Bacterial cultures, especially in bulk quantities must be closely monitored to grow in optimal conditions. These conditions include temperature, pH and oxygenation.
Previous projects have effectively tackled the first two components of this system, but the dissolved oxygen sensing has remained an issue not only in the biomaker challenge-inspired bioreactor but in comparable initiatives as well.
In this project, Open bioeconomy lab team members, Chiara, Anne – Pia and Joseph Wong (PhD Student) from the Department of Chemical Engineering and Biotechnology, University of Cambridge, intends to build a low-cost oxygen sensor for bacterial cultures. The device they aim to build uses a gold wire as a cathode and records current from the oxidation process as the analog input of the sensor. A potentiostat will also be built to control the polarization of the electrode. The overall aim is to have a cheap, open source and sterilizable device able to record the dissolved oxygen in a batch E. coli culture in a bioreactor.
The first objective of the project is to create the electrode system. The oxygen concentration in the culture medium will be measured electrochemically and continuously for at least 24 hours. Gold will be the choice of working electrode that is a stable and inert material to electrochemically reduce oxygen. Gold wire encased in epoxy will be made the working electrode. It will have a fine opening at one end exposing the cross-section of the gold wire to the electrolyte. The opening will be protected by a oxygen permeable membrane resistant to cell attachment and protein aggregation. PTFE and cellulose membrane (dialysis membrane) will be investigated for their applicability to the current set-up. PTFE is cheap, hydrophobic and oxygen permeable while dialysis membrane have different pore sizes to give different diffusivity for different sizes of molecules. The selectively permeable membrane will be held at the opening with an O-ring encapsulating a small volume of electrolyte. A silver rod will be the reference electrode placed in proximity to the working electrode. The current generated by oxygen reduction reaction will be recorded and used to calculate the bulk oxygen concentration. (Fig. 1 and 2)
The second objective of the project is to develop a cheap small-scaled potentiostat. A potentiostat is highly important for controlling electrode polarization and measuring the current output which correlates to dissolved oxygen concentration. Using an op-amp and few electronics could create a simplified version of potentiostat (fig. 3). Oxygen concentration can thus be measured and calculated in real-time by Arduino which can execute the subsequent actions (e.g. introduce additional feeding). A touchscreen user-friendly interface will be set up and programmed for easy control of functions (e.g. calibration and oxygen measurement) and displaying oxygen concentration in real-time.
Outcomes and benefits
The expected outcome of this project is the implementation of an open-source low-cost oxygen sensor module that will serve to measure dissolved oxygen in E. coli cultures. The hardware will be open source and the documentation will include software and protocols for calibration and usage.
Future directions for this project would be to link the data output to a feedback loop and add additional feeds to increase the yield in the bioreactor.
Figure 1: Schematic of the device. The gold wire encased in an epoxy coating and open at the end to a membrane. This sensor is placed close to a silver reference sensor.
Figure 2: Another approach of the device, similar to figure 1 but where the O2 electrode and reference sensor are both in the same casing with the membrane acting as a buffer for the both of them.
Figure 3: Basic schematics of a Potentiostat By Enseeg – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4248644