Polydopamine-Coated Halloysite Nanotubes for Sunlight-Triggered Release of Active Substances

A sunlight-triggered controlled release system comprising environmentally friendly components is presented herein. The combination of a photothermal nanocarrier that can be remotely heated by sunlight and a heat-activatable stopper that can be removed via sunlight-triggered heating of the nanocarriers resulted in a smart controlled release system. Surfaces of halloysite nanotubes (HNTs) were coated with a polydopamine (PDA) layer to create a photothermal nanocarrier that heats up when irradiated with sunlight. The resulting HNT-PDA nanocarriers were impregnated with carvacrol, a volatile essential oil as a model active substance, and were further functionalized with lauric acid, as a stopper layer. The presence of lauric acid hindering the carvacrol release from HNT-PDA nanocarriers was confirmed by differential scanning calorimetry and transmission electron microscopy imaging. HNT-PDA nanocarriers were shown to be heated to 45 °C upon sunlight irradiation for 10 min, demonstrating that their sunlight activation can trigger the melting of the lauric acid stopper. In the absence of the light trigger, the lauric acid stopper slowed down and hindered the carvacrol release. Only 40% of impregnated carvacrol was released from the lauric acid-functionalized nanocarriers in 60 days, whereas all carvacrol molecules were released in 30 days when the lauric acid stopper was not present. Release of carvacrol molecules that were entrapped in the HNT-PDA nanocarriers with the lauric acid stopper was shown to be controlled with sunlight. 10-15% of impregnated carvacrol molecules were released upon sunlight irradiation for 6 h, and carvacrol release stopped when the sunlight irradiation was turned off, where 92% of all carvacrol was released over 6 consecutive light-on/light-off cycles. The presented clay nanotube-based controlled release system comprising a photothermal nanocarrier and a heat-activatable stopper demonstrated excellent sunlight-triggered release behavior and therefore can be utilized in various applications requiring remote control of active substance release.