Quenching Electrochemiluminescence Immunosensor Based on Resonance Energy Transfer between Ruthenium (II) Complex Incorporated in the UiO-67 Metal-Organic Framework and Gold Nanoparticles for Insulin Detection

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.

Amyloid detection in neurodegenerative diseases using MOFs

Neurodegenerative diseases (amyloid diseases such as Alzheimer's and Parkinson's), stemming from protein misfolding and aggregation, encompass a spectrum of disorders with severe systemic implications. Timely detection is pivotal in managing these diseases owing to their significant impact on organ function and high mortality rates. The diverse array of amyloid disorders, spanning localized and systemic manifestations, underscores the complexity of these conditions and highlights the need for advanced detection methods. Traditional approaches have focused on identifying biomarkers using imaging techniques (PET and MRI) or invasive procedures. However, recent efforts have focused on the use of metal-organic frameworks (MOFs), a versatile class of materials known for their unique properties, in revolutionizing amyloid disease detection. The high porosity, customizable structures, and biocompatibility of MOFs enable their integration with biomolecules, laying the groundwork for highly sensitive and specific biosensors. These sensors have been employed using electrochemical and photophysical techniques that target amyloid species under neurodegenerative conditions. The adaptability of MOFs allows for the precise detection and quantification of amyloid proteins, offering potential advancements in early diagnosis and disease management. This review article delves into how MOFs contribute to detecting amyloid diseases by categorizing their uses based on different sensing methods, such as electrochemical (EC), electrochemiluminescence (ECL), fluorescence, Förster resonance energy transfer (FRET), up-conversion luminescence resonance energy transfer (ULRET), and photoelectrochemical (PEC) sensing. The drawbacks of MOF biosensors and the challenges encountered in the field are also briefly explored from our perspective.

A smartphone-assisted electrochemiluminescent detection of miRNA-21 in situ using Ru(bpy)32+@MOF

We have proposed a signal dual-amplification electrochemiluminescence (ECL) strategy based on tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)32+) as chromophores confined with three-dimensional (3D) zinc oxalate metal-organic frameworks (Ru(bpy)32+@MOFs) for the detection of miRNA-21. The three-dimensional chromophore connectivity in zinc oxalate MOFs provided a network among Ru(bpy)32+ units, shielding the chromophores from solvent molecules and resulting in high Ru(II) complex emission efficiency. Additionally, we discovered that magnetic beads (MBs) used as carrier for enriched signals contribute to enhanced ECL intensity of the chromophore. To evaluate its clinical application, we applied this method to determine the concentration of miRNA-21 solutions ranging from 1.56 to 100 nM, obtaining a calibration curve of ECL intensity versus logarithm of concentration (logC) of miRNA-21 with a high correlation coefficient. This work demonstrates the construction of a signal amplification strategy ECL biosensor for miRNA using Ru(bpy)32+@MOF systems and its application in ECL detection for analyte methodology.

Dual-mode fluorescence and electrochemiluminescence sensors based on Ru-MOF nanosheets for sensitive detection of apoE genes

A fluorescence-electrochemiluminescence (FL-ECL) dual-mode sensor for apoE gene detection has been developed, leveraging the unique properties of ruthenium metal organic framework nanosheets (RuMOFNSs). The system utilizes the quenching effect of the Ru(bpy)32+ ECL signal by ferrocene, leading to the synthesis of a multi-electron electrical signal marker, bisferrocene. By immobilizing the P-DNA on RuMOFNSs, bisferrocene quenches both FL and ECL signals. The addition of T-DNA and the consequent formation of double-stranded DNA enable the ExoIII enzyme to excise the bisferrocene fragment, restoring the signals. The sensor demonstrates wide detection linear ranges (1 fM to 1 nM for FL and 0.01 fM to 10 pM for ECL) and remarkable sensitivity (0.048 fM for FL and 0.016 fM for ECL). The dual-mode design offers enhanced reliability through a self-correction feature, reducing false positives. Compared to single-mode sensors, the dual-mode sensor shows significant advantages. Real-world testing confirms the sensor's capacity for robust detection in actual samples, underscoring its promising application in early disease diagnosis. This innovative approach opens up avenues for multi-signal response sensors, offering significant potential for diagnostic technologies.

Electrochemiluminescence resonance energy transfer between a Ru-ZnMOF self-enhanced luminophore and a double quencher ZnONF@PDA to detect NSE

The construction of advanced systems capable of accurately detecting neuron-specific enolase (NSE) is essential for rapidly diagnosing small-cell lung cancer. In this study, an electrochemiluminescence (ECL) resonance energy transfer immunosensor was proposed for the ultra-sensitive detection of NSE. The co-reactants C2O42- and Ru(bpy)32+ were integrated to form a self-enhanced ECL luminophore (Ru-ZnMOF) as the ECL donor. The abundant carboxyl functional groups of Ru-ZnMOF supported antibody 1 via an amidation reaction. Polydopamine-modified zinc dioxide nanoflowers, as ECL acceptors, inhibited Ru-ZnMOF ECL signaling. The linear range of NSE was 10 fg mL-1 to 100 ng mL-1 with a detection limit of 3.3 fg mL-1 (S/N = 3), which is suitably low for determining NSE in real samples.

