Preparation and application of a new ion-imprinted polymer for nanomolar detection of mercury(II) in environmental waters

Abstract

This study introduces a novel ion-imprinted polymer for the ultrasensitive detection of mercury(II) in water. The ion-imprinted polymer was synthesized via a simple bulk polymerization process using methacrylic acid as the functional monomer, ethylene glycol dimethacrylate as the cross-linker, morpholine-4-carbodithioic acid phenyl ester as the chelating agent, and ammonium persulfate as the initiator. The electrochemical mercury(II) sensing capability of the ion-imprinted polymer was studied via the modification of a cost-effective carbon paste electrode. A stripping voltammetric technique was utilized to quantify the analyte ions following open-circuit enrichment. Critical experimental parameters, including the nature and concentration of the eluent, solution pH, preconcentration duration, ion-imprinted polymer dosage, sample solution volume and reduction potential, were systematically studied and optimized. Under optimal conditions, the sensor exhibited a linear response in the range of 1.0 to 240.0 nM, with a low detection limit of 0.2 nM. The sensor demonstrated remarkable selectivity against potential interfering ions, including lead(II), cadmium(II), copper(II), zinc(II), manganese(II), iron(II), magnesium(II), calcium(II), sodium(I) and cobalt(II). The practical applicability of the developed method was successfully validated through the analysis of real water samples, suggesting its potential for environmental monitoring applications.

Introduction

Heavy metal pollution in water is a pervasive environmental issue¹. Among different heavy metals, mercury (Hg) stands out as particularly hazardous in aquatic ecosystems and shows significant toxicity even at low concentrations². Hg exposure can lead to severe health consequences, including respiratory failure, central nervous system issues, and kidney damage³. Hg contamination of environmental waters is a serious problem, resulting from a variety of human activities including agriculture and industry. The release of Hg into the environment through these activities can lead to its accumulation in aquatic ecosystems, where it can have devastating effects on both human health and the environment⁴. On the basis of these points, the US Environmental Protection Agency (EPA) has established guidelines for the maximum allowable concentration of Hg in water, setting a limit of 1 × 10^− 8 M for drinking water⁵. Therefore, screening trace Hg levels in water resources is an important task. Hg(II) contamination levels have been determined via different analytical methods, such as cold vapor atomic absorption spectrometry (CV-AAS)⁶, electrothermal atomic absorption spectrometry (ETAAS)⁷, inductively coupled plasma mass spectrometry (ICP-MS)⁸, spectrofluorimetry⁹, atomic fluorescence spectroscopy (AFS)¹⁰, and neutron activation analysis (NAA)¹¹. Despite their effectiveness, the aforementioned methods are often limited by their time-consuming nature and the need for expensive equipment and complex sample preparation. Electrochemical techniques, however, present a promising alternative, offering a unique combination of simplicity, cost-effectiveness, and remarkable sensitivity^12,13. Electrochemical strategies for Hg(II) analysis offer a range of advantages, including the ability to determine Hg(II) at trace levels, fast analysis time, and potential for real-time measurement^14,15. These advantages are particularly significant in the context of environmental monitoring, where the ability to rapidly and accurately detect mercury contamination is crucial. Among different electrochemical methods, the anodic stripping voltammetry (ASV) technique has proven to be particularly effective for monitoring Hg(II) at trace levels¹⁶. The detection of Hg(II) via ASV has recently been researched through the application of various carbon electrodes, such as screen-printed carbon electrodes (SPCEs)¹⁷, glassy carbon electrodes (GCEs)¹⁸ and carbon paste electrodes (CPEs)¹⁹. CPEs, in particular, have been extensively utilized because of their excellent benefits including low cost, ease of operation, and ability to be easily renewed, as well as their wide potential window, stable response, low background current, and potential for bulk modification and miniaturization²⁰. The modification of CPEs with specific materials can significantly enhance their sensing performance, particularly in terms of the detection limit and sensitivity^21,22.

