Recent Advanced on the MXene–Organic Hybrids: Design, Synthesis, and Their Applications

With increasing research interest in the field of flexible electronics and wearable devices, intensive efforts have been paid to the development of novel inorganic-organic hybrid materials. As a newly developed two-dimensional (2D) material family, MXenes present many advantages compared with other 2D analogs, especially the variable surface terminal groups, thus the infinite possibility for the regulation of surface physicochemical properties. However, there is still less attention paid to the interfacial compatibility of the MXene-organic hybrids. To this end, this review will briefly summarize the recent progress on MXene-organic hybrids, offers a deeper understanding of the interaction and collaborative mechanism between the MXenes and organic component. After the discussion of the structure and surface characters of MXenes, strategies towards MXene-organic hybrids are introduced based on the interfacial interactions. Based on different application scenarios, the advantages of MXene-organic hybrids in constructing flexible devices are then discussed. The challenges and outlook on MXene-organic hybrids are also presented.


Introduction
Recently, with the great potential in the fields of intelligent sensing, health monitoring, and portable display, ever-growing interests have been paid into the field of flexible electronics and wearable devices [1][2][3][4]. However, the fabrication of flexible electronics is usually based on printed electronics technology, as the traditional silicon-based semiconductors have become increasingly difficult to meet the manufacturing requirements. Therefore, increasing research interests have been paid to developing relevant materials in recent years. Instead of the rigid-integrated silicon-based semiconductors, nanotechnology points out a new way towards inorganic flexible devices, which is to buffer the macroscopic deformation by the relative movement of numerous boundaries. At present, the thin-film device constructed by two-dimensional (2D) materials represents one of the most promising candidates [5]. However, the fatigue and phonon scattering at grain boundaries are still unsolved issues [6]. On the other hand, the organic semiconductors built on πconjugated systems are well-known for their good flexibility but suffer from insufficient carrier mobility and aging resistance [7]. Therefore, the combination of advantages from both inorganic and organic materials, namely, the inorganic-organic hybrids, becomes the most obvious choice.
To obtain an ideal material that can fulfill the requirements of electrical, mechanical, and processing properties for printable electronics, the interfacial compatibility between the inorganic and organic components becomes critical [8]. For one thing, a well-compatible interface endows the two components good dispersity, which further affects the uniformity

Structure and Surface Characters of MXenes
The MXenes are a kind of 2D metal carbide/nitride with an alternative atomic arrangement of M n+1 X n (M = transition metals; X = C, N; n = 1, 2, 3, 4). Take the Ti 3 C 2 T x MXene as an example. The Ti 3 C 2 skeleton can be regarded as an extracted layer from fcc metal carbides lattice along (111) plane ( Figure 1). Since the first discovery of Ti 3 C 2 T x MXene in 2011, more than 70 MXenes have been theoretically predicted or synthesized. As mentioned above, the synthesis of MXene follows a "top-down" protocol: selective etching of the A atoms from MAX phase ceramics. However, as the M n+1 X n layers and A atoms are alternatively arranged in MAX phase ceramics, the removal of A atoms will break M-A bonds, thus results in high-active M atoms at the surface. As first presented by Gogotsi et al., the surface terminal groups (T groups, e.g., -F, -O, or -OH) are necessary for stabilizing MXenes due to the high activity of these coordination unsaturated surface metal atoms [22]. Furthermore, owning to these surface terminal groups, the MXenes with typical inorganic carbide skeleton usually presents similar surface physicochemical properties (e.g., hydrophilicity and oxidizability) [23]. Attributing to the surface T groups, the fresh MXene flakes are negatively charged and highly hydrophilic, which will form the colloidal dispersion by surface electrostatic repulsion. Based on the double electrical layer theory, it is easy to speculate that the dispersity of MXene flakes will change along with different solution conditions. Also depends on the average oxidation states of the M and X elements, the Zeta potential of MXenes is varied. In the past several years, the oxidation of MXenes in water also arouses wide concerns, which is found to be isogenesis with their hydrophilic. Still exampled by Ti3C2Tx MXene, although TiO2 species are detected when annealing the MXene under either vacuum, inert atmosphere (Ar, N2), or oxidizing atmosphere (CO2, air), the oxidation can be dramatically slowed down by decrease the environment temperature. However, in an aqueous suspension, nearly half of the flakes degrade in five days, even at room temperature [24,25]. The degradation behavior of MXenes in the aqueous suspension then can be ascribed to the water or the synergy of water and dissolved oxygen molecules. Interestingly, Kathleen et al. found that with oxygen solubility orders of magnitude higher than water, the organic solvent (N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), propylene carbonate (PC), and ethanol) did not cause observable oxidation of MXenes [26]. Therefore, a deeper insight into the relationship between the surface T groups on Ti3C2Tx MXene and their dispersion properties was presented, namely, the surface tension, polarity index, dielectric constant (ε), and cohesive energy density (δ). However, although some of the organic solvents were screened out to be a good medium for Ti3C2Tx MXene dispersion, the dispersibility of Ti3C2Tx MXene in nonpolar organic solvents is still an unsolved issue. What is more, the T groups usually have a random constituent and distribution, which makes the determination of surface properties more difficult.
Enlightened by traditional strategies towards surface compatibility regulation, the surface physicochemical properties of MXenes also can be dramatically altered by replacing the inorganic surface terminal groups with organic species. For example, di(hydrogenated tallow)benzyl methyl ammonium chloride (DHT)-treated Ti3C2Tx MXene has shown a good dispersibility in nonpolar organic solvents [27]. More importantly, the study opens up a new way for MXene-polymer hybrids, which have good interfacial compatibility. In the following section, based on the interaction between organic species and MXenes, the synthesis and corresponding surface physicochemical properties of MXeneorganic hybrids will be briefly discussed. Attributing to the surface T groups, the fresh MXene flakes are negatively charged and highly hydrophilic, which will form the colloidal dispersion by surface electrostatic repulsion. Based on the double electrical layer theory, it is easy to speculate that the dispersity of MXene flakes will change along with different solution conditions. Also depends on the average oxidation states of the M and X elements, the Zeta potential of MXenes is varied. In the past several years, the oxidation of MXenes in water also arouses wide concerns, which is found to be isogenesis with their hydrophilic. Still exampled by Ti 3 C 2 T x MXene, although TiO 2 species are detected when annealing the MXene under either vacuum, inert atmosphere (Ar, N 2 ), or oxidizing atmosphere (CO 2 , air), the oxidation can be dramatically slowed down by decrease the environment temperature. However, in an aqueous suspension, nearly half of the flakes degrade in five days, even at room temperature [24,25]. The degradation behavior of MXenes in the aqueous suspension then can be ascribed to the water or the synergy of water and dissolved oxygen molecules. Interestingly, Kathleen et al. found that with oxygen solubility orders of magnitude higher than water, the organic solvent (N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), propylene carbonate (PC), and ethanol) did not cause observable oxidation of MXenes [26]. Therefore, a deeper insight into the relationship between the surface T groups on Ti 3 C 2 T x MXene and their dispersion properties was presented, namely, the surface tension, polarity index, dielectric constant (ε), and cohesive energy density (δ). However, although some of the organic solvents were screened out to be a good medium for Ti 3 C 2 T x MXene dispersion, the dispersibility of Ti 3 C 2 T x MXene in nonpolar organic solvents is still an unsolved issue. What is more, the T groups usually have a random constituent and distribution, which makes the determination of surface properties more difficult.
Enlightened by traditional strategies towards surface compatibility regulation, the surface physicochemical properties of MXenes also can be dramatically altered by replacing the inorganic surface terminal groups with organic species. For example, di(hydrogenated tallow)benzyl methyl ammonium chloride (DHT)-treated Ti 3 C 2 T x MXene has shown a good dispersibility in nonpolar organic solvents [27]. More importantly, the study opens up a new way for MXene-polymer hybrids, which have good interfacial compatibility. In the following section, based on the interaction between organic species and MXenes, the synthesis and corresponding surface physicochemical properties of MXene-organic hybrids will be briefly discussed.

