Covalent organic frameworks as micro-reactors: confinement-enhanced electrochemiluminescence

An electrochemiluminescent dual-signal immunosensor based on a Ru(bpy)32+-doped silica nanoparticle/copper nanocluster composite for okadaic acid detection

In this study, a sensitive dual-signal electrochemiluminescence (ECL) immunosensor was developed for okadaic acid (OA) detection utilizing copper nanoclusters (CuNCs) and Ru(bpy)32+-doped silica nanoparticles (RuSiNPs). Interestingly, the CuNCs could simultaneously enhance both cathodic (-0.95 V) and anodic (+1.15 V) ECL signals of RuSiNPs, forming a dual-signal ECL sensing platform. Further, RuSiNPs@CuNCs were used as immunomarkers by covalently conjugating them with an anti-OA monoclonal antibody (mAb) to form probes. Finally, dual ECL signals of the immunosensor were fabricated and showed good linear relationships with OA concentrations in the range of 0.05-70 ng mL-1, having a median inhibitory concentration (IC50) of 1.972 ng mL-1 and a limit of detection of 0.039 ng mL-1. Moreover, the constant ratio of the cathodic and anodic ECL peaks achieved self-calibration of the detection signal and improved the reliability of the results. Finally, we successfully applied the ECL sensor to detect OA in spiked oyster samples.

DNA tetrahedral scaffold-corbelled self-feedback circuit for dual-mode ratiometric biosensing with Ru@COF-LZU1 accelerator

Sensitive, reliable, and specific detection of microRNAs (miRNAs) is a key objective for disease diagnosis and prognosis. Here, a ratiometric fluorescent/electrochemiluminescent (FL/ECL) sensor was designed for the dual-mode detection of miRNA-122, a hepatocellular carcinoma biomarker. The strong ECL emission was achieved from imine-linked covalent organic framework (COF-LZU1) accelerator enriched Ru(bpy)32+ molecules (Ru@COF-LZU1), which was applied as a delimited reaction micro-reactor to enhance ECL emission. Impressively, to construct an efficient sensing platform, self-feedback circuit was grafted at the vertex of DNA tetrahedral scaffold (DTS), which could provide a solution-phase-like environment and transform miRNA-122 into abundant single-stranded DNAs on the disposable electrode. Simultaneously, the carboxyfluorescein (FAM) tagged DNA segment was cleaved and released into the reaction solution, bringing in the recovery of FL response (FL on). Finally, the introduction of glucose oxidase (GOD) could generate H2O2 by in situ catalyzing GOD to glucose, resulting in the decrease of ECL signal (ECL off). Relying on FL/ECL ratio value, miRNA-122 was quantified with high sensitivity, well selectivity, stability and favorable practicability, suggesting that the proposed biosensor hold great potential for clinical diagnosis.

Label/immobilization-free Cas12a-based electrochemiluminescence biosensor for sensitive DNA detection

Electrochemiluminescence (ECL) is one of the most sensitive techniques in the field of diagnostics. However, they typically require luminescent labeling and electrode surface biological modification, which is a time-consuming and laborious process involving multiple steps and may also lead to low reaction efficiency. Fabricating label/modification-free biosensors has become one of the most attractive parts for simplifying the ECL assays. In this work, the ECL luminophores carbon dots (CDs) were encapsulated in DNA hydrogel in situ by a simple rolling circle amplification (RCA) reaction. Upon binding of the target DNA, active Cas12a induces a collateral cleavage of the hydrogel's ssDNA backbone, resulting in a programmable degradation of the hydrogel and the release of CDs. By directly measuring the released CDs ECL, a simple and rapid label/modification-free detection of the target HPV-16 was realized. It is noted that this method allowed for 0.63 pM HPV-16 DNA detection without any amplification step, and it could take only ∼60 min for a fast test of a human serum sample. These results showed that our label/modification-free ECL biosensor has great potential for use in simple, rapid, and sensitive point-of-care (POC) detection.

Sulfonic Acid-Functionalized Tetraphenylethylene-Amplified Electrochemiluminescence by Regulating π-π Interaction

The electrochemiluminescence (ECL) effectiveness of the tris(bipyridine) ruthenium(II) (Ru(bpy)32+) system is hampered by aggregation-caused quenching (ACQ) in optoelectronic systems as a result of π-π accumulation of the aromatic ring structure. In this work, a negatively charged tetraphenylvinyl molecule (TPE-2SO3Na, TPE-4SO3Na) was synthesized to modify the electrode interface, and the π-π accumulation between Ru(bpy)32+ molecules was transformed into the π-π interaction between Ru(bpy)32+ and TPE molecules. Interestingly, the ECL signal intensity of the Ru(bpy)32+-tripropylamine (TPA) system in the presence of TPE-2SO3Na was increased by about 15 times due to the π-π action and electrostatic action. In comparison with traditional physical packaging with porous zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), the fabricated electrode interface modification strategy was simple and efficient to avoid π-π accumulation in aqueous solutions. Our success will inspire other researchers to investigate the supramolecular interaction (π-π interaction, electrostatic interaction, hydrophilic interaction, and host-guest interaction) at the electrode interface to amplify the ECL intensities of Ru(bpy)32+.

