The Diversity of MOF Structures and Their Impact on Photoelectrochemical Sensors for Monitoring Environmental Pollution

Abstract

The problem of environmental pollution is one of the most important in the modern world. Pollution causes an increase in human diseases, the extinction of many species of plants and animals, global warming, and many weather anomalies. One of the great challenges for scientists is the development of methods for monitoring and removing the emerging pollutants. This review focuses on Metal–Organic Frameworks (MOFs) and their use as working material to construct different types of sensors for application in environmental pollution monitoring. In particular, the detection of heavy metals (mercury, lead, and arsenic) and organic compounds (drugs, biomolecules, and pesticides) are considered. The collected data show that photoelectrochemical (PEC) sensors based on MOFs are the most fascinating materials due to various combinations (e.g., surface modification) and operational possibilities. PEC sensors achieve enormous sensitivity, which increases even to the pico level, making it the best tool in sensing applications. This review also highlights the main sensor challenges. Most of them are concerned with the possibility of reusing the sensor, its regeneration, and safe disposal. In addition, more attention should be paid to the sensor manufacturing process, which often uses toxic compounds, and research to eliminate them in favor of non-toxic compounds.

1. Introduction

Humankind very intensively explores the natural environment, looking for new materials to meet the requirements of the present. The fight against hunger means that more and more artificial fertilizers and plant protection products are used to increase yields. People want to live longer, which is why we see the intensive development of medicines: the search for new drugs, vaccines, and therapies causes more and more active substances to enter the environment, destroying it; at the same time, the drug resistance of microorganisms increases, and they become more dangerous for humans. The development of new technologies that improve the comfort of our lives and the search for new energy sources that will be more efficient and meet all requirements cause the world to develop and produce more and more waste. Unfortunately, the development of waste management often does not keep pace with technological development. Therefore, monitoring the levels of toxic substances is very important and could avoid humanitarian or ecological disasters. The control of pollution still leaves much to be desired, which is why work on developing modern sensors is no less important than research on the development of new technologies [1,2,3,4]. One of the very promising materials are sensors based on photoelectrochemical detection. The principle of operation of this type of sensor is based on the conversion of light into an electrical signal. Each sensor consists of an excitation light source, a working electrode made from a semiconductor material, and a data acquisition system. As a result of illuminating the electrode, a photocurrent is generated, the value of which changes as a result of contact with the tested analyte. Due to the various redox processes occurring in the system, these sensors are more sensitive than electrochemical or luminescent sensors. Moreover, signal detection is performed using a simple potentiostat and does not require expensive equipment, as in the case of sensors based on spectroscopic detection [4,5].

Advantages of photoelectrochemical sensors:

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PEC sensing is more sensitive than conventional electrochemical methods;

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PEC sensors use light to stimulate photoactive materials;

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PEC sensors have simpler instrumentation;

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PEC sensors have low-cost production.

1.1. MOF Definition

Metal–Organic Frameworks (MOFs) are an intriguing new class of crystalline hybrid materials constructed from metal cations or clusters and organic ligands [1,6]. The vast number of combinations of metal centers and linkers provide structural and functional diversity for MOFs. The MOF materials are characterized by high surface areas, regular pore size, homogeneously dispersed active sites, and uniformly distributed metal centers. What is more, the high porous surface areas of MOFs could load the different guest molecules and/or catalyze the targets with high catalytic activity. The different sizes of pores in MOF materials permit guest molecules to diffuse into the bulk structure. MOFs via hydrogen bonding, π-π interactions, open metal sites, and van der Waals interactions could create specific interactions with the guest molecules [7]. MOFs can also adsorb various molecules on the surface due to appropriate linkers, which very often also have good electro- and photoelectrochemical properties. All these features make MOFs suitable materials for the construction of chemical sensors.

There are many abbreviations used in the naming of MOFs that do not reflect the chemical structure of these compounds. To bring the reader closer to navigating the world of MOFs,

Table 1. A list of the most popular abbreviations, their interpretations, the chemical formulae of clusters, and ligands in MOFs structures.

Abbreviation and Its Interpretation	Examples	Chemical Formula: Cluster (Ligand)	Crystal Structure	Refs.
MOF Metal–Organic Frameworks	MOF-5	Zn4O(BDC)3 (BDC = 1,4-benzenedicarboxylic acid)		[9,12]
	MOF-74	Zn2DOT (DOT = 2,5-dihydroxyterephthalate)		[11,13]
	MOF-545	Zr6O8(H2O)8(TCPP-H2)2 (TCPP = tetracarboxy- phenylporphyrin)		[14]
IRMOF Isoreticular Metal–Organic Framework	IRMOF-1 (MOF-5)	Zn4O(BDC)3·7DEF·3H2O (BDC = 1,4-benzenedicarboxylic acid)		[6]
UiO Universitet in Oslo	UiO-66	Zr6O6(BDC)6 (BDC = 1,4-benzenedicarboxylic acid)		[10,17,24]
	UiO-67	Zr6O6(BPDC)6 (BPDC = biphenyl-4,4′-dicarboxylate)		[10]
	UiO-68	Zr6O6(TPDC)6 (TPDC = [1,1′:4′,1″-terphenyl]-4,4″-dicarboxylate)		[10]
MIL Materials of Institut Lavoisier	MIL-125	Ti8O8(OH)4–(BDC)6 (BDC = 1,4-benzenedicarboxylic acid)		[8]
	MIL-68	[Fe(OH)(COO)2]n (BDC = 1,4-benzenedicarboxylic acid)		[25]
	MIL-101 (Cr)	[Cr3(O)F(BDC)3(H2O)2] (BDC = 1,4-benzenedicarboxylic acid)		[26]
ZIF Zeolite Imidazolate Framework	ZIF-8	Zn(mim)2 (mim = 2-methylimidazolate)		[21]
	ZIF-67	Co(Hmim)2, (Hmim = 2-methylimidazole)		[27]
	ZIF-300	[Zn(2-mim)0.86(5-bbim)1.14] (mim = 2-methylimidazole bbim = 5-bromobenzimidazol)		[28]
HKUST Hong Kong University of Science and Technology	HKUST-1	[(Cu)3(BTC)2] (BTC—benzene tricarboxylate)		[19]

lists the abbreviations, chemical formula, and crystal structures of the most popular MOFs, which are described in a different part of this review [6,8,9,10]. The meaning of the acronym MOF is double: MOF as Metal–Organic Framework describes the general name of compounds with an ordered porous structure. However, if there is a number next to the MOF abbreviation (MOF-5, MOF-74, etc.), it means a specific structure with specific properties [6,9,11,12,13,14]. For example, MOF-74 is constructed from metal(II) oxide clusters, mainly Zn, which can be replaced by other metals such as Cu, Ni, Co, Mn, and Fe. These clusters are connected by 2,5-dioxido-1,4-benzenedicarboxylate (DOT) ligands [11]. IRMOF means Isoreticular Metal–Organic Framework and describes a series of materials with the same underlying topology, usually constructed from the same secondary building units. Different numbers next to the IRMOF are suitable for different substituents in linkers. For example, IRMOF-1 is constructed from Zn-O clusters and [(BDC)₃·7DEF·3H₂O] ligands. The IRMOF-2 has the same cluster but, as a ligand, 2-Br-1,4-H₂(BDC)₃ is used. In the case of the IRMOF-3, the molecule NH₂-H₂(BDC)₃ is a ligand [6,9,15,16]. Many MOFs are described by names with the same acronym, but often their names are not related to a similar structure; instead, they are abbreviated from the names of the universities where they were obtained, for example, MIL, UiO, UAC, UTSA, or HKUST [6,17,18]. The acronym UiO-66 comes from the name of the University of Oslo, where MOF was first synthesized. Structurally, it consists of nodes composed of a metal oxide that is hexoctahedral and allows binding to 12 BDC molecules [17]. The name HKUST is related to Hong Kong University of Science and Technology, where it was synthesized, among others; HKUST-1 is made of paddlewheel units of Cu₂(COO⁻) surrounded by benzene tricarboxylate (BTC) ligands [18,19]. MOFs synthesized at Materials of Institute Lavoisier received the acronym MIL. MIL-125 is a Ti-based MOF derived from Ti metal ions and carboxylate organic ligands [8], but MIL-100 is constructed form aluminum oxide [20]. The acronym ZIF (Zeolite Imidazolate Framework) means the zeolite topology comprises another great class of MOFs. The common feature of ZIF-type structures is a tetrahedra consisting of nitrogen atoms encompassing metal ions (Fe, Co, Cu, Zn, etc.), linked via imidazole rings [6,21,22,23].

