Graphitic Carbon Nitride Nanomaterials-Based Electrochemical Sensing Interfaces for Monitoring Heavy Metal Ions in Aqueous Environments

Abstract

The persistent threat of heavy metal ions (e.g., Pb²⁺, Hg²⁺, Cd²⁺) in aqueous environments to human health underscores an urgent need for advanced sensing platforms capable of rapid and precise pollutant monitoring. Graphitic carbon nitride (g-C₃N₄), a metal-free polymeric semiconductor, has emerged as a revolutionary material for constructing next-generation environmental sensors due to its exceptional physicochemical properties, including tunable electronic structure, high chemical/thermal stability, large surface area, and unique optical characteristics. This review systematically explores the integration of g-C₃N₄ with functional nanomaterials (e.g., metal nanoparticles, metal oxide nanomaterials, carbonaceous materials, and conduction polymer) to engineer high-performance sensing interfaces for heavy metal detection. The structure-property relationship is critically analyzed, emphasizing how morphology engineering (nanofibers, nanosheets, and mesoporous) and surface functionalization strategies enhance sensitivity and selectivity. Advanced detection mechanisms are elucidated, including electrochemical signal amplification, and photoinduced electron transfer processes enabled by g-C₃N₄’s tailored bandgap and surface active sites. Furthermore, this review addresses challenges in real-world deployment, such as scalable nanomaterial synthesis, matrix interference mitigation, and long-term reliable detection. This work provides valuable insights for advancing g-C₃N₄-based electrochemical sensing technologies toward sustainable environmental monitoring and intelligent pollution control systems.

1. Introduction

The contamination of water pollution has become a matter of public concern in environmental issues, especially heavy metal ions (HMIs), which can result in serious injury to public health even at very low levels of exposure [1,2]. Simultaneously, the enrichment and transmission of toxic metals in the food and food chain is one of the main problems in ecology [3,4]. Sources of heavy metal ions include industrial and municipal effluent discharges, as well as mining and smelting. Trace heavy metals such as zinc, copper, and cobalt are essential trace elements for the human body; while others, such as cadmium, mercury, and lead, are highly toxic and cause irreversible damage to the human body [5,6]. In addition, the toxicity of heavy metal pollutants also depends on their main form. For example, Cr(III) is essential for most biological systems, and Cr(VI) is a highly toxic substance that can cause kidney failure, nervous system damage, and even death [7]. The permitted threshold concentrations for common heavy metal ions in drinking water, as established by major regulatory bodies such as the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO), are as follows: lead (Pb) < 5 μg/L; mercury (Hg) < 2 μg/L; cadmium (Cd) < 5 μg/L; arsenic (As) < 10 μg/L; chromium (Cr) < 50 μg/L [8]. Thus, establishing a sensitive, specific, and fast detection method for heavy metal ions is essential to prevent their ecotoxicological hazards.

At present, various methods, such as atomic absorption spectroscopy (AAS) [9], ultraviolet-visible spectrophotometry (UV) [10], X-ray spectroscopy (XRF) [11,12], and inductively coupled plasma mass spectrometry (ICP-MS) [13], have been used for the determination of heavy metal ions. Among these methods, the electrochemical method is favored by researchers for its low cost, easy operation, high stability and sensitivity, and great application prospects [14]. These costly methods demand skilled operators, complex equipment, and multi-step preparation. Limited to quantification, they require chromatography integration for speciation analysis, risking sample alteration during handling/storage [15]. Therefore, developing rapid, low-cost, simple, and reliable technologies for in situ and real-time measurement of heavy metal ions remains an ongoing research focus.

Electrochemical detection methods, with their high sensitivity, simplicity, cost-effectiveness, and ease of integration, have attracted significant attention for enabling rapid on-site analysis of heavy metals in environments [16]. Common techniques include linear sweep voltammetry, cyclic voltammetry, stripping voltammetry, and chronoamperometry. The presence of modified materials for the electrode plays an essential role in determining the detection sensitivity of the electrochemical sensors. In previous research, various available materials such as precious metals (Au, Ag, etc.) [17,18], transition metal oxides (TiO₂, Fe₃O₄, Cu(OH)₂, etc.) [19,20,21], g-C₃N₄ [22,23,24], graphene [25,26], and their nanostructured assemblies [27,28] have been extensively used as electrocatalysts to enhance redox reactions of reactive pollutants through adsorption and catalysis. Among them, graphitic carbon nitride (g-C₃N₄) is a metal-free polymer with good resistance to acid, alkalis, and high temperatures because of the strong covalent bonds between carbon and nitrogen atoms [29]. Compared with other materials, pure g-C₃N₄ has defects of a small specific surface area, low charge mobility, and low utilization of visible light. It is usually modified by doping and changing its morphology to improve the specific surface area, enhance the charge migration, and improve the utilization of visible light [30,31]. In the field of environment and energy, g-C₃N₄ can be used for the photocatalytic degradation of pollutants [32,33], photocatalytic hydrogen production [34,35], carbon dioxide reduction [36,37], supercapacitors [38,39], and the adsorption of heavy metal ions [40]. In recent years, g-C₃N₄ and the nanocomposites based on g-C₃N₄ have been favored by a large number of researchers and also have been applied in electrochemical sensors for detecting heavy metal ions.

Although some review papers have introduced g-C₃N₄-based electrochemical sensors for environmental pollutant sensing [31,41], it is crucial to summarize the characteristics of g-C₃N₄-based electrochemical sensors in detecting heavy metal contaminants by analyzing the sensing behavior at sensitive interfaces and the morphological modulation of g-C₃N₄. Consequently, this review summarizes the detection mechanism of g-C₃N₄ and the properties of g-C₃N₄; the potential application of g-C₃N₄ in heavy metal detection was discussed and prospected. It is hoped that this article can provide some help to the majority of researchers, to make greater breakthroughs and innovations in future research.

2. The Performance Characteristics of C₃N₄

2.1. The Crystal Structure of C₃N₄

The crystal structure model of C₃N₄ was established based on first principles, including the α phase (α-C₃N₄), β phase (β-C₃N₄), cubic phase (c-C₃N₄), quasicubic phase (q-C₃N₄) and graphite-like phase (g-C₃N₄) [42,43,44]. As shown in Figure 1 below, the structure of the graphite-like phase (g-C₃N₄) is the most stable allotrope of carbon nitride and has attracted much attention in recent years [45]. It has a layered structure similar to graphite and contains two allotropes, namely tri-s-triazine (C₆N₇) and s-triazine (C₃N₃) rings. The s-triazine ring is aromatic, so the conjugated two-dimensional polymer of s-triazine will tend to form a p-conjugated plane layer, similar to graphite. Among them, tri-s-triazine is more stable in thermodynamics. Research has revealed that different crystal structures of C₃N₄ may significantly influence its electrochemical sensing performance. For instance, the α-phase demonstrates higher conductivity compared to the β-phase, which can be attributed to the more delocalized π-electrons in the α-phase that facilitate faster electron transfer during redox reactions [46]. The two-dimensional layered structure of graphitic-phase C₃N₄ (g-C₃N₄) promotes ion diffusion and migration, thereby exhibiting superior electrochemical performance in heavy metal detection [47]. These structure-property relationships may provide valuable guidance for material selection in specific sensing applications.

2.2. The Electronic Properties of C₃N₄

By contrast with traditional semiconductors (e.g., TiO₂ and ZnO), g-C₃N₄ owns a two-dimensional (2D) lamellar structure with a π conjugated system and a moderate energy gap (∼2.7 eV) [14], which lead to unique optoelectronic properties, high chemical stability, and good visible-light absorption (approximately at 450∼460 nm). The plane of g-C₃N₄ consists of sp² hybridized CN aromatic heterocycles. Among them, atoms in the layer are arranged in a honeycomb structure with strong covalent bonds and van der Waals forces in between layers. The valence band edge of g-C₃N₄ is mainly composed of N 2p; thus, the photogenic holes are generated at the N position. The conduction band edge is composed of N 2p and C 2p state hybridization, so the photogenerated electron bears high binding energy in the electron-hole pair, which leads to the high photogenerated charge carrier recombination rate of g-C₃N₄ [49,50,51]. Studies have shown that the band structure of the polymer melon structure is non-isotropic, in which there is a direct band gap at Γ and only dispersion in the Γ–X direction (Figure 2a). Furthermore, different thermal condensation temperatures will affect the electronic and optical properties of the synthesized carbon nitride [52]. The absorption edge shifts to longer wavelengths, indicating that the band gap decreases as the condensation temperature increases (Figure 2b).

