Introduction

Persistent organic pollutants (POPs) [1], which are usually derived from various agricultural and industrial activities such as overuse of pesticides and synthesis of fertilizers, have been and continue to be released to the ecological environment and existed in the contaminated water, polluted food and infected air. Due to their environmental persistence, refractory biodegradation, high toxicity and bioaccumulation in the food chain, POPs must involve serious threats to global health problem and have negative side effects on the environment as well as human tissues [2].

The Stockholm Convention (SC) on persistent organic pollutants (POPs) is a worldwide agreement under the United Nation Environmental Program, which aims at protecting environment and human health from toxic as well as persistent pollutants and reducing or eliminating their release ultimately. 22 POPs have been targeted by the SC in 2012 (http://chm.pops.int/Home/tabid/2121/mctl/ViewDetails/Event-ModID/871/EventID/230/xmid/6921/Default.aspx), and all the 22 POPs are classified into Annexes A, B and C (Tables 1 and 2), where Annex A is eliminated from production and use, Annex B is specific and restricted exempted from production and use, and Annex C is unintended-produced POPs [3]. Moreover, under the SC, new kinds of chemicals, such as hexabromocyclododecane and pentachlorophenol, are also evaluated at times by the Persistent Organic Pollutants Review Committee, which implies their appearances in these Annexes of the SC in the near future. Detailed chemical structures of some typical POPs are presented in Fig. 1.

Table 1 POPs listed in the Stockholm Convention (Referring to Ref. [3])
Full size table
Table 2 An overview on reported carbonaceous material-based electrochemical methods for determination of POPs
Full size table
Fig. 1
figure 1

Chemical structures of common POPs (Reproduced from Ref. [3])

Full size image

In order to evaluate the potential negative effects of these POPs on environment and biology, and to get information about the pollution levels, it is extremely necessary and urgent to develop fast, trustworthy and practical methods for the recognition and quantification of POPs compounds. Moreover, high sensitivity is also essential for the detection of POPs, as in most cases POPs present at concentrations in the nanograms/micrograms per litre levels together with relatively higher levels of other chemical pollutants [4]. Great efforts have been made to monitor POPs, however, analysis of POPs in environmental matrices is still a challenging task, quantitative and reliable analysis data are only available for a part of these POPs. These disadvantages are mainly ascribed to the following limitations: (1) the great complexity of the natural environment background; (2) the considerable interferents on POPs analysis especially when their manifold analogs or isomers exist; and (3) the lack of commensurate analytic techniques when the concentrations of some POPs are at trace levels.

Up to now, routine analytic techniques for most of POPs detection are mainly gas chromatography-mass spectrometry, high performance liquid chromatography (HPLC), thin layer chromatography, etc. [5,6,7]. However, these techniques require sophisticated and expensive equipments, expert operators and involve time-consuming sample preparation; additionally, those techniques lack the capability to enforce real-time detection as they cannot be miniaturized, limiting their practical application in environmental monitoring. Electrochemical technique has large amounts of advantages such as rapidity, simplicity, convenience and high sensitivity. Therefore, electrochemical technique is an attractive option for the fast and sensitive determination of POPs.

People have extended progress in the electrochemical detection of POPs using modified electrodes, and many detection results have been obtained [8,9,10,11,12]. However, the most used modified materials are usually non-carbonaceous materials, such as heavy metal nanoparticles, etc., involving high cost and high toxicity. Here we focus our attention on the electrochemistry detection of POPs basing on carbonaceous materials-modified electrode, which have attracted worldwide attention owing to their superior physical and chemical properties including high specific surface area, high thermal/electronic conductivity, excellent chemical/ physical stability, good flexibility and comparatively low cost [13,14,15,16]. The carbonaceous materials modified electrode has high selectivity and detection precision. It can detect the trace POPs, which are difficult to be detected by using conventional electrodes. How to choose the carbonaceous materials of modification of electrochemical working electrodes is a challenge for electrochemical detection of trace POPs. It is a pity that we can rarely list all the relevant works, and only some of the typical papers are selected in this review. We apologize to researchers whose important publications may be left out. Additionally, it is essential to give a brief review on these most commonly used carbonaceous materials before discussing their applications in POPs electrochemical detection, such as carbon nanotubes, graphene, reduced graphene oxides, graphitic carbon nitride and carbon dots.

