4. Material and methods
4.7. Electro-biocatalytic system for formic acid synthesis
The electro-biocatalytic reactor, electrodes, and experimental setup in this study were similar to our previous paper. 1.5 cm × 2.0 cm of copper plates (for S. oneidensis MR-1), carbon felt (for encapsulated MeFDH1) were used as the working electrode. Ag/AgCl electrode was used as the reference electrodes in the cathode section, while coiled platinum wire was utilized as counter electrodes in the anode section. The protons generated from the electrolysis of H2SO4 (1 mM for S.
oneidensis MR-1, 10mM for encapsulated MeFDH1) by the anode were supplied to the cathode through a proton exchange membrane (Nafion 115, 1.0 cm ×1.5 cm). The experiments were conducted under potentiostatic control using MultiEmStat (PalmSens BV, Houten, The Netherlands)
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and Multitrace software control. The cathode was polarized at -0.75 vs. Ag/AgCl reference electrode.8 The current was recorded every 10 seconds for system control as well as Faraday efficiency analysis.
The formic acid synthesis was performed in the cathode reactor by purging gaseous CO2 (99.999%
purity at flow rate of 1 ml/sec). 0.2 M potassium phosphate and 0.2 M MOPS buffer were used for S.oneidensis MR-1 and encapsulated MeFDH1 respectively (adjusted pH = 7.0 by KOH). Whole-cell S. oneidensis MR-1 (wet-cell weight of 0.5 g) and 10 mM methyl viologen (MV) as an electron mediator were added to the above reaction solution to final volume of 10 mL. In case of encapsulated MeFDH1, encapsulated MeFDH1 and 10 mM ethyl viologen (EV) were added to reaction buffer to final volume of 10 ml. The reaction was performed at room temperature (~25 °C) and stirred at 300 rpm.
The synthesized formate was analyzed by HPLC (i.e., Agilent 1200 series with Refractive Index detector (RID) and Rezex ROA-Organic Acid H+ (8%) column using 5 mM H2SO4 (0.6 mL/min) as the mobile phase. To prepare the sample for HPLC analysis, 15 L of H2SO4 (6 M) was added to reaction sample (150 L) to quench the reaction, which was then centrifuged at 10,000 rpm for 1 minute to remove cell pellet. The broth was further filtered with a 0.2 m hydrophilic filter. For each analysis, 20 L samples were injected and formate was detected at 13.8 min in our analysis conditions.
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