Improved Photosynthetic Currents from RuO2 Nanosheet Deposited Electrodes
Authors: Hong Hyeonaug , Lee Jang Mee , Hwang Seong-Ju , Ryu WonHyoung 
Published: February 2020

Thylakoids embedded in the chloroplast of plant absorb solar energy and produce photosynthetic electrons (PEs) from water splitting during photosynthesis. PEs excited by absorbing light energy are transferred via electrical carriers in the thylakoid membrane and finally they are consumed to form NADPH. Recent studies have investigated to harvest these excited PEs from thylakoids more efficiently through various approaches such as physical modification of electrodes, tethering of multi-walled carbon nanotubes to thylakoids, or use of mediators. Although these researches contributed to improve the efficiency of PE harvesting, still further studies are needed to achieve practical uses of the photosynthesis-based energy. In this study, we propose that 2-dimensional (2D) ruthenium oxide (RuO2) nanosheet-deposited electrodes can increase the amount of extracted PEs from thylakoids. With large surface area, 2D RuO2 nanosheets have high electrical conductivity and capacity such that they do not only enhance PE extraction but they also increase electron storage. RuO2 nanosheets were synthesized by following processes. First, layered NaRuO2 was synthesized by a solid state reaction, and then Na2S2O8 solution was added to remove excess Na ions. 1 M HCl solution was treated for protonating Na0.2RuO2 before RuO2 exfoliated to RuO2 nanosheets by additional protonating with tetrabutylammonuim. To purify thylakoids, spinach leaves were blended, filtered and centrifugated to extract chloroplasts which contain thylakoids within their membrane. Then, the chloroplasts were osmotically shocked to destroy chloroplast membranes, and density gradient centrifugation was performed to obtain pure thylakoids. For electrode modification with RuO2, RuO2 nanosheets were deposited on Au electrodes by two different methods: drop casting and electrophoresis. 10 and 50 μL of RuO2 solution with 2 mg/mL concentration was dropped on 1 cm2 Au electrode such that 0.02 and 0.1 mg/cm2 of RuO2 nanosheets were deposited on the electrodes. For electrophoretic deposition, Au electrodes were immersed in an acetone solution with 0.01 mg/mL concentration of RuO2. Because the zeta potential of RuO2 nanosheets is negative, the Au electrode was biased with 20 V in order to deposit RuO2 nanosheets on electrode. The thickness of RuO2 nanosheet film was controlled with deposition time. RuO2 was deposited 70 ~ 90 nm during 30 sec and 1~ 1.6 μm during 300 sec. For PEs extraction, 30 μL of thylakoid solution with 3 mg/mL chlorophyll concentration was dropped and dried on each electrode including bare Au electrodes as a control. Bias potential was maintained at 400 mV verses Ag/AgCl and Pt mesh was used as a counter electrode. Photosynthetic currents evidently increased for Au electrodes modified by RuO2 nanosheets. The PE currents from electrodes coated with RuO2 nanosheets were more than 4 times greater than that of bare Au electrodes. To be specific, in case of 0.02 mg/cm2 drop-cast RuO2 electrodes and electrophoretically-deposited RuO2 samples, PE currents were up to 500 nA/cm2, while PE currents from 0.1 mg/cm2 drop-cast RuO2 electrodes were 350 nA/cm2. Also, electrophoretically deposited RuO2 films were stable, while the drop cast RuO2 films were detached after several hours of solution immersion for electrochemical measurement.