1. Antibacterial and Fungicide Power of Chitosan
It is well known that chitosan can inhibit the growth of bacteria (gram-positive and gram-negative) and fungi. However, this ability can vary according to many factors including the DD, the
MW, and the molecular chain configuration, which affect the physicochemical properties of the macromolecules [
32]. Moreover, differences in the target microorganism can contribute to this variability. Metabolomics analysis in
Listeria innocua has provided evidence that the initial and the most important targets of chitosan are the cell membrane and cell wall. The cationic nature of this macromolecule might enable the interaction with these negatively charged organelles causing its disruption and ultimately contributing to the antibacterial effect of chitosan [
33]. It has been reported that minimum inhibitory concentration values between different species of
Candida spp. vary, probably due to negative charge density and composition of the cell wall [
34]. Furthermore, studies suggested that chitosan uptake and its antifungal activity against
Penicillium expansum (a fungal pathogen) are dependent on clathrin-mediated endocytosis [
35]. To improve the antimicrobial power of chitosan, modifications of the backbone structure of this polysaccharide have been carried out. For instance, chitosan Schiff base derivatives, obtained via coupling chitosan with indole-3-carboxaldehyde, reached inhibition rates of 99% and 92% in gram-positive (
Staphylococcus aureus and
Bacillus cereus) and gram-negative (
Escherichia coli and
Pseudomonas aeruginosa) bacteria, respectively. The authors suggested that this was the result of an increased cationic capacity and better hydrophilicity of the polymer chain, as compared with non-modified chitosan [
36]. On the other hand, in fungi, hydrophobicity is generally associated with better antifungal activity. Substitution reactions with diethylaminoethyl and dodecyl groups on chitosan generated macromolecules with amphiphilic properties, which increased their capacity to form hydrophobic interactions with the fungal cell wall and resulted in a better growth inhibition index of
Aspergillus flavus, a human pathogen that can infect crops; moreover, a lower molecular weight led to higher inhibition [
37]. Similarly, double Schiff bases bearing halogeno-benzenes were tested on
Botrytis cinerea and showed increased antifungal effect (inhibitory indices > 95% at 1 mg mL
−1) due to the strong electron-withdrawing property of halogens and the hydrophobicity, although imine groups might also have contributed to cell death, acting as chelators and affecting the uptake of essential metals [
38]. Furthermore, disk diffusion assays have demonstrated that combination therapy of chitosan with antifungal drugs (e.g., fluconazole) exhibits remarkable synergistic inhibitory effects not only on sensible but also on resistant clinical strains of
Candida species [
26]. On the other hand, a sponge-like material obtained from thymine-modified chitosan derivatives has enhanced the treatment of wounds and tissue regeneration, and when the degree of substitution increased from 0 to 0.62, the minimal inhibitory concentrations (MICs) decreased from 64 to 16 μg mL
−1 for
Acinetobacter baumannii. In this perfect example, the material can confer protection against nosocomial pathogens that commonly infect wounds [
42]. Thus, chitosan is an antimicrobial agent with unique properties toward the development of commercial pharmaceutical formulations.
