Zn(II)/Cd(II) based mixed ligand coordination polymers as fluorosensors for aqueous phase detection of hazardous pollutants

Abstract

Environmental remediation, particularly in the context of water pollution, has been at the centre of a range of research domains over the past few decades. Metal organic frameworks (MOFs) or coordination polymers (CPs) are a relatively new class of infinite materials comprising metal nodes and linkers as repeat units. Exclusive merits of these materials such as tuneable porosity, and physical and chemical properties reflect in their extensive applications in fields such as sensing, adsorption, separation, catalysis, etc. In the present study, we evaluate the application of Zn(II)/Cd(II) based mixed ligand luminescent MOFs/CPs towards addressing the concerns related to water pollution. Firstly, the origin of photoluminescence in MOFs/CPs is discussed, which underlines the merits of Zn(II) and Cd(II) metal ions, mixed linker systems as well as the delicate host–guest interactions between the framework and analyte. The hydrolytic and chemical stability of the luminescent MOFs/CPs is prerequisite for their in-field sensing applications, which are critically studied in the context of Cd(II)/Zn(II) and mixed linker based luminescent MOFs/CPs. Subsequent sections separately underscore the important studies on sensing of organic pollutants viz., nitroaromatics, acetone, acetylacetone and paraquat as well as inorganic pollutants including di- and trivalent metal cations viz, Hg²⁺, Cu²⁺, Pb²⁺, Pd²⁺, Al³⁺, Fe³⁺ and Cr³⁺; and inorganic anions such as cyanide, iodide, chromate, dichromate, manganate etc. Efforts to develop paper strip based versatile sensors for noxious ions are also highlighted.

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Bhavesh Parmar

Bhavesh Parmar was born in Junagadh, Gujarat, India. He obtained his Masters (2012) in Chemistry from the Department of Chemistry, Saurashtra University. He subsequently joined the research group of Dr E. Suresh at CSIR-CSMCRI and is pursuing his doctoral work in the area of structural and functional features of multidimensional coordination polymers. His main area of research interest is the design and synthesis of multidimensional coordination polymers/metal–organic frameworks and their functional aspects such as photoluminescence sensing of organic/inorganic pollutants, CO2 utilization of value-added chemicals and heterogeneous catalysis. While working with Dr E. Suresh, he also gained knowledge and experience in small molecule crystallography.

Kamal Kumar Bisht

Dr Kamal Kumar Bisht was born (1984) in Gangolihat, Uttarakhand, India. He obtained his Masters from Govt PG College, Pithoragarh in 2006 and subsequently joined the research group of Dr E. Suresh at CSIR-CSMCRI and obtained his PhD in 2014 for the design and synthesis of coordination polymers. He joined UPES, Dehradun as Assistant Professor in 2014 and two years later moved to RCU Govt PG College, Uttarkashi through UKPSC. He was recognized by RSC Chem. Soc. Rev. as one of the outstanding reviewers for the journal in 2016 and has been an affiliate of IUPAC since 2018. His research interests lie in CPs, MOFs, hybrid materials and environmental remediation. He presently has around 40 research articles and book chapters to his credit.

Yadagiri Rachuri

Dr Yadagiri Rachuri obtained his BSc from Osmania University and Masters from the Department of Chemistry, Pondicherry University. He joined the research group of Dr E. Suresh at CSIR-CSMCRI in 2012 as a junior research fellow (JRF) and received his Ph.D. on the synthesis, crystal structure and applications of multidimensional coordination polymers in 2018. Subsequently, he had been to South Korea and carried out his first post-doc at Pusan National University under the supervision of Prof. Dae-Won Park on MOFs as heterogeneous catalysts for the conversion of CO2 to cyclic carbonates. Recently, he moved to the King Abdullah University of Science and Technology (KAUST), KSA and is currently pursuing his post-doctoral research with Prof. Kuo-Wei-Huang at the KAUST Catalysis Centre. His research interest focuses on the design and synthesis of metal organic frameworks (MOFs), covalent organic frameworks (COFs) and pincer complexes based on porous organic polymers as heterogeneous catalysts for CO2 conversion and dehydrogenation of formic acid.

Eringathodi Suresh

Dr Eringathodi Suresh received his Ph.D. degree from Maharaja Krishnakumarsinhji Bhavnagar University, Gujarat, India for his work on synthesis and X-ray crystallographic studies on coordination polymers executed at CSIR-CSMCRI, under the guidance of Dr Mohan Bhadbhde. Upon completion of his PhD, he was awarded JSPS postdoctoral fellowship and worked with Prof. Katsuya Inoue, at the Institute of Molecular Science, Okazaki, Japan. He is also the recipient of a CSIR-DAAD fellowship for senior scientists and collaborated with Prof. Christof Wöll at the Karlsruhe Institute of Technology. For the last two decades, he has been working at CSIR-CSMCRI and continuing as a Senior Principal Scientist. The research focus of his team is on the design and synthesis of novel mixed ligand functional coordination polymers for chemical sensing, heterogeneous catalysis and also some basic aspects of chiral coordination polymers.

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Introduction

Over the past few decades, growing attention has been paid to environmental issues, and one of the major challenges is the problem related to the chemical pollutants from industries and other sources. Among a range of water pollutants, nitroorganics, volatile organic compounds (VOCs), and ionic species of different chemicals are of major concern because they are noxious and have human health and environment-related implications. Particularly, many organic nitro compounds and organic solvents are used in many industries, and their eventual discharge into the wastewater leads to environmental problems and adverse effects to aquatic bodies. Hence, the detection and sensing of the mentioned hazardous chemicals, as well as their containment, have remained urgent issues over the past several years. Among the available methods, fluorescent sensors owing to their high sensitivity, fast response, and user-friendly operation have attracted extensive research interest in recent years. On the other hand, chromatographic and other spectroscopic methods used as detection tools are cumbersome and inefficient due to a longer processing time, low sensitivity, and high operational cost. Hence, the development of novel sensor materials with a simple and lucid strategy for the selective and judicious detection of trace levels of pollutants is obligatory to avoid the risk of toxicity.^1–10

So far, there has been considerable progress towards the development of sensors for organic and inorganic pollutants using a variety of materials such as metal organic frameworks (MOF) or coordination polymers (CP), small organic probes, oligomers, nanomaterials, linear conjugated polymers, etc. Luminescent metal organic frameworks (LMOFs) or luminescent coordination polymers (LCPs) have emerged as potential sensor materials owing to their structural diversity, tailorable pore size, and thermal and chemical stabilities. Notably, LMOFs or LCPs incorporating functional linkers and d¹⁰ or Ln metal nodes can be developed as fluorescent sensors. In fact, characteristic luminescence features of LMOFs/LCPs can be modulated by judicious combinations of metal nodes and the linker features such as spacer length, steric hindrance, π-electron availability, and conjugation and the working principle of LCPs as sensors is based on the photoluminescence properties which arise upon the relaxation of the electrons from the excited state to the ground state with photon emission. In sensing application, supramolecular interactions of LCPs with the analyte could increase or decrease the luminescence emission leading to the turn-on or turn-off display respectively.^11–21

The detection of hazardous pollutants by LCPs has often been investigated in organic solvents. The scarcity of aqueous phase detection studies may be associated with the lack of chemical stability of LCPs in water or in the presence of analytes of interest. Hence, the chemical and hydrolytic stability of LCPs/MOFs remains a prerequisite for in-field application towards sensing of noxious analytes. Notably, several strategies to improve the water stability of MOFs/CPs have been reviewed recently.^22,23 There have been a few reviews referring to the designed synthesis of LCPs involving a single ligand moiety for the detection of nitroaromatics (NACs) and organic/inorganic pollutants in organic media.^{1–3,15–18,21} Herein, we present recent developments in the fabrication of hydrolytically stable Zn(II)/Cd(II) based LCPs using a dual ligand strategy. A summary of the updated literature on the utilization of LCP materials for the aqueous phase detection of organic/inorganic pollutants has been presented in tabulated forms, and significant contributions in the selected research field have been discussed. We have mainly focused on the utilisation of the mixed ligand Zn(II)/Cd(II) based LCPs/LMOFs for the aqueous phase detection of organic pollutants (NACs/hazardous organic solvents) and hazardous cationic metal ions (viz., Pb⁺², Hg⁺², Fe⁺³, Pd⁺², Cr³⁺etc.) and anionic species such as CrO₄²⁻, Cr₂O₇²⁻, MnO₄⁻, CN⁻, I⁻, etc.

Photoluminescence and origin of luminescence in LCPs

The emission of light producing from the excited states upon the absorption of energy is termed luminescence. Generally, two types of luminescence called fluorescence and phosphorescence have been observed by radiative relaxation from the excited states accompanying the emission of photons. Fluorescence is a spin-allowed transition in which the light emission occurs between the energy states of the same spin multiplicity between the excited electronic state (S₁) and the ground state (S₀) with lifetimes in a nanosecond range. However, in the case of phosphorescence light emission occurs between states with different spin multiplicities in which transition occurs from a lowest excited triplet electronic state (T₁) to the ground state (S₀). This forbidden electronic transition slows down the emission process resulting in the lifetimes in the range of microseconds to few seconds.^24–26 The use of fluorescent bridging ligands such as aromatic conjugated moiety, photoactive metal ions or combination of both allows the generation of luminescence properties in CPs/MOFs. The encapsulated guest molecules in porous cavities of certain CPs also contribute to the luminescence properties. Hence, the observed luminescence signal in CPs demands consideration of all the photophysical transitions. Generally, luminescence in LCPs/LMOFs arises from the building blocks such as conjugated organic ligands or metal cations/clusters as mentioned above. Organic linkers with aromatic moieties or extended π systems are commonly used for the design and synthesis of CPs due to their rigid molecular backbone. The stacking of the adjacent linkers with an electron rich π cloud can contribute to luminescence and can be classified as linker-based luminescence or LLCT. Close proximity of the confined organic luminophores with well-ordered alignment in MOFs and their intermolecular interaction can alter the luminescence properties compared to the free one. Furthermore, a functionally decorated ligand moiety as a linker in CP/MOF has a tendency to involve supramolecular interaction with the guest molecule and has a profound effect in changing the emission properties of the pristine ligand moiety. A sensible choice of the metal ion/cluster nodes present in the CP/MOF can also lead to a photoactive building unit that enhances the luminescence properties. Particularly, the confinement of conjugated ligands in combination with transition metal ions (d¹⁰) or lanthanide metal ions generates luminescence in CPs and can be designated as luminescent coordination polymers (LCPs), a subclass of CPs. π electron rich aromatic or conjugated ligand moieties by absorbing the radiative excitation energy and d¹⁰/lanthanide metal cations in these hybrid materials are capable of stimulating luminescence emission in LCPs. Electronic properties such as optical absorption and emissions of LCPs can be tuned and have a direct correlation with the band gap. Thus, band gaps can be varied by changing the size of the SBU and the degree of conjugation of the organic linkers having the same metal center and network connectivity or changes to a framework's metal centers leading to atomic compositions of the valence band (or HOMO) and/or conduction band (or LUMO) which directly relates the fluorescence properties of LCPs.^24–33

As mentioned before and previous reports revealed that metal ions/clusters, organic linker moieties, metal–organic charge transfer and guest molecules within porous/non-porous CPs/MOFs all can contribute towards luminescence through a variety of mechanisms. For example, in ligand-centred emission both photon absorption and emission processes occur on the same organic ligand as well as in separate locations within the framework through non-radiative energy and electron transfer processes enlightened by Förster–Dexter theory bridging the gap. The various active mechanisms behind the luminescence stimulation in CPs can be classified into ligand-to-ligand charge transfer (LLCT), ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), metal-to-metal charge transfer (MMCT) and guest centred emission and guest-sensitization mechanisms. In the case of CPs/MOFs with paramagnetic transition metal having unpaired electrons ligand-field transitions (d–d transition) may lead to strong reabsorption and/or quenching of luminescence generated from the organic molecule which can occur via electron or energy transfer through the partially filled d-orbitals. However, paramagnetic metal such as Cu²⁺ based MOFs comprising a π conjugated pentiptycene based ligand showed strong emission under blue light (UV range). Particularly, d¹⁰ metal centres (Zn²⁺/Cd²⁺ and Cu⁺/Ag⁺) are well known for the ligand centred transitions. Completely filled d-orbitals of metals and organic ligands are involved in two types of charge transfer namely ligand to metal charge transfer (LMCT) and metal to ligand charge transfer (MLCT). LMCT is often observed in Zn²⁺ and Cd²⁺ complexes, while MLCT is commonly present in Cu⁺ and Ag⁺ compounds. Interestingly, metal-centred luminescence is often observed when lanthanide/actinide metal nodes are assembled with conjugated organic linkers in the rigid MOF system. Luminescence in lanthanide-based LCPs is accounted for by the delivery of excitation energy through an antenna effect from the triplet excited states to the emissive states in lanthanides by strong photon absorbing ligands with efficient intersystem crossing (ISC).^{24–26,34–43} To enliven the luminescence in the CP/MOF, aromatic di or polycarboxylate with a rigid backbone along with imidazole/pyridine ligands has been commonly used and explored for various applications including the detection of nitroorganic compounds. The various aspects and effects contributing to the luminescence in CPs/MOFs have been schematically demonstrated in Scheme 1.

