Ritsumeikan University

Biodegradable waste from plant and animal sources released into freshwater ecosystems is a significant environmental concern. Nonetheless, current methods for assessing water quality seem more or less impractical due to their complexity and high costs. In a promising development, a team of researchers has successfully constructed a self-sustaining and buoyant biosensor using inexpensive carbon-based materials for monitoring water quality at the inlets of freshwater lakes and rivers. The discharge of organic effluents -- biodegradable waste materials from plants and animals -- into freshwater bodies is a significant environmental concern, affecting the health and sustainability of these aquatic ecosystems. However, the methods currently available for inspecting water quality are complex and costly. In this regard, researchers from Ritsumeikan University, Japan, have recently developed a self-powered, inexpensive, and floating biosensor for monitoring water quality at the input of freshwater lakes and rivers. This paper was made available online on September 9, 2023, and was published in Volume 200 of the Biochemical Engineering Journal on November 1, 2023. "We developed a self-powered, stand-alone, floating biosensor based on a microbial fuel cell (MFC) for early organic wastewater detection. The MFC case was fabricated by a 3D printer and the electrodes were fabricated from low-cost carbon-based materials," remarks Professor Kozo Taguchi from the College of Science and Engineering, Department of Electrical and Electronic Engineering, Ritsumeikan University, who led the study. MFCs generate electricity with the help of electrogenic bacteria. These microorganisms produce an electric current as a result of their biological metabolism. The amount of electricity generated by the MFC is proportional to the concentration of the organic waste that is being consumed by the electrogenic microorganisms. This characteristic feature is, thus, used to design organic waste biosensors powered by MFCs. Using inexpensive carbon-based materials, the Japanese research team developed a self-powered biosensor based on a floating MFC (FMFC) to continually track the level of organic contamination in lakes and rivers. To this end, the team filled the anode (the electrode where oxidation occurs and electrons are given off) of the FMFC with soil containing electrogenic bacteria. The anodic bacteria subsequently decomposed the organic matter present in the water and converted the stored chemical energy into electricity. The electrical output was then used as a measure of the organic waste present in the contaminated water. Although the researchers did not characterize the bacterial communities present in the soil sample, they rationally hypothesized that microorganisms from the genera Geobacter, Shewanella,and Pseudomonas contributed to the electrical activity. Prior studies indicate that paddy soils naturally contain electrogenic bacteria belonging to these genera. Next, the team added a light-emitting diode (LED) to the floatable biosensor assembly. The LED was able to harness the electricity produced by the electrogenic bacteria and visually indicate the level of organic contamination in the water samples under investigation. It started flashing when the chemical oxygen demand (COD) -- a parameter used to measure the level of organic contaminants in water -- exceeded the threshold value of 60 mg/L. In addition, the LED flashed at an increased pace when the COD significantly exceeded the threshold value. Prof. Taguchi adds, "Because the FMFC biosensor produces its own electricity, it requires no external power supply. Moreover, it can be used in early detection systems that monitor influxes of organic wastewater in freshwater bodies." Mr. Trang Nakamoto and Mr. Dung Nakamoto, both from the College of Science and Engineering, contributed to the biosensor development. In summary, the study authors designed, developed, and tested a 3D-printed and cost-effective FMFC biosensor that effectively monitors organic contamination in freshwater bodies. Kudos to the research team for developing a novel biosensor that finds use in monitoring and safeguarding our freshwater ecosystems!

