Chitosan-TiO2: A Versatile Hybrid Composite

In recent years, a strong interest has emerged in hybrid composites and their potential uses, especially in chitosan–titanium dioxide (CS–TiO2) composites, which have interesting technological properties and applications. This review describes the reported advantages and limitations of the functionalization of chitosan by adding TiO2 nanoparticles. Their effects on structural, textural, thermal, optical, mechanical, and vapor barrier properties and their biodegradability are also discussed. Evidence shows that the incorporation of TiO2 onto the CS matrix improves all the above properties in a dose-dependent manner. Nonetheless, the CS–TiO2 composite exhibits great potential applications including antimicrobial activity against bacteria and fungi; UV-barrier properties when it is used for packaging and textile purposes; environmental applications for removal of heavy metal ions and degradation of diverse water pollutants; biomedical applications as a wound-healing material, drug delivery system, or by the development of biosensors. Furthermore, no cytotoxic effects of CS–TiO2 have been reported on different cell lines, which supports their use for food and biomedical applications. Moreover, CS–TiO2 has also been used as an anti-corrosive material. However, the development of suitable protocols for CS–TiO2 composite preparation is mandatory for industrial-scale implementation.


Introduction
In recent years, the development of hybrid composites has increased considerably because they exhibit a wide range of applications, including photo-catalysis, water treatment, and antimicrobial properties [1,2]. Hybrid composites can be made by combining organic-organic (e.g., chitosan-alginate, chitosan-protein, and chitosan-starch), organic-inorganic (e.g., chitosan-TiO 2 , alginate-TiO 2 , and starch-TiO 2 ), and inorganic-inorganic (e.g., TiO 2 -ZnO, TiO 2 -MgO, and TiO 2 -Ag) compounds, and metal oxide; the coating method (Figure 1b), where the metal oxide is deposited in the polymeric surface matrix by electrochemical and/or physical deposition; and the nanocomposite method (Figure 1c), where the polymer and metal oxide are chemically bonded. Nonetheless, the metal oxide can act as crosslinking agent, reinforcing the mechanical properties of polymeric and/or film-forming solution ( Figure 1).
Materials 2020, 13, 811 3 of 28 components) where the technological properties are related with the mixing ratio of the polymer and metal oxide; the coating method (Figure 1b), where the metal oxide is deposited in the polymeric surface matrix by electrochemical and/or physical deposition; and the nanocomposite method ( Figure  1c), where the polymer and metal oxide are chemically bonded. Nonetheless, the metal oxide can act as crosslinking agent, reinforcing the mechanical properties of polymeric and/or film-forming solution ( Figure 1). TiO2 can be incorporated to different organic matrixes as corn and potato starches, carboxymethyl cellulose, sesame, whey proteins, wheat gluten, cellulose, poly-lactic acid, xanthan and gellan gums, and chitosan (alone or in combination) aimed to form hybrid composites with potential industrial applications, as shown in Table 1. Table 1. Application of some polymers and proteins functionalized with TiO2 nanoparticles.

Material
Application Ref.

Corn starch
Bioplastic with potential use as packaging material [23] Carboxymethyl cellulose containing miswak extract Nanocomposite with potential use as food packaging [24] Sesame protein extract Films for food active packaging applications and photo-decolorization purposes [25] Corn starch/PVA Potential application in food and non-food industries as UV shielding packaging materials [26] Chitosan/potato-starch Films with potential use as food packaging [27] Chitosan/starch Films with potential use as food packaging [28] Cellulose Removal of water pollutants [29] Bi2WO6-TiO2/corn starch Films with ethylene scavenging activity, potential use for fruits and vegetables preservation [30] KC/X/G Potential application in food and non-food industries as UV shielding packaging materials [31] Alginate Removal of water pollutants [32] Potato starch Films with potential use as food packaging [33] Amylose-halloysite composite Composite with potential environmental applications as wastewater treatment [34] Wheat/cellulose Films with antibacterial properties [11] Corn starch/PVA Nanocomposite with potential use as packaging material [35] Alginate Medical applications as tissue regenerator [36] Poly(L-lactic acid) Potential use as drug delivery system [37] Whey protein Biopolymer with potential use as packaging material [38] Hydrophilic polyurethane Films with antibacterial properties [39] PVA: Polyvinyl alcohol; KC/X/G: mixture of K-Carrageenan (KC), xanthan gum (X), gellan gum (G).
Amin et al. [23] reported that incorporating TiO2 into corn starch films improved their mechanical (tensile strength) and thermal (glass transition temperature) properties. According to Sreekumar et al. [35], the presence of TiO2 in corn starch films can act as a reinforcement agent, TiO 2 can be incorporated to different organic matrixes as corn and potato starches, carboxymethyl cellulose, sesame, whey proteins, wheat gluten, cellulose, poly-lactic acid, xanthan and gellan gums, and chitosan (alone or in combination) aimed to form hybrid composites with potential industrial applications, as shown in Table 1. Table 1. Application of some polymers and proteins functionalized with TiO 2 nanoparticles.

