Different electrochemical sensor for detection of pollutants and analytical parameters.
Carbon nanomaterials (CNMs), especially carbon nanotubes and graphene, have been attracting tremendous attention in environmental analysis for rapid and cost effective detection of various analytes by electrochemical sensing. CNMs can increase the electrode effective area, enhance the electron transfer rate between the electrode and analytes, and/or act as catalysts to increase the efficiency of electrochemical reaction, detection, adsorption and removal are of great significance. Various carbon nanomaterials including carbon nanotubes, graphene, mesoporous carbon, carbon dots exhibited high adsorption and detection capacity. Carbon and its derivatives possess excellent electro catalytic properties for the modified sensors, electrochemical methods usually based on anodic stripping voltammetry at some modified carbon electrodes. Metal electrode detection sensitivity is enhanced through surface modification of working electrode (GCE). Heavy metals have the defined redox potential. A remarkable deal of efficiency with the electrochemical sensors can be succeeded by layering the surface of the working electrode with film of active electro-catalytic species. Usually, electro catalysts used for fabrication of sensors are surfactants, nano-materials, polymers, carbon-based materials, organic ligands and biomaterials.
- redox active site
- carbon nanotube
- surface functionalization
- organic pollutants
- electrochemical reaction
Sustainable environment requires development of portable sensors for monitoring heavy and toxic metallic pollutants. Nanomaterials and nanostructures play a vital role as an adsorption sites into sensors  that leads to shift sensitivity, selectivity, multiplexed detecting ability towards high performance in terms of capability and portability . Nanomaterials-based sensors exhibit an extremely high surface area, which can increase the number of binding sites  available for the adsorption of metal ions. Heavy metal pollution becomes a concern for global sustainability. Carbon nanomaterials  act as electrochemical sensors because they have higher sensitivities, lower limit detection, and faster electron transfer kinetics than traditional detection electrodes . An electrochemical sensor is an analytical device in which a recognition element is integrated within or intimately associated with a physical transducer  (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 . Functionalized CNTs are good electrochemical sensing materials and can impart strong electro catalytic activity  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 , bacteria, etc., which becomes a serious issues to human health and smooth environment . 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 offers high specific surface areas (more than 2000 m2g−1). However, the grafting of 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  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  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 methods for heavy metal ions detection. The carbon bound Fluorine  exhibited both ionic as well as covalent interface and significantly enhanced the capacitive performance of fluorinated GO compared to pristine GO. Further, the fluorinated GO has high affinity for the simultaneous detection of heavy metal ions Cd2+, Pb2+, Cu2+ and Hg2+ using square wave anodic stripping voltammetry (SWASV) as electrochemical tool. Fluorinated-graphene has gained great attention because of unique properties such as its high temperature resistance and enhanced electro catalytic activity. Electron withdrawing nature is arising from the strong electronegativity of F and electron donating nature from the lone-pair electrons. Graphene or reduced graphene oxide (rGO) are used as an working electrode material; however, low sensitivity and potential interferences lowers their sensing capacity due to inherent irreversible agglomeration of graphene particle which excites researcher to develop green idea in the designing of native grapheme  based detecting electrodes as electrochemical sensor. Low sensitivity and poor selectivity related with the large over potential and the interference from the reduced substance, such as oxygen, H2O, encountered in the nitro aromatic compound (organic pollutants). Nitrophenols readily accumulate in organisms and are difficult to naturally degrade because of the high structural stability. The sensitivity can be improved by incorporating metal nano-particle over the surface of functional sites . Owing to the high specific surface area, chemical stability, high p-conjugation and hydrophilic properties, GO can offer an excellent electrode platform for adsorbing other molecules. High surface to volume ratio with active sites, controlled distribution of pore size, exceptional sorption capacity and high sorption proficiency make CNTs suitable material for the development of electro-analytical systems dedicated for the detection of heavy metal ions. Hence, ionated CNTs play important role in the metal ion sensing due to their better ion exchange capacity. Oxidized CNTs have a great potential for cation uptake compared to non oxidized CNTs. In other words, non oxidized CNTs have tendency towards uptake of anions compared to oxidized CNTs. The presence of an extended π-conjugation in organic conducting polymer (OCPs) confers the required mobility to charges that are present on polymer backbone and makes them electrically conducting . The sensing intensifier played a facilitating role between the GCE surface and the target metal ions by bringing analytes closer to transducer surface resulting in appearance of intense electrochemical signals. CNFs with high length-to-diameter ratio are capable of offering additional active sites for nanoparticle loading or deposition. The carbon nanotubes alone as well as in their oxidized and in their composite forms have tremendous ability to adsorb the heavy metal ions. Unmodified CNTs are unable to chelate metal ions in aqueous solutions and cannot work as good electrode materials. This is due to deficiency of functional group and sufficiency of hydrophobic environment. The effective combination of two carbon nanostructures can not only improve solubility and conductivity but also make up functional deficiencies . Functionalization could significantly assist in the improvement of surface capacitance. Thus, the modified GCE exhibits good electro oxidative activity towards pollutants.
