A review of the advanced developments of electrochemical sensors for the detection of toxic and bioactive molecules

Rasu Ramachandran *a, Tse-Wei Chen b, Shen-Ming Chen *b, Thangaraj Baskar c, Ramanjam Kannan d, Perumal Elumalai e, Paulsamy Raja f, Tharini Jeyapragasam g, Kannaiyan Dinakaran h and George peter Gnana kumar i
aDepartment of Chemistry, The Madura College, Vidya Nagar, Madurai – 625 011, Tamil Nadu, India. E-mail: ultraramji@gmail.com
bElectroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, Republic of China. E-mail: smchen78@ms15.hinet.net
cSchool of Food and Biological Engineering, Jiangsu University, Zhenjiang – 212013, Jiangsu province, P.R. China
dDepartment of Chemistry, Sri Kumaragurupara Swamigal Arts College, Srivaikuntam-628 619, Thoothukudi, Tamil Nadu, India
eCentre for Green Energy Technology, Madanjeet School of Green Energy Technologies, Pondicherry University, Puducherry – 605 014, India
fDepartment of Chemistry, Vivekananda College of Arts and Science, Agastheeswaram, Kanyakumari – 629 004, Tamil Nadu, India
gSethu Institute of Technology, Pulloor – 626 115, Kariapatti, Tamil Nadu, India
hDepartment of Chemistry, Thiruvalluvar University, Vellore – 632 115, Tamil Nadu, India
iDepartment of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India

Received 25th May 2019 , Accepted 3rd September 2019

First published on 6th September 2019


Herein, the design and development of novel sensitive and cost-effective electrochemical sensors based on various classes of nanocomposites fabricated by different methods for the detection of biomolecules (glucose, dopamine, ascorbic acid and uric acid), heavy metal ions (Pb2+, Hg2+, Cd2+ and As3+) and environmental pollutants (hydrazine, nitrobenzene, nitrophenols and pesticides) are reviewed. Furthermore, the recent developments of electrochemical sensors, biosensors and pesticide sensors constructed using different nanocomposite materials having different morphologies for the reactive monitoring of food additives, human health and environmental safety are reviewed and presented. Interestingly, electrochemical techniques have received significant attention due to their remarkable durability; moreover, potential candidates for implementation in the real-time detection of analytes can be found in the recently reported studies. Advanced state-of-the-art nanostructured materials, such as nanorods, nanocubes, nanoneedles, nanoflowers, nanotubes and nanobundles, have the potential for application in electrochemical sensing platforms as they demonstrate high-performance electrochemical sensitivity, selectivity and long-term stability.


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Rasu Ramachandran

Dr Rasu Ramachandran is an Assistant Professor of the Department of Chemistry, The Madura College, Madurai, Tamil Nadu, India. He obtained his doctoral degree from Madurai Kamaraj University, Madurai. He obtained a Master of Philosophy degree from Madurai Kamaraj University, Madurai and a Master of Science degree from The Madura College, affiliated by Madurai Kamaraj University, Madurai, Tamil Nadu, India. He visited the Dr Shen-Ming Chen Laboratory, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Chung-Hsiao East Road Taipei, Taiwan 106 (ROC). His main research areas include the development of carbon-based nanocomposites for electrochemical sensors, biosensors, fuel cells and supercapacitor applications.

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Tse-Wei Chen

Mr Tse-Wei Chen is studying for his Master degree in department of Chemical Engineering and Biotechnology at National Taipei University of Technology. He received B.Sc. (2016) degrees in Chemistry from Fu-Jen Catholic University, Taiwan. He is specialising in nanomaterial synthesis and application for electrochemical sensors and biosensors. His research interest also includes studies on nanomaterials for different applications.

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Shen-Ming Chen

Dr Shen-Ming Chen received his B.S. Degree in Chemistry in 1980 from National Kaohsiung Normal University, Taiwan. He received his M.S. Degree (1983) and Ph.D. degree (1991) in Chemistry from National Taiwan University, Taiwan. He is currently a professor at the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan. His current research interests include electroanalytical chemistry, bioelectrochemistry, fabrication of energy conservation and storage devices and nanomaterial synthesis for electrochemical applications. He has published more than 700 research articles in SCI journals.

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Thangaraj Baskar

Dr Baskar Thangaraj is a Postdoctoral Fellow of the School of Food and Biological Engineering, Jiangsu University, Zhenjiang, China. He received his doctoral degree on bionanocatalysts for biodiesel production from the faculty of Chemical Engineering, Tsinghua University, Beijing, China. He carries two postgraduate degrees M.Sc. Physics and M.Sc. Energy Sciences from Madurai Kamaraj University and a Master of Philosophy degree (Energy Studies) from The Gandhigram Rural Institute, Deemed To Be University in India. He has many research papers in the field of biodiesel and designed reactors, and written a chapter for a book and published several reviews in reputed journals. His basic interest is on heterogeneous nanocatalysts, immobilization of biocatalysts and application of carbon quantum dots.

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Ramanjam Kannan

Dr Ramanjam Kannan obtained his undergraduate degree in Chemical science from Madurai Kamaraj university (India), Postgraduate degree in Chemical science from Gandhigram University (India) and received his Doctoral degree from Anna University (India) in 2013. Then continued his Postdoctoral research in the Prof. Dong Jin Yoo group in Chonbuk National University, RoK. In 2015, He was selected for Korean Research Fellow programme and worked as Assistant Professor in Department of Energy Storage and Conversion, Chonbuk National University, RoK. He is presently working as Assistant Professor at Sri Kumaragurupara Swamigal Arts College, Srivaikuntam, Thoothukudi Dist, India. His research interest include fuel cells, catalysis, biosensor, photochemical reaction etc.

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Perumal Elumalai

Dr Perumal Elumalai received his M.Sc degree in chemistry from Muthurangam Govt. Arts College, Vellore, India in 1997 and PhD degree from the Solid State & Structural Chemistry Unit, Indian Institute of Science, Bangalore, India in 2004. Currently, he is Associate Professor in Department of Green Energy Technology, Pondicherry University, India. His research interests are on high temperature gas sensors, lithium-ion batteries, supercapacitors and electrocatalysis.

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Paulsamy Raja

Prof. P. Raja is working as an Assistant Professor in the Department of Chemistry, Vivekananda College of Arts and Science, Agastheeswaram, Kanyakumari – 629 004, Tamil Nadu, India. He has published in one international journal. He is doing his Doctoral degree under the guidance of Dr Rasu Ramachandran, Department of Chemistry, The Madura College, Maudrai, Tamil Nadu, India. His main research areas are carbon based nanocomposite electrode materials for electrochemical applications.

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Tharini Jeyapragasam

Dr J. Tharini, She completed her doctoral degree from Madurai Kamaraj University, School of Chemistry, Madurai. She obtained her doctorate degree from madurai kamaraj university under the guidance of Prof. Dr R. Saraswathi, Madurai Kamaraj University, Madurai. She was teaching at Sethu Institute of Technology, Pullor. Currently she is doing independent research in Rajarajan Institute of Science, Madurai. Her research interests include electrochemical sensor, biosensor, adsorption and kinetic studies.

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Kannaiyan Dinakaran

Dr K. Dinakaran is working as an Associate Professor in the Department of Chemistry, Thiruvalluvar University, Vellore-632 115, India. He has published more than 55 articles in reputed scientific journals. His main research areas include the development of advanced polymer based nanocomposite materials for biosensors applications.

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George peter Gnana kumar

Dr G. Gnana kumar is working as an Assistant Professor in the Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, India. He has published more than 110 articles in reputed scientific journals and has contributed chapters to books. His main research areas include the design and development of graphene-based electrode materials, graphene supported electrocatalysts and polymer nanocomposites applicable for a number of electrochemical applications, including electrochemical sensors, fuel cells, solar cells, and lithium–air batteries.


