The precise guidance to different ions across the biological channels is essential for many biological processes. An artificial nanopore system will facilitate the study of the ion-transport mechanism through nanosized channels and offer new views for designing nanodevices. Herein we reveal that a 2.5 nm-sized, fullerene-shaped molecular cluster Li48+m K12(OH)m [UO2(O2)(OH)]60-(H2O)n (m ≈ 20 and n ≈ 310) (U60) shows selective permeability to different alkali ions. The subnanometer pores on the water-ligand-rich surface of U60 are able to block Rb(+) and Cs(+) ions from passing through, while allowing Na(+) and K(+) ions, which possess larger hydrated sizes, to enter the interior space of U60. An interestingly high entropy gain during the binding process between U60 and alkali ions suggests that the hydration shells of Na(+)/K(+) and U60 are damaged during the interaction. The ion selectivity of U60 is greatly influenced by both the morphologies of the surface nanopores and the dynamics of the hydration shells.
An actinyl peroxide cage cluster, Li48+m K12 (OH)m [UO2 (O2 )(OH)]60 (H2 O)n (m≈20 and n≈310; U60 ), discriminates precisely between Na(+) and K(+) ions when heated to certain temperatures, a most essential feature for K(+) selective filters. The U60 clusters demonstrate several other features in common with K(+) ion channels, including passive transport of K(+) ions, a high flux rate, and the dehydration of U60 and K(+) ions. These qualities make U60 (a pure inorganic cluster) a promising ion channel mimic in an aqueous environment. Laser light scattering (LLS) and isothermal titration calorimetry (ITC) studies revealed that the tailorable ion selectivity of U60 clusters is a result of the thermal responsiveness of the U60 hydration shells.
The Schulze-Hardy rule is a well-established observation in colloid science (can be derived from the DLVO theory) that demonstrates the relationship between the critical coagulation concentration (CCC) of colloids and the valence of extra counterionic electrolytes (z), with a simple mathematical relationship of CCC≈z . Here the Schulze-Hardy Rule is expanded to much smaller, nano-scaled soluble macroions in aqueous solution, by examining the stability of the macroions in the presence of additional electrolytes. The CCC values of the macroions follow the general trend of CCC≈z but the n value is significantly dependent on the surface charge density of the macroions, ranging from n=2 at very low surface charge density to n=6 at a high surface charge density. In addition, different cations with the same valence showed clear different impacts on the CCC values, with an interesting trend being connected to the Hofmeister series originally discovered in protein solutions.
The predesigned metal-organic macrocycle ZnQDB(NO) (Zn-QDB) was observed to self-assemble into a hollow, spherical, single-layered "blackberry"-type structure. The self-assembly behaviors of the Zn-QDB are significantly influenced by additional small ions. Specifically, the cations exhibit strong co-ion effects on the interaction between cationic macrocycles which are different from the previously reported co-ion effects of simple anions on anionic polyoxometalates. This unusual phenomenon is due to the unique cation-π interaction between small cations and electron-rich cavity of Zn-QDB, as confirmed by UV-vis, H NMR, and fluorescence spectra. The variation of hydrodynamic radius (R) of assemblies with the changes of solution ionic strength and the type of cations reveals the competition between counterion-mediated attraction and cation-π interaction during the self-assembly process. Furthermore, the cooperativity of cation-π interaction and π-π stacking play a vital role in enhancing the stability of the supramolecular structure.
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