University of Michigan: Synthetic Nano-Cartilage could be key to safe ‘structural batteries’ – Extending Battery Capability


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Your knees and your smartphone battery have some surprisingly similar needs, a University of Michigan professor has discovered, and that new insight has led to a ‘structural battery’ prototype that incorporates a cartilage-like material to make the batteries highly durable and easy to shape. Credit: Evan Doughtry

Your knees and your smartphone battery have some surprisingly similar needs, a University of Michigan professor has discovered, and that new insight has led to a “structural battery” prototype that incorporates a cartilage-like material to make the batteries highly durable and easy to shape.

The idea behind structural batteries is to store energy in structural components — the wing of a drone or the bumper of an electric vehicle, for example. They’ve been a long-term goal for researchers and industry because they could reduce weight and extend range. But structural batteries have so far been heavy, short-lived or unsafe.

In a study published in ACS Nano, the researchers describe how they made a damage-resistant rechargeable zinc battery with a cartilage-like solid electrolyte. They showed that the batteries can replace the top casings of several commercial drones. The prototype cells can run for more than 100 cycles at 90 percent capacity, and withstand hard impacts and even stabbing without losing voltage or starting a fire.Military drone images

“A battery that is also a structural component has to be light, strong, safe and have high capacity. Unfortunately, these requirements are often mutually exclusive,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research.

Harnessing the properties of cartilage

To sidestep these trade-offs, the researchers used zinc — a legitimate structural material — and branched nanofibers that resemble the collagen fibers of cartilage.

“Nature does not have zinc batteries, but it had to solve a similar problem,” Kotov said. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries. It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.”

In our bodies, cartilage combines mechanical strength and durability with the ability to let water, nutrients and other materials move through it. These qualities are nearly identical to those of a good solid electrolyte, which has to resist damage from dendrites while also letting ions flow from one electrode to the other.

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Read More: A Way to Make Batteries in Almost Any Shape (Form)

Dendrites are tendrils of metal that pierce the separator between the electrodes and create a fast lane for electrons, shorting the circuit and potentially causing a fire. Zinc has previously been overlooked for rechargeable batteries because it tends to short out after just a few charge/discharge cycles.

Not only can the membranes made by Kotov’s team ferry zinc ions between the electrodes, they can also stop zinc’s piercing dendrites. Like cartilage, the membranes are composed of ultrastrong nanofibers interwoven with a softer ion-friendly material.

In the batteries, aramid nanofibers — the stuff in bulletproof vests — stand in for collagen, with polyethylene oxide (a chain-like, carbon-based molecule) and a zinc salt replacing soft components of cartilage.

Demonstrating safety and utility

To make working cells, the team paired the zinc electrodes with manganese oxide — the combination found in standard alkaline batteries. But in the rechargeable batteries, the cartilage-like membrane replaces the standard separator and alkaline electrolyte. As secondary batteries on drones, the zinc cells can extend the flight time by 5 to 25 percent — depending on the battery size, mass of the drone and flight conditions.

Safety is critical to structural batteries, so the team deliberately damaged their cells by stabbing them with a knife. In spite of multiple “wounds,” the battery continued to discharge close to its design voltage. This is possible because there is no liquid to leak out.

For now, the zinc batteries are best as secondary power sources because they can’t charge and discharge as quickly as their lithium ion brethren. But Kotov’s team intends to explore whether there is a better partner electrode that could improve the speed and longevity of zinc rechargeable batteries.

The research was supported by the Air Force Office of Scientific Research and National Science Foundation. Kotov teaches in the Department of Chemical Engineering. He is also a professor of materials science and engineering, and macromolecular science and engineering.

Story Source:

Materials provided by University of MichiganNote: Content may be edited for style and length.


