Nanostructured Titania Coatings for Biological Applications: Fabrication and Characterization - Biological and Biomedical Coatings: Processing and Characterization - page 218

hydrophobic cap prevents uncontrolled leaching of the hydrophilic drugs into the aque-
ous environment. By exploiting the photocatalytic properties of TiO 2 for UV induced chain
scission of the attached organic monolayer, the cap can be removed and highly controlled
release of drugs is achieved. Figure 5.14 schematically describes the procedures to fabricate
amphiphilic TiO 2 nanotubes and the drug loading methods. The procedures begin with
an anodization step followed by hydrophobic surface modification. Afterward, a second
tube (hydrophilic) layer is grown underneath the first one by a second anodization pro-
cess. The first layer is grown in an electrolyte containing glycerol/water/NH 4 F to a thick-
ness of about 750 nm and a diameter of 90 nm. Subsequently, a hydrophobic monolayer
of octadecylphosphonic acid (OPDA) is attached to the tube wall. The nanotube is then
anodized again in an ethylene glycol/NH 4 F electrolyte. The second layer consists of 1-μm-

Nanotube fabrication:

Ti

TiO 2 /Ti

Anodization

Hydrophobic

Anodization

Hydrophilic Amphiphilic TiO 2
nanotube layer

Alkyl modified TiO 2
nanotube layer

Drug delivery methods:
_ HRP
_ OPDA
_ APTES
_ HRP attached by APTES

I

II

III

TiO 2 nanotubes

IV

FIGURE 5.14
Scheme of procedure for fabricating amphiphilic TiO 2 nanotube layers and four methods for drug loading
using horseradish peroxide (HRP) as a hydrophilic model drug (I) immersion without any TiO 2 surface modi-
fication (for reference), (II) immersion after OPDA modification in upper nanotube layer (hydrophobic cap),
(III) covalently attached HRP over entire nanotube layers, and (IV) OPDA cap in upper nanotube layer and
covalently attached HRP in lower nanotube layer. (From Song et al., J. Am. Chem. Soc ., 131, 4230-4232, 2009. With
permission.)