MXene-Organic Hybrids through Covalent Interaction
Benefiting from the surface atomic layers of early transition metals with empty d orbital, the MXenes skeleton usually presents a strong affinity to electron donors. When the organic molecules with electron-donating groups are introduced, the original T groups can be replaced. To date, numerous small organic molecules are found to be able to covalently connect on MXenes.
Silane (RSiX n ), one of the most commonly used organic coupling agents, has been widely applied for surface modification of inorganic materials. For silane, the R represents the functional group, such as an amino, alkane, aromatic group, epoxy group, and so on; X stands for hydrolysis group, containing Cl, OMe, OEt, etc. [28]. During the so-called "hydrolytic condensation" process ( Figure 2a), X groups will first be hydrolyzed to form a siloxane (-SiOH) structure. Later, the siloxane goes through a dehydration condensation reaction to form oligomers. The -OH groups in oligomers will react with -OH groups on MXenes, forming an intermediate state. Finally, the Ti-O-Si covalent bonds are formed by dehydration condensation [28,29]. Meanwhile, different properties inherited from R groups can be introduced into the inorganic phase. In 2018, Zhao et al. reported a trimethoxy (1H,1H,2H,2H-perfluorodecyl) silane (PFDTMS) modified Ti 3 C 2 T x MXene membrane prepared via a simple wet route [29], in which the water contact angle increased from 38.8 • to 102.0 • after PFDTMS grafting. Later, Sehyeong et al. have systematically studied the surface polarity regulation of Ti 3 C 2 T x MXene by silane with different R groups. After replacing the surface T groups of Ti 3 C 2 T x MXene with lipophilic octyltriethoxysilanes (OTS) [30], the OTS@MXene showed a water contact angle of 102.6 • and exhibited excellent dispersion stability in non-polar hexane for more than four weeks. When methyltriethoxysilane (MTS), propyltriethoxyislane (PTS), and hexyltriethoxysilane (HTS) was selected, the surface hydrophobicity of these silane-MXene hybrids would decrease with shortening the alkyl chain. Beyond the randomly surface grafting, Zhang et al. designed a well-ordered vertically aligned Janus MXene-based aerogel, which has different wettability belong to two sides named VA-MXA [31]. After modifying the Ti 3 C 2 T x MXene by fluorinated alkyl silane under vacuum conditions, the water contact angle of the upper layer increased from 44.6 • to 133.2 • . Also, the excessive concentration of silane clogged the available surface and decreased the hydroxyls on the surface of Ti 3 C 2 T x , which can bring novel surface physicochemical properties [28]. In Tran and his co-workers' work [32], the N,N-(diethylamino)dithiocarbamoyl-benzyl(trimethoxy) silane (SBDC) was used to functional Ti 2 CT x . Interestingly, the SBDC-MXene hybrid was applied as both substrate and iniferter, which allowed further polymerization of thermoresponsive polymer (poly(2-(dimethylamino)ethyl methacrylate (PDMAEMA)) on the surface of Ti 2 CT x. On the other hand, the silane molecules can not only change the surface hydrophilcity, but also alter the surface charged state (zeta potential) of MXenes. For instance, the aminosilane with positively charged amino group was demonstrated to have a critical role on the surface charge of aminosilane-Ti 3 C 2 T x hybrids [33,34]. As reported by Hossein et al. the [3-(2aminoethylamino)propyl]-trimethoxy silane (AEAPTMS)-Ti 3 C 2 T x hybrids had a positive zeta potential of +62 mV at pH = 2.58, which is the highest zeta potential reported for MXenes to date. Similar to silane, the alkylphosphoric acid can also form covalent bonds with MXene through dehydration condensation reaction. Kim et al. demonstrated a simultaneous interfacial chemical grafting and phase transfer method for alkylphosphoric acid (C n PA, n = 3, 6, 8, 10, 12) grafting on the Ti 3 C 2 T x flakes [35]. During the reaction, Ti-O-P bonds formed by interfacial nucleophilic addition and sequential condensation reaction between hydroxyl groups from Ti 3 C 2 T x and phosphate group from C n PA. The pH and concentration of C n PA are found to influence the interfacial chemical reaction and phase transfer. Whereas the dispersion stability of the Ti 3 C 2 T x -C n PA hybrids in nonpolar organic medium can be attributed to the steric stabilization and strong nonpolar interaction of long alkyl chains. Very recently, Sun et al. proposed new evidence on the existence of hydrogen bonds between Ti 3 C 2 T x nanosheets and C 8 PA [36]. Therefore, it seems that the surface interaction between MXenes and coupling agents may be more complicated than we think.
Diazonium salt is another commonly used reagent to covalently connect organic functional groups on MXenes. In 2020, Zhang et al. first proposed a detailed reaction mechanism between aryl diazonium salts and Ti 3 C 2 T x MXene, in which the aromatic primary amine can undergo a diazotization reaction and the produced diazonium salts will covalently bond with MXenes. As shown in Figure 2b, the detailed reaction mechanism can be divided into three steps [37,38]: (1) an electron was transferred from Ti 3 C 2 T x MXene to aryl diazonium salt, which resulted in the cleavage of diazonium to nitrogen and aromatic free radical; (2) the aromatic free radical received an additional H atom from Ti 3 C 2 T x MXene, generating a Ti-O radical on the surface of MXene; (3) strong Ti-O-C covalent bonds formed between Ti-O radical and aromatic free radical. In fact, sulfanilic acid diazonium salts were introduced to get large-scale delaminated Ti 3 C 2 T x multilayers as early as 2016 [39][40][41]. Compared with the pristine Ti 3 C 2 T x , the phenylsulfonic acid grafted Ti 3 C 2 T x MXene becomes amphiphilic material, which shows an increased solubility and excellent stability in water for more than one month. However, in consideration of the potential aromatic π interaction and the oxygen from the sulfonic group, the works did not give a detailed mechanism and solid evidence on the formation of MXene-aryl (Ti-O-C) linkages. Recently, Muhammad et al. compared the noncovalent and covalent interaction mechanism of 1-aminoanthraquinone (AQ) on Ti 3 C 2 T x MXene (denoted as AQ@Ti 3 C 2 T x and AQ-Ti 3 C 2 T x , respectively) [42]. It is found that after the diazonium reaction, the amino group disappeared, and the covalent bond formed between AQ and Ti 3 C 2 T x . As observed, the covalently attached AQ on Ti 3 C 2 T x MXene offered better charge transportation, thus electrochemical performance. According to the abovementioned works, the diazonium reaction can be a useful way for the covalent connection of organic molecules onto the MXene nanosheets and further expands the applications of MXene-organic hybrids.
Taking advantage of the electrical conductivity and layered structure of MXenes, the MXene-polymer hybrids usually show improved properties when compared with the pure polymer, such as mechanical properties [43][44][45][46], flame retardancy [47,48], and responsiveness [32,49]. In early 2015, an MXene-organic hybrid with CO 2 and temperature dual-responsive property was reported by Chen et al. [49]. The hybrid consisted of V 2 CT x MXene and self-initiated photo grafting and photopolymerized (SIPGP) poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. The hydroxyl groups on V 2 CT x MXene were acting as the photoactive sites, which allowed further growth of polymer brushes via SIPGP. When pre-modified MXene by SBDC [32], PDMAEMA brushes can also be connected onto the MXene surface. Zhang et al. [50] utilized in situ free radical polymerization to prepare Ti 3 C 2 T x /polyacrylamide (PAA) hybrid with acrylamide monomer and potassium persulfate as redox initiator, in which the Ti 3 C 2 T x MXene was introduced as the crosslinker.

