Herein, we report a facile approach for constructing a calixarene-based electrochemical heavy metal sensor (Calix/MPA/Au) via a one-pot reaction for the detection of Ni(II) and Zn(II) ions. The surface elemental properties and analytical performance of the Calix/MPA/Au sensor were characterized by X-ray photoelectron spectroscopy (XPS) and differential pulse voltammetry (DPV). Under optimum conditions, the sensor exhibited detection limits of 1.5 and 0.34 mg/L at linear ranges of 2.85–6.65 and 0.13–1.68 mg/L for the Zn(II) and Ni(II) ions, respectively. The developed sensor exhibited a better electrochemical performance in the detection of Zn(II) and Ni(II) ions owing to the favourable host–guest interactions between the hydroxyl groups-functionalized lower rim of dicarboxyl-calixarene and the metal ions. The RSD of the five independent Calix/MPA/Au electrode for Zn(II) and Ni(II) ions was calculated to be 16.3 and 16.1%, respectively. Despite the lower sensitivity of the modified electrode towards Ni(II) ions, this finding proves the high selectivity of the calixarene as a detection probe towards the fitted size of guest ion, hence promising to be assembled and explored as a solid-state based-supramolecular host molecule for tracing metal ions.
A platform for sensing Zn(II) and Ni(II) ions based on calixarene (Calix/MPA/Au) was developed.
The detection was investigated using different electrodes; Au, MPA/Au, and Calix/MPA/Au, demonstrating Calix/MPA/Au had the highest current response towards targets due to host–guest interaction.
The developed sensor exhibited a limit of detection of 1.5 and 0.34 mg/L for Zn(II) and Ni(II) ions, respectively.
Ni(II) and Zn(II) ions, which are naturally found in the Earth's crust, soil, and sediment, pose an environmental and biological risk to the ecosystem when existing in an elevated concentration. According to reports, the corrosion of pipes, water tanks, and pipe coatings is the primary cause of pollution in drinking water sources, notably contamination with metallic elements (ATSDR 2005; Clark et al. 2015; World Health Organization 2021). Accredited agencies such as the U.S. Environmental Protection Agency (USEPA) and the World Health Organization (WHO) have established the safe limits for Zn(II) and Ni(II) ion concentrations in water for public drains at 5 and 0.1 mg/L, respectively. Due to their persistence, severe toxicity, and indegradability, heavy metal ions have been categorized as hazardous pollutant agents (Järup 2003; Saha et al. 2016). It has been disclosed that excessive exposure to Zn(II) results in complications such as nausea, vomiting, stomach pain, cough, chest pain, dyspnoea, pneumotoria, and acute pnuemonitis due to irritation of the respiratory tract, whereas allergic reaction or dermatitis is the most common adverse effect of Ni(II) exposure (ATSDR 2005; Hannachi et al. 2010; Ding et al. 2015). In the worst-case scenario, an excess of Zn(II) might cause neurological diseases such as Parkinson's disease, hypoxic ischaemia, and Parkinson's disease and anaemia (Zhou et al. 2012). Comparable in toxicity to Zn(II), Ni(II) ion has been found to induce long-term health issues such as respiratory disorders, nasal sinus, lung cancer, and pneumonitis (Patil & Salunke-Gawali 2018).