2. Biomedical Application of Chitosan: Encapsulation of Active Molecules
2.1. Phytochemical Protection Using Chitosan: Nutraceutical Formulations
Natural products such as plant-derived nutraceuticals, commonly used as functional ingredients in food, represent a suitable option to enhance treatments for cancer, diabetes, bacterial infections, and other diseases [
44,
45]. However, nutraceuticals exhibit limited gastrointestinal permeability and are susceptible to certain conditions such as degradation reactions and changes in pH and temperature [
46]. These disadvantages affect the performance of the active agents during their oral administration. In this sense, colloidal carriers based on chitosan have been developed to protect these substances, obtaining micro- and nano-particles with biocompatible and biodegradable characteristics [
47]. The interactions between the aminopolysaccharide and nutraceutical compounds can evolve from hydrogen bonds and electrostatic interactions. Specifically, it has been reported that the electrostatic interaction between curcumin and chitosan nanoparticles has a strong correlation with the number of intermolecular hydrogen bonds [
48]. As it is well known, -NH
2 groups onto this polysaccharide allow its chemical derivatization and grafting to improve its physicochemical properties [
49], where one of the objectives is to enhance the water solubility of this macromolecule, increasing the permeability of hydrophobic nutraceuticals, rising the potential of loaded particles as carriers for oral delivery [
50]. Regarding mucins, which are glycoproteins that are the major constituents of the intestinal mucus layer, their interaction with the chitosan backbone depends on the properties of both (mucins and the polymer) and can occur via electrostatic interactions, hydrogen bonding and hydrophobic interactions [
24]; the amino groups of chitosan can bind to the sialic acid of mucins resulting in a better mucoadhesion [
25], as well as the change in the surface’s charge (from negative to positive) of chitosan could trigger an increase in the adhesion of loaded particles on the intestinal mucosa [
25,
51]. In this topic, several studies have reported the development of nutraceutical formulations for oral administration using chitosan and modified chitosan. For instance, researchers reported that apocynin (an anti-inflammatory phytochemical extracted from roots of
Apocynum cannabinum) can be encapsulated in a w/o/w emulsion with non-modified chitosan, producing microparticles with suitable sizes around 326 nm (PDI = 0.201) and an encapsulation efficiency of 45%, and providing controlled drug release under in vitro gastrointestinal conditions (9.7% and 28.8% at a pH of 1.2 and 6.8, respectively). Furthermore, good stability under storage and large periods of absorption by the oral route were registered [
52]. Also, the same guest molecule (apocynin) was loaded in a platform from modified chitosan oligosaccharide, resulting in a carrier system with good stability and enhanced efficacy in longer-time oral administration for gastric ulcers [
53]. Similarly, nanogels from chitosan grafted by
ρ-coumaric acid loaded with
Syzygium aromaticum essential oil (particle size = 255 nm) exhibited a slow in vitro release at pH 7.4 (87.5% after 16 days); in addition, the system resulted in increased antioxidant activity and great potentials for boosting the antibacterial activity of the native essential oil [
54]. Another example is the preparation of chitosan-polycaprolactone nanoparticles loaded with thymoquinone (particle size = 182 nm), which showed a suitable control release of up to 24 h in simulated intestinal fluids, providing excellent mucoadhesion properties and improved oral bioavailability [
25]. In other work, when compared to chitosan, copolymers based on chitosan and polyethylene glycol methyl methacrylate (PEGMA) loaded with phenolic compounds (extracted from oregano) resulted in lower particle size (458 nm), more controlled release patterns in response to pH changes, and higher protection for the active agents in simulated gastric conditions. However, the carrier system prepared with non-modified chitosan (particle size = 1106 nm) had better loading efficiency and was more stable (zeta potential = 50 mV) than the block copolymers-based system (zeta potential = −15 mV) [
55]. As described, chitosan is a powerful material that helps protect nutraceuticals from premature degradation derived from different factors (e.g., light, pH, and temperature) and improves the bioavailability of these active compounds.
2.2. Chitosan in Synthetic Drug Encapsulation: Anticancer Drug Formulations
Cancer is a leading cause of death worldwide, and even though chemotherapy is one of the most effective methods for treating cancer, its clinical application needs to be improved due to the antineoplastics’ cytotoxicity, and the fact that many chemo-drugs are poorly water-soluble, lack of targeted delivery, have side effects, and experience drug resistance [
