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. 2026 Sep 30;16(45):58672–58683. doi: 10.1039/d6ra05132d

Detection of Cd2+ and Pb2+ in egg yolk and maize samples by a novel SDA/MWCNTs-based electrochemical sensor

Jayagopi Gayathri a,✉, Sivakumar Sivalingam a,✉
PMCID: PMC13624723  PMID: 42819707

Abstract

A novel Schiff base-functionalized, multiwalled carbon nanotube-modified, paraffin-impregnated graphite electrode (SDA/MWCNTs/PGE) was developed for simultaneous electrochemical detection of Cd2+ and Pb2+ in food analyses. MWCNTs improved the electroactive surface area and electron-transfer kinetics. Hydroxyl and azomethine donor groups in the SDA ligand provided efficient coordination sites for metal-ion binding. FT-IR, H1 NMR, and FESEM techniques were used to characterise the fabricated sensor, indicating successful synthesis and surface modification. With a limit of detection of 0.33 ppb for Cd2+ and 0.07 ppb for Pb2+, the sensor exhibited a linear range of 1.3–101 ppb under optimised square-wave anodic stripping voltammetry (SWASV) conditions. With relative standard deviation values <3%, the sensor exhibited good repeatability, stability, and excellent selectivity for common interfering metal ions. By analysing egg yolk and maize samples, the practical application of the suggested sensor was confirmed. Results showed good agreement with data from atomic absorption spectroscopy and satisfactory recoveries. The developed SDA/MWCNTs/PGE sensor offers a straightforward, affordable, and dependable platform for monitoring traces of heavy metals in food matrices.

A novel SDA/MWCNTs/PGE sensor enables simultaneous detection of Cd2+ and Pb2+ in egg yolk and maize samples.graphic file with name d6ra05132d-ga.webp

1. Introduction

Cadmium (Cd2+) and lead (Pb2+) are two of the most hazardous heavy metal ions because of their persistence, non-biodegradability, and tendency to bioaccumulate in organisms.1–4 Numerous industrial and agricultural processes, such as mining, electroplating, battery production, pigment manufacturing, fertiliser application, pesticide use, and wastewater discharge, constantly release these metals into the environment.5–7 Cd2+ and Pb2+, when released into soil and aquatic ecosystems, can enter the food chain and accumulate in edible crops and animal-derived products, posing major health risks to humans.

Lead toxicity can adversely impact neurological development, haematological processes, reproductive health, and the central nervous system.8–10 Cadmium exposure has been linked to kidney failure, skeletal damage, pulmonary diseases, and carcinogenic effects.11–13 Long-term exposure to these hazardous metals can have serious toxicological effects even at negligible levels.14,15 Therefore, it is crucial for environmental protection and food safety surveillance to develop sensitive, selective, and accurate analytical methods for monitoring Cd2+ and Pb2+.16,17

Egg yolk and maize are among the food commodities most vulnerable to heavy metal contamination due to contaminated soil, irrigation water, feed ingredients, and environmental exposure.18,19 Egg yolks contain proteins and lipids that can accumulate dangerous metal ions.22–24 Maize readily collects toxic metals from contaminated agricultural soils.20,21 The accumulation of Cd2+ and Pb2+ in the human body due to ongoing ingestion of contaminated food products can result in kidney damage, neurological disorders, bone illnesses, cardiovascular problems, and developmental abnormalities.25 As a result, determining Cd2+ and Pb2+ levels accurately in food matrices continues to be a significant analytical difficulty.

Heavy metal ions have been determined using traditional analytical methods, such as atomic absorption spectroscopy (AAS),26 inductively coupled plasma mass spectrometry (ICP-MS),27 atomic emission spectroscopy (AES),28 and colorimetric methods.29 These well-established methods provide high analytical accuracy, sensitivity, and selectivity, and can enable multi-element determination. However, their routine application may require sophisticated instrumentation, extensive preparation of samples, and laboratory-based operation. Electrochemical sensing methods have, therefore, attracted considerable interest owing to their relatively simple operation, low cost, rapid response, and suitability for trace-level metal-ion detection.30–32 However, the electrode composition, surface modification, accumulation circumstances, and sample matrix have significant impacts on the analytical performance of electrochemical sensors. Some platforms may also involve relatively complex fabrication procedures or specialized materials. The development of simple, repeatable, sensitive, and selective electrochemical systems for heavy-metal determination is driven by these factors.

