Metal Organic Framework (MOF): Synthesis and Fabrication for the Application of Electrochemical Sensing

1. Introduction

Metal-organic framework (MOF) is a category of porous materials that have drawn greater attention a few decades back. The MOF are primarily hybrid materials having organic and inorganic moieties. The organic linker in its structure comprises organic ligands, with the metal cluster acting as inorganic components [1]. The target MOF is distinguished or embedded by the functional groups among metal nodes and organic linkers, supporting the selectivity and reliability of materials for specific purposes, including electrochemical sensing [2]. The MOF showed varied applications in the area of gas adsorption [3], catalysis [4], optical storage media [5], drug delivery [6], sensing, separation [7], redox-active electrode materials [8], etc. due to its micro-to-meso porous structure, high surface area, and flexible structures. Increasing the electrocatalytic signal by immobilizing the metal nanoparticles within the pores of MOFs is advantageous, resulting in suitable and sensitive sensing [9].

The MOFs showed numerous advantages over conventional porous materials due to the high surface area, pore working capabilities, and regulated shape and size of pores due to using suitable organic linkers in the material synthesis [9]. Due to their distinctive properties, MOFs are known hybrid materials with possible applicability in electrochemical sensing techniques for determining or detecting various micro-pollutants and heavy metals in aquatic environments [10]. Initially, the MOFs were derived using the divalent cations of transition metals viz, zinc (Zn²⁺), cobalt (Co²⁺), copper (Cu²⁺), etc. including well known MOFs such as MOF-5, HKUST-1, ZIF-8 etc. The use of divalent transition ions makes easy crystallization of synthesized MOF; however, divalent cations showed several disadvantages, such as low hydrothermal stability that impacts the annealing of materials and even some of the applications [11]. Therefore, the selection of metal cations in the synthesis of MOF is crucial. Further, the thermodynamic studies indicated that higher-charged metal cations possess strong metal-ligand bonds, significantly enhancing MOF’s hydrothermal stability [12]. The stability of the MOF increases with higher valency ions such as iron (Fe), aluminum (Al), zirconium (Zr), and titanium (Ti), which indicated that metal ions with greater valency are more stable than those of divalent metal cations such as zinc (Zn), copper (Cu) and cobalt (Co). In addition, aluminum is abundant on the Earth’s crust, cheap, and lightweight, enabling large-scale production of MOF for varied applications such as removing pollutants, sensors, and storing gases [13].

Voltammetry and amperometry are the most reliable and efficient electroanalytical techniques for the qualitative and quantitative determination of several analytes at trace levels due to their reasonably high sensitivity, robust instrumentation, and relatively cost-effective compared to other spectroscopic or chromatographic techniques. The core component of the electroanalytical techniques is the working electrode at which the analytes react electrochemically. The electrodes fabricated with suitable materials have ability a selective oxidation or reduction of analyte [14]. Many typical electrodes have poor surface kinetics, significantly decrease selectivity and sensitivity. This is known to cause a variety of key difficulties. Analytes often exhibit a broad peak on standard electrodes, with no peak seen at lower concentrations [15].

Similarly, the MOFs have a practical choice because of their ability to selectively analyze the analytes of interest, enabling them to detect them efficiently. The application of MOFs in electroanalytical methods is a relatively newer approach, and several challenges have been found in designing an attractive and intriguing framework for detecting the target analytes or biomolecules in aqueous media [16]. The MOFs are the designed materials, and properties alter as per the specific applications; hence, the MOFs are the futuristic engineered materials with varied applications in diverse research areas. Furthermore, MOFs, in general, possess unique properties such as high specific surface area, pore volume, molecular sizes of the pores, and flexibility of the framework that is responsive (adaptive) towards the target adsorbate ions/molecules. The application of MOFs in modifying carbon-based electrode studies for sensing and detecting several analytes in the literature. However, designing and fabricating the miniaturized device development utilizing the engineered metal-organic frameworks is the futuristic application of MOFs. The main goal of this review is to highlight the synthesis of various kinds of MOFs and focus on the application of the synthesized MOFs in sensing using electroanalytical techniques to detect potential water contaminants at trace levels.

2. Synthesis of MOF

Metal-organic frameworks (MOFs), built from inorganic nodes and organic linkers, have drawn much interest because of their structural variety, rarity of properties, and capacity to customize for specific functions. The organic linkers and metal centers design the structure of MOFs appropriately; the organic linkers participate in shear connections, and the metal centers act as joints [17]. There are several methods to synthesize the MOFs, which are briefly discuss below.