Metal-organic framework-based nanomaterials as opto-electrochemical sensors for the detection of antibiotics and hormones: A review

Increasing trace levels of antibiotics and hormones in the environment and food samples are concerning and pose a threat. Opto-electrochemical sensors have received attention due to their low cost, portability, sensitivity, analytical performance, and ease of deployment in the field as compared to conventional expensive technologies that are time-consuming and require experienced professionals. Metal-organic frameworks (MOFs) with variable porosity, active functional sites, and fluorescence capacity are attractive materials for developing opto-electrochemical sensors. Herein, the insights into the capabilities of electrochemical and luminescent MOF sensors for detection and monitoring of antibiotics and hormones from various samples are critically reviewed. The detailed sensing mechanisms and detection limits of MOF sensors are addressed. The challenges, recent advances, and future directions for the development of stable, high-performance MOFs as commercially viable next-generation opto-electrochemical sensor materials for the detection and monitoring of diverse analytes are discussed.

Electrochemiluminescence resonance energy transfer between Ru(bpy)32+@Cu3(HHTP)2 and GO-Au composites for C-reactive protein detection

Designing innovative electrochemiluminescence (ECL) immunosensors is critical for the detection of biomarkers with a low concentration and the precise evaluation of clinical diseases. Herein, a Cu₃(hexahydroxytriphenylene)₂ (Cu₃(HHTP)₂) nanoflake-based sandwich-type ECL immunosensor was constructed for C-Reactive Protein (CRP) detection. The Cu₃(HHTP)₂ nanoflake, an electronically conductive metal-organic framework (MOF), has a periodically arranged porous structure with a cavity size of 2 nm, which not only accommodates a large amount of Ru(bpy)₃²⁺ but also confines the spatial diffusion of active species. Therefore, the Ru(bpy)₃²⁺-loaded Cu₃(HHTP)₂ nanocomplex (Ru@CuMOF) as an ECL emitter exhibits an enhanced ECL efficiency. The ECL resonance energy transfer (ECL-RET) was accomplished by combining Ru@CuMOF used as a donor with gold nanoparticles-functionalized graphene oxide nanosheets (GO-Au) utilized as an acceptor. This should be ascribed to the fact that the ECL emission spectrum of Ru@CuMOF shows the strongest signal intensity at 615 nm, overlapping with the absorption spectrum of GO-Au at 580-680 nm. Targeted detection of CRP in human serum samples was achieved by the sandwich-type immunosensor based on the ECL-RET mechanism with a 0.26 pg mL^-1 detection limit. The electro-activated hybrids of Cu₃(HHTP)₂ and ECL emitters provide a new sensing strategy for the high-sensitivity detection of disease markers.

Au@NiFeMOFs as the signal quencher of Au@g-C3N4NSs composite for sensitive "on-off" electrochemiluminescence immunosensing of beta-2-microglobulin

In this study, an electrochemiluminescence resonance energy transfer (ECL-RET) immunosensor was constructed to detect beta-2-microglobulin (B2M). As a donor-acceptor pair, a carbon nitride nanosheet modified with gold nanoparticles (Au@g-C₃N₄NSs) and a nickel- and iron-based organic framework modified with gold nanoparticles (Au@NiFeMOFs) were prepared. The sandwich immunosensor was successfully constructed so that ECL-RET occurred between Au@NiFeMOFs and Au@g-C₃N₄NSs. The ECL intensity of the immunosensor decreased with the increase the B2M concentration due to the low conductivity of B2M. The linear range of the ECL-RET immunosensor was from 10 fg/mL to 10 ng/mL, and the limit of detection was 2.3 fg/mL (S/N = 3). The developed immunosensor had high sensitivity, high specificity, and excellent stability. It could realize the sensitivity test of B2M and provide a novel idea for the detection of biomarkers.

Metal-Organic Frameworks as Sensors for Human Amyloid Diseases

Metal-organic frameworks (MOFs) are versatile compounds with emergent applications in the fabrication of biosensors for amyloid diseases. They hold great potential in biospecimen protection and unprecedented probing capabilities for optical and redox receptors. In this Review, we summarize the main methodologies employed in the fabrication of MOF-based sensors for amyloid diseases and collect all available data in the literature related to their performance (detection range, limit of detection, recovery, time of analysis, among other parameters). Nowadays, MOF sensors have evolved to a point where they can, in some cases, outperform technologies employed in the detection of several amyloid biomarkers (amyloid β peptide, α-synuclein, insulin, procalcitonin, and prolactin) present in biological fluids, such as cerebrospinal fluid and blood. A special emphasis has been given by researchers on Alzheimer's disease monitoring to the detriment of other amyloidosis that are underexploited despite their societal relevance (e.g., Parkinson's disease). There are still important obstacles to overcome in order to selectively detect the various peptide isoforms and soluble amyloid species associated with Alzheimer's disease. Furthermore, MOF contrast agents for imaging peptide soluble oligomers in living humans are also scarce (if not nonexistent), and action in this direction is unquestionably required to clarify the contentious link between the amyloidogenic species and the disease, guiding research toward the most promising therapeutic strategies.

