**5.1 Influence of surface group (chromophores) and sensing sites**

The electro catalytic properties of Carbon nanodots material depend on the presence of functional groups on the surface electrode because the material is a great electron acceptor and donor electron with the presence of some functional groups such as hydroxyl groups. Calixarenes [39] have three-dimensional spherical basket, cup or bucket shapes. **Figure 4(a)** depicts structural integrity of Calixarenes [39]. The spherical core volume is utilized in ion selective electrodes and membranes. It can capture stationary phases. The macrocyclic ring structure is efficient ionophores for metal ions viz. Na+ , Cd2+, Pb2+ and Fe3+. Coordination depends on macrocyclic ring size and ionic size of metal ion. Calixarenes can coordinate with the metal ions to increase the sensitivity of the electrochemical sensors. The metal ions, Fe(III), Cd(II), and Pb(II) gave a linear relationship with their concentrations at 1.0–10 nM on the CA/RGO/GCE.


#### **Table 1.**

*Different electrochemical sensor for detection of pollutants and analytical parameters.*

*Analytical Chemistry - Advancement, Perspectives and Applications*

or 'stripped' with a sharp and intense peak by applying an oxidizing potential. During removal of pollutants, the peak current is measured as a function of time or function of the potential between the indicator (sensor) and reference electrodes. The redox probe is introduced as the inner reference to provide a built-in correction towards the signal transduction. The peak current ratio of analyte signal to probe signal is employed as the detected signal for analyte determination. The potential is varied as a square wave superimposed on a linear sweep. The potential separation between the stripping peaks can clear enough to distinguish the various heavy metal ions. The detection is expressed as sensing signals. The stripping peak currents are controlled by the amount of target metal ions adsorbed on the electrode surface. Striping peak current is directly proportionate to concentration of analyte. The SWASV is more prone over other voltammetry technique because of excellent sensitivity and unique ability to detect metals simultaneously. SWASV includes two independent procedures: deposition and stripping. First, in the deposition process (electrochemical reduction), metal ions can be reduced under a certain potential from the analyte solution to the working electrode. Inversely, when anodic potential is applied, the reduced metals are oxidized to their ions. Interference ions reduce the peak current for detected analyte during electrochemical analysis. Peak current varies with concentration of analyte and it increases linearly up to optimum concentration range which is also referred to as linear range concentration profile. Square-wave anodic stripping voltammetry is commonly used for metal detection due to its high sensitivity and low (nM–pM) detection limits. **Figure 3** indicates

*General scheme of electrochemical sensing and detection of inorganic pollutants (heavy metal ions)* 

schematic sensing analysis and detected signal for pollutants [15].

The adsorption activity is related to the number of active functional groups on the surface of the carbon nanomaterials with highly oxidized surfaces showed a greater adsorption affinity for the stabilizers. Electro catalytic activity is related with hydrophobic or hydrophilic, positive or negative redox active groups of carbon nanomaterials**.** *Li et al.* reported MWCNTs are highly efficient to remove perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from aqueous solution in light of their environmental persistence [36]. Bismuth modified CNT polystyrene Sulfonate (PSS) composite electrode for simultaneous detection of Pb(II) and Cd(II) by anodic stripping voltammetry. The designed composite electrode shows synergistic effect of bismuth and Polystyrene sulfonate. Since, polymeric dopant acts as cation-exchanger and CNT as an efficient signal transducer for sensitive and simultaneous detection of lead and cadmium. The detection limits were estimated to be 0.04 ppb for Pb(II) and 0.02 ppb for Cd(II) at 2 min accumulation. The presence

