Recent Developments in Polymer Nanocomposite-Based Electrochemical Sensors for Detecting Environmental Pollutants

Abstract

The human population is generally subjected to diverse pollutants and contaminants in the environment like those in the air, soil, foodstuffs, and drinking water. Therefore, the development of novel purification techniques and efficient detection devices for pollutants is an important challenge. To date, experts in the field have designed distinctive analytical procedures for the detection of pollutants including gas chromatography/mass spectrometry and atomic absorption spectroscopy. While the mentioned procedures enjoy high sensitivity, they suffer from being laborious, expensive, require advanced skills for operation, and are inconvenient to deploy as a result of their massive size. Therefore, in response to the above-mentioned limitations, electrochemical sensors are being developed that enjoy robustness, selectivity, sensitivity, and real-time measurements. Considerable advancements in nanomaterials-based electrochemical sensor platforms have helped to generate new technologies to ensure environmental and human safety. Recently, investigators have expanded considerable effort to utilize polymer nanocomposites for building the electrochemical sensors in view of their promising features such as very good electrocatalytic activities, higher electrical conductivity, and effective surface area in comparison to the traditional polymers. Herein, the first section of this review briefly discusses the most important methods for polymer nanocomposites synthesis, such as in situ polymerization, direct mixing of polymer and nanofillers (melt-mixing and solution-mixing), sol–gel, and electrochemical methods. It then summarizes the current utilization of polymer nanocomposites for the preparation of electrochemical sensors as a novel approach for monitoring and detecting environmental pollutants which include heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, nitrite, and hydrazine in different mediums. Finally, the current challenges and future directions for the polymer nanocomposites-based electrochemical sensing of environmental pollutants are outlined.

Graphical Abstract

1. INTRODUCTION

Over the past years, we have observed the increased global concern regarding the environmental contaminations and universal health influences caused by toxic and poisonous pollutants with heavy industrialization, agriculture activities, and the rapid urbanization. In addition, an acute global issue for half of the world population has been contaminated drinking water.¹ In fact, people are constantly subjected to multiple adverse health impacts due to toxicity from these pollutants, fundamentally by ingesting the polluted drinking water and foodstuffs and inhaling the surrounding air with a higher concentration of contaminants.² Exposure to toxic pollution may lead to numerous health consequences such as mental and physical dysfunctions, organ and neurological disorders, cancers, lower life expectancy, and the weakened body’s immune system, and in a many cases it results in mortality.^3–8 The mentioned concerns have emphasized the prominence of devising reliable, highly sensitive, user-friendly, and inexpensive instruments to quantify pollutants at low concentration levels.

Some conventional analytical technologies such as gas chromatography/mass spectrometry (GC/MS),⁹ atomic absorption spectroscopy (AAS),¹⁰ high-performance liquid chromatography (HPLC),¹¹ spectrofluorimetry,¹² capillary electrophoresis,¹³ and flow injection chemiluminescence,¹⁴ etc. have been used to detect environmental pollutants. However, such analytical procedures have been laborious, expensive, needing advanced skills for their operation, and difficult to deploy due to the bulky size of the devices. In contrast, a quickly emerging class of electrochemical sensors shows promise as essential analytical detection devices because of their high sensitivity, selectivity, speed, flexibility, and cost-effectiveness.

Researchers describe a chemical sensor as one of the devices providing continual knowledge of its context. In an ideal situation, the chemical sensor would provide a specific kind of response with a direct relation to the amount of certain chemical samples. In fact, each chemical sensor comprises a transducer with the ability to transform a response into a visible signal on the contemporary instruments and a chemically selective layer, isolating the analyte response from its immediate context. These may be categorized based on their properties such as optical, electrical, thermal, or mass sensors. In addition, they have been devised for detecting and responding to an analyte in a liquid, gaseous, or solid state.¹⁵ In comparison to the mass, thermal, and optical sensors, the electrochemical sensors have been particularly attractive owing to the respective noticeable experimental simplification, detectability, and affordability.^16–19 Moreover, the electrochemical sensors enjoy a prominent position among the contemporary sensors on the commercial level with major uses in industrial, clinical, agricultural, and environmental analysis.^20–22

Electrochemical sensors typically function based on a redox reaction containing a target analyte in an electrolyte on a working electrode, resulting in the changes of an electrical signal; the signal generated is proportional to the concentration of the analyte. With regard to the signal that should be recorded, scientists classified the electrochemical techniques into amperometric, conductometric, impedimetric, potentiometric, and voltammetric methods.²³ In recent times, the amperometric and voltammetric methods have played an important role in the field of electroanalysis.^24–28

Amperometry is an electrochemical method in which the potential utilized to a sensing electrode is monitored instrumentally, and the current occurrence in oxidation/reduction on the surface of the electrode is measured as the analytical signal. The applied potential can be stepped to and held at a steady value, and the resulting current can be recorded as a function of time.

Voltammetry was identified by the Czech scientist Jaroslav Heyrovsky in 1920s. Contrary to the constant potential in the amperometric method, the voltammetric techniques utilize measuring current at different potentials in current voltage curves. Voltammetric procedures such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), square-wave voltammetry (SWV), linear sweep voltammetry (LSV), and stripping voltammetry, etc., have been regarded as a well-known device to determine the trace concentrations of critical environmental contaminants due to their high accuracy and sensitivity.

On the basis of the electrochemical sensors, the electrode surface alone has been considered as one of the robust tools to quantify an analyte. However, the bare electrodes suffer from several shortcomings such as nonreproducibility of the surface behaviors, slower kinetics of the electrochemical reactions for a number of compounds on the electrode surface and overpotential requirements. Thus, such variables restrict their capability and utilization to attain the analytical objectives. It is notable that the use of chemical and physical modification in the electrodes’ surfaces can eliminate such issues. In fact, the electrode modification has been viewed as one of the processes in which the bare electrodes’ surface would be covered by a chemical or biological compound included in the electrode matrix and altering its electrochemical attributes.^29–33

The fusion of nanotechnology and electrochemical procedures would result in introducing robust and reliable electrical instruments for effectively processing and controlling the contamination. To develop highly sensitive electrochemical sensors, experts in the field have utilized many nanomaterials to modify the surfaces of the electrodes.^34–36

In general, nanotechnology implies a field in the applied science and technology, which controls matter on the molecular or atomic levels in scales between 1 and 100 nm, and production of the tools in the mentioned range for dimension. Nanoparticles (NPs), with size features ranging from 1 to 100 nm, show distinctive physical and chemical properties such as unique electronic, magnetic, and optical properties, and chemical versatility.³⁷ These distinctive features have rendered them as an excellent emerging sensing platform in the disciplines of analytical chemistry and electrocatalysis during the past two decades.³⁸ While NPs contribute differently to diverse electrochemical sensors, the use of an NP-modified electrode enjoys many benefits based on electro-analysis outputs:

• efficient catalysis

• quick mass transfer

• large sensor surface areas

• reasonable control on the microenvironment of the electrode

Emerging polymers have been considered as a turning point in the contemporary analytical sciences. They are commonly pursued as signal-promoting elements in electro-analytical operations as a result of the respective prominent characteristics such as higher conductivity, lighter weight, more flexibility, simplified procurement, and greater active sites as well as strong adherence to the surface of the electrodes.³⁹ Polymers do have limited applications, due to such factors as surface poisoning, low selectivity, low sensitivity due to the adsorbed interference, and intermediates from other species.⁴⁰

Nanotechnology exposes inimitable opportunities to construct advanced combinations of nanoscale materials and polymeric materials to prepare polymer nanocomposites having unique properties. Toyota Research Group invented Nylon-6/clay nanocomposites in 1990 declaring a new horizon in the research area of polymer composites.⁴¹ Polymer composites comprising polymer and filler have been receiving significant attention lately. When a copolymer or polymer is utilized as a matrix or when nanofillers get dispersed in the polymer matrix,^42–44 it constructs a polymer nanocomposite. The properties of these composites are often dominated by the filler. Polymers are generally reinforced by variable fillers to improve their limitations and expand their applications. The application of nanofillers to enhance physical and mechanical properties of polymers has provided an alternative fabrication route for the conventional polymer nanocomposites.^45,46 This is because surface interactions with the adhesion, bonding, dispersion, matrix, particle motion, etc., depend to a large extent on the size of the filler particles. Some of these effects have profound impact on the polymer nanocomposite properties because they generally increase with a decrease in filler size. For instance, nanoscale fillers have very large surface to volume ratio and because properties such as adhesion, catalytic reactivity, chemical reactivity, electrical resistivity, and gas storage depend on the nature of the interface, these properties change dramatically. Therefore, polymer nanocomposites have many unique attributes, such as catalytical,⁴⁷ electrochemical,^48,49 mechanical,⁵⁰ magnetic,⁵¹ optical,⁵² and biological properties,⁵³ etc. It is notable that polymer nanocomposites have proven their prowess as an excellent transducer in electroanalytical sensors, because of their characteristics and synergic effects from the specific components in comparison with those obtained by their individual counterparts, such as morphological variations, facile synthesis, and smart responsibility.⁴³ Their inimitable properties offer advantages, such as enhanced electrical conductivity, low cost, high specific surface area, rapid electrode kinetics, controllable chemical/electrochemical properties, high chemical stability in aqueous solutions, biocompatibility, and environmental stability. Therefore, recent development of polymer nanocomposites-based graphene (Gr), metal and metal oxide NPs, and carbon nanotubes (CNTs) have been attracting much interest for their use in electrochemical sensors.

This review focuses on the recent developments in polymer nanocomposite-based electrochemical sensors for the detection of environmental pollutants to protect the environment and human health. We first cover a brief description for the synthesis method of polymer nanocomposites. Then, variable electrochemical sensors based on polymer nanocomposites developed over the past decade are discussed for the analysis of diverse environmental pollutants (e.g., heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, nitrite, and hydrazine). Finally, a summary of current challenges of polymer nanocomposites-based sensors, and an outlook of their future progress as emerging sensing materials, is explored.

2. METHODS FOR THE SYNTHESIS OF POLYMER NANOCOMPOSITES

The preparation of polymer nanocomposites using a proper processing technique is important to obtain their high efficiency. Several procedures have been developed for the synthesis of polymer nanocomposites. The most important methods are listed here.

2.1. In-Situ Polymerization.

The in situ polymerization method is the traditional technique for processing polymer nanocomposites.^54–57 The principle of this synthesis method includes dispersion of the nanofiller in monomer solution followed by the polymerization of the resultant mixture in the presence of appropriate initiator such as heat, radiation, and oxidizing agent. The in situ polymerization aims to improve the properties of polymer nanocomposites due to good dispersion and distribution of the nanofillers in the polymer matrix. Furthermore, partially exfoliated structures are attainable via this method because of the homogeneous dispersion and intercalation of fillers in the polymer matrix. Despite the advancement of the in situ method in meliorating efficient dispersion of nanofillers in polymer matrices, this technique has several drawbacks namely complex procedures and processing steps and the requirement of expensive reactants. Further, the technique is not appropriate for all types of polymers, only those thermally unstable and insoluble in the solvent, and is suitable only for a limited number of elastomers.

A typical protocol was reported by Tamboli et al. for the synthesis of nanowires of silver–polyaniline (PANI) nanocomposite via in situ polymerization using a AgNO₃ precursor and ammonium persulfate (APS) as an oxidizing agent; dodecylbenzenesulfonic acid (DBSA) was utilized as a protonic acid during the polymerization process.⁵⁸ The synthesis of the Ag–PANI nanocomposite was carried out by direct addition of AgNO₃ to double distilled water followed by the addition of DBSA under stirring. The precooled solution of the aniline monomer was added to the aforementioned solution at room temperature. Finally, APS was added dropwise to initiate the polymerization of aniline, and the reaction was continued with stirring at room temperature, to ensure complete polymerization of aniline. The ensuing greenish black precipitate was centrifuged and washed with distilled water to separate the impurities, such as excess acid, oxidant, and unreacted monomer, and eventually the product was dried.

2.2. Direct Mixing of Polymer and Nanofillers.

Direct mixing of nanofillers and a matrix polymer is a top-down method based on the breakdown of agglomerated fillers in the mixing process. This approach is appropriate for synthesizing polymer-based nanocomposites having submicrometer- or nanosized fillers with a dimension 1 or 2 orders of magnitude larger than the fillers dispersed in the hybrid materials. There are two main pathways for mixing the polymer and fillers:

2.2.1. Melt-Mixing.

Melt-mixing is a blending procedure of the nanofiller and the polymer melt to prepare the polymer nanocomposites. It has the advantage of being environmentally benign due to the lack of solvent utilization. Furthermore, it is compatible with industrial processes such as injection and extrusion molding, which makes it very convenient to apply the existing infrastructure and, thus, very economical. However, the elevated temperatures deployed during the process to melt the polymer can alter the surface modification of the filler.⁵⁹

As an example, Al-Saleh et al. reported the preparation of Gr nanoplatelet-polystyrene (Gr NP/PS) nanocomposites which were prepared by melt-mixing of Gr NP and PS using a small batch mixer at 190 °C for 10 min at a mixing speed of 100 rpm.⁶⁰ Then, the nanocomposites were placed in a compression molding machine after melt compounding to prepare 1.0 mm-thick rectangular plates; the molding being conducted under 25 MPa pressure at 220 °C for 10 min.

