# Electrochemical detection of lead, cadmium, and mercury with selective preconcentration on ligand-modified electrodes

Gaël Levanen, Florence Razan, Sarah Bretin, Stéphanie Betelu, Karine Michel, Florence Geneste

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## ► To cite this version:

Gaël Levanen, Florence Razan, Sarah Bretin, Stéphanie Betelu, Karine Michel, et al., Electrochemical detection of lead, cadmium, and mercury with selective preconcentration on ligand-modified electrodes. Journal of Electroanalytical Chemistry, 2025, 991, pp.119189. 10.1016/j.jelechem.2025.119189. hal-05073360

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Submitted on 20 Jun 2025

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# Electrochemical detection of lead, cadmium, and mercury with selective preconcentration on ligand-modified electrodes

Gaël Levanen<sup>a</sup>, Florence Razan<sup>b</sup>, Sara Bretin<sup>b</sup>, Stéphanie Betelu<sup>c</sup>, Karine Michel<sup>c</sup>, Florence Geneste<sup>a, *,</sup>

## ARTICLE INFO

**Keywords:**

Electrochemical sensors

Preconcentration

Ligand

Complexation

Metallic trace elements

Heavy metals

## ABSTRACT

Detecting Metallic Trace Elements (MTEs), especially toxic ones like cadmium, lead, and mercury, remains a key research focus. While methods using aptamers, molecularly imprinted polymers (MIPs), and metal-organic frameworks (MOFs) are gaining attention, electrochemical sensors with organic ligands remain widely studied due to their simple preparation, durability, and reusability. Metal-selective ligands are also increasingly integrated with MOFs and MIPs for enhanced performance. This review examines recent advancements in electrochemical sensors based on metal-ligand complexation over the past seven years, with a focus on ligand design role in sensor efficiency. The first part addresses generic ligands and strategies for achieving sensor selectivity and sensitivity. The subsequent sections review the literature on specific sensors designed for cadmium, lead, and mercury detection. Special consideration is given to sensor stability and repeatability, key parameters for practical applications. The discussion also covers sensor performance in real samples and interference studies, highlighting their relevance for environmental monitoring.

## 1. Introduction

Heavy metals with environmental concentrations below 100 mg kg⁻¹ are referred to as Metallic Trace Elements (MTEs) [1]. MTEs are naturally present in the asthenosphere, lithosphere, hydrosphere, and atmosphere, which are interconnected compartments undergoing continuous exchange [2,3]. Human activities, particularly their expansion and intensification, have significantly increased the dispersion of MTEs in natural environments [3,4]. Industrial activities represent the primary anthropogenic sources of MTE emissions [5,6]. Metals are described as persistent because they do not degrade; however, their physicochemical forms can change rapidly [3]. Speciation refers to the distribution of the different physicochemical forms of metals in a specific environment [7].

In aqueous environments, MTEs exist in particulate form (with a minimum particle size arbitrarily defined as 0.45 µm [8]), dissolved form, or colloidal form. Their behavior is governed by complexation/dissociation, precipitation/dissolution, adsorption/desorption, and ion-exchange reactions [9]. Speciation depends on multiple factors,

including the metal nature and concentration, the composition and concentration of organic and inorganic constituents, physicochemical conditions (e.g., pH, ionic strength, temperature, oxygen levels, turbidity), and the activity of living organisms [2,10]. These parameters influence the mobility and bioavailability of MTEs—two critical factors for assessing their environmental impact [11]. In risk assessment, the concentration of free metal ions has been identified as a key parameter for evaluating MTE bioavailability and toxicity to plants and living organisms [12]. Techniques capable of preconcentrating and measuring the labile fractions of trace metals are needed as complements to widely used laboratory methods such as inductively coupled plasma mass spectrometry (ICP-MS) [13,14], and atomic absorption spectroscopy (AAS) [15,16], which are primarily employed for total MTE quantification.

Since the 1970s, electrochemistry has shown great promise in addressing this need. Electrochemical equipment and sensors are portable, easy to operate, highly sensitive, and offer rapid analysis times. Mercury-based sensors have historically been popular for the point detection of trace metals using anodic stripping voltammetry (ASV)

<footnote><sup>a</sup> Univ Rennes, CNRS, ISCR, UMR 6226, F-35000 Rennes, France</footnote>

<footnote><sup>b</sup> ENS Rennes, SATIE, UMR-CNRS 8029, Campus de Ker Lann 35170 Bruz, France</footnote>

<footnote><sup>c</sup> BRGM (French Geological Survey), 3 Avenue Claude Guillemin, 45060 Orléans Cedex 2, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: florence.geneste@univ-rennes.fr (F. Geneste).</footnote>
[17–24]. This process involves preconcentrating metal ions through electrodeposition onto a mercury film, forming an amalgam, before reoxidizing them. Mercury enables the simultaneous measurement of multiple ions due to its high hydrogen overvoltage. Automated flow-cell procedures further enhanced the repeatability and selectivity of these methods [25–30]. However, European Directives (2008/105/EC and 2011/65/EU) have banned mercury in measuring devices. This regulatory shift has driven the development of mercury-free sensors [31–33].

The development of mercury-free sensors requires an electrode material with a high hydrogen overvoltage (usually carbon [34]), and the modification of its surface to enhance its affinity for metal ions. The modification of electrode surfaces with ion-specific receptors has emerged as a key strategy to achieve selective preconcentration of analytes, thereby improving electrochemical signals and ensuring selectivity [35,36]. Various complexing materials have been employed to modify electrodes, including molecularly imprinted polymers (MIPs) [37,38], aptamers [39,40], metal-organic frameworks (MOFs) [41,42] and organic ligands [43–45]. Organic ligands are particularly advantageous due to their low cost and durability, as they often enable the fabrication of stable, regenerable sensors. They also enable the detection of free ions and labile complexes capable of forming complexes with immobilized receptors, thereby enhancing speciation analysis. The detection can be performed in real environment without a preliminary purification step.

Cadmium (Cd), lead (Pb), and mercury (Hg) are among the most concerning MTEs due to their extreme toxicity. Consequently, their permissible concentrations in drinking water are stringently regulated: European directives of 2001 set limits of 10, 1, and 5 µg L⁻¹ for Pb, Hg, and Cd, respectively (equivalent to 48, 5, and 45 nM) [46]. The U.S. Environmental Protection Agency (EPA) specified in 2012 upper limits of 5 µg L⁻¹ (45 nM) for Cd(II), 15 µg L⁻¹ (72 nM) for Pb(II), and 2 µg L⁻¹ (10 nM) for Hg(II) [47]. The World Health Organization (WHO) has established maximum contaminant levels of 5 µg L⁻¹ (45 nM) for Cd, 10 µg L⁻¹ (96 nM) for Pb, and 1 µg L⁻¹ (5 nM) for Hg in drinking water in 2004 [47,48].

Although electrochemical sensors for highly toxic metals such as lead, mercury, and cadmium remain a focus of research, recent reviews on this topic are limited and often emphasize the support material used [34,49]. This short review specifically addresses the performance of electrochemical sensors, focusing on the nature of ligand receptors for these three elements. Given the increasing demand for reliable, stable, on-site and low-cost sensors for MTE, particularly in light of ever-stricter

regulatory standards (for example, the lead concentration limit in drinking water is set to reach 5 µg L⁻¹ by 2036 [50]), we have chosen to focus this review on the latest advancements in this field. It examines the parameters that contribute to optimal sensor performance, particularly sensitivity, selectivity, repeatability, and regenerability. The review is structured according to the nature of the ligands used as MTE receptors. The first part concerns generic ligands including basic small molecules and ligands used for the simultaneous detection of several cations. Ligands used to develop specific sensors for Cd²⁺, Pb²⁺ and Hg²⁺ are then treated separately.

## 2. I. Principe of sensing with ligand-modified electrodes

The general principle of sensors based on ligand-modified electrodes is illustrated in Fig. 1.

The first step involves complexing the target cation with a chelating agent that has been previously immobilized on the electrode surface. Various immobilization techniques can be employed, including covalent grafting (electrografting, metal-S bond, functionalized polymer etc.), adsorption, or the fabrication of composite electrodes. This step enables the preconcentration of the cation at the electrode surface, which is critical for enhancing the electrochemical signal. Selectivity can be tuned by exploiting the affinity between the chelating agent and the cation. For instance, based on the Hard and Soft Acids and Bases (HSAB) principle [51], ligands containing functional groups classified as strong bases (e.g. RNH₂, CH₃COO⁻) will have a greater affinity for strong acids (e.g. Fe³⁺, Al³⁺), while weaker bases (e.g., RSH, R₃P) will preferentially interact with weaker acids (e.g., Hg²⁺, Cd²⁺). However, a lot of metal ions are considered borderline (e.g., Pb²⁺, Zn²⁺, Fe²⁺, Co²⁺, Ni²⁺) and may exhibit intermediate behavior. Other factors influencing ligand-metal affinity include the denticity of the ligand and geometric complementarity between the metal ion and the binding site. The pH of the analysis solution must be carefully controlled to ensure optimal complexation between the chelating agent and the metal ion. Acidification is commonly used for sensor regeneration, as it promotes decomplexation of the metal by protonating the ligand binding sites.

Following the decomplexation step, the electroactive metal ion is analyzed. Different sensor configurations are employed. In one approach, the electrode is removed from the sample matrix and transferred to a separate electrolyte for analysis, where detection is achieved either through reduction peak measurement or anodic stripping voltammetry (ASV). This method targets only the ions that were selectively

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** General principle of the sensors using ligand-modified electrodes.</figcaption></figure>
complexed, relying on the high affinity and selectivity of the ligand. An
alternative strategy involves performing the analysis directly in the
sample matrix using ASV. This is preferable when the complexation
constant between the ligand and the metal ion is low, allowing for more
dynamic interactions. Anodic stripping analysis is the most used elec-
trochemical method. It consists of an electrodeposition step, in which
both the deposition potential and duration significantly influence signal
intensity. Since each metal ion has its own deposition potential
depending on the complex formed, this parameter can be used to
minimize interferences. For example, mercury can form amalgams with
metals such as lead and cadmium, and its co-deposition may interfere
with the stripping signals of these metals. This final step is typically
carried out using linear sweep voltammetry (LSV), differential pulse
voltammetry (DPV), or square wave voltammetry (SWV), with pulsed
techniques generally preferred due to their ability to minimize capaci-
tive currents.