Material
Application Ref.
Corn starch Bioplastic with potential use as packaging material [23] Carboxymethyl cellulose containing miswak extract Nanocomposite with potential use as food packaging [24] Sesame protein extract Films for food active packaging applications and photo-decolorization purposes [25] Corn starch/PVA Potential application in food and non-food industries as UV shielding packaging materials [26] Chitosan/potato-starch Films with potential use as food packaging [27] Chitosan/starch Films with potential use as food packaging [28] Cellulose Removal of water pollutants [29] Bi 2 WO 6 -TiO 2 /corn starch Films with ethylene scavenging activity, potential use for fruits and vegetables preservation [30] KC/X/G Potential application in food and non-food industries as UV shielding packaging materials [31] Alginate Removal of water pollutants [32] Potato starch Films with potential use as food packaging [33] Amylose-halloysite composite Composite with potential environmental applications as wastewater treatment [34] Wheat/cellulose Films with antibacterial properties [11] Corn starch/PVA Nanocomposite with potential use as packaging material [35] Alginate Medical applications as tissue regenerator [36] Poly(L-lactic acid) Potential use as drug delivery system [37] Whey protein Biopolymer with potential use as packaging material [38] Hydrophilic polyurethane Films with antibacterial properties [39] PVA: Polyvinyl alcohol; KC/X/G: mixture of K-Carrageenan (KC), xanthan gum (X), gellan gum (G).
Amin et al. [23] reported that incorporating TiO 2 into corn starch films improved their mechanical (tensile strength) and thermal (glass transition temperature) properties. According to Sreekumar et al. [35], the presence of TiO 2 in corn starch films can act as a reinforcement agent, improving its mechanical (tensile strength) and physicochemical properties (degree of swelling in water). Similarly, a maize starch/PVA composite film with TiO 2 incorporated had an increase of 100% in the tensile strength and water vapor permeability (50% decrease), but these effects were dose-dependent with an optimum TiO 2 concentration of 0.05% wt.-the film also exhibited UV protection [26]. In potato starch films, the functional properties (tensile strength and elongation at break and moisture content) were improved by the incorporation of TiO 2 nanoparticles [33]. Besides, it has been reported that Bi 2 WO 6 -TiO 2 /starch nanocomposite films exhibited good ethylene degradation (in vitro) in the presence of UV irradiation [30]. Similar trends were reported when TiO 2 was incorporated into a combined starch-CS composite by González-Calderon et al. [27] and Li et al. [28], who found that incorporation of nano-TiO 2 on a starch-CS films improves the mechanical, physicochemical, thermal, and UV barrier properties of the composite. Most of the positive effects promoted by the incorporation of TiO 2 on the starch coatings/films have been attributed to the strong interfacial interaction between TiO 2 and the amorphous region of the starch chain [23]. However, it must be considered that an excessive concentration of nano-TiO 2 on the biopolymer matrix could cause the aggregation of inorganic particles onto the surface of the composite, thus affecting the mechanical and vapor-related properties [26].
Ahmadi et al. [24] fabricated a carboxymethyl cellulose film containing miswak (Salvadora persica L.) extract and TiO 2 nanoparticles. The authors stated that the presence of TiO 2 (2% wt.) exhibited a marked positive effect on the mechanical and physical properties of the film, it showed enhanced UV barrier properties and antimicrobial activity against S. aureus and E. coli. Similarly, Balasubramanian et al. [31] found that incorporating TiO 2 enhanced the elastic, stretchable, and UV protective properties of films formed by a mixture of K-carrageenan, xanthan gum, and gellan gum; it exhibited partial microbial activity against S. aureus, but its good thermal stability was mainly attributed to the incorporation of TiO 2 , which can act as a physical cross-linker agent. Furthermore, El-Wakil et al. [11] developed a composite by a casting/evaporation method using a mixture of wheat gluten, cellulose nanocrystals, and TiO 2 nanoparticles for a potential food packaging material. They reported that mechanical properties (tensile strength and young modulus) were enhanced by the presence of TiO 2 (0.6% w/w), and the final composite exhibited antimicrobial activity against E. coli and S. aureus. Nonetheless, the incorporation of TiO 2 on a hydrophilic polyurethane film provided antimicrobial activity against E. coli and C. albicans [39].
Fathi et al. [25] develop a nanocomposite film based on sesame proteins incorporated with TiO 2 nanoparticles and reported that the mechanical properties as tensile strength, water vapor transmission, and water solubility were influenced by theTiO 2 content as occurring with the starch-based composites with TiO 2 added, and suggested that the TiO 2 content should not exceed 3% w/w in the whole composite to obtain good mechanical properties in the protein-TiO 2 film. This behavior was previously evidenced by Zhou et al. [38], who informed that the addition of small amounts of TiO 2 (<1% wt.) improved the tensile properties of whey protein films, but decreased the moisture barrier properties, which may be attributed to the interactions between TiO 2 and whey protein through electrostatic interactions; with high amounts of TiO 2 (>1% wt.) the moisture barrier properties increased but tensile strength decreased, a possible reason for this behavior is related to the aggregation of TiO 2 in the whey protein matrix. Furthermore, it has been reported that sesame protein isolate-TiO 2 composite exhibited photo-degradation (120 min of UV treatment) of methylene blue dye (72%), which is a persistent organic water pollutant [25]. Similar effects were observed by Zheng et al. [34] using an amylose-hallosyte-TiO 2 composite for methylene blue dye (90%) degradation under UV-irradiation, also adsorptive properties of the composite were enhanced by including TiO 2 . Hammad et al. [29] and Gjipalaj and Alessandri [32] mentioned that cellulose-TiO 2 and alginate-TiO 2 are a feasible alternative for the removal of water pollutants like orange-G, methyl orange, and methylene blue dyes due to its adsorptive and photocatalytic properties.
Buzarovska and Grozdanov [37] developed a biodegradable poly(L-lactic acid) (PLA)/TiO 2 nanocomposite and evaluated their thermal properties and potential use as drug delivery system via in vitro photo, hydrolytic, and enzymatic degradation. As expected, the thermal properties of the composite were enhanced by the presence of TiO 2 but in a concentration-dependent manner. Similarly, the photo and hydrolytic degradation exhibited an important influence of the TiO 2 content. Nonetheless, the degradation of the PLA-TiO 2 composite was faster than pure PLA; however, the opposite effect was observed on enzymatic degradation, suggesting that nanoparticles help with the diffusion of large molecules, such as α-amylase. On the other hand, Rani et al. [36] mentioned that an alginate-TiO 2 needle composite exhibited a potential use for a tissue engineering application. The fabricated composite exhibited no sign of toxicity on osteosarcoma, fibroblast and human mesenchymal stem cell lines, with well controlled swelling and degradation in comparison with control scaffold.
In general, the incorporation of TiO 2 improves the physicochemical, mechanical and thermal characteristics of films-based protein and/or polysaccharides, and promote antimicrobial and UV barrier properties, giving them a great potential for food, pharmaceutical or biomedical, and environmental applications.

Chitosan-TiO 2 (CS-TiO 2 ) Composite
The CS-TiO 2 hybrid composite is recognized as a promising material with a wide range of applications, mainly by its structural, textural, mechanical, and optical properties; its capacity to function as a water and oxygen barrier; and by its thermal stability and biodegradability. In this context, the effect of TiO 2 incorporation on a CS matrix on the above properties has been extensively studied (Table 2). UV-Vis Composite exhibited a strong absorption range at 300-500 nm. [42] Zeta-potential CS-Ag-TiO 2 coating exhibited good stability (z-potential of 33 mV) after 60 days of storage. [43,44] Textural properties Ads-Des isotherm Composite is classified as a macroporous material (isotherm Type II). [45] SSA Composite's SSA is dependent of the CS:TiO 2 ratio. A major presence of CS promotes a decrease in SSA. [46] Pore volume Decrease in pore volume in CS from 0.25 cm 3 g −1 to 0.15 cm 3 g −1 in CS-TiO 2 composite. [44] Pore size TiO 2 decreases the pore size of the composite. [47] Thermal properties DSC The presence of TiO 2 enhances the thermal stability of CS. [48][49][50][51][52][53]

