Recent Progress in Metal-Organic Framework Based Fluorescent Sensors for Hazardous Materials Detection

Abstract

Population growth and industrial development have exacerbated environmental pollution of both land and aquatic environments with toxic and harmful materials. Luminescence-based chemical sensors crafted for specific hazardous substances operate on host-guest interactions, leading to the detection of target molecules down to the nanomolar range. Particularly, the luminescence-based sensors constructed on the basis of metal-organic frameworks (MOFs) are of increasing interest, as they can not only compensate for the shortcomings of traditional detection techniques, but also can provide more sensitive detection for analytes. Recent years have seen MOFs-based fluorescent sensors show outstanding advantages in the field of hazardous substance identification and detection. Here, we critically discuss the application of MOFs for the detection of a broad scope of hazardous substances, including hazardous gases, heavy metal ions, radioactive ions, antibiotics, pesticides, nitro-explosives, and some harmful solvents as well as luminous and sensing mechanisms of MOF-based fluorescent sensors. The outlook and several crucial issues of this area are also discussed, with the expectation that it may help arouse widespread attention on exploring fluorescent MOFs (LMOFs) in potential sensing applications.

1. Introduction

Today, excessive emissions of toxic and hazardous substances have become a pressing global issue, as these highly soluble and mobile substances spread easily into the atmosphere, land, and water, resulting in a serious threat to the ecosystem [1]. So far, various luminescence-based chemical sensors have been created for specific hazardous substances detection on the basis of host-guest interactions, resulting in the detection of target molecules down to the nanomolar range [2,3,4,5,6]. Among these luminescence-based probes, sensors based on metal-organic frameworks (MOFs) are more attractive in terms of their structural features, functional composition, and the interactions among MOFs and analytes [7]. The accurate framework of MOFs, in terms of their inherent crystallinity, facilitates a full insight into the sensing behavior of MOFs-analytes during the sensing process. Importantly, the proper selection of the organic and metal compositions provides diverse compositions and porosities enabling simple modulation of MOFs-analyte interactions to facilitate target recognition [8]. LMOFs can be obtained by selecting ligands with fluorescent properties, photoactive metal ions, and modified groups with fluorescence functionalization [9]. For example, the integration of fluorescent linkers bearing aromatic groups or conjugated π-systems and photoactive metal ions into MOFs to generate fluorescent MOFs (LMOFs) can dramatically modulate the luminescence features of MOFs, thus promising different sensing applications [10,11]. Beyond that, modifying groups with fluorescence functionalization and modulating the charge transfer between bridging ligands and metal center ions can also enhance the fluorescence properties of LMOFs. Notably, the pore features of LMOFs (including size, shape, pore environment, etc.) can be engineered and tuned to regulate sensing interactions, which often facilitates sensing performance [12].

In recent years, LMOFs, as one of the more widely used branches of MOFs, have flourished in the fields of optical security displays, biomedical imaging and sensing, and illumination decoration due to their optical tunability and fluorescence diversity, particularly in the field of toxic and hazardous substance detection [13]. Here we highlight the latest advances in sensing applications of MOFs-based fluorescent sensors for hazardous materials, including harmful gases, heavy metal ions, radioactive ions, and a range of harmful organic pollutants, with an emphasis on the influence of composition or structure on the sensing capabilities of LMOFs, as shown in Figure 1. Various luminescence sensing mechanisms and structure-performance relationships are also briefly described. Additionally, the outlooks and several crucial issues of this area are also noted with the expectation of stimulating more attention on investigating the potential of LMOFs for sensing applications.

Figure 1.

Schematic illustration of emission modes, detection mechanisms, and the applications of MOF-based fluorescent sensors for hazardous materials detection.

2. Luminous Characteristics of MOF-Based Sensors

The unique pore structure of MOFs restricts the nonradiative leap of organic ligands and promotes the charge transfer between the ligand and the central ion, which results in photoemission and increases the quantum yield and fluorescence efficiency [14]. Therefore, factors like the central metal ion or metal cluster, the structure of the ligand, and the pore size can affect the fluorescence properties of LMOFs. This section will feature an introduction to the luminescence emission patterns and detection mechanism of LMOFs.

2.1. Luminous Principle

As a novel platform for the development of traditional photoluminescence materials as well as practical luminescent applications, the fluorescence emission patterns of LMOFs can be classified into the following types, as shown in Figure 2: (1) Emission based on metal ions or metal clusters. LMOFs constructed by lanthanide (Ln³⁺) ions are the most widely used luminescent materials. The f-f electron transition of Ln³⁺ ions is forbidden by Laporte, which makes lanthanide LMOFs have a long fluorescence lifetime and narrow and sharp fluorescence peak shape. All Ln³⁺ ions except La³⁺ and Lu³⁺ can emit significant blue, red, green, orange, and other fluorescent colors. However, due to the forbidden 4f-4f electron leap of lanthanide ions, it becomes particularly difficult to be excited directly, so the sensitization of organic ligands is required, for example the “antenna effect” [15,16,17]. First, the organic linkers of lanthanide MOFs absorb energy under the stimulation of excitation light of certain energy, then the ligands transfer the energy of the excited state to the excitation energy level of Ln³⁺ ions, and finally, the energy of the excited state of Ln³⁺ ions is radiated in the form of photons, emitting characteristic fluorescence based on lanthanide metal ions. Therefore, an organic ligand with a suitable match to the lowest excited state energy level of the rare earth ion must be selected for efficient energy transfer. (2) Emission based on the organic ligand: There are three types of ligand-based luminescent emission, including single ligand emission, intra-ligand charge transfer (ILCT), and inter-ligand charge transfer (LLCT) [18]. In most of the LMOFs reported in the literature, organic ligands with a large number of conjugated structures (e.g., aromatic structures or π-π conjugated systems) can emit energy directly as photons upon excitation by UV or visible light [19]. Single ligand emission and ILCT generally occur in single-ligand MOFs systems, whereas LLCT requires the presence of mixed ligands in the MOFs structure. (3) Charge transfer between the central metal and the organic ligand: The charge transfer is divided into ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge transfer (MLCT), which mainly depends on the relative height of the lowest excited state energy level of the organic ligand and the lowest excited state energy level of the metal ion. If the lowest excited state energy of the organic ligand is lower than the lowest excited state energy of the metal ion, the energy of the metal ion is excited by the outside world and the energy is transferred to the ligand excited state, and the electrons of the ligand excited state then return to the ground state and emit fluorescence, which is the charge transfer from metal to the ligand. Conversely, the charge transfer from the ligand to the metal center occurs. (4) Emission based on the introduced guest molecule: Due to the diverse pore structure and large porosity of MOFs, it is possible to encapsulate the guest chromophores into the pores of MOFs, and the resulting domain-limiting effect can effectively improve the luminescence properties of the system. In addition, it is also possible to incorporate guest molecules into the MOFs surface by post-modification, which is a reliable way to obtain idealized LMOFs.

Figure 2.

Emission modes of LMOFs.

2.2. Detection Mechanism

The adjustable pore size of LMOFs allows them to be used as sieves to selectively allow specific molecules into the pore, and binding sites such as functional groups are abundant on the surface to target specific molecules, all of which can influence their fluorescence emission performance for specific detection of the analyte. LMOFs with large pores can selectively adsorb organic molecules through a domain-limiting effect to achieve detection results [20]. The detection mechanisms of LMOF-based chemosensors could be divided into the following parts, as shown in Figure 3 [21]: (1) Photoinduced electron transfer (PET). Electron transfer between the excited states of the LMOFs (the body) and the analyte (the guest) is induced by light. When the system is excited by light, electrons are transferred from the electron donor to the excited state of the chromophore, at which point the electron donor fluorescence is too weak to be observed. When the analyte binds to the fluorescent probe, the PET process is blocked and the fluorescence emission of the fluorescent chromophore is restored. (2) Excited state intra-molecular proton transfer (ESIPT): When a certain wavelength of excitation light is irradiated to the recognition system, H⁺ ions located in the excited state can be transferred between the proton donor and the acceptor. (3) Fluorescence resonance energy transfer (FRET): A competitive absorption mechanism whereby the excitation or emission energy of a fluorescent probe is absorbed when the absorption spectrum of the analyte overlaps with the excitation or emission spectra of the LMOFs, resulting in luminescence quenching of MOFs. (4) Quencher detachment (QD): Usually, the energy of the C-H bond or -OH vibrations present in organic ligands or solvent molecules is lost through this non-radiative leap, allowing the fluorescence of LMOFs to undergo a burst. When the recognizers are combined with these readily vibrating groups, the energy loss is reduced, allowing the fluorescence to be restored. (5) Object exchange: The fluorescence of MOFs is also burst when the analyte exchanges or binds to a chromophore in the structure of the LMOFs. (6) Structural collapse of LMOFs: During the detection process, the main framework of LMOF collapses due to the presence of the identifier, causing a change in its fluorescence properties and thus causing a fluorescence burst.

Figure 3.

Detection mechanisms of LMOFs.

3. LMOFs for Hazardous Materials Detecting

3.1. Hazardous Gases

The rise of global temperature is mainly the result of discharge of harmful gases (CO₂, SOx, NOx, NH₃, etc.) into the atmosphere, and these mostly come from human activity, including industry and transportation. These gases can be further shifted to acid/alkaline contaminants, resulting in ozone destruction, photochemical smog, and acid rain [22]. Therefore, a powerful test for these contaminants is required.

Numerous traditional techniques have been adopted for harmful gas detection, like electrochemistry, infrared spectroscopy, GC-MS, and field-effect transistors. Complex instrumentation, large amounts of time, and the need for expensive and bulky equipment limit their widespread use. In addition, some of them cannot tolerate the interference of carbon monoxide and water. Therefore, it is essential to develop dependable harmful gas sensors. Sensors in view of LMOFs have been extensively applied to hazardous gas detection [23]. The hazardous gas detection ability and detection mechanisms of various LMOF-based sensors are summarized in Table 1.

Table 1.

The gas detection ability and detection mechanisms of various LMOF-based sensors.

