Anions are prevalent in nature and have important roles in many biological,
medical, industrial, and environmental processes. These processes lead to the release of
anions in the environment, which act as pollutants at higher concentrations. The proper
management of these anions requires adequate detection techniques. Anion sensing, a
branch of supramolecular chemistry, deals with chemosensors that are capable of
selective recognition and detection of anions through optical or electrochemical
response. Further, these compounds are also used for the construction of sensory
devices and the extraction and separation of anions. Chemosensors are very useful for
the detection of potentially toxic (e.g., fluoride, cyanide) and environmentally
hazardous (e.g., phosphate, nitrate) anions as well as in medical diagnostics.
Consequently, anion sensing has become one of the most active areas of
supramolecular chemistry. The design and synthesis of anion-selective receptors and
sensors are challenging, as compared to cation counterparts, due to their different sizes,
shapes, high hydration energies, and pH-dependent properties. Three approaches have
been used for the detection of anions by chemosensors viz. binding site-signalling
subunit approach, displacement approach, and chemodosimeter approach. This chapter
focuses on small molecular optical chemosensors and the mechanisms adopted for the
detection of anions.
Keywords: Anion-π interaction, Calix [4] pyrrole, Chemodosimeter,
Colourimetric, Displacement approach, Electrostatic interaction, Fluorescence
sensors, Halogen-bonding, Hydrogen-bonding, Hydrophobic interaction, Lewis
acid, Metal complexes, Molecular assembly, Naked-eye detection, Non-covalent
interactions, Optical sensor, Recognition, Self-assembly.