Metal–Organic Frameworks as Materials for Applications in Sensors for Toxicants

Abstract

Application of porous coordination polymers (PCPs), which include metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) in sensors for detection of toxicant pollutants in water is discussed. Particular attention is given to electrochemical and photoluminescent sensors because PCPs/MOFs demonstrate good selectivity towards adsorption of molecules in combination with outstanding luminescent properties and electroconductivity in composite materials. The use of PCPs/MOFs as pre-concentrators of the compounds to be analyzed is also outlined. The review covers the results described in the literature over the past 5 years in such diverse fields as the determination of metal ions and anions, drugs, mycotoxins, pesticides, explosives, bacteria, etc. Thus, the review demonstrates the proliferation of MOF applications and the universal nature of sensors based on them.

1. Introduction

Porous coordination polymers (PCPs) are crystalline or mesocrystalline coordination polymers composed of organic linkers and organic (PCPs, COFs) or inorganic nodes (MOFs) [1]. They represent a class of micro-mesoporous polymer materials with designed physical and chemical properties. About 10,000 papers that are related to PCPs have appeared annually since the first publication in 1989. Omar Yaghi coined the term MOF [2]. Later, a new and special class of metal–organic frameworks was prepared on the basis of imidazolate building units that are somewhat similar to zeolites and which are named zeolitic imidazolate frameworks ZIFs (Zeolitic Imidazolate Frameworks) [3,4].

The modular nature (a combination of inorganic and organic components) [5,6,7] of PCPs is ideal for designing the structures and functions of these materials so as to make them suitable for specific applications. The most outstanding feature of PCPs is their extremely high porosity. The length of ligands and the nature and coordination of inorganic building blocks determine the geometry of pores, with sizes ranging from 0.4 nm to about 5 nm. The large pores can accommodate bulky molecules like polysaccharides and proteins [8]. Some illustrative examples of PCP structures that are most interesting from the point of view of their application in sensors are presented in Figure 1. The molecular sieve effect can be used in sensors based on PCPs.

PCP materials with a high hydrothermal and thermal stability, of up to 400–450 °C, are known in the field. The porosity determined as the values of the specific surface areas of PCPs extends from a few m²/g to over 5500 m²/g (5640 m²/g for MOF-177 [9] and 5900 m²/g for MIL-101 [10]. The maximum predicted surface areas may reach up to 15,000 m²/g [11].

Figure 1. Structures of selected PCP (MOF) materials: (a) HKUST-1 (MOF-199). Reprinted with permission from [12]. (b) MOF-177. Reprinted with permission from [13]. (c) MIL-53 (Cr). Reprinted with permission from [13]. (d) MIL-[Cr₃OF(bdc)₃]. Reprinted with permission from [14]. (e) ZIF-8. Reprinted with permission from [15]. (f) JUC-48 [Cd₃(BPDC)₃(DMF)] 5DMF 18H₂O. Reprinted with permission from [16].

Figure 1. Structures of selected PCP (MOF) materials: (a) HKUST-1 (MOF-199). Reprinted with permission from [12]. (b) MOF-177. Reprinted with permission from [13]. (c) MIL-53 (Cr). Reprinted with permission from [13]. (d) MIL-[Cr₃OF(bdc)₃]. Reprinted with permission from [14]. (e) ZIF-8. Reprinted with permission from [15]. (f) JUC-48 [Cd₃(BPDC)₃(DMF)] 5DMF 18H₂O. Reprinted with permission from [16].

The chemistry and methodology of synthesis of PCPs are presented in a number of recent reviews, for instance [17,18,19,20]. Post-synthetic functionalization of PCPs, including the approaches of “click chemistry”, is rationalized by S. Cohen et al. [21,22]. A variety of functional groups can be introduced in PCPs by using the methodology of organic syntheses (amino, amido, mercapto groups, etc.). Some of these functional groups improve the stability of PCPs or increase the adsorption of analytes by sensor materials. Diverse nanocomposite materials based on PCPs, including host–guest assemblies, membranes (ceramic or mixed matrix membranes), and core–shell structures attract special interest as potentially efficient sensor materials [23].

The nature of coordination polymers or metal–organic frameworks governs their physical properties, including magnetic susceptibility, conductivity, and the optical characteristics that are essential for sensor applications. Over 70,000 MOFs have been synthesized and their structure has been established [24,25,26]. In MOFs, the most widely used metal ions are Cu(II), Mn(II), Zn(II), Ni(II), and lanthanides. The percent of free volume inside PCPs with respect to the overall volume reaches 91.1% in IRMOF-16 (organic linker, tetraphenyl-5,5-dioctylcyclopentadiene (TPDC)), most zeolites have no more than 30–40% of their overall volume accessible [27]. An important advantage of PCPs over traditional sorbents is the uniformity of the pore size. The shape of pores in PCPs can be varied [28]. While zeolites typically have cylindrical, elliptical and slit pores, PCPs can also have square, rectangular, and triangular pores [29,30,31]. This diversity of pore geometries is important for selective sensing [32,33,34,35].

D. De Vos and coauthors [36], while studying adsorption of linear and branched molecules in a Cu-based MOF, found the “molecular recognition” effect of MOF, i.e., the predominant adsorption of certain isomers over others, for instance, p-dichlorobenzene compared with m-dichlorobenzene. It is clear that the opportunities provided by this new class of hybrid inorganic–organic material in sensors are unique, as the choice of the organic linker and inorganic nodes provides the possibility to tune the textural parameters and design of the structure for a particular sensor application (Figure 2) [37].

PCPs can be applied as either optical (luminescence) [38,39,40,41] or electrochemically active sensor materials based on the variation in the electroresistivity due to adsorption on the surface of the electroconductive material (e.g., SnO₂) modified with PCP [42,43]. A number of publications have considered the detection of contaminants in the environment, with some of these papers discussed in a review [44]. In this review, we will focus on the most impressive sensing applications of PCPs and MOFs towards different classes of analytes published mainly in the last 5 years. The biosensors based on PCPs and MOFs have also been reviewed by Lei et al. [37].

2. Luminescent PCP/MOF Materials

Luminescent methods for studying materials are relatively cheap, ergonomic, and provide reliable information about the electronic structure of substances. They have high sensitivity and selectivity. The studies of the properties of PCPs applied in sensors include their luminescence, such as the quenching of Zn(BDC) MOF luminescence after adsorption of ethylamine [45,46]. The emission band at 446 nm was attributed to the fluorescence from intra-ligand emission excited states. Such d¹⁰ metals as Zn⁺² or Cd⁺² are of particular interest, because they exhibit strong fluorescence.

Four-fold catenated diamondoid Zn(II) urocanate (4/5-imidazoleacrylate), having one-dimensional pores, exhibits strong blue fluorescent emission at 499 nm [47]. A similar composition, Zn(II) imidazole-4,5-dicarboxylate, having a helical structure, demonstrates a strong luminescence at 420 nm [48]. MOFs prepared using Zn ions and 2,2′-diimidazole-derived dicarboxylate anions [49] show fluorescence at 414 nm. The optical properties of Zn MOFs are also considered in [50].

The interesting feature of PCPs and MOFs is their ability to change their structure upon adsorption of specific molecules [51]. The interpenetrated framework can adopt the most suitable conformation to maximize the overall binding interactions with the analyte. In such a case, the emission band position is determined by the strength of the host–guest charge transfer interaction. The band gap of PCPs and MOFs can be tailored to absorb visible light via the substitution of oxygen in their frameworks for sulfur and selenium [52]. Porous coordination polymers also demonstrate the so-called breathing effect upon adsorption of certain molecules [53].

The luminescent properties of Cd(II) are similar to Zn(II) in the MOF materials. MOF structures containing rigid Cd(II) and Zn(II) tricarboxylates and flexible imidazolates [54] show fluorescence at 425 nm due to π*-π transitions in the ligands. The luminescence of binuclear and pentanuclear (adipic acid and triazole derivatives) Cd(II)-MOFs has been reported [55,56]. The ligand-to-metal charge transfer emission at 364 nm and 460–495 nm was observed for Cd-MOF based on cyclohexanetricarboxylate and pyrazinedicarboxylate [57,58].

Lanthanide-based MOFs are luminescent materials suitable for sensor applications. Dy and Sm pyridinedicarboxylates [59] demonstrate 3–4 emission bands with maxima in the ranges 480–623 nm for Dy and 560–710 nm for Sm. Eu- and Tb-containing MOFs exhibit strong photoluminescence peaks at 580–695 nm (Eu) and 490–620 nm (Tb) [60,61].

The luminescence of Pb-pyridineacrylate was found [62] to result in two emission bands at 418 and 438 nm due to charge transfer. Phosphorescence of MOFs containing Pb(II), Co(II), Ni(II) pyridinedicarboxylates has also been explored (the red-shifted band at 475 nm for Pb-MOF) [63].

Some Ag-PCP materials with pyrimidine linkers revealed [64] luminescence with the emission bands at 327–360 nm assigned to the optical properties of the organic ligands. [Zn₂(bpdc)₂(bpee)]2DMF (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridyle thylene) was modified with AgNO₃ for the sensing of olefins via the luminescence of silver ions [65].

The JUC-48 material, Cd₃(bpdc)₃(DMF)] 5DMF 18H₂O, or JUC-48 complexed with xanthene Rh6G die, showed good luminescent properties [66,67]. A comprehensive review of the luminescent properties of PCPs/MOFs is presented by Cui et al. [68]. In most cases, composite materials containing PCPs or MOFs as matrices or hosts are studied.

Some of the MOF-based materials demonstrated a multi-targeted sensing activity in the fluorescence mode of operation. Two MOFs were synthesized from 2,6-bis(1-imidazolyl) naphthalene and two different transition metal ions: [Cd(L)(HBTC)] and [Cd(L)(HTTC)]⋅1.5H₂O (L-2,6-bis(1-imidazolyl)naphthalene, HTTC-3,5-bis(p-carboxyphenyl)benzoic acid [69]. The materials demonstrated high sensitivity and selectivity towards Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻, acetone, and nitroimidazole antibiotics present in water.

Similarly, Hasi et al. developed luminescent Cd(II) complexes [Cd(HL)(4,4′-bipy)(H₂O)]_n and Cd(II)-MOF, {[Cd₃(L)₂(bip)₂(H₂O)₃]⋅5H₂O}_n containing rigid tricarboxylic acid ligands (H₃L = 3-[(1-carboxynaphthalen-2-yl)oxy]phthalic acid, 4,4′-bipy = 4,4′-bipyridine, bip = 1,4-di(pyridine-4-yl) benzene) for sensing metal cations, inorganic anions, and organic solvents [70]. Among the analytes studied, a range of inorganic and organic molecules should be noted: Ag⁺, Cr₂O₇²⁻, CrO₄²⁻, nitrobenzene, and nitromethane. The fluorescence quenching caused by the presence of these analytes is explained by weak interactions, competitive energy absorption and electron transfer.

3. Electrochemical Sensors Based on PCPs/MOFs

The other type of sensors is based on semiconducting PCPs or MOFs containing semiconducting Zn, In, W, Sn and Ti oxides or organic semiconducting components, with electroresistivity or electroconductivity as a measurable parameter. A great deal of papers are related to the electrochemical sensors based on bulk oxides (ZnO, SnO₂, In₂O₃) [71,72], but the number of papers focused on PCP-containing electrochemical sensors is limited, because most PCPs are insulators. A. Corma et al. [73] reported the semiconductor properties of Zn-MOF-5. Several recent papers have demonstrated that some other PCPs/MOFs are capable of manifesting a sufficient electroconductivity [74,75,76,77], such as HKUST-1 and Cu₃(btc)₂, containing adsorbed molecules of TCNQ (7,7,8,8-tetracyanoquinodimethane).

Electrocatalysts representing metal nanoparticles supported on PCPs/MOFs may be applied in sensors. An Au-SH-SiO₂@Cu-MOF on glassy carbon demonstrated electrocatalytic activity in the oxidation of hydrazine in a neutral phosphate solution [78]. This material showed a low detection limit, improved sensitivity, and enhanced stability. A similar material was considered for electrocatalytic oxidation and determination of L-cysteine [79]. The detection limits were estimated at 8–10 nM for both analytes.

Y-doped MOF-5 was applied as a sensor material for cataluminescence detection [80] of isobutanol with a detection limit of 3.7 mg L⁻¹.

For an MIL-101-carbon paste electrode, an increased electrocatalytic activity with respect to the oxidation of dopamine and uric acid was found with oxidative potentials of 218 mV (dopamine) and 336 mV (uric acid) [81].

The use of PCPs and MOFs in electrochemical sensors can be enhanced by the use of their composites with conducting materials (indium or tin oxides, carbon nanotubes, etc.). Nanocomposites of multi-walled carbon nanotubes and Cu₃(BTC)₂ were tested for their ability to sense traces of lead in water with differential pulse anodic stripping voltammetry using the lab-on-valve operation [82]. The detection limit was claimed to be extremely low (0.79 nM).

Some of the MOF-based materials demonstrated a multi-targeted sensing activity in the electrochemical mode of operation. For instance, R. Fischer’s group designed a new porphyrinic metal–organic framework (PP-MOF) by supporting Mn-PCN-222 on indium tin oxide [83]. The voltammetric sensor is capable of detecting redox active analytes, including inorganic ions (Cr₂O₇²⁻), heavy metal ions (Cu²⁺, Cd²⁺), organic pollutants (nitroaromatics, phenols, quinone-hydroquinones), biological species (ascorbic acid, uric acid), and azo dyes.

Zhang et al. have reported simultaneous detection of several metal ions (Cd²⁺, Pb²⁺, and Hg²⁺) and herbal medicines in water using a methylene blue-functionalized metal–organic framework, UiO-66-NH₂, combined with multi-walled carbon nanotubes [84]. The low detection limits were 11.08 nM for Cd²⁺, 5.47 nM for Pb²⁺, and 6.42 nM for Hg²⁺.

4. Other Methods of Sensing

Another method of sensor operation is based on the detection of the response of magnetic properties of PCPs sensitive to host–guest interactions, temperature, and the presence of adsorbed molecules. Cu₃(PTMTC)₂(py)₆(CH₃CH₂OH)₂(H₂O) (MOROF-1), having large pores (2.8–3.1 nm) and magnetic ordering, exhibits a reversible and selective analyte-induced ‘shrinking–breathing’ effect, changing the magnetic properties of MOF; this type of magnetic sponge-like behavior may be used in magnetic sensors [85].