1.2. The Classification of MOF-Based Sensors

A sensor is defined as a machine that identifies variations in electrical, physical, chemical, or other quantities and, therefore, generates an output as an affirmation of variances and helps to find out specific chemicals at trace level [29]. Sensitivity and selectivity are the main parameters to describe each sensor. Nevertheless, simplicity, rapidity, stability, repeatability, and cost-effectiveness are also very important in sensing applications. In general, the sensitive and selective detection of analytes has extensive applications in different fields comprising chemical threat detection [30], healthcare monitoring [31,32], food quality control [33,34], environmental pollutants monitoring [35,36], forensic analysis [37], industrial process management [38], etc. Classical sensors possess certain disadvantages like minimal selectivity, short lifetime, poor sensitivity, low stability, and slow response time. MOFs are stable materials and, due to their porous structure, are characterized by high sensitivity and reactivity. Therefore, they seem to be ideal materials for building sensors. Using an output signal determination method, we classified the MOF-based sensors into six groups, as presented in Figure 1: optical [29,39,40,41,42], piezoresitivity [43,44,45], mechanochemical [29,46,47,48], magnetic sensors [45,49,50,51], electrochemical [7,52,53], and photoelectrochemical [2,54,55,56] sensors.

1.2.1. MOF Optical Sensors

In MOF sensors, based on optical properties, we can distinguish colorimetric and luminescence sensors. The first group, colorimetric sensors based on MOF structures, seems to be the simplest one from the point of view of use, because their operation is based on colour change [29,39,41,42,57,58,59]. In the literature, we can find many examples of applications of colorimetric sensor based on MOFs. One of them is a system based on [Zn(OBA)(H₂DPT)_0.5] DMF (TMU-34), which can be used to detect chloroform in the liquid and gas phase, changing its colour from yellow to pink (Figure 2a). This change is related to the conversion of dihydrotetrazine to tetrazine because of its interaction with chloroform. This sensor is reversible and selective as it also works in the presence of other volatile organic compounds [39]. Meanwhile, Yeh et al. developed a colorimetric method for hexavalent chromium ion detection using paper-based devices containing Cu-PyC MOF modified with nanozymes. In the presence of chromium ions, as a result of reacting with 3,3′,5,5′-tetramethylbenzidine (TMB), a bluish green colour was observed within 3 min (Figure 2b). This sensor has a low detection limit (0.051 μmol/L) and is selective in the presence of the most popular 18 cations and 9 anions, making this MOF promising for chromium detection in real samples [60].

The second group of optical sensors is luminescence sensors. Metal–Organic Frameworks seem to be very promising materials for applications as luminescence sensors because they have linkers that contain conjugated π and n systems and show very strong fluorescence signals [29,42]. What is more, their porous structure allows them to easily adsorb small molecules that significantly affect the fluorescence signal [61]. MOF-luminescence sensors can be used to detect various compounds (metals, organic molecules, and biomarkers) and can act as “turn-on” and “turn-off” sensors. An additional advantage of these sensors is their reversibility. A significant group of MOFs are optical sensors for monitoring environmental pollutants, especially heavy metals (lead, mercury, arsenic, and chromium compounds), due to their negative impact on health and environment [62,63].

The NH₂-MIL-125-based sensor, which can detect Pb²⁺, is a good example of an “on-off” fluorescence sensor. It is very sensitive (LOD = 7.7 pM) and stable in water solutions. With increasing Pb²⁺ concentration, the fluorescence signal decreases. However, the analyte can be easily removed by reaction with EDTA and the sensor can be reused [62]. Another interesting example for lead detection is application of bimetallic Eu-Tb MOF with 1,4-benzenedicarboxylate as a ligand. The colour of luminescence of this MOF changed from green to red, depending on the ratio of terbium and europium used. However, the Pb²⁺ presence causes a change in emission colour from red-orange to green [63].

MOF-luminescence sensors are also very promising for detecting arsenic compounds. Kumar at al. developed a mixed-metal (Co/Mo-MOF) sensor based on MOF [(Mo₂O₆)(4,4′–bpy)]_n) as a new sensing platform for As(V) with a low limit of detection 0.02 ppb. Moreover, the developed sensor was able to detect arsenic compounds using spectroscopic and paper methods [64]. Tian et al. showed that, upon excitation of Eu-MOF in the presence of arsenate, the ions obtained a luminescence emission spectrum with high intensity at 427 nm. The obtained method has high sensitivity and allows for detection at even 17.8 nmol/L of As(V). This value is much below the WHO (World Health Organization) recommendation, set at 72 nmol/L in drinking water [65]. Li et al. synthesized stable isostructural alumina-based MOFs, i.e., Al(CTTA) (BUT-18) and Al(CETA) (BUT-19), for selective detection in water samples of two organic arsenic compounds, roxarsone (ROX) and nitarsone (NIT). Due to the high porosity of BUT-18 and BUT-19 and their solid-state fluorescent properties, it is possible to detect organic arsenate. The LOD of ROX is 15.7 ppb (BUT-18) and 13.5 ppb (BUT-19), whereas the LOD of NIT is 32.2 ppb and 13.3 ppb, respectively. The high selectivity of Al-MOFs is explained by specific luminescence properties. This work indicates that MOFs are favorable materials for the selective detection of organic arsenic, which is potentially useful in monitoring drug residues in waste waters [66]. Meanwhile, Ghosh et al. reported a new luminescent iMOF-4C based on a Zn(II) Metal–Organic Framework. The produced MOF exhibited a fluorescence “turn-on” response toward HAsO₄²⁻ and HAsO₃²⁻ with high sensitivity and low detection limits [67].