Square-wave anodic stripping voltammetry (SWASV) is widely employed for trace metal determination because its pre-concentration step can provide high sensitivity and a low limit of detection (LOD).33–35 The analytical performance of electrochemical sensors can be improved through surface modification with functional ligands and nanostructured materials.36,37 Multiwalled carbon nanotubes (MWCNTs) have been widely used as electrode-modifying materials because their large specific surface area, conductive network, adsorption capability, and chemical stability can facilitate analyte accumulation and electron transfer.38–42 However, MWCNTs alone may not provide sufficient chemical selectivity toward specific metal ions. Functionalization with suitable chelating ligands can, therefore, introduce additional binding sites and enhance the interaction between the electrode surface and target analytes. Recent studies have reported a variety of modified electrodes for the simultaneous determination of Cd2+ and Pb2+, including carbon-based nanocomposites, MXene-based materials, metal/metal-oxide nanostructures, polymers, and other functional materials.17,30,37,44–48 These studies have demonstrated that the electrode surface can improve the sensitivity and selectivity of heavy-metal detection. However, analytical performance varies depending on the electrode, modification strategy, accumulation conditions, and sample matrix. Some approaches also involve relatively complex composite preparation or specialized materials. Studies from our research team have demonstrated the application of MWCNT-based Schiff-base-modified electrodes for Cd2+ and Pb2+ detection in different matrices.49,50

Although our studies demonstrated the application of MWCNT-based Schiff-base-modified electrodes for Cd2+/Pb2+ and Hg2+ detection,42,49,50 the application of the present SDA/MWCNT-modified paraffin-impregnated graphite electrode (SDA/MWCNTs/PGE) for simultaneous Cd2+ and Pb2+ determination in egg yolk and maize has not been investigated previously. The SDA ligand contains hydroxyl and azomethine functional groups that can provide binding sites for Cd2+ and Pb2+.

In the present study, a combination of SDA and MWCNTs was, therefore, employed to construct an SDA/MWCNTs/PGE sensor for simultaneous determination of Cd2+ and Pb2+ using SWASV. The analytical performance of the proposed sensor was evaluated in terms of sensitivity, LOD, selectivity, repeatability, stability, and recovery, and its applicability was demonstrated using egg yolk and maize samples.

2. Experimental

2.1. Chemicals and equipment

Merck (India) supplied acetic acid, salicylaldehyde, 1,2-diaminobenzene, lead acetate, cadmium acetate, sodium acetate, and MWCNTs (particle size: 110 nm; purity >90%). All chemicals and reagents were of analytical reagent (AR) grade and used without further purification. The manufacturer did not provide comprehensive details about the MWCNTs, such as the precise number of walls, nanotube length, or supplier grade. Sodium acetate, sodium hydroxide, and acetic acid were used to create a 0.1 M acetate buffer solution (ABS), and the pH was adjusted between 4.0 and 6.5 as required. Throughout the investigation, solutions were prepared using double-distilled (DD) water.

A field-emission scanning electron microscope (Sigma 360, Carl Zeiss, Germany) was used to characterise the surface morphology of the electrodes. SWASV measurements were performed using a CHI660B electrochemical workstation (CH Instruments, USA) with a standard three-electrode cell configuration—a platinum wire counter electrode, a saturated calomel electrode (SCE, saturated KCl) as the reference electrode, and bare or modified electrodes as the working electrode—to conduct electrochemical and voltammetric measurements. Each experiment was carried out at room temperature.

2.2. Composition of SDA

Salicylaldehyde and 1,2-diaminobenzene underwent a condensation reaction to produce the Schiff base ligand N,N′-bis(salicylaldehyde)-1,2-diaminobenzene (SDA) as previously reported.42 In short, 1,2-diaminobenzene (5 mmol) and salicylaldehyde (13.10 mmol) were dissolved in methanol and continuously agitated for 30 min at room temperature. A yellow precipitate that matched the SDA ligand was produced by the reaction. To create a pure crystalline product, the final product was filtered, repeatedly washed with methanol, dried at room temperature, and recrystallized from methanol. The SDA yield was between 70% and 75%.

2.3. Formation of an electrode modified by SDA/MWCNTs

Paraffin wax-impregnated graphite electrodes (PGE) were made using a previously described method.42 Briefly, 1 mg of MWCNTs were ultrasonically dispersed in 1 mL of ethanol for 45 min to form SDA/MWCNTs. The polished PGE surface was then drop-cast with 10 µL of the MWCNT suspension and left to dry. The SDA/MWCNTs-modified electrode was then created by drop-casting 10 µL of a 1 mM SDA solution made in acetonitrile onto the MWCNTs/PGE surface and allowing it to dry naturally.

2.4. Preparation of egg yolk and maize samples

Cd2+ and Pb2+ levels were measured using samples of egg yolk (sample A) and maize (sample B). Before digestion, the materials were cleaned thoroughly, dried, and homogenised.43 Precisely 0.5 g of each homogenised sample was transferred to a digestion vessel, followed by the addition of 5 mL of concentrated HNO3. The mixture was left to predigest for 30 min at room temperature and subsequently heated at 120 °C for 2 h to obtain a clear digest. After cooling, the digested solution was diluted with DD water to a final volume of 25 mL. Subsequently, Cd2+ and Pb2+ standard solutions were added to the digested and diluted sample solutions to obtain final spiked concentrations of 20 and 35 ppb for recovery tests. Thus, Cd2+ and Pb2+ were spiked after completion of the digestion and dilution steps, and immediately before SWASV. The prepared samples were evaluated using SWASV for Cd2+ and Pb2+, and the outcomes were compared with AAS results.