2.1. Hydrothermal/Solvothermal Synthesis

Hydrothermal synthesis is typically adopted to produce sustainable metal-ligand bonds across the framework, which yields the most comprehensive dynamical products [18]. Long-term heating of the reaction mixture at high pressures and temperatures requires hydrothermal synthesis of MOF [19]. Based on the material's dissolution rate in hot water under intense vapor pressure upheld at a temperature variation between the reactor's opposite nodes yields single crystals. Moreover, the 'solvothermal' process utilizes suitable solvents besides water in the material synthesis. The method results from the growth of high-quality crystals and is appropriate for materials with vapor pressure close to their melting points [20]. Figure 1 illustrates a schematic of the steps in the synthetic approach using hydrothermal and solvothermal processes.

The hydrothermal method synthesizes the Cu (4, 4′-bpy)NO₃(H₂O) crystals with rectangular parallelepiped-shaped, utilizing the 4, 4′-bipyridine as a nitrogen donor aromatic ligand [21]. Similarly, the hydrothermal method synthesizes the Cu (copper) benzene-1, 3, 5-tricarboxylate (TMA) [Cu₃(TMA)₂(H₂O)₃]_n complex known as HKUST-1 using the carboxylic as functional group [11]. Transition metals (cobalt, nickel, and zinc) based MOFs were obtained using the hydrothermal route by simple metal ions self-assembling at the adaptive bis-(imidazole) binding sites [22]. The hydrothermal reaction yielded a polycrystalline Fe-MIL-100 powder with a significantly large attainable and enduring porosity, demonstrating an intriguing Friedel-Crafts reaction with catalytic activity employing the redox properties of Fe(III) [23]. Ni(II) is a typical transition metal ion with varied interest because of its inexpensive cost, high abundance, superior catalytic activity, and electrochemical characteristics [24,25]. Ni-MOFs are synthesized by the hydrothermal process using Ni(II) and the 1,3,5-benzene tricarboxylate as a ligand [26]. Nickel-based compounds with the proper design showed considerable potential for usage in the electrochemical industry as electrode surface modifications and electrocatalysts [27,28,29].

2.2. Ultrasonic methods

The ultrasonic-assisted synthesis offers relatively environment-friendly conditions for MOFs synthesis in ambient reaction conditions (i.e., ambient temperature and atmospheric pressure) with less reaction time. Furthermore, the ultrasonic-assisted synthesis approaches avoid safety concerns, providing an opportunity to expand on the twelve principles of green chemistry [30]. Among several MOF synthetic techniques, the ultrasonication method is affordable and environmentally benign. It could produce a high yield while operating under ambient temperature and pressure in a solvent-free reaction [31].

Compared to other methods, ultrasonic-aided MOF synthesis is practically viable, containable, and quickly produces the product with a significant yield [32]. Ultrasonic cavitation for producing MOFs is relatively new and has recently attracted greater attention [33]. In 2009, Khan et al. [34] conducted the first study on the impact of ultrasonic irradiation on the Cu-BTC MOF. The ultrasonically assisted synthesis of MOFs significantly reduces the reaction time compared to the electronically controlled and microwave heating methods. Additionally, a limited sonication enabled the shrinking of size of the MOF particles; however, an increased sonication time from 6 to 45 min caused aggregation of the MOFs [35].

On the other hand, Taghipour et al. reported the possible role of solvent in the yield of MOF production using the ultrasound method [36]. The effect of the solvents (binary and ternary mixtures) on the textural properties of Cu-BTC investigates the ultrasonic parameters, such as sonication duration and power, thoroughly explored in the synthesis of MOFs (Figure 2). A ternary mixture of solvents viz., water, ethanol, and N-dimethylformamide (DMF) sonicated for 120 min at a power of 750 W resulted in an enhanced yield of Cu-MOF [36].