To date various modifiers have been employed for bulk modification of CPEs for the electrochemical monitoring of inorganic ions, biologically important molecules, and pharmaceuticals²³. In recent years, ion imprinting strategies have been widely used for the fabrication of effective heavy metal electrochemical sensors²⁴. Ion imprinting technology has emerged as a promising approach for the creation of highly selective ion-imprinted polymers (IIPs) that are capable of preconcentrating target ions with high selectivity^25,26. In general, IIPs are synthesized through the creation of specific recognition centers within synthetic polymers, which are typically achieved through bulk polymerization^27,28. The target metal ions are then eliminated from the polymeric networks by leaching with mineral acid, resulting in imprinted sites that are tailored to the shape and size of the target ion²⁹. Considering these points, the incorporation of IIPs into CPEs has been recognized as a potent tool for enhancing the performance of electrochemical sensors for heavy metal monitoring^30–32.

In this investigation, a novel Hg(II)-IIP material was prepared via bulk polymerization, utilizing methacrylic acid (MAA) as the functional monomer, ethylene glycol dimethacrylate (EGDMA) as the cross-linker, morpholine-4-carbodithioic acid phenyl ester (MCP) as the chelating agent, and ammonium persulfate (APS) as the initiator. The synthesized polymer was then employed to modify a simple CPE, which was subsequently used for the electrochemical analysis of Hg(II) via an open-circuit procedure. The results of this study offer a valuable tool for detecting toxic Hg(II).

Experimental

Reagents and instruments

Chemicals including MAA, EGDMA, APS, paraffin oil, graphite powder, dimethylsulfoxide (DMSO), mercury(II) nitrate monohydrate (Hg(NO₃)₂⋅H₂O and acetonitrile (AN) were supplied by Merck (Darmstadt, Germany). The MCP ligand was synthesized in our laboratory according to a previously reported method³³. Stock solutions of various heavy metals were prepared using nitrate salts. All other chemicals used were of analytical grade and purchased from Merck (Darmstadt, Germany).

Voltammetric experiments were conducted via a Metrohm 757 Voltammetric Analyzer (VA) Computrac polarograph (Metrohm, Herisau, Switzerland) in a three-electrode configuration. Before each measurement, the test solutions were degassed via nitrogen (N₂) steam for 5 min. Fourier transform infrared (FT-IR) data were obtained via a Bruker Alpha spectrometer (Bruker Optics, Billerica, MA, USA) with a wavenumber range of 400–4000 cm^− 1. The morphology of the synthesized polymer was evaluated via a Hitachi S-4800 scanning electron microscope (SEM, Hitachi High-Technologies, Tokyo, Japan). The thermal stability of the polymer was assessed via a Perkin-Elmer TGA instrument (model: STA1640, Waltham, USA) under an inert atmosphere. The Brunauer‒Emmett–Teller (BET) adsorption technique was employed to analyze the surface areas of the prepared materials via a Belsorp-Mini II (Microtrac Bel, Tokyo, Japan) adsorption analyzer.

Synthesis of hg(II)-IIP

The synthesis of Hg(II)-IIP was carried out via a bulk polymerization protocol³⁴. To initiate the complex formation process, a solution was prepared by dissolving 1.0 mmol of Hg(NO₃)₂⋅H₂O and 2.0 mmol of the MCP ligand in a 50.0 mL mixture of AN: DMSO with a volume ratio of 4:1. The solution was then stirred at room temperature for 1 h to ensure the formation of the Hg(II)-MCP complex. Subsequently, 4.0 mmol of MAA, 40.0 mmol of EGDMA, and 0.2 mmol of APS were added to the solution. The mixture was then stirred briefly to ensure the uniform distribution of the polymerization components. To prevent the interference of dissolved oxygen (O₂) in the polymerization process, the mixture was purged with a N₂ atmosphere for 10 min. Subsequently, the mixture was heated in a water bath at 60 °C for 24 h to complete the polymerization reaction. After polymerization, the excess solvent was removed via rotary evaporation. The resulting polymer was then ground into a powder and thoroughly rinsed with ethanol repeatedly to remove any residual substances. The removal of entrapped Hg(II) ions from the polymer matrix was carried out through acid treatment (HCl, 2.0 M). The ETAAS analysis revealed that three times washing with acidic solutions (50.0 mL) were necessary to achieve complete removal of the Hg(II) ions. After template removal, the polymer was thoroughly washed with deionized water until a neutral pH was achieved. Finally, the rustling IIP product was dried overnight at 60 °C. For comparative purposes, a non-imprinted polymer (NIP) was also synthesized via the same procedure, but the addition of Hg(II) during the polymerization step was omitted. A representative route for the preparation of the Hg(II)-IIP modifier is presented in Fig. 1.