Tailored Cis-Trans Isomeric Metal-Covalent Organic Frameworks for Coordination Configuration-Dependent Electrochemiluminescence

Precise manipulation of the coordination configuration within substances can modulate the band structure and catalytic properties of the target material. Metal-covalent organic frameworks (MCOFs), a crystal material amalgamating the benefits of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), can integrate a predetermined coordination environment into the frameworks for amplifying the catalytic effect. In this study, we delicately synthesize isomeric MCOFs using bis(glycinato)copper as the aminoligand via kinetically and thermodynamically favorable pathways to yield cis-MCOF and trans-MCOF products, respectively, thereby introducing a cis-trans isomeric coordination field into the framework. Moreover, the twisted skeleton derived from the flexibility of amino acid and β-ketoenamine linkages endows trans-MCOF with surprising water dispersibility. Compared to cis-MCOF, the trans isomerism displays a significant enhancement in cathodic electrochemiluminescence via the catalysis of Cu nodes toward K2S2O8. The density of states analysis shows that the d-band center of trans-MCOF is closer to the Fermi level, leading to more stable adsorption binding to promote the catalysis. This study is the first report on constructing predesign coordination configuration MCOFs via an easy-handling method, which gives the guidelines for the design of amino acid-based MCOF materials.

The dual ECL signal enhancement strategy of Pd nanoparticles attached covalent organic frameworks and exonuclease cycling reaction for the ultrasensitive detection of progesterone

Nowadays, novel and efficient signal amplification strategy in electrochemiluminescence (ECL) platform is urgently needed to enhance the sensitivity of biosensor. In this work, the dual ECL signal enhancement strategy was constructed by the interactions of Pd nanoparticles attached covalent organic frameworks (Pd NPs@COFs) with tris (bipyridine) ruthenium (RuP) and Exonuclease III (Exo.III) cycle reaction. Within this strategy, the COFs composite was generated from the covalent reaction between 2-nitro-1,4-phenylenediamine (NPD) and trialdehyde phloroglucinol (Tp), and then animated by glutamate (Glu) to attach the Pd NPs. Next, the "signal on" ECL biosensor was constructed by the coordination assembly of thiolation capture DNA (cDNA) onto the Pd NPs@COFs modified electrode. After the aptamer recognition of progesterone (P4) with hairpin DNA 1 (HP1), the Exo. III cycle reaction was initiated with HP2 to generate free DNA, which hybridized with cDNA to form double-stranded DNA (dsDNA). For that, the RuP was embedded into the groove of dsDNA and achieved the ultrasensitive detection of P4 with a lower limit of detection (LOD) down to 0.45 pM, as well as the excellent selectivity and stability. This work expands the COFs-based materials application in ECL signal amplification and valuable DNA cyclic reaction in biochemical testing field.

Molecular imprinting-based Ru@SiO2-embedded covalent organic frameworks composite for electrochemiluminescence detection of cyanidin-3-O-glucoside

Cyanidin-3-O-glucoside (C3G), a natural antioxidant, plays multiple physiological or pathological roles in maintaining human health; thereby, designing advanced sensors to achieve specific recognition and high-sensitivity detection of C3G is significant. Herein, an imprinted-type electrochemiluminescence (ECL) sensing platform was developed using core-shell Ru@SiO2-CMIPs, which were prepared by covalent organic framework (COF)-based molecularly imprinted polymers (CMIPs) embedded in luminescent Ru@SiO2 cores. The C3G-imprinted COF shell not only helps generate a steady-enhanced ECL signal, but also enables specific recognition of C3G. When C3G is bound to Ru@SiO2-CMIPs with abundant imprinted cavities, resonance energy transfer (RET) behavior is triggered, resulting in a quenched ECL response. The constructed Ru@SiO2-CMIPs nanoprobes exhibit ultra-high sensitivity, absolute specificity, and an ultra-low detection limit (0.15 pg mL-1) for analyzing C3G in food matrices. This study provides a means to construct an efficient and reliable molecular imprinting-based ECL sensor for food analysis.

Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment

Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10-10 m2 s-1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10-11 m2 s-1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10-10 m2 s-1) and low-porosity (DB = 4.5(1) × 10-12 m2 s-1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.

Confinement-enhanced electrochemiluminescence by Ru(dcbpy)32+-functionalized γ-CD-MOF@COF-LZU1 porous hybrid material as micro-reactor for CYFRA 21-1 detection

The improvement of electrochemiluminescence (ECL) performance relies on the electron transfer efficiency between luminophore and coreactant. An ultrasensitive ECL micro-reactor with confinement-enhanced performance was prepared by using the covalent organic framework-LZU1-functionalized metal-organic framework (MOF@COF-LZU1) as a platform to assemble enormous N,N-dibutyl-2-hydroxyethylamine (DBAE) and tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) [Ru(dcbpy)32+] into its pore channels. Compared to individual substances of γ-CD-MOF and COF-LZU1, the synergistic effects can conduce to the enhancement of the intensity, durability and sensitivity of the micro-reactor. Besides, COF-LZU1 can provide a mild environment to accommodate a certain amount of DBAE by concentrating them from the aqueous solution into its hydrophobic cavities and boost the oxidation efficiency of DBAE to generate more DBAE●+ and profited the survival of DBAE●, leading to an improved reaction efficiency with the Ru(dcbpy)32+ intermediate. Thanks to the confinement-enhanced strategy, engineered as high-functioning luminescent materials, Ru@γ-CD-MOF@COF-LZU1 micro-reactors decorated with Au NPs can facilitate electron transfer and capture primary antibodies (Ab1). Moreover, Au-Pd-Pt noble metal aerogels (NMAs) functionalized MoS2 NFs (Au-Pd-Pt NMAs@MoS2 NFs) were chosen as base material due to its large specific surface areas, high porosity, and excellent electrical conductivity. Based on above merits, the sensor demonstrated a sensitive response to CYFRA 21-1 detection in a linear concentration gradient from 10 fg/mL to 50 ng/mL with a detection limit of 0.0055 pg/mL (S/N = 3). The COF-LZU1 decorated ECL micro-reactors were constructed based on the signal amplification strategies to realize accurate CYFRA 21-1 detection.

