Halloysite nanotubes: isocyanate moiety as a suitable monodentate anchoring group

Halloysite nanotubes (HNTs) are unique and versatile naturally occuring nanomaterials composed of double layer of aluminium, silicon, hydrogen and oxygen, with aluminol (Al-OH) groups in the internal surface and Si-O groups on the external surface. The outer surface is negatively charged and inner lumen surface positively charged. They exhibit several advantages with respect to other types of nanotubes. For example, they are inexpensive, exhibit high specific surface area, large aspect ratio, and good biocompatibility. For these reasons, HNTs are attractive nanomaterials in the vast range of biological and nonbiological applications including anticancer therapy, sustained delivery for certain agents, as template or nanoreactor for biocatalyst, and others applications. As for other types of nanomaterials, the controlled and efficient functionalization is the prerequisite for their application for example as nanovectors of drugs or biomolecules. Taking advantage of the hydroxyl groups exposed on the outer silica surface, we have explored an innovative method to load organic molecules on it. In this study, we have explored the possibility of using the isocyanate moiety as a suitable anchoring group and we have loaded bifunctional linkers endowed with orthogonal groups, one of which was an isocyanate moiety. As a result of the interaction between this group and silanol residues, a stable carbamate-like bond was formed. A series of functionalized-HNTs have therefore been obtained, which have been completely characterized by means of FT-IR, DLS, zeta potential, TGA, TEM. (Fig. 1).