Conventional analytical techniques for determining the concentration of heavy metal ions include inductively coupled plasma/mass spectrometry (ICP-MS), inductively coupled plasma/atomic emission spectrometry (ICP-AES), atomic absorption spectroscopy (AAS), X-ray fluorescence, ion chromatography, and ultraviolet-visible spectrometry (UV-VIS). Nevertheless, despite their high sensitivity, precision, and versatility in terms of their ability to trace various metal ions, these techniques had several limitations, such as complex analytical processing, time-consuming and tedious sample preparations, costly equipment, the need for trained personnel, and their inapplicability for on-site applications (Dechtrirat et al. 2018; Eddaif et al. 2019; Eddaif et al. 2020). In contrast to conventional elemental analysis techniques for tracing heavy metals, electrochemical methods have garnered considerable interest in recent decades as an alternative analytical route due to their relative ease of operation, rapid response, cost-effective fabrication, low-cost instrumentation at high accuracy and precision, and wide linearity of sample concentrations (Afkhami et al. 2017; Mei & Ahmad 2021). In addition, other distinctive characteristics of this analysis include the flexibility to miniaturize electrodes and facilitate electrode modification (Kudr et al. 2014). Voltammetry is an alternative electrochemical technique that has been extensively employed for the detection of heavy metals in natural water samples because of its excellent stability for in situ measurement and quantitative analysis (Afkhami et al. 2013). For instance, cyclic, staircase, and pulse are among the most convenient types of approaches used. Differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV) are the most common procedures used to trace heavy metals by using a wide range of working electrodes. Both of these methods have comparable sensitivity but are distinct in the current response measurement. In contrast to the DPV approach, the SWASV method has been demonstrated to be the most sensitive and effective in detecting adsorbed electroactive organic compounds (Lovrić 2010).
The chemical nature of the electrode surface, which relates to the notion of chemically modified electrodes (CME), has a significant impact on the sensitivity of the current response generated by the chosen electrochemical technique. In general, the chemical characteristics and binding properties of a surface electrode are determined by the outermost attached molecule (Murray et al. 1987). By altering the surface by irreversible adsorption, it is possible to create self-assembled layers or covalent bonding electrodes with unique features. The modification technique increased the electron transfer kinetics, allowing them to serve as a conducting substrate. Working electrode surfaces such as the carbon paste electrode (CPE), glassy carbon electrode (GCE), and screen-printed electrode (SPE) that have favourable redox behaviour, rapid electron transfer, a broad potential window, and low toxicity are the most common materials that have been modified using the CME technique (Sajid et al. 2016). SPEs are of particular interest since they have the potential to be used in mass manufacturing at low cost and could be integrated into portable devices that need just modest sample sizes and minimal reagent usage (Dechtrirat et al. 2018). Particularly, the SPCE gold-based construction has garnered attention because of its high electrode transfer, wide anodic potential interval, stability, and chemical reaction resistance (Li & Miao 2013). Gold is the most practical metal surface for SAMs owing to the inclusion of diverse functional groups, which typically results in the production of a well-defined monolayer. In 1983, Nuzzo and Allara were among the pioneering researchers to demonstrate well-directed self-assembly of dialkyl sulphide derivatives on gold surfaces (Bain et al. 1989; Wink et al. 1997). Due to its resistance to surface alteration, gold substrate appears to be a promising platform for a variety of applications, such as the fabrication of electrochemical sensors.
To maximize the efficiency of electrochemical sensors, selective materials that are employed to build the electrode interfaces have become a key step for the development of high-performance sensors. Calixarene, a macrocyclic chemical first found in 1872 as a byproduct of baeklite synthesis, has sensing applications of interest because of its adaptability in tolerating different kinds of guest ions (Agrawal et al. 2010; Vicens & Böhmer 2012; Düker et al. 2014). Intrinsically, the variability of host–guest interactions is contributed by the presence of interactions such as ion–dipole, cation–II, anion–II, CH–II, hydrogen bonds, stacking, and van der Waals interactions (Buschmann et al. 2001; Udachin et al. 2001; Saiapina et al. 2016). Calixarene contains an open, rigid, and preorganized scaffold comprised of upper and lower rims that are typically formed via the reaction of p-substituted phenol with formaldehyde. Calixarene may also be modified to have varied sizes and numerous functions, making it a molecule worthy of exploration (Zaghbani et al. 2011; Vicens & Böhmer 2012). The functional group attached to the calixarene, such as the phenolic hydroxyl group, contributes to the stability of complex forms by providing a polar environment that is beneficial for zwitterion stability (Arnaud-Neu & Schwing-Weill 1997). Besides, the phenolic hydroxyl groups provide an ideal site for the incorporation of other functional groups including ketone, amide, ester, or carboxylic acid groups, making the calixarene-functionalized hydroxyl group an excellent starting material. Depending on the functional group attached at its rim, the complexity of the compound may vary, increasing in the following sequence: ester < ketone < amide < carboxylic acid (Arnaud-Neu & Schwing-Weill 1997). Published early in 2001 by Honeychurch, two types of calixarene derivatives, which are thiolated 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(2-mercaptoethoxy) calix arene and 25,26,27,28-tetrahydroxy-calixarene on screen-printed carbon electrodes, were developed for lead ion detection. This is one of the earlier studies involving the fabrication of a solid-state SPCE sensor using calixarene as the modifier. The LOD was determined to be 5 ng/mL using the DPASV approach. Due to the matrix effect, the performance of the calixarene sensor in real samples was degraded with LOD of 14 ng/mL (Honeychurch et al. 2001).