Electrophoretic deposition of Ru(bpy)32+ in vertically-ordered silica nanochannels: A solid-state electrochemiluminescence sensor for prolidase assay

Prolidase (PLD) plays a crucial role as a dipeptidase in various physiological processes, specifically involved in the cleavage of proline-containing dipeptides for efficient recycling of proline. The accurate determination of PLD activity holds significant importance in clinical diagnosis. Herein, a solid-state electrochemiluminescence (ECL) biosensor was developed to address the urgent need for PLD assay. The Ru(bpy)32+ was electrophoretically deposited within the nanochannels of vertically-ordered mesoporous silica film (VMSF) on indium tin oxide (ITO) electrodes. The Ru(bpy)32+-deposited VMSF/ITO (Ru-VMSF/ITO) exhibited a remarkable ECL response towards proline, attributed to the enhanced concentration of the reactants and improved electron transfer resulting from the nanoconfinement effect. As PLD specifically enzymolyzed the Gly-Pro dipeptide to release proline, a proline-mediated biosensor was developed for PLD assay. Increased PLD activity led to enhanced release of proline into the porous solid-state ECL sensors, resulting in a more robust ECL signal. There was a linear relationship between ΔECL intensity and logarithmic concentration of PLD in the range of 10-10000 U/L, with a detection limit of 1.98 U/L. Practical tests demonstrated the reliability and convenience of the proposed bioassay, making it suitable for widespread application in PLD assays.

Dynamic surface reconstruction of individual gold nanoclusters by using a co-reactant enables color-tunable electrochemiluminescence

Here we report for the first time the phenomenon of continuously color-tunable electrochemiluminescence (ECL) from individual gold nanoclusters (Au NCs) confined in a porous hydrogel matrix by adjusting the concentration of the co-reactant. Specifically, the hydrogel-confined Au NCs exhibit strong dual-color ECL in an aqueous solution with triethylamine (TEA) as a co-reactant, with a record-breaking quantum yield of 95%. Unlike previously reported Au NCs, the ECL origin of the hydrogel-confined Au NCs is related to both the Au(0) kernel and the Au(i)-S surface. Surprisingly, the surface-related ECL of Au NCs exhibits a wide color-tunable range of 625-829 nm, but the core-related ECL remains constant at 489 nm. Theoretical and experimental studies demonstrate that the color-tunable ECL is caused by the dynamic surface reconstruction of Au NCs and TEA radicals. This work opens up new avenues for dynamically manipulating the ECL spectra of core-shell emitters in biosensing and imaging research.

Universal Signal Switch Based on a Mesostructured Silica Xerogel-Confined ECL Polymer for Epigenetic Quantification

The development of a highly accurate electrochemiluminescence (ECL) signal switch to avoid nonspecific stimulus responses is currently a significant and challenging task. Here, we constructed a universal signal switch utilizing a luminophore-quencher pair of mesostructured silica xerogel-confined polymer and gold nanoparticles (Au NPs) that can accurately detect low-abundance epigenetic markers in complex sample systems. Notably, the ECL polymer encapsulated in mesostructured silica xerogel acts as a luminophore, which demonstrated a highly specific dependence on the Au NPs-mediated energy transfer quenching. To demonstrate the feasibility, we specifically labeled the 5-hydroxymethylcytosine (5hmC) site on the random sequence using a double-stranded (dsDNA) tag that was skillfully designed with the CRISPR/Cas12a activator and recombinant polymerase amplification (RPA) template. After amplification by RPA, a large amount of dsDNA tag was generated as the activator to initiate the trans-cleavage activity of CRISPR/Cas12a and subsequently activate the signal switch, allowing for precise quantification of 5hmC. The ECL signal switch improves the stability of the luminophore and prevents nonspecific stimulus responses, providing a new paradigm for constructing high-precision biosensors.

A novel paper-based electrochemiluminescence biosensor for non-destructive detection of pathogenic bacteria in real samples

The demand for sensitive, portable, and non-destructive analysis of pathogenic bacteria is of significance in point-of-care diagnosis. Herein, we constructed a smart electrochemiluminescence (ECL) biosensor by integrating a flexible paper-based sensing device and a disposable three-electrode detecting system. Staphylococcus aureus (S. aureus)-responsive cellulose paper was prepared by employing aptamer as recognition element and a probe DNA (probe DNA-GOD) tagged with glucose oxidase (GOD) as a signal amplification unit. The formation of aptamer-S. aureus complex mediated the quantitative release of probe DNA-GOD. The remaining probe DNA-GOD on the paper-based aptasensor was then activated by glucose, which resulted in a significant decrease in ECL signal. To further improve the ECL performance of biosensor, a large number of Ru(bpy)32+ molecules were embedded into porous zinc-based metal-organic frameworks (MOFs) to form Ru(bpy)32+ functionalized MOF nanoflowers (Ru-MOF-5 NFs). Such biosensor enabled accurate, non-destructive, and real-time monitoring of S. aureus-contaminated food samples, opening a new avenue for sensitive recognition of pathogenic bacteria.

Advances of Electrochemical and Electrochemiluminescent Sensors Based on Covalent Organic Frameworks

Covalent organic frameworks (COFs), a rapidly developing category of crystalline conjugated organic polymers, possess highly ordered structures, large specific surface areas, stable chemical properties, and tunable pore microenvironments. Since the first report of boroxine/boronate ester-linked COFs in 2005, COFs have rapidly gained popularity, showing important application prospects in various fields, such as sensing, catalysis, separation, and energy storage. Among them, COFs-based electrochemical (EC) sensors with upgraded analytical performance are arousing extensive interest. In this review, therefore, we summarize the basic properties and the general synthesis methods of COFs used in the field of electroanalytical chemistry, with special emphasis on their usages in the fabrication of chemical sensors, ions sensors, immunosensors, and aptasensors. Notably, the emerged COFs in the electrochemiluminescence (ECL) realm are thoroughly covered along with their preliminary applications. Additionally, final conclusions on state-of-the-art COFs are provided in terms of EC and ECL sensors, as well as challenges and prospects for extending and improving the research and applications of COFs in electroanalytical chemistry.