Analyte	MOF	Detection Limit	Water Stability	Detection Mechanism	Ref
CO2	[Zn7(ip)12](OH)2	-	High	QD	[27]
	UiO-66-ONa	3.5 × 10−7 M	High	QD	[28]
H2S	UiO-66@NH2	118 μM	High	Target-mediated azide to amine reduction	[37]
	UiO-66@NO2	188 μM	High	Target-mediated nitro group to amine reduction	[38]
	Zn-MOF	28.3 μM	High	Target-mediated nitro group to amine reduction	[39]
	Azide functionalized Ce-MOFs Nitro functionalized Ce-MOFs	12.2 μM 34.8 μM	High	Target-mediated azide/nitro groups to amine reduction	[40]
	Al(OH)(IPA-N3)	2.65 μM	High	Target-mediated N3 to amine reduction	[41]
	Zr6O4(OH)4((NDC-(NO2)2)6	20 μM	High	Target-mediated nitro group to amine reduction	[42]
	Al(OH)(BDC-N3)	90.47 nM	High	Target-mediated N3 to amine reduction	[43]
	Zr6O4(OH)4(BDC- (NO2)2)6	14.14 μM	High	Target-mediated nitro group to amine reduction	[44]
	Eu3+/Cu2+@UiO-66- (COOH)2	-	High	Quench the broad LC emission through its superior affinity for Cu2+ ions	[47]
	Eu3+/Ag+@UiO-66- (COOH)2	23.53 μM	High	-	[48]
	UiO-66-MA	3.3 nM	High	Michael addition to H2S	[49]
SO2	MOF-5-NH2	0.168 ppm	High	-	[52]
NO	Cu-TCA Eu-TCA	0.1 mM 140 μM	High	Cu2+ ions quench the ligand-based fluorescence Formation of coordination bonds between the europium and NO	[58]
	UiO-66@NH2	0.575 μM	High	-	[59]
NO2	{[Tb2(NBDC)3(DMF)4]·2DMF} {[Eu2(NBDC)3(DMF)4]·2DMF}	1.8 ppm 2.2 ppm	High	QD	[61]
	Fe2IIIMII, M = Co, Mn, and Zn	500 ppb	High	QD	[62]
NH3	Mg(H2DHBDC) Zn2(TCPE)	-	High	FRET	[66]
	Eu3-functionalization MOF	2.4 ppm	High	PET	[67]
	(Zn2(bpdc)2(bpee)	50 ppm	High	FRET	[68]

3.1.1. Carbon Dioxide (CO₂)

CO₂, as a major greenhouse gas, not only causes global warming and ocean acidification, but also causes symptoms when present above safe CO₂ levels, such as headache, energy loss, fatigue, inquietude, jelly legs, and pulmonary ventilation problems [24,25]. Molecular systems such as LMOFs are reliable materials that can be applied to CO₂ sensing [26]. They are made by powerful interactions between metal ions or clusters with easy-to-modify organic linkers and additionally exist with a high surface area and well-defined accessible pores. For example, Qi et al. developed a sensor that relied on the changes in luminescence characteristics of [Zn₇(ip)₁₂](OH)₂ (ip = 1H-imidazo[4,5-f] [1,10] phenanthroline) [27]. Stability tests show that the framework of [Zn₇(ip)₁₂](OH)₂ could maintain structural integrity in water, MeOH, or EtOH at room temperature. The photoluminescent property of [Zn₇(ip)₁₂](OH)₂ is intensely influenced by the existence of CO₂ molecules. Low-pressure CO₂ adsorption transfers the discharge color from powerful cyan photoluminescence (λ_max = 487 nm) to yellowish orange (λ_max = 540 nm). The luminescent mechanisms showed that once loaded guest molecules into [Zn₇(ip)₁₂](OH)₂, the strain added on the crystal structure of the framework prevents linker-linker interactions, transferring the λ_max of its fluorescent discharge.

Recently, through the integration of sodium phenolic agents in the framework of UiO-66, Tan et al. obtained a new fluorescent MOF probe, namely UiO-66-ONa, as shown in Figure 4a [28]. The fluorescence lifetimes of UIO-66-NH₂ were 0.611 (84.41%) and 2.44 ns (15.59%), whereas those of UIO-66-ONa were 1.59 (85.13%) and 6.37 ns (14.87%). UiO-66-ONa shows a very good sensitivity to CO₂ because of the conversion of the sodium phenol group into an OH group. Once exposed to CO₂ gas, the fluorescence spectra of UiO-66-ONa and UiO-66-OH are shifted by 50 nm (Figure 4b,c). As a consequence, the concentration of CO₂ in the surrounding atmosphere could be detected through gauging the fluorescence spectrum of the MOF powder suspension. The detection limit of CO₂ was as low as 3.5 × 10⁻⁷ M, and the specific reaction between the sodium phenolic group and CO₂ in solution resulted in its outstanding selectivity over the interference of other ordinary gases, such as CO, N₂, Ar, and O₂. The superiority of this material includes excellent chemical selectivity to CO₂, a well-studied and relatively simple synthesis procedure, and wonderful stability. Stability tests show that the emission intensity did not change significantly after three quenching-recovery cycles, indicating the high reusability of UIO-66-ONa. However, using a suspension is impractical for equipment integration and recycling, requires exposure to NaOH to convert UiO-66-OH to UiO-66-ONa, and indicates that the MOF probe is pH independent, which could limit its efficiency. Further development could conquer the aforementioned limitations, and the concept of a CO₂ sensor built on a well-known and robust MOF, which can be manufactured by applying the simple synthesis process and has the essential potential for chemical adjustability, is very possible.

Figure 4.

(a) The post-synthesis process of UIO-66-ONa. (b) Fluorescence spectra of UIO-66-ONa after the gradual bubbling of CO₂. (c) Fluorescence spectra of UIO-66-OH and UIO-66-ONa before and after bubbling of CO₂. Adapted from [28].

3.1.2. Sulphuretted Hydrogen (H₂S)

H₂S, which smells like rotten eggs, binds to Fe²⁺ ions in mitochondrial cytochrome enzymes and attacks the central nervous system [29,30]. The U.S. Safety and Health Administration states the permissible exposure to H₂S should not exceed 10 ppm during an 8 h workday [31,32]. Therefore, the detection of trace levels of H₂S is of great importance [33,34,35,36]. The original example of the MOF-based fluorescent probe for detecting H₂S was reported in 2014, namely UiO-66@NH₂, which suggested an extreme turn-on luminescent response to H₂S in living cells [37]. After post-modification of UiO-66@NH₂ using an azidation agent, a highly stable MOF-based sensor UiO-66@N₃ was obtained. Very weak fluorescent properties in UiO-66@N₃ were observed because of the existence of an electron-deficient azide agent. However, once UiO-66@N₃ was treated with Na₂S in an HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) aqueous buffer (10 mM, pH 7.4), a powerful discharge (16-fold enhancement) was produced, which was attributed to target-mediated azide to amine reduction, resulting in the luminescence of UiO-66@NH₂. Subsequently, the same group prepared a nitro-functionalised UiO-66@NO₂ as a fluorescence turn-on probe for H₂S detection, where UiO-66@NO₂ can be easily gained in a simple synthetic step by adding 2-nitroterephthalic acid as the organic linker [38]. Recent years have seen the conjunction of azide/nitro moieties with other MOFs, with various luminescent H₂S sensors being generated, including azide-appended Zn-MOF [39], Ce-MOFs (Ce-UiO-66@N₃, Ce-UiO-66@NO₂) [40], Al(OH)(IPA-N₃) [41], Zr₆O₄(OH)₄((NDC-(NO₂)₂)₆ [42], Al(OH)(BDC-N₃) [43], and Zr₆O₄(OH)₄(BDC-(NO₂)₂)₆ [44]. All of these functionalized MOF-based probes showed wonderful selectivity and a quick response time for H₂S.

The binding reaction between S²⁻ and Cu²⁺ can also be adopted for detecting H₂S [45,46]. For example, the authors of [47] prepared a nano-MOF probe for detecting H₂S, namely Eu³⁺/Cu²⁺@UiO-66-(COOH)₂. This MOF-based probe showed two different emissions: 615 nm of Eu³⁺ emission and 393 nm of ligand centered emission. It is noteworthy that H₂S can enhance and quench the two distinct emissions of the nano-MOF probe, probably ascribed to the affinity between H₂S and Cu²⁺. Subsequently, the same group created a MOF-based fluorescent logic platform Eu³⁺/Ag⁺@UiO-66-(COOH)₂ for H₂S detection [48]. Recently, Yang et al. fabricated a dual functional UiO-MOF sensor by facile post-modification UiO-66-MOF with maleic anhydride (MA), namely UiO-66-MA, as shown in Figure 5a [49]. UiO-66-MA can selectively undergo the Michael addition to H₂S, accompanied by linear fluorescence turn-on behavior. This MOF-based sensor is extremely selective and sensitive with a detection limit of 3.3 nM, the lowest recorded in all MOF-based H₂S sensing studies (Figure 5b,c). Mechanistic experiments show that UiO-66-MA exhibits dual sensing function for H₂S and Cu²⁺ in a tandem process in view of the combined rules about Michael addition and S-Cu integration, as shown in Figure 5d.

Figure 5.

(a) The post-synthesis process of UiO-66-MA. (b) Fluorescence titration of UiO-66-MA with H₂S (0-0.625 μM); inset: images of UiO-66-MA under day light (up) and UV light (down, 365 nm) in the presence of 0, 0.04, 0.08, 0.125, 0.165, 0.25, 0.35, and 0.625 μM of H₂S for 1–8, respectively. (c) Fluorescence titration of UiO-66-MA/H₂S with growing concentration of Cu²⁺; inset: images of UiO-66-MA/H₂S under day light (up) and UV light (365 nm, down) in the presence of 0, 0.02, 0.07, 0.12, 0.17, 0.22, and 0.35 μM of Cu²⁺ for 0–6, respectively. (d) A possible tandem process in view of the Michael addition, and S-Cu integration was calculated by DFT. Adapted from [48].

3.1.3. Sulfur Dioxide (SO₂)

The excess emission of SO₂ results in environmental pollution and affects human systems such as the respiratory and nervous systems [50,51]. On study exhibited a sensing paper created from MOF-5-NH₂ that could selectively identify SO₂ in various gases (H₂S, CO₂, NH₃, N₂, NO₂, and so on) [52]. The detection limit was lower than 0.168 ppm. More significantly, the color change of MOF-5-NH₂ toward SO₂ could be monitored with the naked eye, demonstrating a simple method for SO₂ monitoring. Study [53] designed and synthesized a lanthanide MOF (Ln-MOF) that was made up of a Ce⁴⁺ ion, a fluorescent Tb³⁺ ion, and m-phthalic acid (PA) for the test of SO₂ and its derivatives, as shown in Figure 6a. This Ln-MOF based probe is rapidly responsive and steady luminescent for SO₂ and SO₃²⁻, and it also suggests high selectivity and wonderful sensitivity, as shown in Figure 6b.

Figure 6.

(a) Diagram of the Ce⁴⁺/Tb³⁺ based Ln-MOF for sensing SO₂ and sulfite via the turn-on of luminescence induced by energy transfer (ET) triggered by redox-reaction. (b) The luminescence of Ln-MOF increases with the concentration (0, 0.05, 0.1, 0.2, 0.5, 0.8, 1, 5, 20, 50, 100, 150, 200, and 300 mM) of SO₃²⁻; the inset shows the linear relationship of the luminescence intensity of Ce-PA-Tb at 545 nm with the concentration of SO₃²⁻. Adapted from [53].

