Introduction

Heavy metal ions (e.g. those of Cd, Pb, and Hg), and semimetals (e.g. As) are highly toxic and may cause serious damage to health. Owing to growing concerns in this area, many international and national organizations have defined maximum contaminant levels for drinking water. For example, the maximum permissible levels set by the World Health Organization (WHO) for Cd and Pb are 0.003 and 0.010 mg L-1, respectively [1]. Electrochemical sensors are very promising for heavy metal monitoring, as they offer desirable characteristics such as sensitivity, selectivity, inexpensiveness, robustness, and field-deployability [2].

Graphene and its derivatives, including graphene oxide (GO), reduced graphene oxide (rGO), and three-dimensional (3D) graphene, are widely used in electrochemistry [3]. Graphene consists of a one-atom-thick planar sheet comprising a closely packed honeycomb carbon lattice. By definition, graphene contains only sp2 carbons without oxygen (or nitrogen). Pristine graphene has a high electron transfer rate, a large surface area (2630 m2 g-1) [4], and a high conductivity (64 mS cm-1) [5, 6]. Although some studies [7, 8] claim that pristine graphene was used, in fact they used graphite, GO or rGO. GO, a monolayer of graphite oxide, contains many defects and numerous oxygen functional groups, mostly in the form of hydroxyl and epoxy groups on the basal plane, with smaller amounts of carboxyl, carbonyl, phenol, lactone, and quinone groups at the sheet edges [9]. Compared with pristine graphene, the polar oxygen functional groups provide GO with good dispersibility in many polar solvents, particularly in water (concentration < 1 mg mL-1). In addition, the oxygen functional groups serve as sites for immobilizing various electroactive species via covalent or noncovalent bonds. However, the large amount of oxygen functional groups in GO causes some loss of electrical conductivity, which may limit the direct application of GO in electrically active materials and devices [9].

Reduced graphene oxide (rGO) is obtained by the chemical/electrochemical reduction of GO. The charge transportation ability of rGO is enhanced compared with that of GO owing to the removal of some oxygen-containing functional groups and partial remediation of the sp2 conducting structure. Moreover, the chemical and electrical properties of rGO are tunable and the content of oxygen functional groups and defects is sufficient for facilitating analyte adsorption [10]. Although rGO and its composites have been widely employed as sensing materials [11], the dispersion ability remains challenging.

In addition to these two-dimensional (2D) nano-carbon materials, 3D graphene has received considerable interest owing to its outstanding properties, such as an interconnected porous structure, an enormous specific surface area, good mechanical stability, and flexibility to tailorable surface chemistry [12]. Based on these advantages, 3D-graphene-based sensors have been exploited for the electrochemical sensing of heavy metal ions. However, 3D graphene shows an inferior sensing ability for heavy metal ions [13] owing to the restricted diffusion of aqueous analytes in the hydrophobic 3D connected framework [12]. To improve the sensing ability, various active species have been introduced on the 3D framework of graphene [12]. An excellent review related to 3D-graphene-based electrochemical sensors has been published by Baig et al. [14]. A summary of the merits and disadvantages of graphene, GO, rGO, and 3D graphene are listed in Table 1.

Table 1 Summary of the merits and disadvantages of graphene, GO, rGO and 3D graphene for heavy metal ion sensing
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In general, the direct use of pristine graphene or graphene derivatives for electrochemical sensing suffers from low sensitivity, interference from other substrates, and easy agglomeration. Therefore, sensors based on graphene-derived nanomaterials have been widely investigated [15, 16]. Although Gan et al. has addressed the preparation of sensors based on various 2D nanomaterials and their sensing properties for heavy metal ions [17], a systematic investigation of the mechanisms by which graphene-derived nanomaterials achieve improved detection of heavy metal ions is still needed. Our review is organized according to the type of sensing elements, which are classified as heteroatom-doped graphene, metal-modified graphene, metal oxide-modified graphene, organically modified graphene, polymer-modified graphene, and ternary graphene-based nanocomposites. Corresponding hybrid materials are also introduced.

