Fast, Sensitive, and Selective Ion-Triggered Disassembly and Release Based on Tris(bipyridine)ruthenium(II)-Functionalized Metal–Organic Frameworks

Recent Trends and Advances in Porous Metal-Organic Framework Nanostructures for the Electrochemical and Optical Sensing of Heavy Metals in Water

With the expansion and advancement in agricultural and chemical industries, various toxic heavy metals such as lead, cadmium, mercury, zinc, copper, arsenic etc. are continuously released into the environment. Intake of sources contaminated with such toxic metals leads to various health issues. Keeping the serious effects of these toxic metal ions in view, various organic-inorganic nanomaterials based sensors have been exploited for their detection via optical, electrochemical and colorimetric approaches. Since a chemical sensor works on the principle of interaction between the sensing layer and the analytes, a sensor material with large surface area is required to enable the largest possible interaction with the target molecules and hence the sensitivity of the chemical sensor. However, commonly employed materials such as metal oxides and conducting polymers tend to feature relatively low surface areas, and hence resulting in low sensitivity of the sensor. Metal-Organic Frameworks (MOFs) nanostructures are another category of organic-inorganic materials endowed with large surface area, ultra-high and tunable porosity, post-synthesis modification features, readily available active sites, catalytic activity, and chemical/thermal stability. These properties provide high sensitivity to the MOF based sensors due to the adsorption of large number of target analytes. The current review article focuses on MOFs based optical and electrochemical sensors for the detection of heavy metals.

The Application of Metal-Organic Frameworks in Water Treatment and Their Large-Scale Preparation: A Review

Over the last few decades, there has been a growing discourse surrounding environmental and health issues stemming from drinking water and the discharge of effluents into the environment. The rapid advancement of various sewage treatment methodologies has prompted a thorough exploration of promising materials to capitalize on their benefits. Metal-organic frameworks (MOFs), as porous materials, have garnered considerable attention from researchers in recent years. These materials boast exceptional properties: unparalleled porosity, expansive specific surface areas, unique electronic characteristics including semi-conductivity, and a versatile affinity for organic molecules. These attributes have fueled a spike in research activity. This paper reviews the current MOF-based wastewater removal technologies, including separation, catalysis, and related pollutant monitoring methods, and briefly introduces the basic mechanism of some methods. The scale production problems faced by MOF in water treatment applications are evaluated, and two pioneering methods for MOF mass production are highlighted. In closing, we propose targeted recommendations and future perspectives to navigate the challenges of MOF implementation in water purification, enhancing the efficiency of material synthesis for environmental stewardship.

A sandwich electrochemiluminescence immunoassay based on 1T-MoS2@dual MOFs for detecting CA153

A "signal-on" electrochemiluminescence (ECL) immunosensor has been proposed for detecting carbohydrate antigen 153 (CA153) based on the dual MOFs sandwich strategy. The conductive and porous substrate consisting of 1T-MoS2 and two-dimensional conductive metal-organic framework (MOF, Ni-HAB) was anchored onto the glassy carbon electrode (GCE) to label the capture antibody (Ab1), and the luminescence-functionalized MOF (Ru(bpy)32+@UiO-66-NH2) was utilized to immobilize the detection second antibody (Ab2) to construct a "signal-on" responsive sandwich-type electrochemiluminescence immunoassay. Meanwhile, tripropylamine (TPA) acts as the co-reactant and provides a luminescence system for Ru(bpy)32+@UiO-66-NH2. The luminescence-functionalized MOFs showed excellent ECL activity owing to the tunable structure of MOFs. The remarkable enhancement in ECL intensity was obtained by the immunoreaction of antigen and antibody. Under the optimized conditions, the biosensor exhibited a detection limit of 0.0001 U mL-1 (S/N = 3) with a wide range from 0.001 to 50 U mL-1. The proposed ECL immunosensor was applicable for detecting human serum samples with a recovery of 99.83 ∼ 101 % (RSD < 5 %). This work demonstrates that the advantage of multifunctional MOFs could be applied to construct highly selective ECL immunosensor, and it may facilitate the diagnosis of breast cancer in clinics.

