**3. Functionalized carbon nanomaterials and its sensing capacity**

The functionalization of MWCNT [26] will give more active surface area and also the ionic interaction with anions would be more compared to the pristine MWCNT. The enhanced surface area and ionic interaction are very important for the real sample analysis at nanomolar concentrations, especially for the detection of harmful analytes. HOOC-MWCNTs [11] modified glassy carbon electrode (GCE) exhibited high sensing and adsorption capacity towards binary and ionic pollutants. The cyclic voltammetry resolve clear anodic peaks of SO3 2− and NO2 − . The anodic peak currents were gradually increases with concentration of ions. The peak separation between sulfite and nitrite are comparatively higher to probe the sensing of anions in nanomolar concentrations, it was found to be around 420 mV by cyclic voltammetry (CV) technique. This potential difference is highly attractive to determine the sulfite and nitrite simultaneously. The HOOC-MWCNTs decorated GCE had low limit of detection (LOD) of 215 nM and 565 nM for SO3 2− and NO2 − . The electrochemical sensing and detection was found to be two electron transfer oxidative reaction. The sulphate and nitrate ions were produced over the nano surface. *Sablok et al.* reported amine functionalized reduced grapheme oxide/ carbon nanotube decorated novel electrochemical sensor for ultra-trace detection of Trinitrotoluene (TNT) up to 0.01 ppb with good reproducibility (n = 3). The sensing capacity was enhanced due to formation of charge transfer complex between electron rich surface of sensor and electron deficient ring of TNT. The binding of electron-deficient TNT to the amine [27] groups on the nano-sensor surface modulate electrical and optical properties of nano sensing elements. *Devi et al.* reported GCE/rGO-SH/Au-NPs [28] electrode as fascinating electrochemical sensor to analyze mercury (Hg2+) ions in the aqueous solution. The working electrode capture Hg2+ ions electrochemically and consequently get adsorbed on the redox active core surface followed by electrochemical oxidation by differential pulse voltammetry (DPV) with the increased oxidation current at +0.172 V. Moreover, this sensor platform revealed linear response for Hg(II) detection from 1–10 μM in phosphate buffer saline (PBS) solution and the detection limit was found to be 0.2 μM (S/N = 3). *Wang et al.* designed a GCE with MWCNT-CO-PANi [29] as a electrochemical sensor for detection of Pb2+ because the porous structure of conducting PANI surface can retard the bulk surface active compounds from reaching the sensing surface and thus minimizes the passivation of the working electrode. In addition, the PANI matrix offers binding capacity which can firmly hold the MWCNTs on the electrode surface. *Dai et al.* reported the improvement in stripping peak signals of heavy ions on PA/PPy/GO can be attributed to the high surface area of GO and the excellent electrical conductivity of PPy could enhance the electron transfer during the detection processes and peak intensity collaborated with number of functional groups with large negative charges on PA and GO is beneficial to improving the adsorption capacity of heavy metal ions. Phytic acid [30] consists of six membered rings with six phosphate group and two hydroxyl groups, could enhance complexation ability. **Figure 1(a)** shows strip peaks with resolved potential which demonstrates suitability of electrochemical sensor [30]. *Zhang et al.* reported size controlled AuNPs (5–15 nm)/CNFs/GCE electrochemical sensor for simultaneous tracing of

*Analytical Chemistry - Advancement, Perspectives and Applications*

**2.1 Nanomaterials and its chemical functionalization**

porosity, and mechanical rigidity.

highly redox active core center leading to increasing the sensitivity and selectivity of the sensor. Thus, a highly active site has great affinity towards molecules result in molecule gets adsorbed on the surface of electrode to undergo a redox reaction. The conducting and chelating group has marked effect on the designation of sensor. Nanomaterials provide a special platform for the purification of contaminated water due to the high surface area of nano-sorbents and their capability of chemical modification and easier regeneration. NPs, QDs with some functionalization are used as tools, immobilization platforms [20] or electro active labels to improve the sensing performance exhibiting higher sensitivity and stability. The nano-particles and quantum dots [20] structures from the electrodes have significantly made a contribution to increasing the electro-catalytic properties because the functionalization of the structures could improve the high surface area, conductivity, stability,