MATERIALS AND METHODS
Chemicals and reagents
All chemicals purchased were of analytical grade and directly used as received. 3-Mercaptopropionic acid (MPA), dimethyl sulphoxide (DMSO) and phosphate-buffered saline (PBS) solution were purchased from Sigma-Aldrich. Potassium chloride (KCl), sulphuric acid, nickel chloride (NiCl2), and copper (II) sulphate (CuSO4) were purchased from R&M chemicals. Ethylene diamine (EDA), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxy succinimide (NHS), and potassium ferricyanide (K3[Fe(CN)6]) were separately purchased from Acros Organic, Fluka, Alfa Aesar, and Bendosen, respectively. Zinc acetate trihydrate [Zn(C2H3O2)2.3H2O], lead chloride (PbCl2), and ethanol 97% were purchased from HmbG. All aqueous solutions were prepared in deionized water (18.2 MΩ cm at 25 °C). The dicarboxyl-calixarene used in the experiment was provided by Dr Mary Deasy, a chemist from the Institute of Technology, Tallaght, Dublin, Ireland. Screen-printed gold electrodes (SPGEs)/Ink AT/Work in solution (DRP-C220AT) with dimensions of 3.4 × 1.0 × 0.05 cm (length × width × height) and a working surface dimension of 4 mm were purchased from Metrohm Malaysia Sdn Bhd while a gold electrode with a diameter of 1.6 mm was purchased from BASi, which has been used in a few parameters in the optimization step.
Fabrication of Calix/MPA/Au electrode
The fabrication process of the Calix/MPA/Au electrode was performed in accordance with our previously reported findings (an electrochemical sensing platform for the detection of lead ions based on dicarboxyl-Calix  arene). In general, the working gold electrodes were pretreated by immersion in an electrochemical cell containing 0.5 M sulphuric acid solution, followed by 20 cyclic voltammetry (CV) cycle sweeps at a potential window of 0–1.3 V to acquire a clean gold platform. The self-assembly of the MPA monolayer was then performed by dipping the gold surface into 1 mM ethanolic MPA solution, which was then subsequently left in the dark for 24 h. The fabricated electrode (MPA/Au) was immediately used in the next experimental step. The prepared MPA/Au electrode was then fabricated with the dicarboxyl-calixrene modifier. A flowchart depicting the procedure is shown in Figure 1. The carboxyl terminus of MPA/Au was modified by drop-casting a solution of ethylene diamine, EDA (10 mM in DMSO) and dicarboxyl-calixarene (2.0 mg/mL in chloroform) in the presence of EDC/NHS linker (2 mM EDC; 5 mM NHS in DMSO) that had previously been prepared through a one-pot reaction and left for about 1, 2, 3, 4, and 5 h. The Calix/MPA/Au electrode was then dried over a flow of nitrogen and kept at room temperature for detection studies.
The DPV was measured using anodic stripping voltammetry (ASV) mode between –1.2 and 0 V at a scan rate of 100 mV/s, with a conditioning time and equilibrium time of 5 s, followed by a deposition potential at –1.2 V and a deposition time of 120 s.
