Review—Nanocomposite-Based Sensors for Voltammetric Detection of Hazardous Phenolic Pollutants in Water

Usually experiments with cyclic voltammetry (CV) can be used to study the kinetics of the electrocatalytic reactions of the analytes on modified electrodes and also to determine the active surface area of the nanocomposite-modified electrodes. Generally, CV measurements are performed for the modified electrodes in the presence of a redox probe such as [Fe(CN)₆]^3−/4− at different scan rates (Fig. 1). Tohidinia et al.⁵⁹ performed CV measurements of the GPE/PQ–BNWs, GPE/BNWs, GPE/PQ and GPE and analyzed the anodic peak current (I_pa) of the respective CVs attained in the presence of 1 mM [Fe(CN)₆]^3−/4− as shown in Figs. 1a–1d. The anodic and the cathodic peak currents can be plotted against either the scan rate or the square root of the scan rate. If the plot portrays a linear dependence of peak current with scan rate then it usually means it is a surface-controlled process, however, if the plot displays a linear dependence of the peak currents with the square root of the scan rate, as shown in Fig. 1e then it usually means a diffusion-controlled process.^60,61

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Furthermore, for an electrochemically reversible electron transfer process which follows a typical diffusion-controlled electrochemical behavior, the following Randles–Sevcik equation^59,60,62 can be used to calculate the active surface area of the electrode:

$$\begin{eqnarray}&&{I}_{pa}=\left(2.69\times {10}^{5}\right){n}^{3/2}{D}^{1/2}CA{v}^{1/2}\end{eqnarray} \tag{ 1 }$$

where ${I}_{pa}$ refers to the anodic peak current, n is the electron transfer number, A is the surface area of the electrode, D is the diffusion coefficient of the redox species such as [Fe(CN)₆]^3−/4−, C is the concentration of freely diffusing redox species and v is the scan rate. Usually for 1 mM [Fe(CN)₆]^3−/4−, it is assumed that n = 1 and D = 7.6 × 10⁻⁶ cm s⁻¹; the active surface areas of the nanocomposite-modified electrodes are obtained from the slope of the ${I}_{pa}$–${v}^{1/2}$ relation. For example, Tohidinia et al.⁵⁹ used the aforementioned equation and calculated the active surface area for unmodified GPE, GPE/PQ and GPE/PQ–BNWs (nanocomposite modified electrode), the active surface area for the electrodes was found to be 0.0233, 0.0226 and 0.0386 cm², respectively. Therefore, the results emphasized that PQ-BNWs nanocomposite modified electrode, significantly enhanced the effective area of the working electrode and which resulted in improved conductivity of the sensor. Specifically, this indicated that the effective surface area of GPE/PQ–BNWs was about 66% and 71% more than GPE and GPE/PQ, respectively.⁵⁹

Similarly, for a nanocomposite-modified electrode following a typical surface-controlled electrochemical behavior, the following equation can also be used to calculate the active surface area of the electrode^60,61:

$$\begin{eqnarray}&&{I}_{pa}=\displaystyle \frac{{n}^{2}{F}^{2}}{4RT}vA\,{{\rm{\unicode{x00413}}}}^{* }\end{eqnarray} \tag{ 2 }$$

where ${\unicode{x00413}}^{* }$ is the surface covered by the adsorbed species in mol cm⁻², and the other symbols have their usual meanings.

Since the protons take part in the electrode reactions, the electrolyte acidity plays a vital role in the electrooxidation of the phenolic pollutants. Usually, an increase in the pH of the solution would result in shifting of the peak potential towards lower potential, which indicates the involvement of the protons during the electrode reactions. For example, Fig. 2a shows the effect of pH value on the electrochemical behavior of 4-NP at the surface of Cu−Zn/GO/GCE (nanocomposite modified electrode) investigated using CV in the pH range of 2.0 to 11.3 using Britton-Robinson (BR) buffer. In Fig. 2a, it is shown that in the selected pH range the oxidation peak potentials of BPA shifts negatively with increasing pH, suggesting that the protons are actively participating in the electrochemical redox process.⁶³ Moreover, a plot of anodic peak potential vs the pH can also be constructed and the slope of the plot usually follows a linear regression, as shown in Fig. 2b. The obtained slope can be used for the estimation of the ratio of number of protons and electrons. Generally, the anodic peak potentials for the analytes at a specific pH can then be calculated using the following relationship^59,64:

$$\begin{eqnarray}&&d{E}_{p}/dpH=-\displaystyle \frac{2.303mRT}{nF}pH\end{eqnarray} \tag{ 3 }$$

where, m and n are the number of protons and electrons involved in the reaction, respectively, and the corresponding Nernstian slope $\left(-\tfrac{2.303mRT}{nF}\right)$ value is known to be 0.059 $\left(\tfrac{m}{n}\right)$ V/pH.