Scheme 1 Schematic representation for various pathways of luminescence origin in LCPs/LMOFs.^18,58

LCPs as a prospective aqueous phase fluorosensor

π-Electron-rich luminescent organic molecules and polymers are commonly used for sensing hazardous molecules by turn on/turn off fluorescence behaviour. However, the multi-step synthetic protocol and lack of porous nature, recyclability and chemical stability of these materials are some shortcomings. LCPs have numerous key advantages over other potential luminescent probes. The tuneable nature of fluorescence properties by the prudent choice of the metal nodes and conjugated organic linkers and porosity of the inorganic–organic hybrid materials uses LCPs for sensing applications. The crystalline nature of LCPs shows a precise crystal structure by SXRD analysis with exact knowledge of the atomic position and their supramolecular interaction can give insight into the detection of analytes and the chemical stability of these materials offer an added advantage. Porosity in LCPs can provide easy access to the analytes increasing the interaction between the analytes and framework enhancing the sensing ability. The porous environment of the MOFs such as hydrophobicity, polarizability, polarity, acidity and proton affinity can be finely tuned by various chemical manipulations to suit the guest molecules. By tailoring the pore environment sensitive and selective capture of the targeted analytes can be achieved. It is also demonstrated that the functionalization of the pore channels by photochromic or conjugated organic ligands and post-synthetic modification in LCPs can improve the analyte detection. Permanent porosity and cooperative luminescence properties are particularly useful in the development of luminescence sensing materials including LCPs for a variety of sensing applications. The chemical sensors based on LCPs have drawn immense attention for their significant advantages, including short response time, high sensitivity and selectivity and reusability.^{24–26,32,33}

Good emission properties, tunability of the framework topology and viability of selective supramolecular host–guest interactions between frameworks and target analytes are important assets of LCPs towards the detection of trace amounts of hazardous molecules including NACs. Sensing applications by LCPs in the organic/non-aqueous phase are well documented in the literature, but detection remains scantly explored in water may be due to the chemical instability of LCPs in aqueous media.^44–54 Since most of the MOFs are hydrophilic in nature the hydrolytic stability of these materials is a major concern for the practical implementation of these materials including sensing application. It is worth mentioning here the studies on the relative stability of notable MOFs in water and humid environments by Kaskel, and Matzger et al. followed by a few reports providing insight into the development of MOFs with hydrolytic stability and their aqueous phase sensing application.^55,56 A recent comprehensive review by Walton et al. clearly validated the progress in the design strategy and structural features of water stable MOFs.²² Coordinatively saturated metal nodes/metal clusters and the presence of a hydrophobic ligand moiety can resist the approach of water molecules towards the coordination sphere imparting hydrolytic stability in LCPs. Considering the environmental issues and human health it is mandatory to develop new user-friendly materials such as LCPs as fluorosensors with good aqueous phase stability for practical utility in the detection of various hazardous analytes including explosives.

Functional aspects: sensing and detection of pollutants by LCPs

Organic pollutants

Water contamination and environmental problems have become major issues, as large volumes of effluents with high concentrations of toxic organic pollutants are being discharged into water bodies as a result of industrialization and modern living. Governments and research groups all over the world are actively engaged in the remediation of organic waste water pollution and billions have been disbursed towards environmental protection. The detection of toxic organic pollutants such as different organic precursors and solvents used in the synthesis of value-added products, dyes, nitroaromatic compounds (NACs) etc. has always been an important challenge vis-à-vis environmental protection, human health, homeland security, and safety. For example, ketones are important raw materials which have been widely used in industrial production and their toxicity is a substantial threat to human health.^57–61

On the other hand, amines are important precursors in chemical industry; 4-nitroaniline (4-NA) is one of the integral chemicals utilized in the synthesis of azo dyes, pesticides, pharmaceuticals, photostabilizers, etc. and their accumulation in water and soil can cause health problems such as respiratory arrest, diarrhoea, skin eczema, and anaemia. Aniline based compound, 2,6-dichloro-4-nitroaniline, is a pesticide used to control diseases in fruits, vegetables and crops and is harmful to human health as well.^62,63 Another class of compounds, nitro-aromatic compounds (NACs), which include 2,4,6-trinitrophenol (TNP), trinitrotoluene (TNT), 2,4-dinitrophenol (2,4-DNP), dinitrotoluene (DNT), p-nitrophenol (4-NP), 1,3-dinitrobenzene (1,3-DNB), nitrotoluene (NT), and nitrobenzene (NB), are not only strong explosives but also used extensively as raw materials in dyes, fireworks, and the pharmaceutical and leather industries.^15–18 Furthermore, improper disposal of these pollutants causes soil as well as aquatic pollutions and efficient and selective detection of these harmful chemicals in the aqueous phase is a challenge. Hence, the development of luminescent MOFs (LMOFs) with sensing properties has influenced researchers in detecting the toxic environmental contaminants. Luminescence-based detection has gained attention in recent years due to its user-friendly methodology, good sensitivity, quick response time and easy portability. The sensing mechanism mainly relies on electron rich LCPs that undergo quenching on interactions with the electron deficient analyte molecules especially in the case of NACs. This section comprises sensing and detection of various organic pollutants which include mainly different nitroaromatic compounds (NACs), organic solvents such as acetone, acetylacetone, etc. and herbicide/antibiotic molecules such as paraquat and nitrofurazone (NZF) using Zn(II)/Cd(II) mixed ligand LMOFs in water. A comprehensive survey on the sensing and detection of organic pollutants in water medium, which mainly include NACs using Zn(II)/Cd(II) mixed ligand coordination polymers as fluorosensors is summarised in Table 1.

Table 1 List of Zn(II)/Cd(II) mixed ligand LCP/LMOF sensors for aqueous-phase detection of organic pollutants

Sr no.	LCPs/LMOFs^a	Organic analyte	K sv (M^−1)	LOD (M)	Ref.
a BPDC = biphenyl dicarboxylic acid; ad = adenine; ppvppa = N-(pyridin-2-yl)-N-(4-(2-(pyridin-4-yl)vinyl)phenyl)pyridin-2-amine; NDC = naphthalenedicarboxylic acid; Ur = urotropine; L1 = 4-amino-3,5-bis(4-imidazol-1-ylphenyl)-1,2,4-triazole; 5-asba = 2-amino-5-sulfobenzoic acid; bimbu = 1,4-bis(1H-imidazol-1-yl)butane; L2 = 4-pyridyl carboxaldehyde isonicotinoylhydrazone; BDC = benzene-1,4-dicarboxylic acid; BrIP = 5-bromoisophthalic acid; TIB = 1,3,5-tris(imidazol-1-ylmethyl)benzene or 1,3,5-tris(1-imidazolyl)benzene; bda = 2,2′-biphenyl dicarboxylic acid; L3 = 5-aminoisophthalic acid; bpy = 4,4′-bipyridine; azp = 4,4′-azobipyridine; TPOM = tetrakis(4-pyridyloxy-methylene) methane; L5 = 4,4′,4′′,4′′′-(1,4-phenylenebis(2-phenylethene-2,1,1-triyl))tetrabenzoic acid; L6 = 6-(4-carboxyphenyl)-2-naphthoic acid; tpbn = N,N′,N′′,N′′′-tetrakis(2-pyridylmethyl)-1,4-diaminobutane; tphn = N,N′,N′′,N′′′-tetrakis(2-pyridylmethyl)-1,6-diaminohexane; IPA = isophthalic acid; L7 = 3-pyridylcarboxaldehyde nicotinoylhydrazone; L8 = 3-pyridin-3-yl-N-[5-(3-pyridin-3-yl-acryloylamino)-naphthalen-1-yl]-acrylamide; CHD = 1,4-cyclohexane dicaboxylic acid; PDA = 1,4-phenylenediacetic acid; Py2TTz = 2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole; SDB = 4,4′-sulfonyldibenzoate; L9 = N4,N4′-di(pyridin-4-yl)biphenyl-4,4′-dicarboxamide; BTC = 1,3,5-benzenetricarboxylic acid; L10 = 3,5-dibromosalicylaldehyde salicylhydrazone; dpp = 1,3-di(4-pyridyl)propane; cpta = 2-4(-carboxyphenoxy)terephthalic acid; TPA = 3,3′-thiodipropionic acid; bib = 1,4-bis(imidazol-1-yl)benzene; BIYB = 4,4-bis(imidazol-1-ylmethyl)biphenyl; mbhna = 4,4′-methylene-bis[3-hydroxy-2-naphthalene carboxylic acid; tdz = thiadiazole dicarboxylate; MI = 2-methylimidazole; XN = 4′-(4-pyridine)4,2′:2′,4′′-terpyridine; BINDI = N,N′- bis(5-isophthalic acid)naphthalenediimide; bpa = 1,2-bis(4-pyridyl)ethane; bpe = 1,2-bis(4-pyridyl)ethylene; cptpy = 4-(4-carboxyphenyl)-2,2′:4′,4′′-terpyridine; BCA = bis(4-carboxybenzyl)amine; bimb = 1,4-bis(imidazol-1-ylmethyl)benzene; btdpe = 4,4′-bis(4H-1,2,4-triazol-4-yl)diphenyl ether; bidpe = 4,4′-bis(imidazolyl)diphenyl ether; L11 = 5,5′-(1,4-xylylenediamino) diisophthalic acid; opda = 1,2-phenylenediacetic acid; ppda = 1,4-phenylene-diacetic acid; NH2bdc = 2-amino-1,4-benzenedicarboxylic acid; tpt = 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine; tdc = 2,5-thiophenedicarboxylic acid; abpt = 3,5-di(2-pyridyl)-4-amino-1,2,4-triazole.
1	{[Zn8(ad)4(BPDC)6O·2Me2NH2]·G}n (1)	TNP	4.60 × 10^4	1.29 × 10^−8	64
2	[Cd(ppvppa)(1,4-NDC)]n (2)	2,4-DNP	1.18 × 10^2	—	65
3	[Zn4(DMF)(Ur)2(NDC)4]n (3)	TNP	1.08 × 10^5	7.10 × 10^−6	66
4	{[Cd(NDC)(L1)]2·H2O}n (4)	TNP	3.70 × 10^4	—	67
5	{[Cd(5-asba)(bimbu)]}n (5)	NB	1.60 × 10^4	—	68
		4-NA	9.80 × 10^4	—
		2-M-4-NA	8.10 × 10^4	—
		4-NP	6.10 × 10^4	—
6	{[Zn(BDC)(L2)]·xG}n (6)	TNP	1.16 × 10^4	3.60 × 10^−5	69
	{[Cd(BDC)(L2)]·xG}n (7)	TNP	1.35 × 10^4	1.50 × 10^−5
7	[Cd(5-BrIP)(TIB)]n (8)	TNP	2.68 × 10^4	2.70 × 10^−5	70
8	{[Cd2(tib)2(bda)2]·G}n (9)	Acetone	—	—	71
9	{[Zn2(L3)2(bpy)]·(DMF)·(H2O)2}n (10)	TNP	1.53 × 10^4	3.48 × 10^−6	72
	{[Zn2(L3)2(azp)]·(DMF)2·(H2O)}n (11)	TNP	3.11 × 10^4	1.82 × 10^−6
10	{Zn2(H2L5)2(bpy)2(H2O)3·H2O}n (12)	TNP	1.36 × 10^4	4.9 × 10^−5	73
11	[Zn(μ2-1Had)(μ2-SO4)]n (13)	TNP	3.14 × 10^4	4.00 × 10^−10	74
12	{[Zn3(L6)3(Ur)2]·2DMF·3H2O}n (14)	TNP	5.10 × 10^4	8.46 × 10^−6	75
13	{Zn2(tpbn)(2,6-NDC)2}n (15)	TNP	5.90 × 10^3	4.75 × 10^−5	76
	{[Zn2(tphn)(2,6-NDC)2]·4H2O}n (16)	TNP	2.46 × 10^3	8.11 × 10^−5
14	{[Zn(IPA)(L7)]}n (17)	TNP	2.16 × 10^4	1.20 × 10^−7	77
	{[Cd(IPA)(L7)]}n (18)	TNP	1.52 × 10^4	6.00 × 10^−8
15	{Zn4(TPOM)(1,4-NDC)4}n (19)	4-NA	7.87 × 10^4	6.40 × 10^−7	78
		2,6-DCNA	2.74 × 10^4	1.39 × 10^−6
16	[Cd(L8)(IPA)]n (20)	4-NP	1.86 × 10^4	—	79
	{[Zn2(L8)(L3)2]·3H2O}n (21)	TNP	4.19 × 10^4	6.90 × 10^−5
		4-NP	2.93 × 10^4	—
	{Cd(L8)(PDA)(H2O)2}n (22)	4-NP	1.89 × 10^4	—
	{[Cd3(L8)(CHD)3(DMF)2]·2DMF}n (23)	4-NP	3.11 × 10^4	—
17	{[Zn2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n (24)	TNP	3.25 × 10^4	9.30 × 10^−5	80
		NZF	1.72 × 10^4	9.10 × 10^−5
	{[Cd2(Py2TTz)2(BDC)2]·2(DMF)}n (25)	TNP	4.06 × 10^4	9.00 × 10^−5
		NZF	4.53 × 10^4	8.50 × 10^−5
18	{[Cd3(SDB)3(TIB)](H2O)2(1,4-dioxane)(G)x}n (26)	TNP	2.43 × 10^4	1.50 × 10^−5	81
19	{Zn(L9)(L3)(H2O)}n (27)	TNP	—	—	82
	{Zn(L9)(IPA)(DMF)(H2O)1.5}n (28)	NZF
	{Zn(L9)(HBTC)(H2O)2}n (29)
20	[Zn3(L10)2(dpp)2]n (30)	TNT	1.15 × 10^4	4.99 × 10^−5	83
21	{[Zn2(μ3-OH)(cpta)(4,4′-bpy)]·H2O}n (31)	4-NP	4.55 × 10^4	4.01 × 10^−6	84
22	[Cd(TPA)(BIB)]n (32)	NB	1.70 × 10^4	2.70 × 10^−7	85
	[Cd(TPA)(BIYB)]n (33)	2,4-DNP	1.60 × 10^5	1.30 × 10^−7
23	{[Zn2(tpbn)(mbhna)2]·4H2O.1.5DMF}n (34)	4-NA	2.23 × 10^4	5.37 × 10^−5	86
	{[Cd2(tpbn)(mbhna)2]·2DMF}n (35)	4-NA	2.87 × 10^4	1.29 × 10^−5
24	{[Cd2(tdz)2(4,4′-bpy)2]·6.5H2O}n (36)	TNP	4.86 × 10^4	6.30 × 10^−6	87
25	[Zn(NDC)(MI)]n (37)	TNP	4.32 × 10^4	5.80 × 10^−6	88
		2,6-DCNA	2.95 × 10^4	6.00 × 10^−8
26	{[Zn2(XN)2(IPA)2]·2H2O}n (38)	Acetylacetone	2.30 × 10^3	2.49 × 10^−6	89
27	[{Zn(BINDI)0.5(bpa)0.5(H2O)}·4H2O]n (39)	TNP	4.90 × 10^4	2.61 × 10^−6	90
	[{Zn(BINDI)0.5(bpe)}·3H2O]n (40)	TNP	1.29 × 10^4	5.63 × 10^−6
28	[Zn2(cptpy)(btc)(H2O)]n (41)	Paraquat	1.03 × 10^3	9.73 × 10^−6	91
29	[Zn2(H2BCA)2(o-bimb)2(H2O)2]n (42)	Acetone	3.70 × 10^4	9.00 × 10^−8	92
	{[Zn(H2BCA)(m-bib)]·H2O}n (43)	Acetone	2.00 × 10^4	1.30 × 10^−7
30	{[Zn(L11)0.5(bimbu)]·2H2O·0.5(CH3)2NH}n (44)	NB	3.38 × 10^3	8.89 × 10^−6	93
	{[Zn(L11)0.5(bimb)]·4H2O}n (45)	NB	2.85 × 10^3	1.05 × 10^−5
	{[Zn(L11)0.5(btdpe)]·H2O}n (46)	NB	3.75 × 10^3	8.00 × 10^−6
	[Zn(L11)0.5(bidpe)]n (47)	NB	4.59 × 10^3	6.54 × 10^−6
31	[Zn(opda)(bib)]n (48)	2-NP	1.33 × 10^7	2.10 × 10^−7	94
31	[Zn(ppda)(bib)(H2O)]n (49)	2-NP	1.42 × 10^7	1.86 × 10^−7	94
32	[Zn(NH2-bdc)(4,4′-bipy)]n (50)	TNP	3.10 × 10^4	3.20 × 10^−5	95
33	{Zn2(tpt)2(tdc)2·H2O}n (51)	TNP	7.80 × 10^4	2.56 × 10^−6	96
34	[Cd2(2-abpt)(Hbtca)(H2btca)0.5(H2O)]n (52)	TNP	1.63 × 10^4	3.11 × 10^−7	97