2.2.2. Solution-Mixing.

Solution-mixing is a very common method for preparing polymer nanocomposites on a laboratory scale.⁶¹ The polymer is dissolvable in a solvent such as an acetone, cyclohexane, dimethylformamide, tetrahydrofuran, toluene, etc. The process generally contains dispersion of nanofillers in an appropriate solvent and mixing them with the polymer solution. The resulting nanocomposites are recovered from solvent via evaporation of the solvent or by the solvent coagulation method; selection of the solvent is mostly governed by the solubility of the polymer matrix. The same solvent could be applied both, for the nanofiber and matrix.⁶² One of the advantages of this approach is the feasibility to attain good quality dispersion of the nanofillers in a suitable solvent. The obstacle is that this processing method cannot be used for insoluble polymers.⁶³ Additionally, the elimination of organic solvent has environmental implications, which restricts the successful transition of this preparation technique from the laboratory to the industrial scale production.

In a typical example, Marroquin et al. reported the synthesis of a material based on chitosan (CHIT) in which the Fe₃O₄/multiwalled carbon nanotube (MWCNT)/CHIT nanocomposites were prepared by solution mixing.⁶⁴ Fe₃O₄ and MWCNT were ultrasonicated for 1 h in distilled water before adding CHIT and acetic acid. The mixture was magnetically stirred followed by ultrasonication. The mixture was degassed and vacuum-dried to generate the nanocomposite.

2.3. Sol–Gel.

The sol–gel process (Figure 1) is a wet chemical technique used to synthesize polymer nanocomposites which are located on thin films.^65,66 A relatively homogeneous distribution of NPs in the polymer matrix can be accomplished. In brief, sol–gel synthesis begins from the inorganic salt with the polymer monomers or highly reactive metal alkoxide which are used as precursors. Thereafter, the starting materials are mixed in the liquid phase, and it is important to control the molar ratio of the ingredients precisely; the subsequent hydrolysis reaction delivers the sol. After thermal treatment at low temperature, this sol would be changed to solid gel.⁶⁷ As to the polymer nanocomposites which are prepared by this procedure, inorganic nanometer units, segments, and organic polymers are joined by hydrogen or covalent bonding.⁶⁸ The advantages of this method include uniformity and high purity, easily controllable reaction, low sintering temperature, good forming ability, etc. However, there are several limitations in terms of actual application, for example, most of precursors are toxic and expensive. It is very demanding to prepare crystalline composites comprising polymers and inorganic oxide NPs, as the polymers are generally unaffordable at high temperature, which is the incumbent condition to make inorganic NPs form a crystal structure, and the process of sol–gel usually requires an extended period of a few days to weeks. In particular, the contraction brittle fracture due to the volatility of the solvents, small molecules, and water is the biggest problem for this method.

Figure 1.

Preparation route for the sol–gel synthesis method.

As a typical example, Jaseela et al. prepared TiO₂ poly(vinyl alcohol) (PVA) nanocomposite via the sol–gel route.⁶⁹ Triethanolamine was added to the mixture followed by the dropwise addition of water to acquire the homogeneous sol. After the addition of PVA in the above sol and stirring, the solution at room temperature afforded the TiO₂–PVA nanocomposite. After washing with a water–ethanol mixture, the product was dried in an oven. Then the TiO₂–PVA nanocomposite was subjected to heat treatment in a muffle oven.

2.4. Electrochemical Method.

Electrochemical synthesis is a facile chemical preparation procedure which is achieved by electrochemical workstation.^70,71 There are three electrodes including the counter electrode, working electrode, and reference electrode in the electrochemical process. As to the electrocatalytic applications, this synthesis procedure is the best way to prepare the polymer nanocomposite films which can be formed directly on the electrode surface.⁷² The electrochemical synthesis can be controlled simply by the applied potential or current density, and the amount of product can also be regulated by the integrated charges used. In general, this technique has many advantages, such as short reaction time and operational simplicity, and being easier to control and eco-friendly. In addition, the electrochemical method avoids application of the oxidant, thus achieving greater purity of the desired product. However, the use of electrochemical methods for large-scale production is limited because the working electrode has a limited surface area.

In a representative example, Gao et al. reported the dip-coating of reduced graphene oxide sheets (GNS) followed by electrodeposition of PANI nanofibers to prepare a layer-by-layer-GNS/PANI multilayer composite (Figure 2).⁷³ Graphite oxide (GO) was synthesized using natural graphite by the Hummers method. Exfoliation of GO was accomplished by ultrasonication of the dispersion using an ultrasonic bath. Then, glucose was applied as reductant to prepare GNS. For the synthesis of GNS/PANI nanocomposite, stainless steel sheets were prepared as deposition substrates which were partially dipped into the GNS suspension and dried smoothly to form a GNS layer. The electro-polymerization was utilized to synthesize PANI nanofibers on the surface of the GNS film using a three-electrode electrochemical cell; a saturated calomel electrode (SCE) and platinum foil were applied. The stainless steel sheet coated with GNS film was directly utilized as working electrode for the deposition of PANI. Aniline (0.1M) and sulfuric acid (1 M) mixed solution was applied as electrolyte. Aniline monomers could be in situ electro-polymerized on GNS by cyclic voltammetry for two cycles at a scan rate of 5 mV s⁻¹ with the applied potential of −0.2 to 1.2 V vs SCE. GNS/PANI nanocomposite could also be constructed via the electrostatic attraction between positively charged aniline monomer ions and negatively charged GNS nanosheets. Then the multilayer GNS/PANI composite could be assembled on the substrate by the deposition of the two components. The repeated electro-polymerization process (10 times) constructed the GNS/PANI multilayer composite. The coated samples were rinsed with ethanol and dried in an oven.

Figure 2.

(A) Schematic illustration for the fabrication of GNS/PANI nanocomposite: (i) preparation of GNS by glucose reduction of GO;(ii) GNS deposition on stainless steel substrate; (iii) electrodeposition of PANI nanofibers on GNS nanosheets. (B) Structure of PANI. Reproduced with permission from ref 73. Copyright 2013 Elsevier.

3. POLYMER NANOCOMPOSITES-BASED ELECTROCHEMICAL SENSORS FOR THE DETECTION OF ENVIRONMENTAL POLLUTANTS

Recently, environmental pollution is one of the most serious global challenges. Environmental pollution affects biodiversity, ecosystems, and human health worldwide by contaminating soil and water. This issue cannot generally be resolved through conventional tools and traditional strategies. Therefore, the development of simple, fast, and sensitive analytical methods to quantify pollutants in different media (e.g., food, soil, water, air, and biological samples) at trace level quantities has become a critical issue for both analytical chemists and environmental scientists. Chemical contaminants including heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, nitrite, and hydrazine as major common pollutants are identified to be of great concern due to their toxicity, carcinogenicity, or endocrine disrupting effects. The appropriate applications of electrochemical methods have been proven to be very useful for resolving the environmental analysis issues due to high sensitivity, accuracy, and precision as well as a large linear dynamic range, with relatively simple and low-cost procedures. However, analysis by the unmodified electrodes is not generally feasible to acquire the quantitative results for the pollutant compounds due to the low level of environmental pollutants and the complex speciation in their sample matrices. Therefore, the modification of the electrode surface plays a crucial role in electrochemical sensor development. The efficient combination of different nanoscaled materials with proper conductive polymers opens a new avenue for utilizing novel polymer nanocomposites as applicable materials for constructing electrochemical sensing platforms for environmental pollutants detection with high performance. Indeed, this is a growing and demanding research area in analytical science. In this section, the application of polymer nanocomposites in electrochemical sensors for the detection of environmental pollutants namely heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, nitrite and hydrazine in different mediums is briefly introduced.

3.1. Polymer Nanocomposite-Based Electrochemical Sensors for Heavy Metal Ions Detection.

According to the studies, heavy metal pollution could be of significant concern for environmental and human health due to its dramatic proliferation caused by both anthropogenic and natural sources, for example, automobile exhaust, industrial discharge, and mining.⁷⁴ Moreover, some heavy metals are considered to be strongly carcinogenic even at insignificant amounts. Unlike organic pollutants, heavy metals are nonbiodegradable and have a tendency to aggregate in the food chain, which would severely impact people and environment health.⁷⁵ Therefore, devising sensitive and selective procedures for initial identification of the trace level of heavy metals contamination in a variety of chemical systems such as the living system and the whole environment would be of paramount importance.

3.1.1. Hg²⁺.

Matlou et al. prepared the single-walled carbon nanotube poly(m-amino benzenesulfonic acid) (SWCNT-PABSA) modified gold (Au) electrode by the self-assembled monolayers (SAMs) procedure for the detection of Hg²⁺.⁷⁶ A thiol comprising dimethyl amino ethanethiol (DMAET) was utilized for facilitating the SWCNT-PABSA molecules assembly over the surface of the electrode of Au. Moreover, Au-DMAET-(SWCNT-PABSA) has been utilized to detect Hg in water through the square wave anodic stripping voltammetry (SWASV) analyses. Furthermore, this sensor exhibited an acceptable sensitivity and a limit of detection (LOD) equal to 0.0634 μM. Also, the sensor showed a linear concentrations range between 20 and 250 ppb. It is notable that the analytical utility of the recommended technique with the sensor electrode was examined with real water samples; then, inductively coupled plasma mass spectrometry (ICP-MS) was used to validate the technique.

Zuo et al. recently devised an electrochemical sensor to detect Hg²⁺ at trace levels that was constructed using a poly(3,4-ethylene-dioxythiophene) nanorods/GO nanocomposite modified glassy carbon electrode (PEDOT/GO/GCE) (Figure 3).⁷⁷ In addition, differential pulse stripping voltammetry (DPSV) has been utilized for detecting lower concentrations of Hg²⁺ on the PEDOT/GO/GCE. Furthermore, the experimental conditions such as the accumulation time (360 s), deposition potential (−0.2 V), and pH-values (0.1 M acetate buffer solution (ABS) in pH 5.0) have been optimized. On the basis of the optimized conditions, an acceptable linear association has been observed between the peak currents and Hg²⁺ concentration in the range between 10.0 nM and 3.0 mM. The LOD has been determined as 2.78 nM at the ratio of signal-to-noise of 3. Ultimately, the utility for detecting Hg²⁺ in the tap water samples was substantially approved.

Figure 3.

Preparation steps of PEDOT/GO/GCE and electrochemical detection of Hg²⁺. Reproduced with permission from ref 77. Copyright 2016 Elsevier.

3.1.2. Cd²⁺.

The preparation of a mulberry-shaped poly(2-methylaniline) (P2MA)-Ce₂(WO₄)₃@CNT nanocomposite via sol–gel polymerization method was accomplished by Khan et al.⁷⁸ This composite was characterized by various spectroscopic methods (scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), and thermogravimetric analysis). A very simple P2MA-Ce₂(WO₄)₃@CNT nanocomposite was utilized for detection of cadmium ions using a Nafion(NF) bonded glassy carbon electrode (GCE). The sensitivity has been computed from the calibration curve; that is ~5.138 μA μM⁻¹ cm⁻². Additionally, the linear dynamic range of the P2MA-Ce₂(WO₄)₃@CNT/NF/GCE sensor was utilized from 1.0 to 0.1 M (linearly, r²: 0.9917), in which LOD has been computed at about 0.11 nM (S/N) = 3. Also, the P2MA-Ce₂(WO₄)₃@CNT/NF/GCE sensor was the most selective toward the Cd²⁺ ions compared to other ions.

3.1.3. Pb²⁺.

In their study, procurement and utilization of the nanocomposite based on the ion imprinted polymer (IIP) (methacrylic acid)-1-(2-pyridylazo)-2-naphthol (PAN)/MWCNT was accomplished by Tarley et al. as one of the newly devised electrochemical selective sensing platforms of Pb²⁺ in the water specimens.⁷⁹ On the basis of the outputs, the polymer applied synergistic impact of the imprinting effect, MWCNTs, and ligand PAN to enhance the selectively and sensitivity of detecting Pb²⁺. Moreover, the limit of quantitation (LOQ) and LOD were equal to 0.50 μg L⁻¹ and 0.16 μg L⁻¹. Furthermore, interday technique accuracy was 1.72 and 1.86% (relative standard deviation (RSD)) and intraday accuracy was 2.53 and 1.68% for 10.0 μg L⁻¹ and 5.0 of Pb²⁺. Finally, the effective utilization of this sensor for the synthetic urine and water samples was observed with reasonable recovery values between 95 and 103%.

Zuo et al. fabricated one of the sensitive Pb(II) sensors-based on the Co₃O₄/reduced GO (RGO)/CHIT nanocomposite-modified GCE.⁸⁰Figure 4 indicates the procurement procedure of Co₃O₄/RGO/CHIT/GCE and electrochemical determination of Pb(II). The calibration plot for Pb(II) has been linear in the concentration ranges between 1.0 and 200.0 nM. In addition, the combined reasonable conductivity and higher surface areas of RGO, powerful Pb(II) absorption capability of the Co₃O₄ NPs, and very good Pb(II) complex-forming capability of CHIT provided a sensitive Pb(II) sensor with the LOD equal to 0.35 nM. Then, a CHIT with positive charge could interact with the RGO with a negative charge to enhance the Co₃O₄/RGO nanocomposite stability. Finally, this hybrid nanocomposite Pb(II) sensor provided encouraging opportunities to analyze vegetables.

Figure 4.

A schema of the production process of Co₃O₄/RGO/CHIT/GCE and electrochemical determination of Pb(II). Reproduced with permission from ref 80. Copyright 2017 Elsevier.