Fluorescence sensing and device fabrication with luminescent metal-organic frameworks

Metal-organic frameworks (MOFs) are a novel class of hybrid porous multi-functional materials consisting of metal ions/clusters and organic ligands. MOFs have exclusive benefits due to their tunable structure and diverse properties. Luminescent MOFs (LMOFs) exhibit both porosity and light emission. They display abundant host and guest responses, making them conducive to sensing. Currently, LMOF sensing research is gaining more depth, with attention given to their device and practical applications. This work reviews recent advancements and device applications of LMOFs as chemical sensors toward ions, volatile organic compounds, biomolecules, and environmental toxins. Furthermore, the detection mechanism and the correlation between material properties and structure are elaborated. This analysis serves as a valuable reference for the preparation and efficient application of targeted LMOFs.

Dual-quenching mechanisms in electrochemiluminescence immunoassay based on zinc-based MOFs of ruthenium hybrid for D-dimer detection

The successful application of electrochemiluminescence (ECL) in immunoassay for clinical diagnosis requires improving sensitivity and accuracy. Herein was reported an ECL analytical model based zinc-based metal-organic frameworks of ruthenium hybrid (RuZn MOFs) as the signal emitter. To enlarge the output difference, the quenching effect of three different noble metal nanoparticles included palladium seeds (Pd_seeds), palladium octahedrons (Pd_oct), and Pt-based palladium (Pd@Pt_oct) core-shell were researched. Among them, Pd@Pt_oct core-shell possessed higher activity and improved durability than Pd-only (NPs), they could load more protein macromolecules amicably and stabilized in the analysis system. Furthermore, since the charge redistribution owing to the hybridization of the Pt and Pd atoms in Pd@Pt_oct, it could generate the electron flow maximumly from the emitter RuZn MOFs to Pd@Pt_oct and result in the enhancement of quenching ECL. And the UV absorption of noble metal nanoparticles overlapped with the ECL emission of RuZn MOFs to varying degrees, which caused the behavior of resonance energy transfer (RET) reaction at the same time. This would greatly promote the sensitivity of this ECL system compared with the traditional single quenching mechanism. Based on this, a signal-off immunsensor was constructed to sensitive detection of D-dimer with linearity range from 0.001 to 200 ng mL^-1, limit of detection (LOD) was 0.20 pg mL^-1 and provide a further theoretical basis for the clinical application of ECL technology.

Metal organic frameworks as promising sensing tools for electrochemical detection of persistent heavy metal ions from water matrices: A concise review

Water bodies are being polluted rapidly by disposal of toxic chemicals with their huge entrance into drinking water supply chain. Among these pollutants, heavy metal ions (HMIs) are the most challenging one due to their non-biodegradability, toxicity, and ability to biologically hoard in ecological systems, thus posing a foremost danger to human health. This can be addressed by robust, sensitive, selective, and reliable sensing of metal ions which can be achieved by Metal organic frameworks (MOF) based electrochemical sensors. In the present era, MOFs have caught greater interest in a variety of applications including sensing of hazardous pollutants such as heavy metal ions. So, in this review article, types, synthesis and working mechanism of MOF based sensors is explained to give general overview with updated literature. First time, detailed study is done for sensing of metal ions such as chromium, mercury, zinc, copper, manganese, palladium, lead, iron, cadmium and lanthanide by MOFs based electrochemical sensors. The use of MOFs as electrochemical sensors has attractive success story along with some challenges of the area. Considering these challenges, we attempted to highlight the milestone achieved and shortcomings along with future prospective of the MOFs for employing it in electrochemical sensing devices for HMIs. Finally, challenges and future prospects have been discussed to promote the development of MOFs-based sensors in future.

