# New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes
## 4 Overview of MWCNT Applications in Electrochemical Sensors
[51] associated square-wave adsorptive stripping voltammetry (SWadSV) and a modified sensor (\(\beta\)-cyclodextrinrins/MWCNT/GCE) to determine trace concentrations of the pesticide dichlorophen. The excellent electroanalytical performance of the system, mainly in the presence of nanotubes (LOD = 4.4 \(\times\) 10\({}^{-8}\) mol L\({}^{-1}\)), allowed to monitor this vermicide in river water (Pilica River, in Poland) with good selectivity and absence of natural and anthropogenic interfering compounds. [PERSON] and [PERSON] [52] developed a simple and sensitive sensor for the voltammetric determination of the herbicide clopyralid in urine, river water, sugar beet, wheat, and herbicide formulations (Phaeton\({}^{\text{\textregistered}}\) and Lontrel\({}^{\text{TM}}\)). This novel sensor was built by modification of a GCE with a nanocomposite containing acid-activated MWCNT (oxygen-functionalized nanostructures with very low _R\({}_{tc}\)_) and fumed silica, which, when associated with DPV, attained a LOD of 0.8 nmol L\({}^{-1}\), confirming the sensitivity of the proposed method. [PERSON] and [PERSON] [53] reported a strategy to develop a voltammetric sensor for diazinon (an organophosphate insecticide) based on TiO\({}_{2}\) nanoparticles (TiO\({}_{2}\)-NP) and MWCNT nanocomposite assembled onto a GCE. Under optimum conditions, TiO\({}_{2}\)-NP@MWCNT/GCE reached a LOD of 3.0 nmol L\({}^{-1}\), using both CV and SWV, and this performance was explored for the electroanalysis of the pesticide in well and tap water. Other mentioned advantages were the fast response time, good repeatability, stability, besides the fast and inexpensive electrode modification. Sensitive electrochemical sensors that associate the unique properties of MWCNT and metallic nanoparticles were also proposed by [PERSON] et al. [54] and [PERSON] et al. [55], aiming to electroanalysize trichlorofen insecticide (LOD = 4.0 \(\times\) 10\({}^{-7}\) mol L\({}^{-1}\), using Nafion\({}^{\text{\textregistered}}\)/TiO\({}_{2}\)-NP@MWCNT@carboxymethyl chitosan/GCE) and simazine herbicide (LOD = 2.0 \(\times\) 10\({}^{-11}\) mol L\({}^{-1}\), using MIP/platinum nanoparticles@polyoxometalate@f-MWCNT/GCE), respectively, reinforcing the positive synergistic effect between these nanostructures for the improvement of the electroanalytical methodologies. The wide active area and low \(R_{tc}\) observed for MWCNT, and related (nano)composites, have been providing important advances on the single and simultaneous electroanalytical quantification of metallic cations (Cd\({}^{2+}\)[56], Pb\({}^{2+}\)[56], Mn\({}^{2+}\)[57], and Na\({}^{+}\)[58]) and anions (SO\({}_{3}\)\({}^{2-}\)[59], NO\({}_{2}\)\({}^{-}\)[60], and BrO\({}_{3}\)\({}^{-}\)[60]) in varied and complex matrices (wastewater [57], drinking water [56], and groundwater [58; 59]). Trace concentrations of persistent organic pollutants (sunset yellow [61], tartrazine [62], and luteolin [62] dyes), as far as gases (chlorine [63], carbon dioxide [64], and methanol vapor [65]) and industrial by-products (glycerol [66], bisphenol A [67], hydrazine [68], and hydrogen peroxide [69]) are now being monitored with MWCNT-based sensors in a quick, reproducible, reliable, and cost-effective way when compared to the traditional analytical protocols, including those based on electrochemical devices from previous generations. Figure 6: Detailed procedure diagram for fabrication of the HIV-p24/CTS/MWCNT/GCE sensor (adapted by permission from Reference [42]). \begin{table}
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