We designed an in situ surface-enhanced Raman spectroscopy (SERS) system to detect pH change near the electrode surface. An optically transparent substrate covered with Ag nanoparticles (NPs) that induce SERS was positioned vertically to the electrode. A probe molecule that protonates/deprotonates according to the change in pH of surroundings were distributed nearby the NPs, so that the SERS spectrum of the probe indicated the pH changes in the vertical direction to the electrode due to hydrogen consumption. Flexible choices of probe molecules allow the system to be versatile. The system operated effectively in Ni electrodeposition as a case example.

Background Natural rubber (cis-1,4-polyioprene, NR) is an indispensable industrial raw material obtained from the Pará rubber tree (H. brasiliensis). Natural rubber cannot be replaced by synthetic rubber compounds because of the superior resilience, elasticity, abrasion resistance, efficient heat dispersion, and impact resistance of NR. In NR production, latex is harvested by periodical tapping of the trunk bark. Ethylene enhances and prolongs latex flow and latex regeneration. Ethephon, which is an ethylene-releasing compound, applied to the trunk before tapping usually results in a 1.5- to 2-fold increase in latex yield. However, intense mechanical damage to bark tissues by excessive tapping and/or over-stimulation with ethephon induces severe oxidative stress in laticifer cells, which often causes tapping panel dryness (TPD) syndrome. To enhance NR production without causing TPD, an improved understanding of the molecular mechanism of the ethylene response in the Pará rubber tree is required. Therefore, we investigated gene expression in response to ethephon treatment using Pará rubber tree seedlings as a model system. Results After ethephon treatment, 3270 genes showed significant differences in expression compared with the mock treatment. Genes associated with carotenoids, flavonoids, and abscisic acid biosynthesis were significantly upregulated by ethephon treatment, which might contribute to an increase in latex flow. Genes associated with secondary cell wall formation were downregulated, which might be because of the reduced sugar supply. Given that sucrose is an important molecule for NR production, a trade-off may arise between NR production and cell wall formation for plant growth and for wound healing at the tapping panel. Conclusions Dynamic changes in gene expression occur specifically in response to ethephon treatment. Certain genes identified may potentially contribute to latex production or TPD suppression. These data provide valuable information to understand the mechanism of ethylene stimulation, and will contribute to improved management practices and/or molecular breeding to attain higher yields of latex from Pará rubber trees.

l-Menthyl α-d-glucopyranosyl-(1→4)-α-d-glucopyranoside (l-α-MenG2), an α-maltoside of l-menthol, was synthesized through a three-step enzymatic reaction. We found that l-α-MenG2 possesses the properties of a low-molecular-weight gelator. Aqueous solutions containing l-α-MenG2 at concentrations above 30 g L−1 show a thermally reversible sol–gel transition. The sol–gel transition temperature of the aqueous l-α-MenG2 solution increases with l-α-MenG2 concentration: 12 °C at 30 g L−1 and 24 °C at 250 g L−1. d-Menthyl and d-isomenthyl α-maltosides were also synthesized enzymatically, but their aqueous solutions showed no sol–gel transition.