Fig. 1.

Schematic representation of the fabrication of Hg(II)-IIP.

Manufacturing of imprinted sensor

The preparation of the modified carbon paste involved thorough dry-mixing of 67 mg of graphite powder and 8 mg of Hg(II)-IIP. A total of, 25 mg of mineral oil was subsequently added to the mixture and then homogenized for 30 min to achieve a uniform paste. The achieved paste was packed into a glass tube with an inner diameter of 2 mm, ensuring the absence of air gaps. The electrical contact was established via a copper wire. The surface of the electrode was also smoothed with weighing paper before each electrochemical test.

Hg(II) detection procedure

The Hg(II) detection procedure involved a preconcentration step followed by electrochemical analysis, as detailed below. The preconcentration of Hg(II) ions was achieved by immersing the Hg-IIP modified CPE in 20.0 mL of a sample solution (pH 4.0) containing a known concentration of Hg(II). The solution was stirred at 200 rpm for 8 min to ensure the efficient adsorption of Hg(II) ions onto the modified CPE surface. After preconcentration, the electrode was immersed in deionized water for 10 s to eliminate any loosely bound or unabsorbed ions. The electrode was then transferred to a voltammetric cell containing 20.0 mL of 0.1 M HCl as the supporting electrolyte for subsequent electrochemical analysis. Hg(II) was detected via differential pulse anodic stripping voltammetry (DPASV). Initially, the enriched Hg(II) ions on the electrode surface were reduced at a potential of -0.5 V for 30 s. This reduction step ensured the conversion of Hg(II) to elemental mercury (Hg⁰), which was then reoxidized during the anodic stripping phase, providing the analytical signal. The electrochemical measurement of Hg(II) was conducted by screening the DPASV responses in the potential range of -0.3 V to + 0.4 V at room temperature (23 ± 2 °C).

Sample collection

Water samples including dam (Chalidarreh dam lake, Mashhad, Iran), aqueduct (Ardiz, Sabzevar, Iran), tap (Sabzevar drinking water, Sabzevar, Iran), and river (Kardeh River, Mashhad, Iran) samples, were collected from various sources and stored in clean PET bottles to prevent contamination. The samples were filtered through 0.45 μm (Whatman ^® GDX) membrane filters to remove the suspended particles. The filtrate solutions were then acidified to a pH of 4.0 using 0.1 M HCl to increase the stability of the Hg(II) in the solution. To analyze the real samples, known amounts of Hg(II) standards were spiked into separate aliquots of the water samples at three different concentrations (10.0, 25.0, and 50.0 nM) and each concentration was analyzed in five replicates to ensure the reproducibility of the analyses.

Results and discussion

Polymer characterization

The morphological structures of the imprinted materials before and after treatment with HCl are depicted in Fig. 2A and B, respectively. SEM analysis of the leached Hg(II)-IIP sample revealed a porous surface morphology after the removal of the template ions. Furthermore, the removal of Hg(II) ions can result in the formation of nanosized cavities within the polymer matrix. These cavities are crucial for capturing Hg(II) ions during the preconcentration process because of their high surface area.

Fig. 2.

SEM images of the Hg(II)-IIP sample (A) before and (B) after Hg(II) leaching.

