Fluorescence Microscopic Investigations of Molecular Dynamics in Self-Assembled Nanostructures

Self-Assembled Bolaamphiphile-Based Organic Nanotubes as Efficient Cu(II) Ion Adsorbents

Self-assembled organic nanotubes (ONTs) have been actively examined for various applications such as chemical separations and catalysis owing to their well-defined tubular nanostructures with distinct chemical environments at the wall and internal/external surfaces. Adsorption of heavy metal ions onto ONTs plays an essential role in many of these applications but has rarely been assessed quantitatively. Herein, we investigated interactions between Cu2+ and single-/quadruple-wall bolaamphiphile-based ONTs having inner carboxyl groups with different inner diameters, COOH-ONT10nm and COOH-ONT20nm. We first examined the effects of Cu2+ on their nanotubular structures using SAXS, STEM, and AFM. COOH-ONT10nm was stable in aqueous Cu2+ solution in contrast to COOH-ONT20nm owing to the presence of polyglycine-II-type hydrogen bonding networks within its wall. Subsequently, we studied the Cu2+ adsorption behavior of COOH-ONT10nm by monitoring the concentration of unbound Cu2+ using linear sweep anodic stripping voltammetry. The Cu2+ adsorption was quick, attributable to efficient Cu2+ partitioning through the open ends of the ONT, followed by fast Cu2+ diffusion in the uniform, relatively large nanochannel. More importantly, the Cu2+ adsorption capacity and affinity of COOH-ONT10nm were measured under different pH conditions using the Langmuir adsorption model. The adsorption capacity was similar at the pH range examined, showing the participation of approximately 25% of the inner carboxyl groups in the adsorption. The adsorption affinity increased with pH, indicating the essential role of the deprotonated carboxyl groups in Cu2+ adsorption. Most interestingly, the Langmuir adsorption constant was significantly higher than those of previously reported synthetic adsorbents and planar monolayer based on carboxyl binding sites. The high Cu2+ affinity of the ONT was attributable to the highly dense binding sites on the well-defined nanoscale concave structure of the inner channel. These results provide a valuable guideline for designing self-assembled nanomaterials for efficient chemical separations, detection, and catalysis.

Single-Molecule Fluorescence Investigations of Solute Transport Dynamics in Nanostructured Membrane Separation Materials

Many materials used for membrane separations are composed of nanoscale structures such as pores and domains. Such nanostructures often control the solute permeability and selectivity of the separation membranes. Thus, for future development of highly efficient separation membranes, it is important to understand the structural and chemical properties of these nanostructures and also their influences on solute transport dynamics. For the last two decades, single-molecule fluorescence techniques have been used to measure the detailed dynamics of solute molecules diffusing in various nanostructured materials, giving valuable insights into molecular transport mechanisms influenced by nanoscale material heterogeneity. This Perspective discusses recent single-molecule fluorescence studies on solute diffusion in materials relevant to membrane separations, including dense polymer films and nanoporous materials. These studies have revealed the formation and properties of nanostructures and unique transport dynamics of solute molecules manipulated by their confinement and partitioning to the nanostructures, which play key roles in membrane separations. This Perspective will also point out scientific challenges toward a thorough understanding of molecular-level mechanisms in membrane separations.