2.5. Cd2+ and Pb2+ stripping voltammetric determination

Cd2+ and Pb2+ levels were measured simultaneously using SWASV. SWASV measurements were carried out using a CHI660B electrochemical workstation (CH instruments) equipped with a three-electrode system. Under constant stirring, the SDA/MWCNTs-modified electrode was submerged in 0.1 M ABS (pH 5.5) containing the target metal ions. Cd2+ and Pb2+ were accumulated onto the electrode surface at the optimised deposition potential for a predefined deposition duration during the pre-concentration stage. The collected metal ions were then electrochemically reduced to Cd (0) and Pb (0), their metallic forms.

The deposited metals were re-oxidized to Cd2+ and Pb2+ during the anodic stripping scan, resulting in distinct stripping peaks for each analyte. MWCNTs exhibit high electrical conductivity and a large specific surface area, which can facilitate efficient electron transport. If incorporated into an electrode matrix, their interconnected conductive network can enhance charge-transfer processes and improve the current response of the modified electrode. The high electrical conductivity and large specific surface area of MWCNTs, along with the strong complexation ability of the SDA Schiff base ligand towards Cd2+ and Pb2+, were responsible for the improved electrochemical performance of the SDA/MWCNTs-modified electrode shown in Scheme 1.

Scheme 1. Proposed electrochemical sensing of Cd2+ and Pb2+ using the SDA/MWCNTs/PGE electrode.

Scheme 1

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3. Results and discussion

3.1. FT-IR spectroscopy of the SDA ligand

FT-IR spectroscopy confirmed the successful synthesis of N,N′-bis(salicylaldehyde)-1,2-diaminobenzene (SDA). The stretching vibration of the phenolic hydroxyl (–OH) group was responsible for the broad absorption band seen in the spectra at 3414 cm−1 in Fig. S1. The production of the Schiff-base framework through the condensation reaction between salicylaldehyde and 1,2-diaminobenzene was confirmed by the distinctive absorption peak at 1614 cm−1, which corresponds to the azomethine (C Created by potrace 1.16, written by Peter Selinger 2001-2019 N) stretching vibration. Furthermore, the C–O stretching vibration of the phenolic group was identified as the source of the absorption band at 1290 cm−1. The synthesis of the SDA ligand was demonstrated by the presence of azomethine and hydroxyl functionalities, which also offer possible coordination sites for heavy-metal ion binding. For SDA-based Schiff-base ligands, the observed spectrum characteristics corresponded well with previously published data.42

3.2. 1H NMR spectroscopy of the SDA ligand

Using 1H NMR spectroscopy, the molecular structure of the synthetic SDA ligand was further confirmed. The presence of intramolecular hydrogen bonding within the ligand structure was indicated by a unique resonance signal at 13.07 ppm that was attributed to the phenolic hydroxyl proton (Fig. S2). The benzene rings of the ligand were represented by aromatic proton signals that emerged in the range 6.89–7.38 ppm. Additionally, the azomethine (CH Created by potrace 1.16, written by Peter Selinger 2001-2019 N) proton was identified as the source of a sharp singlet at 8.62 ppm, providing compelling evidence for Schiff-base formation. The effective synthesis of the target ligand was confirmed by the obtained spectral measurements, which were consistent with values reported in the literature (ref. 42) and the expected molecular structure of SDA.

3.3. Surface morphology of metal-ion-loaded electrodes

Field-emission scanning electron microscopy (FESEM) was used to examine the surface morphology of the modified electrodes following the accumulation of Cd2+ and Pb2+ ions. SDA/MWCNTs showed a heterogeneous flake-like morphology with a rough surface texture (Fig. 1B), whereas the MWCNTs-modified electrode showed an agglomerated platelet-like structure (Fig. 1A). Functionalization of the MWCNT surface was shown by a discernible morphological change that occurs after SDA-immobilization.

Fig. 1. FESEM and EDS elemental mapping of the electrode surfaces after Cd2+–Pb2+ accumulation: (A) Cd2+–Pb2+-loaded MWCNTs electrode. (B) Cd2+–Pb2+-loaded SDA/MWCNTs-modified electrode. (C) EDS spectrum of the Cd2+–Pb2+-loaded SDA/MWCNTs electrode. (D–H) Elemental maps showing the distribution of C, O, Pb, and Cd on the Cd2+–Pb2+-loaded SDA/MWCNTs electrode.

Fig. 1

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We predicted that the rough and irregular structure of the SDA/MWCNTs composite would offer more accessible adsorption sites for metal-ion accumulation as well as a greater electroactive surface area. During the pre-concentration stage, the hydroxyl and azomethine groups of the SDA ligand operate as electron-donating coordination centres, enabling significant interactions with Cd2+ and Pb2+ ions (Fig. 1C and D). EDS elemental mapping further confirmed the presence and distribution of C, O, Pb, and Cd on the Cd2+–Pb2+-loaded SDA/MWCNTs electrode (Fig. 1D–F). The detection of Pb and Cd after accumulation supported the involvement of the functionalized electrode surface in metal-ion adsorption. These morphological and elemental results suggested that SDA functionalization contributed to enhanced metal-ion accumulation and, consequently, improved electrochemical sensing performance.

