Enhancing Trace Pb²⁺ Detection via Novel Functional Materials for Improved Electrocatalytic Redox Processes on Electrochemical Sensors: A Short Review

Abstract

The efficient detection of lead ions (Pb²⁺) is significant for environmental protection and public health. Electrochemical detection has emerged as one of the most promising technologies due to its low detection limits, high sensitivity, and cost-effectiveness. However, significant challenges remain, including issues related to sensitivity, selectivity, interference, and the stability of electrode materials. This review explores recent advancements in the field, focusing on integrating novel catalytic materials and innovative sensor construction methods. Particular emphasis is placed on enhancing the electrocatalytic redox processes on sensor surfaces using advanced nanomaterials such as MXenes, ferrite-based nanomaterials, carbon nanomaterials, and metal–organic frameworks (MOFs). Additionally, the role of biomaterials and enzymes in improving electrochemical sensors’ selectivity and anti-interference capabilities is discussed. Despite the impressive low detection limits achieved, real-world applications present additional challenges due to the complex composition of environmental samples. The review concludes with future perspectives on overcoming these challenges by leveraging the unique properties of catalytic materials to develop more effective and reliable electrochemical sensors for trace Pb²⁺ detection.

1. Introduction

Pollution by Pb (II) ions is mainly caused by battery manufacturing [1,2], fossil fuel mining [3,4], and farmland sewage irrigation [5,6]. Lead ions (Pb²⁺) are difficult to degrade through biological processes and can accumulate in the food chain via biological amplification [7,8]. They are a significant threat to the natural environment and human health. Due to their high toxicity, Pb²⁺ ions can cause severe damage to the nervous system, hematopoietic system, and many important organs; they may also lead to cancer [9,10,11]. Thus, it is significant to realize the detection of trace amounts of Pb²⁺ ions. At present, there are many technologies to realize the detection of Pb²⁺ ions, such as atomic absorption spectrometry (AAS) [12,13], atomic fluorescence spectrometry (AFS) [14], inductively coupled plasma emission spectrometry (ICP-OES) [15,16], and electrochemical detection [17,18].

As shown in Figure 1, atomic absorption spectrometry, atomic fluorescence spectrometry, and inductively coupled plasma emission spectrometry have excellent detection capabilities [19,20]. Their detection limits can reach ppm or ppb levels. Atomic absorption spectrometry can even simultaneously determine the presence of multiple heavy metal elements, including lead [21,22]. However, these techniques necessitate the use of complicated and expensive devices [23], and the detection process is complex and depends on manual operations, which makes it difficult to realize the automatic detection of Pb²⁺ ions. In contrast, the electrochemical detection method is considered one of the most promising technologies for Pb²⁺ detection [24,25]. It offers several advantages, including high sensitivity, fast detection speed, and low detection limits, with the minimum limit reaching ppb levels. Furthermore, electrochemical detection systems are more cost-effective and accessible, allowing for the potential development of portable and automated detection devices.

Currently, numerous methods for detecting Pb²⁺ rely on electrochemical technology, among which anodic stripping voltammetry (ASV) is the most commonly used. This method can be summarized as the process in which Pb²⁺ ions are first deposited onto the electrode surface and then subsequently dissolved [26,27,28]: Pb²⁺ in the water sample is deposited onto the electrode surface to form an insoluble substance through cathodic polarization. This layer is then oxidized and dissolved when an opposite potential is applied to the electrode surface. The concentration of Pb²⁺ in the solution can be calculated based on the magnitude of the oxidation current peak generated. This method relies on the interaction between the analyte and the electrode surface, where the efficiency of electron transfer and the strength of the generated signal are critical factors [29]. The electrocatalytic oxidation and reduction of Pb²⁺ ions play a vital role in this process [30,31], and effective catalytic materials can significantly enhance the rate of the electrocatalytic redox reactions of Pb²⁺ ions, leading to more pronounced and distinguishable signals. Catalysts work by lowering the activation energy required for these reactions, thereby increasing the reaction rate and improving the sensitivity and selectivity of the electrochemical sensor [32]. Advanced nanomaterials, with their high surface area and unique electronic properties, are well suited for this purpose. They can facilitate the better adsorption and interaction of Pb²⁺ ions with the electrode surface, enhancing signal amplification and more accurate detection. Despite the effectiveness of ASV in controlled laboratory settings, real-world applications present additional challenges. Real-world environments contain various interfering substances that can affect the adsorption, transmission, and reaction processes of Pb²⁺ on the electrode surface [33,34], leading to Pb²⁺ signal peaks weakening or overlapping with other spurious peaks, ultimately complicating the analysis of electrochemical signals.

