Recent advances in Metal-Organic Framework derived nanocomposites in gas and chemical sensors: a review

Abstract

Purpose

This systematic review aims to elucidate recent developments in MOF-derived nanocomposites for gas and chemical sensors. The goal is to explore the potential of MOF-based devices, specifically miniaturized gas sensors, in addressing critical requirements such as sensitivity, selectivity, stability, low power consumption, cost-effectiveness, and extended lifetime. It focuses applications ranging from air quality monitoring and industrial gas leaks to medical diagnostics.

Materials and Methods

The review critically evaluates the last decade's progress in creating MOF-based sensors across diverse analytes. The specific attributes of MOFs, including their enormous specific surface area, tunable pore size/geometry, host-guest interactions, and structural diversity, which make them promising candidates for gas sensing applications. The limitations of MOFs, such as mechanical strength, stability issues, and single-function constraints, and proposes the combination of MOFs with functional materials as a viable strategy to overcome these drawbacks are discussed.

Results

Various types of MOF-based nanocomposites employed in sensing capacities are scrutinized, showcasing their ability to enhance the recognition of diverse species, including hazardous phenolic compounds, heavy metal ions, and aromatic hydrocarbons. The integration of these MOFs into different sensor platforms, such as field-effect transistors, resistive sensors, and optical sensors are discussed in detail.

Conclusions

The potential applications and performance attributes of these MOF-nanocomposites are thoroughly examined, offering guidance for researchers and engineers of sensor technologies. The review identifies key areas for future research, addressing challenges such as mechanical strength, stability enhancement, and exploration of multifunctional MOF nanocomposites to further advance the capabilities of gas and chemical sensors.

Keywords:

• Metal-organic frameworks (MOFs)
• sensor
• nanocomposites
• gas sensing
• chemosensors
• electrochemical sensor
• electrical conductivity

1. Introduction

Air quality remains a significant issue in numerous nations, as ensuring a pure air supply is crucial for both human well-being and environmental integrity (Ghorani-Azam, Riahi-Zanjani, and Balali-Mood 2016). Detecting the presence of gases is crucial due to the varying levels of atmospheric pollutants that pose potential risks to human health and are significant in industrial or medical processes (Manisalidis et al. 2020; Tahir et al. 2019). The air we inhabit is shared by humans, plants, and animals, making their safety a paramount concern. Hence, it is imperative to identify and monitor these gases to ensure the well-being of all inhabitants in our environment. Physical organs have created a variety of sensing systems to interpret exterior inputs into associated signals. These systems rely on thermodynamics, photochemistry, molecular recognition, and Nernst potential (Maksud et al. 2023; Robert, Paul, and Jason 2020). Researchers have created a number of instruments that can measure physical and chemical characteristics, such as motion, pressure, temperature, magnetism, light, colour, odour, and even structural fingerprints. The interpretation of physical and chemical stimuli, encompassing touch, pressure, vibration, vision, auditory cues, taste, and smell, stands as a sophisticated and powerful means by which living organisms engage with their environment (Francis, Johan, and Håkan 2014; Jun 2021; Adimule et al. 2023c). Gas sensors are among the devices that are commonly used in both industrial and residential settings to identify gases that are poisonous, combustible, or greenhouse (Adimule, Manhas, and Sharma 2023).

Gas sensors can be categorised into various types based on their transducing mechanism and/or architecture. These classifications include Chemiresistive, capacitive/impedimetric, field-effect transistor (FET)/Kelvin probe (KP)-based, electrochemical, gravimetric, and optical sensors (Feng et al. 2014; Xianghong et al. 2017). The effectiveness of gas sensors, including sensitivity, selectivity, response/recovery time, reproducibility, and stability, is predominantly influenced by the inherent characteristics of the sensing material. These characteristics encompass factors such as porosity, chemical composition, dimensionality, conductivity, doping, defects, and piezoelectricity (Nirav et al. 2016). Additionally, the interactions between the material and analyte, the fabrication methods employed, and the configuration of the transducer play significant roles in determining sensing performance. The latest gas sensors utilising the mentioned materials exhibit both notable advantages and drawbacks. There is a strong demand for materials possessing enduring porosity, a combination of inorganic and organic traits, and adjustable physicochemical properties to address the existing limitations in gas sensor technology (Manjunatha et al. 2023b; Nirav et al. 2018). We employ porous materials on a daily basis, including porous glass, porous metal, porous ceramic, porous zeolites, porous activated charcoal, and porous polymer foams (Patrick 2017). The field of porous materials, particularly nanoporous materials, has experienced tremendous development in the past 20 years due to its well-known characteristics and wide variety of applications (Yanrong et al. 2016). Nanoporous materials have been at the forefront of development in the porous materials field. Their nanoscale pore dimensions confer unique properties, such as enhanced surface areas and tuneable porosity, enabling novel applications. The past two decades have seen substantial progress in the synthesis methods of nanoporous materials, leading to fine-tuned pore structures and improved properties (Yallur et al. 2023; Adimule, Santosh, and Yallur 2022; Adimule et al. 2023a). This progress has spurred innovations in diverse areas including energy storage, sensors, drug delivery, and environmental remediation. The burgeoning field of porous materials faces ongoing challenges and opportunities (Rangappa et al. 2022). Despite extensive research on gas sensors made up of metal oxide semiconductors, their existing bottlenecks of relatively low selectivity and high operating temperatures (100−500 °C) still pose a great challenge (Bhimaraya et al. 2023a; Charithra et al. 2022). To overcome these circumstances, various strategies have been developed to improve the gas sensing properties, including the exploration of new and novel materials. Researchers are focusing on enhancing the stability, selectivity, and scalability of these materials for real-world applications. Exploration of novel synthesis routes, advanced characterisation techniques, and the integration of porous materials with emerging technologies, such as nanotechnology and biotechnology, continue to drive the field forward (Raril and Manjunatha 2020). Metal-organic frameworks (MOFs) constitute a unique category of crystalline nanoporous materials formed through coordination bonds between inorganic secondary building units (SBUs) and organic ligands (Norbert and Shyam 2012). MOFs offer a new level of design flexibility in terms of topology, porosity (including aperture size and geometry), and the ability to customise responsiveness to external stimuli. This sets them apart from traditional polymers, zeolites, and other inorganic materials (Suresh et al. 2023; Xiao-Lan et al. 2013). When gases come into contact with the organic linkers and/or secondary building units of MOFs, they cause changes in various physical properties, such as conductivity (σ), permittivity (ε), work function (Φ), and refractive index (n) (Tigari et al. 2023). The transducer then transforms at least one of these properties, leading to modifications in electric parameters, including capacitance (C), inductance (L), and resistance (R) (Figure 1). The sensor is connected to a circuit, resulting in the generation of the readout. Signals may take the form of current (i), voltage (V), or potential (E), and their frequency, magnitude, and/or phase can be measured accordingly (Manjunatha et al. 2023a; John et al. 2016).

