Functionalized graphene sensors for real time monitoring fermentation processes : electrochemical and chemiresistive sensors

We developed a reference-less, chemiresistive, solid-state pH sensor to determine the acidification of the fermentation liquid in real time during the growth of Lactococcus lactis. One of the crucial findings of this work was that the ERGO-PA could not be used as such. It appeared that it was necessary to protect the sensor area with a Nafion coating to measure the pH in the fermentation broth. Most likely, the change in the concentration of redox-active components in the fermentation broth influences the conductivity of the ERGO-PA. Nafion formed a cation-selective membrane on top of the ERGO-PA allowing protons to diffuse to the selective layer of the sensor but not the redox-active components in the fermentation medium. We also reported a new approach to measure the dissolved oxygen concentration (DO) in a fermentation broth. The functionality of the sensor to measure DO was demonstrated during the growth of the obligate aerobic actinomycete Amycalotopsis methanolica in miniaturized 3D-printed bioreactors. For this oxygen-sensing application, the required modifications were obtained by doping hydrothermally reduced graphene oxide with nitrogen and boron atoms (N,B-HRGO). Further, these chemiresistive sensors are housed in the 3D printed bioreactor lid and used to measure pH, DO, and biomass in 3 ml fermentation broth. Additionally, the pH-sensor was equipped with a small heating element and a temperature sensor and that could be used for temperature control of the fermentation liquid. The setup was demonstrated to measure the pH, DO, temperature and biomass concentration in four parallel bioreactors.