School of Chemistry and Chemical Engineering, Guangxi Minzu University, Key Laboratory of Chemistry and Engineering of Forest Products, State Ethnic Affairs Commission, Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi Collaborative Innovation Center for Chemistry and Engineering of Forest Products, Key Laboratory of Guangxi Colleges and Universities for Food Safety and Pharmaceutical Analytical Chemistry, Nanning 530008, China
Electrochemiluminescence (ECL) combines the characteristics of electrochemistry and chemiluminescence. Because of its high sensitivity, wide linear range and low background interference, electrochemiluminescence has attracted the attention of many researchers in analytical science. Although the traditional luminescent materials of ECL have high luminous efficiency, they still have some disadvantages, such as high price, lower sample load, etc. g-C3N4 is a kind of metal-free semiconductor nanomaterial, which is based on triazine ring or heptazine ring as the basic structural unit. g-C3N4 is a two-dimensional graphite-like layered structure bonded by vander Waals forces between layers and C—N covalent bonds within layers. Since g-C3N4 was first discovered to have ECL performance in 2012, it had been widely used in the development and fabrication of ECL sensors, due to its advantages, such as stable property, unique band structure, good biocompatibility, better environmental performance, easy to function, inexpensive ingredients and simple preparation process. The research progresses of g-C3N4 in ECL sensor construction in recent years were reviewed, according to the mechanism of ECL luminescence, the effect of sensor, the signal type of sensor,and the different types of detection objects. And the challenges and prospects of g-C3N4 in ECL development were described as well.
Contents
1 Introduction
2 Classification according to luminescence mechanism
2.1 ECL cathodic co-reactant
2.2 ECL anode co-reactant
2.3 Other co-reactant
3 Classification according to sensor effect
3.1 Enhanced ECL sensor
3.2 Quenching ECL sensor
3.3 ECL sensors for both enhancement and quenching
4 Classification according to the signal type of the ECL sensor
Electrochemiluminescence (ECL) refers to the formation of excited products by electron transfer after the substance on the surface of the electrode is excited by the power supply, and then the excited products release energy and return to the ground state in the form of luminescence, which has the characteristics of both electrochemistry and chemiluminescence. Although Harvey's group observed the phenomenon of luminescence in electrolysis experiments as early as the 1920s, it was not until 40 years later that ECL was reported in detail in the relevant literature. Nearly 100 years have passed since the discovery of ECL, and the technology has developed rapidly (Figure 1), and its application has expanded from the initial routine analysis of inorganic and organic substances to the fields of biology and medicine[1⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~16]. Compared with other detection methods, ECL has many advantages. First, the sensitivity is higher and the linear range is wider; Condly, no external light source is needed, the influence of optical factors such as scattered light and the like is avoided, and the background interference is small; Finally, through the electrochemical reaction, the material without luminescent properties can be converted into a chemiluminescent substance, which enlarges the determination range of the substance.
In recent years, our group has constructed a series of electrochemiluminescence sensors by combining nanomaterials, MOFs, quantum dots, aptamers, etc., and used them for ultrasensitive detection of drugs, antibiotics, biomolecules, pesticides, metal ions, etc[17⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~30]. Traditional ECL luminescent reagents (such as bipyridyl ruthenium, luminol, etc.) Have been widely used because of their high luminescent efficiency and good stability, but they have the disadvantages of high price and low loading rate of the target, so it is also the direction of our research group to explore some environmentally friendly and non-toxic ECL luminescent materials with good luminescent efficiency and sample loading rate.
