CNCs are derived from native cellulose fibers using strong acid hydrolysis, enzymatic breakdown, oxidation, and hydrothermal hydrolysis. These processes target and break down the less ordered amorphous and paracrystalline regions of the cellulose, leaving behind rigid and crystalline CNCs with a rod-like or needle-like structure. In addition to their renewable, biodegradable, biocompatible, lightweight, and non-toxic nature, CNCs possess distinctive features, such as a high aspect ratio, liquid crystalline behavior with a chiral nematic phase, remarkable mechanical strength, and adaptable surface chemistry. The rod-like shape and repulsive interactions of surface-modified or surface-charged CNCs cause them to self-organize into an ordered structure, forming a liquid crystalline phase when suspended (Fig. 1(b)). CNCs also have a high degree of crystallinity, which results in a specific modulus ranging from 60 to 120 GPa·cm3.g-1, roughly 4–5 times higher than that of steel. Unlike inorganic nanoparticles, CNCs have numerous active functional groups on their surface, including hydroxyl and aldehyde groups, enabling various chemical modifications to fine-tune their properties for specific applications [15].

Despite their impressive mechanical and physicochemical properties, CNCs often aggregate in many organic solvents and face difficulties in dispersing within hydrophobic polymer matrices, limiting their broader applications. To address this challenge, surface modifications are applied to improve their hydrophobicity. These modifications typically involve attaching various chemical groups to the CNC surface through covalent bonds or electrostatic interactions (Fig. 2) [18].

Physical modification of CNCs, primarily achieved through electrostatic interactions, provides an appealing alternative to chemical modifications because of its simplicity and the absence of solvents. However, research in this area has been limited in recent years [18]. By using this method for CNCs, physical modifications can change their configurational and anatomical characteristics, which in turn affects how well they bond with a polymer matrix. Common methods for physical modification include surface adsorption, plasma treatment, and ion beam treatment [19]. Surface adsorption, which uses surfactants, relies on interactions like hydrogen bonding, charge interactions, and van der Waals forces [20]. However, without strong covalent bonds, surfactant molecules may gradually detach from the matrix [21]. Plasma surface modification is a widely recognized, efficient, and cost-effective approach for enhancing CNCs and polymer surface properties. Plasma treatment improves the interaction between CNCs and the matrix while preserving the bulk material's properties. It involves generating a gas in which some atoms have lost electrons alongside free radicals in a vacuum, with the possibility of either inducing chemical changes within the outermost layers of the material or depositing a thin polymeric film. Low-temperature plasma treatment is known to improve surface properties by interacting with ions and free radicals. Submerged liquid plasma is an emerging method for modifying powders like cellulose, with the advantages of low cost and eco-friendliness [19]. Ion beam treatment, which can induce structural changes in CNCs and other materials, varies in effectiveness depending on temperature, ion type, and beam frequency. Low-energy ion implantation can disrupt cellulose's hydrogen bonding, impacting its crystallinity, while high-energy implantation may reduce crystallinity over time [22].

Recent research has explored different physical modification methods for CNCs. Bai et al. used ethyl lauroyl arginate as a cationic surfactant to modify CNCs, successfully stabilizing emulsified sunflower oil for over a month. The stabilization depends on the type of complex formed and the aqueous solution, which contains surfactant molecules that are not incorporated into aggregates or other structures [23]. Miri et al. combined CNCs with graphene oxide to create hybrid nanofillers that showed higher deformation resistance for poly(vinyl alcohol) (PVA) composites after modification, significantly improving Young's modulus, tensile strength, toughness, thermal properties, and moisture resistance. On the other hand, physical modifications may not always be as effective [24]. For instance, modifying oxidized CNCs with lysozyme via electrostatic interactions resulted in weaker bio-nanostructures compared to chemical modifications [25]. Toncheva et al. found that using physical interactions to assemble silver nanoparticles with CNCs led to nanoparticle aggregation, limiting the enhancement of material properties when compared to chemical bonding [26].

