Luminescent Metal–Organic Frameworks for Electrochemiluminescent Detection of Water Pollutants

Citation: Sentic, M.; Trajkovic, I.; Manojlovic, D.; Stankovic, D.; Nikolic, M.V.; Sojic, N.; Vidic, J. Luminescent Metal–Organic Frameworks for Electrochemiluminescent Detection of Water Pollutants. Materials 2023, 16, 7502. https://doi.org/10.3390/ ma16237502 Academic Editor: Roberta G. Toro Received: 26 October 2023 Revised: 28 November 2023 Accepted: 1 December 2023 Published: 4 December 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Luminescent Metal–Organic Frameworks for Electrochemiluminescent Detection of Water Pollutants Milica Sentic 1, Ivana Trajkovic 1, Dragan Manojlovic 2, Dalibor Stankovic 2, Maria Vesna Nikolic 3, Neso Sojic 4and Jasmina Vidic 5,* 1Institute of Chemistry, Technology and Metallurgy, National Institute of the Republic of Serbia, University of Belgrade, Njegoševa 12, 11001 Belgrade, Serbia; [email protected] (M.S.); [email protected] (I.T.) 2Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia; [email protected] (D.M.); [email protected] (D.S.) 3Institute for Multidisciplinary Research, University of Belgrade, Kneza Viseslava 1, 11030 Belgrade, Serbia; [email protected] 4 Bordeaux INP, ISM, UMR CNRS 5255, University of Bordeaux, 33607 Pessac, France; [email protected] 5INRAE, AgroParisTech, Micalis Institute, UMR 1319, UniversitéParis-Saclay, 78350 Jouy-en-Josas, France *Correspondence: [email protected] Abstract: The modern lifestyle has increased our utilization of pollutants such as heavy metals, aromatic compounds, and contaminants that are of rising concern, involving pharmaceutical and personal products and other materials that may have an important environmental impact. In particular, the ultimate results of the intense use of highly stable materials, such as heavy metals and chemical restudies, are that they turn into waste materials, which, when discharged, accumulate in environmental water bodies. In this context, the present review presents the application of metal–organic frameworks (MOFs) in electrochemiluminescent (ECL) sensing for water pollutant detection. MOF composites applied as innovative luminophore or luminophore carriers, materials for electrode modification, and the enhancement of co-reaction in ECL sensors have enabled the sensitive monitoring of some of the most common contaminants of emerging concern such as heavy metals, volatile organic compounds, pharmaceuticals, industrial chemicals, and cyanotoxins. Moreover, we provide future trends and prospects associated with ECL MOF composites for environmental sensing. Keywords: metal organic frameworks; luminescence; electroluminescent detection; water pollutants 1. Introduction Pollution from different sources due to extensive urbanization and industrialization is placing the global water budget under pressure by reducing the water available for beneficial use [ 1 ]. The demand for water has increased tremendously in agricultural, industrial, and domestic domains, resulting in the huge impact of natural and anthropogenic substances that are constantly released into the environment. Prime agricultural land is being lost to urbanization, threatening the aquatic and terrestrial ecosystems due to the increased use of chemicals. Anthropogenic sources of pollution are growing in number in line with progressing human development. Industrial, agricultural (pesticides and fertilizers), and mining activities, accompanied by construction, fuel and coal burning, traffic emissions (i.e., exhaust gases), and sewage waste, are the most customary pollution sources. Amid different pollutants, even low concentrations of heavy metals have raised health concerns owing to their capability for hazardous bioaccumulation through food chains via the formation of metal–organic complexes [ 2 ]. The greatest number of non-degradable potentially toxic elements (PTEs) include Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Mercury (Hg), Nickel (Ni), Lead (Pb), and Zinc (Zn), pose a serious threat to the ecosystem [ 3 ] when they are above the maximum allowable limits and are listed as Materials 2023,16, 7502. https://doi.org/10.3390/ma16237502 https://www.mdpi.com/journal/materials Materials 2023,16, 7502 2 of 20 priority pollutants requiring control by the U.S. Environmental Protection Agency and the EU Commission. These elements are found in varying amounts throughout the Earth’s crust as naturally occurring components in the water–sediment environment. Their geochemical levels in sediments are, thus, expected to be relatively low [ 4 ]. However, when heavy metals are discharged into aquatic environments from anthropogenic sources, they accumulate between the aqueous phase and the sediments. Volatile organic compounds (VOCs) are persistent and important organic pollutants that can lead to groundwater contamination. VOCs include chlorinated solvents ( e.g., hydrogen sulfide, trichloroethene, carbon tetrachloride) and petroleum hydrocarbons, especially BTEX (benzene, toluene, ethyl-benzene, and mixtures of o-, mand p-xylene) compounds. These volatile aromatic compounds are severely toxic to aquatic organisms if contact is maintained. They are generated by the incomplete combustion of organic