Overcoming the trade-off in reverse osmosis membranes through homologous matching

Domain: neuroscience
Published: 2026-02-03
Editorial outcome: minor_revision

Abstract
The development of cellulose triacetate (CTA)-based reverse osmosis membranes offers a sustainable approach to alleviating the global freshwater crisis, yet overcoming the inherent permeability–selectivity trade-off remains a significant challenge. Herein, we propose a homologous matching strategy to address the trade-off by incorporating carbon dots (M-CDs) derived from m-phenylenediamine (MPD) into the interfacial polymerization process between CTA and polyamide (PA). Systematic characterization and molecular dynamics simulations reveal that M-CDs, which are structurally analogous to the MPD monomer, promote monomer diffusion, regulate cross-linking density, and refine the microstructure of the PA layer. At an optimal M-CDs concentration, the resulting membrane achieves simultaneous enhancements in both salt rejection (99.1% vs. 96.5%) and water flux (18.3 vs. 15.2 L·m-2·h-1), thus surpassing conventional CTA membranes. The incorporation of M-CDs results in a thinner, denser, and more hydrophilic barrier layer with reduced pore size and narrowed distribution, as confirmed by post-annealing structural analysis. Moreover, hydrogen bonding between M-CDs and MPD improves chlorine resistance, maintaining high performance even after exposure to 2000 ppm NaClO solution. Molecular dynamics further illustrate that M-CDs promote water cluster transport while hindering ion penetration, thereby effectively mitigating the trade-off. The innovative use of homologous carbon dots to optimize the CTA–PA interface through structural matching, offering inspiring avenues for developing advanced bio-derived desalination technologies.

Introduction
The growing global population and escalating water pollution have exacerbated the imbalance between clean water supply and demand, posing a significant barrier to sustainable development. Given that 97% of the Earth’s water is saline and unfit for direct use, purifying seawater and brackish water has become essential to alleviate freshwater scarcity1–4. Among various purification approaches, membrane-based desalination has gained prominence and currently contributes to approximately 53% of the world’s drinking water production5,6. Compared to alternatives like nanofiltration, forward osmosis, and electrodialysis, pressure-driven membrane processes, such as reverse osmosis (RO), are particularly favored due to their high selectivity, effective purification performance, and operational simplicity7–9.

Cellulose triacetate (CTA) has emerged as a promising material for RO membranes, owing to its abundance, biodegradability, and environmental compatibility. However, CTA-based RO membranes still face critical limitations, including the inherent trade-off between salt rejection and water permeability, poor chlorine resistance, and susceptibility to compaction.

Membrane performance is largely governed by structural characteristics such as pore uniformity, thickness, and surface properties10. Recent advances in nanotechnology offer pathways to overcome these limitations by incorporating functional nanomaterials to optimize membrane architecture and enhance performance11–13. Through careful design, the pore structure, layer thickness, hydrophilicity, and surface charge of nanomaterials can be precisely tuned, thereby improving selectivity and antifouling properties14,15. Although several materials, such as MXenes16, metal–organic frameworks17,18, graphitic carbon nitride19, mesoporous hollow nanospheres20, and nanomaterial sublayer21, have demonstrated the potential in enhancing permeate flux, the scalable and defect-free integration of these nanomaterials remains a challenge. In many cases, issues such as nanomaterial aggregation and interfacial defects during membrane fabrication can compromise the salt rejection, preventing the simultaneous achievement of high flux and high rejection that has been reported in ideal, lab-scale demonstrations. Additionally, nanomaterials inherently face challenges, such as poor dispersion, dimensional instability, and inadequate compatibility with composites. These issues can lead to reduced membrane permeability owing to pore blockage by composites or increased membrane thickness, which creates impeded transmission paths. Moreover, the recombination of nanomaterials can create larger pore sizes or free volume within the membrane, thereby increasing the permeability of both solvents and solutes. Attributed to the solution diffusion mechanism, it leads to the higher overall permeability but lower selectivity, facing the trade-off in reverse osmosis membranes22.

Carbon dots (CDs), a zero-dimensional carbon-based nanomaterial with dimensions typically below 10 nm, offer a promising alternative for membrane regulation23. Their tunable surface functional groups, excellent dispersibility, and biocompatibility make them particularly suitable for integration into various membrane types, including nanofiltration (NF), RO, and ultrafiltration membranes (UF)24,25. CDs can be uniformly embedded in polymer matrices to form defect-free selective layers, enhancing water permeability and antifouling performance without compromising structural integrity. Depending on the application, CDs can be strategically incorporated into dense selective layers, supporting layers, or throughout the entire membrane structure26. Previous studies have demonstrated that embedding CDs into polyamide (PA) layers can reduce transport resistance, mitigate concentration polarization, and improve both desalination efficiency and permeability27,28. Amine-rich CDs can further create dense, functional separation layers that are capable of discriminating between monovalent and multivalent ions29. Nonetheless, CDs-enhanced membranes are still subject to challenges such as the permeability–selectivity trade-off, weak interfacial adhesion, and limited long-term stability, particularly when considering environmental sustainability30. In our earlier work, we have employed the mechanistic insights of biomimetic nanochannels of microstructures on enhancing desalination from molecular simulations, as well as the trade-off relations between permeability and selectivity, facilitating the overcoming of the trade-off in reverse osmosis membranes through homologous matching of additives31–34.

To address these challenges, this study introduces an approach that utilizes the capabilities of CDs to modify dense selective or substrate layers. Particularly, CDs derived from phenylenediamine and wood were used to enhance the properties of the CTA substrate and the phenylenediamine-derived PA selective layer through a homologous matching strategy to achieve high-efficiency desalination. These CDs act as nano-intercalators, facilitating in-situ polymerization of the PA layer on the CTA surface to form a high-performance composite RO membrane. The nanoscaled dimensions and abundant functional groups of the CDs help to shorten the water transport pathway and reduce transmembrane resistance by modulating the interfacial polymerization process. Moreover, the modified CDs (M-CDs) serve as enhancers and interfacial bridges between CTA and PA, improving the overall compatibility and performance of the composite membrane. As a result, the optimized membrane achieves a salt rejection rate of 99.1% with a permeate flux of 18.3 L·m−2·h−1. It also exhibits excellent chlorine resistance, with a 16% increase in flux after exposure to 2000 ppm NaClO, and maintains nearly 100% performance stability over 700 minutes of continuous operation. These results demonstrate the superiority of the proposed membrane over existing renewable desalination membranes. By incorporating CDs via a homologous matching strategy, this work highlights the potential of biomass-derived nanomaterials in sustainably enhancing the performance of CTA-based RO membranes for desalination.

Results
Innovative strategy for interfacial layer assembly through homologous matching of CDs and PA derived from PD
To address the limitations of traditional CTA-RO membranes, this study introduces a homologous matching strategy to fabricate a series of CTA-based RO membranes (O-CDs-0.1, M-CDs-0.1, and P-CDs-0.1). Carbon dots (CDs: O-CDs, M-CDs, and P-CDs) derived from different phenylene diamine (PD) precursors were synthesized via a simple hydrothermal method (Fig. 1a). And different CDs were incorporated as nano-intercalations into the corresponding aqueous monomers (OPD, MPD, and PPD) for the in-situ interfacial polymerization of the PA layer on the CTA surface (Fig. 1b). The results revealed that CDs enhanced the low-resistance water transport channels due to their nanoscale size, abundant functional groups, and excellent hydrophilicity (Fig. 1c). Notably, the membrane incorporated with M-CDs (CTA/M-CDs/PA) demonstrated a simultaneous enhancement in permeability (from 15.2 to 18.3 L·m−2·h−1) and salt rejection (from 96.5% to 99.1%), effectively mitigating the inherent trade-off effect (Fig. 1e, f). This superior performance is attributed to the homologous matching between M-CDs and the MPD monomer. Their structural similarity ensured seamless integration at the interface, moderated the polymerization kinetics to form a denser and more uniform PA network, and optimized the incorporation of nanochannels for efficient water transport. This mechanistic insight into the synergistic interaction among the CTA base, the homologous M-CDs, and the PA layer is further supported by molecular dynamics simulations (Fig. 1d).