Coreactant-free dual-emitting conjugated polymer for ratiometric electrochemiluminescence detection of SARS-CoV-2 RdRp gene

Constructing mono-luminophor-based electrochemiluminescence (ECL) ratio system is a great challenge due to the limitations of the luminescent species with dual-signal-output, luminescence efficiency and coreactant. This work developed carboxyl-functionalized poly[9,9-bis(3'-(N,N-dimethylamino) propyl)-2,7-fluorene]-alt-2,7-(9,9 dioctylfluorene)] nanoparticles(PFN NPs) as dual-emitting luminophors, which can synchronously output strong cathodic and anodic ECL signals without any exogenous coreactants. The inherent molecular structure enabled efficient intramolecular electron transfer between tertiary amine groups and backbone of PFN to generate strong cathodic and anodic ECL emission. Particularly, H+ in aqueous solution played an irreplaceable role for cathodic ECL emission. The silver nanoparticles (AgNPs) were developed as signal regulator because of their excellent hydrogen evolution reaction (HER) activity, which significantly quenched the cathodic signal while kept the anodic signal unchanged. The dual-emitting PFN NPs cleverly integrated signal regulator AgNPs and bicyclic strand displacement amplification (SDA) to construct a coreactant-free mono-luminophor-based ratiometric ECL sensing for SARS-CoV-2 RdRp gene assay. The strong dual-emitting of PFN NPs and excellent quenching effect of AgNPs on cathodic emission endowed the biosensor with a high detection sensitivity, and the detection limit was as low as 39 aM for RdRp gene. The unique dual-emitting properties of PFN NPs open up a new path to construct coreactant-free mono-luminophor-based ECL ratio platform, and excellent HER activity of AgNPs offers some new thoughts for realizing ECL signal changes.

Vesicles as a Multifunctional Microenvironment for Electrochemiluminescence Signal Amplification

Vesicles as a typical interface-rich microenvironment can promote the reaction rate and the intermediate stability, which are promising for introduction in electrochemiluminescence (ECL) signal amplification. In this work, a kind of multilamellar vesicle obtained from sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was used to modify the electrode surface. The AOT vesicle-modified microenvironment could significantly enhance the ECL performances for the luminol/O2 system in a neutral medium. The mechanism study demonstrated that the nanoscale multilamellar vesicles could maintain the vesicle structure on the electrode surface, which substantially improved the electron transfer and reaction rate, luminescence efficiency of the excited-state 3-aminophthalate anion, and stability of the superoxide anion radical. Alternatively, such a multifunctional microenvironment was also able to enhance the ECL signals from the tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+)/tripropylamine (TPrA) system. Moreover, another dodecyl dimethyl(3-sulfopropyl) ammonium hydroxide inner salt (DSB)-based vesicle was constructed to further verify the versatility of the vesicle-modified microenvironment for ECL signal amplification. Our work not only provides a versatile microenvironment for improving the efficiency of various ECL systems but also offers new insights for the microenvironment construction using the ordered assemblies in ECL fields.

Iron Single-Atom Catalyst-Enabled Peroxydisulfate Activation Enhances Cathodic Electrochemiluminescence of Tris(bipyridine)ruthenium(II)

The tris(bipyridine)ruthenium(II) (Ru(bpy)₃²⁺)-tripropylamine anodic electrochemiluminescence (ECL) system has been widely applied in commercial bioanalysis. However, the presence of amine compounds in the biological environment results in unavoidable anodic interference signals, which hinder further extensive use of the system. In contrast, the cathodic Ru(bpy)₃²⁺ ECL system can overcome these limitations. The Ru(bpy)₃²⁺/peroxydisulfate (S₂O₈^2-, PDS) ECL system has been extensively employed due to its ability to produce a sulfate radical anion (SO₄^•-) with strong oxidation ability, which enhances the ECL signal. However, the symmetrical molecular structure of PDS makes it challenging to be activated and causes low luminescence efficiency. To address this issue, we propose an efficient Ru(bpy)₃²⁺-based ternary ECL system that uses the iron-nitrogen-carbon single-atom catalyst (Fe-N-C SAC) as an advanced accelerator. Fe-N-C SAC can efficiently activate PDS into reactive oxygen species at a lower voltage, which significantly boosts the cathodic ECL emission of Ru(bpy)₃²⁺. Benefiting from the outstanding catalytic activity of Fe-N-C SAC, we successfully established an ECL biosensor that detects alkaline phosphatase activity with high sensitivity, demonstrating the feasibility of practical application.

An electrochemiluminescence sensor based on CsPbBr3 -zquantum dots and poly (3-thiophene acetic acid) cross-linked nanogold imprinted layer for the determination of benzo(a)pyrene in edible oils

A novel quench molecularly imprinted electrochemiluminescence sensor (MIECLS) based on a covalent organic framework composite (COF-300-Au) with enhanced electrochemiluminescence (ECL) signal from CsPbBr₃ quantum dots and cross-linked 3-thiopheneacetic acid functionalized AuNPs (3-TAA@AuNPs) was developed for the detection of the environmental pollutant benzo(a)pyrene (BaP). A composite material constructed of COF-300-Au with a large specific surface area served as the sensor's support substrate, providing more CsPbBr₃ and imprint recognition sites. Electropolymerization was then employed to form an AuNPs three-dimensional imprinting layer with polythiophene cross-linked using BaP as a template and 3-TAA@AuNPs as a functional monomer. A specific cross-linked imprinting recognition effect was recorded on BaP along with the quenching effect of quinones. The density functional theory (DFT) evaluation of the binding mechanism between 3-TAA@AuNPs and BaP revealed powerful MIECLS toward the detection of BaP at concentrations ranging from 10^-14 to 10^-5M, with a detection limit of as low as 4.1 × 10^-15 M.