matter, which is often found in discharges and petroleum products (vehicle exhaust, coal burning and residential heating, waste incineration, petroleum refining processes, and aluminum production). Groundwater contaminated with VOCs can potentially affect freshwater aquatic ecosystems as it can discharge for a long period of time to surface water bodies. Gasoline can contain large amounts of BTEX (up to 40%), and therefore, BTEXs are used as indicators of the gasoline contamination of sediment samples when gasoline contamination is suspected. Aquatic ecosystems are also under threat from the bioaccumulation of other sustainable freshwater contaminants, which are classified as contaminants of emerging concern (CECs) [ 5 – 7 ]. Under this broad family of chemical pollutants, we find synthetic chemicals that could have an impact on human health or ecology. Endocrine-disrupting chemicals or endocrine disruptors (EDCs) and non-steroidal anti-inflammatory drugs (NSAIDs) represent two main CEC subgroups. These pollutants can originate from agriculture, urban runoff, or ordinary household products (disinfectants, fragrances, pesticides) and pharmaceuticals dispatched to sewage treatment plants and then discharged. One main concern with CECs is that existing traditional wastewater treatment processes are ineffective in their removal. The most common include hormones endocrine-disrupting chemicals (estrone, 17ß-estradiol, 17 α -ethynylestradiol, testosterone), disinfection by-products, fluorinated substances (bisphenols, phthalates, synthetic estrogens), pesticides (glyphosate and organophosphorus pesticides), and antibiotics. Finally, waterborne pathogenic bacteria and the mass occurrence of cyanobacteria blooms due to anthropogenic activities in freshwaters, including drinking water reservoirs, threaten human health and the environment because of their toxin-producing qualities [ 8 ]. The most frequently occurring and studied biologically active cyanobacteria toxins are anatoxins (ATXs) and microcystins (MCs), whose toxicity is a major cause of concern in the scientific community and the World Health Organization [ 9 , 10 ]. Anatoxin-a(S) represents an extremely potent natural neurotoxin generated by freshwater cyanobacteria, while exposure to microcystins, which are chemically very stable, leads to liver dysfunction, hemorrhage, and, in acute doses, causes cancer. Due to chronic low-dose exposure, microcystins are cancer promoters. The stability of MCs is attributed to their cyclic structure, which remains unchanged after a few hours in boiling water and even for several years at room temperature if they are in a dry state. Indeed, microcystins are not readily removed from drinking water via conventional treatment methods, which indicates the importance of toxin detection and monitoring in freshwater [11,12]. With such a vast variety of pollutants, the control of water–sediment environments remains a priority and a problem, which has been highlighted in a considerable number of scientific publications [ 13 ]. While plans to minimize global environmental pollution exist, the contamination of water and its sediments is stressing the urgency of technological advances in materials for pollutant-sensitive detection and their elimination. New sensing materials and methods showing outstanding performance, reflected by high sensitivity and selectivity, rapid detection, and ease of use, in comparison with traditional expensive chromatography with complex pretreatment and long test times, are decidedly needed. Materials 2023,16, 7502 3 of 20 A wide range of microand nanomaterials, including nanocarbon materials (carbon nanotube and graphene), metals and metal oxides, semiconducting materials, quantum dots, and polymers with different characteristics, have been applied in environmental monitoring sensors [ 14 – 17 ]. Among these advanced novel materials, metal–organic frameworks (MOFs), also recognized as porous coordination polymers (PCPs), have attracted intense attention due to their excellent physicochemical characteristics owing to the coexistence of crystallinity and porosity [ 16 , 18 – 20 ]. The rational design of MOFs could especially provide innovative emitters or luminophore carriers for the hybrid analytical method of electrochemiluminescence (ECL). ECL combines light-emission detection with orthogonal electrochemical initiation [ 21 – 23 ]. The classical and most exploited ECL system is composed of tris(bipyridine)ruthenium (Ru(bpy) 32+ ) as the luminophore and tri-n-propylamine (TPrA) as the sacrificial co-reactant [ 24 – 30 ]. This ECL system can be significantly improved by utilizing nanomaterials [ 31 – 42 ]. Accordingly, ECL-based MOFs provide a new prospect for highly sensitive and targeted bioanalysis combined with functional nanomaterial design and controllable and tunable photophysical and photochemical properties through the structural modifications of organic linkers, metal clusters, and guest species [ 43 ]. Among other benefits, ECL MOFs enable the re-use of potassium persulfate as a non-toxic coreactant in comparison to TPrA while providing the same or better sensitivity, thus making the whole system more environmentally friendly. In addition, the nanoconfinement