# Novel Applications of Microbial Fuel Cells in Sensors and Biosensors
## 1 Introduction  
Bio-electrochemical systems (BESs) are promoting the development of innovative technologies that combine biological redox catalytic activity with abiotic electrochemical reactions and physics [1; 2; 3]. BESs are normally classified by their applications, such as the generation of chemicals (e.g., formate or methane [4; 5]) and energy [6], as well as water desalinization [7]. Regarding the matter in question, BESs that generate energy are defined as bio-electrochemical fuel cells, which are subdivided into enzymatic fuel cells (EPCs) [8; 9] and microbial fuel cells (MFCs) [10]. Specifically, MFCs use microbes to catalyze electrochemical reactions for electricity production. As shown in Figure 1, MFCs generally consist of anodic and cathodic chambers separated by a proton exchange membrane (PEM) [11], although single chamber MFCs, with or without an air-cathode, also exist [12; 13; 14]. Microorganisms oxidize the organic or inorganic material in the anodic chamber to produce protons and electrons. The electrons generated are captured at the anode and transferred to the cathode through an external circuit. Hence, MFCs, using microorganisms as electrocatalys, are capable of generating electricity directly from a large variety of organic and inorganic compounds [15; 16; 17]. Unlike the practically limitless number of potential anodic reactions in MFCs, the number of cathodic reactions is quite limited, and mostly consists of oxygen reduction to form water or hydroxyl ions [18; 19; 20]. Nevertheless, it must be mentioned that there is still much room for improvement in the reactor configurations and electrolyte design of MFCs, in terms of effective applicability and the reduction of production costs [21]. Derived from the versatility in the nature of feedstock used as an anodic fuel, an innovative dual-purpose has been lately introduced for MFCs as an energy production technology for wastewater treatment. In fact, MFCs could provide a self-sustainable or even a net positive energy output while removing pollutants, with a competitive life-cycle assessment [22; 23; 24; 25; 26; 27]. For example, an MFC system has been implemented in a constructed wetland very recently. A constructed wetland (CW) is defined as a treatment system that involve natural processes to improve water quality [28]. From this study, it has been demonstrated that an MFC/CW system has a low environmental impact, but its operating cost is 1.5 times more expensive than that for standard CW systems [29]. Other recent research projects aim towards the application of MFCs in the provision of power for remote electronic devices such as sensors and biosensors [30], the matter at issue in this review. In general, a sensor can be defined as an electronic device capable of detecting properties and events occurring in its surrounding environment, and quantify their changes [31]. The measured input information is converted into an output electrical voltage or current that is eventually sent to other electronic devices, most usually a computer processor. Based on tremendous efforts done by researchers over the last few decades, measuring water quality such as pH, temperature, and concentrations of heavy metals are some essential applications of wireless sensors nowadays [32]. As shown in Figure 2, in the specific case of biosensors, a biological or biomimetic component acts as a sensitive detector for an analyte of interest. The signal resulting from the interaction with the analyte is transformed, by a physico-chemical component or transducer, into a physical quantifiable signal compatible with an electronic signal processor. The function of a biosensor depends on the biochemical specificity of its biologically active material or bioreceptor [33]. Depending on the type of receptor, biosensors can be classified as enzymatic biosensors [34], immunosensors [35], genosensors or DNA biosensors [36], or tissue-based  
Figure 1: Schematic diagram of a MFC. Figure 2: Schematic diagram of a biosensor. biosensors [35].