Journal Reference:

  1. Mingqiang Wang, Ahmet Emre, Siu On Tung, Alycia Gerber, Dandan Wang, Yudong Huang, Volkan Cecen, Nicholas A. Kotov. Biomimetic Solid-State Zn2 Electrolyte for Corrugated Structural BatteriesACS Nano, 2019; DOI: 10.1021/acsnano.8b05068
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Lithium ion Battery Tech gets a ‘Cool’ rival: Frozen Liquid Air – Could LAES ‘de-throne’ the King?


schematic-of-liquid-air-energy-storage-laes-system

In a bid to help scale renewable energy, many companies are working on new ways to store energy long-term. But the plain old battery is still king. Can ultra-cold liquid air make all the difference?

Elon Musk’s Tesla took less than 100 days to install its Hornsdale Power Reserve – the world’s largest lithium ion battery – in dusty, sunny South Australia, following a Twitter bet. UK-based Highview Power has been a bit slower than that. After years of delays, its Liquid Air Energy Storage (LAES) plant near Manchester has come online.

It’s the world’s first grid-scale liquid air energy storage plant – and with off-the-shelf components, it’s relatively easy and cheap to build and to scale. Air is cooled down, made liquid, and stored in tanks for weeks until you need electricity again. Sounds pretty cool, doesn’t it?

While it’s certainly a moment of success for alternative energy storage, don’t break out the confetti yet: lithium ion isn’t about to give up its crown, says Dan Finn-Foley, a senior analyst at GTM Research. In the US alone, li-ion battery technology accounts for more than 95 per cent of annual storage deployments. But batteries, even the most efficient ones, fail to store energy for longer than a few hours.

So where does it leave solar and wind power, with their need to smooth out the supply peaks and troughs?

“Alternative energy storage could be a holy grail for the grid, a missing link that could get us towards renewables much faster,” explains Ravi Manghani, the director of the energy storage section of GTM Research.

One thing is certain: without reliable energy storage technology, the world will struggle to wean itself off dirty coal and other fossil fuels. If an economy and society wants to rely on renewables on a massive scale, it needs a backup solution. Renewables are growing fast – last year, 29 per cent of all electricity in the UK was generated by renewable energy plants; in Germany, it was 33 per cent.

But the sun doesn’t shine at night, and wind doesn’t always blow. Right now, the storage market is dominated by lithium ion battery technology, but despite Tesla’s worldwide total of one gigawatt-hour of energy storage, the available batteries can last about eight hours tops. “We absolutely must install multiple days worth of energy storage – we can’t get away with four to six hours only,” says Manghani.

Storing electricity for longer

Hornsdale Power Reserve 100 MW storage system can provide 129 megawatt-hours of electricity and is connected to the Hornsdale Wind Farm. Its primary aim is to increase grid stability during system contingencies events like extremely hot summer afternoons, or when a large gas plant will trip – it improves the grid’s ability to cope with small blips in energy generation, which typically means replacing about one to one and half hour of energy supply.

“It’s designed to handle very short duration contingency needs,” says Finn-Foley. That’s why batteries simply can’t provide peak power, or compete with and replace so-called ‘peaker’ plants – power plants like natural gas power stations that are only switched on to fill the gap at times of peak energy demand. They also can’t help extend the use of solar power to later in the day. “You’ll need 10 to 12 hours of continuous discharge duration, which means you’ll need four times the battery or more,” says Finn-Foley.

That’s where alternative energy storage technologies could change things.

Currently, the best long-duration energy storage solutions are thermal storage, pumped hydro, compressed air energy storage – and the newest kid on the block, liquid air energy storage. There are also alternative battery technologies such as flow batteries, which researchers believe may one day scale up to discharge energy for longer than lithium ion.

At the end of the day, though, it all comes down to cost. And developing and operating novel tech is not cheap. The cost of lithium ion batteries, meanwhile, keeps on plummeting, thanks to the ever-surging demand for consumer electronics and electric cars, with all the giga and megafactories mushrooming around the globe. Over the past few years, li-ion battery prices dropped by more than 60 per cent – and are expected to fall by another 40 per cent by 2022.

These cost drops are impressive – but while batteries are good for providing power over short timescales, they quickly get very expensive for storing large amounts of energy over hours and days.

