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Posted: Apr 25, 2012
Nanomaterials: Peeling back the sheets
(Nanowerk News) Two-dimensional (2D) nanomaterials known as nanosheets have attracted a great deal of attention in recent years because of their large surface-to-volume ratio and unconventional properties. Graphene, for example, has found use in a wide range of applications in electronics as it displays both insulating and semiconducting properties.
Scientists have developed a variety of techniques for making nanosheets, but the fabrication of freestanding organic nanosheets remains a challenge. The current technology could either build small 2D fragments from small molecular units in solution or confine molecules or fragments in 2D geometries. Sometimes the 2D fragments might even aggregate into three-dimensional (3D) frameworks.
Yugen Zhang, Jackie Ying and co-workers at the A*STAR Institute of Bioengineering and Nanotechnology have now discovered an easy way of making organic nanosheets — by peeling layers off a porous polymer (see paper in Chemical Communications: "Mesoscopic organic nanosheets peeled from stacked 2D covalent frameworks"). They used a porous polyisocyanurate, which contains many highly reactive, terminal isocyanate groups within its outer layers. These terminal groups can react with amine-functionalised small molecules under relatively mild conditions to give nanosheets that are subsequently isolated by filtration.
"Conventional methods for making organic nanosheets involve the deposition of thin layers of materials onto a solid support," says Zhang. "Our method does not require the use of a support and is the first of its kind for making organic nanosheets."
A range of amine-containing molecules can be used to make differently functionalised nanosheets — for example, aminopropanol and D-glucosamine. The latter creates blue fluorescent nanosheets that show good water solubility. The thickness of the amorphous nanosheets is around three nanometres, and the longer the reaction time, the thinner the sheets and the smaller the dimensions of the sheets. The researchers have obtained nanosheets with thicknesses ranging from 40 to 150 nanometres.
The researchers tested how the fluorescent nanosheets perform as bioimaging materials. They found that the nanosheets had very low cytotoxicity and remained stable in water for up to four months. In addition, the fluorescent nanosheets gave off a blue light after they entered the cytoplasm of biological cells. Furthermore, the nanosheets could act as carriers for delivering hydrophobic molecules, such as the dye molecule known as 'Nile red', into the cytoplasm of cells.
In the future, the researchers aim to make various nanostructured materials using this method and study the materials' properties. "The nanosheets could also potentially be used in sensing and conducting materials," says Zhang.