**2.2. Sea/island cross-section conjugation**

their use in nanofiber production, CDs and their derivatives have become popular in mem-

Recently, workers prepared electrospun nanofibers using chitosan and incorporated silver and iron nanoparticles for water disinfection processes. These nanofibers were later effectively modified using CDs and cellulose to increase their thermal and chemical stability [16]. Somewhere else, thermally and mechanically stable βCD/cellulose acetate nanofibers were synthesized using an environmentally benign procedure and used for enhanced antimicrobial treatment of water [17]. In membrane technology, Adams et al. utilized CD molecules as modifying agents for the preparation of polysulfone-polyurethane (PSF-PU) composite nano-

In this chapter, electrospun CD-based materials are discussed in view of water treatment, their properties and advantages toward improving current water treatment methods by removing EMPs in waste and treated water. We also critically investigate CD-based membrane techniques in terms of their production and characterization methods with focus placed on their application in water treatment. Other applications of these CD-based nanocomposites such as drug delivery, antimicrobial uses, biomedical uses, filtration, photocatalysis and environ-

Various methods are used in the production of nanofibers today. These include, among others, polymer blending, sea/island cross-section conjugation and electrospinning techniques [19, 20]. These technologies have several disadvantages that include sizes in the microscale instead of nanoscale and low tensile strength. They also form nonwoven sheets that need further treatment using organic solvents [21]. However, several researchers have described electrospinning as the best nanofiber fabrication method compared to the other methods. In

The polymer blend method is a method that uses two or more polymers to produce materials with superior properties [22]. This method is divided into three main categories, which are: miscible, immiscible and compatible polymer blending. To produce fibers with nanoscale diameters and uniform continuous length in large scale, this method is often coupled with the electrospinning method. In this way, blended polymer solutions can be electrospun to

Miscible polymer blends are characterized by a homogeneous morphology/mixture on the segment level; however, the local chain dynamic may exhibit different dependences on temperature and blend composition [25]. The presence of nanoheterogeneities has been observed in miscible polymer blends where Lodge and McLeish have described this as "self-concentration" [26]. This illustrates that high glass transition temperature components often have segmental dynamics much closer to the bulk blend, while the low glass transition temperature is closer to the pure component [25]. However, miscible polymers often have one glass transition

the next sections, we explore the various methods of nanofiber production.

filtration membranes, which were used for the removal of undesirable salts [18].

brane technology.

138 Cyclodextrin - A Versatile Ingredient

mental protection are covered.

**2.1. Polymer blend method**

**2. Fabrication methods of nanofibers**

produce fibers with desired properties [23, 24].

Sea or island cross-section conjugation is a type of a conjugate spinning method used to fabricate fibers with diameters of less than 1 μm with a predetermined component arrangement in its cross-section. Two polymer components of a conjugate type are elongated and extruded together from a spinneret. These polymers then combine in the back of a spinning nozzle. The produced conjugate fibers with two components are then split into filaments. This technique involves spinning a bicomponent filament consisting of polyester, polyethylene, nylon or polypropylene used as an island component and a polymer like polystyrene is used as a sea component. The fabric is then exposed to a solvent, thermal or mechanical treatment whereby the immiscible components separate as the polystyrene sea component dissolves in a solvent,

**Figure 3.** Schematic representation of immiscible polymer solution with varied concentrations of each polymer.

**Figure 4.** Illustration of the effects of compatibilizer or filler on compatible polymer blends.

which is a nonsolvent for the polyester island component after conventional processing into fibers. This results in individual polyester island filaments. The ratio of the two components in the ultrafine filament yarn, the shape and the number of the resulting individual segments can be varied depending on the design of the spinneret [20, 21].

### **2.3. Electrospinning technique**

The electrospinning technique is a versatile, flexible and cost-effective method for producing nanofibers. It has become very attractive and common in the synthesis of nanofibers for various applications. Electrospinning is often preferred over other methods since it readily produces nanofibers from a number of materials, which include polymers, ceramics, composites and semiconductors [35, 36]. Electrospun nanofibers can be easily modified to improve certain properties. This can be achieved during electrospinning or by posttreatment methods [36–38].

