pH-Dependent biodegradable silica nanotubes derived from Gd(OH)3 nanorods and their potential for oral drug delivery and MR imaging

We report a pH dependence of degradable silica nanotubes, which dissolved to the biodegradation product monosilicic acid, Si(OH)4. The silica nanotubes, potentially acting as oral-based administration carriers, were resistant to dissolution in the extreme acidic condition of pH 1, but degraded quickly at pH 8, and the degradation rate can be tuned by tailoring the thickness of silica nanotubes with thicker nanotubes dissolving more slowly. Because Gd(OH)3 nanorods were used as templates, the silica nanotubes could be further developed as MR imaging contrast agents as well as drugs carriers. The released Gd3+ ions resulting from the etching of Gd(OH)3 nanorods were chelated by the pre-modified DOTA, yielding Gd-DOTA complexes grafted onto silica nanotubes. The Gd-DOTA grafted silica nanotubes loaded with doxorubicin revealed enhanced T1 imaging contrast and anticancer activity.