Cation–π-induced mixed-matrix nanocomposite for the detection and removal of Hg²⁺ and azinphos-methyl towards environment remediation

Abstract

The unregulated use of pesticides, which constitutes organophosphates, demands their continuous monitoring from a human health perspective. The development of efficient, reliable and affordable methods for the effective quantification, removal and detoxification of pesticides is indeed a significant challenge in the fields of agriculture, environmental science and public health. Herein, we designed a simple approach for the construction of a functionalised electrochemical material that includes the following steps: (i) the cation–π induced non-covalent functionalization of multiwalled carbon nanotubes (MWCNTs) with an organic cation IL, and (ii) the complexation of IL@MWCNTs with Hg²⁺ to accelerate electron transfer, apparently enhancing the response of Hg/IL@MWCNTs towards azinphos-methyl, as revealed by cyclic voltammetry. Hg/IL@MWCNTs/GCE exhibits electrocatalytic behaviour towards azinphos-methyl (AZM) with a low detection limit of 1.10 μM and a wide linear range (0.20–180 μM). The degradation of the AZM pesticide was supported by ³¹P NMR titration and mass spectrometry, which confirmed the conversion of AZM into its non-toxic products. Taking into account the aforementioned findings, the functionalised IL@MWCNT composite was fabricated into an ultrathin polyamide layer on a PES support membrane via interfacial polymerisation for practical application. The developed nanocomposite membrane removes the Hg²⁺ metal ion and azinphos-methyl pesticide from contaminated water with a removal efficiency of 95% and 90%, respectively.

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Water impact

Micropollutants, such as organophosphates and heavy metals, pose a potential risk upon their discharge into the environment. In this context, herein, we developed an organic cation-modified MWCNT-based nanocomposite derived membrane for the detection and removal of Hg²⁺ and azinphos-methyl with a removal efficiency of 95% and 90%, respectively, to reduce environmental pollution.

Introduction

Water is essential for the existence of life on Earth, but the population growth and urbanization have degraded water quality via the increasing consumption of various industrial chemicals, such as heavy metals, pesticides, herbicides and insecticides.¹ According to reports by the World Health Organisation (WHO), nearly 829 000 people die from diarrhoea every year as a consequence of a lack of sanitation and unsafe water for drinking.² In the year 2020, approximately one in four people lacked access to safe drinking water in homes, which indicates that a substantial portion of the global population faced challenges in obtaining water that met the necessary safety and quality standards. The COVID-19 pandemic has highlighted the urgent need for addressing these water and sanitation issues to improve public health and resilience against future health emergencies, yet millions of people around the world lack access to a safe water supply.³ Over the last few decades, toxic organophosphorus pesticides have been extensively utilised in the agriculture, animal husbandry and aquaculture industries to ensure the quality and quantity of food products via the protection of animals or plants from insects, rodents, nematodes, bacteria, fungi, etc.² The origin of these chemicals entering into the surface water include domestic discharge, wastewater treatment plants, industrial discharge, agriculture and air (via dust and rain). Moreover, the worldwide consumption of pesticides is estimated to have reached approximately 3.5 million tons in the year 2020.⁴ In particular, azinphos-methyl is one of the toxic organophosphorus pesticides for non-target organisms, which include insects, birds, human beings, fish and aquatic invertebrates used to control the growth of pests on apple production, vegetables, rice production, nuts, crop field, fruits and shade trees.⁵ Apart from this, azinphos-methyl is widely used as an ingredient for the production of commercially available pesticides, such as Guthion, Cotnion-methyl, Gusathion, Cotnion, Gusathion-M, Carfene, Crysthyron, Bay 17147, Metriltrizotion and R-1852. Azinphos-methyl can enter the body via ingestion, dermal absorption, inhalation, or eye contact, which may lead to bleeding, muscle contraction, pupil constriction, tears, blurred vision, etc.⁶ The mechanism of action involves inhibition of acetylcholinesterase (AChE) through phosphorylation of a serine residue in the enzyme's active site. The accumulation of acetylcholine at the synaptic cleft due to inhibition of acetylcholinesterase leads to overstimulation of acetylcholine receptors, causing paralysis and death in insects.⁷ Thus, it becomes essential for environmental researchers to invent novel methods for the detection and degradation of toxic pesticide waste into non-hazardous compounds, so that the former may not cause any threat to the environment.

Conventional analytical techniques, such as enzyme-linked immunosorbent assays (ELISAs),⁸ high-performance liquid chromatography (HPLC),⁹ gas chromatography-mass spectrometry (GC-MS),¹⁰ liquid-chromatography-mass spectrometry (LC-MS),¹¹ surface enhanced Raman spectrometry,¹² and metal–organic frameworks,¹³ are sensitive and accurate, and are used routinely for the quantification of toxicants in water or food. However, it requires time-consuming sample preparation and relies upon sophisticated instruments and skilled manpower, requiring the transportation of samples to the laboratory for analysis. Thus, there is a need for the development of newer techniques, which are fast, economical, sensitive, as well as selective for the trace level detection of pesticides. Acting as one of the “rising star” metals, metal oxides, conducting polymers, graphene, and carbon nanomaterials have received considerable attention due to the large surface area, extraordinary electrical performance, and unique optical property, and have been applicable in light-emitting diodes, sensors, bioimaging, and others.¹⁴ A perusal of the literature revealed that single-walled (SWCNTs) or multiwalled carbon nanotubes (MWCNTs) have been used extensively for the electrochemical sensing and removal of toxic analytes in healthcare and environmental applications.¹⁵ For instance, Xu et al. have developed a CuMOF-SWCNTs@AuNPs-based electrochemical sensor for the monitoring of imatinib in human serum,¹⁶ whereas Bruzaca et al. developed fMWCNT-Nafion_0.5%/GCE for the recognition of imidacloprid.¹⁷ Sun et al. designed a three-dimensional SERS substrate on a CNTs/Ag@Au/SiO₂ composite for fipronil and imidacloprid pesticides.¹⁸ Furthermore, Singh and co-workers have reported various MWCNTs-modified composites for the recognition of toxic pollutants.^19–22 Apart from this, ionic liquids exhibit good cation–π interaction with MWCNTs, thus effectively preventing damage to the extended conjugation and conductive properties of the MWCNTs.^22–25 The literature studies reveal that there is no such material reported which is capable of simultaneously removing mercury metal and the azinphos-methyl pesticide from contaminated water. Keeping this in mind, the advantages of MWCNTs and cation–π interaction with the ionic liquid, a cost-effective and catalytically efficient material has been designed for the dual-response electrocatalytic quantification and mitigation of Hg²⁺ and azinphos-methyl towards environment remediation (Fig. 1). The functionalised IL@MWCNTs material, along with p-phenylene diamine (PPD), was added into the aqueous phase to fabricate an ultrathin polyamide layer on the PES support membrane by interfacial polymerisation. The developed nanocomposite membrane revealed the removal of the Hg²⁺ metal ion and azinphos-methyl pesticide from contaminated water with removal efficiencies of 95% and 90%, respectively.