Effects of CDs synthesized from different carbon sources on CTA-based membranes
Eucalyptus powder and PD derivatives (OPD, MPD, and PPD) were used as dual carbon sources to develop sustainable CDs (O-CDs, M-CDs, and P-CDs), with typical nanoscale sizes and abundant functional groups. The synthesis of different wood-derived CDs was confirmed through fluorescence spectroscopy, ultraviolet absorption spectroscopy, and fluorescence behaviors (Fig. S1 and S2)35,36. High-resolution transmission electron microscopy (HRTEM) images and particle size distributions revealed that O-CDs, M-CDs, and P-CDs exhibited uniform particle sizes, with lattice spacings of 1.53±0.21, 2.22±0.56, and 1.88±0.48 nm, respectively (Fig. S3). The small particle size and graphene-like spacing of the synthesized CDs facilitated their effective incorporation into CTA-based composite RO membranes, thereby modifying their internal microstructures and mitigating defects at the nanoparticle-selective separation layer interface37,38. This was further confirmed through scanning electron microscopy (SEM) analysis of CDs-embedded O-CDs-0.1, M-CDs-0.1, and P-CDs-0.1 membranes. In the presence of CDs, the surfaces of these membranes exhibited irregular and wrinkled pits. This phenomenon can be attributed to the thermal effects resulting from the reaction between amino and acyl chloride groups during the interfacial polymerization of the aqueous phase (PD) and organic phase (trimesoyl chloride, TMC). This reaction caused solvent evaporation, leading to the formation of a nodular and wrinkled morphology during condensation (Fig. 2a). Among the various membranes, the M-CDs-0.1 membrane exhibited a denser and smoother modified surface. This difference may be attributed to the varying diffusion rates of different PD monomer structures in the aqueous solution compared with the organic phase. Cross-sectional SEM images (inserted in Fig. 2a) revealed that M-CDs-0.1 featured a thinner selective layer and a more pronounced fold morphology, enabling M-CDs to facilitate the formation of shorter water channels with reduced resistance across the RO membrane. This was due to the meso-diamine structure of M-CDs nanoparticles, which enables them to function more effectively than O-CDs and P-CDs nanoparticles as mass transport mediums at the interface of aqueous and organic solutions. Moreover, the contact angles of these membranes were assessed (Fig. 2b). Compared with the CTA-RO membrane, the CDs-embedded membranes exhibited significantly lower contact angles, indicating enhanced hydrophilicity. This improvement in hydrophilicity contributed to the increased permeability of the RO membranes. Consequently, the M-CDs-0.1 membrane, assembled with an M-CDs-modified selective layer, achieved a salt rejection rate of up to 99.1% and a permeate flux of 18.32 L·m−2·h−1 (Fig. 2c, d). The superior performance of the M-CDs-0.1 membrane can be ascribed to the homologous matching effects of M-CDs within the CTA/M-CDs/PA RO membrane system. This enhancement was due to the relatively low steric hindrance of MPD and the linear chemical molecular structure of PA molecules synthesized from MPD and TMC (Fig. 2e and S4). Furthermore, the effects of different aqueous monomer types on the chemical structure (Figs. S5 and S6) and separation performance (Fig. S7) of CTA/PA membranes were investigated (Note S1). The results revealed that the PA separation layer formed through the interfacial polymerization reaction between the aqueous monomer (MPD) and organic monomer (TMC) exhibited the highest compatibility with the CTA-based membrane surface. This layer featured optimal separation performance, further supporting the previous findings.

Effects of different M-CDs concentrations on CTA-based membranes
The evolution of the membrane’s microstructure with increasing M-CDs content is shown in Fig. 3a. At higher M-CDs concentrations, the membranes develop irregular and fluctuating ridge-valley structures. This alteration may be attributed to the disruption of the cross-linking reaction between MPD and TMC during the interfacial polymerization process, and the presence of M-CDs leads to the formation of additional diffusion channels within the nascent PA network39. Particularly, the increased presence of MPD molecules in the aqueous phase enables deeper penetration into the nascent PA layer, facilitating further reactions with TMC molecules in the organic phase. This process continues until the PA layer reaches a sufficient thickness and density, which inhibits further MPD diffusion and terminates the cross-linking reaction. Cross-sectional SEM images confirm that the membrane modified with 0.05 wt% M-CDs exhibits a relatively thin and dense barrier layer. As the M-CDs concentration increased, the surface thickness of the membrane also increased (Fig. 3a). This indicates that the functionalized hydrophilic groups (−OH and −COOH) on the nanoparticles facilitate molecular transport and the diffusion of MPD to the interface. Despite the thinner permeation-selective layer of the M-CDs-0.05 membrane, the membrane does not achieve the same retention capacity as M-CDs-0.1 owing to its shorter water transport path and lower transmembrane resistance. Among the modified membranes, M-CDs-0.1 exhibits the highest surface roughness, with Ra and Rq values of 10.5 and 14.7, respectively (Figs. S8 and S9), and the low contact angle of 42.6° (Fig. S10). It has been demonstrated that the optimal incorporation of M-CDs can significantly enhance both salt rejection and permeate flux of CTA-based membranes (Fig. 3b).

To gain deeper insight into the molecular mechanisms underlying the influence of M-CDs on PA formation, molecular dynamics simulations were conducted (models are shown in Fig. 1d). The cross-linking process between MPD and TMC was simulated in two stages (Fig. 3c): an initial rapid reaction phase with abundant reactants at the interface, followed by a slowdown due to increased steric hindrance and reduced availability of reactive sites, which is consistent with the experimental observations and prior studies40,41. At low concentrations, M-CDs had minimal impact on the cross-linking reaction. However, as the concentrations of M-CDs increased, both the reaction rate and the ultimate degree of cross-linking decreased significantly. In contrast, plain CDs exhibited a weaker effect. Similarly, the diffusion of MPD toward the interface was notably slowed with higher M-CDs concentrations, although CDs also reduced diffusion to a lesser extent (Fig. 3d). Notably, more MPD molecules accumulated around M-CDs than around plain CDs (Fig. 3e–3f), owing to attractive interactions between M-CD-adsorbed MPD and free MPD in the aqueous phase, which further restricted molecular diffusion. Post-annealing analysis of the cross-linked membrane structures revealed average pore sizes in the range of 3–4 Å (Fig. 3g). The smallest pore size (3.12 Å) and the narrowest distribution (smallest FWHM) were achieved at an M-CDs mass fraction of 6.89% (Fig. 3h). These results indicate that an appropriate amount of M-CDs can refine the membrane microstructure, leading to more uniform and smaller pores, which are critical for enhancing separation performance.

Mechanism of enhanced salt rejection and permeation rates of M-CDs in CTA-based membranes
The enhanced salt rejection and permeation capabilities of M-CDs incorporated into CTA-based membranes were systematically investigated through microstructural and physicochemical characterizations. SEM and AFM analyses of the pristine CTA-RO, M-CTA, and M-CDs-0.1 membranes revealed that both modified membranes exhibited a wrinkled PA layer morphology (Fig. 4a and S11). After embedding M-CDs into the intermediate layer between the CTA support and the PA active layer, the surface morphology of M-CTA and M-CDs-0.1 membranes became notably distinct from that of the pristine CTA-RO. Importantly, the incorporation of M-CDs contributed to a more uniform surface microstructure of the PA layer compared to M-CTA. The observed decrease in contact angles can be attributed to the abundant polar oxygen-containing functional groups of M-CDs and the intrinsic hydrophilicity of amino groups, which is consistent with the chemical structure analysis based on the FTIR spectra (Fig. S12)42. Moreover, the hydrophilic nature of M-CDs facilitated the transport of MPD molecules, enabling their outward dispersion at the interface. However, the formation of hydrogen bonds between MPD and M-CDs molecules interfered with this process, leading to reduced crosslinking in the PA layer. Consequently, a greater number of unreactive acyl chloride groups remained on the surface of the M-CDs-0.1 membrane, along with a higher proportion of unreacted hydrophilic carboxyl and amine groups. These factors contributed to the improved hydrophilicity of the M-CDs-0.1 membrane. The significant increase in surface roughness from 3.18±0.21 (CTA-RO) to 10.5±0.53 (M-CDs-0.1) (Fig. 4a and Table S1) is consistent with the reduction in the contact angle from 68.2° (CTA-RO) to 42.1° (M-CDs-0.1) (Fig. S13). The increase in hydrophilicity and permeability from 15.2 (M-CTA) to 18.3 L·m−2·h−1 (M-CDs-0.1) resulted from the incorporation of the CDs-modified CTA matrix and PA layer (Figs. S14 and S15).