Electrospinning can produce nanofibers of long length, diversified composition, high surface area-to-volume ratio, uniform diameter, flexible surface functionalities and superior mechanical properties [39]. In electrospinning, nanofibers are formed as the polymer solution is stretched between two surface charges and as the solvent evaporates due to electrostatic repulsion forces [40]. During electrospinning, a polymer solution in a syringe is stretched to the collector in a cone shape (Taylor cone) under high voltage. The collector screen used can be either a stationery flat screen or a rotating drum collector. The type of collector can greatly influence the morphology of the nanofibers [41]. The diameter, morphology and distribution of electrospun nanofibers depend on the applied voltage, solution viscosity, tip to collector distance, temperature and flow rate. **Figure 5** shows a setup of an electrospinning instrument with a flat stationary collector. The setup consists of a high-voltage supply, polymer solution in a syringe and the collector screen [42].

*2.3.1. Mechanism for the formation of electrospun CD nanofibers and influencing factors*

optimization (b) of all parameters. Reproduced with permission from [43].

Electrospinning CDs into nanofibers is still a challenging task because of their small cyclic structure. However, electrospinning these glucose derivatives would result in nanofibrous mats with excellent properties such as high surface area-to-volume ratio and high possibilities of specific surface functionalization [44, 45]. A number of factors can affect the electrospinning mechanism of CDs. Besides the fact that these cyclic oligosaccharides cannot be easily stretched into nanofibers, factors such as solvent type, concentration, copolymers and compatibility play a critical role in the successful electrospinning of CD nanofibers. Fortunately, CDs are soluble in most organic solvents such as water, dimethylacetamide, dimethyl sulfoxide

**Figure 6.** Microscopic representation of electrospun nanofibers without proper optimization (a) and with proper

**Figure 5.** Schematic illustration of electrospinning setup with flat stationery collector. Reproduced with permission from [42].

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When the electrospinning parameters and polymer solution properties are not properly optimized, beaded fibers such as those depicted in **Figure 6(a)** can be obtained. On the other hand, when all the properties and parameters are precisely optimized, bead-free nanofibers such as those displayed in **Figure 6(b)** can be obtained [43]. In electrospinning, it is important to optimize all parameters including the polymer solution properties such as the concentration before spinning large quantities.

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which is a nonsolvent for the polyester island component after conventional processing into fibers. This results in individual polyester island filaments. The ratio of the two components in the ultrafine filament yarn, the shape and the number of the resulting individual segments

The electrospinning technique is a versatile, flexible and cost-effective method for producing nanofibers. It has become very attractive and common in the synthesis of nanofibers for various applications. Electrospinning is often preferred over other methods since it readily produces nanofibers from a number of materials, which include polymers, ceramics, composites and semiconductors [35, 36]. Electrospun nanofibers can be easily modified to improve certain properties. This can be achieved during electrospinning or by posttreatment methods

Electrospinning can produce nanofibers of long length, diversified composition, high surface area-to-volume ratio, uniform diameter, flexible surface functionalities and superior mechanical properties [39]. In electrospinning, nanofibers are formed as the polymer solution is stretched between two surface charges and as the solvent evaporates due to electrostatic repulsion forces [40]. During electrospinning, a polymer solution in a syringe is stretched to the collector in a cone shape (Taylor cone) under high voltage. The collector screen used can be either a stationery flat screen or a rotating drum collector. The type of collector can greatly influence the morphology of the nanofibers [41]. The diameter, morphology and distribution of electrospun nanofibers depend on the applied voltage, solution viscosity, tip to collector distance, temperature and flow rate. **Figure 5** shows a setup of an electrospinning instrument with a flat stationary collector. The setup consists of a high-voltage supply, polymer solution

When the electrospinning parameters and polymer solution properties are not properly optimized, beaded fibers such as those depicted in **Figure 6(a)** can be obtained. On the other hand, when all the properties and parameters are precisely optimized, bead-free nanofibers such as those displayed in **Figure 6(b)** can be obtained [43]. In electrospinning, it is important to optimize all parameters including the polymer solution properties such as the concentration

can be varied depending on the design of the spinneret [20, 21].

**Figure 4.** Illustration of the effects of compatibilizer or filler on compatible polymer blends.

**2.3. Electrospinning technique**

140 Cyclodextrin - A Versatile Ingredient

in a syringe and the collector screen [42].

before spinning large quantities.