FT-IR analysis (Fig. 3A) confirmed the presence of various functional groups within Hg(II)-IIP before and after acid treatment. The FT-IR spectra show characteristic peaks corresponding to carboxylic acid groups (C = O stretch at 1722 cm⁻¹), C-H stretches (2900–3100 cm⁻¹), and hydroxyl groups (O-H stretch at 3450 cm⁻¹). The peaks at 1254 and 1434 cm^− 1 are attributed to C-O and C = C stretching bonds, respectively. Additionally, peaks associated with C = S and C-N stretches (approximately 1134 and 1639 cm⁻¹, respectively) are implicated in the interaction with Hg(II) ions. Notably, shifts in the C = S and C-N stretching frequencies are observed after Hg(II) leaching, suggesting the successful creation of selective recognition sites for target ions during the imprinting process³⁵.

Fig. 3.

(A) FT-IR spectra and (B) TGA data for the Hg(II)-IIP sample (a) before and (b) after removal of the template ion.

TGA was used to explore the thermal stability of the imprinted samples. As shown in Fig. 3B, the significant weight loss between 390 °C and 480 °C is attributed to the degradation of the polymer network. Notably, the leached IIP sample exhibited a significantly greater weight loss (approximately 100%) than the unleached IIP sample because of the absence of target ions in the polymer particles. These findings support the successful preparation of Hg(II)-IIP and subsequent elution of Hg(II) ions from the imprinted polymer particles.

The N₂ adsorption‒desorption isotherms for IIP before and after Hg(II) leaching are presented in Fig. 4. Both samples exhibit Type IV isotherms according to the IUPAC classification, indicating a mesoporous structure³⁶. The leached IIPs demonstrate greater N₂ uptake compared to the unleached sample, indicating a marked increase in porosity and surface area. This enhancement can be attributed to the removal of template ions during the leaching process, which unblocks the imprinted cavities and creates additional accessible pore volume³⁷. The surface area and pore size analysis of the IIP material before and after Hg(II) leaching are given in Table 1. These results are consistent with the findings obtained from SEM analysis.

Fig. 4.

N₂ adsorption and desorption isotherms for the IIP material before and after Hg(II) leaching.

Table 1.

Surface properties of the IIP material before and after hg(II) leaching.

Parameter	Units	Before leaching	After leaching
BET surface area	m2 g− 1	29.5	48.4
Average pore diameter	nm	11.22	7.54

Adsorption capacity

Adsorption experiments of Hg(II) on the Hg(II)-IIP and NIP samples were conducted in aqueous solutions with various initial concentrations (ranging from 10 to 200 mg/L) at pH 4.0. As shown in Fig. 5, the amount of Hg(II) adsorbed per unit mass of sorbent increases with increasing initial Hg(II) concentration for both the Hg(II)-IIP and NIP samples. The adsorption capacities of the Hg(II)-IIP and NIP samples were found to be 29.4 and 17.3 mg/L, respectively, for three replicate experiments. The higher adsorption capacity of Hg(II)-IIP can be attributed to the presence of many imprinted centers, which have a greater affinity for Hg(II) ions.

Fig. 5.

Hg(II) adsorption capacity of (a) Hg(II)-IIP and (b) NIP samples.

Electrochemical characterization

The cyclic voltammetry (CV) technique was utilized to verify the complete removal of the template Hg(II) ions from the Hg(II)-IIP. Figure 6A presents the CV responses of the leached Hg(II)-IIP modified CPE (curve a) and the untreated imprinted Hg(II)-IIP modified electrode (curve b). The voltammogram for the leached Hg(II)-IIP modified electrode demonstrated the complete disappearance of the characteristic peaks of Hg(II) after the stripping process. This finding indicates that washing the IIP material with HCl effectively and completely eliminates the Hg(II) ions from the modifier. Furthermore, no electrochemical oxidation or reduction peaks were observed in the potential range from − 0.6 V to + 0.6 V during the electrochemical experiments. This confirms the operational stability of the prepared polymer within the studied potential window.

Fig. 6.

(A) CV responses of imprinted sensors (a) after and (b) before removal of the template ion. (B) DPASV voltammograms of (a) bare CPE, (b) NIP-modified CPE, (c) MCP-modified CPE and (d) Hg(II)-IIP modified CPE after preconcentration with 75.0 nM Hg(II).