Fig. 1 Scheme for the design of the IL@MWCNT composite and its role in electrochemical quantification as well as the catalytic detoxification and removal of Hg²⁺ and AZM.

Material and methods

2-Aminobenzimidazole, ethyl bromoacetate, hydrazine hydrate, 4-pyridine carboxaldehyde, metal salts, multi-walled carbon nanotubes and pesticides utilised in different experiments were purchased from Spectrochemical India and Sigma Aldrich. A JEOL instrument was utilised for ¹H NMR and ¹³C NMR characterisation of the synthesised organic cation (IL) operating at 400 MHz and 100 MHz frequency in d₆-DMSO solvent using tetramethylsilane as reference. High-resolution mass spectrometry (HRMS) was recorded on the Xevo G2-XS QTOF (WATERS) instrument. The electrochemical studies were performed using a Metrohm Autolab (AUT86921) workstation with the modified Hg/IL@MWCNTs/GCE as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. The morphology of the synthesized material was examined using field emission scanning electron microscopy (FE-SEM), elemental dot mapping and energy-dispersive X-ray spectroscopy (EDAX) on the JSM-7610F Plus instrument. The oxidation state and constituent elements of the composite material were analysed using XPS (Thermo-Fisher Scientific, model – ESCALAB Xi +) instrumentation. A Perkin Elmer spectrophotometer (model – Veriton S8620G) was utilised for FTIR analysis in the 400–4000 cm⁻¹ range. The thermal stability was evaluated using a Pyris 1 thermogravimetric analysis (TGA) instrument (PerkinElmer, USA) with a temperature increment of 10 °C min⁻¹ under air atmosphere in the range of 0 °C to 700 °C for thermograms. Raman spectra were recorded on a LABH Rev-UV-Open instrument utilising 532 nm wavelength.

Synthesis of the organic cation (IL)

The organic cation (IL) was synthesised via three-step procedure, as shown in Fig. S1.† Initially, KP-1 was prepared using a procedure already reported in the literature.²⁶ Furthermore, precursor A was prepared by dissolving KP-1 and ethyl-2-bromoacetate in ACN solvent under refluxing conditions for 24 h. The white precipitates thus formed were filtered out and washed two to three times with ACN to remove the excess ethyl-2-bromoacetate. Then, precursor A was treated with hydrazine hydrate in EtOH with constant stirring at ambient temperature for 2 h to give white solid (KP-2) precipitates, which were filtered, followed by washing with EtOH to remove the excess hydrazine. Furthermore, a condensation reaction between the KP-2-based hydrazide (0.500 g, 1.90 mmol) and 2-hydroxynapthaldehyde (1.100 g, 2 mmol) was performed in 15 mL MeOH under refluxing for 24 h to give the final organic cation (IL), which was characterized using various spectroscopy techniques (Fig. S2–S10†), such as ¹³C NMR, ¹H NMR spectroscopy, and high-resolution mass spectrometry (HRMS).

Preparation of IL@MWCNTs and modification of the glassy carbon electrode

The IL@MWCNTs composite was prepared using a synthetic method already reported in the literature.²⁷ A stable IL@MWCNTs dispersion was obtained using 2 mg of organic cation (IL) and 1 mg of MWCNTs in 5 ml of MeOH, followed by 24 h stirring.²⁸ The resulting samples were sonicated, followed by centrifugation for 35 min at 8000 rpm. The solvent was removed and solids re-dispersed in MeOH for further characterizations. The bare glassy carbon electrode (GCE) was polished with alumina powder, followed by ultrasonication in ethanol. Afterward, 2 mg of an electrocatalyst was dispersed in 1 ml MeOH with constant ultrasonication at a frequency of 37 kHz for 45 minutes. Then, 5 μL of catalyst suspension was drop-casted onto the surface of a cleaned GCE, followed by drying at room temperature. The electrochemical studies were conducted within the potential range of −1.5 V to 1.5 V at a scan rate of 100 mV s⁻¹ in phosphate-buffered solution (pH 7.4, 0.1 M). The electrochemical impedance spectrum (EIS) was evaluated within the frequency range of 3 × 10⁶–50 Hz in a 5 mM mixture containing [Fe(CN)₆]^3−/4− and 0.1 M KCl. Both LSV and DPV experiments were performed at a scan rate of 100 mV s⁻¹ within the potential range of 0.3 V to −1.5 V.

Preparation of the composite membranes

The polyamide PA/IL@MWCNTs nanocomposite membranes were prepared using a vacuum filtration-assisted procedure, followed by interfacial polymerisation (IP).²⁹ The as-prepared IL@MWCNTs suspension was filtered on a PES substrate with varying compositions of 2.5 wt%, 5 wt% and 7.5 wt%, respectively (Fig. S11†). After filtration, the IL@MWCNTs layer was deposited onto a polyethersulfone support and dried at 40 °C with subsequent soaking in 1.0 wt% p-phenylene diamine (PPD) aqueous solution (30 ml, 5 min), and kept for drying at room temperature to completely evaporate the surface water. The membrane thus obtained was immersed in 0.1 wt% trimesoyl chloride (TMC) in n-hexane solvent (40 ml, 3 min) for the interfacial polymerisation reaction to take place. Finally, the developed PA/IL@MWCNTs nano-composite membrane was dried at 40 °C overnight. The polyamide (PA) membrane on bare PES was prepared following the same procedure as above, except omitting the vacuum filtration step of MWCNTs.

