Abstract
Different metal ions and anions are crucial for the life of organisms, but their excessive intake or deficiency cause various physiological abnormalities or diseases. Toxic metal ions such as Hg2+, Cr6+, Cd2+, explosive nitroaromatic compounds, and volatile organic compounds are widely used in agriculture, rocket fuels, and industry, but they cause severe effects on human health and environmental change. Hence, sensing and identification of these analytes are important for life, health, and environment protection. Recently, metal-organic frameworks (MOFs) have been extensively explored as luminescent sensors due to their unique crystallinity, structural diversity, tunable porosity, and functional groups. This chapter reviewed the origin of luminescent of MOFs and its applications as luminescent sensors for detecting different environmentally toxic analytes such as metal ions, anions, nitroexplosives, and volatile organic compounds. In addition, the detection mechanism of MOFs and its structural dependent properties are also illustrated.
TopIntroduction
Metal-Organic frameworks (MOFs) are class of highly porous crystalline solids constructed from metal ions (primary building unit or PBU) or metal clusters (secondary building unit or SBU) and organic ligands (organic linkers) via coordination bonds, Figure 1a (Keeffe & Yaghi, 2012). Depending on the binding sites of the organic ligands, the MOFs may be arranged themself to one, two or three dimensional networks (Tranchemontagn et al., 2009). Owing to its inherent tunable pores and high porosity, the MOFs have been successfully applied in gas storage and separation, heterogeneous catalysis, drug delivery, sensing etc (Collin & Zhou, 20017; Li et al., 2011; Lee et al., 2009; Hu et al., 2014; Horcajada et al., 2010).
Figure 1. Schematic illustration of formation of metal-organic frameworks, assembled from organic linkers and metal ions
With the increasing population and industry development, environmental pollution has become a serious problem for the ecosystem and public health. Many types of pollutants, e.g., water and air pollution of metal ions, anions, nitroexplosives and organic compounds are associated with health risks (Lustig et al., 2017). Hence, sensing and identifying these analytes are important for life, health, environment protection and industry. Chromatography and its coupled techniques are widely used methods for determining environmentally toxic analytes (Petrovic et al., 2010; Nuijs et al., 2011). However, chromatography-based techniques require costly equipment, complex samples preparation and time consuming. Thus, new sensing techniques, which possess the advantages of high sensitivity, rapid detection and suitability for onsite, real-time, and continuous monitoring of environmental toxic analytes, are necessary. Luminescent materials possess all these requirements. Hence, luminescent materials can be used for detecting environmentally toxic analytes.
Key Terms in this Chapter
Analytes: Cations, anions, nitro, and organic compounds that is of interest in an analytical procedure due to their toxicity.
Fluorescence Resonance Energy Transfer: It is an effective energy transfer process between chemical species and sensor due to their significant overlap of absorption and fluorescence spectra which assist in detecting analytes.
Adsorption: It is processes of removal of toxic analytes from solution with metal organic frameworks by attaching to their pore or surface via host-guest interaction.
Photoinduced Electron Transfer: It is a process that involves the transfer of electron between metal organic frameworks and analytes on excitation with suitable light source, changing their optical properties.
Metal-Organic Frameworks: It is a class of highly porous materials composed of metal ions/metal clusters and organic linkers/organic ligands that may be one, two- or three-dimensional networks.
Fluorescence Quenching and Enhancement: Emissive properties of the metal organic frameworks decreases or increases in the presence of analytes such as metal ions, anions, nitro explosives, and organic compounds, etc.
Luminescence: It is the emission of light after absorption of energy from suitable source such as x-rays, electron beam, ultraviolet light (UV). It has two types of emission, fluorescence and phosphorescence.
Environmental Pollution: It is defined as contamination of various components of earth which possesses a serious problem for the ecosystem and public health.