New Generation of Electrochemical Sensors Based on Multi-Walled Carbon Nanotubes

Abstract

Multi-walled carbon nanotubes (MWCNT) have provided unprecedented advances in the design of electrochemical sensors. They are composed by sp² carbon units oriented as multiple concentric tubes of rolled-up graphene, and present remarkable active surface area, chemical inertness, high strength, and low charge-transfer resistance in both aqueous and non-aqueous solutions. MWCNT are very versatile and have been boosting the development of a new generation of electrochemical sensors with application in medicine, pharmacology, food industry, forensic chemistry, and environmental fields. This work highlights the most important synthesis methods and relevant electrochemical properties of MWCNT for the construction of electrochemical sensors, and the numerous configurations and successful applications of these devices. Thousands of studies have been attesting to the exceptional electroanalytical performance of these devices, but there are still questions in MWCNT electrochemistry that deserve more investigation, aiming to provide new outlooks and advances in this field. Additionally, MWCNT-based sensors should be further explored for real industrial applications including for on-line quality control.

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 sp² carbon units oriented as one (single-walled; SWCNT) or multiple concentric tubes (multi-walled; MWCNT) of rolled-up graphene, separated by ≈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 π- and σ-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 m² g⁻¹) and exceptional electrical conductivity (current densities as high as 10⁹ 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 [1,2,3,6,7,8,9,15,16,17,18]. Other promising innovations can also be found in review papers that have been published over the last decade [6,7,8,9,15,16,17,18,19,20]. 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 [2,6,7,15,16,17,18,19,20]. 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 [4,6,7,8,9]. 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.

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.

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 °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, SiO₂, 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 °C) compared to EAD and LA [23,24,25]. The CVD method also allows greater control of selectivity/type, homogeneity and size of the nanotubes (a few tens to hundreds of micrometers) in comparison to the others, although these characteristics are strongly dependent on the nature and structure of the catalysts, and the operational conditions of the synthesis. All these aspects affect the density and speed of electron transfer through the nanostructures that, in turn, directly impact the performance of the electrochemical sensors.

3. MWCNT Electrochemical Properties

MWCNT-based electrochemical sensors have remarkable advantages for analytical applications, such as enhanced detection sensitivity, low charge-transfer resistance (R_tc), electrocatalytic effect, and reduced fouling. It is also speculated that traces of metallic catalysts can add remarkable electroanalytical characteristics (positive synergistic effect) to the nanotubes, playing a supporting role for the development of the devices [6,7,15,16,17,18,19,20,21,22,23,24,25,26]. There are still controversies about the electron transfer mechanism along the structural network: Some researchers believe that the sidewalls of the CNT are inert (i.e., the edge–plane-graphite-like open ends and defect sites control the phenomenon) [26,27,28], but some studies conducted with two-dimensional SWCNT have shown evidence of charge-transfer on the basal-plane [7,23,25]. Although the uncertainties about the fundamental contributions of MWCNT in electrochemical sensors, the number of MWCNT successful applications in highly complex systems does not stop growing [2,9,17,26].

Understanding the size-dependent features is an indispensable requirement to explore the fascinating properties of nanotubes in analytical tools and to move the electroanalytical researches forward. In this sense, it must be assumed that the electronic band structure of CNT as a hexagonal lattice, where each carbon atom is covalently bonded to three others via sp² molecular orbitals (the fourth valence electron in a p_z-orbital hybridizes with all others to form a delocalized π-band) [27]. Like this, an even number of electrons are contained in the basic nanotube structure, whose conduction character will depend on its behavior as metal or semiconductor. This property can be assessed through the tight-binding electronic structure calculations, considering the translational vector (C_h) that connects two equivalent crystallographic sites (unitary vectors a₁ and a₂) and its coefficients (n₁ and n₂), according to the following Equation (1):

$$C_h=n_1{a}_1+n_2{a}_2$$

(1)

so that a metallic behavior is observed when the result is an integer multiple of three, while a non-metallic/semiconductor profile is observed for other cases; a direct consequence of the different conduction band gaps [6]. The electron-transfer along the MWCNT are also influenced by the tube diameter (d_t), chiral angle ($\theta$), and basic translation vector (T), as shown in Figure 3. Thus, the manner in which a graphene sheet is rolled determines the main electronic properties of the resulting nanotube [6,15,23,27].