Graphite-phase nitrogen carbide (g-C3N4) is a metal-free semiconductor nanomaterial, which mainly uses triazine ring or heptazine ring as the basic structural unit (Fig. 2), and forms a graphite-like two-dimensional layered structure through the van der Waals force between layers and the combination of C — N covalent bonds within layers. Compared with the triazine ring structure, the g-C3N4 with the heptazine ring structure as the basic structure unit has higher chemical stability, stronger corrosion resistance, better luminescence performance, larger specific surface area and easier modification and functionalization, so the g-C3N4 with the heptazine ring structure has wider application. g-C3N4 has been used in photoelectrocatalysis for a long time because of its unique band structure, good biocompatibility, cheap raw materials and simple preparation. It was not until 2012, when Cheng et al. Discovered that g-C3N4 could generate a cathodic ECL signal near 460 nm through electron transfer under the K2S2O8 of coreactant, that g-C3N4 was first used in the detection of ECL[31]. The following year, Cheng et al. Found that g-C3N4 could produce anodic ECL signal under the co-reactant triethanolamine (TEA)[32]. So far, the g-C3N4 has been tracked and explored by the majority of ECL researchers[33⇓⇓~36]. There are many synthesis strategies of g-C3N4 nanosheets, including epitaxial growth, vapor deposition and ultrasonic exfoliation, but in view of the fact that the "bottom-up" epitaxial growth method and vapor deposition method require high preparation environment and the size of the obtained nanosheets is limited, the "top-down" ultrasonic exfoliation method is the most commonly used in the construction of ECL sensors[37,38]. The preparation process is as follows: firstly, taking cyanamide, dicyandiamide or melamine and the like as a precursor, obtaining a blocky C3N4 through heating and condensation, and then obtaining a g-C3N4 nanosheet through an ultrasonic stripping method.
2 Classification according to the mechanism of ECL
The luminescence mechanism of ECL mainly includes co-reactant type, ion fuming type, hot electron induction and electrostatic interaction. In the application of g-C3N4, the most common one is the co-reactant type, which can generate electron transfer on the surface of the electrode to obtain an oxidized or reduced radical intermediate, and then react with the corresponding reduced or oxidized luminophor on the electrode to obtain the excited state of the luminophor, and then release energy to return to the ground state in the form of luminescence. For the same luminophor, if different co-reactants are selected, the ECL signals obtained are also different in different degrees, so in order to improve the ECL signals of the luminophor, the selection of co-reactants is very important.
2.1 Commonly used cathode ECL co-reactant
Under negative voltage, the g-C3N4 produces a cathodic ECL signal, and K2S2O8 and H2O2 are commonly used as co-reactants. Taking the co-reactant K2S2O8 as an example, the cathode ECL emission mechanism of g-C3N4 is as follows:
g-C3N4+ e- → g-C3,
S2 + e- → + ,
+g-C3 → g-C3+,
and/or
g-C3N4+ → g-C 3+,
g-C3+g-C3 → g-C 3+ g-C3N4,
finally,
g-C3→ g-C3N4+ hν
Based on the electron transfer between g-C3N4 and arsenite, and the specific adsorption of arsenite by DNA rich in G/T bases ( (GT) 19), Yu Ludan et al. Assembled (GT) 19 onto a Au-g-C3N4, and constructed an ultrasensitive ECL sensor for arsenite detection based on g-C3N4 with a detection limit of 0.07 mg·m-3 in the potential range of -1.5 ~ 0 V by S2O82- of the coreactant[39]. Xinli Guo constructed an ECL immunosensor with a sandwich structure for the detection of α-fetoprotein (AFP) using Au-g-C3N4 as a signal probe and (NH4)2S2O8 and H2O2 as co-reactants in the potential range of − 1.6 to 0 V[40]. Wu et al. Immobilized the Ab1 of squamous cell carcinoma antigen (SCCA) on two-dimensional Fe3O4@GO using S2O82- as a coreactant, and then labeled the Ab2 of SCCA with two-dimensional Au-g-C3N4.Through the formation of a Faraday cage complex between the antigen and the antibody, the two two-dimensional light-emitting materials are overlapped, and the g-C3N4 on the surface of the electrode participating in the reaction is increased, so that an ultrasensitive cathode ECL sensor is prepared for the detection of SCCA[41]. Wang Yan found that hydroquinone (HQE) could quench the ECL signal of g-C3N4 under the action of coreactant K2S2O8, and combined with MIP technology, a highly sensitive MIP-ECL sensor for the determination of HQE was constructed[42]. Wu et al. Constructed an ultrasensitive ECL sensor for quantitative detection of methotrexate (MTX) using S2O82- as a Co-reactant, Fe-Co-Co Prussian blue analogue as a novel Co-reaction promoter, and Fe-Co-Co PBAs@C-g-C3N4 as an ECL probe[43].
