Nanopore fluctuations turn tiny mass differences into large effects

A research group including Professor Shinji Saito at the Institute for Molecular Science and the Graduate University for Advanced Studies (SOKENDAI), along with Distinguished Professor Susumu Kitagawa at Kyoto University iCeMS, theoretically clarified the physical principles converting subtle differences in molecular mass into large differences in transport speed in Soft Porous Crystals (SPCs) possessing flexibly fluctuating nanopores. The findings were published in Nature Communications.

Among metal-organic frameworks (MOFs), SPCs, which are flexible porous materials, feature entrances and internal structures of pores that fluctuate over time. In 2022, Kitagawa and colleagues experimentally demonstrated that using these SPCs resulted in diffusion speeds of light water and heavy water, which are nearly identical in size, shape, and chemical properties, differing by more than two times, making it possible to separate the two.

Separating light water and heavy water is extremely difficult due to the very small differences in molecular size and chemical properties. When molecules pass through a narrow space, the diffusion coefficient is proportional to the inverse of the square root of the molecular mass. Commonly, the ratio of diffusion coefficients between light water and heavy water was thought to remain within a difference of approximately 5% at most. The physical origin of the more than two-fold difference in diffusion speed discovered by Kitagawa and colleagues remained unclarified. No general theory has been established to explain how flexibly fluctuating nanopores generate mass-selective transport.

The research group re-conceptualized fluctuations in nanopores as "time-varying energy barriers" and constructed a theoretical framework to statistical-mechanically analyze the molecular transport process. By framing the phenomenon in this manner, it became possible to extend the concept of "resonant activation"—previously known in single-barrier passage—to "long-distance transport over periodic energy landscapes" unique to nanopores, demonstrating that transport is significantly accelerated at specific speeds of barrier fluctuation.

In nanopores, the diffusion coefficient depends on mass. The research group revealed that this mass dependence manifests as a difference in passage time over fluctuating energy barriers. Lighter molecules can pass through a temporarily open, low-barrier state in a shorter time. When the speed of nanopore fluctuation meets appropriate conditions, only the transport of lighter molecules is selectively promoted. As a result, they clarified the mechanism through which transport of lighter molecules is selectively promoted under conditions where the timing of nanopore "opening and closing" matches the passage time of lighter molecules.

Furthermore, they conducted quantum-chemical calculations for two SPCs, [Cu(IDB)] and [Cu(OptZ)], on which Kitagawa's group performed experiments. By incorporating the analysis results into their independently constructed theoretical framework, they explained the experimental fact that only [Cu (IDB)] exhibits high selectivity based on energy landscapes obtained from quantum-chemical calculations without using adjustable parameters.

This shows that the theory does not stop at an after-the-fact explanation for individual materials but captures a universal mechanism common to fluctuating nanopores. Generally, fluctuation tends to be thought of as something difficult to control that alters or disturbs structures and properties, but this theory shifts the perspective on such "fluctuation" and redefines its role in nanomaterial design.

In this study, they revealed that fluctuation itself is a new design dimension for achieving separations that are difficult to reach with static structures. Furthermore, by adjusting the ease and speed of pore fluctuation through ligand substitution or actively altering nanopore fluctuation using external fields such as electric fields, they provided theoretical design guidelines for creating highly selective separation technologies and new transport functions not only for water isotope molecules but also for molecules with extremely similar physicochemical properties.

Journal Information
Publication: Nature Communications
Title: Fluctuation-driven mass-selective transport in dynamic nanopores
DOI: 10.1038/s41467-026-75540-5