Engineered 2D Nanoplates for Pickering Emulsification and Functional Materials

Abstract

Layered zirconium phosphate (ZrP), particularly α-ZrP, is crystalline two-dimensional material with a hexagonal shape. Plenty of free hydroxide groups are available on the surface, which provides ZrP a great potential for surface modifications. The surface has been engineered to be either hydrophobic or thermosensitive. Upon surface functionalization, the applications of ZrP disks have been greatly enhanced in Pickering emulsification as well as functional materials.

First of all, after the surface modification with hydrophobic dodecyl chains, ZrP disks were exfoliated into amphiphilic Janus and Gemini ZrP-C18 nanoplates. Both Janus and Gemini nanoplates served well as Pickering emulsifiers of oil-in-water emulsions. Firstly, cancer drug nanoparticles were produced through a surfactant-free emulsification procedure using the ZrP-C18 nanoplates as Pickering emulsifier. The drug nanoparticles fabricated were submicron and relatively uniform, which were ideal designs for delivery. The cytotoxicity assay confirmed the high delivery efficiency of the docetaxel nanoparticles. Secondly, nano-encapsulated phase changing materials (NEPCMs) were fabricated using amphiphilic ZrP-C18 platelets via a two-step Pickering emulsification procedure. The resultant capsules were submicron in size with remarkable uniformity in size distribution. The method’s simplicity and low energy consumption suggest promise for scale-up and mass production.

Furthermore, ZrP was modified with the thermoresponsive polymer PNIPAM (poly N-isoproprylacrylamide) via covalent bonding using pre-irradiation method. ^60Co γ- rays irradiation produced peroxide groups on the surface which, upon heating, initiated free radical polymerization and subsequent attachment of PNIPAM. Asymmetric Janus and Gemini ZrP-PNIPAM nanoplates were obtained by exfoliation of the layered ZrP-PNIPAM. The ZrP-PNIPAM nanoplates proved to be thermosensitive Pickering emulsifiers for controlled-release applications. Similarly, the exfoliated ZrP nanoplates were modified into ZrP-PNIPAM. The ZrP-PNIPAM monolayer aqueous suspensions proved to be good candidates to study the isotropic (I) to nematic (N) phase transition of discotic liquid crystal. The I-N transition vs thickness-over-diameter aspect ratio was investigated by adjusting temperature for a single set of samples due to the thermo-responsive property of PNIPAM on the surface. The soft ZrP-PNIPAM monolayers self-assemble into nematic liquid crystals in a wider aspect ratio than do hard disks.