*2.3. Electrochemical Biosensor*

Due to the direct conversion of a biological event to an electrical signal, electrochemical biosensors provide an appealing technique for analyzing the content of a biological sample. The measurement of electrical characteristics in biosensing, for extracting information from biological systems, is generally electrochemical in nature, with a bio-electrochemical component serving as the major transduction aspect (Figure 4, bottom right). While biosensing devices use a variety of recognition components, electrochemical detection approaches mostly involve enzymes. This is mainly owing to their unique binding properties and biocatalytic activity. In bio-electrochemistry, the reaction under examination would typically create a quantifiable current (amperometric), a measurable potential or charge buildup (potentiometric), or a measurable impedance (impedimetric). The electrodes are essential components for the operation of electrochemical biosensors since reactions are generally observed near the electrode's surface. Depending on the electrode's parameters, the material, the surface modification, or the electrode's size have a significant impact on the capability of detection. In general, three electrodes, namely, the reference electrode, counter or auxiliary electrode, and working electrode are needed for electrochemical sensing. To maintain a known and constant voltage, the reference electrode is kept away from the reaction site. The counter electrode creates a link to the electrolytic solution so that a current may be supplied to the working electrode, while the working electrode acts as the transduction element in the biological reaction. These electrodes ought to be chemically stable and conductible to achieve a faithful analysis.

#### *2.4. Field-Effect Transistor (FET)-Based Biosensor*

FET biosensors, which have the characteristics of being quick, inexpensive, and straightforward, stood out among a wide spectrum of electrical sensing devices as one of the most promising options for biosensing (Figure 4, top right). This cutting-edge technology, which has evolved since 1970 [11] in various forms, is the easiest method for the quick and accurate detection of numerous analytes. Specific probes on the conducting channel of FET-based biosensors can be embedded to provide real-time and label-free analysis. A FET is a type of solid-state device that controls the semiconductor's electron conductivity between its source and drain terminals by the application of a third gate electrode, via an insulator. To recognize specific analytes, biological receptors are immobilized on the sensing channels, which are linked to the source and drain electrodes. After exposing the biosensor to target analytes and forming specific biological complexes, the transducer system converts biochemical changes into a measurable signal. The addition of charged biomolecules to the surface of the gate dielectric is equivalent to the application of voltage by the use of a gate electrode and results in threshold voltage variations. Therefore, the FET biosensors' underlying method relies on the conductance of the species that have been adsorbed. The two main types of FETs are n-type and p-type devices, wherein electrons and holes, respectively, serve as the principal charge carriers. An n-type FET sensor will respond by increasing the conductance if the target molecule is positively charged as a result of electron aggregation. Conversely, the conductance will be reduced if the target is a

molecule with a negative charge. When it comes to the p-type FET system, the opposite tendency is applicable.

**Figure 4.** (**Top left**) A schematic representation of an electrochemical immunosensor (Reprinted with permission from Ref. [12]); (**top right**) schematic diagram of a field effect transistor (FET)-based biosensor with a source and drain (Reprinted with permission from Ref. [13]); (**bottom left**) schematic diagram of optical biosensor constitution (Reprinted with permission from Ref. [14]); (**bottom right**) the main constituents of a nanomaterial-based electrochemical biosensor (Reprinted with permission from Ref. [15]).

#### **3. CNTs-Based Biosensors for the Detection of Human Viruses**
