Delivering quantum dots to lubricants: Current status and prospect

Very recently, two-dimensional quantum dots (2D QDs) have been pioneeringly investigated as lubricant additives, which exhibit superior friction-reducing and wear resistance. Compared with 2D nanoparticles, 2D QDs possess small size (∼10 nm) and abundant active groups. These distinguished advantages enable them to quickly disperse into common lube mediums and maintain long-term storage stability. The good dispersion stability of 2D QDs not only effectively improves their embedding capacity, but also enables continuous supplements of lubricants during the sliding process. Therefore, 2D QDs are attracting increasing research interest as efficient lubricants with desirable service life. In this review, we focus on the latest studies of 2D QDs as liquid lubricant additives (both in polar and nonpolar mediums), self-lubricating solid coatings and gels, etc. Various advanced strategies for synthesis and modification of 2D QDs are summarized. A comprehensive insight into the tribological behavior of a variety of 2D QDs together with the associated mechanism is reviewed in detail. The superior lubricating performances of 2D QDs are attributed to various mechanisms, including rolling effect, self-mending performance, polishing effect, tribofilm formation, nanostructure transfer and synergistic effects, etc. Strategies for friction modulation of 2D QDs, including internal factors (surface modification, elemental doping) and extrinsic factors (counter surfaces, test conditions) are discussed, special attentions for achieving intelligent tribology toward superlubricity and bio-engineering, are also included. Finally, the future challenges and research directions regarding QDs as lubricants conforming to the concept of “green tribology” toward a sustainable society are discussed.


Introduction
Tribology including friction and wear, has become a serious and challenging issue closely linked to energy crisis, carbon footprint, and equipment life [1-3]. Accordingly, tremendous efforts have been involved in developing highly efficient lubricants to reduce friction and wear. Lubricants mainly include solid lubricants and liquid lubricants. Thereinto, solid lubricants have been extensively adopted to minimize friction and wear for some specific conditions, such as heavy loads, high speeds, etc. [4,5]. However, their tribological properties are sensitive to external environment, and the solid lubricant coatings will be worn out after serving for a long time. Historically, a variety of natural oils, such as castor oils, olive oils, coconut oils, lard oils, sperm oils, etc., are used to reduce friction and wear by creating a low-shear *Corresponding author: Yu TIAN, E-mail: tianyu@tsinghua.edu.cn stability through physical adsorptions (such as van der Waals) or interfacial tribo-chemical reaction [19]. Moreover, 2D QDs are small enough to in-situ fill the scratches of the rubbing surfaces, exhibiting self-healing/mending effects [20][21][22]. Consequently, the above-mentioned distinguished advantages of 2D QDs endow them to realize attractively efficient lubrication performances compared with traditional nanoparticles.
This review focuses on a comprehensive discussion of the latest research progress on 2D QDs in tribology. To begin, the advanced fabrication and versatile modified strategies of 2D QDs will be introduced. The particular advantages of 2D QDs are discussed in tribology fields, including good dispersion stability and diverse functionality. These distinguished advantages enable 2D QDs exceptional lubrication performances. Subsequently, the tribological behaviors of 2D QDs and corresponding mechanisms are discussed, which mainly include rolling effects, formation of protective films, mending effects, polishing effects, and synergistic effects. Then, recent advances for modulating the frictional performances are summarized. Finally, the prospects of extensively exploring QDs as lubricants toward sustainable future are discussed.

Why QDs?
QDs feature small size and abundant functional groups that have opened up new possibilities for effectively decreasing friction and wear. This review summarizes the latest advances of exploring QDs as attractive lubricant additives, solid coatings, and gels along with modulations strategies (mainly including surface functionalization, and elemental doping) as presented in Fig. 1.

Design diversity and modification
Since the first discovery of carbon QDs (CQDs) in 2004 [23], a growing number of literatures have emerged to exploiting fabrication methods of QDs, which can be mainly divided into two strategies: "top-down" and "bottom-up" approaches. The former one relies on breaking down larger bulks into smaller ones through dozens of chemical, electrochemical, or physical www.Springer.com/journal/40544 | Friction approaches, such as acidic oxidation, hydrothermal method, laser ablation or electrochemical exfoliation, etc. [24,25]. While the ''bottom-up'' methods synthesize QDs by carbonization of some inexpensive molecular precursors, such as citrate acid, glutamic acid, and carbohydrates, etc. [26][27][28]. Furthermore, modification strategies, e.g., surface functionalization and elemental doping, have been employed to elaborately design QDs to drive further task-specific multi-functionality [29,30].