Observing the referred parameters on the tubulene lattice, d_t values can be determined by Equations (2) and (3), which relate this variable with the circumferential length of the nanotube (L) and a hexagonal network constant (a):

$$d_t=\frac{L}{\pi }=\frac{C_h}{\pi }=\frac{a{\left(n_1^2+n_1{n}_2+n_2^2\right)}^{\frac{1}{2}}}{\pi }$$

(2)

considering $a_{°}=1.42 Å$ for carbon nanotubes [6]:

$$a=\sqrt{3}{a}_{°}=2.49 Å$$

(3)

Regarding the $\theta$, the expression that describes its magnitude can be written as follows:

$$\cos \theta =\frac{2n_1+n_2}{2\sqrt{n_1^2+n_1{n}_2+n_2^2}}$$

(4)

The vector T is used to specify the orthogonal orientation to the C_h axis of the nanotube, and its value can be calculated by:

$$T=\frac{\left(2n_2+n_1\right)a_1-\left(2n_1+n_2\right)a_2}{d_k}$$

(5)

where $d_k$ ranging from d (when $n_1-n_2$ and $d_k$ is not a multiple of 3d) to 3d (when $n_1=n_2$ and $d_k$ is a multiple of 3d) for chiral tubulenes, considering d the greatest common divisor for the coefficients $n_1$ and $n_2$ [6,15,23,27].

In addition, a translation vector (τ) is also necessary to delimit the unitary cell of the nanotube. It can be represented by Equations (6)–(8),

$$\mathsf{\tau }=t_1{a}_1+t_2{a}_2$$

(6)

$$t_1=\frac{(2n_2+n_1)}{d_r}$$

(7)

$$t_2=\frac{-(2n_1+n_2)}{d_r}$$

(8)

knowing that $d_r$ (d–3d) is the greatest common divisor of the respective coefficients. In summary, the limits of the 1D nanotube unit cell (plane segment OAB’BO) are demarcated by the vectors C_h and T, whereas a₁ and a₂ define the area of the 2D graphene unit cell [6,27]. This sequence of equations is very important for the development of electrochemical sensors because it allows to determine the number of electric and phononic bands in the carbon nanostructures, i.e., the magnitude of the electrical conduction through the nanomaterial.

The influence of the band structure on the nanotube conduction bands is defined in the context of the Fermi velocity (${\mathsf{\upsilon }}_F$).

$${\mathsf{\upsilon }}_F=\frac{\text{ћ}{k}_F}{m}$$

(9)

where ħ is the reduced Planck constant, $k_F$ is the Fermi wavevector, and $m$ represents the mass of the nanostructure under study. These values are not well-defined in cases where the Fermi surface is non-spherical, but it is estimated to be ${\mathsf{\upsilon }}_F~{10}^6 {\mathrm{ms}}^{-1}$ for semiconductors or even higher for conductors [27]. This prevision corroborates the observation recorder in the density of states diagram for SWCNT with different chirality, since at the Fermi energy level (E_F) there is a finite electronic transition for metallic structures (first van Hove optical transition), but a zero-band gap for semiconductors (second van Hove optical transition). A general description of the band structures and density of states for metallic and semiconductor nanotube is presented in Figure 4. Thus, if we imagine that MWCNT are composed by several coaxial SWCNT, it might be expected that they are not strictly a 1D-type conductor [7,27]. Likewise, when the nanotubes have different chirality, the band structure undergoes alterations, and the resultant coupling between the electronic states at the E_F and phonon modes may cause superconductivity [4,7,27]. Despite the similarity with metallic materials, the carrier density in carbon nanotubes (1D quantum wires) is much lower and novel physical electron-electron phenomena (such as spin-charge separation and suppression of the electron tunneling density of states) should be considered to justify the possible discrepancies, as predicted by Luttinger liquid theory [23,26,27,28].

