Facile fabrication of polydopamine nanotubes for combined chemo-photothermal therapy

We report a novel synthesis scheme to produce polydopamine (PDA) nanotubes using halloysite clay nanotubes (HNT) as a sacrificial template. Polydopamine nanotubes were fabricated via the formation of a polydopamine coating on HNT from a dopamine solution followed by template removal. Michael Addition was used to decorate the outer surface with polyethylene glycol. Removal of the template material resulted in a bilayered organic nanotube consisting of an inner layer of polydopamine adjacent to the lumen and an outer polyethylene glycol surface. The tubes had an aspect ratio of 5.3 ± 0.6, a diameter on the order of 70 nm, and wall thicknesses on the order of 15 nm. The PDA nanotubes exhibited photothermal conversion upon irradiation at near infrared wavelengths; a temperature increase of 30 °C was observed when a 250 μg ml^-1 sample was subjected to 1 W cm^-2 density irradiation at 808 nm wavelength. Analysis indicated that the prepared tubes had slightly higher photothermal conversion efficiency than PDA nanospheres. An antitumor drug, doxorubicin, was used as a model to test the potential for drug delivery of the organic nanotubes. The nanotubes exhibited higher loading capacities than PDA nanospheres and were capable of controlled release at pHs relevant to human serum and tumors (pH 7.4 and 5.0 respectively). Drug release rates varied as a function of system pH and application of near infrared irradiation. Release of doxorubicin was enhanced at a pH of 5.0, and retarded at a pH of 7.4. Irradiation with 808 nm wavelength light also increased the release of doxorubicin.