Other samples of optical sensors were developed for the detection of chromium compounds: Cr(VI), CrO₄²⁻, and/or Cr₂O₇²⁻ ions. For sensing, mainly Zn-based luminescent Metal–Organic Frameworks are applied [68,69,70,71,72,73]. Wang et al. reported a luminescent cationic-europium(III)-based MOF for the detection of Cr₂O₇²⁻ ions from water [74]. The LOD value depends on the environment and can equal 0.56 (deionized water), 1.75 (water), and 2.88 ppb (lake water) [35]. Lin et al. synthesized Hf-BITD, allowing for the detection of Cr(VI) oxyanion across a broad range of pH values (0–12). It characterizes a linear correlation in luminescence quenching in the range 0–80 µmol/L for CrO₄²⁻ (LOD of 0.38 nmol/L) and 0–50 µmol/L for Cr₂O₇²⁻ (LOD of 0.33 nmol/L) [75]. Sun et al. developed Eu-MOF in which, as linkers, 4,4′,4″-triazine-2,4,6-tribenzoic acid was used for Fe³⁺ (LOD 1.12 × 10⁻⁶ mol/L), Cr₂O₇²⁻ (LOD 1.95 × 10⁻⁶ mol/L), CrO₄²⁻ (LOD 1.89 × 10⁻⁶ mol L⁻¹), and aspartic acid (Asp) (LOD 2.20 × 10⁻⁶ mol L⁻¹) detection in water systems [76]. Liu et al. showed that luminescent Zr(IV)-MOF is an efficient sensor for detecting Cr₂O₇²⁻ (LOD 0.38 μmol/L). Moreover, JLU-MOF60 allows for efficient photoreduction of Cr(VI) to Cr(III) in aqueous solution [76]. Chen, Liu et al. synthesized a two-dimensional, luminescent MOF, i.e., NH₂-CuMOFs, via a mechanism integrating chemical oxidation–reduction and inner filter effect. This mechanism allows for Cr (VI) quantitation in the linear range of 0.1–20 μmol/L (LOD of 18 nmol/L) [77]. Cui et al. developed MOFs with a combination of Eu-mtb and poly(vinylidene fluoride). The high porosity and uniform distribution of Eu-mtb allows fast enrichment of Cr₂O₇²⁻ from solutions. Using this MOF, it is possible to achieve fast removal (via adsorption, with a half time equal to 1.62 min) and fluorescence sensing (inner filter effect) of Cr(VI) oxyanions with a LOD of 5.73 nmol/L [78]. Liu et al. also proposed a colorimetric sensor based on Cu-MOF. Using UV–Vis spectrophotometry, it is possible to quantitatively determine Cr(VI) in a wide linear range of 0.2–100 μmol/L and with a fast response of 1 min (LOD of 0.023 μmol/L). The next sample of MOF was a gold nanoparticle and Pb (II)-β-cyclodextrin (Pb-β-CD) Metal–Organic Framework. In this case, gold ions are directly reduced to AuNPs on the surface of Pb-β-CD (Au@Pb-β-CD) and are responsible for enhanced electrochemiluminescence (ECL) emissions. The developed ECL sensor has a linear response in the range of 0.01–100 μmol/L and an LOD of 3.43 nmol/L [79]. MOF-808 is constructed from six connected zirconium clusters and 1,3,5-benzene tricarboxylic acid, with a crystal size of 40–1000 nm and high surface area. The Zr clusters induce an interaction with K₂Cr₂O₇ and anionic dyes (sunset yellow and quinoline yellow) leading to better pollutant detection in aqueous solutions [80].

1.2.2. MOF Piezoresistive Sensors

The piezoresistive sensor is a device for sensing pressure and strain in materials. The application of composite conductive nanomaterials is helpful to obtain high-performance and multifunctional flexible piezoresistive sensors. For practical applications, the piezoresisitve sensor was applied for monitoring body physiological signals such as speech and artery pulses, indicating that the sensor is suitable for health monitoring [43,44,45]. The use of MOFs as piezoresistive sensors is possible due to their structural flexibility. This property causes, under the influence of a stimulus, mainly small molecules to become adsorbed in the pores; the volume of the MOF structure changes, which is measured as a change in the electrical signal from surface strain. A spectacular example of the operation of such a MOF-based sensor was MIL-88, described in 2007, which increased its volume by 270% because of pyridine adsorption [29,81].

1.2.3. MOF Mechanochemical Sensors

The mechanochemical sensor is based on a mass change during the selective absorption of analytes [46]. The process of detection is carried out using quartz crystal microbalance (QCM) and a system based on surface acoustic wave (SAW) technology, allowing it to measure mechanical stress. The action of the mass-sensitive sensor is based on the observation of slight mass changes by the development of a measurable electric signal [29,47,48]. QCM-based sensors are mainly used for the detection of gaseous and volatile organic compound pollutants. This type of sensor is used to detect benzene, which causes carcinogenic, anesthetic, and neurotoxic effects. MOF-14 (built from Cu²⁺ ions and ligand 1,3,5-benzenetribenzoate) exhibits high sensing performance to benzene vapor with a detection limit at the level of 150 ppb, even in the presence of other benzene derivatives, i.e., toluene [47]. Devkota et al. used a ZIF-8-based (Zn as a node; 2-methylimidazolate as ligands) SAW system for CO₂ and methane detection [82].

1.2.4. MOF Magnetic Sensors

In many examples of MOF structures, the nodes are 3D transition metals (V, Cr, Mn, Fe, Co, Ni, and Cu) with connections to suitable diamagnetic organic linkers (such as oxo, cyano, azido bridges, and polycarboxylic ligands) that have magnetic properties [45,49,50,51]. Metal–Organic Framework materials show magnetic behavior because of the framework structure, which may involve layered geometry with a shorter conjugated distance between metal clusters. The Ni-Glutarate-based MOF [Ni₂₀(H₂O)₈(C₅H₆O₄)₂₀·40H₂O] showed ferromagnetic behavior with a Curie temperature of 4K due to weak ferromagnetic interactions with the Ni-O-Ni angle. HKUST-1 is antiferromagnetic at high temperatures; below 65K, it shows weak ferromagnetism. Jain et al. reported 11 MOFs of the general formula [(CH₃)₂NH₂]M(HCOO)₃ (M = Mn, Fe, Co, and Ni) as multiferroics [83]. There are also indications that this type of structure may be an effective sensor for the determination of arsenic compounds [18].

1.2.5. MOF Electrochemical Sensors

Electrochemical sensors are mainly based on amperometry, potentiometry, impedance spectroscopy (impedimetric sensor), electrochemiluminescence, and photoelectrochemistry methods [2,29,52,56,84,85,86]. The first electrochemical sensors were based on voltamperometric and amperometric techniques, but more sophisticated sensors are based on changes in the impedance signal. In electrochemical sensors, constant potential drives the reaction of the analyte on the electrode to generate an output current. Analyte molecules are involved in the redox reaction at the working electrode of an electrochemical cell, modulating the electrical current. In electrochemical methods, the surface of the working electrode is modified using MOFs, leading to enhance sensitivity during analyte detection. The first example of such sensors was Cu-MOF, which was made from the rigid ligand BIB (1,4-bisimidazolebenzene) and the flexible ligand H₂adp (adipic acid) under hydrothermal conditions. Analysis of the electrochemical behavior of the Cu-MOF-modified electrode revealed a single-electron redox process of the Cu^III(OH)-MOF/Cu^II(H₂O)-MOF couple and electrocatalytic activity towards H₂O₂ oxidation in alkaline electrolyte solution. The changes were observed in a linear range from 0.1 μmol/L to 2.75 μmol/L H₂O₂ with a detection limit of 0.068 μmol/L [87].