TGA
Composite film exhibited lesser degradation than a CS-film. [4] Optical properties Color CS film-forming solution turned whiter when TiO 2 was added, affecting the color and transparency of the composite film. [15] Light transmittance Presence of TiO 2 reduces the optical transmittance of the composite film. [54][55][56] Mechanical properties Thickness TiO 2 promotes an increase in the composite film thickness. [57] Young's modulus TiO 2 improves the flexibility of the composite (increase of 11.8-fold in Young's modulus). [23] Tensile strength TiO 2 improves the elongation at break in 70%.
In general, the structural properties of CS are improved by the TiO 2 incorporation, exhibiting good dispersion and uniformity on the surface of the obtained CS films by becoming rougher and more porous [40,41], but it promotes a slight increase in thickness (70 µm) compared with CS (66 µm). On the other hand, the crystalline peak of CS (2θ = 20.40 • ) was broader by the effect of TiO 2 , which resulted in a decrease in the crystallinity of the CS-TiO 2 composite, indicating a major interaction between TiO 2 and CS [40]. Similar results were observed during the ultraviolet-visible (UV-Vis) spectroscopy studies, where the CS-TiO 2 exhibited a strong absorption band at 300-500 nm (with CS, no absorbance occurred at 350-800 nm) promoted by the incorporation of TiO 2 in the composite through hydrogen bonding [42]. Furthermore, the CS-Ag-TiO 2 and CS-halloysite-TiO 2 composites showed good stability (zeta potential 31-39 mV) during storage [43,44]. The CS-TiO 2 composite showed a type II isotherm indicating that the material is macroporous in nature [45]. Nonetheless, the textural (specific surface area (SSA), pore volume, and pore size) properties of the CS-TiO 2 composite were dose-dependent influenced by TiO 2 (e.g., major concentration of TiO 2 promotes an increase in SSA in the hybrid composite) [44][45][46][47]. These changes in structural and textural properties enable CS-TiO 2 composites to be a good candidate for photocatalysis studies.
The thermal stability of CS-TiO 2 composites has been assessed through DSC/TGA studies. Li et al. [9] reported major thermal stability of CS-TiO 2 composite film because the presence of TiO 2 possibly prevents the thermal decomposition of the composite, mainly by intramolecular interactions [47]. Similar findings were reported by Habiba et al. [48] and Archana et al. [49], who stated that CS started to decompose at 200 • C, while CS-PVA-TiO 2 started to decompose at 250 • C. This behavior was attributed to the presence of TiO 2 . Moreover, Zhang et al. [50] reported an increase in the glass transition temperature (Tg = 111.13 • C) and enthalpy (318.8 J/g) values in CS-whey protein film (104.46 • C and 258 J/g, respectively) by the incorporation of TiO 2 [48][49][50][51][52][53]. According to Vallejo-Montesinos et al. [52] smaller amounts of TiO 2 may promote endothermic transitions to become a narrower composite, while higher amounts of TiO 2 may produce aggregates in the composite surface, which can affect their thermal stability [53].
Optical properties (opacity and light transmittance) indicate how much light that passes through a material [30]. It has been reported that TiO 2 promotes a marked change in color, opacity and light transmittance of CS-based composites. [52]. Díaz-Visurraga et al. [15] reported that the film-forming solution of chitosan-TiO 2 turned whiter when TiO 2 was added, while the films presented a reduced optical transmittance (brightness and transparency decreased, while opacity increased) in a dose-dependent response [28,30,40,53,54], these changes may be linked to their inherent characteristics and optical properties of TiO 2 [54]. These optical transmittance modifications have been exploited for the development of packaging and textile industrial applications for their antimicrobial and UV-barrier effect [5,55,56].
The functionalization of the CS matrix through the incorporation of TiO 2 has gained attention since it can impart the CS-TiO 2 composite with an enhancement of its mechanical properties [27] because it can act as a reinforcing agent [26], with a slightly increased film thickness (70 µm) compared with CS (66 µm) [57]. Amin and Panhius [23] reported that TiO 2 (30% vs. 70% CS) significantly improves the mechanical properties (an increase of Young's modulus in 11.8-fold, a six-fold increase for both tensile strength and toughness) of the CS film. Similar trends were reported during the evaluation of CS-TiO 2 mechanical properties [27,[58][59][60][61]. Al-Sagheer and Merchant [62] mentioned that the viscoelastic properties for hybrid composites, particularly CS/TiO 2 -based composites, depend on the bonding between the polymer-particle interfaces. However, an excessive TiO 2 content may promote negative effects on mechanical properties of the CS-TiO 2 composite [27,40], mainly attributed to the inhomogeneous dispersion and agglomeration of TiO 2 in the film, which cause irregularities in the surface structure of the composite, leading to a reduction of the mechanical properties [60][61][62][63]. The use of TiO 2 as a CS reinforcing agent has been extensively applied for the development of food packaging [5,40] and wound healing [4,59] materials, as well as for dye removal and photocatalytic applications [16,64].
Water vapor permeability (WVP) and oxygen vapor permeability (OVP), as well as water solubility, swelling capacity, and biodegradability rates, are the most important properties that should be considered during the development of packaging and/or wound-healing materials for food and biomedical applications [40,57,59,60]. In general, water and oxygen permeability values of CS films depend on various interrelated factors such as the structural characteristics of the polymeric chains, hydrogen bonding features and other intermolecular interactions, degree of cross-linking, and dispersion of nanoparticles into the polymeric matrix [1]. Delgado-Alvarado et al. [65] reported that the functionalization of a CS matrix with TiO 2 nanoparticles reduces the WVP of CS-TiO 2 films. These results are in line with those of Siripatrawan and Kaewklin [40] and Qu et al. [55] in CS-TiO 2 and zein-CS-TiO 2 films. The authors suggested that TiO 2 may promote a modification of the CS-TiO 2 film matrix, forming a dense structure, which act as obstacles to water vapor diffusion through the film [28,50]. Similar trends were observed in the OVP properties through the incorporation of nano-TiO 2 to CS films according to Peng et al. [59] and Lian et al. [60], who stated that the presence of TiO 2 significantly improves the film functionality by reducing the exchange of oxygen through the CS-TiO 2 composite film [66]. Besides, no significant differences in water solubility were observed between CS and CS-TiO 2 films [57]. The presence of TiO 2 reduces the swelling degree of CS-TiO 2 in a dose-dependent response [49,63]. Alex et al. [67] reported that TiO 2 -CS-chondroitin 4-sulphate composite showed a maximum swelling ratio of 28%. These results are similar to those reported by Kavitha et al. [68], who observed no significant changes in the swelling capacity above a 2:1 (CS:TiO 2 ) ratio because TiO 2 can control the swelling capacity of CS-TiO 2 composite [47] and moderately prevents the mass loss of the composite [51]. According to Amin and Panhuis [23], the swelling decrease in CS-TiO 2 films could be attributed to the presence of TiO 2 into the composite and by its ability to participate in hydrogen bonding with CS. Furthermore, CS-TiO 2 exhibited moderate biodegradability (<3%), and favorable time-dependent biodegradability (0.0521 mg mL −1 h −1 on the first day; 0.0006 mg mL −1 h −1 post-covering) [59]. In this context, the CS-TiO 2 composite exhibits great potential for food and biomedical applications [40,59].
Additionally, the structural changes of CS by the presence of nano-TiO 2 has been extensively evaluated by Fourier transform infrared (FTIR) studies. Figure 2a shows a typical FTIR spectra of a CS-TiO 2 hybrid composite (consistent with reported FTIR spectra in literature for CS-TiO 2 composites [16,40,48]), where the most representative FTIR band positions of the chitosan-TiO 2 hybrid composite are listed in Table 3. between CS and CS-TiO2 films [57]. The presence of TiO2 reduces the swelling degree of CS-TiO2 in a dose-dependent response [49,63]. Alex et al. [67] reported that TiO2-CS-chondroitin 4-sulphate composite showed a maximum swelling ratio of 28%. These results are similar to those reported by Kavitha et al. [68], who observed no significant changes in the swelling capacity above a 2:1 (CS:TiO2) ratio because TiO2 can control the swelling capacity of CS-TiO2 composite [47] and moderately prevents the mass loss of the composite [51]. According to Amin and Panhuis [23], the swelling decrease in CS-TiO2 films could be attributed to the presence of TiO2 into the composite and by its ability to participate in hydrogen bonding with CS. Furthermore, CS-TiO2 exhibited moderate biodegradability (<3%), and favorable time-dependent biodegradability (0.0521 mg mL −1 h −1 on the first day; 0.0006 mg mL −1 h −1 post-covering) [59]. In this context, the CS-TiO2 composite exhibits great potential for food and biomedical applications [40,59]. Additionally, the structural changes of CS by the presence of nano-TiO2 has been extensively evaluated by Fourier transform infrared (FTIR) studies. Figure 2a shows a typical FTIR spectra of a CS-TiO2 hybrid composite (consistent with reported FTIR spectra in literature for CS-TiO2 composites [16,40,48]), where the most representative FTIR band positions of the chitosan-TiO2 hybrid composite are listed in Table 3. Fourier transform infrared spectra of TiO2, chitosan, and chitosan-TiO2 hybrid composite (a) (the CS-TiO2 composite was prepared in our laboratory following the methodology of Siripatrawan and Kaewklin [40] by using 1 g chitosan with medium molecular weight (Sigma-Aldrich, St. Louis, MO, USA) dissolved in 1% acetic acid solution, the nano-TiO2 (previously characterized in reference [14]) was 1% of the total chitosan weight, and 15% of glycerol weight of the total solids), and schematic representation of CS-TiO2 hybrid composite structure (b).