that occurs in such mesoporous materials based on the intensity of ECL has been imaged and spatially resolved with a remarkable spatial resolution. Liu’s group showed that the ECL signals were very stable even in biological media, allowing single biomolecule imaging [ 43 ]. The high sensitivity of ECL sensors based on MOF luminophores allows the efficient detection of water pollutants, which are typically present in low amounts in water bodies. This set off the recent development of novel MOF materials for ECL and new applications, especially sensing and imaging [35,39,44–50]. In this review, we present recent developments in the development of luminescent MOF-based ECL sensors for water pollutant detection. First, we provide a brief description of the ECL method. Second, we introduce different MOFs as carriers of ELC emitters, including luminol, Ru(bpy) 32+ , and their derivatives. Third, we outline promising applications of MOF-based ECL in water monitoring. Finally, we discuss some perspectives on the synthesis and applications of MOF luminophores for ECL sensors. 2. ECL Sensors Current (bio)sensors for the detection of pollutants aim to replace classical detection techniques based on liquid and gas chromatography or mass spectrometry by providing the coupling of the accuracy of measurements equivalent to such instrumental methods with the portability, affordability, and simplicity of analysis. As already mentioned, ECL is an electrochemical process in which an electron-transfer reaction taking place at an electrode surface triggers light emission. In co-reactant ECL, the emission of light is generated via a charge transfer between the electrochemical reaction intermediates of both the emitter and co-reactant (Figure 1). The most widely used system applied for analytical purposes is composed of the luminophore species Ru(bpy) 32+ , or its derivative, and TPrA as a co-reactant. An ECL co-reactant can be defined as a reagent that, following oxidation or reduction, is able to decompose, producing highly reactive reductive or oxidative species (Figure 1), which can proceed through an electron transfer reaction with an oxidized or reduced luminophore in order to generate ECL. Combining electrochemistry and luminescence in this smart way enables the unique advantages of ECL compared to other optical sensing methods, as a light source is not necessary, and this makes the detection set-up simpler and most importantly, no background signal is generated from scattered light, and luminescent impurities. As ECL does not require a light source, it simplifies the detection apparatus, and, most importantly, invalidates background signals from scattered light and luminescent impurities, thus providing improved sensitivity. However, some specific ECL configu- Materials 2023,16, 7502 4 of 20 rations employing semiconductors and named photo-induced ECL require an excitation light to photo-generate holes and electrons that trigger the emission of ECL [ 48 – 50 ]. These properties have resulted in ECL becoming a significant detection method in analytical chemistry and microscopy [ 23 , 30 , 51 – 60 ]. Commercial ECL systems for clinical diagnostics using standard ECL pair, Ru(bpy) 32+ , or one of its derivatives, and TPrA run over 1.3 billion tests per year [26,60]. Materials 2023, 15, x FOR PEER REVIEW 4 of 20 Figure 1. Overview of the ECL process for the most common system used, consisting of the luminophore, Ru(bpy) 32+ , and the tri-n-propylamine (TPrA) co-reactant. The ECL reagents are generated in situ at the electrode using cyclic voltammetry or chronoamperometry by sweeping the applied potential. Upon the oxidation of both the luminophore and co-reactant, the formed activated species further interact to form the excited state Ru(bpy) 32+∗ resulting in an ECL emission. Ru 2+ represents Ru(bpy) 32+ . The most widely used system applied for analytical purposes is composed of the luminophore species Ru(bpy) 32+ , or its derivative, and TPrA as a co-reactant. An ECL coreactant can be defined as a reagent that, following oxidation or reduction, is able to decompose, producing highly reactive reductive or oxidative species (Figure 1), which can proceed through an electron transfer reaction with an oxidized or reduced luminophore in order to generate ECL. Combining electrochemistry and luminescence in this smart way enables the unique advantages of ECL compared to other optical sensing methods, as a light source is not necessary, and this makes the detection set-up simpler and most importantly, no background signal is generated from scattered light, and luminescent impurities. As ECL does not require a light source, it simplifies the detection apparatus, and, most importantly, invalidates background signals from scattered light and luminescent impurities, thus providing improved sensitivity. However, some specific ECL configurations employing semiconductors and named photo-induced ECL require an excitation light to photo-generate holes and electrons that trigger the emission of ECL [48–50]. These properties have resulted in ECL becoming a significant detection method in analytical chemistry and microscopy [23,30,51–60]. Commercial ECL systems for clinical diagnostics using standard