3.1.4. Cr(VI).

Hussein et al. fabricated poly(aniline-co-o-toluidine)/GO nanocomposite termed [PANI-co-PoT/GO] by a well-known in situ oxidative polymerization technique with ultrasonic assistance. However, one of the key objectives of their research was developing greater knowledge in electro-selectively applying the Au NPs as coatings.⁸¹ Thus, the Au NPs/PANI-co-PoT/GO nanocomposite modified Au electrode was effectively procured via an electroadsorption process as one of the electrochemical sensors to detect Cr(VI). Therefore, with the use of the SWV technique, the modified electrode linearly respond to Cr(VI) using a concentration range from 5–500 μM with the LOD equal to 0.0215 μM. This sensor had acceptable sensitivity, stability, selectivity and to detect Cr(VI) in the real samples.

3.1.5. As³⁺.

Dash and Munichandraiah devised the PEDOT modified Pd nanodendrite electrode, which was used for electro-analysis of As(III) in 1 M HCl solution.⁸² Moreover, the electrodeposition procedure was used to grow Pd nanodendrites on a porous thin film of PEDOT. According to CV investigations, the Pd-PEDOT/carbon paper electrode (CE) exhibited enhanced electrocatalytic activities toward the As(III)/As(0) redox reaction in comparison to Pd/CE. Then, researchers performed differential pulse anodic stripping voltammetry (DPASV) to analyze the As(III) ion at pH =1.0. It has been shown that the PEDOT-modified Pd electrode had higher sensitivity to detect As(III) (1482 μA cm⁻²μM⁻¹). Furthermore, wider detection ranges to 10 μM and LOD = 7 nM (0.52 ppb) have been observed with a predeposition time equal to 120 s. Finally, Cu(II) ions demonstrated no interference when this test was performed.

3.1.6. Cd(II) and Pb(II).

Chamjangali et al. designed a voltammetric sensor based on GCEs coated with the MWCNT/poly(pyrocatechol violet) composite and bismuth film (GCE-MWCNT/poly(PCV)/Bi) to detect lead and cadmium as environmental contaminants.⁸³ The presence of the Bi film improved the accumulation of analyte as a result of its capability for forming a fused alloy with lead and cadmium. This electrode linearly responded to Pb²⁺ and Cd²⁺ in an acceptable concentration ranging between 1.0 and 300.0 and 1.0–200.0 μg L⁻¹. The LODs respectively were 0.20 μg L⁻¹ and 0.40 μg L⁻¹ for Cd²⁺ and Pb²⁺. As demonstrated by the findings, the sensor showed the lowest level of interferences from most of the common ions as a result of specific and selective interactions between modifier functionality and the cadmium and lead ions. However, the electrode response has been fixed for not less than 3 weeks of the consequent operations. Finally, this procedure has been utilized for simultaneously detecting the Cd²⁺ and Pb²⁺ content in the water samples.

Promphet et al. prepared one of the electrochemical sensors based on the Gr/PANI/PS nanoporous fiber modified screen printed electrode (SPE) for simultaneous detection of Cd²⁺ and Pb²⁺.⁸⁴ To examine its application, the SWASV was utilized to simultaneously detect Cd²⁺ and Pb²⁺ in the presence of bismuth (Bi³⁺) on the Gr/PANI/PS nanoporous fiber-modified SPE. The anodic current linearly related to the metal ion concentrations within the ranges of 10 and 500 μg L⁻¹ LOD (S/N = 3) equal to, respectively, 4.43 μg L⁻¹ and 3.30 μg L⁻¹ for Cd²⁺ and Pb²⁺. Additionally, researchers examined the impacts of common cation and anion interference that has been usually observed in environmental waters, accompanied by acceptable outputs. Ultimately, the designed electrode experienced a successful application to simultaneously detect Cd²⁺ and Pb²⁺ in real river water samples.

One of the electrochemical sensors for the simultaneous detection of the Cd(II) and Pb(II) ions, developed with a phytic acid (PA) functionalized polypyrrole (PPy)/GO modified electrode, was reported by Dai et al.⁸⁵ The PA/PPy/GO modified GCE enjoyed higher electrochemical conductivity, notably increased peak current in comparison to PA/GO and PPy/GO modified electrodes. This sensor has been utilized to electrochemically analyze distinctive trace metal ions via DPV. Therefore, this modified electrode has been utilized for measuring Pb(II) and Cd(II) with the linear working ranges from 5 to 150 μg/L. The LOD (S/N = 3) was equal to 0.41 μg/L for Pb(II) and 2.13 μg/L for Cd(II). In addition, in order to evaluate the practical application of the PA/PPy/GO/GCE, it was used to determine Pb(II) and Cd(II) in tap water samples.

Baghayeri et al. introduced a GCE coated with poly(amidoamine) dendrimer functionalized magnetic GO (GO-Fe₃O₄–PAMAM) that has been utilized to simultaneously detect Pb(II) and Cd(II) in environmental water.⁸⁶ The SWASV was utilized to detect the targeted metal ions. The electrochemical sensor was followed by a linear voltammetric response in the ranges between 0.4 and 120 μg L⁻¹ for Pb(II) and 0.2 and 140 μg L⁻¹ for Cd(II) with a coefficient of determination greater than 0.99. In addition, a lower LOD (130 ng L⁻¹) was observed for Pb(II) and 70 ng L⁻¹ for Cd(II). In addition, probable interferences from a number of metal ions such as Cu(II), Tl(I), Hg(II), Zn(II), Mg(II), Co(II), Mn(II), Ca(II), Fe(III), Ni(II), Cr(III), and In(III) have been investigated and it has been found that there has been no prominent interference. Ultimately, the current assay has been utilized to simultaneously detect the two cations in the real water samples; ICP-MS was used to verify the outputs.

Maleki et al. synthesized the second-generation (G2) PAMAM functionalized magnetic NPs (Fe₃O₄@G2-PAMAM) which has been utilized for concurrent electrochemical determination of Cd²⁺ and Pb²⁺ ions in the environmental water.⁸⁷ The modified magnetic carbon paste electrode (MCPE) showed very good electrochemical features to simultaneously detect Cd²⁺ and Pb²⁺ with the use of SWASV. The linear range of 0.5–80 ng mL⁻¹ for both metal ions was acquired at the Fe₃O₄@G2-PAMAM/MCPE. The observed LOD-values (0.17 ng mL⁻¹ for Pb and 0.21 ng mL⁻¹ for Cd) satisfied the threshold limit (10 ng mL⁻¹ for Pb and 3 ng mL⁻¹ for Cd) confirmed by the World Health Organization (WHO) for the normal volume of lead and cadmium in drinking water. Finally, they substantially dealt with the utility of Fe₃O₄@G2-PAMAM/MCPE to measure Cd²⁺ and Pb²⁺ ions in diverse real water samples (lake water, river water, and wastewater).

3.1.7. Hg(II) and Pb(II).

Wei et al. utilized PPy/carbonaceous nanospheres (PPy/CNSs) modified SPE in order to selectively detect the Hg(II) and Pb(II) ions.⁸⁸ The optimization of the experimental conditions was performed using SWASV. Supporting Pb and Hg were deposited at the potentials of −1.0 V and −0.3 V for 120 s by the reduction of Hg(II) and Pb(II) in 0.1 M acetate buffer (pH 3.0). Considering the PPy and CNSs benefits in the electrochemical determination, they obtained very good sensitivity (0.113 μA nM⁻¹ for Hg(II) and 0.501 μA nM⁻¹ for Pb(II)). In addition, LODs (0.0041 nM for Pb(II) and 0.0214 nM for Hg(II) 3σ technique) have been very low in comparison to the guideline values for drinking water presented by the WHO. Finally, researchers substantially confirmed the analytical application of the designed electrode for detecting metal ions in a real sample.

3.1.8. Pb(II) and Cu(II).

Wang and co-workers described a new method for ultratrace detection of Pb(II) and Cu(II) with the use of Au nanocomposite electrode modified with NH₂-MIL-53(Al)/PPy.⁸⁹ The PPy nanowires were fabricated by a chemical polymerization method, and NH₂-MIL-53(Al) was deposited on the PPy through an in situ electrochemical process. The electrochemical behavior of the modified electrode toward Pb(II) and Cu(II) was investigated by DPV. Because of the synergistic effect of NH₂-MIL-53(Al) and PPy, the designed electrode displayed proper sensing performance to Pb(II) and Cu(II) in the range of 1–400 μg L⁻¹ with an LOD of 0.315 μg L⁻¹ and 0.244 μg L⁻¹, respectively. NH₂-MIL-53(Al)/PPy/Au also demonstrated strong anti-interference ability and good stability. In addition, this new sensor could be utilized for the determination of Pb(II) and Cu(II) in tap water samples.

3.1.9. Cd(II), Cu(II), and Pb(II).

Guo et al. used DPASV to devise the RGO–CHIT/poly-l-lysine nanocomposite modified GCE to simultaneously detect trace Pb(II), Cd(II), and Cu(II).⁹⁰ The RGO and CHIT hybrid matrix were coated over the surface of GCE. Afterward, poly-l-lysine (PLL) films were procured by electro-polymerization with the CV technique for preparing the RGO–CHIT/PLL modified GCE (RGO–CHIT/PLL/GCE). It should be noted that three suitable sharp stripping peaks could be observed for Pb(II), Cu(II), and Cd(II). In addition, the stripping peak currents showed a suitable linear proportionality between 0.05 μg L⁻¹ and 10.0 μg L⁻¹ with the LOD equal to 0.01 μg L⁻¹ for Cd(II), 0.02 μg L⁻¹ for Cu(II), and 0.02 μg L⁻¹ for Pb(II). Therefore, the LOD for Pb(II), Cu(II), and Cd(II) highly declined in comparison to the guideline values for drinking water presented by the WHO. Ultimately, researchers achieved substantial application for the analysis of tap water samples.

3.1.10. Cu(II), Hg(II), and Pb(II).

Deshmuket al. reported a ethylenediaminetetraacetic acid_–PANI/SWCNTs (EDTA-PANI/SWCNTs) nanocomposite modified electrode to electrochemically detect lead(II), mercury(II), and copper(II) ions by DPV.⁹¹ In fact, a stainless steel electrode was rectified with the use of the PANI/SWCNTs nanocomposite followed by additional modification of the PANI/SWCNTs nanocomposite with the EDTA chelating ligand (Figure 5); the LOD for EDTA-PANI/SWCNTs/stainless steel toward the Pb(II), Hg(II), and Cu(II) was 0.08 ± 0.01 μM, 1.65 ± 0.01 μM, and 0.68 ± 0.01 μM, respectively. Furthermore, the simultaneous and selective detection of Cu(II), Pb(II), and Hg(II) ions was carried out in the presence 2 mM Co(II), Ni(II), and Cd(II) ions. The DPV voltammogram exhibited well-constructed and separated stripping peaks only for all Cu(II), Hg(II), and Pb(II); no rectified effect was observed after the addition of 2 mM of Cd(II), Co(II), and Ni(II).

Figure 5.

Diagram for the generation process of the EDTA-PANI/SWCNTs on a stainless steel electrode and utilization of the EDTA-PANI/SWCNTs/stainless steel electrode in detecting the heavy metal ions with DPV. Reproduced with permission from ref 91. Copyright 2018 Elsevier.

3.1.11. Cd(II), Pb(II), and Zn(II).

Ruecha et al. described a sensitive electrochemical sensor with the use of a Gr/PANI nanocomposite for simultaneously detecting the Cd(II), Zn(II), and Pb(II) ions.⁹² According to the analyses, the Gr/PANI-modified electrode had higher electrochemical conductivity and generated a 3-fold enhancement in the anodic peak current against the unmodified electrode. Moreover, the linear working range between 1 and 300 μg L⁻¹ has been observed between the metal ion concentration and anodic current with the LOD equal to 1.0 μg L⁻¹ for Zn(II), and 0.1 μg L⁻¹ for both Pb(II) and Cd(II) (S/N = 3). Researchers found that the Gr/PANI-modified electrode selectively determined the targeted metals in the presence of common metal interference such as Cu(II), Fe(II), Fe(III), Mn(II), Ni(II), and Co(III) ions. Ultimately, their technique could be utilized to detect Cd(II), Zn(II), and Pb(II) ions in the serum of humans with the use of the standard addition procedure. Table 1 lists applications of polymer nanocomposites-based electrochemical sensors for detecting heavy metal ions in different environmental matrices.

Table 1.