Results and discussion

Development of the organic cation (IL)-modified MWCNTs and its characterisation

A stable IL@MWCNTs dispersion was obtained using 2 mg of organic cation (IL) and 1 mg of MWCNTs in 5 ml of MeOH, followed by 24 h stirring. The solvent was removed and the remaining solids re-dispersed in MeOH, forming a stable dispersion that avoids agglomeration for over a week, as supported by morphological FE-SEM characterisation. It was observed that a continuous coated layer of IL was formed on the surface of MWCNTs relative to the bare MWCNTs displaying smooth surface. These results indicated that IL was successfully wrapped onto the MWCNTs surface (Fig. 2A and B). The elemental composition of the multi-walled carbon nanotubes was analysed using EDAX before and after their modification with an ionic liquid (IL) (Fig. S12 and S13†). The elemental analysis results provide concrete evidence of the IL@MWCNTs composite formation, demonstrating the presence of C, N and O elements. Fig. 2C shows the stacked FTIR spectra of IL, bare MWCNTs and IL@MWCNTs. The peaks at 1700 cm⁻¹ (ester), 1643 cm⁻¹ (–C=O), 1618 cm⁻¹ (–C=N), 1578 cm⁻¹, 1469 cm⁻¹ (–C=C) are attributed to the presence of functional groups in IL. Moreover, the modification of MWCNTs with IL results in an observable variation in the vibrational spectra, which reveals up-shifted peaks, indicating the cation–π stacking interaction and hydrophobic interactions. These findings suggest a specific interaction between the organic cation (IL) and the MWCNTs, providing insight into the structural and chemical modification induced by the organic cation.

Fig. 2 (A) FE-SEM images of bare MWCNTs and (B) IL@MWCNTs. (C) FTIR spectra of IL, MWCNTs and IL@MWCNTs. (D) TGA and DSC analysis of IL@MWCNTs.

The bare MWCNTs did not show signature peaks of functional groups at any wavenumber. Moreover, TGA-DSC analysis was performed to assess the thermal stability and functionalization of various modified materials (Fig. 2D and S14†). For TGA, samples were heated in air at a 10 °C min⁻¹ ramp rate ranging from 0–700 °C. The TGA-DSC curve of the bare MWCNTs shows weight loss stability up to 354.65 °C. In contrast, modification of MWCNTs with IL alters the stability of IL@MWCNTs, as organic ligands degrade at higher temperature. The IL@MWCNTs show stability over a temperature range of 500–600 °C. All these results indicate the surface decoration of MWCNTs with IL.

Metal binding studies with the IL@MWCNTs-modified electrode

Multiwalled carbon nanotubes (MWCNTs) are often employed as electrode modifiers to enhance the performance for various applications, such as sensing, energy storage and catalysis, due to its high electrical conductivity, more specific surface area and good mechanical strength.³⁰ Furthermore, the electrochemical detection of metal ions was evaluated using cyclic voltammetry to gain insight into the electrocatalytic behaviour of IL@MWCNTs. The carbon electrode was modified using IL@MWCNTs (2 : 1), and CV was recorded using phosphate-buffered solution (pH = 7.4, 0.1 M) at a scan rate of 100 mV s⁻¹. The electrochemical detection ability of the modified IL@MWCNTs electrode was evaluated towards various metal ions (5 mM), such as Li⁺, Co²⁺, Na⁺, Ag⁺, K⁺, Ca²⁺, Fe³⁺, Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Ce³⁺, Hg²⁺, and Al³⁺ (Fig. 3A). The cyclic voltammogram reveals that IL@MWCNTs show selectivity towards Hg²⁺ out of various other analysed metal ions. The CV profiles of the modified electrodes were investigated, which revealed an incremental increase in the peak current of Hg/IL@MWCNTs relative to IL, IL@MWCNTs and bare MWCNTs (Fig. S19A†). The results revealed that the peak current is enhanced after the introduction of mercury into IL@MWCNTs, increasing the electron transfer in the fabricated material. New anodic and cathodic peaks of IL@MWCNTs, emerge at 0.33 V (57.49 μA, peak current) and −0.64 V (−55.89 μA, peak current), respectively. A sharp redox potential peak for IL@MWCNTs appeared after interaction with the Hg²⁺ metal ion, corresponding to the reversible transformation of Hg²⁺ to Hg⁰ by the non-bonding electrons of the acylhydrazone moiety present in the organic cation (IL). This redox behaviour of the electrocatalyst can be described by eqn (1) and (2). Moreover, the electrochemical recognition of the Hg²⁺ ion was quantified using the modified IL@MWCNTs/GCE electrode on CV and DPV, respectively, via the gradual addition of the Hg²⁺ ion in a concentration range from 0.4 to 120 μM (Fig. 3B and F). This caused an incremental increase in the cathodic and anodic peak currents with the linear calibration plot showing Pearson's R² value of 0.9614 (I_pa), 0.9715 (I_pc) for CV, and 0.97638 for DPV, respectively (Fig. 3C and inset Fig. 3F). The limit of detection as obtained by 3σ-method is 0.651 μM. To evaluate the electro-kinetics behaviour of the Hg²⁺ ion at the modified IL@MWCNTs/GCE electrode, CV studies were performed at varying scan rates (20–200 mV s⁻¹) using 120 μM of Hg²⁺ in PBS buffer solution (Fig. 3D). The slope of the linear plot consisting of the anodic and cathodic peak current versus the square of the scan rate illustrates that the recognition of the Hg²⁺ metal ion at the IL@MWCNTs/GCE modified electrode is a diffusion-controlled phenomenon (Fig. 3E).

