Toward functionalization of ZnO nanotubes and monolayers with 5-aminolevulinic acid drugs as possible nanocarriers for drug delivery: a DFT based molecular dynamic simulation.

We have investigated the interactions between a 5-aminolevulinic acid (ALA) drug and ZnO nanostructures including ZnO monolayers and ZnO nanotubes (ZnONTs) using density functional theory (DFT) calculations. In the context of the dispersion corrected Perdew-Burke-Ernzerhof (PBE) approach, the energetics, charge transfer, electronic structure and equilibrium geometries have been estimated. As ALA is adsorbed onto/into the ZnONTs and on the ZnO monolayer with interaction energies () of -2.55/-2.75 eV and -2.51 eV, respectively, the calculated values and bonding distances (∼2 Å) reveal that the interaction type is chemisorption. The ZnO nanostructures showed promising performance in the ALA drug functionalization, taking into account the interaction energy values. The band gap almost remains unchanged for both of the substrates under consideration after ALA adsorption, and the semiconductor properties of the substrates are preserved, according to the analyzed density of states (DOSs) spectra. The interaction nature of the ALA-ZnO nanostructures according to the atom in molecule (AIM) analysis was found to be polar attraction with partial covalent bonding between O and Zn. Our DFT based molecular dynamic (MD) simulation results demonstrate that, in the aqueous solution, ALA moves toward the interior sidewall of the ZnONTs and ZnO nanosheet surface and binds to the Zn atom through its O (carbonyl/hydroxyl groups) and N atoms and the hydroxyl H atom was dissociated and binds to the O atom of the ZnO surface. However, in the case of ALA adsorption onto the outer surface of ZnONTs, only the O atoms of carbonyl groups bind to the Zn atom and the structure of the drug remains undestroyed during the adsorption. The current findings shed light on the polar drug adsorption/encapsulation behavior on/into ZnO nanostructures, which may encourage further use of ZnO-based nanomaterials in the field of drug delivery and bio-functionalized nanomaterials.