Tris(bipyridine)ruthenium(II)-functionalized metal-organic frameworks for the ratiometric fluorescence determination of aluminum ions

A novel ratiometric fluorescence probe was designed for the determination of Al³⁺ by self-assembling of NH₂-MIL-101(Fe) and [Ru(bpy)₃]²⁺. Under the excitation wavelength of 360 nm, the NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ presented a dual-emitting luminescent property at 440 and 605 nm, respectively. In the presence of Al³⁺, the blue fluorescence of NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ at 440 nm was enhanced remarkably, while the red emission at 605 nm was almost not influenced. Therefore, taking the fluorescence at 440 nm as the report signal and 605 nm as the reference signal, quantitative determination was achieved for Al³⁺ concentration in the ranges 0.2-25 μM and 25-250 μM. The limit of detection (LOD) and limit of quantification (LOQ) were calculated to be 73 nM and 244 nM, respectively. The sensing mechanisms were studied by theoretical calculation and optical spectra. The analysis of real food samples confirmed the suitability of the proposed method. More importantly, portable fluorescent test papers were successfully manufactured to provide a strategy for visual, rapid, and on-site detection of Al³⁺.

Dumbbell Plate-Shaped AIEgen-Based Luminescent MOF with High Quantum Yield as Self-Enhanced ECL Tags: Mechanism Insights and Biosensing Application

It is widely known that high-performance electrochemiluminescence (ECL) emitters play a crucial part in improving the detection sensitivity of the ECL strategy. Through the combination of aggregation-induced emission luminogens (AIEgens), 1,1,2,2-tetra(4-carboxylbiphenyl)ethylene (H₄ TCBPE) with Zr(IV) cations, a dumbbell plate-shaped metal-organic framework (MOF) with high luminous efficiency is synthesized as ECL tags. The resultant MOF exhibits stronger ECL activity than those of H₄ TCBPE monomers and aggregates. Herein, this phenomenon is defined as the coordination-triggered electrochemiluminescence (CT-ECL) enhancement effect. Furthermore, the nearly matched ECL and photoluminescence (PL) spectra imply the bandgap emission mechanism. Remarkably, polyethyleneimine (PEI) as the coreactant is covalently connected with MOF to form the uniquely self-enhanced ECL complex of Zr-TCBPE-PEI, where the robust ECL signal is captured owing to the intramolecular-like coreaction acceleration. Based on the resonance energy transfer (RET) behavior, the AuPd@SiO₂ composite is designed as the high-efficiency quencher. In this manner, an innovative and ultrasensitive ECL sensor is constructed for neuron-specific enolase (NSE) detection through sandwich-type immunoreaction, with the detection limit down to 52 fg ml^-1 . The present study has gone some way toward designing MOF-based self-luminescent ECL materials, thus paving a new avenue to expand the late-model ECL emitters for immunoassay.

Metal-organic framework application for mercury speciation using solid phase extraction followed by direct thermal release-electrothermal atomization atomic absorption spectrophotometric detection (ETA AAS)

Metal-organic frameworks (MOFs) are increasingly used in analytical chemistry for pre-concentration of trace elements followed by their determination using modern analytical techniques. However, there are a limited number of publications concerning the use of MOFs for speciation purposes, while their structural and functional features are perspective for the element species selective extraction and pre-concentration. It is known that mercury refers to the most hazardous elements which species demonstrate different toxicity, migration routes and bioavailability as well. Consequently the development of new approaches for mercury speciation in environments remains an actual objective of analytical chemistry. In present work a new methodology for inorganic and organic mercury speciation in water was proposed. This approach is based on pre-concentration using solid phase extraction (SPE) followed by their determination directly from the solid phase with the application of the thermal release - electrothermal-atomic-absorption technique (TR-ETA-AAS). An original SPE-procedure based on the use of UIO-66 [Zr₆O₄(OH)₄(bdc)₆] in two different modes (non-modified and modified with cysteine) as a sorbent was designed. As a result of SPE as well as TR-ETA-AAS optimization the detection limits (LOD) for all listed species at the level of 0.06 μg L^-1 have been achieved. It was also shown that the presence of the other elements (K, Na, Ca, Mg at the level of 100 μg L^-1, and Mn, Fe, Cr, Al, Zn, Cd, Pb - of 25 μg L^-1) does not affect the results obtained. The developed assay demonstrates a high efficiency, low LODs, wide linear range and admissible analysis duration. The reliability of the data obtained was confirmed by the standard addition approach and by a comparison with the results of independent analytical methods.