Figure 2.
Fourier transform infrared spectra of TiO 2 , chitosan, and chitosan-TiO 2 hybrid composite (a) (the CS-TiO 2 composite was prepared in our laboratory following the methodology of Siripatrawan and Kaewklin [40] by using 1 g chitosan with medium molecular weight (Sigma-Aldrich, St. Louis, MO, USA) dissolved in 1% acetic acid solution, the nano-TiO 2 (previously characterized in reference [14]) was 1% of the total chitosan weight, and 15% of glycerol weight of the total solids), and schematic representation of CS-TiO 2 hybrid composite structure (b).  [42,50,69] According to Siripatrawan and Kaewklin [40], the incorporation of TiO 2 into the chitosan matrix mainly occurs in the amorphous region of chitosan, strengthening the hydrogen bond (3300 cm −1 ) in the CS-TiO 2 complex [69,71] (Figure 2a). It has been reported that the electron doublet that is free on the nitrogen atom (-NH 2 ) and -OH functional groups on chitosan structure is the responsible for interaction and may serve as a coordination bond (3450 cm −1 ) and reaction site for the adsorption of transition metals and/or other compounds [8,76] (Figure 2b). Saravanan et al. [16] suggested that the stretching vibrations of C-O, amino, and hydroxyl groups are strongly attached to TiO 2 nanoparticles which promote the formation of CS-TiO 2 composite via electrostatic interaction of N-H-O-Ti bonds (3350 cm −1 ). Similarly, Kavitha et al. [70] detected O-Ti-O bonds formed between TiO 2 and CS (385-900 cm −1 ). In addition, a new band appeared at 1000 cm −1 which corresponds to the Ti-O-C bond, demonstrating that CS and TiO 2 have been chemically bonded and not only adsorbed into the CS matrix [75] as a result of a crosslinking process and the complex composite structure [60,77]. Nonetheless, during the CS-TiO 2 preparation, the tendency of TiO 2 to agglomerate is prevented because the chitosan masked the Van der Waals influence on TiO 2 nanoparticles [15,70], due to the immobilization of nano-TiO 2 in the chitosan matrix [41].
Raut et al. [10] mentioned that the incorporation of TiO 2 :Cu-doped did no altered the characteristic structure of CS and reported that the metal-oxygen-metal inorganic union was bonded with CS macromolecules by hydrogen bonding as well as covalent bonding in the CS-TiO 2 :Cu composite; however, in the same study, Raut et al. [10] informed about some minimal changes in the other peaks using X-ray photoelectron spectroscopy studies, which manifest new covalent bonds formation. Zhang et al. [50] reported that CS-TiO 2 exhibited a shift of 21 cm −1 compared to CS (at 1631 cm −1 ), confirming the interaction between CS and TiO 2 nanoparticles [74]. On the other hand, it has been reported the presence of titanates (by the effect of acidic conditions during preparation) in CS-TiO 2 composite (1735 cm −1 ) [71]. This fact may promote a decrease in the crystallinity of the CS matrix (decrease in intensity of -OH and -NH bands), probably to the effect of matrix alteration and interfacial interaction between organic and inorganic phases [5]. However, all effects of TiO 2 on the CS structure are dependent of factors as size and concentration of TiO 2 nanoparticles [16,42,55], but also by the available free amino group resulting for the interaction of TiO 2 with CS [76]. However, the creation of a stable CS-TiO 2 composite is CS and TiO 2 concentration and pH dependent (favorable pH 4.8) [51].

Applications of CS-TiO 2 Composite
The CS-TiO 2 hybrid composite has been widely used for various technological applications such as antimicrobial agents, packaging, films and/or coating materials for food preservation, wound healing and skin regeneration, glucose, α-fetoprotein detection, and water pollutant degradation due to their prospective benefits, as discussed below. Figure 3 illustrated the main applications of CS-TiO 2 hybrid composite, which include photocatalytic properties and barrier, antimicrobial, and wastewater treatment applications. healing and skin regeneration, glucose, α-fetoprotein detection, and water pollutant degradation due to their prospective benefits, as discussed below. Figure 3 illustrated the main applications of CS-TiO2 hybrid composite, which include photocatalytic properties and barrier, antimicrobial, and wastewater treatment applications.

Antimicrobial Activity
CS and TiO 2 NPs exhibit antimicrobial activity against bacteria, yeast, and molds [4,40,78,79], which could be enhanced by their combination, via CS-TiO 2 composite (Table 4). The film exhibited antimicrobial activity against Gram-positive and Gram-negative bacteria, and fungi. [40] Sthapylococcus aureus, Pseudomona aureginosa CS-TiO 2 :Ag composite CS-TiO 2 :Ag exhibited major antibacterial activity than CS-TiO 2 composite against E. coli, S. aureus and P. aureginosa. [72] Sthapylococcus aureus CS-TiO 2 membrane Enhanced antibacterial activity against S. aureus by CS-TiO 2 compared to CS films. [4] Escherichia coli CS-Ag-TiO 2 coating It has been reported that the antimicrobial activity of CS was improved by the incorporation of TiO 2 into a polymeric matrix [4,54,74], and a similar behavior was reported using a combination of CS with zein (Zein/CS) [55], and carboxymethyl cellulose (CS/CMM) [64] adding nano-TiO 2 . Li et al. [9] reported that CS-TiO 2 composites exhibited a higher antimicrobial activity against Xanthomonas oryzae pv. Oryzae than the two individual components under light and dark conditions. Longo et al. [82] explained that TiO 2 could be present or trapped on the composite surface, thus enhancing their antimicrobial activity due to the generation of electron-hole pairs without the need of light-induced radiation (Figure 3b). Similar results were reported by Li et al. [81] and Xiao et al. [6] using a CS-Ag-TiO 2 composite against E. coli, S. aureus, P. aureginosa, and C. albicans, probably for the synergistic effect of the three components, which suggests that microbial inactivation is linked with electrostatic interactions between the cell membranes and the composite, followed by oxidative stress caused by photo-generated reactive radicals, causing cell death (Figure 3d).
Siripatrawan and Kaewklin [40] observed that CS-TiO 2 nanocomposite films exhibited antibacterial activity against Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria and fungi (Aspergillus and Penicillium). Similarly, Díaz-Visurraga et al. [15] prepared a semitransparent composite film using CS and TiO 2 nanotubes by casting plate. They highlighted that the hybrid composite exhibited antimicrobial activity against S. enterica, E. coli, and S. aureus; where the antimicrobial effect was strainand TiO 2 concentration-dependent. Additionally, studies using transmission electron microscopy (TEM), have revealed that CS-TiO 2 films promote bacterial cell disruption, membrane deformation and structural changes of the cell wall. Kavitha et al. [68] mentioned that the CS-TiO 2 film exhibited moderate antibacterial activity against S. aureus, but no against E. coli and K. pneumoniae. On the other hand, Kim et al. [78] found that when CS-TiO 2 composites are used, the death cell ratio depends on several factors (pH, temperature, UV light, and TiO 2 concentration) and their potential interactions. They reported that, at constant TiO 2 concentration, the death ratio of S. choleraesuis increased when pH and temperature decrease, and, at low pH values, the death ratio increased when TiO 2 concentration increased. The antimicrobial activity is affected by the TiO 2 concentration even in the presence of UV-radiation (30 min). At concentrations of 1 mg TiO 2 mL −1 , 48% of S. choleraesuis lost cell viability, while at 0.25 mg mL −1 , the killing efficiency decreased by 22%.
Furthermore, the antimicrobial activity of CS-TiO 2 composites could be enhanced by the modification of the TiO 2 band gap associated with the presence of other compounds in the TiO 2 matrix [14]. Raut et al. [10] informed that the incorporation of TiO 2 :Cu-doped nanoparticles (1 mg mL −1 ) into CS matrix (CS-TiO 2 :Cu) increased up to 200% the photocatalytic antimicrobial activity of CS-TiO 2 composite against E. coli and S. aureus. After 3 h of treatment, more than 85% of the bacteria cells lose their viability, and the presence of Cu in TiO 2 nanoparticles decrease the band gap of TiO 2 , and reduced the recombination of electron-hole pair rate, resulting in a better photocatalytic performance through Fenton chemistry. Similar trends were observed by Li et al. [72] against E. coli, S. aureus and P. aureginosa using a CS-TiO 2 :Ag composite. They concluded that the increase of antibacterial activity is attributable to the increase of the surface area of TiO 2 by silver doping. Kamal et al. [64] suggested that the antibacterial mechanism of the hybrid composites could be associated with the interaction of electrostatic charges between CS-TiO 2 (positive) and bactericidal membranes (negative), which may promote a membrane cell alteration, blocking nutrient intake and affecting the viability and normal cell growth. Shi et al. [79] reported that gauzes impregnated with a CS-TiO 2 emulsion were effective to inhibit the growth of E. coli (99.9%), A. niger (100%), and C. albicans (78.3%). The gauzes could also be reused up to eight times without loss of this antibacterial ability. Similar trends were reported by Xiao et al. [41], who observed that a CS-Fe-TiO 2 composite coating showed favorable shelf life (up to one year) without losing its antibacterial (E. coli) and antifungal (C. albicans and A. niger) activities. Qian et al. [42] found that the incorporation of CS-TiO 2 composite on cotton fibers exhibited antimicrobial activity against E. coli (99.9%), S. aureus (99.9%), and A. niger (97.4%). Moreover, Xu et al. [80] observed that A. niger is less susceptible to a graphene oxide (GO)-CS-TiO 2 composite than B. subtilis at the same experimental conditions. They also stated that a GO-CS-TiO 2 composite can selectively increase the permeability of microbial cell membranes, promoting a leakage of cell contents, mainly by the direct physical interaction between the GO-CS-TiO 2 composite and the microorganisms. Additionally, it has been reported that nano-TiO 2 /CS/PVA ternary composite exhibited enhanced antimicrobial activity against E. coli and S. aureus than the individual components [81].
The CS-TiO 2 composite exhibit enhanced antimicrobial activity against Gram-positive and Gram-negative bacteria, yeast, and molds, which are favorable in many industrial domains such as the food industry for developing food packaging material and for biomedical applications such as wound-healing material.

