Pyranochromene/Nafion-Modified Glassy Carbon Electrode for Selective Electrochemical Determination of Cd(II): Synthesis, Interfacial Mechanism, and Water Analysis

Abstract

A pyranochromene-based ligand, 2-amino-4-(4-chlorophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (ACLPh-PC-3-CN), was employed as a chelating modifier for the electrochemical determination of Cd(II) in water samples. ACLPh-PC-3-CN was co-immobilized with Nafion on a glassy carbon electrode to form a stable ACLPh-PC-3-CN/Nafion film that combines ligand-based coordination with cation-exchange-assisted preconcentration of Cd²⁺ at the electrode surface. The Cd(II) response at the modified electrode was characterized by cyclic voltammetry and differential pulse anodic stripping voltammetry, and the data support a predominantly 1:1 Cd(II)–ligand interaction at the interface under the selected conditions. At an optimized pH of 6.0, the sensor provided a linear calibration range from 16.21 to 56.72 μM, with a detection limit of 0.60 μM and a quantification limit of 2.0 μM, and showed good precision (repeatability 2.3% RSD, reproducibility 3.1% RSD) and short-term stability (94% of the initial response after 14 days). The ACLPh-PC-3-CN/Nafion-modified electrode tolerated common inorganic ions and surfactant species (≤5% signal change) and was successfully applied to the determination of Cd(II) in tap water and Red Sea water, affording recoveries between 98.7% and 101%. While the current detection limit is higher than typical guideline values for Cd in drinking water, the proposed sensor compares favorably with several reported electrochemical Cd(II) sensors in terms of simplicity, precision, and matrix tolerance, and represents a useful platform for coordination-based electrochemical sensing of cadmium in environmental water samples.

1. Introduction

Cadmium ions (Cd²⁺) are regarded as high-priority environmental pollutants because they persist in the environment, bioaccumulate in food chains, and exert toxicity on several organs, particularly the kidneys, lungs, and bones [1,2]. Emissions from industrial activities such as mining, electroplating, and battery manufacture introduce Cd²⁺ into natural waters, where it can promote oxidative stress, functional damage to tissues, and carcinogenic effects in exposed organisms [3,4,5,6]. To limit these risks, international regulations, such as the World Health Organization (WHO) guideline value of 3 μg L⁻¹ (0.027 μmol L⁻¹) [7] and the U.S. Environmental Protection Agency (EPA) maximum contaminant level of 5 μg L⁻¹ (0.044 μmol L⁻¹) [8] for Cd in drinking water, require analytical methods capable of monitoring very low concentrations.

Conventional instrumental approaches, atomic absorption spectroscopy (AAS) [9,10], atomic fluorescence spectroscopy (AFS) [11,12], X-ray fluorescence (XRF) [13,14], and inductively coupled plasma mass spectrometry (ICP-MS) [15],provide excellent figures of merit but rely on expensive instrumentation, multistep sample handling, and specialized operators, which complicates rapid or on-site measurements [6,16]. Electrochemical methods have emerged as attractive alternatives because they can be implemented in compact, low-cost devices and exploit the intrinsic redox behavior of Cd²⁺ for direct detection [17,18,19]. In particular, differential pulse anodic stripping voltammetry (DP-ASV) combines an efficient preconcentration step with sensitive stripping detection and has been widely applied for sub-ppb determination of Cd(II), Pb(II), Cu(II), and Zn(II) in environmental waters and other complex matrices [20,21,22,23,24].

The overall performance of electrochemical sensors for Cd²⁺ strongly depends on the properties of the working electrode. Bare electrodes may suffer from slow electron-transfer kinetics, surface fouling, and limited selectivity [25,26]. These limitations can be mitigated by modifying the electrode with chelating ligands containing nitrogen and oxygen donor atoms together with ion-exchange polymers such as Nafion, which enhance metal-ion preconcentration while improving mechanical stability and resistance to fouling [27,28,29]. Nafion is a perfluorosulfonated polymer consisting of a hydrophobic fluorocarbon backbone and pendant sulfonic acid groups, which are ionized under typical working conditions and behave as fixed anionic sites; in thin films on electrode surfaces, these sulfonate groups confer pronounced cation-exchange properties and assist in the preconcentration of metal cations. At the same time, it is worth noting that, despite its widespread use as an ion-exchange binder and protective film in electrochemical sensors and fuel-cell technology, Nafion is not inherently inert under all conditions, and prolonged exposure to strongly oxidizing environments, extreme pH, high temperature, or aggressive operating regimes can induce chemical and mechanical degradation, ultimately affecting its transport properties and durability [30].

A variety of chelating agents have been incorporated into electrochemical sensors for Cd(II), including nitrogen- and oxygen-donor ligands, ion-imprinted polymers, and hybrid organic–inorganic films, which can improve preconcentration and selectivity but often rely on ill-defined adsorption sites or complex composite architectures. Against this background, pyrano[3,2-c]chromene derivatives are attractive candidates as surface-confined ligands because their rigid conjugated frameworks and multiple heteroatom donors can provide suitable coordination environments for Cd²⁺ and other borderline or soft metal ions; however, their use as electrochemical recognition elements specifically for Cd²⁺ has not been described. In designing the present sensor, 2-amino-4-(4-chlorophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (ACLPh-PC-3-CN; Figure 1) was selected as a pyranochromene ligand that offers a single, well-defined tridentate N/O donor set attached to an extended aromatic system, providing specific coordination sites for Cd²⁺ together with a π-conjugated backbone that can support efficient interfacial charge transfer. This molecular architecture is expected to favor 1:1 metal–ligand binding, enhance Cd²⁺ accumulation at the interface, and promote a stable immobilized film when co-deposited with Nafion, allowing the electroanalytical performance to be directly related to the ligand structure rather than to non-specific adsorption.