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TNP sensing in the aqueous phase by a dual ligand 3D MOF {[Zn₈(ad)₄(BPDC)₆O·2Me₂NH₂]·G}_n (1) also identified as bio-MOF-1 containing BPDC (biphenyl dicarboxylic acid) and ad (adenine) as a fluorosensor has been reported by Ghosh et al. Water stable (1) shows tremendous selectivity and sensitivity for TNP among other NACs with K_sv value 4.60 × 10⁴ M⁻¹ and limit of detection (LOD) value 1.29 × 10⁻⁸ M. Free amino functionality on the adenine linker is highly influencing the sensitivity of (1) for TNP which is further supported by adenine-TNP co-crystal generation.⁶⁴ A mixed linker two-dimensional luminescent coordination polymer [Cd(ppvppa)(1,4-NDC)]_n (2) (where ppvppa = N-(pyridin-2-yl)-N-(4-(2-(pyridin-4-yl)vinyl)phenyl)pyridin-2-amine; 1,4-NDC = 1,4-naphthalenedicarboxylic acid) reported by Lang et al. shows selective sensing of 2,4-DNP with good recyclability. The luminescence of compound (2) is ppvppa ligand centric and offers blue fluorescence in the solid-state as well in different organic solvents including water. Quenching properties vary in the range of 91–72% for 2,4-DNP, 4-NP, and 4-NA with 2,4-DNP showing the highest quenching among different pools of NACs with K_sv value 1.18 × 10² M⁻¹.⁶⁵ In another report by Ghosh and co-workers, the synthesis and sensing application of [Zn₄(DMF)(Ur)₂(NDC)₄]_n (3) composed of π-electron rich naphthalene carboxylate (NDC) and urotropine (Ur) ligands was demonstrated. A tertiary amine decorated porous MOF (3) showed excellent fluorescence properties in water and selectively detected explosive molecule TNP from a variety of NACs with K_sv 1.08 × 10⁵ M⁻¹ and LOD 7.10 × 10⁻⁶ M respectively. Interaction of the TNP hydroxyl group with an aliphatic amine on Ur also identified in the TNP-Ur co-crystal is one of the reasons behind the accurate detection of TNP via the energy transfer process (Fig. 1).⁶⁶ Similarly Su et al. reported a self-penetrating 3D chiral coordination polymer {[Cd(NDC)(L₁)]₂·H₂O}_n (4) (where L₁ = 4-amino-3,5-bis(4-imidazol-1-ylphenyl)-1,2,4-triazole) with good fluorescent properties. The crystal structure indicates helical chains in chiral CP (4) generated by a V-shaped N-donor linker L₁, which also contains an uncoordinated N-atom and amino functionality.

Fig. 1 (a and b) Emission spectra of (3) dispersed in water, upon incremental addition of aqueous TNP solution (1 mM) and quenching efficiency plot for different NACs; (c) response of (3) aqueous dispersions toward the additions of various NAC solutions under UV light (λ_max = 365 nm) (a = TNT, b = NM, c = RDX, d = 2,4-DNT, e = 2,6-DNT, f = NB, g = DMNB, h = TNP); (d) crystal structure of the co-crystal 1 Ur-TNP revealing intermolecular H-bonding interactions. Reproduced with permission from ref. 66. Copyright 2015 American Chemical Society.

The luminescence properties of amino-functionalized (4) in water were successfully utilized for the selective and recyclable sensing of TNP with K_sv value 3.70 × 10⁴ M⁻¹ and the proposed sensing mechanism involves the combination of photoinduced electron-transfer (PET) and resonance energy transfer (RET) processes.⁶⁷ Zhang et al. reported the photoluminescence sensing of various nitro derivatives NB, 4-NA, 2-M-4-NA and 4-NP using 3D Cd(II) MOF {[Cd(5-asba)(bimbu)]}_n (5) constructed from multifunctional 2-amino-5-sulfobenzoic acid (5-asba) and N-donor linker (bimbu). Multi-responsive (5) detects NACs with very good quenching constant values 1.60 × 10⁴ M⁻¹ for NB, 9.80 × 10⁴ M⁻¹ for 4-NA, 8.10 × 10⁴ M⁻¹ for 2-M-4-NA and 6.10 × 10⁴ M⁻¹ for 4-NP in water suspension.⁶⁸ Versatile synthesis of acyl amide decorated isostructural two 3D Zn(II)/Cd(II) coordination polymers reported by our group exhibits water stability and photoluminescence properties. Both mixed linker LCPs, {[M(BDC)(L₂)]·xG}_n, comprising Zn²⁺ (6) or Cd²⁺ (7), benzene-1,4-dicarboxylic acid (BDC) and N-donor ligand 4-pyridyl carboxaldehyde isonicotinoylhydrazone (L₂) cover a 2-fold interpenetrated pillared layered pcu topology. Florescence quenching performance in water towards TNP among different pools of NACs showed good selectivity and sensitivity with K_sv values 1.16 × 10⁴ M⁻¹ and 1.35 × 10⁴ M⁻¹ and limit of detection values 3.60 × 10⁻⁵ M and 1.50 × 10⁻⁵ M for (6) and (7) respectively. In addition to the PET and RET mechanism, quenching behaviour is also supplemented by H-bonding interaction between the amide group of L₂ and the hydroxyl group of TNP via co-crystal formation.⁶⁹ In another example, two dimensional Cd(II) MOFs [Cd(5-BrIP)(TIB)]_n (8) synthesized by our group using tripodal N-donor 1,3,5-tris(imidazol-1-ylmethyl)benzene (TIB) and 5-bromoisophthalate (5-BrIP) showed excellent photoluminescence properties in the solid state and in water. Fluorescence properties of the chemically stable (8) are exploited for the selective detection of TNP from a large pool of other nitroaromatics. Excellent sensing behaviour of (8) for TNP is visible from K_sv and LOD values (2.68 × 10⁴ M⁻¹ and 2.70 × 10⁻⁵ M). LMOF (8) coated on the paper strip demonstrates a fast response of TNP for the practical application of explosive detection (Fig. 2).

Fig. 2 (a) Quenching efficiency of (8) for different nitro analytes; (b) Stern–Volmer traces of various nitro analytes (0–200 μL; 2 mM) in a water suspension of (8) (2 mg/2 mL); (c) digital photographs of paper strips coated with (8) after adding various nitro analytes under UV light (365 nm); (d) the paper strip dipped in 2 mM aqueous solution of TNP for 10 s showed full darkening of paper strip of (8). Reproduced with permission from ref. 70. Copyright 2016 Royal Society of Chemistry.

Electrostatic interaction between TNP and TIB linkers as well as the energy transfer phenomenon favours the quenching mechanism.⁷⁰ Similarly, the tripodal TIB ligand based water stable MOF {[Cd₂(tib)₂(bda)₂]·G}_n (9) synthesized by Bu et al. demonstrated excellent acetone sensing via fluorescence quenching. The crystal structure of LMOF (9) exposed a double helical motif generated via 2,2′-biphenyl dicarboxylate (bda) coordination, which enhances the water stability. The aqueous solution of acetone (0.7 vol%) completely quenched the emission intensity of (9) amid other organic solvent molecules.⁷¹

Neogi et al. hydrothermally synthesized two luminescent MOFs {[Zn₂(L₃)₂(bpy)]·(DMF)·(H₂O)₂}_n (10) and {[Zn₂(L₃)₂(azp)]·(DMF)₂·(H₂O)}_n (11) comprising 5-aminoisophthalate (L₃) and N-donor pillar bpy/azp. Both pillar-layer LMOFs showed high chemical stability in water and possess good luminescence behaviour in the aqueous phase. They utilized both (10) and (11) for the selective and sensitive detection of high explosive TNP molecules with K_sv values 1.53 × 10⁴ M⁻¹ and 3.11 × 10⁴ M⁻¹ and lower detection limit values 3.48 × 10⁻⁶ M and 1.82 × 10⁻⁶ M respectively compared to other NACs. The detection mechanism revealed both static and dynamic quenching behaviour with a resonance energy transfer (RET) phenomenon. Strong non-covalent interactions observed in the co-crystal of TNP and N-donor azp also correlated to support the quenching mechanism.⁷² Highly fluorescent {Zn₂(H₂L₅)₂(bpy)₂(H₂O)₃·H₂O}_n (12) constructed from an active tetraphenylene derivative L₅ (4,4′,4′′,4′′′-(1,4-phenylenebis(2-phenylethene-2,1,1-triyl))tetrabenzoic acid) and bpy as a result of aggregation-induced emission (AIE) is reported by Zhao and co-workers. LMOF (12) showed good water stability and high quantum yield (43%) which is successfully applied for the chemical sensing of the TNP molecule showing quenching constant 1.36 × 10⁴ M⁻¹ and detection limit 4.9 × 10⁻⁵ M. The electron-donating nature of (12) is effective for the detection of electron-deficient nitro explosives with good sensitivity. The electron transfer process which is enlightened by photoinduced electron transfer (PET) and also fluorescence resonance energy transfer (FRET) reinforced by the non-linear Stern–Volmer plot for the quenching process is advocated.⁷³ Luminescent neutral Zn(II) coordination polymer [Zn(μ₂-1Had)(μ₂-SO₄)]_n (13) synthesized from biomolecule adenine (ad) reported by our group possesses excellent fluorescence properties in solid-state as well as in water suspension. The luminescence behaviour of (13) is successfully exploited for the ultrasensitive detection of TNP with K_sv 3.14 × 10⁴ M⁻¹ and a remarkable LOD value of 4.00 × 10⁻¹⁰ M.

The quenching mechanism is explained by strong electrostatic interaction between the free nitrogen atoms of adenine and TNP molecule which is responsible for electron/energy transfer processes. A possible hydrogen bonding interaction between the amino group of the adenine linker and the acidic TNP analyte also plays important roles in high sensitivity. An LCP (13) coated paper strip also has been developed for the visible detection of nitro derivatives which shows the quenching effect under irradiated UV light suggesting the practical applicability of these materials (Fig. 3).⁷⁴ Bharadwaj et al. reported the synthesis and fluorescence sensing ability of porous 2D coordination polymer {[Zn₃(L₆)₃(Ur)₂]·2DMF·3H₂O}_n (14) constructed from the naphthalene based linker L₆ (6-(4-carboxyphenyl)-2-naphthoic acid) and urotropine (Ur). Fluorescent naphthalene ligand dependent excellent emission displayed by LCP (14) is efficient to recognise the TNP explosive in the aqueous phase with quenching constant 5.10 × 10⁴ M⁻¹ and detection limit 8.46 × 10⁻⁶ M.⁷⁵

Fig. 3 (a) Emission spectra of aqueous dispersed (13) after addition of aqueous solutions of NACs; (b) emission quenching (%) of (13) upon addition of different NACs; (c) Stern–Volmer plots for (13) upon gradual addition of NACs; (d) digital images of (13)-coated test strips after they were dipped in aqueous solutions of NACs. Reproduced with permission from ref. 74. Copyright 2017 American Chemical Society.

Similarly, Mandal and co-workers demonstrated the TNP sensing ability of mixed ligand 2D LMOFs namely {Zn₂(tpbn)(2,6-NDC)₂}_n (15) and {[Zn₂(tphn)(2,6-NDC)₂]·4H₂O}_n (16). Both mixed ligand LMOFs comprise naphthalene dicarboxylate and pyridine-based N-donor linkers tpbn (N,N′,N′′,N′′′-tetrakis(2-pyridylmethyl)-1,4-diaminobutane)/tphn (N,N′,N′′,N′′′-tetrakis(2-pyridylmethyl)-1,6-diaminohexane). The selective and sensitive sensing of TNP with Stern–Volmer constant and LOD values by (15) and (16) were 5.90 × 10³ M⁻¹, 2.46 × 10³ M⁻¹ and 4.75 × 10⁻⁵ M, 8.11 × 10⁻⁵ M respectively.⁷⁶ Mechanochemically synthesized isostructural 2D dual ligand coordination polymers {[Zn(IPA)(L₇)]}_n (17) and {[Cd(IPA)(L₇)]}_n (18) comprising isophthalate (IPA) and Schiff base N-donor pillar L₇ (3-pyridylcarboxaldehyde nicotinoylhydrazone) reported by our group displayed excellent water stability and luminescence properties in the aqueous phase. Both LCPs selectively detect the TNP molecule with high quenching constant values of 2.16 × 10⁴ M⁻¹ (17) and 1.52 × 10⁴ M⁻¹ (18) among other nitroanalytes. The sensitivity of both LCPs for TNP was demonstrated by excellent limit of detection values of 1.20 × 10⁻⁷ M (17) and 6.00 × 10⁻⁸ M (18). For understanding of the sensing mechanism, spectral overlap between the UV absorption of NACs and emission of LCPs displaying a clear overlap for TNP suggesting energy transfer processes is responsible for fluorescence quenching. Fluorescence microscopy images captured under a visible/UV filter for pristine LCPs and TNP added LCPs displayed complete quenching of the emission intensity (Fig. 4).⁷⁷

Fig. 4 (a and b) Quenching (%) of aqueous suspensions of (17) and (18) for different NACs. (c and d) Stern–Volmer (SV) plots for various NACs in aqueous suspensions of (17) and (18); (e) spectral overlap between aqueous phase UV spectra of NACs and fluorescence spectra of (17) and (18); (f) microscopy images under a white light/UV filter for both CPs and under a UV filter after the addition of TNP. Reproduced with permission from ref. 77. Copyright 2017 American Chemical Society.

