Polarization Controlled Assembly of Ultrathin Thiorphan Nanostructures on ZnO Surface Facets

Abstract

Despite the importance of thiorphan as a small molecule with vital biological roles, its interactions with zinc oxide (ZnO) nanomaterials that are prospective in drug delivery and theranostic applications have not yet been sufficiently explored.Here the impact of surface polarity of different ZnO facets on thiorphan adsorption is studied both experimentally by atomic force microscopy (AFM) and angle resolved X-ray photoelectron spectroscopy (XPS) and theoretically by force field molecular dynamics (FFMD) and density functional tight binding simulations (DFTB). Polar ZnO surfaces cause the formation of thiorphan nanodots, where the size of the nanodots depends on the direction of dipoles: small (4 nm) nanodots are formed on Znface ZnO, while large (25 nm) nanodots are formed on O-face ZnO. Nonpolar ZnO surfaces cause self-assembly into layered nanoislands with characteristic 4 nm layer thickness, which subsequently merge into rigid nanolayers. The self-assembly is shown to be controlled solely by the effect of surface dipole electric field orientation and magnitude, whereas effects of surface chemistry or solution are negligible. The results thus also show a way for controlling the assembly of thiorphan and other molecular nanomaterials for diverse applications.

AFM-IN-SEM analyses of thiorphan assembly on ZnO polar and nonpolar surfaces

“…The most representative areas are shown here. Thiorphan assembly on the polar Zn-face ZnO surface results in formation of small (4 nm) nanodots [11]. Such small objects should not be detectable by SEM.…”

“…200 µL of thiorphan in ethanol solution was added to each cover and eppendorf tube was closed. After 48 h deposition the ZnO samples were rinsed with ethanol, DI water and dried with nitrogen [11].…”

“…In spite of its practical importance, thiorphan adsorption on surfaces was studied only recently [11]. By using AFM thiorphan adsorption has been found to be guided by surface polarization of ZnO surface, which results in formation of thiorphan nanodots on polar ZnO surfaces and nanoislands on nonpolar ZnO surfaces [11]. These highresolution AFM data were corroborated experimentally by angle resolved X-ray photoelectron spectroscopy and theoretically by force field molecular dynamics and density functional tight binding simulations [11].…”

“…Thiorphan ((DL-3-mercapto-2-benzylpropanoyl)-glycine) is a small bioactive molecule, which was first introduced in 1980 [8] and currently has many possible applications in medicine [9,10]. In spite of its practical importance, thiorphan adsorption on surfaces was studied only recently [11]. By using AFM thiorphan adsorption has been found to be guided by surface polarization of ZnO surface, which results in formation of thiorphan nanodots on polar ZnO surfaces and nanoislands on nonpolar ZnO surfaces [11].…”

Stability and potential degradation of the α′,β′-epoxyketone pharmacophore on ZnO nanocarriers: insights from reactive molecular dynamics and density functional theory calculations

“…[23][24][25][26][27] The typical surfaces (or low-dimensional structural motifs) exposed by ZnO, which could be possibly doped or functionalized to tune drug-carrier interaction, were considered for designing and optimizing delivery or sensor devices. [27][28][29][30] It is known that ZnO surfaces are highly reactive, able to induce dissociation of surfactants like carboxylic acids 31 and small molecules used in drug loading and delivery process, such as ethanol 32 and water. 33 The effects of water-ZnO surface interactions on the oxide surface properties can impact the adsorption of cargo species.…”