3.1.4. Nitrogen Oxides (NOx)

Nitrogen oxides are polluting gases that can contribute to photochemical smog and acid rain to threaten the environment and biology; they are also significant substances that can affect physiological and pathophysiological processes (e.g., vasodilation, neurotransmission, and immune responses) [54,55,56,57]. By integrating the bright blue emitter of the triphenylamine molecule, two LMOFs of Eu-TCA and Cu-TCA (H₃TCA = tricarbonyltriphenylamine) were successfully synthesized by [58]. The luminescence of Cu-TCA is enhanced by treatment with 0.1 mM NO, which reduces the Cu²⁺ center to Cu⁺ and can be recovered in aqueous solution. Furthermore, Cu-TCA enables bioimaging applications containing live cells. Additionally, the significant emissions of triphenylamine (430 nm) and lanthanide ion (610 nm) in an Eu-TCA sensor allow it to serve as a ratiometric luminescence probe for NO detection, in pace of the test limit of 140 μM. Study [59] obtained another amine-modified MOF for the detection of NO in aqueous solutions. The addition of 1.0 equivalents of NO reduced the luminescence of UiO-66@NH₂ by more than 80%, whereas in the presence of 2.0 equivalents of H₂O₂, O₂, OH, ClO^–, NO^3–, and NO₂, the luminescence was not significantly reduced. The results suggest that this amine-modified MOF can be applied as an extremely selective luminescence probe for NO detection.

As another member of the NOx family, NO₂ is a hazardous chemical with short- and long-term toxic effects on human health. Schulz et al. characterized a three-dimensional zirconium-based MOF, called [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃ (FA = formate, cal = 1,3-alt-25,26,27,28-tetrakis[(carboxy)methoxy]calixarene), to selectively detect NO₂ with calixarene linkers, as shown in Figure 7a. [60]. The distinct and reversible color change from white to blue makes the MOF-based probe a visual sensor for detecting of NO₂ at room temperature, as shown in Figure 7b. Alternatively, two isostructural lanthanide-based MOFs are reported as fluorescent sensors to detect NO₂ gas [61]. The amino group in the MOFs could act as a sensitive recognition center for NO₂ molecules. Density functional theory indicated that the shift of energy between the organic linkers and Ln was extremely reliant on the existence of NO₂, leading to an unprecedented photoluminescent sensing mechanism. Moreover, the authors coated the [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃ on glass slides and investigated them by SEM. Subsequently, they used these coated glass slides for first measurements of NO₂ in air to evaluate the potential of the novel calixarene-based MOF as a sensor material. The sensing device response to a step function of NO₂ concentration showed a delay of a few seconds caused by the dead volume of the test setup. The detection of NO₂ is also possible when humid air is used. No color change was observed when exposing the coatings to NO gas, highlighting the selectivity of the [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃. Heterometallic redox-active MOF in open metal sites and mixed metal nodes may produce specific properties in surfaces and synergistic effects to enhance gas-sensitive performance. For example, Khan et al. obtained a series of hetero-metallic MOFs (Fe₂^IIIM^II, M = Co, Mn, and Zn) for NO₂ detection by partially replacing Fe atoms in PCN-250 with the transition metals Co, Mn, and Zn [62]. The morphological and electronic energy band structural properties are controlled by partial metal substitution of Fe while maintaining the PCN-250 framework. The NO₂ sensing performance at room temperature varies considerably, with Fe₂Mn PCN-250 showing a greater magnitude of response structure to ppb-level NO₂ gases compared to pure Fe₃ PCN-250 and other heterometallic MOFs, mainly due to the supreme binding energy of NO₂ gases.

Figure 7.

(a) The structure of [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃. (b) The luminescence intensity changes of [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃ before and after bubbling of NO₂. Adapted from [60].

3.1.5. Ammonia (NH₃)

As one of the most widely used industrial gases, NH₃ is an unstable, corrosive, and colorless gas with an irritating odor. NH₃ is very toxic, and a minor leak may lead to serious ecological disaster [63]. The National Institute for Occupational Safety and Health (NIOSH) has set a safety alert for NH₃ exposure: NH₃ concentrations must not exceed 300 ppm during momentary exposure. Moreover, the U.S. Occupational Safety and Health Administration (OSHA) stipulates that airborne NH₃ concentrations must not be higher than 50 ppm during long-term exposure due to occupational necessity [64].

To date, various LMOFs have been carried out to detect trace amounts of NH₃ [65]. For example, two LMOFs of Mg(H₂DHBDC) (H₂DHBDC²⁻ = 2,5-dihydroxybenzene-1,4-dicarboxylate) and Zn₂(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) were reported in [66] for NH₃ detection. The results showed that these two LMOFs could selectively detect NH₃ at 100 °C. In contrast, their detection of NH₃ at room temperature did not exhibit selectivity, indicating that temperature is a significant parameter for luminescence sensing. Although the mechanism of the above consequences is still unknown, this task provides a new way to gain luminescent tests with high selectivity. Additionally, study [67] also highlighted a turn-on luminescent test for NH₃ applying LMOFs structured by Eu^3$#x2212; functionalization MOF and exchange of linker, respectively. Significantly, all of them have the ability to test for NH₃ in biological systems. Recently, ref. [68] obtained a LMOF (Zn₂(bpdc)₂(bpee), H₂bpdc = 4,4′-biphenyldicarboxylic acid, bpee = 1,2-bipyridylethene) containing mixed matrix membranes (MMMs) by a simple solvent-free method to detect trace amounts of amines in gas phase. The structure of (Zn₂(bpdc)₂(bpee) and sensing mechanism are shown in Figure 8a. After exposing the LMOF-containing MMMs to amines, the bpee exchange by the amine occurred, triggering an absorption and luminescence response ascribed to the release of bpee molecules inside the LMOF. With the increase of NH₃ content from 280 ppb to 28 ppm, the absorption opening is consistent with the gradual evolution of the PL spectrum from broad band with a maximum of 459 nm to narrow PL with a peak of 403 nm, as shown in Figure 8b.

Figure 8.

(a) The structure of 3D (Zn₂(bpdc)₂(bpee) and the schematic representation of the 2D structure potentially obtained after the NH₃ detection. (b) The luminescence intensity changes of (Zn₂(bpdc)₂(bpee) immersed in aqueous solutions of NH₃ with different concentrations. Adapted from [68].

3.2. Hazardous Metal Ions

3.2.1. Heavy Metal Ions

There are three representative environmental pollutants caused by industrial effluents: lead (Pb), mercury (Hg), and cadmium (Cd) [69]. Even when detected in minute quantities, they can do deadly damage to biological systems. To date, several facile and novel analytical methods for the detection of heavy metal ions have been established, including atomic absorption spectroscopy, cold vapor atomic fluorescence spectrometry, inductively coupled plasma mass spectrometry, electrochemical methods, high performance liquid chromatography, and gas chromatography. However, expensive equipment and complicated sample preparation processes limited their widespread use. Thus detection technology with high precision at the ppb level is critical for environmental monitoring, yet remains a challenge. Fluorescent MOF-based sensing is widely used in the field of chemical analysis and detection due to its high sensitivity, quick response, and easy operation. The heavy metal ion detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 1 of Table 2.

Table 2.

The hazardous metal ion detection ability and detection mechanisms of various LMOF-based sensors.

Entries	Analyte	MOF	Detection Limit	Detection Mechanism	Refs
Entry 1	Pb2+	[Tb(L)(H2O)5]n	10−7 M	QD	[70]
		{[Cd(BIPA)(IPA)]·DMF}n	7.5 × 10−7 M	QD	[71]
		{[Cd(BIPA)(HIPA)]·DMF}n	5.0 × 10−7 M
		Zn-TCPP-MOF	4.99 × 10−8 M	-	[73]
		SO3H-UiO-66(Zr)	25.2 ppm	QD	[74]
	Hg2+	UiO-66@butyne	10.9 nmol/L	QD	[79]
		NH2-MIL-53(Al)	0.15 μM	QD	[80]
		Zn2(NDC)2(DPTTZ)	-	FRET	[81]
		[Ni3(BTC)2(H2O)3]·(DMF)3(H2O)3	0.02 μM	QD	[83]
	Cd2+	Zr-based LMOF	0.002 μM	PET	[85]
		[Zn2(tdca)2(bppd)2]·2DMF	0.132 μM	PET	[86]
Entry 2	UO22+	Tb-MOF	0.9 μg/L	PET	[89]
		[Eu2(MTBC)(OH)2(DMF)3(H2O)4]·2DMF· 7H2O	309.2 μg/L	PET	[90]
		(CH3)2NH2[Ln2(BTC)(AC)3(FM)]	4.12 μM	QD	[91]
		[M(HBTC)(BMIOPE)·DMF·H2O]n	2.47 × 10−5 M	FRET	[93]
		Cd-MOF	8.5 μg/L	PET	[94]
	Th4+	[Eu2(FDC)3(DMA)2]·4H2O	3.49 × 10−5 mol/L	PET FRET	[95]
		[Ln2(NH2-BDC)2.5(CH3COO)(DMA)(H2O)]·DMA	3.40 μM	QD	[96]
	99TcO4−	[1H6(ReO4)](CF3-SO3)5·7H2O	-	PET	[97]
		Cd(II)-MOFs	0.68 × 104 M−1	-	[98]
	ReO4−	[Zr6O4(OH)4(NH3+-BDC)6]Cl6·xH2O (MOR-1)	0.36 ppm	PET	[99]
		H16[Zr6O16(H2PATP)4]Cl8·xH2O (MOR-2)	0.20 ppm
		[Ag(1,2,4,5-p4b)](SbF6) (TJNU-302)	-	FRET ESIPT	[100]

