# Novel Applications of Microbial Fuel Cells in Sensors and Biosensors
## 4 MFC-Based Biosensors for BOD Detection  
Biochemical oxygen demand (BOD) is a well-known essential parameter for assessing water quality. Concretely, a five-day BOD test method (BOD\({}_{5}\)) has typically been used as a standard method to determine biodegradable organics concentration in wastewater [11; 138]. The test consists in measuring the amount of dissolved oxygen (DO) before and after incubation of a water sample for five days at 20 \({}^{\circ}\)C [139]. This method is time and labor consuming, and requires strong experience to get reproducible results. In that sense, BOD assessment by MFC-based biosensing appears as a simpler alternative method, less time consuming, and with higher reproducibility and accuracy than a five-day BOD test. Hence, the use of MFCs for measuring BOD is being gradually introduced during the last few years. The BOD measurement in an MFC is based on the proportionality existing between the current efficiency of the cell (also known as Coulomb- or Faraday-efficiency) and the amount of oxidable organic matter contained in the feedstock/fuel used to operate the MFC system [140]. Accuracy and reproducibility, fast response (rapid monitoring), long-term operational stability, and minimum maintenance service are the most important advantages of an MFC-based BOD biosensor compared with conventional ones [141]. For example, [PERSON] et al. developed an MFC-based device for remote BOD detection displaying over 5 years of stable current generation [118]. Temperature, pH, conductivity, and inorganic solid content have been reported to significantly affect the sensitivity of the sensor. Nevertheless, there are many other critical factors affecting the stability and sensitivity of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter Measured** & **Power, Voltage, or Current** & **Detection Range** & **Reference** \\ \hline \multirow{8}{*}{BOD5} & 0.26–0.90 mA & 32–1280 mg L\({}^{-1}\) & [119] \\  & 0.063–0.55 mA & & [120] \\  & 72 mW m\({}^{-2}\) & 17–183 mg L\({}^{-1}\) & [121] \\  & 3.7–5.2 mA & 20–200 mg L\({}^{-1}\) & [48] \\  & 0.05–1.1 mA & 2.6–206 mg L\({}^{-1}\) & [122] \\  & 0.7–1.9 mA & 50–100 mg L\({}^{-1}\) & [123] \\  & 0.2–1.7 mA & - & [124] \\  & 0.05–8 \(\mu\)A & 2–10 mg L\({}^{-1}\) & [125] \\  & 0.0015–0.2 mA & & [126] \\ \hline Organophosphorus & 0.005–0.042 mA & 1–10 mg L\({}^{-1}\) & [127] \\ \hline Cd(II) and Pb(II) & 0.005–0.035 mA & 0.1–1 mg L\({}^{-1}\) & [127] \\ \hline \multirow{2}{*}{Ni} & 0.15–2.25 mA & 10–30 mg L\({}^{-1}\) & [128] \\  & 0.022–0.132 A m\({}^{-2}\) & 0–88 mg L\({}^{-1}\) & [128] \\ \hline Na dodecyl sulfate & 0.85–1.7 mA & 10–50 mg L\({}^{-1}\) & [128] \\ \hline Bentazon & 0.9–1.4 mA & 1–3 mg L\({}^{-1}\) & [128] \\ \hline Cu & 0.7–1.5 A m\({}^{-2}\) & - & [129] \\ \hline Formaldehyde & 0.05 \(\pm\) 0.04–0.1 \(\pm\) 0.03 mA & - & [130] \\ \hline \multirow{2}{*}{_E. Coli_} & 0.1–0.38 mA & - & [122] \\ \cline{2-2}  & 0.6–12.4 A m\({}^{-2}\) & 0–13 nmol\({}^{-1}\) & [131] \\ \cline{2-2}  & 0–0.30 mA & - & [132] \\ \hline \multirow{2}{*}{Oxygen Dissolved} & 5.6–462 mA m\({}^{-2}\) & 0–8.8 mg L\({}^{-1}\) & [133] \\  & 9.5–17 mW m\({}^{-3}\) & - & [134] \\  & 0–0.092 mW & 0–8 mg L\({}^{-1}\) & [135] \\ \hline Volatile fatty acids & 0.22–1.29 mA & 0–40 mg L\({}^{-1}\) & [136] \\ \hline Anaerobic digestion & 0.01–0.095 mA & - & [137] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of viable reported applications of MFC-based biosensors. MFC-based biosensors for BOD monitoring, such as the dissolved oxygen concentration in the cathode chamber, external resistance, effluent flow rate, and any other factor, as long as it will influence the electricity generation of the MFC [30; 48]. Restricted diversity of bacteria and organic substances present, as well as low metabolic rates, are the main disadvantages of MFC-based BOD biosensors [15; 142].