Some electrochemical studies showed a lower R_tc for MWCNT containing structural defects and/or traces of metallic impurities (catalyst nanoparticles), so that the current flow occurs mainly through the outer most nanotube cylinder [26,27,28]. Only one of the concentric tubes needs to exhibit this behavior for the overall electronic properties to be essentially metallic-low quantum capacitance values [7,27]. It is believed that if d_t is smaller than the elastic mean free path, the one-dimensional ballistic transport predominates, but the opposite characterizes a two-dimensional diffusive current flow [7,26,27,28]. In addition, nanotubes with larger d_t have a greater density of structural defects, besides facilitating their modification and functionalization, which are interesting requirements for the development of electroanalytical devices [2,6,7,8,9]. The mean length of the nanotubes also has a significant effect on the electron-transfer rate; they are inversely proportional properties [22,24,27]. Vertically aligned structures also exhibit superior performance compared to those randomly immobilized [2,16,25,26,27,28].

Therefore, MWCNT provide special characteristics in electrochemical sensors both in aqueous and non-aqueous electrolytes (electrocatalytic properties, high and fast electron-transfer, signal amplification and stability, among other particularities observed in different prototypes) when compared with the conventional carbonaceous electrodes (such as glassy carbon, highly-oriented pyrolytic graphite, carbon microfiber, and boron-doped diamond) [26,27,28]. However, the true reasons and fundamental electrochemical properties responsible for the reported performances (both when nanotubes are used alone or in (nano)composites) are still not fully elucidated and demand further research [2,4,6,9,17].

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 (µmol L^–1 to pmol L^–1) of numerous analytes (active principle and metabolites of drugs, personal care products, flame retardants, gases, surfactants, 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 (π) and conduction (π*) bands with very low bandgap (a partial σ–π hybridization), which favors a fast electron transfer. The density of states in MWCNT structure depends on diameter (≈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 ≥95%)—based solid electrodes, as shown in

Table 1. Configuration, analytical performance and application of electrochemical sensors based on multi-walled carbon nanotubes.