59]. A drug delivery system intended for cancer should be biocompatible and maintain the drug’s therapeutic activity, delivering the antineoplastic to the target tissue in a controlled way. That could allow achieving the desired concentration while reducing systemic side effects and the therapeutic dose [
21,
60]. Among drug delivery systems, nanosized carriers are an outstanding approach because they have shown high drug loading efficiency and could penetrate tissues accumulating around the tumor [
21]. For that, chitosan has been widely studied developing pH-responsive drug delivery systems, which swell in an acidic medium due to the protonation of amino groups onto the polysaccharide. Thus, this biocompatible polymer is suitable for drug delivery in cancer, where the tumor microenvironment exhibits a low pH due to the anaerobic metabolism [
60]. In contrast to healthy tissues, during their progression state, tumors may exhibit a higher or lower temperature as a result of increased vascularization or reduced metabolic activity, respectively [
61]. Hence, thermosensitive nanocarriers containing chitosan have been designed for cancer therapy. For that, chitosan grafted with poly(
N-vinylcaprolactam) or poly(
N-isopropylacrylamide) are the most-used copolymers in trials where the temperature is a triggering factor for drug release [
62,
63]. Additionally, dual- and triple-stimuli responsive materials are created for achieving a targeted and controlled drug release. Examples of dual-responsive chitosan-based materials include those that combine temperature and pH-sensitive systems [
64,
65], as well as those that use pH and electric field-sensitive polymers [
66] for triggering drug release. Hydrogen bonds and electrostatic interactions play a key role in polymer-drug interactions [
19]. In this topic, nanoparticles from modified chitosan encapsulating chemotherapeutics such as doxorubicin (encapsulation efficiency up to 85%), 5-Fluorouracil (encapsulation efficiency up to 86%), oxaliplatin, methotrexate, and paclitaxel (encapsulation efficiency up to 79%) have been developed and evaluated as a potential treatment for cancer [
67,
68,
69]. Furthermore, chitosan-based micelles (particle size = 211 nm; drug loading capacity = 54%) have presented outstanding pH-triggered doxorubicin release with negligible premature drug leakage in 60 h, providing better tumor cell growth inhibition than the free drug [
70]. Also, micelles (average size < 200 nm; zeta potential = 43 mV) obtained with amphiphilic chitosan grafted with O-methyl-O′-succinyl polyethylene glycol (mPEG) and oleic acid were developed for oral administration of camptothecin (CPT; drug loading around 8%), helping to decrease colorectal cancer (CRC) progression. This platform improved the aqueous solubility of CPT and protected it from gastrointestinal conditions, resulting in anticancer activity against CRC cell lines (such as Caco-2 and HT29), and a significant decrease in tumor growth and inflammation was observed [
71]. As an example of triple-stimuli responsive materials, a cascade-responsive nano-platform (particle size < 200 nm) was developed for breast cancer therapy. The system combined the thermosensitive characteristic of poly(
N-vinylcaprolactam), the acidic pH response of chitosan, and the cell-penetrating peptide, attaining selective nanoparticle penetration in tumor cells for doxorubicin release; as a result of in vitro and in vivo trials, the formulation that had doxorubicin was selectively taken up by cancerous cells [
67]. In other work, CS-LO-PEG-HER NPs were prepared from chitosan (CS), L-lysine α-oxidase (LO), polyethylene glycol 600 (PEG), and herceptin (HER); these nanoparticles presented cytotoxicity in BT474-xenograft tumor mice by promoting reactive oxygen species, mitochondrial membrane potential loss, and nucleus damage, resulting in a significant tumor cell reduction and avoiding damage in the kidney, liver, and spleen [
72]. Additionally, chitosan functionalization has been exploited for tumor targeting focused on overexpressed surface molecules or receptors on the cancer cell membrane, and single receptor targeting has been developed [
74,
75]; however, a dual receptor is preferred to enhance penetration. For example, folate receptors (FR) and epidermal growth factor receptors (EGFR) are key markers for tumor tissues. An FR ligand is folic acid (essential for cell growth and DNA replication), and an EGFR ligand is cetuximab. For lung carcinoma treatment, docetaxel-loaded chitosan nanoparticles, decorated with the dual receptor targeting of FR and EGFR, showed improved bioavailability and half-life, achieving longer circulation time and sustained release [
21]. In summary, chitosan-based materials have been tested and resulted in outstanding candidates for drug delivery systems toward controlled-targeted drug release, improving the bioavailability and reducing systemic side effects of antineoplastics.