Good compatibility
The polar groups (including hydroxyl groups, carboxyl groups, etc.) of QDs enable them with good affinity in most polar solvents, including water, polyethylene glycol (PEG), and ionic liquids (ILs), etc. Moreover, QDs with high chemical activity are conducive to be modified with target groups to improve colloidal stability and homogeneous distribution in nonpolar mediums, thus switching their affinity from hydrophilicity to hydrophobicity. Accordingly, QDs would exhibit switchable dispersibility and long-term stability in both polar and nonpolar mediums [31,32]. For instance, Ye et al. modified CQDs with an organic agent via a chemically covalent grafting, and the modified CQDs in nonpolar polyalphaolefin (PAO) oil exhibited desired long-term stability and excellent lubrication performances [33].

Improved embedding/film-forming ability and selfmending properties
Owing to the small size and high surface area characters, 2D QDs are easy to enter into contact areas and form complete protection films via strong physical adsorption. Besides, the abundant active groups of 2D QDs enable them to react with freshly rubbing surfaces. This enables 2D QDs to exhibit improved embedding/film-forming abilities. Meanwhile, QDs can be deposited and/or adsorbed in bulges, cracks, and scratches of the rubbing surfaces, exhibiting "self-mending" effects. For instance, Huang et al. pointed out that the CQDs/CuS x nanocomposites in liquid paraffin oil exhibited excellent tribological properties and self-healing abilities on metal-metal surfaces by combining the advantages of graphitic CQDs and highly active CuS x nanoparticles [34].

Multifunctionality: Anticorrosion and oxidation resistance
For practical industrial applications, the corrosion resistance, especially for liquid lubricants, is a critical evaluation criterion. Recently, continuously encouraging progress has been reported that 2D QDs could form high-quality protective films on the tribo-pairs to prevent the penetration of oxygen and water molecules, thus are effectively impervious to corrosive gases and liquids [35]. For instance, Zhu et al. reported that introduction of carbon dots (CDs) could significantly improve the anticorrosion performances of polymer matrixes [36]. Cui (Fig. 2(a)) [39]. For comparison, graphene oxide (GO) was also prepared via prolonging the hydrolysis time of AC from 2 to 5 h. As shown in Fig. 2(b), the friction coefficient (COF) of CDs aqueous solution is 0.2671, which is much lower than that of GO (~0.3125) suspension under the identical test conditions (10 N, 0.1 mg/ml). Likewise, the wear rate (WR) of CDs is 1.5×10 -6 mm 3 /(N·m), which is obviously lower than those of GO and bare AC solutions. The efficiency of CDs solution is contributed to the synergistic effects of rolling effects, formation of high-quality tribo-films, and self-repairing effects ( Fig. 2(c)). The load-carry capacity of lubricants is of great importance for practical engineering applications. Recently, Qiang et al. proved that load-bearing capacity of water-based lubricants can be obviously improved by introduction of an appropriate amount of GQDs [40]. Additionally, compared with GO nanosheets, GQDs dispersions exhibited better tribological performances in terms of a relatively a lower COF value of 0.23 and a longer service life (≥ 3,600 s) under a high load of 100 N. The superior tribological behaviors of GQDs can be described as follows: GQDs could effectively assemble on the steel-steel rubbing surfaces via physical adsorption or forming stable tribo-films, www.Springer.com/journal/40544 | Friction and the excellent dispersion stability would ensure the continuous supply of GQDs in the rubbing interfaces during the sliding process. In contrast, GO nanosheets featured with microscale size would be easily pushed away from the contacting areas, resulting in a sharp increase of COF.

