A review on the determination heavy metals ions using calixarene-based electrochemical sensors

4.1.1.1 Potentiometric analysis of Ag⁺

Ion-selective electrodes based on calixarene have been widely employed in the potentiometric detection of Ag⁺. Calixarene derivatives 1, 2, 3, 4, 5, and 6 (as shown in Fig. 3) were found selective toward Ag⁺. Glassy carbon electrode (GCE) based on calix[4]arene 1 has good long-term stability, with only less than 5 % fluctuation in potential response when detecting silver ions after the electrode was stored in air for 1 month. This sensor also showed good response performance and properties, and hence can be used to detect silver ions at the sub-micromolar level. Ag⁺-ISE was developed by directly coating the surface of GCE with tetrahydrofuran solution containing PVC, 25,27-dihydroxy- 26,28-bis[5-(4-methyl-6-hydroxypurimidine)thiaamyloxy] calix[4]arene, sodium tetraphenyl borate and dioctyl phthalate (Lu, 2004).

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Fig. 3

The structure of 1) 25,27-dihydroxy- 26,28-bis[5-(4-methyl-6-hydroxypurimidine)thiaamyloxy] calix[4]arene (Lu, 2004) 2) calixarene derivative (Chen, 2000) 3) 1,3-Bis(2-benzothiazolyl)thioalkoxy-p-tert-butylcalix([4]arenes (Zeng, 2000) 4) calix[4]arene derivatives functionalized by two hydroxy and two benzothiazolylthioethoxy groups (Chen et al., 2000) 5) 25,27-dihydroxy-26,28-bis(3-benzothiazolyl -thiopropoxyl)-5,11,17,23-tetra-tert-butylcalix[4]carene (Ligand 1, L1), 25,27-dihydroxy-26,28-bis(3-benzothiazolylthiopropoxyl)-calix[4]arene (Ligand 2, L2) (Chen, 2001) and 6) 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-calix[4]arene-thiacrown-4 (Demirel, 2006).

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Calix[4](arene derivative 2 containing N-atoms of modified electrodes displayed good selectivity and sensitivity, a good detection limit, and a good linear response in detection. The electrode retained its performance even after it was repeatedly calibrated (7 times in 1 month). Calixarene derivative 2 containing N-atoms was used to detect Ag⁺ because the soft heavy metal ions (Ag⁺, Pb²⁺, and Hg²⁺) demonstrated a great affinity toward soft coordination centers such as nitrogen, sulfur and selenium atoms. For electrode preparation, a calixarene carrier, poly(vinyl chloride) (PVC), plasticizer, and potassium tetrakis (4-chlorophenyl-borate) (KTClPB) were mixed and dissolved in tetrahydrofuran (THF) to form a PVC-THF syrup. The syrup was poured into a glass mold and evaporated at room temperature overnight. A flexible, transparent membrane (0.2–0.4 mm thick) was obtained and cut into a disk (6 mm diameter) using a cork borer, and then pasted onto the PVC tip clipped at the end of electrode body, while the Ag-AgCl wire was immersed in a 0.01 M silver nitrate, internal solution. Lastly, the modified electrodes were pre-conditioned by immersion in 0.01 M silver nitrate solution for 2 h prior to detection purposes (Chen, 2000).

In the work of Weng et al. (Zeng, 2000); calix[4]arene derivative 3 came with functional groups containing S and N atoms. The study expected the derivative to improve selectivity towards soft heavy metal ions such as Ag⁺, Pb²⁺ and Hg²⁺ over alkali metal ions. Then, 1,3-Bis(2-benzothiazolyl)thioalkoxycalix[4](arenes 1 (n = 1) and 2 (n = 2) as ISEs ionophores were found to be selective toward Ag⁺ with little interference from Hg²⁺ and had better Nernstian response, 58 mV/decade^-1 as compared to the membrane without ionophores with slope less than Nernstian, 41 mV/decade^-1. The study introduced benzothiazolyl units into a calix[4]arene because the introduction of heterocyclic groups (pyridyl, bipyridyl, bithiazolyl) in both the upper and lower rims improved cation selectivity, enzyme mimicking abilities, and other physical properties.

Calix[4]arene derivative 4 functionalized by two hydroxy and two benzothiazolylthioethoxy groups-based polymeric membranes showed more selectivity towards Ag⁺ and against Hg²⁺, with a selectivity coefficient of (logK_Ag/Hg <  − 2.6) compared to Ag₂S-based electrodes, and a selectivity coefficient of (logK_Ag/Hg > 0) (Ma’mun et al., 2018). Of the calix[4]arene derivatives substituted by the benzothiazolyl unit, ligand 1 (L1) and 2 (L2) in 5 (as shown in Fig. 3) incorporated with the Ag⁺ membrane electrodes, showed a better Nernstial slope. L1 and L2 consisting of two (benzothiazolyl)thioproxy groups substituted calix[4]arene and had better sensitivity due to the good coordination geometrical arrangement of N or S atoms in L1 and L2 that affected the complexing ability towards Ag⁺, followed by the potential response of these electrodes. Therefore, dithia macrocycles-based electrodes have a high selectivity toward Ag⁺, not only because of their macrocyclic cavity size but also because of the coordination geometrical arrangement of sulfur group. In the pH study, the modified electrode possess drastic drop in current response at pH outside the range of pH 2.5 – 7.0, which is likely due to the formation of silver hydroxides and response to hydrogen ions at high and low pH, respectively (Chen, 2001).

A new 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-calix[4]arene-thiacrown-4-based PVC membrane potentiometric sensor 6 was found to increase electrode performance. However, this sensor depends on the stoichiometry of the complex formed between the neutral carrier and the target analytes. In this study, these electrodes containing 8.6 mg of calix[4]arene in the membrane demonstrated better performance than those containing 4.3 mg of the neutral carrier, and those containing the same mass ratio of PVC to plasticizer. The modified electrode was highly selective towards Ag⁺ (with a soft acid character) and exhibited the most potential response. The intermediate acid character of the metal ions showed negligible response. Hard acid character metal ions (alkali and alkaline earth ions), also known as soft bases, have negligible response because of their weak interaction with sulfur atoms (Demirel, 2006).