Further insights and evidence were gained through molecular dynamics simulations. The hydration swelling of the dry membrane increased its thickness from ~4 nm to ~6 nm. Under an operating pressure of 600 MPa for a 200 mM NaCl solution, the pure PA membrane showed poor permeability and a salt rejection of only 32.09%. With the incorporation of M-CDs, the separation performance of the membrane improved significantly. Concretely, as the M-CDs content increased from 0 to 6.89 wt%, the permeate flux increased from 2.24 to 16.52×10⁴ L·m−2·h−1·MPa−1, and the salt rejection rose from 32.09% to 76.55%, effectively overcoming the permeability-selectivity trade-off (Figs. 4b, 4c). However, further increasing the M-CDs concentration led to a decline in performance. In contrast, the unmodified CDs also improved membrane properties, but a clear concentration-dependent trend was not observed.

According to Elimelech’s “solution-friction model”, water transport occurs mainly in the form of clusters, and membrane permeability is governed by pore size, solvent molecular dimensions, and viscosity43. The pure PA membrane after full swelling exhibited a water content of 19.02%, consistent with values reported in the literature (10–23%)44,45 and close to the value of 18.9% reported by Zhang et al.46. The incorporation of M-CDs and CDs increased the membrane water content, enhancing water permeability (Fig. 4d). However, high concentrations of M-CDs reduced water content due to their higher density compared to the PA matrix, which restricted the overall swelling of the membrane. Density distribution analysis along the pressure gradient direction revealed a distinct dense/porous bilayer structure in M-CDs-incorporated membranes (Figs. 4e-4f). This structure originated from the cross-linking reaction between M-CDs and TMC, leading to M-CDs enrichment on the aqueous side and formation of a dense layer that enhanced salt rejection. At low concentrations, M-CDs promoted membrane swelling and formed a dense cross-linked network, improving both permeability and selectivity. At high concentrations, however, the reduced spacing between the M-CDs severely restricted chain mobility, resulting in the formation of rigid, high-density regions and defects, which in turn degraded performance47,48. In contrast, unmodified CDs did not participate in cross-linking and could diffuse freely at the oil–water interface, thus failing to form a dense layer; their distribution within the membrane likely influences separation performance (Fig. 4h–4k).

Further analysis of water–water (O–O) and Na⁺–water (Na⁺–O) coordination numbers showed that the O–O coordination number in the membrane ranged from 5 to 8, consistent with Elimelech’s computational results43. The Na⁺–O coordination number was lower than the O–O value, indicating that Na⁺ ions should undergo partial dehydration before entering membrane channels (Fig. 4g). The incorporation of M-CDs increased the O–O coordination number while decreasing the Na⁺–O value. This observation suggests that water transport pathways were either extended or optimized, while ion transport was further hindered, thereby supporting the proposed mechanism.

Figure 4l illustrates the enhanced salt rejection and permeation mechanism of M-CDs in CTA-based membranes. During the interfacial polymerization process, M-CDs in the aqueous MPD monomer solution permeated the pores at the CTA-RO membrane interface and reacted with the TMC organic monomers to form M-CDs nanoaggregates. After heat treatment, the irreversible shrinkage of the composite membrane ensured the stable fixation of these nanoaggregates at the interface between the CTA matrix and the PA separation layer. This resulted in the formation of a dense modified PA layer, which significantly enhanced the salt retention capacity of the membrane. Therefore, the M-CDs-0.1 membrane exhibited a salt retention rate of up to 99.1% and a permeate flux of 18.32 L·m−2·h−1, outperforming the M-CTA RO membrane. More importantly, M-CDs enhanced the salt rejection rate of the M-CDs-0.1 membrane and maintained the permeate flux, thereby mitigating the typical trade-off between salt rejection and permeate flux observed in conventional CTA-based RO membranes.

Contributions of M-CDs to high chlorine resistance and long-term stability of CTA-based RO membranes
Generally, the interfacial polymerization process for creating a PA layer involves diffusion and reaction steps. First, the monomer MPD, which is embedded within the matrix pores and on the surface, diffuses towards the reaction interface. At this interface, MPD reacts with the TMC monomer in the reaction medium to form a complete PA selective layer. This study adheres to the established reaction mechanism and incorporates M-CDs into the interfacial polymerization process. These M-CDs modulate the reaction rate, thereby influencing the thickness and microstructure of the PA layer. Consequently, the M-CTA membrane exhibits a denser selective layer structure, which effectively rejects most small molecules and monovalent salts. The addition of nanoscale M-CDs with surface electronegativity at the CTA interface positively affects both the interfacial polymerization rate and the formation of the PA layer (Fig. 5a). Figure 5b shows the changes in the surface charges of the CTA-RO and M-CDs-0.1 membranes at a pH of 7.0. The electronegativity observed on the CTA-RO membrane surface is consistent with the findings in the previous literature. In contrast, the M-CDs-0.1 membrane, which incorporates M-CDs within the PA layer, exhibits a significantly higher negative charge. This enhanced negativity can be attributed to the carboxyl and amino groups present in M-CDs. Notably, the high electronegativity from the carboxyl groups generated during the interfacial polymerization reaction leads to a significantly lower contact angle for the M-CDs-0.1 membrane compared with the M-CTA membrane. Additionally, considering the negatively charged Cl− in NaCl solutions, the increased electronegativity facilitates electrostatic repulsion between the salt-rejecting components and the negatively charged ions at the RO membrane filtration interface. Consequently, the M-CDs-0.1 membrane exhibits a superior salt rejection rate during the RO process. Moreover, the increased negative charge on the M-CDs-0.1 membrane surface enhances its ability to attract water molecules through hydrogen bonding, leading to an increase in permeate flux.

The contributions of M-CDs to enhancing the desalination efficiency of CTA-based RO membranes are illustrated in Fig. 5c. The electrostatic attraction between M-CDs leads to the adsorption of some nanoparticles onto the CTA substrate surface, thereby partially blocking the pores. This blockage hinders the diffusion of MPD, which limits the extent of the interfacial polymerization reaction with TMC. During the interfacial polymerization process, the adsorbed M-CDs occupy space and may react with TMC, competing with the reaction between MPD and TMC. This competition reduces the degree of the interfacial polymerization reaction and hinders the formation of a well-defined PA network structure. With the addition of M-CDs nanoparticles, the surface roughness of the M-CDs-0.1 membrane increases owing to their interaction during the interfacial polymerization process. The abundance of hydrophilic groups on M-CDs nanoparticles expands the reaction zone between the two immiscible phases (aqueous and organic), enabling MPD molecules to penetrate deeper into the organic phase. Consequently, the M-CDs-0.1 membrane exhibits a more nodular and wrinkled morphology, which enhances the network structure of its PA layer. This refined structure enhances permeability and ensures the high retention capabilities of the membrane.

The relationship between permeate flux and salt rejection may result from the complexities of the mass transfer mechanisms and the microstructure of the RO membrane. The unmodified M-CTA membrane exhibited a permeate flux of 15.2 L·m−2·h−1 and a salt rejection rate of 96.5% when treating the 3000 ppm NaCl solution at 2 MPa. In contrast, under optimal loading conditions, the performance of M-CDs-0.1 membrane exceeded that of the M-CTA membrane, achieving a permeate flux of 18.3 L·m−2·h−1. This indicates a significant increase of 20.4% in permeate flux and a higher retention rate of 99.1% (Fig. 5d, e). This phenomenon can be elucidated through the mass transfer mechanisms across membranes. The mechanisms involve three fundamental steps: the adsorption of water molecules, their diffusion under pressure through the densest structure layer, and subsequent desorption. The SEM analysis indicates that the introduction of M-CDs leads to a rougher surface on the dense layer and a thinner separation layer. These alterations facilitate the efficient transfer of water molecules, thereby enhancing the permeate flux49. Additionally, the increased number of oxygen-containing groups on the membrane surface reduces the water contact angles, thereby enhancing hydrophilicity and further increasing permeate flux. The steric hindrance created by M-CDs limits the penetration of MPD, which delays the formation of the PA layer. Consequently, as the thickness of the active layer decreases, the mass transfer resistance of the membrane significantly decreases, leading to improved permeate flux. However, excessive amounts of M-CDs may result in thicker, denser membranes and agglomeration, which can reduce membrane permeability. Notably, the in situ interfacial polymerization process mainly occurs within the pores of the CTA substrate and the surfactant layer, while post-heat treatment stabilizes the embedding of M-CDs nanoaggregates within these pores. Overall, the strategic surface presence of M-CDs significantly contributes to the partial mitigation of the trade-off in the CTA-based membranes.