### 3. II Generic ligands

The primary role of generic ligands is to complex metal ions and
preconcentrate them on the electrode surface, to increase the sensitivity
of the sensor and to achieve low detection limits. Table 1 summarizes the
properties of electrochemical sensors based on generic ligands from
recent literature.

In most cases, the complexation of ions by ligands enables limits of
detection (LOD) to be achieved below the standards imposed by the
European Union for cadmium and lead, which are 48 and 45 nM,
respectively [46] (Table 1). Some LOD values are up to 1000 times lower
than the lowest value in the reported linearity range, and some ranges
are wide (1000–10,000 nM), which may justify a closer examination of
the accuracy of reported LOD values. Mercury, on the other hand, is
more stringent, with a value of 5 nM, which is more difficult to achieve,
as can be seen from Table 1, where four of the eight mercury ligands
studied failed to meet this target [52,55,60,61].

The electrochemical analysis methods used for this type of sensor are
often square wave anodic stripping voltammetry (SW-ASV) or differ-
ential pulse anodic stripping voltammetry (DP-ASV). Pulsed methods
reduce the proportion of capacitive current compared with the Faradic
one, making them highly sensitive [81]. A preconcentration step at
open-circuit during t<sub>c</sub> (Table 1) allows the selective complexation of the
ions at the electrode surface before the analysis. This step is sometimes
performed at the same time as the electrodeposition of the metal ions (tc
is not given in that case).

Many sensors use glassy carbon as an electrode for its inertness and
wide potential window (Table 1). It can also be easily functionalized by
oxidized carbon species, allowing for the covalent or adsorption-based
immobilization of ligands [73,78,80]. Graphene derivatives have also
been widely used for their excellent characteristics, such as their large
surface area and high electrical conductivity [56,57,63]. They possess
oxide functions that can be easily functionalized, such as OH groups [57]
or COOH groups [63]. Interestingly, Al-Gahouari et al. [65] combined
the properties of graphene oxide (GO) with those of carbon nanotubes
(CNTs). They showed that this mixed system exhibited a higher current
intensity than GO alone or CNTs deposited on glassy carbon electrodes
and attributed this positive effect to the reduction of aggregation phe-
nomena, resulting in a more conductive material with a higher surface
area. The immobilization of ligands was also achieved using porous
materials such as clay and silica due to their high surface area and hy-
droxyl groups that allow for the covalent immobilization of a large
number of ligands [53,66]. Finally, other approaches involved metallic
nanoparticles, whose large surface-to-volume ratio and good conduc-
tivity improve the current response [58,62,79].

Many molecules have been used as complexing agents. Generic li-
gands are simple molecules, chosen for their abundance, accessibility
and low cost. Thus, there are many studies involving a single amino acid
such as cysteine [64–68] or arginine [63], and short peptides such as

glutathione [62]. These molecules are mainly used for complexing a
broad spectrum of MTEs. These generic ligands are sometimes coupled
to other complexing agents. Indeed, the denticity of the ligand is also a
factor that contributes to the selectivity and stability of a complex. For
example, Priya et al. [68] coupled cysteine with κ-carrageenan bearing
sulfonate groups, on a graphene oxide surface that offered excellent
electronic properties [82]. It gave rise to a considerable increase in the
sensitivity of the analytical method by a factor of 10 to 100 compared
with other cysteine-based sensors. Other authors associated β-cyclo-
dextrin, a host-guest inclusion complex, with cysteine to improve the
sensor selectivity [80].

A comparison of the H₂pyrabza [69] and H₂bpabza [70,71] ligands (Fig. 2) studied by Sangeetha Selvan et al. highlights the importance of structural modifications and their influence on MTE measurements. For lead analysis, the first ligand had a detection limit of 1.1 nM, with an RSD (Relative Standard Deviation) of 3.0 %, compared with 2.70 nM (0.4 nM) and 1.7 % (3.1 %) for the second, according to the modified electrode. The modification of pyrroles with pyridine groups and the addition of amide groups appeared to have only a small impact on lead LOD. Both ligand-modified electrodes allowed lead and cadmium detection with good selectivity towards other metal ions (Fig. 2). The selectivity of these sensors for mercury was lower, with a stronger influence of the interfering cations studied.

Tesfaye et al. used HDPBA (Fig. 3a) for the selective detection of
mercury, lead and cadmium, achieving detection limits of 1.28 nM, 9.4
pM and 32 pM, respectively [74–76]. These limits were well below
potability standards, and linearity ranges were extended for each of the
analytes (e.g. 20–10,000 nM for Hg²⁺). While the same ligand was used
for each ion, interference studies seemed to indicate particularly good
selectivity for lead (× 30/Cd²⁺, × 50/Hg²⁺), cadmium (× 10/Pb²⁺, ×
20/Hg²⁺) and also mercury (× 2/Pb²⁺, but interfered with cadmium at
the same concentration) (Fig. 3a). This selectivity seemed to derive
essentially from the specific electrochemical deposition conditions for
each analyte (–1.0 V for Cd²⁺, –0.8 V for Hg²⁺, –0.9 V for Pb²⁺ vs Ag/
AgCl). In those conditions, there was no open-circuit preconcentration
step by ion complexation within the ligand before the application of the
electrodeposition process. It was therefore possible to bring selectivity to
the sensor by adjusting the electrodeposition potential. Kokab et al. also
used the same ligand BOC6 (Fig. 3b) for the detection of mercury,
thallium and arsenic [73], and for the selective detection of lead [72].
These analyses were carried out without an open-circuit preconcentra-
tion step before the application of the deposition potential (–1.2 V vs
Ag/AgCl for Hg²⁺ and –0.7 V vs Ag/AgCl for Pb²⁺). Results showed a
very good selectivity of the sensor for mercury (× 11,000/Pb²⁺)
(Fig. 3b) [73].

Therefore, a way of increasing the selectivity of analyses without
modifying the ligand was to adjust the electrodeposition and measure-
ment potentials to limit the reduction of interfering MTEs. The study by
Tesfaye et al., using an electrode modified with the same ligand for the
analysis of three different MTEs, highlighted this procedure. The authors
used a reduction potential of –0.8 V for mercury analysis since the
reduction of lead occurred at –0.9 V and that of cadmium at –1.0 V.
They noted, however, that cadmium interfered with mercury at the same
concentration. Thus, limiting the reduction potential avoided electro-
deposition of an interfering metal, but did not prevent its complexation
with the ligand. However, the selective electrodeposition of a metal
allowed its corresponding ions to be re-complexed with free ligands,
which accentuated its electrochemical signal. In the case of a selective
preconcentration with a ligand at open circuit, selectivity is only pro-
vided by the complexation reaction with the ligand.