The electrochemical responses of the various modified CPEs toward Hg(II) were also examined via the DPASV method. Bare CPE, MCP modified CPE, NIP modified CPE and Hg(II)-IIP modified CPE were selected to study the performance of the electrodes for Hg(II) detection in open-circuit mode. DPASV voltammograms of various electrodes toward 75.0 nM Hg(II) are shown in Fig. 6B. The unmodified electrode (bare CPE, curve a) exhibited no response toward the target ion at such trace concentration. The stripping response at the Hg(II)-IIP modified CPE (curve d) exhibited a substantial increase in peak current, with values approximately 3 and 6 times greater than those observed at the MCP-modified CPE (curve c) and NIP-modified CPE (curve b), respectively. The notable increase in the stripping signal at the Hg(II)-IIP modified CPE can be attributed to the high affinity of imprinted modifier for Hg(II) ions, which facilitates efficient binding and detection.

Possible sensing mechanism

The possible pathways for Hg(II) analysis via the MCP-modified CPE can be described as follows;

1. Preconcentration step in open-circuit mode via complex formation between Hg(II) ions and the MCP ligand in the IIP network:

   1

2. Reduction step at a potential of -0.5 V:

   2

3. Stripping step via DPASV scan from − 0.3 V to + 0.4 V:

   3

   where the resulting oxidation peak constitutes the analytical signal.

Factors affecting sensor performance

To achieve the best sensor performance, a systematic evaluation of key experimental parameters including the eluent nature and concentration, the solution pH, the preconcentration time, the amount of IIP, the extraction volume and the reduction potential, was conducted.

To determine an appropriate eluent for Hg(II) ions, different acidic solutions such as HNO₃, HCl, and H₂SO₄ (with a 2.0 M concentration) were tested to remove the template ions from the Hg-IIP sample. HCl can accomplish the quantitative elution of the adsorbed Hg(II) ions on the IIP particles. After the selection of the proper leachant, various concentrations of HCl solutions (i.e., 0.25, 0.5, 1.0, and 2.0 M) were employed to explore the optimum elution concentration for leaching Hg(II) ions from the imprinted sites. According to the ETAAS data, the desorption of Hg(II) ions from IIP particles was not complete at lower concentrations of HCl. Therefore, 2.0 M HCl was used as the optimum eluent concentration in this study.

The influence of pH on the preconcentration solution was recognized as a crucial parameter in the accumulation of heavy metals. Consequently, the voltammetric responses of Hg(II) were examined various pH values (from 1.0 to 6.0). The findings (Fig. 7A) indicated that the stripping response reached a maximum at pH 4.0. The decrease in stripping current at lower pH values is due to the lower Hg(II) preconcentration in the presence of hydrogen ions. The reason for the lower responses at higher pH values might be the formation of hydroxyl complexes (such as HgOH(I) and Hg(OH)₂)³⁸. Consequently, a pH of 4.0 was selected for subsequent Hg(II) determination experiments.

Fig. 7.

Impact of (A) pH, (B) IIP amount, (C) preconcentration time, (D) reduction potential and (E) sample volume on the stripping response of the imprinted sensor to 75.0 nM Hg(II).

The effect of the IIP content on the response of the established sensor was examined by varying the amount of IIP (4.0-12.0% w/w) in a solution containing 75.0 nM analyte. According to Fig. 7B, the Hg(II)-IIP modified CPE responses first increase with increasing the dosage of the IIP from 4.0 to 8.0% (w/w) and then hindered upon further increase of the modifier content. On the basis of these findings, the optimized electrode paste composition was determined to be 8.0% Hg(II)-IIP, 67.0% graphite powder, and 25.0% paraffin oil (w/w).

The influence of preconcentration time on the stripping current of the Hg(II)-IIP modified CPE was examined. The findings showed that the measured peak current increased with increasing preconcentration time, reaching a plateau at longer times (Fig. 7C). This suggests that the uptake of Hg(II) is the modified electrode is rapid because of the high ratio of imprinted centers. Consequently, a preconcentration time of 8 min was chosen for all subsequent electrochemical studies to ensure complete Hg(II) enrichment.