ECL pair, Ru(bpy) 32+ , or one of its derivatives, and TPrA run over 1.3 billion tests per year [26,60]. Organometallic compounds are in focus for the development of ECL-emitting species due to their ECL nature. In order to advance the sensitivity, stability, and reproducibility of ECL biosensors, it is particularly important to optimize luminophores in terms of robust ECL signals and stability. Conventional luminophores, including luminol, Ru(bpy) 32+ , gC 3 N 4 , and derivatives, all have excellent ECL responses, but their utilization in an ECL sensor can be affected by their stability in water solutions, or reduced contact probability with the co-reactant. Ru(bpy) 32+ species and their derivatives are considered to be the most efficient luminophores as they possess excellent electrochemical and spectroscopic properties. Ru(bpy) 32+ can be applied in both water and organic solutions depending on the counter-ion, and also immobilized on the electrode surface. For instance, Ru(bpy) 32+ is stable in the solution, and electrochemical oxidation can generate the reactive species Ru(bpy) 32+ (Figure 1). Classical anodic co-reactants are tertiary, secondary, and primary alkyl amine groups (especially TPrA or DBAE) and oxalate [29,51–53]. For instance, after its heterogeneous oxidation at the electrode surface or homogeneous oxidation via Ru(bpy) 32+ , the TPrA ●+ cation radical deprotonates rapidly to form the reducing neutral radical, TPrA ● . Ru(bpy) 33+ is then reduced exergonically via TPrA ● , forming the excited state Ru(bpy) 32+∗ (Equation (1)), which decays to the ground state and emits orange-red light [54–57]. Figure 1. Overview of the ECL process for the most common system used, consisting of the luminophore, Ru(bpy) 32+ , and the tri-n-propylamine (TPrA) co-reactant. The ECL reagents are generated in situ at the electrode using cyclic voltammetry or chronoamperometry by sweeping the applied potential. Upon the oxidation of both the luminophore and co-reactant, the formed activated species further interact to form the excited state Ru(bpy) 32+∗ resulting in an ECL emission. Ru 2+ represents Ru(bpy)32+. Organometallic compounds are in focus for the development of ECL-emitting species due to their ECL nature. In order to advance the sensitivity, stability, and reproducibility of ECL biosensors, it is particularly important to optimize luminophores in terms of robust ECL signals and stability. Conventional luminophores, including luminol, Ru(bpy) 32+ , g-C 3 N 4 , and derivatives, all have excellent ECL responses, but their utilization in an ECL sensor can be affected by their stability in water solutions, or reduced contact probability with the co-reactant. Ru(bpy) 32+ species and their derivatives are considered to be the most efficient luminophores as they possess excellent electrochemical and spectroscopic properties. Ru(bpy) 32+ can be applied in both water and organic solutions depending on the counter-ion, and also immobilized on the electrode surface. For instance, Ru(bpy) 32+ is stable in the solution, and electrochemical oxidation can generate the reactive species Ru(bpy) 32+ (Figure 1). Classical anodic co-reactants are tertiary, secondary, and primary alkyl amine groups (especially TPrA or DBAE) and oxalate [ 29 , 51 – 53 ]. For instance, after its heterogeneous oxidation at the electrode surface or homogeneous oxidation via Ru(bpy) 32+ , the TPrA •+ cation radical deprotonates rapidly to form the reducing neutral radical, TPrA • . Ru(bpy) 33+ is then reduced exergonically via TPrA • , forming the excited state Ru(bpy) 32+∗ (Equation (1)), which decays to the ground state and emits orange-red light [54–57]. Ru(bpy)3+ 3+TPrA• Ru(bpy)+ 3+TPrA•+)→Ru(bpy)2+∗ 3+products (1) (2) Ru(bpy)+ 3+Ru(bpy)3+ 3→Ru(bpy)2+* 3+Ru(bpy)2+ 3(3) Furthermore, the excited state of Ru(bpy)2+ 3 can be produced via three different routes: (1) Ru(bpy) 33+ with reduction by TPrA • as explained above, given in Equation (1); (2) Ru(bpy) 3+ with oxidation by TPrA •+ radical cation (Equations (2) and (3)); and (3) the Ru(bpy)33+ and Ru(bpy)3+annihilation reaction (Equation (3)). Thus, in ECL reactions, light emissions lead to the regeneration of ruthenium complexes, making the ECL methodology reusable and highly attractive from an analytical point of view. Therefore, they act as labels for ECL bioassays. In addition, several advan- Materials 2023,16, 7502 5 of 20 tages exist for immobilizing luminophores, and they include improved sensitivity due to the concentration of emitter centers in the detection region near the electrode’s surface and reduced chemical consumption, particularly impacting flow systems. Hence, the integration of luminophores in materials with a high porosity and specific surface area, such as MOF, can significantly increase the ECL sensor’s analytical performance. Even if TPrA is an effective co-reactant for Ru(bpy) 32+ ECL, there are well-known backdowns associated with its properties. Crucially, TPrA is highly toxic (LD 50 oral: 98 mg/kg, LC 50 inhalation: 1500 mg/m 3 ) and very volatile. The co-reactant ECL technology is an essential part of all commercially available ECL analytical instrumentations. Co-reactants are more comfortable to work with not only in aqueous media but also in physiological conditions (pH~7.4). Finding new co-reactants with a high ECL efficiency for bioassays is a constant driving force in this area [ 25 , 51 , 58 – 61 ]. Among others, persulfate is the first example of a co-reactant ECL system produced by applying cathodic potential [ 62 ]. The application of MOFs as luminophore carriers for pollutant detection enabled the use of persulfate as a less toxic and efficacious co-reactant, reaching extremely low LOD in water pollutant detection such as femtomolar, and making the whole system more environmentally friendly [ 63 ]. Nevertheless, ECL MOFs are, therefore, promising materials for the development of clinical diagnostic assays using a non-toxic ECL system. 