Environmental Applications
Recently, a growing number of organic pollutants have been discharged into all kinds of open waters [16]. Therefore, great emphasis has been put on water treatment, mainly regarding pollutant degradation [64]. Heterogeneous photocatalysis based on semiconductor photocatalysts is a viable alternative because it is an environmentally friendly and cost-effective method for the water pollutant degradation [74]. Nonetheless, in recent works, researches have demonstrated that the incorporation of TiO 2 into a polymeric matrix, such as CS and thus forming a hybrid composite (CS-TiO 2 ), provided a major stabilization of the semiconductor material (TiO 2 ), which resulted in higher pollutant degradation than the individual components. This may be attributable to the adsorbent and chelating properties of CS, which exhibited great potential for water treatment, and to the photocatalytic properties of nano-TiO 2 [83] as illustrated in Figure 3e. Table 5 shows the photocatalytic activity of a CS-TiO 2 composite for different water pollutant degradation.

Cr(VI) CS-TiO 2 composite CS (3.22% w/v), TiO 2 solution
The CS-TiO 2 composite was quite effective for adsorption and detoxification of Cr(VI) in water with a maximum adsorption capacity of 171 mg g −1 for Cr(VI). [50] As CS-TiO 2 composite CS (1 g), TiO 2 (0.42g g −1 CS) Exhibited good Arsenic removal from water, but the effectiveness is TiO 2 concentration dependent. [85] Congo red CS-TiO 2 glass photocatalyst CS (0.83% w/v), TiO 2 (0.83% w/v) The study suggests a new method that has the advantages of photodegradation and adsorption in the abatement of various wastewater pollutants. [8] Methyl orange CS-TiO 2 composite CS (0.5 g 25 mL −1 ), TiO 2 (0.2 g 25 mL −1 ) Exhibited degradation of methyl orange and adsorption on Ni 2+ ions. [86] Cd, 2,4-DCP MICT composite CS (2.5 g 500 mL −1 ), TiO 2 (1 g TiO 2 ), Fe 3 O 4 (1.25 g) The composite is effective for cadmium adsorption (256 mg g −1 ) and 2,4-dichlorophenol degradation (98%), and could be used up to five cycles with preserving 69% of its adsorption properties. [73] Methyl orange TiO 2 -CS-rGO CS (2% w/v), 4 mL of TiO 2 suspension (50 mg mL −1 ), rGo (1% w/w) Hybrid composite showed photocatalytic degradation (97%) of methyl orange. [58] Methylene blue CS-TiO 2 composite CS (1% w/v), TiO 2 (1 g in 20 mL of CS solution) Composite is effective for methylene blue (100%) degradation under UV-light, but the effectiveness was catalyst concentration-dependent. [45] As(III), As(V) CS-TiO 2 :Cu composite CS (1% w/v), TiO 2 (0.6% w/w), Cu (0.7% w/w) The composite showed good photo-oxidation and selective removal for As(III) and As(V). [87] Methyl orange CS-TiO 2 composite CS (1% w/v), TiO 2 (2% w/w) CS is an excellent support for TiO 2 immobilization, which exhibited a complete photodegradation of methyl orange and alizarin red S after 3 h of treatment.   The membrane exhibited high efficiency on 2-4-DCP removal from aqueous solution. Zhang et al. [50] synthetized a CS-TiO 2 composite through a metal-binding reaction using glutaraldehyde as a crosslinking agent and used it for the detoxification of hexavalent chromium from water. The CS-TiO 2 composite exhibited good adsorption capacity of chromium (171 mg g −1 ), and reduced Cr(VI) to Cr(III), a less toxic form of Cr, suggesting that the removal mechanism could involve the Cr(VI) adsorption by electrostatic interactions (Ti 4+ and HCrO 4− ) and ligand exchanges (Cl − and HCrO 4− ), promoting a Cr(VI) reduction to Cr(III) and re-adsorption of Cr(III) onto CS-TiO 2 composite. It has also has been reported that oxygen presented in the solution can acts as a "hole scavenger" and generates H 2 O, besides reduced Cr(VI), as reported by Farzana and Meenakshi [75], who found that the removal efficiency of Cr(VI) was initial-concentration dependent-at higher concentrations of Cr(VI) (125 mg L −1 ) a reduction of 74% was achieved, while at low initial concentrations, the efficiency increased considerably (96.7%).
Razzaz et al. [17] functionalized CS nanofibers with nano-TiO 2 by a coating and entrapped method for the removal of heavy metal ions-Cu(II) and Pb(II)-from water. Both CS-TiO 2 composites exhibited good adsorption capacity for Cu(II) (710 and 579 mg g −1 , respectively) and Pb(II) (526 and 475 mg g −1 , respectively) after 30 min at 45 • C. Nonetheless, the composites prepared by the entrapped method exhibited better results after five adsorption/desorption cycles than the composite prepared by the coating method. The CS-TiO 2 composite showed selective sorption in order of Cu(II) > Pb(II), and the authors discuss that the adsorption effectiveness is related to the metal ions concentration and the adsorption ion capacity of the hybrid composite. Furthermore, it has been reported that the ternary CS-hemicellulose-TiO 2 composite exhibited enhanced properties for the adsorption of heavy metals ions as Ni(II), Cd(II), Cu(II), Hg(II), Mn(II), and Cr(VI) from an aqueous solution due to chelating groups present in the structure, without significant loss of its adsorptive capacity up to five cycles [90]. Similarly, Alizadeh et al. [93], using a cross-linked magnetic EDTA/CS/TiO 2 composite, reported a maximum adsorption capacity of Cd(II) of 209 mg g −1 and phenol degradation efficiency of up to 90% after up to five cycles of reuse. The process of adsorbing Cd(II) on EDTA/CS/TiO 2 has been associated to the presence of functional groups of CS and a synergistic effect with cross-linked magnetic EDTA/TiO 2 . Moreover, Xie et al. [91] developed an electrode by modifying nano-TiO 2 /CS composite film onto a glassy carbon electrode for the determination of Cd(II) with a detection limit of 2.0 × 10 −10 mol L −1 Cd for 180 s accumulation, representing a simple, effective, faster and economical alternative method for Cd detection.
CS-TiO 2 has also been used to oxidize As(III) to As(V) (a less toxic and more easily sequestered arsenic form) in the presence of UV light and oxygen (adsorption capacity of 6.4 mg As(V) g −1 and 4.92 mg As(III) g −1 ) by a hybrid composite [85,96]. In addition, the TiO 2 -enabled CS cross-linked with copper (CS-TiO 2 :Cu-dopped) is an alternative for arsenite and arsenate water removal via photo-oxidation and/or adsorption process, which was mainly associated to the synergistic effect between TiO 2 :Cu loaded into a CS matrix. Furthermore, during the degradation process, CS-TiO 2 :Cu photo-oxidizes As(III) to As(V) through ROS generation, and As(V) is chelated due to the presence of Cu into the composite, which acts as an As(V) chelating agent. Moreover, it has been reported that a CS-TiO 2 :Cu composite exhibited better adsorption properties than traditional adsorbents (Cu, TiO, Al 2 O 3 ) for As(III) and As(V) removal in systems buffered at pH 6 [87]. These results are in agreement with those of Yazdani et al. [83] during arsenate (73%) and arsenite (84%) removal from an aqueous solution using a CS-TiO 2 -feldspar composite. They found that UV irradiation increased As(V) removal regardless pH conditions. According to Wu et al. [90], the dispersion stability of the nano-TiO 2 in the polymeric matrix is a determinant factor to improve the 3D network and surface area of a hybrid composite for metal ions adsorption.