Excessive use of fossil fuels leads to undesired carbon dioxide (CO2) generation, accelerating climate change. One way to tackle this is by converting CO2 into value-added chemicals. On this front, researchers have recently utilized a novel redox couple, for the purpose. Climate change is a global environmental concern. A major contribution to climate change comes from excessive burning of fossil fuels. They produce carbon dioxide (CO2), a greenhouse gas responsible for global warming. In this light, governments globally are framing policies to curb such carbon emissions. However, merely curbing carbon emissions may not be enough. Managing the generated carbon dioxide is also necessary. On this front, scientists have suggested chemically converting CO2 into value-added compounds, such as methanol and formic acid (HCOOH). Producing the latter requires a source of hydride ion (H-), which is equivalent to one proton and two electrons. For instance, the nicotinamide adenine dinucleotide (NAD+/NADH) reduction-oxidation couple is a hydride (H-) generator and reservoir in biological systems. Against this backdrop, a group of researchers led by Professor Hitoshi Tamiaki from Ritsumeikan University, Japan, have now developed a novel chemical method that reduces CO2 to HCOOH using NAD+/NADH-like ruthenium complexes. Their work was published in the journal ChemSusChem on 13 January 2023. Prof. Tamiaki explains the motivation behind their research. "Recently, a ruthenium complex with an NAD+ model -- [Ru(bpy)2(pbn)](PF6)2 -- was shown to undergo photochemical two-electron reduction. It produced the corresponding NADH-type complex [Ru(bpy)2(pbnHH)](PF6)2 under visible light irradiation in the presence of triethanolamine in acetonitrile (CH3CN)," he elaborates. "Further, the bubbling of CO2 into the [Ru(bpy)2(pbnHH)]2+ solution regenerated [Ru(bpy)2(pbn)]2+ and produced formate ion (HCOO-). However, its yield was quite low. Therefore, transferring H- to CO2 required an improved catalytic system." Consequently, the researchers explored various reagents and reaction conditions to facilitate CO2 reduction. Based on those experiments, they proposed a photoinduced two-electron reduction of the [Ru(bpy)2(pbn)]2+/[Ru(bpy)2(pbnHH)]2+ redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Moreover, water (H2O), instead of triethanolamine, in CH3CN further improved the yield. In addition, the researchers explored the underlying reaction mechanism using techniques like nuclear magnetic resonance, cyclic voltammetry, and UV-Vis spectrophotometry. Based on this, they proposed the following: First, the photo-excitation of [Ru(bpy)2(pbn)]2+ produces [RuIII(bpy)2(pbn•?)]2+* radical, which undergoes reduction by BIH to give [RuII(bpy)2(pbn•?)]2+ and BIH•+. Following this, H2O protonates the ruthenium complex, generating [Ru(bpy)2(pbnH•)]2+ and BI•. The obtained product undergoes disproportionation to generate [Ru(bpy)2(pbnHH)]2+ and gives back [Ru(bpy)2(pbn)]2+. Then, the former is reduced by BI• to produce [Ru(bpy)(bpy•?)(pbnHH)]+. This complex is an active catalyst and transfers H- to CO2, producing HCOO- and formic acid. The researchers showed that the proposed reaction demonstrated a high turnover number -- moles of CO2 converted by a mole of catalyst -- of 63. Excited by these findings, the researchers hope to develop a new methodology of energy conversion (sunlight to chemical energy) for the production of novel renewable materials. "Our method would also decrease the total amount of CO2 gas on Earth and help maintain the carbon cycle. Thus, it could reduce global warming in the future," adds Prof. Tamiaki. "Further, the novel organic hydride transfer technology will provide us with invaluable chemical compounds."

Photosynthesis in plants and some bacteria relies on light-harvesting (LH) supramolecules, which come in different structures. So far, these LH molecules have not been artificially prepared. In a recent study, scientists managed to synthesize LH nanorings via self-assembly of chlorophyll derivatives and examined the external conditions that drove their formation. Their findings could help us study artificial photosynthesis and possibly pave the way for novel materials for LH devices like solar cells. Nearly all the chemical energy available to Earth's lifeforms can be traced back to the sun. This is because light-harvesting (LH) supramolecules (two or more molecules held together by intermolecular forces) enable plants and some types of bacteria (typically at the base of the food chain) to leverage sunlight for driving photosynthesis. For these supramolecules to be effective, they need to have multiple pigments, such as chlorophyll, arranged in special structures that vary among species. For instance, green photosynthetic bacteria have LH antennas in which chlorophyll molecules form spiral structures that, in turn, aggregate into large tubular supramolecules. In contrast, purple photosynthetic bacteria, such as Rhodobacter sphaeroides, exhibit different types of LH antennas in which chlorophyll pigments are arranged into ring-shaped architectures. While researchers have managed to recreate the tubular chlorophyl aggregates in the lab with a self-assembly approach, their ring-shaped counterparts have been not artificially reproduced so far. In a recent study published in Chemical Communications on 26 January 2023, a team of scientists from Japan managed to address this knowledge gap. They discovered that mixing a chlorophyll derivative with naphthalenediamide in an organic solvent led to the formation of dimers that spontaneously self-assembled into ring-shaped structures, each several hundred nanometers in diameter. The team included Professor Hitoshi Tamiaki from Ritsumeikan University and Assistant Professor Shogo Matsubara from Nagoya Institute of Technology. Surprised by their initial discovery, the team sought to better understand the formation of the ring-shaped nanostructures and their properties. Upon closer inspection using atomic force microscopy, they observed that chlorophyll dimers, molecules composed of two chlorophyll units linked by naphthalene, initially self-assembled into stable wavy nanofibers. Upon heating these nanofibers at 50°C, they disassembled into smaller nanoring precursors whose ends eventually joined together to form the desired nanorings. Interestingly, this nanofiber-nanoring transformation was dependent on external stimuli. Temperature was observed to play a major role, as well as dimer concentration. Prof. Tamiaki explains: "At low concentrations, ring-shaped aggregates were obtained by a preferential end-to-end joining of a single fiber supramolecule. In contrast, end-to-end linkage between different nanofibers was prevalent at higher concentrations and gave rise to network nanostructures." Overall, the findings of this study reveal a straightforward way to synthesize the LH supramolecule that has eluded scientists for a long time. Excited about the results, Dr. Matsubara remarks: "The self-assemblies we synthesized enable efficient sunlight absorption along with excitation energy migration and transfer. Mimicking the arrangement of chlorophyll pigments observed in nature is critical to not only understand natural photosynthesis but also construct artificial LH systems for devices such as solar cells." Moreover, the structural change from nanofiber to nanoring triggered by external stimuli could help realize novel smart materials with tunable properties. The team commented that further investigations on the optical properties of the self-assembled nanorings are underway. To know what interesting applications this knowledge will lead us to, stay tuned for future publications!