Different electrochemical sensing platforms possess distinct physical and electrochemical properties, which not only directly influence the sensitivity and detection limits but also constrain the applicability of the final electrochemical sensors. Four representative electrochemical sensing platforms in the current field of electrochemical detection are introduced below: Glassy carbon electrodes (GCEs) are widely used due to their excellent conductivity and broad potential window, as demonstrated by Ulaganambi et al. [35], who modified GCEs with human immunoglobulin G (hIgG) and carbon nanocomposites, creating the hIgG@CNT-F/GCE sensor, shown in Figure S1a. Screen-printed carbon electrodes (SPCEs) present advantages over traditional GCEs, including lower cost and ease of mass production, as demonstrated by Kongkaew et al. [36], who fabricated SPCEs from waste DVDs and carbon ink, further modified with gold nanoparticles (AuNPs), as illustrated in Figure S1b. Electrochemical transistors (ECTs) are gaining popularity for their inherent signal amplification, bionic ability, and mechanical flexibility. Ji et al. [37] developed a synaptic ECT-based flexible and bionic pH sensor, depicted in Figure S1(c-1,c-2), achieving high sensitivity and ultrafast response times. Meng et al. [38] introduced vertically aligned carbon nanofibers (VACNFs) for the development of dopamine microsensors, achieving unprecedented sensitivity and low detection limits, as shown in Figure S1(d-1,d-2,d-3). For a detailed discussion and comparison of the above electrochemical sensing platform studies, see Supplementary Materials Text S2. The development of current advanced electrochemical sensing platforms not only aims for miniaturization, microfabrication, and diverse form factors to suit a broader range of applications but also strives for improved sensitivity and detection limits. When the application scope is limited to the detection of trace lead ions, these advancements are essential to effectively monitor and analyze trace substances such as Pb²⁺ ions in complex real-world environments. In addition, based on the Web of Science Core Collection database, we retrieved 1697 publications from the past decade related to the theme of this review. The research areas, languages, and major concepts of these publications are visually presented in the treemap charts in Figures S2–S4, respectively. From these analyses, we can derive: Figure S2 highlights the predominant research areas, with the highest number of publications in chemistry (1562), instruments and instrumentation (1283), and electrochemistry (965). Figure S3 shows that the vast majority of publications are in English (1679), followed by a few in Chinese (17) and unspecified languages (8). Figure S4 details the major concepts covered, with the most publications in biomaterials (238); equipment, apparatus, devices and instrumentation (196); and methods and techniques (159). This comprehensive analysis underscores the multidisciplinary and global nature of research in the electrochemical detection of Pb²⁺ ions, addressing various practical and theoretical aspects.

The choice of materials and the modifications applied to these electrochemical platforms play a crucial role in enhancing their performance. Modifying electrode surfaces with functional materials, such as nanocomposites and biomolecules, can significantly improve the sensitivity, selectivity, and overall detection capabilities of electrochemical sensors. This is evident in the examples provided, where the use of carbon nanomaterials, gold nanoparticles, and advanced fabrication techniques can lead to notable improvements in sensor performance. Traditional materials often lack sensitivity, selectivity, and interference resistance. Developing and integrating novel catalytic materials is essential to overcome these limitations. Future research must focus on exploring the catalytic properties of emerging nanomaterials, such as hybrid composites combining the strengths of metal oxides, MXenes, carbon nanomaterials, and biological macromolecules [39,40,41,42]. These advanced materials have the potential to significantly improve detection performance by enhancing electrocatalytic processes.

Cost is another significant issue. Achieving superior detection performance often requires complex electrode structures and the use of precious metals [43,44], which can be prohibitively expensive. Replacing these with cheaper functional materials [45], such as carbon nanomaterials and inexpensive metal/metal oxide nanomaterials, offers a potential solution. However, the stability of these materials, mainly when used in chemically modified electrodes, remains a concern. Developing functional polymeric coatings [30,46] (e.g., Nafion and polyaniline) and new modification techniques can help address these stability issues, making the sensors more reliable and cost-effective.

Interference from other substances in complex environmental samples poses a further challenge [45,47]. The complex composition of substances in real-world environments can significantly interfere with detecting trace amounts of Pb²⁺ ions. Biomaterials, with their unique spatial structures and abundant active groups, can improve the selectivity of electrochemical sensors. However, the thermal stability of some biomaterials is relatively low, and the structure of electrochemical nucleic acid sensors can be complex and costly. To enhance sensor performance, future research should focus on screening and identifying biomaterials with a higher affinity for Pb²⁺ ions and greater stability, integrating novel nanomaterials with excellent electrical properties into biomaterial-based sensing platforms and reducing overall sensor costs while maintaining high detection capabilities.

In summary, the electrochemical detection of Pb²⁺ ions requires significant advancements in catalysis, cost-efficiency, and interference management to achieve reliable and practical sensor performance. The subsequent sections of this review will delve into the key advancements and future perspectives in this field, highlighting the roles of various nanomaterials and innovative sensor construction methods in addressing these challenges.

2. Improving the Efficiency of Electrocatalytic Redox Reactions Using Inorganic Nanomaterials for Enhanced Pb²⁺ Detection

The main challenge in the electrochemical detection of trace amounts of Pb²⁺ ions lies in identifying and amplifying target electrochemical signals [48]. A critical approach to overcoming these challenges is to focus on the electrocatalytic redox reactions of Pb²⁺ ions on the sensor surface. By using appropriate materials, the efficiency of the reaction, along with the surface adsorption and enrichment of Pb²⁺ ions, can be significantly increased, leading to improved detection performance.

