Thi Hiep Han

Environmental factors affecting indole production in Escherichia coli

A variety of both Gram-positive and Gram-negative bacteria produce large quantities of indole as an intercellular signal in microbial communities. Biosynthesis of indole is well-studied, and while carbon sources and amino acids are important environmental cues for indole production in Escherichia coli, other environmental factors affecting indole production for this strain are less clear. This study demonstrates that the environmental cue pH is an important factor for indole production that further controls biofilm formation of E. coli. Moreover, E. coli produced a higher level of extracellular indole in the presence of the antibiotics ampicillin and kanamycin, and the increased indole enhanced cell survival during antibiotic stress. Additionally, we found here that temperature is another important factor for indole production; E. coli produces and accumulates a large amount of indole at 50 °C, even at low cell densities. Overall, our results suggest that indole is a stable biological compound, and E. coli may utilize indole to protect itself against other microorganisms.

Three-dimensional, highly porous N-doped carbon foam as microorganism propitious, efficient anode for high performance microbial fuel cell

Three-dimensional (3D) N-doped open-porous carbon foam was fabricated using the simple procedure of calcining a melamine sponge. The properties of the fabricated carbon foam and its performance in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 (S. oneidensis) were compared with those of commercial graphite felt. The MFC with the carbon foam anode produced approximately 2 times higher power density than the commercial graphite felt. The superior performance of the as-prepared carbon foam in MFC was attributed to the higher surface area (687.19 m2 g−1) and open-porous scaffold structure. Moreover, the appearance of the hydrophilic functional groups such as CN–C, N–CO on the surface of the as-prepared carbon foam facilitated extracellular electron transfer, resulting in a decrease in charge transfer resistance and an increase in biocompatibility. Owing to the excellent biocompatibility, a large amount of microbial biomass colonized both the surface and inside the carbon foam, which helped enhance the performance of the MFC.

Metal-Free Carbon-Based Materials: Promising Electrocatalysts for Oxygen Reduction Reaction in Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising green approach for wastewater treatment with the simultaneous advantage of energy production. Among the various limiting factors, the cathodic limitation, with respect to performance and cost, is one of the main obstacles to the practical applications of MFCs. Despite the high performance of platinum and other metal-based cathodes, their practical use is limited by their high cost, low stability, and environmental toxicity. Oxygen is the most favorable electron acceptor in the case of MFCs, which reduces to water through a complicated oxygen reduction reaction (ORR). Carbon-based ORR catalysts possessing high surface area and good electrical conductivity improve the ORR kinetics by lowering the cathodic overpotential. Recently, a range of carbon-based materials have attracted attention for their exceptional ORR catalytic activity and high stability. Doping the carbon texture with a heteroatom improved their ORR activity remarkably through the favorable adsorption of oxygen and weaker molecular bonding. This review provides better insight into ORR catalysis for MFCs and the properties, performance, and applicability of various metal-free carbon-based electrocatalysts in MFCs to find the most appropriate cathodic catalyst for the practical applications. The approaches for improvement, key challenges, and future opportunities in this field are also explored.

Indole oxidation enhances electricity production in an E. coli-catalyzed microbial fuel cell

Microbial fuel cells (MFCs) generate electricity from the oxidation of dissolved organic matter. A variety of Gram-positive and Gram-negative bacteria, including Escherichia coli, produce a large quantity of indole, which functions as an extracellular signal molecule. This work explored the role of indole in a mediatorless E. coli catalyzed MFC. Although the presence of indole alone did not affect power generation, indole oxidation by the indole-oxidizing enzyme toluene-o-monooxygenase (TOM) enhanced power density by 9-fold. Open circuit voltage and polarization curve showed that indole oxidation by TOM produced a maximum power density of 5.4 mW/m2 at 1,000 ohm. Cyclic voltammetric results suggested that indole oxidation resulted in the production of redox compounds. This study provides a novel means of enhancing power generation in E. coli-catalyzed MFCs.

