The catalytic effect of TiO2 nanosheets on extracellular electron transfer of Shewanella loihica PV-4

Electron transfer kinetics of Shewanella loihica PV-4 at the up-growing TiO2 nanosheet (TiO2-NS) modified carbon paper (CP) electrode was investigated. The effect of TiO2-NSs, which speeds up the interfacial electron transfer of outer membrane c-type cytochromes (OMCs), was revealed for the first time. TiO2-NSs with a polar surface modified hydrophobic CP into super-hydrophilic TiO2-NS/CP. The favorable interaction between PV-4 and TiO2-NSs not only enhanced microbial adhesion, but also altered the redox nature of OMCs. The mid-point potential of OMCs at TiO2-NS/CP was shifted to a more negative potential, indicating a higher thermodynamic driving force for the protein to release electrons. Moreover, electron transfer from OMCs to TiO2-NSs was also benefited from the positive shift of flat-band potential V-fb owing to reduced pH at the electrode/microorganism interface, as well as good electrical conductivity of TiO2-NSs. As a result, the electron transfer rate constant k(et) of OMCs at the TiO2-NS/CP anode was about three times faster than that at the CP anode. The accelerated electron transfer kinetics as well as 15% increase of biomass together accounted for a 97% increase of the maximum output power density in the MFC. The result expanded our knowledge about the role of a designed TiO2 nanostructure in microbial electron transfer that can be applied in other bio-electrochemical systems.