The long-term salt interception capability and osmotic stability of CTA-based membranes are crucial factors determining their service life. In this test, the M-CDs-0.1 membrane was continuously operated for 700 min at an operating pressure of 2 MPa, and its permeate flux and salt rejection were systematically recorded (Fig. 5d, e). During these long-term experiments, the M-CDs-0.1 RO membrane exhibited excellent stability owing to the use of CTA-RO as the modified base, which inherently provided high mechanical strength. Furthermore, the excellent hydrophilicity and nanoscale properties of M-CDs created ideal conditions for constructing a highly permeable PA layer. The excellent compatibility of M-CDs maintained the structural integrity of the active layer, while their nanoscale size facilitated a uniform distribution of M-CDs interlayers on the membrane surface. The M-CDs-0.1 RO membrane exhibited excellent performance in both permeate flux and salt rejection. Thus, M-CDs effectively functioned as microstructural water transport channels on the membrane surface, thereby significantly enhancing the desalination performance of CTA-based RO membranes.

Chlorine resistance is a crucial factor in evaluating the stability of RO membranes in practical applications. After treatment with NaClO solution, significant differences were observed between the M-CTA and M-CDs-0.1 RO membranes (Fig. 5f). The permeate flux of the M-CTA RO membrane increased to 1.2 times its original value, while the permeate flux of the M-CDs-0.1 RO membrane increased to 1.16 times its original value. However, the salt rejection rate of the M-CTA RO membrane significantly decreased to 0.95% of its original level. Conversely, the M-CDs-0.1 RO membrane exhibited a slight decrease in salt rejection rate, indicating that the M-CDs modified membrane exhibited superior resistance to chlorine degradation. The lower chlorine resistance of the M-CTA RO membrane can be attributed to the degradation of the amide bond50,51. Particularly, active chlorine atoms underwent substitution reactions with free hydrogen atoms on the amide bond, causing the chlorine atoms to migrate to the ring and bind with PA. This disrupted the cross-linking network structure of the amide and weakened hydrogen bonding, generating more water transport channels and resulting in improved permeability. The incorporation of M-CDs nanoparticles during the formation of the PA layer in the M-CDs-0.1 RO membrane enhanced its chlorine resistance owing to several factors. First, the interaction between the PA layer and M-CDs strengthened hydrogen bonding between the two groups, thereby preventing active chlorine atoms from substituting for free hydrogen. Second, the abundance of hydrophilic groups on the M-CDs surface enhanced the hydrophilicity of M-CDs-0.1, thereby acting as a protective layer for the PA layer52,53. Additionally, zeta potential analysis revealed that M-CDs-0.1 exhibited a higher surface negative charge than RO membranes without M-CDs. This increased negative charge enhanced the Cl− barrier, as electrostatic repulsion between Cl− and the negatively charged surface reduced the likelihood of chlorine reaching the PA surface. This interaction further decomposed Cl− and strengthened the Cl− barrier54,55. Moreover, the presence of M-CDs provided additional sites for active chlorine attack, while the amino groups in the M-CDs nanoparticles served as sacrificial groups. This interaction slowed down the hydrolysis of the PA network and significantly improved the chlorine resistance of M-CDs-0.156,57.

Interaction mechanism between CTA, wood-derived CDs, and PA in CTA-based membranes
The chemical functional groups in CDs were confirmed through FTIR spectroscopy (Fig. S12). The observed adsorption peaks at 3502, 1721, and 1670 cm−1 correspond to the tensile vibrations of O–H (hydroxyl), C=O (carbonyl), and N–H (amide) bonds, respectively58. These peaks indicate that the surfaces or edges of the prepared CDs contain oxygen-containing functional groups, as analyzed from a molecular chemistry perspective. FTIR spectra (Fig. S16) of the M-CTA membrane exhibit distinct characteristic peaks at 3410, 1720, and 1380 cm−1, corresponding to the tensile vibrations of O–H, C=O, and C–N functional groups, respectively. The results indicate that a PA layer has been successfully formed on the CTA base surface, in which CDs are embedded. Similarly, the P-CTA and O-CTA membranes exhibit the same chemical structural features (Fig. S5). The incorporation of M-CDs into the PA layer leads to a more pronounced −OH absorption peak, further confirming the integration of M-CDs at the interface between the CTA and PA layers. This observation is consistent with the results from the addition of P-CDs and O-CDs (Fig. S12). The FTIR spectra of M-CTA, O-CTA, and P-CTA membranes exhibit distinct stretching vibration peaks at 1630 and 1560 cm−1, corresponding to C=O and N–H groups. These peaks are indicative of the PA layer, indicating that the introduction of CDs into the membrane does not alter the chemical structure of the PA layer. This observation suggests that the addition of CDs to the aqueous solution does not negatively affect the polymerization process of the PA layer, thereby preserving its integrity and functionality.

Moreover, the FTIR spectrum of the M-CDs-0.1 membrane features a vibration peak at 3430 cm−1, indicating the presence of −OH, C=O, and −NH groups within the membrane. Compared with the M-CTA membrane, the spectrum of the M-CDs-0.1 membrane displays enhanced absorption peaks for C=O, −NH, and −OH groups. This observation indicates that the carboxyl and hydroxyl functional groups from M-CDs are successfully integrated into the M-CDs-0.1 membrane, thereby improving its hydrophilicity and permeability. Furthermore, as the concentration of M-CDs increases, the −OH absorption peak is significantly enhanced, along with a weaker C–C tensile vibration at 1610 cm−1 and a more pronounced C=O tensile vibration at 1710 cm−1. These changes can be attributed to the presence of hydroxyl, carboxyl, and carbonyl groups within the M-CDs nanoparticles (Fig. S17). The presence of these hydrophilic groups is crucial for the successful incorporation of M-CDs into the M-CDs-modified PA layer, contributing to the observed increase in the hydrophilicity of the M-CDs-0.1 surface. However, owing to the relatively low quantity of CDs added to the PA layer, some of the characteristic peaks in the M-CDs spectrum may not be easily identifiable and could be masked by the dominant peaks from the CTA and PA molecules.

To further investigate the chemical composition, confirm the successful integration of CDs, and elucidate the interactions among the three components, X-ray diffraction (XRD) and XPS tests were conducted. The XRD patterns (Figs. S18 and S19) reveal a consistent diffraction pattern across all membranes, exhibiting a peak at 2θ=22.6°. This peak originates from the inherent lattice properties of carbon-based materials, indicating their graphitic carbon nature59. This observation is consistent with the highly disordered structure of carbon, evidenced by a blurred, non-dispersive pattern centered at 2θ=23.2°, similar to the characteristic diffraction pattern of CTA-II. Furthermore, the overall broadening of the diffraction peaks confirms the successful embedding of M-CDs onto the membrane surface.

The XPS results confirm that the M-CTA RO membrane contains C, O, and N elements, which mainly originate from the PA layer formed through a reaction between MPD and TMC on the membrane surface (Table S2 and Fig. 6a–d). Notably, both M-CDs-0.1 and P-CDs-0.1 membranes exhibit significantly higher oxygen content than M-CTA (Fig. S20). This increase can be attributed to the successful incorporation of CDs, which introduces additional oxygen-containing groups, thereby enhancing membrane hydrophilicity60. The P-CDs-0.1 membrane features a lower N content than M-CDs-0.1, suggesting a reduced reaction level between P-CDs and the TMC monomer. Additionally, the N 1s spectrum (Fig. S21) reveals that P-CDs-0.1 contains a higher −NH2 content, indicating that a greater amount of the MPD aqueous solution is retained on the membrane surface. This phenomenon indicates lower reactivity between P-CDs and MPD, which hinders the formation of a uniform and dense selective layer, resulting in reduced permeability of the RO membrane.

Furthermore, the introduction of M-CDs and P-CDs leads to a lower C/N ratio for the M-CDs-0.1 and P-CDs-0.1 membranes compared with the M-CTA membrane, indicating a decrease in the degree of cross-linking. This reduction in cross-linking can be attributed to several factors. The nanoaggregates formed during the in situ polymerization of TMC and CDs remain on the membrane surface and infiltrate the membrane voids, further reducing the cross-linking between the organic phase TMC and aqueous phase monomers. Moreover, the M-CDs-0.1 membrane exhibits elevated −NH− content and excess unreacted −NH2 groups. This may result from residual aqueous monomers, as some amine groups in these monomers react with the carbonyl groups of CDs, leading to a decrease in the cross-linking density of the PA layer and promoting the formation of rapid ion transport channels61. Notably, the electrostatic attraction between the negatively charged −OH/−COOH groups in CDs and positively charged amine groups in MPD molecules hinders the migration of MPD toward TMC molecules, thereby reducing the interfacial cross-linking degree. Additionally, the reduced cross-linking of the PA layer may be attributed to the water molecules surrounding the −COOH groups in the M-CDs nanoparticles, which hinder the secure attachment of PA macromolecules around CDs62. Consequently, this process leads to the formation of interfacial spaces, resulting in a looser PA structure. Furthermore, the elemental analysis of C 1s and O 1s spectra (Fig. S21) reveals that the M-CDs-0.1 membrane exhibits a higher total oxygen content (34.52%) and a C=O content (61.14%), consistent with the improved hydrophilicity confirmed by the contact angle analysis.

