Recent advances in electrochemiluminescence based on polymeric luminophores
Sijia Zhou , Tianyi Zhou , Yuhua Hou , Wang Li , Yanfei Shen , Songqin Liu , Kaiqing Wu , Yuanjian Zhang
Various types of efficient energy conversion are essential for scientific and technological development, such as photoluminescence (PL), chemiluminescence (CL), biogenesis (BL), and electrochemiluminescence (ECL) [1-3]. ECL, also known as electrogenerated chemiluminescence, is an analytical technique that uses electrical energy to generate chemiluminescence [4-7]. The first report concerning light emissions during electrochemical reactions can be traced back to the 1920s [8]. Since Hercules and Bard et al. described detailed ECL studies in the 1960s, ECL has been developed for more than half a century [9]. Specifically, ECL is generated by free radicals on the electrode surface that undergo exergonic electron transfer reactions to form excited states and produce luminescence [10]. Because the emitted light comes from the redox reactions on the electrode surface, ECL requires no external light source and almost zero background, which eliminates high-energy laser excitation and auto-photoluminescence [11]. This excellent technology not only has the advantages of electrochemistry with controllable reactions and simple operation, but also possesses the merits of conventional CL with a wide dynamic range and high sensitivity [12,13]. Generally, ECL mechanisms can be categorized into two types: annihilation and co-reactant mechanisms [14,15]. Although the annihilation ECL is the origin of the early study, the co-reactant one showed stronger and more stable emission in a narrower potential range, resulting in better potential for application [16]. To date, ECL has been recognized as a versatile analytical technique and successfully used in the fields of life analysis, environmental monitoring, and food inspection [17,18].
In general, efficient ECL methods rely on extraordinary ECL emitters [19-21]. These emitters, with their high luminescence efficiency, are the most important core in ECL analysis systems and directly dominate the signal transduction efficiency of the designed biosensors, leading to more reliable and precise results [22]. After aromatic hydrocarbons were first used as luminophores, ongoing research has focused on exploring new materials and techniques to further improve the luminescence efficiency of ECL emitters, thereby advancing the field of biosensor technology [23]. Hitherto, as a typical ECL emitter, the tris(2,2′-bipyridyl) ruthenium(Ⅱ) chloride ([Ru(bpy)3]2+) has been widely utilized in the vast majority of commercial ECL sensing [24]. However, there are still some challenges, such as low ECL efficiency (ΦECL), difficulty in adjusting the emission wavelength, and so on [25,26]. With the development of nanoscience and polymer chemistry, many novel emitters have emerged, significantly enriching the existing pool of ECL emitters [27]. ECL luminophores can be classified into several types according to their composition: small organic molecules, nano-emitters, and polymeric emitters.
Polymeric emitters, represented by frameworks, polymer dots, and graphitic-phase carbon materials, are potentially ideal materials for ECL [28,29]. Owing to their large conjugated discrete structure, they possess distinct advantages of strong emission signals, high photothermal stability, and electrical conductivity [30]. In recent years, polymeric emitters have been proven to possess exceptional mechanical properties and tailored processing during synthesis in polymer chemistry [31,32]. Moreover, polymers are sensitive to very small perturbations in analytical chemistry, where their transport properties, conductivity, and energy migration rate can amplify the sensitivity of sensing [33]. Coupled with the rise in polymer chemistry and materials chemistry over the last century, polymeric emitters have been extensively reviewed and considered as the next generation of promising materials for ECL.
Although significant progress has been made in the combination of polymers and ECL, there is no comprehensive review describing the development of polymeric emitters in the field of ECL, especially in terms of structural synthesis, electrochemical behavior characteristics, and the underlying mechanisms of enhanced ECL. Therefore, it is highly desirable to obtain a basic principle and a thorough comprehension of the ECL properties of polymeric luminophores to guide further research.
In this review, we present a general overview of the recent advances in the development of highly efficient polymeric emitters (Fig. 1), including the classification of each type and feasible synthesis methods. Strategies for the enhancement of ECL with the underlying kinetic mechanisms are also discussed, rooted in a straightforward unique structural modulation. Moreover, as several polymers have been discovered in recent years, promising prospects and perspectives have been discussed.
Frameworks are emerging porous polymers, consisting of organic bridging ligands linked by coordination bonds or hydrogen bonds [34-37]. Owing to their assembled infinite reticulate structure, frameworks are recognized as a class of polymeric luminophores [38,39]. Depending on the linking molecule, frameworks can be divided into metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs) [40]. In the field of ECL, frameworks have received extensive attention owing to their high porosity, tunable pore size, and large surface area for biosensing [22,41].
After being discovered and named by Yaghi et al. in the 1990s, MOFs have been gradually applied for ECL and have quickly become very attractive ECL luminophores because of their abundant active sites and excellent transfer capability [42-45]. As an emerging class of frameworks, MOFs are constructed using organic ligands and metal ions or clusters, and can be designed with arbitrary shapes and sizes (Fig. 2A) [46]. Moreover, they can also be functionalized post-synthetically, leveraging their frameworks for various applications, whether involving conjugated chemistry or surface ligands (Fig. 2B).
Generally, MOFs can be synthesized by several well-established synthetic methods, such as hydrothermal/solvothermal, electrochemical, microwave-assisted, and mechanochemical synthesis [47-50]. The hydrothermal/solvothermal method is a common method for the preparation of MOFs because its high-temperature and high-pressure environment can provide more opportunities for MOFs with defined structures [51]. Although this method is relatively time-consuming, the synthesized MOFs have a well-defined backbone and porous structure, which facilitates charge transfer and understanding of the ECL mechanism. For instance, Fu et al. prepared Zr-MOFs using the solvothermal method, where ZrOCl2 was used [52]. The Zr-MOFs exhibited a porous structure and reduced the aggregation quenching effect (ACQ). Xiao et al. mixed Zn2+ and perylene-3,4,9,10-tetracarboxylate (PTC) to synthesize Zn-PTC MOFs via a hydrothermal reaction [53]. The coordinative immobilization of PTC distinctly increased the ECL intensity owing to the reduced ACQ. To save time, an electrochemical strategy was introduced, which reduced the synthesis time to hours [54]. Because the metal ions and ligands could be controlled by the applied electrode potential, Ru(bpy)32+, trimesic acid (H3BTC), and Zn2+ were simultaneously deposited onto the electrode to form Ru-MOF thin films. This not only allowed for the rapid preparation of MOFs but also for high-quality photoelectrodes. Other rapid and efficient synthesis methods for MOFs have also been developed.