Applications of Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Various Heavy Metal Ions in Environmental Samples

electrochemical sensor	method	analyte	detection limit	linear range	ref
DMAET-SWCNT-PABSA/Au	SWASV	Hg2+	0.0634 μM	20 ppb-250 ppb	76
PEDOT/GO/GCE	DPSV	Hg2+	2.78 nM	10.0 nM-3.0 mM	77
P2MA-Ce2(WO4)3@CNT/NF-GCE	LSV	Cd2+	0.11 nM	1.0 nM −0.1 M	78
IIP/PAN/MWCNT/GCE	DPASV	Pb2+	0.16 μg L−1	0.50−22.0 μg L−1	79
Co3O4/RGO/CHIT/GCE	DPASV	Pb2+	0.35 nM	1.0−200.0 nM	80
Au NPs/PANI-co-PoT/GO/Au	SWV	Cr(VI)	0.0215 μM	5−500 μM	81
Pd-PEDOT/CE	DPASV	As3+	7 nM (0.52 ppb)	0.01 to 0.16 μM	82
GCE-MWCNT/poly(PCV)/Bi	DPASV	Cd2+	0.20 μg L−1	1.0−300.0 μg L−1	83
		Pb2+	0.40 μg L−1	1.0−200.0 μg L−1
Gr/PANI/PS/SPE	SWASV	Pb2+	3.30 μg L−1	10−500 μg L−1	84
		Cd2+	4.43 μg L−1	10−500 μg L−1
PA/PPy/GO/GCE	DPV	Cd(II)	2.13 μg/L	5−150 μg/L	85
		Pb(II)	0.41 μg/L	5−150 μg/L
GO-Fe3O4−PAMAM/GCE	SWASV	Pb(II)	130 ng L−1	0.4−120 μg L−1	86
		Cd(II)	70 ng L−1	0.2−140 μg L−1
Fe3O4@G2-PAMAM/MCPE	SWASV	Pb2+	0.17 ng mL−1	0.5−80 ng mL−1	87
		Cd2+	0.21 ng mL−1	0.5−80 ng mL−1
PPy/CNSs/SPE	SWASV	Pb(II)	0.0041 nM	1−7 nM	88
		Hg(II)	0.0214 nM	5−35 nM
NH2-MIL-53(Al)/PPy/Au	DPV	Pb(II)	0.315 μg L−1	1−400 μg L−1	89
		Cu(II)	0.244 μg L−1	1−400 μg L−1
RGO−CHIT/PLL/GCE	DPASV	Cd(II)	0.01 μg L−1	0.05−10.0 μg L−1	90
		Pb(II)	0.02 μg L−1	0.05−10.0 μg L−1
		Cu(II)	0.02 μg L−1	0.05−10.0 μg L−1
EDTA-PANI/SWCNTs/stainless steel	DPV	Cu(II)	0.08 ± 0.01 μM	2 mM to 1.2 μM	91
		Pb(II)	1.65 ± 0.01 μM	1.8 mM to 4 μM
		Hg(II)	0.68 ± 0.01 μM	2 mM to 2 μM
Gr/PANI/SPE	SWASV	Zn(II)	1.0 μg L−1	1−300 μg L−1	92
		Cd(II)	0.1 μg L−1	1−300 μg L−1
		Pb(II)	0.1 μg L−1	1−300 μg L−1

3.2. Polymer Nanocomposite-Based Electrochemical Sensors for the Detection of Pesticides.

As demonstrated by the relevant research, pesticides, including herbicides, fungicides, and insecticides are extensively applied in agriculture throughout the world to control pests in agricultural crops to enhance the agricultural production yield. Nonetheless, such a widespread utilization would lead to significant risks to the safety of the environment and human health. The precise assessment of their pollution status in the environment is needed for appropriate management and monitoring because of short-term health issues (i.e., eye and skin irritation, headache, nausea, and dizziness) and chronic diseases (i.e., asthma, diabetes, cancer, and neurological disorders) related to pesticide use.⁹³ Consequently, simple, sensitive, and economical analyses of the trace amounts of the pesticides in environmental samples have been laborious tasks.

3.2.1. Fenitrothion (FNT).

Surucu et al. reported the electrochemical behavior and voltammetric determination of FNT on the basis of a pencil graphite electrode (PGE) modified with the RGO/poly(E)-1-(4-((4-(phenylamino)-phenyl)diazenyl)phenyl)ethanone (DPA) film.⁹⁴ Analyses reflected the very good function of the modified PGE to detect FNT. Then, the SWV current response of RGO/DPA/PGE for FNT made a linear change with concentration ranges between 0.096 and 1.912 M with an LOD value of 3.480 × 10⁻⁹ M; analyses indicated suitable selectivity of RGO/DPA/PGE in the interference examinations and its application to detect FNT. Furthermore, the procedure resulted in the proper recovery of FNT from the spiked tomato specimens in a range between 96.4 and 106.9%, which reflected its utility to detect FNT in real environmental samples for the agricultural and industrial sectors.

3.2.2. Malathion (MLT).

Bolat and Abaci presented nonenzymatic electrochemical sensing of the MLT pesticide in apple and tomato samples on the basis of the Au NPs-CHIT-ionic liquid (IL) hybrid nanocomposite.⁹⁵ PGEs have been adequately modified with an Au NPs-CHIT-IL hybrid for nonenzymatic electrochemical sensing of MLT directly. Thus, researchers demonstrated that the nanocomposite film expanded the electrodes’ surface area and created a very good ground to achieve the MLT electrochemical responses regardless of the acetylcholinesterase enzyme. Moreover, voltammetric current response displayed two linear dynamic ranges from 0.89 to 5.94 and 5.94 to 44.6 nM, which showed two binding sites with the LOD value of 0.68 nM. This technique has been exploited in analyzing real samples of tomato and apple, and outputs showed the possibility of the Au NP-CHIT-IL-modified electrodes to simply, rapidly, ultra-sensitively, and inexpensively detect MLT.

3.2.3. Paraquat.

Jeyapragasam and co-workers prepared poly(o-phenylenediamine) (PoPD)-MWCNT nanocomposite by the electropolymerization of o-phenylenediamine onto the functionalized MWCNT which acted as good modifiers for the electrochemical sensing of paraquat.⁹⁶ The PoPD-MWCNT/GCE showed the highest cathodic current toward paraquat compared to GCE, MWCNT/GCE, and PoPD/GCE. The sensor calibration plot for paraquat was constructed using SWV in the linear range from 8.8 × 10⁻⁷ to 2.5 × 10⁻⁸ M. The developed sensor resulted in an LOD value of 1.4 × 10⁻⁹ M. Finally, this sensor has been substantially utilized for determining paraquat in cabbage samples.

3.2.4. Simazine (SMZ).

Zhang et al. presented their newly devised electrochemical sensor on the basis of the molecularly imprinted polymer membranes (MIPM) of poly(o-amino-thiophenol) with the Au NPs modified Au electrode to sensitively and selectively detect SMZ in the environmental samples.⁹⁷ The nanoscaled MIPM with the increased specific surface area was procured by self-assembling the o-amino-thiophenol (ATP) and electrodeposition of ATP@Au NPs in the presence of the template SMZ. The linear dependency of the peak current on the SMZ concentration was in the range between 0.03 and 140 μM with an LOD of 0.013 μM; the SMZ imprinted sensor presented outstanding lengthy stability and reasonable repeatability. Additionally, it was successfully utilized for SMZ measurement in multiple real samples (tap and river water, and soil samples) so that the spiked recovery changed from 91.4% to 96.8%, which reflected an encouraging potential for functional applications.

3.2.5. Imidacloprid (IMI).

The IMI electrochemical detection with the use of the poly(carbazole)/chemically RGO modified GCE was devised by Lei et al.⁹⁸ They used a simplified process to make polycarbazole (PCz) and PCz/chemically RGO (CRGO) modified GCEs and utilized them as the sensing electrodes for voltammetric exploration of the IMI insecticides. Moreover, CV and DPV have been utilized as the sensing procedures. According to the analyses, the two modified electrodes enjoyed acceptable electrocatalytic activities for reducing IMI. In addition, the presence of CRGO augmented the electrochemical signals of the modified GCE, increasing the sensor sensitivity. Furthermore, LOQ and LOD values for CV, respectively, have been 0.74 and 0.22 μM, whereas for DPV, they respectively equaled 1.52 and 0.44 μM.

3.2.6. Phoxim.

Wu et al. designed a newly fabricated nonenzyme amperometric platform on the basis of poly(3-methyl-thiophene)/nitrogen doped graphene rectified GCE (P3MT/NG/GCE) to detect the trace amounts of the pesticide phoxim.⁹⁹ The linear variation of the reduction peak current of the CV curve was observed as the phoxim concentration changed over the two linear ranges between 0.02 and 0.2 μM and 0.2 and 2.0 μM so that lower LOD was 6.4 nM with regard to the declined linear range (S/N = 3). Moreover, the sensor had very good selectivity to phoxim in the wide range of probable interferences such as the ions, traditional environment contaminants, biomolecules, and the commonly applied organophosphatepesticides; environmental phoxim measurements were used to evaluate the reasonable functionality of the sensor.

3.2.7. Methyl Parathion (MP).

Xu et al. developed their newly devised poly(malachite green)/graphene nanosheets–NF (PMG/GrNSs–NF) composite film modified GCE for the electrochemical determination of MP.¹⁰⁰ p-Nitrophenol, the alkaline hydrolysis product of MP, was substantially detected at a comparatively declined potential with the use of the proposed electrode as a result of very good electrocatalytic activities of the PMG/GrNSs–NF composite film. In a composite film, GrNSs promoted the PMG stability and the rate of the electron transfer. On the basis of the optimized conditions, chrono-amperometric response current was proportional to the MP concentration in ranges between 0.02 and 1.5 μM with the LOD of 2.0 nM. Consequently, the authors utilized their sensor to detect MP in real samples (cabbage, river water, tap water) wherein reasonable outputs with recovery have ranged between 97.20 and 104.53%.

Zeng et al. developed a voltammetric sensor to determine MP using the MWCNTs–poly(acrylamide) (MWCNTs–PAAM) nanocomposite film modified GCE.¹⁰¹ Considering the experimental outputs, MWCNTs–PAAM/GCE displayed increased absorption capacity and stronger affinity toward the MP in comparison to a number of metal ions and nitroaromatic compositions present in the environmental samples. Notably, the absorbed content of the MP on the MWCNTs–PAAM/GCE approximated the equilibrium value during a 5 min absorption time. Afterward, a linear calibration curve for MP was discerned in a concentration range between 5.0 × 10⁻⁹ and 1.0 × 10⁻⁵ M with an LOD of 2.0 × 10⁻⁹ M. Finally, researchers demonstrated that MWCNTs–PAAM/GCE is one of the appropriate sensing tools for quickly, sensitively, and selectively detecting MP in the samples of environmental water.

Zhao et al. presented a molecularly imprinted polymer–IL–Gr composite film coated GCE (MIP–IL–Gr/GCE) for electrochemical determination of MP.¹⁰² The researchers produced the electrode by coating a GCE with the IL–GO mix, accompanied by MIP suspension. In the next stage, the obtained electrode was conditioned at −1.3 V (versus SCE) in the Na₂SO₄ solution for making the GO turn to G. Then, MIP was procured via free radical polymerization of methacrylic acid as a functional monomer, MP as the template, ethylene-glycol dimethacrylate as the cross-linking reagent, and 2,2’-azobis(isobutyronitrile) as an initiator. The MP peak current linearly ranged between 0.010 and 7.0 μM with a sensitivity equal to 12.5 μA/μM and LOD of 6 nM (S/N = 3). This method displayed higher stability and selectivity and was utilized for detecting MP in the samples (cabbage, apple peel) so that the recovery ranged between 97 and 110%. Table 2 lists the applications of polymer nanocomposites-based electrochemical sensors for detection of pesticides in different environmental matrices.

Table 2.

Applications of Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Various Pesticides in Environmental Samples

electrochemical sensor	method	analyte	detection limit	linear range	ref
RGO/DPA/PGE	SWV	fenitrothion	0.096−1.912 μM	3.480 × 10−9 M	94
Au NPs-CHIT-IL/PGE	SWV	malathion	0.68 nM	0.89−44.6 nM	95
PoPD-MWCNT/GCE	SWV	paraquat	1.4 × 10−9 M	8.8 × 10−7 to 2.5 × 10−8 M	96
MIP/ATP@Au NPs/ATP/Au	CV	simazine	0.013 μM	0.03 to 140 μM	97
PCz/CRGO/GCE	CV	imidacloprid	0.22 μM	3 to 10 μM	98
P3MT/NG/GCE	CV	phoxim	6.4 nM	0.02−2.0 μM	99
PMG/GrNSs−NF/GCE	chronoamperometry	methyl parathion	2.0 nM	0.02 to 1.5 μM	100
MWCNTs−PAAM/GCE	DPV	methyl parathion	2.0 × 10−9 M	5.0 × 10−9 to 1.0 × 10−5 M	101
MIP-IL-Gr/GCE	DPV	methyl parathion	6 nM	0.010−7.0 μM	102

3.3. Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Phenolic Compounds.

The detection of phenolic compounds in food, medical, environmental ground, and surface water has been a key issue because of the respective toxicity and persistency in the environment.¹⁰³ Some phenol derivatives are from industrial waste due to their use as intermediates in the production drugs, explosives, wood preservatives, and in leather treatments. These polluting chemicals are readily adsorbed by the skin and mucous membrane of human and animal. Moreover, their toxicity has a direct influence on multiple tissues and limbs, mainly the lung, liver, kidney, and the genitourinary system.¹⁰⁴ The permissible level established by the United States Environmental Protection Agency (EPA) has been 1 mg/L for most of the common phenolic compounds.¹⁰⁵ Therefore, numerous researchers have strived to address the simplified and efficacious detection of the phenolic compounds.

3.3.1. Phenol.

Yang et al. reported phenol electrochemical oxidation at a polydiphenylamine (PDPA)-MWCNT composite film modified GCE.¹⁰⁶ The researchers used CV to polymerize PDPA on the MWCNT modified GCE in H₂SO₄(0.1 M). The composite film of PDPA-MWCNT exhibited suitable electrocatalytic behavior for oxidizing phenol at pH 7 where it displayed clear anodic peak at 0.65 V (versus the Ag/AgCl electrode). Results showed increase in the peak current by 2.36 and 8.48 times over that of the PDPA/GCE and MWCNT/GCE electrode with the use of MWCNT in the PDPA-MWCNT film. Peak current experienced a linear enhancement as the phenol concentration increased. Furthermore, linear ranges from 9.8 and 80 μM have been observed for the amperometric detection of phenol at the composite film modified electrode.