According to Table 1, generic ligands are mainly intended for the
analysis of lead, mercury and cadmium, although they are capable of
complexing a wide spectrum of metal ions. Simultaneous measurement
of several analytes is a strategy that can limit the risk of interference and
thus, in a way, bring selectivity to the sensor. Thus, copper is often
analyzed simultaneously with other MTEs due to its high reduction
<table><caption>**Table 1**
Multi-functional ligands for the detection of MTEs, and their characteristics.a)</caption><thead><tr><th>MTE<sup>a</sup></th><th>Ligand</th><th>Electrode type</th><th>Analysis<sup>d</sup></th><th>Procedure<sup>e</sup></th><th>Sensitivity (μA/μM)</th><th>LOD<sup>f</sup> (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>g</sup></th><th>Regeneration</th><th>Studied interferents<sup>h</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>Cd<br>(II)<br>Pb<br>(II)<br>Hg<br>(II)<br>Cu<br>(II)<br>Zn<br>(II)<br>Sr<br>(II)</td><td>1-(3-chlorophenyl)-3-dodecanoylthiourea (Cl-DPTU)</td><td>Glassy carbon</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.0 V<br>t<sub>d</sub>: 100 s</td><td>-</td><td>Cd: 6.45<br>Pb: 11.0<br>Hg: 9.15</td><td>Cd: 1-5000<br>Pb: 1-5000<br>Hg: 1-5000</td><td>Cd: 1.68<br>%<br>Pb: 2.31<br>%<br>Hg: 2.05<br>%</td><td>New electrode for each analysis</td><td>Ni<sup>2+</sup>, Co<sup>2+</sup>, Cr<sup>3+</sup>, Co<sup>3+</sup>, SO<sub>4</sub><sup>2-</sup>, Leucine, Tryptophan, 3-bromo benzaldehyde, 3-hydroxy benzaldehyde [C] ≥5:1</td><td>[52]</td></tr><tr><td>Cd<br>(II)<br>Pb<br>(II)<br>thoxysilane (AAAPTS)</td><td>3-[2-(2-aminoethylamino)ethylamino]propyl-trime</td><td>Clay/carbon (covalent)</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.1 V<br>t<sub>d</sub>: 240 s</td><td>Cd: 18.6<br>Pb: 28.8</td><td>Cd: 6.23<br>Pb: 5.31</td><td>Cd: 18-890<br>Pb: 10-480</td><td>Pb: 4.5<br>%<br>Cd: 3.8<br>%</td><td>-</td><td>Ni<sup>2+</sup>, Co<sup>2+</sup>, Mn<sup>2+</sup><br>[C] 20:1<br>Hg<sup>2+</sup>, Zn<sup>2+</sup><br>[C] 10:1<br>Cu<sup>2+</sup> [C] 5:1</td><td>[53]</td></tr><tr><td>Cd<br>(II)<br>Hg<br>(II)<br>Cu<br>(II)<br>Zn<br>(II)</td><td>Alanine</td><td>Glassy carbon (covalent)</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.3 V<br>t<sub>d</sub>: 140 s</td><td>Cd: 20.26<br>Hg: 19.85</td><td>Cd: 5.77<br>× 10<sup>-3</sup><br>Hg: 5.89<br>× 10<sup>-3</sup></td><td>Cd: 7.5-7500<br>Hg: 7.5-7500</td><td>Cd: 2.05<br>%<br>Hg: 1.73<br>%</td><td>-</td><td>Na<sup>+</sup>, As<sup>3+</sup>, Ag<sup>+</sup>, Cs<sup>+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Sr<sup>2+</sup>, Co<sup>2+</sup>, Pb<sup>2+</sup>, Cl<sup>-</sup>, EDTA, SDS, glucose, CTAB, citric acid, 2-amino-4-nitrophenol, 3-chloro-5-nitrophenol [C] 267:1</td><td>[54]</td></tr><tr><td>Pb (II)<br>Hg<br>(II)</td><td>Aminotetrazole</td><td>MgO@SiO<sub>2</sub> (covalent)</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.0 V<br>t<sub>d</sub>: 120 s</td><td>Pb: 15<br>Hg: 8</td><td>Pb: 19<br>Hg: 41</td><td>Pb: 500-10,000<br>Hg: 500-10,000</td><td>Pb: 2.52<br>%<br>Hg: 2.01<br>%</td><td>-</td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Cd<sup>2+</sup>, Ni<sup>2+</sup>, Fe<sup>3+</sup> [C] 10:1</td><td>[55]</td></tr><tr><td>Cd<br>(II)<br>Pb<br>(II)<br>Fe<br>(III)</td><td>Azo-Calix[4]arene</td><td>RGO<sup>b</sup>/ Glassy carbon (covalent)</td><td>SWV</td><td>t<sub>c</sub>: 25 min</td><td>Cd: 0.2496<br>Pb: 0.2</td><td>Cd: 20 × 10<sup>-3</sup><br>Pb: 20 × 10<sup>-3</sup></td><td>Cd: 0.1-10<br>Pb: 0.1-10</td><td>0.89<br>%</td><td>-</td><td>Mg<sup>2+</sup>, Mn<sup>2+</sup>, Zn<sup>2+</sup><br>[C] 1:1</td><td>[56]</td></tr><tr><td>Cd<br>(II)<br>Pb<br>(II)</td><td>Benzothiazole-2-carboxaldehyde</td><td>GO<sup>b</sup>@Fe<sub>3</sub>O<sub>4</sub> (covalent)</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.2 V<br>t<sub>d</sub>: 180 s</td><td>Cd<sub>1</sub>: 2060<br>Cd<sub>2</sub>: 58.94<br>Pb<sub>1</sub>: 2050<br>Pb<sub>2</sub>: 67.42</td><td>Cd: 126.0<br>× 10<sup>-3</sup><br>Pb: 60.0<br>× 10<sup>-3</sup></td><td>Cd<sub>1</sub>: 0.9 × 10<sup>-3</sup>-217 × 10<sup>-3</sup><br>Cd<sub>2</sub>: 0.22 × 10<sup>-3</sup>-1.3 × 10<sup>-3</sup><br>Pb<sub>1</sub>: 0.003-0.55<br>Pb<sub>2</sub>: 0.55-3.4</td><td>-</td><td>Oxidation at 0.3 V for 150 s in 0.1 M HCl</td><td>Cr<sup>3+</sup>, Cu<sup>2+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Ni<sup>2+</sup>, Na<sup>+</sup> [C] 40:1</td><td>[57]</td></tr><tr><td>Cd<br>(II)<br>Pb<br>(II)</td><td>Calix[4]arene</td><td>Mn<sub>3</sub>O<sub>4</sub> nanoparticles/ Glassy carbon</td><td>DP-ASV</td><td>t<sub>c</sub>: 2 min<br>E<sub>d</sub>: -1.2 V<br>t<sub>d</sub>: 4 min</td><td>-</td><td>Cd: 33.8<br>Pb: 80.1</td><td>Cd: 890-8900<br>Pb: 480-4830</td><td>Pb: 3.3<br>%<br>Cd: 3.7<br>%</td><td>-</td><td>NH<sub>4</sub><sup>+</sup>, Ba<sup>2+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Li<sup>+</sup>, Ag<sup>+</sup>, Cl<sup>-</sup>, F<sup>-</sup>, SO<sub>4</sub><sup>2-</sup> [C] 1000:1<br>Co<sup>2+</sup>, Co<sub>2</sub>O<sub>4</sub><sup>2-</sup>, CO<sub>3</sub><sup>2-</sup><br>[C] 40:1<br>Hg<sup>2+</sup>, Bi<sup>3+</sup>, Sn<sup>2+</sup>, Fe<sup>3+</sup> [C] 20:1</td><td>[58]</td></tr><tr><td>Pb (II)<br>Co<br>(II)<br>Cu<br>(II)</td><td>EDTA</td><td>Carbon graphite paste</td><td>SWV</td><td>t<sub>c</sub>: 10 min [-1.5; 1.5] V</td><td>Non-linear</td><td>Pb: 2.33</td><td>Non-linear</td><td>-</td><td>-</td><td>-</td><td>[59]</td></tr><tr><td>Hg<br>(II)</td><td>EDTA</td><td>Carbon paste electrode</td><td>SWV</td><td>t<sub>c</sub>: 5 min [-1.5; 1.5] V</td><td>10.5</td><td>Hg: 16.6</td><td>500 × 10<sup>-3</sup>-3500 × 10<sup>-3</sup></td><td>Hg: 3.10<br>%</td><td>-</td><td>-</td><td>[60]</td></tr><tr><td>Pb (II)<br>Hg<br>(II)</td><td>EDTA</td><td>PANI/ SWCNTs<sup>c</sup> (covalent)</td><td>DPV</td><td>t<sub>c</sub>: -<br>Pb: [-0.54; 0.54]</td><td>Non-linear</td><td>Pb: 1650</td><td>Non-linear</td><td>-</td><td>-</td><td>Co<sup>2+</sup>, Ni<sup>2+</sup>, Cd<sup>2+</sup><br>[C] 1:1</td><td>[61]</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued)**</caption><thead><tr><th>MTE<sup>a</sup></th><th>Ligand</th><th>Electrode type</th><th>Analysis<sup>d</sup></th><th>Procedure<sup>e</sup></th><th>Sensitivity (µA/µM)</th><th>LOD<sup>f</sup> (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>g</sup></th><th>Regeneration</th><th>Studied interferents<sup>h</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>Cu<br/>(II)</td><td></td><td></td><td></td><td>-0.4] V<br/>Hg: [0.0;<br/>0.3] V</td><td></td><td>Hg:<br/>680</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cd<br/>(II)<br/>Pb<br/>(II)</td><td>Gluthathion</td><td>Fe<sub>3</sub>O<sub>4</sub><br/>nanoparticles<br/>(covalent)</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.4 V<br/>t<sub>d</sub>: 210 s</td><td>Cd: 8.72<br/>Pb: 15.58</td><td>Cd:<br/>1.52<br/>Pb:<br/>0.878</td><td>Cd:<br/>4.45-890<br/>Pb:<br/>2.41-483</td><td>Cd:<br/>3.25<br/>%<br/>Pb:<br/>3.73<br/>%</td><td>-</td><td>NH<sub>4</sub><sup>+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>,<br/>Na<sup>+</sup>, Ca<sup>2+</sup><br/>[C] 700:1<br/>Hg<sup>2+</sup>, Zn<sup>2+</sup>,<br/>Cu<sup>2+</sup>, Fe<sup>2+</sup>,<br/>Fe<sup>3+</sup>, Al<sup>3+</sup>, Ni<sup>2+</sup><br/>[C] 300:1</td><td>[62]</td></tr><tr><td>Pb (II)</td><td>L-Arginine</td><td>RGO<sup>b</sup><br/>(covalent)</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -0.8 V<br/>t<sub>d</sub>: 60 s</td><td>Non-linear</td><td>Pb:<br/>60 ×<br/>10<sup>-3</sup></td><td>Non-linear</td><td>3.39<br/>%</td><td>Stirring in<br/>0.1 M EDTA</td><td>Co<sup>2+</sup>, Cu<sup>2+</sup>,<br/>Ni<sup>2+</sup>, Hg<sup>2+</sup>,<br/>Zn<sup>2+</sup><br/>[C] 33:1</td><td>[63]</td></tr><tr><td>Pb (II)</td><td>Cysteine</td><td>Glassy carbon<br/>(covalent)</td><td>LS-ASV</td><td>t<sub>c</sub>: 7 min<br/>E<sub>d</sub>: -0.9 V<br/>t<sub>d</sub>: 3 min</td><td>Pb: 6.86<br/>Pb:<br/>3.33</td><td>Pb:<br/>3.33</td><td>Pb:<br/>24.1-603</td><td>-</td><td>-</td><td>Hg(II) [C] 7:1<br/>Cu(II) [C] 2:1<br/>Cd(II), Ni (II),<br/>Rh(II), Ru(II), Zn<br/>(II), Ir(IV),Co<br/>(II), As(III)<br/>[C] 0.3:1</td><td>[64]</td></tr><tr><td>Pb (II)</td><td>L-Cysteine</td><td>RGO<sup>b</sup>/<br/>MWCNTs<sup>c</sup><br/>(covalent)</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.2 V<br/>t<sub>d</sub>: 180</td><td>Pb: 47.4<br/>Pb:<br/>0.483</td><td>Pb:<br/>0.483</td><td>Pb:<br/>0.97-193</td><td>2.15<br/>%</td><td>Oxidation at<br/>0.4 V for 60 s<br/>under stirring</td><td>Na<sup>+</sup>, Ca<sup>2+</sup>, Co<sup>2+</sup>,<br/>Cd<sup>2+</sup>, K<sup>+</sup>, Fe<sup>3+</sup>,<br/>Pb<sup>2+</sup>, Mg<sup>2+</sup>,<br/>NO<sub>3</sub><sup>-</sup><br/>[C] 1:1</td><td>[65]</td></tr><tr><td>Cd<br/>(II)<br/>Pb<br/>(II)</td><td>L-Cysteine</td><td>Mesoporous<br/>silica<br/>(covalent)</td><td>SWV</td><td>t<sub>c</sub>: 5 min<br/>[-0.2;<br/>-1.2] V</td><td>Cd<sub>1</sub>:<br/>210.47<br/>Cd<sub>2</sub>:<br/>320.80<br/>Pb<sub>1</sub>:<br/>296.18<br/>Pb<sub>2</sub>:<br/>190.94</td><td>Cd<sub>1</sub>:<br/>1.96<br/>Cd<sub>2</sub>:<br/>2.05<br/>Pb<sub>1</sub>:<br/>1.74<br/>Pb<sub>2</sub>:<br/>3.67</td><td>Cd<sub>1</sub>:<br/>44.5-711.7<br/>Cd<sub>2</sub>:<br/>89.0-711.7<br/>Pb<sub>1</sub>:<br/>24.1-386.1<br/>Pb<sub>2</sub>:<br/>48.3-386.1</td><td>Cd<sub>1</sub>:<br/>3.22<br/>%<br/>Cd<sub>2</sub>:<br/>5.57<br/>%<br/>Pb<sub>1</sub>:<br/>2.74<br/>%<br/>Pb<sub>2</sub>:<br/>1.76<br/>%</td><td>-</td><td></td><td>Zn<sup>2+</sup>, Cu<sup>2+</sup>,<br/>Co<sup>2+</sup>, Fe<sup>3+</sup><br/>[C] 10:1</td><td>[66]</td></tr><tr><td>Cd<br/>(II)<br/>Pb<br/>(II)</td><td>L-Cysteine</td><td>RGO<sup>b</sup>/Bi/<br/>screen-printed<br/>carbon<br/>electrode<br/>(covalent)</td><td>SW-ASV</td><td>Cd:<br/>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.3 V<br/>t<sub>d</sub>: 240 s<br/>Pb:<br/>t<sub>c</sub>: -<br/>E<sub>d</sub>: -0.9 V<br/>t<sub>d</sub>: 180 s</td><td>Cd: 14.61<br/>Pb: 11.24</td><td>Cd:<br/>8.72<br/>Pb:<br/>2.70</td><td>Cd:<br/>89.0-445<br/>Pb:<br/>48.3-241</td><td>Cd:<br/>4.39<br/>%<br/>Pb:<br/>2.98<br/>%</td><td>-</td><td></td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>,<br/>Fe<sup>3+</sup>, Mn<sup>2+</sup>,<br/>Mg<sup>2+</sup>, Al<sup>3+</sup>,<br/>CO<sub>3</sub><sup>2-</sup>, NO<sub>3</sub><sup>-</sup>,<br/>Cl<sup>-</sup>, SO<sub>4</sub><sup>2-</sup><br/>[C] 100:1</td><td>[67]</td></tr><tr><td>Cd<br/>(II)<br/>Pb<br/>(II)</td><td>κ-Carraghenane/<br/>L-Cysteine</td><td>GO<sup>b</sup>/<br/>Carrageenan</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.0 V<br/>t<sub>d</sub>: 120 s</td><td>Cd: 1390<br/>Pb: 1320</td><td>Cd:<br/>5.16<br/>Pb:<br/>5.21</td><td>Cd:<br/>44.5-445<br/>Pb:<br/>24.1-241</td><td>Cd:<br/>3.2<br/>%</td><td>-</td><td></td><td>Cl<sup>-</sup>, SO<sub>3</sub><sup>-</sup>, SO<sub>4</sub><sup>2-</sup>,<br/>NO<sub>3</sub><sup>-</sup><br/>[C] 100:1<br/>Cu<sup>2+</sup>, Cd<sup>2+</sup>,<br/>Hg<sup>2+</sup>, Zn<sup>2+</sup>,<br/>Pb<sup>2+</sup><br/>[C] 1:1</td><td>[68]</td></tr><tr><td>Pb (II)<br/>Hg<br/>(II)</td><td>N,N'-bis(pyrrole-2-ylmethylene)-2-aminobenzylamin (H<sub>2</sub>bpabza)</td><td>Graphite/<br/>carbon<br/>nanotubes</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.2 V<br/>t<sub>d</sub>: 180 s</td><td>Pb: 1545<br/>Hg: 1258</td><td>Pb:<br/>1.1<br/>Hg:<br/>0.36</td><td>Pb: 3.3-66<br/>Hg: 3.3-66<br/>Hg:<br/>0.36</td><td>Pb:<br/>3.0<br/>%</td><td>-</td><td></td><td>Cd<sup>2+</sup>, Zn<sup>2+</sup>,<br/>Cu<sup>2+</sup>, Fe<sup>3+</sup>,<br/>Ni<sup>2+</sup>, Ag<sup>+</sup>, Sn<sup>2+</sup><br/>[C] 10:1</td><td>[69]</td></tr><tr><td>Pb (II)<br/>Hg<br/>(II)</td><td>N,N'-bis(pyrrole-2-ylmethylene)-2-aminobenzylamine (H<sub>2</sub>bpabza)</td><td>Carbon paste<br/>electrode/<br/>MWCNTs</td><td>SW-ASV</td><td>t<sub>c</sub>: 180 s<br/>E<sub>d</sub>: -0.8 V<br/>t<sub>d</sub>: -</td><td>Pb: 0.005<br/>Hg: 0.005</td><td>Pb:<br/>0.4<br/>Hg:<br/>1.5</td><td>Pb: 9.7-676<br/>Hg: 9.9-698<br/>Hg:<br/>1.5</td><td>Pb:<br/>3.1<br/>%</td><td>-</td><td></td><td>Cu<sup>2+</sup>, Cd<sup>2+</sup>,<br/>Zn<sup>2+</sup>, Sn<sup>2+</sup>,<br/>Fe<sup>2+</sup>, Ni<sup>2+</sup>,<br/>Mn<sup>2+</sup>, Co<sup>2+</sup><br/>[C] 0.55:1</td><td>[70]</td></tr><tr><td>Cd<br/>(II)<br/>Pb<br/>(II)</td><td>N,N'-bis(2-pyridinecarboxamide)-2-aminobenzylamine (H<sub>2</sub>pyrabza)</td><td>Carbon paste<br/>electrode/<br/>MWCNTs</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -1.2 V<br/>t<sub>d</sub>: 180 s</td><td>Cd: 405<br/>Pb: 1021</td><td>Cd:<br/>0.92<br/>Pb:<br/>2.70</td><td>Cd: 8.3-115<br/>Pb: 8.3-115<br/>Pb:<br/>2.70</td><td>Pb:<br/>1.7<br/>%</td><td>-</td><td></td><td>Hg<sup>2+</sup>, Zn<sup>2+</sup>,<br/>Cu<sup>2+</sup>, Fe<sup>2+</sup>,<br/>Ni<sup>2+</sup>, Ag<sup>+</sup>, Sn<sup>2+</sup><br/>[C] 10:1</td><td>[71]</td></tr><tr><td>Pb (II)</td><td>N-{4-[2-(1,3-benzoxazolyl)]phenyl)-3,5-N,N'-</td><td>Glassy carbon<br/>electrode</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br/>E<sub>d</sub>: -0,7 V<br/>t<sub>d</sub>: 60 s</td><td>-</td><td>Pb:<br/>15 ×<br/>10<sup>-3</sup></td><td>Pb: 5-50</td><td>-</td><td>-</td><td>Ca<sup>2+</sup>, Co<sup>2+</sup>,<br/>Cd<sup>2+</sup>, Zn<sup>2+</sup>,<br/>Ni<sup>2+</sup>, Sr<sup>2+</sup>, Na<sup>+</sup><br/>(continued on next page)</td><td>[72]</td></tr></tbody></table>
<table><caption>**Table 1 (continued)**</caption><thead><tr><th>MTE<sup>a</sup></th><th>Ligand</th><th>Electrode type</th><th>Analysis<sup>d</sup></th><th>Procedure<sup>e</sup></th><th>Sensitivity (μA/μM)</th><th>LOD<sup>f</sup> (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>g</sup></th><th>Regeneration</th><th>Studied interferents<sup>h</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>Hg (II)</td><td>bis(4-hexyloxybenzoyl) benzamide (BOC6)</td><td>Glassy carbon electrode</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.2 V<br>t<sub>d</sub>: 135 s</td><td>~3</td><td>Hg: 2.52<br>× 10<sup>-6</sup></td><td>75-175</td><td>1.71 %</td><td>-</td><td>K<sup>+</sup> [C] 20:1<br>2A4NP, 3C5NP, Ca<sup>2+</sup>, Co<sup>2+</sup>, Sr<sup>2+</sup>, Cs<sup>+</sup>, Pb<sup>2+</sup>, Zn<sup>2+</sup>, K<sup>+</sup>, EDTA, SDS, glucose, CTAB, amino acids, citric acid [C] 11,000:1</td><td>[73]</td></tr><tr><td>Tl (I)</td><td>N-{4-[2-(1,3-benzoxazolyl)]phenyl}-3,5-N,N'-bis(4-hexyloxybenzoyl) benzamide (BOC6)</td><td>Carbon paste electrode</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -0.90 V<br>t<sub>d</sub>: 270 s</td><td>Pb: 87.90</td><td>Pb: 9.4 × 10<sup>-3</sup></td><td>Pb: 0.1-2500</td><td>Pb: 4.30 %</td><td>4-5 analyses in 0.1 M sodium acetate</td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, NH<sub>4</sub><sup>+</sup>, Cl<sup>-</sup>, NO<sub>3</sub><sup>-</sup> [C] 100:1<br>Co<sup>2+</sup>, Hg<sup>2+</sup>, Ni<sup>2+</sup>, Fe<sup>2+</sup>, SO<sub>4</sub><sup>2-</sup>, [C] 50:1<br>Cu<sup>2+</sup> [C] 20:1<br>Cd<sup>2+</sup> [C] 30:1</td><td>[74]</td></tr><tr><td>Hg (II)</td><td>N1-hydroxy-N1,N2-diphenylbenzamidine (HDPBA)</td><td>Carbon paste electrode</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -0.8 V<br>t<sub>d</sub>: 210 s</td><td>Hg: 37.34</td><td>Hg: 1.28</td><td>Hg: 20-10,000</td><td>Hg: 3.7 %</td><td>4-5 analyses in 0.1 M NH<sub>4</sub>Cl</td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, NH<sub>4</sub><sup>+</sup>, Cl<sup>-</sup>, NO<sub>3</sub><sup>-</sup> [C] 100:1<br>Co<sup>2+</sup>, Ni<sup>2+</sup>, SO<sub>4</sub><sup>2-</sup>, [C] 50:1<br>Fe<sup>3+</sup>, Pb<sup>2+</sup> [C] 20:1<br>Cu<sup>2+</sup> [C] 1:1<br>Cd<sup>2+</sup> [C] 1:1</td><td>[75]</td></tr><tr><td>Cd (II)</td><td>N1-hydroxy-N1,N2-diphenylbenzamidine (HDPBA)</td><td>Carbon paste electrode</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.0 V<br>t<sub>d</sub>: 270 s</td><td>Cd: 2661</td><td>Cd: 32 × 10<sup>-3</sup></td><td>Cd: 0.3-100</td><td>3.8 %</td><td>4-5 analyses in 0.1 M Britton-Robinson buffer</td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup>, Ni<sup>2+</sup>, Fe<sup>3+</sup>, SO<sub>4</sub><sup>2-</sup>, Hg<sup>2+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Pb<sup>2+</sup> [C] 7:1</td><td>[76]</td></tr><tr><td>Pb (II)</td><td>Benzoic acid</td><td>Graphitic carbon (covalent)</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -0.8 V<br>t<sub>d</sub>: 5 min</td><td>Pb: 0.1045</td><td>Pb: 256</td><td>Pb: 10 × 10<sup>3</sup>-1000<br>× 10<sup>3</sup></td><td>Pb: 0.69 %</td><td>-</td><td>K<sup>+</sup>, Na<sup>+</sup>, Mg<sup>2+</sup>, Mn<sup>3+</sup>, Li<sup>+</sup>, Ba<sup>2+</sup>, Cr<sup>3+</sup>, Hg<sup>2+</sup>, Cd<sup>2+</sup>, Cl<sup>-</sup>, F<sup>-</sup>, I<sup>-</sup>, SO<sub>4</sub><sup>2-</sup> [C] 1000:1<br>Zn<sup>2+</sup>, Ni<sup>2+</sup>, Fe<sup>2+</sup>, Fe<sup>3+</sup>, Al<sup>3+</sup>, CO<sub>3</sub><sup>2-</sup>, NO<sub>3</sub><sup>-</sup>, NO<sub>2</sub><sup>-</sup>, SCN<sup>-</sup> [C] 500:1<br>Ca<sup>2+</sup>, Co<sup>2+</sup>, C<sub>2</sub>O<sub>4</sub><sup>2-</sup>, S<sub>2</sub><sup>-</sup> [C] 100:1</td><td>[77]</td></tr><tr><td>Cd (II)</td><td>Thiacalixarene</td><td>Glassy carbon</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.1 V<br>t<sub>d</sub>: 180 s</td><td>Cd: 160.87<br>Pb: 237.83</td><td>Cd: 35.6<br>Pb: 12.1</td><td>Cd: 890-8900<br>Pb: 483-9650</td><td>&lt;5 %</td><td>30 s in acetic acid (pH 5.0)</td><td>K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Al<sup>3+</sup>, Cl<sup>-</sup>, Br<sup>-</sup>, NO<sub>3</sub><sup>-</sup>, SO<sub>4</sub><sup>2-</sup> [C] 500:1</td><td>[78]</td></tr><tr><td>Pb (II)<br>Cu (II)</td><td>Thiolated calix[4]arene</td><td>Gold nanoparticles/ screen-printed carbon electrode</td><td>DP-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1.2 V<br>t<sub>d</sub>: 120 s</td><td>Pb: 3.32</td><td>Pb: 38.5</td><td>Pb: 970-4830</td><td>3.08 %</td><td>-</td><td>Cu<sup>2+</sup>, Cd<sup>2+</sup>, Ni<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup> [C] 1:1</td><td>[79]</td></tr><tr><td>Pb(II)<br>Hg (II)</td><td>β-cyclodextrin<br>L-cysteine</td><td>Glassy carbon (covalent)</td><td>SW-ASV</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: -1 V<br>t<sub>d</sub>: 900 s</td><td>Pb: 18.1<br>Cd: 24.8</td><td>Pb: 0.53<br>Cd: 1.13</td><td>Pb: 1.60-700<br>Cd: 3.42-700</td><td>Pb: 3.85 %</td><td>-</td><td>K<sup>+</sup>, Li<sup>+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Na<sup>+</sup> [C] 10:1</td><td>[80]</td></tr></tbody></table>