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If the experimental slopes obtained from the plots of anodic peak potentials values vs the pH for are close to the theoretical Nernstian slope value then the probable electrochemical reaction for the phenolic analyte should involve equal number of protons and electrons at the nanocomposite-modified electrode reactions. Moreover, an optimum pH for the study can be chosen based on the plot of anodic peak vs the pH values. For example, in Fig. 2b it was shown that while using Cu−Zn/GO/GCE the oxidation peak current BPA increases with increasing pH value until it reaches to its maxima at pH 7.1 (the optimum pH), and then the oxidation peak currents decrease when the pH increases further. This phenomenon of increase and decrease in peak currents at different pH values can be attributed to the fact that protons are involved in the electrochemical reaction. A shortage of protons at high pH values prevents the oxidation of BPA which leads to a decreased current peak intensity, moreover diphenols such as BPA can form anions which can contribute to the decreased current peak intensity observed in Fig. 2b.⁶⁵ Therefore, pH effect study can be helpful in providing important information about the electrooxidation mechanisms of the redox reactions occurring on the surface of nanocomposite-modified electrodes. Several studies in literature have illustrated different electrooxidation mechanisms for each of the phenolic pollutants discussed in this review, which are shown in Schemes 2–6.

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In electrochemical studies usually when the performance of the nanocomposite-modified electrodes is investigated, the calculation of the standard heterogeneous electron transfer rate constant, denoted by k° or k_s, becomes very significant.⁶⁹ These values provides the information about the speed of the electron transfer between the nanocomposite-modified electrode surface and the electroactive species. Researchers have used voltammetric techniques since 1956–57 to report the rate constant values. Since the Marcus theory was reported CV has been a popular choice in determining the k° or k_svalues. Models developed by Nicholson^70,71 or Laviron^72,73 are the most commonly used theories to estimate the rate constants for the electrode materials.

Several studies in literature have determined the k° or k_s values for phenolic pollutants using the Laviron's theory.^63,74–78 Briefly, using CV the modified electrode is utilized in investigating the effect of the scan rate on the anodic and cathodic peak potential for the phenolic pollutants. For reversible electrode processes, the oxidation peak potential will shift positively and the reduction peak potential will shift negatively with increasing scan rate. For example, Fig. 3a shows the effect of scan rate to the CV response of CC at modified carbon paste electrode (CPE) with CePO₄ that is reported by Dang et al.⁷⁹ It is shown in Fig. 3a that the oxidation peak potential shifted positively and the reduction peak potential shifted negatively as the scan rate increases. But for irreversible electrode processes only one of the reduction or oxidation peak is observed, which also shifts negatively or positively with increasing scan rate. According to the Laviron's theory,⁷² for a reversible electrochemical process when the ${\rm{\Delta }}{E}_{p}$ is greater than $0.2/nV,$ the graph of ${E}_{pa}\,or\,{E}_{pc}=f\left(\mathrm{log}\,v\right)$ will give two straight lines where the slope will be equal to $-2.3RT/\alpha nF$ for the cathodic peak (${E}_{pc}$) and $-2.3RT/\left(1-\alpha \right)nF$ for the anodic peak (${E}_{pa}$). The charge transfer coefficient, α, can then be determined from the slopes of the two straight lines of ${E}_{p}$ vs $\mathrm{log}\,v$ based on the Eq. 4⁷⁷:

$$\begin{eqnarray}&&\mathrm{log}\,\displaystyle \frac{{k}_{a}}{{k}_{c}}=\,\mathrm{log}\,\displaystyle \frac{\alpha }{1-\alpha }\,or\,\displaystyle \frac{{k}_{a}}{{k}_{c}}=\displaystyle \frac{\alpha }{1-\alpha }\end{eqnarray} \tag{ 4 }$$

where the ${k}_{a}$ and ${k}_{c}$ are the slopes of linear regression obtained from the plot of anodic and cathodic peak potential vs $\mathrm{log}\,v,$ respectively. For example, Fig. 3b shows the linear relationship of E_pa and E_pc of CC with logν, which are used to derive the values of ${k}_{a}$ and ${k}_{c}.$ Therefore, after solving for α using Eq. 4, the standard heterogeneous electron transfer rate constant, k° or k_s, can be calculated using Eq. 5 as follows:

$$\begin{eqnarray}\begin{array}{lll}\mathrm{log}\,{k}_{s} & = & \alpha \,\mathrm{log}\left(1-\alpha \right)+\left(1-\alpha \right)\mathrm{log}\,\alpha \\ & & -\,\mathrm{log}\,\tfrac{RT}{nFv}-\tfrac{\alpha \,\left(1-\alpha \right)nF{\rm{\Delta }}{E}_{p}}{2.3RT}\end{array}\end{eqnarray} \tag{ 5 }$$

where n is the number of electrons participating in the electrode reaction, ${\rm{\Delta }}{E}_{p}$ is the peak-to-peak potential separation and rest of the symbols have their usual meanings. The number electrons taking part in the reaction are usually estimated based on the results of the pH effect studies and past research. For totally irreversible electrode processes, the dependence of the anodic peak potential $\left({E}_{pa}\right)$ vs the $\mathrm{ln}\,v$ is represented by the following Laviron's equation⁷³:

$$\begin{eqnarray}&&{E}_{pa}={E}_{0}+\left(\tfrac{RT}{\alpha nF}\right)\mathrm{ln}\left(\tfrac{RT{k}^{0}}{\alpha nF}\right)+\left(\tfrac{RT}{\alpha nF}\right)\mathrm{ln}\,v\end{eqnarray} \tag{ 6 }$$

where ${E}_{0}$ is the formal redox potential, ${k}^{0}$ is the standard heterogeneous rate constant, n is the number of electrons transferred, and rest of the symbols have their usual meanings. For example, as shown in Fig. 4a shows that an increase in the scan rates caused the E_pa of BPA on Cu−Zn/GO/GCE to shift towards a more positive potential.⁶³ Moreover, as shown in Fig. 4b the slope from the linear plot of ${E}_{pa}$ vs $\mathrm{ln}\,v$ can be used to determine the value of $\alpha n.$ Moreover, once the number of electrons involved in the oxidation process is available from pH effect studies and past research, the value of α can be calculated by equating the slope to the value of $\alpha n.$ According to Bard and Faulkner,⁶¹ $\alpha$ can also be assumed to be 0.5 for a totally irreversible process, therefore, the number of transferred electrons (n) that are taking part in the electrochemical oxidation of the analyte can be calculated.

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Another equation which is also used for calculating the standard heterogenous rate constants (k_s) for totally irreversible electrode processes at the surface of modified electrodes is Velasco equation, as shown below^80,68:

$$\begin{eqnarray}&&{k}_{s}=1.11{D}^{1/2}{\left({E}_{p}-{E}_{p/2}\right)}^{-1/2}{v}^{1/2}\end{eqnarray} \tag{ 7 }$$

where, D is the apparent diffusion coefficient, E_p and E_p/2 are the oxidation peak potential and half-wave oxidation peak potential, respectively, and v is the scan rate. Here again CV is performed in the presence of the analytes at a low scan rate (between 25 to 50 mV) and the oxidation peak potential and half wave oxidation peak potential are used to determine the ${k}_{s}.$ However, this equation requires the apparent diffusion coefficient for each of the analyte, and this is obtained from the chronoamperometric studies discussed in the following section. But,it should be noted that the diffusion coefficients determination by electrochemical means have some problems in precision.⁸¹

Diffusion coefficients for the electroactive analytes can be determined using chronoamperometry. For chronoamperometry the potentials of the working electrode potentials are usually set at a constant potential close to the anodic peak potential and the current responses from different concentrations of the analytes are recorded. From the study of electrochemical detection of 4-NP by Rajkumar et al.⁶⁸ typical chronoamperograms and the Cottrell plots for 4-NP are shown in Fig. 5. Figure 5a shows the chronoamperometric response for S-GCN/SPCE in N₂-saturated 0.1 M ABS (pH 5.5) in the presence and absence of 4-NP. While Fig. 5b displays a significant increase in the current responses with increasing 4-NP concentrations, which emphasizes the occurrence of the facial electron transfer during the electrocatalytic reduction of 4-NP over S-GCN/SPCE. The diffusion coefficients for the electroactive analytes can be calculated using the Cottrell equation as follows⁸²:

$$\begin{eqnarray}&&I=nFA{D}^{1/2}{C}_{b}{\pi }^{-1/2}{t}^{-1/2}\end{eqnarray} \tag{ 8 }$$

where n is the number of electrons transferred, F is Faraday's constant (96485 C mol⁻¹), A is the effective surface area of the electrode, D is the diffusion coefficient, C_b is the bulk concentration (mol cm⁻³), t is time. The Cottrell plot displayed as the graph of I vs t^−1/2 shows that the relationship of current with time is linear (Fig. 5c). This linearity is resulted due to the diffusion control mechanism followed by the electrochemical reaction under mass transport limited conditions. Furthermore, the slope of the linear region which equals to nFAD^1/2C_bπ^−1/2 can be used to calculate the value of D for the analytes by plotting them against the different concentrations that have been tested as shown in Fig. 5d.

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Overall, this section provides a brief overview of the electrochemical surface characterization for different nanocomposite-modified electrodes that can be performed by determining the effective surface area, diffusion coefficients, heterogeneous rate constants and many other aspects of the electrode reactions and electron transfer that are critical to the sensor's performance. The following sections will discuss and provide a brief overview of the novel modifiers and comparison studies on the analytical performance of the recently reported voltammetric sensors. Moreover, since the environmental/sample matrix utilized in detection of the phenolic pollutants can affect the analytical scope of the sensor, the type of environmental/sample matrix used for recovery experiments in detection of various phenolic pollutants will also be highlighted.

Structural isomers of hazardous organic pollutants pose a significant risk to the environment, therefore, quick and sensitive determination of the pollutants is required during the monitoring.^83–85 Due to their low cost, widespread applications and large-scale production, the DHB isomers are extensively found in the environment. They are utilized as engineering solvents or as raw ingredients in different industries^85,86 and are also used in manufacturing of synthetic fibers, hair dyes, rubber, cosmetics and pharmaceuticals.^86–90 The remediation of DHB isomers is of significance due to their low degradability, high toxicity and high oxygen demand in the aquatic environment. In addition, the national environmental agencies from United States, European Union and Canada have categorized them as primary wastewater contaminants due to the aforementioned reasons.⁹¹ Wastewater effluents containing these isomers with concentrations ranging from 1 to 1000 mg l⁻¹ from different industrial processes have been reported.⁸⁶ Several conventional analytical methods have been reported for determination of DHB isomers such as phosphorescence,⁸³ spectrophotometry,⁸⁵ colorimetric,⁸⁷ chromatography,⁹² electrochemistry,^59,62 chemiluminescence,⁹³ fluorescence,⁹⁴ capillary electrophoresis.⁹⁵ However, the electrochemical methods are most preferred choice due to its sensitivity, selectivity, low cost, potential for miniaturization and rapid detection.⁶² Recently, various nanocomposites have been used as modifiers on different kinds of electrode materials for electrochemical detection of one, two or all three DHB isomers.

Li et al.,⁹⁶ developed copper-based metal-organic frameworks-graphene nanocomposites having good electronic conductivity and stability for electrochemical detection of CC and HQ, however, resorcinol was not detected and interference studies with other compounds have not been reported. Balram et al.,⁹⁷ prepared an ecofriendly reduced graphene oxide (rGO) and synthesized Marsh marigold-like zinc oxide (ZnO) nanoaggregates. Then, they used both of those materials to sonochemically prepare novel nanocomposites using ultrasonication and utilized them to modify the glassy carbon electrode (GCE). Although the other two isomers, CC and RS, were used during the interference studies, the simultaneous detection of all the isomers was not performed. Zhou et al.⁹⁸ developed a cost-effective non-enzymatic cobalt oxide (Co₃O₄) carbon core–shell nanocomposite by exploiting glucose as the carbon source using a hydrothermal method. This kind of nanocomposite can greatly reduce the fabrication cost for potential commercialization, however, the researchers did not perform any interference studies with resorcinol or other phenolic compounds generally found in polluted aquifers. Therefore, the analytical applications of these sensors had a limited scope.