Sensors using heteroatom-doped graphene and GO

Heteroatom (N, S, F, etc.) doped graphene can exhibit various new or improved electromagnetic, physicochemical, optical, and structural properties [18]. For example, Xing et al. synthesized N-doped graphene via a one-step electrochemical strategy. The incorporation of pyridine-like N and pyrrole-like N in graphene was found to greatly enhance the performance for electrochemical determination of Cd2+, Pb2+, Cu2+, and Hg2+ compared with rGO. The detection limit were estimated to be 0.05 μM for Cd2+ and Hg2+, and 0.005 μM for Pb2+ and Cu2+. [19]. Liu et al. reported a nanocarbon paste electrode modified with N-doped graphene for trace Pb and Cd determination using square wave anodic stripping voltammetry. The presence of N atoms in graphene increased the number of catalytically active sites and enhanced the electron transfer ability of the modified electrode [20]. In the presence of dibenzyl disulfide and a silica template, Manna et al. synthesized S-doped porous rGO by thermal annealing, and the material was used for efficient removal and electrochemical determination of Hg2+. As shown in Fig. 1, the presence of a large amount of thiophenic S and the porous structure provided a detection limit as low as 0.5 nM [21]. Antony et al. reported fluorinated GO for the simultaneous detection of Cd2+, Pb2+, Cu2+ and Hg2+ using square wave anodic stripping voltammetry. The incorporation of F into GO improved the sensitivity owing to the interactions between electron-donating F and electron-deficient heavy metal ions [22].

Fig. 1
figure 1

The morphology of S-doped porous rGO and application in Hg2+ electrochemical sensing and removing. Reproduced from [21] with permission of ACS

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Sensors using metal-modified graphene

Graphene modified with noble metal nanoparticles (NPs) has been used for heavy metal ions sensing because noble metal NPs exhibit high catalytic activity via the size effect. Moreover, graphene can transfer electrons acquired from catalytic process of the metal NPs to electrodes, which may accelerate the catalytic process [23]. Among NPs that form nanocomposite with graphene, Au NPs are the most widely applied for the detection of metal ions because they offer the advantages of high chemical stability and easy preparation processes [15]. For example, Au NPs decorated graphene synthesized via electrodeposition method was used for sensitive determination of Hg2+. Compared with their bulk electrode counterpart, Au NP modified electrodes are promising because they can eliminate the memory effect and increase the sensitivity for heavy metal ion detection [15]. The detection limit for Hg2+ was estimated to be 0.03 pM, which is well below the guideline value set by the WHO [24]. In addition to Hg2+ electroanalysis, Au NPs/rGO nanocomposites have also been widely used for the analysis of As3+. For instance, Liu et al. utilized an Au NPs-electroreduced graphene oxide (ERGO) composite film for the determination of As3+. The obtained good sensitivity (limit of detection = 2.7 nM) was attributed to the formation of Au–As intermetallic compounds that enhance the efficiency for cathodic preconcentration of As(0) [25]. Moreover, the effect of the supporting electrolyte (0.20 M aqueous HClO4, 0.20 M aqueous HCl, or 0.10 M aqueous H2SO4) on the magnitude of the detected signal was also evaluated. The detection performance in 0.20 M aqueous HCl was better than that in the other two supporting electrolytes, which was attributed to improved electron kinetics resulting from the complexation of Cl- ions with As3+. However, the detection of inorganic As in highly acidic media could cause problems such as hydrogen evolution or undesirable corrosion reactions [26].

In addition to Au NPs, graphene decorated with Ag NPs has also been used in heavy metal ion sensing. For example, Sang et al. synthesized Ag NPs/rGO via an in situ method. A nanocomposite-modified glassy carbon electrode was used for simultaneous electrochemical sensing of Cd2+, Pb2+, Cu2+ and Hg2+, and this modified electrode showed excellent selectivity [27]. However, expensive materials like Au and Ag are of limited practicality when fabricating macroelectrodes, which require large amount of material, owing to cost considerations [28].

Alternatives like Bi NPs or Sn NPs have also been used in heavy metal ion sensing. For example, Sahoo et al. prepared Bi NPs modified rGO sheets via an in situ method [29]. Lee et al. decorated rGO with Sn NPs via an electrodeposition method and realized electrochemical sensing of Cd2+, Pb2+ and Cu2+ using square wave anodic stripping voltammetry [30]. Sn has similar electroanalytical properties to Bi, but it is less toxic and cheaper [31].

