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In order to distinguish between the porosity on different length scales, we define nanoporosity as the presence of pores between sintered nanoparticles (10–100 nm), and microporosity as the pores created by microsphere templates (10–40 µm). ITO can be structured using a templating approach and has previously been used as porous glass in electrochemical studies of enzymes 7, 8. ITO is commonly used as thin film (tens of nanometres thick) in display applications and was shown to be biocompatible 4, 5, 6. ITO is one of the best performing transparent electrode materials, as it has a large optical bandgap, making it transparent to visible light, while the high levels of tin doping cause a metal-like conductivity. Doped metal oxides are popular transparent electrode materials for a wide range of electronic applications.
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PCC 6803 were each placed on a non-porous indium tin oxide (ITO) electrode, a thick ‘nanoporous’ ITO nanoparticle film, and a ‘microporous’ inverse-opal structure made from the same nanoparticles, and their photocatalytic current generation was investigated. Two photosynthetic microorganisms Nostoc punctiforme and Synechocystis sp. To achieve this goal, we have compared three different electrode morphologies of the same translucent material. In this study, we tested the effect of electrode porosity at different length scales on the performance of bioelectrochemical devices. Although photosynthetic microorganisms are expected to operate with a quantum efficiency of five to ten percent internally, electrode interfaces and microbial electron export pathways currently limit device efficiencies to much lower values 1. Furthermore, because of a lack of transparency of most anodes, there has been little work on the benefits of using porous electrodes in microbe-based devices that rely on light absorption, referred to as ‘biophotovoltaics’, except for one study using larger, eukaryotic, algal cells 3. These complicated correlations currently limit the understanding of design rules for electrochemical bio-interfaces. The electrode porosity usually has a strong effect on device efficiency 2, but the associated change in volume, surface area and organism contact area can rarely be disentangled from the variation of materials themselves that are used to achieve the different morphologies.
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Bio-anodes in the best studied bioelectrochemical technology, microbial fuel cells, are commonly carbon or metal based, and a large diversity of morphologies has been used 2. In such devices, bio-anodes are the electrodes that collect electrons from the living bio-catalyst. Several microorganisms are able to generate electrons that can be collected and utilised in external circuits 1. Our results highlight the importance of electrode nanoporosity in the design of electrochemical bio-interfaces. Electrodes with large enough mesopores for the cells to inhabit show only a small advantage over purely nanoporous electrode morphologies, suggesting the prevalence of a redox shuttle mechanism in the electron transfer from the bacteria to the electrode over a direct conduction mechanism. In addition, the photo response is substantially faster compared to non-porous anodes. PCC6803 on structured indium-tin-oxide electrodes, an increase in current generation by two orders of magnitude is observed compared to a non-porous electrode. For the cyanobacteria Nostoc punctiforme and Synechocystis sp. Here, we report the enhancement of bioelectrochemical photocurrent harvesting using electrodes with porosities on the nanometre and micrometre length scale. Devices harvesting solar energy by this mechanism are currently limited by the charge transfer to the electrode. Some photosynthetically active bacteria transfer electrons across their membranes, generating electrical photocurrents in biofilms.