CDs have inspired considerable research efforts to develop a novel type of nanoscale carbon-based material, because of their exceptional properties and potential applications. Currently, CDs and doped CDs are synthesized via two methods, namely the “top-down” and the “bottom-up” methods. Several synthesis techniques have been developed over the past 20 years, including arc-discharge, laser ablation, sonochemical, microwave, hydrothermal, and electrochemical methods as well as strong acid oxidation. First, small organic molecules were polymerized, followed by carbonization in order to create pristine CDs. A general classification of the synthetic methods for metal and non-metal-doped CDs are as follows: (i) top-down methods and (ii) bottom-up methods. The top-down methods involve the breakdown of larger carbon structures using chemical oxidation, discharge, laser ablation, electrochemical, or ultrasonic processes. The top-down approach has a number of advantages, including being simple to use and environmentally friendly as well as having the ability to generate many types of small fluorescent CD particles. It is also possible to generate noble CDs without ligands using the top-down approach. Despite its advantages, the top-down approach has some disadvantages, such as harsh reaction conditions, high material costs, and long reaction times. CDs and CD-based materials are difficult to obtain in the proper particle size and shape, which is a major disadvantage of this approach. In the bottom-up approach, carbon-based molecules are converted into compact discs of carbon materials of the desired size. A number of bottom-up methods have been developed, including sonochemical, microwave, hydrothermal thermal decomposition of organic molecules, pyrolysis of carbon-based materials, and solvothermal reaction synthesis [32]. Furthermore, CDs can be carefully developed and controlled with respect to their composition, structure, morphology, and particle size by choosing appropriate chemical precursors, functional molecules, and synthesis methods [48]. Given the progress made in the synthesis of CDs, researchers have begun to optimize and design the surface groups and/or carbon core structures of CDs in order to exploit their novel properties for targeted applications [49]. In order to achieve the aforementioned goals, two representative strategies have been utilized, namely surface functionalization and elemental doping. Due to the designability of CDs, they possess superior physicochemical, optical, and mechanical properties. At the same time, the range of applications for CDs is expanding based on a variety of surface functionalization techniques. In addition to their varied applications in sensors, designs for drug delivery, photovoltaic solar cells, different catalysis, and finally lubricant additives, CDs have recently drawn considerable attention as good lubricant performance additives owing to their many desirable properties, such as ultrafine small (~4–7 nm) and even particle size, high dispersion stability in different solvents, adaptable hydrophilicity, and molecular chemical inertness.

Synthesis approaches for CDs are facile and fully mature; they do not require sophisticated equipment or any harsh synthesis conditions [28,50,51,52,53]. The tribological performance of CDs could be quickly modulated by engineering their carbon core structure and surface functionality by several organic molecules, which are based on a well-designed “bottom-up” approach [54]. In order to achieve the above purposes, it is necessary to select appropriate precursors, functional molecules, and CD synthesis methods. Besides the aforementioned merits, CDs are environmentally friendly [48,55], which is an important parameter to be considered when applying nanoscale carbon materials to lubricants. CDs are mainly composed of carbon, oxygen, nitrogen, and hydrogen as with other carbon-based nanomaterials; therefore, they do not pose a threat to the environment [48]. Thus, CDs have a number of advantages, such as the use of inexpensive and non-toxic raw materials, simple synthesis, bioimaging, biomedical applications, renewable resources, simple operations, and environmental friendliness.

CDs nanomaterials can be used to modify the tribological properties of lubricants due to their homogeneous size distribution and other physical and chemical characteristics [56] Moreover, the size, shape, structure, surface functional groups, and concentration of materials play an important role in determining the tribological properties of CD material-based lubricants [57]. The application of CDs and doped CD materials as lubricant additives is still in its infancy, despite the fact that CDs and doped CD materials demonstrate a number of inimitable characteristics [58]. As lubricant additives, CDs and doped CD materials have enjoyed a relatively short history of about seven years since they were first used as a lubricant additive to base oils in the year 2015 [59]. Due to their outstanding properties, considerable attestation has been given to synthesized CDs, as well as doped CDs, as additives to lubricating base oils over the past few years. In this review article, we summarize the recent progress of the utilization of CDs and CD-based nanomaterials in lubricating base oils of various types such as mineral oils, water-based lubricants, and several synthetic oils [60]. Recently, it was found that CDs and doped CDs materials can be used as lubricant additives directly without any complicated and time-consuming post treatment or functionalization [60]. The synthesis and functionalization of CDs and doped CD materials can be achieved by a facile, simple “bottom-up” approach, not requiring any sophisticated equipment or harsh synthesis conditions, and not even tedious pretreatments and post-treatments. Surface functionalization will improve the physicochemical, mechanical, and tribological properties of CDs and doped CD materials. Unquestionably, CDs and doped CD materials will benefit from the surface functional groups of CDs and doped CD materials to improve their compatibility and lubricity. There are several examples of CDs and doped CD nano-lubricants that have been described in the literature.