3.4. Electrochemical evaluation of SDA/MWCNTs

The reversible [[Fe(CN)6]3−/4−] redox probe was used in cyclic voltammetry (CV) and EIS to examine the electrochemical properties of the bare PGE, MWCNTs/PGE, and SDA/MWCNTs/PGE electrodes reported by us.42 The correlated voltammograms are shown in Fig. S3A. CV measurements were carried out in 1 mM ([Fe(CN)6]3−/4−) containing 0.1 M KCl at a scan rate of 50 mV s−1. The redox response of the bare PGE was broad and weak, with a peak-to-peak separation ((ΔEp)) of 267 mV. The anodic peak current (Ipa) for PGE increased from ∼40 µA to 75 µA after modification with MWCNTs, and the ΔEp dropped to 220 mV. The surface area and high electrical conductivity of MWCNTs promote electron transfer and offer enhances the active sites.

The SDA/MWCNTs/PGE electrode showed a more noticeable improvement. Alongside a further drop in (ΔEp) to 180 mV, the anodic peak current increased to ∼205 µA. Better electron-transfer kinetics at the SDA/MWCNTs-modified electrode were shown by the sharper and more intense redox peaks. The combined presence of SDA and MWCNTs created a more advantageous electrochemical interface, as shown by the decreased (ΔEp) and significantly increased peak current when compared with those of PGE and MWCNTs/PGE. The SDA functional groups offered more interaction sites at the modified electrode interface. The MWCNTs enhanced electron transfer and expanded the accessible electroactive surface area. As a result, with respect to the ([Fe(CN)6]3−/4−) redox probe, the SDA/MWCNTs/PGE electrode performed better electrochemically.

The electrochemical properties and charge-transfer behaviour of PGE, MWCNTs/PGE, and SDA/MWCNTs/PGE electrodes have been examined previously using electrochemical impedance spectroscopy (EIS) with the [Fe(CN)6]3−/4− redox probe (Fig. S3B). The findings showed that adding SDA/MWCNTs significantly reduced the charge-transfer resistance when compared with bare PGE and MWCNTs/PGE, suggesting the improved electron-transfer kinetics of the modified electrode.42

Thus, the enhanced interfacial electron-transfer properties of the SDA/MWCNTs-modified electrode could be partially responsible for the better electrochemical response seen in the current stripping-voltammetric tests for Cd2+ and Pb2+.

3.5. Electrochemical mechanism of the sensing of Cd2+ and Pb2+

SWASV was used to assess the electrochemical performance of bare PGE, MWCNTs/PGE, and SDA/MWCNTs/PGE electrodes for the simultaneous measurement of Cd2+ and Pb2+ (Fig. 2). Different stripping peaks corresponding to Cd2+ and Pb2+ oxidation were seen under ideal experimental conditions.

Fig. 2. Comparison of the stripping responses for 27.6 ppb Cd2+ and Pb2+ at PGE, MWCNTs/PGE, and SDA/MWCNTs/PGE electrodes in 0.1 M ABS (pH 5.5). With a pre-concentration time of 240 s, deposition potential of −1.2 V, amplitude of 25 mV, and scan rate of 50 mV s−1, the SDA/MWCNTs/PGE electrode demonstrated improved stripping currents toward both metal ions.

Fig. 2

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The advantageous effect of SDA functionalization was demonstrated by SDA/MWCNTs/PGE, which showed much higher stripping peak currents than bare PGE and MWCNTs/PGE electrodes. The synergistic effect of MWCNTs and the SDA ligand was responsible for this improvement. Azomethine nitrogen and phenolic oxygen donor atoms in the SDA ligand efficiently coordinated Cd2+ and Pb2+ during the pre-concentration process, raising the local concentration of analytes at the electrode surface. In addition, MWCNTs offered a highly conductive network that enhanced charge-transport kinetics and increased electron transfer.

Higher analytical sensitivity and better stripping signals were produced by the combination of selective metal-ion complexation and improved electron-transfer efficiency. These results verified that SDA functionalization offered an efficient sensing interface for trace-level detection of Cd2+ and Pb2+ and greatly enhanced the electrochemical performance of the MWCNT-modified electrode.