Electrochemiluminescence nanoemitters for immunoassay of protein biomarkers

The family of electrochemiluminescent luminophores has witnessed quick development since the electrochemiluminescence (ECL) phenomenon of silicon nanoparticles was first reported in 2002. Moreover, these developed ECL nanoemitters have extensively been applied in sensitive detection of protein biomarker by combining with immunological recognition. This review firstly summarized the origin and development of various ECL nanoemitters including inorganic and organic nanomaterials, with an emphasis on metal-organic frameworks (MOFs)-based ECL nanoemitters. Several effective strategies to amplify the ECL response of nanoemitters and improve the sensitivity of immunosensing were discussed. The application of ECL nanoemitters in immunoassay of protein biomarkers for diagnosis of cancers and other diseases, especially lung cancer and heart diseases, was comprehensively presented. The recent development of ECL imaging with the nanoemitters as ECL tags for detection of multiplex protein biomarkers on single cell membrane also attracted attention. Finally, the future opportunities and challenges in the ECL biosensing field were highlighted.

Metal-organic frameworks: A promising option for the diagnosis and treatment of Alzheimer's disease

Beta-amyloid (Aβ) peptide is one of the main characteristic biomarkers of Alzheimer's disease (AD). Previous clinical investigations have proposed that unusual concentrations of this biomarker in cerebrospinal fluid, blood, and brain tissue are closely associated with the AD progression. Therefore, the critical point of early diagnosis, prevention, and treatment of AD is to monitor the levels of Aβ. In view of the potential of metal-organic frameworks (MOFs) for diagnosing and treating the AD, much attention has been focused in recent years. This review discusses the latest advances in the applications of MOFs for the early diagnosis of AD via fluorescence and electrochemiluminescence (ECL) detection of AD biomarkers, fluorescence detection of the main metal ions in the brain (Zn²⁺, Cu²⁺, Mn²⁺, Fe³⁺, and Al³⁺) in addition to magnetic resonance imaging (MRI) of the Aβ plaques. The current challenges and future strategies for translating the in vitro applications of MOFs into in vivo diagnosis of the AD are discussed.

Nanotechnology‐based electrochemical biosensors for monitoring breast cancer biomarkers

Breast cancer is categorized as the most widespread cancer type among women globally. On-time diagnosis can decrease the mortality rate by making the right decision in the therapy procedure. These features lead to a reduction in medication time and socioeconomic burden. The current review article provides a comprehensive assessment for breast cancer diagnosis using nanomaterials and related technologies. Growing use of the nano/biotechnology domain in terms of electrochemical nanobiosensor designing was discussed in detail. In this regard, recent advances in nanomaterial applied for amplified biosensing methodologies were assessed for breast cancer diagnosis by focusing on the advantages and disadvantages of these approaches. We also monitored designing methods, advantages, and the necessity of suitable (nano) materials from a statistical standpoint. The main objective of this review is to classify the applicable biosensors based on breast cancer biomarkers. With numerous nano-sized platforms published for breast cancer diagnosis, this review tried to collect the most suitable methodologies for detecting biomarkers and certain breast cancer cell types.

Enhancing Electrochemiluminescence Efficiency through Introducing Atomically Dispersed Ruthenium in Nickel-Based Metal-Organic Frameworks

The successful application of electrochemiluminescence (ECL) in various fields required continuous exploration of novel ECL signal emitters. In this work, we have proposed a pristine ECL luminophor named NiRu MOFs, which owned extremely high and stable ECL transmission efficiency and was synthesized via a straightforward two-step hydrothermal pathway. The foundation framework of pure Ni-MOFs with the initial structure was layered-pillared constructed by the coordinated octahedrally divalent between nickel and terephthalic acid (BDC). The terephthalates were coordinated and pillared directly to the nickel hydroxide layers and the three-dimensional framework was formed, which had a weak ECL response strength. Then, the ruthenium pyridine complex was recombined with pure Ni-MOFs to produce NiRu MOFs and part of the introduced ruthenium was atomically dispersed in the layered-pillared structure through an ion-exchange method, which led to the ECL luminous efficiency being significantly boosted more than pure Ni-MOFs. In order to verify the superiority of this newly synthesized illuminant, an ECL immunoassay model has been designed, and the results demonstrated that it had extremely strong and steady signal output in practical application. This study realized an efficient platform in ECL immunoassay application with the limit of detection of 0.32 pg mL^-1 for neuron-specific enolase (NSE). Therefore, the approach which combined the pristine pure Ni-MOFs and the star-illuminant ruthenium pyridine complex would provide a convenient and meaningful solution for exploring the next-generation ECL emitters.

