*1.1. Carbon Nanotubes and Their Applicability in Biosensing*

The carbon nanotube (CNT) is the 1D allotrope of carbon; the experimental evidence was first reported by L.V. Radushkevich and V.M. Lukyanovich [1] from the Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, in 1952. However, after Ijima's paper [2] in 1991, research interest in CNT escalated rapidly. Structurally, CNTs can be divided into two major categories (Figure 1) based on the number of graphitic layers, namely, single-wall CNT (SWCNT) and multi-wall CNT (MWCNT). Depending on the direction of the roll-up, SWCNTs can have different structures, namely, the zigzag, armchair, or chiral formations. While depending on the nature of the wrapping i.e., whether a graphitic sheet is rolled around itself multiple times (Swiss roll) or if the graphitic sheets are arranged as concentric cylinders (Russian doll), MWCNT can also be categorized, as in Scheme 1.

**Figure 1.** The nanostructure of multi-wall carbon nanotube (MWCNT) (**left**) and single-wall carbon nanotube (SWCNT) (**right**) (adapted with permission from Ref. [3]).

**Scheme 1.** Different categories of carbon nanotube (CNT).

CNT can be synthesized through various means, such as arc discharge, laser ablation, chemical vapor deposition (CVD), etc. However, because of the scalability, CVD-based approaches appear to be the most appropriate for large-scale CNT synthesis [4]. A wide range of prospective applications in important industrial fields, including nanoelectronics and biotechnology are promised by the distinctive mix of electrical, thermal, mechanical, and chemical characteristics that CNTs display. Additionally, among the many nanomaterials, CNTs are particularly intriguing because they provide an exceedingly tiny inner hollow core, virtually a one-dimensional space, for material storage. Thus, a novel structure can also be formed by filling the core of a CNT with the components necessary for specific applications.

Previous research has discovered a linkage between biomolecules in living beings and illnesses. The monitoring of aberrant physical parameters and the early diagnosis of illnesses help to minimize mortality and ensure organisms' physical health. Conventional laboratory approaches for assessing pathogenic variables are typically time-consuming, expensive, and complicated. Biosensors can facilitate the reliable and rapid analysis of metabolites in the body to help the current therapeutic procedure. CNTs have unique physicochemical and photoelectric qualities that can improve the performance of biosensors, such as a greater surface area for better catalyst adhesion; CNT-modified electrodes offer quicker electron transfer, resulting in enhanced sensitivity of detection for biosensors. CNT's unrivaled electronic features, such as quantum wire-like behavior, ballistic-type electronic conduction, remarkable thermal properties derived from phonon quantization, excellent flexibility, and high breaking stress despite its low density make CNT one of the best transducer materials for the transmission of signals related to the recognition of analytes, metabolites, or disease biomarkers. The curvature of the tube contributes to CNTs' high reactivity and sensitivity to chemical or environmental interactions. Moreover, as the carbon atom near the end of an open-ended tube has only two bonds, foreign molecules can easily enter the structure, thereby helping in the preferential addition of one or more species for functionalization. More importantly, from the viewpoint of biosensors, CNTs can act effectively as scaffolds for the immobilization of biomolecules at their surface. These fascinating characteristics have led to CNTs being widely used in biosensor applications.

#### *1.2. Biosensors*

According to the IUPAC, a biosensor (Figure 2) can be defined as "a device that uses specific biochemical reactions mediated by isolated enzymes, immunosystems, tissues, organelles, or whole cells to detect chemical compounds, usually by electrical, thermal, or optical signals" [4]. In 1962, the first biosensor for monitoring blood glucose was reported by Clark et al. [5]; later, a biosensor was also developed for the detection of the virus [6]. In 1998, Davis et al. [7] were able to immobilize the proteins on CNTs; afterward, in 1999, Balavoine et al. [8] were successful in developing the first biosensor using CNT. The timeline of biosensor development is represented in Figure 3.

**Figure 2.** The schematic structure and operating principle of a biosensor.

**Figure 3.** Biosensor development timeline (Reprinted with permission from Ref. [9]).

To date, different varieties of biosensors have been fabricated (Scheme 2) based on the analyte and transducer used. However, the type of biosensors that are used for human virus detection falls within the scope of discussion in this review.

**Scheme 2.** Classification of biosensors, based on analyte and transducer use (adapted with permission from Ref. [10]).

#### **2. Types of Biosensors Used for Virus Detection**

#### *2.1. Immunosensors*

Because of its capacity to handle information, the immune system is an appealing subject in scientific studies. The major purpose of an immune system, as part of the system's defensive mechanism, is to accredit and ascertain all cells and molecules in the assembly and classify these biological substances as either toxic or non-toxic. When exposed to foreign substances (i.e., antigens), specialized immune system cells make immunoglobulins (i.e., antibodies) that attach to these antigens precisely. An immunosensor (Figure 4, top left), an affinity-based biosensing device, exploits the concept of immunology and employs an antibody for the specific molecular identification of antigens that are immobilized on a transducer surface, and then develops a stable immunocomplex. The immunocomplex is calculated and quantified by connecting the antibody and antigen interactions to the surface of a transducer. The transducer detects the response and transforms it into an electrical signal, which may then be processed, recorded, and examined. The detection of the target analyte in immunosensors might be direct, by witnessing the production of immunocomplexes, or indirect, by using a label. Immunosensors can be categorized into several categories, based on various methodologies, such as electrochemical, impedimetric, potentiometric, amperometric, voltammetric, conductometric, capacitive, and surface plasmon resonance (SPR)-based methodologies.

### *2.2. Optical Biosensor*

An optical biosensor, a compact analytical instrument, combines an optical transducer system with a biorecognition-sensing element (Figure 4, bottom left). An optical biosensor's

primary goal is to provide a signal that is proportional to the concentration of the material being analyzed (the analyte). Optical detection is made possible by using the interplay between the optical field and a biorecognition element. Label-free and label-based optical biosensing are the main two categories of optical biosensors. In a label-free mode, the interaction between the substances is analyzed, and the transducer directly generates the measured signal. In contrast, label-based sensing makes use of a label to assess the biorecognition event and generates an optical signal using a colorimetric, fluorescent, or luminescent approach. However, in some cases, such as antibody-antigen interactions, when a label is coupled with one of the bio-reactants, then this labeling might modify the binding characteristics, introducing systematic inaccuracy into biosensor analysis.