Lead Ion (Pb²⁺): Pb²⁺, as the leading heavy pollutant ion, can produce fatal effects to human health and lead to a series of incurable diseases. Therefore, it is of great significance to explore a quick and precise method to detect Pb²⁺ ions. A millimeter level lanthanide-based MOF sensor was synthesized by Ji et al. for detecting the Pb²⁺ ion, namely [Tb(L)(H₂O)₅]_n (H₂L = 3,5-dicarboxyphenol) [70]. Tb-MOF demonstrated a quite sensitive green luminescence provoked by the efficient antenna effect of the ligands, and the detection limit for Pb²⁺ ions was as low as 10⁻⁷ M. The reason why Tb-MOF became a trustworthy and simple-to-operate luminescent sensor and is used for pollutant detection is the ideal line type relation between the luminescence intensity of the crystal and the concentration of the Pb²⁺ ions. Subsequently, the authors of [71] fabricated three Cd(II) iso-frameworks to detect a trace quantity of Pb²⁺ by reacting the BIPA ligand (BIPA = bis(4-(1H-imidazol-1-yl)phenyl)amine) and different carboxylic ligands (H₂IPA = isophthalic acid, H₂HIPA = 5-hydroxyisophthalic acid, H₂NIPA = 5-nitroisophthalic acid) with Cd(II), namely {[Cd(BIPA)(IPA)]·DMF}_n, {[Cd(BIPA)(HIPA)]·DMF}_n, and {[Cd(BIPA)(NIPA)]·2H₂O}_n. The fluorescence detection experiments showed that {[Cd(BIPA)(IPA)]·DMF}_n and {[Cd(BIPA)(HIPA)]·DMF}_n are dual-reactive photoluminescent sensors for Hg²⁺ and Pb²⁺ ions with low detection concentration and high quenching constant. Wang et al. tested two kind of luminescence-active transition MOFs, Zn-MOF and Cd-MOF, and carried out the sensing experiments [72]. The detection results prove that even with variable frameworks, both of these two transition MOFs present multiple-target detection for Pb²⁺ with high sensitivity, fair anti-interference capacity, and great recyclability. Competitive energy absorption between MOFs and the analytes made a contribution to the recognition mechanism. Aggregation-induced emission (AIE) in fluorescent probe can also be adopted to detect metal ions in aqueous solutions. For example, by reacting with zinc (Zn²⁺) ions, H₂TCPP and TPE-2COOH, zinc porphyrin-based MOF (Zn-TCPP-MOF) with AIE was successfully obtained by Wang et al. [73]. Excellent fluorescent property of Zn-TCPP-MOF allows it to be used as a fluorescent probe to detect Pb²⁺ ions in aqueous solution. In this experiment, the Zn-TCPP-MOF with a bright red color converted to being colorless after the mixture of Pb²⁺ ions, which can be observed without any devices. At the same time, when the content of Pb²⁺ ions is at quite a low level, a perfect linear relationship between fluorescence quenching efficiency and concentration was illustrated. When compared with other MOFs, the Zn-TCPP-MOF detected Pb²⁺ ions in water, at very low concentrations, as well as at its fully lower limit of detection (LOD) of 4.99 × 10⁻⁸ M. The SO₃H-UiO-66(Zr) MOF system, functionalized with an SO₃H group and usually installed in the end-face of an optical fiber, implemented the function of detecting Pb²⁺ in water at 25.2, 43.5, and 64.0 ppm levels [74]. The proposed removal mechanism is based on the adsorption of [Pb(OH₂)₆]₂₊ in water on SO₃H-UiO66(Zr) because of a strong affinity between functionalized MOF and lead, as shown in Figure 9.

Figure 9.

Diagram of adsorption of [Pb(OH₂)₆]²⁺ on SO₃H-UiO-66(Zr) in water. Adapted from [74].

Mercury Ion (Hg²⁺): The mercury ion Hg²⁺, a highly toxic heavy metal, can be converted to methylmercury, a powerful neurotoxin that can accumulate through the food chain and be ingested by humans to cause serious symptoms. These matters have contributed significantly to the rapid revolution in Hg²⁺ ion detection technology [75,76,77].

Based on the modifiable host-guest interactions among LMOFs and analytes, many MOFs with nitrogen centers, amine groups, and alkyne groups have been engineered to detect Hg²⁺ [78]. For example, ref. [79] fabricated a butyne-modified luminescent MOF probe with high chemical and hydrolytic stability in water, namely UiO-66@butyne, to detect Hg²⁺ through the oxymercuration process. UiO-66@butyne was obtained by the reaction of a Zr ion and 2,5-bis (but-3-yn-1-yloxy) terephthalic acid, in which the butyne moiety acted as the recognition site for Hg²⁺. The MOF also showed high selectivity for Hg²⁺ with a significant LOD of 10.9 nmol L⁻¹ and a rapid response time of 3 min. Zhang et al. obtained a series of amino-functionalized LMOFs with high oxidation state central metals (Al³⁺, Zr⁴⁺, Cr³⁺, Fe³⁺, and Ti⁴⁺), enabling trace detection of Hg²⁺ through strong coordination of amino groups with Hg²⁺ [80]. Additionally, the intrinsic fluorescence intensity of MOFs is dominated by the effect of LMCT. Particularly, NH₂-MIL-53(Al) demonstrated an outstanding ability for Hg²⁺ detection with a wide response range (1–17.3 μM), quite low detection limit (0.15 μM), excellent selectivity, broader pH adaptation (4.0–10.0), and strong anti-interference capacity. Khatun et al. produced a thiazolothiazole-based LMOF with Hg²⁺-sensing capabilities, namely Zn₂(NDC)₂(DPTTZ), in which naphthalene dicarboxylate (NDC) struts served as antenna chromophores and energy donors and N,N′-di(4-pyridyl)thiazolo-[5,4-d]thiazole (DPTTZ) pillars as complementary energy acceptors and light emitters [81]. In the presence of Hg²⁺, the photoluminescence of Zn₂(NDC)₂(DPTTZ) MOF underwent a significant red-shift to 450 nm followed by quenching. The average fluorescence lifetimes of this material in the presences of different concentrations of Hg²⁺ were calculated as 9.57, 7.89, and 7.25 ns, respectively. Recently, a fluorescence “turn-on” probe (BA-Eu-MOF) with boric acid (BA) containing ligand 5-boronobezene-1,3-dicarboxylic acid (5-bop) was prepared to detect Hg²⁺ and CH₃Hg⁺ by Wang et al., as shown in Figure 10a [82]. The BA-Eu-MOF demonstrated a feeble red emission in water due to the passivating “antenna” effect of the ligand caused by the electron withdrawing phenomenon of the BA moiety. A transmetalation reaction (Hg²⁺ or CH₃Hg replaced the BA agent) took place based on the effects described above, as shown in Figure 10b. In this way, the “antenna” effect of the ligand was touched, resulting in the improvement of red emission. With the red emission increased for improved concentration of Hg²⁺ and CH₃Hg, the chromatism can also be seen by the naked eye under 365 nm ultraviolet light. Because of the porous property of the MOF and its inherent surface effect, along with the particular transmetalation reaction between the BA agent and Hg²⁺ or CH₃Hg, the well optimized nano-probe presented some excellent characteristics for simultaneous Hg²⁺ and CH₃Hg detection, such as brief preparation, easy operation, “turn-on” signal output, great selectivity, and high sensitivity. Helal et al. obtained a fluorescein hydrazide-appended Ni(MOF) [Ni₃(BTC)₂(H₂O)₃]·(DMF)₃(H₂O)₃ for probing Hg²⁺ ions [83]. It has been proven that this composite, in the state of aqueous emulsion, could generate a new peak in absorption at 583 nm, with a chromogenic change to pink observed by the naked eye after a mixture with Hg ions. Moreover, this compound enhances its fluorescence with a chromatic transform to green fluorescence upon hybridization with the Hg ions. The binding constant was found to be 9.4 × 10⁵ M⁻¹, with a detection restriction of 0.02 μM or 5 ppb. The sensor also demonstrated that it is reversible and could be used seven times. Moreover, the detection of Hg²⁺ ions in water samples of groundwater, tap water, and drinking water was also tested.

Figure 10.

(a) Synthesis process of BA-Eu-MOF. (b) Schematic diagram of the sensing process of BA-Eu-MOF toward Hg²⁺ and CH₃Hg⁺ ions based on transmetalation. Adapted from [82].

Cadmium Ion (Cd²⁺): Cd²⁺ is extremely poisonous to both the environment and human physiology. Long-term inhalation of cadmium can produce chronic poisoning and cause kidney damage. So far, much attention has been paid on the use of LMOFs for probing Cd²⁺. By incorporated perovskite quantum dots (CH₃NH₃PbBr₃) into MOF-5, study [84] formed a compound of CH₃NH₃PbBr₃@MOF-5 that exhibited good moisture resistance properties even after direct exposure to water for 30 days and could sense Cd²⁺ by enhanced luminescence. By utilizing the pyrrole Lewis base site as a luminescent moiety, Moradi et al. obtained a porphyrinic zirconium-based LMOF for highly selective sensing of Cd²⁺, in which Cd²⁺ can interact with pyrrole Lewis base site, resulting in a luminescent quenching [85]. Recently, Mandal et al. synthesized a porous MOF-based probe [Zn₂(tdca)₂(bppd)₂]·2DMF by using the ligand thiophene-2,5-dicarboxylic acid (H₂tdca), co-ligand N,N′-bis(4-pyridylmethylene)-1,4-benzenediamine (bppd), and Zn(NO₃)₂. It was found that [Zn₂(tdca)₂(bppd)₂]·2DMF is an excellent fluorescence probe for the detection of toxic Cd²⁺ ion selectivity and sensitivity, as shown in Figure 11 [86]. Although an increasing number of LMOF-based sensors have been reported as possible probes for the detection of Cd²⁺ ions, these were mostly employed in DMFs, restricted by their poor water stability. Further explorations are needed to detect Cd²⁺ in water [87].

Figure 11.

Structure of [Zn₂(tdca)₂(bppd)₂]·2DMF, UV-vis spectra of [Zn₂(tdca)₂(bppd)₂]·2DMF in acetonitrile suspension upon addition of Cd²⁺ at room temperature (λ_ex = 330 nm), and photographs of the iodine release process in acetonitrile and hexane. Adapted from [86].

3.2.2. Radioactive Ions

Trace amounts of nuclear waste-related metal ions (UO₂²⁺, Th⁴⁺, TcO₄⁻, and ReO₄⁻) detected in the environment is a thorny issue due to their detrimental influence on human health and the environment [88]. Nuclides are usually present in the form of anions and cations. The current treatment methods for radioactive ion detection can be generally classified into physical methods, chemical methods, and other methods, specifically including co-precipitation, ion exchange, adsorption, electrocoagulation, solvent extraction, redox, biological methods, and membrane separation technology. Compared with other technologies, fluorescence detection technology, especially fluorescent MOF-based sensing, has more advantages, such as simple equipment setup, easy operation, high safety, high efficiency, low consumption, good regeneration performance, and less harmful secondary products. Recently, LMOFs with good chemical stability and abundant functional groups have been developed to effectively detect these radioactive ions in an aqueous environment. The radioactive ion detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 2 of Table 2.