MXene-Organic Hybrids through Electrostatic Interaction
In addition to utilizing T groups of MXenes to covalently graft organic molecules by chemical reactions, the electrostatic interactions on negatively charged MXenes surface can also be utilized. Cui et al. synthesized the (3-aminopropyl) triethoxylsilane (APTES) grafted Si nanoparticles first [51], in which the covalently X groups on silane were consumed. However, by directly using the positively charged Y group (here, amino), the grafted Si nanoparticles were still self-assembled with Ti3C2Tx through electrostatic interaction. Xu et al. reported microcontact printing to get ultrathin MXene micropatterns [52]. The MXene ink was applied to a polydimethylsiloxane (PDMS) film with patterns of alternating grooves. Subsequently, transfer printing it onto APTES-modified coverslips. In contrast, Ti3C2Tx adsorbed onto coverslips by electrostatic interaction with amino groups.

MXene-Organic Hybrids through Electrostatic Interaction
In addition to utilizing T groups of MXenes to covalently graft organic molecules by chemical reactions, the electrostatic interactions on negatively charged MXenes surface can also be utilized. Cui et al. synthesized the (3-aminopropyl) triethoxylsilane (APTES) grafted Si nanoparticles first [51], in which the covalently X groups on silane were consumed. However, by directly using the positively charged Y group (here, amino), the grafted Si nanoparticles were still self-assembled with Ti 3 C 2 T x through electrostatic interaction. Xu et al. reported microcontact printing to get ultrathin MXene micropatterns [52]. The MXene ink was applied to a polydimethylsiloxane (PDMS) film with patterns of alternating grooves. Subsequently, transfer printing it onto APTES-modified coverslips. In contrast, Ti 3 C 2 T x adsorbed onto coverslips by electrostatic interaction with amino groups.
As another kind of common-used organic molecules, alkylammonium salts including dodecyl trimethyl ammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), stearyl trimethyl ammonium bromide (STAB), octadecyl trimethyl ammonium bromide (OTAB), dioctadecyl dimethyl ammonium chloride (DDAC), dihexadecyl dimethyl ammonium bromide (DDAB), and DHT are usually used to insert in MXene layers through electrostatic adsorption. After pillared by alkylammonium, the Ti 3 C 2 T x MXene presented a dramatic increase in interlayer spacing [53,54]. Obviously, the interlayer spacing increased along with the increasing length of alkyl chains [55]. Bian et al. proposed that the positively charged ammonium group might mainly be anchored on the surface -O groups from Ti 3 C 2 T x MXene [56], and the intercalation of alkylammonium can also affect the wettability of MXene (Figure 3a). Maybe because of the competitive adsorption of cations, the CTA cation pillared Ti 3 C 2 T x MXene can only form a stable oil-in-water emulsion in neutral or basic condition. While in acidic conditions, the MXene will aggregate at the oil/water interface. In a current study reported by Michael et al., the DHT not only improved the organophilic nature of Ti 3 C 2 T x , produced stable colloidal suspensions of Ti 3 C 2 T x in nonpolar solvents but also shielded the MXene flakes from oxidation in water [27].
anomaterials 2021, 11, x FOR PEER REVIEW 7 of 2 by alkylammonium, the Ti3C2Tx MXene presented a dramatic increase in interlayer spac ing [53,54]. Obviously, the interlayer spacing increased along with the increasing length of alkyl chains [55]. Bian et al. proposed that the positively charged ammonium group might mainly be anchored on the surface -O groups from Ti3C2Tx MXene [56], and th intercalation of alkylammonium can also affect the wettability of MXene (Figure 3a) Maybe because of the competitive adsorption of cations, the CTA cation pillared Ti3C2T MXene can only form a stable oil-in-water emulsion in neutral or basic condition. Whil in acidic conditions, the MXene will aggregate at the oil/water interface. In a current stud reported by Michael et al., the DHT not only improved the organophilic nature of Ti3C2T produced stable colloidal suspensions of Ti3C2Tx in nonpolar solvents but also shielded the MXene flakes from oxidation in water [27]. Amino acids are also studied as the organic part towards MXene-organic hybrids Elumalai et al. proposed that the electrostatic interaction between -OH and -F groups on Ti3C2Tx MXene and the zwitterion consisted of amino acid molecules might be the drivin force for the spontaneous intercalation [58]. The MXene hybrids contained negativel charged glycine (Gly), phenylalanine (Phe), tryptophan (Trp), and histidine (His) can form stable suspensions in water due to the interparticle electrostatic repulsion. Whereas aro matic (Phe, Trp, His) intercalation may also produce rutile TiO2@Ti3C2Tx by sonication. On the contrary, combined with first principle calculations (DFT), recently, Chen et al. studied the interaction between Gly and double-layer structure Ti3C2O2 [59]. From their analysis the Ti and N atoms from Ti3C2O2 and Gly, respectively, were likely to share electrons and lead to the formation of Ti-N bonding. Obviously, the ionizable organic molecules ar another good choice for preparing MXene-organic hybrids with controllable properties However, the unclear interaction mechanism makes it an urgent task to be solved. Amino acids are also studied as the organic part towards MXene-organic hybrids. Elumalai et al. proposed that the electrostatic interaction between -OH and -F groups on Ti 3 C 2 T x MXene and the zwitterion consisted of amino acid molecules might be the driving force for the spontaneous intercalation [58]. The MXene hybrids contained negatively charged glycine (Gly), phenylalanine (Phe), tryptophan (Trp), and histidine (His) can form stable suspensions in water due to the interparticle electrostatic repulsion. Whereas aromatic (Phe, Trp, His) intercalation may also produce rutile TiO 2 @Ti 3 C 2 T x by sonication. On the contrary, combined with first principle calculations (DFT), recently, Chen et al. studied the interaction between Gly and double-layer structure Ti 3 C 2 O 2 [59]. From their analysis, the Ti and N atoms from Ti 3 C 2 O 2 and Gly, respectively, were likely to share electrons and lead to the formation of Ti-N bonding. Obviously, the ionizable organic molecules are another good choice for preparing MXene-organic hybrids with controllable properties. However, the unclear interaction mechanism makes it an urgent task to be solved.
Carey and his coworkers prepared the 12-aminolauric acid (ALA), or DHT-modified Ti 3 C 2 T x first (denoted as ALA-Ti 3 C 2 T x and DHT-Ti 3 C 2 T x ) [60,61]. After the modification, MXene changed from hydrophilic to organophilic, so the intercalation of the ε-caprolactam monomer and 6-aminocaproic acid catalyst was easier [60]. Additionally, uniform Ti 3 C 2 T x MXene/epoxy hybrids can be synthesized by ALA-Ti 3 C 2 T x and DHT-Ti 3 C 2 T x . The authors proposed that ALA and DHT can speed up the curing reaction of epoxy in the interlayer space, which further enlarged the interlayer space of the MXene layers [61].
Similarly, the aniline monomers are believed to electrostatically adsorb on and between the multilayer MXene nanosheets [62]. Fu et al. reported a graphene-encapsulated Ti 2 CT x @polyaniline (PANI) hybrid named GMP for supercapacitors [63]. The adsorbed aniline monomers can be chemical oxidative polymerized with ammonium persulfate as the oxidant to form Ti 2 CT x @PANI hybrid ( Figure 4). For graphene encapsulation, CTAB was utilized to lower the surface energy and change the zeta potential of Ti 2 CT x @PANI. Recently, there is also evidence that the surface functional groups on MXenes will facilitate nucleation during the PANI polymerization [64].