2.3. Microwave-aided synthesis

Microwave-aided synthesis is the most efficient approach for carrying out many reactions. Compared to conventional solvothermal synthesis, microwave irradiation reduces the reaction time and enhances the crystal growth of porous materials that take several days or weeks in conventional methods [37]. The microwave irradiation energy, time of exposure, solvent concentration, and solvent systems are the key parameters regulating the yield and crystal growth of the MOFs, and microwave irradiation showed a positive impact on material characteristics and properties [38]. Microwave-assisted synthesis acknowledges rapid heating, fast kinetics, phase purity, increased yield, improved dependability, and repeatability over hydrothermal synthesis [39,40,41,42]. Additionally, it offers an effective method to regulate the distribution of macroscopic morphology, particle size, and phase selectivity during the synthesis of inorganic solids and nanocomposite materials. Although the synthesis is substantially faster, the characteristics of the crystals produced by the microwave-assisted approach are on par with those produced by the conventional solvothermal process [43,44,45]. However, microwave-assisted methods have received greater interest investigating the effect of irradiation period, power, temperature, solvent concentration, and metal ion/organic linker ratio, for example, on the synthesis of MOF-5 by employing the microwave-assisted technique [46]. The nanocrystal crystallization process optimizes for several parameters: time, temperature, and power such as 1 h at 130 °C and 600-1000 W, respectively. This study showed that the crystal formation occurred under microwave irradiation of 15 min and produced a high-quality crystal between 30 min and 24 h. Similarly, microwave-assisted synthesis produces the Zr-based MOF (Zr-fum-fcu-MOF) having the octahedral shape at the reaction temperature of 100 °C. The schematic of the synthesis process is shown in Figure 3 [47].

2.4. Electrochemical synthesis

The electrochemical process of MOF synthesis possesses several advantages over conventional MOF synthesis, including relatively short reaction time, relatively simple equipment setup, real-time MOF structure modification, quick synthesis, no need for precursor metal salts, and direct accumulation of MOFs on the preferred substrates, etc. [48,49]. In addition to its simple process, the electrochemical synthesis of MOFs has provided numerous favorable circumstances, such as random and quick synthesis with lesser use of linker and solvent, excellent yield, and low energy consumption [50]. The mild reaction conditions, which perform at ambient temperature and pressure, are the most attractive feature of electrochemical synthesis. Despite these advantages, it remains a less utilized approach, particularly in synthesizing functionalized framework materials [51]. This approach also constantly altered the real-time response, enabling the direct output of crack-free nanostructures in the absence of a pre-treated surface at high temperatures.

On the other hand, relatively high reaction temperatures, longer reaction times, and thermal-induced cracking on the films are shown in the solvothermal or hydrothermal techniques [52]. Zn-based MOF was synthesized electrochemically using the electrochemical method, and the physicochemical parameters, viz., reaction time, electrolyte quantity, current, and voltage, are optimized for greater yield of the solid. The results indicated that both reaction time and current density significantly impacted the purity and yield, and it was observed that an applied current of 60 mA and a reaction time of 2 h yielded 87% of the product [53]. Figure 4 shows the instrumentation of a simple electrochemical synthesis of Zn₃(BTC)₂-MOF [54].

2.5. Mechanochemical synthesis

Mechanochemical synthesis is one of the most exciting chemical modifications employed to obtain high purity with enhanced yield of several MOFs [55]. Most coordination polymerization processes involving multisite ligands with metal ions proceed readily in a suitable solution environment. The solvent-free or solid-state formation of MOFs without any toxic or hazardous solvents has progressively received attention in recent years due to significant advances in mechanochemical synthesis [50]. Coordination polymerization, broadly, involves the reaction in the presence of solution, multisite ligands, and metal ions. Nonetheless, the mechanochemical synthesis of MOFs is less solvent-intensive, solvent-free, or a solid-state organic process devoid of unpleasant and harmful solvents [56].

Furthermore, compared to the diffusion and solvothermal methods, this technique is advantageous for large-scale MOF manufacturing in a shorter reaction time and at room temperature [57,58]. The solid-solid reaction has the potential to synthesize a large-scale production of materials and provides simplicity in handling since it directly produces the products in powdered forms. Although mechanochemical synthesis is a "solvent-free" or "solvent-less" process, a solvent-based purification step is still required. Despite this, the mechanochemical synthesis is reasonably environment friendly and is anticipated to be commercially exciting for MOFs production [59]. The liquid-assisted mechanochemical synthesis produced the MOF-5, showing that the solid possessed a relatively low BET area and contained many amide precursor by-products [60]. A copper-based MOF with a product yield of 97% was synthesized in the mechanochemical process using the Cu(OAc)_2.H₂O and H₃btc. A dark blue color solid was obtained at 30 Hz and 20 min of reaction time [55]. The framework structures produced by the mechanochemical method are easily separable from the host molecules, providing repeatable free pore access for additional uses [61]. The synthesis of HKUST-1 and MOF-14 showed the method’s applicability in efficient MOFs production [55]. Figure 5 demonstrates the mechanochemical method of synthesizing MOF.

