Functionalization strategies to facilitate multi-depth, multi-molecule modifications of nanostructured oxides for triggered release applications

Abstract

Metal-oxide surfaces have successfully been modified to elicit surface hydrophobicity via facile application of self-assembled monolayers (SAMs) using carbohydrate molecules comprising phosphonic acid groups. In this work, we shed light on the role of the functionalization technique used for surface modification of zirconia nanotubes (ZrNT) and compare the extent of achievable hydrophobic effect as a function of application technique, immersion in bulk solution (BI) and micro-contact printing (μCP), for different molecular chain lengths. Qualitative and quantitative evaluation of ZrNT surfaces modified via μCP and BI in SAM solutions was performed combining static water-contact angle measurements, XPS and ToF-SIMS depth-profiling to provide insights into effective multi-functional depth modifications. Deviating modification depths of the nanostructured oxide could be achieved with these methods. The use of sequential functionalization strategies via BI and μCP bears a ‘proof of principle’ to create nanostructures that are able to demonstrate both multi-molecule and multi-depth modifications. Multifunctional ZrNTs, modified with two molecules in selected depths, were obtained by the combination of both modification methods and evaluated via dye release experiments aimed to simulate drug release behavior. The dispensation of loaded dye was investigated in various solvents. While no dye could be detected in aqueous environment, a complete release of the dye was achieved in ethanol, caused by a degradation of the μCP-ODPA layer. Our results indicate that the release of the cargo of multi-functional oxide nanostructures can be triggered, e.g., by alteration of the chemical environment.