Hg²⁺ + 2e⁻ → Hg⁰ Reduction (1)

Hg⁰ → Hg²⁺ + 2e⁻ Oxidation (2)

Furthermore, the X-ray photoelectron spectroscopy (XPS) elemental survey scan in the range of 0–1200 eV clearly reveals the presence of C 1s, N 1s, O 1s and Hg 4f elements on the surface of MWCNTs, indicating the formation of Hg/IL@MWCNTs (Fig. S15†). On deconvolution, the high-resolution C 1s spectrum contains four different peaks at distinct binding energy values. The peaks at 284.7, 285.3, 287.1 and 288.5 eV can be attributed to C–C/C=C, C–N/C–O, C=N and C=O, respectively, which indicates the presence of a conjugated structure (Fig. 4A).^31,32 The O 1s deconvoluted spectrum consists of three peaks at 533.4, 532.4 and 530.9 eV assigned to O–C–O, O–C=O and C=O, respectively (Fig. 4B). Similarly, the deconvoluted N 1s XPS spectrum (Fig. 4C) contains four peaks, which are assigned as follows: 398.9 eV for pyridinic nitrogen, 399.7 eV due to amide, 400.3 eV assigned to pyrrolic nitrogen, and 402.3 eV due to the quaternary nitrogen (N⁺).³³ Similarly, the two doublets of Hg 4f (4f_7/2 and 4f_5/2) can be divided into the main four peaks at 99.6 eV, 104.37 eV, 103.8 eV and 100.3 eV, respectively (Fig. 4D).^34,35 The two peaks at 99.6 eV and 103.8 eV are assigned to Hg⁰, and other peaks at 100.3 eV and 104.37 eV assigned to the Hg²⁺ species, revealing the mercury redox couple formation with IL@MWCNTs/GCE. Moreover, the atomic % for each element as given in the peak table, along with previously discussed results, confirms the formation of the Hg/IL@MWCNTs composite material (Fig. S16†). Raman spectroscopy is a versatile technique for the characterization of carbon-based materials. Thus, the Raman spectra of the bare MWCNTs, IL@MWCNTs and Hg/IL@MWCNTs composites were analysed. The characteristic bands in MWCNTs, D band (defect), G band (graphite band) and G' band (D overtone), were observed at 1347.16 cm⁻¹, 1579.44 cm⁻¹ and 2694.05 cm⁻¹, respectively (Fig. 4E). The intensity ratios of the D and G bands (I_D/I_G) provide evidence regarding the sp³-hybridised carbon atoms and number of structural defects present in the MWCNTs sample reveals non-covalent functionalization. Thus, higher the intensity ratios of the I_D/I_G bands, greater will be the % degree of functionalization.³⁶ The relative intensity ratios of G′/G, D/G and G′/D were separately evaluated. The I_D/I_G ratios for the pristine MWCNTs, IL@MWCNTs and Hg/IL@MWCNTs composites are 0.91, 1.02 and 1.07, respectively (Fig. S17†). The increase in the I_D/I_G ratio of the modified MWCNTs relative to pristine MWCNTs gives evidence of IL decorated on the surface of MWCNTs. Moreover, the I_G′/I_D ratio is 0.91, 0.84 and 0.78 for pristine MWCNTs, IL@MWCNTs and Hg/IL@MWCNTs, respectively. The I_G/I_G′ ratio for pristine MWCNTs, IL@MWCNTs and Hg/IL@MWCNTs is 0.90, 0.86 and 0.83, respectively. The decrease in the I_G′/I_G and I_G′/I_D ratios of the modified MWCNTs relative to pristine MWCNTs revealed a high lattice defects density in the non-covalent modified MWCNTs. Moreover, the broad band observed at 3460 cm⁻¹ in the FT-IR spectra of the organic cation IL is indicative of the presence of the phenolic ν_O–H group. The absence of this characteristic band in the FT-IR spectrum of the metal complex shows that there is a coordination bond formation between the metal ion and phenolic hydroxyl group. An increase in the absorption frequency of the ν(C–O) stretching vibration, observed around 1300 cm⁻¹, further supports the involvement of the oxygen atom of the phenolic group in coordination with the metal ion. This shift to higher wavenumbers (about 14 cm⁻¹) indicates a change in the bond strength or environment of the oxygen atom upon coordination. The intense band at 1619 cm⁻¹ in the IR spectrum of the organic cation IL corresponds to the azomethine ν(C=N) stretching frequency. The observed shift of the intense band corresponding to a lower frequency (40 cm⁻¹) supports the coordination of the azomethine nitrogen to the metal centre. The presence of the band at 3223 cm⁻¹ due to the amide ν(N–H) stretching frequency appears at almost the same position in both spectra, thus indicating that the coordination of the Hg²⁺ ion does not affect the vibrational frequency of the amide group. The further observed shift of the amide carbonyl ν(C=O) stretching frequency to a lower frequency of 55 cm⁻¹ strongly supports the coordination of the oxygen atom of the amide carbonyl ν(C=O) with the metal ion (Fig. S18†) Furthermore, the FE-SEM image of the synthesised Hg/IL@MWCNTs revealed a node-like structure relative to IL@MWCNTs (Fig. 4F). The EDS and elemental mapping results provide insight into the composition and distribution of the C, O, N and Hg elements in the Hg/IL@MWCNTs composite material (Fig. S19†). Therefore, the above results indicate the successful modification of IL@MWCNTs with mercury.

Fig. 3 (A) Cyclic voltammetry behaviour of the modified IL@MWCNTs/GCE electrode towards various metal ions (5 mM) in PBS (pH = 7.4, 0.1 M). (B) Cyclic voltammetry response at IL@MWCNTs/GCE with the gradual addition of Hg²⁺ (0.40 to 120 μM) at a scan rate of 100 mV s⁻¹. (C) Linear regression graph of CV showing the linear change in the peak current of IL@MWCNT/GCE with an increasing concentration of Hg²⁺. (D) CV profiles of IL@MWCNTs/GCE at varying scan rates (20–220 mV s⁻¹) in phosphate-buffer solution consisting of 120 μM Hg²⁺. (E) Linear calibration plot of the peak current versus the square root of the scan rate, and (F) DPV response of IL@MWCNTs/GCE with the addition of Hg²⁺ ions (0.40 to 120 μM). The inset shows the linear regression plot of DPV.