Gold nanoparticle-based signal amplified electrochemiluminescence for biosensing applications

Since the content levels of biomarkers at the early stage of many diseases are generally lower than the detection threshold concentration, achieving ultrasensitive and accurate detection of these biomarkers is still one of the major goals in bio-analysis. To achieve ultrasensitive and reliable bioassay, it requires developing highly sensitive biosensors. Among all kinds of biosensors, electrogenerated chemiluminescence (ECL) based biosensors have attracted enormous attention due to their excellent properties. In order to improve the performance of ECL biosensors, gold nanoparticles (Au NPs) have been widely utilized as signal amplification tags. The introduction of Au NPs could dramatically enhance the performance of the constructed ECL biosensors via diverse ways such as electrode modification material, efficient energy acceptor in ECL resonant energy transfer (ECL-RET), reaction catalyst, surface plasmon resonance (SPR) enhancer, and as nanocarrier. Herein, we summarize recent developments and progress of ECL biosensors based on Au NPs signal amplification strategies. We will cover ECL applications of Au NPs as a signal amplification tag in the detection of proteins, metal ions, nucleic acids, small molecules, living cells, exosomes, and cell imaging. Finally, brief summary and future outlooks of this field will be presented.

Ratiometric electrochemical immunoassay for procalcitonin based on dual signal probes: Ag NPs and Nile blue A

In order to determine procalcitonin, a sandwich-type ratiometic electrochemical immunosensor was developed by differential pulse voltammetry (DPV). Due to high chemical stability and good biocompatibility, graphitic carbon nitride (g-C₃N₄) could be used as feasible supporter to carry silver nanoparticles (Ag NPs) with an obvious oxidative peak (measured typically at + 0.3 V vs. SCE). Ag NPs loaded onto g-C₃N₄ were not only beneficial to prevent the agglomeration of Ag NPs, but also favorable to improve the electron transfer velocity of g-C₃N₄. Moreover, the g-C₃N₄-Ag NPs as the matrix could immobilize primary antibody by Ag-N bond. Nile blue A (NBA), an excellent redox probe based on the redox reaction with two-electrons, provides a current signal at - 0.38 V (vs. SCE). Zr-based metal organic framework (UiO-67), an ideal framework material with large specific surface area and high porosity, could absorb the substantial water-soluble NBA by electrostatic adsorption. The UiO-67 modified by NBA (NBA-UiO-67) owned admirable biocompatibility and was a qualifying marker to load the secondary antibody. For the immunosensor, the current ratio of NBA to Ag NPs (I_NBA/I_{Ag NPs}) was increased as the concentrations of PCT increased. Under the optimum conditions, the linear range of the immunosensor was 0.005 to 50 ng/mL; the detection limit was 1.67 pg/mL (S/N = 3), which reflected the excellent analytical performance of the sensor. The proposed immunosensor strategy is a simple and dependable platform, with great application potential in biometric analysis.

H2O2-activated independently bidirectional regulation for a sensitive and accurate electrochemiluminescence ratiometric analysis

A novel ECL ratiometric sensor was developed based on H2O2 activated independently bidirectional regulation strategy.

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles

Relaxation dynamics of plasmons in Au-SiO₂ core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.

A DNAzyme-Based Dual-Stimuli Responsive Electrochemiluminescence Resonance Energy Transfer Platform for Ultrasensitive Anatoxin-a Detection

An effective and precise electrochemiluminescence resonance energy transfer (ECL-RET), including the efficient regulation over the proximity of a donor and an acceptor and the reliable stimuli responsive as well as the avoidance of undesirable probes leakage, etc., is significant for the development of an accurate and sensitive ECL detection method; yet, the current literature in documentation involves only a limited range of such ECL-RET systems. Herein, we propose an ECL-RET strategy with dually quenched ultralow background signals and a dual-stimuli responsive, accurate signal output for the ultrasensitive and reliable detection of anatoxin-a (ATX-a). The dual quenching is accomplished by an integrated ECL-RET probe of metal organic frameworks (MOFs) encapsulated into Ru(bpy)₃²⁺ (Ru-MOF) (donor) coated with silver nanoparticles (AgNPs) shell (acceptor 1) and close proximity with DNA-ferrocene (Fc) (acceptor 2). Multistimuli responsive DNAzyme facilitated the accurate signal switch by both target ATX-a and hydrogen peroxide (H₂O₂). Because of the specific recognition of the aptamer toward ATX-a, an intricate design of the DNA sequence enabled the exposure of the Ag⁺-dependent DNAzyme sequence and H₂O₂ in situ generated Ag⁺ triggering a catalytic cleavage reaction to freely release the two ECL-RET energy acceptors, thus switching the ECL signal significantly and achieving ultrasensitive detection. It is noteworthy that AgNPs are key in this ECL-RET strategy, serving both as the gate-keepers for avoiding ECL probes leakage and also the ECL energy acceptors, and mostly importantly serving as the redox substrate for the subsequent DNAzyme catalytic signal switch. The proposed ECL-RET aptasensor for ATX-a detection displayed splendid monitoring performance with a quite low detection limit of 0.00034 mg mL^-1. This sensor not only led to the development of a dual-quenching ECL-RET system but also provided meaningful multistimuli responsive ECL biosensing platform construction, which shows a promising application prospect in complicated sample analysis.