The different types of MOF have different routes of synthesis. The organic linker should be chosen wisely depending on the central metal atom. Table 1 displays the different types of MOF synthesized using different routes under varying conditions.

3. Design and fabrication of MOF-based sensors

Due to their high porosity, enormous surface area, and cavity structures, MOFs are valuable in electrochemical sensor applications. These unique properties provide MOFs with a high-level catalytic feature. Moreover, the robust use of these materials on the electrode surface enables efficient development of electrochemical sensors. The MOF materials' high porosity and surface area enhance these sensors' detection sensitivity [86]. MOFs show low electronic conduction properties due to their poor overlapping between the electronic states and frontier orbitals of ligand and metal ions [87,88]. Therefore, the direct use of pure MOFs on electrodes or other electroanalytical methods is limited [89,90,91]. A recent report uses the carbon-based materials viz., fullerenes (C60), graphene, and multi-wall carbon nanotubes (MWCNTs) for the modification of nickel-based MOF, which shows an increased electrical conductivity with decreased charge transfer resistance, high porosity, and large surface area. These composite materials showed enhanced applications in sensor developments [92].

3.1. Carbon based electrode modification by MOF

Among several electrode modifiers used in electrochemical processes, MOFs have shown potential due to their large surface area, copious adsorption sites, and versatile functionality that helps trace and efficiently detect several analytes, including heavy metal ions [93,94]. In general, the carbon allotropes deployed in the fabrication of electrodes are graphite or derived materials intended to modify the carbon paste electrode (CPE) and glassy carbon (GC) for solid carbon electrodes. The conductivity of carbon-based materials allotropes is enhanced primarily due to sp² hybridized bonds and six-membered aromatic rings [95]. It is essential to pre-modify carbon-based electrodes to follow a specific protocol for introducing MOF modifiers at the electrode surface. In the case of glassy carbon electrodes, the modification involves a direct coating of the polished surface by MOF. In contrast, the modifier was first synthesized in a carbon paste electrode by mixing graphite powder with MOF using a specific organic binder [96].

3.1.1. MOF modified carbon paste electrode (CPE)

One of the most often utilized electrodes recently is the carbon paste electrode because of its easy and versatile fabrication [97]. The carbon paste electrodes are inexpensive and made by blending graphite with MOF along with a suitable organic binder. High-purity graphite powder with a particle size of 1 to 10 micrometers fabricates electrodes [98]. Further, the purity of the graphite powder greatly influences the electrochemical performance of the electrodes [99]. In addition, the organic binder helps the modifier (MOF) bind at the electrode surface, which must be stable, insoluble, and impurities-free. Paraffin oils, aliphatic and aromatic hydrocarbons, silicone oils and greases, halogenated hydrocarbons, and similar derivatives are popularly used as CPE binders for CPEs [100]. Graphite powder has high electrical conductivity; hence, it is easy to fabricate the MOF-modified carbon paste electrodes in the electrochemical processes [101]. In typical CPEs, the modifier (MOF) acts as a Lewis acid in catalysis, which further helps decrease the sensor's overpotential and charge transfer resistance. Also, the distinct pore size elevates the selective determination of various analytes in the complex matrix [102].

Carbon paste electrodes are fabricated using graphite powder as a modifier and organic binder. The mixture is mixed in an agate mortar to obtain a well-distributed suspension placed into an electrode frame. The electrical connections often utilize the copper, titanium, or platinum wires. Teflon tube having a diameter of Ca. 0.5 cm is used as the electrode frame. The graphite powder and paraffin binder were mixed at 7:3 and then introduced into the electrode frame, and a copper wire was used as electrical contact [103]. The duplicating paper polishes the fabricated electrode surface.

Additionally, two approaches demonstrate the carbon paste electrode fabrication, viz., the in-situ and ex-situ approaches [104]. The in-situ fabrication utilizes the modifier (MOF) with graphite powder. The disadvantage of in-situ approach is the excess of graphite powder that impact adversely the specific properties of the MOFs or even destroys the physical structure of MOF [105]. On the other hand, the ex-situ approach utilizes the modifier to mix the MOF with the graphite powder, resulting in composite material [106]. The ex-situ approach is reliable since it provides a more accessible and efficient modification of CPE for various electrochemical applications. A simple schematic of an electrochemical approach is shown in Figure 6.