Figure 1. MOF-based gas sensors exhibit a distinctive logical architecture (Sharath et al. 2009).

The distinctive characteristics mentioned above make MOFs well-suited materials for gas sensing applications. Advancements in integrating MOFs onto diverse solid-state surfaces (Eman and Ezzat 2023) have significantly advanced the creation of devices based on MOFs. These include but are not limited to electrically-transduced gas sensors (such as chemiresistor, capacitors, FETs, mass-sensitive, and electrochemical sensors), memristors, interferometers, and micro-ring/surface plasmon resonators (Chang et al. 2022; Adimule, Yallur, and Keri 2022). Initially, the enduring porosity of MOFs provides a significant specific surface area and possible active sites, such as coordinatively unsaturated open metal sites and terminated functional groups. These attributes are advantageous for increased gas absorption and transport, facilitating the preconcentration of the intended analyte and contributing to heightened sensitivity. Secondly, the customised pore size and geometry, along with the physicochemical environment of Metal-Organic Frameworks (MOFs) (including acidity/alkalinity, hydrophilicity/hydrophobicity, and electron-rich/electron-deficient properties), facilitate selective adsorption of specific analytes. This results in heightened selectivity and sensitivity. Thirdly, the well-defined crystallinity and predictable periodic arrangement of atoms/molecules within MOFs enable potential structural identification and property correlation at the molecular/atomic level in connection with host-guest interactions. Additionally, the fourth point emphasises that, with the exception of instances involving strong chemisorption and redox reactions, MOFs exhibit reversible gas uptake and release, contributing to the advantageous regenerability of MOF-based sensors. This stands in contrast to zeolite-based devices, which typically entail a higher energy penalty. Lastly, the generally elevated thermal and chemical stability of most MOFs ensures an extended operational lifetime for MOF-based sensors. The presence of different electroactive components, a bigger surface area, a greater number of electrochemical active sites overall, and improved ion transport pathways allow for the achievement of these features. The creation of enhanced electrochemical sensing capabilities may also result from the well-considered design of MOF structures. On the other hand, this study delivers an overview of the potential of hybrid MOFs for real-world sensing applications, highlighting current constraints and problems that must be overcome while highlighting recent advances in the most pertinent to sensor disciplines. This review summarises the recent development of MOFs and hybrid MOF-based gas sensors. The review has been separated into following sections as a result. The primary characteristics of MOFs that will be employed as sensing materials are introduced. Second, a section that reports several typical different techniques to synthesis the hybrid MOFs using various metal precursor and organic linkers. The utilisation of hybrid MOFs as sensing materials in a range of application areas, such as food safety, environmental monitoring, and biological objectives, will be discussed last. The difficulties and opportunities for hybrid MOF sensors in the upcoming years are thoroughly evaluated.

1.1. Characterisations of MOFs

MOFs provide for versatile structural design since they are created by the three-dimensional crystalline assembly of inorganic metal ions and organic ligands. By choosing alternative building blocks, specific pore diameters, surface areas, and functions may be tuned. Numerous researchers are interested in MOFs due of their highly customisable features. More than 20,000 distinct MOF structures have been identified and are currently being investigated (Hareesha and Manjunatha 2020). MOFs may be created utilising a variety of different synthetic techniques, including slow diffusion (Dongxiao et al. 2023), hydrothermal (solvothermal) (Pemmatte et al. 2021), electrochemical (Vadia Foziya, Naved, and Suresh 2022), mechanochemical (Arul et al. 2020), microwave-aided heating (Bhimaraya et al. 2023b), and ultrasonic (Pushpanjali and Manjunatha 2019), depending on the final architectures and attributes. As a result, MOFs and hybrid MOFs have found extensive use in numerous applications, ranging from molecular storage and separation (Yue et al. 2018), catalysis (Tian et al. 2020), drug delivery (Chen et al. 2018), to chemo/bio-sensing based on luminescence (Rehman et al. 2020). Over the past several years, a number of outstanding evaluations on the potential of hybrid MOFs for various sensing applications have been published, but each takes a different approach. While some reviews focus on very specific MOF or composite types (like carbon dots@ MOFs (Adimule et al. 2021a) or lanthanide MOFs (Wei et al. 2022)), others are more narrowly focused on specific sensor types (like electrochemical (Suhail et al. 2022) or luminescent (Li et al. 2021)), analytes (like food contaminants (Cha et al. 2023), gases (Kolleboyina et al. 2022), neurotransmitters (Xiao et al. 2018), or cancer biomarkers (Yap, Fow, and Chen 2017)), or application fields (like biomedical, food) (Cheng, Kathryn, and Wenbin 2012). Metal/metal oxide nanoparticles, carbon-based materials (reduced graphene oxide (rGO), graphene oxide (GO), quantum dots (QDs), enzymes, and heteropoly acids are some examples of the functional components utilised to create MOF nanocomposites for photoelectrochemical detection (Zhichao et al. 2021). The IUPAC definition of MOF states that it is ‘a coordination network with organic ligands containing potential voids’ (Figure 2) (Patrícia et al. 2015). A coordination network is defined as ‘a coordination compound extending through repeating coordination entities in 1 dimension, but with cross-links between two or more individual chains, loops, or spiro-links, or a coordination compound extending through repeating coordination entities in 2 or 3 dimensions,’ to use the other definitions (Evtugyn et al. 2020). Since nearly all coordination compounds may be associated with the phrase ‘coordination compounds,’ the name formerly used for similar structures—‘hybrid organo-inorganic materials’—is seen as being ambiguous. Even if the aforementioned description does not presuppose a crystalline nature of the MOF structure, it appears crucial to separate them from the broader term ‘coordination polymer’ (Fangzhou et al. 2021; Emmett, Chengshuang, and Matteo 2020).

Figure 2. Illustration of several MOFs’ nanoporous architectures as produced by various research groups. Reproduced from Patrícia et al. (2015) with permission from royal Society of Chemistry.