In recent years, there have been many reported works on the hybrid engineering, defect engineering, etc., of catalytic materials to improve the performance of electrochemical sensors. Hybrid engineering involves combining different materials or nanostructures to create composites with enhanced properties. As shown in Figure 2a, several types of materials that are currently receiving widespread attention and application, such as carbon-based nanomaterials, metal nanomaterials, and inorganic composite materials, each have unique characteristics and advantages that contribute to the overall performance of electrochemical sensors. The radar chart illustrates the electron conductivity, catalytic activity, preconcentration capacity, selectivity, and stability of each material type, highlighting their respective strengths in enhancing sensor performance. For specific discussion and comparison, see Supplementary Materials Text S1. On the other hand, defect engineering focuses on introducing defects into the crystal structure of materials to modulate their electronic properties and create additional active sites. Common methods for creating these defects include heat treatment in a specific gas atmosphere and chemical etching. Defects such as vacancies, interstitials, or substitutional atoms can significantly enhance catalytic performance by altering the electronic structure and increasing the reactivity of the material. By optimizing the defect concentration and distribution, the efficiency of Pb²⁺ detection can be further improved. As depicted in Figure 2b, hybrid engineering and defect engineering offer substantial benefits in improving the performance of electrochemical sensors.

Unlike traditional materials, nanomaterials have unique physical and chemical properties due to surface effects, small-size effects, and quantum effects [49,50,51,52]. The extremely high specific surface area of these nanomaterials [53,54] not only helps to capture trace amounts of substances in the environment but also facilitates the combination of nanomaterials with other functional materials through physical or chemical methods [55,56,57]. This provides more possibilities for designing and developing new electrochemical sensors utilizing inorganic nanomaterials.

So far, research on electrochemical signal amplification based on nanomaterials has achieved certain milestones. Various nanomaterials, including MXene-based nanomaterials, ferrite-based nanomaterials, and metal–organic frameworks (MOFs), have shown great potential in enhancing the detection of trace amounts of Pb²⁺ ions by electrochemical sensors. These materials enhance electrochemical performance by increasing selectivity towards Pb²⁺ ions, optimizing reaction pathways, and enhancing charge transfer. This amplifies the target signals and reduces interference from other substances, making the detection process more accurate and reliable.

Titanium carbide (Ti₃C₂T_x) is known for its exceptional electrical conductivity, mechanical stability, and tunable surface properties. It is an ideal substrate for preparing high-performance electrochemical sensors. Because bismuth (Bi) not only has excellent resistance to dissolved oxygen but also has low toxicity and a relatively low price, these merits make bismuth an ideal electrochemically active material instead of traditional precious metals [58,59,60] (e.g., gold and platinum) and toxic metals [61,62] (e.g., mercury). As shown in Figure 3a, He’s team [63] deposited BiNPs on titanium carbide (Ti₃C₂T_x) sheets and obtained a nanocomposite with excellent conductivity; the nanocomposite was then used to fabricate a new electrochemical sensor. Benefiting from the excellent electrical and physical characteristics of BiNPs, the electrochemical sensor realized the detection of trace amounts of Pb²⁺ ions; the linear range of Pb²⁺ was 0.06–0.6 μM, with a detection limit of 10.8 nM. Similar to Ti₃C₂T_x, CoFe₂O₄ nanomaterials not only exhibit excellent performance in electrocatalysis but can be synthesized through various methods, including solution-based, thermal decomposition, and hydrothermal. This enables precise control over their morphology and surface structure, facilitating the enrichment of Pb²⁺ ions on the surface of electrochemical sensors and further promoting redox reactions involving Pb²⁺ ions.

He et al. [64] reported the fabrication of a BiNPs@CoFe₂O₄ nanocomposite that was used to modify GCE; the preparation process is illustrated in Figure 3b. The fabricated electrochemical sensor achieved a detection limit of 7.3 nM, and the linear range was 0.06–0.6 μM. The combination of metal oxides and metal nanoparticles improved the detection limit of the fabricated electrochemical sensor compared to the pure-BiNP-modified GCE. This may be due to the synergistic effects of metal oxide and metal nanomaterials, including high specific surface area and excellent electrical properties. The electrochemical sensor also displayed a good anti-interference ability toward other metal ions (e.g., Cd²⁺).

MOFs, such as ZIF-67, also play a pivotal role in the electrochemical detection of trace amounts of Pb²⁺ ions. These materials possess highly porous structures and high specific surface areas, providing abundant active catalysis sites. The inherent tunability of MOFs allows for the optimization of their catalytic properties, making them highly effective in promoting the electrocatalytic redox reactions of metal ions. By facilitating the adsorption and subsequent redox processes of Pb²⁺ ions, MOFs enhance the sensitivity and selectivity of electrochemical sensors, enabling more accurate detection even at deficient concentrations. For example, Zhang’s team [65] modified a glassy carbon electrode (GCE) with zeolite imidazolium ester skeleton material (ZIF-67) and multi-walled carbon nanotubes (MWCNTs), as shown in Figure 3c. Nafion was used to immobilize the nanomaterials on the electrode surface. This resulted in a modified electrode with multilayer structures and a detection limit as low as 1 nM, with a significantly extended linear range from 1.38 nM to 5 μM. The high electron transfer ability of CNTs, combined with the catalytic activity of ZIF-67 and the enrichment effect of Nafion on Pb²⁺, finally enhanced the overall performance of the sensor. The sensor’s ability to function normally even when the concentration of interfering ions was 50 times higher than that of Pb²⁺ ions demonstrates the effectiveness of these functional materials in providing selectivity and resistance to interference. The above data are summarized in

Table 1. Electrochemical sensor parameters for the detection of Pb²⁺.