[36–38].

**Figure 5.** Schematic illustration of electrospinning setup with flat stationery collector. Reproduced with permission from [42].

**Figure 6.** Microscopic representation of electrospun nanofibers without proper optimization (a) and with proper optimization (b) of all parameters. Reproduced with permission from [43].

#### *2.3.1. Mechanism for the formation of electrospun CD nanofibers and influencing factors*

Electrospinning CDs into nanofibers is still a challenging task because of their small cyclic structure. However, electrospinning these glucose derivatives would result in nanofibrous mats with excellent properties such as high surface area-to-volume ratio and high possibilities of specific surface functionalization [44, 45]. A number of factors can affect the electrospinning mechanism of CDs. Besides the fact that these cyclic oligosaccharides cannot be easily stretched into nanofibers, factors such as solvent type, concentration, copolymers and compatibility play a critical role in the successful electrospinning of CD nanofibers. Fortunately, CDs are soluble in most organic solvents such as water, dimethylacetamide, dimethyl sulfoxide and dimethylformamide, with water and DMF being the most used solvents [46, 47]. **Figure 7** demonstrates that indeed the type of solvent used to prepare the solution has a pivotal effect on the formation and surface roughness of nanofibers. The most important factors to look at when choosing a solvent include the conductivity, density, solubility with other solvents and viscosity.

When electrospinning CDs, concentration also plays a critical role. At high concentrations, cyclic molecules like CDs and phospholipids can form aggregates and have sufficient electrostatic and intermolecular interactions [44, 48, 49]. The aggregation and molecular interaction act as chain entanglements making the molecules overlap and entangle like polymers in dilute solutions. **Figure 8** clearly depicts the low- and high-concentration effects on electrospun phospholipids and CDs. When using copolymers with CDs, the compatibility and dissolution of the two should be checked since this will affect the intramolecular interactions of the two polymers as well as the formation and morphology of the ultimate nanofibers [36, 47, 50, 51].

#### *2.3.2. Polymer-free cyclodextrin nanofibers*

As highlighted earlier, electrospinning cyclic polymers such as CDs is very challenging. However, CDs can form aggregates in their concentrated solution via intermolecular hydrogen bonding and interactions resulting in chain entanglements making it possible to electrospin CDs [44, 52].

Celebioglu and Uyar reported the first successful electrospinning of carrier polymer-free CD nanofibers. In their report, highly concentrated solutions of methyl-βCD (140 and 160% w/v) were prepared in water and DMF. Electrospinning these solutions yielded nanofibers with diameter ranges of 20–100 and 100–1200 nm using water and DMF, respectively [44, 53]. At concentrations lower than 140% w/v, beaded nanofibers were obtained. They also reported another successful electrospinning of hydroxypropyl-βCD (HP-βCD) and HP-βCD inclusion complex with triclosan. Bead-free HP-βCD and HP-βCD/triclosan were obtained at higher concentrations of 160% w/v. **Figure 9** shows the SEM images of polymer-free HP-βCD and HP-γCD nanofibers dissolved in DMF and water [52]. Celebioglu used HP-βCD and HP-γCD for the entrapment of volatile organic compounds (VOCs), aniline and benzene [45]. The results indicated that CD nanofibers had better performances compared to powdered CDs, while βCD nanofibers performed better than γCD nanofibers. The performance was dependent on the type of CDs, solvent and VOC type. Therefore, electrospinning of CDs and CD derivative nanofibers strongly depends on the type of CDs, solution concentration, solvent used and intermolecular interactions [46]. In **Table 1**, we show electrospun CD nanofibers prepared using different types of CD derivatives in different solvents without an additional

**Figure 8.** SEM micrographs revealing the concentration effect on electrospun phospholipids (a 35% and b 50%) and CD

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nanofibers (c and d) at low and high concentrations. Reproduced with permission from [49].

In order to improve the propensity of electrospinning CDs into excellent nanofibrous mats and take advantage of the CD properties, CDs can be blended with other polymers such as polyethylene terephthalate (PET), polyvinyl alcohol (PVA), polycaprolactone (PCL), poly(methyl methacrylate) (PMMA), polyvinylpyrrolidone (PVP), polylactic acid (PLA) and cellulose. This

polymer being used and the effect on the fiber sizes thereof.