Apart from metal NPs, metal films have also been hybridized with graphene to construct sensors for heavy metal determination. For example, Ping et al. fabricated an electrochemical sensing platform based on a screen-printed electrode modified with an electrochemically rGO. After in situ plating with a Bi film, the electrode was used for the simultaneous determination of Cd2+ and Pb2+ [32]. The mechanism of Cd2+ and Pb2+ detection at the surface of Bi based electrode involves the capacity of Bi to form a “fused alloy” with heavy metal ions [33]. Compared with a previously widely used Hg film, Bi is less toxic to the environment and has excellent mechanical stability [33]. Unfortunately, compared with the Hg-modified electrode, the Bi-modified electrode has various limitations, such as a narrow potential window (below the oxidation potential of Bi) and easy oxidation upon contact with air [34].

Sb film electrodes exhibiting similar electroanalytical performance to the Bi film electrodes have also been applied to heavy metal sensing. For example, Ruengpirasiri et al. used GO-Sb film-modified electrode for the simultaneous determination of Cd2+, Pb2+, Cu2+, and Hg2+ [35]. In situ preparation of Sb films can be conducted in a wider pH range than Bi films because bismuth hydroxide will form at pH 4, which results in irreproducible measurements [36, 37]. Thus, Sb film electrodes are a valuable and complementary alternative to Bi film electrodes for measurement under an oxidative potential or in acidic media (e.g. determination of Cu2+ and Hg2+). However, the toxicity of Sb metal ions, especially Sb3+, cannot be ignored completely [36]. The overview on metal-modified graphene as sensing material for electrochemical sensing of heavy metal ions was displayed in Table 2.

Table 2 Overview on metal-modified graphene as sensing material for electrochemical sensing of heavy metal ions
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Sensors using metal-oxide-modified graphene

As an alternative to metal NPs and metal films, metal oxides have frequently been used in heavy metal ion sensing owing to their large surface areas and high electrocatalytic activities. The sensing mechanism of metal oxide for heavy metal ion is strong adsorption ability, or electrocatalytic activity, or both simultaneously [28, 55,56,57]. However, most metal oxides have inferior conductivities and stabilities, which are unfavorable for electron transfer during the detection process and decrease the long-term stability of the electrode. However, when a metal oxide is combined with graphene, the nanocomposite is expected to provide a new electrochemical platform for heavy metal ion sensing [58]. Up to now, Fe3O4, ZnO, MnO2, Cu2O, Fe2O3, SnO2, TiO2, and Co3O4-based graphene nanocomposites have been successfully applied to the detection of heavy metal ions in aqueous solution. The morphology and average size of the metal oxide can greatly influence the performance of the modified electrode. As shown in Fig. 2, Sun et al. synthesized rGO decorated with three different shapes of Fe3O4 via a one-step in-situ co-precipitation method. The sensitivity for analysis of Pb2+ decreased in the following order: band Fe3O4/rGO > spherical Fe3O4/rGO > rod Fe3O4/rGO [59]. Karthik et al. synthesized Co-doped ZnO/rGO as a heavy metal ion sensor for Cd2+ and Pb2+. Compared with ZnO/rGO, Co-doped ZnO/rGO exhibited better catalytic activity toward Cd2+ and Pb2+ sensing with detection limits of 8.36 nM for Cd2+ and 4 nM for Pb2+ [60].

Fig. 2
figure 2

Preparation processes of three shapes of Fe3O4/rGO by adjusting the mole ratio of Fe2+/Fe3+ via in-situ co-precipitation method. Reproduced from [59] with permission of Elsevier

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Compared with monometallic oxides, bimetallic oxides exhibit better electrochemical activity owing to electron hopping between different valence states of metals in oxygen sites [61]. Huang et al. compared the detection performance of NiCo2O4 with those of Co3O4 and NiO. NiCo2O4 exhibited better performance for electrochemical determination of Pb2+ and Cu2+ than the other two materials [62]. Based on this concept, Xiong et al. used a 1,6-hexanediamine (HDA)- functionalized MgFe2O4/rGO composite for the electrochemical determination Cu2+. The amino group in HDA has high activity for coordination with heavy metal ions. The detection limit was estimated to be 0.2 nM with a sensitivity of 0.0172 μA nM-1 [63]. The same group investigated polyethyleneimine (PEI) (or ethanediamine (EDA)) functionalized CoFe2O4/rGO composite for electrochemical detection of ultra-trace Cu2+, and explored the interaction mechanism. Cyclic voltammetry and X-ray photoelectron spectroscopy results indicated that the interaction between the composite and Cu2+ involved an adsorption control process [64]. Zhou et al. synthesized GO incorporating mesoporous MnFe2O4 for the electrochemical determination of Pb2+. The mesoporous structure of MnFe2O4 increased the specific surface area of GO and enhanced the electrochemical activity toward Pb2+ analysis [65]. The overview on metal oxide-modified graphene as sensing material for electrochemical sensing of heavy metal ions was shown in Table 3.

