Halloysite Nanotube/Polydopamine Nanohybrids as Clay-Based Photothermal Agents for Antibacterial Applications

Nanoparticles with light-to-heat conversion properties play vital roles in the design of photothermal materials that can be remotely heated via light activation. Halloysite nanotubes (HNTs), versatile natural clay nanoparticles, were converted into efficient photothermal agents by functionalizing them with polydopamine, a polymer with light-to-heat conversion properties. By varying the polydopamine functionalization reaction conditions such as dopamine concentration, reaction time, and the nature of the HNTs, HNT-polydopamine (HNT-PDA) nanohybrids comprising different amounts of polydopamine were obtained. HNT-PDA nanohybrids presented significant temperature elevations when irradiated with 808 nm laser light, reaching 250 °C in 2 min, and were demonstrated to be effective photothermal agents, whose light-to-heat conversion properties and the degree of light-activated temperature elevations can be easily tuned by controlling the polydopamine content through reaction conditions. The photothermal effect of HNT-PDA nanohybrids was demonstrated to be stable over multiple laser light-activation cycles, allowing their reusability. In addition to infrared laser light activation, HNT-PDA nanohybrids were also shown to be activated by other light sources of more practical importance such as a solar simulator, an infrared incandescent lamp, and a light-emitting diode (LED) lamp, demonstrating their versatility as photothermal agents. As one of the potential applications of HNT-PDA nanohybrids, their light-activated antibacterial activity was evaluated. The viability of Staphylococcus aureus (S. aureus) treated with HNT-PDA nanohybrids was reduced by 6.3 log when irradiated with infrared laser light for 5 min, whereas bacteria not treated with the nanohybrids stayed alive under the same irradiation conditions. Comprising natural, nontoxic, cost-effective components, HNT-PDA nanohybrids are promising nanoparticles as versatile clay-based photothermal agents that can be utilized in various photo-driven applications.