2.3. Surface Physicochemical Properties of C₃N₄

Compared with most carbon materials, g has electron-rich properties, basic surface functional groups, and H-bonding motifs due to the presence of N and H atoms. Therefore, it is considered a potential candidate to supplement carbon in material applications [53,54,55]. Due to the influence of incomplete polycondensation, there are some defects on the surface of g-C₃N₄, namely the basic primary and/or secondary amine groups at the end of g-C₃N₄ (Figure 3). Compared with the ideal defect-free g-C₃N₄, g-C₃N₄ with surface defects has excellent electron-rich properties, leading to the formation of a variety of functions. In addition, the abundant basic functional groups on the surface of g-C₃N₄ can absorb acidic and toxic molecules through electrostatic action [55]. However, the hydrophobicity of g-C₃N₄ leads to weak interfacial interactions, which further inhibit surface electron transfer and catalytic reactions [56]. To address this issue, materials such as nonmetallic elements, metal nanoparticles, carbon nanotubes, and graphene-based flakes can be doped between the layers of g-C₃N₄ to significantly enhance its hydrophilicity [57]. Zhu et al. [58]. in situ grew CeO₂ nanocrystals on the surface of two-dimensional MoS₂/g-C₃N₄ nanosheets. The ternary composite material significantly enhanced the hydrophilicity of the substrate. In addition, the hydrophilicity can be improved by introducing oxygen-containing functional groups on the g-C₃N₄ structure, which can enhance the interfacial coupling and photocatalytic activity [59]. Shen et al. [60] introduced π-electron-rich domains and hydrophilic hydroxyl groups into the g-C₃N₄ structure to form tartaric acid-modified g-C₃N₄ (TA-CN). In this system, the hydroxyl groups regulate reaction kinetics through their hydrophilicity, enhance the selective adsorption of reactants, and stabilize intermediates.

2.4. Photoelectrochemical Properties of C₃N₄

Graphitic carbon nitride (g-C₃N₄) is defined as an emerging semiconductor for applications in photocatalytic and photoelectrochemical analysis. As a metal-free photocatalyst, g-C₃N₄ has been shown to possess the band characteristics of metal oxide semiconductors with a band gap of about 2.7 eV and spectral response up to 460 nm, indicating visible light absorption [62]. Thus, g-C₃N₄ is regarded as a promising semiconductor for light-emitting devices, photocatalysis, photoelectrodes, energy conversion, photodegradation, and PEC sensors [63,64,65,66]. This work was proposed by Du et al. who synthesized 3D branched crystalline carbon nitride with 1D nanoneedles [67]. It has a high specific surface area, fast photo-generated charge transfer, and excellent light-gathering performance. More importantly, the electron-hole pair separation and crystallinity are improved. In addition, Bu et al. proved the photocurrent conversion performance and photocathodic protection performance of g-C₃N₄ in 304 stainless steel under the irradiation of visible light or white light [68]. Su et al. used mpg-C₃N₄ as a photocatalyst, activated O₂ under visible light irradiation, and achieved high selective catalysis of benzaldehyde [69]. In addition, the photocatalytic performance of visible light-sensitive metal-free mesoporous graphite carbon nitride can be used to establish a system to degrade drugs for the treatment of cardiovascular diseases [70].

2.5. Electrochemical Properties of C₃N₄

As a multifunctional electrocatalyst, the g-C₃N₄ semiconductor is useful in electrochemical detection and biosensing due to its high activity, large surface area, and fast electron transfer rate. Among the multifunctional electrocatalysts, pyridine N atoms in g-C₃N₄ with strong electron acceptors are considered as the active sites for electrochemical reactions [71]. g-C₃N₄ has been used in electrochemical sensing applications in the analysis of water environment pollutants. For example, g-C₃N₄ ultrathin nanosheets displayed higher surface reactive sites and better electrical conductivity and exhibited good selectivity and repeatability in Cd²⁺ detection [24,72]. In addition, the electrochemical sensing device based on g-C₃N₄ can also achieve biomarker analysis. g-C₃N₄ was used in the development of a glucose sensor as well as for the detection of dopamine and H₂O₂. Furthermore, g-C₃N₄ has been utilized in DNA analysis through stacked g-C₃N₄ nanofibers modified sensor detection of free DNA bases, cell analysis in the area of recognition and quantification of specific cells, analysis of heavy metals ions, as well as the detection of high-energy explosives and nerve agents.

3. Electrochemical Detection of Heavy Metal Ions

3.1. Electrochemical Detection Methods

The electrochemical detection method has a short analysis time, low power cost, and high sensitivity, and can be detected in situ [73]. Therefore, it has aroused great interest in the detection of heavy metal ions. Electrochemical sensing of HMI involves the use of biosensing electrodes that are employed to pass current to the aqueous solution and generate some useful and measurable electrical signal corresponding to the electrochemical reactions within the solution due to the presence of metal ions. The general experimental device for electrochemical detection of heavy metal ions usually consists of an electrolytic cell composed of an ion conductor (electrolyte) and an electronic conductor (electrode) [74]. In this case, the aqueous solution composed of HMI acts as an electrolyte. The battery potential is measured at the interface between the electrode and the electrolyte solution. Usually, electrochemical detection uses a three-electrode system, including a working electrode (WE), a reference electrode (RE), and a counter electrode (CE) (Figure 4). In the electrochemical detection process, different types of electrochemical techniques are classified according to the different types of detection signals generated by the presence of heavy metal ions in the solution matrix, such as current, potential, conductivity, electrochemical impedance, electrochemiluminescence, etc. [75,76]. Through customized functionalization and assembly, these materials can be easily assembled on the electrode surface for the manufacture of sensing electrodes and the sensitive and selective detection of heavy metal ions. The assembly of different materials such as metals, metal films, metal oxides, nanomaterials, carbon nanotubes, polymers, microspheres, and biological materials can further improve electrochemical performance. Based on these detection signals, electrochemical sensing can be divided into the amperometric method, potentiometric method, electrochemical impedance method, capacitance method, and electrochemiluminescence method. In most of these techniques, the changes in other parameters are measured by controlling any of the current or electric potentials.

3.2. Possible Mechanisms of Heavy Metal Ion Detection Based on g-C₃N₄ Nanomaterials

Stable and reliable heavy metal ion response signals are an important guarantee for electrochemical analysis methods to meet actual application requirements. The focus is on exploring the interaction between heavy metal ion forms and electrode modification materials, as well as their diffusion and transformation processes on sensitive interfaces. In the detection process of heavy metal ions, good adsorption performance can significantly improve the electrochemical detection performance. Based on this, nanomaterials with high specific surface area, excellent catalytic activity, electrical conductivity, and surface modifiability show significant advantages in electrochemical detection [78]. While a large number of binding sites and functional groups dominate the high sorption capacity of g-C₃N₄ and g-C₃N₄-based materials through the formation of strong surface complexes [79]. The response mechanism of heavy metal ions was studied in detail by X-ray photoelectron spectroscopy. Figure 5 shows that C≡N and N–H functional groups may interact with Pb (II) and Cu (II) to form complexes. The delocalized π-electron system of the triazine ring (C₃N₃) and heptazine ring (C₆N₇) can act as Lewis bases, and metal ions act as Lewis acids. Therefore, through the Lewis acid−base interaction, the adsorption of heavy metal ions on the g-C₃N₄ is also ascribed to the strong surface complexation between the g-C₃N₄ and metal ions [80]. Figure 5a,b also shows that Cu(II) can form a complex with the g-C₃N₄ more powerfully than Pb(II).