The effect of the reduction potential (from − 0.3 to -0.7 V vs. Ag/AgCl) on the stripping peak current for Hg(II) was systematically studied. As illustrated in Fig. 7D, increasing the reduction potential above − 0.5 V led to a decrease in the stripping signal, which can be attributed to the occurrence of hydrogen evolution at these negative potentials³⁹. Therefore, a reduction potential of -0.5 V was selected for the next experiments, as it produced well-defined peaks with an optimal current.

The extraction volume plays a critical role in determining the performance of IIP based sensors^40,41. Therefore, the effect of sample volume on the quantitative adsorption of Hg(II) was examined, and the results are shown in Fig. 7E. The signal current increases with increasing extraction volume from 5.0 to 20.0 mL and then decreases with increasing of the Hg(II) solution volume. Accordingly, 20.0 mL was chosen as the optimal sample volume in the present study.

Analytical determination

To obtain a calibration curve, we recorded DPASV voltammograms for different concentrations of Hg(II) under the previously optimized conditions and the results are illustrated in Fig. 8A. The obtained stripping currents were used to construct a linear calibration plot (Fig. 8B). The Hg(II)-IIP modified CPE exhibited a linear range from 1.0 to 240.0 nM, which was described by the linear regression equation I (µA) = 0.1422 (µA nM) C_Hg (nM) + 0.4662 and a high correlation coefficient of 0.9995. The established assay has a low detection limit of 0.2 nM (based on the S_b/m = 3 method⁴²), which is lower than the USEPA guidelines for Hg(II) in drinking water. The analytical performance of the proposed Hg(II) sensor was also compared against that of recently reported electrochemical sensing protocols^43–48. Table 2 highlights the key performance metrics for various Hg(II) sensors, including the detection limit, linear range, electrode type, electrode modifier, and detection method. Notably, the detection limit of the Hg(II)-IIP modified CPE is superior to other methods, demonstrating its excellent performance for Hg(II) detection.

Fig. 8.

(A) DPAS responses of the Hg(II)-IIP modified CPE to various concentrations of Hg(II) (1.0 nM to 240.0 nM). (B) Calibration curve for Hg(II) determination via the imprinted sensor under optimal conditions.

Table 2.

Comparison of various voltammetry techniques for the determination of hg(II).

Electrode/Method	Electrode modifier	Detection limit	Linear range	References
CPEa/DPASVb	Hg-IIP using pyrrole	130 nM	0.46–100 µM	[43]
CPE/DPVc	Hg-IIP using 1,5-dipenylcarbazone	70 nM	0.1–40 µM	[44]
GCEd/DPV	Hg-IIP using porphyrin and MWCNTs	5 nM	0.01–700 µM	[45]
CPE/DPV	Hg-IIP using 4-vinyl pyridine	0.52 nM	2.5–500 nM	[46]
GCE/SWASVe	Sulphur containing carboxy methyl/Hg-IIP	0.5 nM	99.7–4000 nM	[47]
GCE/DPV	Hg-IIP based on thiourea derivative functionalised graphene quantum dot	23.5 nM	0.05–230 µM	[48]
CPE/DPASV	Hg(II)-IIP using MCP	0.2 nM	1.0-240.0 nM	This work

a: Carbon paste electrode; b: Differential pulse anodic stripping voltammetry; c: Differential pulse voltammetry; d: Glassy carbon electrode; e: Square wave anodic stripping voltammetry.

Reproducibility, repeatability and stability of the Hg(II) sensor

The reproducibility of the proposed sensor was studied under the optimized conditions. The reproducibility of the proposed sensor was validated by testing the fabricating six sensors using the same ion-imprinting procedure and testing them independently. The relative standard deviation (RSD) value for different sensors was 3.9% using six independent Hg(II)-IIP modified CPEs.

To ensure the repeatability of the sensor, a series of consecutive measurements using the same sensor was conducted under identical experimental conditions. An RSD value of 2.8% was obtained for repeated measurements (six times) using a single electrode. In addition, the inter-day precision was analyzed over six consecutive days, with each day involving the preparation and analysis of fresh samples. The resulting RSD values were 3.4%, demonstrating that the method maintains consistent performance across different days. These findings highlight the potential of the sensor as a reliable tool for Hg(II) monitoring in real water samples.

