Cocatalyst decoration has demonstrated to be effective in boosting CO2 photocatalytic conversion, yet the critical role of sing-atomic state on CO2 photocatalytic reduction, distinguished from the oxide and elemental states, remains a mystery. Herein, palladium single-atoms, palladium oxides, and palladium nanoparticles were homogeneously anchored on g-C3N4 to investigate their CO2 photocatalytic reduction behaviors. And the mechanism behind their significantly different activities, especially the role of Pd-SA in boosting CH4 production were comprehensively revealed. Credit: Chinese Journal of Catalysis (2023). DOI: 10.1016/S1872-2067(22)64199-8
Using solar energy and photocatalysts to convert CO2 into high value-added chemicals can simultaneously alleviate the greenhouse effect and energy crisis. Single atom cocatalysts decoration has been demonstrated to be an effective strategy to improve the CO2 photocatalytic reduction efficiency.
Unfortunately, when unraveling the mechanism behind performance promotion, most studies mainly focus on clarifying the superior physicochemical and photoelectrical properties of SACs in comparison with the substrate. The critical role of the sing-atomic state distinguished from those oxide and elemental states was often neglected and remains a mystery.
Recently, a research team led by Prof. Zhongbiao Wu and Haiqiang Wang from Zhejiang University, China, comprehensively investigated the effect of Pd chemical states on CO2 photocatalytic reduction of g-C3N4 (CN) under visible light irradiation, especially the critical role of Pd-SA in boosting CH4 production. The results were published in the Chinese Journal of Catalysis.
Performance tests showed Pd species decoration improved the CH4 production of CN, with Pd/CN-SA exhibiting the optimum yields (2.25 μmol g-1), markedly higher than that of PdOx/CN (1.08 μmol g-1) and Pd/CN-NP (0.44 μmol g-1). After comprehensive mechanism analysis with various characterization techniques, in-situ FTIR spectra and DFT calculations, it was found that the conducive activation of CO2, negative conduction band potentials, and excellent •H utilization efficiency, collaboratively contributed to the superior CO2 reduction performance of Pd/CN-SA, especially in the remarkably boosted CH4 production.
In addition, despite the larger electron density of Pd/CN-NP and PdOx/CN, the moderate reduction ability of their photogenerated electrons restricted the further reduction of adsorbed CO2 species and CO intermediate, limiting the enhancement of CO2 reduction activity. Furthermore, the CH4 evolutions of Pd/CN-NP and PdOx/CN were also limited by the poor •H supply and inferior •H utilization efficiency, respectively.
The new insights may advance the understanding of CO2 reduction process and inspire the design of efficient photocatalysts for CO2 photocatalytic conversion.
More information: Qian Li et al, Effect of palladium chemical states on CO2 photocatalytic reduction over g-C3N4: Distinct role of single-atomic state in boosting CH4 production, Chinese Journal of Catalysis (2023). DOI: 10.1016/S1872-2067(22)64199-8
Crystals of the newly synthesized compounds, anthraquinodimethane derivatives, which have different colors depending on their molecular structure. Credit: Yusuke Ishigaki
Newly synthesized organic molecules can be tuned to emit different colors depending on their molecular structures in crystal form.
Molecular switches are chemicals with molecular structures that can be shifted between two or more stable configurations in response to changes in their environment. They are of great interest in the development of molecular computers, molecular machines and drug delivery systems. Compounds with conformational isomers—identical molecular formulas but different molecular structures—can make very effective molecular switches.
Researchers at Hokkaido University and Kyushu University have developed a technique to synthesize potential molecular switches from anthraquinodimethanes (AQDs), a group of overcrowded organic molecules. The study, led by Associate Professor Yusuke Ishigaki at Hokkaido University and Associate Professor Toshikazu Ono at Kyushu University, was published in the journal Materials Chemistry Frontiers.
“AQDs are a type of overcrowded ethylene, molecules with carbon-carbon double bonds surrounded by large chemical groups,” explains Ono. “They have two common isomers, the folded and twisted forms. They are especially interesting as molecular switches, as their sterically hindered double bond can provide isomers absorbing and emitting different wavelengths of light.”
The folded and twisted isomers absorb different wavelengths of light. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
AQDs generally adopt the most stable folded or twisted form, making it difficult to isolate pure samples of any other isomer to study its properties. The researchers surmounted this obstacle by designing flexible AQD derivatives that can more easily and stably form different isomers.
The synthesized derivatives were not only able to stably form twisted and folded isomers, but also other isomeric forms, when recrystallized in different solvents. The researchers performed detailed analysis of the derivatives to fully understand their properties.