Typical carbonaceous materials used in electrochemical detection

Carbon nanotubes

Carbon nanotubes (CNTs), classic one-dimensional (1D) carbon nanomaterial, were first discovered in 1991 by Iijima on a cathode through a carbon-are discharge method [17]. CNTs are basically depicted as graphite sheets rolled into cylindrical form with multiple walls (MWCNTs) or single wall (SWCNTs) and capped with half shape of fullerene structure [18]. Owing to the π-delocalized curved surface together with unique length-to-diameter ratio, the theoretical value of their specific surface area reaches to 1315 m2 g−1 for SWCNTs [19]; moreover, through multiple reciprocities with other target analytes (e.g., dispersion force, electrostatic force, dative bond, π stacking and hydrophobic interaction), CNTs are also exceptionally good adsorbent and activator compared with the typical carbon materials, including graphite and graphitized carbon [20]. CNTs chemistry has already been the subject of several reviews [21,22,23]. Since the walls of pristine CNTs are unreactive, functionalization of their walls is powerful to fulfill their enormous potential [24]. Indeed, functionalization is an effective way to vary the retention and selectivity of pure CNTs, and thus various CNTs-based nanomaterials with improved detection capability can be obtained [25, 26].

Graphene and reduced graphene oxides

Graphene (GR), two-dimensional (2D) sp2-hybridized carbon sheet, has drawn great attention owing to its many remarkable optical, electrical and mechanical properties as well as potential use in many fields such as composite materials, sensors and electronics [27,28,29]. As a new form of carbonaceous material discovered by Novoselov and Geim in 2004, GR shows a great influence on the worldwide scientific community [30]. Due to the amazing theoretical value of its specific surface area (2630 m2 g−1) and a large number of adsorption sites [31], GR also can be an ideal candidate for physico-chemical analysis. Various processes for the functionalization of GR and GR composites have been gathered in previous reviews [32,33,34]. Moreover, reduced graphene oxide (rGO), oxygen-rich carbon material containing many sp3 carbons, also have been prepared and used in many previous works [35, 36]. Different from GR, rGO is not perfectly planar and shows excellent flexility; besides, due to the numerous defects and oxygen functions on its surface as well as edge, a large number of adsorptive and active sites also exist on rGO, leading to its potential use in electrochemical detection. There are some affairs should be noticed when using GR or rGO, which has been elucidated as below.

  1. (1)

    Diversity. Either GR or rGO shows significant differences in size, shape and thickness, which probably bring in a large number of uncertain factors to the experimental analysis. From the practical applications’ point of view, the reproducibility of the final experimental results cannot be guaranteed. Therefore, it is well advisable to obtain GR or rGO with characters alike by means of controlled synthesis as well as purification after synthesis.

  2. (2)

    Metallic impurities. The residual metallic impurities (e.g., Fe, Co and Ni) contained in GR or rGO perhaps come from natural graphite or metal catalysts used during the synthesis process [37]. These metallic impurities are so not easy to be removed, thus becoming a trouble to practical applications. So, related discrimination and purification is extremely necessary.

  3. (3)

    Agglomeration. Due to π stacking and van der Waals force, either GR or rGO have a strong tendency to agglomerate. The agglomeration can seriously weaken the excellent capabilities of individual GR or rGO. To lessen the agglomeration effectively, modification (e.g., interspersion of other nanomaterials into GR sheets), immobility (e.g., immobilization on platform) and stabilizer (e.g., addition of surfactant) have been widely used.

  4. (4)

    Irreversible adsorption. Target analytes might irreversibly adsorb onto the surface of GR and rGO, which would cause nonrecycle and low recovery of their related materials. Thus irreversible adsorption should be prevented during real applications.

Graphitic carbon nitride

Graphitic carbon nitride (g-C3N4) is a novel conjugated polymer mainly composed of carbon and nitrogen atoms with an interlayer distance of ca. 3.3 nm [38,39,40]. It possesses a 2D framework of s-triazine or tri-s-triazine connected via tertiary amines, and the tri-s-triazine structure is more favorable in consideration of thermodynamic stability [41, 42]. Up to now, g-C3N4 is mostly prepared from melamine, dicyandiamide or cyanamide polymerization through various treatments (e.g., solvothermal reaction, thermal nitridation and chemical/physical vapor deposition) [43, 44]; however, in most cases, bulk g-C3N4 is first obtained according to the synthesis routes as mentioned above, which is difficult to use directly because of its poor dispersity. Various works have been reported on exfoliating bulk g-C3N4 into ultrathin g-C3N4 nanosheets, including thermal oxidation, liquid exfoliation, ultrasonic and chemical exfoliation [45,46,47,48,49,50]. Benefiting from the lone pair of nitrogen atoms and the relatively narrow bandgap (~ 2.7 eV), g-C3N4 has been successfully applied in optics/electricity fields such as photocatalysis, electrocatalysis and photoelectronic devices.

