Influences of Hydrogen Bonding-Based Stabilization of Bolaamphiphile Layers on Molecular Diffusion within Organic...

This paper reports molecular diffusion behavior in two bolaamphiphile-based organic nanotubes having inner carboxyl groups with different inner dimeters (10 and 20 nm) and wall structures, COOH-ONT_10nm and COOH-ONT_20nm, using imaging fluorescence correlation spectroscopy (imaging FCS). The results were compared to those previously obtained in a similar nanotube with inner amine groups (NH₂-ONT_10nm). COOH-ONT_10nm, as with NH₂-ONT_10nm, were formed from a rolled bolaamphiphile layer incorporating triglycine moieties, whereas COOH-ONT_20nm consisted of four stacks of triglycine-free bolaamphiphile layers. Imaging FCS measurements were carried out for anionic sulforhodamine B (SRB), zwitterionic/cationic rhodamine B (RB), and cationic rhodamine-123 (R123) diffusing within ONTs (1-9 μm long) at different pH (3.4-8.4) and ionic strengths (1.6-500 mM). Diffusion coefficients (D) of these dyes in the ONTs were very small (0.01-0.1 μm²/s), reflecting the significant contributions of molecule-nanotube interactions to diffusion. The D of SRB was larger at higher pH and ionic strength, indicating the essential role of electrostatic repulsion that was enhanced by the deprotonation of the inner carboxyl groups. Importantly, the D of SRB was virtually independent of nanotube inner diameter and wall structure, indicating the diffusion of the hydrophilic molecule was controlled by short time scale adsorption/desorption processes onto the inner surface. In contrast, pH effects on D were less clear for relatively hydrophobic R123 and RB, suggesting the significant contributions of non-Coulombic interactions. Interestingly, the diffusion of these molecules in COOH-ONT_20nm was slower than in COOH-ONT_10nm. Slower diffusion in COOH-ONT_20nm was attributable to relatively efficient partitioning of the hydrophobic dyes into the bolaamphiphile layers, which was reduced in COOH-ONT_10nm due to the stabilization of its layer by polyglycine-II-type hydrogen bonding networks. These results show that, by tuning the bolaamphiphile structures and their intermolecular interactions, unique environments can be created within the nanospaces for enhanced molecular separations and reactions.