For carbon-neutral transition, technologies are required which enable the efficient conversion of CO2 into valuable feedstocks for chemicals and fuels (e.g., CO, methane, and ethylene). Electrochemical reduction of CO2 (CO2ER) is one of the most promising pathways due to its high product selectivity as well as high activity at mild temperature and pressure, powered by electricity from renewable sources. Among the CO2 reduction products with high selectivity through CO2ER, CO is a useful feedstock to produce methane, methanol, and olefins through conventional catalytic reactions. It is reported that noble metal CO2ER catalysts such as Au and Ag exhibit high CO selectivity[1]. Toward social implementation of CO2ER technology on a commercial scale, earth-abundant elements are more favorable as a catalyst. In recent years, nitrogen (N)-doped carbon materials have attracted much attention for their high CO selectivity comparable to Au and Ag. Previous studies indicate the N-doped carbon catalysts with high content of pyridinic N show high CO generation activity[2]. However, the specific role of pyridinic N (how N contributes to the activity during CO2ER) is still unclear. To establish a design strategy for the efficient N-doped carbon catalyst, the catalytic mechanism involving the pyridinic N must be clarified. In this study, carbon nanotube (CNT) is selected as a model carbon material whose commercial-scale production methods are relatively advanced among carbon materials. With electrochemical and spectroscopic measurements, the local pH dependence of the CO2ER catalytic performance of N-doped CNT (NCNT) was investigated, and the role of the pyridinic N on the CO2ER activity was discussed. The NCNT was prepared by pyrolysis of 1,10-phenanthroline on a multi-walled carbon nanotube as previously reported[2]. CO generation activity was examined in two types of electrochemical systems; the liquid phase electrolysis using an H-cell with a three-electrode system including 4 cm2 NCNT-loaded glassy carbon as a working electrode, and the gas phase using a polymer electrolyte fuel-cell type reactor including membrane-electrode assembly (MEA) with 5 cm2 active area. In the liquid phase electrolysis with CO2 gas bubbling, NCNT showed high faradaic efficiencies toward CO in 1.0 M KHCO3 electrolyte (pH 7.37), while no CO was detected in 1.0 M KHSO4 (pH 0.55) at any applied potentials. In the case of the gas phase reaction, NCNT loaded on a carbon paper (a gas diffusion electrode) with a Nafion® ionomer (Nafion® DE2020CS, The Chemours Company, Delaware, U.S.) showed poor CO selectivity at any cell voltage in both cases using a proton and an anion exchange membrane, while the ionomer substitution to Sustainion® XA-9 (Dioxide Materials, Inc., Florida, U.S.), an anion exchange type, increased the faradaic efficiency toward CO remarkably. From those results, it was confirmed that the local pH near the catalyst surface affects the CO generation activity of NCNT. The local pH dependence on the CO generation activity could be attributed to the CO2 concentration in the electrolyte as well as the protonation of the pyridinic N of the catalyst as previously reported in the system of O2 electroreduction[3]. To observe changes in the local pH and the chemical state of pyridinic N directly, spectroscopic analyses were conducted. We applied our previously reported in situ surface-enhanced Raman spectroscopy measurement system[4] to this reaction to detect the local pH near NCNT surface in the liquid phase as well as the adsorbate species. Surface X-ray photoelectron spectroscopy was also performed to study the changes in the chemical state of N. Those spectroscopic analyses suggested that the factor which affects the CO2ER activity is not only the difference in dissolved CO2 concentration induced from the less acidic condition but also the change in the pyridinic N state. References [1] P. De Luna, C. Haun, D. Higgins, S. A. Jaffer, T. F. Jaramillo, E. H. Sargent, Science 364, eaav3506 (2019). [2] C. Ma, P. Hou, X. Wang, Z. Wang, W. Li, P. Kang, Appl. Catal. B Environ. 250 347-354 (2019). [3] K. Takeyasu, M. Furukawa, Y. Shimoyama, S. K. Singh, J. Nakamura, Angew. Chem. Int. Ed. 60, 5121 (2021). [4] K. Ide, M. Kunimoto, S. Yoshida, M. Yanagisawa, T. Homma, Electroanalysis (in press). Fig. 1. Schematic illustration of experimental setup for in situ SERS measurement of the liquid phase CO2ER with a three-electrode system. Figure 1

Nitrogen-doped carbon nanotubes (NCNTs) have been considered a promising catalyst for the electrochemical reduction of CO 2 (CO 2 ER) to generate CO. Although pyridinic N sites have been suggested to be the active center of NCNTs, their behavior in the reaction remains unclear because of the lack of experimental evidence. Herein we focused on the pH dependence of CO 2 ER activity of NCNT and investigated the effects of local pH at the electrode surface to estimate the catalytic role of the pyridinic N. The results of the in situ local pH measurements using surface-enhanced Raman spectroscopy (SERS) revealed that CO 2 ER activity disappears in an acidic environment at pH below 4. SERS detected no CO species at the surface during the reaction in the acidic electrolyte, and ex situ X-ray photoelectron spectroscopy indicated the protonation of the pyridinic N. These results suggest the protonation of pyridinic N, the active site of NCNT, inhibits the CO 2 adsorption and the following reduction to define the catalytic activity.