Exploring the interaction mechanism between CTA, CDs, and PA within CTA-based membranes is crucial and requires an in-depth study. Molecular dynamics simulations of CTA, M-CDs, and PA molecules are conducted (Fig. 6e, f and Figs. S22–S24). M-CDs are embedded at the interface between CTA and PA, which serves as a key connector for facilitating transitions and optimizing the overall structure. The mean square displacement (MSD) analysis of the azimuthal shift reveals insightful trends. As CTA molecules are isolated, the MSD value is at its lowest, indicating restricted molecular mobility due to their tightly packed arrangement. However, upon the polymerization of PA on the CTA surface, interactions at the interface layer enhance system flexibility, resulting in an increased overall MSD value. Additionally, the incorporation of M-CDs into the PA layer creates additional binding sites, which enhance intermolecular interactions. Consequently, the MSD value decreases and falls between the values observed for the isolated CTA molecules and polymerized PA scenarios. This finding indicates that M-CDs significantly influence the dynamics and interactions within the membrane structure.

A comprehensive analysis was performed to assess the interaction energies between CTA, M-CDs, and PA (Fig. 6f). Notably, CTA and M-CDs exhibit the strongest interaction owing to the ability of the hydroxyl groups in CTA to form stable hydrogen bonds with the functional groups on the M-CDs surface. Moreover, Van der Waals forces may contribute to this interaction, particularly owing to the favorable alignment of distances between the surface functional groups of M-CDs and the hydroxyl groups in CTA. In the selective layer, M-CDs and PA exhibit a stronger interaction than that between CTA and M-CDs. This enhanced interaction is due to the strong hydrogen bond donor and acceptor properties of the amide bond in PA, which facilitate the formation of numerous hydrogen bonds with the functional groups on the M-CDs surface. Although CTA and PA molecules can form hydrogen bonds through their hydroxyl and amide groups, the extent and strength of these bonds are limited when compared with those involving M-CDs owing to spatial constraints. Nevertheless, the significance of the hydrogen bonds formed between CTA and PA molecules should not be underestimated. The strong interaction between CTA and M-CDs indicates the significant potential of M-CDs to enhance the overall properties of composite membranes, including high-efficiency desalination, chlorine resistance, and long-term stability. Conversely, the weaker interaction between CTA and PA suggests that the formation of a stable composite structure between CTA and PA without M-CDs is challenging. Therefore, M-CDs can enhance the properties of the membrane and serve as a crucial connector between CTA and PA, thereby optimizing the performance of CTA-based membranes.

Methods
Material and chemical reagents
Cellulose triacetate (CTA, 43–49 wt% acetyl) was purchased from Wengjiang Reagent Co., Ltd. (Guangdong, China). Eucalyptus chips were provided by a wood-working factory (Guangxi, China). Sodium chloride (NaCl, 99 wt%), hexane (≥99.5 wt%), trimesoyl chloride (TMC, 99 wt%), dimethyl sulfoxide (DMSO, 99 wt%), o-phenylenediamine (OPD, AR), m-phenylenediamine (MPD, AR), p-phenylenediamine (PPD, AR), and lauryl sodium sulfate (SDS, AR) were provided by Aladdin Reagent Co., Ltd. (Shanghai, China). Other chemicals used are described in the Supplementary Information.

Preparation of CDs derived from eucalyptus powder and phenylenediamine. First, eucalyptus chips were dried in the oven for 24 h, then grinded and sifted to obtain 300 mesh of eucalyptus powder. 3 g of different carbon sources (OPD, MPD, PPD) were added to the inner lining of a reactor containing 50 ml deionized water, and then 0.3 g of eucalyptus powder was added as another carbon source. After fully stirring for 1 h, the mixture was placed in a high-pressure reactor at 180 °C for 24 h to obtain a brown solid-liquid mixture. The obtained solid-liquid mixture was filtered through a 0.22 μm cellulose filter membrane to filter out the macromolecular particles. To further clarify the filtrate, high-speed centrifugation was performed. Finally, the small molecular precursors were removed by rotating dialysis to obtain pure carbon dots, which were labeled as O-CDs, M-CDs, and P-CDs, matching different carbon sources of OPD, MPD, and PPD, respectively.

Interfacial layer assembly of CTA/wood-derived CDs/PA RO membranes
The CTA-RO membrane was assembled through the phase inversion process, leveraging the non-solvent induced phase separation (NIPS) system, with details provided in the Supplementary Information63. Then, a PA layer was synthesized on the surface of the CTA-RO base membrane via conventional interfacial polymerization reaction. Concretely, the CTA-RO base membrane was immersed in a 2 wt% OPD/MPD/PPD solution containing 0.1 wt% CDs for 5 min, followed by a 3-minute air exposure to remove excess OPD/MPD/PPD solution. After heat treatment in a water bath at 85 °C for 10 min, the treated surface of CTA-RO base membrane was immersed in an n-hexane solution containing 0.15 wt% TMC for 2 min to ensure interfacial polymerization reaction with OPD/MPD/PPD aqueous solution on the surface of CTA-RO base membrane, and then to remove excess TMC solution. The CTA/PA RO membranes prepared by different aqueous solutions were designated as O-CTA, M-CTA, and P-CTA, respectively, to explore the influences of variational aqueous monomers on the performances of the membranes. Subsequently, in order to modify the PA layer, the synthesized O-CDs, M-CDs, and P-CDs were added to the OPD, MPD, and PPD aqueous solutions at a concentration of 0.1 wt%, respectively. In particular, a series of interfacial layer assemblies of CTA/wood-derived CDs/PA RO membranes was fabricated using the same process outlined above, labeled as O-CDs-0.1, M-CDs-0.1, and P-CDs-0.1, respectively, where “0.1” denotes the concentration of the synthesized CDs in the respective MPD aqueous solution. Moreover, the effect of CDs concentration (0.05 wt%, 0.1 wt%, 0.15 wt%) on the properties of the composite membranes was also further discussed, named as M-CDs-0.05, M-CDs-0.1, and M-CDs-0.15, respectively.

Characterization
The microsize and micromorphology of the CDs were observed using transmission electron microscopy (TEM, FEI Tecnai, USA), and the diameter distribution of the CDs was further analyzed with ImageJ. The chemical structure was examined by Fourier transform infrared spectroscopy (Nicolet iS 50, Thermo Fisher, USA) to test the FTIR spectra of the samples in the range of 4000–600 cm−1. An X-ray diffractometer (SmartLab 9kw, Nikaku, Japan) was adopted to record the XRD pattern of the sample to analyze the crystal characteristics. The lyophily of the sample was tested by a contact angle device (DSA100, KRUSS, Germany). The surface and cross-sectional micro-morphologies of the membranes were characterized using scanning electron microscopy (SEM, Sigma 300, Zeiss, Germany). The surface roughness of membranes was characterized by atomic force microscopy (AFM, Bruker size icon). Molecular dynamics simulations of systems containing CTA, M-CDs, and PA molecules were conducted following a 7-step compression and relaxation scheme, as outlined in Table S3 (detailed in the Supplementary Information). Molecular dynamics simulations of membrane formation and separation processes were performed using the LAMMPS package. System visualization was carried out with VMD and OVITO, while molecular coordinate files were generated using Moltemplate. Crosslinking reactions were modeled using the REACTER protocol. Additional details regarding the simulation methodologies are provided in the Supplementary Information.