In a crystalline state, each of these isomers absorbs and emits distinct frequencies of light, which is due to the differences in the distribution of electrons in the isomer molecules. Interestingly, the light absorption and emission changed when the crystals were ground into amorphous solid, and following treatment with appropriate solvents can produce original or other crystals with a variety of colors.
When ground into amorphous solid and treated with appropriate solvents the light absorption and emission changes. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
The methyl derivative of the new compound has four different isomers, with different crystal structures each. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
When ground into amorphous solid and treated with appropriate solvents the light absorption and emission changes. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
The methyl derivative of the new compound has four different isomers, with different crystal structures each. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
“This work is the first report on the isolation of multiple isomeric forms of AQD,” Ishigaki concluded. “Their absorption and emission of different light frequencies, and more importantly, the ability to modulate the absorption and emission by external stimuli, make these compounds excellent candidates for the development of molecular switches.”
More information: Kazuma Sugawara et al, Exceptionally flexible quinodimethanes with multiple conformations: polymorph-dependent colour tone and emission of crystals, Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
Identifying the compounds that give fermented coffee, pictured brewing here, its unique flavor and aroma could allow more people to enjoy it. Credit: Samo Smrke
Specialty coffees are gaining traction in coffeehouses around the world—and now a fermented version could bring a fruity taste to your morning cup of joe. This new kind of beverage has a raspberry-like taste and aroma, but what causes these sensations has been a mystery. Today, scientists report six compounds that contribute to the fermented coffee experience. The work could help increase production of the drink and make it more readily available for everyone to enjoy.
The researchers will present their results at the spring meeting of the American Chemical Society (ACS). ACS Spring 2023 is a hybrid meeting being held virtually and in-person March 26–30.
“There are now flavors that people are creating that no one would have ever associated with coffee in the past,” says Chahan Yeretzian, Ph.D., the project’s principal investigator. “The flavors in fermented coffee, for example, are often more akin to fruit juices.”
This unusual type of beverage provides a unique flavor experience for consumers, and the growing demand for it means that fermented coffee beans can fetch a high price, potentially benefiting farmers. And the process by which the beans are prepared requires much less water than traditional methods, making it a more environmentally friendly alternative to a standard cup of coffee.
But despite this drink’s growing popularity, the compounds that cause its distinctive flavor were unknown. And with fermented coffee becoming more popular in competitive events, some people have been concerned that the lack of knowledge about fermented coffee may make it difficult to distinguish between the genuine product and regular joe that has been illicitly adulterated. So, Yeretzian and colleagues from the Coffee Excellence Center at Zurich University of Applied Sciences sought to identify the compounds that are responsible for these new and exciting flavors. And because flavor and smell are intimately linked, studying the beverages’ scents could help the team gain a better understanding of how fermented coffee’s complex flavor is created.
To single out the compounds unique to fermented coffee’s aromas, researchers took arabica beans and divided them into three groups. One was prepared using a wash process, which is likely how the average afternoon pick-me-up brew is made. Here, a gelatinous substance known as mucilage is stripped from the coffee bean, which is washed with water before being dried.
The researchers prepared the second group using the pulped natural process—another common approach—in which the skin is removed from the bean, but the mucilage is left intact.
Finally, the team fermented beans in the third group using carbonic maceration, a process often used in winemaking. This method was first introduced to the specialty coffee world in 2015, when the winning contestant in the World Barista Championship used it to prepare their entry. With this process, whole coffee fruits are fermented in stainless steel tanks and infused with carbon dioxide to lower the pH of the fermentation. Unlike the other brews, the coffee made with fermented beans was described as smelling intense, like raspberries with a hint of rose.
Next, the researchers brewed coffee using each type of bean and analyzed the samples with gas chromatography (GC) sniffing, also called GC olfactometry. First, the GC instrument separated individual components in the air above each sample. Then, as the compounds left the instrument, they went to a mass spectrometer for identification, and to someone sitting at the outlet to describe what they smelled.
“Because the chemical signature doesn’t tell us how a compound smells, we have to rely on the human nose to detect the scent as each compound comes out of the chromatography instrument individually,” says Yeretzian. This methodology can be tricky because there is a subjective element to it. “We’re using people to detect scents, and everybody perceives flavors a little differently,” says Samo Smrke, Ph.D., a research associate in the lab who is presenting the results. “But in this case, the panel was very consistent in the smells they described. So, what is traditionally considered a challenge was actually not an issue because the aromas were so clear.”