3. MOFs for ECL Sensors Metal–organic frameworks represent an attractive group of highly ordered crystalline coordination polymers shaped via the coordination of metal ions/clusters and organic bridging linkers/ligands. Taking into account the unique structures and properties of MOFs, which include high surface area, tailorable pore size, the high density of active sites, and high catalytic activity, different MOF-based sensing platforms have been designed for environmental contaminant detection and purification involving anions, heavy metal ions, organic compounds, and gases. Figure 2shows the articles published for “Environmental pollutant” and “MOF for Environmental Pollutant”, along with the future trends according to their publication rate in the last twenty years. Materials 2023, 15, x FOR PEER REVIEW 6 of 20 (a) (b) Figure 2. Environmental pollutants and metal–organic frameworks in the literature in the period 2002–2022. (a) Values were obtained by searching “Environmental pollutants” and (b) “Environmental pollutants-Metal–organic frameworks” in Scopus (solid lines). Trends were obtained via fitting a tendency curve and projecting it for the next 4 years (dotted lines). Due to their high chemical stability, MOFs have been exploited not only as a promising sensing material but also as superior adsorbents of different environmental pollutants from both soil and water. In comparison with MOFs, different porous sorbents like zeolites, activated carbon, and others have several disadvantages, including material stability, high density, a lack of structural tenability, and low uptake capacity or selectivity. MOFs of different sizes and morphologies can be controllably produced using various synthesis methods such as sonication, electrochemical, hydro/solvothermal, mechanochemical, microwave, etc., [64–67]. The diverse MOFs obtained, which can be a class of 2D or 3D microporous materials, have emerged as prominent materials for water contaminant research. In these materials, porous structures are assembled using metal cation salts or clusters linked with polydentate organic ligands with coordination-type connections. They can also be combined with other materials, such as nanoparticles, to form advanced nanocomposite materials. For instance, MOFs combined with conductive nanoparticles show exceptional electron conductivity, while MOFs alone have poor conductivity. In this review, we explore the relationship between the characteristics of ECL-active MOFs and their application for the detection of water pollutants (Figure 3). Electroactive luminophores can be easily incorporated in MOFs due to their nanoscale and ordered porosity or through metal ion chelation to generate ECL-active MOFs [68]. Luminophores can be incorporated in MOFs during their synthesis or via post-synthesis modifications. The integration of MOFs is achieved through functional nanomaterial design by monitoring and tuning photophysical and photochemical properties and changing the structure of organic linkers, metal clusters, and guest species. For instance, when Ru complexes are integrated into 2D MOF nanosheets during the synthesis, a significant improvement in ECL luminescence efficiency was obtained [69]. In comparison to Ru complexes alone, the resulting emitter within the composite has a high level of mobility inside frameworks with restricted intramolecular rotation and exhibits enhanced charge delocalization. Moreover, an additional increase in ECL efficiency can be obtained by integrating Ru complexes in MOFs doped with other transition metals. The doping of MOFs with transition metals improves their electrical conductivity. Zhao and coworkers [70] showed that introducing a ruthenium pyridine complex in Ni-MOFs to produce NiRu-MOFs can lead to a significant boost in ECL efficiency compared to pure Ni-MOFs. Another approach consists of the encapsulation of the luminophore during the growth of MOF. For instance, Dong et al. [71] encapsulated Ru(bpy)32+ within mesoporous and hollow MIL-101(Al)–NH2 to which the co-reactant, poly(ethylenimine), was covalently linked. The co-reactant prevented luminophore leakage and enabled a self-enhanced ECL Figure 2. Environmental pollutants and metal–organic frameworks in the literature in the period 2002–2022 . ( a ) Values were obtained by searching “Environmental pollutants” and ( b ) “Environmental pollutants-Metal–organic frameworks” in Scopus (solid lines). Trends were obtained via fitting a tendency curve and projecting it for the