MOFs play various roles in ECL processes, including ECL carriers, emitters, and donors or acceptors in ECL resonance energy transfer (ECL-RET) [55]. Owing to their high porosity and large specific surface area, MOFs can potentially function to load classical luminophores, including [Ru(bpy)3]2+, quantum dots (QDs), metal nanoclusters, and polymer dots (Pdots) [56-60]. For example, 2-amino terephthalic acid (2-NH2-BDC) has been employed in the synthesis of isoreticular metal-organic framework-3 (IRMOF-3) [59]. IRMOF-3 facilitated the loading of a significant amount of CdTe QDs through encapsulation and served as a co-reactant accelerator that promoted the conversion of the co-reactant (Fig. 3A). The electrochemical behavior of the loaded luminophores was explored, showing that CdTe emitters exhibit a notably enhanced ECL intensity in the MOFs (Fig. 3B). Concurrently, the cyclic voltammetry (CV) diagrams also show a significant increase, indicating the enhancement of the reaction rate between the emitters and the co-reactant in the MOFs (Fig. 3C). Yang and co-workers synthesized the Zn-MOF@luminol complex. Because of the large amount of luminol loaded and acting as a co-reactant accelerator in the luminol-H2O2 ECL system, a strong ECL signal was achieved [61]. MOFs can also function as ECL emitters, where many small molecules have been proven to be luminescent ligands, such as pyrene, perylene, rubrene, and N-(4-aminobutyl)-N-ethylisoluminol (ABEI) [62-65]. An abundance of ABEI was used to form the MOF skeleton structure (ABEI@Fe-MIL-101), resulting in an enhanced ECL intensity (Fig. 3D) [66]. Here, MOFs were employed as luminophores and exhibited electrochemical properties inherent to their synthetic ligands, as shown in the ECL and CV diagrams (Fig. 3E). A broadened scanning range revealed an intensified ECL intensity, because the generated O2•− could react with the oxidized ABEI to generate more ABEI in the excited state (Fig. 3F). Furthermore, tetraphenylethylene (TPE), porphyrin, and their derivatives have been promising ligands for the synthesis of MOF emitters in recent years [67-72]. ECL-RET is a process in which energy is transferred from a donor to an acceptor. Owing to their broad absorption spectra that overlap with those of other molecules, MOFs are considered typical donors and acceptors in ECL-RET. Fe3O4@PDA-CuxO-Ab2 was synthesized as a signal-quenching probe for the ECL system, where RET occurs from Fe3O4@PDA-CuxO to Ru@MIL-101 [73]. An ECL biosensor for procalcitonin (PCT) detection was developed. g-C3N4 was used as both the substrate and donor, while Ru@MOFs were used as acceptors to detect the Aβ protein (Fig. 3G) [74]. The absorption spectra of Ru@MOFs overlapped with the ECL emission spectra of g-C3N4, indicating that energy was transferred from g-C3N4 to Ru@MOFs, resulting in a dual-wavelength ECL (Figs. 3H and I). Concurrently, Pd NPs@NH2−MIL-53, Fe-MIL-88 MOFs, MIL-125, etc., have been used in ECL-RET for practical applications [75-77].
Typically, when used as ECL emitters, MOFs are commonly employed to enhance ECL because rigidifying the backbone can constrain intramolecular rotations and motions, suppressing nonradiative transitions [78-80]. Furthermore, because the electronic properties of MOFs are influenced by the metals within the framework, extensive work has been undertaken on the modulation of metal ions to innovate the ECL mechanisms of MOFs. For instance, lanthanide-based MOFs are prospective ECL emitters with long-lifetime excited states, excellent PL efficiency, and low absorption coefficients [55]. Wei and co-workers invented a series of Eu-based MOFs, where the Eu3+ luminescence could be sensitized effectively by different ligands [81]. These Eu-MOFs were synthesized using Eu(Ⅲ) ions and 5-boronoisophthalic acid (5-bop) and can be excited efficiently at a single excitation. Because the electron-deficient 5-bop decreased the energy transfer efficiency from ligands to Eu(Ⅲ) ions, a red ECL emission was achieved by Eu(Ⅲ) ions. Furthermore, a near-infrared Eu-MOF was synthesized using an energy-transfer strategy [82]. The electrons tended to transfer from the singlet state to the triplet state and sensitize Eu(Ⅲ) via nonradiative energy transmission to enhance the ECL emission. The rational selection of metals has proven to be an effective approach for modulating the ECL properties of MOFs [83-86]. Impressively, diverse ligands within MOFs often play distinct roles in the ECL mechanism, offering unique opportunities for tailored luminescence properties. An electroactive MOF (E-MOF) was designed as an efficient crystalline ECL emitting material, which was synthesized using hydroquinone and phenanthroline as oxidative and reductive couples (Fig. 4A) [78]. These electroactive mixed ligands of hydroquinone and phenanthroline contributed to the highly oxidative and reductive redox properties of E-MOFs. Mechanistic studies showed that the self-enhancing ECL originated from the accumulation of E-MOF•+ during the step pulse via pre-reduction which resulted in a significant increase in the intensity of the cathodic ECL (Fig. 4B). This novel E-MOF provides a new platform for the design of efficient MOF-based molecular-crystal ECL emitters. Moreover, a crystalline m-MOF was integrated with two ligands (one as a luminophore and the other as a co-reactant) by a mixed-ligand method to achieve self-enhanced ECL without additional co-reactants (Fig. 4C) [79]. Because of the incorporation of the two ligands into the framework and the intrareticular charge transfer (IRCT) between these ligands, the ECL intensity of m-MOF was significantly enhanced. As the consequence of its integration within the m-MOF and charge transfer between the ligands, the ECL intensity of the m-MOF exhibited a remarkable enhancement. During the second oxidation, stepwise ECL emission was observed, which was attributed to localized excitation within the ligand units (Fig. 4D). This novel ECL system provides proof-of-concept for the design of self-enhanced ECL and opens new avenues for innovative luminescent materials.
Although MOFs have attracted much attention in recent years, the potential ECL quenching effect caused by metal ions remains an important issue that hinders the development of MOF-based ECL emitters [41]. Therefore, metal-free COFs have been developed, which were composed of light elements (B, C, N, O, etc.) [87]. Generally, COFs share similar synthetic methods with MOFs, and hydrothermal/solvothermal methods are the most common synthetic methods for COFs. Owing to the robust covalent bonds and the absence of metals, COFs typically exhibit more difficult preparation, lower crystallinity, lower density, and a more stable structure [88]. Using a variety of organic linkers with different symmetric combinations and following the principles of reticular chemistry, many different types of 2D COFs can be constructed (Fig. 5). The remarkably rigid structure theoretically enables precise control over structures, ranging from the nanoscale to the macroscale [89,90]. For instance, Zhuo et al. prepared a type of imine-linked COF (COF-LZU1) via the reaction between 1,4-diaminobenzene and 1,3,5-triformylbenzene dissolved in 1,4-dioxane [91]. Since COFs and tri-n-propylamine (TPrA) are both lipophilic, a large amount of TPrA can be encapsulated and enriched in the hydrophobic cavity of COF-LZU1, reducing the distance between Ru(bpy)32+ and the co-reactants. These exceptional properties make COFs often function as carriers and accelerators in ECL [92,93]. To date, COFs have been widely studied with excellent prospects in many fields; however, there are few reports on COFs as ECL emitters.