There is one major advantage to GC sniffing. The human nose can sometimes detect scents from compounds that are at such a low concentration, they’re unable to be picked up by mass spectrometry. In this case, although six compounds appeared to contribute to the intense fruity flavor and the raspberry scent of the fermented coffee, the team was only able to identify three of them: 2-methylpropanal, 3-methylbutanal and ethyl 3-methylbutanoate.
In the future, the researchers hope to identify the remaining compounds, as well as judge the intensity of different flavors and scents. Additionally, the researchers would like to know more about how these unique compounds form. Potential factors include farming practices, the variety of coffee beans, the microclimate of specific farms and the microbes present during fermentation.
“There’s still quite a lot of unknowns surrounding this process,” says Smrke. A better understanding of the sources of these compounds could help the team standardize production methods, making it easier to produce fermented coffee at larger scales and allowing even more people to enjoy this distinctive flavor.
All biological amino acids on Earth appear exclusively in their left-handed form, but the reason underlying this observation is elusive. Recently, scientists from Japan uncovered new clues about the cosmic origin of this asymmetry. Based on the optical properties of amino acids found on the Murchison meteorite, they conducted physics-based simulations, revealing that the precursors to the biological amino acids may have determined the amino acid chirality during the early phase of galactic evolution.
If you look at your hands, you will notice that they are mirror images of each other. However, no matter how hard you try to flip and rotate one hand, you will never be able to superimpose it perfectly over the other. Many molecules have a similar property called “chirality,” which means that the “left-handed” (L) version of a molecule cannot be superimposed onto its “right-handed” (D) mirror image version. Even though both versions of a chiral molecule, called “enantiomers,” have the same chemical formula, the way they interact with other molecules, especially with other chiral molecules, can vary immensely.
Interestingly, one of the many mysteries surrounding the origin of life as we know it has to do with chirality. It turns out that biological amino acids (AAs)—the building blocks of proteins—on Earth appear only in one of their two possible enantiomeric forms, namely the L-form. However, if you synthesize AAs artificially, both L and D forms are produced in equal amounts. This suggests that, at some early point in the past, L-AAs must have come to dominate a hetero-chiral world. This phenomenon is known as “chiral symmetry breaking.”
Against this backdrop, a research team led by Assistant Professor Mitsuo Shoji from University of Tsukuba, Japan, conducted a study aimed at solving this mystery. As explained in their paper published in The Journal of Physical Chemistry Letters, the team sought to find evidence supporting the cosmic origin of the homochirality of AAs on Earth, as well as iron out some inconsistencies and contradictions in our previous understanding.
“The idea that homochirality may have originated in space was suggested after AAs were found in the Murchison meteorite that fell in Australia in 1969,” explains Dr. Shoji. Curiously enough, in the samples obtained from this meteorite, each of the L-enantiomers was more prevalent than its D-enantiomer counterpart. One popular explanation for this suggests that the asymmetry was induced by ultraviolet circularly polarized light (CPL) in the star-forming regions of our galaxy. Scientists verified that this type of radiation can, indeed, induce asymmetric photochemical reactions that, given enough time, would favor the production of L-AAs over D-AAs. However, the absorption properties of the AA isovaline are opposite to those of the other AAs, meaning that the UV-based explanation alone is either insufficient or incorrect.
Against this backdrop, Dr. Shoji’s team pursued an alternate hypothesis. Instead of far-UV radiation, they hypothesized that the chiral asymmetry was, in fact, induced specifically by the CP Lyman-α (Lyα) emission line, a spectral line of hydrogen atom that permeated the early Milky Way. Moreover, instead of focusing only on photoreactions in AAs, the researchers investigated the possibility of the chiral asymmetry starting in the precursors to the AAs, namely amino propanals (APs) and amino nitriles (ANs).
Through quantum mechanical calculations, the team analyzed Lyα-induced reactions for producing AAs along the chemical pathway adopted in Strecker synthesis. They then noted the ratios of L- to D-enantiomers of AAs, APs, and ANs at each step of the process.
The results showed that L-enantiomers of ANs are preferentially formed under right-handed CP (R-CP) Lyα irradiation, with their enantiomeric ratios matching those for the corresponding AAs. “Taken together, our findings suggest that ANs underlie the origin of the homochirality,” remarks Dr. Shoji. “More specifically, irradiating AN precursors with R-CP Lyα radiation lead to a higher ratio of L-enantiomers. The subsequent predominance of L-AAs is possible via reactions induced by water molecules and heat.”
The study thus brings us one step closer to understanding the complex history of our own biochemistry. The team emphasizes that more studies focused on ANs need to be conducted on future samples from asteroids and comets to validate their findings. “Further analyses and theoretical investigations of ANs and other prebiotic molecules related to sugars and nucleobases will provide new insights into the chemical evolution of molecules and, in turn, the origin of life,” concludes Dr. Shoji.