The stability of the IIP sensor was also evaluated over a long-term period. The Hg(II) sensor was remarkably stable, maintaining its performance for at least 4 weeks with minimal variation (less than ± 5.0%) in the stripping signal.

Interference study

To evaluate the anti-interference ability of the sensor, we studied potential interference from common ions such as Pb(II), Cd(II), Cu(II), Zn(II), Mn(II), Fe(II), Mg(II), Ca(II), Na(I) and Co(II). The selectivity of the Hg(II)-IIP modified CPE was evaluated by calculating the peak current ratio (I_s/I₀), where I_s is the DPASV response of analyte ions (25.0 nM) in the presence of interfering metal ions, and I₀ is the current in their absence (Fig. 9). The results show that even a 50-fold excess of heavy metals caused minimal changes in the electrochemical current of the Hg(II) sensor. For comparison, the selectivity of a NIP based sensor was also tested with the same interferences. The NIP modified CPE exhibited a lack of selectivity, leading to a pronounced dependence of the analytical signal on the background matrix composition. This highlights the advantage of the selective binding sites within Hg(II)-IIP, which effectively recognize and capture target ions.

Fig. 9.

Anti-interference study using Hg(II)-IIP and NIP based sensors under optimal conditions.

Sensor applicability

To explore the practical implementation of the assay in complex matrices, the sensor was employed to detect Hg(II) in real water samples (dam, aqueduct, tap, and river water). The water samples were spiked with Hg(II) at three concentrations (10.0, 25.0, and 50.0 nM), and the Hg(II) content was determined via the proposed electrochemical strategy (n = 5 for each sample). The sensor applicability results (Table 3) show that the relative recoveries (RR) ranged from 96.8 to 103.6%. In addition, inter- and intra-day precision (RSD) percentages were reported respectively within a range of 2.5–3.4% and 2.1–3.6%. In addition, according to the one-sample t-test analysis (at the 95% confidence level), the observed deviations from 100% recovery can be attributed to random variation within the finding data. The achieved data demonstrate the effectiveness of the present assay for Hg(II) monitoring in various environmental waters, highlighting its potential for real-world applications.

Table 3.

Applicability of the imprinted sensor for the analysis of hg(II) in real water samples.

Sample	Added (nM)	Found (nM)	RR (%)
Dam water	10.0	9.8	98.0
	25.0	24.2	96.8
	50.0	51.2	102.4
Aqueduct water	10.0	10.1	101.0
	25.0	25.9	103.6
	50.0	48.7	97.4
Tap water	10.0	9.9	99.0
	25.0	24.4	97.6
	50.0	49.1	98.2
River water	10.0	10.2	102.0
	25.0	25.2	100.8
	50.0	48.6	97.2

Conclusions

This study presents the fabrication of a novel Hg(II)-IIP modifier for the selective and sensitive electrochemical detection of Hg(II) in aqueous media. The method enables Hg(II) detection at nanomolar concentrations, surpassing conventional methods that often require preconcentration steps and expensive equipment to achieve similar sensitivity. Under the optimized experimental conditions, the established sensor demonstrated excellent analytical performance, with a wide linear range of 1.0-240.0 nM and a remarkably low detection limit of 0.2 nM. Notably, this detection limit is significantly below the maximum contaminant level for mercury in drinking water set by the USEPA. The imprinted polymer cavities recognize Hg(II) ions in a selective manner, even in the presence of other common metal ions. The practical applicability of the sensor was confirmed via the analysis of analyte in various water samples including dam, aqueduct, tap, and river water samples. In conclusion, the proposed electrochemical assay, which uses Hg(II)-IIP as a modifier, is a highly effective method for the determination of Hg(II) in aqueous environments.

Author contributions

The electrochemical part of this work was designed by Ebrahim Shamsabadi who interpreted the results; the works related to the synthesis and identification of imprinted materials were conducted by Alireza Motavalizadehkakhky; and Hashem Akhlaghi and Mehdi Baghayeri evaluated all experimental data and wrote the manuscript.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.