Chemistry

Work on tiny dots that light up TV screens and help doctors see the blood vessels that feed tumors has earned three scientists the 2023 Nobel Prize in chemistry.  

Chemist Moungi Bawendi, chemist Louis Brus and physicist Alexei Ekimov split the prize for the discovery and synthesis of quantum dots, the Royal Swedish Academy of Sciences announced October 4.

Chemist Moungi Bawendi (left), chemist Louis Brus (middle) and physicist Alexei Ekimov (right) have split the 2023 Nobel Prize in chemistry for “the discovery and development of quantum dots, nanoparticles so tiny that their size determines their properties.”MIT, Columbia University, Nexdot

“Quantum dots are a new class of materials, different from molecules,” said Heiner Linke, a member of the Nobel committee. Just adjusting the size of these nanoparticles, roughly a few billionths of a meter across, can change their properties — optical, electric, magnetic, even melting points — thanks to quantum mechanics (SN: 6/29/15). 

That’s also true of color. “If you want to make different colors with molecules, you would choose a new molecule, a new set of atoms” arranged in a different structure, Linke said. But quantum dots of different colors have the exact same arrangement of atoms. The only difference is particle size.

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When quantum dots are irradiated by light, electrons within get energized, eventually releasing that energy as fluorescent light. The smaller the dots are, the more they compress the wave function of an electron, increasing its energy so that the dot appears blue. Larger dots appear red. 

Dots of the same size made from different materials may also emit slightly different wavelengths of light, says Jean-Marc Pecourt, a chemist at CAS, a division of the American Chemical Society. Quantum dots are usually made from semiconductor materials, such as graphene, selenite or metal sulfides, Pecourt says. So by adjusting materials or the size of quantum dots, chemists can alter their properties for a wide variety of uses.

The idea that the size of these nanoparticles could alter their properties was predicted nearly a century ago, but at the time it seemed impossible to reproduce that effect in the real world. To do that, researchers would need a perfectly crystalline material, and would need to control the size of the nanomaterial very precisely, sculpting it atom layer by atom layer.  

Then, in the early 1980s, Ekimov and Brus independently showed that it could be done. Ekimov, now at Nanocrystals Technology, Inc., in Briarcliff Manor, N.Y., demonstrated this in glass, adding copper chloride to produce tiny crystals and revealing that the color of the glass was linked to the size of those crystals. Brus, of Columbia University, made a similar discovery, but in a different context: He demonstrated the link between size and color for nanoparticles floating freely in a solution and in gaseous compounds (SN: 10/3/92). 

Those discoveries triggered intense interest in how to harness these little dots for a variety of applications. But manufacturing them would require being able to control the size of the particles to precise specifications. 

A decade later, Bawendi, of MIT, developed a method to precisely control the speed of the crystals’ growth in a solution, figuring out how to stop them right when they reach a desired size. He did this by first injecting chemical reagents into the solution that instantaneously formed the tiny crystals and then promptly adjusting the temperature of the solution, halting their growth. 

“I’m deeply honored and surprised and shocked by the announcement this morning,” Bawendi said October 4 during an MIT news conference. “I’m especially honored to share this with Lou Brus, who was my postdoctoral mentor [from] whom I learned so much. I tried to emulate his scholarship and his mentoring style as a professor myself when I came to MIT.” 

Bawendi started working on quantum dots after he met Brus at Nokia Bell Labs, headquartered in Murray Hill, N.J. The researchers needed high quality quantum dots to study the physics of the nanoparticles, Bawendi said. “It wasn’t because I wanted to make the best quantum dots possible for application, it was because we needed to make the best possible quantum dots to study them.” It took years of trial and error to work out the method, he said.