1. Introduction

Hydrazine has emerged as a highly promising material for various applications such as in explosives, rocket propellants, fuel cells, pesticides, emulsifiers and catalysts.1 The laboratory usage of hydrazine can cause environmental hazards because of its high toxicity, which can lead to blood abnormalities as well as damage to the internal organs (liver and kidneys).2,3 Hydrazine plays a vital role in the construction of polymers, pharmaceuticals and pesticides.4 The hydrazine threshold limit has been set as 10 ppb, and hydrazine has been classified as a probable human carcinogen and neurotoxin and graded as B2 by the United State Environmental Protection Agency (USEPA) and World Health Organisation (WHO);5 the widely used carcinogenic hydrazine has been analysed by sensitive and selective analytical techniques to detect its low concentration levels (nanomolar to picomolar). Several analytical methods, such as electroanalytical,6 chromatography,7 chemiluminescence8 and titrimetric methods,9 have been successfully applied for the determination of hydrazine. Among the analytical techniques, the electrochemical method has unexpected promise for the detection of hydrazine contaminants because of its high sensitivity, selectivity and better detection values.10,11 Generally, hydrazine oxidation is a slow process and requires the modification of the electrode surface with metal nanoparticles that increase the electrocatalytic activities of the electrode.12 Liu et al.13 have recently highlighted environmental affected toxic compound, the reported limit of detection value is 0.02 μM. Bansal and co-workers used zirconia nanoparticle-supported gold (ZrO2/Au) composites, which showed excellent quantitative estimation of hydrazine from real water samples.14 An environmentally friendly gold-supported copper dendritic nanostructured (D-AuCuNS) electrode was prepared by Gowthaman et al.15 for the ultrasensitive and selective detection of hydrazine from water samples in 0.2 M PBS, which displayed the detection limit value of 0.12 nM. The modification of tyrosinase and carbon-based electrodes endowed the electrochemical sensor with high electrochemical activity and good electrode stability. Moreover, the inhibitory effect of tyrosinase could lead to an amperometric response with low micromolar levels and good precision.16 The increased responsiveness of electrochemical sensors based on polypyrrole-decorated stainless steel electrodes appears to be a very attractive mode of sensing; the gold nanoparticle-based polypyrrole (Au/Ppy) electrode can offer an increased surface area and lead to the formation of homogeneously distributed particles that result in good electrocatalytic properties.17 The various advantages of citrate-stabilized gold nanosheet (GNS) electrodes may boost the ultrasensitive electrocatalytic activity towards the oxidation of hydrazine.18 The application of a modified palladium nanoparticle/reduced graphene oxide (PdNP/rGO) composite in an electrochemical sensor has been reported by the Krittayavathananon group,19 who has developed a chronoamperometry technique for the quantitative determination of hydrazine in 0.2 M PBS (Fig. 1). The as-prepared novel Pd/NPs/rGO composite showed a sharp peak for the oxidation of hydrazine. The limit of detection (LOD) values were optimized through the chronoamperometry technique, and the reported LOD value was 7 nM. Poly(alizarin yellow R)-supported Ag@C-core–shell nanospheres are a special type of electrode materials that have the specialized characteristics of lower oxidation potential, high sensitivity and easy accessibility; however the uniqueness of the fabricated composites lies in their great electrocatalytic properties for the monitoring of real water samples.20 Moreover, an electrochemically reduced graphene oxide (CNT-ErGO) composite film was deposited on a gold nanoparticle (AuNP)-supported electrode, and the resulting composite surface made the sensor highly sensitive towards hydrazine; in addition, the very high LOD of up to 0.065 μM was achieved.21 Among these approaches, electrochemical deposition is a rapid method and easily modifies gold nanoparticle-sustained SWCNT nano horn composites for extensive use in the development of a high-performance hydrazine sensor.22 Considering their potential applications, carbon-coated Cu/CuO nanocomposites have been prepared by a cost-effective calcination method. The fabricated nanocomposites were very effective in the determination of hydrazine.23 Among these electrodes, the AuNP-decorated AC-60-modified screen-printed carbon electrode (SPCE) composite showed enhanced electrocatalytic properties with lower overpotential for hydrazine oxidation.24 Xu et al.25 have developed a sensitive and selective hydrazine sensor-based metal oxide (CuxO)-decorated three-dimensional poly(3,4-ethylenedioxythiophene) (3D-PEDOT) hybrid composite. The composite has received significant attention due to its enhanced conductivity and ten-fold lower LOD.
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Fig. 1 (a) Cyclic voltammograms of PdNPs/rGO in 100 mM hydrazine in 0.2 M PBS (pH 7.4) at a scan rate of 0.1 V s−1; (b) chronoamperograms of PdNPs/rGO in 0.2 M PBS (pH 7.4) with successive injections of hydrazine at different concentrations (0.1–1000 μM) at a rotation speed of 2000 rpm. Reprinted (adapted) with permission from ref. 19. Copyright (2014) American Chemical Society.

Generally, nitrobenzene (NB) can be used as a starting material for the synthesis of dyes, pesticides and explosives. However, due to the highly toxic nature and adverse effects of NB on human health, the detection and monitoring of the concentration of NB is of prime importance.26 In this regard, nitrogen-doped hollow carbon nanospheres (NHCPs) have been prepared by a template method for the highly sensitive determination of NB.27 Similarly, a reduced graphene oxide-supported nickel oxide tetraphenylporphyrin (GRGO/Ni-TPP) composite has been prepared by an environmentally friendly method; the as-prepared nanocomposite has large surface area, and it can be used for the detection of NB. The sensor system displayed a linear range between 0.5 and 878 μM, and the LOD value was 0.14 μM.28 The well-known nitrophenolic compounds are widely used in the creation of chemical intermediates, pesticides, and synthetic dyes, petroleum and textile industries. These compounds can permeate soil, water and are highly toxic to humans and animals. The European Union Agency has listed phenolic compounds as environmental pollutants of significant concern, and methods are needed to detect of these compounds in the environment.29,30

Basically, pesticides are organic compounds that are applied in the field of agriculture for the control of pests; however, pesticide residues can enter the soil, water and food and cause contamination. Moreover, there is a great scope for the development of electrochemical portable devices for the monitoring of toxic pesticides.31–33 Various nanocomposite electrode materials have been widely used for the detection of non-enzymatic glucose sensors. Studies have been reported on the use of stretchable capillary microfluidics-integrated nanoporous gold,34 PtNiNP-graphene,35 CuO/PANI-NF/FTO,36 and NiO/PANI37 for the sensing of non-enzymatic glucose molecules. In addition, a Ni(OH)2-decorated sulfur-doped carbon nanoparticle (SDCN) matrix was developed, and the performance of the biosensors based on it was examined; the results indicated an excellent sensitivity (LOD = 28 nM) of these biosensors and good electrode stability.38 Moreover, PANI@CuNi nanocomposites have been particularly used as electrode materials owing to their excellent electrochemical activity and effectiveness in real sample analysis;39 on the other hand, copper-doped zinc oxide (Cu–ZnO) nanoparticles have been highlighted as having more electroactive sites and providing more electron transfer for glucose oxidation.40 The directly grown of 3D porous (Ni–Co)3S4-supported rGO-PEDOT hybrid film has significant attention for the construction of non-enzymatic glucose sensors with the detection limit value of 0.503 μM.41 Recently, L-cysteine-mediated nanostructured Ag–Au bimetallic fractals have been reported by Qureshi et al. for the sensitive detection of DA. In this process, the thiol group helps to stabilize the MNPs.42 An amide-functionalized graphene–chitosan matrix was developed for the electrocatalysis of neurotransmitters. This matrix served a cost-effective alternative to precious metal catalysts and exhibited excellent sensitivity and selectivity towards DA and UA.43 Wang et al. have developed flower-like manganese oxide/MWCNT on electrochemically pre-treated GC electrodes to enhance the sensibility (oxidation current) with respect to the analytes.44

Heavy metals at high concentrations are pollutants that are extremely harmful due to their environmental impact and the resultant impairment of human health. Therefore, it is necessary to identify an appropriate tool for the quick detection and monitoring of these pollutants.45 The electrochemical method, as an alternative to conventional spectroscopic techniques, has been accepted as an efficient method for the sensing of heavy metal ions due to its selective sensitivity, rapid analysis, low energy consumption, portability and low cost.46 Many solid electrodes made of gold, gold wire, platinum and certain chemically modified metals are being used to detect the abovementioned pollutants. Working electrodes made of gold are observed to be superior due to their high affinity for metal ions, which can improve the pre-concentration effect. Gold-based electrodes are in limited use as sensors since undesirable gold amalgam may be formed, which is known to destroy the surface features of the electrodes.47 Gold nanoparticles (AuNPs) are, however, widely used in analytic chemistry. Modified electrodes have been observed to be effective sensors. They serve as random arrays of microelectrodes and show distinct advantages over conventional macro electrodes. They are characterised by increased mass transport, decreased resistance in solution, high sensitivity and better signal-to-noise ratios.48 In addition, surface-modified electrodes are miniaturized devices due to the small amounts of analytes (typically less than 100 μL).49 The electrochemical performances of different modified electrodes, electrochemical methods, various kinds of analytes, pH and the detection limits reported in the literature are also provided in Table 1.