What is liquid air energy storage?

Enter LAES. First dreamt up in the 1970s in the UK and then toyed with in the 1980s and 90s by Hitachi and Mitsubishi (without any proper pilot plants though), this tech has the potential to scale up at low cost, says professor Yulong Ding at the University of Birmingham, who together with Highview developed the technology.

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LAES works by using electricity from the grid to cool atmospheric air until it liquifies, and then storing it in big tanks at low pressure at –196C – at a fraction of the air’s original volume. “The working principle is quite similar to a domestic fridge – just the temperature and pressure ranges are different,” says Ding. The air can stay in the tanks for weeks and even months, dissipating slowly – and the better the insulation, the slower it will vanish. “It can easily be kept in tanks for about two months,” adds Ding.

When you need to generate electricity, you just have to heat the air to ambient temperature. In the process it will expand a whopping 700 times, creating a lot of air pressure that can be used to spin a turbine in the same way that, say, steam would in conventional generators – and produce electricity.

Because it’s so similar to a traditional fridge, the individual components of LAES for cooling, storing, and re-pressurising gases can be bought quite cheaply off the shelf. “These are well-understood, decades and centuries-old processes that are highly cost-efficient,” says Finn-Foley. The only novel bit here, says Ding, is the integration of the different parts in the most-optimised way.

LAES is not that efficient, though: Tesla’s battery in Australia is 88 per cent efficient, while LAES is 60 to 70 per cent, says Manghani. But as batteries can only store energy for a few hours, if they need to supply energy for longer, they quickly get very costly.

LAES also cannot respond to grid signals in a matter of milliseconds like batteries do. On the upside, the liquid air project can provide energy in bulk, around a day’s worth of it (although the pilot can store just 5 MW of electricity – enough to power roughly 5,000 homes for about three hours; on a commercial scale, Manchester’s LAES plant could have the capacity of 50 MW).

Still, as the liquid air energy storage is so cheap and can scale easily, it could, potentially, fill a crucial gap in the successful energy ecosystem geared towards renewables. Why, then, is it just the UK looking into it? Jonathan Radcliffe, an energy researcher at the University of Birmingham, has a simple answer: because of the UK’s ambitious plans for electricity generation from offshore wind in the 2020s. Also, he adds, “as an island, we have fewer connections to other electricity networks that could help balance supply and demand”.

Manghani is even more prosaic: the world isn’t ready for LAES just yet. Even at the scale of current use of renewables in countries like Germany and Australia, “there is no market out there that needs such longer duration of storage solutions,” he says – experimental plants like LAES are looking for a problem that doesn’t yet exist. But in a decade from now, once solar panel arrays and wind turbines produce more than 60 or 70 per cent of our energy, long-duration storage will be crucial. And we can’t wait a decade to start finding a viable solution, says Manghani – we have to get ready now.

De-Throning the king?

Highview claims that overall, LAES plants will be cheaper than lithium ion; if that’s confirmed at scale: “I expect the technology to go global quickly,” says Finn-Foley. But first, it has to start competing in multiple markets and applications, and existing regulations, as well as incentives to invest in energy storage, are a challenge.

laes iii application-comparison-for-various-energy-storage-technologies-with-the-addition-of-ptes

The LAES plant “will need to operate for some time to demonstrate that they have truly worked out the kinks, says Finn-Foley. It also has to prove viability, which is tricky for a project that is supposed to run for decades. “Batteries degrade and must be replaced – but proving a forty-year lifetime is hard to do until you’ve run it for 40 years,” he adds.

But in the end of the day, alternative technologies aren’t trying to usurp li-ion’s throne, but “carve out their own kingdom, with applications and use cases that they think they can do better,” he says. “So far they have been unsuccessful, but a pilot project proving cost-effectiveness is a crucial step.” For the next five years though, he says, “lithium ion will keep the crown”.