Dual-signal electrochemiluminescence immunosensor for Neuron-specific enolase detection based on "dual-potential" emitter Ru(bpy)32+ functionalized zinc-based metal-organic frameworks

Neuron-specific enolase (NSE) is the preferred marker for monitoring small cell lung cancer and neuroblastoma. We devised a dual-signal ratiometric electrochemiluminescence (ECL) sensing strategy for sensitive detection of NSE. In this work, Ru (bpy)₃²⁺ functionalized zinc-based metal-organic framework (Ru-MOF-5) nanoflowers (NFs) with plentiful carboxyl groups provide an excellent biocompatible sensing platform for the construction of immunosensor. Importantly, Ru-MOF-5 NFs possess stable and efficient "dual-potential" ECL emission of cathode (-1.5 V) and anode (1.5 V) in the existence of co-reactant K₂S₂O₈. Simultaneously, the cathode ECL emitter ZnO-AgNPs are employed as the secondary antibody marker, whose participation amplify the cathode ECL signal as well attenuate the anode ECL emission of Ru-MOF-5 NFs. By monitoring the ECL dual-signal of -1.5 V and 1.5 V and calculating their ratios, a ratiometric strategy of quantified readout proportional is implemented for the proposed immunosensor to precise analyze NSE. Based on optimization conditions, the ECL immunosensor displays the wide linear range of 0.0001 ng/mL to 200 ng/mL and the minimum detection limit is 0.041 pg/mL. The "dual-potential" ratiometric ECL immunosensor effectively reduces system error or background signal by self-calibration from both emissions and improves detection reliability. The dual-signal ratiometric strategy with satisfactory reproducibility and stability provides further development possibilities for other biomolecular detection and analysis.

A novel electrochemiluminescence sensor based on resonance energy transfer from MoS2QDs@g-C3N4 to NH2-SiO2@PTCA for glutathione assay

In this work, a solid-state electrochemiluminescence (ECL) sensor based on resonance energy transfer (RET) was proposed using MoS2QDs@g-C3N4 as a donor and NH2-SiO2@PTCA as an acceptor. Herein, MoS2QDs could significantly facilitate the stability and efficiency of the ECL of g-C3N4. PTCA provided a large platform to anchor NH2-SiO2 nanoparticles. The prepared MoS2QDs@g-C3N4 exhibited good spectral overlap with the UV-vis absorption spectrum of NH2-SiO2@PTCA. Based on this, we designed an "off-on" ECL sensing strategy for sensitive and selective detection of glutathione (GSH). Under the best conditions, the linear range of the sensor for GSH detection was from 0.001 to 100 μM with a detection limit of 0.63 nM (S/N = 3). More importantly, GSH in commercial samples can be detected using the proposed sensor, which indicated its superior detection capabilities and potential application value in commercial medicines.

An enzyme-free electrochemiluminescence insulin probe based on the regular attachment of ZnO nanoparticles on a 3-D nickel foam and H2O2 as an efficient co-reactant

In this study, a highly sensitive, fast, and enzyme-free electrochemiluminescence (ECL) probe based on the decoration of zinc oxide nanoparticles on nickel foam is proposed for insulin determination. A silica film was employed as a size adjusting agent for the modification of the nickel foam surface with ZnO nanoparticles (ZnO NPs). The ECL of the ZnO NP/Ni foam was investigated in a natural medium in the presence of hydrogen peroxide (H₂O₂) as an efficient co-reactant. With increasing insulin concentration, a remarkable improvement in ECL signal was observed, which proved the enhancing effect of insulin on the ECL emission. The characterization of the ZnO-NP/Ni-foam electrode was performed via electrochemical impedance spectroscopy, Brunauer-Emmett-Teller (BET) surface area measurement, X-ray diffraction, field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray analysis techniques. The fabricated electrode was applied for the trace analysis of insulin using the ultrasensitive ECL method in a phosphate buffer solution. Under the optimal conditions, the results showed excellent performance during insulin determination with a wide linear range of 3.57 × 10^-15 M to 2.94 × 10^-9 M, a low detection limit of 1.00 × 10^-16 M, and a relative standard deviation of 1.03%. The proposed ECL sensor with excellent reproducibility, long-term stability, and high selectivity was used for insulin determination in real serum samples with acceptable outcomes.

