Intelligent transport of molecular species across different barriers is critical for various biological functions and is achieved through the unique properties of biological membranes1,2,3,4. Two essential features of intelligent transport are the ability to (1) adapt to different external and internal conditions and (2) memorize the previous state5. In biological systems, the most common form of such intelligence is expressed as hysteresis6. Despite numerous advances made over previous decades on smart membranes, it remains a challenge to create a synthetic membrane with stable hysteretic behaviour for molecular transport7,8,9,10,11. Here we demonstrate the memory effects and stimuli-regulated transport of molecules through an intelligent, phase-changing MoS2 membrane in response to external pH. We show that water and ion permeation through 1T′ MoS2 membranes follows a pH-dependent hysteresis with a permeation rate that switches by a few orders of magnitude. We establish that this phenomenon is unique to the 1T′ phase of MoS2, due to the presence of surface charge and exchangeable ions on the surface. We further demonstrate the potential application of this phenomenon in autonomous wound infection monitoring and pH-dependent nanofiltration. Our work deepens understanding of the mechanism of water transport at the nanoscale and opens an avenue for the development of intelligent membranes.
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The data that support the plots within this paper and other findings of this study are available from the corresponding authors on request. Data related to AIMD and CMD simulations are available from A.M. (firstname.lastname@example.org) and N.Z. (email@example.com), respectively.
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This work was supported by the Royal Society, the Leverhulme Trust (no. PLP-2018-220), the Engineering and Physical Sciences Research Council (no. EP/P00119X/1), Graphene Flagship, Carlsberg Research Laboratory and the European Research Council (contract no. 679689). C.Y.H. acknowledges support from the China Scholarship Council. We thank the XAFS station (no. BL14W1) of the Shanghai Synchrotron Radiation Facility and R. Qin and W. Zhang for XAFS measurements. We thank V. G. Kravets, Y. Su and K.-G. Zhou for discussions. N.Z. acknowledges support from the National Key Research and Development Program (no. 2021YFC2901300). K.S.N. acknowledges support from the Ministry of Education, Singapore, through the Research Centre of Excellence award to the Institute for Functional Intelligent Materials (I-FIM, project no. EDUNC-33-18-279-V12) and the Royal Society (UK, grant no. RSRP \R \190000).A.M. acknowledges support from the European Research Council under the European Union’s Seventh Framework Programme (no. FP/2007-2013)/ERC Grant Agreement no. 616121 (HeteroIce project). We are grateful for computational support from the UK Materials and Molecular Modelling Hub, which is partially funded by EPSRC (grant nos. EP/P020194/1 and EP/T022213/1), for which access was obtained via the UKCP consortium and funded by EPSRC grant no. EP/P022561/1. We acknowledge use of the facilities at the Henry Royce Institute and UCL Grace High Performance Computing Facility (Grace@UCL), and associated support services.
The authors declare no competing interests.
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Extended data figures and tables
(a) Variation in water permeance in different reports of 1T′ MoS2 membranes with different thicknesses. Ref. (A)23; (B)21; (C)26; (D)28. The absence of thickness-permeance correlation may be attributed to the imperfect stacking of MoS2 sheets in the membranes. (b) Schematic showing the water permeation pathway in a perfectly stacked, lamellar and (c) imperfectly stacked and disordered MoS2 membranes. In a lamellar membrane, water molecules and ions flow through the interlamellar channels of MoS2. On the other hand, in imperfect stacking, the voids between the poorly stacked layers (framework defects) dominate the transport. (d) SEM cross-sectional image showing the closely packed lamellar structure of the 1T′ MoS2 membrane used in this study. Scale bar; 500 nm.
X-ray diffraction of 1T′ MoS2 membranes treated with aqueous solutions with pH changing from 1 to 12.1 (a) and 12.1 to 1 (b). (c) Schematic showing interlayer spacing (d) and available free space (δ) in base and acid treated MoS2 membranes.
(a) Water vapour adsorption isotherm of MoS2 membranes treated at different pH from 1 to 12.1. P/P0 is the ratio of vapour pressure to saturated vapour pressure at a fixed temperature. Acid-treated MoS2 membranes show negligible water uptake even at a P/P0 of 0.99. Upon increasing the pH of the treatment solution from pH 1 to 12.1 in a stepwise manner, we observe no discernible change in water vapour adsorption charesteristic of MoS2 membranes upto pH 10. Further increase in pH to 11.8 resulted in slight increase in water vapour uptake of MoS2 membranes to approx. 10 wt% at P/P0 = 0.99. However, after treating the membrane with pH 12.1 solution, we observe a huge increase in water vapour adsorption to ~38 wt% at P/P0 = 0.99. It is to be noted that the adsorption isotherm of the 1T′ MoS2 treated with base is of type II, which is typical for monolayer-multilayer adsorption in non-porous solids. The isotherm consists of two distinct steps. At low humidity, water molecules get adsorbed on to the surface forming multilayers of water inside the capillary. Then, at higher humidity, there occurs a cooperative process involving the interaction between adsorbate molecules, which leads to capillary condensation, and thus a sudden increase in water uptake. (b) Water vapour adsorption isotherm of MoS2 membranes treated at different pH from 12.1 to 1. While reducing pH of base treated membrane from 12.1 to 1, we observes a dramatic decrease in water vapour uptake below pH 4 (c) Water uptake at P/P0 = 0.99 as a function of the pH treatment, closely mimicking the hysteretic response in membrane permeability. The dashed line is a guide to the eye and arrows indicate the direction of pH change.
Schematic representation of the mechanism of hysteretic permeation of water/ion through 1T′ MoS2 membrane as a function of pH. The arrows indicate the direction of pH change. The hysteretic response of water permeation through the 1T′ MoS2 membranes can be attributed to two counteracting mechanisms involving lithium intercalation and replacement of intercalated Li+ ions by H+. During acid treatment of lithiated MoS2 membranes, the H+ ions replace the interlayer Li+ cations of negatively charged MoS2 sheets by forming S-H bonds on the MoS2 basal plane. This eliminates the excess negative charges on the MoS2 membrane and results in permeability switching. We have observed that the replacement of interlayer cations requires an H+ concentration above 0.1 mM, corresponding to pH 4. In the reverse process, lithium intercalation involves cleaving of S-H bonds, which can only happen at high pH due to the weak acidity of S-H bonds, thus creating a hysteretic response in water permeation.
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Hu, C.Y., Achari, A., Rowe, P. et al. pH-dependent water permeability switching and its memory in MoS2 membranes. Nature 616, 719–723 (2023). https://doi.org/10.1038/s41586-023-05849-4