Covalent organic framework: A state-of-the-art review of electrochemical sensing applications

Covalent organic framework (COF), a kind of porous polymer with crystalline properties, is a periodic porous framework material with precise regulation at atomic level, which can be formed by the orderly connection of pre-designed organic construction units through covalent bonds. Compared with metal-organic frameworks, COFs exhibit unique performance, including tailor-made functions, stronger load ability, structural diversity, ordered porosity, intrinsic stability and excellent adsorption features, are more conducive to the expansion of electrochemical sensing applications and the universality of applications. In addition, COFs can accurately integrate organic structural units with atomic precision into ordered structures, so that the structural diversity and application of COFs can be greatly enriched by designing new construction units and adopting reasonable functional strategies. In this review, we mainly summarized state-of-the-art recent advances of the classification and synthesis strategy of COFs, the design of functionalized COF for electrochemical sensors and COFs-based electrochemical sensing. Then, an overview of the considerable recent advances made in applying outstanding COFs to establish electrochemical sensing platform, including electrochemical sensor based on voltammetry, amperometry, electrochemical impedance spectroscopy, electrochemiluminescence, photoelectrochemical sensor and others. Finally, we discussed the positive outlooks, critical challenges and bright directions of COFs-based electrochemical sensing in the field of disease diagnosis, environmental monitoring, food safety, drug analysis, etc.

Recent progress in assembly strategies of nanomaterials-based ultrasensitive electrochemiluminescence biosensors for food safety and disease diagnosis

The Electrochemiluminescence (ECL)-based biosensors have received considerable attention in food contaminants and disease diagnosis, due to their fascinating advantages such as low cost, fast analysis speed, wide linear range, high sensitivity, and excellent anti-interference ability. Meanwhile, with the vigorous development and improvement of nanotechnology, biosensor assembly strategies tend to diversify and be multifunctional. This review focuses on the representative ECL biosensors in food safety and disease diagnosis reported by our research group and other research groups based on nanomaterials assembly strategies in recent years. According to the different roles of nanomaterials played in the constitution of ECL biosensors, nanomaterials would be divided into the following two categories to be summarized: (1) Nanomaterials for signal amplification. (2) Nanomaterials as ECL emitters. Finally, this review prospects the perspectives on the future development direction of ECL biosensor in food safety and disease diagnosis.

Direct Visualization of Nanoconfinement Effect on Nanoreactor via Electrochemiluminescence Microscopy

Nanoconfinement in mesoporous nanoarchitectures could dramatically change molecular transport and reaction kinetics during electrochemical process. A molecular-level understanding of nanoconfinement and mass transport is critical for the applications, but a proper route to study it is lacking. Herein, we develop a single nanoreactor electrochemiluminescence (SNECL) microscopy based on Ru(bpy)₃ ²⁺ -loaded mesoporous silica nanoparticle to directly visualize in situ nanoconfinement-enhanced electrochemical reactions at the single molecule level. Meanwhile, mass transport capability of single nanoreactor, reflected as long decay time and recovery ability, is monitored and simulated with a high spatial resolution. The nanoconfinement effects in our system also enable imaging single proteins on cellular membrane. Our SNECL approach may pave the way to decipher the nanoconfinement effects during electrochemical process, and build bridges between mesoporous nanoarchitectures and potential electrochemical applications.

Construction of an aggregation-induced electrochemiluminescent sensor based on an aminal-linked covalent organic framework for sensitive detection of glutathione in human serum

We demonstrate the construction of an aggregation-induced electrochemiluminescent (AIECL) sensor for glutathione (GSH) assay by integrating an aminal-linked covalent organic framework (A-COF) with manganese dioxide (MnO₂) nanosheets. The AIECL of the A-COF is quenched by the MnO₂ nanosheets via electrochemiluminescent resonance energy transfer (ERET) from the excited A-COF to MnO₂. The presence of GSH can reduce the MnO₂ nanosheets into Mn²⁺, restoring the AIECL emission of the A-COF. This AIECL sensor has the characteristics of fast response, high sensitivity, and good selectivity toward GSH, and it can accurately measure GSH in human serum.

Progress and Prospects of Electrochemiluminescence Biosensors Based on Porous Nanomaterials

Porous nanomaterials have attracted much attention in the field of electrochemiluminescence (ECL) analysis research because of their large specific surface area, high porosity, possession of multiple functional groups, and ease of modification. Porous nanomaterials can not only serve as good carriers for loading ECL luminophores to prepare nanomaterials with excellent luminescence properties, but they also have a good electrical conductivity to facilitate charge transfer and substance exchange between electrode surfaces and solutions. In particular, some porous nanomaterials with special functional groups or centered on metals even possess excellent catalytic properties that can enhance the ECL response of the system. ECL composites prepared based on porous nanomaterials have a wide range of applications in the field of ECL biosensors due to their extraordinary ECL response. In this paper, we reviewed recent research advances in various porous nanomaterials commonly used to fabricate ECL biosensors, such as ordered mesoporous silica (OMS), metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and metal–polydopamine frameworks (MPFs). Their applications in the detection of heavy metal ions, small molecules, proteins and nucleic acids are also summarized. The challenges and prospects of constructing ECL biosensors based on porous nanomaterials are further discussed. We hope that this review will provide the reader with a comprehensive understanding of the development of porous nanomaterial-based ECL systems in analytical biosensors and materials science.