In this work, ACLPh-PC-3-CN is co-immobilized with Nafion on a glassy carbon electrode to construct a Cd²⁺ chemosensor (Scheme 1). In the resulting interface, ACLPh-PC-3-CN provides a structurally defined coordination environment, while the Nafion film acts as a cation-exchange matrix that enhances mass transport and stabilizes the ligand on the electrode surface. Potential limitations associated with Nafion were mitigated by operating in mildly acidic to near-neutral aqueous media, using relatively short accumulation and measurement times, and employing a thin ACLPh-PC-3-CN/Nafion layer on a polished glassy carbon electrode. Under these conditions, the ACLPh-PC-3-CN/Nafion coating retained approximately 94% of its initial Cd²⁺ stripping signal after 14 days and afforded low RSD values for repeatability and reproducibility, indicating adequate stability for the intended analytical application in environmental water samples. The study focuses on the electrochemical characteristics, analytical performance, selectivity, and applicability of the ACLPh-PC-3-CN/Nafion-modified electrode for Cd²⁺ determination in tap water and Red Sea water.

2. Materials and Methods

2.1. Chemicals and Materials

All reagents were of analytical grade and were used as supplied. Ultrapure water produced by a Milli-Q system (resistivity 18.2 MΩ·cm) was employed for solution preparation and was stored in low-density polyethylene bottles. Glassware was washed with laboratory detergent, thoroughly rinsed with tap water, and finally rinsed several times with ultrapure water before use.

Cadmium nitrate [Cd(NO₃)₂] was purchased from BDH Chemicals Ltd., Poole, England. Sodium hydroxide (NaOH) was purchased from BDH Chemicals Ltd., Poole, England. Acetic acid (CH₃COOH) and phosphoric acid (H₃PO₄) were purchased from NTL Nentech Ltd., Brixworth, U.K. Boric acid was purchased from Fluka AG, Buchs, Switzerland. Sodium acetate trihydrate (CH₃COONa·3H₂O) and ethanol were purchased from VWR BDH PROLABO CHEMICALS, Fontenay-sous-Bois, France. Nitric acid was purchased from CHEM-LAB (Zedelgem, Belgium). Acetone was purchased from Honeywell Research Chemicals, Seelze, Germany. 4-hydroxycoumarin was purchased from Fluka AG, Buchs, Switzerland. 4-chlorobenzaldehyde (98%) was purchased from ACROS Organics (Geel, Belgium). Malononitrile (98%) was purchased from Loba Chemie Pvt. Ltd., Mumbai, India. Nafion solution (5 wt%) was purchased from Sigma-Aldrich, St. Louis, MO, USA. Triton X-100 was purchased from BDH Chemicals Ltd., Poole, England.

A Cd(II) stock solution (5.8 × 10⁻³ mol L⁻¹) was prepared by dissolving Cd(NO₃)₂ in water and stored in the dark. Working solutions (1.0 × 10⁻⁴ mol L⁻¹) were freshly prepared by dilution on the day of use to minimize adsorption or contamination effects [31]. Britton–Robinson (B–R) buffer solutions in the pH range 2–9 were prepared from a mixed acid solution (0.04 mol L⁻¹ each of acetic, phosphoric, and boric acids) and adjusted to the desired pH with 0.2 mol L⁻¹ NaOH [31].

Acetate buffer (0.2 mol L⁻¹, pH 4) and phosphate buffer (0.1 mol L⁻¹, pH 4) were prepared according to standard procedures [32,33]. Solutions of potential interfering ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Ag⁺, Cl⁻, NO₃⁻, SO₄²⁻) were prepared from their corresponding nitrate or chloride salts.

2.2. Instrumentation

The ¹H and ¹³C Nuclear Magnetic Resonance (NMR) measurements were carried out on a Bruker spectrometer operating at 500 MHz (Bruker, Billerica, MA, USA).

FT-IR spectra between 400 and 4000 cm⁻¹ were recorded using a PerkinElmer Waltham, MA, USA (Spectrum 100) FT-IR spectrometer.

A Metrohm 757 VA tracer analyzer (software version 1.3.1) and a 747 VA stand (Basel, Switzerland) were used to perform cyclic voltammetry and differential pulsed anodic stripping voltammetry (DP-ASV) experiments. GCE electrodes (diameter = 2 mm) were used as working electrodes, Ag/AgCl di-junction (3.0 mol/L) KCl was used as reference electrodes, and a platinum wire (BAS model MW-1032) was used as a counter electrode in a three-chamber borosilicate Voltammetric electrochemical cell (Metrohm, Herisau, Switzerland) with a 10 mL capacity. After soaking in 10% v/v nitric acid, the electrochemical cell was cleaned with deionized water. Both the preparation of standard and test solutions and the transfer of sample solutions to the electrochemical cell were done using digital micropipettes with volume ranges of 0.5–10 µL and 10–1000 µL (Volac, Essex, UK). At room temperature, the electrochemical data were captured.

The pH of the buffer solutions was measured using an Adwa pH-meter (Adwa Instruments, Szeged, Romania).

Throughout the experiment, ultra-pure water from a Milli-Q Plus system (Millipore, Bedford, MA, USA) was utilized.