Sensor	Modification Procedure	Analyte(s)	Technique(s)/Detection Limit	Application	Stability	Reference
Pharmaceuticals
Ni-CTS/MWCNT/GCE	GCE modified with MWCNT and Ni-CTS complex through drop coating and self-assembly, respectively	metronidazole	DPV/0.025 μmol L−1	tablet and biological samples	81% after one month	[29]
Co-Pht@f-MWCNT/Au-NP/GCE	suspension of Co-Pht and f-MWCNT immobilized on Au-NP modified GCE by drop coating	acetaminophen	SWV/0.135 μmol L−1	commercial formulations	n.r.	[30]
MWCNT(shorter diameter)/GCE	MWCNT (diameter × length: 6–9 nm × 5 μm) dropped on GCE	ibuprofen	CV/1.90 μmol L−1	tablet and liquid commercial formulations	n.r.	[31]
Three bilayer MWCNT@CTS/FTO	FTO coated with nanostructured Layer-by-Layer films of MWCNT@CTS	17-α-ethinylestradiol	SWV/0.09 μmol L−1	synthetic urine samples	n.r.	[32]
MWCNT@IL/GCE	immobilization of MWCNT and IL (1-octyl-3-methyl-imidazolium hexa-fluorophosphate) nanocomposite on GCE	propranolol	LSV/n.r.	commercial reagent and wastewater	n.r.	[33]
MWCNT/GCE	suspension of MWCNT dropped on GCE	hydrochlorothiazide and triamterene	ASV/2.8 × 10−8 and 2.9 × 10−8 mol L−1 for hydrochlorothiazide and triamterene, respectively	hemodialysis samples	n.r.	[34]
Au@Ag-NP/MWCNT-SGSs/GCE	layer-by-layer assembly of Au@Ag-NP and MWCNT-SGSs on GCE	mangiferin and icariin	DPV/0.017 μmol L−1 for both compounds	Rhizoma anemarrhenae, Artemisia capillaris Herba and Epimedium macranthum samples	≤95.1% after one month	[35]
3D-Grf@MWCNT/GCE	electrodeposition of 3D-Grf@MWCNT suspension on GCE	natamycin	LSV/1.0 × 10−8 mol L−1	red wine and beverage samples	94.6% after two weeks	[36]
Fe3O4-NP@MWCNT/PDDA/GCE	casting of PDDA modified GCE with Fe3O4-NP@MWCNT hybrid film	omeprazole	LSV/15 nmol L−1	tablet, capsules, wastewater, serum, and urine	92.1% after three weeks	[37]
Calixarene/MWCNT/SPE	dip coating of graphite-based SPE in composite matrix of calixarene and MWCNT	gentamicin sulphate	potentiometry/7.5 × 10−8 mol L−1	dosage forms and spiked surface water samples	n.r.	[38]
Biologically active molecules
Ni-NP@f-MWCNTs/GCE	drop coating of hybrid film (Ni-NP and f-MWCNT) on GCE	glucose	CV and amperometry/0.021 μmol L−1	human blood serum samples	practically constant signal after 1000th cycle	[39]
Cu@Ni-NP/MWCNT/GCE	immobilization of hybrid film of Cu@Ni-NP and MWCNT on GCE	guanine and adenine	DPV/0.17 μmol L−1 and 0.33 μmol L−1 for guanine and adenine, respectively	ds-DNA from mice brain tissues	96.7% for 30 days	[40]
Gold electrode	measurements with unmodified gold electrode, keeping f-MWCNT additive and DNA sequence in electrolyte solution	breast cancer marker (5′-GTG TTG TCT CCT AGG TTG GCT CTG-3′; 24-base fraction of the p53 gene)	DPV/141.2 pmol L−1	solution containing the complementary sequence (5′-CAG AGC CAA CCT AGG AGA CAA CAC-3′)	n.r.	[41]
HIV-p24/MIP /MWCNT/GCE	HIV-p24 crosslinking MIP (acrylamide functional monomer, N,N′-methylenebisacrylamide as crosslinking agent and ammonium persulphate as initiator) immobilized on MWCNT/GCE	HIV-p24 protein	DPV/0.083 pg cm−3	real human serum samples	98.6% after 10 days	[42]
MIP/MWCNT/Cu	MIP (itaconic acid monomer, ethylene glycol dimethacrylate cross-linker and α,α′-azobisisobutyronitrile as initiator) on MWCNT modified Cu electrode	hemoglobin	potentiometry/1.0 µg mlL−1	human bile juice and urine samples	6 months without significant change in the electrode performance	[43]
Cu-MP@polyethylenimine /MWCNT/GCE	Cu-MP dispersed in polyethylenimine and dropped on MWCNT/GCE	amino acids, albumin and glucose	SWV and amperometry/0.10–0.37 μmol L−1 for the amino acids (L-cystine, L-histidine and L-serine); 1.2 mg mL−1 for albumin; and 182 nmol L−1 for glucose	pharmaceutical products and beverages	7.7% RSD after 10 successive calibration plots using the same surface	[44]
MIP/Au-NP/MWCNT/GCE	Au-NP electrodeposited on MWCNT/GCE, and assembled with MIP (tetrabutylammonium perchlorate)	cholesterol	DPV/3.3 × 10−14 mol L−1	n.r.	91.7% after one month	[45]
MWCNT/SPE	MWCNT films casted onto SPE	bilirubin	CV/9.4 µmol L−1	n.r.	n.r.	[46]
Grf-Ox@MWCNT/GCE	drop coating of Grf-Ox@MWCNT suspension on GCE	catechol and dopamine	CV/n.r.	n.r.	n.r.	[47]