The essence of electrochemical detection is the electrocatalytic reaction process at the sensing interface, which will affect the enrichment/reduction of heavy metals and the corresponding dissolution/oxidation signal. Therefore, elucidating the electrocatalytic mechanism between the sensing interface and the target pollutant is essential for the development of an effective functional sensing interface to achieve high sensitivity and selective detection of heavy metals. Hu et al. [81] prepared a metal-free g-C₃N₄/CB material for the detection of heavy metal ions. The detection sensitivity of the g-C₃N₄/CB electrochemical sensor under visible light irradiation is greatly improved under dark conditions (Figure 5c). Stripping voltammetry includes two processes: enrichment and dissolution. By applying an applied voltage, the target pollutant in the solution is enriched on the electrode surface, and a reverse voltage is applied to dissolve the metal or insoluble film enriched on the electrode surface [82]. The g-C₃N₄ material is excited under visible light irradiation to generate electrons (e⁻) and holes (h⁺) in the conduction band and valence band, respectively. Photogenerated electrons act as a reducing agent to promote the reduction of heavy metal ions to the metal state on the electrode surface. In the electrochemical dissolution process, photogenerated holes are beneficial to the oxidation of heavy metals into corresponding ions (Figure 5d).

In summary, the intrinsic properties of g-C₃N₄ and the potential electrochemical detection mechanisms indicate that g-C₃N₄ nanomaterials are promising candidates for constructing electrochemical sensors toward heavy metal ions. Especially, its tunable surface chemistry, unique electronic structure, and high stability ensure its environmental monitoring application.

4. Application of g-C₃N₄ in Heavy Metal Detection

Bulk g-C₃N₄ has been considered to have a similar layered structure to graphite; with its unique electronic energy band structure, high thermochemical stability, and low-cost precursor materials, g-C₃N₄ has attracted people’s attention in the photocatalytic degradation of environmental organic pollutants and electrochemical sensor detection of heavy metal ions [83,84]. However, the high recombination rate of photoexcited charge carriers and the small specific surface area in the bulk g-C₃N₄ system led to low photocatalytic activity [85,86]. Therefore, many approaches have been used to address these limitations, such as changing the morphology of g-C₃N₄ or a composite with other nanoparticles as a template. Mainly, the excellent adsorption and catalytic properties of g-C₃N₄ nanomaterials brought new insights to utilize them as an electrochemical modifier for heavy metal ion detection. In the following sub-sections, recent advances in g-C₃N₄ and g-C₃N₄-based nanocomposites for heavy metal ion sensing are discussed, and their sensing performances are presented in

Table 1. Comparison of the determination performance of electrochemical sensors based on graphite phase carbon nitride in the determination of heavy metal ions.

Electrodes	HMIs	Methods	Range of Linearity (μM)	Sensitivity (μA·μM−1)	LOD (μM)	Ref.
g-C3N4/GC	Pb(II)	SWASV	0.5~3.5	10.887	0.228	[87]
	Cu(II)		0.5~4.5	4.794	0.103
	Hg(II)		0.5~4.5	18.180	0.217
g-C3N4/Nafion/GC	Cd(II)	DPASV	0.001~100	/	0.5 × 10−3	[88]
g-C3N4 nanosheet/GC	Cd(II)	SWASV	0.05~0.7	22.668	3.94 × 10−3	[24]
Ultrathin g-C3N4/GC	Cd(II)	SWASV	0.001~0.1	43.9	0.35 × 10−3	[72]
g-C3N4/Pt/PAn NCs/GC	Hg(II)	DPASV	0.5~1	1.0787	9 × 10−3	[89]
g-C3N4 nanosheet/GC	Pb(II)	DPASV	0.0075~1.0	33.08	1 × 10−3	[90]
S-g-C3N4/GC	Pb(II)	DPV	0.075~2.5	0.220	3 × 10−3	[91]
Au/g-C3N4/rGO/GC	Pb(II)	DPASV	0~0.1	0.618	0.1 × 10−3	[92]
AuNPs/mpg-C3N4/GC	Cr(VI)	LSV	0~19.3	0.110	0.283	[7]
AuNPs/mpg-C3N4/GC	MeHg(I)	DPASV	0~0.116	61.4	0.47 × 10−3	[93]
S-g-C3N4/Au electrode	MeHg(I)	DPASV	0~0.116	110.0	0.81 × 10−3	[94]
S-g-C3N4 tube bundles/graphene nanosheets	Pb(II)	SWASV	0.025~8.5	21.684	0.78 × 10−3	[95]
	Hg(II)		0.05~7.5	15.71	1.15 × 10−3
	Cd(II)		0.05~5	10.132	2.30 × 10−3
g-C3N4@FeP-C/GC	Cu(II)	DPASV	0.05~20	0.125	0.0167	[96]
g-C3N4/CNT/NH2-MIL-88(Fe)/GC	Cd(II)	SWSV	0.12~6.0	3.66	39.6 × 10−3	[97]
	Pb(II)		0.02~6.0	19.15	7.6 × 10−3
	Cu(II)		0.04~6.0	12.15	11.9 × 10−3
	Hg(II)		0.03~6.0	15.10	9.6 × 10−3
rGO/g-C3N4/GC	Pb(II)	SWASV	0.00001~1	195.22	1.07 × 10−6	[98]
Au/N-deficient-g-C3N4/GC	Pb(II)	SWASV	0.2~0.8	184.4	29 × 10−3	[99]
g-C3N4/rGO/GC	Pb(II)	SWASV	0.05~1.45	13.6	0.72 × 10−3	[100]
CuS/g-C3N4/GC	Pb(II)	SWASV	0.05~5.000	10.811	4 × 10−3	[101]
Bi/g-C3N4/SPE	Pb(II)	SWASV	0.267~1.067	4.98	0.156	[23]
	Cd(II)		0.145~0.531	1.787	0.039
Ti3C2(HF)/Fe3O4/g-C3N4/GC	Pb(II)	DPASV	0.005~0.5	57.23	0.12	[102]
	Cd(II)			44.14
	Hg(II)			48.24
Pd/g-C3N4/GC	Hg(II)	DPV	0.05 × 10−3~0.025	42.281	0.45 × 10−3	[103]
Fe2O3/g-C3N4/GC	Pb(II)	PEC	3.0 × 10−4~4.8	40.6	3.8 × 10−5	[104]
g-C3N4/CNT/NH2-MIL-88(Fe)/GC	Pb(II)	SWSV	0.02~6.00	19.15	7.6 × 10−3	[97]
	Cu(II)		0.04~6.00	12.15	11.9 × 10−3
	Hg(II)		0.03~6.00	15.10	9.6 × 10−3
	Cd(II)		0.12~6.00	3.66	39.6 × 10−3
pg-C3N4/CoMn2O4/GC	Pb(II)	SWASV	0.2~4.4	1.660	1.4 × 10−3	[105]
	Cd(II)		0.5~7.0	5.178	2.1 × 10−3
Pt/g-C3N4/Polythiophene/GC	Hg(II)	DPV	0.01~5	76.15	9 × 10−6	[89]
PEDOT/g-C3N4/GC	Pb(II)	DPV	0.04~11.6	8.8135	4.21 × 10−3	[106]
	Cd(II)	DPV	0.06~12	3.0645	1.4 × 10−3
Porous-g-C3N4/O-MWCNTs/SPE	Pb(II)	DPV	0.17 × 10−3~0.53	/	0.038 × 10−3	[107]
	Cd(II)		0.038~2.233	/	0.266 × 10−3
	Zn(II)		0.064~3.089	/	0.917 × 10−3
	Hg(II)		0.024~0.459	/	0.199 × 10−3
g-C3N4/CB composite electrode	Cd(II)	DPASV	0~0.7	11.0	2.1 × 10−3	[81]
	Pb(II)		0~0.3	9.0	0.26 × 10−3
	Hg(II)		0~0.5	4.0	0.22 × 10−3
g-C3N4/Ti3C2Tx/GC	Cd(II)	SWASV	0.05~1.5	40.97	1 × 10−3	[108]
	Pb(II)		0.05~1.5	49.91	0.6 × 10−3
P-CN_T60/MWCNT/CFE	Cu(II)	DPSV	6.6 × 10−6~8.5	0.094	1.0 × 10−7	[109]
	Pb(II)		0.81 × 10−3~8.5	0.030	1.8 × 10−5
	Hg(II)		0.22 × 10−6~8.5	0.021	8.0 × 10−6
poly(melamine)/g-C3N4/SPE	Pb(II)	DPV	0.1~1	/	8 × 10−3	[110]
	Cd(II)		0.1~1	/	20 × 10−3
poly(BPE)/g-C3N4/GC	Cd(II)	DPV	0.12~7.2	7.636	9.7 × 10−3	[111]
	Pb(II)		0.08~7.2	11.140	3.27 × 10−3

.