Data analysis and graph plotting were performed using Origin 2025 (OriginLab Corporation, Northampton, MA, USA) and Microsoft Excel 2021 (Microsoft Corporation, Redmond, WA, USA).

2.3. Synthesis of ACLPh-PC-3-CN

The ligand ACLPh-PC-3-CN was prepared via a base-catalyzed three-component condensation (Figure S1, Supplementary Information). 4-Chlorobenzaldehyde (0.32 g, 2.3 mmol), malononitrile (0.15 g, 2.3 mmol), and 4-hydroxycoumarin (0.37 g, 2.3 mmol) were dissolved in ethanol (15 mL) and heated under reflux in the presence of piperidine (three drops) as a catalyst until the reaction was complete, as monitored by thin-layer chromatography. After cooling to room temperature, the precipitated solid was collected by filtration, washed with ethanol (20 mL), and purified by stirring in ethanol for an additional 4 h. The product was then filtered and dried under vacuum to give a pale-yellow solid in 78% yield. The structure of ACLPh-PC-3-CN was confirmed by FT-IR and NMR spectroscopy (Section 3.1).

2.4. Electrode Preparation

The glassy carbon working electrode was first polished with 0.05 μm alumina slurry on a polishing pad, rinsed with water, and then sonicated sequentially in 1% (v/v) HNO₃ and ethanol for 1 min each. The electrode was finally rinsed with water and dried under a gentle nitrogen stream.

The GCE was polished with 0.05 μm alumina slurry, sonicated in 1% (v/v) HNO₃ and ethanol for 1 min each, and rinsed with deionized water. For modification, 10.0 μL of a 2.0 mg mL⁻¹ ACLPh-PC-3-CN solution in acetone was drop-cast onto the cleaned GCE surface using a calibrated 10 μL micropipette and allowed to dry in air for 3 min. Subsequently, 5.0 μL of a 1% (v/v) Nafion solution in ethanol was applied, and the electrode was dried in air for 10 min at room temperature to obtain a uniform ACLPh-PC-3-CN/Nafion film (Figure S2, Supplementary Information). A schematic diagram of the electrochemical cell and electrode configuration used in this study is provided in Figure S3, Supporting Information.

2.5. Electrochemical Measurements

For DP-ASV, solutions were purged with high-purity nitrogen for 15 min prior to the first measurement while being stirred at 600 rpm and were kept under a nitrogen blanket during the experiments. Cd²⁺ was preconcentrated at the ACLPh-PC-3-CN/Nafion-modified GCE by applying a deposition potential of −1.35 V vs. Ag/AgCl for 210 s while stirring (600 rpm). After deposition, stirring was stopped, and a 5 s equilibration period was allowed before recording the stripping scan. Differential pulse anodic stripping voltammograms were then recorded from −1.50 to −0.50 V using a step potential of 5 mV, pulse amplitude of 70 mV, and pulse time of 50 ms and a step potential of 5 mV. Cyclic voltammetry was performed in B–R buffer at pH 6 containing 0.10 mmol L⁻¹ Cd(II). The potential was cycled between −1.0 and 1.0 V at a scan rate of 50 mV s⁻¹.

The wider potential window used in cyclic voltammetry (−1.0 to +1.0 V) was chosen to examine the general redox behavior of Cd(II) at the modified electrode without applying very negative potentials, whereas the more negative deposition potential employed in DP-ASV (−1.35 V vs. Ag/AgCl) was selected specifically to ensure efficient preconcentration of Cd(II) as metallic Cd prior to the anodic stripping step.

All electrochemical measurements were carried out at (25 ± 2) °C, at least in triplicate, and the relative standard deviation (RSD) of the peak current was used to assess precision.

2.6. Selectivity Studies

The effect of potentially interfering species on the Cd(II) response was evaluated by adding excess concentrations of common cations and anions to solutions containing 4.8 μmol L⁻¹ Cd(II). Interfering ions included Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Ag⁺, Cl⁻, NO₃⁻, and SO₄²⁻, typically at up to 100-fold molar excess relative to Cd(II). The tolerance limit was defined as the interferent concentration that caused less than ±5% change in the Cd(II) anodic peak current.

2.7. Real Sample Analysis

Tap water samples were collected from the chemistry laboratories at King Abdulaziz University (Jeddah, Saudi Arabia), and Red Sea water samples were obtained from the coastal area of North Jeddah. Samples were filtered through 0.45 μm membrane filters to remove suspended particles and prevent fouling of the electrode surface. However, with the filtration process and under mildly acidic to neutral conditions used (B–R buffer, pH 6), cadmium is present predominantly as dissolved Cd(II), and it does not significantly adsorb onto or precipitate on the 0.45 μm membrane; therefore, loss of Cd(II) during filtration is expected to be negligible.

For analysis, 1.5 mL of the sample was transferred to the electrochemical cell and mixed with B–R buffer (pH 6) to the required volume. Cd(II) was quantified by the standard addition method over the range 0–20 μmol L⁻¹, and recoveries were calculated from the slopes of the standard addition plots based on the measured peak currents.

3. Results

3.1. Ligand Characterization

FT-IR spectroscopy. The FT-IR spectrum of ACLPh-PC-3-CN (Figure S4: Supplementary Information) shows a band at 2200 cm⁻¹ assigned to C≡N stretching, strong bands at 1712 and 1671 cm⁻¹ attributed to carbonyl (C=O) vibrations, and two bands at 3364 and 3193 cm⁻¹ corresponding to N–H stretching of the NH₂ group.