Metal-organic frameworks-based sensitive electrochemiluminescence biosensing

Metal-organic frameworks (MOFs) as porous materials have attracted much attention in various fields such as gas storage, catalysis, separation, and nanomedical engineering. However, their applications in electrochemiluminescence (ECL) biosensing are limited due to the poor conductivity, lack of modification sites, low stability and specificity, and weak biocompatibility. Integrating the functional materials into MOF structures endows MOF composites with improved conductivity and stability and facilitates the design of ECL sensors with multifunctional MOFs, which are potentially advantageous over their individual components. This review summarizes the strategies for designing ECL-active MOF composites including using luminophore as a ligand, in situ encapsulation of luminophore within the framework, and post-synthetic modification. As-prepared MOF composites can serve as innovative emitters, luminophore carriers, electrode modification materials and co-reaction accelerators in ECL biosensors. The sensing applications of ECl-active MOF composites in the past five years are highlighted including immunoassays, genosensors, and small molecule detection. Finally, the prospects and challenges associated with MOF composites and their related materials for ECL biosensing are tentatively proposed.

Recent Progress on Luminescent Metal-Organic Framework-Involved Hybrid Materials for Rapid Determination of Contaminants in Environment and Food

The high speed of contaminants growth needs the burgeoning of new analytical techniques to keep up with the continuous demand for monitoring and legislation on food safety and environmental pollution control. Metal-organic frameworks (MOFs) are a kind of advanced crystal porous materials with controllable apertures, which are self-assembled by organic ligands and inorganic metal nodes. They have the merits of large specific surface areas, high porosity and the diversity of structures and functions. Latterly, the utilization of metal-organic frameworks has attracted much attention in environmental protection and the food industry. MOFs have exhibited great value as sensing materials for many targets. Among many sensing methods, fluorometric sensing is one of the widely studied methods in the detection of harmful substances in food and environmental samples. Fluorometric detection based on MOFs and its functional materials is currently one of the most key research subjects in the food and environmental fields. It has gradually become a hot research direction to construct the highly sensitive rapid sensors to detect harmful substances in the food matrix based on metal-organic frameworks. In this paper, we introduced the synthesis and detection application characteristics (absorption, fluorescence, etc.) of metal-organic frameworks. We summarized their applications in the MOFs-based fluorometric detection of harmful substances in food and water over the past few years. The harmful substances mainly include heavy metals, organic pollutants and other small molecules, etc. On this basis, the future development and possible application of the MOFs have prospected in this review paper.

Sensitive immunosensor based on high effective resonance energy transfer of lucigenin to the cathodic electrochemiluminescence of tris(bipyridine) Ru(II) complex

Electrochemiluminescence resonance energy transfer (ECL-RET) has been attracting much focus as an effective approach for great ECL enhancement. Here, we found that lucigenin (Luc) could serve as a new energy transfer donor and greatly improve the cathodic ECL of bis(2,2'-bipyridyl)(4'-methyl-[2,2']bipyridinyl-4-carboxylicacid) ruthenium(II) (Ru(Bpy)₂(Mcbpy)²⁺, acceptor). Then, both Luc and Ru(Bpy)₂(Mcbpy)²⁺ were largely co-immobilized onto the PdCu nanocrystals and polyethyleneimine (PEI) modified single-walled carbon nanohorns (SWCNHs-PdCuNCs-PEI) through π-π stacking and crosslinking reaction, respectively. By this way, the excellent electrocatalytic behavior and high loading capability for both Luc and Ru(Bpy)₂(Mcbpy)²⁺ of SWCNHs-PdCuNCs-PEI effectively facilitated the ECL reaction. Particularly, the co-immobilization strategy making the donor (Luc)/acceptor (Ru(Bpy)₂(Mcbpy)²⁺) pairs co-exist in the same nano-composite could obviously increase the ECL-RET efficiency by shortening the electron-transfer path and reducing energy loss, further significantly improving the ECL signal. Combining the obtained nano-composite (Luc-SWCNHs-PdCuNCs-PEI-Ru(Bpy)₂(Mcbpy)²⁺) with sandwiched immunoreaction, an ECL immunosensor was constructed for β2-microglobulin (β2-M) measurement. And as a result, it exhibited excellent performance in sensitivity, stability and selectivity. The establishment of the new effective donor/acceptor pairs for ECL-RET and the co-immobilization strategy of making those donor/acceptor pairs largely co-exist in the same nano-composite would greatly improve the ECL efficiency and motivate the wider application of ECL technology.