4.1. g-C₃N₄ Nanofibers/Tubes

Low-dimensional functional nanomaterials have attracted much attention due to their high specific surface area and excellent electronic and optical properties [112,113]. Compared with bulk g-C₃N₄, one-dimensional structures such as g-C₃N₄ tubes or rods have a higher aspect ratio, which can guide the electrons to move along the axial direction and limit the lateral transfer of electrons, thereby inhibiting photo-generated carrier recombination [114,115]. At the same time, this structure usually has a larger specific surface area, so that its surface has more reactive active sites, which is conducive to the improvement of catalytic reaction performance [116]. In addition, the highly ordered array of nanotube rod structures can provide an efficient electron migration path for electron-hole transport. The one-dimensional structure of g-C₃N₄ is synthesized using chitosan as a template [117]. Lv et al. [118] successfully prepared g-C₃N₄ nanofibers by hydrolysis under alkaline conditions. Figure 6a is the proposed preparation mechanism of g-C₃N₄ nanofibers; the g-C₃N₄ nanofibers show an ultrathin structure and are up to a micron in length (Figure 6c). Compared with bulk g-C₃N₄, the prepared g-C₃N₄ nanofibers have higher fluorescence quantum yield and larger specific surface area. In addition, g-C₃N₄ nanofiber fluorescent probes have achieved high selectivity and sensitivity for DA detection, with a minimum detection limit of 17 nM (Figure 6d,e). Tian et al. [119] quickly prepared ultrathin C₃N₄ nanofibers with a diameter of 5–10 nm from top to bottom through alkaline catalytic hydrolysis. The morphology of g-C₃N₄ nanostructures can be controlled by reaction time. A fluorescence sensor was constructed using the prepared ultrathin C₃N₄, which can detect Fe³⁺ rapidly and selectively. Zheng et al. [120] synthesized helical g-C₃N₄ rods by nanometer casting using mesoporous silica as templates. Compared with bulk g-C₃N₄, the spiral structure of g-C₃N₄ promotes the charge separation and mass transfer of carbonitride semiconductors, thus significantly improving the catalytic efficiency of water decomposition and carbon dioxide reduction. Dimitra et al. [121] synthesized rod graphitic carbon nitride (RGCN) by a chemical method, which showed obvious selectivity to Cu²⁺ ions. The influence of morphology on the Cu²⁺ sensor was further investigated, and the thin sheet-like graphitic carbon nitride (SGCN) was synthesized. Compared with SGCN, RGCN shows a stronger Cu²⁺ sensing capability due to its porous rod structure and high nitrogen content. Wang et al. [95] reported a highly sensitive electrochemical sensor based on a sulfur-doped C₃N₄ bundle with hierarchical pores/graphene composites (STB/Gs-x) for the detection of trace heavy metal ions in the environment. The sulfur-containing GCN tube bundle (STB) was constructed by a one-step method. The focus is on the synergistic effect of the layered porous tube bundle structure and S atom doping on the detection of HMIs, which further solves the deficiencies of b-g-C₃N₄ in the detection of electrochemical HMIs.

4.2. g-C₃N₄ Nanosheets

In recent years, inspired by the methods for preparing graphene and graphene-like materials, the bulk g-C₃N₄ can be exfoliated into a single or few layers of graphene-like C₃N₄ [122,123]. The thickness of nanosheets is generally at the nanometer or sub-nanometer level, and due to their unique two-dimensional anisotropic structural characteristics and quantum-filling effects, nanosheets have new physical and chemical properties [124,125]. It was found that 2D layered materials showed superior electrochemical performance in detecting heavy metal ions compared with 3D block materials [126]. The bulk C₃N₄ has a stacked layered structure in which each layer is composed of C–N covalent bonds with strong bonding force, but the bond energy between the layers is the van der Waals force with weak bond energy. Therefore, it is feasible to obtain C₃N₄ nanosheets by bulk C₃N₄ [127]. Nanosheets have the following advantages: (1) Having a large specific surface area can provide rich reactive sites. (2) With a short bulk diffusion length, the recombination of photo-excited charge carriers can be reduced. (3) Has a large band gap, which helps to enhance the redox capacity of charge carriers. (4) The photophysical behavior of the photo-excited carriers has changed compared with the bulk C₃N₄, which can extend the life of the charge carriers.

The preparation methods of g-C₃N₄ nanosheets can be divided into “top-down” and “bottom-up” [128]. The ultrasonic liquid exfoliation method and the secondary thermal oxidation etching method are the most commonly used to prepare g-C₃N₄ nanosheets from the “top-down” methods. In each of these methods, it is synthesized from a block structure by a two-step process. Bulk C₃N₄ was prepared by thermal polymerization and then transformed into nanosheets by exfoliating and etching. This approach offers advantages such as simple operation, low cost, and the ability to retain the inherent structure of g-C₃N₄. However, it faces challenges in precisely controlling the thickness and size distribution of the nanosheets during exfoliation. Additionally, the exfoliated nanosheets may restack, reducing the exposure of active sites. In contrast, “bottom-up” methods directly synthesize nanosheet structures through the condensation polymerization of molecular precursors (e.g., melamine and urea), including techniques such as hard-template, soft-template, and molecular self-assembly approaches. These methods provide advantages such as controllable structures, high specific surface area, and fewer defects. Nevertheless, they suffer from limitations like complex procedures and low yield.

Graphite carbon nitride has undergone sufficient protonation and ultrasonic treatment to synthesize g-C₃N₄ nanosheets. Its properties are similar to two-dimensional films, with enhanced ion conductivity and a large specific surface area. Density functional theory (DFT) calculations show that the high level of protonation makes g-C₃N₄ have better conductivity [129]. Hatamie et al. [90] prepared a large number of bulk g-C₃N₄ via high-temperature polymerization of melamine and obtained g-C₃N₄ nanometer tablets by sonication-assisted liquid exfoliation. Using g-C₃N₄ nanosheets to modify glassy carbon electrodes can achieve trace detection of lead ions in a water environment; the detection limit is 1 ng·L⁻¹. Using the same method, Zhang et al. [130] prepared ultrathin g-C₃N₄ (Utg-C₃N₄) nanosheets with a thickness of about 8 mm. Compared with g-C₃N₄, the Utg-C₃N₄-modified GC electrode has an enhanced electrochemical response to Hg²⁺, which could be ascribed to the strong affinity between utg-C₃N₄ and Hg²⁺ through its single bond –NH and single bond –NH₂ groups (Figure 7a). This allows for the detection of Hg²⁺ in aqueous solutions with high sensitivity and selectivity (Figure 7b,c). In addition, studies have shown that graphene-like carbonitrides can be synthesized by liquid phase exfoliation in 1,3-butanediol (Figure 7d). The obtained graphene-like C₃N₄ has a two-dimensional thin-layer structure with a thickness of about 3–6 atoms and a specific surface area of 32.54 m²g⁻¹ (Figure 7e–h). The photocurrent response, electron transfer ability, and photocatalytic activity are all enhanced. In another work, Liu et al. [24] successfully prepared the activated ultrathin g-C₃N₄ nanosheets by bulk graphitic carbon nitride (g-C₃N₄) to liquid ultrasonic exfoliation and protonation treatment. An electrochemical sensing interface was developed using activated g-C₃N₄ nanosheets modified with a glassy carbon electrode for the determination of Cd²⁺, with the obtained sensitivity and detection limit being 22.668 µA/µM and 3.9 nM, respectively. In addition, high spiked recoveries were obtained in the detection of Cd²⁺ in natural water and rice samples using the prepared a g-C₃N₄ modified electrode. This result can be attributed to the ultrathin layer and high surface for the effective accumulation of metal ions required for enhanced sensitivity.