A mixed ligand luminescent Zn(II) based 3D MOF {Zn₄(TPOM)(1,4-NDC)₄}_n (19) constructed from tetrapodal semi-rigid N-donor linker TPOM (tetrakis(4-pyridyloxy-methylene)methane) and 1,4-naphthalene dicarboxylate (NDC) has been reported by Mandal and co-workers for amine sensing ability. MOF (19) with good chemical stability and recyclability in aqueous medium revealed excellent sensing ability towards nitro aniline derivatives 4-NA and 2,6-DCNA (pesticide) with K_sv values of 7.87 × 10⁴ M⁻¹ and 2.74 × 10⁴ M⁻¹ and LOD values of 6.40 × 10⁻⁷ M and 1.39 × 10⁻⁶ M respectively.⁷⁸ In a recent report, four mixed ligand luminescent coordination polymers containing a Cd(II)/Zn(II) metal node, naphthalene core based acylamide functionalized N-donor linker L₈ (3-pyridin-3-yl-N-[5-(3-pyridin-3-yl-acryloylamino)-naphthalen-1-yl]-acrylamide and different carboxylate derivatives have been unveiled. All LCPs [Cd(L₈)(IPA)]_n (20), {[Zn₂(L₈)(L₃)₂]·3H₂O}_n (21), {Cd(L₈)(PDA)(H₂O)₂}_n (22) and {[Cd₃(L₈)(CHD)₃(DMF)₂]·2DMF}_n (23) have been successfully utilized for the aqueous phase detection of nitrophenol derivative 4-NP with very good quenching constants 1.86 × 10⁴ M⁻¹, 2.93 × 10⁴ M⁻¹, 1.89 × 10⁴ M⁻¹ and 3.11 × 10⁴ M⁻¹ respectively. LCP (21) showed good sensing ability towards TNP also with a quenching constant of 4.19 × 10⁴ M⁻¹ with a detection limit of 6.90 × 10⁻⁵ M. Non-linear Stern–Volmer behaviour suggests static and dynamic quenching and spectral overlap between the emission fluorophore and analyte absorption band supporting the resonance energy transfer (RET) quenching mechanism in operation.⁷⁹ Doubly interpenetrated 3D fluorescent Zn(II)/Cd(II) MOFs {[Zn₂(Py₂TTz)₂(BDC)₂]·2(DMF)·0.5(H₂O)}_n (24) and {[Cd₂(Py₂TTz)₂(BDC)₂]·2(DMF)}_n (25) assembled using thiazole based pyridyl linker Py₂TTz (2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole) and benzene dicarboxylate (BDC) reported by Zang and co-workers were revealed as potential fluorosensors for TNP and antibiotic NZF.

Crystallographic analysis revealed that both LMOFs possess three dimensional nets with 2-fold interpenetration and pcu topology. Good fluorescence properties of (24) and (25) in the aqueous phase selectively detect trace amounts of explosive molecule TNP with LOD values 9.30 × 10⁻⁵ M and 9.00 × 10⁻⁵ M as well as antibiotic NZF with LOD values 9.10 × 10⁻⁵ M and 8.50 × 10⁻⁵ M respectively. Excellent K_sv values for TNP 3.25 × 10⁴ M⁻¹ (24) and 4.06 × 10⁴ M⁻¹ (25) and for NZF 1.72 × 10⁴ M⁻¹ (24) and 4.53 × 10⁴ M⁻¹ (25) demonstrate the selectivity among other NACs/antibiotics. The sensing mechanism is corroborated with density functional theory (DFT) calculations via HOMO–LUMO energy differences and is also supported by life time decay profiles for static–dynamic quenching and spectral overlap experiments for energy transfer processes (Fig. 5).⁸⁰ Cd(II) based luminescent dual ligand 3D MOF {[Cd₃(SDB)₃(TIB)](H₂O)₂(1,4-dioxane)(G)_x}_n (26) hydrothermally synthesized using angular dicarboxylate (SDB) and flexible imidazole containing an N-donor linker and the sensing application have been reported by us recently. LMOF (26) has been proved to be an excellent material for the selective adsorption and detection of the nitroaromatic pollutant TNP. The aqueous solution of TNP completely quenches the fluorescence intensity of 26 and exhibits non-linear Stern–Volmer behaviour with K_sv value 2.43 × 10⁴ M⁻¹. Static as well as dynamic quenching behaviour was evaluated by lifetime decay experiments before and after the addition of TNP to the aqueous suspension of (26) which showed no change in florescence lifetime supporting the non-linear SV plots. High sensitivity of (26) towards the detection of TNP is evident from the good limit of detection value of 1.50 × 10⁻⁵ M compared to other NACs. This fact is further supported by the visually observable cessation of fluorescence emission of the (26)-coated paper stripes in presence of TNP as shown in Fig. 6.⁸¹

Fig. 5 (a and b) Quenching (%) for (24) upon addition of different NACs and antibiotics; (c and d) Comparisons of HOMO and LUMO energies for linker Py2TTz with selected NACs and antibiotics. Reproduced with permission from ref. 80. Copyright 2018 American Chemical Society.

Fig. 6 (a) Fluorescence titration experiment for (26) upon gradual addition of TNP solution in water; (b) Stern–Volmer plot for (26) upon incremental addition of aqueous solution of TNP; (c) lifetime decay profiles of (26) in the presence and absence of TNP solution in water; (d) digital photograph of (26)-coated paper strips after being dipped in aqueous solutions of NACs. Reproduced with permission from ref. 81. Copyright 2018 American Chemical Society.

Li et al. employed a series of newly synthesized Zn-MOFs to be scanned as potential explosive/antibiotic sensors by combining ligands based on the amide functionalized linker L₉ (N4,N4′-di(pyridin-4-yl)biphenyl-4,4′-dicarboxamide) in combination with different carboxylates. Thus, three luminescent MOFs {Zn(L₉)(L₃)(H₂O)}_n (27), {Zn(L₉)(IPA)(DMF)(H₂O)_1.5}_n (28) and {Zn(L₉)(HBTC)(H₂O)₂}_n (29) for the aqueous phase detection of TNP from various other nitroaromatics and NZF among other antibiotics have been deliberated via a fluorescence quenching phenomenon.⁸² All of the aforementioned MOFs showed pronounced fluorescence quenching upon interaction with TNP and NZF compounds compared to other NACs/antibiotics. One of the rare reports by Wu et al. demonstrates aqueous phase TNT sensing by a dual ligand Zn(II) coordination polymer [Zn₃(L₁₀)₂(dpp)₂]_n (30) constructed by an acyl amide linker L₁₀ (3,5-dibromosalicylaldehyde salicylhydrazone). LCP (30) detects the highly explosive TNT with good sensitivity (K_sv value 1.15 × 10⁴ M⁻¹ and LOD value 4.99 × 10⁻⁵ M respectively).⁸³ Three dimensional LMOF {[Zn₂(μ₃-OH)(cpta)(4,4′-bpy)]·H₂O}_n (31) synthesized using a V-shaped semi-rigid linker cpta (2-4(-carboxyphenoxy)terephthalic acid) in conjunction with N-donor (4,4′-bpy) reported by Wang and co-workers has exhibited excellent sensing ability towards nitrophenol derivative 4-NP with K_sv 4.55 × 10⁴ M⁻¹ and LOD value 4.01 × 10⁻⁶ M in water.⁸⁴ Fu et al. provided an in depth spectroscopic study of the fluorescence quenching mechanisms of two new Cd(II) based dual linker fluorescent coordination polymers [Cd(TPA)(BIB)]_n (32) and [Cd(TPA)(BIYB)]_n (33) for the aqueous detection of nitrobenzene (K_sv = 1.70 × 10⁴ M⁻¹; LOD = 2.70 × 10⁻⁷ M) and 2,4-DNP (K_sv = 1.60 × 10⁵ M⁻¹; LOD = 1.30 × 10⁻⁷ M).⁸⁵

Two dual ligand luminescent MOFs {[Zn₂(tpbn)(mbhna)₂]·4H₂O.1.5DMF}_n (34) and {[Cd₂(tpbn)(mbhna)₂]·2DMF}_n (35) comprising naphthalene/hydroxyl functionalized dicarboxylate mbhna (4,4′-methylene-bis[3-hydroxy-2-naphthalene carboxylic acid) along with an N-donor ligand showed amine sensing ability especially 4-NA with K_sv values 2.23 × 10⁴ M⁻¹, 2.87 × 10⁴ M⁻¹ and LOD values 5.37 × 10⁻⁵ M, 1.29 × 10⁻⁵ M respectively. The sensing mechanism is elucidated by spectral overlap and HOMO–LUMO energy calculations.⁸⁶ Three dimensional MOF {[Cd₂(tdz)₂(4,4′-bpy)₂]·6.5H₂O}_n (36) constructed from flexible thiadiazole dicarboxylate (tdz) and bpy exhibits excellent luminescence properties in the aqueous phase. The high chemical stability of LMOF (36) is utilized for TNP detection showing good quenching constant 4.86 × 10⁴ M⁻¹ and detection limit 6.30 × 10⁻⁶ M amid other nitroaromatics.⁸⁷ Zn(II) based fluorescent 2D coordination polymer [Zn(NDC)(MI)]_n (37) synthesized using mixed linker naphthalene dicarboxylate and 2-methylimidazole displayed excellent stability and emission properties in water. The aqueous phase suspension of LCP (37) attested selective and recyclable detection of explosive molecule TNP and pesticide 2,6-DCNA with exceptional SV constant values 4.32 × 10⁴ M⁻¹ and 2.95 × 10⁴ M⁻¹ respectively. The sensitivity of (37) towards pollutant recognition is well supported by lower detection values 5.80 × 10⁻⁶ M (TNP) and 6.00 × 10⁻⁸ M (2,6-DCNA). The fluorescence quenching phenomenon is explained on the basis of simultaneous effects of PET and FRET processes (Fig. 7).⁸⁸

Fig. 7 (a and b) Reduction in emission intensity of (37) dispersed in water after incremental addition of TNP and 2,6-DCNA; (c and d) the Stern–Volmer plot of (37) for TNP and 2,6-DCNA. Reproduced with permission from ref. 88. Copyright 2019 Royal Society of Chemistry.

Selective and sensitive sensing of the acetylacetone solvent molecule was achieved using a thermally and chemically stable luminescent Zn(II) MOF {[Zn₂(XN)₂(IPA)₂]·2H₂O}_n (38) with a good K_sv value of 2.30 × 10³ M⁻¹ and an LOD value of 2.49 × 10⁻⁶ M.⁸⁹ Nagaraja et al. reported the solvothermal synthesis of a Zn²⁺ MOF with the formula [{Zn(BINDI)_0.5(bpa)_0.5(H₂O)}·4H₂O]_n (39) and [{Zn(BINDI)_0.5(bpe)}·3H₂O]_n (40) constructed from electron rich dicarboxylate BINDI (N,N′-bis(5-isophthalic acid)naphthalenediimide) and N-donor ligands and their luminescence properties. Water stability and aqueous phase fluorescence properties of both LMOFs have been exploited for the selective and sensitive sensing of TNP with excellent K_sv and limit of detection values (K_sv 4.90 × 10⁴ M⁻¹ (39), 1.29 × 10⁴ M⁻¹ (40); LOD 2.61 × 10⁻⁶ M (39), 5.63 × 10⁻⁶ M (40)).⁹⁰ 3D LMOF [Zn₂(cptpy)(btc)(H₂O)]_n (41) was hydrothermally synthesized by the combination of terpyridine based N-donor cptpy (4-(4-carboxyphenyl)-2,2′:4′,4′′-terpyridine) and benzene tricarboxylate exhibiting excellent luminescence sensing behaviour towards herbicide molecule paraquat (PAQ). The sensing ability of (41) for PAQ by a Stern–Volmer constant was 1.03 × 10³ M⁻¹ with a lower detection value of 9.73 × 10⁻⁶ M.⁹¹ Secondary amine based flexible dicarboxylate bis(4-carboxybenzyl)amine (BCA) containing 3D LCPs [Zn₂(H₂BCA)₂(o-bimb)₂(H₂O)₂]_n (42) and {[Zn(H₂BCA)(m-bib)]·H₂O}_n (43) has shown excellent sensory material properties towards acetone in water. The quenching constant values 3.70 × 10⁴ M⁻¹ (42), 2.00 × 10⁴ M⁻¹ (43) and limit of detection values 9.00 × 10⁻⁸ M (42), 1.30 × 10⁻⁷ M (43) clearly demonstrated the potential of both LCPs for aqueous phase acetone detection.⁹²

Fan and co-workers described the potential of dual ligand diisophthalate based LMOFs {[Zn(L₁₁)_0.5(bimbu)]·2H₂O·0.5(CH₃)₂NH}_n (44), {[Zn(L₁₁)_0.5(bimb)]·4H₂O}_n (45), {[Zn(L₁₁)_0.5(btdpe)]·H₂O}_n (46) and [Zn(L₁₁)_0.5(bidpe)]_n (47) for selective nitrobenzene (NB) sensing. All LMOFs showed very good K_sv values in the range of 2.85–4.59 × 10³ M⁻¹ and LOD values in the range of 0.10–8.89 × 10⁻⁶ M for NB detection in water.⁹³ Two 3D dual linker LCPs [Zn(opda)(bib)]_n (48) and [Zn(ppda)(bib)(H₂O)]_n (49) displayed selective sensing of nitrophenol derivative 2-NP in aqueous media. Both LCPs showed outstanding K_sv values 1.33 × 10⁷ M⁻¹ (48), 1.42 × 10⁷ M⁻¹ (49) and excellent LOD values 2.10 × 10⁻⁷ M (48), 1.86 × 10⁻⁷ M (49) for 2-NP recognition.⁹⁴ Amino functionalized Zn(II) LMOF [Zn(NH₂-bdc)(4,4′-bipy)]_n (50) demonstrated unique sensing behaviour towards the TNP molecule via fluorescence turn on (ethanol) and turn off (water). The aqueous phase detection ability of (50) for TNP was established by a quenching constant (K_sv) value of 3.10 × 10⁴ M⁻¹ and LOD value 3.20 × 10⁻⁵ M.⁹⁵ Of late, Zhang et al. reported mixed ligand LCP {Zn₂(tpt)₂(tdc)₂·H₂O}_n (51) for sensitive sensing of TNP in water with an ultra-high quenching constant value of 7.80 × 10⁴ M⁻¹ and a detection value of 2.56 × 10⁻⁶ M.⁹⁶ Similarly, dual linker Cd(II) LMOF [Cd₂(2-abpt)(Hbtca)(H₂btca)_0.5(H₂O)]_n (52) was synthesized from triazole containing abpt (3,5-di(2-pyridyl)-4-amino-1,2,4-triazole) employed as a fluorosensor for TNP in water with very good quenching constant of 1.63 × 10⁴ M⁻¹ and detection limit of 3.11 × 10⁻⁷ M.⁹⁷ The aqueous phase sensing and detection of organic pollutants such as solvents and precursors used for the synthesis of value-added chemicals and nitro organic compounds which are not only industrially important molecules but also hazardous and explosive in nature is synchronised in this inclusive review using a Zn(II)/Cd(II) based fluorosensor.