In environmental water samples the three DBH isomers usually co-exist as pollutants.^87,99,100 This could impact the sensitivity and the selectivity of the sensor as some electrode modifiers respond with broad overlapping redox potentials of CC and HQ at conventional electrodes and they cannot overcome the electrochemical inactivity of resorcinol.^101,102 Therefore, the need for novel nanocomposites which can assist in simultaneous determination of these isomers becomes necessary.^62,99 Recently, Yang et al.,¹⁰³ fabricated a new nanocomposite by first developing a nanoribbon from multi-walled carbon nanotube (MWCNT) and graphene oxide and then hydrothermally reducing the nanocomposite to form MWCNTs incorporated with rGO nanoribbons. The resulting nanocomposite had a very wide linear range compared to some other recently reported nanocomposites such as hollow nitrogen-doped mesoporous carbon spheres decorated graphene,⁹⁹ poly(quercetin)-bismuth nanowires,⁵⁹ chitosan supported functionalized-MWCNT,¹⁰⁴ MWCNT-titanium dioxide (TiO₂) in chitosan matrix,¹⁰⁰ and nanoraspberry-like copper-reduced graphene oxide.⁶² Meng et al.,⁶⁶ had developed a carbon cloth-zinc oxide nanorod nanocomposite (Fig. 6) for simultaneous detection of DHB isomers, however the detection limit is too high and the linear range is too narrow compared to the other recently developed nanocomposites (Table I). The selection of cyclic voltammetry (CV) as the quantification method compared to more sensitive differential pulse (DPV) or square-wave voltammetric (SWV) techniques might have been a limiting factor for such results.

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Conflicting reports have also reported that unlike CC and HQ, the two-electron and two-proton oxidation mechanism does not help in generating a stable quinone with RS due to its meta structure.^124,125 Recent studies by Meng et al.,⁶⁶ Yang et al.,¹⁰³ Kokulnathan et al.,¹⁰⁴ and Fotouhi et al.,¹⁰⁰ reported that all three DHB isomers underwent two-electron and two-proton processes as their electrooxidation mechanisms (Scheme 2). However, other researchers such as Tohidinia et al.,⁵⁹ Edris et al.,¹⁰⁵ Nady et al.,¹²⁶ and Sabbaghi et al.,⁶² have reported contrasting electrooxidation mechanism (Scheme 3). For RS, the resonance between the hydroxyl group and the π electrons is compromised due to its meta-structure which impedes its oxidation and usually requires overpotential to undergo oxidation. The RS undergoes keto-enol tautomerism reaction mechanism, which would lead to polymerization involving the hydroxyl groups of the RS. The reaction mechanism shown in Scheme 3 as reported by Tohidinia et al.,⁵⁹ Edris et al.,¹⁰⁵ Nady et al.,¹²⁶ and Sabbaghi et al.,⁶² indicate that initially one electron and one proton are exchanged and this results in formation of three RS radicals which would further react with another RS to form three types (ether, carbon, or peroxide) of dimers.^59,62 Past research has shown that these three dimers are oxidized more easily compared to the RS monomers.^59,62,127 The carbon- or ether-linked dimers are known to be more predominant as the peroxide linked dimers are not stable and break shortly once they formed.¹²⁸ Based on previous studies, carbon linkage is predominantly observed in the phenol-containing polymers, while it is the ether linkage, which is predominant for the highly substituted phenol-containing polymers.⁵⁹ However, in terms of real life relevance, unfortunately not much has been reported in literature for the products formed during the proposed electrooxidation mechanism of RS. Overall, all the studies^{62,105,126,106} proposing the alternative electrooxidation mechanism for RS have reported only on the preference of different products that can be produced but the significance of them have not been reported. Therefore, future studies are needed for studying the effects of the by-products on the environment and human health. One way to study that would be by performing additional mechanistic studies to improve our understanding of the electrooxidation processes of DHB isomers.

Overall, it is also significant to develop new nanocomposites as modifiers for the detection of the DHB isomers due to the issues pertaining with the facile synthesis, eco-friendliness of the constituent materials, detection limits and the linear range of the existing modifiers. A summary of recent nanocomposite-based simultaneous or individual voltammetric detection of DHB isomers have been compiled in Table I.

