Metal-Organic Frameworks (MOFs) are a novel and exciting platform for sensing technology. Among its applications, we can find the detection of toxic gases, explosives, water pollutants, and biomolecules such as DNA. The variety of molecules detected requires different properties of the sensing material. Therefore, these sensors work under several working principles giving sensors in four categories:
Each principle is described in detail in the following sections and we briefly go over the advantage of MOFs for sensing applications compared to other materials.
The optical sensors are the most used among the different MOF-based sensors. The design of MOFs for sensing applications can be directed towards different mechanisms, taking advantage of its highly ordered structure. The main optical phenomenon in MOF sensors is the luminescence, in which an emitting component undergoes an interaction with the analyte, causing a signal in the form of light emission used for quantification. The emitting component can be a metal ion, a guest species, a ligand, or a product of a catalyzed reaction.
The emitting components are incorporated in the structure of the MOF at different steps, during the synthesis or in a post-synthesis modification. During the synthesis, metal ions (i.e. metal lanthanide ions) and ligands (aromatic organics) are usually incorporated. While post-synthesis modification allows the addition of guest species (i.e. quantum dots, metal complexes, organic dyes), and catalytic components (i.e. nanomaterials).
The result of the interaction between the analyte and the sensing material can be detected as an increase or decrease of the luminescence (the decrease in intensity of the luminescence is also known as quenching), or as a change of color. As will be expected, the detection methods are based on spectrophotometric techniques.
Optical MOF-based sensors have been used for the determination of a wide variety of molecules like metal ions (Cu2+, Ag+, Zn2+, Cd2+, Hg2+, Na+, NH4+, Ca2+, Mg2+, Sr2+, Ba2+, and Pb2+ ), nitroaromatic explosives, and biomolecules (glucose, nitrite, Adenosine Triphosphate or ATP).
MOFs are also known for their adsorption capabilities of different gases; taking advantage of this property, we find the electromechanical sensors. In this case, the gravimetric changes are created by the adsorption of the analyte, which induces changes in vibrational frequencies or other mechanical signals for detection.
Due to the nature of the working principle, technologies like microcantilever (MCL), quartz crystal microbalance (QCM), and surface acoustic wave (SAW) are used for the fabrication of gas and humidity sensors. The changes in the mass of the sensing film influence the oscillating frequency of the cantilever, the frequency of the resonant vibrations traveling perpendicular to the surface of the quartz crystal, or the frequency of the acoustic waves traveling parallel to the surface, resulting in signal generation and detection.
Electromechanical sensors have been applied mainly for the detection of volatile organic compounds and the humidity level of gas samples.
The isolating nature of MOFs was making the possibility of MOF-based electrochemical sensors less likely. However, recent advances have shown the ability to create conductive MOF materials. Also, the combination with nanomaterials has been used as an alternative to increasing the application of MOFs as electrochemical sensors. The nanomaterials used include carbon-based materials (like carbon nanotubes and graphene oxide), and metal nanoparticles (i.e. palladium, gold, and ruthenium).
The mechanisms involved in MOF electrochemical sensors include electrocatalytic activity (oxidation/reduction reaction) of the metal clusters or the organic ligands. For instance, Cu-based MOFs showed enzyme-like catalytic activity for the oxidation of 3,3′,5,5′-Tetramethylbenzidine or TMB in the presence of hydrogen peroxide. Another mechanism is the implementation of MOFs as a carrier of metal ions.
In some cases, the functional groups in organic ligands can adsorb the target molecule, which acts as a preconcentration step for further determination after a catalyzed reaction. Among the applications of MOF-based electrochemical sensors is the detection of molecules like hydrogen peroxide, glucose, metal ions, nitrite, ascorbic acid, cysteine, hydroquinone, urea, etc.
This category arises as a combination of two phenomena, in which an electrochemical reaction is driven by a photon excitation, the process is also known as photocatalysis. In this case, the MOF material participates as a catalyst for the redox reaction, and the product of the reaction is the signal for detection.
The current response of the sensors can be obtained in both directions, some analytes can increase the current, and others can decrease it. This type of sensors has been used for the detection of molecules like hydrogen peroxide, glyphosate, and ascorbic acid.
Advantages of using MOFs as a sensing platform
Regardless of the type of sensor, MOF materials as a sensor platform offer several advantages over conventional methods for detection in gases and liquids including:
- Easy manufacture
- Chemical and thermal stability
- Room temperature operation
- High selectivity and sensitivity
The properties of the MOFs can be tuned through a conscious design of its chemical structure, pore size, and reactivity toward specific target molecules.
Metal-Organic Frameworks offer new alternatives for sensing technology while meeting the most important requirements like chemical and thermal stability. Such requirements are mandatory for real-world applications. MOF-based sensors can be designed for simple and straightforward operation with high selectivity and sensitivity. With more than 88,000 MOFs that have been already synthesized, the possibilities for the development of new sensor materials are limitless.