<footnote><sup>a</sup> MTEs studied in the article. Only characteristics related to Cd<sup>2+</sup>, Pb<sup>2+</sup> and Hg<sup>2+</sup>, will be represented. <sup>b)</sup> (R)GO (Reduced) graphene oxide <sup>c)</sup> Polyaniline (PANI), single walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs) <sup>d)</sup> Square wave (SW), Differential pulse (DP), Linear sweep (LS), anodic stripping voltammetry (ASV) <sup>e)</sup> t<sub>c</sub>: complexation time, E<sub>d</sub>: applied reduction potential vs Ag/AgCl, t<sub>q</sub>: applied potential time, [E<sub>1</sub>;E<sub>2</sub>] potential range for DPV or SWV analyses. <sup>f)</sup> Values above the limits set by European directives are in red <sup>g)</sup> Reproducibility <sup>h)</sup> [C] is the ratio of interferent concentration to analyte concentration tested, Ethylenediaminetetraacetic acid (EDTA), Sodium dodecyl sulfate (SDS), Cetrimonium bromide (CTAB).</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 2. H₂pyrabza (a) [71] and H₂bpabza (b) [69,70] ligands and signal change in the presence of interferents at a concentration × times higher than the analyte.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 3. HDPBA [74–76] (a) BOC6 [72,73] (b) ligands and signal change in the presence of interferents at a concentration × times higher than the analyte.</figcaption></figure>