4.1.1.2 Potentiometric analysis of Cd²⁺, Cs⁺, Hg²⁺, Ni²⁺

A cadmium-selective sensor containing p-tert-butylcalix[6]arene 7 (as shown in Fig. 4) showed a fast response time of 35 s in detecting cadmium ions (Cd²⁺) and had a good lifetime; it can be used for 4 months and more without undergoing any significant change in potential response. Based on the literature, the loss of ionophores from the membrane while in contact with aqueous solution is the main factor contributing to a sensor’s limited lifetime. This research revealed that sufficient amounts of plasticizer and lipophilicity of ionophores could ensure long lifetimes and stable potentials. This sensor was successfully applied to industrial waste water samples, with the results showing good agreement with standard atomic absorption spectroscopy (AAS) technique (Gupta, 2014).

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Fig. 4

The structure of p-tert-butylcalix[6]arene (Gupta, 2014).

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Calix[6]arene tetraester I 8 (as shown in Fig. 5) displayed better detection of Cs⁺ with a detection limit below 10⁻⁶ M. The membrane (0.3 mm) for cesium ion detection was developed via a mixture of poly(vinyl chloride) (PVC): plasticizer in the ratio of 1:2, 1 mg ionophore and potassium tetrakis(p-chlorophenyl)borate (KTpClPB) (50 mol.% of ionophore if needed) in an appropriate volume of tetrahydrofuran (THF) followed by mechanical stirring. Lastly, the tetraester was casted in glass rings and dried at room temperature for 24 h. Calix[6]arene tetraester I without para t-butyl substituents was found to have superior characteristics in comparison to those containing para t-butyl substituents. Hence, this study concludes that, in PVC membrane electrodes, the ionophore structure is the major factor determining selectivity aside from the type of plasticizer used (Oh, 2000).

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Fig. 5

The structures of calix[6]arene tetraester I (Oh, 2000).

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Fig. 6 shows that calixarene derivatives 9, 10, and 11 have been found to be highly selective toward Hg²⁺. Calix[4]arene derivative 9 was synthesized through the condensation of calixamine and a Schiff base product. The response time of sensor 9 towards Hg²⁺ was rapid—within 20 s. In the pH study, the potential was maintained from pH 1.3 to pH 4 and dropped after pH 4 because of the formation of Hg(OH)⁺ ions. This work reported p-tert-butyl calix[4]crown with imine units, as the mercury (II) ion-selective electrode is superior in slope, response time, selectivity values, linear concentration range, and has a lifetime of more than 3 months. However, Ag⁺ with a high selectivity coefficient value strongly interferes with the detection of Hg²⁺ ions (Mahajan, 2004).

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Fig. 6

The structures of 9) p-tert-butyl calix[4]crown with imine units (Mahajan, 2004) 10) p-tert-butyl-calix[4]arenethioether derivative (Tyagi et al., 2010), and 11) 5,11,17,23,29,35-Hexa[(1-thiazole) azo]-37,38,39,40,41,42-hexahydroxy calix[6]arene (Lu et al., 2003).

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Calix[4]arenethioether derivative 10 acts as an ionophore for the ISE and displayed high selectivity towards Hg²⁺ ions, with advantages such as fast response, simplicity, low detection limit, reproducibility, and fair stability. The mechanism of the electrode response to Hg²⁺ involves the interaction of Hg²⁺ (electron acceptor) with O and S atoms in the calix[4]arenethioether derivative molecule (electron donor). This sensor showed a high response time of 14 s and could work satisfactorily in both aqueous and partially non-aqueous media up to 40 % (v/v) acetone, ethanol, and methanol, with a long lifetime of up to 7 – 9 months. The electrode was also successfully applied in real applications, including in amalgam samples, industrial wastewater, and a solder sample, displaying satisfactory agreement with the result obtained from atomic absorption spectroscopy (Tyagi et al., 2010).

In the detection of Hg²⁺ using calix[6]arene derivative 11, two Nernstian responses were obtained, at a slope of 61.1 mV per decade (at pH 6.5) and 28.7 mV per decade (at pH 4), due to the different forms of Hg²⁺ at different pH values. At pH 5.5 to pH 7.0, the ISE showed a twice Nernstian response typical for a monovalent cation, i.e., a slope of 55 mV per decade. At pH 3.0 to pH 5.0, ISE showed a Nernstian response with a slope of 25.6 mV per decade, which is a typical value for a divalent cation. Ionophore 11 is rich in intermediate donor atom-groups (nitrogen atom and azo group) and soft donor atoms; therefore, interferences from Ag⁺, Cu²⁺ and Pb²⁺, which are soft and intermediate acidic ions, are more likely to form complexation with donor N and S atoms attached to the calixarene. This proposed electrode proved applicable for real samples, such as in the detection of mercury ions in industrial wastewater, lake water, and synthetic solution (Lu et al., 2003).

p-(2-thiazolazo)calix[4]arene 12 (as shown in Fig. 7), an ionophore with an azo group on the upper rim of calixarene is an excellent neutral carrier for the solid state Ni (II) sensor and is applicable for determining Ni²⁺ ion concentration in environmental analysis (electroplating waste), and food samples (chocolate), where the results exhibited close agreement with those analyzed using atomic absorption spectroscopy (AAS). When the sensor was equilibrated with Ni²⁺ ions, the highest emf response was obtained. This result reveals that the ionophore is more selective towards Ni²⁺ in comparison with other metal ions, and the resulting ligand–metal ion complex experienced fast exchange kinetics. This sensor fabrication is different from the conventional ISE, where 10 μL of the membrane solution was directly modified on a cleaned and polished graphite electrode, and then evaporated at room temperature. This research found that the addition of plasticizers shortened the response time, returning best results of 10–15 s in comparison with the response time of 180 s (at high concentration level) and 300 s (at low concentration level) of the membrane without any plasticizer (Kumar, 2012).