Uranium Ion (UO₂²⁺): UO₂²⁺ has been extensively used in the development of nuclear technology, and there have been some unpredictable leaks over the last few decades. Very trace amounts of UO₂²⁺ in a human being can cause cancer and acute kidney and liver damage. Therefore, it is important to explore a new approach with high selectivity, high detection sensitivity, and operational simplicity to detect uranium. Liu et al. reported on a Tb-MOF with large channels to accommodate UO₂²⁺. Using the Tb-MOF, a luminescent detection of UO₂²⁺ can be realized in water and the LOD is 0.9 μg L⁻¹ [89]. Compared with the above study, the LOD limit in water demonstrates a better practical application. Subsequently, they synthesized another Ln-MOF based sensor for detecting UO₂²⁺, namely [Eu₂(MTBC)(OH)₂(DMF)₃(H₂O)₄]·2DMF·7H₂O (MTBC = 4′,4′,4′,4′-methanetetrayltetrakis-[1,1′-biphenyl]-4-carboxylic acid) [90]. The detection limit towards UO₂²⁺ ions is 309.2 μg/L. Recently, many La-based LMOFs (CH₃)₂NH₂[Ln₂(BTC)(AC)₃(FM)], (Ln = Pr, Ce, Nd, Eu, Sm, Gd, Tb, Ho, Er, and Yb, H₃BTC = 1,3,5-benzenetricarboxylic acid) were obtained by adding H₃BTC and Ln(NO₃)₃·6H₂O with mixed formic acid (FM) into glacial acetic acid (AC) [91]. The authors indicated that (CH₃)₂NH₂[Eu₂(BTC)(AC)₃(FM)] had rapid detection capabilities for UO₂²⁺ in an aqueous solution with a lower detection limit of 4.12 μM. At the same time, the quenching rate for UO₂²⁺ can reach 98.01%. In addition to La-based LMOFs, a carboxyl-functionalized 3D Zn-based LMOF [Zn(HL)(bipy)_0.5(H₂O)]·2H₂O ([H₃L = 9-(4-carboxy-phenyl)-9H-carbazoly-3,6-dicarboxylic acid, bipy = 4,4′-bipyridine]) was fabricated by Hou et al. with sensing ability for UO₂²⁺ ions in an aqueous solution by fluorescence quenching [92]. Chen and Wang obtained a series of 2D isomorphous MOFs [M(HBTC)(BMIOPE)·DMF·H₂O]_n (M = Zn, Zn_0.7Co_0.3, Zn_0.5Co_0.5, Zn_0.3Co_0.7, Co, BMIOPE = 4,4′-bis(2-methylimidazol-1-yl)diphenyl ether, H₃BTC = 1,3,5-benzenetricarboxylic acid) to discuss the optical detection of UO₂²⁺ in an aqueous solution [93]. Seven isostructural MOFs indicating novel 2D→2D supramolecular entanglement featuring catenane-like interlocking of tricyclic cages by the assembly of two tripyridinium-tricarboxylate ligands and different metal ions were reported by Guo et al., as shown in Figure 12a [94]. The tripyridinium-afforded and metal-modulated photoresponsive properties of Cd-MOF allow it to be used as a potential fluorescence sensor for sensitive and selective detection of UO₂²⁺ in water, as shown in Figure 12b. The photoresponse in both light absorption (color) and emission has the appeal for applications in dual-output optical devices. Mechanistic experiments showed that network entanglement dictates close donor-acceptor contacts, which make fluorescence originating from ILCT. These contacts also allow photo-induced electron transfer, which is the basis for the photochromic and corresponding fluorescence reactions. Metal dependence in fluorescence and photochromism can be bound up with energy transfer through metal-centered d-d conversions. Importantly, LMOF-based sensors are highly capable of detecting UO₂²⁺ ion in natural water systems such as lake and sea water, which provides an excellent development strategy for UO₂²⁺ detection.

Figure 12.

(a) Structure of L and Cd-MOF. (b) Fluorescence response of Cd-MOF to different metal ions. Adapted from [94].

Thorium Ion (Th⁴⁺): The thorium ion Th⁴⁺ is a radioactive contaminant and can give rise to some incurable diseases, such as bone and lung cancers. It is essential for both nuclear science and environmental protection to explore a precise and quick way to detect thorium ions. However, only two examples of LMOF-based probes for Th⁴⁺ ion detecting have so far been published. In 2019, the authors of [95] designed a 3D MOF, [Eu₂(FDC)₃(DMA)₂]·4H₂O, with excellent hydrolytic stability by using 2,5-furan dicarboxylic acid (2,5-H₂FDC) as an organic linker. [Eu₂(FDC)₃(DMA)₂]·4H₂O presented excellent sensing ability toward the Th⁴⁺ ion in water with the ⁵D₀ lifetime as long as 1.087 ms. The experimental detection limit of 3.49 × 10⁻⁵ mol L⁻¹ was realized. Additionally, 80–95% of Th⁴⁺ ions were recognized by this Eu-MOF with a mixture of other metals. The sensing mechanism indicated that the competitive absorption of the excitation light significantly contributed to the luminescence quenching, whereas the high uptake and sensing capacity toward Th⁴⁺ were mainly caused by host-guest interaction. In 2021, Li et al. proposed a series of 3D LMOFs, [Ln₂(NH₂-BDC)_2.5(CH₃COO)(DMA)(H₂O)]·DMA, Ln = Pr, Nd, Sm, Eu, Gd, and Tb, NH₂-BDC = 2-aminoterephthalic acid, DMA = N,N-Dimethylacetamide) by hydrothermal synthesis, as shown in Figure 13a [96]. A series of post-synthesis modified materials with great fluorescence and quite stable structure were finally synthesized by an aldimine reaction, due to the existence of uncoordinated amino groups in the organic ligands. These post-synthesis modified materials were capable of being used as potential fluorescence sensors for Th⁴⁺, UO₂²⁺, and Cr₂O₇²⁻ detection, and the calculated result proved that these modified probes are more sensitive to the detection of analytes (Th⁴⁺, UO₂²⁺, and Cr₂O₇²⁻) than coordination polymers, as shown in Figure 13b–d. Moreover, the fluorescence lifetime of post-synthesis modified material was 1.39 ms; when analytes was added, the fluorescence lifetimes were 1.33 ms (Th⁴⁺), 1.36 ms (UO₂²⁺), and 1.33 ms (Cr₂O₇²⁻), respectively.

Figure 13.

(a) Synthetic route of the post-synthesis modified PSM-4. (b) Fluorescence spectra of PSM-4 for the detection of Th⁴⁺. (c) Fluorescence spectra of PSM-4 for the detection of UO₂²⁺. (d) Fluorescence intensity of PSM-4 with the addition of different anions. Adapted from [96].

Pertechnetate Ion (TcO₄⁻): The ion ⁹⁹TcO⁴⁻ has a long half-life period and high radioactivity, and it is not only a potential radiation hazard, but also has redox activity and high environmental mobility. Therefore, the selective and efficient detection of ⁹⁹TcO⁴⁻ has been an urgent issue in the field of environmental radiochemistry. However, very few LMOFs have been discovered for detecting TcO⁴⁻. The first LMOF-based sensor for TcO⁴⁻ detection in water was proposed by Amendola et al. in 2014 [97]. Subsequently, a study utilized tris(4-imidazolylphenyl)amine (TIPA) to synthesize a series of Cd(II)-MOFs with pores occupied by different anions [98]. The experimental results of Cd(II)-MOFs proved that only the Cd(II)-MOF with ClO⁴⁻ can be quenched by ⁹⁹TcO⁴⁻, along with a quenching constant value of 0.68 × 104 m⁻¹. The strong interaction and similar sorption wavelength between Cd(II)-MOFs and guest anions may be responsible for the presented phenomenon.

Perrhenate Ion (ReO₄⁻): One of the most difficult anions to handle in nuclear contaminated waste, ReO₄⁻ is almost identical in structure, charge density, size, and physicochemical properties with ⁹⁹TcO⁴⁻. The first example of using LMOF-based probes to detect the ReO₄⁻ ion was reported by Rapti et al. They reported two Zr-based MOFs [Zr₆O₄(OH)₄(NH₃⁺-BDC)₆]Cl₆·xH₂O (MOR-1) and H₁₆[Zr₆O₁₆(H₂PATP)₄]Cl₈·xH₂O (MOR-2) to achieve adsorption and detection of ReO₄⁻ and TcO₄⁻ [99]. MOR-1 and MOR-2 showed great absorbability for ReO₄⁻ and TcO₄⁻ anions. Importantly, both of these two compounds exhibit selective luminescence sensing properties for ReO₄⁻. An additional study explored a cationic framework of [Ag(1,2,4,5-p4b)](SbF6) (TJNU-302), exhibiting an apparent emission peak at 416 nm under excitation at 324 nm, as shown in Figure 14a,b [100]. After the mixture of ReO⁴⁻, a relative high quenching percentage of almost 86% was achieved. This compound also obtained the highest detection limit value of 90 μm compared to other reported ReO⁴⁻ sensors. Moreover, the sensing behavior of the TJNU-302 is independent of the pH of the solution, further indicating its hydrolytic stability. Even in the simulated Hanford Roche melter circulation stream, quenching percentages of 4.6–27.7% were still observed, indicating that TJNU-302 has good practical applications as a luminescence sensor for ReO⁴⁻.

Figure 14.

(a) Structure of TJNU-302. (b) UV photos of TJNU-302 crystals before and after immersion in KReO₄ water solution and emission spectra of TJNU-302 in water solutions containing different anions. Adapted from [100].

3.3. Hazardous Organic Pollutants

3.3.1. Antibiotics

Antibiotics, including aminoglycosides, beta-lactams macrolides, amino alcohols, tetracyclines (TC), lincosamide peptides, antifungals, and antineoplastics, are drugs with immunosuppressive effects. Abuse of antibiotics can lead to super antibiotic resistance. In addition, antibiotics can deteriorate and cause organic pollution, which does not naturally degrade. As the concentration of antibiotics in wastewater is relatively low, the detection of antibiotics is usually done with trace analysis, often using instruments with high sensitivity. The main techniques used by various research institutes for the detection of antibiotics in livestock wastewater are chromatography and its coupling techniques, enzyme immunoassay, and capillary electrophoresis. However, all these methods are time consuming, expensive, and require complex equipment and trained personnel. Alternately, optical sensing and adsorption-based methods have been considered as promising technologies in the detection and removal of antibiotics and organic explosives, respectively, because of some advantages such as easy operation, low energy use, high efficiency, and so on.