Phenazine methosulfate/3-aminophenylboronic acid/f-MWCNT/GCE	drop coating of phenazine methosulfate and 3-aminophenyl boronic acid on f-MWCNT/GCE	NADH	amperometry/0.16 μmol L−1	human serum	96.7% after five consecutive measurements	[48]
HPU/β-CD/MWCNT@Nafion®/GCE	layer-by-layer of HPU, β-CD and composite film (MWCNT@Nafion®) on GCE	uric acid	amperometry/n.r.	n.r.	n.r.	[49]
Microorganisms
MWCNT@Nafion®/GCE	dip coating of composite suspension (MWCNT@Nafion®) on GCE	Enterotoxigenic Escherichia coli F4 (K88)	SWV/6 × 104 CFU mL−1	swine stool samples	n.r.	[50]
Pesticides
β-CD/MWCNT/GCE	β-CD and MWCNT composite suspension dropped on GCE	dichlorophen	SWAdSV/ 4.4 × 10−8 mol L−1	river water	93.9% after one week	[51]
Fumed silica/acid-activated MWCNT/GCE	drop coating of a nanocomposite suspension (Fumed silica and acid-activated MWCNT) on GCE	clopyralid	DPV/0.8 nmol L−1	urine, river water, sugar beet, wheat, and herbicide formulations (Phaeton and Lontrel)	91% after three weeks	[52]
TiO2-NP@MWCNT/GCE	TiO2-NP@MWCNT nanocomposite dropped on GCE	diazinon	CV an SWV/3.0 nmol L−1	well and tap water	89% after 28 days	[53]
Nafion®/TiO2-NP@MWCNT @carboxymethyl chitosan/GCE	Nafion® assembled on composite film (TiO2-NP@MWCNT @carboxymethyl chitosan) previously immobilized on GCE	trichlorfon	DPV/4.0 × 10−7 mol L−1	apple, mushroom, and cucumber	98% after one week	[54]
MIP/Pt-NP@polyoxometalate@f-MWCNT/GCE	MIP (pyrrole in the presence of the analyte) assembled on hybrid film (Pt-NP@polyoxometalate@f-MWCNT) immobilized on GCE	simazine	DPV/2.0 × 10−11 mol L−1	wastewater samples	96.9% after 45 days	[55]
Metallic cations
BiF/Grf-Red/MWCNT/SPE	layer-by-layer of BiF, Grf-Red and MWCNT on SPE (gold support)	Cd2+ and Pb2+	SWV/0.6 ppb for Cd and 0.2 ppb for Pb	drinking water	n.r.	[56]
Mn2+-imprinted polymer /IL@CTS@MWCNT/GCE	thermal immobilization of Mn2+-imprinted polymer on composite layer (IL@CTS@MWCNT) dropped on GCE	Mn2+	SWAdSV/0.15 μmol L−1	wastewater	94.8% after two weeks	[57]
β-NiOx/MWCNT-modified CPE	electrodeposition of hybrid film of β-NiOx on MWCNT-modified CPE	Na+	SWV/9.86 × 10−8 mol L−1	groundwater	practically constant signal for more than five hundred consecutive cycles	[58]
Anions
f-MWCNT/GCE	drop coating of f-MWCNT (COOH-functionalized structures) as suspension on GCE	SO32− and NO2−	DPV/215 nmol L−1 for SO32− and 565 nmol L−1 for NO2−	groundwater	≥96.4% after one week	[59]
Ag-NP@MWCNT/GCE	drop coating of nanocomposite (Ag-NP@MWCNT) on GCE	BrO3−	amperometry/n.r.	n.r.	n.r.	[60]
Dyes
Grf-Ox@MWCNT/GCE	suspension of Grf-Ox@MWCNT immobilized on GCE by drop coating	sunset yellow and tartrazine	LSV/0.025 µmol L−1 for sunset yellow and 0.010 µmol L−1 for tartrazine	orange juice	89–93% after one month	[61]
Poly(crystal violet)/MWCNT/GCE	electropolymerization of crystal violet on MWCNT/GCE	luteolin	DPV/5.0 × 10−9 mol L−1	Chrysanthemum samples	93% after one month	[62]
Gas/Vapor
Hexa-decafluorinated zinc phthalocyanine @f-MWCNT/SPE (gold support)	drop coating of the composite (Hexa-decafluorinated zinc phthalocyanine@f-MWCNT) on SPE	Cl2	resistance/0.06 ppb	n.r.	n.r.	[63]
SbSI@CNTs/Au-microelectrode	ultrasonic bonding of SbSI@CNTs composite on Au-microelectrode	CO2	amperometry/n.r.	n.r.	n.r.	[64]
MWCNT@polyaniline/FTO	drop coating of MWCNT@polyaniline nanocomposite on FTO	methanol vapor	resistance/≈50 ppm	n.r.	the signal remained almost constant for up to 20 days	[65]
Industrial by-products
CuO-NP/MWCNT/GCE	electrodeposition of CuO-NP on MWCNT/GCE	glycerol	amperometry/5.8 × 10−6 g dm−3	biodiesel samples	n.r.	[66]
Au-NP/Grf-Red@MWCNT/GCE	electrodeposition of Au-NP on Grf-Red@MWCNT/GCE	bisphenol A	DPV/1.0 × 10−9 mol L−1	river water and shopping receipt samples	98% after 30 days	[67]
Pd-NP/Grf-Red@MWCNT/GCE	electrodeposition of Pd-NP on Grf-Red@MWCNT/GCE	hydrazine	amperometry/0.15 µmol L−1	tap water spiked with hydrazine	n.r.	[68]
Prussian blue/CTS@MWCNT /GCE	electrodeposition of Prussian blue complex on GCE Modified with CTS@MWCNT nanocomposite	hydrogen peroxide	amperometry/0.10 µmol L−1	routine analysis in pure electrolyte	90.5–92.6% after two weeks	[69]