CQDs: Switchable polar and nonpolar lubricant additives
The polar nature endows QDs excellent hydrophilicity, but indeed poses the question of their compatibility with nonpolar lubricating oils, such as PAO and mineral oils. For practical purposes, the hydrophobic nano-additives are largely needed because most of the vehicles and machines are lubricated by engine oils. Fortunately, QDs with abundant functional groups could easily switch from hydrophilicity to hydrophobicity by grafting target molecules or groups. From the viewpoint of the switchable compatibility required for lubricant additives, the modification strategies of QDs were listed in Table 1.
In 2018, Shang et al. synthesized hydrophilic nitrogen-doped CDs (N-CDs) with via a "bottom-up" method, and then modified the obtained N-CDs with hydrophobicity via a covalently grafting of alkyl chains from oleyl amine (OA) [41]. The hydrophilic and hydrophobic N-CDs disperse homogeneously in polar solvents, including PEG, H 2 O, N,Ndimethylformamide (DMF), and nonpolar mediums (including PAO, toluene, and petroleum ether), respectively ( Fig. 3(a)). Furthermore, the mean COF and wear volumes (WV) lubricated by hydrophilic N-CDs/PEG suspension account for 24.1% and 17.2% of those lubricated by PEG, demonstrating the improvement in the friction-reducing and anti-wear (AW) property with the addition of N-CDs. Likewise, the hydrophobic N-CDs in nonpolar PAO oil also exhibited improved tribological performances compared with those lubricated by pure PAO. As shown in Figs. 3(b) and 3(c), both the hydrophilic and hydrophobic N-CDs exhibited better tribological performances than traditional lubricant additives, such as bis(salicylato) borate ionic liquid (IL) and zinc dialkyldithiophosphates (ZDDP), respectively. In a follow-up study, He et al. prepared powdery GQDs by a one-pot gaseous detonation method, and further investigated their tribological performances in 150SN mineral oil. A low COF of 0.031 was achieved at an appropriate additive content of 0.8 wt% against a high load of 392 N, which is correspondingly improved by 65% of that lubricated by neat 150SN mineral oil. Additionally, GQDs play a vital role in reducing wear scar diameter (WSD) and depth [42]. Ye et al. designed and fabricated multi-functionalized CQDs (CQDs-N) through a one-pot hydrolysis method using organic 4-aminodiphenylamine (ADPA) as a multifunctional modifier [43]. The obtained CQDs-N could be suspended homogeneously in PEG solution with long-term storage stability (~5 months) owing to the strong interactions between oxygen groups deriving from CQDs surfaces and PEG chains. As expected, the CQDs-N in PEG exhibited lowest COF and WSD compared with CQDs and bare PEG, even under an extremely high load of 588 N.
Moreover, doping with metal ions offers another effective tool for improving the tribological behaviors of 2D QDs. The presence of metal ions endows 2D QDs  [44]. Specifically, the load-carrying capacity was significantly enhanced from 120 N to no less than 500 N. Furthermore, the Zn-CDs exhibited good dispersion stability and lubricating performances in PEG via an anion replacing with N(CF 3 SO 2 ) 2-. Table 2 summarizes the recent advances of CDs and GQDs as lubricant additives in both polar and nonpolar mediums.

Solid coating
Solid lubricant coatings can provide lubrication function for sliding tribopairs under boundary lubricating conditions, which is an efficient strategy to facilitate precise devices to run smoothly and safely [45,46]. The good dispersion stability of 2D QDs is an important advantage for solution-based coatings.
Recently, the solution-based coatings, including selfassembly approaches (e.g., solvent vaporization) and electrophoretic deposition (EPD), have been intensively used to realize large-scale and uniform solid coatings of 2D QDs. In 2020, Qiang et al. deposited GQDs coatings (GQDCs) on silicon (Si) substrate via an EPD approach, the thickness of GQDCs can be wellcontrolled via varying the deposition voltage [47]. Compared with GO coatings (GOCs) and bare Si substrate, the lowest COF and AW performance of GQDCs was immediately achieved owing to the synergistic effects of laminar slip effect and the formation of protective films. Moreover, the counterpart materials also influenced the interfacial nanostructures transformation and bonding state, thus affecting the friction properties. For instance, Yin et al. coated GQDs on different hydrogenated carbon films, involving graphite-like carbon (GLC), diamond-like carbon (DLC), and polymer-like carbon (PLC), and compared their tribological performances against self-mated counterparts or neat steels [48]. The lower COF values of self-mated counterparts are observed than those sliding against bare steel tribocouples. Specifically, the tribo-system containing GQDs coating on DLC surface is able to achieve superlubricity, which might attribute to the synergy of GQDs coating and the hydrogenated DLC film. However, the service life of this trio-system is very short (~115 s) due to their high sensitivity to environmental humidity.