Table 1 Summary of the various classes of electrode materials for the sensing of toxic and bioactive molecules
Electrode materials Analyte Medium/pH Technique/methods Detection limit Ref.
Lutetium bisphthalocyanide Caffeic acid 3.6 Cyclic voltammetry 3.12 × 10−5 M 50
Ultrafine wavy PtRu NWs Dopamine 7.2 Amperometry 50 nM 51
Ferrocyanyl tethered (Fc-D) Hydrazine 9.0 Cyclic voltammetry 100 fg mL−1 52
SWCNT/pyrene cyclodextrin p-Nitrophenol 5.0 Differential pulse voltammetry 0.12 ppb 53
Propylene carbonate Hydroquinone 7.0 Cyclic voltammetry 4.9 ppm 54
MWCNT-Nafion/GCE Eu3+ 4.5 Square wave voltammetry 0.37 nM 55
MnFe2O4@Cys/GCE Pb(II) 5.0 Square wave voltammetry 0.5 μM 56
Screen printed electrode SPE/SWCNTs/BiF Cd(II) 4.5 Square wave voltammetry 0.2 μg L−1 57
Pb(II) 0.4 μg L−1
Ppy/rGO Pb2+ 1.0 Square wave anodic stripping voltammetry (SWASV) 0.3 nM 58
Calixarene functionalized rGO (CA/rGO/GCE) Pb2+ 6.0 SWASV 2 × 10−11 M 59
Carbon fiber rod (CFR) Pb2+ 4.2 Cyclic voltammetry 0.1 μg L−1 60
Cd2+ 0.2 μg L−1
Zn2+ 1.0 μg L−1
GO-MWCNTs hybrid nanocomposite Pb2+ 4.5 Differential pulse anodic stripping voltammetry (DPASV) 0.2 mg L−1 61
Cd2+ 0.1 μg L−1
Glutathione coated magnetic nanoparticles (GSH@Fe3O4) Pb2+ 4.5 SWASV 0.182 μg L−1 62
Cd2+ 0.172 μg L−1
MnFe2O4/GO Pb2+ 5.0 SWASV 0.0883 μM 63
3D graphene foam/sodium dodecyl benzene sulfonate hemimicelle Pb2+ 5.5 SWASV 0.0145 nM 64
Screen printed electrode (Bioassay) Carbomate 7.4 DPV 10 ng g−1 65
Organophosphate 30 ng g−1
Magnetic bead quantum dots anti-butyryl cholinesterase (MB-QD-anti-BChE) Organophosphate 7.4 SWV 0.05 nM 66
Tyrosinase enzyme based electrode Diazinon 7.0 Amperometry 75 nM 67
Printed graphene Organophosphate 8.0 Amperometry 3 nM 68
Lateral flow test strip electrode (FTSES) Organophosphate 7.4 SWASV 0.02 M 69
Porous NiTe2 nanosheet array Glucose 13.0 Amperometry 0.12 μM 70
CuS dendrite Glucose 13.0 Amperometry 0.05 μM 71
Cu2O NPs/3D graphene Glucose 13.0 Amperometry 0.14 ± 0.01 μM 72
Cu2O/AuCu/Cu composite H2O2 7.0 Amperometry 0.14 μM 73
ZnCr2O4/MWCNTs H2O2 7.4 Amperometry 0.11 μM 74
3D mesoporous samarium oxide hydrangea microspheres H2O2 10.0 Amperometry ∼1.0 μM 75
Palladium nanocluster based composite (PdNCs/PPy) Nitrate 7.0 DPV 0.744 μM 76
0.453 μM
Electrochemically reduced holey graphene electrode (ERHG/GCE) Nitrate 7.4 Amperometry 0.054 μM 77
Cu/MWCNT/RGO Nitrate 3.0 SWV 30 nM 78
20 nM
Nafion/Ti3C2Tx (MXene) Bromate 1.0 DPV 41 nM 79


Bare electrodes have been commonly used in the fields of electrochemical sensors, biosensors, pesticide sensors and energy-storage devices. However, unmodified electrodes are likely to undergo chemical fouling and agglomeration and have less electrode surface area, catalytic poisoning nature, low mechanical strength and poor chemical stability; nanocomposites exhibit excellent opportunity and application in various technologically important fields such as in energy generation and storage, small and medium electronics and low-to-high-temperature applications. The presence of two or more materials will provide synergistic enhancement in their physical and chemical properties as compared to the case of a single material. Moreover, structural flexibility and processability to the desired size and shape related to their application can be achieved by nanocomposites; in addition, the related limitations have been overcome by the modification of nanocomposite electrodes, which are low cost and exhibit significant improvement of the current response, large electrode surface area, and excellent electrode durability.19,21,25,27,28,35,39

In general, commercial electrode (screen printing electrode and indium tin oxide) materials are costly and lead to certain problems; these issues have been overcome using developed probes (carbon-based materials, metal oxides and conducting polymers) due to their cost-effectiveness, environmental friendliness and reusability; a new revolution in nanotechnology is required for the construction of 3-dimensional-based electrode catalysts to increase the electrode surface-to-volume ratio, sensitivity, and selectivity and improve the LOD as well as the electron-transfer process. This is an emerging area in the fields of energy conversion, signal processing, robotic science and medical imaging.

2. Environmental pollutant molecules

2.1. Nitrobenzene

Nitrobenzene (NB) is a hazardous organic material that finds applications in various fields such as in petroleum refineries, pesticides, herbicides, insecticides, dyes, shoe polishes, and soaps.80 Its high concentration in water can affect human health and environment. The maximum concentration of NB allowed in drinking water is 1 mg L−1.81 Therefore, conversion methods (chemical and electrochemical reduction process) have been used for the reduction of nitrobenzene, which is required for environmental protection and public health.82 Various analytical techniques such as ultraviolet spectrophotometry (UV), high-performance liquid chromatography (HPLC), fluorescence quenching, gas chromatography (GC)83–86 and electrochemical87,88 methods have been used for the quantitative analysis of NB. Among these, the electrochemical method has been used as a reliable technique for the determination of both organic and inorganic materials. This method is considered simple, cost-effective, highly sensitive and easy to operate on a long term basis.89 Rastogi et al.90 reported a method for the study of the mechanistic and kinetic parameter-oriented electrocatalytic reduction of NB using a palladium nanoparticle-supported biodegradable polymer (guar gum-grafted polyacrylamide) and silica (Pd-GG-g-PAM-silica) nanocomposite. A simple one-step microwave-assisted method has been developed to produce a Pt ensemble@macroporous carbon hybrid nanocomposite, which is sensitive, anti-interference and highly stable for NB determination by an electrochemical sensor.91 On the other hand, a silver nanoparticle-decorated nitrogen-doped ordered mesoporous carbon electrode was fabricated by a one-pot strategy to detect the concentration of NB in 0.5 M KCl by differential pulse voltammetry (DPV).92 Moreover, a de-agglomerated alumina-polished glassy carbon (γ-Al2O3/GCE) electrode has been employed for the electrochemical reduction of NB because of its practical applications, high sensitivity and cost-effectiveness.93 Velmurugan et al.94 have reported that the β-cyclodextrin (β-CD)-adsorbed graphene oxide (GO) disposable composite can greatly enhance the rate of the electrochemical reduction of NB. Recently, α-Fe2O3 nanorods have gained significant attention as an efficient electrode material for the electrochemical sensing of NB.95 The successful modification of a nickel organic framework-capped polyvinylpyrrolidone (Ni-MOF-PVP) electrode has received significant attention for the sensitive analysis of NB.96 The 2D network structure of the tetra-pyridyl functionalized Ag(I)-calix[4]arene coordination polymer was employed as a fascinating platform for the effective determination of degraded NB.97 Zhang et al.98 have demonstrated the use of a cobalt-based macroporous carbon (Co-MOF) composite as an excellent electrochemical sensing platform for monitoring the reduction of NB for the first time. Recently, Gowthaman et al.99 prepared Au–Ag spherical-shaped nanoparticles by a chemical reduction method for the efficient electrochemical reduction of hydrogen peroxide and nitrobenzene. The headspace adsorptive carboxylated MWCNTs-GCE electrode has been fabricated through a drop-casting method, and it has received significant attention for the detection of nitrobenzene and nitro toluene in real sample analysis.100 The possible electrochemical generation of a microporous polymer network (MPN) electrode with an increased BET surface area (1300 F g−1) as well as a stable network has been considered for boosting the performance of electrochemical sensor devices (Fig. 2); the resulting cathodic current response has displayed an improved performance by up to two orders of magnitude, and the electron-transfer process becomes a diffusion-controlled process.101
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Fig. 2 Linear scan voltammograms of PTPTCz-modified GC electrodes for 3.0 μM (a) nitrobenzene (NB) and (b) 1,3,5-trinitrobenzene (TNB) in aqueous 0.2 M KCl and 0.1 M PBS (pH 7.4) solution at various scan rates. Reprinted (adapted) with permission from ref. 101. Copyright (2015) American Chemical Society.