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Similarly, the aniline monomers are believed to electrostatically adsorb on and between the multilayer MXene nanosheets [62]. Fu et al. reported a graphene-encapsulated Ti2CTx@polyaniline (PANI) hybrid named GMP for supercapacitors [63]. The adsorbed aniline monomers can be chemical oxidative polymerized with ammonium persulfate as the oxidant to form Ti2CTx@PANI hybrid ( Figure 4). For graphene encapsulation, CTAB was utilized to lower the surface energy and change the zeta potential of Ti2CTx@PANI. Recently, there is also evidence that the surface functional groups on MXenes will facilitate nucleation during the PANI polymerization [64]. When turning to the other common-used conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT), Gogotsi and his co-workers revealed the mechanism of chargetransfer-induced polymerization of EDOT on the surface of Ti3C2Tx MXene [65]. Verified by the theoretical calculations, the parallel orientation of EDOT monomer on Ti3C2Tx surface presented the most stable configuration with the lowest binding energy of −1.02 eV. After adsorption, 0.34 electrons were transferred from EDOT to Ti3C2O2, which initiated the in situ polymerization of EDOT on Ti3C2O2. With the energy supplement via electrochemical reaction, Qin et al. demonstrated a one-step in situ electrochemical polymerization for Mo1.33C/PEDOT and Ti3C2Tx/PEDOT films with controlled thickness and micropattern [66]. However, due to the insoluble nature of PEDOT, the commercial product usually contains a mixture of poly(styrene sulfonate) (PSS). In consideration of the bicom- When turning to the other common-used conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT), Gogotsi and his co-workers revealed the mechanism of chargetransfer-induced polymerization of EDOT on the surface of Ti 3 C 2 T x MXene [65]. Verified by the theoretical calculations, the parallel orientation of EDOT monomer on Ti 3 C 2 T x surface presented the most stable configuration with the lowest binding energy of −1.02 eV. After adsorption, 0.34 electrons were transferred from EDOT to Ti 3 C 2 O 2 , which initiated the in situ polymerization of EDOT on Ti 3 C 2 O 2 . With the energy supplement via electrochemical reaction, Qin et al. demonstrated a one-step in situ electrochemical polymerization for Mo 1.33 C/PEDOT and Ti 3 C 2 T x /PEDOT films with controlled thickness and micropattern [66]. However, due to the insoluble nature of PEDOT, the commercial product usually contains a mixture of poly(styrene sulfonate) (PSS). In consideration of the bicomponent structure and the ionic chain nature, the ex-suit blending method was more popular than in-suit polymerization towards MXene-PEDOT:PSS hybrids and the electrostatic interaction between PSS and MXene promoted the formation of 3D interconnected and reticulated structure. The hydrogen from -SO 3 H on PEDOT:PSS with -OH,-F on Ti 3 C 2 T x and the electrostatic interaction between PSS and MXene were also found to jointly shield the coulombic attraction between PEDOT and PSS, which enhanced the inter-chain charge transport between PEDOT chains [67]. The Ti 3 C 2 T x /(PEDOT:PSS) film can be fabricated by drop-casting [68], vacuum-assisted filtration [69], and simply filtration [70,71]. Recently, MXene/(PEDOT:PSS) hybrid was also fabricated into a fiber-shaped supercapacitor by a one-step wet-spinning approach [72].
By the ex-situ blending method, plenty of MXene/polymers hydrides can be prepared. Firstly reported by Gogotsi et al., charged poly-diallyl dimethyl ammonium chloride (PDDA) and electrically neutral polyvinyl alcohol (PVA) were blended with Ti 3 C 2 T x by vacuum-assisted filtration [73]. Subsequently, the dispersibility of MXene sheets in the PVA matrix and the dielectric performance of MXene/PVA hybrids in X-band frequency was studied [74]. Boota et al. synthesized polyfluorene (PFO) derivatives containing nonpolar, polar nitrogen-containing, and charged nitrogen-containing lateral chains by Suzuki polycondensation reaction (Figure 3b) [57]. They have pointed out that both the electrostatic interactions and hydrogen bonds may be involved between MXene and PFO derivatives. However, the bulky methyl groups from polar polymers with charged nitrogen-containing ends prohibited the formation of the hydrogen bond. A physical vapor deposition technique, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), was performed by Ajnsztajn et al. to produce Ti 3 C 2 T x /PFO transparent hybrid electrode [75]. Via RIR-MAPLE, the film exhibited the minimal phase segregation.