2.2 Commonly used anode ECL co-reactant
At positive voltage, the g-C3N4 produces an anodic ECL signal, and triethylamine, TEA, tripropylamine, and 2- (dibutylamino) ethanol are commonly used as co-reactants. Taking the co-reactant triethanolamine (TEA) as an example, the emission mechanism of g-C3N4 anode electrochemiluminescence is as follows:
g-C3N4- e- → g-C 3
TEA - e- → TE
TE - e- → TEA ·
g-C3 + TEA· → g-C 3+ TEA oxidant
g-C3→ g-C3N4+ hν
Liu et al. Constructed an ultrasensitive sensor for the determination of acetylcholinesterase (AChE) by using triethylamine as a co-reactant and g-C3N4 as a luminophor in the presence of AChE, which can hydrolyze the substrate acetylthiocholine to produce acetic acid to consume triethylamine and reduce the ECL anode signal of g-C3N4[44]. Liu et al. Constructed an ultrasensitive ECL sensor for dopamine specific detection using triethylamine as a co-reactant and g-C3N4 as an ECL anode emitter[45]. Based on the fact that dopamine (DA) can quench the anodic ECL signal of g-C3N4, Wang Yan constructed a MIP-ECL sensor with specific recognition ability for DA by using triethylamine as co-reactant[42]. Liu et al. Constructed an anodic ECL sensor for the determination of microRNA-21 with triethanolamine as a co-reactant and Cu@Cu2O as a co-reaction promoter, and the detection limit was 48 amol/L[46].
2.3 Other novel co-reactant
Some traditional co-reactants are easy to evolve hydrogen or volatilize on the electrode surface, resulting in unstable ECL signals, so exploring new and efficient co-reactants is the direction of researchers'efforts. Poly (L-lysine) (PLL) was used as an anodic coreagent of Ag-g-C3N4 to enhance the ECL signal of Ag-g-C3N4. Based on the quenching effect of glutathione (GSH) on the intermediate radical of PLL, a highly sensitive ECL sensor for GSH detection was constructed[47]. Using potassium persulfate and tetrabutylammonium bromide as double coreactants, Wang et al. Obtained two ECL peaks with strong g-C3N4 at 2.82 V and 1.73 V, respectively, in the range of -3 ~ 3 V, and used the signal at 1.73 V as an internal standard to construct a bimodal ECL sensor for the determination of ascorbic acid in human serum samples with a detection limit of 0.1 nmol/L[48].
3 Classification according to the effect of the sensor
3.1 Enhancement
Although g-C3N4 has been widely used in ECL, its luminous efficiency is not as high as that of traditional ECL materials, which also restricts its development in ECL to some extent. Therefore, improving the ECL signal of g-C3N4 is also the direction that researchers have been working on. Guo Xinli modified the glassy carbon electrode with 2-aminoterephthalic acid electropolymerization film, fixed the Ab1, and used K2S2O8 and H2O2 as co-reactants.Carcinoembryonic antigen (CEA) can form a sandwich immunocomplex with Ab1 and g-C3N4 labeled Ab2 on the electrode, thus constructing an enhanced ECL immunosensor for sensitive detection of CEA[40]. Wang et al synergistically enhanced the ECL signal of g-C3N4 with K2S2O8 and H2O2 double coreactive, when squamous cell antigen (SCCA) was present.The g-C3N4 loaded with Ab1 can form a sandwich structure with the Ab2 carried by Ag NPs through SCCA, resulting in a strong ECL signal, and the determination principle is shown in Figure 3A[49]. Wang et al. Used β-cyclodextrin modified g-C3N4 as the signal probe of ECL, based on the inhibition of acetylcholinesterase activity by organophosphorus pesticides (OPs), which can reduce the consumption of co-reactant triethylamine, thereby enhancing the ECL signal of g-C3N4, and established an ECL sensor for sensitive determination of OPs, and the detection principle is shown in Fig. 3B[50].
3.2 Quenching type
Li Ying et al. Constructed an ECL sensor for the determination of PKA based on the fact that Au NPs have strong absorption at the ECL emission peak of g-C3N4, which can quench the ECL signal of g-C3N4, and with the increase of protein kinase A (PKA) activity, the sulfhydryl phosphate bound to the electrode surface increases, which can capture more Au NPs and quench the ECL signal of g-C3N4[51]. Zuo et al. Constructed a novel ECL sensor for the determination of dopamine (DA) based on the bimolecular recognition strategy and the quenching effect of polyaniline (PANI) on the ECL emission of g-C3N4, and the construction principle is shown in Fig. 4[52]. Li et al. Adsorbed Ab1 on Ag@g-C3N4 through Ag NPs, with the help of Au's ability to adsorb proteins.An ECL sensor for detection of carcinoembryonic antigen A was constructed by immobilizing Fc-COOH-labeled Ab2 on Au-doped Al2O3 and utilizing the quenching effect of Au@Al2O3-Fc-COOH@Ab2 on Ag@g-C3N4ECL[53].