 

Promising New Research for High Performance Lithium Batteries – Engineering 2D Nanofluidic Channels


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Abstract: In article number 1703909, Gang Chen, Guihua Yu, and co-workers present a novel concept of 2D nanofluidic lithium-ion transport channels based on stacked Co3O4nanosheets for high-performance lithium batteries. This unique nanoarchitecture exhibits exceptional capacity and outstanding long-term cycling stability for lithium-ion storage at high-rates in both half- and full-cells.

 

Despite being a promising electrode material, bulk cobalt oxide (Co3O4) exhibits poor lithium ion storage properties. Nanostructuring, e.g. making Co3O4 into ultrathin nanosheets, shows improved performance, however, Co3O4-based nanomaterials still lack long-term stability and high rate capability due to sluggish ion transport and structure degradation.

Nanofluidic channels possess desired properties to address above issues. However, while these unique structures have been studied in hollow nanotubes and recently in restacked layered materials such as graphene, it remains challenging to construct nanofluidic channels in intrinsically non-layered materials.
Motived by the large number of non-layered materials, e.g. transition metal oxides, which hold great promise in battery applications, scientists aim to extend the concept of nanofluidic channels into these materials and improve their electrochemical properties.
Nanofluidic channels feature a unique unipolar ionic transport when properly designed and constructed. By controlling surface charge and channel spacing, enhanced and selective ion transport can be achieved in these channels by constructing them with dimensions comparable to the double Debye length and opposite surface charge with respect to the transporting ion.
In a new study published in Advanced Materials (“Engineering 2D Nanofluidic Li-Ion Transport Channels for Superior Electrochemical Energy Storage”), researchers have developed a Co3O4-based two-dimensional (2D) nano-architecture possessing nanofluidic channels with specially designed interlayer characteristics for fast lithium ion transport, leading to exceptional performance in lithium ion batteries ever reported for this material.
“Such constructed 2D nanofluidic channels in non-layered materials manifest a general structural engineering strategy for improving electrochemical properties in a vast number of different electrode materials,” Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute, University of Texas at Austin. “The enhanced and selective ion transport demonstrated in our study is of broad interest to many applications where fast kinetics of ion transport is essential.”
Illustration of lithium ion transport in the 2D nanofluidic channels
Illustration of lithium ion transport in 2D nanofluidic channels. (Reprinted with permission by Wiley-VCH)
On the one hand, an intercalated molecule acts as interlayer pillar in the stacked oxide, constituting transport channels with proper spacing. On the other hand, negatively charged functional groups anchored on the nanosheets surface facilitate transport of positively charged lithium ions inside the channels.
“Satisfying aforementioned conditions for unipolar ionic transport, combined with other advantageous features – extra storage capacity contributed by the surface functional groups, buffered structural stress from the interlayer spacing, and shortened lithium ion diffusion distance due to the ultrathin nanosheet morphology – the resulting nanoarchitecture exhibit exceptional electrochemical performance as tested in lithium-ion batteries,” notes Yu.
In a next step, the researchers are going to extend the concept of 2D nanofluidic channels to other electrode materials with or without layering structures. With ability to further tune interlayer spacing, they expect some promising energy storage applications in beyond-lithium-ion batteries.
It might also be interesting to examine this structural engineering strategy in other applications, for example, catalysis.
Design and LIBs application of Co3O4 nanosheets with 2D nanofluidic channels
Design and LIBs application of Co3O4 nanosheets with 2D nanofluidic channels. (a) The synthetic route from Co-based layered hydroxide precursor to Co3O4 nanosheets with 2D nanofluidic channels. (b) Cycling performance of a full cell (anode: Co3O4 nanosheets /cathode: commercial LiCoO2). (Reprinted with permission by Wiley-VCH)
Constructing 2D nanofluidic channels for energy storage application is still in its infancy and the success of using non-layered materials demonstrated in this study promises a bright future in this direction with a broader material coverage.
“We are also taking this research direction even further by looking into the transport and storage properties for energy storage systems based on larger charge-carrying ions, such as Na+ and Mg2+, ” concludes Yu. “In order to realize that, an important challenge is to tune the channel spacing in a controlled manner. It is also imperative to investigate structural stability and scalability of this specially designed nanoarchitecture for its utilization in practical applications.”
@Michael Berger © Nanowerk

