## 2 Applications of Biosensors in Bacteria Detection
### Recognition of Bacteria Using Biosensors
#### 2.1.3 Electrochemical Biosensors  
Electrochemical methods (\(\sim\)21%) are the second most applied detection principle after the SPR (surface plasmon resonance) technique [21]. They are dominated by voltammetric and amperometric methods, but these types of sensors include impedimetric and potentiometric sensing techniques as well [23]. These sensors analyze the impedance of bacterial cells when they are attached to or associated with the electrodes. Electrochemical methodologies have lower manufacturing costs and ease of system miniaturization and integration. Electrochemical biosensors, more specifically, impedimetric sensors, can take the leading position in this area, however, the optimization, miniaturization, and clinical trials need to be performed before any item is sent into the market [23]. Among others, their advantages are the potential use in point-of-care testing thanks to the low cost and miniaturizability [23]. For detecting whole bacteria, impedimetric biosensors are a very good choice because they are relatively cheap and highly sensitive label-free systems. Furthermore, they are easy to miniaturize, so they may be used as point-of-care systems. In these types of biosensors, the analyte-bioreceptor interaction causes a change in electron transfer resistance and capacitance across a working electrode surface. The analyte binding raises with higher concentration, thus, the impedance across the electrode surface changes and it is detected by a transducer. The impedance increases or decreases depending on the analyte [23]. In general, bioreceptors are antibodies [119; 120; 121] and aptamers [122; 123], but they may be other molecules as well which are able detect proteins or whole bacteria and viruses [23; 124; 125]. The main advantage is that there are no requirements for the analyte to be an enzymatic substrate or for formation of electroactive species. However, no impedance biosensors have had widespread commercial success, doubtless due to disadvantages like irreproducibility, high limits of detection, and problems with nonspecific binding [23]. In case of whole bacterial detection, viable _E. coli_ cells in mixed populations of living and dead cells have been reported by [PERSON] and co-workers [23; 126]. Differentiating living cells from dead cells can be beneficial when the number of viable cells shows the true pathogenic count. In general, living cells are voluminous compared to dead cells. The interference with the electric field is higher in case of the viable cells than that of the dead cells due to the higher cell volume of living bacteria, and this difference can be detected by impedance and capacitance measurements [23; 126]. Robotic layer-by-layer construction and miniaturization of these techniques may develop sensor performance with sufficiently high reproducibility for commercialization [23]. Volumetric biosensors measure the current change beside the applied potential caused by the analyte. The advantages of this technique are rapid detection and high sensitivity, but the analysis and setups are complicated [127]. Bioreceptors used in this technique are antibodies [128], bacteriophage [129], aptamers [130], enzymes [131], and peptides [132] for bacterial cell detection. Potentiometric biosensors apply ion-selective electrodes to examine the potential of a solution based on specific interactions with ions in the solution. This technique measures the change in potential that occurs upon analyte interaction at the working electrode [23]. Most of the potentiometric biosensors include membrane-based ion-selective electrodes (ISEs) and ion-selective field-effect transistors (ISFETs) [133]. Potentiometry is widely used in the biosensor field; however, potentiometric biosensors for the detection of whole bacterial cells are few. This is probably because potentiometry cannot give specific signals for large analytes such as bacteria (a false positive signal may arise as a result of a nonspecific ion flux) [23].