The gravimetric method of sensing based on the use of a nanoporous material supported on the surface of a quartz resonator (piezoresistive cantilever) is efficient for sensing molecules in the gas phase, but this method can hardly be used for sensors of molecules in solutions [86,87,88]. Beyond the microcantilever method of sensing, other mechanical methods can be used, such as a quartz crystal microbalance, a microresonator, and surface acoustic wave sensors coated with PCP thin layers capable of enhancing the adsorption of the gas to be analyzed [89,90]. However, these methods cannot be used for sensors of analytes in solutions.

Some metal nanoparticles (Au, Ag), when supported on PCPs or MOFs, can demonstrate absorption bands in the UV–visible range due to the surface plasmon resonance, and the positions and intensities of these bands are sensitive to the interaction with molecules that can be strongly adsorbed on such metal nanoparticles [91]. Additionally, the method based on monitoring the changes of the refractive index of PCP upon adsorption of molecules can be used for sensing [92].

Table 1. Chemical analytes studied using PCP/MOF materials in sensor applications.

Class of Target Chemicals	Representative Examples Available in the Literature
Cations	Fe3+, Al3+, Cu2+, Zn2+, Cd2+, Hg2+, Ag+, Pb2+, Ca2+, Co2+, Ni2+, Cr3+, Mg2+, Mn2+, Bi3+, Pb2+, Cd2+, Ln3+ (Eu3+, Tb3+), Pd2+, K+, etc.
Anions	F−, Cl−, Br−, I−, CN−, SCN−, N3−, NO2−, NO3−, N(CN)2−, CO32−, PO43−, ClO4−, ClO−, BF4−, B4O72−, MnO42−, CrO42−, Cr2O72−, etc.
Inorganic molecules	O2, H2, NH3, NO, NO2, HCl, H2O, H2S, I2, CO, CO2, N2H4, etc.
Organic molecules	Alkanes, alkenes, alcohols, amines, aminoalcohols, aminoacids, nitro aromatic compounds, aldehydes, formaldehyde, dimethyl ether, ketones, phenol, chloroform, DMF, explosives, phosphor-organic compounds, antibiotics, mycotoxins, peracids, oxidized edible oil etc.

summarizes the different types of chemical analytes that have been investigated in the literature using PCP materials in sensor applications. The table includes cations, anions, inorganic molecules, organic molecules, including explosives and phosphor-organic chemicals like Vx gases.

5. Detection of Metal Ions

Sensors to determine the presence of lead, cadmium, chromium, copper, and other toxic metal ions in water are in high demand. A recent review [93] summarizes the latest achievements in this field and provides a comparison of diverse MOF-based materials for sensing metal ions.

5.1. Lead Detection

Jain et al. developed a carbon quantum dot metal–organic framework-based catalytic nuclei acid fluorescent system for the highly sensitive and selective detection of Pb²⁺ in aqueous solutions [94]. The authors used a DNAzyme system that is active only in the presence of lead(II) ions. The main components of the material are boron- and nitrogen-containing carbon dots with carboxyl functionalized Tb-MOF as a fluorescent agent and the quencher-modified catalytic NH₃-GR5 DNAzyme as a bioreceptor species. The systems demonstrate dual fluorescence with emission in the blue and green regions. This biosensor is characterized by the low detection limit of Pb²⁺ of 0.96 ppb in a broad detection range of 2 to 1000 nM.

NH₂–Cu₃(btc)₂ was applied to modify electrodes for the detection of lead. Zn₄O(BDC)₃ supported on a carbon paste electrode was used for lead detection by differential pulse stripping voltammetry [95]. The detection limit was 4.9 × 10⁻⁹ mol L⁻¹.

Olorunyomi et al. [96] used NH₂-UiO-66 modified with gold nanoparticles (1.4 wt % Au) and thiol-functionalized carbon dots by imitating metal–protein interactions to reach high sensitivity and selectivity to Pb²⁺ ions by using a fluorescence method of sensing. The composites demonstrated an extremely high sensitivity to Pb²⁺. A limit of detection of 80 parts per trillion (ppt) was reached, and the material was tested for five cycles. Surprisingly, the fluorescence response of the material can be reversed from emission quenching to enrichment by varying the Au content.

Detection of trace lead ions in water was studied using substitution-triggered chemiluminescence enhancement with Cu-MOF-74 [97]. Pb²⁺ ions replace Cu ions in Cu-MOF-74 and the released Cu ions amplify the chemiluminescence emission of the luminol–hydrogen peroxide component. A linear range of 0.5–50 nM and a detection limit of 0.3 nM were determined for this system.

3D printed composite of UiO-66-NH₂ with rGO as a fiber electrochemical sensor [98] showed excellent Pb²⁺ sensing activity with enhanced conductivity and demonstrated a rather low detection limit of 0.041 μM.

Liu et al. proposed the simultaneous detection of cadmium and lead ions [99]. The amino-functionalized Bi-containing material, NH₂-Bi-MOF, in combination with graphene aerogel, was prepared by ultrasonic deposition. The developed electrochemical sensor demonstrated excellent sensitivity with good accuracy and repeatability.

Another option for electrochemical sensors of Cd²⁺ and Pb²⁺ ions has been developed by Zheng et al. [100]. The two-dimensional material developed by the authors was a composite of polyvinylpyrrolidone/graphene and iron-based metal–organic frameworks manufactured by the ultrasonic method. The composite demonstrates enhanced conductivity and a high specific surface area. The simultaneous detection of Cd²⁺ and Pb²⁺ ions is characterized by linear ranges of 0.030–1.0 μmol/L for Cd²⁺ and 3.0 nmol/L−1.0 μmol/L for Pb²⁺, whereas the limits of detection for these metal ions were 10 nmol/L and 1.0 nmol/L, respectively.

5.2. Copper Detection

A novel Zr−tetraphenylporphyrin tetrasulfonic acid hydrate metal−organic framework (ZTM) was synthesized for the detection of lead and copper in water samples [101]. The material demonstrated strong red fluorescence with the quantum yield reaching 12.22%. Furthermore, the authors designed a ratiometric fluorescent probe by adding green fluorescein isothiocyanate to the material to avoid environmental interferences. The fluorescence emission from MOF at 667 nm was found to be quenched and the fluorescence emission from fluorescein at 515 nm was enhanced, as a result, the color changed from red to green under UV irradiation, depending on the concentration of copper(II) ions in the solution. This fluorescent probe is characterized by the linear detection ranges of 0.1 to 5 μM and 5 to 50 μM and the low detection limit of 5.61 nM.

A composite UIO-66@FS represents the dye fluorescein sodium molecule (FS) with a metal–organic framework. UIO-66 exhibits fluorescence emission with the quantum yield of 62.03%. It was applied for the detection of Cu²⁺ ions in aqueous media. The fluorescence is quenched by Cu²⁺ ions. The system has the advantages of high selectivity, excellent sensitivity, fast response, wide linear range (2.5 μM–500 μM), and low detection limit (0.246 μM) [102].

A porphyrin-based metal–organic framework Al-TCPP (H₂TCPP-meso-tetrakis (4-carboxyphenyl) porphyrin) has been proposed by Liu et al. for the selective sensing of copper ions by monitoring the fluorescence intensities of two emission peaks at 467 and 646 nm, with the latter band being quenched in the presence of copper ions [103]. An extremely low limit of detection was found at 5.28 nM, one of the lowest reported values for Cu²⁺ sensors. The charge transfer from TCPP ligands to Cu²⁺ ions was confirmed by MAS NMR and XPS methods. The sensors (paper strips) were used to determine the copper contents in samples of water, tea and serum.

Copper and iron ions in water can be detected simultaneously using a luminescent lanthanide-functionalized hydrogen-bonded organic framework hydrogel, Tb and Zn-functionalized hydrogen-bonded organic framework (HOF) as the fluorescence functional core, and sodium alginate as the hydrogel matrix [104]. Limits of detection of 1.275 μM for Cu²⁺ and 0.549 μM for Fe³⁺ were reported. The material was applied for the detection of these ions in river water samples. The authors considered disruption of the “antenna”, absorption competition quenching, and ion exchange as the main mechanisms resulting in the fluorescence quenching.

5.3. Cerium Detection

A novel two-dimensional chain material with excellent chemical and thermal stability, based on Eu-MOF [Eu₂(tcpa)(Htcpa)₂], was prepared from 3,4,5,6-tetrachloro-1,2-benzenedicarboxylic acid (H₂TCPA) and Eu nitrate. Upon UV excitation (292 nm), four fluorescence emission bands were visualized at 420, 595, 620 and 705 nm, with the first band intensity enhancing with the addition of cerium ions, and that at 620 nm was quenched [105]. The material demonstrates a good linear relationship in the range of 0.05–15 μM and a detection limit of 16 nM. The mechanism of the Ce³⁺-quenching effect for Eu-MOF fluorescence is the inner filter effect.

5.4. Mercury Detection

A very selective and sensitive Hg²⁺ ion electrochemical sensor based on a composite of silver nanoparticles with zinc benzenedicarboxylate was prepared by drop-casting the Ag@ZnBDC material onto a glassy carbon electrode. The cyclic voltammetry and differential pulse voltammetry were used for sensing mercury ions at the concentration of 1 μM. The limit of detection was found to be 4.16 nM in the linear range of 1–10 nM [106].

A composite containing blue fluorescent carbon dots, red fluorescent InP/ZnS quantum dots, and ZIF-8 was designed and used for the visual detection of Hg²⁺ ions and cysteine. The composite provides a limit of detection of 8.68 nM for Hg²⁺ ions and 37.96 nM for cysteine [107].

5.5. Iron Detection

A hybrid lanthanide metal–organic framework based on porphyrin was designed for the detection of iron ions [108]. The material is characterized by a low detection limit (0.3 μM) and a wide linear range (1–90 μM), which meet the requirements of water quality monitoring.

Mao et al. [109] proposed a multifunctional fluorescence sensor on the basis of a Zn(II) metal–organic framework prepared from zinc nitrate and pyridine-2,5-dicarboxylic acid for the fast and sensitive detection of Fe³⁺ and Al³⁺ ions in aqueous solutions by fluorescence quenching. The limits of detection of Fe³⁺ and Al³⁺ ions were found to be 0.18 μM and 0.064 μM, respectively. The mechanism of the Zn-MOF response was found to be based on the inner filter effect.

He et al. [110] proposed a Eu³⁺ in-situ functionalized Gd-BTC material for selectively detecting acetone and Fe³⁺ ions in aqueous solutions by the fluorescence method. A quenching effect was observed in response to both analytes, with a detection limit as low as 6.8 × 10⁻⁶ M. The energy absorption competition between the Eu³⁺ luminescence centers and the analytes was considered as the mechanism behind the quenching effect.

A fluorescent ZIF-90@FSS material was designed by combining ZIF-90 with sodium fluorescein [111]. In the range of 0–150 ng/mL, a low detection limit of 0.26 ng/mL was demonstrated. The probe was tested using actual samples and living cells. The time of analysis was about 2 min.

There are examples of using MOF materials for detecting several metal ions. For instance, MOF-808(Zr) and MOF -808(Zr)-2D carbon nanocomposites were evaluated for their electrochemical sensing of Cu, Cd, Pb, and Hg ions [112].

5.6. Cadmium Detection

A Ca-MOF was proposed as an electrochemical sensing material with an LOD of 640–1400 μg/L using the anodic stripping voltammetry method [113]. An electrochemical detector based on graphite paste modified with Ca-MOF was manufactured and tested. This sensor detected not only Cd, but also Pb, Cu, Zn ions with a very low LOD of 0.64–1.4 mg/L.

A new MOF material modified with ferrocenecarboxylic acid on thermally reduced graphene oxide (trGNO/Fc-NH₂-UiO-66) turned out to be even more efficient with LOD as low as 8.5 × 10⁻³ using ratiometric electrochemical sensing [114]. The sensor can be used for simultaneous detection of Cd²⁺, Pb²⁺, and Cu²⁺ ions. The LOD values of the electrochemical detector were found to be 0.8 nM for Cu²⁺, 0.6 nM for Pb²⁺, and 8.5 nM for Cd²⁺.

UiO-66 materials modified with ethylenediamine and polyaniline capable of coordinating heavy metal ions via amino groups were found to show excellent selectivity to Cd²⁺ ions with the possibility to simultaneously detect Cd²⁺, Pb²⁺, and Cu²⁺ ions [115,116,117].

Zn-MOF was also able to sensing cadmium ions via the fluorescence method and the LOD value of 5 × 10⁻⁸ M was determined [118].

6. Small Molecules as Analytes

MOF UiO-66-NH₂ was used to create a sensor for ammonia in aqueous solutions. The UiO-66 MOF consists of Zr₆O₄(OH)₄(CO₂)₁₂ clusters and 2-aminoterephthalic acid (BDC-NH₂) linkers. The voids of this MOF can be easily filled with an internal reference molecule, oxazine perchlorate derivative ((5E)-N-Ethyl-9-(ethylamino)-10-methyl-5H-benzo[a]phenoxazin-5-iminium perchlorate), for more reliable quantification of the ammonia content. When excited with UV light at 360 nm in the presence of ammonia, the sensor produces fluorescence at 429 and 652 nm, and the fluorescence intensity at 652 nm for the internal reference molecule remained unchanged. The fluorescence intensity at 429 nm gradually increased with the increase of aqueous ammonia concentration. The detection limit of ammonia was 83.5 nM. The comparison of the specific surface areas of UiO-66-NH₂ and UiO-66-NH₂ modified with oxazine perchlorate derivative, 1224.6 m²/g and 830.8 m²/g, respectively, proved that the voids in UiO-66-NH₂ were partially filled with oxazine perchlorate derivative [119].

A photoelectrochemical (PEC) sensor based on a modified NH₂-UiO-66@Bi₂O₃ photoelectrode matrix is developed for the detection of H₂S. After reaction with H₂S, Bi₂O₃ is transformed on the photoelectrode surface into Bi₂S₃, which has excellent absorption capacity for visible light and opens a new pathway for electron-hole separation, thereby causing effective photocurrent enhancement. It is important to note that the PEC sensor, being highly sensitive and simple, exhibits a wide linear response range in the H₂S concentration changing from 2.5 μM to 100 μM with a detection limit of 0.78 μM [120].