More information: Mitsuo Shoji et al, Enantiomeric Excesses of Aminonitrile Precursors Determine the Homochirality of Amino Acids, The Journal of Physical Chemistry Letters (2023). DOI: 10.1021/acs.jpclett.2c03862
Sandro Botticelli, the Lamentation of Christ, with indication of sampling positions for binding medium analyses. Credit: Bavarian State Painting Collections, Munich.
A team of chemical engineers affiliated with several institutions in Europe has determined why Old Masters of the 16th, 17th, and early 18th centuries added egg yolk to their oil-based paints. In their study, reported in the journal Nature Communications, the group added fresh egg yolk to the types of oils used by Renaissance painters and tested them to see changes it made to their properties.
For many years, historians have known that the Old Masters of the Renaissance, such as Sandro Botticelli or Leonardo da Vinci, added proteins such as egg yolk to their oil-based paints—creating a medium known as egg tempera—but no one knew why. In this new effort, the chemists set themselves the task of solving the mystery.
The work involved creating two types of oil-based paints, both with yolk added. One mixture consisted of nothing but yolk and oil. The other had yolk, oil and pigments to add coloring. The team also created similar paints without using egg yolk. The group then used the paints to create paintings that could be used for testing purposes. Such tests included taking measurements of moisture amounts and movement, oxidation, time to dry and heat capacity.
The researchers found that adding yolk helped to create stronger bonding between pigment particles, which made for stiffer paint. And that, they noted, would have been ideal for use in impasto painting, where the ink is applied thickly to give it a three-dimensional quality. The group also found that adding yolk reduced wrinkling of the paint, helping it to retain the shape applied by the painter—and helped to protect the paint against exposure to high humidity. Antioxidants in the yolk also helped to prevent yellowing.
Adding egg yolk also allowed the painter, the team found, to add more pigment to the oil, which in turn, helped to create more vivid images.
The research group found that there was one a downside to adding yolk—the paint takes longer to dry, which means the painter has to wait longer to add another coat.
More information: Ophélie Ranquet et al, A holistic view on the role of egg yolk in Old Masters’ oil paints, Nature Communications (2023). DOI: 10.1038/s41467-023-36859-5
Nickel-iron hydrogenase, described by researchers as “one of nature’s most complicated and beautiful enzymes,” may be crucial in the world’s push toward a renewable energy economy. Credit: Mirica group, University of Illinois at Urbana-Champaign
An ancient biological enzyme known as nickel-iron hydrogenase may play a key role in producing hydrogen for a renewables-based energy economy, researchers have found. Careful study of the enzyme has led chemists from the University of Illinois Urbana-Champaign to design a synthetic molecule that mimics the hydrogen gas-producing chemical reaction performed by the enzyme.
The researchers reported their findings in the journal Nature Communications.
Currently, industrial hydrogen is usually produced by separating hydrogen gas molecules from oxygen atoms in water using a process called electrolysis. To boost this chemical reaction in the industrial setting, platinum metal is used as a catalyst in the cathodes that direct the reaction. However, many studies have shown that the expense and rarity of platinum make it unattractive as the world pushes toward more environmentally sound energy sources.
On the other hand, nature’s nickel-iron hydrogenase enzyme produces hydrogen using earth-abundant metals in its core, said chemistry professor Liviu Mirica, who led the study with graduate student Sagnik Chakrabarti.
“The nickel at the core of the natural enzyme produces hydrogen by reducing protons in water,” Chakrabarti said. “During the catalytic process, the nickel center goes through paramagnetic intermediates, meaning that the intermediates have an unpaired electron—which makes them extremely short-lived.”
Synthetic chemists have made nickel compounds that produce hydrogen for over a decade, Mirica said. While some of these compounds are very efficient at producing hydrogen, the vast majority of them operate via intermediates that are not paramagnetic.
“Researchers are trying to mimic exactly what nature does because it is efficient, and maximizing efficiency is a key challenge to overcome when engineering energy sources,” Mirica said. “Being able to reproduce the paramagnetic intermediate steps that occur in the natural enzyme is what our group is trying to achieve—to increase efficiency and mimic nature.”
To achieve this, the team designed an organic molecule called a ligand that contains electron-donating atoms like nitrogen and sulfur, and can hold the nickel in place and support the two relevant paramagnetic states that produce hydrogen. The key design element that sets this molecule apart from other catalysts is the presence of a carbon-hydrogen bond near the nickel center that is broken and formed again during catalysis. This was crucial in stabilizing the aforementioned paramagnetic states.