Electrochemical Sensor	LOD (S/N = 3)	Linear Range	Ref.
Based on metal/metal oxide nanomaterials
BiNPs@Ti3C2Tx@GCE	10.8 nM	0.06–0.6 μM	[63]
BiNPs @CoFe2O4@GCE	7.3 nM	0.06–0.6 μM	[64]
ZIF-67/MWCNT/Nafion/GCE	1 nM	1.38 nM to 5 μM	[65]
Based on carbon nanomaterials
HD-CNTf	0.45 nM	0.48–144.6 nM	[66]
L-cys/GR-CS/GCE	2.17 nM (0.45 μg/L)	5.02–300 nM (1.04–62.1 μg/L)	[67]
PrGO/AuNPs/Sal-Cys/GCE	0.04 nM	1–10 nM	[68]
BiNPs modified aerogel	0.63 pM (0.13 ng/L)	0.02–2 nM (5–500 ng/L)	[69]
Based on renewable biomaterials
CS/MWCNTs/GR/AuNPs/Nafion/GCE	0.283 nM	1 nM–50 μM	[70]
MWCNT-β-CD (physical modification)/SPE	0.004 μM (0.9 ppb)	0.015–0.50 μM (3.1–103.3 ppb)	[71]
MWCNT-β-CD (chemical modification)/SPE	0.011 μM (2.3 ppb)	0.030–0.50 μM (6.2–103.5 ppb)	[71]
Based on nucleic acids
Tetrahedron-based DNAzyme sensors	0.01 μM	0.01–100 μM	[72]
MCH/P/AuNPs/RGO/GCE	0.015 nM	0.05–400000 nM	[73]
The sensor based on DDFA strategy	0.048 pM	0.2 pM–100 nM	[74]

Note: The grey background in the table indicates more detailed classifications based on the types of materials used to construct the electrochemical sensors.

.

In summary, the utilization of inorganic nanomaterials, such as MXene-based nanomaterials, ferrite-based nanomaterials, and metal–organic frameworks (MOFs), has significantly advanced the field of electrochemical detection of trace amounts of Pb²⁺ ions. These materials offer unique properties that enhance the efficiency of electrocatalytic redox reactions, thereby improving the identification and amplification of target electrochemical signals. The high specific surface area, excellent electrical conductivity, and tunable surface chemistry of these nanomaterials facilitate the effective adsorption and enrichment of Pb²⁺ ions on sensor surfaces. In addition, other inorganic nanoparticles, such as chromium (Cr³⁺)-doped magnesium aluminate (MgAl₂O₄) nanoparticles, have shown promise in electrochemical applications. A recent study [75] synthesized chromium (Cr³⁺)-doped MgAl₂O₄ using a low-temperature self-ignition solution combustion method, demonstrating excellent photocatalytic activity and significant electrochemical behavior. The proton diffusion coefficient of the optimized NP electrode was found to be substantially greater than that of the pure MgAl₂O₄, suggesting its potential application as an anodic electrode. These advanced materials collectively contribute to the improved performance and broadened applicability of electrochemical sensors in detecting trace amounts of Pb²⁺ ions.

3. Advantages of Carbon Nanomaterials in Achieving Lower Detection Limits for Pb²⁺ Ions

Despite the impressive low detection limits (LODs) achieved by the inorganic functional materials mentioned above, often in the nanomolar (nM) range [63,64], there remains a significant challenge. The LODs are typically not within the linear range of detection, and the LOD values are often far below the minimum value of the linear range. This discrepancy can hinder the practical application of these materials in real-world scenarios where consistent, linear responses across a broader concentration range are essential for accurate quantification.

On the other hand, carbon nanomaterials offer numerous advantages, such as excellent conductivity, low manufacturing cost, and accessible surface modification [76,77]. These properties make carbon nanomaterials highly attractive for electrochemical sensing applications. They can be used directly as functional materials with electrochemical activity or as substrates for other functional materials, thereby enhancing the overall performance of electrochemical sensors. As a typical one-dimensional nanomaterial, carbon nanotubes (CNTs) exhibit stable chemical and electrochemical properties and superior strength, conductivity, and thermal conductivity [78,79,80]. These characteristics make CNTs ideal candidates for catalytic materials in electrochemical sensors. Techniques such as acidification and gas-phase oxidation can decorate the surface of CNTs with numerous chemically active sites [81,82]. These active sites facilitate the interaction between CNTs and trace amounts of target substances in the environment, resulting in improved electrochemical performance. Similarly, metal nanomaterials also provide significant benefits in electrochemical sensing. Their high surface area and excellent catalytic properties enhance the sensitivity and selectivity of sensors. For instance, noble metals such as gold and platinum have been extensively used due to their superior electrocatalytic activity and stability.

There has been considerable research to leverage the excellent properties of various one-dimensional and two-dimensional carbon nanomaterials. By addressing their inherent drawbacks through modification and rational design, these materials can further promote the redox reactions of trace Pb²⁺ ions and enhance the conduction of the resulting electrical signals. This has led to the precise detection of Pb²⁺ ions at nanomolar levels.