The MOF structures are made up of metal atoms or metal clusters (secondary building blocks) connected by polytopic ligands like terephthalic acid or imidazole fragments (Figure 3). Typically, the structure of metal clusters is polyhedral. Different coordination numbers are possible depending on the linkers.

Figure 3. Chemical compositions of the organic linkers used in MOF synthesis.

The majority of MOFs that are often discussed in the construction of electrochemical sensors, and particularly biosensors, are built using carboxylate-based linkers (Evtugyn et al. 2020). The quality of the bonds between the metal-based nodes and the carboxylate groups determines how resistant they are to hydrolysis. Numerous documented instances of MOFs based on different metals mention UiO-66, which has linear dicarboxylate linkers and hexa-zirconium nodes (Amir et al. 2021). They are redox inactive and exhibit excellent chemical stability in both acidic and neutral conditions, thus they must first be labelled with the proper groups, such as porphyrin fragments (Mathieu, Muwei, and Hong-Cai 2014). As an alternative to the abovementioned carboxylate-based MOFs, MOFs based on nitrogen-containing linkers are being studied. Electrochemical detection techniques are most frequently used with members of the ZIF (zeolitic imidazolate framework) family. Due to the creation of very tiny and hydrophobic pores, they are assembled with Zn²⁺ and Co²⁺ cations and are quite stable even in basic environments. As an alternative, bimetallic heterostructured MOFs can be created by sequentially building the right layers or by combining two independent materials (MOFs-on-MOFs). These heterostructured materials are of interest because of the wider variety of the materials properties and the positive interactions between the two elements in the electrocatalysis and analyte identification processes (Jenna and Maarit 2022). Combining two metals results in products with higher hydrolytic stability against mechanical mixing of relevant MOFs taken individually. Indeed, more than 100,000 distinct MOF structures have been documented to far, opening considerably more opportunities for the creation of sensing materials.

2. Fabrication of MOF derived films for sensors

Significant progress has been made in the past two decades particularly to design, synthesis, and characterisation of new MOFs in powder form with interesting physicochemical features. To broaden their use in cutting-edge technologies, such as membrane-based separation and chemical sensing based on electrical devices, MOFs must be shaped onto solid-state substrates. MOF films on various surfaces may now be produced using a number of reported techniques (Adimule et al. 2023d). A few of these methods are: layer-by-layer or stepwise growth; electrochemical deposition; direct hydro/solvothermal growth; direct deposition assisted by spin/dip-coating; interfacial synthesis approach followed by a transfer; and template-mediated growth/transformation (Young-Moo et al. 2022). The widespread use of MOFs in industrial settings is required by the large-scale fabrication of MOF films. Nevertheless, it is still difficult to create large area MOF films with favoured orientation, adjustable thickness, high homogeneity, and significant mechanical properties. This is due to the fact that most MOFs are formed by the hydro/solvothermal technique, and the volume of the vessel that is often used limits the area of MOF films created by the direct growth/deposition method. Meanwhile, the production of large-area MOF films is further limited by the poor processability of most MOF powders. Large-area MOF films may be produced using a number of techniques, including electrochemical (Jinxuan and Christof 2017), spin coating (Sabzehmeidani and Kazemzad 2023), doctor-blading or spray coating (Somjit et al. 2021), template-mediated growth/transformation (Asmaa et al.), and the interfacial synthesis method (Adimule, Bhowmik, and Gowda 2021). The chronology for the development of MOF integration into chemical sensors is depicted in Figure 4 (Weina et al. 2023). The gap in selectivity could be attributed to several factors inherent to MOFs. Firstly, the porous nature of MOFs allows for the adsorption of a variety of gas molecules, leading to a lack of specificity. The structural diversity of MOFs, arising from different combinations of metal ions and organic linkers, can result in multi-interaction sites, accommodating various gas molecules. Additionally, the reversible nature of gas adsorption-desorption in MOFs might contribute to a broad range of gases being accommodated. Moreover, the dynamic nature of gas interactions within the MOF structure, influenced by factors such as pore size, surface chemistry, and host-guest interactions, can lead to a more generalised sensing behaviour rather than a specific response to a particular gas. This lack of specificity might be considered a trade-off for the versatility and tuneability that MOFs offer. Over the past 15 years, MOF-based chemical sensors have seen a remarkable transformation that has been largely attributed to the creation of novel materials and equipment. This development shows how MOFs have the potential to improve chemical sensors’ performance and are becoming increasingly important. Conventional signal transduction systems have been used in the development of MOF-based chemical sensors, which either use MOF directly or couple with other functional materials. Designing and creating sensory materials benefits from an understanding of sensing mechanisms.

Figure 4. Timeline of the development of the integration of MOFs into chemical sensors. Adapted with permission from min Tu et al. Copyright 2023 Elsevier.

3. Critical sensor parameters

Parameters such as sensitivity, selectivity, res/rec time, and limit of detection (LOD) are pivotal to assess the sensing efficiency of sensors (Meng et al. 2022). Sensor parameters can be optimised by various technological approaches. Sensors are specialised devices designed to detect either the presence (qualitative sensors) or the concentration (quantitative sensors) of specific target analytes, whether they are individual compounds, groups of substances, or even multiple analytes at once (multiplexed sensors) (Xian et al. 2021). The response is an alert in one or more of the sensor properties such as mass, electrical conductivity, and capacitance. Today, chemical sensors represent a class of devices of outmost importance both from the scientific and applicative point of view. Since 1991 the International Union of Pure and Applied Chemistry (IUPAC) provided the definitions and classification of chemical sensors: ‘A chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal’ (Fenghe et al. 2020). While designing a sensor, it’s essential to focus on three key aspects often referred to as the ‘3 S’ rule: sensitivity (indicative of the calibration curve’s steepness), selectivity (the capability to differentiate the target analyte), and stability/reusability (alterations in accuracy over time or repeated use cycles). The main benefits of using MOFs for creating sensing materials come from their distinctive and highly modifiable physical, chemical, and structural (and thus functional) properties (Ke et al. 2021), which include their regular porosity and modifiable pore size, multivariate structures with multiple metals (either mono-, bi-, or tri-metallic systems), and/or organic linkers, as well as employing conformationally flexible linkers and/or geometrically versatile inorganic building units. The necessity for creating novel sensing materials may be addressed by including all attributes, including high sensitivity, high selectivity, short reaction time, increased stability, and reusability (Gustafson and Wilmer 2018). Most hybrid MOF sensors described so far are electrochemical or based on optical transduction techniques (such as luminescent, colorimetric, or plasmonic). The hybrid MOF characteristic that changes during contact with the analyte might be quantified as a sensing signal (Figure 5).