CS-TiO 2 has an excellent photo-catalytic degradation efficiency against methyl orange (MO) dye [16]. Zhu et al. [84] mentioned that CdS nanocrystals deposited on TiO 2 /crosslinked CS composite (CdS/TiO 2 /CS) is a viable alternative for decolorization treatments of water contaminated with dyes due to its photocatalytic and adsorbent properties. The composite exhibited a good methyl orange adsorption character (MO, 2.66 mg g −1 ) and high photocatalytic degradation potential of MO-solution (99.1%) after simulated solar light radiation (210 min). The photocatalytic activity (89%) was maintained after five batch reactions. Moreover, a CdS/TiO 2 /CS composite could be also used for Congo red degradation [77]. Afzal et al. [94] reported that TiO 2 -supported CS exhibited a complete degradation of MO within 90 min of UV exposure. Similarly, Zainal et al. [8] mentioned that during MO-removal (87%) from water using a CS-TiO 2 composite, the reactive -NH 2 and -OH functional groups of CS and TiO 2 are directly responsible for the photodegradation-adsorption process during abatement of various wastewater pollutants. During MO degradation (90%) using CS-TiO 2 composite, Li et al. [86] found that the MO removal was the result of an additive effect of both CS and TiO 2 and not only by the adsorptive properties of CS. Furthermore, Xiao et al. [6] reported that a CS-TiO 2 composite showed enhanced photocatalytic selectivity for MO compared with CS, and the hybrid composite could be reused up to 10 cycles while preserving 60% of its efficiency. According to Zhu et al. [97], a TiO 2 /ZnO/CS composite exhibited high photocatalytic degradation of MO (>97%) solution, which was mainly attributed to the adsorption ability of the composite and its high surface area. In addition, the decolorization process was accelerated under simulated solar irradiation. Zhu et al. [97] mentioned that the addition of ZnO into the TiO 2 matrix (TiO 2 :ZnO) and then to the CS matrix has a beneficial role in improving the charge separation and extending the response of the composite to visible light. Additionally, it has been reported that a CS-TiO 2 -rGo composite performs well as a MO degrader (97%) [58]. Meanwhile, Dhanya and Aparna [88] reported that CS may be a good and economically support for TiO 2 immobilization (by hydrogel method), and the resultant composite exhibited a 100% of degradation of MO and alizarin red dyes after 3 h of treatment under UV light (293 nm) irradiation. Furthermore, this photocatalyst can be recycled up to three times without efficiency losses (65%). The efficiency for dye decoloration is pH-dependent (pH 11 < pH 9 < pH 7 < pH 5 < pH3). Similarly, Habiba et al. [71] observed a complete degradation of MO using a Chitosan/PVA/Na-Titanate/TiO 2 composite under UV irradiation. According to Habiba et al. [71], the deacetylation degree (DD) of CS plays a key role for MO removal, mainly by the available active sites, where CS with high DD exhibited better adsorption capacity for MO removal than medium DD [48].
Haldorai and Shim [74] reported that a CS-TiO 2 composite was effective for the degradation (90%) of methylene blue (MB) under UV-light illumination without significant losses of catalytic activity after five cycles of reuse. In addition, the obtained composite could be an excellent candidate as an environment-friendly photocatalyst due to its low cost and catalytic efficiency. Similar trends were reported by Nithya and Jothivenkatachalam [45], who observed a complete (100%) degradation of MB (initial concentration 5 mg L −1 ) using CS-TiO 2 composite at 0.3 g L −1 under UV-light and suggested that MB degradation could be accelerated under alkaline conditions (pH 11).
Chen et al. [73] fabricated a thiourea magnetic ion-imprinted CS-TiO 2 composite for simultaneous cadmium removal and 2,4-dichlorophenol (2-4-DCP) degradation. The prepared composite exhibited good cadmium adsorption (256 mg g −1 ) and 2,4-DCP degradation (98%) capabilities (at initial concentration of 10 mg L −1 ). Several intermediates (4-chlorophenol, 1,4-benzoquinone, phenol, and cyclohexanol) were formed during the degradation process in the presence of UV light, and the authors suggested that the major decomposition pathway of 2,4-DCP is linked to the reductive dechlorination and reaction with hydroxyl radicals. Liu et al. [95] evaluated the adsorptive removal of 2,4-DCP from water using a CS/activated carbon fiber (ACF)/TiO 2 membrane and reported that the membrane possesses high filtration properties (in static and dynamic modes) and high adsorptive removal of 2,4-DCP using both flux modes. Furthermore, the membrane adsorption capacity can be regenerated by oxidizing adsorbed 2,4-DCP via in situ or off-site photocatalysis and/or Fenton oxidation. Xiao et al. [89] incorporated Ag + ions into the surface of CS-TiO 2 composites and found and enhanced efficient catalytic activity in the reduction of 4-NPh (hazardous material) to 4-APh (commercial compound) by NaBH 4 , with 100% conversion reached in 120 min without catalytic activity losses after five continuous cycles.
Zhou et al. [92] prepared a nano-TiO 2 /CS/poly(N-isopropyl acrylamide) composite hydrogel for the removal of ionic dyes as acid fuchsin (AF). The hydrogel composite exhibited high photocatalytic degradation of AF with a removal rate of 90% under UV radiation after 160 min. However, the effectiveness of the composite was pH-and temperature-dependent, exhibiting a better performance at pH 4 and 20 • C. Authors also reported a decrease in the removal rate of AF when temperature increased (40 • C), which may be associated with the gradual hydrogen bond weakening between amine groups of AF and hydrophilic groups (-NH 2 , -OH, and -CONH-) of the hydrogel composite. According to Longo et al. [82], these reactions can occur under coordinated cationic Ti surface sites, and the reaction efficiency is directly proportional to the number of adsorptive/photoactive sites of the composite [48]. On the other hand, for thymol violet removal from water, a TiO 2 /CS/carboxymethyl cellulose composite seemed to be a promising alternative [64].
In summary, research-based reports support the assertion that CS-TiO 2 is an attractive material with high performance and a low-cost and is efficient, reusable, and environmentally friendly for heavy metal ion removal and pollutant degradation from water.