By making it possible to manufacture quantum dots, Bawendi’s method opened up a world of possible uses for the nanoparticles. Quantum dots make it possible to very precisely change the color of LED lights and dramatically improve their efficiency. Dots that glow with fluorescent light, injected into the body and attached to immune cells that swarm to cancerous tissues, can help surgeons distinguish even hard-to-see tumors (SN: 8/3/04). The ability to be tuned to absorb different wavelengths of light could also allow the manufacture of customized solar cells that are highly efficient in different light conditions. The dots might also be used to build quantum computers, Pecourt says (SN: 2/14/18). 

Biomedical engineer and chemist Warren Chan says the prize is well deserved. “They’re the ones who built the foundation,” says Chan, of the University of Toronto. “I’m really happy that the field is getting credit for really changing the world, not just in quantum dots, but in a lot of different areas.” 

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One of the first applications came in the late 1990s when Chan and colleagues used quantum dots to tag cells in the lab, he says. “The surface modifications that were used for integrating quantum dots for applications were then also adapted for other types of nanoparticles.”

The Nobel committee looks not only at past contributions, but also the effect a discovery may have on the future, Chan says. The ability to tune nanoparticles by changing their size or surface properties could open a wide variety of possibilities that have not yet been explored. Chan and colleagues are now using quantum dots to detect infectious diseases, including HIV, influenza and hepatitis B.

“I was absolutely thrilled to see this,” says Judith Giordan, president of the American Chemical Society. “We have three people recognized who brought this technology from a dream, a hope, a theoretical construct … all the way through synthesis and manufacture.”

Earlier this week, the development of mRNA vaccines — widely speculated as a candidate for the 2023 chemistry Nobel Prize — received the Nobel in medicine or physiology instead (SN: 10/2/23). 

“Sometimes chemistry gets a bad rap,” Giordan says. “But here are two magnificent examples of how chemistry has solved problems in the world.”

The three winners will share the prize of 11 million Swedish kronor, or about $1 million. More

Luis Melecio-Zambrano is the summer 2023 science writing intern at Science News. They are finishing their master’s degree in science communication from the University of California, Santa Cruz, where they have reported on issues of environmental justice and agriculture. More

A new material design could reduce pollution where the rubber meets the road.

Strategically adding weak points along microscopic chains called polymers actually makes them harder to tear, researchers report in the June 23 Science. Because polymers are used in car tires, the findings could help reduce plastic pollution as tires wear down over time.

When tires scrape against the road, they drop tiny particles of rubber and plastic polymers, which pollute waterways and contaminate the air (SN: 11/12/18). Every year, tires release an estimated 6 million metric tons of these microplastics into the environment. Stronger polymers that break apart less easily could limit the amount of particles shed annually. More

For photosynthesis, one photon is all it takes.

Only a single particle of light is required to spark the first steps of the biological process that converts light into chemical energy, scientists report June 14 in Nature.

While scientists have long assumed that the reactions of photosynthesis begin upon the absorption of just one photon, that hadn’t yet been demonstrated, says physical chemist Graham Fleming, of the University of California, Berkeley. He and colleagues decided “we would just look to see was it really true that one photon was enough to start the whole thing off.”

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The sunlight that falls on Earth’s surface seems brilliant to human eyes. But on small scales, that translates to a dribble of photons. Only a few tens of photons of the appropriate wavelengths of sunlight fall on a square nanometer per second, the scale of the tiny chlorophyll and bacteriochlorophyll molecules that are central to photosynthesis in plants and bacteria.

Many laboratory experiments on photosynthesis use lasers, much more powerful light sources, to kick off the reactions. Instead, Graham and colleagues used a source of light that produces just two photons at a time. One photon served as a herald, going off to a detector to let researchers know when two photons were released. The other photon went into a solution containing photon-absorbing structures from the photosynthetic bacterium Rhodobacter sphaeroides. These structures, called light-harvesting 2 complexes, or LH2, are made up of two rings of bacteriochlorophyll and other molecules.