Fig. 4 (A–D) High-resolution XPS spectra of C 1s, O 1s, N 1s and Hg 4f of the synthesised solid-state Hg/IL@MWCNTs material. (E) Raman spectra of MWCNTs, IL@MWCNTs and Hg/IL@MWCNTs, and (F) FE-SEM image of Hg/IL@MWCNTs.

Moreover, the interfacial charge transfer characteristics of the modified (GCE, IL@MWCNTs and Hg/IL@MWCNTs/GCE) electrodes were evaluated using electrochemical impedance spectroscopy (EIS) in a mixed solution of [Fe(CN)₆]^3−/4− and 0.1 M KCl (Fig. S20B†). The Hg/IL@MWCNTs modified GCE was observed to exhibit a small semi-circle relative to the bare GCE and IL@MWCNTs. The smaller semicircle results in a lower charge transfer resistance in Hg/IL@MWCNTs relative to bare GCE (R_ct = 1999.48 Ω) and IL@MWCNTs/GCE (R_ct = 1492.13 Ω), suggesting that the mercury-doped IL@MWCNTs is beneficial to improve the electrical conductivity relative to other electrodes. Moreover, in order to support the EIS data, the electrochemical behaviour of Hg/IL@MWCNTs/GCE was evaluated utilising cyclic voltammetry at scan rates varying from 20–200 mV s⁻¹ in a mixed 5 mM [Fe(CN)₆^3−/4−] solution, revealing the enhancement in the anodic and cathodic currents with an observable shift in the potential towards higher value (Fig. S20C and E†). Moreover, the linear calibration plot of the redox current vs. the square root of the scan rate reveals diffusion-controlled phenomena (Fig. S20F†). The electro-active surface areas (ESA) of the modified electrodes were calculated using the Randles–Sevcik equation, which is expressed as:³⁷

I_p = 2.69 × 10⁵n^3/2ACD^1/2v^1/2 (3)

where, I_pa = anodic peak current, D = diffusion coefficient of the ferro-ferricyanide solution (D = 6.70 × 10⁻⁶ cm² s⁻¹), A = electrochemical active surface area (ESA), n = no. of electrons transferred (n = 1), C = concentration of ferro-ferricyanide solution (C = 5 mM L⁻¹) and v = scan rate. Thus, the calculated surface area for Hg/IL@MWCNTs/GCE was found to be 0.200 cm², which is higher relative to those of the bare GCE (0.050 cm²) and IL@MWCNTs/GCE (0.178 cm²) electrodes (Fig. S20D†). Numerous sensing probes have been reported in the literature for the detection of heavy metal ions. However, they lack the capability to extract these ions from solution.^38,39 Thus, we endowed IL@MWCNTs with both adsorptive and separable properties to remove mercury ions from aqueous solution (Fig. S21A†). The maximum adsorption capacity of IL@MWCNTs for Hg²⁺ is 152.7 mg g⁻¹. The adsorption isotherm results were fitted using the Freundlich adsorption isotherm,⁴⁰ supporting the multilayer adsorption mechanism (Fig. S21B†). Moreover, a leaching study^41,42 was conducted to assess the stability of IL@MWCNTs after Hg²⁺ adsorption. A quantity of 20 mg of IL@MWCNTs adsorbed with Hg²⁺ was suspended in varying pH solutions for 12 h. The suspension was separated by filtration, and the leached amount of mercury in the supernatant was analysed by atomic adsorption spectrometry (AAS) (Fig. S22†).

Electrochemical detection of azinphos-methyl (AZM) using Hg/IL@MWCNTs/GCE

A perusal of the literature revealed that different kinds of metal complexes have been utilised for the recognition of pesticides/herbicides. Singh and co-workers have reported various metal complexes for the recognition of pesticides/herbicides.^43–47 The strong electrochemical characteristics of IL@MWCNTs/GCE towards Hg²⁺ metal ion inspired us to use the Hg/IL@MWCNTs material for the sensing of the azinphos-methyl pesticide. The bare glassy carbon electrode (GCE) was modified by drop casting an 8 μl suspension of Hg/IL@MWCNTs/GCE, followed by drying at room temperature overnight. The modified Hg/IL@MWCNTs electrode was activated in PBS (0.1 M, pH = 7.4) using CV at a scan rate of 50 mV s⁻¹ for 50 cycles before using for electrochemical applications.