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Fig. 7

The structure of p‐(2‐thiazolazo)calix[4]arene (Kumar, 2012).

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4.1.1.3 Potentiometric analysis of Pb²⁺

Calixarene derivatives 13, 14, 15, and 16 (as shown in Fig. 8) are selective toward Pb²⁺. To prepare an electrode with ionophore 13, platinum wires were used as substrates to coat the slurries calix[6]arene, PVC poly(vinylchloride) and anion excluder NaTPB (sodium tetraphenyl borate) dissolved in THF (tetrahydrofuran). Sensor 13 could be used for over 5 months without exhibiting any remarkable effect on the membrane potential, but the electrode response dropped after 5 months, possibly due to the aging of the PVC ionophore, the matrices, and the plasticizers. The sensor was successfully used in determining Pb²⁺ in synthetic samples such as alloy samples, battery waste samples, effluent water, and in electroplating bath solutions. The results acquired using this proposed sensor are in good agreement with both AAS and titrimetric techniques. The coated-wire ISE was selected in this work because it requires a very small volume of sample. It is also low cost, simple in design, and allows for the possibility of microfabrication and miniaturization. It also has mechanical flexibility in which the electrode can be used at any angle and in the absence of an internal solution (Bhat et al., 2004). 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(diphenylphosphinoylmethoxy)calix[4]arene 14 used as an ionophore, was directly coated on a graphite electrode to detect lead ions. In the graph of the potential response of the electrode for the Pb²⁺-ion-selective electrode, a plateau was observed at high concentrations, possibly due to the saturation of test solution/membrane interface by the lead complexes. The sensor exhibited a lifetime of over eight weeks. Additionally, the slope of the potential versus lead ion concentration remained constant throughout the assessment period. This sensor was tested in various synthetic water samples; however, there was some inaccuracy in measurement that could have been due to the presence of calcium. The electrode could be successfully used to determine Pb²⁺ when performing the potentiometric titration of lead solution with EDTA (Yaftian, 2006).

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Fig. 8

The structure of 13) 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexahydroxycalix[6]arene (Bhat et al., 2004) 14) 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(diphenylphosphinoylmethoxy)calix[4]arene (Yaftian, 2006) 15) 5,11,17,23-tetrakis[(p-Carboxyphenyl)azo]-25,26,27,28-tetrahydroxy calix[4]arene (Lu et al., 2002), and 16) general structures of butylcalix[n]arene ethyleneoxydiphenylphosphine (n = 4, 5 and 6 for ligands L1–L3, respectively) (Cadogan, 1999).

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The ionophore of a calix[4]arene derivative containing carboxyphenyl azo group 15, with a polymeric membrane, was found selective towards lead ions. The binding mechanism was studied using molecular modeling in which lead ion complexed with the heteroatom group (N = N) on the upper rim of the calixarene derivatives and displayed a cone conformation. The molecular modeling was studied using Hyperchem molecular modeling package (version 6.01), and a 3D energy-minimized structure recommend that lead ions possible to accommodate medium cavity calixarene as shown in Fig. 9. The lead ions were found to bind electrostatically to the polar cavity containing four azo-bond rings (Lu et al., 2002).

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Fig. 9

Side view of the 3D structure of the ligand, i.e., the Pb²⁺ complex. The energy-minimized structure suggests that the lead ion is located in a medium cavity (hydrogen atoms omitted for clarity) (Lu et al., 2002).

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Three difference ring sizes of calixarene phosphine oxide deriviative 16 were used to fabricate ion-selective electrodes with selectivity towards Pb²⁺ ions. An increase in the number of repeating units, n, in macrocycles, was found to enhance the selectivity. Ca²⁺, the most important interfering element, experienced decreased impact with increased cavity size from tetramer (n = 4), pentamer (n = 5), and then to hexamer (n = 6). All these calixarenes were found to have similar modes of complexation. The models obtained using Monte Carlo conformational searches indicated that the ions were complexed within the medium region between two types of oxygen atoms, namely, phosphine oxide and the phenoxy in all cases (Cadogan, 1999).

4.1.1.4 Potentiometric analysis of Sr²⁺, (UO₂)²⁺

A strontium (II)-selective sensor based on 4-tert-butylcalix(8)arene-octaacetic acid octaethyl ester 17 (as shown in Fig. 10) in a poly(vinyl chloride) (PVC) matrix showed greatly improved selectivity towards strontium ions in comparison with existing sensors. This sensor is an extended work from 4-tert-butylcalix(8)arene which exhibited improved pH range, selectivity, and response time. Based on the literature, the ester derivatives of calix[n]arenes (n = 4, 6) revealed better selectivity compared to the parent calixarene because of the change in conformation, cavity size, and also increased binding sites. In the pH study, the sensors displayed constant potential in detecting Sr²⁺ ions from pH 3 to pH 10. The changes in potential at pH lower than pH 3 and higher than pH 10 are due to the cofluxing of hydrogen ions and the formation of SrOH⁺ ions. The response time for this electrode to detect Sr²⁺ was 10 s. This electrode has a lifetime of 4 months. This electrode could be used as an indicator electrode in Sr²⁺ ion sensing via potentiometric titration (Jain et al., 2004).

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Fig. 10

The structure of 4-tert-butylcalix(8)arene-octaacetic acid octaethyl ester (Jain et al., 2004).

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Calixarene derivative 18 (as shown in Fig. 11) was incorporated in a poly(vinyl chloride) (PVC) membrane for uranium complexation. The extraction process was first carried out by immersing the membrane in uranyl species for 65 h, and then followed by uranium analysis. Extraction in this project entailed the ability of the membrane-bound calixarene to complex with the UO₂²⁺. This study showed that the presence of plasticizer (2-nitrophenyl octylether (NPOE)) did not interfere with the ISE measurement, as there was no extraction of ions when no calixarene was present. The selectivity of sensor 18 to uranium in simulated waste solution was also studied. The preparation of diluted uranium solution with waste solution generated a response at lower concentration; however, no response was observed when the uranium solutions were prepared directly using waste solution. Under these conditions, this experiment showed that calix[4]arene might have reacted with other background ions, as the electrode was not selective towards (UO₂)²⁺ prepared directly using waste solution (Duncan and Cockayne, 2001).