MXene-Organic Hybrids through Hydrogen Bonds and Other Supermolecular Interactions
Except for the strong inorganic-organic correlation provided by covalent and electrostatic interaction, the supermolecular interactions are also critical to the formation of MXene-organic hybrids. In 2016, a Ti 3 C 2 T x /polypyrrole (PPy) hybrid with highly aligned PPy chains in between the Ti 3 C 2 T x layers was prepared by unconventional oxidant-free polymerization (Figure 5a) [76]. Specifically, the key factor in the alignment process was the hydrogen bond, which might originate from the N-H group of the pyrrole ring and terminating oxygen or fluorine on the Ti 3 C 2 T x surface. What is more, the fluorine on Ti 3 C 2 T x will be doped in PPy chains, which can further enhance their electrochemical activity. Different from the oxidant-free chemical polymerization, Zhu et al. [77] fabricated Ti 3 C 2 T x /PPy hybrid film via electrophoretic deposition and electrochemical polymerization. The pronounced acidic character of the Ti 3 C 2 T x surface generated hydrogen bonding between Ti 3 C 2 T x and PPy during electrochemical polymerization. However, for the most traditional pyrrole polymerization [78], FeCl 3 ·6H 2 O was applied as an oxidant, and the strong oxidizing Fe 3+ ions will result in the oxidation of Ti 3 C 2 T x . Therefore, Zhang et al. only found the 3D TiO 2 @NC/Fe 7 S 8 hybrids by in situ polymerization of pyrrole monomer with alkalized Ti 3 C 2 T x .
As confirmed by previous studies, quinones are important redox-active centers for the chemical reaction [79]. However, there are only a limited amount of quinones reserved in polydopamine (PDA), which is positively correlated to the oxidation state of PDA [80]. Interestingly, Ti 3 C 2 T x /PDA hybrids were found to be an excellent precursor for o-benzoquinone after low-temperature heat treatment at 300 • C [79]. After that, Li et al. further prepared vertically oriented ordered mesopores (OM) PDA/Ti 3 C 2 T x hybrid (OMPDA/Ti 3 C 2 T x ) by using the PS-b-PEO block polymer as a soft-template (Figure 5b) [81]. In their report, the hydrogen bond also played a crucial role in the assembly of dopamine/PS-b-PEO micelles and Ti 3 C 2 T x . Recently, Du and coworkers utilized Ti 3 C 2 T x @PDA prepared by in situ polymerization as starting materials to hybrid with poly(ethylene glycol) (PEG)-based polyurethane (PU) [82]. The PDA is found to be a medium molecule that facilitated the interfacial compatibility of Ti 3 C 2 T x flakes in PEG-based PU, where the Ti 3 C 2 T x @PDA covalently bonded with PU during the process of copolymerization.
Beyond anchoring the organic species on the MXene surface, the hydrogen bond also functions as a flexible junction between the inorganic MXene and organic part. Hybrid consisting of Ti 3 C 2 T x and polyacrylamide (PAM) was fabricated by Niu et al. [50]. In 2020, the exfoliated Ti 3 C 2 T x nanosheets were reported for the first time that acted as a crosslinker instead of traditional organic molecules. In the hybrid, hydrogen bonding between the -CONH 2 groups from PAM chains and the hydrophilic groups (-OH and -F) from the Ti 3 C 2 T x nanosheets were observed. Zhang et al. simply prepared MXene hydrogel hybrids by mixing Ti 3 C 2 T x MXene and commercial polyvinyl alcohol (PVA) hydrogel [83]. They indicated the secondary crosslinking between MXene and PVA and a dense network yielded by polymer chain entanglements. Xuan et al. improved this method by added MXene flakes into a homogeneous PVA solution [84], with the borate solution acting as the crosslinking agent, MXene bonding with PVA chains, and tetrafunctional borate ion covalently. A similar interaction was also found in hydrophobically associated dry polyacrylamide (HAPAM) hydrogel [85]. In HAPAM hydrogel, hydrogen bonds between MXene and amide group of HAPAM became additional crosslinking points, which contributed to the hybrid double-network with N,N'-methylene diacryl amide (MBAA) crosslinker and affected the self-healing ability and temperature sensibility. When there were two or more different kinds of monomers or polymers together with MXene nanosheets, MXene-copolymeric hydrogels can also be prepared [86][87][88]. MXene and amide group of HAPAM became additional crosslinking points, which con tributed to the hybrid double-network with N,N'-methylene diacryl amide (MBAA) cross linker and affected the self-healing ability and temperature sensibility. When there were two or more different kinds of monomers or polymers together with MXene nanosheets MXene-copolymeric hydrogels can also be prepared [86][87][88]. On the aspect of ex situ blending, Le et al. intertwined Ti3C2Tx MXene particles in tangled PPy nanowires through hydrogen bond [89]. The intercalated PPy chains between the conductive Ti3C2Tx sheets prevented the dense stacking of Ti3C2Tx. Meanwhile, the in tercalation further improved the structural stability of PPy backbones. Moreover, Ti3C2T MXene and PANI hybrid was also prepared by mechanical blending [90]. The lightweigh Ti3C2Tx/PANI hybrids for EMI shielding was obtained by simply mixing PANI powder with MXene powder. The authors suggested that an equal concentration of both materials would be beneficial for enhanced interfacial interaction between them.
Except for hydrogen, the polymer chain entanglements, ionic interactions, covalen bonding were also reported, and the role of MXene sheets in hydrogel varies [86]. MXene polymer hydrogel was gradually developed with improved performance. The typical in teractions between MXene and organic modified species are summarized in Table 1. On the aspect of ex situ blending, Le et al. intertwined Ti 3 C 2 T x MXene particles in tangled PPy nanowires through hydrogen bond [89]. The intercalated PPy chains between the conductive Ti 3 C 2 T x sheets prevented the dense stacking of Ti 3 C 2 T x . Meanwhile, the intercalation further improved the structural stability of PPy backbones. Moreover, Ti 3 C 2 T x MXene and PANI hybrid was also prepared by mechanical blending [90]. The lightweight Ti 3 C 2 T x /PANI hybrids for EMI shielding was obtained by simply mixing PANI powder with MXene powder. The authors suggested that an equal concentration of both materials would be beneficial for enhanced interfacial interaction between them.
Except for hydrogen, the polymer chain entanglements, ionic interactions, covalent bonding were also reported, and the role of MXene sheets in hydrogel varies [86]. MXene polymer hydrogel was gradually developed with improved performance. The typical interactions between MXene and organic modified species are summarized in Table 1. Table 1. Summary of the interactions between MXene and organic species.