Electrochemical sensors were also designed for metal ions (lead and arsenic) detection. Wang et al. modified carbon paste electrodes by MOF-5. Resultantly, lead ions are adsorbed on the MOF’s surface from real water samples, leading to metal accumulation and signal enhancement. The linear range of detection varied from 1.0 × 10⁻⁸ to 1.0 × 10⁻⁶ mol/L and the LOD was 4.9 × 10⁻⁹ mol/L [88]. Another approach to electrochemical detection involves the use of a more complex MOF structure, e.g., MOF with a DNA-modified iron-porphyrin structure (GR–5/(Fe–P)_n-MOF). Due to the mimic peroxidase property of (Fe–P)_n-MOF, an enzymatically amplified electrochemical signal was obtained allowing for selective lead ion detection in the range from 0.05 to 200 nmol/L with an LOD of 0.034 nmol/L [89]. Shirsat et al. synthesized Au nanoparticles and single-walled carbon nanotube (SWNT) nanocomposite, which were then incorporated to MOF-199 (copper benzene tricarboxylate). The obtained Au/SWNTs@ MOF-199 exhibited excellent sensing performance towards lead ions in the range of 1 pmol/L–10 mmol/L, with an LOD of 25 pmol/L. Moreover, this sensor response appeared within a few seconds [90]. Electrochemical sensors were also developed for As(III) ion detection. For this purpose iron-based MOF containing MXene (Fe-MOF/MXene) was prepared. Modification to surface electrodes using such MOFs shows very high sensitivity of 8.94 µA/ng·L⁻¹·cm² and an LOD of 0.58 ng/L [91].

Another sample of electrochemical sensor application is nitrite ion (NO₂⁻) detection. Nitrites are very common in food and water but in a larger amount can cause cancer diseases. Sensors based on Cu-MOF-modified Au nanoparticles or rGO showed the amperometry response immediately after the nitrite injection due to the oxidation of NO₂⁻ to NO₃⁻. Both these sensors have a low limit of detection: 82 nmol/L for Cu-MOF-AuNPs [92] and 33 nmol/L for Cu-MOF/rGO [93]. These examples prove that the amperometric detection of nitrite is very promising.

Electrochemical MOF sensors could also be used to detect CO₂. Tanase et al. discussed the application of Zn-MOF-74 and NdMo-MOF with high absorption capability for CO₂ sensing using electrochemical impedance spectroscopy. An MOF-based sensor was prepared by drop casting the MOFs suspension onto the platinum electrode. Increasing the concentration of CO₂ causes an increase in both impedance signals, real and imaginary. The changes in impedance were observed under ambient conditions. Moreover, the Zn-MOF-74-based sensor reacted quickly to changes in relative humidity and CO₂ content [53]. MOF-impedimetric sensors can be used to effectively determine pesticides such as imazalil [85], parathion [94], and deoxynivalenol [95].

1.2.6. MOF Photoelectrochemical Sensors

The best selectivity and sensitivity of electrochemical sensors are based on the photoelectrochemical effect, because combining the optical and electrical signals generates a photocurrent as the sensor responds. In a photoelectrochemical system, three main factors are important: an excitation light source, a detection system, and a signal-reading device. In MOF photoelectrochemical sensors, the photoactive materials are MOFs. In the next paragraphs we describe, in more detail, the photoelectrochemical properties of MOF and sensors based on them.

2. Photoelectrochemistry of MOF Materials

The photoelectrochemistry process is defined as the interaction of photoactive materials under applied bias and during light illumination, which generates electron excitation following charge transfer from photoexcited materials. Photoelectrochemistry is also defined as an interdisciplinary field, consisting of chemistry, physics, spectroscopy, and electronics, and researches the process of electron transfer at solid–liquid interfaces [2]. The photoelectrochemical system consists of three electrodes—working electrode with photoactive materials, counter electrode, and reference electrode—immersed in electrolyte solution. Usually, semiconductors are used as photoactive materials; in our case, they are MOFs structures. MOFs are very good candidates for photoactive materials because of their large surface areas. From the point of view of photoelectrochemistry, the band gap value, charge separation efficiency, photoexcited carrier lifetimes, ground- or excited-state conductivity, recombination pathways, and band alignment are the most important properties of MOFs [2,56,96]. We can improve these properties by modification of the MOF structures. We can distinguish between three main types of MOF modification influence based on their photoelectrochemical properties: structure modification, surface modification, or heterojunction formation (Figure 3).

2.1. Modification of MOF Structure

Classical, unmodified MOFs are usually insulators with a large band gap, which allow only a narrow range of light to be absorbed. Many attempts have been made to reduce their band gap to achieve semiconducting electronic properties [96]. One of them is selecting a suitable transition metal element as a MOF metal node. A node with an unoccupied d-orbital, with an energy level below the organic linker LUMO, narrows the band gap. The best metals are indium (MIL-68) [97], titanium (MIL-125) [8], iron (MIL-88B, MIL-100) [43], and zirconium (UiO-66, PCN-222) [17,31,98].

Another attempt to control the MOF electronic properties is introducing a guest metal. The general rules in changing the metal node are that the dopant metal should form a more reducible metal oxide than the host metal, which translates into a smaller band gap and lower absolute position of the conduction band edge. For example, for MIL-125-NH₂ with titanium nodes, the replacement of Ti by V, Sn, W, and Nb is interesting. Another example is Zr replacement in UiO-66-NH₂ by Ti, Nb, and W. Such bimetallic MOFs showed a higher intensity of photocurrent in comparison to MOFs with only one type of metal [2,24].

Another attitude to improve the electronic structure of MOFs is ligands modification. The most promising ligands are porphyrins, phthalocyanines, imidazolate ligands, and thiophenes. All these ligands characterize good photosensitive properties: ultrafast electron injection, high light absorption, efficient charge separation, and good chemical stability under illumination [2,56]. Additional modification of ligands could be made by introducing various functional groups (e.g., -NH₂, -OH, -CH₃, and halogens). Adding chemically soft side groups or increasing the softness of a ligand by reducing its the ionization potential (IP) and increasing the electron affinity (EA) allows us to raise the ligand-centered valence band (VB) position and decrease conduction band (CB) energy, and vice versa [2,96].

2.2. Surface Modifications of MOFs

In order to increase the sensitivity and selectivity of photoelectrochemical sensors, their surface is often modified with various biologically active compounds and materials, including nucleic acids (DNA and RNA), enzymes, nanoparticles, and so on [99,100,101]. Antibody and aptamer-based photoelectrochemical sensors have been designed and utilized in various detection methods to specifically recognize antigens using antibodies as biorecognition elements.

Thanks to very specific bonds between biomolecules and the analyte, the selectivity of this type of sensors increases significantly and they can often be used in the natural environment, e.g., it is possible to detect the carcinogenic aflatoxin B1 in corn grains. The sensitivity of this type of biosensors is also very high. The above-mentioned sensor based on DNA-modified Er-MOF and aptasensor has the ability to detect aflatoxin B1 at the fM level [99].

The surfaces of the MOFs can also be modified with conductive polymers that positively affect the photoelectrochemical properties by increasing the range of light absorption and/or increasing the efficiency of the generated photocurrents. As an example, we can use the Fe-MOF–PEDOT system, which serves as a sensor for detecting pesticides. The advantage of this system is that PEDOT, as a conductive polymer, increases the conductivity of this system. Moreover, because of such a connection, we obtain an n-p heterojunction (FeMOF is n-type and PEDOT is p-type), which provides better charge separation and prevents recombination. The absorption range in the visible light range is also increased compared to the unmodified MOF [102].

2.3. Multicomponent Heterostructures Formations

The third type of improvement of photoelectrochemical properties is the creation of various multicomponent structures. The combination of the MOF structure with a classic semiconductor or metal nanoparticles positively affects the properties of the sensors.