**5. Adsorption sites and its electrochemical sensitivity**

**78**

**Figure 3.**

*through SWASV.*

**Figure 4.** *Structure of organic compounds with core functional and sensing unit.*

Different binding energies of functional groups have a great impact on absorption; weaker binding energies facilitate easier desorption [27]. As one of the best hydrophilic functional groups, the amidoxime groups modified on the electrode surface largely intensified the adsorption of heavy metals and lowered the impedance of the material when compared with an unmodified electrode. Amidoxime [49] group for functionalizing the carbon felt electrodes because of its superior adsorption ability for metal ions resulting from their coordination active sites. **Figure 4(b)** shows the structure of amidoxime [49]. The amidoxime group can coordinate with cations to form stable pentacyclic compounds, suggesting that this coordination bond should be stronger than other kinds of monodentate groups. The organic ligands having the amide functional moiety revealed strong and selective complexing ability towards metal ions when used to fabricate electrochemical sensor. The presence of carboxylate and pyridinium functional groups as negative and positive charge bearers over the surface of CNMs enhances the affinity of electrochemical sensing of cations and anions, respectively. The potential to modify carbon nanotubes with multiple chelating molecules with different selectivity towards various analytes attract designing of fancy sensor [12, 13].

#### **5.2 Sensing reaction over electrochemical integrity**

Composite phase provides more recognition sites on the surface of the electrode to achieve high affinity for the binding of inorganic pollutants. Conducting polymer enhances the collective capacity of carbon nanomaterials towards metal ions [33].

a.Accumulation/adsorption at working electrode

Gly/RGO/PANi + M2+ (solution phase) + 2e− → M0 ----Gly/RGO/PANi

b.Anodic striping (electro analytical operation)

M0 ----Gly/RGO/PANi → Gly/RGO/PANi + M2+ (solution phase) + 2e−

Highly efficient ionophores was developed for removing Cd2+ and Pb2+ using (PyTS-CNTs) [8]. The small conjugative surface is recalled 1, 3, 6, 8-pyrenetetrasulfonicacid sodium salt as the sensing material. The working window was from 1.0 μg/L

**81**

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

to 110 μg/L for both Pb2+ and Cd2+ ions. The limit of detection (LOD) was 0.02 and 0.08 μg/L, respectively. Functional groups greatly improve the hydrophobicity and ion exchange capacity, leading to strong interactions with pollutants. **Figure 5(a)** depicts the structural entity as a sulfonated salt of pyrene which can detect the pollutants through adsorption and ion exchange phenomenon [9]. **Figure 5(b)** demonstrates thiol and Au NPs anchor the adsorption and electrochemical reduction through

**(a)** *Structure of functionalized pyrene.* **(b)** *Schematic representation for adsorption and electro-reduction of* 

Analytical performances of sensors are proportional to the surface concentration of the receptors. Cross-linking can enhance the electrochemical properties of electrochemical sensor. The CNT possess –COOH group. The cross linked and grafted CNT improve adsorption, adhesion, completion, chelation and ion exchange phenomenon along with fast charge transport [6]. This enhances selectivity and sensitivity. Cross linking increases the capturing integrity over the surface of carbon nanotube [43]. The chemical modifications of the chitosan by covalently attaching of selected molecules to the amino or hydroxyls groups can improve the ion-transport and ion-exchange proprieties of the biopolymer. Strong electron transmission from substituent (bridge) to carbon nanomaterial enhances the kinetics of sensing phenomenon [34]. *Janegitz et al.* developed functionalized carbon nanotubes paste electrode (CNPE) modified with cross-linked chitosan [crosslinked with glutaraldehyde (CTS-GA)] for determination of Cu(II) in industrial wastewater [44]. Chitosan is a cross linked [50] biopolymer having enriched –NH2 and –OH functionality.

The pore radius and pore volume decides enhanced redox peaks with much higher current and the kinetics of electro analytical process otherwise; signal distortion appears during detection of environmental pollutants. Different structures

The decrease in stripping current is attributed to the inadequate accumulation of the metal ions at lower negative potential and the initiation of hydrogen evolution reaction at a higher negative potential that may damage the surface of the electrode. The optimum deposition potential will resolve the ions efficiently and selectively.

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

enhanced charge transport [30].

*Hg(II) at rGO-SH/Au-NPs functionalized sensor.*

**Figure 5.**

**5.3 Influence of cross linked structure and core integrity**

**6. Factors influencing electrochemical detection**

have different activation energy of adsorption and binding.