Peracetic acid is extensively used for decontamination of medical devices and agricultural tools, as a decolorizing agent in the textile industry, therefore, peracetic acid may be released into wastewater. In addition, it is used in the synthesis of explosives and drugs, which makes its trafficking control important. A facile and rapid method for the quantitative determination of peracetic acid (PAA) is needed. Probes of the composition [RE(bpm)₂](NO₃)₃ and [RE(bpy)₂](NO₃)₃, where RE = Eu, Tb, bpm = 2,2-bipyrimidine and bpy = 2,2 -bipyridine, were introduced into the micropores of bio-MOF-1 via ion exchange with different relative doping levels [121]. Bio-MOF-1 has the composition [Zn₈(ad)₄(BPDC)₆O₂(Me₂NH₂)⁺]•G, ad = adenine, BPDC = 4,4-biphenyldicarboxylic acid, G = N,N-dimethylformamide and water. The interaction between PAA and the probe ligands (bpm and bpy) excites the triplet state of the ligand, which in turn leads to a change in the intensity of the luminescence emission lines of Tb(III) and Eu(III), and the change in luminescence intensities served as a ratiometric signal for the determination of PAA. Porous bio-MOF-1 improves the working conditions of rare earth probes by dispersing the probe molecules throughout the voids of bio-MOF-1, which leads to uniform quenching of luminescence. For the Eu:Tb(bpm)@MOF frameworks, four metal concentrations were tested to find the optimal doping level: Eu:6Tb(bpm)@MOF, Eu:8Tb(bpm)@MOF, Eu:10Tb(bpm)@MOF, and 0.1Eu:0.8Tb(bpm)@MOF. X-ray diffraction study of Eu:Tb(bpm)@MOF and Eu:Tb(bpy)@MOF showed that they are single phases. The specific surface area of bio-MOF-1 was 760 m²/g, and after loading of the probes into bio-MOF-1, the micropores were occupied and the surface area decreased to 530 m²/g for Eu:6Tb(bpm)@MOF. The interaction of PAA with the ligands (bpm and bpy) regulated the ligand triplet level (T1), which resulted in the change of the Tb(III) and Eu(III) emission line intensities (increasing Tb(III) emission and decreasing Eu(III) emission). For bpm-based samples, the linear calibration curves were obtained with the maximum sensitivity of 8.01, response time of ∼36 s, LOD of 0.3 μM and working region of 0–70 μM. Such promising PAA sensing efficiency was observed only for bpm-based samples, while bpy-based samples showed poor sensing efficiency. It was suggested that the presence of several N atoms in the bpm ligand plays a key role in controlling the sensitivity of the probe to PAA.

A flexible substrate sensor for monitoring NO₂ was developed. Bacterial cellulose (BC) has a high mechanical strength and porous macrostructure. According to the authors, the MOF composition with electrically conductive BC is the best choice for obtaining flexible electronics with both gas and stress/strain sensitivity. Taking into account the outstanding adsorption capabilities of zeolitic imidazolate frameworks-8 (ZIF-8) towards NO₂ and the strong binding of polypyrrole (PPY), the authors fabricated a new dual-functional flexible NO₂ and stress/strain sensor, designated as Def-ZIF-8/PPY/BC, which was obtained by growing defective ZIF-8 (Def-ZIF-8) on BC in the presence of PPY. Due to partial hydrolysis, the ZIF-8 framework became defective, which resulted in a significant improvement in the room temperature sensitivity to NO₂ and antimicrobial activity. According to the powder XRD patterns of Def-ZIF-8/PPY/BC, the crystalline structure of ZIF-8 is destroyed after formation of the defects, but the specific surface area of Def-ZIF-8/PPY/BC is 163.4 m²/g. The defects allow the sensor based on Def-ZIF-8/PPY/BC to demonstrate a response intensity to 80 ppm NO₂ that is 2.57 times higher than that of the flexible sensors based on ZIF-8/PPY/BC at room temperature. In terms of stress/strain measurement performance, the flexible sensor based on Def-ZIF-8/PPY/BC demonstrates high sensitivity (6.44 kPa⁻¹) with fast response and recovery time (0.7/0.4 s) and exceptional cyclic stability (8000 cycles). It also exhibits remarkable antibacterial properties (over 99% against E. coli, S. aureus, and P. aeruginosa [122].

Oxidation of edible oils significantly reduces their quality, so it is necessary to quickly and reliably determine the degree of oxidation in each batch of oil. The sensor contained ZIF-8 crystals 60–70 nm in size and CsPbBr₃ crystals grown in the voids of the framework crystals and not exceeding 5 nm [123]. Comparison of the specific surface areas of ZIF-8 (1659.38 m²/g) and CsPbBr₃@ZIF-8 (629.40 m²/g) showed that not all framework voids are filled with nanocrystals. As a result of storage for 14 days, CsPbBr₃@ZIF-8 retained about 90% of the fluorescence intensity, while CsPbBr₃ nanocrystals retained about 50%. About 50% of the fluorescence intensity of CsPbBr₃@ZIF-8 was retained under 12-h UV irradiation, while CsPbBr₃ retained only 5%. CsPbBr₃@ZIF-8 in water retained about 50% of the fluorescence intensity after seven days, while the fluorescence of CsPbBr₃ was almost completely quenched on day 7. A linear dependence of the oil acidity (in the range of 0.04–6.0 mg KOH/g) and the relative fluorescence intensity of the sensor was obtained. The LOD did not exceed 0.06 mg KOH/g.

A ratiometric fluorescent sensor based on UiO-66-NH₂, an excellent water-stable zirconium MOF, was carefully designed and successfully applied to evaluate the aqueous ammonia level in hair dye and household water samples [124]. The detection limit was as low as 83.5 nM. In addition, a novel detection mechanism based on the reaction of Brønsted acid sites was demonstrated. The recovery rate was ranged from 98.2 to 102.9%.

7. Sensing of Inorganic Anions

Inorganic anions are among the most common water and food pollutants. Groundwater across much of Europe contains nitrate (NO₃⁻) concentrations exceeding the World Health Organization (WHO) guideline for drinking water quality (50 mg/L). The primary source of nitrate contamination is agricultural activity [125]. In biological systems, NO₃⁻ can be reduced to NO₂⁻, which is considerably more toxic due to its ability to oxidize iron in hemoglobin, impairing oxygen transport [126]. High levels of NO₃⁻ and PO₄³⁻ in surface waters promote excessive algal and plant growth (eutrophication), which depletes dissolved oxygen and can lead to massive die-offs of aquatic organisms, further deteriorating water quality [127]. In several regions, including India, Poland, Kenya, and China, elevated levels of fluoride (F⁻) have been reported in groundwater. Chronic exposure to high fluoride levels can result in dental fluorosis or even skeletal fluorosis [128]. Thus, the detection of inorganic anions in aqueous systems is a timely research challenge (

Table 2. Sensing of inorganic anions using MOF materials.

MOF-Based Sensor	Pollutant	Method	Detection Limit, µM	Linear Range, µM	Ref.
Bi-MOF/CNFs	NO2−	Amp.	0.000184	0.002–2000	[129]
CoN-PCRs	NO2−	Amp.	0.14	0.2–4000/4000–10,000	[130]
CNF/PANI/ZIF-8	NO2−	Vol.	8.1	16–835	[131]
Ru@NH2-UiO-66	NO2−	Flu.	0.5	0.6–1.5	[132]
CDs/NH2-UiO-66	NO2−	Flu.	0.76	0.5–2	[133]
Eu@CMERI	F−	Flu.	1.5	-	[134]
NH2-MIL-101(Fe)	CO32−	Flu.	0.014	-	[135]
NH2-MIL-101(Fe)	PO43−	Flu.	0.131	-	[135]
amEu-ENX/NH2BDC	ClO−	Flu.	0.03	0.05–40	[136]
[Zn(L)(H2O)]⋅DMF	Cr2O72−	Flu.	0.23	50–600	[137]
[Zn(L)(H2O)]⋅DMF	CrO42−	Flu.	2.06	20–800	[137]

Amp.—amperometry, Vol.—voltammetry, Flu.—fluorescence.

).

Electrochemical sensors are well-suited for the detection of NO₂⁻ due to its inherent redox activity. MOFs owing to their high surface area, tunable pore structure, and chemical functionality, offer versatile platforms for the design of electrode materials in such sensing systems. Aul et al. [129] used a Bi³⁺-based MOF with biphenyl-4,4′-dicarboxylic linkers to fabricate a composite with carbon nanofibers by ultrasonication. The composite was deposited onto a glassy carbon electrode and utilized in an amperometric electrochemical cell. Detection of NO₂⁻ and NO was performed at pH = 7, with broad oxidation peaks observed at E = +0.62 V. For NO₂⁻, a wide linear detection range was achieved (2 nM–2 mM). The presence of a 15-fold excesses of common cations and a 25-fold excesses of anions increases the analytical signal for NO₂⁻, while no response was detected in the absence of NO₂⁻, indicating high selectivity of the electrode. Storage for over 7 days had no significant effect on the electrode performance, demonstrating excellent stability.

A composite based on mixed metal (Co, Zn) imidazolate framework (ZIF) with polyvinylpyrrolidone was employed for the preparation of a carbon material by pyrolysis at 800 °C under Ar [130]. The resulting carbon material containing metallic cobalt nanoparticles was supported onto the surface of a magnetic glassy carbon electrode. In a 0.1 M phosphate buffer solution (pH 4), NO₂⁻ exhibited a broad oxidation peak at E = +0.9 V. The analytical response displayed two distinct linear ranges: 0.2–4000 µM and 4–10 mM. The addition of a 50-fold excesses of NaCl, CaCl₂, MgCl₂, and glucose did not exhibit any significant impact on the NO₂⁻ signal. After 20 days of the electrode storage at 4 °C, the analytical signal decreased by 20%, indicating acceptable stability of the sensing system.

The electrochemical performance of the imidazolate framework ZIF-8(Zn) was enhanced by incorporating it into a ternary composite with carbon nanofibers (CNF) and polyaniline (PANI) under ultrasonic treatment (Figure 3) [131]. The CNF/PANI/ZIF-8 composite was applied onto a glassy carbon electrode and employed for NO₂⁻ detection using square wave voltammetry in an acetate buffer solution (pH 4). The ternary composite exhibited an oxidative peak at E = +1.0 V, which was absent for CNF/PANI or ZIF-8. A linear response to NO₂⁻ was observed over the concentration range of 16–835 µM, with a detection limit of 8.1 µM. Over a 60-day period, the sensor signal changed by only 6%, confirming its high long-term stability. A significantly simpler and more accessible approach for NO₂⁻ detection can be realized by fluorescence analysis, including visual observation or quantification using any smartphone camera. In a study [132], the Zr⁴⁺-based MOF (NH₂-UiO-66) constructed from 2-aminobenzene-1,4-dicarboxylic linkers was synthesized in the presence of tris(2,2′-bipyridyl)ruthenium(II). Under UV illumination (365 nm), NH₂-UiO-66 exhibited intense blue fluorescence (474 nm), while the Ru(II) complex emitted red fluorescence (608 nm). In the presence of NO₂⁻ (in 0.06 M H₂SO₄), selective interaction with the amino groups of the MOF framework led to a decrease of the blue emission, which makes the red signal visible. This blue to red fluorescence change serves as an effective visual indicator, allowing for quantitative NO₂⁻ detection via smartphone imaging. The method achieved a detection limit of 0.5 µM and exhibited a linear response in the range of 0.6–15 µM, with acceptable correlation maintained up to 45 µM—a range that overlaps well with the threshold limit value of NO₂⁻ in drinking water (65 µM).

A more cost-effective reference fluorophore for fluorescence detection can be provided by carbon dots (CDs). In a study [133], Rhodamine B based CDs were grafted onto NH₂-UiO-66 by ultrasonic treatment of their dispersion. NO₂⁻ detection was performed after incubating the sample in 1.5 mM H₂SO₄ for 60 min at 70 °C. With increasing NO₂⁻ concentration, the fluorescence (365 nm) intensity of NH₂-UiO-66 (454 nm) gradually decreased, while the emission of the CDs (581 nm) remained constant. The system exhibited a linear response in the range of 0.5–20 μM with a detection limit of 0.76 μM. In contrast, the use of NH₂-UiO-66 fluorescence without CDs resulted in a significantly narrower linear range (0.5–10 μM) and lacked visual detectability. High concentrations of common inorganic cations and anions showed no significant impact, indicating excellent selectivity of the CDs/NH₂-UiO-66 sensor.

Fluorometry can also be used for the detection of F⁻ in water using an Eu³⁺ based MOF constructed from a 2,6-naphthalenedicarboxylic linker (Eu@CMERI) [134]. This material exhibits intense red fluorescence (615 nm), which is selectively quenched in the presence of 10⁻⁴ M F⁻. The quenching response is highly selective toward F⁻ compared to other common anions. Eu@CMERI in the composition of paper test strips was successfully applied for F⁻ detection in various environmental samples, including bovine serum, well, river water, and toothpaste.

NH₂-MIL-101(Fe), a Fe³⁺ based MOF constructed from 2-aminobenzene-1,4-dicarboxylic acid linkers exhibits no significant fluorescence under UV irradiation (360 nm) [135]. However, in the presence of CO₃²⁻ and PO₄³⁻ ions, the material displays blue fluorescence, with detection limits of 14.13 nM and 131 nM, respectively. The calculated absolute quantum yields were 38.8% for CO₃²⁻ and 23.3% for PO₄³⁻. This anion-induced fluorescence activation phenomenon suggests potential applications of NH₂-MIL-101(Fe) as a UV-responsive anti-counterfeiting tag.

Amorphous MOF based on Eu³⁺ ions, 2-aminobenzene-1,4-dicarboxylic acid (abdc), and enoxacin as co-ligands was developed for the detection of hypochlorite (ClO⁻) [136]. The material exhibits dual-emission fluorescence, with blue emission (427 nm) attributed to the abdc linker and red emission (611 nm) resulting from Eu³⁺ sensitized by coordinated enoxacin. In the presence of ClO⁻, a selective decrease of the blue emission occurs, enabling ratiometric detection. The sensor shows a linear response in the range of 15–40 μM with a detection limit of 0.50 μM. This sensing system was successfully applied for the detection of ClO⁻ in chlorinated tap water.

