May 9, 2019
picture of leaf with CO2 written on it
Collaboration by Liu, Sletten teams leads to incorporation of perfluorocarbon emulsions into an electrochemical system for a microbial solution to CO2 fixation.
By Roselyn Rodrigues
Carbon dioxide (CO2) is a gas that is produced by cars, boats, planes, concrete production and numerous other sources. For years, scientists have discussed the effect that carbon dioxide has on the environment, with most scientists agreeing that our environment would benefit from the removal or reduction of carbon dioxide from the atmosphere. One popular method studied to remove carbon dioxide from the air is called carbon dioxide fixation, where inorganic CO2 is converted to more useful organic compounds. In nature, the process of CO2 fixation is most famously achieved by plants and through photosynthesis, a process of converting CO2 to food through absorbing sunlight.
Other living organisms such as microbes (bacteria) can also convert CO2 and other atmospherically abundant gases into environmentally benign, useful chemicals. These biological processes inspired the development of an artificial photosynthesis process that mimics the natural photosynthesis. In the artificial photosynthesis, renewable electricity generated from solar panels is used to power the living organisms to synthesize chemicals from water and air.
The figure at left is a schematic representation of the cathodic side of the reactor where H+ is converted to H2 which is then transferred to the microbes for the catalytic conversion of CO2 into commodity chemicals. Scenario 1 depicts the previously studied system in which the concentration of H2 is represented by [H2]1 and the kinetic rate of H2 transfer to the microbes is represented by k1. Scenario 2 depicts the work reported in the paper, where the nanoemulsions are added to the system to generate a new concentration of [H2]2 and kinetic rate of transfer k2.  (Chong Liu group UCLA)
Chong Liu, UCLA assistant professor of chemistry who holds the Jeffrey and Helo Zink Endowed Professional Development Chair in Chemistry, is a pioneer in this concept since his time as a graduate student at UC Berkeley, and later a postdoctoral fellow at Harvard University. While at Harvard, Liu was the first author on a 2016 Science paper, which demonstrated that it is not only possible to use renewable electricity to power living organisms, but the artificial process is about 10 times more efficient than plants at removing carbon dioxide from the air. Yet, in spite of such a high efficiency, the throughput of the whole device is limited. The problem in applying these processes in a controlled way is that some of the necessary substances for the CO2 fixation (such as H2 and N2) don’t stay dissolved in water. With a limited supply of these molecules in solution, there is a maximum output that can be achieved by the artificial photosynthesis system.
This was the challenge that Liu sought to address upon his appointment as an assistant professor in 2017. Less than two years later Liu’s group, in collaboration with assistant professor of chemistry and biochemistry Ellen Sletten’s group, has reported an enhanced electricity-driven microbial CO2 fixation system in a recent issue of the journal Nature Catalysis
For months, Roselyn Rodrigues, Liu’s first graduate student, made several attempts to improve the throughput of the system, but to no avail. Many of the potential solutions to address the challenge were not biologically compatible. Serendipitously, Liu attended a research lecture by Sletten, who holds the John McTague Career Development Chair and specializes in organic chemistry and chemical biology. There, she described her research with perfluorocarbons, which are organic molecules comprised of carbon and fluorine — molecules with exceptional gas solubility and other interesting properties.
Sletten’s research group, building off previous work using perfluorocarbons for oxygen delivery, had found a way to create nanoscale droplets of perfluorocarbons suspended in water as nanoemulsions, with the use of a polymer as a stabilizer. Sletten’s group could precisely control the size and other properties of the nanoemulsions by tuning the concentration and physical properties of the polymer, referred to as the surfactant, as well as the concentration and type of perfluorocarbon. In the research of Sletten’s group and other laboratories, these emulsions are used for cell imaging, where researchers aim to get these emulsions to bind to cells in the body for cancer detection, in addition to therapeutic delivery. Pictured above: Professor Chong Liu, first author Roselyn Rodrigues, and Professor Ellen Sletten.
Liu thought that perfluorocarbon nanoemulsions might work with bacteria due to their good biocompatibility and high gas solubilities. To the researchers’ surprise, not only were the bacteria surviving, but the perfluorocarbons were also shown to enhance bacterial growth. With the introduction of nanoemulsions, the bacteria were getting more hydrogen from electricity and were therefore able to fix CO2 into more acetic acid, their target compound.
“We thought that if perfluorocarbon nanoemulsions work for oxygen, they will also allow hydrogen gas to stay in the liquid so the bacteria can use it,” said first author Rodrigues. “We are using the bacteria almost like a catalyst to convert carbon dioxide into a useful chemical. By using the exceptional gas solubility properties of the perfluorocarbon nanoemulsions, prepared by the Sletten group, we are able to increase the amount of hydrogen that goes into the bacteria while minimizing electrical energy input. This results in more efficient production of chemicals in a shorter period of time and greatly enhances the bacteria’s ability to reduce CO2 into useful chemicals by decreasing energy demands of the system and making the gases readily available to the microbes.”
Rodrigues shared her research experience in a recent blog at Nature Research Chemistry Community. 
Thanks to Sletten’s lecture that Liu attended, the collaboration between these two groups successfully demonstrated a novel use for perfluorocarbons. Perfluorocarbons have long been employed in other areas that demand high gas solubility, but their application for microbial CO2 fixation or use as a means to enhance the rates of electrochemical reactions had yet to be fully studied. Additional collaborative researches using these emulsions are ongoing.
From Liu’s group is first author, graduate student Roselyn M. Rodrigues; co-authors and graduate students Xun Guan and Jesus Iñiguez; and undergraduate student researcher Shuyan Huang.  From Sletten’s group are contributions from graduate student co-author Daniel A. Estabrook and undergraduate student researcher John O. Chapman. Co-senior authors of the research article are Liu and Sletten.
Roselyn Rodrigues, UCLA Department of Chemistry & Biochemistry,