In a study by Gupta et al. [66], an electrochemical sensor based on highly densified carbon nanotube fiber (HD-CNTf) demonstrated the detection of Pb²⁺ ions at ppm levels, as shown in Figure 4a, achieving a LOD of 0.45 nM and a linear range of 0.48–144.6 nM. The π-electron-conjugated structure and high specific surface area of carbon nanotubes can enhance their electron transfer ability and provide more active sites, which is crucial for promoting the redox reaction of Pb²⁺ on their surface. This effective catalysis promotes the oxidation or reduction reactions of Pb²⁺ ions, enabling the electrochemical sensor to detect low concentrations of Pb²⁺ ions. CNTs are also commonly combined with other functional materials to modify bare electrodes, enhancing detection performance, extending service life, and improving selectivity. Furthermore, combining CNTs with other advanced materials can further optimize their catalytic properties.

The two-dimensional structure of graphene endows it with unique physical and chemical properties [83]. The material not only has high specific surface area but also possesses excellent mechanical and electrochemical properties. The high specific surface area of graphene, due to its planar two-dimensional structure, exposes more active sites on its surface, facilitating electrocatalytic reactions. Additionally, the two-dimensional structure allows electrons to travel freely across the plane [84,85], offering more direct and efficient electron transfer paths than carbon nanotubes, thereby reducing electron transfer resistance and enhancing the rate and efficiency of electrocatalytic reactions. Moreover, graphene’s rich π-electron-conjugated structure and active edge sites confer high electrochemical activity, promoting electrochemical reactions and improving catalytic performance. As shown in Figure 4b, Zhou et al. [67] prepared a new electrochemical sensor based on L-cysteine and graphene-modified GCE. The prepared sensor showed good performance and stability in detecting Pb²⁺ ions, achieving a LOD value of 2.17 nM and a linear range of 5.02–300 nM.

However, the π–π interactions and van der Waals forces within graphene can hinder the dispersion of nanoparticles, ultimately reducing the actual surface area available for catalytic activity [86]. Furthermore, when graphene layers stack, the interlayer distance increases, negatively affecting electron transfer efficiency. This stacking results in stronger π–π interactions and van der Waals forces, leading to electron scattering and a higher energy barrier for electron transfer between layers. Consequently, the electron mobility decreases, limiting the overall electrochemical performance. The increased interlayer distance also extends the path electrons must travel, increasing resistance and reducing the efficiency of electrochemical reactions.

To address these issues, Priya et al. [68] synthesized the salicylaldehyde L-cysteine (Sal-Cys) ligand. They combined it with gold nanoparticles (AuNPs) to modify partially reduced graphene oxide (PrGO). As shown in Figure 4c, these mixed materials were used to modify GCE for Pb²⁺ detection. Introducing AuNPs improved electrical conductivity and increased the surface area of graphene on the electrode. The modified electrochemical sensor enabled susceptible detection of Pb²⁺ ions, exhibiting a detection limit as low as 0.04 nM with a linear range of 1–10 nM. The AuNPs enhanced the electron transfer pathways and provided additional catalytic sites, thereby overcoming the limitations posed by graphene stacking and significantly boosting the sensor’s performance. The transition from conventional one-dimensional and two-dimensional nanomaterials, such as graphene and carbon nanotubes (CNTs), to highly porous three-dimensional aerogels is of great significance. This transition provides greater surface area and more accessible active sites, significantly enhancing the sensitivity and selectivity of sensors for detecting trace amounts of Pb²⁺ ions. Similarly aiming to improve the detection ability of electrochemical sensors, Zhang et al. [69] prepared a novel mixed aerogel with CNTs and graphene oxide (GO), and then loaded BiNPs on the surface of the aerogel to construct an electrochemical sensor that can be used for Pb²⁺ detection, as shown in Figure 4d. In addition to the excellent electrical conductivity of the nanomaterials, the porous, three-dimensional structure of the aerogel greatly improved the specific surface area of the electrode materials, endowing the electrochemical sensor with better performance. The limit of detection reached 0.63 pM with a linear range of 0.02–2 nM.

The combined use of carbon nanomaterials and other catalytic materials highlights the potential for synergistic effects, leading to superior electrochemical sensor performance. This approach addresses inherent material limitations, optimizes catalytic properties, and ultimately improves the detection capabilities for trace substances like Pb²⁺ ions. The data summarized in Table 1 underscore the advancements and effectiveness of these innovative modifications in electrochemical sensing.

4. Enhancing Selective Adsorption and Anti-Interference in Pb²⁺ Detection via Functional Group Modification of Electrodes

A significant challenge in the electrochemical detection of Pb²⁺ ions is that many metal ions and even some organic substances have similar redox potentials. When a specific potential is applied to the electrode, multiple substances can produce overlapping oxidation peaks, complicating the accurate interpretation of results. This issue is particularly pronounced when detecting Pb²⁺ ions at low concentrations [87,88].