Figure 5. Schematic representation of a sensor’s primary components (sampling, recognition, and transducer), demonstrating how MOFs may serve as sorbent materials, sensing/recognition materials, or transducing elements in each component. (Reproduced with the permission from zuliani et al. Anal bioanal chem (2023) (Zuliani, Khiar, and Carrillo-Carrión 2023).

3.1. Sensitivity

The signal resulting from the interaction between the detector and the target analyte becomes evident and detectable within the designated detection period (Xu et al. 2022). Sensitivity can be defined depending on sensor types as follows.

3.1.1. Chemo-resistive sensors

Chemo-resistive sensors are a type of chemical sensor that operates based on the principle of change in electrical resistance in response to changes in the presence of specific target chemicals or gases (Zonta et al. 2023). These sensors are widely used for detecting and monitoring the concentration of various analytes, such as gases, volatile organic compounds (VOCs), and other substances, in the surrounding environment. The basic working principle of chemo-resistive sensors involves a sensing material, often referred to as a ‘sensing element’ or ‘sensing layer,’ which undergoes a change in its electrical conductivity or resistance when it interacts with the target analyte (Mahesh et al.). This change in conductivity or resistance can be attributed to several mechanisms, including adsorption or absorption of the analyte on the sensor’s surface, changes in the electronic properties of the sensing material, or alterations in the physical structure of the material. Common sensing materials used in chemo-resistive sensors include metal oxides (e.g. tin dioxide, zinc oxide), organic polymers, carbon nanotubes, graphene, MOFs and other nanomaterials. The choice of sensing material depends on the target analyte and the desired sensor performance characteristics (Naresh and Lee 2021). Chemo-resistive sensors have experienced significant advancements in recent years, including improvements in sensitivity, selectivity, and stability, driven by advancements in nanotechnology and materials science. These sensors play a crucial role in enhancing our ability to monitor and respond to changes in the chemical composition of our environment. $$\text{S=}\frac{R_{\mathit{\text{Air}}}}{R_{\mathit{\text{Gas}}}}\text{for reducing gases}$$ $$\text{S=}\frac{R_{\mathit{\text{gas}}}}{R_{\mathit{\text{Air}}}}\text{for oxidising gases}$$ $$\text{S percentage}=\frac{R_{\mathit{\text{Gas}}-}{R}_{\mathit{\text{Air}}}}{R_{\mathit{\text{Gas}}}}\times 100$$

Applications: Chemo-resistive sensors find applications in various fields, including environmental monitoring (e.g. air quality monitoring), industrial safety (e.g. detection of toxic gases), medical diagnostics (e.g. breath analysis for disease detection), and consumer electronics (e.g. gas leak detectors).

3.1.2. Key features of chemo-resistive sensors include

Sensitivity: Chemo-resistive sensors can exhibit high sensitivity to specific target gases or chemicals, allowing them to detect low concentrations of analytes.

Selectivity: The sensing material’s properties can be tailored to respond selectively to specific analytes, reducing interference from other substances present in the environment.

Response Time: Chemo-resistive sensors generally offer relatively fast response times, making them suitable for real-time monitoring applications.

Miniaturisation: These sensors can be miniaturised and integrated into compact devices, enabling their use in portable and wearable applications.

Ease of Integration: Chemo-resistive sensors can be easily integrated with electronic circuits, allowing for straightforward data acquisition and processing.

3.1.3. Electric resistive sensor

An electric resistive sensor, also known as a resistive sensor or a resistive transducer, is a type of sensor that operates based on changes in electrical resistance (Adimule et al.). It utilises the principle that the resistance of certain materials changes in response to external factors like physical deformation, temperature variations, or the presence of specific analytes. The resistance change is then converted into a measurable signal.

The formula to calculate the resistance of a resistive sensor is the well-known formula for resistance in a simple electrical circuit (Gómez-Ramírez et al. 2019): $$\text{R =}\mathrm{\rho }\text{*}\left(\text{L/A}\right)$$ where: R is the resistance of the sensor (measured in ohms, Ω),

ρ is the resistivity of the material (measured in ohm-metres, Ω·m),

L is the length of the sensor material (measured in metres, m),

A is the cross-sectional area of the sensor material (measured in square metres, m²).

A resistive sensor’s resistance changes can be measured directly, and this change can be correlated to the external factor being sensed. For example, in a strain gauge, the mechanical deformation of the sensor causes a change in its resistance, which can be used to measure the amount of strain or pressure applied. Resistive sensors find applications in various fields such as pressure sensing, temperature sensing, position sensing, and more (Adimule et al. 2021b). However, they might have limitations in terms of accuracy, sensitivity, and stability compared to other sensor types like capacitive or piezoelectric sensors. $$\text{S=}\frac{I_{\mathit{\text{Gas}}}}{I_{\mathit{\text{Air}}}}$$

3.1.4. Capacitive sensor

A capacitive sensor is a type of sensor that measures changes in capacitance, which is the ability of a system to store an electric charge (Adimule et al. 2021c). These sensors work based on the principle that the capacitance between two conductive surfaces changes when there is a variation in the distance or dielectric material between them. In simpler terms, a capacitive sensor uses the changes in electrical properties between its components to detect the presence, position, or properties of an object. These sensors find applications in various fields, such as touchscreens, proximity sensing, level measurement, and even in some biomedical devices (Madappa et al. 2023). $$\text{S\%=}\frac{{(C}_{\mathit{\text{final}}}-C_{\mathit{\text{initial}}})}{C_{\mathit{\text{initial}}}{(T}_{\mathit{\text{final}}}-T_{\mathit{\text{initial}}})}\times 100\mathrm{\%}$$ where, C_initial and C_final are the capacitance at the initial (T_initial) and the final (T_final) temperatures, respectively.