Detection technology of antibiotic based on LMOFs was initially explored in 2016 [101]. The LOD for nitrofurazone (NZF) and 2,4,6-trinitrophenol (TNP) could be up to 58 and 90 ppb, respectively, when used with the sensors based on this Zr(IV)-MOF. Enlightened by this method, an increasing number of LMOFs have been shown to be useful for the detection of antibiotics, particularly TCs and nitrofurans (NFs). The antibiotics detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 1 of Table 3. The authors of the article had also synthesized a series of Cd(II)-based LMOFs to detect antibiotics, namely [Cd₂Na(L)(BDC)_2.5]·9H₂O, [Cd₂(L)(2,6-NDC)₂]·DMF·5H₂O and [Cd₂(L)(BPDC)₂]·DMF·9H₂O (L = N¹-(4-(1H-1,2,4-triazole-1-yl)benzyl)-N¹-(2-aminoethyl)ethane-1,2-diamine) [102]. The LODs of these MOFs sensors for NZF detection are approximately 162, 75, and 60 ppb, respectively. Both fast reaction characteristics and high sensitivity for trace amounts of antibiotic were demonstrated by the experimental results of these MOFs. Since then, a series of chemical sensors based on Cd-MOFs have been reported for the detection of explicit antibiotics. For example, a PET-based photochromism 2D Cd-based LMOF was obtained from a tris(pyridinium)-based hexacarboxylate zwitterionic organic linker [103]. The 2-fold 2D→2D parallel entanglement structure determined the tight interlayer contact between the carboxylate (electron donor) and pyridinium (acceptor), which in turn gives the MOF its reversible photochromic properties. In addition, the fluorescence property of Cd-LMOF in water dispersion is highly selective for nitrofuran antibiotics with high selectivity. Recently, various Cd-containing LMOFs sensors were generated, including {[Cd(HL)(bic)(H₂O)]·2H₂O}, {[Cd₃(L)₂(bbi)₂(H₂O)₂]·2H₂O}, [Cd₃(L)₂(bib)₂(H₂O)₂] and [Cd₃(L)₂(idy)₂], where bic = 3,6-bis(imidazol-1-yl)-9H-carbazole, bbi = 1,1′-(1,4-butanediyl)bis(imidazole), bib = 1,4-bis(1-imidazoly)benzene and idy = 2-imidazol-1-ylpyridine [Cd(opda)(mbib)(H₂O)], [Cd(opda)(pbib)(H₂O)], [Cd(ppda)(mbib)], and [Zn₂(mpda)₂(mbib)₂]·2H₂O, (H₂opda = 1,2-phenylenediacetic acid, H₂mpda = 1,3-phenylenediacetic acid, H₂ppda = 1,4-phenylenediacetic acid, mbib = 1,3-bis(imidazolyl)benzene, pbib = 1,4-bis(imidazolyl)benzene) [Cd(BPDC)(BP4VA)·2DMF]_n (9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA), and biphenyl-4,4′-dicarboxylic acid (H₂BPDC) [104,105,106]. All of these functionalized MOF-based probes showed excellent selectivity and fast response time against antibiotics. Moreover, inconspicuous effect caused by various substances (such as metal ions, acid, base, and some antibiotics other than fluoroquinolone) appeared in the fluorescence detection results when these MOFs were applied to fluoroquinolone antibiotic sensing; moreover, the MOFs sample after a one time detection experiment could be reused without a distinct loss of function. Gai et al. demonstrated that Zn(II)-based MOFs {[Zn₂(bcob)(OH)(H₂O)]·DMA}n (ROD-Zn1) and {[Zn(Hbcob)]·(solvent)}_n (ROD-Zn2) (H₃bcob = 1,3-bis((4′-carboxylbenzyl)oxy)benzoic acid) with rod second building units (SBUs) were also expected to be applied for detecting and removing antibiotics existing in water [107]. In addition, fluorescent arrays formed by MOF functionalization can also be used as probes for the rapid and selective detection of antibiotics. For example, by encapsulating the gust dye 4-(4-diethylaminostyryl)-1-methylpyridinium (DEASM) into the frameworks of MOFs, a series of dynamic breathing LMOFs dual-emission fluorescent arrays were obtained by Xing et al. for antibiotics detection in water [108]. The resulting dual-emission fluorescent arrays show biphasic behavior with linear color tuning from green to red and controlled sensitivity to the ratiometric luminescence response to pH, supported by dimensionally relevant energy transfer. Moreover, luminescence intensity of a dual-emission fluorescent array has no distinct change with good photostability after two weeks of storage in air and deionized water. The results of a Boolean logic-gate strategy illustrate dual-emission fluorescent composites could be a well-defined logic device for monitoring and analyzing NZF levels with a simple and facile method. Xie et al. fabricated a Eu³⁺ and Tb³⁺ co-doped LMOF based ratiometric array for identification and determination of antibiotics [109]. The reaction between MOFs and different antibiotics made an impact on the ratio of fluorescence intensity at 545 nm and 616 nm (F₅₄₅/F₆₁₆), after these various responses were differentiated by principal component analysis (PCA), eight kinds of 25 antibiotics were effectively distinguished with the existence of interfering substances. Both high accuracy (98%) for the recognition of 48 unknown sample existed in the water and the outstanding quantitative ability for the mixture of antibiotics were demonstrated in this test. Finally, the sensor array was proved to be practical for actual sample analysis. This strategy not only provides an effective method for the comprehensive identification and detection of antibiotics, but also provides a new opportunity for the development of a sensor array based on ratio signal.

Table 3.

The hazardous organic pollutant detection ability and detection mechanisms of various LMOF-based sensors.

Entries	Category	MOF	Sensing Target	Detection Limit	Detection Mechanism	Ref
Entry 1	Antibiotics	Zr(IV)-MOF	NZF TNP	58 ppb 90 ppb	PET FRET	[101]
		[Cd2Na(L)(BDC)2.5]·9H2O [Cd2(L)(2,6-NDC)2]·DMF·5H2O [Cd2(L)(BPDC)2]·DMF·9H2O	NZF	162 ppb 75 ppb 60 ppb	PET FRET	[102]
		Cd-based LMOF	NFZ NFT	0.20µM 0.26 µM	PET	[103]
		[Cd(opda)(mbib)(H2O)] [Cd(opda)(pbib)(H2O)] [Cd(ppda)(mbib)] [Zn2(mpda)2(mbib)2]·2H2O	CEF TEC	0.278 μM for CEF; 0.384 μM for TEC 0.379 μM for CEF; 0.0.189 μM for TEC 0.52 μM for CEF; 0.421 μM for TEC 0.171 μM for CEF; 0.137 μM for TEC	PET FRET	[104]
		{[Cd3(L)2(bbi)2(H2O)2]·2H2O}	NZF	1.83 ppM	FRET	[105]
		{[Zn2(bcob)(OH)(H2O)]· DMA}n (ROD-Zn1) {[Zn(Hbcob)]·(solvent)}n (ROD-Zn2)	TC	0.11 μmol 0.12 μmol	PET FRET	[107]
		Eu3+ and Tb3+ co-doped LMOFs	Minocycline Norfloxacin	1.23 μM 0.06 μM	FRET PET	[109]
		Zr-based MOF (PCN-128Y)	TC	30 nM	FRET PET	[111]
		Al-MOF@Mo/Zn-MOF	DOX TET OTC CTC	0.56 nM 0.53 nM 0.58 nM 0.86 nM	FRET PET	[112]
		[Zn3(μ3-OH)(HL)L(H2O)3]· H2O	Sulfonamide	-	PET	[113]
		In(III)-MOFs	Norfloxacin Enrofloxacin Ciprofloxacin	56.7 ppb 79.0 ppb 72.9 ppb	FRET PET	[114]
Entry 2	Pesticides	MOF-5	Nitro-OPs	5 ppb	-	[117]
		Eu3+@1	p-nitrophenol 3-methyl-4-nitrophenol	0.36 μg mL−1 0.41 μg mL−1	Competitive absorption	[118]
		Zr-based MOF	Malathion	4.9 × 10−15 M	-	[119]
		AuNCs@ZIF-8	OPs	0.3 μg/L	-	[120]
		Mn/Fe-MIL(53)	Methyl parathion Chlorpyrifos	2.8 nM 0.95 nM	-	[121]
		[Cd(NH2-bdc)(azp)]·DMF	Glyphosate	25 nM	Structural collapse of LMOFs	[122]
		[Tb(L)2NO3]n	Glyphosate	0.0144 μM	PET	[123]
		[Zn2(cptpy)(btc)(H2O)]n	Paraquat	9.73 × 10−6 mol/L	FRET	[124]
		NU-901 NU-901-sbdc	Paraquat	2.0 nM 3.3 nM	PET	[126]
		E@D1 E@D3	Nitenpyram	0.94 μM 1.18 μM	PET	[127]
Entry 3	Nitro- Explosives	[Zn2(oba)2(bpy)]·3DMA	Nitrobenzene	-	PET FRET	[132]
		[Cd(NDC)0.5(PCA)]-Gx	TNP	3.5 × 104 M−1	PET FRET	[133]
		M-TABD-MOFs	NTO	4 × 10−8 mol/L	Structural collapse of LMOFs	[135]
Entry 4	Hazardous solvents	[Mn6(L1)2(H2O)5]n	Acetophenone	-	QD	[139]
		{[Cd2(μ7-L)(μ3-OH)(H2O)2]·1.3H2O}n	Aacetylacetone	35 μM	QD	[140]
		ZrIV-based UiO-66	Aacetylacetone	1.23 ppm	Object exchange PET	[141]
		Mg-NDI	Organic amine	-	PET	[145]
		Zr-BTDB fcu-MOF	Methylamine	66 nM	-	[146]

Apart from nitrofuran antibiotics, LMOFs have also been used in the detection of tetracycline (TC), sulfonamide, fluoroquinolones, and nitroimidazole antibiotics [110]. In 2018, Zhou et al. explored the first example water-stable LMOF-based probe of Zr-based MOF (PCN-128Y) based on tetraphenylethylene (TPE)-based ligand (H₄ETTC) for detecting TC in water, with an experimental detection limit of 30 nm [111]. Theoretical/experimental studies prove that the luminescence quenching can be attributed to a combined function of the strong absorption of TC at the excitation wavelength and the PIET process from the ligand of PCN-128Y to TC. In addition, other MOF-based probes have also been proposed for TC detection, such as nanosensor and hybrid heterostructures. Specifically, the energized moieties capable of forming host-guest interactions with TC have been widely used to integrate with MOFs. On the basis of the results noted above, Li et al. constructed a nano-MOF (Al-MOF@Mo/Zn-MOF) containing amino moiety [112]. The hydrogen-bonding and π–π interactions among Al-MOF@Mo/Zn-MOF and TCs can be formed due to the strong chelating capable of the metal nodes of Al, Zn, and Mo to the analytes. The luminescent intensities at 425 nm decreased gradually with the increase of TC content. The LOD of Al-MOF@Mo/Zn-MOF for DOX, TET, OTC, and CTC achieved 0.56, 0.53, 0.58, and 0.86 nm, respectively. These values are almost the lowest when compared to other reported MOF-based detectors. The quenching mechanism of Al-MOF@Mo/Zn-MOF participated in a PET process through H-bonding interaction and the competitive absorption of excitation energy. Zhu et al. proposed a LMOF, namely [Zn₃(μ₃-OH)(HL)L(H₂O)₃]·H₂O (H₃L = 5-(4-carboxy-phenoxymethyl)-isophthalic acid), with the character of highly ordered structure [113]. The experimental data showed that this MOF possesses great stability in simulated waste water and expectable sensitivity and rapid response to a series of sulfonamide antibiotics by fluorescence quenching as a biosensor. Using H₃CTTA as organic linker (H₃CTTA = 5′–(4-carboxyphenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid), another study fabricated an In(III)-MOF with rod-shaped SBU, which links the CTTA³⁻ linker into a 3D framework [114]. In(III)-MOF displayed an apparent emission at 380 nm when excited at 280 nm. The detected results showed that In(III)-MOF can selectively sensor fluoroquinolones (norfloxacin, ciprofloxacin, and enrofloxacin) with LOD of 56.7, 72.9, and 79.0 ppb, respectively, as shown in Figure 15. The UV–Vis spectra manifested that the absorption peak of these three antibiotics at 280 nm were high-degree overlapped the excitation band of In(III)-MOF. Therefore, the principle of the sensing process is likely to be involved in a competitive absorption and FRET among the MOF framework and the antibiotic. LMOF based sensors exhibited outstanding performance in antibiotic detection. However, because of the poor structure stability in water and the difficulty in pore modification, the device application of such materials is limited to some extent.

