*4.2. CNC*/*GO*

GO, as one of the most important derivatives of graphene, contains a high density of oxygenfunctional groups, which can allow covalent, ionic or hydrogen interactions with numerous polymeric matrices, paving the way to several technological applications. It displays interesting features such as high specific surface area, high binding potential, high hydrophilicity, high dispersibility, superior mechanical properties and surface functional groups that can be employed as attachment sites [149]. However, to fully exploit the potential of GO, the production of GO-based composites is significant. Various GO-based nanocomposites have become increasingly mature in several fields. Recently, studies on CNC/GO hybrids have been actively conducted. CNC with outstanding features such as high Young's modulus, high crystallinity, high surface chemical activity and tailorable surface characteristics can offer strong, non-toxic and flexible advanced GO-based hybrids, which further inherit the features of both CNC and GO for which desired properties can be obtained. CNC/GO-related material composites have been investigated both on their own and after incorporation of other components such as metals, ceramics or polymers to modulate their final properties for specific uses. These hybrids found applications in nano paper, food packaging, biomedical, energy storage, sensors, decontamination, catalysis, adsorption, shape memory devices, foams, fire retardants and insulating materials, to cite a few.

The aqueous GO solution with CNC is considered the common approach used to prepare CNC/GO composites. Kafy et al. have produced CNC/GO film as humidity sensor using this blending method, followed by the drying process [150]. They synthesized CNC using the conventional H2SO4 hydrolysis method, whereas they produced GO by way of the modified Hummer's method. Then, they mixed the two suspensions at a desired ratio followed by homogenization. After that, the solution was poured in a petri dish and dried. The obtained hybrid exhibited a good dispersion of GO in CNC matrix for which high dielectric constant and low dielectric loss have been revealed, owing to the special polarization dipoles in CNC. These authors prepared a renewable, flexible and cheap sensor using this hybrid and an interdigital transducer patterned electrode deposited on a polyethylene terephthalate (PET) substrate. This sensor displayed a good sensitivity to humidity even under different temperatures. Similarly, Chen et al. produced CNC/GO using an aqueous suspension of CNC with either GO suspension or GO powder (Figure 7) [151]. The mixing process generated a stable solution for the first, whereas a metastable solution was obtained when GO powder was used. The drying process carried out via vacuum-assisted self-assembly technique (VASA), engendered non-iridescence and iridescence films, respectively, for the hybrids containing GO suspension and GO powder. It is worthy to note that CNC-based iridescent films found applications in optical functional materials. It was demonstrated that self-organized film was obtained from stable solution, while the separated structure was generated from the metastable solution. Interestingly, the later film, which displayed iridescent optical properties, consisted of self-assembled liquid crystals phase of CNC with embedded GO sheets. The authors claimed that such iridescent hybrid can be applied in security materials, reflective filters, sensors and other photonic materials.

**Figure 7.** Schematic illustration of the preparation process of GO/CNC hybrid films with or without iridescence. Reproduced with permission from Reference [151]. Copyright ©2014, The Royal Society of Chemistry.

Although the water-based dispersion is the widely adopted method to produce CNC/GO, it represents an unavoidable issue of the higher resistance. Valentini et al. have developed a method to produce CNC/GO with reduced electrical resistivity [129]. They employed the same approach of the mixing of the CNC suspension with the GO solution but assisted by an external electric field. This latter induced de-oxygenation of GO and hence its conductivity can be recovered to some extent. Such electrical conductivity was rather moderate, because of the presence of CNC as an insulating matrix. In a separate work of the same authors, the above approach, which is based on the drop-casting of an aqueous solution of CNC/GO between two metal electrodes, was found to be efficient to produce a resistive memory device based on CNC/GO thin hybrid [152]. Such thin film-based device exhibited a transition between low and high conductivity states upon changing the polarity of the applied external electric field. The authors claimed that such an achievement could promote the development of post-silicon electronic devices based on the integration of CNC/GN thin hybrids. Recently, Pan et al. developed a new method to produce chiral smectic structures through self-assembling 2D GO and 1D CNC nanorods [153]. Such a structure is closely dependent on the ratio of nanorods and nanosheets as well as the concentration of the composite colloid. The authors initially mixed CNC and GO suspensions at low concentration (<1%) and incorporated cross-linked polyacrylate hydrogel to concentrate the blend suspension. The CNC/GO was recovered by spin-coating of the colloid on PES substrate and dried at 60 ◦C. This method was considered timesaving compared to traditional approaches. It was demonstrated that such advancement can pave the way to develop optical metamaterials for optical modulation and mechanochromic sensors.