Nanoscale metal-organic frameworks in detecting cancer biomarkers

Following the efficient performance of metal-organic frameworks (MOFs) as recognition elements in gas sensors, biosensors based on MOFs are now being investigated to capture and quantify potential cancer biomarkers, such as circulating tumor cells (CTCs), nucleic acids and proteins. The current status of MOF-based biosensors in the detection of early stages of cancer is in its infancy, although it has significantly emerged since the beginning of this decade. That said, salient research has been conducted in the past five years to utilize the distinctive porous crystalline structure of MOFs for highly sensitive and selective detection of cancer biomarkers. In this pursual, MOFs designed with bimetallic assembly, doped with magnetic nanoparticles, coated with polymers, and even conjugated with peptides or oligonucleotides have shown promising outcomes in detecting CTCs, nucleic acids and proteins. In particular, aptamer-conjugated MOFs are able to perform at a lower limit of detection down to the femtomolar, implying their efficacy for the point of care testing in clinical trials. In this way, aptasensors based on aptamer-conjugated MOFs present a newer sub-branch, to be coined as a MOFTA sensor in the current review. Considering the emerging progress and promising outcomes of MOFTA sensors as well as a variety of MOF-based techniques of detecting cancer biomarkers, this review will highlight their significant advances and related aspects in the recent five years on the context of detecting CTCs, nucleic acids and proteins for the early-stage detection of cancer.

Noble Metal Nanostructured Materials for Chemical and Biosensing Systems

Nanomaterials with unique physical and chemical properties have attracted extensive attention of scientific research and will play an increasingly important role in the future development of science and technology. With the gradual deepening of research, noble metal nanomaterials have been applied in the fields of new energy materials, photoelectric information storage, and nano-enhanced catalysis due to their unique optical, electrical and catalytic properties. Nanostructured materials formed by noble metal elements (Au, Ag, etc.) exhibit remarkable photoelectric properties, good stability and low biotoxicity, which received extensive attention in chemical and biological sensing field and achieved significant research progress. In this paper, the research on the synthesis, modification and sensing application of the existing noble metal nanomaterials is reviewed in detail, which provides a theoretical guidance for further research on the functional properties of such nanostructured materials and their applications of other nanofields.

Metal-Organic Frameworks with Double Channels for Rapid and Reversible Adsorption of 1,2-Ethylenediamine and Gases

Selective liquid and gas adsorptions are important for environmental control and industrial processes. Here, unique porous lanthanide-organic frameworks of [Ln₂(1,3-pdta)₂(H₂O)₂]_n^2n- {Ln = La (1), Ce (2), Pr (3), and Nd (4), 1,3-pdta = CH₂[CH₂N(CH₂CO₂H)₂]₂} are template-synthesized by 1,2-ethylenediamine and fully characterized, which possess hydrophobic and hydrophilic open channels simultaneously. The skeletons are stable up to 200 °C. Obvious downfield shifts have been observed for 1,2-ethylenediamine in the confined channel with solid-state ¹³C NMR measurement. The ammonium salt is directly used for the removal of 1,2-ethylenediamine in water. Its saturated adsorption capacity is reached in <1 min and can be regenerated easily with a similar uptake capacity. Moreover, the materials can also selectively adsorb O₂, CH₄, and CO₂, respectively, which is useful for CO₂/CH₄, CO₂/H₂, and O₂/N₂ separation. The combined hydrophobic and hydrophilic open channels of the lanthanides make them promising functional materials for the elimination of 1,2-ethylenediamine and gas separations.

Enhanced biosensing strategies using electrogenerated chemiluminescence: recent progress and future prospects

An overview of enhancement strategies for highly sensitive ECL-based sensing of bioanalytes enabling early detection of cancer.

Metal–organic frameworks: a future toolbox for biomedicine?

The present review focuses on the use of Metal–Organic Frameworks, (MOFs) highlighting the most recent developments in the biological field and as bio-sensors.

Electrochemiluminescence immunoassay for the N-terminal pro-B-type natriuretic peptide based on resonance energy transfer between a self-enhanced luminophore composed of silver nanocubes on gold nanoparticles and a metal-organic framework of type MIL-125