*2.3.3. Copolymerized cyclodextrin nanofibers*

**Figure 7.** (A) AFM image and (B) fiber axis cross-section profile of the MβCD nanofiber prepared in water. (C) AFM image and (D) fiber axis cross-section profile of the MβCD nanofiber dissolved DMF. Reproduced with permission from [44].

**Figure 8.** SEM micrographs revealing the concentration effect on electrospun phospholipids (a 35% and b 50%) and CD nanofibers (c and d) at low and high concentrations. Reproduced with permission from [49].

Celebioglu and Uyar reported the first successful electrospinning of carrier polymer-free CD nanofibers. In their report, highly concentrated solutions of methyl-βCD (140 and 160% w/v) were prepared in water and DMF. Electrospinning these solutions yielded nanofibers with diameter ranges of 20–100 and 100–1200 nm using water and DMF, respectively [44, 53]. At concentrations lower than 140% w/v, beaded nanofibers were obtained. They also reported another successful electrospinning of hydroxypropyl-βCD (HP-βCD) and HP-βCD inclusion complex with triclosan. Bead-free HP-βCD and HP-βCD/triclosan were obtained at higher concentrations of 160% w/v. **Figure 9** shows the SEM images of polymer-free HP-βCD and HP-γCD nanofibers dissolved in DMF and water [52]. Celebioglu used HP-βCD and HP-γCD for the entrapment of volatile organic compounds (VOCs), aniline and benzene [45]. The results indicated that CD nanofibers had better performances compared to powdered CDs, while βCD nanofibers performed better than γCD nanofibers. The performance was dependent on the type of CDs, solvent and VOC type. Therefore, electrospinning of CDs and CD derivative nanofibers strongly depends on the type of CDs, solution concentration, solvent used and intermolecular interactions [46]. In **Table 1**, we show electrospun CD nanofibers prepared using different types of CD derivatives in different solvents without an additional polymer being used and the effect on the fiber sizes thereof.

#### *2.3.3. Copolymerized cyclodextrin nanofibers*

and dimethylformamide, with water and DMF being the most used solvents [46, 47]. **Figure 7** demonstrates that indeed the type of solvent used to prepare the solution has a pivotal effect on the formation and surface roughness of nanofibers. The most important factors to look at when choosing a solvent include the conductivity, density, solubility with other solvents and

When electrospinning CDs, concentration also plays a critical role. At high concentrations, cyclic molecules like CDs and phospholipids can form aggregates and have sufficient electrostatic and intermolecular interactions [44, 48, 49]. The aggregation and molecular interaction act as chain entanglements making the molecules overlap and entangle like polymers in dilute solutions. **Figure 8** clearly depicts the low- and high-concentration effects on electrospun phospholipids and CDs. When using copolymers with CDs, the compatibility and dissolution of the two should be checked since this will affect the intramolecular interactions of the two polymers as well as the formation and morphology of the ultimate nanofibers

As highlighted earlier, electrospinning cyclic polymers such as CDs is very challenging. However, CDs can form aggregates in their concentrated solution via intermolecular hydrogen bonding and interactions resulting in chain entanglements making it possible to electro-

**Figure 7.** (A) AFM image and (B) fiber axis cross-section profile of the MβCD nanofiber prepared in water. (C) AFM image and (D) fiber axis cross-section profile of the MβCD nanofiber dissolved DMF. Reproduced with permission from [44].

viscosity.

142 Cyclodextrin - A Versatile Ingredient

[36, 47, 50, 51].

spin CDs [44, 52].