It was reported that poor dispersion of reinforcements at the nanoscale in addition to the weak interfacial interactions can negatively affect the material strength, toughness and other properties. Thus, several physical or chemical modifications can be employed to overcome such issues. In the case of CNC/GO composites, several approaches have been proposed to improve their efficiency for numerous applications. The common ones used to enhance the interfacial of such hybrids were based on the modification of CNC surface features, whereas few modifications have been simultaneously applied to CNC and GO.

One of the interesting production methods was that developed by Xiong et al. to manufacture ultra-robust transparent CNC/GO membrane with high electrical conductivity [154]. These authors improved the interfacial interactions of anionic CNC, prepared by H2SO4 hydrolysis and anionic GO sheets obtained through the modification of CNC with 10 wt.% cationic polyethyleneimine to introduce positive surface charge functionalities. This modification enhanced the ionic interactions between the strongly positively charged polymer and negatively charged flexible GO, which consequently improved the layer-by-layer assembly, carried out on a sacrificial layer of cellulose acetate on a silicon wafer, to design laminated nanohybrids with high flexibility, outstanding mechanical strength, high optical transparency along with excellent toughness. The authors claimed that such CNC/GO hybrids could be used for a wide range of technological applications, encompassing wearable electronic devices, biofluid separation, electromagnetic interference shielding and ballistic protection. The authors also employed the same approach to produce CNC/RGO hybrids after the electrochemical reduction of the former membrane [155]. In another work, Kabiri et al. produced acetylated CNC (CNCA), which was further used to prepare well dispersed CNCA/GO hybrid by a solvent casting method (Figure 8A) [156]. It was stipulated that the modification of CNC will promote its interfacial adhesion and miscibility with GO via hydrogen bonding. It was proved that composite supplemented with 0.8M of GO offered better thermal stability, interesting mechanical properties with an increase in the tensile strength of 61.92% with respect to CNCA. Moreover, the barrier characteristics against water were improved. The authors claimed that such a composite could find potential application in electrical and electrochemical fields.

**Figure 8.** (**A**) Schematic interaction of CNCA/GO. Reproduced with permission from Reference [156]. Copyright ©2014, Springer; (**B**) Schematic diagram of CTA-NCC/GO nanocomposite. Reproduced with permission from Reference [157]. Copyright ©2019, Elsevier.

Recently, Daniyal et al. have prepared hexadecyltrimethylammonium bromide (CTA) modified CNC/GO thin film and assessed its potential in sensing copper and nickel ions based on surface plasmon resonance (SPR) technique [157,158]. The authors initially prepared CTA-CNC solution and then 0.1 wt.% of GO was dispersed within the solution and sonicated at 70 ◦C for 1 h (Figure 8B). The obtained solution of CTA-CNC/GO was spin-coated and deposited as a thin layer on the glass substrate modified with a thin gold film. The authors demonstrated that the presence of CTA improved the sensitivity of the SPR. They revealed that the combination of SPR and CTA-CNC/GO has the potential to be employed as effective sensors, which can detect copper and nickel ions. In another research work, Beyranvand et al. produced hydrogel based on CNC/GO hybrid as a new adsorbent for methylene blue. During the preparation, azide-functionalized CNC was synthesized after CNC tosylation [159]. Then CNC-N3/GO was obtained via nitrene chemistry [160]. The production process of CNC-N3/GO, as well as its mechanism of action, is illustrated in Figure 9. It was demonstrated that the prepared hybrid was an excellent adsorbent of methylene blue owing to the higher adsorption capacity, reasonable contact time and recyclability. More recently, Zheng et al. have synthesized a modified CNC/GO hybrid as an efficient adsorbent of Dy (III). The authors used the evaporation-induced self-assembly (EISA) method to spontaneously form an imprinted film. Beforehand, they carried out an in situ selective oxidation of CNC using 2,2,3,3-tetramethylpiperidine-1-oxyl (TEMPO) for which C6 hydroxyl group was primarily oxidized to the carboxyl group (-COOH) [161]. It was found that such modification improved the stability of the TEMPO-modified CNC through strong electrostatic repulsion on one hand and on the other hand, it offered more surface active sites for the adsorption of Dy (III). The latter was further improved by the introduction of GO, which created extra bonding sites to Dy (III) and enhanced the adsorption capacity of TEMPO-CNC/GO hybrid. These authors reported that the developed green hybrid was efficient and had a strong regeneration performance.

