**6. Cyclodextrin-based polymers as chemical sensors**

Due to widespread applicability of *π*-conjugated polymers in electroluminescence, light-emitting diodes (LEDs), electrical conductivity, and chemical sensors, researchers are curious worldwide to explore fine-tuning of their electrical and optical properties by virtue of stimuli, such as pH, metal ion, and redox reactions [71]. In this context, Harada's research group has constructed *β*-CD functionalized poly(phenylene ethynylene)-based *π*-conjugated fluorescent polymer (**49**), which is water soluble and displays blue fluorescence in DMF and green fluorescence in aqueous solutions (**Figure 13**) [72]. Upon the addition of a competitive guest molecule known as 1-adamantanecarboxylic acid (**51**) to **49**-based intermolecular aggregates (**50**), fluorescence color variation from green to blue was observed by the authors (**Figure 13**). This can be ascribed to the fact that 1-adamantanecarboxylic acid (**51**) complexation with *β*-CD units of **49** results in the dissociation of various intermolecular *π*-stacking interactions of polymeric backbone. In fact, the repulsive interactions between the anionic counterparts of 1-adamantanecarboxylic acid (**51**) hinder the polymeric chains to come into the aggregation. By adding electron-accepting adamantane-functionalized viologen derivative (**53**) to the *π*-conjugated fluorescent polymer (**49**), large fluorescence quenching was seen due to the formation of inclusion complex between *β*-CD moiety of polymer (**49**) and adamantane group of viologen derivative (**53**). Further, the host-guest interaction, assisted in upholding viologens on the polymeric side chain, results in an adept electron transfer between polymeric backbone to viologen unit of **54** (**Figure 14**).

As coumarin and pyrene scaffolds are of great importance, owing to their vital role in biological systems and sensing arena as well [73]. In this regard, Ueno's group has synthesized *β*-CD-peptide hybrid polymeric conjugate (**55**), having pyrene as a donor moiety and coumarin as an acceptor one (**Figure 15**) [74, 75]. The coumarin moiety is encapsulated within the hydrophobic cavity of *β*-CD and thus offers strong fluorescence to the hybrid polymeric conjugate (**55**) *via* fluorescence resonance energy transfer (FRET) from donor pyrene unit to acceptor coumarin unit ("FRET-ON" response). It has been remarked that the addition of competitive guest molecule, namely hyodeoxycholic acid (**56**), leads to the decrease in fluorescence by virtue of the exclusion of coumarin moiety from inside of the *β*-CD hydrophobic cavity to outside. This, in turn, leads to the association between coumarin and pyrene units and offers the "FRET-OFF" response (**Figure 15**) [74]. Inouye and Fujimoto have developed methylated *β*-CD-DNA hybrid polymeric conjugate (**58**) sensor for porphyrin derivatives (**Figure 16**) [76]. They observed that **58** captures

### **Figure 13.**

*Schematic illustration of fluorescence color variation from green to blue upon addition of 1-adamantanecarboxylic acid (51) to the intermolecular aggregates (50).*

*meso*-tetraphenylporphyrin sulfonate (**59**) in a 2:1 stoichiometric ratio. This, in turn, induces the formation of DNA duplex structure and results in excimer emission (**Figure 16**).

On the other hand, quite recently, Badiei and co-workers have established a *β*-CDbased cross-linked polymer, the CD-nanosponge (**62**) with pyromellitic anhydride (**61**) cross-linker for the selective and sensitive detection of diclofenac among various other interfering analytes, such as ibuprofen, morphine, amphetamine, and codeine (**Figure 17**) [77]. For diclofenac, they have observed a detection limit of 0.92 μM and linear range of 133 μM. Interestingly for real-world applications, the established *β*-CD-based fluorescence probe has the utility to determine the concentration of diclofenac in commercially accessible pharmaceutical tablets.