Bisphenol A (2,2-bis(4-hydroxyphenyl)propane) (BPA) is a major phenolic monomer compound with a widespread use in industrial manufacturing of polycarbonate plastics, epoxy resins and in the plastic industry for production of various other household materials.¹²⁹ BPA is reported to be a xenoestrogen, an endocrine disrupting compound.¹³⁰ Even at low concentrations, exposure to BPA is carcinogenic and can cause abnormal hormone responses resulting in endocrine disorders.^131–133 Owing to its negative implications on humans, aquatic biota and wildlife, this compound was banned in Canada in 2010.¹³¹ Due to its extensive usage in industrial and domestic settings, it is approximated that roughly 2000 ton of BPA is released annually into the environment.¹³⁴ Moreover, it is vital to highlight that the BPA is not degraded and treated by the wastewater treatment plants, therefore, it is found in natural waters, drinking freshwater sources, seawater, riverbeds, air, landfills, and different food matrices.^129,133 The use of BPA in baby teethers and plastic bottles is strictly prohibited as infants and children are the most vulnerable to BPA. According to United States Food and Drug Administration, 50 μg/kg/day is apparently a safe and tolerable dose,¹³⁰ while European Food Safety Authority has set the tolerable and safe dose limit at 4 μg·kg⁻¹ body weight per day.¹³⁵ Therefore, it is essential to develop sensing techniques that are rapid, affordable, sensitive, selective and have convenient sample preparation and facile surface renewal, which allows real-time on-field detection of BPA. Usually a bare electrode would have limitations in terms of sensitivity and selectivity, and it would also be prone to surface fouling. Therefore, electrode modifications with nanocomposites can solve the problem of overvoltage and slow electrode kinetic processes.^133,136

Among various advanced detection techniques enzymatic electrochemical sensors incorporating nanocomposites are emerging as a sensitive, selective and simple biosensing devices.^129,137 Recently, Wu et al.¹³⁷ used SiO₂ nanospheres and modified their surface with gold-platinum (Au-Pt) nanoparticles (APS) to form Au-Pt@SiO₂ nanocomposites. These nanocomposites were then incorporated with tyrosinase (Tyr) and then along with gold nanoparticles modified graphene nanosheet (Au-GN) as electrode substrate were drop casted on GCE to fabricate a tyrosinase modified Au-Pt@SiO₂/Au-graphene electrode (Tyr/APS/Au-GN) (Fig. 7). An excellent detection limit of 7.88 nM was achieved with a wide linear range from 0.04–43.8 μM. Although this electrode was used in detection of BPA in food matrices using DPV and SWV, the applicability can be expanded to water samples as other researchers have used nanocomposite-modified Tyr-based enzymatic electrochemical sensors for BPA detection in water samples. For example, Liu et al.,¹²⁹ developed tyrosinase enzymatic electrochemical sensor by incorporating biochar nanoparticle (BCNPs)-tyrosine nanocomposite and mixing it with Nafion before drop-casting it on GCE. The highly conductive BCNPs were the transducer and signal enhancer in this study, and a tyrosinase enzyme was used as signaling component due to its catalytic bioactivity towards BPA. The Nafion helped in achieving a smooth and homogenous surface on the electrode surface. The ortho-hydroxylation oxidase properties of Tyr oxidized the BPA to polyhydric phenol and o-quinone, which is electroactive (Fig. 8).¹²⁹ The amperometric detection of BPA was then performed by reducing the o-quinone at the electrode surface and measuring the reducing current signal directly related to concentrations of BPA.¹²⁹ However, some drawbacks of such enzymatic electrochemical sensors relate to its high cost, complexity and multi-step immobilization and variability of samples.¹³⁰ Enzymatic electrochemical sensors also suffer from the burial of the redox active sites in the enzyme and also from the orientation of the enzymes on the electrode surfaces which would hinder the electron transfer processes.¹³⁸ Therefore, novel nanocomposite-based electrochemical aptasensors have been emerging as an excellent alternative that is highly sensitive, selective, stable, biocompatible with potential for miniaturization. Aptamers are single-stranded DNA or RNA oligonucleotides that have high specificity and sensitivity towards the target analyte. They are advantageous over other immunosensors or antibodies because they are developed with an in vitro technique called systematic evolution of ligands by exponential enrichment (SELEX).^103,139 Moreover, they can be modified as per the needs of the research, are repeatable and comparatively cost-effective.¹⁴⁰ It has been reported elsewhere that small molecules like BPA are easily captured by the aptamers, as their shorter oligonucleotide length assists in capturing by accurately differentiating the functional groups of analogous structures.¹⁴¹ Several recent studies have used aptasensors for electrochemical sensing of BPA. For example, Abnous et al.,¹³⁹ developed an aptasensor based on nontarget-induced aptamer length extension activated by terminal deoxynucleotidyl transferase and the establishment of a physical barrier on the electrode surface. The sensor is capable of excellent sub-nanomolar detection limit of 15 pM and wide linear range from 0.08 to 15.0 nM. Yao et al.,¹⁴² first developed water-soluble nitrogen, sulfur, phosphorus co-doped carbon dots (N,S,P-CDs) using hydrothermal method from cucumber juice. The N,S,P-CDs were then drop coated on the GCE to increase the active surface area and increase the sensitivity of the sensor. The gold nanoparticles (AuNPs) were then dropped on the GCE modified with N,S,P-CDs to form nanocomposites. In the last step the thiolated anti-BPA aptamers were attached onto the gold nanoparticle surface. The long-chain extension of the anti-BPA aptamers allowed for sensitive and selective detection of BPA by allowing or disallowing the electrochemical redox probe to reach the electrode surface (Fig. 9). With regards to the BPA detection, the linear range was from 0.01 μM to 120 μM and the detection limit was as low as 0.53 nM. However, there are still some limitations for electrochemical aptasensors, one of them is the degradation of the aptamers with nucleases. Another challenge pertains to the limited knowledge of the aptamer immobilization approaches that can significantly affect the detection capabilities of the sensor.¹⁴³