3.1.5. Electrochemical sensor

These sensors provide the ability to quickly and efficiently identify analytes that are rapidly reduced or oxidised by monitoring changes in the measured current, electric potential, or other electrical signals (Charithra and Manjunatha 2019). Electrochemical devices are well-known and widely used equipment that are dependable, appealing, adaptable, very sensitive, and simple to use. Electrochemical sensors have gotten a lot of interest in numerous analytical research disciplines due to their benefits. Modern analytical chemistry uses these sensory systems often for a variety of applications due to their technological simplicity, high mobility, sensitivity, cost-effectiveness, and possibility for decentralisation in in-field analysis. The MOFs must be deposited or immobilised on the electrode surface since electrochemical reactions can only take place there. The process of creating MOFs with electrocatalytic activity entails adding redox-active sites to the metal nodes or organic ligands, such as by combining active metal sites with nitrogen-containing ligands (like porphyrin- and bipyridine-based ligands) or by functionalising ligands with electron-donating or electron-withdrawing groups (Nidhi et al. 2023). The preparation of conductive or electrochemically active MOFs, MOFs supported on conductive substrates, and MOFs hybridised with active materials are the three primary methods suggested for enhancing the electrocatalytic activities of MOFs in their pure form. Additionally, it has been demonstrated that nanoMOFs, Hybrid MOFs and ultrathin 2D MOF nanosheets exhibit much increased electrochemical activity (Zhang et al. 2016). This is due to the existence of more exposed or easily accessible active sites as well as enhanced analyte transport to the catalytic sites.

3.2. Selectivity

Sensor selectivity refers to the sensor’s ability to detect a particular target analyte accurately and specifically within a mixture of various analytes. Attaining a high level of selectivity remains a significant challenge, particularly for sensors based on Metal-Organic Framework Nanostructures (MOFs) (Adimule et al. 2024). The task of distinguishing a single analyte distinctly amidst a range of analytes is often achieved through methods such as temperature modulation or arrays of sensors. Additionally, the introduction of functionalised MONs with dopants and hybrid configurations can lead to substantial improvements in enhancing the sensor’s selectivity.

3.3. Stability

Stability plays a crucial role in the expansion of sensors within the market and can be comprehended from two distinct angles. Initially, active stability pertains to ensuring the reproducibility of sensor characteristics over a minimum period of two years under operational conditions. The second facet of stability involves the sensor’s ability to uphold both its sensitivity and selectivity attributes over time.

4. Key features of MOFs for sensing

The majority of the metal ions utilised in MOFs are electrochemically active, they offer considerable promise in the field of electrochemistry in addition to the applications such as gas separation/storage, water purification, batteries, supercapacitors, and catalysts. Crucially, MOFs may modify the locations of atomic metal centres, reducing mass consumption and improving electrode interface. The drawback is that single MOFs still have some constraints that keep them from having the most influence on the development of chemical sensors. These limits include limited electrocatalytic activity, poor mechanical stability, and poor electrical conductivity. In light of these considerations, there is interest in enhancing these features by mixing MOFs with bi metals, polymers, metal oxides, carbon materials. The composites might be provided exceptional selectivity and conductivity by including such components. Sensitivity, selectivity, response time, stability, reusability, and the incorporation of suitable signal transduction capabilities (e.g. optical, electrical/electrochemical, photoelectrochemical, mechanical, thermal, mass, magnetic, acoustic) are the essential features of MOF-based sensors. The two primary methods for improving the selectivity of MOF sensing materials are physical–chemical interactions and size exclusion, often known as molecular sieving via pores. By carefully choosing the metal ions and organic linkers (e.g. node and linker sizes and geometries, linker appendages and their directional orientation), the MOF porosity can be adjusted.

This changes the pore and aperture sizes as well as the material’s hydrophilic-hydrophobic character, which increases the molecular adhesion to the target analyte. When it comes to the use of physical–chemical interactions as a tactic to boost sensitivity and modulate selectivity, functional groups (such as amines, carboxylic acids, and hydroxyls) are frequently incorporated into the framework to encourage binding with the target analyte through covalent bond formation, hydrogen bonding, electrostatic interactions, and electron donor/acceptor interactions. The degree to which the analyte binds to the MOF will determine the sensitivity; a stronger binding will result in lower detection limits, even if the sensitivity is somewhat dependent on the signal transduction mechanism. It is also possible to modify the analyte-MOF's strength by varying the pores’ size, hydrophilic-hydrophilic character, presence of certain functional organic groups, and functionalization with biomolecules. As indicated in Table 1, there are a variety of methods for increasing selectivity and sensitivity, each with their own set of benefits and drawbacks that should be considered when designing a sensor based on its intended use. For instance, highly selective but expensive sensors might make sense in a particular situation involving biomedical applications.

Table 1. Advantages and disadvantages of the main strategies for enhancing selectivity and sensitivity. Reproduced with the permission from 2023 Springer.

5. MOF-based devices for gas sensors

The development of chemically altered electrodes is still an active field that requires ongoing investigation of various materials and molecules to guarantee the highest quality of electrochemical sensor characteristics. Various nanostructured materials have been included into contemporary sensor designs as better adsorbents for electrode modification and electrocatalysts. These include carbon-based materials (such as carbon nanotubes and graphene), metal nanoparticles (NPs) (like Ag and Au NPs), metal oxides (like CuO, MnO₂, SnO₂, ZnO, InO₂, and WO₃), conductive polymers (like polyaniline, poly(3,4-ethylenedioxythiophene) and polypyrrole), and frameworks (like MOFs and covalent-organic frameworks, COFs). Many researchers are interested in these materials because of their inherent benefits and potential for future developments. But because of the limits of current technology, electrochemical sensors made of these materials have difficulties and restrictions in terms of performance. Table 2 provides a comparative evaluation of the benefits and drawbacks of common nanomaterials.

Table 2. A comparison of the benefits and drawbacks of various sensor materials.