Interaction Type Typical Modifier Binding Forms
Covalent interaction

MXene-Organic Hybrids for Flexible Supercapacitors
MXenes have been the center of general scientific attention since it was first synthesized in 2011 [91,92] because of their large specific surface areas, good hydrophilicity, and excellent metallic properties. It is believed that the interlayer space of MXenes can accommodate various cations, which is particularly well-suited for energy storage devices, e.g., supercapacitors and metal-ion batteries [93]. In a previous study, a high capacity of about 900 F cm −3 can be achieved in Ti 3 C 2 T x MXene [22], which was higher than all the reported carbon-based capacitors [31]. However, the inevitable restacking of MXene sheets inhibited the diffusion and mass transfer process and hampered the access of electrolyte to the surface area. For this purpose, enlarging the interlayer space and simultaneously keeping the contact of MXene sheets become rather important. Here, modifying the surface of the MXenes by organic species is a pursuable method, which could increase the ion adsorption and reaction sites, and further enhance their capacitive performance.
Qin et al. evaluated the capacitive properties of the Mo 1.33 C MXene/PEDOT:PSS film [71], which presented a volumetric capacitance of 1310 F cm −3 at 2 mV s −1 in a three-electrode setup. When been fabricated into a flexible solid-state supercapacitor, the volumetric capacitance can still reach 568 F cm −3 at a current density of 0.5 A cm −3 . In addition, an ultrahigh energy density of 33.2 mW h cm −3 and a power density of 19.47 W cm −3 were observed in the device. Further validated by Li et al., the insertion of conductive PEDOT chains resulted in the increasing interlayer spacing of Mo 1.33 C MXene sheets and more surface redox processes, and thus enhanced the electrochemical capacitance [70]. The conductive PEDOT chains were believed to be a bridge in forming the multidimensional electronic transport channels, which promoted the electrochemical reaction process. Gund et al. [94] prepared, flexible Ti 3 C 2 T x /PEDOT:PSS hybrids (Figure 6a) with optimal mass ratios of 1:2. Benefiting from the porous architecture, the hybrid experienced a rapid charge transport within the electroactive quinoid structure of PEDOT:PSS, and a strong surface interaction, exhibiting an areal capacitance of 2 mF cm −2 and volumetric capacitance of 83 F cm −3 at a scan rate of 1000 V s −1 . After been assembled into a symmetric electrochemical capacitor, the areal and volumetric capacitance reached 2 mF cm −2 and 83 F cm −3 at the same scan rate with long-term durability over 30,000 cycles. Nanomaterials 2021, 11, x FOR PEER REVIEW 13 of 25  Besides PEDOT:PSS, the conductive polymer, such as PPy and PANI, are also widely utilized in supercapacitors [66]. Zhu et al. found that the PPy/l-Ti 3 C 2 T x film delivered an areal capacitance of 203 mF cm −2 when compared with that of the pristine Ti 3 C 2 T x MXene (150 mF cm −2 ) and maintained nearly 100% of the initial values after 20,000 charging/discharging cycles (Figure 6b) [77]. A symmetric solid-state flexible supercapacitor was fabricated using the PPy/l-Ti 3 C 2 T x films and PVA-H 2 SO 4 solid-state electrolyte. As shown in Figure 6b, the capacitance was stable after 10,000 cycles, and no sharp fluctuation was found during bending. The enhanced performance was ascribed to the synergy of l-Ti 3 C 2 T x with unfolded PPy molecules and the strong hydrogen bonds between the two materials. Therefore, the hybrid of Ti 3 C 2 T x MXene and PPy chains not only improved the structural stability of PPy backbones but also provided more pathways for charge carriers. Boota et al. [76] further improved the volumetric capacitance up to 1000 F cm −3 by adjusting the ratio of PPy and Ti 3 C 2 T x in the hybrid film. It was found that the increase in capacitance was ascribed to the aligned PPy chains and the intercalated pyrrole molecules between the Ti 3 C 2 T x monolayers, which improved the electronic conductivity, reversibility in a redox reaction, and ion transportation. In addition, different morphology and structures of the PPy/MXene were also designed by other teams, such as a 3D Ti 3 C 2 T x @PPY nanowire network [89], organ-like Ti 3 C 2 T x /PPy [53], carambola-like MXene/PPy [95] and MXene/(BC)@PPy [96]. Xu [64] reported a Ti 3 C 2 T x /PANI hybrid as an electrode with the capacitance 556.2 F g −1 at 0.5 A g −1 , which had a capacitance retention of 78.7% at 5 A g −1 . This improvement was derived from the numerous channels for the electrolyte ions to ingress and a decreased charge transfer resistance. VahidMohammadi et al. prepared a freestanding flexible MXene/PANI electrode with a thickness of 4 µm [97]. With a mass loading of 1.34 mg cm −2 , and the hybrid exhibited a specific capacitance of about 503 F g −1 at a scan rate of 2 mV s −1 with 98.3% capacitance retention after 10,000 cycles, validating good cycling stability. Fu et al. synthesized a graphene-encapsulated MXene Ti 3 C 2 T x @PANI (GMP) hybrid material and assembled it into an asymmetric pouch-type supercapacitor [63]. It was found that the GMP electrode exhibited excellent performance with a gravimetric capacitance of 635 F g −1 . The cycling stability of the GMP electrode and integrated supercapacitor was 97.54% and 94.25% at 10 A g −1 after 10,000 cycles (Figure 7a), respectively. The robust hierarchical nanostructure and complementary synergistic effect facilitated the electrochemical process. As shown, an LED was successfully lightened by a two-series connected pouch-type GMP||graphene asymmetric supercapacitor (p-ASCs). Yun [98] first reported the layer-by-layer (LbL) assembly of the PANI nanofibers (PNFs)/Ti 3 C 2 T x with a composition of 77 wt % PNFs and 23 wt % MXenes. With a thickness of 2 µm, the PNF/Ti 3 C 2 T x hybrid showed an optimal performance with an areal capacity of 17.6 µA h cm −2 , areal energy of 22.1 µW h cm −2 , and an areal power of 1.5 mW cm −2 . The contribution of the PNFs and MXenes in the hybrid electrode was distinctly studied, and the charge storage process was consisting of a faradaic part and a non-faradaic part. On account of the easily oxidized surface, the MXenes are always used as negative electrodes. However, in a recent study, the 3D porous Ti 3 C 2 T x MXene was reported to act as a positive electrode [99]. The surface of the MXene was covered by positively charged PANI to enhance the antioxidation of MXene, and polystyrene (PS) spheres were introduced as a sacrificial template to create a 3D open structure of the hybrid (Figure 7b). It was found that the hybrid exhibited an ultrahigh performance with specific capacitances of 1632 F cm −3 at 10 mV s −1 , 1302 F cm −3 at 1000 mV s −1 and outstanding capacitance retention of 51% at a high scan rate of 5000 mV s −1 , when the PANI content was 40%. The capacitance retention was 65.4% even the current density reached to 1000 A g −1 . Based on the superior performance, an asymmetric device was fabricated to practically evaluate the feasibility of the PANI@M-Ti 3 C 2 T x electrode. The energy densities of the M-Ti 3 C 2 T x ||PANI@M-Ti 3 C 2 T x were 50.6 mWh cm −3 at 1.7 W cm −3 and 24.4 mWh cm −3 at 127 W cm −3 , respectively, and sharply increased to 1.381 kW cm −3 at 31 mWh cm −3 with a lessened cathode mass loading. Therefore, the open framework from 2D to 3D and the antioxidation of the MXenes was an innovative strategy to improve the capacitive properties.   The glycine molecules were successfully grafted onto the surface of the Ti 3 C 2 T x MXene via Ti-N bonding and showed enhanced capacitance [59]. The as-prepared d-Ti 3 C 2 T x /glycine reached 324 F g −1 at 10 mV s −1 and 140 F g −1 at 1 V s −1 , respectively, and doubled the capacitance retention of the pristine Ti 3 C 2 T x . The LBL self-assembly approach could appropriately expand the interlayer spacing of the MXene sheets without weakening the interaction force between the MXenes and tris-(2-aminoethyl) amine [100]; hence, it was broadly employed. Wu et al. [101] reported a freestanding hybrid film through a combination of a decentralized conjugated polymer chain with Ti 3 C 2 T x MXene to enhance flexible cycle performance. The hybrid film not only exhibited an area capacitance of 284 mF cm −2 at 50 mA cm −2 and 100% capacitance retention after 10,000 cycles but also showed good stability during the 0-90 • static bending test for 10,000 cycles. Another conjugated polymer with high charge mobility and tunable bandgap properties, (poly(9,9dioctylfiuorene), PFO), has been proposed to be an electrochemically active material in 2017 [57]. Only the PFOs with charged nitrogen groups (denoted as P3) was attached to the negative Ti 3 C 2 T x layers due to the electrostatic interaction. The P3@Ti 3 C 2 T x showed an improved capacitance of 380 F g −1 at 2 mV s −1 and long-term stability. Later, the authors successfully functionalized the Ti 3 C 2 T x MXene by incorporating 1-aminoanthraquinone (AQ) [42]. The AQ-Ti 3 C 2 T x film showed a gravimetric capacitance of~300 F g −1 at 5 mV s −1 , which was higher than the pristine Ti 3 C 2 T x MXene. The outstanding capacitance could be attributed to enhanced charge transportation endowed by the AQ molecules intercalation and covalently grafting onto the surface of Ti 3 C 2 T x MXene surface. The performance of MXene/polymer hybrids for supercapacitors are summarized in Table 2.

