# New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes
## 4 Overview of MWCNT Applications in Electrochemical Sensors
Using differential pulse voltammetry (DPV), Ni-CTS/MWCNT/GCE also showed electrocatalytic effect toward metronidazoleae reduction, making the method highly selective to its analysis in pharmaceutical and biological samples, with a low detection limit (LOD; 0.025 umol L\({}^{-1}\)) and suitable reproducibility and stability. [PERSON] et al. [30] reported a simple methodology to quantify acetaminophen (a compound with analgesic and antipyretic action) in commercial formulations, using a GCE modified with gold nanoparticles (Au-NP), functionalized MWCNT (\(f\)-MWCNT) and cobalt phthalocyanine (Co-Pht) as working sensor. Using\(f\)-MWCNT, a lower system capacitance and better stability of the sensor was observed. The quantification of acetaminophen on Co-Pht/\(f\)-MWCNT/Au-NP/GCE was carried out by square-wave voltammetry (SWV) that, in turn, showed a sensitivity (LOD = 0.135 umol L\({}^{-1}\)) and reproducibility similar to the standard ultraviolet-visible spectrophotometric procedure (at the wavelength of 257 nm). [PERSON] et al. [31] studied the superior electrochemical activity of MWCNTs with shorter dimensions (diameter \(\times\) length: 100-170 nm \(\times\) 5-9 um and 6-9 nm \(\times\) 5 um) towards the oxidation of propionic acid derivatives, which are drugs with analgesic, antipyretic, and anti-inflammatory properties. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetric (CV) data showed better electrocatalysis effect, \(R_{tc}\) and signal amplification for the sensor constructed with the shorter diameter nanostructures due to the increased defect density revealed by Raman spectroscopic measurements. The developed sensor (MWCNT (diameter \(\times\) length: 6-9 nm \(\times\) 5 um)/GCE) was adapted to a flow-injection analysis system (FIA) and provided a sensitivity and accuracy to determine ibuprofen (LOD = 1.9 umol L\({}^{-1}\))by amperometry comparable to that obtained by capillary electrophoresis. [PERSON] et al. [32] used a fluorine doped tin oxide (FTO) electrode, coated with nanostructured layer-by-layer films of CTS@MWCNT (Figure 5), as a sensor for 17-\(\alpha\)-ethinylestradiol, that is a synthetic estrogen widely used as an oral contraceptive. The electron-transfer on the sensor reduced dramatically in the presence of the nanotubes, improving its electroanalytical performance and enabling assays at very low concentrations. Using the irreversible and adsorption-controlled oxidation process of the estrogen onto the three-bilayer CTS@MWCNT/FTO, it was possible to determine it by SWV (LOD = 0.09 umol L\({}^{-1}\)) without significant effect of the most common interfering compounds present in human urine (urea, uric, and ascorbic acids, glucose, NaCl, KCl, and NH\({}_{4}\)Cl). [PERSON] et al. [33] used a nanocomposite based on MWCNT and 1-octyl-3-methyl-imidazolium hexa-fluorophosphate ionic liquid (IL) to modify a GCE, to develop a chiral electrochemical sensor for the enantiomeric recognition of propranolol, using the linear sweep voltammetry (LSV) technique. The authors did not present a LOD but declared the success of the proposed method to evaluate the enantiomeric purity of reagents and wastewater treatment efficiency. Other promising MWCNT-based electrochemical devices were also reported to successfully analyze hydrochlorothiaz azide and triamterene [34], mangiferin and icarin [35], natamycin [36], omeprazole [37], gentamicin [38], and many other drugs [2; 6; 9; 17; 18; 19; 20], attesting their electroanalytical performance and effectiveness. The growing trend of using miniaturized systems for the electroanalysis of bioactive molecules in living organisms through less invasive tests has positive repercussion and wide acceptance [2; 17]. From this perspective, glucose sensors have been receiving special attention, since these analytic tools can be used as an effective way to diagnose diabetes, hypoglycemia, and other metabolic diseases in real time, using human blood and other secreted fluids as study matrix. [PERSON] et al.