Fan et al. [137] used Zn²⁺ based MOF constructed from 4-[6-(4-carboxyphenyl)pyrazin-2-yl]benzoic acid that exhibited blue fluorescence (413 nm) under UV excitation (325 nm). The fluorescence intensity is selectively quenched in the presence of riboflavin, as well as dichromate (Cr₂O₇²⁻) and chromate (CrO₄²⁻) ions. The sensor material can be reused up to five times without significant loss of the performance.

Detection of As(III) as an arsenite ions in water media became possible using a As(III)-MIM@MOF/AuNPs/GCE composite sensor, which exhibits a wide linearity range (1.0 × 10⁻¹¹ to 3.0 × 10⁻⁵ mol/L) and a low detection limit (3.0 × 10⁻¹³ mol/L) [138]. The sensitivity was enhanced by inclusion of MOF/Au nanoparticles.

A hybrid material containing graphene oxide, UiO-67 and Pt nanoparticles was used for detection of As(III) in water samples [139]. The high specific surface area of UiO-67 is beneficial for adsorption and concentration of As(III), whereas graphene oxide provides improved electron transfer, and platinum nanoparticles are considered as selective recognition sites for As(III) via the electrocatalytic action. The wide linear range of 2.7–33.4 nM and the low detection limit of 0.48 nM, which is lower than the standard of drinking water used by the World Health Organization (10 μg/L). The material shows no significant interference of other analytes, good reproducibility and reliability.

8. Sensing of Antibiotics and Pharmaceuticals

8.1. Antibiotics

Over the past 5 years, intensive work has been carried out on the development of MOFs for the detection of antibiotics in food and water. First of all, it is worth noting the development of sensors (e.g., optical, electrochemical, or colorimetric) based on MOFs for the detection of traces of tetracycline (TC) antibiotics [140,141,142,143]. Tetracycline antibiotics are widely used as broad-spectrum antibiotics, but their excessive use leads to environmental pollution and threatens human health. Existing detection methods (mass spectrometry, etc.) are complex and cumbersome and do not allow for rapid detection. Therefore, it is of utmost importance to develop a detection technology for the rapid, on-site detection of full-spectrum TCs residues. During TC-MOF interactions (coordination with exposed metal sites via electrostatic interactions, hydrogen bonding, acid-base bonding, π-complexation and hydrophobicity), the change of electrical, optical, and spectral signals can be potentially used for the detection of the target analyte [140]. In luminescent/fluorescent sensors, several mechanisms can explain the fluorescence quenching of MOF-based sensors, such as Förster resonance energy transfer (FRET), inner filter effect (IFE), photoinduced electron transfer (PET) and excitation light competition between MOF and analyte [140]. Luminescent metal–organic frameworks (LMOFs) as sensors can effectively detect pollutants such as heavy metals or antibiotics. Main characteristics of monofunctional sensors as well as bifunctional ones as well as the proposed mechanisms of quenching are given in

Table 3. Characteristics of monofunctional MOF-based sensors for TC determination in food samples.

Type of MOF-Based Sensors	MOF (Metal/Ligand)	Mechanism of Detection	Samples	Linear Range	LOD (nM)	Response Time (s)	Ref.
Monometallic	Eu(NO3)3·6H2O/H3BDC	IFE and PET	Milk and beef	0–140 μM	9.8	60	[141]
	Tb(NO3)3·6H2O/H2L	IFP	Raw milk and pasteurized milk	0–100 μmol/L 100–244 μmol/L	84	120	[142]
	InCl3/H2sbdc	FRET	Milk, pork and fish	0–30 μm	280	-	[143]
	Tb(NO3)3·6H2O/H3L1	IFE	Tap water	0.06–10 mg/mL	18	-	[144]
Modified	AlCl3·6H2O/NH2-BDC	IFE and PET	Milk	0.001–53.33 μM	0.53	480	[145]
	Zn(NO3)2·H2O/2-MIM	Chelation of Zn2+ and TC, π-π stacking and H-bonding 2-MIM and TC	Drink water, milk, beef samples	0.01–1 μM 5–30 μM	2.4	180	[146]
	ZrCl4/PMA	IFE and PET	Milk, pork, honey and tap water	0.01–200 μM	12.8	60	[147]
	Zn(CH3COO)2·H2O/2-MIM	IFE	Drinking water and tap water	0–40 μM 40–150 μM	110	-	[148]
	ZnCl2/2-MIM and l-histidine	Electrostatic interaction and H-bonding	Milk and honey	0.1–20 μM	28.6	60	[149]
	Zn(CH3CO2)2·H2O/BDC	Antenna effect	Milk and pork	0–20 μM	18.5	120	[150]
	AlCl3/NH2-BDC	Electrostatic/π-π stacking and Van der Waals	Beef, chicken, milk, honey	0.02–0.375 ng/mL	0.12	-	[151]
	AlCl3·6H2O/NH2-BDC	IFE and PET	Milk	0.00–72.33 μM	26.16	30	[152]
	ZrCl4/BDC	-	Honey and milk	0.1 and 6 μM	17.9	60	[153]
	ZrCl4/p-CBA	Energy transfer from UiO-66-(COOH)2/p-CBA to TC	Pig kidney	0–50 μM	26	30	[154]
	FeCl3·6H2O/H2BDC	-	Tap water	0.0643–1.53 μmol/L	26	-	[155]
	AlCl3·6H2O/NH2-H2BDC	-	Tap water	0.5–60 mM	160	60	[156]
Composite	FeCl3·6H2O/NH2BDC	-	Tap water	0.1–105 nM	0.1	120	[157]
	ZrCl4/H4BTCC	IFE and Antenna effect	Milk and Honey	0–4 μM	3.16	14	[158]
	Zn(NO3)2·H2O/2-MIM	-	Raw and pasteurized cow Milk	20–650 nM	4.8	300	[159]
	ZIF-8@PCN-128Y	-	Milk and beef	0.4–200 μM	60	25	[160]
	In(NO3)3·H2O/H2BDC	-	Milk	0.01–500 nmol/L	0.0033	-	[161]
	ZIF-8	Electron transfer	Milk	1 pM–100 nM	0.0001	-	[162]

H₃BDC: Trimesic acid; IFE: Inner filter effect; PET: Photoinduced electron transfer; FRET: Förster resonance energy transfer; H₂sbdc: 5,5-dioxo-5H-dibenzo[b, d]thiophene-3,7-dicarboxylic acid; H₂L: 5-(pyridin-4-ylmethoxy) isophthalic acid; PMA: Benzene-1, 2, 4, 5-tetracarboxylic acid; NH₂-BDC: 2-Aminoterephthalic acid; BDC: 1,4-benzene dicarboxylic acid; MIM: 2-methylimidazole; H₃L₁: 1,1′:3′,1″- terphenyl]-4,4″,5′-tricarboxylic acid; p-CBA: 4-Carboxybenzaldehyde.

and

Table 4. Characteristics of bifunctional MOF-based sensors for TC determination in food samples.

Type of MOF-Based Sensors	MOF (Metal/Ligand)	Mechanism	Samples	Absorption Capacity (mg/g)	Linear Range	LOD (nM)	Response Time (s)	Recovery (%)	Ref.
Bimetallic	ZrCl4 and Eu(NO3)3/BDC	FRET	Pork	289	0.001–0.5 μg/mL	2.07	360	93.16–104.16	[163]
Modified	EuCl3·6H2O/H2BIPA and 2-MIM	Antenna effect	Tap water, milk	387.14	0.05–60 μM	3	300	94.89–112.06	[164]
	AlCl3·6H2O/NH2-BDC	IFE and PET	Milk	533	0.004–38.5 μg/L	2.7	480	87.07–116.44	[165]
Composite	Zn(NO3)2·6H2O/2-MIM	Antenna effect	Milk, honey and tap Water	377.07	0–70 mM	17	120	96.9–104.6	[166]
	Zn(NO3)2·6H2O/2-MIM	IFE and interaction between TC and Zn2+	Milk	1974.03	0.5–50 μM	6.56	60	94.83–101.79	[167]
	ZrCl4/BPDC	IFE and FRET	Water, fish and sheep muscle	427.35	0.08–20 μg/L	141.78	900	87.12–107.52	[168]

IFE: Inner filter effect; PET: Photoinduced electron transfer; FRET: Forster resonance energy transfer; 2-MIM: 2-methylimidazole; BDC: 1,4-benzene dicarboxylic acid; NH₂-BDC: 2-Aminoterephalatic acid; BPDC: biphenyl-4,4′-dicarboxylic acid.

using the example of sensing of antibiotics [140].

A novel 2D (two-dimensional) Cd-LMOF LCU-125 was fabricated by introducing rich amide active functional sites to serve as a multi-sensitive luminescent sensor [169]. As expected, LCU-125 could selectively and sensitively detect Fe³⁺, Cr₂O₇²⁻ and OTC (oxytetracycline) through the quenching effect in both water and biological buffer solution. Specifically, the limits of detection (LOD) of Fe³⁺, Cr₂O₇²⁻ and OTC were as low as 0.0441/0.0084 μM, 0.0849/1.38 μM, and 1.46 μM (OTC in the buffer), respectively. The authors [170] developed a novel selective sensitive chemiluminescence sensor using blue nitrogen-doped carbon dots (BCDs) and europium metal–organic framework (Eu-MOF) (Scheme 1). The BCDs/Eu-MOF sensor is designed to detect the full spectrum of TCs, where TCs can “switch off” the blue fluorescence of BCDs via IFE while sensitizing Eu³⁺ resulting in intense red fluorescence. This enables the fluorescence of the sensor to achieve a visual continuous color change from blue to violet to red with a low LOD (1.37 to 6.49 nM) with a response time of less than 1 s. Furthermore, the detection method demonstrates an effective detection capability of TCs in real complex samples. In addition, the sensor can be encapsulated in a non-fluorescent glass capillary and combined with a smartphone color recognition application to form a portable capillary chemiluminescence sensor detection kit, which can quickly and sensitively detect TC residues with a LOD of only 1.90 nM using TC as a reference. The developed selective chemiluminescence sensor is characterized by a rapid response, simplicity, high sensitivity and ease of operation, thereby providing a reliable quantitative detection method for TC in the environment.

The luminescent principle of the sensor for detection of tetracycline residues and other ions in aqueous media was described by Yuana et al. [171]. In this work, a 1D Eu coordination polymer {[Eu₂(pyia)₃(H₂O)₄](bpy) 3H₂O}_n (ECP1) was successfully prepared by the hydrothermal reaction and used as a detection reagent for identification of cations, anions, antibiotics, and pesticides. In contrast to fluorescence quenching sensors of many lanthanide complexes, Cu²⁺, B₄O₇²⁻, tetracycline, and prochloraz (Pro-imidazole fungicide) induced fluorescence enhancing properties of ECP1, which significantly contributed to achieving the lowest detection limits of pollutants at the ppm level. The fluorescence enhancement was due to the synergy of the static response, photoexcited electron transfer, and interactions between ECP1 and the analytes.

A number of works are devoted to the development of MOF-based sensors for the detection of antibiotics and other classes of drugs [172,173,174,175,176,177]. Thus, Yonga et al. [172] presented a luminescent Zn-MOF constructed by a thiazolo [5,4-d]thiazole ligand, which has a chromophore group that serves as a sensor for the detection of cobalt(II) and antibiotics. The crystal structure of {[Zn₂(OBA)₂(4bpyttz)]2DMF} (1) (4bpyttz = 2,5-bis(pyridin-4-yl)thiazolo [5,4-d]thiazole, H₂OBA = 4,4-oxybis(benzoic acid)) showed a three-dimensional (3D) network structure of the 6-c framework with a novel topological type. It has a unique columnar-layered structure and a novel topology due to the flexible dicarboxylic acid. The open active sites in the MOFs arising from the vacancy of N atoms in the chromophoric ligands can be effectively utilized for analyte trapping. Moreover, the luminescence sensing studies showed that 1 exhibits high selectivity and multireactivity for multiple ions, mainly for Co²⁺, and nitrofuran antibiotics, mainly for nitrofurazone (NFZ) and nitrofurantoin (NFT)) in aqueous media, causing turn-off effects. The detection limits were determined to be 1.14 μM, 0.14 μM, and 0.072 μM for 1, respectively. In addition, fluorescent test papers and composite films prepared from MOFs were used for the visual detection of Co²⁺, NFZ, and NFT, thereby expanding the potential applications of the system. In addition, the potential luminescence quenching mechanism of 1 toward Co²⁺ ions and nitrofuran antibiotics could be deduced through various characterization methods.

Zhang et al. [173] developed an approach to use lanthanide MOFs (Ln-MOFs) for the determination of antibiotics. Such framework structures are interesting due to the unique photoactivity of Ln³⁺ ions, such as characteristic narrow and strong emissions, high quantum yields, long lifetimes, large Stokes shifts, etc. In this work, a dual-emitter Ln-MOF Tb-TCPB (H₃TCPB = 1,3,5-tris(1-(2-carboxyphenyl)-1H-pyrazol-3-yl)benzene) is used for the ratiometric determination of antibiotics (Figure 4). Tb-TCPB can specifically detect NFZ and furazolidone through the ligand to Tb³⁺ ratio of emission intensities with high sensitivity and selectivity, low detection limits, and recyclability.

The authors [174] synthesized colorless flower-like crystals of [Zn(Cei)] using bis(2-carboxyethyl)isocyanurate (H₂Cei) by a simple process at room temperature and tested their photoluminescence sensing and photocatalytic degradation of antibiotics. Photoluminescence characterization of [Zn(Cei)] showed that it is very sensitive and selectively detects norfloxacin and nitrofurantoin in an aqueous solution at 0.323 and 0.590 ppm, respectively. Also, [Zn(Cei)] showed excellent photocatalytic activity against sulfonamides ornidozole and chloramphenicol with degradation efficiencies of 90% and 86% within 120 min under UV irradiation, respectively, good stability and reusability of the photocatalyst.

Ofloxacin (OFL), a widely used antibiotic, is of particular concern due to its potential to contaminate milk and surface water. The authors developed [175] a novel aptasensor-based personal glucometer using a porous spherical cerium metal–organic framework (Ce-MOF) as a loading platform for glucose oxidase (GOx) and oligonucleotide sequences (Ce-MOF-GOx-cDNA). The hybrid probe formed by conjugating Ce-MOF-GOx-cDNA with aptamer-modified magnetic beads enabled the specific recognition of OFL through base pairing. The sensor showed a detection range of 50 pg/mL to 500 ng/mL with a detection limit of 40 pg/mL under optimal conditions. The process showed excellent selectivity, stability and reproducibility. Testing of real samples in spiked milk and surface waters showed recovery rates of 99.5–108%, with relative standard deviations below 4.7%.