3.1.2. MOF modified glassy carbon electrode (GCE)

Glassy carbon electrodes showed outstanding physical properties, viz., withstand at higher temperatures, hardness, low density, low electrical resistance, low thermal resistance, extreme chemical resistance, and gas and liquid impermeability. A restrained pyrolysis of phenol-formaldehyde resin in an inert atmosphere produces a glassy carbon electrode. Pyrolysis is the thermal decomposition of an organic precursor or hydrocarbons. Glassy carbon is usually obtained at a temperature >2000⁰C since pyrolysis composites have low thermal conduction, resulting in a thermal drop within the sample [107]. On the other hand, the nanoscale fabrication of glassy carbon is achieved relatively at a lower pyrolysis temperature, i.e., Ca. 900⁰C associated with oxygen impurities [108].

In the fabrication of the working electrode, the glassy carbon electrode was first polished using an alumina slurry on a polishing cloth, followed by the tip of the electrode was dipped in deionized water and ethanol, followed by sonication for 10 min, and the electrode was dried slightly above the room temperature. The modifier (MOF) was mixed with ethanol and then sonicated for 20 min to get a fine mixture of the solution, which was then dropped on the glassy carbon electrode's tip and again dried at ambient temperature. Finally, the Nafion solution was cast on the MOF-modified electrode and dried at ambient temperature [109].

4. Application of MOF in electrochemical sensing

In electrochemical sensors using MOF-modified electrodes in amperometric and voltammetric methods gained attention, particularly for detecting several pharmaceuticals, heavy metals, biomolecules, and other micro-pollutants. The MOF-based sensors showed a significantly low detection limit (LOD) and enhanced selectivity compared to the sensors based on other materials.

4.1. Biomolecules sensing

Biomolecules are the essentials of living organisms that control various biochemical functions of the body. Therefore, the quantitative determination of biomolecules is necessary for chemical pathology or even food chemistry. The electrochemical technique proves to be prominent in quantifying various biomolecules in different matrices [110]. Detecting biomolecules employs different methods; however, the electrochemical methods are promising and show potential in the trace detection of several biomolecules [111].

The electrochemical detection of biomolecules utilizing electrodes fabricated in two ways: i) Enzyme-modified electrodes and ii) Non-Enzymatic or electrodes modified with novel materials (MOF, nanomaterials, metal oxides, etc.). In order to obtain an enhanced selectivity for an analyte, the methods primarily utilize a variety of enzymes to modify the carbon-based electrodes. Nevertheless, enzyme-modified electrodes showed several limitations in the sensing process since some enzymes are often less mobile on the electrode surface and are stable only at specific temperatures and pH levels [112,113]. These limiting factors prompted the use of non-enzymatic modified sensors. The enzyme replaces this electrode type with novel materials, especially MOF, which has an exceptional physicochemical property with high sensitivity and selectivity towards various analytes [103].

The MOFs synthesized using transition metals like Fe²⁺, Cu²⁺, Co³⁺, and Ni²⁺ are promising in the low-level electrochemical detection of several biomolecules viz., glucose, ascorbic acid, urea, and H₂O₂ [114]. The detection of glucose is a widely accepted electrochemical method; Cu-MOF@Pt was effectively used to determine glucose in human serum specimens, with excellent recovery and repeatability [115]. The glassy carbon electrode was modified with the nickel-based MOF and employed in cyclic voltammetry (CV) to detect glucose. NiO and Ni/NiO/CNTs display well-defined NiO redox peaks in the absence of glucose and increased peak currents after glucose addition. Furthermore, in the presence and absence of glucose, Ni/NiO/CNTs exhibit significantly greater peak current than NiO, indicating enhanced electrochemical performance. As glucose concentrations rise, the oxidation peak currents rise, with a minor positive shift in peak potential and a drop in the reduction peak currents. This indicates that Ni/NiO/CNTs have outstanding electrocatalytic activity on glucose oxidation. The mechanism involving the oxidation of glucose may be written as [116]:

Ni(OH)₂ + OH^- → NiOOH + H₂O + e^-

(1)

NiOOH + glucose → Ni(OH)₂ + glucolactone

(2)

On the other hand, the copper and nickel MOF-based electrodes were fabricated and utilized in the electrochemical detection of glucose. A redox couple of Cu²⁺/Cu³⁺ and Ni²⁺/Ni³⁺ occurs in an alkaline medium, which in turn leads to the oxidation of transition metals used (i.e., copper or nickel) as shown in Equation (1) and Equation (2), which further catalyzes the oxidation of glucose. The reaction mechanism is shown in Figure 7.

