These reverse osmosis membranes are layers of material with an active aromatic polyamide layer that allows water molecules through, but screens out 99 to 99.9 percent of the salt.

“As water stresses continue to grow, better membrane filtration materials are needed to enhance water recovery, prevent fouling, and extend filtration module lifetimes while maintaining reasonable costs to ensure accessibility throughout the world,” said Enrique Gomez, professor of chemical engineering, Penn State. “Knowing what the material looks like on the inside, and understanding how this microstructure affects water transport properties, is crucial to designing next-generation membranes with longer operational lifetimes that can function under a diverse set of conditions.”

Gomez and his team looked at the internal structure of the polyamide film using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography. HAADF-STEM’s image intensity is directly proportional to the density of the material, allowing mapping of the material to nanoscale resolution.

“We found that the density of the polyamide layer is not homogeneous,” said Gomez. “But instead varies throughout the film and, in this case, is highest at the surface.”

This discovery changes the way the engineers think about how water moves through this material, because the resistance to flow is not homogeneous and is highest at the membrane surface.

HAADF-STEM allowed the researchers to construct 3D models of the membrane’s internal structure. With these models, they can analyze the structural components and determine which characteristics must remain for the membrane to function and which could be manipulated to improve membrane longevity, antifouling, and enhance water recovery.

Another characteristic revealed through HAADF-STEM was the presence, or rather absence, of previously reported enclosed voids. Researchers thought that the membranes fine structure would contain enclosed void spaces that could trap water and alter flow patterns. The 3D models show that there are few closed voids in the state-of-the-art material studied.

“Local variations in porosity, density and surface area will lead to heterogeneity in flux within membranes, such that connecting chemistry, microstructure and performance of membranes for reverse osmosis, ultrafiltration, virus and protein filtration, and gas separations will require 3D reconstructions from techniques such as electron tomography,” the researchers report in a recent issue of Proceedings of the National Academy of Sciences.

The researchers would like to push the resolution of this technique to below 1 nanometer resolution.

“We don’t know if sub nanometer pores exist in these materials and we want to be able to push our techniques to see whether these channels exist,” said Gomez. “We also want to map how flow moves through these materials to directly connect how the microstructure affects water flow, by marking or staining the membrane with special compounds that can flow through the membrane and be visualized in the electron microscope.”

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Recent research in electrocatalytic CO₂ conversion points the way to using CO₂ as a feedstock and renewable electricity as an energy supply for the synthesis of different types of fuel and value-added chemicals such as ethylene, ethanol, and propane. But scientists still do not understand even the first step of these reactions — CO₂ activation, or the transformation of the linear CO₂ molecule at the catalyst surface upon accepting the first electron. Knowing the exact structure of the activated CO₂ is essential because its structure dictates both the end product of the reaction and its energy cost. This reaction can start from many initial steps and go through many pathways, giving typically a mixture of products. If scientists figure out how the process works, they will be better able to selectively promote or inhibit certain pathways, which will lead to the development of a commercially viable catalyst for this technology.

Columbia Engineering researchers announced today that they solved the first piece of the puzzle?they have proved that CO₂ electroreduction begins with one common intermediate, not two as was commonly thought. They applied a comprehensive suite of experimental and theoretical methods to identify the structure of the first intermediate of CO₂ electroreduction: carboxylate CO₂? that is attached to the surface with C and O atoms. Their breakthrough, published online today in PNAS, came by applying surface enhanced Raman scattering (SERS) instead of the more frequently used surface enhanced infrared spectroscopy (SEIRAS). The spectroscopic results were corroborated by quantum chemical modeling.

“Our findings about CO₂ activation will open the door to an incredibly broad range of possibilities: if we can fully understand CO₂ electroreduction, we’ll be able to reduce our dependence on fossil fuels, contributing to the mitigation of climate change,” says the paper’s lead author Irina Chernyshova, associate research scientist, department of earth and environmental engineering. “In addition, our insight into CO₂ activation at the solid-water interface will enable researchers to better model the prebiotic scenarios from CO₂ to complex organic molecules that may have led to the origin of life on our planet.”

