The incorporation of carbon Nano-onions (CNOs) into polymeric materials represents a significant stride in materials engineering, leveraging their unique spherical, multi-layered structures to boost the mechanical, thermal, and conductive properties of polymers. Figure 1 shows the chemical structure of CNOs. Functionalized CNOs can be tailored through chemical modifications to enhance their compatibility with various polymer matrices, a practice that has seen increasing utilization across diverse fields. This editorial article examines the role of these functionalized nanoparticles in revolutionizing polymer composites, specifying briefly the techniques for their functionalization, the general types of polymer matrices used, their effects on composite properties, and their broadening range of applications.

Functionalizing CNOs involves attaching specific functional groups such as carboxyl, benzoyl, hydroxyaryl, or sulfophenyl, which are instrumental in enhancing their dispersibility and interfacial bonding within polymer matrices [1]. Figure 2 displays the functional groups: carboxyl, benzoyl, hydroxyaryl, and sulfophenyl. Techniques such as chemical oxidation, which introduces carboxyl groups [2]; benzoylation, which adds benzoyl groups [3]; hydroxyarylation, which adds hydroxyaryl groups [4]; and phenylation followed by sulfonation, which adds sulfophenyl groups [5] are commonly employed to increase the utility of CNOs in polymer composites [6]. These modifications facilitate stronger interactions between the CNOs and the polymer chains, resulting in composites that exhibit superior mechanical and thermal properties.

Selecting the appropriate polymer matrices is crucial for optimizing the performance of CNO-filled composites. Common matrices include chitosan [2,3,7] and poly(ε-caprolactone) (PCL) [8,9]. Figure 3 displays the chemical structures of chitosan and PCL. Biopolymers such as chitosan exhibit good solubility in conventional organic solvents compared to chitin [10]. PCL is renowned for its biocompatibility and biodegradability, underlining its importance in specialized medical applications [11]. Incorporating CNOs into these polymers often results in significant property enhancements. By modifying the characteristics of CNOs, their interactions with these polymers can be finely tuned to create materials that are not only structurally robust but also functionally adaptable to complex environments.

Functionalized CNOs (f-CNOs) primarily contribute to the mechanical strengthening of polymer composites through effective stress transfer mechanisms at the nano-level [12]. These enhancements are further supported by improvements in thermal stability [5] and electrical conductivity [9], which are essential for maintaining operations under varying working conditions. Additionally, the presence of functional groups on the CNOs enhances the interactions between these f-CNOs and the polymer chains, leading to increased dispersion of f-CNOs within the polymer matrix [1,9]. This uniform distribution helps prevent the aggregation of nanoparticles, thus enhancing the properties and improving the composite’s overall performance and durability over extended periods.

The strategic incorporation of f-CNOs into polymer matrices unlocks numerous applications across various industries. In the field of biomedicine, these composites are used to develop innovative regenerative tissue scaffolds [2,7], drug delivery systems [8,12], and biomaterials [9], capitalizing on the biocompatibility and enhanced mechanical properties provided by f-CNOs. Energy storage is another critical area, with f-CNOs-filled composites being explored for developing next-generation batteries and supercapacitors [3,4], which offer higher efficiency and extended life cycles. In additional applications, these composites are employed to produce electrode materials for electrochemical sensors, proton exchange membranes for fuel cells [5], and polymer nanocomposites for thermal management [1].

The functionalization of CNOs and their incorporation into polymeric materials represent a dynamic and rapidly evolving field in materials science, promising to meet the demands for materials that combine high performance with functional versatility. As research continues to advance, new methods of CNO functionalization and improved techniques for composite processing are expected to emerge, further broadening the applications of these materials. The ongoing development and optimization of CNO-filled composites not only pave the way for innovative applications in medicine and technology but also hold the potential for significant environmental and economic impacts by offering sustainable alternatives to conventional materials.