Figure 15.

Quenching efficiencies of the tested antibiotics at different concentrations on the fluorescence of In(III)-MOF at room temperature; inset: the structure of In(III)-MOF. Adapted from [114].

3.3.2. Pesticides

Excessive and inappropriate use of pesticides in agricultural production results in high pesticide residue levels, which not only directly pollute water and the environment, but also seriously endangers human health [115]. Due to the high toxicity of pesticides, accurate and sensitive detection of pesticide residues has become imperative to protect the environment and food resources. The detection of pesticides has long been achieved using conventional methods such as gas chromatography (GC), high-performance liquid chromatography (HPLC), capillary electrophoresis, potentiometry, and flow injection spectrophotometry. However, use of these methods has often been limited by a number of disadvantages, including high analytical costs, time-consuming procedures (in sample preparation and pretreatment), and sophisticated instrumentation. In light of the limitations associated with the conventional methods, there has been a growing demand for quick and reliable methods for detection of various pesticides in environmental samples. This demand has been partially met using chromogenic and luminescent chemosensors.

Since one study in 2014 demonstrated the original case of an LMOF-based probe for pesticide detection with fast response and high selectivity, various MOF-based fluorescent probes have been used to detect different pesticides, in particular organophosphates (OPs) and non-organophosphates (non-OPs) [116]. The pesticides detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 2 of Table 3.

Organophosphates (OPs)

Organophosphates are widely used in agriculture as a very important class of pesticides, generally including parathion, methyl parathion, and fenitrothion. The rapid and selective detection of OPs is significant due to their relatively high performance and moderate environmental persistence. The -NO groups in OPs can act as Lewis acid-base sites, interacting with the electron-rich centers of LMOFs, leading to luminescence detection. In 2014, Kumar et al. discussed the effect of a range of nitro-OPs on MOF-5 and found that all of the nitro-OPs similarly quenched the luminescence of MOF-5 [117]. In contrast, no quenching behavior was evident for other non-nitro organic compounds (e.g., malathion, dichlorvos, and monocrotophos). It was observed that the luminescence quenching of MOF-5 is related to the presence of nitro-organic groups in this solution. Subsequently, ref. [118] proposed a functionalised MOF to detect nitro-OPs of p-nitrophenols in water with a simultaneous detection limit of 0.36 μg mL⁻¹. The luminescence spectra of the probe barely changed although the temperature varied from 15 °C to 30 °C. The possible mechanism is the competitive uptake among linkers and metabolites. Ma et al. obtained a Zr-based MOF acetylcholinesterase (AChE) biosensor with large surface area and high-dispersion Pt nanoparticles for the detection of Ops [119]. Due to the combined influence of electron conduction channels and the increase in adsorption dots as well as the ultra-high surface dimension to enhance AChE immobilization, the proposed biosensors demonstrated high sensitivity to malathion; the presented biosensors achieved the fairly low detected limit of 1 × 10⁻¹⁴ M to 1 × 10⁻⁹ M and 4.9 × 10⁻¹⁵ M. Fluorescent probes based on enzyme-mimicking activities are burgeoning as prospective candidates for the colorimetric detection of OPs.

By integrating the enzymolysis product from AChE and choline oxidase (CHO) on AuNCs@ZIF-8, a dual function probe with a fluorescence lifetime and quantum yield of 6.83 μs and 4.63% was obtained by Cai et al. [120]. The authors used colorimetric strips constructed from the fluorescent material to visualise semi-quantitative detection. In addition, they developed a smartphone app to make the visualisation more accurate and enable real-time regulation of pesticide contamination. Similarly, Luo et al. introduced Mn ions into Fe-based MOF (Fe-MIL(53)) via a one-pot hydrothermal reaction strategy, resulting in a bimetallic Mn/Fe-MIL(53) MOF nanoenzyme that achieves selectivity and sensitivity for OPs detection, as shown in Figure 16a [121]. The proposed Mn/Fe-MIL(53) MOF nanozyme can be used for a quantitative analysis of methyl parathion and chlorpyrifos in the concentration range of 10-120 nM and 5-50 nM, respectively. The detection limits of 0.95 nM (3 S/N) for chlorpyrifos and 2.8 nM for methyl parathion were also realized, as shown in Figure 16b,c. Good recoveries were obtained when applied in actual sample assays. In addition, this work establishes a solid foundation for improving the catalytic performance of MOF nanozymes, which is of great significance for biosensing.

Figure 16.

(a) Representation of the sensing process of Mn/Fe-MIL(53) MOF nanoenzyme for OPs detection. UV–vis spectrum of nanoenzyme-TMB catalyzed system corresponding to different dosages of methyl parathion (b) and clorpyrifos (c). Adapted from [121].

Nonorganic Phosphates (Non-OPs)

Non-OP pesticides are general insecticides, including glyphosate, paraquat, endiquat, and so on. Wiwasuku et al. constructed a microcrystalline Cd(II)-MOF enhanced with open amino and azo agents, called [Cd(NH₂-bdc)(azp)]·DMF (NH₂-H₂bdc = 2-amino-1,4-benzenedicarboxylic acid, azp = 4,4′-azopyridine), as a fluorescent sensor in the off state using a PET process [122]. As the first MOF-based probe for detecting non-OP pesticides, [Cd(NH₂-bdc)(azp)]·DMF shows highly selective and sensitive visual turn-on fluorescence detection of glyphosate and Cr³⁺ with LOD of 25 nM and 0.6 μM, respectively. The amino group in [Cd(NH₂-bdc)(azp)]·DMF plays a key role in the selective binding of Cr³⁺, facilitating the sensitivity and selectivity of the detection. DFT indicated that the mechanism of fluorescence enhancement was related to the inhibition of the PET process stimulated by the framework of [Cd(NH₂-bdc)(azp)]·DMF dissociation in the presence of glyphosate and Cr³⁺. Recently, Li et al. reported a case of a water-stable 3D MOF, [Tb(L)₂NO₃]_n (HL = 3.5-bis(triazol-1-yl)benzoic acid), that can distinguish various pesticides under a 2D decoding map with ratio fluorescence [123]. [Tb(L)₂NO₃]_n is an ideal fluorescent probe for the detection of glyphosate with high selectivity and sensitivity. The detection limit for glyphosate was 0.0144 μM. In addition to glyphosate, LMOF-based sensors can also selectively detect paraquat in aqueous systems. For example, Chen et al. constructed a bifunctional LMOF-based sensor [Zn₂(cptpy)(btc)(H₂O)]_n for paraquat detection in water by hydrothermal reaction of Zn²⁺ ions with a mixture of two ligands, Hcptpy and H₃btc (Hcptpy = 4-(4-carboxyphenyl)-2,2′:4′,4′′-terpyridine; H₃ btc = 1,3,5-benzenetricarboxylic acid) [124]. This bifunctional LMOF-based sensor could be stable in air for more than 3 months and in aqueous solutions (pH 3–11) for at least 1 week. Parmar et al. evaluated Zn(II)/Cd(II) based luminescent MOFs/CPs with mixed ligands to address problems associated with water pollution [125]. The advantages of Zn(II) and Cd(II) metal ions, the hybrid linker system, and the subtle host-guest interactions between the framework and analyte allow this fluorescent system to detect organic contaminants. Zhao et al.has developed a pyrene-based MOF probe with anionic ligands for the analysis of paraquat in aqueous and organic phases. The electrostatic attraction between the anionic ligands in the framework of the pyrene-based MOF probe improves the sensing performance [126]. In addition, the system achieved the identification of paraquat in a range of electron deficient agrochemicals and reached the detection limit at the nM level. Wei et al. obtained an EY@Zr-MOF for detecting nitenpyram in which the fluorescent molecule of eosin Y (EY) was implanted into Zr-MOF. EY@Zr-MOF manifested the dual emission signals emitted by the Zr-MOF and EY molecules at 430 and 560 nm, respectively, as shown in Figure 17a [127]. The colors of EY@Zr-MOF could be gradually transformed from bright brown to orange when increasing the loading molar ratio of EY, leading to E@D1 (0.023 mol%), E@D2 (0.18 mol%), and E@D3 (0.22 mol%), as shown in Figure 17b. After the mixture of 20 pesticides, both E@D1 and E@D3 exhibited selective detection of nitenpyram by the changes in luminescence ratios, as shown in Figure 17c,d.

Figure 17.

(a) Structure diagram of the as-prepared EY@DUT-52 sample. (b) Optical pictures of DUT-52 and three EY@DUT-52 samples under sunlight (the solvent in the vial is ethanol). (c) Fluorescent emission spectra of E@D1/ethanol suspensions with different concentrations of nitenpyram. (d) Fluorescent emission spectra of E@D3/ethanol suspensions with different concentrations of nitenpyram. Adapted from [127].

3.3.3. Nitro-Explosives

Nitro explosives, such as 2,4,6-trinitrophenol (TNP), 2,4,6-trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX), 2,4-dinitrotoluene (2,4-DNT), and so on, are widely used in military production and industrial blasting. These compounds are not only explosive, but also toxic, hard to degrade, carcinogenic, and can cause vomiting, convulsions, neurological disorders, and even death after introduction into the human body [128,129]. Therefore, the development of rapid trace detection technology for nitro explosives is of great significance to prevent terrorist attacks, maintain public safety, and safeguard human health. However, the low vapor pressure and chemical reactivity make nitro explosives difficult to be detected. Current detection methods typically involve canines or sophisticated instruments. Both techniques are expensive and may not always be easily accessible. Fluorescence quenching employing conjugated polymers is a simple and promising alternative procedure that is based on the donor-acceptor electron-transfer mechanism.

