Proof of Concept

Our goal is to enable Shewanella to enrich silver ions using silver-binding proteins and convert silver ions into silver nanoparticles, thereby creating a fast pathway for electron transfer and enhancing electricity production. After successfully introduced Atox1 into Shewanella，we conducted a series of experiments and modeling efforts to proof that our concept is feasible。

Silver Nanoparticles Detection

After our successful transformation into Atox1, we cultured the engineered Shewanella using 1 mM \(\ce{AgNO3}\) and detected AgNPs in the engineered Shewanella under transmission electron microscopy. Denser clusters of silver nanoparticles could be detected on Shewanella with Atox1. This proves that the silver ion-binding protein works successfully. We used the engineered Shewanella to build half-cells for testing. The experimental results demonstrated that our engineered Shewanella had higher current densities than wild ones.

Half-cell Experiment

In the half-cell experiment, we build a three-cell system and designed four working electrodes.

They are ordinary carbon paper, ordinary carbon paper + reduced graphene oxide, and further, adding chemically synthesized nano-silver particles, and bio-synthesized nano-silver particles in this experiment. The I-t curve was measured at a constant voltage of 0.2V. The electrode with improved stains obtains a higher peak current than the ordinary electrode.

In addition, comparing the chemically synthesized silver nanoparticles with the biosynthetic ones, we use 5% as much silver as is used in our reference to achieve a similar max current intensity.

The experimental results of the half-cell show that our idea is successful. The in situ synthesis of silver nanoparticles does boost the energy density, which proves our concept.

MFC Model

Due to the lack of time to experiment, we built a two-chamber cell model of the microbial fuel cell. In the model, we considered the effect of silver nanoparticles on the model parameters. The simulation results show that our modified Shewanella can promote electricity production in a complete battery. learn more information in Model。

Future Experiments

Based on the model and our interviews with the sewage treatment plant, we plan to build a closed full-cell device. We plan to use rGO as the anode of the battery and \(\ce{Fe(SCN)3}\) as the cathode of the cell. We will perform three experiments using a full-cell device.

Experiment 1: Full Cell

Test the voltage and current densities achievable by our engineered Shewanella using a full-cell device. To apply our device in an actual sewage treatment plant, we plan to measure the voltage and current density of the microbial fuel cell. The test results can help us design downstream circuit components to charge the appropriate battery. Once the hardware is complete, we can test the stability during the device's operation, proving that our device will work for a long time.

Experiment 2: Sewage

Use sewage or simulated sewage to test the performance of the MFC. In the experiments, we used lactic acid as the single substrate. However, the composition of sewage substrates in the real world is very complex. We plan to use sewage or simulated sewage to demonstrate the ability of our design to increase power generation in complex environments.

Experiment 3: Silver

Use different silver ion concentrations to test the safety of the device. Although our engineered Shewanella works in a closed device in an anaerobic environment, we still need to consider the leakage of silver ions/AgNPs or engineered bacteria. We plan to use lower concentrations of silver ions for the biosynthesis of silver nanoparticles by Shewanella. If lower concentrations of silver ions still achieve better results, we can use lower concentrations of silver ions to further improve the safety of our device. At the same time, the use of low-concentration silver ions can reduce the treatment cost after the device is decommissioned.

Device Design

The device prototype we designed consists of five parts: MFC, MCU, power management, sensor, and battery. MFCs are responsible for converting chemical energy in organic matter into electrical energy. Power management can stabilize the voltage output by the MFC within the power supply range of the battery, keeping the current and voltage stable. It can also supply power to the MCU. The battery stores the electrical energy output by the MFC. The MCU collects information such as temperature, pH, current, and voltage of the MFC cathode, anode, or power management by sensors. The MCU can report the collected data to the users and control the input of substrates to the MFC.