Carbon dots

Carbon dots (CDs) have attracted much attention due to their unique combination of many crucial merits, including biocompatibility, small dimension, well photostability as well as chemical inertness, tunable electrochemiluminescence/photoluminescence character and excellent up-conversion property [51,52,53]. Generally speaking, all the small carbon nanomaterials, which mainly consist of carbon and possess one dimension less than 10 nm in size at least, can be called CDs. Due to a great diversity of CDs, we mainly focus on the following three types of CDs in this review: carbon nanoparticles (CNs, which do not have a crystal lattice), carbon quantum dots (CQDs, which have an obvious crystal lattice) and graphene quantum dots (GQDs, which possess one/several layers of graphene together with functional groups at the edge).

Application of carbonaceous materials

Great efforts have been made for the detection of POPs based on electrochemical techniques. To be electrode materials, carbonaceous materials possess various fascinating properties such as excellent electrical conductivity, superior electrocatalytic activity, large specific surface area as well as good biocompatibility [54]. Moreover, the combination of carbonaceous materials and other raw materials have attracted much interest, which possesses not only properties of the individual components, but also a synergistic effect. Therefore, how to obtain the high-performance modified electrode has become a great challenge for the electrochemical detection of trace POPs.

In this part, we focus on the carbonaceous material-based hybrids as templates, carriers, immobilizers and transducers for the construction of electrochemical sensors/biosensors by employing some typical examples.

Applications of carbon nanotubes

The CNTs-based electrochemical sensors have been applied to detect trace POPs compounds successively.

Anirudhan and Alexander utilize the reaction between multiwall carbon nanotubes (MWCNTs) and glycidyl methacrylate (GMA) to produce a MWCNT/g-GMA successfully, and then the MWCNT/g-GMA further reacts with allylamine to form a new MWCNT/g-GMA-CH=CH2 molecularly imprinted polymer (Fig. 2a) [55]. At a particular pH value (pH = 3), the negatively charged chlorine atoms of lindane (γ-hexachlorocyclohexane, γ-HCCH) interacts with protonated –COOH groups of the molecularly imprinted polymer through an electrostatic interaction; additionally, when γ-HCCH is selectively adsorbed in the network of the molecularly imprinted polymer, the potential alters as the concentration of γ-HCCH changes, thus potentiometric sensing of γ-HCCH can be achieved. Under optimum conditions, the electrochemical sensor shows a wide linear range from 1 × 10−10 to 1 × 10−3 mol L−1 and a comparatively low detection limit of 1 × 10−10 mol L−1.

Fig. 2
figure 2

a TEM images of multiwall carbon nanotubes (MWCNTs), MWCNT/g- glycidyl methacrylate (GMA), MWCNT/g-GMA-CH=CH2 molecularly imprinted polymer (Reproduced from Ref. [55]); b The possible oxidation mechanism of PCP on the ZnSe quantum dots/MWCNTs modified glassy carbon electrode (Reproduced from Ref. [56])

Full size image

Feng and coworkers report a ZnSe quantum dots decorated multiwall carbon nanotubes (ZnSe QDs/MWCNTs) electrochemical sensor for the sensitive detection of pentachlorophenol (PCP) [56]. The behaviors of PCP on the ZnSe QDs/MWCNTs surface indicate that the ZnSe QDs/MWCNTs hybrid serves as a favorable access for electron transfer between the electrode and analyte, and realizes a two-electron and two-proton electrocatalytic oxidation toward PCP (Fig. 2b). Differential pulse voltammetry (DPV) is used for the determination of PCP, the peak current of PCP is proportional to the concentration at the range from 8.0 × 10−8 to 4.0 × 10−6 mol L−1 with a detection limit 2.0 × 10−9 mol L−1. Moreover, the DPV signal of PCP are much larger than those of other structure-similar chlorophenols such as 2-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol, indicating highly selective detection of PCP with this sensor in practical samples.

