**2. Chromophore-appended cyclodextrins as classical chemical sensors**

Among a variety of chemical sensors, the optical chemosensors are truly interesting and advantageous [17]. This can be ascribed to the fact that optical variations *viz.* color, absorption, and/or emission developing after the recognition of the targeted guest analyte by a typical host molecule are most of the time directly visible through naked eye. In these optical chemosensors, chromophores are attached with basic sensing scaffolds in order to utilize their optical variation, impending for the determination of successful recognition/sensing event [18].

Keeping in consideration, the fact that microenvironment of the utilized chromophores offer changes in color as well as fluorescence pattern, and to get optimum output, researchers globally have functionalized the CDs with a variety of dyes besides the fluorescent moieties [19, 20]. Using chromophore-appended CDs, the detection of a range of hydrophobic guest molecules inside the hydrophobic cavities has extensively been studied in recent years. However, a plethora of chromophore-appended CDs have been constructed and their sensing activities have also been accomplished by several research groups worldwide. But, the pioneering work in this field has been revealed by Ueno and teammates; they reported many CD-based fluorescent chemical sensors through the installation of diverse fluorophores (dansyl, pyrene, anthracene, *etc*.) in CDs *via* flexible linker [21, 22]. By means of this rigid spacer, no self-inclusion complex formation has been noticed. This in turn exposes the fluorophore to a hydrophilic environment and leads to the fluorescence quenching of the CDs. Consequently, the addition of hydrophobic guest leads to its inclusion in the hydrophobic cavity of CDs, thereby bringing the appended fluorophore to a more hydrophobic environment in comparison with the free state of CD-fluorophore conjugates. Hence, enhancement in the fluorescence leads to the "turn-on" fluorescence response (**Figure 2b**) [21].

On the other hand, the same group has also constructed various colorimetric indicator dyes (*viz.* phenolphthalein, methyl red, *p*-nitrophenol, alizarin yellow) appended CD-based chemosensors, wherein the dye moiety is included in the hydrophobic cavity of CDs and generates the self-inclusion complex well isolated from the exterior aqueous hydrophilic media [23–25]. In this self-inclusion complex state, the color changes of the appended dye moiety through protonation/deprotonationassisted pH variation are suppressed (**Figure 3**). The consequent addition of competitive hydrophobic guest molecule leads to the segregation of appended dye moiety from the interior of CD hydrophobic cavity to the exterior hydrophilic environment.

#### **Figure 2.**

*Pictorial representation of the turn-off (a) and turn-on (b) fluorescent CD-based chemical sensors developed by Ueno and co-workers.*

#### **Figure 3.**

*Schematic representation of the* p*-methyl red appended CD chemical sensor.*

In this manner, the appended dye moiety displays normal color variations upon changing the pH through the protonation or deprotonation tactic (**Figure 3**) [26, 27].

Sulfur dioxide is widely used as a preservative and antioxidant in the food and beverage industries. Thus, constructing sulfur dioxide sensors is of utmost significance in food and analytical chemistry. In this regard, Levine and co-workers have *Cyclodextrin-Based Sensors for the Recognition of Small Molecules DOI: http://dx.doi.org/10.5772/intechopen.108500*

**Figure 4.**

*Schematic depiction of chemical reaction involved in the attachment of* β*-CD (2) with Whatman filter paper.*

modified the Whatman filter paper with *β*-CD (**2**) and manganese in order to develop a colorimetric sensor for sulfur dioxide in an aqueous solution (**Figure 4**) [28]. It has been revealed that the developed sensor is sensitive (limit of detection up to 33 ppm), practical, and broadly applicable in the rapid detection of sulfur dioxide *via* naked eye color change. Besides, the redox reaction of the manganese has been found responsible for the perceived naked eye color variations and other UV-Vis spectral variations. For practical applications, these studies pave the way toward the construction of CD-based novel sulfur dioxide sensors for their employment in beverage and food industries.