In the beginning, Huang et al. developed CuSx nanocomposites found to possess effective lubrication additives and metal-related wear restoration properties, which can be applied to CDs with multi-layered graphene structures and CuSx nanoparticles with high chemical activity [59]. HuaPing reported the synthesis of water-soluble CDs and their usage in lubricant additives with deionized water [54]. There was a decrease in the COF (coefficient of friction) in both types of tribological pairs after the addition of CDs. There were maximum reductions in COF of 30% and 14% for the Si₃N₄-steel and Si₃N₄-Si₃N₄ contacts, respectively [54]. Recent studies have investigated the tribological behavior of functionalized hybrid doped CDs, particularly metallic Ga-doped CDs (Ga@CDs) and nonmetallic nitrogen doped CDs (N@CDs) materials, and compared them with the properties of pristine CDs [47]. As part of this study, Tomala et al. described a simple method for synthesizing metallic and nonmetallic doped CDs, and they examined their tribological characteristics as a potential candidate for lubricant additives. The doping of CDs with different atoms/elements may be able to give them required properties for anti-wear and extreme-pressure performance [47]. Furthermore, Zhiqiang et al. also synthesized nickel-doped compact discs (NI-CDs) using citric acid and nickel acetate [61]. Based on observations, CD particles and Ni-CD nanoparticles could enhance the lubricant additive properties of PEG-200 molecules [61]. Ni-CDs particles, however, increase the lubrication properties more than CD nanoparticles alone. In tests with an applied load of 8 N and a correspondence speed of 25 mm/s over one hour, PEG-200-containing 2 wt.% Ni-CDs showed a reduced friction coefficient and wear rate of 35.5% and 36.4%, respectively, compared to PEG20-containing particles of pure CDs [61].

In recent years, Tang et al. has worked in the area of lubricant additives because of their unexpected tribological additive properties, especially in terms of friction reduction and found anti-wear properties of CDs and metal hybrid CDs [56]. Metal-doped/hybrid CDs are a novel type of CD, whose friction-reducing and anti-wear properties are more attractive for lubricant additives. Thus, sequences of CDs and hybrid/doped CDs with various metal ions have been synthesized via one-step pyrolysis [56] These metal-doped CDs have tribological properties in the following order: Zn-CDs > Cu-CDs >> Mg-CDs > Fe-CDs > U-CDs [56].

A recent publication by Therinchet et al. reported the development of carbon dots for additives in lubricants by either using several ionic liquids as a carbon source or by using glutathione as a carbon source and then designing an approach to obtain carbon dots with large organic cations from the ionic liquid [62]. Researchers have observed that CDs originating directly from several organic ionic liquids, particularly methyl-tri-octylammonium chloride, are excellent candidates as additive lubricants in several different compositions of base oils (0.1%, w/v) and lubrication regimes, reducing COF by ~30% and wear scar by more than 60% [62]. X is the symbol for the anionic surrounding of polyelectrolyte outer shells as well as the hexafluorophosphate (PF₆-) compound, the bis(trifluoromethane)sulfonimide (NTf₂-) molecules, the oleate (OL-) reagents, and the bis(salicylato) borate (BScB-) moiety. Consequently, the hydrophobic CD-PEI-X showed outstanding dispersibility and long-lasting stability in PEG-200 liquid solvent (base oil) due to the favorable compatibility of these anions with PEG molecules. Additionally, functionalized CD-PEI-X has been used as a lubricant additive in PEG-200 due to their tribological behavior [49]. A critical parameter in determining the tribological behavior of nanoadditives is the concentration of additives. 

In comparison with PEG200 functionalized CDs-PEL-OL/PEG200 dispersion was stable during the friction test but showed a slight fluctuation during the selected interval of the steady period (6000–6100 s).  The morphological and size distribution features of different CDs were established by transmission electron microscopy (TEM) micrograph [49] The results of this analysis demonstrate the feasibility and versatility of functionalizing surface molecules of CDs rapidly and effectively. The results also reveal the important role of the surface group as a shell and carbon as cores in the lubrication process, allowing for the development of functionalized CD-based lubricant additives [49]. Wolk et al. produced functionalized graphene oxide quantum dots (GQD, similar to CDs) by surface functionalization of a few upper layers of graphene oxide and measured them using SEM, AFM, and HRTEM [63]. In addition, the excellent solubility provides effective and rapid spray application of the surface-functionalized CDs to steel surfaces. Corrosion behavior of surface-functionalized CDs was studied with a steel specimen exposed to 20 spray cycles over 1 h in seawater [63]. The corrosion attack on the steel sample was relatively small after 20 spray cycles. An investigation of the potential of functionalized CDs as lubricants was conducted by spraying the suspension of dodecyl amine CDs on a fresh, new steel surface and measuring the lubricant properties with different levels of coverage [63]. The surface coverage was determined by the number of spray cycles, and as the number of spray cycles is increased, the COF value decreases. Thus, the macroscopic friction properties were examined with a novel Thwing-Albert FP-2250 friction test instrument. On steel, a developed CD-coated film of surface-functionalized graphene oxide quantum dots with dodecyl amine reduced the COF value from 0.17 to 0.11 and demonstrated significant corrosion inhibitory properties [63].

Furthermore, Shang et al. synthesized CDs by one-pot pyrolysis of citric acid (TDCA) [64]. Studies have been conducted on the relationship between microstructures, compositions, and tribological performance [64]. By increasing the load, more TDCA accumulated on the interface layer and the surface of carbon layer was disposed to degrade and form an orderly structure, which protected the interface layer from unwanted friction and wear [64]. Liu et al. has also modified CDs with PEG-400 by pyrolyzing a combination of gluconic acid and molecules of PEG-400 [65]. In particular, addition of 0.20 wt. % of PEG-200 functionalized CDs (CDs-PEG-200) to a base solvent resulted in the most significant reductions in COF value and wear volume: up to 84% and 91%, respectively, at a constant applied load (40N) [65].