# New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes
## 1 Introduction  
Several nanomaterials have been exploited for different purposes, but carbon nanotubes (CNT) and their composites emerged as the most exciting and versatile [1; 2; 3; 4; 5]. CNT are carbon allotropes with intermediate properties between graphite and fullerenes [6]. They are composed by sp\({}^{2}\) carbon units oriented as one (single-walled; SWCNT) or multiple concentric tubes (multi-walled; MWCNT) of rolled-up graphene, separated by \(\approx\)0.35 nm [4; 6; 7; 8]. Both nanostructures had positive impacts for the applied sciences, but the latter has gained more attention due to the higher yield and lower production cost per unit, thermochemical stability, and ability to maintain or improve its electrical properties when submitted to different functionalization processes [9]. A general MWCNT representation and the possible \(\pi\)- and \(\sigma\)-covalent bonds established along the nanostructure are displayed in Figure 1. MWCNT also present remarkable active surface area, chemical inertness, high strength, and low charge-transfer resistance in both aqueous and non-aqueous solutions, making them suitable for numerous applications as nano-sized metallic and/or semiconducting components in transistors [10], energy storage [11], hybrid conductors [12], supercapacitors [13], thin-film coatings [2; 5; 6], field emitters [14], (bio)fuel cells [1; 2; 3; 7], photovoltaic devices [1; 3], electromagnetic shields [1; 7], and electrochemical probes [1; 2; 5; 6; 7]. MWCNT provide a series of functionalities to the electrochemical sensors, making them fantastic devices for analytical purposes. Electrochemical sensors are by definition, according the International Union of Pure and Applied Chemistry, \"electrochemical devices that transform the effect of the electrochemical interaction analyte-electrode into a useful analytical signal\" [15]; these effects may be promoted electrically or may be caused by a spontaneous interaction at the zero-current condition [15]. Thus, electrochemical sensors may be organized in four main subgroups: voltammetric, potentiometric, potentiometric solid electrolyte gas sensors, and chemically sensitized field effect transistor [15]. MWCNT associate high surface area (about 1600 m\({}^{2}\) g\({}^{-1}\)) and exceptional electrical conductivity (current densities as high as 10\({}^{9}\) A cm\({}^{-2}\)), which suit them to the miniaturization and portability of the systems [14]. Nowadays, electrochemical MWCNT-based sensors have demonstrated remarkable applications in medicine, clinical diagnostics, pharmacology, food industry, sanitary surveillance, occupational safety, forensic chemistry, and environmental analysis [15; 16; 17; 18; 1; 1; 3; 6; 7; 8]. Other promising innovations can also be found in review papers that have been published over the last decade [6; 17; 18; 19; 20; 7; 8]. The assays are performed by various electrochemical techniques (voltammetry, amperometry, potentiometry, impedance spectroscopy, and their associations), which are selected according to the nature of the target-redox process [15; 16; 17; 18; 2; 2; 6; 7; 2; 2; 2; 20]. MWCNT bring together high electronic conductivity, large specific surface area, chemical stability, biocompatibility, and ease of modification, improving the intensity and resolution at which the electroanalytical signals are observed on the working platforms [15; 16; 17; 18]. However, the successful application requires a refined control of the sensor architecture and physicochemical properties, as well as the functionalization and/or surface modification of the carbon nanotubes [4; 6; 17; 18; 9]. Thus, in this study, these critical variables will be highlighted and discussed, addressing exciting issues from the MWCNT synthesis to their application in electrochemical biosensors design. With this goal in mind, the available literature concerning this subject in Thomson Reuters, _Web of Science_ was reviewed from 2013 to March 2018.