In 2020, Li et al. first explored COF-based ECL emitters by adding aggregation-induced luminescent (AIE) groups via solvothermal synthesis [94]. The COFs exhibited strong anodic ECL with H2O2 as the co-reactant and a cobalt tetroxide (Co3O4) nanozyme as the co-reaction accelerator. Subsequently, an increasing number of COFs have been thoroughly investigated for their ECL behavior. For example, HHTP-HATP-COF was synthesized from hexaaminotriphenylene (HATP) and hexahydroxy-triphenylene (HHTP) after pre-reduction (Fig. 6A) [95]. The cathode ECL mechanism of HHTP-HATP-COF prepared in this study was revealed by determining whether S2O82– was electroreduced by comparing the reduction peaks in the CVs (Fig. 6B). Owing to the appropriate cavities and high conductivity accelerating the charge transfer, ECL emission was greatly enhanced (Fig. 6C). It was used to construct an ultrasensitive biosensor for thrombin detection as a highly efficient ECL beacon, together with an aptamer/protein DNA walker, showing broad linearity (100 amol/L to 1 nmol/L) with a detection limit of 62.1 amol/L. In addition, a pyrene-based carbon-conjugated COF was synthesized through the condensation of tetrakis (4-formylphenyl) pyrene (TFPPy) and 2,2′-(1,4-phenylene)diacetonitrile [96]. Upon introduction of Bu4NPF6 into the system as a co-reaction accelerator, the cathode ECL signal was further amplified. Apart from cathodic ECL, Zhang et al. established COF-based anode ECL systems [97]. They constructed an aminal-linked COF (A-COF) via condensation of (4′,4′′,4′,4′-(1,2-ethenediylidene)tetrakis [1,1′-biphenyl]-4-carboxaldehyde (ETBC) and piperazine [98]. This A-COF was also used to construct an aggregation-induced ECL (AIECL) sensor (Fig. 6D) [99]. The CV curve showed a distinct oxidation wave with an onset potential of +0.9 V, indicating the oxidization of TPrA (Fig. 6E). The ECL intensity of the system containing TPrA was significantly higher than that of the system without TPrA (Fig. 6F).
Some strategies have been proven to eliminate the ACQ effect and even realize the transformation from ACQ to AIE [102]. The above pyrene-based COF was proven to achieve an enhanced ECL, which was attributed to the topological attachment of 2,2′-(1,4-phenylene)diacetonitrile (PDAN) and TFPPY in COFs, leading to a greatly reduced ACQ [96]. Reasonable utilization of intramolecular framework structure regulation is an advantage of COF-based ECL emitters. Recently, a new method has been developed to achieve highly efficient charge transfer. By restricting the donor and acceptor to the tight electron configurations of COFs, efficient ECL emission was achieved owing to the intramolecular electronic networks [103]. Subsequently, Lei et al. proposed a mechanism for the donor-acceptor (D-A) unit in COFs [100]. A t-COF was developed by integrating triazine and triphenylamine as donor and acceptor units (Fig. 7A). The obtained COFs exhibited a 123-fold ECL intensity compared with the benzene-based COF, which was realized by tunable IRCT. Theoretical calculations simulated the charge density difference between the first excited state and the ground state. The electron density loss on the triazine unit and charge density gain on the triphenylamine unit confirmed charge transfer between the triphenylamine and triazine units. When holes and electrons are doped, efficient charge transfer can be identified by the movement of the highest occupied state and the lowest unoccupied state (HOS/LUS) to the Fermi level. As the pH was varied from 6.0 to 8.0, the oxidation potential of TPrA shifted to a negative potential, while the oxidation potential of t-COF remained constant. In an alkaline environment, the oxidation of TPrA occurred earlier than that of t-COF, thus providing a coreactant-mediated oxidation mechanism that led to ECL I. In contrast, under acidic conditions, TPrA and t-COF were simultaneously oxidized on the electrode surface, resulting in a direct oxidation mechanism and the emergence of ECL II (Fig. 7B). The reaction mechanism within the structure is depicted schematically as illustrated (Fig. 7C). This work provides profound insights into the D-A mechanism of COF-based ECL emitters. Along this line, in terms of rapid IRCT, a crystalline co-reactant-embedded COF (C-COF) provides a more efficient intrareticular charge transfer (Fig. 7D), which effectively avoids the inherent defects of the diffusion distance of the co-reactant radicals as well as the short lifespan, thereby realizing an extraordinary 1008-fold enhancement of ECL intensity (Fig. 7E) [101]. A bimodal oxidation ECL mechanism attributed to the antedating oxidation of the co-reactant and the self-oxidation of C-COF in the framework was also proposed. For one thing, the co-reactant N, N–diethyl ethylenediamine (DEDA) can reduce the C-COF to form C-COF•– through IRCT at low potentials and react to generate the excited state of COF* for the first ECL emission. For another, the C-COF can be directly oxidized to COF•+ and DEDA•+ at high potentials, leading to the second ECL (Fig. 7F). This post-synthetic coreactant-embedded emitter represents an innovative and universal avenue for the advancement of ECL, offering promising new directions for luminescent material design and applications.
The concept of HOFs using hydrogen bonding interactions was proposed in the 1990s, almost the same time as MOFs and synthesized in 2011 [104]. In contrast to the two frameworks mentioned above, the interaction and stability of the HOFs skeleton are relatively weak, yet the unique structure shows promising enhancements to ECL because of its reversible synthesis, structural flexibility, and adequate active sites for the transport of energy and electrons [105,106]. Generally, HOFs can be formed by self-assembly via non-covalent forces between polar molecules owing to spontaneous preorganization interactions among polar molecules, where a suitable temperature increase is required to improve the efficiency of the reaction in some cases [107].
Feng and co-workers first reported that HOFs consisting of only organic units linked by intermolecular hydrogen bonds also had ECL properties (Fig. 8A) [108]. Triazinyl-based HOFs (HOFs-Tr) synthesized by self-assembly polymerization showed highly enhanced ECL, whereas their monomers had little ECL emission (Fig. 8B). HOFs-Tr with a polymeric structure exhibited pronounced CV curves, which indicated better charge transfer. The as-built sensor displayed a broadened linear range of 1 nmol/L–10 µmol/L (R2 = 0.994) and a limit of detection (LOD) down to 0.28 nmol/L, which also displayed practical applications in the analysis of Kana in the milk and diluted human serum samples. Since then, HOF-based ECL sensing systems have been extensively developed. Subsequently, Zhang et al. synthesized HOF-101 with distinct electrochemical behaviors owing to its different synthetic structures, demonstrating cathodic ECL properties (Fig. 8C) [109]. The long-range ordered structure contributed to a stable and strong ECL signal. The ECL signal of HOF-101 can be effectively quenched by zeolitic imidazolate framework (ZIF)-67 through a synergistic mechanism that encompasses both ECL resonance energy transfer with HOF-101 and steric hindrance imparted by its structure (Fig. 8D). Additionally, the anodic ECL emission of luminol in a dissolved O2 system can be enhanced by ZIF-67, leveraging its ordered and porous crystalline lattice along with atomically dispersed Co2+ ions. These attributes not only enrich our understanding of the interaction between ZIF-67 and HOFs but also open up new avenues for the rational design of advanced ECL-based sensors and devices.