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In the recent years, molecular imprinting approach has received great attention for detection of chemical contaminants, where the molecular identification in a composite is achieved through the target analyte supported assembly.^144–147 In this technique, the target analyte is used as a template by the polymeric network for fabricating the recognition imprints of the analyte during the course of the composite formation. Therefore, Ali et al.,¹⁴⁷ coupled the molecular imprinting method with electrochemical analyses for developing a molecularly imprinted polymer nanocomposite for rapid, selective and sensitive detection of BPA. The nanocomposite is constituted of polyacrylate, RGO and β-cyclodextrin that are covalently linked while forming a 3D network (Fig. 10). This 3D network of the nanocomposite assists in selective capture of BPA as it has molecular imprints of the BPA resulting via the host-guest complexation amongst BPA and β-cyclodextrin. The linear concentration range for detection of BPA in water samples was 20 nM—1.0 μM, while the detection limit was 8 nM. Pei et al.,¹⁴⁶ developed a molecularly imprinted TiO₂ single crystal having selective inorganic-framework molecular detection capability, with a linear range of 10.0 nM—20.0 μM. Although these molecularly imprinted polymer nanocomposite sensors are highly selective and help in attaining low detection limit, compared to the other recently reported nanocomposite-based electrochemical, especially voltammetric, sensors in literature, they work usually within a very narrow linear concentration range (Table II). Finally, a recent study by Ulubay Karabiberoğlu⁶³ proposed that the BPA undergoes two-electron and two-proton processes as their primary electrooxidation mechanism at Cu−Zn/GO/GC nanocomposite-modified electrode (Scheme 4).Although it should be noted that the number of protons or electrons involved in the electrooxidation mechanism can change depending the nanocomposite used for modifying the electrode. Another study by Yan et al.⁶⁷ reported that BPA undergoes four-electron and four-proton processes as their primary electrooxidation mechanism at nanoporous gold modified glassy carbon electrode (Scheme 5).

