Organosilane functionalization of halloysite nanotubes for enhanced loading and controlled release.

The surfaces of naturally occurring halloysite nanotubes were functionalized with γ-aminopropyltriethoxysilane (APTES), which was found to have a substantial effect on the loading and subsequent release of a model dye molecule. APTES was mostly anchored at the internal lumen surface of halloysite through covalent grafting, forming a functionalized surface covered by aminopropyl groups. The dye loading of the functionalized halloysite was 32% greater than that of the unmodified sample, and the release from the functionalized halloysite was dramatically prolonged as compared to that from the unmodified one. Dye release was prolonged at low pH and the release at pH 3.5 was approximately three times slower than that at pH 10.0. These results demonstrate that organosilane functionalization makes pH an external trigger for controlling the loading of guest on halloysite and the subsequent controlled release.