Extreme operating conditions & superlubricity
With the rapid development of manufacturing, most of the modern equipment is required to work availably at severe conditions (including elevated temperature, high speed, heavy loads, and oxidizing atmosphere, etc.). Thus, it is essential to develop special lubricant additives available at extreme operating conditions. Moreover, superlubricity is leading to a revolution in engineering technology, which is an ideal state that friction between two sliding surfaces is negligible [49]. It is still a challenge to achieve superlubricity at a macro-scale and reveal its underlying mechanisms.

MoS 2 and WS 2 QDs: Elevated temperature and heavy load
As emerging 2D materials, MoS 2 and WS 2 nanosheets have demonstrated outstanding tolerance to hightemperatures and heavy loads owing to their lamellar structures and unique physicochemical properties. However, the realization of dispersing MoS 2 and WS 2 nanosheets in common lube oils is still severely restricted by their solid characters. To address this problem, oil-soluble MoS 2 nanosheets have been reported via a surface modification [50]. The modified MoS 2 nanosheets exhibited superior extreme pressure properties (no less than 2,000 N) compared to other Mo-contained lubricant additives, e.g., molybdenum dialkyldithiophosphate (MoDDP) (600 N), and fullerene-like MoS 2 nanoparticles (IF MoS 2 ) (300 N), and micro MoS 2 (200 N).
Notably, the discovery of MoS 2 and WS 2 QDs opens up new possibility of lubricant additives at extreme  Fig. 4(a)), resulting in lower frictionreducing and AW behaviors under the identical conditions. XPS analysis proved that MoS 2 /WS 2 QDs could bond with Fe atoms by Fe-S bonds, or iron oxide layer by M-O and S-O bonds, confirming the formation of tribo-films on the worn surfaces ( Fig. 4(b)). The possible lubrication mechanism was described as follows: MoS 2 and WS 2 QDs are more conducive to enter into the contacting areas, creating high-quality boundary films and in-situ "filling" the asperity valleys (Fig. 4(c)). In contrast, MoS 2 and WS 2 nanosheets tend to be squeezed out of the contacting areas, resulting in gradually deteriorating lubrication performances [51]. Furthermore, Gong et al. revealed that MoS 2 /WS 2 QDs could disperse stably in commercial IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF 6 ) (≥ 2 months).
Compared with neat ILs, the presence of MoS 2 /WS 2 QDs would dramatically improve the friction-reducing and AW performances even under severe conditions (500 N, 150 °C), as shown in Fig. 4(d) [52]. It is quite attractive that MoS 2 QDs would also disperse stably in nonpolar paraffin oil and exhibit an extremely low COF of 0.061, which is much lower than that of bare paraffin oil (0.169) [53]. Table 3 summarizes the recent advances involving commonly used MoS 2 /WS 2 QDs as lubricant additives.

BP QDs: Superlubricity
As emerging 2D materials, BP with armchair-zigzag orientation and attractive physicochemical properties www.Springer.com/journal/40544 | Friction has been explored to achieve superlubricity through experiments and simulation [54,55]. For instance, BP nanosheets attached with hydroxyl groups can deliver robust superlubricity [56]. Compared to nanosheets, BP QDs are promising to deliver superlubricity owing to the competitive advantages of QDs. Ren et al. investigated an initial lubricating performance of BP QDs-ethylene glycol aqueous (EG aq ) suspension [57].
As shown in Figs. 5(a)-5(c), a "robust" and "durable" macroscale superlubricity state of BP QDs-EG aq suspension was observed, which exhibited an extremely low COF value in the range of ~0.001−0.003 for more than 2 h. As depicted in Fig. 5(d), the superlubricity and excellent AW performances of BP QDs-EG aq suspension were attributed to the synergy of rolling effects, interlayer sliding, hydrogen bond layers  between BP QDs, and formation of protective films consisted of oxidized BPQDs (P x O y ). The tribological performances of BP QDs as water-dispersible lubricant additives were also assessed by Tang et al. [58]. Compared with bare base liquid (2.0 wt% TEA aqueous solution), the load-supporting capacity of 0.005 wt% BP QDs dispersion was dramatically improved from 120 N to over 300 N. The obtained BP QDs could maintain long-term period (~120 min) of stable lubrication effect at a relatively high load of 80 N. Additionally, the anti-friction and AW performances of small BP QDs are considerably better than those of the large-sized BP powder. These results can be explained that the BP QDs can easily to react with the freshly rubbing surfaces to form a robust boundary tribo-film, while BP powder might be squeezed from rubbing surfaces during the friction process.