Self-enhanced electrochemiluminescence of luminol induced by palladium-graphene oxide for ultrasensitive detection of aflatoxin B1 in food samples

In this work, a novel and credible electrochemiluminescence immunoassay (ECLIA) was constructed for the ultrasensitive and highly selective detection of aflatoxin B₁ (AFB₁). Amino-functionalized 3D graphene hydrogel (NGH) served as the ECL platform with the self-enhanced ECL of luminol-palladium-graphene oxide (lum-Pd-GO) acting as a marker for the antibodies against AFB₁. Pd-GO was synthesized by a self-redox method; it promotes the formation of reactive oxygen species, which are important to the ECL of luminol, from dissolved oxygen. The π-π conjunction between luminol and GO shortens their electron transfer distance, resulting in an amplified ECL signal (∼8.5 times larger than conventional luminol ECL). Moreover, 3D NGH, with its good conductivity, large surface area, and sufficient amino groups, was used to anchor gold nanoparticles (AuNPs), which subsequently immobilized bovine serum albumin (BSA)-AFB₁ through Au-S bonds. The resultant, competitive ECLIA gave a relative low detection limit of 5 × 10^-3 μg kg^-1 and exhibited a broad linear relationship over the range of 0.05-50 μg kg^-1. Finally, the proposed ECLIA was successfully used to analyze AFB₁ contents in food samples. ECLIA: electrochemiluminescence immunoassay; AFs: Aflatoxins; HPLC: high-performance liquid chromatography.

Application of Nanomaterials for Coping with Mycotoxin Contamination in Food Safety: From Detection to Control

Mycotoxins, which are toxic secondary metabolites produced by fungi, are harmful to humans. Mycotoxin-induced contamination has drawn attention worldwide. Consequently, the development of reliable and sensitive detection methods and high-efficiency control strategies for mycotoxins is important to safeguard food industry safety and public health. With the rapid development of nanotechnology, many novel nanomaterials that provide tremendous opportunities for greatly improving the detection and control performance of mycotoxins because of their unique properties have emerged. This review comprehensively summarizes recent trends in the application of nanomaterials for detecting mycotoxins (fluorescence, colorimetric, surface-enhanced Raman scattering, electrochemical, and point-of-care testing) and controlling mycotoxins (inhibition of fungal growth, mycotoxin absorption, and degradation). These detection methods possess the advantages of high sensitivity and selectivity, operational simplicity, and rapidity. With research attention on the control of mycotoxins and the gradual excavation of the properties of nanomaterials, nanomaterials are also employed for the inhibition of fungal growth, mycotoxin absorption, and mycotoxin degradation, and impressive controlling effects are obtained. This review is expected to provide the readers insight into this state-of-the-art area and a reference to design nanomaterials-based schemes for the detection and control of mycotoxins.

Sustained-release nanocapsule based on a 3D COF for long-term enzyme prodrug therapy of cancer

A well-designed three-dimensional (3D) covalent organic framework (COF) was constructed as a nanocapsule for the encapsulation of horseradish peroxidase (HRP), which could realize sustained release of HRP to prolong the duration of the therapeutic agents and promote long-term enzyme prodrug therapy.

Pyrenecarboxaldehyde encapsulated porous TiO2 nanoreactors for monitoring cellular GSH levels

Recently, ternary electrochemiluminescence (ECL) system has become a hot research topic due to its great potential for improving ECL efficiency by promoting the generation of intermediates. However, it is still a great challenge to increase the utilization rate of intermediates in a ternary ECL system. Herein, we propose a strategy to increase the utilization rate of intermediates by designing pyrenecarboxaldehyde (Pyc) encapsulated porous titania (pTiO₂) nanospheres (Pyc@pTiO₂) as ECL nanoreactors for an integrated ternary (luminophore/coreactant/co-reaction accelerator, Pyc/S₂O₈^2-/TiO₂) ECL system construction. Specifically, pTiO₂ acted as an ECL co-reaction accelerator, in which Pyc could obtain electrons from the conduction band of TiO₂ to produce more SO₄˙^-, increasing its emissions. Simultaneously, pTiO₂ could provide confined reaction spaces to effectively shorten the diffusion distance, extend the lifetime of free radicals, increase the utilization rate of intermediates and improve the efficiency of the ternary ECL system. As a proof of concept, the Pyc@pTiO₂ nanoreactors-based sensing platform was successfully constructed to sensitively monitor cellular GSH levels. Overall, this work for the first time proposed an avenue to increase the utilization rate of intermediates in a ternary ECL system, which opened a new route for ECL biosensing in cell analysis applications.

Hydrophobic Localized Enrichment of Co-reactants to Enhance Electrochemiluminescence of Conjugated Polymers for Detecting SARS-CoV-2 Nucleocapsid Proteins

The enrichment of co-reactants is one of the keys to improving the sensitivity of electrochemiluminescence (ECL) detection. This work developed a novel hydrophobic localized enrichment strategy of co-reactants utilizing the inner hydrophobic cavity of β-cyclodextrin (β-CD). Pt nanoparticles (Pt NPs) were grown in situ on the coordination sites for metal ions of β-CD to prepare the β-CD-Pt nanocomposite, which could not only enrich co-reactant 3-(dibutylamino) propylamine (TDBA) highly efficiently through its hydrophobic cavity but also immobilize TDBA via the Pt-N bond. Meanwhile, the carboxyl-functionalized poly[2,5-dioctyl-1,4-phenylene] (PDP) polymer nanoparticles (PNPs) were developed as excellent ECL luminophores. With SARS-CoV-2 nucleocapsid protein (ncovNP) as a model protein, the TDBA-β-CD-Pt nanocomposite combined PDP PNPs to construct a biosensor for ncovNP determination. The PDP PNPs were modified onto the surface of a glassy carbon electrode (GCE) to capture the first antibody (Ab1) and further capture antigen and secondary antibody complexes (TDBA-β-CD-Pt@Ab2). The resultant biosensor with a sandwich structure achieved a highly sensitive detection of ncovNP with a detection limit of 22 fg/mL. TDBA-β-CD-Pt shared with an inspiration in hydrophobic localized enrichment of co-reactants for improving the sensitivity of ECL detection. The luminophore PDP PNPs integrated TDBA-β-CD-Pt to provide a promising and sensitive ECL platform, offering a new method for ncovNP detection.

