L-phenylalanine (L-Phe, $\ge$98%), cadmium sulfate (CdSO₄, $\ge$99.9%), potassium ferricyanide (K₃[Fe(CN)₆]), potassium ferrocyanide (K₄[Fe(CN)₆]), potassium chloride (KCl, $\ge$99%), and other metal salts were purchased from Sigma-Aldrich (Shanghai, China) or Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl, 36%~38%), sulfuric acid (H₂SO₄, 98%), and ethanol were obtained from Kermel Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were analytical grade and used as received. Alumina polishing powders (1.0, 0.3, and 0.05 µm) were from Chenhua Instrument Co., Ltd. (Shanghai, China). All solutions were prepared using ultrapure water (Milli-Q, resistivity $\ge$18.2 M$\mathrm{\Omega }$$\cdot$cm).

Electrochemical measurements were performed with a CHI 660E workstation (CH Instruments, Shanghai, China). A conventional three-electrode system was used: a bare or modified glassy carbon electrode (GCE, 3 mm diameter) as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE, 0.241 V vs. SHE) as the reference. All potentials are reported vs. SCE. pH measurements were made with a PHSJ-3F pH meter (INESA Scientific Instrument Co., Ltd., Shanghai, China).

The pretreatment procedure involved successively polishing the bare GCE with aqueous alumina slurries of decreasing particle size (1.0, 0.3, and 0.05 µm) on a micro-cloth pad until a mirror finish was attained. It was then subjected to ultrasonic cleaning in ethanol and ultra-pure water for 2 minutes each to remove any adsorbed alumina particles. Electrochemical activation was performed in 0.5 M H₂SO₄ by cycling from –1.0 to +1.0 V until a stable baseline was obtained. Electropolymerization of L-Phe was conducted from a 2 mM solution (1 mM HCl + 0.1 M KCl) by applying 8 cyclic voltammetry cycles between –1.0 and +1.0 V at 100 mV s^-1. The formation of a light-yellow film indicated polymerization. The modified PPhe/GCE was then thoroughly rinsed with water, air-dried, and subsequently employed for electrochemical measurements.

The electropolymerization of L-Phe proceeds via electrochemical oxidation of the amine group, generating radical intermediates that couple to form a polymeric network, similar to mechanisms reported for other aromatic amines [31]. HCl provides a protonic medium to facilitate polymerization, while KCl acts as the supporting electrolyte for ionic conductivity.

For electrochemical characterization, the modified electrodes were subjected to cyclic voltammetry (CV) analysis in a solution containing a 5.0 mM 1:1 mixture of K₃[Fe(CN)₆] and K₄[Fe(CN)₆], with 0.1 M KCl as the supporting electrolyte. For the detection of Cd²⁺, differential pulse anodic stripping voltammetry (DPASV) was employed. The analysis followed a standard two-step procedure: (1) Pre-concentration/deposition: the PPhe/GCE was immersed in the sample solution, and a deposition potential of –1.0 V was applied for 120 s under stirred conditions. This step reduces Cd²⁺ ions and accumulates elemental Cd (0) onto the electrode surface. (2) Stripping scan: after a 10 s quiet time without stirring, the stripping step was carried out by applying a differential pulse voltammetry (DPV) scan from –1.0 V to –0.6 V, employing a pulse amplitude of 50 mV, a pulse width of 50 ms, and a potential step of 5 mV. Quantification was based on the current of the anodic stripping peak (observed around –0.8 V), which signifies the oxidation of Cd (0) to Cd²⁺.

Water samples were collected from Luming Lake (Xuchang, Henan, China), filtered through a 0.45 µm membrane. Acidification with 0.01 M HCl converts Cd²⁺ into its free, electroactive form by dissociating complexes with natural organic matter. Dilution (four-fold) reduces ionic strength and mitigates matrix adsorption on the electrode surface, which is a common practice to ensure reliable quantification in complex environmental samples. Cd²⁺ was determined by the standard addition method using DPASV under optimized conditions.