2.2. Nitrophenol

Nitrophenols are widely used in various industrial fields such as in the fields of pesticides, dye-stuffs, pharmaceuticals, and explosives.102 Industrial effluents containing phenolic compounds are inevitably released into the soil, leading to serious environmental contamination; 4-nitrophenol at high concentrations is harmful to human health because of its carcinogenicity and damage to the kidneys, liver and the central nervous system. It also causes health issues such as nausea, headaches, cyanosis and drowsiness.103 Therefore, the allowable limit set by the United States Environmental Protection Agency (EPA) for 4-NP contaminants is 60 ppb in drinking water.104 Several methods, such as high-performance liquid chromatography (HPLC),105 capillary zone electrophoresis,106 flow injection analysis,107 gas chromatography-mass spectrometry (GC-MS)108 and fluorescence,109 are used for the quantification of 4-NP. However, these techniques are highly expensive, less sensitive, easily interfered by other compounds and time-consuming. Electrochemical methods offer great potential for the study of analytes due to their low-cost and ease of operation. Sulfur-doped graphitic carbon nanosheet (S-GCN)-modified screen printed electrodes have been successfully applied in the determination of 4-NP, which showed good electrocatalytic activity, a better LOD (0.0016 μM), a wide linear response range from 0.05 to 90 μM and greater sensitivity (Fig. 3).110 Hu et al.111 have successfully fabricated nitrogen-doped hierarchical porous carbon with a chitosan (CTS/NPS/ITO) composite matrix for electrochemical application in real water sample analysis. Recently, novel structural analogues and a convenient strategy for molecularly imprinted polymer electrodes modified with a vinyl group-functionalized graphene composite have been considered to be a good candidate for the highly sensitive electrochemical sensing of 4-NP.112 Wei et al.113 have proved that the polyfurfural film-based glassy carbon electrode is a remarkable and promising material for the simultaneous determination of nitrophenol isomers. An electrochemical sensor made of a molecularly imprinted polymer-supported graphene oxide (MIP/GO) composite revealed excellent sensitivity toward p-NP in real water sample analysis.114 The study of Wiench et al.115 has also showed that GCE/rGO is a novel platform for the sensing of 4-NP. This platform displayed good sensitivity, reliable recoveries and long-term durability in the real sample analysis. Moreover, a new kind of inorganic–organic coating containing Prussian blue, polyazulene, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-[(E)-2-azulene-1-yl)vinyl]thiophene hybrid has been developed for the electrochemical reduction of 4-NP and dopamine (DA) oxidation.116
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Fig. 3 (a) Schematic of the preparation route of S-GCN; (b) the DPV response of the S-GCN/SPCE under consecutive additions of 4-NP within a total dosage range of 0.05–90 μM. Reprinted (adapted) with permission from ref. 110. Copyright (2018) American Chemical Society.

3. Electrochemical sensing of pesticides

Generally, pesticides are chemical compounds that can be used to destroy or control pests such as mice, insects, weeds, microorganisms and fungi. Most of the pesticides are hazardous in nature and can kill the organisms by causing deadly diseases. Some of the toxic or hazardous pesticides that are used in the agricultural fields are organophosphates, parathion, carbamate, dichlorodiphenyltrichloroethane (DDT), mercury and arsenic derivatives.117–120 Pesticides can exhibit harmful effects and may cause serious effects in human beings such as infertility, neurological diseases, cancer and respiratory problems. As per the UK advisory board, the set limit for pesticide residues in foodstuffs and drinking water is in the range from 0.3 to 400 μL−1.121

3.1. Organophosphates

Due to the highly neurotoxic nature of organophosphates (OPs), they have been commonly used as pesticides and chemical-based nerve agents to cause and threaten homeland security, animals, human health, ecosystems.122 OPs are the derivatives of amides and phosphonic acids, which can regulate household and agricultural pests.123 Even trace amounts of OPs are highly toxic, and they are mainly classified on the basis of their toxicity such as toxicity class I for highly toxic and toxicity class II for moderately toxic OPs.124 Immobilization is a process in which the enzyme is strongly bound to the modified electrode surface for the fast determination of pesticides. Acetylcholinesterase has been widely used for the detection of pesticides. A schematic of the electrochemical biosensors based on AChE is shown in Scheme 1.125
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Scheme 1 Schematic of the acetylcholinesterase mechanism.

Cancar et al.126 have constructed novel acetylcholinesterase biosensors and showed that conjugated polymers are an excellent platform for the detection of OP pesticides. Another biosensor based on the ZrO2/AuNP electrode was developed for the detection of OPs in a 0.1 M KCl aqueous solution. The sensor showed a very low detection limit of 1 ng mL−1 using SWV.127 Highly sensitive immune-captured MWCNT–Au nanocomposites wrapped with a screen-printed carbon electrode (SPCE) were immobilized with AChE for the biomonitoring of exposed OP pesticides.128 Recently, a new strategy for the construction of electrochemical sensors has been established using a Coomassie Brilliant Blue dye surface-confined carbon black nanoparticle (GCE/CBnano@CoomBB)-modified electrode. The composite exhibited an agglomerated particle-like structure with strong interactions between CBnano and CoomBB. The efficient electrochemical performance also included the LOD = 20 μL (Fig. 4).129


image file: c9qi00602h-f4.tif
Fig. 4 (a) The TEM image of CBnano@CoomBB; (b) the effect of the concentration of triazophos in pH 7 PBS; (c) amperometric it response of GCE/CBnano@CoomBB. Reprinted (adapted) with permission from ref. 129. Copyright (2018) American Chemical Society.

3.2. Methyl parathion

Over the past two decades, parathion has been used as a pesticide, which is highly toxic and readily absorbed through the skin.130 Due to the presence of methyl parathion in water, soil and food, their bioaccumulation affects human health and environment.131 A novel electrochemical co-deposition method has been developed for the fabrication of a nanosilver/Nafion composite, which exhibits excellent electrocatalytic activity and good reproducibility.108,132 Gong and Wang133 developed an electrochemically fabricated AChE-supported gold nanoparticle and polypyrrole nanowire (AChE–Au–PPy/GCE) composite for the rapid detection of toxic compounds. Multi-walled carbon nanotube-based cerium oxide gold nanoparticle-supported (MWCNTs–CeO2–Au) composites have received significant interest because of their unique features such as cost-effectiveness, efficient enrichment and electrochemical detection of MP.134 A polyethyleneimine and silica gel composite-modified electrode has been employed to detect parathion from real spiked vegetable samples.135 Nickel oxide-supported screen-printed electrodes have been widely used to develop non-enzyme based electrochemical sensors. They have attracted significant attention, and their nitro groups are reduced in the estimated pesticide molecule.136 The crumpled and wrinkled structure of a graphene–chitosan (GR–CS) composite has received significant attention because its sheet-like structure is connected to its electrochemical properties.137 Oliveira et al.138 reported the electrochemical quantification of methyl parathion using a rich functional group-based carbon paste electrode-modified biochar composite. The acid-treated composites could achieve high sensitivity with good accuracy for the detection of MP in tap water. Vital functions of the Fe3O4@Au magnetite nanocomposites have attracted enormous interest as for the development of highly sensitive and selective on-site detection of MP.139 Tan et al.140 have reported the preparation of a cationic water-soluble pillar[5]arene-modified reduced graphene (CP5-rGO/GCE) nanocomposite, which exhibits excellent electrocatalytic activity and is a promising tool for the rapid, facile and sensitive study of MP in soil and wastewater samples. The crumpled structure of the electrochemically deposited graphene/gadolinium Prussian blue (GdHCF/GNs) nanocomposite has been considered as a potential electrode material for the quantification of MP in environmental water samples.141 Similarly, the crumpled and granular, spherical, eco-friendly reduced graphene oxide-supported palladium tetraphenylporphyrin (RGO/Pd-TPP) nanocomposites have emerged as new rising star materials because of their effective electrocatalytic activity towards the reduction of MP and oxygen reduction (ORR) (Fig. 5). The MP electron-transfer mechanism was also estimated from the K–L plot and is described in eqn (3).142
 
image file: c9qi00602h-u1.tif(3)

image file: c9qi00602h-f5.tif
Fig. 5 (a) & (b) Low- and high-magnification TEM images of RGO/Pd-TPP and (c) the DPV response for the addition of different concentrations of MP in a 0.05 M PB solution. Reprinted (adapted) with permission from ref. 142. Copyright (2017) American Chemical Society.