Materials	Downsides	Benefits	References
Metal Oxides	Detection limits tend to be high Poor conductivity Poor cycling stability	Cheap and easy to prepare Plenty of active sites Good mechanical properties Good designability	(Wang et al. 2010; Goel et al. 2023)
Conducting Polymers	Poor solubility Additional dopant required to improve conductivity	Easily and stably modified surface of electrodes Tuneable structure High electrical conductivity of the material after doping	(Wang et al. 2020; Adimule, Nandi, and Jagadeesha Gowda 2021)
Metal Nanoparticles	Precious metals are expensive Poor dispersibility and solubility Insufficient stability of transition metals Prone to agglomeration	High surface energy Extremely large surface area Excellent conductivity High quantity of active sites	(Kumar et al. 2021; Białas et al. 2022)
COFs	Poor conductivity and dispersibility Shape and size are difficult to control effectively	Very low density Large specific surface area Tuneable structure Higher stability Abundant active sites	(Sidi et al. 2020; Hessamaddin et al. 2023)
MOFs	Weak conductivity Poor stability in harsh environments	Abundant active sites Tuneable structure Easy to introduce other functional groups Large surface area and porosity	(Leelasree, Sanket, and Himanshu 2022; Joshi et al. 2018)

The exponential growth in research in this field over the past several years is proof that the study of MOF-based sensors has produced a number of novel findings. But because of the weak separation of electrons and poles, the majority of pure MOFs have significant restrictions. One of the ideas is to combine MOFs with other guest elements to provide the composite outstanding stability in addition to its light-absorbing properties. In order to overcome the drawbacks of pure MOFs, MOF-based composite materials with excellent electrical conductivity and powerful catalytic activity frequently combine the benefits of both MOFs and guest materials. When taking into account the different types of MOFs mentioned in the previous study findings, it can be seen that they mostly fall under the categories of lanthanide MOFs (Ln-MOF), ZIF, MIL, UiO, and HKUST, and to a lesser degree, TCPP-based MOFs, IRMOF, and NU (Manjunatha 2018). Young-Kyu Han et al. (Haldorai et al. 2018) reported using a single precursor (zeolitic imidazolate framework-67) to create a nanoporous carbon/cobalt oxide (NPC/Co₃O₄) composite. The composite electrode’s differential pulse voltammetric response (at 0.12 V vs. SCE) is linear in the 2-240 M AA concentration range, with a 20 nM detection limit. The electrode has great sensitivity (0.13 A M cm⁻²), consistent repeatability, and superior selectivity. Mahmut Ozacar et al. (Didem et al. 2020) reported reduced graphene oxide (rGO)/Zn-MOF-74 hybrid nanomaterial coated with glucose oxidase (GO_x) on platinum nanoparticles (Pt NPs) The linear measurement range for glucose for the GO_x-rGO/Pt NPs@Zn-MOF-74 coated electrode was 0.006 to 6 mM, with a detection limit of 1.8 M (S/N: 3) and sensitivity of 64.51 μA mM⁻¹cm⁻² (Figure 6).

Figure 6. Development of reduced GO/Pt nanoparticles/Zn-MOF-74 nanomaterials. (B) Nyquist plot of the electrodes and the randles circuit as well as cyclic voltammograms of the electrodes in 0.1 M PBS containing 5 mM [Fe(CN)₆]3-/4- and (C) 0.1 M KCl, with a scan rate of 100 mVs⁻¹. Reproduced with the permission from 2019 Wiley.

A one-pot approach was used to create two-dimensional Au nanocluster-embedded zirconium-based metal-organic framework nanosheets (abbreviated as 2D AuNCs@521-MOF). As determined by electrochemical impedance spectroscopy and differential pulse voltammetry, respectively, the results showed that the 2D AuNCs@521-MOF-based aptasensor had high sensitivity for detecting cocaine within the broad concentration range of 0.001-1.0 ngmL⁻¹ and the low limit of detection of 1.29 pM (0.44 pgmL⁻¹) and 2.22 pM (0.75 pgmL⁻¹). In addition to having a high specific surface area, excellent electrochemical activity, and physicochemical stability, the optimised 2D AuNCs@521-MOF nanosheets also showed a remarkable bio-affinity towards phosphate groups found in biological molecules (Figure 7).

Figure 7. Diagram illustrating the creation of electrochemical biosensing based on AuNCs@Zr-MOF-based nanosheets for the detection of cocaine. This diagram shows the synthesis of the 2D AuNCs@Zr-MOF nanosheets, the immobilisation of the aptamer strands, and the detection of cocaine. Adapted from Wei et al. (2020).

Zhaosheng Li et al. (Wen et al. 2017) reported a hybrid material (polyoxometalate-based metal–organic framework (POMOF/rGO) integrating POMOF and rGO is obtained via a simple one-pot approach. The synthesised POMOF/rGO hybrid material exhibits its extremely promising capabilities for dopamine detection and the result should encourage the further development of POMOF-based sensors since it is a non-noble metal catalyst built from function and structure-matched building blocks. The obtained electrochemical sensor exhibited satisfactory stability and reproducibility with a wide linear detection range from 1 × 10⁻⁶ to 2 × 10⁻⁴ m and low detection limit of 80.4 × 10⁻⁹ m (Figure 8).

Figure 8. (a) Schematic illustration of the assembly and sensing of DA in POMOFs/RGO. (b) Scheme for the preparation of Fe₃O₄@ZIF-8/RGO, and its application for the determination of DA. (c) Illustration for the sonication-assisted preparation of Cu(tpa)-GO and its application for the simultaneous determination of ACOP and DA. Adapted from Yan et al. (2018).

Polymer/MOF nanocomposites received a lot of attention because they combine the benefits of both flexible polymer materials and extremely porous MOFs. There are several instances where MOF and polymers have been combined. Polymers and MOFs are frequently combined together in membranes with mixed matrix. In composite materials, polymer chains are used to cross-link MOF particles, and some of the repeating units in the chains serve as ligands for the MOF structure (Adimule et al. 2023b; Aisha and Maliha 2022). Usually, post-synthetic modification is used to build up the polymer covering. There are non-covalent and covalent coating techniques that have been developed for MOF nanoparticles. The primary components of non-covalent strategies are hydrogen bonds or electrostatic interactions. ‘Grafting to’ and ‘grafting from’ techniques are two categories of covalent approaches. ''Grafting to’' refers to the reactivity of end-functionalised polymers with functional groups on the MOF, the coordinatively unsaturated metal sites, or groups on the ligands, and ''Grafting from’' refers to polymerisation from active sites on the MOF (Xue et al. 2020). Saleh T. Mahmoud et al (Ashraf et al., 2021). reported MOF-5 microparticles embedded on a conductivity-controlled chitosan (CS) organic membrane. By combining an organic membrane with a glycerol ionic liquid (IL) at various concentrations, the conductivity of the membrane may be adjusted. At ambient temperature, the sensor demonstrated excellent detection sensitivity for H₂S gas at concentrations as low as 1 ppm. The MOF-5/CS/IL gas sensor has remarkable sensing stability, averaging at 97% detection with 50 ppm of H₂S gas, a rapid reaction time (8 s), a recovery time of less than 30 s, and very desired detection selectivity (Figure 9).

Figure 9. Scheme of a flexible and highly sensitive metal-organic framework for H₂S gas sensor. Copyright © 2021 the authors. Published by American chemical Society.