Electrogenerated Chemiluminescence Biosensor Based on Functionalized Two-Dimensional Metal-Organic Frameworks for Bacterial Detection and Antimicrobial Susceptibility Assays

The emergence of antibiotic resistance has prompted the development of rapid antimicrobial susceptibility testing (AST) technologies to guide antibiotic prescription. A novel electrochemiluminescence (ECL) biosensor developed can quantitatively measure the binding between the lectin and lipopolysaccharide (LPS) on Gram-negative bacteria for bacterial determination and to characterize the antimicrobial activities of β-lactam and non-β-lactam antibiotics to normal and antibiotic-resistant bacteria. The biosensor utilizes ruthenium complex tagged concanavalin A (Ru-Con A) coated on NH₂-MIL-53(Al) interface for LPS binding measurements. The decreased ECL signal obtained was directly proportional to increasing Escherichia coli (E. coli) BL21 concentrations. The sensitivity displayed logarithmic dependence in the range of (50-5.0) × 10⁴ cells/mL, with a detection limit of 16 cells/mL. The minimum inhibitory concentration (MIC) values of antibiotics for normal E. coli BL21 were 0.02-0.2, 2-4, 0.002-0.02, and 0.2-1 mg/L for levofloxacin hydrochloride (LVX), tetracycline (TCY), imipenem (IPM), and cefpirome (CPO), respectively. The increased MIC values (8-16 and 4 mg/L for IMP and CPO, respectively) in New Delhi metallo-β-lactamase-1 expressing E. coli BL21 (NDM-1-E. coli BL21) indicated greater resistance to β-lactams in NDM-1-E. coli BL21 compared with normal E. coli BL21. Therefore, the changed ECL signal because of binding between LPS with the lectin has a relation with the type of antibiotic and bacterial strains, making the ECL biosensor promote clinical practicability and facilitate antibiotic stewardship.

Electrochemiluminescence from the Graphene- and Fullerene-Like Nanostructures of Glassy Carbon Microspheres and Its Application in Immunoassay

Glassy carbon (GC) as a well-known electrode material has recently been proposed to consist of fullerene-like nanostructures. In order to verify the nanostructures in GC, find more physiochemical properties of GC, and develop sensors based on GC-related carbon nanomaterials, we investigated the morphologies and surface states of GC microspheres (GCMs) and their HNO₃-oxidized products (ox-GCMs) with scanning electron microscopy (SEM), electrochemiluminescence (ECL), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron-paramagnetic resonance (EPR) spectroscopy. Our research results reveal that ox-GCMs rather than raw GCMs have abundant surface states, including many carboxyl groups (-COOH), surface defects (or carbon edges), and C-related dandling bonds. The surface states with a band gap of 2.14 eV endow ox-GCMs with strong cathodic ECL activity in the presence of peroxydisulfate (S₂O₈^2-). The ECL behaviors and maximum emission wavelength (580 nm) of ox-GCMs are very similar to those of small-sized graphene quantum dots and fullerene-like nanosheets, verifying that GCMs are essentially 3-D nanomaterials consisting of graphene or fullerene-like carbon nanostructures. It is for the first time that a microsized carbon material was reported to have good ECL activity in aqueous media. Possible mechanisms for surface state formation and ECL reactions are proposed for ox-GCMs, and a promising application of ox-GCMs in ECL immunosensing has been demonstrated by determining prostate specific antigen (PSA) as a model cancer biomarker.

Fe3O4 NP@ZIF-8/MoS2 QD-based electrochemiluminescence with nanosurface energy transfer strategy for point-of-care determination of ATP

Herein, Fe₃O₄ NP@ZIF-8/MoS₂ QD-based electrochemiluminescence (ECL) biosensor with nanosurface energy transfer strategy was successfully developed for point-of-care determination of ATP. With the porous structure and poor electron transfer ability, Fe₃O₄ NP@ZIF-8 complex was first used as an excellent catalyst in ECL. The complex catalyzed the coreactant for more free radicals and hindered the quenching effect of Fe₃O₄ nanoparticles (NPs) on quantum dots (QDs). In ECL-nanosurface energy transfer (NSET) system, through the specific binding of complementary DNA linked to MoS₂ QDs (QDs-cDNA) and aptamer linked to Au NPs, interaction between the point dipole of MoS₂ QDs and the collective dipoles of Au NPs quenched ECL signal. When ATP was captured by aptamer, the ECL-NSET system was taken apart, which resulted in the recovery of ECL signal. Moreover, changes of the ECL imaging can be captured by a smartphone, which enabled point-of-care determination of ATP from 0.05 nmol L^-1 to 200 nmol L^-1 with LOD of 0.015 nmol L^-1. With superior specificity and stability, the sensing system showed significant potential about the application of catalysts coated with ZIF and NSET in point-of-care ECL determination.