Table 3 Overview on metal oxide-modified graphene as sensing material for electrochemical sensing of heavy metal ions
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Sensors using organically modified graphene

The modification of graphene with organic molecules is believed to increase the sensitivity and selectivity of graphene-based electrochemical sensors for heavy metal ion through two different recognition mechanisms, namely, chemical affinity and cavity entrapment (or both simultaneously). Various kinds of organic molecule including small organic molecules (containing electron-rich groups such as –OH, –SH, and –NH2) and caged molecules (calixarenes and cyclodextrins) have been investigated [1]. For instance, Muralikrishna et al. synthesized L-cysteine functioned GO by reacting the carboxyl groups in graphene with the amino group in L-cysteine. This material was used for the simultaneous electrochemical determination of Cd2+, Pb2+, Cu2+, and Hg2+. The oxygen–containing groups of GO and the electron donor group in L-cysteine facilitated the adsorption process of heavy metal ions [81]. Yuan et al. reported high–density 2-amino-5-mercapto-1,3,4-thiodiazole (AMT)–grafted GO prepared via an amidation reaction between GO and AMT under strong basic conditions. The high grafting density was attributed to the high density carboxyl groups on GO. The detection signal during electroanalysis of Cu2+ was amplified by the abundant N, O, and S donor atoms of AMT.

To exploit the coordination between heavy metal ions and N atoms in piperazine, our group synthesized piperazine–grafted GO through nucleophilic ring–opening of epoxy groups on GO with the amino groups of piperazine. After chemical reduction by ascorbic acid, the modified glassy carbon electrode was used for the detection of Hg2+ with a detection limit of 0.2 nM [82]. Based on the same synthetic mechanism, Zhou et al. reported cysteamine-functionalized GO for the selective determination of Hg2+. In addition to interacting with the Au electrode surface through the formation of Au–S bonds, the residual mercapto groups in cysteamine can selectively interact with Hg2+ [83]. Göde et al. functionalized rGO with calixarenes using 1-ethyl-3-(3-dimethylaminoprophy) carbondiimide hydrochloride (EDC) and N-hydroxy succinimide (NHS) to activate the carboxylic acid (–COOH) groups on rGO. The nanocomposite was used for simultaneous determination of Fe3+, Cd2+, and Pb2+. As shown in Fig. 3, the 3D basket, cup, or bucket shapes of calixarenes can effectively entrap metal ions and the oxygen–containing groups can form complexes with the metal ions, thus increasing the sensitivity and selectivity of the sensor for these metal ions [84].

Fig. 3
figure 3

Preparation of calixarene/rGO/GCE and nano-sensing of the guest metal ions. Reproduced from [84] with permission of Elsevier

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Yu et al. fabricated N-[(1-pyrenyl-sulfonamido)-heptyl]-gluconamide (PG) modified graphene for ultrasensitive and selective sensing of heavy metal ions. Owing to the large π system of pyrene, a stable interaction can occur between the pyrene residue and graphene. Whereas functional groups such as hydroxyls and imines in glucose can act as coordination sites for Hg2+ during the detection process [85]. Magerusan et al. used an N–doped graphene/chitosan nanocomposite for selective Pb2+ detection. The electron-donating functional groups such as hydroxyls and amines in chitosan and the N doping groups in rGO can easily coordinate electron-deficient heavy metal ions. Moreover, positively charged chitosan can interact with negatively charged rGO to increase the stability of the nanocomposite [86]. Yang et al. constructed an As3+ sensor with excellent selectivity using an Au microelectrode decorated with amino-functionalized GO. Benefited from the synergetic effect of the strong adsorption capability of NH2-GO and the excellent electrocatalytic ability of Au microwire, resulting in a low detection limit of 2.16 nM [87]. The overview on organically modified graphene as sensing material for electrochemical sensing of heavy metal ions were listed in Table 4.