Nanomaterials have one dimension <100 nm [1] and possess physico-chemical properties dictated by their unusually small size, large surface area, shape and chemical composition. Nanomaterials usually require the surface functionalization for specific detection of metal ions. The p-type (anion doped) CNTs can behave as an electron deficient surface which can easily adsorb reductive molecule (NO2) on its surface. The electrochemical sensitivity can be enhanced through attachment of active redox center either via physical or chemical forces over the reactive surface of carbon nanotube. Non-covalent functionalization normally involves physical forces (ion dipole, dipole–dipole, electrostatic force) for the binding of CNTs with catalysts (e.g., metal nanoparticles and metal oxides). Covalent functionalization [21] involves chemical forces (chemical reaction) for tagging of functional group with CNTs. In other words, it is realized through covalent attachment of chemical groups on the conjugated surfaces (edge, plane core) of CNTs. The number of oxygenated functional groups (e.g., –OH, –CO, and –COOH) created during calcinations, purification and isolation processes. As a result, controlled functionalities are susceptible to determine the sorption capacity of CNTs. These chemical groups greatly reduce the hydrophilicity and improve the capacity of ion exchanging behavior, leading to strong interactions with pollutants (e.g., heavy metal ions and organic compounds). Especially, the hydrophilic –OH and –COOH groups on the surface of CNTs exhibit superior sorption phenomenon towards low molecular weights and polarity. Their large surface area as pore volumes, functional surface groups and two basal planes are quite useful for the adsorption of pollutants. CNTs have been exploited in multiple electrochemical sensors because of their ability to facilitate electron transfer reactions [22] with electroactive species in solution and the electrode interface. *Thiruppathi et al.* reported functionalities of a carbon surface may assist the heavy metal ion adsorption properties. To improve their conductivity, FGO and GO were electrochemically reduced at −1.2 V for 300 s in a 0.1 M acetate buffer (pH = 5.0).Fe3O4 possessed electrostatic adsorption interaction with lead, and amine [13] acted as a better ligand displaying good chelation with lead. *Xiong et al.* designed amine –Fe3O4 modified glassy carbon electrode [23] as electrochemical sensor for detection of Pb(II) with a detection limit of 0.15 μM and 10.07 μA/μM sensitivity. Graphene-based nano-adsorbents are excellent advanced materials for the removal of the organic contaminants [24] from the water because of their nano-scaled size, high surface area, and ability to interact via pi-pi stacking. F-doped carbon nanomaterials have gained great attention because of unique properties such as its high temperature resistance, capacitance and enhanced electrocatalytic activity. The cross linked and bridged group exhibited high affinity and

**74**

Cd(II), Pb(II) and Cu(II) with SWASV and detected three signal at −0.8, −0.5 and 0 V over a linear range of concentration 0.1 μM - 1 μM at a deposition potential of −1.8 V. *Mohamed Shaban* reported a porous Anodic Alumina (PAA) membrane was functionalized with CoFe2O4 nanoparticles and used as a substrate for the growing of very long helical-structured Carbon Nanotubes (CNTs) with a diameter less than 20 nm. The designed electrode was found to be suitable for detection of Hg2+, Cd2+ and Pb2+ ions**. Figure 1(b)** indicates concentration dependent profile which demonstrates maximum range of concentration of detection for which sharp and intense peak appeared sensitively [30].

### **3.1 Design of selective electrochemical sensor**

Ferrocene (Fc) functionalized MWCNTs works as a ratio metric electrochemical sensor. The Fc-MWCNTs/GCE modified sensor was used for detection of o-nitrophenol and p-nitrophenol present in water as toxic pollutants. When Fc-MWNTs/GCE [31] was dipped in 50 μM of o-NP and p-NP, the reduction peak of Fc remained fixed, but two well-separated peaks at about −0.66 V and-0.79 V could be detected which correspond to the reduced peaks of o-NP and p-NP, respectively. The process implies that the modified Fc can effectively separate the reduction peaks of o-NP and p-NP by about 0.13 V, which makes suitable it to detect o-NP and p-NP individually and simultaneously. **Figure 2(a)** demonstrates ferrocene functionalized MWCNTs as a ratio-metric and selective sensor [32]. **Figure 2(b)** indicates suitability of modified sensor and influence of metal nanoparticle on sensitivity [30].

