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1Metal-Organic FrameworksAn Introduction to Advanced Sensing Applications
Felipe de Souza1 and Ram K. Gupta1,2
1Kansas Polymer Research Center, Pittsburg State University, Pittsburg, Kansas, USA
2Department of Chemistry, Pittsburg State University, Pittsburg, Kansas, USA
DOI: 10.1201/9781003188148-1
1.1Introduction
Nano-sized materials has become a significant research area that is present in nearly all scientific fields such as chemistry, physics, materials science, engineering, medicine, and the like. Nanotechnology can be applied to electronics, drug delivery systems, biosensors, energy storage, catalysis, fuel cells, and many others. Nanomaterials can show enhanced properties in comparison to their bulky counterparts.
Nanomaterials can be classified into four categories based on their dimension: zero-dimension (0-D), one-dimension (1-D), two-dimension (2-D), and three-dimension (3-D). 0-D are materials that present all three dimensions within the nanoscale such as nanoparticles. 1-D materials have at least one dimension less than 100 nm. Applications of these include nanorods, nanowires, or nanotubes. 2-D materials have one of their dimensions out of the nanoscale. Applications of these include a single nanolayer, nanofilms, or nano-coatings. Finally, 3-D materials are usually comprised of an array of nanomaterials that are combined to have a three-dimensional structure. Application of these include nano-layered structures, nano nets which are arrays of nanotubes in the vertical plane and nanofilms in the horizontal plane, or nanosized active sites uniformly distributed, with a 3-D structure. Based on that kind of 3-D structure, was the development of metal-organic frameworks (MOFs) which are structured with a high degree of crystallinity based on coordination complexes that consist of an inorganic core, usually a transition metal, bonded with an organic ligand forming a 3D structure.
Due to the large number of materials available, there are around 2,000 types of MOFs. Such a wide variety is a consequence of several variations that can be made when one is designing their synthesis. This includes variations in starting materials, size, geometry, and functionality. The type of chemical bonding is based on the positively charged empty d orbitals in a transition metal bonding with one negatively charged organic group such as carboxylates, phenolates, phosphonates, or imidazolates for instance. Through this, robust and crystalline structures can be obtained that present a relatively high surface area that can vary from 1,000 up to 10,000 m2/g, which is larger than other materials such as activated carbon-based compounds and zeolites. This aspect of MOFs makes them desirable for applications related to electrocatalysis, CO2 reduction, fuel cells, and energy storage, for example. Part of the reason why MOFs can cover a wide range of applications is their tunable surface characteristics. By varying experimental conditions, their surface characteristics can be easily tuned. Through that, the pore sizes can be controlled which allows them to interact with specific molecules or biomolecules, adsorb small molecules, or even function as active sites to catalyze certain reactions. Their chemical and thermal stability can be also improved as they can be further functionalized after their synthesis. Other unique properties of MOFs can be observed when lanthanides are used, which can fluoresce when irradiated with UV light. Further, the bonding strength between the metallic and organic ligands can be redesigned to make them biodegradable. Importantly, their structure can be precisely controlled thus facilitating their interaction with guest molecules in terms of catalysis when acting as an active site or for identification of small molecules or biomolecules [1].
An example of these properties is the applications of MOFs as gas absorbents because their pore size can be controlled by means of the synthesis temperature, to selectively absorb CO2 or CH4 [2, 3]. Hence, MOFs can be used as sensors for many gases. Taking these properties in another direction, MOFs can also be used as effective capacitors due to their high surface area that allows an easy diffusion of ions within the frameworks, leading to high capacitance. Moreover, the frameworks can be functionalized with other elements such as N or P-based ligands to induce pseudo-capacitance, which further improves their energy storage capabilities. For this reason, the redox properties and exposed active sites allow MOFs to function as electrocatalysts for H2O splitting to produce H2 and O2. MOFs have also found application in the biomedical field because, when they are on the nanoscale, they can be used as carriers to deliver drugs, contrasting agents for imaging, photo-induced therapy, or chemotherapy [4, 5].
1.2 Synthesis of MOFs
The synthesis of MOFs can be performed through several physical or chemical routes. Both routes can be subdivided into top-down or bottom-up approaches. A top-down approach is based on using a bulky starting material that is broken into smaller pieces until it reaches the nanoscale. In the physical route, it can be performed through mechanical milling, for instance. The main disadvantage of this process is the wide distribution of particle sizes. Physical exfoliation is another procedure that falls within a top-down category as the bulky material can be peeled off into thinner films such as the use of adhesive tape to peel off the graphene layer from graphite. Even though this method is effective it is also tedious as it may require repetitive peelings with the tape followed by dissolution to claim the nanomaterial.