The CV response of Hg/IL@MWCNTs/GCE was recorded by adding 150 μM of various pesticides/insecticides (such as chlorpyrifos, imidacloprid, isoproturon, phosmet, butachlor, glyphosate, azamethiphos, ethion, diethyl-cyano-phosphate and azinphos-methyl) (Fig. 5A). None of the pesticides/insecticides showed variation in the cathodic and anodic peak potentials, except for azinphos-methyl (AZM). The gradual addition of AZM exhibited a corresponding shift in the anodic potential from 0.386 V to 0.339 V, along with an enhancement in the peak current from 14.64 μA to 16.97 μA. Moreover, a prominent ratiometric response was observed with a decrease in the cathodic peak current and potential from −20.23 μA to −8.74 μA and −0.66 V to −0.37 V, respectively, with the appearance of distinct peaks at −1.23 V at −30.79 μA (Fig. 5B). The remarkable electrocatalytic characteristic of Hg/IL@MWCNTs/GCE can be attributed to several factors: (i) MWCNTs act as a strong backbone, providing better conductivity for fast electron transfer, (ii) incorporation of the Hg²⁺ metal ion helps the electron transfer, thus improving the electrochemical performance and also increasing the catalytic binding sites, and (iii) cation–π stacking interactions between the organic cation (IL) and MWCNTs expand the conductivity of the carbon nanotubes. Furthermore, AZM quantification using Hg/IL@MWCNTs was achieved by cyclic voltammetry with varying AZM concentrations from 0.20 μM to 180 μM (Fig. 5B). The results showed a ratiometric response of the modified Hg/IL@MWCNTs electrode towards AZM at a potential of −1.23 V, as the new cathodic peak current appears with the gradual increase in the AZM concentration. The calibration plot shows a linear relationship with the increase in the AZM concentration (Fig. 5C). The linear calibration values thus obtained can be written using the equation, I_p,c1 [μA] = 2.23[AZM]/μM − 1.32 with R² = 0.9780 and I_p,c2 [μA] = −4.40[AZM]/μM − 1.31 with R² = 0.9920. The limit of detection, as calculated by the 3σ method, is 1.10 μM. In addition to this, DPV and LSV studies were carried out for AZM quantification at Hg/IL@MWCNTs/GCE in phosphate-buffered solution. Both LSV and DPV studies of Hg/IL@MWCNT/GCE after the gradual addition of AZM with varying concentrations from 0.20 μM to 180 μM exhibit a ratiometric increase and decrement in the cathodic current intensity at −1.23 V and −0.66 V, respectively, supporting the CV results (Fig. 5D and F). The linear calibration plot of the LSV and DPV equation can be given as I_p,c1 [μA] = 4.161[AZM]/μM − 1.328 with R² = 0.95919, I_p,c2 [μA] at −0.006 V = −5.94[AZM]/μM − 1.27 with R² = 0.9791 (Fig. 5E and inset Fig. 5F).

Fig. 5 (A) Cyclic voltammetry response of Hg/IL@MWCNTs/GCE towards various insecticides/pesticides (150 μM), using PBS (pH = 7.4, 0.1 M). (B) Cyclic voltammetry response at Hg/IL@MWCNTs/GCE with the gradual addition of AZM (0.20 to 180 μM) at a scan rate of 100 mV s⁻¹ (inset is zoom view of AZM reduction with Hg/IL@MWCNTs/GCE). (C) Linear calibration plot of the cathodic and anodic peak current v/s AZM concentration at CV. (D–F) LSV and DPV studies at Hg/IL@MWCNTs/GCE with varying AZM concentrations (0.20 to 240 μM) at a scan rate of 100 mV s⁻¹ (inset is linear calibration plot of cathodic peak current v/s AZM concentration at DPV).

Scan rate, pH studies, stability and reproducibility

To explore the electrocatalytic behaviour of AZM at Hg/IL/MWCNTs, a cyclic voltammetry study was carried out with varying scan rates (20–220 mV s⁻¹) in 180 μM AZM solution consisting of 0.1 M phosphate-buffered solution. A linear increase in the cathodic current intensity versus the square root of the scan rate, along with the shift in the cathodic peak potential towards more negative values was observed (Fig. 6A). Meanwhile, the linear calibration equation of I_p,c [μA] = −1.104v^1/2 (V s⁻¹)^1/2 − 4.110 with R² = 0.9885 shows a diffusion-controlled phenomenon (inset Fig. 6A).⁴⁸ The effect of pH on the electroreduction of the AZM pesticide at Hg/IL@MWCNTs was investigated in the pH range of 2–12, depicting a slight change in the current intensity (Fig. S23†). Selectivity is an important factor to assess the effectiveness of the Hg/IL@MWCNTs composite material. With higher selectivity, the capability of the detection method against various interference agents will be greater. Thus, for real-time applications, we checked the selectivity of Hg/IL@MWCNTs with various interferents, such as metals (Na⁺, Ca²⁺, K⁺, Mg²⁺), bio-thiols (cysteine, glutathione), amino acids (tryptophan, iso-leucine, histidine), vitamins (vitamin B12, vitamin D, vitamin K and vitamin C), polysaccharides (glucose, pectin, fructose, starch and cellulose) and nitroaromatic compounds (4-aminophenol, 4-nitrobenzene, 4-nitroaniline, 4-nitrophenol and 2-aminophenol) (Fig. 6C and S24A–C†).⁴⁷ Thus, 180 μM of AZM was dissolved in a PBS buffer solution having 10-fold excess concentration. The DPV results confirm that Hg/IL@MWCNTs exhibit excellent anti-interference ability for the selective recognition of AZM in the presence of various kinds of interfering species. Moreover, the cyclic voltammogram was recorded five times in 180 μM AZM solution at a scan rate of 100 mV s⁻¹ to investigate the repeatability of the electrochemical reduction. The results showed good reproducibility of Hg/IL@MWCNTs with RSD 3.14% (Fig. 6D). Furthermore, stability test studies were carried out by recording the CV profile within a number of days (Fig. S24D†). No variation in the peak current intensity was observed after 21 days at room temperature. The current remained at 93.7% of the original value after 21 days, displaying the excellent stability of the composite material.

Fig. 6 (A) CV plot after the addition of 180 μM of AZM at Hg/IL@MWCNTs with varying scan rates (20–220 mV s⁻¹). The inset plot shows the calibration plot of current with the square root of the scan rate in a phosphate-buffered (0.1 M, pH = 7.4) solution. (B) Linear calibration plot of anodic peak showing the linear change in peak current of Hg/IL@MWCNT/GCE with varying pH range (2 to 12) in 0.1 M PBS at 100 mV s⁻¹ scan rate. (C) Relative error percentage bar graph for the interference study (inset is zoomed view of relative % error for various interfering analytes), and (D) stability studies containing 180 μM AZM at Hg/IL@MWCNTs/GCE in phosphate buffer solution (0.1 M, pH = 7.4; the inset is bar graph showing the stability response for AZM detection with number of days).