potential. The simultaneous measurement of copper and other MTEs therefore reduces the risk of interference.

In most articles, sensors were also tested with real samples to assess the effect of the matrix on sensor performance. In the majority of these studies, the samples involved deionized water and environments close to the ideal medium, such as tap water and drinking water. However, more complex matrices were also investigated, including lake water, pond water, river water, rainwater, well water, seawater, milk or even industrial wastewater [53,55,57,58,62,63,66,68,69,72–75,77]. For these aqueous samples, the pretreatment before analysis involved a filtration step to remove suspended particles and a dilution of the solution in a buffered medium. Some articles focused on solid samples from soil, vegetables, food, and tobacco [70–72,74–77]. In this case, the sample treatment involved extracting heavy metals from the samples in their ionic form through acid-digestion. In most articles, the real effluent matrix was spiked with heavy metals. Furthermore, standard addition was often used at concentrations between 0.1 and 150 µM or even at concentrations closer to the LOD, ranging from 25 to 60 nM [63,69,72,74]. The % recovery was generally very good, ranging between 93 % and 107 %, often with RSD below 2 %, and was comparable to traditional heavy metal analysis methods such as AAS [58,60,69,75,76], ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) [79] and ICP-MS [80]. The challenge with this technique lies in the fact that real matrices often contain a large number of ions, making it difficult to test for interference from all ions and compounds present in the solution. Therefore, interference studies are

essential for determining the operational limits of each sensor.

Although generic ligands can offer promising sensitivities and LOD for MTE analysis, they are subject to interferences that influence measurements. In the context of MTEs analysis, the sensor needs to give quantitative information on a specific ion, which is why the use of selective ligands appears to be an interesting solution. In the following sections, ligands developed over the past few years for the specific detection of cadmium, lead and mercury will be presented and compared.

## 4. III-Selective ligands for cadmium detection

Cadmium is considered a soft acid according to the HSAB (hard and soft (Lewis) acids and bases) theory [51] and has a strong affinity for soft bases such as nitrogen-containing ligands. It is illustrated in Table 2 by the systematic presence of at least one nitrogen atom in the structure of the ligands used as specific receptors.