3.6. Cd2+ and Pb2+ measured with SWASV

SWASV was used to simultaneously measure Cd2+ and Pb2+ under optimal experimental circumstances (Fig. 2). In order to pre-concentrate 27.6 ppb of Cd2+ and Pb2+ onto the electrode surface, the electrode was immersed in a 0.1 M ABS (pH 5.5) and continuously agitated for 240 s. After the modified electrode was removed and placed in a fresh background electrolyte, a deposition potential of −1.2 V was applied for 90 s. Stripping voltammograms were then captured in the potential range of −1.2 to 0.0 V at a frequency of 40 Hz, amplitude of 25 mV, and potential increment of 4 mV. The SDA/MWCNTs/PGE electrodes demonstrated much greater stripping peak currents for Cd2+ and Pb2+ as compared with MWCNTs/PGE and bare PGE electrodes. The increased electrochemical response could have been caused by the SDA ligand on the electrode surface, which effectively interacted with Cd2+ and Pb2+ during the pre-concentration stage. Additionally, the effective surface area and electron transfer rate of the electrode were enhanced by the addition of MWCNTs. These combined effects greatly improved the sensitivity and electroanalytical performance of the modified electrode towards the detection of Cd2+ and Pb2+.

3.7. Enhancement of experimental conditions

3.7.1. Impact of the supporting electrolyte

The supporting electrolyte affects ionic conductivity, metal-ion transport, and the electrode/electrolyte interface environment, its characteristics are critical to the pre-concentration and stripping processes. Thus, SWASV was used to examine the impact of several 0.1 M supporting electrolytes, such as phosphate buffer solution (PBS), sulfuric acid (H2SO4), potassium nitrate (KNO3), and ABS, on the simultaneous detection of Cd2+ and Pb2+. Fig. 3A displays the comparable stripping voltammograms acquired in the presence of 41.3 ppb Cd2+ and Pb2+.

Fig. 3. How the stripping response of 41.3 ppb Cd2+ and Pb2+ at the SDA/MWCNTs/PGE electrode is affected by experimental circumstances. The anodic stripping peak currents of Cd2+ and Pb2+ are affected by (A) various supporting electrolytes (0.1 M PBS, KNO3, H2SO4, and ABS), (B) pH, and (C) pre-concentration time.

Fig. 3

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The greatest stripping peak currents for Cd2+ and Pb2+ were produced by 0.1 M ABS among the electrolytes tested, indicating better analytical performance. The advantageous ionic strength and buffering ability of acetate ions (which enable effective mass transport and encourage the development of established stable bonds between the SDA ligand and target metal ions) were responsible for the increased reaction. Additionally, acetate buffer offers an appropriate electrochemical environment that improves electron-transfer kinetics at the modified electrode surface and reduces competing side reactions. PBS, H2SO4, and KNO3, on the other hand, produced relatively smaller peak currents, perhaps as a result of less ideal interfacial conditions for metal-ion build-up. As a result, 0.1 M ABS was chosen as the best supporting electrolyte and used in all of the studies that followed.

3.7.2. Effect of pH

The protonation state of the functional groups in the SDA ligand and the speciation of metal ions are strongly influenced by the pH of the solution. Therefore, the effect of pH on the electrochemical responses of Cd2+ and Pb2+ was investigated using 0.1 M acetate buffer containing 41.3 ppb of each analyte over a pH range of 4.5–6.0 (Fig. 3B).

As the pH increased, the stripping peak currents of Cd2+ and Pb2+ increased, reaching their maximum values at pH 5.5. Lower accumulation efficiencies were caused by excessive protonation of the azomethine nitrogen and phenolic oxygen donor sites at lower pH levels, which decreased the availability of coordination centres for metal-ion binding. As the pH increased, deprotonation of these functional groups improved the preconcentration and stripping responses by promoting complex formation between the SDA ligand and the target metal ions. Peak current did, however, decrease beyond pH 5.5, which could be explained by the start of metal-ion hydrolysis and decreased accessibility of electroactive species. As a result, the ideal pH for the ensuing electrochemical measurements was determined to be 5.5.

3.7.3. Effect of the pre-concentration time

Although the pre-concentration time directly affects the amount of analyte accumulated on the electrode surface before electrochemical stripping, it is a crucial factor in stripping voltammetry. In a 0.1 M acetate buffer solution with 41.3 ppb Cd2+ and Pb2+, the impact of deposition duration was examined over a range of 60–300 s (Fig. 3C).

As the pre-concentration time increased from 60 to 240 s, the stripping peak currents gradually increased, indicating enhanced accumulation of Cd2+ and Pb2+ at the SDA/MWCNTs/PGE interface. This increase indicated the affinity of the functional groups of the Schiff base for metal ions and showed effective accumulation properties of the modified electrode. In contrast to 240 s, a noticeable decrease in peak current was seen at 300 s. Further deposition might become less effective after a significant portion of the available sites are occupied due to adsorption–desorption equilibrium and limits on the movement of metal ions from the bulk solution to the electrode surface. Furthermore, competition for the available binding sites may result from the simultaneous accumulation of Cd2+ and Pb2+, which would prevent additional accumulation of the individual metal ions. Stripping-voltammetric measurements of metal ions have shown similar decrease behaviour at extended deposition durations.42 The ideal pre-concentration time for additional analytical studies was determined to be 240 s based on these results.