They decided to use SERS rather than SEIRAS for their observations because they found that SERS has several significant advantages that enable more accurate identification of the structure of the reaction intermediate. Most importantly, the researchers were able to measure the vibrational spectra of species formed at the electrode-electrolyte interface along the entire spectral range and on an operating electrode (in operandi). Using both quantum chemical simulations and conventional electrochemical methods, the researchers were able to get the first detailed look at how CO₂ is activated at the electrode-electrolyte interface.

Understanding the nature of the first reaction intermediate is a critical step toward commercialization of the electrocatalytic CO₂ conversion to useful chemicals. It creates a solid foundation for moving away from the trial-and-error paradigm to rational catalyst design. “With this knowledge and computational power,” says the paper’s co-author Sathish Ponnurangam, a former graduate student and postdoc in Somasundaran’s lab who is now an assistant professor of chemical and petroleum engineering at the University of Calgary, Canada, “researchers will be able to predict more accurately the reaction on different catalysts and specify the most promising ones, which can further be synthesized and tested.”

“The Columbia Engineering experiments provide such detail that we should be able to obtain very definitive validation of the computational models,” says William Goddard, Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at CalTech, who was not involved with the study. “I expect that together with our theory, the Columbia Engineering experiments will provide precise mechanisms to be established and that examining how the mechanisms change for different alloys, surface structures, electrolytes, additives, should enable optimization of the electrocatalysts for water spitting (solar fuels), CO₂ reduction to fuels and organic feedstocks, N2 reduction to NH3 to obtain much less expensive fertilizers, all the key problems facing society to obtain the energy and food to accommodate our exploding population.”

Electrocatalysis and photocatalysis (the so-called artificial photosynthesis) are among the most promising ways to achieve effective storage for renewable energy. CO₂ electroreduction has been capturing the imagination of researchers for more than 150 years because of its similarity to photosynthesis. Just as a plant converts sunlight into chemical energy, a catalyst converts electrons supplied by renewable energy into chemical energy that is stored in reduced products of CO₂. In addition to its application for renewable energy, electrocatalysis technology may also enable manned Mars missions and colonies by providing fuel for the return trip and carbonaceous chemicals from the CO₂ that makes up 95 percent of that planet’s atmosphere.

“We expect our findings and methodology will spur work on how to make go faster and at a lower energy cost not only electrocatalytic but also photocatalytic CO₂ reduction,” says Ponisseril Somasundaran, LaVon Duddleson Krumb Professor of Mineral Engineering, Department of Earth and Environmental Engineering. “In the latter case, a catalyst reduces CO₂ using direct sun light. Even though these two approaches are experimentally different they are microscopically similar?both start with activation of CO₂ upon electron transfer from a catalyst surface. At this point, I believe that both these approaches will dominate the future.”

The team is now working to uncover the subsequent reaction steps?to see how CO₂ is further transformed?and to develop superior catalysts based on earth-abundant elements such as Cu (copper) and Sn (tin).

The finding, published in the journal of the Proceedings of the National Academy of Sciences, could lead to new ways of treating UTIs and other infections without antibiotics.

Antibiotic overuse can lead to resistance and impacts on the natural bacteria that share our bodies, called the microbiome, said study co-leader Lynette Cegelski, an associate professor of chemistry at Stanford’s School of Humanities and Sciences. “There are many ways to target disease and if you just target a specific bacterium’s virulence strategies, you could still prevent infection and also eliminate the total insult to your microbiome,” she added.

Produced by plants, algae and some bacteria, cellulose is the most abundant organic polymer on Earth. It’s also one of the best studied, since it’s used to produce everything from paper to ethanol fuel. It was thus surprising when Cegelski’s team announced earlier this year that they had discovered a chemically unique form of cellulose, called pEtN (for the chemical group phosphoethanolamine), in the bacteria E. coli.

That finding, which was published in Science, showed that pEtN cellulose is a crucial component in the bacteria’s biofilm, a slimy secretion that bacteria use to share nutrients and to protect themselves against antibiotics and the immune system attacks of their host.