The electrochemical behavior of bare GCE and PPhe/GCE was first examined using CV in 5 mM [Fe(CN)₆]³⁻^/4⁻ containing 0.1 M KCl. As depicted in Fig. 1a, both electrodes exhibited well-defined reversible redox peaks. The peak currents at PPhe/GCE were approximately 1.5 times higher than those at bare GCE. This marked augmentation of the peak current unequivocally indicates that the electropolymerized PPhe film effectively enhances the electrode performance. The electroactive area, calculated via the Randles–Ševčík equation, was ~1.4 times larger than that of the bare GCE (Supplementary Fig. 1).

Fig. 1.

CV characterization of bare GCE and PPhe/GCE in [Fe(CN)₆]^3-/^4- and Cd²⁺ solutions. (a) CVs of bare GCE and PPhe/GCE in 5 mM [Fe(CN)₆]^3-/^4- (0.1 M KCl). (b) CVs of the same electrodes in 100 µM Cd²⁺ (0.1 M KCl, 0.01 M HCl). (c) CVs of PPhe/GCE in 100 µM Cd²⁺ at different scan rates. (d) Linear relationship between anodic peak current (i) and the square root of scan rate (v^1/2). CV, cyclic voltammetry; PPhe/GCE, L-phenylalanine on a glassy carbon electrode; KCl, potassium chloride; HCl, Hydrochloric acid; Cd²⁺, cadmium ions.

CV responses in 100 µM Cd²⁺ solution (0.1 M KCl, 0.01 M HCl) are presented in Fig. 1b. A distinct anodic stripping peak appeared around –0.8 V for both electrodes. The peak current at PPhe/GCE was approximately double that at bare GCE.

The effect of scan rate on the stripping peak at PPhe/GCE was investigated (Fig. 1c). The anodic peak current increased with increasing scan rate. A linear relationship was observed between the peak current (i) and the square root of scan rate (v^1/2) (Fig. 1d), yielding the regression Eqn. 1 with a correlation coefficient of 0.9905.

(1)$$i (\mu \mathrm{A})=2.0517 v^{1/2} – 2.0424$$

In-situ monitoring of the electropolymerization process (Supplementary Fig. 2) showed a characteristic current decay with increasing scan number, indicating progressive film growth. Comparison with other amino-acid-based polymers (Supplementary Fig. 3) revealed that the Cd²⁺ stripping peak current at PPhe/GCE was approximately 1.3 and 1.7 times higher than that at polyarginine (PArg) and polyglutamate (PGA) modified GCEs, respectively.

Key experimental parameters affecting the sensor’s performance were systematically optimized using DPASV for 100 µM Cd²⁺. All optimization experiments (Supplementary Fig. 4) were performed in triplicate (n = 3).

The electropolymerization potential window was evaluated (Fig. 2a). The highest oxidation peak current (74.07 µA) was obtained with the window of –1.0 to +1.0 V, compared to 37.52 µA for –0.9 to +1.0 V and 27.27 µA for –1.0 to +0.9 V.

Fig. 2.

Optimization of detection parameters for 100 µM Cd²⁺ at PPhe/GCE. Optimization of detection conditions for 100 µM Cd²⁺ at PPhe/GCE: (a) Electropolymerization potential window. (b) Number of polymerization cycles. (c) HCl concentration. (d) Supporting electrolyte (0.1 M). (e) Deposition potential. (f) Deposition time.

The number of polymerization cycles was optimized (Fig. 2b). The peak current increased with cycle number up to 8 cycles (74.09 µA) and then decreased upon further cycling (12 and 16 cycles).

The effect of HCl concentration (0.005–0.04 M) is shown in Fig. 2c. The current increased from 0.005 M to 0.01 M (74.06 µA) and then declined at higher concentrations.

The Different supporting electrolytes (0.1 M) were tested in 0.01 M HCl (Fig. 2d). KCl yielded the highest oxidation peak current (74.10 µA), outperforming NaCl, K₂SO₄, and Na₂SO₄ by factors of approximately 1.2, 1.4, and 1.8, respectively.

Deposition potential was varied from –0.8 V to –1.0 V (Fig. 2e). The peak current increased with more negative potential, reaching 85.83 µA at –1.0 V, which was 1.5 times higher than at –0.9 V and 1.9 times higher than at –0.8 V.