The liquid phase stripping method has the advantages of simple and direct operation, but its yield is very low, and it is time-consuming. Thus, a new synthesis idea was developed: the precursors of g-C₃N₄ were pretreated and thermally polymerized from bottom to top to directly synthesize nano-flake g-C₃N₄. Sadhukhan et al. [131] synthesized two-dimensional carbon nitride by a microwave-assisted method. The g-C₃N₄ nanosheets were formed by evaporation-induced self-assembly of nitride carbon points on the fixed substrate by microwave radiation of formamide. In addition, a g-C₃N₄ nanosheet-modified glassy carbon electrode can be used for the highly sensitive detection of mercury ions in an aqueous solution, with a minimum detection limit of 9.1 × 10⁻¹¹ M. Using melamine as a raw material and acetic acid as a bubble template, Wang et al. [72] prepared graphitized carbon nitride ultrathin nanosheets by thermal polymerization taking melamine as raw materials and deionized water and acetic acid as bubble templates. During the thermal polymerization process, acetic acid decomposes to produce a large number of bubbles, which is beneficial to the formation of CN nanosheets (Figure 7i,j). BET data (Figure 7k) show that the specific surface area of ultrathin nanosheets is 10 times that of g-C₃N₄, which indicates that there are more active sites to adsorb Cd²⁺. Then, an electrochemical-sensitive interface based on a g-C₃N₄ nanosheet was constructed for the detection and analysis of cadmium ions in an aqueous environment (Figure 7l). The results showed that the electrochemical sensor had a low detection limit (0.35nm) and had potential application potential in the detection of heavy metals. Zou et al. [91] reported that the sulfur-doped carbon nitride nanosheet-modified glassy carbon electrode (S-g-C₃N₄/GCE) has the best large specific surface area and good lead ion (Pb²⁺) detection ability. The results of DFT and XPS show that S doping mainly replaces the N atoms in the g-C₃N₄ molecular framework.

Figure 7. (a) Illustration of the electrochemical sensing of Hg²⁺ ions using a utg-C₃N₄/GC electrode. Electrochemical impedance spectra (b) and cyclic voltammograms (c) of bare GC, bg-C₃N₄/GC, and utg-C₃N₄/GC electrodes [130]. (d) Schematic illustration of liquid-exfoliation process from the bulk g-C₃N₄ to graphene−like C₃N₄. (e) AFM image of the prepared graphene-like C₃N₄. (f) The corresponding height image of the five randomly chosen graphene-like C₃N₄ sections. (g) TEM image of the graphene-like C₃N₄. (h) HRTEM image of the graphene-like C₃N₄ [132]. SEM and TEM images of NSCN (i,j). (k) N₂ adsorption–desorption isotherms of CN and NSCN. (l) The detection of Cd²⁺ on bare GC, CN-GC, and NSCN-GC (the concentration of Cd²⁺ is 10 μM) [72].

Figure 7. (a) Illustration of the electrochemical sensing of Hg²⁺ ions using a utg-C₃N₄/GC electrode. Electrochemical impedance spectra (b) and cyclic voltammograms (c) of bare GC, bg-C₃N₄/GC, and utg-C₃N₄/GC electrodes [130]. (d) Schematic illustration of liquid-exfoliation process from the bulk g-C₃N₄ to graphene−like C₃N₄. (e) AFM image of the prepared graphene-like C₃N₄. (f) The corresponding height image of the five randomly chosen graphene-like C₃N₄ sections. (g) TEM image of the graphene-like C₃N₄. (h) HRTEM image of the graphene-like C₃N₄ [132]. SEM and TEM images of NSCN (i,j). (k) N₂ adsorption–desorption isotherms of CN and NSCN. (l) The detection of Cd²⁺ on bare GC, CN-GC, and NSCN-GC (the concentration of Cd²⁺ is 10 μM) [72].

4.3. The Mesopores g-C₃N₄

The attractive properties of mesoporous materials, including a high specific surface area and a controllable mesoporous structure, make them attractive subjects for many applications. Recent developments in mesoporous materials have improved the performance of these sensors by the immobilization of receptors to large and accessible surface areas and well-defined pores, which facilitate a high adsorption capacity for chromogenic/fluorescent molecules and the effective transport of analytes and hence detection limits below tens of nanomolar concentrations. In 2005, mesoporous carbon nitrides with different pore diameters were synthesized for the first time [133]. Compared with bulk g-C₃N₄, it significantly increased the specific surface area while maintaining the same physical and chemical properties, thereby improving its application in catalysis and adsorption. On the one hand, the presence of mesopores increases the specific surface area of g-C₃N₄ and provides more active reaction sites for metal ions. On the other hand, the hollow structure facilitates the transfer of electric charges and improves the sensitivity of electrochemical reactions.

The template method is one of the most effective methods to prepare MCN with controllable pore size, morphology, and structure. It is mainly divided into the hard template method and the soft template method. The hard template method is used to synthesize MCN mainly by immersing the precursor in the silica template such as SBA-15 [134], FDU-12 [135], KIT-6 [136], etc., polymerizing and crosslinking the carbon-containing precursor and the nitrogen-containing precursor at high temperature to form mesoporous CN material, and then cleaning with HF to remove the silica template [137]. The soft template method is a method in which surfactants, amphiphilic block polymers, or ionic liquids are used as templates for synthesis. The soft-template synthesis is mainly through the formation of different emulsions to obtain the micellar phase used as a template. The g-C₃N₄ precursor undergoes condensation around the soft template, and the soft template is removed by calcination polymerization [138]. Wang et al. [139]. prepared mesoporous graphite-carbonitride (mpg-C₃N₄) by the hard template method and proposed the use of mpg-C₃N₄ and β-cyclodextrin (β-CD) nanocomposites to modify glassy carbon working electrodes for the detection of trace amounts of TNT. It can be observed from the TEM image that mpg-C₃N₄ has a stacked lamellar structure and tiny mesopores (Figure 8a,b). BET measurement results show that g-C₃N₄ does not have obvious pores. For mpg-C₃N₄ samples, the pores are mainly distributed in the range of 3–20 nm (Figure 8c). The sensitivity of mpg-C₃N₄/β-CD nanocomposite for detecting TNT is 0.2 μA/μM, and the detection limit is 68ppb (Figure 8d,e). The mpg-C₃N₄/β-CD nanocomposite is used to capture nitroaromatic hydrocarbons near the electrode–solution interface to promote electrochemical reduction. Zhao et al. [140]. used a one-step soft template method to synthesize hollow mesoporous g-C₃N₄ spheres with high specific surface area and high porosity. It utilizes the supramolecular assembly of cyanuric acid and melamine through hydrogen bonds and the structure-oriented properties of ionic liquids. And the corresponding formation mechanism is placed in Figure 8h. It can be seen from Figure 8f,g that there are a large number of mesopores in the structure of CN, which may be caused by ionic liquid micelles, NH_3, and CO₂ gases. In addition, the porous structure and ultrathin nanosheets facilitate the exposure of active sites, thereby increasing the interface charge transfer rate.