In a normal photosynthesis reaction, LH2 absorbs a photon and passes its energy to another LH2 complex, and then another, like a game of hot potato. Eventually the energy reaches another type of ring, called the light-harvesting 1 complex, or LH1, which then passes it to the reaction center where the energy is finally converted into a form that the bacterium can use.

In the experiment, there was no LH1, so the LH2 instead emitted a photon of a different wavelength than the first, a sign that energy had been transferred from the first ring of LH2 to the second, a first step of photosynthesis. The researchers detected that second photon, and by comparing the detection times to those of the initial herald photons, confirmed that the LH2 needed to absorb only one photon to kick things off.

Plants and bacteria use different processes for photosynthesis, but the initial steps are similar enough that in plants, too, a single photon would set off the initial steps, Fleming says. However, in plants, multiple independently absorbed photons are needed in order to complete the reaction.

The role of single photons isn’t surprising, says biochemist Richard Cogdell of the University of Glasgow in Scotland. The important thing the researchers have done, he says, is to demonstrate the new technique. “By doing this you’re able to essentially interrogate what will be happening in nature,” he says.  

Some scientists suspect that photosynthesis relies on quantum physics (SN: 2/3/10). While it’s not clear whether the new technique could resolve the role of quantum effects, it could help scientists disentangle natural effects from artifacts of using intense sources of light in studies of photosynthesis.

“You can really work out what’s happening in the early reactions in photosynthesis as it were outside,” says Cogdell, “[as if] you could shrink yourself down and watch these photons moving around.” More

Art historians often wish that Renaissance painters could shell out secrets of the craft. Now, scientists may have cracked one using chemistry and physics.

Around the turn of the 15th century in Italy, oil-based paints replaced egg-based tempera paints as the dominant medium. During this transition, artists including Leonardo da Vinci and Sandro Botticelli also experimented with paints made from oil and egg (SN: 4/30/14). But it has been unclear how adding egg to oil paints may have affected the artwork.  

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“Usually, when we think about art, not everybody thinks about the science which is behind it,” says chemical engineer Ophélie Ranquet of the Karlsruhe Institute of Technology in Germany.

In the lab, Ranquet and colleagues whipped up two oil-egg recipes to compare with plain oil paint. One mixture contained fresh egg yolk mixed into oil paint, and had a similar consistency to mayonnaise. For the other blend, the scientists ground pigment into the yolk, dried it and mixed it with oil — a process the old masters might have used, according to the scant historical records that exist today. Each medium was subjected to a battery of tests that analyzed its mass, moisture, oxidation, heat capacity, drying time and more.

In both concoctions, the yolk’s proteins, phospholipids and antioxidants helped slow paint oxidation, which can cause paint to turn yellow over time, the team reports March 28 in Nature Communications. 

In the mayolike blend, the yolk created sturdy links between pigment particles, resulting in stiffer paint. Such consistency would have been ideal for techniques like impasto, a raised, thick style that adds texture to art. Egg additions also could have reduced wrinkling by creating a firmer paint consistency. Wrinkling sometimes happens with oil paints when the top layer dries faster than the paint underneath, and the dried film buckles over looser, still-wet paint.

The hybrid mediums have some less than eggs-ellent qualities, though. For instance, the eggy oil paint can take longer to dry. If paints were too yolky, Renaissance artists would have had to wait a long time to add the next layer, Ranquet says.

“The more we understand how artists select and manipulate their materials, the more we can appreciate what they’re doing, the creative process and the final product,” says Ken Sutherland, director of scientific research at the Art Institute of Chicago, who was not involved with the work.

Research on historical art mediums can not only aid art preservation efforts, Sutherland says, but also help people gain a deeper understanding of the artworks themselves. More

MIT chemist Admir Masic really hoped his experiment wouldn’t explode.

Masic and his colleagues were trying to re-create an ancient Roman technique for making concrete, a mix of cement, gravel, sand and water. The researchers suspected that the key was a process called “hot mixing,” in which dry granules of calcium oxide, also called quicklime, are mixed with volcanic ash to make the cement. Then water is added.