Doping MOF with semiconductors: inorganic metal oxides (BiVO₄ [103,104], TiO₂ [105], ZnO [106], and Cu₂O [107]), sulfides (CdS) [31,55,103,104,108], nanoparticles (Au) [109,110], or g-C₃N₄ [104,111] can enhance the photoelectrochemical properties of MOFs. Introducing metal oxides or sulfides to MOF surfaces generates trap states, improves the charge separation, and decreases the recombination process. Semiconductors can also improve the range of light absorption, for example, the combination of CdS with Zr-MOF: MOF-808, NU-1000, and PCN-222 have a considerable red shift and enhanced visible light absorption. In comparison to pure CdS with CdS@Zr-MOFs, such multicomponents have high surface areas, uniform active sites, and more precise structures. What is more, the regular pores of CdS@Zr-MOFs can also promote the diffusion of substrates and products. All these features improve photocatalytic performance [31,112]. Heterostructures made from MOFs and semiconductors can also influence the direction of generated photocurrents. CdS is a well-known n-type semiconductor; after illumination, it generates only anodic photocurrent, but in the composite with PCN-224, a cathodic photocurrent was observed [31].

Plasmonic organometallic structures are hybrid materials consisting of metal nanoparticles (mainly Au, Ag, and Pd) embedded on the surface or in the pores of MOFs [32,113]. These materials, due to mutual interactions, show an improvement in their optical and photoelectrochemical properties [109,110,114]. Metal nanoparticles generate the so-called “hot electrons” coming from the conduction band during plasmon decay. These electrons can then take part in electrochemical processes or amplify the generated photocurrents. Positive charges of lower energy, so-called “hot holes”, can also be generated in the system [114]. Both electrons and holes can take part in anodic or cathodic reactions, making them ideal for use as photoelectrochemical sensors. SPR derived from metal nanoparticles in combination with the MOF structure improves light absorption and facilitates electron transfer, making photoelectrochemical sensors even more sensitive and allowing the detection of analytes with lower concentrations. This effect was used to build a photoelectrochemical sensor for tetracycline detection. MOF-derived In₂O₃@g-C₃N₄ modified with gold nanoparticles was used to build the photosensitive material [97].

MOFs with nanoparticles can find various applications beyond photoelectrochemical application, for example, ZIF-8 in combination with silver nanoparticles (Ag@ZIF-8) have been used to separate alcohols from an aqueous environment. The ZIF-8 structure ensured very good adsorption of alcohols, while the presence of Ag nanoparticles enabled their recycling by triggering the release of alcohol from the ZIF-8 structure [113]. Another example is the controlled release of drugs using MOF-nanoparticle systems with ZIF-8 and gold nanoparticles, which can locally release large amounts of heat. Drug release was controlled by changes in pH [32].

3. Mechanisms of Photocurrent Generation in Photoelectrochemical Sensors

The operation of photoelectrochemical sensors begins with the absorption of light, which causes the excitation of electrons and the formation of holes. In the next stage, the charge is separated and photoinduced carriers can undergo electrochemical reactions or a recombination of carriers. At this stage, an anode or a cathode photocurrent is observed, depending on the type of semiconductor; in this case, we are dealing with MOFs. The next step is to add the analyte to be tested to the solution. Then, as a result of the interaction of the analyte with the photoactive substance, we observe a change in the intensity or direction of the generated photocurrent. Based on the change in the generated signal, we can qualitatively or quantitatively determine the presence of the analyte being tested (Figure 4).

Depending on the structure of the photoactive substance in the sensor, the mechanism of action will be different and dependent on various factors. Below, we will briefly try to present the most important mechanisms of operation of photoelectrochemical sensors based on MOFs, describing specific examples.

The first MOF material that exhibited the photoelectrochemical behavior was MOF-5 ([Zn₄O(BDC)₃]), which is a three-dimensional cubic porous framework with [Zn₄O]⁶⁺ clusters linked together through 1,4-benzenedicarboxylate (BDC²⁻) ligands. The band gap of this material was 3.3 eV. An electrode made by the fabrication of MOF-5 thin films on GCE (glassy carbon electrode) was used to detect ascorbic acid (AA) and showed strong antioxidant properties with an oxidation potential of −0.185 V (vs. SCE). When the electrode made from MOF-5 was illuminated with light with energies higher than that of the band gap, the electrons were excited from the VB to the CB, forming electron–hole pairs. The injection of CB electrons into the GCE yields the photocurrent, whereas AA provides the electrons to the VB holes, completing the photocurrent generation cycle and preventing the recombination of excited electrons (Figure 5). By measuring the photocurrent originating from the electron transfer process, the concentration of AA at that point may be determined. A strongly enhanced photocurrent response was observed for the MOF-5 thin film in the presence of the addition of 1 mM AA. A linear increase in photocurrent in the system was observed in the AA concentration range from 0.05 to 1.4 mM [12].

Another example of using a simple MOF structure as a photoelectrochemical sensor for the determination of α-caseins is the MOF designated as PCN-222, built from Zr−O clusters linked with TCPP ligands. This substance deposited on ITO was used as an electrode to build a sensor. In an oxygen-saturated solution, it was observed that the tested system generates cathode photocurrents. In addition, to strengthen the intensity of the photocurrent and, thus, increase the sensitivity of the sensor, dopamine was added to the solution, which is a very efficient hole scavenger [98]. The system built, in this way, was used as a sensor to detect α-casein, a popular protein that occurs in milk and can be a strong allergen. The addition of α-casein to the PEC system reduced the intensity of the generated photocurrent. This is because the interaction of the protein with the electrode blocks interface electron transfer and its reactions with oxygen and dopamine (Figure 6). The limit of detection for the α-casein sensor was estimated at 0.13 μg/mL, which is comparable to another ELISA method (0.05 μg/mL). However, it should be emphasized that the PEC-based sensor is simpler and cheaper [98].

A large group of photoelectrochemical sensors are based on MOF and metal nanoparticles composites [97,115]. The presence of nanoparticles on the surface of the MOF structure causes an increase in the photoelectrochemical activity of the material. This is because the metal nanoparticles (Au and Ag) have the appearance of the Surface Plasmonic Resonance (SPR) effect [116]. For example, in noble metal nanoparticles, hot charge carriers are generated by localized surface plasmon resonance (LSPR), a collective oscillation of free electrons on a confined surface induced by an electromagnetic wave [114]. The mechanism of photoelectrochemical sensors is also slightly different. The MOF-metal nanoparticle system absorbs light, which, in the MOF structure, causes the transfer of electrons from the valence band to the conduction band and further to the electrode, generating a photocurrent. At the same time, “hot electrons” are generated in the nanoparticles, which are transported to the conduction band and further to the electrode, amplifying the signal generated by the MOF (Figure 7a). Moreover, in the system, a Schottky junction is formed, and it could be suppressing the backflow of charge carriers and positively influence sensors sensitivity.

As an example, we can describe a tetracycline sensor based on Au-(In₂O₃@g-C₃N₄) (AuInCN) composite. The mechanism of operation of this sensor uses the synergy effect of both components (MOF and NPs) and is shown in Figure 7b. When the system is illuminated, electrons are simultaneously excited to the conduction band in In₂O₃ and g-C₃N₄. Since the position of VB and CB g-C₃N₄ is more negative than in In₂O₃, there is a transfer of electrons from CB (g-C₃N₄) to CB (In₂O₃). Photogenerated holes in VB (In₂O₃) are transported to VB (g-C₃N₄). Concurrently “hot electrons” are generated in AuNPs and transferred to the g-C₃N₄ conduction band, increasing the photocurrent value. In the presence of the analyte tetracycline, an increase in the intensity of the photocurrent is observed, because holes in the valence band can be easily oxidized by tetracycline molecules, thus preventing electron recombination [97]. Sensors for the detection of SARS-CoV2 spike glycoprotein (S protein) based on Au NPs/Yb-TCPP decorated with DNA aptamer against S protein is another example of using nanoparticles to enhance photocurrent intensity. In this case, it is very important because, after contact with the sensor, the glycoprotein signal decreases, due to high steric hindrance and low conductivity of the S protein (Figure 7c). Another advantage of using nanoparticles is the ability to attach the aptamer to gold [115].

