**5. Cyclodextrin-based electrochemical sensors**

Owing to the chief and portable instrumentation, rapid analysis, and high selectivity, as well as sensitivity, in recent years, electrochemical sensing has engrossed a significant courtesy in the recognition of biomolecules and environmentally hazardous pollutants [51–53]. Cyclodextrin-based functional materials have proven to be highly useful in the domain of electrochemical sensing in past decade [54, 55]. These functional materials mainly include CD-based carbon nanomaterials: carbon nanotubes (CNTs), graphene, and conducting polymers. Nowadays, developing the CD-based conducting polymers for the purpose of electrochemical sensing is considered a hot subject of research interest [56]. This is due to the fact that CD-based conducting polymers pasted on electrodes *via* electrooxidation process of monomers offer high stability, good catalytic ability, and electronic features [57]. To this regard, Bouchta *et al.* have fabricated gold electrode with poly(3-methylthiophene)-based *γ*-CD through electropolymerization process for the electrochemical determination of dopamine, chlorpromazine, 3,4-dihydroxyphenyl alanine, etc. [58]. On the other hand, Luong and co-workers have doped a diamond electrode with boron and sulfobutylether functionalized *β*-CD along with a composite film of polypyrrole and poly (*N*-acetyltyramine) for the selective electrochemical determination of neurotransmitter dopamine among other interfering analytes, such as ascorbic acid and uric acid [59].

In the context of CD-based carbon nanomaterials, Huang's research group has modified glassy carbon electrode (GCE) by single-walled CNT (SWCNT) and pyrene functionalized *β*-CD (**42**) in order to determine the 3,3<sup>0</sup> ,4,4<sup>0</sup> -tetrachlorobiphenyl (**41**) *via* electrochemical impedance method (**Figure 10**). It was noticed by the authors that the pyrene moiety aids in attaching the **42** onto the SWCNT (**43**) sidewall through ππ-stacking interactions, and the guest molecule **41** gets encapsulated by the hydrophobic cavity of **42** [60]. Furthermore, they also reported the electrochemical sensing

#### **Figure 10.**

*Schematic illustration of* π-π*-stacking adsorption of pyrene functionalized* β*-CD (42) onto the side wall of SWCNT (43) along with the structure of 3,3*<sup>0</sup> *,4,4*<sup>0</sup> *-tetrachlorobiphenyl (41) guest molecule.*

of *p*-nitrophenol using same SWCNT (**43**)-based pyrene functionalized *β*-CD nanohybrids. The nanohybrid traps *p*-nitrophenol in the hydrophobic cavity of **42** with high selectivity and sensitivity with a detection limit of 0.00086 μM [61].

Recognition of chiral enantiomers *via* CD-based electrochemical sensors is of immense importance in the medical and pharmaceutical sciences [62]. Yang and coworkers have recently modified the surface of GCE by hydroxypropyl *β*-CD grafted cellulose, multi-walled CNTs (MWCNTs), and copper ions in order to develop a sensitive electrochemical sensor for the recognition of chiral enantiomers of tryptophan (*D*-Trp/*L*-Trp) [63]. It has been perceived that the fabricated electrochemical sensor has higher affinity toward *L*-Trp in comparison with the *D*-Trp (**Figure 11**). Additionally, the developed electrochemical sensor has been successfully utilized to monitor the quantity of *D*-Trp in racemic mixture. These studies thus pave the way toward the development of realistic chiral platforms for the recognition of diverse chiral molecules. On the other hand, a *β*-CD-based sensitive electrochemical sensor for the recognition of endocrine disrupting agent known as bisphenol A, in an aqueous solution, has been reported through the pasting of MWCNTs (**46**) and graphene oxide (**48**) on screen-printed carbon electrode (SPE) (**Figure 12**) [64]. This versatile system follows a diffusion-controlled mechanism in the sensitive electrochemical sensing of bisphenol A in drinking water with a detection limit of up to 6 nM. These studies thus offer a promising role in the determination of water quality *via* bisphenol A monitoring.

#### **Figure 11.**

*Schematic illustration of electrochemical recognition of the chiral enantiomers of tryptophan (Trp)* via *hydroxypropyl* β*-CD-based electrochemical sensor.*

### **Figure 12.**

*Schematic illustration for the development of bisphenol A electrochemical sensor* via *modification of SPE with* β*-CD and graphene oxide-functionalized MWCNTs.*

Over the past several years, diverse CD-based potentiometric sensors working through electrochemical means have fruitfully been developed, which find significant applications in medicine, agriculture, environmental monitoring, pharmaceutical

sciences, and industries [65, 66]. In this context, Lenik and teammates have developed functionalized *β*-CDs-based potentiometric sensors for the determination of useful pharmaceutical drugs known as naproxen and ketoprofen. It has been observed that the guest naproxen molecule is partially or completely encapsulated within the cavity of host functionalized *β*-CD [67, 68]. On the other hand, Amorim *et al.* have also used functionalized *β*-CDs in the fabrication of potentiometric sensors for psychiatric drug molecules *viz.* diazepam and midazolam [69]. On the other hand, Khaled's research group has fabricated carbon paste electrodes with *β*-CD based polyvinyl chloride in order to determine diverse acetylcholine derivatives *viz.* butrylcholine, acetylthiocholine, and acetylmethylcholine [70].