Plausible mechanism of AZM binding

The plausible mechanism of AZM binding with Hg/IL@MWCNTs was supported using various techniques, such as XPS, LSV, CV, and DPV, showing the probable interaction mechanism. Step (i): after the addition of the Hg²⁺ solution into the IL@MWCNTs mixture, the successful adsorption of Hg²⁺ was observed on the surface of IL@MWCNTsvia the acylhydrazone moiety of IL, leading to the formation of a strong redox couple involving the organic cation (Fig. 7A). The XPS spectrum and CV results provide evidence of the co-existence of both Hg²⁺ and Hg⁰ in the system (Fig. 4D and 3A). The synergic effect produced by the transfer of an extra electron in the solid state is reflected in the well-defined anodic and cathodic peaks observed in the CV. Step (ii): cyclic voltammetry results revealed that after the addition of AZM, a decrease in the cathodic peak current of Hg⁰ occurred with the appearance of a new cathodic peak for AZM, corresponding to –N=N– bond reduction.^49,50

Fig. 7 (A) Scheme for the plausible binding mechanism of AZM with Hg/IL@MWCNTs. XPS spectrum after adding AZM into Hg/IL@MWCNTs: (B) full region XPS survey spectrum, (C) high-resolution XPS spectrum of Hg 4f, and (D) FE-SEM image after AZM addition on Hg/IL@MWCNTs.

The XPS spectrum was also recorded after adding AZM (Fig. 7B). The full range spectrum reveals the presence of Hg 4f, S 3d, C 1s, N 1s, O 1s and P 3d elements. The high-resolution Hg 4f spectrum (Fig. 7C) after AZM addition reveals the specific binding energies of 100.15 eV (4f_7/2) and 103.19 eV (4f_5/2) corresponding to Hg in the +2 oxidation state.²⁷ Moreover, after the addition of the AZM pesticide, the morphological variation clearly shows the dense layer deposition of the pesticide on the surface of Hg/IL@MWCNTs (Fig. 7D).

Degradation of AZM and determination in real water samples

The mass spectrum of AZM after treatment with Hg/IL@MWCNTs reveals the presence of various peaks, i.e., 3-methyl-2,3-dihydrobenzo[d][1,2,3]triazin-4(1H)-one, which corresponds to the m/z peak at 162.93, and another peak for O,O-dimethyl S-hydrogen phosphorodithioate with the m/z value at 158.91. Moreover, additional fragments of anthranilic acid and 4-oxo-1,2-dihydrobenzo[d][1,2,3]triazine-3(4H)-carbaldehyde corresponding to m/z values at 136.96 and 178.89, respectively, were observed as degraded products⁵⁰ of the AZM pesticide (Fig. S25†). Apart from this, ³¹P NMR titration was carried out to investigate the mechanism of the hydrolysis reaction of the azinphos-methyl pesticide in the presence of the Hg/IL@MWCNTs catalyst. The signal at δ 95.60 corresponding to the azinphos-methyl pesticide is shifted upfield to δ 26.95 ppm using Hg/IL@MWCNTs as the catalyst, which corresponds to the hydrolysed product,^45–47O,O-dimethyl S-hydrogen phosphorodithioate (Fig. S26†). In order to validate and verify the practical application, the developed composite (Hg/IL@MWCNTs/GCE) was used for AZM recognition in a tap water sample collected from the institute laboratory, river water collected from the Sutlej River (Rupnagar City, Punjab), and agriculture run-off collected from nearby fields in Rupnagar. Firstly, a phosphate-buffered solution (pH = 7.4) was prepared using the collected real water samples, and then the prepared PBS was spiked with varying AZM concentrations (10, 20 and 30 μM). Subsequently, the CV, LSV and DPV responses were recorded. It can be observed from Table S1† that the labelled samples exhibit good percentage recovery (above 90%). The results revealed that the prepared solid composite exhibited good accuracy, possessing broad application for monitoring AZM in real water samples.

Development of modified PA/IL@MWCNTs nanocomposite membranes for the removal of Hg²⁺ and AZM from contaminated water

Polymeric composite membranes have indeed been reported as effective materials for the removal of organic pollutants from wastewater, particularly in ultra-filtration (UF) and nanofiltration (NF) processes. Of the various available polymers, polyethersulfone (PES) and polyamide (PA) emerge as leading polymer choices for membrane formation due to their high permeability, thermal and chemical stability, and good film forming characteristics.^51,52 Researchers have explored the incorporation of functionalised carbon nanotubes (CNTs) into polymer matrices to enhance membrane hydrophilicity and mitigate fouling in water treatment applications.⁵³ Thus, inspired by these characteristics, the IL@MWCNTs was used to synthesise a membrane for subsequent removal of Hg²⁺ ion and AZM from wastewater by regulating the interfacial polymerisation process. Prior to the interfacial polymerisation reaction, a layer consisting of the IL@MWCNTs was first positioned on a PES support via vacuum-assisted filtration to avoid leaching of the metal ion and pressure on the membrane.^54,55 The as-prepared IL@MWCNTs suspension was filtered on a PES substrate with varying compositions of 2.5 wt%, 5 wt% and 7.5 wt%. The contact angle analysis was conducted to examine the impact of IL@MWCNTs incorporation on the wettability of the prepared polyamide membranes (Fig. S27†). It can be seen that the contact angle decreased upon the doping of IL@MWCNTs, improving the hydrophilicity of membranes relative to the pristine PA. Thus, the nanocomposite membranes benefited from the hydrophilic groups of IL@MWCNTs, which enhanced the water affinity with the membrane surface. The hydrophilicity of the nanocomposite membranes is in accordance with those of the –OH, –NH functional groups present on the non-covalently modified MWCNTs, which improve the water cluster formations and hydrophobicity of the PES/PA membrane.⁵⁶ Moreover, nitrogen adsorption/desorption analysis was carried out using a Quantachrome Instrument in order to investigate the internal structure of the composite membranes. The type IV isotherms observed for all samples indicate the presence of mesopores. Pure PA, PA/IL@MWCNTs-2.5, PA/IL@MWCNTs-5 and PA/IL@MWCNTs-7.5 exhibit specific surface areas of about 6.98, 61.31, 74.18 and 87.68 m² g⁻¹, respectively (Fig. 8A). It was found that the modified polyamide membrane PA/IL@MWCNTs exhibit enhanced BET surface area relative to pristine PA, which could be due to the presence of IL@MWCNTs that led to the creation of meso (medium-sized pores) and macro pores (large-sized pores) within PA/IL@MWCNTs.⁵⁵ Apart from this, the Barrett–Joyner–Halenda (BJH) plot was used to gain insight into the porous structure (pore size and pore volume). The pore volumes of the differently modified membranes ranged up to 0.294 cm³ g⁻¹, which decreased with the increase in concentration of IL@MWCNTs (Table S2†). A very low concentration of Hg²⁺ ions (20 mg L⁻¹ aqueous solution of Hg (NO₃)₂) was employed to investigate the PA/IL@MWCNTs ability for Hg²⁺ ion filtration. All the Hg²⁺ metal ion removal experiments were performed at room temperature on a membrane of 3 × 3 cm². The concentration of Hg²⁺ ion was estimated using atomic absorption spectroscopy (AAS) employed to determine the mercury ion rejection.⁵³