Evaluation of membrane separation performance
Two key properties of RO membranes, i.e., permeate flux (Jw, L·m−2·h−1) and salt rejection (R, %), were tested using a lab-made cross-flow device and calculated by the corresponding equations (S1-S2). All membranes were measured at least 3 times under identical conditions, including an operating pressure of 2 MPa, NaCl concentration of 3000 ppm, effective membrane area of 15.89 cm2, a test temperature of 25±2.0 °C, and a transverse flux of 1.0 L/min. For the evaluation of chlorine resistance of membranes, the membranes were immersed in the prepared 2000 ppm sodium hypochlorite (NaClO) solution for 2 h, rinsed several times, and then tested for separation performance. The chlorine resistance was assessed based on the permeate flux and NaCl rejection of the RO membranes before and after treatment with the NaClO solution. Additionally, the long-term salt rejection and osmotic stability of the RO membranes were also tested under the same conditions, with the only variation being the duration of the test.

Data availability
The data that support the findings of the study are included in the main text and supplementary information file. Source data are provided with this paper or on Figshare (https://doi.org/10.6084/m9.figshare.31081366). All data are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors developed an innovative use of homologous carbon dots to optimize the interfacial layer between cellulose triacetate and polyamide through homologous matching. It increases both salt rejection from 96.5 % to 99.1 % and permeate flux from 15.2 to 18.3 L·m−2 h−1, far breaking through the “trade-off” in conventional cellulose membranes. It is interesting and can be consider for publication after the following revisions.\n1. Suggest shortening abbreviations, for example, O-CCDs-0.1, M-CCDs-0.1, P-CCDs-0.1 can be revised to O-CDs-0.1, M-CDs-0.1, P-CDs-0.1.\n2. Remove the parentheses from the serial numbers of Figures to match the publishing style of NATURE COMMUNICATIONS.\n3. The dimensions of O-CDs, M-CDs, and P-CDs should be the average size, please be precise.\n4. Some of the fonts in Figure 2f should be enlarged for easier viewing.\n5. Introduction is too lengthy and thus should be rewritten concisely.\n6. It is not clear why CTA is used as a substrate rather than conventional polysulfone and polyethersulfone supports.\n7. How to obtain the particle size distributions of O-CDs, M-CDs, and P-CDs needs to be specified.\n8. More discussion should be carried out in terms of the homologous matching effects of M-CDs on CTA/M-CDs/PA RO membranes.\n9. The unit of pressure (Mpa) in the text should be corrected to \"MPa\".\n10. Figure 5, the particle size is recommended to be expressed as the average particle size.\n11. There are some writing errors in this article, for example, “This indicates that the CTA base surface successfully forms a PA layer, with CDs embedded within the layer”.\n12. More discussion should be carried out in terms of the enhanced salt rejection and permeation mechanism of M-CDs in CTA-based membranes.", "reviewer_2": "The authors used a homologous matching strategy to fabricate a series of CTA-based RO membranes for desalination. Although the flux and salt rejection are OK, the experimental characteristics and molecular dyanmics simulation can not provide the forming mechanism of such membranes. The pore structure and the performance of the membrane are also not clearly demonstrated. I think this manuscript should not be recommended for publication in “Nature Communications.”\nThe suggestion is as follows:\n1. In lines 72-76, the authors say that “the membranes still exhibit insufficient salt rejection capacity owing to nanomaterial accumulation and the presence of defects”. However, for “MOF, acidified graphitic carbon nitride nanosheets, mesoporous hollow nanospheres” reported in reference, the flux and rejection of beyond 20 L·m-2 h-1 and 99.0%, respectively. Such membrane performance is more excellent than the flux and rejection reported by authors. The authors state that “insufficient salt rejection capacity owing to nanomaterial accumulation” is a mistake.\n2. In lines 149-150 , the authors state that “inherent nanopores into membranes filled with M-CDs significantly improved both permeability (from 15.2 to 18.3 L·m-2 h-1) and salt rejection rates (from 96.5% to 99.1%)”. I understand these membranes should be porous membrane. However, the authors do not nearly provide the pore size information about this membrane, which can obtained by the Positron Annihilation Lifetime Spectrum or the molecular weight cutoff measurement (MWCO) based on the apparent rejections of PEGs. The lack of the pore structure hinders the authors to get the hindrance effect for ion rejection or rapid water transport.\n4. In the experimental part, the author explores the effect of varying CD concentrations on the formation of membrane morphology. However, the molecular dynamics simulation only considers the effect of the existence or non-existence CDs. The design of molecular dynamics simulation is insufficient. And a series of CDs should be considered by molecular dynamics simulation.\n5. The description of molecular dynamics simulation is inadequate. The content of CTA, CDs, PA, and water content is lacking. Hence, It is not convincing that the setting of the simulation system can reflect the experimental phenomena. Also, there is no verification of molecular dynamics. Hence, comparing the density, pore size, and crosslink degree between experiment and simulation is necessary.\n6. Typically, the polymer-based membrane rejects ions based on size, Donnan, and Steric effect, but the author cannot discuss this in this work. Recently, Solution-Friction has demonstrated rapid water transport well according to the molecular dynamics simulation (Sci. Adv., 2023, 9, eadf8488). Hence, I suggest that the authors supplement the corresponding experimental characteristics and molecular dynamics simulation to demonstrate the underlying mechanism of salt rejection and fast water transport.\n7. There are many works using the NaCl rejection to evaluate the purification of seawater and brackish water. But the authors used the NaClO for evaluation. Why consider using the NaClO?"}, "author_response": "Responses to reviewers’ comments\nManuscript Number: NCOMMS-24-76359\nTitle: Overcoming the “trade-off” in reverse osmosis membranes through homologous\nmatching\nReviewer #1:\nComments to the Author: In this manuscript, the authors developed an innovative use of\nhomologous carbon dots to optimize the interfacial layer between cellulose triacetate and\npolyamide through homologous matching. It increases both salt rejection from 96.5 % to 99.1\n% and permeate flux from 15.2 to 18.3 L·m−2 h−1, far breaking through the “trade-off” in\nconventional cellulose membranes. It is interesting and can be consider for publication after\nthe following revisions.\nResponses: Thank you for your professional comments. We have carefully revised the\nmanuscript point-by-point according to your suggestions.\nComment 1: Suggest shortening abbreviations, for example, O-CCDs-0.1, M-CCDs-0.1,\nP-CCDs-0.1 can be revised to O-CDs-0.1, M-CDs-0.1, P-CDs-0.1.\nResponses: Thank you for your professional comment. The abbreviations have been\nshortened as suggested in the revised manuscript.\npublishing style of NATURE COMMUNICATIONS.\nResponses: Thank you for your professional comment. The parentheses from the serial\nnumbers of the figures have been deleted as suggested.\nComment 3: The dimensions of O-CDs, M-CDs, and P-CDs should be the average size,\nplease be precise.\nResponses: Thank you for your professional comment. The dimensions of O-CDs, M-CDs,\nand P-CDs are indeed average sizes, which have been revised as suggested in Page 8.\nComment 4: Some of the fonts in Figure 2f should be enlarged for easier viewing.\nResponses: Thank you for your suggestions. The fonts in Figure 2f have been enlarged for\neasier viewing as suggested.\nComment 5: Introduction is too lengthy and thus should be rewritten concisely.\nResponses: Thank you for your professional comment. The Introduction section has been\nshortened in the revised manuscript.\nComment 6: It is not clear why CTA is used as a substrate rather than conventional\npolysulfone and polyethersulfone supports.\n(CTA) as the substrate was primarily based on its superior hydrophilicity and chlorine\nresistance compared to conventional polysulfone (PSU) and polyethersulfone (PES) supports.\nComment 7: How to obtain the particle size distributions of O-CDs, M-CDs, and P-CDs\nneeds to be specified.\nResponses: Thank you for your professional comment. The particle size distributions of the\nO-CDs, M-CDs, and P-CDs were obtained through statistical analysis of their respective\nhigh-resolution transmission electron microscopy (HRTEM) images. Specifically, the sizes of\nindividual particles from multiple HRTEM images for each type of CDs were measured using\nimage analysis software (Image J). The measured sizes were then used to generate the\nparticle size distribution histograms presented in the manuscript. This method provides a\ndirect and reliable measurement of the core size distribution of the O-CDs, M-CDs, and\nP-CDs.\nComment 8: More discussion should be carried out in terms of the homologous matching\neffects of M-CDs on CTA/M-CDs/PA RO membranes.\nResponses: Thank you for your professional comment. As recommended, we have now\nincluded a more detailed and in-depth discussion on the homologous matching effects of\nM-CDs in the revised manuscript (Page 6).