Zhang et al. [69,176] proposed to use two novel metal–organic frameworks synthesized from 2,6-bis(1-imidazolyl)naphthalene and transition metal ions. The materials were comprehensively characterized by single crystal X-ray diffraction, powder X-ray diffraction, elemental analysis, infrared spectroscopy, and thermogravimetric analysis, further confirming their high purity, thermal stability, and water resistance. In addition, the fluorescence sensing of MOFs has been extensively tested and was shown to be highly sensitive and selective to nitroimidazole antibiotics, particularly 1,2-dimethyl-5-nitroimidazole, as well as Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and acetone.

One of the quinoxalines, olaquindox (OLA) is widely used in livestock and aquaculture due to its potent antimicrobial effects and ability to improve protein digestion. Studies of OLA metabolites excreted in pig urine using ¹⁴C-labeled compounds, thin-layer chromatography and high-voltage electrophoresis revealed the presence of the parent drug. The European Union and many other countries have banned the addition of OLA to animal feed. A nanoreactor based on a self-assembling method has been successfully used for rapid and accurate detection of OLA [177]. The voids of the ZIF-8 framework were used as nanoreactors. The nanoreactors contained gold clusters, horseradish peroxidase, invertase, and antibodies. On this basis, a highly sensitive lateral flow fluorescence immunoassay was developed. Based on the catalytic action of horseradish peroxidase on tetramethylbiphenyl, the authors developed a visual colorimetric detection method without an excitation light source. A smartphone was used for accurate detection. In addition, after the catalytic reaction between the substrate solution and nanoreactor, a personal glucometer acted as an electrochemical sensor. The results show the lowest LOD of the assay for OLA of 0.08 μg/L and good specificity. Peak recoveries in environmental water and swine urine samples ranged from 85.2% to 112.9%.

8.2. Pharmaceuticals

Non-steroidal anti-inflammatory drugs (NSAIDs), as a water pollutants, pose a potential hazard to the environment and human health. Sha and Yan [178] used a dual emission sensor Eu@MIL-53-TDC (TDC = 2,5-thiophenedicarboxylic acid) for practical purposes by introducing Eu³⁺ ions into MIL-53-TDC through post-coordination modification. One source of dual emission is fluorescence emission of MIL-53-TDC and another emission produced by sensitization of Eu³⁺ ions through the antenna effect. MIL-53-TDC is characterized by good stability to water. The TDC ligand was chosen for two reasons: it is as an antenna ligand, which can sensitize Eu³⁺ ions to emit characteristic fluorescence; and it is a binding site into which the sulfur atoms on the thiophene ring can bind Eu³⁺ ions via coordination. Therefore, the Eu³⁺ hybrid materials MIL-53-TDC (Eu@MIL-53-TDC) were developed as a fluorescent sensor for the detection of two kinds of NSAIDs, S-ibuprofen (S-IBP) and diclofenac (DCF). The concentration range of S-IBP and DCF detected by the prepared sensors is 0.001–0.07 μM (LOD = 0.5 μM) and 0.0005–0.1 μM (LOD = 0.2 μM), respectively. Moreover, this sensor not only can achieve rapid (3 min) and sensitive analysis of these two NSAIDs, but also has satisfactory recovery rate for the detection of S-IBP and DCF in serum and tap water.

Intensive use of malachite green (MG) has resulted in significant water pollution, and its metabolite, leucomalachite green (LMG), poses serious health hazards. Rapid detection of drug residues in aquaculture is critical for food safety. Fu et al. [179] synthesized a dual-emission fluorescent sensor RhB@Tb-MOF (RhB = rhodamine B) based on fluorescence resonance energy transfer (FRET).

MG significantly quenches the fluorescence of RhB and affects the fluorescence of Tb-MOF to varying degrees, making it a suitable ratiometric fluorescent sensor for the sensitive detection of MG with LOD of 9.6 × 10⁻³ μg/mL. Although LMG did not affect the fluorescence of RhB@Tb-MOF, it formed a ratio with the fluorescence of RhB@Tb-MOF, and the LOD was 3.5 × 10⁻³ μg/mL (Figure 5). In addition, increasing the concentration of MG changed the color of the sensor system from pink to blue, with a colorimetric LOD of 0.15 μg/mL. Thus, a colorimetric/fluorescence dual-mode sensor for the detection of MG and LMG was developed. The same problem related to the detection of MG was solved by Jiang et al. [180]. UiO-66-NH₂@RhB MOF with a dual feedback signal was used for the visual detection of MG by integrating it with a smartphone. The quenching of red fluorescence was attributed to the internal filtration effect between UiO-66-NH₂@RhB and MG, while blue fluorescence was enhanced due to the hydrogen bonding interaction between the amino group of the fluorescent probe and the OH group of MG. The ratiometric sensor exhibits superior color resolution and improved visualization compared with the monochromatic fluorescence sensor. The fluorescence color showed a distinct transition from red to blue when exposed to UV light at 365 nm. Using smartphone applications, the concentration of MG can be accurately determined by calculating the RGB values of the captured images. The method has the advantages of on-site analysis, rapid analysis, low cost, high sensitivity and visualization for the detection of MG in real water and fish samples.

Levofloxacin (LEV) is used in pharmaceuticals for the treatment of bacterial infections, but is rarely metabolized by the human body and hence is largely excreted. This leads to its accumulation in wastewater, which poses a risk to human health and the environment. Zhou et al. [181] proposed a modified screen-printed electrode (SPE) using a Cu–MOF derivative as an electrochemical sensor for the detection of LEV. It had a large specific surface area and highly dispersed active sites, which facilitated the complete contact with LEV. The electrochemical behavior of LEV was evaluated using cyclic voltammetry in the potential range of −0.2 to 1 V. The linear response range was 0.1–100 μM, the sensitivity was 1855 μM⁻¹ cm⁻², and the detection limit was 0.016 μM. The detection limit and sensitivity of differential pulse voltammetry were 0.17 μM and 183 μA mM⁻¹ cm⁻², respectively. The detection limit and sensitivity of chronoamperometry were 0.037 μM and 825 μA mM⁻¹ cm⁻², respectively. The method demonstrated significant improvement in the selectivity and stability. The use of the electrochemical sensor containing Cu-MOF is a simple and rapid method for determining LEV in medicine and food industries.

Another example of an electrochemical approach to creating an electrochemical sensor for detecting the analgesic acetaminophen (AP) or paracetamol is presented by Afzal et al. [182]. The sensor was based on a composite of UiO-66-NH₂ and MXene (Ti₃C₂). UiO-66-NH₂ was synthesized in situ on MXene via a one-step hydrothermal process with varying MXene loadings, followed by calcination at 300 °C under an argon flow. This treatment induced the formation of TiO₂ on the MXene surface and increased the interlayer spacing, which improved its electrochemical performance. The resulting UiO-66-NH₂@Ti₃C₂-C electrode exhibited remarkable electrochemical activity due to the large surface area and excellent conductivity of MXene. The fabricated sensor demonstrated a simple but effective approach for the rapid and quantitative detection of AP with a linear detection range of 0.032–160 μM and a low detection limit of 10 nM. Moreover, the sensor was successfully applied to detect AP in various water samples.

9. Detection of Mycotoxins

Mycotoxins are harmful to human health at very low concentrations. However, their high chemical stability makes it difficult to prevent mycotoxins from entering food products. Fast and reliable detection of mycotoxins at each stage of agricultural production and processing of raw materials is necessary, but due to the low concentration in samples it is necessary to resort to a long, expensive and labor-intensive solid-phase extraction.

Composites of MOF and melamine foamed sponges have been proposed to produce robust sensing materials. Melamine foam sponge (MF) is a commercially available flexible sponge with uniform open macropores for adsorption and deposition of nanoparticles including MOFs without compromising the adsorption properties of MOF crystals. A robust and inexpensive fluorescent Zr-LMOF/MF composite for rapid (10 min) concentration and detection of mycotoxins is reported [183]. Such a dual-function composite is sensitive to fluorescence quenching by mycotoxins. Zr-LMOF is a microporous crystalline material with cavities <1 nm. Zr⁴⁺ ions are coordinated by eight carboxylate groups to form a cluster ([Zr₆(μ³-O)₄(μ³-OH)₄]₁₂⁺). The cluster is connected by 1,2,4,5-tetra(4-carboxyphenyl)benzene (H₄TCPB) bridges to form a porous framework permeated by rhombic channels of ~5.3 × 10.5 Å and ~2.4 × 3.5 Å. The specific surface area of the composite is 1390 m²/g. Zr-LMOF was deposited on the MF sponge using a hydrothermal method, namely, the composite was left in an aqueous solution of aflatoxin B1 (AFB1) for 2 min then heated at 85 °C in a desiccator for 2 min to remove residual water from the macropores. The spectra were excited by radiation at 300 nm and the emission intensity of the composite was recorded at 392 nm. The composite also exhibited strong adsorption effect to other mycotoxins including aflatoxin B2, aflatoxin G1, aflatoxin G2, aflatoxin M1, and ochratoxin A.

10. Sensing of Phenols and Nitroaromatic Compounds

Nitroaromatic compounds (NACs) are widely used industrial chemicals involved in the manufacture of explosives, dyes, herbicides, pharmaceuticals, and corrosion inhibitors. Due to their large-scale production, NACs are frequently detected as contaminants in industrial wastewater. These compounds exhibit high toxicity toward aquatic organisms and are of particular environmental concern. In the human body, NACs can be reduced to genotoxic intermediates, contributing to their recognized carcinogenic potential [184]. Phenols represent another major class of highly toxic aromatic pollutants. Their primary sources include petrochemical and coking industries, as well as coal mining, where phenol concentrations in wastewater can reach up to 1.6 g/L. Owing to their acute toxicity, mutagenicity, and carcinogenicity, phenolic compounds pose significant risks to human health and the environment [185]. Given their widespread occurrence and hazardous nature, the development of sensitive and selective sensing platforms for the detection of NACs and phenols in aqueous environments remains a pressing and important analytical challenge. Diverse MOFs are widely used in this application (

Table 5. The use of MOFs for sensors of nitroaromatics and phenols.

MOF-Base Sensor	Pollutant	Method	Detection Limit, µM	Linear Range, µM	Ref.
ZIF-67/RGO/SPCE	4NP	Vol.	0.0003	0.09–13.6	[186]
Ni-BDC/MWCNTs	4CP	Vol.	0.0165	0.1–50/50–500	[187]
Cu-abdc	TNP	Flu.	0.08	0.5–30	[188]
Eu@MOF-253	TNP	Flu.	0.01	0.01–100	[189]
Eu@MOF-253	2,4DNP	Flu.	0.01	0.01–25	[189]
SM-2	TNP	Flu.	0.0005	0.005–0.03	[190]
PUC2	TNT	Flu.	0.145	20–100	[191]
Zn-BDC	4NP	Flu.	0.68	2.5–45	[192]
Zn-BDC	TNP	Flu.	0.92	2.5–80	[192]
ssDNA@Ni-MOF	BPA	Flu.	0.34	15–40	[193]

4NP—4-nitrophenol, 4CP—4-chlorophenol, TNP—2,4,6-trinitrophenol, 2,4DNP—4,6-dinitrophenol, TNT—2,4,6-trinitrotoluene, BPA—Bisphenol A, Vol.—Voltammetry, Flu.—Fluoromety.

).

The electrochemical reducibility of NACs enables their detection using electrochemical sensing platforms. The authors [185] proposed screen-printed carbon electrodes modified with a ternary nanocomposite of ZIF-8/Ag/reduced graphene oxide for the detection of 4-nitrophenol (4NP). The sensor exhibited a well-defined reduction peak at −0.80 V. The most intense redox response was observed at neutral pH (pH = 7), where 4-NP exists in its uncharged molecular form, resulting in a remarkably low detection limit of 0.3 nM within the linear concentration range of 0.09–13.06 µM. Presence of a 10-fold excess of phenol, catechol, 2-aminophenol, 3-nitrophenol, 2-chlorophenol, or hydroquinone, as well as a 300-fold excess of inorganic cations or anions, led to a signal deviation of less than 4.3%, demonstrating the excellent selectivity of the sensing platform.

Fluorometric detection of NACs can be achieved by MOFs fluorescence quenching, offering a straightforward sensing approach [188]. 2D-MOF Cu-abdc (Cu²⁺ aminobenzene-1,4-dicarboxylate) nanosheets exhibited blue fluorescence (430 nm) under UV irradiation (350 nm). The fluorescence was quenched in the presence of 2,4,6-trinitrophenol, attributed to strong hydrogen bonding interactions between the NO₂ groups of TNP and the NH₂ groups of Cu-abdc. The optimal analytical response was observed at pH 7 and in 15 s after addition of the analyte to the Cu-abdc suspension. The sensor demonstrated high selectivity not only over common inorganic cations and anions but also toward structurally similar organic pollutants such as phenol, nitrobenzene, 2-nitrotoluene, 2,4-dinitrobenzene, and 4-nitrotoluene. Moreover, the sensor was successfully applied for the detection of TNP in river water samples.

Simultaneous fluorometric detection of TNP and 2,4DNP was achieved using a MOF-253 (based on Al³⁺ and 2,2′-bipyridine-5,5′-dicarboxylic linkers) modified with Eu³⁺ ions (Figure 6) [189]. Under UV irradiation (310 nm), the sensor exhibited a ligand broad emission band (364 nm), along with two Eu³⁺ emission lines (591 and 612 nm). Among a broad range of NACs tested including nitrobenzene, 4-nitrotoluene, 1,2-dinitrobenzene, 2,4-dinitrobenzene, 1,4-dinitrobenzene, 4-nitrophenol, trinitrotoluene, and trinitrophenol, fluorescence quenching at 364 nm was observed exclusively for TNP and 2,4DNP. The analytical signal was based on the emission intensity at 365 nm, with detection limits for both analytes reaching as low as 10 nM. The sensing mechanism is primarily attributed to the internal filtration effect and electron transfer. The sensor could be reused up to four times or stored for 30 days without significant loss of performance.