Biological materials [89,90], such as polymers, nanomaterials, and composites, typically possess specific spatial structures and functional groups [91,92,93,94]. For example, cyclodextrin molecules have a hollow, cage-like structure with numerous hydroxyl groups on the surface. These hydroxyl groups can bind with Pb²⁺ ions through hydrogen bonding, allowing for chemical modification or specific interactions with other molecules. Chitosan, a natural polysaccharide, contains alternating amino and hydroxyl groups [95,96,97]; the amino groups can bind with Pb²⁺ ions through coordination bonds, forming a well-ordered dimer chain structure. These specific spatial structures and functional groups endow some biological materials with extensive application potential in the field of electrochemical detection. Additionally, certain biological materials possess nano- or microporous structures [98,99], help increase the sensor’s surface area, thereby enhancing their adsorption capacity for Pb²⁺ ions.

In practical applications for fabricating electrochemical sensors, control over the usage and spatial structure of biological materials on the sensor surface can be achieved through several methods: (1) Hybridization: Biological materials can be blended with carbon-based nanomaterials or metal nanomaterials through methods such as physical blending or chemical conjugation to control the precise amount of biological material. This approach not only improves the inherent poor conductivity of biological materials but also provides a synergistic effect that enhances the Pb²⁺ detection capability. (2) Layer-by-layer (LbL) assembly technology: This method allows for the sequential deposition of biological and conductive materials, creating multilayered structures with specific characteristics. By varying the number of layers and assembly conditions, the sensitivity, selectivity, and stability of the sensor can be tuned. (3) Ion imprinting technology: This technique creates selective binding sites in biological materials. By incorporating Pb²⁺ ions into the biological material matrix during synthesis and subsequently removing them, cavities that match the size and coordination environment of Pb²⁺ ions are formed. These imprinted sites provide high selectivity and affinity for Pb²⁺ ions, significantly improving the sensor’s selectivity.

Through these techniques, the integration of biological materials into the design of electrochemical sensors can achieve the more precise and reliable detection of Pb²⁺ ions, even at trace levels. This integration not only enhances the selectivity and anti-interference properties of the sensors but also leverages the environmental and biocompatibility benefits of renewable biological materials.

As the deacetylation product of chitin, chitosan contains abundant active groups (-NH₂, -OH). Additionally, chitosan exhibits good film-forming properties. Wu et al. [70] prepared a new electrochemical sensor based on ion imprinting technology, as shown in Figure 5a. Through mixing Pb²⁺ into the chitosan-based film on the surface of the electrode, followed by the elution process, many specific cavities can form in the chitosan-based film. The cavity structure increased the selectivity of the electrode to Pb²⁺ and improved the anti-interference ability of the electrochemical sensor. They also studied the effect of different pH values on the electrochemical performance of the sensor and found that the maximum current response occurred at pH 5.25. They also evaluated the impact of interfering metal ions (such as Cr⁶⁺, Cu²⁺, Ni²⁺, Co²⁺, Fe²⁺, Zn²⁺, Mn²⁺, and Cd²⁺) on the selectivity of the Pb²⁺ sensor, finding that these interfering metal ions had a negligible effect on the DPV peak current of Pb²⁺, indicating the excellent selectivity of the sensor. The sensor exhibited a low detection limit of 0.283 nM and a wide linear range of 1 nM–50 μM. Most biomaterials have poor conductivity [70,100]. Therefore, they are often used as functional and high-conductivity materials. There are two ways to combine biomaterials with other materials: physical and chemical methods. Different treatments may cause differences in many ways. β-cyclodextrin has a good affinity for Pb²⁺; Alam et al. [71] modified MWCNTs with β-cyclodextrin through physical (non-covalent modification) and chemical (covalent modification based on the Steglich esterification reaction) methods, respectively. The modified MWCNTs were then loaded on the surface of a screen-printed electrode (SPE) to detect Pb²⁺. It was found that the electrochemical sensors exhibited different electrochemical characteristics when different modification methods were adopted: (1) The electrochemical sensor based on the physical modification showed higher performance; the limit of detection was 0.004 μM, with a limit of detection of 0.004 μM and a linear range of 0.015–0.50 μM. (2) The electrochemical sensor based on the chemical modification showed lower performance, with a limit of detection of 0.011 μM and a linear range of 0.030–0.50 μM. The chemical modification resulted in an increase in an increased LOD of the electrochemical sensor. Additionally, the reliability of the sensor was enhanced; the electrode can be reused more times without losing sensitivity. It was speculated that there was less β-cyclodextrin on the electrode surface obtained by chemical modification. However, the binding was more stable, and β-cyclodextrin molecules did not easily detach from the electrode surface, as shown in Figure 5b. The electrochemical measurements were carried out in a 0.1 M acetate buffer at pH 5. The selectivity of the MWCNT-βCD(Phys)/SPE electrode towards Pb²⁺ was measured in the presence of other metal ions, such as Cd²⁺, Zn²⁺, Cu²⁺, Hg²⁺, and Ni²⁺. The Pb²⁺ peak showed less than a 5% decrease in peak current in the presence of these ions. More detailed studies on other potentially interfering substances, including organic compounds and different ionic strengths, would be beneficial to understand the robustness of the sensor. The above data are summarized in Table 1.