Adriana et al. develop a MWCNT/epoxy (MWCNT/EP) electrode for the voltammetric/amperometric detection of PCP in aqueous solutions [57]. Around +0.80 ~ +0.97 V vs. SCE, PCP is oxidized on the MWCNT/EP electrode, but no reduction peak can be detected, which reveals that the anodic oxidation of PCP on the MWCNT/EP electrode is irreversible. Moreover, the oxidation currents of PCP increase successively with its concentrations in a wide linear range from 0.2 to 12 μmol L−1 based on different electrochemical determination methods such as cyclic voltammetry, DPV, square wave voltammetry and multiple-pulsed amperometry techniques (Fig. 3).

Fig. 3
figure 3

a Cyclic voltammograms of the MWCNT/epoxy (MWCNT/EP) composite electrode recorded in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of different PCP concentrations: (2) 2 μM, (3) 4 μM, (4) 6 μM, (5) 8 μM, (6) 10 μM; b DPVs of the MWCNT/EP composite electrode (modulation amplitude 0.2 V, step potential 0.02 V) in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of different PCP concentration: (2) 2 μM, (3) 4 μM, (4) 6 μM, (5) 8 μM, (6) 10 μM, (7) 12 μM; c SWVs of the MWCNT/EP composite electrode (modulation amplitude of 0.1 V, step potential of 0.01 V and frequency 10 Hz) in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of different PCP concentrations: (2) 2 μM, (3) 4 μM, (4) 6 μM, (5) 8 μM, (6) 10 μM, (7) 12 μM; d Multiple-pulsed amperograms (MPAs) of the MWCNT/EP electrode in 0.1 M Na2SO4 supporting electrolyte and in the presence of different PCP concentrations: 2, 4, 6, 8, 10 and 12 μM recorded at (1) E = +1.25 V; (2) E = +0.97 V and (3) E = −0.1 V vs. SCE. (Reproduced from Ref. [57])

Full size image

Application of (doped-) graphene and reduced graphene oxides

As novel carbon nanomaterials, (doped-) GR and rGO are in the spotlight in adsorption and catalytic reactions ever since their discoveries. Till now, pure GR and nitrogen–doped GR [58], rGO decorated by other materials [59,60,61] and GR with cyclodextrin [62,63,64] have shown their fascinating applications in electrochemical sensing of POPs, improving the selectivity of the electrochemical sensors.

Yu et al. have successfully applied pure GR as well as nitrogen–doped GR (NG) for the sensitive detection of hexachlorobenzene (HCB) with a linear range from 3 to 10 μg L−1 and a detection limit of 1.72 μg L−1 [58]. Due to the particular electronic interaction between lone-pair electrons of nitrogen and π-system of graphitic carbon [65], NG not only exhibits higher electrocatalytic activity for the reduction of HCB compared with pristine GR, but also shows good adsorption ability towards HCB, decreasing the detection limit of the sensor. DPV results confirm the electrochemical reduction process of HCB (Fig. 4).

Fig. 4
figure 4

a Schematic drawing of the assembly of (nitrogen–doped GR/chitosan)3.5 / glassy carbon electrode ((NG/CS)3.5/GCE) and schematic representation of the electrochemical responses of hexachlorobenzene (HCB) at (NG/CS)3.5/GCE; b DPV behaviors of HCB and pentachlorobenzene (QCB) at (NG/CS)3.5/GCE (a): without and (b): with 3 mg/L HCB; (c): without and (d): with 3 mg/L QCB; c DPV behaviors of 3 mg/L HCB at (a): bare GCE, (b): (G/CS)3.5/GCE, (c): (NG/CS)3.5/GCE. (Reproduced from Ref. [58])