MXene-Organic Hybrids for Flexible Metal-Ion Batteries
When getting into the field of metal-ion batteries, similar requirements as which in supercapacitors, including large specific surface area and excellent conductivity, are needed to be fulfilled. However, beyond these demands, the intrinsic electrochemical activity of the host materials is more crucial. Therefore, based on the abundant surface redox sites, the potential of MXene-organic hybrids in metal-ion batteries has been evaluated in recent years.
Siriwardane et al. [106] demonstrated that the pre-intercalated Ti 3 C 2 O 2 by 1,4benzoquinone (C 6 H 4 O 2 ) or tetrafluoro-1,4-benzoquinone (C 6 F 4 O 2 ) organic molecules facilitated the ion transportation within the interlayer space, thus resulting in a high lithium storage capacity and fast kinetics. Chen et al. prepared a Ti 3 C 2 T x /PEDOT hybrid by in situ polymerization triggered by electron transfer to improve lithium-ion storage capability [65]. On this basis, a vertically ordered mesoporous PDA was attached on the surface of Ti 3 C 2 T x MXene by Li and co-workers [81], which provided more channels for ion diffusion and electron transfer. A negligible volume expansion and the unvaried morphology of the mesoporous PDA and an ultrathin solid electrolyte interphase layer steadily surrounding the active material were found in the in situ transmission electron microscopy analysis. Dong et al. [79] reported PDA-derived material (denoted as PDA300) for the first time, which exhibited an ultrahigh capacity of 977 mAh g −1 at 50 mA g −1 compared with that of the original PDA (100 mAh g −1 ). A hybrid consisted of PDA300 and Ti 3 C 2 T x was fabricated (Figure 8a) and showed a high specific capacity of 1190 mAh g −1 at 50 mA g −1 , excellent rate capability of 552 mAh g −1 at 5 A g −1 , and good cycling stability (82% retained after 1000 cycles). Although the hybrid was not fabricated by in situ polymerization, the unsaturated carbon-carbon bonds in the PDA300 and the unique structure of the highly conductive Ti 3 C 2 T x MXene jointly enhanced the outstanding performance in lithium-ion batteries (LIBs). The same polymerization approach was also available to synthesize a 3D hierarchically porous PANI/Ti 3 C 2 T x network [107]. The mechanism of Na + storage in the PANI/Ti 3 C 2 T x network is shown in Figure 8b. The larger interlayer space and negatively charged surface of Ti 3 C 2 T x nanosheets generated plenty of channels for Na + diffusion. Meanwhile, the 2D electron-transfer platforms provided by highly conductive Ti 3 C 2 T x nanosheets guaranteed the excellent electrical conductivity of the PANI/Ti 3 C 2 T x network. Interestingly, the combination of MXene flakes and PANI generated a tough but pliable network to accommodate the volume change during Na + diffusion, which also had good tolerance to temperature change. Therefore, the PANI/Ti 3 C 2 T x network exhibited an excellent capacity at the broad temperature range from −30 • C to +50 • C. Furthermore, the PANI/Ti 3 C 2 T x network also presented an excellent specific capacity of 254 mA h g −1 after 100 cycles at 100 mA g −1 and an ultrastable cycling capacity of 135.4 mA h g −1 at a high current density of 2000 mA g −1 after 10,000 cycles. The full cell assembled by PANI/Ti 3 C 2 T x anode and Na 3 V 2 (PO4) 3 cathode delivered a high discharge capacity of 140.1 mA h g −1 after 100 cycles.

MXene-Organic Hybrids for Flexible Sensor
When turning to sensors, benefiting from the anisotropic conductivity of MXenes, the electroresponse of MXene-based hybrids can be easily regulated by changing the testing direction. However, also due to the in-plane metallic conductivity of MXene sheets, the deformation and variation of surface adsorption species show a limited impact on the internal resistance. Therefore, an interfacial engineering strategy towards tuning the electrical properties of the entire material system becomes more powerful. In addition, alt-