3.3.2. 2-Aminophenol (2-AP) and 4-Aminophenol (4-AP).

Yin et al. devised and utilized a Gr–CHIT composite film modified GCE for the voltammetric detection of 4-AP.¹⁰⁷ According to the results obtained from the 0.1 M pH 6.3 phosphate buffer solution (PBS), the 4-AP redox peak currents notably increased, oxidation overpotential lowered, and a great decline was observed in the peak to peak separation at the Gr–CHIT composite film modified GCE in comparison to the bare GCE and CHIT modified GCE, which reflected electrocatalytic activities of graphene toward 4-AP. The current of the oxidation peaks corresponded to the range of 4-AP concentration between 0.2 and 550 μM with the correlation coefficient equal to 0.9930 and LOD of 0.057 μM (S/N = 3). According to this newly designed procedure, 4-AP could be readily detected in water specimens and paracetamol pills via the standard addition, which reflected the devised sensor utilization for detecting 4-AP in the environment and drug.

Fan et al. devised one of the electrochemical sensors on the basis of the Gr–PANI nanocomposite for voltammetric detection of 4-AP;¹⁰⁸ CV was used to investigate the electrochemical behaviors of the 4-AP at the Gr–PANI composite film modified GCE where the analyses showed the increase of voltammetric response at Gr–PANI modified GCE via 4-AP. Moreover, it exhibited an acceptable analytical function to detect 4-AP with an LOD of 6.5 × 10⁻⁸ M and higher sensitivity of 604.2 μA mM⁻¹. In addition, it could be possible to simultaneously detect the 4-AP and paracetamol with no interferences from them in the larger dynamic range. Finally, this Gr–PANI nanocomposite could present a novel ground to devise the graphene-based electrochemical sensors and biosensors.

In another study, one of the electrochemical sensors based on the RGO/poly-l-glutathione nanocomposite (RGO/P-L-GSH), developed for the determination of 4-AP, was reported by Ezhil Vilian et al.¹⁰⁹ Researchers electrodeposited the P-LGSH on the RGO to generate the RGO/P-L-GSH composite matrix on a GCE (RGO/P-L-GSH/GCE). In addition, their electrochemical sensor superbly responded to 4-AP over a wide broad range between 0.4 and 200 μM (R² = 0.9934) with the decreased LOD = 0.03 μM (S/N = 3) and sensitivity = 27.2 μA μM⁻¹ cm⁻². However, the sensor demonstrated suitable anti-interference capability in the presence of hydrogen peroxide, glucose, paracetamol, fructose, l-isoluecine, ethanol, l-cysteine, l-histidine, l-serine, dopamine, l-tyrosine, urea, phenylalanine, K⁺, Ca²⁺, Na⁺, F⁻, SO₄²⁻, Cu²⁺, Fe³⁺, Zn²⁺, SO₄²⁻, SO₃²⁻, Cl⁻, ascorbic acid, and folic acid. Finally, researchers experienced successful utilization of their procedure for producing a sensor to detect 4-AP in water and juice samples.

In their research, Yi et al. reported a strongly sensitive and simultaneous electrochemical detection of 2-AP and 4-AP.¹¹⁰ By simple electro-polymerization of l-arginine (l-Arg) and β-cyclodextrin (β-CD) on the CNTs@graphene nanoribbons (CNTs@GNRs) surface, the P-β-CD-l-Arg/CNTs@GNRs composites-modified GCE was achieved, which subsequently was applied for electrochemical sensing of the 4-AP and 2-AP (Figure 6). However, P-β-CD-l-Arg/CNTs@GNRs/GCE showed synergistic impacts from β-CD (that has a high host–guest recognition and enrichment ability), l-Arg (very good electrocatalytic activities), and CNTs@GNRs (major electrochemical features and larger surface areas). The P-β-CD-l-Arg/CNTs@GNRs/GCE was employed for the simultaneous determination of 2-AP and 4-AP with the linear response in the ranges between 25.0 and 1300.0 nM.

Figure 6.

Procurement procedure for P-β-CD-l-Arg/CNTs@GNRs/GCE and sensing 2- and 4-AP with electrochemical technique. Reproduced with permission from ref 110. Copyright 2016 Elsevier.

3.3.3. Nitrophenol (NP).

Ndlovu et al. reported the electrochemical detection of ortho-nitrophenol (o-NP) by SWV with the use of a nanocomposite of the G2 poly(propyleneimine) dendrimer and the Au NPs modified GCE (PPI-Au NPs/GCE).¹¹¹ Therefore, a simplified and reproducible one-step electrocodeposition of PPI dendrimer and Au NPs on the GCE was carried out with a typical reduction peak at ca-650 mV for o-NP with the greater currents. Then, they utilized the nanocomposite electrode as a sensor to detect the organic pollutant of the model with highly acceptable reproducible and stable features. It was found that the reduction peak current had a direct relationship with o-NP concentrations in a range from 6.1 × 10⁻⁷ M to 6.25 × 10⁻⁴ M with the LOD of 4.5 × 10⁻⁷ M.

Tang et al. procured the Gr-CHIT composite film-modified GCE (Gr-CHIT/GCE) for the simultaneous detection of o-NP and para-nitrophenol p-NP)¹¹² and investigated the electrochemical behavior of p-NP and o-NP by CV; the modified GCE displayed very good electrocatalytic activities to reduce p-NP and o-NP. Using LSV method, p-NP exhibited the linear determination ranges from 0.1–140 μM with an LOD of 0.09 μM (S/N = 3) at G-CHIT/GCE, while 1–240 μM for o-NP with the LOD equal to 0.1 μM. Finally, Gr-CHIT was demonstrated as an encouraging sensor material for simultaneously detecting the nitrophenol isomers. The excellent recovery of the spiked samples indicated that the as-prepared electrode was practical for the detection of nitrophenol isomers in real samples.

An electrochemical sensor for sensitively detecting the o-NP was constructed by Arfin et al. using the GO-poly-(ethylenimine) dendrimer (GO–PEI)-modified GCE which showed very good electrochemical behaviors toward o-NP redox via a combination of the PEI and GO features.¹¹³ The peak currents linearly related to o-NP concentration in a range between 5 and 155 μM. According to the analyses, the LOD of o-NP has been as low as 0.10 μM characterized by the signal-to-noise (S/N = 3) properties. In addition, the RSD for three times the detection of o-NP has been 0.12%, revealing acceptable features of the sensor such as its ability to be reproducible, selective, and stable. Finally, researchers reported successful utilization for the sensor to treat the industrially polluted waters in the course of removing the heavy metals such as Ni²⁺, Fe³⁺, Pb²⁺, Cu²⁺, and Zn²⁺ ions.

Wei and co-workers developed ZnFe₂O₄/PANI@RGO aerogel as a convenient electrochemical sensor for the detection of p-NP.¹¹⁴ The synthesized ZnFe₂O₄/PANI@RGO aerogel exhibited excellent electrochemical sensing for p-NP and efficient catalytic reduction. In addition, ZnFe₂O₄/PANI@RGO modified GCE demonstrated a linear concentration range between 1 and 100 μM, excellent sensitivity of 36.898 mA mM⁻¹ cm⁻², and an LOD equal to 0.083 μM for p-NP. The ZnFe₂O₄/PANI@RGO/GCE offered long-term stability and excellent anti-interference ability for p-NP detection. In addition, the detection reliability of tap water was estimated, and an average recovery of 101.16% was maintained.

3.3.4. Chlorophenols (CPs).

Wang et al. constructed a strongly selective electrochemical sensor to detect 4-chlorophenol (4-CP). Therefore, to attain higher selectivity, they dealt with the synthesis of the MIP of 4-CP and blended it with poly(diallyldimethylammonium chloride) (PDDA)-functionalized Gr(PDDA-Gr).¹¹⁵ This MIP/PDDA-Gr/GCE exhibited a higher sensing function toward 4-CP. On the basis of the optimum conditions, their sensor linearly responded to 4-CP concentration in the wider ranges between 0.8 to100 μM with an LOD of (S/N=3) of 0.3 μM. Furthermore, the imprinted sensor had very good specific recognition capability for 4-CP, avoiding the interference of the phenolic compounds with the same structure. Finally, its successful utilization for detecting 4-CP in tap and lake water samples was illustrated with satisfactory analytical results.

Zhu et al. constructed a novel electrochemical sensing platform to quantitatively determine and assess the toxicity of of 2,4-dichlorophenol (2,4-DCP) with the use of poly(eosinY)/hydroxylated MWCNT modified electrode (PEY/MWCNTs–OH/GCE).¹¹⁶ Researchers observed linear enhancement of the oxidation peak current as the concentration was enhanced from 0.005 to 0.1 μM and 0.2 to 40.0 μM with an LOD of 1.5 nM. In addition, PEY/MWNTs–OH/GCE displayed very good electrocatalytic activities onto the intracellular electro-active samples. They attributed two sensitive electrochemical signals to adenine/hypoxanthine and guanine/xanthine in the human hepatoma (HepG2) cells, which were determined concurrently, and successful application of the sensor was demonstrated for the evaluation of 2,4-DCP toxicity to the HepG2 cells. According to the analyses, the IC50-values based on two electrochemical signals were 252.83 and 201.07 μM.

3.3.5. Dihydroxybenzenes Isomers.

Yin et al. prepared Gr–CHIT composite film modified GCE (Gr–CHIT/GCE).¹¹⁷ Their fabricated electrode had very good electrochemical catalytic activity toward oxidation of resorcinol (RS), catechol (CC), and hydroquinone (HQ) and their simultaneous detection in the water specimens. The oxidation overpotentials of RS, CC, and HQ notably declined and accordingly the oxidation current enhanced significantly in comparison to currents observed at the bare GCE and CHIT modified GCE. They utilized DPV to simultaneously detect the RS, CC, and HQ in their ternary mix. Moreover, the peak to peak potential separation between RS and HQ, HQ and CC, and CC and RS, respectively, was 0.484, 0.096, and 0.388 V. Furthermore, calibration curves for RS, HQ, and CC, respectively, ranged from 1 × 10⁻⁶ to 5.5 × 10⁻⁴ M, 1 × 10⁻⁶ to 3 × 10⁻⁴ M, and 1×10⁻⁶ to 4 × 10⁻⁴ M with an LOD of 7.5 × 10⁻⁷ M(S/N = 3).

Xu et al. developed a sensitive amperometric sensor for the selective detection and electrocatalytic oxidation of HQ in cosmetics on the basis of superb electrocatalytic properties of CNT-doped PEDOT conducting polymer.¹¹⁸ The electrochemical oxidization of HQ at the PEDOT/CNT modified CPE was investigated using CV and DPV which reflected the highly declined oxidation potential of HQ on the PEDOT/CNT/CPE in comparison to the bare electrode. The DPV current of the sensor was linear with the HQ concentration in the ranges between 1.1 and 125 μM with an LOD of 0.3 μM. Thus, this sensor could readily quantify HQ in the presence of conventional interference and could be used in real cosmetics specimens with reasonable accuracy.

Nambiar et al. designed the specific Au atomic cluster–PEDOT nanocomposite modified Au electrode (AuAC/PEDOT/Au) for ultrasensitive voltammetric determination of CC.¹¹⁹ The addition of copper(II) resulted in the enhancement of the CC electrocatalytic oxidation by forming copper(I). Figure 7 presents a schematic illustration of the preparative procedure for the AuAC/PEDOT/Au electrode which offers a broader calibration range between 1 × 10⁻⁴ and 10 μM with the minimum LOD of 6.3 pM for CC. The electrochemical sensor displayed acceptable selectivity reproducibility (1.23% for 1 nM of CC), and it has been applied for routinely detecting and quantifying catechol in natural water specimens.

Figure 7.

Schematic representation for the fabricated catechol electrochemical sensor. Reproduced with permission from ref 119. Copyright 2013 Royal Society of Chemistry.

Tehrani et al. devised a voltammetric sensor to simultaneously detect CC and HQ using Gr and poly(4-vinylpyridine) modified GCE (Gr-P4VP/GCE)¹²⁰ where the electrode applied a synergistic impact of Gr and P4VP on the electrocatalytic oxidation of HQ and CC in sodium PBS at 2.5 pH. Then, CV and DPV were deployed to resolve the anodic peak potential of Epa, providing simultaneous detection of the two compounds. Thus, Gr-P4VP/GCE had very good sensitivity for measuring CC and HQ with an LOD of 26 and 8.1 nM, respectively, and had no impact on the conventional interference by nitrophenol, phenol, chloro-phenols, and aminophenols. Finally, it has been used to simultaneously detect the CC and HQ spikes in tap and lake waters with reasonable outputs.

Li et al. prepared a GCE modified with poly(sulfosalicylic acid) (PSA) and PDDA-Gr via a convenient self-assembly procedure for the simultaneous determination of CC and HQ.¹²¹ Under optimal conditions, the modified GCE exhibited two complete redox waves for HQ and CC in CV with a peak potential separation equal to 111 mV, ensuring the anti-interference potential of the electrochemical sensor and providing the simultaneous detection of the dihydroxybenzene isomers in real specimens. The results showed a linear anodic peak current of CC within a concentration range between 1 × 10⁻⁶ and 4 × 10⁻⁴ M in the presence of 3 × 10⁻⁵ M HQ and an LOD of 2.2 × 10⁻⁷ M (S/N = 3). Additionally, the HQ anodic peak current was found to be linear in a concentration range between 2 × 10⁻⁶ and 4 × 10⁻⁴ M in the presence of 2 × 10⁻⁵ M CC with an LOD of 3.9 × 10⁻⁷ M (S/N = 3) and it was utilized for concurrently detecting HQ and CC in tap water with reasonable outputs.