2. Nanomaterials extended electrochemical sensing platforms
Electrochemical Carbon Nanotube Filter Oxidative Performance  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 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  or electro active labels to improve the sensing performance exhibiting higher sensitivity and stability. The nano-particles and quantum dots  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  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  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  with electroactive species in solution and the electrode interface.
3. Functionalized carbon nanomaterials and its sensing capacity
The functionalization of MWCNT  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  modified glassy carbon electrode (GCE) exhibited high sensing and adsorption capacity towards binary and ionic pollutants. The cyclic voltammetry resolve clear anodic peaks of SO32− 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 SO32− 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.
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  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 . Figure 2(b) indicates suitability of modified sensor and influence of metal nanoparticle on sensitivity .
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  is functionalized for improving its solubility and also remove the catalytic impurities for enhancing the electrochemical properties by 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.
4. Analytical role of square wave anodic stripping voltammetry (SWASV)
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 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 schematic sensing analysis and detected signal for pollutants .
5. Adsorption sites and its electrochemical sensitivity
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
|CNHs/GO||4-NCB||54.47 μA/μM||10 nM||SWASV|||
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  have three-dimensional spherical basket, cup or bucket shapes. Figure 4(a) depicts structural integrity of Calixarenes . 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.
Different binding energies of functional groups have a great impact on absorption; weaker binding energies facilitate easier desorption . 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  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 . 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 .
Accumulation/adsorption at working electrode
Gly/RGO/PANi + M2+ (solution phase) + 2e− → M0----Gly/RGO/PANi
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) . 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 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 . Figure 5(b) demonstrates thiol and Au NPs anchor the adsorption and electrochemical reduction through enhanced charge transport .
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 . This enhances selectivity and sensitivity. Cross linking increases the capturing integrity over the surface of carbon nanotube . 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 .
6. Factors influencing electrochemical detection
6.1 Nature and structure of sensor
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 have different activation energy of adsorption and binding.
6.2 Deposition potential
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.
6.3 Deposition time
The stripping peak currents show a linear increase with a prolonged period holding the maximum peak current which indicates the saturation of all the possible attachment sites on the functionalized electrode by the adsorption of the heavy metal contaminants.
6.4 pH (buffer capacity) of medium
The pH-value have significant influence onto the size of the square wave voltammetric peaks and also assists in the hydrolysis of metal ions, therefore, it is crucial to choose a suitable pH-value for the sensing of metal ions. At the high concentration of hydrogen ion, the intensity of peak current is reduced due to protonation of hydrophilic groups on the surface of sensing material which leads to the decrease in the attachment sites for the adsorption of the heavy metal ions. Proper pH maintains originality of electron rich functional group over the sensing integrity.
6.5 Supporting electrolyte
Supporting electrolytes are introduced to purge off the electro-migration effect. Therefore, the stripping voltammetric response of the peaks of current for the metal ions determination was also assessed by varying the nature of the stripping medium.
7. Conclusion and perspectives
In summary, non modified electrochemical sensor exhibited week binding and week adsorption capacity. The introduction and modification of surface functional groups was explored to improve the chemical selectivity and charge density at the active surface. The detection capacity can be improved through attachment of functional group having high affinity towards environmental pollutants. The electrochemical detection depends on the nature and structure of sensing electrode. The sensitivity and selectivity are critical core sites which enhances electrochemical analysis. CNTs enable faster transfer of electrical signal due to its high conductivity and conjugated polymers provide advanced affinity towards metal ions. Functionalized CNPs can result in highly sensitive redox sensors for a number of analytes. It can be demonstrated that the modified electrode showed excellent electro catalytic activity, increase the rate of electron transfer electron and the adsorption of the pollutants (inorganic/organic)molecules on the surface of the electrode.
The authors are thankful to the authorities of L.N.T. College and R.B.B.M. College Muzaffarpur (S.K.) and Nitishwar Mahavidyalaya, Muzaffarpur (A.N.S.) for providing necessary facilities related to proceed the work. We are also grateful to the authorities of B.R.A. Bihar University, Muzaffarpur as well as Higher education department, Govt. of Bihar, Patna for their kind support in terms of academic and research development.
Conflict of interest
The authors have declared no conflict of financial interest.