Deposition time was investigated from 60 to 150 s (Fig. 2f). The current increased up to 120 s (85.81 µA) and then slightly decreased at 150 s.

Under the optimized conditions, DPASV responses for Cd²⁺ at various concentrations were recorded (Fig. 3a). Well-defined stripping peaks increased with Cd²⁺ concentration. The calibration plot (Fig. 3b) exhibited a linear relationship from 0.50 to 500 µM, with a regression Eqn. 2 and a correlation coefficient (R) of 0.9962.

Fig. 3.

Analytical performance of PPhe/GCE for Cd²⁺ detection. (a) Differential pulse anodic stripping voltammetry (DPASV) curves for increasing concentrations of Cd²⁺ (0.50, 1, 2, 5, 10, 20, 40, 60, 80, 100, 200,300, 400, 500, 600, 700 and 800 µM) at the PPhe/GCE in 0.1 M KCl and 0.01 M HCl solution. (b) Corresponding calibration plot of peak current versus Cd²⁺ concentration.

(2)$$i (\mu \mathrm{A})=0.6020 c (\mu \mathrm{M})+1.0930$$

The limit of detection (LOD) was calculated to be 0.115 µM based on 3N/S (where N is the standard deviation of the blank, n = 12, and S is the slope of the calibration line). A comparison of the analytical performance of PPhe/GCE with other reported electrodes for Cd²⁺ detection [32, 33, 34, 35, 36, 37] is summarized in Table 1.

The selectivity of PPhe/GCE was evaluated by adding a five-fold excess (500 µM) of interfering ions (Ni²⁺, Mn²⁺, Mg²⁺, Fe³⁺) to 100 µM Cd²⁺. As shown in Fig. 4a,b, the signal changes were within $\pm$5%.

Fig. 4.

Selectivity and reproducibility of PPhe/GCE. (a) DPASV curves and (b) current histogram for 100 µM Cd²⁺ with a 5-fold excess of interfering ions. (c) DPASV responses and (d) current histogram for 100 µM Cd²⁺ obtained with five independently prepared PPhe/GCEs.

Reproducibility was assessed using five independently prepared PPhe/GCEs. The DPASV responses and corresponding current histogram are shown in Fig. 4c,d, yielding a relative standard deviation (RSD) of 2.4% (n = 5). Repeatability tested with a single electrode over 20 consecutive scans gave an RSD 5%. Long-term stability was examined by storing the electrode at 4 °C for 21 days, the sensor retained $>$90% of its initial response.

The practical applicability of PPhe/GCE was tested by analyzing Cd²⁺ in lake water samples. No Cd²⁺ peak was detected in the original unspiked sample. Recovery tests were performed by spiking the samples with 5, 50, and 200 µM Cd²⁺. As presented in Table 2, the recoveries ranged from 96.2% to 102.2%, with RSD values below 1.5% (n = 3).

The results demonstrate that the PPhe-modified electrode significantly enhances the electrochemical response toward Cd²⁺. The increased peak currents in both [Fe(CN)₆]³⁻^/4⁻ and Cd²⁺ solutions (Fig. 1a,b) indicate that the electropolymerized PPhe film effectively enlarges the electroactive surface area and facilitates electron transfer. The ~1.4–fold increase in electroactive area and the ~2–fold enhancement in Cd²⁺ stripping current suggest that the film provides abundant active sites for analyte accumulation.

The superior performance of PPhe/GCE can be attributed to the synergistic combination of multiple functional groups within the polymer film. The amino (–NH₂) and carboxyl (–COOH) groups along the polymer backbone serve as excellent ligands for Cd²⁺ coordination [28], enabling effective preconcentration of the analyte onto the electrode surface. Additionally, the aromatic benzene rings may contribute through $\pi$–cation interactions [29], further enhancing the accumulation efficiency. This dual–mode interaction is supported by the comparative study with other amino–acid–based polymers (Supplementary Fig. 3), where PPhe outperformed polyarginine and polyglutamate–polymers lacking aromatic moieties–by factors of 1.3 and 1.7, respectively. The characteristic current decay observed during electropolymerization (Supplementary Fig. 2) confirms the formation of polymeric film growth.