next 4 years (dotted lines). Due to their high chemical stability, MOFs have been exploited not only as a promising sensing material but also as superior adsorbents of different environmental pollutants from both soil and water. In comparison with MOFs, different porous sorbents like zeolites, activated carbon, and others have several disadvantages, including material stability, high density, a lack of structural tenability, and low uptake capacity or selectivity. MOFs of different sizes and morphologies can be controllably produced using various synthesis methods such as sonication, electrochemical, hydro/solvothermal, mechanochemical, microwave, Materials 2023,16, 7502 6 of 20 etc., [ 64 – 67 ]. The diverse MOFs obtained, which can be a class of 2D or 3D microporous materials, have emerged as prominent materials for water contaminant research. In these materials, porous structures are assembled using metal cation salts or clusters linked with polydentate organic ligands with coordination-type connections. They can also be combined with other materials, such as nanoparticles, to form advanced nanocomposite materials. For instance, MOFs combined with conductive nanoparticles show exceptional electron conductivity, while MOFs alone have poor conductivity. In this review, we explore the relationship between the characteristics of ECL-active MOFs and their application for the detection of water pollutants (Figure 3). Electroactive luminophores can be easily incorporated in MOFs due to their nano-scale and ordered porosity or through metal ion chelation to generate ECL-active MOFs [ 68 ]. Luminophores can be incorporated in MOFs during their synthesis or via post-synthesis modifications. The integration of MOFs is achieved through functional nanomaterial design by monitoring and tuning photophysical and photochemical properties and changing the structure of organic linkers, metal clusters, and guest species. For instance, when Ru complexes are integrated into 2D MOF nanosheets during the synthesis, a significant improvement in ECL luminescence efficiency was obtained [ 69 ]. In comparison to Ru complexes alone, the resulting emitter within the composite has a high level of mobility inside frameworks with restricted intramolecular rotation and exhibits enhanced charge delocalization. Moreover, an additional increase in ECL efficiency can be obtained by integrating Ru complexes in MOFs doped with other transition metals. The doping of MOFs with transition metals improves their electrical conductivity. Zhao and coworkers [ 70 ] showed that introducing a ruthenium pyridine complex in Ni-MOFs to produce NiRu-MOFs can lead to a significant boost in ECL efficiency compared to pure Ni-MOFs. Another approach consists of the encapsulation of the luminophore during the growth of MOF. For instance, Dong et al. [ 71 ] encapsulated Ru(bpy) 32+ within mesoporous and hollow MIL-101(Al)–NH 2 to which the co-reactant, poly(ethylenimine), was covalently linked. The co-reactant prevented luminophore leakage and enabled a self-enhanced ECL response. Post-synthesis modifications of MOFs are possible due to well-defined pore sizes and their charge via some linkers [44,68,72,73]. Materials 2023, 15, x FOR PEER REVIEW 7 of 20 response. Post-synthesis modifications of MOFs are possible due to well-defined pore sizes and their charge via some linkers [44,68,72,73]. Figure 3. Overview of MOF structures and their applications for water pollutant assessment. Although ECL-based MOFs just recently opened a new horizon for highly sensitive targeted bioanalysis, the performance of each of these MOFs alone does not meet the requirements for signal amplification. Recently, luminescent MOFs, which belong to the group of multifunctional MOFs, were designed as highly crystallized ECL emitters in an aqueous medium [74]. These MOFs demonstrated exceptional performance with surface state models in both the co-reactant and annihilation of ECL in the aqueous medium. In comparison with individual elements, the framework structure of multifunctional MOFs significantly upgrades the emission of ECL. A high-stability self-enhanced ECL emission can be achieved and realized by the accumulation of MOF cation radicals via pre-reduction electrolysis. These MOFs enable a proof of concept that molecular crystalline materials can be applied as new ECL emitters. Table 1 shows some recent examples of ECL-active MOFs applied in the sensing of potentially toxic species, the linear detection range (LDR), the type of co-reactants, and the type of the real sample’s medium. Sensor performances were in accordance with provisional guideline values for pollutant concentrations in drinking water regarding the limit of detection for the adequate analytical detection method as recommended in the drinking-water quality guidelines provided by WHO [75]. Figure 3. Overview of MOF structures and their applications for water pollutant assessment. Although ECL-based MOFs just recently opened a new horizon for highly sensitive targeted bioanalysis, the performance of each of these MOFs alone does not meet the Materials 2023,16, 7502 7 of 20 requirements for signal amplification. Recently, luminescent MOFs, which belong to the group of multifunctional