Shun Mao et al. (Xian et al. 2023) reported an extremely sensitive electrochemical sensor for the detection of trichloroacetic acid (TCAA) is created using a composite made of iron (II) phthalocyanine (PcFe) and a Zn-based metal organic framework (ZIF-8). ZIF-8, which functions as an electrode material, has a wide surface area and a porous structure that demonstrate high absorbability; in contrast, PcFe (II), which serves as the sensing element, reduces from PcFe (II) to PcFe (I) throughout the sensing process. The sensor has a high sensitivity (826 μA/μM) and a limit of detection (LOD) of 1.89 nM, which is better than previous TCAA sensors that have been described (Figure 10).

Figure 10. ZIF-8 and PcFe@ZIF-8 synthesis process schematics. A more detailed look at the molecular structure of PcFe. the trichloroacetic acid measurement electrochemical sensor system. Copyright © 2019 American chemical Society.

Liping Guo et al. (Deng et al. 2017) prepared the zirconium-based MOF (UiO-66)/mesoporous carbon (MC) composite was synthesised using conventional hydrothermal method. For the simultaneous and sensitive measurement of hydroquinone (HQ), catechol (CT), and resorcinol (RS) dihydroxy benzene isomers, a new electrochemical sensor based on UiO-66/MC was built. The suggested sensor has outstanding electrocatalytic activity when it comes to the oxidation of HQ, CT, and RS. In the concentration ranges of 0.5-100 μM, 0.4-100 μM, and 30-400 μM, the electrochemical sensor exhibits a broad linear response with detection limits of 0.056 μM, 0.072 μM, and 3.51 μM (S/N = 3) for HQ, CT, and RS, respectively (Figure 11). The sensor also has excellent anti-interference capabilities, robust repeatability, and electrochemical stability in addition to exceptional sensitivity.

Figure 11. The design and recognition method of the sensor zirconium-based MOF (UiO-66) and mesoporous carbon (MC) to fabricate a novel electrochemical sensing are represented schematically with the permission from 2017 Elsevier.

The performance of electrochemical sensors based on MOFs-graphene composite was good when utilised to detect a variety of analytes, including acetaminophen, dopamine, H₂O₂, ascorbic acid, dihydroxy benzene isomers, heavy metals, 2,4,6-trinitrophenol, and 4-nonylphenol (Yan-Yan et al. 2017). A unique and prospective electrochemical sensing platform is the MOF-graphene composite. Xue Wang et al. (Xue et al. 2020) created cerium-centered metal-organic framework electrochemically reduced graphene oxide composite (Ce-MOF-ERGO) coupled with cetyltrimethylammonium bromide (CTAB) signal amplification, a sensitive electrochemical sensor for the quick measurement of bisphenol A (BPA). With a detection limit of 1.9 nM, the suggested electrochemical sensor demonstrated a linear response for BPA in the concentration range of 3 nM to 10 μM. Due to the Ce-MOF-ERGO composite’s high conductivity, significant surface area, and catalytic activity, the proposed sensor performed exceptionally well (Figure 12).

Figure 12. Schematic representation of the electrochemical detection procedure and the manufacturing of Ce-MOF-ERGO/GCE. Adapted from 2020 Springer nature.

A soft-lithographic method for creating 2D photonic structures based on metal organic frameworks (MOFs) at submicrometer sizes was described by Marco Faustini and colleagues. Because of its chemical stability and vapour-selective adsorption capabilities, zinc, or nanometric zeolitic imidazole framework material ZIF-8, is selected as the sensible MOF material. Nanopatterned colloidal ZIF-8 homo- and ZIF-8/TiO₂ heterostructures are the two distinct systems that are created (Figure 13). Furthermore, these photonic MOF-heterostructures are shown to be highly affordable sensor platforms that work with smartphone technology. First, isopropyl alcohol adsorption/desorption cycling is used as a model example to assess the sensors’ performance. At quantities below the human legal exposure limit, selective detection of styrene in the presence of interfering water is established. To validate the sensor performances, in situ ellispometric investigations are also performed.

Figure 13. Soft-lithographic approaches for the fabrication of a) nanopatterned ZIF-8 and b) ZIF-8/TiO₂ heterostructures. Reproduced with permission from Copyright 2016 Wiley-VCH GmbH (Dalstein et al. 2016).

Gang Xu and colleagues (Ming-Shui et al. 2016) detailed the preparation of a ZnO@ZIF-CoZn gas sensor, involving the application of a ZIF-CoZn thin film onto a ZnO nanowire array through a straightforward solution method (refer to Figure 14). The gas sensor utilising this innovative material exhibited notable improvements in comparison to the MOX sensor lacking a MOF sheath: (1) achieving selectivity between acetone and humidity with only a 7.4% standard deviation in a wide relative humidity range for the first time; (2) a roughly 20-fold enhancement in response; (3) a two-orders-of-magnitude improvement in the detection limit; (4) acceleration of response and recovery behaviours by 48% and 470%, respectively; and (5) a reduction in operating temperature by approximately 125 °C. It is noteworthy that MOF materials offer significant design flexibility in terms of structure and properties, unique gas selectivity based on factors such as size, shape, chirality, and exceptional catalytic activity.

Figure 14. Schematic illustration of the preparation of ZnO@ZIF-CoZn gas sensors. Reproduced with permission from Copyright 2016 Wiley-VCH GmbH.

By combining the right materials with the frameworks, it may be possible to overcome the limitation on conductivity imposed by the enormous unoccupied volume of MOFs. To overcome this obstacle and serve as efficient electrochemical sensors for environmental and biological targets, several MOF composite-based platforms have been created. The summary of the numerous types of MOF-based sensors, their integration techniques, and sensing capabilities are shown in Table 3. As a contrast, noble-metal nanoparticles (NPs) (such as Au, Ag, Pd, and Pt), conducting polymers, Metal NPs, Carbon compunds can be predicted in sensing, imaging, cancer treatment, optical data storage, and catalysis due to their superior physicochemical characteristics. However, because of their high surface energy, aggregation limits the aforementioned performances. In order to tackle this challenge through numerous strategies, the researcher mixes them with MOFs. As a result, composites might serve as superior molecular recognition sensors.

Table 3. The summary of the numerous types of MOF-based sensors, their integration techniques, and sensing capabilities.