Tian et al. synthesized one-dimensional (1D) PEDOT-Gr composites with the help of in situ electro-polymerization of 3,4 ethylene-dioxythiophene on the Gr.¹²² The 1D PEDOT-Gr composite showed outstanding electrocatalytic activity toward dihydroxybenzene isomers and NO₂^– ions. Simultaneous detection of the four analytes namely CC, HQ, nitrite, and RS, was recorded by DPV using the PEDOT-Gr/Ta electrode which exhibited the peak to peak separation of 392 mV between CC and RS, 108 mV between HQ and CC, as well as 188 mV between RS and nitrite, while the LOD respectively was 0.08, 0.06, 7, and 0.16 mM for CC, HQ, nitrite, and RS. Furthermore, 1D PEDOT-Gr composite displayed higher electrocatalytic activities and selectivity, and faster response ability.

Liu et al. prepared poly(N-methylthionine) (PNMTh) at RGO electrode in one of the strong acid solutions with the use of electrochemical polymerization; PNMTh/RGO nanocomposite served as electrochemical sensing materials for sensitively detecting HQ.¹²³ As a result of the synergistic effects between RGO (large specific surface area and higher conductivity) and PNMTh (redox properties), the PNMTh/RGO nanocomposite efficiently enhanced the electron transfer between the electrode and HQ, thus reducing the overpotential of the HQ oxidation. In addition, the linear regression area ranged between 1.0 and 1000.0 μM, the LOD was 0.75 μM, and importantly catechol had no interference with HQ detection.

Cheng et al. constructed an electrochemical sensor by electrodeposition of dendritic Au nanostructures (DGNs) and electropolymerization of PEDOT consecutively on GCE (PEDOT/DGNs/GCE). The constructed sensor with a dendritic structure, large surface area, and good conductivity, displayed high sensitivity for the detection of dihydroxybenzene isomers¹²⁴ with a lower LOD of 9.5 × 10⁻⁷ M for HQ, 1.6 × 10⁻⁶ M for CC, and 1.3 × 10⁻⁶ M for RS. Finally, their sensor showed acceptable results for a successful application to determine dihydroxybenzene in cosmetic samples with satisfactory results.

3.3.6. Naphthol Isomers.

Li et al. fabricated a polymer film incorporated with the Au NPs modified electrode¹²⁵ which entailed the eletrodeposition of Au NPs and electro-polymerization of 3-methylthiophene (3MT) over the GCE; semi-derivative voltammetry was used for simultaneously detecting naphthol isomers at the P3MT nano-Au/GCE in the presence of surfactant, cetyltrimethylammonium bromide (CTAB). Additionally, the researchers evaluated the contribution of assorted surfactants, and the respective mechanisms were comprehensively illustrated. Under the optimized conditions, the linear calibration range for detecting naphthols was between 7.0 × 10 and 7 M to 1.5 × 10 and 4 M for 1 naphthol and 1.0 × 10⁻⁶ and 1.5 × 10⁻⁴ M for 2-naphthol with an LOD of 1.0 × 10⁻⁷ and 3.0 × 10⁻⁷ M (S/N = 3), respectively.

3.3.7. Bisphenol A (BPA).

Yin et al. proposed a GCE, which was modified with the PAMAM dendrimers and composite Au NPs as well as the silk fibroin (PAMAM-Au NPs-SF/GCE)¹²⁶ which showed a differentiated electrochemical response to BPA. The electrochemical impedance spectroscopy (EIS) was utilized to characterize the surface where the electrode showed improvement in the absorption capacity and increase in the response to BPA in comparison to a surface with no modification; a linear response in the concentration ranges between 1 nM and 1.3 μM with a correlation coefficient of 0.9991 and LOD of 0.5 nM at an S/N of 3 was discerned. Thus, the procedure could be used for detecting BPA in water samples with the recovery ranging between 97% and 105%.

Yin et al. presented a simplified and sensitive electro-analytical technique to detect BPA with the use of PAMAM and Fe₃O₄ magnetic NPs modified GCE.¹²⁷ However, in comparison to the bare electrode, the PAMAM–Fe₃O₄ modified electrode notably improved the BPA oxidation peak current and declined the oxidation overpotential, thus reflecting on the remarkable improvement in the detection of BPA sensitivity via the modified electrode. A linear increase of the oxidation current was observed as the BPA concentration range was enhanced from 1 × 10⁻⁸ to 3.07 × 10⁻⁶ M with a correlation coefficient of 0.9996 and LOD of 5 × 10⁻⁹ M. Then, the current approximated 95% of the steady-state current in ~6 s. Finally, the procedure was utilized with encouraging outputs for detecting BPA in milk samples.

Zhang et al. devised a ratiometric electrochemical sensor for strong sensitive and selective detection of BPA (Figure 8).¹²⁸ Their assay strategy was established on the competitive host–guest interactions between poly-β-cyclodextrin/electro-reduced graphene (Pβ-CD/ERGr) and Rhodamine B (RhB) probe or BPA target molecule. In fact, RhB is capable of entering the β-CD hydrophobic inner cavity and displayed a clear oxidation peak on the Pβ-CD/ERGr modified GCE. Consequently, in the presence of BPA, RhB molecules were displaced by BPA as the host–guest interaction between BPA and β-CD was more powerful in comparison to the interaction between RhB and β-CD. Hence, RhB (I_RhB) oxidation peak current decreased with the increase in the oxidation BPA (I_BPA) peak current. Furthermore, the logarithmic value of I_BPA/I_RhB was linear with the logarithm of BPA concentration in a range between 1 and 6000 nM and LOD of 52 pM (S/N = 3). Finally, the developed electrode was used to detect BPA in real-water samples.

Figure 8.

A schematic of the ratiometric electrochemical sensor to detect BPA. Reproduced with permission from ref 128. Copyright 2015 Elsevier.

Peng et al. described an electrochemical sensor to simultaneously detect BPA and HQ.¹²⁹ This electrochemical sensor was devised on a CPE that was doped with Gr and coated with poly(melamine) (PME/Gr-CPE). The modified electrode exhibited very good conductivity, electro-oxidative activities, and absorptive abilities with DPV peaks separated for BPA and HQ by ~300 mV. It was found that the amperometric response (at 0.5 V for HQ and 0.8 V for BPA both versus SCE) was linear for BPA in a concentration ranging between 9.0 μM and 1.0 mM, and for HQ between 7.0 μM and 1.0 mM with LOD values of 10.5 and 74.0 nM, respectively. Finally, the sensor was reproducible with high stability and sensitivity, and could be utilized for the simultaneous detection of HQ and BPA in spiked tap water and wastewater.

Li et al. proposed a simplified electrochemical sensor with the RGO-Ag/PLL modified GCE for BPA detection,¹³⁰ where the synthesized RGO-Ag/PLL nanocomposites displayed higher electrocatalytic activities toward BPA electrochemical oxidation. Additionally, DPV was applied as an analytical procedure to quantitatively detect BPA as the electrochemical sensor linearly responded to BPA in ranges between 1 and 80 μM with an LOD of 0.54 μM at the signal-to-noise ratio of 3. The BPA sensor exhibited excellent selectivity, stability, and reproducibility and was finally utilized for detecting BPA in the drinking water satisfactorily.

Bolat et al. presented an electrochemical for the detection of BPA based on a convenient procedure¹³¹ where CTAB and MWCNTs were blended via electro-polymerization on the PGE surface through CV to produce the poly(CTAB)-MWCNTs composite. Analysis showed a significant increase of the BPA electrochemical signal at poly(CTAB)-MWCNTs modified PGE with an LOD value of 134 pM with an acceptable regression between the BPA peak current (R² = 0.993) and the concentration with the use of the SWV in ranges between 2 and 808 nM. The technique was used to detect BPA in the bottled water, a baby’s bottle, and a teether from local markets with the greatest level of recovery.

Ramu and colleagues synthesized the Fe₃O₄–PEI nanocomposite for the sensitive electrochemical detection of BPA in real samples.¹³² The Fe₃O₄–PEI/carbon electrode showed high electrochemical sensitivity and good catalytic activity toward BPA. According to the analyses, the linear ranges of BPA were equivalent to 0.01 to 50 μM. The LOD was 3.2 nM. The fabricated electrode successfully detected BPA in milk and water samples with high recovery percentages of 100.9% and 100.5%, respectively. Table S1 in the Supporting Information lists the recent applications of polymer nanocomposites-based electrochemical sensors for the detection of phenolic compounds in different environmental matrices.^106–132

3.4. Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of the Nitroaromatics.

Nitroaromatic compounds have been associated with contaminants, toxicity, carcinogenesis, mutagenesis, and treatment actions, have appeared as intermediates in the syntheses of complicated organic molecules, and are found to be responsible for major environmental pollution issues. Therefore, the development of specific and convenient analytical methods for nitroaromatic compounds is very significant.

3.4.1. Nitrobenzene (NBz).

Xu et al. developed a selective and sensitive electrochemical sensor for detection on a CPE modified nanocomposite procured from the conducting polymer PEDOT and MWCNT¹³³ which displayed excellent catalytic activity toward the electrochemical reduction of NBz; the NBz reduction potential was significantly lowered with an increase in the reduction current. The modified CPE could determine NBz in a concentration range between 0.25 and 43 μM with an LOD of 83 nM. The highly stable sensor endowed with reapplicable potential had no interference by other widely found nitrocompounds and could be utilized for the detection of NBz in wastewater specimens.

Ren et al. devised a sensitive electrochemical sensor to detect nitrobenzene on immobilized silver NPs (AgNP)-PME-MWCNT on a GCE.¹³⁴ It has been found that the modified electrode had an effective electrocatalytic contribution to reduce NBz and remarkably declined the nitrobenzene overpotential; therefore, the peak current enhanced greatly compared with the bare GCE or the other modified electrodes. The sensor had a wider linear calibration range between 20 and 1000 μM and between 1000 and 6000 μM, with lower LOD of 0.55 μM to detect NBz. The modified electrode enjoyed acceptable stability, reproducibility, and selectivity and was successful applied as a sensor for detecting NBz in the river water and tap water samples with a recovery between 97.2 to 104.6%.

Kariuki et al. developed an electrochemical sensing platform for NBz with the use of the silver NPs (Ag NPs) inserted in a poly(amic) acid (PAA) polymer matrix (PAA-Ag NPs)¹³⁵ and the PAA-Ag NPs-modified GCE exhibited encouraging potential as an electrochemical sensor. It exhibited wider linear dynamic ranges between 10 and 600 μM with a correlation coefficient of 0.9735, LOD of 1.6 at 8 μM, and sensitivity of 7.88 μA/μM. This sensor showed the least interference impacts from structurally similar nitroaromatic compositions and metal samples namely 2-nitrophenol (2 NP), 4-nitroaniline (4-NA), dinitrobenzene (DNBz), Pb²⁺, and Cd²⁺.

3.4.2. 2,4,6-Trinitrotoluene (TNT).

Cotchim et al. used a PME/GO layer modified GCE for the adsorption and electrochemical determination of 2,4,6-TNT.¹³⁶ Then, the electrochemical behavior and surface morphology of the PME/GO/GCE were established by FT-IR, SEM, CV, and adsorptive cathodic stripping voltammetry (AdCSV). The produced PME/GO/GCE exhibited magnificent adsorption and electrochemical reduction of TNT via the AdCSV method with two linear ranges, 1–90 mg L⁻¹ and 100–1000 mg L⁻¹, an LOD of 0.34 mg L⁻¹ and LOQ of 1.14 mg L⁻¹, respectively; good repeatability, high sensitivity, and selectivity were discernible which prompted its successful application for detecting TNT in soil samples with reasonable recovery that ranged from 93 to 99%.

Mahmoud et al. developed a flexible electrochemical sensor on poly(styrene-co-acrylic acid) PSA/SiO₂/Fe₃O₄/Au NPs/lignin modified GCE for selectively detecting traces of TNT in an aqueous medium with enhanced sensitivity.¹³⁷ Figure 9 depicts the fabrication steps of lignin-modified magnetic microspheres (L-MMS). The LOD was equal to that of TNT detected by amperometric i–t curve, ~35 pM. Notably, higher sensitivity and very good catalytic activities of the modified electrode may be ascribed to the lignin layer, and the strongly packed Fe₃O₄/Au NPs on the surface of the electrode. Furthermore, total recovery of TNT from tap water and seawater matrices was found to be 98% and 96%, respectively. Moreover, the electrode film was highly stable following five iterated absorption/desorption cycles, and the newly devised electrochemical sensing provided a very sensitive, selective, and flexible assay for in situ determination of TNT in the complicated water media.

Figure 9.

Synthesis of PSA/SiO₂/Fe₃O₄/Au NPs/L-MMS.

