**2. Nanomaterials extended electrochemical sensing platforms**

Electrochemical Carbon Nanotube Filter Oxidative Performance [19] as a Function of Surface adsorption. The presences of surface resident reactive groups, or edge-plane like sites that are situated at the ends of their structures, and at defect sites, are responsible for the excellent electro catalytic activity of carbon nanomaterials. Nanoparticles exhibited high surface to volume ratio with functional and

*Analytical Chemistry - Advancement, Perspectives and Applications*

offers high specific surface areas (more than 2000 m<sup>2</sup>

organic, inorganic or biomaterials into mesoporous carbon produces different functional groups and binding capacity which further improving their analytical performances. CNMs have received significant attention as candidate materials for detecting [10, 11] NOx, NH3, CO, SO2 etc. For example, sensing of nitrogen oxide (NOx), a major air pollutant emitted from power plants, which causes neurodegenerative diseases. The interfacial interaction can be enhanced by the surface-functionalization of nanotubes. The polar groups [12] on the nanotube surface increase the adsorption affinity of the electron-donor or acceptor pollutants and consequently offer better response. The detection of mercury ion at the Au-NPs interface is more sensitive and selective because they can form amalgam only with Hg compared to other metal ions. The electrochemical sensing performance had a relationship with the adsorption capacity, which excites the design of new sensing materials. The amino group on the surface of functionalized CMS [13] is bringing increased attractive force in adsorption of heavy metal ions. Though increasing the deposition time improves the sensitivity, it also lowers the detection limit because of the surface saturation at high metal ions concentrations. Carbon nanomaterials endowed with unique physiochemical properties were found to be most suitable for electrochemical detection of heavy metal due to their ease to modify, high sensitivity, good selectivity and high reproducibility. Unmodified CNTs are unable to chelate metal ions in aqueous solutions and cannot work as good electrode materials for the ASV analyses. The hydrophilic hybrid nanocomposites are able to adsorb heavy metal ions from aqueous solution due to the rich chelating groups. Carbon nano tubes (CNTs) exhibited effective adsorbent as well as sorbents for heavy metal ions. Therefore, it is reasonable to construct electrochemical sensors using the CNT or graphene-functionalized redox electrodes entity for detection of heavy metal because they are capable to detect simultaneously a majority of heavy metal ions with high resolution for defined and measured concentrations. The stripping techniques and particularly square wave and differential pulse anodic stripping voltammetry ensured alternative and extensive explored sensitive electrochemical

have higher sensitivities, lower limit detection, and faster electron transfer kinetics than traditional detection electrodes [4]. An electrochemical sensor is an analytical device in which a recognition element is integrated within or intimately associated with a physical transducer [5] (an electrode) that transfers the analytical signal to an electronic circuit for the purpose of detecting a target analyte. The development of active electro catalysts plays a key role in the design of efficient, reliable, stable, and innovative sensing devices. Electrochemical detection is highly favored by the characteristics of rapid detection, high sensitivity and selectivity, high adsorption capability and large surface area [6]. Functionalized CNTs are good electrochemical sensing materials and can impart strong electro catalytic activity [7] to electrochemical reaction for most environmental pollutants such as heavy metal ions, organic pollutants containing electro active group. Environmental pollution is considered as a worldwide public problem, including heavy metals, inorganic/ organic compounds, toxic gases, pesticides, antibiotics [8], bacteria, etc., which becomes a serious issues to human health and smooth environment [9]. The catechol (1, 2-dihydroxybenzene) is a phenolic compound which is extensively used in dye, petroleum refinery, plastic, antioxidant, cosmetics, medicines. The high toxicity and low degradability cause eczematous dermatitis, depression of the central nervous system (CNS) and a prolonged rise of blood pressure. With industrial development, many metal ions have discharged into natural environment. Unfortunately, metal ions, especial heavy metal ions, are easily caused soil and water polluted. Ordered mesoporous carbon have well-ordered and tunable porous structures and surface which have pore sizes in the range of 2–50 nm, Porosity

g−1). However, the grafting of

**72**

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, porosity, and mechanical rigidity.

#### **2.1 Nanomaterials and its chemical functionalization**

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

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

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

ent functional groups which can tailor the sensing behavior.

ants. The cyclic voltammetry resolve clear anodic peaks of SO3

GCE had low limit of detection (LOD) of 215 nM and 565 nM for SO3

attract environmental pollutants more efficiently. Strength of binding varies with functional group. The Hydrophilic HOOC-MWCNTs [25] can improve MWCNTs in synergistically electrocatalytic ability and adhesive ability. The introduction of organic, inorganic or biomaterials into ordered mesoporous carbon produces differ-

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 pollut-

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

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

2− and NO2

− . The

2− and NO2

− .

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

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

attract environmental pollutants more efficiently. Strength of binding varies with functional group. The Hydrophilic HOOC-MWCNTs [25] can improve MWCNTs in synergistically electrocatalytic ability and adhesive ability. The introduction of organic, inorganic or biomaterials into ordered mesoporous carbon produces different functional groups which can tailor the sensing behavior.