An ultra-high-sensitivity electrochemiluminescence aptasensor for Pb2+ detection based on the synergistic signal-amplification strategy of quencher abscission and G-quadruplex generation

Signal amplification provides an effective way to improve detection performance. Herein, an ultrasensitive electrochemiluminescence (ECL) aptasensor for Pb²⁺ detection was developed based on a dual signal-amplification strategy of the abscission of a quencher and the generation of a G-quadruplex by one-step and simultaneous way. Nitrogen-doped carbon quantum dots linked with complementary DNA (cDNA-NCQDs) at the sensing interface was applied as the quencher of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl)ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine system to minimize the ECL signal due to the intermolecular hydrogen bond-induced energy-transfer process. Upon the addition of Pb²⁺, its specific binding with the aptamer triggered the abscission of cDNA-NCQDs, accompanied by the formation of G-quadruplex on the surface of the electrode, both of which amplified the intensity of the light emission. The ECL amplification efficiency induced by the above two mechanisms (78.6%) was valuably greater than that of their sum value (69.3%). This synergistic effect resulted in high detection sensitivity of the ECL aptasensor, which allowed to thereby obtain Pb²⁺ measurements in the range of 1 fM - 10 nM with an ultra-low detection limit of 0.19 fM. The Pb²⁺-mediated synergistic signal-amplification ECL strategy can provide a new approach for integrating various amplification strategies.

Conductive Covalent Organic Frameworks with Conductivity- and Pre-Reduction-Enhanced Electrochemiluminescence for Ultrasensitive Biosensor Construction

Covalent organic frameworks (COFs) have attracted widespread attention in the electrochemiluminescence (ECL) field owing to their high load capacity of ECL luminophores and porous structures, but their ECL performance is still limited by the intrinsic poor conductivity (generally <10^-8 S m^-1). To address this shortcoming, we used 2,3,6,7,10,11-hexaaminotriphenylene (HATP) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to synthesize a conductive COF (HHTP-HATP-COF, conductivity = 3.11 × 10^-4 S m^-1). Compared with HATP, HHTP, and low-conductive HHTP-DABZ-COF, HHTP-HATP-COF exhibited superior ECL performance, not only because HHTP-HATP-COF possessed massive ECL luminophores but also because its conductive porous framework accelerated charge transport in the whole framework and improved the utilization ratio of ECL luminophores. More interestingly, the ECL intensity of the HHTP-HATP-COF/S₂O₈^2- system was further improved after pre-reduction electrolysis due to the accumulation of HHTP-HATP-COF cation radicals. The experimental results showed that the ECL intensity of the HHTP-HATP-COF/S₂O₈^2- system after pre-reduction was about 1.64-, 3.96-, 6.88-, and 8.09-fold higher than those of HHTP-HATP-COF/S₂O₈^2-, HHTP-DABZ-COF/S₂O₈^2-, HHTP/S₂O₈^2-, and HATP/S₂O₈^2- systems, respectively. Considering the superior ECL property of the HHTP-HATP-COF/S₂O₈^2- system after pre-reduction, it was used as a high-efficient ECL beacon together with an aptamer/protein proximity binding-induced three-dimensional bipedal DNA walker to construct an ultrasensitive biosensor for thrombin detection, which displayed broad linearity (100 aM to 1 nM) with a detection limit of 62.1 aM. Overall, the work offered effective ways to increase ECL performance by the enhancement of conductivity and by the pre-reduction, proposing new ideas to design high-efficiency COF-based ECL materials and endowing conductive COFs with ECL biosensor application for the first time.

LAMP-H+-responsive electrochemical ratiometric biosensor with minimized background signal for highly sensitive assay of specific short-stranded DNA

Herein, the sequence-specific short-stranded biomarker DNA (hDNA, 21-nt) is acted as targeting out-primer to implement the loop-mediated isothermal amplification for releasing hydrogen ions (LAMP-H⁺). Using LAMP-H⁺ as signaling transducer, we report a highly sensitive electrochemical ratiometric biosensor for hDNA with minimized background signal, which is achieved via magnetic separation using AuNPs-modified Fe₃O₄ (Au@Fe₃O₄) as micro-reactor. In Au@Fe₃O₄, a double-stranded complex of a pH-responsible strand (I*) and a substrate strand (S*) is bound via Au-N bonds, where the treatment with LAMP-H⁺ leads to I* folding into i-motif conformation and S* dehybridization. The S* further hybridizes a catalytic strand (C*) to assemble Mg²⁺-DNAzymes that are cleaved by Mg²⁺, releasing C* for repeated formation and robust nicking of Mg²⁺-DNAzymes. The resultant output fuel strands (F*) are introduced in a modified electrode to drive the strand displacement of two hairpins individually labeled with two electron mediators. Through F*-mediated recycled amplification, the ratio of their electrochemical currents changed in opposite is highly sensitive to the varied hDNA down to 2.1 fM. By integrating LAMP-H⁺-stimulated i-motif switching with Mg²⁺-DNAzyme cleavage, this logic transduction of LAMP-H⁺(i-motif/Mg²⁺-DNAzyme)F* efficiently minimizes the inherent background of traditional LAMP-based assays. Resultantly, our electrochemical ratiometric strategy would be applicable to diverse short-stranded DNAs or even RNAs as targeting primers of LAMP.