Table 4 Overview on organically modified graphene as sensing material for electrochemical sensing of heavy metal ions
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Sensors using polymer modified graphene

Polymers with a high number of reactive sites allows for analyte preconcentration on the electrode surface and are thus expected to increase the sensitivity when used for heavy metal ion sensing. Among the various types of polymers, conducting polymers have received much attention owing to their superior electrical conductivities and anti-fouling capabilities [115]. Moreover, the morphology of the conducting polymers (fiber, wire, film, or particle) and the dopants are related to the detection performance (sensing range, limit of detection, and response/recovery time) of the modified electrode [116, 117]. Conducting polymers including polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT) have been widely used in heavy metal determination. For example, in our previous work, we synthesized PEDOT nanorods/GO nanocomposite via interfacial polymerization as a new electrode material for electrochemical detection of Hg2+. The specific doping and de-doping properties of PEDOT could be controlled by varying the deposition potential, providing a selective sensing platform for Hg2+ determination. Moreover, in the nanocomposite, the PEDOT nanorods can function as electro-active sites to facilitate electron transfer during the determination process [118]. Dai synthesized PPy/GO nanocomposites via in situ chemical oxidation polymerization, and phytic acid molecules were functionalized with nanocomposites through electrostatic attraction. Owing to the presence of phosphoric acid groups in phytic acid and N-containing groups in PPy, the sensor was utilized for the simultaneous determination of Cd2+ and Pb2+ with detection limits of 19 and 1.98 nM, respectively [119]. Muralikrishna et al. described PANI/GO hydrogels for highly sensitive electrochemical determination of Pb2+. The hydrogels were synthesized through in situ polymerization of aniline in the presence of GO nanosheets followed by hydrogel formation at an elevated temperature [120].

Besides conducting polymers, other electroactive polymers including Nafion, poly(dimethylsiloxane) (PDMS), polydopamine, poly-L-lysine (PLL), polyallylamine, and polyethyleneimine have also been used in heavy metal ion sensing. For example, Li et al. reported a Nafion–graphene nanocomposite for ultrasensitive determination of Cd2+, with a detection limit of 0.044 nM [121]. The addition of Nafion can increase the mechanical robustness of the electrode and avoid interference from anionic in the sample (NO3-, SO42-, or CO32-). Chałupniak et al. prepared a microfluidic lab-on-a-chip platform for heavy metals preconcentration and electrochemical detection based on a GO-PDMS nanocomposite. The use of GO–PDMS significantly improve the sensitivity for the electrochemical detection of heavy metals with a low detection limit of 0.34 pM [122]. Guo et al. prepared an electrode modified with an GO and chitosan hybrid matrix through drop casting, and a PLL film was coated on the electrode through electropolymerization via a cyclic voltammetry method. The amino and hydroxyl groups in this system effectively coordinated metal ions. Moreover, the PLL films which had excellent permselectivity, good stability, strong adherence to the electrode surface, and an increased amount of active sites enhanced the electrocatalytic activity of the modified electrode. When used for the simultaneous electrochemical detection of Cd2+, Pb2+, and Cu2+, detection limits of 0.089, 0.097, and 0.31 nM, respectively, were obtained [123]. Liu et al. constructed polyallylamine-hydrochloride-functionalized via a non-covalent method. The –NH2 functional groups of polyallylamine hydrochloride improved the performance or trace detection of Cu2+, with a relatively low detection limit of approximately 0.35 nM [124]. Through nucleophilic substitution reactions between the surface-exposed epoxy groups in GO and the active amine groups in PEI, Hu et al. synthesized PEI-rGO nanocomposites. When combined with Nafion, the hybrid modified electrode showed selectivity for Cu2+ electrochemical determination, with a detection limit of 0.3 μM [125]. In conclusion, polymer-modified interfaces have many outstanding merits, but the application of these systems is still limited by potential swelling or denaturation of the polymers during prolonged accumulation times and slow diffusion across the films [126]. In addition, the overview on polymer modified graphene as sensing material for electrochemical sensing of heavy metal ions was showed in Table 5.