The sensors prepared from these cadmium-specific ligands have shown interesting analytical performance with lower LOD values than the limits set by European directives (Table 2). The strong affinity of cadmium with imines was illustrated by the low detection limits of 140, 32 and 53 pM obtained with the ligands MPEBSH [83], BZNA [84] and Hzb [86], respectively (Fig. 4), and the wide ranges of linearity in their calibration curves. These multidentate ligands also contain several oxygens and nitrogen atoms, allowing Cd²⁺ complexation at multiple sites and stabilizing the complexes. The signal obtained with electrodes
<table><caption>**Table 2**

Selective ligands for cadmium detection and sensor performance.a)</caption><thead><tr><th>Ligand</th><th>Analysis<sup>a</sup></th><th>Electrode type</th><th>Procedure<sup>b</sup></th><th>Sensitivity (μA/μM)</th><th>LOD (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>c</sup></th><th>Regeneration</th><th>Interference study<sup>d</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>(E)-4-Methyl-N0-(1-(pyridin-2-yl)ethylidene)benzenesulfonohydrazide (MPEBSH)</td><td>LSV</td><td>Glassy carbon</td><td>[0; 1.5] V</td><td>2.22 (μA/μM/cm²)</td><td>0.14 × 10<sup>-3</sup></td><td>0.35–350</td><td>2.30 %</td><td>–</td><td>Al<sup>3+</sup>, As<sup>3+</sup>, Ba<sup>2+</sup>, Ca<sup>2+</sup>, Ce<sup>2+</sup>, Cr<sup>3+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Ni<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Mg<sup>2+</sup>, Sn<sup>2+</sup>, Sb<sup>3+</sup> (analyzed without cadmium)</td><td>[83]</td></tr><tr><td>1,1'-((biphenyl-4,4'-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene) dinaphthalen-2-ol (BZNA)</td><td>LSV</td><td>Glassy carbon</td><td>[0; 1.5] V</td><td>~ 0.1</td><td>0.032</td><td>0.1–100</td><td>3.58 %</td><td>–</td><td>Hg<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Pb<sup>2+</sup>, Co<sup>2+</sup>, Ca<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Ni<sup>2+</sup> (analyzed without cadmium)</td><td>[84]</td></tr><tr><td>6-furfurylaminopurine</td><td>EIS</td><td>Glassy carbon</td><td>-1.2 V ± 10 mV 100–100,000 KHz</td><td>–</td><td>0.396</td><td>1–1000</td><td>4.10 %</td><td>–</td><td>Cu<sup>2+</sup>, Zn<sup>2+</sup>, Pb<sup>2+</sup> (analyzed without cadmium)</td><td>[85]</td></tr><tr><td>2-oxo-2-phenyl-N-(4-(trifluoromethyl)phenyl) acetohydrazonoyl bromide (HzB)</td><td>LSV</td><td>Glassy carbon</td><td>[0; 1.5] V</td><td>1.6</td><td>0.0534</td><td>0.01–10,000</td><td>1.10 %</td><td>–</td><td>Na<sup>+</sup>, Fe<sup>3+</sup>, Ba<sup>2+</sup>, K<sup>+</sup>, CO<sub>3</sub><sup>2-</sup>, SO<sub>4</sub><sup>2-</sup>, NO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup>, EtOH, methanoic and ethanoic acids</td><td>[86]</td></tr><tr><td>N-[(Benzyloxy)carbonyl]-L-alanyl-L-prolyl-L-leucine-N-cyclohexylcyclohexanamin (Cbz-APL)</td><td>SW-ASV</td><td>Glassy carbon</td><td>t<sub>c</sub>: - E<sub>d</sub>: - 1.3 V t<sub>d</sub>: 135 s</td><td>16</td><td>4.34 × 10<sup>-6</sup></td><td>0.0001–15</td><td>1.05 %</td><td>Re-immobilization of the functionalized layer</td><td>K<sup>+</sup>, Al<sup>3+</sup>, Ag<sup>+</sup>, Cs<sup>+</sup>, Pb<sup>2+</sup>, Ca<sup>2+</sup>, Co<sup>2+</sup>, Cr<sup>3+</sup>, Cu<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Sr<sup>2+</sup>, Tl<sup>+</sup>, amino acids, citric acid, EDTA, SDS, CTAB, glucose, 2-amino-4-nitrophenol, 3-chloro-5-nitrophenol [C] 10,000:1</td><td>[87]</td></tr><tr><td>1,2-bis-[o-aminophenylthio] ethane (APTE)</td><td>LSSV</td><td>Glassy carbon (covalent)</td><td>t<sub>c</sub>: 5 min E<sub>d</sub>: - 1.45 V t<sub>d</sub>: 5 min</td><td>3.0 ± 0.1</td><td>35</td><td>50–80</td><td>&lt; 5 %</td><td>Oxidation at 0.05 V for 5 min</td><td>Pb<sup>2+</sup>, Cu<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Cr<sup>2+</sup>, Mn<sup>2+</sup>, Fe<sup>3+</sup>, Co<sup>2+</sup></td><td>[88]</td></tr></tbody></table>

a Electrochemical impedance spectroscopy (EIS), square wave (SW), anodic stripping voltammetry (ASV), linear sweep voltammetry (LSV), linear sweep stripping voltammetry (LSSV) b) t<sub>c</sub>: complexation time. E<sub>d</sub>: applied reduction potential vs. Ag/AgCl. t<sub>d</sub>: applied potential time, [E<sub>1</sub>;E<sub>2</sub>] potential range for LSV analysis c) Reproducibility d) Signal attenuation observed at a given time. e) [C] is the ratio of interferent concentration to analyte concentration tested, Ethylenediaminetetraacetic acid (EDTA), Sodium dodecyl sulfate (SDS), Cetrimonium bromide (CTAB).

<figure><img src="image_8.png" /></figure>

Signal 20-30% lower with Al<sup>3+</sup>, As<sup>3+</sup>, Ba<sup>2+</sup>, Ca<sup>2+</sup>, Ce<sup>2+</sup>, Cr<sup>3+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Ni<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Mg<sup>2+</sup>, Sn<sup>2+</sup>, Sb<sup>3+</sup>

<figure><img src="image_9.png" /><figcaption>**Fig. 4.** MPEBSH [83] (a), HzB [86] (b) and BZNA [84] (c) ligands for the selective detection of cadmium.</figcaption></figure>

<figure><img src="image_10.png" /></figure>

Signal 30-80% lower with Co<sup>2+</sup>, Ba<sup>2+</sup>, Mg<sup>2+</sup>, Ag<sup>+</sup>, As<sup>3+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup>, Hg<sup>2+</sup>, Cr<sup>3+</sup> and Al<sup>3+</sup>

Signal 55-85% lower with Hg<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Pb<sup>2+</sup>, Co<sup>2+</sup>, Ca<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, and Ni<sup>2+</sup>

modified by these ligands for different interferents was lower than that of cadmium, highlighting the strong affinity of Cd<sup>2+</sup> with these ligands.

In parallel, a sensor based on 6-furfurylaminopurine as a specific receptor [85] (Fig. 5) was developed for impedance detection of cadmium, achieving a detection limit of 396 pM with linearity from 1 to 1000 nM. Its better selectivity towards cadmium was shown compared with three interferents Cu(II), Zn(II) and Pb(II).

Finally, a remarkable detection limit of 4.34 fM was obtained by SW-

ASV on a Cbz-APL-modified electrode (Fig. 6) [87]. Its linearity range extended from 0.1 pM to 15 nM. This ligand showed very good selectivity in the presence of interferents 10,000 times more concentrated than cadmium, making it a very good candidate for selective cadmium analysis. Only thallium, whose oxidation peak was very close to that of cadmium, led to an increase in cadmium signal.

The sensors prepared from the cadmium-specific ligands studied in Table 2 yielded results comparable to those of generic ligands for the
<figure><img src="image_11.png" /></figure>

Signal 60-90% lower with Cu²⁺, Zn²⁺, Pb²⁺

<caption>**Fig. 5.** 6-furfurylaminopurine ligand [85] for the selective detection of cadmium.</caption>

analysis of real samples (% recovery between 94 and 102 % for spiked Cd²⁺ solutions with concentrations around 10–100 nM). Furthermore, the electrode modified with the 6-furfurylaminopurine ligand (Fig. 5) achieved a high % recovery for spiked Cd²⁺ solutions at very low concentrations (2 nM), highlighting the interest of this sensor.

It is interesting to note that sensors based on metal complexation on ligands had a very good long-term stability of several weeks to several months, even when stored in air at room temperature, which is a great advantage for this type of sensor [83,85,86,88]. However, their regeneration after analysis has not often been studied. In the case of sensors prepared by electrografting of 1,2-bis-[o-aminophenylthio]ethane (APTE) onto glassy carbon, our group has shown that simple electrochemical oxidation for 5 min was sufficient to regenerate the electrode with good repeatability of the electrochemical signal [88]. In addition, a comparison of repeatability measurements with sensors prepared by a multilayer and monolayer electrografting process for APTE immobilization clearly showed an improvement in repeatability with the monolayer process. This highlights the importance of the immobilization procedure on the electrochemical signal. We also noted that the selectivity of the sensor for Cd²⁺ was directly linked to the presence of APTE on the electrode surface. These results underline the role of the ligand for the selectivity of this type of sensor which includes an open circuit complexation step.

## 5. IV-Selective ligands for lead detection

The borderline “hard-soft” character of lead (II) does not allow a marked donor atom preference [51]. As can be seen in Table 3, few works have been done in recent years on the development of lead-selective ligands. 1-dodecanoyl-3-phenylthiourea [89] (DPT; Fig. 7a) was immobilized by adsorption, a physical process, that consists of dissolving the ligand into solution, before being deposited by drop-coating on the working electrode.

<figure><img src="image_12.png" /><figcaption>**Fig. 6.** Cbz-APL ligand [87] for the selective detection of cadmium.</figcaption></figure>

<table><caption>**Table 3**
: Selective ligands for lead detection and their characteristics.a)</caption><thead><tr><th>Ligand</th><th>Analysis<sup>a</sup></th><th>Electrode type</th><th>Procedure<sup>c</sup></th><th>Sensitivity (µA/µM)</th><th>LOD (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>d</sup></th><th>Regeneration</th><th>Interference study<sup>e</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>1-dodecanoyl-3-phenylthiourea (DPT)</td><td>SW-ASV</td><td>Glassy carbon</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: - 1.1 V<br>t<sub>d</sub>: 120 s</td><td>–</td><td>3.35</td><td>11 × 10<sup>3</sup>–45 × 10<sup>3</sup></td><td>2 %</td><td>–</td><td>Cu<sup>2+</sup>, Cd<sup>2+</sup>, Hg<sup>2+</sup>,<br>Zn<sup>2+</sup>, Ag<sup>+</sup>, Al<sup>3+</sup>, Co<sup>2+</sup><br>[C] 2:1<br>[C] 5:1</td><td>[89]</td></tr><tr><td>β-Cyclodextrin (β-CD)</td><td>DP-ASV</td><td>MWCNT/<br>SPE<sup>b</sup><br>(covalent)</td><td>t<sub>c</sub>: -<br>E<sub>d</sub>: - 0.8 V<br>t<sub>d</sub>: 600 s</td><td>20<br>8</td><td>4.3<br>11.1</td><td>15–20,000<br>30–500</td><td>5 %</td><td>Stirring at 0.31 V<br>for 300 s</td><td>Cu<sup>2+</sup>, Cd<sup>2+</sup>, Hg<sup>2+</sup>,<br>Zn<sup>2+</sup>, Ni<sup>2+</sup><br>[C] > 2:1</td><td>[90]</td></tr></tbody></table>

This ligand enabled lead analysis with a detection limit of 3.35 nM, 8 times lower than the standard imposed by the European Union.

