3.2.2. Grafting of Cyclodextrins onto Textile Substrates

The most common procedure in the application of cyclodextrin into textiles is esterification, which can be done using modified cyclodextrins (Figure 5), or through a reaction using dimethylol urea [128], citric acid [111], BTCA [78,129], or other products.

Esterification can be defined as a nucleophilic substitution reaction of the acyl group catalyzed by a mineral acid, involving a carboxylic acid and an alcohol [130]. From there, a proton transitions from one oxygen to another, resulting in a second tetrahedral intermediate, and converts the -OH group into a leaving group, culminating in the loss of a proton that regenerates the acid catalyst, originating the ester [131].

Figure 6 shows the procedure for incorporating MCT-β-CD into cellulosic fiber. The interaction occurs due to the availability of the chlorine group present in MCT-β-CD and the hydroxyl group of cellulose, thus representing a second order nucleophilic substitution reaction [132].

MCT-β-CD is fixed on cellulosic fibers in alkaline conditions and, due to the covalent bond between the cellulosic chain and MCT-β-CD, the durability of β-CD in textile products is excellent [23,133].

Ibrahim et al. [134] also used MCT-β-CD for the functionalization of wool by a method of fixation in foularding. Due to the presence of -OH groups in the protein, it is also possible to perform nucleophilic substitution. As with polyamide fabrics and polyester/cotton blends, this β-CD derivative has also been grafted, making the fabric antibacterial and a receptor for drugs and essential oils, in addition to improving thermal stability and dyeability [128,130].

The MCT compound was also used to make polyester a functional fabric, made from alkaline hydrolysis, which created reactive hydroxyl groups on the surface of the polyester fibers able to react with MCT-β-CD covalently [39]. From the interaction with the cyclodextrins, the modified polyester can adsorb bioactive molecules [112].

Cyclodextrin compounds treated with itaconic anhydride can bind to cellulosic and polyamide fibers. In the case of cellulosic fibers, the fabric must be treated with a mixture of nitric acid (1%) and cholic ammonium nitrate to generate free radicals and, after drying, the cotton is treated with a derivative of CD itaconate, which is able to covalently bond to cellulosic fibers [5,122].

In addition to the processes using modified cyclodextrins, esterification between cyclodextrins and textile fibers can be achieved. In this case, the esterification reaction requires a crosslinking agent such as citric acid, BTCA, or other polycarboxylic acids [135]. The disadvantage of using citric acid is the yellowing of the cellulosic fabric in the curing phase [136]. This process includes two steps; in the first, a cyclic anhydride is formed between two groups of adjacent carboxylic acids and, in the second, the esterification reaction occurs between the acid anhydrides previously formed and the hydroxyl groups of the macromolecules of the fiber and of the cyclodextrins, to form ester bonds [23]. *Molecules* **2020**, *25*, x 12 of 29

**Figure 6.** Nucleophilic substitution reaction of MCT-β-CD with cellulose. **Figure 6.** Nucleophilic substitution reaction of MCT-β-CD with cellulose.

MCT-β-CD is fixed on cellulosic fibers in alkaline conditions and, due to the covalent bond between the cellulosic chain and MCT-β-CD, the durability of β-CD in textile products is excellent Figure 7 illustrates the bonding between CDs, through BTCA as a crosslinking agent, and -OH groups of fibers.

[23,133]. Ibrahim et al. [134] also used MCT-β-CD for the functionalization of wool by a method of fixation in foularding. Due to the presence of -OH groups in the protein, it is also possible to perform For the esterification reaction to occur, both sodium hypophosphite and the cure are used as catalysts [38]. The same process can be performed on other fibers that have -OH groups, such as cellulose, silk, polyamide, and wool [78].

nucleophilic substitution. As with polyamide fabrics and polyester/cotton blends, this β-CD derivative has also been grafted, making the fabric antibacterial and a receptor for drugs and essential oils, in addition to improving thermal stability and dyeability [128,130]. Regarding the insertion of cyclodextrins into polyester fibers, they can be functionalized by forming a network of CDs that cover the fiber, forming a reticulated coating between β-CD and BTCA through a polyesterification reaction [38,137].

The MCT compound was also used to make polyester a functional fabric, made from alkaline hydrolysis, which created reactive hydroxyl groups on the surface of the polyester fibers able to react with MCT-β-CD covalently [39]. From the interaction with the cyclodextrins, the modified polyester As shown in Figure 5d, the hydroxyl groups of the CD can be oxidized by enzymes, converting them into aldehyde groups, which are able to react with the amine groups of the wool fibers through a Schiff-based reaction [127]. Figure 8 shows this reaction.

can adsorb bioactive molecules [112]. Cyclodextrin compounds treated with itaconic anhydride can bind to cellulosic and polyamide fibers. In the case of cellulosic fibers, the fabric must be treated with a mixture of nitric acid (1%) and cholic ammonium nitrate to generate free radicals and, after drying, the cotton is treated with a In this way, the application of CDs in fibrous polymers occurs. The substrate undergoes a change at the surface that can transform it, in the future, into functionalized fabrics after the complexation of the bioactive molecules by the CDs present on the surface of the materials.

derivative of CD itaconate, which is able to covalently bond to cellulosic fibers [5,122].

first, a cyclic anhydride is formed between two groups of adjacent carboxylic acids and, in the second,

In addition to the processes using modified cyclodextrins, esterification between cyclodextrins

the esterification reaction occurs between the acid anhydrides previously formed and the hydroxyl

groups of the macromolecules of the fiber and of the cyclodextrins, to form ester bonds [23].

