Self-Assembly of Asymmetrically Functionalized Titania Nanoparticles into Nanoshells

Abstract

Titania (anatase) nanoparticles were anisotropically functionalized in water-toluene Pickering emulsions to self-assemble into nanoshells with diameters from 500 nm to 3 μm as candidates for encapsulation of drugs and other compounds. The water-phase contained a hydrophilic ligand, glucose-6-phosphate, while the toluene-phase contained a hydrophobic ligand, n-dodecylphosphonic acid. The addition of a dilute sodium alginate suspension that provided electrostatic charge was essential for the self-limited assembly of the nanoshells. The self-assembled spheres were characterized by scanning electron microscopy, elemental mapping, and atomic force microscopy. Drug release studies using tetracycline suggest a rapid release dominated by surface desorption.

Keywords: janus particles; nanoshells (hollow spheres); pickering emulsion; self-assembly.

Figure A1

The material after stirring.

Figure A2

Titania sonicated in water or toluene compared with titania sonicated in the presence of ligands. The addition of ligands stabilizes the particles and less precipitation has occurred six hours after sonication.

Figure A3

Competition between assembly of nanoshells and sheets when higher amounts of titania and alginate were used.

Figure A4

PXRD pattern of the hydrothermally synthesized titania. Peaks labeled “a” belongs to the anatase phase.

Figure A5

TEM micrograph of hydrothermally synthesized titania.

Figure A6

Change in morphology from spherical assemblies to oval assemblies at high loadings of drugs (2 mM). Star shaped assemblies are also emerging, mostly consisting of titanium, sodium and phosphorous according to EDS. These structures are presumably formed through evaporation induced self-assembly (EISA) during drying on the carbon tape (on SEM sample holder) from suspended NPs and sodium ions directed by the organic ligands.

Figure 1

Scanning electron micrographs of self-assembled titania nanoshells. (a) Lower magnification micrograph of nanoshells dispersed over a matrice. (b) Micrograph of titania nanoshells of rather narrow size distribution. Average diameter calculated from 40 random nanoshells was ca. 1 μm (standard deviation (SD) 760 nm). (c) An example of nanoshells with higher size distribution. (d) Micrograph with several broken nanoshells. Average diameter calculated from 40 random nanoshells was ca. 680 nm (SD = 410 nm). The inset in (d) shows several incomplete nanoshells at 9000× magnification.

Figure 2

Elemental mapping of self-assembled nanoshells, showing the presence of titanium, oxygen, and phosphorous. Analysis of (a) an aggregate of self-assembled hollow spheres and (b) an individual hollow sphere.

Figure 3

Atomic force micrographs of titania nanoshells. (a) Micrographs of a few larger nanoshells. (b) An aggregate of smaller nanoshells average diameter is 420 nm (SD = 80 nm). (c) Magnified micrograph of three small nanoshells average diameter is 416 nm (SD = 55 nm). (d) Magnified micrograph of the surface of a nanoshell. Nanoshell topography is shown in (e), and (f), and a three-dimensional (3D) image of a surface in (g).

Figure 4

Proposed structure of self-assembled nanoshells. The inner side of the spheres are coated with the hydrophobic ligand (DPA), facing encapsulated toluene, and the outer side is coated with the hydrophilic ligand (G6P), facing the water phase. The alginate polymer helps stabilizing the nanoshells and may interact with the hydrophilic ligand via hydrogen bonding, and potentially via electrostatic interaction with the titania surface.

Figure 5

Release profile from the material for tetracycline in physiological sodium chloride at room temperature. Error bars are standard deviation.