3. The Role of Chitosan in Food: Material to Extend the Shelf Life
3.1. Coatings from Chitosan for Fruits and Vegetables
At the present time, besides the consumer interest in food quality and safe foods with new functionalities, it is required new materials with antimicrobial properties protecting fruits and vegetables during storage, which has extended the research concerning coating materials that can be fully eaten [
76,
77]. In addition, consumers demand foods with environmentally friendly packaging, forcing the industry to innovate and develop new packaging strategies. Edible coatings are thin layers made from edible materials that are formed into solid sheets and then applied over the food product [
78]; these coatings help prevent moisture loss and microbial development, establishing a semi-permeable safety barrier and maintaining the product’s structural integrity. Eventually, they could contain antioxidants and antimicrobials as to avoid deterioration in food products [
79]. Chitosan is considered a suitable material for the purposes of coating formation for fruits and vegetable protection because it is biodegradable and biocompatible, has biocidal activity and gas barrier properties, and yields edible coatings with excellent adhesiveness and cohesion with smooth surfaces for food products [
80]. Authors have stated that suitable chitosan-based films do not alter the appearance, flavor, aroma, or texture of fruits (e.g., strawberries) [
81]; in addition, this material helps control the oxidative stress, preserving a proper balance of reactive oxygen species in fruit cells [
82]. Furthermore, chitosan allows an easy combination with additional components toward film formation, such as other polysaccharides, plasticizers, proteins, or lipids. Hence, this outstanding polymer favors the formation of coating and films with good mechanical properties that have selective permeability to oxygen [
83]. Parameters such as tensile strength and elongation at break can be adjusted by using an optimal content of chitosan [
84]. Researchers have indicated that a higher chitosan content (more free amino groups of the polymer) in composite films led to a more compact structure, decreased permeability, and enhanced antioxidant activity [
85]. As illustrative examples, from chitosan-based films plasticized with spermidine and/or glycerol, authors reported that the incorporation of spermidine increased markedly the elongation at break, just as proper concentrations of both spermidine and glycerol enhanced the extensibility and plasticity; also, the gas permeability (GP) was reduced (2.40 cm
3 mm m
−2 day
−1 kPa
−1) but the water vapor permeability (WVP) was higher (0.37 cm
3 mm m
−2 day
−1 kPa
−1), as compared with single chitosan (GP = 15.81 cm
3 mm m
−2 day
−1 kPa
−1; WVP = 0.05 cm
3 mm m
−2 day
−1 kPa
−1) [
88]. Similarly, researchers carried out the preparation of alginate/chitosan-mixed edible films as a coating on figs (
Ficus carica), and found that the coating preserves bioactive compounds and the antioxidant capacity of the product during storage [
89]. In other work, Pavinatto et al. studied chitosan-based coatings for the mechanical and biological protection of strawberries, where glycerol was used to enhance elasticity and hydrophobic character. The results showed that fungal growth in coated strawberries was not detected (after 7 days at 23 °C) when chitosan/glycerol-30% films were used, but uncoated strawberries were completely taken up by fungi [
81]. In the same sense, an edible antimicrobial coating was produced from chitosan modified with monomethyl fumaric acid (CS-MFA) for fresh strawberries; when compared with the non-modified polymer and the control samples, CS-MFA decreased the weight loss, total aerobic count, and the count of yeast and molds [
90]. Thus, chitosan based-coatings are highly promising materials for extending the shelf life of fruits during storage.
3.2. Biodegradable Plastics Containing Chitosan: Food Packaging
Biodegradable plastics are materials that can be broken down into water and CO
2 by naturally occurring activities of bacteria, fungi, and algae. Thus, the degradation rate depends on the environments where they end up (e.g., soil or marine water) [
92]. In this topic, several variables affect the biodegradability, including the raw material, chemical composition, final product structure, and the environmental conditions in which the product is expected to biodegrade [
93]. Poly(lactic acid) (PLA), obtained from renewable sources, seems to be one of the most promising biodegradable materials for replacing plastics derived from petroleum, because this polymer provides similar or better properties than conventional plastics [
94,
95]. However, despite the biological compatibility and high transparency of PLA, properties such as high flammability, poor ultraviolet resistance, and brittleness need to be addressed [
96,
97]. To this end, chitosan possesses additional advantages such as antimicrobial activity, the possibility of chemical modification from its reactive amino groups, and excellent functional properties when combined with other materials [
98,
99,
100]. Adding different content (0–5.0%
w/
w) of chitosan nanoparticles to PLA by twin-screw extrusion benefits the properties of the resulting composite by enhancing the elongation and the impact strength; however, the tensile strength and thermal stability are decreased [
101]. Another important aspect of these plastics is biodegradability; in this regard, a film based on PLA and chitosan was fabricated by a non-solvent induced phase separation method. The synthesized film presented a porous structure where the pore size can be changed by modifying the PLA/chitosan ratio, and more importantly, the degradation rates were proportional to the pore size; therefore, tunable degradation rate can be obtained [