Biomedical Applications
Miyazaki et al. [22] highlighted that organic-inorganic composites-and CS-TiO 2 in particularexhibited interesting biomedical applications. It has been recognized that the incorporation of TiO 2 into a CS biopolymer is a strategy to enhance the biological activities of CS [3]. Table 6 shows the potential biomedical applications of a CS-TiO 2 composite. Behera et al. [4] evaluated the effect of a CS-TiO 2 composite membrane on the proliferation and survival of L929 fibroblast cells as potential wound dressing and skin regeneration material. The results showed no cytotoxicity effects on L929 cells growth by the effect of the composite and the CS-TiO 2 membrane can scavenge the reactive oxygen species (ROS) over-production in the L929 cell line, suggesting that the composite exhibited good antioxidant capability. Furthermore, the cells under CS-TiO 2 showed a favorable effect on the cell cycle progression (G0/G1 phase), thus promoting the proliferation and survival of fibroblast cells. Peng et al. [59] reported that a CS-TiO 2 composite showed a reduced effect on the pro-inflammatory TNF-α and anti-inflammatory cytokine IL-6 when comparing the control to commercial products (Duoderm); nonetheless, CS-TiO 2 exhibited a better wound healing effect than the commercial product during an in vivo trial. Similar results were reported by Archana et al. [49], who observed that CS-pectin-TiO 2 composite exhibited good in vitro results by inducing blood coagulation and showed good hemostatic properties and no cytotoxic toward L929 or NIH3T3 mouse fibroblast cells. Additionally, the in vivo open excision-type wound healing study in adult male albino rats revealed that CS-pectin-TiO 2 (99% of closure) had better results than CS (95%) and gauze (91%) after 16 days. Similarly, Woo et al. [101] fabricated a wound dressing material with a bilayer composite of nano-TiO 2 combined with a CS membrane and a sub-layer of human adipose-derived extracellular matrix (ECM) sheet, and reported that, from an in vivo experiment using rats, the TiO 2 -CS-ECM had faster regeneration of the granulation tissue and epidermis with less scar formation in comparison with control wounds. It has been suggested that the wound-healing effect of the CS-TiO 2 composite is related to the regulation of re-epithelialization and inflammation process [76].
Zhang et al. [102] reported that the self-assembly CS/gelatin composite coating on icariin(ICA) -modified TiO 2 nanotubes (by physical adsorption) exhibited osteoblast bioactivity; which can modulate primary osteoblasts due to the ICA-Ti surface facilitating the initial adhesion, promoting osteoblastic proliferation and up-regulating the expression of bone-related genes (osteopontin, type I collagen and osteoprotegerin), while down-regulating RANKL mRNA expression, indicating a potential application on Ti implants to improve biocompatibility and osseointegration. Alex et al. [67] mentioned that TiO 2 -CS-Chondroitin 4-sulphate composite exhibited great potential for bone implant applications because it showed excellent bioactivity, biocompatibility and high cell viability at low concentrations on human MG-63 cell line. Likewise, Safari et al. [103] evaluated the in vitro drug-delivery properties of a CS/dopamine(DOP)/TiO 2 system (the solution was at pH 7.4) and concluded that the incorporation of TiO 2 on a CS/DOP system enhanced the drug entrapment, and provided a controlled drug-release effect (16 h) compared with CS/DOP system (10 min). This fact was attributable to the strong hydrogen bonding interactions between CS/DOP systems with -OH groups on the TiO 2 surface.
Additionally, Huang et al. [98] developed an amperometric immunobiosensor for α-fetoprotein detection (AFP oncofetal glycoprotein, which is a clinical cancer biomarker) using an Au/CS/TiO 2graphene composite based platform. The biosensor was a novel strategy for AFP detection with good bioactivity, sensitivity, and selectivity with good storage stability. Nonetheless, an Au/CS/TiO 2graphene composite is an interesting alternative to the development of biosensors aimed to the detection of diverse antigens or compounds [104]. Similarly, Al-Mokaram et al. [99] fabricate a non-enzymatic glucose biosensor based on electrochemically prepared polypyrrole(Ppy)-CS-TiO 2 nanocomposite film and reported that the device showed good sensitivity over a linear range of 1-14 mM and a detection limit of 614 µM of glucose. The biosensor also exhibited high selectivity and extended useful life with no interference effect in comparison with Ppy-CS without TiO 2 , where TiO 2 increased the electrocatalytic activity of the electrode surface. Furthermore, the Ppy-CS-TiO 2 nanocomposite can be used as a glucose sensor due to the ability of composite for glucose oxidation [99]. These results are in line with those of Zhang et al. [100], who developed a glucose biosensor on a TiO 2 -multiwall carbon nanotubes-chitosan composite and functionalized with Au nanoparticles, which performed as a good glucose detector with a linear range of 6 µM to 1.2 mM and a detection limit of 0.1 µM. This was attributed to the increase in the surface area of the composite by the presence of TiO 2 and Au nanoparticles, which facilitated the electron transfer to the electrode surface.
In general, the CS-TiO 2 composite exhibits great potential for biomedical applications mainly by its low cytotoxic effects on different cell culture lines [4,49,59]. Jayakumar et al. [47] reported that a CS-TiO 2 composite scaffold did not exhibit cytotoxicity on cell lines such as osteoblast-like cells (MG-63), fibroblast cells (L929), and human mesenchymal stem cells (hMSCs) and highlighted the potential use of a CS-TiO 2 composite for biomedical applications. Raut et al. [10] observed that CS-TiO 2 :Cu-doped exhibited low toxicity toward mammalian cells with more than 85% of cell viability after 72 h of exposure. Similar results were reported by Xu et al. [80], who observed a low toxicity degree in mammalian somatic and plant cells of GO-CS-TiO 2 coating. Moreover, Kavitha et al. [46] and Li et al. [9] reported no significant cytotoxic effects of CS-TiO 2 on gastric carcinoma cell line and L929 cell line using a TiO 2 -CS-PVA composite, respectively. However, the preparation method of hybrid composite has a higher impact on the biocompatibility between CS and TiO 2 , which is linked with its biological activity [46]. Therefore, the development of suitable protocols for CS-TiO 2 composite preparation is needed in order to optimize the CS-TiO 2 fabrication.