Alam et al. chose to explore two methods of electrode functionalization with β-cyclodextrin (β-CD) (Fig. 7b) [90]. In the first one, the molecule was just adsorbed on the electrode surface. The second method involved covalent grafting of β-CD by an ester bond formed between the carboxylic acid of the oxidized carbon surface, and the hydroxyl groups of the ligand. It is interesting to note that the adsorption-modified electrode showed better sensitivity (× 2.5) and a detection limit 2.5 times lower than the covalent-grafting-modified electrode. The authors explained this result by the lower amount of immobilized ligands on the electrode surface when covalent grafting was performed. However, they also mentioned the uncertain reliability of the adsorption-functionalized electrode, suggesting that covalent grafting was more appropriate to ensure good repeatability.

It is interesting to compare the structures of these two ligands. The first one is linear, consisting of a long carbon chain, an electronically rich head, a phenyl and 4 heteroatoms with free doublets enabling lead complexation. The second is an oligosaccharide, rich in hydroxyl groups, forming a cavity for lead insertion. β-CD contains several heteroatoms favoring the multidentate complexation of large metal ions such as Pb²⁺, which increases the selectivity for larger relative to smaller metal ions. According to Shah [89], the signal obtained by DPT began to weaken in the presence of interferents whose concentration was 5 times greater than that of lead. β-CD remained selective for lead even when interfering cations were present at twice the concentration.

Although these ligands were specifically used for lead detection in these articles, other sensors were developed from them [80,91,92], which was consistent with their moderate selectivity (> 90 % for simple matrices such as tap water, lake water, and drinking water for Cd²⁺ solutions spiked at 100–170 nM).

## 6. V-Selective ligands for mercury detection

Mercury (II) is a metal ion with a strong affinity for soft donors such as sulfur and nitrogen-containing ligands, by HSAB theory [51]. Although its detection with specific ligands was more investigated than lead in recent years, designing a high-performance sensor is difficult due to the much lower and more difficult-to-achieve detection limit target (5 nM) [46]. Of the seven ligands presented in Table 4, only five managed to pass the 5 nM threshold, or to come very close to it.

Except for H4TSDB [91], these ligands are composed of sulfur atoms,

< 5% signal change for × 10000 K⁺, Zn²⁺, As³⁺, Ag⁺, Cs⁺, Ca²⁺, Sr²⁺, Co²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Pb²⁺, amino acid, citric acid, EDTA, glucose, SDS, CTAB, 2-amino-4-nitrophenol, and 3-chloro-5-nitrophenol

<footnote><sup>a</sup> Square wave (SW), anodic stripping voltammetry (ASV), differential pulse, (DP) <sup>b)</sup> Multiwalled carbon nanotubes/screen-printed electrode (MWCNT/SPE) <sup>c)</sup> t<sub>c</sub>: complexation time. Ed: applied reduction potential vs. Ag/AgCl. t<sub>d</sub>: applied potential time <sup>d)</sup> Reproducibility <sup>e)</sup> [C] is the ratio of interferent concentration to analyte concentration tested.</footnote>
<figure><img src="image_13.png" /><figcaption>**Fig. 7.** Specific ligands for lead detection: DPT [89] (a) and β-CD [90] (b).</figcaption></figure>

whose presence enables the complexation of mercury on the electrode surface. TMT [95], DAN [94] and FcTSC [92] sensors (Fig. 8) reached LOD of 5.3, 3.19 and 4 nM respectively, while with ATC4A [93], a ligand from the calixarene family, a remarkable limit of 1 pM was reached. Although its structure did not contain sulfur atoms, H4TSDB (Fig. 8) allowed the achievement of a performant sensor with a LOD of 0.8 nM [91]. However, its selectivity towards mercury was limited since the interference study was only carried out at interferent concentrations lower than those of mercury.

DAN (Fig. 8b), used for the selective detection of mercury, is very similar to DPT (Fig. 7a), used for the selective detection of lead. These two molecules are identical apart from the presence of two electron-withdrawing nitro groups on the aromatic ring of DAN. The interference studies performed in the presence of interfering ions in concentrations five times higher than the concentration of the target ion led to good selectivity even in the presence of Pb²⁺ for DAN and Hg²⁺ for DPT. It would have been interesting to measure the complexation constants of lead and mercury with these two ligands to understand the effect of the nitro-withdrawing groups on the sensor selectivity. This measurement was performed with N,N'-(disulfanediylbis(2,1-phenylene))bis(1-(thiophen-2-yl)methanimine), which exhibited a binding constant of 3.52 × 10⁴ M⁻¹ and 2.67 × 10⁴ M⁻¹ for Hg²⁺ and Cu²⁺, respectively, attributed to a structural change during the recognition process by the authors [98].

The calixarene ATC4A modified electrode not only exhibited low LOD as mentioned above but also a very high selectivity for mercury [93]. Interference studies with metal ions showed selectivity for mercury even in the presence of metal interferents (Ag⁺, Fe²⁺, Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺) 250 times more concentrated. Unfortunately, lead and cadmium were not included as interferents in this study.

The other sensors with ligands CPCHA, NDBD and DDBDD (Table 4) appeared less efficient for mercury detection, either because of their low selectivity [99] or because of their too-high LOD value [96,97]. For example, mercury detection was studied on DDBDD-modified electrodes using cyclic voltammetry by Kaur et al. [96]. While the ligand seemed promising in terms of selectivity, the electrode performance should be studied using a more sensitive method such as SW-ASV or DP-ASV to enhance its analytical performance.

The capability of the sensors from Table 3 for analyzing real samples was tested. The matrices were generally tap water, lake water, seawater, well water, stream water, river water, and milk. The samples were sometimes treated with an acidic solution to prevent bacterial growth, were filtered, and were diluted to adjust the pH of the solution [95,98,99]. A known amount of Hg²⁺ was generally added at the micromolar level (2–550 µM) or the nanomolar level (20–135 nM) in some articles [91,92,95], and the recovery percentage obtained was

approximately 96–105 % using the standard addition method and 89–107 % using the calibration curve method [94]. These results were sometimes confirmed by analyses with AAS [91,92] or microwave plasma atomic emission spectroscopy (MPAES) [98].

## 7. Conclusions

In conclusion, the modification of electrodes by ligands leads to the complexation of MTEs at their surface. This preconcentration step enables their detection at low concentrations and provides an additional parameter, alongside the metal deposition potential, to enhance the sensor selectivity when the complexation is preferential for a specific metallic cation. This complexation is linked to the presence of electron-rich heteroatoms, such as nitrogen, oxygen and sulfur. The corresponding sensors are reliable, with repeatability deviations generally below 5 %.

