Chemists Write Invisible Gradients Into Crystal Membranes, Unlocking Directional Ion Transport and Osmotic Power

Chemists have found a way to write an invisible chemical gradient into a crystalline membrane — and the result behaves like a one-way street for ions. A team in China has transformed uniformly built covalent organic framework membranes into materials whose pore-wall chemistry changes continuously from one face to the other, all while keeping the underlying crystal perfectly aligned. The built-in asymmetry lets the membranes distinguish direction, selectively transport cations with near-unity selectivity even against a 1,000-fold concentration difference, and convert osmotic energy with a 12-fold efficiency gain over compositionally uniform analogues. The work, published in Nature Synthesis, establishes diffusion-limited topotactic conversion as a general platform for programming continuous chemical gradients into crystalline reticular membranes.

The central challenge the researchers set out to solve is subtle but fundamental. Covalent organic frameworks, or COFs, are porous crystalline polymers assembled from molecular building blocks through reticular chemistry — the principle that frameworks can be designed by linking chosen units through strong covalent bonds. To achieve long-range crystalline order, chemists typically rely on reversible bond formation, which allows the system to make and break connections until it settles into the thermodynamically most stable, perfectly ordered arrangement. But that same reversibility has a downside: it drives the material towards spatial uniformity. Any chemical difference introduced during assembly tends to be averaged out across the crystal, making it synthetically difficult to embed continuous chemical gradients into a single-phase crystalline membrane.

The team, led by Yunqing Su, Zhuozhi Lai, Sai Wang, Banglin Chen and Qi Sun, with contributors from the Hangzhou Institute of Advanced Studies at Zhejiang Normal University and the Key Laboratory of Biomass Chemical Engineering of the Ministry of Education at Zhejiang University, sidestepped this problem by decoupling two steps that normally happen together. First, they built a uniformly imine-linked, vertically aligned COF membrane using reversible chemistry — the standard route to crystalline order. Then, instead of trying to introduce chemical diversity during assembly, they subjected this precursor membrane to a diffusion-limited solid-state linkage conversion. In this second step, the imine bonds in the framework are progressively converted into β-ketoenamine linkages, a more robust and chemically distinct bond type, while the parent topology, membrane continuity and crystalline alignment are preserved throughout.

The key insight is that the conversion is limited by how fast the reacting species can diffuse through the solid framework, not by how fast the chemical reaction itself proceeds. Because diffusion through a crystalline solid is slow and directional, the conversion front advances gradually from one face of the membrane towards the other. By carefully controlling the reaction time, the researchers can kinetically trap intermediate conversion states — snapshots of the process frozen in place. The result is a through-thickness chemical gradient written into the aligned nanochannels of the membrane: one face remains rich in imine linkages while the opposite face becomes enriched in β-ketoenamine linkages, with a continuous transition between the two extremes across the membrane thickness.

The researchers confirmed this gradient using a battery of characterization techniques. Solid-state carbon-13 nuclear magnetic resonance spectroscopy tracked the progressive emergence of characteristic carbonyl and carbon–carbon double bond-to-nitrogen resonances alongside the attenuation of imine-related signals, supporting the gradual imine-to-β-ketoenamine conversion during topotactic exchange. Depth-resolved nitrogen 1s X-ray photoelectron spectroscopy, collected during argon ion sputter profiling, revealed a continuous through-thickness change in the relative abundance of β-ketoenamine and imine linkages — the ratio of the corresponding spectral components decreased from approximately 2.70 to 0.33 with increasing sputtering time. This profile is consistent with enrichment of β-ketoenamine linkages towards the tube-wall-facing region of the membrane and supports the formation of a kinetically arrested linkage-composition gradient rather than an abrupt chemical junction.

Crucially, the strategy is not limited to a single pair of linkage chemistries. The team demonstrated the approach across compatible reticular chemistries, producing gradient-encoded β-ketoenamine-, imide-, enaminone- and olefin-linked frameworks. This breadth matters because different linkage types confer different chemical properties — stability, polarity, charge distribution — and being able to program gradients across multiple linkage families opens the door to designing membranes with tailored functional landscapes for specific applications, from selective separations to energy conversion.

To understand how the gradient affects ion transport, the researchers used imine-to-β-ketoenamine conversion as a model system. The resulting axial chemical asymmetry creates a spatially varying ion–wall interaction landscape inside the aligned nanochannels. When ions move through the membrane in one direction, they encounter a different sequence of chemical environments than when they move in the opposite direction. This direction-dependent interaction landscape enables direction-dependent ion transport — the membrane can distinguish which face is exposed to a given solution and respond accordingly. Computational simulations of the potential of mean force for potassium ion translocation confirmed that the continuous-gradient membrane shows a strongly orientation-dependent transport profile, with a more favourable energy landscape when the β-ketoenamine-rich surface faces the potassium chloride reservoir. Notably, the simulations also showed that a continuous linkage gradient provides a more favourable potassium transport landscape than an abrupt chemical junction between two uniform segments, underscoring the advantage of the smooth gradient over a simple layered structure.

The practical consequences are striking. The gradient-encoded membranes achieve near-unity cation selectivity even under a 1,000-fold concentration gradient — meaning they can almost perfectly distinguish cations from anions even when one side of the membrane is exposed to a solution 1,000 times more concentrated than the other. This level of selectivity, maintained across such an extreme concentration difference, is remarkable for a synthetic membrane and approaches the kind of discrimination found in biological ion channels. The membranes also deliver a 12-fold enhancement in osmotic energy-conversion efficiency relative to compositionally uniform analogues. Osmotic energy conversion — harvesting power from the salinity difference between fresh water and seawater, sometimes called blue energy — is a promising renewable technology, and membranes that can selectively and directionally transport ions are central to its efficiency. The gradient-encoded COF membranes represent a significant step forward in this area.

Equally important is the chemical durability of the resulting materials. The membranes retained structural integrity after exposure to 12 molar hydrochloric acid — an extraordinarily harsh condition that would dissolve or degrade most synthetic membranes. This robustness stems from the β-ketoenamine linkages introduced during topotactic conversion, which are far more resistant to acid and base than the parent imine bonds. The combination of acid stability and gradient-encoded functionality means these membranes can operate in strongly acidic environments where many other advanced materials would fail, opening possibilities for applications in acid recovery, industrial wastewater treatment and electrochemical devices that operate under corrosive conditions.

This study establishes diffusion-limited topotactic conversion as a platform for programming continuous chemical gradients in crystalline reticular membranes. By decoupling lattice formation from subsequent chemical diversification, the approach overcomes the fundamental tension between reversible assembly — needed for crystalline order — and chemical asymmetry — needed for directional function. The ability to write smooth, continuous gradients into aligned nanochannels, rather than abrupt junctions between uniform segments, provides a more favourable transport landscape and superior performance. As reticular chemistry continues to mature, strategies like this one that treat the crystal lattice as a programmable substrate — capable of being re-written after assembly without destroying its order — are likely to become central to designing next-generation membranes for energy conversion, selective separations and iontronic devices that operate under conditions where conventional materials cannot survive.

Subject of Research: Materials Science

Article Title: Topotactic synthesis of gradient-encoded covalent organic framework membranes

Article References: Topotactic synthesis of gradient-encoded covalent organic framework membranes. (n.d.). https://doi.org/10.1038/s44160-026-01192-4

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01192-4

Keywords: covalent organic frameworks, topotactic synthesis, membrane technology, ion transport, osmotic energy conversion, reticular chemistry, Nature Synthesis, Topotactic, synthesis, gradient-encoded, covalent, organic