Chemically treating is a commonly used method for permanently altering the functional groups and bonding structure of CNCs. These nanocrystals feature numerous hydroxyl groups and, in oxidized forms, aldehyde or carboxyl groups, which serve as active sites for modification [18]. It is essential to ensure that the primary structure of the nanocrystals remains intact during surface modification, requiring the use of gentle chemical techniques to preserve their integrity. To enable CNCs to better interact with water-repelling polymeric structures or liquid-based formulations, their hydrophilicity and miscibility with polar environments should be enhanced. Chemical modifications of CNCs can address this by utilizing two primary approaches: (I) small molecule functionalization, which includes oxidation, esterification, cationization, acetylation, silylation, and sulfonation, and (II) grafting polymers onto the CNCs surface through “grafting onto”, “grafting from”, and “grafting through” methods using radical and RDRP processes, namely nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT). In the “grafting onto” method, pre-synthesized polymers are linked to the CNC's surface using coupling reactions. In contrast, “grafting from” features synthesizing polymer chains directly from initiator-anchored CNCs, and “grafting through” entails functionalizing CNCs with polymerizable groups (e.g., acrylic) and then performing in situ polymerization. Surface-modified CNCs are desirable for various functions in medical care, catalysis, optics, remediation, electrical fields, textiles, and paper production [12,19,27,28]. For many of these applications, functionalizing CNCs with stimuli-responsive polymers that react to environmental triggers, like temperature, pH, light, or CO2, is particularly beneficial. These modifications are mainly achieved through RDRP methods.

Surface functionalization of CNCs has become a key technique for tailoring their properties, including hydrophobicity, hydrophilicity, and photoresponsivity. While many methods have been used to modify CNCs' surfaces, recent studies suggest that introducing cationic charges to CNCs could open additional possibilities, particularly for applications in gene delivery, targeted drug delivery, and the development of replacement tissues. The natural surface of CNCs is characterized by an abundance of negatively charged ions, resulting from sulfuric acid hydrolysis, limiting their interaction with biological molecules that carry a negative charge, like nucleic acids and proteins. Cationizing CNCs could enhance these interactions and expand their applications in various biomedical fields (Fig. 3(a)). Given that cationic CNCs are biocompatible, non-toxic, and renewable, they offer great potential in therapeutic applications. Several techniques for cationizing CNCs have been explored. One method involves etherification reactions to introduce positive charges, while other approaches involve the incorporation of cationic porphyrin-based CNCs with photo-bactericidal properties or grafting cationic imidazolium or pyridinium groups onto the CNCs' surface [27].

Hasani et al. were the pioneers in documenting the process of giving CNCs a positive charge. They used an etherification reaction for cationization of CNCs derived from cotton filter hydrolysis. In this case, 2,3-(epoxypropyl)trimethylammonium chloride (EPTMAC) was used as the cationizing substance at high pH values. The reaction involved the nucleophilic addition of the hydroxyl functionalities on the CNCs surface to the epoxy group of EPTMAC (Fig. 3(b)). This process resulted in the creation of hydroxypropyltrimethylammonium chloride CNCs, and the cationization process was validated using electrophoretic mobility measurements and conductometric titrations. Nanoscale imaging showed that the morphology of the CNCs remained unchanged, although the surface charge density decreased after modification [29]. Feese et al. introduced another approach by modifying CNCs with a cationic porphyrin via the “Click” reaction, which forms a stable triazole ring by linking azide and alkyne groups. In this study, CNCs derived from cotton fibers were first azidated and then reacted with cationic porphyrin. The resulting cationic CNCs were able to photo-inactivate bacteria, such as Staphylococcus aureus and Mycobacterium smegmatis, highlighting their potential as biodegradable and renewable materials for antimicrobial applications (Fig. 3(c)) [30]. Thielemans et al. developed a one-pot heterogeneous synthetic method to prepare cationic pyridinium-grafted CNCs. In this process, pristine CNCs were processed using 4-(1-bromoethyl)benzoic acid alongside pyridine, introducing cationic pyridinium units onto CNCs (Fig. 3(d)). The process involved esterification and nucleophilic substitution, and the purified cationic CNCs were tested for their enhanced surface properties. These surface functionalization techniques expand the potential applications of CNCs, particularly in areas requiring interaction with negatively charged biomolecules or in antimicrobial and biomedical fields [31].