Au-NP: gold nanoparticles; Ag-NP: silver nanoparticles; Cu-MP: copper microparticles; CuO-NP: copper oxide nanoparticles; Pd-NP: palladium nanoparticles; CTS: chitosan; Co-Pht: cobalt phthalocyanine; IL: ionic liquid; HPU: hydrothane polyurethane; β-CD: β-cyclodextrin; BiF: bismuth film; MIP: molecular imprinted polymer; MWCNT: multi-walled carbon nanotubes; f-MWCNT: functionalized multi-walled carbon nanotubes; 3D-Grf: three-dimensional graphene; Grf-Ox: graphene oxide; Grf-Red: reduced graphene; HIV-p24: retrovirus of the AIDS; GCE: glassy carbon electrode; SPE: screen-printed electrode; CPE: carbon paste electrode; FTO: fluorine doped thin oxide electrode; LSV: linear sweep voltammetry; CV: cyclic voltammetry; DPV: differential pulse voltammetry; SWV: square-wave voltammetry; ASV: adsorptive stripping voltammetry; SWAdSV: square-wave adsorptive stripping voltammetry; n.r.: no reported.

. 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. Mao et al. [29] fabricated an electrochemical sensor for metronidazole (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. Using differential pulse voltammetry (DPV), Ni-CTS/MWCNT/GCE also showed electrocatalytic effect toward metronidazole reduction, making the method highly selective to its analysis in pharmaceutical and biological samples, with a low detection limit (LOD; 0.025 μmol L⁻¹) and suitable reproducibility and stability. Holanda 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 μmol L⁻¹) and reproducibility similar to the standard ultraviolet-visible spectrophotometric procedure (at the wavelength of 257 nm). Montes et al. [31] studied the superior electrochemical activity of MWCNTs with shorter dimensions (diameter × length: 100–170 nm × 5–9 μm and 6–9 nm × 5 μm) 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 × length: 6–9 nm × 5 μm)/GCE) was adapted to a flow-injection analysis system (FIA) and provided a sensitivity and accuracy to determine ibuprofen (LOD = 1.9 μmol L⁻¹) by amperometry comparable to that obtained by capillary electrophoresis. Pavinatto 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-α-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 μmol L⁻¹) without significant effect of the most common interfering compounds present in human urine (urea, uric, and ascorbic acids, glucose, NaCl, KCl, and NH₄Cl). Chen 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 hydrochlorothiazide and triamterene [34], mangiferin and icariin [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. Başkaya et al. [39] fabricated a non-enzymatic glucose sensor using a hybrid film (composed by f-MWCNT and highly monodisperse nickel nanoparticles) immobilized on GCE, which was able to determine this target in human blood serum samples by amperometry, with sensitivity (LOD = 0.021 μmol L⁻¹) and stability similar to one of commercial enzymatic biosensors. The results also support the positive synergistic effects that are observed when different nanomaterials are associated into the same matrix, enhancing the capacity and versatility of the electrochemical sensors. Purine bases are also crucial biomolecules to the immune function and abnormal changes causes serious disorders, such as cancer, epilepsy, lupus erythematous, renal calculi, and cardiovascular alterations. Wang et al. [40] prepared a nanocomposite by mixing MWCNT and copper-nickel hybrid nanoparticles (Cu@Ni-NP), which was deposited onto a GCE surface, with the goal of monitoring adenine and guanine in ds-DNA from mice brain tissues. Using DPV under the optimized conditions, the Cu@Ni-NP/MWCNT/GCE sensor achieved a LOD of 0.17 μmol L⁻¹ and 0.33 μmol L⁻¹ for guanine and adenine, respectively, attesting to its potential to quantify purine bases in complex samples. Likewise, sensors for point-of-care detection of DNA damage in medical diagnostics are also on the rise. Li and Lee [41] investigated the performance of f-MWCNT (functionalization with oxygen-containing groups to improve the immobilization and stability of the bioelement) as an additive in electrochemical DNA sequence differentiation ability. The electroanalytical signal achieved by DPV was highly selective and modulated between the probe (5′-GTG TTG TCT CCT AGG TTG GCT CTG-3′; 24-base fraction of the p53 gene, used as a marker for breast cancer) and its complementary sequence (5′-CAG AGC CAA CCT AGG AGA CAA CAC-3′). The detection capacity (LOD = 141.2 pmol L⁻¹) was more than two orders better than the previously reported for graphene functionalized (f-Grf) based sensors, even in the presence of single (5′-CAG AGC CAA CCT CGG AGA CAA CAC-3′) and all-base mismatched pairs (5′-ATA TCG ACC TTG GCC GAG ACG GTG-3′). Great effort has also been directed towards the development of electrochemical devices capable of early detection of immune deficiency diseases, such as acquired immune deficiency syndrome (AIDS) caused by the HIV-retrovirus. Ma et al. [42] proposed a device composed by a GCE modified with MWCNT and molecularly imprinted polymers (MIPs) based on CTS for the determination of HIV-p24 protein in real human serum samples (Figure 6). Using DPV and the developed sensor (HIV-p24/CTS/MWCNT/GCE), a low limit of detection (0.083 pg cm⁻³), good selectivity, repeatability, reproducibility, stability, and accuracy were reached for the proposed electroanalysis; validation was performed by comparative studies with enzyme-linked immunosorbent assay (ELISA). The outstanding ability of MWCNT-based electrochemical sensors to simulate redox reactions that occur in vivo has also been demonstrated with the electroanalysis of hemoglobin [43], amino acids [44], cholesterol [45], bilirubin [46], neurotransmitters, and related compounds [47,48,49], and even microorganisms [50], in mammalian fluid samples, achieving great results and contributing to the early diagnosis and treatment of several related diseases.

