Abstract
Metal-organic frameworks (MOFs) are versatile materials that are of interest due to their application and properties. MOFs are highly crystalline and porous materials; they are composed of organic bridging ligands, acting as linkers, and a three-dimensional (3D) network of metal ions that are secondary building units. Since the MOFs have a high surface area, high porosity, tunable topography, and their structures are quite diverse, these materials are used in process of separation/purification, gas/energy storage, drug delivery, catalysis, and chemical sensors. Since the MOFs can be modified to selectively adsorb chemical species, they can be used as sensitive layer for modification of sensors. This process allows the sensor to detect the target analyte. Quartz crystal microbalances (QCMs) are highly sensitive mass sensors. In this chapter, the authors review the literature related to QCMs modified with MOFs. In particular, the relationship between target analyte, class of MOF, and instrument used for measurement of frequency variations.
TopIntroduction
The importance of a material comes from its functionality for solving a particular problem. The emergence of novel materials that can help to remedy certain issues requires processes, where it is necessary to have a good control of the final features of material, namely, textural, structural and chemical properties. For many applications as catalysis, sensing, and adsorption, porosity is a key point to development of good devices. In this sense, zeolites had been used as starting point for the development of new ideas and improvements. In this context, advancements starting from zeolites had been good, but such an approach presents some limitations due to its inorganic nature. The above, open the way for the study of a new kind of materials which combine of zeolitic features and the functionality brought from the inclusion of different organic species to the system. Therefore metal‑organic frameworks are particularly suitable for that. Metal-organic frameworks (MOFs) stand for a new kind of crystalline porous materials also defined in a more general way as “coordination polymer”, “coordination framework”, “metallosupramolecular network” and “hybrid materials” (Batten et al., 2013; Kepert 2010). The MOFs structures are constructed by the linkage of metal ions or ion cluster through organic ligands to form different dimensional structures, namely, one‑dimensional (1D), two-dimensional (2D) or three-dimensional (3D) architectures (Bataille et al., 2012; Xia et al., 2017; Chen et al., 2013). The use of MOFs is as ancient as the use of Prussian blue (more than 300 years), but the study of coordination polymers or MOFs as such is reported during the forties (Griffith, 1943) and the big contribution to the field was by the work of Robson and co-workers, who introduce the “node-and-spacer” approach (Hoskins et al., 1990), Kitagawa et al. (Kitagawa et al., 1991) and Yaghi et al. (Yaghi et al., 1995). The combination of metal ions or cluster ions with organic compounds resulting in crystalline structures opens a wide range of possibilities of features such as structure, size and shape of pore of final product. The MOFs, as well as zeolites, display very good features such as high adsorption capacities, big surface area, and pore volume. Although, the MOFs porosity is better than displayed by zeolites, since those are constructed from tethaedral units which can give rise to a finite number of Secondary building units (SBU), while MOFs can form SBUs from metal atom or cluster and one-, two- or three-dimensional extended inorganic substructures (Butova, 2016) which give rise to a wide range of possibilities with regard of topologies of the final material.
Nomenclature
As was already mentioned MOF is the abbreviation generally used to refer to this kind of crystalline materials. When the abbreviation MOF is followed by a number it represents a specific framework, as examples: MOF-74, MOF-177, MOF-253. When a structure has the same symmetry that other MOF but different composition is known as an isoreticular framework and is denoted adding the prefix IR to the MOF. In the same way, some MOFs are denoted with the letters of the place where they were discovered, as is the case of MIL, HKUST or UiO (Barthelet et al., 2002; Chui et al., 1999; Kandiah et al., 2010) which correspond to Materials of Institut Lavoisier, Hong Kong University of Science and Technology, and Universitetet i Oslo, respectively. In this regard, some efforts have been made to develop a more standardized nomenclature for all kind of synthesized MOF (O’Keeffe et al., 2008; Tranchemontagne et al., 2009) which could minimize the problem of give the correct name to new structures synthesized in the future.
Key Terms in this Chapter
Frequency Shift: This is the variation of the frequency of a signal. Usually, this term is used when a sensor with frequency output has a frequency variation after it is stimulated.
Sensitive Layer: This is the name of a thin layer of a material that is intended to interact with a specific chemical specie. Usually, this layer is grown or synthesized in a sensor, this process gives the sensor the ability to interact with a particular analyte.
QCM: Acronym of Quartz Crystal Microbalance. This is a sensor made of a thin quartz disk with a metal electrode on each side.
MOF: Acronym of Metal-Organic Framework. This is a crystalline material that is made of a metal center with an organic linker.
Analyte: A chemical substance that is of interest to detect.