Acetylation, in which acetyl groups are placed on the cellulose surface, enhances its various properties. This is typically done by treating cellulose with acetic anhydride in acetic acid, which acetylates the cellulose hydroxyl groups and decomposes cellulose chains through hydrolysis. This reaction not only modifies the surface but also reduces the diameter of the resulting CNCs. Other methods for acetylation include using aqueous emulsions of alkenyl succinic anhydride as a template or reacting CNCs with vinyl acetate with the addition of potassium carbonate, although the use of 4-dimethylaminopyridine (DMAP) is known to be particularly effective in driving the acetylation reaction. The resulting acetylated CNCs exhibit increased hydrophobicity, which enhances their ability to dissolve in many different liquids, and consequently extends their utilization [32].

Esterification is a technique used to add ester units to the CNCs through condensation reactions between the cellulosic hydroxyl groups and esterifying reagents [32]. One of the most commonly studied esterification methods is acetylation, which involves replacing some of the hydroxyl groups of CNCs with acetyl groups. This process typically uses acetic anhydride (Ac2O) as the acyl donor, with dry acetic acid and small amounts of sulfuric or perchloric acid as catalysts (Fig. 4(a)). The acetylation process primarily targets the more accessible hydroxyl groups of CNCs [33]. The degree of substitution (DS), which measures how many hydroxyl groups are replaced by acetyl groups per anhydroglucose unit, is a key factor in quantifying the extent of acetylation [34]. A DS of around 1 facilitates better distribution in polar liquids, while a DS closer to 3.0 enhances its compatibility with non-polar media and provides stronger connections with hydrophobic matrices, leading to improved mechanical properties in composites. The DS can be regulated by adjusting the ratios of acetic anhydride and acetic acid during the reaction [35]. Zhou et al. showed that despite utilizing a high molar proportion of Ac2O to acetic acid, the DS reached only 2.61, indicating that acetylation does not fully replace all hydroxyl groups [35]. Barbosa et al. achieved a higher acetylation efficiency with sulfuric acid-hydrolyzed CNCs, reaching a DS of about 2.1, as the sulfate ester groups on the surface facilitated better acetylation by reducing electrostatic repulsion and making more hydroxyl groups accessible. Ultrasonication was found to improve acetylation efficiency without compromising the crystallinity of the CNCs. Other researchers have also developed methods to esterify CNCs [36]. Tang et al. achieved modified CNCs, which were esterified through a process involving acetic and sulfuric acids and ultrasonication, resulting in a yield of 85 % and a DS of 0.46 after 5 h at 70 °C, while preserving the high crystallinity of the CNCs [37]. Braun and Dorgan demonstrated that a single-phase method using a mixed acid system could esterify CNCs' hydroxyl groups while retaining their crystalline structure (Fig. 4(b)). This approach improved the distribution of the esterified CNCs in organic solvents like ethyl acetate and toluene, enhancing their compatibility with matrices made of hydrophobic polymers as nanoscale additives [38].