One of the advantages of this type of sensor is its stability, from a few days to several months, but this parameter was not always studied. Electrode regeneration was also a key step in the use of functionalized electrochemical sensors. It allowed the metal ion to be decomplexed from the ligand, making the sensor usable again for further measurements with good repeatability. The regeneration method depended on the electrode modification process. For carbon-paste electrodes, electrode regeneration consisted of removing the functional layer, based on a mixture of ligand and carbon paste, before redepositing a new one on the electrode surface [52,73,99]. For covalently immobilized ligands, regeneration was based on three different methods, and sometimes a combination of some of them. The first involved the use of acid-base solutions, or more generally acidic solutions, to modify complexation equilibria. The acidic medium allowed the protonation of the amino groups of the ligand, leading to the release of ions into the solution. The second method used electrochemical techniques, such as chro-noamperometry. It consisted of applying a fixed potential to the modified electrode to oxidize MTEs in an MTE-free solution under strong stirring to modify the chemical equilibrium and release the metal ions in the solution. Finally, regeneration was possible by dipping the modified electrode in a solution containing a strong complexing agent. The principle was based on the superior complexing power of the agent in solution, once again, to shift the chemical equilibrium and force metal ions off the electrode surface. This regeneration step was linked to the complexing strength between the metal ion and the ligand. It had to be high enough to obtain good sensor selectivity but low enough to allow its regeneration. The effectiveness of the electrode regeneration also depended on the modification procedure. Sensors with adsorbed molecules could not be regenerated without redepositing the ligand on the electrode surface. It also depended on analysis and regeneration
<table><caption>**Table 4**
Selective ligands for mercury detection and sensor performance.a)</caption><thead><tr><th>Ligand</th><th>Analysis<sup>a</sup></th><th>Electrode type</th><th>Procedure<sup>c</sup></th><th>Sensitivity (µA/µM)</th><th>LOD<sup>d</sup> (nM)</th><th>Linearity range (nM)</th><th>RSD<sup>e</sup></th><th>Regeneration</th><th>Interference study<sup>f</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>Amino-thiacalix[4]arene (ATC4A)</td><td>DP-ASV</td><td>Screen-printed electrodes</td><td>t<sub>c</sub>: 120 s<br>E<sub>d</sub>: -0.4 V<br>t<sub>d</sub>: 30 s</td><td>-</td><td>0.001</td><td>0.002–0.020</td><td>3 %</td><td>-</td><td>Ag<sup>-</sup>, Fe<sup>2+</sup>, Ni<sup>2+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup><br>[C] 250:1<br>K<sup>+</sup>, Na<sup>+</sup>, Cs<sup>+</sup><br>[C] 580:1<br>Ca<sup>2+</sup>, Co<sup>2+</sup>, Ba<sup>2+</sup>, Mg<sup>2+</sup>, Cr<sup>3+</sup>, As<sup>3+</sup>, Fe<sup>3+</sup>, Be<sup>2+</sup><br>[C] 450:1<br>C<sub>2</sub>O<sub>4</sub><sup>-</sup>, CO<sub>3</sub><sup>-</sup>, SO<sub>4</sub><sup>-</sup>, SO<sub>3</sub><sup>-</sup>, NO<sub>3</sub><sup>-</sup>, NO<sub>2</sub><sup>-</sup>, Cl<sup>-</sup>, F<sup>-</sup>, I<sup>-</sup><br>[C] 410:1</td><td>[93]</td></tr><tr><td>1-(2,4-dinitrophenyl)-dodecanoylthiourea (DAN)</td><td>DP-ASV</td><td>Glassy carbon</td><td>-</td><td>18.44</td><td>3.19</td><td>100 × 10<sup>3</sup>–1000<br>× 10<sup>3</sup> and<br>10–110</td><td>3.5<br>%</td><td>-</td><td>Co<sup>2+</sup>, Cu<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, Zn<sup>2+</sup>, Fe<sup>3+</sup><br>[C] 5:1</td><td>[94]</td></tr><tr><td>2,4,6-trimercapto-1,3,5-triazine (TMT)</td><td>DP-ASV</td><td>Gold Nanoparticles<br>/ carbon graphite<br>paste<br>(covalent)</td><td>t<sub>c</sub>: 4 min<br>E<sub>d</sub>: -0.3 V<br>t<sub>d</sub>: 80 s</td><td>330 and 60</td><td>5.3</td><td>16–200<br>200–2000</td><td>1.8<br>%</td><td>0.2 M HNO<sub>3</sub> for 3 min</td><td>Cd<sup>2+</sup>, Pb<sup>2+</sup>, Zn<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup></td><td>[95]</td></tr><tr><td>1,7-diamino-3,9-dibutyl benzo[1,2,3-de:4,5,6-d'e']diquinoline-2,8(3H,9H)-dione (DDBDD)</td><td>CV</td><td>-</td><td>[-0.3; 0.4]<br>V<br>20 mV/s</td><td>-</td><td>610</td><td>-</td><td>-</td><td>No regeneration</td><td>Ni<sup>2+</sup>, Co<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Pb<sup>2+</sup><br>[C] 10:1</td><td>[96]</td></tr><tr><td>N, N<sup>′</sup> di (2-hydroxy-5-[(4-nitrophenyl) diazenyl]benzaldehyde) benzene-1,2-diamine (NDBD)</td><td>Chronoamperometry</td><td>MWCNT<sup>b</sup>/<br>glassy carbon</td><td>E: 0.1 V</td><td>514.6</td><td>60</td><td>1000–25,000</td><td>3 %</td><td>-</td><td>Co<sup>2+</sup>, Cu<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, Cr<sup>3+</sup>, Ni<sup>2+</sup>, Mn<sup>2+</sup> (6,3 %), PO<sub>4</sub><sup>-</sup>, Cl<sup>-</sup><br>[C] 2:1</td><td>[97]</td></tr><tr><td>N,N<sup>′</sup>,N<sup>′′</sup>,N<sup>′′′</sup>-Tetrasalicylidene-3,3'-diaminobenzidine (H<sub>4</sub>TSDB)</td><td>SW-ASV</td><td>Carbon paste</td><td>t<sub>c</sub>: 3 min<br>E<sub>d</sub>: -0.1 V<br>t<sub>d</sub>: 1 min s</td><td>15</td><td>0.8</td><td>2.4–220</td><td>3 %</td><td>0.01 M EDTA pH 4.5 and<br>washing with water</td><td>Zn<sup>2+</sup>, Cu<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, Ni<sup>2+</sup>, Mn<sup>2+</sup><br>[C] 0.7:1</td><td>[91]</td></tr><tr><td>N,N<sup>′</sup>-(disulfanediylbis(2,1-phenylene))bis(1-(thiophen-2-yl)meth animine)</td><td>DPV</td><td>Carbon paste<br>electrode</td><td>-</td><td>-</td><td>56.8</td><td>1990–41,800</td><td>-</td><td>-</td><td>Na<sup>+</sup>, Li<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup>, Ni<sup>2+</sup>, Pb<sup>2+</sup>, Co<sup>2+</sup>, Fe<sup>2+</sup>, Cr<sup>3+</sup>, Al<sup>3+</sup>, Fe<sup>3+</sup><br>[C] 1:1</td><td>[98]</td></tr><tr><td>N-p-Chlorophenylcinnamohydroxamic acid (CPCHA)</td><td>SW-ASV</td><td>Carbon paste<br>electrode</td><td>t<sub>c</sub>: 210 s<br>E<sub>d</sub>: -0.6 V<br>t<sub>d</sub>: 60 s</td><td>0.334</td><td>12.9</td><td>1000–25,000</td><td>3.80<br>%</td><td>Renewal by polishing the<br>electrode surface and<br>redepositing fresh carbon<br>paste</td><td>NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup>, Ni<sup>2+</sup><br>[C] 100:1<br>SO<sub>4</sub><sup>-</sup><br>[C] 10:1<br>Co<sup>2+</sup>, Cd<sup>2+</sup><br>[C] 5:1<br>Cu<sup>2+</sup><br>[C] 20:1<br>Pb<sup>2+</sup><br>[C] 1:1<br>Fe<sup>3+</sup><br>[C] 1:1</td><td>[99]</td></tr><tr><td>Ferrocene-based thiosemicarbazone</td><td>DPV-ASV</td><td>Glassy carbon<br>(covalent)</td><td>t<sub>c</sub>: 20 min<br>E<sub>d</sub>: -1.2 V<br>t<sub>d</sub>: 120 s</td><td>-</td><td>4</td><td>5–1000</td><td>5 %</td><td>-</td><td>Zn<sup>2+</sup>, Cd<sup>2+</sup>, Pb<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup><br>[C] 5:1</td><td>[92]</td></tr></tbody></table>

<footnote><sup>a</sup> Differential pulse (DP), anodic stripping voltammetry (ASV), cyclic voltammetry (CV), square wave (SW) <sup>b</sup> Multiwalled carbon nanotubes (MWCNT) <sup>c</sup> t<sub>c</sub>: complexation time. E<sub>d</sub>: applied reduction potential vs. Ag/AgCl. t<sub>d</sub>: applied potential time, [E<sub>1</sub>;E<sub>2</sub>] potential range for CV analysis <sup>d</sup> Values above the limits set by European directives are in red <sup>e</sup> Reproducibility <sup>f</sup> [C] is the ratio of interferent concentration to analyte concentration tested.</footnote>
<figure><img src="image_14.png" /></figure>

< 5% signal change for Cd²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Mn²⁺

<figure><img src="image_15.png" /></figure>

< 5% signal change for a mixture of × 5 Zn²⁺, Cd²⁺, Pb²⁺, Ni²⁺, and Cu²⁺

<figure><img src="image_16.png" /></figure>

< 5% signal change for × 5 Co²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Fe³⁺

<figure><img src="image_17.png" /></figure>

< 5% signal change for × 0.67 Zn²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mn²⁺ and Cu²⁺

<caption>**Fig. 8.** Specific ligands for mercury detection: TMT [95] (a) DAN [94] (b) FcTSC [92] (c) and H₄TSDB [91] (d).</caption>

conditions, which could be a source of sensor deterioration. Indeed, the regeneration method had to be compatible with the functionalization of the electrode. Various parameters needed to be taken into account, such as the pH and the electrochemical oxidation conditions, which could damage the organic layer of the working electrode. Thus, the ligand had to be designed to allow both the selective complexation of the target ion and its regeneration. While strong complexation enhanced sensor selectivity, it also required stronger regeneration conditions that might have damaged the surface. An alternative strategy involved incorporating protonable functions, such as amines, to weaken the complexation constant by adjusting the pH when needed.

While some ligands were described as selective for a given analyte, this aspect was not always thoroughly discussed in articles. Parameters related to selectivity, such as complexation constants and species predominance diagrams, could provide useful insights into ligand selectivity. When an open-circuit preconcentration step was not used, selectivity was primarily determined by the applied reduction potential rather than the ligand itself. The ligand mainly functions as a complexing agent, facilitating analyte accumulation on the electrode surface and enhancing measurement sensitivity. The design of new ligands enabling selective complexation of metal ions continues to be explored in the literature [100–103], which should help the development of selective electrochemical sensors. Moreover, the consistently good results obtained with real samples demonstrate that these methods possess sufficient selectivity to detect Cd²⁺, Hg²⁺, and Pb²⁺ pollutants in a wide range of environmental liquid and solid samples.

Finally, the many advantages of ligands, coupled with electrochemical techniques, are a real asset for the detection of MTEs. The overall equipment and sensor design are much less expensive than conventional analysis techniques while enabling on-site analysis of a sample in just a few minutes. Selective electrochemical sensors are therefore prime candidates for offering an alternative to conventional laboratory analysis techniques, which are generally more time-consuming and costly. Moreover, the selective complexation of ions is a highly attractive technique for the analysis of not only electroactive cations but also non-electroactive ions in the presence of a redox probe, as previously demonstrated [104]. This technique could be of interest in the analysis of emerging pollutants such as fluoride ions [105–107].

### CRediT authorship contribution statement

**Gaël Levanen:** Writing – original draft, Visualization, Validation, Investigation. **Florence Razan:** Supervision, Project administration, Funding acquisition. **Sara Bretin:** Visualization, Validation. **Stéphanie Betelu:** Writing – original draft, Visualization, Validation. **Karine Michel:** Project administration, Funding acquisition. **Florence Geneste:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by the French National Agency (ANR-18-CE42-0011, Programme AAPG 2018).

### Data availability

Data will be made available on request.

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