102]. The degradation behavior under different times of standard weathering conditions has been analyzed for a film containing polyethylene, PLA, and chitosan prepared by extrusion. The films containing a mixture of synthetic and natural polymers are more susceptible to degradation in comparison to films without chitosan. Moreover, the incorporation of a compatibilizer (poly (ethylene-g-maleic anhydride)) into the films increases the degree of homogeneity and favors film degradation without a significant effect on their thermal stability [
103]. Chitosan microspheres and phytic acid with core-shell structure have been developed and employed as additives for PLA composites, which improved flame retardancy, mechanical properties, UV resistance, and degradation capacity in soil. To explain the accelerated degradation, the authors proposed that the water is easy to gravitate and attack the chain of PLA, and both the additive and water assisted the microbial reproduction, and these processes simultaneously erode the film [
96]. In other work, Chang et al. prepared chitosan/PLA plastic films by extrusion and demonstrated that covering fish fillet with a 0.5% chitosan–PLA film reduced the number of several microbes (e.g., mesophiles, psychrophiles, coliforms,
Pseudomonas,
Aeromonas, and
Vibrio) and the total volatile basic nitrogen value in the grouper fillets, when stored at 4 °C [
104]. Similarly, composite films from nanochitosan in PLA matrix, using polyethylene glycol as a cross-linking agent and polyvinyl alcohol as a plasticizer, were found to be useful for the packaging of fresh prawn as it extended its shelf life. In this case, the quality parameters of the product were acceptable until 15 days of storage wherein the use of chitosan (1%) effectively reduced the microbial growth. Furthermore, the author indicated that both the film thickness and the chitosan incorporation influenced the permeability of the film [
105]. Tan et al. developed biodegradable plastics from chitosan-reinforced starch-based films; they investigated the effects of processing parameters, such as the polymer concentration, glycerol loading, and temperature, on mechanical properties. As a result, a tensile strength of 5.19 MPa and elongation at break of 44.6% were attained using reaction conditions involving 5 wt.% starch, 40 wt.% glycerol, and 20 wt.% chitosan at 70 °C [
18]. Researchers have also demonstrated that chitosan can be uniformly integrated into polyethylene terephthalate (PET), a typical packaging material for disposable soft drink bottles, through the extrusion process. The best performance in miscibility and degradation was reached with a mixture 95/5 (PET/chitosan) in weight ratio [
106]. Thus, the incorporation of chitosan into plastic materials modifies characteristics like UV resistance, transparency, and antimicrobial activity, among others. Moreover, the resulting plastic is more easily degraded; nevertheless, these biodegradable materials are often not as biodegradable as required. It is unlikely to find a unique solution in terms of designing a single polymer, which degrades easily in a wide variety of ecosystems.
4. Agriculture: The Role of Chitosan in Plant Growth
One of the goals of sustainable development is to ensure food for all people worldwide. Thus, it is necessary to improve food and agriculture systems. In this regard, chitosan has been registered with EPA (US Environmental Protection Agency) as a fungicidal and antimicrobial agent, as well as a plant growth regulator (PGR) within the minimum risk pesticide list [
107]. The advantage of chitosan appreciated by farmers is its contribution to promoting plant growth, eliciting plant resistance against biotic and abiotic stress, and activating symbiotic signaling between plants and beneficial microorganisms [
108]. Chitosan and its fragments have been shown to act as defense elicitors for diseases, mainly fungal infections, as they are recognized by the plant as stress signals. For instance, plants increase hormones and phenolics production when chitosan is applied to plants’ roots [
109]. Foliar spraying of chitosan solutions also impacts the infection by fungal pathogens; as an example, it has been reported that
Botrytis cinerea infection is affected by the increase of plant resistance when chitosan is applied. This effect is related to callose deposition and accumulation of jasmonic acid (JA) in leaf tissues [
110]. Chitosan has also been studied as a growth promotor. Studies on ornamental plants like
Dendrobium orchids indicated that this polysaccharide increases floral production by affecting chloroplast gene expression [
111]. For that, its oligosaccharides of low molecular weight chains are recognized to be more active [
112]. When applied to baby leaf red perilla (a culinary vegetable), chitosan promoted plant height, fresh weight, and antioxidant levels, acting as a biostimulant for plant growth and quality [
113]. For the in vitro germination of plants, the supplementation of media with plant growth regulators is used to stimulate seed germination and organ development. Studies have shown that chitosan and its oligomers can be used as alternatives to the commonly used plant growth regulators including auxins and cytokinins [
15]. In this regard, germination studies conducted on orchids confirmed that chitosan acts as an in vitro growth stimulator for meristemic tissues, accelerating protocorm formation up to 15 times compared to control plants, and demonstrating the relationship between the polymer molecular weight and its effectiveness [