Food Preservation Applications
Most of the food applications described in the literature for CS-TiO 2 composites are focused on developing food packaging alternatives and edible coatings for fruit preservation (Table 7). It has been reported that the inclusion of TiO 2 into food packaging materials could prevent degradation associated with UV light because TiO 2 acts as a filter against radiation, thus protecting the food inside the package [15], as illustrated in Figure 3c. Coated fruits exhibited less than 5% weight loss and maintained color attributes compared to uncoated fruits. PPO activity was diminished in coated fruits. [80] Stauntonive fruit CS-TiO 2 coating CS (1% w/v), TiO 2 (0.03% w/w) The fruit treated with the composite coating exhibited a reduction on the CO 2 transmission coefficient compared to the CS-treated, without significant changes in quality parameters during 45 days of storage. [105] Gingko biloba seeds CS-TiO 2 coating films CS (1% w/v), TiO 2 (0.02% w/v) The composite preserved the quality parameters (firmness and antioxidant activity) of gingko seeds and prevented the mildew apparition. Kaewklin et al. [5] developed an active packaging with CS-TiO 2 nanocomposite aimed to enlarge the shelf life of tomato fruit, founding that tomato packaged in the CS-TiO 2 film had lower quality changes (firmness, weight loss, color, total soluble solids, lycopene, and ascorbic acid content) than those in the CS control group after 15 days of storage at 20 • C. These results were attributed to the ethylene photo-degradation achieved by the presence of TiO 2 (UV-irradiated at 320-390 nm during 180 min) into the film and consequently delayed the ripening process and storage-wise changes on quality parameters of tomatoes. Tian et al. [106] mentioned that a CS-TiO 2 coating film was effective for Gingko biloba seeds preservation without a significant reduction of firmness and antioxidant capacity, and also, prevented the mildew apparition and reduced some fruit metabolic processes (respiration rate and ethylene production), which delayed the senescence of gingko seeds, concluding that CS-TiO 2 film is an attractive method for food preservation. Similar results were reported on ethylene photo-degradation (in vitro and TiO 2 dose-dependent) by Siripatrawan and Kaewklin [40] and Zhang et al. [54] using a CS-TiO 2 nanocomposite and CS-TiO 2 -black plum peel extract (BPPE) films, respectively. The authors suggested that the incorporation of BPPE into CS-TiO 2 composite improves the antioxidant capacity of the resulting packaging material [57].
Zhang et al. [54] reported that a CS-TiO 2 film was effective to extend the shelf life of red grapes storage at 37 • C up to 15 days in comparison with a CS-based film, proving the improvement of wettability and mechanical properties of CS film through the addition of TiO 2 nano-powder. Yuan et al. [105] formulated a CS/nano-TiO 2 composite coating and evaluated their effect in extending the postharvest shelf life of Stauntonvine fruit. The fruit treated with the composite coating exhibited a reduction of the CO 2 transmission coefficient (27 g d −1 ) compared to the CS coating (35 g d −1 ); preventing the loss of ascorbic acid and changes in pH, titratable acidity, and total soluble solids after 45 days of storage at room temperature. This behavior was related to the modification on the internal atmosphere (CO 2 and O 2 ratio) between the fruit and the composite coating. Likewise, Xu et al. [80] mentioned that a GO-CS-TiO 2 coating was effective to extend the shelf life of strawberries and mangoes without significant weight loss (<5%), and maintaining good appearance, color, and brightness, which was attributable to the low polyphenol oxidase enzyme activity and the activation of the fruit antioxidant enzymes system, including superoxide dismutase activity. On the other hand, Lin et al. [44] mentioned that a CS-Ag-TiO 2 coating showed good adherence (an important parameter for edible coatings) when used on cantaloupe rind.
Despite the observation that CS-TiO 2 exhibited great potential to be used for food packaging purposes (good adherence in food surface, prevented microbial infection, and ethylene scavenging activity), it is necessary to assess the possible TiO 2 migration from CS-TiO 2 films into the food matrix [44]. García et al. [107] recently informed that migration studies in other types of hybrid composites (CS-ZnO and CS-Ag composites) have demonstrated that only a negligible amount of the nanomaterial migrates from the package to the food simulants or foods, suggesting that consumer exposure to these nanomaterials and associated health risks are minimal. However, further studies are needed to evaluate the potential migration of TiO 2 from hybrid composites to food matrices and its toxicity potential. Table 8 shows other potential applications of CS-TiO 2 , such as anti-metal corrosive material, as well as for the development of textile materials with protective effects against solar radiation. The treated cotton (CS-TiO 2 ) showed excellent protection against UV radiation in comparison to a cotton-TiO 2 system.
Chen et al. [56] integrated a CS-TiO 2 composite into a cotton matrix by a dip-pad-dry cure process for sun blocking applications and reported that the treated cotton fabrics demonstrated lower photocatalytic activity in comparison to cotton fabrics treated with pure TiO 2 in a typical photocatalytic test. The authors highlighted that the CS-TiO 2 /cotton showed excellent UV protection and suggested that the nanocomposite could be used on materials that are in direct contact with the human body. Similar trends were reported by Hsieh et al. [108] using wool fabrics cured with citric acid and TiO 2 -CS.
Bajali and Sethuraman [109] studied the effect of a CS-doped hybrid/TiO 2 (1:1 molar relation) nanocomposite by a sol-gel method and a self-assembled method on the corrosion resistance of aluminum in NaCl medium (3.5%) through electrochemical impedance studies and potentiodynamic polarization. The presence of the nanocomposite improved the corrosion resistance of aluminum (efficiency 90%) compared to undoped CS (66%). The aluminum corrosion resistance may be attributable to the formation of a dense network structure formed by the Al¬-O-Ti-O-Si-O-Si bond. Meanwhile, Ledwing et al. [110] achieved the optimization of electrophoretic deposition parameters for manufacturing of TiO 2 /CS composite coating on X2CrNiMo17-12-2 stainless steel. This hybrid coating (4-8 µm thickness film) improved the corrosion resistance of steel (−165 to −110 Ecorr) compared to uncoated steel (−291 Ecorr), and the composite coating exhibited a fully uniform distribution of TiO 2 (3 g L −1 ) nanoparticles on the chitosan (0.5 g L −1 ) surface. The increase in the corrosion resistance may be due to a significant reduction in the exposure area between metal and Ringer's solution. On the other hand, the adhesion of TiO 2 /CS coating on steel was dependent on the CS/TiO 2 ratio in the coating.

Conclusions
Evidence shows that the incorporation of TiO 2 into the chitosan (CS) matrix is a viable way to enhance the technological and biological properties of both compounds. In general, CS-TiO 2 exhibits better physicochemical, mechanical, and thermal characteristics than pure chitosan, with high biocompatibility and low cytotoxic effects on different cell lines. It is an attractive material with high performance, is low-cost, efficient, reusable, and environmentally friendly for diverse industrial applications based on its antimicrobial and photocatalytic characteristics.
CS-TiO 2 hybrid composite development is an active research area for biomedical and environment remediation applications and, hence, shows great value for industrial applications. A crescent research area for the application of CS-TiO 2 is food science and preservation, where their major challenges are the safe implementation of a hybrid composite and their public acceptance. However, additional research is needed to standardize and further develop the CS-TiO 2 hybrid composites because their technological and biological activities depend on their preparation method.