To date, various LMOFs have already been proposed to detect explosives through PET and FRET mechanisms, where the electrostatic interactions and electron transfer perform a significant role in the explosives sensing process [130]. The nitro-explosive detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 3 of Table 3. In 2009, the first example of using LMOF to detect explosives was reported by Lan et al., namely [Zn₂(bpdc)₂(bpee)], providing a new application of microporous MOFs [131]. Subsequently, in order to diversify aromatic explosives with electron-withdrawing groups or electron-donating groups in the vapor, they constructed another microporous LMOF of [Zn₂(oba)₂(bpy)]·3DMA [132]. The detection mechanism is also given, which shows that only explosives with nitro electron-withdrawing groups can significantly decrease the luminescence intensity of Zn-MOF. On photo-excitation, the energy in the conduction band (CB) of Zn-MOF and the LUMO band of the explosive can produce electron transfer in the explosives sensing process. If explosives with electron withdrawing groups are present, a quenching effect occurs, which is based on the transfer of electrons from the CB of Zn-MOF to the LUMO of nitroaromatics. Importantly, this work proves that the detection capability is related to suitable porosity, strong overlap, and the driving force for electron transfer between the LMOF and analyte molecules. Since then, more and more scientists are focusing on detecting explosives using LMOF-based sensors. One study verified the selective detection of TNP by the MOF-based sensor for the first time; [Cd(NDC)_0.5(PCA)]-Gx (G = guest molecule, NDC = 2,6-naphthalenedicarboxylic acid, PCA = 4-pyridinedicarboxylic acid) showed a strong emission at 384 nm when it was excited at 340 nm [133]. Luminous intensity of this Cd-MOF can be quenched by nearly 78% when dispersed in TNP solution, as shown in Figure 18. Importantly, this work allows for the detection of explosives in aqueous solutions, showing great application prospect in the future.

Figure 18.

Quenching efficiencies of tested nitro-explosives at different concentrations on the fluorescence of [Cd(NDC)_0.5(PCA)]-Gx at room temperature; inset: the structure of [Cd(NDC)_0.5(PCA)]-Gx. Adapted from [133].

In addition to the sensing principle of energy transfer in LMOFs, the AIE mechanism was also used to sense explosives [134]. Guo et al. produced three M-TABD-MOFs (M = Mg²⁺, Ni²⁺, and Co²⁺) using TABD-COOH (4,4-(Z,Z)-1,4-diphenylbutane-1,3-diene-1,4-diyl) dibenzoyl as AIE linkers [135]. Usually, the coordination between metal and carboxylate agent would be weakened by the heterocyclic explosive, which contains C=N and/or N=N band, and as a result, partial linkers will be replaced. Thus, MOFs sensors with AIE connectors for the detection of explosives can be designed where the AIE molecules can be released and reassembled to form emissive aggregates. Because of the emulative replacement of the ligand, it is still hard to maintain the integrity of the framework TABD-MOF after the mixture of NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one). Therefore, to release free TABD-COOH molecules into solution, trace amounts of TABD-MOFs can be deposited on the paper strips. The dissociated molecules TABD-COOH can be aggregated again after evaporation of the THF solvent to enable sustainable detection.

3.3.4. Hazardous Solvents

Hazardous solvents, highly toxic substances that can cause cancer, deformities, and blood poisoning, generally include toxic organic solvents and amine-based organic molecules [136,137]. Up to now, several methods such as high performance liquid chromatography, nuclear magnetic resonance spectroscopy, mass spectrometry, and infrared and ultraviolet (UV) spectroscopy have been available for probing hazardous solvents. Recently, MOFs as luminescent sensors to recognize acetylacetone have also been explored. The hazardous solvents detection ability and detection mechanisms of various LMOF-based sensors are summarized in Entry 4 of Table 3. Early in 2011, Lu et al. produced a solvatochromic MOF {[(WS₄Cu₄)I₂(dptz)₃]·3DMF}_n (dptz) for sensing solvents [138]. They found that the absorption bands of {[(WS₄Cu₄)I₂(dptz)₃]·3DMF}_n were shifted to blue in UV–Vis spectra when the polarity of the solvent increases.

Subsequently, a growing number of LMOF-based probes have been exploited for the identification of poisonous organic solvents. For example, Li et al. reported a 3D porous MOF of [Mn₆(L₁)₂(H₂O)₅]_n, as shown in Figure 19a [139]. Solvent recognition experiments showed that the color variations of this MOF could be distinctly observed with the naked eye when treated with ketone molecules, as shown in Figure 19b. The possible induction mechanism can be attributed to the hydrogen bonding interaction between guest ketone molecules and μ₂-bridged oxygen, which can lead to the deformation of the octahedral coordinated Mn²⁺ center, as shown in Figure 19c. Li et al. developed an ultra-stable tetrazolium Cd-based MOF {[Cd₂(μ₇-L)(μ₃-OH)(H₂O)₂]·1.3H₂O}_n containing a new bi-functional ligand H₃L (H₃L = 4′-(1H-tetrazole-5)-biphenyl-2,3-dicarboxylic acid) under the solvo-thermal conditions [140]. Fluorescent characterization shows that this ultra-stable MOF allows highly sensitive real-time discrimination of acetylacetone with good reusability (high quenching efficiency K_sv for acetylacetone: 2.5 × 10⁵ M⁻¹) and very low detection limit values (for acetylacetone: 35 μM). Recently, Ghosh et al. obtained a Zr^IV-based UiO-66 LMOF with sulfonamide functionality for the detection of acetylacetone [141]. The thermally activated MOF showed highly selective and sensitive fluorescent turn-on properties for superoxide (O₂^•−) in MeOH and acetylacetone in DMF medium. The detection limit (98 nM) of O₂^•− using this probe is much lower than that of known MOF-based chemical sensors for O₂^•−. The MOF is unique in that it is able to detect acetylacetone up to the limit of 1.23 ppm, which is similar to that of the previously published MOF chemo-sensors for acetylacetone.

Figure 19.

(a) Structure of [Mn₆(L₁)₂(H₂O)₅]_n. (b) Photographs of different solvent-loaded [Mn₆(L₁)₂(H₂O)₅]_n. (c) Perspective view of the hydrogen-bonding interaction between the acetophenone molecule (blue color) and host framework. Adapted from [139].

In addition to ketones, ammonia solvents can also be detected by LMOF-based probes [142,143,144]. For example, one study designed a porous MOF of Mg-NDI using chromophore naphthalenediimide (NDI) for detecting solid-state organic amine (like aniline, ethylenediamine, triethylamine, and hydrazine) [145]. The rapid color change from yellow to dark black of Mg-NDI also can be seen by the naked eye. A further study achieved the detection of aliphatic amines in aqueous solution by Zr-BTDB fcu-MOF (H₂BTDB = 4,4′-(2,1,3-benzothiadiazole-4,7-diyl) bis[benzoic acid]) based on the underlying fcu topology, as shown in Figure 20a,b [146]. Due to the strong hydrogen bonding between protonated methylamine and N atoms of thiadiazole, the dihedral angle (thiadiazole unit and benzene) of the linker (H₂BTDB) becomes smaller from 22 to 12 when H₂BTDB is attached to two or four methylamine, as shown in Figure 20c,d. Therefore, this hydrogen bond can inhibit the rotation of thiadiazole core in linker H₂BTDB and reduces the pathway for non-radiative recombination. Importantly, the hydrogen bond interaction between ligands and amines results in an ultra-low detection limit of 66 nm, as shown in Figure 20e.

Figure 20.

(a) Schematic representation of the synthesis of Zr-BTDB-fcu-MOF (1). (b) PL spectra for (1) (red) and linker (black). (c) Fluorescence intensity of (1) aqueous suspension upon addition of 3 μM of of MA (c), aniline (d) (λmax = 515 nm). (e) Molecular structures of H₂BTDB, H₂BTDB with two and four MA molecules. Red arrows represent the dihedral angles between benzene and thiadiazole-based units, and red dashed lines indicate the hydrogen-bond lengths (dH) formed by H of MA and N of thiadiazole Adapted from [146].

4. Conclusions and Outlook

Excessive emissions of toxic and hazardous substances have become a pressing global issue, as these highly soluble and mobile substances can easily spread into the atmosphere, land, and water, causing a serious threat to ecosystems. All current technologies for the sensing of toxic and hazardous substances have their own advantages and limitations due to their specific sensing applications. Recent years have seen that MOFs-based fluorescent probes have shown outstanding advantages in the fields of hazardous substance identification and detection. This study presents a summary of some of the latest developments in LMOF-based chemical sensors for detecting harmful gases, heavy metal ions, radioactive ions, and a range of harmful organic pollutants, with an emphasis on the influence of composition or structure on the sensing performance of MOF-based probes. A variety of luminescence sensing mechanisms and structure-performance relationships are also outlined. Despite significant advances and outstanding achievements of LMOF based sensors that have been achieved, there are still many challenges that need to be addressed.

First, adding the structural/luminescence stability of LMOFs in environmental media is a challenging mission, as hazardous substances are mostly discharged into relatively sophisticated environmental media. Water and air stability are prerequisites for MOF-based sensors to detect hazardous substances, which will ensure structural integrity and recoverability in sewage systems or complex environments with uncompromised performance. So far, very few families of MOFs such as the ZIFs, Al-based MOFs, MIL MOFs, and Zr-based MOFs, made up of high-valent metallic centers, imidazolate, terephthalate, and additional hydrophobic linkers based LMOFs, have demonstrated exceptional stability under wet conditions. Significant attempts within the MOF community to improve MOF stability include the construction of MOF composites with hydrophobic materials having special stability even under extreme acidic and alkaline environments. Second, a further developing characterization technique to verify and validate each of mechanism outlined above is beneficial for the potential of suggesting and developing new mechanisms. The exact synthesis methods, the in situ monitoring techniques, and the theoretical computations should be synergized to learn the structure–function relationships between LMOFs and the corresponding sensing performances. Third, more wearable and hand-held MOF sensors should be developed to enable fast and easy detection of diverse analytes as future MOF devices may play a key role in the revolution in healthcare, smart farming, environmental monitoring, defense, and artificial intelligence.

Briefly, in spite of the various challenges, MOFs still remain one of the most attractive classes of sensor materials, with great potential for applications in the detection of hazardous substances. With sensible design and optimization, MOF-based materials will continue to progress, offering unbelievable opportunities not only in the sensing field, but in numerous other fields as well.

Author Contributions

D.Z. and Z.-Z.C. conceived and wrote the paper; S.Y., W.-J.J., Z.-H.C. and D.-L.L. provide partial figures; Y.-L.L. made suggestions to improve the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research APC was funded by the Scientific Research Foundation of Ningbo University, grant number 421911882.

Conflicts of Interest

The authors declare no conflict of interest.