“One of the key takeaways from our work is that by using the specially designed ligand in the manner we did, we have successfully united ideas from two fields of inorganic chemistry—bioinorganic and organometallic chemistry—to make nickel complexes that behave similarly to the active site of one of nature’s most beautiful and complicated enzymes,” Chakrabarti said.
In the recent past, several unusual enzymes have been found that feature metal-carbon bonds in their active sites, the researchers said. Such design principles in synthetic complexes could lead to further insights into how nature performs chemistry with small molecules like hydrogen.
Former Illinois researchers Soumalya Sinha, Giang N. Tran and Hanah Na contributed to this study.
More information: Sagnik Chakrabarti et al, Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst, Nature Communications (2023). DOI: 10.1038/s41467-023-36609-7
The graph in the center of this image shows the hybrid photocatalysts, labeled as 1 through 4, and the stark reduction in hexvalent chromium in the research when compared to other photocatalysts. Credit: Polyoxometalates, Tsinghua University Press
Toxic heavy metals found in wastewater have health and safety ramifications for communities affected by pollution. Hexavalent chromium is a dangerous, cancer-causing byproduct of industrial processes that is known to cause birth defects, severe diarrhea, and is linked to kidney, bladder, and liver cancers. Famously, it was the center of the lawsuit dramatized in the film “Erin Brockovich.”
Researchers are trying to find effective ways to remove hexavalent chromium from wastewater and a recently published paper shows how photocatalytic technology may be a solution. Photocatalysis is when light and a catalyst are used to speed up chemical reactions.
The paper was published in Polyoxometalates.
“Rapid industrialization causes an increased release of wastewater containing heavy metal ions. Hexavalent chromium, which has high carcinogenicity and teratogenicity, is widely found in wastewater and can easily enter food chains,” said Yuan-Yuan Ma, a researcher at Hebei Normal University in Shijiazhuang, China. Photocatalysis technology is an appealing solution for removing heavy metals from wastewater because it is sustainable, cost-effective, and environmentally friendly.
“This green approach for the removal of heavy metal ions uses sustainable light energy via hourglass-type phosphomolybdate-based crystalline photocatalysts and develops a strategy for the regulation of photocatalytic performance by adjusting the central metal ions in hourglass-type phosphomolybdate clusters,” said Ma. Researchers chose this particular type of photocatalyst because of its molecular properties and well-defined hourglass-type structure, which give it a wide light absorption ability and the band structure necessary to reduce the levels of hexavalent chromium.
Researchers tested four “hybrid” photocatalysts and compared them to six other photocatalysts. The hybrids had slightly different compositions, but all had the same hourglass-type structure that could be maintained up to 200 degrees Celsius. They had wide visible-light absorption and similar energy band structures. Researchers labeled these as Hybrid 1, 2, 3, and 4. After 80 minutes of exposure to a 10W LED light, hybrid 1 and 3 had around a 90% reduction in hexavalent chromium, while 2 and 4 had around a 74% and 71% reduction in hexavalent chromium respectively.
The hybrids generally performed better than any of the tested photocatalysts. Hybrids 1 and 3, which performed best, both were Mn{P4MO6}2-based hybrids. Hybrids 2 and 4 were Co{P4MO6}2-based. Researchers suspect that the better performance was due to structural differences that gave hybrids 1 and 3 a narrower band gap. “The semiconductor photocatalysts in photocatalytic processes can absorb photons matched with their band gap energy, leading to the conversion of toxic hexavalent chromium to less toxic chromium,” said Ma.
Looking ahead, researchers will focus on improving the design of the photocatalysts, while also planning for how to best bring this technology to a real-world wastewater setting. “Designing effective photocatalysts is the first step to solve heavy metal pollution in water,” said Ma. “We will design more efficient photocatalysts and apply the developed photocatalysts to actual industrial wastewater. We will also expand the treatment range of polluted water sources and strive to realize the practicality of the photocatalyst materials used.”
More information: Xiao-Yu Yin et al, Photoactive hourglass-type M{P 4Mo 6} 2 networks for efficient removal of hexavalent chromium, Polyoxometalates (2023). DOI: 10.26599/POM.2023.9140027
Converting CO2 to Ethanol on Partially Oxidized Ag Nanowires with High Selectivity and Activity. Credit: Science China Press
In a study, published in the journal Science China Chemistry and led by Prof. Pingping Fang (School of Chemistry, Zhejiang University) and Prof. Jianfeng Li (College of Chemistry and Chemical Engineering, Xiamen University), experiments were performed by using an Xplora Raman spectrometer with a 50x microscope objective and an excitation wavelength of 638 nm from a He–Ne laser.