U.S. Department of Energy’s Lawrence Berkeley National Laboratory: Scientists grow atomically thin transistors and circuits


auto thin trans 072016 berkeleylabsThis schematic shows the chemical assembly of two-dimensional crystals. Graphene is first etched into channels and the TMDC molybdenum disulfide (MoS2) begins to nucleate around the edges and within the channel. On the edges, MoS2 slightly …more

In an advance that helps pave the way for next-generation electronics and computing technologies—and possibly paper-thin gadgets —scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) developed a way to chemically assemble transistors and circuits that are only a few atoms thick.

What’s more, their method yields functional structures at a scale large enough to begin thinking about real-world applications and commercial scalability.

They report their research online July 11 in the journal Nature Nanotechnology.

The scientists controlled the synthesis of a transistor in which narrow channels were etched onto conducting graphene, and a semiconducting material called a transition-metal dichalcogenide, or TMDC, was seeded in the blank channels. Both of these materials are single-layered crystals and atomically thin, so the two-part assembly yielded electronic structures that are essentially two-dimensional. In addition, the synthesis is able to cover an area a few centimeters long and a few millimeters wide.

“This is a big step toward a scalable and repeatable way to build atomically thin electronics or pack more computing power in a smaller area,” says Xiang Zhang, a senior scientist in Berkeley Lab’s Materials Sciences Division who led the study.

Zhang also holds the Ernest S. Kuh Endowed Chair at the University of California (UC) Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley. Other scientists who contributed to the research include Mervin Zhao, Yu Ye, Yang Xia, Hanyu Zhu, Siqi Wang, and Yuan Wang from UC Berkeley as well as Yimo Han and David Muller from Cornell University.

Their work is part of a new wave of research aimed at keeping pace with Moore’s Law, which holds that the number of transistors in an integrated circuit doubles approximately every two years. In order to keep this pace, scientists predict that integrated electronics will soon require transistors that measure less than ten nanometers in length.

Transistors are electronic switches, so they need to be able to turn on and off, which is a characteristic of semiconductors. However, at the nanometer scale, likely won’t be a good option. That’s because silicon is a bulk material, and as electronics made from silicon become smaller and smaller, their performance as switches dramatically decreases, which is a major roadblock for future electronics.

Researchers have looked to two-dimensional crystals that are only one molecule thick as alternative materials to keep up with Moore’s Law. These crystals aren’t subject to the constraints of silicon.

In this vein, the Berkeley Lab scientists developed a way to seed a single-layered semiconductor, in this case the TMDC molybdenum disulfide (MoS2), into channels lithographically etched within a sheet of conducting graphene. The two atomic sheets meet to form nanometer-scale junctions that enable graphene to efficiently inject current into the MoS2. These junctions make atomically thin transistors.

“This approach allows for the chemical assembly of electronic circuits, using two-dimensional materials, which show improved performance compared to using traditional metals to inject current into TMDCs,” says Mervin Zhao, a lead author and Ph.D. student in Zhang’s group at Berkeley Lab and UC Berkeley.

Optical and electron microscopy images, and spectroscopic mapping, confirmed various aspects related to the successful formation and functionality of the two-dimensional transistors.

In addition, the scientists demonstrated the applicability of the structure by assembling it into the logic circuitry of an inverter. This further underscores the technology’s ability to lay the foundation for a chemically assembled atomic computer, the scientists say.

“Both of these two-dimensional crystals have been synthesized in the wafer scale in a way that is compatible with current semiconductor manufacturing. By integrating our technique with other growth systems, it’s possible that future computing can be done completely with atomically thin crystals,” says Zhao.

