Asymmetric Modification of Carbon Nanotube Arrays with Thermoresponsive Hydrogel for Controlled Delivery

In this work, bipolar electrochemistry is used to perform wireless indirect electrodeposition of two different polymer coatings on both sides of carbon nanotube arrays. Using a thermoresponsive hydrogel on one side and an inert insoluble polymer on the other side, it is possible to generate, in a single step, a nanoporous reservoir with Janus character closed on one side by a thermoresponsive membrane. The thermoresponsive polymer, poly(N-isopropylacrylamide) (pNIPAM), is generated by the local reduction of persulfate ions, which initiates radical polymerization of NIPAM. Electrophoretic paint (EP) is chosen as an inert polymer. It is deposited by precipitation because of a local decrease in pH during water oxidation. Both polymers can be deposited simultaneously on opposite sides of the bipolar electrode during the application of the electric field, yielding a double-modified Janus object. Moreover, the length and thickness of the polymer layers can be controlled by varying the electric field and the deposition time. This concept is applied to vertically aligned carbon nanotube arrays (VACNTs), trapped inside an anodic aluminum oxide membrane, which can further be used as a smart reservoir for chemical storage and release. A fluorescent dye is loaded in the VACNTs and its release is studied as a function of temperature. Low temperature, when the hydrogel layer is in the swollen state, allows diffusion of the molecule. Dye release occurs on the hydrogel-modified side of the VACNTs. At high temperatures, when the hydrogel layer is in the collapsed state, dye release is blocked because of the impermeability of the pNIPAM layer. This concept paves the way toward the design of advanced devices in the fields of drug storage and directed delivery.