Recently, HOFs have been successfully employed for sensing ions, nucleic acids, and enzymes [108-110]. Nevertheless, limited by their low porosity and energy transfer, HOFs suffer from weak ECL. Xiao et al. proposed the concept of porosity and aggregation-induced enhanced ECL (PAIE-ECL) based on a pyrene-based HOF (Py-HOF) prepared using 1,3,6,8-tetrakis(p-benzoic acid) pyrene (H4TBAPy) [111]. The rational assembly of hydrogen bonds could constrain intramolecular movements to decrease the nonradiative transition, which could effectively enhance ECL. The H4TBAPy units within the Py-HOF were stacked in a face-to-face sliding manner to form J-aggregates, contributing to the enhancement of the ECL. Moreover, Lei et al. emphasized the significance of the charge transfer process in the annihilation reaction in HOF structures [112]. They used 1,3,6,8-tetra(4-carboxylphenyl) pyrene as a ligand to synthesize HOF-101 owing to multiple hydrogen bonds and π interactions (Fig. 9A). By leveraging their densely stacked structures, HOF-101 can amplify the ECL signal via the intrareticular electron coupling (IREC) pathway. Theoretical model simulations illustrated the charge density difference between S1 and S0, indicating mutual electron density depletion and accumulation of vertically stacked units, and finally achieved ECL via IREC (Fig. 9B). The subsequent development of HOF-101 has led to its application in ultrasensitive detection of oxytetracycline (OXY) [113]. The remarkable ECL intensity of this sensing system enables direct visualization with the naked eye. In recent studies, the adjustment of pore sizes within isoluminol amide HOFs (ILu-HOFs) was demonstrated to effectively accelerate the reaction rates of the co-reactant, ultimately leading to a noTable 23.4 times enhancement in ECL (Fig. 9C) [107]. Linear sweep voltammetry (LSV) curves demonstrate a favorable proportionality between the peak current and scan rate, exhibiting a notable enhancement in the slope value. Moreover, the electron paramagnetic resonance (EPR) plots showed that ILu-HOFs generated more co-reactant radicals, attributed to porous networks with a pore diameter to accelerate the mass transfer exchange of HOFs and co-reactant radicals (Fig. 9D).
In the realm of ECL, HOF have demonstrated their utility as efficient emitters [108]. Their unique crystalline porous structure allows for the precise control of intramolecular charge transfer, enhancing ECL performance [112]. The high surface area and porous structure provide ample space for loading ECL reagents, enhancing the sensitivity and stability of ECL systems. Furthermore, the tunable nature of HOF offers versatility in tailoring their properties for specific ECL applications [109]. By carefully selecting and engineering the building blocks of HOF, researchers can modulate their optical and electrochemical properties and optimize them for use in ECL devices. Despite this progress, challenges remain in the application of HOF in the ECL field. The hydrophobicity limits their solubility and dispersion in aqueous media, whereas a high excitation potential may require the use of more energetic excitation sources. These issues need to be addressed through further research and development to unlock the full potential of HOF in ECL applications.
Since the first report on the ECL phenomenon of poly(9,9-dioctylfluorene-co-benzothiadiazole) dots in acetonitrile solution, polymer dots have received considerable attention as effective ECL emitters in recent years [114]. Pdots possess special mechanical properties of polymers, unique low toxicity, and easy regulation of photoelectric properties [115]. The intrinsic advantage of Pdots is that their polymer backbone can be combined with other molecules, creating unique directions [116,117]. Recently, some polyfluorene polymers, such as poly[(9,9-dioctylindole-2,7-diyl) cothiadiazole)] (PFBT), have been used for commercial ECL biosensing. Based on these available polyfluorene polymers, various Pdots have been explored as ECL emitters [118]. Broadly, Pdots can be structurally classified into two subclasses: conjugated polymer dots (conjugated Pdots) and carbonized polymer dots (carbonized Pdots).
The luminescence of conjugated Pdots is related to the π-conjugation in the molecular skeleton [119]. When the conjugated structure comprises repeatedly interconnected monomers, the band gap of the conjugated Pdots diminishes because of the intricate interactions among the electron orbital domains [120]. Consequently, these conjugated Pdots exhibit the characteristic properties of semiconductors. Represented by nano-precipitation, conjugated Pdots can be prepared using different methods, including mini-emulsions, self-assembly, and hydrothermal/solvothermal methods [121]. Owing to their structural controllability and short preparation time, conjugated Pdot-based ECL emitters are commonly prepared via nanoprecipitation.
The typical electrochemical properties of Pdots were initially reported, while prior studies have left unresolved questions regarding the characteristics of Pdots, such as low hydrophilicity, high excitation potential, and weak stability [122]. Chi and co-workers pioneered the application of a reprecipitation method for the first hydrophilic Pdots (CP-dots) in aqueous solution by capping poly[2–methoxy-5-(2-ethylhexyloxy)-1,4-phenylvinylene] (MEH-PPV) with triton X-100 (Fig. 10A) [123]. The electrochemical behavior and ECL performance of the aqueous CP-dots were studied in detail, and the mechanism was explained by the oil/water interface of the materials and electrodes (Figs. 10B and C). The hydrophilic group on the interface was able to inject electrons into the CP-dots and simultaneously produce ECL. Since then, conjugated Pdots have been successfully applied for sensing the water phase. Similarly, cyano groups were attached to the copolymers to enhance their hydrophilicity [124]. The obtained Pdots showed enhanced ΦECL because the electron-withdrawing cyano groups increased the electron affinity of the polymer. In another study, silole-containing Pdots (SCP dots) were synthesized to achieve low oxidation potentials (Fig. 10D) [125]. A robust anodic ECL emission could be observed at a potential peak (+0.78 V with SCP dots in solution at bare GCE and TPrA as co-reactants, while the SCP-dot-modified GCE peaked at +1.1 V (Figs. 10E and F). Hereafter, Luminol-doped polymer dots (L-Pdots) and diethylamine-coupled Pdots (N-Pdots) were designed to devise a strategy for potential- and color-resolved ECL with emission at 450 nm and 675 nm while the potential of +0.6 V. and +1.0 V, respectively [126]. The excitation potential peak for Pdots was further narrowed down to +0.45 V [127]. Nevertheless, weak noncovalent interactions may easily contribute to the shedding of emitters from the core, which prevents conjugated Pdots from qualified stability. Xu and co-workers synthesized novel carboxyl Pdots composed of fluorene derivatives and benzothiadiazole [128]. Based on the rational design of the linkage between polymers and carboxyl groups on the side chains by nanoprecipitation, the Pdots exhibited favorable stability. Conjugated Pdots with reversible redox properties have also been introduced to address the challenges of irreversible redox reactions [129]. Benzothiadiazole (BT) in the Pdots had a rigid structure and excellent electrochemical behavior, which facilitated the preparation of Pdots to avoid disruption of the conjugated structure as much as possible, thus favoring reversible reactions. These studies pave the way for solving problems in the practical application of Pdots.