*2.3.2. Polymer-free cyclodextrin nanofibers*

In order to improve the propensity of electrospinning CDs into excellent nanofibrous mats and take advantage of the CD properties, CDs can be blended with other polymers such as polyethylene terephthalate (PET), polyvinyl alcohol (PVA), polycaprolactone (PCL), poly(methyl methacrylate) (PMMA), polyvinylpyrrolidone (PVP), polylactic acid (PLA) and cellulose. This

**Figure 9.** SEM images of (a-i) HP-βCD/powder, (a-ii) HP-βCD/water-nanofiber, (a-iii) HP-βCD/DMF-nanofiber, (b-i) HP-γCD/ powder, (b-ii) HP-γCD/water-nanofiber and (b-iii) HP-γCD/DMF-nanofiber. Reproduced with permission from [45].

successfully encapsulated organic vapors such as aniline and styrene. The organics were inclusion complexed by βCD cones in solution [46]. CD/polymer nanofibers have also been used as reducing and stabilizing agents for other nanoparticles. Celebioglu et al. demonstrated this when they used polyvinyl alcohol/HP-βCD (PVA/HP-βCD) nanofibers as reducing and stabilizing agents for Ag nanoparticles. In this case, PVA was used as a primary

**Figure 10.** SEM micrographs for electrospun (a) PS and (b) PS-βCD nanofibers. Reproduced with permission from [46].

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In water treatment, materials with high adsorption capacity are useful when it comes to the

as adsorbents for the removal of indigo carmine dye in wastewater. The nanofibers were found to have adsorption capacities of up to 495 mg/g and equilibrium was reached in less than 40 min, due to the presence of CDs [61]. Zhang et al. prepared composite nanofibrous membranes from PVA/βCDs using electrospinning for molecular entrapment of organics. It was found that these nanofibrous membranes could effectively and readily capture organic molecules. This was attributed to the inclusion complexation of organic molecules by the CDs. It was further suggested that these kinds of membranes can also be applied in areas such as drug delivery, separation/purification and electrochemical sensors, among

Research shows that copolymerized CD nanofibers and their derivatives find application in various areas due to the properties and advantages induced by the incorporation of CDs.

Blending electrospun CD nanofibers with nanomaterials results in unique properties from large surface area of nanofibers and excellent properties of the nanomaterials to specific structural and functional properties [67]. Nanomaterials supported on other materials have the disadvantage of low efficiency because of small interface surface available compared to powder photocatalysts. However, powdered nanomaterials cause secondary contamination with low recovery and require further treatment after usage. The high efficiency of powdered

**Table 2** shows various copolymerized CD nanofibrous mats and their applications.

/βCD nanofibers

removal of pollutants such as dyes. Teng et al. used mesoporous PVA/SiO<sup>2</sup>

agent, while HP-βCDs were used as secondary agents [56].

*2.3.4. CD nanofibers incorporated with nanoparticles*

others [62].


**Table 1.** Electrospun polymer-free CD nanofibers, conditions of preparation and size of nanofibers.

greatly improves the general properties of the nanofibers and expands their possible areas of application [19, 20, 50, 51, 56]. For example, **Figure 10** depicts electrospun CD-modified PS nanofibrous mats. Nanofibrous mats or membranes with high permeability are excellent candidates as microporous support substrates for thin film composite membranes with high flux and for application in microfiltration processes [57–60]. Copolymerization of cross-linked or modified CDs with other polymers reduces their solubility in water and makes them excellent candidates for water treatment applications.

Uyar and coworkers electrospun poly(methyl methacrylate) functionalized with CDs (PMMA/βCDs) for the treatment of organic vapors. It was found that PMMA/βCD nanofibers Cyclodextrin-Based Nanofibers and Membranes: Fabrication, Properties and Applications http://dx.doi.org/10.5772/intechopen.74737 145

**Figure 10.** SEM micrographs for electrospun (a) PS and (b) PS-βCD nanofibers. Reproduced with permission from [46].

successfully encapsulated organic vapors such as aniline and styrene. The organics were inclusion complexed by βCD cones in solution [46]. CD/polymer nanofibers have also been used as reducing and stabilizing agents for other nanoparticles. Celebioglu et al. demonstrated this when they used polyvinyl alcohol/HP-βCD (PVA/HP-βCD) nanofibers as reducing and stabilizing agents for Ag nanoparticles. In this case, PVA was used as a primary agent, while HP-βCDs were used as secondary agents [56].