Electrochemically synthesized sulfur-doped graphene as a superior metal-free cathodic catalyst for oxygen reduction reaction in microbial fuel cells

Platinum nanoparticles (PtNPs) have long been regarded as the benchmark catalyst for the oxygen reduction reaction (ORR) in the cathode of microbial fuel cells (MFCs). On the other hand, the practical applications of these catalysts are limited by the high cost and scarcity of Pt. Therefore, developing an alternative catalyst to PtNPs for efficient ORR activity is essential for meeting the future demands for practical applications in MFCs. In this study, sulfur-doped graphene (S-GN) was synthesized via the environmental friendly, economical and facile one pot electrochemical exfoliation of graphene in a unique combination of electrolytes, which both catalyzed the exfoliation reaction and acted a sulfur source. The initial activity of S-GN as an ORR active catalyst was examined by cyclic voltammetry (CV), which showed that the as-synthesized S-GN exhibited better ORR activity than the plain material. Furthermore, the application of S-GN as a cathode material was also studied in MFCs. The results showed that the MFC equipped with the S-GN cathode produced a maximum power density of 51.22 ± 6.01 mW m−2, which is 1.92 ± 0.34 times higher than that of Pt/C. The excellent performance of S-GN as a cathode catalyst in MFCs could be due to the doping of graphene with heteroatoms, which increased the surface area and improved the conductivity of graphene through a range of interactions. Based on the above MFC performance, the as-synthesized S-GN catalyst could help reduce the cost and scale up the design of MFCs for practical applications in the near future.

Electrochemically active biofilm-assisted biogenic synthesis of an Ag-decorated ZnO@C core–shell ternary plasmonic photocatalyst with enhanced visible-photocatalytic activity

Colonies of electrochemically active microorganisms called electroactive biofilms (EABs) have potential applications in bioenergy and chemical production. In the present study, an EAB was used as a reducing tool to synthesize Ag-decorated ZnO@C core–shell (Ag–ZnO@C) ternary plasmonic photocatalysts. A simple thermal decomposition route was followed to synthesize ZnO@C nanoparticles using a zinc aniline nitrate complex. The simultaneous adsorption of Ag+ in the carbon shell of the ZnO@C particles during reduction using an EAB allowed the direct contact among Ag nanoparticles, the ZnO core, and the carbon shell. Therefore, the synthesized Ag–ZnO@C ternary photocatalysts showed a stronger interconnection among all the components, which allowed the easy transfer of photogenerated charges and provided enhanced charge carrier separation. Optical characterization showed that the enhanced absorption of visible light along with a decrease in the band gap and a red shift in the valence band maximum occurred due to the decoration of Ag-nanoparticles on ZnO@C. Ag–ZnO@C exhibited higher photocatalytic activity for the degradation of rhodamine blue and 4-nitrophenol under visible light irradiation than ZnO@C and bare ZnO without any significant loss after five successive cycles. Finally, a possible photocatalytic mechanism for charge transfer was proposed to explain the enhanced photocatalytic performance of the Ag–ZnO@C ternary photocatalyst. This study provides insights into the ternary photocatalytic system with a core–shell material and offers a biogenic route for the facile fabrication of Ag–ZnO@C photocatalysts.

Development of Suitable Anode Materials for Microbial Fuel Cells

Microbial fuel cells (MFCs) and related bioelectrochemical systems (BESs) have shown impressive developments for many purposes over the past decade (Kalathil et al. 2012; Han et al. 2013, 2014, 2016). Even with the noticeable improvements in power density, the large-scale application of MFCs is still limited due to the low power generation and high cost (Wei et al. 2011). To take this technology from laboratory-scale research to commercial applications, the cost and the performance of these systems need to be optimized further. The anode electrode plays an important role in the performance and cost of MFCs. The electrode materials in MFCs have some general and individual characteristics. In general, electrode materials must have good conduction, excellent biocompatibility, good chemical stability, high mechanical strength and low cost. The anode material design has attracted an enormous number of studies over the past decade.