to see which biological structures you are looking at.”

Energy-dispersive X-ray Analysis

The researchers used a detector that utilized energy-dispersive X-ray analysis, developed for materials science. The Delft team leader Jacob Hoogenboom says,

“We purchased the detector to study extremely small structures for the semiconductor industry. We were already working with the UMCG on other projects. They had used comparable techniques to colour in biological samples, but this only produced two colours. So we thought we’d study them with this detector, too.”

The detector can identify each separate building block of molecules, including nitrogen, phosphorus, sulphur, iron and other metals. Giepmans says,

“DNA contains a lot of phosphorus, for instance. If we map the phosphorus in a cell and assign it a color, we can see where the DNA is.”

Diabetes Research

The researchers applied the technique to their own field of research, type 1 diabetes.

“We looked at the cells in the pancreas of a rat that was sensitive to type 1 diabetes. We could clearly identify the different cells in the pancreas. Insulin-producing cells acquired a colour from the sulphur, because insulin contains a lot of sulphur, whereas cells that produce glucagon took on another colour, because that hormone contains other elements.”

Tissue was identified in Groningen and sent to Delft, where the new detector was used to analyse certain regions. This led to surprising observations.

“In this rat, we could see substances in parts of the pancreas where they are not usually found,” Giepmans explains.

The UMCG now has its own ‘color EM’ detector, and Giepmans is already receiving cell material from home and abroad so that he can test the new technique.

The researchers are not the first to color elements using an electron microscope.

“In a previous study, they could only color two substances. We can now measure and color many different elements at the same time. I knew that it must be possible. I dreamt about it for a long time, but it only got off the ground when we started working with Delft and used their detector on our tissue. What is perhaps best about this technique is that it is affordable. It really is a new microscopy tool that we are already using for many research groups.”

Funding support came from the European Union and the Netherlands Organization for Scientific Research.

Original Study: Multi-color electron microscopy by element-guided identification of cells, organelles and molecules
Image: TU Delft