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.

An AIEgen-based 2D ultrathin metal-organic layer as an electrochemiluminescence platform for ultrasensitive biosensing of carcinoembryonic antigen

In this work, a novel two-dimensional (2D) ultrathin metal-organic layer (MOL) based on the aggregation-induced emission (AIE) ligand H4ETTC (H4ETTC = 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carboxylic acid))) was developed and used to construct a novel electrochemiluminescence (ECL) aptasensor for ultrasensitive detection of carcinoembryonic antigen (CEA). The newly synthesized AIE luminogen (AIEgen)-based MOL (Hf-ETTC-MOL) yielded a higher ECL intensity and efficiency than did H4ETTC monomers, H4ETTC aggregates and 3D bulk Hf-ETTC-MOF. This improvement occurred not only because the ETTC ligands were coordinatively immobilized in a rigid MOL matrix, which restricted the intramolecular free rotation and vibration of these ligands and then reduced the non-radiative transition, but also because the porous ultrathin 2D MOL greatly shortened the transport distances of ions, electrons, coreactant (triethylamine, TEA) and coreactant intermediates (TEA˙ and TEA˙+), which made more ETTC luminophores able to be excited and yielded a high ECL efficiency. On the basis of using the Hf-ETTC-MOL as a novel ECL emitter and rolling circle amplification (RCA) as a signal amplification strategy, the constructed ECL aptasensor exhibited a linear range from 1 fg mL-1 to 1 ng mL-1 with a detection limit of 0.63 fg mL-1. This work has opened up new prospects for developing novel ECL materials and is expected to lead to increased interest in using AIEgen-based MOLs for ECL sensing.

Electrochemiluminescence biosensor for DNA hydroxymethylation detection based on enzyme-catalytic covalent bonding reaction of -CH2OH and thiol functionalized Fe3O4 magnetic beads

5-Hydroxymethylcytosine (5 hmC) is a novel epigenetic modification that plays an important role in mammalian nuclear reprogramming, regulation of gene activity, and initiation of DNA demethylation. In this paper, an electrochemiluminescence sensor was constructed for 5 hmC detection based on thiol functional Fe₃O₄ magnetic beads and covalent chemical reaction of -CH₂OH in 5 hmC. First, Fe₃O₄ magnetic beads were prepared and modified with thiol. Then, 5 hmC was captured on the surface of the magnetic beads by the reaction between -CH₂OH of 5 hmC and -SH of the thiol-functionalized Fe₃O₄ under the catalysis of DNA methyltransferase (M. HhaI). After that, through a series of reactions, phos-tag-biotin, avidin, and bis(hexafluorophosphate) (Ru (bpy)₂ (phen-5-NH₂) (PF₆)₂) (Ru) were further successively immobilized on the surface of the magnetic beads. More importantly, these reactions were carried out in a solution to ensure the activity of the biomolecules, and further to ensure that the reaction proceeded sufficiently. Finally, an ECL signal was generated by the introduction of Ru. The concentration of 5 hmC presented a good linear relationship with the ECL signal intensity in the range of 0.01-500 nM, and the detection limit was 2.86 pM. Moreover, we also used this method to study the 5 hmC content change in rice seedlings treated with antibiotics and heavy metal composite pollutants, and in chicken embryo fibroblast cell infected with and without avian leukosis virus subgroup J.