An SM-2 (Zn²⁺-based MOF with pyridine-2,5-dicarboxylate linkers) framework exhibiting strong fluorescence can be used for fluorometric detection of TNP and Cr³⁺ ions [190]. Upon UV excitation (272 nm), SM-2 displayed a broad emission peak (354 nm). Among various NACs (TNP, 2-nitrophenol, 2-nitroaniline, 3-nitroaniline, and 1,3-dinitrobenzene), the most significant fluorescence quenching was observed for TNP, with an exceptionally low detection limit of 0.5 nM. Moderate quenching was also noted for 2-nitrophenol. In addition to organic analytes, the sensor exhibited quenching for Cr³⁺ and a weaker response detected for Hg²⁺.

Another important NAC 2,4,6-trinitrotoluene (TNT) can be fluorometrically detected using PUC-2 (based on Zn²⁺ with 2-aminobenzene-1,4-dicarboxylic and 1-(3-aminopropyl)imidazole linkers) (Figure 7) [191]. The UV-induced (328 nm) blue fluorescence (431 nm) of PUC2 is selectively quenched by TNT in comparison to other explosives, with a detection limit of 145 nM. The pronounced fluorescence quenching is attributed to photoinduced electron transfer, with an additional enhancement through Forster resonance energy transfer. The sensor response remains stable over a broad pH range of 4–11.

An example of a framework for fluorometric detection of a broad range of NACs is Zn-BDC (based on Zn²⁺ and benzene-1,4-dicarboxylate) [192]. Upon UV excitation (330 nm), Zn-BDC exhibits blue fluorescence (421 nm), which is efficiently quenched by all tested NACs (4-nitroaniline, nitrobenzene, 4-NP and TNP). Notably, low detection limits of 0.68 µM and 0.92 µM were achieved for 4-NP and TNP, respectively. In addition, the sensor shows sensitivity toward Fe³⁺ and Cu²⁺ ions, as well as to nitrobenzene in the vapor phase. The fluorescence quenching was attributed to a charge transfer mechanism. The sensor retains functionality after three sensing cycles or one day in the analyte solution.

In a fluorescent sensor system, MOF can play a structure-directing and protective role without directly participating in the photophysical process. For example, the Ni-MOF (based on Ni²⁺ and 2,5-dihydroxybenzene-1,4-dicarboxylate linkers) adsorbs single-stranded DNA (ssDNA) in the presence of Mg²⁺ ions, resulting in fluorescence quenching of DNA (Figure 8) [193]. Upon exposure to bisphenol A (BPA), the ssDNA@Ni-MOF system selectively restores fluorescence compared to other pollutants (phenol, sulfamethoxazole, dimethylacetamide, and PbSO₄). The sensor exhibits a detection limit of 0.34 µM for BPA.

11. Detection of Pesticides and Herbicides

11.1. Pesticides

The paper [194] reviews the advances in MOF-based sensors for pesticide detection made in the previous five years (Figure 9). Optical and electrochemical sensors can reduce the cost and time of sample preparation and provide on-site pesticide detection. New biosensors offer unrivaled mechanical, electrical, optical, and magnetic properties. The specificity and sensitivity of pesticide detection can be increased by several orders of magnitude, allowing femtometer-scale detection. Bioelectrodes made from plant tissues are reported to have a longer lifetime than pure enzyme biosensors. New technologies such as molecular imprinting, aptamers and polymer biochips for detection of multiple pesticides will become important due to their advantages. Aptamers can be formulated against multiple target compounds such as heavy metals, pesticides, antibiotics, amino acids, nucleotides and biopolymers such as nucleic acids, carbohydrates and proteins. They have a long shelf life and good detection range. Thus, future research in the field of aptamers should focus on screening aptamers for detection of pesticides with high specificity and affinity.

Let us review the recent advances in the field of MOF-based sensors for pesticides. For example, a unique 3D framework {[(CH₃)₂NH₂]₂[Cd₃(BCP)₂]10H₂O 3.5DMF}n was synthesized and characterized [195]. It can serve as a chemical sensor for the sensitive detection of dinotefuran in water, and the LOD can reach 2.09 ppm. This compound also exhibits satisfactory selectivity towards other pesticides and excellent anti-interference efficiency with common ions in water. In addition, it has attractive reusability and is stable after five detection cycles. The mechanistic study implied that fluorescence sensing involves competitive energy absorption and static quenching processes. This is the first report of MOF-based chemical sensors for the detection of dinotefuran, and this MOF may be a potential probe for monitoring the dinotefuran level in hydrographic environment.

Thiabendazole (TBZ) is widely known as a pesticide and/or fungicide for fruits and vegetables. A highly selective and sensitive fluorescent sensor for monitoring TBZ in oranges based on Tb³⁺ functionalized Zr-MOF (Tb³⁺@Zr-MOF) was constructed [196]. Tb³⁺@Zr-MOF showed attractive properties in TBZ sensing, including a wide linear range (0–80 μM), high selectivity, low LOD (0.271 μM), and fast response time (1 min). Moreover, the probe was used to determine TBZ in real orange samples, in which good recoveries ranging from 98 to 104% were obtained. The whole detection process of TBZ in real orange samples combined with QuEChERS method took only 35 min.

Wang et al. [197] reported a promising electrochemical sensor for the determination of catechol (CC), a key phenolic isomer of dihydroxybenzene. The authors presented a composite sensor based on N,S-MXene quantum dots (N,S-MQDs) modified with reduced graphene oxide (rGO) to anchor a defective bimetallic organic framework (D-FeCu-MOF) for the rapid determination of CC in water. The device features are as follows: D-FeCu-MOF provides reversible redox properties, Lewis sites for analyte interaction, and superior electron transfer capability with reduced resistance compared to FeCu-MOF (11,413–7794 Ω), which is attributed to the presence of bimetallic sources, functional groups, and defects. The N,S-MQD@rGO matrix has a high dispersion and porosity, large surface area, abundant active sites and high conductivity (69.18 Ω) along with good water solubility. As a result, the D-FeCu-MOF/N,S-MQD@rGO sensor exhibits excellent sensitivity for CC detection with wide linear ranges of 0.004–520 μM and low detection limit of 0.0014 M. The sensor exhibits great selectivity in the presence of potential interferences, satisfactory recovery (98–103%) in real sample tests, with its potential for environmental monitoring.

A smartphone-based ratiometric fluorescence sensor for intelligent field imaging and diquat (DQ) detection was developed [198]. The 3D Zn-MOFs are prepared by a solvothermal method using a rigid ligand 4,4′,4″-s-triazine-2,4,6-triyltribenzoic acid (H₃TATB), a flexible ligand 1,4-bis((1H-imidazol-1-yl)methyl)benzene (bimb), and a transition metal (Zn²⁺), which shows good structural and thermal stability. The Zn-MOF is a 3D framework crystallizing in a triclinic crystal system in the Pī space group. Then, RhB@Zn-MOF is prepared by introducing rhodamine B (RhB) into the Zn-MOF framework. The inner filter effect quenches the red fluorescence and the fluorescence resonance energy transfer enhances the blue fluorescence, achieving high sensitivity and visual detection of diquat. The sensor has a low detection limit of 10.5 nM and is linear in the concentration range of 0–70.00 μM. Compared with Zn-MOF (detection limit of 16.90 nM), the introduction of RhB significantly improved the accuracy and sensitivity of diquat detection.

To prevent the threat to environmental safety and human health from organophosphorus pesticide (OP) residues, the development of rapid and accurate monitoring methods is of critical importance. Gu et al. [199] proposed a novel quantum dot functionalized metal–organic framework composite (IRMOF-3/CdTe), which achieved ultra-strong cathodic electrochemiluminescence (ECL) emission in aqueous media. IRMOF-3 could effectively catalyze sulfate radical generation, while protecting the metastable intermediates from environmental quenching, synergistically improving the ECL signal intensity. By optimizing the ratio of IRMOF-3/CdTe and combining the ECL intensity and electrochemical impedance spectroscopy, IRMOF-3 was found to have dual effects on the ECL performance of CdTe. Based on the efficient ECL performance of IRMOF-3/CdTe and the inhibitory effect of OP on the acetylcholinesterase (AChE) activity, a highly sensitive ECL enzyme biosensor was constructed for the detection of profenofos (Pff). The fabricated biosensor exhibited a wide detection range (134 fM−1.34 mM) and low detection limit (44.7 fM) and was successfully applied to detect Pff residues in real samples (vegetables, milk, and Yangtze River water).

A rapid and reliable SERS method based on Ag@ZIF-8@Au platforms was developed for monitoring acetamiprid residues in the environment [176]. In this method, a MOF material ZIF-8 is coated with silver nanoparticles and distributed between AgNPs and AuNPs to enhance the Raman signal, which can enrich the pesticide moles in the hot spot region provided by the noble material and help to avoid the oxidation of silver nanoparticles. A high sensitivity (LOD 9.027 × 10⁻¹⁰ M for acetamiprid and SERS enhancement factor 4.3 × 10⁷), excellent reproducibility (6.496% or 7.198% RSD for 30 random spots) and excellent stability (3.127% RSD for 6 weeks) were achieved using the proposed method. Acetamiprid with concentrations ranging from 10⁻⁴ to 10⁻⁹ M was successfully detected by SERS. Linear detection models of acetamiprid in different environmental matrices (lake water, tea leaves, tea plantation soil, oranges and orange orchard soil) were established.

Excess of pesticide chlorpyrifos (CPF) causes serious harm to food and soil safety, so it is important to control its content. A fluorescent sensor (ZIF-8@CuNCs) composed of copper nanoclusters (CuNCs) wrapped with zeolitic imidazolate framework ZIF-8 was prepared byy a simple one-pot strategy to achieve selective detection of CPF [200]. The detection mechanism is as follows: the fluorescence of ZIF-8@CuNCs can be quenched by 3,5,6-trichloro-2-pyridinol, a hydrolysate of CPF in an alkaline media, through the inner filter effect. Thus, the enzyme-free CPF detection method using ZIF-8@CuNCs was developed. The detection range is 0–50 μg/mL, the detection limit is 0.43 μg/mL, and the recovery range of CPF from food and soil is 90.6–112.9%. ZIF-8@CuNCs was successfully combined with a smartphone to realize rapid visualization and quantitative analysis of CPF.

11.2. Herbicides

Glyphosate, a powerful herbicide widely used in the world, poses potential hazards to human health by accumulating in the food chain. Due to the lack of chromophores and fluorophores, rapid visual detection of glyphosate has always been difficult. Wan et al. [201] constructed a paper-based geometric field enhancement device visualized by amino-functionalized bismuth metal–organic framework (NH₂–Bi-MOF) for sensitive fluorescence detection of glyphosate (Figure 10). The fluorescence of the synthesized NH₂–Bi-MOF was enhanced by interaction with glyphosate. The field enhancement of glyphosate was realized by coordinating the electric field and electroosmotic flow. The method showed a linear range of 0.80–200 μmol/L under optimal conditions with a signal enhancement of about 12,500 times achieved in only 100 s of electric field enhancement. It was applied to soil and water with the recovery ranging from 95.7% to 105.6%.

Paraquat (PQ) is widely used worldwide as a fast-acting herbicide and is highly toxic. A composite of gold nanoparticles coated with ZIF-8 and parasulfonylcalix[4]arene (pSC4) (pSC4-AuNPs@ZIF-8) was prepared by a one-step method [202]. The electrochemical biosensor analysis of PQ was performed using the modified pSC4-AuNPs@ZIF-8 glassy carbon electrode through the host-guest recognition of PQ and pSC4. The detection limit of PQ was 0.49 pM.

Detection of the herbicide 4-chloro-2-methylphenoxyacetic acid (CMPA) is of importance due to potential health risks associated with adverse effect on liver and kidney functions. To address this issue, a carbon paste-based sensor incorporating a composite material was developed [203]. The composite sensor is based on a porous cobalt-1,4-benzenedicarboxylate metal–organic framework and exfoliated montmorillonite nanolayers (Co-MOF/MMt). Compared with the Co-MOF material, the Co-MOF/MMt nanocomposite showed an improved specific surface area of 1004 m²/g with a crystal size of 30 nm. This sensor enables the voltammetric detection of CMPA in soil samples using linear adsorption anodic stripping voltammetry, facilitating early and accurate monitoring of herbicide. The Co-MOF/MMt nanocomposite was synthesized using a hydrothermal method via precipitation of Co-MOF in the presence of MMt. The resulting modified carbon paste sensor containing 1.0% Co-MOF/MMt nanocomposite showed superior electrochemical properties compared to the unmodified carbon paste sensor. Under standard operating conditions, the developed sensor demonstrated detection limits of 0.03 nM and 0.1 nM over two wide linear ranges (0.03–0.10 nM–0.10–1.0 nM) and (0.1–1.0 nM–1.0–7.0 nM).

12. Detection of Chloroorganic Compounds

Inhalation of certain chlorinated volatile organic compounds (CVOCs), such as dichloromethane, chloroform, carbon tetrachloride, and 1,2-dichloroethane, has been linked to chronic cardiovascular disease, cognitive impairment, reproductive harm, and cancer risk. A new luminescent LMOF based on Cu(I) clusters of the composition {[Cu₃I₃(TPSA)]⋅CH₃CN}n, where N,N′,N″-tri(3-pyridine)thiotriamide phosphate (TPSA) is the phosphor, was synthesized [204]. The asymmetric part of the unit cell of the LMOF crystal contains a {Cu₃I₃S} core, a TPSA ligand and an acetonitrile molecule. The {Cu₃I₃S} core consists of three tetra-bonded copper ions, two μ²-I ions, one μ³-I ion and one μ²-S atom. Each ligand is bound to four copper cores to form a three-dimensional framework. The structure contains one-dimensional open channels with a pore size of 4.0 Å. Due to this unusual photophysical properties, LMOF can be also used as a fluorescent gas sensor for CH₂Cl₂, CHCl₃, and 1,2-dichloroethane at low concentrations, in which the detection limits are 0.4 Pa, 3.1 Pa, and 1.2 Pa, respectively, with a response time of several seconds.

4-Chlorophenol (4-CP) has strong biological toxicity, teratogenic, carcinogenic, and mutagenic effects, it accumulates in the body. A rapid, simple, efficient and accurate method for detecting 4-CP and a new method for online monitoring of this toxicant is developed [186]. The sensor is obtained by self-assembly of a mixture of sheet-like Ni-BDC (BDC: 1,4-dicarboxybenzene) scaffolds and multi-walled carbon nanotubes (MWCNTs) (Ni-BDC/MWCNT) at room temperature. Due to the large specific surface area, high electrochemical activity of Ni-BDC and high electrical conductivity of MWCNT, the Ni-BDC/MWCNT sensor had a wide linear range (0.1–50 and 50–500 μM), low detection limit (16.5 nM), perfect reproducibility and high stability, as well as good selectivity.