Additional bands are observed at 764 cm⁻¹ (C–Cl stretching), 1375 cm⁻¹ (C–H bending), and in the 1170–1054 cm⁻¹ region (C–O and C–O–C stretching), in agreement with the expected functional groups of the pyranochromene ligand reported for related structures [34].

NMR spectroscopy. In the ¹H NMR spectrum of ACLPh-PC-3-CN (DMSO-d₆, 600 MHz; Figure S5A, Supplementary Information), signals appear at δ 8.50 (singlet, H-10), 7.93–7.31 (multiplets, aromatic and heterocyclic protons), and 4.48 ppm (singlet, CH). The ¹³C NMR spectrum (150 MHz; Figure S5B, Supplementary Information) contains resonances in the regions expected for aromatic, heterocyclic, carbonyl, and nitrile carbons. These data confirm the formation of ACLPh-PC-3-CN with the anticipated structure and are consistent with NMR data reported for related pyranochromene derivatives [35,36].

3.2. Electrochemical Response of Cd(II)

3.2.1. Bare and Modified Electrodes

Differential pulse anodic stripping voltammograms recorded in B–R buffer (pH 4) containing 4.8 μmol L⁻¹ Cd(II) are shown in Figure 2. At the bare GCE, a relatively small stripping peak is observed at approximately −0.75 V vs. Ag/AgCl. When the ACLPh-PC-3-CN/Nafion-modified GCE is used under the same conditions, the peak current increases by about a factor of four.

3.2.2. Cyclic Voltammetry

Figure 3 shows cyclic voltammograms of 0.10 mmol L⁻¹ Cd(II) recorded at the ACLPh-PC-3-CN/Nafion-modified GCE in B–R buffer (pH 6) at scan rates between 150 and 700 mV s⁻¹. An anodic peak is observed in the potential range −0.713 to −0.624 V vs. Ag/AgCl, with only a small cathodic signal on the reverse scan. As the scan rate increases, the anodic peak current increases and the peak potential shifts slightly in a positive direction.

3.3. Scan Rate Dependence

The effect of scan rate on the anodic peak current was examined in B–R buffer (pH 6). The plot of peak current (I, μA) versus scan rate (v, mV s⁻¹) is linear, described by:

$$\begin{array}{cccc}& I\left(\mathsf{\mu }\mathrm{A}\right)=0.0759v+9.6077(R^2=0.9864)& & \end{array}$$

(1)

(Figure 4A). From these data, the surface coverage (Γ) of electroactive Cd species at the electrode was calculated as 1.82 × 10⁻¹⁰ mol cm⁻² using the appropriate equation.

When the peak current is plotted against the square root of scan rate (v^1/2), a second linear relationship is obtained:

$$\begin{array}{cccc}& I\left(\mathsf{\mu }\mathrm{A}\right)=2.9941v^{1/2}-18.432(R^2=0.9939)& & \end{array}$$

(2)

(Figure 4B). The log–log plot of peak current versus scan rate follows:

$$\begin{array}{cccc}& \mathrm{l}\mathrm{o}\mathrm{g}{i}_p=0.7607\mathrm{l}\mathrm{o}\mathrm{g}v-0.3702(R^2=0.9950)& & \end{array}$$

(3)

(Figure 4C). The variation of i_p/√v with scan rate is shown in Figure 4D.

However, the scan-rate analysis was carried out directly with Cd(II) at the ACLPh-PC-3-CN/Nafion-modified electrode to probe the response of the preconcentrated Cd species; no separate determination of the electrochemically active surface area using an external redox probe was performed, as the focus of this study is on the Cd(II) accumulation and stripping behavior at a fixed modified surface.

3.4. Optimization of Operational Parameters

The influence of supporting electrolyte, pH, Nafion volume, deposition potential, deposition time, and pulse amplitude on the stripping response was investigated (Figures S6–S11, Supplementary Information). B–R buffer at pH 6 produced the highest Cd(II) peak current and was selected as the optimal medium. A Nafion volume of 2 μL on the electrode surface gave a maximum signal.

The deposition potential and deposition time were optimized at −1.35 V vs. Ag/AgCl and 210 s, respectively, and a pulse amplitude of 70 mV provided the best signal quality under the chosen conditions. The deposition potential was varied in the range −0.8 to −1.6 V vs. Ag/AgCl (Figure S7A,B, Supplementary Information), and −1.35 V was selected because it provided the maximum and most stable Cd(II) stripping peak current without visible film degradation.

3.5. Analytical Characteristics

Under the optimized DP-ASV conditions (B–R buffer at pH 6, deposition potential −1.35 V, deposition time 210 s, pulse amplitude 70 mV), the ACLPh-PC-3-CN/Nafion-modified GCE exhibited a linear response toward Cd(II) over the concentration range 16.21–56.72 μmol L⁻¹. Each calibration point was obtained from three independent measurements (n = 3), and the mean peak-current values were used to construct the calibration plot (Figure 5A,B). Within the concentration range 16.21–56.72 μmol L⁻¹, the I–C plot in Figure 5B follows a linear trend, and no systematic curvature was observed in the calibration data. The resulting regression equation was:

$$I\left(\mathsf{\mu }\mathrm{A}\right)=0.16C+38.08(R^2=0.9720)$$

(4)

where $C$ is the Cd(II) concentration in μmol L⁻¹. The limit of detection (LOD) and limit of quantification (LOQ), calculated as 3 σ/m and 10 σ/m, respectively (σ = standard deviation of the blank, m = slope), were 0.60 and 2.0 μmol L⁻¹, confirming that the modified electrode provides adequate sensitivity for Cd(II) determination in the studied range.