Inorganic pollutants

The importance of inorganic ionic species in different areas of chemistry such as biology, medicine, catalysis and environment has triggered the momentum for their selective recognition and sensing in recent years. Anions and cations play a significant role in different capacities in chemical, biological and environmental processes, establishing their importance and are widely distributed in nature and organisms.^98,99 During the industrial production of value-added chemicals and as a catalyst, molecules with inorganic components are extensively used and some of these hazardous untreated inorganic species released to soil and aquatic bodies can cause environmental pollution. Due to the wide-ranging utilization as a strong oxidant, metal derived oxoanions such as Cr₂O₇²⁻, CrO₄²⁻, MnO₄⁻, etc. which are noxious inorganic pollutants have been extensively used in various industrial processes. For example, Cr₂O₇²⁻ has been widely used in industrial processes, including electroplating, paint manufacturing, tanning, and steel fabrication etc., and the improper disposal of industrial waste containing chromium can contaminate soil and water sources harming the environment. A trace amount of highly toxic carcinogenic Cr(VI) can induce DNA damage and allergic reactions such as ulcers, renal failure, lung cancer, etc. eventually causing a high threat to human health.^100–102 A requisite amount of Fe³⁺ ions plays an important role in the physiological and biochemical processes of the human body and the deficiency can cause serious diseases such as anaemia or excess can induce damage to liver and kidneys.^103,104 As is well known, Hg²⁺ and Pb²⁺ are harmful and dangerous inorganic pollutants, which are difficult to degrade and having a bio-accumulative effect on the ecosystem causing environmental issues and human health. It has been documented in the literature that the accumulation of Hg²⁺ and Pb²⁺ through food chains can cause serious damage to central nervous and immune systems, protein synthesis and so on.¹⁰⁵ On the other hand, the risk of aluminum contamination is threatening to human health considering its usage and distribution causing toxicity to human health and can induce diseases such as softening of the bones, osteoporosis, rickets, etc. including Alzheimer's disease.^106,107 The mentioned concerns which have relevance in environmental protection and human health have driven research efforts to develop new materials as chemical sensors for detecting the minute concentrations of toxic ions in soils and water bodies. Luminescent coordination polymers (LCPs) and luminescent metal organic frameworks (LMOFs) have emerged as excellent chemical sensors for the detection of pollutants or analytes causing threat to environment/human health and the present section deals with a comprehensive review of sensing and detection of inorganic pollutants in the aqueous phase since 2015. The classification of inorganic pollutants mainly includes different hazardous metal cations such as Hg²⁺, Fe³⁺, Pb²⁺, Pd²⁺, Cr³⁺, Al³⁺, Cu²⁺ and anionic species viz. CrO₄²⁻, Cr₂O₇²⁻, MnO₄⁻, CN⁻, I⁻. Aqueous phase detection by Zn(II)/Cd(II) mixed ligand LMOFs of the mentioned pollutants has been summarised in Table 2 and abridged in this section.

Table 2 List of Zn(II)/Cd(II) mixed ligand LCP/LMOF sensors for aqueous-phase detection of inorganic pollutants

Sr no.	LMOFs/LCPs^a	Inorganic analyte	K sv (M^−1)	LOD (M)	Ref.
a L12 = hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid; bipy = 2,2′-bipyridine or 4,4′-bipyridine; bibp = 4,4′-di(1H-imidazol-1-yl)-1,1′-biphenyl; tib = 1,3,5-tri(1H-imidazol-1-yl)benzene; L13 = 4,4′-((1,2-phenylenebis(methylene))bis(oxy))dibenzoic acid; bpe = (E)-1,2-di(pyridin-4-yl)ethene; 2-NH2bdc or ATA = 2-amino-1,4-benzenedicarboxylic acid; hfipbb = 4,4-(hexafluoroisopropylidene)bis(benzoic acid); trz = triazole; 5-asba = 2-amino-5-sulfobenzoic acid; bimbu = 1,4-bis(1H-imidazol-1-yl)butane; L4 = benzo-(1,2;4,5)-bis(thiophene-2′-carboxylic acid; BBI = 1,1′-(1,4-butanediyl)bis(imidazole); TPOM = tetrakis(4-pyridyloxy-methylene) methane; BPDC = biphenyl dicarboxylic acid; ad = adenine; DAAC = 3,6-diaminoacridinium cation; 2,5-tdc = 2,5-thiophenedicarboxylic acid; 3-abit = 4-amino-3,5-bis(imidazol-1-ylmethyl)-1,2,4-triazole; L14 = 4,4′-(2,5-bis-(methylthio)-1,4-phenylene)dipyridine; SDBA = 4,4′-sulfonyldibenzoic acid; 4-tkpvb = 1,2,4,5-tetrakis(4-pyridylvinyl)benzene; 5-tert-BIPA = 5-tert-butylisophthalic acid; IPA = isophthalic acid; L7 = 3-pyridylcarboxaldehyde nicotinoylhydrazone; qptc = terphenyl-2,5,2′5′-tetracarboxylic acid; tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene; ndc = naphthalenedicarboxylic acid; OBA = 4,4′-oxybis(benzoic acid); DPT = 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine; L2 = 4-pyridyl carboxaldehyde isonicotinoylhydrazone; bptc = 3,3′,5,5′-biphenyltetracarboxylic acid; phen = 1,10-phenanthroline; abpt = 4-amino-3,5-bis(3-pyridyl)-1,2,4-triazole or 3,5-di(2 -pyridyl)-4-amino-1,2,4-triazole; bimb = 1,4-bis(imidazol-1-ylmethyl)benzene; 1,3-bitl = 1,3-bis(1-imidazoly)toluene; dpb = 3,5-di(4′-carboxyl-phenyl)benzene acid; L3 = 5-aminoisophthalic acid; L8 = 3-pyridin-3-yl-N-[5-(3-pyridin-3-yl-acryloylamino)-naphthalen-1-yl]-acrylamide; L10 = 3,5-dibromosalicylaldehyde salicylhydrazone; dpp = 1,3-di(4-pyridyl)propane; HTABDC = 4,4′-((Z,Z)-1,4-diphenylbuta-1,3-diene-1,4-diyl)bis(2-hydroxybenzoic acid); DDB = 3,5-di(2′,4′-dicarboxylphenyl)benozoic acid; bib = 1,4-bis(imidazol-1-yl)benzene; BIPA = bis(4-(1H-imidazol-1-yl)phenyl)amine; HIPA = 5-hydroxyisophthalic acid; NIPA = 5-nitroisophthalic acid; PDA = 1,4-phenylenediacetate; dpdc = 3,3′-diphenyldicarboxylate; btb = 1,4-bis(1,2,4-triazol-1-yl)butane; cpta = 2-4(-carboxyphenoxy)terephthalic acid; TPA = 3,3′-thiodipropionic acid; BIYB = 4,4-bis(imidazol-1-ylmethyl)biphenyl; L15 = 1,5-bis(2-methylbenzimidazol-1-yl)pentane; hfpd = 4,4′-(hexafluoroisopropylidene)diphthalic acid; tbta = tetrabromoterephthalic acid; bpydb = 4,4′-(4,4′-bipyridine-2,6-diyl)dibenzoic acid; atz = 3-amino-1,2,4-triazole; cpota = 2-(4-carboxyphenoxy)terephthalic acid; L16 = pyridine 4-carboxylic acid; BDC = benzene-1,4-dicarboxylate; CNA = 4-carboxycinnamic acid; bpeb = 2-(4-((E)-2-(pyridine-2-yl)vinyl)styryl)-pyridine or 1,4-bis[2-(4-pyridyl)ethenyl]benzene; cptpy = 4-(4-carboxyphenyl)-2,2′:4′,4′′-terpyridine; btc = 1,3,5-benzenetricarboxylic acid; BCA = bis(4-carboxybenzyl)amine; L11 = 5,5′-(1,4-xylylenediamino)diisophthalic acid; btdpe = 4,4′-bis(4H-1,2,4-triazol-4-yl)diphenyl ether; bidpe = 4,4′-bis(imidazolyl)diphenyl ether; L17 = 4,4′-((1,3-phenylenebis(methylene))bis(oxy))dibenzoic acid; DBPT = 3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1H-1,2,4-triazole; DiPyDz = 1,2-di(pyridin-4-yl)diazene; afsba = 2-amino-5-sulfobenzoic acid; bbtz = 1,4-bis(triazol-1-ylmethyl)benzene; L18 = 4-(tetrazol-5-yl)phenyl-4,2′:6′,4′′-terpyridine; bpta = N^1,N^4-di(pyridin-4-yl)terephthalamide; opda = 1,2-phenylenediacetic acid; ppda = 1,4-phenylene-diacetic acid; tpt = 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine; btca = 1,2,4,5-Benzenetetracarboxylic.
1	[Cd3(L12)(bipy)2(DMA)4]n (53)	CrO4^2−	—	—	108
	[Cd6(L12)2(bibp)2(DMA)4]n (54)	Cr2O7^2−
	[Cd3(L12)(tib)(DMF)2]n (55)
2	[Zn2(L13)2(bpe)2(H2O)2]n (56)	Fe^3+	2.39 × 10^3	2.50 × 10^−5	109
3	[Zn(2-NH2bdc)(bibp)]n (57)	Hg^2+	4.55 × 10^3	—	110
		Cr2O7^2−	6.55 × 10^6
	[Cd(2-NH2bdc)(tib)(H2O)4(DMA)0.5]n (58)	Hg^2+	—	4.20 × 10^−8
4	[Zn5(hfipbb)4(trz)2(H2O)2]n (59)	Fe^3+	—	2.00 × 10^−4	111
5	{[Cd(5-asba)(bimbu)]}n (5)	Fe^3+	1.78 × 10^4	1.87 × 10^−4	68
6	[Zn(L4)(BBI)·(H2O)2]n (60)	Hg^2+	9.39 × 10^3	—	112
		Cr2O7^2−	1.16 × 10^4
	[Cd(L4)(TPOM)0.75]·xG (61)	Cu^2+	1.78 × 10^4	—
		Cr2O7^2−	1.34 × 10^4
7	[Zn8(ad)4(BPDC)6O.2Me2NH2]·G@DAAC (62)	CN^−	—	1.93 × 10^−8	113
8	[Zn(μ2-1Had)(μ2-SO4)]n (13)	Hg^2+	7.7 × 10^3	7.00 × 10^−8	76
9	{[Zn(2,5-tdc)(3-abit)]·H2O}n (63)	CrO4^2−	5.43 × 10^3	—	114
		Cr2O7^2−	5.08 × 10^3
10	{[Cd(L14)(BPDC)]·2H2O}n (64)	Fe^3+	3.63 × 10^4	2.21 × 10^−6	115
		CrO4^2−	—	—
		Cr2O7^2−	6.40 × 10^3	3.76 × 10^−5
	{[Cd(L14)(SDBA)(H2O)]·0.5H2O}n (65)	Fe^3+	3.59 × 10^4	7.14 × 10^−6
		CrO4^2−	—	—
		Cr2O7^2−	4.97 × 10^3	4.86 × 10^−5
11	[Cd(4-tkpvb)(5-tert-BIPA)]n (66)	Hg^2+	1.94 × 10^4	1.50 × 10^−7	116
		CrO4^2−	4.68 × 10^4	8.00 × 10^−8
		Cr2O7^2−	2.50 × 10^4	1.20 × 10^−7
12	{[Zn(IPA)(L7)]}n (17)	CrO4^2−	1.00 × 10^3	1.83 × 10^−5	77
		Cr2O7^2−	1.37 × 10^3	1.20 × 10^−5
	{[Cd(IPA)(L7)]}n (18)	CrO4^2−	1.30 × 10^3	2.52 × 10^−6
		Cr2O7^2−	2.91 × 10^3	2.26 × 10^−6
13	[Me2NH2]4[Zn6(qptc)3(trz)4]·6H2O (67)	Cr^3+	4.39 × 10^4	1.00 × 10^−6	117
14	{[Zn2(tpeb)2(2,3-ndc)2]·H2O}n (68)	Cr^3+	—	2.19 × 10^−9	118
		CrO4^2−	—	7.23 × 10^−9
		Cr2O7^2−	—	8.58 × 10^−9
15	[Zn(OBA)(DPT)0.5]·DMF (69)	Hg^2+	3.73 × 10^3	1.80 × 10^−6	119
16	{[Zn(ATA)(L2)]·H2O}n (70)	CrO4^2−	1.48 × 10^3	2.50 × 10^−7	120
		Cr2O7^2−	2.62 × 10^3	4.30 × 10^−7
		Fe^3+	0.55 × 10^3	3.76 × 10^−6
		Pd^2+	4.18 × 10^4	2.00 × 10^−7
	{[Cd(ATA)(L2)]·2H2O}n (71)	CrO4^2−	0.97 × 10^3	1.80 × 10^−7
		Cr2O7^2−	3.11 × 10^3	1.90 × 10^−7
		Fe^3+	3.83 × 10^3	1.77 × 10^−6
		Pd^2+	7.87 × 10^4	1.00 × 10^−7
17	{[Cd2(bptc)(2,2′-bipy)2(H2O)2]}n (72)	Fe^3+	8.61 × 10^3	1.02 × 10^−5	121
		CrO4^2−	7.95 × 10^3	7.79 × 10^−6
		Cr2O7^2−	1.17 × 10^4	7.38 × 10^−6
	{[Cd2(bptc)(phen)2]·4H2O}n (73)	Fe^3+	3.07 × 10^3	2.17 × 10^−5
		CrO4^2−	1.09 × 10^3	1.06 × 10^−4
		Cr2O7^2−	2.09 × 10^3	5.89 × 10^−5
	{[Cd2(bptc)(4,4′-bipy)(H2O)2]·4H2O}n (74)	Fe^3+	6.21 × 10^3	2.03 × 10^−5
		CrO4^2−	5.38 × 10^3	1.60 × 10^−5
		Cr2O7^2−	9.34 × 10^3	1.36 × 10^−5
18	{[Zn2(1,4-ndc)2(3-abpt)]·2DMF}n (75)	Fe^3+	2.38 × 10^4	—	122
		Al^3+	6.98 × 10^4	—
	{[Cd(1,4-ndc)(3-abit)]·H2O}n (76)	Fe^3+	9.54 × 10^3	—
		Al^3+	3.84 × 10^4	—
19	{[Cd(dpb)2(bimb)2]·0.5H2O}n (77)	Cr2O7^2−	6.25 × 10^3	—	123
	[Cd(dpb)(tib)]n (78)	Fe^3+	1.21 × 10^4	—	123
		Cr2O7^2−	4.89 × 10^4	—
	[Zn(Hdpb)2(4,4′-bibp)]n (79)	Fe^3+	9.38 × 10^3	—
		Cr2O7^2−	4.35 × 10^4	—
	[Cd(dpb)(1,3-bitl)]n (80)	Cu^2+	4.89 × 10^4	—
		Cr2O7^2−	1.04 × 10^4	—
20	{[Zn2(L8)(L3)2]·3H2O}n (21)	Fe^3+	4.47 × 10^3	1.29 × 10^−6	79
21	[Zn3(L10)2(dpp)2]n (30)	Cu^2+	9.70 × 10^3	1.05 × 10^−6	83
22	Zn(HTABDC)(bpy)·DMF (81)	Al^3+	—	—	124
23	{[Cd2(HDDB)(bib)1.5(H2O)]·2.5H2O}n (82)	CrO4^2−	4.70 × 10^3	—	125
		Cr2O7^2−	2.70 × 10^4	—
	[Cd2(HDDB)(m-bimb)]·H2O}n (83)	CrO4^2−	2.50 × 10^3	—
		Cr2O7^2−	1.80 × 10^4	—
	{[Cd2(DDB)(p-bimb)]·2.5H2O·0.5(p-H2bimb)}n (84)	CrO4^2−	6.00 × 10^3	—
		Cr2O7^2−	2.80 × 10^4	—
24	{[Cd(BIPA)(IPA)]·DMF}n (85)	Hg^2+	9.21 × 10^3	5.0 × 10^−7	126
		Pb^2+	—	7.5 × 10^−7
	{[Cd(BIPA)(HIPA)]·DMF}n (86)	Hg^2+	1.28 × 10^4	2.50 × 10^−7
		Pb^2+	—	5.00 × 10^−7
	{[Cd(BIPA)(NIPA)]·2H2O}n (87)	Cu^2+	—	—
		Fe^3+	—	—
25	[Cd2.5(PDA)(trz)3]n (88)	I^−	1.80 × 10^4	6.30 × 10^−7	127
26	[Zn(dpdc)(btb)0.5]n (89)	Fe^3+	—	—	128
		Al^3+	—	—
	[Cd(dpdc)(btb)0.5]n (90)	Fe^3+	—	—
27	{[Zn2(μ3-OH)(cpta)(4,4′-bipy)]·H2O}n (31)	Cr^3+	9.47 × 10^3	5.55 × 10^−6	84
		Cr2O7^2−	5.45 × 10^3	6.91 × 10^−6
28	[Cd(TPA)(BIB)]n (32)	Hg^2+	5.70 × 10^4	2.10 × 10^−7	85
		Cr2O7^2−	3.60 × 10^4	1.10 × 10^−6
	[Cd(TPA)(BIYB)]n (33)	Hg^2+	4.60 × 10^4	1.90 × 10^−7
		Cr2O7^2−	1.40 × 10^7	2.40 × 10^−7
29	[Cd2(L15)2(hfpd)]n (91)	Fe^3+	7.90 × 10^4	3.10 × 10^−7	129
	[Zn(L15)(tbta)]n (92)	Fe^3+	8.60 × 10^4	8.50 × 10^−7
30	[Zn2(mtrb)2(btec)]·H2O}n (93)	Fe^3+	1.92 × 10^3	6.28 × 10^−7	130
		CrO4^2−	1.62 × 10^3	5.72 × 10^−7
		Cr2O7^2−	4.40 × 10^3	3.05 × 10^−7
31	[Zn3(bpydb)2(atz)2(DMF)](DMF)6}n (94)	Fe^3+	1.15 × 10^4	—	131
32	[Cd3(cpota)2(phen)3]n·5H2O (95)	CrO4^2−	6.90 × 10^3	3.70 × 10^−7	132
		Cr2O7^2−	1.21 × 10^4	4.18 × 10^−7
33	[Zn2(BDC)1.5(L16)(DMF)]·1.5DMF (96)	MnO4^−	3.20 × 10^5	3.00 × 10^−8	133
		CrO4^2−	6.10 × 10^5	3.00 × 10^−8
		Cr2O7^2−	1.00 × 10^6	2.00 × 10^−8
34	[Cd(2-bpeb)0.5(CNA)(H2O)]n (97)	MnO4^−	3.55 × 10^4	7.79 × 10^−5	134
		Cr2O7^2−	7.61 × 10^3	3.67 × 10^−4
	[Cd(2-bpeb)0.5(NDC)]n (98)	MnO4^−	2.16 × 10^4	1.31 × 10^−4
		Cr2O7^2−	3.10 × 10^3	9.20 × 10^−4
	[Zn(2-bpeb)(BDC)]n (99)	MnO4^−	1.04 × 10^4	2.70 × 10^−4
		Cr2O7^2−	1.88 × 10^3	1.44 × 10^−3
35	[Zn2(cptpy)(btc)(H2O)]n (41)	Fe^3+	3.08 × 10^3	4.33 × 10^−6	91
36	[Zn2(H2BCA)2(o-bimb)2(H2O)2]n (42)	CrO4^2−	—	1.30 × 10^−7	92
		Cr2O7^2−	6.6 × 10^4	7.00 × 10^−8
	{[Zn(H2BCA)(m-bib)]·H2O}n (43)	CrO4^2−	—	1.40 × 10^−7
		Cr2O7^2−	5.3 × 10^4	7.00 × 10^−8
37	{[Zn(L11)0.5(bimbu)]·2H2O·0.5(CH3)2NH}n (44)	Fe^3+	6.28 × 10^4	4.80 × 10^−7	93
		CrO4^2−	5.04 × 10^4	6.00 × 10^−7
		Cr2O7^2−	5.68 × 10^4	5.30 × 10^−7
37	{[Zn(L11)0.5(bimb)]·4H2O}n (45)	Fe^3+	4.07 × 10^4	7.40 × 10^−7	93
		CrO4^2−	6.76 × 10^4	4.40 × 10^−7
		Cr2O7^2−	7.35 × 10^4	4.10 × 10^−7
	{[Zn(L11)0.5(btdpe)]·H2O}n (46)	Fe^3+	5.10 × 10^4	5.90 × 10^−7
		CrO4^2−	6.47 × 10^4	5.40 × 10^−7
		Cr2O7^2−	5.55 × 10^4	4.80 × 10^−7
	[Zn(L11)0.5(bidpe)]n (47)	Fe^3+	4.67 × 10^4	6.40 × 10^−7
		CrO4^2−	5.54 × 10^4	5.40 × 10^−7
		Cr2O7^2−	6.20 × 10^4	4.80 × 10^−7
38	{[Zn(L17)(bpeb)]·(DMA)2.5}n (100)	CrO4^2−	0.82 × 10^4	4.40 × 10^−5	135
		Cr2O7^2−	2.27 × 10^4	1.60 × 10^−5
39	{[Cd1.5(DBPT)(DiPyDz)(H2O)]·3.5H2O}n (101)	Fe^3+	4.78 × 10^5	1.94 × 10^−5	136
40	[Zn(afsba)(bbtz)1.5(H2O)2]·2H2O}n (102)	Fe^3+	4.25 × 10^4	1.29 × 10^−6	137
		CrO4^2−	2.06 × 10^4	1.90 × 10^−6
		Cr2O7^2−	4.42 × 10^4	1.18 × 10^−6
41	[Zn(oba)2(bipy)2]n (103)	Fe^3+	2.98 × 10^4	—	138
		CrO4^2−	—	—
		Cr2O7^2−	9.26 × 10^3	—
42	{[Zn2(L18)2(BDC)]·2H2O}n (104)	Fe^3+	6.40 × 10^3	3.84 × 10^−6	139
		CrO4^2−	1.30 × 10^4	2.10 × 10^−6
		Cr2O7^2−	6.05 × 10^3	3.80 × 10^−6
43	[Zn2(oba)2(bpta)]·(DMF)3}n (105)	Fe^3+	6.50 × 10^4	3.00 × 10^−6	140
		Al^3+	1.40 × 10^3	1.20 × 10^−6
44	[Zn(opda)(bib)]n (48)	Cr2O7^2−	1.54 × 10^7	2.99 × 10^−7	94
	[Zn(ppda)(bib)(H2O)]n (49)	Cr2O7^2−	9.77 × 10^8	4.37 × 10^−7
45	[Zn(NH2-bdc)(4,4′-bipy)]n (50)	CrO4^2−	4.56 × 10^3	2.21 × 10^−6	95
		Cr2O7^2−	7.62 × 10^3	1.30 × 10^−6
46	{Zn2(tpt)2(tdc)2·H2O}n (51)	Fe^3+	4.60 × 10^4	4.72 × 10^−6	96
47	[Cd2(2-abpt)(Hbtca)(H2btca)0.5(H2O)]n (52)	Cu^2+	3.83 × 10^4	1.39 × 10^−6	97
48	{[Zn(L19)(bimb)]·2H2O}n (106)	Fe^3+	1.66 × 10^5	1.80 × 10^−7	141
		CrO4^2−	4.94 × 10^4	6.10 × 10^−7
		Cr2O7^2−	—	—