3.3. Carbamate

Carbamates are mainly used for agricultural pest control and are classified into three types: N-methylcarbamate, thiocarbamate and dithiocarbamate;143 they are among the most carcinogenic pesticides, cause endocrine disruption and are environmentally hazardous; as such, they must be monitored in water and food as enforced by the United States Environmental Protection Agency (USEPA).144 A biosensor has been developed based on the immobilization of acetylcholinesterase on a polyaniline-modified MWCNT core–shell electrode for the detection of carbonyl with a detection limit of 1.4 μmol L−1 in real sample (cabbage, broccoli and apple) analysis.145 Ni et al.146 have studied four carbamates (propoxur, isoprocarb, carbaryl and carbofuran) in three different water samples (tap water, lake water and pond water) obtained from different places in Nanchang. The immobilization of AChE on 3-carboxyphenylboronic/reduced graphene oxide-modified gold nanocomposite electrodes has been extensively investigated as an attractive pathway for the electrochemical detection of organophosphates and carbamates with good repeatability and high sensitivity.147 Silva and Cesarino148 prepared an immobilized GCE/rGO/AChE biosensor, optimized the electrochemical (DPV) detection limit value up to the nanomolar level and monitored carbonyl compounds in tomato samples. Another pesticide sensor was developed using boron-doped diamond thin-film electrode for the detection of N-methylcarbamate pesticide, with a lower detection limit of 5–20 ng mL−1 (Fig. 6).149 Kelgraf-F-graphite represents a class of carbon-based electrode materials that have been extensively used in electrochemical studies for the detection of carbamate pesticides in real sample analysis due to their high electrocatalytic activity (the achieved LOD is 40 pg).150
image file: c9qi00602h-f6.tif
Fig. 6 (a) Cyclic voltammograms for the phenolic derivatives of 50 μM carbaryl at a fresh diamond electrode in 0.1 M NaClO4 containing 0.02 M NaOH. The pH was adjusted to 3.2 by adding glacial acetic acid. (b) Background voltammograms at the used diamond electrode (a) before and (b) after electrochemical treatment at 2.5 V vs. SCE for 10 min; (c) FIA-ED results for the diamond electrode using 20 μL injections of 50 μM pre-hyrolyzed carbaryl. Applied potential, 0.9 V vs. Ag/AgCl. Reprinted (adapted) with permission from ref. 149. Copyright (2002) American Chemical Society.

4. Bioactive molecules

4.1. Glucose

Recently, glucose sensors have been significantly developed for the detection of glucose levels in human blood because millions of people are suffering from diabetes.151 Diabetes can produce metabolic diseases, and the measurement of glucose level in blood is significant in in vitro or in vivo medical applications. Numerous methods, such as optical, fluorescence, acoustic and electrochemical methods, have been used for the detection of glucose molecules.152–155 In particular, electrochemical techniques have received significant attention for their cost-effectiveness, ease of operation, high sensitivity, selectivity and remarkable stability during the electrochemical sensor applications.156 Wang et al.157 designed and developed an IrO2@NiO core–shell nanocomposite, which could be a promising candidate for non-enzymatic glucose sensing. Notably, an electroactive cobalt phosphate nanostructured electrode has been prepared by a scalable crystalline method, which can be used for the study of glucose in human serum.158 Recently, a free-standing three-dimensional graphene-based cobalt oxide (3D graphene/Co3O4)-supported nanowire composite has been investigated for energy storage device (1552 F g−1) as well as a state-of-the-art non-enzymatic glucose sensor applications with the lowest detection limit value of 25 nM (Fig. 7).159 Multi-functional and low-cost 3D-nickel phosphate nano-supported micro flake nickel foam (Ni3(PO)4·8H2O/NF) electrode materials are most often used for multiple applications in electrochemical energy storage devices and non-enzymatic glucose sensor applications.160 Among them, the nickel ferrite-based chitosan (CHIT/NiFe2O4NPs) composite has been explored as a more efficient candidate owing to its high sensitivity, remarkable stability and excellent catalytic activity towards glucose oxidation.161
image file: c9qi00602h-f7.tif
Fig. 7 (a) Low-magnification SEM images of the graphene/Co3O4 nanowire composite; detection of glucose in a 0.1 M NaOH solution using the 3D graphene/Co3O4 composite electrode. (b) CV curves measured at different scan rates (5, 10, 20, and 50 mV s−1). (c) Amperometric response to the addition of 10 μL of different analytes to 20 mL of electrolyte (0.1 M NaOH). AA = ascorbic acid; UA = uric acid. Reprinted (adapted) with permission from ref. 159. Copyright (2012) American Chemical Society.

4.2. Dopamine, ascorbic acid and uric acid

Dopamine (DA) is a major active neurotransmitter and has an important role in the message transport and reward pathway processes. The recent progress in the electrooxidation of DA has been discussed in detail. Carbon nanostructures have been developed by a number of groups for the selective and sensitive detection of dopamine (DA). Carbon nanoball-aggregated network-based aerogels were derived from citrus maxima peels by the Zhou group162 and used as electrode modifiers to electrochemically detect dopamine (DA) and common interfering agents such as ascorbic acid (AA), hydrogen peroxide (H2O2) and uric acid (UA). Due to the higher surface area and pore volumes of 446.39 m2 g−1 and 87 nm, respectively, the carbon nanoballs made better electrocatalytic platforms for the sensitive detection (lowest detection) of hydrogen peroxide (H2O2) at 3.53 μ mol−1.163 Very few authors have tested the carbon paste electrochemical (CPE) platform for DA detection. Holmberg et al.164 have developed an electrochemical sensor for DA by a poly(diallyldimethylammonium chloride) graphene oxide (GO)-multi-walled carbon nanotube (CNT)-combined CPE. This electrode showed excellent electrocatalytic activity towards DA and 5-hydroxytripelennamine with the lower detection limits of 16 nm and 9.8 nm, respectively, due to the synergistic polymer–GO–CNT interactions, which facilitated the lower detection of neurotransmitters. Polyacrylonitrile-derived stress -activated pyrolytic carbon nanofibers infused with CNT were developed by Holmberg et al. and used for the electrocatalytic detection of DA in the presence of AA and UA. This sensible electrochemical activity was achieved in the presence of nitrogen (a heteroatom) with edge plane deformation in the carbon nanostructures. Similarly, Behan et al.165 also developed N-doped carbon nanostructures and found that N-doped carbon showed enhanced catalytic activity towards DA. Theoretical (DFT) and electrochemical results have showed that the enhanced electrochemical response is due to the greater adsorption of DA at the N-atom in the carbon than that in the case of the non-doped carbon. Wei et al.166 have studied phosphorus and nitrogen-doped plasma-etched graphene, which showed excellent catalytic activity towards DA. Moreover, this effect is accounted by the hetero-atoms such as phosphorus and nitrogen, which synergistically enhances the electrocatalytic activity through the selective adsorption of the analyte.

The catecholamine electrocatalysis on basal plane highly oriented pyrolytic graphite (HOPG) and graphite-HOPG electrodes has been reported by Alvarez and Ferapontava.167,168 They tested the catecholamine electrocatalysis through the ‘negative electrocatalysis’ concept introduced by Compton et al.;169 with the difference in over-potential, the interference compounds may be removed or by-pass the analysis of the main analyte. The same author expressed that the HOPG/graphite electrode sensed the DA oxidation with excellent activity and selectivity towards the detection of DA. Commercial graphene, having edge defects, is expected to enhance the catalytic activity. Fu et al.170 have used these graphene inks as electrocatalysts after the removal of impurities by simple water washing through the immersion method. The commercial graphene-ink-casted electrode showed excellent electrocatalytic activity for the simultaneous detection of DA, AA and UA. The lower detection limits of 17.8 μM, (AA), 1.46 μM (DA) and 0.29 μM (UA) were achieved.