Full size image

In 2012, Yu et al. have found that the binding of some POPs such as PCBs to the cavities of β-cyclodextrin leads to readily measurable conductivity decreases associated with the formation of guest − host complexes (Fig. 5 (A)) [66]. Inspired by this, based on the host-guest interaction, several works have been reported on the combination of GR and cyclodextrin for POPs sensing by the DPV electrochemical method. For example, Zheng et al. modify reduced graphene oxide with β-cyclodextrin polymer (β-CDP), which serves as an electrochemical detection platform for PCBs with ferrocene as redox indicator (Fig. 5 (B)) [62]. Compared with ferrocene, PCBs possess higher affinity towards β-CDP and can replace the ferrocene in the host-guest cavity formed by β-CDP and ferrocene, which results in lower detection limit through selective host-guest interaction; additionally, ferrocene is a commonly used redox probe and thus can be easily determined by DPV method, the new DPV sensor shows good detection performance towards PCBs with a low detection limit of 5.0 × 10−13 mol L−1. Similar results have also been reported by Chen’s group [63, 64]. Novel electrochemical sensor has been developed for the detection of POPs based on β-cyclodextrin noncovalently functionalized graphene sheets with the help of 3,4,9,10-perylene tetracarboxylic acid (CD-PTCA-GR) [63]. The authors point out that due to the excellent physical/chemical performances of graphene, the high supramolecular recognition and enrichment properties of cyclodextrin, CD-PTCA-GR and POPs (e.g. PCP) can form an inclusion complex with guest molecules. Moreover, the resulted DPV sensors have greatly improved sensing performance basing on the oxidation of these POPs during the DPV process.

Fig. 5
figure 5

a Schematic of an electrical nanogap device modified with carbon dots (CDs) for polychlorinated biphenyls (PCBs) detection. (a) Electrical nanogap device is constructed by a nanogapped gold particle film on an interdigitated microelectrode with a 2.5-μm gap. (b) Conceptual illustration of specific inhibition of charge transport. Functionalized point “traps” made through the modification of HS-β-CDs between the gaps in an electrical nanogap device. Cavities of HS-β-CDs act as specific grabbers to PCBs. Charge transport can be inhibited by electrical barriers (i.e., PCB molecules) in the transport channels (Reproduced from Ref. [66]); b (a) Schematic illustration of the immunosensor preparation process, (b) How the β-CD interacts with PCB via the host-guest interaction (Reproduced from Ref. [62])

Full size image

Furthermore, as a newly powerful sensing technique, electrochemiluminescence (ECL) detection combines the advantages of chemiluminescence and electrochemistry, and can not only offer cheap, control and portable instrumentation but also show low background noise and wide response range, which can potentially proffer low detection limit and good selectivity [67, 68]. Works on rGO-based ECL detection of PCP also have been reported successively [59,60,61, 69]. Luo et al. have fabricated a stable and effective ECL sensor by immobilizing Au nanoclusters (Au NCs) on the rGO, with S2O82− as coreactant [59]. rGO facilitates both Au− and SO4− production, and the yield of the excited Au NCs (Au*+) has been enhanced. The interaction between Au*+ and PCP enables a sensitive change in the ECL intensity with a low detection limit of 1.0 × 10−14 mol L−1 in a linear range from 1.0 × 10−14 mol L−1 to 1.0 × 10−10 mol L−1 towards PCP (Fig. 6a), other compounds which cannot be oxidized by the Au*+ show little interference on the detection of PCP. Wu et al. develop a novel ECL platform based on copper oxide nanowires coupled with reduced graphene oxide (CuO NWs/rGO) (Fig. 6b) [60]. CuO nanowire is used as electroluminescent for the first time, and the rGO can greatly enhances the ECL signal. In the presence of the coreactant S2O82−, the CuO NWs/rGO ECL sensor achieves sensitive and selective detection of PCP with a wide linear range from 1.0 × 10−14 to 1.0 × 10−9 mol L−1 and an unprecedented detection limit of 0.7 × 10−14 mol L−1. Liang et al. report a novel strategy for the design of CdS quantum dots/reduced graphene oxides/carbon nanotubes (CdS/rGO/CNTs) hybrid. Based on the CdS/rGO/CNTs hybrid, an advanced ECL sensor is also fabricated for the real-time detection of PCP with low detection limit and high selectivity (Fig. 6c) [61]. Yang’ group have developed a novel electrochemiluminescence (ECL) sensor based on carbon quantum dots (CQDs) immobilized on rGO for the determination of chlorinated phenols (CPs) in water, using PCP as the indicator for CPs (Fig. 6d) [69]. The ECL sensor enables the real-time detection of PCP with detection limit reaching 1.0 × 10−12 mol L−1 and shows high selectivity to CPs, especially to PCP. Detailed detection mechanism also has been discussed in this work.