112]. Chitosan nanoparticles obtained with TPP have also been tested as elicitors for germination but these have shown a phytotoxic effect at lower concentrations (5–20 mg L
−1) than the bulk chitosan (100 mg L
−1), causing a dramatic growth cessation. Bulk chitosan achieved higher antioxidant levels; nevertheless, the nano-chitosan was the most effective elicitor for organogenesis [
114]. Similarly, the results of chitosan microparticles supplementation improved the promotion of the foliar area and root and shoot biomass than bulk chitosan in tomato seeds [
115]. Chitosan oligomers can enhance the activity of enzymes involved in primary (e.g., nitrate reductase, ribulose-1,5-bisphosphate carboxylase/oxygenase, and carbonic anhydrase – all of these are involved in photosynthesis) and secondary (e.g., phenylalanine ammonia lyase and L-tryptophan decarboxylase, for phenolics and alkaloid biosynthesis, respectively) metabolic pathways [
116,
117]. The secondary metabolites “terpenes” are economically attractive for their curative and industrial uses, and their production in plants also increases with the application of chitosan [
116]. In the same way, authors studied the foliar application of a mixture of semisynthetic chitosan derivatives to induce tolerance to water deficit (for 15 days) in maize, finding that the mixture of derivatives increased the content of phenolic compounds and the activity of enzymes involved in their production, increasing dehydroascorbate reductase, total soluble sugars, total amino acids, starch, grain yield, and harvest index [
119]. In this topic, materials from chitosan can also be tailored for encapsulation and slow release of plant growth regulators (e.g., pesticides and fertilizers), being a polymeric matrix that provides different benefits, such as protection of guest compounds from adverse environmental conditions (pH, light, temperatures) and protection of plant cells from hazardous effects thus avoiding a burst release of active ingredients [
120]. For instance, Feng et al. studied coumarin-containing light-responsive carboxymethyl chitosan nanocarriers for controlled release of pesticides, and found good bioactivity on the target plant (cucumber) with no impact on the non-target plant (wheat) using 2,4-dichlorophenoxyacetic acid as model pesticide [
23]. Based on the abovementioned, chitosan application in crops, medicinal, and ornamental plants influences plant defense and plant growth by inducing enzymatic genes for primary and secondary metabolism, and it is a promising way for increasing the yield of economically valuable secondary products. An interesting recent approach is genetic engineering using nanochitosan for a sustainable increase in crop productivity. Results showed that nanochitosan enhanced anti-pathogenic and plant growth-promoting activity. Nanochitosans are also promising materials in agriculture for the controlled release of pesticides, nutrients, fertilizers, and plant hormones [
121,
122].
5. Textile Industry: Development of (Cosmeto-)Textiles Containing Chitosan
The demand for textile goods with antimicrobial activity is continuously growing, and a number of chemicals are used to fulfill this task; nevertheless, the change from toxic to non-toxic chemicals, producing eco-friendly materials, is preferred. For instance, medical staff, law enforcement officers, and firefighters, among other occupations, should be correctly protected from biological agents to avoid the propagation of infectious microorganisms [
123]. In this sense, chitosan is a versatile polymer with broad applications in the textile industry [
1]. The main characteristics of chitosan appreciated by textile industrialists are its biodegradability, antistatic activity, chelating property, deodorizing property, ability to form films, chemical reactivity and encapsulating capability, ability to control the strength and rigidity of fibers and dyeing, thickening properties, and its ability to heal wounds, the latter being of interest in the biomedical field (surgical threads and sanitary fibrous products) [
124,
125]. Specifically, chitosan can be used to concede excellent properties like antimicrobial activity to commercial textiles; this power against several bacteria and fungi is due to its polycationic nature [
126]. Authors have performed textile physical tests of chitosan-based fibers, comparing the maximum tensile force and maximum knot breaking strength after a knot formation for the fibers from chitosan and ionic liquid 1-butyl-3-methylimidazolium acetate, where both parameters were adjusted by controlling the chitosan content [
127]. Furthermore, cotton and wool textiles pretreated with chitosan present good affinity to anionic dyes, high dye uptake, and color strength due to the high proportion of amino group on chitosan, which provided more adsorption sites for anionic dyes through van der Waals forces and electrostatic attraction [
128]. This property of chitosan is highly relevant in two different aspects; first, the amount of dye required in the process is less, and therefore, it is more economically viable; second, the dye deposited into the environment is reduced. When dyes are released into the environment, they can generate lethal wastes, and also, they are extremely mutagenic and carcinogenic. Besides, their perseverance endangers productive agricultural land and aquatic life, and even a small dye concentration adversely affects gas solubility and the transparency of water [
22]. Chitosan is incorporated in textile products as fiber after undergoing a series of processes. However, the fibers have some limitations related to poor mechanical properties, high electrostatic charge, and high cost [