The electrochemical behaviour of Calix/MPA/Au electrode towards heavy metal ions monolayer was performed through differential pulse voltammetry (DPV) of ASV mode. The modified electrode was immersed in a cell containing 10 mL of 1 μM of analyte in 0.1 M KCl solution. Prior to the DPV measurement, the electrochemical procedure was set to a preconcentration step to enhance the detection signal. The deposition potential was performed at the potential range of –1.3 to –1.0 V for the detection of Ni(II) ion and a potential range of –1.4 to –1.0 V for Zn(II) ion. The parameter was studied in the presence of 1 μM metal analytes in 0.1 M KCl at pH 7 as electrolyte solution for about 120 s. The deposition time of analytes Zn(II) and Ni(II) were varied at five different deposition times; 30, 60, 90, 120, and 150 s employing 0.1 M KCl at pH 7 as supporting medium at the potential of –1.2 V. Following that, the potential was scanned in the range of –1.2 to –0.4 V for Ni(II) ions and –1.3 to – 0.4 V for Zn(II) ion with set deposition potential of –1.2 V for 120 s at a scan rate of 100 mV/s with a conditioning time and equilibrium time of 5 s to acquire a detection peak. All the experiments were conducted at room temperature.
XPS analysis was conducted using an X-ray Microprobe Phi Quantera II with a spectrometer equipped with a monochromated Al Kα scanning X-ray source with an energy of 1,486.6 eV. The wide and high-resolution scans were executed using a beam size of 300 μm with 50 W power and pass energies of 280 and 112 eV, respectively. Prior to elemental analysis, all the binding energies (BEs) were referenced to the carbon C-C component at 285 eV, while the background subtraction was performed with the Casa XPS software using either a linear or Shirley-type method, based on the peak shape fit. The defined peaks were acquired by fitting with the Gauss–Lorentz profile, and the elemental composition data of the electrode surface was displayed. Voltammetric measurements were performed employing an electrochemical system comprised of the AUTOLAB instrument Model uAutolab Type III (Eco Chemie B. V., Netherlands). Reference Ag/AgCl (3.0 M KCl) and counter platinum electrodes were required to run the three-electrode system analysis utilizing the modified gold electrode as the working electrode, whereas a cable connector (CAC) was required for the gold screen-printed electrode (SPE) measurement. The CV and DPV voltammograms were analysed using NOVA 1.11 software.
RESULTS AND DISCUSSION
Surface analysis by X-ray photoelectron spectroscopy
Optimization of experimental parameters for Ni(II) and Zn(II) detection
Next, the influence of the supporting electrolyte was studied in three types of electrolytes: 0.1 M potassium chloride (pH 5.6), 0.1 M acetate buffer (pH 4.5), and 0.1 M phosphate buffer saline (pH 7) solutions containing 1 μM of metal-ion analyte. Figure 3(b) shows the magnitude of the anodic peak responses of Zn(II) and Ni(II) ions. A high background current was observed in the presence of acetate and phosphate-buffered saline, resulting in the low intensity of the detection peak for both metal ions. Thus, KCl solution was selected as the optimal supporting electrolyte as the highest current magnitude was recorded (Liu et al. 2013). Meanwhile, the bar chart illustrated in Figure 3(c) represents the current response of Zn(II) and Ni(II) detection in the pH dependence study. 0.1 M KCl as the supporting electrolyte solution in the pH range of 3.0–10.0 with the presence of a 1 μM metal-ion solution was employed. The pH of the solution was adjusted by adding 0.1 M sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions to acquire acidic and basic conditions. The results showed that the potential response of the Calix/MPA/Au electrode achieved the best current signal under neutral conditions (pH 7) for both metal ions. At pH < 7, the decrease in current could be ascribed to the partial protonation of the tailored modifier, whereas the formation of hydroxo complexes may have contributed to the decrease in the peak current under more basic conditions (Ganjali et al. 2010; Kempegowda & Malingappa 2012; Sharma et al. 2013; Huang et al. 2014; Liu et al. 2015).