Explore further: Excitonic dark states shed light on TMDC atomic layers

More information: Large-scale chemical assembly of atomically thin transistors and circuits, Nature Nanotechnology, DOI: 10.1038/nnano.2016.115

Read more at: http://phys.org/news/2016-07-scientists-atomically-thin-transistors-circuits.html#jCp

Read more at: http://phys.org/news/2016-07-scientists-atomically-thin-transistors-circuits.html#jCp

Breakthrough in Nanotechnology is a BIG DEAL for Electronics


201306047919620University of Akron researchers have developed new materials that function on a nanoscale, which could lead to the creation of lighter laptops, slimmer televisions and crisper smartphone visual displays.

 

Known as “giant surfactants” – or surface films and liquid solutions – the researchers, led by Stephen Z.D. Cheng, dean of UA’s College of Polymer Science and Polymer Engineering, used a technique known as nanopatterning to combine functioning molecular nanoparticles with polymers to build these novel materials.

The giant surfactants developed at UA are large, similar to macromolecules, yet they function like molecular surfactants on a nanoscale, Cheng says. The outcome? Nanostructures that guide the size of electronic products.

More efficient designs possible Nanopatterning, or self-assembling molecular materials, is the genius behind the small, light and fast world of modern-day gadgetry, and now it has advanced one giant step thanks to the UA researchers who say these new materials, when integrated into electronics, will enable the development of ultra-lightweight, compact and efficient devices because of their unique structures.

During their self-assembly, molecules form an organized lithographic pattern on semiconductor crystals, for use as integrated circuits. Cheng explains that these self-assembling materials differ from common block copolymers (a portion of a macromolecule, comprising manyunits, that has at least one feature which is not present in the adjacent portions) because they organize themselves in a controllable manner at the molecular level.

“The IT industry wants microchips that are as small as possible so that they can manufacture smaller and faster devices,” says Cheng, who also serves as the R.C. Musson and Trustees Professor of Polymer Science at UA.

He points out that the current technique can produce the spacing of 22 nanometers only, and cannot go down to the 10 nanometers or less necessary to create tiny, yet mighty, devices. The giant surfactants, however, can dictate smaller-scale electronic components.

“This is exactly what we are pursuing – self-assembling materials that organize at smaller sizes, say, less than 20 or even 10 nanometers,” says Cheng, equating 20 nanometers to 1 /4,000th the diameter of a human hair.

Team work has commercial applications

An international team of experts, including George Newkome, UA vice president for research, dean of the Graduate School, and professor of polymer science at UA; Er-Qiang Chen of Peking University in China; Rong-Ming Ho of National Tsinghua University in Taiwan; An-Chang Shi of McMaster University in Canada; and several doctoral and postdoctoral researchers from Cheng’s group, have shown how well-ordered nanostructures in various states, such as in thin films and in solution, offer extensive applications in nanotechnology.

The team’s study is highlighted in a pending patent application through the University of Akron Research Foundation and in a recent journal article, “Giant surfactants provide a versatile platform for sub-10-nm nanostructure engineering” published in Proceedings of the National Academy of Sciences of the United States of America (110, 10078-10083, 2013).

“These results are not only of pure scientific interest to the narrow group of scientists, but also important to a broad range of industry people,” says Cheng, noting that his team is testing real-world applications in nanopatterning technologies and hope to see commercialization in the future.

Tetrapod Quantum Dots Light the Way


QDOTS imagesCAKXSY1K 8Fluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced optomechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile strength of polymer fibers with minimal impact on the fiber’s mechanical properties.

In a study led by Paul Alivisatos, Berkeley Lab Dir. and the Larry and Diane Bock Prof. of Nanotechnology at the Univ. of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium-sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.

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Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs  (upper) and stressed (lower).

Image:  LBN Laboratory.

“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20% by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”

Alivisatos is the corresponding author of a paper describing this research in NANO Letters. Co-authors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.

Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.

“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”

The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be achieved with electrospinning rather than diffusion.

When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.

“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber—it is no longer pure polylactic acid but instead a composite—we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”

The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.

“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”

For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.

“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.

“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”

Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.

“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”

Source: Lawrence Berkeley National Laboratory