The mechanism of oxidation of glucose on the electrode surface is given as [118,119]:

M(OH)₂ + OH^- → MOOH + H₂O + e^-

(1)

MOOH + glucose → M(OH)₂ + glucolactone

(2)

Glucolactone (Hydrolysis) → Gluconic Acid

(3)

The first step oxidizes the M²⁺ to M³⁺, and these species take part at the electrode surface in glucose oxidation. The most crucial species for glucose oxidation is M³⁺, which also serves as the primary electron transfer mediator. M²⁺ in the MOF undergoes oxidation to produce M³⁺ throughout the potential scan between 0.3 and 0.5 V. M³⁺ preferentially oxidizes the glucose to produce gluconolactone, which produces gluconic acid by hydrolysis [119]. The Differential pulse voltammogram produced for each 50 μM addition of glucose in 0.1 M NaOH is shown in Figure 8a. Adding of 50 μM glucose to 0.1 M NaOH results in an oxidation peak at +0.38 V. This demonstrates that the GC/CuO electrode is ideal for determining glucose. The amperometric i-t curve for glucose produced at a GC/CuO electrode in 0.1 M NaOH stirred solution at an applied voltage of 0.50 V is shown in Figure 8b. The initial current response of the GC/CuO electrode was due to 5 μM glucose, and the continued addition of 5 μM glucose in each step with a sample interval of 50 sec enhances the current response [117].

A chromium-based metal-organic framework (MIL-101) modified with platinum nanoparticles (PtNPs) detects simultaneously the Xanthine, uric acid, and dopamine in spiked serum samples using the differential pulse voltammetry (DPV) [120]. The sensor has an extensive linear range (0.5 - 162 µM), a low detection limit (0.42 µM), and excellent selectivity, according to differential pulse voltammetry. This determines dopamine, uric acid, Xanthine, and hypoxanthine simultaneously at working potentials of 0.13, 0.28, 0.68, and 1.05 V (vs. Ag/AgCl) and to quantify Xanthine in spiked serum samples. DPV curves of simultaneous determination of dopamine (DA), uric acid (UA), Xanthine (XA), and hypoxanthine (HXA) are shown in Figure 9.

Further, Table 2 includes several MOF-based materials used in the electrochemical detection of various biomolecules in real and complex samples. The LOD and Linear range is also included to demonstrate the detection of these biologically important molecules.

4.2. Detection of hydrogen peroxide

Hydrogen peroxide (H₂O₂) has physical and chemical significance in different research fields, viz., pharmaceutical industry, food and chemical industry, environment, and biological samples [109,131,132]. In the food industry, bleaching or disinfection utilizes hydrogen peroxide during food processing. However, higher concentrations of hydrogen peroxide residues in food products negatively affect human health. Excessive levels of retained hydrogen peroxide in food showed several harmful effects, which include cancer, rapid aging, coronary artery disease, severe gastrointestinal issues, and neurological illnesses [133]. The necessity of detecting hydrogen peroxide is manifold, and the miniaturized device for on-site detection is a prerequisite for various biological or environmental samples. Several detection methods have been employed for many decades. However, the electrochemical methods are promising because of their robustness in operation, selectivity, dependability, and sensitivity towards the analyte, and the on-site detection with a miniaturized device added to the method's suitability [134].

Cobalt-based MOF (Co(pbda)(4,4-bpy)·2H₂O]_n) was incorporated on GCE and used for the detection of hydrogen peroxide (H₂O₂) at 0.1 M NaOH solution employing cyclic voltammetry (CV) [132]. The cyclic voltammogram of the Co-MOF modified GC electrode exhibits three reduction peaks: (a) in the absence of H₂O₂, (b) with the addition of 0.1 M NaOH solution containing 1 mM H₂O₂ and (c) with the addition of 0.1 M NaOH solution containing 2 mM H₂O₂ at approximately -0.40 V. The amperometric response indicates that the Co-MOF modified GC electrode shows enhanced electrocatalytic response towards the H₂O₂ reduction current, and the calibration line obtains for a wide concentration range of H₂O₂ 5 µM to 9.0 mM. The method provides a low detection limit of 3.76 µM and a high sensitivity of 83.10 A µmM⁻¹ cm² at an applied potential of -0.40 V. Additionally, and the detection method demonstrates favorable selectivity and long-term stability. Furthermore, the mechanism indicated that the Co-MOF is highly efficient in intrinsic peroxidase-like activity, which allows it to catalyze H₂O₂ decomposition to form hydroxyl radical, which then oxidizes the peroxidase substrate (terephthalic acid) to produce colour. Figure 10 shows the reduction of H₂O₂ at an applied potential of -0.4V in the absence of H₂O₂, along with the addition of 0.1M NaOH solution containing 1 and 2 mM H₂O₂. It also depicts the Co-MOF-modified GCE's amperometric response at a potential range of −0.4 to −0.6V [118]. Table 3 comprises the use of different MOFs in the electrochemical detection of hydrogen peroxide in various samples. The limit of detection and experimental conditions with the experimental concentration range of H₂O₂ is included in Table 3.