\nmembrane performance. This enhancement is attributed to the structural and chemical\nhomology between MPD-derived carbon dots (M-CDs) and the MPD monomer, which\noperates through three key mechanisms.\n(i) Enhanced Interfacial Compatibility. The shared molecular backbone promotes seamless\nintegration of M-CDs into the PA network, reducing interfacial defects.\n(ii) Modulation of Polymerization Kinetics. The M-CDs act as molecular regulators,\nmoderating the cross-linking rate to facilitate the formation of a more densely cross-linked\nand uniform PA structure.\n(iii) Creation of Optimized Water Transport Pathways. The homologous matching ensures\nthe efficient incorporation of the CDs' nanopores and hydrophilic groups into the PA matrix,\ncreating low-resistance channels for water without sacrificing selectivity.\nThank you again for your insightful comment.\nComment 9: The unit of pressure (Mpa) in the text should be corrected to \"MPa\".\nResponses: Thank you for your professional comment. The unit of pressure (Mpa) in the text\nhas been corrected to “MPa”.\nComment 10: Figure 5, the particle size is recommended to be expressed as the average\nparticle size.\nas the average particle size as suggested in Figure 5a.\nComment 11: There are some writing errors in this article, for example, “This indicates that\nthe CTA base surface successfully forms a PA layer, with CDs embedded within the layer”.\nResponses: Thank you for your professional comment. “This indicates that the CTA base\nsurface successfully forms a PA layer, with CDs embedded within the layer” has been revised\nas suggested in Page 22.\nComment 12: More discussion should be carried out in terms of the enhanced salt rejection\nand permeation mechanism of M-CDs in CTA-based membranes.\nResponses: Thank you for your professional comment. More discussion has been carried out\nin terms of the enhanced salt rejection and permeation mechanism of M-CDs in CTA-based\nmembranes in “Mechanism of enhanced salt rejection and permeation rates of M-CDs in\nCTA-based membranes” section in the revised manuscript.\nComments to the Author: The authors used a homologous matching strategy to fabricate a\nseries of CTA-based RO membranes for desalination. Although the flux and salt rejection are\nOK, the experimental characteristics and molecular dyanmics simulation can not provide the\nforming mechanism of such membranes. The pore structure and the performance of the\nmembrane are also not clearly demonstrated. I think this manuscript should not be\nrecommended for publication in “Nature Communications.”\nResponses: Thank you for your professional comments. We have carefully revised the\nmanuscript point-by-point according to your suggestions.\nComment 1: In lines 72-76, the authors say that “the membranes still exhibit insufficient salt\nrejection capacity owing to nanomaterial accumulation and the presence of defects”.\nHowever, for “MOF, acidified graphitic carbon nitride nanosheets, mesoporous hollow\nnanospheres” reported in reference, the flux and rejection of beyond 20 L·m-2 h-1 and 99.0%,\nrespectively. Such membrane performance is more excellent than the flux and rejection\nreported by authors. The authors state that “insufficient salt rejection capacity owing to\nnanomaterial accumulation” is a mistake.\nResponses: We sincerely thank the reviewer for this insightful comment and for pointing out\nthis imprecise generalization in our original manuscript. We fully agree with the reviewer that\nseveral advanced nanomaterials, including MOF, acidified graphitic carbon nitride\nnanosheets, and mesoporous hollow nanospheres, have indeed been successfully fabricated\n6\ninto RO membranes with both high flux and exceptional rejection in the pioneering studies.\nOur intention was to highlight the common fabrication challenges faced in the broader field,\nrather than to negate the excellent performance achieved in those specific instances. To\naddress this concern and to more accurately reflect the current state of research, we have\ncarefully revised the relevant paragraph in the manuscript. The revised text acknowledges the\nhigh potential of these nanomaterials, while also explaining that scalable, defect-free\nintegration can be challenging and often leads to suboptimal performance in many cases. This\nformulation better distinguishes between the ideal performance achieved under optimized\nconditions and the practical hurdles that remain. The changes made can be seen on Page 3 of\nthe revised manuscript. We believe the revised version is more precise and constructive.\nOnce again, we are grateful for the reviewer's valuable feedback, which has helped us\nimprove the quality of our work.\nComment 2: In lines 149-150 , the authors state that “inherent nanopores into membranes\nfilled with M-CDs significantly improved both permeability (from 15.2 to 18.3 L·m-2 h-1) and\nsalt rejection rates (from 96.5% to 99.1%)”. I understand these membranes should be porous\nmembrane. However, the authors do not nearly provide the pore size information about this\nmembrane, which can obtained by the Positron Annihilation Lifetime Spectrum or the\nmolecular weight cutoff measurement (MWCO) based on the apparent rejections of PEGs.\nThe lack of the pore structure hinders the authors to get the hindrance effect for ion rejection\nor rapid water transport.\nstructure, which is indeed essential for understanding the separation mechanism. In direct\nresponse to this comment, we have utilized our molecular dynamics (MD) simulations to\nquantitatively analyze the nanoscale pore structure of the formed membranes.\nAs now detailed in the revised manuscript (specifically in the new Fig. 3g and 3h and the\naccompanying discussion on Page 14-15), we performed a post-annealing statistical analysis\nof the cross-linked membrane structures generated by our simulations. The results clearly\nshow that the average pore sizes of the PA layers fall within the range of 3–4 Å. Most\nimportantly, the membrane formed with an optimal M-CDs mass fraction of 6.89% exhibits\nthe smallest average pore size (3.12 Å) and the narrowest pore size distribution (smallest\nFWHM).\nThis simulated pore size information provides a direct molecular-level explanation for the\nenhanced performance:\nThe refined and smaller pores (approaching the hydrated radius of ions like Na⁺ (~3.6 Å))\ncreate a stronger steric hindrance and sieving effect, which is the primary reason for the sharp\nincrease in salt rejection.\nThe more uniform pore network (narrower distribution), combined with the inherent\nhydrophilicity of the M-CDs, facilitates more efficient and rapid water transport through the\nmembrane, leading to the observed increase in permeability.\nWe agree with the reviewer that experimental techniques like PALS or MWCO are\nvaluable, and we acknowledge that the simulated pore size is a representative statistical\n8\nvalue. However, we believe that this MD-based analysis now provides the crucial pore\nstructure information that directly links the microstructural refinement by M-CDs to the\nmacroscopic performance enhancement, thereby fully addressing the reviewer's insightful\nconcern.\nComment 3: In the section “Effects of different M-CDs concentrations on CTA-based\nmembranes”, the authors state that “Morphological transformation may be attributed to the\ndisruption of the crosslinking reaction between MPD and TMC monomers during the IP\nprocess, facilitated by the presence of M-CDs.” Also, Fig. 6(g) and (h) represent that\nintroducing CDs causes moderate diffusion and strong interaction between CTA and M-CDs.\nRegretfully, how such diffusion and interaction phenomena can not be directly associated\nwith the crosslinking reaction. Hence, the forming mechanism of such membrane is lacking;\nthus, the morphological transformation cannot be explained. I think there exists a gap\nbetween experiments and molecular dynamics simulation.\nResponses: We are very grateful to the reviewer for this insightful comment, which has\nhelped us identify and bridge a key gap in our original mechanistic explanation. We agree\nthat it was insufficient to merely suggest a \"disruption\" of the reaction. In response, we have\nconducted a comprehensive molecular dynamics (MD) simulation study to directly probe the\ninfluence of M-CDs on the interfacial polymerization (IP) process, and the revised\nmanuscript now provides a clear, step-by-step mechanistic narrative.\ndiffusion and interaction phenomena to the cross-linking reaction and its final outcome. The\nmechanism can now be explained as follows:\n(1) M-CDs Modulate Monomer Diffusion: The simulations quantitatively show that\nM-CDs, due to their homologous structure, act as molecular anchors for MPD monomers.\nThis is evidenced by the significant accumulation of MPD around M-CDs (Fig. 3f-g), which\nslows the diffusion of MPD toward the organic interface (Fig. 3e).\n(2) Altered Diffusion Directly Governs Cross-linking Kinetics: The controlled, slower\nrelease of MPD to the reaction front directly impacts the cross-linking reaction with TMC. As\nshown in Fig. 3d, higher M-CDs concentrations lead to a decreased reaction rate and a lower\nultimate degree of cross-linking. This provides the direct causal link that was previously\nmissing: the strong MPD-M-CD interactions dictate diffusion, which in turn governs the\nreaction kinetics.\n(3) The Resulting Microstructure Explains Morphology and Performance: This modulated\nreaction kinetics results in a PA layer with a refined microstructure. Post-simulation analysis\nof the cross-linked structures reveals that the membrane formed with the optimal M-CDs\nconcentration possesses the smallest pore size (3.12 Å) and narrowest pore size distribution\n(Fig. 3g-h). This denser, more homogeneous network is the direct microstructural\nmanifestation of the altered IP process and explains the simultaneous enhancement in\npermeability and rejection.