# New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes
## 2 MWCNT Synthesis Methods  
Regarding synthesis methods, there are different paths that can be followed, such as electrical arch-discharge (EAD), laser ablation (LA), and numerous types of chemical vapor deposition (CVD), so that MWCNT conductor character varies according to the diameter and degree of helicity [4; 7; 21]. An overall layout of each system and its associated processes are shown in Figure 2. The yield rate of all aforementioned methods is more than 75% under appropriate operating conditions [22], but some questions about the structural quality, diameter uniformity, chirality, practicality, number of unitary operations, and production cost continue to divide opinions about the best strategy [4; 21; 22; 23; 24]. Researchers are continually trying to explore each possibility to optimize or innovate them for several new applications. Figure 2: General layout of an (**a**) electrical arch-discharge (adapted by permission from Reference [23]), (**b**) laser ablation (adapted by permission from Reference [23]) and (**c**) chemical vapor deposition apparatus (adapted by permission from Reference [25]) used to synthesize multi-walled carbon nanotubes. EAD is relatively less expensive, offers better yield quality, but involves high temperature for synthesis [21; 22; 23]. Triggering an electrical arc-discharge between two electrodes, a plasma composed by carbon and metallic catalysts vapor (iron, nickel, cobalt, yttrium, boron, cerium, among others) is formed under a rare gas atmosphere (helium or argon) [21; 22; 23]. The vapor content is a consequence of the energy transferred to the catalysts-doped anode, causing its erosion, while MWCNT are formed as a hard and consistent deposit on the cathode [23]. The LA method also represents a promising alternative to synthesize high-quality carbon nanotubes at room temperature, although it demands high instrumental and operational costs [21; 22; 23]. In this technique, a pulsed or continuous laser beam is focused on catalyst-based graphite pellet, which is placed at the center of a quartz tube filled with an inert gas and kept at 1200 \({}^{\circ}\)C. The radiant energy is enough to sublimate the carbon substrate, and this vapor is swept by the gas flow towards the conical water-cooled copper collector. MWCNT deposits are formed in the majority on the same collector, but appreciable amounts are also found on the quartz tube walls and graphite pellet backside. Both EAD and LA also produce in parallel other carbon phases and all products contain metallic impurities from the catalysts [23]. The small fractions of remaining metals do not compromise the performance of the electrochemical devices, and there are several purification processes proposed in the literature and by some commercial companies to remove undesirable phases [22; 23; 24]. Nevertheless, these procedures are based on oxidation under strong acidic conditions, which may significantly affect the integrity of the nanostructures. The synthesis performed by CVD overcomes the aforementioned difficulties, guarantees large-scale production and, for these reasons, it has become more widely used to obtain MWCNT [22; 23; 24; 25]. This technique involves either a heterogeneous (if a solid substrate is used) or a homogeneous process (supposing that reactions takes place in the gas phase) [23; 25]. The main synthetic route explores the catalytic decomposition of a carbon-containing source on substrates containing transition metals and their composites (Au, Ag, Cu, Cr, Co, Fe, Mn, Mo, Ni, Pt, Pd, SiO\({}_{2}\), SiC, and ceramics), which enables the growth of aligned and dense arrays of nanotubes [23; 24]. Alternative energy sources such as plasma and optical excitation can also be employed, giving the possibility to synthesize MWCNT at low-temperatures (450-1000 \({}^{\circ}\)C) compared to EAD and LA [23; 24; 25].