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Fig. 11

The structure of 5,11,17,23-tetra-tert-butyl-25,27-bis(hydroxy)-26-(ethoxycarbonylmethoxy)-28-(diethyl carbamoyl-methoxy) calix[4]arene (Duncan and Cockayne, 2001).

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Table 1 lists a summary of various calixarene derivative electrochemical sensors and the associated potentiometric method used for heavy metal ion detection.

4.5.1.1 Differential pulse anodic stripping voltammetry analysis of Ag⁺, Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺

Glassy carbon electrode coated with Langmuir-Blodgett film of p-tert-butylthiacalix[4]arene, LB-TCA/GCE 27 (as shown in Fig. 23) was investigated and found highly selective towards Ag⁺ using differential pulse anodic stripping voltammetry. An LB film modified electrode was used in this work, as it enhances the measuring sensitivity of Ag⁺. Langmuir-Blodgett technique is useful for forming sequential layers of ultrathin organic films and able to precisely control the thickness and order of a film at the molecular scale. The GCE was modified using direct coating method. The effect of accumulation time of Ag⁺ was studied. The result shows peak current increased from 0  to 20 min, and remain constant after 20 min because of the surface saturation. This sensor (LB-TCA/GCE) demonstrated good repeatability. Besides, the response of Ag⁺ on LB-TCA/GCE poses little effect after 30 days. A 50-fold Pb²⁺ and Cu²⁺ concentration was found to interfere with the detection of Ag⁺ as they competed with Ag⁺ to form complexes with TCA at the GCE surface. This research suggested adding a shelter reagent to eliminate the interference from Cu²⁺, Hg²⁺ and Pb²⁺. Real samples, including tap, lake, and synthesized water, were analyzed via the standard addition method and the result showed a good recovery percentage (Wang, 2009).

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Fig. 23

The structure of p-tert-butylthiacalix[4]arene (TCA) (Wang, 2009).

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Calix[4]arene 28 (as shown in Fig. 24), labelled as CAL, was modified on SPCEs for cadmium ions determination by anodic stripping voltammetry using open circuit accumulation. The CAL-SPCEs were developed by dropping 2 μL, 10 mg mL⁻¹ of CAL in dichloromethane (DCM) solution onto the electrode surface via micropipette, followed by solvent evaporation at room temperature. The principle of the method comprises the chelation of Cd²⁺ ions with calixarene modified SPCE at the open circuit. At the closed circuit, the accumulated Cd²⁺ ions are reduced to Cd⁰ and form a thin film on SPCE surface. Finally, reduced Cd⁰ undergo electrochemically strip back into the solution in the form of Cd²⁺ using differential pulse waveform. The resulting oxidation peak current was then displayed as the analytical signal. This sensor successfully accumulated Cd²⁺ at pH 8, 15 min accumulation time with a detection limit 2.8 ng mL⁻¹. Furthermore, it is the first report calixarene-based sensor for the trace determination of cadmium ions in real environmental water samples, namely river water (Honeychurch, 2002).

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Fig. 24

The structure of (5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(2-mercaptoethoxy)-calix[4]arene) (Honeychurch, 2002).

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Calix[4]arene 29 (as shown in Fig. 25) modified carbon paste electrode was used to determine Cu²⁺ by the accumulation of Cu²⁺ ions at an open circuit in the stirred solution with 10 min preconcentration time. The results revealed that an increase in calix[4]arene ligand percentage increases the accumulated amount of metal ions. However, poor reproducibility was observed in higher percentage of ligand. The increase in accumulation time led to an increase in analytical signals, thus resulting in lower detection limits of Cu²⁺. The copper signals also increased with stirring rate, and slow down until maximum accumulation of Cu²⁺ reached. The sensitivity of the sensor is retained for three months without significant changes, proving the chemical stability of the calixarene ligand. To prevent oxidative or hydrolytic cleavage of the ligand in atmospheric conditions, it was recommended that the sensor be stored in a desiccator. In a real application, a standard addition method was applied to tap water samples. The dilution of the samples gave a closer response to the pure aqueous solution as the matric effects and ionic strength of the solution had decreased, thus enhancing the accumulation of Cu²⁺ (Canpolat, 2007).

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Fig. 25

The structure of p-tert-butyl-2,4-dimethoxy-1,3[(dimethyl-thiocarbamoyl)oxy] calix[4]arene (Canpolat, 2007).

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Calixarene derivatives 30, 31, and 32 (as shown in Fig. 26) are selective toward Hg²⁺. Amino-thiacalix[4]arene 30 modified graphitic carbon on screen-printed graphite electrodes was developed using a microwave-irradiation process. The carboxylic groups were induced on the electrode via microwave-irradiation process, providing an ease platform for the anchoring of amino-thiacalix[4]arene via amide bond formation on screen-printed electrodes. Amino-thiacalix[4]arene was synthesized as mercury sensing due to the potential of carbonyl oxygen and amide nitrogen as donor atoms. The interaction mechanism is predicted to be through the thiolate ion and the lone pairs of electrons present on the nitrogen and oxygen atoms (as shown in Fig. 27). Hence, due to its features, the covalently modified amino-thiacalix[4]arene interface was able to detect mercury at trace level concentrations (pico molar). In the recovery study, known concentrations of standard solution were spiked into the samples and the results validated the employment of the proposed sensors in real applications (Adarakatti et al., 2017).

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Fig. 26

The structure of 30) amino-thiacalix[4]arene (Adarakatti et al., 2017) 31) amino-calix[4]arene (Prashanth and Pandurangappa, 2016) 32) p-allylcalix[4]arene (Dong, 2006).

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Fig. 27

Schematic representation of the binding and complexation modes of the modifier with the analyte (Adarakatti et al., 2017).