The preparation and design of molecularly imprinted polymers (MIPs) is a multidisciplinary field, which encompasses various aspects of molecular recognition, biomimetic biology and polymer chemistry. MIPs preparation involves arranging functional monomers around a template, followed by polymerization with the presence of cross-linkers and a suitable initiator through covalent, semi-covalent or non-covalent intermolecular interactions and finally template removal. Such an approach has been recently explored to produce CNC/GO-based composites as molecular imprinted electrochemical sensors, which exhibited outstanding features. For instance, Anirudhan et al. have prepared MIP of silylated GO and chemically modified CNC using a drop cast method for the selective sensing of cholesterol [162]. The authors incorporated ZnO to CNC to enhance their conductivity. The electrochemical studies were carried out using cyclic voltammetry and differential pulse voltammetry. This sensor achieved good stability and reproducibility, low detection limit and wide linear range. The optimum pH, equivalent to the blood pH, was 7.4 and the optimum response time was only 10 min. In another work,

Wang et al. manufactured a CNC/GO-based MIP for the selective extraction and fat adsorption of synthetic antibiotics (fluoroquinolones, FQs), which can accumulate as residues in river water, causing a hazard for living organisms [163]. The preparation process of the magnetic@GO-grafted-CNC@MIP is depicted in Figure 10. It was found that the utilization of CNC and GO as substrates can improve the properties such as the stability, selectivity and affinity of MIPs compared to the conventional ones. The authors demonstrated that the prepared hybrid displayed an ultra-fast adsorption profile for FQs with high recognition and large detection limit range. They claimed that this method is accurate, effective, sensitive and simple, thereby appropriate for the detection of residual FQs in water sample.

**Figure 9.** Schematic representation of the production procedure, as well as the exhibition of the carboxylate and phenoxide adsorption, GO sites of the hybrids towards methylene blue. Reproduced with permission from Reference [159]. Copyright ©2019, American Chemical Society.

**Figure 10.** Schematic procedure of the preparation of the magnetic@GO-grafted-CNC@MIP. Reproduced with permission from Reference [163]. Copyright ©2017, Springer.

The simultaneous incorporation of CNC and GO to numerous polymers such poly(3 hydroxybutyrate-co-3-hydroxy valerate) [164], poly-N-isopropyl acrylamide [165], poly(3,4 ethylenedioxythiophene) [166], poly(vinylidene fluoride) [167], polyacrylamide [168], poly(εcaprolactone) [169], polylactic acid [170], poly(vinyl alcohol) (PVA) [171] and chitosan [172] was adopted as an efficient method to produce composites with excellent features since these nanofillers offer outstanding synergetic effects. Some surface modifications can be applied to CNC or GO to improve their dispersion and compatibility within the polymeric matrices. This type of nanocomposite found a wide range of applications in biosensing [165], plastic masks [172], tissue engineering [168,169], wastewater treatment [167], food packaging [173], supercapacitors [166], to cite a few. For instance, El Miri et al. evaluated the synergetic effect of CNC/GO as a functional hybrid to enhance the properties of PVA nanocomposites (Figure 11I). The nanocomposites were prepared via solvent casting method. The authors demonstrated that the tensile strength, toughness and Young's modulus were respectively enhanced by 124%, 159% and 320% compared to the neat PVA. The strong interfacial interactions and the synergetic effect of 1D elongated CNC and 2D exfoliated GO, which improved the dispersion and avoided the agglomeration of the nanofillers, were also highlighted compared to the incorporation of pure CNC or GO. Such nanocomposite may find application in food packaging materials. In another recent study, Kumar et al. produced hybrid hydrogels containing polyacrylamide-sodium carboxymethylcellulose (PMC), GO and CNC via in situ free-radical polymerization (Figure 11II) [168]. The obtained composite displayed outstanding mechanical performance, self-healing behavior and shape-recovery feature. The authors claimed that such highly hydrated hybrid hydrogel with tailorable properties might provide a 3D microenvironment for tissue engineering applications.

**Figure 11.** (**Ia**) Scanning electron microscopy (SEM) micrographs of PVA nanocomposites with CNC, GO and their hybrid (C: G-2:1 and C:G-1:2) and (**Ib**) schematic representations of the dispersion state of (i) GO and (ii) C: G hybrid within the PVA polymeric matrix. Reproduced with permission from Reference [171]. Copyright ©2016, Elsevier; (**II**) Schematic of the formation of the hydrogel: (**A**) Before and after heat treatment of PMC-GO1/CNC10.0 hybrid solution and (**B**) A suggested mechanism of physical and chemical interactions in the hybrid hydrogel system. Reproduced with permission from Reference [168]. Copyright ©2018, Elsevier.
