**1. Introduction**

The excessive consumption of fossil-based resources and resulting environmental problems issues coupled with the constancy growing global population requests the improvement of living standard and accelerating technology development. It has stimulated and attracted researchers worldwide to develop a sustainable bio-based alternative that can compete in performance with petroleum-based products expected to be employed in a wide range of applications [1–7]. Cellulose, as the most abundant bio-based material from the biosphere, has attracted more and more attention in different fields and could serve as a prominent alternative to the exhaustible fossil resources, owing to its renewability, biodegradability, biocompatibility, non-toxicity and environmental friendliness [8–12]. The advantages of cellulose can be also pushed forward through the exploration of its nonmetric size, which generates nanocellulose (NC), considered as a promising class for future materials due to its outstanding physicochemical properties [13–16]. NC displays low density, specific barrier properties

and low thermal expansion coefficient, high strength, excellent stiffness, elongation morphology, inertness, large surface area and aspect ratio, abundance and ease of bio-conjugation [11,17–19]. The presence of several reactive chemical groups on its surface allows it to be modified by physical adsorption, covalent bonding or surface grafting to further extend its performance [20]. Research activities concerning NC had attracted growing interest over the past decade as reflected by the rapid increase of scientific publications and patents granted internationally [21]. According to Markets and Markets, the NC market is forecasted to achieve USD 783 Million by 2025 [13] and thus, NC production will have a high economic impact [22]. Moreover, an interesting review has been recently published by Charreau et al. dealing with the analysis of the evolution of patents involving nanocellulose since 2010 [23], demonstrating the increasing industrial interest in the this, which enabled the setting-up of the first facilities producing commercial quantities of NC.

Numerous nanocellulose types with outstanding features can be produced from different cellulosic sources employing various approaches [17,24–26]. NC can be divided into two main categories, that is, nanostructured materials (cellulose microfibrils and microcrystalline cellulose) and nanofibers (cellulose nanocrystals, cellulose nanofibrils and bacterial cellulose) [13]. Due to their excellent inherent characteristics, cellulose nanocrystals (CNC), as a subclass of NC, is commonly produced from cellulosic fibers and fibrils after the elimination of the amorphous regions by acid hydrolysis [23,27,28]. CNC have aroused wide scientific and technological interest from both academicians and industrials and can be utilized as an independent functional material, template support, stabilizer, filler or reinforcing agent [29–31]. CNC-based nanomaterials have been extensively investigated due to their unique physicochemical, mechanical, thermal, rheological and optical features. CNC could confer excellent properties to hybrids or nanocomposites (metallic, ceramics and polymeric) even at low concentration for different applications such as medical, pharmaceutics, catalysis, oil/water separation, decontamination, flame retardancy, electronic and optical devices, energy storage, sportswear, light weight armor systems, food packaging, to cite a few [10,11,13,14,20,32–39].

The combination of CNC and nanocarbons, such as fullerenes, nanotubes (single-walled, doublewalled, few-walled or multi-walled), nanodiamonds and graphene-based materials (graphene, graphene oxide, reduced graphene oxide, graphene quantum dots), has recently emerged as a new class of hybrid materials for which a synergetic effect has been revealed in a wide range of applications, spanning from sensing and biosensing to catalysis, photonics and optics, energy and environment, water treatment, medical and optoelectronics. Other nanocarbons such as carbon black, activated carbon, carbon quantum dots and carbon nanofibers are less frequently used as CN-based hybrids [12,20,40–43].

Graphene-based nanomaterials (GNM), which have been considered as emerging and high efficient two-dimensional (2D) nanomaterials, play a crucial role in various research area since the discovery of graphene in 2004 [44,45]. They find applications in several fields such as thin-film transistors, ultra-sensitive chemical sensors and transparent conductive films, biomedical, microelectronics, composites, among others [46–49]. Recent investigations by Yang et al. provided general reviews of the whole graphene patenting activities and especially focused on the study of sustainable competitive advantages in the biomedical field [50,51]. A comprehensive review dealing with graphene, its related materials and properties have also been published [52]. Although the present development of industrialscale graphene is still widely at the Research and Development (R&D) stage, the global graphene market reached ca. USD 78.7 million in 2019, with the request in nanocomposites, energy storage materials and semiconductor electronics, which are also underpinning future growth rate estimates of >30% per year and expected to reach >USD 221.4 million by 2025 [53,54]. Nowadays, graphene oxide (GO) materials account for >30% of the global graphene market share as progresses in GO, permitting for numerous of possibly scalable approaches to reach mass production of chemically modified graphene with a wide range of applications [54,55]. However, despite that many technically feasible approaches are currently being developed to produce efficient GNM, there are still numerous practical obstacles that need to be overcome. For instance, GNM are more frequently produced from aqueous dispersions but can easily aggregate. Such agglomeration behavior can reduce the surface area

and negatively impact the mechanical, electrical and optical properties. Therefore, the incorporation of CNC not only surpasses such drawback through its excellent dispersive features but also confers further benefits to the produced GNM/CNC hybrids such as flexibility, stretchability, in addition to the improvement of the adsorption capacity, photothermal activity, stability, intrinsic luminescence and fluorescence, optical transparency and thermal conductivity [36,40,42,56].

Owing to the benefits of CNC and GNM materials as well as the numerous research works published during the last few years worldwide, a timely update on recent advancements in the field of CNC/GNM hybrid-based materials is an urgent need for both academic and industrial scientists. In this overview, we thoroughly review the recent progress made in the preparation, modification, properties and current applications of CNC/GNM hybrids in various fields. This work highlights a comprehensive overview with a forward-looking approach on CNC/GNM hybrids for numerous utilizations, which have emerged in the past five years. For the reader's comfort and to maintain lucidity, first, some of the basic concepts dealing with CNC and GNM, their preparation and features to further elucidate their unique attributes, are discussed in brief. We will then focus on state-of-the-art cellulose nanocrystals-graphene based materials, which have mainly emerged since 2015. Few articles before 2015 are succinctly summarized in some sections.
