# Novel Applications of Microbial Fuel Cells in Sensors and Biosensors
## 2 MFC Power Supply for Sensors and Biosensors
### Effect of pH  
As in every biological system, the pH is an essential factor in MFCs, affecting both anodic microbial activities and cathodic reactions [76; 77]. Protons, produced by the oxidation of organic matter in the anode compartment of an MFC, pass into the cathode compartment through the PEM and usually react with oxygen generating water molecules [58]. Accumulation of protons, due to continuous operation and/or limited proton diffusion and migration through the membrane, will lead to acidification of the reaction medium in the microbial anode chamber. On the other hand, production and migration of protons from the anode also influences the pH in the cathode compartment. In fact, the continuous intake of protons by oxygen reduction reaction (ORR), under unbalanced replenishment of protons, favors the alkalinization [60; 78; 79; 80]. Eventually, energy generation decreases due to the pH increment in the cathode chamber, according to the Nernst equation for ORR [81]. Indeed, low operational pH benefits oxygen reduction, increasing the current output from an MFC. However, too low pH in the anode may negatively affect the biofilm performance and stability, decreasing bacterial activity, and consequently, the electron and proton generation. Despite the influence on the biofilm formation, the specific response to the pH variations, which strongly depends on the type of microbe and conditions needed for its growth, will affect other primary parameters, such as ion concentration, membrane potential, and proton motive force [68; 76; 82; 83; 84; 85]. Thus, the performance for energy generation in conventional double-chamber MFCs depends on the pH gradient between anode and cathode compartments, each of them usually presenting a different optimum pH. On the other hand, the performance of single-chamber MFCs, with only one electrolyte, is determined by mixed effects of the electrolyte pH on both anodic and cathodic reactions [58]. In general, the operational pH range for most MFCs is reported to be between 6 and 9, with an optimum usually at neutral pH [58; 68]. Nevertheless, optimal performance can be found at higher pH ranges, between 8 and 10, for air-cathode MFCs [58; 86]. Changes in current densities above a factor of 4 have been reported for pH variations of 1 unit in MFCs [68]. Therefore, active pH control by means of chemical buffers, such as phosphate [64], bicarbonate [87], borax [88], or synthetic zwitterionic compounds [89], have been proposed in order to keep steady-state current production. However, in spite of keeping a constant pH, the use of chemical buffers interferes with the MFC performance due to interactions with the electroactive microorganisms, electrodes, and/or proton exchange membrane [58; 64]. Therefore, the unavoidable strong impact on the power output, along with an inherent increase of operational cost derived from their use, make chemical buffers not feasible for real applications. A step forward is the harnessing of local sources of carbon dioxide for the in situ generation of a carbonate/bicarbonate buffer, by reaction with hydroxide ions in the MFC cathode, proposed as a promising cost-efficient alternative for overcoming the pH-buffering challenge in MFCs [76; 77; 90; 91; 92]. Alternatively, self-sustaining pH control MFCs have been built as hybrid systems between single and dual chamber MFCs [93]. In detail, an aerobic microbial oxidation of acetate by the biofilm attached to the air cathode of a single chamber MFC (SC-MFC), was responsible for the self-sustaining removal of accumulated protons in the effluent of the dual chamber MFC (DC-MFC). It has been demonstrated that the hybrid exhibited the highest electricity output performance and the most effective conversion of acetate into electricity at high power levels, compared with the MFC consisting of an isolated single or dual chamber.