There are some works [205], in which substances were obtained that cannot be called MOFs due to their small specific surface area, for example, Cu−NH₂BDC, where NH₂BDC is 2-amino-terephthalic acid, has a BET specific surface area of 14.7 m²/g, and the properties of a perfluorooctanoic acid sensor, based on Cu−NH₂BDC should be explained by the properties of the salt, not the framework.

13. Sensing of Radioactive Impurities

Technetium-99 is one of the most problematic radionuclides with a long half-life (2.13 × 10⁵ years), which is quite stable in the anionic form (pertechnetate, TcO₄). A method for monitoring technetium in water is needed. Tetraphenylethylene (TPE) with the effect of aggregation-induced emission (AIE) was used to synthesize a substance that concentrates and detects pertechnetate ions, in which molecules that are weakly luminescent or do not luminesce, intensely fluoresce in an aggregate state. Based on TPE, tetrakis(4-pyrimidylphenyl)ethene was prepared and used as an organic linker to construct a cationic water-stable MOF with silver ions ZJU-X8 [206]. There are three coordination centers of silver ions in ZJU-X8, one of which is 6-coordinated with two nitrogen atoms from pyrimidyl units and four oxygen atoms from nitrate anions. In the 7-coordinated mode, the silver ion is coordinated with one DMSO molecule and two pyrimidyl groups and three nitrates. In the 3-type centers, the silver atoms have a CN of 5. These coordination centers, connected by linkers, create a cationic layered structure. TcO₄ can strongly attach to the available silver sites in ZJU-X8 after anion exchange, which changes the overall electronic structure of AIE molecules. Due to this property, ZJU-X8 showed a unique fluorescence color change after replacing nitrates with TcO₄ from bright blue to yellowish green under UV irradiation. The relationship between the emission wavelength and the concentration of ReO₄ in the solution is close to linear (R² = 0.99), and the calculated LOD and limit of quantification are 10.2 and 34 ppm, respectively.

In recent years, a large amount of radioactive uranium has been released into the environment, mainly due to uranium mining, nuclear industry emissions and military activities. ²³⁸U has a long half-life of 4.5 × 10⁹ years and can exhibit combined chemotoxicity and radiotoxicity, causing DNA damage, nephropathy, osteosarcoma and other diseases. Not only ultra-sensitive and rapid on-site detection but also effective concentration and removal of uranium from water is of great significance to human health, environmental safety, and sustainable development. To solve the problem of uranium determination in water and its concentration and extraction from water, novel cost-effective dual-functional luminescent mixtures Tb-MOF/Tb-AG were successfully prepared by combining Tb-MOF and alginate gel (AG) using terbium(III) as a connecting node [207]. Due to the presence of luminescent centers and binding sites for uranium, the obtained Tb-MOF/Tb-AG mixtures can be used as an effective adsorbent and visual sensor, exhibiting a remarkable adsorption capacity of 549 mg/g and ultra-high sensitivity to uranium with a detection limit of only 1.2 ppt. The treated Tb-MOF/Tb-AG sorbent can be reused for uranium adsorption with satisfactory adsorption capacity. Tb-MOF was synthesized by heating a mixture of Tb(NO₃)₃⋅6H₂O (0.1 mmol), 2,4,6-tri[(pcarboxyphenyl)amino]-1,3,5-triazine (H₃TATAB) (0.05 mmol), H₂O (2 mL), and DMF (3 mL) in an autoclave at 100 °C for 72 h. To obtain the granular sorbent, the Tb-MOF suspension in water was slowly dropped into the SA aqueous solution and stirred to obtain the Tb-MOF sol. Then, the Tb-MOF sol was injected into the Tb(NO₃)₃⋅6H₂O (5 wt%) aqueous solution, and spherical granules of Tb-MOF/Tb-AG mixtures with a uniform size of about 3.6 mm were obtained. It is noteworthy that the obtained LOD of 1.2 ppm is low. The maximum allowable content of uranium in drinking water is 0.13 μmol/L (30 ppb) according to the requirements of the U.S. Environmental Protection Agency. The obtained LOD is lower than this threshold by 4 orders of magnitude, therefore the presented new sorbent and indicator Tb-MOF/Tb-AG is sensitive enough to detect traces of uranium. The specific surface area of Tb-MOF/Tb-AG is 21 m²/g, but the adsorption capacity of Tb-MOF/Tb-AG was much greater than that of Tb-MOF, which was explained by the greater hydrophilicity of the Tb-MOF/Tb-AG composite than Tb-MOF, as well as chelation, hydrogen bonding and ion-exchange interactions between uranium and Tb-MOF and Tb-AG fragments in the Tb-MOF/Tb-AG composite.

Based on the concepts of molecular recognition, a new method that demonstrates high selectivity and sensitivity for the detection of uranyl ions in water using MOFs was developed. The MOF Tb@UiO-66-(COOH)₂ containing 1,2,4,5-benzenetetracarboxylic acid as a linker and terbium ions in the MOF nodes was used as a sensor [208]. Tb@UiO-66-(COOH)2 can specifically recognize uranyl ions with ultra-high sensitivity. The stronger affinity of carboxyl groups in UiO-66-(COOH)₂ for uranyl ions compared to that of Tb³⁺ ions leads to the replacement of Tb³⁺ by uranyl, which causes the quenching of the luminescence of Tb³⁺ ions. The LOD were determined to be 8.26 nM and 5.68 nM in lake water and seawater, respectively, which are much lower than the uranium contamination standard in drinking water defined by US EPA.

In the nuclear fuel cycle, iodine isotopes are generated in reactors and can be released into the environment, posing a long-term risk to the environment. Therefore, detection of these released harmful and persistent iodine species is essential. MOFs of the composition Ln(BTC)(H₂O), denoted as Ln-BTC, where BTC is 1,3,5-benzenetricarboxylic acid (H₃BTC), adsorb about 340 mg/g of iodine at 80 °C in saturated iodine vapor [209]. The crystal structure after calcination of Ln-BTC did not change, but the iodine uptake capacity increased to about 530 mg/g, while the MOF structure was preserved. Highly sensitive and selective fluorescence/electrochemical detection of iodine using Ln-BTC MOFs under competitive adsorption of water vapor is demonstrated. Iodine quenches fluorescence of Ln-BTCs via a photoinduced electron transfer mechanism, after adsorption of 2% iodine, the fluorescence quenches two orders of magnitude. The conductivity of Ln-BTCs increases 107 times after capturing iodine and 104 times at 18% relative humidity.

14. Detection of Amines

Ammonia and amines are common contaminants in food and water, typically formed during the decomposition of proteins in biological processes. Another source of these pollutants is agriculture, where they are used as fertilizers and for plant protection [210]. The concentration of ammonia in water is a crucial for assessing the nitrogen cycle, pollution levels, and the ecological status of aquatic environments. Thus, the development of sensors for detection of ammonia and amines is of considerable importance (

Table 6. Sensing of amines by MOF-based materials.

MOF-Base Sensor	Pollutant	Method	Detection Limit	Linear Range	Ref.
PE@MOF-5	NH3(gas)	VC	49.6 ppb	5.4–80.47 ppm	[212]
AN@NH2-UiO-66	NH3(liquid)	VC	1.43 mM	-	[213]
CDs@ZIF-8(In)	TEA(liquid)	Flu.	58.7 µM	0.001–5.8 mM	[214]
CDs@DZIF-8(In)	TEA(gas)	RS	1 ppm	1–100 ppm	[214]
H-AlMOF@AuNP	CAB	Vol.	80 aM	0.3 fM–10 pM	[215]
H-AlMOF@AuNP	CAB	Amp.	300 aM	0.7 fM–10 pM	[215]

TEA—triethylamine, CAB—carbendazim, VC—visual colorimetry, RS—resistive sensor, Flu.—fluoromety, Vol.—voltammetry, Amp.—amperometry.

) [211].

MOF-5 (based on Zn²⁺ and benzene-1,4-dicarboxylate linkers) was used as a matrix for immobilizing natural acid-base indicators extracted from Solenostemon scutellarioides (L.) plants as NH₃ sensors [212]. The test strips were evaluated for visual and smartphone-assistant analysis. The system selectively detects NH₃ with a detection limit of 49.6 ppb, does not show any significant effect of water vapor and other common contaminants such as acetone, ethanol, methanol, and chloroform. The test strips were successfully applied for monitoring the freshness of food products, including chicken, veal and fish.

Fang et al. [213] studied NH₂-UiO-66 incorporated into colorimetric films based on sodium alginate and anthocyanins extracted from red cabbage. The colorimetric films containing NH₂-UiO-66 exhibited much faster color changes when exposed to an NH₃ solution and maintained their sensing properties for a longer duration compared to films without MOF. The role of MOF in the colorimetric films is to stabilize anthocyanins through strong hydrogen bonding and π–π interactions, as well as to provide protection from light-induced degradation. These films are sensitive not only to NH₃ but also to dimethylamine and trimethylamine, while showing resistance to interference from water, alcohols, aldehydes, and hydrocarbons. The colorimetric films were successfully applied to monitor the freshness of shrimp, detecting spoilage prior to visible changes.

MOFs can demonstrate potential as a versatile multimode sensor applicable to both liquid- and gas-phase analyses. Yang et al. [214] explored ZIF-8 post-synthetically modified by partial substitution of Zn²⁺ with In³⁺, followed by deposition of carbon dots (CDs). The resulting composite CDs@ZIF-8(In) was applied in fluorometric detection of triethylamine (TEA). Upon exposure to TEA, both fluorescence emission peaks of the composite (at 440 and 610 nm) exhibited dose-dependent enhancement. In contrast, the fluorescence response to ammonia was significantly weaker, while no enhancement was observed in the presence of other potential interferents (n-butylamine, alcohols, acetone, hydrocarbons). Furthermore, pyrolyzed CDs@ZIF-8(In), denoted as CDs@DZIF-8(In), was employed in a resistive gas sensor for TEA, demonstrating high sensitivity and selectivity.

MOFs can be integrated into multicomponent molecular ensembles enabling the detection of ultra-trace levels of contaminants. An H-AlMOF@AuNP system comprising an Al-based MOF, hemin, and dopamine provide electrocatalytic reduction of H₂O₂ on an SPE electrode (Figure 11) [215]. This platform enables the determination of carbendazim using differential pulse voltammetry and chronoamperometry, achieving detection limits of 80 and 300 attomol/L, respectively, which represents a remarkable performance. The sensor was successfully applied to analysis of tap water, apple juice, and tomato juice.

15. Sensing of Explosives

Some examples of the use of MOF-based materials for detection of nitroaromatic compounds, including TNT have been considered already in Section 10. Here we will discuss briefly a few more reports. [Cd(atc)(H₂O)₂]_n (NMOF 1) (atc = 2-aminoterephthalic acid) is highly sensitive in sensing nitroaromatic compounds by fluorescence quenching [216].

Photoluminescence of a nanocomposite of luminescent Eu-containing framework and CdSe quantum dots was used for sensitive detection of trinitrotoluene [217]. The linear limits of photoluminescence quenching based detection of trinitrotoluene is 5–1000 ppb with the lowest detection limit of 3 ppb. Detection of trinitrotoluene was selective in the presence of other aromatic molecules (phenol, o-cresol, toluene, benzene, nitrobenzene, and nitrophenol). Detection of explosives is a serious problem and PCPs can serve as pre-concentrators of explosives [218]. This approach was manifested using hexahydro-1,3,5-trinitro-1,3,5-triazine as an explosive compound [219]. PCPs were shown to concentrate it up to 3000 times compared to gas phase detectors. The selectivity to hexahydro-1,3,5-trinitro-1,3,5-triazine vs. a hydrocarbon was up to 5000 [220].

16. Detection of Bacteria

Bacillus anthracis is a Gram-positive bacterium that poses a lethal threat to living organisms and is considered a biological weapon. It can cause anthrax, which has a high lethality rate and a short duration of effective treatment. Therefore, early detection of B. anthracis is crucial for effective treatment of anthrax. Based on theoretical calculations (DFT), a model was proposed to predict the effect of functional groups of PTA-X linkers (PTA: p-phthalic acid, X = NH₂, CH₃, H, OH) on the luminescent properties of Ln-MOFs, which is necessary for the design of linkers for single- and dual-emission Ln-MOFs. The model was verified by synthesizing eight Ln-MOFs and measuring their fluorescence spectra. As a result, Tb-PTA-OH was identified as the only dual-emission Ln-MOF. It was applied for the detection of DPA (DPA—dipicolinic acid, Bacillus anthracis biomarker). A mini-device visual color detection method for DPA based on a smartphone was developed, and a novel detection mechanism and selectivity mechanism of DPA “triple gate” were proposed. The so-called “triple gates” are the molecular orbital energy, excited state energy level, and excited state transition intensity, which are used to determine whether the target analyte or interfering compound satisfies the requirements for sensitizing Tb³⁺ luminescence. Only substances that have passed the “triple gate” can be selectively detected. Good linearity was obtained in the concentration range of DPA (0.05–6 μM). The LOD of DPA is 13.4 nM [221].

