Influences of Hydrogen Bonding-Based Stabilization of Bolaamphiphile Layers on Molecular Diffusion within Organic Nanotubes Having Inner Carboxyl Groups

Abstract

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 (NH 2 -ONT 10nm ). COOH-ONT 10nm , as with NH 2 -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 μm 2 /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.

Fluorescence Microscopic Investigations of Molecular Dynamics in Self‐Assembled Nanostructures

“…Meanwhile, the permeability of each ONT ( D ONT ) can be assessed by averaging D local values on the ONT. ONTs in a batch exhibited similar D ONT regardless of their lengths (1–9 μm), verifying the homogeneity of their solute releasing properties [35,51] . The D ONT values for individual ONTs can be further compiled to obtain ensemble‐averaged diffusion coefficients ( D FCS ).…”

“…Rhodamine (Figure 2a) [35,49,51] and perylene diimide (PDI) (Figure 2b) [38,40,42,44,46,47] dyes having different net charges and hydrophobicity have been used to investigate the effects of steric, electrostatic, and hydrophobic interactions on their diffusion dynamics. Among them, SRB has permitted us to investigate the morphology and permeability of PEO microdomains in PS‐ b ‐PEO films owing to its selective distribution to these microdomains [39,41,43,45] .…”

“…We investigated the diffusion of three differently‐charged rhodamine dyes (Figure 2a) in individual ONTs (Figure 1c) filled with different aqueous solutions using imaging FCS (see Section 2.5 ). Figure 11a ‐c shows the effects of solution pH on the D FCS of anionic SRB, cationic/uncharged‐zwitterionic RB, and cationic R123 in single‐wall NH 2 −ONT 10nm , singe‐wall COOH−ONT 10nm , and quadruple‐wall COOH−ONT 20nm [35,51] . Basically, the diffusion of these molecules was controlled by electrostatic interactions that were manipulated by the protonation/deprotonation of the inner −NH 2 and −COOH groups, as revealed by the pH dependent changes in D FCS .…”

“…Herein, we introduce our approaches to unveiling the detailed dynamics of fluorescent solute molecules in self‐assembled nanostructured materials, including surfactant‐filled silica mesopores (Figure 1a), cylindrical block copolymer (BCP) microdomains (Figure 1b), and bolaamphiphile‐based organic nanotubes (ONTs) (Figure 1c), using advanced fluorescence methods built on SMT and FCS [12,34,35] . We have established a number of measurement and analysis methods including (1) orthogonal regression methods (ORM) for the quantitative analysis of 1D trajectories [36,37] and nanostructure characterization; [38–44] (2) FRAP‐SMT for direct comparison of ensemble and single‐molecule diffusion behavior; [45] (3) SMT with simultaneous single‐molecule emission polarization measurements (SMT‐SMEP) for quantitative measurements of translational and orientational molecular dynamics; [46–48] and (4) two‐color SMT for simultaneous measurements of the distribution and diffusion of single solvatochromic molecules [48] .…”

“…We have established a number of measurement and analysis methods including (1) orthogonal regression methods (ORM) for the quantitative analysis of 1D trajectories [36,37] and nanostructure characterization; [38–44] (2) FRAP‐SMT for direct comparison of ensemble and single‐molecule diffusion behavior; [45] (3) SMT with simultaneous single‐molecule emission polarization measurements (SMT‐SMEP) for quantitative measurements of translational and orientational molecular dynamics; [46–48] and (4) two‐color SMT for simultaneous measurements of the distribution and diffusion of single solvatochromic molecules [48] . We also employed imaging FCS for assessing molecular diffusion in individual ONTs [35,49–51] . These methods have elucidated the structural and chemical properties of nanostructures, and also have provided invaluable insights into the detailed molecular diffusion dynamics in the nanostructures.…”