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Posted: Nov 14, 2006
Photocatalytic growth process for metallic nanocages could double as biomolecular nanotagging
(Nanowerk Spotlight) Back in March Nanowerk Spotlight reported on work by Sandia researchers who developed a range of novel platinum nanostructures with potential applications in fuel and solar cells (see: Novel platinum nanostructures). Through the use of liposomal templating and a photocatalytic seeding strategy the Sandia team produced a variety of novel dendritic platinum nanostructures such as flat dendritic nanosheets and various foam nanostructures (nanospheres and monoliths). In an intriguing follow-up report on the growth of hollow platinum nanocages, they now show for the first time a one-to-one correspondence between the porphyrin photocatalyst molecules and the seed particles that go on to grow the dendrites. This indicates that the whole process might be used for nanotagging biological molecules and other structures that have been labeled with a photocatalytic porphyrin.
Hollow nanospheres are of interest because of their tunable structural features, including shell thickness, interior cavity size, and chemical compositions. In particular, metallic hollow nanospheres have drawn much attention because of their high surface area, low density, material economy, cost reduction, and in some cases surface permeability compared with solid nanospheres. These unique properties lead to a diverse range of reported biomedical, catalytic, and optical applications.
The synthesis of metallic hollow nanospheres is limited mainly to two approaches. One is the deposition of metal onto solid nanospheres of silica, latex, metal, and other materials, with subsequent removal of the templating core, typically by replacement reactions or corrosion. The other approach is the co-assembly of metallic nanoparticles with organic molecules into hollow nanospheres. A disadvantage of both approaches is that the hollow nanospheres are mainly composed of discrete nanoparticles, making them relatively unstable and apparently precluding the possibility of making hollow nanospheres larger than 100 nm in diameter with thin 2–3-nm shells.
(left) SEM and (right) STEM image of the hollow Pt nanocages (Source: Sandia/Dr.Shelnutt)
"The nanocages are different from the foam-like platinum (Pt) nanostructures and the globular platinum dendrites that we reported earlier" Shelnutt explains the recent findings to Nanowerk. "For the earlier Pt materials, large continuous nanosheets were grown on the liposome templates. In our new work, we initiated many dendritic growth centers in each liposome, then grew very small uniform-sized dendritic sheets at these growth centers letting them grow only until they touch and join with neighboring dendrites. These linked small dendrites (not much bigger than the seed particles) form a network of the Pt dendrites that preserves the spherical shape of the liposome even after the surfactant that forms the liposome is removed."
The Pt nanocages may be suitable for many applications (e.g., as catalysts and electrocatalysts) for which platinum is required to be in an open, low-density, high surface area form. Additional synthetic control over the nanocages can be realized by variation of parameters, such as light exposure, porphyrin concentration, and the size of the templating liposomes. The same method could also be used to make nanocages composed of other metals and alloys.
Besides the novel platinum nanosphere (i.e. the nanocage), photocatalytic nanotagging is a very interesting aspect of this work.
If there is a one-to-one correspondence between a porphyrin molecule and the seed (and thus Pt dendrite) that it produces and, in addition, if the seed particle remains at the location of the porphyrin molecule, then the Pt nanoparticles could be grown and used as nanotags to reveal the location of the porphyrin in the membrane.
The techniques used in this case to produce nanospheres might also be generalized to porphyrin-labeled molecules (e.g., lipids, drug molecules, proteins, or other biological structures) for growing nanometer- sized tags, which can be imaged using electron microscopy to give a high resolution picture of the spatial distribution of the porphyrin-labeled species.
Shelnutt gives an example:
"You could label a drug molecule by attaching a porphyrin, then bind the drug to its biological receptor, say on the surface of a cell. Then, adding the Pt complex and light to grow a nanometer-sized Pt particle (or small dendrite), which can be imaged with electron microscopy to reveal the location of the drug receptor molecules."
This type of nanotagging technique might be used in non-biological applications as well.