Graphene quantum dots composited/functionalized with chitosan and beta-cyclodextrin were utilized as electrode modifiers on GC electrodes for the determination of biomolecules such as L-cysteine, DA, UA, L-tyrosine, L-phenylalanine and AA detection.171 A hydroquinone [2-(5-ethyl 2,4-dihydroxyphenyl)-5,7 dimethyl 4-H pyridine]-based (carbon paste electrode) CPE was fabricated and tested for DA, epinephrine and Acetaminophen detection. The fabricated electrode showed a lower overpotential of 279 mV for epinephrine oxidation and additionally showed good response for the simultaneous detection of DA and norepinephrine.172,173 A one-pot synthesis of reduced graphene oxide supported Prussian blue nanocomposite was developed by Santos et al.174 for the study of ascorbic acid, dopamine and uric acid. The author examined the efficiency and effect of the cobalt oxide percentage on electrocatalysis and concluded that about 16 wt% cobalt oxide on the CPE exhibited the best performance of 1–20 μM of DA. A photochemically derived Prussian Blue/GO nanocomposite was used as an electrode for the detection of DA, AA, UA and H2O2. Herein, PB helped to reduce GO and simultaneously form nanocubes. These composites had the lower detection levels of 34.7 μM, 26.2 μM, and 8.0 μM for AA, DA and UA, respectively. Recently, the Tharmaraj group175 developed a poly(pyrrole)-Ag-PVP nanohybrid-based electrochemical sensor for the selective detection of DA. The authors developed the composite electrode by an in situ process, where silver nitrate acted as an oxidant and Ag derived from this acted as an electrode modifier that resulted in excellent selectivity for DA detection (Fig. 8). The two-electron transfer reaction mechanism for the selective and sensitive reversible electrochemical behaviour of dopamine using the PPy-Ag-PVP composite is presented in eqn (4).


image file: c9qi00602h-f8.tif
Fig. 8 (a) HR-TEM image of PPy-Ag-PVP, (b) DPV response of PPy-Ag-PVP, and (c) amperometric it curve for the determination of DA at the Py-Ag-PVP (2)-modified GCE in a 0.2 M PB solution (pH 7.2). Reprinted (adapted) with permission from ref. 175. Copyright (2017) American Chemical Society.

Multi-metallic nanoparticles of platinum–cobalt (Pt–Co) nanowires were developed by the Guo group,176 and the author expressed that the nanoparticles automatically manipulated their surface into a nanostructure and resulted in urchin-like nanostructures. Due to their

 
image file: c9qi00602h-u2.tif(4)
structural features, they presented a better response towards peroxide, hydrazine, DA and Acetaminophen. The Tig group177,178 developed a nanocomposite of GO/Ag/L-arginine composite for AA and DA detection; they also developed a Au–GO–poly(2,6-pyridinedicarboxylic acid) composite electrode for AA, DA and UA sensing with excellent peak separation and a lower detection of 1.764 μM, 0.017 μM and 0.16 μM for DA, AA and UA, respectively. Metal oxides and metal sulphides were also used as electrode modifiers for the electrochemical sensing applications. Recently, Yang et al.179 reported a molybdenum sulphide (MoS2) poly-PABSA nanocomposite prepared by a potentiostatic method and was used it to detect DA. The PABSA–MoS2 nanocomposite possesses a negative charge with large surface area, and its synergistic electrocatalytic activity towards DA is observed.

Similarly, the Yang group180 reported a Bi2S3–rGO composite electrode by a one-pot hydrothermal process, where the synergistic effect of Bi2S3–rGO resulted in enhanced DA electrooxidation with a lower detection limit of 12.3 nM. Liu et al.181 developed 2D nanostructures composed of a C3N4 carbon matrix containing the Co3O4/Fe3O4 nanocomposite, which exhibited excellent electrocatalytic activity for AA, DA and UA oxidation. The electrochemically reduced manganese oxide-MnO2 nanowires composite was developed by Liang et al.,182 which showed 13 times higher sensitivity towards DA detection. The lower detection limit of 1.0 nm was obtained at the modified electrode. Moreover, the modified system showed the selective detection of DA even at 100 fold AA and UA.

5. Heavy metal ions

Cadmium(II), lead(II), copper(II), arsenic and mercury(II) are the heavy metal ions that pose a serious threat to human health and the environment due to their toxicity. These ions are non-biodegradable, which can progressively concentrate in food products.183 Conventional analytical techniques, such as atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICPMS), cold vapour generation coupled to atomic fluorescence spectrometry (CV-AFS), high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS), surface-enhanced Raman spectrometry (SERS), X-ray absorption spectroscopy (XPS) and inductively coupled plasma atomic emission spectrometry (ICPAMS), have been used to analyse these ions, which are expensive and time-consuming, and thus their extensive applications are limited.184,185 Therefore, it is necessary to develop highly sensitive, rapid and low-cost methods for the analysis of heavy metal ions. The electrochemical method is the best choice to overcome certain of the problems encountered in conventional analytical techniques since it is relatively simple, cheap and readily available.

5.1. Hg2+ ions

Mercury is one of the major environmental pollutants in the world, and it originates from either natural geothermal sources or volcanic eruptions and anthropogenic activities. Mercury ions in any form are toxic, affecting the normal functions of the brain, nervous system, immune system, etc. Although the metal form of mercury is relatively harmless, it undergoes unpredictable chemical changes and makes biological interactions.185,186 A novel volumetric sensor based on amino-thiacalix[4]arene-modified graphitic carbon has been employed for the quantification of the Hg(II) ions. The sensor shows linearity in the range 2–20 pM with the detection limit of (3σ) 1 pM. The modified electrode maintained its stability over a long period without loss in its activity at ambient temperature.187 Dithizone-MCF (siliceous mesostructured cellular foams)-modified carbon paste electrode is a good sensor for detecting mercury ions because dithizone contains sulphur, which acts as a good ligand to form complexes with mercury ions. The modified electrode system response to Hg2+ is linear at 0.01–0.5 nmol L−1 (with sensitivity at 7.53 μA nmol−1 L−1) and at 0.5–10.0 nmol L−1. The effect of pH on the extraction was investigated, and its effect on the Hg2+ extraction in the electrode was also studied. The electrode response decreases at extreme pH values. At higher pH values (>4.0), the voltammetric signal of the electrode decreases due to the hydrolysis of mercury ions. Lower ranges of pH cause the protonation of the heteroatoms of dithizone present in the MCF; this decreases the complexing capability of dithizone-MCF toward the Hg2+ ions. A good response was achieved at pH 4.0.184 The electrodeposited graphene-Au (Ge-EAu)-modified electrode is used as a working electrode for attomolar mercury detection. The Ge-EAu-modified electrodes with nano Au carriers have amplified signals, which improve the response signal intensity several times. The current response decreases by 70 percent through Eau-modified electrodes than the case of graphene-Eau-modified electrodes.188 A composite of graphene aerogel (GA) and the metal–organic framework (MOF) of UiO-66-NH2 crystals on the GA matrix was developed for the detection of multiple heavy metal ions in aqueous solutions. GA not only serves as the backbone for UiO-66-NH2 but also improves the conductivity of the composites by accelerating the electron transfer in the matrix. UiO-66-NH2 acts as a binding site for heavy metal ions due to the interaction between hydrophilic groups and metal cations.189 The gold atomic cluster-chitosan (AuAC-Chit)-modified electrode is highly sensitive and selective for the electrochemical detection of mercury ions, which offers a broader calibration range of 10−14–10−7 M with the limit of detection (LOD) of 0.8 × 10−14 M and the limit of quantification (LOQ) of 6.6 × 10−14, much lower than the guideline value of 1 × 10−8 M specified by the United States Environmental Protection Agency (USEPA), bearing a good accuracy of 1.06 percent for 10−13 M of mercury ions.190 The veil-like graphene was electrodeposited with gold nanoparticles, and the as-prepared graphene-based electrode was used to detect the mercury ions at concentrations ranging from 1.0 aM to 100 nM. In this study, SWV is an attractive technique in the identification of mercury ions with the LOD of 0.001 aM (Fig. 9).191 Zhang et al.192 developed a highly sensitive sensor for heavy metal ions using mercury-specific oligonucleotides, three-dimensional gold nanoclusters and an anionic intercalator. The mercury ions were detected in the range from 0.05 to 350 nM, and the limit of detection was 0.01 nM.
image file: c9qi00602h-f9.tif
Fig. 9 (a) Square wave voltammograms of a mercuric sensor for various concentrations of Hg2+ ions in 20 mL of Tris containing 10 mM KCl (10 mM, pH 7.4). (b) Interference testing under optimal experimental conditions; 10 nM of Hg2+, 500 nM of K+, Ba2+, Ca2+, Cd2+, Co2+, Cr2+, Cu2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, Al3+, Fe3+, and their mixture containing 10 nM of Hg2+ were respectively measured. Reprinted (adapted) with permission from ref. 191. Copyright (2015) American Chemical Society.