Sensing device	MOF Material	Integration technique	Target analyte	Detection range	Sensitivity	Response/recovery time (Seconds)	Ref.
Amperometric oxidase-based biosensor	rGO/Pt NPs/Zn-MOF-74	Drop casting	Glucose	0.006 to 6 mM	64.51 μAmM^-1 cm^-2	40	(Didem et al. 2020)
Electrochemical Sensor	2D AuNCs@ 521-MOF	Drop casting	Cocaine	0.001–1.0 mg·mL^−1	(0.437 pg·mL^–1	–	(Wei et al. 2020)
Differential pulse voltammetry (DPV) method	POMOF/ rGO/GCE	Hydrothermal	Dopamine	1 × 10^−6 to 2 × 10^−4 m	0.373 mA × 10^−3 m^−1 cm^−2	–	(Wen et al. 2017)
Electrochemical sensors	MOF-5/Chitosan/IL	Drop casting	H2S	100 ppm	–	<8 s	(Ashraf et al. 2021)
Electrochemical sensors	Fe (II) phthalocyanine (PcFe) and Zn-(ZIF-8)	Hydrothermal	Trichloroacetic acid	0.02-1 μM	826 μΑ/μM	–	(Xian et al. 2023)
Electrochemical sensor	Zr-MOF (UiO-66)/mesoporous carbon (MC)	Hydrothermal method	Dihydroxybenzene isomers (DBIs) of hydroquinone (HQ), catechol (CT) and resorcinol (RS)	30 to 400 μM	HQ = 0.360, 0.147 μA μM^-1 CT = 0.142 0.034, μA μM^-1 RS = 0.018 μA μM^-1	–	(Deng et al. 2017)
Electrochemical sensor	Ce-MOF-ERGO	Electrochemical reduction	Bisphenol A (BPA) based	3 nM to 10 μM	5.0 μmol L^−1	150 s	(Xue et al. 2020)
2D Photonic crystals	ZIF-8/TiO2	Soft-lithographic approaches	Styrene/water	1%–10% P/P0	–	–	(Dalstein et al. 2016)
Chemiresistive sensors	ZnO@ZIF-CoZn	In situ solvothermal	acetone	0.25–100 ppm	–	42/62	(Ming-Shui et al. 2016)
3D Photonic crystals	MOF-SiO2	In-situ solvothermal	CS2	100–10,000 ppm	–	–	(Lu et al. 2011)
Chemicapacitive sensors	Mg-MOF-74	In situ solvothermal	CO2, benzene	200–5,000 ppm, 2–100 ppm,	–	200/500	(Yuan et al. 2019)
Chemiresistive sensors	SnS2@ZIF-8	In situ solvothermal	NO2	0.2–1 ppm	–	369/902 s	(Kim et al. 2023)
FET sensors	SWCNT@ Cu3(HHTP)2	Dielectrophoresis	Carbohydrates	–	–	–	(White et al. 2021)
Diode sensors	Ag/Cu3(HITP)2/n-Si/Al	Layer-by-layer (LBL) assembly	NH3	25–100 ppm	–	144/450	(Cao et al. 2020)
Electrochemical sensor	ZIF-8-on-ZnO-on-rGO	Chemical vapour deposition	Dopamine	1 × 10^-9 –1 × 10^-5 mM	–	5	(Xu et al. 2023)
Electrochemical sensors	Cu-MOF	Drop casting	Glucose	–	–	–	(Ling et al. 2018)
Electrochemical sensors	Ni3(HHTP)2	Drop casting	Dopamine	1 × 10^-5 −0.2 mM	–	–	(Ko et al. 2020)

6. Conclusion and future prospects

Hybrid MOFs, a brand-new kind of crystalline molecular material, feature exceptional chemical and physical qualities such pore diameters that can be tailored, a large amount of unique surface area, and a programmable structure. Hybrid MOFs are used widely in a variety of sectors, including gas storage, separation, catalysis, biomedical imaging, and sensing, as a result of their outstanding features. However, there are still several drawbacks to MOFs and hybrid MOFs, which restrict their use. These drawbacks include a lack of mechanical strength, weak stability, and a single function. As a consequence, novel MOF composites with performance that surpasses that of the original components may be produced by combining MOF and functional materials. Due to its selective adsorption and separation, catalysis, magnetism, and luminescence properties, MOF nanocomposites are widely used in a variety of sensors, including electrochemical and optical ones. We covered many MOF-based composite types used in the sensing process in this review.

Additionally, it is not always clear how the characteristics of MOFs and the detected sensory responses relate to one another specifically. Through the combination of many in situ analytical methods, such as in situ infra-red spectroscopy or X-ray diffraction, a deeper knowledge of MOF sensing processes will be possible in the future. In order to increase sensing performances, new desirable MOF materials may still be designed based on the correlations between structure and property, even if there are a lot of new sensory MOF materials that need to be invested in for sensing. The challenges of improving selectivity, sensitivity, response/recovery time, reusability, and stability properties still need to be addressed in order to accelerate the practical implementation of MOFs in chemical sensors.

As a consequence, novel MOF composites with performance that surpasses that of the original components may be produced by combining MOF and functional materials. The functions of selective adsorption and separation, catalysis, magnetism, and luminescence in MOF nanocomposites allow for a wide range of applications in sensors, including electrochemical and optical ones. It is necessary to create or implement pertinent algorithms to improve data processing and pre-train MOF-based sensors. Therefore, it is feasible to afford MOF-based sensors that have self-calibration, anomaly detection, and machine learning-based lifespan prediction.

The sensing capacities of these MOF-nanocomposites may be improved for recognising a range of species, such as temperature, dangerous phenolic compounds, heavy metal ions, anions, aromatic hydrocarbons, and anions, as has been mentioned in various research. Future studies may use a range of functionalised MOFs to improve the recognition performance of composite-based sensors. MOF-nanocomposites can enhance the sensing capabilities for detecting a variety of species, including heavy metal ions, anions, aromatic hydrocarbons, hazardous phenolic chemicals, and temperature. In the upcoming years, the development of MOF-based gas sensors with enhanced application, sensing performance, and touchable commercialisation will be accelerated.

Acknowledgments

Authors are thankful to KLE Technological University Dr. M. S. Sheshgiri Campus, Belagavi Angadi Institute of Technology and Management, Belagavi and School of Advance science, KLE Technological University, Hubballi, India for providing all necessary facilities required to carry out this work.

Disclosure statement

The authors declare that they have no conflict of interest.