All-sealed paper-based electrochemiluminescence platform for on-site determination of lead ions

Lab-on-paper (LOP) devices are urgently required for the rapid development of point-of-care diagnoses and environmental assays. Herein, an all-sealed paper-based electrochemiluminescence (ECL) platform was developed to achieve lead ions (Pb²⁺) sensitive analysis via incorporating convenient plastic package technology. Benefiting from transparent plastic encapsulation, the sealed devices effectively avoided the interference of O₂. Meanwhile, myrica rubra-like Pt nanomaterials (MPNs) prepared by an economical and easy-to-operate ultrasound method were employed to catalyze H₂O₂ decomposition. With the help of Pb²⁺-specific DNAzymes, the oligonucleotide probe functionalized via MPNs could be detached from the device in the presence of target, resulting in the reduced ECL intensity. Moreover, the combination of modified paper electrode with functional regions separated by multiple layers of wax enhanced the practicability of the LOP device for rapid detection. Under the optimal conditions, the all-sealed platform achieved wide linear relationship ranging from 0.01 nM to 0.05 μM with a low detection limit of 0.004 nM for sensitive detecting Pb²⁺. It is believed that this platform could provide a robust, simple and versatile strategy for sensitive determination of heavy metal ions, and be applied in on-site contamination analysis in the future.

Bifunctional Moderator-Powered Ratiometric Electrochemiluminescence Enzymatic Biosensors for Detecting Organophosphorus Pesticides Based on Dual-Signal Combined Nanoprobes

The bifunctional moderator is urgently needed in the field of ratiometric electrochemiluminescence (ECL) sensing since it can mediate simultaneously two ECL signals to conveniently realize their opposite change trend. This work designed a novel dual-signal combined nanoprobe with carboxyl-functionalized poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadazole)] nanoparticles (c-PFBT NPs) as the anodic ECL probe and L-cysteine capped CdS quantum dots (L-CdS QDs) as the cathodic ECL probe, which performed a dual-signal output capability without any additional coreactants. More importantly, hydrogen peroxide (H2O2) produced in situ by enzyme-catalyzed reaction was developed as a bifunctional moderator for simultaneously regulating two signals. The dual-signal combined nanoprobe (c-PFBT NPs@CdS QDs) served as the matrix to immobilize acetylcholinesterase (AChE) and choline oxidase for organophosphorus (OPs) analysis. In the absence of OPs, H2O2 was produced by catalyzing the substrate acetylthiocholine (ATCl) with enzymes and it quenched the anodic ECL signal from c-PFBT NPs and simultaneously promoted the cathodic ECL signal from L-CdS QDs. When OPs was present, the activity of AChE was inhibited, the anodic signal would increase, and the cathodic signal would accordingly decrease. The integration of the bifunctional moderator H2O2 and dual-signal combined nanoprobe c-PFBT NPs@CdS QDs not only provides an attractive ECL platform for enzymatic sensing involving the generation or consumption of H2O2 but also paves a new pathway for other ratiometric ECL systems involving enzyme catalytic amplification for detecting antigens, antibodies, DNA, RNA, etc.

Highly Stable Covalent Organic Framework Nanosheets as a New Generation of Electrochemiluminescence Emitters for Ultrasensitive MicroRNA Detection

A pyrene-based sp² carbon-conjugated covalent organic framework (COF) nanosheet (Py-sp²c-CON) with strong and stable electrochemiluminescence (ECL) emission was constructed by C═C polycondensation of tetrakis(4-formylphenyl)pyrene (TFPPy) and 2,2'-(1,4-phenylene)diacetonitrile, which was employed as a highly efficient ECL emitter to fabricate an ECL biosensor for the first time. The Py-sp²c-CON exhibited higher ECL intensity and efficiency than those of TFPPy, bulk Py-sp²c-COF, and imine-linked pyrene COF, not only because the pyrene luminophores and aggregation-induced emissive luminogens (cyano-substituted phenylenevinylene) were topologically linked into Py-sp²c-CON, which greatly increased the immobilization amount of luminophores and decreased the aggregation-caused quenching effect and nonradiative transition but also because the porous ultrathin structure of Py-sp²c-CON effectively shortened transport distances of an electron, ion, and co-reactant (S₂O₈^2-), which made more ECL luminophores be activated and thus efficiently increased the utilization ratio of luminophores. More interestingly, when Bu₄NPF₆ was introduced into the Py-sp²c-CON/S₂O₈^2- system as a co-reaction accelerator, the ECL signal of Py-sp²c-CON was further amplified. As expected, the average ECL intensity of the Py-sp²c-CON/S₂O₈^2-/Bu₄NPF₆ system was about 2.03, 5.76, 24.31, and 190.33-fold higher than those of Py-sp²c-CON/S₂O₈^2-, Py-sp²c-COF/S₂O₈^2-, TFPPy/S₂O₈²,^- and imine-linked pyrene COF/S₂O₈^2- systems. Considering these advantages, the Py-sp²c-CON/S₂O₈^2-/Bu₄NPF₆ system was employed to prepare an ECL biosensor for microRNA-21 detection, which exhibited a broad linear response (100 aM to 1 nM) and a low detection limit (46 aM). Overall, this work demonstrated that sp² carbon CONs can be directly used as a high-performance ECL emitter, thus expanding the application scope of COFs and opening a new horizon to develop new types of ECL emitters.

Nanocage-confined electrochemiluminescence for the detection of dopamine released from living cells

Herein we report an electrochemiluminescent nanocage array (ENA) in which luminophores are physically confined. The ENA can function as a solid sensor for the detection of dopamine released from living cells on the basis of its quenching effect on the ECL signal.