The main cause of foodborne illness is contamination by pathogenic bacteria such as Staphylococcus aureus (S. aureus) and Salmonella. Food products can be contaminated with pathogenic bacteria, so aptasensors are needed. Since their discovery in the early 1990s, aptamers (oligonucleotide or peptide molecules that specifically bind to certain target molecules) served as an alternative to antibodies required for target recognition by a biosensor. Aptamers have stability, variability, low cost and high specific affinity for a particular target molecule. They are widely used as a recognition component in biosensors called aptasensors. The authors [222] obtained and applied a MOF nanocomposite with such electrochemical and catalytic properties that enabled a tandem sensor system for fast and simple bacterial detection. The gold nanoparticle-modified framework containing copper ions (Au/CuMOF) cannot only exhibit a clear electrochemical potential peak from coordinated Cu²⁺, but also catalyze a color reaction, which opens the way to fabricate a tandem sensor for bacterial detection. S. aureus is used as a target model. Different concentrations of S. aureus lead to different amounts of Au/CuMOF to be attached to the magnetic beads or introduced into the mother liquor, which in turn leads to differences in the current strength and adsorption. Utilizing an electrochemical workstation and spectrophotometer as detection tools, the tandem sensor allows quantification of S. aureus with high accuracy and sensitivity, enabling food monitoring. CuMOF consists of 1,3,5-benzenetricarboxylate ions (TMA) and Cu²⁺ ions with the composition [Cu₃(TMA)₂(H₂O)₃]_n. The specific surface area (BET) of CuMOF is 692.2 m²/g. The achievable porosity is 40.7%, which is comparable to zeolites. Gold nanoparticles were deposited on the surface of CuMOF crystals. The ssDNA-Au/CuMOF bioconjugates were formed on Au/CuMOF surfaces through specific interaction of -SH groups of ssDNA with gold nanoparticles. The MB-Apt bioconjugate was obtained via the specific combination of streptavidin STV-MB with a biotin tag modified at the S. aureus aptamer end. Due to the chelated copper ions and the high catalytic activity of Au/CuMOF towards the 3,3′,5,5′ tetramethylbenzidine (TMB) substrate, remarkable electrochemical and colorimetric signals were obtained. The method demonstrated a wide linear range from 10 to 108 CFU/mL with a LOD of only 5 CFU/mL, good selectivity and ability to resist interferences in complex samples. It was also convenient due to the ease of separation of the MB bioconjugate and the use of disposable screen-printed carbon electrodes. By substituting aptamers and DNA sequences, the detection of other targets including bacteria could be realized in food analysis [223].

17. Smartphone-Based Methods and Nanozimes

A composite containing blue fluorescent carbon dots, red fluorescent InP/ZnS quantum dots, and ZIF-8 was manufactured and applied in a fluorescence sensor for Hg²⁺ ions and cysteine [107]. The application of this composite with the color recognition and appropriate software for a smartphone provides the real-time and rapid sensing of mercury and cysteine. There are many examples in literature and our review in the above chapters.

“Nanozymes are nanomaterials that catalyze the conversion of enzyme substrates to products and follow enzymatic kinetics (e.g., Michaelis–Menten) under physiologically relevant conditions, even though the molecular mechanisms of the reactions could be different between nanozymes and the corresponding enzymes” [224]. They are not inferior in selectivity and surpass in activity natural enzymes, for example, natural peroxidase. They are easier to produce and are undemanding to storage conditions compared to natural compounds. Nanozymes replacing oxidase and peroxidase are the most common. They can produce large amounts of reactive oxygen species and act as an excellent oxidizer. Many MOFs exhibit nanozyme properties. A nanozyme sensor (NH₂-UiO-67(Zr/Cu)) with peroxidase properties and a smartphone-enabled portable device was applied for rapid in situ detection of H₂O₂ [225]. Tetramethylbenzidine (TMB) in the presence of H₂O₂ and peroxidase-like enzymes forms a blue oxidation product (oxTMB), which enables colorimetric detection of H₂O₂. This makes TMB in combination with NH₂-UiO-67(Zr/Cu) an ideal indicator for rapid in situ detection of H₂O₂. The colorimetric measurements are complemented by fluorescence spectra. NH₂-UiO-67(Zr/Cu), being a nanozyme, serves as a source of fluorescence. In the presence of H₂O₂, the fluorescence of NH₂-UiO-67(Zr/Cu) is quenched at 405 nm, which enables fluorescence detection by measuring the degree of quenching. To improve the sensitivity, a portable 3D printed device was created, equipped with two light sources: a daylight LED for colorimetric detection by monitoring the color changes of the solution and a UV LED for fluorescence detection by monitoring the fluorescence changes of NH₂-UiO-67(Zr/Cu) excited at 365 nm. The smartphone converts the color information into RGB values. This smartphone device provides excellent selectivity and interference resistance in H₂O₂ detection, simplifying the analytical process and providing high flexibility for on-site applications. This H₂O₂ detection method is of important for environmental and process control, food safety.

A paper-based single and dual colorimetric smartphone sensor was developed to detect PO₄³⁻ and Cu²⁺ ions using BDC-Co-BIm MOF (BDC is a linker, benzene-1,4-dicarboxylic acid, and BIm is benzimidazole) as a peroxidase-type nanozyme [226]. The detection time of PO₄³⁻ and Cu²⁺ ions was 10 and 5 min, LOD was 0.09 and 0.35 μM, respectively. BDC-Co-BIm MOF catalyzes oxidation of TMB by hydrogen peroxide, forming blue oxidation products (oxTMB). When PO₄³⁻ ions are introduced into the blue oxidized environment, they complex with Co²⁺ ions, causing the blue solution to become lighter. Subsequently, addition of Cu²⁺ ions results in a stable covalent complex Cu₃(PO₄)₂, significantly restoring the blue color. This mechanism enables sensitive and selective detection of PO₄³⁻ and Cu²⁺ ions using BDC-Co-BIm MOF. However, BDC-Co-BIm MOF is not a framework but rather a dinuclear coordination compound. The description of the synthesis and X-ray diffraction experiments are not informative and inconclusive.

Fast, sensitive, and user-friendly colorimetric As(V) detection based on Fe-MOFs was developed [227]. The framework compound NH₂-MIL-88(Fe), consisting of linkers, 2-amino terephthalic acid, and iron atoms at the framework sites, exhibited peroxidase-like activity, which allowed the oxidation of colorless TMB to blue oxTMB in the presence of H₂O₂. Using the strong As–O–Fe coordination, NH₂-MIL-88(Fe) rapidly adsorbed As(V), reducing its peroxidase-like activity and causing a decrease in UV–visible absorption and decolorization of the blue color of the solution. This MOF-based colorimetric strategy enabled the detection of As(V) within 15 min using the rapid adsorption of As(V) onto unsaturated metal centers, namely, iron atoms. The developed method enables the detection of As(V) at concentrations as low as 2.78 μg/L over a wide linear range of 5.00–600.00 μg/L.

A bactericidal, insecticidal veterinary drug, malachite green (MG) is widely used in the aquaculture industry, despite the categorical ban due to its toxicity. When MG enters a biological organism, it is quickly reduced to leucomalachite green (LMG) and accumulates in tissues under the action of reductase. LMG is more toxic than MG. The total amount of MG and LMG was determined by oxidation of LMG to MG by nanozymes. For this purpose, a new nanozyme, the metal–organic framework ZIF-67 (PSS/ZIF-67) with the addition of poly(sodium 4-styrene sulfonate) (PSS), as a surfactant, was designed and manufactured [228]. The addition of PSS significantly enhanced the nanozyme activity of ZIF-67. PSS/ZIF-67 showed excellent oxidase-like activity and could rapidly and selectively oxidize LMG into MG within 1 min. The total amount of MG and LMG was determined colorimetrically in 10 min. The colorimetric determination has a linear relationship with the total concentration of LMG and MG in the range of 2.0–15.0 μM, and LOD of 0.25 μM. Using the test strip technology and a mobile phone, a portable visual platform for the quantitative determination of total MG and LMG in fish was obtained. This method provides a simple and rapid determination of total MG residue, and also avoids interference.

Two copper-containing 2D MOFs (Cu₃-TDH and Cu₃-BDU) with excellent laccase-like properties were synthesized as nanozymes for phenol oxidation and simultaneous identification and detection of multiple phenolic pollutants [229]. Cu₃-TDH and Cu₃-BDU were prepared by condensation of terephthalic dihydrazide (TDH) and 1,1′-(1,4-phenylene)diurea (BDU) with a coordination cyclic compound, each complex consisting of alternating three Cu(I) atoms and three 1H-pyrazole-4-carbaldehyde molecules lacking hydrogen atoms at the nitrogen atoms, i.e., the complex contains three Cu-N covalent bonds and three Cu-N coordination bonds. According to the authors, Cu₃-TDH and Cu₃-BDU demonstrated the specific surface area, 90.86 and 75.01 m²/g, respectively.

18. Prospects and Outlook

Table 7 presents the most sensitive PCP/MOF materials showing the record low detection limits to the most important analytes. Noteworthy, the values of detection limits presented in Table 7 exceed the parameters achievable by using conventional sensor materials. The analysis of the state of the art [230, 231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259] shows that the scope of the use of PCPs/MOFs in sensors is a broad field for the future research. The unique properties of PCPs/MOFs include high specific surface areas and pore volumes, adjustable molecular sizes of the pores, flexibility of the framework that is responsive toward the adsorbate as well as by using a “molecular recognition” approach [241]. The above factors contribute to the effective penetration of degradable molecules to the corresponding sites of the frameworks, while creating luminescence centers with high intensity. An increase in the specific MOF surface area also leads to an increase in the concentration of luminescence centers. It is important that the pore sizes of the frameworks affect the selectivity of the process of creating luminescence centers. The further improvements are based on the development of more efficient methods of deposition of PCPs layers onto the surface of appropriate substrates [248] and original compositions and preparation methods of nanocomposites with the goal to reach a synergy between the PCP and the other components of the nanocomposite [252,254]. New methodologies should also include photoelectrochemical sensors.

Another approach is based on the application of MOFs as precursors of oxide nanoparticles with a variety of morphologies (nanocubes, nanocages, hierarchical structures, core-shell materials). Presumably, these materials preserve the structure and developed porosity of the initial MOF, which provides potential in sensor applications, especially in gas sensors [240,243,244,247,255,258].

Table 7. The most illustrative examples of the sensitive PCP/MOF materials demonstrating the record low detection limits towards the essential analytes.

Metal–organic framework	Sensor Type/Sensing Mechanism	Target Chemical	Detection Limit	Reference
Cations
Tb(BTB)(DMF) 1.5DMF 2.5H2O	Luminescence	Fe3+	10−2 mmol/L	[230]
Eu3+@UiO-66(Zr)-COOH	Luminescence	Cd2+	6 × 10−2 mmol/L	[231]
Adenine-La-MOF	Luminescence	Hg2+	0.2 nmol	[232]
Eu3+@MIL-121	Luminescence	Ag+	0.1 mmol	[233]
DNA-functionalized Fe(porphyrin)-PCP	Luminescence	Pb2+	0.034 nmol	[234,235]
Eu3+/CDs@UiO-66	Luminescence	Cu2+	51 nmol	[236]
Anions
[Ln2ZnL3(H2O)4](NO3)2·12H2O)n (Ln = Eu, Tb; L = 4,4′-dicarboxylate-2,2′-dipyridine anion)	Luminescence	I−	1 ppb	[237]
({RuII(tBubpy)(CN)4[CuII(dien)]2}(ClO4)2	Luminescence	CN−	30 ppb	[238]
UiO-66-NH2	Luminescence	ClO−	40 nmol	[250]
UiO–66–NH2@Eu–MOF	Luminescence	PO43−	0.67 μmol/L	[239]
Molecules
Layers of Cu-BTC	Surface acoustic waves	H2O	<1 ppm	[249]
Cu3(HITP)2	Chemiresistive	NH3	0.5 ppm	[242]
MFM-300 (In-MOF)	Capacitive	SO2	5 ppb	[251]
Au-SH-SiO2NPs on Cu-MOF	Electrochemical	N2H4	10 nmol	[78]
Organic molecules
ZnO@ZIF-67	Chemiresistive	CH2O	50 ppm	[256]
Co(Im)2	Chemiresistive	Me3N	2 ppm	[245]
UiO-66-NH2	Work function	Dimethylmethylphosphonate	3 ppb	[246]
NH2-MIL-53(Al) in Matrimid	Work function	Alcohols	2 ppm	[253]
IRMOF-3	Luminescence	Trinitrophenol	0.1 ppm	[257]

Table 7. The most illustrative examples of the sensitive PCP/MOF materials demonstrating the record low detection limits towards the essential analytes.

Metal–organic framework	Sensor Type/Sensing Mechanism	Target Chemical	Detection Limit	Reference
Cations
Tb(BTB)(DMF) 1.5DMF 2.5H2O	Luminescence	Fe3+	10−2 mmol/L	[230]
Eu3+@UiO-66(Zr)-COOH	Luminescence	Cd2+	6 × 10−2 mmol/L	[231]
Adenine-La-MOF	Luminescence	Hg2+	0.2 nmol	[232]
Eu3+@MIL-121	Luminescence	Ag+	0.1 mmol	[233]
DNA-functionalized Fe(porphyrin)-PCP	Luminescence	Pb2+	0.034 nmol	[234,235]
Eu3+/CDs@UiO-66	Luminescence	Cu2+	51 nmol	[236]
Anions
[Ln2ZnL3(H2O)4](NO3)2·12H2O)n (Ln = Eu, Tb; L = 4,4′-dicarboxylate-2,2′-dipyridine anion)	Luminescence	I−	1 ppb	[237]
({RuII(tBubpy)(CN)4[CuII(dien)]2}(ClO4)2	Luminescence	CN−	30 ppb	[238]
UiO-66-NH2	Luminescence	ClO−	40 nmol	[250]
UiO–66–NH2@Eu–MOF	Luminescence	PO43−	0.67 μmol/L	[239]
Molecules
Layers of Cu-BTC	Surface acoustic waves	H2O	<1 ppm	[249]
Cu3(HITP)2	Chemiresistive	NH3	0.5 ppm	[242]
MFM-300 (In-MOF)	Capacitive	SO2	5 ppb	[251]
Au-SH-SiO2NPs on Cu-MOF	Electrochemical	N2H4	10 nmol	[78]
Organic molecules
ZnO@ZIF-67	Chemiresistive	CH2O	50 ppm	[256]
Co(Im)2	Chemiresistive	Me3N	2 ppm	[245]
UiO-66-NH2	Work function	Dimethylmethylphosphonate	3 ppb	[246]
NH2-MIL-53(Al) in Matrimid	Work function	Alcohols	2 ppm	[253]
IRMOF-3	Luminescence	Trinitrophenol	0.1 ppm	[257]

One more class of materials demonstrating similarities to MOFs are metal organic gels. The have no regular crystalline structure but are characterized by high surface areas and pore volumes with adjustable pore sizes. A metal organic gel of the Fe-ndc type demonstrated good sensitivity in detecting aromatic and nitro compounds with enhancement (turn-on) or quenching (turn-off) of luminescence, with the increase of the luminescence intensity reaching 862% [259].

In general, hybrid, core-shell, MOF-on-MOF, MOF@MOF, catenated and 2D structured MOF/COF/PCP materials and composites based on these components deserve special attention as they combine advantages of the components and should be designed to fulfill the highest possible sensing characteristics.