“Due to the proper adsorption energy of CO on the partially oxidized Ag NWs, it is of great significance to correlate the high ethanol selectivity for CO2 electroreduction with the partially oxidized active center. Ag catalysts have been shown to exhibit high selectivity for CO2 electroreduction to CO at low over potentials with depressed H2 evolution. CO is an important intermediate for C-C coupling, therefore, Ag has the potential to exhibit high selectivity for ethanol products by tuning the adsorption energy of CO. This study provides a new insight to design efficient catalysts and investigate the mechanisms to improve the selectivity,” Fang says.
Interestingly, high ethanol FE was obtained on partially oxidized Ag NWs for CO2 electroreduction, and operando EC-SERS combined with DFT calculation explained the mechanisms why partially oxidized Ag NWs exhibited so high ethanol selectivity. The ethanol FE can reach as high as 85% on partially oxidized Ag NWs at −0.95 V vs. RHE. Operando EC-SERS found the high coverage of CO can greatly facilitate the ethanol formation on partially oxidized Ag NWs during CO2 electroreduction.
DFT calculation results show that the adsorption energy of CO on the partially oxidized Ag NWs is higher than that on Cu, and the reaction free energy of CO coupling with *CHO to *COCHO intermediate on partially oxidized Ag NWs is smaller than that on Cu surface, which explains the high ethanol selectivity very well.
Therefore, experiment results, operando EC-SERS and DFT calculations together prove that such partially oxidized Ag NWs can provide high ethanol selectivity for CO2 electroreduction. These results provide new clues for designing Ag based catalysts to improve the ethanol selectivity and mechanism studies.
More information: Qiong Liu et al, Converting CO2 to ethanol on Ag nanowires with high selectivity investigated by operando Raman spectroscopy, Science China Chemistry (2022). DOI: 10.1007/s11426-022-1460-7
An illustration of how next-generation bioimager BIGTUNA simultaneously uses multiple approaches to significantly improve metabolic mapping. Credit: Nathan Johnson, Pacific Northwest National Laboratory
Microbes may be among the smallest living things on Earth, but bioimaging to understand the chemistry that fuels these organisms could reveal important clues about the intricacies of gene function and the health of the planet. Because of this, scientists have long sought ways to eavesdrop on conversations between living microbes in their environment.
This has been exceptionally difficult, in part because microbes communicate using molecules instead of words. Deciphering conversations means identifying small, specific, and quickly changing molecules called metabolites, something even the most powerful instruments struggle to attempt. But a team of researchers at Pacific Northwest National Laboratory (PNNL) have spent the last decade continuously developing a next-generation bioimaging instrument that is making progress toward this goal.
The Chemical Dynamics Initiative (CDi), an internal PNNL investment, supported PNNL chemist Patrick El Khoury and his team as they developed the technology to measure phenomena in the quantum realm. Here the team imaged subatomic waves of energy called phonons as they formed, beat, and dissipated in a single trillionth of a second.
“Similar technologies can be used to image phonons and metabolites in real space and real time,” said El Khoury. “The fundamental advances required in both areas comprise a challenge worthy of a national laboratory and continued investments.”
Now researchers are taking the technologies to the next level as they use bioimaging to map metabolites exchanged by live microbes.
Bioimaging to fish out whispers in a crowd
The bioimager is known as BIGTUNA, short for BioImaginG Technology Using Nano-optical Approach. The keys to BIGTUNA are its multiple optical capabilities, each providing complementary information about the position and composition of molecules in a study sample. Many laser sources focus on the tip of a very sharp nanosized needle. Researchers position the needle’s tip in the sample area they want to examine, then use the light focused on the tip of the needle to measure the sample’s physical and chemical features. Through this, researchers identify molecules and understand how they interact.
One of the custom-built nano-optical system precursors to the BIGTUNA bioimaging device. Credit: Patrick El-Khoury, Pacific Northwest National Laboratory
Chemical bioimaging with light has been done for a hundred years, but never at this molecular scale.
“Some methods illuminate a relatively large area, but these far-field approaches are like listening in to a crowd and expecting to understand individual conversations,” said PNNL chemist Scott Lea. To overcome this challenge, researchers focused on combining a wide range of near-field techniques to capture and characterize the maximum information in an area as small as a few molecules.
“If we don’t have multiple streams of data coming from multiple techniques, we only get partial information,” said El Khoury. “And in addition to developing the techniques, we developed our understanding of optical selection rules to maximize the information we get from one sample in one set-up.”