Fig. 6
figure 6

a The electrochemiluminescence (ECL) intensity responses of Au nanoclusters/rGO (Au NCs/rGO) in 0.067 M PBS (pH 7.0) containing 100 mM S2O82− at different concentration of Pentachlorophenol (PCP) (×10−13 M): (a) 0.1, (b) 1.0, (c) 10, (d) 50, (e) 100, (f) 1000. (Reproduced from Ref. [61]); b (a) Top-view SEM image of copper oxide nanowires/rGO (CuO NWs/rGO) on a Ti ribbon, with its cross-sectional SEM image inset, (b) TEM image of CuO NWs/rGO, (c) TEM image of a CuO NW, and (d) HR-TEM image of the CuO NW shown in (c). (Reproduced from Ref. [62]); c The ECL intensity response of CdS/rGO/CNTs in 100 mM S2O82− solution (pH 7) at different concentration of PCP (×10−12 M): (a) 1, (b) 5, (c) 10, (d) 50, (e) 100, (f) 500, (g) 1000, the insert image is the calibration curve for PCP determination (Reproduced from Ref. [61]); d SEM image of carbon quantum dots/rGO (Reproduced from Ref. [69])

Full size image

Application of graphitic carbon nitride

Graphitic carbon nitride (g-C3N4), a novel catalyst, has shown excellent electrocatalytic properties in the past few years [70,71,72,73]. Since the ECL behaviors of g-C3N4 were first reported by Cheng et al. in 2012 [74], a large number of g-C3N4 ECL sensing systems have been developed and the possible reaction mechanisms are also proposed for many of these ECL systems [75,76,77,78]. As for environmental detection, previous attentions have been mainly focused on the metal ions [74, 78,79,80]. For example, Cheng et al. have found that g-C3N4 modified carbon paste electrode generate a strong cathodic ECL signal, using K2S2O8 as coreactant (Fig. 7a) [74]. The ECL signal can be efficiently quenched when Cu2+ are added, thus g-C3N4 is utilized to fabricate an ECL sensor for the detection of Cu2+ with a detection limit of 0.9 nmol L−1. Possible ECL detection mechanism for Cu2+ is also proposed in this work (Fig. 7b).

Fig. 7
figure 7

a ECL-potential (curves a, b) and CV (curves c, d) curves of the graphitic carbon nitride (g-C3N4)-modified carbon paste electrode in 0.10 M K2SO4 without (curves a, c) and with (curves b, d) 3.0 m M K2S2O8 . Inset displays the enlarged view of curve a; b The possible ECL reaction mechanisms. (Reproduced from Ref. [74])

Full size image

However, as for POPs detection, to the best of our knowledge, related works are particularly rare [81]. Yang’s group has prepared a g-C3N4/rGO hybrid where GR serves as both immobilization platform and ECL signal amplifier for g-C3N4 (Fig. 8a)84; moreover, owing to the quenching effect of PCP on the ECL signal of g-C3N4/rGO, the g-C3N4/rGO ECL sensor can detect PCP with unprecedented detection limit reaching 1.0 × 10−11 mol L−1 in a wide linear range from 1.0 × 10−11 to 1.0 × 10−7 mol L−1. The possible ECL detection mechanism also has been proposed in detail (Fig. 8b). Electrons can be transferred from the electrode to g-C3N4 and dissolved O2 through rGO, leading to g-C3N4• − and O2• − production respectively (Reactions (1) and (2)). Then O2• − injects hole into the highest occupied molecular orbital of g-C3N4• −, producing the excited state species g-C3N4* (negatively charged) which emits light (Reactions (3) and (4)). And after PCP is added, PCP can be absorbed onto the surface of g-C3N4 and reduced by g-C3N4* (negatively charged), resulting in ECL quenching and the generation of 2,3,5,6-tetrachlorophenol (2,3,5,6-TCP) during the reduction process of PCP (Reaction (5)).

Fig. 8
figure 8

a Illustration for preparing g-C3N4/rGO hybrid; b Illustrative ECL detection mechanism for PCP with g-C3N4. (Reproduced from Ref. [81])

Full size image

Application of carbon dots

Carbon dots (CDs) possess fascinating optical properties such as chemiluminescence as well as photoluminescence, and ECL especially [82,83,84,85,86]. As a substitute to dye-based probes and commonly toxic ECL quantum dots (QDs) such as CdTe and CdSe QDs, CDs have attracted much attention for their excellent ECL properties, well biocompatibility and non-toxicity. Up to now, CDs are mainly prepared through oxidation strategy by treating various carbon sources (e.g., graphite, GO and soot from natural gas/burning candle) [87,88,89,90,91]. However, similar to g-C3N4, these prepared CDs with high ECL efficiency have been mainly used to detect metal ions in environment by metal ions-inducing ECL inhibiting in the CDs/K2S2O8 system [92]. As for POPs detection, related works are still in their infancy [93].