---

Highly luminescent mixed ligand Cd(II)-MOFs [Cd₃(L₁₂)(bipy)₂(DMA)₄]_n (53), [Cd₆(L₁₂)₂(bibp)₂(DMA)₄]_n (54) and [Cd₃(L₁₂)(tib)(DMF)₂]_n (55) consisting of a semi rigid linker L₁₂ (hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid) in combination with different N-donor spacers has been synthesized using the solvothermal method by Sun et al. All three LMOFs possess a 3D structure and are utilized for the selective detection of chromate anions among other interfering anions via fluorescence quenching. Chromate anions completely quenched the luminescence intensity of LMOFs in the aqueous phase which is explained by the weak interaction of analytes with frameworks and energy adsorption competition between analytes/LMOFs.¹⁰⁸ On the other hand, the Zn(II) coordination polymer [Zn₂(L₁₃)₂(bpe)₂(H₂O)₂]_n (56) reported by Lang et al. comprising flexible dicarboxylate L₁₃ (4,4′-((1,2-phenylenebis(methylene))bis(oxy))dibenzoic acid) and N-donor linker bpe ((E)-1,2-di(pyridin-4-yl)ethene) is known for exclusive sensing of the Fe³⁺ cation. The fluorescence intensity of LCP (56) was quenched by Fe³⁺ in aqueous solution with K_sv value 2.39 × 10³ M⁻¹ and a limit of detection value 2.50 × 10⁻⁵ M.¹⁰⁹ Wen et al. reported two amino decorated luminescent dual ligand MOFs [Zn(2-NH₂bdc)(bibp)]_n (57) and [Cd(2-NH₂bdc)(tib)(H₂O)₄(DMA)_0.5]_n (58) constructed from 2-aminotherephthalate (2-NH₂bdc) and imidazole based N-donor spacers. LMOF (57) is applicable as an excellent bifunctional fluorosensor for selective and sensitive detection of the Hg²⁺ cation and Cr₂O₇²⁻ anion via a quenching phenomenon (Fig. 8). The selectivity and sensitivity of 57 towards respective analytes is validated by high K_sv values 4.55 × 10³ M⁻¹ (for Hg²⁺) and 6.55 × 10⁶ M⁻¹ (for Cr₂O₇²⁻). Similarly, LMOF (58) is also an excellent fluorosensor for Hg²⁺ with a limit of detection value 4.20 × 10⁻⁸ M in the aqueous phase.¹¹⁰

Fig. 8 (a and b) Quenching effect on the fluorescence intensity of (57) by various cations/anions in the aqueous phase; (c and d) fluorescence spectra of (57) in the presence of increasing concentration of Hg²⁺ and Cr₂O₇²⁻. Inset: Visual changes of quenching effect under UV light (365 nm). Reproduced with permission from ref. 110. Copyright 2015 American Chemical Society.

Doubly interpenetrated 3D mixed linker MOF [Zn₅(hfipbb)₄(trz)₂(H₂O)₂]_n (59) synthesized from V-shaped dicarboxylate (hfipbb = 4,4-(hexafluoroisopropylidene)bis(benzoic acid)) and triazole ligands exhibited excellent luminescence properties and chemical stability in water due to the presence of a hydrophobic CF₃ functional group. The luminescence properties of (59) have been successfully utilized for the fluorescence sensing of Fe³⁺ in water with LOD value 2.00 × 10⁻⁴ M. The quenching effect underlined by overlapping of the absorption spectrum of Fe³⁺ with the emission spectrum of LMOF (59) suggests the competition for excitation energy between the analyte and host resulting in a decrease of emission intensity.¹¹¹

As discussed earlier, LMOF (5) was also utilized as a luminescent sensor for inorganic pollutant cation Fe³⁺ with quenching constant 1.78 × 10⁴ M⁻¹ and detection limit 1.87 × 10⁻⁴ M in water as well as in HEPES buffer solution for simulative physiological conditions towards biological application.⁶⁸ In another report, Wen and co-workers constructed two mixed linker luminescent MOFs [Zn(L₄)(BBI)·(H₂O)₂]_n (60) and [Cd(L₄)(TPOM)_0.75]·xG (61) using thiophene based carboxylate (L₄ = (benzo-(1,2;4,5)-bis(thiophene-2′-carboxylic acid) and N-donor ligands (BBI = 1,1′-(1,4-butanediyl)bis(imidazole), TPOM = tetrakis(4-pyridyloxy-methylene)methane). Both LMOFs showed excellent photoluminescence properties in the aqueous phase and was exposed as a competent sensory material for inorganic cations/anions (Cu²⁺, Hg²⁺, Cr₂O₇²⁻). LMOF (60) selectively detected the contaminates Hg²⁺ and Cr₂O₇²⁻ with K_sv values 9.39 × 10³ M⁻¹ and 1.16 × 10⁴ M⁻¹ respectively. Similarly, LMOF (61) showed excellent sensing ability for Cu²⁺ and Cr₂O₇²⁻ with quenching constant values 1.78 × 10⁴ M⁻¹ and 1.34 × 10⁴ M⁻¹ respectively. The diffusion of guest metal ions into the pores of LMOFs and non-covalent interaction of analyte/LMOFs paves the way for the energy transfer mechanism for the quenching phenomenon.¹¹² Ghosh et al. demonstrated a chemodosimetric approach for the recognition of a highly toxic cyanide (CN⁻) anion using a cationic dye incorporated MOF [Zn₈(ad)₄(BPDC)₆O.2Me₂NH₂]·G@DAAC (62). In the MOF (62) guest solvent molecules were replaced by a cationic dye molecule (3,6-diaminoacridinium cation (DAAC)) which is selectively reacted with cyanide anion turning on the fluorescence properties of MOF. A superb sensing ability of (62) supported by a limit of detection value of 1.93 × 10⁻⁸ M as well as biocompatibility of the MOF is useful for in vitro sensing of the cyanide anion in aqueous media.¹¹³ Previously discussed LCP [Zn(μ₂-1Had)(μ₂-SO₄)]_n (13) also showed an admirable fluorescence quenching response towards highly toxic metal cation mercury (Hg²⁺) in water with quenching constant 7.7 × 10³ M⁻¹ amid other interfering cations. The sensitivity of 13 is demonstrated by excellent detection limit 7.00 × 10⁻⁸ M for mercury which is further sustained by LCP coated test paper strip detection (Fig. 9). The quenching mechanism is enlightened by a probable interaction of the Hg²⁺ cation with free heterocyclic nitrogen and amino functionality on an adenine ligand.⁷⁴

Fig. 9 (a) Emission quenching (%) of (13) upon addition of different cations; (b) emission behavior of (13) dispersed in water toward different concentrations of Hg²⁺; (c) Stern–Volmer plots for (13) upon gradual addition of Hg²⁺ solution; (d) digital images of (13)-coated test strips after they were dipped in aqueous solutions of cations. Reproduced with permission from ref. 74. Copyright 2017 American Chemical Society.

