# New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes
## 4 Overview of MWCNT Applications in Electrochemical Sensors  
Different and exciting properties are obtained with macro-, micro-, and nano-structured carbon derivatives [4; 17], but the implementation of the latter in the configuration of electrochemical sensors brought new opportunities to detect trace concentrations (\(\mu\)mol L\({}^{-1}\) to pmol L\({}^{-1}\)) of numerous analytes (active principle and metabolites of drugs, personal care products, flame retardants, gases, surfactants,  
Figure 4: Idealized models of band structures and density of states for (**a**) metallic and (**b**) semiconducting nanotubes. industrial additives, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), hormones, pesticides, among others) in complex matrices (natural waters, soils, foodstuffs, biological fluids, fuels, wastes, and even in living organisms) [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. This trend led to the development of the newest category of electroanalytical devices (third generation/way), i.e., nanotools for nanoscale analysis [2; 16]. The basic mechanism for the operation of these tools is based on the conversion of redox processes, registered at the sensor/electrolyte interface, onto quantitative or semi-quantitative information. MWCNT had an important role to reach such advances, and this fact can be easily attested by the exceptional growth of scientific reports in this field [2; 3; 4; 5; 6; 8; 16; 17]. MWCNT's large active surface area, facile electron-transfer with a variety of molecules, and their use in nano-composites (mixed with conductive polymers, nanoparticles and active biomolecules), as well as their high potential for device miniaturization, have been enhancing the sensitivity of current methodologies and providing new sensing opportunities [2; 17; 19; 20; 21; 22]. The multichannel electron ballistic transport and the presence of holes/defects along the structures, especially when functionalized with carboxyl groups and their derivatives, further enhance the previously mentioned properties [2; 4; 5; 6; 7; 8; 9]. When converting a graphene sheet to a nanotube, one electron of the carbon atoms remains in \(p\)-orbitals, organized between valence (\(\pi\)) and conduction (\(\pi^{\star}\)) bands with very low bandgap (a partial \(\sigma\)-\(\pi\) hybridization), which favors a fast electron transfer. The density of states in MWCNT structure depends on diameter (\(\approx\)2-100 nm), chirality/configuration (concentric, herringbone, and bamboo) and nature (semiconductor, conductor, and superconductor) [1; 7; 15]. In addition, the strength of C\(-\)C bonds is among the most robust and, when they are organized in the nanotube form, these interactions also provide flexibility and resistance to rupture, increasing the versatility and number of possible applications of these nanomaterials [4; 7; 16]. Electrochemical sensing of a wide variety of chemically, biologically, and environmentally important analytes was investigated on MWCNT (chemical purity \(\geq\)95%)--based solid electrodes, as shown in Table 1. Numerous traditional and modern pharmaceuticals, for instance, are electroactive on these sensors and relevant contributions have been reported, either in stationary or in flow-through systems. [PERSON] et al. [29] fabricated an electrochemical sensor for metronidazoleae (antibacterial and anti-inflammatory compound employed for the treatment of protozoal diseases) based on MWCNT and chitosan (CTS)-nickel complex modified glassy carbon electrode (GCE) by the self-assembly technique. The polymer matrix facilitates the immobilization of the nanocomposite on the electrode support, while the association between nickel and carbon nanomaterials increases the current intensity of the electroanalytical signal.