Li et al. have prepared CDs from activated carbon powder by chemical oxidation (Fig. 9a); besides, strong and stable ECL of CDs is also achieved in a CDs/ K2S2O8 system [93]. As PCP can effectively quench the ECL signal of CDs, a novel ECL sensor has been developed for PCP detection. Comparing with other methods, the ECL sensor shows better reproducibility, wide-range linearity and a low detection limit of 1.3 × 10−12 g L−1. The ECL mechanism for PCP detection has been explained in detail (Fig. 9b). Similar results are also obtained by Yang’s group [69]. In their report, after immobilizing CDs on rGO, a powerful K2S2O8-based ECL sensor has been obtained for the determination of chlorinated phenols (CPs) using PCP as an important indicator, which enables the unprecedentedly sensitive and selective detection of PCP with the detection limit of 1.0 × 10−12 mol L−1 (Fig. 9c). rGO can facilitate both C•− and SO4•− production, resulting in a high yield of C*+; while PCP is added, PCP can be absorbed onto the CDs surface and be oxidized by C*+, leading to a decrease in ECL intensity (Fig. 9d).

Fig. 9
figure 9

a ECL of carbon quantum dots (C QDs) in 0.1 M PBS (2 M NaNO3 + 20 mM S2O82−) in the absence (a) and presence (b) of 10−8 g L−1 PCP; b Schematic showing the ECL detection of PCP with CDs in S2O82− solution. (Reproduced from Ref. [93]); c ECL intensity response of C QDs/rGO in 100 mM S2O82− solution (pH 7) at different concentrations of PCP (×10−12 M): (a) 1, (b) 10, (c) 100, (d) 1000, and (e) 10,000; the inset plot is the calibration curve for PCP determination; d Illustrative ECL Detection Mechanism for PCP with C QDs/rGO in S2O82− solution. (Reproduced from Ref. [69])

Full size image

Conclusions and future perspectives

In conclusion, the application of carbonaceous materials has become one of the most important tendencies in electrochemical detection of the POPs compounds targeted by the Stockholm Convention. Carbonaceous materials play important roles in decreasing detection limit, improving selectivity, greening fabrication process and shortening analytical time, which would provide better chances to satisfy the increasingly urgent demands for the fast as well as sensitive recognition and quantification of POPs. Therefore, carbonaceous materials doubtless have a promising future in this area.

It also should be noticed that the application of carbonaceous materials is still at an early stage, and a large amount of challenges nevertheless exist in the analysis of 22 POPs under the Stockholm Convention. Limited by the relatively low production yield and high production cost, the synthesis technique for carbonaceous materials is still a block which confines their wide use. More importantly, to obtain more reliable and reproducible results for practical analysis application, especially for detection of POPs at trace levels, carbonaceous materials with higher standard such as higher purity, better monodispersity and higher stability are urgently needed. In addition, as a great deal of very complex matrices might be included in real environmental samples, the potential of carbonaceous materials in analyzing more complex samples rather than simple water samples also should be further explored. For this purpose, smart carbonaceous materials with improved selectivity to the analytes and excellent anti-interference to the complex matrices in samples must be developed. Moreover, the electrochemical detection of POPs based on carbonaceous materials has been limited to a small part of POPs until now, and many important POPs under the Stockholm Convention still cannot be recognized and quantified by the carbonaceous materials so far. Additionally, new kinds of POPs also continue to be evaluated and appeared in the Annexes of the SC, leading to the increasing requirements for new detection methods. Thus, more attempts must be made for extending the application of carbonaceous materials in electrochemical detection of more POPs, newly evaluated POPs under the Stockholm Convention especially.

Furthermore, compared with applying carbonaceous materials directly as an alternative to common materials in routine analytical techniques, it is very attractive interesting to combine carbonaceous materials with emerging microtechnology or nanotechnology (e.g., microextraction, nanoelectrode and ECL) to solve current analytical problems. It is reasonable to forecast a dramatic and wide-reaching change in POPs analysis application brought by carbonaceous materials in the near future.