In some cases, the design of conjugated polymer backbones incorporating AIE-active moieties or featuring an energy donor-acceptor structure has been demonstrated as an effective strategy to enhance the ECL of Pdots [130-134]. For example, two types of AIE-active conjugated Pdots have been designed and synthesized. Among them, iridium complex end-capped poly-tetraphenylethene (PTPE) Pdots showed the highest ECL efficiency, which was mainly attributed to their fully conjugated structure with good film conductivity, thus facilitating intramolecular electron transfer [133]. Gao et al. developed a novel an aggregation-induced delayed ECL (AIDECL) active organic dot (OD), for the first time by integrating D-A pair of dimethylacridine-benzophenone, and the ODs obtained enjoy a 6.8-fold ECL efficiency relative to the control AIDECL-active ODs [134]. The reversible Pdots synthesized from BT exhibited significantly enhanced ECL [129]. The tetraphenylethene derivative, a prototypical AIE-active moiety, facilitated highly efficient luminescence in the aggregated states. A molecular engineering strategy has also been introduced to achieve significant AIECL enhancement by adding a moderate proportion of carboxyl groups [135]. The carboxyl Pdots showed enhanced ECL and low oxidation potential, which originated from the enhanced proton-coupled electron transfer (PCET) in the carboxyl nanoparticles. In addition, the rational design strategies of intramolecular resonance energy transfer in D-A system were also considered [58,136]. Moreover, a type of Pdot synthesized from poly(TMTPA-DCB) showed desirable annihilation and co-reactant ECL behaviors with thermally activated delayed fluorescence (TADF Pdots) properties (Fig. 11A) [137]. The ECL emission of TADF Pdots adhered to the “S-route”, theoretically enabling the harvesting of 100% excitons for radiative transition in ECL (Fig. 11B). This superior performance was attributed to the TADF property and effective shielding of the oxygen-quenching effects on triplet excitons. Furthermore, suitable structures of conjugated Pdots can be designed to bind to the co-reactants, greatly increasing the charge transfer efficiency and improving the co-reactant-based ΦECL. Chen and co-workers developed a new hydrophobic localized enrichment strategy for co-reactants using the hydrophobic part of the β-cyclodextrin (β-CD) cavity [138]. Through its hydrophobic cavity, this structure not only efficiently enriched the co-reactant 3-(dibutylamino) propylamine (TDBA) but also immobilized the TDBA through the Pt-N bond, thus achieving an excellent ECL intensity. Notably, a co-reactant-embedded tertiary amine (TEA)-PFBT Pdot ECL system was introduced by attaching a TEA to the side chain of PFBT (Fig. 11C) [139]. The rapid intramolecular electron transfer and conjugated superstructures contributed to the unprecedented ECL strength, which was 132 times stronger than that of the mixture. The ECL emission and intramolecular charge-transfer mechanisms of the TEA-Pdots are shown in Fig. 11D. Based on this, a single-cell protein imaging strategy was proposed. This work provides a new way to design efficient co-reactant embedded Pdots and new insights into exploring the ECL of Pdots.
Compared to conjugated Pdots, carbonized Pdots exhibit superior water solubility and chemical stability [140]. Carbonized Pdots feature a hybrid structure encompassing a carbonic core surrounded by numerous polymer chains and functional groups on the surface [141]. This core arises from the dehydration and carbonization of the interconnected polymer chain network [142]. Notably, incomplete carbonization may result in the retention of shorter polymer chains or functional groups. Unlike carbon quantum dots (CDs), which are nanoparticles, carbonized Pdots are polymeric materials with macromolecular structures [143]. This polymeric structure confers high thermal stability and oxidation resistance upon carbonization of the carbonized Pdots. Typically, carbonized Pdots are synthesized through hydrothermal/solvothermal methods, including dehydration, condensation, carbonization, or assembly routes, utilizing fluorescent nonconjugated polymers as building blocks [144].
Reports on the ECL of carbonized Pdots remain sparse, and their application in ECL is emerging. Recently, carbonized Pdots, named polymerized carbon dots (PCDs), doped with N and O and featuring a large conjugated π-system exhibited superior electrical conductivity, a narrow band gap, and robust radiative transitions (Fig. 12A) [145]. These attributes endow them with a high ΦECL, enabling enhanced detection sensitivity and long-wavelength ECL emission. The potential-resolved ECL and CV curves exhibited typical luminescent properties characteristic of carbonized Pdots (Fig. 12B). Recently, a type of carbonized Pdot named L-CPD was successfully synthesized, boasting a remarkably low cathode excitation potential of −0.95 V (Fig. 12C) [146]. Here, zeolitic imidazolate framework-8 (ZIF-8) served as a nanoconfinement carrier, encapsulating the L-CPDs, while the Platinum composite iron oxide (Fe3O4/Pt) functioned as a co-reaction accelerator, significantly minimizing the nonradiative energy loss in the luminophore (Fig. 12D). Based on this substantially enhanced ECL signal, a biosensor was fabricated to detect cytokeratin 19 fragment antigen 21–1 (CYFRA21–1).
In the pursuit of understanding these remarkable materials, Yang et al. made numerous fascinating advancements [147]. Because the luminescent centers of carbonized PDs are attributed to carbon cores or surface chromophores, they typically exhibit intense fluorescent emission within a narrow range. The emission can be enhanced via structural crosslinking and physical immobilization of the polymer chains, indicating the remarkable tunability of their structural properties [148]. Nevertheless, there is a paucity of systematic reports on the ECL properties of carbonized PDs, suggesting a potentially promising research direction for future exploration.
Graphite-phase carbon materials are often employed as substrates for enhancing ECL because of their exceptional charge-transfer properties attributed to π-electron delocalization [149]. Although most graphite-phase carbon materials, such as graphene, are conductors and inherently lack ECL, they can be modulated to exhibit the properties of semiconductors using various techniques, including elemental doping, size control, and topological engineering [150-152]. Both classical and novel graphite-phase carbon materials are discussed below.
As a typical carbon-based material, carbon nitride (CN) has been studied for over a hundred years, which was first reported by Berzelius and Liebig as “melon”, and was considered as one of the oldest synthetic polymers [153]. Graphitic C3N4 (g-C3N4) is a conjugated semiconductor with triazine or heptazine rings as the basic structural unit, which has a layered structure similar to graphite (Fig. 13) [154]. The rigid, covalently bonded two-dimensional structure, along with its unique photophysical properties, positions carbon nitride as a promising candidate for novel ECL reagents.
The synthesis of carbon nitride involves diverse methods including thermal condensation, solid-phase reactions, solvothermal synthesis, and electrochemical deposition. Thermal condensation is the most straightforward method involving heating-induced condensation of precursors to yield g-C3N4 materials. Utilizing cyanamide as an illustrative example, bulk CN can be obtained simply by heating the precursor and maintaining this temperature for hours [155-157]. By manipulating the precursor types and reaction conditions, it is possible to tailor g-C3N4 materials with diverse structures and properties, thereby offering a robust platform for material design and optimization. In addition, by harnessing the inherent advantages of carbon nitride, including its controllable structure and facile functionalization, pioneers have demonstrated a diverse array of structural modulation techniques [158]. These include chemical tailoring, doping, heterojunction recombination, and noncovalent functionalization, each offering unique opportunities to fine-tune the properties of the material and expand its application horizons [159]. Since its initial application in photocatalysis, CN has garnered significant attention from the scientific community and has been successfully applied in catalysis, energy storage, degradation, and sterilization [154].