**Figure 7.** Direct connection of the β-CD to the textile fiber via crosslinking. **Figure 7.** Direct connection of the β-CD to the textile fiber via crosslinking.

For the esterification reaction to occur, both sodium hypophosphite and the cure are used as Table 3 shows some studies that used cyclodextrin for the functionalization of finished textiles.


catalysts [38]. The same process can be performed on other fibers that have -OH groups, such as cellulose, silk, polyamide, and wool [78]. **Table 3.** Studies that used cyclodextrin to graft finishes in textiles.


*Molecules* **2020**, *25*, x 14 of 29

**Table 3.** *Cont*.

**Figure 8.** Functionalization of wool fibers with β-CD after oxidation**. Figure 8.** Functionalization of wool fibers with β-CD after oxidation.

#### In this way, the application of CDs in fibrous polymers occurs. The substrate undergoes a change *3.3. New Trends in Textile Finishes Using Cyclodextrins*

Antimicrobial

Cotton

at the surface that can transform it, in the future, into functionalized fabrics after the complexation of the bioactive molecules by the CDs present on the surface of the materials. Table 3 shows some studies that used cyclodextrin for the functionalization of finished textiles. **Table 3.** Studies that used cyclodextrin to graft finishes in textiles. The use of citric acid as a reticulating agent was also a strategy adopted by Castriciano et al. [154] to design polypropylene fabric finished with hydroxypropyl β-CD. After complexation with tetra-anionic 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine (TPPS), the textile device was evaluated as a biocidal agent via antimicrobial Photodynamic Therapy (aPDT)—an alternative treatment to overcome

**Fiber Effect Active Molecule Reference**

Fragrance, antimicrobial Essential Oils [74,145]

Insect repellent Cypermethrin and Prallethrin [146]

Octenidine dihydrochloride [138]

ZnO, TiO2 and Ag nanoparticles [142]

Phenolic compounds [76,141]

Ketoconazole [115]

Miconazole nitrate [143]

Triclosan [144]

Silver [139,140]

Polyamide

Tencel

Polyester

Cotton and

Cotton, wool and

the drug resistance associated with the indiscriminate use of antibiotics. The base of aPDT is the irradiation of a photosensitizer (PS) in the presence of oxygen, to generate reactive oxygen species (ROS) which attack the microorganisms at the target site (Figure 9). The PP-CD/TPPS fabric, containing 0.022 ± 0.0019 mg cm−<sup>2</sup> of the TPPS, was capable of photokilling 99.98% of Gram-positive S. aureus, with low adhesion of bacteria to the textile. The aPDT approach was also used by Yao et al. [155] to develop biocidal materials based on beta cyclodextrins modified with hyaluronic acid (HA) for coating purposes. After the inclusion of PS methylene blue (MB), HA-CD/MB was tested against S. aureus, eradicating 99% of the bacteria at 0.53 ± 0.06 µg cm−<sup>2</sup> . The use of aPDT in textile finishing may represent a new class of smart textiles with high anti-microorganism potential. evaluated as a biocidal agent via antimicrobial Photodynamic Therapy (aPDT)—an alternative treatment to overcome the drug resistance associated with the indiscriminate use of antibiotics. The base of aPDT is the irradiation of a photosensitizer (PS) in the presence of oxygen, to generate reactive oxygen species (ROS) which attack the microorganisms at the target site (Figure 9). The PP-CD/TPPS fabric, containing 0.022 ± 0.0019 mg cm-2 of the TPPS, was capable of photokilling 99.98% of Grampositive S. aureus, with low adhesion of bacteria to the textile. The aPDT approach was also used by Yao et al. [155] to develop biocidal materials based on beta cyclodextrins modified with hyaluronic acid (HA) for coating purposes. After the inclusion of PS methylene blue (MB), HA-CD/MB was tested against S. aureus, eradicating 99% of the bacteria at 0.53 ± 0.06 μg cm-2 . The use of aPDT in textile finishing may represent a new class of smart textiles with high anti-microorganism potential.

anionic 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine (TPPS), the textile device was

Wool Insect repellent Citronella essential oil [78]

Polyester Insect repellent Citronella essential oil [38]

polyester Fragrance <sup>β</sup>-citronellol, camphor, menthol, cis-

*Molecules* **2020**, *25*, x 15 of 29

antioxidant properties Melatonin [77]

For coetaneous affections Hydrocortisone acetate [147]

Antibiotics Ciprofloxacin [148]

Sunscreen Octyl methoxycinnamate [149]

Antibiotics Ciprofloxacin [150]

Vanillin, benzoic acid and Iodine, *N,N*-diethyl-m-toluamide and dimethyl-phthalate

Curcumin [151]

4-tert-butylbenzoic acid [152]

jasmone and benzyl acetate [153]

[44]

Perfume, moisturize and UV-protect. 2-ethoxynaphtalene (neroline) [119]

Nocturnal regulation of sleep and

Fragrance, antimicrobial and insect repellent

Antimicrobial

*3.3. New Trends in Textile Finishes Using Cyclodextrins*

**Figure 9.** Finishing textiles with photodynamic potential. **Figure 9.** Finishing textiles with photodynamic potential.