The occurrence of pesticides in foodstuffs, natural water, soils, among other matrices of environmental relevance, constitute a worldwide concern. Sipa et al. [51] associated square-wave adsorptive stripping voltammetry (SWAdSV) and a modified sensor (β-cyclodextrins/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 × 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. Özcan and Gürbüz [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^® and Lontrel™). 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. Ghodsi and Rafati [53] reported a strategy to develop a voltammetric sensor for diazinon (an organophosphate insecticide) based on TiO₂ nanoparticles (TiO₂-NP) and MWCNT nanocomposite assembled onto a GCE. Under optimum conditions, TiO₂-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 Wei et al. [54] and Ertan et al. [55], aiming to electroanalysize trichlorfon insecticide (LOD = 4.0 × 10⁻⁷ mol L⁻¹, using Nafion^®/TiO₂-NP@MWCNT@carboxymethyl chitosan/GCE) and simazine herbicide (LOD = 2.0 × 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 (Cd²⁺ [56], Pb²⁺ [56], Mn²⁺ [57], and Na⁺ [58]) and anions (SO₃²⁻ [59], NO₂⁻ [60], and BrO₃⁻ [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.

5. Final Remarks

MWCNT and their (nano)composites have been the origin for exciting and versatile sensing platforms due to their inherent advantages, namely large active surface area, electrocatalytic properties, high and fast electron-transfer, signal enlargement, reduction of the overpotential, chemical inertness, among others. Their incorporation into electrochemical sensors has been improving the electroanalytical performance of these devices and simultaneously widening their scope and range of applications. However, some limitations related to the high cost of production of high purity MWCNT have been restricting their large-scale and industrial utilization. Thus, improvements on the current synthetic routes and purification methods or/and development of novel low cost and effective techniques are clearly needed. In addition, comprehensive characterization of the toxicity of MWCNT is another requirement to expand their application to in vivo assays. These achievements will contribute to the construction of reliable and reproducible MWCNT-based electrochemical sensors enlarging sensors exploitation in real in vivo monitoring and in on-line manufacturing applications.