MXene-Organic Hybrids for Flexible Sensor
When turning to sensors, benefiting from the anisotropic conductivity of MXenes, the electroresponse of MXene-based hybrids can be easily regulated by changing the testing direction. However, also due to the in-plane metallic conductivity of MXene sheets, the deformation and variation of surface adsorption species show a limited impact on the internal resistance. Therefore, an interfacial engineering strategy towards tuning the electrical properties of the entire material system becomes more powerful. In addition, although the coating of MXenes on various 3D skeletons can generate a large variety of MXene-based hybrid sensors, here we only focus on the MXene-organic hybrids that are consisting of certain organic components and through strong interfacial bonding. Liao and co-works [87] fabricated a flexible wearable strain sensor assembled with Ti 3 C 2 T x MXene nanocomposite organohydrogel (MNOH), which was prepared by immersing MXene nanocomposite hydrogel (MNH) in ethylene glycol solution (Figure 9a). The MNOH maintained flexibility and reversible bending capability and successfully lighted a LED at an extremely low temperature of −40 • C on account of the strong hydrogen bonds and excellent conductivity, while the fracture occurred, and the LED were not worked when the MXene hybrid hydrogel was at −25 • C. Meanwhile, the MNOH also owned a good self-healing capacity and showed no fracture even at a stretching condition, and the lighted LED further demonstrated the self-healing performance. In addition, the MNOH-based strain sensor could monitor human activities by connecting to a wireless transmitter, such as finger bending with different angles and saliva swallowing. Besides the flexible, selfhealing, and wearable properties, long-time storage, and superior self-adhesive Ti 3 C 2 T x MNOH were manufactured by Wu's team [88]. After been stored for 10 days at 20 • C and 55% humidity, the organohydrogel retained 90% moisture and showed no shape change and no fracture upon twisting or bending. The MNOH supported a weight of 105 g without any adhesives and was used repeatedly with no significant loss of the adhesion strength. The muscle movement caused by breathing was monitored via the sensor attached to the chest, which further validated its high sensitivity. High-strength also could be associated with hybrid double-network hydrogens [85], which could be stretched to over 14 times of the pristine length and achieve a 0.4 MPa tensile strength meanwhile reserve the self-healing ability, temperature-sensitive ability, and excellent conductivity. A network-structured Ti 3 C 2 T x MXene/PU mat (network-M/P mat) not only provided a high sensitivity with the gauge factor up to 228 and a lower detection limit with 0.1% but also exhibited a large and tunable sensing range up to 150% [108] (Figure 9b), excellent stability over 3200 cycles and multiple functions towards strain from various direction and deformation. Cai et al. recently developed a PDMS/Ti 3 C 2 T x MXene hybrid as electronic skin for intelligence daily health monitoring [109]. As a common flexible substrate, the PDMS can be easily patterned with surface wrinkles. When blended with MXenes, the hybrid presented a sensitive electroresponse to the applied pressure and can be assembled as a self-powered tactile sensor. As shown in Figure 9c, for monitoring (i) pulse, (ii) heartbeat, (iii) breath, (iv) flexion and extension of biceps, and (v) grab of five different weights, the sensor revealed different waveforms for different cases, which provided more information for medical professionals to monitor patient status in real time.
the PDMS can be easily patterned with surface wrinkles. When blended with MXenes, t hybrid presented a sensitive electroresponse to the applied pressure and can be assembl as a self-powered tactile sensor. As shown in Figure 9c, for monitoring (i) pulse, (ii) hea beat, (iii) breath, (iv) flexion and extension of biceps, and (v) grab of five different weigh the sensor revealed different waveforms for different cases, which provided more inf mation for medical professionals to monitor patient status in real time.  Chen [49] reported that a V 2 C@(poly(2-(dimethylamino)ethyl methacrylate), PDMEMA) hybrid could undergo hydrophobic-hydrophilic transitions in water for the first time, and the transmittance and conductivity of the hybrid were changed along with external temperature due to the thermal behavior of PDMEMA (Figure 10a). The opposite state of the PDMEMA chains at different temperatures resulted in the change of the V 2 C@PDMEMA suspension in water, with the corresponding transmittance changed from 15% to 75%. The change of conductivity of 76 µS cm −1 was also monitored in the V 2 C@PDMEMA suspension by cycled bubbling CO 2 and N 2 as CO 2 stimulation induced the uncharged-charged transitions of PDMEMA (Figure 10a). Later, Tran et al. [32] found that the existence of the covalent bonding between the MXene and PDMEMA and investigated the mechanism by density functional theory. The hybrid also presented a switchable behavior at below or above the lower critical solution temperature of PDMEMA. The DFT further elucidated that the polymer state (swelling or shrinking) at different temperatures changed the interlayer distance of the MXene, which resulted in the varied conductivity.
15% to 75%. The change of conductivity of 76 μS cm −1 was also monitored in the V2C@PDMEMA suspension by cycled bubbling CO2 and N2 as CO2 stimulation induced the uncharged-charged transitions of PDMEMA (Figure 10a). Later, Tran et al. [32] found that the existence of the covalent bonding between the MXene and PDMEMA and investigated the mechanism by density functional theory. The hybrid also presented a switchable behavior at below or above the lower critical solution temperature of PDMEMA. The DFT further elucidated that the polymer state (swelling or shrinking) at different temperatures changed the interlayer distance of the MXene, which resulted in the varied conductivity. The sensors not only respond to temperature or physical parameters such as strain but also can be triggered by some chemical molecules. For example, An et al. [111] firstly found that Ti3C2Tx MXene/polyelectrolyte multilayer films prepared by LBL assembly could immediately respond to H2O molecules. The intercalation or deintercalation of H2O changed the interlayer space between two MXene layers and resulted in a changed tunneling resistance between MXene sheets. Later, Neampet [112] and co-workers further enriched the sensor applying range by preparing a lactate oxidase immobilized on The sensors not only respond to temperature or physical parameters such as strain but also can be triggered by some chemical molecules. For example, An et al. [111] firstly found that Ti 3 C 2 T x MXene/polyelectrolyte multilayer films prepared by LBL assembly could immediately respond to H 2 O molecules. The intercalation or deintercalation of H 2 O changed the interlayer space between two MXene layers and resulted in a changed tunneling resistance between MXene sheets. Later, Neampet [112] and co-workers further enriched the sensor applying range by preparing a lactate oxidase immobilized on Pt//PANI/Ti 3 C 2 T x MXene hybrid for a modified screen-printed carbon electrode, which shows good response to H 2 O 2 and lactate. Besides, oxygen-containing volatile organic compounds, such as ethanol and acetone, could be detected by chemosensitive sensors, which were made by fluoroalkyl silane molecules modified Ti 3 C 2 T x [113]. The synergy of the strong adsorption energy of ethanol on Ti 3 C 2 T x -F and the local structure deformation contributed to the high sensitivity, good repeatability as well as long-term stability. By utilizing the conductivity of MXenes, Li et al. presented a flexible PANI/Ti 3 C 2 T x MXene hybrid for selective NH 3 gas monitoring [114]. The in situ polymerization of PANI with Ti 3 C 2 T x MXene generated a homogeneous Schottky junction. At this interface, Ti 3 C 2 T x induced an improved protonation of PANI, which resulted in high selectivity. As reported by Zhou and the co-workers, the PPy@Ti 3 C 2 T x hybrid can be a good platform for osteopontin detection (Figure 10b) [110]. After loading the phosphomolybdic acid (PMo 12 ), the final hybrid delivered an extremely low detection limit of 0.98 fg mL −1 , as well as high selectivity and stability. The superior performance was ascribed to the abundant surface active sites (Ti 4+ from MXenes and Mo 4+ from PMo 12 ) and the conducting network of PPy@Ti 3 C 2 T x hybrid. When directly blended with polymer, the MXene-based hybrids also can be a good candidate for advanced sensors. Recently, Wang et al. reported a PEDOT:PSS/Ti 3 C 2 T x hybrid for room-temperature methanol detector [68]. As a p-type semiconductor, the adsorption of electron-donating gas neutralized the holes in PEDOT:PSS, thus raising the resistance. While for the PEDOT:PSS/Ti 3 C 2 T x hybrid, the conductive mechanism changed to a summation of a mixed pathway, in which the MXene-MXene connection was the easiest to destroy. Therefore, the PEDOT:PSS/Ti 3 C 2 T x hybrid with a suitable polymer/MXene ratio revealed a large response to methanol.

MXene-Organic Hybrids for Other Applications
The diversified properties of the modified MXenes have also been applied in the fields of photocatalysis [115], molecular separation [78,116], seawater desalination [29], flame retardant [48], and others. Li et al. [115] prepared a modified Ti 3 C 2 T x MXene by a chlorophyll derivative called zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide (Chl), and the hybrid exhibited a high HER performance with 52 ± 5 µmol h −1 g cat −1 at the optimal Chl/Ti 3 C 2 T x ratio. The aggregated exciton transfer in Chl and the resultant charge separation at the interface of Chl/Ti 3 C 2 T x after the efficient light-harvesting improved performance together. Liu et al. [117] found that a 100 nm thickness of Ti 2 CT x -(hyperbranched poly-ethyl enimine, HPEI)/(tri-mesoyl chloride, TMC) membrane with ordered 2D nanochannels could extract 99 wt% water from the 10 wt% water/isopropanol mixtures. Shen et al. [116] prepared a thinner nanofilm about 10 nm to separate gas molecules. The chemically tuned Ti 3 C 2 T x MXene allowed CO 2 permeation, while the H 2 selective transport in the pristine MXene nanofilms. Zhao et al. [29] proposed a hydrophobic MXene membrane consisting of trimethoxy (1H,1H,2H,2H-perfluorodecyl) silane (PFDTMS) modified Ti 3 C 2 T x nanosheets and commercial filter membrane to issue salinity water desalination. The fabricated membrane not only possessed high-efficiency with a solar evaporation rate of 1.31 kg m −2 h −1 and 71% solar steam conversion efficiency, but also stably worked over 200 h. A poly(diallyldimethylammoniumchloride) (PDDA) modified Ti 3 C 2 T x MXene adding to thermoplastic polyurethane matrix obviously decreased the peak heat-release rate of 50% and lessened total smoke production of 47% [48].

Outlook
In consideration of the fascinating electric, mechanical properties, and processability of MXene-organic hybrids, the combination of MXene with suitable organic counterparts will offer a broad space for fabricating flexible electronics. To get more specific guidance for materials design, a deeper understanding of the interaction between MXene and the organic components is necessary. In this review, we mainly focus on the surface characters of pristine MXene and the accompanying physicochemical interaction between MXene and the organic components in the hybrid. The applications of MXene-organic hybrids in the fields of flexible devices are also briefly summarized.
There is no doubt that the research on MXene-organic hybrids is still at its early stage, but the rapid development of wearable devices for intelligent sensing, health monitoring, and portable display will boost the demands of relevant materials in the near future. Moreover, although the importance of interfacial behaviors between the MXene and the organic component has been widely concerned, there is still plenty of work that needs to be done, especially concerning the detailed roles of different surface terminal groups on the interaction with organic molecules and the mechanism of carriers' transition on the MXene-organic interface. We believe that with the highly compatible interface, the MXene-organic hybrids will fully integrate the advantages from both MXene and organic molecules, thus opening a new territory for next-generation flexible wearing devices.