Hot mixing, they thought, would ultimately produce a cement that wasn’t completely smooth and mixed, but instead contained small calcium-rich rocks. Those little rocks, ubiquitous in the walls of the Romans’ concrete buildings, might be the key to why those structures have withstood the ravages of time.

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That’s not how modern cement is made. The reaction of quicklime with water is highly exothermic, meaning that it can produce a lot of heat — and possibly an explosion.

“Everyone would say, ‘You are crazy,’” Masic says.

But no big bang happened. Instead, the reaction produced only heat, a damp sigh of water vapor — and a Romans-like cement mixture bearing small white calcium-rich rocks.

Researchers have been trying for decades to re-create the Roman recipe for concrete longevity — but with little success. The idea that hot mixing was the key was an educated guess.

Masic and colleagues had pored over texts by Roman architect Vitruvius and historian Pliny, which offered some clues as to how to proceed. These texts cited, for example, strict specifications for the raw materials, such as that the limestone that is the source of the quicklime must be very pure, and that mixing quicklime with hot ash and then adding water could produce a lot of heat.

The rocks were not mentioned, but the team had a feeling they were important.

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“In every sample we have seen of ancient Roman concrete, you can find these white inclusions,” bits of rock embedded in the walls. For many years, Masic says, the origin of those inclusions was unclear — researchers suspected incomplete mixing of the cement, perhaps. But these are the highly organized Romans we’re talking about. How likely is it that “every operator [was] not mixing properly and every single [building] has a flaw?”

What if, the team suggested, these inclusions in the cement were actually a feature, not a bug? The researchers’ chemical analyses of such rocks embedded in the walls at the archaeological site of Privernum in Italy indicated that the inclusions were very calcium-rich.

That suggested the tantalizing possibility that these rocks might be helping the buildings heal themselves from cracks due to weathering or even an earthquake. A ready supply of calcium was already on hand: It would dissolve, seep into the cracks and re-crystallize. Voila! Scar healed.

But could the team observe this in action? Step one was to re-create the rocks via hot mixing and hope nothing exploded. Step two: Test the Roman-inspired cement. The team created concrete with and without the hot mixing process and tested them side by side. Each block of concrete was broken in half, the pieces placed a small distance apart. Then water was trickled through the crack to see how long it took before the seepage stopped.

“The results were stunning,” Masic says. The blocks incorporating hot mixed cement healed within two to three weeks. The concrete produced without hot mixed cement never healed at all, the team reports January 6 in Science Advances.

Cracking the recipe could be a boon to the planet. The Pantheon and its soaring, detailed concrete dome have stood nearly 2,000 years, for instance, while modern concrete structures have a lifespan of perhaps 150 years, and that’s a best case scenario (SN: 2/10/12). And the Romans didn’t have steel reinforcement bars shoring up their structures.

More frequent replacements of concrete structures means more greenhouse gas emissions. Concrete manufacturing is a huge source of carbon dioxide to the atmosphere, so longer-lasting versions could reduce that carbon footprint. “We make 4 gigatons per year of this material,” Masic says. That manufacture produces as much as 1 metric ton of CO2 per metric ton of produced concrete, currently amounting to about 8 percent of annual global CO2 emissions.

Still, Masic says, the concrete industry is resistant to change. For one thing, there are concerns about introducing new chemistry into a tried-and-true mixture with well-known mechanical properties. But “the key bottleneck in the industry is the cost,” he says. Concrete is cheap, and companies don’t want to price themselves out of competition.

The researchers hope that reintroducing this technique that has stood the test of time, and that could involve little added cost to manufacture, could answer both these concerns. In fact, they’re banking on it: Masic and several of his colleagues have created a startup they call DMAT that is currently seeking seed money to begin to commercially produce the Roman-inspired hot-mixed concrete. “It’s very appealing simply because it’s a thousands-of-years-old material.” More