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Amino-calix[4]arene 31 modified a graphitic composite glassy carbon electrode as a sensing probe for Hg²⁺. The graphite material was underwent microwave irradiation to induce COOH functional groups on its surface in the presence of low concentrated nitric acid. Therefore, the introduction of carboxyl groups on graphite materials are easier for further immobilization of amino-calixarene moieties to graphite surfaces through amide bond formation. Preconcentration time was studied in this work to obtain better current response because it promotes the accumulation of mercury ions on the modified electrode interface, known as the chelating phenomena. This proposed interface was successfully applied to real sample matrices such as chrome plating, lake water, and industrial effluents. Moreover, this proposed interface was validated as an alternative sensor, which displayed better results compared to other existing sensors (Prashanth and Pandurangappa, 2016).

The fabricated electrode of Langmuir-Blodgett film with calixarene 32 showed superior sensitivity and reproducibility for both preconcentration and stripping analysis of Hg²⁺ ions in water. The various steps involved in analysis process are as follows:

i) Preconcentration step: $$\text{allyl}\left(\text{surface}\right)\text{+}{\text{Hg}}^{\text{2+}}\stackrel{}{\to }\text{allyl}{\text{-Hg}}^{\text{2+}}$$

ii) Reduction step: $$\text{allyl-}{\text{Hg}}^{\text{2+}}\left(\text{surface}\right)\stackrel{}{\to }\text{allyl +}{\text{Hg}}^{\text{0}}\text{(surface)}$$

iii) Stripping step: $${\text{Hg}}^{\text{0}}\left(\text{surface}\right)\stackrel{}{\to }{\text{Hg}}^{\text{2+}}\left(\text{solution}\right)\text{+ 2e}$$

In mercury detection, the LB film coated electrode was found to give higher peaks compared to the direct coating method of calixarene 32. This may occur because the number of active groups outside of the surface of the electrode is higher than the electrodes in the direct coated method. The allyl groups at the upper rim of calixarene 32 is important in mercury ion recognition. This is because this study found that the modification of p-tert-butylcalixarene on LB film is not be able to recognize soft metal ions, meaning that benzene rings were not involved in the mercury recognition process. This fabricated electrode has great potential for practical sample analysis such as tap, lake, and river water with good recoveries for the determination of mercury (Dong, 2006).

Calixarene 33 and 34 (as shown in Fig. 28) were selective toward Pb²⁺. Calix[4]arene-tren (Calix-tren) 33 modified glassy carbon (GC) electrode was fabricated for the quantification of lead ions. The modification was performed by immersing the clean bare GCE into the modifier solution containing 0.2 mg/mL Calix-tren in chloroform, 10.0 mM ethylene diamine (EDA), 2.0 mM 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), and 5.0 mM N-hydroxy succinimide (NHS) in dimethyl sulfoxide (DMSO) for 1 h, followed by air drying. The results revealed that this modified sensor provides a better electron transmission pathway compared to bare glassy carbon electrodes. The detection limit of the sensor towards lead ions is 0.11 µM. It was found that the presence of competitive ions caused little effect on lead ion determination. This developed sensor was successfully applied in the analysis of real samples of wastewater. The proposed sensor has advantages of being low cost, good selectivity, and reproducibility in lead ions quantification under the optimum conditions of pH 7, −1.2 V, and 120 s (Kocer, 2019).

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Fig. 28

The structure of 33) Calix[4]arene-tren (Calix-tren) (Adarakatti et al., 2017) 34) thiacalixarene (TCA) (Wang, 2012).

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The combination of thiacalix[4]arene 34 with superior selective recognition and multi-walled carbon nanotubes (MWCNTs) with outstanding electronic properties in electrode modification displayed outstanding selectivity and sensitivity toward Pb²⁺ ions. The introduction of carbon nanotubes (CNTs) as a new coating material in this study is due to their high conductivity which can counteract the poor conductivity of calixarenes, significantly improving the electrochemical signals, and minimizing the detection limit. The electrode was modified by introducing TCA-MWCNTs into acetone, followed by MCNTs sonication to aid the dissolution of the functionalized MNCTs. In the last step, the sonicated solution was casted directly onto the electrode using micro-injector. The TCA functionalized MCNT-modified electrodes are highly selective towards Pb²⁺ ions with a detection limit of 4 × 10⁻¹¹ M. The low detection limit may be ascribed to the excellent electronic properties of MWCNT and the outstanding recognition ability of the TCA ligand. The separate signal produced when analysing the solution containing both interfering ions, Sn²⁺ and Pb²⁺ may also be due to the excellent recognition of thiacalixarene 34. In this study, the mechanism of the stripping voltammetric measurement was proposed. Initially, at open circuit conditions, Pb²⁺ ions are accumulated from the solution phase onto the electrode surface via selective complexation with TCA. Then, the Pb²⁺ on the electrode surface were reduced to Pb⁰ at applied voltage. The Pb²⁺ was then electrochemically stripped back into solutions by scanning toward the positive potential (Wang, 2012).

4.5.1.2 Simultaneous detection of Cd²⁺ and Pb²⁺ using differential pulse anodic stripping voltammetry analysis

There are several calixarene derivatives that are selective toward Cd²⁺ and Pb²⁺ including 35, 36, 37, and 38 (as shown in Fig. 29). The amino-calix[4]arene-derivative 35 was modified on a graphitic carbon electrode to perform simultaneous quantification of Cd²⁺ and Pb²⁺ ions at a picomolar level. The selectivity of the amino-calixarene-modified electrode was primarily examined using cyclic voltammetry (CV) to understand the potential affinity of sensors towards Cd²⁺ and Pb²⁺. The peak potential displayed in the stripping analysis shows that it is located at a more negative potential than the cyclic voltammetry because of the complexation phenomenon between metal ions and ligand molecules (CO-NH groups in the modifier molecule) (as shown in Fig. 30). The excellent detection limits using this developed sensor may be due to the complexation of Cd²⁺ and Pb²⁺ with the hydroxyl groups (lower rim) and amide groups (upper rim) of calixarene during the stripping study. This developed sensor has been successfully applied to determine Cd²⁺ and Pb²⁺ levels in alloy materials, battery effluents, and sewage water sample matrices (Adarakatti and Malingappa, 2016).