3.3 Both enhancement and quenching
Xia et al. Synthesized a graphene g-C3N4 by electrochemical reduction method, and the ECL signal of the composite material was stronger than that of the g-C3N4 itself. Since pentachlorophenol could quench the ECL signal of the GO/g-C3N4, an ECL sensor for detecting pentachlorophenol was constructed[54]. Hu et al. Enhanced the ECL signal of g-C3N4 through the synergistic effect of GO-g-C3N4 and MWCNTs-Au NPs, and the ECL signal of g-C3N4 was significantly reduced when diclofenac (DCF) was added, thus preparing a novel ECL immunosensor for detecting DCF, and its detection principle is shown in Figure 5A[55]. Xu et al. Quenched the ECL signal of g-C3N4 by the photoinduced electron transfer effect of Cu2+, and then introduced pyrophosphate anion PPi to release Cu2+ from the Cu2+-g-C3N4 system.An electrochemiluminescence sensor was constructed to detect pyrophosphate ions by forming a Cu2+-PPi complex to restore the ECL signal of the g-C3N4, and the mechanism is shown in Figure 5B[56]. Xu et al. Prepared a C3N4QDs@C3N4 nanosheet composite, which had a stronger ECL signal compared with a single g-C3N4. The Fc-labeled aptamer was bound to QD @ CNNS through π-π conjugation and electrostatic interaction, which quenched the ECL signal. When platelet-derived growth factor and aptamer formed a complex, the ECL signal could be restored. The action principle of the sensor is shown in Figure 5C[57].
4 Classification according to the signal type of the sensor
4.1 Single signal ECL transducer
The traditional ECL method usually adopts the single signal mode. Sha et al. Used glucose (Glu) as the recognition element of concanavalin A, MWCNT as the carrier material of MnO2, synthesized MnO2-MWCNTs-COOH@Glu, and combined with concanavalin A (Con A) as the energy acceptor to quench the ECL signal of g-C3N4[58]. Fu Xiaolong grows MnO2 nanosheets in situ on g-C3N4, and energy resonance transfer can occur between MnO2 and g-C3N4 to quench the ECL signal of g-C3N4[59]. In the presence of GSH, GSH can react with MnO2 to restore the ECL signal of g-C3N4, thus constructing an ECL sensor sensitive to GSH. Fu et al. Prepared C60/g-C3N4 composite through π-π bonding, and constructed an ECL sensor for sensitive detection of melamine based on the quenching effect of melamine on g-C3N4ECL. Compared with other methods, the linear range of the sensor for melamine detection was improved by 6 orders of magnitude, and the detection limit was 1.3×10-13mol/L. The detection mechanism is shown in Fig. 6[60]. Most ECL biosensors based on g-C3N4 are signal off type, but signal off is prone to false positive. In order to avoid this situation, Wu Qian synthesized MnO2/g-C3N4 composite based on the mechanism that MnO2 can quench g-C3N4ECL signal, and constructed a "off-on" type ECL biosensor for the detection of L-cysteine (CySH)[47]. The signal of the reduction of MnO2 to Mn2+,ECL was recovered in the presence of CySH, and the sensor showed good selectivity and wide linear range for CySH with a detection limit of 0.18 nmol/L.
4.2 Dual signal ECL sensor
Compared with the traditional single signal sensor, the ratiometric sensor can overcome the interference of the background signal, thereby improving the accuracy of the detection results. According to the different ways of signal output, dual-signal ECL sensors mainly include dual-wavelength ratio type and dual-potential ratio type. To construct a dual-signal ECL sensor, the following requirements usually need to be met: (1) two ECL luminescent materials need to share the same co-reactant; (2) In order to better distinguish the two luminescence signals, the excitation potentials of the ECL should be sufficiently different.