#### **3.2 Electro active carbon nanomaterials and its high surface density**

Carbon nanomaterials mainly include zero-dimensional fullerene (C60) and carbon dots (CDs), one-dimensional carbon nanotubes (CNTs) and carbon nanohorns (CNHs), Carbon nanofiber, two- dimensional graphene and its derivatives, and ordered mesoporous carbon (OMC). The hydrogen bonding interaction between the oxygenated groups of CNMs and hydroxyl groups has been utilized for the adsorption of pollutants containing functional groups (e.g., amine, hydroxyl and carboxyl groups. The CNF [33] is functionalized for improving its solubility and also remove the catalytic impurities for enhancing the electrochemical properties by

#### **Figure 1.**

**(a)** *DPV of PA/GO, PA/PPy, PPy/GO and PA/PPy/GO modified electrodes in 0.1 M acetate buffer solution (pH 4.5) containing 50* μ*g/L Cd(II) and Pb(II).* **(b)** *Calibration curve for Pb2+ determination, from down to up0.2, 0.5, 1, 5, 15, 20, 30, 40, 60,80, 100, and 130* μ*g/L Pb2+ in 0.1 M acetate buffer (pH 4.5) at PPy/CNFs/ CPE under the optimized experimental.*

**77**

peaks at ordinary electrode [35].

**(SWASV)**

**4. Analytical role of square wave anodic stripping voltammetry** 

Electrochemical techniques have the capability to maintain environmental interfacial processes at high rates and efficiencies by directionally and accurately controlling the electron transfer processes. An electrochemical technique where the analyte of interest is first electrodeposited onto the sensing electrode and removed

*Application of Carbon Nanomaterials Decorated Electrochemical Sensor for Analysis…*

the generation of more anchoring sites and surface reactive groups (carboxylic acid, hydroxyl, and carbonyl groups) on the open end and side walls of CNF. *Ramaraj et al.* reported BiFeO3-F-CNF modified GCE is an effective electrode for electrochemical detection of catechol. The lowest value of ΔEp (0.106 V) and higher redox peak current response are indicating that BiFeO3-CNF/GCE has faster electron transfer kinetics than that of other modified electrodes. The electrode is fabricated to anchor more recognition sites on the surface of the electrode and to achieve high affinity for the chemical adsorption of heavy metal ions. Polyaniline is combined with the rGO and glycine for strengthening the collective capacity for metal ions through nitrogen functionalities for example amine (-NH-) and imine (=N-) functional groups [13, 33]. Chitosan (CS) is a polysaccharide and its chemical modification can introduce new chelating groups along the CS chains, which can not only prevent its dissolution in acidic solutions but also improve the adsorption capacity and selectivity of an existing group for a specific metal ion. The MWCNT can be adhered through thiol functionalized Chitosan [21] which enhances the surface density to capture heavy metal ions. MWCNTs can lead to formation of good conduction pathway in the CS-SH film for better electro-analysis. *Li et al.* reported the simultaneous stripping analysis of Cd2+ and Pb2+ at the nitrogen doped carbon quantum dots modified grapheme oxide NCQDs-GO/GCE. The recorded ASV curves depict individual and highly resolved peaks at around −0.75 V for Cd2+ and − 0.50 V for Pb2+, respectively. The peak to peak separation potential is about 0.250 V, which is large enough to recognize selective detection of Cd2+ and Pb2+ simultaneously. GO are served as a novel support to load nitrogen doped carbon quantum dots (NCQDs) and improve the conductivity and electron transfer rate of the hybrid. The AuNPs [34] modified working electrode can provide more electro active sites and faster electron transfer rate, all of which contribute to enhance the sensitivity of the simultaneous determination of Hydroquinone and catechol. It is difficult to simultaneously determine catechol and hydroquinone due to their overlapping

**(a)** *Ferrocene functionalized MWCNT,* **(b)** *SWASVs for simultaneous detection of Cd2+, Pb2+ and Cu2+ with* 

*DOI: http://dx.doi.org/10.5772/intechopen.96538*

*the modified AuNPs/CNFs/GCE, CNFs/GCE and bare/GCE.*

**Figure 2.**

*Application of Carbon Nanomaterials Decorated Electrochemical Sensor for Analysis… DOI: http://dx.doi.org/10.5772/intechopen.96538*

**Figure 2.**

*Analytical Chemistry - Advancement, Perspectives and Applications*

peak appeared sensitively [30].

nanoparticle on sensitivity [30].

**3.1 Design of selective electrochemical sensor**

Cd(II), Pb(II) and Cu(II) with SWASV and detected three signal at −0.8, −0.5 and 0 V over a linear range of concentration 0.1 μM - 1 μM at a deposition potential of −1.8 V. *Mohamed Shaban* reported a porous Anodic Alumina (PAA) membrane was functionalized with CoFe2O4 nanoparticles and used as a substrate for the growing of very long helical-structured Carbon Nanotubes (CNTs) with a diameter less than 20 nm. The designed electrode was found to be suitable for detection of Hg2+, Cd2+ and Pb2+ ions**. Figure 1(b)** indicates concentration dependent profile which demonstrates maximum range of concentration of detection for which sharp and intense