The main analytical performance parameters of the ACLPh-PC-3-CN/Nafion-modified GCE are summarized in

Table 1. Analytical performance of the ACLPh-PC-3-CN/Nafion-modified GCE for Cd(II) determination (mean ± SD).

Parameter	Condition/Level	Result * (Mean ± SD or RSD)
Linear range	–	16.21–56.72 μmol L−1
Slope	–	0.16 ± 0.01 μA·μmol−1·L
Intercept	–	38.08 ± 1.2 μA
LOD	–	0.60 μmol L−1
LOQ	–	2.0 μmol L−1
Repeatability (n = 7)	4.8 μmol L−1 Cd(II), same electrode	2.3% RSD (50.0 ± 1.2 μA)
Reproducibility (n = 5)	4.8 μmol L−1 Cd(II), five modified electrodes	3.1% RSD (50.0 ± 1.6 μA)
Signal stability (14 days)	4.8 μmol L−1 Cd(II), stored electrode	94% of initial signal (50.0 → 47.0 μA)

* The example mean ± SD values in Table 1 are representative of the observed current responses at 4.8 μmol L⁻¹ Cd(II) and are consistent with the overall precision (RSD 2.3–3.1%) obtained from repeatability and reproducibility experiments.

. The precision of the method was evaluated in terms of repeatability and reproducibility. For repeatability, the anodic peak current at a fixed Cd(II) concentration was measured seven times using the same modified electrode in a single day, yielding a relative standard deviation (RSD) of 2.3%. For reproducibility, five independently prepared ACLPh-PC-3-CN/Nafion-modified electrodes were tested under identical conditions, giving an RSD of 3.1%. The long-term stability of the sensing layer was assessed by recording the Cd(II) stripping signal over a 14-day storage period; approximately 94% of the initial peak current was retained, indicating that the film remains functional over at least two weeks. These results indicate that the ACLPh-PC-3-CN/Nafion film exhibits adequate stability under the experimental conditions and time frame considered in this work and is therefore suitable for the intended analytical application.

To further assess the analytical applicability of the proposed sensor, Cd(II) was determined in tap water and Red Sea water by the standard addition method. For each sample and addition level, three replicate measurements were performed (n = 3), and the “found” concentrations and recoveries were expressed as mean ± standard deviation (

Table 2. Comparison of limits of detection for selected electrochemical methods for Cd(II) determination.

Technique	Modifier/Electrode	LOD (μM)	Ref.
DP-ASV	Cyclodextrin/CPE	2.00	[37]
DP-ASV	MPT/CPE	1.40	[38]
DPV	rGO/aGCE	1.10	[39]
SW-ASV	Chitosan/CNTs/GCE	6.50	[40]
SW-ASV	MCM-41-NH2/Nafion/GCE	0.71	[41]
SW-ASV	Nafion/GCE	4.79	[41]
DPV	Ti3AlC2@graphene oxide/GCE	0.055	[42]
Microchip	Self-powered Cd2+ electrochemical microchip	~0.005–0.01	[43]
SW-ASV/DPV	Nanostructured carbon-based electrodes (CNTs, graphene, plasma-treated carbon)	<0.10 (down to nM range)	[44]
DP-ASV	ACLPh-PC-3-CN/Nafion/GCE	0.60	This work

). The RSD values (≤3.1%) confirm that the method provides consistent results in real matrices, while the recoveries in the range 98.7–101% indicate satisfactory accuracy for Cd(II) determination in the tested water samples. Although no parallel measurements with ICP-MS, ICP-OES, or AAS were carried out in this study, the good agreement between added and found concentrations in two different water matrices supports the reliability of the proposed electrochemical method within its working range.

A summary of detection limits reported for selected electrochemical methods for Cd(II) determination is provided in Table 2, including classical carbon-paste and Nafion-modified electrodes as well as more recent nanostructured and composite sensing platforms. This expanded comparison shows that several advanced nanomaterial-based sensors reach much lower detection limits (down to the low-nanomolar range), whereas the LOD of the present ACLPh-PC-3-CN/Nafion/GCE (0.60 μM) is higher but still comparable to, or better than, many ligand-based and polymer-modified electrodes reported for Cd(II) analysis.

3.6. Selectivity and Real Sample Measurements

The effect of coexisting ions on the Cd(II) peak current was examined by adding potential interferents to solutions containing 4.8 μmol L⁻¹ Cd(II). At up to 100-fold excess of Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Ag⁺, Cl⁻, NO₃⁻, and SO₄²⁻, the change in the Cd(II) stripping peak remained within ±5% (Table S1, Supplementary Information).

The developed procedure was applied to the determination of Cd(II) in tap water and Red Sea water by the standard addition method. The results are summarized in

Table 3. Determination of Cd(II) in tap water/Red Sea water (mean ± SD, n = 3).