4.2.1 Dual wavelength ratio type
Dual-wavelength ratiometric ECL sensor means that two luminescent materials have different ECL signals at different wavelengths, and the detection of the target is realized through the linear relationship between the ratio of the two ECL signals and the concentration of the target. Feng et al. Used K2S2O8 as a co-reactant. Based on the mechanism of energy resonance transfer, g-C3N4 had a strong ECL signal at 460 nm, and ruthenium bipyridine could absorb visible light at 460 nm and emit ECL at 620 nm. Based on this, an ECL sensor of g-C3N4 and Ru(bpy)32+ ratio type was constructed, and microRNA 21 was quantified by measuring the ECL460 nm/ECL620 nm. The measurement principle is shown in Figure 7A[61]. Fu Xiaolong doped the Cd2+ into the g-C3N4, and when the target H2S existed,S2- can combine with Cd2+ to form CdS, which can quench the ECL signal of g-C3N4 at 435 nm.A dual-wavelength ratiometric ECL sensor was designed to detect the H2S accurately and quantitatively by ECL435 nm/ECL515 nm[59]. Wang et al. Regarded g-C3N4 and Ru @ MOF as the donor and acceptor of energy, respectively. With the increase of Aβ concentration, the ECL of g-C3N4 at 460 nm gradually decreased, while the ECL of Ru @ MOF at 620 nm gradually increased. According to the ratio of ECL460nm/ECL620nm, a dual-wavelength ratiometric ECL sensor for sensitive detection of amyloid Aβ was designed[62]. Zhang et al. Used g-C3N4 as ECL emitter to combine Au-g-C3N4 and Ab1, and polyaniline (PANI) and N- (aminobutyl) -N- (ethylisominol) (ABEI) modified Ab2, when calcitonin (CT) was present.PANI can quench the ECL signal of g-C3N4 at 460 nm and enhance the luminescence of ABEI at 475 nm, and the ECL460 nm/ECL475 nm has a good linear relationship with the concentration of CT[63].
4.2.2 Bipotential ratio type
Bipotential ratio ECL sensor means that two luminescent materials get different ECL signals at different potentials, and the detection of the target is realized through the linear relationship between the ratio of the two signals and the concentration of the target. Wang et al. Used potassium persulfate and tetrabutylammonium bromide as co-reactants, and found that the g-C3N4 could produce strong ECL signals at 2.82 and 1.73 V. According to the good linear relationship between the concentration of ascorbic acid (AA) and the I(ECL2.82V/ECL1.73V), a bipotential ratiometric ECL sensor for sensitive determination of AA was constructed, as shown in Fig. 7B[48]. Based on the fact that g-C3N4 can produce ECL signals at both anodic + 1.48 V and cathodic -1.2 V, and dopamine (DA) can quench its anodic signal, but has almost no effect on the cathodic signal, Wang Yan found that the concentration of DA has a good linear relationship with the ECL ratio of g-C3N4 at these two different potentials by using the cathodic signal as a reference, and constructed a double-potential ratiometric ECL sensor for accurate determination of DA[42]. Wang et al. Used g-C3N4 as the ECL luminophor of the reduction-oxidation type and Ag-PAMAM-luminol as the ECL luminophor of the oxidation-reduction type, and constructed a dual-potential ratiometric sensor for accurately measuring the concentration of HL-60 cancer cells by the decrease of ECL signal of g-C3N4 and the enhancement of ECL signal of luminol at two different potentials[64].
5 Classify according to the type of detected object
The detection objects based on g-C3N4ECL sensors have expanded from inorganic salts and small organic molecules to gene nucleic acid, biological immunity and cell detection.
5.1 Ions and small molecules
The ECL sensor of g-C3N4 can be used for ultrasensitive detection of ions and small molecules, such as heavy metals, small pharmacognostical molecules and toxicants[31,65⇓⇓⇓~69][42,49,52,70⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~82][83⇓⇓⇓~87]. It can be seen from Table 1 that compounding g-C3N4 with quantum dots, metal or metal oxide, graphene, carbon nanotubes and other materials can not only increase the functional modification sites of the target object to be detected, but also broaden the linear range of detection, obtain a lower detection limit, improve the sensitivity of the method, and realize the ultrasensitive detection of the target object.