In water treatment, materials with high adsorption capacity are useful when it comes to the removal of pollutants such as dyes. Teng et al. used mesoporous PVA/SiO<sup>2</sup> /βCD nanofibers as adsorbents for the removal of indigo carmine dye in wastewater. The nanofibers were found to have adsorption capacities of up to 495 mg/g and equilibrium was reached in less than 40 min, due to the presence of CDs [61]. Zhang et al. prepared composite nanofibrous membranes from PVA/βCDs using electrospinning for molecular entrapment of organics. It was found that these nanofibrous membranes could effectively and readily capture organic molecules. This was attributed to the inclusion complexation of organic molecules by the CDs. It was further suggested that these kinds of membranes can also be applied in areas such as drug delivery, separation/purification and electrochemical sensors, among others [62].

Research shows that copolymerized CD nanofibers and their derivatives find application in various areas due to the properties and advantages induced by the incorporation of CDs. **Table 2** shows various copolymerized CD nanofibrous mats and their applications.

### *2.3.4. CD nanofibers incorporated with nanoparticles*

greatly improves the general properties of the nanofibers and expands their possible areas of application [19, 20, 50, 51, 56]. For example, **Figure 10** depicts electrospun CD-modified PS nanofibrous mats. Nanofibrous mats or membranes with high permeability are excellent candidates as microporous support substrates for thin film composite membranes with high flux and for application in microfiltration processes [57–60]. Copolymerization of cross-linked or modified CDs with other polymers reduces their solubility in water and makes them excellent

**Figure 9.** SEM images of (a-i) HP-βCD/powder, (a-ii) HP-βCD/water-nanofiber, (a-iii) HP-βCD/DMF-nanofiber, (b-i) HP-γCD/ powder, (b-ii) HP-γCD/water-nanofiber and (b-iii) HP-γCD/DMF-nanofiber. Reproduced with permission from [45].

**w/v)**

Water, DMF and DMAc 100–160 Electrospinning 250–1860 [55]

HP-βCDs Water 50–70 Electrospinning 933–990 [54]

HP-βCDs Water 100–160 Electrospinning 200–1600 [52] M-βCDs Water and DMF 100–160 Electrospinning 20–1200 [44]

**Table 1.** Electrospun polymer-free CD nanofibers, conditions of preparation and size of nanofibers.

**Method Size (nm) Ref**

120–160 Electrospinning 80–940 [53]

Uyar and coworkers electrospun poly(methyl methacrylate) functionalized with CDs (PMMA/βCDs) for the treatment of organic vapors. It was found that PMMA/βCD nanofibers

candidates for water treatment applications.

**CD-type Solvent Conc. (%** 

DMSO

α and βCDs Water, DMF, DMAc and

M-βCDs, HP-βCDs and

144 Cyclodextrin - A Versatile Ingredient

HP-γCDs

Blending electrospun CD nanofibers with nanomaterials results in unique properties from large surface area of nanofibers and excellent properties of the nanomaterials to specific structural and functional properties [67]. Nanomaterials supported on other materials have the disadvantage of low efficiency because of small interface surface available compared to powder photocatalysts. However, powdered nanomaterials cause secondary contamination with low recovery and require further treatment after usage. The high efficiency of powdered


**Table 2.** Electrospun copolymerized CD nanofibers, solvent type, sizes and their applications.

nanomaterials is outweighed by the recovery, recyclability and reusability of supported nanomaterials [68]. **Figure 11** shows SEM and TEM images of electrospun HP-βCD containing Au nanoparticles which also shows the d-spacing of 0.235 nm between Au {111} planes (**Figure 11**) [69].

CD-TiO<sup>2</sup>

nanotubes. These CD-TiO<sup>2</sup>

**Figure 12.** SEM images illustrating the 3D nature of CD-directed TiO<sup>2</sup>

of organic materials [74–76].

Reproduced with permission from [73].

nanofiber catalysts have shown enhanced photodegradation

nanoparticles forming nanofibers (a) A small

Using CDs, Zhao and Chen have also demonstrated the preparation of ZnO nanofibers and multipetals (**Figure 13**). When analyzed, the ZnO nanomaterials were decorated with CD molecules [77]. In another study, CD-ZnO nanofibers were synthesized under mild conditions of thermal decomposition [78, 79]. In their study, zinc acetate was coated with CD molecules and ZnO synthesis took place within the CD molecules resulting in CD-ZnO nanofibers. Rakshit and Vasudevan prepared CD-ZnO fibers with high degradation performance of Nile red [80].

aggregate attached on the side of the nanofiber. (b) The aggregate smoothes and continues to grow into a new fiber.

