Diffusion Behavior of Differently Charged Molecules in Self-Assembled Organic Nanotubes Studied Using Imaging Fluorescence Correlation Spectroscopy

Abstract

The diffusion behavior of fluorescent molecules within bolaamphiphile-based organic nanotubes (ONTs) was systematically investigated using imaging fluorescence correlation spectroscopy (imaging FCS). Anionic sulforhodamine B, zwitterionic/cationic rhodamine B, or cationic rhodamine 123 was loaded into ONTs having cylindrical hollow structures (ca. 10 nm in inner diameter) with amine and glucose groups on the inner and outer surfaces, respectively. Wide-field fluorescence video microscopy was used to acquire imaging FCS data for dye-doped ONTs in aqueous solutions of different ionic strengths (1−500 mM) at different pH (3.4−8.4). The diffusion behavior of these dyes was discussed on the basis of their apparent diffusion coefficients (D) that were determined by autocorrelating the time transient of fluorescence intensity at each pixel on an ONT. Molecular diffusion in the ONTs was significantly slowed by the molecule−nanotube interactions, as shown by the very small D (10 −1 to 10 −2 μm 2 /s). The pH dependence of D revealed that dye diffusion was basically controlled by electrostatic interactions associated with the protonation of the amine groups on the ONT inner surface. The pH-dependent change in D was observed over a wide pH range, possibly because of electrostatically induced variations in the pK a of the densely packed ammonium ions on the ONT inner surface. On the other hand, the influence of ionic strength on D was relatively unclear, suggesting the involvement of non-Coulombic interactions with the ONTs in molecular diffusion. Importantly, individual ONTs of different lengths (1−5 μm) afforded similar diffusion coefficients for each type of dye at each solution condition, implying that the properties of the ONTs were uniform in terms of solute loading and release. These results highlight the characteristics of the molecular diffusion behavior within the ONTs and will help in the design of ONTs better suited for use as drug vehicles and contaminant adsorbents.

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 ).…”

“…Specifically, we have investigated solute diffusion along flow‐aligned rod‐shaped surfactant micelles and silica mesopores in microchannels (Figure 1a), [38,40,42,52] surfactant‐templated silica mesopores in substrate‐supported thin films (Figure 1a), [36,44,46–48] cylindrical microdomains in thin polystyrene‐ block ‐poly(ethylene oxide) (PS‐ b ‐PEO) films (Figure 1b), [39,41,43,45] and substrate‐immobilized ONTs (Figure 1c) [35,49–51] . Rod‐shaped micelles with diameters of 3–10 nm can be obtained by adjusting material compositions to give a hexagonal mesophase [38,53] .…”

“…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 .…”

“…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.…”