It was not until 2012 that Xiao and co-workers proposed for the first time that g-C3N4 could be successfully applied as a cathode ECL emitter (Fig. 14A) [160]. They conducted a thorough investigation of the CV and ECL curves of CN using K2S2O8 in the cathode ECL (Fig. 14B). g-C3N4 was thus employed to fabricate an ECL sensor that showed high selectivity for Cu2+ determination with a limit of detection (LOD) of 0.9 nmol/L. Subsequently, the anode ECL behavior was examined using TEA as a co-reactant (Figs. 14C and D) [161]. Since the anodic ECL signal could be efficiently quenched by rutin, a facile anodic ECL sensor for the detection of rutin was successfully developed with a linear response in the range of 0.20–45.0 µmol/L and a low detection limit of 0.14 µmol/L (at signal-to-noise of 3).
A comparative analysis of the spectral characteristics of the ECL and PL spectra of CN revealed a striking resemblance, indicating that the excited states of CN arising from both ECL and photoexcitation are identical. This detailed analysis offers insights into the electrochemical behavior and luminescence properties of CN in the presence of these co-reactants. Since then, sensing systems that use g-C3N4 as a novel ECL emitter have been developed. Nonetheless, the low ΦECL of bulk CN has severely limited the development of ECL. A green liquid exfoliation route was introduced to obtain CN nanosheets (CNNS), which endowed them with high ECL intensity and great potential for hydrophilic electrodes [162,163]. Subsequently, nanomaterial modification composites, metal doping, and defect engineering strategies have been introduced to enhance the ECL intensity and efficiency of carbon nitride [164-169].
Notably, the flexible two-dimensional structure of CN can be employed as a model for investigating ECL enhancement mechanisms. Numerous backbone modulation strategies have been developed because of the controllability of conjugated structures. Within this realm, strategies involving vacancy modulation and element doping have offered fresh insights into enhanced ECL [167-170]. For instance, a nitrogen vacancy (NVs) engineering strategy was developed to enhance ECL efficiency and stability [171]. The proposed ECL mechanism for CN—NVs is as follows: First, a strong oxidant SO4•– was produced during the electrochemical reduction process from the S2O82– co-reactant (Eq. 1). Subsequently, electrons were injected into the conduction band (CB), which flowed to the surface NV states to form (CN•–)VN (Eq. 2). Subsequently, SO4•– radicals can react with (CN•–)VN to produce the excited state CN* (Eq. 3) while injecting holes into the surface NV-related lower energy state (Eq. 4). Finally, the excited state CN* decayed back, accompanied by photon production (Eq. 5).
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Mechanism studies revealed that NVs can not only promote electron transfer in the ECL process, but also serve as electron traps to suppress electrode passivation. This nitrogen-vacancy carbon nitride exhibited a 70 times enhancement in ECL efficiency and an exceptionally stable signal intensity. Owing to the limited understanding of the interface and orbital delocalization mechanisms, visible ECL emission remains a challenge. This constraint underscores the need for further exploration and understanding of these fundamental processes to enhance the luminescence properties of this material. In 2019, our group synthesized CN films in situ on fluorine-doped tin dioxide (FTO) electrodes using a microwave-assisted method that effectively eliminated grain boundaries and enabled accelerated electron and hole separation (Fig. 15A) [172]. The unique layer could enhance the charge mobility, resulting in the cathodic ΦECL reaching seven times that of Ru(bpy)32+/K2S2O8. Subsequently, the profound effect of orbital delocalization on ΦECL was explored, and the cathodic ΦECL was increased to 170 times that of the benchmark using CN with high delocalization (Fig. 15B) [173]. Soon after, a robust, dense, and highly transparent CN film was prepared using a two-step crystallization method [174]. The cathodic efficiency increased to 2200 times that of the benchmark because this film inhibited the direct reduction of the co-reactant at the electrode, and the extremely thin layer reduced the self-absorption (Fig. 15C). Since then, CN have been successfully applied for visual detection. Accompanied by the improvement in ECL performance, the lack of research on CN intrinsic mechanisms is still in its infancy. Recently, our group revealed that the shallow electron-trapped state serves as a pivotal determinant of ΦECL [175]. Through strategic doping of noble metals, a timescale coordination strategy was introduced (Fig. 15D). This innovative approach culminated in a remarkable enhancement of ECL, surpassing the benchmark by more than 3000 times. These remarkable CN photoelectrodes hold promise for the visualization of NO2− detection. Consequently, this finding established a new record for cathodic ΦECL in CN, setting a new milestone in the co-reactant pathways.
Beyond the exploration of the ECL enhancement mechanism, the tunable polymeric backbone structure provides a platform for the design of unique sensing systems [176]. The degree of polymerization could be modulated by controlling the temperature (Fig. 16A) [177]. CNNS with different chemical structures exhibited different ECL responses to metal ions, which realized the regulation of ECL signals attributed to the inner filter effect and induced electron transfer effect (Fig. 16B). Compared with traditional metal ion sensors, the ECL fingerprint spectrum of a single metal ion with multiple sensing signals can effectively avoid false-positive detection and improve the accuracy of the sensor (Fig. 16C). The introduction of defects into the polymeric backbone enables the tuning of the emission wavelength from blue to green (Fig. 16D) [178]. The constructed wavelength-resolved ECL biosensor exhibited ultrasensitive microRNA (miRNA) detection capabilities (Figs. 16E and F). However, CN exhibits inherent interfacial inertness, which poses unique challenges for its surface modification. Facilitated by π-π stacking interactions, a facile method was presented for the exfoliation of bulk CN into nanosheets through mechanical grinding (Fig. 16G) [179]. The resulting CNNS retained the inherent optoelectronic properties of the bulk material while offering an amenable interface for the conjugation of biomolecules with enhanced functionalities, paving the way for its utilization in advanced biosensing applications (Fig. 16H). This system was applied to the detection of transferred DNA (tDNA), and the results demonstrated its capability within a broad range from 10–6 mol/L to 10–13 mol/L, with a very low detection limit of 3.6 × 10–14 mol/L.