4.3. Organic pollutant sensing

Aromatic organic compounds such as phenol, nitrobenzene, hydrazine, chlorinated phenols, polyaromatic hydrocarbons, pesticides, nitrites, and pharmaceuticals have shown more significant environmental concerns, contaminating the water bodies and severely affecting the human health. The detection and elimination of these pollutants from the aquatic environment has been a concern of environmentalists for the last few decades. Yadav et al. demonstrated the synthesis of zinc (II) based MOF (MOF-5) decorated with the Au(NPs), and the material modifying the glassy carbon electrode and detects the nitrobenzene and nitrite in an aqueous medium [149]. Figure 11(1) shows cyclic voltammograms of nitrite (1.0 mM) using the GC/Au-MOF-5, GC/MOF-5, and GC electrodes in 0.1 M phosphate buffer (PBS) (pH 7.0) at 20 mV/s. An oxidation peak of nitrite appeared at an applied potential of 0.85V using the GC/Au-MOF-5 electrode. However, the reverse scan reveals the reduction peak at 0.46 V.

On the other hand, the nitrite oxidizes at 0.93V and 1.01V using the GC and GC/MOF-5 electrodes, respectively. The results imply a significant increase in peak currents at low applied potentials using the GC/Au-MOF-5 electrode. Therefore, Au(NPs) in Au-MOF-5 showed an efficient electrocatalytic activity for nitrite oxidation. Similarly, the cyclic voltammograms of nitrobenzene (NB) using the GC, GC/MOF-5, and GC/Au-MOF-5 electrodes in an N₂ saturated atmosphere and employing 0.1 M pH 7.0 phosphate buffer, shown in Figure 11(2). In the absence of NB, no peak current appeared in the voltammograms; the presence of NB exhibits a distinct reduction of nitrobenzene, and the reduction peak is observed at an applied potential of -0.77, 0.79, and 0.77 V for the GC, GC/MOF-5 and GC/Au-MOF-5 electrodes, respectively [149].

Many nations have limited the use of patulin residues due to their severe toxicity and ubiquitous occurrence in foods. The Meals and Drug Administration of the United States and China mandates a maximum residual of patulin of 50 µM in fruit juice and processing products. In contrast, the European Union mandates a limit of 10 µM in newborn and child meals [150]. Because of the widespread occurrence and high toxicity of patulin in food, a rapid and sensitive detection method averts the possible harmful effects of patulin on human health [132]. Copper-based MOF decorated with the Au(NPs) was used to modify the GCE, which electrochemically detects patulin in apple juice. The square wave voltammograms of patulin show that raising the concentration of patulin from 0.001 nM to 1.0 nM and 1.0 nM to 250.0 nM increased the cathodic signal. The LOQ was 0.001 nM, and the LOD was 3.33 10-4 nM. RSDs of 1.65% and 0.83% were found for six replicate measurements of 1.0 nM and 250.0 nM patulin, respectively [151].

Similarly, the HKUST-1 MOF incorporated with Au(NPs)-GCE detects electrochemically the paracetamol. The MOF/AuNP modified GCE detects the paracetamol efficiently, and a wide linear range of paracetamol concentration (0.01 µM to 100 µM) provides the detection limit of 0.0011 µM [152]. Cu-BTC nanocrystals/CNTs modified GC electrode showed an improved oxidation current for the metformin detection. Peak current demonstrated good linearity with concentrations ranging from 0.5 μM to 25 μM with a detection limit of 0.12 μM under optimal conditions. Further, the detection method implied for the accurate detection of metformin in pharmaceutical samples [153]. The EPC-modified GCE showed good sensitivity and low detection limit (i.e., 2.9 µM) in detecting chloramphenicol residual in honey [154]. Table 4 displays the list of different types of MOFs used to detect organic contaminants.