According to previous reports, the pore structure of these mesoporous materials is conducive to enhancing the light-harvesting capacity and the ability to adsorb pollutants and significantly improving the photocatalytic activity. Therefore, the preparation of mesoporous g-C₃N₄ may be a promising method to solve the defects of the g-C₃N₄ body.

4.4. g-C₃N₄-Based Nanocomposites

4.4.1. Metal Nanoparticles/g-C₃N₄ Nanocomposites

Metal nanoparticles such as gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have the characteristics of small size and a large specific surface area. At the same time, their unique optical, electronic, magnetic, and antibacterial properties have also received extensive attention [141]. The precursors and synthesis methods of metal nanoparticles play an important role in their performance. In general, metal salts are used as precursors to be reduced to the corresponding metal nanoparticles. The stabilizer is attached to the surface of the metal nanoparticles, effectively inhibiting the agglomeration of the metal nanoparticles. The metal nanoparticle-modified electrode can control the functionalization of the required groups and increase the mass transfer rate and specific surface area, thereby enhancing the dynamic performance of the electrode.

The g-C₃N₄ nanocomposite materials based on metal nanoparticles are considered to be effective, strong, and stable electrochemical sensing materials. Among them, AuNPs/g-C₃N₄ nanocomposites are the most widely used electrochemical modification materials. Xiao et al. [99] found that in Au/N-deficient-C₃N₄, N vacancy defects and Au nanoparticles have a synergistic catalytic effect, which can be used for high-sensitivity electrochemical detection of Pb(II) and has anti-interference properties. Figure 9a–d show the SWASV responses for the detection of Pb(II), Hg(II), Cu(II), and Cd(II). It can be concluded that the sensitivity of Au/N-deficient-C₃N₄/GCE towards Pb(II) is the best (Figure 9e). Figure 9f describes the electrocatalytic mechanism of Pb(II) on the sensitive interface modified by three different nanomaterials and clearly illustrates the significance of the synergistic catalytic effect of N vacancies and ~5 nm Au nanoparticles. The N vacancies in g-C₃N₄ could greatly improve the selective adsorption of Pb(II), and ∼5 nm Au nanoparticles enhanced the catalysis of materials. Chen et al. [93] developed a green method to synthesize AuNPs/mpg-C₃N₄ nanocomposites for constructing an anti-interference electrochemical sensing interface toward methylmercury, which might be used in the field of the fast screen of methylmercury in food and the environment. In one of their works, the electrochemical sensor constructed as an electrode modification material can directly, quickly, and accurately analyze Cr(VI) in wastewater samples from factories such as leather wastewater and electroplating wastewater [7]. The electrochemical test results yielded 376 ppb of Cr(VI) in TW, which is similar to the ICP-OES result (386 ppb) with a recovery of 97.4%. It is shown that the constructed electrochemical sensor is suitable for the analysis of real water samples in complex situations and has a strong potential for practical application. Mesoporous graphite phase carbon nitride and chloroauric acid were used as photocatalysts and raw materials, respectively, and successfully reduced gold nanoparticles to the surface and interlaminar gaps of mesoporous graphite phase carbon nitride under the irradiation of ultraviolet light (Figure 9g). Figure 9h shows the LSV response of different material modifications to Cr(VI). The peak current of Au/mpg-C₃N₄/GCE is almost double that of AuNPs/GCE. As shown in Figure 9i,j, a calibration curve in the range 100~1000 ppb is developed using an Au/mpg-C₃N₄ sensor by the electrochemical method, with the obtained sensitivity and detection limit of Cr(VI) being 0.002196 µA/ppb and 14.7 ppb, respectively. The excellent performance is probably due to the charge transfer between gold nanoparticles and g-C₃N₄, which induces a synergistic catalytic effect. The optimal UT-g-C₃N₄/Ag hybrids displayed a faster electron transfer rate in comparison to UT-g-C₃N₄ and bulk g-C₃N₄. The preparation of Pt/g-C₃N₄/polythiophene nanocomposites (Pt/g-C₃N₄/PTh) as an electrochemically sensitive material for the determination of Hg²⁺ is presented by Mahmoudian et al. [89]. The results showed an increase in the active sites due to the presence of the g-C₃N₄ which significantly enhanced the absorption of Hg²⁺. The presence of PTh and PtNPs compensated for the lower electrical conductivity of the g-C₃N₄ in the nanocomposite. Nanometer zero-valent iron (nZVI) is one of the most widely used nanomaterials for groundwater and hazardous waste treatment due to its high reduction rate and removal capacity [142]. g-C₃N₄ can effectively disperse nZVI while adjusting its microstructure, further achieving efficient adsorption and reduction of target pollutants. The results show that G-nZVI has a better affinity for Pb(II) than naked nZVI, mainly depending on the change in microstructure to improve the adsorption capacity of heavy metal ions and synergistically enhance the reaction activity and stability [143]. Meanwhile, the high catalytic capacity of metal nanoparticles is a key factor in increasing the electrochemical performance of metal nanoparticles/g-C₃N₄ nanocomposites. There will be good prospects in the future for metal nanoparticles/g-C₃N₄ nanocomposites in electrochemical redox reactions.

4.4.2. Metal Oxide Nanomaterials/g-C₃N₄ Nanocomposites

Metal oxide nanomaterials have low environmental toxicity and strong biocompatibility. Due to their strong adsorption capacity, larger specific surface area, and better electron transfer kinetics, they have good physical and chemical properties and high-efficiency catalysis. Their original form and combination with other materials have excellent electrochemical properties and are suitable for use as electrode modification materials to achieve the purpose of capturing target pollutants in water with high selectivity. Among the electrode modification materials currently used for electrochemical analysis of heavy metal pollutants, MnO₂ [144], ZnO [145], CuO [146], Fe₃O₄ [147], ZrO₂ [148], TiO₂ [149], etc., are the most commonly used nano metal oxide materials.

The incorporation of metal oxide materials increases the oxygen vacancies and defects of the composite material, creating key active sites for the catalytic reaction [150]. Wang et al. [151] present a novel method to prepare mesoporous materials via in situ self-assembly of graphitic carbon nitride nanosheets and SiO₂ nanoparticles. Combining the advantages of g-C₃N₄ nanosheets and a mesoporous structure, the as-prepared materials exhibit superior adsorption capabilities for heavy metal ions and organic pollutants. The band gap of g-C₃N₄ can be reduced by doping CuO, and the electron transfer resistance of the interface can be enhanced. At the same time, the strong coordination mode between the N and Cu atoms at the interface of the CuO-g-C₃N₄ framework heterojunction is beneficial to improving the electrochemical performance [146]. The CuO/g-C₃N₄ material prepared by Atacan has remarkable repeatability and good stability for H₂O₂ electrochemical sensors [152]. As a common semiconductor material, copper sulfide (CuS) can be used as a functional component to improve the electrical properties of matrix materials because its conductivity is comparable to that of metals. Ning et al. [101] successfully prepared a new type of electrochemical sensor based on CuS/g-C₃N₄/GCE, which can effectively amplify the SWASV signal of Pb²⁺ (Figure 10b,c). The limit of detection (LOD) for Pb²⁺ is calculated to be 4.00 nM (S/N = 3). Figure 10a shows the possible reduction mechanism of Pb²⁺ at the CuS/g-C₃N₄ interface. The higher response of Pb²⁺ may be due to the outstanding advantages of this multifunctional composite material: (i) the complementary effect of “dispersibility/electrical conductivity”. (ii) A synergistic enrichment effect of “coordination/adsorption” [101]. In addition, binary transition metal oxides have better catalytic activity and conductivity than single metal oxides. Wang et al. [105] integrated pg-C₃N₄ and CoMn₂O₄ into nanocomposite materials and developed a heavy metal ion detection probe with high sensitivity and selectivity (Figure 10d,e). It was observed by SEM and TEM that pg-C₃N₄ was wrapped on the surface of CoMn₂O₄ (Figure 10f,g). The pg-C₃N₄/CoMn₂O₄ nanocomposite has abundant accessible active sites and a high-efficiency electron transport path, which can realize the selective capture of Cd(II) and Pb(II) (Figure 10h,i). Furthermore, the precise coupling of g-C₃N₄ with CuCo₂O₄ can enhance the performance for Hg²⁺ reduction through the heterostructure formation [153]. Li et al. [154] combined low-pressure ultraviolet (LPUV) photolysis with a ZnO/g-C₃N₄ photocatalyst to enhance the SWASV signal of heavy metal ions (HMIs). In the analysis of HMIs in real soil extracts, the sensing system achieved an accuracy of 94.9% and 99.8% for the detection of Cd²⁺ and Pb²⁺, respectively, which verified the feasibility and validity of the proposed method for environmental applications.