129]. To overcome these limitations and enhance the performance of the manufactured yarns, distinct approaches have been addressed during its synthesis, such as the blending ratio of chitosan with a second material, blending methods, the solvent used, and others [
130]. During textile production, a sizing agent is needed to protect against breaking the fibers and filament yarns in the weaving machine; for that, chitosan has been proposed as a highly compatible alternative to synthetic sizing agents. The economic and ecological advantages of applying chitosan in sizing were demonstrated with the weaving efficiency increase (based on the reduction in yarn breakage), reduction in wastewater (from the use of less sizing agent), and eco-friendly textile production (from substances which are easily biodegradable) [
131]. In this topic, a water-soluble chitosan derivative,
O-acrylamidomethyl-
N-[(2-hydroxy-3-dimethyldodecylammonium) propyl], was synthesized and applied to cotton samples. The treated cotton fabrics were able to maintain antimicrobial properties against
Escherichia coli even after 30 home launderings. Furthermore, salt-free reactive dyeing of the treated fabric showed good dyeing properties and washing fastness [
135]. Researchers have also tested chitosan microcapsules containing an antifungal agent (clotrimazole) with potential applications onto socks or bandages as a treatment for athlete’s foot; the performance of these microcapsules was evident after studying the in vitro inhibition of
Trichophyton rubrum growth and cytotoxicity (in skin cell lines). The authors suggested that the system could continuously release antifungal agents in a controlled manner under pressure [
136]. As demonstrated by these studies, chitosan is a promising polymer in the development of (cosmeto-)textiles due to its versatility.
6. Synthesis Processes: Chitosan in Catalytic Scaffolds
Catalysis is a technology developed to increase the rate of chemical reactions and/or establish mild reaction conditions using catalysts. Ninety-five percent of chemicals in the industry come from catalytic processes [138]. In this field, heterogeneous catalysis offers advantages over its counterpart (homogeneous), such as easier separation of catalyst from the reaction mixture, reusability, good stability, and low toxicity of catalyst, among other factors [139]. Regarding solid catalysts, the supported ones are the most commonly used, and they are typically formed by nanometric particles of at least a metal that is dispersed on the support’s surface [140,141]. In this topic, chitosan is an attractive material to be used as support in supported catalysts. The presence of amino and hydroxyl groups onto the polymer chain provides a number of structural modifications that can improve thermal and mechanical properties [142]. Besides, chitosan functional groups represent possible binding sites that could strongly interact with metal ions and metal nanoparticles [31,143], which is a key requirement for a catalyst support. For metal surfaces, their interaction with the -NH2 and -OH groups of chitosan can be attributed to the strong chelating interaction between the lone-pair electrons of O or N atoms and empty d-orbits of the metal [30]; this is an advantage in processes such as the preparation of 3D-macroporous scaffolds with in situ formed metal nanoparticles (catalysis) [31]. Recently, the excellent properties of TiO2 as a photocatalyst for the degradation of different organic compounds in an aqueous medium have been reported, but one of the challenges for its technical implementation is the difficulty in its separation from water. Therefore, to address this challenge, Bergamonti et al. employed a 3D-printed chitosan scaffold as a support for TiO2 and obtained an active photocatalyst for amoxicillin degradation. The size of the TiO2 nanocrystals (approximately 20 nm for the anatase phase and 25 nm for the rutile phase) was not affected by its immobilization within the 3D chitosan scaffold [144]. Furthermore, chitosan-based scaffolds supporting metal nanoparticles have been claimed to be active for catalytic reduction in pollutants; for that, Pt and Pd nanoparticles were in situ formed into walls of 3D-macroporous scaffolds (cryogels), resulting in materials catalytically active to 4-nitrophenol reduction [31]. Another interesting catalytic topic involves the conversion of biomass to high-value chemicals or fuels, where lignin, cellulose, and hemicelluloses are the three major components of lignocellulosic biomass. Recently, photocatalysis has emerged as a promising method for that purpose [145], and it has been reported that chitosan-based catalysts are efficient photocatalysts. Li et al. reported a feasible path for lactic acid production via photocatalytic reformation of biomass, promoted by an alkaline chitosan hydrogel hybridized with CuO [146]; these authors attributed the efficiency to the CuO ability for visible-light adsorption (CuO band gap is ca. 1.7), and to the improved stability of CuO provided by chitosan; additionally, the visible-light adsorption of CuO was not affected after hybridization with chitosan. Chitosan also plays a very important role in catalysis as a green solvent or green electrolyte. It generates green solvents, increasing the surface exchange capabilities and utilization of ionic liquids, contributing to the implementation of green chemistry principles by minimizing the amount of required products and the utilization of renewable raw materials. These solvents open up new fields of non-aqueous biocatalysis and biotechnology (enzymology). Hence, chitosan represents a potentially sustainable and versatile material for catalytic applications.