**Figure 11.** (a) SEM and (b) TEM images of electrospun HP-βCD nanofibers incorporated with Au nanoparticles. Inset

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HR-TEM image of a single Au-nanoparticle indicating the d-spacing between the planes [69].

CDs can also act as electron donors and molecular recognition agents when incorporated with photocatalytic nanoparticles [76]. CD chemisorption onto photocatalytic nanomaterials improve their stability against aggregation and enhance their charge transfer reactions.

In all these studies, CD-ZnO nanofibers were prepared using self-assembly processes.

CD nanofibers incorporated with nanoparticles have been prepared via electrospinning and selfassembly. These two techniques produced hierarchical structures of nano- to macroscale catalysts such as CD-TiO<sup>2</sup> and CD-ZnO [70, 71]. CD-TiO<sup>2</sup> nanofibers prepared by Yoon and Nichols displayed a hierarchical structure of nanoparticles interconnected into a 3D network as demonstrated in **Figure 12**. The 3D structure was found to have high surface area and large porosity [72, 73]. CD-TiO<sup>2</sup> composites have been found to be stable over a wide range of pH. Somewhere else, it was found that CDs guide the assembly of TiO<sup>2</sup> nanomaterials into nanowires hosted in a CD nanotube structure. In this process, CD molecules coat TiO<sup>2</sup> nanomaterials and produce long Cyclodextrin-Based Nanofibers and Membranes: Fabrication, Properties and Applications http://dx.doi.org/10.5772/intechopen.74737 147

**CD type Copolymer Solvent Method Size** 

toluene

ethanol

PLA DMF, DMSO

Water and chloroacetic acid

and chloroform

PET TFA and DMF Electrospinning 12.4–

PS DMF Electrospinning 300–

PMMA DMF Electrospinning 625–

**Table 2.** Electrospun copolymerized CD nanofibers, solvent type, sizes and their applications.

and CD-ZnO [70, 71]. CD-TiO<sup>2</sup>

else, it was found that CDs guide the assembly of TiO<sup>2</sup>

CD nanotube structure. In this process, CD molecules coat TiO<sup>2</sup>

βCDs PMMA DMF and

βCDs PVP Water and

βCDs Carbonaceous nanofiber membrane

146 Cyclodextrin - A Versatile Ingredient

βCDs Chitosan and PVA

α, β and γCDs

α, β and γCDmenthol IC

α, β and γCDs

α, β and γCDs

α, β and γCDs

(**Figure 11**) [69].

lysts such as CD-TiO<sup>2</sup>

[72, 73]. CD-TiO<sup>2</sup>

HP-βCDs PVA Deionized water Electrospinning 290–485 Reducing and stabilizing

βCDs PCL DMF and DCM Electrospinning 336–389 Drug delivery of naproxen [50]

βCDs PVC DMF and THF Electrospinning 420–450 Membrane modification [19]

βCDs Cellulose DMF Electrospinning 100–800 Antibacterial activity (*E. coli*

Coprecipitation and electrospinning

nanomaterials is outweighed by the recovery, recyclability and reusability of supported nanomaterials [68]. **Figure 11** shows SEM and TEM images of electrospun HP-βCD containing Au nanoparticles which also shows the d-spacing of 0.235 nm between Au {111} planes

CD nanofibers incorporated with nanoparticles have been prepared via electrospinning and selfassembly. These two techniques produced hierarchical structures of nano- to macroscale cata-

displayed a hierarchical structure of nanoparticles interconnected into a 3D network as demonstrated in **Figure 12**. The 3D structure was found to have high surface area and large porosity

composites have been found to be stable over a wide range of pH. Somewhere

Zein DMF Electrospinning 90–185 — [5]

Water Electrospinning 120–144 Membrane filtration of

**(nm)**

Electrospinning 625–977 Organic vapor waste

Electrospinning 450 Stabilizing and reducing

15.3

4250

140– 1580

1024

Electrospinning 84–285 Drug delivery [64]

**Application Ref.**

[56]

[51]

[63]

[20]

[47]

[7]

[65]