Metal oxide nanoparticle/g-C₃N₄ nanocomposites have emerged as high-performance materials for the electrochemical detection of heavy metal ions, leveraging the synergistic interplay between g-C₃N₄’s rich surface chemistry and the catalytic/conductive properties of metal oxides. Among them, metal oxides can enhance redox activity and electron transfer kinetics, enabling ultrasensitive detection via stripping voltammetry [155]. Functional oxides can improve selective adsorption of target ions through electrostatic interactions, ion exchange, or surface complexation. Magnetic oxides enable easy electrode regeneration via external magnetic fields, addressing fouling issues and extending sensor lifespan.

4.4.3. Carbonaceous Nanomaterials/g-C₃N₄ Nanocomposites

Carbon-based nanomaterials have attracted widespread attention in the field of electrochemical sensors due to their unique properties. Carbon-based nanostructured materials include fullerenes, carbon nanotubes (CNTs), graphene (G) and its derivatives (graphene oxide (GO)), nano-diamonds (NDs), and carbon-based quantum dots (CQDs). Carbon nanotubes (CNTs) have high tensile strength and strong hardness, and due to their one-dimensional nanotube structure, their electrical and thermal conductivity have been significantly improved, even surpassing some conductive metals [156]. These excellent properties have important significance for improving the sensitivity of electrochemical sensors.

In the research conducted by Wang et al. [95], the synergistic effect between sulfur-doped C₃N₄ tube bundles (STBs) with hierarchical pores and graphene nanosheets (Gs) is beneficial for ultra-trace heavy metal ion (HMI) detection and makes the STB/Gs composite a promising sensitive electrochemical sensor for HMIs detection. It can be seen from the TEM image (Figure 11 a–d) that there are many small holes in the tube wall, and these pores promote the rapid diffusion of the HMI solution in the tube bundle structure. The STB/Gs-2 composite is shown in Figure 11e–h. The STB could be seen scattered on the surface of Gs. When STB active sites adsorbed HMIs, Gs could rapidly transfer electrons between STB active sites and electrodes to realize the rapid electrochemical REDOX reaction of HMIs deposition and stripping processes [95]. The SWASV curves of the STB/Gs-2-modified electrode were demonstrated in Figure 11j. The STB/Gs-2-modified electrodes showed that the LOD was 1.17, 0.38, and 0.61 nM for simultaneous detection and 2.30, 0.78, and 1.15 nM for individual detection of Cd²⁺, Pb²⁺, and Hg²⁺. Thin g-C₃N₄ nanosheets were prepared by acid–base corrosion and an ultrasonic-assisted method, using g-C₃N₄ NS/RGO to construct an electrochemical sensing interface for the detection of Pb²⁺ and Cd²⁺ [157]. The reduced graphene oxide sheet acts as a conductive channel to promote the electron–hole separation and transfer rate in g-C₃N₄ NS, thereby improving the photocurrent intensity and photocatalytic performance. Studies have shown that the composite of g-C₃N₄ and CB can create a new synergistic effect while maintaining the characteristics of each material, which can promote the effective detection of toxic heavy metals in the solution [81]. Under visible light conditions, the detection sensitivity of the g-C₃N₄/CB electrochemical sensor is greatly improved than under dark conditions. The photo-assisted electrochemical sensor has good detection stability and reproducibility and can detect multiple different heavy metal ions at the same time, and the detection limit (LOD) is very low. The performance of carbon nanotubes largely depends on the way they are rolled. Among them, multi-walled carbon nanotubes (MWCNTs) have more excellent properties. In the research of Ramalingam et al. [107], three-dimensional nanocomposites composed of porous graphitic carbon nitride nanosheets (p-g-C₃N₄-NSs) and oxidized multiwalled carbon nanotubes (O-MWCNTs) were prepared by chemical oxidation (Figure 11k,l). One-step oxidation has two main effects. One is to cause the formation of acidic functional groups on both the basal plane of g-C₃N₄ and MWCNTs, and the other is that O-MWCNTs bind in situ to the porous structure of p-g-C₃N₄. In Figure 11m, the nanocomposite material used to modify the screen-printing electrode (SPE) to have good sensitivity and selectivity to heavy metal ions Cd(II), Hg(II), Pb(II), and Zn(II) can be seen. Recently, MOFs have been used as electrode materials to detect HMIs on account of their excellent ability to bind heavy metal ions, high specific surface area, and easy modification. Chen et al. combined the advantages of g-C₃N₄ and ZIF-8 and developed a g-C₃N₄@ZIF-8 nanocomposite material, which is suitable as a sensitive material for detecting metal ions and solvents [158]. Chen et al. [97] modified GCE with g-C₃N₄/CNT/NH₂-MIL-88(Fe) as a modifier, and used square wave stripping voltammetry to simultaneously detect Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺. The doping of g-C₃N₄ in the composite, rich in N-containing functional groups, participates in the adsorption of metal ions on the surface of the electrodes. The porous composite provides accommodation room for metals generated by electro-reduction. The detection limit for Cd²⁺, Pb²⁺, Cu^2+, and Hg²⁺ is 39.6 nM, 7.6 nM, 11.9 nM, and 9.6 nM, respectively. In addition, Ti₃C₂T_x and g-C₃N₄ are potential complementary materials for the preparation of stable and conductive composite materials, which exhibit excellent electrochemical performance. The protonated g-C₃N₄/Ti₃C₂T_x electrode exhibited remarkable sensitivities for the simultaneous detection of Cd²⁺ (0.05∼1.50 μM) and Pb²⁺ (0.05∼1.50 μM), and the detection limits of Cd²⁺ and Pb²⁺ were 0.001 and 0.0006 μM, respectively [108].

In summary, carbon-based materials enhance the electrochemical performance of g-C₃N₄ by addressing its inherent limitations, including low electrical conductivity and limited active site accessibility. Key advancements include the following: (1) enhanced conductivity: integration with graphene or CNTs creates conductive networks that accelerate electron transfer kinetics, improving sensitivity in techniques; (2) active site engineering: functionalized carbon materials (e.g., oxygen/nitrogen-doped graphene) coupled with g-C₃N₄ increase adsorption capacity and catalytic activity for target ions via π–π interactions, electrostatic binding, or chelation. (3) Stability: carbon coatings (e.g., graphitic shells) protect g-C₃N₄ from oxidative degradation in harsh environments, ensuring long-term operational reliability.