From the above research, the abundant active groups in some biological materials can enrich Pb²⁺ ions or contribute to forming a selective membrane that acts as a barrier to other ions in the mixture. These characteristics enhance the collection and amplification of weak electrical signals, ultimately endowing the sensors with high sensitivity and selectivity. However, due to their poor conductivity, biomaterials are often combined with high-conductivity materials to fabricate high-performance electrochemical sensors. By incorporating these biological materials into the design of electrochemical sensors, it is possible to achieve the more precise and reliable detection of Pb²⁺ ions, even at trace levels. This integration not only improves the selectivity and anti-interference properties of the sensors but also leverages the environmental and biocompatibility benefits of renewable biomaterials.

This chapter discusses two methods for combining biological materials with other functional materials: physical blending and chemical conjugation. Physical blending is relatively simple and cost-effective. However, it may result in weaker interactions between the biological and functional materials, potentially leading to sensor stability issues. Chemical conjugation involves forming covalent bonds between biological materials and nanomaterials, resulting in more stable and robust composites. However, the latter process is typically more complex and costly, making it less suitable for large-scale industrial applications. Overall, for industrial applications, physical blending is the more feasible and cost-effective method than chemical conjugation. It provides a balance between performance and practicality, making it suitable for large-scale production. To address the issues associated with physical blending, non-covalent interactions (such as electrostatic forces and hydrogen bonding) can be leveraged to strengthen the bonding between different materials. Additionally, selecting appropriate surfactants or adhesives during sensor fabrication can enhance the interface adhesion between the two materials.

5. Amplifying Electrochemical Signals with Nanozyme Materials for Trace Pb²⁺ Detection through Biocatalytic Reactions

As previously mentioned, the limitations of traditional materials become particularly evident when detecting deficient concentrations of Pb²⁺ ions and in complex environments. Researchers have focused on the unique properties of biological macromolecules to address these challenges [101,102].

The unique three-dimensional architecture of enzymes and nucleic acids offers numerous active sites for the enrichment of Pb²⁺ ions and facilitates their corresponding redox reactions [103,104]. This specificity in catalyzing the oxidation and reduction of Pb²⁺ ions is crucial for amplifying the electrochemical signals, making the detection more precise. By enhancing the interaction between Pb²⁺ ions and the sensor surface, these biological macromolecules significantly boost the efficiency of lead ion detection. This capability is crucial for amplifying weak electrochemical signals and distinguishing the target ions from other interfering substances in complex mixtures. As a result, these biological macromolecules can provide the entire electrochemical sensing system with greater anti-interference potential. Incorporating these biological materials into the design of electrochemical sensors allows for breakthroughs in both the linear range and LOD. The high catalytic activity and specificity of enzymes and nucleic acids enable the precise and reliable detection of Pb²⁺ ions at trace levels, overcoming the limitations faced by traditional materials. This integration not only enhances the accuracy of detection but also extends the applicability of the sensors to a wider range of concentrations.

There are two main categories of electrochemical nucleic acid sensors for Pb²⁺ detection [105,106]: (1) The nucleic acid probes on the electrode surface bind directly to Pb²⁺ and exhibit changes in morphological structure. (2) The nucleic acid probes on the electrode surface are cleaved by a nuclease activated by Pb²⁺. In both cases, the interactions lead to changes in the electrochemical properties of the electrode surface. The transducer converts these changes into quantitative electrical signals, which can be used to determine the concentration of Pb²⁺ ions. Compared to traditional electrochemical sensors, electrochemical nucleic acid sensors have low detection limits and exhibit excellent selectivity due to their catalytic properties. Guo et al. [72] developed an electrochemical sensor based on nucleic acids and nucleases. Both ends of a multi-segment DNA strand were attached to the electrode surface, and methylene blue formed a stable tetrahedral structure in three-dimensional space. Pb²⁺ acts as a cofactor to enhance the activity of DNAzyme, causing DNA strand breaks. The distance between methylene blue and the electrode surface changes, along with the electrical signal. The sensor exhibited a detection limit of 0.01 μM and a linear range of 0.01–100 μM.

Lai et al. [73] fixed reduced graphene oxide (RGO) on the surface of a glassy carbon electrode (GCE) and modified it with gold nanoparticles. They then self-assembled ferrocene-labeled single-stranded DNA on the electrode surface by forming Au-S bonds. Pb²⁺ acts as a cofactor to catalyze deoxyribozyme 8–17 (DNAzyme) to cut specific DNA fragments at deficient concentrations. As a result, the ferrocene loaded on the electrode surface leaves, and the change is converted into electrical signals, as shown in Figure 6a.

Cai’s team [74] proposed a “dual DNA enzyme feedback amplification” (DDFA) strategy to detect Pb²⁺ at ultra-low concentrations, as shown in Figure 6b. Signal amplification was achieved through the following pathway: “Pb²⁺ activated DNAzyme → DNA strand cleavage → specific DNA fragments amplification.” The sensor had a meager detection limit of 0.048 pM and a linear range from 0.2 pM to 100 nM.

Electrochemical nucleic acid sensors have extremely high sensitivity and selectivity, primarily due to their catalytic interactions with Pb²⁺ ions. However, these sensors can be expensive and require strict environmental conditions, making them more suitable for laboratory use. Nonetheless, their ability to catalyze specific reactions and convert these interactions into measurable signals makes them a powerful tool for precisely detecting trace Pb²⁺ ions.