4. MOF-Photoelectrochemical Sensors for Environmental Pollution Monitoring

Modern countries have, for many years, been pursuing a policy aimed at reducing greenhouse gases, focusing on the development of alternative energy sources and investing in the development of green energy (development of wind farms). This policy leads to the emergence of new directives related to the environment and its protection. Zero-waste policies also place an emphasis on recycling, introducing new production methods that will be more ecological and energy-saving. In this section, we are limited to describing only the most common environmental pollutants mentioned in this review in the context of described sensors based on MOFs. Moreover, the origins, the sources of pollutant emissions, their permissible content in the environment, and the health impacts are also underlined.

4.1. Heavy Metal Compounds

In the first part of our review, we described many examples of MOF-based sensors that are used to detect heavy metal ions. Most of them are optical sensors with fluorescence detection. Only a few examples of photoelectrochemical sensors based on MOF structures can be found in the literature, which are used to monitor contamination with heavy metals such as mercury, arsenic, or lead. All examples of such sensors found are listed in

Table 2. Examples of photoelectrochemical sensors for heavy metal compounds.

Sensing Materials	Analyte	Linear Detection Range	Sensitivity/ Detection Limit	Reference
HMA/Eu3+-CdS	Hg2+	0.1 pM to 1.0 μM	0.067 pM	[55]
TFPB−APTU COF	Hg2+	10 pM to 100 μM	0.006 nM	[117]
PCN-224/rGO	p-ASA	10 ng/L to 10 mg/L	5.47 ng/L	[118]
Cu-modified NH2-MIL-125	Pb2+	10 fM–1 µM	6.8 fM	[119]

and described below.

4.1.1. Mercury

Mercury has a very toxic effect on the environment and human health. Especially dangerous is the ability of mercury to accumulate in food, mainly in fish [120,121]. Mercury is an element that can exist in a pure metallic form (Hg) or compounds like non-soluble Hg₂Cl₂ (calomel) and those that are soluble in water (HgCl₂, HgBr₂, or Hg(OH)). Mercury compounds have a strong chemical and biological activity. This means mercury can easily penetrate the environment, threatening animals and plants. Another dangerous compound is organic methyl mercury (CH₃Hg), which drives the major human exposure route via fish consumption, particularly large marine fish, e.g., tuna and swordfish [120].

Monitoring trace amounts of mercury ions (Hg²⁺) is also possible using photoelectrochemical sensors based on MOFs. The biosensor for detecting mercury was built from hydrophobically modified alginate (HMA) combined with an europium(III) ion, which creates a MOF structure, and CdS nanoparticles forming a photoactive substrate. This sensor structure provided a friendly platform for bioconjugation and ensured greater sensitivity. By adding a thymidine-rich DNA probe to this system, steric hindrance structures are formed in the presence of mercury, which reduces the PEC response of the sensor. The described photoelectrochemical detection system enabled the selective detection of Hg²⁺ with high sensitivity (see Table 2). The use of semiconductor CdS quantum dots as light harvesters and MOF-like composites as sensitizers expands possible design concepts for photoelectrochemical sensor systems [55].

Another example of using a sensor sensitive to mercury ions and based on a MOF-like structure is a spherical covalent organic framework (TFPB-APTU COF), which can be used simultaneously to detect and remove mercury from solutions. The big advantage of this system is the two active centers that can chelate mercury. One of them is the sulfur atom, which easily reacts with mercury, and the other active center is the amino group. The formed coordination compounds create steric hindrances that, as in the previous example, reduce the photocurrent signal and, thus, increase the sensitivity of the sensor. The TFPB-APTU COF system has the ability to detect Hg²⁺ in a wide concentration range from pM to µM (see Table 2). An additional advantage of this system is the ability to remove mercury from the environment. Due to its spherical structure, the high removal capacity is 2692 mg/g. Moreover, this sensor is characterized by a high ability to regenerate [117].

4.1.2. Arsenic Compound

Arsenic compounds are also very toxic and naturally occur in nature as both organic and inorganic compounds. Arsenic is an element with four oxidation states (−3, 0, +3, and +5) and usually occurs as an element accompanying copper, nickel, cobalt, iron, or mixed sulphide ores and form minerals like enargite (Cu₃AsS₄), tennantite (Cu₁₂As₄S₁₃), cobaltite (CoAsS), arsenopyrite (FeAsS), realgar (As₄S₄), and orpiment (As₂S₃) [122]. Arsenic also exists as an inorganic arsenide compounds like arsenate AsO₃³⁻ and arsenite AsO₄³⁻ in aqueous environments [123].

Considering that most arsenic compounds are soluble in water, photoelectrochemical detection can be used to monitor its content. As an example, we can describe a PEC sensor based on the PCN-224 structure modified with reduced graphene oxide (rGO) flakes, which was used to detect p-arsanilic acid (p-ASA). The addition of rGO to the MOF improved the efficiency of the generated photocurrent by 12 times due to effective charge separation. The sensor showed strong affinity for p-ASA due to the formation of a coordination bond between Zr-O-As atoms and π–π interactions. The described PEC sensor was successfully applied to monitor p-ASA in simulated natural water and swine fecal lixivium. This sensor shows that not only MOFs modified with biomolecules show high sensitivity and selectivity in pollution control [118].

4.1.3. Lead

Another commonly used heavy metal is lead, which occurs naturally in the ores of galena (PbS), cersite (PbCO₃), and anglesite (PbSO₄). Since 1923, lead in the form of tetraalkyl lead has been used in gasoline to improve the octane number of fuels and improve engine performance. Since then, lead has been released in the transport sector, which causes many diseases and is especially dangerous for children. In 2021, the last country, Algeria, stopped supplying leaded gasoline [124]. Currently, lead is mainly used in the production of car batteries (80%) and replacing it with other compounds is still a big challenge for scientists [125].

Lead poisoning is eliminated through the development of modern sensors, including those based on MOFs, which allow for the effective detection of this element. One example is a biosensor based on the NH₂-MIL-125 structure, into which copper(II) ions were introduced during the calcination process. The resulting Cu₂O-CuO-TiO₂ heterojunction was used to build an efficient photocathode. With Pb²⁺, multiple single strands of DNA (S1) can be released based on a Pb²⁺-assisted cyclic amplification strategy and hybridized with hairpin DNA (HP1) on a modified electrode. The sensor constructed in this way has a very wide detection range of lead ions with a lower detection limit of 6.8 fM. Moreover, this sensor was tested in natural water and showed very good recovery [119].

4.2. Organic Pollutants—Generated Mainly by Pharmaceutical, Paper, and Textile Industry

Every year, the pharmaceutical, paper, and textile industries produce huge amounts of waste, mainly containing aromatic compounds, which are harmful to people and the environment and cause various diseases. In many countries, there are legal restrictions that very rigorously define the methods of dealing with this type of waste, and various organizations define acceptable standards. Unfortunately, there is still a significant problem with controlling the level of various organic pollutants. Therefore, the topic is a constant challenge for scientists who work on modern monitoring systems.