Silylation with silanes is a widely used technique for enhancing the hydrophobicity of CNCs, creating hydrophobic CNCs by replacing surface hydroxyl groups with different silanes. By using methyl trimethoxysilane as the silane coupling agent, the resulting hydrophobic CNCs exhibit a higher water contact angle and improved non-polar solvent dispersion compared to the neat CNCs. Traditional silylation methods typically require organic solvents and can be complicated during solvent exchange processes [39]. To address these challenges, a more environmentally friendly, solvent-free process has been developed utilizing 3-aminopropyltriethoxysilane (APTES) silane coupling agent. This method maintains the crystallinity and morphology of CNCs while enhancing their thermal stability [40]. APTES is favored for silylation due to its affordability [41]. During the silylation process, the alkoxy groups of APTES are hydrolyzed into silanol, which then bonds to the CNCs surface through silylation reactions [40]. Pei et al. used n-dodecyldimethyl chlorosilane as a silane coupling agent in toluene, which resulted in the generation of stable suspensions of partly silylated CNCs in THF and chloroform [42]. Similarly, cotton CNCs were silylated with isocyanatepropyl triethoxysilane (IPTS) in DMF, improving their dispersion in polymer matrices and expanding their potential for use in composite materials [43].

Sulfonation is a modification process that replaces some of the hydroxyl groups of CNCs with sulfonic acid groups, introducing anionic charges to the surface. The degree of substitution varies depending on several factors, including the proportion of sulfuric acid, the reaction temperature, the source of CNCs, and the production method. Sulfuric acid is a key chemical in the CNCs production process and surface modification at high temperatures, as it not only facilitates the hydrolysis of cellulose but also forms sulfate half-esters by interacting with the hydroxyl groups of CNCs. The sulfonation process results in the CNC's surface being covered with negatively charged sulfate groups. This negative charge enhances the CNC's ability to disperse in water and contributes to the formation of stable colloidal suspensions. In high-concentration suspensions, CNCs may self-align into a chiral nematic phase, a characteristic behavior driven by their unique structural properties. The sulfate groups, which also help stabilize these colloidal suspensions, can be retained in optically active films upon controlled drying. While the sulfate groups improve water dispersion, they also reduce the thermal stability of CNCs, particularly when the surface is extensively functionalized. Neutralizing the surface with sodium hydroxide can improve thermal stability. Additionally, hydrolysis is carried out by a mixture of sulfuric and hydrochloric acids, along with sonication, to produce CNCs with a lower density of sulfate groups, which enhances their thermal stability compared to those modified solely with sulfuric acid [44].

Oxidation reactions are often used to introduce functional groups like carboxylic acids and aldehydes onto the surface of cellulose. One of the most popular methods for this modification is 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation. This strategy particularly targets the hydroxyl groups attached to the sixth carbon of the cellulose chain, converting them into carboxylic acid units, while leaving the hydroxyl groups on the second and third carbons unaffected [45]. For optimal selectivity, the reaction is typically conducted at a pH between 9 and 11, as the process is slow and lacks selectivity at a pH below 8 [32]. In general, the process proceeds via the addition of TEMPO as a stable nitroxyl radical along with sodium bromide to a cellulose suspension, followed by the introduction of a secondary oxidizing agent like sodium hypochlorite or sodium chlorite. This process occurs at room temperature and proceeds along a controlled pathway, resulting in the targeted oxidation of primary alcohols to carboxyl units (Fig. 5) [45]. In a study comparing the oxidation efficiency of TEMPO and its derivatives, Iwamoto et al. found that 4-acetamide-TEMPO and 4-methoxy-TEMPO demonstrated a higher oxidation capability for wood cellulose in comparison with TEMPO. Furthermore, Isogai et al. introduced an electron-mediated oxidation method for softwood bleached kraft pulp, using TEMPO and 4-acetamido-TEMPO as catalysts, in the absence of relying on chlorine-based oxidants. This method helped maintain the structural integrity of cellulose, though it required a longer reaction time (48 h) compared to conventional oxidation techniques [46].

