It is important to thoroughly understand the diffusion behavior of molecules in organic nanostructures for their applications to controlled drug delivery. In this study, we systematically investigated the diffusion behavior of differently charged dye molecules in individual bolaamphiphile-based organic nanotubes (ONTs) using imaging fluorescence correlation spectroscopy (imaging FCS). The ONTs had amino and glucose groups on the inner and outer surfaces, with inner diameters of ca. 10 nm and lengths of 2-5 µm. Anionic sulforhodamine B (SRB), zwitterionic/cationic rhodamine B (RB) or uncharged/cationic rhodamine-123 (R-123) was loaded into ONTs immobilized on glass coverslips, and their diffusion was measured in aqueous phosphate buffered saline (PBS) at different pH and ionic strength conditions. The diffusion coefficient (D) of fluorescent molecules in individual organic nanotubes was determined from the fluorescence time transient recorded at each of the pixels of the nanotube. Anionic SRB showed a decrease in D with decreasing pH from 8.4 to 4.4, and then a slight increase in D at pH 3.4. The decrease in D was attributable to the enhancement of electrostatic attraction to the protonated inner amino groups, as supported by the observation of strong charge screening at higher PBS concentration. The higher D at pH 3.4 was not expected and might reflect SRB protonation inside the nanospace. In contrast, zwitterionic/cationic RB (pKa ~ 3) and uncharged/cationic R-123 (pKa ~ 6.8) exhibited an increase in D from 6.4 to 3.4 and 8.4 to 3.4 respectively, reflecting an enhanced electrostatic repulsion between the cationic dyes and protonated amines on the inner tube surface. Interestingly, D for these dyes was not strongly influenced by PBS concentration, suggesting a significant contribution by hydrophobic effects to the diffusion behavior of these more hydrophobic molecules. It should be noted that similar D values were observed regardless of ONT length, reflecting the uniform cylindrical structures and chemical environments of the ONTs. These observations will help design ONTs better suited for controlled delivery and release of charged drugs.

Diffusion behavior of charged dye molecules in self-assembled organic nanotubes studied using imaging fluorescence correlation spectroscopy

256th National Meeting (2018)

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