In the most recent iteration of this project, the researchers zoomed out to a larger area, although still only a thousandth the thickness of a strand of hair. At this slightly farther distance, they identified the most promising approaches to capture information about the patterns of molecular bonds and the distribution of electrons. These new nano-optical measurements are addressing a much smaller number of molecules; therefore, the researchers must continue developing new theories that describe nanoscopic interactions of light and matter.
Combining these conceptual and technological developments will allow the researchers to move beyond model systems they studied using early incarnations of BIGTUNA. The chemical signals in these model systems were much stronger than chemical signals from the metabolites involved in microbial communications. In addition to having weaker signals, biological samples are also susceptible to damage by light, which is why BIGTUNA’s noninvasive approach makes it ideal to develop for bioimaging applications. Including state-of-the art data and computational techniques from PNNL data scientists Sarah Akers and Edo Aprà will help automate where and how the instrument balances exploration with the sensitivity of a living system.
Bioimaging to tune in to talking microbes
As an initial foray into biology, researchers are focusing BIGTUNA’s bioimaging power on a community of symbiotic microbes that live in deep ocean sediments. One microbe reduces sulfur, the other oxidizes methane, a powerful greenhouse gas.
Image of the microbe community (purple and green) within sediment particles (yellow) that researchers are studying through bioimaging with BIGTUNA. Credit: Patrick El Khoury, Pacific Northwest National Laboratory
Previous approaches to unraveling microbial interactions have mainly focused on identifying influential genes or on examining isolated enzymes and pathways. The approaches often include fixing, freezing, or combining the biological system. But these approaches lose out on time-dependent or space-specific details. And the researchers can’t look at the flow of metabolites to get a predictive understanding of how and why microbes interact.
Even so, PNNL collaborator and CalTech geologist Victoria Orphan has theories about how these symbiotic microbes share metabolites. Bioimaging with BIGTUNA could produce the first close-up view of the metabolites in action as the instrument sends light through the sample and measures what gets absorbed or scattered. Researchers use the information to identify metabolites and create a detailed record of microbial intercellular communication pathways. In turn, this knowledge could help researchers understand the degree to which microbes respond to environmental changes.
A new generation of nano-optics
“Possibilities for BIGTUNA extend far beyond the realm of bioimaging,” said Peter Sushko, CDi’s chief scientist. “Because this highly adaptable instrument can obtain detailed information describing atomic motion and electronic processes, it will be useful in seeking answers to a broad range of questions that are of interest to chemists, physicists, and materials scientists as well.”
Potential applications include quantum materials, catalysis, and human health, in addition to the current work in microbial systems. In that realm, planned future developments could incorporate environmental controls to further generalize the approach.
A portion of the blueprint for BIGTUNA was designed under PNNL’s CDi, a five-year internal investment in capabilities to better understand and predict the evolution of complex chemical systems in real-world or operational environments.
The graphic illustrates five different effects of Ir-based catalysts, which were studied for future improvements for usage in proton exchange membrane water electrolyzers (PEMWEs). Credit: Nano Research Energy, Tsinghua University Press
Hydrogen production powered by wind and solar energy is still too expensive if it is to play a role in the clean transition via energy storage and to help decarbonize hard-to-electrify sectors. Much effort in reducing its cost focuses on enhancing production efficiency by improving the performance of iridium-based catalysts that can speed up the oxygen-related part of the electrochemical reaction involved in splitting water into its component parts, hydrogen and oxygen.
A new review of the state of the field discusses its recent progress and challenges and identifies research gaps that need to be filled before such catalysts can achieve commercial viability.
The review paper was published in the journal Nano Research Energy.
Cleanly produced hydrogen is essential in the transition away from fossil fuels in order to avoid dangerous climate change, both as an energy carrier to be used on its own or as a component of a synthetic fuel for those sections of the economy such as long-haul shipping and aviation that are hard to electrify.
But such clean hydrogen production—which is performed via electrolysis, using electricity to split water into its component elements, hydrogen and oxygen—is extremely energy intensive. This energy intensity of electrolysis in turn makes clean production of hydrogen very expensive, and thus uncompetitive with fossil fuels.
If this were not enough of a challenge, using wind and solar energy as the source of clean electricity to power the electrolysis—a form of hydrogen production termed ‘green hydrogen’—places a significant burden on the electrolyzers because these energy sources are intermittent. The sun doesn’t always shine and the wind doesn’t always blow.
This means that sometimes there is little to no current and at other times, there can be a big spike of current, which places stress on the electrolyzers, again pushing up costs. However, proton exchange membrane water electrolyzers (PEMWE) are a very promising option here, as PEMWEs can operate at high current densities such as those posed by these spikes.