3.8. Analytical performance: LOD, calibration curve, and linear range

Under ideal experimental conditions, SWASV was used to assess the analytical performance of the SDA/MWCNTs/PGE sensor for the simultaneous measurement of Cd2+ and Pb2+. Fig. 4A displays representative stripping voltammograms obtained at various analyte concentrations, and Fig. 4B displays the related calibration curves.

Fig. 4. Analytical performance of the SDA/MWCNTs/PGE electrode for the detection of Cd2+ and Pb2+. (A) Square-wave anodic stripping voltammograms for concentrations of Cd2+ and Pb2+ in 0.1 M acetate buffer solution (pH 5.5) ranging from 1.3 to 101.1 ppb. (B) Linear calibration curves derived from the stripping peak currents, indicating that the modified electrode was appropriate for the simultaneous measurement of Cd2+ and Pb2+ trace levels. The pre-concentration period was 240 s, the deposition potential was −1.2 V, and the scan rate was 50 mV s−1.

Fig. 4

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Over the concentration range of 1.3–101 ppb, increased predictably with increasing concentrations of Cd2+ and Pb2+, indicating a satisfactory quantitative response of the proposed sensor. The peak current and analyte concentration exhibits a good linear relationship, with a coefficient of determination (R2) of 0.98 for both Cd2+ and Pb2+. The data are shown as mean ± standard deviation (SD), and each measurement was done in triplicate (n = 3). To show the measurement variability, Fig. 4B includes the relevant error bars.

The 3σ/S method of calculation was used to determine the LOD, where S is the slope of the relevant calibration curve, and σ is the standard deviation of the blank response. Eqn (1) was used to obtain the LOD.

LOD = 3σ/S 1

For Cd2+ and Pb2+, the calibration equations were Y = 0.062X + 0.186 and Y = 0.074X + 0.390, respectively. As a result, the slopes of the calibration curves for Cd2+ and Pb2+ were 0.062 and 0.074, respectively. The calculated LODs were 0.07 ppb for Pb2+ and 0.33 ppb for Cd2+.

The higher stripping current and calibration slope observed for Pb2+ compared with Cd2+ at the same nominal concentration may have been associated with differences in their electrochemical stripping behaviour and their interactions with the functional groups present on the SDA/MWCNT-modified electrode. Similar differences in the stripping responses of Cd2+ and Pb2+ have been reported for other modified electrodes,53 indicating that the relative sensitivity toward these ions is strongly dependent on the electrode surface and experimental conditions. The LOD was found to be 0.33 ppb for Cd2+ and 0.07 ppb for Pb2+, demonstrating the high sensitivity of the proposed sensor for detecting trace levels of toxic heavy metals.

These values are much below the permissible levels set for food safety monitoring and significantly lower than those of several previously reported electrochemical sensors (Table 1). Following each measurement, the electrode surface was restored by immersing it in a 0.01 M EDTA solution for 3 min. By creating stable, water-soluble metal–EDTA complexes, EDTA eliminated the accumulated metal ions and restored the active sensing surface. After cleaning with DD water, the regenerated electrode was reused without any noticeable performance degradation.

Table 1. Comparison of the analytical properties of the SDA/MWCNTs-modified electrode with previously published electrochemical methods for the detection of Cd2+ and Pb2+.

Modified electrode Techniques Real samples Linear range µg L−1 LOD µg L−1 Reference
Pb2+ Cd2+ Pb2+ Cd2+
SDA/MWCNTs/PGE SWASV Maize and egg yolk samples 1.3–101 1.3–101 0.07 0.33 This work
G-COOH-MWCNTs/ZnOa DPV Sea water 25–450 25–450 0.535 0.354 45
g-C3N4/MWCNTsb/GCE SWV River and sea waters 1036–10360 562–5620 0.0984 0.0471 46
ZIF-7@PANIc/GCE DPV Water samples 0.002–1 0.02–30 2.96 10.6 47
MgO/ZnO modified CPEd ASV Milk samples 100–800 100–800 0.142 0.45 48
ZnO/ErGO-GCEe ASV Aquatic sources 2.5–200 2.5–200 1.69 0.45 49
SABA/MWCNTsf/PGE SWASV Rice water, tobacco extract and raw milk 0.1–170 0.1–170 0.02 0.012 50
TSAB/MWCNTg/PGE SWASV Raw milk, honey and groundnut shell 1.1–16 1.1–16 0.18 0.13 51
CUiO-66/Bi/GCEh SWASV Real water environment 0.178–1.067 0.178–1.067 5.98 41.57 52
Bi/GDY/GCEi DPASV Real seawater sample 0.010–100.0 0.010–100.0 0.146 0.171 53
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a

Graphene framework -carboxylated multiwalled carbon nanotubes/ZnO nanoparticles.

b

Graphitic carbon nitride (g-C3N4) nanosheets were anchored onto multi-walled carbon nanotubes (MWCNTs).

c

Zeolite imidazolate framework ZIF-7 and polyaniline (PANI).