The N-terminal pro-B-type natriuretic peptide (NT-proBNP) is a marker of heart failure. A novel sandwich type electrochemiluminescence (ECL) immunoassay is described for the NT-proBNP. The method is based on ECL resonance energy transfer (RET) between silver nanocubes that were covered with semicarbazide-modified gold nanoparticles (AgNC-sem@AuNPs) as the donor, and a Ti(IV)-based metal-organic framework of type MIL-125 as the acceptor. The ECL signal was strongly amplified by increasing the luminous efficiency. ECL-RET occurs due to the partial overlap between the ECL emission of the AgNC-sem@AuNPs (emission wavelength at 470 nm to 900 nm) and the visible absorption spectrum of MIL-125 (absorption wavelength at 406 nm to 900 nm). This results in the quenching of ECL. The AgNC-sem@AuNPs were placed on the electrode. The antibody was immobilized on AgNC-sem@AuNPs via Au-NH₂ bond, and MIL-125 was utilized as a label for the secondary antibody. The assay works in the 0.25 pg mL^-1 to 100 ng mL^-1 concentration range and has a 0.11 pg mL^-1 lower detection limit (at S/N = 3). Graphical abstract Schematic representation of self-enhanced luminescence mechanism (semicarbazide (Sem) as co-reaction accelerator) and Electrochemiluminescence resonance energy transfer (ECL-RET): silver nanocubes (AgNCs) as the energy donor and MIL-125 as the energy acceptor.

Improving the photocatalytic hydrogen evolution of UiO-67 by incorporating Ce4+-coordinated bipyridinedicarboxylate ligands

UiO-67 is a Zr-based metal-organic framework (MOF) containing an organic linker namely, the dianion of biphenyl-4,4'-dicarboxylic acid (bpdc). Ce⁴⁺ metal ions (0.02 Ce to Zr atom ratio) were incorporated into UiO-67 via partially replacing bpdc with the dianion of 2,2'-bipyridine-5,5'-dicarboxylic acid (bpydc); thus, the latter forms a bpydc-Ce complex. The resulting product (i.e., UiO-67-Ce) demonstrated a photocatalytic hydrogen evolution rate that was over 10 times higher than that of UiO-67. Through this modification, a new energy transfer channel is opened up. The energy transfer between the bpdc and bpydc-Ce ligands (i.e., from excited bpdc to bpydc-Ce) weakened the recombination of the charge carriers, which was confirmed by photoluminescence, emission lifetime, and transient absorption measurements. This study presents a new way to construct highly efficient MOF photocatalysts.

Signal-Off Electrogenerated Chemiluminescence Biosensing Platform Based on the Quenching Effect between Ferrocene and Ru(bpy)32+-Functionalized Metal-Organic Frameworks for the Detection of Methylated RNA

N⁶-methyladenine (m⁶A), one of the most common chemical modifications of eukaryotic RNA, participates in many important biological processes. An effective strategy for the quantitative determination of m⁶A is of great significance. Herein, we used methylated microRNA-21 (miRNA21) as the model target to propose a simple and sensitive electrogenerated chemiluminescence (ECL) biosensing platform to detect a specific m⁶A RNA sequence. This strategy is based on the fact that the anti-m⁶A-antibody can specifically recognize and bind to the m⁶A site in the RNA sequence, resulting in a quenching effect between Ru(bpy)₃²⁺-functionalized metal-organic frameworks and ferrocene. Luminescent metal-organic frameworks (Ru@MOFs) not only act as ECL indicators but also serve as nanoreactors for the relative ECL reactions owing to their porous or multichannel structure, which overcomes the fact that Ru(bpy)₃²⁺ is easily released when used for aqueous-phase detection, thus enhancing the ECL efficiency. Moreover, the ECL method has fewer modification steps and uses only one antibody to recognize the target RNA sequence, which simplifies the operation process and reduces the detection time, presenting a wide linear range (0.001-10 nM) for m⁶A RNA determination with a low detection limit (0.0003 nM). Additionally, this developed strategy was validated for m⁶A RNA detection in human serum. Thus, the ECL biosensing method provides a new method for m⁶A RNA determination that is simple, highly specific, and sensitive.

Intracellular Imaging of Glutathione with MnO2 Nanosheet@Ru(bpy)32+-UiO-66 Nanocomposites

Fluorescent detection of glutathione (GSH) in the living system has attracted much attention, but current fluorescent probes are usually exposed to the exterior environment, leading to photobleaching and premature leakage and subsequently limiting the sensitivity and photostability. Herein, luminescent metal-organic frameworks [Ru(bpy)₃²⁺ encapsulated in UiO-66] coated with manganese dioxide nanosheets [MnO₂ NS@Ru(bpy)₃²⁺-UiO-66] were prepared by an in situ growth method and further explored to construct a GSH-switched fluorescent sensing platform. Because of the splendid fluorescence quenching ability, special probe leakage blocking role and distinguished recognition of the MnO₂ NS, and the improved fluorescence of Ru(bpy)₃²⁺ by UiO-66, a low background, highly sensitive and selective detection of GSH with a low limit of detection as 0.28 μM was realized. At the same time, the preparation of MnO₂ NS@Ru(bpy)₃²⁺-UiO-66 nanocomposites is simple and less toxic, and there was no notable loss of cell survivability after being exposed to MnO₂ NS@Ru(bpy)₃²⁺-UiO-66 below the concentrations of 120 μg mL^-1 for 24 h. Consequently, the results coming from this effort suggest that the new sensing platform will have a great potential in the detection of GSH in living cells.