4.4.4. Conductive Polymer/g-C₃N₄ Nanocomposites

Conducting polymers (CPs), such as polyaniline (PANI), polypyrene (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT), have good electrochemical stability and high electrical conductivity, and are widely used in energy storage and electrochromic devices, ion sensing, and biosensors [159,160,161]. The excellent properties of polymer nanocomposites help promote the ability of electron transfer in electrochemical reactions, and they are an ideal material for improving the sensitivity of electrochemical detection [162,163]. In general, conducting polymers are not used alone but are compounded with other materials such as metals, metal oxides, and carbon materials. Wu et al. [106] compounded PEDOT and g-C₃N₄ to prepare a feasible method for the determination of heavy metal ions. The enhanced electrochemical performance of composite materials mainly depends on the strong synergy of the components. First of all, the use of the BCP method is conducive to the uniform combination of PEDOT and g-C₃N₄ flakes and strengthens the strong π–π interaction between them. Secondly, the fast charge transfer ability and large surface area of g-C₃N₄ promote the coordination of g-C₃N₄ with metal ions. In the work of Eswaran et al. [110], a new, low-cost, high-efficiency nano-engineered poly(melamine)/graphitic-carbon nitride nanonetwork (PM/g-C₃N₄)-modified screen-printed carbon electrode (SPE) for electrochemical monitoring of toxic HMIs in environmental water is proposed. Using the prepared PM/g-C₃N₄/ASPE as an environmental sensor, differential pulse voltammetry was used for selective and synchronous electrochemical detection of Pb²⁺ and Cd²⁺ ions. The sensor has good sensitivity and selectivity to Pb²⁺ and Cd²⁺, with sensitivities of 0.008 μM and 0.02 μM, respectively (Figure 12E–G). In addition, the composite of a D-A-D-conjugated polymer and g-C₃N₄ can be used as a good electrode modification material for constructing electrochemical sensors for the high-efficiency detection of heavy metal ions. In the work reported by Ding et al. [111], poly(2,5-bis(3,4-ethylenedioxythienyl)pyridine)/graphitic carbon nitride composites (poly(BPE)/g-C₃N₄) were prepared by an in situ chemical polymerization method and used for simultaneous detection of Cd²⁺ and Pb²⁺. The combination of poly(BPE) and g-C₃N₄ improves the conduction path on the electrode surface, and the conjugation effect between them enhances the adsorption of metal ions. In addition, the prepared modified electrodes can be used to detect Cd²⁺ and Pb²⁺ in tap water samples with recoveries of 98.64~106.74% and 99.81~113.15%, respectively. According to the report of Teng et al. [164], EDTA has been covalently immobilized onto CN-NS using N-(trimethoxysilylpropyl) ethylene-diamine triacetic acid sodium salt as a coupling reagent, which has been designed to improve the accumulation performance for Pb (II) onto the electrode surface. Studies have shown that the electrochemical response signal of Pb (II) on EDTA-CN-NS is significantly stronger than that of unfunctionalized CN-NS, and its LOD is 5.7 × 10⁻¹³ mol/L. The simultaneous electrochemical detection of Pb (II), Cu (II), and Hg (II) using the EDTA-CN-NS/Nafion/GCE was illustrated as well. In short, conductive polymer-modified graphitic carbon nitride nanocomposites have emerged as promising platforms for the electrochemical detection of heavy metal ions, leveraging the synergistic advantages of both components. Conductive polymers enhance the electrical conductivity, catalytic activity, and stability of g-C₃N₄, while the latter provides abundant active sites and tunable surface chemistry.

The doping of g-C₃N₄ is a widely studied strategy to enhance its physicochemical properties for applications in photocatalysis, energy storage, and environmental remediation. The effects of doping of different materials on the properties of g-C₃N₄ mainly include the following aspects:

(1)

Enhanced photocatalytic performance

By introducing external dopant atoms into g-C₃N₄, the electronic structure and energy levels can be tuned to enhance photoresponsivity and improve charge separation. Non-metallic doping creates new energy levels within the band gap, which broadens the spectral response and reduces the electron–hole pair complexation rate. Therefore, through strategies such as non-metal doping, co-doping, and vacancy engineering, the photocatalytic performance of g-C₃N₄ can be enhanced by improving light absorption, facilitating charge separation and transport, and prolonging charge carrier lifetimes [165];

(2)

Improved conductivity and charge transfer capability

The introduction of dopant atoms, on one hand, modifies the electronic structure of g-C₃N₄, significantly reducing the carrier migration barrier and thereby enhancing its intrinsic conductivity [166]. On the other hand, it creates additional active sites, introduces defects, and further improves conductivity;

(3)

Improved thermal and chemical stability

Dopant atoms can be evenly dispersed in the C₃N₄ lattice as point defects, which can play a role similar to that of “pinning” and inhibit the expansion of lattice defects, thus improving the chemical stability of the material [167].

In summary, doping tailors g-C₃N₄’s properties by modifying its electronic structure, surface chemistry, and morphology, making it a versatile material for sustainable technologies. Future work should focus on optimizing dopant selection and understanding structure–property relationships at the atomic level.

5. Conclusions and Prospects

In environmental monitoring, electrochemical sensing methods are one of the most powerful tools. Generally, improving the stability and reliability of the electrochemical sensing interface requires a detailed investigation of the design and development of sensing materials. To achieve this goal, sensing materials need to have excellent adsorption and catalytic properties. In this feature article, we have presented an overview of g-C₃N₄ and g-C₃N₄-based nanocomposites: (1) characteristics specific for g-C₃N₄, (2) electrochemical detection of heavy metal ions, and (3) the application of g-C₃N₄ and g-C₃N₄-based nanocomposites in heavy metal detection. G-C₃N₄ has proven to be one of the most promising candidate materials for electrochemical sensing for designing and fabricating advanced composite catalysts for various applications.

Multifunctional materials will become a feasible direction for environmental safety monitoring in the future. In recent years, various nanomaterials such as precious metals, alloys, carbon materials, metal oxides, and their composites have been widely used in the electrochemical detection of heavy metal ions. Furthermore, in situ field analysis requires the development of portable integrated electrochemical devices and further advancement toward constructing highly integrated miniaturized electrochemical platforms. Currently, progress has been made in portable electrochemical sensors, where g-C₃N₄ nanocomposites combined with screen-printed electrodes have been preliminarily developed for environmental pollutant monitoring. Meanwhile, utilizing USB-powered electrochemical workstations and capsule-shaped electrochemical workstations configured with carbon electrodes, gold electrodes, and graphene electrodes modified with g-C₃N₄ nanocomposites enables the detection of specific biomarkers. The detection signals can be wirelessly transmitted via Bluetooth to mobile phones or other display terminals, demonstrating promising potential for real-time monitoring of toxic contaminants in aquatic environments.

Though a large number of studies have been devoted to this field and some significant results have been achieved, some important issues are still worth studying. Firstly, there are still some problems to be solved in the large-scale preparation of g-C₃N₄ with controllable morphology. Current synthesis methods for g-C₃N₄ nanostructures (e.g., quantum dots and porous frameworks) often lack reproducibility and scalability. Standardized protocols and cost-effective fabrication techniques (e.g., green chemistry approaches and roll-to-roll manufacturing) are urgently needed for industrial adoption. Secondly, complex environmental matrices (e.g., organic matter and competing ions) often compromise sensor selectivity and accuracy. Developing surface modification strategies or selective recognition elements (e.g., aptamers and ion-imprinted polymers) is essential to enhance anti-interference capabilities. Lastly, balancing ultra-trace detection limits (sub-ppb) with high specificity in multi-ion systems remains challenging, necessitating advanced signal amplification mechanisms or machine learning-assisted data processing. Therefore, future efforts should prioritize surface engineering, interdisciplinary integration (e.g., AI-guided material design), and pilot-scale validation to bridge the gap between laboratory research and practical environmental monitoring applications.

Furthermore, g-C₃N₄-based sensors offer high sensitivity and selectivity for HMIs, but challenges in scalability, stability, and device integration hinder commercialization. For example, high-quality g-C₃N₄ requires complex synthesis steps, such as thermal polymerization and exfoliation, which are energy-intensive. Therefore, the production of defect-free nanosheets on an industrial scale remains challenging; prolonged exposure to harsh electrochemical conditions (e.g., acidic media) can degrade g-C₃N₄ composites, reducing sensor lifespan; most g-C₃N₄ sensors rely on glassy carbon electrodes (GCEs). Adapting them to wearable or field-deployable formats requires miniaturization and compatibility with flexible substrates, which are underdeveloped. Based on this, innovations can be made in material design (such as defect engineering and hybrid composite materials) and synthesis methods (such as microwave-assisted routes), striving to accelerate the transition from lab-scale prototypes to market-oriented devices.