Zn(II) based 2D luminescent MOF {[Zn(2,5-tdc)(3-abit)]·H₂O}_n (63) synthesized by a combination of ligands namely 2,5-thiophenedicarboxylic acid (2,5-tdc) and 4-amino-3,5-bis(imidazol-1-ylmethyl)-1,2,4-triazole (3-abit) showed good water stability and selective sensing properties for inorganic anions (CrO₄²⁻; Cr₂O₇²⁻) in water. The corresponding detection ability was determined by quenching constant values 5.43 × 10³ M⁻¹ (CrO₄²⁻) and 5.08 × 10³ M⁻¹ (Cr₂O₇²⁻).¹¹⁴ Zheng et al. explored two mixed linker Cd(II) based LMOFs {[Cd(L₁₄)(BPDC)]·2H₂O}_n (64) and {[Cd(L₁₄)(SDBA)(H₂O)]·0.5H₂O}_n (65) constructed using N-donor linker L₁₄ (4,4′-(2,5-bis-(methylthio)-1,4-phenylene)dipyridine) and dicarboxylates SDBA/BPDC. Both LMOFs were productively employed as selective fluorosensors for chromate anions and Fe³⁺ cations in the aqueous phase. Sensing ability for Cr₂O₇²⁻ was well explored by Stern–Volmer constants 6.40 × 10³ M⁻¹ (64), 4.97 × 10³ M⁻¹ (65) and LOD values 3.76 × 10⁻⁵ M (64), 4.86 × 10⁻⁵ M (65) respectively. Similarly, both LMOFs detect Fe³⁺ with K_sv 3.59 × 10⁴ M⁻¹ (64), 3.63 × 10⁴ M⁻¹ (65) and LOD values 2.21 × 10⁻⁶ M (64), 7.14 × 10⁻⁶ M (65) respectively. The quenching mechanism is proposed based on competition between analytes and LMOFs for absorption energy due to a similar absorbance region.¹¹⁵ Lang and co-workers constructed a dual ligand luminescent coordination polymer [Cd(4-tkpvb)(5-tert-BIPA)]_n (66) from N-donor 4-tkpvb (1,2,4,5-tetrakis(4-pyridylvinyl)benzene) and dicarboxylate 5-tert-BIPA (5-tert-butylisophthalic acid). LCP (66) acts as an aqueous phase multi-responsive fluorosensor for chromate anions and mercury cations. Excellent quenching constants 1.94 × 10⁴ M⁻¹ (Hg²⁺), 4.68 × 10⁴ M⁻¹ (CrO₄²⁻), 2.50 × 10⁴ M⁻¹ (Cr₂O₇²⁻) and detection limit values 1.50 × 10⁻⁷ M (Hg²⁺), 8.00 × 10⁻⁸ M (CrO₄²⁻), 1.20 × 10⁻⁷ M (Cr₂O₇²⁻) for LCP (66) demonstrated the selective and sensitive detection of these analytes. The sensing mechanism was explicated by the interaction of analytes with a free pyridyl moiety and energy transfer between analyte/LCP.¹¹⁶ Formerly mentioned multi-responsive LCPs (17) and (18) also displayed selectivity towards chromate/dichromate anions with quenching constants 1.00 × 10³/1.37 × 10³ M⁻¹ (17) and 1.30 × 10³/2.91 × 10³ M⁻¹ among other inorganic anions (Fig. 10). Detection limits for chromate/dichromate anions were 1.83 × 10⁻⁵/1.20 × 10⁻⁵ M (17) and 2.52 × 10⁻⁶/2.26 × 10⁻⁶ M (18) suggesting good sensitivity towards these analytes. Competition for excitation energy between analytes/LCPs and energy transfer supported by spectral overlap between analytes/LCPs are the main justification for the sensing mechanism.⁷⁷

Fig. 10 (a and b) Quenching efficiency of (17) and (18) for different anions; (c, d, e and f) Fluorescence titration of (17) and (18) (dispersed in water) for different concentrations of chromate anions. Reproduced with permission from ref. 77. Copyright 2017 American Chemical Society.

Mixed linker 3D luminescent MOF [Me₂NH₂]₄[Zn₆(qptc)₃(trz)₄]·6H₂O (67) constructed using π-conjugated dicarboxylate qptc (terphenyl-2,5,2′5′-tetracarboxylic acid) and N-heterocycle triazole exhibits high fluorescence properties towards sensing of the chromium cation (Cr³⁺) in water with K_sv 4.39 × 10⁴ M⁻¹ and LOD value 1.00 × 10⁻⁶ M. Supramolecular interaction of the analyte with uncoordinated carboxylate oxygen on LMOF (67) is responsible for the fluorescence quenching mechanism.¹¹⁷ Fluorescence sensing for chromate anions as well as chromium cations in the aqueous phase by Zn(II) based luminescent coordination polymer {[Zn₂(tpeb)₂(2,3-ndc)₂]·H₂O}_n (68) engaging N-donor tpeb (1,3,5-tri-4-pyridyl-1,2-ethenylbenzene) and naphthalene dicarboxylate (2,3-ndc) is reported by Lang et al. Excellent sensitivity supported by a higher limit of detection values 2.19 × 10⁻⁹ M (Cr³⁺), 7.23 × 10⁻⁹ M (CrO₄²⁻), 8.58 × 10⁻⁹ M (Cr₂O₇²⁻) and quenching is inferred by an energy transfer phenomenon via a spectral overlap of analyte/LCP.¹¹⁸ Mixed linker fluorescent MOF [Zn(OBA)(DPT)_0.5]·DMF (69) synthesized by Morsali and co-workers from flexible dicarboxylate OBA (4,4′-oxybis(benzoic acid)) and tetrazine functionalized N-donor DPT (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine) showed excellent turn on fluorescence sensing properties for Hg²⁺ in water with enhancement constant 3.73 × 10³ M⁻¹ and lower detection value 1.80 × 10⁻⁶ M.¹¹⁹ Two 3D dual ligand LMOFs {[Zn(ATA)(L₂)]·H₂O}_n (70) and {[Cd(ATA)(L₂)]·2H₂O}_n (71) constructed using amino functionalized dicarboxylate ATA (2-amino-1,4-benzenedicarboxylic acid) and Schiff base N-donor pillar L₂ (4-pyridyl carboxaldehyde isonicotinoylhydrazone) by us possess excellent photoluminescence in the solid-state as well as in water revealing a selective multisensory material towards inorganic pollutants (CrO₄²⁻, Cr₂O₇²⁻, Fe³⁺ and Pd²⁺). Water stable LMOFs showed high quenching ability for CrO₄²⁻/Cr₂O₇²⁻ anions with K_sv values 1.48 × 10³/2.62 × 10³ M⁻¹ (70), 0.97 × 10³/3.11 × 10³ M⁻¹ (71) and detection limit values 2.50 × 10⁻⁷/4.30 × 10⁻⁷ M, 1.80 × 10⁻⁷/1.90 × 10⁻⁷ M (71) respectively (Fig. 11). Similarly, both LMOFs also demonstrated sensing ability for Fe³⁺/Pd²⁺ cations with K_sv values 0.55 × 10³/4.18 × 10⁴ M (70), 3.83 × 10³/7.87 × 10⁴ M⁻¹ (70), 3.76 × 10⁻⁶/2.00 × 10⁻⁷ M⁻¹ (71) and LOD values 3.76 × 10⁻⁶/2.00 × 10⁻⁷ M (70), 1.77 × 10⁻⁶/1.00 × 10⁻⁷ M (71) respectively. The sensing mechanism was explained by the interaction of analytes with a free amino group/amide group on linkers and energy transfer via spectral overlap between analyte/LMOFs.¹²⁰

Fig. 11 (a and b) Fluorescence quenching effect of different aqueous anion solutions on LMOFs (70) and (71); (c–f) luminescence titration of (70) and (71) by increasing concentration of CrO₄²⁻/Cr₂O₇²⁻ solutions in water. Reproduced with permission from ref. 120. Copyright 2017 American Chemical Society.

Three cadmium based dual linker LCPs {[Cd₂(bptc)(2,2′-bipy)₂(H₂O)₂]}_n (72), {[Cd₂(bptc)(phen)₂]·4H₂O}_n (73) and {[Cd₂(bptc)(4,4′-bipy)(H₂O)₂]·4H₂O}_n (74) were synthesized from 3,3′,5,5′-biphenyltetracarboxylate (bptc) in combination with different pyridyl based N-donor spacers. All LCPs selectively detect chromate anions and Fe³⁺ cations in the aqueous phase with quenching constant values nearer to 10³ M⁻¹ and LOD values nearer to 10⁻⁵ M (Table 2, entry 17).¹²¹ Zhang et al. synthesized two LMOFs {[Zn₂(1,4-ndc)₂(3-abpt)]·2DMF}_n (75) and {[Cd(1,4-ndc)(3-abit)]·H₂O}_n (76) and successfully utilized as multi-responsive fluorosensors for Fe³⁺/Al³⁺ in the aqueous phase with excellent quenching constant values (Table 2, entry 18).¹²² Similarly, Fan and co-workers reported a series of LCPs {[Cd(dpb)₂(bimb)₂]·0.5H₂O}_n (77), [Cd(dpb)(tib)]_n (78), [Zn(Hdpb)₂(4,4′-bibp)]_n (79) and [Cd(dpb)(1,3-bitl)]_n (80) constructed from V-shaped 3,5-di(4′-carboxyl-phenyl)benzene acid (dpb) in combination with various imidazole based N-donor linkers. All LCPs exhibit fluorescence properties in water and developed as multisensory materials towards various inorganic pollutants (Table 2, entry 19).¹²³

LCP (21) formerly discussed as a fluorosensor for organic pollutants is also exploited as a luminescent sensory material towards the Fe³⁺ cation with K_sv value 4.47 × 10³ M⁻¹ and LOD value 1.29 × 10⁻⁶ M in water.⁷⁹ Similarly, LCP (30) demonstrated excellent aqueous phase sensing behaviour for Cu²⁺ with quenching constant 9.70 × 10³ M⁻¹ and a detection limit of 1.05 × 10⁻⁶ M.⁸³ Aggregation-induced emission (AIE) active MOF Zn(HTABDC)(bpy)·DMF (81) containing 4,4′-((Z,Z)-1,4-diphenylbuta-1,3-diene-1,4-diyl)bis(2-hydroxybenzoic acid) (HTABDC) and bipyridyl linker demonstrated fluorescence turn on sensing of the Al³⁺ cation in water.¹²⁴ Three mixed ligand LMOFs {[Cd₂(HDDB)(bib)_1.5(H₂O)]·2.5H₂O}_n (82), [Cd₂(HDDB)(m-bimb)]·H₂O}_n (83) and {[Cd₂(DDB)(p-bimb)]·2.5H₂O·0.5(p-H₂bimb)}_n (84) synthesized from 3,5-di(2′,4′-dicarboxylphenyl)benzoic acid (DDB) and imidazole based N-donor spacers have been exploited for the aqueous phase detection of chromate anions. The emission intensity of LMOFs selectively quenches by chromate and dichromate solution with the K_sv value in the range 10³–10⁴ M⁻¹ (Table 2, entry 23).¹²⁵ Zheng et al. reported the synthesis and luminescence properties of three dual linker Cd(II) MOFs {[Cd(BIPA)(IPA)]·DMF}_n (85), {[Cd(BIPA)(HIPA)]·DMF}_n (86) and {[Cd(BIPA)(NIPA)]·2H₂O}_n (87) coined from angular isophthalic acid derivatives and imidazole based N-donor spacer 1,4-bis(imidazol-1-ylmethyl)benzene (bimb). All LMOFs showed different degrees of quenching and enhancement behaviour for different metal ions in the aqueous phase. LMOFs (85) and (86) remained sensitive for Hg²⁺via a turn off and Pb³⁺via a turn on phenomenon (Fig. 12), whereas LMOF (87) showed quenching ability towards Fe³⁺ and Cu²⁺ (Table 2, entry 24).¹²⁶ Mahata and co-workers reported iodide (I⁻) sensing by mixed ligand LMOF [Cd_2.5(PDA)(trz)₃]_n (88) containing 1,4-phenylenediacetic acid (PDA) and 1,2,4-triazole in water.

Fig. 12 (a) Quenching percentage of (85–87) in the presence of different metal ion solutions, (b) digital images blank (85–87) and after addition of different metal ion solutions under UV light (310 nm). Reproduced with permission from ref. 126. Copyright 2018 American Chemical Society.