d

Magnesium oxide/zinc oxide (MgO/ZnO) nanocomposite-modified carbon paste electrode (CPE).

e

A glassy carbon electrode (GCE) modified by ZnO- electrochemically reduced graphene oxide (ZnO/ErGO).

f

Salicylidene-2-amino benzyl alcohol (SABA)/multiwalled carbon nanotubes (MWCNTs) modified electrode.

g

N′,N″,N‴,N′′′′-Tetra (salicylidene)-amino benzene-modified multi-walled carbon nanotube electrode (TSAB/MWCNT).

h

CUiO-66: carbonized Zr-MOF, Bi: bismuth.

i

Bi: bismuth, GDY: graphdiyne GCE (glassy carbon electrode), CPE (carbon paste electrode), PGE (paraffin wax graphite electrode).

3.9. Ability to eliminate interference

An essential requirement for useful electrochemical sensors used for real-sample analysis is selectivity. Thus, in the presence of common coexisting metal ions, such as Cu2+, As3+, Ni2+, Sn2+, and Fe2+, the anti-interference ability of the SDA/MWCNTs/PGE sensor was evaluated. Under ideal experimental conditions, the study was conducted in 0.1 M ABS (pH 5.5) containing 41.3 ppb of Cd2+ and Pb2+. The stripping peak currents of Cd2+ and Pb2+ no changes in the presence of 10 ppb of interfering ions, with Cd2+ and Pb2+ present at an equimolar concentration ratio (1 : 1), as shown in Fig. 5. The following formula was used to determine the relative signal changes:Change in relative signal (%) = [(Ii/I0) − 1] × 100where I0 and Ii are the stripping currents that were produced when interfering ions were present and absent, respectively.

Fig. 5. Interference investigation for the simultaneous measurement of 41.3 ppb Cd2+ and Pb2+ in an equimolar concentration ratio (1 : 1 M) of 10 ppb of possibly interfering metal ions (Cu2+, As3+, Ni2+, Sn2+, and Fe2+) in 0.1 M acetate buffer solution (pH 5.5).

Fig. 5

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Excellent selectivity towards Cd2+ and Pb2+ was demonstrated, with the measured fluctuations staying within 5% (Table 2). The preferential coordination of the SDA ligand towards target analytes and the adequately separated stripping potentials of the interfering ions, which avoid peak overlap during voltammetric analyses, may have been responsible for the excellent selectivity. These findings showed that the sensor could accurately detect Cd2+ and Pb2+ in complicated sample matrices with little interference.

Table 2. The manner in which the stripping response of Pb2+ (41.3 ppb) in 0.1 M acetate buffer solution (ABS) using the SDA/MWCNTs-modified electrode was affected by interfering metal ions (Cu2+, As3+, Ni2+, Sn2+, and Fe2+).

Interference ions Cd2+ Pb2+
Peak current (µA) Relative signal changes (%) Peak current (µA) Relative signal changes (%)
No interference ions 0.22 — 0.50 —
Cu2+ 0.221 0.45 0.491 1.80
As3+ 0.218 0.91 0.488 2.40
Ni2+ 0.217 1.36 0.486 2.80
Sn2+ 0.221 0.45 0.485 3.00
Fe2+ 0.221 0.45 0.487 2.60
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4. Studies on reproducibility and stability

Over a period of 1 week, the operational stability of the SDA/MWCNTs/PGE sensor was assessed using 41.3 ppb Cd2+ and Pb2+ in a 0.1 M ABS (pH 5.5). The stripping peak currents stayed almost constant during the storage time, as seen in Fig. 6, demonstrating good retention of the electrochemical activity of the modified electrode.

Fig. 6. Stability of the SDA/MWCNTs and square wave anodic stripping voltammograms of 41.3 ppb of Cd2+ and Pb2+ using a freshly made electrode and one that was kept at room temperature for 1 week.

Fig. 6

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The good stability of the sensing platform was confirmed by the computed relative standard deviation (RSD) values of 2.1% for Cd2+ and 1.9% for Pb2+. The substantial immobilisation of SDA on the MWCNT surface and the characteristic chemical robustness of the composite material were responsible for the exceptional stability.

Five independently fabricated SDA/MWCNTs/PGE electrodes were used to further evaluate the reproducibility of the sensor production procedure. With RSD values of 2.0% and 2.3% for Cd2+ and Pb2+, respectively, the corresponding stripping responses (Fig. 7) showed outstanding agreement. The strong fabrication repeatability and dependability of the suggested sensing platform were demonstrated by the low RSD values, emphasising its suitability for traditional analytical applications.

Fig. 7. Reproducible nature of five SDA/MWCNTs/PGE electrodes made under the same conditions for the measurement of 41.3 ppb Cd2+ and Pb2+.