Recently, a novel allotrope of graphite, graphdiyne (GDY), has emerged as a two-dimensional layered material composed of carbon atoms with both sp and sp2 hybridization (Fig. 17A) [180,181]. GDY possesses numerous remarkable and intriguing properties, including highly conjugated and exceptionally large π-structures, infinitely distributed pores, and a high degree of charge distribution inhomogeneity [182]. These unique characteristics endow GDY with potential for diverse applications in gas separation, catalysis, water remediation, and sensing [183]. It has been demonstrated to be accessible through various synthetic methods, including total organic synthesis, surface-confined chemical reactions, and solution-phase polymerization reactions [184]. Despite their established properties, reports on the ECL of GDY remain relatively sparse. Until 2022, Mao et al. reported that GDY, without any functionalization or treatment, exhibited a completely different ECL behavior from other carbon allotropes (Fig. 17B) [180]. The ECL emission spectrum of GDY peaked at 705 nm, which is in the near-infrared region and is very different from its fluorescence spectrum. Mechanistic studies showed that the emission process was attributed to the surface state mechanism, with a ΦECL of 424% compared to that of Ru(bpy)32+/K2S2O8 (Fig. 17C). This revealed a new ECL property of GDY and laid the foundation for its development in emerging applications. GDY has emerged as a novel luminescent material and has been applied in ECL biosensing [185-187]. Recently, GDY was used as a substrate for the deposition of Pd nanoparticles (PdNPs) to improve the conversion efficiency of dissolved O2 to reactive oxygen species (ROS) [188]. The electrochemical reaction of the co-reactant involves an oxidation–reduction reaction (ORR), and the acceleration of this process can facilitate the enhancement of ECL signals (Fig. 17D). The CV curves exhibited distinct peaks indicative of intense electrochemical reactions in the presence of dissolved O2, which correlated with the enhanced ECL intensity (Fig. 17E). Based on this, anti-oxidants could quench luminol ECL by depleting ROS, and a platform of Pd/GDY to detect intracellular antioxidants was developed (Fig. 17F). This work provides new guidance for enhancing ECL emission by the design and preparation of new catalysts for ORR and broadens the application of GDY in ECL.
These results demonstrate the advantages of using GDY as the ECL emitter. It is foreseeable that the ECL generated in the near-infrared region may possess greater potential than visible-light emission because of its lower background interference, improved tissue penetration, and reduced photochemical damage. However, it is worth noting that the high ECL excitation potential of GDY may pose a challenge for biosensing applications [180]. Strategies for adjusting these bands may be introduced in the future. Meanwhile, investigating the surface-state emission mechanism of GDY could also be crucial for modulating its ECL properties.
In this review, we have summarized polymeric ECL emitters in detail, including their structural and synthetic methodologies. In addition, the characteristic ECL behavior of each polymeric luminophore, along with typical strategies for enhancing ECL from a mechanistic perspective, are also thoroughly discussed. Crucially, the ECL properties of certain polymers, as a novel class of luminophores, have only recently been discovered, indicating their vast potential and promising prospects for future exploration [189-191]. As an emerging class of ECL emitters, polymers stand out for their well-defined and tunable molecular structures, which create infinite possibilities for designing novel ECL systems. The tunable structure offers advantages such as abundant active sites, large surface area, low-voltage excitation, and excellent hydrophilicity. Despite great advances in the field of ECL based on polymeric emitters, there are still impediments to building a bridge between laboratory research and commercial applications. To develop polymeric emitters into prospective next-generation ECL sensors, some important issues should be addressed to promote innovation in ECL biosensing, which are described as follows.
(ⅰ) Improvement of intrinsic ECL kinetics. Efficient ECL emitters possess adjustable structures as the core for sensing, which is a unique advantage of polymers. The tunable structure provides an opportunity to establish rational energy dissipation pathways of ECL because most of the energy is consumed via nonradiative transitions. Rational energy dissipation pathways can provide new insights into ECL mechanisms.
(ⅱ) Designation of extrinsic ECL interface. Most bio-applications take place in the aqueous phase; however, the excessive molecular structure may result in poor water solubility and biological inertness. It is possible to ameliorate their weaknesses by introducing specific groups to the interface of polymeric emitters, such as reasonably increasing the number of hydrophilic groups and coupling biomolecules.
(ⅲ) Extension to more applications. With the interdisciplinary integration of electrochemistry, spectroscopy, polymer chemistry, and nanotechnology, ECL has now been included, but is not limited to applications in other fields, such as ECL imaging, ECL-photodynamic therapy, encryption of anti-counterfeiting information, and evaluation of oxygen reduction reactions [192]. In these aspects, polymeric emitters are relatively backward compared to other ECL emitters but have promising prospects. Although still in its infancy, it has already extended traditional ECL to a bright new realm, which calls for a more comprehensive exploration of ECL.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Sijia Zhou: Writing – review & editing, Writing – original draft. Tianyi Zhou: Writing – review & editing, Writing – original draft. Yuhua Hou: Writing – review & editing, Writing – original draft. Wang Li: Writing – original draft. Yanfei Shen: Writing – review & editing. Songqin Liu: Writing – review & editing. Kaiqing Wu: Writing – review & editing. Yuanjian Zhang: Writing – review & editing, Project administration, Funding acquisition, Conceptualization.
This work was supported by the National Natural Science Foundation of China (Nos. 22174014 and 22074015), China Postdoctoral Science Foundation (No. 2023M740595), Postdoctoral Fellowship Program of CPSF (No. GZC20230427), and Jiangsu Funding Program for Excellent Postdoctoral Talent (No. 2023ZB353).
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Figure 1 Classification of polymeric luminophores. Reproduced with permission [46,89,107,125,146,155,180]. Copyright 2018, Wiley-VCH. Copyright 2019, American Chemical Society. Copyright 2016, American Chemical Society. Copyright 2023, Elsevier. Copyright 2018, Royal Society of Chemistry. Copyright 2022, Wiley-VCH.
Figure 2 (A) Synthesis scheme of MOFs from metal ions and organic ligands with tunable size and morphology and (B) universal functionalization approaches with different conjugation strategies and multiple surface ligands. Reproduced with permission [46]. Copyright 2018, Wiley-VCH.
Figure 3 (A) Synthesis scheme of IRMOF-3 loading CdTe QDs emitters. (B) ECL and (C) CV curves of (a) CdTe, (b) CdTe with IRMOF-3, (c) CdTe with S2O82−, and (d) CdTe with S2O82− and IRMOF-3. Reproduced with permission [59]. Copyright 2018, American Chemical Society. (D) Synthesis scheme of ABEI@Fe-MIL-101 for ECL emitters. (E) ECL (a1) and CV (b1) curves of ABEI@Fe-MIL-101 at potentials of 0 V to 0.7 V. (F) ECL (a2) and CV (b2) curves of ABEI@Fe-MIL-101 at potentials of −1 to 0.7 V. Reproduced with permission [66]. Copyright 2017, Royal Society of Chemistry. (G) Synthesis scheme of Ru@MOFs as acceptors in ECL-RET. (H) Overlap of (a) absorption spectrum of Ru@MOF, (b) ECL emission spectrum of g-C3N4,. (I) ECL curves of Ru@MOF and g-C3N4 with different concentrations of Aβ (ng/mL). Reproduced with permission [74]. Copyright 2019, American Chemical Society.
Figure 4 (A) Scheme of E-MOF structures. (B) Mechanism of enhanced ECL from the accumulation of E-MOF. Reproduced with permission [78]. Copyright 2020, Wiley-VCH. (C) Scheme for the structure of m-MOF. (D) Mechanism of a stepwise ECL emission of IRCT in m-MOF. Reproduced with permission [79]. Copyright 2021, American Chemical Society.
Figure 5 Scheme of different combinations used for various COF constructions (stick on the left is the ligands and the net on the right is the synthesized COFs). Reproduced with permission [89]. Copyright 2019, American Chemical Society.