**6.1 Nature and structure of sensor**

**6.2 Deposition potential**

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

**Figure 5.**

*Analytical Chemistry - Advancement, Perspectives and Applications*

Different binding energies of functional groups have a great impact on absorption; weaker binding energies facilitate easier desorption [27]. As one of the best hydrophilic functional groups, the amidoxime groups modified on the electrode surface largely intensified the adsorption of heavy metals and lowered the impedance of the material when compared with an unmodified electrode. Amidoxime [49] group for functionalizing the carbon felt electrodes because of its superior adsorption ability for metal ions resulting from their coordination active sites. **Figure 4(b)** shows the structure of amidoxime [49]. The amidoxime group can coordinate with cations to form stable pentacyclic compounds, suggesting that this coordination bond should be stronger than other kinds of monodentate groups. The organic ligands having the amide functional moiety revealed strong and selective complexing ability towards metal ions when used to fabricate electrochemical sensor. The presence of carboxylate and pyridinium functional groups as negative and positive charge bearers over the surface of CNMs enhances the affinity of electrochemical sensing of cations and anions, respectively. The potential to modify carbon nanotubes with multiple chelating molecules with different selectivity

Composite phase provides more recognition sites on the surface of the electrode to achieve high affinity for the binding of inorganic pollutants. Conducting polymer enhances the collective capacity of carbon nanomaterials towards metal ions [33].


Highly efficient ionophores was developed for removing Cd2+ and Pb2+ using (PyTS-CNTs) [8]. The small conjugative surface is recalled 1, 3, 6, 8-pyrenetetrasulfonicacid sodium salt as the sensing material. The working window was from 1.0 μg/L


towards various analytes attract designing of fancy sensor [12, 13].

**5.2 Sensing reaction over electrochemical integrity**

*Structure of organic compounds with core functional and sensing unit.*

a.Accumulation/adsorption at working electrode

b.Anodic striping (electro analytical operation)

Gly/RGO/PANi + M2+ (solution phase) + 2e− → M0

**80**

M0

**Figure 4.**

**(a)** *Structure of functionalized pyrene.* **(b)** *Schematic representation for adsorption and electro-reduction of Hg(II) at rGO-SH/Au-NPs functionalized sensor.*

to 110 μg/L for both Pb2+ and Cd2+ ions. The limit of detection (LOD) was 0.02 and 0.08 μg/L, respectively. Functional groups greatly improve the hydrophobicity and ion exchange capacity, leading to strong interactions with pollutants. **Figure 5(a)** depicts the structural entity as a sulfonated salt of pyrene which can detect the pollutants through adsorption and ion exchange phenomenon [9]. **Figure 5(b)** demonstrates thiol and Au NPs anchor the adsorption and electrochemical reduction through enhanced charge transport [30].

#### **5.3 Influence of cross linked structure and core integrity**

Analytical performances of sensors are proportional to the surface concentration of the receptors. Cross-linking can enhance the electrochemical properties of electrochemical sensor. The CNT possess –COOH group. The cross linked and grafted CNT improve adsorption, adhesion, completion, chelation and ion exchange phenomenon along with fast charge transport [6]. This enhances selectivity and sensitivity. Cross linking increases the capturing integrity over the surface of carbon nanotube [43]. The chemical modifications of the chitosan by covalently attaching of selected molecules to the amino or hydroxyls groups can improve the ion-transport and ion-exchange proprieties of the biopolymer. Strong electron transmission from substituent (bridge) to carbon nanomaterial enhances the kinetics of sensing phenomenon [34]. *Janegitz et al.* developed functionalized carbon nanotubes paste electrode (CNPE) modified with cross-linked chitosan [crosslinked with glutaraldehyde (CTS-GA)] for determination of Cu(II) in industrial wastewater [44]. Chitosan is a cross linked [50] biopolymer having enriched –NH2 and –OH functionality.