5.2 Nucleic acid detection
g-C3N4 ECL sensor has been widely used in gene, tumor markers and other nucleic acid detection, which provides more ideas for clinical medical examination. Wang et al. Immobilized carbon nanotubes (CNT) on GCE, prepared Au/Pt-CNT by electrochemical deposition, then immobilized the capture probe S1, and then combined the signal probe S3 with Au-g-C3N4NHs.A sandwich-type DNA complex was formed by hybridization of target DNA S2 with S1 and S3, thereby constructing an ECL biosensor for detecting DNA of a specific sequence, and the action mechanism is shown in Fig. 8[88]. Pan Changgang linked walker DNA to the Au-Fe3O4, and then blocked it with blocker DNA. When the target miRNA-155 existed, the blocking of blocker DNA could be destroyed, and product DNA was obtained under the Mn2+. The single-stranded product DNA could bind to the g-C3N4 through π-π bonds to enhance the ECL emission of luminol and H2O2, thus preparing a wash-free ECL sensor for the detection of miRNA-155[89]. Sui Chengji used MoS2 as a co-catalyst and TiO2 as a phosphate group trapping agent to prepare a TiO2/MoS2/C3N4 composite material. When 5-hydroxymethylcytosine (5hmC) existed, it could compound with ferrocene carboxylic acid polymer to form 5hmC-PFc, which could inhibit the ECL signal of g-C3N4, and prepare an ECL sensor for detecting 5hmC[90].
5.3 Protein immunity
The deterioration of histiocytic lesions has a development process, only through highly sensitive immunoassay technology, can the lesions be detected in the embryonic stage in time, so that patients can be treated as soon as possible. g-C3N4 has been used in ECL immunoassay by researchers because of its good biocompatibility and easy functional modification, as shown in Table 2.
6 Conclusion and prospect
g-C3N4 has been widely used in the field of ECL in recent years because of its good photoelectric properties, low biological toxicity, stable properties, cheap and readily available raw materials and simple preparation process. However, there are still some shortcomings and limitations, for example, the luminescent efficiency of g-C3N4 itself is not high, and other materials need to be further modified to improve its luminescent properties. Only a part of the g-C3N4 on the electrode participates in the electrochemiluminescence, and the utilization rate is not high; Due to the steric hindrance of the electrode surface, the detection object is still relatively single. In view of the above problems, the main outlook is as follows.
(1) Develop more different kinds of two-dimensional materials combined with g-C3N4. Because the surface of the two-dimensional material has a large number of defect sites, functionalization modification and electron transfer are easier; and meanwhile, a plurality of two-dimensional materials are overlapped to form a Faraday cage structure, so that the utilization rate of the g-C3N4 on the surface of an electrode is improved, and the sensitivity of ECL is further improved.
(2) Single particle ECL detection and cell ECL imaging have great development prospects. Single cell analysis is of great significance in the early diagnosis and treatment of diseases, but it is still challenging to detect the chemical components of single cells because of the small size of cells, the difficulty of sampling and the low content of components in single cells. In addition, the steric hindrance of cells and the high insulation of cell membrane limit the application of ECL technology in cell analysis, so solving the problems of steric hindrance and conductivity of cells is the research direction of researchers.
(3) Simultaneous detection of two or more targets. In addition to the bipotential structure described above, ECL sensors can also be constructed in combination with bipolar electrodes (BPE) to reduce mutual interference between targets and materials.
(4) Synthesize C3N4 with different morphologies from the perspective of materials, increase the defects and luminescence efficiency of the surface of materials, and make them easier to functionalize and have higher luminescence efficiency. At present, two-dimensional graphitic nitrogen carbide is the most widely used in ECL, but few other dimensions of C3N4 are emerging in ECL. More one-dimensional C3N4 nanobelts and zero-dimensional CN QDs and their composites can be developed by breaking the C — N bond.
(5) Explore more co-reactants or co-reaction promoters. At present, the type of a co-reactant is single, and the co-reactant is easily damaged in the reaction process, so that the signal is unstable; and the working principle of the co-reactant can be combined, and a co-reaction promoter such as an oxide and the like is utilized, so that the ECL signal and stability of the g-C3N4 are improved.
(6) in combination with other methods. Although the traditional ECL detection technology based on photomultiplier tube has high sensitivity and low detection limit, it still has the shortcomings of insufficient luminescence stability and poor selectivity, which can be complemented by the combination technology.
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