\nestablish a clear causal chain: Homologous M-CDs → Strong interaction with/sequestration\nof MPD → Slower diffusion → Moderated reaction kinetics → Formation of a denser, more\nuniform PA microstructure → Enhanced desalination performance and morphological\ntransformation. We thank the reviewer again for prompting this significant improvement in\nour work.\nComment 4: In the experimental part, the author explores the effect of varying CD\nconcentrations on the formation of membrane morphology. However, the molecular dynamics\nsimulation only considers the effect of the existence or non-existence CDs. The design of\nmolecular dynamics simulation is insufficient. And a series of CDs should be considered by\nmolecular dynamics simulation.\nResponses: Thank you for your professional comment. Accordingly, we have performed\nadditional molecular dynamics simulations to investigate the effect of CD concentration. The\nnew results have been shown in the revised manuscript (Fig. 3 and Fig. 4, Effects of different\nM-CDs concentrations on CTA-based membranes and “Mechanism of enhanced salt rejection\nand permeation rates of M-CDs in CTA-based membranes” sections). This provides a\ndefinitive molecular-scale rationale for the existence of an optimal CD loading for our\nexperimental findings, greatly enriching our discussion.\nCTA, CDs, PA, and water content is lacking. Hence, it is not convincing that the setting of the\nsimulation system can reflect the experimental phenomena. Also, it is not convincing that the\nsetting of the simulation system can reflect the experimental phenomena. Also, there is no\nverification of molecular dynamics. Hence, comparing the density, pore size, and crosslink\ndegree between experiment and simulation is necessary.\nResponses: We sincerely thank the reviewer for these critical and constructive suggestions\nregarding the molecular dynamics (MD) simulation methodology (Page 30 and Supporting\nInformation) and have incorporated discussions to validate our models (Fig. 3 and Fig. 4).\nThe key additions and clarifications are as follows:\n(1) Detailed System Composition: The revised manuscript now explicitly specifies the\ncomposition and construction details of all simulation systems.\n The initial number and configuration of MPD and TMC monomers for the\npolymerization simulation.\n The molecular structure and force field parameters for the CTA substrate.\n The atomic structure, functional groups, and number of molecules for the M-CDs and\nplain CDs.\n The exact water content (number of water molecules) and the size of the simulation box\nfor each system.\n12\n(2) Verification via Quantitative Comparison: While obtaining absolute experimental\nvalues for the density and cross-linking degree of the ultrathin PA layer is extremely\nchallenging, we have strengthened the validation of our MD models through a quantitative\nand comparative approach.\n Pore Size: As presented in the new Fig. 3g-h, the simulated pore size range of 3-4 Å for\nthe PA matrix is in excellent agreement with the widely accepted effective pore size of\npolyamide RO membranes reported in the literature (typically 3-5 Å).\n Cross-linking Degree: Although the absolute experimental cross-linking degree is\ndifficult to measure, the trend observed in our simulations—where the cross-linking\ndegree decreases with increasing M-CDs concentration—provides a perfect\nmolecular-level explanation for the experimental observation of a more refined and\ndenser membrane structure at the optimal M-CDs loading. This inverse correlation\nvalidates that our simulation correctly captures the modulating effect of M-CDs on the\npolymerization kinetics.\n System Representativeness: The simulation boxes were constructed to be large enough to\ncapture the key interfacial phenomena and were run for sufficient time to reach\nequilibrium states.\nWe believe that these substantial additions to the methodology and the direct comparison\nof key structural properties (pore size) with established literature values significantly enhance\nthe reliability and persuasiveness of our molecular dynamics simulations. They robustly\n13\nbridge the gap between the simulated models and the experimental phenomena we observed.\nThank you again for prompting us to improve this crucial aspect of our work.\nComment 6: Typically, the polymer-based membrane rejects ions based on size, Donnan,\nand Steric effect, but the author cannot discuss this in this work. Recently, Solution-Friction\nhas demonstrated rapid water transport well according to the molecular dynamics simulation\n(Sci. Adv., 2023, 9, eadf8488). Hence, I suggest that the authors supplement the\ncorresponding experimental characteristics and molecular dynamics simulation to\ndemonstrate the underlying mechanism of salt rejection and fast water transport.\nResponses: We are deeply grateful to the reviewer for this insightful suggestion, which has\nguided us to significantly deepen the mechanistic discussion of our membrane's performance.\nIn direct response, we have conducted additional molecular dynamics (MD) simulations and\nanalyses, framed within the solution-friction model as recommended, to provide a more\ncomprehensive and modern interpretation of the underlying mechanisms. These substantial\nadditions are now detailed in the revised manuscript (Fig. 3 and Fig. 4, Effects of different\nM-CDs concentrations on CTA-based membranes and “Mechanism of enhanced salt rejection\nand permeation rates of M-CDs in CTA-based membranes” sections).\nOur new analyses reveal the following key insights, which bridge our experimental\nobservations with the solution-friction theory:\n Performance Metrics under Pressure: The MD simulations under an applied pressure of\n14\nenhancement. With an optimal M-CDs content of 6.89 wt%, the membrane's permeate\nflux increased dramatically from 2.24 to 16.52 × 10⁴ L·m⁻²·h⁻¹·MPa⁻¹, while the salt\nrejection rose from 32.09% to 76.55%, directly overcoming the trade-off effect.\n Water Content and Transport Pathways: The analysis of membrane water content, a key\nparameter in the solution-friction model, shows that the incorporation of M-CDs\nincreases the water content of the PA matrix, thereby enhancing water permeability. This\naligns with the model's view that water transport is governed by the interplay of pore\nsize, solvent dimensions, and viscosity.\n Formation of a Dense/Porous Bilayer Structure: A critical finding from our density\ndistribution analysis is that M-CDs promote the formation of a distinct dense/porous\nbilayer structure. The dense layer, enriched with M-CDs on the aqueous side, enhances\nsalt rejection through intensified steric and Donnan exclusion, while the porous layer\nfacilitates water transport. This structural optimization directly reduces\nsolution-membrane friction for water while increasing it for ions.\n Molecular-Level Coordination Environment: Further validation comes from the analysis\nof water-water (O–O) and Na⁺-water (Na⁺–O) coordination numbers. The incorporation\nof M-CDs increases the O–O coordination number (indicating more extensive or\noptimized water clustering for efficient transport) while decreasing the Na⁺–O\ncoordination number (suggesting ions require more significant dehydration, a\nhigh-energy process, to enter the membrane channels). This provides direct\nmolecular-level evidence for the reduced friction for water and increased friction for\n15\nions.\nSo, the revised manuscript now provides a multi-faceted mechanism: the improved\nperformance is attributed not only to the classical size-sieving and Donnan effects but, more\nimportantly, to the M-CDs induced microstructural changes that optimize the\n\"solution-friction\" landscape—facilitating water cluster transport while imposing a greater\nenergy barrier and frictional resistance on ions. We sincerely thank the reviewer for\nsuggesting this powerful framework, which has greatly strengthened the theoretical\nfoundation of our work.\nThe related reference (Sci. Adv., 2023, 9, eadf8488) has also been cited as Ref. [43] in the\nrevised manuscript.\nComment 7: There are many works using the NaCl rejection to evaluate the purification of\nseawater and brackish water. But the authors used the NaClO for evaluation. Why consider\nusing the NaClO?\nResponses: Thank you for your professional comment. For the evaluation of chlorine\nresistance of membranes, the membranes were immersed in the prepared 2000 ppm sodium\nhypochlorite (NaClO) solution for 2 h, rinsed several times, and then tested for separation\nperformance. The chlorine resistance was assessed based on the permeate flux and NaCl\nrejection of the RO membranes before and after treatment with the NaClO solution.\n16"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The comments have been addressed. It can be accepted.", "reviewer_2": "The authors have addressed my question in the revision. I recommend acceptance. This study successfully developed cellulose triacetate (CTA)-based reverse osmosis membranes by modulating the concentration of M-CDs in the laboratory, which simultaneously enhanced both seawater permeability and salt rejection. Furthermore, MD simulations revealed that the membrane exerts a steric hindrance mechanism against Na+, providing insights for the future development of desalination membranes."}, "author_response": ""}]

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