In the bottom-up approach, smaller starting materials are combined until they form a larger structure. One example of this case is the laser evaporation method in which a starting metal oxide is vaporized when exposed to a strong laser beam and condensed onto a substrate [6]. The parameters such as atmosphere composition and laser power can vary the size and property of nanoparticles. For example, magnetic nanomaterials can change their phase depending on these conditions or a larger number of defects on the surface area can be induced to enhance the catalytic activity [7, 8]. Slow solvent evaporation is a simpler precipitation method that is performed at room temperature in the presence of one or more volatile solvents to make the process cheaper against the cost of the long evaporation time [9]. Metal can also be converted from its ionic state that agglomerates into nanoparticles, which is another type of bottom-up approach. It can be performed through thermolysis of LiN3, for example, by heating it to 400 °C which causes it to decompose, releasing N2 gas (since its decomposition temperature is close to 370 °C). This process causes a decrease of pressure in the system which creates a driving force for the agglomeration of Li atoms that forms 5 nm nanoparticles [10].
There are several chemical routes available for the synthesis of MOFs such as co-precipitation, sol-gel, hydro or solvothermal, chemical vapor deposition, pyrolysis, microwave-assisted, microemulsion, ion exchange, intercalation, chemical exfoliation, sonochemical, mechanochemical, and electrochemical, to name a few. Co-precipitation is an easy method that can yield uniform-sized nanoparticles through the precipitation of a precursor in aqueous media. It consists of mixing two or more soluble salts and then using precipitating agents or suitable conditions to precipitate them. To avoid the formation of clusters, stirring is usually required along with a controlled temperature, as both parameters can affect the nanoparticles’ sizes and shapes [6]. Slightly higher temperatures induce higher crystallinity as nanoparticles tend to have more energy to move and properly arrange. Commonly employed reducing agents are NH3, NaOH, NaBH4, LiBH4, N2H4·2HCl, N2H4·H2O, among others which can also control the pH of the system to promote the precipitation of a specific metal. Based on that, there is a sequence of events for the synthesis of MOFs or nanomaterials. This sequence is: nucleation, growth, coarsening, agglomeration, and stabilization. Controlling the agglomeration is an important step to keep the particles within the nanoscale, otherwise undesired levels of agglomeration may occur due to the high surface energy of small nanoparticles. Hence, proper concentration of reducing or capping agents is important to control the size and shape.
A hydrothermal method is an efficient approach where the starting materials are exposed to high temperature and pressure in an autoclave to yield particles of regular size under relatively lower temperatures and a facile procedure. The variation in parameters such as solvents, pH, time, and temperature can greatly influence the MOF’s properties. A solvothermal method is like a hydrothermal method except that instead of water, organic solvents such as dimethylformamide, acetonitrile, diethyl formamide, acetone, and so on, are used. It can lead to drastic changes in the morphology and in the properties of MOFs [9].
The sonochemical approach consists of ultrasonic radiation in a solution containing the starting materials that creates bubbles leading to high temperatures and pressures. When a bubble bursts, it provokes chemical excitation for the reaction to occur. Synthesis of CoS2, CdSe, ZnSe, alloys, and oxides has taken place by this method [6]. Microwave-assisted synthesis is a convenient method as the conversion of high-frequency electromagnetic waves into heat, interacts with the solution containing the starting materials to create the driving force for the reaction. It is a method with a facile approach and a much shorter reaction time than traditional hydro or solvothermal methods. Additionally, it yields nanostructures with a narrow size distribution.
Electrochemical synthesis is based on dissolving metal ions along with organic ligands and electrolytes which are exposed to a specific voltage to carry on the reaction. This is a convenient process as it can be performed in a short time and with high reproducibility. Mechanochemical synthesis is another attractive method as chemical bonds are created with the input of mechanical force under small or no solvent content. On top of being a cheap method, it also yields different types of materials that can range from 1D, 2D, or 3D materials such as frameworks of zeolitic imidazolates [9].
1.3 Chemistry and Applications of MOFs
Properties such as high surface area, highly organized structure, exposed active sites, and facile functionalization are signatures of MOFs. Such properties endorse their use in several fields related to gas adsorption and their identification, sensors, batteries, supercapacitors, electrocatalysts, and biosensors/trackers. Such properties are related to the tunable chemical structure of MOFs which can be synthesized through a variety of transition metals and organic ligands. MOFs can function as adsorbents for gases which can reduce the cost since several types of gases used in the industry require highly pressurized containers and many steps for compression that make the process expensive and create the potential danger of explosions or leakage. Hence, a simpler procedure to store gases that do not require pressurized systems is desired. Also, due to the growing concern regarding large emission of greenhouse gases, it is considered important to introduce an efficient technology to address this situation. MOFs can be used as efficient nanocontainers for many gases since, by adjusting the synthesis procedure their properties can be tuned to adsorb different gases in MOFs. For example, for absorption of CO2...