Strategies for friction modulation 4.1 Impacts of substrates on interfacial friction: Tribo-pair effects
In general, friction behaviors at contact surfaces were largely governed by physical conditions of contact surfaces (roughness, hardness, etc.) and chemical interactions between the sliding interfaces. Though QDs exhibited promising lubricating behaviors, their lubrication effects and mechanisms vary with different tribopairs. The non-uniform protective film formed by QDs on the worn surface is usually not tough enough due to the weak interactions between QDs and sliding surfaces, and will eventually lose lubrication under harsh conditions (e.g., high load, high temperature, etc.). Therefore, further studies are necessary particularly focusing on the tribopairs effects. Physically, amorphous carbon (a-C) coating is considered as an effective approach to achieve ultralow COF and excellent wear-resistance owing to their chemical inertness, low adhesiveness, and high smoothness. Tang et al. employed CDs aqueous solutions to enhance the lubricity for a-C/a-C contacts [59], and found that the introduction of 0.1 wt% CDs added to water exhibited a much low COF of 0.03 owing to the synergistic effect of CDs and the smooth a-C surfaces. Another effective approach for achieving super low friction is choosing appropriate tribological counterparts. The commonly used bearing steels are prone to react with most nanolubricants, leading to unexpected corrosive attack. Recently, polymers with suitable elastic modulus have been gradually employed as tribopairs. Among diverse polymers, polytetrafluoroethylene (PTFE) with self-lubrication and suitable Young modulus is considered as applicable tribopairs to conduct promising lubrication performances. Yin et al. suggested that the unprecedented lubrication of 2D Ti 3 C 2 /GQDs coating against PTFE surfaces should be assigned to the synergy of shielding effect, self-lubrication, rolling effect, slipping effect, and intercalation effect [45].

QDs/2D nanomaterials: Synergistic effects
Despite various successful reports in the literatures, commercialization of QDs as lubricants is still a great challenge. One of the major challenges is QDs will lose their ''nanorolling effect" under high-load conditions due to their ultrasmall size. Different from the high load bearing capacity of 2D nanosheets [60], the 2D QDs could be completely extruded out of the contacting surfaces under ultrahigh loads and thus lost the high load lubricating performances. Facing the challenge, our group proposed a universal strategy to improve the load-capacity of CQDs in synergy with 2D nanosheets [61]. The aqueous dispersions of CQDs decorated 2D nanosheets (h-BN, MoS 2 , MoSe 2 , WS 2 , and graphene) were prepared using CQDs as efficient stabilizers and exfoliation agents through π−π interactivity ( Fig. 6(a)). These CQDs decorated 2D nanosheets suspensions exhibited stable and desirable tribological performances compared with those of bare CQDs suspensions (Fig. 6(b)). Not only that, these CQDs/2D nanosheets suspensions also exhibited improved load-carrying capacity from 100 to 300 N, which could be attributed to the synergetic effects of 2D nanosheets and CQDs as depicted in Fig. 6(c).
Recently, chemical modified graphene (GO), is considered one of the main strategies to achieve stable graphene relevant aqueous solutions [62]. However, their tribological properties are severely limited due to the strong van der Waals force and π-π stacking interactions of GO sheets, leading to the severe concerns of dispersibility and storage stability. Considering the conjugated π structure, CQDs are expected to form strong noncovalent π-π interactions with GOs, preventing the stacking of GO nanosheets. In 2018, Shang et al. reported that CQDs/GO hybrid could dissolve in PEG with ultrahigh dispersion stability after 6 months [63]. The COF and wear volume of CQDs/GO hybrid in PEG were 0.039, 3.11×10 -3 mm 3 , which accounted for 32.5% and 16.5% of those lubricated by neat PEG under a high load of 588N. The better lubricating properties of QDs/GO hybrid in PEG are contributed to synergistic effect of sphere-like CQDs and lamellar GOs layers.