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Fig. 29

The structure of 35) amino-calixarene (Adarakatti and Malingappa, 2016) 36) calix[4]arene (Adarakatti et al., 2019) 37) p-tert-butylthiacalix[4]arene (Zheng, 2007) 38) calix[6]arene (Xiao-bo, 2004).

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Fig. 30

Schematic representation of binding and complexation mode of modifier with analyte (Adarakatti and Malingappa, 2016).

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Calix[4]arene 36 functionalized Mn₃O₄NPs were used as electrochemical sensors for the simultaneous measurement of Cd²⁺ and Pb²⁺ ions at a nanomolar level by differential pulse anodic stripping voltammetry. In cyclic voltammogram of glassy carbon electrode (GCE), calix[4]arene functionalized Mn₃O₄NPs modified GCE poses a higher peak current than the calix[4]arene 36 and Mn₃O₄ modified GCE under identical conditions, and at a similar anodic and cathodic peak potential. The improvement in electrochemical response is due to the functional groups with cavity and a high surface to volume ratio. The wide surface area and high porosity of calix[4]arene allow Cd²⁺ and Pb²⁺ ions to access cavities and wide internal surfaces boosted the complexation between Cd²⁺ and Pb²⁺ ions and the surface of the calix[4]arene functionalized Mn₃O₄NPs. For the electrode modification, 5 µL of modified suspension volume was selected as the optimum volume because higher amount of modifier suspension will increase the resistance of modified film against mass transfer of analytes and electron transfer. In the calibration plot of Pb²⁺, the result demonstrated an incomplete oxidation process, which may due to the insufficient or incomplete optimized parameter for Pb²⁺ to get oxidized. The application of this sensor to detect Cd²⁺ and Pb²⁺ is verified in the water samples of wood’s alloy and lead-acid battery, and it is in good agreement with the standard protocol (Adarakatti et al., 2019).

A disposable sensor, glassy carbon electrode modified Langmuir-Blodgett film of p-tert-butylthiacalix[4]arene (LB_TCA-GCE) 37 was fabricated for quantification of cadmium and lead ions. The voltammogram shows two-well-defined stripping peaks appearing at the LB_TCA-GCE. The large difference between Cd²⁺ and Pb²⁺ stripping peak potential (210 mV) proposes LB_TCA-GCE suitable to perform simultaneous detection (as shown in Fig. 31). The recognition of metal ions can be explained by the hard and soft acid and base (HSAB) principle. Accordingly, oxygen and sulfur atoms attached to calixarene are soft bases, Cd²⁺ and Pb²⁺ are soft acids, because of that the presence of Cd²⁺ and Pb²⁺ giving notable signals on the LB_TCA-GCE electrode. This method holds promise for simultaneous detection of Cd²⁺ and Pb²⁺ in natural water samples with superb average recoveries and good matching with the results obtained using FAAS technique (Zheng, 2007).

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Fig. 31

Differential pulse voltammograms of 1 × 10⁻⁶ mol l⁻¹ Pb²⁺ and Cd²⁺ in 0.1 mol l⁻¹ acetate buffer at different glassy carbon electrodes: (a) bare glassy carbon electrode; (b) LB_TCA modified glassy carbon electrode. Accumulation time, 5 min; accumulation potential, −1.2 V; differential pulse amplitude, 50 mV; scan rate, 50 mV s⁻¹; pulse duration, 40 ms (Zheng, 2007).

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In this present work, calix[6]arene modified carbon paste electrodes (CAL-CPEs) 38 were used to detect Pb²⁺ and Cd²⁺ simultaneously with high sensitivity, low detection limit, good reproducibility, and able to determine Pb²⁺ and Cd²⁺ in water samples. The results demonstrated the difference of stripping peak potential between Pb²⁺ and Cd²⁺ is 240 mV, which revealed the potential of this modified electrode to perform simultaneous detection of Pb²⁺ and Cd²⁺ using DPSV. The weight ratio of calix[6]arene (20%) to carbon powder was the optimum loading. The higher loading of calix[6]arene decreased the peak current as higher loading calix[6]arene causing fewer ions to arrive into the inner binding sites for the reduction to M⁰. The notable drop in signal is related to the reduction of conductivity of modified electrode because of the rising of resistance at the CAL-CPE. The calix[6]arene modified carbon paste electrode has great potential for practical sample analysis as its recoveries are fine enough for practical use and the results were very close with the results analyzed by atomic absorption spectroscopy (AAS) (Xiao-bo, 2004).

4.5.1.3 Simultaneous detection of Cd²⁺and Tl⁺ using differential pulse anodic stripping voltammetry analysis

Langmuir-Blodgett film of p-allylcalix[4]arene 39 (as shown in Fig. 32) (AllylCA-GCE) was designed for the simultaneous detection of cadmium and thallium ion traces by using differential pulse anodic stripping voltammetry. LB film was developed by spreading the solution (1–2 mg of compound 39 dissolved in dichloromethane) onto a pure water surface via syringe, followed by 30 min evaporation of spreading solvent. Finally, the LB film was obtained through the horizontal deposition method and coated on a glassy carbon electrode. The mechanism involved in ion detection is the chelation of Cd²⁺ and Tl⁺ ions from solution on the surface of AllylCA-GCE, followed by the reduction of accumulated Cd²⁺ and Tl⁺ ions, and eventually the reductive materials were electrochemically stripped back into the electrolyte solution. KH₂PO₄ buffer (potassium dihydrogen phosphate buffer) was employed throughout the experiment as the highest oxidation peak current was obtained, the lowest background current, and the simultaneous oxidation peak shape was well separated. This research suggested that the sensitivity of detection for low concentrations of analytes can be enhanced by increasing the preconcentration time. The proposed voltammetric sensor can be applied to determine Cd²⁺ and Tl⁺ ions in lake and tap water (Dong, 2006).

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Fig. 32

The structure of p-tert-butylcalix[4]arene and p-allylcalix[4]arene (Dong, 2006).