Ferrocene (Fc) functionalized MWCNTs works as a ratio metric electrochemi-

Carbon nanomaterials mainly include zero-dimensional fullerene (C60) and carbon dots (CDs), one-dimensional carbon nanotubes (CNTs) and carbon nanohorns (CNHs), Carbon nanofiber, two- dimensional graphene and its derivatives, and ordered mesoporous carbon (OMC). The hydrogen bonding interaction between the oxygenated groups of CNMs and hydroxyl groups has been utilized for the adsorption of pollutants containing functional groups (e.g., amine, hydroxyl and carboxyl groups. The CNF [33] is functionalized for improving its solubility and also remove the catalytic impurities for enhancing the electrochemical properties by

**(a)** *DPV of PA/GO, PA/PPy, PPy/GO and PA/PPy/GO modified electrodes in 0.1 M acetate buffer solution (pH 4.5) containing 50* μ*g/L Cd(II) and Pb(II).* **(b)** *Calibration curve for Pb2+ determination, from down to up0.2, 0.5, 1, 5, 15, 20, 30, 40, 60,80, 100, and 130* μ*g/L Pb2+ in 0.1 M acetate buffer (pH 4.5) at PPy/CNFs/*

cal sensor. The Fc-MWCNTs/GCE modified sensor was used for detection of o-nitrophenol and p-nitrophenol present in water as toxic pollutants. When Fc-MWNTs/GCE [31] was dipped in 50 μM of o-NP and p-NP, the reduction peak of Fc remained fixed, but two well-separated peaks at about −0.66 V and-0.79 V could be detected which correspond to the reduced peaks of o-NP and p-NP, respectively. The process implies that the modified Fc can effectively separate the reduction peaks of o-NP and p-NP by about 0.13 V, which makes suitable it to detect o-NP and p-NP individually and simultaneously. **Figure 2(a)** demonstrates ferrocene functionalized MWCNTs as a ratio-metric and selective sensor [32]. **Figure 2(b)** indicates suitability of modified sensor and influence of metal

**3.2 Electro active carbon nanomaterials and its high surface density**

**76**

**Figure 1.**

*CPE under the optimized experimental.*

**(a)** *Ferrocene functionalized MWCNT,* **(b)** *SWASVs for simultaneous detection of Cd2+, Pb2+ and Cu2+ with the modified AuNPs/CNFs/GCE, CNFs/GCE and bare/GCE.*

the generation of more anchoring sites and surface reactive groups (carboxylic acid, hydroxyl, and carbonyl groups) on the open end and side walls of CNF. *Ramaraj et al.* reported BiFeO3-F-CNF modified GCE is an effective electrode for electrochemical detection of catechol. The lowest value of ΔEp (0.106 V) and higher redox peak current response are indicating that BiFeO3-CNF/GCE has faster electron transfer kinetics than that of other modified electrodes. The electrode is fabricated to anchor more recognition sites on the surface of the electrode and to achieve high affinity for the chemical adsorption of heavy metal ions. Polyaniline is combined with the rGO and glycine for strengthening the collective capacity for metal ions through nitrogen functionalities for example amine (-NH-) and imine (=N-) functional groups [13, 33]. Chitosan (CS) is a polysaccharide and its chemical modification can introduce new chelating groups along the CS chains, which can not only prevent its dissolution in acidic solutions but also improve the adsorption capacity and selectivity of an existing group for a specific metal ion. The MWCNT can be adhered through thiol functionalized Chitosan [21] which enhances the surface density to capture heavy metal ions. MWCNTs can lead to formation of good conduction pathway in the CS-SH film for better electro-analysis. *Li et al.* reported the simultaneous stripping analysis of Cd2+ and Pb2+ at the nitrogen doped carbon quantum dots modified grapheme oxide NCQDs-GO/GCE. The recorded ASV curves depict individual and highly resolved peaks at around −0.75 V for Cd2+ and − 0.50 V for Pb2+, respectively. The peak to peak separation potential is about 0.250 V, which is large enough to recognize selective detection of Cd2+ and Pb2+ simultaneously. GO are served as a novel support to load nitrogen doped carbon quantum dots (NCQDs) and improve the conductivity and electron transfer rate of the hybrid. The AuNPs [34] modified working electrode can provide more electro active sites and faster electron transfer rate, all of which contribute to enhance the sensitivity of the simultaneous determination of Hydroquinone and catechol. It is difficult to simultaneously determine catechol and hydroquinone due to their overlapping peaks at ordinary electrode [35].