GQDs/2D nanocomposites: Microstructure transformation
As discussed above, the sliding-induced microstructure transformation of carbon-based nanomaterials is an important way for modulation of their lubricating performances, especially under high loads and fast sliding speeds. For instance, Wang et al. addressed that fullerene-like carbon would convert to graphene during the sliding process, thus exhibiting super-low COF values [64]. However, 2D graphene would be severely ripped under fast sliding speed, and eventually lost its lubrication effects. Therefore, it is important to deep understanding the structural evolution, interfacial interactions, and lubrication mechanisms of nanolubricants towards practical applications. Very recently, molecular dynamic (MD) simulation has been employed to reveal and predict the frictional mechanisms of diverse nanoparticles during the frictional process [65]. Ma et al. verified that the a-C films underwent shear-induced sp 3 -to-sp 2 structure transformation by using the MD simulations [66]. To achieve desirable lubrication performances, Yin et al. confirmed that the QDs with attractive interfacial activities can deliver promising lubrication behaviors via the strong intermolecular interactions with the rubbing surfaces and formation of lubricative tribofilms on the rubbing surfaces, which was supported by the MD simulation [48]. From the experimental aspect, Zhang et al. evaluated the tribological revolutions of graphene/GQDs suspensions in YG8 hard alloy contacts under a high sliding speed of 1,450 rpm [67]. The sulfurized isobutene (SIB) was employed as an extreme pressure (EP). Surprisingly, the graphene/ GQDs/SIB performed exceptional effectiveness in reducing friction and wear. They attributed the attractive lubricating performances to the synergistic effect of SIB-assisting microstructures transformation from GQDs to fullerene QDs and the tribo-films formation during the severely sliding process (Fig. 7).

QDs/ILs: Improved durability and embedded stability
Currently, ILs have received increasing attention owing to their good dispersibility, inherent polarity, and strong interfacial bonding [68]. Additionally, the structures and constituent ions of ILs are tailorable, which enable them either to adapt to various frictional contacts, or to meet some task-specific functions. However, the cost of ILs is relatively high, and some ILs containing halogen groups usually cause metal corrosion. Considering the excellent lubricating properties of QDs, integrating QDs with ILs, especially with halogen-free ILs [69], has becoming increasingly popular. Some strategies to synthesize ILs with QDs are summarized in Table 5, including one-pot pyrolysis, covalent grafting, and assembly, etc. Their lubricating behaviors will be discussed in the following section.

i) QDs/imidazolium-based ILs
Among diverse ILs, imidazolium-based ILs have attracted growing attention due to their relatively high thermal stability, and flexibility of molecular design. Tang

ii) QDs/Quaternary ammonium-based ILs
Recently, quaternary ammonium based ILs has attracted increasing attention owing to their exceptional advantages, such as low cost, easy synthesis, and good oil solubility. As pointed out previously, ILs or polyelectrolytes with abundant groups exhibit stable and extreme low COF due to the strong interactions/interplay between ions pairs of ILs and charged rubbing surfaces [76,77]. Therefore, developing polymerized ILs or di-ionic ILs will provide more sufficient anchors on the rubbing surfaces, thus deliver more desirable lubricating behaviors. Mou et al. reported the polymerized quaternary ammonium/CDs hybrids (CDs-PILs) [78], and the obtained CDs-PILs in PEG200 performed high-performance lubricating performances. As shown in Fig. 8

iii) QDs/ILs analogue (deep eutectic solvent, DES)
Very recently, deep eutectic solvents (DES), a new family of synthesis-free and biocompatible ILs  [80]. The DES/CQDs with sphere-in-shell structure exhibited superlubricity with an extremely low COF of 0.006 for Al 2 O 3 contacts owing to the advantages of synergistic effects of interlayer shearing, sliding, and rolling effects.