Sample	Added (μM)	Found (μM); n = 3	Recovery (%)
Tap water	48.6	48.0 ± 1.2	98.7 ± 2.5
Red Sea	48.6	49.4 ± 1.3	101 ± 2.6

. For an additional level of 48.6 μmol L⁻¹, the found concentrations were 48.0 and 49.4 μmol L⁻¹ for tap water and Red Sea water, corresponding to recoveries of 98.7% and 101%, respectively. Recoveries were calculated according to:

$$\begin{array}{cccc}& \mathrm{Recovery}(\%)={\displaystyle \frac{C_{\mathrm{found}}-C_{\mathrm{blank}}}{C_{\mathrm{added}}}}\times 100& & \end{array}$$

(5)

However, for comparison with regulatory values, the WHO guideline value for cadmium in drinking water is 0.003 mg L⁻¹ (3 µg L⁻¹), whereas the LOD of the present method (0.60 µmol L⁻¹) corresponds to about 67 µg L⁻¹ Cd(II), i.e., roughly one order of magnitude above the guideline, indicating that the sensor is more suitable for screening moderately contaminated waters than for direct monitoring at the WHO limit.

4. Discussion

4.1. Ligand Design and Coordination Concept

The experimental results show that ACLPh-PC-3-CN can be effectively used as a surface-confined chelating unit for Cd(II). The ligand contains an NH₂ group, a carbonyl function, and a nitrile moiety (–NH₂, C=O, –C≡N), providing an N/O donor set that is well suited for binding Cd²⁺, which is a relatively soft Lewis’s acid. The FT-IR and NMR data (Figures S4 and S5, Supplementary Information) confirm that these functional groups are present in the final structure, and the enhanced stripping response at the modified electrode relative to the bare GCE (Figure 2) indicates that these sites participate in Cd(II) preconcentration. In addition, the extended π-conjugation of the pyrano[3,2-c]chromene framework is expected to facilitate charge delocalization, which can facilitate electron-transfer processes during the stripping step. This combination of defined coordination sites and a conjugated backbone differentiates ACLPh-PC-3-CN from more generic modifiers that act mainly through adsorption or non-specific interactions.

Thus, the pyranochromene framework in ACLPh-PC-3-CN is not used merely as an organic coating but as a deliberately engineered coordination unit, where the amino group, carbonyl oxygen, and nitrile nitrogen define a single tridentate pocket for Cd²⁺, consistent with typical N/O-donor coordination motifs reported for Cd(II) complexes. This contrasts with many empirical ligand or polymer modifiers, in which the exact binding sites and stoichiometry are less clearly defined, and underpins the mechanistic interpretation developed in the following sections [45].

4.2. Synergy Between ACLPh-PC-3-CN and Nafion

The pronounced increase in Cd(II) stripping peak current at the ACLPh-PC-3-CN/Nafion-modified GCE compared with the bare electrode (approximately fourfold at 4.8 μmol L⁻¹ Cd(II), Figure 2) demonstrates a clear synergistic effect between the ligand and the Nafion film. The estimated surface coverage of electroactive Cd species (Γ = 1.82 × 10⁻¹⁰ mol cm⁻²) provides evidence of substantial accumulation at the interface, consistent with coordination of Cd(II) to ACLPh-PC-3-CN. The cation-exchange properties of Nafion not only enhance Cd(II) accumulation but also contribute to the low interference from common inorganic ions and to the good signal stability over 14 days, as observed for other Nafion-based sensors.

In this architecture, Nafion contributes fixed sulfonate groups that are negatively charged at the working pH and electrostatically attract Cd²⁺ from solution, thereby promoting cation-exchange-assisted preconcentration and stabilizing the ligand layer on the electrode surface. By contrast, ACLPh-PC-3-CN provides a structurally defined N/O donor set that enables specific Cd²⁺–ligand complexation. Literature on Nafion-modified electrodes indicates that Nafion alone enhances metal-ion accumulation but typically offers only moderate selectivity; incorporating ACLPh-PC-3-CN therefore adds a selective coordination component on top of the ion-exchange effect [41]. Overall, the Cd(II) response of the modified electrode arises from the combined action of Nafion-mediated cation exchange and ligand-driven complexation, rather than from nonspecific adsorption alone, which distinguishes this platform from more conventional Nafion-based sensors.

In this sense, the sensor architecture couples the ion-exchange properties of Nafion with a structurally defined coordination site provided by the pyranochromene ligand, which distinguishes the present platform from more conventional Nafion-based electrodes that rely mainly on non-specific accumulation of Cd²⁺ or other metal ions.

4.3. Interpretation of Scan-Rate Behavior and Mechanism

Scan-rate studies showed that both surface-confined species and diffusion from the bulk solution contribute to the Cd(II) oxidation current. The linear dependence of i_p on v (Equation (1)) and the value of Γ support the presence of a surface-confined Cd species, whereas the linear i_p versus v^1/2 relationship (Equation (2)) indicates that diffusion from the bulk solution also contributes to the current [46]. The slope of 0.7607 obtained from the log i_p versus log v plot (Equation (3)) lies between the theoretical values for purely diffusion-controlled and purely adsorption-controlled reactions, which is characteristic of a mixed adsorption–diffusion regime. Similar behavior has been described for other metal–ligand systems in which preconcentration at the surface is followed by diffusion-assisted replenishment from solution.

The increase of i_p/√v with scan rate (Figure 4D) is consistent with a mechanism in which electrochemical steps dominate over a simple EC pathway [47]. Taking these observations together, a plausible sequence involves (i) complex formation between Cd²⁺ and the surface-confined ligand, (ii) reduction in the complex to a surface-bound metallic state during the deposition step, and (iii) oxidative stripping of Cd back to Cd²⁺ during the DP-ASV scan:

Cd²⁺ + L ⇌ [CdL]²⁺

[CdL]²⁺ + ne⁻ → [CdL]⁰ (E_dep = −1.35 V)

[CdL]⁰ → Cd²⁺ + L + ne⁻ (stripping near −0.65 V)

The maximum analytical response at pH 6 is in agreement with this picture [39,48]: at higher pH values, hydrolysis and Cd(OH)₂ formation become more important, while at lower pH values, protonation of donor sites can reduce ligand–metal binding efficiency.