Preconcentration effects on the detection of Ni(II) and Zn(II) ions
Analytical performance of the Calix/MPA/Au electrode towards Zn(II) and Ni(II) ions
Comparative study of Zn(II) and Ni(II) ion detection at the Au, MPA/Au, and Calix/MPA/Au interfaces
Interference, cycle number, reproducibility, and stability studies of the Calix/MPA/Au electrode
Figure 7(b) depicts the bar chart of Zn(II) and Ni(II) ions current density as a function of DPV cycles. The maximum current was observed in the first DPV measurement cycle. Owing to the fragility of the Calix/MPA/Au electrode, which may have had physical damage, no further treatment was performed to regenerate the modified electrode, thus resulting in the degradation of sensor performance as more cycles were applied. A further increase in cycle number resulted in the anodic current suppression since there were fewer binding sites accessible on the modified surface due to surface saturation. This demonstrated that the detection of Zn(II) ions by the Calix/MPA/Au electrode was best at the first usage. Meanwhile, a different trend in current response was observed for Ni(II) detection using the Calix/MPA/Au electrode. The anodic current reached its maximum reading on the 2nd cycle, indicating that the modifier on the electrode surface can capture a maximum amount of Ni(II) ions on the second scan. The highest current response was achieved as there were more available sites on the dicarboxyl-calixarene rims that could fit the smaller Ni(II) ions compared with the Zn(II) ions. However, the bulky alkyl group of calixarene was likely to hinder the oxidation and reduction of the captured metal ions in the rims. This explained the current response of Ni(II) ions, which was much smaller than the current response of Zn(II) ions that bound directly to the hydroxyl groups because of their larger size.
The reproducibility of the Calix/MPA/Au electrode was determined on the basis of the RSD values of five independently developed sensors for Zn(II) and Ni(II) cations, which were found to be 16.3 and 16.1%, respectively, as shown in Figure 7(c). The fabrication process involving low concentrations of the bulky modifier (dicarboxyl-calixarene) could have contributed to the high RSD values since the low uniformity of the interface reduced the availability of active sites for metal-ion binding. Moreover, the efficiency of the modified electrode to trace metal ions depended on factors such as the solubility and pH of the prepared solutions, which affected the electroactive species (metal cations) ionic activity present in the solution. The deterioration of the modified electrode surface remains one of the challenges in fabricating electrochemical sensors for industrial purposes. The dependence of the current response on the stability of the Calix/MPA/Au electrode was studied, and readings were obtained periodically after 14 and 30 days. The bar chart in Figure 7(d) illustrates a significant decrease in peak current response up to 92.86% for the Calix/MPA/Au electrode in the presence of Zn(II) ions after a month. A gradual decrease was also observed during the detection of Ni(II) ions over the same period as the peak was suppressed to 84.85%. The deterioration of the modified electrode stability was ascribable to the degradation of the MPA monolayer, which was first fabricated onto the SPGE surface. This was supported by data obtained from a study done by Mani et al. in 2008, who reported that the degradation of the formed thiol derivative monolayer on metal surfaces such as gold and titanium was likely to occur after seven days due to the oxidation process (Mani et al. 2008). Thus, a freshly prepared Calix/MPA/Au electrode is recommended to acquire the best sensitivity in the detection of selected metal ions.
In this work, we have successfully incorporated an electrochemical heavy metal-ion sensor for Zn(II) and Ni(II) utilising a screen-printed gold electrode based on calixarene. The macrocyclic molecule whose lower rim was functionalized with a hydroxyl group could well be presented as a potential sensing material that can accommodate metal ions of favourable size via favourable host–guest interaction. The inclusion of calixarene has been demonstrated to enhance the sensitivity for metal-ion detection particularly relative to conductive bare SPGE (Au) and MPA/Au electrodes. The results revealed that the Calix/MPA/Au sensor was considerably sensitive and selective in recognizing Zn(II) ions based on the LOD of 1.5 g/mL in a concentration range of 2.85–6.65 g/mL, which passed the threshold limit. Nevertheless, despite the fact that the modified electrode could only detect Ni(II) concentrations as low as 0.34 g/mL, which is slightly above the permissible limit, the selective properties of the macrocyclic compound in accommodating and forming a complex with the fitted metal ion as a guest ion was still a significant discovery of supramolecular compound as solid-state metal-ion sensor.
The authors would like to thank the Ministry of Higher Education for financial support via the Fundamental Research Grant Scheme (FRGS/2/2014/ST01/UPM/02/3) and Universiti Putra Malaysia for the Research University Grant Scheme (GP-IPS/2017/9647500) in funding this research project.
AVAILABILITY OF DATA AND MATERIAL
All datasets used in this study are included in the manuscript.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.