Polydopamine-Modified Nanochannels in Vertically Aligned Carbon Nanotube Arrays for Controllable Molecule Transport

We investigate the transport behavior of hydrophilic calcein molecules in the intertube nanochannels of vertically aligned carbon nanotube arrays (CNTAs). The intertube nanochannels with an average tube-to-tube distance of 18 nm are formed via a capillary-induced densification treatment. During this densification treatment, loosely packed carbon nanotubes (CNTs) in pristine arrays are drawn together like a “zipping” effect. The wettability of nanochannels in the densified CNT array (DCNTA) is greatly improved after a noncovalent modification by incorporating a hydrophilic polydopamine (Pdop) coating on the sidewalls of CNTs through the strong π–π interactions of dopamine and CNTs. Polydopamine incorporated into DCNTA makes the transport of hydrophilic calcein molecules in the nanochannels of DCNTA more efficient. The overall transport rate of calcein in Pdop-modified DCNTA (Pdop-DCNTA) is significantly enhanced as compared to DCNTA without the chemical modification. Accordingly, the apparent diffusion coefficient of calcein in the Pdop-DCNTA membranes significantly increases. After a 24 h modification, the apparent diffusion coefficient of calcein in Pdop-modified DCNTA is 1.20 × 10–4 cm2/s, and the corresponding value for unmodified DCNTA is close to 0 because the hydrophilic calcein molecules hardly diffuse through the unmodified DCNTA. The wettability of DCNTA, overall transport rate ,and apparent diffusion coefficient of calcein in DCNTA can be well controlled by Pdop content in DCNTA or chemical modification time. Moreover, the overall molecule transport rate normalized by the concentration of calcein in feeding solution (Cf) is insensitive to Cf.