5.2. Cd2+ ions

The utilization of cadmium (Cd) is significantly increasing in various industrial operations such as nickel–cadmium rechargeable batteries, corrosion resistance in electroplating, photoconductive surfaces in television picture tubes, photovoltaic cells, and fluorescent probes in fluorescence microscopy; however, cadmium is seriously toxic to the environment. Abnormal doses of Cd are known to cause renal abnormalities, lung cancer, bone demineralization, breast cancer, etc. In addition, the regular inhalation of Cd is likely to cause fever, edema or even death.193 Nanostructured Mg–Al-layered double hydroxides (Mg–Al-LDHs) are used to detect Cd(II) ions, which are highly selective and sensitive in the detection of Cd2+ by anodic stripping voltammetry (ASV). The advantage of Mg–Al-LDHs is that they do not require any modification, and the surface of Mg–Al-LDHs has certain hydroxyl (Sur-OH) functional groups that can react with metal ions through chemical bonding to form inner-sphere complexes. For the time being, there are few deprotonated hydroxyl groups (Sur-O) that may create outer-sphere complexes with metal ions.194,195 A 3-mercaptopropionic acid (MPA)-functionalized gold electrode was used to modify glutathione (GSH) via carbodiimide coupling to fabricate the MPA–GSH-modified electrode. This electrode detects cadmium ions with the detection limit of 5 nM. The effect of accumulation time on the response of MPA–GSH-modified electrodes was tested at various concentrations of Cd2+ (18, 98 and 418 nM). At the higher concentration of 418 nM, the current reaches a plateau at shorter accumulation times. The major significance of this sensor is its long-term stability; if the electrodes are used once a day (for more than 14 days), they may be regenerated at least 16 times on continuous use.196 An electrochemical sensor was fabricated with reduced graphene oxide (rGO)/gold nanoparticle (AuNP)/tetraphenylporphyrin (TPP) (rGO/AuNPs/TPP) nanoconjugates for the detection of Cd2+ ions. rGO was employed to accelerate the incorporation of cadmium ions and tetraphenylporphyrin. Furthermore, the AuNPs enhance the sensitivity of electrochemical detection. This sensor shows high sensitivity, from 50 nM to 300 μM, with low-detection limit and high selectivity. The stability and reproducibility of the sensor was maintained for up to 5 repeated cycles.197 A semiconductor and a gas sensing material of tin oxide (SnO2) was combined with a reduced graphene oxide by a one-step wet chemical method. Reduced graphene oxide (RGO) was modified by the Hummers’ method, which resulted in a well-defined peak at high current and large 2-D electrical conductivity. The modified SnO2/reduced graphene oxide nanocomposite electrode has higher current than the other electrodes of SnO2 and the glassy carbon electrode (GCE). The modified electrode offers the necessary conduction pathways on the electrode surface and better electrochemical catalytic behaviour, effecting the promotion of the electron transfer process on the electrode surface of SnO2/RGO. In addition, the modified electrode has a huge capacitive current as compared to the other electrodes. These advantages are reflected in the modified electrode for potential application in electrochemical supercapacitors.45 A SnO2 quantum dot-based electrode (SnO2/Nafion/Au) has been used for sensing the cadmium ions. The impedance studies indicate that the modified SnO2/Nafion/Au electrode has a reduced impedance value than the bare Au electrode. The modified electrode enhances the sensitivity towards cadmium ions as SnO2 acts as an efficient electron mediator for electrochemical sensing. The fabricated sensor with the modified SnO2/Nafion/Au electrode shows a high sensitivity of ∼77.5 × 102 nA ppm−1 cm−2 with a low detection limit of ∼0.5 ppm and response time of <2 s.193 Moreover, an AlOOH-reduced graphene oxide nanocomposite was prepared directly from graphene oxide (GO) and aluminium nitrate in the presence of urea by a hydrothermal method (Fig. 10a). Aluminum ions (Al3+) are adsorbed on the surface of GO due to the electrostatic forces between the metal ions and the negatively charged oxygen-containing functional groups such as carboxyl, hydroxyl and epoxy groups of GO. As is well-known, urea can release CO2 and OH when the temperature of the solution exceeds 80 °C, and then, Al3+ ions react with the OH ions to form aluminium hydroxide Al(OH)3 on the surface of the GO nanosheets. The nanoplates of AlOOH are grown on the surface of graphene nanosheets along with a reduction process of pristine GO to graphene via a hydrothermal treatment at 180 °C for 10 h. The reactions involved in the formation of AlOOH are presented as follows:
 
CO(NH2)2 + 3H2O → 2NH4+ + 2OH (5)
 
Al3+ + 3OH → Al(OH)3 (6)
 
Al(OH)3 → AlOOH + H2O (7)

image file: c9qi00602h-f10.tif
Fig. 10 (a) Schematic of the one-pot synthesis of AlOOHRGO nanocomposites; (b) SWASV response of the AlOOH–RGO nanocomposite-modified GCE at 0, 1.0, 1.5, and 2.0 μM Cd(II) in the presence of 1.0 μM Pb(II) in 0.1 M NaAc–HAc solution (pH 6.0); at 0, 1.0, 1.5, and 2.0 μM Pb(II) in the presence of 1.0 μM Cd(II) in 0.1 M NaAc–HAc solution (pH 6.0). Reprinted (adapted) with permission from ref. 198. Copyright (2012) American Chemical Society.

The AlOOH nanoplates are inserted into the graphene nanosheets, which effectively prevent the restacking of the as-reduced graphene nanosheets and the hybrid AlOOH–RGO nanocomposites. The electrochemical performance of the AlOOH–RGO nanocomposites is greater than that of AlOOH or RGO as graphene nanosheets have higher active surface area. Mutual interference is a general problem existing in the simultaneous detection of several metal ions. Mutual interference occurs between the two metal ions Cd(II) and Pb(II) at the AlOOH–RGO nanocomposite modified electrode. The peak current of Cd(II) linearly increases, whereas the peak current of Pb(II) almost remains the same when the concentration of Pb(II) is kept constant and the concentration of Cd(II) is changed (Fig. 10b). Similarly, the peak current of Pb(II) linearly increases, whereas the peak current of Cd(II) remains almost the same. The Cd(II) concentration is also constant, whereas that of Pb(II) varies. The results showed that there was no mutual interference between Cd(II) and Pb(II) at the AlOOH–RGO nanocomposite-modified electrode during the simultaneous detection of these two target metal ions.198

A highly sensitive, robust and low-cost microfluidic electrochemical carbon-based sensor (μCS) was developed by Shen et al.49 This sensor was fabricated to overcome the challenging issues of low limits of detection and transfer the sensing performance in a miniature device. The above portable sensor was applied to detect cadmium ions by square-wave anodic stripping voltammetry with a detection limit of 1.2 μg L−1. The μCS maintained their stability and reusability for up to 10 repetitive cycles for the detection of the metal ions. The micro-patterned reduced graphene oxide/carbon nanotube/bismuth composite electrode has high sensitivity (262 nA ppb−1 cm−2) in the detection of cadmium ions with a sensitivity of 0.66 ppb.199

5.3. Pb2+ ions

Lead ions (Pb2+) are a major pollutant due to their potential bioaccumulation and toxicity, and these ions are released from lead-based paints and contaminate water, soils and foodstuffs.200 Lead causes serious damage to the environment and human health with the rapid expansion of industrial production. Lead poisoning is the cause of many health issues, such as memory loss, anemia, muscle paralysis and high blood pressure, in human beings. The accumulation of lead creates serious damage to the brain and central nervous system due to its neurotoxic effects. In addition, it hampers the physical and mental development of children.201–203 A novel electrochemical sensor has been developed based on a conducting polymer-coated nanostructured porous gold electrode using a peptide probe as a recognition element for Pb2+. The presence of a porous nanostructure has improved the electrode surface area due to the reduction of the porous gold electrode. The fabricated sensor has good sensitivity for detecting as low as 1 nM of Pb2+ under low pH conditions with a broad linear dynamic detection range between 1 nM and 10 mM. Moreover, this sensor is very specific in selectively, securing Pb2+ without showing cross-reactivity towards Pb2+ ion detection. The electrode maintains its stability for up to 5 cycles with ethylenediaminetetraacetic acid (EDTA).201