A huge amount of environmental pollution comes from industry. The production of plastic materials is one of the most polluting to the environment. One very popular and very toxic substance is dibutyl phthalate (DBP), which is used as a plasticizer in everyday items. The DBF concentration can be monitored by the MOF-COF hybrid system based on NH₂-UiO-66/TpPa-1-COF. In addition, the surface of the sensor has been modified with a molecularly imprinted polymer that facilitates the binding of DBF to the sensor. The DBF concentration can be monitored by the MOF-COF hybrid system based on NH₂-UiO-66/TpPa-1-COF. In addition, the surface of the sensor is modified with a molecularly imprinted polymer that facilitates the binding of DBF to the sensor. As a result, the interaction blocks electron transfer, and the observed signal decreases with the increasing concentration of the analyte. This sensor can operate in a wide range of concentrations of DBF (see

Table 3. Examples of photoelectrochemical sensors for waste from industry.

Sensing Materials	Analyte	Linear Detection Range	Sensitivity/ Detection Limit	Reference
NH2-UiO-66/TpPa-1-COF	dibutyl phthalate (DBP)	1.0 × 10−10–1.0 × 10−4 M	3.0 × 10−11 M	[126]

). Moreover, this sensor has been tested in contaminated drinking water and chicken samples, and the obtained results are consistent with the results of the analysis using the HPLC method [126].

4.2.1. Pesticides

Plant protection products, both fertilizers and insecticides or fungicides, are a large group of compounds that can pollute the environment and affect human health. Therefore, in this area, monitoring of these pollutants is necessary. There are several examples of photoelectrochemical sensors based on the MOF structure that can be used for this purpose (see Table 4).

One example is a sensor based on NH₂-MIL-125(Ti)/TiO₂ for the detection of clethodim, which is a herbicide used to control grasses in vegetables and other crops. Unfortunately, this substance has a negative effect because it inhibits the activity of acetyl CoA carboxylase, whose function is to regulate the metabolism of fatty acids. The operation of this sensor is based on the interaction of light with the NH₂-MIL-125(Ti)@TiO₂ system; during this process, holes are created, which can then produce hydroxyl radicals from water or OH- ions on the surface. In turn, hydroxyl radicals take part in the reaction with clethodium, oxidizing it to a radical. Due to this, the charge separation in the system increases and an increase in the photocurrent intensity is observed. The detection limit of this sensor is 0.01 µM and is comparable to chromatographic methods, but its advantage is the ease of measurement and fast detection time of several seconds [127].

Another example is a more advanced sensor based on the OECT-PEC architecture. As an organic electrochemical transistor (OECT), a conducting polymer PEDOT was used, combined with FeMOF as a photoelectrochemical active element. This sensor, equipped with a specific aptamer, was used to detect malathion (MAL). Malathion is a well-known pesticide often used in agriculture, residential landscaping, and public recreation areas and is used to kill mosquitoes. As a result of the interaction of the sensor with MAL, a decrease in the signal is observed with increasing pesticide concentration. This sensor is selective even in the presence of different pesticides (diazinon (DIA), edifenphos (EDI), profenofos (PRO), and omethoate (OMT)), both individually and as a mixture [102].

Table 4. Examples of photoelectrochemical sensors for pesticides.

Sensing Materials	Analyte	Linear Detection Range	Sensitivity/Detection Limit	Reference
Cu-BTC MOF/g-C3N4	glyphosate	1.0 × 10−12–1.0 × 10−8 M	1.3 × 10−13 M	[128]
FeMOF@PEDOT	malathion	0.1–10 µg/L	0.03 ng L−1	[102]
NH2-MIL-125(Ti)@TiO2	clethodim	0.2–25 µM	0.01 µM	[127]

Table 4. Examples of photoelectrochemical sensors for pesticides.

Sensing Materials	Analyte	Linear Detection Range	Sensitivity/Detection Limit	Reference
Cu-BTC MOF/g-C3N4	glyphosate	1.0 × 10−12–1.0 × 10−8 M	1.3 × 10−13 M	[128]
FeMOF@PEDOT	malathion	0.1–10 µg/L	0.03 ng L−1	[102]
NH2-MIL-125(Ti)@TiO2	clethodim	0.2–25 µM	0.01 µM	[127]

4.2.2. Drugs and Biomolecules

The most effective drugs help us recover, but, unfortunately, they also have many side effects that affect our health. Moreover, they have a detrimental effect on flora and fauna, and it should be constantly monitored. Too high concentrations of drugs in the environment can have a very negative impact on animals, even leading to the extinction of some species. The accumulation of different types of antibiotics, on the other hand, can lead to the worldwide spread of bacterial resistance. This phenomenon is very dangerous, because drug-resistant bacteria that lead to death are already appearing today.

Antibiotics are a very important group of drugs; here, we will focus on photoelectrochemical sensors for their detection (see Table 5) [31,108]. Dong et al. described a photoelectrochemical sensor based on MOF (PCN-224) coupled with CdS for doxorubicin hydrochloride (Dox) and gentamicin sulfate (CN) [31]. Dox is an effective drug with anticancer properties. Therapy with Dox has a lot of side effects: cardiotoxicity, suppression of bone marrow hematopoiesis, or hair loss. CN, in turn, is a popular antibiotic against Gram-negative and -positive bacteria, but its excess causes tinnitus, dizziness, or hematuria. An electrode made from CdS@PCN-224 under visible light illumination generated a cathodic photocurrent, but in the presence of Dox or CN, with an increasing concentration of analyte, the photocurrent intensity decreased. This PEC sensor has a wide linear range and detection limit of 3.57 nm for Dox and 0.158 nM for CN. For practical application, a test with milk and different concentrations of analyte was performed [31]. Another example may be a sensor based on a CdS/Eu-MOF composite, whose surface has been modified with an aptamer, that selectively binds ampicillin, even in the presence of other antibiotics (kanamycin, streptomycin, bleomycin, and tetracycline). Detection of the antibiotic content is based on an increase in the intensity of the photocurrent in the presence of ampilicin, which is oxidized on the electrode. The sensor has been tested in milk samples and is suitable for use in the food industry; according to the US Food and Drug Administration (FDA), the tolerance or safe level of AMP in milk is 28.6 nM [108].

Photoelectrochemical sensors based on MOFs are used to monitor various types of drugs, for example, a NiPc-Cu MOF has been used to detect N-acetyl-L-cysteine (NAC), which acts as an antioxidant to protect cells from poisoning. This sensor generates photocurrents which, as a result of interaction with the analyte, are weakened; thus, it is possible to detect NAC in very low concentration (see Table 5) [129].

Another challenge of the modern world is the detection and control of the level of various types of biomolecules, such as proteins or lipids, which can be used as markers of cancer or viral diseases (SARS-CoV-2) [109,110,115]. An example of a tumor marker for which a photoelectrochemical sensor has been developed is α-fetoprotein (AFP), considered a marker of liver tumors. The sensor based on the Eu-MOF@AuNP structure modified with AFP antibodies is very sensitive (Table 5). The method of detection consisted of reducing the intensity of photocurrents as a result of the interaction of AFP with the antibody. A very similar sensor for detecting AFP was also developed on the basis of AuNPs@Zn-MOF [110]. In both cases, modification of the MOF structure with Au nanoparticles increased the sensitivity of the sensor [109,110,115].

Table 5. Examples of using MOF-based sensors for drugs and biomolecules.