MOFs, were designed as highly crystallized ECL emitters in an aqueous medium [ 74 ]. These MOFs demonstrated exceptional performance with surface state models in both the co-reactant and annihilation of ECL in the aqueous medium. In comparison with individual elements, the framework structure of multifunctional MOFs significantly upgrades the emission of ECL. A high-stability self-enhanced ECL emission can be achieved and realized by the accumulation of MOF cation radicals via pre-reduction electrolysis. These MOFs enable a proof of concept that molecular crystalline materials can be applied as new ECL emitters. Table 1shows some recent examples of ECL-active MOFs applied in the sensing of potentially toxic species, the linear detection range (LDR), the type of co-reactants, and the type of the real sample’s medium. Sensor performances were in accordance with provisional guideline values for pollutant concentrations in drinking water regarding the limit of detection for the adequate analytical detection method as recommended in the drinking-water quality guidelines provided by WHO [75]. Table 1. MOF-based ECL sensing of water pollutants. MOF Type Analyte Limit of Detection (LOD) Type of MOF Synthesis Linear Detection Range (LDR) Co-Reactants Medium Reference Ru-Zn: MOF Ru(bpy)32+ 1,3,5-benzentriic acid Ag+/Hg2+ 0.00298–0.00032 pM Electrodeposition, electrochemical synthesis 0.001–1000 pM/ 0.01–10,000 pM K2S2O8Seawater, water [76] Ag-MOF@CS @(Au-NPs) Hg2+ 66 fM Ultrasonic, solvothermal 300 fM–1 µMK2S2O8Water, lake water [63] NH2-SiO2/Ru(bpy)32+- UiO66 Pb2+ 1.0 ×10−7µM1.0 ×10−6– 1.0 ×10−2µMTEA Water, tap water [77] Ru-MOFs H2S2.5 ×10−12 mol L−11.0 ×10−11 mol L−1– 1.0 ×10−4mol L−1 NBD-amine 7-nitro-1,2,3benzoxadiazole amine Water, human serum samples [78] S2-Fc/S3/S1-AgNPs @Ru-MOF Anatoxin-a 0.034 µg/mL Solvothermal 0.001–1 mg/mL TPrA Lake and river water [79] Ru-Cu MOF Microcystin-LR 0.143 pg/mL Ultasonication 0.0001–50 ng/mL TPrA Tap water [80] Hf-MOF/Ir2PD/APS/ ITO Acetamiprid 0.0025 nM Directional self-assembling 0.01–10 nM TPrA Pakchoi [81] CdTe@ZnNi-MOF Chlorpyrifos 6.23 ×10−17 MBlending 1.0 ×10−14– 1.0 ×10−9MLuminol-O2Vegetables [82] Co-Ni/MOF Chloramphenicol 2.9 ×10−14 MSolovothermal 1.0 ×10−13– 1.0 ×10−6M BP/PTCNH2)/S2O8with K2S2O8 Tap water [83] Hollow Cu/Co-MOF Acetamiprid and malathion 0.015 pM/ 0.018 pM In situ, solvothermal 0.1 µM–0.1 pM Luminol H2O2, K2S2O8 Apple and tomato [84] UCNPs/Pt@MOF Diethylstilbestrol 3.8 fg/mL Layer-by-layer growth method 0.1 pg/mL to 30 ng/mL CBS H2O2 Tap and river water [85] Ru(bpy)32+/UiO-67 Diethylstilbestrol 3.27 fg/mL Solvothermal 0.01 pg/mL to 50 ng/mL TPrA Urine [86] Eu(II)-MOFs Trenbolone 4.42 fg/mL 10 fg/mL–100 ng/mL TPrA River water [87] CDs@HKUST-1 Catechol 3.8 ×10−9mol/L Hydrothermal synthesis 5.0 ×10−9–2.5 ×10−5mol/L K2S2O8Tea sample [88] NH2-Zr-MOF DEHP 2.43 ×10−13 mg/mL 1.0 ×10−12– 1.0 ×10−4mg/mL K2S2O8 River and urban drinking water [89] Ru-MOF 5-fluorouracil 0.031 pg/mL Ultasonication 0.0001–100 ng/mL K2S2O8Serum [90] PCN-222@CdSe p-PNP 0.03 ppb Solvothermal 100 ppm to 0.1 ppb K2S2O8Lake and tap water [91] PtNPs@Ce-MOFs Trenbolone 3.61 fg/mL One-pot solvothermal 10 pg/mL–100 ng/mL K2S2O8River water [92] 4. Applying ECL-Active MOFs in Water Pollutant Sensing 4.1. ECL MOF Sensors for Heavy Metals Detection Waterbody contamination with heavy metals is a critical issue that adversely affects humans, plants, and animals. Heavy metal pollution has been found in sediments of rivers, lakes, and other waters and is a reason for significant concern because of their enrichment, concealment, persistence, and toxicity. A large number of scientific reports deal with the improvement of systems for the detection of heavy metals compared to time-consuming classical analytical techniques, including atomic absorption spectrometry and inductively Materials 2023,16, 7502 8 of 20 coupled plasma—optical emission spectrometry. The application of MOFs as platforms for sensing and capturing heavy metals has significantly increased due to their high surface area, tunable pore chemistry, and fast adsorption kinetics. Currently, photoluminescence is the most exploited method for detecting heavy metals with MOFs, and in this case, interactions between the pollutants and MOFs modify luminescent properties [ 93 , 94 ]. Among the luminescent MOFs, MOFs with luminophores that exhibit ECL signals have drawn a lot of attention as emitters for the sensitive detection of heavy metals. Shan et al. [ 77 ] reported on an ECL sensor for the highly sensitive and selective detection of Pb 2+ based on Ru(bpy) 32+ and the encapsulated UiO66 metal–organic framework. The nanocomposite contained Ru(bpy) 32+ —UiO66 MOF and –NH 2 —group functionalized silica nanoparticles (NH 2 -SiO 2 ). The large surface area of NH 2 -SiO 2 ensured an excellent platform for ECL sensing. The encapsulation of Ru(bpy) 32+ in UiO66 MOF significantly enhanced the ECL efficiency of the suggested sensor. The good linear relationship of the quenched ECL intensity within Pb 2+ concentrations in the range from 1.0 ×10−6to 1.0 ×102µM with a detection limit of 1.0 × 10 −7µ M was obtained under optimal conditions. The oxidation-initiated reductive excitation pathway (vide supra) represented the