[66]

[9]

agent for Ag antibacterial

nanoparticles

treatment

phenolphthalein

and *S. aureus*)

agent for Au nanoparticles

Phenanthrene removal in aqueous solutions

Enhancement of durability and stability of fragrances

Antibacterial growth (*E. coli*

Molecular filters and water

nanofibers prepared by Yoon and Nichols

nanomaterials into nanowires hosted in a

nanomaterials and produce long

and *S. aureus*)

treatment

**Figure 11.** (a) SEM and (b) TEM images of electrospun HP-βCD nanofibers incorporated with Au nanoparticles. Inset HR-TEM image of a single Au-nanoparticle indicating the d-spacing between the planes [69].

**Figure 12.** SEM images illustrating the 3D nature of CD-directed TiO<sup>2</sup> nanoparticles forming nanofibers (a) A small aggregate attached on the side of the nanofiber. (b) The aggregate smoothes and continues to grow into a new fiber. Reproduced with permission from [73].

CD-TiO<sup>2</sup> nanotubes. These CD-TiO<sup>2</sup> nanofiber catalysts have shown enhanced photodegradation of organic materials [74–76].

Using CDs, Zhao and Chen have also demonstrated the preparation of ZnO nanofibers and multipetals (**Figure 13**). When analyzed, the ZnO nanomaterials were decorated with CD molecules [77]. In another study, CD-ZnO nanofibers were synthesized under mild conditions of thermal decomposition [78, 79]. In their study, zinc acetate was coated with CD molecules and ZnO synthesis took place within the CD molecules resulting in CD-ZnO nanofibers. Rakshit and Vasudevan prepared CD-ZnO fibers with high degradation performance of Nile red [80]. In all these studies, CD-ZnO nanofibers were prepared using self-assembly processes.

CDs can also act as electron donors and molecular recognition agents when incorporated with photocatalytic nanoparticles [76]. CD chemisorption onto photocatalytic nanomaterials improve their stability against aggregation and enhance their charge transfer reactions.

functionalities, which greatly improve the hydrophilicity and permeability of membranes [86]. Mixed matrix membranes (MMMs) and thin film composite (TFC) membranes are two types of membranes where CDs and their derivatives have been used as modifying agents to

Cyclodextrin-Based Nanofibers and Membranes: Fabrication, Properties and Applications

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Mixed matrix membranes (MMMs) are known for their high flux and low pressure drops. MMM high flux capacity and selectivity are often a result of functionalized modifying agents [87]. MMMs are used mostly for the removal of heavy metals, natural organic matter (NOM), EMPs and disinfection by products such as trihalomethanes, haloacetic acids, trihaloacetaldehydes, haloacetones and trihalonitromethanes in water [88]. Adams et al. prepared MMMs using polysulfone/βCD-polyurethane (PSf/βCD/PU) for the selective removal of Cd2+ ions and improved structural properties of PSf MMMs. Upon studying their characteristics, it was found that βCD-polyurethane enhanced the water sorption and hydrophilicity and achieved 70% removal of Cd2+ ions [18]. Adams et al. used the same material (PSf/βCD/PU) in 2014 to study the effect of βCD/PU on the rejection of NOM and fouling resistance of PSf MMMs. It was concluded that βCD/PU improved the effective pore sizes and molecular-weight cut-off of PSf membranes due to their conical structure and larger pore sizes, which allows water molecules to pass easily [89]. Other workers used ceramic membranes modified with cross-linked silylated dendritic polymers and CDs for the removal of organic pollutants in water. The modified membranes removed pollutants such as monocyclic aromatic hydrocarbons (93%), pesticide (43%), polycyclic aromatic hydrocarbons (99%) and trihalogen methanes (81%). The high removal percentage was attributed to the dendritic polymers and CDs [90]. **Figure 14** shows

**Figure 14.** SEM images comparing morphologies for (a) PSf and (b) PSf/βCD outside surface and cross-section.

improve their total performance.

**3.1. Mixed matrix membranes**

Reproduced with permission from [91].

**Figure 13.** Micrographs showing multipetals of (a–c) ZnO nanomaterials prepared with βCD and (d) ZnO nanofibers prepared without βCD. Reproduced with permission from [77].