Recent advances in two-dimensional-material-based sensing technology toward health and environmental monitoring applications

Monitoring harmful and toxic chemicals, gases, microorganisms, and radiation has been a challenge to the scientific community for the betterment of human health and environment. Two-dimensional (2D)-material-based sensors are highly efficient and compatible with modern fabrication technology, which yield data that can be proficiently used for health and environmental monitoring. Graphene and its oxides, black phosphorus (BP), transition metal dichalcogenides (TMDCs), metal oxides, and other 2D nanomaterials have demonstrated properties that have been alluring for the manufacture of highly sensitive sensors due to their unique material properties arising from their inherent structures. This review summarizes the properties of 2D nanomaterials that can provide a platform to develop high-performance sensors. In this review, we have also discussed the advances made in the field of infrared photodetectors and electrochemical sensors and how the structural properties of 2D nanomaterials affect sensitivity and performance. Further, this review highlights 2D-nanomaterial-based electrochemical sensors that can be used to check for contaminations from heavy metals, organic/inorganic compounds, poisonous gases, pesticides, bacteria, antibiotics, etc., in water or air, which are severe risks to human wellbeing as well as the environment. Moreover, the limitations, future prospects, and challenges for the development of sensors based on 2D materials are also discussed for future advancements.

In Situ Growing Triethanolamine-Functionalized Metal-Organic Frameworks on Two-Dimensional Carbon Nanosheets for Electrochemiluminescent Immunoassay

A facile strategy for in situ growing triethanolamine (TEOA)-functionalized metal-organic framework (TEOA@MOF) on the two-dimensional graphene oxide (GO) or g-C₃N₄ nanosheets via the self-assembly technique was introduced. In this method, Zn²⁺ was first attached on the carbon nanosheets by electrostatic interaction; then, trimesic acid (H₃btc) acted as the complex agent and TEOA as a base for the deprotonation of H₃btc and a template, which leads to in situ growing the MOF on the carbon nanosheets obtaining a sandwich-like structure. Different types of surface analysis techniques were employed to characterize the GO-TEOA@MOFs and g-C₃N₄-TEOA@MOFs nanomaterials fabricated. The GO-TEOA@MOFs or g-C₃N₄-TEOA@MOFs nanomaterial-modified electrode brings out obviously enhanced electrochemiluminescence (ECL) behaviors due to numerous TEOA in the framework structures. Specifically, both TEOA and GO can serve as the co-reactants for the ECL system of Ru(bpy)₃²⁺ and have the synergic effect of enhancing the signal. Based on the GO-TEOA@MOFs modified electrodes, we developed a sensitive and rapid label-free ECL immunoassay strategy for human copeptin, and the linear range was 5 pg mL^-1 to 500 ng mL^-1 as well as the limit of detection was 360 fg mL^-1. This work exhibits excellent specificity and good stability of the prepared immunosensor in the practical sample determination, demonstrating it can serve as a very promising method for the clinical diagnostics of acute myocardial infarction disease.

NanoPOP: Solution-Processable Fluorescent Porous Organic Polymer for Highly Sensitive, Selective, and Fast Naked Eye Detection of Mercury

Fluorescence-based detection is one of the most efficient and cost-effective methods for detecting hazardous, aqueous Hg²⁺. We designed a fluorescent porous organic polymer (TPA-POP-TSC), with a "fluorophore" backbone and a thiosemicarbazide "receptor" for Hg²⁺-targeted sensing. Nanometer-sized TPA-POP-TSC spheres (nanoPOP) were synthesized under mini-emulsion conditions and showed excellent solution processability and dispersity in aqueous solution. The nanoPOP sensor exhibits exceptional sensitivity (K_sv = 1.01 × 10⁶ M^-1) and outstanding selectivity for Hg²⁺ over other ions with rapid response and full recyclability. Furthermore, the nanoPOP material can be easily coated onto a paper substrate to afford naked eye-based Hg²⁺-detecting test strips that are convenient, inexpensive, fast, highly sensitive, and reusable. Our design takes advantage of the efficient and selective capture of Hg²⁺ by thiosemicarbazides (binding energy = -29.84 kJ mol^-1), which facilitates electron transfer from fluorophore to bound receptor, quenching the sensor's fluorescence.