When sodium periodate (NaIO4) oxidizes CNCs, aldehyde functionalities are generated for crosslinking or further modification, thereby expanding the range of CNC applications. The hydroxyl groups of cellulose can be selectively oxidized to 2,3-dialdehyde functionalities by using NaIO4. In this reaction, two aldehyde groups are produced per oxidized unit (Fig. 2). To prevent light-induced decomposition of periodate, this reaction is carried out under mild conditions in an aqueous medium and in the dark. Temperature, reaction time, and NaIO4 concentration can control the degree of oxidation. Reactive aldehyde groups on the surface of CNCs can participate in Schiff base reactions with primary amine-containing components, including chitosan, amino acids, or polyethyleneimine. During this reaction, which arises from imine linkages, stable covalent bonds form between CNCs and functional molecules, thereby expanding CNCs applications for heavy-metal adsorption, hydrogel formation, and biomolecule immobilization [48,49]. While TEMPO-mediated oxidation generates carboxylic acids, sodium periodate oxidation modifies hydroxyl groups, producing aldehyde functionalities.

The functionalization of CNCs with polymers is essential in modulating their intrinsic physicochemical and mechanical characteristics and, consequently, their applicability across diverse domains. A primary consequence of polymer grafting is the enhanced colloidal stability and improved dispersion of CNCs within polymer matrices. This modification reduces agglomeration phenomena, facilitating a homogeneous distribution and consequently optimizing the thermal and mechanical functionality of the resultant composites. The establishment of polymeric chains at the CNCs interface promotes augmented interfacial interactions with polymer matrices, resulting in improved stress transduction and heightened mechanical robustness of the composite. The design of stimuli-responsive materials is enabled through the suitable selection of functional polymers, allowing for composites to exhibit tailored responses to environmental triggers, for instance, variations in pH, temperature, or gaseous environments. Surface modification ensures enhanced bio-compatibility while preserving the advantageous attributes of CNCs [50].

For changing surface properties of CNCs via polymer grafting, there are three primary strategies of “grafting onto”, “grafting from”, and “grafting through” (Fig. 6). The “grafting onto” method (Fig. 6(a)) involves anchoring a synthesized polymer chain to the surface of the substrate through covalent bonds. It offers the benefits of a well-defined molecular weight and narrow dispersity of the polymer. However, the approach can be limited by steric hindrance, resulting in a lower grafting density on the surface. This can reduce the efficiency of polymer surface functionalization, especially in the case of high molecular weight polymer chains. The “grafting from” (Fig. 6(b)) technique is known as surface-initiated polymerization (SIP), which involves initiating polymerization directly from initiation sites located on the surface of the substrate. The “grafting from” procedure typically yields a higher grafting density, as the polymer chains grow directly from the surface, reducing steric hindrance. However, controlling the molecular weight of the grafted polymers and achieving uniform polymerization can be more challenging in contrast to the “grafting onto” procedure. In the “grafting through” procedure, solid substrates are functionalized with polymerizable groups (e.g., acrylic groups), and then in situ polymerization is carried out to form grafted polymer chains. This method combines elements of “grafting onto” along with “grafting from,” providing flexibility in terms of grafting density and polymerization control. These surface modification techniques are critical for optimizing CNCs for specific applications, as they allow the adjustment of properties, such as hydrophobicity, chemical reactivity, and responsiveness to external stimuli, ultimately broadening the range of uses for CNC-based materials in various industries [12].

Surface modification of CNCs with polymers reacting to environmental changes offers a robust methodology for fabricating sophisticated substances with adaptable characteristics. This method improves the CNC's ability to blend, remain stable, and spread evenly within various polymer mixtures, while also giving them the capacity to react dynamically to external factors like acidity levels, heat, light, or specific chemical indicators. Chemical bonding of smart polymers to the CNC's surface results in materials that exhibit regulated expansion, reversible organization, or selective interactions with their surroundings. In summary, this technique broadens the range of potential applications for CNCs, establishing it as a pivotal strategy in the fabrication of future-oriented, smart, and environmentally sound nanomaterials [[51], [52], [53]]. Surface energy and wettability of CNCs change after polymer grafting, as shown in Fig. 6(c). This scheme highlights the influence of converting hydroxyl groups to other functional groups or replacing them with polymer chains via small-molecule modification or polymer grafting, respectively, thereby reducing surface energy and changing the nature of CNCs from hydrophilic to hydrophobic.