R(%) = (1 − C_p/C_f) × 100 (4)

where C_p and C_f are mercury ion concentration (mg L⁻¹) in permeate and feed, respectively. The PA/IL@MWCNTs-7.5 membrane exhibited the highest Hg²⁺ ion removal (95%) (Fig. 8B). Moreover, it has been observed that PA, PA@MWCNTs, and PA@IL exhibit 20%, 75% and 55% removal efficiency, respectively. Furthermore, the membrane polluted with Hg²⁺ ions was utilised for the removal and degradation of the AZM pesticide. Before that, a stability analysis of the PA/IL@MWCNTs-7.5 + Hg²⁺ membrane was conducted, and the effect of pH on the adsorption of Hg²⁺ ions was analysed in the first step. Fig. S28† shows the sudden increase in the adsorption of Hg²⁺ in the 2–5 pH range. After that, the adsorption remains constant at pH 5–11, reaching a maximum value.^57,58 The adsorption is dependent on the electrostatic attraction, and also on the metal chelation by carboxyl and hydroxyl groups present on nanocomposite membrane. Less adsorption of the metal ion is observed at pH < 4 due to competition between H⁺ and Hg²⁺ ions, and the chemical equilibrium strongly depends on the H⁺ concentration (pH). Furthermore, the PA/IL@MWCNTs-7.5 + Hg²⁺ membrane was treated with 100 ml of AZM pesticide solution having a concentration of 20 mg L⁻¹. The degradation efficiency (Table S3†) and rejection percentage were then calculated using eqn (4), which was found to be 90% relative to other composite membranes with an adsorption capacity of 130 mg g⁻¹. The UV-visible spectrophotometer was used to obtain the filtered AZM concentration at 225 nm (Fig. 8C and D). As evident in the FE-SEM images, conventional interfacial polymerisation was carried out on top of an unmodified PES support, leading to the formation of thin-film composite (TFC) membranes. The FE-SEM image shows the surface morphology of the membrane, which is typical for an interfacial polyamide polymerisation comprising multi-layered ridge-and valley.^59,60 The membrane morphology changed after the addition of MWCNTs (Fig. S29A and B†). Furthermore, after the filtration of the Hg²⁺ and AZM contaminated water from the composite PA membrane, the deposition of particles on the surface of the membrane was observed (Fig. S29C†). It should be noted that no Hg²⁺ ion is detected in the solution after the filtration or adsorption of AZM under any of the conditions. This indicates that no leaching of Hg²⁺ ion occurs after filtration of AZM (Fig. S30†). Thus, organic cation-modified MWCNTs is an effective and efficient material for the detection, degradation and removal of Hg²⁺ and AZM relative to literature reports (Table S4†).

Fig. 8 (A) N₂ adsorption–desorption isotherm of differently modified membranes. (B) Removal efficiency of differently modified composite membranes for mercury. (C) UV-visible absorption spectrum showing the filtration of AZM, and (D) bar graph showing the removal percentage of AZM.

Conclusion

Herein, an IL@MWCNTs conductive hybrid material was designed, which involves non-covalent cation–π induced functionalization of multiwalled carbon nanotubes (MWCNTs) with an organic cation (IL). The material was then employed for metal binding studies, showing a selective response towards mercury. Later, the solid-state Hg/IL@MWCNT material was characterized using FE-SEM, EDAX, TGA-DSC, XPS, and FTIR techniques, which provided comprehensive insights into its structural, morphological, compositional, thermal and chemical properties. Benefitting from this, the Hg/IL@MWCNTs/GCE exhibited electrocatalytic behaviour towards azinphos-methyl (AZM) with a low detection limit of 1.10 μM and wide-concentration linear range (0.20–180 μM). The degradation of AZM was supported by ³¹P NMR titration and mass spectrometry, confirming the formation of non-toxic products. Moreover, the functionalised IL@MWCNTs material, along with p-phenylene diamine (PPD), was added into the aqueous phase to fabricate an ultrathin polyamide layer on the PES support membrane by interfacial polymerisation. The developed nanocomposite membrane revealed the removal of the Hg²⁺ metal ion and azinphos-methyl pesticide from contaminated water with a removal efficiency of 95% and 90%, respectively.

Author contributions

Kamalpreet Kaur: investigation, data curation, writing – original draft. Gagandeep Singh: methodology, validation, writing – review & editing. Navneet Kaur: conceptualization, supervision. Narinder Singh: conceptualization, supervision, resources.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Kamalpreet Kaur is thankful to U.G.C. (New Delhi) for a Senior Research Fellowship [17/(CSIR-UGC NET June 2019)]. The authors also acknowledge the financial support in the form of a sponsored project from the Science and Engineering Research Board (SPG/2021/002995-G).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterisation of materials (¹H NMR, ¹³C NMR, HRMS, UV-visible absorption bar diagrams, FTIR). See DOI: https://doi.org/10.1039/d4ew00114a

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