LMOF (88) showed excellent quenching ability for iodide with K_sv value 1.80 × 10⁴ M⁻¹ and lower detection limit 6.30 × 10⁻⁷ M among other anions. The sensing mechanism was explained by the molecular interaction of iodide with LMOF and unsaturation present in the penta-coordinated Cd(II) metal centre.¹²⁷ Two luminescent MOFs [Zn(dpdc)(btb)_0.5]_n (89) and [Cd(dpdc)(btb)_0.5]_n (90) prepared using diphenyldicarboxylate (dpdc) and N-donor spacer 1,4-bis(1,2,4-triazol-1-yl)butane (btb) showed aqueous luminescence sensing behaviour towards Fe³⁺ and Al³⁺ cations via interaction with free N-atoms on the framework.¹²⁸ LMOF (31) also acts as a multi-responsive sensing material for Cr³⁺/Cr₂O₇²⁻ with Stern–Volmer constant value 9.47 × 10³/5.45 × 10³ M⁻¹ and LOD value 5.55 × 10⁻⁶/6.91 × 10⁻⁶ M in water.⁸⁴ As mentioned in earlier discussions, LMOFs (32) and (33) are also exploited for the sensing application of Hg²⁺/Cr₂O₇²⁻ in aqueous media with K_sv value 5.70 × 10⁴/3.60 × 10⁴ M⁻¹ (32), 4.60 × 10⁴/1.40 × 10⁷ M⁻¹ (33) and detection limit values of 2.10 × 10⁻⁷/1.10 × 10⁻⁶ M (32), 1.9 × 10⁻⁷/2.4 × 10⁻⁷ M (33) respectively. The interaction of a thio group present on LMOFs with Hg²⁺ and energy transfer between the accepter (chromate anion) and donor (LMOFs) are responsible factors for the sensing mechanism.⁸⁵ LCPs [Cd₂(L₁₅)₂(hfpd)]_n (91) and [Zn(L₁₅)(tbta)]_n (92) reported by Cui et al. showed Fe³⁺ sensing behaviour with excellent quenching constant 7.90 × 10⁴/8.60 × 10⁴ M⁻¹ and lower detection value 3.10 × 10⁻⁷/8.50 × 10⁻⁷ M,¹²⁹ whereas LCP [Zn₂(mtrb)₂(btec)]·H₂O}_n (93) was exploited as a multifunctional sensory material towards Fe³⁺ and chromate anions in water (Table 2, entry 30).¹³⁰ Similarly, dual ligand LMOF [Zn₃(bpydb)₂(atz)₂(DMF)](DMF)₆}_n (94) showed luminescence sensing of Fe³⁺ in water with quenching constant value 1.15 × 10⁴ M⁻¹.¹³¹ 3D microporous mixed ligand LMOF [Cd₃(cpota)₂(phen)₃]_n·5H₂O (95) synthesized from 2-(4-carboxyphenoxy)terephthalic acid (cpota) and 1,10-phenanthroline showed luminescence behavior in water. LMOF (95) is applicable for the sensing of hexavalent chromate anions via emission quenching with an energy transfer mechanism (Table 2, entry 32).¹³²

Recently, Morsali et al. reported a water stable 3D dual ligand LMOF [Zn₂(BDC)_1.5(L₁₆)(DMF)]·1.5DMF (96) constructed from benzene dicarboxylate (BDC)/pyridine carboxylate (L₁₆) and developed a fluorescent chemo sensor for the selective detection of Cr^VI and Mn^VII oxoanions in the aqueous phase with good recyclability. The emission intensity of LMOF (96) was quenched by MnO⁴⁻/CrO₄²⁻/Cr₂O₇²⁻ with excellent K_sv values 3.20 × 10⁵/6.10 × 10⁵/1.00 × 10⁶ M⁻¹ and demonstrated higher LOD values 3.00 × 10⁻⁸/3.00 × 10⁻⁸/2.00 × 10⁻⁸ M. Absorbance bands of the analytes hugely overlapping with the absorbance/emission band of the LMOF clearly suggest the quenching mechanism based on the energy transfer phenomenon.¹³³ Similarly, three water stable LCPs [Cd(2-bpeb)_0.5(CNA)(H₂O)]_n (97), [Cd(2-bpeb)_0.5(NDC)]_n (98) and [Zn(2-bpeb)(BDC)]_n (99) synthesized using N-donor spacer 2-(4-((E)-2-(pyridin-2-yl)vinyl)styryl)-pyridine (2-bpeb) along with different dicarboxylates showed sensing affinity towards MnO⁴⁻/Cr₂O₇²⁻ anions in water. All three LCPs possess selectivity for MnO⁴⁻/Cr₂O₇²⁻ with quenching constant values 3.55 × 10⁴/7.61 × 10³ M⁻¹ (97), 2.16 × 10⁴/3.10 × 10³ M⁻¹ (98), 1.04 × 10⁴/1.88 × 10³ M⁻¹ (99) and detection limit values of 7.79 × 10⁻⁵/3.67 × 10⁻⁴ M (97), 1.31 × 10⁻⁴/9.20 × 10⁻⁴ M (98), 2.70 × 10⁻⁴/1.44 × 10⁻³ M (99) respectively. As discussed earlier, the quenching mechanism is supported by competition for excitation energy and energy transfer between analyte/LCPs.¹³⁴ LMOF (41) mentioned in our earlier readings also had the ability for selective and sensitive sensing of Fe³⁺ cations in water. The sensing ability of LMOF (41) is well supported by an energy transfer phenomenon between the donor and accepter with quenching constant 3.08 × 10³ M⁻¹ and LOD value 4.33 × 10⁻⁶ M.⁹¹ Multi-responsive LCPs (42) and (43) also demonstrated excellent fluorescence quenching properties towards chromate anions with K_sv values 6.6 × 10⁴, 5.3 × 10⁴ M⁻¹ and LOD values 7.00 × 10⁻⁸, 7.00 × 10⁻⁸ M respectively.⁹² Likewise, multifunctional LMOFs (44–47) showed aqueous phase sensing for Fe³⁺ and chromate oxyanions with sensible quenching constant and limit of detection values (Table 2, entry 37). Spectral overlap between analyte/LMOFs suggests that the energy transfer mechanism as well as similar absorbance bands also supported competition for excitation energy resulting in a decrease of emission of LMOFs.⁹³

Dual linker 3D pillar-layered LCP {[Zn(L₁₇)(bpeb)]·(DMA)_2.5}_n (100) constructed from 4,4′-((1,3-phenylenebis(methylene))bis(oxy))dibenzoic acid (L₁₇) and N-donor pillar bpeb showed excellent luminescence behaviour in aqueous media and successfully applied for sensitive chromate detection among other interfering anions. The quenching ability of LCP (100) for CrO₄²⁻/Cr₂O₇²⁻ was demonstrated by Stern–Volmer constant values 0.82 × 10⁴/2.27 × 10⁴ M⁻¹ and LOD values of 4.40 × 10⁻⁵/1.60 × 10⁻⁵ M. LCP (100) dispersed in water as well as coated on a paper strip showed visible luminescence reduction upon the addition of chromate anion solution under 365 nm UV radiation (Fig. 13). The sensing mechanism involves the energy transfer via spectral overlap of a wide absorbance band of the analyte with an excitation/emission band of LCP.¹³⁵ Liu and co-workers synthesized LMOF {[Cd_1.5(DBPT)(DiPyDz)(H₂O)]·3.5H₂O}_n (101) containing triazole based dicarboxylate DBPT (3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1H-1,2,4-triazole) and N-donor pillar 1,2-di(pyridin-4-yl)diazene (DiPyDz). LMOF (101) possesses luminescence sensing ability for Fe³⁺ cations in water with a quenching constant of 4.78 × 10⁵ M⁻¹ and an LOD value of 1.94 × 10⁻⁵ M.¹³⁶ A series of mixed ligand Zn-based LCPs involving aromatic dicarboxylate and N-donor ligands such as [Zn(afsba)(bbtz)_1.5(H₂O)₂]·2H₂O}_n (102), [Zn(oba)₂(bipy)₂]_n (103) and {[Zn₂(L₁₈)₂(BDC)]·2H₂O}_n (104) with hydrolytic stability have been reported. All three LCPs (102–104) showed excellent luminescence properties in water and were successfully utilized as selective sensory materials for the detection of Fe³⁺ cations/chromate oxyanions (Table 2, entries 40–42). The sensing mechanism for all three complexes involves the energy transfer phenomenon between the accepter analyte and donor frameworks.^137–139 Mixed linker Zn(II) MOF [Zn₂(oba)₂(bpta)]·(DMF)₃}_n (105) containing flexible dicarboxylate (oba) and amide functionalized N-donor pillar bpta (N¹,N⁴-di(pyridin-4-yl)terephthalamide) verified the selective and sensitive fluorescence sensing ability for Fe³⁺ (turn off) and Al³⁺ (turn on) cations in aqueous media (Table 2, entry 43).¹⁴⁰

Fig. 13 (a) Digital images of (100) in the presence of different anions under a 365 nm UV lamp; (b) fluorescence spectra of (100) dispersed in an aqueous solution of different anions; (c) images of (100) coated test paper after immersion in different concentrations of chromate anion solutions under 365 nm UV light. Reproduced with permission from ref. 135. Copyright 2019 Royal Society of Chemistry.

LCPs (48) and (49) mentioned earlier as sensors of organic pollutants are also applicable as dichromate sensors in water. Fluorescence quenching experiments for both LCPs in the presence of different anions showed the selectivity towards dichromate anions with excellent K_sv values 1.54 × 10⁷ M⁻¹ (48) and 9.77 × 10⁸ M⁻¹ (49), whereas the sensitivity of LCPs demonstrated lower detection limit values of 2.99 × 10⁻⁷ M (48), 4.37 × 10⁻⁷ M (49). The quenching mechanism was supported by the energy transfer process through spectral overlap between the analyte and LCPs.⁹⁴ Likewise, amino functionalized dual linker Zn(II) based LMOF (50) discussed earlier is also valid as a multi-responsive sensory material towards Cr^VI oxyanions in aqueous media. Chromate oxyanions selectively reduced the emission properties of LMOF (50) which can also be visible under UV radiation (Fig. 14). Fluorescence titration showed linear behaviour of the Stern–Volmer plot and the respective calculated quenching constant 4.56 × 10³ M⁻¹ (CrO₄²⁻) and 7.62 × 10³ M⁻¹ (Cr₂O₇²⁻) and excellent LOD values of 2.21 × 10⁻⁶ M (CrO₄²⁻) and 1.30 × 10⁻⁶ M (Cr₂O₇²⁻) (Fig. 14). The sensing mechanism is clarified by the resonance energy transfer effect and competitive absorption between chromate anions and LMOF (50).⁹⁵ LMOFs (51) and (52) also showed the multi sensing properties for Fe³⁺ and Cu²⁺ cations among other interfering cations in water respectively in addition to the sensing of organic pollutants (Table 2, entries 46 and 47).^96,97

Fig. 14 (a) Quenching percentage of (50) dispersed in different anion solutions; (b) digital images of (50) dispersed in different anion solutions under UV irradiation; (c and e) fluorescence spectra of (50) at different concentrations of chromate anion solutions; (d and f) linear fitting of the Stern–Volmer plot for (50) in the presence of chromate anions. Reproduced with permission from ref. 95. Copyright 2019 Elsevier B.V.

In recent report, Long et al. demonstrated the synthesis and sensing properties of a dual ligand luminescent coordination polymer {[Zn(L₁₉)(bimb)]·2H₂O}_n (106) towards Fe³⁺ cations and chromate anions. The selective detection ability of LCP (106) for Fe³⁺/CrO₄²⁻ is also supported by excellent corresponding quenching constants 1.66 × 10⁵/4.94 × 10⁴ M⁻¹ and LOD values 1.80 × 10⁻⁷/6.10 × 10⁻⁷ M. Competition for excitation energy between the analyte and LMOF due to a large overlap of absorbance/emission bands is specified to be the probable reason for selective sensing.¹⁴¹ This section of the review gives a snapshot of various Zn(II)/Cd(II) based mixed ligand fluorosensors which are highly stable in water and showed selective and sensitive detection of various inorganic pollutants in the aqueous phase. The detection of analytes by LMOFs is centred by turn on and turn off fluorescent response and can be accounted for by the interaction of analytes with the fluorophore. The detection of analytes by the LMOF is a well established mechanism based on photoinduced electron transfer (PET), resonance energy transfer (RET) and competition of the excitation energy by the analyte and fluorophore. In most cases of the present investigation, fluorescence quenching is responsible for the detection of both organic and inorganic pollutants and a few and the above-mentioned established mechanisms are responsible for the efficient sensing of diversity of analytes.^18,26 Extensive literature search has been done and most of the results are not only tabulated in the format, we also elaborated the ligands used for the construction of the LCPs, probable sensing mechanism in operation and limit of detection of the pollutants by a variety of LCPs fabricated for fluorescence sensing application.

Conclusions

The past two decades have witnessed rapid development of LMOF/LCP based fluorosensors for numerous chemical species. The studies have shown that the judiciously chosen metal nodes and linkers could not only yield LMOFs/LCPs suitable for chemical sensing but can also impart aquatic and chemical stability to the framework, which paves the way for their in-field applications. Critical evaluation of relevant research testifies that the photoluminescence of MOFs is not only sensitive to the nature of their constituent metal nodes and linkers but also to the structure of the chemical entities in their surroundings by virtue of delicate host–guest interactions. This fact has been exploited by researchers to develop fluorosensors suitable for sensitive detection of noxious organic and inorganic pollutants species present in polluted water. In this review we have provided an up-to-date account of Zn(II)/Cd(II) and mixed linker based LMOFs having potential to detect organic pollutants viz., nitroaromatics, acetone, acetylacetone, and paraquat as well as inorganic pollutants including di- and trivalent metal cations viz, Hg²⁺, Cu²⁺, Pb²⁺, Pd²⁺, Al³⁺, Fe³⁺ and Cr³⁺; and inorganic anions such as cyanide, iodide, chromate, dichromate and manganate anions in aqueous medium. The origin of fluorescence in LMOFs comprising Zn(II)/Cd(II) and mixed linkers and their aquatic stability is also discussed. It has been seen that the Zn(II)/Cd(II) and mixed linker based LMOF offer advantages such as aquatic stability and also a rapid, selective, and sensitive response towards the analyte. The selectivity of LMOF/LCP based chemosensors may be attributed to the intricate surface and pore geometries of the framework. This also justifies the scarcity of MOF based sensors for very small ions such as fluoride as well as very large biomolecules, viz., DNA, and enzymes. With the pace at which the concerned research field is developing, it is fairly logical to anticipate Zn(II)/Cd(II) and mixed linker based LMOFs/LCPs for selective and sensitive fluorosensing of biomaterials in the coming times.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The registration number of this publication is the CSIR-CSMCRI – 158/2019. Financial support from CSIR-SRF (BP) is gratefully acknowledged.

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Footnote

† Current address: KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.

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