Electrolysis is a chemical reaction composed of two parts, or ‘half reactions.’ One is the hydrogen evolution reaction (HER), which generates the hydrogen, and the other is the oxygen evolution reaction (OER), which produces the oxygen. But it is actually the latter reaction that is most important with respect to the energy efficiency of the overall process and thus production of clean hydrogen.
And so to reduce the energy demands and thus the cost of clean production of hydrogen, a lot of research has focused on superior catalysts—chemicals that speed up a chemical reaction—for the OER part of the process and that pair well with PEMWEs.
However, the severe corrosion in the acidic environment of PEMWEs makes most non-precious metal-based catalysts—for example using cobalt, nickel, or iron—unstable. But iridium-based catalysts exhibit much better catalytic stability in these harsh acidic conditions.
A number of recent studies have reported significant advances in the development of iridium-based catalysts for green hydrogen production, including the use of new synthesis methods and the optimization of catalyst structures and compositions.
However, there are still several research challenges that need to be addressed to fully realize the potential of iridium-based catalysts for green hydrogen production. One major challenge is the high cost of iridium—and high costs are precisely what novel catalysts were intended to avoid.
“To overcome this, researchers are exploring new synthesis methods and alternative catalyst materials that can replace iridium or reduce the amount of iridium required,” said Chunyun Wang, of the School of Chemistry and Chemical Engineering at Yangzhou University and lead author of the review. “Some novel and effective options have emerged recently, such as iridium oxides, perovskites, pyrochlores, and single-atom catalysts.”
“And so we thought it was about time that we paused and assessed the state of play in iridium-based catalysts for green hydrogen production with a review paper,” added Alex Schechter, a chemist with Ariel University in Israel and co-author of the review paper. “The benefit of this is to pool information across many different teams of researchers and, crucially, identify research gaps.”
The review focuses in particular on how the catalysis operates (the catalytic mechanism), design of catalysts, and strategies for synthesis of catalysts. In particular, the analysis looks at different attributes of catalysts that affect their promotion of the catalysis process including geometric effects, electronic effects, synergistic effects, defect engineering and support effects, and how different research teams have dealt with each option to try to improve performance.
Geometric effects in essence describe the shape, structure and size of the catalyst molecule, including which of its crystal planes are exposed, and how atomic arrangements might be ordered or disordered. All of this significantly affects catalyst performance. Electronic effects refer to the structure of electrons associated with the relevant molecules.
Synergistic effects are those where two or more ingredients come together to produce a superior result than either one on its own. Defect engineering involves efforts to design the surface chemistry of catalysts via voids, dislocations, vacancies and so on—deliberately introducing imperfections—so as to increase the number of places where the chemical reaction can take place (active sites). And support effects come from metals that interact with and support the catalyst.
The reviewers concluded after surveying their field that the most successful strategy for improving the performance of iridium-based catalysts includes defect engineering, adjusting synergistic effects and altering geometric effects. The number of exposed active sites can be increased by constructing a porous structure and introducing supports for the catalyst that promote transfer of both mass and electrons. And enhanced metal-support interaction can increase the long-term stability of the catalysts.
Despite the considerable research success, the field still faces challenges. Many high-performance iridium-based catalysts have been developed, but most of them can only be synthesized on a small scale of just a few grams or even hundreds of milligrams in the laboratory. Complex preparation processes thus must be simplified.
In addition, lab conditions are a bit too ideal compared to actual catalytic systems, and so real-world conditions need to be part of any follow-up research. This includes looking at realistic electrolyzer temperature, current density, and product delivery, amongst other aspects, that will enable evaluation of performance catalysts in practical applications.
And beyond the catalysts themselves, other components need to be optimized as well, including the development of electrode plates with high corrosion resistance and low cost, proton exchange membranes with high proton transport capacity.
The reviewers stressed however that none of these challenges are deal-breakers for iridium-based catalysts for green hydrogen production. Instead these represent possible avenues for new research that may deliver the breakthroughs this process requires to achieve commercial viability.
More information: Chunyan Wang et al, Iridium-based catalysts for oxygen evolution reaction in acidic media: Mechanism, catalytic promotion effects and recent progress, Nano Research Energy (2023). DOI: 10.26599/NRE.2023.9120056
When ground into amorphous solid and treated with appropriate solvents the light absorption and emission changes. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
The methyl derivative of the new compound has four different isomers, with different crystal structures each. Credit: Materials Chemistry Frontiers (2023). DOI: 10.1039/D2QM01199A