6. Conclusions and Future Perspectives

6.1. Conclusions

In the electrochemical detection of Pb²⁺ ions, various materials and sensor construction methods have been explored to significantly enhance performance by promoting the electrocatalytic process. The following summary highlights the key advancements and contributions of these innovative approaches:

(1)

Inorganic nanomaterials, such as MXenes, ferrite-based nanomaterials, and MOFs, enhance the electrochemical detection of Pb²⁺ by improving electrocatalytic redox reactions. They offer high surface area, conductivity, and tunable chemistry, leading to better signal amplification and ion enrichment. Despite their low detection limits, challenges with the linear detection range remain.

(2)

Carbon nanomaterials, such as CNTs and graphene, are highly conductive, cost-effective, and easily modifiable. Combining them with other catalytic materials optimizes sensor performance, enhancing the detection of trace Pb²⁺ ions through synergistic effects.

(3)

Materials rich in various active groups, including nucleic acids, chitosan, and cyclodextrin, enhance electrochemical sensors’ selectivity and anti-interference capabilities by explicitly interacting with Pb²⁺ ions. These environmentally friendly materials improve sensor accuracy and reliability at trace levels.

(4)

Biological macromolecules, such as enzymes and nucleic acids, offer high catalytic activity, enriching Pb²⁺ ions and facilitating redox reactions. This enhances signal amplification and detection precision. Though expensive and best suited for labs, these sensors provide high sensitivity and selectivity.

In conclusion, integrating various functional materials and innovative sensor construction methods significantly enhances the electrochemical detection performance for Pb²⁺ ions by promoting the electrocatalytic redox process. Metal oxide nanomaterials, MXene-based nanomaterials, carbon nanomaterials, biological materials, and enzyme-based approaches each contribute unique advantages, addressing different sensitivity, selectivity, and interference challenges. By leveraging their catalytic properties, these advanced materials pave the way for more effective and reliable monitoring of lead contamination.

6.2. Future Perspectives

Electrochemical sensing holds great promise as an ideal detection technology for trace amounts of Pb²⁺ ions. However, the application of electrochemistry in detecting trace heavy metal ions still faces several challenges:

(1)

Catalytic advancements: Catalysis will play a crucial role in the future of electrochemical detection. Developing and integrating novel catalytic materials will be essential for overcoming current limitations in sensitivity, selectivity, and interference. Future research should focus on exploring the catalytic properties of emerging nanomaterials, such as hybrid composites that combine the strengths of metal oxides, MXenes, carbon nanomaterials, and biological macromolecules. These materials can enhance the electrocatalytic redox reactions of Pb²⁺ ions, leading to improved detection performance.

(2)

Cost issues: Better detection performance often involves using complex electrode structures and precious metals, which can be costly. Replacing these with cheaper functional materials, such as carbon nanomaterials and inexpensive metal/metal oxide nanomaterials, can reduce costs. However, the stability of these materials, particularly on chemically modified electrodes, remains a concern. Developing functional polymeric coatings (e.g., Nafion and polyaniline) and new modification techniques can help address these stability issues, making the sensors more reliable and cost-effective.

(3)

Interference issues: The complex composition of substances in real-world environments can significantly interfere with detecting trace amounts of Pb²⁺ ions. Biomaterials, with their unique spatial structures and abundant active groups, can improve the selectivity of electrochemical sensors. However, the thermal stability of some biomaterials is relatively low, and the structure of electrochemical nucleic acid sensors can be complex and costly.

Electrochemical sensing technology has the following advantages over other existing technologies:

(1)

Real-time monitoring: One of the significant advantages of electrochemical sensors is their capability for real-time monitoring. Unlike traditional techniques that require sample collection and laboratory analysis, electrochemical sensors can provide immediate feedback on Pb²⁺ levels. This functionality is achieved by applying a specific voltage pattern to the electrochemical sensor, which controls the repeated adsorption–reduction–oxidation–dissolution process of lead ions on its surface. By monitoring the peak current generated during these processes, it is possible to obtain real-time measurements of Pb²⁺ concentrations.

(2)

Miniaturization and portability: Electrochemical sensors can be miniaturized and made portable, allowing for on-site testing. This portability is a significant advantage over other techniques such as AAS or ICP-OES, which are typically confined to laboratory settings due to their size and complexity.

(3)

Integration with Electronic Devices: Electrochemical sensors can be easily integrated with electronic devices, including smartphones and IoT (Internet of things) systems. This integration facilitates data collection, analysis, and remote monitoring, providing a technological edge over traditional methods that require separate, often cumbersome, data handling processes.

(4)

Cost-effectiveness: By developing cost-effective synthesis methods for nanomaterials and optimizing sensor designs to use minimal amounts of expensive components, electrochemical sensors can be made more affordable. This cost-effectiveness, combined with high performance, makes them suitable for widespread use, especially in resource-limited settings.

By addressing these areas, the field of electrochemical detection of Pb²⁺ ions can advance significantly, leading to more effective, reliable, and widely applicable sensor technologies. The inherent advantages of electrochemical sensors, such as real-time monitoring, miniaturization, portability, integration with electronic devices, and cost-effectiveness, position them as superior alternatives to traditional detection methods, offering substantial improvements in both technological and scientific aspects.