Intelligent lubrication
With the rapid development of industrial technology, newly developed tribo-systems are needed to meet the continuously rising stringent requirements for mechanical systems [81,82]. In pursuit of high lubricating performances, core-shell nanoparticles have been employed which possess excellent lubricity, superior mechanical properties and flexibility owing to the advantage of synergy [83]. In 2019, He et al. reported core-shell structured PEG modified CDs (PEG/CDs) through a facile and productive ultrasonic treatment [84]. The PEG/CDs exhibited stable and super-low COF of 0.02 assigning to the surface passivation of PEG and rolling effects of CDs. For some specific industrial applications, it is essential to develop intelligent lubrication systems with real-time self-repair, self-storage, and self-diagnosis [85]. For instance, a real-time and self-powered system was developed to monitor the lubricating oil condition aiming to online evaluate the reliability of working machines [86]. Moreover, developing new materials and approaches towards the biological lubrication systems is of major importance for human's well-being, including artificial joints, artificial teeth, artificial heart valves and other important parts that are easily damaged [87,88].
Inspired by the artificial joints, microencapsulation has been implemented to achieve biological lubricity with controllable release of lubricants and long service life. Additionally, lubricants in gelation formation can prevent the oil creeping and evaporating loss, and thus are beneficial for maintenance and operation. Recently, eco-friendly CDs/PEG/chitosan (CDs/PEG/CS) gels were prepared and stored in bio-inspired grooved surfaces based on cartilage-embedded structures [89]. The CDs/PEG/CS gel solution exhibits superior lubricity for UHMWPE contacts and long service lifetime owing to the synergy of bio-inspired structure and strong interactions between CDs and PEG.

"Green nanotribology": Summary and prospects
In future, tribology will gradually play an increasingly important role in manufacturing, power output equipment, transportation, biological and other fields [2, 90,91]. With regard to this situation, a concept of "green tribology" is proposed by Zhang towards a sustainable society in 2008 [92]. Guided by the viewpoint of green tribology, one of the most important approaches is to develop bio-/eco-lubricants with exceptional friction-reducing and wear resistance performances [93].
As we summarized in this review, the discovery of 2D QDs has become a burning research topic for green tribology, especially since the possibility of their large-scale and cost-effective manufacturing [94]. Recent studies have verified that 2D QDs-based lubricants could provide efficient lubrication functions in forms of liquid nanoadditives, solid coatings and gels, etc. The corresponding lubrication mechanisms are mainly categorized into nanolubrications (ballbearing effects, self-mending effects, polishing effects, and formation of high-quality protective films) and synergistic effects.
Additionally, the rapid realization of superlubricity of 2D QDs, especially at macro-scale and in harsh working conditions, enables them to spark more broad application fields, including aerospace industry, and marine engineering, etc. Moving forward, there are still many exciting opportunities and challenges for further breakthroughs to discover QDs in the tribology field. As depicted in Fig. 9, we list some frontiers as follows: i) From chemistry and physics perspectives, in-depth theoretical models performed by artificial intelligence (AI) (e.g. machine learning) [95,96], and empirical experiments [97] are needed to delicately predict, design, and fabricate QD-based lubricants with desirable multi-functionality. Besides 2D QDs, more efforts should be concentrated to exploit other QDs with potential lubrication performances, such as ceramic QDs, metal oxide QDs, etc.  ii) The in-situ characterization of interfacial phenomenon occurring at the microscale, even at the atomic and molecular scales, is needed to disclose the underlying lubricating mechanisms of nanolubricants, combined with advanced characterization technology such as atomic force microscope (AFM), surface force apparatus (SFA), scanning tunneling microscope (STM), and confocal microscopy, etc. [98].
iii) Moving forward, from an engineering perspective, the widely tunable physicochemical, optical and electrical properties of QDs will drive further exciting opportunities of tribology-oriented applications. For instance, owing to the fluorescence properties, the large area QDs coatings on metals or ceramics will intuitively real-time monitor the crack propagation for specific engineering fields.
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To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Canada, she is now working as a professor at School of Mechanical Engineering, Nanjing University of Science and Technology. Her current research interests mainly focus on nanotribology, including the design and manufacturing of functional nanomaterials and their lubrication mechanisms. She has published more than 60 peer-reviewed papers.

References
Tianhao LI. He is now a master student at Herbert Gleiter Institute of Nanoscience, Department of Materials Science and Engineering, Nanjing University of Science and Technology. His current research focuses on fabrication of diverse 2D nanomaterials relevant lubricant additives.