The linear relationship between peak current and the square root of scan rate (Fig. 1d) confirms that the electrode process is diffusion–controlled [38], a typical characteristic of stripping analysis. This finding indicates that the PPhe film, while offering strong preconcentration capability, does not impede the mass transport of Cd²⁺ from the bulk solution to the electrode interface.

The optimization results (Fig. 2) provide insight into the factors governing sensor performance. The optimal electropolymerization window of –1.0 to +1.0 V (Fig. 2a) ensures complete polymerization and the formation of a conductive film with a high density of accessible binding sites. Restricted windows may yield incomplete polymerization or less favorable film morphologies, compromising electron transfer and analyte accumulation.

The dependence on polymerization cycles (Fig. 2b) reflects a trade-off between film thickness and accessibility. With too few cycles (5 cycles), surface coverage is insufficient, limiting the number of active sites. Conversely, excessive cycling (12 or 16 cycles) produces an overly thick film that may hinder charge transfer and Cd²⁺ diffusion to the electrode surface. 8 cycles provide an optimal balance, maximizing available binding sites while maintaining efficient electron transfer.

The presence of acid in the electrolyte may influence the doping state and conductivity of the polyphenylalanine film (Fig. 2c), as acidic media are known to affect the electrochemical activity of conducting polymers. At the optimal concentration of 0.01 M HCl, the film likely exhibits enhanced conductivity, facilitating electron transfer during the stripping step. Higher acidities may alter the film structure or lead to competing reactions that diminish the signal.

Among supporting electrolytes, KCl yielded the highest response (Fig. 2d). This is consistent with the high mobility of K⁺ ions and the ability of Cl^– to form soluble chlorocomplexes (e.g., CdCl₂) during the stripping step, which sharpens the voltammetric peaks. Sulfate electrolytes may form less soluble cadmium species or alter the double-layer structure, impairing stripping efficiency.

The increase in stripping current with more negative deposition potential (Fig. 2e) reflects enhanced reduction of Cd²⁺. The optimal potential of –1.0 V provides sufficient overpotential for efficient deposition while avoiding hydrogen evolution, which could destabilize the electrode surface. The deposition time of 120 s (Fig. 2f) allows adequate preconcentration without reaching surface saturation, which would lead to a signal plateau or decline due to the formation of a resistive cadmium layer.

The PPhe/GCE sensor exhibits a wide linear range (0.50–500 µM) and a low detection limit (0.115 µM) for Cd²⁺ (Fig. 3). As shown in Table 1, these analytical figures are competitive with or superior to many previously reported Cd²⁺ sensors [32, 33, 34, 35, 36, 37]. Notably, the detection limit is lower than that of sensors based on carbon composites and comparable to more complex systems.

In addition, the fabrication process of PPhe/GCE is simpler and more environmentally friendly than many alternative approaches. Unlike sensors requiring multi-step synthesis of nanomaterials, the one-step electropolymerization of L-phenylalanine is both time-efficient and green. This combination of high performance and simple fabrication underscores the promise of amino-acid-derived polymers as advanced sensing materials.

The excellent selectivity of PPhe/GCE against common interfering ions (signal changes $\pm$5%) suggests that the PPhe film offers specific recognition of Cd²⁺, likely due to the preferred coordination geometry of its functional groups with Cd²⁺ compared to other metal ions. The high reproducibility (RSD 2.4%) and repeatability (RSD 5%) confirm the robustness and consistency of the fabrication method (Fig. 4). The long-term stability (21 days, signal $>$90%) and regeneration capability further enhance its practicality for routine use.

The successful determination of Cd²⁺ in spiked lake water samples with recoveries of 96.2%–102.2% and low RSDs (Table 2) demonstrates that the sensor performs reliably even in complex environmental matrices. These results validate its potential for real-world environmental monitoring applications.

The present work primarily focuses on demonstrating the analytical utility and performance of the PPhe-based electrochemical platform for cadmium detection. While the sensor exhibits promising characteristics, a few aspects may be considered in subsequent studies to further support its development. For instance, future work could explore the fine structural features of the polymer film through advanced characterization techniques.