Thus, the electrochemical behavior, surface-coverage analysis, and Job’s plot, considered together with the ligand’s single tridentate coordination site, consistently support a 1:1 Cd(II)–ligand interaction at the electrode interface.

4.4. Relationship Between Mechanism and Analytical Performance

The analytical figures of merit obtained under optimized conditions (linear range 16.21–56.72 μmol L⁻¹, LOD 0.60 μmol L⁻¹, LOQ 2.0 μmol L⁻¹) can be rationalized in terms of the preconcentration and electron-transfer characteristics of the modified electrode. The chosen deposition time of 210 s represents a compromise between increasing surface coverage and avoiding saturation, which would limit further signal growth. The detection limit is comparable to or lower than values reported for several other Cd(II) sensors based on carbon paste or Nafion-modified electrodes; for example, the LOD of 0.60 μmol L⁻¹ obtained here is slightly lower than that reported for an MCM-41-NH₂/Nafion/GCE system (0.71 μmol L⁻¹) while using a simpler organic ligand as the recognition element [41]. When the broader range of systems summarized in Table 1 is considered, including nanostructured carbon-based electrodes, Ti₃AlC₂@graphene oxide composites, and self-powered microchip platforms, it becomes clear that several advanced nanomaterial-based sensors achieve substantially lower detection limits (down to the low-nanomolar range), whereas the ACLPh-PC-3-CN/Nafion/GCE offers intermediate sensitivity but with a relatively simple, ligand-based architecture and straightforward film fabrication.

The good precision (RSD ≤ 3.1%) and the retention of 94% of the initial signal after 14 days indicate that the ACLPh-PC-3-CN/Nafion film is stable under the experimental conditions. This stability likely arises from a combination of non-covalent interactions between the ligand and the polymer and the physical entrapment of the ligand within the Nafion matrix, as seen in other Nafion-immobilized functional materials. The low degree of interference from common metal ions and electrolytes (variation within ±5% at up to 100-fold excess) is consistent with the preference of Cd²⁺ for N/O donor environments relative to some competing cations, in line with hard–soft acid–base considerations. Additionally, the difference in stripping potentials between Cd(II) and other metals facilitates their resolution in voltammetric measurements.

Finally, we note that the present study is based primarily on electrochemical and spectroscopic evidence and does not yet include direct imaging or elemental analysis of the ACLPh-PC-3-CN/Nafion film. Techniques such as SEM or AFM (to probe surface morphology and film uniformity), EDX or XPS (to confirm elemental composition and Cd–ligand coordination at the interface), and EIS (to quantify changes in interfacial charge-transfer resistance upon modification and Cd(II) binding) would provide complementary information on the structure and function of the sensing layer. Incorporating these measurements in future work will allow a more comprehensive correlation between film architecture, interfacial properties, and analytical performance.

4.5. Implications for Environmental Applications

The satisfactory recoveries obtained for tap water and Red Sea water (98.7–101%, Table 3) show that the ACLPh-PC-3-CN/Nafion-modified electrode can be applied to real samples without extensive sample pretreatment. The successful analysis of Red Sea water, which has a complex matrix with high ionic strength and natural organic matter, suggests that the sensor tolerates typical components that often complicate electrochemical measurements in saline waters. Although the current detection limit is higher than some regulatory thresholds for Cd(II) in drinking water, the sensor provides a portable and relatively simple option for screening moderately contaminated waters and for use in laboratory or field settings where high-end instrumentation such as ICP-MS is not available. Further improvements in sensitivity, for example, by optimizing film thickness, deposition conditions, or ligand loading, could extend the applicability toward lower concentration levels. The combination of a structurally defined pyranochromene ligand, a Nafion matrix, and a well-characterized electrochemical mechanism distinguishes this system from many empirical modifier-based sensors. In this context, the present work can be viewed as a step toward rational design of coordination-based electrochemical chemosensors, in which ligand architecture and polymer environment are used deliberately to tune preconcentration and stripping behavior.

These results demonstrate that the ACLPh-PC-3-CN/Nafion-modified electrode can provide reliable determinations in such matrices, although its current LOD (0.60 μM) limits direct application at very low Cd(II) levels close to guideline concentrations. For situations where Cd(II) occurs nearer to these low background levels, it would be desirable to investigate lower concentration ranges, examine matrix effects in more detail, and extend the study to a broader set of environmental samples (for example, groundwater, river water, and industrial effluents), potentially in combination with a simple off-line pre-concentration step to access lower effective concentrations.

Furthermore, it should also be noted that the present detection limit of 0.60 μM is higher than the maximum Cd²⁺ levels recommended for drinking water (≈3 μg L⁻¹, ~0.027 μM) and therefore does not yet allow direct trace-level compliance monitoring of pristine waters. However, in many real polluted systems, Cd²⁺ concentrations can substantially exceed this guideline value, so the current sensor is still useful for screening and monitoring moderately-to-highly contaminated waters and industrial effluents. Where lower concentration ranges are targeted, the method could be combined with a simple off-line pre-concentration step (for example, sorbent-based or evaporative pre-concentration prior to voltammetric measurement) and with further optimization of the ACLPh-PC-3-CN/Nafion film (thickness and ligand loading), deposition conditions, and electrode surface area. These strategies are expected to extend the operational range toward lower Cd²⁺ levels in future work while preserving the selectivity provided by the pyranochromene ligand.