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4.5.1.4 Simultaneous detection of Cd²⁺, Cu²⁺ and Pb²⁺using differential pulse anodic stripping voltammetry analysis

Thiacalix[4]arene (TC4A) 40 (as shown in Fig. 33 a) modified electrode was successfully applied in the simultaneous detection of Cu²⁺, Cd²⁺, and Pb²⁺ in a sample solution and individual detection in a river water sample. TC4A was directly coated onto the glassy carbon electrode and dried at room temperature, forming a continuous uniform film on the electrode surface. The immobilization of TC4A onto the electrode surface is through electrostatic action and Van der Waals force. The coating of TC4A on the glassy carbon electrode was able to detect Cu²⁺, Cd²⁺ and Pb²⁺ with DPASV curves present at − 0.03 V, − 0.79 V and − 0.55 V, respectively, and the peak current value increased linearly with the concentration (as shown in Fig. 33 b). The complexation ability of TC4A to gather three kinds of heavy metal ions in the solution and deposited it on the TC4A-GCE surface with large quantities may contribute to the simultaneous detection of these target analytes (Cu²⁺, Cd²⁺, and Pb²⁺). The number of heavy metal ions deposited at the TC4A-GCE surface was limited because of the competitive reaction of different heavy metal ions in which complexation priority of this sensor with heavy metal is based on electron arrangement. In a real application, the results obtained by this proposed method and AAS are in good agreement (Liu et al., 2019).

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Fig. 33

a) The structure of thiacalix[4]arene (TC4A). b) DPASV responses of the TC4A-GCE for the simultaneous determination of different concentrations of Cu (II), Cd (II), and Pb (II) mixed heavy metal ions in a 0.1 mol L⁻¹ pH 5.0 acetic acid solution. The concentrations of the heavy metal ions from curve-1 to curve-10 were: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 mg L⁻¹ for Cu (II) and Pb (II); and 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 mg L⁻¹ for Cd (II), respectively. The experiment was performed under the optimal conditions: deposition potential 1.1 V, deposition time 180 s, pulse amplitude 50 mV (Liu et al., 2019).

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4.5.1.5 Simultaneous detection of Cu²⁺, Hg²⁺ and Pb² using differential pulse anodic stripping voltammetry analysis

Calix[4]arene bulk 41 (as shown in Fig. 34 a) modified screen-printed electrodes (SPCCEs) as a one-shot disposable electrochemical sensor was successfully applied in the simultaneous determination of toxic metal ions including copper, mercury, and lead ions in environmental samples. Multiple peaks were observed in the simultaneous quantification of Cu²⁺, Hg²⁺ and Pb²⁺ (as shown in Fig. 34 b) due to non complexed analytes trapped on the modified SPCCEs surface or the leaching of the modifier molecule, while the different of intercept of Cu²⁺ and Hg²⁺ in comparison with Pb²⁺ illustrated kinetic limitation. A very tiny volume of real samples (15 – 25 μL) was used in the application study of real sample matrices. This fabricated sensor was tested in sensing target ions present in environmental samples (industrial effluents and wastewater samples) and further validation was conducted using the atomic absorption spectrometric technique. The results verify that this proposed protocol is in good agreement with the standard method (Adarakatti, 2017).

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Fig. 34

a) The structure of 25, 26, 27, 28-tetrahydroxy-calixarene. b) Overlaid anodic stripping voltammograms of lead (II), copper (II), and mercury (II) with increasing concentration range from 100 to 2400 μg/L. Conditions: accumulation potential − 1.1 V, deposition time 180 s, potential window − 1.0 V to 0.6 V (Adarakatti, 2017).

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Table 4 illustrates the summary list of various calixarene derivatives electrochemical sensor in relation to differential pulse anodic stripping voltammetry method in heavy metal ion detection.

4.5.2.1 Square wave anodic stripping voltammetry analysis of Cu²⁺ and Pb²⁺

Calixarene 42 and 43 (as shown in Fig. 35) are selective toward Cu²⁺. The modified electrode with calix[4]arene derivative 42 and its optimal experimental conditions was successfully applied in various bivalve mollusks samples with satisfactory results. The modified electrodes were fabricated by drop-coating calixarene derivative 42 on the electrode surface, followed by solvent evaporation at room temperature for 5 min. The preconcentration capacity of the electrochemical sensor was enhanced owing to the chemical linkage of organometallic fragment cyrhetrenyl to lower rim of calixarene macrocycles, thus providing a new platform to carry out rapid and sensitive analyses in quantified Cu²⁺ in marine resources. The increase in current of the modified electrode for Cu²⁺ quantification may be due to the increase in the number of active sites caused by the two OH groups of the calix[4]arene and two imine unsaturated spacers that connect the two fragments, which enhanced the electrostatic interaction with Cu²⁺ ions. In Cu²⁺ detection, the optimum current was observed at pH 4. However, the current decreased at lower pH which probably due to the decomposition of the compound. This proposed method is effective in detecting Cu²⁺ in real bivalve mollusks samples and proposes that the matrix effect does not affect the response of modified electrodes (Pizarro, 2019).

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Fig. 35

The structure of 42) p-tert-butylcalix[4]arene-bis-cyrhetrenylimine (Cy₂(Calix[4]) (Pizarro, 2019) 43) tetraoxocalix[2]arene [2]triazine (TOCT) (Zou, 2012).

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Langmuir-Blodgett film of tetraoxocalix[2]arene[2]triazine (TOCT) 43 was modified on electrodes to analyse traces of copper ions in water samples using a square wave stripping voltammetry (SWSV). From the voltammogram result, a sharp peak was observed using TOCT-GCE. The peak current was further improved when LB_TOCT-GCE, Langmuir-Blodgett film of tetraoxocalix[2]arene[2]triazine modified glassy carbon electrode (two layers) was used in the analysis under the same experimental conditions. This may be due to the homogenous and stable surface of LB_TOCT-GCE that is favorable for the accumulation and selective recognition of copper ions. Furthermore, the cavity and multi-dimensional structure of TOCT on modified electrodes are favorable for copper ion accumulation as TOCT greatly improves the electrode surface area. This proposed method is found to be applicable to quantify copper ions in lake water, drinking water, and city wastewater samples by using a standard addition technique (Zou, 2012).