A Sensitive Electrochemiluminescence Sensor for Brilliant Blue FCF Using Ru(bpy)32+ Immobilized Zn-MOF

A novel electrochemiluminescence (ECL) sensor for detection of brilliant blue FCF (BB) has been developed using Ru@Zn-MOF/nafion modified GCE (glass carbon electrode) in this research. Different from conventional method for usage of Ru(bpy)₃²⁺ in solution-phase, Ru(bpy)₃²⁺ here was immobilized on a zinc-metal-organic-framework (Zn-MOF). After adding BB, a significant quenching phenomenon of ECL intensity was observed. The behavior of BB on the quenching effect of Ru(bpy)₃²⁺/Zn-MOF in different conditions was investigated thoroughly and the detection limit was achieved to 2.5 × 10^-8 M in an optimized condition. Furthermore, the interference of some conventional ions and amino acids to the detection of BB was also investigated. Additionally, the composite showed a good effect on the detection of BB in commercial samples. The proposed sensor provided a promising platform for food safety analysis, environmental monitoring and clinical testing.

Enhancing Luminol Electrochemiluminescence by Combined Use of Cobalt-Based Metal Organic Frameworks and Silver Nanoparticles and Its Application in Ultrasensitive Detection of Cardiac Troponin I

Electrochemiluminescence (ECL) has emerged as one of the most important methods for in vitro diagnosis and detection, but it is still limited in sensitivity for ultrasensitive biodetections. Fast and ultrasensitive detection of biomolecules is critical, especially for the clinical detection of cardiac troponin I (cTnI) for cardiac infarction diagnosis. In this study, an effective tactic was developed to enhance ECL efficiency of the luminol system, by combined use of Co²⁺-based metal organic frameworks (MOF), zeolitic imidazolate frameworks (ZIF-67), and luminol-capped Ag nanoparticles (luminol-AgNPs). The integration leads to a pronounced ∼115-fold enhancement in luminol ECL. On the basis of this fascinating sensing platform, a robust label-free ECL immunosensor was constructed for ultrasensitive detection of cTnI, the main marker of myocardial infarction, with good stability and a detection limit as low as 0.58 fg mL^-1 (S/N = 3).

Selective detection and removal of mercury ions by dual-functionalized metal–organic frameworks: design-for-purpose

In this study, through introducing a new functional group into the structure, the performance and efficiency of MOFs as a sensor for heavy metal cations have been improved.

Simple One-Pot Preparation of a Rapid Response AIE Fluorescent Metal-Organic Framework

Luminogenic materials, particularly those that have turn-on response by sensing the analytes, are highly regarded as optical instruments, sensing material, fluorescent probes, etc. However, most of these materials are only usable in dilute form and often show the self-quenching effect at high concentrations. The use of light-emitting AIE-based materials (aggregation-induced emission) is the solution of this problem. The rigid structure of these active fluorescence ligands, which contains several aromatic rings attached to each other, does not lose its fluorescence properties by increasing the concentration. Unlike other AIE ligands, which have a complex or multistep synthetic route, here, we present a simple one-pot method for in situ synthesis of the AIE ligand and the metal-organic framework (MOF) contained therein. Presence of metal nodes having varied outer-shell electron configurations affects the fluorescence intensity of these materials and, thus, both high and low emissive "turn on" MOFs were readily acquired. Based on the possible interactions between the free nitrogens on the ligand and the phenolic compounds, the MOFs enable highly selective and sensitive detection of phenol derivatives in several seconds with low detection limits (less than 65 nM for 4-aminophenol and 120 nM for phenol) through turn-on emission fluorescence.

Fabrication of Tris(bipyridine)ruthenium(II)-Functionalized Metal-Organic Framework Thin Films by Electrochemically Assisted Self-Assembly Technique for Electrochemiluminescent Immunoassay

A simple strategy for one-step fabrication of tris(bipyridine)ruthenium(II) (Ru(bpy)₃²⁺)-functionalized metal-organic framework (Ru-MOF) thin films using a self-assembly approach assisted by an electrochemical way was introduced. In this protocol, the electrochemically driven cooperative reaction of Ru(bpy)₃²⁺ as an electrochemiluminescent (ECL) probe and a structure-directing agent, trimesic acid (H₃btc) as a ligand, and Zn(NO₃)₂ as the Zn²⁺ source leads to an one-step and simultaneous synthesis and deposition of the MOF onto the electrode surface. Characterization of the Ru-MOF thin films was performed with scanning electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy. Scanning ion conductance microscopy was specially applied in situ to image the topography and thickness of the Ru-MOF thin films. The Ru-MOF thin films as a sensing platform show excellent ECL behavior because of plenty of Ru(bpy)₃²⁺ molecules encapsulated in the frameworks. On the basis of the Ru-MOF modified electrodes, an ultrasensitive label-free ECL immunosensing method for the human heart-type fatty-acid-binding protein has been developed with a wide linear response range (150 fg mL^-1-150 ng mL^-1) and a very low limit of detection (2.6 fg mL^-1). The prepared immunosensor also displayed excellent stability and good specificity in the test of practical samples.