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Nitrogen containing aromatic compounds such as para-nitrophenol (4-nitrophenol, 4-NP) have a widespread use in industrial sectors. They are routinely used in the production of paints, plasticizers, leather, pesticides, explosives, dyes, drugs, and so forth.^163,164 Even at low concentrations, the nitro-aromatic compounds are considered to be carcinogenic, mutagenic and teratogenic pollutants and therefore, they pose a significant risk to human health and aquatic biota.¹⁶⁵ Moreover, they have high polarity and are poorly biodegradable due to the presence of strong electron withdrawing groups, which causes 4-NP to persist in the environment (soil and water) for a long time after its release as wastewater effluents from the industries where it is used.¹⁶⁶ Moreover, due to its bioavailability and prevalent use, 4-NP is found in freshwater environment and it has been classified as primary pollutant by the United States Environmental Protection Agency.¹⁶⁷ Several conventional methods have been utilized for detection and quantification of 4-NP from contaminated aquifers including, capillary electrophoresis,¹⁶⁸ liquid chromatography–mass spectrometry,¹⁶⁹ gas chromatography–mass spectrometry,¹⁷⁰ flow-injection reflectometry,¹⁷¹ and fluorescence.¹⁷² Electrochemical techniques have received great attention as complementary pre-screening tools to the aforementioned conventional techniques in detection of the environmentally hazardous 4-NP.^{78,164,166,173} Since the surface properties and the sensor constituents are extremely important in eliciting electrochemical response with high sensitivity several nanocomposite-based sensors have been recently developed.^{78,165,173,174} Li et al.¹⁷³ have fabricated a nanocomposite with cetyltrimethyl ammonium bromide (CTAB) and graphene nanosheets using a simple wet-ball-mill synthesis approach for detection of 4-NP. The exfoliation efficiency of the graphene nanosheets was greatly enhanced by the use of CTAB during the synthesis. They reported that the sensitivity of the electrochemical sensor was directly proportional to the ball-milling time, with optimized time of 12 h. The linear range for their sensor was from 7.19 μM to 107.83 μM, while the estimated limit of detection was 3.02 μM.¹⁷³ Although, the nanocomposite fabrication method is facile and straight-forward, compared to the other recently developed nanocomposite-based electrochemical sensors by Kumar et al.¹⁶³ , the sensitivity of sensor in terms of the detection limit by Li et al.¹⁷³ was too low. Kumar et al.¹⁶³ electrodeposited copper nanospheres and metallized a chemically modified electrode to develop a novel nanocomposite-based electrochemical sensor for the detection of 4-NP. They were in turn inspired by the copper deposition on self-assembled monolayer¹⁷⁵ and curcumin conjugation¹⁷⁶ studies available in literature. Kumar et al.¹⁶³ fabricated a sustainable nanocomposite by first synthesizing the curcumin-cysteine (CC) composite using Fischer esterification method. Next, the dithiol terminated CC composite was used for modifying the gold electrode through self-assembly approach. Finally, copper nanospheres conjugated with diketo moiety of curcumin and the electrochemical sensor was used for chronoamperometric sensing of 4-NP in the concentration range of 0.1 μM–900 μM and 970 μM–1760 μM, with a detection limit down to 0.057 μM. This detection limit becomes highly relevant as the US EPA and the European Commission have set the safety limit for 4-NP in drinking limit to be 0.43 μM and 0.0007 μM respectively.¹⁶³

A recent study by Kim et al.¹⁶⁵ developed nanocomposites from the electrocatalysts of Au and zinc sulfide (ZnS) nanorods for trace detection of 4-NP. The synthesis of Au-ZnS nanocomposites was performed using a simple photo-assisted reduction step. The gold nanoparticles were utilized for improving the electron transfer processes and were photo-deposited on the ZnS rods to form Au-ZnS hybrids (AZS) (Fig. 11). This study also tested the effect of different weight percent of gold used for depositing and decorating the ZnS nanorods. The best results in terms of linear range and the detection limit for the electrochemical sensor was achieved when the loading amount for the gold salt was 8%. Another significant advantage of Au in the AZS hybrids was the electron reduction at the interface of Au-ZnS nanocomposite during anodic potential scan. This allows the oxidation of 4-NP at lower oxidation potential (+0.1 V).¹⁶⁵ Moreover, the electrooxidation current was linearly proportional between the concentration range of 0.15 μM–2 μM, with a detection limit of 0.32 μM.

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Kubendhiran et al.¹⁶⁶ used ultrasonication method to produce a non-covalent carbon black and β-cyclodextrin (β-CD) nanocomposite (Fig. 12). This nanocomposite was used to modify screen-printed electrodes for determination of 4-NP. The conductivity of the carbon black improved the electrochemical sensing property, while the β-CD increased the polarity of the nanocomposite and improved the supramolecular recognition of 4-NP.¹⁶⁶ The nanocomposite-based electrochemical sensor in this case had excellent sensitivity and robustness depicted by wide linear range from 0.125–255.80 μm with a detection limit of 0.040 μm. This was one of the first work using a facile ultrasonication approach to achieve a stable dispersion of the carbon black using β-CD. This will create new opportunities for creation of cost-effective nanocomposites using greatly conductive carbon black for detection of different nitroaromatic compounds.¹⁶⁶ Rajkumar et al.,⁶⁸ have recently proposed an electrooxidation mechanism in the presence of S-GCN nanosheets which included a multi-step irreversible catalytic reduction of 4-NP. It was proposed that 4-NP is inclined to lose four electrons and four protons in the first step first to form 4-hydroxylaminophenol. In the following step the 4-hydroxylaminophenol followed a reversible two-electron and two-proton redox reaction to form 4-nitrosophenol. Again, it should be noted that this study did not discuss the real-life implications of the by-products, therefore, it is recommended that future studies should be performed for improving the mechanistic insights about 4-NP electrooxidation (Scheme 6).

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The analytical performance results of nanocomposite-based electrochemical, especially voltammetric, sensors for the detection of 4-NP are summarized in Table III.