Fluorescent sensors have good selectivity and high selectivity. A fluorescent sensor was made of 1-vinylimidazole. The polymeric sensor response was linear over a concentration range from 4.83 × 10−8 to 4.83 × 10−7 mol L−1. It has high sensitivity with a detection limit as low as 1.87 × 10−8 mol L−1 and a selectivity of over four-thousand fold.204 A miniaturized sensor was made on a micro-patterned reduced graphene oxide/carbon nanotube/bismuth composite electrode for lead ion detection. This sensor has a high sensitivity of 926 nA ppb−1 cm−2 with the favourable detection limit of 0.2 ppb. This sensor shows sharp and high peaks for the target metal ions due to the excellent properties of the rGOCNT10-1 film and the good stripping characteristics of Bi. In this composite material, bismuth has attractive electrochemical characteristics such as stripping behaviour, broad linear range, and good signal-to-background ratio. Moreover, reduced graphene oxide (rGO) has high surface area. The major disadvantages of rGO are the easy formation of aggregates and restacking during processing and the less potential accessible surface area of the electrode, which is much lower as compared to the surface area of other electrodes. The problem can be overcome by the addition of carbon nanotubes between the graphene sheets to prevent their restacking. Though the composite electrode enhances the sensing properties, it increases the cost of the sensor due to the expense of rGO and CNT.205 An ionic liquid/poly-L-cysteine (IL/PLC) composite on flexible and hierarchical porous laser-engraved graphene electrode (LEGE) was used to detect lead ions. The as-prepared composite material has a large specific surface area, strong electron transfer ability, abundant active sites and special complexing capability toward Pb2+ due to the combination of the large specific surface area of LEGE. The specific complexing ability of PLCs and the good conductivity of IL endowed the newly designed sensor with an ultralow detection limit of 0.17 μg L−1 (S/N = 3).206

Porous activated carbon (PAC) is a versatile material in chosen fields such as environment, energy and gas sensing. It is richly used in catalysis, gas adsorption/separation, fuel and energy storage, fuel cells, dye-sensitized solar cells (DSSCs), electrochemical sensors, and chemical/biological sensing of toxic metal ions. Biomass-derived PAC is of low-cost and is easily derived from industrial and biomass wastes. Biomass-derived PACs from pomegranate and orange fruit peels were incorporated with palladium nanoparticles (PdNPs) by a hydrothermal carbonization method (Fig. 11a), which has desirable textural properties and porosities favourable for the dispersion of PdNPs with 3–4 nm on the graphitic PAC substrate. The synthesized Pd/PAC was used to modify glassy carbon electrodes (GCEs), which are employed as electrochemical sensors for the detection of toxic heavy metal ions. It has a superior performance for both individual and simultaneous detections. Pd/PAC-modified GCE performs well for the simultaneous detection of Cd2+, Pb2+, Cu2+, and Hg2+ ions by differential pulse voltammetry (DPV) with rapid response as compared to other techniques, particularly stripping voltammetry. The DPV profiles are given in Fig. 11b for the Pd1.5/PAC-900-modified GCE under different metal ion concentrations (0.5–8.9 μm) in the presence of a real milk sample, which exhibits four well-resolved response concentrations at 2.4 μm, 2.9 μm, 2.9 μm and 10–20 μL accounting for Cd2+, Pb2+, Cu2+ and varied amounts of Hg2+ ions, respectively.207


image file: c9qi00602h-f11.tif
Fig. 11 (a) A schematic of the applications of Pd/PAC materials for the detection of toxic heavy metals; (b) DPV curves observed for the Pd1.5/PAC-900-modified GCE in the presence of a mixture of a real milk sample with metal ion analytes: Cd2+ (2.4 μM), Pb2+ (2.9 μM), Cu2+ (2.9 μM), and varied amounts of Hg2+ (10–20 μL) (Reprinted (adapted) with permission from ref. 207. Copyright (2016) American Chemical Society.).

5.4. As3+ ions

Arsenic (As) is an abundant element and it is available in composites for many industrial applications. However, arsenic ions are highly toxic, especially in the form of As3+. These ions can have severe health effects, especially dermal changes, cardiovascular, gastrointestinal, genotoxic, mutagenic and carcinogenic effects.208,209 Nowadays, arsenic contamination has affected a large number of people across the world and has been reported in 20 countries where arsenic levels in drinking water are above the World Health Organization's arsenic guideline value of 10 μg L−12. Many methods are available for detecting arsenic levels, among which biosensors provide an effective, rapid and portable method.210 The determination of arsenic is carried out by anodic stripping voltammetry (ASV) and differential pulse anodic stripping voltammetry (DPASV) using various electrode materials such as Hg, Pt and Au. Among these, Au offers a highly sensitive response towards arsenic oxidation as compared to the other electrode materials investigated and has higher hydrogen overvoltage and reversibility as compared to Pt. Gold is a superior substrate for the working electrode as compared to Hg and Ag as the arsenic stripping peak appears as a shoulder on the oxidation waves of Hg and Ag. Gold-modified glass carbon electrodes are being developed for the detection of As3+ ions in 1 M HCl solution.208 The limited resources and high cost of Au is the major barrier and hence there is an urgent need for the development of a substitute for pure Au catalysts. Au-based bimetallic nanoparticles overcome the problem of pure Au due to their unique structures and compositions, which would improve the catalytic performance. Au-based bimetallic nanoparticles with different morphological structures including Au–Pt, Au–Pd, Au–Ag, Au–Cu and Au–Fe are being investigated. Among these, Au–Cu bimetallic nanoparticles are proved to perform better and have attracted widespread interest since the cost of Cu is relatively low. Au–Cu bimetallic nanoparticles with different compositions are used as a sensing material substrate for the analysis of As(III) in optimal conditions. It has a sensitivity of 1.63 μA ppb−1 cm−2 with a low LOD of 2.09 ppb obtained on Au89Cu11 bimetallic nanoparticles-modified GCE. Au89Cu11 has a higher sensitivity than other compositions of Au93Cu7 and Au79Cu21.211

The Au-supported tellurium (Au/Te) crystalline hybrid with a 3D structure has a high sensitivity of 6.35 μA ppb−1 in the electrochemical detection of As(III). The addition of Te enhances the sensitivity due to its anisotropic property with unique helical chain 3D nanostructures. The sensitivity of the hybrid electrode is more than 2 orders better than that of the pure Au electrode.212 A dumbbell-like structure of Au/Fe3O4 magnetic nanoparticles was modified on the screen-printed carbon electrode for serving as an efficient sensing interface for As(III) detection with a high sensitivity of 9.43 μA ppb−1 and a low detection limit of 0.0215 ppb. The characterisation results indicate that the Fe(II)/Fe(III) cycle is involved in the As3+ detection on the interface of the dumbbell-like Au/Fe3O4 nanoparticles. A more distorted local coordination of Fe–O and Fe–Fe in 10 nm Fe3O4 NPs provides higher adsorption and surface atom activity. The typical square wave anodic stripping voltammetry (SWASV) responses and corresponding linear calibration plots of peak current against As(III) concentrations at the Au/Fe3O4 screen-printed carbon electrode (SPCE) in various concentration ranges from 0.1 to 10 ppb are graphically represented in Fig. 12a and b. The peak shape is well-defined and the detection range in low concentrations (0.1–10 ppb) with a consistent sensitivity of 9.43 ppb−1 is more attractive. The sensitivity is sharply decreased in lower concentration ranges, which may be due to the shorter concentration time and weaker synergistic effect.213


image file: c9qi00602h-f12.tif
Fig. 12 (a) Typical SWASV responses of As(III) at Au/Fe3O4 SPCE across different concentration ranges. (b) Corresponding linear calibration plots of peak current against As(III) concentrations from 0.1 to 10 ppb. Insets in panels a and b show the enlarged views that correspond to a range of 0.1–2 ppb. Reprinted (adapted) with permission from ref. 213. Copyright (2018) American Chemical Society.

6. Conclusion

In this review article, we have discussed cost-effective and environmentally friendly nanoscale electrochemical sensors for the monitoring of biomolecules and toxic industrial waste compounds; sensors (electrochemical sensors, biosensors and pesticide sensors) are notable due to their unique electrochemical properties (high surface to volume ratio, high sensitivity and long durability) and simultaneous detection of different analytes. Electrochemical techniques (cyclic voltammetry, SWV, DPV and amperometry) can effectively serve as sensitive methods for the detection, degradation and removal of environmental pollutants; nanostructured electrode materials find wide application in the detection of pesticides. A newly developed inexpensive and environmentally friendly nanocomposite matrix exhibited great potential for the sensing of bioactive molecules (bio-sensing devices) such as glucose, dopamine, ascorbic acid and uric acid; electrochemical sensitive techniques could also be used as a powerful tool for the simultaneous detection of heavy metal ions (Pb2+, Cd2+, Hg2+ and As3+). Modified nanocomposites have attained significant attention, and their extraordinary sensitive and selective electrochemical properties may also be improved during the study of target analytes.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We acknowledge the financial support received from Science & Engineering Research Board (SERB), a statutory body of the Department of Science & Technology, Government of India (Diary No: SERB/F/8355/2016-17 Dated, Feb-2017, File No. EEQ/2016/000427).

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