UC Davis

Surface-molecule interactions play an essential role in loading capacity and release kinetics in nanostructured materials with high surface area-to-volume ratio. Engineering the surfaces via immobilizing functional moieties is, therefore, a versatile means to enhance the performance of drug delivery platforms with nanostructured components. Nanoporous gold (np-Au), with its high effective surface area, well-established gold-thiol chemistry, and tunable pore morphology, is an emerging material not only for drug delivery applications but also as a model system to study the influence of physicochemical surface properties on molecular loading capacity and release kinetics. Here, we functionalize np-Au with self-assembled monolayers (SAMs) of alkanethiols with varying functional groups and chain lengths and use fluorescein (a small-molecule drug surrogate) to provide insight into the relationship between surface properties and molecular release. The results revealed that electrostatic interactions dominate the loading capacity for short SAMs (two carbons). As the SAM length increases, the loading capacity displays a nonmonotonic dependence on chain length, where medium-length SAMs (six carbons) allow for higher loading, plausibly due to denser SAM surface packing. For longer SAMs (11 carbons), the steric hindrance due to long chains crowds the pores, thereby hampering fluorescein access to the deeper pore layers, consequently reducing loading capacity.