ECL excitation route of Ru(bpy) 32+ . The co-reactant used in the generation of ECL was triethylamine (TEA) dissolved in a buffer solution. The sensor operated at pH 7.5. At a pH lower than 7.5, the signal was low due to the inhibition of the co-reactant deprotonation, while an alkaline environment caused the precipitation of Pb 2+ . The potential ECL mechanism of Ru(bpy) 32+ /TEA was proposed, suggesting a similar route as in the standard system of Ru(bpy) 32+ /TPrA. Regarding its application, the proposed ECL sensor displayed its detection ability with an average concentration of Pb(II) of 9.99 nM in tap water. This was in accordance with the WHO guidelines, prescribing 1 µ g/L as LOD via AAS and a practical quantification limit in the region of 1–10 µ g/L [ 75 ]. Moreover, the sensor showed good stability since the ECL intensity was stable for 14 scan cycles. In another study, Jin et al. [ 95 ] synthesized a silver–MOF composite (Ag-MOF) with terephthalic acid. The silver ion was used as the ECL luminophore for an aptamer sensor to detect mercury ions in water. To improve the ECL stability of the Ag-MOF, chitosan and gold nanoparticles (Au NPs) were additionally attached to the composite. The ECL response was obtained using K2S2O8as a co-reactant. Recently, a di-functional ECL sensor utilizing Ru-MOFs and the strand-displacementamplification reaction was proposed for the ultrasensitive detection of two heavy metal ions, Hg 2+ and Ag + , using K 2 S 2 O 8 as a co-reactant [ 76 ]. Several improvements were proposed. First, the electrochemical method was applied using Ru(bpy) 32+ and 1,3,5-benzentriic acid to prepare Ru(bpy) 32+ -functionalized MOFs (Ru-MOFs) under mild conditions. In most developments, the processes applied to synthesize functionalized MOF take a long time as they include complex reaction steps and harsh conditions. Secondly, the detection step included dissolved oxygen in the reaction system and utilized its significant quenching effect on the ECL signal generated by Ru(bpy) 32+ [ 96 ]. For this, a low concentration of hemin was used as a quencher O 2 [ 97 , 98 ] in order to ultimately and indirectly enhance the ECL signal. Finally, carboxyl groups in Ru-MOFs films formed on a glassy carbon electrode (GCE) were activated using EDC/NHS to immobilize the DNA H1 oligomer in the solution containing hemin. Hemin was bound to the guanine-rich part of DNA H1, forming a G-quadruplex structure (Figure 4). The ECL signal of Ru(bpy) 32+ for trace amounts of Ag + was recorded after adding EDTA, while Hg 2+ was detected in the solution containing cytosine-rich cDNA to mask Ag + . This ECL sensor operated at pH 7.4 and at 37 ◦C showed the relative standard deviation (2.62–3.37%) and recovery rates ( 93.43–105.49% ) when applied in seawater. These values were in acceptable ranges regarding the standard regulative for these two heavy metals. The sensor had reliable storage stability when placed at 4 ◦C and provided an unchanged signal for 30 cycles of ECL responses. Materials 2023,16, 7502 9 of 20 Figure 4. Schematic diagram of ECL ultrasensitive sensing of Hg 2+ ( A ) and Ag + ( B ). Adapted with permission from [76]. 4.2. ECL MOF Sensors for CEC Detection Contaminants of emerging concern (CEC) is a definition for different substances present in the environment that were not detected before, or at least not in significant amounts [ 99 ]. Nowadays, various CECs, including personal care products, pharmaceuticals, hormones, and industrial chemicals released into the environment. Their potential toxicity points out that additional detection methods and new regulations are needed to enable their control and prevention. Besides presenting risks to human health, the release of various CECs in nature, even at trace level concentrations, has negative effects on animals and the environment. For instance, steroid hormones can induce feminization or masculinization in aquatic fauna, while traces of antibiotics contribute to bacteria becoming more resistant. In the context that most CECs have unknown toxicity, the United States Environmental Protection Agency and the European Commission have instigated EDSP and REACH programs, respectively, to investigate the toxicity and endocrine disruptive properties of different CECs. By contrast, many developing and underdeveloped countries still do not have such programs despite the increasing presence of pharmaceuticals, chemicals, and hormones in domestic sewage and surface water sources. The quantitative analysis of CECs requires sophisticated and sensitive analytical instruments, as mentioned above. MOF ECL sensors could be the answer to such limitations. In most of them, recognizing the need to target CEC relies on the utilization of specific antibodies or aptamers that are immobilized in MOFs. For example, a new competitive ECL immunosensor platform has been designed for the detection of diethylstilbestrol (DES) via the encapsulation of Ru(bpy) 32+ in UiO-67 MOF [ 86 ]. The interaction with DES resulted in the activation of Ru-UiO-67 MOF as a luminophore and enhanced ECL signal emissions. DES is a synthetic non-steroidal estrogen causing reduced fertility upon in utero exposure. 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