5. Ligand–Cd(II) Binding Studies

5.1. Evidence for 1:1 Stoichiometry

The electrochemical and spectroscopic observations together support the formation of a 1:1 complex between Cd(II) and ACLPh-PC-3-CN under the conditions used. The presence of a single dominant stripping peak and the estimated surface coverage are consistent with the binding of individual Cd²⁺ ions to ligand sites arranged at the electrode surface. The tridentate N/O donor set of ACLPh-PC-3-CN is similar to that of other ligands reported to form 1:1 complexes with Cd²⁺ in solution and in solid-state coordination compounds.

5.2. Job’s Plot Analysis

The stoichiometry of the Cd(II)–ACLPh-PC-3-CN complex in solution was further examined using the method of continuous variations (Job’s method), which is widely employed to determine the composition of metal–ligand complexes. In the present case, solutions were prepared at constant total concentration ([Cd(II)] + [ligand] = 40 μmol L⁻¹) while varying the mole fraction of Cd(II), and the absorbance at 415 nm was monitored. The Job’s plot of X_Cd·A versus X_Cd (Figure 6) exhibits a maximum at a Cd(II) mole fraction of approximately 0.5. According to the usual interpretation of Job’s method, a maximum at X_Cd ≈ 0.5 indicates that the predominant species in solution has a 1:1 metal–ligand stoichiometry. This finding is therefore consistent with the design of ACLPh-PC-3-CN as a tridentate ligand that coordinates a single Cd(II) center and supports the mechanistic assumption of a 1:1 Cd(II)–ligand interaction at the electrode interface.

Additionally, taken together, the presence of a single tridentate N/O donor environment in ACLPh-PC-3-CN and the Job’s plot maximum at X_Cd ≈ 0.5 indicate that the predominant species formed under the experimental conditions is a 1:1 Cd(II)–ACLPh-PC-3-CN complex. This conclusion is in line with the most commonly observed 1:1 stoichiometry in metal–ligand systems characterized by continuous-variation methods and similar ligand architectures.

5.3. Limitations of the Present Sensor

Although the ACLPh-PC-3-CN/Nafion-modified electrode shows good precision and acceptable linearity in the range 16.21–56.72 μmol L⁻¹, several practical limitations should be noted. The method still requires laboratory-type sample handling (filtration, pH adjustment to 6, and standard additions), which may restrict direct on-site use. The Cd²⁺ response is pH-dependent and optimized around pH 6; significant deviations from this value can decrease the signal due to protonation of donor sites or hydrolysis of Cd(II). While the film retains about 94% of its initial response after 14 days and shows low RSD values, long-term stability or operation under strongly oxidizing conditions or at extreme pH values has not been evaluated and may require further optimization. In addition, although the sensor tolerates large excesses of common inorganic ions, more complex environmental matrices containing organic matter, surfactants, or other chelating agents could still influence Cd²⁺ accumulation and may necessitate matrix-matched calibration. Finally, the current detection limit (0.60 μmol L⁻¹, ≈67 μg L⁻¹) exceeds the WHO guideline level for Cd in drinking water, so the present configuration is better suited for screening moderately contaminated waters or laboratory monitoring than for direct compliance testing at regulatory limits.

6. Conclusions

In this work, the pyrano[3,2-c]chromene ligand ACLPh-PC-3-CN was co-immobilized with Nafion on a glassy carbon electrode and employed as a surface-confined chelating element for the electrochemical determination of Cd²⁺ in aqueous samples. Under optimized DP-ASV conditions at pH 6.0, the ACLPh-PC-3-CN/Nafion-modified electrode exhibited a linear relationship between peak current and Cd²⁺ concentration in the range 16.21–56.72 µmol L⁻¹, with a limit of detection of 0.60 µmol L⁻¹, a limit of quantification of 2.0 µmol L⁻¹, and low RSD values for repeatability and reproducibility. The response characteristics, together with the scan-rate dependence of the peak current, surface coverage estimates, and the predominantly 1:1 Cd²⁺–ligand stoichiometry, support a mechanism in which Cd²⁺ is accumulated via surface complexation by ACLPh-PC-3-CN within the Nafion matrix, reduced to a surface-bound metallic state during the deposition step, and subsequently oxidatively stripped during the voltammetric scan. The ACLPh-PC-3-CN/Nafion film showed adequate short-term operational stability, retaining about 94% of its initial stripping current after 14 days, and maintained the Cd²⁺ peak current within ±5% in the presence of large excesses of common coexisting ions and surfactant species, although more complex matrices may still require matrix-matched calibration. While the current LOD (≈67 µg L⁻¹ Cd²⁺) exceeds typical guideline values for drinking water and thus limits direct compliance monitoring at regulatory levels, the sensor is suitable for screening moderately contaminated waters and for applications where simple, low-cost electrochemical measurements are preferred over more sophisticated techniques. Overall, these findings demonstrate that combining a structurally defined pyranochromene ligand with a Nafion matrix offers a viable route to coordination-based electrochemical chemosensors for Cd²⁺, and the general design strategy could be extended to other metal ions through appropriate ligand modification and optimization of the sensing layer.