Table 5 Overview on polymer modified graphene as sensing material for electrochemical sensing of heavy metal ions
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Sensors using ternary graphene-based nanocomposite

Compared with binary graphene-based nanocomposites, ternary or quaternary graphene nanocomposites such as metal-conducting polymers, metal-carbon nanotube (CNT) or conducting polymer-CNT hybrid with graphene show better performance [23]. For example, Dong et al. constructed an Au NPs/PANI/graphene modified electrode for sensitive detection of Pb2+. Compared with Au NPs, PANI, or graphene modified glassy carbon electrodes, the ternary hybrid showed improved detection performance, which was attributed to the synergetic effects of these three materials [138]. Moreover, PANI can function as a protective layer for Au NPs, avoiding interparticle aggregation via van der Waals attraction [1].

Wang et al. reported that the passivation of modified electrodes is problematic real sample analysis because various surface-active species may be adsorbed on the electrode. However, fouling of the electrode can be effectively alleviated by modification of the electrode with a dialysis membrane layer, such as Nafion, PLL, or cellulose acetate. Commonly used membrane modification methods usually involve a solvent evaporation procedure, which results in unsatisfactory homogeneity and reproducibility of the membrane. Therefore, they adopted an electrodeposition method to modify rGO/glassy carbon electrode with p-aminobenzene sulfonic acid. Compared with the afore-mentioned modification method, this electropolymerization method had the advantages of strong adhesion, controllable film thickness, uniform structure, and good stability. After in-situ plating a stannum film, the sensor was used for the sensitive determination of trace Cd2+ [139].

Cui et al. prepared thiazole-derivative functionalized graphene decorated with SnO2 NPs and compared the influence of different halogen anions (F, Cl, or I) on the detection performance of the composite. The F@SnO2/thiazole derivative-functionalized graphene exhibited superior performance for the detection of Cu2+ than the other two materials [140]. Recently, sensors based on a flexible substrate have gained attention, owing to their potential application as wearable sensors to monitor heavy metal ion in sweat, saliva, tears, or other body fluids. For example, Xuan et al. fabricated a fully integrated, miniaturized, and flexible electrochemical sensor based on a micro-patterned rGO and CNT composite on a flexible Au substrate as a working electrode (Fig. 4). After plating with a Bi film, the sensor exhibited separated and well-defined stripping peaks for Cd2+ and Pb2+ [141].

Fig. 4
figure 4

Photographs of the fabricated miniaturized, integrated, and flexible heavy metal ion sensor with micro-patterned rGO and a CNT composite working electrode. Photo images (a, d) of a fabricated flexible heavy metal ion sensor, (b) microscope image of 3 electrodes, and (c) working electrode. (Gap size: 50 μm, total effective working electrode area: 1.5 mm2, total working electrode thickness: ∼1 μm). Reproduced from [141] with permission of Elsevier

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Sensors based on films have also received attention owing to their potential as disposable electrodes for heavy metal ion sensing. For example, Dong et al. synthesized a sandwich structured ionic liquid-CNT-graphene film via an effective inkjet printing method for electrochemical determination of Cd2+ and Pb2+. The sensor exhibited high sensitivity, a wide linear range, and a low detection limit owing to the synergetic effects of these materials including fast charge transferability, sufficient surface active sites, and a large surface area [142]. Table 6 shows the overview on ternary or quaternary graphene-based nanocomposite as sensing material for electrochemical sensing of heavy metal ions.

Table 6 Overview on ternary or quaternary graphene-based nanocomposite as sensing material for electrochemical sensing of heavy metal ions
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Conclusions and perspectives

Graphene-based nanocomposites have been widely investigated as chemical sensors with high sensitivity and selectivity. We reviewed the sensing principles of graphene-based hybrids, including heteroatom-doped graphene, metal-modified graphene, metal-oxide-modified graphene, organically modified graphene, polymer-modified graphene, and ternary graphene based nanocomposite, which provide sensitive, selective and stable platforms for heavy metal ions determination. On one hand, searching for new materials and new methodologies to control the morphology and structure of sensing materials to fabricate new sensors is an important direction for graphene-based sensors. On the other hand, optimizing the performance of current sensor systems, including sensitivity, selectivity, and stability, is of equal importance. Furthermore, the development of flexible or wearable sensors for detecting heavy metal ions in real samples or human body fluids is an important endeavor.