Highly Stable Mesoporous Luminescence-Functionalized MOF with Excellent Electrochemiluminescence Property for Ultrasensitive Immunosensor Construction

In this work, a novel mesoporous luminescence-functionalized metal-organic framework (Ru-PCN-777) with high stability and excellent electrochemiluminescence (ECL) performance was synthesized by immobilizing Ru(bpy)₂(mcpbpy)²⁺ on the Zr₆ cluster of PCN-777 via a strong coordination bond between Zr⁴⁺ and -COO^-. Consequently, the Ru(bpy)₂(mcpbpy)²⁺ could not only cover the surface of PCN-777 but also graft into the interior of PCN-777, which greatly increased the loading amount of Ru(bpy)₂(mcpbpy)²⁺ and effectively prevented the leaching of the Ru(bpy)₂(mcpbpy)²⁺ resulting in a stable and high ECL response. Considering the above merits, we utilized the mesoporous Ru-PCN-777 to construct an ECL immunosensor to detect mucin 1 (MUC1) based on proximity-induced intramolecular DNA strand displacement (PiDSD). The ECL signal was further enhanced by the enzyme-assisted DNA recycling amplification strategy. As expected, the immunosensor had excellent sensitivity, specificity, and responded wide linearly to the concentration of MUC1 from 100 fg/mL to 100 ng/mL with a low detection limit of 33.3 fg/mL (S/N = 3). It is the first time that mesoporous Zr-MOF was introduced into ECL system to assay biomolecules, which might expand the application of mesoporous metal-organic frameworks (MOFs) in bioanalysis. This work indicates that the use of highly stable mesoporous luminescence-functionalized MOFs to enhance the ECL intensity and stability is a feasible strategy for designing and constructing high-performance ECL materials, and therefore may shed light on new ways to develop highly sensitive and selective ECL sensors.

Zr-Based MOFs integrated with a chromophoric ruthenium complex for specific and reversible Hg2+ sensing

A novel metal-organic framework of RuUiO-67 was successfully fabricated and exploited as a chemical sensor for the colorimetric detection of Hg2+. The chromophoric Ru complex in RuUiO-67 was designed as a Hg2+ recognition site and a signal reporter. The elaborated probe exhibited a rapid colorimetric response, high selectivity, and sub-micromolar sensitivity for Hg2+ detection.

Metal-Organic Framework-Based Sensors for Environmental Contaminant Sensing

Increasing demand for timely and accurate environmental pollution monitoring and control requires new sensing techniques with outstanding performance, i.e., high sensitivity, high selectivity, and reliability. Metal-organic frameworks (MOFs), also known as porous coordination polymers, are a fascinating class of highly ordered crystalline coordination polymers formed by the coordination of metal ions/clusters and organic bridging linkers/ligands. Owing to their unique structures and properties, i.e., high surface area, tailorable pore size, high density of active sites, and high catalytic activity, various MOF-based sensing platforms have been reported for environmental contaminant detection including anions, heavy metal ions, organic compounds, and gases. In this review, recent progress in MOF-based environmental sensors is introduced with a focus on optical, electrochemical, and field-effect transistor sensors. The sensors have shown unique and promising performance in water and gas contaminant sensing. Moreover, by incorporation with other functional materials, MOF-based composites can greatly improve the sensor performance. The current limitations and future directions of MOF-based sensors are also discussed.

Photonic functional metal–organic frameworks

The recent progress in photonic MOFs for luminescence sensing, white-light emission, photocatalysis, nonlinear optics, lasing devices, and biomedicine is summarized.