Figure 6 (A) Synthesis scheme of HHTP-HATP-COF. (B) CV curves of (a) bare glassy carbon electrode (GCE) without S2O82−, (b) bare GCE with S2O82−, (c) HHTP-HATP-COF/GCE in N2-saturated phosphate buffered saline (PBS) with S2O82−. (C) ECL curves of (a) bare GCE without S2O82−, (b) bare GCE with S2O82−, (c) HHTP-HATP-COF/GCE in N2-saturated PBS with S2O82−, (d) HHTP-HATP-COF/GCE after pre-reduction in N2-saturated PBS with S2O82−. Reproduced with permission [95]. Copyright 2022, American Chemical Society. (D) Synthesis scheme of A-COF. (E) CV (black line) and ECL curves (red line) of A-COF modified GCE with TPrA. (F) ECL curves of bare GCE (black line) and A-COF-modified GCE (blue line) without TPrA. Reproduced with permission [99]. Copyright 2022, Royal Society of Chemistry.
Figure 7 (A) Synthesis scheme of three tris(4-formylphenyl)amine (TFPA)-based COFs. (B) Dual-peak ECL patterns of t-COFs modified GCEs containing 20 mmol/L TPrA. (C) Mechanism of the double ECL emission via intrareticular charge transfer. Reproduced with permission [100]. Copyright 2021, Nature Publishing Group. (D) Synthesis scheme of C-COF. Top views of a graphical representation of the rectangular grid showing staggered A − B stacking of COF and C–COF. C, gray; N, blue; O, red; H, white, and even-numbered layers, light blue. (E) ECL curves of COF-modified GCEs containing 0 µmol/L (a), 3 µmol/L (b) and 20 mmol/L (c) N,N–diethyl ethylenediamine (DEDA), and (d) C–COF-modified GCEs. Inset: Enlarged ECL curves. (F) ECL intensity of IRCT enhanced mechanism compared with the intermolecular mechanism. Reproduced with permission [101]. Copyright 2024, Wiley-VCH.
Figure 8 (A) Synthesis scheme of HOFs-Tr. (B) ECL curves of (a) HOF-Tr and (b) phenyDAT as the monomer with TPA. Reproduced with permission [108]. Copyright 2021, American Chemical Society. (C) Schematic for the synthesis of HOF-101. (D) ECL curves of (a) bare GCE, (b) HOF-101/GCE, (c) ZIF-67/HOF-101/GCE, and (d) α-Glu/ZIF-67/HOF-101/GCE. Reproduced with permission [109]. Copyright 2024, American Chemical Society.
Figure 9 (A) Scheme of HOF-101 with a channel structure and interlayer stacking. (B) IREC pathway-driven ECL mechanism of HOF-101. Reproduced with permission [112]. Copyright 2022, Royal Society of Chemistry. (C) Synthesis scheme of ILu-HOFs accelerating the reaction. (D) Simulated spatial structure of HOFs with O2•− and H2O2. Reproduced with permission [107]. Copyright 2023, American Chemical Society.
Figure 10 (A) Scheme of electron/hole injections into hydrophilic CP-dots. (B) ECL and (C) CV curves of (a) Triton X-100-capped CP-dots and (b) bared CP-dots with TPrA. Reproduced with permission [123]. Copyright 2015, American Chemical Society. (D) Schematic for the synthesis of SCP dots. (E) ECL (a) and CV (b) curves of SCP dots modified GCE with TPrA. (F) ECL (a) and CV (b) curves of 20 µg/mL SCP dots in buffer at bare GCE with TPrA. Reproduced with permission [125]. Copyright 2016, American Chemical Society.
Figure 11 (A) Synthesis scheme of the TADF Pdots. (B) Comparison of energy transfer in the FL-ECL (left) and TADF-ECL (right) systems. Reproduced with permission [137]. Copyright 2022, American Chemical Society. (C) Schematic for the synthesis of TEA-Pdots. (D) Comparison of intramolecular electron transfer (left) and intermolecular electron transfer between TEA-Pdots and co-reactant (right). Reproduced with permission [139]. Copyright 2021, Wiley-VCH.
Figure 12 (A) Synthesis scheme of PCDs. (B) ECL curves of GCE in (a) PBS, (b) PBS with S2O82−, PCDs/GCE in (c) PBS. (d) PBS with S2O82−. Reproduced with permission [145]. Copyright 2024, Elsevier. (C) Synthesis scheme of L-CPDs. (D) ECL curves at low cathode excitation potentials. Reproduced with permission [146]. Copyright 2023, Elsevier.
Figure 14 (A) Scheme of synthesis of cathode ECL mechanism of CN. (B) ECL curves (a) without and (b) with S2O82− and CV curves (c) without and (d) with S2O82−. Reproduced with permission [160]. Copyright 2012, American Chemical Society. (C) Synthesis scheme of anode ECL mechanism of CN. (D) ECL curves (a) without and (b) with TEA and CV curves (c) without and (d) with TEA. Reproduced with permission [161]. Copyright 2013, American Chemical Society.
Figure 15 (A) Scheme of synthesis of CN photoelectrode of microwave-assisted method. Reproduced with permission [172]. Copyright 2020, Wiley-VCH. (B) Scheme of the relationship between ΦECL and orbital delocalization. Reproduced with permission [173]. Copyright 2022, Wiley-VCH. (C) Synthesis scheme of CN photoelectrode by a two-step crystallization method. Reproduced with permission [174]. Copyright 2023, Wiley-VCH. (D) Scheme of timescale coordination strategy for enhanced ΦECL. Reproduced with permission [175]. Copyright 2023, Nature Publishing Group.
Figure 16 (A) Scheme of exfoliating bulk CN. Photographs: bulk CN-T (T = 400, 450, 500, 550, 600, 650) and the corresponding CNNS-T solution irradiated with a UV lamp. (B) ECL emission spectra of different structures of CNNS. (C) Different ECL responses of CNNS-400, CNNS-500, and CNNS-650 to metal ions. Reproduced with permission [177]. Copyright 2016, American Chemical Society. (D) Proposed structure of CN with defects by the copolymerization of urea with other organic monomers. (E) Adjustable ECL emission of CN with different defects and (F) corresponding photographs. Reproduced with permission [178]. Copyright 2023, American Chemical Society. (G) Schematic of exfoliation and modification of CN via noncovalent π-π stacking interactions. (H) Schematic representation of functionalization and biomolecule immobilization. Reproduced with permission [179]. Copyright 2017, American Chemical Society.
Figure 17 (A) Schematic of the structure of pristine GDY. (B) ECL and CV curves of GDY with/without S2O82−. (C) Schematic of ECL emission of GDY. Reproduced with permission [180]. Copyright 2022, Wiley-VCH. (D) CV curves of Pd/GDY-modified GCE saturated with N2, air, and O2. (E) ECL curves of the bare, GDY, PdNPs, and Pd/GDY modified GCE with luminol. (F) Schematic for enhanced ECL with Pd/GDY. Reproduced with permission [188]. Copyright 2024, American Chemical Society.
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