Shamsipur et al. used the electrochemically reduced graphene oxide-PAMAM hybrid (ErGO-PAMAM)-modified GCE (ErGO-PAMAM/GCE) for the fabrication of a TNT electrochemical sensor¹³⁸ which had high sensitivity to TNT relative to those of GCE, GO/GCE, GO-PAAM/GCE, and ErGO/GCE. As demonstrated by the outputs, ErGO-PAMAM/GCE had higher response to the TNT in comparison to the remaining graphene-based modified electrodes and the sensor displayed acceptable response to the TNT with a linear range between 0.05 and 1.2 ppm and LOD of0.0015 ppm.

Sağlam et al. developed a electrochemical sensor to detect the nitroaromatic explosive substances on Au NP-modified GCE coated with the poly(o-phenylenediamine-aniline film) (GCE/P(O-PDA-CO-ANI)-Au nano electrode).¹³⁹ The calibration curve of the current intensity vs concentration linearly ranged at 2.5 to 40 mg L⁻¹ for 2,4,6-TNT with an LOD of 2.1 mg L⁻¹, 2 to 40 mg L⁻¹ for 2,4-dinitrotoluene (DNT) with an LOD of 1.28 mg L⁻¹, and 5 to 100 mg L⁻¹ for tetryl. The coefficients of variation of intra-assay and inter-assay measurements, respectively, were 0.6% and 1.2% (N = 5), which denoted higher reproducibility of the assay. The technique experienced a successful application for analyzing nitro-aromatics in military explosives, namely, comp B, octol, and tetrytol.

Yang et al. prepared molybdenum disulfide–PABSA (MoS₂–PABSA) nanocomposite with the help of a simplified electro-polymerization process to detect TNT.¹⁴⁰ The nanocomposite possesses a very good electrochemical detection function for aromatic ring compositions such as TNT with robust π–π interactions with MoS₂–PABSA sensing platform; a significantly sensitive determination for TNT, LOD of 0.043 ppb (0.187 nM), and a linear range between 0.1 and 200 ppb was observed. Their study revealed that the MoS₂–PABSA was a worthy electrochemical sensing platform for additional simplified and sensitive determination of several other aromatic ring compositions. Table 3 lists applications of polymer nanocomposite-based electrochemical sensors for the detection of nitroaromatic compounds in various environmental matrices.

Table 3.

Applications of Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Various Nitroaromatic Compounds in Environmental Samples

electrochemical sensor	method	analyte	detection limit	linear range	ref
PEDOT/MWCNT/CPE	amperometry	nitrobenzene	83 nM	0.25−43 μM	133
AgNP/PME/MWCNT/GCE	DPV	nitrobenzene	0.55 μM	20−6000 μM	134
PAA-Ag NPs/GCE	DPV	nitrobenzene	1.68 μM	10−600 μM	135
PME/GO/GCE	AdCSV	2,4,6-trinitrotoluene	0.34 mg L−1	1−1000 mg L−1	136
PSA/SiO2/Fe3O4/Au NPs/lignin/GCE	amperometry	2,4,6-trinitrotoluene	35 pM	0.05−10 nM	137
ErGO-PAMAM/GCE	DPV	2,4,6-trinitrotoluene	0.0015 ppm	0.05 to 1.2 ppm	138
		2,4,6-trinitrotoluene	2.1 mg L−1	2.5−40 mg L−1
P(O-PDA-CO-ANI)-Au NPs/GCE	CV	2,4-dinitrotoluene	1.28 mg L−1	2−40 mg L−1	139
		tetryl	3.8 mg L−1	5−100 mg L−1
MoS2−PABSA/GCE	DPV	2,4,6-trinitrotoluene	0.043 ppb	0.1−200 ppb	140

3.5. Polymer Nanocomposites-Based Electrochemical Sensors for Detection of Nitrite.

The nitrite ions (NO₂^–) are major prevalent poisonous inorganic contaminants found in soil, food, physiological systems, and water. Further, the increased concentration of nitrite ions may be derived from caustic radioactive wastes.¹⁴¹ Excessive nitrite ions are harmful to humans, and the increased concentration of nitrite in drinking water would be a severe hazard for the health of infants. Additionally, nitrite can combine with hemoglobin in the blood of humans forming methemoglobin, causing a phenomenon that is usually called “blue baby syndrome”. Further, it is possible to convert nitrite into nitrosamine that has been implicated in cancer and high blood pressure.¹⁴² Due to increasingly stringent environmental protection to protect public health, the quantitative monitoring of nitrite in food and drinking water has become an important task.

Ye et al. fabricated a nitrite sensor on a Gr/PPy/CHIT nanocomposite film-modified GCE (Gr/PPy/CHIT/GCE).¹⁴³ It was found that −NH₃⁺ in the structure of CHIT attracted the ions with negative charge such as NO₂^–so that it could accumulate the nitrite ions on the surface of electrode; CV and EIS were used to examine the behavior of the electron transfer of the modified electrode in [Fe(CN)₆]^3–/^4– redox probe. Moreover, DPV and amperometry were utilized to study the electrochemical features of the sensor which displayed reasonable stability and reproducibility to detect nitrite. The linear response ranged between 0.5 and 722 μM with an LOD of 0.1 μM (S/N = 3) to detect nitrite, and this sensor was utilized for the determination of nitrite in lake water and river water samples with satisfactory results.

Wang et al. examined the oxidative electro-chemistry of nitrite on the GCEs modified with cobalt NPs, Gr, and PEDOT (Co NPs-PEDOT-Gr/GCE)¹⁴⁴ because of the synergistic effect of the nanoscale properties of Co NPs and the promoted electron transferrance by PEDOT-Gr composite, the overpotential for nitrite electro-oxidation was reduced at the Co NPs-PEDOT-Gr/GCE (~500 mV lower than unmodified GCE); the peak current linearly related to the nitrite concentration in the range from 0.5 μM to 240 μM, with an LOD of 0.15 μM. The modified electrodes enabled the determination of nitrite at low potentials where the noise level and interferences by other electro-oxidizable compounds are weak.

Ning et al. used a simplified chemical reduction technique to synthesize silver NPs stabilized by PAMAM dendrimer.¹⁴⁵ The newly devised electrochemical sensor detected nitrite on the basis of PAMAM nanocomposite. Figure 10 depicts the construction process of a nitrite electrochemical sensor based on Ag–PAMAM nanocomposite which displayed very good electrocatalytic function to oxidize nitrite; the sensor showed a broad linear range between 4.0 μM and 1.44 mM at 6.0 pH of the PBS. Additionally, this sensor enjoyed attributes such as higher sensitivity (265 μAmM⁻¹ cm⁻²) and a shorter response time of three seconds. Moreover, an LOD of 0.4 μM and the sensor ability to detect nitrite have been observed in tap water and milk samples.

Figure 10.

A schematic for the procurement procedure of Ag-PAMAM/GCE. Reproduced with permission from ref 145. Copyright 2014 Elsevier.

Xu et al. constructed a sensitive electrochemical nitrite sensor based on poly(diallyldimethylammonium chloride) (PDDA) functionalized RGO (PDDA-RGO).¹⁴⁶ Modification with PDDA endows a positive surface charge and RGO with good dispersity facilitating adsorption of nitrite as investigated by an amperometric method. The PDDA-RGO modified electrode exhibits lower overpotential equal to 0.75 V, with a wider linear range from 0.5 to 2 mM and a lower LOD of 0.2 μM for nitrite sensing. Additionally, the recoveries for NO₂⁻(10 μM) spiked into drinking water is in the range of 95.4% to 106%, thus illustrating the possibility of the proposed sensor to sense nitrite in real samples.

Lin et al. described the synthesis of a nanocomposite with PEDOT doped with the NPs (Au NPs) by electropolymerizing the EDOT in a solution comprising colloidal Au NPs.¹⁴⁷ According to the outputs, nanocomposite exhibited reasonable conductivity as well as a three-dimensional microporous network structure as a result of the presence of the Au NPs; they function as the conductive dopant and as a template to grow the microporous polymer. The GCE modified with nanocomposite exhibited very good electrocatalytic activities to oxidize nitrite. Finally, in the case of the best operation at a voltage of 0.8 V (vs Ag/AgCl), it could determine nitrite in a concentration ranging between 0.2 and 1400 μM with the LOD of 60 nM.

Liu et al. reported a sensitive nitrite sensor on the basis of the PDDA-coated Fe_1.833(OH)_0.5O_2.5-decorated NG ternary hierarchical nanocomposite (Fe_1.833(OH)_0.5O_2.5/NG@PDDA.¹⁴⁸ The sensor exhibited very good analytical functions to detect nitrite linearly within two distinct concentration ranges between 0.1 and 347 μM as well as 347 and 1275 μM with an LOD of 0.027 μM (S/N = 3). Additionally, the electrochemical sensor showed acceptable stability, reproducibility, and very good antiinterference capability versus the metal ions and electro-active samples and exhibited reasonable recovery to detect nitrite in drinking water.

Madhuvilakku et al. designed a GCE modified with RGO/molybdenum disulfide/PEDOT nanocomposite (RGO/MoS₂/PEDOT/GCE) for the detection of nitrite in water and milk.¹⁴⁹ Electrochemical results indicated that the RGO-MoS₂–PEDOT modified GCE exhibited improved electrochemical activity in 0.1 M PBS (pH = 4) compared to bare GCE, RGO-MoS₂/GCE, and PEDOT/GCE. The modified electrode demonstrated good sensitivity (874.19 μA Mm⁻¹cm⁻²), low LOD (0.059 μM, S/N = 3), and a wide linear range(0.001–1 mM) of nitrite with high selectivity, reproducibility, and long-term stability. The practical usability of the sensor is verified by the successful detection of nitrite ion in water and milk samples with outstanding correlation.

Sahoo and co-workers synthesized RGO/MnFe₂O₄/PANI nanocomposite for the detection of nitrite in a neutral medium.¹⁵⁰ The prepared RGO/MnFe₂O₄/PANI nanocomposite provided enhanced surface area for excellent preconcentration of nitrites and promoted fast charge transfer between the electrode surface and nitrite. It also demonstrated good catalytic activity toward the oxidation of nitrite. Moreover, a linear calibration plot was observed within the concentration ranges between 0.05 and 12000 μM of nitrite with an LOD of 0.015 μM. The presented sensing platform was found to have very good reproducibility and selectivity and was applied to detect nitrite in rainwater samples.

Table 4 lists applications of polymer nanocomposites-based electrochemical sensors for the detection of nitrite in various environmental matrices.

Table 4.

Applications of Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Nitrite in Environmental Specimens

electrochemical sensor	method	analyte	detection limit	linear range	ref
Gr/PPy/CHIT/GCE	amperometry	nitrite	0.1 μM	0.5−722 μM	143
Co NPs-PEDOT-Gr/GCE	amperometry	nitrite	0.15 μM	0.5 μM to 240 μM	144
Ag−PAMAM/GCE	amperometry	nitrite	0.4 μM	4.0 μM to 1.44 mM	145
PDDA-RGO/GCE	amperometry	nitrite	0.2 μM	0.5 μM to 2 mM	146
PEDOT/AuNP/GCE	amperometry	nitrite	60 nM	0.2 to 1400 μM	147
Fe1.833(OH)0.5O2.5/NG@PDDA/GCE	amperometry	nitrite	0.027 μM	0.1−1275 μM	148
RGO/MoS2/PEDOT/GCE	DPV	nitrite	0.059 μM	0.001−1.0 mM	149
RGO/MnFe2O4/PANI/GCE	DPV	nitrite	0.015 μM	0.05−12000 μM	150

3.6. Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Hydrazine.

Hydrazine (N₂H₄) has extensively been applied in various fields, for example, pharmaceuticals, industrial factories, as corrosive inhibitor, and in photography chemicals.^151–153 However, it is well-known that N₂H₄ is a very toxic substance, identified as a carcinogen and mutagenic by the EPA;¹⁵⁴ exposure causes damage to lungs, the liver, nervous system, and the kidney. In addition, N₂H₄ is absorbed easily when acute exposure to N₂H₄ occurs, which can lead to death.¹⁵⁵ Consequently, a technique for the easy, accurate, and sensitive determination of N₂H₄ is very significant for both environmental safety and the health of humans.

Peng et al. conducted an electrochemical detection of hydrazine based on synthesized sulfur-poly(pyrrole-Co-aniline) (S/PPyA) nanofiber.¹⁵⁶ A copolymer poly(pyrrole-co-aniline) PPyA was synthesized via the copolymerization reaction between aniline and pyrrole. In the next stage, an S/PPyA composite was obtained via the treatment of a mixture of PPyA and sublimed sulfur at 160 °C for 24 h. Afterward, S/PPyA was applied to modify GCE, and electrochemical activities of the as-synthesized S/PPyA/GCE to oxidize hydrazine were addressed; a lower LOD of 0.18 μM (S/N =3) and faster response time of three seconds were discerned.

Rahman et al. reported an interesting hydrazine sensor based on the silver NPs decorated PANI tungstophosphate nanocomposite¹⁵⁷ accumulated on a GCE causing the improvement in the electrochemical sensor to dissolve hydrazine with the help of I–V (current–voltage). The operation of the sensor at +0.5 V displayed a rapid response time (10 s), good sensitivity (~12.5 μA·mM⁻¹·cm⁻²), and extended stability. Its response to hydrazine was found to be linear in the concentration ranging from 10 nM to 10 mM with an LOD of 2.8 nM at the signal-to-noise ratio equal to 3.

