Multi-Stimuli-Responsive Janus Hollow Polydopamine Nanotubes

A tubular-shaped Janus nanoparticle based on polydopamine that responds to near-infrared, magnetic, and pH stimuli is reported. The robust tubular polydopamine structure was obtained by optimizing the halloysite template-to-dopamine ratio during synthesis. The inner and outer surfaces of the tube were exposed at different steps of the template-sonication--etching process, enabling the differential surface modification of these surfaces. Poly(ethylene glycol) (PEG) and poly(N-isopropylacrylamide) (PNIPAM) were grafted to the outer and inner surface of the nanotube, respectively. The PEG-coated surface limited aggregation of the nanoparticles at elevated temperatures. The PNIPAM-coated interior enhanced doxorubicin loading and endowed the nanoparticle with temperature-responsive behavior. The deposition of precipitated Fe₃O₄ nanoparticles further modified the nanoparticles. The resulting magnetic Janus nanoparticles responded to pH, temperature, and magnetic fields. Temperature changes could be induced by near-infrared laser, and all three stimuli were found to influence release rates of adsorbed doxorubicin from the nanoparticles. The interaction of the stimuli on release kinetics was elucidated using a linear mixed model; reduced pH and NIR irradiation enhanced release while applying a static magnetic field retarded release. Furthermore, the mechanism was shifted toward Fickian behavior by applying a static magnetic field and low pH conditions. However, NIR irradiation only shifted the behavior toward Fickian behavior at low pH.