Fig. 7

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4.1. Determination of Cd2+ and Pb2+ in egg yolk and maize samples

The applicability of the proposed SDA/MWCNTs/PGE sensor for the determination of Cd2+ and Pb2+ was evaluated using (sample A) egg yolk and (sample B) maize samples in Fig. 8A and B. The procedure described in section 2.4 was used to homogenise, acid-digest, and dilute the materials. Under optimal conditions, including an acetate buffer pH of 5.5, the resultant solutions were examined. To evaluate the accuracy of the proposed method, known amounts of (20 and 35 ppb) Cd2+ and Pb2+ were spiked into the prepared sample solutions, and the recoveries were calculated from the measured concentrations. Table 3 summarises the analytical results obtained from the recovery experiments. The proposed SDA/MWCNTs/PGE sensor exhibited satisfactory recoveries of 97.1–103.1% for Cd2+ and 100.5–104.0% for Pb2+, with RSD values of 1.2–2.1% and 1.2–2.2%, respectively. These results indicated good accuracy and precision with limited influence of the food matrix on the electrochemical determination of Cd2+ and Pb2+.

Fig. 8. The SDA/MWCNTs/PGE electrode was used to measure Cd2+ and Pb2+ in actual samples. The suitability of the sensor for trace metal analyses in complex food matrices was demonstrated by the SWASV responses recorded in acetate buffer solution (pH 5.5) containing 20 ppb and 35 ppb Cd2+–Pb2+ mixtures at an accumulation potential of −1.2 V and an accumulation time of 240 s for (A) egg yolk and (B) maize samples.

Fig. 8

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Table 3. Validation of the suggested SDA/MWCNTs sensor by comparison with AAS for measurement of Cd2+ and Pb2+ in actual samples (n = 3).

Samples M2+ (SWASV) AAS
Added ppb Found ppb RSD (%) Recovery (%) Found ppb RSD (%) Recovery (%)
Sample-A (egg yolk) Cd2+ 20.0 20.1 1.4 100.5 20.0 1.2 100.0
35.0 34.0 1.9 97.1 35.0 2.1 100.0
Pb2+ 20.0 20.3 1.2 101.5 21.0 2.5 105.0
35.0 35.2 1.7 100.5 36.0 2.2 103.0
Sample-B (maize) Cd2+ 20.0 20.0 1.6 100.0 20.0 1.3 100.0
35.0 36.1 2.1 103.1 35.0 1.5 100.0
Pb2+ 20.0 21.0 2.0 104.0 21.0 2.4 105.0
35.0 36.3 2.2 103.7 36.0 2.2 103.0
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To further evaluate the reliability of the proposed sensor, the results obtained by SWASV were correlated with those obtained using AAS, as presented in Table 3. AAS showed RSD values of 1.2–2.5%, with recoveries ranging from 100.0–105.0%. The close agreement between the SWASV and AAS results demonstrated the satisfactory analytical performance of the proposed sensor for the determination of Cd2+ and Pb2+ in egg yolk and maize samples.

5. Conclusions

SWASV was effectively used to develop an SDA/MWCNTs/PGE electrode for the simultaneous electrochemical measurement of Cd2+ and Pb2+. MWCNs and the Schiff-base ligand SDA acted synergistically to create numerous active sites for metal-ion complexation and accelerate electron transfer at the electrode surface. As a result, when compared with the original electrode, the modified electrode showed noticeably improved stripping responses towards Cd2+ and Pb2+.

With an LOD of 0.33 ppb for Cd2+ and 0.07 ppb for Pb2+, the suggested sensor demonstrated a linear response over the concentration range of 1.3–101 ppb for both metal ions under ideal experimental conditions.

Conflicts of interest

There are no conflicts of interest to declare.

Supplementary Material

RA-016-D6RA05132D-s001

Acknowledgments

The Dean of Research and Development at Vel Tech Rangarajan Dr Sagunthala R & D Institute of Science and Technology (Chennai, India) is greatly appreciated by the authors for providing the instrumentation and characterisation facilities required to complete this research project.

Data availability

The data generated specifically for the present study, including Cd2+ and Pb2+ stripping voltammetric measurements, calibration data, selectivity/interference studies, reproducibility, stability, and real-sample analysis, are in the manuscript.

Supplementary information (SI): FT-IR, 1H-NMR, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) characterization data for the SDA/MWCNT electrode have been reported in our related study on Hg(ii) sensing (https://doi.org/10.1039/d4ra00210e). See DOI: https://doi.org/10.1039/d6ra05132d.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA05132D-s001
RA-016-D6RA05132D-s001.pdf (380.9KB, pdf)

Data Availability Statement

The data generated specifically for the present study, including Cd2+ and Pb2+ stripping voltammetric measurements, calibration data, selectivity/interference studies, reproducibility, stability, and real-sample analysis, are in the manuscript.

Supplementary information (SI): FT-IR, 1H-NMR, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) characterization data for the SDA/MWCNT electrode have been reported in our related study on Hg(ii) sensing (https://doi.org/10.1039/d4ra00210e). See DOI: https://doi.org/10.1039/d6ra05132d.

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