Sensing Materials	Anayte	Linear Detection Range	Sensitivity/ Detection Limit	Reference
CdS@PCN-224	Doxorubicin hydrochloride	10 nM and 1 µM	3.57 nM	[31]
CdS@PCN-224	Gentamicin sulfate	10 nM and 1 µM	0.158 nM	[31]
Eu-MOF	Ampicillin	0.1–200 nM	0.01 nM	[108]
MOF-derived In2O3@g-C3N4	Tetracycline	0.01–500 nM	3.3 pM	[97]
Mg-PTCA MOF	miRNA 21	10 aM to 10 pM	2.8 aM	[128]
Au NPs/Yb-TCPP	SARS-CoV 2 spike glycoprotein	0.5–8 μg/mL	72 ng/mL	[115]
Cu-MOF@CuPc-TA-COF	HIV-1 DNA	1 fM to 1 nM	0.07 fM	[130]
NiPc-Cu MOF	N-acetyl-L-cysteine	0.0125 to 42.5 μM	5.2 nM	[129]
Eu-MOF@AuNPs	α-fetoprotein	0.002–15.0 ng/mL	0.16 pg/mL	[109]
AuNPs@Zn-MOF	α-fetoprotein	0.005–15.0 ng/mL	1.88 pg/L	[110]
Ce-Por-MOFs/ AgNWs	ronidazole	0.1–104 nM	0.038 nM	[131]

Table 5. Examples of using MOF-based sensors for drugs and biomolecules.

Sensing Materials	Anayte	Linear Detection Range	Sensitivity/ Detection Limit	Reference
CdS@PCN-224	Doxorubicin hydrochloride	10 nM and 1 µM	3.57 nM	[31]
CdS@PCN-224	Gentamicin sulfate	10 nM and 1 µM	0.158 nM	[31]
Eu-MOF	Ampicillin	0.1–200 nM	0.01 nM	[108]
MOF-derived In2O3@g-C3N4	Tetracycline	0.01–500 nM	3.3 pM	[97]
Mg-PTCA MOF	miRNA 21	10 aM to 10 pM	2.8 aM	[128]
Au NPs/Yb-TCPP	SARS-CoV 2 spike glycoprotein	0.5–8 μg/mL	72 ng/mL	[115]
Cu-MOF@CuPc-TA-COF	HIV-1 DNA	1 fM to 1 nM	0.07 fM	[130]
NiPc-Cu MOF	N-acetyl-L-cysteine	0.0125 to 42.5 μM	5.2 nM	[129]
Eu-MOF@AuNPs	α-fetoprotein	0.002–15.0 ng/mL	0.16 pg/mL	[109]
AuNPs@Zn-MOF	α-fetoprotein	0.005–15.0 ng/mL	1.88 pg/L	[110]
Ce-Por-MOFs/ AgNWs	ronidazole	0.1–104 nM	0.038 nM	[131]

5. Advantages/Disadvantages and Challenges in Photoelectrochemical MOF-Based Sensors

Summing up the examples of the use of MOF structures for the synthesis of photoelectrochemical sensors described above, it can be concluded that MOFs are materials that are very well suited for this purpose. Among their most important features is the porous structure capable of adsorbing various molecules in the pores or on the surface, as well as the chemical stability of the structure under different conditions. Through various modifications of the structure of MOFs, it was possible to solve many problems that, at first glance, disqualified the use of MOFs. The following should be mentioned here:

(1)

Narrow range of light absorbance: surface modification to increase the range of light, introduction of nanoparticles, and modification of linkers;

(2)

Low conductivity: introduction of nanoparticles, modification of linkers, creation of bimetallic systems, or creation of composites with semiconductors or carbon nanostructures;

(3)

Poor solubility: synthesis of materials directly on the electrode;

(4)

Low selectivity: enhanced using specific biomolecules (DNA, enzymes, and aptamers).

The principle of operation of the photoelectrochemical sensor is very simple and consists in direct observation of the generated signal. To interpret the results, you do not need a complicated analysis of the obtained data, only appropriate calibration of the device.

Regardless of the type of MOF used to build the photoelectrochemical sensor, from the user’s point of view, we can generally divide it into two modes of operation: enhancing or weakening the sensor’s operation. In the case of sensors whose operation involves a decrease in the intensity of the generated signals, the strategy of modification with metal nanoparticles has proven to be very effective, significantly strengthening the generated signal and increasing the sensitivity and operating range of the sensor. In most examples, surface modification of metal nanoparticle MOFs resulted in an increase in sensor sensitivity by about an order of magnitude and the ability to detect the analyte at the nano or pico level.

At present, health and the environment are the most important aspects, which should be take into account during the production of new materials. In this context, the life cycle of MOF sensors (see Figure 8) should be considered.

The life cycle of each MOF sensor includes the following steps: design, production, delivery, application, and disposal and/or recovery. Designing sensors based on MOF is a complex process in which we must first define the need and determine how to meet the need (future applications). The selection of starting materials (composition, necessary facilities, hardware resources, human resources, etc.) is also very important, as well as the development of an optimal method of material synthesis with high efficiency and low production costs.

The next stage is production. The selection of optimal production conditions (ways to obtain high process efficiency through the synthesis method used, production conditions, etc.) is also important. At this stage, costs are generated mainly by reagents used for synthesis, hardware resources (production hall or small laboratory, depending on the scale; equipment; etc.), energy consumption, and human resources (cost related to the quantity and qualifications of personnel). There is also an aspect related to health and environmental protection, such as answering the following questions: How much waste will be generated? Are the reagents used toxic? Are the by-products safe for humans during the production chain (from unpacking the ingredients needed for synthesis to receiving final product)? What actions should be taken to minimize the risk of possible exposure of humans and the environment during sensor production?

Once we receive the desired product, it must be delivered to the customer. It is worth determining the stability of the product, taking into account transport conditions (e.g., the influence of temperature, changes in humidity during transport, appropriate packaging, etc.) and safety.

The next step is the application of the product. The purpose of each product is its practical use. This process should be safe for people and the environment. Moreover, well-designed MOF-based sensors should have the following properties: high sensitivity, high selectivity, fast response, and the possibility of regeneration. Sensors should also be non-toxic, reusable, and cheap. In this context, certain criteria may be missing in terms of the benefits obtained in the contamination detection process. One example is the use of chromium, which is not safe for the environment, in the MOF production process to detect other compounds [132].

The last step is related to what will happen to the sensor after it loses its original properties. The ideal method would be to be able to choose between two methods, i.e., safe disposal and/or recovery. This part is most important considering the potential for toxic sensor components to be released into the environment during long-term storage. So, this part presents more challenges related to MOF-based sensors. Moreover, the number of MOFs produced in recent years is so large that it is impossible to verify their negative effects on an ongoing basis. The very aspect of determining the toxicity of such structures is also problematic. There are currently no guidelines on the health and environmental impacts of MOFs.

6. Conclusions

MOFs are a unique class of materials that, at least, contains one metal as a node and an organic compound as a linker. The connection of these compounds via a selected route leads to obtaining a highly crystalline and porous structure, stability, and tunable physicochemical properties. The obtained properties are often a synergetic effect of metal and organic compounds. Depending on the structure, MOFs might have a different application, such as pollutant sensing. This review shows some of the sensors that are mostly used for inorganic compound detection, e.g., Pb, As, and Cr, for which mostly optical sensors are applied. Comparatively, electrochemical sensors are promising tools for Pb, NO_x, CO₂, CO, and As detection. As was also shown, photoelectrochemical sensors, due to their advantages, find application mostly in organic compounds (pesticides, drugs, and biomolecules) detection. Awareness of health and the environment forces the search for new materials that will show high sensitivity and a low detection limit for analytes. A big challenge in the production of sensors is the safety of people and the environment at every stage of their life cycle and the elimination of toxic ingredients during their production. In addition, the development of methods for the recovery or disposal of MOFs should be conducted in harmony with nature.