Kaur et al. illustrated nanomolar concurrent detection of hydrazine and phenylhydrazine based on Cu NPs-decorated PANI-zeolite nanocomposite-modified GCE (Cu NPs(5%)-PANI-Nano-ZSM-5/GCE);¹⁵⁸ higher electrocatalytic activities, greater stability, sensitivity, and selectivity were observed with wider linear ranges between 4 and 800 μM with the LOD of 1 nM for phenyl-hydrazine and hydrazine. The analytical function of the devised sensor was established for the determination of phenyl-hydrazine and hydrazine in distinctive water bodies with reasonable outputs.

Peng et al. synthesized polydopamine-RGO (PDA-RGO) nanocomposites in which the GO sheets have been functionalized with PDA via self-polymerization.¹⁵⁹ Hydrazine could be detected by the electrochemical feature of the PDA-RGO, and hydrazine electrochemical oxidation was notably enhanced by the PDA-RGO nanocomposite as a result of the greater accessible surface areas of the electrode. Finally, a rapid amperometric response was noticed for the electrochemical sensor nanocomposite to measure hydrazine in a wider linear range from 0.03 to100 μM in which the LOD value was 0.01 μM.

Faisal et al. developed an electrochemical method for the effective and rapid determination of hydrazine using polythiophene (PTh)/ZnO nanocomposite modified GCE;¹⁶⁰ the synthesis of PTh/ZnO nanocomposite for the detection of hydrazine via amperometry is depicted in Figure 11. Because of the synergistic catalytic effect of metal oxide and conducting polymer, the PTh/ZnO-modified GCE based sensor presented improved performance toward the detection of hydrazine when compared to the bare GCE or pure ZnO. Hydrazine was detected over the concentration range of 0.5–48 μM, with high sensitivity (1.22 μA μM⁻¹ cm⁻²) and lower detection limit (0.207 μM). The amperometric analysis clarified that this proposed hydrazine sensor presented rapid (<5 s) response with appropriate operational and long-term stability, repeatability, and outstanding selectivity.

Figure 11.

Sol–gel synthesis of PTh/ZnO nanocomposite with the amperometric detection of hydrazine on modifed GCE. Reproduced with permission from ref 160. Copyright 2018 Elsevier.

Afshari et al. prepared graphitic carbon nitride/PANI/silver nanocomposites (PANI/g-C₃N₄/Ag NPs) by electrochemical polymerization and followed their performance by potentiometric methods.¹⁶¹ Then, the fabricated PANI/g-C₃N₄/Ag NPs electrode was utilized for the electrochemical oxidation of hydrazine in the alkaline media; it displayed a wider linear range from 5–300 mM and an LOD of 300 μM (S/N = 3). Moreover, the amperometric currents are highly stable in the presence of interfering species. The PANI/g-C₃N₄/Ag NPs electrode could selectively detect hydrazine in the presence of interfering compounds, for example, fructose, galactose, glucose, and hydroxylamine.

Kaladevi and co-workers reported the synthesis of silver NPs-decorated PANI/RGO nanocomposite by interfacial polymerization under static conditions. The electrocatalytic oxidation of hydrazine was investigated at the surface of the modified electrode (AgNPs@PANI-RGO/GCE).¹⁶² The electrochemical behavior of AgNP@PANI-RGO was studied by CV and EIS. The amperometry method was applied for the quantitative detection of hydrazine. Furthermore, hydrazine detection linearly ranged between 0.4 to 2.2 μM with the LOD equal to 70 nM. The results showed that the proposed sensor possesses good accuracy toward the trace travel detection of hydrazine in tap water, lake water, and drinking water.

Saeb and colleagues provided a sensitive electrochemical sensor for hydrazine oxidation, by titanium oxide@PANI@Au NPs (TiO₂@PANI@Au nanocomposite) on the GCE surface.¹⁶³ The fabricated sensor displayed high electrocatalytic activity to hydrazine oxidation. The DPV technique was used to construct the calibration plot for hydrazine. According to the results, it was possible to detect hydrazine in the linear ranges of 0.9 × 10⁻⁶ to 1.2 × 10⁻³ M with an LOD to 0.15 ±0.02 μM by this sensor. Additionally, the TiO₂@PANI@Au/GCE presented good stability and repeatability in hydrazine detection. Finally, the designed electrochemical sensor was successfully applied to detect hydrazine in various water samples (e.g., power plant sewage water and hookah water).

Table 5 lists the applications of polymer nanocomposites-based electrochemical sensors for the detection of hydrazine in various environmental matrices.

Table 5.

Applications of Polymer Nanocomposites-Based Electrochemical Sensors for the Detection of Hydrazine in Environmental Samples

electrochemical sensor	method	analyte	detection limit	linear range	ref
S/PPyA/GCE	amperometry	hydrazine	0.18 μM	1 μM to 1 mM	156
tungstophosphate-PANI/Ag/GCE	amperometry	hydrazine	2.8 nM	10 nM to 10 mM	157
Cu NPs(5%)-PANI-Nano-ZSM-5/GCE	DPV	hydrazine	1 nM	4 nM-800 μM	158
		phenylhydrazine	1 nM	4 nM-800 μM
PDA-RGO/GCE	SWV	hydrazine	0.01 μM	0.03 to 100 μM	159
PTh/ZnO/GCE	amperometry	hydrazine	0.207 μM	0.5−48 μM	160
PANI/g-C3N4/Ag NPs/GCE	CV	hydrazine	300 μM	5−300 mM	161
AgNPs@PANI-rGO/GCE	amperometry	hydrazine	70 nM	0.4−2.2 μM	162
TiO2@PANI@Au/GCE	DPV	hydrazine	0.15 ± 0.02 μM	0.9 × 10−6-1.2 × 10−3 M	163

4. CHALLENGES AND PERSPECTIVES

Although polymeric nanocomposite-based electrochemical sensors offer great potential toward the sensitive detection of environmental pollutants, they also pose significant challenges. It is often difficult to achieve a uniform dispersion of nanofillers in the polymer matrix, and to enable a strong interaction at the interface between the nanofiller and the polymer matrix. The large scale production of polymer nanocomposites is an important issue because of the complicated fabrication steps involved. Besides the construction complexity, one should consider the suitable selection of fabrication methods and appropriate operating conditions. The realization of advanced, highly sensitive, selective, accurate, portable, and integrative detection equipment is in the preliminary stages and still remains between laboratory prototypes and their widespread commercial applications. It is necessary to make extensive collaborative effort between multidisciplinary science and engineering disciplines to design and fabricate easy to use sensors especially for the environment.

In future development, researchers should focus on novel synthetic methods and an efficient combination of novel NPs and nanostructures with good conductive polymers that can produce polymer nanocomposites for appliances in electrochemical devices as elements to improve the selectivity and sensitivity for the electrochemical sensing of environmental pollutants. The improvement in the uniform dispersion of nanofillers in the polymer matrix by physical treatment, surfactants, and chemical functionalization of the surface of nanofillers may open new avenues for applying novel polymer nanocomposites in fabricating novel electrochemical sensing platform. Moreover, the biopolymer nanocomposites based electrochemical sensors and biosensors are expected to be a promising platform for the detection of environmental pollutants due to their nontoxicity, biocompatibility, biodegradability, multiple functional groups, pH-dependent solubility in aqueous media, good adhesion, and broad chemical modification capacity.

5. CONCLUSIONS

The unique properties of polymer nanocomposites such as high electrical and thermal conductivity, chemical stability, and mechanical strength with the tunable characteristics of polymers in terms of their chemical structure, lightweight, simplified procurement, high active sites, and strong adherence to the surface of the electrodes expose them as ideal materials for the development of practical chemical sensors. In this review article, we have summarized the common synthesis methods of polymer nanocomposites and their recent progressing applications for the electrochemical detection of a wide range of environmental pollutants. As demonstrated in Tables 1–5 and in the Supporting Information, polymer nanocomposites as a sensing platform have exhibited high performance in the construction of electrochemical sensors for the detection of environmental pollutants including heavy metal ions, pesticides, phenolic compounds, nitroaromatics, nitrites, and hydrazine. Although the tables also provide a comparison study for the environmental pollutants detection by the various electrochemical sensors, they are not an accurate source to verify which sensor functions more efficient than another on the basis of LOD, wide linear range, and the type of polymer nanocomposite used for analysis. Indeed, there are several influential factors that should be considered for a true comparison besides those shown in the tables, including ease of modification procedure, response time, total cost of modification, simplicity of sample pretreatment before analysis, selectivity, stability, repeatability, and reproducibility of electrochemical sensor, etc. The present review provides a comprehensive overview on the electrochemical sensing applications of polymer nanocomposites for environmental pollutants detection and aims to open new horizons for the practical use of these nanostructured materials toward fabrication of novel sensing platforms with improved properties.

ABBREVIATIONS

GC/MS

gas chromatography/mass spectrometry

AAS

atomic absorption spectroscopy

HPLC

high-performance liquid chromatography

CV

cyclic voltammetry

SWV

square-wave voltammetry

DPV

differential pulse voltammetry

LSV

linear sweep voltammetry

NPs

nanoparticles

Gr

grapheme

CNTs

carbon nanotubes

PANI

polyaniline

APS

ammonium persulfate

DBSA

dodecylbenzene sulfonic acid

NP

nanoplatelet

PS

polystyrene

MWCNT

multi-walled carbon nanotube

PVA

poly(vinyl alcohol)

GNS

reduced graphene oxide sheets

GO

graphene oxide

SCE

saturated calomel electrode

SWCNT

single-walled carbon nanotube

PABSA

poly(m-amino benzene sulfonic acid)

SAMs

self-assembled mono-layers

DMAET

dimethyl amino ethane thiol

SWASV

square wave anodic stripping voltammetry

LOD

limit of detection

ICP-MS

inductively coupled plasma mass spectrometry

PEDOT

poly(3,4-ethylene-dioxythiophene

GCE

glassy carbon electrode

DPSV

differential pulse stripping voltammetry

P2MA

poly(2-methylaniline)

SEM

scanning electron microscopy

XRD

X-ray diffraction

FT-IR

Fourier transform-infrared spectroscopy

IIP

ion imprinted polymer

PAN

1-(2-pyridylazo)-2-naphthol

LOQ

limit of quantitation

RSD

relative standard deviation

RGO

reduced GO

PANI-co-PoT

poly(aniline-co-o-toluidine)

CE

carbon paper electrode

DPASV

differential pulse anodic striping voltammetry

poly(PCV)

poly(pyrocatechol violet)

SPE

screen printed electrode

PA

phytic acid

PPy

poly-pyrrole

PAMAM

poly(amidoamine)

G2

second-generation

MCPE

magnetic carbon paste electrode

WHO

World Health Organization

CNSs

carbonaceous nanospheres

PLL

poly-l-lysine

EDTA

ethylenediaminetetraacetic acid

SS

stainless steel

FNT

Fenitrothion

PGE

pencil graphite electrode

DPA

poly(E)-1-(4-((4-(phenylamino)phenyl)diazenyl)-phenyl)ethanone

MLT

malathion

IL

ionic liquid

PoPD

poly(o-phenylenediamine)

SMZ

simazine

MIPM

molecularly imprinted polymer membranes

ATP

o-aminothiophenol

IMI

imidacloprid

PCz

polycarbazole

CRGO

chemically rgo

P3MT

poly(3-methyl-thiophene)

NG

nitrogen doped grapheme

MP

methyl parathion

PMG

poly(malachite green)

GrNSs

graphene nano-sheets

PAAM

poly(acrylamide)

MIP

molecularly imprinted polymer

PDPA

polydiphenylamine

2-AP

2-aminophenol

4-AP

4-aminophenol

PBS

phosphate buffer solution

P-l-GSH

poly-l-glutathione

l-Arg

l-arginine

β-CD

β-cyclodextrin

GNRs

graphene nano-ribbons

NP

nitrophenol

_o-NP

ortho-nitrophenol

PPI

poly(propyleneimine)

_p-NP

para-nitrophenol

PEI

poly(ethyleneimine)

4-CP

4-chlorophenol

PDDA

poly(diallyldimethylammonium chloride)

2,4-DCP

2,4-dichlorophenol

PEY

poly(eosin Y)

MWCNTs–OH

hydroxylated MWCNT

RS

resorcinol

CC

catechol

HQ

hydroquinone

AuAC

gold atomic cluster

P4VP

poly(4-vinylpyridine)

PSA

poly(sulfosalicylic acid)

1D

one-dimensional

PNMTh

poly(n-methylthionine)

DGNs

dendritic gold nanostructures

3MT

3-methylthiophene

CTAB

cetyl-trimethyl ammonium bromide

BPA

bisphenol A

SF

silk fibroin

EIS

electrochemical impedance spectroscopy

Pβ-CD

poly-β-cyclodextrin

ERGr

electro-reduced grapheme

RhB

rhodamine B

PME

poly(melamine)

NBz

nitrobenzene

PAA

poly(amic) acid

2-NP

2-nitrophenol

4-NA

4-nitroaniline

DNBz

dinitrobenzene

TNT

2,4,6-trinitrotoluene

AdCSV

adsorptive cathodic stripping voltammetry

PSA

poly(styrene-co-acrylic acid)

L-MMS

lignin modified magnetic microspheres

ErGO

electrochemically reduced graphene oxide

P(O-PDA-CO-ANI)

poly(o-phenylenediamine-aniline

DNT

2,4-dinitrotoluene

PDDA

poly(diallyldimethylammoniumchloride)

S/PPyA

sulfur-poly(pyrrole-co-aniline)

PPyA

poly(pyrrole-co-aniline)

PDA

polydopamine

PTh

polythiophene

g-C₃N₄

graphitic carbon nitride