A calix[4]arene 44 (as shown in Fig. 36) modified sensor was developed by coating calixarene solution and Nafion on a glassy carbon electrode surface, followed by 2 h of solvent evaporation at room temperature. Prior to utilization, this sensor was conditioned in 0.05 M, pH 4.0 potassium phthalate buffer overnight for analytes detection. The modified electrode is applicable for multipurpose usage by cleaning the coating layer of the electrode at a potential of + 0.3 V for 1 min, and then treated in 0.1 M EDTA solution for 2 min to detach the residual metal ions from the polymer layer. In this work with calixarene 44, ASV contains the accumulation of lead ions onto the electrode surface by complexation with a ligand (ionophore) in an open circuit. Acetate buffer was selected as blank solution in the experiment because the best peak shape and largest stripping current were obtained while performing detection in acetate buffer. In this study, the number of the binding sites on the electrode surface could be enhanced while applying a ligand incorporated-chemically inert polymer. This is the first polymer/ligand modified bismuth-film electrode used to detect Pb²⁺ ions using adsorptive stripping voltammetry. This electrode was successfully used to evaluate the lead concentration in environmental water samples such as tap, and rainwater with good recovery (Torma, 2009).

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Fig. 36

The structure of calix[4]arene tetrathioamide ligand (Torma, 2009).

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4.5.2.2 Simultaneous detection of Pb²⁺ and Tl⁺ using square wave anodic stripping voltammetry

The Langmuir-Blodgett film of a p-tert-butylcalix[4]arene derivative 45 (as shown in Fig. 37)–modified glassy carbon electrode was fabricated for the simultaneous detection of Pb²⁺ and Tl⁺ using square wave anodic stripping voltammetry. Langmuir-Blodgett film of p-tert-butylcalix[4]arene derivative modified glassy carbon electrode (LB_DCA-GCE) displayed good response in tracing Pb²⁺ and Tl⁺ while no obvious peaks were observed at the bare GCE. One layer of LB_DCA-GCE showed a higher peak current than two layers of LB_DCA-GCE in the determination of target ions. This is because LB_DCA acts as a bulwark and the R_et (reaction resistance) of one layer of LB_DCA (1.07 kW) is smaller than that of the two layers (3.02 kW). The DCA attached with LB_DCA does not complexation with Pb²⁺ and Tl⁺ but LB_DCA functions as a new electrode material by providing a site for Pb²⁺ and Tl⁺ reduction. The mechanisms involved are as follows:

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Fig. 37

The structure of 25,27-Dimethoxy-26-(N-trichloroacetyl)carbamoyloxy-p-tert-butylcalix[4]arene (DCA) (Zou, 2009).

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i) Reduction step (accumulation): $${\text{LB}}_{\text{DCA}}\left(\text{surface}\right)\text{+}{\text{M}}^{\text{n}\text{+}}\text{(solution) + n e}\stackrel{}{\to }{\text{LB}}_{\text{DCA}}\text{-}{\text{M}}^{\text{0}}\text{(surface)}$$

ii) Oxidation step (stripping): $${\text{LB}}_{\text{DCA}}\text{-}{\text{M}}^{\text{0}}\left(\text{surface}\right)\stackrel{}{\to }{\text{LB}}_{\text{DCA}}\text{(surface) +}{\text{M}}^{\text{n}\text{+}}\left(\text{solution}\right)\text{+ n e}$$

The applicability of the proposed method for Pb²⁺ and Tl⁺ detection in tap water and lake water samples was found successful yielding high accuracy and good recovery results (Zou, 2009).

Table 5 provides a summary of electrochemical sensors based on various calixarene derivatives in the determination of Cu, Pb, and Tl metal ions using square wave anodic stripping voltammetry.

4.7.1.1 Cyclic voltammetry analysis of Hg²⁺

This sensor 50 (as shown in Fig. 45) with calix[4]arene conjugated ferrocenyl dendrimers 1, 2, and 3 is highly sensitive to Hg²⁺ with the lowest concentration 2 × 10⁻⁵ M (1, 2), 1 × 10⁻⁵ M (3) using cyclic voltammetry. From the cyclic voltammograms, the redox sensing of Hg²⁺ shifted the CV curve to a less positive side (as shown in Fig. 46) which demonstrated the ion-pairing reaction between mercury ions and the –OH group attached at the lower rim of calixarene and one N atom of each triazoyl groups. The ferrocenyl dendrimers (as shown in Fig. 45) were able to detect Hg²⁺ ion selectively in river water and other water resources containing competitive ions such as Ag⁺, Cu²⁺, and Zn²⁺ (Saravanan et al., 2017).

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Fig. 45

The structure of p-tert-butylcalix[4]arene cored triazolyl ferrocene conjugates 1 – 3 (Saravanan et al., 2017).

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Fig. 46

Cyclic voltammetric redox sensing of Hg²⁺ on dendrimer 1, 2, and 3 (2 × 10^-5 M): (a) dendrimer 1 with Hg²⁺ in CH₃CN (2 × 10^-5) (b) dendrimer 2 with Hg²⁺ in CH₃CN and (c) dendrimer 3 with Hg²⁺ in CH₃CN (Saravanan et al., 2017).

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All in all, the work focuses on the detection limit of the calixarene derivatives-based electrochemical sensor. For the sensing parameters, other than detection limit (LOD), parameters such as linear detection range, selectivity, sensitivity, stability and reproducibility also significant in analytes detection (Gu, 2020; Liu et al., 2020; Gu, 2020). These parameters were discussed further in the works of (Gu, 2020). In the work of Liu et al. (Liu et al., 2020) , the structure of MOF-based materials have impact on the sensitivity, selectivity, stability, and accuracy of sensor. Moreover, the size and morphology also affect the performance and applicability of MOFs (Gu, 2020). Hence, in this field, the design of calixarene is also crucial to yield an excellent reproducibility and long shelf life sensors for heavy metal determination.