**3. Metallocyclodextrin-based chemical sensors**

The metallocyclodextrin-based CDs have been developed by various research groups and utilized in the field of chemical sensors [29, 30]. To this line, ligands consisting of the metal binding sites, for example, crown ether, diethylenetriaminepentaacetate (DTPA), and ethylenediaminetetraacetate (EDTA), have successfully been reported. Noticeably, among the various metal ions, lanthanide metal ions (Eu2+ & Tb2+) are primarily used in the fabrication of metallocyclodextrin-based chemical sensors by virtue of the fact that they exhibit strong fluorescence and also showed the longer lifetimes [31]. Out of various sensing mechanisms, absorption energy transfer emission (AETE) has been found responsible for the sensing of metallocyclodextrins. This sensing mechanism preliminary involves the excitation of light harvesting guest molecule *via* absorption of photon energy followed by the transfer of energy to a photoactive metal ion (Eu2+/Tb2+) and subsequent emission from these metal ions. It has been revealed that the complexes of Eu2+ and Tb2+ ions with the appended CDs *viz.* crown ether-CD (**19**) and DTPA-CD (**20**) conjugate display slight fluorescence in aqueous solution due to dearth of aromatic hydrocarbons acting as light-harvesting groups (**Figure 5**) [32]. However, the addition of aromatic guest/light-harvesting molecules, such as benzene, toluene, and biphenyl, leads to the inclusion of these molecules into the inner hydrophobic cavity of metallocyclodextrins, thereby displaying fluorescence enhancement *via* AETE, and, hence, offers a unique approach to develop the turn-on fluorescent chemical sensors (**Figure 5**). On the other hand, Reinhoudt and teammates have constructed *β*-CD dimer (**22**) in which two *β*-CD units

#### **Figure 5.**

*Diagrammatic illustration of crown ether appended CD (19), DTPA appended CD (20), and EDTA linked* β*-CD dimer (22). Moreover, the mechanistic overview of AETE sensing phenomenon involving light harvesting guest molecules is also shown.*

are linked through EDTA (**Figure 5**) [33]. From the experimental studies, it has been noticed that the complex formation occurs between **22** and lanthanide metal ions, which upon the addition of biphenyl-linked adamantane dimer results in the inclusion of adamantyl ends in the hydrophobic cavities of *β*-CD dimer (**22**). Consequently, AETE has been noticed from the biphenyl group of adamantane dimer to the lanthanide metal ion complexed with *β*-CD dimer (**22**). In this way, the overall lanthanide metallocyclodextrin-based assembly functions like a turn-on fluorescent chemical sensor. Additionally, polypyridine, as well as hepta bipyridine, appended CDs forms the complexes with lanthanide metal ions and acts as the chemical sensors toward targeted guest molecules *via* AETE sensing phenomenon [34, 35].

Liu *et al.* have studied the transition metal cation ligand-appended CDs as fluorescent chemical sensors [36]. They have synthesized *β*-CD dimer (**23**) in which two *β*-CD units are joined through the biquinolino subunits (**Figure 6**), and by virtue of this group, *β*-CD dimer (**23**) forms a complex with Cu(II) transition metal ion. Upon the resulting addition of steroid guest molecule, a 1:1 sandwich-type inclusion complex was formed, displaying the enhancement in fluorescence and hence acting as an efficient fluorescent chemical sensor. Moreover, the same group has also synthesized quinoline functionalized *β*-CD-based selective fluorescent sensor (**24**) for Zn(II) ion, among several other interfering metal ions, such as Ca(II) and Mg(II) (**Figure 6**). For real-world uses, the sensor (**24**) might prove highly valuable as an imaging agent for Zn(II) in living cells or tissues [37]. In a separate report, Yang *et al.* have established a selective and sensitive *β*-CD-based fluorescent sensor (**25**) for the recognition of Zn

#### **Figure 6.**

*General structures of* β*-CD dimer (23) connected by the biquinolino group, quinoline functionalized* β*-CD sensor (24) for Zn(II) ion and alkylated* β*-CD/tetraphenylporphyrin-based 2:1 host-guest complex (25).*

(II) ion (**Figure 6**). The developed fluorescent sensor (**25**) is composed of alkylated *β*-CD and tetraphenylporphyrin units in the stoichiometric ratio of 2:1. In this case, fluorescent enhancement has been revealed upon the selective complexation of Zn(II) with *meso*-tetraphenylporphyrin among various other interfering metal ions in aqueous medium [38].