4.4. Heavy metals sensing

Heavy metals such as lead, mercury, cadmium, chromium, and arsenic are potential heavy metals and seriously threaten human health and the marine environment [164]. These heavy metals contaminate the environment due to many geological and human-caused phenomena, including agricultural, industrial, and domestic wastes [165]. The detection of heavy metals in different environmental matrices using various MOF-based sensors has recently gained attention [144]. Although various detection techniques have been developed in the past, electrochemical analysis is one of the prominent and robust techniques due to its accuracy and reliability. The exceptional features of MOF, i.e., high porosity, homogeneous structures, large surface area, and ease of functionalization, received greater attention in the efficient and selective use in sensor developments for detecting several heavy metals [166]. Many MOFs show low conductivity due to the bulk organic linkers coordinated to the core metal atom [167]. Therefore, MOFs are typically coupled with high-conductivity elements such as metal oxides, metal nanoparticles, and carbon compounds to overcome the limitations. Furthermore, MOFs with high conductivity are utilized as alternative materials for the trace detection of several heavy metal toxic ions. Moreover, the repeatability of the signal due to the micro-sized MOFs is one of the challenges in the detection system [14,168,169].

Cd²⁺ and Pb²⁺ were simultaneously and selectively detected in water samples using the Yb-MOF-modified electrode [170]. Under ideal circumstances, the Yb-MOF/GCE electrode detects the Cd²⁺ and Pb²⁺ independently at values ranging from 0 to 40 ppb. I (µA) = 0.2045 x C (ppb) - 0.0838 (Cd²⁺) and I (µA) = 0.1971 x C (ppb) + 0.0686 (Pb²⁺) are the recorded current responses on DPASV curves that grow linearly with increasing ion concentration. For Cd²⁺ and Pb²⁺, the detection limits (LOD) were 7.40 ppb and 2.02 ppb, respectively. The use of Yb-MOF/GCE for the simultaneous electrochemical detection of Cd²⁺ and Pb²⁺ in the concentration range of 0 to 50 ppb. I (µA) = 0.4676 x C (ppb) + 0.0414 (R² = 0.997) for Cd²⁺ and I (µA) = 0.4082 x C (ppb) - 0.26 (R² = 0.998) for Pb²⁺, as shown in Figure 12, increased linearly with the ion concentrations. For Cd²⁺ and Pb²⁺, the sensitivities were 469 µA ppm⁻¹ cm⁻² and 397 µA ppm⁻¹ cm⁻², respectively. The detection limits for Cd²⁺ and Pb²⁺ were 3.0 ppb and 1.6 ppb, respectively. Pb²⁺ was more favorably absorbed and reduced on the porous structure of the Yb-MOF framework than Cd²⁺ [171]. The recent advancements in the electrochemical detection of heavy metals using MOFs are illustrated in Table 5, along with the optimized pH and potential.

5. Conclusion and Future perspective

MOFs and MOF-derived materials are suitable modifiers of electrodes for detecting different pollutants in aquatic environments. MOFs possess distinctive properties such as high specific surface area, pore working capabilities, and regulated shape and size of pores. Moreover, using suitable organic linkers in the material synthesis makes the material a flexible framework for the target adsorbate ions/molecules. Therefore, ample opportunities lie for advancement in the development of MOFs-based sensing devices by improving the sensitivity and stability of the material. Further study on developing MOF and MOF-derived materials for practical applications could open newer avenues for sensing in environmental applications.

Several researchers reported ground-breaking achievements in the applications of MOFs for electrochemical sensors in the recent past; however, advancements towards the MOFs developments for targeted sensors applications. Compared to carbon-based materials, MOF-derived composites face lesser sensitivities, poor stability, and less repeatability. Therefore, synthesizing advanced MOFs materials could enable viable alternatives for miniaturized device development. Similarly, there are several challenges in the development of sensors using MOF and MOF-based materials; the following are the critical issues of employing MOF-based materials in electrochemical sensors:

• Challenges encompass the synthesis of advanced materials to control the shape and size of MOFs; this may result in uniform growth of nanostructures with a significant increase in surface area.
• Homogeneous dispersion of active metals on the surface of MOF-derived carbons remains challenging.
• Although many MOFs are used to fabricate electrodes for heavy metal detection, the insights into electrochemical sensing mechanisms could enable greater implications for device development.
• MOF stability in an aqueous media remains challenging; studies on coupling hydrophobic ligands with high valence metal ions could provide newer research areas for suitable applications.
• Since MOFs' pore width and geometry play an essential role in the highly selective determination of food contaminants, synthesizing functionalized MOFs could enable required selectivity towards the target analyte species in complex matrices.