7. Water Treatment Using Chitosan: Flocculation
Water treatment/remediation is an issue of great interest to ensure the water supply for future generations. In recent years, the increase in contamination of natural water reservoirs (e.g., rivers and lakes) has been exposed [151]. As is well known, water treatment plants are inefficient in removing certain substances such as metals, drugs, dyes, plastics, and pesticides because they were not designed to remove these pollutants [152,153]. Furthermore, the inappropriate management of effluents triggers freshwater contamination resulting in an ecological disturbance and representing a public health risk [154,155]. Therefore, it is essential to improve the processes involved in the treatment trains. Chitosan, a low-cost and versatile biopolymer, can be used for environmental applications, including water and wastewater treatment (biocoagulation, bioflocculation, and biosorption), membrane filtration (polymer-assisted ultrafiltration), sludge dewatering, and odor reduction. Among them, flocculation deserves particular attention; for example, for the removal of pollutants present in water from aquaculture [156]. The flocculation process is widely used at the industrial scale, where eco-friendly flocculants such as chitosan with high effectiveness, manufactured from renewable sources, and ease of use are desired [157]. It is relevant to highlight that unmodified chitosan has presented a higher performance potential than poly(aluminum chloride) (PAC) when pH regulation and the removal of heavy metal ions from wastewater were studied, which could be attributed to the great effectiveness of this aminopolysaccharide in the removal of dissolved/dispersed organic matter (combining coagulation and flocculation), in addition to its high chelating ability [158]. Moreover, turbidity reduction using samples from rivers and wastewater has been quite comparative for both flocculants, obtaining bigger and more compact floc with chitosan; further, in the case of metal ion removal, chitosan has shown more affinity to certain metals. However, an optimal dose (e.g., from 0.5 to 15 mg L−1) is usually required [16,158]. Particularly, for water samples having pH values from 6.5 to 8, electrostatic interactions between chitosan (isoelectric point within the pH range 7–8) and contaminants are limited [158]. On the other hand, the simultaneous addition of PAC and chitosan to low turbid water resulted in efficient turbidity removal of 87%, indicating a synergistic effect between the two polymers [159]. The use of modified chitosan, such as chitosan-based graft copolymers, can result in water-soluble materials that exhibit a wider flocculation window, a range of concentrations yielding high effectiveness, and higher flocculation performance toward the removal of turbidity, small molecules, and heavy metal ions [160]. Also, the modified chitosan provides a synergistic effect with FeCl3 yielding higher turbidity and orthophosphate removal (>93%) with greater efficiency over unmodified chitosan [13]; for microplastic (polystyrene), the system tannic acid–chitosan conjugates and FeCl3 has shown higher removal efficiency (84%) aided by metal-polymer coordination bonds, as compared to single chitosan (54%) and tannic acid–chitosan (52%) in absence of Fe3+ [162]. Besides, graft copolymers offer excellent sludge dewatering (improved sediment consolidation) [165]. For their application on an industrial scale, the process using chitosan and its derivatives could be performed in the same infrastructure of treatment plants as for PAC, given that their dosage can also be carried out in a liquid formulation. Eventually, a chitosan-PAC mixture could be a feasible option in terms of its effectiveness, availability, and cost of materials.
Regarding the performance in other treatment methods, chitosan-based membranes have been designed for small substances entrapment via filtration, and in specific trials up to 81.21% of dye rejection (reactive black 5) and 78% of heavy metal removal (manganese) have been reached [166]. Hence, it is clear that this polysaccharide can help in the production of drinking water and water remediation by different strategies.
This entry is adapted from the peer-reviewed paper 10.3390/polym15030526
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