In the new study, Cegelski’s group showed that pEtN also plays an important role in UTIs. UTI is one of the most common infectious diseases worldwide and E. coli is the one of the main culprits.

“Our experiments here reveal a specific function for the cellulose in which it serves a mortar-like role to enhance the adhesion strength of bacteria with bladder epithelial cells,” said study co-leader Gerald Fuller, the Fletcher Jones II Professor in the School of Engineering.

Help from an unexpected source

The discovery was the result of a chance encounter at a Stanford lunch event, where Fuller learned that Cegelski’s group was seeking a way to measure how strongly a layer of E. coli bacteria sticks to a layer of isolated bladder cells. “Oh, I can help you,” Fuller volunteered.

A few years earlier, Fuller had developed an instrument, called the Live-Cell Monolayer Rheometer (LCMR), to investigate the adhesion of contact lenses to corneal cells. “We wanted to know whether cells from your eye adhere to the underbelly of contact lenses when you take them off, and we found that indeed they do,” Fuller said.

The LCMR was designed to bring a glass plate into contact with a single thin layer of live cells and then maintain a constant pressure between them. An attached microscope allows scientists to monitor cells during experiments, and a heated bottom plate helps keep the cells alive.

The scientists realized that with a few tweaks, Fuller’s instrument could achieve what Cegelski had in mind. “For these experiments, we improved the microscopy to better visualize the contact between the bacteria and bladder cell layer during measurements,” study first author Emily Hollenbeck, a former joint-graduate student in both Cegelski and Fuller’s groups. “This was an important check to ensure we were always measuring adhesion forces between these very thin layers.”

Getting the bacteria to grow as a single layer also proved challenging, and it took Hollenbeck several tries before she was successful. “Creating uniform layers of bacteria and bladder cells was highly important for obtaining reproducible LCMR measurements,” Hollenbeck said. “It was especially challenging to create these uniform layers out of materials that are continually growing and adapting to their environment.”

Breaking the cycle of infection

Previous studies had already noted that pEtN cellulose and cell-surface fibers called curli are braided together to create E. coli biofilms. And curli have been implicated in kidney infections and sepsis. “The more severe the infections, the more likely those bacteria were making curli,” Cegelski said.

What Cegelski wanted to understand was the relative contributions of pEtN and curli to the adhesion of bacteria to host cells and how they worked together during infection. To answer these questions, her group designed a series of experiments to test the adhesion strength of the curli and the cellulose separately as well as together.

In one experiment, they attached E. coli whose biofilms contained both cellulose and curli to the top plate of the LCMR and then brought it into contact with a bottom plate containing bladder cells. The scientists then quickly sheared the top plate horizontally by a tiny amount, and the resulting level of adhesive stress gave them a quantitative measure of the bacteria’s “stickiness.”

They repeated the experiment with genetically engineered E. coli whose biofilms contained only cellulose, and again with a top plate that contained only curli and no bacteria.

What they found was that the bacteria that produced both curli and cellulose exhibited the highest adhesion strength, followed by the curli alone, and lastly the cellulose alone. “Without cellulose, the cells break away very easily from the bacteria,” Fuller said. “The cellulose acts like a glue to really help hold everything together.”

The finding suggests that an approach to treating UTIs that does not involve antibiotics may be possible. “Attacking the cellulose could be a great alternative to traditional antibiotics as preventing bacterial adhesion could help break the cycle of infection,” Hollenbeck said. “This type of treatment also avoids the ‘life-or-death’ pressure of traditional antibiotics that lead to drug resistant mutations.”

Other Stanford coauthors on the study, entitled “Phosphoethanolamine Cellulose Enhances Curli-Mediated Adhesion of Uropathogenic Escherichia coli to Bladder Epithelial Cells,” include Alex Antonoplis, Chew Chai, and Wiriya Thongsomboon.

The research was funded by the Stanford Center for Molecular Analysis and Design, the Achievement Rewards for College Scientists Foundation, and the National Science Foundation.